U.S. patent application number 11/661122 was filed with the patent office on 2007-11-22 for hydrogen absorbing electrode and nickel metal-hydridge battery.
This patent application is currently assigned to GS YUASA CORPORATION. Invention is credited to Toshinori Bandou, Shuichi Izuchi, Hiroaki Mori, Kazuya Okabe, Kouichi Sakamoto.
Application Number | 20070269717 11/661122 |
Document ID | / |
Family ID | 35967383 |
Filed Date | 2007-11-22 |
United States Patent
Application |
20070269717 |
Kind Code |
A1 |
Bandou; Toshinori ; et
al. |
November 22, 2007 |
Hydrogen Absorbing Electrode and Nickel Metal-Hydridge Battery
Abstract
The object of the present invention is to provide a nickel
metal-hydride battery excellent in cycle performance, high-rate
discharging ability, and output power performance, by utilizing a
hydrogen absorbing electrode comprising a hydrogen absorbing alloy
powder as an active material, which is excellent in resistance to
corrosion and high-rate discharging performance. Provided are a
hydrogen absorbing electrode comprising 100 parts by weight of a
hydrogen absorbing alloy powder which contains, as a main
component, a rare earth element and a transition metal element, and
has a saturation mass susceptibility of 1.0 to 6.5 emu/g, and 0.3
to 1.5 part by weight of an oxide or hydroxide of a rare earth
element, the oxide or hydroxide has as a main component one or two
or more rare earth elements selected from a group consisting of Dy,
Ho, Er, Tm, Yb, and Lu and is in the form of powder whose average
diameter is equal to or less than 5 .mu.m, and a nickel
metal-hydride battery comprising a nickel electrode as a positive
electrode and a hydrogen absorbing electrode as a negative
electrode.
Inventors: |
Bandou; Toshinori; (Kyoto,
JP) ; Sakamoto; Kouichi; (Kyoto, JP) ; Mori;
Hiroaki; (Kyoto, JP) ; Okabe; Kazuya; (Kyoto,
JP) ; Izuchi; Shuichi; (Kyoto, JP) |
Correspondence
Address: |
KANESAKA BERNER AND PARTNERS LLP
1700 DIAGONAL RD
SUITE 310
ALEXANDRIA
VA
22314-2848
US
|
Assignee: |
GS YUASA CORPORATION
1 inobaba-cho, Nishinosho, Kisshoin Minami-ku
Kyoyo-shi
JP
601-8520
|
Family ID: |
35967383 |
Appl. No.: |
11/661122 |
Filed: |
August 10, 2005 |
PCT Filed: |
August 10, 2005 |
PCT NO: |
PCT/JP05/14964 |
371 Date: |
February 26, 2007 |
Current U.S.
Class: |
429/218.2 ;
429/122 |
Current CPC
Class: |
H01M 10/345 20130101;
C01B 3/0057 20130101; H01M 50/538 20210101; H01M 4/48 20130101;
H01M 10/30 20130101; Y02E 60/32 20130101; H01M 4/364 20130101; H01M
4/242 20130101; H01M 50/107 20210101; H01M 4/383 20130101; Y02E
60/10 20130101; H01M 4/26 20130101; H01M 4/52 20130101 |
Class at
Publication: |
429/218.2 ;
429/122 |
International
Class: |
H01M 4/58 20060101
H01M004/58; H01M 6/00 20060101 H01M006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 26, 2004 |
JP |
2004-246550 |
Jul 4, 2005 |
JP |
2005-195372 |
Claims
1. A hydrogen absorbing electrode comprising 100 parts by weight of
a hydrogen absorbing alloy powder which contains, as a main
component, a rare earth element and a transition metal element and
has a saturation mass susceptibility of 1.0 to 6.5 emu/g, and 0.3
to 1.5 parts by weight of an oxide or hydroxide of a rare earth
element wherein said oxide or hydroxide has as a main component one
or two or more rare earth elements selected from a group consisting
of Dy, Ho, Er, Tm, Yb, and Lu and is in the form of powder whose
average diameter is equal to or less than 5 .mu.m.
2. The hydrogen absorbing electrode as described in claim 1 wherein
80 wt % or more of a rare earth element contained in said oxide or
hydroxide of a rare earth element is one or two or more selected
from a group consisting of Dy, Ho, Er, Tm, Yb, and Lu.
3. The hydrogen absorbing electrode as described in claim 2 wherein
80 wt % or more of the rare earth element contained in said oxide
or hydroxide of a rare earth element is Er.
4. The hydrogen absorbing electrode as described in claim 2 wherein
80 wt % or more of the rare earth element contained in said oxide
or hydroxide of a rare earth element is Yb.
5. The hydrogen absorbing electrode as described in claim 1 wherein
said hydrogen absorbing alloy powder is obtained by immersing a
hydrogen absorbing alloy powder containing, as a main component, a
rare earth element and a transition metal element, in an aqueous
alkaline solution at a high temperature so that its saturation mass
susceptibility is 1.0 to 6.5 emu/g.
6. The hydrogen absorbing electrode as described in claim 5 wherein
said hydrogen absorbing alloy powder is obtained by immersing a
hydrogen absorbing alloy powder in an aqueous sodium hydroxide
solution containing sodium hydroxide at a concentration of 28 to 50
wt % and at 90 to 110.degree. C.
7. The hydrogen absorbing electrode as described in claim 1 wherein
said hydrogen absorbing alloy powder has an average diameter of 10
to 30 .mu.m.
8. The hydrogen absorbing electrode as described in claim 1 wherein
said oxide or hydroxide of a rare earth element in the form of
powder has an average diameter equal to or less than 3.5 .mu.m.
9. The hydrogen absorbing electrode as described in claim 8 wherein
said oxide or hydroxide of a rare earth element in the form of
powder has an average diameter of 0.1 to 3 .mu.m.
10. A nickel metal-hydride battery comprising a nickel electrode as
a positive electrode and a hydrogen absorbing electrode as a
negative electrode, wherein said hydrogen absorbing electrode
comprises a mixture of 100 parts by weight of a hydrogen absorbing
alloy powder containing, as a main component, a rare earth element
and a transition metal element and having a saturation mass
susceptibility of 1.0 to 6.5 emu/g, and 0.3 to 1.5 parts by weight
of a powder of an oxide or hydroxide of a rare earth element which
has as a main component one or two or more rare earth elements
selected from a group consisting of Dy, Ho, Er, Tm, Yb, and Lu, and
has an average diameter equal to or less than 5 .mu.m.
11. The nickel metal-hydride battery as described in claim 10
wherein said powder of said oxide or hydroxide of a rare earth
element is a powder of an oxide or hydroxide of a rare earth
element which has as a main component at least one of Er and
Yb.
12. The nickel metal-hydride battery as described in claim 11
wherein said powder of said oxide or hydroxide of element which has
as a main component at least one of Er and Yb has an average
diameter equal to or less than 3.5 .mu.m.
13. The nickel metal-hydride battery as described in claim 10
wherein said hydrogen absorbing alloy powder has a saturation mass
susceptibility of 2 to 6 emu/g.
14. The nickel metal-hydride battery as described in claim 10
wherein 80 wt % or more of a rare earth element contained in said
oxide or hydroxide of a rare earth element is accounted for by Er
or Yb.
15. The sealed nickel metal-hydride battery as described in claim
10 obtained by inserting a rolled electrode assembly having an
upper current collecting plate attached thereon into a cylindrical
container with a bottom, closing an open end of said cylindrical
container by means of a lid, and connecting a sealing plate
composing said lid and said upper current collecting plate via a
current collecting lead whose one end is attached to the internal
surface of said sealing plate and the other end to the upper
surface of said upper current collecting plate, wherein, at least
one out of a welded point between said internal surface of said
sealing plate and one end of current collecting lead and a welded
point between the other end of current collecting lead and said
upper surface of said upper current collecting plate has been
welded by applying current through an interior of said battery by
an external power source between the positive and negative
terminals of said battery after the sealing.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hydrogen absorbing
electrode in which a hydrogen absorbing alloy powder is used as an
active material, and a nickel metal-hydride battery using the
electrode, more specifically to a hydrogen absorbing electrode
excellent in cycle performance and high-rate discharge ability at
low temperature, and a nickel metal-hydride battery using the
electrode which is improved in output power performance and cycle
life.
BACKGROUND ART
[0002] In recent years, electric motor-driven equipment that
include mobile electronic equipment such as mobile computers,
digital cameras, etc. for which compaction of size and weight is
demanded tend to rapidly increase. As a power supply of such
electric motor-driven equipment, a sealed nickel metal-hydride
storage battery is more widely used than a nickel cadmium storage
battery, lead storage battery, etc., because the former provides a
higher energy density per unit volume or unit weight, has a higher
resistance to over-charge and higher resistance to over-discharge,
and are less harmful to the environment than the latter. The
application field of a sealed nickel metal-hydride storage battery
has spread rapidly and it is now used as a power source of hybrid
electric vehicles (HEVs), and even of electric motor-driven tools
and electric toys which require high output power performance
(high-rate discharging ability) from batteries to supply power and
which have been driven heretofore by nickel cadmium batteries.
[0003] However, the aforementioned hydrogen absorbing alloy has a
number of drawbacks: it is vulnerable to corrosion due to
electrolyte; the hydrogen absorbing electrode (negative electrode)
has a poorer high-rate discharging ability and charge receptivity
than a nickel electrode (positive electrode), and thus to maintain
a balance against the positive electrode, the negative electrode
must comprise a hydrogen absorbing alloy having a larger volume
(about 1.5 time) than that of the positive electrode, which makes
it difficult to raise the energy density of the negative electrode.
The hydrogen absorbing alloy, though being highly resistant to
corrosion and having a long life, is slow in activation and, if it
is used neat as an electrode, requires a considerable time for
initial activation before it exhibits a sufficiently high
discharging activity. Specifically, it requires several tens
charge/discharge cycles, or in some cases even several hundreds
charge/discharge cycles, before it becomes sufficiently active.
[0004] To be used as a power supply of HEVs, electric motor-driven
tools and electric motor-driven toys, it is necessary for a nickel
metal-hydride battery to have a better charge/discharge cycle
performance and high-rate discharging ability.
[0005] To solve a problem involved in the delayed activation of a
hydrogen absorbing alloy, although it is highly resistant to
corrosion, many remedial methods have been proposed for promoting
the activity of a hydrogen absorbing alloy powder or hydrogen
absorbing alloy electrode. One of such methods is to subject a
hydrogen absorbing alloy powder to a surface treatment which
consists of immersing the powder in an acidic aqueous solution
having a pH of 0.5 to 3.5 (for example, see Japanese Patent
Application Publication No. 7-73878 (page 3, paragraph 0011)).
[0006] According to this patent document, the acidic treatment
removes the coat of oxides or hydroxides covering the surface of
particles of a hydrogen absorbing alloy powder, and recovers the
clean surface of the powder, which enables the improved activity of
the hydrogen absorbing electrode, and reduces the time necessary
for activation. However, this treatment is not as effective for
improving the cycle life of the battery. This is probably because,
since elements eluted by the acidic treatment are different from
the elements eluted in an aqueous solution of alkali metal which
serves as an electrolyte of a nickel metal-hydride battery, the
hydrogen absorbing alloy powder is corroded by the alkaline
solution when the hydrogen absorbing alloy powder treated by the
acidic treatment is used in the construction of a nickel
metal-hydride storage battery.
[0007] Another method consists of immersing a hydrogen absorbing
alloy powder in an aqueous sodium hydroxide solution containing
sodium hydroxide at a concentration of 30 to 80 wt % at a
temperature equal to or higher than 90.degree. C., thereby
producing an alloy powder which has a high-rate discharging ability
and cyclic performance and is suitable for electrode use (for
example, see Japanese Patent Application Publication No.
2002-256301 (page 3, paragraph 0009)).
[0008] According to the above-cited patent document, treatment with
an aqueous conc. NaOH solution at a high temperature can remove
oxides attached to the surface of the material powder more
efficiently in a shorter period than does the treatment using a KOH
aqueous solution. Furthermore, the treatment impedes the fresh
attachment of oxides to the exposed surface of the alloy powder,
thereby reducing the contact resistance of the powder, and
improving its reactivity. Thus, according to this patent document,
the time spent for procedures necessary for activating a hydrogen
absorbing alloy is reduced, and discharging ability that is
excellent from the initial phase of charge/discharge cycles is
obtained, but the cycle performance is still inadequate. The
hydrogen absorbing electrode produced by the method has a high-rate
discharging ability better than a conventional comparable hydrogen
absorbing electrode, but its high-rate discharging ability does not
necessarily reach a level sufficiently high to meet the stern
standard sought by hybrid electric vehicles (HEVs), electric
motor-driven tools, etc.
[0009] According to the last-mentioned patent document, a hydrogen
absorbing alloy is immersed in an alkaline aqueous solution before
it is installed in a battery, so as to allow, on the surface of the
hydrogen absorbing alloy, a layer to be formed which is stable to
the alkaline aqueous solution, so that the hydrogen absorbing alloy
powder can be protected against corrosion when it is installed in a
battery. However, when the storage battery undergoes
charge/discharge cycles, the alloy experiences a series of cycles
consisting of hydrogen absorption and hydrogen desorption, and in
conjunction with these cycles, the alloy repeats
expansion/shrinkage which causes the alloy powder to have strain,
and breaks the powder into finer particles. Therefore, as a result
of the repeated charge/discharge cycles, the alloy exposes its
fresh surfaces which are then exposed to the electrolyte to be
corroded by the latter, which will lead to the reduction of charge
reserve capacity. Because of this, the method described in the
second patent document is not likely to bring about a significant
improvement in the cycle performance. Furthermore, the repetition
of charge/discharge cycles leads to the rapid decline of high-rate
discharging ability. This is probably based on the following
mechanism: when the battery undergoes a series of charge/discharge
cycles, lighter rare earth elements such as La and the like, and Mn
and Al contained in the hydrogen absorbing alloy are eluted
although small in amount, to deposit, as hydroxides, on the surface
of hydrogen absorbing alloy powder, which interferes with electrode
reactions.
[0010] A third production of a hydrogen absorbing electrode is
proposed which consists of adding, prior to the preparation of a
hydrogen absorbing electrode, an yttrium (Y) compound or a compound
of a lighter rare earth element such as lanthanum (La), cerium
(Ce), praseodymium (Pr), etc. to a hydrogen absorbing alloy powder,
in order to enhance the resistance to corrosion of the hydrogen
absorbing alloy powder while maintaining the output power
performance of the hydrogen absorbing electrode (see, for example,
Japanese Unexamined Patent Application Publication No.
11-260361).
[0011] However, a sufficient life-improving effect could not be
obtained particularly at high temperature probably because the
enhanced corrosion resistance conferred by the yttrium compound to
the hydrogen absorbing alloy powder may not be sufficiently high,
or probably because the corrosion resistance enhancing effect of
the yttrium compound may be impaired by a lighter rare earth
element such as La used in combination. In any case, the
aforementioned means did not bring about a battery possessed of an
excellent high-rate discharging ability and a long cycle life.
[0012] A fourth hydrogen absorbing electrode has been proposed
which is obtained by immersing, in advance, a hydrogen absorbing
alloy powder in an alkaline or weakly acidic aqueous solution, and
adding, to the powder, a rare earth element which is less basic
than La, such as Sm, Gd, Ho, Er, Yb, etc. neatly or in the form of
a compound for mixture (see, for example, U.S. Pat. No. 6,136,473
and Japanese Patent Application Publication No. 9-7588).
[0013] According to the description given in those patent
documents, it is possible to protect a hydrogen absorbing alloy
powder against corrosion which would otherwise result during its
immersion in alkaline electrolyte, and to enhance its durability by
depositing the hydroxides or oxides of a rare earth element as
described above on the surface of the hydrogen absorbing alloy.
However, when the method is actually practiced, it is found in some
cases that the internal resistance of hydrogen absorbing electrode
increases which may lead to the decline of output power
performance. When it is required to add a compound of a rare earth
element in the form of powder to a hydrogen absorbing alloy powder,
the effect of the size of particles of the added compound upon the
performance of the resulting electrode has been neglected, and a
rare earth element compound in the form of coarse particles have
been used. This probably explains the reason why addition of a
particulate compound of a rare earth element did not produce a
satisfactory result as expected.
[0014] For example, to be used for a power supply of an HEVs, a
battery preferably has an output power performance of producing
1400 W/kg or more at 25.degree. C. The output power performance at
25.degree. C. of a conventional cylindrical nickel metal-hydride
battery, however, is as low as 1000 W/kg. As shown in FIG. 4, a
conventional cylindrical nickel metal-hydride battery comprises a
lid which serves as one terminal (positive electrode terminal) out
of the two terminals (the lid comprises a knob-like cap 6, a
sealing plate 0, and a valve 7 provided in a space enclosed by the
cap 6 and sealing plate 0; a gasket 5 is attached to the periphery
of sealing plate 0; and the open end of a cylindrical container 4
with a bottom is folded tightly around the periphery of the lid so
that the lid and the end of container are brought into air-tight
and contact via gasket 5), wherein an upper current collecting
plate 2 (current collecting plate to serve as the positive
electrode) attached to the upper ends of a rolled electrode
assembly 1 is connected to the sealing plate 0 via a ribbon-like
current collecting lead 12 as shown in FIG. 5. In the manufacture
of a conventional battery, one end of ribbon-like current
collecting lead 12 is attached by welding to the internal surface
of sealing plate 0; the other end of current collecting lead 12 is
attached by welding to the upper current collecting plate 2; and
then the lid is applied to the open end of container 4 to close the
open end. Therefore, the current collecting lead 12 should have a
curvature to produce an extra length. Because of this, the current
collecting lead 12 connecting a welded point provided on the
internal surface of sealing plate 0 with another welded point
provided on the upper current collecting plate 2 usually has a
length 6 to 7 times as large as the distance between sealing plate
0 and upper current collecting plate 2, which increases the
electric resistance of the current collecting lead itself. This may
act as one of the causes responsible for the degraded output power
performance of a nickel metal-hydride battery.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0015] The present invention was proposed to give a solution to the
problems described above, and provides a nickel metal-hydride
battery incorporating a hydrogen absorbing electrode which, because
of its containing a hydrogen absorbing alloy powder as an active
material, is highly resistant to the corrosion due to electrolyte
and has an excellent high-rate discharging ability, characterized
in that it is excellent in cycle performance, high-rate discharging
activity, and output power performance.
Means for Solving Problem
[0016] The present invention gives a solution to the above problems
by conferring the following features to a hydrogen absorbing
electrode.
[0017] (1) A hydrogen absorbing electrode comprising 100 parts by
weight of a hydrogen absorbing alloy powder which contains, as a
main component, a rare earth element and a transition metal
element, and has a saturation mass susceptibility of 1.0 to 6.5
emu/g, and 0.3 to 1.5 parts by weight of an oxide or hydroxide of a
rare earth element, wherein said oxide or hydroxide has as a main
component one or two or more rare earth elements selected from a
group consisting of Dy, Ho, Er, Tm, Yb, and Lu and is in the form
of powder whose average diameter is equal to or less than 5 .mu.m
(Claim 1).
[0018] The saturation mass susceptibility described above is
determined as follows: 0.3 g of a hydrogen absorbing alloy powder
is precisely weighed to serve as a sample which is measured using a
vibrating sample magnetometer in magnetic fields up to 5 kOe. When
a hydrogen absorbing electrode prepared according to the invention
is installed in a nickel metal-hydride storage battery, the
electrode will exhibit a saturation mass susceptibility to fall in
the range described above, after it has undergone at least 30
cycles of charges/discharges including activation of the storage
battery.
[0019] (2) The hydrogen absorbing electrode as described in
paragraph (1) wherein 80 wt % or more of a rare earth element
contained in said oxide or hydroxide of a rare earth element is one
or two or more selected from a group consisting of Dy, Ho, Er, Tm,
Yb, and Lu (Claim 2).
[0020] (3) The hydrogen absorbing electrode as described in
paragraph (2) wherein 80 wt % or more of the rare earth element
contained in said oxide or hydroxide of a rare earth element is Er
(Claim 3).
[0021] (4) The hydrogen absorbing electrode as described in
paragraph (2) wherein 80 wt % or more of the rare earth element
contained in said oxide or hydroxide of a rare earth element is Yb
(Claim 2).
[0022] (5) The hydrogen absorbing electrode as described in anyone
of paragraphs (1) to (4) wherein said hydrogen absorbing alloy
powder is obtained by immersing a hydrogen absorbing alloy powder
containing, as a main component, a rare earth element and a
transition metal element, in an aqueous alkaline solution at a high
temperature so that its saturation mass susceptibility is 1.0 to
6.5 emu/g (Claim 5).
[0023] (6) The hydrogen absorbing electrode as described in
paragraph (5) wherein said hydrogen absorbing alloy powder is
obtained by immersing a hydrogen absorbing alloy powder in an
aqueous sodium hydroxide solution containing sodium hydroxide at a
concentration of 28 to 50 wt % and at 90 to 110.degree. C. (Claim
6).
[0024] (7) The hydrogen absorbing electrode as described in anyone
of paragraphs (1) to (6) wherein said hydrogen absorbing alloy
powder has an average diameter of 10 to 30 .mu.m (Claim 7).
[0025] (8) The hydrogen absorbing electrode as described in anyone
of paragraphs (1) to (7) wherein said oxide or hydroxide of a rare
earth element in the form of powder has an average diameter equal
to or less than 3.5 .mu.m (Claim 8).
[0026] (9) The hydrogen absorbing electrode as described in
paragraph (8) wherein said oxide or hydroxide of a rare earth
element in the form of powder has an average diameter of 0.1 to 3
.mu.m (Claim 9).
[0027] (10) A nickel metal-hydride battery comprising a nickel
electrode as a positive electrode and a hydrogen absorbing
electrode as a negative electrode, wherein said hydrogen absorbing
electrode comprises a mixture of 100 parts by weight of a hydrogen
absorbing alloy powder containing, as a main component, a rare
earth element and a transition metal element, and having a
saturation mass susceptibility of 1.0 to 6.5 emu/g, and 0.3 to 1.5
parts by weight of a powder of an oxide or hydroxide of a rare
earth element, which has as a main component one or two or more
rare earth elements selected from a group consisting of Dy, Ho, Er,
Tm, Yb, and Lu, and has an average diameter equal to or less than 5
.mu.m. (Claim 10).
[0028] (11) The nickel metal-hydride battery as described in
paragraph (10) wherein said powder of said oxide or hydroxide of a
rare earth element is a powder of an oxide or hydroxide of a rare
earth element which has as a main component at least one of Er and
Yb (Claim 11).
[0029] (12) The nickel metal-hydride battery as described in
paragraph (11) wherein said powder of said oxide or hydroxide of a
rare earth element which has as a main component at least one of Er
and Yb has an average diameter equal to or less than 3.5 .mu.m
(Claim 12).
[0030] (13) The nickel metal-hydride battery as described in any
one of paragraphs (10) to (12) wherein said hydrogen absorbing
alloy powder has a saturation mass susceptibility of 2 to 6 emu/g
(Claim 13).
[0031] (14) The nickel metal-hydride battery as described in any
one of paragraphs (10) to (13) wherein 80 wt % or more of a rare
earth element contained in said oxide or hydroxide of a rare earth
element is accounted for by Er or Yb (Claim 14).
[0032] (15) The sealed nickel metal-hydride battery as described in
any one of paragraphs (10) to (14) obtained by inserting a rolled
electrode assembly having an upper current collecting plate
attached thereon into a cylindrical container with a bottom,
closing an open end of said cylindrical container by means of a
lid, and connecting a sealing plate composing said lid and said
upper current collecting plate via a current collecting lead whose
one end is attached to the internal surface of said sealing plate
and the other end to the upper surface of said upper current
collecting plate, wherein, at least one out of a welded point
between said internal surface of said sealing plate and one end of
current collecting lead and a welded point between the other end of
current collecting lead and said upper surface of said upper
current collecting plate has been welded by applying current
through an interior of said battery by an external power source
between the positive and negative terminals of said battery after
the sealing.
Effect of the Invention
[0033] According to an aspect of the invention as described in
Claims 1 and 2, it is possible to provide a hydrogen absorbing
electrode for a nickel metal-hydride battery which has a discharge
capacity as good as that of a conventional one and is excellent in
high-rate discharging ability and charge/discharge cycle
performance. (Claim 15)
[0034] According a second aspect of the invention as described in
Claim 3, it is possible to provide a hydrogen absorbing electrode
for a nickel metal-hydride battery which is particularly excellent
in high-rate discharging ability.
[0035] According to a third aspect of the invention as described in
Claim 4, it is possible to provide a hydrogen absorbing electrode
for a nickel metal-hydride battery which is particularly excellent
in charge/discharge cycle performance.
[0036] According to a fourth aspect of the invention as described
in Claims 5 to 7, it is possible to provide a hydrogen absorbing
electrode for a nickel metal-hydride battery which is excellent in
high-rate discharging ability.
[0037] According to a fifth aspect of the invention as described in
Claims 8 and 9, it is possible to provide a hydrogen absorbing
electrode for a nickel metal-hydride battery which is particularly
excellent in charge/discharge cycle performance.
[0038] According to a sixth aspect of the invention as described in
Claims 10 and 11, it is possible to provide a nickel metal-hydride
battery which exhibits an excellent charge/discharge cycle
performance, while maintaining its high output power
performance.
[0039] According to a seventh aspect of the invention as described
in Claim 12, it is possible to provide a nickel metal-hydride
battery which exhibits a particularly excellent charge/discharge
cycle performance.
[0040] According to an eighth aspect of the invention as described
in Claim 13, it is possible to provide a nickel metal-hydride
battery which is excellent both in output power performance and
cycle performance.
[0041] According to a ninth aspect of the invention as described in
Claim 14, it is possible to provide a hydrogen absorbing alloy
powder which is particularly excellent in the resistance to
corrosion.
[0042] According to a tenth aspect of the invention as described in
Claim 15, it is possible to provide a nickel metal-hydride battery
which exhibits a high output power performance in conjunction with
the effects obtained from the use of a hydrogen absorbing electrode
as described in Claims 10 to 14.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 is a schematic diagram for showing the structure of a
nickel metal-hydride battery prepared according to the invention,
and method for connecting one end of an current collecting lead to
an upper current collecting plate by welding.
[0044] FIG. 2 shows an exemplary current collecting lead to be
applied to a nickel metal-hydride battery of the invention.
[0045] FIG. 3 shows an exemplary upper current collecting plate to
be applied to a nickel metal-hydride battery of the invention.
[0046] FIG. 4 is a schematic diagram for showing the sectional
structure of a conventional cylindrical nickel metal-hydride
battery with parts of key point.
[0047] FIG. 5 is a schematic diagram for showing a ribbon-like
current collecting lead.
REFERENCE NUMERALS
[0048] 0: Sealing plate
[0049] 1: electrode assembly
[0050] 2: Upper current collecting plate
[0051] 3: Lower current collecting plate
[0052] 4: Container
[0053] 8: Main lead
[0054] 9: Supplementary lead
[0055] 10, 11, 14: Projections
[0056] 12. Ribbon-like lead
[0057] A, B: Output terminals for an external power source
(electric resistance welder)
[0058] P1: Welded point between current collecting lead and upper
current collecting plate
[0059] P2: Welded point between lower current collecting plate and
the bottom of container
BEST MODE FOR CARRYING OUT THE INVENTION
[0060] The composition of a hydrogen absorbing alloy used for the
construction of a hydrogen absorbing electrode of the invention is
not limited to any specific one. May be used any hydrogen absorbing
alloy that contains, as a main component, a rare earth element such
as La, Ce, Pr, Nd, etc., and a transition metal element including
nickel, and has a crystalline structure represented by AB.sub.5, or
hydrogen absorbing alloy that contains, as a main component, Mg and
nickel, and has a crystalline structure represented by AB.sub.3 or
AB.sub.3.5, or hydrogen absorbing alloy that contains, as
components, Ti, V, and Cr, and has a structure represented by
AB.sub.2.
[0061] When the alloy is a hydrogen absorbing alloy having a
structure represented by AB.sub.5 or MmNi.sub.5 (Mm refers to a
misch-metal representing a mixture of rare earth elements), it is
preferable to substitute part of nickel for Co, Mn, Al, or Cu,
because the resulting alloy will exhibit an excellent cycle life
performance and high discharge capacity.
[0062] It is also preferred to add, as an additive, an oxide or
hydroxide of one or two or more rare earth elements selected from a
group consisting of Dy, Ho, Er, Tm, Yb, and Lu in the form of
powder to a hydrogen absorbing alloy in the form of powder, with
the former being 0.3 to 1.5 parts by weight with respect to 100
parts by weight of the latter. Of those rare earth elements,
addition of Er or Yb is particularly preferred, because an Er-added
hydrogen absorbing electrode exhibits an excellent high-rate
discharging ability, while a Yb-added hydrogen absorbing electrode
exhibits an excellent cycle performance.
[0063] It is also preferred to allow a hydrogen absorbing alloy
powder to contain hydrogen at 5% or more with respect to its full
hydrogen absorbing capacity, because then it is possible to shift
the potential of the hydrogen absorbing alloy powder to a far lower
electronegative potential at which Ni or Co begins to dissolve, and
thus to greatly reduce the elution of Ni or Co when the hydrogen
absorbing alloy is subjected to an immersion treatment. The
hydrogen absorbing capacity mentioned above refers to the amount of
hydrogen absorbed by a hydrogen absorbing alloy powder at an
equilibrium hydrogen pressure (plateau area) of a PCT curve taken
at 60.degree. C. Furthermore, by allowing a hydrogen absorbing
alloy to absorb hydrogen prior to its being subjected to immersion
treatment, it is also possible to efficiently produce surface
layers, probably because the prior absorption of hydrogen by the
hydrogen absorbing alloy leads to the cracking of surfaces of the
hydrogen absorbing alloy powder, and thus to obtain a hydrogen
absorbing alloy powder having a sufficient activity by subjecting
the powder to a minimum immersion treatment, which minimizes the
loss of the content of alloy.
[0064] A conventional hydrogen absorbing alloy has a saturation
mass susceptibility less than 0.1 emu/g. In contrast, a hydrogen
absorbing alloy to be used in a hydrogen absorbing alloy electrode
of the invention has a saturation mass susceptibility of 1.0 to 6.5
emu/g, preferably 2 to 6 emu/g, more preferably 3 to 4 emu/g.
[0065] According to the invention, it is preferable to subject a
hydrogen absorbing alloy powder to a surface reforming treatment,
because it is possible by so doing to raise the saturation mass
susceptibility of hydrogen absorbing alloy powder which is usually
0.1 emu/g or less, to a level equal to or higher than 1 emu/g, and
to obtain, by using such a hydrogen absorbing alloy powder, a
nickel metal-hydride battery exhibiting an excellent output power
performance. It is particularly preferred to raise the saturation
mass susceptibility of a hydrogen absorbing alloy powder to 2 emu/g
or higher, because then it is possible to obtain, by using such a
hydrogen absorbing alloy powder, a battery exhibiting an extremely
excellent output power performance. The reason why a hydrogen
absorbing alloy powder having a saturation mass susceptibility
equal to or higher than 1 emu/g can provide a battery exhibiting an
excellent output power performance remains unclear, but from the
observation that, on the surface of a hydrogen absorbing alloy
powder having its saturation mass susceptibility enhanced, an
Ni-rich phase consisting of a stack of layers 50 nm in thickness is
detected, it seems likely that the Ni-rich phase acts as a catalyst
for promoting the charge-transfer reaction on the surface of the
hydrogen absorbing alloy. It is also possible for said phase to
offer a channel through which accelerate the speed of the hydrogen
diffusion within the internal space of hydrogen absorbing alloy
powder. The Ni-rich laminar phase formed on the surface of a
hydrogen absorbing alloy powder will be referred to as a catalyst
layer hereinafter.
[0066] In this invention, a hydrogen absorbing alloy powder is
immersed in an alkaline aqueous solution having a specified
concentration for a specified period to enhance the saturation mass
susceptibility of the powder. Immersing a hydrogen absorbing alloy
powder in an alkaline aqueous solution at a high temperature allows
the coat of oxides and hydroxides developed on the surface of
hydrogen absorbing alloy powder to be removed by dissolution in a
characteristic manner: rare earth elements, and Al and Mn contained
in the hydrogen absorbing alloy powder which are soluble to the
alkaline solution dissolve readily, and Co which is hardly soluble
to the aqueous sodium hydroxide solution and Ni which is stable to
the solution are left where they are. The Co and Ni when combined
with rare earth elements or elements such as Mn or Al to form
alloys do not exhibit magnetism, but, when they are dissolved to be
isolated elements, they come to exhibit magnetism. Therefore, when
a hydrogen absorbing alloy powder is treated at a high temperature
with an aqueous sodium hydroxide solution, the hydrogen absorbing
alloy powder will have an enhanced saturation mass susceptibility.
Dissolution of rare earth elements to the solution will take place
at every interface between the hydrogen absorbing alloy and the
treatment solution. Thus, if the surface of hydrogen absorbing
alloy powder has cracks, dissolution of rare earth elements will
take place at the surface of those cracks as well as at the normal
surface of the powder. On the surface of cracks as well as on the
normal surface, there develop crusts containing rich amounts of Co
and Ni (conversely, meager amounts of rare earth elements, and Mn
and Al) as compared with the interior of the powder in the form of
a stack of layers which form a catalyst layer.
[0067] Ni is a metal which is originally highly resistant to
corrosion in an alkaline aqueous solution. However, the rich
content of Ni in the catalyst layer does not enhance the resistance
of hydrogen absorbing alloy powder to corrosion as expected,
probably because the catalyst layer is porous. An analysis made on
a battery which has reached the end of cycle life reveals that the
most important factor responsible for the deterioration of the
battery is corrosion of the hydrogen absorbing alloy included in
the negative electrode by the electrolyte.
[0068] To inhibit corrosion due to the reaction of the alloy with
the electrolyte, anti-corrosion inhibitors were added to the
hydrogen absorbing electrode as a trial. It was found that addition
of an oxide or hydroxide of an element selected from so-called rare
earth elements such as Y and lanthanoids, among them particularly
Dy, Ho, Er, Tm, Yb, and Lu is effective for the purpose.
Particularly, when an oxide or hydroxide of Er or Yb was added in
the form of powder or fine powder in particular, the resulting
alloy electrode was found to exhibit a surprisingly high
anti-corrosion activity.
[0069] In this invention, it is possible to obtain a hydrogen
absorbing electrode having a high anti-corrosion activity and
high-rate discharging ability by allowing a hydrogen absorbing
alloy powder to have a saturation mass susceptibility to fall
within a range as specified above, and by adding a compound of a
rare earth element as specified above to the hydrogen absorbing
alloy powder. Immersion treatment using an alkaline solution
provides the formation of a catalyst layer on the surface of a
hydrogen absorbing alloy powder: the catalyst layer then increases
the saturation mass susceptibility of the powder to fall within a
range as specified above, offers a channel through which protons
can enter the interior of the hydrogen absorbing alloy powder, and
serves as a reaction field, thereby enabling an electrode prepared
from the hydrogen absorbing alloy powder to have a heightened
activity, and a battery incorporating the electrode to exhibit an
excellent high-rate discharging ability.
[0070] Incidentally, a battery whose saturation mass susceptibility
is less than 1.0 emu/g may not provide a sufficiently good
high-rate discharging ability because of the inadequate formation
of a catalyst layer. Conversely, a battery whose saturation mass
susceptibility exceeds 6.5 emu/g may have a lowered discharge
capacity because of the reduction of hydrogen available for
absorption/desorption.
[0071] The catalyst layer prevents elements such as rare earth
elements which are readily soluble to the electrolyte, from coming
into direct contact with the electrolyte, and provides a reaction
field of electrode which can take place which are necessary for the
charge-transfer reaction. Accordingly, the catalyst layer
preferably has a uniform thickness and high dense, and forms a
continuous superficial layer. It is more preferred to use, for
immersion treatment, an aqueous NaOH solution than an aqueous KOH
solution, because with the former solution it is possible to obtain
a hydrogen absorbing electrode better both in high-rate discharging
performance and charge/discharge cycle performance. This is
probably because when treated with the former solution, the
catalyst layer more uniform and having a more continuity is formed
on the surface of hydrogen absorbing alloy. The aqueous NaOH
solution is most suitable for the formation of a uniform
breach-free superficial layer of all the alkaline solutions. Use of
the NaOH aqueous solution is also preferred because it contracts
the time necessary for treatment: for example, treatment with an
NaOH aqueous solution proceeds at a speed two times as high as the
treatment with a comparable LiOH aqueous solution.
[0072] Treatment with an aqueous NaOH solution proceeds more
rapidly as the concentration of the solution becomes higher. The
treatment speed begins to increase when the concentration of NaOH
becomes equal to or higher than 28 wt %. The solution containing
NaOH at a concentration of 28 wt % or higher is preferred, because
it can contract the time necessary for treatment. Increasing the
concentration of NaOH over 50 wt %, however, brings about a
problem: when such an aqueous NaOH solution is cooled to room
temperature, sodium hydroxide will precipitate and it becomes
difficult to separate sodium hydroxide from a treated hydrogen
absorbing alloy powder Making allowance for this, it is preferable
to adjust the concentration of NaOH of an aqueous NaOH solution
used for immersion treatment to fall in the range of 28 to 50 wt
%.
[0073] The speed at which immersion treatment proceeds varies
greatly depending on the temperature (temperature of immersion
treatment) of a treatment solution. To increase the speed of
treatment, it is preferred to keep the temperature of treatment
solution equal to or higher than 90.degree. C. It is more
preferable to keep the temperature of treatment solution equal to
or higher than 100.degree. C. because then the speed of treatment
increases dramatically. However, it is preferred to keep the
temperature of treatment solution below its boiling point, because
if the temperature of treatment solution is above the boiling point
of the solution, the reaction speed will become so high that
control of the formation of a catalyst layer will become difficult.
It is more preferred to keep the temperature of treatment equal to
or lower than 110.degree. C.
[0074] If the speed of treatment varies during treatment, it will
cause the formation of a superficial layer on the surface of a
hydrogen absorbing alloy powder to proceed unevenly, which may
exert an adverse effect on the function of a resulting electrode.
To keep the speed of treatment at a steady level, the treatment
solution is preferably stirred during treatment, so that powdery
hydrogen absorbing alloy will not sink to the bottom of a vessel.
By stirring, it is possible to keep uniform the distributions of
alkaline concentration and temperature within the treatment
vessel.
[0075] To form a uniform superficial layer having a uniform
thickness and high dense, it is also preferred to keep the
temperature of treatment solution in the range of 90 to 110.degree.
C. as described above. Specifically, it is preferred to start the
treatment at a temperature within the above range, and keep the
variation of temperature during treatment at .ltoreq..+-.3.degree.
C., more preferably at .ltoreq..+-.2.degree. C. It is preferred to
start the treatment at an NaOH concentration in the range of 28 to
50 wt %, and keep the variation of concentration during treatment
at .ltoreq..+-.5 wt %, more preferably at .ltoreq..+-.3 wt %.
[0076] A hydrogen absorbing alloy powder having a saturation mass
susceptibility of 1.0 to 6.5 emu/g according to the invention has a
superficial layer formed thereon whose thickness is about 50 to 400
nm (based on the observation via a Focused Ion Beam device of a
cross-section of a hydrogen absorbing alloy powder). The time
necessary for immersion treatment is about 0.9 to 5.5 hours. The
time necessary for immersion treatment is not limited to any
specific range, but may be changed as appropriate to allow a
hydrogen absorbing alloy powder to exhibit, after treatment, a
saturation mass susceptibility in the range of 1.0 to 6.5
emu/g.
[0077] When a hydrogen absorbing alloy powder is treated with an
alkaline aqueous solution, rare earth elements such as La residing
on the surface of the powder are once eluted into the solution and
then deposit on the surface of the alloy powder as hydroxides.
[0078] If a hydrogen absorbing alloy powder having the hydroxides
of rare earth elements deposited on its surface is used as it is as
an electrode, the deposit of hydroxides will interfere with the
electron conductions at the interface where the particles of
hydrogen absorbing alloy powder come into contact with each other,
and contact of hydrogen absorbing alloy powder with electrolyte,
thus inhibiting the charge/discharge reaction of hydrogen absorbing
electrode, and thus the apparent capacity of battery will be
reduced. In addition, since the powder has an increased resistance,
the high-rate discharging ability of battery will be attenuated. To
avoid this, it is preferred to remove these hydroxides of rare
earth elements.
[0079] In order to remove the hydroxides of rare earth elements
deposited on the surface of a hydrogen absorbing alloy powder, a
method of separating and removing the hydroxides of rare earth
elements by utilizing the sedimentation speed in the aqueous
solution after the hydrogen absorbing alloy powder is exposed to
ultrasonic wave and the hydroxides of rare earth elements are
peeled from the alloy after the immersion treatment solution is
removed by filtration, (specifically, the method in which flowing
water is carried away from the bottom of a tank and the impurities
of rare earth elements which are more reluctant to precipitate are
mixed into the flowing water to be removed) or a method by
utilizing a difference of particle diameter (the method in which
small-sized particles are removed by filtration because diameters
of the impurities of rare earth elements are smaller than that of
the alloy) is preferable, because composition of a surface of
powder is not changed and resistance of the powder to corrosion is
not changed. Although there is also a method in which a hydrogen
absorbing alloy powder is taken into contact with a diluted
solution of hydrochloric acid or acetic acid and the hydroxides of
rare earth elements are filtrated while being dissolved, the
superficial layer developed during the immersion treatment may be
dissolved by the acid, its composition may be altered and the
corrosion resistance of the powder to alkaline solution is
lowered.
[0080] While a hydrogen absorbing alloy is immersed in an aqueous
alkaline solution, hydrogen gas evolves, and part of the gas is
entrapped by the hydrogen absorbing alloy. If the hydrogen
absorbing alloy entrapping hydrogen is exposed to air, the alloy
may generate heat to get on fire. With the generation of heat,
corrosion of the hydrogen absorbing alloy powder may proceed
rapidly. To avoid this, it is preferred to remove hydrogen from the
alloy.
[0081] Removal of hydrogen from the alloy may be achieved by a
number of methods. First, an oxidizing agent such as H.sub.2O.sub.2
may be used to oxidize hydrogen. However, this method is not
preferred because the oxidizing agent is expensive and method
requires the use of a large amount of such an agent. A second
method is preferred because of its being free from the drawback of
the first method. This method is to bring a hydrogen absorbing
alloy powder treated by immersion treatment into contact with warm
water, then with hydrogen hydrogen peroxide solution to remove
hydrogen entrapped by the powder from the latter. Particularly a
modified method wherein a hydrogen absorbing alloy powder is
exposed to warm water of 80.degree. C. or higher and having a pH of
9 or lower is preferred, because the method allows the majority of
hydrogen entrapped by the alloy to be removed as gas efficiently
and at low cost, and released gas to be available for reuse.
Suitable oxidizing agents are not limited to any specific species,
but hydrogen peroxide is preferred because it does not lead to the
generation of products which may act as impurities affecting the
performance of the alloy. The oxidizing agent, when it is allowed
to contact with the formative layer on the surface of an alloy
powder at 45.degree. C. or higher, will decompose of itself to
release oxygen, thereby lowering the efficiency of the process.
Therefore, it is preferred to use the oxidizing agent cooled to a
temperature equal to or less than 45.degree. C., because then the
agent can efficiently react with hydrogen entrapped by the alloy
powder.
[0082] A hydrogen absorbing alloy powder treated by immersion
treatment, if exposed to air, has its surface oxidized and its
activity will be reduced.
[0083] On the contrary, if a hydrogen absorbing alloy powder does
not have any coat of hydroxides on its surface, its activity will
become so high that it is ready to ignite, and will actually get on
fire during transportation or when it is transferred from one place
to another during processing. Moreover, if a hydrogen absorbing
alloy powder is stored not being dried sufficiently but
moisturized, the capacity of the alloy will greatly be reduced
during storage possibly because the rare earth elements contained
in the alloy will be eluted to exhibit alkalinity, which promotes
the corrosion of the alloy.
[0084] When a hydrogen absorbing alloy powder is exposed to air
kept at 60 to 90.degree. C. to be partially dried, the alloy powder
will be removed of its moisture but have its surface oxidized. This
drying process is preferred, however, because the lowering of
high-rate discharging ability has a limitation, although the
surface of the alloy powder is oxidized. This is probably because a
thin coat of oxides formed on the surface of alloy powder by the
process will be deprived of oxygen to be reconverted to original
elements and removed when a battery incorporating an electrode
comprising the alloy powder is activated via
charging/discharging.
[0085] A hydrogen absorbing alloy powder treated by immersion
treatment was partially dried under the condition as described
above, and thus an alloy powder suitable for the construction of a
negative electrode was obtained that will exhibit no loss of
capacity during prolonged storage, be safe and free from the risk
of getting on fire, and have an excellent high-rate discharging
ability.
[0086] The immersion treatment is also advantageous because the
superficial layer formed on the surface of a hydrogen absorbing
alloy powder after the treatment acts as an activator of the alloy
powder, and thus during the initial phase of use, a battery
incorporating the electrode will exhibit an excellent high-rate
discharging ability.
[0087] However, if a hydrogen absorbing alloy powder is simply
immersed in an alkaline solution, an electrode incorporating the
resulting alloy powder will quickly have its high-rate discharge
ability reduced when it is subjected to charge/discharge cycles.
This is probably because during charge/discharge cycles, rare earth
elements such as La and the like, and Mn and Al contained in the
hydrogen absorbing alloy are eluted although small in amount, to
deposit, as hydroxides, on the surface of hydrogen absorbing alloy
powder, which interferes with electrode reactions. To avoid this,
by adding the oxides or hydroxides of rare earth elements as
specified above to a hydrogen absorbing alloy powder, it is
possible to obtain a hydrogen absorbing electrode capable of
exhibiting an excellent high-rate discharging ability over a
prolonged period. This is probably because addition of the oxides
or hydroxides of rare earth elements will inhibit the elution of
rare earth elements such as La and the like, and Mn and Al from the
hydrogen absorbing alloy.
[0088] As described in Claim 1, the inventors found that addition
of an oxide or hydroxide of a specified rare earth element to a
hydrogen absorbing alloy treated by using an alkaline solution can
dramatically enhance the resistance of the alloy to alkalinity. The
inventors also made an observation that an electrode incorporating
such a hydrogen absorbing alloy as described above exhibits a far
more excellent high-rate discharging ability and enhanced
resistance to alkalinity, depending on the species of rare earth
element added. Specifically, the inventors found that addition of
an oxide or hydroxide of a rare earth element, particularly of Er
will lead to the production of a hydrogen absorbing electrode which
is excellent both in the resistance to alkalinity and in high-rate
discharging ability. The inventors also found that addition of an
oxide or hydroxide of Yb will lead to the production of a hydrogen
absorbing electrode which is markedly excellent in the resistance
to alkalinity.
[0089] A hydrogen absorbing electrode of the invention contains, as
main components, a hydrogen absorbing alloy powder having a
saturation mass susceptibility of 1.0 to 6.5 emu/g, and a compound
of one or more rare earth elements selected from Dy, Ho, Er, Tm,
Yb, and Lu. The ratio of the weight of a rare earth element against
the total weight of the compound of a rare earth element is
preferably 80 wt % or more, particularly preferably 90 w % or more.
By containing a compound of a rare earth element, a hydrogen
absorbing alloy has its corrosion resistance markedly enhanced, and
provides an excellent cycle performance. Of the rare earth elements
cited above, Er is most preferred because addition of Er will lead
to the production of a hydrogen absorbing electrode having an
excellent high-rate discharging ability. Yb is also preferred
because addition of Yb will lead to the production of a hydrogen
absorbing electrode having a highly enhanced resistance to
corrosion.
[0090] Observation by EPMA indicates that Dy, Ho, Er, Tm, Yb, or Lu
added to a hydrogen absorbing alloy powder concentrates on the
surface of the alloy powder, and analysis by X-ray diffraction
reveals that the majority of compounds concentrated on the surface
of the alloy consists of hydroxides, and that the hydroxides are
likely to deposit on the surface of hydrogen absorbing alloy powder
in layers to form a coat thereon (the hydroxides are likely to
penetrate into cracks and holes present on the surface of the alloy
powder). The exact reason why the hydroxides of a rare earth
element deposit on the surface of a hydrogen absorbing alloy powder
is not clear, but may be most likely explained by the clean surface
of the alloy powder exposed by the foregoing alkali treatment, and
strong adsorption of the hydroxides to the surface of the alloy
powder via a mechanism involving the zeta potential of the
hydroxides.
[0091] The mechanism by which the compound of a rare earth element
enhances the corrosion resistance of a hydrogen absorbing alloy
powder is unclear, but is most likely ascribed to the formation of
a coat by the compound of a rare earth element on the surface of a
hydrogen absorbing alloy powder which enhances the resistance of
the alloy powder against corrosion. Of those rare earth elements
cited above, Yb is most effective in the production of a hydrogen
absorbing electrode highly resistant to corrosion because its
hydroxide most readily disperses in an alkaline electrolyte,
thereby enabling the formation of a uniform coat. On the other
hand, addition of a hydroxide of Er is less effective in the
production of a hydrogen absorbing electrode resistant to corrosion
as compared with the addition of a hydroxide of Yb, although the
electrode exhibits an excellent high-rate discharging ability. This
is probably because an hydroxide of Er, although being ready to
disperse in electrolyte, exists as particles whose size is larger
than the corresponding particles of other rare earth elements, and
thus a coat derived therefrom could not block the entry of
electrolyte effectively.
[0092] The mixture ratio between a hydrogen absorbing alloy powder
and a compound of a rare earth element is very important because it
greatly affects the high-rate discharging ability and cycle
performance of a resulting hydrogen absorbing electrode. According
to the invention, preferably 0.3 to 1.5, more preferably 0.7 to 1.5
part by weight of a compound of a rare earth element is mixed with
100 parts by weight of a hydrogen absorbing alloy powder. If a
compound of a rare earth element is added at an amount less than
0.3 part by weight, the resulting hydrogen electrode would not have
its corrosion resistance enhanced. On the contrary, if the addition
amount of a compound of a rare earth element exceeds 1.5 part by
weight, electrode reactions on a resulting hydrogen electrode would
be emphasized so much that the high-rate discharging ability of the
electrode would be markedly damaged. In addition, the hydrogen
over-voltage of the electrode would be reduced so much that the
evolution amount of hydrogen during charging would be
increased.
[0093] Formation of a coat of a hydroxide of a rare earth element
on the surface of a hydrogen absorbing alloy powder may be achieved
by an alternative method: a rare earth element is added as a
constituent to a hydrogen absorbing alloy powder, and the resulting
hydrogen absorbing alloy powder is brought into contact with an
alkaline solution, thereby allowing the rare earth element to elute
in the solution and form its hydroxide there. However, this method
is disadvantageous because, if one of the rare earth elements cited
in this invention is added to a hydrogen absorbing alloy, the
resulting alloy will not have a saturation mass susceptibility
sufficiently enhanced even after it has been immersed in an
alkaline solution. Moreover, when the mixed hydrogen absorbing
alloy is brought into contact with alkaline solution, to allow a
rare earth element to be eluted in the solution, the rare earth
element eluted mainly consists of atoms located on the surface of
the alloy powder, and is so small in quantity that it is not
sufficient to form a coat on the surface of the powder, and thus
will not provide the effect which will be obtained by the addition
of a hydroxide of a rare earth element. As described above, the
latter method in which a rare earth element is added as a
constituent to a hydrogen absorbing alloy powder prior to alkaline
immersion treatment could hardly achieve the object of the
invention. Thus, it is preferred to add an oxide or hydroxide of a
rare earth element to a hydrogen absorbing alloy powder for
mixture.
[0094] As described above, the majority of rare earth elements
added will become, after they are incorporated in a battery,
hydroxides finally. The compound of a rare earth element to be
added to a hydrogen absorbing alloy powder is not limited to any
specific one, but preferably includes those that will not affect
the concentration of electrolyte by reacting with the latter, and
are readily available. Specifically, oxides or hydroxides are
preferred. Particularly, oxides are preferred, because when an
oxide of a rare earth element is added, the oxide is dissolved once
in electrolyte, and then reappears as hydroxide to precipitate
which is so small in size that it forms a uniform coat on the
surface of a hydrogen absorbing alloy powder, thereby emphasizing
the effect characteristic with the addition of a rare earth
element.
[0095] Conventionally, an oxide or hydroxide of Er or Yb is added
neat to a hydrogen absorbing alloy powder for mixture without
pulverization. The oxide or hydroxide of Er or Yb commercially
available consists of powder whose average diameter is in the range
of 8 to 15 .mu.m that is larger than 5 .mu.m. According to the
invention, an oxide or hydroxide of Er or Yb in the form of powder
is preferably pulverized to allow the powder to have an average
diameter equal to or less than 5.0 .mu.m, before it is added to a
hydrogen absorbing alloy powder for mixture. It will be possible by
so doing to obtain a hydrogen absorbing electrode exhibiting a good
cycle performance because of the excellent corrosion resistance
conferred by the oxide or hydroxide of Er or Yb added. More
preferably the average diameter of the oxide or hydroxide of Er or
Yb is made equal to or less than 3.5 .mu.m. Initially we were
afraid that addition of an oxide or hydroxide of Er or Yb in the
form of powder would degrade the output power performance of the
resulting hydrogen absorbing electrode, but after studying the
result of experiment we found that addition of an oxide or
hydroxide of Er or Yb does not affect in any way the output power
performance of the resulting hydrogen absorbing electrode.
[0096] Particularly, in order to lengthen the cycle life of a
hydrogen absorbing electrode, an oxide or hydroxide of Er or Yb to
be added preferably has an average diameter (D50) of 0.1 to 3
.mu.m, more preferably 0.1 to 1 .mu.m. If it is tried to obtain an
oxide or hydroxide of Er or Yb having an average diameter less than
0.1 .mu.m, the production process will be so complicated as to
elevate the production cost. On the contrary, if the average
diameter in exceeds 3 .mu.m, an oxide or hydroxide of Er or Yb will
hardly be adsorbed to the surface of a hydrogen absorbing alloy
powder and a stable coat will never be formed on the surface of the
powder.
[0097] The reason why finely pulverized oxide or hydroxide of Er or
Yb exerts an enhanced effect is unclear, but the most likely
explanation is that, when an oxide or hydroxide of Er or Yb is
pulverized into a fine powder, the dispersion tendency of the
compound is more stressed as compared with the same compound
consisting of a coarse powder, and thus the powder is allowed to
form a more uniform coat on the surface of a hydrogen absorbing
alloy powder. At least part of powdery oxide or hydroxide of Er or
Yb added to a hydrogen absorbing alloy powder for mixture reacts,
after the resulting hydrogen electrode has been installed in a
battery, with electrolyte though at a slow pace and is converted
into hydroxides. It seems likely that during this process, the
hydroxides thus formed will accumulate on the surface of hydrogen
absorbing alloy powder which stays at a electronegative potential.
If the oxide or hydroxide of Er or Yb added consists of powder
having a small diameter, its reaction with electrolyte will be
emphasized, and accumulation of resulting hydroxides onto the
surface of a hydrogen absorbing alloy powder will be enhanced,
which will explain the reason why the addition of oxide or
hydroxide of Er or Yb markedly elevate the corrosion resistance of
hydrogen absorbing electrode.
[0098] The same will also hold true for an oxide or hydroxide of a
rare earth element other than Er and Yb, such as Dy, Ho, Lu, etc.
which is in the form of powder having a small diameter.
[0099] The powdery oxide or hydroxide of a rare earth element to be
added to a hydrogen absorbing alloy powder will be called an
anti-corrosion agent hereinafter.
[0100] When the oxide or hydroxide of Er is compared with the
corresponding compound of Yb, the enhanced anti-corrosion activity
of a resulting hydrogen absorbing electrode due to an Er compound
is somewhat inferior to the corresponding activity due to a
comparable Yb compound. This is probably because the Er compound is
more strongly inhibited of its dispersion within the hydrogen
absorbing electrode than the comparable Yb compound. However, it
was found that addition of an oxide or hydroxide of Er does not
greatly increase the resistance of a resulting hydrogen absorbing
electrode, and the electrode exhibits an output power performance
as high as the electrode having no oxide or hydroxide of Er added.
On the other hand, addition of an oxide or hydroxide of Yb brings
about a more enhanced resistance of a hydrogen absorbing electrode
to corrosion than a comparable compound of Er, probably because it
is more readily dispersed in the electrode than the latter.
However, addition of an oxide or hydroxide of Yb more strongly
lowers the discharging ability of a hydrogen absorbing electrode
probably because the Yb compound is more readily dispersed in the
electrode. Namely, addition of an oxide or hydroxide of Yb reduces
more strongly the discharging ability of a hydrogen absorbing
electrode, and the output power performance of a resulting battery
than the comparable Yb compound probably due to the readier
dispersion of the former in the electrode.
[0101] Since rare earth elements resemble each other so much that
it is difficult to separate them from one another, and thus
isolation of a target rare earth element is often accompanied by
impurities including other rare earth elements. As described above,
however, it was found that the oxides or hydroxides of Dy, Ho, Er,
Tm, Yb, Lu are effective, when added to a hydrogen absorbing alloy
powder, in enhancing the corrosion resistance of the latter. If to
a certain corrosion-resistance enhancer, a rare earth element other
than Dy, Ho, Er, Tm, Yb, and Lu is added, the corrosion-resistance
enhancing effect of that agent may be damaged. To avoid this, it is
preferred according to the invention that the entry of other rare
earth elements than Dy, Ho, Er, Tm, Yb, and Lu should be avoided as
far as possible. However, it was found from a study that, as far as
the content of Dy, Ho, Er, Tm, Yb, or Lu in the total amount of a
rare earth element contained in a corrosion-resistance enhancer is
80 wt % or more (preferably 90 wt % or more), the enhancer can
exert its desired effect to a satisfactory level, even though it is
contaminated with other rare earth elements.
[0102] As described above, addition of an oxide or hydroxide of Er
will enhance the corrosion resistance of a hydrogen absorbing alloy
powder while hardly increasing the reaction resistance of a
resulting hydrogen absorbing electrode. Addition of an oxide or
hydroxide of Yb is effective in markedly enhancing the resistance
of a hydrogen absorbing alloy powder, although it slightly
increases the reaction resistance of a resulting hydrogen absorbing
electrode. As seen from above, Er and Yb compounds exert different
effects. To make the most of those different effects, it is
preferred to avoid Er and Yb compounds to be mixed inadvertently or
at random. It was found from a study that even when Er and Yb
compounds are mixed, and the ratio of Er or Yb (also called the
purity of Er or Yb) against the total amount of rare earth elements
of the mixture is 80 wt % or more, the mixture will exert its
desired effect determined by a predominant rare earth element
species in the mixture. For the reason given above, the purity of a
corrosion-resistance enhancer in terms of its content of Er or Yb
is preferably 80% or more, more preferably 90% or more.
[0103] Addition of a corrosion-resistance enhancer to a hydrogen
absorbing alloy powder may interfere with the formation of a
catalyst layer upon the surface of the powder. If an oxide or
hydroxide of Er or Yb is added to a hydrogen absorbing alloy powder
which does not yet receive a catalyst layer on its surface, the
hydrogen absorbing alloy powder will not be activated, even after
it is put to a activation process. Thus, when a resulting battery
was charged, the electrode did not absorb hydrogen, and the battery
suffered from leakage at an initial stage of operation. To avoid
this, when an oxide or hydroxide of Er or Yb is added to a hydrogen
absorbing alloy powder, it is necessary for the hydrogen absorbing
alloy powder to be subjected in advance to the surface treatment
and then to have a catalyst layer formed on its surface. When a
hydrogen absorbing alloy powder is subjected to the surface
treatment according to the invention, its saturation mass
susceptibility will be increased. However, if the saturation mass
susceptibility of hydrogen absorbing alloy powder exceeds 6 emu/g,
the resulting alloy will have a lowered capacity and cycle
performance probably because of the reduction of hydrogen absorbing
sites. Therefore, it is preferred to set the saturation mass
susceptibility of hydrogen absorbing alloy powder to 1 to 6 emu/g,
more preferably 2 to 6 emu/g. For reference, a hydrogen absorbing
alloy powder prepared from a hydrogen absorbing alloy of the
invention was observed to have a catalyst layer formed on its
surface whose thickness is 50 nm or more.
[0104] The diameter of a hydrogen absorbing alloy powder to be used
as a hydrogen absorbing electrode of the invention is not limited
to any specific range, but it is preferably in the range of 10 to
30 .mu.m. If the average diameter is less than 10 .mu.m, a
resulting electrode will not exhibit a good cycle performance
because its corrosion resistance is unsatisfactory. On the
contrary, if the average diameter exceeds 30 .mu.m, powder
constituting a resulting electrode, when subjected to a series of
charge/discharge cycles, will be cracked to expose its fresh
surfaces which may promote the progression of corrosion of the
electrode. Moreover, if an alloy powder whose average diameter is
equal to or larger than 30 .mu.m is used as an electrode, a
prolonged treatment will be necessary for attaining a desired level
of saturation mass susceptibility and a resulting electrode will
not exhibit a high-rate discharging ability enhanced to a
sufficiently high level.
[0105] To confer an excellent high-rate discharging ability to an
electrode, a hydrogen absorbing alloy powder preferably has an
average diameter of 30 .mu.more less. Furthermore, to confer a
sufficiently high cycle life to an electrode, a hydrogen absorbing
alloy powder preferably has an average diameter of 10 .mu.m or
more, more preferably 20 .mu.m or more.
[0106] In order to provide an oxide or hydroxide of a rare earth
element in the form of powder or a hydrogen absorbing alloy powder
having a desired shape, a mill and sieve are used. For example, a
mortar, ball mill, sand mill, vibration mill, satellite ball mill,
jet mill, counter jet mill, or swirling current jet mill, and a
sieve are used. For milling, wet milling accompanied by the
addition of water or an aqueous solution of an alkali metal may be
used. Sieving may take place by any suitable method: a sieve or
blower sorting machine may be used under a wet or dry condition as
needed.
[0107] The active material of a negative electrode which forms the
predominant constituents of a negative electrode has been described
above. The hydrogen absorbing electrode may include, in addition to
the predominant constituents, other constituents such as an
electric conductor, binding agent, thickener, filler, etc.
[0108] The positive electrode may also contain, in addition to an
active material making the predominant constituents of a positive
electrode, other constituents such as an electric conductor,
binding agent, thickener, filler, etc.
[0109] The electric conductor is not limited to any specific one,
but may be any appropriate conductor as long as it does not exert
an adverse effect on the function of a battery. However, usually
the suitable electric conductor may include one of natural graphite
(scale-like graphite, clay-like graphite), artificial graphite,
carbon black, acetylene black, Ketchen black, carbon whisker,
carbon fiber, gas phase developed carbon, metal powder (copper,
nickel, gold, etc.), and metal fiber, etc., or combination thereof.
Among them, Ketchen black is most preferred because it has a high
electric conductivity and can be easily applied to an external
object as a paste. The addition amount of an electric conductor is
preferably in the range of 0.1 to 2 wt % with respect to the total
weight of a positive or negative electrode, because then it will
not greatly reduce the capacity of an electrode to which it is
added. Ketchen black which has been pulverized into a ultra-fine
powder having a diameter of 0.1 to 0.5 .mu.m is particularly
preferred, because then it will reduce the necessary amount of
carbon.
[0110] Usually the suitable binding agent may include one, or two
or more in combination selected from thermoplastic resins such as
polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene
(PP), and polymers having a rubber-like elasticity such as
ethylene-propylene-dienterpolymer (EPDM), sulfonated EPDM,
styrenebutadiene rubber (SBR), etc. The addition amount of a
binding agent is preferably in the range of 0.1 to 3 wt % with
respect to the total weight of a positive or negative
electrode.
[0111] Usually the suitable thickener may include one, or two or
more in combination selected from polysaccharides such as
carboxymethylcellulose (CMC), methylcellulose (MC),
hydroxypropylmethyl cellulose (HPMC), etc. The addition amount of a
thickener is preferably in the range of 0.1 to 3 wt % with respect
to the total weight of a positive or negative electrode.
[0112] Suitable fillers may include any appropriate filler as long
as it does not exert an adverse effect on the function of a
battery. Usually, the suitable filler may include olefin polymers
such as polyethylene, and carbon. The addition amount of a filler
is preferably less than 5 wt % with respect to the total weight of
a positive or negative electrode.
[0113] A positive or negative electrode is preferably manufactured
by adding an active material, electric conductor, and binding agent
to water, or an organic solvent such as alcohol or toluene for
mixture, applying a resulting liquid mixture to a current
collecting substrate, and drying the assembly. Suitable coating
methods for forming a coat having a desired shape and size may
include, for example, roller coating such as applicator roll,
screen coating, doctor blade method, spin coating, coating by a bar
coater, etc. However, the suitable method is not limited to those
mentioned above.
[0114] Suitable current collecting substrates may include any
appropriate current collecting substrates as long as it does not
exert an adverse effect on the function of a battery. For example,
a nickel plate, or a nickel-deposited steel plate may be employed
with profit. Suitable forms may include substrates such as a foam,
molded bundle of fibers, three-dimensional substrates formed to
convexo-concave, and two-dimensional plates such as a punched steel
plate. The suitable thickness is not limited to any specific one,
but usually 5 to 700 .mu.m is preferred. However, the current
collecting substrate of a positive electrode is preferably obtained
by processing Ni which is resistant to corrosion and oxidization
into a porous substrate because the porous substrate is a structure
advantageous for collecting current. On the other hand, the current
collecting substrate of a hydrogen absorbing electrode is
preferably obtained by depositing nickel onto an iron foil which is
cheap and has an excellent conductivity to enhance the resistance
of the foil to reduction, and punching the resulting foil (punched
substrate). Punching is preferably performed so that holes having a
diameter of 1.7 mm or less are formed thereon, and the total area
of holes accounts for 40% or higher of the total surface area of
the plate. It is possible via this arrangement to secure the
adhesion between an active material of a negative electrode and a
current collecting substrate, even though a binder is used at a
small amount.
[0115] In addition to baked carbon and conductive polymer, an Ni
powder, or carbon or platinum in the form of powder may be applied
to the surface of an Ni-made current collecting substrate in order
to enhance the adhesiveness, conductivity, and oxidation resistance
of the substrate. The additives described above may be applied
after their surface has been subjected to oxidation treatment.
[0116] Suitable materials for a separator of a nickel metal-hydride
battery may include a porous membrane or non-woven fabric alone or
in combination. Suitable materials constituting such a porous
membrane or non-woven fabric may include polyolefin resins
represented, for example, by PE, PP, etc., and polyamide resins
(nylon). The porosity of a material constituting a separator is
preferably equal to or less than 80 vol %, because then the
resulting separator can have a sufficient strength, prevent the
occurrence of electrical short circuit through a penetration of the
separator, and ensure the secure passage of gases. On the contrary,
the porosity of a material constituting a separator is preferably
equal to or more than 20 vol %, in order to keep the electric
resistance of separator at a low level and ensure the high-rate
performance of a battery. The separator is preferably treated to be
hydrophilic. For this purpose, for example, a polyolefin resin such
as polyethylene may be subjected to sulfonation treatment, corona
treatment, or PVA treatment. Or, two or more materials subjected to
different treatments may be combined.
[0117] Any electrolyte commonly used in an alkali battery may be
used. Suitable electrolyte may contain K, Na and Li alone or in
combination in water, but suitable electrolyte is not limited to
the one mentioned above. The concentration of solute in electrolyte
is preferably 5 to 7 mol/dm.sup.3 for potassium hydroxide, and 0.1
to 0.8 mol/dm.sup.3 for lithium hydroxide, because then it is
possible to securely obtain a battery exhibiting a high battery
performance.
[0118] The construction of a nickel metal-hydride battery of the
invention is not limited to any specific one, but generally
suitable batteries may include coin batteries, button batteries,
square-shaped batteries, flat batteries, etc. comprising a negative
electrode, a positive electrode, and a separator having a single or
multiple layer structure. A cylindrical battery in which housed is
a rolled electrode assembly comprising a positive electrode sheet
and a negative electrode sheet with a separator sheet in between
tightly rolled together, is preferred because it can manage with
comparatively few polar plates, and have polar plates having a
comparatively large area.
[0119] A sealed nickel metal-hydride storage battery of the
invention may be preferably obtained, for example, by injecting
electrolyte before or after the positive electrode and separator
and negative electrode are stacked, and finally sealing using a
wrapping member. In the manufacture of a sealed nickel
metal-hydride battery with a electrode assembly obtained by rolling
a stack of a positive and negative electrodes with a separator in
between tightly together, electrolyte is preferably injected into
the electrode assembly both before and after the stack is rolled.
As injecting method of the electrolyte, injecting the electrolyte
under normal pressure may be used, but it is also possible to be
used that the electrolyte is injected under vacuum or under
positive pressure or using centrifugal force.
[0120] Suitable materials of a container of a sealed nickel
metal-hydride battery may include, for example, nickel-plated iron,
stainless steel, polyolefin resins, etc.
[0121] As shown in FIG. 1, for a cylindrical nickel metal-hydride
battery of the invention, a lead connects an internal surface of a
cap 6 which serves as one out of a positive and negative
electrodes, a sealing plate 0, with an upper current collecting
plate 2. According to the invention, as shown in FIG. 1, a
electrode assembly 1 having an upper current collecting plate 2 and
a lower current collecting plate 3 attached thereto is placed in a
metal container 4 with a bottom; a specified amount of electrolyte
is injected into the trough, thereby electrically connecting the
lower current collecting plate 3 with the inner surface of the
bottom of container 4; then one end of the lead is connected to the
internal surface of sealing plate (in the particular example shown
in FIG. 2, the lead consists of a main lead 8 and supplementary
lead 9); a cap 6 with a valve 7 to act as a safety valve is
attached to one electrode of the battery, and a sealing plate 0
with a gasket 5 along its periphery is applied on the upper surface
of upper current collecting plate and the open end of container 4
is folded to hold the gasket tightly; then one output terminal A
(also called electrode rod) of an electric resistance welder is
connected to the external surface of sealing plate 0 (or cap 6)
while the other output terminal B is connected to the outer surface
of the bottom of container 4, and electric current which is
sufficiently strong for welding is passed through the battery,
thereby welding the connection between the lead and the upper
current collecting plate 2.
[0122] As seen from above, since the joint between the lead and the
upper current collecting plate 2 is welded after the sealing plate
is applied to the open end of container, it is not necessary to use
a lead having an extra curvature as is observed in a conventional
battery, in other words, it is possible to connect the two elements
here concerned via a lead shorter than a corresponding lead of a
conventional battery, thus reducing the electric resistance of the
lead as compared with a lead of a conventional battery. To obtain a
battery having an output power performance of 1400 W/kg or more
which is necessary to serve as powder source of HEVs as described
above, the ratio of the length of current collecting lead against
the distance between sealing plate 0 and upper current collecting
plate 2 is preferably 2.1 or less, more preferably 1.7 or less, in
which the length of current collecting lead is defined as the
length of lead from a welded point between a distal end of the
current collecting lead and a welded point P1 between a central end
of the same lead and upper current collecting plate 2. The electric
current passing through a battery for welding as described above is
preferably in the form of alternate pulses because such an electric
current will not decompose the electrolyte during passage.
[0123] An embodiment of the aforementioned lead is shown in FIG. 2.
The lead comprises, for example, a ring-shaped main lead 8 and a
supplementary lead 9. The main lead 8 has, on its distal end
surface, a plurality of projections 11 for ensuring the secure
contact of the lead to the sealing plate after electric resistance
welding is performed, and is connected, through its central end
surface, to the supplementary lead 9. The supplementary lead 9
consists of a plurality of jutted chips 9' which protrude from the
ring-shaped central end of main lead 8 inward (or outward), and
each jutted chip 9' has a knob 10 on its tip for ensuring the
secure contact when electric resistance welding is performed for
connecting the jutted chip to the upper current collecting plate 2.
The jutted chip 9' is slanted downward from the main lead 8 as
shown in FIG. 2, and is elastically responsive to up-to-down and
down-to-up deformations. Even when the electrode assembly 1 have
different heights from one to another (variance in height), and the
jutted chip is pushed upward by a different pressure each time the
electrode assembly is changed from one to another, the jutted chip,
owing to its elasticity, ensures the secure contact between the
knob 10 and the upper current collecting plate 2, which helps to
achieve a secure electric connection of the lead to upper current
collecting plate 2 by electric resistance welding.
[0124] As shown in FIG. 3, the upper current collecting plate 2 has
a disc-like shape which has a through hole at its center with a
plurality of slits 2-2 extending radially therefrom. The slits are
provided to reduce ineffective current when the upper current
collecting plate is connected to the end surface of electrode
assembly by electric resistance welding. Each slit has two parallel
ridges 2-3 along two edges facing to each other which intersect at
right angles with the end surfaces of rolled electrode sheets
constituting the electrode assembly and are brought into electric
contact with the latter. To ensure the intimate contact of said
ridges of slits to the end surface of long side of electrode sheets
over the entire surface of said long side of electrode sheets, the
upper current collecting plate 2 preferably has a radius similar to
that of electrode assembly 1 (however, the periphery of upper
current collecting plate dose not stick out from that of electrode
assembly), and a center corresponding to that of the electrode
assembly.
[0125] The lead is preferably connected to upper current collecting
plate 2 via plural welded points (P1 of the FIG. 1) by welding. The
appropriate number of welded points may vary depending on the size
of a battery, and is not limited to any specific one, but it is
preferably 2 to 16, more preferably 4 to 16. To minimize the
difference of distance between electrode sheets and a given welded
point, the plurality of welded points P2 are preferably arranged,
at an equal distance apart from each other, along the periphery of
one or more circles concentrically provided around the center of
the current collecting plate. The ratio of the distance of a welded
point P1 from the center of upper current collecting plate against
the radius of electrode assembly 1 is preferably selected to be 0.4
to 0.7, because then the welded point takes a position
corresponding to a middle point of the long side of electrode
sheets which may enhance the current collecting function of the
plate and lead to the enhanced output power performance of a
battery.
[0126] A lower current collecting plate 3 and the inner surface of
the bottom of container 4 are preferably connected via a plurality
of welded points P2 diffusely excepting the center of lower current
collecting plate as shown in FIG. 1. The lower current collecting
plate 3 has a disc-like shape with a plurality of slits extending
radially from the center, as does upper current collecting plate 2
described above. To ensure the intimate contact of lower current
collecting plate 3 to the inner surface of the bottom of container
4, the plurality of projections 14 are provided not only on the
peripheral surface of lower current collecting plate 2 but also at
the center of the latter, in opposition to the projections provided
on the upper current collecting plate 2. The appropriate number of
projections 14 arranged on the periphery of lower current
collecting plate may vary depending on the size of a battery, and
is not limited to any specific one, but it is preferably 2 to 16,
more preferably 4 to 16, because then it is possible to reduce the
electric resistance between the lower current collecting plate and
electric container 4. The ratio of the distance of the plurality of
peripherally arranged welded points P2 from the center of lower
current collecting plate against the radius of electrode assembly 1
is preferably selected to be 0.5 to 0.8, because then the welded
points take positions corresponding to middle points of the long
side of electrode sheets which may enhance the current collecting
function of the electrode sheet and lead to the enhanced output
power performance of a battery.
BEST MODE FOR CARRYING OUT THE INVENTION
[0127] The present invention will be further described below with
reference to attached drawings, but the invention is never limited
to those examples, and selection of a testing method, and of active
materials for positive and negative electrodes of a battery,
materials of the positive and negative electrodes, electrolytes,
and materials of a separator, and the shape of a battery may be
made as appropriate.
[0128] (1) Evaluation Based on Test Cell Comprising a Hydrogen
Absorbing Electrode
[0129] In Examples 1 to 23, and Comparative examples 1 to 15 below,
evaluation was made on test cells comprising a hydrogen absorbing
electrode.
EXAMPLE 1
(Surface Modification Treatment of a Hydrogen Absorbing Alloy
Powder)
(Pretreatment: Hydrogen Absorption)
[0130] A hydrogen absorbing alloy in the form of powder having an
average diameter of 30 .mu.m and belonging to an AB.sub.5 type rare
earth element system whose composition is represented by
MmNi.sub.3.55Co.sub.0.6Al.sub.0.3Mn.sub.0.35 was heated at
100.degree. C. for 10 hours under reduced pressure. The powder was
then exposed at 60.degree. C. for 15 minutes to a hydrogen
atmosphere where the partial pressure of hydrogen was kept at 0.1
Mpa, to allow the powder to absorb hydrogen.
[0131] (1st Step: Immersion Treatment)
[0132] A 1 kg of the hydrogen absorbing alloy powder treated by the
hydrogen absorption treatment was immersed in 11 of an aqueous NaOH
solution containing NaOH at 45 wt % and kept at 100.degree. C. The
immersion treatment was allowed to take place for 52 minutes.
During the immersion treatment, the treatment solution was stirred
to prevent the hydrogen absorbing powder from sinking to the
bottom. During the treatment, the environment was controlled so
that the temperature was kept within 100.+-.3.degree. C. and
concentration of NaOH within 45.+-.3 wt %.
[0133] (2nd Step: Separation of Hydroxides of Rare Earth Element
for Removal)
[0134] After immersion treatment, the hydrogen absorbing alloy
powder was separated by pressure filtration from the treatment
solution, to which was added pure water having the same weight with
that of the hydrogen absorbing alloy powder, and the resulting
mixture was exposed for 10 minutes to an ultrasonic wave having the
frequency of 28 kHz. Then, while the mixture was being stirred
gently, pure water was injected below a stirred suspension layer
and excess water was allowed to flow out, thereby purging
hydroxides of rare earth metal liberated from the alloy powder for
disposal. Then, reducing was continued until the pH of spilled
water became pH10 or lower. The remaining mixture was filtered
under pressure.
[0135] (3rd Step: Hydrogen Desorption 1)
[0136] Desorption of hydrogen was achieved by exposing the hydrogen
absorbing alloy powder to water warmed to 80.degree. C.
[0137] (4th Step: Hydrogen Desorption 2)
[0138] The warm water was filtered under pressure, and washed with
water again. The alloy was cooled to 25.degree. C., to which was
added 4% hydrogen peroxide solution having the same weight with
that of the alloy to desorb hydrogen.
[0139] (5th Step: Drying/Surface Oxidation)
[0140] The hydrogen absorbing alloy powder deprived of hydrogen was
exposed to a hot blow of wind kept at 80.degree. C. for 30
minutes.
[0141] (Measurement of Saturation Mass Susceptibility)
[0142] A 0.3 g of the dried hydrogen absorbing alloy powder was
weighed precisely, charged in a sample holder, and placed in a
magnetic field uo to 5 kOe produced by a vibration sample
magnetometer manufactured by Riken Electronics (model BHV-30) for
measurement.
[0143] (Preparation of Hydrogen Absorbing Electrode Plate)
[0144] To 100 parts by weight of a hydrogen absorbing alloy powder
having an average diameter (D50) of 1 .mu.m, was added 1 part by
weight of erbium oxide (Er.sub.2O.sub.3) for mixture. The mixture
was mixed with a styrenebutadiene copolymer at a ratio of
99.35:0.65, which was dispersed in water containing a dispersant to
form a paste. The paste was applied with a blade coater onto a
nickel-plated punched steel plate made of a iron plate. The product
was kept at 80.degree. C. to be dried, and pressed to give a master
plate for hydrogen absorbing electrode which was a square having a
specified thickness and 44 mm.times.44 mm area. Incidentally, of
the total weight of rare earth elements contained in
Er.sub.2O.sub.3 mentioned above, Er accounted for 97%, and
impurities consisted of trace amounts (0.5 to 1.5 wt %) of Dy, Ho,
Tm, and Yb.
[0145] (Preparation of Test Cell Comprising a Hydrogen Absorbing
Electrode)
[0146] The master plate for hydrogen absorbing electrode was cut to
give a plate having an area of 30.times.30 mm, and used as an
electrode plate for a hydrogen absorbing electrode-based test cell
having a capacity of about 470 mAh. To the electrode plate was
connected a lead by spot welding. The hydrogen absorbing electrode
was sandwiched by the separator, and the assembly was further
sandwiched from outside by two nickel electrodes having a capacity
two times as large as that of the hydrogen absorbing electrode to
give a electrode assembly for evaluation of the test cell. The
electrode assembly was placed in an open container, which was
flooded by electrolyte, and a mercury/mercury oxide (Hg/HgO)
reference electrode was inserted into the container flooded by
electrolyte to serve as a hydrogen absorbing test cell (called a
test cell hereinafter).
[0147] (Discharge Test: Determination of Discharge Capacity)
[0148] The test cell prepared as above was charged at 20.degree. C.
via 0.02 ItA to 25% of the capacity of the hydrogen absorbing
electrode, and then charged via 0.1 ItA to 100% of the capacity of
the hydrogen absorbing electrode. After 1 hour pause, the test cell
was discharged via 0.2 ItA until the potential of the hydrogen
absorbing electrode became -0.6 V versus the reference electrode.
Then, the test cell was charged via 1 ItA to 120%, which was
followed by 1 hour pause. Later, the test cell was discharged via
0.2 ItA to -0.6V versus the reference electrode. The above
charge/discharge cycle was repeated four times. Based on the
discharge capacity obtained from the fourth discharge cycle, the
discharge capacity per 1 g of hydrogen absorbing alloy was
determined by calculation.
EXAMPLE 2
[0149] A test cell was prepared in the same manner as in Example 1,
except that the immersion treatment performed in the surface
modification step (1st Step: immersion treatment) for modifying the
surface of a hydrogen absorbing alloy powder took 1.3 hour, and the
discharge capacity of the cell was determined. The cell was made
Example 2.
EXAMPLE 3
[0150] A test cell was prepared in the same manner as in Example 1,
except that the immersion treatment performed in the surface
modification step (1st Step: immersion treatment) for modifying the
surface of a hydrogen absorbing alloy powder took 1.8 hour, and the
discharge capacity of the cell was determined. The cell was made
Example 3.
EXAMPLE 4
[0151] A test cell was prepared in the same manner as in Example 1,
except that the immersion treatment performed in the surface
modification step (1st Step: immersion treatment) for modifying the
surface of a hydrogen absorbing alloy powder took 2.5 hour, and the
discharge capacity of the cell was determined. The cell was made
Example 4.
EXAMPLE 5
[0152] A test cell was prepared in the same manner as in Example 1,
except that the immersion treatment performed in the surface
modification step (1st Step: immersion treatment) for modifying the
surface of a hydrogen absorbing alloy powder took 3.5 hour, and the
discharge capacity of the cell was determined. The cell was made
Example 5.
EXAMPLE 6
[0153] A test cell was prepared in the same manner as in Example 1,
except that the immersion treatment performed in the surface
modification step (1st Step: immersion treatment) for modifying the
surface of a hydrogen absorbing alloy powder took 4.5 hour, and the
discharge capacity of the cell was determined. The cell was made
Example 6.
EXAMPLE 7
[0154] A test cell was prepared in the same manner as in Example 1,
except that the immersion treatment performed in the surface
modification step (1st Step: immersion treatment) for modifying the
surface of a hydrogen absorbing alloy powder took 5.0 hour, and the
discharge capacity of the cell was determined. The cell was made
Example 7.
EXAMPLE 8
[0155] A test cell was prepared in the same manner as in Example 1,
except that the immersion treatment performed in the surface
modification step (1st Step: immersion treatment) for modifying the
surface of a hydrogen absorbing alloy powder took 5.5 hour, and the
discharge capacity of the cell was determined. The cell was made
Example 8.
COMPARATIVE EXAMPLES 1-4
[0156] Test cells having the same composition with that of Example
1 were prepared, except that the immersion treatment for hydrogen
absorbing alloy powder samples lasted 0 or 24 minutes, or 6 or 8
hours, and subjected to the same test. The cells were made
Comparative examples 1 to 4.
[0157] Table 1 lists the test results of hydrogen absorbing alloy
powders representing Examples 1 to 8, and Comparative examples 1 to
4 regarding their saturation mass susceptibility, and discharge
capacity per 1 g of hydrogen absorbing alloy powder. TABLE-US-00001
TABLE 1 Treatment Saturation mass Additive Capacity Classification
time (h) susceptibility (emu/g) Chemical formula Added amount
(mAh/g) Example 1 0.87 1 Er.sub.2O.sub.3 1 Part by weight 315
Example 2 1.3 1.5 Er.sub.2O.sub.3 1 Part by weight 315 Example 3
1.8 2 Er.sub.2O.sub.3 1 Part by weight 315 Example 4 2.5 3
Er.sub.2O.sub.3 1 Part by weight 315 Example 5 3.5 4
Er.sub.2O.sub.3 1 Part by weight 304 Example 6 4.5 5
Er.sub.2O.sub.3 1 Part by weight 290 Example 7 5 6 Er.sub.2O.sub.3
1 Part by weight 277 Example 8 5.5 6.5 Er.sub.2O.sub.3 1 Part by
weight 270 Comparative example 1 0 0.05 Er.sub.2O.sub.3 1 Part by
weight 217 Comparative example 2 0.4 0.5 Er.sub.2O.sub.3 1 Part by
weight 304 Comparative example 3 6 7 Er.sub.2O.sub.3 1 Part by
weight 263 Comparative example 4 8 10 Er.sub.2O.sub.3 1 Part by
weight 220
[0158] According to the results shown in Table 1, the hydrogen
absorbing alloy powder samples representing 1-3 Examples 1 to 4
have the same saturation mass susceptibility of 315 mAh/g which is
the highest of all the test results. Both of Example 2 which has a
saturation mass susceptibility of 4 emu/g and Comparative example 2
which has a saturation mass susceptibility of 0.5 emu/g show high
saturation mass susceptibilities exceeding 300 mAh/g. Comparative
example 1 which received no immersion treatment shows a saturation
mass susceptibility of 217 mAh/g which is extremely low. As is
evident by comparing the results of Examples 6 to 8, and
Comparative examples 3 and 4 with each other, as the saturation
mass susceptibility of a hydrogen absorbing alloy powder increases
above 5 emu/g, its capacity declines with the increment.
[0159] For the test cells, their discharge capacity were determined
for the fourth discharge cycle, and compared with each other. When
the test was performed at such an early phase of charge/discharge
cycle as above, a hydrogen absorbing alloy powder receiving no
immersion treatment like Comparative example 1 has a low capacity,
probably because the coat of oxides on the surface of alloy powder
is not removed sufficiently which will interfere with the electrode
reactions, and because the superficial layer is not formed on the
surface of the powder which leads to the lowered activity of the
powder.
[0160] The reason why the capacity of a hydrogen absorbing alloy
powder decreases with the increment of its saturation mass
susceptibility may be ascribed to the excess elution of rare earth
elements contained in the hydrogen absorbing alloy powder as a
result of immersion treatment which leads to the reduced hydrogen
absorbing ability of the alloy powder.
[0161] When applications involve the low rate discharge (0.2 ItA)
as was employed in the present test of test cells, the hydrogen
absorbing electrode is preferably so constructed as to give a
saturation mass susceptibility of 0.5 to 4 emu/g, more preferably 1
to 3 emu/g.
EXAMPLE 9
[0162] A test cell was prepared in the same manner as in Example 1,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 9.
Incidentally, of the total weight of rare earth elements contained
in Yb.sub.2O.sub.3 mentioned above, Yb accounted for 98.5%, and
impurities consisted of trace amounts (0.5 wt %) of Er, Tm, and
Lu.
EXAMPLE 10
[0163] A test cell was prepared in the same manner as in Example 2,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 10.
EXAMPLE 11
[0164] A test cell was prepared in the same manner as in Example 3,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 11.
EXAMPLE 12
[0165] A test cell was prepared in the same manner as in Example 4,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 12.
EXAMPLE 13
[0166] A test cell was prepared in the same manner as in Example 5,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 13.
EXAMPLE 14
[0167] A test cell was prepared in the same manner as in Example 6,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 14.
EXAMPLE 15
[0168] A test cell was prepared in the same manner as in Example 7,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 15.
EXAMPLE 16
[0169] A test cell was prepared in the same manner as in Example 8,
except that an ytterbium oxide (Yb.sub.2O.sub.3) powder having an
average diameter (D50) of 1 .mu.m was added to a hydrogen absorbing
alloy powder instead of erbium oxide (Er.sub.2O.sub.3), and
subjected to the same test. The cell was made Example 16.
COMPARATIVE EXAMPLES 5-8
[0170] Test cells having the same composition with those of
Comparative examples 1 to 4 were prepared, except that ytterbium
oxide (Yb.sub.2O.sub.3) powders having an average diameter (D50) of
1 .mu.m were added to hydrogen absorbing alloy powders instead of
erbium oxide (Er.sub.2O.sub.3), and subjected to the same test. The
cells were made Comparative examples 5 to 8.
[0171] Table 2 lists the test results of hydrogen absorbing alloy
powders representing Examples 9 to 16, and Comparative examples 5
to 8 regarding their discharge capacity per 1 g of hydrogen
absorbing alloy powder. TABLE-US-00002 TABLE 2 Treatment Saturation
mass Additive Capacity Classification time (h) susceptibility
(emu/g) Chemical formula Added amount (mAh/g) Example 9 0.87 1
Yb.sub.2O.sub.3 1 Part by weight 315 Example 10 1.3 1.5
Yb.sub.2O.sub.3 1 Part by weight 315 Example 11 1.8 2
Yb.sub.2O.sub.3 1 Part by weight 315 Example 12 2.5 3
Yb.sub.2O.sub.3 1 Part by weight 315 Example 13 3.5 4
Yb.sub.2O.sub.3 1 Part by weight 304 Example 14 4.5 5
Yb.sub.2O.sub.3 1 Part by weight 290 Example 15 5 6 Yb.sub.2O.sub.3
1 Part by weight 277 Example 16 5.5 6.5 Yb.sub.2O.sub.3 1 Part by
weight 270 Comparative example 5 0 0.05 Yb.sub.2O.sub.3 1 Part by
weight 202 Comparative example 6 0.4 0.5 Yb.sub.2O.sub.3 1 Part by
weight 304 Comparative example 7 6 7 Yb.sub.2O.sub.3 1 Part by
weight 263 Comparative example 8 8 10 Yb.sub.2O.sub.3 1 Part by
weight 220
[0172] According to the results shown in Table 2, the hydrogen
absorbing alloy powders receiving the addition of ytterbium oxide
(Yb.sub.2O.sub.3) show the results similar to those of hydrogen
absorbing alloy powders receiving the addition of erbium oxide
(Er.sub.2O.sub.3) that are shown in Table 1. From the results shown
in Table 2 too, the hydrogen absorbing electrode is preferably so
constructed as to give a saturation mass susceptibility of 0.5 to 4
emu/g, more preferably 1 to 3 emu/g to use in the low rate
discharge (discharged via 0.2 ItA).
EXAMPLE 17
[0173] A test cell was prepared in the same manner as in Example 5,
except that 0.3 part by weight of Er.sub.2O.sub.3 in the form of
powder having an average diameter (D50) of 1 .mu.m was added to 100
parts by weight of a hydrogen absorbing alloy powder, and subjected
to the same test. The cell was made Example 17.
EXAMPLE 18
[0174] A test cell was prepared in the same manner as in Example 5,
except that 0.5 part by weight of Er.sub.2O.sub.3 in the form of
powder having an average diameter (D50) of 1 .mu.m was added to 100
parts by weight of a hydrogen absorbing alloy powder, and subjected
to the same test. The cell was made Example 18.
EXAMPLE 19
[0175] A test cell was prepared in the same manner as in Example 5,
except that 0.7 part by weight of Er.sub.2O.sub.3 in the form of
powder having an average diameter (D50) of 1 .mu.m was added to 100
parts by weight of a hydrogen absorbing alloy powder, and subjected
to the same test. The cell was made Example 19.
EXAMPLE 20
[0176] A test cell was prepared in the same manner as in Example 5,
except that 1.5 part by weight of Er.sub.2O.sub.3 in the form of
powder having an average diameter (D50) of 1 .mu.m was added to 100
parts by weight of a hydrogen absorbing alloy powder, and subjected
to the same test. The cell was made Example 20.
COMPARATIVE EXAMPLES 9-12
[0177] Test cells having the same composition with that of Example
5 were prepared, except that 0.1, 2, or 3 parts by weight of
Er.sub.2O.sub.3 was added to 100 parts by weight of a hydrogen
absorbing alloy powder, and subjected to the same test. The cells
were made Comparative examples 9 to 12.
[0178] Table 3 lists the test results of hydrogen absorbing alloy
powders representing Examples 17 to 20, and Comparative examples 9
to 12 regarding their discharge capacity per 1 g of hydrogen
absorbing alloy powder. TABLE-US-00003 TABLE 3 Saturation mass
susceptibility Additive Capacity Classification (emu/g) Chemical
formula Added amount (mAh/g) Example 17 4 Er.sub.2O.sub.3 0.3 Part
by weight 304 Example 18 4 Er.sub.2O.sub.3 0.5 Part by weight 304
Example 19 4 Er.sub.2O.sub.3 0.7 Part by weight 304 Example 5 4
Er.sub.2O.sub.3 1 Part by weight 304 Comparative example 9 4 -- 0
304 Example 20 4 Er.sub.2O.sub.3 1.5 Part by weight 304 Comparative
example 10 4 Er.sub.2O.sub.3 0.1 Part by weight 304 Comparative
example 11 4 Er.sub.2O.sub.3 2 Part by weight 246 Comparative
example 12 4 Er.sub.2O.sub.3 3 Part by weight 182
[0179] According to the results shown in Table 3, being discharged
via 0.2 ItA, the hydrogen absorbing electrode gave a large
discharge capacity exceeding 300 mAh/g, when Er.sub.2O.sub.3 was
added at 1.5 or less part by weight with respect to 100 parts by
weight of hydrogen absorbing alloy powder. In contrast, for
Comparative examples 11 and 12 in which Er.sub.2O.sub.3 was added
at 2 and 3 parts by weight, respectively, with respect to 100 parts
by weight of hydrogen absorbing alloy powder, the hydrogen
absorbing electrode gave a small discharge capacity below 250
mAh/g. The low discharge capacity of Comparative example 11 may be
ascribed to the addition of Er.sub.2O.sub.3 (after being
incorporated in a battery, Er.sub.2O.sub.3 was likely to be
converted for the most part into Er(OH).sub.3) which will interfere
with the conductivity of electrons and migration of charge through
the hydrogen absorbing electrode. Er.sub.2O.sub.3 is preferably
added at 1.5 part or less by weight with respect to 100 parts by
weight of a hydrogen absorbing alloy, because as seen from the
results of Comparative examples 11 and 12, if the added amount of
Er.sub.2O.sub.3 exceeds the above range, the resulting electrode
will not only exhibit a low discharge capacity even when discharged
via 0.2 ItA, but also have its capacity greatly reduced when it
discharge at high-rate.
EXAMPLE 21
[0180] A test cell was prepared in the same manner as in Example
13, except that 0.3 part by weight of Yb.sub.2O.sub.3 in the form
of powder having an average diameter (D50) of 1 .mu.m was added to
100 parts by weight of a hydrogen absorbing alloy powder, and
subjected to the same test. The cell was made Example 21.
EXAMPLE 22
[0181] A test cell was prepared in the same manner as in Example
13, except that 0.5 part by weight of Yb.sub.2O.sub.3 in the form
of powder having an average diameter (D50) of 1 .mu.m was added to
100 parts by weight of a hydrogen absorbing alloy powder, and
subjected to the same test. The cell was made Example 22.
EXAMPLE 23
[0182] A test cell was prepared in the same manner as in Example
13, except that 0.7 part by weight of Yb.sub.2O.sub.3 in the form
of powder having an average diameter (D50) of 1 .mu.m was added to
100 parts by weight of a hydrogen absorbing alloy powder, and
subjected to the same test. The cell was made Example 23.
EXAMPLE 24
[0183] A test cell was prepared in the same manner as in Example
13, except that 1.5 part by weight of Yb.sub.2O.sub.3 in the form
of powder having an average diameter (D50) of 1 .mu.m was added to
100 parts by weight of a hydrogen absorbing alloy powder, and
subjected to the same test. The cell was made Example 24.
COMPARATIVE EXAMPLES 13-15
[0184] Test cells having the same composition with that of Example
13 were prepared, except that 0.1, 2, or 3 parts by weight of
Yb.sub.2O.sub.3 powders having an average diameter (D50) of 1 .mu.m
were added to 100 parts by weight of hydrogen absorbing alloy
powders, and subjected to the same test. The cells were made
Comparative examples 13 to 15.
[0185] Table 4 lists the test results of hydrogen absorbing alloy
powders representing Examples 21 to 24, and Comparative examples 13
to 15 regarding their discharge capacity per 1 g of hydrogen
absorbing alloy powder. TABLE-US-00004 TABLE 4 Saturation mass
susceptibility Additive Capacity Classification (emu/g) Chemical
formula Added amount (mAh/g) Example 21 4 Yb.sub.2O.sub.3 0.3 Part
by weight 304 Example 22 4 Yb.sub.2O.sub.3 0.5 Part by weight 304
Example 23 4 Yb.sub.2O.sub.3 0.7 Part by weight 304 Example 13 4
Yb.sub.2O.sub.3 1 Part by weight 304 Example 24 4 Yb.sub.2O.sub.3
1.5 Part by weight 304 Comparative example 9 4 -- 0 304 Comparative
example 13 4 Yb.sub.2O.sub.3 0.1 Part by weight 304 Comparative
example 14 4 Yb.sub.2O.sub.3 2 Part by weight 237 Comparative
example 15 4 Yb.sub.2O.sub.3 3 Part by weight 182
[0186] According to the results shown in Table 4, it is evident
that Yb.sub.2O.sub.3 is preferably added at 1.5 part or less by
weight with respect to 100 parts by weight of a hydrogen absorbing
alloy, as with Er.sub.2O.sub.3.
(2) Evaluation Based on Cylindrical Nickel Metal-Hydride Storage
Battery
[0187] In Examples 25 to 70, Comparative examples 16 to 32, and
Reference examples 1 to 12 below, evaluation was made on test cells
comprising a hydrogen absorbing electrode.
EXAMPLE 25
[0188] (Preparation of Hydrogen Absorbing Electrode)
[0189] A master plate applied to the test cell of Example 1 in
paragraph (1), was cut to a size of 44 mm.times.130 mm which served
as a hydrogen absorbing electrode. The capacity of the hydrogen
absorbing electrode was 2950 mAh.
[0190] (Preparation of Nickel Electrode)
[0191] Nickel sulfate and zinc sulfate and cobalt sulfate were
added at a specified ratio to water to give an aqueous solution, to
which were added ammonium sulfate and an aqueous solution of
caustic soda to produce a ammine complex. To the reaction system,
under vigorous stirring, was further added as fall in drops as an
aqueous solution of sodium hydroxide until the pH of the reaction
system became pH 11 to 12, to thereby produce nickel hydroxide in
the form of highly dense spherical particles in which nickel
hydroxide, zinc hydroxide, and cobalt hydroxide coexisted at the
weight ratio of 88.45:5.12:1.1. This served as a core mother
material of the electrode.
[0192] The highly dense particles of nickel hydroxide were put into
an alkaline aqueous solution whose pH had been adjusted to pH 10 to
13 with sodium hydroxide. To the resulting solution was added as
fall in drops as an aqueous solution containing cobalt sulfate and
ammonia at specified concentrations under stirring. During the
addition, an aqueous solution of sodium hydroxide was added to the
mixture as needed to maintain the pH of the mixture at pH 10 to 13.
A superficial layer comprising mixed hydroxides including the
hydroxides of cobalt was allowed to form on the surface of
particles of nickel hydroxide by maintaining the pH of the mixture
at pH 11 to 12 for 1 hour. The weight ratio of the mixed hydroxides
comprising the superficial layer against the core mother material
(to be referred to simply as core layer hereinafter) was 4.0 wt
%.
[0193] A 50 g of nickel hydroxide powder in the form of particles
whose surface carried a superficial layer comprising mixed
hydroxides was put into an aqueous solution of 30 wt % (10 M/l)
sodium hydroxide at 110.degree. C., and the mixture was stirred
thoroughly. Then, K.sub.2S.sub.2O.sub.8 was added to the mixture in
an amount in excess of an amount equivalent to the amount of the
hydroxides of cobalt contained in the superficial layer, and
evolution of oxygen gas from the superficial layer was confirmed.
The active material particles were separated by filtration, washed
with water, and dried.
[0194] To the active material particles was added an aqueous
solution of carboxymethylcellulose (CMC) to give a paste in which
the active particles and CMC solute were combined at the weight
ratio of 99.5:0.5. The paste was applied to a nickel porous body
having a surface density of 380 g/m.sup.2 (Nickel Cellmet #8
manufactured by Sumitomo Electric Industries). Then, the body was
dried at 80.degree. C., and pressed into a plate having a specified
thickness. The plate had its surface coated with Teflon (registered
mark), and cut to give a rectangular plate of 44 mm in width and
98.5 mm in length (the area of non-coated strips being 4.times.7
mm) that has a capacity of 1800 mAh. This was used as a nickel
positive electrode plate.
[0195] (Preparation of Cylindrical Storage Battery)
[0196] A plate of a hydrogen absorbing electrode prepared as above,
a sheet of non-woven textile having a thickness of 110 .mu.m and
comprising sulfonated polypropylene which serves as a separator,
and a nickel electrode plate prepared as above were combined one
over another, and rolled together to form a roll, into which was
injected an electrolyte obtained by dissolving lithium hydroxide to
an aqueous solution of 6.8 M/l potassium hydroxide to 0.8 M/l.
Thus, an AA type cylindrical nickel metal-hydride storage battery
was obtained that was equipped with a valve capable of opening to a
pressure of 3 MPa.
[0197] (Chemical Activation)
[0198] A cylindrical nickel metal-hydride storage battery prepared
as described above was left at 20.degree. C. for 12 hours.
[0199] Then, the battery was charged to 600 mAh bypassing 0.02 ItA,
and then 0.1 ItA for 10 hours, discharged to 1V via 0.2 ItA,
charged for 12 hours via 0.1 ItA, and discharged to 1V iva 0.2 ItA.
This cycle was repeated twice.
[0200] Then, the battery was charged for 16 hours via 0.1 ItA, and
discharged to 1V via 0.2 ItA. This cycle was repeated twice. The
capacity of the battery obtained after the second discharge was
taken as 100% discharge capacity of the battery obtained through
the passage of 0.2 ItA.
[0201] (Test of High-Rate Discharge at Low Temperature)
[0202] A activated battery was charged for 16 hours via 0.1 ItA,
left at 5.degree. C. for 5 hours, and discharged to 0.8V via 3 ItA.
The capacity of the battery obtained after the discharge was taken
as a discharge capacity of the battery for 3 ItA discharge.
Evaluation of the battery was performed by expressing the 3 ItA
discharge capacity (%) relative to the 0.2 ItA discharge capacity
(100%). For a battery to be used for applications requiring high
output such as HEVs or electric motor-driven toolselectric
motor-driven tools, it must have a relative discharge capacity
equal to or more than 80%.
[0203] A activated battery was charged for 16 hours via 0.1 ItA,
left at 5.degree. C. for 5 hours, and discharged to 0.8V via 5 ItA.
The capacity of the battery obtained after the discharge was taken
as a discharge capacity of the battery for 5 ItA discharge.
Evaluation of the battery was performed by expressing the 5 ItA
discharge capacity (%) relative to the 0.2 ItA discharge capacity
(100%). For a battery to be used for applications requiring high
output such as HEVs or electric motor-driven tools, it is
particularly preferred that the battery has a relative discharge
capacity equal to or more than 80%.
[0204] A activated battery was charged for 16 hours via 0.1 ItA,
left at 5.degree. C. for 5 hours, and discharged to 0.8V via 8 ItA.
The capacity of the battery obtained after the discharge was taken
as a discharge capacity of the battery for 8 ItA discharge.
Evaluation of the battery was performed by expressing the 8 ItA
discharge capacity (%) relative to the 0.2 ItA discharge capacity
(100%). For a battery to be used for applications requiring high
output such as HEVs or electric motor-driven tools, it is
particularly preferred that the battery has a relative discharge
capacity equal to or more than 85%.
[0205] A activated battery was charged for 16 hours via 0.1 ItA,
left at 5.degree. C. for 5 hours, and discharged to 0.8V via 10
ItA. The capacity of the battery obtained after the discharge was
taken as a discharge capacity of the battery for 10 ItA discharge.
Evaluation of the battery was performed by expressing the 10 ItA
discharge capacity (%) relative to the 0.2 ItA discharge capacity
(100%). For a battery to be used for applications requiring high
output such as HEVs or electric motor-driven tools, it is
particularly preferred that the battery has a relative discharge
capacity equal to or more than 80%.
[0206] Incidentally, batteries having relative discharge capacity
less than 30% for all 3 to 10 ItA discharges when examined by a
high-rate discharge test at low temperature as described above,
were determined to be unable to discharge.
[0207] (Charge/Discharge Cycle Test)
[0208] Activated batteries were subjected to charge/discharge cycle
test at 45.degree. C. or 20.degree. C.
[0209] Test under the ambient temperature of 45.degree. C. was
performed as follows. A test battery was charged via 1 ItA until
-.DELTA.V exhibited the variation of 5 mV, and then discharged via
1 ItA to 1.0 V. The above charge/discharge sequence was taken to
form a cycle, and the cycle was repeated continuously until the
discharge capacity of test battery declined below a level equal to
80% of the discharge capacity of the same battery subsequent to the
first charge/discharge cycle, and the number of cycles delivered
heretofore was taken to represent the cycle life of the
battery.
[0210] Test under the ambient temperature of 20.degree. C. was
performed as follows. A test battery was charged via 0.5 ItA until
-.DELTA.V exhibited the variation of 5 mV, and then discharged via
0.5 ItA for 1.6 hours. The above charge/discharge sequence was
taken to form a cycle, and the cycle was repeated continuously
until the discharge voltage of test battery declined to 0.9 V, and
the number of cycles delivered heretofore was taken to represent
the cycle life of the battery.
[0211] The nickel metal-hydride storage battery is very tough on a
hot ambient temperature. The cycle test under the ambient
temperature of 45.degree. C. was undertaken to evaluate the
endurance of a test battery to a high ambient temperature. In the
high ambient temperature test, a battery preferably has a cycle
life equal to or more than 250 cycles, more preferably 300 cycles,
most preferably 400 cycles. The cycle test under the ambient
temperature of 20.degree. C. was undertaken to evaluate the
endurance of a test battery during the use at normal
temperature.
EXAMPLE 26
[0212] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 2 of the paragraph (1) was cut to give
a rectangle having a size of 44.times.130 mm to serve as a hydrogen
absorbing electrode. The battery was activated, and subjected to
test. The battery was made Example 26.
EXAMPLE 27
[0213] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 3 was cut to give a rectangle having a
size of 44.times.130 mm to serve as a hydrogen absorbing electrode.
The battery was activated, and subjected to test. The battery was
made Example 27.
EXAMPLE 28
[0214] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 4 of the paragraph (1) was cut to give
a rectangle having a size of 44.times.130 mm to serve as a hydrogen
absorbing electrode. The battery was activated, and subjected to
test. The battery was made Example 28.
EXAMPLE 29
[0215] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 5 of the paragraph (1) was cut to give
a rectangle having a size of 44.times.130 mm to serve as a hydrogen
absorbing electrode. The battery was activated, and subjected to
test. The battery was made Example 29.
EXAMPLE 30
[0216] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 6 of the paragraph (1) was cut to give
a rectangle having a size of 44.times.130 mm to serve as a hydrogen
absorbing electrode. The battery was activated, and subjected to
test. The battery was made Example 30.
EXAMPLE 31
[0217] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 7 of the paragraph (1) was cut to give
a rectangle having a size of 44.times.130 mm to serve as a hydrogen
absorbing electrode. The battery was activated, and subjected to
test. The battery was made Example 31.
EXAMPLE 32
[0218] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 8 of the paragraph (1) was cut to give
a rectangle having a size of 44.times.130 mm to serve as a hydrogen
absorbing electrode. The battery was activated, and subjected to
test. The battery was made Example 32.
COMPARATIVE EXAMPLES 16-19
[0219] Cylindrical nickel metal-hydride storage batteries having
the same composition with that of Example 25 were prepared, except
that master plates for the hydrogen absorbing electrodes of test
cells of Comparative examples 1 to 4 were used as hydrogen
absorbing electrodes, and subjected to the same test. The batteries
were made Comparative examples 16 to 19.
[0220] Table 5 lists the test results of cylindrical nickel
metal-hydride storage batteries representing Examples 25 to 32, and
Comparative examples 16 to 19. TABLE-US-00005 TABLE 5 Cycle life
Saturation mass Additive High-rate discharging ability at low
temperature(5.degree. C.) 45.degree. C. 20.degree. C.
susceptibility Chemical 5 ItA 3 ItA atmos- atmos- Classification
(emu/g) formula Added amount 10 ItA discharge 8 ItA discharge
discharge discharge phere phere Example 25 1 Er.sub.2O.sub.3 1 Part
by weight Unable to discharge Unable to discharge Unable to 90% 464
1800 discharge Example 26 1.5 Er.sub.2O.sub.3 1 Part by weight
Unable to discharge Unable to discharge Unable to 91% 464 1800
discharge Example 27 2 Er2O.sub.3 1 Part by weight Unable to
discharge Unable to discharge 89% 92% 464 1800 Example 28 3
Er.sub.2O.sub.3 1 Part by weight Unable to discharge Unable to
discharge 90% 92% 464 1800 Example 29 4 Er.sub.2O.sub.3 1 Part by
weight 84% 87% 91% 92% 417 1650 Example 30 5 Er.sub.2O.sub.3 1 Part
by weight 85% 88% 92% 92% 357 1400 Example 31 6 Er.sub.2O.sub.3 1
Part by weight 84% 88% 92% 92% 301 1200 Example 32 6.5
Er.sub.2O.sub.3 1 Part by weight Unable to discharge 88% 92% 92%
280 1050 Comparative 0.05 Er.sub.2O.sub.3 1 Part by weight Unable
to discharge Unable to discharge Unable to Unable to 104 400
example 16 discharge discharge Comparative 0.5 Er.sub.2O.sub.3 1
Part by weight Unable to discharge Unable to discharge Unable to
Unable to 417 1600 example 17 discharge discharge Comparative 7
Er.sub.2O.sub.3 1 Part by weight Unable to discharge 88% 91% 92%
241 950 example 18 Comparative 10 Er.sub.2O.sub.3 1 Part by weight
Unable to discharge Unable to discharge 90% 92% 57 250 example
19
[0221] As shown in Table 5, Comparative examples 16 and 17 have a
comparatively low high-rate discharge ability while Examples 25 to
32, and Comparative examples 18 and 19 have discharge capacity
equal to or more than 90%. However, the cycle performance of
Comparative examples 18 and 19 is poor. The low high-rate discharge
ability at low temperature of Comparative examples 16 and 17 may be
ascribed to the insufficient activation of hydrogen absorbing alloy
powders incorporated in those batteries as is supported by the
observation that the hydrogen absorbing alloy powders exhibit low
saturation mass susceptibilities. The poor cycle performance of
Comparative examples 18 and 19 may be ascribed to the insufficient
insurance of charge reserve at a negative electrode (hydrogen
absorbing electrode) as is suggested by the low discharge capacity
of hydrogen absorbing alloy as is shown in Table 1 (Comparative
examples 3 and 4).
[0222] From the fact that Examples 25 to 32 are excellent both in
high-rate discharge ability at low temperature and cycle
performance, it is known that a hydrogen absorbing electrode
containing, as an additive, Er.sub.2O.sub.3 and having a saturation
mass susceptibility of 1 to 6.5 meu/g is preferably used.
[0223] In view of improving the high-rate discharge ability at low
temperature of a battery, preferred is the use of a hydrogen
absorbing alloy powder having a saturation mass susceptibility of 2
to 6.5 emu/g, more preferably 4 to 6.5 emu/g, most preferably 4 to
6 emu/g. From this, for an Er.sub.2O.sub.3 added hydrogen absorbing
alloy powder to be excellent in both high-rate discharging ability
at low temperature and cycle performance, a hydrogen absorbing
alloy powder preferably has a saturation mass susceptibility of 2
to 6 emu/g, more preferably 2 to 5 emu/g.
[0224] Incidentally, Examples 25 to 32, when subjected to cycle
test at 20.degree. C., exhibit a life over 1000 cycles, that is,
excellent cycle performance.
EXAMPLE 33
[0225] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 9 was cut to give a rectangle having a
size of 44.times.130 mm to serve as a hydrogen absorbing electrode.
The battery was activated, and subjected to test. The battery was
made Example 33.
EXAMPLE 34
[0226] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 10 was cut to give a rectangle having
a size of 44.times.130 mm to serve as a hydrogen absorbing
electrode. The battery was activated, and subjected to test. The
battery was made Example 34.
EXAMPLE 35
[0227] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 11 was cut to give a rectangle having
a size of 44.times.130 mm to serve as a hydrogen absorbing
electrode. The battery was activated, and subjected to test. The
battery was made Example 35.
EXAMPLE 36
[0228] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 12 was cut to give a rectangle having
a size of 44.times.130 mm to serve as a hydrogen absorbing
electrode. The battery was activated, and subjected to test. The
battery was made Example 36.
EXAMPLE 37
[0229] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 13 was cut to give a rectangle having
a size of 44.times.130 mm to serve as a hydrogen absorbing
electrode. The battery was activated, and subjected to test. The
battery was made Example 37.
EXAMPLE 38
[0230] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 14 was cut to give a rectangle having
a size of 44.times.130 mm to serve as a hydrogen absorbing
electrode. The battery was activated, and subjected to test. The
battery was made Example 38.
EXAMPLE 39
[0231] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 15 was cut to give a rectangle having
a size of 44.times.130 mm to serve as a hydrogen absorbing
electrode. The battery was activated, and subjected to test. The
battery was made Example 39.
EXAMPLE 40
[0232] A cylindrical nickel metal-hydride storage battery was
prepared in the same manner as in Example 25, except that a master
plate prepared as in Example 16 was cut to give a rectangle having
a size of 44.times.130 mm to serve as a hydrogen absorbing
electrode. The battery was activated, and subjected to test. The
battery was made Example 40.
COMPARATIVE EXAMPLES 20-23
[0233] Cylindrical nickel metal-hydride storage batteries having
the same composition with those of Comparative examples 16 to 19
were prepared, except that master plates for the hydrogen absorbing
electrodes of test cells of Comparative examples 5 to 8 were used
as hydrogen absorbing electrodes, and subjected to the same test as
was performed for Comparative examples 16 to 19. The batteries were
made Comparative examples 20 to 23.
[0234] Table 6 lists the test results of cylindrical nickel
metal-hydride storage batteries representing Examples 33 to 40, and
Comparative examples 20 to 23. TABLE-US-00006 TABLE 6 Cycle life
Saturation mass Additive High-rate discharging ability at low
temperature(5.degree. C.) 45.degree. C. 20.degree. C.
susceptibility Chemical 5 ItA 3 ItA atmos- atmos- Classification
(emu/g) formula Added amount 10 ItA discharge 8 ItA discharge
discharge discharge phere phere Example 33 1 Yb.sub.2O.sub.3 1 Part
by weight Unable to discharge Unable to discharge Unable to 89% 522
>2000 discharge Example 34 1.5 Yb.sub.2O.sub.3 1 Part by weight
Unable to discharge Unable to discharge Unable to 90% 522 >2000
discharge Example 35 2 Yb.sub.2O.sub.3 1 Part by weight Unable to
discharge Unable to discharge Unable to 91% 522 >2000 discharge
Example 36 3 Yb.sub.2O.sub.3 1 Part by weight Unable to discharge
Unable to discharge 89% 92% 522 >2000 Example 37 4
Yb.sub.2O.sub.3 1 Part by weight Unable to discharge Unable to
discharge 90% 92% 469 1800 Example 38 5 Yb.sub.2O.sub.3 1 Part by
weight Unable to discharge 87% 89% 91% 402 1600 Example 39 6
Yb.sub.2O.sub.3 1 Part by weight Unable to discharge Unable to
discharge 89% 91% 339 1350 Example 40 6.5 Yb.sub.2O.sub.3 1 Part by
weight Unable to discharge Unable to discharge 89% 91% 305 1200
Comparative 0.05 Yb.sub.2O.sub.3 1 Part by weight Unable to
discharge Unable to discharge Unable to Unable to 44 210 example 20
discharge discharge Comparative 0.5 Yb.sub.2O.sub.3 1 Part by
weight Unable to discharge Unable to discharge Unable to Unable to
469 1550 example 21 discharge discharge Comparative 7
Yb.sub.2O.sub.3 1 Part by weight Unable to discharge Unable to
discharge 89% 90% 272 1100 example 22 Comparative 10
Yb.sub.2O.sub.3 1 Part by weight Unable to discharge Unable to
discharge Unable to 90% 64 250 example 23 discharge
[0235] As shown in Table 6, for hydrogen absorbing electrodes made
from hydrogen absorbing alloy powders to which Yb.sub.2O.sub.3 was
added instead of Er.sub.2O.sub.3, similar relationships as are
observed in Er.sub.2O.sub.3 added hydrogen absorbing electrodes are
found among the high-rate discharging ability at low temperature,
cycle performance, and saturation mass susceptibility of those
electrodes. Specifically, from the fact that Examples 33 to 40 are
excellent both in high-rate discharge ability at low temperature
and cycle performance, it is known that a hydrogen absorbing
electrode containing, as an additive, Yb.sub.2O.sub.3 and having a
saturation mass susceptibility of 1 to 6.5 meu/g is preferably
used.
[0236] As seen from the results shown in Table 6, the
Yb.sub.2O.sub.3 added hydrogen absorbing electrode exhibits a more
excellent cycle performance than the Er.sub.2O.sub.3 added hydrogen
absorbing electrode whose test results are shown in Table 5,
although its high-rate discharge ability at lower temperature is
inferior to that of the latter.
[0237] With regard to Yb.sub.2O.sub.3added hydrogen absorbing
electrodes whose results are shown in Table 6, in view of improving
the high-rate discharge ability at low temperature of a battery,
preferred is the use of a hydrogen absorbing alloy powder having a
saturation mass susceptibility of 3 to 6.5 emu/g, more preferably 5
emu/g. In view of improving the cycle performance of a battery,
preferred is the use of a hydrogen absorbing alloy powder having a
saturation mass susceptibility of 1 to 6.5 emu/g, more preferably 1
to 5 emu/g. Particularly, it is known that a battery incorporating
a hydrogen absorbing alloy powder having a saturation mass
susceptibility of 1 to 3 emu/g will exhibit a cycle performance
over 500 cycle when examined by cycle test at an ambient
temperature of 45.degree. C. From this, for a Yb.sub.2O.sub.3 added
hydrogen absorbing electrode to be excellent in both high-rate
discharging ability at low temperature and cycle performance, a
hydrogen absorbing alloy powder preferably has a saturation mass
susceptibility of 3 to 5 emu/g.
[0238] Moreover, Examples 33 to 40, when subjected to cycle test at
20.degree. C., exhibit a life over 1200 cycles, that is, excellent
cycle performance.
EXAMPLE 41
[0239] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 17 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 41.
EXAMPLE 42
[0240] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 18 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 42.
EXAMPLE 43
[0241] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 19 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 43.
EXAMPLE 44
[0242] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 20 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 44.
COMPARATIVE EXAMPLES 24-27
[0243] Cylindrical nickel metal-hydride storage batteries having
the same composition with that of Example 29 were prepared, except
that master plates for the hydrogen absorbing electrodes of test
cells of Comparative examples 9 to 12 were used as a hydrogen
absorbing electrode, and subjected to the same test. The batteries
were made Comparative examples 24 to 27.
[0244] Table 7 lists the test results of cylindrical nickel
metal-hydride storage batteries representing Example 29, Examples
41 to 44, and Comparative examples 24 to 27. TABLE-US-00007 TABLE 7
Saturation mass Additive High-rate discharging ability at low
temperature(5.degree. C.) Cycle life susceptibility Chemical 10 ItA
8 ItA 5 ItA 45.degree. C. Classification (emu/g) formula Added
amount discharge discharge discharge 3 ItA discharge atmosphere
Example 41 4 Er.sub.2O.sub.3 0.3 Part by weight 84% 87% 91% 92% 349
Example 42 4 Er.sub.2O.sub.3 0.5 Part by weight 84% 87% 91% 92% 334
Example 43 4 Er.sub.2O.sub.3 0.7 Part by weight 84% 87% 91% 92% 405
Example 29 4 Er.sub.2O.sub.3 1 Part by weight 84% 87% 91% 92% 417
Example 44 4 Er.sub.2O.sub.3 1.5 Part by weight 80% 84% 89% 90% 422
Comparative 4 -- 0 84% 87% 91% 92% 156 example 24 Comparative 4
Er.sub.2O.sub.3 0.1 Part by weight 84% 87% 91% 92% 160 example 25
Comparative 4 Er.sub.2O.sub.3 2 Part by weight Unable to Unable to
Unable to Unable to discharge 426 example 26 discharge discharge
discharge Comparative 4 Er.sub.2O.sub.3 3 Part by weight Unable to
Unable to Unable to Unable to discharge 410 example 27 discharge
discharge discharge
[0245] As shown in Table 7, Examples 41 to 44 in which
Er.sub.2O.sub.3 was added at 0.3 to 1 part by weight exhibit
discharge capacity of 80% or more for 10 ItA discharge, which is as
good as that of Example 29. They have a cycle life over 300 cycle
when examined at 45.degree. C. Particularly, Examples 43 and 44
exhibit an excellent cycle performance: their life cycle exceeds
400 cycles which is similar to that of Example 29. Comparative
examples 26 and 27 in which Er.sub.2O.sub.3 was added at 2 and 3
parts by weight respectively exhibit high-rate discharge abilities
at low temperature inferior to those of Examples. It was found that
Comparative examples 26 and 27 are less receptive to electricity
during charging performed prior to discharging. This is probably
because Comparative examples 26 and 27 are given Er.sub.2O.sub.3 in
excess of appropriate amounts, which interferes with the electric
conductivity and charge-transfer reactions through hydrogen
absorbing electrodes, thus leading to the impaired high-rate
discharge abilities at low temperature. As seen from this, the
ratio of the added amount of rare earth elements with respect to
the weight of a hydrogen absorbing alloy powder is an important
factor for determining the high-rate discharge ability at low
temperature and cycle performance of a resulting battery.
[0246] From the results shown in Table 7, for an Er.sub.2O.sub.3
added hydrogen absorbing electrode to be excellent in both
high-rate discharging ability at low temperature and cycle
performance, a hydrogen absorbing alloy powder preferably has 0.3
to 1.5, more preferably 0.7 to 1.5 part by weight of
Er.sub.2O.sub.3 added, because then a resulting exhibits excellent
cycle performance.
[0247] In the case of Comparative example 24, an excellent
high-rate discharging ability at the initial of charge/discharge
cycles was indicated as shown in Table 7, however, according to
result of evaluating the high-rate discharging ability at low
temperature in the middle of charge/discharge cycles performed at
45.degree. C., it became unable to discharge at 3 ItA when the
charge/discharge sequence exceeded about 100 cycles, in contrast,
Examples 41 and 42 after 250 cycles, and Examples 29 and 43 and 44
after 300 cycles exhibit cycle performances over 80% for 3 ItA
discharge. As seen from this, although Comparative example 24 in
which a hydrogen absorbing alloy powder was subjected only to
alkaline immersion treatment has its high-rate performance declined
with the progression of charge/discharge cycles, though the
high-rate performance being at a high level at an initial phase of
charge/discharge cycles, batteries prepared in Examples kept their
excellent high-rate abilities from being impaired during the
progression of charge/discharge cycles, that is, maintained their
excellent high-rate abilities over a prolonged period.
EXAMPLE 45
[0248] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 21 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 45.
EXAMPLE 46
[0249] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 22 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 46.
EXAMPLE 47
[0250] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 23 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 47.
EXAMPLE 48
[0251] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that a master plate for the hydrogen absorbing electrode of a test
cell of Example 24 was used as a hydrogen absorbing electrode, and
subjected to the same test. The battery was made Example 48.
COMPARATIVE EXAMPLES 28-30
[0252] Cylindrical nickel metal-hydride storage batteries having
the same composition with that of Example 37 were prepared, except
that master plates for the hydrogen absorbing electrodes of test
cells of Comparative examples 13 to 15 were used as a hydrogen
absorbing electrode, and subjected to the same test. The batteries
were made Comparative examples 28 to 30.
[0253] Table 8 lists the test results of cylindrical nickel
metal-hydride storage batteries representing Examples 45 to 48, and
Comparative examples 24, and 28 to 30. TABLE-US-00008 TABLE 8
Saturation mass Additive High-rate discharging ability at low
temperature (5.degree. C.) Cycle life Classifi- susceptibility
Chemical 5 ItA 45.degree. C. cation (emu/g) formula Added amount 10
ItA discharge 8 ItA discharge discharge 3 ItA discharge atmosphere
Example 45 4 Yb.sub.2O.sub.3 0.3 Part by weight Unable to discharge
Unable to discharge 90% 92% 314 Example 46 4 Yb.sub.2O.sub.3 0.5
Part by weight Unable to discharge Unable to discharge 90% 92% 347
Example 47 4 Yb.sub.2O.sub.3 0.7 Part by weight Unable to discharge
Unable to discharge 90% 92% 420 Example 37 4 Yb.sub.2O.sub.3 1 Part
by weight Unable to discharge Unable to discharge 90% 92% 469
Example 48 4 Yb.sub.2O.sub.3 1.5 Part by weight Unable to discharge
Unable to discharge 85% 89% 474 Comparative 4 -- 0 84% 87% 91% 92%
156 example 24 Comparative 4 Yb.sub.2O.sub.3 0.1 Part by weight 84%
87% 91% 92% 160 example 28 Comparative 4 Yb.sub.2O.sub.3 2 Part by
weight Unable to discharge Unable to discharge Unable to Unable to
480 example 29 discharge discharge Comparative 4 Yb.sub.2O.sub.3 3
Part by weight Unable to discharge Unable to discharge Unable to
Unable to 460 example 30 discharge discharge
[0254] As shown in Table 8, the improvement in high-rate
discharging ability at low temperature and cycle performance of a
hydrogen absorbing electrode is brought about in the same manner as
in the Er.sub.2O.sub.3 added hydrogen absorbing electrode, through
addition of Yb.sub.2O.sub.3 to a hydrogen absorbing alloy powder.
Specifically, to 100 parts by weight of a hydrogen absorbing alloy
powder, Yb.sub.2O.sub.3 is preferably added at 0.3 to 1.5, most
preferably at 0.7 to 1.5 part by weight, because then it is
possible to obtain a hydrogen absorbing electrode exhibiting an
excellent cycle performance. It was found that Comparative examples
29 and 30, like Comparative examples 26 and 27, are less receptive
to electricity during charging performed prior to discharging.
EXAMPLE 49
[0255] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, Dy.sub.2O.sub.3 in
the form of powder having an average diameter (D50) of 1 .mu.m was
used instead of Er.sub.2O.sub.3, and subjected to the same test.
Incidentally, the Dy.sub.2O.sub.3 used here had a purity equal to
or more than 95%. The battery was made Example 49.
EXAMPLE 50
[0256] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, Ho.sub.2O.sub.3 in
the form of powder having an average diameter (D50) of 1 .mu.m was
used instead of Er.sub.2O.sub.3, and subjected to the same test.
Incidentally, the Ho.sub.2O.sub.3 used here had a purity equal to
or more than 95%. The battery was made Example 50.
EXAMPLE 51
[0257] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, Tm.sub.2O.sub.3 in
the form of powder having an average diameter (D50) of 1 .mu.m was
used instead of Er.sub.2O.sub.3, and subjected to the same test.
Incidentally, the Tm.sub.2O.sub.3 used here had a purity equal to
or more than 95%. The battery was made Example 51.
EXAMPLE 52
[0258] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, Lu.sub.2O.sub.3 in
the form of powder having an average diameter (D50) of 1 .mu.m was
used instead of Er.sub.2O.sub.3, and subjected to the same test.
Incidentally, the Lu.sub.2O.sub.3 used here had a purity equal to
or more than 95%. The battery was made Example 52.
COMPARATIVE EXAMPLES 31 AND 32
[0259] Cylindrical nickel metal-hydride storage batteries having
the same composition with that of Example 29 were prepared, except
that during the preparation of a hydrogen absorbing electrode,
Y.sub.2O.sub.3 or Gd.sub.2O.sub.3 in the form of powder having an
average diameter (D50) of 1 .mu.m was used instead of
Er.sub.2O.sub.3, and subjected to the same test. The batteries were
made Comparative examples 31 and 32.
[0260] Table 9 lists the test results of cylindrical nickel
metal-hydride storage batteries representing Examples 29 and 37 and
49 to 52, and Comparative examples 24, and 31 and 32.
TABLE-US-00009 TABLE 9 Saturation mass Additive High-rate
discharging ability at low temperature (5.degree. C.) Cycle life
Classifi- susceptibility Chemical 5 ItA 45.degree. C. cation
(emu/g) formula Added amount 10 ItA discharge 8 ItA discharge
discharge 3 ItA discharge atmosphere Example 49 4 Dy.sub.2O.sub.3 1
Part by weight Unable to discharge 88% 91% 92% 349 Example 50 4
Ho.sub.2O.sub.3 1 Part by weight Unable to discharge 88% 92% 92%
334 Example 29 4 Er.sub.2O.sub.3 1 Part by weight 84% 87% 91% 92%
417 Example 51 4 Tm.sub.2O.sub.3 1 Part by weight Unable to
discharge 88% 92% 92% 313 Example 37 4 Yb.sub.2O.sub.3 1 Part by
weight Unable to discharge Unable to 90% 92% 469 discharge Example
52 4 Lu.sub.2O.sub.3 1 Part by weight Unable to discharge Unable to
89% 90% 386 discharge Comparative 4 -- 0 84% 87% 91% 92% 156
example 24 Comparative 4 Y.sub.2O.sub.3 1 Part by weight Unable to
discharge 87% 90% 92% 244 example 31 Comparative 4 Gd.sub.2O.sub.3
2 Part by weight Unable to discharge Unable to Unable to 88% 161
example 32 discharge discharge
[0261] As shown in Table 9, the batteries of Examples 49, 29, 50,
51, 37 and 52 in which Dy.sub.2O.sub.3, Er.sub.2O.sub.3,
Ho.sub.2O.sub.3, Tm.sub.2O.sub.3, Yb.sub.2O.sub.3, and
Lu.sub.2O.sub.3 were added to respective hydrogen absorbing alloy
powders are superior in cycle performance when examined at
45.degree. C. to the battery of Comparative example 31 in which
Y.sub.2O.sub.3 was added; and
[0262] are superior in both high-rate discharging ability at low
temperature and cycle performance at 45.degree. C. to the battery
of Comparative example 24 in which no rare earth element was added
and Comparative example 32 in which Gd.sub.2O.sub.3 was added. It
is evident, therefore, that addition of these oxides of the rare
earth elements is effective for having the high-rate discharging
ability and improving cycle performance of a nickel metal-hydride
storage battery incorporating a hydrogen absorbing electrode
containing, as an active material, hydrogen absorbing alloy.
[0263] From Table 9, it is known that the battery of Example 29 in
which Er.sub.2O.sub.3 was added exhibits a particularly excellent
high-rate discharging ability at low temperature as compared with
other Examples in which rare earth elements other than
Er.sub.2O.sub.3 were added.
[0264] From Table 9, it is further known that the battery of
Example 37 in which Yb.sub.2O.sub.3 was added exhibits an excellent
cycle performance as compared with other Examples in which rare
earth elements other than Yb.sub.2O.sub.3 were added.
EXAMPLE 53
[0265] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, a mixture of 90
parts by weight of Er.sub.2O.sub.3 and 10 parts by weight of
Gd.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Er.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 53.
EXAMPLE 54
[0266] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, a mixture of 80
parts by weight of Er.sub.2O.sub.3 and 20 parts by weight of
Gd.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Er.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 54.
EXAMPLE 55
[0267] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 37, a mixture of 90
parts by weight of Yb.sub.2O.sub.3 and 10 parts by weight of
Gd.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Yb.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 55.
EXAMPLE 56
[0268] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 37, a mixture of 80
parts by weight of Yb.sub.2O.sub.3 and 20 parts by weight of
Gd.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Yb.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 56.
REFERENCE EXAMPLES 1 AND 2
[0269] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, a mixture of 70
parts by weight of Er.sub.2O.sub.3 and 30 parts by weight of
Gd.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Er.sub.2O.sub.3, and subjected
to the same test. The battery was made Reference example 1.
[0270] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 37, a mixture of 70
parts by weight of Yb.sub.2O.sub.3 and 30 parts by weight of
Gd.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Yb.sub.2O.sub.3, and subjected
to the same test. The battery was made Reference example 2.
EXAMPLE 57
[0271] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, a mixture of 90
parts by weight of Er.sub.2O.sub.3 and 10 parts by weight of
Yb.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Er.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 57.
EXAMPLE 58
[0272] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 29 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 29, a mixture of 70
parts by weight of Er.sub.2O.sub.3 and 30 parts by weight of
Yb.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Er.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 58.
EXAMPLE 59
[0273] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 37, a mixture of 90
parts by weight of Yb.sub.2O.sub.3 and 10 parts by weight of
Er.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Yb.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 59.
EXAMPLE 60
[0274] A cylindrical nickel metal-hydride storage battery having
the same composition with that of Example 37 was prepared, except
that during the preparation of a master plate for a hydrogen
absorbing electrode performed as in Example 37, a mixture of 70
parts by weight of Yb.sub.2O.sub.3 and 30 parts by weight of
Er.sub.2O.sub.3 in the form of powder having an average diameter
(D50) of 1 .mu.m was used instead of Yb.sub.2O.sub.3, and subjected
to the same test. The battery was made Example 60.
[0275] Table 10 lists the test results of cylindrical nickel
metal-hydride storage batteries representing Examples 53 to 60, and
Reference examples 1 and 2. TABLE-US-00010 TABLE 10 Saturation mass
Additive High-rate discharging ability at low temperature
(5.degree. C.) Cycle life Classifi- susceptibility Chemical Weight
5 ItA 3 ItA 45.degree. C. cation (emu/g) formula ratio Added amount
10 ItA discharge 8 ItA discharge discharge discharge atmosphere
Example 53 4 Er.sub.2O.sub.3/Gd.sub.2O.sub.3 90/10 1 Part by weight
84% 87% 91% 92% 401 Example 54 4 Er.sub.2O.sub.3/Gd.sub.2O.sub.3
80/20 1 Part by weight Unable to discharge 88% 91% 92% 333
Reference 4 Er.sub.2O.sub.3/Gd.sub.2O.sub.3 70/30 1 Part by weight
Unable to discharge Unable to 91% 92% 238 example 1 discharge
Example 55 4 Yb.sub.2O.sub.3/Gd.sub.2O.sub.3 90/10 1 Part by weight
Unable to discharge Unable to 90% 92% 443 discharge Example 56 4
Yb.sub.2O.sub.3/Gd.sub.2O.sub.3 80/20 1 Part by weight Unable to
discharge Unable to 90% 92% 301 discharge Reference 4
Yb.sub.2O.sub.3/Gd.sub.2O.sub.3 70/30 1 Part by weight Unable to
discharge Unable to 90% 92% 262 example 2 discharge Example 57 4
Er.sub.2O.sub.3/Yb.sub.2O.sub.3 90/10 1 Part by weight 81% 84% 91%
92% 419 Example 58 4 Er.sub.2O.sub.3/Yb.sub.2O.sub.3 70/30 1 Part
by weight Unable to discharge Unable to 91% 92% 427 discharge
Example 59 4 Yb.sub.2O.sub.3/Er.sub.2O.sub.3 90/10 1 Part by weight
Unable to discharge Unable to 91% 92% 448 discharge Example 60 4
Yb.sub.2O.sub.3/Er.sub.2O.sub.3 70/30 1 Part by weight Unable to
discharge Unable to 91% 92% 435 discharge
[0276] As shown in Table 10, the battery representing Reference
example 1 in which the relative content of Er, out of the rare
earth elements contained in oxides of rare earth elements added to
an hydrogen absorbing alloy powder is 70 wt % is inferior both in
high-rate discharging ability at low temperature and cycle
performance, as compared with Examples 53 and 54 in which the
relative contents of Er are 90 and 80 wt %, respectively.
[0277] The battery representing Reference example 2 in which the
relative content of Yb is 70 wt % is inferior in cycle performance,
as compared with the batteries representing Examples 55 and 56 in
which the relative contents of Yb is 90 and 80 wt %,
respectively.
[0278] As seen from above, it is seen that the effect of adding Er,
Yb is remarkable and it is possible to obtain a hydrogen absorbing
electrode and a nickel metal-hydride storage battery which are
superior in high-rate discharging ability at low temperature and
cycle performance, when the relative content of Er, Yb out of rare
earth elements contained in oxides of rare earth elements added to
an hydrogen absorbing alloy powder is 80 wt % or more, particularly
90 wt % or more In contrast, if the addition amount of Er or Yb is
below 80 wt % of purity, the effect obtainable from the addition of
Er or Yb will hardly be obtained.
[0279] When the oxide of a rare earth element such as
Dy.sub.2O.sub.3, Ho.sub.2O.sub.3, Tm.sub.2O.sub.3, Yb.sub.2O.sub.3,
or Lu.sub.2O.sub.3 is added to a hydrogen absorbing alloy powder,
its addition amount should also be adjusted such that the relative
content of the rare earth element becomes preferably 80 wt % or
more, more preferably 90 wt % or more, although the reason for this
will not be detailed here.
[0280] The battery of Example 57 to 60 in which a mixture of 90
parts by weight of Er.sub.2O.sub.3 and 10 parts by weight of
Yb.sub.2O.sub.3 was added to a hydrogen absorbing alloy powder as
shown in Table 10, is somewhat improved in its cycle performance as
compared with the battery of Example 29 whose test results are
shown in Table 5, indicating the addition of rare earth elements
leading to improvement of the performance of a resulting battery.
In contrast, the battery of Example 58 in which a mixture of 70
parts by weight of Er.sub.2O.sub.3 and 30 parts by weight of
Yb.sub.2O.sub.3 was added did not show a notable improvement in its
high-rate discharging ability at low temperature, an effect
characteristically observed for the addition of a rare earth
element.
[0281] The battery of Example 59 in which a mixture of 90 parts by
weight of Yb.sub.2O.sub.3 and 10 parts by weight of Er.sub.2O.sub.3
was added to a hydrogen absorbing alloy powder is somewhat improved
in its high-rate discharging ability at low temperature, though
somewhat degraded in cycle performance as compared with the battery
of Example 37 whose test results are shown in Table 6, indicating
the addition of rare earth elements leading to improvement of the
performance of a resulting battery. In contrast, the battery of
Example 60 in which a mixture of 70 parts by weight of
Yb.sub.2O.sub.3 and 30 parts by weight of Er.sub.2O.sub.3 was added
did not show a notable improvement in its high-rate discharging
ability at low temperature, an effect characteristically observed
for the addition of a rare earth element. Thus, to obtain effects
characteristic with the addition of Er,Yb, it is preferred to use
Er, Yb alone, even if rare earth elements are mixed and added, the
relative content (weight ratio) of Er,Yb to added rare earth
elements is preferably 90 wt % or more.
EXAMPLE 61
[0282] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 29 was prepared, except that a hydrogen absorbing alloy
powder having an average diameter of 10 .mu.m was used, and
subjected to the same test. The battery was made Example 61.
EXAMPLE 62
[0283] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 29 was prepared, except that during the preparation of a
hydrogen absorbing electrode, a hydrogen absorbing alloy powder
having an average diameter of 20 .mu.m was used, and subjected to
the same test. The battery was made Example 62.
REFERENCE EXAMPLES 3-5
[0284] Sealed nickel metal-hydride storage batteries including
their hydrogen absorbing alloy powders with a saturation mass
susceptibility having the same composition and property with those
of Example 29 were prepared, except that during the preparation of
a hydrogen absorbing electrode, hydrogen absorbing alloy powders
having an average diameter of 5, 40, and 50 .mu.m were used, and
subjected to the same test. The batteries were made Reference
examples 3 to 5.
EXAMPLE 63
[0285] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 37 was prepared, except that during the preparation of a
hydrogen absorbing electrode, a hydrogen absorbing alloy powder
having an average diameter of 10 .mu.m was used, and subjected to
the same test. The battery was made Example 63.
EXAMPLE 64
[0286] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 37 was prepared, except that during the preparation of a
hydrogen absorbing electrode, a hydrogen absorbing alloy powder
having an average diameter of 20 .mu.m was used, and subjected to
the same test. The battery was made Example 64.
REFERENCE EXAMPLES 6-8
[0287] Sealed nickel metal-hydride storage batteries including
their hydrogen absorbing alloy powders with a saturation mass
susceptibility having the same composition and property with those
of Example 37 were prepared, except that during the preparation of
a hydrogen absorbing electrode, hydrogen absorbing alloy powders
having an average diameter of 5, 40, and 50 .mu.m were used, and
subjected to the same test. The batteries were made Reference
examples 6 to 8.
[0288] Table 11 lists the test results of cylindrical nickel
metal-hydride storage batteries representing Examples 61 to 64, and
Reference examples 3 to 8, together with those of Example 29 and
37. TABLE-US-00011 TABLE 11 Average High-rate discharging particle
Saturation mass Additive ability at low temperature (5.degree. C.)
Cycle life diameter Treatment susceptibility Chemical 10 ItA 8 ItA
5 ItA 3 ItA 45.degree. C. Classification (.mu.m) time (emu/g)
formula Added amount discharge discharge discharge discharge
atmosphere Example 61 10 2.0 4 Er.sub.2O.sub.3 1 Part by weight 89%
91% 92% 92% 180 Example 62 20 3.0 4 Er.sub.2O.sub.3 1 Part by
weight 86% 89% 91% 92% 396 Example 29 30 3.5 4 Er.sub.2O.sub.3 1
Part by weight 84% 87% 91% 92% 417 Reference 5 1.0 4
Er.sub.2O.sub.3 1 Part by weight 90% 91% 92% 92% 95 example 3
Reference 40 4.0 4 Er.sub.2O.sub.3 1 Part by weight Unable to
Unable to 90% 92% 230 example 4 discharge discharge Reference 50
6.0 4 Er.sub.2O.sub.3 1 Part by weight Unable to Unable to Unable
to 91% 200 example 5 discharge discharge discharge Example 63 10
2.0 4 Yb.sub.2O.sub.3 1 Part by weight 86% 89% 91% 92% 160 Example
64 20 3.0 4 Yb.sub.2O.sub.3 1 Part by weight 84% 87% 91% 92% 420
Example 37 30 3.5 4 Yb.sub.2O.sub.3 1 Part by weight Unable to
Unable to 90% 92% 469 discharge discharge Reference 5 1.0 4
Yb.sub.2O.sub.3 1 Part by weight 89% 91% 92% 92% 60 example 6
Reference 40 4.0 4 Yb.sub.2O.sub.3 1 Part by weight Unable to
Unable to 90% 92% 230 example 7 discharge discharge Reference 50
6.0 4 Yb.sub.2O.sub.3 1 Part by weight Unable to Unable to Unable
to 91% 200 example 8 discharge discharge discharge
[0289] According to the results shown in Table 11, it is evident
that a hydrogen absorbing alloy powder having an average diameter
of 10 to 30 .mu.m is preferably used, because then it is possible
to obtain a battery excellent both in high-rate discharging ability
at low temperature and cycle performance, and that it is more
preferable to use a hydrogen absorbing alloy powder having an
average diameter of 20 to 30 .mu.m, because then it is possible to
obtain a battery particularly excellent in cycle performance. In
contrast, the batteries of Reference examples 3 and 6 in which a
hydrogen absorbing alloy powder having an average diameter of 5
.mu.m was used is inferior in cycle performance, although their
high-rate discharging ability at low temperature is excellent. On
the other hand, the batteries of Reference examples 4 and 7 in
which a hydrogen absorbing alloy powder having an average diameter
of 40 .mu.m was used, and of Reference examples 5 and 8 in which a
hydrogen absorbing alloy powder having an average diameter of 50
.mu.m was used, were all inferior in high-rate discharging ability
at low temperature, as compared with the batteries of Examples.
REFERENCE EXAMPLES 9 AND 10
[0290] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 5 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous KOH
solution containing 28 wt % KOH at 100.degree. C. for 2 hours was
used, and subjected to the same test. The test cell was made
Reference example 9.
[0291] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 5 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous LiOH
solution containing 10 wt % LiOH at 100.degree. C. for 24 hours was
used, and subjected to the same test. The test cell was made
Reference example 10.
REFERENCE EXAMPLES 11 AND 12
[0292] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 13 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous KOH
solution containing 28 wt % KOH at 100.degree. C. for 2 hours was
used, and subjected to the same test. The test cell was made
Reference example 11.
[0293] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 13 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous LiOH
solution containing 10 wt % LiOH at 100.degree. C. for 24 hours was
used, and subjected to the same test. The test cell was made
Reference example 12.
REFERENCE EXAMPLES 13 AND 14
[0294] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 29 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous KOH
solution containing 28 wt % KOH at 100.degree. C. for 2 hours was
used, and subjected to the same test. The test cell was made
Reference example 13.
[0295] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 29 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous LiOH
solution containing 10 wt % LiOH at 100.degree. C. for 24 hours was
used, and subjected to the same test. The test cell was made
Reference example 14.
REFERENCE EXAMPLES 15 AND 16
[0296] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 37 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous KOH
solution containing 28 wt % KOH at 100.degree. C. for 2 hours was
used, and subjected to the same test. The test cell was made
Reference example 15.
[0297] A test cell including its hydrogen absorbing electrode with
a saturation mass susceptibility having the same composition and
property with those of Example 37 was prepared, except that during
the preparation of a hydrogen absorbing electrode, a hydrogen
absorbing alloy powder which had been immersed in an aqueous LiOH
solution containing 10 wt % LiOH at 100.degree. C. for 24 hours was
used, and subjected to the same test. The test cell was made
Reference example 16.
[0298] Table 12 lists the test results of test cells representing
Reference examples 9 to 12 together with those of Examples 5 and
13, and Table 13 lists the test results of nickel metal-hydride
storage batteries representing Reference examples 13 to 16 together
with those of Examples 29 and 37. TABLE-US-00012 TABLE 12
Saturation mass Additive High-rate discharging ability at low
temperature (5.degree. C.) Cycle life Classifi- Treatment
susceptibility Chemical 8 ItA 5 ItA 45.degree. C. cation solution
(emu/g) formula Added amount 10 ItA discharge discharge discharge 3
ItA discharge atmosphere Example 5 NaOH 4 Er.sub.2O.sub.3 1 Part by
weight 84% 87% 91% 92% 417 Reference KOH 4 Er.sub.2O.sub.3 1 Part
by weight Unable to discharge Unable to 89% 92% 300 example 9
discharge Reference LiOH 4 Er.sub.2O.sub.3 1 Part by weight 86% 88%
91% 92% 340 example 10 Example 13 NaOH 4 Yb.sub.2O.sub.3 1 Part by
weight Unable to discharge 85% 90% 92% 469 Reference KOH 4
Yb.sub.2O.sub.3 1 Part by weight Unable to discharge Unable to
Unable to 91% 350 example 11 discharge discharge Reference LiOH 4
Yb.sub.2O.sub.3 1 Part by weight Unable to discharge 88% 91% 92%
390 example 12
[0299] TABLE-US-00013 TABLE 13 Saturation mass susceptibility
Additive Capacity Classification Treatment solution Treatment time
(emu/g) Chemical formula Added amount (mAh/g) Example 29 NaOH 3.5 4
Er.sub.2O.sub.3 1 Part by weight 304 Reference exampl 13 KOH 2 4
Er.sub.2O.sub.3 1 Part by weight 304 Reference exampl 14 LiOH 24 4
Er.sub.2O.sub.3 1 Part by weight 304 Example 37 NaOH 3.5 4
Yb.sub.2O.sub.3 1 Part by weight 304 Reference exampl 15 KOH 2 4
Yb.sub.2O.sub.3 1 Part by weight 304 Reference exampl 16 LiOH 24 4
Yb.sub.2O.sub.3 1 Part by weight 304
[0300] As shown in Tables 12 and 13, immersion treatment using an
aqueous NaOH solution is more preferred than the treatment using an
aqueous LiOH solution, because the former treatment provides a
nickel metal-hydride storage battery having a more excellent cycle
performance with a far shorter period of immersing treatment. The
treatment is also more preferred than the treatment using an
aqueous KOH solution, because it provides a nickel metal-hydride
storage battery more excellent in high-rate discharging ability at
low temperature and cycle performance.
[0301] In comparison to a hydrogen absorbing alloy powder treated
with an aqueous KOH solution, a hydrogen absorbing alloy powder
treated with an aqueous NaOH solution shows more compact and
uniform surfaces devoid of indentations. These surfaces act as a
protector against the corrosion which would otherwise penetrate the
surface into the interior. This is probably the reason why the
immersion treatment enables the production of a nickel
metal-hydride storage battery excellent in high-rate discharging
ability at low temperature and cycle performance.
EXAMPLE 65
[0302] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 30 was prepared, except that during the preparation of a
hydrogen absorbing electrode, Er.sub.2O.sub.3 in the form of powder
having an average diameter (D50) of 0.1 .mu.m was added to a
hydrogen absorbing alloy powder, and subjected to the same test.
The battery was made Example 65.
EXAMPLE 66
[0303] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 30 was prepared, except that during the preparation of a
hydrogen absorbing electrode, Er.sub.2O.sub.3 in the form of powder
having an average diameter (D50) of 3 .mu.m was added to a hydrogen
absorbing alloy powder, and subjected to the same test. The battery
was made Example 66.
EXAMPLE 67
[0304] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 30 was prepared, except that during the preparation of a
hydrogen absorbing electrode, Er.sub.2O.sub.3 in the form of powder
having an average diameter (D50) of 5 .mu.m was added to a hydrogen
absorbing alloy powder, and subjected to the same test. The battery
was made Example 67.
EXAMPLE 68
[0305] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 38 was prepared, except that during the preparation of a
hydrogen absorbing electrode, Yb.sub.2O.sub.3 in the form of powder
having an average diameter (D50) of 0.1 .mu.m was added to a
hydrogen absorbing alloy powder, and subjected to the same test.
The battery was made Example 68.
EXAMPLE 69
[0306] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 38 was prepared, except that during the preparation of a
hydrogen absorbing electrode, Yb.sub.2O.sub.3 in the form of powder
having an average diameter (D50) of 3 .mu.m was added to a hydrogen
absorbing alloy powder, and subjected to the same test. The battery
was made Example 69.
EXAMPLE 70
[0307] A sealed nickel metal-hydride storage battery including its
hydrogen absorbing alloy powder with a saturation mass
susceptibility having the same composition and property with those
of Example 38 was prepared, except that during the preparation of a
hydrogen absorbing electrode, Yb.sub.2O.sub.3 in the form of powder
having an average diameter (D50) of 5 .mu.m was added to a hydrogen
absorbing alloy powder, and subjected to the same test. The battery
was made Example 70.
[0308] Table 14 lists the test results of Examples 65 to 70
together with those of Examples 30 and 38. TABLE-US-00014 TABLE 14
Hydrogen absorbing alloy Additive Saturation Average mass
susceptibility Average particle diameter Chemical particle diameter
Cycle life Classification (emu/g) (.mu.m) formula (.mu.m) Added
amount 45.degree. C. atmosphere Example 65 5 30 Er.sub.2O.sub.3 0.1
1 Part by weight 393 Example 30 5 30 Er.sub.2O.sub.3 1 1 Part by
weight 357 Example 66 5 30 Er.sub.2O.sub.3 3 1 Part by weight 339
Example 67 5 30 Er.sub.2O.sub.3 5 1 Part by weight 264 Example 68 5
30 Yb.sub.2O.sub.3 0.1 1 Part by weight 442 Example 38 5 30
Yb.sub.2O.sub.3 1 1 Part by weight 402 Example 69 5 30
Yb.sub.2O.sub.3 3 1 Part by weight 381 Example 70 5 30
Yb.sub.2O.sub.3 5 1 Part by weight 286
[0309] It is known from the results shown in Table 14 that the
oxide of a rare earth element, whether it is Er.sub.2O.sub.3 or
Yb.sub.2O.sub.3, having an average diameter (D50) of 0.1 to 3
.mu.m, particularly 0.1 to 1 .mu.m will provide a battery more
excellent in cycle performance at 45.degree. C. than the oxide
having an average diameter of 5 .mu.m. As seen from above, usually
addition of Er.sub.2O.sub.3 or Yb.sub.2O.sub.3 having a small
average diameter (D50) allows the production of a battery excellent
in cycle performance. This is probably because the coat formed by
powder Er.sub.2O.sub.3 or Yb.sub.2O.sub.3 additive having a smaller
diameter on the surface of a hydrogen absorbing alloy powder is
more protective against corrosion. Incidentally, the high-rate
discharging ability of a battery is practically the same,
regardless of the average diameter (D50) of powdery Er.sub.2O.sub.3
or Yb.sub.2O.sub.3 added, although the relevant data are not cited
in Table 14.
[0310] It is also possible, although this is not demonstrated in
connection with any Examples and Reference examples cited above, to
obtain a hydrogen absorbing electrode more excellent both in
high-rate discharging ability and cycle performance by taking
oxides or hydroxides of two or more rare earth elements selected
from a group consisting of Dy, Ho, Er, Tm, Yb and Lu such that the
chosen rare earth elements account for 80 wt % or more, more
preferably 90 wt % or more of the total weight of the oxides or
hydroxides, and adding the oxides or hydroxides to a hydrogen
absorbing alloy powder, as compared with a hydrogen absorbing
electrode for which no compound of rare earth element was
added.
[0311] In Examples 71 to 85, and Comparative examples 33 to 39
described below, the relationship of the average diameter of powder
and an anti-corrosion agent with the performance of a battery
containing such an anti-corrosion agent was studied.
EXAMPLE 71
[0312] (Preparation of Positive Electrode Plate)
[0313] Nickel sulfate and zinc sulfate and cobalt sulfate were
added at a specified ratio to water to give an aqueous solution, to
which were added ammonium sulfate and an aqueous solution of sodium
hydroxide to produce an ammine complex. To the reaction system,
under vigorous stirring, was further added as fall in drops as an
aqueous solution of sodium hydroxide until the pH of the reaction
system became pH 11 to 12, to thereby produce nickel hydroxide in
the form of highly dense spherical particles in which nickel
hydroxide, zinc hydroxide, and cobalt hydroxide coexisted at the
weight ratio of 88.45:5.12:1.1.
[0314] The highly dense particles of nickel hydroxide were put into
an alkaline aqueous solution whose pH had been adjusted to pH 11 to
12 with sodium hydroxide. To the resulting solution was added as
fall in drops as an aqueous solution containing cobalt sulfate and
ammonium sulfate at specified concentrations under stirring. During
the addition, an aqueous solution of sodium hydroxide was added as
fall in drops to the mixture as needed to maintain the pH of the
mixture at pH 11 to 12. A superficial layer comprising mixed
hydroxides including the hydroxides of cobalt was allowed to form
on the surface of particles of nickel hydroxide by maintaining the
pH of the mixture at pH 11 to 12 for 1 hour. The weight ratio of
the mixed hydroxides constituting the superficial layer against the
core mother material (to be referred to simply as core layer
hereinafter) was 4.0 wt %.
[0315] A 50 g of nickel hydroxide in the form of particles whose
surface carried a superficial layer comprising mixed hydroxides was
put into an aqueous solution of 30 wt % (10 mol/dm.sup.3) sodium
hydroxide at 110.degree. C., and the mixture was stirred
thoroughly. Then, K.sub.2S.sub.2O.sub.8 was added to the mixture in
an amount in excess of an amount equivalent to the amount of the
hydroxides of cobalt contained in the superficial layer, and
evolution of oxygen gas from the superficial layer was confirmed.
The active material particles were separated by filtration, washed
with water, and dried.
[0316] To a mixed powder comprising the active material particles
and a Yb(OH).sub.3 powder having an average diameter of 5 .mu.m,
was added an aqueous solution of carboxymethylcellulose (CMC) to
give a paste in which the active particles, Yb(OH).sub.3 powder,
and CMC (solid portion) were combined at the weight ratio of
100:2:0.5. The paste was applied to a nickel porous body having a
surface density of 450 g/m.sup.2 (Nickel Cellmet #8 manufactured by
Sumitomo Electric Industries). Then, the body was dried at
80.degree. C., and pressed into a plate having a specified
thickness. The plate was cut to give a rectangular plate of 48.5 mm
in width and 1100 mm in length with non-coated strips along the
long sides having a width of 1.5 mm that has a capacity of 6500 mAh
(6.5 Ah). This was used as a nickel positive electrode plate.
[0317] (Preparation of a Negative Electrode Plate)
[0318] A hydrogen absorbing alloy in the form of powder having an
average diameter of 20 .mu.m and belonging to an AB.sub.5 type rare
earth element system whose composition is represented by
MmNi.sub.4.0Co.sub.0.55Al.sub.0.35Mn.sub.0.30 (Mm refers to a
misch-metal representing a mixture of La, Ce, Pr, and Nd at a
weight ratio of La:Ce:Pr:Nd=70:22:2:6) was immersed in an aqueous
solution of 48 wt % NaOH (in terms of the specific gravity at
20.degree. C.) kept at 100.degree. C. for 1.3 hour to provide an
alloy with a saturation mass susceptibility of 2 emu/g. Then, the
mixture was filtered under pressure to separate the alloy from the
treatment solution. To the alloy was added pure water having the
same weight, and the resulting mixture was exposed for 10 minutes
to an ultrasonic wave having the frequency of 28 kHz. Then, while
the mixture was being stirred gently, pure water was injected below
a stirred suspension layer and excess water was allowed to flow
out, thereby purging hydroxides of rare earth metal liberated from
the alloy powder for disposal. Then, reducing was continued until
the pH of spilled water became pH10 or lower. The remaining mixture
was filtered under pressure. Desorption of hydrogen was achieved by
exposing the hydrogen absorbing alloy powder to water warmed to
80.degree. C. The warm water was filtered under pressure, and
washed with water again. The alloy was cooled to 25.degree. C., to
which was added 4% hydrogen peroxide solution having the same
weight with that of the alloy to desorb hydrogen. Thus, a hydrogen
absorbing alloy powder for the construction of an electrode was
obtained. The hydrogen absorbing alloy powder had a saturation mass
susceptibility of 2 emu/g.
[0319] Powder of Er.sub.2O.sub.3 commercially available (average
diameter of 10 .mu.m) was wet-ground by a satellite ball mill using
water as a dispersant. The yield was removed of moisture to be
dried, and sieved by a blower sorting machine to provide powder of
Er.sub.2O.sub.3 having an average diameter of 0.3 .mu.m.
[0320] To 100 parts by weight of a hydrogen absorbing alloy powder,
was added 1 part by weight of erbium oxide (Er.sub.2O.sub.3) for
mixture, which was followed by the addition of 0.65 part by weight
of a styrenebutadiene copolymer (SBR), and 0.3 part by weight of
hydroxypropylmethylcellulose (HPMC) for mixture. To the yield was
added a specified amount of water and the mixture was kneaded to
form a paste. The paste was applied with a blade coater onto a
punched steel plate made of a nickel-coated iron plate; The product
was kept at 80.degree. C. to be dried, and pressed to give a master
plate for hydrogen absorbing electrode which was a rectangular
plate of 48.5 mm in width and 1180 mm in length with non-coated
strips along the long sides having a width of 1.5 mm that has a
capacity of 11000 mAh (11.0 Ah). This was used as a negative
electrode plate (hydrogen absorbing electrode). For reference, the
amount of the hydrogen absorbing alloy powder applied to every 1
cm.sup.2 area of the negative electrode was 0.07 g.
[0321] An electrode plate shaped like a square coated with the
active material having a size of 30 mm.times.30 mm was cut from the
negative electrode, and used as a negative electrode plate of a
test cell.
[0322] (Preparation of a Test Test Cell Comprising a Negative
Electrode, and Evaluation Test)
[0323] An open type cell was obtained by sandwiching a negative
electrode plate serving as a test negative test cell by two
separators (made of the same material as that of a separator of a
sealed nickel metal-hydride battery described later) and placing,
outside the separators, two nickel electrode plates having a
capacity two times as large as that of the negative electrode plate
to produce a test electrode assembly, and injecting electrolyte
having the same composition as that of electrolyte used for a
sealed nickel hydrogen batter described later, into the test
electrode assembly. The cell prepared as above was charged at
20.degree. C. via 0.02 ItA to 25% of the capacity of the negative
electrode, and then charged via 0.1 ItA to 100% of the capacity of
the negative electrode. After 1 hour pause, the cell was discharged
via 0.2 ItA until the potential of the negative electrode versus
the reference electrode (Hg/HgO) became -0.6 V. Then, the cell was
charged via 0.1 ItA to 120%, which was followed by 1 hour pause.
Later, the cell was discharged to -0.6V versus the reference
electrode. The above charge/discharge cycle was repeated four
times. Based on the discharge capacity obtained from the fourth
discharge cycle, the discharge capacity per 1 g of hydrogen
absorbing alloy was determined by calculation.
[0324] (Preparation of Sealed Nickel Metal-Hydride Battery)
[0325] Process responsible for the production of a sealed nickel
metal-hydride battery of the invention will be described below with
reference to FIGS. 1, 2, and 3.
[0326] (Attaching Upper and Lower Current Collecting Plates to the
Ends of Electrode Assembly of Rolled Sheets)
[0327] A plate of a negative electrode prepared as above, a sheet
of non-woven textile having a thickness of 120 .mu.m and comprising
sulfonated polypropylene which served as a separator, and a
positive electrode plate were combined, and rolled together to form
a electrode assembly 1 with a radius of 15.2 mm. To the positive
electrode plate protruding from one end of the electrode assembly
1, was connected an upper current collecting plate 2 (current
collecting of positive electrode) by electric resistance welding.
The upper current collecting plate 2 is a nickel-plated steel plate
of 0.3 mm in thickness having a disc-like shape with a radius of
14.5 mm which has a throughhole at its center with eight slits 2-2
extending radially therefrom. Each slit has two parallel ridges 2-3
of 0.5 mm in height (portions to interdigitate with an electrode
substrate) along two edges facing to each other. Another
nickel-plated steel plate was prepared that is a nickel-plated
steel plate of 0.3 mm in thickness having a disc-like shape with a
radius of 14.5 mm with eight slits extending radially therefrom.
This was a lower current collecting plate (negative electrode plate
represented by numeral 3 of FIG. 1). The lower current collecting
plate had a projection at the center, and eight projections along
the periphery of a concentric circle having a radius 11 mm around
the center of lower current collecting plate 3 wherein the
projections served as welding joints for connecting the lower
current collecting plate to the bottom of container. The lower
current collecting plate 3 was connected by electric resistance
welding to the other end protruding from the rolled electrode
assembly 1. The projections were provided such that the central
projection had a height slightly less than that of eight
projections arranged at the periphery of concentric circle.
[0328] A cylindrical container 4 with a bottom made of a nickel
plated steel was prepared. A electrode assembly 1 having an upper
current collecting plate 2 and a lower current collecting plate 3
attached thereto was placed in the container 4 such that the upper
current collecting plate was flush with an open end of container 4
and the lower current collecting plate was brought into contact
with the bottom of container 4. Then, a specified amount of
electrolyte consisting of an aqueous solution comprising KOH at 6.8
mol/dm.sup.3 and LiOH at 0.8 mol/dm.sup.3 was injected into the
trough. After injection, output terminals A, B (also called
electrode rods) of an electric resistance welder are connected one
to the upper current collecting plate 2 and the other to the bottom
of container 4 (negative electrode terminal), and the welder was
adjusted so that charging and discharging may occur at the same
current for the same period. Specifically, the welder was adjusted
to give an alternate current through the positive electrode plate
(6.5 Ah in capacity) at a rate of 0.6 kA/Ah (3.9 kA) for 4.5 msec
with the same amount of current both for charging direction and for
discharging direction. This alternate pulse sequence consisting of
a square charging pulse and discharging pulse was counted as one
cycle, and the welder was set to give two cycles. Through passage
of the current, the lower surface of lower current collecting plate
3 was connected by welding to the inner surface of the bottom of
container via eight projections 14 arranged at the periphery of a
concentric circle 11 cm apart from the center of lower current
collecting plate 3. Then, one output terminal of the welder was
inserted through a round central hole provided at the center of the
electrode assembly until it is brought into contact with the upper
surface of lower current collecting plate, while the other output
terminal was contacted with the outer surface of the bottom of
container. Thus, the central projection provided at the center of
the lower surface of lower current collecting plate 3 was brought
into intimate contact with the inner surface of the bottom of
container, and current was passed by the welder to thereby connect
by welding the center of lower current collecting plate 3 to the
inner surface of the bottom of container 4. The ratio of the
distance of welded points P2 from the center of lower current
collecting plate 3 against the radius of electrode assembly 1 was
0.7.
[0329] (Preparation of Current Collecting Lead, Its Attachment to
Lid, and Sealing the Open End of Container)
[0330] As shown in FIG. 2, an current collecting lead comprised a
ring-shaped main lead 8 and a supplementary lead 9 connected to a
long end of the former (to the lower end in the particular example
shown in FIG. 2). The main lead was made of a strip of nickel plate
having a thickness of 0.8 mm with 16 projections 11 of 0.2 mm
height on one long end and 16 projections of 0.2 mm height on the
other long end. The main lead was a ring obtained by deforming the
strip of nickel plate into a ring having a width of 2.5 mm and
length of 66 mm with an internal diameter of 20 mm (10 mm radius)
(FIG. 2 shows a main lead 8 which has a supplementary lead 9
attached to its lower long side, and thus projections provided to
the other long side are not visible). The supplementary lead 9 was
made of a strip of nickel plate of 0.3 mm thickness, and consisted
of a ring having the same external diameter with that of the main
lead, and 8 jutted chips 9' protruding inward by 1 mm from the
ring. Each jutted chip 9' had a knob 10 on its tip. The jutted chip
9' of supplementary lead 9 was slanted downward from the ring
portion as shown in FIG. 2, and thus was given a spring
function.
[0331] A sealing plate 0 was prepared which was made of a nickel
deposited steel plate and had a disc-like shape with a round
through-hole having a diameter of 0.8 mm formed around the center.
After the inner surface of sealing plate 0 was contacted with one
long end of the main lead, the ring-shaped main lead 8 was
connected by electric resistance welding to the inner surface of
sealing plate 0. Then, in the same manner, the ring portion of
supplementary lead 9 was connected by electric resistance welding
to the other long end of ring-shaped main lead 8. A valve (vent
valve) 7 and a cap 6 were mounted to the outer surface of sealing
plate 0 to form a lid. A ring-shaped gasket 5 was applied onto the
peripheral edge of sealing plate 0 to contain the latter.
Incidentally, the lid (sealing plate 0) had a radius of 14.5
mm.
[0332] The lid having the current collecting lead attached thereto
was placed on electrode assembly 1 so as to bring supplementary
lead 9 into contact with upper current collecting plate 2, and the
open end of container 4 was sealed air-tight, and the battery
assembly was compressed to give a specified overall height. During
this operation, since the jutted chips 9' of supplementary lead 9
were conferred a spring function, it is possible to bring the knobs
10 of supplementary lead 9 into intimate contact with upper current
collecting plate 2, even when the distance between sealing plate 0
and upper current collecting plate 2 varies from one battery to
another.
[0333] (Connection by Welding of Current Collecting Lead to Upper
Current Collecting Plate)
[0334] The output terminals A, B of the electric resistance welder
were attached to the lid (positive electrode terminal) and to the
bottom surface of container 4 (negative electrode terminal), and
was set to pass current pulses having the same intensities both for
charging and discharging directions over the same period.
Specifically, the welder was adjusted to give an alternate current
through the positive electrode plate (6.5 Ah in capacity) at a rate
of 0.6 kA/Ah (3.9 kA) for 4.5 msec with the same amount of current
both for charging direction and for discharging direction. This
alternate pulse sequence consisting of a square charging pulse and
discharging pulse was counted as one cycle, and the welder was set
to give two cycles. During passage of the current, it was checked
that gas having a pressure exceeding the open pressure of the valve
did not evolve. Thus, the lid and the positive electrode current
collecting plate were connected to each other via the ring-shaped
main lead with supplementary lead to produce a sealing nickel
metal-hydride storage battery as shown in FIG. 1. The ratio of the
distance of welded points P1 from the center of upper current
collecting plate 2 against the radius of electrode assembly 1 was
0.6. The ratio of the length of current collecting lead from the
welded points formed on sealing plate 0 to welded points P1 formed
on upper current collecting plate 2 against the distance between
sealing plate 0 and upper current collecting plate 2 was about 1.4.
All the batteries used in Examples and Comparative examples were
172 g in weight.
[0335] (Chemical Activation)
[0336] A sealed storage battery prepared as described above was
left at 25.degree. C. for 12 hours. Then, the battery was charged
to 1200 mAh by passing 130 mA (0.02 ItA), and then 650 mA (0.1 ItA)
for 10 hours. The battery was then discharged to 1V or a cut
voltage via 1300 mA (0.2 ItA). The battery was charged again for 16
hours via 650 mA (0.1 ItA), and discharged to 1V or cut voltage via
1300 mA (0.2 ItA). The cycle consisting of charge/discharge
sequence was repeated four times. To activate the battery, the
battery was charged via 6500 mA (1 ItA) until -.DELTA.V exhibited
the variation of 5 mV, and then discharged via 6500 mA (1 ItA) to
1.0 V or discharge cut voltage. The above charge/discharge sequence
was taken to form a cycle, and the cycle was repeated 10 times.
[0337] (Charge/Discharge Cycle Test)
[0338] Activated batteries were subjected to charge/discharge cycle
test under the ambient temperature of 45.degree. C. A test battery
was charged via 0.5 ItA until -.DELTA.V exhibited the variation of
5 mV, and then discharged via 1 ItA to 1.0 V or discharge cut
voltage. The above charge/discharge sequence was taken to form a
cycle, and the cycle was repeated continuously until the discharge
capacity of test battery declined below a level equal to 80% of the
discharge capacity of the same battery subsequent to the first
charge/discharge cycle, and the number of cycles delivered
heretofore was taken to represent the cycle life of the
battery.
[0339] (Measurement of Output Density)
[0340] To measure the output density of a test battery, the battery
was placed in an atmosphere kept at 25.degree. C. After the end of
discharge, the battery was charged by passing 650 mA (0.1 ItA) for
5 hours, and then discharged by passing 60 A for 12 sec. During
discharging, the voltage of the battery at the 10th second was
measured and the measurement was named the 10th second voltage for
60 A discharge. A charging current of 6 A was passed until the
discharge was completely compensated for. Then, the battery was
discharged by passing 90 A for 12 sec. During discharging, the
voltage of the battery at the 10th second was measured and the
measurement was named the 10th second voltage for 90 A discharge.
The discharge was compensated for by passing a charging current of
6 A. Then, the battery was discharged by passing 120 A for 12 sec,
and meantime the voltage of the battery at the 10th second was
measured and the measurement was named the 10th second voltage for
120 A discharge. The discharge was compensated for by passing a
charging current of 6 A. Then, the battery was discharged by
passing 150 A for 12 sec, and meantime the voltage of the battery
at the 10th second was measured and the measurement was named the
10th second voltage for 150 A discharge. The discharge was
compensated for by passing a charging current of 6 A. Then, the
battery was discharged by passing 180 A for 12 sec, and meantime
the voltage of the battery at the 10th second was measured and the
measurement was named the 10th second voltage for 180 A discharge.
Relationship of the tenth second voltage measurements with
discharging currents was approximated to a line by a least square
method: the line, when plotted on a coordinate system, takes a
voltage value of E0 when the current value is 0 A, and has a
gradient RDC. Then, from the formula below: Output density
(W/kg)=(E0-0.8)/RDC.times.0.8/battery weight (kg) the output
density of a battery was calculated (battery was discharged at
25.degree. C. to 0.8V or cut voltage).
EXAMPLE 72
[0341] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 0.4 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 72.
EXAMPLE 73
[0342] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 1.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 73.
EXAMPLE 74
[0343] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 3.5 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 74.
EXAMPLE 75
[0344] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 5.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 75.
COMPARATIVE EXAMPLE 33
[0345] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 8.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder,
the Er.sub.2O.sub.3 powder being obtained by pulverizing a
commercially available Er.sub.2O.sub.3 powder as described above
with a ball mill. The electrode plate was made Comparative example
33.
COMPARATIVE EXAMPLE 34
[0346] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1 part by weight of a commercially
available Er.sub.2O.sub.3 powder (average diameter of 10 .mu.m) as
described above was added without pulverization to 100 parts by
weight of a hydrogen absorbing alloy powder. The electrode plate
was made Comparative example 34.
EXAMPLE 76
[0347] Powder of Er(OH).sub.3 commercially available (average
diameter of 10 .mu.m) was wet-ground by a satellite ball mill using
water as a dispersant. The yield was removed of moisture to be
dried, and sieved by a blower sorting machine to provide powder of
Er(OH).sub.3 having an average diameter of 0.3 .mu.m. A negative
electrode plate was prepared in the same manner as in Example 71,
except that 1 part by weight of Er(OH).sub.3 in the form of powder
having an average diameter of 0.3 .mu.m was added to 100 parts by
weight of a hydrogen absorbing alloy powder. The electrode plate
was made Example 76.
[0348] A negative electrode plate was prepared in the same manner
as in Example 76, except that 1 part by weight of Er(OH).sub.3 in
the form of powder having an average diameter of 0.5 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 77.
EXAMPLE 78
[0349] A negative electrode plate was prepared in the same manner
as in Example 76, except that 1 part by weight of Er(OH).sub.3 in
the form of powder having an average diameter of 1.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 78.
EXAMPLE 79
[0350] A negative electrode plate was prepared in the same manner
as in Example 76, except that 1 part by weight of Er(OH).sub.3 in
the form of powder having an average diameter of 3.5 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 79.
EXAMPLE 80
[0351] A negative electrode plate was prepared in the same manner
as in Example 76, except that 1 part by weight of Er(OH).sub.3 in
the form of powder having an average diameter of 5.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 80.
COMPARATIVE EXAMPLE 35
[0352] A negative electrode plate was prepared in the same manner
as in Example 76, except that 1 part by weight of Er(OH).sub.3 in
the form of powder having an average diameter of 8.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder,
the Er(OH).sub.3 powder being obtained by pulverizing a
commercially available Er(OH).sub.3 powder as described above with
a ball mill. The electrode plate was made Comparative example
35.
COMPARATIVE EXAMPLE 36
[0353] A negative electrode plate was prepared in the same manner
as in Example 76, except that 1 part by weight of a commercially
available Er(OH).sub.3 powder (average diameter of 10 .mu.m) as
described above was added without pulverization to 100 parts by
weight of a hydrogen absorbing alloy powder. The electrode plate
was made Comparative example 36.
EXAMPLE 81
[0354] Powder of Yb.sub.2O.sub.3 commercially available (average
diameter of 10 .mu.m) was wet-ground by a satellite ball mill using
water as a dispersant. The yield was removed of moisture to be
dried, and sieved by a blower sorting machine to provide powder of
Yb.sub.2O.sub.3 having an average diameter of 0.3 .mu.m. A negative
electrode plate was prepared in the same manner as in Example 71,
except that 1 part by weight of Yb.sub.2O.sub.3 in the form of
powder having an average diameter of 0.3 .mu.m was added to 100
parts by weight of a hydrogen absorbing alloy powder. The electrode
plate was made Example 81.
EXAMPLE 82
[0355] A negative electrode plate was prepared in the same manner
as in Example 81, except that 1 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 0.5 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 82.
EXAMPLE 83
[0356] A negative electrode plate was prepared in the same manner
as in Example 81, except that 1 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 1.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 83.
EXAMPLE 84
[0357] A negative electrode plate was prepared in the same manner
as in Example 81, except that 1 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 3.5 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 84.
EXAMPLE 85
[0358] A negative electrode plate was prepared in the same manner
as in Example 81, except that 1 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 5.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 85.
COMPARATIVE EXAMPLE 37
[0359] A negative electrode plate was prepared in the same manner
as in Example 81, except that 1 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 8.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder,
the Yb.sub.2O.sub.3 powder being obtained by pulverizing a
commercially available Yb.sub.2O.sub.3 powder as described above
with a ball mill. The electrode plate was made Comparative example
37.
COMPARATIVE EXAMPLE 38
[0360] A negative electrode plate was prepared in the same manner
as in Example 81, except that 1 part by weight of a commercially
available Yb.sub.2O.sub.3 powder (average diameter of 10 .mu.m) as
described above was added without pulverization to 100 parts by
weight of a hydrogen absorbing alloy powder. The electrode plate
was made Comparative example 38.
COMPARATIVE EXAMPLE 39
[0361] A negative electrode plate was prepared in the same manner
as in Example 71, except that no oxide or hydroxide of Er and Yb
was added to a hydrogen absorbing alloy powder. The electrode plate
was made Comparative example 39.
[0362] Table 15 lists the test results of Examples 71 to 85, and
Comparative examples 33 to 39 regarding discharge capacity per 1 g
of hydrogen absorbing alloy powder when they are used as a negative
electrode, and regarding cycle performance (cycle life) and output
density when they are used as a sealed nickel metal-hydride
battery. TABLE-US-00015 TABLE 15 Combined addition Oxide/hydroxide
of rare earth element Average particle Chemical diameter Cycle life
Output Example formula (.mu.m) (cycles) (W/kg) Example 71
Er.sub.2O.sub.3 0.3 638 1500 Example 72 Er.sub.2O.sub.3 0.4 609
1500 Example 73 Er.sub.2O.sub.3 1.0 581 1500 Example 74
Er.sub.2O.sub.3 3.5 581 1500 Example 75 Er.sub.2O.sub.3 5.0 413
1500 Comparative example 33 Er.sub.2O.sub.3 8.0 310 1500
Comparative example 34 Er.sub.2O.sub.3 10 306 1500 Example 76
Er(HO).sub.3 0.3 618 1500 Example 77 Er(HO).sub.3 0.5 596 1500
Example 78 Er(HO).sub.3 1.0 577 1500 Example 79 Er(HO).sub.3 3.5
573 1500 Example 80 Er(HO).sub.3 5.0 408 1500 Comparative example
35 Er(HO).sub.3 8.0 310 1500 Comparative example 36 Er(HO).sub.3 10
305 1500 Example 81 Yb.sub.2O.sub.3 0.3 750 1400 Example 82
Yb.sub.2O.sub.3 0.5 694 1400 Example 83 Yb.sub.2O.sub.3 1.0 638
1400 Example 84 Yb.sub.2O.sub.3 3.5 609 1400 Example 85
Yb.sub.2O.sub.3 5.0 413 1400 Comparative example 37 Yb.sub.2O.sub.3
8.0 310 1400 Comparative example 38 Yb.sub.2O.sub.3 10 307 1400
Comparative example 39 No addition -- 300 1500
[0363] From Table 15 it is known that addition of powdery
Er.sub.2O.sub.3, Er(OH).sub.3, or Yb.sub.2O.sub.3 having an average
diameter of 0.3 to 5.0 .mu.m, particularly 0.3 to 3.5 .mu.m is
effective for greatly improving the cycle performance of a battery,
as seen from the comparison with Comparative example 39 in which no
addition of an anti-corrosion agent was undertaken. This is
probably because when an anti-corrosion agent having a small
diameter is added to a hydrogen absorbing alloy powder, it can
disperse readily into the electrode body to distribute uniformly in
it, and when receiving the injection of electrolyte, readily react
with elements of the solution, and, in addition, the anti-corrosion
agent is also distributed uniformly over the surface of hydrogen
absorbing alloy powder. In contrast, when the powdery
anti-corrosion agent has an average diameter of 8 to 10 .mu.m, it
can not disperse readily in the electrode body due to its large
diameter, is so reluctant to the reaction with elements of
electrolyte, that it will hardly distribute uniformly on the
surface of hydrogen absorbing alloy powder. This is the reason why
such an anti-corrosion agent can not exert its function
satisfactorily even after activation (including the time when the
cycle test is performed).
[0364] A battery incorporating an Er.sub.2O.sub.3-added electrode
has a higher output density, although being somewhat inferior in
cycle performance, than a battery incorporating a
Yb.sub.2O.sub.3-added electrode. Conversely, a battery
incorporating a Yb.sub.2O.sub.3-added electrode has a more
excellent cycle performance than a battery incorporating an
Er.sub.2O.sub.3-added electrode, although being somewhat inferior
in output density. This is probably because the Yb.sub.2O.sub.3
powder is more dispersive than the Er.sub.2O.sub.3 powder, and
thus, when added to the electrode body, Yb(OH).sub.3 which is
generated as a result of reaction in the electrode body will cover
more effectively the surface of hydrogen absorbing alloy powder
including its active spots. In view of this, when emphasis is put
on the output power performance, powder of Er.sub.2O.sub.3 is
preferably selected. When stress is given to the cycle performance,
powder of Yb.sub.2O.sub.3 is preferably selected. In the examples
above, the anti-corrosion inhibitors having a purity of 90% were
used. However, from the functional point of view, 100% pure
anti-corrosion inhibitors are more preferred, although they are
more expensive.
[0365] A battery with an Er.sub.2O.sub.3-added electrode exhibits a
more excellent cycle performance than a battery with an Er
(OH).sub.3-added electrode, although being similar in output
density. This is probably because, since Er.sub.2O.sub.3 powder is
more soluble to alkaline electrolyte than Er(OH).sub.3 powder, the
former disperses more uniformly in the electrode body, which will
lead to the more enhanced anti-corrosion activity of hydrogen
absorbing alloy powder. Addition of Yb(OH).sub.3 powder having an
average diameter of 5 .mu.m or less also has an excellent
anti-corrosion enhancing effect, although its details will be
omitted here.
[0366] According to the invention, it is also possible to combine
two kinds of oxides or hydroxides of Er or Yb different in average
diameter, for example, one having a diameter equal to 5 .mu.m or
less, and the other having a diameter over 5 .mu.m (for example,
powder not pulverized), and to add the mixture (having two or more
peaks in particle size distribution) to a hydrogen absorbing alloy
powder. In this case, addition of the mixture is preferably
adjusted such that the powder having an average diameter equal to
or less than 5 .mu.m is added at 0.3 to 1.5 part by weight with
respect to 100 parts by weight of a hydrogen absorbing alloy
powder. However, combinational addition of powder of an oxide or
hydroxide of Er or Yb having an average diameter over 5 .mu.m is
preferably avoided, because it is not only ineffective for
enhancing the anti-corrosion activity of hydrogen absorbing alloy
powder, but also lowers the filling density of hydrogen absorbing
alloy powder within a hydrogen absorbing electrode, thereby
reducing the utility of the electrode. Combinational addition
should be avoided also because coexistence of powder of an oxide or
hydroxide of Er or Yb having an average diameter exceeding that of
hydrogen absorbing alloy powder, will reduce, when the mixture is
processed to give a paste of active material, the dispersibility of
powder within the paste. According to the invention, when powder of
oxide or hydroxide of Er or Yb is used, that powdery additive
preferably has a d90 size (that corresponds to the size of particle
which gives 90% accumulation of particles on a cumulative curve
which is plotted based on the particle size distribution of a given
powder, where 100% corresponds to the assessed volume of the
overall profile of the powder) which is smaller than the average
diameter of a hydrogen absorbing alloy powder to which it is to be
added.
[0367] In Examples 86 to 93, and Comparative examples 40 to 45
described below, the relationship of the added amount of powdery
anti-corrosion agent with the performance of a battery containing
such an anti-corrosion agent was studied.
EXAMPLE 86
[0368] A negative electrode plate was prepared in the same manner
as in Example 71, except that 0.3 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 86.
EXAMPLE 87
[0369] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1.5 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 87.
COMPARATIVE EXAMPLE 40
[0370] A negative electrode plate was prepared in the same manner
as in Example 71, except that 0.1 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Comparative example 40.
COMPARATIVE EXAMPLE 41
[0371] A negative electrode plate was prepared in the same manner
as in Example 71, except that 3 parts by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Comparative example 41.
EXAMPLE 88
[0372] A negative electrode plate was prepared in the same manner
as in Example 71, except that 0.3 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 5.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 88.
EXAMPLE 89
[0373] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1.5 part by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 5.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 89.
COMPARATIVE EXAMPLE 42
[0374] A negative electrode plate was prepared in the same manner
as in Example 71, except that 3 parts by weight of Er.sub.2O.sub.3
in the form of powder having an average diameter of 5.0 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Comparative example 42.
EXAMPLE 90
[0375] A negative electrode plate was prepared in the same manner
as in Example 71, except that 0.3 part by weight of Er(OH).sub.3 in
the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 90.
EXAMPLE 91
[0376] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1.5 part by weight of Er (OH).sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 91.
COMPARATIVE EXAMPLE 43
[0377] A negative electrode plate was prepared in the same manner
as in Example 71, except that 3 parts by weight of Er(OH).sub.3 in
the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Comparative example 43.
EXAMPLE 92
[0378] A negative electrode plate was prepared in the same manner
as in Example 71, except that 0.3 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 92.
EXAMPLE 93
[0379] A negative electrode plate was prepared in the same manner
as in Example 71, except that 1.5 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Example 93.
COMPARATIVE EXAMPLE 44
[0380] A negative electrode plate was prepared in the same manner
as in Example 71, except that 0.1 part by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Comparative example 44.
COMPARATIVE EXAMPLE 45
[0381] A negative electrode plate was prepared in the same manner
as in Example 71, except that 3 parts by weight of Yb.sub.2O.sub.3
in the form of powder having an average diameter of 0.3 .mu.m was
added to 100 parts by weight of a hydrogen absorbing alloy powder.
The electrode plate was made Comparative example 45.
[0382] Table 16 lists the test results of Examples 86 to 93, and
Comparative examples 40 to 45 together with those of Examples 71,
75 and 76, and 81 regarding discharge capacity per 1 g of hydrogen
absorbing alloy powder when they are used as a negative electrode,
and regarding cycle performance (cycle life) and output power
performance when they are used as a sealed nickel metal-hydride
battery. TABLE-US-00016 TABLE 16 Rare earth element added Average
particle diameter Content Capacity Cycle life Output Classification
Chemical formula (.mu.m) (part by weight) (mAh/g) (cycles) (W/Kg)
Example 86 Er.sub.2O.sub.3 0.3 0.3 280 516 1500 Example 71
Er.sub.2O.sub.3 0.3 1 280 638 1500 Example 87 Er.sub.2O.sub.3 0.3
1.5 280 554 1500 Comparative example 40 Er.sub.2O.sub.3 0.3 0.1 280
357 1500 Comparative example 41 Er.sub.2O.sub.3 0.3 3 170 -- --
Example 88 Er.sub.2O.sub.3 5 0.3 280 384 1500 Example 75
Er.sub.2O.sub.3 5 1 280 413 1500 Example 89 Er.sub.2O.sub.3 5 1.5
280 462 1500 Comparative example 42 Er.sub.2O.sub.3 5 3 170 -- --
Example 90 Er(OH).sub.3 0.3 0.3 280 474 1500 Example 76
Er(OH).sub.3 0.3 1 280 618 1500 Example 91 Er(OH).sub.3 0.3 1.5 280
590 1450 Comparative example 43 Er(OH).sub.3 0.3 3 170 -- --
Example 92 Yb.sub.2O.sub.3 0.3 0.3 280 615 1400 Example 81
Yb.sub.2O.sub.3 0.3 1 280 750 1400 Example 93 Yb.sub.2O.sub.3 0.3
1.5 280 652 1400 Comparative example 44 Yb.sub.2O.sub.3 0.3 0.1 280
421 1500 Comparative example 45 Yb.sub.2O.sub.3 0.3 3 160 -- --
[0383] As described above, as the powdery anti-corrosion agent has
a larger particle diameter, it becomes more reluctant to react with
the elements of electrolyte, and thus the agent becomes more
inactive with regard to its anti-corrosion enhancing activity. To
compensate for the drawback, so it had been thought, it might be
effective to increase the addition amount, when powdery
anti-corrosion agent has a comparatively large particle diameter.
Table 16 shows the relation of the addition amount of powdery
Er.sub.2O.sub.3 with the performance of a battery with an
Er.sub.2O.sub.3-added electrode while varying the average diameter
of the powder from 0.3 .mu.m (the minimum size that is found to be
effective for enhancing the anti-corrosion activity of a battery)
to 5 .mu.m (the maximum size that is found effective for enhancing
the anti-corrosion activity of a battery) as shown in FIG. 15. The
result of Comparative example 42, however, shows that a battery
with an Er.sub.2O.sub.3-added electrode does not exhibit normal
discharge performance when the powdery Er.sub.2O.sub.3 is added at
3 parts by weight with respect to 100 parts by weight of a hydrogen
absorbing alloy powder, even when the powdery anti-corrosion agent
has an average diameter of 5 .mu.m. This is probably because the
excess addition of the anti-corrosion agent may increase the
resistance of hydrogen absorbing electrode to reaction, which
impairs the normal discharging activity of the battery. From the
results shown in Table 2, it was found that the anti-corrosion
agent is preferably added at 0.3 to 1.5, more preferably 1 to 1.5
part by weight, regardless of the average diameter of the powdery
anti-corrosion agent, because then it is possible to obtain a
battery exhibiting a good cycle performance.
[0384] As seen from Table 16, a battery incorporating an
Er(OH).sub.3 or Yb.sub.2O.sub.3-added electrode exhibits an
improved cycle performance, when the anti-corrosion agent is added
at 0.3 to 1.5 part by weight, as compared with a battery
incorporating an electrode in Comparative example 39 to which no
anti-corrosion agent shown in FIG. 1 is added. This effect is
particularly notable when the addition amount of the anti-corrosion
agent is 1 to 1.5 part by weight. On the other hand, when
Er(OH).sub.3 or Yb.sub.2O.sub.3 is added at 3 parts by weight
(Comparative examples 43 and 45), the battery exhibits an
inadequate initial activation. This is probably because the battery
has an insufficient capacity owing to the increased resistance to
reaction. When Er.sub.2O.sub.3 or Yb.sub.2O.sub.3 is added at 0.1
part by weight (Comparative examples 40 and 44), the battery
exhibits a rather low capacity. This is probably because a hydrogen
absorbing alloy is quickly corroded during charge/discharge cycles
which leads to the faster decline of capacity, as compared with
Examples. From the results shown in Table 16, it was found that
Er(OH).sub.3 and Yb.sub.2O.sub.3 are preferably added at 0.3 to
1.5, more preferably 1 to 1.5 part by weight. It was known further
that when Yb(OH).sub.3 is added as an anti-corrosion agent, it is
preferably added, like Yb.sub.2O.sub.3, at 0.3 to 1.5, more
preferably 1 to 1.5 part by weight.
[0385] In Examples 94 to 96, and Comparative examples 46 to 49
described below, the relationship of the saturation mass
susceptibility of a hydrogen absorbing alloy powder with the
performance of a battery containing such a hydrogen absorbing alloy
powder was studied.
EXAMPLE 94
[0386] A negative electrode plate was prepared in the same manner
as in Example 73, except that a hydrogen absorbing alloy powder was
immersed in an aqueous solution of 48 wt % NaOH kept at 100.degree.
C. for 0.7 hour to produce a hydrogen absorbing alloy powder having
a saturation mass susceptibility of 1 emu/g, and that the hydrogen
absorbing alloy powder was used to form a negative electrode plate.
The electrode plate was made Example 94.
EXAMPLE 95
[0387] A negative electrode plate was prepared in the same manner
as in Example 73, except that a hydrogen absorbing alloy powder was
immersed in an aqueous solution of 48 wt % NaOH kept at 100.degree.
C. for 2.6 hours to produce a hydrogen absorbing alloy powder
having a saturation mass susceptibility of 4 emu/g, and that the
hydrogen absorbing alloy powder was used to form a negative
electrode plate. The electrode plate was made Example 95.
EXAMPLE 96
[0388] A negative electrode plate was prepared in the same manner
as in Example 73, except that a hydrogen absorbing alloy powder was
immersed in an aqueous solution of 48 wt % NaOH kept at 100.degree.
C. for 4 hours to produce a hydrogen absorbing alloy powder having
a saturation mass susceptibility of 6 emu/g, and that the hydrogen
absorbing alloy powder was used to form a negative electrode plate.
The electrode plate was made Example 96.
COMPARATIVE EXAMPLE 46
[0389] A negative electrode plate was prepared in the same manner
as in Example 73, except that a hydrogen absorbing alloy powder
which was not treated so as to have a catalytic layer formed on the
surface of the powder, was used for the construction of the
electrode plate. The resulting hydrogen absorbing alloy had a
saturation mass susceptibility of 0.06 emu/g. The electrode plate
was made Comparative example 46.
COMPARATIVE EXAMPLE 47
[0390] A negative electrode plate was prepared in the same manner
as in Example 73, except that a hydrogen absorbing alloy powder was
immersed in an aqueous solution of 48 wt % NaOH kept at 100.degree.
C. for 5.3 hours to produce a hydrogen absorbing alloy powder
having a saturation mass susceptibility of 8 emu/g, and that the
hydrogen absorbing alloy powder was used to form a negative
electrode plate. The electrode plate was made Comparative example
47.
COMPARATIVE EXAMPLE 48
[0391] A negative electrode plate was prepared in the same manner
as in Example 83, except that a hydrogen absorbing alloy powder
which was not treated so as to have a catalytic layer formed on the
surface of the powder, was used for the construction of the
electrode plate. The resulting hydrogen absorbing alloy had a
saturation mass susceptibility of 0.06 emu/g. The electrode plate
was made Comparative example 48.
COMPARATIVE EXAMPLE 49
[0392] A negative electrode plate was prepared in the same manner
as in Comparative example 46, except that powder of Er.sub.2O.sub.3
was not added to a hydrogen absorbing alloy powder. The electrode
plate was made Comparative example 49.
[0393] Table 17 lists the results of Examples 94 to 96 together
with those of Examples 73 and 83, and Comparative examples 46 to 49
regarding cycle test and output density measurement. TABLE-US-00017
TABLE 17 Saturation mass susceptibility hydrogen absorbing alloy
Oxide of rare earth element (during preparation of electrode
Content Cycle life Output Classification (emu/g) Chemical formula
(part by weight) (cycles) (W/kg) Example 94 1 Er.sub.2O.sub.3 1 469
1300 Example 73 2 Er.sub.2O.sub.3 1 581 1500 Example 95 4
Er.sub.2O.sub.3 1 559 1650 Example 96 6 Er.sub.2O.sub.3 1 525 1650
Comparative example 46 0.06 Er.sub.2O.sub.3 1 -- -- Comparative
example 47 8 Er.sub.2O.sub.3 1 289 1500 Example 83 2
Yb.sub.2O.sub.3 1 638 1400 Comparative example 48 0.06
Yb.sub.2O.sub.3 1 -- -- Comparative example 49 0.06 None 0 283
1000
[0394] As seen from Table 17, the hydrogen absorbing alloy powder
receiving the addition of Er.sub.2O.sub.3 exhibits a saturation
mass susceptibility of 1 to 6 emu/g, and improved cycle
performance. A hydrogen absorbing alloy powder, after receiving the
addition of Er.sub.2O.sub.3, can have a far higher output density
of 1300 W/kg at 25.degree. C. than a conventional alloy powder even
though its saturation mass susceptibility is as low as 1 emu/g.
However, generally, hydrogen absorbing alloy powders having a
saturation mass susceptibility of 2 to 6 emu/g exhibit excellent
output power performance exceeding 1400 W/kg at 25.degree. C., and
can attain a far more excellent cycle performance than a
corresponding alloy powder having a saturation mass susceptibility
of 1 emu/g. Therefore, the hydrogen absorbing alloy powder
preferably has a saturation mass susceptibility of 2 to 6 emu/g.
This is because such a hydrogen absorbing alloy powder has a
catalytic layer formed on the surface of powder, is ready to be
activated, gives a sufficient capacity from the initial phase of
use life, and can inhibit the progression of corrosion due to the
anti-corrosion agent. Comparative example 47, although it has a
high saturation mass susceptibility, exhibits a poor cycle
performance. This is probably because an excess amount of rare
earth elements contained in the hydrogen absorbing alloy powder
dissolve away during the treatment for the formation of catalytic
layer; thereby the hydrogen absorbing ability of alloy powder is
reduced; the electrode can not have a sufficient capacity; and thus
its cycle life declines.
[0395] Comparative example 49 which does not receive the addition
of anti-corrosion agent is inferior in cycle performance, as
compared with Examples. The former is also inferior in output power
performance. This is probably because the hydrogen absorbing alloy
powder of Comparative example 49 does not have catalytic layer
formed on its surface. It was also found that as seen from
Comparative examples 46 and 48, if a hydrogen absorbing alloy
powder is not treated for the formation of catalytic layer, even
though it receives the addition of anti-corrosion agent, and is
activated, the resulting battery will not operate satisfactorily
because the hydrogen absorbing electrode is not sufficiently
activated. Thus, to ensure that addition of the anti-corrosion
agent leads to the improvement of cycle performance, it is
preferable to add the anti-corrosion agent to a hydrogen absorbing
alloy powder which has a catalytic layer formed in advance on the
surface of the powder.
[0396] In Reference examples 17 and 18, and Comparative example 50
described below, the relationship of the treatment solution used
for the formation of a catalytic layer with the performance of a
resulting battery was studied.
COMPARATIVE EXAMPLE 50
[0397] A negative electrode plate was prepared in the same manner
as in Comparative example 39, except that a hydrogen absorbing
alloy powder was immersed in an acetic acid-sodium acetate buffer
kept at pH3.5, instead of an alkaline aqueous solution, and that a
hydrogen absorbing alloy powder having a saturation mass
susceptibility of 2 emu/g was used. The electrode plate was made
Comparative example 50.
REFERENCE EXAMPLE 17
[0398] A negative electrode plate was prepared in the same manner
as in Example 71, except that a hydrogen absorbing alloy powder was
immersed in an acetic acid-sodium acetate buffer kept at pH3.5,
instead of an alkaline aqueous solution, and that a hydrogen
absorbing alloy powder having a saturation mass susceptibility of 2
emu/g was used. The electrode plate was made Reference example
17.
REFERENCE EXAMPLE 18
[0399] A negative electrode plate was prepared in the same manner
as in Example 95, except that a hydrogen absorbing alloy powder was
immersed in an acetic acid-sodium acetate buffer kept at pH3.5,
instead of an alkaline aqueous solution, and that a hydrogen
absorbing alloy powder having a saturation mass susceptibility of 4
emu/g was used. The electrode plate was made Reference example
18.
[0400] Table 18 lists the results of Comparative example 50 and
Reference examples 17 and 18 together with those of Examples 71 and
95, and Comparative example 39 regarding cycle test and output
density measurement. TABLE-US-00018 TABLE 18 hydrogen absorbing
alloy Saturation mass susceptibility Treatment solution during
preparation of electrode Combined addition Cycle life Output
Classification type (emu/g) Oxide of rare earth element (cycles)
(W/kg) Comparative example 39 Alkaline 2 No addition 300 1500
Comparative example 50 Acidic 2 No addition 286 1500 Example 71
Alkaline 2 Er.sub.2O.sub.3 581 1500 Example 17 Acidic 2
Er.sub.2O.sub.3 508 1500 Example 95 Alkaline 4 Er.sub.2O.sub.3 559
1650 Reference example 18 Acidic 4 Er.sub.2O.sub.3 469 1520
[0401] As seen from Table 18, a battery incorporating a hydrogen
absorbing alloy powder which has been treated with an alkaline
aqueous solution shows a better cycle performance than a comparable
battery incorporating a hydrogen absorbing alloy powder which has
been treated with an acetic acid-sodium acetate buffer. This is
probably because, even if a hydrogen absorbing alloy powder has a
catalyst layer formed on its surface, part of the catalyst layer
will be torn off from the surface of the powder when it is immersed
in an acetic acid-sodium acetate buffer. From this, to ensure the
catalytic function, a hydrogen absorbing alloy powder is preferably
treated with an alkaline aqueous solution.
[0402] Although this is not directly related with the examples
cited in the table, it is possible to obtain a hydrogen absorbing
electrode and battery that are excellent in high-rate discharging
ability and cycle performance, by adding a compound (a combination
of two compounds one containing Er and the other Yb, or a complex
containing both Er and Yb) containing both Er and Yb at 80 wt % or
more, particularly 90 wt % or more to a hydrogen absorbing alloy
powder which will be incorporated into the electrode and
battery.
[0403] In Reference example 19, and Comparative example 51
described below, the relationship of the current collecting
structure with the performance of a battery was studied.
REFERENCE EXAMPLE 19
[0404] A battery was prepared as in Example 95, except that
connection between the lower current collecting plate and the
bottom of container was achieved only through the welded point at
the center of the lower current collecting plate. The battery was
made Reference example 19.
COMPARATIVE EXAMPLE 51
[0405] A battery was prepared as in Example 78 except for the
following points. The current collecting lead was a ribbon-like
current collecting lead according to the structure of a
conventional sealed nickel metal-hydride battery. The lower current
collecting plate was connected to the inner surface of the bottom
of container only via the point at the center of the lower current
collecting plate. The ribbon-like current collecting lead consisted
of a strip of nickel plate of 0.6 mm in thickness, 15 mm in width,
and 25 mm in length. Before the lid was mounted to the battery body
(prior to sealing), the ribbon-like current collecting lead was
connected to the inner surface of the sealing plate and to the
upper surface of upper current collecting plate each via four
welded points. The length of the current collecting lead from its
contact point to the sealing plate as far as its contact point to
the upper current collecting plate was 7 times as large as the
distance between the sealing plate and upper current collecting
plate. The battery was made Comparative example 51.
[0406] Table 19 lists the test results of Reference example 19 and
Comparative examples 51 together with those of Example 95 regarding
output density. TABLE-US-00019 TABLE 19 Welded points between lower
current Output density Classification Shape of current collecting
lead collecting plate and bottom of container (W/kg) Example 95
Ring-like main lead + supplementary lead Center of lower current
collecting plate + 8 points 1650 Example 19 Ring-like main lead +
supplementary lead Center of lower current 1490 collecting plate
only Comparative Ribbon-like current collecting lead Center of
lower current 920 example 51 collecting plate only
[0407] As seen from Table 19, the batteries of Example 95 and
Reference example 19 have higher output densities than the battery
of Comparative example 51. With the battery of Example 95,
connection of the current collecting lead to the upper current
collecting plate was achieved by welding after sealing which
deprives the current collecting lead of the need for an extra
curvature, thereby enabling the reduced electric resistance of the
current collecting lead. In contrast, with the battery of
Comparative example 51 which was prepared according to a
conventional method, one end of the current collecting lead was
connected in advance by welding to the inner surface of sealing
plate, and the other end was connected later by welding to the
upper current collecting plate. Then, the lid was mounted to the
open end of container. This sequence of procedures requires the
addition of an extra curvature to the current collecting lead, and
since the current collecting lead has such a large electric
resistance that the additional electric resistance will lead to the
reduced output of the battery. The battery of Comparative example
51 has an output density well below 1400 W/kg at 25.degree. C., and
thus it is not suitable, for example, to be used as a power source
of HEVs. Furthermore, according to a conventional battery, it is
customary to connect the lower current collecting plate to the
inner surface of the bottom of container only via the welded point
provided at the center of the lower current collecting plate as in
Reference example 19. However, connection via multiple welded
points in addition to the central point as in Example 95 will be
more advantageous, because it will allow a battery to provide a
higher output. It is possible according to the invention to obtain
a battery excellent in output power performance, particularly by
preparing a hydrogen absorbing electrode according to the invention
and combining it with a current collecting structure as shown in
Example 95 and Reference example 19.
INDUSTRIAL APPLICABILITY
[0408] As detailed above, the present invention provides a nickel
metal-hydride storage battery incorporating a hydrogen absorbing
electrode containing a hydrogen absorbing alloy powder as an active
material, or a battery which is excellent both in cycle performance
and output power performance, and can be applied widely to various
industries.
* * * * *