U.S. patent application number 10/798031 was filed with the patent office on 2004-09-02 for hydrogen absorbing alloy and nickel-metal hydride rechargeable battery.
This patent application is currently assigned to Shin-Etsu Chemical Co., Ltd.. Invention is credited to Maeda, Takao, Shima, Satoshi, Shinya, Naofumi.
Application Number | 20040170520 10/798031 |
Document ID | / |
Family ID | 26524615 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040170520 |
Kind Code |
A1 |
Maeda, Takao ; et
al. |
September 2, 2004 |
Hydrogen absorbing alloy and nickel-metal hydride rechargeable
battery
Abstract
An object of the present invention is to provide a hydrogen
absorbing alloy which can improve a high rate discharge property
while suppressing particle size reduction, exhibits cycle life
characteristics equal to or higher than those of conventional
alloys even when its cobalt content is decreased, and has a high
capacity. Specifically, the present invention provides a hydrogen
absorbing alloy having a CaCu.sub.5 type crystal structure in its
principal phase, wherein the La content in the alloy is in the
range of 24 to 33% by weight and the Mg or Ca content in the alloy
is in the range of 0.1 to 1.0% by weight, as well as the aforesaid
alloy wherein the Co content in the alloy is not greater than 9% by
weight.
Inventors: |
Maeda, Takao; (Fukui-ken,
JP) ; Shima, Satoshi; (Fukui-ken, JP) ;
Shinya, Naofumi; (Fukui-ken, JP) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
Shin-Etsu Chemical Co.,
Ltd.
|
Family ID: |
26524615 |
Appl. No.: |
10/798031 |
Filed: |
March 11, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10798031 |
Mar 11, 2004 |
|
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|
09631491 |
Aug 3, 2000 |
|
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6733724 |
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Current U.S.
Class: |
420/441 ;
420/900 |
Current CPC
Class: |
H01M 10/345 20130101;
H01M 4/385 20130101; Y10S 420/90 20130101; H01M 4/383 20130101;
Y02E 60/10 20130101 |
Class at
Publication: |
420/441 ;
420/900 |
International
Class: |
C22C 019/03 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 1999 |
JP |
221990/1999 |
Jun 23, 2000 |
JP |
2000-189040 |
Claims
1. A hydrogen absorbing alloy having a CaCu.sub.5 type crystal
structure in its principal phase, comprising La in the range of 24
to 33% by weight in the alloy, and Mg or Ca in the range of 0.1 to
1.0% by weight in the alloy.
2. A hydrogen absorbing alloy according to claim 1, further
comprising 9% by weight or less of Co in the alloy.
3. A hydrogen absorbing alloy according to claim 1, further
comprising 6% by weight or less of Co in the alloy.
4. A hydrogen absorbing alloy according to claim 1, wherein the Co
content is 6 to 9% by weight, and the atomic ratio B/A is 5.0 to
5.25, where A represents a rare earth element including La, and B
represents a rare earth element, transition metal or Al.
5. A hydrogen absorbing alloy according to claim 1, further
comprising one or more selected from the group consisting of Ti, Zr
and V.
6. A hydrogen absorbing alloy having a CaCu.sub.5 type crystal
structure in its principal phase, comprising Mg and having a-axis
length of 4.990 to 5.050 .ANG. and c-axis length of 4.030 to 4.070
.ANG. for the lattice constants in the CaCu.sub.5 type crystal
structure.
7. A hydrogen absorbing alloy according to any one of claims 1 to 4
having a-axis length of 4.990 to 5.050 .ANG. and c-axis lenth of
4.030 to 4.070 .ANG. for the lattice constants in the CaCu.sub.5
type crystal structure.
8. A method for manufacturing a hydrogen absorbing alloy having a
CaCu.sub.5 type crystal structure in its principal phase,
characterized in that a Mg-supply material is added to dissolution
of component elements for hydrogen absorbing alloy in an amount of
0.1 to 1.0% by weight in an entire hydrogen absorbing alloy.
9. A method for manufacturing a hydrogen absorbing alloy according
to claim 8, characterized in that at least Ni and Co are melted in
a melting vessel, and then the Mg-supply material is added to the
melting vessel.
10. A method for manufacturing a hydrogen absorbing alloy according
to claim 8 or 9, characterized in the Mg-supply material is
selected from metallic Mg and Mg alloy with melting point of
650.degree. C. or higher.
11. A nickel-metal hydride rechargeable battery using the hydrogen
absorbing alloy of any one of claims 1 to 7 for an electrode
thereof.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to hydrogen absorbing alloys, and
more particularly to a hydrogen absorbing alloy which can be used
to form negative electrodes for use in nickel-metal hydride
rechargeable (secondary) batteries.
[0003] 2. Description of the Related Art
[0004] Conventionally, misch metal (hereinafter referred to as
"Mm") comprising a mixture of rare earth elements such as La, Ce,
Pr, Nd and Sm, and nickel-base alloys formed by replacing a part of
Ni with various elements are widely used as hydrogen absorbing
alloys for forming negative electrodes for use in nickel-metal
hydride rechargeable batteries.
[0005] It is known that, among others, cobalt-containing alloys are
capable of absorbing a relatively large amount of hydrogen, are
less liable to particle size reduction in their hydrogen-loaded
state, have excellent corrosion resistance in alkalis, and are
effective in prolonging the lives of nickel-metal hydride
rechargeable batteries when they are used for the negative
electrodes thereof.
[0006] On the other hand, it is also known that lower cobalt
contents are more desirable for an improvement of a high rate
discharge property. The reason for this is believed to be that a
decrease in cobalt content promotes particle size reduction and
hence causes an increase in surface area per unit weight.
SUMMARY OF THE INVENTION
[0007] In order to solve these problems of the prior art, the
present invention provides a hydrogen absorbing alloy which can
improve a high rate discharge property while suppressing particle
size reduction, exhibits cycle life characteristics equal to or
higher than those of conventional alloys even when its cobalt
content is decreased, and has a high capacity.
[0008] The present invention is based on the discovery that, when a
hydrogen absorbing alloy has a relatively high La content and
contains an alkaline earth metal (i.e., Mg or Ca) in a relatively
small amount above impurity levels, the alloy can improve a high
rate discharge property in spite of suppressed particle size
reduction while maintaining its high capacity, and can suppress
particle size reduction even when its cobalt content is decreased
to less than the conventionally known level.
[0009] Specifically, the present invention relates to a hydrogen
absorbing alloy having a CaCu.sub.5 type crystal structure in its
principal phase, wherein the La content in the alloy is in the
range of 24 to 33% by weight and the Mg or Ca content in the alloy
is in the range of 0.1 to 1.0% by weight.
[0010] In a preferred embodiment, the present invention also
relates to the aforesaid alloy wherein the cobalt content in the
alloy is not greater than 9% by weight.
[0011] When-the hydrogen absorbing alloy of the present invention
is used for the negative electrode of an alkaline rechargeable
battery, it can increase the capacity of the battery, can improve
the high rate discharge property thereof, and can suppress particle
size reduction even at low cobalt contents to cause a reduction in
battery cost.
BRIEF DESCRIPTION OF THE DRAWING
[0012] FIG. 1 is a x-ray diffraction pattern for the hydrogen
absorbing alloy of Example 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] In the AB.sub.5 type hydrogen absorbing alloy of the present
invention, 0.1 to 1.0% by weight of Mg or Ca is contained in order
to improve the high rate discharge property while suppressing
particle size reduction. Moreover, the La content in the alloy is
set at 24 to 33% by weight in order to increase the amount of
hydrogen absorbed and control the equilibrium pressure of hydrogen.
Thus, as contrasted with conventional alloys, the hydrogen
absorbing alloy of the present invention has a high capacity, can
improve the high rate discharge property while suppressing particle
size reduction, and can enhance resistance to particle size
reduction even at low cobalt contents.
[0014] Such hydrogen absorbing alloys may, for example, be
expressed in terms of the following chemical formulas:
La.sub.uR.sub.vMg.sub.wNi.sub.xCo.sub.yM.sub.z, and
La.sub.uR.sub.vCa.sub.wNi.sub.xCo.sub.yM.sub.z
[0015] wherein R is a rare earth element other than La, M is at
least one element chosen from the group consisting of Mn, Al, Si,
Sn, Fe, Cu, Ti, Zr, and V or the like, the content of La is
preferably 24 to 33% by weight, R 15% by weight, Mg or Ca 0.1 to
1.0% by weight, Ni 50 to 60% by weight, Co 9% by weight or less,
and M 3 to 10% by weight. Here, the compositional ratios of
elements are expressed in terms of atomic ratios (u, v, w, x, y and
z). These atomic ratios can be obtained by dividing the
percentage-by-weight for each element by the respective atomic
weight and then by normalizing the resulting figures using the sum
of constitutional ratios of La and R, which are classified as "A"
elements. Thus, u plus v equals 1 by definition. Because R is a
rare earth element which is other than La, and M is at least one
element chosen from the group of Mn, Al, Si, Sn, Fe, Cu, Ti, Zr, V
or the like, the weighted averages of atomic weights are used for R
and M. Excluding Mg and Ca, which are added in minor amounts, as
well as unavoidable impurities, the ratio of elements belonging to
"B" to those belonging to "A" is calculated as a B/A ratio
according to the following equation: B/A ratio =(x+y+z)/(u+v).
[0016] Furthermore, in the AB.sub.5 type hydrogen absorbing alloy
of the present invention, the remainder of the moiety A comprises
one or more rare earth elements other than La, and the remainder of
the moiety B comprises one or more transition metals such as Ni, Co
and Mn and/or Al or the like. The atomic ratio of B to A, B/A, is
preferably 4 to 7, more preferably 5 to 7, further more preferably
5 to 6.
[0017] The AB.sub.5 type hydrogen absorbing alloy used in the
present invention is preferably a hydrogen absorbing alloy having a
CaCu.sub.5 type crystal structure in its principal phase. As used
herein, the expression "hydrogen absorbing alloy having a
CaCu.sub.5 type crystal structure in its principal phase" refers to
a hydrogen absorbing alloy in which, although segregation phases
are partly recognized by metallographic observation of a section,
the diffraction pattern recorded by XRD exhibits a CaCu.sub.5 type
alloy phase.
[0018] The hydrogen absorbing alloy of the present invention is
characterized in that its Mg or Ca content is in the range of 0.1
to 1.0% by weight. If its Mg or Ca content is less than 0.1% by
weight, the effect of suppressing particle size reduction will be
insufficient. If its Mg or Ca content is greater than 1.0% by
weight, the amount of hydrogen absorbed will be decreased to an
undue extent.
[0019] When the Co content is lowered, the equilibrium pressure of
hydrogen at the time of absorption or desorption of hydrogen is
elevated. Accordingly, the La content is set at 24 to 33% by weight
in order to maintain the equilibrium pressure of hydrogen at the
same level as those of conventional alloys and to maintain or
improve the high capacity. In the present invention, it is
especially preferable to add Mg.
[0020] Moreover, the present invention involving the addition of a
relatively small amount of Mg or Ca as described above makes it
possible to achieve a long life at cobalt contents of not greater
than 9% by weight, preferably 7% by weight or less, and more
preferably 6% by weight or less, as contrasted with the prior art
in which it has been unachievable.
[0021] The addition of a small amount of one or more selected from
the group consisting of Ti, Zr and V to the Mg- or Ca-containing
hydrogen absorption alloy can enhance the initial characteristics
or cycle life characteristics. The amount of the addition is as
small as 0.5% by weight or less based the Mg- or Ca-containing
hydrogen absorbing alloy.
[0022] Moreover, the Mg-containing hydrogen absorbing alloy has a
CaCu.sub.5 type crystal structure in its principal phase, wherein
the length for a-axis (a-axis =b-axis) is in the range of 4.990 to
5.050 .ANG., the length for c-axis is in the range of 4.030 to
4.070, regarding the lattice constants thereof. Comparing the
lattice constants in these range between Mg-free and Mg-containing
hydrogen absorbing alloys, the addition of Mg tends to increase the
lattice constants. It has been particularly found that increase for
c-axis is larger than that for a-axis so that the ratio of length
of c-axis to length of a-axis, c/a, becomes larger.
[0023] It has been found that increase in the ratio c/a results in
less liability to particle size reduction so as to produce a
battery with longer cycle life. The reason for this is believed to
be that the larger face distance between the face perpendicular to
c-axis, which are the faces for closest packing of crystal,
suppress the extension of the lattice. Consequently, the stress is
restrained and the developing distances for cracks become smaller.
Thus, the less liability to the particle size reduction for the
hydrogen absorbing alloy comprising 0.1 to 1.0% by weight of Mg is
thought to be derived from c-axis having longer extension than
a-axis.
[0024] Further, it has been fount that, the hydrogen absorbing
alloy, having 24 to 33% by weight of La, 6 to 9% by weight of Co
and the atomic ratio B/A of 5.0 to 5.25, and which Mg is added to
in an amount of 0.1 to 1.0% by weight, can result in a battery with
higher capacity such as 340 mAh/g or more, keeping cycle life
unchanged. In this case, the ratio B/A means the sum of atomic
ratios of, for example, Ni, Co, Mn and Al, excluding the elements
in very small amounts such as Mg and Ca, by designating the sum of
atomic ratios of rare-earth metals such as La, Ce, Pr and Nd for
one.
[0025] The hydrogen absorbing alloy of the present invention can be
manufactured by a dissolution method such as arc dissolution and
high frequency dissolution, casting in a mold, table-casting, a
rapid roll quenching method, gas atomization, disk-atomization or a
spin-cup method, or a combination thereof.
[0026] The hydrogen absorbing alloy of the present invention may be
prepared in the following manner.
[0027] Predetermined amounts of various elements may be weighed out
and melted in a high-frequency furnace having an atmosphere of an
inert gas (at 200 to 1,500 Torr) such as Ar gas. In the case of an
element (e.g., Mg or Ca) having a high vapor pressure, it may be
added directly by itself, or in the form of an alloy formed of such
an element and one or more other elements constituting the alloy.
In the melting method, it may be preferable that Mg or Ca is not
added until metals with high melting point such as Ni or Co are
melted in order to prevent added components from evaporating or
assure the safe operation. The resulting melt may be cast in a mold
made of iron at a temperature of 1,300 to 1,600.degree. C. to form
an ingot. Or the other methods mentioned above may be also used. In
case of special need, the ingot may be heat-treated at a
temperature of 800 to 1,200.degree. C. for 5 to 20 hours in an
inert atmosphere (at 600 to 1,500 Torr), for example, of Ar
gas.
[0028] Using a jaw crusher, a roll mill, a hummer mill, a pin mill,
a ball mill, a jet mill, a roller mill and the like, the hydrogen
absorbing alloy prepared in the above-described manner may be
ground to an average particle diameter of 4 to 70 .mu.m in an inert
atmosphere, for example, of Ar. Moreover, reduction in particle
size by hydrogen absorption and desorption, so-called hydrogenation
method, may be used. Thus, there can be obtained a hydrogen
absorbing alloy in accordance with the present invention.
[0029] The hydrogen absorbing alloy powder thus obtained may be
formed into electrodes according to any well-known method. This can
be accomplished, for example, by mixing the alloy powder with a
binder selected from polyvinyl alcohol, cellulose derivatives
(e.g., methylcellulose), PTFE, polyethylene oxide, high polymer
latices and the like, kneading this mixture into a paste, and
applying this paste to an electrically conducting three-dimensional
support (e.g., foamed nickel or fibrous nickel) or an electrically
conducting two-dimensional support (e.g., punching metal). The
amount of binder used may be in the range of 0.1 to 20% by weight
per 100% by weight of the alloy.
[0030] Moreover, if necessary, an electrically conducting filler
such as carbon-graphite powder, Ni powder or Cu powder may be added
in an amount of 0.1 to 10% by weight based on the alloy.
[0031] Alkaline batteries using the hydrogen absorbing alloy of the
present invention for the negative electrodes thereof have a long
cycle life and exhibit an excellent high rate discharge property
and low-temperature discharge characteristics, even when the alloy
has a low cobalt content.
[0032] The present invention is further illustrated by the
following examples. However, these examples are not to be construed
to limit the scope of the invention.
EXAMPLE 1 AND COMPARATIVE EXAMPLE 1
[0033] Mm or rare earth elements such as La, Ce, Pr and Nd,
metallic elements such as Ni, Co, Mn and Al, and Mg were weighed
out so as to give each of the compositions shown in Table 1. Mg was
used in the form of a MgNi.sub.2 (m.p. 1100.degree. C.) alloy.
These materials were melted in a high-frequency melting furnace,
and the resulting melt was cast in a mold made of iron to form an
ingot. As for the Mg-free alloy, the ingot is formed without using
the Mg-Ni alloy.
[0034] This ingot was heat-treated at 1,050.degree. C. for 6 hours
in an atmosphere of Ar. Thereafter, using a grinder, this ingot was
ground to an average particle diameter of 33 .mu.m so as to obtain
a hydrogen absorbing alloy powder. Analysis of this allow powder by
XRD revealed that it had a CaCu.sub.5 type crystal structure (FIG.
1).
[0035] 10 g of this powder was mixed with 2.5 g of a 3 wt % aqueous
solution of polyvinyl alcohol (with an average degree of
polymerization of 2,000 and a degree of saponification of 98 mole
%) to prepare a paste. This paste was filled into a porous metallic
body of foamed nickel in an amount of 30% by volume, dried, and
then pressed into a plate having a thickness of 0.5-1.0 mm.
Finally, a negative electrode was made by attaching a lead wire
thereto.
[0036] A positive electrode, which comprised a sintered electrode,
was bonded to the aforesaid negative electrode with a polypropylene
separator interposed therebetween. This assembly was immersed in a
6N KOH electrolyte to construct a battery.
[0037] Each of the batteries so constructed was tested in the
following manner. First of all, at a temperature of 20.degree. C.,
the battery was charged to 120% at 0.3C (90 mA/g) based on the
capacity of the negative electrode, rested for 30 minutes, and then
discharged at 0.2C (60 mA/g) until the battery voltage reached 0.6
V. When this cycle was repeated twenty times, the greatest
discharge capacity was regarded as the "capacity" of the alloy.
Subsequently, this battery was charged to 120% at 0.3C, and
discharged at 2.0C (600 mA/g). The capacity measured in this manner
was regarded as the "high rate discharge capacity". Thereafter, in
order to observe the degree of particle size reduction, the
negative electrode was disassembled, placed in water, and exposed
to ultrasonic waves from an ultrasonic horn so as to separate the
alloy powder from the current collector. The particle size
distribution after repeated charging and discharging was measured
by means of a Microtrack analyzer to determine the average particle
diameter D.sub.50 (.mu.m). The results thus obtained are shown in
Table 1. It is to be-understood that, when the frequency of
occurrence of each particle diameter in the measured particle size
distribution is cumulatively added from the smaller to the larger
side, the particle diameter corresponding to 50% of the entire
distribution is defined as D.sub.50.
1 TABLE 1 High rate Average particle discharge diameter after Alloy
composition (wt %) Capacity capacity size La Ce Pr Nd Mg Ni Co Mn
Al (mAh/g) (mAh/g) reduction (.mu.m) Example 1 25.04 3.16 1.90 1.30
0.27 56.61 5.31 4.58 1.82 305 220 25.31 Comparative 25.45 3.21 1.94
1.32 0.00 53.76 8.64 3.77 1.92 302 162 23.14 Example 1
[0038] As shown in Table 1, Mg-containing alloy has a better high
rate discharge property and less liability to particle size
reduction.
EXAMPLES 2-5 AND COMPARATIVE Example 2
[0039] The compositions shown in Table 2 were employed for the
formation of alloys in the same manner as Example 1 and capacities
were measured in the same matter as Example 1 to examine the
relationship between the La content and the capacity when magnesium
is contained in the alloys. The results thus obtained are shown in
Table 2. It can be seen from Table 2 that, in order to obtain an
alloy having a high capacity, the La content in the alloy must be
not less than 24% by weight.
2 TABLE 2 Alloy composition (wt %) Capacity La Ce Pr Nd Mg Ni Co Mn
Al (mAh/g) Example 2 25.56 3.87 1.30 1.33 0.17 58.86 2.71 3.79 2.42
306 Example 3 25.06 3.79 1.27 1.30 0.16 58.92 2.66 4.46 2.37 297
Example 4 24.86 3.76 1.26 1.29 0.27 59.22 2.64 3.69 3.02 293
Example 5 24.69 3.74 1.25 1.28 0.27 58.81 2.62 5.00 2.34 289
Comparative 23.80 6.25 1.32 1.35 0.29 57.44 2.77 4.38 2.41 275
Example 2
EXAMPLES 6-8 AND COMPARATIVE Example 3
[0040] Employing the compositions shown in Table 3, alloy powders
were prepared in the same manner as in Example 1. Then, electrode
tests were carried out in the same manner as in Example 1 to
determine the respective capacities. The results thus obtained are
shown in Table 3. It can be seen from Table 3 that Mg contents of
greater than 1.0% by weight cause an undue reduction in
capacity.
3 TABLE 3 Alloy composition (wt %) Capacity La Ce Pr Nd Mg Ni Co Mn
Al (mAh/g) Example 6 26.59 3.87 1.30 1.33 0.17 58.94 2.71 3.80 2.30
306 Example 7 25.53 3.86 1.29 1.33 0.28 58.80 2.71 3.79 2.42 301
Example 8 24.97 3.78 1.27 1.30 0.55 58.69 2.65 4.44 2.36 286
Comparative 24.99 3.15 1.27 1.30 1.09 58.74 2.65 4.45 2.37 270
Example 3
EXAMPLES 9-12 AND COMPARATIVE EXAMPLES 4-7
[0041] Employing the alloy compositions shown in Table 4, electrode
tests were carried out in the same manner as in Example 1.
Thereafter, each negative electrode was disassembled, placed in
water, and exposed to ultrasonic waves from an ultrasonic horn so
as to separate the alloy powder from the current collector. The
particle size distribution after repeated charging and discharging
was measured by means of a Microtrack analyzer to determine the
average particle diameter D.sub.50 (.mu.m). On the basis of the
average particle diameter of an alloy containing no Mg, the effect
of Mg addition, i.e. the improvement of particle size reduction,
was calculated as R1 (%) according to the following equation.
R1(%)={(D.sub.50 (.mu.m) of Mg-containing alloy)/(D.sub.50 (.mu.m)
of Mg-free alloy)}.times.100 (%)
[0042] Since the degree of particle size reduction varies greatly
with the Co content, the improvement of particle size reduction is
independently shown with respect to each Co content. The D.sub.50
is defined in such a way that, when the particle size distribution
of the hydrogen absorbing alloy is measured and the frequencies of
detection of various particle diameters are cumulatively added from
smaller-diameter to larger-diameter particles, the particle
diameter corresponding 50% of all particles is represented by
D.sub.50.
4 TABLE 4 Improvement of particle size Alloy composition (wt %)
reduction La Ce Pr Nd Mg Ni Co Mn Al R1 (%) Example 9 25.53 3.86
1.29 1.33 0.28 58.80 2.71 3.79 2.42 129.2 Example 10 24.97 3.78
1.27 1.30 0.55 58.69 2.65 4.44 2.36 134.7 Comparative 25.60 3.87
1.30 1.33 0.00 58.96 2.72 3.80 2.42 100.0 Example 4 Comparative
25.56 3.87 1.30 1.33 0.08 58.86 2.71 3.79 2.42 104.0 Example 5
Example 11 25.51 3.86 1.29 1.32 0.28 56.19 5.41 3.78 2.35 121.7
Comparative 25.58 3.87 1.30 1.33 0.00 56.34 5.43 3.79 2.36 100.0
Example 6 Example 12 25.38 3.84 1.29 1.32 0.28 53.62 8.61 3.76 1.91
110.5 Comparative 25.45 3.85 1.29 1.32 0.00 53.77 8.64 3.77 1.92
100.0 Example 7 Example 12 25.37 3.84 1.29 1.32 0.28 53.20 9.42
3.39 1.91 103.0 Comparative 25.44 3.85 1.29 1.32 0.00 53.09 9.45
3.65 1.92 100.0 Example 8
[0043] It can be seen from Table 4 that the addition of Mg
suppresses particle size reduction at the same Co content, and this
effect becomes more pronounced as the Co content is decreased. It
can also be seen that, at a low Mg content, for example, of less
than 0.1% by weight, the improvement of particle size reduction is
as low as 5% or less. Moreover, it can also be seen that, at a high
Co content, for example, of greater than 9% by weight, the effect
of Mg addition is lessened. In commercially available nickel-metal
hydride rechargeable batteries having a high capacity, the Co
content is usually not less than 9%. However, it can be seen that
the present invention exhibits a significant effect at Co contents
of not greater than 7%.
EXAMPLES 14-17 AND COMPARATIVE EXAMPLES 8-11
[0044] Employing the alloy compositions shown in Table 5, the alloy
powders were prepared in the same manner as in Example 1 except the
following: metallic Mg (m.p. 650.degree. C.) was used instead of
the Mg-Ni alloy, and the mixture of Ni, Co, Mn, Al and some of
rare-earth elements were melted in advance, and then after
confirming the melting, the other of rare-earth elements and
metallic Mg were added. As for the Mg-free alloys, the melting was
carried out without addition of the metallic Mg.
[0045] The capacity in Table 5 was measured as follows. After
dry-mixing hydrogen absorbing alloy 0.5 and Ni power 1.5 in the
weight ratio, the mixture was molded in a mold with a diameter of
20 mm to produce an electrode. The battery was charged to 125% at
0.5C (150 mA/g), rested for 10 minutes, and then discharged at 0.5C
(150 mA/g) until the voltage difference based on mercury reference
electrode (Hg/HgO) reached 0.6v. After this cycle was repeated ten
times, the capacity was measured (as pellet capacity).
[0046] Moreover, Cycle life was measured as follows. Using the
above-mentioned sample battery having the paste electrode, at a
temperature of 20.degree. C., the battery was charged to 120% at
0.3C (90 mA/g) based on the capacity of the negative electrode,
rested for 30 minutes, and then discharged at 0.2C (60 mg/g) until
the battery voltage based on the positive electrode reached 0.8V.
This cycle for charge and discharge was repeated two hundred times,
and the maintenance of discharge capacity (cycle life) was
calculated using the next equation.
Maintenance(%) ={(discharge capacity after 200 cycles)/(discharge
capacity after 20 cycles)}.times.100
[0047] Further, using the above-mentioned sample battery having the
paste electrode, at a temperature of 20.degree. C., the battery was
charged at 0.3C (90 mA/g) based on the capacity of the negative
electrode, rested for 30 minutes, and then discharged at 0.2C (60
mA/g) until the battery voltage reached 0.8V. After this cycle was
repeated twenty times, in order to observe the degree of particle
size reduction, the battery was disassembled and the alloy powder
for the negative electrode was exposed to ultrasonic waves from an
ultrasonic horn so as to separate the alloy powder from the current
collector. The particle size distribution after repeated charging
and discharging was measured by means of a Microtrack analyzer to
determine the average particle diameter D.sub.50 (.mu.m). The
improvement of particle size reduction R1 was calculated.
[0048] The diffraction patterns for the alloys shown in Table 5
were measured using a X-ray diffraction method for powder. The
lattice constants were calculated based on the measurement data
using a method of least squares.
5 TABLE 5 Improve- ment of particle Capac- size Length Length ity
Cycle reduction of of Elongation Elongation Alloy composition (wt
%) (mAh/ life R1 a-axis c-axis of a-axis of c-axis La Ce Pr Nd Mg
Ni Co Mn Al g) (%) (%) (.ANG.) (.ANG.) (.ANG.) (.ANG.) Example 14
25.07 3.79 1.27 1.30 0.27 57.88 3.99 4.46 1.95 330 90 117 5.020
4.063 0.002 0.007 Comparative 25.14 3.80 1.28 1.31 0.00 58.04 4.00
4.48 1.95 320 82 100 5.018 4.056 Reference Reference Example 8
Example 15 25.04 3.79 1.27 1.30 0.27 56.36 5.31 4.83 1.82 335 93
114 5.024 4.061 0.001 0.006 Comparative 25.11 3.80 1.27 1.30 0.00
56.51 5.33 4.84 1.83 330 85 100 5.023 4.055 Reference Reference
Example 9 Example 16 28.07 3.86 0.00 0.00 0.28 55.81 6.77 3.66 1.55
350 83 115 5.033 4.048 0.000 0.008 Comparative 28.15 3.87 0.00 0.00
0.00 55.97 6.79 3.67 1.55 330 70 100 5.033 4.040 Reference
Reference Example 10 Example 17 28.07 3.86 0.00 0.00 0.28 54.59
8.12 3.53 1.55 348 90 107 5.034 4.046 0.001 0.006 Comparative 28.15
3.87 0.00 0.00 0.00 54.74 8.14 3.54 1.55 325 80 100 5.033 4.040
Reference Reference Example 11
[0049] As shown in Table 5, forcusing on the effects of the
addition of Mg, the addition of Mg increases capacity, cycle life,
and improvement of particle size reduction. Comparison of the
lattice constants shows that the addition of Mg tends to increase
the c-axis more remarkably than a-axis. This is thought to be one
of the reasons why the higher capacity and increased cycle life are
attained. The results for Examples 16 and 17 show the specific
increase of discharge capacity, although increase of cycle life
therefor is fair.
EXAMPLES 18 TO 32, COMPARATIVE EXAMPLES 12 TO 19
[0050] Employing the alloy compositions shown in Table 6, the alloy
powders were prepared using MgNi.sub.2 (m.p. 1100.degree. C.) in
the same manner as in Example 1 except the following: the mixture
of Ni, Co, Mn, Al and some of rare-earth elements were melted at
first. Then, after the confirmation of the melting, the other of
rare-earth elements and the Mg-Ni alloy were added for melting. As
for the Mg-free alloys, the melting was carried out without
addition of the metallic Mg.
[0051] Pellet capacity and maintenance of discharge capacity (cycle
life) were obtained as the same manner as described above. After
the average particle diameter D.sub.50 was obtained as the same
manner as described above, the improvement of particle size
reduction was calculated as R2 (%) according the following
equation. The R2 shows the inhibition effect against particle size
reduction for the alloys other than the alloy for Example 16 on the
basis of the average particle diameter of the alloy of Comparative
Example 16.
R2(%)={(D.sub.50 (.mu.m) of Mg-containing alloy)/(D.sub.50 (.mu.m)
of the alloy for Example 16 alloy)}.times.100 (%)
6 TABLE 6 Improvement of Cycle particle size Alloy composition (wt
%) Ratio Capacity life reduction R2 La Ce Pr Nd Mg Ni Co Mn Al B/A
(mAh/g) (%) (%) Example 18 28.74 3.22 0.00 0.00 0.28 53.30 8.81
3.79 1.86 5.20 340 93 137 Example 19 28.74 3.22 0.00 0.00 0.28
53.98 8.13 3.79 1.86 5.20 350 90 130 Example 20 28.74 3.22 0.00
0.00 0.28 54.66 7.45 3.79 1.86 5.20 350 88 130 Comparative 28.82
3.23 0.00 0.00 0.00 52.77 9.51 3.80 1.87 5.20 315 90 130 Example 12
Comparative 28.82 3.23 0.00 0.00 0.00 53.45 8.83 3.80 1.87 5.20 320
83 130 Example 13 Comparative 28.82 3.23 0.00 0.00 0.00 54.13 8.15
3.80 1.87 5.20 323 77 125 Example 14 Comparative 28.82 3.23 0.00
0.00 0.00 54.81 7.47 3.80 1.87 5.20 328 73 115 Example 15
Comparative 28.82 3.23 0.00 0.00 0.00 55.49 6.79 3.80 1.87 5.20 330
89 100 Example 16 Example 21 28.74 3.22 0.00 0.00 0.28 52.63 9.48
3.79 1.86 5.20 325 97 140 Example 22 28.74 3.22 0.00 0.00 0.28
55.33 6.77 3.79 1.86 5.20 350 83 120 Example 23 30.34 0.97 0.32
0.33 0.28 53.98 8.13 3.79 1.86 5.20 350 88 130 Example 24 28.74
2.58 0.32 0.33 0.28 53.97 8.13 3.79 1.86 5.20 350 90 130 Example 25
27.14 3.86 0.65 0.33 0.28 53.97 8.13 3.79 1.86 5.20 345 92 130
Example 26 23.93 6.44 0.97 0.66 0.28 53.94 8.12 3.79 1.86 5.20 340
92 130 Comparative 32.03 0.00 0.00 0.00 0.00 54.15 8.15 3.80 1.87
5.20 350 75 110 Example 17 Comparative 21.13 7.31 1.23 6.27 0.00
51.03 7.69 3.58 1.76 5.20 330 92 130 Example 18 Comparative 20.79
9.04 1.30 1.00 0.00 54.07 8.14 3.80 1.86 5.20 320 93 130 Example 19
Example 27 27.89 3.84 0.00 0.00 0.28 56.15 6.70 3.62 1.53 5.25 345
85 121 Example 28 28.07 3.86 0.00 0.00 0.28 55.86 6.74 3.64 1.54
5.20 350 85 118 Example 29 28.26 3.89 0.00 0.00 0.28 55.56 6.79
3.67 1.55 5.15 350 83 114 Example 30 28.46 3.91 0.00 0.00 0.28
55.26 6.83 3.69 1.56 5.10 355 80 110 Example 31 28.65 3.94 0.00
0.00 0.28 54.95 6.88 3.72 1.57 5.05 355 75 107 Example 32 28.85
3.97 0.00 0.00 0.29 54.64 6.93 3.75 1.59 5.00 360 70 105
[0052] As shown in Table 6, the alloys keeping La content of 24 to
33% by weight and Co content of 6 to 9% by weight in addition of
0.1 to 1.0% by weight of Mg with the B/A atomic ratio of 5.0 to
5.25, enables the achievement for the higher capacity such as 340
mAh/g or more of capacity, although the cycle life therefor is as
usual.
* * * * *