U.S. patent application number 11/657870 was filed with the patent office on 2007-05-31 for performance enhancing additive material for the nickel hydroxide positive electrode in rechargeable alkaline cells.
Invention is credited to Boyko Aladjov, Subhash K. Dhar, Stanford R. Ovshinsky, Bora Tekkanat, Srinivasan Venkatesan, Meera Vijan, Hong Wang.
Application Number | 20070120091 11/657870 |
Document ID | / |
Family ID | 33310432 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070120091 |
Kind Code |
A1 |
Ovshinsky; Stanford R. ; et
al. |
May 31, 2007 |
Performance enhancing additive material for the nickel hydroxide
positive electrode in rechargeable alkaline cells
Abstract
A conductive additive for the positive nickel electrode for
electrochemical cells which provides increased performance by
suppressing an oxygen evolution reaction occurring parallel to the
oxidation of nickel hydroxide, increasing conductivity of the
electrode and/or consuming oxygen produced as a result of the
oxygen evolution reaction.
Inventors: |
Ovshinsky; Stanford R.;
(Bloomfield Hills, MI) ; Aladjov; Boyko;
(Rochester Hills, MI) ; Venkatesan; Srinivasan;
(Southfield, MI) ; Tekkanat; Bora; (Ann Arbor,
MI) ; Vijan; Meera; (West Bloomfield, MI) ;
Wang; Hong; (Troy, MI) ; Dhar; Subhash K.;
(Bloomfield, MI) |
Correspondence
Address: |
ENERGY CONVERSION DEVICES, INC.
2956 WATERVIEW DRIVE
ROCHESTER HILLS
MI
48309
US
|
Family ID: |
33310432 |
Appl. No.: |
11/657870 |
Filed: |
January 25, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10428547 |
May 2, 2003 |
7201857 |
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11657870 |
Jan 25, 2007 |
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10378586 |
Mar 3, 2003 |
7172710 |
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10428547 |
May 2, 2003 |
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Current U.S.
Class: |
252/182.1 |
Current CPC
Class: |
H01M 4/48 20130101; H01M
10/345 20130101; H01M 4/364 20130101; H01M 4/50 20130101; H01M 4/32
20130101; H01M 4/52 20130101; H01M 4/62 20130101; Y02E 60/10
20130101; H01M 10/4235 20130101 |
Class at
Publication: |
252/182.1 |
International
Class: |
H01M 4/88 20060101
H01M004/88 |
Claims
1. An active material composition for a nickel positive electrode
comprising: a nickel hydroxide material; and an additive material
comprising a solid solution, said solid solution comprising two or
more metals.
2. The active material composition according to claim 1, wherein
said solid solution additive lacks nickel.
3. The active material composition according to claim 1, wherein
said solid solution additive includes two or more metals having an
oxidation state of +2 or higher.
4. The active material composition according to claim 1, wherein
said solid solution additive includes two or more metals having an
oxidation state of +3 or higher.
5. The active material composition according to claim 1, wherein
said solid solution additive is a metal oxide, said metal oxide
incorporating said two or more metals.
6. The active material composition according to claim 1, wherein
said solid solution additive does not form a solid solution with
said nickel hydroxide material.
7. The active material composition according to claim 1, wherein
the structure of said solid solution additive differs from the
structure of said nickel hydroxide material.
8. The active material composition according to claim 1, wherein
said solid solution additive comprises cerium.
9. The active material composition according to claim 1, wherein
said solid solution additive comprises three or more metals.
10. The active material composition according to claim 1, wherein
said nickel hydroxide material further comprises Co.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present invention is a continuation of, and is entitled
to the benefit of the earlier filing date and priority of,
co-pending U.S. patent application Ser. No. 10/428,547, to
Ovshinsky et al., which is assigned to the same assignee as the
current application, entitled "PERFORMANCE ENHANCING ADDITIVE
MATERIAL FOR THE NICKEL HYDROXIDE POSITIVE ELECTRODE IN
RECHARGEABLE ELECTROCHEMICAL CELLS", filed May 2, 2003, the
disclosure of which is hereby incorporated by reference. The
present invention is a continuation-in-part of, and is entitled to
the benefit of the earlier filing date and priority of, co-pending
U.S. patent application Ser. No. 10/378,586, to Ovshinsky et al.,
which is assigned to the same assignee as the current application,
entitled "PERFORMANCE ENHANCING ADDITIVE MATERIAL FOR THE NICKEL
HYDROXIDE POSITIVE ELECTRODE IN RECHARGEABLE ELECTROCHEMICAL
CELLS", filed Mar. 3, 2003, the disclosure of which is hereby
incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention generally relates to electrodes
utilizing performance enhancing additives. More particularly, the
present invention relates to utilizing a performance enhancing
additive to increase power output in a rechargeable electrochemical
cell by reducing positive electrode resistance.
BACKGROUND
[0003] The recent trend for portable devices has increased the
needs and requirements for high energy density and high power
density rechargeable batteries. High energy density and high power
density are also important criteria for batteries used for electric
or hybrid vehicles.
[0004] Nickel hydroxide has been used for years as an active
material for the positive electrode of alkaline electrochemical
cells. Examples of such nickel-based alkaline cells include nickel
cadmium (Ni--Cd) cells, nickel-iron (Ni--Fe) cells, nickel-zinc
(Ni--Zn) cells, and nickel-metal hydride (Ni-MH) cells. The energy
density of nickel-based electrochemical cells may be increased by
closely packing the nickel hydroxide active material into an
electrically conductive substrate such as a porous foam. However,
there are limitations on the amount of pressure used to increase
packing density. Application of too much pressure causes expansion
of electrode plates and compresses the separators placed between
the positive and negative electrodes. The overcompression of the
cells limit the wettability as well as the amount of electrolyte in
separators by squeezing out the absorbed electrolyte, which in turn
deteriorates the performance of these cells.
[0005] In general, nickel-metal hydride (Ni_MH) cells utilize a
negative electrode comprising a metal hydride active material that
is capable of the reversible electrochemical storage of hydrogen.
Examples of metal hydride materials are provided in U.S. Pat. Nos.
4,551,400, 4,728,586, and 5,536,591 the disclosures of which are
incorporated by reference herein. The positive electrode of the
nickel-metal hydride cell comprises a nickel hydroxide active
material. The negative and positive electrodes are spaced apart in
the alkaline electrolyte.
[0006] Upon application of an electrical current across a Ni-MH
cell, the Ni-MH material of the negative electrode is charged by
the absorption of hydrogen formed by electrochemical water
discharge reaction and the electrochemical generation of hydroxyl
ions: ##STR1## The negative electrode reactions are reversible.
Upon discharge, the stored hydrogen is released to form a water
molecule and release an electron.
[0007] The charging process for a nickel hydroxide positive
electrode in an alkaline electrochemical cell is governed by the
following reaction: ##STR2##
[0008] After the first charge of the electrochemical cell, the
nickel hydroxide is oxidized to form nickel oxyhydroxide. During
discharge of the electrochemical cell, the nickel oxyhydroxide is
reduced to form beta nickel hydroxide as shown by the following
reaction: ##STR3##
[0009] The charging efficiency of the positive electrode and the
utilization of the positive electrode material is affected by the
oxygen evolution process which is controlled by the reaction:
2OH--.fwdarw.H.sub.20+1/2 0.sub.2+2e- (4) During the charging
process, a portion of the current applied to the electrochemical
cell for the purpose of charging, is instead consumed by a parallel
oxygen evolution reaction (4). The oxygen evolution reaction
generally begins when the electrochemical cell is approximately
20-30% charged and increases with the increased charge. The oxygen
evolution reaction is also more prevalent with increased
temperatures. The oxygen evolution reaction (4) is not desirable
and contributes to lower utilization rates upon charging, can cause
a pressure build-up within the electrochemical cell, and can upon
further oxidation change the nickel oxyhydroxide into its less
conductive forms. One reason both reactions occur is that their
electrochemical potential values are very close. Anything that can
be done to widen the gap between them (i.e., lowering the nickel
reaction potential in reaction (2) or raising the reaction
potential of the oxygen evolution reaction (4)) will contribute to
higher utilization rates. It is noted that the reaction potential
of the oxygen evolution reaction (4) is also referred to as the
oxygen evolution potential.
[0010] Furthermore, the electrochemical reaction potential of
reaction (4) is highly temperature dependent. At lower
temperatures, oxygen evolution is low and the charging efficiency
of the nickel positive electrode is high. However, at higher
temperatures, the electrochemical reaction potential of reaction
(4) decreases and the rate of the oxygen evolution reaction (4)
increases so that the charging efficiency of the nickel hydroxide
positive electrode drops.
[0011] One way to increase the electrochemical potential of
equation (4) is by adding certain additives with the nickel
hydroxide active material when forming the positive electrode
material. U.S. Pat. Nos. 5,466,543, 5,451,475, 5,571,636, 6,017,655
6,150,054, and 6,287,726 disclose certain additives which improve
the rate of utilization of the nickel hydroxide in a wide
temperature range. The present invention discloses an improved
additive which enhances performance of the positive electrode by
reducing the resistance within the nickel electrode and
simultaneously increasing the oxygen evolution potential.
SUMMARY OF THE INVENTION
[0012] The present invention discloses an active material
composition for a nickel positive electrode comprising a nickel
hydroxide material and an additive material comprising a metal
oxide. The metal oxide may be one or more of a single metal oxide
containing a metal from the group comprising Ce, Ti, Mo, V, W, Sn,
Mn, In, Y, Sm, and Nb, or a binary or higher non-stoichiometric,
solid solution, oxide containing two metals from the group
comprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm, Nb, Ce, and Mm.
[0013] Preferably, the metal oxide includes cerium and/or titanium.
Cerium oxide, as an additive to the positive electrode, may be
further doped with a divalent oxide, a trivalent oxide, a
tetravalent oxide, or combinations thereof to improve the overall
electrode performance. The titanium oxide may also be comprised of
two or more sub-oxides having the formula TiO.sub.x, wherein x may
range from 0.65 to 1.25.
[0014] The single oxide preferably has the formula
A.sub.nO.sub.2n-1, wherein 4.0.ltoreq.n.ltoreq.10 and A is a metal
from the group comprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm, or Nb.
The binary or higher non-stoichiometric, solid solution, oxide
preferably has the formula B.sub.x(A.sub.nO.sub.2n-1).sub.1-x,
wherein 0.0<x<1.0, 4.0.ltoreq.n.ltoreq.10, A is one or more
metals from the group comprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm,
or Nb, and B is one or more metals from the group comprising Ti,
Mo, V, W, Sn, Mn, In, Y, Sm, Nb, Ce, and Mm.
[0015] The active material composition may comprise 78.6 to 85.6
weight percent nickel hydroxide, 3.0 to 6.0 weight percent cobalt,
3.0 to 8.0 weight percent cobalt oxide, 3.0 to 10.0 weight percent
of additive material, and 0.4 weight percent binder material.
Preferably, the active material comprises 5.0 to 9.0 weight percent
of the additive material.
DETAILED DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1, shows the conductivities of various titanium oxides
ranging from pure metallic Ti to TiO.sub.2.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention discloses a positive electrode active
material comprising a nickel hydroxide material and a performance
enhancing additive material. The performance enhancing additive
material in accordance with the present invention may be comprised
of one or more single, binary, or ternary or higher oxides. The
additives in accordance with the present invention enhance
performance in nickel hydroxide positive electrodes by suppressing
oxygen evolution by increasing the oxygen evolution potential. The
additive material may also further improve performance of the
positive electrode by increasing conductivity within the positive
electrode and/or consuming at least some of the evolved oxygen
within the electrochemical cell.
[0018] In the embodiments of the present invention, the positive
electrode active material generally comprises 73.6 to 88.1 weight
percent nickel hydroxide, 3.0 to 6.0 weight percent cobalt, 3.0 to
8.0 weight percent cobalt oxide, 0.5 to 15.0 weight percent of
additive material, and 0.4 weight percent binder material.
Preferably, the additive material is present in the range of 5.0 to
9.0 weight percent.
[0019] The nickel hydroxide material may be any nickel hydroxide
material known in the art. It is within the spirit and intent of
this invention that any and all kinds of nickel hydroxide materials
may be used. Examples of possible nickel hydroxide materials are
provided in U.S. Pat. Nos. 5,348,822, 5,637,423, and 6,086,843 the
disclosure of which are herein incorporated by reference.
[0020] The binder materials may be any material, which binds the
active material together to prevent degradation of the electrode
during its lifetime. Binder materials should be resistant to the
conditions present within the electrochemical cells. Examples of
binder materials, which may be added to the active composition,
include, but are not limited to, polymeric binders such as
polyvinyl alcohol (PVA), fluoropolymers, carboxymethyl cellulose
(CMC), hydroxycarboxymethyl cellulose (HCMC), and mixtures thereof.
An example of a fluoropolymer is polytetrafluoroethylene (PTFE).
Other examples of additional binder materials, which may be added
to the active composition, include elastomeric polymers such as
styrene-butadiene rubber latex. Furthermore, depending upon the
application, additional hydrophobic materials may be added to the
active composition.
[0021] In a first embodiment of the present invention, the additive
material may be cerium oxide. Cerium oxide is essentially
nonconductive material, however, cerium oxide increases the oxygen
evolution potential, thereby suppressing the parallel oxygen
evolution reaction, and consumes at least some of the oxygen formed
by the parallel oxygen evolution reaction, thus preventing further
oxidation of the nickel positive electrode materials into
nonconductive oxides.
[0022] In a second embodiment of the present invention, the
additive material may comprise a single oxide formed from a metal
selected from the group comprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm,
or Nb. These oxides are electroconductive and are able to reduce to
some extent the overall resistance within the nickel positive
electrode by increasing the conductivity and suppressing the
parallel oxygen evolution reaction within the nickel positive
electrode. Oxides of V, Ti, Mo, or W may form oxides in the Magneli
Phase having the general formula A.sub.nO.sub.m wherein n is in the
range of 4 to 50 and A is a metal chosen from the group of Ti, Mo,
V, and W. When A is Ti or V, m equals 2n-1, and when A is Mo or W,
m equals 3n-1 or 3n-2. Metallic oxides in the Magneli phase may
exhibit exceptional electroconductive properties as compared to
other metallic oxides. Depending on the metal valency, these metals
may form oxides comprising one or more sub-oxides having different
conductivities. For example, Ti forms an oxide comprised of
multiple sub-oxides having different conductivities. Titanium
sub-oxides having the formula TiO.sub.x, with
0.65.ltoreq.x.ltoreq.1.25 have high conductivities and are
preferred for use in the present invention. A plot showing the
relationship between conductivity and stoichiometry of several
different titanium oxides is shown in FIG. 1.
[0023] In a third embodiment of the present invention, the additive
material may be a mixed oxide which is a non-stoichiometric, solid
solution, binary or higher oxide. The non-stoichiometric, solid
solution, binary oxides are formed from oxides of two or more
metals selected from the group consisting of Ce, Mn, Co, Ni, Sm, Y,
Nb, In, Ti, V, W, Mo, and Mm, wherein Mm is a Misch metal alloy
formed from two or more of the rare earth metals. As a general
rule, these oxides are represented by a formula MO.sub.2 where M
represents the sum total of all metals present in the oxide
including the dopants. If it is a true binary, MO.sub.2 will
represent the total of two metals present in the alloy. Non
stoichiometry in the oxides can be due to oxygen vacancy or metal
vacancy in the oxide. In typical examples of TiO.sub.2, in one case
there is oxygen vacancy and in the other case there is oxygen
excess. Both cause disturbances in the d-orbitals causing disorder
in their lattices. This disorder (or non stoichiometry) results in
enhanced conductivity and other benefits. When the binary or higher
oxides are formed with V, Ti, Mo, or W, the oxides may fall within
the Magneli phase. The formula for binary or higher oxides in the
Magneli phase is B.sub.x(A.sub.nO.sub.m).sub.1-x, wherein
0.0<x<1.0, 4.0.ltoreq.n.ltoreq.10, A is one or more metals
selected from the group of Ti, Mo, V, W, and B is one or more
metals chosen from the group of Ti, Mo, V, W, Sn, Mn, In, Y, Sm,
Nb, Ce, and Mm. When A is either V or Ti, m is equal to 2n-1 and
when A is either Mo or W, m is equal to 3n-1 or 3n-2. Some elements
such as Nb, when included in binary oxide increase the thermal and
electrochemical stability to the binary oxide thereby preventing
further oxidation of the binary oxide to lesser conductive forms.
Preferably the binary oxides include cerium thereby providing
increased conductivity, increased oxygen evolution potential, and
consumption of at least some of the evolved oxygen within the
nickel positive electrode. Niobium and cerium also increase the
oxygen vacancies within the oxides. Examples of some of the
preferred non-stoichiometric, solid solution, binary compounds
including cerium or niobium are Ce.sub.0.4Ti.sub.3.6O.sub.7,
Ce.sub.2Ti.sub.4O.sub.10, Ce.sub.2Ti.sub.5O.sub.12,
CeTi.sub.4O.sub.9, CeTi.sub.5O.sub.11, and
Nb.sub.0.1Ti.sub.0.9O.sub.2.
[0024] In a fourth embodiment of the present invention, the
additive material may be a compositional mixture of two or more
oxides selected from cerium oxide, single oxides, and binary or
higher oxides as described above.
[0025] The additive material suppresses the oxygen evolution
reaction by increasing the electrochemical over potential of the
oxygen evolution reaction thereby making the nickel hydroxide
oxidation the more favorable reaction within the electrode. Without
addition of the additive material the overlapping range of the
potential of the oxygen evolution reaction and the potential for
the nickel hydroxide oxidation occurring during charging of the
positive electrode is very large. The additive material raises the
potential of the oxygen evolution reaction thereby reducing the
overlapping range between the fully charged positive electrode
potential and the potential for the oxygen evolution reaction.
[0026] As compared to other additives used to suppress the parallel
oxygen evolution reaction occurring within the positive electrode,
such as Ca(OH).sub.2, the additive materials in accordance with the
present invention have a much higher conductivity resulting in
lower overall resistance within the electrode. The excellent
conductivity of the additive materials create increased
conductivity within the positive electrode thereby providing higher
power output for the resulting electrochemical cell.
[0027] Additive materials including cerium oxide provide an
additional benefit by being able to consume at least some of the
evolved oxygen via a redox couple mechanism. By consuming at least
some of the evolved oxygen via an oxidation reaction, the redox
couple prevents further oxidation of nickel oxyhydroxide and oxides
contained in the additive material to less conductive forms. Cerium
oxide, acts as a redox couple
(Ce.sub.2O.sub.3.revreaction.CeO.sub.2) by consuming oxygen
produced by the parallel oxygen evolution reaction occurring during
charging of the electrochemical cell. Oxygen is consumed by the
cerium oxide redox couple via an oxidation reaction during charging
and is released by the redox couple via a reduction reaction to
form hydroxide ions during discharging. The redox couple reaction
for cerium oxide is shown as: ##STR4##
[0028] The mechanism behind the extraordinary ability of ceria
(CeO.sub.2) to store, release, and transport oxygen can be
explained by oxygen-vacancy formation and migration coupled with
the quantum process of electron localization. Ceria releases oxygen
under reduction conditions forming a series of reduced oxides with
stoichiometric cerium oxide (Ce.sub.2O.sub.3) as an end product,
which in its turn easily takes up oxygen under oxidizing
conditions, turning the Ce.sub.2O.sub.3 back into ceria. The
CeO.sub.2--Ce.sub.2O.sub.3 transition is entirely reversible.
[0029] Cerium is the first element in the periodic table with a
partially occupied f orbital. This leads to many features of
elemental cerium, such as the g--a iso-structural transition, where
at a critical pressure the volume of the unit cell suddenly
collapses preserving the face centered cubic (fcc) structure. The
reason for this drastic change in volume at the transition point
can be explained by the delocalization (or metallization) of the 4f
electron under pressure.
[0030] This characteristic of cerium appears to be equally
justified for the insulating cerium oxides, as cerium formally has
the valance 4+ in CeO.sub.2, the most oxidized form of cerium, and
3+ in Ce.sub.2O.sub.3, the other extreme final state of the
transition. In CeO.sub.2, all four valence electrons of Ce,
6s.sup.25d.sup.14f.sup.1, nominally leave the host atoms and
transfer into the p bands of two oxygen atoms, while in
Ce.sub.2O.sub.3 the Ce f electron is fully localized. The oxygen p
band has two extra electrons provided by cerium. These electrons
are left behind when an oxygen atom leaves its lattice position.
The oxygen-vacancy formation process is essentially facilitated by
a simultaneous condensation of these two electrons into localized
f-level traps on two cerium (3+) atoms. Such a description of the
two forms of cerium oxide, that the localization-delocalization of
the Ce 4f electron is involved in the CeO.sub.2--Ce.sub.2O.sub.3
transition, can also be supported from structural point of view on
the microscopic level.
[0031] It is possible to choose a common unit cell for both cerium
oxides. The C-type structure of Ce.sub.2O.sub.3, which is the end
product of reduction process of CeO.sub.2, can be constructed out
of eight unit cells of CeO.sub.2, with 25% oxygen vacancies ordered
in a particular way. The addition or removal of oxygen atoms
involves a minimal reorganization of the skeleton arrangement of
cerium atoms. This structural property definitely facilitates the
excellent reversibility of the reduction-oxidation process. The
condensation of the f electron into core state of a Ce atom (i.e.
its localization) leads to 10% volume increase. In other words, as
far as the cerium atoms are concerned, the reduction-oxidation
transition can be viewed upon as an almost isostructural transition
accompanied by a 10% volume change, in resemblance with the g--a
transition in pure fcc cerium showing a volume discontinuity of
about 16%.
[0032] Clearly, on the microscopic level, the removal of an oxygen
atom is made possible due to ability of the cerium atom to easily
and drastically adjust its electronic configuration to best fit its
immediate environment. Thus, the process of oxygen-vacancy
formation is closely coupled with the quantum effect of
localization/delocalization of the 4f electron of cerium. This is
the basis for the oxygen storage capacity of cerium oxide.
[0033] To help promote the beneficial effect of the cerium oxide
redox couple, the cerium oxide may be doped with divalent or
trivalent oxides (or some of the oxides described above) to create
additional structural defects causing more oxygen vacancies within
the redox couple containing electrode. These additives do not
change the fundamental character of the reactions but will improve
their relative rates. In this aspect the additives could even be
characterized as "promoters". Solid solutions of cerium oxide with
some oxides such as Y or La can be readily formed. The resulting
intentionally designed oxygen vacancies are mobile and form the
dominant point defect involved in transport behavior; oxygen
diffusion is very fast whereas the cation diffusion is slow.
[0034] When forming the positive electrode, the positive electrode
active material is prepared and affixed to a current collector
grid. The additive materials may be chemically impregnated into the
active material, mechanically mixed with the active material,
co-precipitated into or onto the surface of the active material
from a precursor, distributed throughout the active material via
ultrasonic homogenation, deposited onto the active material via
decomposition techniques, or coated onto the active material. The
positive electrode active material may be formed into a paste,
powder, or ribbon. The positive electrode active material may also
be pressed onto the current collector grid to promote additional
stability throughout the electrode. The current collector grids in
accordance with the present invention may be selected from, but not
limited to, an electrically conductive mesh, grid, foam, expanded
metal, perforated metal, or combination thereof. The most
preferable current collector grid is an electrically conductive
mesh having 40 wires per inch horizontally and 20 wires per inch
vertically, although other meshes may work equally well. The wires
comprising the mesh may have a diameter between 0.005 inches and
0.01 inches, preferably between 0.005 inches and 0.008 inches. This
design provides optimal current distribution due to the reduction
of the ohmic resistance. Where more than 20 wires per inch are
vertically positioned, problems may be encountered when affixing
the active material to the substrate. One current collector grid
may be used in accordance with the present invention, however the
use of two current collector grids may further increase the
mechanical integrity of the positive nickel electrode.
EXAMPLE 1
[0035] Several test cells using positive electrodes in accordance
with the present invention were constructed and tested against a
cell using a standard (control) positive electrode. To form the
standard positive electrode, a standard positive electrode paste
was formed from 88.6 weight percent nickel hydroxide material with
co-precipitated zinc and cobalt from Tanaka Chemical Company, 5.0
weight percent cobalt, 6.0 weight percent cobalt oxide, and 0.4
weight percent polyvinyl alcohol binder. The paste was then affixed
to a current collector grid to form the standard positive
electrode. Three additional positive electrodes were constructed
similarly by replacing 3.0 weight percent of the nickel hydroxide
material with cerium oxide, 5.0 weight percent nickel hydroxide
with cerium oxide, and 10.0 weight percent nickel hydroxide with
cerium oxide respectively.
[0036] A positive limited tri-electrode battery cell (test battery
cell) was formed using two hydrogen storage alloy negative
electrodes, a nickel hydroxide positive electrode and an auxiliary
Hg/HgO reference electrode. Each one of these electrodes are
contained in a non conducting but porous separator bag to prevent
shorting. The hydrogen storage alloy negative electrode includes an
active electrode composition formed by physically mixing 97 wt % of
a hydrogen storage alloy, 1.0 wt % carbon, and 2.0 wt % binder. The
active electrode composition is made into a paste and applied onto
a current collector grid to form the negative electrode.
[0037] After the initial formation procedure and two regular
charge/discharge cycles, the control cell (utilizing standard
positive nickel electrode) and the test cells (utilizing positive
nickel electrode with cerium oxide) are each discharged to 50%
depth of discharge at constant discharge current. The control cell
and the test cells are then subjected to a sequence of 10 and 30
second discharge pulses of increasing magnitude (0.5 amp, 1 amp,
1.5 amp, etc.). The potential change (.DELTA.V) of the positive
electrode after 10 and 30 seconds is measured relative to the
Hg/HgO reference electrode. The positive electrode potential values
in respect to Hg/HgO reference electrode (at the end of each of the
discharge current pulses) were plotted versus the value of the
discharge currents for both the control cell and the test cells.
The slopes of the linear portion of the plots represent the
resistance of each positive nickel electrode. The electrodes
containing the cerium oxide showed reduced resistance as compared
to the standard positive electrode tested under the same
conditions. The resistance gradually decreased with the increase of
cerium oxide up to 10 percent by weight of the positive electrode
active material. The results for these tests are shown in Table 1.
TABLE-US-00001 TABLE 1 Resistance at the Resistance at the end of a
10 end of a 30 Sample Second Pulse Second Pulse Standard Electrode
.065 Ohm .072 Ohm Electrode w/3 wt % CeO.sub.2 .059 Ohm .065 Ohm
Electrode w/5 wt % CeO.sub.2 .049 Ohm .054 Ohm Electrode w/10 wt %
CeO.sub.2 .047 Ohm .052 Ohm
EXAMPLE 2
[0038] Several test cells using a positive electrodes in accordance
with the present invention was constructed and tested against a
cell using a standard (control) positive electrode. To form the
standard positive electrode, a standard positive electrode paste
was formed from 88.6 weight percent AP64 nickel hydroxide material
produced by Ovonic Battery Company with co-precipitated zinc and
cobalt, 5.0 weight percent cobalt, 6.0 weight percent cobalt oxide,
and 0.4 weight percent polyvinyl alcohol binder. The paste was then
affixed to a current collector grid to form the standard positive
electrode. Three electrodes in accordance with the present
invention were constructed similarly by replacing some of the
nickel hydroxide material with an additive material in accordance
with the present invention. One electrode was constructed with 5.0
weight percent of the nickel hydroxide material being replaced with
commercially available EBONEX (the registered trademark of
Atraverda Limited) titanium oxide. A second electrode was
constructed with 5.0 weight percent of the nickel hydroxide
material being replaced with synthesized Magneli phase type
titanium oxide with the overall formula Ti.sub.4O.sub.7. A third
electrode was constructed with 5.0 weight percent of the nickel
hydroxide material being replaced with a first sample of
Ce.sub.0.4Ti.sub.3.6O.sub.7 produced using a one step synthesis
process. A fourth electrode was constructed with 5.0 weight percent
of the nickel hydroxide material being replaced with a second
sample of the same formulation Ce.sub.0.4Ti.sub.3.6O.sub.7 produced
using a two step synthesis process.
[0039] A sample of Magneli phase type titanium oxide
(Ti.sub.4O.sub.7) was synthesized by combining an appropriate
amount of TiO.sub.2 powder and Ti powder and heating the well mixed
homogeneous mixture for 4 hours at 1300.degree. C. in a hydrogen
environment. A first sample of Ce.sub.0.4Ti.sub.3.6O.sub.7 was
produced utilizing a one step process including combining
appropriate amounts of CeO.sub.2, TiO.sub.2, and Ti, mixing the
materials into a homogeneous mixture, and heating the mixture for 4
hours at 1200.degree. C. in a hydrogen environment. A second sample
of Ce.sub.0.4Ti.sub.3.6O.sub.7 was produced utilizing a two step
synthesis process including 1) combining TiO.sub.2 and Ti and
heating for 4 hours at 1300.degree. C. in a hydrogen environment to
form Ti.sub.4O.sub.7, and 2) adding CeO.sub.2 and heating for an
additional 4 hours at 1300.degree. C. in a hydrogen environment to
form Ce.sub.0.4Ti.sub.3.6O.sub.7.
[0040] As described in example 1, a positive limited tri-electrode
battery cell (test battery cell) was prepared using two hydrogen
storage alloy negative electrodes, a nickel hydroxide positive
electrode and an auxiliary Hg/HgO reference electrode. Each one of
these electrodes are contained in a non conducting but porous
separator bag to prevent shorting. The hydrogen storage alloy
negative electrode includes an active electrode composition formed
by physically mixing 97 wt % of a hydrogen storage alloy, 1.0 wt %
carbon, and 2.0 wt % binder. The active electrode composition is
made into a paste and applied onto a current collector grid to form
the negative electrode.
[0041] After the initial formation procedure and two regular
charge/discharge cycles, the control cell (utilizing standard
positive nickel electrode) and the test cell (utilizing positive
nickel electrode with additive material in accordance with the
present invention) are each discharged to 50% depth of discharge at
constant discharge current. The control cell and the test cells
were then subjected to a sequence of 30 second discharge pulses of
increasing magnitude (0.5 amp, 1 amp, 1.5 amp, etc.) The potential
change (.DELTA.V) of the positive electrode after 10 and 30 seconds
is measured relative to the Hg/HgO reference electrode. The
positive electrode potential values in respect to Hg/HgO reference
electrode (at the end of each of the discharge current pulses) were
plotted versus the value of the applied discharge currents for both
the control cell and the test cell. The slopes of the linear
portion of the plots represents the resistance of each positive
nickel electrode. The electrodes containing the additives in
accordance with the present invention showed reduced resistance as
compared to the standard positive electrode tested under the same
conditions. The electrodes containing the
Ce.sub.0.4Ti.sub.3.6O.sub.7 formed by the two step process
exhibited the best results. The results for these tests are shown
in Table 2. TABLE-US-00002 TABLE 2 Resistance at the Resistance at
the end of a 10 end of a 30 Sample Second Pulse Second Pulse
Standard Electrode .065 Ohm .071 Ohm Electrode w/5 wt % Ebonex .056
Ohm .062 Ohm Ti.sub.4O.sub.7 Electrode w/5 wt % .046 Ohm .053 Ohm
synthesized Ti.sub.4O.sub.7 additive Electrode w/5 wt % .051 Ohm
.059 Ohm Ce.sub.0.4Ti.sub.3.6O.sub.7 (1 step process) Electrode w/5
wt % .041 Ohm .047 Ohm Ce.sub.0.4Ti.sub.3.6O.sub.7 (2 step
process)
[0042] It is to be understood that the disclosure set forth herein
is presented in the form of detailed embodiments described for the
purpose of making a full and complete disclosure of the present
invention, and that such details are not to be interpreted as
limiting the true scope of this invention as set forth and defined
by the appended claims.
* * * * *