Note: Descriptions are shown in the official language in which they were submitted.
-- 11426
ll~Z~ZO
FIELD OF THE I~VENTION
The invention relates to a silver oxide alkaline
cell employing a divalent silver oxide containing positive
electrode and wherein a minor amount of a cadmiu~ compound,
such as cadmium oxide, is incorporated into the positive
electrode and/or the electrolyte so as to improve the chemical
stability of the divalent silver oxide in contact with the
alkaline electrolyte. In addition, the cadmium compound
c n be em~loyed as an overbalance depolarizer in a divalent
silver oxide/zinc/alkaline cell so as to substantially
prevent hydrogen gassing from the cathode upon complete
discharge of the cell~
BACKGROUND OF THE I~VENTION
The battery has become a primary power source
for many portable electronic devices such as radios, hearing
aids, watches, calculators and the like. In order to
maintain the overall electronic device as compact as
possible the electronic devices are u~ually designed with
cavities to accommodate~iniature cells as their source of
power. The ca~ities are usually made so that a cell can
be snugly positionet therein thus making electronic contact
with appropriate terminals within the device. A major
potential problem in the use of a high energy density cell
such as a divalent silver oxide/zinc/alkaline cell, is that
if the cell bulges, it usually becomes wedged within the
cavity of the device which sometimes can result in damage
to the device. In addition, when the cell bulges it may
disturb the seal wheseupon the electrolyte might escape to
cause damage to the device andlor oxygen from the atmosphere
may enter which could cause wasteful corrosion of the anode.
2.
1142fi
2n
On the Dther hand, lf the ~eal of the cell is m2intained, high
$nternal g~s pressure may develop which coult cause not only
bulging of the cel} but even pos~ible disassembly of the cell.
Although divalent cilver oxide is a good hi8h ca?acity
positive active material when used in alkaline cells, it is
rather unstable when in contact wieh an aqueous alkaline elec-
trolyte. Specifically, divalene 6ilver oxide is a highly oxi-
dizing material nd as 6uch it i6 capable of decomposing the
water in an aqueous al~aline electrolyte there~y yielding oxy-
gen gas, In ddition, divalent ~ilver oxide will liberate oxy-
gcn when lt decomposes to form monovalent ~ilver oxide when in
contact with the queous alkaline electrolyte. Divalent silver
oxide can also attack cellulosic materials in the cell, such
as the separator, to form carbonate ion at the expense of even
more electrolyte. These are undesirable processes because they
lead to bulging of the cell, deterioration of its parts and
loss of ~ervice.
U. S. Patent 3,853,623 discloses one approach to
~tabilize divalent ~ilver oxide in silver oxide/zinc/ alkaline
cell through the use of gold ions incorporated inro the alkaline
clec roly~e on the posi~ive ~ide of .he cell's separaror or
gold oxide added ta the positive active material of ~he cell.
Canadian a~lication ~erial no. 323,519
discloses an alkaline ~ilver oxide cell employing
divalent 6ilver oxide-containing electrode wherein an alu~-
lnum additive i~ incorporated in the cell to improve the
~hemical ctability of the divalent silver oxide when in contact
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'~ i 11426
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with the cell's alkaline electrolyte.
It is an object of the present invention to
provide a divalent silver oxide cell thAt will minimize
internal gas pressure buildup so as to effectively eliminate
distortion of the cell's housing.
It is another object of the present invention to
provide a divalent silver oxide cell wherein a cadmium
compound, such as cadmium oxide, is incorporated into the
cell's electrolyte and/or the positive electrode so as to
effectively control gas pressure builtup within the cell
and thereby effectively eliminate distortion of the cell's
housing.
It is another ob;ect of the present invention to
incorporate a cadmium compound, such as cadmium oxide.in
a divalent silver oxide cell to chemically stabilize the
divalent silver oxide when in contact with the cell's
aqueous electrolyte.
It is another object of the present invention
to incorporate a cadmium compound, such as cadmium oxide,
in a divalent silver oxide/zinc cell as an
overbalance depolarizer to effectively prevent hydrogen
gassing from the cathode upon complete discharge of the
cell.
The foregoing and additional objects will become
more fully apparent from th~ following description.
SUMMARY OF THE I~VENTION
The invention relates to a silver oxide cell having
a negative electrode, An aqueous alkaline electrolyte, a
positive electrode comprising a major portion of divalent
-~ 11426
~422;~:~
,
silver oxide and a separator between said negative and
positive electrodes,the im~rovement comprising the
incorporation of a minor amount of a cadmium compound, such
as ca~mium oxide, into the positi~e electrode and/or into
the electrolyte of the cell to ~mprove the stability of
the divalent silver oxide-containing electrode in
contact with the aqueous alkaline electrolyte.
As used herein, a positive electrode or a silver
electrode shall mean an electrode wherein the active cathode
material is divalent silver oxide (AgO) or an electrode
wherein the major active material is divalent silver oxide
in conjunction with an amount below 50% by weight of
novalent silver oxide CAg20) and/or some other electro-
chemically active positive material.
The cadmium compound, such as cadmium oxide,
may be added in the solid positive electrode andlor
dissol~ed in the electrolyte. The ca~mium compound added
to the positive electrode may be either admixed with the
divalent silver oxide or incorporated within the divalent
silver oxide crystallites themselves through coprecipitation
or absorption dur$ng synthesis. Admixture of the cadmium
compound and d$valent silver oxide material has the
advantage of greater flexibility in the choice of the
divalent silver oxide material with regard to particle
size, purity and the like.
The minor amount of the cadmium compound additive
for use in the positive electrode of this invention to
improve the stability of the divalent silver oxide when
~ Z 2~ 11426
in contact with ~he cell's aqueous alkaline electrolyte
should be between about 0.01 and about 5 per cent based on
the dry weight of the divalent silver oxide in the positive
electrode and prefèrably between about 0.1 and about 0.3
weight Per cent based on the try weight of the divalent silver
oxide in the positive electrote. An amount of the cadmium
compound less than, for example, about O.Olweight per cent,
would not provide sufficient material to effectively improve
the stability of the divalent silver oxide material when in
contact with the aqueous alkaline electrolyte. An amount of
the cadmium compound more than, for example, about 5 weight
per cent added to the positive electrode would not further
improve the stability of the divalent silver oxide when in
contact with the aqueous alkaline electrolyte.
The cadmium compound to be added to the electrolyte
in accordance with this invention should be added in an amount
between about 0.0001 per cent and up to saturation of the
csdmium compound in the electrolyte. For example, when using
a 33 per cent potassium hydroxide solution, the upper limit
would be about 0.00125 per cent based on the weight of the
electroly~e. An amount of the cadmium oxide less than, for
example, about 0.0001 per cent would not provide sufficient
material to effectively improve the stability of the divalent
silver oxide material when in contact with the aqueous
alkaline electrolyte.
In addition to its use as a stabilizing agent,
cadmium oxide may also be used in larger amounts ranging
6.
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1~4Z~:20
11426
fr~m a~out 5 per cent to about 25 per cent by weight of the
dry constituçnts of the positive electrode, as an overbalance
depolarizer in a divalent silver oxide cell. An overbalance
depolarizer is that part of the positive electrote in excess
of the stoichiometric amount neeted to balance the negati~e
electrode. Its purpose is to augment the positive electrode
capacity sufficiently to prevent hydrogen gassing from the
cathode upon complete discharge of the cell (anode-limited),
Like divalent ~ilver oxide, a cadmium compound, such as
cadmium oxide, has limited solubility in alkaline environments,
discharges at a potential positive to hydrogen, and its
volumetric energy density is sufficiently high so as not to
impair the over-all energy density of the silver oxide/anode
system. As an overbalance substitute for divalent silver
oxide, its advantages are that it iæ far less costly and
imparts chemical stability to the divalent silver oxide
material when in contact with aqueous alkaline electrolyteO
It is al90 within the scope o~ this invention to
additionally dd zinc oxide or aluminate ions to the elec-
trolyte, and/or zinc oxide or aluminum oxide to the positiveelectrode. The zinc oxide and aluminate ions can be added
to the electrolyte in a range between about 0.5 and about
5 per cent based on the weight of the electrolyte. When
incorporatet in the positive electrode, the zinc oxide and
aluminum oxide can be added in an amount between about 0.5
and about 5 and about 0.001 and about 0.1 weight per cent,
respectively, baset on the dry weight of the divalent silver
ll~ZZZC~
11426
oxide in the positive electrode. Zinc oxide has been found
to be more desirable than aluminum oxide. Unexpectedly,
the combination of cadmium oxide in the positive electrode
and zinc oxide or aluminate ions in the electrolyte produced
a synergistic effect on the stability of divalent silver oxite
in contact with the a~ueous alkaline electrolyte.
It is also within the scope of this invention tO
additionally add a minor amount of a stabilizer, a flow agent
ant/or a lubricat~ng agent to the active positive mix to
further alter the physical characteristics of the active
po~itive mix for molding purposes to produce various size
and type electrodes. Examples of some of these additives
are ethylene bis-stearamide, ~inc stearate, lead stearate,
calcium 8tearate and the like.
The silver oxide electrodes of this in~ention may ,
be employet in an aqueous cell system using an anode such as
zinc, cadmium, in~ium or the li~e. The electrode couple so
selectet can be employed with a compatible electrolyte and
preferably an alkaline electrolyte. Examples of sultable
electrolytes include aqueous solutions of alkaline earth
metal hytroxides, 8uch as strontium hydroxide and alkali
metal hytroxides, exemplified by sodium hydroxide, potassium
hydroxide, lithium hydroxide, rubidium hydroxide and cesium
hydroxide. Com~atible mixtures of the preceding may be
utilized. Preferably, the electrode of this invention should
be porous so that the walls of the pores and interstices of
the electrode can become wetted by the electrolyte.
22ZO
11426
EXAMPLE I
Several lots of test cells were produced each
using a divalent silver oxide-containing posieive pellet
having a density of 90 grams per cubic inch, a negative
electrode of zinc, and a 33z KOH electrolyte. The positive
electrode was positioned in a cathote collector cup with
a zinc screen disposet between ehe inner surface of the cup
and the positive pellet as disclosed in U. S. Patent No.
3,920,478. Next a dual separator consisting of a cellulosic
barrier film and an absorbent layer, having a gold layer on
that side adjacent the cathode, was disposed on top of the
positive pellet followed by the zinc electrode, An anode
cup containing the zinc electrode was then placed over the
cathode Sup and sealed thereto in a conventional manner.
The exact constituents of the positive electrode
ant any attitives to the electrolyte are shown in Table 1.
The cells were stored for three month~ under
various temperature conditions. Thereafter the cells were
mea~ured for any bulging ant the results obtained for each
test lot were veraged and are shown in Table 2 along w~th
the maximum cell bulge observed in each lot. In addition,
the service life to a 1.3 volt cutoff was observed and is
also shown in Table 2,
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11426
The resulta of the data shown in Table 2 illu~trate
that using the teachings of this invention, a cadmium compound
additive with or without zinc oxide and aluminum addi~ives can
be offectively used to stabilize divalent silver oxide when in
contact with an aqueous a~kaline electrolyte ~o as to reduce cell
bulging, Aa also illustrated, the combination of a cadmium com-
pound in the po~itive olectrode and zinc oxide or aluminate ions in
the eloctrolyte produced a synergistic effect on the stability of
divalent silver oxide in contact with aqueous alkaline electrolyte.
That ia, the addition of both a cadmium compound in the positive
clectrode and zinc oxide or aluminate ions to the elqctrolyte reduced
bulge more than the composite of either material acting alone. This
i9 ~hown fo~ example by comparing te-t Lot 23 with Lots 16, 17,
and 19 for ZnO addition to the electrolyte, and by comparing Lot 22
with Lot~ 16, 18, and 19 for aluminate addition to the electrolyte.
EXAMPLE II
.
Several test lots of cells were produced as described in
Example I except that the cathode and electrolyte compositions employed
are as shown in Table 3. The reoults of the data shown in Table 3
illu~trate the benefits as expressed above in Example I in that the
cadmium compound additive reduces cell bulging. Both Example I
and Example ~T ~how that the addition of the cadmium compound by
admixture give~ slightly better bulge control than by coprecipitation.
14.
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