BACKGROUND OF THE INVENTION

This invention relates to electrochemical cells and, more particularly, to sealed nickel-zinc cells. Many kinds of portable equipment, such as, for example, tape and video recorders, power tools, calcul¬ ators, radios, electric razors, television sets, tele¬ phone pagers, microcomputers, and the like, have been developed over the last several years; and their use has become relatively widespread. In some situations, a primary battery or cell has been utilized as the power source. However, while primary cells offer the advan¬ tage of relatively high energy densities, these are relatively expensive because of the continual need for replacement. For this reason, it has been useful to employ a rechargeable cell as the power source. Various types of lead-acid cells have been utilized, particularly where cost is the primary consideration. However, such cells have relatively low energy densities and cycle life capabilities, as well as requiring relatively long times for charging.

The inadequacies of primary and lead-acid cells have led to the use of nickel-cadmium cells in some

applications. Such cells, while relatively expensive in comparison to lead-acid cells, offer relatively high energy densities and an extremely long cycle life. Moreover, this type of cell is capable of being employed at relatively high charge/discharge rates.

Despite the advantages provided by nickel-cadmium cells, there is a continuing demand for many applica¬ tions for a power source capable of achieving even higher energy densities and operating at higher working voltages. This situation has led to the investigation of nickel-zinc rechargeable cells for these applica¬ tions. The nickel-zinc system is well known and, at least potentially, offers substantial advantages. In comparison to nickel-cadmium cells, nickel-zinc cells have higher working or operating voltages (viz. about 1.65 volts) and potentially can provide significantly higher energy densities • U.S. Patents 3,951,687 and 4,037,033 disclose configurations for nickel-zinc cells. Despite this promise, the commercial use of nickel- zinc cells for the portable applications described here¬ in has been extremely limited. This is principally due to the inability to deal adequately with the rela¬ tively high internal pressures inherent in this system. Thus, while the cadmium electrode in a nickel-cadmium cell is marginally thermodynamically stable with respect to reduction of aqueous alkaline battery electrolytes with consequent evolution of hydrogen gas, the zinc electrode in a nickel-zinc system is unstable. The nickel-zinc system accordingly tends to evolve hydrogen gas under all conditions of service, viz., charge, dis¬ charge, overcharge and open-circuit stand.

To be commercially useful, a sealed nickel-zinc cell must therefore possess the ability to compensate for the hydrogen gas evolved so that the cell under con- ditions of use should not vent. If the cell vents.

decreased performance can result if enough electrolyte is lost. Moreover, discharge of alkaline electrolyte into the environment could be harmful to electronic or other components in the area where the cell is employed. The internal pressures which can be tolerated depend upon the strength of the containers utilized. For small cylindrical cells that typically use container materials which will rupture at about 450 to 500 p.s.i.g. (at such pressures the container top typically separates from the container), safety considerations dictate that venting means be employed which will vent at internal pressures of about 250 p.s.i.g. or so. In prismatic cells, the plastic container materials typically used require that such cells vent at significantly lower internal pres- sures, viz. - about 25 p.s.i.g. or so.

The internal pressures developed in sealed nickel- zinc cells under conditions of use can readily exceed 250 p.s.i.g. under various conditions. A considerable amount of effort has been expended to develop a nickel- zinc system capable of operation at satisfactorily low internal pressures.

Japanese Publication Nos. 135433/78 and 138021/78 make reference to hydrogen gas having been absorbed in nickel-zinc batteries by various methods, including (1) producing water by the chemical reaction of hydrogen and oxygen using a gas-phase catalyst such as platinum, pal¬ ladium or silver, (2) chemically absorbing hydrogen selectively, using manganese dioxide and a silver or palladium catalyst, (3) producing a metal hydride such as titanium, and (4) producing water electrochemically using a supplementary electrode. It is noted that, how¬ ever, since the absorption of hydrogen gas is insuffi¬ cient, sealing of the battery has been, difficult because of the build-up of inner pressure, expansion of the cell casing and leakage of the electrolyte solution.

These patent applications suggest the use of a sup¬ plementary electrode for the absorption of hydrogen gas. This approach is considered to be relatively complex and, accordingly, relatively expensive. Moreover, this approach apparently does not address the problem of pressure buildup when a cell is on stand.

It has also been suggested to utilize manganese dioxide to absorb hydrogen. Kozawa, in an article entitled "HYDROGEN GAS REMOVER MADE OF M 0 ₂ ", New Mater. Processes Electrochem. Technol. 1981 1-NMNTDE, describes various approaches and resulting data for removing hydrogen gas by this technique. Applications of this technique are said to include the removal of hydrogen gas in sealed cells and in lead-acid batteries, as well as the removal of hydrogen gas generated from a battery power pack which operates adjacent electronic equipment. It is noted that hydrogen gas is often generated from dry and rechargeable batteries and alters the charac¬ teristics of the capacitors and resistors in the elec- tronic circuit, which problem can be eliminated by using a hydrogen remover at the top of the battery pack.

The difficulty with the use of this technique is that this is typically an irreversible system. Once the manganese dioxide (serving as the source of oxygen) is used up, no more hydrogen absorption will take place.

It is accordingly necessary to make sure there is enough material to absorb the desired amount of hydrogen gas which is evolved. Moreover, in a nickel-zinc cell, for example, the hydrogen which is absorbed will be incap- able of being restored to the cell upon charging. And, in the extreme case, the negative zinc electrodes can become essentially completely discharged while the nic¬ kel positive electrodes are almost completely charged. In this condition, the cell simply cannot be recharged.

U.S. 4,224,384 to Kozawa et al. discloses a silver catalyzed— anganese dioxide mixture useful as a hydrogen gas absorber. As noted therein, alkaline silver oxide- zinc cells were in the past assembled with the addition of small amounts of manganese dioxide to the silver oxide cathode for the purpose of extending the discharge capacity. It was observed that such cells had some hydrogen absorption properties ascribed to the silver oxide, since it was well known that manganese dioxide was incapable of reacting with hydrogen at room temper¬ ature. It was believed, applicants stated, that silver oxide with a small amount of manganese dioxide as a hydrogen absorber would be undesirably expensive and would not absorb hydrogen in sufficient amounts at a sufficiently rapid rate.

The Kozawa et al. patent discloses forming a silver catalyzed-manganese dioxide mixture into a shaped article of a desired configuration. The shaped article can then be used in electrochemical cells to absorb hydrogen gas as described in U.S. 3,893,870. The hydrogen gas absorber is stated to be not only relatively inexpensive, but, in addition, to absorb hydrogen gas at a relatively fast rate and to have a relatively high total capacity for absorbing hydrogen gas. Such absorbers can be regenerated, by, for exam¬ ple, after having been saturated with hydrogen, exposing the absorber to air at room temperature for 1 to 4 days. It is not explained how such regeneration could be effected in a cell, and it would seem that this would have to be carried out outside of the cell. This approach may accordingly lack the ease of use required in a practical commercial application.

Another proposal is disclosed in the co-pending Gibbard et al. application identified herein, and assigned to the assignee of the present invention. The

SUBSTITUTE SHEET

OM

•Ary WI

nickel-zinc system disclosed therein includes various aspects, one of which is a fuel cell cathode unconnected to the remainder of the system. Utilization of this approach has been found to substantially reduce the internal pressure developed under typical cycling conditions. However, the performance upon prolonged stand and high rate charge/discharge conditions can certainly be improved. Under either of these condi¬ tions, internal pressure can develop to the point where the cells will vent.

Accordingly, despite the prior efforts to provide a commercially viable sealed, nickel-zinc cell, such cells still have made little, if any, inroad as replace¬ ments for the various power sources now employed for portable equipment applications. There certainly exists the need to provide a commercially attractive and cost-effective sealed nickel-zinc cell capable of obviating undue internal pressure buildup under all conditions the cell would be exposed to in its service life.

It is accordingly a principal object of the present invention to provide a sealed, secondary alkaline cell of the type where hydrogen buildup is a problem which is capable of effectively obviating undue pressure buildup.

A still further object is to provide a cell of the foregoing type that is simple in construction and which is capable of being economically manufactured.

Another object of this invention lies in the provision of a cell of the foregoing type having the capability of being both charged and discharged at relatively high rates without undue internal pressure buildup.

Yet another object of this invention is to provide a cell of the foregoing type capable of remaining on

open-circuit stand for extended periods of time without undue pressure buildup.

Other objects and advantages of the present inven¬ tion will become apparent from the following detailed description, and from the drawings in which:

FIGURE 1 is a side elevation of a nickel-zinc cell embodying the present invention and partially cut away to show the internal configuration;

FIG. 2 is a cross-sectional view taken generally along lines 2-2 of FIGURE 1 and further illustrating the internal configuration of a cell according to the present invention, and

FIG. 3 is a graph of internal pressure versus time and showing the improved performance of a cell in accordance with the present invention.

While the invention is susceptible to various modifications and alternative forms, there is shown in the drawings and will herein be described in detail, the preferred embodiments. It is to be understood, however, that it is not intended to limit the invention to the specific forms disclosed. On the contrary, it is intended to cover all modifications and alternative forms falling within the spirit and scope of the inven¬ tion as expressed in the appended claims. For example, while the present invention will be primarily described in connection with a rechargeable nickel-zinc sealed cell, the invention is equally applicable to use in conjunction with nickel-cadmium cells. Indeed, the present invention may be advantageously utilized in any sealed, secondary alkaline cell (prismatic or cylindri¬ cal configuration) having a restricted amount of elec¬ trolyte (viz. - not having so much electrolyte that an efficient oxygen recombination reaction will be pre¬ vented, as is known in the sealed cell art), and a

nickel-oxygen containing positive electrode, where internal pressure due to hydrogen buildup is a problem. In general, the present invention is predicated on the discovery that the incorporation of a suitable catalyst for the oxidation of hydrogen gas, for example, Ag,0, into the positive electrodes of a nickel-zinc cell will provide a cell capable of operation without undue hydrogen pressure being built up under virtually all conditions likely to be encountered in use. Even when employed in applications which involve relatively high rate discharges, the internal hydrogen pressure buildup is moderate. Moreover, the balance of the cell chemis¬ try is maintained so that the cell may be readily re¬ charged without loss of capacity, even at relatively rapid rates without undue pressure buildup. The cells may also be allowed to stand on open circuit for pro¬ longed periods with no difficulty due to pressure build¬ up, since the internal pressure tends to decrease on stand. In short, the present invention provides a power source for portable equipment which satisfies the var¬ ious requirements in a fashion that will allow the cells to be expeditiously made in commercial production.

Turning now to the illustrative embodiment, there is shown in the drawings a rechargeable, sealed nickel- zinc cell incorporating the present invention, the cell being generally designated at 10. The particular con¬ figuration of the cell is only exemplary, and may be modified as desired. The cell 10 comprises an outer housing 12 defining a cell 14. The cup-shaped housing 12 has an open end 16 which is closed by closure 18 sealingly mounted upon open end 16 by an annular insula¬ tor 20. A perforator disc 22 is secured to the open end 16 of the outer housing 12 by an annular retainer 24 and is provided with a piercing tab 26 adapted to pierce closure 18 in the event the closure is urged

outwardly, as by internal pressure buildup within the sealed battery. If desired, a resealable vent could be employed; and many such vent constructions are known. As best seen in FIG. 2, a cell element shown generally at 28 is contained in cell 14 in the form of a wound roll comprising a negative electrode layer 30, a positive electrode layer 32, and a separator shown generally at 34 intermediate the electrode layers. In addition, a wicking layer 36 for absorbing electrolyte is provided on the side of the separator 34 adjacent the positive electrode layer 32. While the use of such a wicking layer is optional, this has been found to be advantageous. A wicking layer could be likewise pro¬ vided adjacent the negative electrode layer 30, if desired; but this has been found not to offer any advan¬ tages.

As best seen in FIGURE 1, perforator disc 22 cooperates with closure 18 in defining the negative terminal of the housing. More specifically, a first connecting means tab 38 is electrically connected to the negative electrode layer 30, extending outwardly from the roll into electrically connected association with closure 18.

Outer housing 12 suitably comprises a metal can which defines the positive terminal of the battery.

Thus, as is illustrated in FIGURE 1, a second connecting tab means 40 is electrically connected with positive electrode layer 32 and housing 12.

When utilized in a cylindrical cell, as is shown in the illustrative embodiment, the positive and nega¬ tive electrode layers and the separator should be sufficiently flexible so that a wound element can be provided. The manufacturing techniques to provide suitable positive and electrode layers of adequate flexibility are well known.

The negative zinc electrodes may thus be made by conventional techniques. As one example, a powdered mixture of the desired materials and a binder can be rolled onto a suitable current collector, such as, for example, a copper screen. While the mixture can com¬ prise both zinc oxide and zinc, it is preferred to utilize a mixture incorporating little or no zinc metal. It has thus been found that, if the cell is allowed to stand in a completely discharged condition for an extended period, the presence of zinc metal would tend to result in undesired pressure buildup, perhaps causing the cell to vent.

A variety of binder materials for fabricating zinc electrodes is known and may be employed. The binder material used should desirably be inert in the cell environment and is preferably utilized in an amount just sufficient to hold the mixture together, providing a positive bond as well to the current collector. An elastomeric, self-cured carboxylated styrene-butadiene latex is an example of a suitable binder material.

Specific illustrative examples of this type are AMSCO RES 4150 and 4816, manufactured by the AMSCO Division of Union Oil Company. It has been found satisfactory to utilize this binder in an amount, for example, of about 4%, based upon the total weight of the negative electrode mixture.

If desired, the negative electrode may contain other ingredients, as is known. For example, it has been found useful to include a minor amount of cadmium metal which increases the electronic conductivity of the negative electrode. This apparently acts to stabil¬ ize the negative electrode against shape change as well as reducing the rate of evolution of hydrogen. Cadmium oxide may likewise be employed to serve as the source of the cadmium metal; however, it has been found that

-li¬ the use of cadmium oxide may result in a small loss in capacity in comparison to what occurs whencadmium metal is employed. Similarly, bismuth oxide, Bi ₂ 0 ₃ , may be . included. This is believed to increase the hydrogen over-voltage of the negative electrode, resulting in the di unition of hydrogen evolution. As a specific example, it has been found useful to include, based upon the total weight of the negative electrode mixture, cadmium or cadmium oxide in an amount of about 5 to 6% by weight, and bismuth oxide in an amount of about 7 to 8% by weight.

Considering the positive electrode layer, this may be made by any of the conventionally known techniques. Thus, the use of both sintered and non-sintered elec- trodes is known and either may be utilized. Indeed, it is satisfactory, for example, to employ the techniques used in making nickel electrodes for nickel-cadmium cells.

In accordance with the present invention, the posi¬ tive electrode is modified to incorporate a catalyst for the oxidation of hydrogen in an amount sufficient to catalyze the oxidation of enough of the hydrogen gas evolved during use to prevent undue pressure buildup. It has thus been found that the hydrogen evolved in the cell is oxidized at the positive electrode to effectively provide an internal pressure in the cell that is well below the level at which venting would occur with con¬ ventional container materials and closure means. More¬ over, this oxidation reaction results in conversion of the NiOOH active material in the nickel-zinc system to the discharged species, Ni(0H) ₂ , so that the cell chem¬ istry remains at least essentially in balance. This allows the cell to be readily recharged without loss in capacity.

Any catalyst capable of catalyzing the oxidation of hydrogen gas which results in the concomitant reduction

SUBSTITUTE SHEET

of the nickel, oxygen-containing electrode may be utilized. It is preferred to utilize silver. More particularly, it is believed that elemental silver will be at least principally converted in the cell environ- ment to g ₂ 0, which serves as the active catalytic spe¬ cies for the oxidation of hydrogen in service. This conversion would likewise take place if Ag ₂ 0 or other silver-containing compounds, serving as percursors, were to be initially incorporated into the positive electrode. It may well be that, to some extent, ele¬ mental silver or other catalytically active silver specie are also present. Accordingly, as used herein, the term silver is intended to refer to elemental silver as well as to g ₂ 0 and any other silver-containing co - pound present in the cell environment which catalyzes the oxidation of hydrogen. It may perhaps be adequate to employ platinum or palladium. However, these mater¬ ials have relatively low hydrogen over-voltages; and, if used, steps should be taken to insure that migration to the negative electrode and depositing thereon, which could well cause massive hydrogen evolution and venting of the cell, is avoided.

The catalyst may be incorporated into (i.e. - physically associated with) the nickel electrode by any means desired which results in the catalyst being located in position to oxidize adequately the evolved hydrogen. As a specific example, when silver is the catalyst, this may be satisfactorily incorporated into the nickel electrode by the following procedure. The nickel electrodes are soaked in an aqueous solution of silver nitrate at ambient temperatures for a time suf¬ ficient to impregnate the electrode (e.g. - 20 minutes has been found satisf ctory), removed and then blotted dry. The electrodes are then placed in a KOH solution (a 10% by weight aqueous solution being suitable) to

precipitate silver as Ag ₂ 0, then removed, drained, and repeatedly washed with water (preferably deionized) until free of KOH (as determined by a negative response in a phenolphthalein test). Following vacuum drying at an elevated temperature for several hours (16 hours has been satisfactory), the electodes are ready for use.

The amount of catalyst present should, in a func¬ tional sense, be sufficient to maintain the internal pressure at the level desired. The amount necessary to effect this result may vary somewhat, depending upon the size of the cell. For sub-C size cells, as an exam¬ ple, it has been found satisfactory to utilize silver in a range of from about 0.2% to about 0.9%, based upon the total weight of the nickel electrode. As is well known, in nickel-zinc systems using con¬ ventional aqueous solutions, such as potassium hydrox¬ ide, as an electrolyte, the zinc specie(s) formed during discharge is soluble in the electrolyte to a significant extent. Some of the active zinc material thus tends to enter the electrolyte while the system is being dis¬ charged, as well as while the system stands in a dis¬ charged condition. Upon recharging of the battery system, the zinc specie(s) in the electrolyte returns to the zinc electrode but can alter the electrode struc- ture. The active zinc material can thus migrate from the edges or periphery of the electrode structure and collect in the central regions of electrode, resulting in an irreversible loss of capacity. This phenomenon has been often termed "shape change". Because of this phenomenon, the cell element util¬ ized in the present invention should be positioned in the cell in a fashion which will at least minimize, and hopefully eliminate, shape change. It has been found satisfactory, when a cylindrical cell is involved, sim- ply to wind the element such that the element is under

compression while in position within the cell. This assists in minimizing shape change as a problem.

As is likewise well known, the replating or redep- osition of zinc often occurs in the form of treed or branched crystals having sharp points (dendrites) which can readily bridge the gap between the plates or elec¬ trodes of opposite polarity, thereby causing short circuits and the destruction of the cell. Accordingly, the material used for the separator should be a membrane having a relatively fine, uniformly sized pore structure which allows electrolyte permeation therethrough while preventing dendrite penetration. Still further, the material employed should possess chemical stability in the cell environment. Additionally, suitable materials should possess sufficient flexibility and strength characteristics to adequately endure any shape change and/or electrode expansion that might take place during service. A large number of materials have been proposed for use and are well known, as are their methods of manufacture.

As one illustrative example, the separator may comprise a commercially available "Celgard" polypropyl¬ ene film (Celanese Fiber Company). It has been found particularly desirable to utilize two layers of such material (each layer about one mil thick being adequate) to form the separator layer 34, the individual layers being shown generally at 42 and 44 (FIG. 2). The use of two layers allows the large pores or holes, due to imperfections produced during manufacture or subse- quently, in each layer to be non-aligned with respect to each other to minimize problems with dendrites. Of course, a single layer or more than two layers may like¬ wise be employed if desired.

Any conventional alkaline electrolyte used with a nickel-zinc system may be employed. As one example, it

is satisfactory to utilize an aqueous potassium hydrox¬ ide solution containing about 25% by weight potassium hydroxide. It is desirable to utilize initially an electrolyte saturated with Zn(OH) ₂ so as to prevent initial dissolution of zinc oxide into the electrolyte. As is known in the sealed cell art, the amount of elec¬ trolyte used should be restricted sufficiently so that an effective oxygen recombination reaction will be pro¬ vided. In the illustrative embodiment, the necessary electrolyte can be added to the open space in the core of the wound cell element 28 prior to the sealing of the cell.

As has been previously referred to, a wicking layer 36 is provided on the side of the separator layer adjacent the positive electrode. Any alkali-resistant material capable of absorbing electrolyte can be util¬ ized. In general, a non-woven fabric of a synthetic resin, such as polypropylene, may be employed. One illustrative example of a suitable polypropylene wicking sheet is "Webril 1488" non-woven fabric (Kendall Com¬ pany) having a thickness of about 3 mils.

With respect to the first connecting tab 38, this should be made of a conductive material having a hydro¬ gen overvoltage characteristic at least approximately as high as that of zinc. An illustrative example is a nickel element, plated with copper and then overplated with silver. The closure 18 may suitably comprise a steel sheet plated with nickel which is, in turn, cov¬ ered with copper plating, and then covered with silver plating. The second connecting tab 40 may comprise, for example, a nickel element which is electrically con¬ nected to the nickel plating 46 of outer housing 12.

A wide variety of materials are known for the connecting tabs and the housing for nickel-zinc systems, and such materials may be utilized in the nickel-zinc

cell of the present invention. The particular materials of construction may accordingly vary rather widely.

Further, if desired, a water sealant coating, as is known, may be applied to the metal or other surfaces in the cell. A suitable sealant is the styrene-buta- diene material described herein as the binder for the negative electrode mixture. As shown in FIGURE 1, a coating 48 has been applied to the exposed surfaces of the closure 18 and the first connecting tab 38. This may be applied by brushing on to a thickness, for example, of about 1 mil.

In addition, to insure that adequate insulation is provided between the cell element 28 and the terminals, insulators 50, 50' may be included, if desired. While shown as spatially removed from the cell element 28 for simplicity of illustration, insulator 50 may suitably rest upon separator layers 42, 44, which desirably terminate somewhat above the upper end of the elec¬ trodes• The nickel-zinc cell of the present invention may be utilized in either a prismatic or cylindrical design, as is desired for the particular application. Likewise, the capacity of the cell may vary within wide limits, the size being dictated by the requirements of the par- ticular end use application. As one example, a cylin¬ drical sub-C size cell for use in cordless or portable power tools may suitably have a capacity of, for exam¬ ple, 1.2 Ampere-Hours.

In use, the sealed nickel-zinc cells of the present invention are characterized by relatively low internal pressures in use and on stand and are capable of being operated under relatively high rate charge and discharge without building up pressures that would cause venting. For example, in a cylindrical sub-C cell (a cell size often used for many portable equipment applications)

having a capacity of 1.2 Ampere-Hours, the cells of this invention should be capable of providing a life of . 100 to 200 cycles, even when discharged at rates up to 7C. More moderate discharge rates, viz. - C/2 or lower, should allow the cells to provide a cycle life of up to 500 or more. The cells of this invention may be charged at rates up to 1C or so at ambient temperatures (provided that the cell voltage does not exceed 2.05 volts) without undue pressure buildup. 0 Such cells are thus suitable for use in high rate, intermittent discharge applications (e.g. - discharge for 6 seconds at 8 amperes, followed by 12 seconds of rest — repeated until the cell is discharged) such as is involved in, for example, portable power tool appli- 5 cations. Indeed, applications of this sort are consid¬ ered to be relatively demanding; and the characteristics of the cells of this invention should accordingly amply satisfy the commercial requirements for a wide variety of portable equipment applications. _Q As a specific example, a sub-C size cylindrical cell described herein may utilize conventional nickel electrodes employed for nickel-cadmium cells (typical thickness about 22-25 mils) and negative electrodes having the following percentages by weight: 82.42 zinc 5 oxide; 7.88 Bi ₂ 0,, 5.36 cadmium and 4.34 binder (self- cured carboxylated styrene butadiene binder previously described). The negative electrode may have a thickness about half that of the positive electrode. The ratio of negative active material to positive active mater- 0 ial is suitably about 4:1 (this ratio may be varied as desired although it is preferred to utilize a substan¬ tial excess of negative active material). The elec¬ trolyte may be a 25% by weight aqueous KOH solution saturated with Zn(OH) ₂ and present in the cell in an 5 amount of about 3 cubic centimeters. The cells may be

desirably assembled with the electrodes in a discharged condition.

The following Example is illustrative of the present invention, and not in 1.Imitation thereof.

EXAMPLE

Cells according to the present invention were assembled and compared to control cells in a charge- discharge regime, as well as upon open circuit stand.

Two cylindrical sub-C size cells in accordance with the present invention were asembled utilizing the parameters for the specific example set forth above. The separator utilized was two layers of "Celgard" polypropylene film (each about 1 mil in thickness), and a "Webril" polypropylene wicking layer (about 3 mils in thickness) was employed. The cell configuration was as is shown in FIGS. 1 and 2.

The positive electrodes were impregnated with silver by the following procedure. The nickel elec¬ trodes were immersed in a 1 molar solution of silver nitrate in deionized water at ambient temperature under vacuum for 20 minutes. The electrodes were then removed, blotted dry and placed in a 12% by weight KOH aqueous solution for 20 minutes. The electrodes were then removed, drained and repeatedly washed in deionized water until free of KOH (as determined by a negative response in the phenolphthalain test). The electrodes were then vacuum dried at 50°C for 16 hours.

Two control cells, identical in construction to the cells of this invention described herein, were prepared, except that the positive electrodes were not impregnated with silver.

The assembled cells were then tested in a charge/ discharge regime as follows. In the first three cycles, the cells of this invention were subjected to a 4 hour charge at 300 milliamps, followed by a discharge at 600

milliamps until the cell voltage declined to 1.1 volts. The control cells were subjected in the first three cycles to what is considered to be a more moderate charge, 300 milliamps for three hours. The control cells were then discharged using the same regime as for the cells of the present invention. For the fourth cycle, the cells were given a 16 hour charge (1.2 Ampere-Hours received over that time period), and then were discharged using the same regime as for the first three cycles. Thereafter, the cells were charged for 4 hours at about 350 milliamps, discharged using the regime previously described, and this cycling continued until the total time elapsed in the cycling was about 140 hours. The cells were then discharged for about 30 min¬ utes, resulting in a removal of about 25% of the capac¬ ity, and were allowed to stand on open circuit for about 20 hours. The cells were then exposed to what was bas¬ ically an overcharge regime, involving charge at about a C/14 rate for about 50 hours and were then allowed to stand on open circuit.

The internal pressure of each cell as a function of time was measured, and the average of the two cells of this invention and that of the control cells is shown in FIG. 3. The average internal pressure for the control cells is represented by curve A, while the average internal pressure for the cells of this inven¬ tion is represented by curve B. The period designated by C is essentially the time at which the cells were on open circuit.

As can be seen, the internal pressure in the cells of this invention did not develop any general increasing tendency over the cycling regime, but, rather, developed a relatively level internal pressure of about 40 p.s.i.g. or so. In contrast, the internal pressure in

the control cells tended to rise during the cycling regime. It is believed that this tendency would even¬ tually result in the control cells venting. Moreover, while upon open circuit, the internal pressure in the cells of this invention tended to decline, the contrary was true in the control cells.

During the overcharge regime (not shown in FIG. 3), the internal pressure in all cells tended to increase, although the increase in the case of the cells of this invention was more moderate. Moreover, upon being allowed to again remain on open stand, the internal pressure in the cells of this invention declined to about 39 p.s.i.g.

It should be appreciated that FIG. 3 has been some- what simplified. More particularly, all of the cells exhibited transient pressure excursions (increases) near the end of charge in each cycle. This is believed due to oxygen evolution at the positive electrode. For simplicity, such excursions are not shown in FIG. 3, which graph is believed to reflect the pressure due to hydrogen evolution.

In the case of the control cells, the extent of the transient pressure excursions became more signif¬ icant as the cycling progressed. It is believed pos- sible that longer cycling could well have resulted in the venting of the cell, simply due to such excursions. This should not occur in the case of the cells of this invention, as the increase in pressure due to such excursions was quite moderate, without any tendency for the extent of the excursions to markedly increase as cycling progressed.

Thus, as has been seen, the present invention provides a sealed nickel-zinc cell which copes with the inherent hydrogen evolution occuring in the system in an effective and straightforward manner. More particularly,

the hydrogen evolved is oxidized in a fashion which maintains the balance of the cell chemistry. The cells of this invention may accordingly be effectively and rapidly fully recharged. The effectiveness of the hydrogen oxidation reaction allows the cells to be used under even relatively high rate charge/discharge condi¬ tions. Moreover, the cells may be allowed to stand on open circuit for extended periods of time with no tendency for internal pressure buildup whatever. The attributes of the cells of this invention make such cells highly desirable for use in a wide variety of applications.