CA2037744A1 - Rechargeable alkaline manganese cell having improved capacity and improved energy density - Google Patents

Abstract

RECHARGEABLE ALKALINE MANGANESE CELL HAVING
IMPROVED CAPACITY AND IMPROVED ENERGY DENSITY

ABSTRACT OF THE DISCLOSURE:

A rechargeable alkaline manganese cell is provided, having improved capacity and improved energy density.
The cell is anode limited, and each of the anode and cathode is physically dimensioned so that the anode has a capacity in the range of from about 45% to about 100%
of the capacity of the cathode. The gravimetric energy density of the cell exceeds 70 Wh/kg, and the volumetric energy density of the cell exceeds 200 Wh/litre. Each of the anode and cathode may comprise additional additives. For example, the cathode may include additional hydrophobic materials and a porous additive such as carbon black so as to improve gas transport of hydrogen gas into the cathode where it is oxidized; and the anode may comprise hydrogen gassing inhibitors as well as oxygen gas recombining agents.

Description

2~37744 RECHARGEABLE ALKALINE MANGANESE ~ELL ~IAYING
IMPROVED CAPACITY AN~ IMP~OVED ENE~GY DENS~TY

FIELD OF THE INVENTION:

This invention relates to rechargeable alkaline manganese dioxide cells, and especially provides such cells having improved capacity and improved energy densities. The usual embodiment of such cells is contemplated as a "bobbin" type cylindrical cell, but spirally wound cells and flat plate cells may also be provided in keeping with the present invention. A typical cell, in any event, comprises a manganese dioxide cathode, a zinc anode, a separator between them, a container for the cell, a closure member to seal the cell, and an alkaline electrolyte --usually potassium hydroxide.

BAC~GROUN~ O~ TH~ INVENTION:

Manganese dioxide electrodes as used in rechargeable alkaline manganese dioxide cells are reversible only if the manganese dioxide cathodes are discharged to the point where the MnO2 is converted to Mn203. It has been well established that if the discharge continues beyond that level, an irreversible phase change occurs 90 that the manganese dioxide cathode is no longer rechargeable. Under certain conditions, it is now possible that MnO2 electrodes for rechargeable alkaline cells can be rendered reversible within the two electron range.
It has always been desirable to produce rechargeable manganese dioxide cells with zinc anodes -- especially cylindrical cells having conventional cylindrical configurations -- to have high energy density. This has not been particularly 2~377~

successful, and numerous difficulties have been encountered.
Several approaches have been provided to ensure reversibility and rechargeability of MnO2 cells, including the provision of electronic means so as to prevent overdischarge of the MnO2;
designing the cell so as to be anode limited; and modifying the MnO2 particularly by the addition of heavy metals thereto.
Indeed, rechargeable alkaline MnO2/Zn cells have been available, at least in the North American market, since the late 1960's. However, those cells were not generally successful, and by the mid 1970's they were removed from the market. At least in part, the lack of success of those cells was due to the fact that they were generally assembled in batteries and not available in single cell configurations, and that they were required to be monitored very carefully to determine the end of the useful discharge capacity. Such monitoring was by timing the operation of the cells, or by the e~pensive incorporation of electronic control means to determine the point of discharge beyond which further discharge could not be tolerated. Moreover, the cells were quite modest in terms of their density capabilities, and "D"
cells having nominal 2 Ah capacities had energy densities of, at best, about 52 Wh/litre or 18 Wh/Kg.
To overcome those difficulties, cells were then developed by which anode limitation of the cells was imposed; meaning that the capacity of the anode was severely limited so that it became impossible to discharge the manganese dioxide cathodes to more than about 40% of their theoretical capacity. By these means, the rechargeability of the MnO2 was assured. By providing cells ~1~377~

having severe anode limitation characteristics, however, the cells were thereby prejudiced by having quite low energy densities, and therefore the cells were not widely accepted in the market.
Several specific patents which address some of the issues above, and the approaches to preserve the rechargeability of cells, are discussed below:
AMANO et al, U.S. Patent 3,530,496, issued September 22, 1970, provide alkaline MnO2/Zn cells which are rechargeable, but wher0 the depth of discharge of the manganese dioxide cathode is severely regulated by limiting the available capacity of the zinc anode to less than 40% of the available capacity of the MnO2 cathode. Indeed, Amano et al suggest that anode limitation i5 preferably iIl the range of 20% to 30% of the MnO2 cathode capacity, to achieve optimum performance of the cells. In practise, the theoretical capacity of the cell is not realized, except at very low drain rates.
The Amano et al cells achieve their zinc anode limitations by providing cathodes which are essentially equal in their dimensions to those of primary alkaline cells, and then reducing the zinc capacity by placing a cylindrical gelled zinc anode adjacent to the MnO2 cathode and separated from it by suitable two component separators; and then by filling the center of the cell with gelled electrolyte that does not have any active anode material added to it. It should be noted that Amano et al prefer amalgamated copper particles to be included in the anode so as to 2~377~

enhance its conductivity. They also provide a zinc oxide reserve mass, and they must use a perforated coated screen current collector rather than a single nail as might otherwise be used in primary alkaline cells -- and as used in the present invention.
~ ORDESCH, in U.S. Patent 4,091,178, issued May 23, 1978, provides a rechargeable Mn02/Zn in which the anode capacity is specifically limited to about 33% of the capacity of the cathode.
Kordesch also provides a charge reserve mass in which a quantity of zinc oxide is placed equal to at least 50% of the anode discharge capacity. Once again, because there is an excessive capacity of Mn02, as well as additional ZnO, the energy density of the Kordesch cell is quite low.
DZIECIUCH et al, were granted U.S. Patent 4,451,543 on May 29, 1984. That patent teaches a rechargeable Mn02/Zn cell where the Mn02 is doped with heavy metals such as bismuth or lead. The intention is that up to 50% of the theoretical two electron capacity of the MnO~ can be reached. However, the Mn02 cathode comprises relatively high quantities of carbon, which results in the cathodes having a low specific density and a low cell energy density. Still further, it must be noted that in practical cells the second electron reduction step of the Mn02 occurs at a too low voltage, namely below 0.9 volts. It is questioned, therefore, whether such cells as are provided by Dzieciuch et al are capable of delivering even a relatively substantial portion of their theoretical capacity above the 0.9 volt cutoff voltage that is generally required in uses such as electronic toys, small battery driven appliances, and the like.

~3~74~ -Clearly, the intent of the present invention is to provide cells having a high initial capacity, high discharge ~oltage, high cumulative capacity, an extended cycle life, and cells that are capable of maintaining high drain rates over most of their lifetime. Moreover, cells according to the present invention must be capable of being easily and economically manufactured, with production costs substantially in the order of the production costs of high quality primary al~aline cells.

BRX~F DRSCRIPTION OF THE DRAWINGS:
The present invention is described hereafter in greater detail, in association with the accompanying drawings, in which:
Figure 1 is a cross-section of a typical cylindrical rechargeable alkaline manganese dioxide cell with a zinc anode, according to the present invention;
Figure 2 shows curves comparing the capacity of active material of manganese dioxide cathodes and zinc electrodes intended for use in cells according to the present invention, where the data were determined in half cell tests; and Figure 3 shows the results of cycling tests using deep discharge cycles, for two different cell configurations having differing ratios of anode capacity to cathode capacity.

DESCRIPTION OP THE PREFERRED EMBODIMENTS:
In general, the cell design of the present invention embodies a cathode whose dimenc;ions and volume are enlarged when compared to those of a primary cell. Thus, the diameter of the anode in a cylindrical cell -- or the volume of an anode in flat ~37744 plate cells -- is reduced. Maximum space utilization is achieved, and rechargeable cells having increased energy capacities when compared to prior art cells are provided.
A typical cylindrical cell is shown in Figure 1, at 10. The cell comprises a container 12, within which is a cathode 14 and an anode 16. Between the cathode and the anode there is located a separator 18. The cell is sealed by a closure member 20; and a current collector 22 in the form of a nail extends through the closure member 20 into the anode 16. The nail 22 contacts a metal cap 24 placed (or welded) across its head and across the closure member 20, to provide a negative contact for the cell 10.
At the other end of the cell, a pip 26 is formed to provide a positive contact for the cell; and it is in.sulated from the anode 16 by an insulating washer or cup 28.
Certain specific options and alternative compositions and embodiments of cells according to the present invention are now discussed:
In general, the container or can 12 is a nickel plated deep drawn steel can, although other suitable metal cans may be used.
So as to improve the contact and conductivity between the cathode 14 and the can 12, and thereby so as to reduce the internal resistance of the cell, the internal surface of the container 12 may be coated with a conductive coating such as LONZA (TM).
Moreover, by using the conductive coating on the interior surface of the container 12, the risk of iron leaching from the can into the cell, which could result in increased hydrogen gassing of the anode, is reduced.

2 0 3 ~

Referring to the cathode, which is manganese dioxide, it is noted that alkaline battery grades of commercially available electrolytic manganese dioxide are utilized. The usual cathode composition provides for about 5% to about 15% by weight of carbon, which may be graphite or carbon black, so as to enhance the conductivity of the cathode, together with a binder. Typical binder and cathode compositions are taught in United States Patent 4,957,827 to RORDESCH et al, and assigned to a common assignee with the present invention. The cathode may also contain reinforcing agents such as graphite fibres, as taught in co-pending U.S. application Serial No. 07/400,712, filed August 30, 1989, in the name of Tomantschger and Michalowski, and assigned commonly with the present invention. Otherwise, an organic binder such as PTFE or polyethylene, or the like, may be used.
Hydrogen gas recombination within the cathode is, of course, to be accomplished in a rechargeable alkaline cell, and to promote such hydrogen gas recombination the cathode composition may include hydrogen recombination catalysts such as those taught in commonly owned U.S. Patent application Serial No. 07/520,820, filed July 9, 1990. Still further, so as to provide for overcharge capability, an oxygen evolution catalyst as taught in U.S. Patent 4,957,827, referred to above, may be utilized.
Whatever catalyst is selected is chosen so as to be stable over a wide voltage range -- typically from 0.9 volts versus Zn to 2.0 volts versus Zn -- and also over a wide temperature range --typically from -40'C to +70 C -- without any significant deterioration in performance of the cell. Such catalysts may be ~377~

oxides, spinels, or perovskites of nickel, cobalt, iron, manganese, chromium, vanadium, titanium, and silver. Also, as taught in U.S. Patent 4,957,827, the oxygen evolution catalyst may be placed on the outer surface of the cathode.
So as to ensure hydrogen gas porosity of the cathode, and thereby access of hydrogen gas into the cathode where the hydrogen will be oxidized, the cathode composition preferably contains both carbon black as well as the hydrophobic binder.
Still further, for purposes of hydrogen gas porosity and accessibility, the cathode composition may further comprise from about 0.1% to 5.0% of a hydrophobic material such as PTFE, polyethylene, or polypropylene, together with an additional porous additive such as from about 0.1% to 5.0% of carbon black.
Such additives improve the gas transport characteristics of the cathode, and thereby enhance the hydrogen recombination rate of the cathode.
Placement of the cathode 14 into the container 12 may be accomplished by molding the cathode into discrete pellets, and inserting them into the can. This may be followed by the additional step of recompacting the cathode once it is placed in the can. Alternatively, the cathode may also be extruded directly into the can 12.
The anode 16 comprises powdered zinc together with the suitable gelling agent such as carboxy methyl cellulose, starches, and their derivatives. The anode 16 may include additives to reduce or inhibit hydrogen gassing, such as small : ~0377~

amounts of mercury, gallium, indium, or cadmium. Certain commercially available organic hydrogen gassinq inhibitors such as GRILLIN (TM) may also be used.
The choice of gelling agents will depend on its gelling characteristics, the temperature and chemical stability of the gelling agents -- and, indeed, of the production circumstances --as well as the capability of the gelling agent to release hydrogen gas that may be produced in the cell. Zincate mobility within the cell may be reduced by the use of additives such as compounds of magnesium, barium, and calcium, typically their oxides, or their hydroxides. Oxygen gas recombining agents may also be added to the zinc anode as are taught in commonly owned United States patent application Serial No. 07/478,638, filed February 12, 1990.
The electrolyte is an aqueous alkali metal hydroxide solution, usually 4N to 12N potassium hydroxide solution. The electrolyte may cont:ain additives such as dissolved zinc oxide so as to reduce the gassing of the active zinc within the anode, and so as to permit overcharge of the cell without damage to it.
The plastic closure member 20 normally contains a safety vent (not shown) which may be simply a rupturable membrane, or a resealable vent. The plastic closure member is molded from a thermoplastic material having enhanced hydrogen permeation rates, such as polypropylene, talc filled polypropylene, and nylon.
The design of the separator 18 is such that it is a complex structure that exhibits the characteristic of being permeable to the passage of gasses such as hydrogen and oxygen that are produced in the cell on overcharge, standby, or discharge; and 2~377~

that it is impermeable to zinc dendrites so as to preclude the possibility of zinc dendrites causing a short circuit within the cell. Thus, the separator tube comprises an absorber layer and a barrier layer. The absorber layer may be made of rayon or polyvinyl acetate fibre; and the barrier layer may consist of cellulose, CELLOPHANE (TM), polyamide, polyethylene, and the like. Certain commercially available separator materials such as CELGARD (TM) and PERMION (TM), maybe used.
Certain experimental data which establish the criteria and rating limitations of the present invention are now discussed:
First, means must be determined so that different designs of cells, and their practical and theoretical energy capacities, may be compared. Of course, it is desirable that energy densities of different cells might be compared in actual discharge experiments; however, in practice, great difficulties in making such comparisons are! encountered. For example, it is well known that the drain rate of a cell, expressed for example as mA/g of active material, may affect the utilization of the theoretical energy capacity for a given electrode composition. In essence, the practical capacity of an electrode approaches the theoretical capacity of the electrode only in circumstances where a very low drain rate can be maintained. Such circumstances may be, for example, electrical clocks where the clock runs continuously for a year or more on a single AA cell; but most battery powered devices such as radios, tape players, electric toys, and the like, require considerably higher drain rates.

~0377~'1 In any event, the degree of utilization of the electrode also depends on the composition of the electrvde; for example, whether the electrode includes conductive additives, as well as the particle size of the active material of the electrode, the electrolyte concentration, and so on. Thus, specific cells can be designed and optimized for high drain rate circumstances, or for low drain rate circumstances.
That fact of different cell designs to accommodate different drain rates thereby makes it necessary to perform tests on commercially available cells over all drain rates that may be encountered in practical applications; and then to apply wei~hting factors for each drain rate so as to attempt to determine which cell has demonstrated the best overall performance. However, depending on the cell balance, the drain rate that is based on the active material content of the electrodes of the cell, will be different for different designs.
A specific example follows:

Ex amp l e I:
The following compositions were used to construct half-cells, both as to the cathode composition and the anode composition. Each cell is expressed in parts by weight:

Table 1: Composition of Mn02 and Zn Electrodes Used In the Half Cell Tests Cathode ComPosition Anode ComPoSition Mn02 84.5 Zn(3%Hg) * 65.50 Graphite 9.00 CMC/CARBOPOL (1/1) 1.00 9N XOH 6.50 9N KOH, 5M ZnO33.50 (TM) 20377~

Half cell experiments were then carried out, at various drain rates expressed in terms of mA/g. The capacity of the active material for the zinc and the manganese dioxide was determined in mAh/g, at various drain rates. The results are shown in Figure 2, where curve 40 shows the theoretical zinc capacity and curve 42 shows the actual determined capacity of zinc to a cutoff voltage of -435 mV versus a Hg/HgO reference electrode voltage. Likewise, curve 44 shows the theoretical capacity of manganese dioxide for the one electron (le~) discharge, and curve 46 shows the measured capacity at various drain rates to a cutoff voltage of -435 mV versus a Hg/HgO
reference electrode voltage. It is clear from Figure 2 that only the theoretical energy content or capacity of respective cells provides a reliable means of comparing cell designs.

ExamDle II:
Having determined that the best comparison between cell designs is only in respect of their theoretical energy capacity, various cell designs of AA test cells were fabricated in keeping with the teachings of the three prior art patents noted above.
Then, their theoretical energy capacities were analyzed, with the result being shown in Table 2, below. It should be noted that the comparisons are made on the basis of each of the cells having a volume of 7.5 ml, a weight of 22.5 g, an operating voltage of 1.25 volts, and with 3.6 ml of active material. The theoretical capacities are in practise, and as discussed above, achievable only at low discharge rates. Table 2 also provides the ;:
' theoretical gravimetric and volumetric energy densities of the respective prior art cells; and included in Table 2 in all categories shown in that Table is a cell in keeping with the present invention.

Table 2: AA Cell Design Comparisons KordeschAmanoDzieciuch Present 4,091,178 3,530,496 4,451,543 Desiqn Mn02[Ah] 2.89 2.97 1,32 2.76 Zn[Ah] 0.85 1.19 1.62 1.93 Cell[Ah] 0.85 1.19 1.32 1.93 Zn:Mn02 0.30 0.40 1.20 0.70 Ratio theor.
Energy Density ~Sh/kg] 47 <66 69 107 [Wh/l] 142 <198 176 322 Table 3, below, is the composition of the cathode and anode used in the cell of the present invention as specified in Table 2 above.

Table 3: Composition of Present Design Test Cells Used in Example II
Cathode_Composition Anode ComPosition Mn02 83.03 Zn(1.0%Hg) * 65.50 Graphite 9.00 CMC/CARBOPOL (1/1) 1.00 Carbon 0.37 9NKOH, 5MZnO 33.50 Carboflex (TM) 1.00 *
9 N KOH 6.50 (TM) ~0377~4 Exam~le III:
Using the cathode and anode compositions as described above, the present invention was applied to AAA, AA, C, and D cel 15 having conventional cylindrical cell configurations. The capacity in ampere-hours of cells in each size was determined, as noted below in Table 4, and the cells were constructed having the respective ratios of the zinc anode to the MnO2 cathode as noted in Table 4.

Table 4: Energy Densities of Cylindrical ~AM Cells According to the Present Design AAA AA C D
Capacity ~Ah] 1.02 1.93 5.73 11.46 Zn:MnO2 Ratio 0.95 0.70 0.64 0.55 theor.
Energy Density [Wh/kg~ 116 107 116 104 [Wh/l] 364 322 311 298 The theoretical energy densities, both gravimetric and volumetric were then determined, as also noted in Table 4. It will be seen that the gravimetric energy densities generally range from about 100 Wh/kg to about 120 Wh/kg; and that the volumetric energy densities generally ranged from about 275 Wh/litre to about 375 Wh/litre.

Exam~le IV.
Finally, cells in keeping with the present invention and having cathode and anode compositions as noted below in Table 5 were constructed. However, one set of AA cells was constructed having an anode:cathode ratio of about 37%; and the other set of ~03774~

AA cells was constructed having an anode:cathode ratio of about 70%. Those cells were then subjected to deep discharge tests at 3.9 Ohms, to a 0.9 volt cutoff. The results of those tests are shown in Figure 3.

Table 5: Composition of Present Design Test Cells used in Example IV
Cathode ComPosition Anode ComPosition Mn02 83.03 Zn(0.15%Hg) * 65.50 Graphite 9.00 CMC/CARBOPOL (1/1) 1.00 C-Fibre 1.00 9NKOH, 5MZnO 33.50 Carbon Black0.37 Ag20 0.10 (TM) 9N XOH 6.50 It will be seen from Figure 3 that the cells having an anode:cathode ratio of 37% are shown in curve 50; and cells having an anode:cathode ration of 70% are shown in curve 52. The average cell life of the cells shown in curve 50 was 8 cycles above the 300 mAh capacity cutoff. The average cell life of the cells shown in curve 52 was more than 40 cycles before the cutoff of 300 mAh capacity was reached. It will also be noted that the cumulative capacity of the cells in curve 52 for the first 25 cycles was about 19.4 Ah; whereas the cumulative capacity for the first 25 cycles of the cells in curve 50 was only 8.3 Ah. Thus, over the first 25 cycles, the cumulative capacity of cells in keeping with the present invention and having an anode:cathode ratio of 70~ as compared with cells having an anode:cathode ratio of 37%, was exceeded by more than 100%. Clearly, the cells with , , 20377~4 an anode:cathode ratio of 37% are emulative of prior art cells, particularly such as those taught by Amano et al and Kordesch, as discussed above.
There has been described improved rechargeable alkaline manganese cells having significantly better gravimetric and volumetric energy densities than prior art cells, and having higher capacity than prior art cells. Various examples have been shown, with discussion of a number of different specific compositions. Other compositions and assemblies of cells in keeping with the present invention can be determined and effected without, however, departing from the spirit and scope of the present invention as defined by the appended claims.

b:2350-020111speci~iclPeb.26.911sll

Claims (14)

1. A rechargeable alkaline manganese cell comprising a metal container, a zinc anode, a manganese dioxide cathode, a separator between said anode and said cathode, a closure element to seal said cell, a current collector passed through said closure element and into said anode, and an alkaline electrolyte;
wherein said zinc anode consists mainly of gelled powdered zinc, said manganese dioxide cathode consists mainly of manganese dioxide together with from about 5% to 15% by weight of conductive carbon additive and a binding agent, and said electrolyte consists mainly of an aqueous alkaline metal hydroxide;
wherein said alkaline manganese dioxide cell is anode limited in that the anode has a capacity which is in the range of from about 45% to about 100% of the capacity of said cathode;
and wherein the gravimetric energy density of said cell exceeds 70 Wh/Kg and the volumetric energy density of the cell exceeds 200 Wh/litre.

2. The rechargeable alkaline manganese cell of claim 1, wherein said electrolyte is 4N to 12N potassium hydroxide.

3. The rechargeable alkaline manganese cell of claim 2, wherein said electrolyte has a small quantity of zinc oxide dissolved therein.

4. The rechargeable alkaline manganese cell of claim 2, wherein said current collector is a single nail extending into said anode.

5. The rechargeable alkaline manganese cell of claim 4, wherein the manganese dioxide of said cathode is made from electrolytic manganese dioxide, together with a hydrogen recombination catalyst.

6. The rechargeable alkaline manganese cell of claim 5, wherein said manganese dioxide cathode further contains carbon fibre, a hydrophobic binder chosen from the group consisting of PTFE and polyethylene, and a further additive chosen from the group consisting of a hydrophobic additive in the amount of 0.1%
to 5.0% by weight of one of the group consisting of PTFE, polyethylene, and polypropylene together with a porous additive in the amount of 0.1% to 5.0% by weight of carbon black.

7. The rechargeable alkaline manganese cell of claim 6, wherein said cathode is placed in said container by being molded into discrete pellets, by being molded into discrete pellets and then recompacted after placement in said container, or by being extruded into said container.

8. The rechargeable alkaline manganese cell of claim 5, wherein said gelled powdered zinc anode contains a gelling agent chosen from the group consisting of carboxy methyl cellulose, starches, and derivatives thereof.

9. The rechargeable alkaline manganese cell of claim 8, wherein said anode further comprises a small quantity of a hydrogen gassing inhibitor additive chosen from the group consisting of mercury, gallium, indium, cadmium, and commercially available organic hydrogen gassing inhibitors.

10. The rechargeable alkaline manganese cell of claim 8, wherein said anode contains a small quantity of an additive to reduce zincate mobility, wherein said additive is chosen from the group consisting of magnesium, barium, and calcium, their oxides, and their hydroxides.

11. The rechargeable alkaline manganese cell of claim 5, wherein said metal container is a nickel plated steel can.

12. The rechargeable alkaline manganese cell of claim 11, wherein said container is coated on its interior surface with a conductive coating.

13. The rechargeable alkaline manganese cell of claim 5, wherein said closure element has a safety vent formed therein, and is chosen from the group consisting of thermoplastic materials having enhanced hydrogen permeation characteristics consisting of polypropylene, talc filled polypropylene, and nylon.

14. The rechargeable alkaline manganese cell of claim 5, wherein said separator is a complex flexible structure which is gas permeable at least to gaseous hydrogen and oxygen, but impermeable to zinc dendrites.