HK1132585B - Novel silver positive electrode for alkaline storage batteries - Google Patents

Description

The present invention relates to the field of silver positive electrode alkaline batteries.

It relates in particular to a new silver positive electrode technology, its application to alkaline accumulators, in particular in combination with a zinc negative electrode, to the separators and the electrolyte used, and to the operation of the silver-zinc accumulator (AgZn) thus constituted, both in open and watertight mode.

The electrochemical couples using a silver electrode (silver-zinc, silver-cadmium, ...) have been known since the 19th century.

The effective use of alkaline silver positive electrode secondary systems did not really develop until after 1940, after Henri Georges ANDRÉ developed a silver-zinc accumulator using cellophane separators functioning as semi-permeable membranes, and zinc electrodes whose porosity was sought to be increased.

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The low cycling ability of the AgZn accumulator is mainly attributed to the behavior of the zinc electrode in an alkaline medium.

The reactions at the anode level are as follows in an alkaline accumulator: - What?

In fact, it is generally the recharging of the zinc electrode from its oxides and hydroxides and zincates that leads to the formation of deposits of a structure modified from their original form, often described as dendritic, spongy or powdery.

Successive recharging thus leads to rapidly anarchic zinc growths or spikes through the separators and to short-circuiting with the opposite polarity electrodes.

As for the powder or sponge deposits, they do not allow the reconstitution of electrodes suitable for satisfactory and durable operation, since the adhesion of the active substance is insufficient.

Furthermore, the reduction of metallic zinc oxides, hydroxides and zincates at the anode level during the charging phases is also characterized by changes in the morphology of the electrode itself. Depending on the mode of operation of the accumulators, different types of changes in the shape of the anode are observed, due to an uneven redistribution of zinc during its formation. This may be translated locally into a harmful densification of the anodic active mass at the electrode surface, most often at its central zone.

These major handicaps, reducing the number of cycles to only a few dozen - insufficient to give a secondary system any real economic value - have led to a great deal of work being done to improve the deposition characteristics of zinc in the recharge, with a view to increasing the number of charge cycles - discharges - that the generator could accept.

A key innovation was described in the patent application for French patent No 99 00859 (publication No 2.788.887), supplemented by that for French patent application No FR 2 828 335, the developed zinc anode technology which can allow several hundred cycles to be performed in a wide range of operating modes and up to very high discharge depths, by means of means designed to increase the efficiency of use of the active material by improving the percolation of loads within it.

The origin of this invention is based on the observation that insufficient drainage of charges within the active material leads to the formation of zinc deposits during refills at sites representing only a limited percentage of the total active mass.

The technology described in the above document shows that this mechanism can be greatly reduced when the same total quantity of zinc can be deposited on a much larger surface by multiplying the deposition sites in the entire electrode volume by a large number.

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Unlike previous technology in this field, this new electrode can operate smoothly in a concentrated alkaline medium, without the use of multiple separator layers designed to delay zincate diffusion and dendritic growth.

Under these conditions, nickel-zinc accumulators incorporating this new anode technology have a reduced internal resistance and can respond to high power demands without the appearance of passivation of the zinc electrode.

In the case of silver-zinc accumulators corresponding to the traditional state of the art, the separators play a dual role: prevent the migration of zincate ions and delay the dendritic growth of zinc during loading,stop the migration of soluble silver ions (Ag+ and Ag2+) and metallic silver particles forming in successive cycles.

Cellophane is traditionally the preferred material for AgZn battery separators because it offers the best tradeoff between cost and performance for a system with low cycling capacity.

To achieve operating times of several dozen cycles, at least four layers of cellophane membrane are required, as well as layers of fibre separator, which is to promote the retention of electrolyte between the electrodes (electrolyte reservoir function).

Substitutes for cellophane have been proposed: silver-treated cellophane, microporous polypropylene separator incorporating cellulose acetate, polyvinyl alcohol, etc., without however reducing the number of separator layers required for acceptable operation of the AgZn accumulator.

Because of the short life of the AgZn system, which seemed impossible to escape due to the well-known rapid degradation of the zinc anode, relatively little work was devoted to the silver cathodes used in this accumulator to improve its operation.

These cathodes are usually made by sintering metallic silver powder or silver monoxide (Ag2O), the latter being reduced to metallic silver during the sintering phase.

Such electrodes have high specific capacities (up to 300 Ah/kg and 1500 Ah/dm3), but to obtain a few dozen cycles of operation of the AgZn accumulator, these positives must be combined with highly overcapacitive zinc anodes, in order to avoid imposing large discharge depths on them.

The French patent application No FR 2 537 784 concerns an electrode for an electrochemical generator containing an active substance based on silver oxide powder and an electronic conductor in the form of film of a chosen body of silver, graphite, nickel and cobalt, the latter being intimately mixed with the powder, the whole being plasticized by means of a binder.

US Patent No. 3 282 732 concerns a process for preparing a divalent silver oxide electrode for a battery, involving the treatment of a silver grid with an aqueous solution of carboxy-methyl-cellulose, then the immediate application of divalent silver oxide to the grid, the mechanical upgrading of the treated grid and the application of a pressure of at least 1000 pounds per square inch (psi, or 69 bars) to the treated grid.

US patent No. 3 223 555 relates to a watertight battery with electrode stacking assembly.

US Patent No 4 835 077 relates to a cathode material containing silver oxide comprising AgO, Ag5Pb2O6, Ag2PbO2 and at least 5% by weight of the cathode material, Ag2O.

US Patent No 5 981 105 covers an electrode for use in an alkaline mineral electrolyte electrochemical cell, consisting essentially of an active material, a depolarizer made of a monovalent metal oxide, a current collector and a binder, and this electrode is free of electronic conductive additives or semiconductors.

Japanese patent application No JP 55 133765 proposes a positive electrode cast by pressurized casting of a positive mixture and addition of zinc oxide powder in an amount of 0.49 to 9.1% by weight.

The authors of this patent application have logically sought to make the AgZn system benefit from the considerable progress made in zinc anode technology, with the technology described in the above-mentioned patent 99 00859.

Such a zinc negative electrode provides essential answers to the problems and constraints of the AgZn battery, due to its long cycling life, without morphological change, and the high cost of the battery. The use of high-speed electrical power supplies, including high load and discharge rates, up to very high discharge depths (avoiding high anodic overcapacities),concentrated electrolytes (essential for cellophane use),and without the need for multiple separator layers.

The authors of the present invention have, however, shown that the mere combination of a conventional silver electrode, as described above, and a zinc electrode made according to the French patents FR 2788887 and FR 2828335, did not allow a satisfactory number of cycles to be obtained, owing to the short life of such a silver cathode: rapid gradual degradation in cycling, with significant migration of soluble species permeating the opposite electrode and the separators, until short-circuiting.

The authors thus made accumulators by placing a silver electrode of conventional design with a collector consisting of a perforated sheet of silver, with a nominal capacity of 0.75 Ah, between two zinc electrodes of the technology described in FR 2788887 and FR 2828 335 above.

The electrolyte was 10 N potash, saturated with zinc oxide, and containing 10 g/litre of lithium (LIOH).

The accumulators were cycled at 0.2 C5 A, with a discharge depth of 70% calculated on the nominal capacity of the silver cathode.

The object of the present invention is to define a novel embodiment technology for the silver positive electrode, which for the first time allows the cycling ability of the silver positive electrode to be greatly increased and consequently the cycling life of the AgZn accumulator itself to be increased.

According to the present invention, it is proposed to make a silver electrode which is of a plasticized type, and using a three-dimensional collector, the active mass being advantageously able to induce a porous and wetting agent of the electrode.

The work on defining a new silver electrode technology aimed in particular at designing a cathode which could operate under good conditions of homogeneity of electric field and ion diffusion within it, in order to optimise the conditions for the electrochemical reduction of the oxidized silver species, when recharged, to lead to the most complete and homogeneous metallic silver deposit possible within the electrode.

It is for the purpose of achieving such operating conditions that a three-dimensional collector with high porosity and large developed surface area has been used, on the one hand, and cathodic additives consisting of metal oxides, acting as a porophore and wetting agent for the electrode, and also capable of fixing soluble ions Ag+ and Ag2+.

The authors found that certain metal oxides used led to a major change in the charging and discharging mechanism of the silver electrode, together with a significant increase in the cycling time of the battery.

The three-dimensional collector used is preferably of the type of cross-linked alveolar metal foam; the cathodic additive is preferably made of zinc oxide, calcium oxide and/or titanium dioxide.

The scope of the invention will be better understood by the following embodiments, which describe the ways of implementing the silver electrode according to the invention, as well as that of silver-zinc accumulators incorporating zinc anodes made according to the descriptions of applications FR 2788887 and FR 2828 335.

The following table shows the data for the calculation of the average annual average price of the products:

For the three-dimensional cathode-ray collector, nickel foam of grade 90 PPI, with a surface density of 500 g/m2 and a thickness reduced by compaction or rolling from 1.6 mm to 1.0 mm, is conveniently coated by depositing - especially electrolytically - a thin layer of silver, the thickness of which is at least on the order of the micron.

To test the exclusive contribution of such a collector to cathode behaviour, the traditional method of preparing silver electrodes is followed: a paste is formed by mixing silver monoxide (Ag2O) powder, the particles of which have a diameter of 40 microns or less, with water and carboxy-methylcellulose, and the porosity of the foam is filled.

After drying, the electrode is heat treated under a reducing atmosphere at about 700°C to reduce the oxide and fry the resulting silver powder and the collector.

The resulting electrode is placed between two zinc electrodes made according to the description of FR 2788 887 and FR 2828 335, the anodes being overcapacitive compared to the silver cathode.

The separators consist of a layer of polyethylene microporous membrane (Celgard 3401) placed on the zinc electrodes, a polyamide fibre separator (Viledon) serving as an electrolyte reservoir and a layer of grafted polyethylene separator (Shanghai Shilong Hi-Tech Co) placed in contact with a silver electrode.

The electrolyte is a 10 N potash saturated with zinc oxide and containing 10 g/litre of lithine and 0.5 g/litre of aluminium.

The silver electrode has a nominal capacity of 0,76 Ah, corresponding to an efficiency of not more than 50% of the theoretical capacity

It is cycled in an open accumulator at 0.2 C5 A, with a discharge depth of 70% calculated on the nominal capacity of the silver cathode.

The electrode sintered on foam metal is found to maintain a good stability of capacity for almost 100 cycles, at about 80% of its initial capacity, but the capacity then drops rapidly.

The usefulness of a three-dimensional collector with high porosity and large developed surface area, particularly in this example of a metallic cross-linked foam type, is clearly shown by a doubling of the measured life for a cycling stop at 50% of the initial rated capacity.

The grade of the foam, and therefore the size of its pores, can be chosen from a wide range, preferably from 45 to 100 degrees, depending on the thickness of the cathode and the power densities to be obtained from the system.

The deposit of silver on a nickel foam allows, in all the test applications, a marked improvement in the cathode yields, this silver coating increasing the surge of oxygen at the electrode.

Variations in the design have shown that other types of collectors with high porosity three-dimensional structures, such as three-dimensional metal weaves (especially of the type whose structure can be produced on Raschel loom) or metal felt, can also be used effectively as electrode supports.

It should also be noted that such a three-dimensional manifold can be made according to the invention from any metal compatible with the potential use of the cathode, possibly coated with a silver layer.

Example 2 (comparative)

A plated-type silver electrode is made by filling a 90 PPI nickel foam coated with silver by electrolytic deposition as described in Example 1 with a paste consisting of silver monoxide powder (Ag2O) with particles of less than 40 microns in diameter, a hydrophobic binder, e.g. PTFE added at 3% by weight to the active substance, and water as solvent.

After drying, the electrode is compressed to 2000 kPa. The resulting cathode is used as a positive electrode in an AgZn cell, identical to that described in Example 1.

The nominal capacity is 0.8 Ah, or 98% of the theoretical capacity.

Cycling shall be stopped after 120 cycles when the electrode reaches 50% of its initial rated capacity after being increased by 80% at 45 cycles and 62% at 100 cycles.

The organic binder used for the implementation of the cathode-ray tube can be PVDF, a styrene-butadiene copolymer (SBR), an acrylonitrile-butadiene copolymer (NBR) or a mixture thereof.

The binding agent can be added advantageously at a rate of approximately 1 to 10% by weight of the active substance, and preferably at a rate of 2 to 6%.

The following table shows the data for the calculation of the average annual average cost of production:

A silver electrode of the plasticized type is made, the active substance being introduced as silver monoxide (Ag2O), as in example 2, but with the addition of zinc oxide (ZnO) powder to the paste at a rate of 3% by weight of the active substance.

After drying, the electrode is compressed to 2000 kPa. The resulting cathode is used as a positive electrode in an AgZn cell identical to that described in examples 1 and 2.

The nominal capacity is 0.8 Ah, or 98% of the theoretical capacity.

Cycling is stopped after 140 cycles when the electrode reaches 50% of its initial rated capacity, after being increased by 80% at 48 cycles, and 73% at 100 cycles.

The following table shows the data for the calculation of the average annual average cost of production:

A plated silver electrode is made, the active substance being introduced as silver monoxide (Ag2O), as in example 3, but the amount of zinc oxide in powder rising from 3 to 30% by weight in relation to the active substance.

After drying, the electrode is compressed to 2000 kPa. The resulting cathode is used as a positive electrode in an AgZn cell identical to the one described in the previous examples.

The nominal capacity is 0.78 Ah, or 96% of the theoretical capacity.

Cycling is stopped after 135 cycles when the electrode reaches 50% of its initial rated capacity, after being increased by 80% at 95 cycles, and by 76% at 100 cycles.

The benefit of the presence of zinc oxide is measured in the light of the comparative results of examples 2 to 4 by increasing the stability of the capacity level (by 10 to 15% higher after 100 cycles), these improved capacities being translated into a gain in life for a cycling stop at 50% of the initial rated capacity.

Analysis of the results of variants of the work showed that it is advantageous to use zinc oxide in amounts between 1.5% and 50% by weight of the active substance, preferably between 5% and 35%, for a significant effect without excessive deterioration of the specific capacities.

The following table shows the data for the calculation of the average annual average cost of production:

A plated-type silver electrode is made by filling a 90 PPI nickel foam coated with silver by electrolytic deposition as described in Example 1 with a paste consisting, for the active material, of silver metal powder of an advantageous particle size of between 0.2 and 40 microns, the average particle diameter being preferably about 2 microns, a hydrophobic binder consisting of PTFE added at a ratio of 3% by weight to the silver mass, and water as solvent.

After drying, the electrode is compressed to 2000 kPa. The resulting cathode is used as a positive electrode in an AgZn cell identical to the one described in the previous examples.

The nominal capacity is 0.7 Ah, or 65% of the theoretical capacity. The electrode is cycled at 0.2 C5 A, with a depth of discharge of 70%. The cycling is stopped after 250 cycles, when the electrode reaches 50% of its initial nominal capacity, after being increased by 80% to 175 cycles.

Various applications have shown that finer silver powders yield the best yields, while larger powders favour the strongest power demands.

The following table shows the data for the calculation of the average annual average price of the products:

A plated silver electrode is made in the manner described in Example 5, but with the addition of zinc oxide to the active mass at a rate of 30% by weight of the silver mass.

After drying, the electrode is compressed to 2000 kPa. The resulting cathode is used as a positive electrode in an AgZn cell identical to the one described in the previous examples.

The nominal capacity is 0.6 Ah, or 72% of the theoretical capacity. The electrode is cycled at 0.2 C5 A with a depth of discharge of 70%.

More than 500 cycles are obtained, with the capacity returned after these 500 cycles still being 60% of the rated capacity, after being increased by 80% to 215 cycles.

Example 7 (invention)

A plated silver electrode is made in the manner described in Example 5, but with the addition of titanium dioxide to the active mass at a rate of 30% by weight of the silver mass.

After drying, the electrode is compressed to 2000 KPa. The cathode obtained as a positive electrode in an AgZn cell is identical to that described in the previous examples.

The nominal capacity is 0.8 Ah, or 85% of the theoretical capacity.

More than 350 cycles are obtained, with the capacity returned after these 350 cycles still being 80% of the nominal capacity.

Example 8 (invention)

A plated silver electrode is made as described in Example 5, but with the addition of 18% and 12% by weight of zinc oxide and titanium dioxide respectively to the active mass of the silver.

After drying, the electrode is compressed to 2000 KPa. The resulting cathode is used as a positive electrode in an AgZn cell identical to the one described in the previous examples.

The nominal capacity is 0.9 Ah, or 90% of the theoretical capacity.

More than 500 cycles are obtained, with the capacity returned after these 500 cycles still being 80% of the rated capacity.

Examples 5 to 8 show that the use of silver powder as active material brings about a significant gain in stability of cycling capacity, leading to a significant increase in the life of the silver cathode and the AgZn accumulator.

These characteristics are enhanced by the presence of metal oxides acting as porphores. These additives, consisting inter alia of zinc oxide or titanium dioxide, can be effectively added in ranges of quantities similar to those described in examples 3, 4 and 6 to 8, preferably between 1.5 and 50% by weight of the active substance, and preferably between 5 and 35%. The porphorus additives mentioned in the example may also be mixed in varying proportions, the sum of the two additives being preferably 1.5 to 50% by weight of the active substance, and preferably 5 to 35%.

Titanium dioxide, in addition to significantly improving the cycling ability of the silver electrode, changes the flow of the discharge curves of the AgZn accumulator as shown in the single figure.

The first landfill level has almost completely disappeared in favour of a second one.

Although the authors of the present invention cannot present a theory explaining the phenomenon, it appears that the disappearance of this first bearing does not affect the performance of the silver electrode, since it is observed that in the presence of titanium dioxide the recovered capacity of the electrode is equal to or greater than 90% of the theoretical capacity.

It is as if the successive reduction reactions of silver oxide were carried out in a single step according to the reaction: - What? AgO + H2O + 2e- → Ag + 2OH-

The following table shows the data for the calculation of the average annual average price of the product concerned:

A silver electrode is made according to example 6 and a zinc electrode is made according to a method described in French patent application FR 2828335. The electrodes are cut to the appropriate dimensions to be placed in an R6 (or AA) size accumulator bucket after spiraling. The ratio of the positive and negative electrode capacities is 1.

The accumulator shall be filled with an electrolyte consisting of 10 N potash, saturated with zinc oxide, and containing 20 g/l of lithium and 0,5 g/l of aluminium.

A hydrogen-oxygen gas recombination catalyst is attached to the inside of the lid in accordance with the procedure described in French patent application FR 2858464. The assembly is then closed and the accumulator is cycled after formation at 0.25 C (C/4) under load and 0.5 C (C/2) under discharge.

The initial rated capacity of the cells thus produced, the optimization of which has not been sought here, which is 1.10 to 1.15 Ah depending on the accumulator, is maintained at 100% at 180 cycles and remains at 90% after 250 cycles and 79% after 390 cycles.

Cycling of R6 elements in watertight mode at C/4 load and discharge at room temperature, with constant current without voltage limitation, with 10% overload, shows that with the gas recombination device mentioned above, the internal pressure remains limited to a maximum of 600 kPa, which is perfectly compatible with mounting in cylindrical buckets.

For the above-mentioned unoptimised capacity of 1,10 Ah, measured in R6 format, and taking into account the average discharge voltage of 1,60 Volt and the corresponding mass of 22,2 g, the nominal mass and volume energies are 74 Wh/kg and 198 Wh/litre respectively.

Various operation and storage tests at temperatures far from the ambient have shown the excellent performance of silver-zinc accumulators according to the invention.

The following results obtained in R6 type watertight accumulators are given as an illustration of the performance level achieved by the implementation of the present invention: For a charge and a discharge both at C/5 and 55°C, the capacity returned is 88% of the rated capacity, for a shutdown voltage of 1,0 Volt.After a charge in C/5 at room temperature, a storage of 72 hours at 55°C, the C/5 discharge at room temperature returns 84% of the rated capacity, for a shutdown voltage of 1,0 Volt.After a charge in C/5 at room temperature, a storage of 72 hours at -20°C, the C/4 discharge at room temperature returns 96% of the rated capacity, for a shutdown voltage of 1,0 Volt.

These examples show that a plated-type silver electrode made by filling a metal foam using a paste containing metal silver particles as the active substance and a powder of titanium dioxide and possibly zinc oxide of calcium oxide acting as a porophore and fixing agent for soluble ions Ag+ and Ag2+, has a cycling ability and efficiency much higher than that of fritted-type electrodes and also improved compared to those of plated electrodes whose active substance is introduced as silver oxide.

Under comparable conditions, and without going beyond the scope of the present invention, it is possible to combine one or more silver cathodes according to the invention with one or more cadmium anodes, particularly of the plasticised type with a metal foam collector, to produce alkaline silver-cadmium (AgCd) accumulators capable of operating efficiently in open and watertight modes and in particular of constituting excellent power systems.

The use of titanium dioxide alone or mixed with zinc oxide also improves the performance of the silver electrode and greatly increases its cycling ability.

Titanium dioxide also modifies the electrochemical reduction process of the silver electrode, by reducing, or removing, the first discharge plate corresponding to the reduction of silver oxide to silver monoxide.

The authors pointed out that the use of silver monoxide powder as cathodic active material in the invention's three-dimensional plasticised electrodes generally yielded higher initial rated capacities than silver metal powder, but they also pointed out, as is also widely shown by the comparative study of the results in the examples, that the use of silver metal powder yielded greater stability of cycling capacity and longer service life.

They then made cathodes according to the invention, combining metallic silver and silver monoxide in varying proportions, and were able to show that such combinations make it possible to obtain electrodes that are both high performing in terms of specific capacities and lifetimes.

Combinations of Ag-Ag2O, varying in proportions, may be used, depending in particular on the applications concerned and the type of characteristic sought as a priority in each.

The combination of silver cathodes according to the invention and zinc anodes made according to the technology described in the French patents FR 2788857 and FR 2 828 335 makes it possible to produce silver-zinc alkaline accumulators in open and watertight configurations, prismatic and cylindrical formats, which exhibit excellent cycling ability and use potash-based electrolytes with initial concentrations greater than or equal to 7 N. This ability to cycle for a long time is confirmed by negative and positive electrode capacitance ratios which vary widely according to the advantages of mounting at a unit of about 1.50 in connection with various thickness and capacitance definitions of the electrodes.

Claims (18)

1. Silver electrode for alkaline secondary electrochemical generators where the active mass is prepared in the form of a plasticised paste in which the active matter is incorporated in the form of metallic silver particles and/or silver monoxide particles, the paste also comprising an organic binder and a solvent, and the active mass comprising a porophore additive introduced in the form of metal oxides, said silver electrode being characterized in that the porophore additive is formed of titanium dioxide optionally with zinc oxide or calcium oxide.
2. Silver electrode for alkaline secondary electrochemical generators according to claim 1, characterized in that the porophore additive further comprises zinc oxide.
3. Silver electrode for alkaline secondary electrochemical generators according to claim 1, characterized in that the porophore additive is formed of titanium dioxide and calcium oxide.
4. Silver electrode for alkaline secondary electrochemical generators according to any one of claims 1 to 3, characterized in that is produced by impregnating a highly porous three-dimensional collector with the plasticized paste of active mass, these then being dried and compacted.
5. Silver electrode for alkaline secondary electrochemical generators according to any one of claims 1 to 4, characterized in that the silver powder has a particle size distribution between 0.1 and 40 microns.
6. Silver electrode for alkaline secondary electrochemical generators according to any one of claims 1 to 5, characterized in that the silver monoxide powder has a particle size distribution lower than or equal to 40 microns.
7. Silver electrode for alkaline secondary electrochemical generators according to any one of claims 1 to 6, characterized in that the porophore additive formed of metal oxides represents 1.5 to 50% by weight of the active matter.
8. Silver electrode for alkaline secondary electrochemical generators according to any one of claims 1 to 7, characterized in that the highly porous three-dimensional collector is a cross-linked cellular metal foam.
9. Silver electrode for alkaline secondary electrochemical generators according to claim 4, characterized in that the highly porous three-dimensional collector is a metal fabric or a metal felt
10. Silver electrode for alkaline secondary electrochemical generators according to claim 9, characterized in that the highly porous three-dimensional collector is made of any metal compatible with the usage potentials of the cathode.
11. Silver electrode for alkaline secondary electrochemical generators according to claim 10, wherein the highly porous three-dimensional collector is made of silver or nickel optionally coated with a silver layer.
12. Silver electrode for alkaline secondary electrochemical generators according to any one of claims 1 to 11, characterized in that the organic binder is PTFE, PVDF, a styrene-butadiene copolymer (SBR), an actylonitrile-butadiene copolymer (NBR) or a mixture thereof.
13. Silver electrode for alkaline secondary electrochemical generators according to any one of claims 1 and 12, characterized in that the binder represents from 1 to 10% by weight of the active matter.
14. Secondary electrochemical generator comprising one or more silver positive electrodes according to any one of claims 1 to 13, characterized in that the alkaline electrolyte is made from potassium hydroxide having an initial concentration greater than or equal to 7 N.
15. Secondary electrochemical generator according to claim 14, characterized in that the negative electrode(s) is/are (a) zinc anode(s).
16. Secondary electrochemical generator according to claim 14, characterized in that the negative electrode(s) is/are (a) cadmium anode(s).
17. Secondary electrochemical generator according to claim 15, characterized in that the negative electrode(s) is/are (a) zinc anode(s) produced according to the technology described in French patent application no. 99 00859 published under no. FR 2 788 887.
18. Secondary electrochemical generator according to any one of claims 14 to 17, characterized in that it functions in a tight manner, a catalyst arranged inside the housing enabling the oxygen and hydrogen formed during the cycle of said generator to be recombined in a catalytic manner.