Conventional alkaline cells, such as the generally disk-shaped, miniature alkaline zinc/silver oxide cells and the miniature zinc-air cells, are commonly known for supplying a battery voltage. Examples of conventional miniature alkaline zinc/silver oxide and zinc-air cells are disclosed in US-A-3,925,102 and US-A-5,306,580, respectively. Alkaline cells such as these generally have an anode containing zinc powder, as well as a binding agent, electrolyte solution, and other additives. The alkaline cell also has a cathode which commonly contains oxides of manganese, silver, mercury etc., and a separator disposed between the anode and cathode.

It has been found that conventional miniature alkaline cells can experience a sharp drop in pulse voltage when operated with pulsed discharges. For example, a miniature alkaline zinc/silver oxide cell (No. 357), with a rated minimum voltage of 1.1 volts, subjected to a 1.3K ohm background load with a 22 ohm discharge load for one second, every two hours, initially provides acceptable pulses. However, when the cell has only been partially discharged, such as after a twenty percent discharge, there can be a temporary, sudden and severe drop in pulse voltage supply. Although the cell voltage may frequently recover as the cell is further discharged, the temporary voltage reduction can often be sufficient to render the cell unacceptable for certain applications, thereby substantially reducing the effective service performance and overall usefulness of the cell.

This severe drop in pulse voltage is believed to be caused by the sudden precipitation of zinc oxide, especially near the separator interface, as the anode solution becomes super- saturated with zinc oxide. During discharge of an alkaline cell, the concentration of zinc oxide in the potassium hydroxide electrolyte continuously increases until the zinc oxide precipitates.

This precipitation creates a film which causes increased impedance in the anode, thereby potentially leading to a severe voltage drop when the cell has been only partially discharged.

The resulting increased impedance can temporarily reduce the supply voltage below an acceptable cut-off, thereby rendering the alkaline cell ineffective for use in a given application and considerably shortening the service life of the cell, at least for that application.

One approach to reducing or eliminating the severe voltage drop and to extend the effective service life of the cell is to increase the amount and the availability of electrolyte. This improves transport of zinc oxide away from the region of generation, minimising concentration gradients and, thus, reducing precipitation of zinc oxide as a film. While this approach reduces the severity of the above-identified problem, the addition of increased amounts of electrolyte can significantly lower the volume available for active materials and lower cell capacity.

Accordingly, the need to find new ways to increase service performance remains a primary goal of cell designers.

We have now, surprisingly, found that it is possible to avoid the problem of voltage dip by seeding the anode, thereby encouraging even deposition of zinc oxide and avoiding super- saturation.

Thus, in a first aspect, the present invention provides an alkaline cell comprising an anode which contains zinc and electrolyte solution, characterised in that the anode further comprises a zinc oxide precipitation agent.

In an alternative aspect, there is provided an alkaline cell comprising: a cathode; an anode containing zinc and electrolyte solution; a separator disposed between the cathode and the anode; and a precipitating agent disposed within the anode for seeding precipitation of zinc oxide so as to prevent a sudden decrease in supply voltage of the alkaline cell during a partial discharge state.

The performance of alkaline cells is improved by seeding the anode with a precipitating agent to prevent super-saturation of zinc oxide in the anode. By preventing super-saturation of zinc oxide, a lower zinc oxide concentration is maintained which can eliminate sudden precipitation of zinc oxide film and, hence, eliminate the occurrence of severe voltage drops

during partial discharge of the cell. Provided that the agent is evenly distributed throughout the anode, then this also encourages disperse precipitation, rather than precipitation at the separator interface.

The agent to precipitate zinc oxide may be any that is suitable, such as are described hereinunder, but is most conveniently zinc oxide itself. It is well recognised that a solution can become super-saturated without the solute crystallising out, if there are no seed crystals or no suitable surface to key crystallisation. Seed crystals eventually form, whereupon rapid deposition of excess solute occurs, sometimes virtually instantaneously. In the case of alkaline cells, it is likely that seed crystals form at the separator interface, as this is where the highest concentrations of zinc oxide occur. Rapid deposition at the separator interface then, effectively, blocks the separator.

Thus, it will be appreciated that the precipitation, or keying, agent needs to be present in an amount sufficient to key crystallisation, or precipitation, of zinc oxide once the anode becomes saturated with zinc oxide, thereby to avoid the problems associated with super- saturation. Where zinc oxide is the agent used, then it will be appreciated that this should be added to the anode in sufficient quantity that some remains in the solid phase. The amount necessary to achieve this will depend on various parameters, all of which will be readily apparent to one skilled in the art.

Parameters determining the amount of zinc oxide that needs to be incorporated into the anode include such considerations as the nature, concentration and amount of solvent, as well as the temperature at which the cell is made, storage temperature and operating temperature.

It will also be appreciated that, at no time, should the zinc oxide in the anode ever become fully dissolved, if this can be avoided, as it may be difficult to return the excess to the solid phase. Even if this could be achieved, such as by freezing, it may be that the re-created solid phase is undesirably concentrated in a particular area, such as at the separator interface, for example.

It is possible for only a very small excess of zinc oxide to be incorporated into the anode, such as is barely sufficient to leave a small amount of solid phase zinc oxide. However, it is preferred that the amount of zinc oxide used is sufficient to allow some to remain solid even at elevated temperatures and/or to allow for fluctuations in the amount of solvent. Use of such amounts of zinc oxide thereby avoids all of the zinc oxide entering solution at any one time.

It will further be appreciated that similar considerations apply to any other seeding agent that is soluble in the electrolyte, and that do not readily, evenly precipitate out of solution.

While zinc oxide has been found to serve well as a precipitating agent to achieve the desired nucleating effect, other precipitating agents may be used, such as other metal oxides or mixed metal oxides, particularly those having a crystal structure similar to zinc or zinc oxide. It is further preferred that such precipitating materials have relatively low solubility so that only a relatively small amount is required to achieve the nucleating effect.

Where the keying agent is not soluble in the anode preparation, then concentration considerations are different. If the precipitating agent is a solid, such as an insoluble metal oxide, then this is preferably evenly dispersed throughout the anode structure. It is not necessary for the amount of agent to be high, provided that it is sufficient to cause precipitation of zinc oxide, once the concentration of zinc oxide reaches saturation levels.

Further discussion of the precipitating agent will generally be with reference to zinc oxide, although it will be appreciated that such discussion is equally applicable to other agents, where appropriate.

In general, it will be appreciated that the amount of zinc oxide that is required may vary depending on the anode preparation and blending techniques, as well as temperature and concentration of KOH. For example, gel blending in a blender may lead to an increase in solubility of zinc oxide in comparison to a dry blending process. Likewise, an increase in process or application temperature may further increase the zinc oxide solubility, thereby requiring an increase in the amount of zinc oxide necessary to maintain solid zinc oxide in the anode mix.

More specifically, using 45% KOH electrolyte, for example, a level of 10% zinc oxide in the anode will generally be sufficient to maintain the existence of solid zinc oxide particles.

This level rises with increasing concentration of KOH, owing to the increased solubility of zinc oxide in higher concentrations of KOH. When the concentration of zinc oxide is adequate, the anode gel generally has a thick milky appearance, with solid zinc oxide visible. We have found that, generally, electrolyte in the range of 30% to 50% KOH requires a minimum of 4% to 10% zinc oxide by weight of electrolyte.

It may also be beneficial to add components to the anode mixture to increase the conductivity of the zinc oxide precipitated, so as to reduce any impedance increase during discharge. For example, doping the zinc oxide with metallic zinc, aluminium, chromium, tin, or gallium can significantly increase conductivity. These and other modified zinc compounds and alloys may further improve the cell discharge characteristics.

From the foregoing, it will be appreciated that it is generally preferred that the precipitating agent is solid, and particularly that the precipitating agent comprises zinc oxide additive.

In addition, in a preferred embodiment, there is provided a cell comprising: a cathode ; an anode containing zinc powder and electrolyte solution; a separator disposed between the cathode and the anode; and solid zinc oxide disposed throughout the anode in an amount such that at least some of the zinc oxide remains solid for seeding precipitation of additional zinc oxide so as to prevent sudden precipitation of the zinc oxide.

It will be appreciated that the present invention further provides anodes comprising a zinc oxide precipitation agent, such as defined herein.

In a preferred embodiment, there is provided an anode for use in an alkaline cell, said anode comprising: zinc powder; an electrolyte solution; and

a solid precipitating agent mixed with the electrolyte solution and zinc powder such that at least some of the solid precipitating agent is solid in the electrolyte solution, said solid precipitating agent providing substantially uniform precipitation in the anode so as to prevent a voltage decrease during a partial discharge of the cell.

It will be appreciated that, in the anodes of the invention, the solid precipitating agent preferably comprises zinc oxide. It is also preferred that the solid precipitating agent is uniformly mixed throughout the anode. Suitable anodes further comprise a binder agent and/or a gelling agent.

It will be appreciated that the essential feature of the present invention is the precipitating agent. Otherwise, the present invention is not limited to the type of cell, provided that it is an alkaline cell with a zinc-containing anode. Any such cells are suitable for use with the present invention, although we particularly prefer to use the present invention with miniature or button cells.

Likewise, there is no particular restriction on how the keying agent is incorporated into the anode, and it may be added at any suitable time. However, in general, it will be added during mixing the ingredients of the anode in such a way as to ensure even distribution throughout the anode. Various ways of assembling the anode into the cell are known, and are compatible with the present invention. Some such methods are exemplified hereinbelow.

The other constituents of the cell may also be those generally found in alkaline cells with zinc-containing anodes, and there is no particular restriction on such components.

The present invention will now be further illustrated by reference to the following drawings, in which: Figure 1 is a cut-away, cross-sectional view of a miniature alkaline zinc/silver oxide cell having an anode seeded with solid zinc oxide in accordance with the present invention; and Figure 2 is a graph comparing a conventional pulsed voltage response with the voltage response of a cell in accordance with the present invention.

A preferred embodiment of the present invention is described herein in conjunction with a miniature alkaline zinc/oxide cell, such as the type disclosed in US-A-3,925,102. However, it will be appreciated that the present invention is equally applicable to other battery cells containing an alkaline electrolyte and a zinc anode, such as a cylindrical alkaline cell, a miniature alkaline cell, or a miniature alkaline zinc-air cell, such as is disclosed in US-A-5,306,580.

In Figure 1, a miniature alkaline battery cell 10 ofthe zinc/silver oxide type is illustrated. The cell 10 has an anode 12, which is also referred to as the negative electrode, a separator 14, and a cathode 16, which is also referred to as the positive electrode, housed within a two-part container having a cathode container 18 and an anode cup 20. The cathode container 18 has a flange 22 which is crimped inwardly against a U-shaped flange of anode cup 20 via grommet 24 during assembly to seal the cell 10. The cathode container 18 may be made of any suitable substance, such as nickel-plated steel, nickel or stainless steel, while the anode cup 20 may suitably be made of copper-clad steel, for example. The grommet 24 may be made of a suitable resilient electrolyte-resistant material such as neoprene or nylon.

The cathode 16 of the cell 10 may include densely compressed pellets of divalent silver oxide powder or a mixture of divalent silver oxide powder and monovalent silver oxide powder, dispensed in the cathode container 18. Other types of cells may employ various other active materials in the cathode 16. For example, miniature alkaline zinc/air cells typically have removable tabs covering openings formed in the cathode container to receive atmospheric air as an active material.

The separator 14 may suitably be a three-layer laminate having two outer layers of radiation-grafted polyethylene and an inner layer of cellophane or the like. Also disposed between anode 12 and separator 14 is a layer of electrolyte-absorbent material 26 which may include, for example, various cellulosic fibres.

The anode 12 includes zinc powder, a binding agent, and an electrolyte solution blended together and dispensed within the anode cup 20. The electrolyte solution may include potassium hydroxide, sodium hydroxide, or mixtures thereof, in aqueous solution, and may either be added to the zinc powder and binding agent, before dispensing as a gel in the anode

cup 20, or may be added to a powdered or pressed tablet of combined zinc and binding agent in the anode cup 20, for example.

In this embodiment, and in accordance with the present invention, prior to partial discharge of the cell, zinc oxide is uniformly distributed throughout the anode 12 in an amount, in excess of the soluble amount, such that a portion of the zinc oxide remains as a solid. The presence of solid phase zinc oxide in the anode mix can be established by the thick milky appearance of the mix, with individual solid zinc oxide particles being visible through a microscope. By blending particles of solid zinc oxide uniformly throughout the anode mix, the solid zinc oxide acts as a seed for precipitation of more zinc oxide, and thereby prevents localise super-saturation of zinc oxide. This effectively prevents the occurrence of sudden precipitation of the zinc oxide and thereby prevents severe pulse voltage dips.

The anode mix can be processed with solid zinc oxide as a precipitating agent in various ways. Three, exemplary anode mix processes are provided as follows: Anode Mix Process A (1) Blend together dry ingredients of the anode including zinc powder, binding agent, and solid zinc oxide particles.

(2) If required, add mercury to the dry ingredients and blend together.

(3) Add aqueous electrolyte solution to the dry ingredients and blend to form a gel until the desired consistency is reached. Heat may be applied as necessary, depending on the binder, electrolyte, viscosity desired, etc.

(4) Cool the anode gel.

(5) Dispense the anode gel in the anode cup of the cell during assembly.

Anode Mix Process B (1) Blend together dry ingredients of the anode including zinc powder and binding agent.

(2) If required, add mercury to the dry ingredients and blend.

(3) Add aqueous electrolyte solution to the dry ingredients and blend to form a gel until the desired consistency is reached. Heat may be applied as necessary, depending on the binder, electrolyte, viscosity desired, etc.

(4) Cool the anode gel.

(5) Add enough solid zinc oxide particles to the gel mixture and blend so that some solid zinc oxide remains in the gel. This could be accomplished straight away, or after ageing the gel.

(6) Dispense the anode gel in the anode cup of the cell during assembly.

Anode Mix Process C (1) Blend together dry ingredients of the anode including zinc powder, binding agent, and solid zinc oxide particles to form an anode mix.

(2) If required, add mercury to the dry ingredients and blend.

(3) Dispense the anode mix as a powder or in a pressed tablet form in the anode cup of the cell during assembly.

(4) Dispense aqueous electrolyte solution in the anode cup of the cell.

In each of the aforementioned anode mix processes, particles of zinc oxide are preferably uniformly dispersed throughout the anode mix in an adequate amount such that at least some of the zinc oxide remains solid. The solid zinc oxide acts as a precipitating agent so that subsequent precipitation of zinc oxide will be uniformly distributed throughout the anode, instead of being concentrated at the separator interface or some other local region of the anode.

In each of these processes, the amount of solid zinc oxide used is greater than the amount soluble in the anode mix. In process B, the dissolution of zinc oxide is reduced by the action of the binder, while process C minimises dissolution since there is no agitation and the action of the binder will reduce dissolution of the zinc oxide.

EXAMPLE A miniature alkaline zinc/silver oxide cell was assembled with solid zinc oxide particles uniformly added to the anode mix in a solid state according to the present invention. The anode gel was prepared with 8% zinc oxide by weight of electrolyte. The gel formulation was 65% zinc powder, 3.9% mercury, 1.0% CMC (carboxymethylcellulose) binder and 30.1% of 45% KOH electrolyte solution. The zinc oxide amounted to approximately 2.4% of the total anode gel by weight. The solid zinc oxide was added by hand kneading it into the anode gel.

At this level, solid phase zinc oxide was readily seen to be visibly present in the anode gel. The zinc oxide used is commonly available and has a particle size of less than one um, as observed with a scanning electron microscope.

With particular reference to Figure 2, the pulsed voltage response 30 is shown for a miniature alkaline zinc/silver oxide cell having the solid zinc oxide disposed in the anode as provided above, and compares the response to the pulsed voltage response 28 of a miniature cell of the conventional type. The voltages were plotted in response to a pulsed voltage test applying a 1.3K ohm background load with a 22 ohm discharge load applied for one second, every two hours. The conventional cell response 28 initially exhibits an acceptable supply voltage in excess of a 1.1 volt cut-off. However, after only a partial discharge of the cell, the conventional cell experiences a sudden voltage dip which drops to approximately 0.95 volts at 38 hours. In contrast to conventional response 28, the cell employing the anode with solid zinc oxide of the present invention eliminated the existence of any such severe voltage dip during a partial cell discharge. As a consequence, the improved anode cell of the present invention maintained a battery supply voltage 30 well in excess of the 1.1 volt cut-off voltage, up until the cell was near full discharge.