GB2379326A - Optimised alkaline electrochemical cells - Google Patents

Abstract

Alkaline electrochemical cells having a porous cathode comprising manganese dioxide, wherein the cathode porosity is equal to or greater than 27%, the anode porosity is equal to or greater than 69%, the anode comprises zinc, and the calculated overall tap density of the zinc is less than 3.2 g/cc, perform well on constant wattage discharge. The zinc in the cell anode may comprise at least 4%, by weight, uniformly shaped zinc particles with low apparent density.

Description

OPTIMISED ALKALINE ELECTROCHEMICAL CELLS

The present invention relates to alkaline electrochemical cells having porous cathodes comprising manganese dioxide.

Alkaline electrochemical cells have been commercially available for well over twenty years. In many modern applications, alkaline cells vastly outperform traditional zinc carbon cells, and are the batteries of choice for most consumers.

0 The competition to produce the best alkaline battery continues to be fierce, but a large number of restrictions exist, not least of which is the size of any given cell.

Conventional sizes for primary alkaline batteries include AAA, AA, C, D and 9V (LR03, LR6, LR14, LR20 and 6LR61, respectively), and standard dimensions are 5 laid down for all of these types. Thus, whichever battery is chosen must fit within a given volume, thereby limiting the maximum amount of ingredients which it is possible to put into any given cell.

Working against these constraints, battery manufacturers have, for example, 20 substantially reduced the thickness of the cell walls, reduced the thickness of the seals, and changed the nature of the labelling of the cell, all in order to optimise the use of the internal volume of the cell.

When it becomes impractical to further increase the volume of the cell 25 ingredients, battery manufacturers then have the problem of trying to further enhance performance and battery life through enhancing and/or changing the ingredients used, but there must, ultimately, be a limit.

In US Patent No. 5,283,139 ('139 or US '139 hereafter), there is disclosed a cell 30 in which increased performance is achieved by increasing the density of both the anode and the cathode, without increasing the amount of aqueous, potassium hydroxide M&C GBP83394 SP1368. 1

electrolyte. If the volume of a given active ingredient cannot be increased, then increasing its density is a logical, straightforward means for increasing the discharge capacity of the cell.

5 Nevertheless. there remains a desire to provide better and hefter electrochemical cells. In co-pending GB Patent Application No. 0015003.7, we demonstrate that, surprisingly, and contrary to expectations, substantial enhancement of cells, above and 10 beyond those prepared in accordance with US '139, is possible, with performances being increased by as much as 15%, or more. by optimising the water ratios, in contrast to US '139.

Prior to '139, US-A-5,489,493 disclosed alkaline electrochemical cells 5 comprising a cathode comprising manganese dioxide, in which the cathode is composed of a mixture of a minor amount of highly porous manganese dioxide and a major amount of low porosity manganese dioxide. The highly porous manganese dioxide is exemplified as chemical manganese dioxide (CMD) and is distributed throughout the cathode in order to provide ion diffusion paths through the cathode. However, the use 20 of CMD is not desirable in all cases, since CMD has both a lower peroxidation and a lower density than electrolytic manganese dioxide (EMD). Consequently, CMD has a lower theoretical capacity than EMD on a volumetric basis.

In WO 00/30193, there is disclosed the possibility of using a semi-solid cathode 2s material. This semi-solid material has a high porosity and high electrolyte content, and primarily serves to reduce cathode polarization effects. The drawbacks to this construction include the fact that there is a substantially reduced capacity in the cell and that the MnO2: C ratio is very low but, more importantly, that this cathode material becomes very difficult to handle, so that it is impractical to make a cell containing such 30 material using conventional manufacturing processes and equipment. The capacity and performance of such cells is also severely compromised by comparison with US '139.

M&C GsP83394 SPI368.1

WO 98/50969 discloses the use of uniform zinc particles in the anode, which increases performances for anodes having porosities of up to and beyond 80%. Such porosities tend to separate zinc particles and increase impedance, especially in a lm 5 drop test. This disclosure teaches that increased anode porosities increase performance,

provided that there is flaked zinc. High anode porosities are easily achievable, even at relatively high zinc densities, as zinc is extremely dense, and experimental data show good results with anode porosities between 75 and 80%.

lo Demand for batteries also becomes ever more specialized, with cells being increasingly required to perform well in niche areas, such as for intermittent use, or in constant current (Amps), or constant power (Watts) conditions. In wattage tests, it is known that the higher the voltage, the lower current being drawn. Accordingly, adding voltage boost agents such as Ag2O or Serrate is theoretically attractive, but these 5 additives bring their own problems, such as expense, handling and instability.

In standard alkaline cells, the only practical way to increase the running voltage of the cell, therefore, is to lower its internal resistance. However, efforts to improve the electronic conductivity of the anode and cathode to improve wattage performance have 20 met with little or no success, as the change in other parameters has counteracted the improvement in wattage performance. Any improvement gained this way is negligible compared with improvements to be gained by improving other parameters.

Surprisingly, we have now discovered that when the porosity of both the cathode 25 and the anode are at optimum efficiency levels, then factors affecting internal conductivity have a disproportionately beneficial effect on the performance of the cell on constant power (wattage) discharge, particularly on high power discharge, if the zinc is properly selected to maintain sufficient electrical conductivity in the anode. Thus, cells of the present invention can provide even better performance than available from 30 following the teaching of US '139, despite actually reducing quantities of active materials in order to achieve the higher porosities.

M&C GBP83394 SP1368.1

Thus, in a first aspect, the present invention provides an alkaline electrochemical cell having a manganese dioxide based cathode and a zinc based anode, the cathode having a porosity equal to or greater than 27% and the anode having a porosity equal to 5 or greater than 69%, the zinc of the anode consisting of zinc having a calculated overall tap density of less than 3.2 g cc.

By "manganese dioxide based" and "zinc based" is meant that manganese dioxide and zinc are the primary active electrode materials in the cathode and anode, I o respectively.

It is necessary that the overall tap density of the zinc be less than 3.2 g/cc in order to maintain sufficient electrical conductivity within the highly porous anode.

Otherwise, the zinc particles become separated, interrupting the conductive matrix within the anode. In general, as the anode porosity is increased, the tap density of the zinc must be reduced.

One way to control the overall tap density is to use a mixture of high and low density zincs. Accordingly, in another aspect of the invention, the zinc comprises a 20 mixture that includes at least 4%, by weight, uniformly shaped, low-density zinc particles having a tap density of less than 2.5 gJcc. The present invention will be further illustrated by reference to the accompanying drawings, in which: Fig. 1 is a graph showing anode porosity vs. the amount of zinc flake for cells made according to the invention; and 25 Fig. 2 is a graph showing the maximum overall zinc tap density as a function of anode porosity for cells made according to the invention.

Cells made according to the present invention have a highly porous cathode and a highly porous anode. The particles of zinc in the anode must have sizes and shapes 30 that allow the zinc to form an electrically conductive matrix even though the amount of zinc relative to the anode volume is relatively small. When zinc particles have such M&C GBP83394 SPI368.1

s shapes and sizes, the zinc has a relatively low tap density. The overall tap density of the zinc is calculated from the measured tap densities of the different types of zinc contained in the anode. The overall tap density equals 100/ (dn/wn), where tin is the tap density of zinc type n and wn is the weight % of zinc type n, based on the total amount 5 of zinc particles in the anode. If a single zinc type is used, the overall tap density is equal to the measured tap density of a sample of that zinc.

For anodes with a porosity of 69%, it is preferable that the overall tap density of the zinc be no greater than 3.2 g/cc. For lower porosity anodes, the preferred maximum lo overall tap density is lower, by 0.06 g/cc for every 1% increase in anode porosity above 69%. It is more preferred that when the anode porosity is 69% the overall tap density of the zinc is no greater than 3.13 g/cc, and for lower porosity anodes the overall tap density is 0.085 g/cc lower for every 1% increase in anode porosity above 69%. When the anode porosity is about 71%, the preferred overall tap density of the zinc is from 5 2.83 to 2.96 g/cc, inclusive.

For anode processing and cost reasons, it may be desirable to use a blend of conventional, irregularly shaped, high-density zinc powder and uniformly shaped, low-

density zinc particles to obtain the desired overall zinc tap density.

"Uniformly shaped zinc particles" means that the individual particles of zinc have substantially consistent morphology. This is in contrast with typical zinc powders with consistently irregular shapes, few of which are similar to each other. To have a uniform shape, each of the particles' shape factor must be substantially similar to all 25 other particles. Therefore, for example, if the uniform shape is flakes, then all of the particles must be flakes. In order to produce uniformly shaped particles, control of the forming or classification process should be used to ensure the desired consistent particle shape. The shapes of zinc particles can be determined by examining the particles with a scanning electron microscope.

M&C GsP83394 SP1368.

Zinc flake is useful, but other 2-dimensional and 3-dimensional shapes are also useful for reducing the overall tap density of the zinc in the anode. Such low-density zinc overcomes the problem of loss of electrical path in the zinc with separation of the zinc particles at higher anode porosities. Zinc flake is a preferred form of uniformly shaped, s lov - density zinc and will generally be referred to herein, although it will be understood that reference to zinc flake, or flake, includes reference to other forms of low-density zinc, unless otherwise indicated or apparent.

In general, a zinc flake content of about 1. % w/w per 1% increase in porosity lo of the anode over 67% is sufficient, although it will be appreciated that the skilled person may employ as much flake as desired. More preferably, zinc flake content is about 2% w/w per 1% increase in porosity of the anode over 67%. Levels beyond this are acceptable, but generally yield little further improvement in performance, high levels of flake are more difficult to handle, and zinc flake is more expensive.

More particularly, a level of about 8% to 11% flake, based on the total amount of zinc particles in the anode, has been found to be useful in combination with an anode porosity of about 71%, for example, a level of about 8% flake being both commercially and practically convenient.

Although the effect of increasing conductivity in the anode has been found to be of importance, we have also found that conductivity of the cathode has a significant effect on the performance of cells of the invention in wattage tests.

2s Conductivity of the cathode may be enhanced by appropriate selection of the carbon content. In the art, carbon content has generally been kept to a minimum so that, what is necessary is that the amount of MnO2 be reduced as far as possible to reduce inactive content in the cathode, while maintaining a sufficient level of conductivity.

This applies to other cell types but, in the present invention, it has been found that the 30 MnO2: C weight ratio is preferably no higher than about 26:1. A ratio of between 20:1 and 25:1 is more preferred, especially between 22:1 and 24:1, with about 23:1 being M&C GBP83394 SP 1368.1

currently preferred when using Superior GAIT graphite and GHIJ EMD, for example.

When using other carbon sources or graphites and/or other brands of MnO2, these ratios may vary, and suitable ratios are readily determined by those skilled in the art, such as in accordance with the Examples hereinbelow.

The term "porosity", as used herein, relates to the volumetric amount of non-

solids in the electrode in question. Solids are those components that are insoluble under conditions pertaining in the assembled cell. In the anode, the solids will generally only comprise zinc and indium hydroxide, where present. The other anode components are 1 o usually soluble in the electrolyte solution, including gellants. Components that are soluble in the electrolyte need not be considered as solids when calculating porosity.

Where a portion of an ingredient is insoluble, such as where the electrolyte is saturated, it is not necessary to include the insoluble portion as a solid when calculating the electrode porosity; the entire amount of that ingredient is excluded. In any event, the 5 amount of gellant in the anode is generally so small that, to most intents and purposes, it can be discounted when calculating porosity. In the cathode, the solids will generally effectively comprise only the MnO2 and carbon (conventionally graphite). For practical considerations, although cathode binders are usually insoluble and, therefore, count as solids, the amount of any binder is generally so small that it has no significant effect on 20 the calculated porosity.

The porosities of the electrodes are important in the present invention. If the porosities are below the levels of the invention, then the beneficial effects of increasing the conductivity of the electrodes is not seen, or is masked. Above the minimum levels 25 of the present invention, there is seen improvement of the performance of the cell, with both electrode discharge efficiency and duration increasing. However, there appears to be little additional improvement to be gained by increasing anode porosity beyond about 71%. Above this level, the increase in efficiency of the anode by increasing porosity is counteracted by the necessary reduction in capacity of the cell, so that little 30 or no advantage is seen when increasing the porosity further, a plateauing and subsequent gradual drop in performance being observed. This effect is not so marked M&C GBP83394 SP1368. 1

with the cathode but, again, there is increasingly little to be gained by increasing cathode porosity beyond 28%, although the intended use of the cell is relevant. Much above 33%, there is little or no reward for increasing cathode efficiency, in terms of overall performance.

Another factor that, while significant, is not crucial to the present invention is electrode capacity. This is the theoretical capacity of the electrode in ampere hours (Ah). Unless otherwise indicated or apparent, the specific capacity values used herein are, for manganese dioxide, 0. 285 Ah/g for a 1 electron discharge, and, for zinc, 0.820 0 Ah/g.

As a guide, it is preferred that the ratio of cathode capacity: cell volume for cells of the present invention be in the range of 0.44 to 0.49 Ah/cm3, when cathode capacity is calculated in accordance with the 1 electron principle described hereinbelow.

5 When using the principle assumed in US '139, then this ratio is approximately 0.57 to 0.63 Ah/cm3.

In a preferred embodiment, then, there is provided a cell of the invention wherein the ratio of cathode capacity: cell volume is in the range of 0.44 to 20 0.49 Ahlcm3, when cathode capacity is calculated in accordance with the 1 electron principle. Thus, in the context of an AA, or LR6, cell, the advantages of the present invention are particularly observed when the capacity of the cathode is equal to or 25 greater than 2.7 Ah. Below this point, performance tends only to be equivalent to that of the art, although advantages may be observed in reduced quantities of active ingredients. Accordingly, in a preferred embodiment, there is provided an LR6 cell of the 30 invention which has a cathode capacity of at least 2.7 Ah.

M&C GBPS3394 SPl368.

Also in relation to LR6 cells, and taking the above ratios into account, it becomes substantially difficult to exceed a cathode capacity of 3.1 Ah. In general, the range of 2.7 to 3.0 Ah provides very significantly enhanced LR6 cell performance in high drain applications, and is particularly preferred.

As noted above, the intended end-use of the cell has an impact on the choice of cell parameters. For example, if the cell is intended for use in a continuous power drain situation, then it is desirable to have a cathode porosity in excess of 30%, and preferably between 30 and 36%, with 32 to 34 /0 being more preferred, especially about 33%.

lo Anode porosity is generally substantially independent of the intended end use, and any porosity of 70%, such as 71% as mentioned above, is useful. However, the capacity of the cathode is preferably between about 2.7 and 2.9 Ah, inclusive, for AA, or LR6 cells, in such continuous power drain applications, and EMI):C is, independently, preferably between 20:1 and 25:1.

For intermittent power drain situations, as expected, the capacity of the cell becomes a factor, so that the balance swings back. The porosity of the cathode is preferably between about 28 and 30 /0, inclusive, and the capacity of the cathode is preferably between 2.9 and 3.0 Ah, inclusive, for an AA or LR6 cell. EMD:C is, 20 independently, preferably between 20:1 and 23:1. Indeed, this embodiment provides excellent results in most if not all wattage tests currently used, and forms a particularly preferred embodiment of the present invention. In accordance with the principles enumerated above, it will be appreciated that this embodiment extends to all cell types, as do all other embodiments described herein, unless otherwise apparent.

As In general, it will be appreciated that, with increasing porosity of either electrode, the capacity of that electrode will necessarily drop, assuming constant volume of the electrode. However, above the minimum levels of the invention, the loss in capacity is compensated by the increase in performance of the cell for a short range.

30 The increase in porosity appears to improve the efficiency of the relevant electrode, and the combination of the improvements in both electrodes seems to allow the M&C C}BP83394 SP1368.1

JO electrochemical cell reaction to proceed more freely, thereby more than compensating for loss in electrode capacity.

Cells of the invention with porosities generally above those indicated as 5 preferred are also included herein as, although such cells have only similar, or even lower, performance than cells of the art, they still provide performance recognised as being useful, while containing significanl]y reduced quantities of active ingredients, thus being of benefit in reducing manufacturing costs.

0 Cells of the present invention are of particular use in high drain applications, such as digital cameras, video cameras and photoflash devices. They are especially preferred for devices that discharge the cells in a nearly constant power mode, such as MP3 players, for example.

s It is particularly preferred that, at a depth of discharge of one electron, i. e. after the cell has discharged to a level of 1 electron as described below, any cell has a calculated KOH concentration of about 50% wiw. Above 50%, KOH crystallizes out rapidly, removing both KOH and water from the electrolyte solution, as each molecule of KOH takes two molecules of water of crystallization. (:ells having a calculated final 20 concentration of KOH much above 50%, therefore' fail quickly.

Cells having a calculated final concentration of KOI] much below 50% will generally be inefficient, with water taking up room better taken up by active ingredients.

In addition, if KOH is too dilute, then the inefficiency can be further compounded, as 25 detailed below. With the porosities of the present invention, however, and using electrolyte concentrations used in the art, final KOH concentrations can fall below 50% without necessarily wasting space. Such cells, whilst generally being encompassed by the present invention, have been found to be not operating at maximum efficiency, despite the electrolyte only taking space that cannot be used by the active ingredients, 30 optimal efficiency only occurring when the final KOH concentration is calculated to be 50%. M&:C GBP83394 SPI368.1

What we have found is that, for cells where maximal efficient anode porosity has been reached, final KOH concentrations of less than 50 /0 lead to a voltage drop, so that cells "fail" earlier. In other words, in a continuous test, for example, cells reach the 5 deemed failure point of 1V sooner than cells with a 1 electron final KOH concentration of 50%. In fact, it has also been noted that cells with a final KOH of less than 50% also exhibit earlier zinc passivation.

In cells having anode porosities lower than those of the present invention, 10 increased KOH concentration, both to start and at calculated 1 electron discharge, leads to improved performance. However, the performance never exceeds that of the present invention and, because the porosities are lower, the amount of active material is necessarily greater. The porosities of the present invention, therefore, allow the use of significantly smaller amounts of active material but provide enhanced performance Is characteristics.

Without being bound by theory, it is believed that the increased KOH concentration at lower anode porosities serves to drive the reaction scheme (I), shown below, forward where, otherwise, the reduced porosity serves to hinder the reaction by 20 restricting ionic movement. With the porosities of the present invention, however, there is no significant hindrance and, once an anode porosity of about 70% or 71% is achieved, the cell reaction becomes cathode limited.

During the cell reaction, when current is flowing, the concentration of KOH 25 rises appreciably in the cathode, while falling in the anode. If the anode porosity is too low, then the ion exchange necessary to compensate this effect does not happen sufficiently quickly, exacerbating the concentration effect. If the concentration of KOH drops to 0% anywhere in the anode, then the zinc in that area is liable to passivation, severely compromising performance.

M&C GBP83394 SP1368.1

In general, it appears that, below an anode porosity of about 70%, changes affecting the anode are significant, and it can be considered that the performance of the cell is anode limited. Above this range, it appears that cathode effects are more important. Without being bound by theory, it appears that the reason for a final KOH 5 concentration of 50% being the most preferred is because KOH crystallises out in the cathode, rather than the anode and that, above 70% anode porosity, the cathode becomes determinative. Below 70% anode porosity, higher KOH serves to partially compensate for reduced porosity, thereby partially overriding the adverse effect on the cathode.

This effect is still apparent above an anode porosity of 70%, but the effect on the 10 cathode becomes more important, so that crystallization of KOH in the cathode dictates the maximum final concentration of KOH.

Thus, there is further provided a cell as defined above and comprising electrolyte containing KOH, which, prior to discharge, has a KOH concentration is selected such that, after the cell has been discharged to a depth of discharge of one electron, the calculated KOH concentration is about 50% wfw. Preferably, the amount of electrolyte is such that, at a calculated level of one electron discharge of the manganese dioxide, the calculated concentration of potassium hydroxide is between 49.5 and 51.5% (w/w solution).

Previously, it had not been possible to glean information, such as the impact of porosity changes, from data obtained from experimental cells, owing to the complications presented by the uncertainty of the relationship of each constituent to the other. In particular, the only previous certainty was that there should be sufficient water 25 present, in the form of potassium hydroxide solution (referred to as "electrolyte" herein and in the industry) to enable the electrochemical reaction to take place. This is illustrated in US '139, where every attempt is made to maximise the quantity of'tactive" ingredients, without taking any particular note of the amount of electrolyte.

30 We now know that it is important that the final concentration of the KOH not exceed about 50% after one electron discharge, and that this rule applies, regardless of M&C GBP83394 SP1368.1

the extent to which the cell is intended for discharge, or the rate at which it is discharged. Deviation from this rule has very significant repercussions on cell performance, and distorts experimental results obtained from changing other parameters, to the extent that it is impossible to derive meaningful conclusions when it

s is not observed.

Recognising that the final concentration of KOH should be 50% after I electron discharge provides a firm rule for standardizing electrochemical cells, so that it is now possible to establish relationships between all of the other ingredients of 0 electrochemical cells. Values for the variables in the complex equation which makes up electrochemical cells can now be established, given that there is now available a fixed standard by which the remainder can be measured.

Previously, without the realisation that it was crucial that the end concentration IS of KOH be in the region of 50% to maximise the performance of any given cell, varying the porosity of either the cathode or the anode had the inevitable result of varying the final concentration of KOH, so that the fmal performance of the cell was not only related to the porosity chosen, but also to the final KOH concentration. Thus, the results were meaningless, and it was not possible to establish the ideal porosity of either 20 the cathode or the anode.

However, provided that cells are manufactured which adhere to the principle that the final KOH concentration should be in the region of 50% after one electron discharge, then it is now possible to establish optima for the other cell parameters, so 2s that there is a readily attainable ideal of porosity for both cathode and anode. Once these are established, then it becomes possible, where it was previously impossible, to devise a cell that is efficient in wattage discharge tests by increasing internal conductivity. M&C GBP83394 SP1368.1

In general, it is preferred that the cells of the invention contain no added mercury and comprise an aqueous potassium hydroxide electrolyte, a zinc-containing anode and a manganese dioxide-containing cathode.

s Although not essential to the invention, it is preferred that the concentration of potassium hydroxide in the electrolyte is between about 34 and 37% (w/w solution) prior to discharge. By "prior to discharge" we mean that the cell is in the condition intended for use by the user, after all manufacturing steps. Manufacturing may include a small amount of discharge, e.g., during electrical testing. However, what is generally 10 considered to be more important is for the flea] concentration to be in the region of 50%, with the starting concentration being appropriately selected to achieve this end.

It will be appreciated that the amount of MOON and zinc will generally be kept at a constant capacity ratio, so that a change in one will result in a concomitant change in 5 the other. The ratio may be any that a givenmanufacturer might want to employ, and is anywhere in a range of about 1. 2: l to about 1.4: l anode to cathode (A: C ratio). It is assumed herein that, when discussing varying parameters of either electrode, the overall ratio of anode capacity to cathode capacity is maintained.

20 The porosity of either electrode may be increased in any suitable manner. In the case of the anode, for example, this may be achieved by increasing the internal diameter of the cathode, thereby increasing the volume of the anode, while keeping the same amount of zinc. If the volume of the anode is not increased, then the amount of zinc and, concomitantly, the amount of cathode material, must be reduced in order to reduce 2s the anode porosity, unless a deliberate decision is taken to vary the A: C ratio. It will be appreciated that it is an advantage of the present invention that the overall amount of active electrode material can be reduced while increasing cell performance.

The cathode may be made porous in many ways. One method in the art employs 30 CMD. Cells having cathodes containing CMD generally perform worse in high drain, continuous tests than cells containing only EMD. However, surprisingly, we have now M&C GBP83394 SP1368. i

found that, if CMD is used in the present invention, with a porous anode, then the drawbacks of CMD are ameliorated. While cells containing CMD do not perform quite as well as cells whose porosity is raised to the same level by another means, the difference is only small. What is particularly notable is the considerable extra strength 5 imparted to the cathode, which is a substantial advantage in assembly.

Other methods may also be used to prepare porous cathodes. Preferred methods are those that will lead to a generally homogeneous texture. This can generally be achieved by reducing the amount of MnO2 or carbon in the cathode mix. However, 10 there can be a problem when MnO2 is insufficiently compacted, in that the structural integrity of the cathode will tend to be compromised, unless otherwise compensated.

Likewise, reduction in the amount of carbon can not only reduce structural integrity, but can also result in reduced conductivity.

5 Reducing the amount of MnO2 reduces capacity and, if there is less material to form the cathode, then the density of the cathode must also be reduced. This can lead to manufacturing problems with pellets, for example, being more likely to crumble. This can be overcome in various ways, such as by using crushed ice or solid KOH during construction, or maximising MnO2 and reducing the carbon content, such as by using 20 carbon black. In all such cases, the cathode can still be compacted to the normal extent, but leaving the cathode more porous.

Where solid KOH is used, subsequent addition of water or KOH solution to yield desired electrolyte levels has, surprisingly, been found not to give rise to excessive 25 heating, and has yielded strong pellets. Carbon black provides weak pellets but with good conductivity. Surprisingly, we have found that, if it is pre-mixed with between about and 30% electrolyte, usefully about 10 to 15% electrolyte, then strong pellets can be formed. All of these methods are individually preferred, and each assists in providing sufficiently solid cathode material after compaction to be used in cell 30 manufacture.

M&C GsP83394 SP1368.1

The general, alkaline cell reaction scheme is as follows: Zn 2MnO2 H2O ZnO + 2MnOOH (I) 5 Although the manganese compound is shown as MnO2 as is conventional, it is well understood by those skilled in the art that manganese dioxide is non-stoichiometric, and the actual formula is approximately MnO 96 Accordingly, the actual number of electrons involved in this reaction is approximately 0.925. This is referred to herein, as is also conventional, as "1 electron", or l e.

The reaction shown in scheme (I) above, generally referred to as the first electron reaction" is not necessarily the only reaction to occur, and it may be followed by a second electron reaction, in which the MnOOH is converted to Mn(OH)2. This second reaction is usually only significant where cells are severely depleted. It appears Is to have very little, or no, effect on the requirement for a 50% final KOH concentration after 1 e discharge (first electron reaction), and cells optimised for 50% KOH after 1 e perform better, regardless of whether they proceed to the second electron reaction.

Thus, herein, only the first electron reaction is taken into account. Any references herein to the "point of completion" mean the point at which the first electron reaction, 20 or 0.925 electron in stoichiometric terms, has gone to completion (led, Mn+3 925 is reduced to Mn; I).

By way of illustration, on a discharge plot, the ampere hours are measured under the discharge line which, in the case of manganese dioxide, originally tends to form a 25 very substantial shoulder and then plateau's off very sharply, after which point it is occasionally possible to observe removal of the second electron. One electron discharge corresponds to the juncture of the bottom of the shoulder and the beginning of the plateau. Although this plateau occurs at a voltage below that considered as failure for most cells, the drop to this point is generally steep, and the calculations of 1 electron 30 for the purposes of the present invention are unaffected.

M&C&BP83394 SP1368 1

It can be seen from the above equation that there must be sufficient water present to allow the reaction to go sufficiently to completion to be considered full discharge. The above reaction scheme takes place in the presence of a strongly alkaline solution, a solution of potassium hydroxide being currently preferred by cell 5 manufacturers.

From the above, it will be understood that the "final concentration" of KOH (i.e., at the end of the first electron discharge) is a calculated one. However, applying reaction scheme (I) above, the final whole cell concentration of KOH is readily lo calculable, provided that the initial concentration of KOH is known.

In practice, calculating the final KOH concentration, based on this principle, means that it is neither necessary to discharge a cell by 1 electron, nor to measure final KOH concentration, whether in the anode, cathode, or both.

Accordingly, preferred cells can be designed and manufactured with considerable ease, as starting amounts of active materials are readily assembled and adjusted to yield a suitable, final, calculated concentration of KOH at 1 electron discharge. As noted above, the final concentration of potassium hydroxide (KOH) has a very substantial impact on the performance of the cell. It is particularly desirable that, at the point of completion, the concentration of KOH, as calculated for the whole cell, should not exceed a value of greater than about 51.5%. More preferably, this should be 25 no greater than 51%, with about 50.6% being around the optimum value. Values of less than 50.6% are acceptable, including about 49.5%, but much less than this also reduces the possibility for enhancement, as discussed below.

As noted above, by "porosity" is meant the relative amount, v/v, of the electrode 30 in question that is not taken up with solids. As the solids content, volurnewise, is generally easier to calculate than the nonsolids, and also because porosity includes any M&C GBP83394 SP1368.1

trapped air, for example, then the calculation to determine percent porosity is generally expressed as [(V: - VS) / V7] * 1 00

wherein Vz is the measured total volume of the electrode and Vs is the volume of the s solid component.

The volume of the solid component is not, generally, measured directly, but calculated as the product of weight over density. For the purposes of porosity, it will be appreciated that a given solid substance is quite likely already to possess a certain lo degree of porosity, such as chemical manganese dioxide (CMD) which can have porosities in excess of 50%, for example.

Thus, in order to more reliably calculate electrode porosity, the theoretical porosity of the substance is used. This is calculated based on molecular structure and IS 3-D arrays, and takes no account of any porosity that might result from the method of manufacture. Accordingly, for these purposes, both EMD and CMD are considered to possess the same theoretical density. If the actual, apparent density of the substance were employed in the porosity calculations, then the resulting, calculated porosity of the electrode would take no account of porosity introduced with the solids, and would, at 20 best, be misleading and, at worst, meaningless.

Theoretical densities assumed for the electrodes in the present invention are as follows: M&C GBP83394 SP1368.1

Cathode Component Theoretical Density Wt per 100g Vol. per 100g EMD 4 53 (d) w v = w /4.53 CMD 4 53 (d2) w2 v2 = w2/4.53 Graphite 2.25 (d3) w3 V3 = w3/2.25 Coathylene 0.92 (d4) w4 V4 = w4/0.92 40% KOH 1.39 (as) ws V5 = w5/1.39 Another components d6 etc. w6 etc. v6 = w6/d6 etc. Coathylene is polyethylene Anode Component Type Theoretical Wt per 100g Vol per 100g Density Zinc + Solid 7.14 (d7) W7 V7= W7/7.16 Carbopol 940 Liquid 1.41 (d8) w8 v = w /1.41 Indium hydroxide Solid 4.60 (as) w9 vg = w9/4.60 ZnO Liquid 5.61 (d o) wl0 v 0 = w o/5.61 36% KOH Liquid 1.35 (d) w v, = w /1. 35 Component x d,2 w 2 v 2 = w 2/d 2 etc. s In which CX', '?' and 'etc., allow for any further component(s), which may be solid or liquid. Accordingly, the theoretical volume of the cathode is the sum of all of the 10 ingredients = V = (v:v6) = (v + V2 + V3 + V5 + V6 etc.).

Likewise, the theoretical volume of the anode = VT = V7 + VS + V9 + VIO + Vl1 + Vl2 M&C GsP83394 SP1368.1

In the case of the cathode, the theoretical N olume is substantially the same as the actual volume, so that it is not necessary to build in any compensatory factors.

However, should the actual cathode volume be different from that calculated, then it is the porosity of the actual cathode that prevails.

Is For the avoidance of doubt, the actual cathode volume can be calculated from knowing the height of the cathode (H), and the internal and external diameters of the cathode (ID and OD, respectively). In the present invention, it is preferred to manufacture the cell using a stack of cathode pellets, so that H = Height of stack of pellets In a specific example, which is for illustration only, cathode diameters are as follows: Pellet as manufactured In canl Cathode OD 1.345 = 00p 1.335 = 0I) c1 Cathode ID 0.900 = IDp 0.885 = ID 15 Thus, Actual Volume = VA = H.. (OD While, Theoretical Volume = VA = H CODA'- IDp: In the above case, whether the cathode pellet is as manufactured or "in can", the product of oD2 ID2 is 0.999. This is because, in this instance, and as preferred in the present invention, the pellets are designed to be interference- fitting within the can, so that, on insertion, the pellets are compressed. Because this does not affect the volume 25 then there must be a concomitant reduction in the internal diameter to compensate for the reduction in external diameter, in order that the volume remain unchanged.

In the cathode, the Theoretical Volume of Solids = Vs = vat + V2 + V3 + V4 M&C GBP83394 SPl 368.1

Thus, Cathode Porosity = (is) x l OO VA and this is the porosity to which the present invention pertains.

In the anode, Via = Volume of Liquids = vet + vie + vat Vs = Volume of Solids = V7 + V9 so that the Theoretical Anode Porosity = (V_ - V.il x 100 = VL X 1 00 10 VT VT

and it is the theoretical porosity to which the anode paste is made up, and to which the present invention pertains.

5 In the case of the anode, there tends to be a substantial difference between the theoretical volume and the actual volume depending, to a certain extent, on the method used to fill the anode basket. In the embodiment under discussion, the basket comprises the separator fitted into the anode cavity in the cathode.

20 Methods used to fill the anode basket are generally one of two. The first is top filling, the second bottom filling. The former involves dropping in the anode paste generally from the vicinity of the top of the basket. The latter generally involves inserting a dispensing tube into the basket and injecting anode paste at a rate equivalent to withdrawal of the tube, withdrawal of the tube being generally effected or assisted by 25 the force of the expulsion of the paste from the tube.

With top filling, more air tends to be trapped in the anode than with bottom filling. In any case, the trapped air, or anode deadspace, is usually at least 5% v/v and anywhere up to about 17%. Using bottom filling, the margins are between about 5% 30 and 10% while, with top filling, the margins are between about 8% and 17%.

M&C GBP83394 SP1368.1

The porosity of the anodes of the present invention is not dependent on the anode deadspace, and a simple core of the anode will substantially yield the porosity to which the anode w as made. Thus, the porosity of the present invention applies to the anode paste before being placed in the cell.

In a cell "off the shelf,', there will be an anode deadspace as noted above, and generally in the region of about 10%. In order to establish the porosity of the anode, in accordance with the present invention, the most accurate method is to take a core sample, and perform the analysis described below. As a rougher guide, however, the lo anode deadspace found in most cells is about 10%. Variations from this amount provide porosities largely within experimental error, as an anode deadspace of about 10% gives an overall increase in anode porosity of about 3% compared with an anode deadspace of 0%. Thus, if an anode deadspace of about 10% is assumed, and standard bottom filling in a manufacturing facility yields about 9% anode deadspace, while 15 standard top filling in such a facility yields an anode deadspace of about 12 or 13%, then it will be appreciated that, assuming a deadspace of about 10% will yield a porosity tolerance of l%.

In making up cells of the present invention, the theoretical volume of the 20 components of the anode is first calculated, per l OOg of total components. The volume of the anode basket is then established, which will vary from the internal space defined by the cathode according to the volume of separator material used. This volume is then reduced by 10% to adjust for anode deadspace, and this is the volume of anode paste used. Thus, if porosity is simply taken as a measure of the total solids in relation to the volume in the basket, then the resulting, apparent porosity of the anode, assuming 10% deadspace, will be about equal to [theoretical porosity/(100-10)] * 100. In other words, apparent porosity theoretical porosity + 11% 30 As a rough guide, then, the actual porosity of the anode from a cell off the shelf will be equal to about the apparent porosity divided by 1.1 1. However, as noted, this will M&C &BP83394 SP1368.1

depend on the deadspace of the cell. As noted above, the porosity to which the present invention pertains is the porosity of the anode itself, and not the porosity of the anode + deadspace. 5 The anode fill volume, in the present example, despite being reduced by 10%, generally results in a fill of anode paste to generally the same height as the top of the cathode pellets. It will be appreciated that the amount of 10% may need to be modified according to anode fill techniques employed by those skilled in the art. In practice, the deadspace is filled with electrolyte, whether this enters after filling, or whether 0 electrolyte is already present in the basket prior to filling, as part of the overall electrolyte needed in the cell of the invention. In any event, the anode deadspace is taken up with electrolyte, either straightaway, or after dispersion of the air.

In any event, the level of the anode paste should be about the same height as the 15 cathode material. If the heights are different, especially if the anode is lower than the cathode, then high drain performance is adversely affected. Thus, a tolerance of no greater than 2. 5% in height differential is envisaged, in relation to cathode height. If there is a differential, then it is preferred that the anode be higher than the cathode, but preferably only by a small margin, and preferably no more than 2.5%.

It will be appreciated that the amount of anode paste, after the 10% adjustment, will need to contain the appropriate amount of zinc to maintain the anode: cathode Ah ratio which, in the present example, is assumed to be 1.33. Where other ratios are applied, then suitable adjustments to volumes, for example, need to be made, but the 25 principles of the invention remain unchanged.

In a cell "off the shelf", porosities may be determined readily. Essentially, it is necessary to first determine the volumes of the electrodes, then to establish their solids content. In the case of determining the KOH content, this can be established by 30 assaying the various components of the cell and then combining the results.

M&C GBP83394 SP1368.1

The amount of water can be established by the use of a modified Dean & Stark method. Apparatus is available from Quickest & Quartz Ltd., for example. The sample is covered with dry toluene and reflexed for 45 minutes, ensuring that a majority of the condensation takes place in the water-cooled condenser. Water is collected in a 5 measuring cylinder or cuvette disposed under the condenser to catch the run-off. This method is modified by bubbling CO2 gas through the boiling toluene, in order to convert KOH to K2CO3, otherwise not all water can be collected, as some stays behind with the KOH as water of crystallization.

0 The amount of OH- is readily determined by soshleting each component separately with water to obtain a solution containing KOH and water. All samples are combined, made up to a known volume, and then titrated for OHby standard methods.

For example, HCl of known molarity, together with phenolphthalein as an indicator, may be used. In this method, it is assumed that all OH- is KOH, and weights are 15 calculated accordingly.

Together with the volume of water and the amount of MnO (calculated as described below), it is then within the abilities of the skilled person to establish that a given cell satisfies the criteria of the present invention.

Returning to electrode porosities, and as noted above, these are calculated essentially as follows: [(Total volume - Solids volume) / (Total volume)] * 100 2s More specifically, the volumes of the electrodes may be determined in any suitable manner. It is preferred to establish the volume in situ, and this is preferably achieved by the use of X-rays, which give a clear indication of the internal proportions of the cell, especially anode and cathode height and width. This done, the cell can then be cut open' and the electrodes separated.

M&C GBP83394 SPI368.1

As a generality, for example, with the cells illustrated in the accompanying Examples, what we have found is that, in the anode, only the zinc needs to be considered while, in the cathode, only the manganese dioxide (EMD and CMD, where present) and carbon (usually graphite) need to be considered, when determining 5 porosities. The remaining components are either present in vanishingly small quantities, are both not particularly dense and present in small quantities, or form part of the electrolyte, so that even if account is made for these components, the difference they make is lost in the margins of error.

lo Accordingly, in the anode: Measure dimensions of internal volume of anode basket Measure height of anode in basket from X-ray of cells Remove all anode material and wash zinc with water to remove gellant and electrolyte 5 Wash with ammonium hydroxide solution to leave just zinc Weigh zinc Volume of zinc = weight of zinc / 7.14 Porosity = [(. 9*Volume basket - volume zinc)/(.9*volume basket)] * 100 It will be appreciated that the 0.9 accounts for the 10% deadspace. If necessary, the 20 deadspace may be calculated by careful washing of the anode pellet to remove gelled electrolyte, and determining the remaining volume of the anode.

In the cathode: Measure dimensions of cathode from X-ray and observation before removing 25 cathode from can (Cathode OD, Cathode ID, Cathode Height determined) Wash cathode with water to leave EMD/CMD, graphite and binder. Binder ignored as minor component and does not significantly affect cathode volume (less than error resulting from measurement) Weigh solids 30 Dissolve MnO2 out of solids by a mixture of 50% w/v aqueous HCl to leave graphite residue M&C GBP83394 SP1368.1

Weigh graphite MnO2 weight = solids weight - graphite weight Volume of MnO, = Weight of MnO2 / 4.53 Volume of graphite = Weight of Graphite / 2.25 s Porosity of cathode = [(cathode vol. - MnO2 vol. - Carbon vol.) /cathode vol.] * 100 It will be appreciated that more sophisticated chemical or mechanical methods may be used, if desired, and are well within the ability of a person skilled in the art.

10 It will be apparent that the zinc component, for example, comprises more than one component (powder and flake) as may the manganese dioxide (EMI) and CMI}), but this has no practical effect on determination of porosity.

It will also be appreciated that the density of the KOH solution, or electrolyte, 5 will vary according to KOH content. However, KOH solution density is not important to the present invention. In general, densities of compounds can be found in the Handbook of Chemistry Physics.

Cells of the present invention as illustrated herein are assumed to have a volume 20 of 6.2 ml, and to be AA cells, unless otherwise stated. However, it will be appreciated that the present invention extends to all cells, including AARON, AAA, AA, C, D and 9V, for example, as well as other cell types, and it will also be appreciated that suitable adjustments for capacity may need to be made. However, the principles of the present invention remain unaffected, regardless of cell type.

2s For example, the present invention may be applied in the same way, using the same ratios of cathode to anode volume, to other well known standard or non-standard cell sizes, such as AAAA whose available internal volume is approximately 1.35 my AAA whose available internal volume is between about 2.65 and 2.81 ml, C whose 30 available internal volume is approximately 20.4 ml and D whose available internal volume is approximately 43.7 ml.

M&C GBP83394 SP1368.1

Manganese dioxide of battery grade should be used in the present invention, and this may be from chemical, electrolytic or natural sources, with electrolytic most preferred, followed by chemical. Manganese dioxide exists in a number of different 5 crystalline structures, commonly called, "a", " 3", "a", "a" etc. We prefer to use the by form, and any MnO2 calculations herein are based upon the use of this form. Where manganese dioxide of another crystalline structure is used, the point of completion of the reaction may need to be calculated on the basis of different assumptions, in particular in relation to the meaning of "1 electron". In particular, the endpoint may be lo taken to be at the juncture between the main discharge curve and the plateau occurring at a voltage of less than l.O. In the case of the form of MnO2, this endpoint is calculated as being when all manganese is Mn+3 0.

Where other materials or reactions (for example, co-cathodes) that consume 5 water in the course of discharge are present in the cell, allowance should be made for the water consumed by these materials or reactions. Materials involved in the cell reactions that do not consume water may be disregarded for the purposes of these calculations. 20 Similar considerations also apply to the concentration of KOH at the beginning, before the cell has been discharged. Reaction Scheme I (supra) shows that the electrode reaction consumes one molecule of water for every two molecules of manganese dioxide consumed. However, a different reaction applies much below about 36% KOH.

25 Much below about 36 % KOH, the reaction scheme changes to Zn + 2MnO2 + 2H2O Zn(OH)2 + 2Mn00H (II) Thus, until the amount of KOH increases to about 36%, the reaction is inefficient, as 30 more water than is necessary is being consumed, so that the point of completion of 50.6% is either reached too early, or more water than is necessary is present in the cell, M&C GBP83394 SP1368.1

thereby excluding other acti\ e ingredients. The former is generally more undesirable than the latter, as full discharge cannot be achieved.

Zinc oxide (ZnO) has previously been incorporated into the cell, either in the electrolyte, which is not especially efficient, or into the cathode, as it was found that the ZnO in the anode primarily served to plate the current collector, thereby protecting it. It has been conventional to add around 3% wfw zinc oxide to the whole cell. However, it is more efficient to add it to the anode only, in which case about 0. 05% w/w in the anode achieves similar results.

When considering starting concentrations of KOH, it is generally assumed, herein, that cells start with 0% ZnO concentration in the cathode. However, it has been established that ZnO affects the starting concentration of KOH, insofar as a higher starting KOH concentration is needed to still reach a final concentration of about 50%.

15 Accordingly, if 3% ZnO w/w whole cell (by incorporation with electrolyte) is used, then the starting KOH concentration is preferably about 37% wfw solution, if it is 2% w/w, then between 36 and 37% w/w solution is preferred, and at 1%, 35-37% w/w solution is the preferred range.

20 As noted above, the concentration of KOH rises during the life time of the cell, with an effective cut-off at just over 50.6% KOH, after which the cell fails quickly. The more water there is in the cell, the higher the starting concentration of KOH there can be, assuming a point of completion of 50.6%. If the initial amount of water is reduced, then the concentration of KOH must also be reduced' if it is intended not to exceed the 25 point of completion of 50.6%.

The calculation of final KOH will depend on the starting characteristics of the cell. When a cell is first constructed, the following are know-e: The weight. of MnO2 in the cell we The initial weight of electrolyte in the cell w2 The average initial KOH. concentration in the cell z /0 M&C GBP83394 SPl368.1

The final KOH concentration (i.e., at the end of the 1 e discharge) is calculated based on the assumption that all of the MnO2 is discharged to MnOOH It does not matter whether this is true, as it has been found that the advantages of designing a cell to have a final KOH concentration of about 50% are still obtained, regardless of the fmal depth of discharge of the cell.

Which cell reaction applies is dependent on the average initial KOH: 2MnO2 + lH2O + Zn = 2MnOOH + ZnO (I) 0 2MnO2 + 2H2O + Zn = 2MnOOH + Zn(OH) 2 (II) The calculations herein assume 100% Reaction (I) occurs when initial OH- is > 8N and that 100% of Reaction (II) occurs when initial OH- is < 6N. It will also be appreciated that the exact initial KOH concentration upon which the 8N and 6N calculations are based will depend on what else is dissolved in the KOH, such as ZnO or silicate, for 5 example.

For example, everything else being equal, at 0.05% w/w ZnO in the anode, > 8N OH- corresponds to >36% initial KOH, and <6N corresponds to <29%; at 3% ZnO dissolved in the KOH throughout the cell, then >8N OH- corresponds to >38% initial 20 KOH, and <6N OH- corresponds to <31% initial KOH; while, if nothing else is dissolved in the electrolyte (only KOH), then > 8N OHcorresponds to >34% initial KOH and <6N corresponds to <27% initial KOH.

It is assumed that for any pH between 6-8N OH- that the change from Reaction 25 (I) to Reaction (II) occurs linearly.

Thus: Calculation of final KOH cone.

100% Reaction (I) 0% Reaction (II) >8N OH % of Reaction (I) = a% = 100% % of Reaction (II) = (100-a)% = 0% M&C GBP83394 SP1368.1 0% Reaction (I) 100% Reaction (II) <61's,T OH % of Reaction (I) = am = 0%

% of Reaction (II) = ( 1 00-a) /0 = 100% 50% Reaction (I) 50% Reaction (II) 7N OH % of Reaction (I) = a% = 50% % of Reaction (II) = (100-a)% = 50% Ott. of H20 consumed by Reaction (I) = t(No. of electrons) x (a7100) x (0.5 x Mol. Wt 5 Water)] / (Mol. Wt MnO) x we = Nv3 Wt. of H2O consumed by Reaction (II) = t(No. of electrons) x (100-a)/100] x t(1.0 x Mol. Wt Water) / (Mol. Wt MnO2) x we] = w4 The above equations are empirical, but results generally accord with these equations.

No. of electrons = 0.925 Mol. Wt of Water = 18 Mol. Wt of MnO2 = 86.93 Final Wt of electrolyte = W2 - w3 -W4 = Ws Wt. of KOH solid = zoo /100 x w2 = w6 Final KOH concentration = W6 / W5 x 100 What is surprising is that there is no need to take any account whatever of the 2nd electron reaction. This reaction generally takes place after the first electron reaction, and can yield extra power from the cell. However, for optimization of the cell, it has 5 now clearly been established that no account need be taken of the 2nd electron reaction.

It has been found that particularly useful separators for use in the present invention employ separators comprising a copolymer of: (1) an ethylenicaily unsaturated carboxylic acid of formula (I): M&C &BP83394 SP1368.1

R1> <R3

R: A-COOH (1)

(where: R1, R2 and R3 are the same as or different from each other and each represents a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms or an aryl group; and A represents a direct bond or an alkylene group having up to 8 carbon atoms) or a salt or ester thereof; and (2) an aromatic compound of formula (II): R4 R5 <R6 R7 (where: R4, R5 and R6 are the same as or different from each other and each represents a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms or an aryl group; 10 and R7 represents a sulphonate or carboxylate group and balancing cation) or the separator comprises a homopolymer of said aromatic compound of formula (II). In general, it is preferred that A is a direct bond and Ri R7 are all hydrogen.

The copolymer may be used by itself as a separator, in which case it is IS preferably used to form the separator in situ in the cell, or it may be used as a coating on a porous substrate (for example traditional separator paper), in which case it can allow thinner paper andlor fewer layers to be used.

Particularly preferred copolymers are those comprising acrylic or methacrylic 20 acid and a styrenesulphonate, and most preferred is a copolymer of acrylic acid and a styrenesulphonate, optionally with one or more other monomers, but preferably without.

Most preferred is a copolymer of acrylic acid and sodium styrenesulphonate.

Alternatively, a homopolymer of sodium styrenesulphonate may be used.

M&C GBP83394 SP1368.1

Where the copolymer or homopol mer alone is to be used as a separator, it is preferably sprayed as a solution or dispersion in situ in the cell. Thus, the cell is partially assembled, one of the anode and cathode being inserted into the cell housing.

The solution or dispersion of the copolymer or homopolymer is applied, e. g. by 5 spraying, onto that anode or cathode and allowed to dry, and then the other of the cathode and the anode is inserted into the cell, and the cell is completed.

Alternatively, and as used in the Examples herein, the copolymer or homopolymer is supported on a porous substrate of the type commonly used as a I O separator in electrochemical cell technology, also referred to herein as separator paper, although the substrate need not actually be paper. The copolymer or homopolyrner may be applied as a coating to one or both sides, but preferably only one, for ease of application, or it may be soaked into the substrate. In either case, it is applied as a solution or dispersion and then dried (by removal of solvent, e.g. by evaporation), 15 typically by steam drum drying, or coagulated as described above.

The apparatus used for coating may be any conventional coating apparatus, and many forms of such apparatus are available commercially. The apparatus used herein was a Dixon Pilot Coater, manufactured by T.H. Dixon & Co. Ltd., Letchworth, Herts, 20 England, and this, or equivalent full scale apparatus, may be used in practicing the present invention.

In particular, the advantage of this type of separator is that a single layer of separator paper, coated or impregnated with this copolymer or homopolymer, is the 25 only separator that is required to form a useful cell resistant to shorts. The art uses double layers of separator which, especially in smaller cells, takes up valuable space which could otherwise be given over to active material.

Any suitable or conventional separator material may be employed in the present 30 invention. Examples of suitable materials include the mixtures of polyvinyl alcohol (vinylon), and mercerised hardwood fibre sold as VLZ75 and VLZ105 (respectively M&C GBP83394 SP1368.1

about 75 and 105 1lm thick) by Nippon Kodoshi Corporation (NKK), the similar material sold by Hollingsworth and Vose and the mixture of Iyocell rayon fibre, polyvinyl alcohol fibre, matrix fibre and binder fibre sold by Freudenberg.

5 Accordingly, in a preferred embodiment, there is provided an electrochemical cell comprising a single layer of separator paper, coated and/or impregnated with a copolymer, or homopolyrners, as defined above.

It will be appreciated that many factors must be taken into account when 0 constructing an alkaline electrochemical cell. In the present invention, it is assumed that the cell generally conforms to the principles of cell manufacture, and it is envisaged that the cathode is of a solid nature, rather than semi-solid. Any standard ingredients may be used in cells of the present invention, including binders and anti-gassing agents, for example. The accompanying Examples were formulated without binders.

The invention is further illustrated by the following Examples. The initial concentration of KOH ranges between 36 and 42% but is selected so that the final concentration, after 1 e discharge, is 50.6%. Likewise, the Ah, and any reference to Ah (Ampere hours) herein, is calculated based on the assumption that 1 e reaction goes to 20 completion (0.925 e) but that the second electron reaction does not take place, so that capacity is predicated on the first, and main, reaction only. Unless otherwise specified, Ah relates to the capacity of the cathode. The separator used was a layer of VLZ75 paper coated with a copolymer of acrylic acid and sodium styrenesulphonate in a ratio of 20:80 w/w at a rate of 30 gem. The MnO2 used was GlIU EMD and the graphite was 25 Superior GA17.

In the Examples, the cells were subjected to the following tests, using a standard test machine Model No. BT2043 from Arbin Instruments, 3206 Longmire Drive, College Station, TX77845, USA, and software MITS97, also from Arbin Instruments.

M&C GBP83394 SPI368.1

lA/Cont./l VO In this test, the electrochemical cells were discharged through a resistance of 1 f2 at a constant current of 1 A continuously, until an endpoint voltage of 1 V was reached. The results are reported in minutes (m), in common with the other tests.

l W/Cont./ I VO In this test, the electrochemical cells were discharged at a rate of l W continuously, until an endpoint voltage of 1 V was reached. The results are reported in minutes (m).

lo 1 WO/30rnl1 2h 1 VO The test cell is drained at a rate of 1.0 Watt for 30 minutes, then allowed to stand at idle for l 1 hours and 30 minutes. This cycle is then repeated until the cell reaches the l.OV end-point. 15 lW5/3s/7s/lh ld/'lVO The test cell is drained at a rate of 1.5 watts for 3 seconds then allowed to stand at idle for 7 seconds. This cycle is repeated continuously for 1 hour. The cell is then allowed to stand at idle for 23 hours. The whole cycle is then repeated until the cell reaches the l.OV end-point.

EXAMPLE 1

25 Effects of Zinc Tan Density and Flake Content on Continuous Drain Performance The effects of zinc tap density and flake content were measured on cells which otherwise conformed with the present invention. The results are shown in Table 1, below. The overall tap density was calculated as described above. The tap density of 30 each zinc type was measured using the following procedure, though any suitable method that would be recognised by the skilled person as producing the same results may be used: M&C OBP83394 SPl368.l

À Weigh an empty 5 ml graduated measuring cylinder À Add zinc to the measuring cylinder and reweigh À Tap the measuring cylinder with a rubber bung until the zinc settles to a stable level 5 À Read the volume of the zinc from the graduations on the measuring cylinder À Determine the tap density of the zinc by dividing the net weight of the zinc in the measuring cylinder by the observed volume of the tapped zinc Table 1

Cathode Cathode Anode Flake Zn Tap A/C 1 AlCont. 1 W/Cont. Difference Porosity Ah Porosity Content Density ratio 1 VO 1 VO nun.

g/cc min. niin.

28% 3.1 71% 0% 3.40 l.l9 51 55 4 28% 3.1 71% 5% 3.11 l.lD 56 61 5 28% 3.1 71% 8% 2.96 1.19 55 62 7

28% 3.1 71% 11% 2.83 l.l9 54 63 9 29% 2.9 71% 0% 3.40 1.33 53 57 4

29% 2.9 71% 3% 3.22 1.33 56 62 6

29% 2.9 71% 5% 3.11 1.33 59 66 7

29% 2.9 71% 8% 2.96 1.33 58 66 8

It can be seen that, with decreasing overall zinc tap density, the performance in both the 1 Amp continuous and 1 Watt continuous tests improved. Reduced overall zinc tap densities were obtained by using a mixture of zinc powder and zinc flake. Zinc 5 powder typically has a tap density of about 3.2 to 3.7 glcc. The zinc powder used in this example was a barium-indiurn-aluminiurn alloy, supplied by Union Miniere of Brussels, Belgium, and had a tap density of 3.4 g/cc. The flake zinc was product number 5454.3 from Transmet Corp. of Columbus, Ohio, USA, and had a tap density of 1.2 g/cc. At an overall tap density of 3.11 g/cc, corresponding to a flake level of 5 /0, based on the total 20 weight of zinc, performance in the 1 Amp continuous test peaks, while, in the 1 Watt continuous test, performance continues to improve above 5%, with no indication that there is a drop-off. In addition, those cells with higher cathode porosity also show M&C GBP83394 SP1368.1

higher results in both tests, thereby emphasising the importance of maximising cathode efficiency. No advantage for a lop A/C ratio is seen.

5 EXAMPLE 2

Effect of EMD:C ratio on Continuous Drain Performance The effect of the EMD: C ratio was determined, using cells with >28% cathode 10 porosity, 71% porosity anode paste containing zinc comprising 8 weight % flake, and having an A/C ratio of 1.33. The overall tap density of the zinc was 2.96 'cc. The discharge test results are shown in Table 2, below.

Table 2

I Cathode EMD:C | lA / Cont. | 1W / Cont. Difference Ah ratio,l 1 VO 1 VO min. min. min. 2.7 15:1 1 54 65 11

2.8 20:1 1 56 66 10

2.8 23:1 38 68 10

_ 2.9 25:1 58 68 10

2.9 30:1 59 64 5

It can be seen from Table 2 that cell discharge capacity generally tends to increase with increasing EMD:C ratio. As the EMD:C ratio increases, the lAICont.

performance increases, as expected, with increasing cathode capacity. However the 20 IWlCont. performance does not show the same relationship, with a peak performance at 23-25:1 EMD:C ratio. Thus, while l Arnp continuous performance is primarily dependent on capacity, I Watt continuous performance shows some dependence on capacity, but conductivity of the cathode is an important factor.

M&C GBP83394 SP1368.1

EXAMPLE 3

Effect of Different Factors on Continuous Drain Performance The effect of varying various cell parameters is illustrated in Table 3, below. In each case, anode porosity was 71 %.

Table 3

Zn Tap 1 WO/Cont.

Cathode CathodeEMD:C A/C Ratio Flake Density lVO Porosity % /O g/cc min. > 30% 2.7-2.9 <26 >1.25

33 2.9 25 1.33 8 2.96 69

36 2.7 20 1.33 11 2.83 68

36 2.7 20 1.33 8 2.96 67

34 2.8 23 1.33 8 2.96 67

<30% 2.7-2.9 <26 >1.25

29 2.9 20 1.33 8 2.96 66

29 2.9 23 1.33 8 2.96 64

28 2.9 20 1.33 10 2.87 65

28 2.7 15 1.33 8 2.96 65

>30% <2.7 <26 >1.25

34 2.6 23 1-;33 8 2.96 62

<30% 2.7-2.9 >26 >1.25

29 2.9 30 1.33 8 2.96 65

>30% >2.9 <26 <1.25

30 3.0 25 1.22 8 2.96 64

C30% >2.9 <26 >1.25

28 3.0 20 1.26 8 2.96 63

M&C GBP83394 SP1368 1

<30% 1 >2.9 <26 1 <1.25

. 29 3.0 23 1.24 8 2.96 60

28 3.1 25 1.19 11 2.83 64

28 3.1 25 1.19 8 2.96 62

28 3.0 20 1.22 8 2.96 62

l >30% >2.9 >26 <1.25

29 3.1 30 1.19 11 2.83 62

_ <30% >2.9 >26 <1.25

_, i 28 3.1 30 1.18 8 2.96 61 j The above Table 3 is divided into 9 regions, for ease of reference.

Region l represents a generally preferred range in respect of cells intended for l Watt continuous use: 5 Cathode Porosity Cath. Ah EMD:C A/C ratio Region 1 >30% >2.7-2.9 <26 >1.25 One change from Region 1 Region 2 <30% >2.7-2.9 <26 >1.25 Region 3 >30% c2 7 <26 >1.25 0 2 changes from Region 1 Region 4 <30% >2.7-2.9 >26 >1.25 Region 5 >30% >2.9 <26 <1.25 Region 6 <30% >2.9 <26 >1.25 At least 3 changes from Region 1 5 Region 7 <30% >2.9 <26 <1.25 Region >30% >2.9 >26 >i.25 Region 9 <30% >2.9 >26 <1. 25 In the above Table 3, it can be seen that the best results for the continuous 20 constant power drain test are achieved when a balance is struck between capacity, efficiency and conductivity. This is in contrast to cells intended for continuous constant current discharge, which require only a balance between capacity and efficiency.

M&C GBP83394 SP1368.1

Where any one component is not optimal, it can be seen that a slight excess of another can compensate, such as where the cathode porosity is 33%. When it rises to 36%, efficiency improves at the expense of capacity, but can be partly compensated for by an increase in EMD:C ratio.

EXAMPLE 4

Effect of Different Factors on Intermittent Power Drain Performance The effect of varying certain parameters was measured on cells having an anode porosity of 71 %. The results are shown in Table 4, below.

Table 4

Cathode Cathode EMD:C A/C Ratio Flake Zn Tap lWS/3s/7s Porosity Ah o/o Density lh/ld/lVO % a/cc min. _ >28% >2.9 <24 >1.2S

*29 2.9 23 1.33 8 2.96 43

29 2.9 20 1.33 10 2.87 43

28 2.9 20 1.33 10 2.87 43

28 3.0 20 1.26 8 2.96 43

>28% >2.9 <24 <1.2S

29 3.0 23 1.24 8 2.96 41

28 3.0 20 1.22 8 2.96 39

28 2.9 15 1.24 8 2.96 38

>28% >2.9 >24 >1.2S

28 3.0 25 1.26 8 2.96 37

29 2.9 30 1.33 9 2.92 34

>28% <2.9 <24 >1.2S

34 2.8 23 1.33 8 2.96 32

M&C GBP83394 SP1368.1

>28% >2.9 >24 <1.25

30 3.0 25 1.22 8 2.96 37

28 3.1 25 1.19 2.92 34

28 3.1 30 1.18 8 2.96 36

.. <28% >2.9 <24 >1.25

26 3.1 20 1.19 9 2.92 33

The above Table 4 is divided into seven regions. Region 1 is the generally preferred range for the test illustrated in Table 4.

Cathode Porosity Cath. Ah EMD:C AIC ratio s Region 1 >28% >2.9 <24 >1.25 One change from Region 1 Region 2 >28% >2.9 <24 <1.25 Region 3 >28% >2.9 > 24 >1.25 Region 4 >28% <2.9 <24 >1.25 0 2 changes from Region 1 Regions > 28% >2.9 >24 <1.25 Region 6 <28% >2.9 <24 <1.25 The highest performance is obtained for the combination of >2.9 Cathode Ah, <24 EMD:C Ratio and > 1.25 AIC ratio for lW5/3s/7s/lh/ld/lV0 intermittent drain 15 power test.

EXAMPLE 5

20 Comparison of 3 Types of Wattage Test In this Example, data for 3 constant wattage tests are compared. In each case, anode porosity was 71 %. The data for the three tests were summed and expressed as a percentage of the sum of the target results obtained by a standard commercial cell in the 2s same 3 tests. The comparative standard commercial cell had a cathode porosity of 28%, M&C GBP83394 SP1368.1

cathode capacity of 2.7 Ah, AIC ratio of 1.33, anode porosity of 69%, zinc tapped density of 3.40 g/cc, and no zinc flake. The results are shown in Table 5, below.

Table 5

Cathode Cathode Zn Tap lW/Cont./ 1 W0130rn/ 1 W5/3sl7s/ onto Porosity Cathode EMD:C ID A/tiC Flake Density 1 VO 1 2h/lVO 1 h/1 d/1 VO of total /O Ah rnm a o g/cc rnin. rnin. rain. Targets Target 37 37 18 100% 28-30% 2.9-3.0 <24 >1.25

29 2.9 23 8.90 1.33 8 2.96 64 64 43 195%

29 2.9 20 8.90 1.33 10 2.87 66 58 43 191%

28 2.9 20 9.00 1.33 10 2.87 64 57 43 189%

28 3.0 20 8.73 1.26 8 2.96 63 57 43 188%

28-30% 2.9-3.0 <24 <1.25

29 3.0 23 8.73 1.24 8 2.96 60 57 41 181%

28 3.0 20 8.73 1.22 8 2.96 61 57 39 179%

28-30% 2.9-3.0 >24 >1.25

29 3.0 309.00 1.33 8 2.96 64 58 34 173%

28-30% 2.9-3.0 >24<1.25

29 - 3.0 25 8.73 1.22 8 2.96 65 57 39 182%

30 3.0 25 8.73 1.23 8 2.96 63 60 37 179%

>30% <2.9 <24 >1.25 _

34 2.8 23 8.73 1.29 8 2.96 67 58 34 175%

36 2.7 20 8.73 1.33 8 2.96 68 58 34 176%

_ 28-30% >3.0 >24 <1.25

29 3.1 30 8.73 1.18 8 2.96 61 57 36 174%

28 3.1 25 8.73 1.19 8 2.96 63 57 34 171%

<28% >3.0 <24 <1.25

26 3.1 20 8.73 1.19 9 2.92 61 57 33 171%

M&C G8P83394 SP1368.

Table 5 is divided into 7 regions. Region 1 is the generally preferred range for cells optimised for all three of the tests: Cathode Porosity Cath. Ah EMD:C A/C ratio Region 1 28-30% 2.9-3.0 <24 >1.25 s One change from Region 1 Region 2 28-30% 2.9-3.0 <24 cl.25 Region 3 28-30% 2.9-3.0 > 24 >1.25 2 changes from Region 1 Region 4 28-30% 2.9-3.0 <24 <1.25 0 Region 5 >30% <2.9 <24 >1.25 At least 3 changes from Region 1 Region 6 2830% >3.0 <24 <1.25 Region 7 <28% >3.0 <24 <1.25 Is Although it can be seen that individual high results can be achieved with higher cathode porosities, overall a cathode porosity in the region of 28-30% in combination with an EMD:C ratio of below 24%, an overall zinc tap density of 2.96 g/cc, and a flake content of about 8%, based on the total weight of zinc, provides a cell which performs excellently in all the power drain tests employed herein.

EXAMPLE 6

Flake Levels Appropriate for Different Cell TYnes 2s In accompanying Figure 1, there is demonstrated the approximate effect of zinc flake, according to the intended use of the cell. In this graph, the required weight percent zinc as flake for maximum discharge performance is shown as a function of the anode porosity for three different modes of discharge [constant resistance, constant 30 current (amps), and constant power (watts)]. For example, at a level of 71% anode porosity, if the cell is intended for use in applications represented by the industry M&C GBP83394 SP1368.1

standard (ANSI/IEC) constant resistance type discharge tests, then only very low levels of flake are of any benefit, typically no more there 1 to 2%. Where the cell is intended for use in constant current drain situations, an optimum flake level is about 4 weight % of zinc. This corresponds to an overall zinc tap density of 3.1 g/cc. However, where 5 the cell is intended for use in constant power requirement scenarios, a flake level of about 8% (overall zinc tap density of about 2.96) is optimum.

Interestingly, whatever the final intended use, the graph in Figure 1 clearly demonstrates that there is no advantage to adding flake, unless the anode porosity lo exceeds 67%, or thereabouts.

Similarly, the graph in Figure 2 shows the relationship between overall zinc tap density and anode porosity. At 71% anode porosity the overall zinc tap density can be as high as almost 3.3 g/cc without sacrificing performance on constant resistance tests, 5 whereas, on constant current discharge the best performance is with a tap density of no more than about 3.1 g/cc, and on constant power discharge the maximum overall tap density of the zinc is about 2.96 g/cc.

M&C G8P83394 SP1368.1

EXAMPLE 7

Typical Formulation of the Invention Can AA Graphite Coating yes Internal Volume cm3 6.33 Ingredients Volume cm3 6.20 Cathode Mix EMD % 94 30

Graphite % 4.10 40% KOH % 1.60

s Cathode Pellet Weight g2.70 Height cm 1.080 Cathode OD cm 1.345 Cathode ID cm 0.890 No. of pellets 4 Cathode ID in can cm 0. 85 Separator No. of Layers 1 Type VLZ75/AA:SSA M&C G8P83394 SP1368.1

Anode Paste Zinc % 68.00 Carbopol % 0.400 In(OH)3 % 0.015 ZnO % 0 034 Electrolyte % 31.55 KOH Conc. % 38.5 % of zinc as flake % 10 Anode Paste Wt g 6.90 Electrolyte Addition Electrolyte Conc. %38.5 Pre-Addition g1.49 Post-Addition g0.21 Calculated Items Cathode Ah Ah2.90 Cathode Porosity %29.2 EMD:C ratio 23 Anode Ah Ah3.86 Anode Porosity %70.9 Anode/Cathode Ah ratio 1.33 Final KOH Conc. %50-51 Cathode Ah/Ingredients Vol ratio 0. 468 M&C G8P83394 SP1368.1

Claims (22)

CLAIMS:

1. An alkaline electrochemical cell having a manganese dioxide based cathode and a zinc based anode, the cathode having a porosity equal to or greater than 27% and the s anode having a porosity equal to or greater than 69%, the zinc of the anode consisting of zinc having a calculated overall tap density of less than 3.2 g/cc.

2. A cell according to claim 1, wherein the maximum calculated overall tap density ofthe zinc is 3.19 g/cc when the anode porosity is 69% and 0. 06 g/cc lower for every lo 1% increase in anode porosity over 69%.

3. A cell according to claim 1, wherein the maximum calculated overall tap density of the zinc is 3.13 g/cc when the anode porosity is 69% and 0. 085 g/cc lower for every 1% increase in anode porosity over 69%.

4. A cell according to any preceding claim, wherein the overall tap density of the zinc is from 2.83 to 2.96 inclusive and the anode porosity is about 71%.

5. A cell according to any preceding claim, wherein the zinc comprises at least 4%, 20 by weight, uniformly shaped, low-density zinc particles having a tap density less than 2.5 g/cc.

6. A cell according to claim 5, wherein the uniformly shaped, low-density zinc is zinc flake.

7. A cell according to claim 6, wherein the flake thickness dimension is at least 10 times smaller than the flake length and width dimensions.

8. A cell according to any of claims 5 to 7, wherein the uniformly shaped, low 30 density zinc forms at least 1.5% w/w per 1% porosity of the anode over 67%.

M&C GBPS3394 SP1368.1

9. A cell according to claim 8, wherein the uniformly shaped, low-density zinc forms at least 2% w/w per 1% porosity of the anode over 67%.

l O. A cell according to any preceding claim, wherein about 8% to 11%, by weight, s of the zinc is zinc flake and the anode porosity is about 71%.

11. A cell according to any preceding claim, wherein the MnO2: C weight ratio is no higher than about 26:1.

10

12. A cell according to claim 11, wherein the MnO2: C weight ratio is between 20:1 and25:1.

13. A cell according to claim 12, wherein the MnO2: C weight ratio is between 22:1 and 24:1.

14. A cell according to claim 13, wherein the MnO2: C weight ratio is about 23:1.

15. A cell according to any preceding claim, wherein the anode porosity is about 71%.

16. A cell according to any preceding claim, wherein the cathode porosity is between 28 and 36%, inclusive.

17. A cell according to any preceding claim, wherein the ratio of cathode capacity: 2s cell volume is in the range of 0.44 to 0.49 Ah/cm3, when cathode capacity is calculated based upon an assumed reduction of all manganese in the manganese dioxide to Mn+3 0.

18. A cell according to claim 17, which is an LR6 cell and has a cathode capacity of 2.7 Ah to 3.0 Ah.

M&C G8P83394 SP1368.1

19. A cell according to claim 18, which has a cathode porosity in excess of 30%, cathode capacity of between about 2.7 and 2.9 Ah, inclusive, and MnO2:C ratio of between 20:1 and 25:1.

5

20. A cell according to claim 19, which has a cathode porosity of between about 28 and 30%, inclusive, cathode capacity between 2.9 and 3.0 Ah, and MnO2:C ratio between 20:1 and 23:1.

21. A cell according to any preceding claim, wherein the manganese dioxide 0 comprises electrolytic manganese dioxide.

22. A cell according to any preceding claim, said cell comprising electrolyte containing KOH, and which, prior to discharge, has a KOH concentration selected such that, after the cell has been discharged to a depth of discharge equivalent to that required 5 to reduce all manganese in the manganese dioxide to Mn+3 0, the calculated KOH concentration is about 50% w/w, preferably between 49.S and 51.5% (w/w solution).

M&C Gsp83394 SP1368.1