GB2377077A

GB2377077A - Optimised alkaline electrochemical cells

Info

Publication number: GB2377077A
Application number: GB0115587A
Authority: GB
Inventors: Christopher Fred Randell
Original assignee: Ever Ready Ltd
Current assignee: Ever Ready Ltd
Priority date: 2001-06-26
Filing date: 2001-06-26
Publication date: 2002-12-31
Also published as: GB0115587D0

Abstract

Alkaline electrochemical cells having a cathode porosity equal to or greater than 33% have substantially increased performance characteristics. The cathode may be manganese dioxide, the anode may be zinc and the electrolyte may by KOH.

Description

OPTIMISED ALKALINE ELECTROCHEMICAL CELLS The present invention relates to alkaline electrochemical cells having cathodes comprising manganese dioxide.

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

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 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, substantially reduced the thickness of the cell walls, reduced the thickness of the seals, and changed the nature of the labelling of the cell. In addition, they have optimised the use of the internal volume of the cell such that, typically, 95% of the internal volume of the cell is taken up with cell ingredients.

Having reached the volumetric limit on 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 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 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.

Nevertheless, there remains a desire to provide better and better 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 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 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 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 material. This semi-solid material has a high porosity and high electrolyte content, and primarily serves to reduce cathode polarisation effects. The drawbacks to this construction include the fact that there is a substantially reduced capacity in the cell and that the Mn02 : 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 material. The capacity and performance of such cells is also severely compromised by comparison with US'139.

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%. This disclosure teaches that increased anode porosities increase cell performance, provided that there is a certain amount of flaked zinc comprised in the anode. Accordingly, recent research has concentrated on increasing the porosity of the anode as much as possible, rather than looking at the cathode.

EP-A-747981 discloses cells having cathodes with porosities of 15 to 35%. The method for achieving this is similar to WO 00/30193, above, and suffers from the same problems.

Surprisingly, what we have now discovered is that the porosity of the cathode actually is important, provided that it is high enough.

Thus, in a first aspect, the present invention provides an alkaline electrochemical cell having a porous cathode comprising manganese dioxide, characterised in that the cathode has a porosity equal to or greater than 33%.

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. Components which 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 the cathode, the solids will generally effectively comprise only the Mn02 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 the calculated porosity.

In one embodiment, it is preferred that the Mn02 of the cathode be comprised substantially entirely of EMD.

Although the various means for increasing the porosity of the cathode may result in some localised variation in porosity within the cathode, it is preferred that conditions be selected such that any variation is kept to a minimum. It will be appreciated that porosity figures quoted herein are mean figures obtained by calculation.

For practical reasons, it may be difficult to exceed 45% porosity in the cathode, as decreasing material not only reduces the discharge capacity of the cell, thereby necessarily affecting the maximum performance, but decreasing solids material can also reduce the mechanical strength.

Thus, in a preferred embodiment, there is provided a cell as defined above, having a cathode porosity of between 33 and 45%. A more preferred range is greater than 35% to 40% and, more particularly, 36% to 39%.

As noted above, with increasing porosity of the cathode, the theoretical capacity of the cathode and the actual discharge capacity of the cell will necessarily drop, assuming constant volume of the electrode. However, above the minimum levels of the invention, the loss in theoretical cathode capacity is at least partially compensated by the increase in efficiency of the cathode. It is particularly those cells where the increase in cathode efficiency is useful, that are preferred in the present invention and, in the range of about 35 to 38% cathode porosity, cells of the invention are generally capable of outperforming cells of the art, regardless of efficiency.

It will be appreciated that cells having only similar, or even lower, performance than cells of the art are also envisaged, as such cells provide performance recognised as being useful, while containing significantly reduced quantities of active ingredients, thus being of benefit in reducing manufacturing costs.

As used herein, electrode capacity is the theoretical capacity of the electrode in ampère hours (Ah). Unless otherwise indicated or apparent, the specific capacity values

used herein are, for manganese dioxide, 0. 285 Ahlg for a 1 electron discharge, and, for zinc, 0.820 Ah/g.

From the point of view of performance in an AA, or LR6, cell, the advantages of the present invention are particularly observed wherein the capacity of the cathode is equal to or greater than 2.6 Ah. Below 2.6 Ah, 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 AA, or LR6, cell of the invention which has a cathode capacity of at least 2.6 Ah.

As a guide, it is preferred that the ratio of cathode capacity: cell volume for cells of

3 the present invention is in the range of 0. 42 to 0. 47 Ahlcm3, when cathode capacity is calculated in accordance with the 1 electron principle described hereinbelow. When using 3 the principle assumed in US'139, then this ratio is approximately 0. 54 to 0. 61 Ah/cm3.

In a preferred embodiment, then, there is provided a cell of the invention wherein 3 the ratio of cathode capacity : cell volume is in the range of 0. 42 to 0. 47 Ahlcm3, when cathode capacity is calculated in accordance with the 1 electron principle.

Cells of the present invention are of particular use in high drain applications, such as digital cameras, video cameras and photoflash devices. For low drain applications, cells of the invention still provide an advantage, although the reduction in capacity with increasing porosity has greater significance, so that this must be factored in accordingly when providing cells for such applications.

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% w/w. Above 50%, KOH crystallises out rapidly, removing both KOH and water from the electrolyte solution, as each molecule of KOH takes two molecules of water of crystallisation. Cells having a calculated final concentration of KOH much above 50%, therefore, fail quickly.

The primary reaction in an alkaline cell can be represented by the equation:

The cathode half cell reaction is then:

while the corresponding anode half-cell reaction is:

This half cell reaction assumes that there is sufficient water available. In any event, when the solution becomes saturated with zincate, the zinc hydroxide slowly dehydrates, and this can be represented by the reaction:

Accordingly, the total cell reaction to a discharge of 1 electron per mole of Mn02 can be represented as the equation:

Without being bound by theory, it appears that, especially in high drain situations, the half cell reactions do not have sufficient opportunity to equilibrate, so that, in the cathode for example, water is consumed while OH-is generated. The 50% above which KOH crystallises can then be reached substantially before the cell would otherwise fail under low-drain conditions.

However, it is believed that the cells of the present invention are better under high drain conditions because the increased porosity is more conducive to equilibration of the

two half cell reactions and, at the same time, effectively provides a greater reservoir, reducing the rate at which the electrolyte in the cathode approaches the 50% KOH saturation level.

In addition, cells having a calculated final concentration of KOH much below 50% will generally be inefficient, with water taking up room better taken up by active ingredients, although if KOH is too dilute, the inefficiency can be further compounded, as detailed below.

Thus, there is further provided a cell as defined above and comprising electrolyte containing KOH, which, prior to discharge, has a KOH concentration selected such that, after the cell has been discharged to a depth of discharge of one electron, the calculated KOH concentration is about 50% w/w. 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 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"active"ingredients, without taking any particular note of the amount of electrolyte. The previous understanding of electrochemical cells was purely empirical, even though the electrode reaction was known.

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 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 is not observed.

Recognising that the final concentration of KOH should be 50%, after I electron discharge, provides a firm rule for standardising electrochemical cells, so that it is now possible to establish relationships between all of the other ingredients of 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 of KOH be in the region of 50% to maximise the performance of any given cell, varying porosity had the inevitable result of varying the final concentration of KOH by varying the initial amount of electrolyte, so that the final performance of the cell was not only related to the porosity chosen, but also to the final KOH concentration. Thus, the results were meaningless.

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 has now proven possible to ascertain the effect on cell performance of varying only one parameter, such as cathode porosity.

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.

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 considered to be

more important is for the final 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 ofMnOz and zinc will generally be kept at a constant capacity ratio, so that a change in one will result in a concomitant change in the other. The ratio may be any that a given manufacturer might want to employ, and is anywhere in a range of about 1.2 : 1 to about 1.4 : 1 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.

The porosity of the cathode may be increased in any suitable manner. One method is to reduce the amount ofMnOx or carbon in the cathode mix. However, there can be a problem when Mn02 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.

Reducing the amount ofMn02 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, or maximising Mn02 and reducing the carbon content, such as by using 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 heating, and has yielded strong pellets. Carbon black provides weak pellets but with good conductivity but, surprisingly, we have found that, if it is pre-mixed with between about 5 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 manufacture.

Carbon content is generally kept to a minimum so that, what is necessary is that the amount of Mn02 be reduced relative to volume of cathode in order to provide higher porosity. For any given combination ofMnO and graphite types and morphologies, a minimum constant volume percent carbon content in the cathode can be established which will provide adequate electrical conductivity in the cathode over the porosity range of the invention. Volume percentages can be converted to weight percentages using the real densities of the materials. In an example, when using GHU EMD and Superior Graphite GA 17 expanded graphite, an EMD : graphite weight ratio of at least 20: 1 is preferred.

In general, the level of the anode paste should be about the same height as the 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 difference of no greater than 2.5% in nominal height 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 also be appreciated that the amount of anode paste 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 principles of the invention remain unchanged.

As noted above, the primary alkaline cell reaction scheme is as follows:

Although the manganese compound is shown as Mn02, 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 Mn01. 96. Thus, the valency of the manganese drops from 2 x 1.96 = 3.92, for Monos 96, to 3.0 for MnOOH. Accordingly, the actual number of electrons involved in this reaction is approximately 0.92 or, as has more accurately been determined, 0.925. This number is important in calculating capacity but, for simplicity, is referred to herein, as is conventional in the art, as"1 electron", or 1 e.

The reaction shown in scheme (1) above, generally referred to as the"first electron reaction", 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, and is of little interest except in low drain, intermittent usage. It appears 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, or 0. 925 electron in stoichiometric terms, has gone to completion (i. e., Mon+3925 is reduced to Mn).

By way of illustration, on a discharge plot of voltage as a function of time, the ampere hours are measured by calculating the area under the discharge line which, in the case of manganese dioxide, originally tends to form a 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 for the purposes of the present invention are unaffected.

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 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 calculable, provided that both the initial concentration of KOH and the electrolyte volume are 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 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 cathode that is not taken up with solids. As the solids content, volumewise, is generally easier to calculate than the non-solids, and also because porosity includes any trapped air, for example, then the calculation to determine percent porosity is generally expressed as

wherein Vs is the measured total volume of the cathode and Vs is the volume of the 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 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 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 best, be misleading and, at worst, meaningless.

Theoretical densities assumed for the electrodes in the present invention are as follows: Cathode

Component Theoretical Density Wt perlOOg Vol. perlOOg EMD 4. 53 (di) vi= wl/4. 53 CMD 4. 53 (d2) W2V2 = W2/4. 53 Graphite 2. 25 (d3) W3 V3 =W3/2. 25 Coathylene&commat;0. 92 (d4) W4 V4 = W4/0. 92 40% KOH 1. 39 (ds) 'v Vs =W5/1. 39 Another components d6 etc. We etc. V6 = w6/d6 etc.

# = 100

Coathylene&commat; is polyethylene Accordingly, the theoretical volume of the cathode is the sum of all of the ingredients = Vs = L (VI : V6) = (VI + V2 + V3 + V5 + V6 etc. ).

Essentially, the theoretical volume 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.

For the avoidance of doubt, the actual cathode volume of a cylindrical cell 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 can Cathode OD 1. 345 = ODp 1. 335 = ODe Cathode ID 0. 900 = IDp 0. 885 =IDc

2 Thus, Actual Volume = VA = H. 7T. (OD-ID) 4 While, Theoretical Volume = VA = H. 1t. (ODQ2 - IDQj 4

In the above case, whether the cathode pellet is as manufactured or"in can", the product ofOD-ID 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 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.

For cells with non-cylindrical cathodes (e. g., prismatic cells, cells with spiral wound or flat electrodes, cathodes with tapered walls, and cathodes with non-cylindrical anode cavities), the method of calculating the volume is adapted to the specific design of the cell. For the avoidance of doubt, if there are other components within the cathode (e. g., current collectors), or if there are gaps or other components between sections of the

cathode (e. g., layers of separator between stacked cathode pellets), then those gaps and/or other components do not form part of the cathode volume.

In the cathode, the Theoretical Volume of Solids = Vs = Vi + V2 + V3 + V4

Thus, Cathode Porosity = (YA-V ,) x loo VA and this is the porosity to which the present invention pertains.

It is preferred that the level of the anode paste should be about the same height as the 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 will need to contain the appropriate amount of zinc to maintain the anode: cathode Ah ratio which is assumed to be 1.33 herein, unless otherwise specified. Where other ratios are applied, then suitable adjustments to volumes, for example, need to be made, but the principles of the invention remain unchanged.

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

The amount of water can be established by the use of a modified Dean & Stark method. Apparatus is available from Quickfit & Quartz Ltd. , for example. The sample is covered with dry toluene and refluxed for 45 minutes, ensuring that a majority of the condensation takes place in the water-cooled condenser. Water is collected in a 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 crystallisation.

The amount of OH-is readily determined by soxhleting 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 OH-by standard methods. For example, HC1 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 calculated accordingly.

Together with the volume of water and the amount ofMn02 (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 cathode porosity, and as noted above, this is calculated essentially as follows: [ (Total volume-Solids volume) / (Total volume)] * 100 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 may be used to give a clear indication of the internal proportions of the cell, especially cathode height and width. This done, the cell can then be cut open, and the electrodes separated.

As a generality, for example, with the cells illustrated in the accompanying Examples, what we have found is that, in the cathode, only the manganese dioxide (EMD and CMD, where present) and carbon (usually graphite) need to be considered, when determining 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.

Accordingly, in the cathode: Measure dimensions of cathode from X-ray and observation before removing 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 . Dissolve Mn02 out of solids by a mixture of 50% w/v aqueous HC1 to leave graphite residue * Weigh graphite * Mn02 weight = solids weight-graphite weight * Volume of Mn02= Weight of Mn02/4. 53 * Volume of graphite = Weight of Graphite/2.25 Porosity of cathode = [ (cathode vol.-Mn02 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.

It will be apparent that the manganese dioxide component may comprise more than one type of Mn02 (EMD and CMD), but this has no practical effect on determination of porosity.

It will also be appreciated that the density of the KOH solution, or electrolyte, 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 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 AAAA, 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.

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 ml, AAA whose available internal volume is approximately 2.65 ml, C whose available internal volume is approximately 20.4 ml and D whose available internal volume is approximately 43.7 ml.

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 crystalline structures, commonly called,"oc","P","7","8"etc. We prefer to use the y form, and any Mn02 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 taken to be at the juncture between the main discharge curve and the plateau occurring at a voltage of less than 1.0. In the case of the y form of Mn02, this endpoint is calculated as being when all manganese is Mont30.

Where other materials or reactions (for example, co-cathodes) that consume 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 that do not consume water may be disregarded for the purposes of these calculations.

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.

Much below about 36 % KOH, the reaction scheme changes to

Thus, until the amount of KOH increases to about 36%, the reaction is inefficient, as 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, thereby excluding other active 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% w/w 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%.

Accordingly, if 3% ZnO w/w whole cell (by incorporation with electrolyte) is used, then the starting KOH concentration is preferably about 37% w/w 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.

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 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 known: The weight. ofMn02 in the cell Wl The initial weight of electrolyte in the cell W2 The average initial KOH. concentration in the cell zi% The final KOH concentration (i. e. , at the end of the 1 e discharge) is calculated based on the assumption that all of the Mn02 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 final depth of discharge of the cell.

Which cell reaction applies is dependent on the average initial KOH:

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 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 KOH, and < 6N OH-corresponds to < 31 % initial KOH; while, if nothing else is dissolved in the electrolyte (pure KOH), then > 8N OH-corresponds 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 (I) to Reaction (II) occurs linearly.

Thus: Calculation of final KOH conc.

100% Reaction (I) 0% Reaction (II) > 8N OH- % of Reaction (I) = a% = 100% % of Reaction (II) = (100-a) % = 0% 0% Reaction (I) 100% Reaction (II) < 6N OH- % of Reaction (I) = a% = 0% % of Reaction (II) = (100-a) % = 100% 50% Reaction (I) 50% Reaction (II) 7N OH- % of Reaction (I) = a% = 50% % of Reaction (II) = (100-a) % = 50% Wt. of H2O consumed by Reaction (I) = [(No. of electrons) x (a/100) x (0.5 x Mol. Wt Water)]/ (Mol. Wt Mn02) x wi-ws Wt. ofHzO consumed by Reaction (II) = [(No. of electrons) x (100-a)/100] x [ (1. 0 x Mol.

Wt Water)/ (Mol. Wt Mn02) x wi] = 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 Mn02 =86. 93 Final Wt of electrolyte = W2-W3-W4 = W5 Wt. of KOH solid = z1/100 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 optimisation of the cell, it has 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 ethylenically unsaturated carboxylic acid of formula (I) :

(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 alkylen group having up to 8 carbon atoms) or a salt or ester thereof; and (2) an aromatic compound of formula (II) :

(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; 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 R'-R are all hydrogen.

The copolymer may be used by itself as a separator, in which case it 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 and/or fewer layers to be used.

Particularly preferred copolymers are those comprising acrylic or methacrylic 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.

Where the copolymer or homopolymer 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 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 separator in electrochemical cell technology, also referred to herein as separator paper, although the substrate need not actually be paper. The copolymer or homopolymer 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), 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, England, and this, or equivalent full scale apparatus, may be used in practising 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 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 invention. Examples of suitable materials include the mixtures of polyvinyl alcohol (vinylon), and mercerised hardwood fibre sold as VLZ75 and VLZ105 (respectively about 75 and 105 05 m thick) by Nippon Kodoshi Corporation (NKK), the similar material sold by Hollingsworth and Vose and the mixture oflyocell rayon fibre, polyvinyl alcohol fibre, matrix fibre and binder fibre sold by Freudenberg.

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 homopolymers, as defined above.

It will be appreciated that many factors must be taken into account when 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.

In the following Examples, the Ah, and any reference to Ah (Ampere hours) herein, is calculated based on the assumption that 1 e reaction goes to 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 gsm. The Mn02 used was GHU EMD and the graphite was Superior Graphite GA17 expanded graphite.

FORMULATION EXAMPLE

Can LR6 (AA) Graphite Coating yes Internal Volume cm3 6.33 Ingredients Volume cm 6.20 Cathode Mix EMD % 94.30 Graphite % 4.10 40% KOH % 1.60 Cathode Pellet Weight g 2.85 Height cm 1.080 Cathode OD cm 1.345 Cathode ID cm 0.820 No. of pellets 4 Cathode ID in can cm 0.805 Separator No. of Layers 1 Type VLZ75/AA : SSA Anode Paste Zinc % 75. 900 Carbopol % 0. 300 In (OH) 3 % 0.017 ZnO % 0. 037 Electrolyte % 23.740 KOH Conc. % 36 % of zinc as flake % 0

Anode Paste Wt g 6.56 Electrolyte Addition Electrolyte Conc % 36 Pre-Addition g 1.73 Pst-Addition g 0.26 Calculated Items Cathode Ah Ah 3.07 Cathode Porosity % 33.0 EMD : C ratio 23 Anode Ah Ah 4.08 Anode Porosity % 62.7 Anode/Cathode Ah ratio 1.33 Final KOH Cone. % 50-51 Cathode Ah/Ingredients Vol ratio 0.495

EXAMPLE 2 Cells were made up as specified below, and subjected to the following test, 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. lA/Cont./lVO In this test, the electrochemical cells were discharged at a constant current of 1 A continuously, until an endpoint voltage of 1 V was reached. The results are reported in minutes (m).

Cathode Porosity Initial Final Porosity Ah EMD: C Type ID A/C KOH KOH Sep 1A > 33 3. 07 20 23% TR + Coath8. 051. 33 36 50-51 Standard 37 > 33 3.07 20 Standard 8. 051. 33 36 50-51 Standard 44 > 33 3.07 20 6.6% solid KOH 8. 051. 33 36 50-51 VLZ75/AA : SSA 47 > 33 3.06 20 Standard 8.051. 33 36 50-51 VLZ75/AA : SSA 48 TR-Sedema TR CMD made by Erachem Europe SA, rue de la Carbo BP9-B-7333 TERTRE, Belgium.

Coath-Coathylene&commat; (polyethylene)

Claims

CLAIMS: 1. An alkaline electrochemical cell having a porous cathode comprising manganese dioxide, characterised in that the cathode porosity is equal to or greater than 33%.
2. A cell according to claim 1, wherein the cathode porosity is no greater than 45%.
3. A cell according to claim 1 or 2, wherein the cathode porosity is greater than 35%.
4. A cell according to any preceding claim, wherein the ratio of cathode capacity: cell

3 volume is in the range of 0. 42 to 0. 47 Ah/cm3.
5. A cell according to any preceding claim which is an LR6 cell, and wherein the capacity of the cathode is between 2.6 Ah and 2.9Ah.
6. A cell according to any preceding claim and comprising electrolyte containing KOH, which, prior to discharge, has a KOH concentration selected such that, after the cell has been discharged to reduce the manganese in the manganese dioxide to Mn , the calculated KOH concentration is about 50% w/w.
7. A cell according to claim 6, wherein 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).
8. A cell according to any preceding claim, wherein the concentration of potassium hydroxide in the electrolyte is between about 34 and 37% (w/w solution) prior to discharge.
9. A cell according to any preceding claim, wherein the weight ratio of total water to manganese dioxide, calculated on the assumption of a 36% KOH concentration, is from 0.205 to 0.225.

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10. A cell according to any preceding claim, wherein the Mn02 of the cathode is substantially entirely EMD.
11. A cell according to any preceding claim, wherein the cathode is cylindrical.
12. A method for the manufacture of a cell according to any preceding claim, wherein the porosity of the cathode in the cell is increased by reducing the amount ofMnOz in the cathode mix but maintaining cathode pellet size, prior to assembly of the cell.
13. A method according to claim 12, wherein crushed ice or solid KOH is incorporated into the cathode mix and the mix compressed to form a pellet.
14. A method according to claim 12, wherein carbon black is incorporated into the cathode mix and the mix compressed to form a pellet, the carbon black having first been pre-mixed with between about 5 and 30% electrolyte, usefully about 10 to 15% electrolyte.