GB2376340A

GB2376340A - Method of making cathodes

Info

Publication number: GB2376340A
Application number: GB0113988A
Authority: GB
Inventors: Robert Batey; Christopher Fred Randell
Original assignee: Ever Ready Ltd
Current assignee: Ever Ready Ltd
Priority date: 2001-06-08
Filing date: 2001-06-08
Publication date: 2002-12-11
Also published as: GB0113988D0

Abstract

An electrochemical cell having an anode, a cathode and an electrolyte is made by inserting an anode and a cathode, in any order, into a housing, adding electrolyte, and then completing the cell. In accordance with the present invention, the cathode includes a solid material which is changed to a non-solid state upon the addition of electrolyte to the cell and the electrolyte is added to the cell after the insertion of the cathode, so as to increase the porosity of the cathode, said solid material being frozen carbon dioxide, frozen water, a frozen solution of the electrolytic solid or the electrolytic solid.

Description

METHOD OF MAKING CATHODES The present invention relates to a novel method of making porous cathodes, especially for use in electrochemical cells.

Electrochemical cells, commonly known as batteries, are used in a vast and ever increasing number of devices in the modem world. For some of these applications, it is desirable that the cell should exhibit maximum performance, i. e. last for as long as possible, in which case it is necessary to put as much active material in the cell as possible. On the other hand, for other applications, the important factor is the efficiency of the cell, i. e. the ability to get out of the cell as much as possible of the theoretical maximum energy. In this case, there is less need to cram the cell with active materials, and we have recently discovered that the use of a porous cathode will enhance the efficiency of such cells. However, we have found that there is a number of practical problems to be overcome if such porous electrodes are to be used, and it is an aim of the present invention to provide a solution to these problems.

The sizes of electrochemical cells are strictly controlled, and it is important, whatever else is done, that the cell should conform to internationally agreed standards as to its dimensions. If a cell does not so conform, it is likely to be unsaleable or confined to niche markets.

Electrochemical cells are made by the billions every year, using high speed assembly machinery, and so it is necessary that the various components of the cell, in addition to meeting the inherent requirements needed for a functioning cell, and in addition to the need to conform to size standards, should also be able to withstand high speed processing. Cathode materials are conventionally first formed into the desired shape and this is then inserted into the can housing the cell components. It is self-evident that a porous cathode will have less mechanical strength than a solid one, and we have found that porous cathodes, when pre-formed outside the cell, cannot be inserted into the cell by the conventional types of machine without damage or total disintegration. However, we have found that the use of porous cathodes may give several advantages in performance of electrochemical cells, especially alkaline manganese electrochemical cells. Nonetheless, because of the problems outlined

above, until now, it has not been possible to use a porous cathode in a practical cell construction.

Attempts have been made to include extra binders in the cathode, but this adds to the cost and the binder is an extra inactive material to be included, which is undesirable. Moreover, if sufficient binder is incorporated to have an effect, it adds its own processing problems. Another way of reducing porosity would be to include in the cathode mix a proportion of chemical Mn02 (CMD), which may be naturally porous, in addition to the electrochemically produced Mn02 (EMD) which is the preferred material for cathode mixes in alkaline manganese cells. CMD has a high porosity and low bulk density relative to EMD, and so the resulting pellets are more porous and less dense than those using EMD alone. However, although the pellets that are produced are strong, performance on a 1 A continuous discharge test is poor.

It would also be expected that performance on other tests would be poor as CMD is known to be less effective than EMD in electrochemical cells.

If an attempt is made to produce low density pellets by using less compaction, whilst performance on 1A continuous discharge is unaffected, the pellets that are produced are too weak to handle.

We have now discovered that a porous cathode of the desired strength can be made by incorporating in the cathode mix a solid material which is changed to a nonsolid state upon the addition of electrolyte to the cell.

Thus, the present invention consists in a method of making an electrochemical cell having an anode, a cathode and an electrolyte, which comprises inserting an anode and a cathode, in any order, into a housing, adding electrolyte, and then completing the cell, characterised in that the cathode includes a solid material which is changed to a non-solid state upon the addition of electrolyte to the cell, and that the electrolyte is added to the cell after the insertion of the cathode, said solid material being frozen carbon dioxide, frozen water, a frozen solution of the electrolytic solid or the electrolytic solid.

In battery technology, it is common to use the word'electrolyte'to refer to the solution which is included in an electrochemical cell to conduct electricity, rather than

the solid material which is dissolved to form this solution. In the present specification, we adhere to this convention, and, accordingly, the word'electrolyte'as used herein refers to this solution. Where it is necessary to refer to the solid which is dissolved, this is referred to as the'electrolytic solid'.

In changing to a non-solid state, the originally solid material leaves voids or pores, which increase the porosity of the cathode. However, because that material was solid when the cathode was inserted into the cell housing, it is possible to make a cathode assembly which retains all of the strength and other desirable properties of conventional cathodes.

The porosity of the cathode may be varied over a wide range by the method of the present invention. However, for practical use, we prefer that the porosity of the cathode, determined as described below, should be at least 26%.

The term"porosity", as used herein, relates to the volumetric amount of nonsolids 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 usually soluble in the electrolyte solution, including gellants.

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 any event, the 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 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.

Enhanced performance characteristics are readily observable with increasing porosity of either electrode, although there is little improvement unless both porosities are at least at a certain minimum level of the present invention. At these levels, there is immediate and rapid improvement of the performance of the cell, with both

electrode efficiency and lifetime dramatically increasing. In particular, there appears to be little improvement to be gained by increasing cathode porosity beyond 27% and, particularly, beyond 28% unless anode porosity is at least 69% and, particularly, at least 70%, and vice versa. Accordingly, cells in which the cathode porosity is at least 28% and anode porosity is at least 70% are preferred.

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 a cathode porosity of 26% and an anode porosity of 69%, the loss in capacity is more than compensated by the increase in performance of the cell for a short range, and this increase in performance is highly significant. Indeed, without being bound by theory, the porosities of the cathode and the anode actually appear to potentiate each other, above the limits specified above. 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 electrochemical cell reaction to proceed more freely, thereby more than compensating for loss in capacity.

The point above which little or no benefit can be seen will depend on the overall characteristics of the cell. However, in general, this point appears to be around 71% to 76% and, particularly, 72% to 74% for the anode, and around 31% for the cathode. Once the cell has a cathode porosity of 31% and an anode porosity of 74%, any increase in efficiency of the cell to be obtained by increasing either figure is generally outweighed by loss in performance.

It will be appreciated that cells having higher porosities are envisaged by the present invention, to the extent that those cells whose performance is superior to cells of the art are covered. Such cells will generally be within a range of about 5% extra porosity over the maxima noted above, before loss of capacity reduces performance to levels found in the art. However, cells having increased porosity and 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.

Several methods are available to achieve the results of the present invention.

For example, the solid material incorporated into the cathode mix may be a substance that is liquid or gaseous under normal ambient conditions (say 180C and atmospheric pressure) but whose temperature has been reduced so that it is frozen solid. Such a material is carbon dioxide, dry ice. Upon the addition of electrolyte, which would normally be at ambient temperature, the solidified gas will revert to the gaseous state and will escape the cathode through the pores naturally present, leaving larger pores where it once was. This has the advantage that it leaves behind nothing that would react with or interfere with the components of the cell.

If dry ice is to be used, the amount used by volume will normally be directly equivalent to the porosity which it is desired to achieve, preferably from 26% to 31 % by volume of the cathode, so as to leave the required porosity, and more preferably from 27 to 30% by volume. Of course, greater or smaller amounts could be used, but these would result in greater or lesser porosity than we recommend. If, in addition to the incorporation of dry ice, other steps, whether in accordance with the present invention or not, are taken to increase porosity, then correspondingly lower amounts of dry ice would be employed. This would apply, in particular, if CMD, which may be naturally porous, is employed in addition to EMD.

Another option is to use ice or frozen electrolyte as the solid material which is changed to a non-solid state upon the addition of electrolyte to the cell. Upon the addition of electrolyte, which will commonly be an aqueous solution of an alkali, to the cell, the ice or frozen electrolyte will melt and will merely dilute or add to the electrolyte. Where ice is used, it will, therefore, be necessary that the electrolyte added to the cell should be more concentrated than is required in the finished cell, to allow for its dilution by the melted ice.

If ice is to be used, the amount is preferably from 7 to 10% by volume of the cathode, so as to leave the required porosity, more preferably from 8 to 9% by volume, which would equate to a porosity of from 30 to 27%. Of course, greater or smaller amounts could be used, but these would result in greater or lesser porosity than we recommend. If frozen electrolyte is to be used, the amount is preferably from 9 to 14% by volume of the cathode, so as to leave the required porosity, more preferably from 10.5 to 12% by volume, which again would equate to a porosity of

from 30 to 27%. We prefer to use frozen electrolyte or, as suggested below, the electrolytic solid or a similar compound, because when potassium hydroxide is eventually added there is ion-exchange of protons in the MnO. In certain circumstances this can give an acid wave through the cathode, which results in corrosion at the can wall. Using potassium hydroxide or potassium carbonate in the cathode means that ion-exchange takes place uniformly with no acid wave and therefore no corrosion of the can wall.

A further, and preferred, alternative is to use the electrolytic solid as the solid material which is changed to a non-solid state upon the addition of electrolyte to the cell. Then, when the electrolyte is added to the cell, the electrolytic solid dissolves in the electrolyte, leaving pores or voids. The electrolyte added should, in this case, be more dilute than is needed in the final cell, and it may even be that water alone need be added. This has the advantage that no material which would not normally be in the cell need be used, and that any risk of an acid wave will be avoided.

The electrolytic solid used in the majority of alkaline manganese electrochemical cells is potassium hydroxide, and this is preferably used as the electrolytic solid and as the solid material which is changed to a non-solid state upon the addition of electrolyte. It would, however, be possible to have different alkaline materials for these two purposes, although there is little or nothing to be gained from so doing.

Although potassium hydroxide is the electrolytic solid of choice for alkaline manganese cells, for other types of cell, it may be possible or desirable to use other alkaline materials as either or both of the electrolytic solid and the solid material which is changed to a non-solid state upon the addition of electrolyte. Examples include other alkali metal hydroxides such as sodium hydroxide and lithium hydroxide, and other alkali metal compounds such as potassium carbonate. Potassium carbonate may also be used as the solid material for alkaline manganese cells.

The final concentration of alkali metal hydroxide in the finished cell before discharge (the concentration changes as the cell is discharged) is preferably from 34% to 42% (w/w solution), more preferably from 35% to 37% (w/w solution) and most preferably about 36% (w/w solution). In particular, we prefer that the total amount of

the alkali metal hydroxide solution used should be in accordance with the guidance given in British Patent Application No. 0015003. 7, so that 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 after this discharge is between 49.5 and 51.5%, most preferably about 50.6%, (w/w solution). If, as is preferred, the alkali used as the electrolytic solid is KOH, in calculating the amount of KOH to be added. care needs to be taken to make allowance for or exclude moisture picked up from the air, in view of the deliquescent nature of KOH.

If the preferred volume of pores is to be introduced by the method of the present invention and the preferred amount and concentration of electrolyte are to be used, then the preferred concentration of electrolyte to be added to the partially assembled cell to dissolve the solid KOH or other electrolytic solid is preferably such as to give a calculated concentration of potassium hydroxide after discharge of between 49.5 and 51.5%, most preferably about 50.6%, (w/w solution).

In addition to the solid material which is changed to a non-solid state upon the addition of electrolyte, the cathode of the present invention may contain all the other components of a cathode, which will vary depending upon the nature of the electrochemical cell, as is well known in the art. For example, for an alkaline/manganese cell, the cathode would normally contain: manganese dioxide, commonly in an amount of from 79 to 85% by weight; carbon, which serves as an electronic conductor, in an amount of from 7 to 10% by weight; electrolyte, for example aqueous KOH as noted above, for example in an amount from 7 to 10% by weight; and, if desired, a binder to keep the ingredients together. However, these components and quantities are given by way of example only and may be varied as desired depending upon the requirements of the electrochemical cell being constructed.

In addition to the cathode, a typical electrochemical cell would include an anode, a separator between the cathode and anode, a current collector, a can housing all of these components and a seal assembly to seal the cell. All of these components and their assembly are well known to those skilled in the art, and are described, for example, in'Handbook of Batteries'Second Edition by David Linden, published by McGraw-Hill, 1995, the disclosures of which are incorporated herein by reference.

Broadly, for an alkaline manganese cell, there will be an anode and a cathode separated by a separator, and contained within a can, sealed with an appropriate seal.

In addition, there will be an electrolyte, normally an aqueous solution of an alkali, e. g. an alkali metal hydroxide, such as potassium hydroxide, in a concentration from 33 to

A') 0/-'rk-cli t x iE,. Li L/U. iL. A. 1, final weight %. The amount of potassium hydroxide will preferably be such as to give a final potassium hydroxide concentration after discharge of the cell to the one electron level of from 50 to 51%, most preferably about 50. 6%.

The anode may be in the form of a paste containing as the main active component zinc. In addition, it will generally contain a proportion of the electrolyte, normally an aqueous solution of potassium hydroxide, to form a paste. We prefer to incorporate a binder, such as a carbomer, for example Carbopol&commat; 940 (from B. F.

Goodrich Specialty Chemicals, Cleveland, Ohio, U. S. A. ), and other ingredients, such as zinc oxide and/or a gassing inhibitor, e. g. indium hydroxide, may also be included, if desired, as is well known in the art.

The separator is required in order to prevent physical and electrical contact between the cathode and anode, while permitting ionic transfer. Moreover, it is necessary that the separator should prevent growth of zinc oxide deposits (dendrites) which could lead to shorting and thus abrupt and premature failure of the cell.

Various materials, commonly referred to as'paper'are normally used to make such separators, typically paper sheets or cellophane films disposed between the electrodes.

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: R, R 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) :

(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 R 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 or homopolymer 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.

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 um thick) by Nippon Kodoshi Corporation (NKK), the similar material sold as by Hollingsworth and Vose and the mixture of lyocell rayon fibre, polyvinyl alcohol fibre, matrix fibre and binder fibre sold by Freudenberg.

As noted above, by"porosity"is meant the relative amount, v/v, of the electrode in question 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 Vx is the measured total volume of the electrode 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.

The theoretical densities assumed for the cathode in the present invention are as follows: Cathode

Component Theoretical Density Wt per lOOg Vol. per lOOg EMD 4.53 (dol) w ; = wl/4. 53 CMD 4.53 (d2) W2 V2=W2/4. 53 Graphite 2.25 (d3) W3 V3 = w3/2. 25 Coathyleneg 0.92 (d4) W4 v4=w4/0. 92 40% KOH 1.39 (ds) w Vs = wsl1. 39 Another components d6 etc. W6 etc. V6 = W6/d6 etc.

#=100 Coathyleneg is polyethylene

Anode

Component Type Theoretical Wt per lOOg Vol. per lOOg Density zinc Solid 7.14 (d7) w7 v7=w7/7. 16 Carbopol940 Liquid 1.41 (dg) w & vs=W8/1. 41 Indium hydroxide Solid 4.60 (d9) w9 V9 = w9/4. 60 ZnO Liquid 5.61 (duo) w10 v10=w10/5.61 36% KOH Liquid 1. 35(d11) w11 v11=w11/1. 35 Component x ? d12 w12 v12=w12/d12 etc.

In which'x','7'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

ingredients = Vx = X (vl : V6) = (VI + V2 + V3 + v$ + V6 etc.).

Likewise, the theoretical volume of the anode =

In the case of the cathode, 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 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 = ODc Cathode ID 0.900 = IDp 0.885 = IDe

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

In the cathode, the Theoretical Volume of Solids = V5 = vs + V2 + V3 + V4 Thus, Cathode Porosity = () x 100 ,) x 100 VA

and this is the porosity to which the present invention pertains.

In the anode, VL = Volume of Liquids = vg + VIO + vn Vs = Volume of Solids = V7 + V9 so that the Theoretical Anode Porosity = íYI - V J x 100 = V L x 100 ~,) x 100=-YLx 100 VT VT

and it is the theoretical porosity to which the anode paste is made up.

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.

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 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% and 10% while, with top filling, the margins are between about 8% and 17%.

The porosity of the anodes used in the electrochemical cells 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 was made. Thus, the porosity 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, the most accurate method is to take a core sample, and perform the analysis described below. As a rougher guide, however, the 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 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 - 1%.

In making up cells of the present invention, the theoretical volume of the components of the anode is first calculated, per 100g 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%, and this is the amount of anode paste used. Thus, if porosity is simply taken as a measure of the total solids in relation to the volume of the basket, then the resulting, apparent porosity of the anode, assuming about 10%

deadspace, will be about equal to theoretical porosity/ (100-10)] *100. In other words,

apparent porosity z theoretical porosity ±11%

As a rough guide, then, the actual porosity of the anode from a cell off the shelf will be equal to the apparent porosity divided by 1.11. However, as noted, this will depend on the deadspace of the cell. As noted above, the porosity is the porosity of the anode itself, and not the porosity of the anode + deadspace.

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 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 being 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 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 anode 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 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 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 K2C03, 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, HCI of known molarity and together with phenolphthalein as an indicator may be used. In this method, it is assumed that all OH-is KOH, and weights calculated accordingly.

Together with the volume of water and the amount of Mn02 (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.

Turning to electrode porosities, these are 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 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.

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 porosities.

The remaining components are either present in vanishingly small quantities, or are not particularly dense and present in small quantities, or form a 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 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 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 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 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 z 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 zinc component, for example, may comprise more than one component (powder and flake) as may the manganese dioxide (EMD and CMD), but this has no practical effect on determination of porosity. Such considerations also apply to other components, such as the binder used.

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

The invention is further illustrated by the following non-limiting Examples. In the Examples, the electrochemical cells used are of internationally recognised size LR6 (AA), being the most common size electrochemical cell in use today. This has a nominal internal volume of 6.3 ml and an internal volume available for ingredients of approximately 6.2 ml-the actual volume available may vary somewhat from this

value depending upon the exact construction of the cell. However, the results reported here are fully scaleable to other cell sizes, making appropriate allowance, as is well known in the art, for cathode inner and outer diameter and cell height. 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, LR03 (AAA) whose available internal volume is approximately 2.65 ml, LR14 (C) whose available internal volume is approximately 20.4 ml and LR20 (D) whose available internal volume is approximately 43.7 ml.

In all the completed electrochemical cells used in these Examples, the cell used had an anode having the composition shown below and had a separator comprising a single layer of VLZ75 (a known separator paper, about 75 u. m thick) available from Nippon Kodoshi Corporation (NKK), coated with an 80: 20 (molar) copolymer of acrylic acid and sodium styrenesulphonate in an amount of 40 gsm. The interior of the can had a coating of graphite.

The anodes used had the following composition:

Zinc * 75.200 weight % Carbopol 940 0. 310 weight % In (OH) 3 0. 017 weight % ZnO 0.037 weight % Electrolyte ** 24.440 weight % * The zinc powder was a barium-indium-aluminium alloy (available from Union Miniere, Brussels, Belgium).

** The electrolyte was a 40% w/w aqueous solution of potassium hydroxide.

2 % of the weight of zinc used was as flake, the remainder was a powder. The zinc powder was a barium-indium-aluminium alloy. Zinc flake is available from Transmet Corporation of Columbus, Ohio, U. S. A.

The ratio of anode to cathode in each case was such as to give a ratio of anode Ah (Ampere hours) to cathode Ah of 1.33 (calculated on the assumption that the first electron reaction goes to completion but that the second electron reaction does not take place; i. e. , the manganese is reduced to an average ofMn ).

All cells were designed to give a final KOH concentration at the deemed failure point, and on the assumption of a 1 electron discharge of the manganese dioxide (to Mn+3 0), of 50-51%, and all had an initial KOH concentration of 36% (w/w solution).

All cells had a cathode inner diameter (ID) when in the can of 8.05 mm which, because of the compression resulting from fitting the cathode into the can was somewhat less than the ID of 8.20 mm out of the can. The cathode pellets thus had an outer diameter of 13.45 mm, and a height of 10.80 mm, with 4 such pellets being stacked in each AA cell. The weight of each cathode pellet was 3.01 g.

The CMD used was Sedema TR, available from Erachem Europe S. A. , Rue de la Carbo BP9-B-7333 Tertre Belgium.

Where a binder was used, this was coathylene, a polyethylene binder available from Plast-Labor S. A. , Bulle (FR) Suisse.

EXAMPLE 1 Preparation and testing of cathode mixes Cathode mixes suitable for use in an alkaline/manganese electrochemical cell were produced by mixing the components shown in Table 1 in a conventional manner.

In the case of Experiments 1 to 4, the KOH was added as an aqueous solution in the amount shown (by weight of the finished cathode pellet) and at the concentration shown (w/w solution). In the case of Experiment 5, the KOH was added as a solid, which, in the form in which it is normally commercially available, naturally contains only 85% KOH. The balance is minor quantities of impurities together with some water, since KOH is highly deliquescent. All cathode mixes were prepared in such a way as to give a calculated cathode porosity of 35% by volume. The cathodes were then used in conventional alkaline/manganese cells as described above.

The cathode pellets produced were tested for strength by the method shown below, and the results are reported in Table 1 as relative values based on the formulation of Experiment 1 as 100%.

Test method for measuring strength.

The pellets were carefully placed in pairs with the pellets of each pair exactly 2 cm apart and balanced on their curved edges. Then a shim of known weight was placed on top of each pair of pellets. Further different weights were then added on top of each shim until the cathode pellets broke. The average total weight of the shim & extra weights is the weight recorded in Table 1.

The assembled cells were tested for performance using the lAmp continuous test (I A/Cont./I VO). In this test, the electrochemical cells were discharged through a resistance of 1 Q continuously, until an endpoint voltage of 1 V was reached. The results are reported in minutes (m). The results are also shown in Table 1.

TABLE 1

Expt. EMD CMD Graphite KOH Cone. Binder Cathode Strength Cathode A/C Anode 1A/Cont./IVO No. % Porosity Porosity g ID Ratio 1 93.7% 0% 4.7% 1.6% 40 0% 33% 225 8.05 1.33 63% 44m 2 93.1% 0% 4.7% 1.6% 40 0.6% 33% 350 8.05 1.33 63% 3 68.9% 23.0 4.7% 2.8% 40 0.6% 33% 1200 8. 05 1. 33 63% 37m % 4 69. 3% 23. 1 4. 7% 2. 9% 40 0% 33% 800 8.05 1.33 63% % 5 89. 5% 0% 3.9% 6.6% 85* 0% 33% 630 8.05 1.33 63% 47m

From the above results, it can be seen that the inclusion of 0. 6% coathylene binder increased the pellet strength by a factor of 1.5. The inclusion of a significant proportion of CMD increased pellet strength substantially, but at the cost of significantly worse performance. However, the inclusion of 6.6% solid KOH increased pellet strength by a factor of 3. 5, with no degradation, indeed a small improvement, in performance, despite the slightly lower content of EMD compared with Experiment 1.

EXAMPLE 2 Relative pellet strength vs. porosity Conventional cathode pellets (zero binder, zero solid KOH), Groups 1-14, were made as described in Example 1, except that the porosity was varied by varying the degree of compaction. Similar cathode pellets including 0.6% by weight coathylene binder were made at different porosities, Groups 15-17. In Groups 18-19 cathode pellets of the present invention including about 7% by weight solid KOH were also made. The strength of these pellets was measured by the method described in Example 1, and the results are shown in the following Table 2. The results are also summarised in the graph forming the accompanying drawing.

Table 2

Experiment ID (mm) Porosity Relative No. (%) Strength Group. 1-14 NO BINDER 1 9. 33 24. 1 1230 2 9. 33 25. 8 908 3 9. 33 26. 5 828 4 9. 33 27. 4 630 5 9. 53 28. 8 500 6 8. 2 31. 3 480 7 9. 2 31. 3 375 8 9. 53 31. 3 275 9 8. 73 33. 8 255 10 9. 2 33. 8 225 11 9. 53 33. 8 155 12 8. 2 33 280 13 8. 73 36. 3 150 14 8. 2 37. 1 165 Group 15-17 0.6% Coathylene Binder 15 9. 33 26. 5 1200 16 8. 73 30. 0 600 17 8. 2 33. 0 350 Group 18 6.6% Solid KOH 18 9. 0 29. 0 1050 Group 19 7.0% Solid KOH 19 8. 2 1630

From the results of Groups 1 to 14, it can be seen that the strength of the pellet generally increases as the porosity decreases in the conventional pellets. The addition of a binder, even in the relatively low amount of 0.6% by weight as in Groups 15-17,

substantially increases the strength. However, in accordance with the present invention (Groups 18-19), the inclusion of solid KOH also substantially increases pellet strength.

EXAMPLE 3 Temperature rise on dissolving solid KOH Because the dissolution of solid KOH in water is exothermic, it might be thought that the rise in temperature caused by dissolving solid KOH in the cathode pellets of the present invention could cause problems. Accordingly, the following experiment was carried out.

Cathode pellets were prepared as described in Example 1 containing 8% solid KOH. These were then inserted into a standard LR6 size battery can, a single layer separator was added as described above, and the starting temperature was measured.

1.17 g of tap water was added to each can, and the temperature was again measured.

Both temperature measurements were taken between the separator and the cathode.

The starting temperature was 22. 4 C. The final temperature taken immediately after addition of water was 23. 7 C, an increase of 1. 3 C. This is a relatively small increase and is such as not to cause problems during cell assembly.

Claims

Claims 1. A method of making an electrochemical cell having an anode, a cathode and an electrolyte, which comprises inserting an anode and a cathode, in any order, into a

housing, adding electrolyte, and then completing the cell, characterised in that the - C,-e,----t, IM ALI Lil L LAI cathode includes a solid material which is changed to a non-solid state upon the addition of electrolyte to the cell, and that the electrolyte is added to the cell after the insertion of the cathode, said solid material being frozen carbon dioxide, frozen water, a frozen solution of the electrolytic solid or the electrolytic solid.
2. A method according to Claim 1, in which said solid material which is changed to a non-solid state upon the addition of electrolyte to the cell is carbon dioxide.
3. A method according to Claim 1, in which said solid material which is changed to a non-solid state upon the addition of electrolyte to the cell is water.
4. A method according to Claim 1, in which said solid material which is changed to a non-solid state upon the addition of electrolyte to the cell is the electrolytic solid.
5. A method according to Claim 1, in which said electrolytic solid is potassium hydroxide.