GB2181884A - Sheet electrode for electrochemical cells and manufacture thereof - Google Patents

Abstract

A sheet electrode for an electrochemical cell comprising a support which is composed of a mat of non- metallic fibres which are electrically non-conducting and an electrode material which impregnates the support. The invention also provides a method of making a sheet electrode for an electrochemical cell, the method comprising the steps of: a) providing an electrode support comprising a mat of non-metallic fibres which are electrically non-conducting; and b) impregnating the support with an electrode material.

Description

SPECIFICATION Sheet electrode for electrochemical cells and manufacture thereof The present invention relates to a sheet electrode for electrochemical cells, a method of making a sheet electrode for an electrochemical cell and an electrochemical cell incorporating a sheet electrode. In particular, the sheet electrode is a flexible cathode sheet for use in non-aqueous electrochemical cells, such as lithium cells. More particularly, thepresent invention relates to the production of a cathode for use in thin lamella lithium cells.

It isknown in the productionof thin lamella electrochemical cells for the casing of the cell to be comprised of two stainless steel plates whichare joined together by aseal. One of theelectrodes of the cell is formed by coating the electrode material ontoone of the stainless steel plates which constitutes one half ofthe casing of the cell. When the electrode is acathode ina lithium cell it requiresa PTFE binder in order to hold the cathode material together onthe stainless steel plate. The cathode suffers from thedisadvantage that the PTFE renders thecathode difficult towet by the electrolyte and this can deterioratethe performance of the cell.Furthermore, the PTFE binder requires a high temperature sintering treatment once the cathode has been formed on theplate, typicallyat a temperature of around 330 C. This is inconvenient tocarry out in a production process. Inaddition, since the cathode must be formedon part of the casingof the cell, thecathodemust be formed individually and shortly beforeassembly of thecell sothat thecathode material does not deteriorate priorto assembly of the cell due to theeffectof moisture in the atmosphere.

It has also been proposed to coat a pasteof a cathode material for an electrochemical cell onto a metal mesh. This can sufferfrom the disadvantage that the metal mesh is not very flexible, which is required for thin lamella cells.

Also, the cathode material can flake off when the meshis flexed. Furthermore, the ends of the wires in the metal mesh can act as barbs which extend across the thin electrolyte layer and short circuit the cell. In addition, the metal meshes cannot be made sufficiently thin for incorporation into thin lamella cells.

A further proposal for making an electrode for an electrochemical cell has been to form carbon fibre mats, known in the art as carbon fibre paper. However, these mats are relatively expensive to manufacture, are difficult to put together and are not very flexible and so are not particularly suitable for use in thin lamella cells.

It is also known to form an electrode for an electrochemical cell by depositing, by electroless deposition, nickel metal onto cellulosic paper.

The present invention provides a sheet electrode for an electrochemical cell comprising a support which is comprised of a mat of nonmetallic fibres which are electrically non-conducting and an electrode material which impregnates the support.

The present invention also provides a method of making a sheet electrode for an electrochemical cell, the method comprising the steps of: a) providing an electrode support comprising a mat of non-metallic fibres which are electrically non-conducting; and b) impregnating the support with an electrode material.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is an exploded view of an electrochemical cell incorporating a sheet cathode made in accordance with the present invention; and Figure 2 shows discharge curves for lithium cells made in accordance with the present invention.

Referring to Figure 1, there is shown a thin rectangular lamella cell for use as ultra thin battery in a credit card. A typical battery has dimensions of 16.5x34.5x0.5 millimetres.

The cell comprises an upper casing part 1 of stainless steel which has disposed around the outside of the lower face thereof an annular band of hot melt adhesive 2. A lithium anode 3 is disposed beneath the upper casing part 1 and is positioned and dimensioned so that it is covered by the centre portion of the upper casing part 1 which is not provided with the band of hot melt adhesive 2. In a preferred cell the lithium metal anode had a thickness of 50-150 microns and an area of 1.8cm2. A separator 4, such as glass-fibre sheet, containing a non-aqueous electrolyte is disposed beneath the lithium anode 3 and above a sheet cathode 5 of the present invention. The sheet cathode may be a single or multi-layer sheet.The non-aqueous electrolyte preferably has lithium perchlorate (LiClO4) as the solute and in the preferred cell has a volume of from 30 to 80 microlitres. In the preferred cell the sheet cathode is from 150 to 250 microns thick and has an area of 2.2 cm2. Beneath the sheet cathode 5 is disposed a stainless steel lower casing part 7 which is similar to the upper casing part 1, and is provided on its upper surface with an annular band of hot melt adhesive 6.

When the cell is formed, the various components are assembled together in the order shown between the upper casing part 1 and the lower casing part 7. The cell is sealed by heat sealing the two hot melt adhesive bands 2 and 6 together. In the resultant cell, the upper and lower casing parts 1, 7 act as a current collector for the lithium anode and for the cathode sheet 5 respectively. The cell is assembled in a dry air atmosphere having less than 10 ppm water vapour so as not to deteriorate any of the cell components, in particular the lithium anode, by reaction with water.

The sheet cathode 5 of the invention and its manufactures will now be described in detail.

First, there is provided a mat of non-metallic fibres such as polymeric fibres or glass fibres.

Suitable polymeric fibres are composed of polyethylene terephthalate, nylon, cellulose or polyvinyl alcohol. Preferably, the mat consists of a non-woven polyester tissue. The mat may be composed of fibres of one or more materials.

The mat desirably has high porosity and good strength and may be made any appropriate thickness, shape or size. The pore size of the mat is within a predeterminable size and is preferably from 30 to 75 microns. The mat must also be chemically stable in non-aqueous electrolytes and solvents and in manganese dioxide. The mat must in addition be thermally stable up to about 1 700C since during production of the cathode the mat is dried at an elevated temperature which must not result in deterioration of the mat.

The mat is put into a tray and a slurry containing a mixture of the materials to form the cathode 5 is poured over the mat. The cathode material mixture comprises an active cathode material, an electrically conductive material and, optionally, a binding material.

The size of the particles of the materials of the slurry is carefully controlled in response to the pore size of the mat so that the particles can penetrate and impregnate the pores of the fibrous mat. There is a strong inter-relationship between the size of the pores in the mat and the size of the particles in the slurry. Typ ically, the pore size of the mat is around 50 microns and the particle size of the active cathode material is from 4 microns to 100 microns, preferably from 30 to 75 microns, and the particle size of the electrically conductive material is from submicron (colloidal) size to 50 microns, preferably from 5 to 20 mi crons. This ensures that the particles can fully penetrate the mat and impregnate substantially all the pores of the mat.It is generally preferred to have the particle size of the electri cally conductive material to be less than that of the active cathode material so that the active cathode material particles are surrounded by the smaller particles of the electrically con ductive material whereby the resultant sheet cathode has sufficient electrical conductance.

The smaller electrically conductive particles tend to coat the larger active cathode material particles and the matrix of the electrically con ductive particles is sufficiently continuous to render the impregnated support mat electri cally conductive.

The slurry is prepared by suspending the active cathode material, the electrically conductive material and, when appropriate, the binding material in the desired proportions in a liquid. When the sheet cathode is for use in a lithium cell, the active cathode material is heat treated (350 C, 16hr) electrolytic manganese dioxide or chemical manganese dioxide. The purpose of the heat treating step is to remove from the manganese dioxide irreversably, chemically bound water and also to modify its crystal structure from the y-phase to a mixed y/fl phase. The electrically conductive material is selected from one or more of graphite, acetylene black, carbon black and colloidal graphite.The binding material may be one or more of polyacrylamide, polyacrylic acid, methyl cellulose, hydroxypropylmethylcellulose, polyethylene oxide, polyvinyl acetate, ethylene vinylacetate, ethylene acrylic acid, polyvinyl methyl ether, polyvinyl alcohol, polyvinyl pyrollidone and cellulose. The liquid used to make the slurry may be ethanol, toluene or water.

The amounts and proportions of the various constituents of the slurry may be varied in response to the differing types of the constituents used and the differing fibrous mats which are to be impregnated and the differing cells into which the resultant cathode is to be incorporated. In particular, it may be possible to omit the liquid from the slurry. Furthermore, the binder may be omitted, although in order to prevent dust from being given off from the cathode mat the material impregnated into the mat preferably includes from 1 to 10 weight % binder, more preferably from 3 to 4 weight% binder.

In addition, the impregnation of the fibrous mat may be carried out in a multi-stage manner, with first some constituents being impregnated and then the remaining constituents being impregnated. For example, it may be appropriate to employ a slurry containing the conducting material, the binder and the liquid or alternatively a slurry containing the active cathode material, the conducting material and the liquid.

The slurry particles impregnate the pores of the mat almost instantaneously. The mat is then removed from the slurry and is dried at an elevated temperature. Typically, the drying temperature is about 170 C, at which temper ature the manganese dioxide is conditioned to remove physically re-absorbed water therefrom and also water is removed from the binders.

The polymeric material of the mat must be able to withstand this elevated temperature and since polyethylene terephthalate has a high temperature resistance, and is also not deformed during calandering, this makes it the most preferred material for the mat.

The dried mat is then weighed to determine the amount of loading of the mat with slurry particles. If the mat has not been impregnated with sufficient slurry particles, the mat is again submerged in the slurry and re-loaded with slurry particles. The mat is again dried and weighed and this cycle may be repeated as necessary in order to obtain the required loading of the mat.

After the mat has been loaded with slurry particles and dried, it is too thick for use in the thin electrochemical cell. The loaded mat typically has a porosity of around 60%. The mat is calendered, preferably between two opposed rollers, until it is the required thickness and has the required porosity. For the thin electrochemcial cells, the cathode mat desirably has substantially no porosity. For other electrochemcial cells, the porosity of the cathode mat typically is from 20% to 30%.

The porosity of the cathode mat affects the electrical conductivity of the cathode. The electrical conductivity is dependant upon the electronic and ionic conductivities in the mat.

As the porosity of the mat is increased, the electronic conductivity is lowered since there is less direct contact between the particles of the electrically conductive material, but the ionic conductivity is increased since the greater number of pores permits greater ionic mobility.

For an electrochemical cell having a high current rate, the porosity is preferably around 25% at which porosity the electronic and ionic conductivities are balanced so as to maximise the total electrical conductivity of the cathode.

For an electromechemical cell having a low current rate, such as the thin electrochemical cells, the variation of electrical conductivity with porosity is very much reduced and so the porosity can be substantially zero while still providing sufficient electrical conductivity of the cathode for the cell to operate at its low current rate.

The calendering of the mat ensures that the bulk of the slurry material on the mat is impregnated into the pores in the body of the mat. Strictly, the calendering is not essential but if it is not employed the resultant mat is thicker and the slurry material tends to flake off from it. The calendering performs two functions:- it compresses the mat, and it impregnates and increases the keying of the material in the mat. This tends to reduce the material flaking off from the mat and since it is very undesirable for dust to flake off the mat, as much material as possible is impregnated into the mat so as to be firmly held in the pores of the mat. Typically, for a mat having an initial thickness prior to impregnation of 150 microns, the impregnated mat has a thickness of 450 microns prior to calendering and a thickness of 180 microns after the calendering.

When the mat has been calendered to the required thickness, the mat is then cut to the required area and dimensions ready for incorporation into an electrochemical cell. The mat can readily be cut without deforming or disintegrating.

The present invention can also be utilised to make cathodes for air cells having an aqueous electrolyte and in which the cathodic reaction is the reduction of oxygen. The air cathode is made by impregnating a fibrous mat as described above on one side only with a hydrophobic dispersion of PTFE and a conducting species, for example carbon. The dispersion may contain a further binder such as those described above if required. The impregnated mat is then dried. The other side of the mat is then impregnated in the manner described above with a catalyst mixture of, for example, carbon or a mixture of carbon and manganese dioxide, the catalyst mixture containing a nonwater soluble binder such as those described above. The mat is dried again, and subsequently pressed and rolled (i.e. calendered) to the required thickness.The mat can then be cut to the required size and shape for incorporation into an air cell.

The present invention provides a cathode for an electrochemical cell, in particular, a thin lamella electrochemical cell, which has a number of advantages over known cathodes for use in thin lamella electrochemical cells.

First, the use of a fibrous mat in which the cathode material is impregnated results in the cathode material being securely held within the mat without the need for a high strength PTFE binder which is required in the known cathode in which the cathode material is deposited onto a stainless steel plate. This obviates the wetting problems which are associated with the employment of a PTFE binder. It also obviates a high temperature sintering treatment and so permits a wider variety of polymeric fibres to be used since the cathode of the invention need to be heated to only relatively low temperatures (around 170 C).

Furthermore, the cathode of the invention has a high mechanical flexibility without detachment or flaking off of the cathode material. This may be compared to the known metal mesh and carbon fibre mesh which are not flexible. Thus the invention can provide a cathode which does not tend to generate dust during assembly of the cell and this is of great advantage since it is necessary to exclude dust from the sealing region of the cell casing during assembly.

The manufacture of the cathode sheet is particularly suitable for mass production. The fibrous mat can be in a continuous sheet which is fed through the manufacturing stages outlined above. The cathode sheet can be made any desired thickness and can be cut to any desired size or shape. The individual cut cathodes can then be stored for longperiods of time prior to being impregnated into electrochemical cells. The cathode sheets are individual components of the electrochemical cells which can be made in advance and independently of the cell assembly process. This may be compared to the known method of forming a cathode in a half-casing of a cell, in which th cathode must be made in the same manufacturing process and substantially at the same time as the cell assembly.Also, the half-casings must be coated individually, whereas in the present invention large numbers of cathodes can be cut from a single cathode sheet. Thus the present invention can provide an efficient, convenient and cost-effective manufacturing process for the electrochemical cells.

Furthermore, the cathodes of the present invention can be used in any desired type of cell construction e.g. spiral, round, flat, button, etc. This is because the cathode is deformable and can be cut to any desired size and shape.

Thin lamella electrochemical cells incorporating the cathode according to the invention have been found to have substantially the same performance in use as the thin lamella cells which include a cathode which is coated onto a half casing for the cell. In particular, the cells have equivalent lifetimes and storage lives. Such thin lamella electrochemical cells may be incorporated into credit cards to provide power for miniaturised memories, calculators, etc, Those cells would have low current densities, of the order of a few micro amps per square centimetre, and would have a very high Faradaic efficiency, which is the theoretical electrical efficiency from the amounts of the electrochemical components.

For other electrochemical cells which have high current densities, of the order of 10mA/cm2, the use of the electrically nonconducting fibrous support has been found to work surprisingly well and in fact has been found by the present inventors to be superior to electrochemical cells incorporating the known cathodes which are based on metal mesh supports. A 200 micron thick cathode of the present invention in a lithium cell can perform at a current density of 5 milli amps/cm2 in a suitable electrolyte.

The present invention will be further illustrated by the following examples.

Each of the examples relates to a lithium cell such as that illustrated in Figure 1.

In the Examples, the proportions of the constituents of the cathode mix are given in percentages by weight.

Example 1 The fibrous mat consisted of nylon fibres and had a thickness of 150 microns. The cathode mix consisted of 88% chemical manganese dioxide, 9% graphite and 3% methyl cellulose. The cathode slurry consisted of 1 part of the cathode mix suspended in 1 part of water. The cathode slurry was impregnated into the fibrous mat and as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4, the electrolyte being (PC.LiCl04).

The lithium cell had a short circuit current (SCA) of 20 mA, a lifetime under a 33KQload of 140 hours and a cell capacity of 20 mAh.

Example 2 The fibrous mat consisted of polyethylene terephthalate (PETP) fibres and had a thickness of 120 microns. The cathode mix consisted of 87% electrolytic manganese dioxide (EMD), 9% graphite and 4% polyacrylamide.

The cathode slurry consisted of 1 part of the cathode mix suspended in 1.5 parts of water.

The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including 14 bytyrolactone (y-BL)and 1M Lilo4, the electrolyte being (y-BL.LiCl04).

The lithium cell had a short circuit current (SCA) of 50 mA, a lifetime under a 3KQload of 210 hours and a cell capacity of 23 mAh.

Example 3 The fibrous mat consisted of PETP fibres and had a thickness of 120 microns. The cathode mix consisted of 87% EMD having a particle size of less than 50 microns, 9% colloidal graphite and 4% polyacrylamide. The cathode slurry consisted of 1 part of the cathode mix suspended in 1.5 parts of water. The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including 40% PC, 60% 1-2 dimethoxyethane (DME) and 0.7M Lilo4.

The lithium cell had a short circuit current (SCA) of 200 mA, a lifetime under a 3KQload of 21 hours and a cell capacity of 22 mAh.

Example 4 The fibrous mat consisted of nylon fibres and had a thickness of 150 microns. The cathode mix consisted of 87% EMD having a particle size of less than 100 microns, 9% graphite and 4% methyl cellulose. The cathode slurry consisted of 1 part of the cathode mix suspended in 1 part of water. The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4.

The lithium cell had a short circuit current (SCA) of 45 mA, a lifetime under a 3unload of 120 hours and a cell capacity of 25 mAh.

Example 5 The fibrous mat consisted of nylon fibres and had a thickness of 150 microns. The cathode mix consisted of 88% EMD having a particle size of less than 50 microns, 9% acetylene black and 3% methyl cellulose. The cathode slurry consisted of 1 part of the cathode mix suspended in 1 part of water. The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4.

The lithium cell had a short circuit current (SCA) of 80 mA, a lifetime under a 3kflload of 160 hours and a cell capacity of 20 mAh.

Example 6 The fibrous mat consisted of PETP fibres and had a thickness of 120 microns. The cathode mix consisted of 89% EMD having a particle size of less than 50 microns, 9% graphite and 2% polyvinylpyrrolidone. The cathode slurry consisted of 1 part of the cathode mix suspended in 1 part of water. The cathode slurry was impregnated into the fibrous mat as described above. The supported cathode was dried at 170 C.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4.

The lithium cell had a short circuit current (SCA) of 45 mA, a lifetime under a 33unload of 120 hours and a cell capacity of 17 mAh.

Example 7 The fibrous mat consisted of PETP fibres and had a thickness of 120 microns. The cathode mix consisted of 87% EMD having a particle size of less than 50 microns, 9% 9raphite and 4% polyacrylamide. The cathode slurry consisted of 1 part of the cathode mix suspended in 1.5 parts of water. The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4.

The lithium cell had a short circuit current (SCA) of 105 mA, a lifetime under a 33KQload of 240 hours and a cell capacity of 22 mAh.

Example 8 The fibrous mat consisted of PETP fibres and had a thickness of 120 microns. The cathode mix consisted of 87% EMD having a particle size of less than 50 microns, 9% colloidal graphite and 4% polyvinyl alcohol. The cathode slurry consisted of 1 part of the cathode mix suspended in 1 part of water. The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4.

The lithium cell had a short circuit current (SCA) of 100 mA, a lifetime under a 33unload of 250 hours and a cell capacity of 24 mAh.

Example 9 The fibrous mat consisted of PETP fibres and had a thickness of 120 microns. The cathode mix consisted of 87% EMD having a particle size of less than 50 microns, 9% colloidal graphite and 4% polyvinyl methylene.

The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4.

The lithium cell had a short circuit current (SCA) of 90 mA, a lifetime under a 33KQload of 240 hours and a cell capacity of 23 mAh.

Example 10 The fibrous mat consisted of PETP fibres and had a thickness of 120 microns. The cathode mix consisted of 86% EMD having a particle size of less than 50 microns, 9% graphite and 3% polyacrylamide and 2% methyl cellulose. The cathode slurry consisted of 1 part of the cathode mix suspended in 1 part of water. The cathode slurry was impregnated into the fibrous mat as described above.

The resultant cathode was incorporated into a lithium cell having an electrolyte including propylene carbonate (PC) and 1M Lilo4.

The lithium cell had a short circuit current (SCA) of 40 mA, a lifetime under a 33unload of 190 hours and a cell capacity of 23 mAh.

Figure 2 shows three typical discharge curves represented as output voltage E in volts against time tin hours, for lithium cells constructed in accordance with Examples 3 and 7. Curve 2a is for a cell of Example 7 discharged through a 33unload which results in a Faradaic efficiency of 90%. Curve 2 is a similar curve for the same cell when discharged through a 10KQload. In this case, the cellhas a Faradaic efficiency of 80%. Curve 2c shows a discharge plot for a cell of Example 3 discharged through a 200load, which gives a Faradaic efficiency of 60%.

Claims (28)

1. A sheet electrode for an electrochemical cell comprising a support which is comprised of a mat of non-metallic fibres which are electrically non-conducting and an electrode material which impregnates the support.

2. A sheet electrode according to Claim 1 wherein the fibrous mat comprises non-woven polymer or glass fibres.

3. A sheet electrode according to Claim 2 wherein the polymer fibres are composed of polyethylene terephthalate, nylon, cellulose, or polyvinyl alcohol.

4. A sheet electrode according to any one of Claims 1 to 3 wherein the sheet electrode has a thickness of from 150 to 250 microns.

5. A sheet electrode according to any foregoing Claim wherein the electrode material comprises an active cathode material and an electrically conductive material.

6. A sheet electrode according to Claim 5 wherein the active cathode material is selected from heat treated electrolytic manganese dioxide and chemical manganese dioxide.

7. A sheet electrode according to Claim 5 or Claim 6 wherein the electrically conductive material is selected from one or more of graphite, acetylene black, carbon black, and colloidal graphite.

8. A sheet electrode according to any one of Claims 5 to 7 wherein the electrode material further comprises a binding material selected from one or more of polyacrylamide, polyacrylic acid, methyl cellulose, hydroxypropyl methyl cellulose, polyethylene oxide, polyvinyl acetate, ethylene vinyl acetate, ethylene acrylic acid, polyvinylmethyl ether, polyvinyl alcohol, polyvinyl pyrrolidone and

9. A sheet electrode according to any one of Claims 5 to 8 wherein the fibrous mat has pores therein which have a size of from 30 to 75 microns, the active cathode material comprises particles of from 30 to 75 microns in size and the electrically conductive material comprises particles of from 5 to 20 microns in size.

10. A sheet electrode for an electrochemical cell substantially as hereinbefore described with reference to Figure 1.

11. A sheet electrode for an electrnchemical cell substantially as hereinbefore described with reference to any one of Examples 1 to 10.

12. A method of making a sheet electrode for an electrochemical cell, the method comprising the steps of: a) providing an electrode support comprising a mat of non-metallic fibres which are electrically non-conducting; and b) impregnating the support with an electrode material.

13. A method according to Claim 12 wherein fibrous mat comprises non-woven polymer or glass fibres.

14. A method according to Claim 13 wherein the polymer fibres are composed of polyethylene terephthalate, nylon, cellulose, or polyvinyl alcohol.

15. A method according to any one of Claims 12 to 14 wherein the sheet electrode has a thickness of from 150 to 250 microns.

16. A method according to any one of Claims 12 to 15 wherein the electrode material comprises an active cathode material and an electrically conductive material.

17. A method according to Claim 16 wherein the active cathode material is selected from electrolytic manganese dioxide and chemical manganese dioxide.

18. A method according to Claim 16 or Claim 17 wherein the electrically conductive material is selected from one or more of graphite, acetylene black, carbon black and colloidal graphite.

19. A method according to any one of Claims 16 to 18 wherein the electrode material further comprises a binding material selected from one or more of polyacrylamide, polyacrylic acid, methyl cellulose, hydroxypropylmethyl cellulose, polyethylene oxide, polyvinyl acetate, ethylene vinyl acetate, ethylene acrylic acid, polyvinylmethyl ether, polyvinyl alcohol, polyvinyl pyrrolidone and cellulose.

20. A method according to any one of Claims 16 to 19 wherein the fibrous mat has pores therein which have a size of from 30 to 75 microns, the active cathode material comprises particles of from 30 to 75 microns in size and the electrically conductive material comprises particles of from 5 to 20 microns in size.

21. A method according to any one of Claims 12 to 20 wherein the electrode material is impregnated into the support as a slurry and further comprising the step of drying the slurry-impregnated support.

22. A method according to Claim 21 wherein the liquid of the slurry is selected from ethanol, toluene and water.

23. A method according to Claim 21 or Claim 22 wherein after the slurry impregnated support has been dried, the impregnated support is pressed so as to reduce its thickness to a desired value.

24. A method according to Claim 23 wherein the impregnated support is pressed by calendering.

25. A method of making a sheet electrode for an electrochemical cell substantially as hereinbefore described in any one of Examples 1 to 10.

26. A sheet electrode for an electrochemical cell when made by the method of any one of Claims 12 to 25.

27. An electrochemical cell including as a cathode a sheet electrode according to any one of Claims 1 to 11 or Claim 26.

28. An electrochemical cell according to Claim 27 which is a lithium cell.