EXTRUSION OF POLYMER-BASED CELL COMPONENTS

Abstract: A method of making a cell component containing a polymer, the polymer consisting primarily of vinylidene fluoride, in which the polymer is mixed with at least one other ingredient, such as an organic plasticiser, and extruded at a temperature above its melting point. The method can be used to make electrolyte layers in which case the polymer would be mixed with at least a salt, and to make layers of composite material for use as anodes or cathodes, in which case the polymer would be mixed with at least a particulate insertion material. No volatile solvents are required.

Description: Extrusion of Polymer-Based Cell Components This invention relates to a method of making a cell component containing a polymer, to a cell component so made, and to an electrochemical cell incorporating this cell component. For many years it has been known to make rechargeable cells with lithium metal anodes, and cathodes of a material into which lithium ions can be intercalated or inserted. Such cells may use a separator such as filter paper or polypropylene saturated with, as electrolyte, a' solution of a lithium salt (e.g. lithium perchlorate) in an organic liquid such as propylene carbonate. Alternatively they may use a solid-state ion- conducting polymer, for example a complex of a lithium salt with poly (ethylene oxide). A wide variety of intercalation or insertion materials are known as cathode materials, such as TiS 2 , V 6 0 13 and Li χ Co0 2 where x is less than 1; and these materials are often mixed with solid electrolyte material to form a composite cathode. To avoid the problems arising from dendrite growth at the anode, it has been proposed to use an intercalation material such as graphite as the anode material too, and this also may be mixed with solid electrolyte material to form a composite anode. Similar cells can be made in which sodium takes the place of lithium. An alternative type of polymer electrolyte has recently been proposed by Gozdz et al (US 5 296 318) , which comprises a copolymer of 75 to 92% vinylidene fluoride and 8 to 25% hexafluoropropylene, blended with a lithium salt and a compatible solvent such as ethylene carbonate/propylene carbonate mixture and cast from solution in a low boiling-point solvent such as tetrahydrofuran. This is said to provide a stable film -4 -3 -1 with conductivity in the range 10 to 10 S cm . Patent application GB 2 309 701 A describes an electrolyte in which the PVdF/HFP copolymer is replaced by a polymeric chain consisting primarily of vinylidene fluoride onto which is grafted a mono-unsaturated carboxylic, sulphonic or phosphonic acid, ester or amide; as in Gozdz the electrolyte also comprises a salt and an organic solvent such as ethylene carbonate and/or propylene carbonate, and it is cast from solution in a low boiling point solvent such as dimethylacetamide or tetrahydrofuran. Although this process provides good quality electrolytes, it is inconvenient to have to evaporate the large quantities of low boiling point solvent involved. According to the present invention there is provided a method of making a cell component comprising a polymer, the method comprising mixing a polymer consisting primarily of vinylidene fluoride with at least one other ingredient of the component, and extruding the mixture at a temperature above the melting point of the polymer so as to melt cast a non-porous film or sheet of the cell component . The cell component may be a cell electrolyte layer. In this case the mixture to be extruded will include an appropriate salt. It might also contain an inert filler material such as silica or alumina in particulate form. Alternatively the cell component may be a composite cathode or a composite anode layer, in which case an appropriate insertion material in particulate form would be mixed with the polymer prior to extrusion. In either case it is usually also desirable to mix an organic plasticiser with the polymer prior to completion of the extrusion. If the cell is a lithium cell (or a lithium ion cell) then the salt will be a lithium salt, such as lithium perchlorate LiC10 4 . Other suitable salts are LiAsF 6 , iPF 6 , LiBF 4 , LiN (S0 2 CF 3 ) 2 , or LiCF 3 S0 3 ; if the salt is to be extruded then it must be stable at the extrusion temperature. A variety of compatible plasticisers may be used, in particular ethylene carbonate or propylene carbonate; alternative plasticisers are diethoxyethane or diethyl carbonate. Plasticisers such as tetraethylene glycol dimethyl ether (tetraglyme) , or N-methyl- pyrrolidone ( l-methyl-2-pyrrolidone) may also be provided, to ensure that other plasticisers do not crystallise at the desired operating temperature and so to ensure adequate electrical conductivity. Other plasticisers which may be used are phthalates such as dibutylphthalate, esters of aliphatic dibasic acids such as dioctyladipate or dibutylsebacate, alkyl phosphates such as tributylphosphate, and polymeric polyesters of adipic acid such as poly (1, 3 butylene glycol/1, 2 propylene glycol/adipic acid ester) . It will be appreciated that the plasticiser must have a boiling point above the extrusion temperature to ensure that at least the bulk of the plasticiser does not evaporate during extrusion. The polymer comprises a polymeric chain consisting primarily of vinylidene fluoride and may be a homopolymer, i.e. polyvinylidene fluoride (PVdF) , or may be a copolymer or terpolymer of vinylidene fluoride (VdF) and other monomers such as hexafluoropropylene (HFP) , chlorotrifluoroethylene (CTFE) , or tetrafluoroethylene (TFE) , and in these cases the proportion of VdF is preferably no less than 70% by weight. The polymer may have further monomers grafted onto such a polymeric chain, in particular a mono-unsaturated carboxylic, sulphonic or phosphonic acid, ester or amide may be grafted onto the chain. Such a grafted polymer may have improved adhesion to metal components such as current collectors in a cell. The monomers to be grafted must have only one double-bond in the carbon chain R- , and one or more carboxyl groups -COOH, sulphonic acid groups -S0 2 OH, phosphonic acid groups -PO(OH) 2 , ester groups -COOR' , or amide groups -C0NH . Generally smaller monomers, with less than five carbon atoms in the carbon chain R-, are preferable. For example acrylic acid; crotonic acid, vinylacetic acid, methylacrylic acid (isomers of butenoic acid) ; isomers of pentenoic acid such as allylacetic acid, or tiglic acid; or, as examples of monomers with more than one acid group, itaconic acid or maleic acid. The corresponding amides such as acrylamide may also be used. In an ester, the group R' might be methyl, ethyl, hydroxyethyl, or butyl; for example esters such as methyl acrylate or hydroxyethyl methacrylate may be used. Vinyl or styrene sulphonic acid may be used. The most preferred monomers to be grafted are acrylic acid or methacrylic acid. The grafting may be achieved by an irradiation process. For example the polymer chain substrate and the graft monomer material together may be subjected to continuous or intermittent irradiation, or more preferably the substrate may be pre-irradiated before it is brought into contact with the graft monomer material. The radiation may for example be an electron beam, or X-rays or γ-rays. The irradiation activates the substrate, apparently by generating free radicals. The degree of grafting is determined by several factors, the most important being the length of time that the activated substrate is in contact with the graft monomer material, the extent of pre-activation of the substrate by the irradiation, the extent to which the graft monomer material can penetrate the substrate, and the temperature of the substrate and monomer material when in contact. If the graft monomer material is an acid, the degree of grafting can be monitored by sampling the solution containing the monomer, and titrating against a base to find the concentration of the acidic monomer remaining. The degree of grafting in the resulting composition is desirably between 2 and 20% of the final weight, more preferably between 3 and 12%, for example 5% or 10%. In making an electrolyte layer, the mass of polymer is preferably no more than the mass of the other organic ingredients in it, that is to say the mass of any compatible organic additives and of the plasticiser. The ratio between the mass of these organic materials and the mass of the polymer is desirably between about 1 and 10, more preferably between 1.2 and 3.0; if the ratio is less than 1 the electric conductivity becomes rather too low, while if the ratio is above 15 or 20 a coherent film or sheet is not formed. In a second aspect the present invention provides a cell component in the form of a sheet or film of non- porous polymer-containing material made by extrusion as specified above; and in a third aspect it provides an electric cell incorporating at least one such cell component . Thus the cell may incorporate a composite cathode made by extrusion, and/or a composite anode so made, and/or an electrolyte layer between an anode and a cathode. The preferred cell is a rechargeable lithium swing cell (that is, without lithium metal) with a composite anode and a composite cathode each comprising polymer mixed with appropriate insertion materials, each being made by extrusion, and separated by an electrolyte layer which may also be made by extrusion. The composite anode and the composite cathode must each be in contact with a suitable current collector such as copper or aluminium foil or mesh. In one method a composite electrode is extruded in tubular form, a strip of current collector foil is then inserted into the tube, and the tube is then squashed flat so the foil is sandwiched between sheets of composite electrode material. This last step may be performed using heated rollers to ensure the foil is bonded to the composite electrode material. In an alternative method a composite anode, a polymer electrolyte, and a composite cathode are all extruded, are then sandwiched between metal foils, and are bonded together by passing between heated rollers. The invention will now be further and more particularly described, by way of example only. Polymer-Based Electrolyte Layers A polymer-based electrolyte layer must be a good ion conductor, and so incorporates both a plasticiser and a salt. Example 1 : This example uses polyvinylidene fluoride (PVdF) powder, Solvay Solef grade 1010 (trade marks) which is a homopolymer with a melt flow index (measured according to ASTM D1238) at 230°C of 2 g/10 in at a load of 2.16 kg, and a melting point of 174°C. A mixture is made up as follows, the parts being given by mass: 1 part PVdF 0.96 parts lithium trifluoromethane sulphonate (salt) 1.52 parts ethylene carbonate (EC) 0.9 parts propylene carbonate (PC) (plasticiser) These ingredients are well mixed, and then extruded at an extrusion temperature of between 200° and 210°C to form pellets. The pellets are then extruded again to form a strip of polymer electrolyte 200 mm wide and 0.08 mm thick. This electrolyte layer has a conductivity at 20°C of 3 x 10 ~ S cm "1 . Examp1e 2 This example uses Solef grade 6020 PVdF powder, which is a homopolymer with a melt flow index (in each case this parameter is measured according to ASTM D1238) at 230°C of 0.87 g/10 minutes at a load of 21.6 kg, and which has a melting point of 171°C. A mixture was made up as follows, the parts being given by mass: 1 part PVdF 0.26 parts lithium trifluoromethane sulphonate 0.74 parts ethylene carbonate 0.49 parts propylene carbonate These ingredients were mechanically blended to form a homogeneous mixture at a temperature of 40°C. The mixture was then melt blended on a Brabender (trademark) twin screw cavity mixer at 150°C for 10 minutes at 40 rpm. A thin layer of material was then obtained by pressing this composition at 180°C in a hydraulic press for five minutes between sheets of mylar, the load on the platens being about 4.8 tonnes, the resulting layer being of thickness 0.135 mm. (In this, and all the other examples using the hydraulic press, the sample being pressed spread out to an area less than that of a circle of diameter 90 mm) . The layer was then tested and found to have an -4 -1 electrical (ionic) conductivity at 20°C of 4 x 10 S cm . Polymer-based Composite Electrode Layers Where a polymer-based composite electrode layer is to be used in combination with a separator and a liquid electrolyte (for example lithium hexafluorophosphate dissolved in an ethylene carbonate/propylene carbonate mixture) to form a cell, then it is not necessary for the composite layer initially to contain plasticiser. Where a polymer-based composite electrode layer is to be used in combination with a polymer-based electrolyte layer to form a cell, then the composite electrode layer must have adequate ionic conductivity; this may be ensured by including an appropriate salt and plasticiser in the layer as extruded, or by forming the layer without salt or plasticiser and subsequently soaking it in an organic liquid electrolyte. In either .case it is necessary to ensure that the composite electrode layer adheres to the current collector, and this adhesion can be improved by the use of a grafted polymer. Example 3 This example uses grade 21510 Solvay Solef (trade marks) which is a vinylidene fluoride/hexafluoropropylene co-polymer (with the same melt flow index as the grade 1010 homopolymer) , and a melting point of 132°C. PVdF/HFP powder was irradiated by a cobalt-60 gamma source for a total dose of 15 kgray, at a dose rate of 1 kgray hour . The irradiated powder was then placed in a reaction vessel containing a de-oxygenated aqueous solution of acrylic acid (25% by weight) also containing ferrous sulphate (0.02 M) as a homopolymerisation inhibitor. This reaction mixture was held at 60°C and the progress of the reaction with the acrylic acid was monitored at intervals by taking a sample of the mixture and determining the remaining concentration of the acid by titration with sodium hydroxide. After a few hours, when the desired consumption of acrylic acid had been achieved, the resulting graft copolymer powder was washed several times with deionised water, and then dried in a vacuum oven for 24 hours at 50°C. The weight of the powder was found to have increased, the increase being equal to 10% of the final weight, as a consequence of the grafting of acrylic acid into the polymer chain. A mixture is made up as follows: 1 part PVdF-HFP/acrylic acid graft polymer 0.42 parts LiCF 3 S0 3 1.7 parts ethylene carbonate 1.1 parts propylene carbonate 2.85 parts graphite These ingredients are well compounded, and then extruded at an extrusion temperature of between 165° and 205°C to form pellets. The pellets are then extruded again to form a strip of composite anode material 200 mm wide and 0.1 mm thick, onto a copper foil 0.01 mm thick, and laminated between heated nip rollers immediately after extrusion. Example 4 This example uses the Solef grade 21510 PVdF/HFP copolymer as in Example 3, and grafts acrylic acid onto the polymer chain in the same manner, so the increase in weight is again 10% of the final weight. A mixture is made up as follows: 1 part PVdF-HFP/AA graft polymer 0.6 parts LiCF 3 S0 3 2.2 parts ethylene carbonate 1.5 parts propylene carbonate 7 parts LiNi0 2 0.16 parts carbon These ingredients are well mixed, and then extruded to form pellets at an extrusion temperature of between 165° and 205°C. The pellets are then extruded again to form a strip of composite cathode material of thickness 0.1 mm and width 200 mm, and laminated onto an aluminium foil 0.02 mm thick by passing between heated nip rollers immediately after extrusion. The next three examples relate to composite anode layers using un-grafted PVdF. Example 5 This example uses Solef grade 21010 which is a vinylidene fluoride/hexafluorophosphate co-polymer with a melt flow index at 230°C of about 6 g/10 min at a load of 5 kg, and a melting point of 146°C. A mixture is made of 1 part PVdF/HFP and 2.75 parts graphite powder. These were mechanically blended, and then melt blended in a twin screw cavity mixer at 174°C for 10 minutes at 35 rpm. A thin layer was then made by pressing this mixture at 200°C in a hydraulic press as described above in Example 2 for five minutes, the load on the platens being about 6.4 tonnes. The final thickness was 0.10 mm. This anode layer was then tested in a test cell with an ethylene carbonate/dimethyl carbonate electrolyte containing lithium hexafluorophosphate, and a lithium counter electrode. This test cell was cycled between 0.01 and 1.5 volts. The anode was found to intercalate lithium ions, and this intercalation was mostly reversible. Example 6 This example used Solef grade 1015 which is a PVdF homopolymer with a melt flow index at 230 of 0.7 g/min at a load of 10 kg, and a melting point of 172°C. The ingredients used were as follows: 1 part PVdF 0.13 parts stearic acid 5.67 parts graphite 2 parts dibutylsebacate (DBS) The PVdF and stearic acid were mixed together and then introduced into a twin screw extruder with two mixing zones. The graphite was introduced at the start of the screw, being fed from two hoppers, and the dibutylsebacate plasticiser was added by volumetric pump just before the first mixing zone. The temperature throughout the extruder was at 180°C. The extruder produced a layer 0.30 mm thick which was then calendered to produce a film of thickness 0.10 mm. This anode layer was then tested in a test cell with an ethylene carbonate/dimethyl carbonate electrolyte containing lithium hexafluorophosphate, and a lithium counter electrode. This test cell was cycled between 0.01 and 1.5 volts. The anode was found to intercalate lithium ions, and this intercalation was at least partly reversible. Example 7 This example used Solef grade EX1300 which is a PVdF homopolymer with a melt flow index at 230°C of 2.5 g/10 in at a load of 10 kg, and a melting point of about 173°C. A mixture was made up as follows: 1 part PVdF 1.14 parts ethylene carbonate 2.76 parts graphite These ingredients were mechanically blended, and then melt blended in a twin screw cavity mixer at 174°C for 10 minutes at 35 rpm. A thin layer was then made by pressing this mixture at 200°C in a hydraulic press as described above for five minutes, the load on the platens being about 6.4 tonnes. The final thickness was 0.10 mm. This anode layer was then tested in a test cell with an ethylene carbonate/dimethyl carbonate electrolyte containing lithium hexafluorophosphate, and a lithium counter electrode. This test cell was cycled between 0.01 and 1.5 volts. The anode was found to intercalate lithium ions, and this intercalation was mostly reversible. (The plasticiser DBS is commonly used with PVdF, but does not enhance the conductivity; consequently the composition in Example 6 does not perform as well, as an anode, as that in Example 7) . The next example relates to a composite cathode layer using un-grafted polymer. Example 8 This example used Solef grade 21010 which is a VdF/HFP copolymer with a melt flow of index at 230°C of 6 g/10 min at a load of 5 kg, and with a melting point of 146°C. A mixture was made as follows: 1 part PVdF/HFP 0.8 parts ethylene carbonate 0.4 parts conductive carbon 2.12 parts LiMn204 These ingredients were mechanically blended, and then melt blended in a twin screw cavity mixer at 172°C for 10 minutes at 40 rpm. A thin layer was then made by pressing this mixture at 200°C in a hydraulic press as described above for five minutes, the load on the platens being about 8.1 tonnes. The final thickness was 0.10 mm. It will be appreciated that a similar composition may be made with a homopolymer, for example Solef grade EX1300 homopolymer as described in example 7. In making a composite anode or cathode it is generally preferable to include a plasticiser. The proportions (by weight) of the ingredients are preferably: PVdF : 15% to 30% plasticiser: 15% to 30% the remainder being graphite (in the case of an anode) or: carbon : 7% to 15% and insertion material (in the case of a cathode)

Claims: Claims 1. A method of making a cell component comprising a polymer, the method comprising mixing a polymer consisting primarily of vinylidene fluoride with at least one other ingredient of the component, and extruding the mixture at a temperature above the melting point of the polymer so as to melt cast a non-porous film or sheet of the cell component. 2. A method as claimed in Claim 1 wherein the polymer comprises a polymeric chain which is a vinylidene fluoride homopolymer. 3. A method as claimed in Claim 1 wherein the polymer comprises a polymeric chain which is a copolymer or terpolymer of vinylidene fluoride and other monomers. 4. A method as claimed in Claim 2 or Claim 3 wherein the polymer has a mono-unsaturated carboxylic, sulphonic or phosphonic acid, ester or amide grafted onto the chain. 5. A method as claimed in any one of the preceding Claims wherein the mixture to be extruded includes an insertion material in particulate form so as to produce a composite electrode. 6. A method as claimed in any one of Claims 1 to 4 wherein the mixture to be extruded includes a salt, so as to produce an electrolyte layer. 7. A method as claimed in Claim 6 or Claim 7 wherein an organic plasticiser is mixed with the polymer prior to completion of the extrusion. 8. A method as claimed in Claim 7 wherein the ratio of the mass of any organic additives (if present) and the plasticiser, to the mass of the polymer, is between 1 and 10. 9. A method as claimed in Claim 8 wherein the said ratio is between 1.2 and 3.0. 10. A cell component in the form of a sheet or film of non-porous polymer-containing material made by a method as claimed in any one of the preceding Claims. 11. An electric cell incorporating at least one cell component as claimed in Claim 10.

Inventors: KRONFLI ESAM, MATTINGLEY NEVILLE JOHN, KRONFLI, ESAM, MATTINGLEY, NEVILLE, JOHN
Applicants: AEA TECHNOLOGY PLC, KRONFLI ESAM, MATTINGLEY NEVILLE JOHN, KRONFLI, ESAM, MATTINGLEY, NEVILLE, JOHN
Application year: 1998
Main classification: H01M10
IPC codes: B29C47, H01M4, H01M6, H01M10