GB2201154A - Electrolytic polymers - Google Patents

Abstract

A method of producing a polymeric electrolyte comprises melting a polymer or mixture of polymers ("matrix polymer") and dispersing therein a finely divided ionic salt. A preferred-matrix polymer is polyethylene oxide, and the mixture of polymer and salt is preferably ground finely together prior to melting. The electrolytes may be used in electrolytic cells and batteries, and the method provides an alternative to the present method of solvent casting of polymeric electrolytes.

Description

lectrolytic Polymers The present invention relates to polymeric electrolytes and their application in electrolytic cells and high energy density batteries.

The electrolytes that are at present normally used in electrolytic cells and batteries are of the solvent type wherein an ionic species is contained in solution and ;"migration of charged ions takes place through the solution between the anode and cathode of the cell. however electrolyte solutions are disadvantageous in that they are often corrosive and or toxic, and as such this presents problems to their handling and the risk of spillage or leakage of electrolyte.

To overcome these problems there is currently much interest in the field of solid or high viscosity electrolytic polymers as a possible replacement for solution electrolytes. The conduction of charge tnrougfl these polymer electrolytes is thought to be by chain flexing wherein charge transport is via the makinÖ and breaCInOy of ionpolymer interactions as the chain flexes so that the ions move along tne polymer chain.

The conductivity of a particular electrolytic polymer is there fore aided by the ease with which backbone flexing motion of the polymer chain can take place. Therefore if high conductivity is required it is preferable that the polymer be of an amorphous structure and not crystalline to any great extent as crystallinity with its attendant interpolymer and intrapolymer crosslinking would result in a much reduced amplitude of backbone motion than that achieved with little or no crystallinity.

Generally polymeric electrolytes based on polymers such as polyethylene oxide (PEO) exist as a mixture of phases ie an ion conducting amorphous material phase, crystalline PEO and crystalline salt phases. The conductivity of a polymeric electrolyte is relative to the proportion and mixture of phases present. At temperatures below the polymer melting point ion motion is impaired by the presence of large amounts of crystalline material.

Above the polymer Ineltingpoint crystalline material becomes amorphous and as such is able to participate in the conductivity process wiiicij is signified by a sudden increase in the conductivity.

Polymer-salt electrolytes based on alkali metal salt complexes of linear polyetliers have been found to exhibit conducting properties which make them potentially practical materials for solid state batteries. Polyethylene oxide (PEO) salts are particularly promising materials but despite this there is one major drawback to their practical app#lication ie their mechanical properties.

At temperatures above the PEO melting point (which increases with increasing molecular weight) films of PEO and its salts become very flexible and at higher temperatures creep many occur. If this were to occur within a battery cell then there is a risk that the cell will short out.

At present the most widely adopted method for preparation of films of PEO and similar polymers is solvent casting where a solution of the components is left to allow the solvent to evaporate and leave a cast film behind. There are however problems associateù with solvent casting in that the last traces of solvent are very difficult to remove thereby reducing the conductivity and mechanical strength, solvents are generally expensive, phase separation may occur if more than one polymer is present, and the mechanical strength of the filing is inadequate for applications much above the Tg of the polymer electrolyte.

This invention is intended to improve the mechanical stability of PEO electrolytes and similar polymeric electrolytes at temperatures below and above the polymer melting point with minimal if any degradation in the conductivity of the material.

According to the present invention there is provided a method of producing a polymeric electrolyte which includes the steps of melting a polymer or mixture of polymers ("matrix polymer") and dispersing therein a finely divided ionic salt.

Such a method is especially suited to the polymer PEO. The salt flay be made into a finely divided state by known methods eg grinding and it is preferred that the matrix polymer or polymers are made into a finelydivided form prior to melting as this it droves the distribution of the electrolyte through the polymer or polymers.

Prefer & ly both the matrix polymer and electrolyte are ground together to ensure even better distribution of the matrix polymer and elecçrolyte. Division of the matrix polymer is preferably carried out by cooling it to a low temperature so that it becomes brittle and then grinding. Grinding is preferably carried out in a suitable ball mill preferably under nitrogen for a short time, for example 15 minutes. The molecular weight of matrix polymers used is preferably in the range of 1 x 104-1 x 107 for PEO.

The ionic salt of the electrolyte is preferably the salt of an alkali metal or alkaline earth metal, although an ammonium salt or substituted ammonium salt may be used. Lithium salts are particularly preferred although Na,K,Ca or Mg salts may alternatively be used.

The anion of the salt may be any commonly used anion preferably a relatively large anion of a strong acid, eg perchlorate (CID 4), tetrafluoroborate ( BF4 ), hexafluorophosphate (PF6)1 As, trifluoromethane sulphonate (CF3S03), SCN-, CF3C02 or 1 or B(c0H5)-4.

A suitable polymer for a polymer electrolyte preferably has a low melting point and flows easily so that the ionic salt may be dispersed in the polymer giving a homogeneous mixture without the need for a solvent. Various molecular weights of polymer within the aforesaid molecular weight limits may be used, the PE0 polymers being preferred.

The ratio of polymer to salt is variable but in the case of PE0 and LiCP3S03 suitable ratios of ethylene oxide to lithium ions are from 8:1 to 30:1 and are preferably 8:1 to 20:1.

In an important further embodiment of the invention, the matrix polymer and salt may be mixed with a monomer which polymerises to give a polymer ("structural polymer") having a higher Tg than the matrix polymer. The mixture is subjected to appropriate conditions to cause polymerisation of the monomer.It is. preferred to prepare a mixture of the ground matrix polymer, the ground salt and the :monomer prior to polymerisation of the monomer and prior to the melting of the matrix polymer. Polymerisation of the monomer may be carried out in a suitable container such as a polymerisation tube which is preferably degassed and sealed under vacuum. Tne polyinerisation may be achieved by the application o#f a suitable initiator such as -butyl hydroperoxide or azoisobutyronitrile but thermal polymerisation (for example at temperatures of 1200C to 1500C in the case of styrene) is preferred.

Times for thermal polymerisation are variable from 5 hours upwards but 24-43 hours is preferred for styrene as this allows complete poly merisation at temperatures between 1200C and 1500 C. The resulting polystyrene (Tg 1000 C) or other structural polymer acts as a mechanical support system to the polymer electrolyte by giving a random distribut- ion of structural support polymer throughout the polymer electrolyte.

The styrene or other monomers may also polymerise to form graft copolymers with the polymer electrolyte and this helps to prevent phase separation.

The resulting change to the physical properties of a polymeric electrolyte upon polymerisation with a monomer or monomes whose corresponding polymerised form is of a higher Tg than the polymer produced and polymeric electrolyte present. In the case of PE0/polystyrene electrolytes the polystyrene may be present in any amount from 0 to 8090 by volume in the polymeric electrolyte. It has been ound that up to o0',0 of polymer electrolyte can be structural polymer oefore conductivity is significantly affected, that is the conducting volume may be as low as 40#6.

In a further embodiment of tne present invention the polymeric electrolytes whether or not supported by an additional polymeric structure can be annealed to produce an increase in the conductivity of the polymeric electrolytes. This conductivity increase is thought -to be due to tile reduction in the crystallinity of the polymer electrolyte upon heating, none or only some of the crystallinlty returning upon cooling.

In a further aspect of this embodiment the polymeric material may be pressed into a film, heated and then allowed to cool. This is preferably a two stage process in which a portion of the bulk material which may be ground or cut up first, is pressed into a pellet preferably by means of a hydraulic press. The material is softened by heating preferably via heating of the die to a temperature between 5500 and 1300C in the case of a PRO Lie3 polystyrene complex but this is dependant upon the polymer or polymers present and their molecular weight. The temperature has to be carefully controlled to avoid over softening and extrusion of the polymeric material under pressure whilst being high enough to induce flow.After cooling the pellet is repressed, then the load reduced but not removed and the film heated again preferably as before to produce honlogenous films.

Film thickness is dependant upon the press but, typical film thickness may be l50-300)1;fl. However, this is variable and may be as little as loAm.

The polymeric electrolytes produced by the process described herein are novel, and therefore the present invention also provides a polymeric electrolyte produced by the process described herein, ie comprising a dispersion of a finely divided ionic salt (as described above) in a matrix polymer or polymers (as described above) and which may also contain one or more structural polymers (as described above).

Such polymers lay also be produced by different processes to those described herein but the nature and proportion of the various onstituents, ie matrix and structural polymers, salt etc however the polymeric electrolyte is prepared will be as described in relation to the process parameters herein.

From filings prepared as above and subsequent tests for conductivity and physical strength upon them it has been found that (i) electrolytic polyners prepared by the technique as described hereinbe-fore have superior conductivities (by a factor of 5) and mechanical strength than corresponding solvent cast materials especially at higher temperatures (ii) the physical strength of the polymeric electrolyte may be much improved by the addition of one or more structural polymers of higher Tg than the polymer of the electrolyte.

(iii) whilst addition of structural polymers reduces the conductivity of the polymer electrolyte, as much as 40fi,S #by volume of polystyrene may be incorporated into the P > 0 LiCFO before conductivities fell to the level of those for PiO10LiCF3,o 3 03/solvent cast films and for a PEO of molecular weight 4 x 106 with 40, by volume of polystyrene content its mechanical strength as determined by penetrometry is over 5 tines that of PEO10LiCF3S03.

(iv) In the case of polystyrene PEO LiCF3S03, for a concentration range of 0-60 volume percent polystyrene there may be a decrease in conductivity of less than a factor of 10 in the higher temperature range and this compares very favourably with the best previously attained conductivities from films without added structural polymers.

The increased mechanical strength derived from added structural polymers with polymeric electrolytes makes them viable for use in higher temperature batteries and other electrolyte containing apparatus.

(v) The molecular weight of the polymeric electrolyte polymer ie PEO has little effect on the conductivity where molecular weight is in the range 1 x 1G5 to 4 x 106. The higher molecular weight polymeric electrolyte polymer is mechanically stronger than the low molecular weight polymer with or without the presence of polystyrene. The difference in strength of various polymers as determined from penetrometry being between about 20CXó and 3060 depending upon the volume % of polystyrene present.

(vi) The fall in conductivity of PEO-LiC3S03 poly styrene with respect to the polystyrene concentration was only large for concentrations above the percolation limit that is where the polystyrene concentration > 60 volume %.

The decrease is due in part to the fall in the number of carriers per unit volume but also by the increased formation of graft copolymer, the side chains of which hinder the relaxation of the PEG or other polymer backbone which is as aforesaid an integral part of the mechanism of ion transport.

The process for the production of solid or high viscosity electrolytic polymers according to#the invention will now be described by way of example only with reference to the accompanying figures which Figure 1 shows the theoretical values of Haliphatic/Haromatic VSX Polystyrene Figure 2 shows the conductivities of PEO10LiCF3S03 from various film preparations techniques Figure 3 shows the conductivity of PS-0l0LiCF3SO3 Y% Styrene wliere the molecular weight of PEO is 4 x 105 and Y is variable Figure 4 shows the conductivity of PEO10LiCF3S03 Y::-3, Styrene wiiere the molecular weight of PO is 1 x 105 and Y is variable Figure 5 shows conductivity for TO LiCF3S03 with different concentrations of polystyrene over the temperature range 60-l300C Figure 6 shows the conductivities of Figure 3 corrected for non-conducting volume Figure 7 shows the conductivities of Figure 4 corrected for non-conducting volume Figure 8 shows the conductivity of PEO LiCF3 SO Y% Styrene formed at 3 1200C where the molecular weight of PEO is 4 x 106 Figure 9 shows the conductivity of PEO LiCF3 SO3 Y% Styrene formed at at l500C where the molecular wig of PEO is 4 x 106 Purification of Chemicals PEO-based electrolytes were formed using polymers of the follow ing weights: BDH Polyox W301, molecular weight 4 x 106, and PEO (Aldrich), molecular weight 1 x 105. These were used without further purification. Lithium trifluoromethane sulphonate (LiCF3S03 (3M UK Ltd battery grade) was heated under vacuum at 100-1100C for 6-8 hours to remove water.

After cooling under vacuum, the salt was transferred to a drybox where it was ground and stored. Styrene approximately 100cm2 was washed with a 10% w/v solution of sodiu;n hydroxide (5 x 50cm ) in a separating flask to remove the polymerisation inhibiter t-butylcatechol.

The styrene was subsequently washed with approximately 400 mls of distilled water and dried over magnesium sulphate overnight. Styrene was decanted and distilled underreduced pressure. It could be stored indefinitely under refrigeration.

Grinding of Polvmer In later experiments to improve the distribution of electrolyte in the polymer, it was necessary to first reduce the particle size of both polymer and electrolyte. To grind the polymer, a simple ball mill was made which could be sealed and frozen in liquid nitrogen.

Approximately 2.5g of PEO was enclosed in a stainless steel, hemispherically-ended tube 0.25cm thick, inner diameter 2.5cm and length 9cm with 100 stainless steel ballbearings of 0.25cm diameter.

The tube was ix=ersed in liquid nitrogen after sealing with a rubber bung. Once frozen, the tube was mounted on a mechanical shaker and agitated for various lengths of time. The tube was usually refrozen at least once during this period. Photographs taken on a scanning electron microscope (SM) showed that best results were obtained for a grinding period of 10-15 minutes. All samples were ground subsequently for between 12 and 15 minutes. The dimensions of the tube placed a limit on the amount of material which could be successfully ground at any one time. As a result, the milling technique was modified at a later stage. Up to 5g of polymer could be successfully ground in the new tube which was 0.1cm thick, had inner diameter 3.8cm and was 11cm long.Due to the much thinner walls of this tube, it was necessary to agitate the mill over liquid nitrogen.

An arm on the mechanical shaker was specially adapted to secure the tube. A series of trials was carried out to establish an optimum number and size of ball-bearing. With the milling kept to 12-15 minutes, the most successful powdering was achieved with 40-50 ball-bearings of 0.6cm diameter.

Preparation of Films a) Hot Pressed Films Te preparation of films for conductivity measurements was carried cut by a two-sta6e hot pressing process . A small portion of the bulk sample was pressed into a 5ni# diameter pellet by means of a hydraulic press, Specac model P/N 15.00 (Analytical Accessories Ltd). in order to soften the material, the die was heated to a tesperature between. 55 and 1100C, depending on the polystyrene concentration and the PEO molecular weight.The temperature had to be carefully controlled to avoid over-softening and extrusion of the polymer under pressure (3.8-7.6 MPa), whilst being high enough to induce flow. After cooling, the pellet was re-pressed in a 13mm diameter die. After cold pressing at 19 MPa, the die was heated as before under a reduced load of 7.6 SIPa. This technique produced homogeneous 13mm diameter films of the order of 10 -300um in thickness.

b) Solvent Cast Films The most widely adopted method for film preparation has been solvent casting. In order to compare conductivities of hot-pressed and solvent cast material, a number of films of Pi'O10LiCF3S03 were prepared by casting a 4 wtS ace toni trile solution into 15mm diameter glass formers on teflon sheets. Residual solvent was removed and films annealed by heating for 46 hours at 120 C in a dry nitrogen flow.

c) Unannealed films further film of PEO10LiCF3S03 was prepared by hot pressing as in a) but the material was not annealled prior to pressing. Annealing above 10000 brings about the conversion of a crystalline to an amorphous structure which is, to some extent retained after cooling. An unannealed film acted as a 'blanks for examining this effect.

2.4 Penetron#et#v The relative physical strengths of the polymer systems were measured in the temperature range 70-75 0C using a Perkin Elrr Tt4S-1 penetroneter. 5mm diameter pellets were prepared as described earlier. Samples were held in a dry nitrogen atmosphere and a furnace, raised around the sample, was controlled by a modified Pekin Elner DSC-1 differential scanning calorineter. A weighted Inn quartz probe was positioned such that the tip just made contact with the sample surface. The probe penetration was then monitored as a function of sample temperature.The results are shown in Table 2 and show the increase in mechanical strength with the higher molecular weight PEO over the lower molecular weight PEO.

Composition Analysis a) Degree of Polymerisation Approximately 3@ solutions of PlO-polystyrene systems in CDCl3 were prepared and the H nmr spectrum obtained. Three distinct sets of peaks were icentified:aliphatic protons of polystyrene (i.75ppm downfield frcn TMS) aromatic protons (6.7-7.2pp=) and a single peak at 3.74ppm due to the equivalent protons of PEO.

The theoretical values of the aliphatic to aromatic proton ratio were calculated for various percentages of polystyrene in PEO. A plot of ratio against %polystyrene is given in Figure | . The ratio of Hai/Har was obtained for each polymer using the integration of peaks.

This value was used to read off the %polystyrene from the theoretical curve.

The ratio and percentage polystyrene of each polymer is given in Table 1. These values are accurate to within about +5. Some variation around the expected percentages exists: in particular, polymer PEO(K)/120 (wherein K refers to a molecular weight of 1x105) has a polystyrene 13% above its expected 20% and PEO (M) /120 (wherein M refers to a molecular weight of 4x106) 13% below an expected 60w- b) Graft Copclyeer In order to detect the presence of graft copolyrmer, free polystyrene had first to be separated from the polymerised material. Unlike PEO, polystyrene is insoluble in water. However, it was considered possible for sufficiently short crains of polystyrene grafted on to s FEO backbone, to be drawn into solution by the dissolution of the FLO in aqueous media. Polymer systems were agitated in hot water (80 0C) for 6 hours. After filtering, a u.v. spectrum in the range 2CO-4COra was obtained on a Pye Unicam SP8-150 W,VIS spectrophotometer exhibiting a small peak at 270nm giving positive evidence for the presence of graft copolymer in the mixed polymer systems.

A.C. Conductivity Measurements Films were mounted between two stainless steel electrodes within a cell holder.

A Solartron 1170 Frequency Response Analyser (FRA), controlled by a Tektronix 11052 minicomputer was used to obtain the conductivity spectra of polymer films.

The complex impedance of the sample was measured over a frequency range of 1MHz to 1Hz. Essentially, the voltage across the sample and a standard measuring resistor, connected in series, is measured as a ratio. This gives the ratio of the complex impedance of the sample to that of the standard resistor and is expressed in terms of complex variables a and b. These parameters are output to the computer where a program is used to plot the complex plane impedance spectrum. Two main features may be identified: a high frequency semicircle and a straight line section at low frequency, arising from various cell and electrolyte resistance and capacitance combinations. The intersection of the two portions, on the x-axis, gives the zero frequency or d.c.

bulk conductivity. These data, together with temperature measurements, may be used to obtain an Arrhenius type plot.

Differential Scanning calorimetry Thermal behaviour was studied with a Perkin Elder DSC-2 differential scanning calorimeter. The heating rate was either 10 or 20 min-1. The transition temperature was found by intercepting the baseline with the extrapolated line of the peak edge. The enthalpy carve involved in melting PEO was evaluated using indium metal as a standard.

Example 1 Polvmerisation of PEO-LiCF SO -Stvrene with Initiator 2g of PEO and 0.591g LiCF3SO3 were heated together until all the solid had dissolved. Purified styrene (20 by volume) was added with an initiator, t-butyl hydroperoxide, the mixture degassed and the ampoule sealed. This was maintained at 1150C for 1 hour. The material remained clear and a strong smell of styrene was detected on opening the ampoule, which indicated that very little polymerisation had taken place. The polymerisation was repeated using azoisobutyronitrile as initiator and the mixture was heated at 6000 for 1.5 hours. The resulting material was å white solid but still very soft.Even after the remaining styrene had been pumped off, the polymer remained soft indicating that insufficient polymerisation had taken place to improve the physical properties.

Example 2 Thermal Polymerisation of PEO-LiCF3SO -Stvrene 1g of PEO was mixed with the lithium salt by stirring together in a sample bottle with a glass rod until the two solids appeared to be thoroughly mixed. To the sample bottle the determined amount of purified styrene was added and again mixed through the solid. The mixture could then be put into an ampoule ready for degassing and sealing.

Three mixtures were prepared: (1) 12:? PEO to LiCF3SO3 and 30p (by volume) styrene heated at 1250C for 7 hours (2) 8:1 PEO TO LiCF3SO3 and 40% styrene heated to 143 C for 5.5 hours (3) 8:1 PEO to LiCF3SO3 and 40p styrene heated at 1350C for 24 hours No initiator was added to the samples:all were thermally polymerised.

After polymerisation, sample (1) became hard but elastomeric. Sample (2) and (3) were more brittle and could be ground, unlike the former.

Example 3 Conductivity of PEO -LiCF SO -RO Styrene This material could not be ground but was cut into small pieces and subsequently pressed into 5mm diameter pellets under a pressure of 2 tons on a Specac press model P/N 15.00 (Analytical Accessories Ltd). On heating above 650C, the pellets were found to soften. By weighting the pellet between two glass plates and heating to approximately 80 C, under a number of successive pressings, the pellet could be moulded into films of thicknesses ranging between 180 and 350 pm.

The conductivity of these films was measured as previously described . From 20 to 60 0c the films showed conductivities comparable with those of a PEO complex. (Typically, at 30 C,or=2x10 6 ohm-1cm-1). Above 60 C, however, it was difficult to obtain reproducibility due to softening of the film and changes in cell constant.

Example 4 PEO@-LiCF3SO3-40% Stvrene(Shortt#e##rnqi po#ymen'#sation (5.5 hours)# This polymer system was more brittle than the previous one and could be ground before pressing into pellets. The sample became extremely 'gummy' on pressing and all attempts at forming films failed. The pellets were soft enough to press without heating but all broke up on attempting to remove from the glass slide. Pellets left for several weeks did eventually harden.

Example 5 PEO -LiCF3S03-40% Styrene(Long thermal polymerisation (24 hours)) In Example 4 the polymer could be ground and formed into pellets.

This time the pellets were hard They softened less readily on heating and the thinnest film made was 300 m Films were moulded under pressure rather than by heating by placing a 5mm diameter pellet into a 13mm diameter die and taking the pressure up to 8 tons. Films of 180 - 200 Sum thickness could be easily formed in this way. The films were much stiffer and whiter than those produced from the 30% styrene mixture, which is to be expected.

The conductivity of these films was lower by approximately a factor of 10 on PEO complex measurements. Unlike the 30% styrene films, conductivities could be measured and were reproducible over a much larger temperature range (20-1000C). A detectible change in cell constant was only apparent above 1100C.

Example 6 Solubility The solubility of the polymers was tested in various solvents and mixtures thereof. The solvents used were methyl and ethyl acetate, THF, chloroform and acetonitrile. Chloroform was the only solvent used here which was a common solvent for both pure poly(ethylene oxide) and polystyrene. No one solvent dissolved the polymer completely although it was partially soluble in acetonitrile and chloroform. A mixture of 2:1 ThF to chloroform was found to be the best solvent although acetonitrile could also be used as an alternative to chloroform.

Example ? PlO-LiCF#SO -Polystyrene ComDlexes 1. Two average molecular weights of PEO were studied, a) BDH Polyox W301 4x106 b) Aldrich 1x105 2. The ratio of EO units to Li ions was held throughout at 10:1 3. Polymerisations were carried out at two temperatures:120 and 1500C 4. The percentage of styrene of the total weight was 0,20,40 and 60 5. For each polymerisation carried out, a blank containing no electrolyte was polymerised at the same time 6. A polymerisa#tion time was found which was sufficient to allow the reaction to go to completion 7. The particle size of PEO was reduced by milling or grinding.

8. The polymer and electrolyte were milled together to ensure even distribution Example 8 Mixfnp PEO and Electrolvte The dried electrolyte, LiCF3S03, was finely ground by hand in a drybox and mixed into the PEO. This mixture was transferred to the steel mill, frozen in liquid nitrogen and ground for 5 minutes. Once the tube had been brought back to room temperature, it was reopened inside the drybox.

Example 9 Polymerisation of PEO-LiCF#SO with Stvrene Styrene was purified as already described and was used within 24 hours of distillatson. Appropriate amounts of styrene and polymer-electrolyte were mixed thoroughly and transferred to the polymerisation tubes within the drybox. These tubes were wide-necked to facilitate filling and were fitted with greaseless joints. The top was narrowed above the joint to allow the tube to be sealed after degassing.

Polymerisations were carried out at two temperatures: 120 and 1500C. Sufficient time had to be allowed for polymerisation to go to completion. Purified styrene thermally polymerises slowly in vacuo or under nitrogen at a a slow reproducible rate.

the initial rate of polymerisation of the bulk monomer may be expressed as: 11 Initial Rate (% per hour) - 3.55x10 exp(-19,200/RT) From this equation, an initial rate of 8% per hour is predicted for thermal polymerisation of styrene at 1200C. Similarily, at 150 C, the rate would be 42% per hour. These are minimum times and to ensure complete polymerisation samples were heated for 48 hours.

Example 10 ?:#m formation of i5Olu-LìCF3SO > -20 Stvrene Due to the rather hard, rubbery nature of the new electrolyte systems, it was not possible to successfully grind samples. The samples were therefore cut into small fragments. These were pressed on the 15 ton press in a 5mm die to 1.5 tons. The resulting pellets were poor, having distinct boundaries between the large fragments.

In order to soften the material and induce flow, the samples were heated while under pressure. This gave an even-surfaced homogeneous sample. The samples were transferred to a 5= die inside a drybo.

During the heating and pressing, the die was evacuated to water pump pressure. Two close fitting heating rings connected to a power source were placed round the die and the temperature monitored via a Cr/Al thermocouple placed between the rings and the die. Sufficient time was allowed for temperature equilibrium to be established through the die.

Variation in pressure (maximum 2 tons) showed no visible effect on the pellet and subsequently a pressure of#I.5 tons was used for all pressings.

Temperature standardisation was more critical. After 50 minutes pressing at temperatures between 60 and 68 0C, (melting temperature of PEO is 63 0C) the pressed fragments had not completely flowed together. Above 76 0C, the polymer softened sufficiently for it to flow under pressure around the sides of the anvils and the sample was therefore lost. Heating the pellet at 70-720C for 30 minutes produced even surfaces. All samples were held under pressure until the die had cooled to room temperature.

Once a pellet had been formed, it could be pressed out into a thin film. the best results were obtained by hot rather than cold pressing. The 5mm diameter pellets were placed in a 13mm diameter die which was evacuated. First the pellet was warmed to around 55 C before applying a 5 ton load for 1-2 minutes. The pressure was dropped to 2 tons and the temperature increased to 700C (higher temperatures resulted in over-softening the film again). This temperature was maintained for 15-20 minutes and the die allowed to cool before the pressure was released. Films were typically 120-250 ym thick. In general, the films were stored for 1 week to 10 days before conductivity measurements were carried out.

Example 11 Preparation of films of PEO -LiCF3SO3-40d Styrene Difficulty was encountered in preparing pellets and films of these materials by the method described in the previous Example. Some films of polymer containing higher molecular weight(4x106) PEO were made by this method but in general, the film surfaces were poor.

Pellets formed from polymer systems containing lower molecular weight (1x105) PEO broke up on removal from the anvils.

All variations of temperature and pressure failed to produce a film of the lower molecular weight material. - Attempts to grind the polymer at liquid nitrogen temperatures failed even after 30 minutes milling.

Films were successfully made by cutting from the polymerised block a single piece approximately 3mm by 3mm. This was cold pressed in a 5mm die at 2 ton load. Pressure was released and the die heated to 110 C and held at this temperature for 10 minutes. The die was then cooled to approximately for a load of 1.5 tons applied to the pellet and the system cooled to room temperature. There was slight flow of material around the anvils but insufficient material was lost to prevent a good pellet from being moulded. The pellet was removed and placed in a 13mm die, heated to 750C and 2 tons pressure applied.

The system was cooled under pressure. At room temperature, these films were stiff and tolerated only a limited amount of bending before breaking.

Example 12 Preparation of films of PlO 0-LiCF3SO3-6O# Stvrene A section of material, just small enough to fit into the 5mm die, was cut from the main sample. This was cold pressed at a 2 ton load.

Samples of the lower molecular weight PEO were found to be softer and broke up more easily than the equivalent higher molecular weight samples and were subsequently pressed under different conditions.

Example 13 Preparation of filmy containing PEO(4x10 average molecular weights After cold pressing, the samples were heated to 110 C (under a pressure of 1.5 tons) and held at this temperature for 15 minutes.

Once the die had cooled to room temperature, pressure was released and the pellet transferred to a 13mm die. The pellet was cold pressed at 5 tons for 2 minutes after which the pressure was reduced to 2 tons, the system heated for 15 minutes at 1100C and finally allowed to cool to room temperature, still under pressure.

Example 14 Preparation of filmy containing PEO(7xlO Average Molecular Weight) After cold pressing , the pressure was reduced to 1.5 tons and the sample heated at 80 C for 15 minutes. This was cooled to room temperature (under pressure) and the sample transferred to the 13mm die. The pellet was cold pressed for 2 minutes at 5 tons which was subsequently reduced to 0.5 tons before heating the sample to 800C for 15 minutes. The sample was again cooled to room temperature under pressure.

AC conductivity measurements were macie on polystyrene-free PEO- electrolyte filins, prepared by various methods. Figure 2 shows the variation in conductivity over the temperature range 20-1000C. For both molecular weights of PEO, the poorest conductivities were exhibited by cast films wi#th only a slight improvement for unannealed films.

Films prepared by the hot pressing method exhibited consistently higher conductivities than reported elsewhere and by comparing with figures 2 and 3, it can be seen that as much as 40,:0 styrene may be added to the system before conductivities fell to levels of previously reported data.

Conductivities for the mixed polymer materials PEO(M)120- polystyrene (where PSO molecular weight M is 4 x 106) and PEO (K) 123polystyrene (where P-0 molecular weight K is 1 x 103) are shown in figures 3 and 4.

Systems polymerised at 1500C were very similar to those polymerised at 120 C, Fig 9, and the molecular weight of PEO had little effect on conductivity. Figure 5 shows the temperature variation of the conductivity in the range 60-150 C for PEO(M) l20-(40, 60%) styrene.

As the concentration of polystyrene is increased, so there is an accompanying decrease in the volume of the conducting material Figures 6 and 7 show figures 3 and 4 corrected for this volute dilution.

Figure 2 Conductivities of PEO1oLiCF3SO3 from various film preparations

Annealed, hot pressed Unannealled, hot pressed Solvent cast - Data reported by J E Weston and B C H Steele Solid State Ionics (1981) ,2,347 Figure 3 Conductivity of PÒ(M)10LiCF3SO3 Y% Styrene

Figure 4 Conductivity of PEO(K)10LiCF3S03 Y% Styrene

Figure 5 Conductivity in temperature rarBe 60-130 C

PEO10LiCF3S03, 40% Styrene PEO10LiCF 3S03,60% Styrene lure 6 Conductivities of Figure 3 correctec for non-conducting volute figure 7 Conductivities of Figure 4 corrected for non-conoucting volune Table 1 Calculated values of volumes polystyrene Table 2 Relative physical strengths of polymer systecs Table 1 Percentage Polvstyrene in Polymers from 1H NMR POLYMER %STYRENE H(alith)/H(arom) 8 POLYSTYRENE PEO(K)/120 20 4.4 33 PEO(K)/150 n 6.9 22 PlO(M)/120 " 7.2 21 PEO(M)/150 n 8.6 18 PEO(K)/120 40 4.0 35 PEO(K)/150 " 2.5 48 PEO(M)/120 " 4.3 311 PEO(M)/150 " 2.3 48 PEO(K)/120 60 1.3 65 PEO(K)/150 " 1.8 57 PEO(M)/120 " 3.8 47 PEO(M)/150 " 1.6 61 (M) and (K) refer to molecular weight 4X106 and 1x105 respectively 120/150 refer to the polymerisation temperature Table 2 Normalised Penetration Vol% Styrene PEO(M) PEO(K) 0 1 1.29 20 - 0.15 40 0.2 0.26 60 0.07 .80 0.009 - o

Claims (17)

1. Claims 1. A method of producing a polymeric electrolyte which includes the steps of melting a polymer or mixture of polymers ("Matrix polymer") and dispersing therein a finely divided ionic salt.
2. 2. A method according to claim 1 wherein the matrix polymer comprises at least polyethylene oxide.
3. 3. A method according to claim 1 or 2 wherein the matrix polymer is made into a finely divided form prior to melting.
4. 4. A method according to claim 1, 2 or 3 wherein the finely divided ionic salt and a finely divided matrix polymer are produced by grinding.
5. 5. A method according to any one of the preceding claims wherein the ionic salt and matrix polymer are ground together.
6. 6. A method according to any one of the preceding claims wherein division or grinding of the ionic salt and/or matrix polymer is carried out at a temperature at which the matrix polymer becomes brittle.
7. 7. A method according to any one of the preceding claims wherein division or grinding is carried out under nitrogen.
8. 8. A method according to any one of the preceding claims wherein division or grinding is carried out in a ball mill.
9. 9. A method according to claim 7 wherein division or grinding is carried out for 10-15 minutes.
10. 10. A method according to claim 9 wherein the moleculer weight of matrix polymer is in the range 1 x 104 to 1 x 107
11. 11. A method according to any one of claims 1 to 8 wherein the ionic salt is the salt of an alkali metal alkaline earth metal or an ammonium or substituted ammonium salt.
12. 12. A method according to claim 11 wherein the ionic salt is LiCF3s03.
13. 13. A method according to any one of the preceding claims wherein a monomer which can polymerise is included with the polymer or mixture of polymers.
14. 14. A method according to claim 12 wherein the monomer is styrene.
15. 15. A method according to any one of the preceding claims wherein after formation of the polymeric electrolyte it is hot pressed into a pellet by pressing followed by heating and then allowing the polymeric electrolyte to cool.

    16. A method according to claim 15 wherein after cooling the pellet is repressed, then the load reduced but not removed and the film heated again and then allowed to cool.
16. 16. A process according to any one of the preceding claims, substantially as hereinbefore described.
17. 17. A polymeric electrolyte when prepared by a process according to any one of the preceding claims.