GB2346729A - Polymer electrolyte - Google Patents

Abstract

A polymer electrolyte for use in electrochemical cells comprises a polymer combined with a solution of a salt in a plasticising solvent, wherein the polymer is a copolymer of vinylidene fluoride and between 1 and 8% by weight of hexafluoropropylene. The copolymer has a sufficiently high molecular weight to form a mechanically strong polymer film, and so has a low value of melt flow index, which at 230{C and 10 kg is less than 5.0 g/10 min, and preferably less than 1.0 g/10 min. A film of the polymer electrolyte can be cast from a volatile solvent, while the plasticising solvent can be selected to achieve the desired electrical properties. Electrical cells using this electrolyte have a high capacity, which does not decrease rapidly with cycling.

Description

Polvmer Electrolvte This invention relates to a polymer electrolyte for use in electrochemical cells, and to an electrochemical cell incorporating this electrolyte.

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 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 lithium cobalt oxide, and such materials may be mixed with solid electrolyte material to form a composite cathode. The use of an intercalation material such as graphite as the anode material (in place of metallic lithium) has also been suggested, and this also may be mixed with solid electrolyte material to form a composite anode.

Polymer electrolytes comprising a polymer matrix plasticised with a solution of a lithium salt in an organic solvent have also been suggested. For example Gozdz et al. (U. S. 5 296 318 and WO 95/15589) describe compositions comprising a copolymer of 75 to 92% by weight vinylidene fluoride and 8 to 25% hexafluoropropylene; this copolymer can be combined with a lithium salt and a plasticising solvent such as ethylene carbonate/propylene carbonate to provide a stable film with conductivity greater than 10 S cm. This composition may be cast from a suitable solvent. A range of such electrolyte compositions are described in an article by Z. Jiang et al. (Electrochimica Acta, Vol 42, p 2667), the compositions being prepared by thermal extrusion, and being based on either the homopolymer PVdF 741 or the copolymer PVdF/HFP 2822 (Kynar (trade mark) grades). These grades of polymer can be characterised by their melt flow index, measured by the method specified in standard ASTM D 1238, which are respectively 7-15 g/10 min and 3-8 g/10 min measured at 232 C with a 12.5 kg load. GB 2 309 703 (AEA Technology) describes the use in making an electrolyte of a homopolymer of PVdF of exceptionally low melt flow index; such a polymer may be combined with a salt and a plasticising solvent and cast from a suitable low boiling point solvent to produce a good quality electrolyte film.

According to the present invention there is provided a polymer electrolyte comprising a polymer combined with a solution of a salt in a plasticising solvent, wherein the polymer is a copolymer of vinylidene fluoride and between 1 and 8% by weight of hexafluoropropylene, and has a melt flow index, at 230 C and 10 kg, of less than 5.0 g/10 min.

The polymer of the present invention contains less hexafluoropropylene (HFP) than Gozdz et al. teach as being the minimum for formation of a satisfactory film.

Nevertheless it has been found that very good quality films can be made, mechanically strong and with high electrical conductivity. As with the low melt flow index homopolymer PVdF described in the AEA Technology patent mentioned above, the copolymer of the present invention has a sufficiently high molecular weight to form a mechanically strong polymer film, and so a low value of melt flow index. The melt flow index at 230 C and 10 kg is desirably less than 5.0 g/10 min, and preferably less than 1.0 g/10 min. In contrast it is soluble in a greater range of solvents, such as tetrahydrofuran (THF) or acetone. Consequently a film of the polymer electrolyte can be cast from a volatile solvent, while the plasticising solvent can be selected to achieve the desired electrical properties.

A restriction on the use of the homopolymer PVdF described in the AEA Technology patent mentioned above is the limited range of solvents available for the polymer at room temperature: dimethyl acetamide (DMA), dimethyl formamide (DMF), and N-methyl-pyrrolidone (NMP). These are all high boiling point solvents, which therefore require harsh drying conditions if complete removal of the solvent is to be ensured, and such drying conditions would also tend to remove significant quantities of the plasticising solvent. Indeed experiments have indicated that where DMA is used as a casting solvent the residual quantities of DMA are significant, and decomposition of such residual DMA at voltages above 4 V may be a factor in causing capacity decline on cycling in cells containing lithium cobalt oxide composite cathodes.

The invention will now be further and more particularly described, by way of example only, with reference to the following Examples, and with reference to the following drawings in which: Figure 1 shows graphically the variation in specific energy with the number of discharge/charge cycles, for a cell whose active cathode material is lithium nickel oxide; Figure 2 shows graphically the variation of specific energy with number of cycles for a cell whose active cathode material is lithium nickel/cobalt oxide; and Figure 3 shows graphically the variation of specific energy with number of cycles for a cell whose active cathode material is lithium cobalt oxide.

Electrolvte production Example 1 An electrolyte has been made using an electrolyte salt solution (1M LiPF6 in a mixture of three parts ethylene carbonate to two parts propylene carbonate (3EC/2PC)). 15.0 grams of this salt solution were added to 18.0 grams of tetrahydrofuran (THF), and 3.0 grams of a copolymer of vinylidene fluoride and hexafluoropropylene, containing 2% by weight of hexafluoropropylene (PVdF/2HFP), was added while stirring. The mixture was warmed to fully dissolve the polymer.

The volatile solvent THF would boil at about 66 C.

The mixture was cast using a doctor blade above a roller, the electrolyte being coated directly onto a composite electrode at a web speed of 1.0 m/min and with a coating head blade gap in the range 0.35-0.6 mm. The solvent was evaporated in the presence of an air stream while passing through successive drying zones at temperatures of 50 , 55 and 60 C, to evaporate all the THF.

Example 2 An electrolyte has been made using the same electrolyte salt mixture (1M LiPF6 in 3EC/2PC), 20.0 grams of this salt solution being added t-o 28.0 grams dimethyl carbonate (DMC), and 4.0 grams of PVdF/2HFP copolymer added while stirring. The mixture was warmed to fully dissolve the polymer.

The volatile solvent DMC would boil at about 89 C.

An electrolyte film was cast from this DMC mixture in the same way as described in relation to Example 1, using the same drying temperatures to evaporate substantially all the DMC (although the presence of any residual DMC would not be expected to be detrimental to cell performance).

The electrical conductivity of the electrolytes of examples 1 and 2 is similar to that of an electrolyte using homopolymer PVdF, the value at room temperature (say 20 C) being about 2 mS/cm.

Cathode production Example 3 50.0 grams LiNio2 1. 7 g carbon (Ketjenblack EC600) and 60 cm acetone were measured into a plastic container containing alumina balls, and the mixture was ball-milled for 1 hour.

25.0 grams electrolyte salt solution (1M LiPF6 in 3EC/2PC) was added to 36 grams THF, and 3.0 grams PVdF/2HFP copolymer was added while stirring, and the mixture was warmed to fully dissolve the polymer. This polymer and salt solution was then added to the plastic container containing Lino2 and ball-milled for a further hour.

The cathode mixture was then cast, using a doctor blade over a roller with a blade gap 0.65-1.20 mm, onto a 25 Wm aluminium foil coated with a graphite (Acheson dag) coating. The web speed was 0.65 m/min, and the dryer zones were at 50 , 60 , and 80 C to ensure evaporation of the volatile solvents acetone and THF.

Example 4 50. 0 grams LiNio 75 Co 0. 25021 1. 7 g carbon (Ketjenblack 3 EC600) and 60 cm acetone were measured into a plastic container containing alumina balls, and the mixture was ball-milled for 1 hour.

25.0 grams electrolyte salt solution (1M LiPF6 in 3EC/2PC) was added to 40 grams DMC, and 3.0 grams PVdF/2HFP copolymer was added while stirring. The mixture was warmed to fully dissolve the polymer. This polymer solution was then added to the plastic container and ball-milled for a further hour.

This cathode mixture was cast in the same way as described in relation to Example 3, using the same dryer zone temperatures to ensure evaporation of the volatile solvents acetone and DMC.

Anode production Example 5 6.0 grams PVdF/2HFP copolymer was added to 30.0 grams of electrolyte salt solution (1M LiPF6 in 3EC/2PC) and 40 grams THF, and the solution stirred and warmed to dissolve the polymer.

43.2 grams mesocarbon microbeads of particle size 10 Wm, heat treated at 2800 C (MCMB 10-28) were mixed with 4.8 grams graphite powder and then mixed with 24 grams THF to produce a slurry. The polymer solution was slowly stirred into this carbon slurry, and stirred on a hot plate for one hour.

This anode mixture was then cast, using a doctor blade over a roller with a blade gap 0.30-0.65 mm, onto a 10 Wm copper foil. The web speed was 0.65 m/min, and the dryer zones were at 70 , 90 , and 100 C to ensure evaporation of the volatile solvent THF.

Example 6 6.0 grams PVdF/2HFP copolymer was added to 30.0 grams of electrolyte salt solution (1M LiPF6 in 3EC/2PC) and 30 grams DMC, and the solution stirred and warmed to dissolve the polymer.

43.2 grams mesocarbon microbeads (MCMB 10-28) were mixed with 4.8 grams graphite powder and then mixed with 32 grams DMC to produce a slurry. The polymer solution was slowly stirred into this carbon slurry, and stirred on a hot plate for one hour.

This anode mixture was then cast as described in relation to Example 5, using the same dryer temperatures to ensure evaporation of the volatile solvent DMC, but at a web speed of 0.85 m/s.

In each of the above Examples the polymer component was the PVdF/2HFP copolymer, but it will be appreciated that other similar copolymers may be used, in particular copolymers of vinylidene fluoride with 4% or 6% (by weight) of hexafluoropropylene (PVdF/4HFP or PVdF/6HFP respectively). In each case the selected copolymer should have a very high molecular weight, and consequently a low melt flow index. Values of melt flow index for these copolymers (with a load of 21.6 kg) have been measured as 3.1,2.6, and 2.8 g/10 min respectively, and the values with a load of 10 kg would be less than a third of these values.

In the above examples of electrolytes (Examples 1 and 2) the ratio of copolymer to electrolyte salt mixture was about 1: 5, but satisfactory electrolytes have been made with different values of this ratio, for example 1: 4.

Layers of electrolyte, anodes, and cathodes made as described in the above examples can be assembled to form electrical cells. Preferably a layer of electrolyte is cast onto the active surface of the anode, a layer of electrolyte is cast onto the active surface of the cathode, and then the anode/electrolyte is laminated to the electrolyte/cathode. Alternatively a layer of electrolyte is laminated onto the active surface of the anode, a layer of electrolyte is laminated onto the active surface of the cathode, and then the anode/ electrolyte is laminated to the electrolyte/cathode. The layers may be bonded together by heating them to about 110 C and then passing them between rollers pressing the layers together. Alternatively a cell might have just a single layer of electrolyte.

Such cells have been tested by being repeatedly discharged and recharged between voltage limits of about 4.0 volts and 3.0 volts, the first few cycles enabling the cell capacity to be determined, and the subsequent cycles being performed at the C/5 rate. The typical voltage limits depend upon the composition of the cathode, the voltage limits with LiNiO2 being 4.0 to 2.7 V; the voltage limits with LiNil-xco. being 4.1 or 4.2 V to 3.0 V; and the voltage limits for LiCoO2 being 4.1 or 4.2 V to 2.75 V. These cells have typically been found to have a specific energy in the range 6-8 mWh cm, and the specific energy typically decreases by no more than about 20% over 300 cycles. In cells made as described the total thickness of electrolyte is about 0.2 mm (that is to say about 0.1 mm provided by each layer of electrolyte); it has been observed that the thickness of electrolyte in such a cell is not a significant factor in determining the cell's capacity, which indicates that most of the cell's internal resistance is in the electrodes.

Referring to Figure 1 there is shown the variation of specific energy with number of discharge/recharge cycles for a cell in which the polymer was PVdF/6HFP.

The cathode contained lithium nickel oxide (LiNio2) as active material, and was cast using acetone and DMC as solvent. The anode contains MCMB 10-28 with 10% graphite, and was cast using THF as solvent. The electrolyte had a polymer: electrolyte salt solution ratio 1: 4.6 by weight, and was cast from DMC as solvent.

Almost all the values are between 9 and 10 mWh cm, and there is a decrease of about 10% over almost 150 cycles.

(The low values obtained on the 12th-17th cycles are because measurements were made to determine the cell capacity, using higher rates of discharge.) Referring now to Figure 2 there is shown the variation of specific energy with number of discharge/ recharge cycles for a cell in which the polymer was PVdF/2HFP. The cathode contained a mixed nickel/cobalt oxide, LiNio~5Coo. z5O2, and was cast from acetone and DMC as solvent. The anode contained MCMB 10-28 with 10% graphite, and was cast using DMC as solvent. The electrolyte had a polymer: electrolyte salt solution ratio 1: 5 by weight, and was cast from DMC as solvent.

The cell was tested using 4.2 volts as the upper charge voltage limit. The cell capacity is observed to decrease -2 gradually from about 8.8 to 6.9 mWh cm over 350 cycles.

Referring now to Figure 3 there is shown the variation of specific energy with number of discharge/recharge cycles for a cell in which the polymer was PVdF/2HFP. The cathode was lithium cobalt oxide (LiCoO2) and was cast using acetone as solvent. The anode contained MCMB 10-28 with 10% graphite, and was cast using THF as solvent. The electrolyte had a polymer: electrolyte salt solution ratio 1: 4.5 by weight, and was cast from DMC as solvent. The cell was tested using 4.1 volts as the upper charge voltage limit. The cell capacity is observed to decrease gradually from -2 about 7.6 to 6.4 mWh cm over more than 80 cycles.

It will be appreciated that the polymer electrolyte of the invention may differ from those described above while remaining within the scope of the invention. For example the adhesion between the composite anode or cathode and the metal foil current collector may be enhanced by grafting monomers onto the co-polymer chain.

As described in GB 2 309 701 (though in relation to a PVdF homopolymer) these monomers may be a monounsaturated sulphonic acid, phosphonic acid, carboxylic acid, ester, or amide ; generally smaller monomers with less than five carbon atoms in the carbon chain R-, are preferable. For example acrylic acid, various isomers of butenoic acid, or isomers of pentenoic acid may be used.

The degree of grafting is desirably between 2 and 20% of the final weight, more preferably between 3 and 12%, for example 5% or 10%. The grafting can be achieved by an irradiation process or a pre-irradiation process.

Claims (8)

1. Claims 1. A polymer electrolyte comprising a polymer combined with a solution of a salt in a plasticising solvent, wherein the polymer is a copolymer of vinylidene fluoride and between 1 and 8% by weight of hexafluoropropylene, and has a melt flow index, at 230 C and 10 kg, of less than 5.0 g/10 min.
2. 2. A polymer electrolyte as claimed in claim 1 wherein the copolymer has a value of melt flow index which at 230 C and 10 kg is less than 3.0 g/10 min.
3. 3. A polymer electrolyte as claimed in claim 1 or claim 2 wherein the ratio of copolymer to the solution of salt in the plasticising solvent, by weight, is between 1: 2 and 1: 8.
4. 4. A polymer electrolyte as claimed in claim 3 wherein the said ratio is between 1: 4 and 1: 6.
5. 5. A polymer electrolyte as claimed in any one of the preceding claims wherein the plasticising solvent consists essentially of a mixture of ethylene carbonate and propylene carbonate.
6. 6. An electrochemical cell incorporating a polymer electrolyte as claimed in any one of the preceding claims.
7. 7. An electrochemical cell comprising an anode, a cathode, and an electrolyte layer, incorporating a polymer electrolyte as claimed in any one of claims 1 to 5 within the anode, the cathode and the electrolyte layer.
8. 8. An electrochemical cell substantially as hereinbefore described with reference to the accompanying drawings.