COPOLYESTER FILMS FOR USE AS SEPARATORS IN LITHIUM-ION WET CELL BATTERIES

Abstract

Use of a copolyester film in the manufacture of a lithium-ion wet cell battery comprising an anode, a cathode and an electrolyte, wherein the copolyester film comprises a copolyester which comprises repeating units derived from a diol, a dicarboxylic acid and a poly(alkylene oxide)glycol.

Claims

1. Use of a copolyester film in the manufacture of a lithium-ion wet cell battery comprising an anode, a cathode and an electrolyte, wherein the copolyester film comprises a copolyester which comprises repeating units derived from a diol, a dicarboxylic acid and a poly(alkylene oxide)glycol, wherein said copolyester film does not comprise lithium ions, wherein said copolyester film is an oriented film, and wherein during said manufacture said copolyester film is disposed in said battery as a separator between the anode and the cathode.

2. A use according to claim 1 wherein during said manufacture, said copolyester film which does not comprise lithium ions is contacted with said electrolyte prior to said copolyester film being disposed in said battery as a separator between the anode and the cathode.

3. A use according to claim 1 or 2 wherein said film has a thickness of no more than about 25 μm, preferably no more than about 20 μm, preferably no more than about 18 μm, preferably no more than 15 μm.

4. A use according to any preceding claim, wherein said film has a thickness of from about 0.3 μm, preferably from about 0.5 μm, preferably from about 0.9 μm, preferably from about 1.0 μm.

5. A use according to any preceding claim, wherein said copolyester comprises semi-crystalline segments derived from said diol and said dicarboxylic acid, and amorphous segments derived from said poly(alkylene oxide) glycol.

6. A use according to any preceding claim wherein the glycol is an aliphatic glycol, preferably wherein the glycol is selected from C.sub.2, C.sub.3 or C.sub.4 aliphatic diols, preferably wherein the glycol is ethylene glycol.

7. A use according to any preceding claim wherein the dicarboxylic acid is an aromatic dicarboxylic acid, preferably wherein the dicarboxylic acid is selected from naphthalene dicarboxylic acid and terephthalic acid, preferably wherein the dicarboxylic acid is terephthalic acid.

8. A use according to any preceding claim wherein the poly(alkylene oxide) glycol is selected from polyethylene glycol (PEG) and/or polypropylene glycol (PPG), preferably wherein the poly(alkylene oxide) is polyethylene glycol (PEG).

9. A use according to any preceding claim wherein the weight average molecular weight of the poly(alkylene oxide) glycol is from about 200 to about 20000 g/mol, preferably from about 400 to about 3900 g/mol, preferably from about 500 to about 3900 g/mol, preferably from about 500 to about 3800 g/mol, preferably from about 500 to about 3700 g/mol, preferably about 3450 g/mol.

10. A use according to any preceding claim wherein the film further comprises an antioxidant.

11. A use according to any preceding claim wherein the film exhibits a conductivity of at least about 10.sup.−8 S/m, preferably at least about 10.sup.−7 S/m, preferably at least about 10.sup.−6 S/m, preferably at least about 10.sup.−5 S/m, preferably at least about 10.sup.−4 S/m, measured at 25° C.

12. A use according to any preceding claim wherein the film exhibits a shrinkage of less than 5.0% after 30 mins at 100° C. in both dimensions of the film.

13. A use according to any preceding claim wherein the film is a biaxially oriented film.

14. A use according to any preceding claim wherein the film is a self-supporting film.

15. A use according to any preceding claim wherein the film has a crystalline melting point (T.sub.m) of greater than 175° C.

16. A use according to any preceding claim wherein the film has a glass transition point (T.sub.g) of no more than 60° C.

17. A method of manufacturing a lithium-ion wet cell battery comprising an anode, a cathode, a separator between the anode and the cathode and an electrolyte, the method comprising the steps of: (a) preparing or obtaining a separator which is or comprises a copolyester film which does not comprise lithium ions as defined in any of claims 1 to 16; (b) preparing or obtaining an electrolyte; (c) assembling the battery, wherein the battery comprises an anode, a cathode, a separator between the anode and the cathode, and an electrolyte, wherein said separator is obtained from step (a) and wherein said electrolyte is obtained from step (b).

18. A method according to claim 17 wherein said copolyester film which does not comprise lithium ions is contacted with said electrolyte prior to said copolyester film being disposed in said battery as a separator between the anode and the cathode.

19. A method of manufacturing a lithium-ion wet cell battery comprising an anode, a cathode, a separator between the anode and the cathode, and an electrolyte, the method comprising the steps of: (a) preparing or obtaining a separator which is or comprises an oriented copolyester film wherein the oriented copolyester film comprises a copolyester which comprises repeating units derived from a diol, a dicarboxylic acid and a poly(alkylene oxide)glycol; (b) preparing or obtaining an electrolyte; (c) assembling the battery, wherein the battery comprises an anode, a cathode, a separator between the anode and the cathode, and an electrolyte, wherein said separator is obtained from step (a) and wherein said electrolyte is obtained from step (b), and wherein the copolyester film already comprises lithium ions prior to contacting with said electrolyte, wherein said lithium ions are intrinsic lithium ions introduced into the copolyester from which the copolyester film is derived prior to film formation.

20. A method according to claim 19 wherein said copolyester film is as defined in any of claims 3 to 16.

21. A method according to claim 17, 19 or 20 wherein said copolyester film is contacted with said electrolyte either before or after said copolyester film is disposed in said battery as a separator between the anode and the cathode.

22. A method according to claim 19 or 20, or according to claim 21 when dependent from claim 19 or 20, wherein said intrinsic lithium ions are introduced into the copolyester from which the copolyester film is derived prior to film formation during synthesis of said copolyester or during a separate compounding step in which said copolyester is compounded with a lithium salt.

23. A use according to any one of claims 1 to 16, or a method of manufacturing according to any of claims 17 to 22, wherein the electrolyte comprises lithium hexafluorophosphate (LiPF.sub.6), lithium hexafluoroarsenate (LiAsF.sub.6), lithium tetrafluoroborate (LIBF.sub.4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium thiocyanate (LiSCN), lithium trifluoromethanesulfonate (LiCF.sub.3SO.sub.3), lithium bromide (LiBr), lithium iodide (LiI), lithium bis(trifluoromethanesulfonimide), (LiN(CF.sub.3SO.sub.2).sub.2), lithium tris(trifluoromethylsulfonyl)methide (LiC(CF.sub.3SO.sub.2).sub.3), lithium orthosilicate (Li.sub.4O.sub.4Si), lithium trifluoroacetate (LiCF.sub.3CO.sub.2), lithium bis(fluorosulfite)amide (LiN(FO.sub.2S).sub.2)LiClO.sub.4, lithium iron phosphate (LiFePO.sub.4), lithium bis(oxalate)borate (LiBOB) and/or lithium difluorophosphate (LiPO.sub.2F.sub.2).

24. A use according to any one of claim 1 to 16 or 23, or a method according to any of claims 17 to 23, wherein the electrolyte comprises ethylene carbonate and ethyl methyl carbonate.

Description

EXPERIMENTAL

Experiment 1

[0133] A series of copolyesters was made using terephthalic acid, ethylene glycol and polyethylene glycol (PEG3450). PEG3450 was present at a level of 10 to 12 wt % of the copolyester. The copolyesters were made by reacting 2050 kg terephthalic acid, 1050 kg ethylene glycol and 700 kg PEG3450 under pressure (about 40 psi) at high temperature (about 240° C.), along with the addition of an antioxidant (Irganox® 1010, 1300 g) and china clay (5.2 kg). A trace of sodium hydroxide (130 g) was added to prevent the formation of unwanted by-products, and the esterification reaction proceeded without the need of a catalyst. Water was distilled off from the reaction and the reaction stopped once 90% of the theoretical weight of water from the reaction had been collected. Phosphoric acid stabiliser (975 g) was added to neutralise the base. Polycondensation was then effected with an antimony trioxide catalyst (1040 g) at about 280° C., and wherein the pressure above the melt was reduced to less than 1 mm Hg. As the polycondensation reaction proceeded, the viscosity of the batch increased, and once a pre-determined viscosity had been achieved the polymerisation reaction was stopped by restoring the pressure in the vessel back to atmosphere. The copolyester was then extruded as a lace and cast into a water bath, dried and pelletized before being subjected to solid state polymerisation under dynamic vacuum at about 210° C. for about 24 hours.

[0134] These copolyesters were then extruded through a film-forming die on to a water-cooled rotating, quenching drum at various line speeds to yield an amorphous cast composite extrudate. The cast extrudate was heated to a temperature of between about 50 and 60° C. and then stretched longitudinally at a forward draw ratio of about 3.3. The polymeric film was pre-heated to 65 to 70° C. and passed into a stenter oven at a temperature of from 75 to 85° C. where the sheet was stretched in the sideways direction to approximately 4 times its original dimensions. The film was then heat-set under dimensional restraint in a 3-stage crystalliser held at temperatures within the range of 180° C. to 220° C. with dimensional relaxation of up to 5% in the transverse dimension. The final thickness of the film was 10 μm.

[0135] The through-film ionic conductivity of the film was measured in both the dry cell setup (solid state), and the wet cell set-up (electrolyte present) and the results are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Through-film Ionic Operating Temperature Conductivity Setup (° C.) (S m.sup.−1) Dry cell 25 5.0 × 10.sup.−8 Wet cell 25 2.8 × 10.sup.−7 Wet cell 30 3.4 × 10.sup.−6 Wet cell 40 1.6 × 10.sup.−5 Wet cell 50 8.1 × 10.sup.−5 Wet cell 60 2.3 × 10.sup.−4 Wet cell 70 5.1 × 10.sup.−4 Wet cell 75 6.0 × 10.sup.−4

[0136] These results demonstrate that the copolyester films described herein have excellent through-film ionic conductivity at a low thickness of 10 μm. In particular, the copolyester films of the present invention have excellent through-film ionic conductivity when used as a separator in a wet cell battery with a lithium-based electrolyte across a range of commercially important operating temperatures.

Experiment 2

[0137] A lithium-ion wet cell battery was prepared. The film of experiment 1 was soaked in the electrolyte. A composition comprising 1M LiPF.sub.6 in ethylene carbonate:ethyl methyl carbonate (EC:EMC) with 1 wt % vinylene carbonate (VC) was used as the electrolyte. The soaked film was then placed between a graphite anode and a NMC (Nickel Manganese Cobalt 622) cathode to prepare the lithium-ion wet cell battery.