Copolyesters derived from aliphatic glycols, aromatic dicarboxylic acids and poly(alkylene-oxides) and films made therefrom

Abstract

An optionally oriented copolyester film comprising a copolyester which comprises repeating units derived from an aliphatic diol, an aromatic dicarboxylic acid and a poly(alkylene oxide), wherein the copolyester film further comprises lithium ions, and wherein the film has a thickness of no more than about 25 m. The copolyester film is suitable for use a separator in a lithium-ion rechargeable battery.

Claims

1. An oriented copolyester film comprising a copolyester which comprises repeating units derived from an aliphatic diol, an aromatic dicarboxylic acid and a poly(alkylene oxide), wherein the copolyester film further comprises lithium ions, wherein the film is a self-supporting film and wherein the film has a thickness of no more than 25 m; wherein said lithium ions are derived from lithium salts selected from lithium salts of: (i) carbonic acids; (ii) phenolic acids; (iii) the aromatic dicarboxylic acid from which the copolyester is derived; or (iv) an alkoxylate ester of the aromatic dicarboxylic acid from which the copolyester is derived wherein the alkoxylate ester is derived from the aliphatic diol from which the copolyester is derived.

2. A film according to claim 1, wherein the copolyester is block-like, the repeating units of the copolyester having a degree of randomness denoted by a value of B of from about 0.1 to 0.95.

3. A film according to claim 2, wherein the value of B is from about 0.3 to about 0.9.

4. A film according to claim 1, wherein the film has a thickness of no more than about 20 m.

5. A film according to claim 1, where the film comprises no plasticiser or comprises less than 15% of any plasticiser by weight of the film.

6. A film according to claim 1, wherein said copolyester comprises semi-crystalline segments derived from an aromatic dicarboxylic acid and an aliphatic glycol, and amorphous segments derived from poly(alkylene oxide).

7. A film according to claim 1, wherein the aliphatic diol is selected from C.sub.2, C.sub.3 or C.sub.4 aliphatic diols.

8. A film according to claim 1, wherein the weight average molecular weight of the poly(alkylene oxide) is from about 200 to about 20000 g/mol.

9. A film according to claim 1, wherein said lithium ions are present in a polymeric matrix of the film by the interaction between the lithium ions and negatively charged oxygen atoms.

10. A film according to claim 1 wherein the copolyester film comprises a lithium salt held within a polymeric matrix.

11. A film according to claim 1, wherein the Li:O molar ratio is from about 5:1 to about 1:50, wherein the number of O atoms in this ratio is defined as the number of O atoms in the poly(alkylene oxide) residue.

12. A film according to claim 1, wherein the melt viscosity of the copolyester from which the film is derived is from about 100 to 1000 Pa.Math.s at 275 C.

13. A film according to claim 1, which further comprises an antioxidant.

14. A film according to claim 1, which exhibits a conductivity of at least about 10.sup.13 S/cm measured at 100 C.

15. A film according to claim 1, wherein the copolyester is random, the repeating units of the copolyester having a degree of randomness denoted by a value of B of about 1.0.

16. A film according to claim 1, wherein the fraction S.sub.W of repeating units derived from aromatic dicarboxylic acid and poly(alkylene oxide), wherein S.sub.W is defined as the total molecular weight of the amorphous segments as a percentage of the total molecular weight of the copolyester, is in the range of from about 5% to about 70%.

17. A film according to claim 1, wherein the aromatic dicarboxylic acid is selected from naphthalene dicarboxylic acid and terephthalic acid.

18. A film according to claim 1, wherein the poly(alkylene oxide) is selected from polyethylene glycol (PEG) and polypropylene glycol (PPG).

19. A film according to claim 1, wherein said lithium ions are derived from lithium salts selected from lithium salicylate and dilithium bis hydroxy ethyl terephthalate of formula (I), ##STR00003##

20. A film according to claim 1, wherein the aromatic dicarboxylic acid is terephthalic acid and the lithium salt is dilithium terephthalate.

21. A film according to claim 1, wherein the lithium ions are held within a polymeric matrix of the film by virtue of the interaction between the lithium ions and the anion of a lithium salt which is not covalently bound to the copolyester.

22. A film according to claim 1, wherein the lithium ions are present in an amount of no more than about 5 weight % of the film.

23. A film according to claim 1, wherein the melt viscosity of the copolyester from which the film is derived is from about 100 to 1000 Pa.Math.s at a temperature within the range of T.sub.M to T.sub.M+10 C. wherein T.sub.M is the crystalline melting temperature of the copolyester.

24. A film according to claim 1, which exhibits a shrinkage of less than 5.0% after 30 mins at 100 C. in both dimensions of the film.

25. A film according to claim 1, which is a biaxially oriented film.

26. A film according to claim 1, which has a crystalline melting point (T.sub.m) of greater than 175 C.

27. A film according to claim 1, which has a glass transition point (T.sub.g) of no more than 50 C.

28. A lithium-ion rechargeable battery comprising an anode, a cathode and a separator between the anode and the cathode, wherein said separator is the copolyester film defined in claim 1.

29. A method of manufacturing a lithium-ion rechargeable battery, as claimed in claim 28 comprising the steps of: (a) preparing or obtaining a separator using the copolyester film; (b) assembling the lithium-ion rechargeable battery, wherein the battery comprises an anode, a cathode and a separator between the anode and the cathode, wherein said separator is obtained from step (a).

Description

EXAMPLES

(1) An illustrative reaction scheme to prepare copolyesters of the present invention is shown in Scheme 1 below.

(2) ##STR00002##

(3) Scheme 1: Synthesis of the bis(hydroxyalkyl)-ester of an aromatic dicarboxylic acid and subsequent polycondensation in the presence of a poly(alkylene oxide) to give a copolyester as described herein. The lithium salt (not shown in the scheme) is added at the start of the synthesis.

Experiment 1

(4) A preliminary series of copolyesters was made using ethylene glycol, terephthalic acid, polyethylene glycol (PEG4000) and dilithium terephthalate (DLTA), as set out in Table 1, to investigate the effects of PEG and lithium on the melt processability and conductivity of the copolyester. The copolyester compositions were made by reacting terephthalic acid and dilithium terephthalate with ethylene glycol under pressure (about 40 psi) at high temperature (about 240 C.). A trace of sodium hydroxide 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 was added to neutralise the base. Polyethylene glycol was then added and polycondensation was effected with an antimony trioxide catalyst at about 290 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 compositions were then extruded as a lace and cast into a water bath, dried and pelletized. The thermal properties, melt viscosity and conductivity were measured and the results are shown in Table 1 below (in which n/a means not applicable; and n/m means not measured).

(5) TABLE-US-00001 TABLE 1 Soft wt % Li Li T.sub.g; Melt TA EG PEG DLTA in final content in final Li/O T.sub.m; Viscosity Log .sup.DC added added added added polymer in feed polymer molar T.sub.d (Pa .Math. S) at (S cm.sup.1) Sample (g) (g) (g) (g) (wt %) (wt %) (wt %) ratio ( C.) 275 C. at 100 C. PET 2250 1000 0 0 n/a n/a n/a n/a 82.34; 217 14.72 control 249.31; (COMP A) 420 1 4537 2860 0 1513 n/a 2.25 1.65 n/a 78.17; 370 14.30 255.62; 414 2 4537 2860 1030 1513 16.00 1.84 1.52 0.64 24.59; 41 9.92 254.85; 406 3 5551 2640 777 0 n/m 0.00 0.00 n/a 44.21; 155 12.07 243.59; n/m 4 5330 2640 777 220 5.62 0.34 n/m n/m 40.86; 159 11.79 248.38; n/m 5 5050 2640 1000 501 27.00 0.49 n/m n/m 28.24; 121 n/m 226.38; n/m

(6) These initial results demonstrated that lithium ions can be successfully incorporated into a polyethylene terephthalate (PET) polyester to provide a conductive material. Sample 1 shows that lithium incorporation leads to an increase in the melt viscosity, but this increase can be counter-balanced by the decrease in melt viscosity which results from the copolymerisation of PEG into the lithium-containing polyesters (sample 2), while retaining the thermal properties within acceptable levels. However, the inventors have found that if the amount of PEG is too high and/or if the weight average molecular weight (M.sub.W) of PEG is too high, the lower melt viscosity may make it harder to reliably form film in a conventional film-forming apparatus. The inventors also observed that the copolymerisation of PEG increased the melt conductivity of the lithium-containing polymer, which the inventors believe is due to an increase in the mobility of lithium ions within the polymer morphology.

Experiment 2

(7) A second series of copolyesters was made using ethylene glycol, terephthalic acid, polyethylene glycol (PEG1000) and dilithium terephthalate, with the addition of 0.1 g of antioxidant (Irganox 1010). The amounts of the reactants are shown in Table 2, in which BHET refers to bis(hydroxyethyl)terephthalate and DLTA refers to dilithium terephthalate. The copolyester compositions were made generally in accordance with Scheme 1 above via trans-esterification followed by polycondensation, using 0.1 g of an Sb.sub.2O.sub.3 catalyst. The reaction products were subjected to solid state polymerisation over 24 hours at 200 C. under dynamic vacuum. The copolyester compositions were then extruded as a lace and cast into a water bath, dried and pelletized. The T.sub.g, T.sub.m, T.sub.d and melt viscosity were measured prior to solid state polymerisation and the results are shown in the Table, along with the conductivity, extractables content and randomness of the copolyester.

(8) TABLE-US-00002 TABLE 2 Soft wt % Li Li in T.sub.g; Melt Log .sup.DC Degree BHET PEG DLTA in final content final Li/O T.sub.m; Viscosity (S cm.sup.1) Ex- of added added added polymer in feed polymer molar T.sub.d (Pa .Math. S) at at tractables random- Sample (g) (g) (g) (wt %) (wt %) (wt %) ratio ( C.) 275 C. 100 C. (%) ness 6 22.60 17.5 59.90 60.02 12.08 1.84 0.10 14.15; 12 9.33 5.60 0.719 58.76 & 209.79; 401 7 90.20 8.62 1.18 10.10 0.12 0.04 0.03 45.53; 128 12.09 nm n/m 238.89; 404 8 86.58 8.62 4.80 10.60 0.49 0.16 0.14 44.93; 147 12.07 n/m n/m 237.49; 404 9 81.54 8.62 9.84 11.30 1.06 0.32 0.19 45.51; 145 12.28 n/m n/m 238.76; 404

(9) TABLE-US-00003 TABLE 3 Soft wt % T.sub.g; Melt Log .sup.DC Degree TA PEG in final T.sub.m; Viscosity (S cm.sup.1) Ex- of added added polymer T.sub.d (Pa .Math. S) at at tractables random- Sample (kg) (g) (wt %) ( C.) 275 C. 100 C. (%) ness 10 2050 700 15.90 38.80; 50 9.33 2.60 0.719 249.07; 405

(10) As demonstrated by the data in Table 2, copolyesters of the present invention have good thermal properties and through-film ionic conductivity.

Experiment 3

(11) Sample 10 was prepared in a similar manner to that described in Experiment 2. As demonstrated by the data in Table 3, copolyesters of the present invention have good thermal properties and through-film ionic conductivity.

Experiment 4

(12) A third series of copolyesters was made using 1050 kg ethylene glycol, 2050 kg terephthalic acid and 700 kg polyethylene glycol (PEG3450), with the addition of 1300 g of antioxidant (Irganox 1010). The copolyester compositions were made generally in accordance with Experiment 1, using 1040 g of the Sb.sub.2O.sub.3 catalyst. The reaction products were subjected to solid state polymerisation under dynamic vacuum at 210 C. between 0-48 h (see Table 4) and a subsequent Soxhlet extraction in xylenes. The copolyester was determined to have a soft weight, S.sub.W, of 15.80% and a degree of randomness, B, of 0.405. The extractables content, the composition of the extractable content and the PEG copolymerisation were measured and calculated as defined herein and the results shown in Table 4.

(13) TABLE-US-00004 TABLE 4 Ex- SSP tractables Extractables composition (wt %) PEG Reaction Content Cyclic Linear Copoly- Time (h) (%) PET PET PEG merisation(%) 0 2.60 1.07 49.71 49.22 89.34 16 1.05 4.81 71.95 23.24 97.97 24 0.67 3.64 75.00 21.36 98.81 30 0.42 4.19 80.59 15.22 99.47

(14) As shown in Table 4, a negative correlation exists between the length of solid state polymerisation reaction time versus the total amount of extractables obtained and the relative amount of unreacted polyethylene glycol in the extracted material. Thus, solid state polymerisation is preferably performed as an additional polymerisation step to aid the copolymerisation of the polyethylene glycol and polyester units.