NEW POLYMERS FOR BATTERY APPLICATIONS

Abstract

New block polymers are described, as well as processes for preparing them using ring-opening polymerisation and ring-opening copolymerisation techniques. Also described are electrolytes, cathodes and batteries comprising the polymers.

Claims

1. A polymer having a structure according to Formula I:
A-B-A (I) wherein A is a polycarbonate block; A is absent or is a polycarbonate block A; and B is different to A and is a block composed of a poly(ester-co-carbonate) or a polycarbonate.

2. The polymer of claim 1, wherein the polymer has a number average molecular weight (M.sub.n) of 15-100 kg mol.sup.1; or 30-55 kg mol.sup.1.

3. (canceled)

4. The polymer of claim 1, wherein the polymer comprises 16-65 wt % of block A; or 30-40 wt % of block A.

5. (canceled)

6. The polymer of claim 1, wherein A has a glass transition temperature, T.sub.g, that is 60 C.

7. The polymer of claim 1, wherein B has a glass transition temperature, T.sub.g, that is 0 C.

8. The polymer of claim 1, wherein A is a polycarbonate block A, such that the polymer is a tri-block copolymer.

9. The polymer of claim 1, wherein A is absent, such that the polymer is a di-block copolymer.

10. The polymer of claim 1, wherein (1) the polymer is a di-block copolymer and has a molecular weight (M.sub.n) of 20-70 kg mol.sup.1 and comprises 20-60 wt % of block(s) A, or (2) the polymer is a tri-block copolymer and has a molecular weight (M.sub.n) of 45-70 kg mol.sup.1 and comprises 25-65 wt % of block(s) A.

11. The polymer of claim 1, wherein A has a structure according to Formula A-i: ##STR00018## wherein .sup.1 denotes the point of attachment of an oxygen atom, said oxygen atom being a part of B; X.sup.1 is an end group; and L is a linking group separating the two oxygen atoms to which it is attached by either: (i) a distance of 2-3 carbon atoms; or (ii) a distance of 2 carbon atoms, said 2 carbon atoms forming part of a ring.

12. (canceled)

13. The polymer of claim 1, wherein a proportion of the A and/or B block repeating units independently comprises a pendant neutral functional group FG.sub.N selected from P(O)(OH).sub.2, COOH, OH, SO.sub.3H, NH.sub.2, C(O)NH.sub.2, F, CF.sub.3 and CN, and/or a pendant anionic functional group FG.sub.A selected from PO.sub.3.sup.2, PO.sub.2(OH).sup., COO.sup., SO.sub.3.sup., SO.sub.2N.sup.SO.sub.2CF.sub.3, N.sup.SO.sub.2CF.sub.3, (CF.sub.2).sub.2O(CF.sub.2).sub.2SO.sub.3.sup., BO.sub.4.sup., (C.sub.6H.sub.4).sub.4B.sup., (C.sub.6F.sub.4).sub.4B.sup. and CHFCF.sub.2SO.sub.3.sup..

14. The polymer of claim 1, wherein A has a structure according to Formula A-ii: ##STR00019## wherein .sup.1 denotes the point of attachment of an oxygen atom, said oxygen atom being a part of B; X.sup.1 is an end group; and each R.sup.1 is independently absent or a group X(R.sup.2).sub.v, in which each R.sup.2 is independently a pendant neutral functional group FG.sub.N or a pendant anionic functional group FG.sub.A, wherein FG.sub.N is selected from P(O)(OH).sub.2, COOH, OH, SO.sub.3H, NH.sub.2, C(O)NH.sub.2, F, CF.sub.3 and CN, and FG.sub.A is selected from PO.sub.3.sup.2, PO.sub.2(OH).sup., COO.sup., SO.sub.3.sup., SO.sub.2N.sup.SO.sub.2CF.sub.3, N.sup.SO.sub.2CF.sub.3, (CF.sub.2).sub.2O(CF.sub.2).sub.2SO.sub.3.sup., BO.sub.4.sup., (C.sub.6H.sub.4).sub.4B.sup., (C.sub.6F.sub.4).sub.4B.sup. and CHFCF.sub.2SO.sub.3; each v is independently 0 or 1; and each X is (when v is 1) a linking group that links R.sup.2 to the cyclohexyl ring, or is (when v is 0) a terminal group.

15. The polymer of f claim 1, wherein B is a poly(ester-co-carbonate).

16. The polymer of any one of claim 1, wherein B is poly(caprolactone-co-trimethylene carbonate) and wherein block B comprises 70-90 mol % of ester repeating units and 10-30 mol % of carbonate repeating units.

17. A process for the preparation of a polymer, the process comprising the steps of: (a) performing ring-opening polymerisation of: (i) a cyclic carbonate or (ii) a mixture of a cyclic carbonate and a cyclic ester to form a polymeric block B being a polycarbonate or a poly(ester-co-carbonate); and (b) growing a polymeric block A on one or both ends of the polymeric block B by ring-opening copolymerisation of: (i) an epoxide or an oxetane, and (ii) carbon dioxide.

18. The process of claim 17, wherein ring opening polymerisation in step (a) is initiated using a monofunctional initiator and step (b) comprises growing the polymeric block A on one end of the polymeric block B; or the ring opening polymerisation in step (a) is initiated using a difunctional initiator and step (b) comprises growing the polymeric block A on both ends of the polymeric block B.

19. (canceled)

20. The process of claim 17, wherein steps (a) and (b) are conducted in a one-pot manner, and wherein step (a) is terminated and step (b) is commenced by the addition of carbon dioxide.

21. The process of claim 17, further comprising the step of: (c) modifying a proportion of the block A and/or block B repeating units by introducing: a pendant neutral functional group selected from P(O)(OH).sub.2, COOH, OH, SO.sub.3H, NH.sub.2, C(O)NH.sub.2, F, CF.sub.3 and CN; and/or a pendant anionic functional group selected from PO.sub.3.sup.2, PO.sub.2(OH).sup., COO.sup., SO.sub.3.sup., SO.sub.2N.sup.SO.sub.2CF.sub.3, N.sup.SO.sub.2CF.sub.3, (CF.sub.2).sub.2O(CF.sub.2).sub.2SO.sub.3.sup., BO.sub.4.sup., (C.sub.6H.sub.4).sub.4B.sup., (C.sub.6F.sub.4).sub.4B and CHFCF.sub.2SO.sub.3.sup..

22. An electrolyte comprising a mixture of a polymer as claimed in claim 1 and a metal salt, wherein the metal salt is of the formula M.sup.+ X.sup., wherein M.sup.+ is selected from Na.sup.+, Li.sup.+ and K.sup.+, and X.sup. is selected from BF.sub.4.sup., ClO.sub.4.sup., bis(fluorosulfonyl)imide, bis(trifluoromethanesulfonyl)imide, bis(oxalato)borate, a perfluoroalkylsulfonate (e.g., CF.sub.3SO.sub.3.sup.), a polyfluoroalkyl sulfontate, PF.sub.6, AsF.sub.6.sup., cyano(trifluoromethanesulfonyl)imide, bis|(pentafluoroethyl)sulfonyllimide, B(CN).sub.4.sup., 4,5-dicarbonitrile-1,2,3-triazole, perylene diimide, 4,5-dicyano-2-(trifluoromethyl)imidazolium and combinations of two or more thereof.

23. (canceled)

24. A cathode for a battery, the cathode comprising a polymer as claimed in a claim 1.

25. A battery comprising a polymer as claimed in claim 1.

Description

EXAMPLES

[0170] One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying figures:

[0171] FIG. 1. ROP of -CL and TMC followed by ROCOP of VCHO with CO.sub.2 to produce the copolymer PVCHC-PCL/PTMC via switch catalysis. When producing triblock copolymers, R.sub.1=poly(CL-r-TMC) and R.sub.2=(CL-r-TMC-b-VCHC). When producing diblocks, R.sub.1=R.sub.2H. (i) ROP was conducted at 100 C. for 10 minutes in a reaction mixture that is 26 mL, 40% VCHO and 60% toluene by volume. Typical molar ratio: [LZn.sub.2Ph.sub.2]/[CTA]/[TMC]/[CL]/[VCHO]=1/4/375/1500/3000, where [CTA], [-CL], and [TMC] are adjusted to achieve the desired composition and M.sub.n. To produce triblock copolymers, the chain transfer agent (CTA) benzene dimethanol (BDM) was used; methyl benzyl alcohol (Me-BnOH) was used to produce diblocks. The catalyst concentration was 0.92 mM. (ii) The reaction vessel temperature was maintained at 100 C. and the vessel atmosphere was changed to 1 bar CO.sub.2 to initiate a mechanistic switch to VCHO/CO.sub.2 ROCOP.

[0172] FIG. 2. Exemplar polymer characterisation data. (a) Assigned .sup.1H NMR spectrum (CDCl.sub.3) of the purified block polymer PVCHC-PCL/PTMC-PVCHC. (b) .sup.31P{H} spectra (CDCl.sub.3) after reaction of the polymer hydroxyl end groups with 2-chloro-4,4,5,5-tetramethyldioxaphospholone. Top: Tri-block copolymer PVCHC-PCL/PTMC-PVCHC, displaying PVCHC-OH end groups. Bottom: Homopolymer PCL/PTMC, displaying PCL-OH and PTMC-OH end groups. (c) GPC trace for the polymer sample ABA(44, 0.53), conducted with a THF eluent and calibrated to a polystyrene standard.

[0173] FIG. 3. Thermal and mechanical behaviour of the ABA(50, 0.35) polymer electrolyte film, with 17 wt. % LiTFSI. (a) DSC trace for: ABA(50, 0.35) (top) exhibiting a lower T.sub.g1 at 49 C. and a weak upper T.sub.g2 at 89 C.; ABA(50,0 0.35) with 17 wt. % LiTFSI, exhibiting a lower T.sub.g1 at 40 C. and a weak upper T.sub.g2 at 100 C. (b) TGA trace for ABA(44, 0.53) with 17 wt. % LiTFSI. (c) Cyclic tensile testing (20% strain, 10 mm min.sup.1 extension rate). (d) Elastic recovery at zero stress after 20% strain, defined as 100(.sub.20%(0,.sub.20%))/.sub.20%, where .sub.maX and (0, .sub.maX) are the maximum strain and the strain in the cycle at zero stress after the maximum strain .sub.maX, as a function of cycle number..sup.[10]

[0174] FIG. 4. Li-ion conductivity data for ABA(50, 0.35) with 17 wt. % LiTFSI. (a) Li-ion conductivity as a function of temperature, obtained by electrochemical impedance spectroscopy. (b) VFT fit.

[0175] FIG. 5. Plots exploring the relationship of the ionic conductivity of the polymer electrolytes to the polymer's, M.sub.n hard wt., and the wt. % LiTFSI; and further electrochemical data. (a): The ionic conductivity of the materials at 30 C. in relation to the polymer's M.sub.n for both triblock and diblock copolymers, where all polymers have a fixed hard weight fraction of 0.5. (b) The ionic conductivity of the polymer electrolytes at 30 C. in relation to the polymer's hard weight fraction, for both triblock and diblock copolymers, where all polymers have a fixed M.sub.n of approximately 50 kg mol.sup.1. (c) Li-ion conductivity data showing the effect of salt concentration. Samples of ABA(50, 0.35) with 50, 40, 30, 20, and 17 wt. % LiTFSI have been studied. (d) Cyclic voltammetry for ABA(50, 0.35) polymer electrolyte with 17 wt. % LiTFSI with a cell configuration of lithium vs. stainless steel, measured at 60 C. at a sweep rate of 0.5 mV s.sup.1.

MATERIALS AND METHODS

Materials

[0176] All solvents and reagents were purchased and used as obtained from commercial sources (Sigma Aldrich) unless stated otherwise. The synthesis of 4-tert-butyl-2,6-diformylphenol, [H.sub.4L])(ClO.sub.4).sub.2 and H.sub.2L were carried out in air..sup.[11] The synthesis of the catalyst, [LZn.sub.2(Ph).sub.2],.sup.[10]] monomer purification and subsequent polymerisations were carried out under inert conditions using standard Schlenk line techniques and a nitrogen-filled glovebox. Vinyl cyclohexenoxide (VCHO) and -caprolactone (-CL) were dried by stirring over CaH.sub.2, distilled under reduced pressure and stored under nitrogen. 1,4-Benzene dimethanol (BDM) was recrystallized from toluene three times and kept under nitrogen. Trimethylene carbonate (TMC) was purchased from TCI. It was recrystallized from dry Et.sub.2O under a nitrogen atmosphere and dried in vacuo before use.

NMR

[0177] .sup.1H and .sup.31P{H} NMR spectra were obtained using a Bruker AVIII HD 400 NMR spectrometer. .sup.13C{H} NMR spectra were obtained using a Bruker AVII 500 NMR spectrometer.

GPC

[0178] GPC data was obtained using a Shimadzu LC-20AD instrument equipped with a Refractive Index (RI) detector and two PSS SDV 5 m linear M columns. HPLC grade THF was used as the eluent, flowing at 1.0 mL/min at 30 C. A monodisperse polystyrene standard was used for calibration. Samples were passed through 0.2 m syringe filters prior to analysis.

DSC

[0179] DSC of the polymers was conducted using a DSC3+ (Mettler-Toledo Ltd) instrument. A sealed, empty crucible was used as a reference and the DSC was calibrated using zinc and indium. Samples were cooled from 25 C. to 80 C. at a rate of 20 C. min.sup.1 under a N.sub.2 flow (80 mL min.sup.1) followed by a 5 minute isotherm at 80 C. Samples were then heated to 200 C. at a rate of 20 C. min.sup.1; kept at 2000 C. for a further 5 minutes; followed by a cooling-heating procedure from 200 C. to 80 C. at 10 C. min.sup.1. Glass transition temperatures (T.sub.g) were reported as the midpoint of the transition taken from the third second cycle.

TGA

[0180] TGA was conducted on a TGA/DSC 1 (Mettler Toledo Ltd) system. Polymer samples were heated from 30 to 500 C. at a rate of 5 C. min.sup.1, under an N.sub.2 flow (100 mL min.sup.1).

Tensile Measurements

[0181] For tensile testing, dumbbell specimens were cut according to ISO 527-2, specimen type 5B with Zwick ZCPO20 cutting press (length=35 mm, gauge length=10 mm, width=2 mm). Uniaxial extension measurements were carried out on a Shimadzu EZ-LZ Universal testing instrument at an extension rate of 10 mm min.sup.1 to derive stress-strain relationships.

Electrochemical Impedance Spectroscopy

[0182] Ionic conductivity was measured by impedance spectroscopy using an MTZ-35 Impedance Analyzer (Biologic) over the frequency range 10 MHz-0.01 Hz with the amplitude set to 10 mV. The electrolytes were sandwiched between gold electrodes in a Controlled Environment Sample Holder which was then enclosed in an Intermediate Temperature System. Measurements were taken at 10 C. intervals between 2 and 70 C. The samples were equilibrated at each temperature for 20 min before a new recording was made. The resistance was calculated with EBioLabs using a modified Debye equivalent circuit.

Voltammetry

[0183] Linear sweep and cyclic voltammetry were conducted on a VMP2 (Bio-Logic). Between two stainless steel discs, a 4 mm disc of ABA(50, 0.35) and a 3 mm lithium disc were sandwiched. The cell was annealed at OCV for 3 h at 60 C. before cycling was performed at 0.5 mV s.sup.1. All procedures were carried out in an argon-filled glovebox.

Experimental

Synthesis of H.SUB.2.L

[0184] The ligand was synthesised according to the published procedure..sup.[10] [H.sub.4Ln](ClO.sub.4) (5.0 g, 6.7 mmol) and MeOH (500 mL) were added to a round-bottom flask to obtain a red/orange solution. The solution was cooled to 0 C. before the slow addition of NaBH.sub.4 (7.58 g, 200 mmol) to yield a colourless solution. The solution was left stirring at room temperature for 1 h before water was added until precipitation was observed (400 mL). The resultant suspension was left standing for 10 h before being filtered, washed with water and dried under vacuum, at 40 C., to yield a white solid. The precipitate was crystallised from MeOH to yield white crystals (2.56 g, 4.63 mmol, 72%). .sup.1H NMR (400 MHz, CDCl.sub.3, 298 K): (ppm)=6.93 (s, 4H, ArH), 3.73 (s, 8H, CH.sub.2), 2.52 (s, 8H, CH.sub.2), 1.25 (s, 18H, CH.sub.3), 1.01 (s, 12H, CH.sub.3).

Synthesis of [LZn.sub.2(Ph).sub.2]

[0185] The catalyst was synthesised according to the published procedure..sup.[10] Under anaerobic conditions, two separate solutions of [H.sub.2L](0.40 g, 0.7 mmol) in THF (5 mL) and ZnPh.sub.2 (0.25 g, 1.2 mmol) in THF (2 mL) were pre-cooled to 40 C. before being added to a glass vial together to obtain a cloudy solution. The mixture stirred for 25 h at 25 C. and filtered to obtain a white solid (260 mg, 0.311 mmol, 55%).

Switch Catalysis of VCHO, CO.SUB.2., TMC, and -CL

[0186] In the glovebox, TMC was added to 1,4-BDM, followed by -CL, VCHO, toluene, and LZn.sub.2Ph.sub.2. The reaction vessel was sealed, taken outside of the glovebox and heated to 100 C., with rapid stirring. At 90% -CL conversion (approx. 20 min), the reaction vessel atmosphere was changed to 1 bar CO.sub.2. At 15% VCHO conversion (approx. 30 h), the reaction vessel was cooled, opened to the atmosphere, and 0.2 mL of 0.1 M benzoic acid in chloroform was added, stirring. The conversion of -CL and VCHO was determined by .sup.1H NMR spectroscopic analysis of the crude reaction mixture. The reaction mixture was precipitated three times from methanol (3200 mL) to yield a white polymer. The material was dried in vacuo and the block copolymer was isolated as a colourless solid (c.a. 2 g). See FIG. 1a for an exemplar assigned .sup.1H NMR spectrum; see Table 3 for the reagent quantities used to produce various tri- and diblock copolymers.

TABLE-US-00001 TABLE 1 Reagent quantities for the synthesis of triblock and diblock poly(carbonate-b-ester-r-carbonate). LZnPh.sub.2 .sup.b CTA .sup.c -CL .sup.d TMC .sup.d VCHO .sup.d Entry .sup.a (mmol) (mmol) (mmol) (mmol) (mmol) ABA(35, 0.52) 0.024 0.114 15.4 3.8 82.5 ABA(44, 0.53) 0.024 0.091 15.1 3.8 84.1 ABA(50, 0.47) 0.024 0.080 11.2 2.8 74.6 ABA(66, 0.52) 0.024 0.061 15.4 3.8 82.5 ABA(51, 0.26) 0.024 0.078 23.7 5.9 41.3 ABA(47, 0.30) 0.024 0.085 22.4 5.6 47.6 ABA(50, 0.35) 0.024 0.080 20.8 5.2 55.6 ABA(60, 0.62) 0.024 0.067 12.2 3.0 98.4 AB(26, 0.45) 0.024 0.154 17.6 4.4 71.4 AB(32, 0.51) 0.024 0.125 15.7 3.9 81.0 AB(45, 0.47) 0.024 0.089 17.0 4.2 74.6 AB(69, 0.58) 0.024 0.058 15.4 3.8 92.1 AB(37, 0.21) 0.024 0.108 25.3 6.3 33.3 AB(54, 0.33) 0.024 0.074 11.8 2.9 52.4 .sup.a Entries named ABA(X, Y) for triblock copolymers and AB(X, Y) for diblock copolymers, where X is M.sub.n, NMR and Y is the hard wt. fraction; .sup.b Quantity of catalyst, LZn.sub.2Ph.sub.2, in the reaction mixture; .sup.c Quantity of chain transfer agent (CTA). Benzene dimethanol (BDM) used to make triblock copolymers, methylbenzylalcohol (MeBnOH) used to make diblock copolymers; .sup.c Quantity of -CL, TMC, and VCHO in the reaction mixture. The total volume is made up to 26 mL with toluene.

Results and Discussion

Polymer Synthesis

[0187] PVCHC-PCL/PTMC-PVCHC and the corresponding diblock polymer PVCHC-PCL/PTMC were produced by switch catalysis, using a LZn.sub.2Ph.sub.2 catalyst which was synthesised according to the literature..sup.[11] LZn.sub.2Ph.sub.2 is known to be active for both lactone ROP and epoxide/CO.sub.2 ROCOP and is able to polymerise selectively from a monomer mixture..sup.[12] It has phenyl co-ligands: these are unable to initiate polymerization but can react in-situ with an alcohol initiator to produce the initiating species. This produces hydroxy-telechelic polymers only; this is an important attribute when targeting ABA and AB block polymers. A typical polymerisation was conducted using a relative ratio of 1/4/375/1500/3000 of LZn.sub.2Ph.sub.2/chain transfer agent/TMC/-CL/VCHO. The reaction mixture was 26 mL: by volume, approximately 40% VCHO and 60% toluene. The optimal catalyst concentration was 0.92 mM. For the triblock copolymer, 1,4-benzenedimethanol (BDM) was used as the chain-transfer agent; 4-methylbenzyl alcohol (Me-BnOH) was used to produce diblocks. Experiments were stirred at 1600 rpm and heated to 100 C.

[0188] To aid the systematic investigation of properties, two series of triblock polymers have been targeted: those with a fixed hard wt. % and different molecular weight (M.sub.n) (ABA(X, 0.50), and those with a fixed M.sub.n and different hard wt. % (ABA(50, Y)) (Table 1). Both M.sub.n and hard-block content influence achieving phase-separated nanostructures and thus affect the physical properties of the polymer. The ratio of chain transfer agent, -CL, TMC, and VCHO were adjusted to produce the desired polymer weight and composition. By using a mono-functional initiator, a series of AB diblock polymers, featuring the same systematic variations in M.sub.n and hard-block content, were also prepared.

[0189] Initially, ROP of -CL and TMC produced poly(CL-r-TMC) (FIG. 1). By .sup.1H NMR spectroscopy of the crude reaction mixture, it was shown that 90% -CL conversion was reached after 20 minutes. At this point, the reaction vessel atmosphere was changed to 1 bar CO.sub.2 to initiate a mechanistic switch to VCHO/CO.sub.2 ROCOP. This proceeded from the polyester chain ends and produced the ABA triblock copolymer PVCHC-PCL/PTMC-PVCHC (Scheme 1). The ROCOP was allowed to proceed for 30 hours. After that, the reaction mixture was quenched by addition of 0.2 mL of a 0.1 M benzoic acid solution in chloroform: this reacts with the catalyst to stop the polymerization. Crude polymer samples were analysed by .sup.1H NMR spectroscopy to determine monomer conversion of -CL and VCHO by integration of the peaks corresponding to the monomer (=2.70, 4.87 ppm) and polymer (=4.14, 3.20 ppm). This was not possible for TMC as its signals overlapped with -CL. The polymer was purified by precipitation from methanol, dried in vacuo, and characterized using .sup.1H NMR spectroscopy, .sup.31P{H} end-group analysis, diffusion-ordered NMR spectroscopy (DOSY), and gel permeation chromatography (GPC).

[0190] In the .sup.1H NMR spectrum of the purified polymer, the expected signals were observed corresponding to PVCHC and PCL-r-PTMC. Integration of the peaks revealed the molar composition of the polymer (FIG. 2a). As the signals attributed to TMC (4.08-4.25, 1.99 ppm) both show some overlap with -CL and VCHO signals respectively, it is difficult to observe their ratios; however, the polymers are estimated to be 20 mol % TMC, as targeted. The polymerisation is highly selective for polyester and polycarbonate formation as ether linkages were not observed (3.30-3.60 ppm). Due to the high molar masses of the polymers, block junction signals were not detectable. Repeated precipitations of the polymer did not change the polymer composition, supporting the formation of a block copolymer rather than a blend. Chain end-group analysis was conducted by .sup.31P{H} NMR spectroscopy after reaction of the polymers' ,-hydroxyl groups with a phosphorous reagent, according to a literature procedure..sup.[13] The vinyl cyclohexenol end group signal (146.6 ppm) was observed, indicative of selective block copolymer formation (FIG. 2b). The block structure was further evidenced by DOSY. A single diffusion coefficient was observed, indicative of joined blocks of PVCHC and PCL/PTMC; if a homopolymer blend was instead present, two distinct diffusion coefficients would be observed.

[0191] The molecular weight of the polymer was estimated by .sup.1H NMR spectroscopy and GPC, plus the theoretical value can be calculated from the monomer/initiator ratio and the monomer conversions. There was good agreement between DP.sub.calc and DP.sub.NMR, suggesting that there were few impurities such as H.sub.2O to act as additional chain transfer agents (Table 1). GPCs have been conducted with a THF eluent. Monomodal mass distributions were observed and polymers showed high M.sub.n (FIG. 2c). This supports the formation of one type of polymer chain, rather than a mixture of homopolymer and block copolymer. For most samples, there was good agreement between M.sub.n,NMR and M.sub.n,GPC (Table 1). A narrow polydispersity index (D) was not expected for this polymer..sup.[14] -CL provides a primary propagating site for ROP, thus there is a broader dispersity. Some polymers show a disparity between M.sub.n,NMR and M.sub.n,GPCparticularly, ABA(66, 0.52), ABA(50, 0.35), and AB(69, 0.58). This is likely due to a GPC effect: differences in M.sub.n,NMR and M.sub.n,GPC for other polymers have previously been reported..sup.[15] It can be attributed to polycarbonates exhibiting different chain-folding behaviour to polystyrene, which the column is calibrated to. This results in a lower hydrodynamic volume, so a lower measured molar mass value.

TABLE-US-00002 TABLE 2 Characterisation data for the triblock and diblock copolymers DP.sub.PC-DP.sub.PE- DP.sub.PC-DP.sub.PE- M.sub.n, NMR.sup.c M.sub.n, GPC, THF.sup.d DP.sub.PC DP.sub.PC M.sub.n, NMR, ABA.sup.c (kg (kg mol.sup.1) Hard T.sub.g1, T.sub.g2.sup.e T.sub.d, 5%.sup.f Entry.sup.a (calc).sup.b (NMR).sup.c (kg mol.sup.1) mol.sup.1) [] wt. %.sup.c ( C.) ( C.) ABA(35, 0.52) 52-144-52 54-157-54 9-17-9 35 29 [1.69] 52 38, 89 204 ABA(44, 0.53) 70-182-70 70-194-70 12-21-12 44 47 [1.56] 53 40, 102 235 ABA(50, 0.47) 73-274-73 70-250-70 12-27-12 50 40 [2.03] 47 43, 84 242 ABA(66, 0.52) 99-258-99 102-266-102 17-29-17 66 45 [1.58] 52 47, 98 ABA(51, 0.26) 47-366-47 40-352-40 7-38-7 51 28 [1.75] 26 31, 81 ABA(47, 0.30) 46-323-46 42-315-42 7-34-7 47 49 [1.52], 30 49, 95 240 8 [1.18] ABA(50, 0.35) 55-359-55 52-306-52 9-33-9 50 35 [1.89] 35 48, 89 241 ABA(50, 0.62) 85-176-85 93-176-93 16-19-16 50 50 [1.85] 62 48, 94 230 AB(26, 0.45) 88-152 70-139 13-15 26 26 [1.63] 45 49, 92 210 AB(32, 0.51) 106-160 95-152 16-16 32 45 [1.52] 51 37, 88 AB(45, 0.47) 100-236 126-215 12-13 45 47 AB(69, 0.58) 286-280 238-261 88-64 69 69 [1.68] 58 56, 113 231 AB(37, 0.21) 60-340 54-315 9-34 37 37 [1.70] 21 51, 92 216 AB(54, 0.33) 120-369 107-343 18-36 54 21 [1.90] 33 43, 81 .sup.aEntries named ABA(X, Y) for triblock copolymers and AB(X, Y) for diblock copolymers, where X is M.sub.n, NMR and Y is the hard wt. fraction; .sup.bCalculated from the initial [-CL + TMC + VCHO].sub.0/[BDM].sub.0 ratio and the monomer conversions; .sup.cDetermined from the .sup.1H NMR spectra of the purified polymer by integration of the aromatic initiator resonance (7.34 ppm for 1,4-BDM, 7.17 ppm for MeBnOH) against those of PVCHC (5.76 ppm) and PCL/PTMC (2.00, 1.38 ppm); .sup.dM.sub.n of the overall block copolymer determined by GPC with THF eluent, calibrated using PS standards; .sup.eEstimated by DSC (10 C. min.sup.1 heating rate), third heating curve; .sup.fThermal degradation temperature of the polymer with 17 wt. % LiTFSI, determined by TGA. Recorded as the temperature at 5% mass loss.

Thermal Properties

[0192] Thermal data was obtained using DSC. Polymers were fully amorphous and possessed a strong lower T.sub.g at around 40 C., corresponding to the inner poly(CL-r-TMC) block, and a weaker upper T.sub.g at around 100 C., corresponding to the hard poly(VCHO-alt-CO.sub.2) block (FIG. 3a). The T.sub.g values are consistent with the literature, where T.sub.g=106 C. is reported for PVCHC,.sup.[16] and T.sub.g=56 C. for PCL/PTMC..sup.[14] The observation of upper and lower T.sub.g values close to the homopolymer values demonstrated that there is phase separation in the polymer. This is desirable for good mechanical properties: flexibility, extensionability, elastic recovery, high temperature stability, and tensile strength.

[0193] As these polymers are being investigated for battery applications, it is pertinent to study their properties after the addition of lithium salt. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was chosen as this salt is widely used in studies relating to Li-ion batteries..sup.[17] To the polymers, 17 wt. % of LiTFSI was added (relative to the overall polymer M.sub.n) and electrolyte films were prepared by solvent casting..sup.[18]

[0194] The DSC traces of the polymers with salt retain their amorphous nature and exhibit a shift in T.sub.g relative to the pure polymer (FIG. 3a). ABA(50, 0.35) showed an increase in the upper and lower T.sub.g on salt addition. The lower T.sub.g corresponds to the polyester block poly(CL-r-TMC): salt enables transient crosslink formation within this block, as observed in the literature..sup.[17, 19] With regards to the upper T.sub.g, Kimura et al. have shown that the T.sub.g of polyethylenecarbonate-based electrolytes increases from 0 to 10 wt. % salt, likely due to interaction between the carbonyl oxygen and Li ions..sup.[20] This supports the upper T.sub.g, corresponding to the polycarbonate PVCHC, increasing.

[0195] The thermal stability of the polymers have been measured with the addition of 17 wt. % LiTFSI. For all polymers, T.sub.d,5 % is above 200 C. (Table 1): this is sufficiently high for the desired application. The TGA trace shows three different regions: 200-270 C. corresponds to decomposition of the poly(CL-r-TMC) block; 270-385 C. corresponds to decomposition of the PVCHC block; and 385-500 C. to the decomposition of LiTFSI (FIG. 3b).

Mechanical Properties

[0196] The elastic recovery of the ABA(50, 0.35) polymer electrolyte film with 17 wt. % LiTFSI was determined: the ability of the material to return to its original shape after strain. In a battery, polymers will experience stress and strain during charge and discharge cycles thus this is a key parameter. Dumbbell-shaped specimens were cut from the polymer film using a cutting press.

[0197] Three ABA(50, 0.35) samples were tested: the mean is reported and the errors are represented by standard deviations. Each sample was subjected to 10 hysteresis cycles to 20% strain at an extension rate of 10 mm min.sup.1. This strain was used as it is a little above the volume change experienced by high-capacity cathode particles..sup.[21] The first elastic cycle differs to those subsequent due to the initial disentangling of polymer chains. An elastic recovery of 70% was recorded: this was lower than what would be seen from an ideal elastomer but nevertheless demonstrates that the material shows recovery after experiencing strain (FIGS. 4c, 2d).

Ion Transport

[0198] Li-ion conductivity measurements have been obtained for the polymers in the series (Table 3) using electrochemical impedance spectroscopy (EIS) in a 2-electrode cell. Electrolyte films of 250 m thickness were prepared by solvent casting the polymer plus 17 wt. % LiTFSI from a THF solution (20 wt. %) in a Teflon mould. It was dried by solvent evaporation under ambient pressure and an N.sub.2 atmosphere for 48 h, and then in vacuo at 40 C. for a further 72 h. Measurements were taken at 10 C. temperature intervals between 2 and 70 C.

[0199] The triblock and diblock copolymer with the highest ionic conductivity at 30 C. were ABA(50, 0.35) and AB(37, 0.21) respectively: 5.910.sup.6 S cm.sup.1 and 9.810.sup.6 S cm.sup.1. Plots of ionic conductivity against temperature have been produced for each sample and is shown for ABA(50, 0.35) (FIG. 4a). Ionic conductivity increased with temperature: from 5.910.sup.6 S cm.sup.1 at 30 C. to 7.210.sup.5 S cm.sup.1 at 70 C. This is due to increasing polymer chain mobility with increasing temperature, consequently increasing ion mobility.

[0200] The temperature dependence of ionic conductivity can be modelled by the Vogel-Fulcher-Tammann (VFT) fit (FIG. 4b), a modified Arrhenius equation, by plotting equation 1:

[00001]$\begin{array}{cc}=A{T}^{-1/2}\exp \left[-B/\left(T-T_0\right)\right]& \left(1\right)\end{array}$

where A is a constant proportional to the number of carrier ions, B is related to the activation energy of Li.sup.+ movement, and T.sub.0 is the temperature where configurational entropy is zero (T.sub.0T.sub.g50 K).

[0201] For the polymers studied, the plot shows a linear relationship. This indicated an ionic conductivity mechanism where ions hop between vacant coordination sites, aided by the segmental motion of the polymer. This was true for all of the polymers studied. The gradient of the plot allows the activation energy of the electrolyte to be calculated: this was 17.4 kJ mol.sup.1 for ABA(51, 0.35). This value was higher than obtained for poly(CL-r-TMC) (9.4 kJ mol.sup.1), as reported by Mindemark et al..sup.[14] This suggests that the presence of the PVCHC hard block produced an additional energy barrier for ion movement. Here, there was a positive correlation between E.sub.a and InA, akin to the observation made by Balsara et al. for a poly(styrene-b-ethylene oxide) electrolyte..sup.[22] Consequently, a lower E.sub.a does not necessarily correspond to a higher ionic conductivity because there are also fewer carrier ions.

TABLE-US-00003 TABLE 3 Li-ion conductivity data for ABA and AB polymers M.sub.n Hard T.sub.g1, T.sub.g2.sup.c .sub.70 C..sup.di .sub.28 C..sup.dii E.sub.s.sup.e Entry.sup.a (kg mol.sup.1).sup.b wt. %.sup.b ( C.) (S cm.sup.1) (S cm.sup.1) (kJ mol.sup.1) lnA.sup.f ABA(35, 0.52) 35 52 49, 101 5.6 10.sup.5 6.6 10.sup.7 15.3 5.6 ABA(44, 0.53) 44 53 48, 102 2.2 10.sup.3 9.8 10.sup.7 8.7 2.9 ABA(50, 0.47) 50 47 48, 100 4.9 10.sup.5 2.6 10.sup.6 13.0 0.0 ABA(66, 0.52) 66 52 32, 100 5.6 10.sup.3* 5.5 10.sup.5 12.8 6.3 ABA(51, 0.26) 52 26 38, 98 9.9 10.sup.5 1.2 10.sup.6 16.0 7.3 ABA(47, 0.30) 47 30 33, 66 2.6 10.sup.5** 4.3 10.sup.5 11.2 2.5 ABA(50, 0.35) 50 35 40, 100 2.2 10.sup.4* 5.9 10.sup.6 17.0 8.3 ABA(50, 0.62) 50 62 32, 85 3.8 10.sup.5 1.4 10.sup.6 14.6 1.7 AB(26, 0.45) 28 45 52, 96 4.4 10.sup.5 1.5 10.sup.6 15.4 4.0 AB(32, 0.51) 32 51 37, 84 1.9 10.sup.5** 8.4 10.sup.7 23.6 12.5 AB(45, 0.47) 45 47 19, 102 2.2 10.sup.3 2.0 10.sup.7 9.1 3.8 AB(69, 0.58) 69 58 20, 94 1.9 10.sup.5 2.1 10.sup.8 8.8 0.9 AB(37, 0.21) 37 21 5.8 10.sup.5** 9.8 10.sup.6 14.5 2.6 AB(54, 0.33) 54 33 42, 82 1.6 10.sup.4 2.2 10.sup.5 10.0 2.6 .sup.aEntries named ABA(X, Y) for triblock copolymers and AB(X, Y) for diblock copolymers, where X is M.sub.n, NMR and Y is the hard wt. fraction; .sup.bDetermined from the .sup.1H NMR spectra of the purified polymer by integration of the aromatic initiator resonance (7.34 ppm for 1,4-BDM, 7.17 ppm for MeBnOH) against those of PVCHC (5.76 ppm) and PCL/PTMC (2.00, 1.38 ppm); .sup.cLower (T.sub.g1) and upper (T.sub.g2) glass transition temperatures of the polymer with 17 wt. % LiTFSI, estimated by DSC (10 C. min.sup.1 heating rate), third heating curve; .sup.dIonic conductivity was measured using impedance spectroscopy. The ionic conductivity was calculated from the resistance that was obtained by fitting the acquired data to a modified Debye circuit;.sup.[23] .sup.diLi-ion conductivity at 70 C., except for values labelled * which were conducted at 60 C. and ** which were conducted at 50 C., values have a 9% error .sup.diiLi-ion conductivity at 30 C., values have an 11% error; .sup.eActivation energy of Li-ion transfer, obtained from the VFT fit; .sup.fRelative free charge carrier concentration of the polymer electrolyte, obtained from the VFT fit.

[0202] For this set of polymers, ionic conductivity was not significantly affected by the glass transition temperature of either the soft or hard block. When T.sub.g1 was between 53 and 33 C., there was no significant trend between T.sub.g1 and ionic conductivity. Polymer samples AB(45, 0.47) and AB(69, 0.58) had T.sub.g1's of 19 and 20 C. and also had the lowest ionic conductivities (2.010.sup.7 S cm.sup.1, 2.110.sup.7 S cm.sup.1). This suggests that for this system, lowering T.sub.g1 is an effective strategy for improving ionic conductivity until 20-33 C.; below this, other factors become more important..sup.[19]

[0203] The relationship between the ionic conductivity of the materials and their M.sub.n and hard wt. % has been explored. Triblock polymers with a fixed hard weight fraction of 0.5 showed an increase in ionic conductivity at 30 C. as their M.sub.n is increased: a 10-fold increase was observed between ABA(35, 0.52) and ABA(66, 52). The opposite trend was observed for diblocks: as their M.sub.n increased, their ionic conductivity decreased (FIG. 5a). For the triblock copolymers, this could be due to the high M.sub.n polymers having a larger grain size..sup.[19] This is this is an encouraging finding as the higher M.sub.n polymers will likely show improved mechanical properties, such as a higher storage modulus. For the diblocks, higher M.sub.n led to a significantly greater T.sub.gfor AB(69, 0.58), a 36 C. increase in T.sub.g1 is observed relative to the pure polymerthis results in a decrease in ionic conductivity.

[0204] For triblock copolymers with a fixed M.sub.n of approximately 50 kg mol.sup.1, ionic conductivity peaked at a hard weight fraction of 0.35: more or less hard block content resulted in a decreased ionic conductivity. A similar trend was seen for diblock copolymers: the highest ionic conductivity was at a hard weight fraction of 0.33 (FIG. 5b). At this composition, it is likely that the polymer forms a phase which has favourable to ion transport.

[0205] The effect of salt concentration on ionic conductivity has been studied on polymer ABA(50, 0.28). Electrolyte films containing 17, 20, 30, 40, and 50 wt. % LiTFSI have been prepared. The optimum amount of LiTFSI for ionic conductivity was 20 wt. % (FIG. 5c). This is because too little salt results in fewer free charge carriers, whereas too much may result in transient crosslinking of the polymer chains..sup.[17, 24] This can be observed in the DSC traces: generally, as salt concentration decreased, the lower T.sub.g also decreased.

Electrochemical Performance

[0206] The polymers with the highest ionic conductivity at 30 C. were AB(37, 0.21) (9.810.sup.5 S cm.sup.1) and AB(54, 0.33) (2.210.sup.5 S cm.sup.1). ABA(50, 0.35) demonstrated the best properties overall, and were studied further as the lead polymer.

[0207] The electrochemical stability of carbonyl coordinating polymers is often greater than what can be achieved with PEO electrolytes. To assess the electrochemical stability window of this material, linear sweep voltammetry (LSV) was conducted on the ABA(50, 0.35) electrolyte. A lithium|polymer|stainless steel cell configuration was used to evaluate the stability between its open-cell voltage (OCV) and 6 V at 60 C. with a sweep rate of 0.05 mV s.sup.1. Prior to sweeping, the cell was shown to be stable by maintaining the OCV for 3 hours. The polymer is oxidatively stable to above 5 V vs. Li/Li.sup.+; this suggests compatibility with high voltage cathodes. Its stability is comparable to other polyester and polycarbonate-based electrolytes in the literature, for example poly(CL-r-TMC) (>5 V) and poly(styrene-b-CL-r-TMC) (5 V)..sup.[14][25] Cyclic voltammetry was conducted between 3 and 4.5 V on ABA(50, 0.35), using the same cell configuration as in the linear sweep. It showed good oxidative stability across 35 cycles. (FIG. 5d).

CONCLUSIONS

[0208] The synthesis of two different architectures of block copolymers has been achieved by switch catalysis: AB and ABA type polymers have been produced, where A is a polycarbonate block (PVCHC) and B is a poly(ester-co-carbonate) (PCL-r-TMC). Experimental data was consistent with block formation and allowed the hard block wt. % and molar mass values to be measured. Polymer electrolytes have been prepared using LiTFSI salt. All of the polymer electrolytes tested had sufficient thermal stability, with T.sub.d,5 %>200 C., and demonstrated phase separation through the observation of an upper and lower T.sub.g. The materials showed moderate elastic recovery of 70%, as investigated by cyclic tensile testing.

[0209] Li-ion conductivity has been measured using electrochemical impedance spectroscopy. Good conductivity was demonstrated at elevated temperatures (.sub.70 C.=2.210.sup.4 S cm.sup.1 for ABA(50, 0.35)) and moderate performance at ambient (.sub.30 C.=5.910.sup.6 S cm.sup.1 for ABA(50, 0.35)). Trends related to polymer composition and architecture were elucidated. Electrochemical performance has been investigated: linear-sweep voltammetry demonstrates that the electrolyte is stable to >5 V, and cyclic voltammetry has successfully cycled the material 35 times between 3 V and 4.5 V.

[0210] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.

REFERENCES

[0211] [1]H. Government, (Ed.: D. f. Transport), 2021. [0212] [2]X. Wang, R. Kerr, F. Chen, N. Goujon, J. M. Pringle, D. Mecerreyes, M. Forsyth, P. C. Howlett, Adv. Mater., n/a, 1905219. [0213] [3] aL. Long, S. Wang, M. Xiao, Y. Meng, J. Mater. Chem. A 2016, 10038-10069; bT. Famprikis, P. Canepa, J. A. Dawson, M. S. Islam, C. Masquelier, Nature Materials 2019, 18, 1278-1291; cP. Albertus, V. Anandan, C. Ban, N. Balsara, I. Belharouak, J. Buettner-Garrett, Z. Chen, C. Daniel, M. Doeff, N. J. Dudney, B. Dunn, S. J. Harris, S. Herle, E. Herbert, S. Kalnaus, J. A. Libera, D. Lu, S. Martin, B. D. McCloskey, M. T. McDowell, Y. S. Meng, J. Nanda, J. Sakamoto, E. C. Self, S. Tepavcevic, E. Wachsman, C. Wang, A. S. Westover, J. Xiao, T. Yersak, ACS Energy Letters 2021, 6, 1399-1404. [0214] [4] aY. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nat. Energy 2016, 1, 16030; bH. Buschmann, J. Dlle, S. Berendts, A. Kuhn, P. Bottke, M. Wilkening, P. Heitjans, A. Senyshyn, H. Ehrenberg, A. Lotnyk, V. Duppel, L. Kienle, J. Janek, Phys. Chem. Chem. Phys. 2011, 13, 19378-19392. [0215] [5]K. Jeong, S. Park, S.-Y. Lee, J. Mater. Chem. A 2019, 7, 1917-1935. [0216] [6]H. Chen, M. Ling, L. Hencz, H. Y. Ling, G. Li, Z. Lin, G. Liu, S. Zhang, Chem. Rev. 2018, 118, 8936-8982. [0217] [7]D. E. Fenton, J. M. Parker, P. V. Wright, Polymer 1973, 14, 589. [0218] [8] aZ. Geng, N. S. Schauser, J. Lee, R. P. Schmeller, S. M. Barbon, R. A. Segalman, N. A. Lynd, C. J. Hawker, Macromolecules 2020; bE. Tsuchida, H. Ohno, K. Tsunemi, N. Kobayashi, Solid State Ion. 1983, 11, 227-233; cY. Zhao, Y. Bai, W. Li, M. An, Y. Bai, G. Chen, Chem. Mater. 2020. [0219] [9]D. T. H. Jr., N. P. Balsara, 2013, 43, 503-525. [0220] [10]M. R. Kember, P. D. Knight, P. T. R. Reung, C. K. Williams, J. Am. Chem. Soc. 2009, 48, 931-933. [0221] [11]T. T. D. Chen, University of Oxford 2020. [0222] [12] aC. Romain, J. A. Garden, G. Trott, A. Buchard, A. J. P. White, C. K. Williams, Chem. Eur. J. 2017, 23, 7367-7376; bM. R. Kember, J. Copley, A. Buchard, C. K. Williams, Polym. Chem. 2012, 3, 1196-1201. [0223] [13]A. Spyros, D. S. Argyropoulos, R. H. Marchessault, Macromolecules 1997, 30, 327-329. [0224] [14]J. Mindemark, B. Sun, E. Trm, D. Brandell, J. Power Sources 2015, 298, 166-170. [0225] [15]J. Kimpel, University of Oxford 2019. [0226] [16]H. Zhang, B. Liu, H. Ding, J. Chen, Z. Duan, Polymer 2017, 129, 5-11. [0227] [17]J. Mindemark, M. J. Lacey, T. Bowden, D. Brandell, Prog. Polym. Sci. 2018, 81, 114-143. [0228] [18]A. Bergfelt, G. Hernndez, R. Mogensen, M. J. Lacey, J. Mindemark, D. Brandell, T. M. Bowden, ACS Appl. Polym. Mater. 2020, 2, 939-948. [0229] [19]D. M. Pesko, Y. Jung, A. L. Hasan, M. A. Webb, G. W. Coates, T. F. Miller, N. P. Balsara, Solid State Ion. 2016, 289, 118-124. [0230] [20]K. Kimura, J. Motomatsu, Y. Tominaga, J. Phys. Chem. C 2016, 120, 12385-12391. [0231] [21]Y. Shi, X. Zhou, G. Yu, Acc. Chem. Res. 2017, 50, 2642-2652. [0232] [22]M. Chintapalli, T. N. P. Le, N. R. Venkatesan, N. G. Mackay, A. A. Rojas, J. L. Thelen, X. C. Chen, D. Devaux, N. P. Balsara, Macromolecules 2016, 49, 1770-1780. [0233] [23]R. A. Huggins, Ionics 2002, 8, 300-313. [0234] [24]M. A. Ratner, D. F. Shriver, Chemical Reviews 1988, 88, 109-124. [0235] [25]A. Bergfelt, M. J. Lacey, J. Hedman, C. Sngeland, D. Brandell, T. Bowden, RSC Advances 2018, 8, 16716-16725.