Solid polymer electrolyte and its method of manufacture

Abstract

Method of producing a solid polymer electrolyte (3), comprising steps of: - preparing a precursor solution (5) comprising vinylene carbonate monomer, a lithium or sodium salt, a fluoroethylene carbonate plasticiser and a thermal radical initiator, said precursor solution (5) being substantially free of organic phosphorous compounds; - heating said precursor solution (5) to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer by free radical polymerisation, wherein said precursor solution (5) comprises 40-65 wt% vinylene carbonate, 20-35 wt% fluoroethylene carbonate, 15-30 wt% of said lithium or sodium salt, and 0.01-2 wt.%, preferably 0.02-0.5 wt%, of said radical initiator.

Description

DescriptionSOLID POLYMER ELECTROLYTE AND ITS METHOD OF MANUFACTURETechnical Field

[0001] The present invention relates to the field of batteries. In particular it relates to a method of manufacturing a solid polymer electrolyte (SPE) for batteries, and the SPE thereby produced.State of the Art

[0002] Solid-state batteries employing solid electrolytes can avoid some of the drawbacks associated with the low thermal stability and flammability of liquid electrolytes employed in traditional lithium-ion batteries. Solid electrolytes also show promise in enabling the use of lithium metal as an anode, which can result in a significant increase in the energy density of the batteries.

[0003] Compared to an inorganic solid electrolyte, solid polymer electrolytes (SPE) offer low electronic conductivity, high flexibility, and easy processability. In addition, SPE’s can be prepared by in-situ polymerization methods, in which a monomer solution is converted to a SPE inside a battery casing. Compared to conventional SPE preparation methods, this method can provide great simplicity in manufacturing and results in significantly better contact between SPE and the active materials of the cathode and the anode.

[0004] However, SPE’s typically suffer from low ionic conductivity and low oxidative stability, which excludes the possibility of using high-voltage cathodes and cycling the batteries at room temperature.

[0005] Document WO2023 / 109659 describes in-situ polymerisation of 1 ,3-dioxolane monomer into a solid polymer electrolyte (SPE), by in-situ cationic ringopening polymerisation. The resulting SPE offers not only a decent ionic conductivity but also a low interfacial resistance, but due to the existence of the ether functional groups in the SPE, it still suffers from low oxidative stability, which results in poor compatibility with high-voltage cathodes, including lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, etc, and significantly reduces theenergy density of the batteries. In addition, the cationic ring-opening polymerization process results in soft polymer chains due to the polymerisation reaction opening the ring structure of the monomer, which results in poor mechanical properties.

[0006] CN113839096 describes in-situ polymerisation of various monomers in the presence of fluoroethylene carbonate and a lithium salt, specifically lithium bistrifluoroimide, lithium hexafluorophosphate, lithium bisfluorosulfonimide, and lithium oxalyl diglutamate, at room temperature, resulting in low monomer conversion, poor mechanical properties and low polymerisation efficiency, even taking days to achieve a complete polymerisation at room temperature.

[0007] US2022 / 271335 describes flame-resistant solid-state polymer electrolytes which include organic phosphorous compounds as an essential component of their formulation, which may reduce the ionic conductivity and / or the electrochemical stability of the electrolyte.

[0008] CN118054063 discloses, in one example, an SPE precursor solution comprising 12 wt.% vinylene carbonate monomer, 0.4 wt.% LiNOs, 9 wt.% LiPFe, 15 wt.% of a fluoroethylene carbonate plasticiser, and 0.03 wt.% azobisisobutyronitrile as thermal radical initiator. However, this SPE needs to be relatively thick, limiting the energy densities possible, and stable cycling for more than 100 cycles has not been demonstrated.

[0009] CN 114 221 021 discloses, in one example, an SPE precursor solution comprising 30 wt.% vinylene carbonate monomer, LiPFe and LiBF2(C2O4), 7 wt.% of a fluoroethylene carbonate plasticiser, and 0.6 wt.% diphenyl peroxide as thermal radical initiator. Cycling performance can, however, be improved, and the compatibility of the SPE with a lithium metal anode has not been demonstrated.

[0010] Zhang Shengnan et al (2021 ), "Bifunctional In Situ Polymerized Interface for Stable LAGP-Based Lithium Metal Batteries”, ADVANCED MATERIALS INTERFACES, vol. 8, no. 10, 23 March 2021 (2021-03-23), ISSN: 2196- 7350, DOI: 10.1002 / admi.202100072, XP093245084, discloses bifunctional in-situ polymerized interfaces for stable LAGP-based lithium metal batteries. In an example, a precursor solution for such an interface layer is disclosed,which comprises 1.35 g vinylene carbonate monomer, 0.143g LiDFOB, 5 wt.% of a fluoroethylene carbonate plasticiser, and 2 mg AIBN as thermal radical initiator. There is no suggestion that this interface material could serve as an SPE.

[0011] The aim of the invention is hence to at least partially overcome at least some of the above-identified drawbacks of the prior art.Disclosure of the Invention

[0012] More precisely, in a first variant, the invention relates to a method of producing a solid polymer electrolyte (SPE), as defined in claim 1. This method comprises steps of:

[0013] - preparing a precursor solution comprising vinylene carbonate (VC) monomer, a lithium or sodium salt, a fluoroethylene carbonate (FEC) plasticiser and a thermal radical initiator, said precursor solution being substantially free of organic phosphorous compounds, preferably entirely free thereof;

[0014] - heating said precursor solution to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer by free radical polymerisation, thereby creating polyvinyl carbonate (PVCA).

[0015] Said precursor solution comprises 40-65 wt% vinylene carbonate monomer, 20-35 wt% fluoroethylene carbonate, 15-30 wt% of said lithium or sodium salt, and 0.01-2 wt.%, preferably 0.02-0.5 wt%, of said radical initiator.

[0016] This method is extremely simple to carry out, uses readily-available ingredients, and the absence of organic phosphorous compounds ensures good ionic conductivity and electrochemical stability of the resulting SPE.

[0017] Furthermore, the carbonate groups in PVCA ensure high oxidative stability, and the radical polymerization process reserves the ring structure of VC, which offers rigid polymer chains and excellent mechanical properties. In addition, it has been found that the FEC in the SPE can significantly improve the ionic conductivity and the stability of the interphase formed on a lithium metal anode when used in a battery. Due to all these excellent properties ofthe PVCA-based SPE resulting from the method of the invention, it has surprisingly been found that ultrathin SPE layers in batteries, down to even 5 pm or less thickness (the SPE situated between the anode and the cathode being counted for this purpose), can achieve outstanding cycling performance at room temperature.

[0018] Advantageously, said lithium or sodium salt comprises, or indeed may consist of, lithium bis(trifluoromethanesulfonyl)imide or sodium bis(trifluoromethanesulfonyl)imide. Alternatively, said lithium or sodium salt may comprise, or indeed consist of, one or more of M bis(fluorosulfonyl)imide, M hexafluorophosphate, M difluoro(oxalato)borate, M tetrafluoroborate, M bisoxalate borate, M triflate, M perchlorate, M nitrate, where M is Na or Li. Preferably, the lithium or sodium salt is not lithium difluoro(oxalato)borate (LiDFOB).

[0019] Advantageously, said thermal radical initiator comprises, or indeed consists of, benzoyl peroxide (BPO) or azobisisobutyronitrile (Al BN). Alternatively, said thermal radical initiator may comprise, or consist of, one or more of ammonium persulfate, 2,2'-azobis[2-(2-imidazolin-2-yl)- propane] dihydrochloride, tert-butyl hydroperoxide, cumene hydroperoxide, di-tert-butyl peroxide, dicumyl peroxide.

[0020] Advantageously, said precursor solution comprises 50-55 wt% vinyl carbonate, 23-28 wt% fluoroethylene carbonate, 17-22 wt% lithium bis(trifluoromethane)sulfonimide and 0.2-0.6 wt% benzoyl peroxide, the sum of the wt% of these substances adding to substantially 100 wt % (aside from unavoidable impurities which do not affect the method or the properties of the resulting SPE). This formulation has proven very effective as an SPE in batteries, particularly batteries with a NMC622 (Ni-rich Manganese Cobalt) cathode and a Li metal anode, separated by a porous separator such as polyethylene.

[0021] This method of manufacturing an SPE can be integrated into a method of manufacturing a battery, such that the SPE is formed in-situ in the battery. This method comprises steps of:

[0022] - providing a battery casing housing a porous cathode and a porous separator adapted to separate said porous cathode from an anode, whether the anode is conventional and already present within the battery casing or will be formed when the resulting battery is first charged, i.e. a so-called “anode-free” configuration;

[0023] - injecting into said battery casing a precursor solution comprising vinylene carbonate monomer, a lithium or sodium salt, a fluoroethylene carbonate plasticiser and a thermal radical initiator, said precursor solution being substantially (preferably entirely) free of organic phosphorous compounds. The precursor solution hence ends up in intimate contact with the cathode and the porous separator, and also with the anode if present;

[0024] - sealing said battery casing;

[0025] - heating said battery casing and said precursor solution to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer in-situ by free radical polymerisation, the resulting SPE being in intimate contact with the cathode, porous separator and anode (if already present).

[0026] Again, this method is extremely simple, does not require removal of solvents or similar extra steps, and is hence very economical to carry out, all while obtaining an SPE formed in-situ with the advantages mentioned above.

[0027] Advantageously, said cathode comprises at least one of lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminium oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, or sodium chromium oxide, sodium cobalt oxide, sodium manganese oxide, sodium cobalt phosphate, sodium nickel phosphate, sodium iron phosphate, sodium manganese phosphate, sodium iron hexacyanoferrate, sodium manganese hexacyanoferrate, or sodium vanadium fluorophosphate. NMC622 (Ni-rich Manganese Cobalt) has proven to provide excellent results and is preferred.

[0028] Advantageously, said anode is chosen from pure lithium, pure sodium, or an alloy or lithium or sodium with one or more of In, Al, Sn, Pb, Au, Ag, Mg, Pt, Si, Ge, Bi, Sb. Pure lithium is preferred.

[0029] In another alternative variant, the invention relates to a method of producing a solid polymer electrolyte as defined in a further independent claim. This method comprises steps of:

[0030] - preparing a precursor solution comprising:

[0031] - 50-90 wt.%, preferably 50-70 wt.%, further preferably 50-55 wt.% vinylene carbonate;

[0032] - 5-30 wt.% total of at least two lithium or sodium salts, of which 5-25 wt.%, preferably 18-21 wt.% is of a first lithium or sodium salt, together with 0.5-5 wt.%, preferably 2-3 wt.% of LiNOs;

[0033] - 5-30 wt.% total of at least two plasticisers, 5-30 wt.% total, of which 5-25 wt.%, preferably 15-20 wt.% is of a first plasticiser, together with 2-10 wt.%, preferably 5-8 wt.% fluoroethylene carbonate

[0034] - 0.1-4 wt.%, preferably 0.3-1 wt.% of a thermal radical initiator.

[0035] This precursor solution is then heated to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer by free radical polymerisation;

[0036] Said precursor solution is substantially free of organic phosphorous compounds.

[0037] Said first lithium or sodium salt is chosen from one or more of: lithium bis(trifluoromethane)sulfonimide, lithium or sodium bis(fluorosulfonyl)imide, lithium or sodium hexafluorophosphate, lithium or sodium difluoro(oxalato)borate, lithium or sodium bis(oxalato)borate, lithium or sodium perchlorate, lithium or sodium tetrafluoroborate, lithium or sodium trifluoromethanesulfonate, lithium or sodium chloride, lithium or sodium bromide.

[0038] Said first plasticiser is chosen from one or more of: sulfolane, dimethyl sulfoxide, 1 ,2-dimethoxyethane, dioxolane, ethylene carbonate, diethyl carbonate.

[0039] Said thermal radical initiator is preferably chosen from (and is hence not limited to): 2,2'-Azobis(2-methylpropionitrile), benzoyl peroxide, dicumyl peroxide, tert-Butyl peroxybenzoate, 2,2‘-Azobis(2,4-dimethyl-4- methoxyvaleronitrile), 2,2'-Azobis(2,4-dimethylvaleronitrile), azobisisobutyramide.

[0040] In the same manner as for the embodiment of claim 1 discussed above, this method is extremely simple to carry out, uses readily-available ingredients, and the absence of organic phosphorous compounds ensures good ionic conductivity and electrochemical stability of the resulting SPE.

[0041] Furthermore, the carbonate groups in PVCA ensure high oxidative stability, and the radical polymerization process reserves the ring structure of VC, which offers rigid polymer chains and excellent mechanical properties. In addition, it has been found that the FEC in the SPE can significantly improve the ionic conductivity and the stability of the interphase formed on a lithium metal anode when used in a battery. Due to all these excellent properties of the PVCA-based SPE resulting from the method of the invention, it has surprisingly been found that ultrathin SPE layers in batteries, down to even 5 pm or less thickness (the SPE situated between the anode and the cathode being counted for this purpose), can achieve outstanding cycling performance at room temperature.

[0042] Advantageously, said first lithium or sodium salt is lithium bis(trifluoromethane)sulfonimide and / or said first plasticiser is sulfolane and / or said thermal radical initiator is 2,2'-azobis(2-methylpropionitrile). Preferably, all three of these are the case.

[0043] Again, this method of manufacturing an SPE can be integrated into a method of manufacturing a battery, such that the SPE is formed in-situ in the battery. This method comprises steps of:

[0044] - providing a battery casing housing a porous cathode and a porous separator adapted to separate said porous cathode from an anode, whether the anode is conventional and already present within the battery casing or will be formed when the resulting battery is first charged, i.e. a so-called “anode-free” configuration;

[0045] - injecting into said battery casing a precursor solution as defined above in the context of the alternative variant. The precursor solution hence ends up in intimate contact with the cathode and the porous separator, and also with the anode if present;

[0046] - sealing said battery casing;

[0047] - heating said battery casing and said precursor solution to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer in-situ by free radical polymerisation, the resulting SPE being in intimate contact with the cathode, porous separator and anode (if already present).

[0048] Again, this method is extremely simple, does not require removal of solvents or similar extra steps, and is hence very economical to carry out, all while obtaining an SPE formed in-situ with the advantages mentioned above.

[0049] The invention also relates to solid polymer electrolytes (SPE), as produced by the methods disclosed above. In the first variant, this SPE comprises polyvinylene carbonate, a lithium or sodium salt, a fluoroethylene carbonate plasticiser, a thermal radical initiator, and is substantially free (preferably entirely free) of organic phosphorous compounds, that is to say that no organic phosphorous compounds at levels which have an effect on its properties are present. In the second, alternative variant, the SPE comprises polyvinylene carbonate, at least two lithium or sodium salts, at least two plasticisers, and a thermal radical initiator as described above, and is likewise substantially free (preferably entirely free) of organic phosphorous compounds. The best performing SPE’s of the second, alternative variant comprise 23-35 wt.% polyvinylene carbonate, although the full range is 12-85 wt.%.

[0050] The exact lithium or sodium salts and thermal radical initiators are, naturally, as for the methods, and the weight percentages (wt%) of the various components are likewise as for the method, aside from the wt% of polyvinylene carbonate being between 30% to 100% of the wt% of vinylene carbonate in the precursor solution. As a result, some vinylene carbonate monomer may remain in the SPE, which can also boost the ionic conductivity.

[0051] Specifically, in the first variant, an SPE consisting of 15-55 wt% polyvinyl carbonate, 0-40 wt% unconverted vinylene carbonate, 23-28 wt% fluoroethylene carbonate, 17-22 wt% lithium bis(trifluoromethane)sulfonimide, 0.2-0.6 wt% benzoyl peroxide and up to 2% of other substances, such asunavoidable impurities, (the sum of the listed components adding up to 100%) has proven to give excellent performance.

[0052] The SPE of either variant can be integrated into a battery comprising:

[0053] - a battery casing housing a porous cathode and a porous separator adapted to separate said porous cathode from an anode, as described above;

[0054] - a solid polymer electrolyte as described above, according to either variant, said solid polymer electrolyte being in contact with said cathode and said porous separator.

[0055] Advantageously, said solid polymer electrolyte has a thickness of 100 pm or less, preferably 20 pm or less, further preferably 5 pm or less, counting its thickness between said cathode and said anode and hence not counting the SPE polymer which is situated within the porous cathode.Brief Description of the Drawings

[0056] Further details of the invention will become more apparent upon reading the following description, with reference to the appended figures, in which:- Figure 1 is a schematic diagram of a method of manufacturing a battery with a solid polymer electrolyte according to the invention, formed according to the method of the invention;- Figures 2 and 3 are graphs of measurement results of batteries fabricated with a solid polymer electrolyte according to an implementation example of the invention; and- Figure 4 is a graph of measurement results of batteries fabricated with a solid polymer electrolyte according to an alternative implementation example of the invention.Embodiments of the Invention

[0057] Figure 1 illustrates a method of manufacturing a battery 1 incorporating a solid polymer electrolyte (SPE, indicated in lighter grey) 3 manufactured according to the method of the invention. Although the SPE 3 is described here in terms of its use in a battery of any type (e.g. a coin cell, cylindrical cell or similar), it has wider applications and the invention is not to be construed as limited to a battery 1.

[0058] SPE 3 is formed from a precursor solution 5 (indicated in darker grey), which comprises:

[0059] - vinylene carbonate (VC) as monomer, 40-65 wt%;

[0060] - fluoroethylene carbonate (FEC) as plasticiser, 20-35 wt%;

[0061] - a lithium or sodium salt, 15-30 wt%;

[0062] - a radical initiator, 0.01-2 wt%, preferably 0.02-0.5 wt%;

[0063] - Optional additives, such as ionic liquids, carbonate esters, fluorinated carbonate esters, ethers, fluorinated ethers, nitriles, amides, amines, sulfones, in a proportion up to 50 wt%, preferably up to 30 wt%;

[0064] - one or more further monomers, such as acrylic monomers, fluorinated monomers, cationic monomers or anionic monomers, up to 50 wt%, preferably up to 30 wt%;

[0065] - unavoidable impurities in quantities not substantially affecting (preferably not affecting) the properties of the resulting SPE or the progress of the polymerisation reaction: up to 2%.

[0066] The lithium or sodium salt is advantageously a single salt or a combination of salts selected from M bis(trifluoromethanesulfonyl)imide, M bis(fluorosulfonyl)imide, M hexafluorophosphate, M difluoro(oxalato)borate, M tetrafluoroborate, M bisoxalate borate, M triflate, M perchlorate, so M nitrate, in which M is either lithium or sodium. Sodium bis(trifluoromethanesulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide are preferred. Although mentioned as a possibility above, the lithium or sodium salt is preferably not lithium difluoro(oxalato)borate (LiDFOB).

[0067] It should be noted that the precursor solution, and the resulting SPE 3, are substantially free (preferably entirely free) of organic phosphorous compounds, that is to say that any organic phosphorous compounds, if present, are only present in trace quantities that do not influence the polymerisation reaction or the properties of the resulting SPE 3.

[0068] The thermal radical initiator is typically selected from benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), ammonium persulfate, 2,2'-azobis[2-(2- imidazolin-2-yl)- propane] dihydrochloride, tert-butyl hydroperoxide, cumene hydroperoxide, di-tert-butyl peroxide, dicumyl peroxide. BPO or AIBN is preferred.

[0069] In an alternative formulation, the precursor solution 5 comprises:

[0070] - vinylene carbonate monomer, 50-90 wt.%, preferably 50-70 wt.%, further preferably 50-55 wt.%;

[0071] - At least two lithium or sodium salts, 5-30 wt.% total, comprising 5-25 wt.%, preferably 18-21 wt.% of a first lithium or sodium salt, together with 0.5-5 wt.%, preferably 2-3 wt.% of LiNOs;

[0072] - at least two plasticisers, 5-30 wt.% total, comprising: 5-25 wt.%, preferably15-20 wt.% of a first plasticiser, together with 2-10 wt.%, preferably 5-8 wt.% fluoroethylene carbonate (FEC),

[0073] - a thermal radical initiator, 0.1-4 wt.%, preferably 0.3-1 wt.%.

[0074] First lithium or sodium salt is chosen from one or more of: lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium or sodium bis(fluorosulfonyl)imide (LiFSI I NaFSI), lithium or sodium hexafluorophosphate (LiPF6 I NaPF6), lithium or sodium difluoro(oxalato)borate (LiDFOB I NaDFOB), lithium or sodium bis(oxalato)borate (LiBOB I NaBOB), lithium or sodium perchlorate (LiCIO4 I NaCIO4), lithium or sodium tetrafluoroborate (LiBF4 I NaBF4), lithium or sodium trifluoromethanesulfonate (LiOTf I NaOTf), lithium or sodium chloride (LiCI I NaCI), lithium or sodium bromide (LiBr I NaBr). Lithium bis(trifluoromethane)sulfonimide (LiTFSI) is preferred. Preferably, and despite being mentioned above as a possibility, the first lithium or sodium salt is not lithium difluoro(oxalato)borate (LiDFOB).

[0075] First plasticiser is chosen from one or more of: sulfolane (SL), dimethyl sulfoxide (DMSO), 1 ,2-dimethoxyethane (DME), dioxolane (DOL), ethylene carbonate (EC), diethyl carbonate (DEC). Sulfolane (SL) is preferred.

[0076] Thermal radical initiator is preferably chosen from: 2,2'-Azobis(2- methylpropionitrile) (AIBN), benzoyl peroxide (BPO), dicumyl peroxide (DCP),tert-Butyl peroxybenzoate (TPBP), 2,2‘-Azobis(2,4-dimethyl-4- methoxyvaleronitrile) (V-70), 2,2'-Azobis(2,4-dimethylvaleronitrile) (V-65), azobisisobutyramide (AIBME). 2,2'-Azobis(2-methylpropionitrile) (AIBN) is preferred.

[0077] The precursor solution 5 is injected into a prepared battery casing 7, provided with an opening 7a to this effect, said battery casings 7 housing an anode 9, a cathode 11 , and a porous separator 11 serving to keep anode 9 and cathode 11 apart. However, so-called anode-free types of battery, which only use a current collector, are also possible, in which case the lithium or sodium material is plated on the current collector in situ during the first charge process so as to form the anode. Battery casing 7 comprises, or is delimited into, positive and negative terminals as is generally known, the positive terminal being in electrical connection with the cathode and the negative terminal being in electrical connection with the anode, as generally known and which does not need to be illustrated.

[0078] In a conventional configuration, anode 9 is typically of a lithium, lithium alloy, sodium or sodium alloy, the alloys typically containing one or more of In, Al, Sn, Pb, Au, Ag, Mg, Pt, Si, Ge, Bi, Sb.

[0079] Cathode 11 is of a porous type, typically of lithium nickel manganese cobalt oxide (which is preferred, more preferably specifically lithium nickel manganese cobalt oxide 532 or 622), lithium nickel cobalt aluminium oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, or sodium chromium oxide, sodium cobalt oxide, sodium manganese oxide, sodium cobalt phosphate, sodium nickel phosphate, sodium iron phosphate, sodium manganese phosphate, sodium iron hexacyanoferrate, sodium manganese hexacyanoferrate, or sodium vanadium fluorophosphate.

[0080] Porous separator 13 is typically of polyethylene, but other types of separators such as polypropylene, polyimide, PVDF, PVDF-HFP, glass fiber, etc. are possible, as is generally known.

[0081] Once the precursor solution 5 has been injected into the battery casing 7, it permeates through the cathode 11 and porous separator 13, and is in intimate contact with the anode 9 or, in the case of an anode-free configuration, with the part of the battery casing 7 which will have the anode deposited upon it when this is formed during the first charging cycle. The opening 7a in the battery casing 7 is then sealed. Due to the SPE polymer adhering to the surface of the porous separator 13 and filling the voids therein, the thickness of the porous separator 13 determines the thickness of the SPE 3 for the purposes of the present invention, the SPE polymer which penetrates into the cathode not being counted for this purpose. In other words, the thickness of the SPE is considered as being the distance between the cathode 11 and the anode 9 which is filled with the porous separator 13 and SPE polymer.

[0082] The precursor solution 5 is then subject to thermal polymerisation between 50°C and 100°C, for a duration of between 30 minutes and 24 hours, the temperature and duration being selected as required based on the thermal radical initiator and its concentration. This is carried out by heating the entire battery 1 .

[0083] In a first implementation example, an SPE precursor solution 5 was prepared by mixing 2 g VC, 1 g FEC, 0.75 g lithium bis(trifluoromethane)sulfonimide, and 0.015 g benzoyl peroxide. The precursor solution 5 was stirred for 15 min at room temperature in an argon-silled glovebox. 70 pL of the precursor solution was injected into the battery housing 7 of a 2032 coin cell which was then sealed. The battery housing 7 of the coin cell contained a 2 mAh / cm2 NMC622 (Ni-rich Manganese Cobalt) cathode 11 , a Li metal anode 9, and a 16 pm-thick polyethylene porous separator 13, and then the sealed cell was heated at 80 °C overnight for polymerization. After polymerization, the battery1 was cycled at 1 mA / cm2 at 25 °C. The same method was also carried out on with a 5 pm-thick polyethylene porous separator 13. As shown in Figures2 and 3, an initial specific capacity of 170 mAh / g (lower curve) was achieved with a 16 pm porous separator 13 and hence SPE 3 for figure 2 and a 5 pm porous separator 13 and hence SPE 3 for figure 3, and the battery 1 showsexcellent cycling stability for more than 480 cycles. The average Coulombic efficiency (upper curve) is 99.9% in both cases, which suggests excellent electrochemical stability of the SPE 3.

[0084] In an alternative, second implementation example, an SPE precursor solution 5 according to the alternative formulation described above was prepared by mixing 53.0 wt.% VC, 19.9 wt.% LiTFSI, 6.7% wt.% FEC, 0.42 wt.% AIBN, 17.5 wt.% SL, 2.51 wt.% LiNO3, the battery 1 being otherwise prepared as for the first implementation example but with a thinner anode of 25 pm thick lithium metal, the cathode being again 2 mAh / cm2 NMC622, and the porous separator being 16 pm thick. As can be seen from the graph of figure 4, the resulting battery was discharged at 1 mA.cirr2at 25 C, and it showed an excellent initial discharge capacity of around 1.8 mAh, and good stability over almost 300 cycles.

[0085] Although the invention has been described in terms of particular embodiments, variations thereto are possible without departing from the scope of the invention as defined by the appended claims.

Claims

Claims1. Method of producing a solid polymer electrolyte (3), comprising steps of:- preparing a precursor solution (5) comprising vinylene carbonate monomer, a lithium or sodium salt, a fluoroethylene carbonate plasticiser and a thermal radical initiator, said precursor solution (5) being substantially free of organic phosphorous compounds;- heating said precursor solution (5) to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer by free radical polymerisation, wherein said precursor solution (5) comprises 40-65 wt% vinylene carbonate, 20- 35 wt% fluoroethylene carbonate, 15-30 wt% of said lithium or sodium salt, and 0.01-2 wt.%, preferably 0.02-0.5 wt%, of said radical initiator.

2. Method according to the preceding claim, wherein, during said free radical polymerisation, the rings of said vinylene carbonate remain intact.

3. Method according to one of claims 1 or 2, wherein said lithium or sodium salt comprises lithium bis(trifluoromethanesulfonyl)imide or sodium bis(trifluoromethanesulfonyl)imide.

4. Method according to one of claims 1-3, wherein said lithium or sodium salt comprises one or more of M bis(fluorosulfonyl)imide, M hexafluorophosphate, M difluoro(oxalato)borate, M tetrafluoroborate, M bisoxalate borate, M triflate, M perchlorate, M nitrate, where M is Na or Li.

5. Method according to one of claims 1-4, wherein said thermal radical initiator comprises benzoyl peroxide (BPO) or azobisisobutyronitrile (AIBN).

6. Method according to one of claims 1-5, wherein said thermal radical initiator comprises one or more of ammonium persulfate, 2,2'-azobis[2-(2-imidazolin-2-yl)- propane] dihydrochloride, tert-butyl hydroperoxide, cumene hydroperoxide, di- tert-butyl peroxide, dicumyl peroxide.

7. Method according to claim 1 , wherein said precursor solution (5) consists of:- 50-55 wt% vinyl carbonate;- 23-28 wt% fluoroethylene carbonate;- 17-22 wt% lithium bis(trifluoromethane)sulfonimide,- 0.2-0.6 wt% benzoyl peroxide- 0-2 wt% other substances.

8. Method of producing a solid polymer electrolyte (3), comprising steps of:- preparing a precursor solution (5) comprising:- 50-90 wt.%, preferably 50-70 wt,%, further preferably 50-55 wt.% vinylene carbonate;- 5-30 wt.% total of at least two lithium or sodium salts, of which 5-25 wt.%, preferably 18-21 wt.%, is of a first lithium or sodium salt, together with 0.5-5 wt.%, preferably 2-3 wt.% of LiNOs;- 5-30 wt.% total of at least two plasticisers, 5-30 wt.% total, of which 5-25 wt.%, preferably 15-20 wt.% is of a first plasticiser, together with 2-10 wt.%, preferably 5-8 wt.% fluoroethylene carbonate- 0.1-4 wt.%, preferably 0.3-1 wt.% of a thermal radical initiator,- heating said precursor solution (5) to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer by free radical polymerisation; wherein said precursor solution (5) is substantially free of organic phosphorous compounds, wherein said first lithium or sodium salt is chosen from one or more of: lithium bis(trifluoromethane)sulfonimide, lithium or sodium bis(fluorosulfonyl)imide, lithium or sodium hexafluorophosphate, lithium or sodium difluoro(oxalato)borate, lithium or sodium bis(oxalato)borate, lithium or sodium perchlorate, lithium or sodium tetrafluoroborate, lithium or sodium trifluoromethanesulfonate, lithium or sodium chloride, lithium or sodium bromide,wherein said first plasticiser is chosen from one or more of: sulfolane, dimethyl sulfoxide, 1 ,2-dimethoxyethane, dioxolane, ethylene carbonate, diethyl carbonate, and wherein said thermal radical initiator is preferably chosen from: 2,2'-Azobis(2- methylpropionitrile), benzoyl peroxide, dicumyl peroxide, tert-Butyl peroxybenzoate, 2,2‘-Azobis(2,4-dimethyl-4-methoxyvaleronitrile), 2,2'- Azobis(2,4-dimethylvaleronitrile), azobisisobutyramide.

9. Method according to the preceding claim, wherein at least one, preferably all, of the following are true:- said first lithium or sodium salt is lithium bis(trifluoromethane)sulfonimide;- said first plasticiser is sulfolane;- said thermal radical initiator is 2,2'-Azobis(2-methylpropionitrile).

10. Method of manufacturing a battery comprising steps of:- providing a battery casing (7) housing a porous cathode (11) and a porous separator (13) adapted to separate said porous cathode (11) from an anode (9);- injecting into said battery casing (7) a precursor solution (5) comprising vinylene carbonate monomer, a lithium or sodium salt, a fluoroethylene carbonate plasticiser and a thermal radical initiator, said precursor solution (5) being substantially free of organic phosphorous compounds ;- sealing said battery casing (7);- heating said battery casing (7) and said precursor solution (5) to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer by free radical polymerisation, wherein said precursor solution (5) comprises 40-65 wt% vinylene carbonate, 20- 35 wt% fluoroethylene carbonate, 15-30 wt% of said lithium or sodium salt, and 0.01-2 wt.%, preferably 0.02-0.5 wt%, of said radical initiator.11 . Method of manufacturing a battery comprising steps of:- providing a battery casing (7) housing a porous cathode (11) and a porous separator (13) adapted to separate said porous cathode (11) from an anode (9);- injecting into said battery casing (7) a precursor solution (5) comprising:- 50-90 wt.%, preferably 50-70 wt.%, further preferably 50-55 wt.% vinylene carbonate;- 5-30 wt.% total of at least two lithium or sodium salts, of which 5-25 wt.%, preferably 18-21 wt.% is of a first lithium or sodium salt, together with 0.5-5 wt.%, preferably 2-3 wt.% of LiNOs;- 5-30 wt.% total of at least two plasticisers, 5-30 wt.% total, of which 5-25 wt.%, preferably 15-20 wt.% is of a first plasticiser, together with 2-10 wt.%, preferably 5-8 wt.% fluoroethylene carbonate- 0.1-4 wt.%, preferably 0.3-1 wt.% of a thermal radical initiator,- sealing said battery casing (7);- heating said battery casing (7) and said precursor solution (5) to a temperature of between 50°C and 100°C for a duration between 30 minutes and 24 hours so as to polymerise said vinylene carbonate monomer by free radical polymerisation, wherein said precursor solution (5) is substantially free of organic phosphorous compounds, wherein said first lithium or sodium salt is chosen from one or more of: lithium bis(trifluoromethane)sulfonimide, lithium or sodium bis(fluorosulfonyl)imide, lithium or sodium hexafluorophosphate, lithium or sodium difluoro(oxalato)borate, lithium or sodium bis(oxalato)borate, lithium or sodium perchlorate, lithium or sodium tetrafluoroborate, lithium or sodium trifluoromethanesulfonate, lithium or sodium chloride, lithium or sodium bromide, wherein said first plasticiser is chosen from one or more of: sulfolane, dimethyl sulfoxide, 1 ,2-dimethoxyethane, dioxolane, ethylene carbonate, diethyl carbonate, and wherein said thermal radical initiator is preferably chosen from: 2,2'-Azobis(2- methylpropionitrile), benzoyl peroxide, dicumyl peroxide, tert-Butyl peroxybenzoate, 2,2‘-Azobis(2,4-dimethyl-4-methoxyvaleronitrile), 2,2'- Azobis(2,4-dimethylvaleronitrile), azobisisobutyramide.

12. Method according to one of claims 10-11 , wherein said cathode (11) comprises at least one of lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminium oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, orsodium chromium oxide, sodium cobalt oxide, sodium manganese oxide, sodium cobalt phosphate, sodium nickel phosphate, sodium iron phosphate, sodium manganese phosphate, sodium iron hexacyanoferrate, sodium manganese hexacyanoferrate, or sodium vanadium fluorophosphate.

13. Method according to one of claims 10-12, wherein said anode (9) is chosen from pure lithium, pure sodium, or an alloy or lithium or sodium with one or more of In, Al, Sn, Pb, Au, Ag, Mg, Pt, Si, Ge, Bi, Sb.

14. Solid polymer electrolyte (3) comprising:- 12-65 wt.% polyvinylene carbonate;- 15-35 wt.% of a lithium or sodium salt;- 20-35 wt.% of a fluoroethylene carbonate plasticiser;- 0.01-2 wt.%, preferably 0.02-0.5 wt.%, of a thermal radical initiator, wherein said solid polymer electrolyte is substantially free of organic phosphorous compounds.

15. Solid polymer electrolyte (3) according to the preceding claim, consisting of:- 15-55 wt% polyvinyl carbonate;- 0-40 wt % vinylene carbonate;- 23-28 wt% fluoroethylene carbonate;- 17-22 wt% lithium bis(trifluoromethane)sulfonimide,- 0.2-0.6 wt% benzoyl peroxide;- 0-2 wt% other substances.

16. Solid polymer electrolyte (3) comprising:- 12-85 wt.%, preferably 15-55 wt.%, further preferably 23-35 wt.% polyvinylene carbonate;- 5-30 wt.% total of at least two lithium or sodium salts, of which 5-25 wt.%, preferably 18-21 wt.%, is of a first lithium or sodium salt, together with 0.5-5 wt.%, preferably 2-3 wt.% of LiNOs;- 5-30 wt.% total of at least two plasticisers, 5-30 wt.% total, of which 5-25 wt.%, preferably 15-20 wt.% is of a first plasticiser, together with 2-10 wt.%, preferably 5-8 wt.% fluoroethylene carbonate- 0.1-4 wt.%, preferably 0.3-1 wt.% of a thermal radical initiator, wherein said solid polymer electrolyte is substantially free of organic phosphorous compounds, wherein said first lithium or sodium salt is chosen from one or more of: lithium bis(trifluoromethane)sulfonimide, lithium or sodium bis(fluorosulfonyl)imide, lithium or sodium hexafluorophosphate, lithium or sodium difluoro(oxalato)borate, lithium or sodium bis(oxalato)borate, lithium or sodium perchlorate, lithium or sodium tetrafluoroborate, lithium or sodium trifluoromethanesulfonate, lithium or sodium chloride, lithium or sodium bromide, wherein said first plasticiser is chosen from one or more of: sulfolane, dimethyl sulfoxide, 1 ,2-dimethoxyethane, dioxolane, ethylene carbonate, diethyl carbonate, and wherein said thermal radical initiator is preferably chosen from: 2,2'-Azobis(2- methylpropionitrile), benzoyl peroxide, dicumyl peroxide, tert-Butyl peroxybenzoate, 2,2‘-Azobis(2,4-dimethyl-4-methoxyvaleronitrile), 2,2'- Azobis(2,4-dimethylvaleronitrile), azobisisobutyramide.

17. Battery (1) comprising:- a battery casing (7) housing a porous cathode (11) and a porous separator (13) adapted to separate said porous cathode (11) from an anode (9);- a solid polymer electrolyte (3) according to one of claims 14-16, said solid polymer electrolyte being in contact with said cathode and said porous separator.

18. Battery (1 ) according to the preceding claim, wherein said solid polymer electrolyte (3) has a thickness of 100 pm or less, preferably 20 pm or less, further preferably 5 pm or less, between said cathode (11) and said anode (9).

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