Solid electrolyte crosslinked with a vinyl siloxane crosslinking agent

Abstract

The invention relates to a crosslinkable electrolyte formulation comprising at least: ‐ a lithium salt or a mixture of lithium salts; ‐ a hydrocarbon-based molecule comprising two thiol functions; ‐ an unsaturated hydrocarbon-based molecule comprising two C=C double bonds; ‐ a crosslinking agent, said crosslinking agent being a molecule comprising at least three vinyl functions and at least one Si-O-Si siloxane bond; wherein the [C=C double bond] / [thiols] molar ratio is between 1 and 1.1. The invention also relates to a method for preparing a crosslinked electrolyte on the basis of the above crosslinkable formulation.

Description

[0001] Solid electrolyte crosslinked with a vinylsiloxane crosslinker

[0002] Technical field

[0003] The invention relates to the preparation of crosslinked solid electrolytes for lithium-ion batteries based on thiol-ene reactions.

[0004] Prior art

[0005] The reaction used in the invention (thiol-ene reaction) has applications in many fields including batteries.

[0006] The thiol-ene reaction (also alkene hydrothiolation) is a reaction between a thiol and an alkene to form a thioether. This reaction was first reported in 1905, but gained prominence in the late 1990s and early 2000s for its feasibility and wide range of applications, particularly for electrolytes.

[0007] Thiols are excellent nucleophiles through the formation of thiolate anions (RS-) and are also electrophiles via thiyl radicals (RS*). One of the most studied reactions involving thiols is the hydrothiolation of double bonds (C=C). Depending on the nature of the substituents of the double bond, i.e. electron donors or acceptors, and the type of catalyst used, the thiol-C=C reaction can occur following a radical or nucleophilic mechanism.

[0008] In the case of a nucleophilic mechanism, the reaction is the Michael addition on a double bond substituted by an electron-withdrawing group, such as (meth)acrylates (monomers containing an active double bond), and a nucleophilic base is used as a catalyst.

[0009] In the case of a radical mechanism, it is important to avoid electron-withdrawing substituents (monomers containing an active double bond) in order to avoid the polymerization of unsaturated species and uncontrolled side reactions. Preferably, non-activated monomers such as allyl or vinyl ethers are used. Hydrothiolation can then be initiated thermally or photochemically.

[0010] Xuan et al. (2020) Journal of Power Sources, vol. 456, p. 228024. DOI: 10.1016 / j.jpowsour.2020.228024 use a thiol of the type (2,2'-(ethylenedioxy)diethanethiol), which they react with an acrylate function, without crosslinker. The polymers obtained are viscous liquid polymers. The conductivity values ​​described at 25 °C correspond to those of poorly crosslinked materials under equivalent conditions.

[0011] Sarapas et al Macromolecules 2016, 49, 4, 1154-1162, use thiol and vinyl monomers for chain growth, but without crosslinking. The conductivities described at 80 °C are relatively low.

[0012] The publication ACS Macro Lett. 2020, 9, 500-506 describes a (2,2'-(ethylenedioxy)diethanethiol) combined with an allyl ethylene glycol, in association with pentaerythritol tetrakis mercaptopropionate as a crosslinker. The measured conductivity values ​​nevertheless remain low.

[0013] Patent applications US2019237803 AA and US2020411906 AA describe networks formed from the thiol-ene reaction in an aqueous medium. The electrolyte obtained is a polymer gel (i.e., a material trapping a significant amount of liquid), necessarily including water. The examples only relate to gels containing solvents. The faradaic efficiencies (battery charge-discharge) described are very low (around 80%).

[0014] Patent application US2021057753 AA describes an anode for an electrochemical cell comprising an electroactive material comprising lithium and a porous protective layer comprising a thiol-based polymer. The polymer is used as an additive or protective layer for an electrode, but not as an electrolyte.

[0015] Different publications disclose the use of poly-siloxane crosslinkers in solid electrolytes, these electrolytes can be standard (with a Li salt) or "single ion" with a lithium salt grafted to the polymer network, the chemical function grafted on the siloxane is a thiol (Highly conductive, flexible polymer electrolyte membrane based on polyethylene glycol) diacrylate-co-thiosiloxane network; Camilo Piedrahitaa, Victor Kusumab, Hunaid B. Nulwalac, Thein Kyua; Solid State Ionics 322 (2018) 61-68 [https: / / doi.Org / 10.1016 / j.ssi.2018.05.006]; Adv. Energy Mater.2022, 12, 2200013: Polysiloxane-Based Single-Ion Conducting Polymer Blend Electrolyte Comprising Small-Molecule Organic Carbonates for High-Energy and High-Power Lithium-Metal Batteries Hai- Peng Liang, Maider Zarrabeitia, Zhen Chen, Sven Jovanovic, Steffen Merz, Josef Granwehr, Stefano Passerini,* and Dominic Bresser [DOI: 10.1002 / aenm.202200013] JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1548-1557).

[0016] Hao Zhang et al. describe a thiol-ene reaction with a trimethoxysilane monomer that will then be crosslinked by sol-gel, and not a polysiloxane crosslinker already present in the medium at the time of network formation. (Robust, self-healing, superhydrophobic coatings highlighted by a novel branched thiol-ene fluorinated siloxane nanocomposites; Hao Zhang, Yong Ma, Jiaojun Tan, Xinlong Fan, Yibin Liu, Junwei Gu, Baoliang Zhang, Hepeng Zhang, Qiuyu Zhang; Composites Science and Technology 137 (2016) 78-86

[0017] [http: / / dx.doi.Org / 10.1016 / j.compscitech.2016.10.023])

[0018] Summary of the invention

[0019] Surprisingly, the Applicant has noticed that, in non-aqueous formulations of solid electrolytes, crosslinking with a crosslinker comprising a siloxane unit and at least three vinyl functions, preferably at least four vinyl functions, improves the electrochemical properties of the polymer network formed compared to the use of other crosslinkers, and that the use of a hydrocarbon molecule carrying two thiol functions in association with an unsaturated hydrocarbon molecule comprising two C=C double bonds in the presence of said crosslinker, in which the molar ratio [C=C double bond or alkenes] / [thiols] in the formulation is between 1 and 1.1, makes it possible to increase the ionic conductivity and to increase the resistance of the network to chemical degradation. The term [alkenes] / [thiols] molar ratio means the molar ratio of all the C=C double bonds to all the thiol bonds in the formulation.

[0020] In particular, it appears that three organic molecules: triethylene glycol divinyl ether, the linear or cyclic vinylsiloxane crosslinker, and 2,2'-(ethylenedioxy)diethanethiol, can be used as a solid electrolyte for an all-solid Li-ion battery or as a component of the positive electrode (posolyte or catholyte), by including a lithium salt to ensure good transport of lithium ions in the electrolyte and the catholyte. The electrolyte formulation according to the invention makes it possible in particular to obtain conductivities higher than those obtained in the prior art. Throughout the description, a "sacrificial solvent" is a solvent present in the crosslinkable formulation according to the invention, but no longer forming part of the final composition of the crosslinked electrolyte.

[0021] The crosslinkable electrolyte formulation according to the invention comprises at least:

[0022] - A lithium salt or a mixture of lithium salts;

[0023] - A hydrocarbon molecule comprising two thiol functions;

[0024] - An unsaturated hydrocarbon molecule containing two C=C double bonds;

[0025] - A crosslinker, said crosslinker being a molecule comprising at least three vinyl functions and at least one Si-O-Si siloxane bond; in which the molar ratio [C=C double bond] / [thiols] is between 1 and 1.1.

[0026] The quantity of lithium salt(s) may represent between 2 and 40% by mass relative to the total formulation mass, preferably between 10 and 30% by mass relative to the total formulation mass.

[0027] The amount of crosslinker may be between 0.5 and 20% by mass, preferably between 1 and 5% by mass relative to the total mass of the formulation.

[0028] The crosslinkable electrolyte formulation according to the invention may comprise a radical or anionic initiator, preferably radical, in an amount of between 0.01 and 0.05 times the total mass of formulation excluding lithium salt(s).

[0029] The crosslinkable electrolyte formulation according to the invention may comprise a chain terminator making it possible to generate pendant chains, said chain terminator being a molecule carrying a single C=C double bond, without labile protons, in an amount strictly less than the amount of material of the crosslinker, preferably a carbonate carrying a vinyl function, very preferably 4-Vinyl-1,3-dioxolan-2-one, or an allyl alkyl ether, preferably a vinylmethyl or vinyl ethyl ether.

[0030] In one embodiment, the lithium salt may have a C=C bond and also act as a chain terminator.

[0031] The polysiloxane-based crosslinker comprising at least one vinyl function may be chosen from poly-(methyl-vinyl siloxanes (Chem 1) or 2, 4, 6, 8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane (Chem 2) or hexavinyldisiloxane (Chem 3), very preferably 2, 4, 6, 8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane.

[0032] Chem 1

[0033] The hydrocarbon molecule comprising two thiol functions may be 2,2'-(ethylenedioxy)diethanethiol. The hydrocarbon molecule comprising two C=C double bonds may be chosen from oligoethylene glycols bearing two C=C bonds, diallyl ethers and diacrylates or dimethacrylates, preferably triethylene glycol divinyl ether.

[0034] The crosslinkable electrolyte formulation according to the invention may comprise a plasticizer of the oligoethylene glycol type, a low-volatile non-aqueous solvent or ionic liquid, said plasticizer representing less than 30% by mass, preferably less than 25%, very preferably less than 20% by mass, even more preferably less than 15% by mass relative to the total mass of the formulation.

[0035] The invention also relates to the electrolyte obtained by at least partial crosslinking of the crosslinkable electrolyte formulation described above.

[0036] The invention also relates to a process for preparing a crosslinked solid electrolyte in which: a) the crosslinkable electrolyte formulation is crosslinked according to any one of the variants described, in the optional presence of a sacrificial solvent, preferably non-aqueous, by means of a radical or anionic initiator, preferably radical, added or already present in the formulation and by thermal or photo-induced or redox activation to form a polymer conducting lithium ions, b) the possible sacrificial solvent present in said polymer is evaporated in order to obtain a crosslinked solid electrolyte.

[0037] Photo-induced activation can be performed by UV lamp or stereolithography.

[0038] Redox activation can be carried out as follows: the reaction can be initiated using a peroxide (very preferably benzoyl peroxide BPO) and an amine (preferably N,N-dimethyl-p-toluidine or p-tolyldiethanolamine). An inhibitor can be added to slow down the generation of radicals (preferably 4-methoxyphenol or 4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl / TEMPOL). The radicals are generated immediately upon mixing the first two components, which must be added just before forming the electrolyte. The quantities of peroxide, amine and inhibitor are chosen by the person skilled in the art.In one embodiment of the method for preparing the crosslinked solid electrolyte, the crosslinking step a) can be carried out sequentially, i.e. by carrying out several successive crosslinking steps, adding to each additional step a certain quantity of crosslinkable electrolyte formulation (or at least one of its components) and a certain quantity of initiator (also called primer).

[0039] The invention also relates to the crosslinked electrolyte and the use of the crosslinked electrolyte obtained by crosslinking the formulation according to any of the variants described or obtained by the process for preparing a crosslinked electrolyte according to any of the variants described as an all-solid Li-ion battery solid electrolyte or as a component of the positive (posolyte or catholyte) or negative (negolyte or anolyte) electrode of an electrochemical system.

[0040] List of figures

[0041] Figures 1 to 5 illustrate the invention without limitation.

[0042] Figure 1 represents the conductivity (S / cm) as a function of temperature (1000 / T with T in K) of the electrolytes according to the invention and Ref (comparative) in example 1.

[0043] Figure 2 represents the mechanical properties: storage modulus E' and delta tangent measured by DMA of the electrolytes obtained from the formulations according to the invention of example 1.

[0044] Figure 3 represents the mechanical properties: storage modulus E' and delta tangent measured by DMA of the electrolytes obtained from the formulations according to the invention of example 2 (tetrasiloxane crosslinker with or without lithium salt and with (25 or 50% by mass) or without NMP solvent).

[0045] Figure 4 represents the conductivity (S / cm) as a function of the temperature (1000 / T with T in K) of the electrolytes of example 3 (Ref. and according to the invention) and shows the influence of the sacrificial solvent (acetone or toluene) for two formulations according to the invention on the final electrolyte, compared to that obtained according to the prior art.

[0046] Figure 5 represents the conductivity (S / cm) as a function of temperature (1000 / T with T in K) of the electrolytes of Example 4 (Ref. and according to the invention, with or without chain terminator), and shows the influence of the chain terminator on the properties of the final electrolyte. Description of the embodiments

[0047] The invention relates to a crosslinkable electrolyte formulation comprising:

[0048] - A lithium salt or a mixture of lithium salts

[0049] - A hydrocarbon molecule carrying two thiol functions

[0050] - An unsaturated hydrocarbon molecule containing 2 C=C double bonds

[0051] - A polyvinylsiloxane type crosslinker, namely comprising vinyl functions and at least one siloxane unit;

[0052] - possibly a plasticizer of the oligo(ethylene glycol) type, a low-volatile non-aqueous solvent (vapour pressure less than 8 kPa at 20°C) or ionic liquid, said plasticizer representing less than 30% by mass relative to the total mass of the formulation.

[0053] The crosslinkable electrolyte formulation according to the invention may contain a radical or anionic initiator in order to subsequently initiate the polymerization / crosslinking reaction. Alternatively, the radical or anionic initiator may be added to the formulation according to the invention during the polymerization / crosslinking reaction.

[0054] The formulation obtained can be crosslinked by any technique known to those skilled in the art.

[0055] In the electrolyte formulation according to the invention, the molar ratio [double bond C=C] / [thiols] is advantageously between 1 and 1.1, preferably strictly greater than 1, in order to obtain a crosslinkable formulation having the rheological properties allowing good implementation, said crosslinkable formulation making it possible to obtain an electrolyte having ionic conductivity and increased resistance to chemical degradation.

[0056] In particular, the invention can implement the association of three organic molecules: 2,2'-(ethylenedioxy)diethanethiol, triethylene glycol divinyl ether and polymethylvinylsiloxane as crosslinker, in the presence of a lithium salt or a mixture of lithium salts.

[0057] The electrolyte obtained by crosslinking can be used as a solid electrolyte of a Li-battery or as a component of the positive or negative electrode (posolyte or catholyte, negolyte or anolyte), and including a lithium salt to ensure good transport of lithium ions in the electrolyte and the catholyte / anolyte.

[0058] In one embodiment, a chain terminator compound may also be added to the crosslinkable electrolyte formulation of the invention to create pendant chains upon crosslinking. In a variation of this embodiment, a lithium salt having a C=C double bond may act as a chain terminator.

[0059] In order to obtain a crosslinked solid electrolyte according to the invention, the crosslinkable mixture comprising the following three molecules: hydrocarbon molecule carrying two thiol functions, unsaturated hydrocarbon molecule comprising two C=C double bonds and crosslinking agent, is brought into contact with a lithium salt and optionally, a non-aqueous sacrificial solvent, then the crosslinking reaction is triggered by means of an initiator (radical or anionic initiator) by any technique known to those skilled in the art. The crosslinking reaction can be initiated in particular by thermal or photo-induced or redox activation.

[0060] At the end of the reaction, a lithium ion conducting polymer is formed, this polymer may contain a trapped solvent if a solvent is present in the formulation, which can then be evaporated to obtain a solid material.

[0061] In the electrolyte formulations according to the invention, the thiol / double bond ratio is generally stoichiometric or in a slight excess of C=C double bonds; on the other hand, an excess of thiol is not desirable.

[0062] The composition of the formulation can be modified by varying the amount of crosslinker and, if applicable, the monomer forming the pendant chains. The amount of lithium salt as well as radical or anionic initiator can also be modified and a plasticizer can be added.

[0063] The crosslinked solid electrolytes according to the invention comprise at least:

[0064] - A molecule carrying 2 C=C double bonds, advantageously chosen from oligoethylene glycols carrying two C=C bonds (for example divinyls such as triethylene glycol divinyl ether which is the most commercially available molecule, diallyl ethers, diacrylates or dimethacrylates). - A molecule carrying two thiol functions (2,2'-(ethylenedioxy)diethanethiol for example)

[0065] - A lithium salt or a mixture of lithium salts: any lithium salt soluble in the resin formed (LiPFe, LiFSI Lithium bis (fluorosulfonyl)imide, LiTFSi Lithiumbis (trifluoromethanesulfonyl)imide, LiCIO4, ...) or a mixture of several soluble lithium salts may be suitable.

[0066] LiFSI (Lithium bis(fluorosulfonyl)imide) and LiTFSi (Lithiumbis(trifluoromethanesulfonyl)imide) are preferred due to their good conductivity and electrochemical stability properties. The amount of lithium salt or lithium salt mixture preferably varies between 2 and 40% by mass relative to the total formulation mass, for example a content of 20% by mass can be used.

[0067] A crosslinker: any polymer-type molecule carrying at least one Si-O-Si siloxane unit and at least three vinyl functions can serve as a crosslinker for the crosslinkable electrolyte formulation according to the invention, advantageously at a content of between 0.5% by mass and 20% by mass relative to the total mass of the formulation, very preferably between 1 and 15% by mass, even more preferably between 1 and 5% by mass. Optionally, a compound having the function of chain terminator making it possible to generate pendant chains can be added to the formulation: any molecule carrying a single C=C double bond can be suitable, but the presence of ether or poly(ethylene glycol) functions is desirable. The chosen molecule must not contain labile protons and the quantity of material of the chain terminator is advantageously strictly less than the quantity of material of the crosslinker.

[0068] In the crosslinkable electrolyte formulation according to the invention, the ratios between the different components are chosen according to precise criteria making it possible to obtain the rheological properties of the formulation to ensure good implementation, and good conductivity of the crosslinked electrolyte (once the sacrificial solvent has evaporated if there is any) and optimized mechanical properties of the crosslinked electrolyte.

[0069] In particular, the ratios between crosslinker, molecule carrying 2 C=C double bonds, molecule carrying two thiol functions and chain terminator are calculated as follows: The molar ratio [C=C double bond] / [thiols] must be between 1 and 1.1; an excess of thiols is not desirable.

[0070] Optionally, a plasticizer chosen from oligo(ethylene glycol), low-volatile non-aqueous solvents of carbonate or polyether type or ionic liquids can be added to the formulation. The plasticizer must comprise less than 30% by mass, preferably less than 25% by mass, very preferably less than 20% by mass, even more preferably less than 15% by mass relative to the total mass of the formulation. The plasticizer is advantageously chosen to be less volatile than the sacrificial solvent.

[0071] The radical or anionic initiator, preferably radical (AIBN and TPO in the examples), makes it possible to initiate the crosslinking reaction.

[0072] The initiator may be activated thermally (e.g., a diazo or peroxide compound) or by UV radiation or by redox activation. Any initiator known to those skilled in the art, including any commercial initiator, may be chosen.

[0073] The quantity of initiator to be used advantageously represents between 0.01 and 0.05 times the mass of the mixture of crosslinkable reagents (i.e. the mass of the formulation without the lithium salt(s), without plasticizer, and without any solvent), the choice of the exact quantity is adjusted by the person skilled in the art.

[0074] For redox activation, the reaction can be initiated using a peroxide (very preferably benzoyl peroxide BPO) and an amine (preferably N,N-dimethyl-p-toluidine or p-tolyldiethanolamine). An inhibitor can be added to slow down the generation of radicals (preferably 4-methoxyphenol or 4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl / TEMPOL). The radicals are generated immediately upon mixing the first two components, which must be added just before forming the electrolyte. The quantities of peroxide, amine and inhibitor are chosen by the person skilled in the art.

[0075] Furthermore, the invention relates to:

[0076] - an all-solid Li-ion battery solid electrolyte formed from a crosslinkable formulation according to the invention, and - a component of the positive (posolyte or catholyte) or negative (negolyte or anolyte) electrode of an electrochemical system formed from a crosslinkable formulation according to the invention, and

[0077] - a Li-ion battery comprising one of the components listed above.

[0078] Nature of the crosslinker:

[0079] Any molecule carrying at least three vinyl functions and at least one Si-O-Si siloxane bond can serve as a crosslinker.

[0080] The crosslinker in the crosslinkable formulation according to the invention is advantageously chosen from molecules containing at least two Si atoms linked by an oxygen and at least three vinyl functions, preferably a polymethylvinylsiloxane. By vinylsiloxane crosslinker, is meant in the remainder of the description oligomethylvinylsiloxane, linear or cyclic or any crosslinker comprising at least three vinyl functions and at least one Si-O-Si bond.

[0081] The crosslinker that can be used in the formulations according to the invention can in particular be chosen from poly-(methyl-vinyl siloxane)s or 2, 4, 6, 8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane or hexavinyldisiloxane.

[0082] Most preferably, the crosslinker is 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane.

[0083] This molecule has the particular advantage of being commercially available and it does not have easily hydrolyzable functions (ester, amide, etc.) which could possibly weaken the network.

[0084] In the case of a polymethylvinylsiloxane useful for the invention, the chain length is not important as long as it is greater than or equal to 4 monomers (which gives a linear polymer of viscosity 3-7 cSt), that is to say that the average functionality of the monomer is greater than or equal to 4.

[0085] The use of a crosslinker according to the invention makes it possible to obtain a greater number of vinyl functions on the crosslinker and therefore to obtain networks with better mechanical strength for the same quantity of crosslinker used. Another advantage of this family of crosslinkers is that crosslinking via click chemistry is more efficient and the reagents are relatively inexpensive.

[0086] In addition, the use of a vinylsiloxane crosslinker also improves the ionic conductivity of the electrolyte (presented in the examples) probably due to a better mobility of the polymer chains in the network. Without wishing to be bound by any theory, this exacerbated mobility of the vinylsiloxane chains is partly explained by the high angle of the Si-O-Si bond (145°).

[0087] Advantageously, a crosslinker based on a polysiloxane carrying at least three vinyl functions and preferably having a viscosity greater than or equal to 3 cSt can be used, preferably cyclotetravinyl siloxane or a poly-(methyl-vinylsiloxane) [p-MVS] of variable molar mass. In practice, poly-(methyl-vinyl siloxane) are generally marketed according to their viscosity (expressed in centistokes or cSt). Two different polymers, 3-5 cSt and 7-15 cSt, are presented in the examples, but in practice any poly-(methyl-vinyl siloxane) may be suitable. The viscosity is a function of the molar mass of the polymer, i.e. the number of monomer units.

[0088] The crosslinkable formulation according to the invention advantageously comprises at least one “chain terminator” based on a carbonate carrying a vinyl function or an allyl alkyl ether, where the alkyl chain is preferably a methyl or ethyl group.

[0089] The crosslinkable formulation according to the invention can use a “sacrificial solvent” to form a precursor gel of the network, the use of said sacrificial solvent making it possible to compatibilize products which are not miscible alone.

[0090] For example, polymethylvinylsiloxane 7-15 cSt is not miscible with other reagents and requires a sacrificial solvent, for example, acetone, to be prepared. In the absence of a compatibilizing solvent, no crosslinking is observed. The sacrificial solvent is then removed by evaporation to obtain an all-solid electrolyte. If another product used as a plasticizer is added, it must be of low volatility compared to the sacrificial solvent so that it is not evaporated at the same time as the sacrificial solvent.

[0091] The sacrificial solvent may be, but is not limited to: N-methyl pyrrolidone (NMP), acetone, methyl ethyl ketone, toluene, anisole, acetonitrile, chloroform, dichloromethane, diethyl ether, methyl tert-butyl ether, 2-methyltetrahydrofuran (2-MeTHF), dioxane, 1,2-dimethoxyethane (DME), THF, methoxycyclopentane, dimethyl carbonate, methyl isobutyl ketone, alone or in a mixture, hexane (in a mixture), heptane (in a mixture).

[0092] Mixtures of solvents are also possible. In practice, any solvent that makes all the reagents miscible can be suitable. Acetone is particularly preferred for solubilizing the p-MVS 7-15 cSt crosslinker.

[0093] The amount of sacrificial solvent can be up to 75% by mass of the formulation. Preferably, between 10 and 75% by mass of sacrificial solvent is used in the formulation, more preferably between 25% and 50% by mass of sacrificial solvent. The use of a sacrificial solvent does not change the conduction properties of the materials once the solvent has evaporated.

[0094] Alkaline lithium salts can be lithium salts carrying a C=C double bond (vinyl, allyl, acrylate or styrene) on the anion and which can be grafted onto the network to produce so-called "single ion" materials. In this case, the lithium salt is used in addition to or instead of the chain terminator based on a carbonate carrying a vinyl function or an allyl alkyl ether: the reactive groups that can be used are the acrylate and styrene sulfonate group for lithium, sodium and potassium salts.

[0095] Crosslinking activation can be done after adding the radical or anionic initiator, or by using the radical or anionic initiator already present in the formulation by photo-induction or by thermal activation or by redox activation.

[0096] Photo-induction can be carried out using a UV lamp or stereolithographic techniques, including laser, LCD screen, 3D printing (also called additive manufacturing). The invention can be applied in particular to 3D printing of batteries, especially vehicle batteries.

[0097] In the case where the solid electrolyte is in contact with the positive electrode, the crosslinkable formulation according to the invention in the liquid state can be coated on a positive electrode already prepared according to the protocols of a person skilled in the art so that the liquid formulation wets the porosity of the electrode. Finally, crosslinking is obtained after thermal initiation or photoinitiation (UV). In a variant of the invention, the formulations according to the invention can be used to formulate a positive electrode ink (cathode), in particular by adding to the formulation the active material (for example a lithium nickel manganese cobalt mixed oxide or a lithium iron phosphate) and the electronic percolant. This ink, once coated and dried, can constitute a solid positive electrode.

[0098] Characterization techniques

[0099] The ionic conductivity of an electrolyte is measured in S / cm and characterizes the electrolyte's ability to transport ions.

[0100] The ionic conductivity of the electrolyte is measured by electrochemical impedance spectroscopy in a Biologie® CESH cell. The measurement is carried out at several temperatures using a Biologie® ITS thermostated chamber. The impedance spectrum is acquired using a Biologie® MTZ 35 potentiostat between 30 MHz and 0.1 Hz around 0 V and with an amplitude of ± 10 mV. The conductivity value can be determined by fitting the curve with an equivalent circuit of the type (RI + R2 / / CPE1 + CPE2) or visually by taking the value of the real part of the impedance Re(Z) at the minimum reached by the curve between the semicircle and the half-line corresponding to the capacitive part on the Nyquist diagram.

[0101] The cation transport number (t+) corresponds to the fraction of the total conductivity related to the transport of charges by cations and is characterized by impedance spectroscopy on a sample mounted between two non-blocking electrodes (lithium metal) at 60°C with a Biologie VMP3 potentiostat between 1 MHz and 0.1 mHz around 0V with an amplitude of 10 mV. The transport number is obtained by fitting an equivalent circuit (RI + R2 / / CPE2 + R3 / / CPE3 + Wdl) then by applying the formula: t+ = R2 / (Rwdi + R2). This measurement method corresponds to the so-called "Watanabe" method (Solid State Ionics 28-30 (1988) 911-917).

[0102] Solvent swelling test and calculation of soluble fraction are classical methods for characterizing polymer networks in order to have an indirect estimate of their crosslink density as well as their chemical stability. After immersing about 300 mg of polymer network sample for 72 hours at room temperature in a large excess of chloroform (CHCh), the swollen samples are weighed and then dried under vacuum.

[0103] The soluble fraction (ws) is calculated with the following equation: ws = 1 - md / mO with mO the initial mass of the solvent-swollen sample (estimated by linear regression on the curve of masses measured at regular intervals of 0.5 to 5 minutes after removing the sample from the solvent) and md the mass of the sample after extraction of the soluble fraction and drying.

[0104] The swelling rate, which corresponds to the volume gain of the network after immersion in the solvent, (l / q2) is calculated with the following equation: l / q2 = p p (ql - l) / ps where ql is the ratio of the masses of the swollen sample to the dry sample, p s is the density of the solvent (ps = 1.492 g. cm -3 for CHCh) and p p the density of the polymer network.

[0105] The thermomechanical properties of single-ion polymer networks are characterized by DSC, TGA and DMA to determine the glass transition value T, respectively. g and the storage modulus E' values ​​at the glassy plateau (around -70 °C) and at the rubbery plateau (around 20 °C) as well as the alpha transition temperature. This makes it possible to demonstrate the mobility of the networks in relation to their ionic conductivity properties.

[0106] Differential scanning calorimetry (DSC) experiments were performed using a Q200 DSC apparatus (TA Instruments) calibrated with an indium standard. Samples (5–10 mg) were prepared using airtight capsules and measured under a helium flow of 25 mL.min -1 . The samples were first heated to 120 °C with a heating rate of 10 °C. min -1 then kept at this temperature for 2 min. Then the temperature was brought to -80 °C with a cooling rate of 10 °C. min -1 before heating again, after a 2 min isotherm, up to 30 °C with a heating rate of 10 °C. min -1 . The glass transition temperatures (T g) were measured at the mid-height of the transitions appearing during the second heating using TA Thermal Analysis software. Dynamic Mechanical Analysis (DMA) experiments were carried out with a DMA Q800 type apparatus (TA Instrument) in the film tension mode. Rectangular samples of 5 x 1.5 mm 2 surface area and approximately 15 mm in length were tested at a frequency of 1 Hz and an amplitude of 5 pm. A temperature ramp from -80 °C to 30 °C at a rate of 3 °C. min -1 was applied. The Ta values ​​were calculated from the maximum value of Tan 6 corresponding to the inflection point of the storage modulus E'.

[0107] Advantages of the invention

[0108] The crosslinkable electrolyte formulation according to the invention has the advantage, in particular compared to already polymerized polymers, of being liquid at the start, which makes it possible to obtain good cohesion at the interfaces with the electrodes, in particular due to easier filling of the porosity of the materials.

[0109] The use of controlled crosslinking on a mixture of particular monomers and the particular morphology of the three-dimensional network also provide increased mobility of the chains and consequently improved ionic conduction. The performance of the solid electrolyte according to the invention is based in particular on the specific choice of certain very flexible monomers which are good solvents for lithium ions and by controlled crosslinking.

[0110] The rheological behavior obtained for the crosslinked electrolyte samples according to the invention advantageously corresponds to that of a crosslinked polymer.

[0111] Examples

[0112] In the examples, the following nomenclature will be used:

[0113] - PEO: poly(ethylene oxide)

[0114] - LiTFSI: lithium bis(trifluorosulfonylimide)

[0115] - Polysiloxane: poly(methylvinylsiloxane)

[0116] - Tetrasiloxane: 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane

[0117] - TPO: diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide Example 1: basic formulations and effect of the crosslinker

[0118] Three formulations are made using different crosslinkers in the absence of solvent. The crosslinkers are present in the formulation at 5% by mass and the other components are dosed so as to respect the stoichiometry between the alkene functions and the thiol functions.

[0119] 20% by mass of lithium salt LiTFSI is added to the mixture as well as 1% of TPO initiator (calculated on the basis of crosslinkable mixture without taking into account the Li salt). Once the mixture is homogeneous it is crosslinked between a glass plate and a Teflon® plate separated by a spacer of known thickness. Crosslinking is done by UV illumination of 180 mW / cm 2 for 1 minute. Samples are prepared by the same method with a thickness of 1 and 2 mm for DSC and DMA measurements.

[0120] Figure 1 represents the comparison between different crosslinkable formulations: a, b, c using a crosslinker according to the invention and Ref. (comparative, according to the prior art)

[0121] The results obtained show two important trends: for an equivalent quantity of crosslinker, the ionic conductivity of the solid electrolyte is better if a vinylsiloxane crosslinker is used. This result validates the improvement provided by these crosslinkers compared to those known from the prior art. The network conductivity is slightly lower than that of a PEO:LiTFSI 20:1 (mixture of poly(ethylene oxide) and lithium bis-trifluorosulfonylimide with one lithium atom for 20 "ethylene oxide" segments) above 50 °C, but remains significantly better at lower temperatures. Indeed, unlike PEO, the crosslinked polymers obtained according to the invention are amorphous and not semi-crystalline. However, no significant difference is observed between the two crosslinkers which have similar structures (approximately 4 monomer units each), the tetrasiloxane is cyclic while the 3-7 cts polysiloxane is rather linear.

[0122] This example shows the improvement in the conductivity of the electrolyte compared to the electrolytes of the prior art at an equivalent quantity of crosslinker. It also shows the absence of significant difference between the two vinylsiloxane crosslinkers proposed. Table 1 below presents the thermomechanical characteristics of the electrolytes obtained measured respectively by DSC, TGA and DMA according to the different crosslinkers used.

[0123] Table 1

[0124] Figure 2 represents the mechanical characteristics: Storage modulus E' and tan ô measured by DMA according to the different crosslinkers used.

[0125] Example 2:

[0126] Table 2 shows the thermomechanical characteristics for the tetrasiloxane crosslinker with or without lithium salt and with (25 or 50% by mass) or without NMP solvent.

[0127] Table 2

[0128] Figure 3 represents the storage modulus E' and Tangent delta measured by DMA for the tetrasiloxane crosslinker with or without lithium salt and with (25 or 50% by mass) or without NMP solvent. The results in Figure 3 show that the presence of solvent during crosslinking does not significantly impact the mechanical and mobility properties of the network. The elastic modulus is slightly decreased when the network is generated in the presence of solvent. The molecular mobility is, however, slightly improved, since the Ta decreases with the addition of 25% solvent and even more so at 50% solvent. This can be explained by a lesser physical entanglement of the crosslinked chains when they are formed in a dilute medium.

[0129] Example 3:

[0130] Precursor formulations with compositions identical to those presented in Example 1 were prepared; these formulations all contained a solvent. In this example, acetone and toluene were used. In the case of toluene, the amount of solvent used was 50% by mass of the total mixture. For acetone, the amount was not precisely measured, but was estimated to be approximately 50% by mass.

[0131] The samples were crosslinked under the same conditions as in Example 1. The solvent was then evaporated in a reduced pressure oven (0.01 mbar) at 80°C for 16 h.

[0132] Conductivity measurements show that the nature of the solvent or crosslinker has no significant influence on the conductivity of the material (Figure 4).

[0133] Example 4:

[0134] A formulation similar to Example 1 was made using 7-15 cts polysiloxane and acetone as the sacrificial solvent. A chain terminator (4-Vinyl-1,3-dioxolan-2-one) was added, representing 1% by mass of the composition. In an attempt to maintain a similar crosslinking rate, additional polysiloxane is added (1 molar equivalent of crosslinker monomer unit per chain terminator).

[0135] Conductivity measurements taken between 80°C and -20°C (Figure 5) reveal that the addition of a chain terminator slightly increases the ionic conductivity of the material. This phenomenon is probably due to improved mobility of the polymer chains due to the presence of pendant chains.

Claims

Claims 1. Crosslinkable electrolyte formulation comprising at least: A lithium salt or mixture of lithium salts; A hydrocarbon molecule containing two thiol functions; An unsaturated hydrocarbon molecule containing two C=C double bonds; A crosslinker, said crosslinker being a molecule comprising at least three vinyl functions and at least one Si-O-Si siloxane bond; in which the molar ratio [C=C double bond] / [thiols] is between 1 and 1.

1.

2. Crosslinkable electrolyte formulation according to claim 1 in which the quantity of lithium salt(s) represents between 2 and 40% by mass relative to the total formulation mass, preferably between 10 and 30% by mass relative to the total formulation mass.

3. Crosslinkable electrolyte formulation according to one of the preceding claims in which the quantity of crosslinker is between 0.5 and 20% by mass, preferably between 1 and 5% by mass relative to the total mass of formulation.

4. Crosslinkable electrolyte formulation according to one of the preceding claims comprising a radical or anionic initiator, in an amount of between 0.01 and 0.05 times the total mass of formulation excluding lithium salt(s).

5. Crosslinkable electrolyte formulation according to one of the preceding claims comprising a chain terminator for generating pendant chains, said chain terminator being a molecule carrying a single C=C double bond, without labile protons, in an amount strictly less than the amount of material of the crosslinker, preferably a molecule comprising ether or poly(ethylene glycol) functions, very preferably a vinyl ethyl ether or a vinyl methyl ether OR preferably a carbonate carrying a vinyl function, very preferably 4-Vinyl-1,3-dioxolan-2-one or an allyl alkyl ether, preferably a vinyl methyl or vinyl ethyl ether.

6. A crosslinkable electrolyte formulation according to claim 5 wherein the lithium salt comprises a C=C bond and also acts as a chain terminator.

7. Crosslinkable electrolyte formulation according to one of the preceding claims in which the crosslinker is chosen from poly-(methyl-vinyl siloxanes or 2, 4, 6, 8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane or hexavinyldisiloxane, very preferably 2, 4, 6, 8-tetramethyl-2,4,6,8-tetravinyl-cyclotetrasiloxane.

8. Crosslinkable electrolyte formulation according to one of the preceding claims in which the hydrocarbon molecule comprising two thiol functions is 2,2'-(ethylenedioxy)diethanethiol.

9. Crosslinkable electrolyte formulation according to one of the preceding claims in which the hydrocarbon molecule comprising two C=C double bonds is chosen from oligoethylene glycols bearing two C=C bonds, diallyl ethers and diacrylates or dimetacrylates, preferably is triethylene glycol divinyl ether.

10. A crosslinkable electrolyte formulation according to one of the preceding claims comprising a non-aqueous sacrificial solvent.

11. Crosslinkable electrolyte formulation according to one of the preceding claims comprising a plasticizer of the oligoethylene glycol type, low-volatile non-aqueous solvent or ionic liquid, said plasticizer representing less than 15% by mass relative to the total mass of the formulation.

12. Process for the preparation of a crosslinked solid electrolyte in which: a) the crosslinkable electrolyte formulation according to one of the preceding claims is crosslinked, in the optional presence of a non-aqueous sacrificial solvent, by means of a radical or anionic initiator added or already present in the formulation and by thermal or photo-induced or redox activation to form a polymer conducting lithium ions, b) the possible sacrificial solvent present in said polymer is evaporated in order to obtain a crosslinked solid electrolyte.

13. Process for preparing a crosslinked solid electrolyte according to claim 12 in which the photo-induced activation is carried out by UV lamp or stereolithography.

14. Process for preparing a crosslinked solid electrolyte according to claim 12 or 13, characterized in that step a) of crosslinking is carried out sequentially.

15. Use of the crosslinked electrolyte obtained by crosslinking the formulation according to one of claims 1 to 11 or obtained by the process according to one of claims 12 to 14 as a solid electrolyte of an all-solid Li-ion battery or as a component of the positive (posolyte or catholyte) or negative (negolyte or anolyte) electrode of an electrochemical system.

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