Electrochemical element sodium-ion

Abstract

An electrolyte for a sodium-ion electrochemical element, which electrolyte comprises: - one or more solvents, - at least two sodium salts other than sodium difluorophosphate, one of the salts being sodium bis(fluorosulfonyl)imidide, - vinylene carbonate and / or ethylene monofluorocarbonate, - sodium difluorophosphate, and - ethylene sulfate. The use of this electrolyte in a sodium-ion electrochemical element increases the element's cycling lifetime. Abbreviated figure: Figure 1

Description

Title of the invention: Electrochemical element sodium-ion technical field

[0001] The technical field of the invention is that of rechargeable electrochemical elements (or accumulators) of the sodium-ion type. Background

[0002] A sodium-ion electrochemical element is known in the prior art. It generally comprises a positive electrode containing an active material capable of inserting sodium into its structure, a negative electrode containing an active material such as carbon, and an electrolyte. The electrolyte comprises one or more organic solvents in which one or more sodium salts are dissolved. The electrolyte fills the internal volume of the element and allows sodium ions to move between the positive and negative electrodes. The operation of the electrochemical element is based on the principle of the reversible insertion of sodium into the host structure of an electrochemically active material. When the element is charged, the positive active material is oxidized and disinserts sodium from its structure, while the negative active material is reduced and sodium is inserted into its structure.Conversely, during the discharge of the element, the positive active matter is reduced and inserts sodium into its structure, while the negative active matter is oxidized and sodium is disinserted from its structure.

[0003] A sodium-ion element is an attractive alternative to a lithium-ion element, particularly due to the greater abundance of sodium compared to lithium. However, one of the current drawbacks of a sodium-ion element is its shorter cycle life compared to that of a lithium-ion element. One reason for the decrease in capacity of a sodium-ion element during cycling is the decomposition of the electrolyte on the surface of the negative electrode when the latter is made of carbon. A known method for slowing this decomposition is to add one or more additives to the electrolyte that promote the formation of a protective passivation layer (a "SEI" layer for Solid Electrolyte Interface) on the surface of the negative electrode. This passivation layer protects the carbon surface from contact with the electrolyte. Examples of such additives are vinyl carbonate (VC) and ethylene monofluorocarbonate (FEC).To observe a lasting protective effect, the passivation layer must be as stable as possible. Indeed, if it lacks stability, it dissolves again during cycling and a new passivation layer forms, which consumes electrolyte and leads to a decrease in the element's performance. Furthermore, this layer is inherently resistive. If its thickness increases, We observe an increase in the impedance of the element and therefore a decrease in its performance under high current discharge.

[0004] Document CN117096436 describes an electrolyte for a sodium ion, which electrolyte comprises an organic solvent that is a cyclic carbonate, a sodium salt, a first additive, and a second additive. The first additive may have the following formula:

[0005] [Chem.l] R3 Or RI represents OmCnH2n, with 0 <m<l, 0<n<3, R2 represents a halogenated or non-halogenated group in the Ci-C3 range. R3 is chosen from H, F and a halogenated or non-halogenated CrC hydrocarbon group 3- The second additive has the formula NaxPOyFz with l <x<2, 2<y<3 et l<z<2.

[0006] We seek to make available new sodium-ion elements exhibiting a long lifetime. Summary

[0007] To this end, the invention proposes an electrolyte comprising: - one or more solvents, - at least two sodium salts other than sodium difluorophosphate (NaPO2F2), one of the salts being sodium bis(fluorosulfonyl)imidide (NaFSI), - vinylene carbonate (VC) and / or ethylene monofluorocarbonate (FEC), - sodium difluorophosphate (NaPO2F2) and - ethylene sulfate (ESA also abbreviated DTD).

[0008] The invention is based on the discovery that the association of the components NaPO2F2, DTD and NaFSI as co-salt of a sodium salt makes it possible to obtain a passivation layer which is stable over time and has low resistance.

[0009] According to one embodiment, the electrolyte consisting of: - one or more solvents, - at least two sodium salts other than sodium difluorophosphate, one of the salts being sodium bis(fluorosulfonyl)imidide, - vinylene carbonate and / or ethylene monofluorocarbonate, - sodium difluorophosphate, and - ethylene sulfate.

[0010] According to one embodiment, sodium bis(fluorosulfonyl)imidide represents 10 to 50% by mole of the total number of moles of sodium salts.

[0011] According to one embodiment, sodium bis(fluorosulfonyl)imidide represents 20 to 40% by mole of the total number of moles of sodium salts.

[0012] According to one embodiment, said one or more solvents is a mixture of propylene carbonate and ethyl methyl carbonate.

[0013] According to one embodiment, the electrolyte consists of: - propylene carbonate and ethyl methyl carbonate in respective volume proportions of 10-30% / 90-70%, - 0.1 to 0.5 mol.L 1 of sodium bis(fluorosulfonyl)imidide and 0.5 to 0.9 mol.L 1 of sodium hexafluorophosphate, - 1 to 3% by mass of vinylene carbonate and / or ethylene monofluorocarbonate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate, - 0.2 to 2% by mass of sodium difluorophosphate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfony l)imidide and sodium hexafluorophosphate, - 0.5 to 3% by mass of ethylene sulfate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate.

[0014] According to one embodiment, the electrolyte consists of: - propylene carbonate and ethyl methyl carbonate in respective volume proportions of 15-25% / 85-75%, - 0.2 to 0.4 mol.L 1 of sodium bis(fluorosulfonyl)imidide and 0.6 to 0.8 mol.L 1 of sodium hexafluorophosphate, - 1.5 to 2.5% by mass of vinylene carbonate and / or ethylene monofluorocarbonate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate, - 0.5 to 1.5% by mass of sodium difluorophosphate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate, - 0.5 to 1.5% by mass of ethylene sulfate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate.

[0015] According to one embodiment, the mass ratio of ethylene sulfate to vinylene carbonate is less than or equal to 1, preferably less than or equal to 0.5.

[0016] According to one embodiment, the mass ratio of ethylene sulfate to sodium difluorophosphate is greater than or equal to 1.

[0017] The invention also relates to a sodium-ion electrochemical element comprising an electrolyte as described above.

[0018] According to one embodiment, the electrochemical element comprises: - at least one positive electrode comprising a positive active material chosen from: a) an oxide of formula Naw(NixFeyMnzMt)O2 where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci ; b) a polyanion; c) Prussian blue and its analogues; and d) mixtures of these, - at least one negative electrode.

[0019] According to one embodiment, the positive active material is an oxide of formula Naw (NixFeyMnzMt)O2 where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci.

[0020] According to one embodiment, the negative electrode comprises a negative active material which is a hard carbon or a soft carbon or a mixture of a soft carbon and a hard carbon.

[0021] According to one embodiment, - the positive active substance is an oxide with the formula Naw(NixFeyMnzMt)O2 where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci, - the negative active material is a hard carbon.

[0022] According to one embodiment, x<0.5; y <0.5; z<0.5; x+y+z=l and t=0.

[0023] According to one embodiment, x=y=z=l / 3. Brief description of the figure

[0024] [Fig.1] represents the variation in the capacity of elements A, B and C during a cycle at the charge regime of C / 3 and discharge regime of D / 3 at 30°C. Detailed description of the electrolyte embodiments:

[0025] The electrolyte of the sodium-ion element can be liquid or gelled.

[0026] The solvent can be chosen from among saturated cyclic carbonates, carbonates unsaturated cyclics, linear carbonates, alkyl esters, cyclic esters (lactones), linear ethers, cyclic ethers and mixtures thereof.

[0027] Among the saturated cyclic carbonates, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and mixtures thereof may be mentioned.

[0028] Among the linear carbonates, we can mention dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC) and mixtures thereof.

[0029] In a preferred embodiment, the solvents consist of one or more cyclic carbonate(s) and / or one or more linear carbonate(s).

[0030] Preferably, the cyclic carbonate(s) represent 50% or less of the total solvent volume and the linear carbonate(s) represent 50% or more of the total solvent volume. The cyclic carbonate(s) may represent 10 to 40%, 15 to 30%, or 20 to 30% of the total solvent volume, and the linear carbonate(s) may represent 60 to 90%, 70 to 85%, or 70 to 80% of the total solvent volume.

[0031] By alkyl ester is meant a compound of formula RC(=O)-O-R' in which R and R' each independently designate an alkyl group, the compound not comprising an oxygen atom between the alkyl group R and the carbonyl function -C(=O). Examples of alkyl esters include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and mixtures thereof.

[0032] Among the ethers, we can mention tetrahydrofuran (THF), 1,3-dioxolane, dimethyl ether (DME) or diethyl ether (DEE) and mixtures thereof.

[0033] According to one embodiment, the electrolyte is free of ether or alkyl ester. In a preferred embodiment, the solvents consist of PC and EMC.

[0034] The passivation additive is VC or FEC or a mixture of VC and FEC. Preferably, it is VC.

[0035] The mass proportion of VC or FEC or of the mixture of VC and FEC is generally less than or equal to 10% relative to the total mass of said one or more sodium salts and said one or more solvents. It may range from 1 to 3% or from 1.5 to 2.5%.

[0036] The mass proportion of NaPO2F2 is generally less than or equal to 5% relative to the total mass of said one or more sodium salts and said one or more solvents. It may range from 0.2 to 2% or from 0.5 to 1.5%.

[0037] The mass proportion of DTD is generally less than or equal to 5% relative to the total mass of said one or more sodium salts and said one or more solvents. It may range from 0.5 to 3% or from 0.5 to 1.5%.

[0038] Preferably, the mass ratio between DTD and VC or the mass ratio between DTD and FEC or the mass ratio between DTD and (VC+FEC) is less than or equal to 1 or less than or equal to 0.5 or less than or equal to 0.25. If this ratio is greater than 1, the lifetime of the element may be reduced due to a possible redissolution of the passivation layer.

[0039] Preferably, the mass ratio between DTD and NaPO2F2 is greater than or equal to 1 or greater than or equal to 2. Above 1% by mass of NaPO2F2, it may not be completely solubilized in the electrolyte, which may lead to a loss of ionic conductivity and an increase in the resistance of the element.

[0040] According to one embodiment, the electrolyte does not contain any additive other than VC and / or FEC, DTD and NaPO2F2.

[0041] The electrolyte contains at least two sodium salts other than NaPO2F2, at least one of the two salts being NaFSI. NaFSI may represent from 10 to 90% or from 20 to 70% or from 30 to 50% of the total number of moles of sodium salts, including NaPO2F2.

[0042] The sodium salt other than NaFSI and NaPO2F2 may be hexafluorinated or tetrafluorinated. It may be selected from the group consisting of NaPF6, NaBF4, NaAsF6 and NaSbF6. Preferably, it is NaPF6.

[0043] Other sodium salts may also be present in the electrolyte. These may include NaC104, sodium bis(trifluoromethanesulfonyl)imidide NaN(CF3SO2)2 (NaTFSI), sodium tris(fluoromethanesulfonyl)methylide NaC(CF3SO2)3 (NaTFSM), sodium bis(pentafluoroethylsulfonyl)imidide NaN(C2F5SO2)2 (NaBETI), sodium 4,5-dicyano-2-(trifluoromethyl)imidazolide (NaTDI), sodium bis(oxalato)borate (NaBOB), sodium difluoro(oxalato)borate (NaDFOB), sodium tris(pentafluoroethyl)trifluorophosphate NaPF3(CF2CF3)3 (NaFAP), sodium difluorophosphate NaPO2F2 and mixtures thereof.

[0044] The total concentration of sodium salts generally ranges from 0.7 to 2 mol.L 1 or from 1 to 1.5 mol.L1.

[0045] Preferably, the electrolyte does not contain any other sodium salt than NaPF6 and NaFSI.

[0046] A preferred electrolyte composition consists of: - a mixture of propylene carbonate (PC) and ethyl methyl carbonate (EMC) as solvents, - NaPF6 and NaFSI as sodium salts, - VC and / or FEC, - NaPO2F2 and - DTD.

[0047] Another preferred electrolyte composition consists of: - PC / EMC in the respective volume proportions of 10-30% / 90-70%, - 0.1 to 0.5 mol.L 1 of NaFSI and 0.5 to 0.9 mol.L 1 of LiPF6, - 1 to 3% by mass of VC or FEC or (VC+FEC), this mass being expressed relative to the total mass of PC, EMC, NaFSI and NaPF6, - 0.2 to 2% by mass of NaPO2F2, the mass of NaPO2F2 being expressed relative to the total mass of PC, EMC, NaFSI and NaPF6, - 0.5 to 3% by mass of DTD expressed relative to the total mass of PC, EMC, NaFSI and NaPF6.

[0048] Another preferred electrolyte composition consists of: - PC / EMC in the respective volume proportions of 15-25% / 85-75%, - from 0.2 to 0.4 mol.L 1 of NaFSI and from 0.6 to 0.8 mol.L 1 of LiPF6, - 1.5 to 2.5% by mass of VC or FEC or (VC+FEC), this mass being expressed relative to the total mass of PC, EMC, NaFSI and NaPF6, - 0.5 to 1.5% by mass of NaPO2F2, the mass of NaPO2F2 being expressed relative to the total mass of PC, EMC, NaFSI and NaPF6, - 0.5 to 1.5% by mass of DTD expressed relative to the total mass of PC, EMC, NaFSI and NaPF6. Positive electrode:

[0049] The positive electrode includes a current collector, at least one of whose faces is coated with a layer of a composition of positive active materials, which includes one or more active materials and optionally one or more binders and one or more electronically conductive materials.

[0050] The positive current collector is a solid or perforated metal strip which may be made of aluminum or an aluminum alloy or steel or stainless steel. Its thickness may be in the range of 6 to 30 µm or 5 to 20 µm or 10 to 15 µm, preferably 10 to 15 µm.

[0051] The positive active ingredient can be chosen from: a) a lamellar oxide of formula NaMO2 where M denotes at least one transition metal; b) a polyanion; (c) a compound of Prussian blue or its analogues whose formula is AxP[R(CN)6]y .zH2O, where A is an alkali cation, and P and R are divalent or trivalent transition metal cations, 0 <x<2 ; y<l ; 0<z ; et d) des mélanges de ceux-ci.

[0052] Among the compounds of type a), the preferred ones have the formula Naw(NixFeyMnzM t)O2 where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci. Cette matière active est avantageuse en ce qu’elle est dépourvue de cobalt lequel présente une certaine toxicité.

[0053] According to one embodiment, x<0.5, y<0.5 and z<0.5 and x+y+z+t=l. A preferred compound is Na(Ni / 3Fei / 3Mni / 3)O2.

[0054] Among type b compounds, the preferred ones are phosphates, such as Na3V2(PO4)3 and Na3V2(PO4)2F3.

[0055] Among type c compounds), Na2Mn[Fe(CN)6] is preferred.

[0056] An ink is prepared by dispersing one or more positive active substances in a solvent or in a mixture of several organic or aqueous solvents. Optionally, a binder and an electronically conductive material are added to the dispersion. By varying the amount of solvent incorporated into the mixture, the viscosity of the ink can be varied before it is deposited on a face of the current collector. The ink-coated current collector is dried and then laminated to adjust its thickness. After evaporation of the solvent(s), a layer is obtained composed of one or more active substances, the proportions of whose various constituents are typically: - 80 to 98% or 90 to 95% by mass of one or more active ingredients, - 1 to 10% or 2 to 5% by mass of one or more binders, - from 0.1 to 10% or from 2 to 5% by mass of one or more electronically conductive materials.

[0057] The binder can be selected from poly(vinylidene fluoride) (PVDF) and its copolymers, polytetrafluoroethylene (PTFE) and its copolymers, polyacrylonitrile (PAN), poly(methyl or butyl methacrylate), poly(vinyl chloride) (PVC), poly(vinyl formalin), polyester, sequenced polyetheramides, acrylic acid polymers, methacrylic acid polymers, acrylamide polymers, itaconic acid polymers, sulfonic acid polymers, elastomers, and cellulosic compounds such as carboxymethylcellulose (CMC). Elastomers usable as binders can be selected from styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), and hydrogenated butadiene-acrylonitrile (HNBR). Said at least one binder may be dispersible in aqueous media, such as polytetrafluoroethylene (PTFE), the carboxymethylcellulose (CMC), styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), hydrogenated butadiene-acrylonitrile (HNBR) and polyvinyl alcohol (PVA).

[0058] The electronically conductive material is generally chosen from graphite, carbon black, acetylene black, soot, graphene, carbon fibers, carbon nanotubes or a mixture thereof. Negative electrode:

[0059] The negative electrode includes a current collector, at least one of whose faces is coated with a layer of a composition of negative active materials, which includes one or more negative active materials and optionally one or more binders and one or more electronically conductive materials.

[0060] The active material may be hard carbon or soft carbon, or a mixture thereof. Preferably, it is hard carbon because it exhibits higher cycling capacity and stability than soft carbon. Even more preferably, the negative electrode contains no active material other than hard carbon.

[0061] Hard carbon is defined as carbon that cannot be transformed into graphite (non-graphitizable), even when heated to a temperature exceeding 2500 °C. It generally has a disordered, non-crystalline structure. It is obtained by pyrolysis under an inert atmosphere of a precursor at a temperature of approximately 1000 °C. The precursor can be a synthetic polymer such as a phenolic resin, polyaniline (PANI), and polyacrylonitrile (PAN). It can be a biopolymer such as sucrose, glucose, cellulose, cotton, and chitosan. It can be derived from raw biomass such as leaves, algae, wheat straw, and pine.

[0062] Soft carbon is defined as carbon that can be transformed into graphite (graphitizable) by pyrolysis of a precursor at a temperature of approximately 3000°C. This precursor can be polyvinyl chloride (PVC) or petroleum coke.

[0063] The current collector is preferably a two-dimensional conductive support such as a solid or perforated strip, preferably made of aluminum or an aluminum alloy. The current collector may be coated on one or both sides with a layer of carbon. Its thickness may range from 3 to 25 µm, preferably from 10 to 15 µm.

[0064] To obtain the negative active material composition, an ink is prepared by dispersing one or more negative active materials, and optionally one or more binders and one or more electronically conductive compounds, in a solvent or a mixture of solvents, preferably aqueous. The binder(s) may be chosen from the same list as that described in relation to the positive electrode, without necessarily being the same as those of the positive electrode. Similarly, the electronically conductive material(s) may be chosen from the same list as that described in relation to the positive electrode, without necessarily being the same as those of the positive electrode.

[0065] The ink-coated current collector is dried and then laminated to adjust its thickness. After evaporation of the solvent(s), a layer is obtained composed of one or more active materials, the proportions of whose various constituents are typically: - 85 to 98% or 90 to 98% by mass of one or more negative active ingredients, - 1 to 10% or 1 to 5% by mass of one or more binders, - 0 to 5% by mass or 1 to 5% of one or more electronically conductive materials. Separator:

[0066] A separator is generally interposed between a negative electrode and a positive electrode to prevent potential short circuits. It prevents electrical contact between a negative electrode and a positive electrode but nevertheless allows the transport of ions between these two electrodes. The separator material can be selected from the following materials: a polyolefin, for example polypropylene and polyethylene, a polyester, glass fibers bonded together by a polymer, polyimide, polyamide, polyaramid, polyamideimide, and cellulose. The polyester can be selected from polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Advantageously, the polyester, polypropylene, or polyethylene contains or is coated with a material selected from the group consisting of a metal oxide, a carbide, a nitride, a boride, a silicide, and a sulfide. This material can be SiO2 or Al2O3.The separator can be coated with an organic coating, for example comprising an acrylate or PVDF or P(VdF-HFP). A preferred separator is made of polyethylene or is made of a combination of three layers which are polypropylene PP / polyethylene PE / polypropylene PP.

[0067] A preferred electrochemical element sodium-ion comprises: - a negative electrode comprising an active material which is hard carbon, - a positive electrode comprising an active material of the formula Naw(NixFeyMnzMt)O2 where 0.9 <w<l,l ; 0<x<0,5 ; 0<y<0,5 ; 0<z<0,5 avec x+y+z=l ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci ; - an electrolyte consisting of PC, EMC, NaPF6, NaFSI, VC, DTD and NaPO2F2.

[0068] The electrochemical element is manufactured in a conventional manner. It can be in prismatic, cylindrical, pocket or button format. Examples

[0069] Three sodium-ion electrochemical elements A, B, and C were manufactured. In these three elements, the negative electrode contains hard carbon as the active material, and the positive electrode contains Na(Ni / 3Fei / 3Mni / 3)O2 as the active material. Elements A, B, and C differ in the composition of their electrolyte, which is shown in Table 1 below. Solvent percentages are expressed by volume. Additive percentages are expressed relative to the total mass of solvents and NaPF6 and NaFSI, where applicable. The elements were cycled at 30°C under C / 3 and D / 3 regimes. [Tables 1] Element Electrolyte Composition Percentage of Initial Capacity at 30 Cycles A* PC / EMC (20 / 80) NaPF6 1 molLL1 + 2% VC 46 B* PC / EMC (20 / 80) NaPF6 1 molLL1 + 2% VC + 1% NaPO2F2 + 1% DTD 66 C PC / EMC (20 / 80) NaPF6 0.7 molLL1 + NaFSI 0.3 molLL1 + 2% VC + 1% NaPO2F2 + 1% DTD 89 * Element not included in the invention

[0070] The results of the cycling test of these elements are shown in [Fig. 1]. It can be seen that the element which retains its initial capacity the longest during cycling is element C. The electrolyte of element C contains the combination of VC, NaPO2 F2, DTD, NaPF6 and NaFSI.

[0071] A comparison between element C and element B shows the beneficial effect of substituting 30% of the number of moles of NaPF6 salt with NaFSI. The lifetime of element C is longer than that of element B.

[0072] A comparison between element B and element A shows that the addition of NaPO2F2 and DTD slows down the loss of capacity of the element during cycling.

Claims

Demands

1. Electrolyte comprising: - one or more solvents, - at least two sodium salts other than sodium difluorophosphate, one of the salts being sodium bis(fluorosulfonyl)imidide, - vinylene carbonate and / or ethylene monofluorocarbonate, - sodium difluorophosphate, and - ethylene sulfate.

2. Electrolyte according to claim 1, consisting of: - one or more solvents, - at least two sodium salts other than sodium difluorophosphate, one of the salts being sodium bis(fluorosulfonyl)imidide, - vinylene carbonate and / or ethylene monofluorocarbonate, - sodium difluorophosphate, and - ethylene sulfate.

3. Electrolyte according to claim 1 or 2, wherein sodium bis(fluorosulfonyl)imidide represents 10 to 50% by mole of the total number of moles of sodium salts.

4. Electrolyte according to claim 3, wherein sodium bis(fluorosulfonyl)imidide represents 20 to 40% by mole of the total number of moles of sodium salts.

5. Electrolyte according to any one of the preceding claims, wherein said one or more solvents is a mixture of propylene carbonate and ethyl methyl carbonate.

6. Electrolyte according to claim 5, consisting of: - propylene carbonate and ethyl methyl carbonate in respective volume proportions of 10-30% / 90-70%, - 0.1 to 0.5 mol.L 1 of sodium bis(fluorosulfonyl)imidide and 0.5 to 0.9 mol.L 1 of sodium hexafluorophosphate, - 1 to 3% by mass of vinylene carbonate and / or ethylene monofluorocarbonate expressed as a percentage of the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate, - 0.2 to 2% by mass of sodium difluorophosphate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate, - 0.5 to 3% by mass of ethylene sulfate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate.

7. Electrolyte according to claim 6, consisting of: - propylene carbonate and ethyl methyl carbonate in the respective volume proportions of 15-25% / 85-75%, - 0.2 to 0.4 mol.L 1 of sodium bis(fluorosulfonyl)imidide and 0.6 to 0.8 mol.L 1 of sodium hexafluorophosphate, - 1.5 to 2.5% by mass of vinylene carbonate and / or ethylene monofluorocarbonate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate, - 0.5 to 1.5% by mass of sodium difluorophosphate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate, - 0.5 to 1.5% by mass of ethylene sulfate expressed relative to the total mass of propylene carbonate, ethyl methyl carbonate, sodium bis(fluorosulfonyl)imidide and sodium hexafluorophosphate.

8. Electrolyte according to any one of the preceding claims, wherein the mass ratio of ethylene sulfate to vinylene carbonate is less than or equal to 1, preferably less than or equal to 0.

5.

9. Electrolyte according to any one of the preceding claims, wherein the mass ratio of ethylene sulfate to sodium difluorophosphate is greater than or equal to 1.

10. Electrochemical sodium-ion element comprising an electrolyte according to any one of claims 1 to 9.

11. Electrochemical sodium-ion element according to claim 10, comprising: - at least one positive electrode comprising a positive active material chosen from: a) an oxide of formula Naw(NixFeyMnzMt)O2 where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci ; b) un polyanion ; c) le bleu de Prusse et ses analogues ; et d) des mélanges de ceux-ci, - au moins une électrode négative.

12. Electrochemical sodium-ion element according to claim 11, wherein the positive active material is an oxide of formula Naw(NixFey MnzMt)O2 where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci.

13. Electrochemical sodium-ion element according to any one of claims 11 to 12, wherein the negative electrode comprises a negative active material which is a hard carbon or a soft carbon or a mixture of a soft carbon and a hard carbon.

14. Electrochemical sodium-ion element according to claim 13, wherein: - the positive active material is an oxide of formula Naw(NixFeyMnz Mt)O2 where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci, - la matière active négative est un carbone dur.

15. Electrochemical sodium-ion element according to claim 14, wherein x<0.5; y<0.5; z<0.5; x+y+z=l and t=0.

16. Electrochemical sodium-ion element according to claim 15, in which x=y=z=l / 3.

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