Sodium-ion electrochemical element

Abstract

A sodium-ion electrochemical cell comprising an electrolyte comprising vinylene carbonate, hexane-1,3,6-tricarbonitrile, one or more sodium salts and one or more solvents. Abstract Figure: Figure 1

Description

Title of the invention: Sodium-ion electrochemical element 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 from the state of the art. It generally comprises a positive electrode comprising an active material consisting of a material capable of inserting sodium into its structure, a negative electrode comprising an active material consisting for example of 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 electrode and the negative electrode. 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 oxidizes 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 material is reduced and inserts sodium into its structure while the negative active material is oxidized and sodium is disinserted from its structure.

[0003] A sodium-ion cell is an attractive alternative to a lithium-ion cell due to the greater abundance of sodium compared to lithium and the lower cost. However, currently, one of the disadvantages of a sodium-ion cell is a shorter cycle life than a lithium-ion cell.

[0004] Document CN107171020 describes a sodium-ion electrochemical element comprising a non-aqueous electrolyte. This electrolyte contains an additive that contributes to the formation of a stable protective film on the surface of the positive electrode of the cell. This film helps maintain the cell's performance during cycling. The molecule of this additive contains at least two nitrile groups. The additive can be chosen from succinonitrile (or butanedinitrile), glutaronitrile (or pentanedinitrile) and hexanetrinitrile. The negative active material is soft carbon. The positive active material is Na[Cu-Fe-Mn]O2 type O3.

[0005] Document CN 114824474 describes a sodium-ion electrochemical element comprising a non-aqueous electrolyte. The electrolyte contains a first additive which is a nitrile compound which may be chosen from succinonitrile, glutaronitrile and hexanetrinitrile and a second additive which is ethyl 4,4,4-trifluorobutenoate. The The electrolyte solvent can be a cyclic or linear carbonate. The negative active material can be hard carbon.

[0006] Document CN114400377 describes a sodium-ion electrochemical element comprising a non-aqueous electrolyte. The electrolyte contains a first additive which is a nitrile compound which may be chosen from succinonitrile, glutaronitrile and hexanetrinitrile and a second additive which is a thiosulfate. The solvent of the electrolyte may be a cyclic or linear carbonate. The negative active material may be hard carbon.

[0007] Document WO2023 / 109708 describes a sodium-ion electrochemical element comprising a non-aqueous electrolyte. The electrolyte comprises a first additive in the form of a sodium salt such as NaNO3, NaBOB (sodium bis(oxalato)borate), NaDFOB (sodium difluro(oxalato)borate) and a second additive in the form of a co-solvent, such as fluorobenzene and succinonitrile. Hexanenitrile can be used as a co-solvent, therefore in high proportion in the electrolyte. The total concentration of sodium salts ranges from 3 to 15 mol.L *. It is indicated that the positive active material is a graphite-based material.

[0008] New sodium-ion electrochemical elements with a long cycling life are being sought. Summary

[0009] For this purpose, the invention provides a sodium-ion electrochemical element comprising an electrolyte comprising vinylene carbonate (VC), hexane-1,3,6-tricarbonitrile (HTCN), one or more sodium salts and one or more solvents.

[0010] It has been discovered that the combination of vinylene carbonate with hexane-1,3,6-tricarbonitrile makes it possible to extend the life of the element in cycling.

[0011] According to one embodiment, the sodium-ion electrochemical element comprises a 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 thereof.

[0012] 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.

[0013] According to one embodiment, the sodium-ion electrochemical element comprises a negative electrode comprising a negative active material which is a hard carbon or a soft carbon or a mixture of a soft carbon and a hard carbon.

[0014] According to one embodiment, the sodium salt is NaPF6.

[0015] According to one embodiment, the sodium salts are NaPF6 mixed with sodium bis(fluorosulfonyl)imide Na(FSO2)2N (NaFSI).

[0016] According to one embodiment, the solvents are propylene carbonate and methyl ethyl carbonate.

[0017] According to one embodiment, the sodium salt is NaPF6, the solvents are propylene carbonate and methyl ethyl carbonate, 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, et la matière active négative est un carbone dur.

[0018] According to one embodiment, x<0.5; y<0.5; z<0.5; x+y+z=1 and t=0.

[0019] According to one embodiment, x=y=z=l / 3.

[0020] According to one embodiment, the mass proportion of vinylene carbonate ranges from 0.5 to 5% expressed in relation to the total mass of said one or more sodium salts and of said one or more solvents.

[0021] According to one embodiment, the mass proportion of vinylene carbonate ranges from 1 to 3% expressed relative to the total mass of said one or more sodium salts and said one or more solvents.

[0022] According to one embodiment, the mass proportion of hexane-1,3,6-tricarbonitrile ranges from 0.5 to 3% expressed relative to the total mass of said one or more sodium salts and said one or more solvents.

[0023] According to one embodiment, the mass proportion of hexane-1,3,6-tricarbonitrile ranges from 1 to 2% expressed relative to the total mass of said one or more sodium salts and said one or more solvents.

[0024] According to one embodiment, propylene carbonate represents from 10 to 30% of the total volume of solvents, ethyl methyl carbonate represents from 90 to 70% of the total volume of solvents. Brief description of the figure

[0025] [Fig. 1] represents the variation of the ratio between the capacity discharged by elements A, B and C and their initial capacity as a function of the cycle number. Detailed description of the embodiments

[0026] Electrolyte:

[0027] The electrolyte of the sodium-ion element comprises one or more solvents, one or several sodium salts, vinylene carbonate and hexane-1,3,6-tricarbonitrile. The electrolyte can be liquid or gelled.

[0028] The solvent may be chosen from saturated cyclic carbonates, unsaturated cyclic carbonates, linear carbonates, alkyl esters, cyclic esters (lactones), linear ethers, cyclic ethers and mixtures thereof.

[0029] Among the saturated cyclic carbonates, mention may be made of ethylene carbonate (EC), ethylene monofluorocarbonate (FEC), propylene carbonate (PC), butylene carbonate (BC) and mixtures thereof.

[0030] Among the linear carbonates, mention may be made of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC) and mixtures thereof.

[0031] The electrolyte may be free of ethylene monofluorocarbonate (FEC).

[0032] Preferably, the cyclic carbonate(s) represent 50% or less of the total volume of the solvents and the linear carbonate(s) represent 50% or more of the total volume of the solvents. The cyclic carbonate(s) may represent from 10 to 40% or from 20 to 30% of the total volume of the solvents and the linear carbonate(s) may represent from 60 to 90% or from 70 to 80% of the total volume of the solvents. According to one embodiment, the electrolyte comprises a mixture of propylene carbonate (PC) and ethyl methyl carbonate (EMC).

[0033] Among the alkyl esters, mention may be made of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate and mixtures thereof.

[0034] Among the ethers, mention may be made of tetrahydrofuran (THF), 1,3-dioxolane, dimethyl ether (DME) or diethyl ether (DEE) and mixtures thereof.

[0035] According to one embodiment, the electrolyte is free of ether or ester.

[0036] The mass proportion of vinylene carbonate is generally less than 10% 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 5% or from 1 to 3%. The mass proportion of hexane-1,3,6-tricarbonitrile is generally less than 10% 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 1 to 2%.

[0037] According to one embodiment, the electrolyte does not contain any additive other than vinylene carbonate and hexane-1,3,6-tricarbonitrile.

[0038] The sodium salt may be hexafluorinated or tetrafluorinated. It may be selected from the group consisting of NaPF6, NaBF4, NaC104, NaAsF6, NaSbF6. Preferably, it is NaPF6.

[0039] The sodium salt may also be chosen from sodium bis(fluorosulfonyl)imide Na(FSO2)2N (NaFSI), sodium bis(trifluoromethanesulfonyl)imide NaN(CF3SO2 )2 (NaTFSI), sodium tris(fluoromethanesulfonyl)methylide NaC(CF3SO2)3 (NaTFSM), sodium bis(pentafluoroethylsulfonyl)imide 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.

[0040] The total concentration of sodium salts generally ranges from 0.7 to 2 mol.L 1 or from 1 to 1.5 mol.L 1.

[0041] Preferably, the electrolyte contains a mixture of propylene carbonate (PC) and ethyl methyl carbonate (EMC) as solvents, NaPF6 as the sodium salt and vinylene carbonate and hexane-1,3,6-tricarbonitrile as the only additives. Positive electrode:

[0042] The positive electrode comprises a current collector, at least one of the faces of which is coated with a layer of a composition of positive active materials, which comprises one or more active materials and optionally one or more binders and one or more electronically conductive materials.

[0043] 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.

[0044] 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 Prussian blue compound or its analogues having the formula 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) mixtures thereof.

[0045] Among the compounds of type a), the preferred ones have 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. Cette matière active est avantageuse en ce qu’elle est dépourvue de cobalt lequel présente une certaine toxicité.

[0046] Among the compounds of type b), those preferred are phosphates, such as Na3V2(PO4)3 and Na3V2(PO4)2F3.

[0047] Among the compounds of type c), Na2Mn[Fe(CN)6] is preferred.

[0048] An ink is prepared by dispersing one or more positive active materials in a solvent or in a mixture of several organic or aqueous solvents. Even Typically, 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 one side of the current collector. The ink-coated current collector is dried and then rolled to adjust its thickness. After evaporation of the solvent(s), a layer of a composition of one or more active ingredients is obtained, the proportions of the various constituents of which are typically: - from 80 to 98% or from 90 to 95% by mass of one or more positive active ingredients, - from 1 to 10% or from 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.

[0049] The binder may be chosen 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 formal), polyester, block polyetheramides, polymers of acrylic acid, methacrylic acid, acrylamide, itaconic acid, sulfonic acid, elastomers, and cellulose compounds such as carboxymethylcellulose (CMC). The elastomers that can be used as binders may be chosen from styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), hydrogenated butadiene-acrylonitrile (HNBR). Said at least one binder may be dispersible in an aqueous medium, such as polytetrafluoroethylene (PTFE), carboxymethylcellulose (CMC), styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), hydrogenated butadiene-acrylonitrile (HNBR) and polyvinyl alcohol (PVA).

[0050] 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:

[0051] The negative electrode comprises a current collector, at least one of the faces of which is coated with a layer of a composition of negative active materials, which comprises one or more negative active materials and optionally one or more binders and one or more electronically conductive materials.

[0052] The active material may be hard carbon or soft carbon or a mixture thereof. Preferably, it is hard carbon because it has a higher capacity and cycling stability than soft carbon. More preferably, the negative electrode does not contain any active material other than hard carbon.

[0053] Hard carbon is understood to mean carbon that cannot be transformed into graphite (non-graphitizable), even if it is 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 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.

[0054] Soft carbon is understood to mean carbon capable of being transformed into graphite (graphitizable) by pyrolysis of a precursor at a temperature of approximately 3000°C. This precursor may be polyvinyl chloride (PVC) or petroleum coke.

[0055] The current collector is preferably a two-dimensional conductive support such as a solid or perforated strip, preferably made of aluminum or an aluminum-based 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.

[0056] To obtain the negative active material composition, an ink is prepared by dispersing in a solvent or in a mixture of solvents, preferably aqueous, one or more negative active materials, and optionally one or more binders and one or more electronically conductive compounds. 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.

[0057] The ink-coated current collector is dried and then laminated to adjust its thickness. After evaporation of the solvent(s), a layer of a composition of one or more active materials is obtained, the proportions of the various constituents of which are typically: - from 85 to 98% or from 90 to 98% by mass of one or more negative active ingredients, - from 1 to 10% or from 1 to 5% by mass of one or more binders, - from 0 to 5% by mass or from 1 to 5% of one or more electronically conductive materials. Separator:

[0058] A separator is generally inserted between a negative electrode and a positive electrode to prevent possible 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 material of the separator can be chosen 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 may be selected from polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Advantageously, the polyester or 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 may be SiO2 or Al2O3. The separator may 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 the combination of three layers which are polypropylene PP / polyethylene PE / polypropylene PP.

[0059] A preferred sodium-ion electrochemical element comprises: - a negative electrode comprising an active material which is hard carbon, - a positive electrode comprising an active material of 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 comprising a mixture of propylene carbonate (PC) and ethyl methyl carbonate (EMC) as solvents, NaPF6 as sodium salt, vinylene carbonate and hexane-1,3,6-tricarbonitrile as the only additives.

[0060] The electrochemical element is manufactured in a conventional manner. It can be prismatic, cylindrical, pocket or button format. Examples

[0061] Three sodium-ion electrochemical cells A, B and C were manufactured. In these three cells, the negative electrode contains hard carbon as active material and the positive electrode contains Na(Nii / 3Fei / 3Mni / 3)O2 as active material. Cells A, B and C differ in the composition of their electrolyte. This is shown in Table 1 below. The percentages of the solvents are expressed by volume. The percentages of the additives are expressed relative to the total mass of the solvents and NaPF6. The cells were cycled at 25°C at the C / 3 and D / 3 regimes. [Tables 1] Element Electrolyte composition Cycle number corresponding to a percentage of the initial capacity of 90% A* EC / PC / EMC (20 / 20 / 60) NaPF61 moLL1 + 2% FEC 82 B* PC / EMC (20 / 80) NaPF61 moLL1 + 2% VC 195 C PC / EMC (20 / 80) NaPF61 moLL1 + 2% VC+ 1% HTCN 240 * Element outside the invention

[0062] The results of the cycling test of these elements have been represented in [Fig.l]. It can be seen that the element which retains its initial capacity for the longest time during cycling is element C. The electrolyte of element C contains the combination of vinylene carbonate (VC) and hexane-1,3,6-tricarbonitrile (HTCN). The comparison between element C and element B shows the beneficial effect of hexane-1,3,6-tricarbonitrile on the life of the element during cycling.

Claims

Claims

1. A sodium-ion electrochemical cell comprising an electrolyte comprising vinylene carbonate, hexane-1,3,6-tricarbonitrile, one or more sodium salts and one or more solvents.

2. Sodium-ion electrochemical element according to claim 1, comprising a positive electrode comprising a positive active material chosen from: a) an oxide of formula Naw(NixFeyMnzMt)O2where 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.

3. A sodium-ion electrochemical element according to claim 2, 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.

4. A sodium-ion electrochemical element according to one of the preceding claims, comprising a negative electrode comprising a negative active material which is a hard carbon or a soft carbon or a mixture of a soft carbon and a hard carbon.

5. A sodium-ion electrochemical element according to one of the preceding claims, wherein the sodium salt is NaPF6.

6. Sodium-ion electrochemical element according to one of the preceding claims, in which the sodium salts are NaPF6 in admixture with sodium bis(fluorosulfonyl)imide Na(FSO2)2N (NaFSI).

7. A sodium-ion electrochemical element according to one of the preceding claims, wherein the solvents are propylene carbonate and methyl ethyl carbonate.

8. Sodium-ion electrochemical element according to claims 3, 4, 5 and 7, in which: - the sodium salt is NaPF6, - the solvents are propylene carbonate and methyl ethyl carbonate, - the positive active material is an oxide of formula Naw(NixFeyMnzMt)O 2 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.

9. A sodium-ion electrochemical element according to claim 8, wherein x<0.5; y<0.5; z<0.5; x+y+z=1 and t=0.

10. The sodium-ion electrochemical element of claim 9, wherein x=y=z=l / 3.

11. Sodium-ion electrochemical element according to one of the preceding claims, in which the mass proportion of vinylene carbonate ranges from 0.5 to 5% expressed relative to the total mass of said one or more sodium salts and said one or more solvents.

12. A sodium-ion electrochemical element according to claim 11, wherein the mass proportion of vinylene carbonate ranges from 1 to 3% expressed relative to the total mass of said one or more sodium salts and said one or more solvents.

13. Sodium-ion electrochemical element according to one of the preceding claims, in which the mass proportion of hexane-1,3,6-tricarbonitrile ranges from 0.5 to 3% expressed relative to the total mass of said one or more sodium salts and said one or more solvents.

14. A sodium-ion electrochemical element according to claim 13, wherein the mass proportion of hexane-1,3,6-tricarbonitrile ranges from 1 to 2% expressed relative to the total mass of said one or more sodium salts and said one or more solvents.

15. Sodium-ion electrochemical element according to one of claims 8 to 14, in which: - propylene carbonate represents from 10 to 30% of the total volume of solvents, - ethyl methyl carbonate represents from 90 to 70% of the total volume of solvents.

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