WO2022122905A1 - Lithium-sulphur electrochemical element with gelled electrolyte - Google Patents

Lithium-sulphur electrochemical element with gelled electrolyte Download PDF

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WO2022122905A1
WO2022122905A1 PCT/EP2021/084974 EP2021084974W WO2022122905A1 WO 2022122905 A1 WO2022122905 A1 WO 2022122905A1 EP 2021084974 W EP2021084974 W EP 2021084974W WO 2022122905 A1 WO2022122905 A1 WO 2022122905A1
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lithium
carbon
carbonate
mass
sulfur
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PCT/EP2021/084974
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French (fr)
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Florent FISCHER
Julien Demeaux
Olivier Jan
Marlène OSWALD
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Saft
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/133—Electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0565—Polymeric materials, e.g. gel-type or solid-type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0569—Liquid materials characterised by the solvents
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/134—Electrodes based on metals, Si or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1395—Processes of manufacture of electrodes based on metals, Si or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1397—Processes of manufacture of electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/381—Alkaline or alkaline earth metals elements
    • H01M4/382—Lithium
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the technical field of the invention is that of lithium-sulphur electrochemical elements with a gelled electrolyte as well as that of methods for assembling such elements.
  • Lithium/sulphur (Li/S) electrochemical elements comprising a liquid electrolyte are known from the state of the art. They typically comprise at least one positive electrode (cathode) based on elemental sulfur, an organic liquid electrolyte and at least one negative electrode (anode) made of lithium metal or lithium metal alloy.
  • the cathode is usually composite, i.e. it is prepared from elemental sulfur and non-electrochemically active additives.
  • Non-electrochemically active additives mention may be made of an electronic conductor, such as carbon, making it possible to improve the electronic conductivity of the cathode because sulfur is an electronic insulator. Mention may also be made of one or more polymeric binders making it possible to ensure cohesion between the different materials of the cathode. Due to the low atomic mass of lithium and the moderate mass of sulfur, Li/S electrochemical elements are relatively light. They are a promising alternative to lithium-ion cells due to their higher energy density and the low cost of sulfur. They are advantageously used in fields in which a high specific energy is required. This is the case for the aviation and space domains (satellites).
  • the electrochemical element is initially in the charged state.
  • the elemental sulfur of the cathode is reduced to lithium sulphide Li2S and the metallic lithium or the metallic lithium alloy is oxidized at the anode.
  • the following reactions take place at the electrodes:
  • a lithium/sulfur electrochemical cell typically comprises an electrolyte whose solvent is based on ethers.
  • Ethers such as 1,3-dioxolane or tetrahydrofuran have been used for several decades. centuries and allow significant solubilization of lithium polysulphides.
  • DME 1,2-dimethoxyethane
  • the cyclic molecules of sulfur (in the form of octasulfur Ss) are reduced and form linear chains of lithium polysulfides, of general formula Li2S n , n generally ranging from 2 to 8.
  • the first compounds formed during the discharge of the element are the long-chain lithium polysulphides, such as Li2Ss or Li2Se.
  • Long-chain lithium polysulfides are likely to migrate through the electrolyte and reach the lithium anode where they will be reduced to short-chain polysulfides during charging following cell discharge.
  • the short chain polysulphides return to the cathode where they are further reoxidized into long chain polysulphides, and so on.
  • This shuttle mechanism (“shuttle”) of the polysulphides between the anode and the cathode is the cause of a low coulombic efficiency of the element, that is to say a low ratio between the capacity discharged by the element and the capacitance charged in the element during the charge preceding the discharge.
  • this shuttle mechanism causes a strong self-discharge.
  • the use of a lithium anode with a liquid electrolyte leads to an electrolyte deficit during cycling and to a limited cycle life.
  • An electrochemical element of the lithium-sulfur type with a gelled electrolyte is therefore sought, presenting electrical performances almost as good or even better than those obtained for an electrochemical element comprising a liquid electrolyte, especially when the element is used in discharge under a strong current, for example at a discharge rate greater than C/5, C being the nominal capacity of the element.
  • a lithium-sulphur electrochemical element comprising: a) a cathode comprising: i) carbon having a porous structure comprising pores with an average diameter less than or equal to 6 nm; ii) sulfur with a degree of oxidation ranging from 0 to -2 in the pores of the carbon; b) a gelled electrolyte comprising a matrix which is a polymer of poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) in which is incorporated a liquid mixture comprising at least one lithium salt and a solvent, the solvent comprising at least one fluorinated carbonate which can be cyclic or linear; c) an anode comprising lithium metal or a lithium metal alloy.
  • the element according to the invention also has an improved cycle life compared to that obtained with an element whose electrolyte is liquid.
  • said at least one fluorinated carbonate is selected from the group consisting of ethylene monofluorocarbonate (FEC), ethylene difluorocarbonate, ethylene trifluoromethylcarbonate, propylene monofluorocarbonate, difluorocarbonate propylene, propylene trifluorocarbonate and a mixture thereof.
  • FEC ethylene monofluorocarbonate
  • ethylene difluorocarbonate ethylene difluorocarbonate
  • ethylene trifluoromethylcarbonate propylene monofluorocarbonate
  • propylene monofluorocarbonate difluorocarbonate propylene
  • propylene trifluorocarbonate and a mixture thereof.
  • said at least one fluorinated carbonate is ethylene monofluorocarbonate.
  • said at least one fluorinated carbonate represents from 5 to 75% of the volume of the solvent, preferably from 5 to 50%, more preferably from 10 to 30%.
  • said at least one fluorinated carbonate is cyclic and is associated with at least one non-fluorinated linear carbonate.
  • said at least one non-fluorinated linear carbonate is dimethyl carbonate (DMC).
  • the solvent consists of 10 to 30% by volume of said at least one fluorinated cyclic carbonate and 90 to 70% by volume of said at least one non-fluorinated linear carbonate.
  • the lithium salt is chosen from lithium hexafluorophosphate LiPFe, lithium bis(fluorosulfonyl)imide Li(FS ⁇ 2)2N (LiFSI) and a mixture thereof.
  • the poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix represents from 3 to 25% by mass of the mass of the liquid mixture.
  • the porous carbon is in the form of a powder whose particle size is characterized by a median diameter by volume Dv50 ranging from 1 to 10 ⁇ m, preferably ranging from 1 to 5 ⁇ m.
  • the sulfur is elemental solid sulfur; the mass of elemental solid sulfur is 30 to 90% of the sum of the masses of carbon and elemental solid sulfur, and the mass of carbon is 70 to 10% of the sum of the masses of carbon and elemental solid sulfur.
  • the mass of elemental solid sulfur represents from 40 to 60% of the sum of the masses of carbon and elemental solid sulphur, and the mass of carbon is 60 to 40% of the sum of the masses of carbon and elemental solid sulphur.
  • the pores of the cathode have an average diameter less than or equal to 1 nm;
  • the poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix represents from 5 to 15% by mass of the mass of the liquid mixture;
  • the solvent consists of 10 to 30% by volume of ethylene monofluorocarbonate (FEC) and 90 to 70% by volume of dimethyl carbonate (DMC);
  • the lithium salt is chosen from lithium hexafluorophosphate LiPFe, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI) and a mixture thereof.
  • FIG. 1 represents a schematic sectional view of a stack of a cathode, a separator and an anode in an electrochemical element according to the invention.
  • FIG. 2A represents the discharge curves of element A according to the prior art.
  • FIG. 2B represents the discharge curves of element B according to the invention.
  • FIG. 2C represents the discharge curves of element C according to the prior art.
  • FIG. 2D represents the discharge curves of element D according to the invention.
  • FIG. 3 represents the variation of the reversible capacitance of elements A and B during cycling at ambient temperature at C/10.
  • FIG. 4 represents the variation of the reversible capacitance of elements C and D during cycling at room temperature at C/10.
  • the cathode is a composite prepared from electrochemically active elemental solid sulfur and non-electrochemically active compounds.
  • Elemental solid sulfur exists in different molecular forms.
  • the preferred form is alpha sulfur Sa, of formula Sx corresponding to cyclooctasulfur, which is the thermodynamically most stable form.
  • Carbon is one of the non-electroactive constituents of the cathode. It has a porous structure mainly consisting of pores having an average porous average diameter of less than or equal to 6 nm. Preferably, at least 50% or at least 75% or at least 90% of the pore volume consists of pores having an average diameter less than or equal to 6 nm.
  • the carbon can have a porous structure mainly consisting of pores having an average diameter less than or equal to 2 nm.
  • at least 50% or at least 75% or at least 90% of the pore volume consists of pores having an average diameter less than or equal to 2 nm.
  • the pores of the porous structure can have an average diameter ranging from 0.5 nm to 2 nm.
  • at least 50% or at least 75% or at least 90% of the pore volume is constituted by pores having an average diameter ranging from 0.5 nm to 2 nm.
  • micropores The pores having an average diameter less than or equal to 2 nm are called micropores. Those with an average diameter greater than 2 nm and less than 50 nm are called mesopores.
  • the “Barrett-Joyner-Halenda” B JH method makes it possible to estimate the mean pore diameter associated with the mesoporous volume, which will be denoted dmesoporous, from the nitrogen adsorption and desorption isotherms.
  • the Horvath-Kawazoe method makes it possible to estimate the average pore diameter associated with the microporous volume, which will be denoted dmicroporous, from the nitrogen adsorption and desorption isotherms.
  • the mesoporous volume denoted Vmesoporous is measured according to the BJH method from the nitrogen adsorption and desorption isotherms.
  • the microporous volume denoted Vmicroporous is measured according to the "t-plot" method from the nitrogen adsorption and desorption isotherms.
  • the evaluation of the average pore diameter of the carbon is calculated by the average of the microporous and mesoporous diameters weighted by the associated pore volumes according to the following formula: daverage porous ((Vmesoporous * dmesoporous ) + (Vmicroporous * dmicroporous))/ (Vmesoporous + Vmicroporous) Mesopores can contribute 40% or less, or 30% or less, or 20% or less, or 10% or less of the total pore volume.
  • the contribution of the mesopores to the total pore volume be minimized in order to avoid the formation of polysulfides in the electrolyte.
  • polysulfides are not stable in the presence of carbonates and contribute to the unwanted "shuttle" mechanism.
  • the microporous carbon is in the form of particles whose particle size distribution is characterized by a volume median diameter Dvso which may be greater than or equal to 1 ⁇ m and less than or equal to 10 ⁇ m, or which may range from 1 to 5 p.m.
  • volume median diameter Dvso of X pm means that 50% of the volume of the carbon particles consists of particles having an equivalent diameter less than X pm and 50% of the volume of the carbon particles consists of particles having a diameter equivalent greater than X pm.
  • Equivalent diameter” of a particle designates the diameter of a sphere having the same volume as this particle.
  • the particle size distribution of the carbon particles can be determined by laser diffraction.
  • a suitable device for determining the particle size distribution is the Mastersizer 2000 marketed by the company Malvern.
  • the microporous carbon particles are preferably obtained by mechanical grinding of carbon.
  • a mechanical grinding process excludes obtaining carbon nanotubes. Mechanical grinding makes it possible to obtain a powder of particles having a Dvso ranging from 1 ⁇ m to 10 ⁇ m. Choosing this specific range of particle sizes results in an element with a high mass capacity.
  • the pores of the porous structure accommodate the particles of elemental sulfur.
  • a method possibility for the incorporation of particles of elemental sulfur into the pores of the porous carbon structure is as follows.
  • the porous carbon is mixed with solid elemental sulfur.
  • the mass of solid elemental sulfur is 30 to 80% or 40 to 65% or typically about 50% of the sum of the masses of solid elemental sulfur and carbon.
  • the mass of carbon is typically 70 to 20% or 60 to 35% or typically about 50% of the sum of the masses of solid elemental sulfur and carbon.
  • the mixture is heated to a temperature close to 155° C. for about 5 hours, under vacuum, to allow the sulfur molecules to penetrate into the open pores of the carbon. Around 155°C, sulfur in the liquid state has its lowest viscosity.
  • the mixture is then heated under inert gas at a temperature of about 300°C for about 30 minutes, which has the effect of sublimating the sulfur and eliminating the excess.
  • the product obtained is then generally mixed with at least one binder and at least one good electronic conductor compound. Said at least one binder and said at least one electronically conductive compound are non-electroactive.
  • the binder can be chosen from carboxymethylcellulose (CMC), a butadiene-styrene copolymer (SBR), polytetrafluoroethylene (PTFE), polyamideimide (PAI), polyimide (PI), styrene-butadiene rubber (SBR), poly(acrylic acid) (PAA), polyvinyl alcohol, polyvinylidene fluoride (PVDF) and a mixture thereof.
  • CMC carboxymethylcellulose
  • SBR butadiene-styrene copolymer
  • PTFE polytetrafluoroethylene
  • PAI polyamideimide
  • PI polyimide
  • SBR styrene-butadiene rubber
  • PAA poly(acrylic acid)
  • PVDF polyvinylidene fluoride
  • the electronically conductive compound is generally carbon black.
  • a typical composition of the ink deposited on the current collector can be the following:
  • the anode may consist of lithium or of an alloy based on lithium and a chemical element chosen from Si, Sn, In and C.
  • the anode may also consist of a layer of lithium metal on which is deposited a layer of indium, the indium layer being in contact with the layer of solid electrolyte.
  • the anode may consist only of a lithium ribbon.
  • the gelled electrolyte comprises a matrix which is a poly(vinylidene fluoride-co-hexafluoropropylene) polymer in which is incorporated a liquid mixture comprising at least one lithium salt and a solvent, the solvent comprising at least a fluorinated carbonate which can be cyclic or linear.
  • Poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) has the formula: [Chem 1] where x denotes the number of vinylidene fluoride units and y denotes the number of hexafluoropropylene units.
  • the molecular mass by weight of P(VdF-HFP) can vary from 300 Da to 5 MDa. It can be in the range of 300 to 800 Da or in the range of 200 to 400 kDa.
  • the P(VdF-HFP) matrix can represent from 3 to 95% or from 3 to 50%, or from 3 to 20% or from 3 to 10% by mass relative to the mass of the liquid mixture.
  • the liquid mixture comprises said at least one lithium salt and the solvent, the solvent comprising said at least one cyclic or linear fluorinated carbonate.
  • a preferred percentage range is the range from 3 to 25%, more preferably from 10 to 20%. This preferred range makes it possible both to obtain a good resistance of the electrolyte to oxidation at high cathode potentials as well as a good reversible capacity of the element.
  • the resistance of the electrolyte to oxidation may decrease if the percentage of P(VdF-HFP) added to the liquid mixture is less than or equal to 3%.
  • the reversible capacity of the cell containing the electrolyte may decrease if the percentage of P(VdF-HFP) added to the liquid mixture is greater than 25%.
  • a percentage of polymer greater than 25% a poorer impregnation of the electrodes by the polymer can be observed.
  • the polymer may be insufficiently in contact with the porosity of the electrodes.
  • P(VdF-HFP) By comparison with polyvinylidene fluoride PVdF, P(VdF-HFP) exhibits greater solubility with respect to the liquid mixture comprising said at least one lithium salt, the solvent and the ionic liquid. It would seem that the polymer of P(VdF-HFP) is reduced during the operation of the element and by being reduced generates LiF which plays a protective role of the anode in lithium or in lithium alloy and makes it possible to increase the life of the element in cycling, unlike other polymers containing oxygen atoms.
  • the matrix may also comprise a minor amount of one or more polymers in combination with P(VdF-HFP).
  • This or these other polymer(s) may be chosen from a poly(ethylene oxide), poly(vinylidene fluoride) PVDF, a polyacrylate, a poly(imidine), a polymer of butadiene acrylonitrile NBR and a polymer of hydrogenated acrylonitrile butadiene HNBR .
  • P(VdF-HFP) preferably represents at least 50%, preferably at least 90% by mass of the mixture of polymers.
  • the gelled electrolyte composition is obtained by mixing a poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) polymer with a liquid mixture comprising at least one lithium salt, a solvent comprising at least one cyclic or linear fluorinated carbonate.
  • a poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) polymer with a liquid mixture comprising at least one lithium salt, a solvent comprising at least one cyclic or linear fluorinated carbonate.
  • SEI passivation layer
  • the saturated cyclic carbonate can be monofluorinated or difluorinated or trifluorinated. It can be chosen from the group consisting of ethylene monofluorocarbonate (FEC), also designated by the term 4-fluoro-1,3-dioxolan-2-one, ethylene difluorocarbonate, ethylene trifluoromethylcarbonate , also designated by the term 4-trifluoro-1,3-dioxolan-2-one, propylene monofluorocarbonate, propylene difluorocarbonate, propylene trifluorocarbonate and a mixture thereof.
  • FEC ethylene monofluorocarbonate
  • the monofluorinated saturated cyclic carbonate is ethylene monofluorocarbonate.
  • Said at least one fluorinated carbonate can represent from 5 to 75% of the volume of the solvent, preferably from 5 to 50%, more preferably from 10 to 30%.
  • Said at least one fluorinated carbonate can be cyclic and be associated with at least one non-fluorinated linear carbonate.
  • the solvent may consist of 10 to 30% by volume of said at least one fluorinated cyclic carbonate and 90 to 70% by volume of said at least one non-fluorinated linear carbonate.
  • the non-fluorinated linear carbonate can be chosen from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC) and methyl propyl carbonate (PMC).
  • DMC dimethyl carbonate
  • DEC diethyl carbonate
  • EMC methyl ethyl carbonate
  • PMC methyl propyl carbonate
  • the solvent may be free of linear carbonates other than DMC and EMC.
  • the linear carbonate can represent from 50 to 90% or from 60 to 80%, or from 70 to 80% by volume of the total volume of the cyclic and linear carbonates.
  • the solvent may consist of a mixture of FEC and DMC, without any other carbonate.
  • FEC preferably represents from 10 to 30% by volume of the total volume (FEC+DMC) and DMC preferably represents from 90 to 70% by volume of the total volume (FEC+DMC).
  • the solvent does not contain linear ester(s) or cyclic ester(s), also called lactones.
  • the solvent does not contain ether(s).
  • the nature of the lithium salt is not particularly limited. Mention may be made of lithium hexafluorophosphate LiPFe, lithium hexafluoroarsenate LiAsFe, lithium hexafluoroantimonate LiSbFe and lithium tetrafluorocarbonate LiBF4, lithium perchlorate LiC104, lithium trifluoromethanesulfonate LiCFsSOs, bis( lithium fluorosulfonyl)imide Li(FSO2)2N (LiFSI), lithium trifluoromethanesulfonimide LiN(CFsSO2)2 (LiTFSI), lithium trifluoromethanesulfonemethide LiC(CF3SO2)3 (LiTFSM), lithium bis-perfluoroethylsulfonimide LiN(C2F
  • the concentration of said at least one lithium salt can range from 0.75 to 1.5 mol.L' 1 . Preferably, it ranges from 1 to 1.5 mol.L' 1 . More preferably, it is approximately equal to 1 mol.L'. 1
  • the poly(vinylidene fluoride-co-hexafluoropropylene) polymer is incorporated into the liquid mixture.
  • the mixture is stirred for several minutes. It can be heated to a temperature not exceeding 50°C in order to accelerate the swelling of the polymer.
  • a particularly preferred example of a gelled electrolyte composition comprises the poly(vinylidene fluoride-co-hexafluoropropylene) matrix in which is incorporated a liquid mixture comprising LiPFe and/or LiFSI and a solvent consisting of FEC and DMC, the polymer matrix of poly(vinylidene fluoride-co-hexafluoropropylene) representing from 3 to 25% by mass relative to the mass of the liquid mixture.
  • a separator is generally interposed between an anode and a cathode to avoid possible short circuits. It prevents electrical contact between an anode and a cathode 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, polyaramide, polyamideimide and cellulose.
  • the polyester can be chosen from polyethylene terephthalate (PET) and polybutylene terephthalate (PBT).
  • 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 sulphide.
  • This material can be SiCL 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 the combination of three layers which are polypropylene PP/polyethylene PE/polypropylene PP.
  • the gelled electrolyte composition is deposited in contact with the ink previously deposited on the cathode.
  • a separator is then deposited on the ink impregnated with the gelled electrolyte composition.
  • the face of the separator intended to be in contact with the anode is coated with gelled electrolyte composition.
  • An anode is then applied in contact with the gelled electrolyte composition.
  • the gelled electrolyte composition is deposited on the one hand on the ink previously deposited on the cathode and on the other hand on the anode.
  • a separator is inserted between the ink and the anode impregnated with gelled electrolyte composition.
  • the two opposite faces of the separator are soaked with gelled electrolyte composition and the separator is inserted between a cathode and an anode.
  • an assembly is obtained in which the gelled electrolyte composition and the separator are sandwiched between an anode and a cathode.
  • a separator is not necessary if the mass of P(VdF-HFP) represents at least 50% of the mass of the gelled electrolyte composition.
  • the gelled electrolyte composition in this case plays both the role of separator and electrolyte.
  • the element Before starting the “formation” of the electrodes, that is to say performing a first charge/discharge cycle of the element, it may be useful to let the element rest at a temperature above ambient temperature, for example 50 or 60° C., for several hours, for example from 5 to 15 hours, so as to promote the impregnation of the active material of the electrodes by the gelled electrolyte composition.
  • FIG 1 is an exploded schematic representation of a stack of a cathode (2), a separator (3) and an anode (4) in an electrochemical element (1) according to the invention.
  • the cathode (2) has a porous structure (5).
  • the pores of the cathode house sulfur.
  • the gelled electrolyte (6) soaks the pores (5) of the cathode (2) and the pores of the separator (3). It also impregnates the face of the separator in contact with the anode and therefore the anode.
  • the formation of the element can be carried out at a temperature less than or equal to 50° C., for example ranging from 20 to 50° C. An increase in the formation temperature allows better imbibition of the pores of the electrodes by the gelled electrolyte composition.
  • the invention in addition to improving the performance of the element in discharge under high current and its cycle life, provides the following advantages:
  • the association of the carbon cathode having a porous structure comprising pores with an average diameter less than or equal to 6 nm with the electrolyte comprising at least one carbonate and at least one lithium bis(fluorosulfonyl)imide salt (LiFSI ) makes it possible to reduce the quantity of hydrogen sulphide likely to be emitted in the event of accidental opening of the container of the element;
  • the use of a porous carbon whose pores have an average diameter less than or equal to 6 nm makes it possible to reduce the self-discharge of the element;
  • the gelled form of the electrolyte makes it possible to avoid the dissolution of the polysulphides in the liquid electrolyte, this dissolution being responsible for the phenomenon of "shuttles" and the self-discharge of the element in the case of a liquid electrolyte with ether(s) base;
  • the solvent is retained in the P(VDF-HFP) matrix. Therefore, if the container is opened, the amount of solvent escaping from the element is small. Moreover, the quantity of “free” solvent being very limited in comparison with that of an element comprising a liquid electrolyte, the risk of ignition of the element is reduced. This makes it possible to improve the safety of a user placed in the vicinity of the element.
  • Li/S electrochemical cells (2032 format button cells) referenced A to D with a capacity of 5 mAh each were manufactured. They differ in their electrolyte composition. The different compositions of electrolytes used are indicated in Table 1 below.
  • the composite of sulfur and carbon was prepared by mixing carbon with solid elemental sulfur in a ball mill.
  • the carbon used is activated carbon with an average pore diameter of 0.77 nm.
  • the mixture was heated at a temperature of 155°C for about 5 hours, under vacuum to allow the sulfur molecules to penetrate into the pores of the carbon. Heating of the mixture was continued at a temperature of 300°C for 30 minutes under a flow of inert gas in order to remove excess sulfur.
  • the sulfur content in the carbon-sulfur composite prepared is measured by Thermogravimetric Analysis (TGA) using a TA Instrument Q500 device.
  • the amount of sulfur is 1.9 mg per cm 2 of cathode.
  • the anode is a lithium ribbon.
  • a liquid mixture was first prepared by mixing FEC with DMC in a volume ratio of 1 to 4 and LiPFe or LiFSI was dissolved therein at a concentration of 1 mol. . Then, this mixture was heated and the polymer P(VDF-HFP) was incorporated therein in an amount representing 10% of the mass of the liquid mixture. The same volume of electrolyte was introduced into the various elements (either in liquid form for elements A and C, or in gelled form for elements B and D).
  • Elements A to D underwent discharges at rates of C/10, C/5, C/2, C and 2C at ambient temperature down to the cut-off voltage of 1 V.
  • the discharge curves of these elements have been shown in Figures 2A through 2D. If we compare the discharged capacity of element B with that of element A, we see that they are close for a discharge current up to C. They are approximately 600 mAh/g of sulphur. For a discharge current of 2C, the capacity of cell B begins to be significantly lower than that of cell A (510 mAh/g sulfur for cell B compared to 550 mAh/g sulfur for cell A). item A). If we compare the discharged capacitance of element D with that of element C, we see that they are close for a discharge current up to C.
  • the capacitance of the element D is significantly higher than that of element C (590 mAh/g sulfur for element D compared to 540 mAh/g sulfur for element C).
  • Element D which includes a gelled electrolyte, shows performances at least equal or even superior to those of element C. This result is surprising given that a gelled electrolyte generally penalizes the power performance of the element in comparison with a element comprising a liquid electrolyte.
  • the results of Figures 2A to 2D also show the superiority of LiFSI compared to LiPFe in terms of high current discharge.
  • FIG. 3 shows the variation in the percentage of capacity retention of elements A and B as a function of the number of cycles and in FIG. 4 the variation in the percentage of capacity retention of elements C and D.
  • capacity of element B is less than that of element A and that the drop in capacity of element D is less than that of element C.

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Abstract

Lithium-sulphur electrochemical element comprising: a) a cathode comprising: i) carbon with a porous structure which comprises pores with an average diameter less than or equal to 6 nm; ii) sulphur having a degree of oxidation from 0 to -2 in the carbon pores; b) a gelled electrolyte comprising a matrix which is a polymer of poly(vinylidene fluoride-co-hexafluoropropylene) in which is embedded a liquid mixture comprising at least one lithium salt and a solvent, the solvent comprising at least one fluorinated cyclical or linear carbonate; c) an anode comprising lithium metal or a lithium metal alloy.

Description

Description Description
Titre : ELEMENT ELECTROCHIMIQUE LITHIUM- SOUFRE A ELECTROLYTE GELIFIE Title: ELECTROCHEMICAL ELEMENT LITHIUM-SULFUR WITH GELIFIED ELECTROLYTE
Domaine technique de l’invention Technical field of the invention
[0001] Le domaine technique de l’invention est celui des éléments électrochimiques lithium- soufre à électrolyte gélifié ainsi que celui des procédés d’assemblage de tels éléments.The technical field of the invention is that of lithium-sulphur electrochemical elements with a gelled electrolyte as well as that of methods for assembling such elements.
Contexte de l'invention Background of the invention
[0002] Le terme « élément » utilisé dans ce qui suit désigne un élément électrochimique. Les termes « élément » et « élément électrochimique » sont utilisés de manière interchangeable dans ce qui suit. Les éléments électrochimiques lithium/ soufre (Li/S) comprenant un électrolyte liquide sont connus de l’état de la technique. Ils comprennent typiquement au moins une électrode positive (cathode) à base de soufre élémentaire, un électrolyte liquide organique et au moins une électrode négative (anode) en lithium métal ou en alliage métallique de lithium. La cathode est généralement composite, c’est-à-dire qu’elle est préparée à partir de soufre élémentaire et d’additifs non électrochimiquement actifs. Comme additifs non électrochimiquement actifs, on peut citer un conducteur électronique, tel que le carbone, permettant d’améliorer la conductivité électronique de la cathode car le soufre est un isolant électronique. On peut aussi citer un ou plusieurs liants polymériques permettant d’assurer la cohésion entre les différents matériaux de la cathode. En raison de la faible masse atomique du lithium et de la masse modérée du soufre, les éléments électrochimiques Li/S sont relativement légers. Ils constituent une alternative prometteuse aux éléments lithium-ion en raison de leur densité énergétique plus élevée et le faible coût du soufre. Ils sont avantageusement utilisés dans des domaines dans lesquels une énergie massique élevée est requise. Tel est le cas du domaine de l’aviation et du domaine spatial (satellites). [0002] The term "element" used in the following denotes an electrochemical element. The terms “element” and “electrochemical element” are used interchangeably in the following. Lithium/sulphur (Li/S) electrochemical elements comprising a liquid electrolyte are known from the state of the art. They typically comprise at least one positive electrode (cathode) based on elemental sulfur, an organic liquid electrolyte and at least one negative electrode (anode) made of lithium metal or lithium metal alloy. The cathode is usually composite, i.e. it is prepared from elemental sulfur and non-electrochemically active additives. As non-electrochemically active additives, mention may be made of an electronic conductor, such as carbon, making it possible to improve the electronic conductivity of the cathode because sulfur is an electronic insulator. Mention may also be made of one or more polymeric binders making it possible to ensure cohesion between the different materials of the cathode. Due to the low atomic mass of lithium and the moderate mass of sulfur, Li/S electrochemical elements are relatively light. They are a promising alternative to lithium-ion cells due to their higher energy density and the low cost of sulfur. They are advantageously used in fields in which a high specific energy is required. This is the case for the aviation and space domains (satellites).
[0003] Partant d’une cathode de soufre élémentaire et d’une anode de lithium métal ou d’un alliage de lithium, l’élément électrochimique se trouve initialement à l’état chargé. En décharge, le soufre élémentaire de la cathode se réduit en sulfure de lithium Li2S et le lithium métallique ou l’alliage métallique de lithium s’oxyde à l’anode. Les réactions suivantes ont lieu aux électrodes : [0003] Starting from a cathode of elemental sulfur and an anode of lithium metal or a lithium alloy, the electrochemical element is initially in the charged state. In discharge, the elemental sulfur of the cathode is reduced to lithium sulphide Li2S and the metallic lithium or the metallic lithium alloy is oxidized at the anode. The following reactions take place at the electrodes:
Cathode : Ss + 16 e' — > 8 S2' Anode : Li — > Li+ + e' Cathode: Ss + 16 e' — > 8 S 2 ' Anode: Li — > Li + + e'
La réaction globale de décharge de l’élément est : 16 Li + Sx — > 8 Li2S The overall cell discharge reaction is: 16 Li + Sx — > 8 Li2S
[0004] A la différence d’un élément électrochimique lithium-ion, un élément électrochimique li- thium/soufre comprend typiquement un électrolyte dont le solvant est à base d’éthers. Les éthers, tels que le 1,3-dioxolane ou le tétrahydrofurane sont utilisés depuis plusieurs dé- cennies et permettent une solubilisation importante des polysulfures de lithium. Les solvants organiques de type glyme de formule générale H-[O-CH2-CH2]n-OH, tels que le 1,2-diméthoxyéthane (DME), sont aussi fréquemment utilisés comme solvant de l’électrolyte. [0004] Unlike a lithium-ion electrochemical cell, a lithium/sulfur electrochemical cell typically comprises an electrolyte whose solvent is based on ethers. Ethers, such as 1,3-dioxolane or tetrahydrofuran have been used for several decades. centuries and allow significant solubilization of lithium polysulphides. Glyme-type organic solvents of the general formula H-[O-CH2-CH2] n -OH, such as 1,2-dimethoxyethane (DME), are also frequently used as solvent for the electrolyte.
[0005] Au cours de la réduction du soufre qui se produit lors de la décharge de l’élément, les molécules cycliques de soufre (sous forme d’octasoufre Ss) sont réduites et forment des chaînes linéaires de polysulfures de lithium, de formule générale Li2Sn, n allant généralement de 2 à 8. Les premiers composés formés au cours de la décharge de l’élément sont les polysulfures de lithium à chaînes longues, tels que Li2Ss ou Li2Se. Les polysulfures de lithium à chaînes longues sont susceptibles de migrer à travers l’électrolyte et d’atteindre l’anode de lithium où elles seront réduites en polysulfures à chaînes courtes au cours de la charge qui suit la décharge de l’élément. Les polysulfures à chaînes courtes retournent vers la cathode où ils sont de nouveaux réoxydés en polysulfures à chaînes longues, et ainsi de suite. Ce mécanisme de navette (« shuttle ») des polysulfures entre l’anode et la cathode est la cause d’un faible rendement coulombique de l’élément, c’est-à-dire un faible ratio entre la capacité déchargée par l’élément et la capacité chargée dans l’élément au cours de la charge ayant précédé la décharge. De plus, ce mécanisme de navette entraine une forte autodécharge. En outre, l’utilisation d’une anode en lithium avec un électrolyte liquide conduit à un déficit d’électrolyte au cours du cyclage et à une durée de vie limitée en cyclage. [0005] During the reduction of sulfur which occurs during the discharge of the element, the cyclic molecules of sulfur (in the form of octasulfur Ss) are reduced and form linear chains of lithium polysulfides, of general formula Li2S n , n generally ranging from 2 to 8. The first compounds formed during the discharge of the element are the long-chain lithium polysulphides, such as Li2Ss or Li2Se. Long-chain lithium polysulfides are likely to migrate through the electrolyte and reach the lithium anode where they will be reduced to short-chain polysulfides during charging following cell discharge. The short chain polysulphides return to the cathode where they are further reoxidized into long chain polysulphides, and so on. This shuttle mechanism (“shuttle”) of the polysulphides between the anode and the cathode is the cause of a low coulombic efficiency of the element, that is to say a low ratio between the capacity discharged by the element and the capacitance charged in the element during the charge preceding the discharge. In addition, this shuttle mechanism causes a strong self-discharge. In addition, the use of a lithium anode with a liquid electrolyte leads to an electrolyte deficit during cycling and to a limited cycle life.
[0006] Afin de réduire le mécanisme de navette, on a recherché à réduire la mobilité des polysulfures dans l’électrolyte. Un moyen de réduire la mobilité des polysulfures a été d’utiliser un électrolyte gélifié en remplacement de l’électrolyte liquide. Des éléments comportant un électrolyte gélifié sont évoqués par exemple dans les documents EP 1726052, EP 2333886 et US 9722245. Or, il s’avère que les électrolytes gélifiés connus ne permettent pas aux ions qui y sont contenus d’avoir une mobilité suffisante. La mobilité insuffisante des ions pénalise les performances de l’élément, surtout lorsque celui-ci fonctionne sous un courant de décharge élevé. Jusqu’à présent, les performances en décharge à fort courant d’éléments Li/S à électrolyte gélifié étaient inférieures à celles d’éléments Li/S à électrolyte liquide. [0006] In order to reduce the shuttle mechanism, attempts have been made to reduce the mobility of the polysulphides in the electrolyte. One way to reduce the mobility of polysulfides has been to use a gel electrolyte instead of the liquid electrolyte. Elements comprising a gelled electrolyte are mentioned for example in the documents EP 1726052, EP 2333886 and US 9722245. However, it turns out that the known gelled electrolytes do not allow the ions contained therein to have sufficient mobility. The insufficient mobility of the ions penalizes the performance of the element, especially when it operates under a high discharge current. Until now, the high current discharge performances of Li/S cells with gelled electrolyte were inferior to those of Li/S cells with liquid electrolyte.
[0007] On recherche donc un élément électrochimique de type lithium-soufre à électrolyte gélifié présentant des performances électriques quasiment aussi bonnes voire meilleures que celles obtenues pour un élément électrochimique comprenant un électrolyte liquide, surtout lorsque l’élément est utilisé en décharge sous un fort courant, par exemple à un régime de décharge supérieur à C/5, C étant la capacité nominale de l’élément. [0007] An electrochemical element of the lithium-sulfur type with a gelled electrolyte is therefore sought, presenting electrical performances almost as good or even better than those obtained for an electrochemical element comprising a liquid electrolyte, especially when the element is used in discharge under a strong current, for example at a discharge rate greater than C/5, C being the nominal capacity of the element.
Résumé de l'invention Summary of the invention
[0008] A cet effet l’invention propose un élément électrochimique lithium- soufre comprenant : a) une cathode comprenant : i) du carbone présentant une structure poreuse comprenant des pores avec un diamètre moyen inférieur ou égal à 6 nm ; ii) du soufre avec un degré d’oxydation pouvant aller de 0 à -2 dans les pores du carbone ; b) un électrolyte gélifié comprenant une matrice qui est un polymère de poly(fluorure de vinylidène-co-hexafluoropropylène) P(VDF-HFP) dans laquelle est incorporé un mélange liquide comprenant au moins un sel de lithium et un solvant, le solvant comprenant au moins un carbonate fluoré pouvant être cyclique ou linéaire ; c) une anode comprenant du lithium métal ou un alliage métallique de lithium. [0008] To this end, the invention proposes a lithium-sulphur electrochemical element comprising: a) a cathode comprising: i) carbon having a porous structure comprising pores with an average diameter less than or equal to 6 nm; ii) sulfur with a degree of oxidation ranging from 0 to -2 in the pores of the carbon; b) a gelled electrolyte comprising a matrix which is a polymer of poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) in which is incorporated a liquid mixture comprising at least one lithium salt and a solvent, the solvent comprising at least one fluorinated carbonate which can be cyclic or linear; c) an anode comprising lithium metal or a lithium metal alloy.
[0009] L’utilisation concomitante d’une matrice de poly(fluorure de vinylidène-co-hexafluoro- propylène) avec un solvant comprenant au moins un carbonate fluoré et avec une cathode composite à base de soufre et de carbone poreux dont le diamètre moyen des pores est inférieur ou égal à 6 nm permet de manière surprenante d’obtenir des performances en décharge à fort courant quasiment aussi bonnes que celles obtenues avec un électrolyte liquide. [0009] The concomitant use of a poly(vinylidene fluoride-co-hexafluoropropylene) matrix with a solvent comprising at least one fluorinated carbonate and with a composite cathode based on sulfur and porous carbon whose average diameter of the pores is less than or equal to 6 nm surprisingly makes it possible to obtain high current discharge performance almost as good as that obtained with a liquid electrolyte.
[0010] L’élément selon l’invention présente également une durée de vie en cyclage améliorée par rapport à celle obtenue avec un élément dont l’électrolyte est liquide. The element according to the invention also has an improved cycle life compared to that obtained with an element whose electrolyte is liquid.
[0011] Selon un mode de réalisation, ledit au moins un carbonate fluoré est choisi dans le groupe consistant en le monofluorocarbonate d’éthylène (FEC), le difluorocarbonate d’éthylène, le trifluorométhylcarbonate d’éthylène, le monofluorocarbonate de propylène, le difluorocarbonate de propylène, le trifluorocarbonate de propylène et un mélange de ceux-ci. According to one embodiment, said at least one fluorinated carbonate is selected from the group consisting of ethylene monofluorocarbonate (FEC), ethylene difluorocarbonate, ethylene trifluoromethylcarbonate, propylene monofluorocarbonate, difluorocarbonate propylene, propylene trifluorocarbonate and a mixture thereof.
[0012] Selon un mode de réalisation, ledit au moins un carbonate fluoré est le monofluorocarbonate d’éthylène. According to one embodiment, said at least one fluorinated carbonate is ethylene monofluorocarbonate.
[0013] Selon un mode de réalisation, ledit au moins un carbonate fluoré représente de 5 à 75% du volume du solvant, de préférence de 5 à 50%, de préférence encore de 10 à 30%. According to one embodiment, said at least one fluorinated carbonate represents from 5 to 75% of the volume of the solvent, preferably from 5 to 50%, more preferably from 10 to 30%.
[0014] Selon un mode de réalisation, ledit au moins un carbonate fluoré est cyclique et est associé à au moins un carbonate linéaire non fluoré. According to one embodiment, said at least one fluorinated carbonate is cyclic and is associated with at least one non-fluorinated linear carbonate.
[0015] Selon un mode de réalisation, ledit au moins un carbonate linéaire non fluoré est le carbonate de diméthyle (DMC). According to one embodiment, said at least one non-fluorinated linear carbonate is dimethyl carbonate (DMC).
[0016] Selon un mode de réalisation, le solvant est constitué de 10 à 30 % en volume dudit au moins un carbonate cyclique fluoré et de 90 à 70% en volume dudit au moins un carbonate linéaire non fluoré. According to one embodiment, the solvent consists of 10 to 30% by volume of said at least one fluorinated cyclic carbonate and 90 to 70% by volume of said at least one non-fluorinated linear carbonate.
[0017] Selon un mode de réalisation, le sel de lithium est choisi parmi l’hexafluorophosphate de lithium LiPFe, le bis(fluorosulfonyl)imide de lithium Li(FSÛ2)2N (LiFSI) et un mélange de ceux-ci. According to one embodiment, the lithium salt is chosen from lithium hexafluorophosphate LiPFe, lithium bis(fluorosulfonyl)imide Li(FSÛ2)2N (LiFSI) and a mixture thereof.
[0018] Selon un mode de réalisation, la matrice de polymère de poly(fluorure de vinylidène-co- hexafluoropropylène) représente de 3 à 25 % en masse de la masse du mélange liquide. According to one embodiment, the poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix represents from 3 to 25% by mass of the mass of the liquid mixture.
[0019] Selon un mode de réalisation, le carbone poreux est sous la forme d’une poudre dont la granulométrie est caractérisée par un diamètre médian en volume Dv50 allant de 1 à 10 pm, de préférence allant de 1 à 5 pm. According to one embodiment, the porous carbon is in the form of a powder whose particle size is characterized by a median diameter by volume Dv50 ranging from 1 to 10 μm, preferably ranging from 1 to 5 μm.
[0020] Selon un mode de réalisation, le soufre est du soufre solide élémentaire ; la masse de soufre solide élémentaire représente de 30 à 90 % de la somme des masses du carbone et du soufre solide élémentaire, et la masse de carbone représente de 70 à 10 % de la somme des masses du carbone et du soufre solide élémentaire. According to one embodiment, the sulfur is elemental solid sulfur; the mass of elemental solid sulfur is 30 to 90% of the sum of the masses of carbon and elemental solid sulfur, and the mass of carbon is 70 to 10% of the sum of the masses of carbon and elemental solid sulfur.
[0021] Selon un mode de réalisation, la masse de soufre solide élémentaire représente de 40 à 60 % de la somme des masses du carbone et du soufre solide élémentaire, et la masse de carbone représente de 60 à 40 % de la somme des masses du carbone et du soufre solide élémentaire. According to one embodiment, the mass of elemental solid sulfur represents from 40 to 60% of the sum of the masses of carbon and elemental solid sulphur, and the mass of carbon is 60 to 40% of the sum of the masses of carbon and elemental solid sulphur.
[0022] Selon un mode de réalisation, les pores de la cathode présentent un diamètre moyen inférieur ou égal à 1 nm ; la matrice de polymère de poly(fluorure de vinylidène-co-hexafluo- ropropylène) représente de 5 à 15 % en masse de la masse du mélange liquide ; le solvant est constitué de 10 à 30 % en volume de monofluorocarbonate d’éthylène (FEC) et de 90 à 70 % en volume de carbonate de diméthyle (DMC) ;le sel de lithium est choisi parmi l’hexafluorophosphate de lithium LiPFe, le bis(fluorosulfonyl)imide de lithium Li(FSO2)2N (LiFSI) et un mélange de ceux-ci. According to one embodiment, the pores of the cathode have an average diameter less than or equal to 1 nm; the poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix represents from 5 to 15% by mass of the mass of the liquid mixture; the solvent consists of 10 to 30% by volume of ethylene monofluorocarbonate (FEC) and 90 to 70% by volume of dimethyl carbonate (DMC);the lithium salt is chosen from lithium hexafluorophosphate LiPFe, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI) and a mixture thereof.
Brève description des figures Brief description of figures
[0023] [Fig. 1] représente une vue schématique en coupe d’un empilement d’une cathode, d’un séparateur et d’une anode dans un élément électrochimique selon l’invention. [0023] [Fig. 1] represents a schematic sectional view of a stack of a cathode, a separator and an anode in an electrochemical element according to the invention.
[0024] [Fig. 2A] représente les courbes de décharge de l’élément A selon l’art antérieur. [0024] [Fig. 2A] represents the discharge curves of element A according to the prior art.
[0025] [Fig. 2B] représente les courbes de décharge de l’élément B selon l’invention. [0025] [Fig. 2B] represents the discharge curves of element B according to the invention.
[0026] [Fig. 2C] représente les courbes de décharge de l’élément C selon l’art antérieur. [0026] [Fig. 2C] represents the discharge curves of element C according to the prior art.
[0027] [Fig. 2D] représente les courbes de décharge de l’élément D selon l’invention. [0027] [Fig. 2D] represents the discharge curves of element D according to the invention.
[0028] [Fig. 3] représente la variation de la capacité réversible des éléments A et B au cours d’un cyclage à température ambiante au régime de C/10. [0028] [Fig. 3] represents the variation of the reversible capacitance of elements A and B during cycling at ambient temperature at C/10.
[0029] [Fig. 4] représente la variation de la capacité réversible des éléments C et D au cours d’un cyclage à température ambiante au régime de C/10. [0029] [Fig. 4] represents the variation of the reversible capacitance of elements C and D during cycling at room temperature at C/10.
Description des modes de réalisation de l'invention Description of embodiments of the invention
Les différents constituants d’un élément électrochimique selon l’invention vont être décrits dans ce qui suit. The different constituents of an electrochemical element according to the invention will be described in the following.
Cathode : cathode:
[0030] La cathode est un composite préparé à partir de soufre solide élémentaire électrochimi- quement actif et de composés non électrochimiquement actifs. [0030] The cathode is a composite prepared from electrochemically active elemental solid sulfur and non-electrochemically active compounds.
[0031] Le soufre solide élémentaire existe sous différentes formes moléculaires. La forme préférée est le soufre alpha Sa, de formule Sx correspondant au cyclooctasoufre, qui est la forme thermodynamiquement la plus stable. Elemental solid sulfur exists in different molecular forms. The preferred form is alpha sulfur Sa, of formula Sx corresponding to cyclooctasulfur, which is the thermodynamically most stable form.
[0032] Le carbone est l’un des constituants non électroactifs de la cathode. Il présente une structure poreuse principalement constituée de pores ayant un diamètre moyen dmoyen poreux inférieur ou égal à 6 nm. De préférence, au moins 50 % ou au moins 75 % ou au moins 90 % du volume poreux est constitué de pores ayant un diamètre moyen inférieur ou égal à 6 nm. Carbon is one of the non-electroactive constituents of the cathode. It has a porous structure mainly consisting of pores having an average porous average diameter of less than or equal to 6 nm. Preferably, at least 50% or at least 75% or at least 90% of the pore volume consists of pores having an average diameter less than or equal to 6 nm.
[0033] Le carbone peut présenter une structure poreuse principalement constituée de pores ayant un diamètre moyen inférieur ou égal à 2 nm. De préférence, au moins 50 % ou au moins 75 % ou au moins 90 % du volume poreux est constitué de pores ayant un diamètre moyen inférieur ou égal à 2 nm. [0034] Les pores de la structure poreuse peuvent avoir un diamètre moyen allant de de 0,5 nm à 2 nm. Dans un mode de réalisation, au moins 50 % ou au moins 75 % ou au moins 90 % du volume poreux est constitué par des pores ayant un diamètre moyen allant de 0,5 nm à 2 nm. The carbon can have a porous structure mainly consisting of pores having an average diameter less than or equal to 2 nm. Preferably, at least 50% or at least 75% or at least 90% of the pore volume consists of pores having an average diameter less than or equal to 2 nm. The pores of the porous structure can have an average diameter ranging from 0.5 nm to 2 nm. In one embodiment, at least 50% or at least 75% or at least 90% of the pore volume is constituted by pores having an average diameter ranging from 0.5 nm to 2 nm.
[0035] Les pores présentant un diamètre moyen inférieur ou égal à 2 nm sont appelés micropores. Ceux présentant un diamètre moyen supérieur à 2 nm et inférieur à 50 nm sont appelés mésopores. The pores having an average diameter less than or equal to 2 nm are called micropores. Those with an average diameter greater than 2 nm and less than 50 nm are called mesopores.
[0036] La méthode « Barrett-Joyner-Halenda » B JH permet d’estimer le diamètre moyen des pores associé au volume mésoporeux, qui sera noté dmésoporeux, à partir des isothermes d’ adsorption et de désorption à l’azote. La méthode de Horvath-Kawazoe permet d’estimer le diamètre moyen des pores associé au volume microporeux qui sera noté dmicroporeux, à partir des isothermes d’adsorption et de désorption à l’azote. The “Barrett-Joyner-Halenda” B JH method makes it possible to estimate the mean pore diameter associated with the mesoporous volume, which will be denoted dmesoporous, from the nitrogen adsorption and desorption isotherms. The Horvath-Kawazoe method makes it possible to estimate the average pore diameter associated with the microporous volume, which will be denoted dmicroporous, from the nitrogen adsorption and desorption isotherms.
Le volume mésoporeux noté Vmésoporeux est mesuré selon la méthode BJH à partir des isothermes d’adsorption et de désorption à l’azote. Le volume microporeux noté Vmicroporeux est mesuré selon la méthode « t-plot » à partir des isothermes d’adsorption et de désorption à l’azote. The mesoporous volume denoted Vmesoporous is measured according to the BJH method from the nitrogen adsorption and desorption isotherms. The microporous volume denoted Vmicroporous is measured according to the "t-plot" method from the nitrogen adsorption and desorption isotherms.
Lorsque le carbone présente à la fois un volume mésoporeux et microporeux, l’évaluation du diamètre moyen poreux du carbone est calculé par la moyenne des diamètres microporeux et mésoporeux pondérés par les volumes poreux associés selon la formule suivante : dmoyen poreux ((Vmésoporeux * dmésoporeux) + (Vmicroporeux * dmicroporeux))/ (Vmésoporeux + Vmicroporeux) Les mésopores peuvent contribuer à 40% ou moins, ou à 30% ou moins, ou à 20% ou moins ou à 10% ou moins du volume total des pores. Il est préférable que la contribution des mésopores au volume poreux total soit minimisée afin d'éviter la formation de poly- sulfures dans l'électrolyte. Comme mentionné ci-dessus, les polysulfures ne sont pas stables en présence de carbonates et contribuent au mécanisme indésirable de «navette». When the carbon has both a mesoporous and microporous volume, the evaluation of the average pore diameter of the carbon is calculated by the average of the microporous and mesoporous diameters weighted by the associated pore volumes according to the following formula: daverage porous ((Vmesoporous * dmesoporous ) + (Vmicroporous * dmicroporous))/ (Vmesoporous + Vmicroporous) Mesopores can contribute 40% or less, or 30% or less, or 20% or less, or 10% or less of the total pore volume. It is preferable that the contribution of the mesopores to the total pore volume be minimized in order to avoid the formation of polysulfides in the electrolyte. As mentioned above, polysulfides are not stable in the presence of carbonates and contribute to the unwanted "shuttle" mechanism.
[0037] Le carbone microporeux se présente sous forme de particules dont la distribution granulo- métrique est caractérisée par un diamètre médian en volume Dvso pouvant être supérieu- rou égal à 1 pm et inférieur ou égal à 10 pm, ou pouvant aller de 1 à 5 pm. Le terme «diamètre médian en volume Dvso de X pm» signifie que 50 % du volume des particules de carbone est constitué de particules ayant un diamètre équivalent inférieur à X pm et 50 % du volume des particules de carbone est composé de particules ayant un diamètre équivalent supérieur à X pm. Le terme « diamètre équivalent » d'une particule désigne le diamètre d'une sphère ayant le même volume que cette particule. La distribution granulo- métrique des particules de carbone peut être déterminée par diffraction laser. Un appareil approprié pour la détermination de la distribution granulométrique est le Mastersizer 2000 commercialisé par la société Malvern. Les particules de carbone microporeux sont de préférence obtenues par broyage mécanique de carbone. Un procédé de broyage mécanique exclut l’obtention de nanotubes de carbone. Un broyage mécanique permet d’obtenir une poudre de particules présentant un Dvso allant de 1 pm à 10 pm. Le choix de cette plage spécifique de tailles de particules permet d’obtenir un élément présentant une capacité massique élevée. [0037] The microporous carbon is in the form of particles whose particle size distribution is characterized by a volume median diameter Dvso which may be greater than or equal to 1 μm and less than or equal to 10 μm, or which may range from 1 to 5 p.m. The term "volume median diameter Dvso of X pm" means that 50% of the volume of the carbon particles consists of particles having an equivalent diameter less than X pm and 50% of the volume of the carbon particles consists of particles having a diameter equivalent greater than X pm. The term “equivalent diameter” of a particle designates the diameter of a sphere having the same volume as this particle. The particle size distribution of the carbon particles can be determined by laser diffraction. A suitable device for determining the particle size distribution is the Mastersizer 2000 marketed by the company Malvern. The microporous carbon particles are preferably obtained by mechanical grinding of carbon. A mechanical grinding process excludes obtaining carbon nanotubes. Mechanical grinding makes it possible to obtain a powder of particles having a Dvso ranging from 1 μm to 10 μm. Choosing this specific range of particle sizes results in an element with a high mass capacity.
[0038] Les pores de la structure poreuse logent les particules de soufre élémentaire. Un procédé possible pour l’incorporation de particules de soufre élémentaire dans les pores de la structure poreuse du carbone est le suivant. On mélange le carbone poreux avec du soufre élémentaire solide. Typiquement, la masse de soufre élémentaire solide représente de 30 à 80% ou de 40 à 65% ou typiquement environ 50% de la somme des masses de soufre élémentaire solide et de carbone. La masse de carbone représente typiquement de 70 à 20% ou de 60 à 35% ou typiquement environ 50% de la somme des masses de soufre élémentaire solide et de carbone. The pores of the porous structure accommodate the particles of elemental sulfur. A method possibility for the incorporation of particles of elemental sulfur into the pores of the porous carbon structure is as follows. The porous carbon is mixed with solid elemental sulfur. Typically, the mass of solid elemental sulfur is 30 to 80% or 40 to 65% or typically about 50% of the sum of the masses of solid elemental sulfur and carbon. The mass of carbon is typically 70 to 20% or 60 to 35% or typically about 50% of the sum of the masses of solid elemental sulfur and carbon.
[0039] Le mélange est chauffé à une température voisine de 155°C pendant environ 5 heures, sous vide, pour permettre aux molécules de soufre de pénétrer dans les pores ouverts du carbone. Au voisinage de 155°C, le soufre à l’état liquide présente sa viscosité la plus faible. Le mélange est ensuite chauffé sous gaz inerte à une température d’environ 300°C pendant environ 30 minutes, ce qui a pour effet de sublimer le soufre et d’en éliminer l’excès. Le produit obtenu est ensuite généralement mélangé à au moins un liant et au moins un composé bon conducteur électronique. Ledit au moins un liant et ledit au moins un composé conducteur électronique sont non électroactifs. The mixture is heated to a temperature close to 155° C. for about 5 hours, under vacuum, to allow the sulfur molecules to penetrate into the open pores of the carbon. Around 155°C, sulfur in the liquid state has its lowest viscosity. The mixture is then heated under inert gas at a temperature of about 300°C for about 30 minutes, which has the effect of sublimating the sulfur and eliminating the excess. The product obtained is then generally mixed with at least one binder and at least one good electronic conductor compound. Said at least one binder and said at least one electronically conductive compound are non-electroactive.
[0040] Le liant peut être choisi parmi la carboxyméthylcellulose (CMC), un copolymère de buta- diène - styrène (SBR), le polytétrafluoroéthylène (PTFE), le polyamideimide (PAI), le polyimide (PI), le caoutchouc styrène-butadiène (SBR), les poly(acide acrylique) (PAA), l’alcool poly vinylique, le polyfluorure de vinylidène (PVDF) et un mélange de ceux-ci. [0040] The binder can be chosen from carboxymethylcellulose (CMC), a butadiene-styrene copolymer (SBR), polytetrafluoroethylene (PTFE), polyamideimide (PAI), polyimide (PI), styrene-butadiene rubber (SBR), poly(acrylic acid) (PAA), polyvinyl alcohol, polyvinylidene fluoride (PVDF) and a mixture thereof.
[0041] Le composé conducteur électronique est généralement du noir de carbone. [0041] The electronically conductive compound is generally carbon black.
[0042] Le mélange du composite avec ledit au moins un liant et ledit au moins un composé conducteur électronique conduit à une encre que l’on dépose sur l’une ou les deux faces d’un collecteur de courant pouvant être un feuillard en aluminium. Le collecteur de courant enduit d’encre est laminé afin d’ajuster son épaisseur. Une cathode est ainsi obtenue. Mixing the composite with said at least one binder and said at least one electronically conductive compound leads to an ink which is deposited on one or both sides of a current collector which may be an aluminum strip. . The ink-coated current collector is laminated to adjust its thickness. A cathode is thus obtained.
[0043] Une composition typique de l’encre déposée sur le collecteur de courant peut être la suivante : A typical composition of the ink deposited on the current collector can be the following:
- de 30 à 80 % en masse, de préférence de 40 à 70 % de soufre élémentaire solide par rapport à la somme des masses du soufre élémentaire solide, du carbone, du ou des liant(s) et du ou des composé(s) conducteur(s) électronique(s) ; - from 30 to 80% by mass, preferably from 40 to 70% of solid elemental sulfur relative to the sum of the masses of the solid elemental sulfur, the carbon, the binder(s) and the compound(s) electronic conductor(s);
- de 10 à 60 % en masse, de préférence de 15 à 40 % de carbone poreux solide par rapport à la somme des masses du soufre élémentaire solide, du carbone, du ou des liant(s) et du ou des composé(s) conducteur(s) électronique(s) ; - from 10 to 60% by mass, preferably from 15 to 40% of solid porous carbon relative to the sum of the masses of the solid elemental sulfur, the carbon, the binder(s) and the compound(s) electronic conductor(s);
- de 3 à 8 % en masse, de préférence de 5 à 7 % de liant par rapport à la somme des masses du soufre élémentaire solide, du carbone, du ou des liant(s) et du ou des composé^) conducteur(s) électronique(s) ; - from 3 to 8% by mass, preferably from 5 to 7% of binder relative to the sum of the masses of the solid elemental sulfur, the carbon, the binder(s) and the conductive compound(s) ) electronic(s);
- de 5 à 15 % en masse, de préférence de 5 à 10 % de composé conducteur électronique par rapport à la somme des masses du soufre élémentaire solide, du carbone, du ou des liant(s) et du ou des composé(s) conducteur(s) électronique(s). - from 5 to 15% by mass, preferably from 5 to 10% of electronically conductive compound relative to the sum of the masses of the solid elemental sulfur, of the carbon, of the binder(s) and of the compound(s) electronic driver(s).
[0044] La quantité de soufre élémentaire est mesurée sur le composite par analyse thermogravi- métrique après traitement à 300°C. Anode : [0044] The amount of elemental sulfur is measured on the composite by thermogravimetric analysis after treatment at 300°C. Anode:
[0045] L’anode peut être constituée de lithium ou d’un alliage à base de lithium et d’un élément chimique choisi parmi Si, Sn, In et C. L’anode peut aussi être constituée d’une couche de lithium métal sur laquelle est déposée une couche d’indium, la couche d’indium étant au contact de la couche d’électrolyte solide. L’anode peut n’être constituée que d’un ruban de lithium. The anode may consist of lithium or of an alloy based on lithium and a chemical element chosen from Si, Sn, In and C. The anode may also consist of a layer of lithium metal on which is deposited a layer of indium, the indium layer being in contact with the layer of solid electrolyte. The anode may consist only of a lithium ribbon.
Electrolyte gélifié : Gel electrolyte:
[0046] L’électrolyte gélifié comprend une matrice qui est un polymère de poly(fluorure de viny- lidène-co-hexafluoropropylène) dans laquelle est incorporé un mélange liquide comprenant au moins un sel de lithium et un solvant, le solvant comprenant au moins un carbonate fluoré pouvant être cyclique ou linéaire. Le poly(fluorure de vinylidène-co-hexafluo- ropropylène) P(VdF-HFP) a pour formule : [Chem 1]
Figure imgf000008_0001
où x désigne le nombre de motifs de fluorure de vinylidène et y désigne le nombre de motifs d’hexafluoropropylène. [0046] The gelled electrolyte comprises a matrix which is a poly(vinylidene fluoride-co-hexafluoropropylene) polymer in which is incorporated a liquid mixture comprising at least one lithium salt and a solvent, the solvent comprising at least a fluorinated carbonate which can be cyclic or linear. Poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) has the formula: [Chem 1]
Figure imgf000008_0001
where x denotes the number of vinylidene fluoride units and y denotes the number of hexafluoropropylene units.
[0047] La masse moléculaire en poids de P(VdF-HFP) peut varier de 300 Da à 5 MDa. Elle peut se situer dans la plage allant de 300 à 800 Da ou dans la plage allant de 200 à 400 kDa. The molecular mass by weight of P(VdF-HFP) can vary from 300 Da to 5 MDa. It can be in the range of 300 to 800 Da or in the range of 200 to 400 kDa.
[0048] La matrice de P(VdF-HFP) peut représenter de 3 à 95 % ou de 3 à 50 %, ou de 3 à 20 % ou de 3 à 10 % en masse par rapport à la masse du mélange liquide. Le mélange liquide comprend ledit au moins un sel de lithium et le solvant, le solvant comprenant ledit au moins un carbonate fluoré cyclique ou linéaire. Une plage de pourcentage préférée est la plage allant de 3 à 25 %, de préférence encore allant de 10 à 20 %. Cette plage préférée permet à la fois d’obtenir une bonne résistance de l’électrolyte à l’oxydation aux potentiels élevés de cathode ainsi qu’une bonne capacité réversible de l’élément. La résistance de l’électrolyte à l’oxydation peut diminuer si le pourcentage de P(VdF-HFP) ajouté au mélange liquide est inférieur ou égal à 3 %. La capacité réversible de l’élément contenant l’électrolyte peut diminuer si le pourcentage de P(VdF-HFP) ajouté au mélange liquide est supérieur à 25 %. De plus, pour un pourcentage de polymère supérieur à 25 %, on peut observer une moins bonne imprégnation des électrodes par le polymère. Le polymère peut être insuffisamment en contact avec la porosité des électrodes. The P(VdF-HFP) matrix can represent from 3 to 95% or from 3 to 50%, or from 3 to 20% or from 3 to 10% by mass relative to the mass of the liquid mixture. The liquid mixture comprises said at least one lithium salt and the solvent, the solvent comprising said at least one cyclic or linear fluorinated carbonate. A preferred percentage range is the range from 3 to 25%, more preferably from 10 to 20%. This preferred range makes it possible both to obtain a good resistance of the electrolyte to oxidation at high cathode potentials as well as a good reversible capacity of the element. The resistance of the electrolyte to oxidation may decrease if the percentage of P(VdF-HFP) added to the liquid mixture is less than or equal to 3%. The reversible capacity of the cell containing the electrolyte may decrease if the percentage of P(VdF-HFP) added to the liquid mixture is greater than 25%. In addition, for a percentage of polymer greater than 25%, a poorer impregnation of the electrodes by the polymer can be observed. The polymer may be insufficiently in contact with the porosity of the electrodes.
[0049] Par comparaison avec du polyfluorure de vinylidène PVdF, P(VdF-HFP) présente une plus grande solubilité vis-à-vis du mélange liquide comprenant ledit au moins un sel de lithium, le solvant et le liquide ionique. Il semblerait que le polymère de P(VdF-HFP) se réduise au cours du fonctionnement de l’élément et en se réduisant génère du LiF qui joue un rôle protecteur de l’anode en lithium ou en alliage de lithium et permet d’augmenter la durée de vie de l’élément en cyclage, contrairement à d’autres polymères comprenant des atomes d’oxygène. By comparison with polyvinylidene fluoride PVdF, P(VdF-HFP) exhibits greater solubility with respect to the liquid mixture comprising said at least one lithium salt, the solvent and the ionic liquid. It would seem that the polymer of P(VdF-HFP) is reduced during the operation of the element and by being reduced generates LiF which plays a protective role of the anode in lithium or in lithium alloy and makes it possible to increase the life of the element in cycling, unlike other polymers containing oxygen atoms.
[0050] La matrice peut aussi comprendre en quantité minoritaire un ou plusieurs polymères en association avec P(VdF-HFP). Ce ou ces autres polymères peuvent être choisis parmi un poly(oxyde d’éthylène), le poly(fluorure de vinylidène) PVDF, un polyacrylate, un poly(imidine), un polymère de butadiène acrylonitrile NBR et un polymère de butadiène acrylonitrile hydrogéné HNBR. Dans ce cas, P(VdF-HFP) représente de préférence au moins 50 %, de préférence au moins 90 % en masse du mélange de polymères. The matrix may also comprise a minor amount of one or more polymers in combination with P(VdF-HFP). This or these other polymer(s) may be chosen from a poly(ethylene oxide), poly(vinylidene fluoride) PVDF, a polyacrylate, a poly(imidine), a polymer of butadiene acrylonitrile NBR and a polymer of hydrogenated acrylonitrile butadiene HNBR . In this case, P(VdF-HFP) preferably represents at least 50%, preferably at least 90% by mass of the mixture of polymers.
[0051] La composition d’électrolyte gélifié est obtenue en mélangeant un polymère de poly(fluo- rure de vinylidène-co-hexafluoropropylène) P(VdF-HFP) avec un mélange liquide comprenant au moins un sel de lithium, un solvant comprenant au moins un carbonate fluoré cyclique ou linéaire. Sans vouloir être lié par une quelconque théorie, la demanderesse est d’avis que l’utilisation conjointe de P(VdF-HFP) avec au moins un carbonate fluoré permet d’améliorer la qualité de la couche de passivation (SEI) à la surface du lithium ou de l’alliage de lithium en favorisant la conductivité ionique du lithium et en limitant sa conductivité électronique. The gelled electrolyte composition is obtained by mixing a poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) polymer with a liquid mixture comprising at least one lithium salt, a solvent comprising at least one cyclic or linear fluorinated carbonate. Without wishing to be bound by any theory, the applicant is of the opinion that the joint use of P(VdF-HFP) with at least one fluorinated carbonate makes it possible to improve the quality of the passivation layer (SEI) on the surface lithium or lithium alloy by promoting the ionic conductivity of the lithium and by limiting its electronic conductivity.
[0052] Le carbonate cyclique saturé peut être monofluoré ou difluoré ou trifluoré. Il peut être choisi dans le groupe consistant en le monofluorocarbonate d’éthylène (FEC), encore désigné par le terme 4-fluoro-l,3-dioxolan-2-one, le difluorocarbonate d’éthylène, le tri- fluorométhylcarbonate d’éthylène, encore désigné par le terme 4-trifluoro-l,3-dioxolan- 2-one, le monofluorocarbonate de propylène, le difluorocarbonate de propylène, le tri- fluorocarbonate de propylène et un mélange de ceux-ci. De préférence, le carbonate cyclique saturé monofluoré est le monofluorocarbonate d’éthylène. The saturated cyclic carbonate can be monofluorinated or difluorinated or trifluorinated. It can be chosen from the group consisting of ethylene monofluorocarbonate (FEC), also designated by the term 4-fluoro-1,3-dioxolan-2-one, ethylene difluorocarbonate, ethylene trifluoromethylcarbonate , also designated by the term 4-trifluoro-1,3-dioxolan-2-one, propylene monofluorocarbonate, propylene difluorocarbonate, propylene trifluorocarbonate and a mixture thereof. Preferably, the monofluorinated saturated cyclic carbonate is ethylene monofluorocarbonate.
[0053] Ledit au moins un carbonate fluoré peut représenter de 5 à 75% du volume du solvant, de préférence de 5 à 50%, de préférence encore de 10 à 30%. Said at least one fluorinated carbonate can represent from 5 to 75% of the volume of the solvent, preferably from 5 to 50%, more preferably from 10 to 30%.
[0054] Ledit au moins un carbonate fluoré peut être cyclique et être associé à au moins un carbonate linéaire non fluoré. Le solvant peut être constitué de 10 à 30 % en volume dudit au moins un carbonate cyclique fluoré et de 90 à 70% en volume dudit au moins un carbonate linéaire non fluoré. Said at least one fluorinated carbonate can be cyclic and be associated with at least one non-fluorinated linear carbonate. The solvent may consist of 10 to 30% by volume of said at least one fluorinated cyclic carbonate and 90 to 70% by volume of said at least one non-fluorinated linear carbonate.
[0055] Le carbonate linéaire non fluoré peut être choisi dans le groupe consistant en le carbonate de diméthyle (DMC), le carbonate de diéthyle (DEC), le carbonate de méthyle éthyle (EMC) et le carbonate de méthyle propyle (PMC). Le carbonate de diméthyle (DMC) et le carbonate de méthyle éthyle (EMC) sont particulièrement préférés. Le solvant peut être exempt de carbonates linéaires autres que DMC et EMC. Le carbonate linéaire peut représenter de 50 à 90 % ou de 60 à 80 %, ou de 70 à 80 % en volume du volume total des carbonates cyclique et linéaire. The non-fluorinated linear carbonate can be chosen from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC) and methyl propyl carbonate (PMC). Dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) are particularly preferred. The solvent may be free of linear carbonates other than DMC and EMC. The linear carbonate can represent from 50 to 90% or from 60 to 80%, or from 70 to 80% by volume of the total volume of the cyclic and linear carbonates.
[0056] Le solvant peut être constitué d’un mélange de FEC et de DMC, sans autre carbonate. Dans ce cas, FEC représente de préférence de 10 à 30 % en volume du volume total (FEC+DMC) et DMC représente de préférence de 90 à 70 % en volume du volume total (FEC+DMC). The solvent may consist of a mixture of FEC and DMC, without any other carbonate. In this case, FEC preferably represents from 10 to 30% by volume of the total volume (FEC+DMC) and DMC preferably represents from 90 to 70% by volume of the total volume (FEC+DMC).
[0057] De préférence, le solvant ne contient pas d’ester(s) linéaire(s) ou d’ester(s) cyclique(s), encore appelés lactones. De préférence, le solvant ne contient pas d’éther(s). [0058] La nature du sel de lithium n’est pas particulièrement limitée. On peut citer l'hexafluoro- phosphate de lithium LiPFe, l’hexafluoroarsénate de lithium LiAsFe, l’hexafluoroantimo- nate de lithium LiSbFe et le tétrafluorob orate de lithium LiBF4, le perchlorate de lithium LiC104, le trifluorométhanesulfonate de lithium LiCFsSOs, le bis(fluorosulfonyl)imide de lithium Li(FSÛ2)2N (LiFSI), le trifluorométhanesulfonimide de lithium LiN(CFsSO2)2 (LiTFSI), le trifluorométhanesulfoneméthide de lithium LiC(CF3SO2)3 (LiTFSM), le bis- perfluoroéthylsulfonimide de lithium LiN(C2FsSO2)2 (LiBETI), le 4,5-dicyano-2-(trifluo- romethyl) imidazolide de lithium (LiTDI), le bis(oxalatoborate) de lithium (LiBOB), le difluoro(oxalato) borate de lithium (LiDFOB), le tris(pentafluoroethyl)trifluorophosphate de lithium LiPF3(CF2CF3)3 (LiFAP) et les mélanges de ceux-ci. Les sels préférés sont l'hexafluorophosphate de lithium LiPFe et le bis(fluorosulfonyl)imide de lithium Li(FSO2)2N (LiFSI). Preferably, the solvent does not contain linear ester(s) or cyclic ester(s), also called lactones. Preferably, the solvent does not contain ether(s). The nature of the lithium salt is not particularly limited. Mention may be made of lithium hexafluorophosphate LiPFe, lithium hexafluoroarsenate LiAsFe, lithium hexafluoroantimonate LiSbFe and lithium tetrafluorocarbonate LiBF4, lithium perchlorate LiC104, lithium trifluoromethanesulfonate LiCFsSOs, bis( lithium fluorosulfonyl)imide Li(FSO2)2N (LiFSI), lithium trifluoromethanesulfonimide LiN(CFsSO2)2 (LiTFSI), lithium trifluoromethanesulfonemethide LiC(CF3SO2)3 (LiTFSM), lithium bis-perfluoroethylsulfonimide LiN(C2FsSO2) 2 (LiBETI), lithium 4,5-dicyano-2-(trifluoromethyl) imidazolide (LiTDI), lithium bis(oxalatoborate) (LiBOB), lithium difluoro(oxalato) borate (LiDFOB), lithium tris(pentafluoroethyl)trifluorophosphate LiPF3(CF2CF3)3 (LiFAP) and mixtures thereof. The preferred salts are lithium hexafluorophosphate LiPFe and lithium bis(fluorosulfonyl)imide Li(FSO 2 ) 2 N (LiFSI).
[0059] La concentration dudit au moins un sel de lithium peut aller de 0,75 à 1,5 mol.L'1. De préférence, elle va de 1 à 1,5 mol.L'1. De préférence encore, elle est environ égale à 1 mol.L'.1 The concentration of said at least one lithium salt can range from 0.75 to 1.5 mol.L' 1 . Preferably, it ranges from 1 to 1.5 mol.L' 1 . More preferably, it is approximately equal to 1 mol.L'. 1
Fabrication de l’électrolyte gélifié : Production of the gelled electrolyte:
[0060] Pour fabriquer la composition d’électrolyte gélifié, on incorpore le polymère de poly(fluorure de vinylidène-co-hexafluoropropylène) au mélange liquide. Le mélange est agité pendant plusieurs minutes. Il peut être chauffé à une température ne dépassant pas 50°C afin d’accélérer le gonflement du polymère. To manufacture the gelled electrolyte composition, the poly(vinylidene fluoride-co-hexafluoropropylene) polymer is incorporated into the liquid mixture. The mixture is stirred for several minutes. It can be heated to a temperature not exceeding 50°C in order to accelerate the swelling of the polymer.
[0061] Un exemple particulièrement préféré de composition d’électrolyte gélifié comprend la matrice de poly(fluorure de vinylidène-co-hexafluoropropylène) dans laquelle est incorporé un mélange liquide comprenant LiPFe et/ou LiFSI et un solvant constitué de FEC et de DMC, la matrice de polymère de poly(fluorure de vinylidène-co-hexafluoropropylène) représentant de 3 à 25 % en masse par rapport à la masse du mélange liquide. A particularly preferred example of a gelled electrolyte composition comprises the poly(vinylidene fluoride-co-hexafluoropropylene) matrix in which is incorporated a liquid mixture comprising LiPFe and/or LiFSI and a solvent consisting of FEC and DMC, the polymer matrix of poly(vinylidene fluoride-co-hexafluoropropylene) representing from 3 to 25% by mass relative to the mass of the liquid mixture.
Séparateur : Separator:
[0062] Un séparateur est généralement intercalé entre une anode et une cathode pour éviter d’éventuels courts-circuits. Il empêche un contact électrique entre une anode et une cathode mais permet néanmoins le transport des ions entre ces deux électrodes. Le matériau du séparateur peut être choisi parmi les matériaux suivants : une polyoléfine, par exemple le polypropylène et le polyéthylène, un polyester, des fibres de verre liées entre elles par un polymère, le polyimide, le polyamide, le polyaramide, le polyamideimide et la cellulose. Le polyester peut être choisi parmi le téréphtalate de polyéthylène (PET) et le té- réphtalate de polybutylène (PBT). Avantageusement, le polyester ou le polypropylène ou le polyéthylène contient ou est revêtu d’un matériau choisi dans le groupe consistant en un oxyde métallique, un carbure, un nitrure, un borure, un siliciure et un sulfure. Ce matériau peut être SiCL ou AI2O3. Le séparateur peut être revêtu d’un revêtement organique, par exemple comprenant un acrylate ou PVDF ou P(VdF-HFP). A separator is generally interposed between an anode and a cathode to avoid possible short circuits. It prevents electrical contact between an anode and a cathode 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, polyaramide, polyamideimide and cellulose. The polyester can be chosen 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 sulphide. This material can be SiCL or Al2O3. The separator can be coated with an organic coating, for example comprising an acrylate or PVDF or P(VdF-HFP).
Un séparateur préféré est constitué de polyéthylène ou est constitué de l’association de trois couches qui sont polypropylène PP / polyéthylène PE / polypropylène PP. Préparation du faisceau électrochimique : A preferred separator is made of polyethylene or is made of the combination of three layers which are polypropylene PP/polyethylene PE/polypropylene PP. Preparation of the electrochemical bundle:
[0063] Dans une première variante, on dépose la composition d’électrolyte gélifié au contact de l’encre préalablement déposée sur la cathode. On dépose ensuite un séparateur sur l’encre imprégnée de la composition d’électrolyte gélifié. On enduit de composition d’électrolyte gélifié la face du séparateur destinée à être en contact avec l’anode. On applique ensuite une anode au contact de la composition d’électrolyte gélifié. In a first variant, the gelled electrolyte composition is deposited in contact with the ink previously deposited on the cathode. A separator is then deposited on the ink impregnated with the gelled electrolyte composition. The face of the separator intended to be in contact with the anode is coated with gelled electrolyte composition. An anode is then applied in contact with the gelled electrolyte composition.
[0064] Dans une seconde variante, on dépose la composition d’électrolyte gélifié d’une part sur l’encre déposée préalablement sur la cathode et d’autre part sur l’anode. On intercale un séparateur entre l’encre et l’anode imprégnées de composition d’électrolyte gélifié. In a second variant, the gelled electrolyte composition is deposited on the one hand on the ink previously deposited on the cathode and on the other hand on the anode. A separator is inserted between the ink and the anode impregnated with gelled electrolyte composition.
[0065] Dans une troisième variante, on imbibe de composition d’électrolyte gélifié les deux faces opposées du séparateur et on insère le séparateur entre une cathode et une anode. In a third variant, the two opposite faces of the separator are soaked with gelled electrolyte composition and the separator is inserted between a cathode and an anode.
[0066] On obtient à l’issue de ces différentes variantes, un montage dans lequel la composition d’électrolyte gélifié et le séparateur sont pris en sandwich entre une anode et une cathode. On peut noter que la présence d’un séparateur n’est pas nécessaire si la masse de P(VdF- HFP) représente au moins 50 % de la masse de la composition d’électrolyte gélifié. La composition d’électrolyte gélifié joue dans ce cas à la fois le rôle de séparateur et d’électrolyte. At the end of these different variants, an assembly is obtained in which the gelled electrolyte composition and the separator are sandwiched between an anode and a cathode. It can be noted that the presence of a separator is not necessary if the mass of P(VdF-HFP) represents at least 50% of the mass of the gelled electrolyte composition. The gelled electrolyte composition in this case plays both the role of separator and electrolyte.
[0067] Avant de débuter la « formation » des électrodes, c’est-à-dire d’effectuer un premier cycle de charge/décharge de l’élément, il peut être utile de laisser reposer l’élément à une température supérieure à la température ambiante, par exemple 50 ou 60°C, pendant plusieurs heures, par exemple de 5 à 15 heures, de manière à favoriser l’imprégnation de la matière active des électrodes par la composition d’électrolyte gélifié. Before starting the "formation" of the electrodes, that is to say performing a first charge/discharge cycle of the element, it may be useful to let the element rest at a temperature above ambient temperature, for example 50 or 60° C., for several hours, for example from 5 to 15 hours, so as to promote the impregnation of the active material of the electrodes by the gelled electrolyte composition.
[0068] La figure 1 est une représentation schématique éclatée d’un empilement d’une cathode (2), d’un séparateur (3) et d’une anode (4) dans un élément électrochimique (1) selon l’invention. La cathode (2) présente une structure poreuse (5). Les pores de la cathode logent du soufre. L’électrolyte gélifié (6) imbibe les pores (5) de la cathode (2) et les pores du séparateur (3). Il imprègne également la face du séparateur au contact de l’anode et donc l’anode. Figure 1 is an exploded schematic representation of a stack of a cathode (2), a separator (3) and an anode (4) in an electrochemical element (1) according to the invention. The cathode (2) has a porous structure (5). The pores of the cathode house sulfur. The gelled electrolyte (6) soaks the pores (5) of the cathode (2) and the pores of the separator (3). It also impregnates the face of the separator in contact with the anode and therefore the anode.
Formation de l’élément : Element formation:
[0069] La formation de l’élément peut être effectuée à une température inférieure ou égale à 50°C, par exemple allant de 20 à 50°C. Une augmentation de la température de formation permet une meilleure imbibition des pores des électrodes par la composition d’électrolyte gélifié. The formation of the element can be carried out at a temperature less than or equal to 50° C., for example ranging from 20 to 50° C. An increase in the formation temperature allows better imbibition of the pores of the electrodes by the gelled electrolyte composition.
[0070] L’invention, en plus d’améliorer les performances de l’élément en décharge sous fort courant et sa durée de vie en cyclage, procure les avantages suivants : The invention, in addition to improving the performance of the element in discharge under high current and its cycle life, provides the following advantages:
- l’association de la cathode de carbone présentant une structure poreuse comprenant des pores avec un diamètre moyen inférieur ou égal à 6 nm avec l’électrolyte comprenant au moins un carbonate et au moins un sel de bis(fluorosulfonyl)imide de lithium (LiFSI) permet de réduire la quantité d’hydrogène sulfureux susceptible d’être émise en cas d’ouverture accidentelle du conteneur de l’élément ; - l’utilisation d’un carbone poreux dont les pores ont un diamètre moyen inférieur ou égal à 6 nm permet de réduire l’autodécharge de l’élément ; - the association of the carbon cathode having a porous structure comprising pores with an average diameter less than or equal to 6 nm with the electrolyte comprising at least one carbonate and at least one lithium bis(fluorosulfonyl)imide salt (LiFSI ) makes it possible to reduce the quantity of hydrogen sulphide likely to be emitted in the event of accidental opening of the container of the element; - the use of a porous carbon whose pores have an average diameter less than or equal to 6 nm makes it possible to reduce the self-discharge of the element;
- la forme gélifiée de l’électrolyte permet d’éviter la dissolution des polysulfures dans l’électrolyte liquide, cette dissolution étant responsable du phénomène des « navettes » et de l’autodécharge de l’élément dans le cas d’un électrolyte liquide à base d’éther(s) ;- the gelled form of the electrolyte makes it possible to avoid the dissolution of the polysulphides in the liquid electrolyte, this dissolution being responsible for the phenomenon of "shuttles" and the self-discharge of the element in the case of a liquid electrolyte with ether(s) base;
- le solvant est retenu dans la matrice de P(VDF-HFP). Par conséquent, en cas d’ouverture du conteneur, la quantité de solvant s’échappant de l’élément est faible. De plus, la quantité de solvant « libre » étant très limitée en comparaison avec celle d’un élément comprenant un électrolyte liquide, le risque d’inflammation de l’élément est réduit. Ceci permet d’améliorer la sécurité d’un utilisateur placé au voisinage de l’élément. - the solvent is retained in the P(VDF-HFP) matrix. Therefore, if the container is opened, the amount of solvent escaping from the element is small. Moreover, the quantity of “free” solvent being very limited in comparison with that of an element comprising a liquid electrolyte, the risk of ignition of the element is reduced. This makes it possible to improve the safety of a user placed in the vicinity of the element.
[0071] EXEMPLES [0071] EXAMPLES
[0072] Quatre éléments électrochimiques Li/S (piles boutons de format 2032) référencés A à D de 5 mAh de capacité chacun ont été fabriqués. Ils diffèrent par la composition de leur électrolyte. Les différentes compositions d’électrolytes utilisées sont indiquées dans le Tableau 1 ci-après. Four Li/S electrochemical cells (2032 format button cells) referenced A to D with a capacity of 5 mAh each were manufactured. They differ in their electrolyte composition. The different compositions of electrolytes used are indicated in Table 1 below.
[0073] Le composite de soufre et de carbone a été préparé en mélangeant le carbone avec le soufre élémentaire solide dans un broyeur à billes. Le carbone utilisé est du charbon actif avec un diamètre moyen des pores de 0,77 nm. Le mélange a été chauffé à une température de 155°C pendant environ 5 heures, sous vide pour permettre aux molécules de soufre de pénétrer dans les pores du carbone. Le chauffage du mélange s’est poursuivi à une température de 300°C pendant 30 minutes sous un flux de gaz inerte afin d’éliminer l’excès de soufre. La teneur en soufre dans le composite carbone- soufre préparé est mesurée par Analyse Thermogravimétrique (TGA) à l’aide d’un appareil TA Instrument Q500. La quantité de soufre est de 1,9 mg par cm2 de cathode. L’anode est un ruban de lithium. [0073] The composite of sulfur and carbon was prepared by mixing carbon with solid elemental sulfur in a ball mill. The carbon used is activated carbon with an average pore diameter of 0.77 nm. The mixture was heated at a temperature of 155°C for about 5 hours, under vacuum to allow the sulfur molecules to penetrate into the pores of the carbon. Heating of the mixture was continued at a temperature of 300°C for 30 minutes under a flow of inert gas in order to remove excess sulfur. The sulfur content in the carbon-sulfur composite prepared is measured by Thermogravimetric Analysis (TGA) using a TA Instrument Q500 device. The amount of sulfur is 1.9 mg per cm 2 of cathode. The anode is a lithium ribbon.
[0074] Pour obtenir l’électrolyte gélifié, on a d’abord préparé un mélange liquide en mélangeant FEC avec DMC dans un rapport volumique de 1 à 4 et on y a dissous LiPFe ou LiFSI à la concentration de 1 mol.L'1. Puis, on a chauffé ce mélange et on y a incorporé le polymère P(VDF-HFP) en une quantité représentant 10% de la masse du mélange liquide. Le même volume d’électrolyte a été introduit dans les différents éléments (soit sous forme liquide pour les éléments A et C, soit sous forme gélifiée pour les éléments B et D). To obtain the gelled electrolyte, a liquid mixture was first prepared by mixing FEC with DMC in a volume ratio of 1 to 4 and LiPFe or LiFSI was dissolved therein at a concentration of 1 mol. . Then, this mixture was heated and the polymer P(VDF-HFP) was incorporated therein in an amount representing 10% of the mass of the liquid mixture. The same volume of electrolyte was introduced into the various elements (either in liquid form for elements A and C, or in gelled form for elements B and D).
[0075] [Tableau 1]
Figure imgf000013_0001
[0075] [Table 1]
Figure imgf000013_0001
* exemple hors invention * example excluding invention
A) Comparaison des capacités déchargées : A) Comparison of discharged capacities:
[0076] Les éléments A à D ont subi des décharges aux régimes de C/10, C/5, C/2, C et 2C à température ambiante jusqu’à la tension de coupure de 1 V. Les courbes de décharge de ces éléments ont été représentées Figures 2A à 2D. Si on compare la capacité déchargée de l’élément B avec celle de l’élément A, on constate qu’elles sont voisines pour un courant de décharge allant jusqu’à C. Elles sont d’environ 600 mAh/g de soufre. Pour un courant de décharge de 2C, la capacité de l’élément B commence à être notablement inférieure à celle de l’élément A (510 mAh/g de soufre pour l’élément B en comparaison avec 550 mAh/g de soufre pour l’élément A). Si on compare la capacité déchargée de l’élément D avec celle de l’élément C, on constate qu’elles sont voisines pour un courant de décharge allant jusqu’à C. Pour un courant de décharge de 2C, la capacité de l’élément D est notablement supérieure à celle de l’élément C (590 mAh/g de soufre pour l’élément D en comparaison avec 540 mAh/g de soufre pour l’élément C). L’élément D qui comporte un électrolyte gélifié montre des performances au moins égales voire supérieures à celles de l’élément C. Ce résultat est surprenant étant donné qu’un électrolyte gélifié pénalise généralement les performances en puissance de l’élément en comparaison avec un élément comportant un électrolyte liquide. Les résultats des Figures 2A à 2D montrent également la supériorité de LiFSI par rapport à LiPFe en termes de décharge sous fort courant. Elements A to D underwent discharges at rates of C/10, C/5, C/2, C and 2C at ambient temperature down to the cut-off voltage of 1 V. The discharge curves of these elements have been shown in Figures 2A through 2D. If we compare the discharged capacity of element B with that of element A, we see that they are close for a discharge current up to C. They are approximately 600 mAh/g of sulphur. For a discharge current of 2C, the capacity of cell B begins to be significantly lower than that of cell A (510 mAh/g sulfur for cell B compared to 550 mAh/g sulfur for cell A). item A). If we compare the discharged capacitance of element D with that of element C, we see that they are close for a discharge current up to C. For a discharge current of 2C, the capacitance of the element D is significantly higher than that of element C (590 mAh/g sulfur for element D compared to 540 mAh/g sulfur for element C). Element D, which includes a gelled electrolyte, shows performances at least equal or even superior to those of element C. This result is surprising given that a gelled electrolyte generally penalizes the power performance of the element in comparison with a element comprising a liquid electrolyte. The results of Figures 2A to 2D also show the superiority of LiFSI compared to LiPFe in terms of high current discharge.
B) Comparaison des durées de vie en cyclage : B) Comparison of cycle life:
[0077] Les éléments A à D ont subi un cyclage à température ambiante au régime de C/10. On a représenté sur la figure 3 la variation du pourcentage de rétention de capacité des éléments A et B en fonction du nombre de cycles et sur la figure 4 la variation du pourcentage de rétention de capacité des éléments C et D. On constate que la baisse de capacité de l’élément B est moindre que celle de l’élément A et que la baisse de capacité de l’élément D est moindre que celle de l’élément C. Ces essais démontrent donc l’amélioration apportée par le remplacement d’un électrolyte liquide par un électrolyte gélifié comprenant du P(VDF-HFP). La durée de vie est améliorée pour un volume d’électrolyte constant, en raison d’une meilleure interface électrolyte-anode due à l’utilisation du gel. Elements A to D underwent cycling at ambient temperature at C/10. FIG. 3 shows the variation in the percentage of capacity retention of elements A and B as a function of the number of cycles and in FIG. 4 the variation in the percentage of capacity retention of elements C and D. capacity of element B is less than that of element A and that the drop in capacity of element D is less than that of element C. These tests therefore demonstrate the improvement brought about by the replacement of an electrolyte liquid with a gelled electrolyte comprising P(VDF-HFP). The life is improved for a constant electrolyte volume, due to a better electrolyte-anode interface due to the use of the gel.

Claims

Revendications Claims
[Revendication 1] Elément électrochimique lithium-soufre comprenant : a) une cathode comprenant : i) du carbone présentant une structure poreuse comprenant des pores avec un diamètre moyen inférieur ou égal à 6 nm ; ii) du soufre avec un degré d’oxydation pouvant aller de 0 à -2 dans les pores du carbone ; b) un électrolyte gélifié comprenant une matrice qui est un polymère de poly(fluorure de vinylidène-co-hexafluoropropylène) dans laquelle est incorporé un mélange liquide comprenant au moins un sel de lithium et un solvant, le solvant comprenant au moins un carbonate fluoré pouvant être cyclique ou linéaire ; c) une anode comprenant du lithium métal ou un alliage métallique de lithium. [Claim 1] A lithium-sulfur electrochemical cell comprising: a) a cathode comprising: i) carbon having a porous structure comprising pores with an average diameter less than or equal to 6 nm; ii) sulfur with an oxidation state ranging from 0 to -2 in the carbon pores; b) a gelled electrolyte comprising a matrix which is a poly(vinylidene fluoride-co-hexafluoropropylene) polymer in which is incorporated a liquid mixture comprising at least one lithium salt and a solvent, the solvent comprising at least one fluorinated carbonate which may be cyclical or linear; c) an anode comprising lithium metal or a lithium metal alloy.
[Revendication 2] Elément électrochimique selon la revendication 1, dans lequel ledit au moins un carbonate fluoré est choisi dans le groupe consistant en le monofluorocarbonate d’éthylène (FEC), le difluorocarbonate d’éthylène, le trifluorométhylcarbonate d’éthylène, le monofluorocarbonate de propylène, le difluorocarbonate de propylène, le trifluorocarbonate de propylène et un mélange de ceux-ci. [Claim 2] An electrochemical cell according to claim 1, wherein said at least one fluorinated carbonate is selected from the group consisting of ethylene monofluorocarbonate (FEC), ethylene difluorocarbonate, ethylene trifluoromethylcarbonate, ethylene monofluorocarbonate propylene, propylene difluorocarbonate, propylene trifluorocarbonate and a mixture thereof.
[Revendication 3] Elément électrochimique selon la revendication 2, dans lequel ledit au moins un carbonate fluoré est le monofluorocarbonate d’éthylène. [Claim 3] An electrochemical cell according to claim 2, wherein said at least one fluorinated carbonate is ethylene monofluorocarbonate.
[Revendication 4] Elément électrochimique selon l’une des revendications 1 à 3, dans lequel ledit au moins un carbonate fluoré représente de 5 à 75% du volume du solvant, de préférence de 5 à 50%, de préférence encore de 10 à 30%. [Claim 4] Electrochemical element according to one of Claims 1 to 3, in which the said at least one fluorinated carbonate represents from 5 to 75% of the volume of the solvent, preferably from 5 to 50%, more preferably from 10 to 30 %.
[Revendication 5] Elément électrochimique selon l’une des revendications précédentes, dans lequel ledit au moins un carbonate fluoré est cyclique et est associé à au moins un carbonate linéaire non fluoré. [Claim 5] Electrochemical element according to one of the preceding claims, in which the said at least one fluorinated carbonate is cyclic and is associated with at least one non-fluorinated linear carbonate.
[Revendication 6] Elément électrochimique selon la revendication 5, dans lequel ledit au moins un carbonate linéaire non fluoré est le carbonate de diméthyle (DMC). [Claim 6] An electrochemical cell according to claim 5, wherein said at least one non-fluorinated linear carbonate is dimethyl carbonate (DMC).
[Revendication 7] Elément électrochimique selon l’une des revendications 5 à 6, dans lequel le solvant est constitué de 10 à 30 % en volume dudit au moins un carbonate cyclique fluoré et de 90 à 70% en volume dudit au moins un carbonate linéaire non fluoré. [Claim 7] Electrochemical element according to one of Claims 5 to 6, in which the solvent consists of 10 to 30% by volume of the said at least one fluorinated cyclic carbonate and of 90 to 70% by volume of the said at least one linear carbonate non-fluorinated.
[Revendication 8] Elément électrochimique selon l’une des revendications précédentes, dans lequel le sel de lithium est choisi parmi l’hexafluorophosphate de lithium LiPFe, le bis(fluorosulfonyl)imide de lithium Li(FSO2)2N (LiFSI) et un mélange de ceux-ci. [Claim 8] Electrochemical cell according to one of the preceding claims, in which the lithium salt is chosen from lithium hexafluorophosphate LiPFe, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI) and a mixture of these.
[Revendication 9] Elément électrochimique selon l’une des revendications précédentes, dans lequel la matrice de polymère de poly(fluorure de vinylidène-co- hexafluoropropyl ène) représente de 3 à 25 % en masse de la masse du mélange liquide. [Claim 9] Electrochemical cell according to one of the preceding claims, in which the polymer matrix of poly(vinylidene fluoride-co- hexafluoropropylene) represents from 3 to 25% by mass of the mass of the liquid mixture.
[Revendication 10] Elément électrochimique selon l’une des revendications précédentes, dans lequel le carbone poreux est sous la forme d’une poudre dont la granulométrie est caractérisée par un diamètre médian en volume Dvso allant de 1 à 10 pm, de préférence allant de 1 à 5 pm. [Claim 10] Electrochemical element according to one of the preceding claims, in which the porous carbon is in the form of a powder whose particle size is characterized by a median diameter by volume Dvso ranging from 1 to 10 μm, preferably ranging from 1 to 5 p.m.
[Revendication 11] Elément électrochimique selon l'une des revendications précédentes, dans lequel le soufre est du soufre solide élémentaire et: [Claim 11] Electrochemical element according to one of the preceding claims, in which the sulfur is elemental solid sulfur and:
- la masse de soufre solide élémentaire représente de 30 à 90 % de la somme des masses du carbone et du soufre solide élémentaire, - the mass of elementary solid sulfur represents 30 to 90% of the sum of the masses of carbon and elementary solid sulfur,
- la masse de carbone représente de 70 à 10 % de la somme des masses du carbone et du soufre solide élémentaire. - the mass of carbon represents 70 to 10% of the sum of the masses of carbon and elemental solid sulphur.
[Revendication 12] Elément électrochimique selon la revendication 11, dans lequel : [Claim 12] An electrochemical cell according to claim 11, wherein:
- la masse de soufre solide élémentaire représente de 40 à 60 % de la somme des masses du carbone et du soufre solide élémentaire, - the mass of elementary solid sulfur represents 40 to 60% of the sum of the masses of carbon and elementary solid sulfur,
- la masse de carbone représente de 60 à 40 % de la somme des masses du carbone et du soufre solide élémentaire. - the mass of carbon represents 60 to 40% of the sum of the masses of carbon and elemental solid sulphur.
[Revendication 13] Elément électrochimique selon l’une des revendications précédentes, dans lequel : [Claim 13] Electrochemical cell according to one of the preceding claims, in which:
- les pores de la cathode présentent un diamètre moyen inférieur ou égal à 1 nm ; - the pores of the cathode have an average diameter less than or equal to 1 nm;
- la matrice de polymère de poly(fluorure de vinylidène-co- hexafluoropropylène) représente de 5 à 15 % en masse de la masse du mélange liquide ; - the poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix represents from 5 to 15% by mass of the mass of the liquid mixture;
- le solvant est constitué de 10 à 30 % en volume de monofluorocarbonate d’éthylène (FEC) et de 90 à 70 % en volume de carbonate de diméthyle (DMC) ; - the solvent consists of 10 to 30% by volume of ethylene monofluorocarbonate (FEC) and 90 to 70% by volume of dimethyl carbonate (DMC);
- le sel de lithium est choisi parmi l’hexafluorophosphate de lithium LiPFe, le bis(fluorosulfonyl)imide de lithium Li(FSO2)2N (LiFSI) et un mélange de ceux-ci. - the lithium salt is chosen from lithium hexafluorophosphate LiPFe, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI) and a mixture thereof.
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FR3117679A1 (en) 2022-06-17
FR3117679B1 (en) 2023-04-21

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