Electrochemical element comprising a positive electrode based on lithium manganese and iron phosphate

Abstract

An electrochemical lithium-ion element comprising: - a positive electrode comprising one or more positive active materials, one of which is a lithium manganese iron phosphate; - a negative electrode comprising one or more negative active materials, one of which is a silicon carbon composite; - an electrolyte comprising a mixture of solvents and one or more lithium salts dissolved in the solvent mixture, the solvent mixture comprising at least one fluorinated cyclic carbonate, the other solvent(s) being selected from non-fluorinated cyclic carbonates or non-fluorinated linear carbonates, the volume proportion of the fluorinated cyclic carbonate representing 10 to 30% of the total solvent volume, at least one lithium salt being lithium bis(fluorosulfonyl)imidide Li(FSO2)2N. Abbreviated figure: Fig. 1a

Description

Title of the invention: Electrochemical element comprising a positive electrode based on lithium manganese and iron phosphate Technical field of the invention

[0001] The technical field of the present invention is that of electrochemical elements comprising a positive electrode based on lithium manganese and iron phosphate. Background to the invention

[0002] Rechargeable electrochemical cells of the lithium-ion type are very widely used as energy storage devices to power portable electronic devices such as laptops, mobile phones or to power electric or hybrid vehicles.

[0003] Electrochemical elements comprising a positive electrode whose active material is a lamellar lithium oxide and comprising a negative electrode whose active material is graphite are known from the state of the art. They have a high specific energy but they can prove unstable when subjected to overcharging or overheating. This is why attempts have been made to improve their safety of use in the event of overcharging or overheating. For example, it has been proposed to replace part of a lithium oxide of nickel, manganese and cobalt, with a lithium phosphate of manganese LiMnPO4 or a lithium phosphate of iron LiFePO4> or with a lithium phosphate of manganese and iron LiMni xFexPO4 with x <l (LMFP). Le document WO 2016 / 184896 décrit un tel élément. L’élément électrochimique résultant de ce remplacement partiel présente une sécurité d’utilisation satisfaisante.However, the increase in safety of use is achieved at the expense of the specific energy of the element. By increasing the ratio between the quantity of lithium phosphate and the quantity of lithium oxide of nickel, manganese and cobalt, a cell is obtained which has satisfactory safety of use, but whose specific energy does not reach that of an element whose positive electrode contains a lithium oxide of nickel, manganese and cobalt as the only active material.

[0004] [Fig. 1a] compares the specific energies of different elements 1-3 of the prior art. These differ in the composition of their positive electrode. The negative electrode of elements 1-3 is made of graphite. Element 1 comprising a positive electrode based on a lithium oxide of nickel, manganese and cobalt of formula LiNiO.8MnO.1CoO.102 (NMC811) has a specific energy greater than 300 Wh / kg, which is high, but this element has insufficient safety of use in the event of overheating or overload. Element 2 comprising an electrode positive based on a lithium manganese and iron phosphate has a specific energy of only 240 Wh / kg and satisfactory safety of use. Element 3, whose positive electrode comprises a mixture of a lithium manganese and iron phosphate and a lithium nickel manganese and cobalt oxide of formula NMC811, has satisfactory safety of use but a specific energy lower than that of element 1.

[0005] [Fig.lb] is a comparison of the volume energies of the three elements 1, 2 and 3 of the prior art. It can be seen that the volume energies of these three elements are in the same order as that of the mass energies of [Fig.la].

[0006] It is therefore sought to improve the mass energy and / or the volume energy of an element whose positive electrode contains a mixture of LMFP and a lamellar lithium oxide and whose negative electrode contains graphite, while maintaining satisfactory safety for the user. Preferably, it is desired that the improvement in the mass energy and / or the volume energy does not come at the expense of the performance of the element in cycling. Summary of the invention

[0007] To this end, the present invention provides a lithium-ion electrochemical element comprising: - a positive electrode comprising one or more positive active materials, one of which is a lithium manganese and iron phosphate of formula LixMni y zFey MzP04 (LMFP) where 0.8 <x<l,2 ; 0,50<l-y-z<l ; 0<y+z<0,50 ; 0<y<0,5 ; 0<z<0,2 ; M étant un ou plusieurs éléments choisis dans le groupe consistant en Al, B, Mg, K, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W, Mo, Pb et S, - a negative electrode comprising one or more negative active materials, one of which is a composite of silicon and carbon, - an electrolyte comprising a mixture of solvents and one or more lithium salts dissolved in the mixture of solvents, the mixture of solvents comprising at least one fluorinated cyclic carbonate, the other solvent(s) being chosen from non-fluorinated cyclic carbonates or non-fluorinated linear carbonates, the volume proportion of fluorinated cyclic carbonate representing from 10 to 30% of the total volume of the solvents, at least one lithium salt being lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI).

[0008] The invention is based on the discovery that three specific constituents of the element cooperate with each other to improve on the one hand the mass energy and / or the volume energy without excessively penalizing the cycling performance of the element. These three constituents are the electrolyte, the negative active material and the positive active material, as described above. The invention makes it possible, for example, to increase 13% and 22% respectively the mass energy and volume energy of an element comprising a positive electrode comprising a mixture of LMFP and NMC 811 and a graphite-based negative electrode. This improvement is highlighted by the bar corresponding to element 4 of the bar diagrams of figures 1a and 1b.

[0009] According to one embodiment, the silicon and carbon composite consists of a carbon matrix in which silicon particles are incorporated, the largest dimension of the silicon particles being less than or equal to 500 nm.

[0010] According to one embodiment, the silicon and carbon composite consists of a porous carbon matrix, silicon particles partially or totally covering the surface of the pores of the carbon matrix, the diameter of the pores of the carbon matrix ranging from 10 nm to 1 μm.

[0011] According to one embodiment, the porous carbon matrix is ​​made up of carbon particles, the pores of the porous carbon matrix representing from 15 to 90% of the volume of the carbon particles, the volume of the carbon particles being delimited by the outer surface of the particles.

[0012] According to one embodiment, the negative electrode further comprises a second active graphite material, the graphite representing from 10 to 90% of the total mass of the active materials of the negative electrode, the silicon and carbon composite representing from 90 to 10% of the total mass of the active materials of the negative electrode.

[0013] According to one embodiment, the fluorinated cyclic carbonate is ethylene fluorocarbonate (FEC).

[0014] According to one embodiment, the solvent mixture comprises from 1 to 20% by volume of non-fluorinated cyclic carbonate.

[0015] According to one embodiment, the solvent mixture consists of ethylene fluorocarbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC).

[0016] According to one embodiment, the volume proportions of FEC, EC, DMC and EMC are located in the following respective ranges: 15-25%, 5-15%, 40-50% and 20-30%.

[0017] According to one embodiment, the lithium ions from the lithium bis(fluorosulfonyl)imide salt Li(FSO2)2N (LiFSI) represent from 50 to 99%, preferably from 90 to 99% of the total lithium ions provided by the lithium salts.

[0018] According to one embodiment, the lithium bis(fluorosulfonyl)imide salt Li(FSO2)2 N (LiFSI) is combined with at least one other salt chosen from lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4 and lithium difluoro(oxalato)borate (LIDFOB).

[0019] According to one embodiment, the lithium bis(fluorosulfonyl)imide salt Li(FSO2)2 N (LiFSI) is the only lithium salt in the electrolyte.

[0020] According to one embodiment, the total concentration of lithium ions provided by the or lithium salts ranges from 0.9 to 1.3 moles of lithium per liter of solvent(s).

[0021] According to one embodiment, the positive electrode further comprises one or more positive active materials chosen from: a) a lithium oxide of nickel, manganese and cobalt of formula Liw(NixMnyCozMt)O2(NMC) where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci, b) a lithium oxide of nickel, cobalt and aluminum of formula Liw(NixCoyAlzMt)O2(NCA) where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de B, Mg, Si, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, Sr, Ce, Ga, Ta, Nd, Pr, La et des mélanges de ceux-ci, c) a compound of formula Lii+xMi_xO2_yFy of cubic crystal structure where M represents at least one element selected from the group consisting of Na, K, Mg, Ca, B, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb, Mo, Ru, Ag, Sn, Sb, Ta, W, Bi, La, Pr, Eu, Nd and Sm and where 0 < x < 0.5 and 0 < y < 1, d) a lithium oxide of nickel and manganese (NMX) of formula Lia(Nii_x y zMnxCoyMz)O2 with 0.9 <a<l,l ; 0,60<l-x-y-z<0,80 ; 0<x ; 0<y<0,02 ; 0<z ; et M étant choisi dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ga, Ta, Nd, Pr, La et des mélanges de ceux-ci ; e) a lithium oxide of nickel and manganese of formula Liw(NixMnyCozMt)O2 where 1.1 <w<1,6 ; 0<x ; 0,50<y<0,80 ; 0<z<0,02 ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci. Brief description of the drawings

[0022] Embodiments of the invention are described below in more detail with reference to the accompanying figures.

[0023] [Fig. 1a] is a comparison of the mass energies of three elements 1, 2 and 3 of the prior art and an element 4 according to the invention.

[0024] [Fig.lb] is a comparison of the volume energies of three elements 1, 2 and 3 of the prior art and of an element 4 according to the invention.

[0025] [Fig.2a] represents the capacity retention of elements A, B and C as a function of the number of cycles. Elements A, B and C differ in the composition of the electrolyte solvent mixture.

[0026] [Fig.2b] represents the capacity retention of elements B and D as a function of the number of cycles. Elements B and D differ in the composition of the electrolyte salts.

[0027] [Fig.3] represents the capacity retention of elements D and E as a function of the number of cycles. Elements D and E differ in the type of negative electrode composite.

[0028] [Fig.4] represents the capacity retention of the F and G elements as a function of the number of cycles. The F and G elements differ in the composition of the electrolyte salts.

[0029] The cycling of Figures 2a, 2b and 3 consists of charges at the C / 3 regime and discharges at the D / 3 regime at room temperature. The cycling of [Fig.4] consists of charges at the C / 2 regime and discharges at the D / 2 regime at room temperature.

[0030] Description of the embodiments of the invention Negative electrode:

[0031] The Si-C composite is defined as a heterogeneous mixture consisting of silicon and carbon particles, all of the particles having a high capacity for interpenetration and adhesion. It is therefore not silicon carbide SiC, which is a homogeneous solid and in which each silicon atom is linked to a carbon atom by a covalent bond. It is therefore also not an alloy which consists of a metallic mixture of several elements.

[0032] Two types of composite may be used. A first type consists of silicon nanoparticles embedded in a carbon matrix. By nanoparticles is meant particles whose largest dimension is less than or equal to 500 nm. In certain cases, the largest dimension may be less than or equal to 100 nm. The mass of silicon nanoparticles may represent from 5% to 60% of the mass of the carbon matrix. The first type of composite may be obtained by a method comprising the steps of: (a) provide a silicon powder obtained by the plasma-enhanced chemical vapor deposition (PECVD) technique or by CO2 laser, the size of the silicon particles being less than or equal to 500 nm or less than or equal to 100 nm; b) mixing the silicon powder with a carbon polymer; and c) carrying out the pyrolysis of the mixture. A solvent suitable for dissolving the carbon-containing polymer may be added to the mixture in step b), and the mixture is then kept under mechanical stirring until the solvent has evaporated. The solvent may be propylene oxide. The carbon-containing polymer may be polyvinyl chloride (PVC). Pyrolysis may be carried out at a temperature between 750°C and 1000°C.

[0033] The Si-C composite can also be obtained by mechanical grinding of silicon particles and carbon particles, for example using a ball mill. The mechanical grinding is continued for a time sufficient to obtain the required level of adhesion between the silicon particles and the carbon particles. The grinding is stopped after a time from which the start of the formation of covalent bond between carbon and silicon. The presence of silicon carbide can be demonstrated by RAMAN spectroscopy or X-ray diffraction. Typically, grinding in a ball mill can last from 2 to 24 hours or from 2 to 12 hours.

[0034] A second type of composite consists of silicon particles housed in pores of a carbon matrix, the pores being able to be located on the surface of the carbon matrix or in the carbon matrix. The silicon particles can partially or totally cover the surface of the pores of the carbon matrix. The carbon pores can have a diameter ranging from 10 nm to 1 pm. To obtain this second type of composite, a porous carbon matrix is ​​manufactured and then silicon is deposited in the pores of the carbon matrix. The silicon deposition is obtained by decomposing a silane gas of formula SiH4 into the element silicon and hydrogen. The decomposition of the silane is initiated by an increase in temperature, generally above 400°C. The silicon particles can be nanoparticles or have a larger size.

[0035] In these two types of composite, the silicon and carbon used can be independently amorphous or crystalline.

[0036] The mass proportion of silicon in the composite typically ranges from 5 to 65% or from 25 to 55% or from 30 to 50% or from 35 to 50%. The mass proportion of carbon in the composite typically ranges from 35 to 95% or from 45 to 75% or from 50 to 70% or from 50 to 65%.

[0037] According to one embodiment, the Si-C composite does not comprise a coating layer, in particular a carbon coating layer.

[0038] In contrast to the use of silicon particles, the applicant observed that the Si-C composite underwent smaller volume variations than when silicon was used as the negative active material. It is indeed known that when silicon was used as the negative active material, large variations in its volume were observed. These large variations were caused by the alloying of lithium in the structure of the silicon during the charging and discharging of the cell. These repeated volume variations resulted in the external surface of the silicon crumbling and the periodic renewal of a fresh external surface of silicon. This freshly renewed external surface reacted with the electrolyte and decomposed it. This reaction had the effect of consuming electrolyte and reducing the cycling performance of the cell.One of the advantages of the invention is therefore to reduce, thanks to the use of the Si-C composite, the phenomenon of crumbling of the negative active material during operation of the element in cycling.

[0039] The Si-C composite may be combined with one or more other negative active materials, such as graphite. The mass proportion of Si-C composite may range from 10 to 90% or from 30 to 60% of the mass of all negative active materials. The mass proportion of graphite may range from 10 to 90% or from 50 to 70% of the mass of all negative active materials. Preferably, the Si-C composite and the graphite are the only two negative active materials.

[0040] According to one embodiment, the negative electrode does not contain a silicon alloy. According to one embodiment, the negative electrode does not contain silicon oxide SiOx with 0 <x<2. Selon un mode de réalisation, l’électrode négative ne contient pas de particules de silicium libres, c’est-à-dire non liées à des particules de carbone.

[0041] One or more electronically conductive additives may be added to the Si-C composite. These additives may be selected from graphite, carbon black, soot, acetylene black, graphene, carbon fibers, single-walled or multi-walled carbon nanotubes, and mixtures thereof.

[0042] One or more binders may also be added. They may be chosen from poly(vinylidene fluoride) (PVDF) and its copolymers, polytetrafluoroethylene (PTFE) and its copolymers, polyacrylonitrile (PAN), poly(methyl or butyl methacrylate), poly(vinyl chloride) (PVC), poly(vinyl formal), polyester, block polyether-amides, polymers of acrylic acid, methacrylic acid, acrylamide, itaconic acid, sulfonic acid, elastomers, and cellulosic compounds such as carboxymethylcellulose (CMC). The elastomers that can be used as binders can be chosen from styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), hydrogenated butadiene-acrylonitrile (HNBR).

[0043] Preferably, said at least one binder is a binder dispersible in an aqueous medium, such as polytetrafluoroethylene (PTFE), carboxymethylcellulose (CMC), styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), hydrogenated butadiene-acrylonitrile (HNBR) and polyvinyl alcohol (PVA). Preparation of the negative electrode:

[0044] The negative electrode is prepared in a conventional manner. An ink is prepared by dispersing in a solvent or in a mixture of solvents the Si-C composite optionally associated with one or more negative active materials, one or more binders and one or more electronically conductive additives.

[0045] The ink-coated current collector is dried and then rolled so that its thickness is adjusted. This results in a negative electrode. Typical proportions of the components of the negative active ingredient composition layer, after evaporation of the solvent contained in the ink, are: - from 85 to 98% or from 90 to 98% by mass of negative active materials, - from 1 to 10% or from 1 to 5% by mass of binder(s), - from 0 to 5% by mass or from 1 to 5% of an electronically conductive material. Positive electrode:

[0046] The positive electrode of the element comprises a current collector, at least one of the faces of which is coated with a layer of a composition of positive active materials. By "composition of active materials" is meant a composition comprising the mixture of active materials and optionally one or more binders and one or more electronically conductive materials.

[0047] The positive current collector is a solid or perforated metal strip which may be made of aluminum or an aluminum alloy or steel or stainless steel. Its thickness may be in the range of 6 to 30 μm or 5 to 20 μm or 10 to 15 μm, preferably 10 to 15 μm.

[0048] Before the current collector is coated with the active material composition layer, it may be coated, on one or both sides, with a coating intended to improve the electronic conductivity between the active material composition layer and the foil and / or to improve the adhesion of the active material composition layer to the foil. The coating material may be selected from the group consisting of amorphous carbon, graphite, carbon fibers, carbon nanotubes, and mixtures thereof. It is preferably made of amorphous carbon. The coating material may be obtained by coating the foil with a dispersion of the material and then evaporating the solvent from the dispersion, or it may be obtained by sputtering. Only certain portions of the foil may be coated with the coating material.The coated portions may be separated from each other by predetermined or periodic intervals ("intermittent coating"). The intermittent coating is preferably present on both sides of the foil. It facilitates the processing of the electrodes. Alternatively, one or both sides of the foil may have undergone a surface treatment intended to increase the adhesion of the active material composition layer to the foil. This may be a surface treatment creating asperities or micro-roughness, such as chemical etching or laser treatment.

[0049] The mixture of active materials comprises at least one lithium manganese and iron phosphate of formula LixMni y zFeyMzPO4 (LMFP) where 0.8 <x<l,2 ; 0,5<l-y-z<l ; 0<y+z<0,50 ; 0<y<0,5 ; 0<z<0,2 ; M étant un ou plusieurs éléments choisis dans le groupe consistant en Al, B, Mg, K, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W, Pb, Mo et S.

[0050] The stoichiometric index 1-yz of manganese may be at least equal to 0.6 or at least equal to 0.7 or at least equal to 0.8. It may range from 0.7 to 0.9 or from 0.75 to 0.80. Preferably, 0.70 <l-y-z<0,95.

[0051] Examples of LMFP compounds are LiMn0j8Feoj2P04, LiMn0j78Fe0j22PO4, LiMn0j7Fe 0j3PO4, LiMn2 / 3Fei / 3P04jLiMnoj6Feoj4P04 and LiMno.5Fco.5PO4.

[0052] LMFP may be coated with carbon or carbon nanotubes.

[0053] LMFP can be either in the form of disjoint particles, also called primary particles, or in the form of agglomerates of primary particles, also called secondary particles. LMFP can be in the form of secondary particles with a volume median diameter Dv50 in the range of 1 to 15 pm or 1.5 to 4 pm. DV5o is measured by laser diffraction.

[0054] LMFP can further be mixed with one or more positive active ingredients chosen from: a) a lithium oxide of nickel, manganese and cobalt of formula Liw(NixMnyCozMt)O2(NMC) where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci, b) un oxyde lithié de nickel, cobalt et aluminium de formule Liw(NixCoyAlzMt)O2(NCA) where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de B, Mg, Si, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, Sr, Ce, Ga, Ta, Nd, Pr, La et des mélanges de ceux-ci. c) a compound of formula Lii+xMi.xO2.yFy of cubic crystal structure where M represents at least one element selected from the group consisting of Na, K, Mg, Ca, B, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb, Mo, Ru, Ag, Sn, Sb, Ta, W, Bi, La, Pr, Eu, Nd and Sm and where 0 < x < 0.5 and 0 < y < 1, d) a lithium oxide of nickel and manganese (NMX) of formula Lia(Nii.xyzMnxCoyMz)O2 with 0.9 <a<l,l ; 0,60<l-x-y-z<0,80 ; 0<x ; 0<y<0,02 ; 0 <z ; et M étant choisi dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ga, Ta, Nd, Pr, La et des mélanges de ceux-ci ; e) a lithium oxide of nickel and manganese of formula Liw(NixMnyCozMt)O2 where 1.1 <w<1,6 ; 0<x ; 0,50<y<0,80 ; 0<z<0,02 ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci.

[0055] Preferably, it is a lithium oxide of nickel, manganese and cobalt (NMC). Examples of NMC type compounds are LiNii / 3Mni / 3Coi / 3O2, LiNio>6Mn0j2Cooj202, LiNioj8oMnojioCoojio02, LiNio^Mno^Coo^O^ LiNio>87Mnojo6Coojo702 and LiNio^Mno^Co 0.05O2.

[0056] The addition of one or more of the active materials a) to e) to the LMFP makes it possible to increase the mass capacity of the positive electrode.

[0057] LMFP may be the majority in the mixture of positive active ingredients. The proportions of LMFP and the second active ingredient(s) a) to e) may be as follows: - from 95 to 50% by mass of LMFP and from 5 to 50% by mass of the second active ingredient(s) a) to e), or - from 90 to 60% by mass of LMFP and from 10 to 40% by mass of the second active ingredient(s) a) to e), or - from 80 to 70% by mass of LMFP and from 20 to 30% by mass of the second active ingredient(s) a) to e).

[0058] LMFP may be a minority in the mixture of positive active ingredients. The proportions of LMFP and the second active ingredient(s) a) to e) may be as follows: - from 5 to 50% by mass of LMFP and from 50 to 95% by mass of the second active ingredient(s) a) to e), or - from 10 to 40% by mass of LMFP and from 60 to 90% by mass of the second active ingredient(s) a) to e), or - from 20 to 30% by mass of LMFP and from 70 to 80% by mass of the second active ingredient(s) a) to e).

[0059] A possible mixture of active ingredients consists of 35 to 45% by mass of LMFP and 55 to 65% by mass of the second active ingredient(s) a) to e). The second active ingredient is preferably NMC. A possible mixture consists of 40% by mass of LMFP and 60% by mass of NMC.

[0060] The binder(s) used in the composition of positive active materials may be chosen from the same list as that mentioned in relation to the composition of negative active materials. Within this same list, the binder(s) of the positive electrode may be different from the binder(s) of the negative electrode. Similarly, the electronically conductive compound(s) used in the composition of positive active materials may be chosen from the same list as that mentioned in relation to the composition of negative active materials. Preparation of the positive electrode:

[0061] An ink is prepared by dispersing one or more active materials of LMFP type and optionally one or more active materials of type a) to e) in a solvent or in a mixture of several solvents. Optionally, a binder and an electronically conductive material are added to the dispersion. By varying the amount of solvent incorporated in the mixture, the viscosity of the ink can be varied before it is deposited on one face of the current collector. The ink-coated current collector is dried and then rolled so that its thickness is adjusted. After evaporation of the solvent(s), a layer of a composition of active materials is obtained, the proportions of the various constituents of which are typically: - from 80 to 98% or from 90 to 95% by mass of positive active ingredients, - from 1 to 10% or from 2 to 5% by mass of binder(s), - from 0.1 to 10% or from 2 to 5% by mass of electronically conductive material. Electrolyte:

[0062] The electrolyte is liquid and is obtained by dissolving lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI) and optionally other lithium salts in the solvent mixture. The solvent mixture comprises at least one fluorinated cyclic carbonate, the other solvents in the mixture being chosen from non-fluorinated cyclic carbonates and non-fluorinated linear carbonates. Preferably, the fluorinated cyclic carbonate is ethylene fluorocarbonate (FEC). According to the invention, the volume proportion of fluorinated cyclic carbonate represents from 10 to 30% or from 15 to 25% or from 20 to 25% of the total volume of the solvents. Beyond 20% by volume of fluorinated cyclic carbonate, no significant increase in the lifetime of the element is observed.

[0063] According to one embodiment, the solvent mixture comprises from 5 to 70% by volume of one or more non-fluorinated linear carbonates.

[0064] Among the non-fluorinated cyclic carbonates, mention may be made of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and mixtures thereof.

[0065] Among the non-fluorinated linear carbonates, mention may be made of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC) and mixtures thereof. The mixture of DMC and EMC is preferred.

[0066] A preferred mixture of solvents consists of EC, FEC, DMC and EMC. The volume percentage ranges of these different solvents can be: 5-15%, 15-25%, 40-50% and 20-30%.

[0067] According to one embodiment, the mixture of solvents contains neither ester nor ether.

[0068] Lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI) may be combined with one or more salts selected from lithium perchlorate LiC104, lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium hexafluoroarsenate LiAsF6, lithium hexafluorantimonate LiSbF6, lithium trifluoromethanesulfonate LiCF3SO3, lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium tris(fluoromethanesulfonyl)methylide LiC(CF3SO2)3 (LiTFSM), lithium bis(pentafluoroethylsulfonyl)imide LiN(C2F5SO2)2 (LiBeTI), lithium-4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LIDFOB), lithium tris(pentafluoroethyl)trifluorophosphate LiPF3(CF2CF3)3 (LiFAP), lithium difluorophosphate LiPO2F2, and mixtures thereof.

[0069] Preferably, LiFSI is associated with a tetrafluorinated salt such as LiBF4 or with a hexafluorinated salt such as LiPF6. It can be associated with lithium difluoro(oxalato)borate (LIDFOB).

[0070] The lithium ions from lithium bis(fluorosulfonyl)imide LiFSI can represent from 50 to 100% or from 60 to 90% or from 90 to 100% or from 70 to 80% by mole of the total quantity of lithium ions provided by the lithium salt(s).

[0071] The lithium ions from lithium bis(fluorosulfonyl)imide LiFSI may represent from 0 to 50% or from 10 to 40% or from 0 to 10% or from 20 to 30% by mole of the total quantity of lithium ions provided by the lithium salt(s).

[0072] The mass percentage of LiFSI may be less than or equal to the mass percentage of LiPF6.

[0073] When LiFSI is combined with LiPF6, the lithium ions from the LiPF6 can represent from 10 to 90% or from 20 to 80% or from 30 to 70% or from 40 to 50% by mole of the total quantity of lithium ions provided by the lithium salt(s).

[0074] The total concentration of lithium salt(s) may be in the range from 0.75 to 5 mol.L *. It is preferably in the range from 1 to 4 mol.L '. It is even better in the range from 0.9 to 1.3 mol.L *.

[0075] LiFSI may be combined with one or more salts, the concentration of each of the salts other than LIFSI being at least 0.70 mol.L 1 or at least 0.80 mol.L 1 or at least 0.90 mol.L 1 or at least 1 mol.L 1 or at least 1.5 mol.L *. The mass percentage of each of the salts other than LIFSI may be at least 15% or at least 20% or at least 25% or at least 30%. LiPF6 may be one of these salts.

[0076] The electrolyte may contain one or more additives intended to stabilize the passivation layers at the positive and negative electrodes. This may be vinylene carbonate (VC). The mass proportion of each of these additives does not exceed 5% relative to the mass of prepared electrolyte.

[0077] An example element includes: - a positive electrode comprising a mixture containing from 35 to 45% by mass of LMFP and from 55 to 65% by mass of NMC; - a negative electrode comprising the Si-C composite and graphite; - an electrolyte comprising 5-15% EC, 15-25% FEC, 40-50% DMC and 20-30% EMC, by volume to which vinylene carbonate has preferably been added in a mass proportion less than or equal to 5% of the mass of the prepared electrolyte, - a mixture of LiPF6 and LiFSI as electrolyte salts, in a molar ratio which can be 10% LiPF6 and 90% LiFSI. Separator:

[0078] A separator is interposed between a positive electrode and a negative electrode. The material of the separator may be selected from the following materials: a polyolefin, for example polypropylene PP, polyethylene PE, a polyester, polymer-bonded glass fibers, polyimide, polyamide, polyaramid, polyamide-imide, and cellulose. The polyester may be selected from polyethylene terephthalate. (PET) and poly(butylene terephthalate) (PBT). Advantageously, the polyester or polypropylene or polyethylene contains or is coated with a ceramic material selected from the group consisting of metal oxides, oxyhydroxides, carbides, nitrides, borides, silicides and sulfides. This ceramic material may be SiO2 or Al2O3. The separator may be a ceramic-coated polyolefin layer, preferably a polyethylene layer coated with ceramic on both sides. Preparation of the electrochemical beam:

[0079] An electrochemical bundle is formed by inserting a separator between at least one positive electrode and at least one negative electrode. The electrochemical bundle is inserted into the cell container. The cell container may be parallelepipedal or cylindrical in shape. In the latter case, the electrochemical bundle is spiraled to form a cylindrical assembly of electrodes.

[0080] The electrochemical element can also be in pouch format. Examples:

[0081] Various pouch-type AG elements were fabricated and tested under room temperature cycling conditions. Table 1 summarizes the constituents of the various elements.

[0082] [Tables 1] Element Active material of the positive electrode Active material of the negative electrode Solvents Salt / mixture of salts Additives A* 40% LMFP 60%NMC811 40% 1st type of Si-C composite 60% graphite**** EC / PC / DMC / EMC 10 / 20 / 45 / 25** LiPF6 1 mol.L 1 3% VC *** B* 40% LMFP 60%NMC811 40% 1st type of Si-C composite 60% graphite EC / FEC / DMC / EM C 10 / 20 / 45 / 25 LiPF6 1 mol.L 1 3% VC c* 40% LMFP 60%NMC811 40% 1st type of Si-C composite 60% graphite FEC / DMC / EMC 30 / 45 / 25 LiPF6 1 mol.L 1 3% VC D 40% LMFP 60%NMC811 40% 1st type of Si-C composite 60% graphite EC / FEC / DMC / EM C 10 / 20 / 45 / 25 LiPF6 0.1 mol.L 1 LiFSI 0.9 mol.L 1 3% VC E 40% LMFP 60%NMC811 45% 2nd type of Si-C composite 55% graphite EC / FEC / DMC / EM C 10 / 20 / 45 / 25 LiPF6 0.1 mol.L 1 LiFSI 0.9 mol.L 1 3% VC F* 40% LMFP 60%NMC811 Graphite EC / FEC / DMC / EM C 10 / 20 / 45 / 25 LiPF6 0.1 mol.L 1 LiFSI 0.9 mol.L 1 3% VC G* 40% LMFP Graphite EC / FEC / DMC / EM LiPF6 3% VC 60%NMC811 C 10 / 20 / 45 / 25 1 mol.L 1 *: excluding invention **: volume proportions ***: quantity of additive expressed in mass relative to the mass of prepared electrolyte ****; capacity of 1000 mAh / g

[0083] [Fig.2a] represents the capacity retention of cells A, B and C as a function of the number of cycles performed. Cells A, B and C differ in the composition of the electrolyte solvent mixture. Comparing cell B with cell A, it can be noted that replacing non-fluorinated cyclic carbonate PC with fluorinated cyclic carbonate FEC makes it possible to extend the cycling life. Indeed, for an initial capacity retention of 80%, a doubling of the life is observed, increasing from 40 to 80 cycles. Comparing element A with element C, it can be noted that the replacement of EC and PC by FEC also leads to an improvement in the lifetime, the quantities of cyclic carbonate being identical in elements A and C. The value of 20% by volume of fluorinated cyclic carbonate seems to be an optimum beyond which, the presence of fluorinated cyclic carbonate no longer significantly improves the lifetime of the element.

[0084] [Fig.2b] represents the capacity retention of cells B and D as a function of the number of cycles. Cells B and D differ in the composition of the electrolyte salts. Comparing cell B with cell D, it can be noted that replacing 90% of the number of moles of LiPF6 with LiFSI leads to an extension of the lifetime. For an initial capacity retention of 80%, the lifetime is doubled, increasing from 75 to 140 cycles.

[0085] The results of Figures 2a and 2b therefore show that the composition of the solvents and salts of the electrolyte strongly influences the cycling performance of an element comprising a negative electrode based on the Si-C composite.

[0086] [Fig. 3] represents the capacity retention of elements D and E as a function of the number of cycles. Elements D and E differ in the type of composite used in the negative electrode. The results show that the second type of composite in which the silicon particles are deposited on the surface of pores of a carbon matrix leads to a longer cycling life than that obtained when silicon nanoparticles are embedded in a carbon matrix. The second type of composite makes it possible to achieve almost 500 cycles for an initial capacity retention of approximately 80%.

[0087] The effect of partial replacement of LiPF6 by LiFSI was studied in two cases, one in which the negative electrode comprises an active material which is a Si composite C, the other, in which the negative electrode comprises an active material which is graphite. [Fig.4] represents the capacity retention of cells F and G as a function of the number of cycles. The negative electrode of cells F and G is based on graphite. These results are to be compared with [Fig.2b] for which cells B and D tested have a negative electrode based on Si-C composite. No improvement in the lifetime caused by the replacement of 90% of LiPF6 by LiFSI is observed. The result of [Fig.4] shows that the effect of the partial replacement of LiPF6 by LiFSI is dependent on the nature of the negative active material, which indicates the existence of an interaction between the negative electrode and the electrolyte.

Claims

Claims

1. A lithium-ion electrochemical element comprising: - a positive electrode comprising one or more positive active materials, one of which is a lithium manganese and iron phosphate of formula LixMni y zFeyMzPO4 (LMFP) where 0.8 <x<l,2 ; 0,50<l-y-z<l ; 0<y+z<0,50 ; 0<y<0,50 ; 0<z<0,2 ;M being one or more elements selected from the group consisting of Al, B, Mg, K, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W, Pb, Mo and S, - a negative electrode comprising one or more negative active materials, one of which is a composite of silicon and carbon, - an electrolyte comprising a mixture of solvents and one or more lithium salts dissolved in the mixture of solvents, the mixture of solvents comprising at least one fluorinated cyclic carbonate, the other solvent(s) being selected from non-fluorinated cyclic carbonates or non-fluorinated linear carbonates, the volume proportion of fluorinated cyclic carbonate representing from 10 to 30% of the total volume of the solvents, at least one lithium salt being lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI).;

2. An electrochemical element according to claim 1, wherein the silicon and carbon composite consists of a carbon matrix in which silicon particles are incorporated, the largest dimension of the silicon particles being less than or equal to 500 nm.

3. An electrochemical element according to claim 1, wherein the silicon and carbon composite consists of a porous carbon matrix, silicon particles partially or completely covering the surface of the pores of the carbon matrix, the diameter of the pores of the carbon matrix ranging from 10 nm to 1 pm.

4. An electrochemical element according to claim 3, wherein the porous carbon matrix consists of carbon particles, the pores of the porous carbon matrix representing from 15 to 90% of the volume of the carbon particles, the volume of the carbon particles being delimited by the outer surface of the particles.

5. Electrochemical element according to one of the preceding claims, in which the negative electrode further comprises a second active graphite material, the graphite representing from 10 to 90% of the mass total active materials of the negative electrode, the silicon and carbon composite representing 90 to 10% of the total mass of the active materials of the negative electrode.

6. Electrochemical element according to one of the preceding claims, in which the fluorinated cyclic carbonate is ethylene fluorocarbonate (FEC).

7. An electrochemical element according to any preceding claim, wherein the solvent mixture comprises from 1 to 20% by volume of non-fluorinated cyclic carbonate.

8. An electrochemical element according to one of the preceding claims, wherein the solvent mixture consists of ethylene fluorocarbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC).

9. An electrochemical element according to claim 8, wherein the volume proportions of FEC, EC, DMC and EMC are within the following respective ranges: 15-25%, 5-15%, 40-50% and 20-30%.

10. Electrochemical element according to one of the preceding claims, in which the lithium ions from the lithium bis(fluorosulfonyl)imide salt Li(FSO2)2N (LiFSI) represent from 50 to 99%, preferably from 90 to 99% of the total lithium ions provided by the lithium salts.

11. Electrochemical element according to one of the preceding claims, in which the lithium bis(fluorosulfonyl)imide salt Li(FSO2)2N (LiFSI) is combined with at least one other salt chosen from lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4 and lithium difluoro(oxalato)borate (LIDFOB).

12. An electrochemical element according to one of claims 1 to 9, wherein the lithium bis(fluorosulfonyl)imide salt Li(FSO2)2N (LiFSI) is the only lithium salt in the electrolyte.

13. Electrochemical element according to one of claims 1 to 12, in which the total concentration of lithium ions provided by the lithium salt(s) ranges from 0.9 to 1.3 moles of lithium per liter of solvent(s).

14. Electrochemical element according to one of the preceding claims, in which the positive electrode further comprises one or more positive active materials chosen from: a) a lithium oxide of nickel, manganese and cobalt of formula Liw(NixMnyCozMt)O2(NMC) where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges of these, b) a lithium oxide of nickel, cobalt and aluminum of formula Liw(NixCoyAlzMt)O2(NCA) where 0.9 <w<l,l ; 0<x ; 0<y ; 0<z ; 0<t ; M étant choisi dans le groupe constitué de B, Mg, Si, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, Sr, Ce, Ga, Ta, Nd, Pr, La et des mélanges de ceux-ci, c) a compound of formula Lii+xMi_xO2 yFy of cubic crystal structure where M represents at least one element selected from the group consisting of Na, K, Mg, Ca, B, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb, Mo, Ru, Ag, Sn, Sb, Ta, W, Bi, La, Pr, Eu, Nd and Sm and where 0 < x < 0.5 and 0 < y < 1, d) a lithium oxide of nickel and manganese (NMX) of formula Lia(Nii_x_y_zMnxCoyMz)O2 with 0.9 <a<l,l ; 0,60<l-x-y-z<0,80 ; 0<x ; 0<y<0,02 ; 0<z ; et M étant choisi dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ga, Ta, Nd, Pr, La et des mélanges de ceux-ci ; e) a lithium oxide of nickel and manganese of formula Liw(NixMnyCoz Mt)O2 where 1.1 <w<1,6 ; 0<x ; 0,50<y<0,80 ; 0<z<0,02 ; 0<t ; M étant choisi dans le groupe constitué de Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ta, Ga, Nd, Pr, La et des mélanges de ceux-ci.

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