Positive electrode for lithium-ion electrochemical element

Abstract

An electrode comprising a mixture of active materials, which mixture comprises: - a lithium nickel manganese oxide of formula LiaNi1-x-y-zMnxCoyMzO2 where 0.9≤a≤1.1; 0.60≤1-x-y-z<0.80; 0<x; 0≤y≤0.02; 0≤z; M being one or more elements chosen from the group consisting of Al, B, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, W, Mo, S, Sr, Ce, Ga, Ta, Nd, Pr, and La; and - a lithium manganese iron phosphate of formula LixMn1-y-zFeyMzPO4 where 0.8≤x≤1.2; 0.5≤1-y-z<1; 0<y≤0.5; 0≤z≤0.2; M is one or more elements chosen from the group consisting of Al, B, Mg, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W and Mo.

Description

Description Title: Positive electrode for lithium-ion electrochemical cell Technical field of the invention

[0001] The technical field of the present invention is that of active materials intended to be used in the cathode, that is to say the positive electrode, of an electrochemical element of the lithium-ion type, also called a lithium-ion element. Background to the invention

[0002] Lithium-ion electrochemical elements comprising a positive electrode whose active material is based on lithium phosphate of at least one transition metal are known from the state of the art. A lithium phosphate of at least one transition metal typically has the formula LiMPCU where M represents at least one transition metal, such as Mn, Co, Ni, Fe or Mn associated with Fe. When Mn is associated with Fe, the lithium phosphate is abbreviated LMFP in the following.

[0003] Determining the state of charge of a cell whose positive electrode includes LMFP is difficult. Indeed, such a cell has, for states of charge generally between 30 and 80%, a so-called "flat" charge profile. A charge profile is understood to be the curve representing the variation of the cell's voltage as a function of time during charging. In the range of states of charge between 30 and 80%, the cell's voltage increases very little, so that it is difficult to establish a correlation between the cell's voltage and its state of charge. In addition, as the end of charging approaches, i.e. for a state of charge between approximately 95 and 100%, the cell's voltage increases sharply. Indeed, the delithiation of almost all the lithium present in the lithium phosphate leads to a sudden increase in voltage. This sudden increase does not allow a user to be warned early enough of the imminence of an overload.The cell's voltage can quickly reach high values. Prolonged exposure of the cell to overcharging leads to degradation of the electrolyte and a reduction in the cell's lifespan.

[0004] Methods for detecting the end of charge of cells comprising a positive electrode based on LMFP have been sought in order to warn a user sufficiently early of the imminence of the end of charge of the cell. Document FR 3 122 286 teaches for example to use in mixture with LMFP a nickel, manganese and cobalt oxide, abbreviated NMC, rich in nickel or a lithium oxide of nickel, cobalt and aluminum, abbreviated NCA, also rich in nickel. The term nickel-rich designates a stoichiometric index of nickel of at least 0.80 for NMC. It designates a nickel stoichiometric index of at least 0.83 for NCA.

[0005] However, a progressive dissolution of manganese and nickel from the NMC and NCA oxides in the cell electrolyte has been observed. This dissolution results in a progressive reduction in the cell's discharge capacity, and therefore a reduction in its lifespan. We are therefore seeking to reduce the dissolution of manganese and nickel while maintaining the possibility of accurately determining the end of the charge of the cell whose positive electrode is based on LMFP. Summary of the invention

[0006] For this purpose, the invention proposes an electrode comprising a mixture of active materials, which mixture comprises: - a monocrystalline lithium oxide of nickel and manganese (NMx) of formula LiaNii.xy-zMn xC0yMzO2 where 0.9 <a<1 , 1 ; 0,60<1-x-y-z<0,80 ; 0<x ; 0<y<0,02 ; 0<z ; M étant un ou plusieurs éléments choisis 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, et La ; et - a lithium manganese and iron phosphate (LMFP) of formula Li x Mni.y.zFe y MzPO4 where 0.8 <x<1 ,2 ; 0,5<1-y-z<1 ; 0<y<0,5 ; 0<z<0,2 ; M étant un ou plusieurs éléments choisis dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W et Mo.

[0007] It was found that by using monocrystalline NMx and choosing the stoichiometric indices of nickel, manganese and cobalt such that they fall within the following respective intervals: 0.60<1-xyz<0.80; 0 <x et 0<y<0,02, il était possible de réduire la dissolution du nickel et du manganèse tout en conservant, voire en améliorant, la possibilité de détecter avec précision la fin de la charge de l’élément.

[0008] According to one embodiment, the mixture of active ingredients consists of: - from 10 to 70% or from 20 to 40% by mass of the monocrystalline lithium nickel and manganese oxide. - from 90 to 30% or from 80 to 60% by mass of lithium phosphate of manganese and iron.

[0009] According to one embodiment, in the single-crystal lithium nickel manganese oxide, 0.60<1 -xyz or 0.60<1-xyz<0.75.

[0010] According to one embodiment, in the single-crystal lithium nickel manganese oxide, 0.005 <y<0,015 ou y=0.

[0011] According to one embodiment, the single-crystal lithium nickel manganese oxide has a particle size distribution characterized by a volume median diameter DV 50 ranging from 1 to 7 pm, preferably ranging from 1 to 4 pm.

[0012] According to one embodiment, in the lithium manganese and iron phosphate, 0.7<1-yz<0.9.

[0013] The invention also relates to a lithium-ion electrochemical element comprising at least one positive electrode which is the electrode described above and at least one negative electrode.

[0014] According to one embodiment, said at least one negative electrode comprises an active material chosen from graphite, silicon, titanium oxide, a lithiated titanium oxide, a titanium and niobium oxide and a mixture thereof.

[0015] The invention also relates to a method for improving the detection of the end of charge of a lithium-ion electrochemical element comprising a positive electrode comprising a lithium manganese and iron phosphate of formula Li x Mni.y. zFeyMzPO4 where 0.8 <x<1 ,2 ; 0,5<1-y-z<1 ; 0<y<0,5 ; 0<z<0,2 ; M étant un ou plusieurs éléments choisis dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W et Mo, ladite méthode comprenant : a) le remplacement au cours de la fabrication d’une électrode positive d’une partie du phosphate lithié de manganèse et de fer par un oxyde lithié de nickel et de manganèse monocristallin de formule LiaNii.xy-zMn x C0yMzO2 where 0.9 <a<1 , 1 ; 0,60<1-x-y-z<0,80 ; 0<x ; 0<y<0,02 ; 0<z ; M étant un ou plusieurs éléments choisis 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, et La ; b) établissement d’une corrélation entre la tension de l’élément et son état de charge.

[0016] The method can be implemented in the case where the positive electrode of the element is as described above. Brief description of the figures

[0017] Embodiments of the invention are described below in more detail with reference to the accompanying figures.

[0018] [Fig. 1] shows the charge / discharge curves in the first cycle of cells comprising the positive electrodes A to E of the examples.

[0019] [Fig. 2] represents the dQ / dV curves of the active materials of electrodes A and B.

[0020] [Fig. 3] represents the differential scanning calorimetry curves obtained on the mixtures of active materials of electrodes C and D. Description of embodiments of the invention Positive electrode

[0021] At least one positive electrode of the element comprises a current collector of which at least one of its faces 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.

[0022] 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.

[0023] Before the current collector is coated with the active material composition layer, it may be coated on one or both sides with a coating 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 carbon, graphite, carbon fibers, carbon nanotubes, and mixtures thereof. It is preferably made of 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.

[0024] The mixture of active ingredients comprises at least one single-crystal lithium nickel manganese oxide NMx and at least one lithium manganese iron phosphate LMFP. A single crystal is a solid consisting of a single crystal, formed from a single seed. Synonymous terms that may be used are: monolithic material, "single crystal material" in English or monocrystalline material, commonly called "single crystal" by those in the field. The term monolithic or monocrystalline is also used to refer to a morphology in which a secondary particle is ideally made up of a single primary particle.

[0025] NMx is a compound of formula (I) UaNii.xy-zMn xC0yMzO2 where 0.9 <a<1 , 1 ; 0,60<1-x-y-z<0,80 ; 0<x ; 0<y<0,02 ; 0<z ; M étant un ou plusieurs éléments choisis 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 et La.

[0026] The nickel stoichiometric index of at least 0.60 makes it possible to obtain a sufficiently high electrochemical capacity of the positive electrode. According to one embodiment, 0.65<1-xyz<0.80 or 0.70<1-xyz<0.80 or 0.75<1-xyz<0.80. Conversely, a nickel stoichiometric index value greater than or equal to 0.8 is detrimental to the stability of the lithium nickel oxide when it is either exposed to a high temperature or to a potential greater than 4.3 V vs. Li. The choice of a stoichiometric index of between 0.6 and less than 0.8, preferably around 0.75, makes it possible to provide a positive electrode having both a high electrochemical capacity and good stability from a thermal and electrochemical point of view.

[0027] According to one embodiment, the stoichiometric index of manganese is included in the following ranges of values: 0.20 <x<0,40 ou 0,25<x<0,30.

[0028] According to one embodiment, the stoichiometric index of cobalt is included in the following ranges of values: 0.001 <y<0,015 ou 0,005<y<0,010 ou 0,001 <y<0, 005. De préférence, y=0. La faible quantité de cobalt dans NMx, voire l’absence totale de cobalt, répond au problème de la raréfaction du cobalt.

[0029] According to one embodiment, z=0.

[0030] According to one embodiment, y=0 and z=0.

[0031] According to one embodiment, y=0, z=0, 0.20 <x<0,40 et 0,60<1-x<0,80.

[0032] According to one embodiment, y=0, z=0, 0.20 <x<0,30 et 0,70<1-x<0,80.

[0033] Examples of preferred compounds are LiNi0.65Mn0.35O2, LiNi0.7oMn0.3o02 and LiNi0.7sMno.2502.

[0034] The applicant found that the tendency of the NMx compound to release nickel and manganese into the electrolyte was less than for the LiNio,8Mno,iCoo,i02 (NMC811) compound. This advantage can be attributed on the one hand to the monocrystalline nature of NMx and on the other hand to different stoichiometric coefficients of nickel, manganese and cobalt. The invention therefore makes it possible to increase the lifetime of the element compared to an element whose positive electrode would contain NMC811.

[0035] A parameter characterizing the size distribution of single crystals is the volume median diameter D v so of single crystals. The expression "volume median diameter D V5 O" means that, in a given population of particles, 50% of the volume of particles consists of particles having an equivalent diameter less than the value D v5o and 50% of the particle volume consists of particles having an equivalent diameter equal to or greater than the value D v5 o. The term "equivalent diameter" of a particle refers to the diameter of a sphere having the same volume as that particle. Dvso of NMx typically ranges from 1 to 7 pm or 1 to 4 pm. A value greater than 7 pm will usually indicate that it is a polycrystal. Dvso can be measured using the laser diffraction technique.

[0036] The space group of monocrystalline NMx is the R 3m group (trigonal system). It can be verified that the space group of monocrystalline NMx is indeed the R 3m group by means of X-ray diffraction.

[0037] According to one embodiment, the NMx particles are devoid of any coating, in particular a coating which would be made of LMFP or a coating of a metal oxide.

[0038] LMFP is a compound of formula (II) Li xMni.y.zFe y MzPO4 where 0.8 <x<1 ,2 ; 0,5<1-y-z<1 ; 0<y<0,5 ; 0<z<0,2 ; M étant un ou plusieurs éléments choisis dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W et Mo.

[0039] According to one embodiment, 0.5<1-yz<1 or 0.7<1-yz<0.9 or 0.75<1-yz<0.80.

[0040] Examples of LMFP compounds are LiMno.sFeo.sPC, LiMnojsFeo^PC, LiMn0.7Feo,3P04, LiMn2 / 3Fei / sPO4 or LiMno.5Feo,sP04.

[0041] LMFP can be coated with carbon or carbon nanotubes.

[0042] 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 D v50 located in the range of 1 to 15 pm or 1.5 to 4 pm.

[0043] The proportions of LMFP and NMx in the mixture of active ingredients may be as follows: 10 to 70% by mass of monocrystalline NMx, and 90 to 30% by mass of LMFP, or 20 to 40% by mass of monocrystalline NMx, and 80 to 60% by mass of LMFP, or 25 to 35% by mass of monocrystalline NMx, and 75 to 65% by mass of LMFP.

[0044] The invention excludes the LMFP active material constituting a coating of the NMx particles.

[0045] The active ingredient composition may comprise a plurality of single-crystal lithium nickel and manganese oxides of formula (I) and a plurality of lithium manganese and iron phosphates of formula (II). Preferably, the positive active ingredient composition does not contain active ingredients other than compounds of formula (I) or (II).

[0046] The binder generally used in the active material composition strengthens the cohesion between the active material particles and improves the adhesion of the active material composition layer to the current collector. The binder may contain one or several of the following compounds: 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 cellulose 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). The binder can represent from 1% to 10% or from 1 to 5% or from 2 to 5% of the mass of the dry composition of active ingredients.

[0047] The electronically conductive material is generally selected from graphite, carbon black, acetylene black, soot, graphene, carbon fibers, single-walled carbon nanotubes, multi-walled carbon nanotubes, and mixtures thereof. It may represent from 0.1 to 10% or from 0.1 to 5% of the mass of the dry composition of active materials. Preparation of the positive electrode:

[0048] An ink is prepared by dispersing one or more LMFP-type active materials and one or more NMx-type active materials in a solvent or 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 side of the current collector. The ink-coated current collector is dried and then rolled to adjust its thickness. After evaporation of the solvent(s), a layer of a composition of 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. Negative electrode:

[0049] At least one negative electrode comprises a current collector, at least one of two faces of which is coated with a layer of a negative active material composition. The negative active material composition comprises at least one negative active material and optionally a binder and an electronically conductive material. The negative current collector is a solid or perforated metal strip which may be made of copper or a copper-based alloy. Its thickness may range from 3 to 10 μm, preferably from 5 to 8 μm.

[0050] 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 and the method of deposition of the coating may be the same as those described for the current collector of the positive electrode.

[0051] The negative active material is preferably selected from carbon, graphite, coke, carbon black, glassy carbon, silicon, a silicon-carbon composite, a silicon carbide, a titanium oxide, a lithiated titanium oxide, a titanium niobium oxide and a mixture thereof. The titanium oxide may be selected from H2Ti60i3, H2Tii2O25 and TiO2. The lithiated titanium oxide may be selected from Li4Ti50i2, Li2TiO3, Li2Ti3O?, LiThC and Li2Na2Ti60i4. The titanium niobium oxide may be selected from TiNb2O?, Ti2Nb20g and Ti2Nb O2g. Preparation process of the negative electrode:

[0052] The negative electrode is prepared in a conventional manner. An ink is prepared by dispersing in a solvent or a mixture of solvents one or more negative active materials, optionally with a binder. The binder may be such as those described in connection with the positive electrode.

[0053] The ink-coated current collector is dried and then rolled to adjust its thickness, resulting 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 ingredients, - 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. Electrolyte:

[0054] The electrolyte may be liquid. It is obtained by dissolving one or more lithium salts in one or more organic solvents. The solvent may be chosen from saturated cyclic carbonates, unsaturated cyclic carbonates, non-cyclic carbonates, alkyl esters, ethers, nitrile solvents, and tetrahydrothiophene dioxide (sulfolane).

[0055] Saturated cyclic carbonates include ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), butylene carbonate (BC), and mixtures thereof.

[0056] Unsaturated cyclic carbonates include vinylene carbonate (VC).

[0057] Non-cyclic carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), and mixtures thereof.

[0058] Alkyl esters include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and mixtures thereof.

[0059] Ethers include dimethyl ether (DME), diethyl ether (DEE), and mixtures thereof.

[0060] The lithium salt may be selected from lithium perchlorate LiCICU, lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate UBF4, lithium hexafluoroarsenate LiAsF6, lithium hexafluorantimonate LiSbFe, lithium trifluoromethanesulfonate UCF3SO3, lithium bis(fluorosulfonyl)imide Li(FSC>2)2N (LiFSI), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium tris(fluoromethanesulfonyl)methylide LiC(CF3SC>2)3 (LiTFSM), lithium bis(pentafluoroethylsulfonyl)imide LiN(C2F(SC>2)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 UPO2F2, and mixtures thereof.

[0061] The concentration of said at least one lithium salt may be in the range from 0.75 to 1.5 mol / L. It is preferably in the range from 1 to 1.5 mol / L. It is even better in the range from 1 to 1.2 mol / L. Separator:

[0062] 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 polybutylene 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, carbides, nitrides, borides, silicides, and sulfides. This ceramic material may be SiC>2 or AI2O3. The separator may be a layer of polyolefin coated with ceramic, preferably a layer of polyethylene coated with ceramic on both sides. Preparation of the electrochemical beam:

[0063] An electrochemical beam 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 can be parallelepipedal or cylindrical. In the latter case, the electrochemical bundle is spiraled to form a cylindrical assembly of electrodes. Filling and closing the container:

[0064] A cover is assembled onto the cell container, for example by laser welding. The cell container is filled with electrolyte. For this purpose, the cover may include a filling hole through which the electrolyte is introduced. Once the electrolyte has been introduced, this filling hole is closed by a stainless steel ball by electrically welding the ball to the hole. Examples:

[0065] Five positive electrodes AE were prepared. They differ in their active materials. Table 1 shows the active materials used. In this table, LMFP is LiMno,78FeO,22PO4; NMx is monocrystalline LiNio,75Mno,2sO2 and NMC811 is LiNio,8oMno,iCoo,iO2. Table 1 Electrode outside the invention

[0066] The electrodes were exposed to a voltage of 4.5 V vs. Li metal for 7 days. The amount of metal released by the positive electrode and found on the negative electrode was measured after these 7 days of exposure to 4.5 V. Table 2 presents the results of the measurements obtained. Table 2 * Electrode outside the invention ** Quantity of nickel or manganese measured at the negative electrode at the end of the test and expressed as a percentage of the corresponding quantity initially present at the positive electrode.

[0067] It is observed that electrode C containing a mixture of NMx and LMFP is the one for which the quantity of nickel and the quantity of manganese released are the lowest. This result is surprising. Indeed, the percentage of nickel released by electrode C is 40% lower than the percentage of nickel released by electrode D while the stoichiometric index of nickel is 0.75 in NMx and 0.80 in NMC811, i.e. only 6% lower in NMx. Furthermore, we note, concerning manganese, that there is no proportionality between the percentage of manganese released by the electrode and the stoichiometric index of manganese in the lithiated oxide. Indeed, NMx contains 2.5 times more manganese than NMC811. However, the percentage of manganese released by electrode C containing NMx is lower than that released by electrode D containing NMC811.

[0068] The applicant also observed that replacing NMC 811 with NMx in a mixture with LMFP improved the accuracy of determining the cell's state of charge as it approached the fully charged state.

[0069] The charge / discharge curves of the five AE cells with a lithium metal negative electrode were plotted between the voltages of 2.5 V and 4.45 V. They are shown in Figure 1 for the first cycle.

[0070] The voltage of cell B comprising NMC811 varies proportionally with the cell's state of charge up to approximately 80% of full charge. Beyond 80%, the charge / discharge curve exhibits a horizontal plateau which makes it imprecise to determine the cell's state of charge based solely on its voltage. The voltage of cell A comprising NMx also varies proportionally with the cell's state of charge, even beyond 80% of full charge. The use of NMx therefore allows for more precise detection of the end of charge.

[0071] The charge / discharge curve of cell E comprising LMFP alone has two plateaus: a first plateau P1 at approximately 3.50 V attributable to iron and a second plateau P2 at approximately 4.1 V attributable to manganese. Detecting the proximity of the end of the charge is difficult given the sudden rise in voltage at the end of the second plateau P2. Replacing 30% of LMFP with NMC811 (curve D) allows on the one hand to increase the mass capacity of the electrode and on the other hand to gradually increase the voltage as the end of the charge approaches in the form of an inclined part (la) whose middle is located at approximately 4.2 V. Replacing 30% of LMFP with NMx (curve C) also increases the mass capacity compared to LMFP alone, but it also increases the voltage of the inclined part Ib to around 4.3 V instead of 4.2 V for the inclined part la.The gap between the inclined part Ib and the P2 plateau of the manganese at 4.1 V is greater than the gap between the inclined part Ia and the P2 plateau of the manganese at 4.1 V. Because of this greater gap, it is possible to establish a more precise correlation between the voltage of the element and its. state of charge.

[0072] The applicant found that NMx exhibited better oxidation stability than NMC811 when the applied potential exceeds 4.3 V vs. Li. Figure 2 compares the dQ / dV curves of active materials A and B, which are NMx and NMC811, respectively. The high intensity peak at approximately 4.2 V indicates that at this potential, significant structural degradation of NMC811 occurs. This indicates that charging NMC811 at a potential above 4.2 V significantly degrades the structure of the material. In contrast, NMx exhibits a much lower intensity peak than NMC811. In addition, the peak of NMx appears at a potential above 4.2 V. It is therefore possible to charge NMx at a potential of 4.45 V without significantly degrading the structure of the material.This advantage is used in the invention to exploit a wider potential range of up to 4.45 V instead of 4.3 V for NMC811, thus making it possible to increase the accuracy of the determination of the state of charge near the end of the charge. All of these results demonstrate the possibility of reducing the dissolution of nickel and manganese while improving the detection of the end of the charge.

[0073] An additional advantage of replacing NMC811 with NMx is that it provides a greater safety margin for the user of the element in the event of an increase in temperature. The differential scanning calorimetry technique was used to detect the temperature at which the active material mixtures of electrodes C and D start to release heat when they are subjected to an increase in temperature. These results, shown in Figure 3, show that the heat release occurs as early as 200°C for the active material mixture of electrode D outside the invention, whereas it is necessary to reach approximately 230°C to observe a heat release for the active material mixture of electrode C.

[0074] Finally, replacing part of LMFP with NMx increases the specific energy and specific capacity compared to an element whose positive active ingredient would be LMFP alone. This is highlighted by the results in Table 3. [Table 3] Electrode outside the invention

Claims

Claims

1. Electrode comprising a mixture of active materials, which mixture comprises: - a monocrystalline lithium oxide of nickel and manganese of formula Li a Nii-xy-zMn x C0yMzO2 where 0.9 <a<1 , 1 ; 0,60<1-x-y-z<0,80 ; 0<x ; 0<y<0,02 ; 0 <z ; M étant un ou plusieurs éléments choisis 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, et La ; et - a lithium phosphate of manganese and iron of formula Li x Mni-y- z FeyM z PO4 where 0.8 <x<1 ,2 ; 0,5<1-y-z<1 ; 0<y<0,5 ; 0<z<0,2 ; M étant un ou plusieurs éléments choisis dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W et Mo.

2. Electrode according to claim 1, in which the mixture of active materials consists of: - from 10 to 70% by mass of the monocrystalline lithium nickel and manganese oxide; - from 90 to 30% by mass of lithium phosphate of manganese and iron.

3. Electrode according to claim 2, in which the mixture of active materials consists of: - from 20 to 40% by mass of the monocrystalline lithium nickel and manganese oxide; - from 80 to 60% by mass of lithium phosphate of manganese and iron.

4. An electrode according to any preceding claim, wherein in the single-crystal lithium nickel manganese oxide, 0.60<1 -xyz.

5. Electrode according to one of claims 1 to 3, in which in the single-crystal lithium nickel manganese oxide, 0.60<1-xyz<0.

75.

6. Electrode according to one of the preceding claims, in which in the single-crystal lithium nickel manganese oxide, 0.005 <y<0,015.

7. Electrode according to one of claims 1 to 5, in which in the single-crystal lithium nickel manganese oxide, y=0.

8. An electrode according to any preceding claim, wherein the single-crystal lithium nickel manganese oxide has a distribution of particle size characterized by a volume median diameter Dv5o ranging from 1 to 7 pm, preferably ranging from 1 to 4 pm.

9. Electrode according to one of the preceding claims, in which in the lithium manganese and iron phosphate, 0.7<1-yz<0.

9.

10. Lithium-ion electrochemical element comprising: - at least one positive electrode which is an electrode according to one of claims 1 to 9; - at least one negative electrode. [Claim 1 1] The lithium-ion electrochemical element of claim 10, wherein said at least one negative electrode comprises an active material selected from graphite, silicon, titanium oxide, a lithiated titanium oxide, a titanium and niobium oxide and a mixture thereof.

12. Method for improving the detection of the end of charge of a lithium-ion electrochemical element comprising a positive electrode comprising a lithium manganese and iron phosphate of formula LixMni.y.zFeyMzPC where 0.8 <x<1 ,2 ; 0,5<1-y-z<1 ; 0<y<0,5 ; 0<z<0,2 ; M étant un ou plusieurs éléments choisis dans le groupe consistant en Al, B, Mg, Si, Ca, Ti, V, Cr, Co, Cu, Ni, Zn, Y, Zr, Nb, W et Mo, ladite méthode comprenant : a) le remplacement au cours de la fabrication d’une électrode positive d’une partie du phosphate lithié de manganèse et de fer par un oxyde lithié de nickel et de manganèse monocristallin de formule LiaNii.xy-zMn xC0yMzO2 where 0.9 <a<1 , 1 ; 0,60<1-x-y-z<0,80 0<x 0<y<0,02 0<z m étant un ou plusieurs éléments choisis 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, et la b) l’établissement d’une corrélation entre la tension de l’élément son état charge.

13. A method according to claim 12, wherein the positive electrode is according to one of claims 1 to 9.< / a<1>

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