Sodium-ion electrochemical element and method of cycling such an element

Abstract

A method for improving the cycling life of a sodium-ion electrochemical cell comprising a positive electrode based on an oxide of O3 crystallographic structure. This method is based on blocking the transformation from the P3 crystallographic structure to the O3 crystallographic structure after the first transformation from the O3 crystallographic structure to the P3 crystallographic structure, which takes place when the cell is first charged, has occurred.The maintenance of the P3 structure is obtained by taking advantage on the one hand of the property of certain transition metals to favor the formation of the P3 crystallographic structure for charge states close to the fully discharged state of the element and on the other hand of the fact that the irreversibility created by an excess of negative electrode stops the sodiation of the oxide earlier during the discharge and thus blocks the transformation of the P3 crystallographic structure into the O3 crystallographic structure. Abstract figure: Figure 3.

Description

Title of the invention: Sodium-ion electrochemical element and method for cycling such an element Technical field

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

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

[0003] Sodium-ion electrochemical cells have a lower mass capacity and volume capacity than lithium-ion electrochemical cells. However, they represent a future technology for stationary energy storage applications for which the objectives of compactness and lightness are not essential. For these applications, they are more attractive than lithium-ion cells because of their lower manufacturing cost. Furthermore, because sodium is more abundant than lithium, the manufacturing cost of sodium-ion electrochemical cells should be less subject to variations than that of lithium-ion electrochemical cells.

[0004] The active materials commonly used in the positive electrode of a sodium-ion cell are sodium-containing layered oxides of one or more transition metals. The article entitled “Layered oxide cathodes for sodium-ion batteries: phase transition, air stability, and performance” published by Wang et al. in Adv. Energy Mater. 2017, 1701912 presents different formulas of such oxides. This article presents also the main crystallographic structures of these oxides. These are the structures P2, O2, O3 and P3 shown schematically in [Fig.l] of Wang. The structures P2, O3 and P3 are reproduced in [Fig.l] of the present application. The oxides are called lamellar because they are made up of a stack of sheets of formula M02, where M denotes at least one transition element. Each sheet is made up of the association of M06 octahedra sharing their edges. The center of each octahedron is occupied by a transition element M and the six vertices of the octahedron are occupied by an oxygen atom. The sodium atoms are intercalated between the M02 sheets. During the charging of the electrochemical element, they are detached from the sheets. During the discharge of the element, they are reinserted between the sheets.

[0005] Crystallographic structures are designated by the letter O or P according to the coordination environment of a sodium atom intercalated between two sheets. When this is at the center of a prism whose vertices are oxygen atoms, the crystallographic structure is called "P" for prism. When this is at the center of an octahedron whose vertices are oxygen atoms, the crystallographic structure is called "O" for octahedron. The number 2 or 3 following the letter P or O refers to the minimum number of sheets required to describe the hexagonal unit cell. [Fig.l] shows, with regard to the P2 structure, that two sheets are sufficient to describe the unit cell, one of type AB, the second of type BA. The letters A, B and, possibly, C correspond to the relative positions of the oxygen atoms from one plane to another. The sodium atom can occupy two different prismatic positions between the two sheets AB and BA.For the 03 structure, three sheets are sufficient to describe the unit cell, the first of type AB, the second of type CA and the third of type BC. The sodium atom is located at the center of an octahedron inserted either between the AB and CA sheets, or between the CA and BC sheets. For the P3 structure, three sheets are sufficient to describe the unit cell, the first of type AB, the second of type CB and the third of type CA. The sodium atom can occupy a prism inserted either between the AB and BC sheets, or between the BC and CA sheets. The type of structure, 03, P3 or P2 can be determined experimentally by carrying out the diffractogram of the materials using the X-ray diffraction technique.

[0006] Figure 2c of the article by Wang et al. shows an experimental charge and discharge curve for an electrochemical element comprising a positive electrode based on O3-NaxCoO2. The x-axis indicates the value of the stoichiometric index x of sodium in O3-NaxCoO2. The index x varies between 0.825 and 1 during a cycle. It is indicated that the P'3 form is present for x ranging from 0.825 to 0.9. These experimental results are from work dating from 1981. More Recent studies indicate that this range of values ​​is erroneous. In this regard, reference may be made to the article entitled "NaxCoO2 phase stability and hierarchical orderings in the O3 / P3 structure family" by Jonas L. Kaufman and Anton Van der Ven published on January 10, 2019, in the journal Phys. Rev. Materials 3, 015402. This article presents the characterization results of the different crystallographic structures of NaxCoO2. Figure 2b of this article shows that in reality, for a voltage ranging from 2.85 V to 3.5 V, which corresponds to the region of existence of the P'3 structure in the Wang article, the value of x is located between 0.3 and 0.5 and not between 0.825 and 0.90.

[0007] Oxides with the crystallographic structure O3 are the most interesting oxides as positive active material because they have the highest capacity. Indeed, it is possible to manufacture an O3-NaMO2 oxide in which the stoichiometric index of sodium is 1. On the other hand, it is not possible to manufacture a P2-NaxMO2 oxide with a stoichiometric index x of sodium that is greater than about 0.67. In addition, a collapse of the structure of the P2 oxide is observed when, during the charging of the element, the stoichiometric index of sodium falls below the range of values ​​around 0.2 to 0.3. Finally, it is not possible to manufacture a P3-NaxMO2 oxide with a stoichiometric index x of sodium that is greater than 0.60. P3 lamellar oxides are therefore not sufficiently capacitive materials.

[0008] During the charging of the element, the O3 structure transforms into a P3 structure. Conversely, during the discharge, the P3 structure returns to the O3 structure. As an example, we can refer to Figures 5b and 11a of Wang et al., which show the progressive transformation of the O3 structure into the P3 structure during the charging and then the progressive transformation of the P3 structure into the O3 structure during the discharge. At the end of the discharge, only the O3 crystallographic structure is present. The repeated cycles of insertion and deinsertion of sodium in the active material create repeated sliding of the transition metal octahedron sheets and a significant expansion (+7%) of the active material. This results in fragmentation of the active material particles. This fragmentation is the main cause of the loss of capacity of the sodium lamellar oxides of O3 structure during the cycling of the element. [Fig.2] is a scanning electron microscopy photo showing, on an example, the fragmentation of a sodium lamellar oxide of formula O3-NaNii / 3Fei / 3Mni / 3O2 after 100 cycles.

[0009] We therefore seek to improve the cycling life of a sodium-ion electrochemical element comprising a positive electrode based on an oxide with crystallographic structure O3.

[0010] CN 116190633 describes the preparation of a sodium lamellar oxide of formula NaxCuyMnzMaO2 where M is a doping element, 0.60 <x<l,20, 0,05<y<0,4, 0,30<z<0,80, 0<a<0,60, 0,95<y+z+a<l,05. L’oxyde lamellaire obtenu peut présenter une structure cristallographique de type P2 ou de type 03. Il présente une stabilité améliorée vis-à-vis de l’exposition à l’air et à l’humidité. La distance interfeuillet décrite dans ce document est typique des matériaux P2 ou 03 à l’état d’origine, c’est-à-dire avant leur utilisation en élément électrochimique.

[0011] WO 2021 / 150168 describes a stabilized structure P3 of a sodium oxide of formula NaxMyOz where x>0.66, 0.8 <y<l,0, z<2 et M est choisi parmi un ou plusieurs éléments du groupe constitué d'un métal de transition 3d, d'un métal de transition 4d, Al, Mg, B, Si, Sn, Sr et Ca. Bien qu’il soit indiqué que x puisse atteindre une valeur élevée, par exemple 1, une telle valeur de 1 est difficilement atteignable en pratique en raison de la répulsion coulombique entre les atomes de sodium lorsque les espaces interfeuillets sont remplis. En pratique, la capacité électrochimique de l’oxyde de sodium de structure P3 est bien inférieure à la capacité théorique correspondant à un indice stoechiométrique du sodium de 1.

[0012] None of the aforementioned documents deals with a method for improving the cycling life of a sodium-ion electrochemical element comprising a positive electrode based on an oxide of structure O3. Summary

[0013] The invention provides a method for cycling a sodium-ion electrochemical element taking into account the specific electrochemical properties of its positive and negative electrodes, which method comprises: a) providing a sodium-ion electrochemical element comprising: - a positive electrode comprising a lamellar oxide of formula NawMxM't02 where M is at least one transition metal chosen from Ti, V, Cr, Mn, Fe, Co and Ni, M' is chosen from the group consisting of Mg, Ca, Al, Cu, Zn and mixtures thereof, 0.8 <w<l,l ; 0<x ; 0<t, lequel oxyde lamellaire présente avant une première charge de l’élément électrochimique, une structure cristallographique de type 03 ou 0’3, et - a negative electrode comprising a negative active material, the ratio between the electrochemical capacity of the negative active material and the electrochemical capacity of the lamellar oxide ranging from 1.05 to 1.2, b) a first charge of the electrochemical element leading to the transformation of the lamellar oxide from the crystallographic structure of type O3 or O'3 to the crystallographic structure of type P3 or P'3, c) a first discharge of the electrochemical element, and for a discharge depth of the element ranging from 80 to 90%, at least 75% of the lamellar oxide has a crystallographic structure of type P3 or P'3, d) a repetition of charges and discharges, and for a discharge depth of the electrochemical element ranging from 80 to 90%, at least 75% of the lamellar oxide has a crystallographic structure of type P3 or P'3.

[0014] The depth of discharge is the percentage of capacity discharged relative to the capacity discharged by the element when it is discharged to a voltage of IV at 20-25°C.

[0015] The invention is based on maintaining the P3 structure after the first transformation of the O3 structure into the P3 structure, which takes place during the first charge of the element, has occurred. The maintenance of the P3 structure is obtained by taking advantage on the one hand of the property of certain transition metals to promote the formation of the P3 structure for charge states close to the discharged state and on the other hand of the fact that the irreversibility created by the excess negative electrode will stop the sodiation of the oxide earlier during the discharge and thus block the transformation of the P3 structure into the O3 structure. This results in a reduction in the number of transformations of the P3 structure into the O3 structure during the cycling discharges and an improvement in the cycling lifetime of the element.

[0016] According to one embodiment, in steps c) and d) of the method, at least 90% of the lamellar oxide has a crystallographic structure of type P3 or P'3.

[0017] According to one embodiment, 100% of the lamellar oxide has a crystallographic structure of type P3 or P'3.

[0018] According to one embodiment, the negative active material is hard carbon.

[0019] According to one embodiment, in the lamellar oxide, 0 <t.

[0020] According to one embodiment, M represents Mn and Fe; or M represents Mn, Fe and Ni; or M represents Ni; or M represents Ni and Fe; or M represents Cr; or M represents Mn, Co and Ni; or M represents Mn and Ni; or M represents Ni and Ti.

[0021] According to one embodiment, the lamellar oxide is chosen from: NawMnxiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiFex2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawNixiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiCox2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawMnxiNix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; and NawNix[Tix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1.

[0022] According to one embodiment, the ratio between the electrochemical capacity of the negative active material and the electrochemical capacity of the lamellar oxide ranges from 1.12 to 1.17.

[0023] According to one embodiment, the lamellar oxide has, at the end of the first discharge of the electrochemical element and at the end of the subsequent discharges, a crystallographic structure of type P3 and w>0.65 or w>0.70.

[0024] The invention also relates to a sodium-ion electrochemical element comprising: - a positive electrode comprising a lamellar oxide of formula NawMxM'tO2 where M is at least one transition metal chosen from Ti, V, Cr, Mn, Fe, Co, Ni, M' is chosen from the group consisting of Mg, Ca, Al, Cu, Zn and mixtures thereof, 0.8 <w<l,l ; 0<x ; 0<t ; et - a negative electrode comprising a negative active material, the ratio between the electrochemical capacity of the negative active material and the electrochemical capacity of the lamellar oxide ranging from 1.05 to 1.2, characterized in that: a) before a first charge of the electrochemical element, the lamellar oxide has a crystallographic structure of type O3 or O'3, and b) for a discharge depth ranging from 80 to 90% during a first discharge of the electrochemical element and subsequent discharges, at least 75% of the lamellar oxide has a crystallographic structure of type P3 or P'3.

[0025] According to one embodiment, the negative active material is hard carbon.

[0026] According to one embodiment, in the lamellar oxide, 0 <t.

[0027] According to one embodiment, the lamellar oxide is chosen from: NawMnxiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiFex2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawNixiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiCox2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawMnxiNix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; and NawNixiTix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1.

[0028] According to one embodiment, the lamellar oxide has, at the end of the first discharge and at the end of the subsequent discharges, a crystallographic structure of type P3 and w>0.65 or w>0.70. Brief description of the figures

[0029] [Fig.l] schematically represents the crystallographic structures P2, O3 and P3.

[0030] [Fig.2] is a photo taken with a scanning electron microscope of an oxide monocrystalline sodium lamellar of formula O3-NaNii / 3Fei / 3Mni / 3O2 after 100 cycles.

[0031] [Fig.3] schematically represents the principle of the invention. Detailed description of the embodiments Negative electrode#:

[0032] The sodium-ion cell according to the invention is characterized by a high irreversible capacity. The irreversible loss of capacity occurs during the first charge / discharge cycles of the cell. It is partly due to the formation of a passivation layer on the surface of the negative electrode. This passivation layer is formed by decomposition of the electrolyte in contact with the surface of the negative electrode. This decomposition reaction consumes sodium ions. The sodium ions used in the formation of the passivation layer no longer insert themselves into the positive active material. This results in the earlier occurrence of the end of the discharge of the cell. Since the end-of-discharge voltage is obtained earlier during discharge, the transformation of the P3 structure into the O3 structure is prevented.To promote irreversibility, a ratio of the capacity of the negative active ingredient to the capacity of the positive active ingredient ranging from 1.05 to 1.20 is used. It can be greater than 1.10 or greater than 1.12 or greater than 1.15 or greater than 1.17. It can be less than 1.17 or less than 1.15 or less than 1.12 or less than 1.10.

[0033] The negative active material is preferably hard carbon because it has a high irreversibility. Hard carbon is understood to mean carbon that cannot be transformed into graphite (non-graphitizable), even if it is heated to a temperature exceeding 2500°C. It generally has a disordered, non-crystalline structure. It is obtained by pyrolysis under an inert atmosphere of a precursor at a temperature of approximately 1000°C. The precursor may be a synthetic polymer such as a phenolic resin, polyaniline (PANI) and polyacrylonitrile (PAN). It may be a biopolymer such as sucrose, glucose, cellulose, cotton and chitosan. It may be derived from raw biomass such as leaves, algae, wheat straw and pine. For example, the first charge capacity of hard carbon can range from about 300 to 380 mAh / g and the discharge capacity can range from about 247 to 336 mAh / g, i.e. an irreversibility ranging from 11 to 18%.

[0034] Preferably, the negative electrode does not contain any active material other than hard carbon.

[0035] The negative electrode comprises a current collector, at least one of the faces of which is coated with a layer of a composition of negative active materials, which comprises one or more negative active materials and optionally one or more binders and one or more electronically conductive materials.

[0036] The current collector is preferably a two-dimensional conductive support such as a solid or perforated strip, preferably made of aluminum or an aluminum-based alloy. Its thickness can range from 3 to 25 μm, preferably from 10 to 15 μm. The current collector can be coated on one or both of its faces with a layer of carbon.

[0037] To obtain the negative active material composition, an ink is prepared by dispersing in a solvent or in a mixture of solvents, preferably aqueous, one or more negative active materials, and optionally one or more binders and one or more electronically conductive compounds.

[0038] The binder may be chosen from poly(vinylidene fluoride) (PVDF) and its copolymers, polytetrafluoroethylene (PTFE) and its copolymers, polyacrylonitrile (PAN), poly(methyl or butyl methacrylate), poly(vinyl chloride) (PVC), poly(vinyl formal), polyester, block 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 binder may be chosen from styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), hydrogenated butadiene-acrylonitrile (HNBR). Said at least one binder may be dispersible in aqueous medium, such as polytetrafluoroethylene (PTFE), carboxymethylcellulose (CMC), styrene-butadiene (SBR), butadiene-acrylonitrile (NBR), hydrogenated butadiene-acrylonitrile (HNBR) and polyvinyl alcohol (PVA).

[0039] The electronically conductive material is generally chosen from graphite, carbon black, acetylene black, soot, graphene, carbon fibers, carbon nanotubes or a mixture thereof.

[0040] The ink-coated current collector is dried and then laminated to adjust its thickness. After evaporation of the solvent(s), a layer of a composition of one or more active materials is obtained, the proportions of the various constituents of which are typically: - from 85 to 98% or from 90 to 98% by mass of one or more negative active ingredients, - from 1 to 10% or from 1 to 5% by mass of one or more binders, - from 0 to 5% by mass or from 1 to 5% of one or more electronically conductive materials. Positive electrode#:

[0041] The lamellar oxide(s) have the 0'3 or 03 structure before the first charge of the element and have the P'3 or P3 structure at 80-90% depth of discharge during the first discharge of the element and subsequent discharges. Preferably, they also have the P'3 or P3 structure between 90 and 100% depth of discharge.

[0042] The terms 0'3 and P'3 denote distorted 03 and P3 structures respectively. The term distorted structure means a deformation of the structure of the octahedra formed by the 6 oxygen atoms under the Jahn-Teller effect. Two of the six bonds between the transition metal and the six oxygen atoms have a given length and the other four bonds have a different length, thus inducing a deformation of the octahedron. The particular electronic structure of Mn3+, when this element is present, induces a Jahn-Teller effect responsible for the deformation of the structure of the octahedron.

[0043] Preferably, the crystallographic structures are not distorted.

[0044] Transition elements favoring the P3 or P'3 structure are Ni, Mn and Co. Preferably, Ni is partially substituted by Fe.

[0045] Substituents of the transition elements promoting the P3 or P'3 structure are Mg, Ca, Al, Cu, Zn and mixtures thereof. According to one embodiment, the substituents are selected from Mg, Cu, Zn and mixtures thereof.

[0046] [Fig. 3] schematically represents the principle of the invention. During the first charge, the O3 structure of the oxide transforms into a P3 structure. A significant fraction of the sodium ions participate in the formation of the passivation layer on the negative electrode. During the subsequent discharge, the fraction of sodium ions having participated in the formation of the passivation layer is no longer available to insert into the structure of the positive active material. At the end of discharge, the maximum quantity of sodium ions that can insert into the structure of the oxide is approximately 80%, i.e. a corresponding formula of the oxide Na0j8oMO 2. For a non-optimized oxide composition, the oxide has the O3 structure at the end of discharge. The transformation of the P3 structure into the O3 structure is accompanied by an expansion of the volume of active material which is detrimental to the life of the element.On the other hand, for an oxide according to the invention in which the formation of the P3 structure is favored, the P3 structure is preserved even for a state of charge close to the end of the discharge. This prevents stresses from being exerted on the active material.

[0047] The term "end of cell discharge" corresponds to the instant at which the cell voltage reaches IV at 20-25°C. The term "end of cell charge" corresponds to the instant at which the cell voltage reaches 4V at 20-25°C.

[0048] The lamellar oxide may have a P3 structure in the range of values ​​of the index w from 0.3 to 0.9. The lower limit value may be greater than or equal to 0.35 or greater than or equal to 0.40 greater than or equal to 0.45 or greater than or equal to 0.50 or greater than or equal to 0.55 or greater than or equal to 0.60 or greater than or equal to 0.65 or greater than or equal to 0.70 or greater than or equal to 0.75 or greater than or equal to 0.80 or greater than or equal to 0.85. The upper limit value may be less than or equal to 0.85 or less than or equal to 0.80 or less than or equal to 0.75 or less than or equal to 0.70 or less than or equal to 0.65 or less than or equal to 0.60 or less than or equal to 0.55 or less than or equal to 0.50 or less than or equal to 0.45 or less than or equal to 0.40 or less than or equal to 0.35. In one embodiment, 0.60 <w<0,90.

[0049] The table below indicates formulas of lamellar oxides in which the P3 crystallographic structure is preserved even for charge states close to the end of the discharge.

[0050] [Tables 1] Range of values ​​of w within which the oxide has a P3 structure NawMni / 3Fei / 3Nii / 3O2 0.38-0.69 NawMni / 2Fei / 2O2 0.38-0.70 NawMni / 3Fe2 / 3O2 0.38-0.53 NawNiO2 0.50-0.67 NawNii / 2Fei / 2O2 0.50-0.70 NawCrO2 0.50-0.70 NawMni / 3Coi / 3Nii / 3O2 0.50-0.85 NawMni / 2Nii / 2O2 0.45-0.70 NawNii / 2Tii / 2O2 0.50-0.58

[0051] According to the invention, for a discharge depth of the element ranging from 80 to 90%, at least 75% or at least 90% of the mass of the lamellar oxide has a crystallographic structure of type P3 or P'3. Preferably, 100% of the mass of lamellar oxide has a crystallographic structure of type P3 or P'3. In other words, the O3 structure is absent from the positive electrode at the end of discharge.

[0052] Preferably, for a discharge depth of the element ranging from 90 to 100%, at least 75% or at least 90% of the mass of the lamellar oxide has a crystallographic structure of type P3 or P'3. Preferably, 100% of the mass of lamellar oxide has a crystallographic structure of type P3 or P'3.

[0053] The positive electrode comprises a current collector, at least one of the faces of which is coated with a layer of a composition of positive active materials, which comprises one or more of the active materials described above and optionally one or more binders and one or more electronically conductive materials.

[0054] The binder(s) may be selected from the same list as that described in relation to the negative electrode, without necessarily being the same as those of the negative electrode. Similarly, the electronically conductive material(s) may be selected from the same list as that described in relation to the negative electrode, without necessarily being the same as those of the negative electrode.

[0055] 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 pm or 5 to 20 pm or 10 to 15 pm, preferably 10 to 15 pm.

[0056] An ink is prepared by dispersing one or more positive active materials in a solvent or in a mixture of several organic or aqueous solvents. 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 one or more 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 one or more positive active ingredients, - from 1 to 10% or from 2 to 5% by mass of one or more binders, - from 0.1 to 10% or from 2 to 5% by mass of one or more electronically conductive materials. Electrolyte:

[0057] The electrolyte of the sodium-ion element comprises one or more solvents and one or more sodium salts. The electrolyte may be liquid or gelled.

[0058] The solvent may be chosen from saturated cyclic carbonates, unsaturated cyclic carbonates, linear carbonates, alkyl esters, cyclic esters (lactones), linear ethers, cyclic ethers and mixtures thereof.

[0059] Among the saturated cyclic carbonates, mention may be made of ethylene carbonate (EC), ethylene monofluorocarbonate (FEC), propylene carbonate (PC), butylene carbonate (BC) and mixtures thereof.

[0060] Among the linear carbonates, mention may be made of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC) and mixtures thereof.

[0061] Preferably, the cyclic carbonate(s) represent 50% or less of the total volume of the solvents and the linear carbonate(s) represent 50% or more of the total volume of the solvents. The cyclic carbonate(s) may represent from 10 to 40% or from 20 to 30% of the total volume of the solvents and the linear carbonate(s) may represent from 60 to 90% or from 70 to 80% of the total volume of the solvents. According to one embodiment, the electrolyte comprises a mixture of propylene carbonate (PC) and ethyl methyl carbonate (EMC).

[0062] Among the alkyl esters, mention may be made of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate and mixtures thereof.

[0063] Among the ethers, mention may be made of tetrahydrofuran (THF), 1,3-dioxolane, dimethyl ether (DME) or diethyl ether (DEE) and mixtures thereof.

[0064] According to one embodiment, the electrolyte is free of ether or ester.

[0065] The sodium salt may be hexafluorinated or tetrafluorinated. It may be selected from the group consisting of NaPF6, NaBF4, NaC104, NaAsF6, NaSbF6. Preferably, it is NaPF6.

[0066] The sodium salt may also be chosen from sodium bis(fluorosulfonyl)imide Na(FSO2)2N (NaFSI), sodium bis(trifluoromethanesulfonyl)imide NaN(CF3 SO2)2 (NaTFSI), sodium tris(fluoromethanesulfonyl)methylide NaC(CF3SO 2)3 (NaTFSM), sodium bis(pentafluoroethylsulfonyl)imide NaN(C2F5SO2 )2 (NaBETI), sodium 4,5-dicyano-2-(trifluoromethyl) imidazolide (NaTDI), sodium bis(oxalato)borate (NaBOB), sodium difluoro(oxalato)borate (NaDFOB), sodium tris(pentafluoroethyl)trifluorophosphate NaPF3(CF2CF3)3 (NaFAP), sodium difluorophosphate NaPO2F2 and mixtures of these.

[0067] The total concentration of sodium salts generally ranges from 0.7 to 2 mol.L 1 or from 1 to 1.5 mol.L 1. Separator:

[0068] A separator is generally interposed between a negative electrode and a positive electrode to avoid possible short circuits. It prevents electrical contact between a negative electrode and a positive electrode but nevertheless allows the transport of ions between these two electrodes. The material of the separator can be chosen from the following materials: a polyolefin, for example polypropylene and polyethylene, a polyester, glass fibers bonded together by a polymer, polyimide, polyamide, polyaramid, polyamideimide and cellulose. The polyester 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 chosen from the group consisting of a metal oxide, a carbide, a nitride, a boride, a silicide and a sulfide. This material can be SiO2 or Al2O3.The separator may be coated with an organic coating, for example comprising an acrylate or PVDF or P(VdF-HFP). A preferred separator is made of polyethylene or is made of the combination of three layers which are polypropylene PP / polyethylene PE / polypropylene PP.

[0069] The electrochemical element is manufactured in a conventional manner. It can be prismatic, cylindrical, pocket or button format.

Claims

Claims

1. A method of cycling a sodium-ion electrochemical cell, said method comprising: a) providing a sodium-ion electrochemical cell comprising: - a positive electrode comprising a lamellar oxide of formula NawMxM'tO2where M is at least one transition metal selected from Ti, V, Cr, Mn, Fe, Co and Ni, M' is selected from the group consisting of Mg, Ca, Al, Cu, Zn and mixtures thereof, 0.8 <w<l,l ; 0<x ; 0<t, lequel oxyde lamellaire présente avant une première charge de l’élément électrochimique, une structure cristallographique de type 03 ou 0’3 et, - une électrode négative comprenant une matière active négative, le ratio entre la capacité électrochimique de la matière active négative et la capacité électrochimique de l’oxyde lamellaire allant de 1,05 à 1,2,b) a first charge of the electrochemical element leading to the transformation of the lamellar oxide from the crystallographic structure of type O3 or O'3 to the crystallographic structure of type P3 or P'3, c) a first discharge of the electrochemical element, and for a discharge depth of the electrochemical element ranging from 80 to 90%, at least 75% of the lamellar oxide has a crystallographic structure of type P3 or P'3, d) a repetition of charges and discharges, and for a discharge depth of the electrochemical element ranging from 80 to 90%, at least 75% of the lamellar oxide has a crystallographic structure of type P3 or P'3.,

2. A method according to claim 1, wherein in steps c) and d), at least 90% of the lamellar oxide has a crystallographic structure of type P3 or P'3.

3. A method according to claim 2, wherein 100% of the layered oxide has a crystallographic structure of type P3 or P'3.

4. A method according to any one of claims 1 to 3, wherein the negative active material is hard carbon.

5. A method according to any one of claims 1 to 4, wherein in the lamellar oxide, 0 <t.

6. A method according to any one of claims 1 to 4, wherein M represents Mn and Fe; or M represents Mn, Fe and Ni; or M represents Ni; or M represents Ni and Fe; or M represents Cr; or M represents Mn, Co and Ni; or M represents Mn and Ni; or M represents Ni and Ti.

7. Method according to claim 6, wherein the layered oxide is selected from: NawMnxiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiFex2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawNixiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiCox2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawMnxiNix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; and NawNix[Tix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1.

8. Method according to one of the preceding claims, in which the ratio between the electrochemical capacity of the negative active material and the electrochemical capacity of the lamellar oxide ranges from 1.12 to 1.

17.

9. Method according to one of the preceding claims, in which, the lamellar oxide has at the end of the first discharge of the electrochemical element and at the end of the subsequent discharges, a crystallographic structure of type P3 and w>0.

65.

10. Sodium-ion electrochemical element comprising: - a positive electrode comprising a lamellar oxide of formula NawMxM'tO2 where M is at least one transition metal chosen from Ti, V, Cr, Mn, Fe, Co, Ni, M' is chosen from the group consisting of Mg, Ca, Al, Cu, Zn and mixtures thereof, 0.8 <w<l,l ; 0<x ; 0<t ; et - a negative electrode comprising a negative active material, the ratio between the electrochemical capacity of the negative active material and the electrochemical capacity of the lamellar oxide ranging from 1.05 to 1.2, characterized in that: a) before a first charge of the electrochemical element, the lamellar oxide has a crystallographic structure of type 03 or 0'3, and b) for a discharge depth ranging from 80 and 90% during a first discharge of the electrochemical element and subsequent discharges, at least 75% of the lamellar oxide has a crystallographic structure of type P3 or P'3.

11. The sodium-ion electrochemical element of claim 10, wherein the negative active material is hard carbon.

12. Electrochemical element according to one of claims 10 to 11, in which in the lamellar oxide, 0 <t.

13. Electrochemical element according to one of claims 10 to 11, in which the lamellar oxide is chosen from: NawMnxiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiFex2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawNixiFex2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; NawMnxiCox2Nix3O2 with xl<0.5; x2<0.5 and x3<0.5 with (xl + x2 + x3) < 1; NawMnxiNix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1; and NawNix[Tix2O2 with xl>0.5; x2<0.5 with (xl + x2) < 1.

14. Electrochemical element according to one of claims 10 to 13, in which the lamellar oxide has, at the end of the first discharge and at the end of the subsequent discharges, a crystallographic structure of type P3 and w>0.

65.

15. An electrochemical element according to claim 14, wherein w>0.70.

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