Preparation of an active material for a reference electrode

Abstract

Preparation of an active material for a reference electrode The present invention relates to a method for preparing an active material for a reference electrode, comprising at least the steps of: (1) providing at least one electroactive compound in its reduced form, and the associated redox reaction of which has a constant electrochemical potential for intermediate charge states; (2) providing said electroactive compound in its oxidized form; and (3) mixing the reduced and oxidized forms of said electroactive compound in adjusted proportions to obtain a material whose composition corresponds to an intermediate charge state. It also relates to an active material for a reference electrode obtained according to this method, and its use for preparing a reference electrode.

Description

Title of the invention: Preparation of an active material for a reference electrode Technical field

[0001] The present invention relates to the field of electrochemical storage systems, more precisely the field of electrochemical devices of the metal-ion accumulator or battery type, in particular sodium-ion, lithium-ion, potassium-ion, magnesium-ion batteries; lithium-sulfur or lithium-air batteries; and supercapacitors, for example potassium hybrid supercapacitors.

[0002] More specifically, it aims to propose a new method for preparing a reference electrode that is directly functional and usable in a cell of such an electrochemical storage system, without requiring prior activation of the electrode by electrochemical means. Prior art

[0003] Electrochemical devices of the battery type, for example lithium batteries, and supercapacitors, find multiple applications, in particular in the power supply of thin embedded systems, such as credit cards, smart labels, in the power supply of mobile phones, the storage of energy from photovoltaic cells, or even in the power supply of electric vehicles.

[0004] Various electrochemical storage systems or electrochemical generators have been developed, such as metal-ion batteries, including sodium-ion, lithium-ion, potassium-ion, magnesium-ion batteries; lithium-sulfur or lithium-air batteries; and more recently supercapacitors.

[0005] These electrochemical generators, for example lithium, conventionally operate on the principle of insertion and deinsertion (or intercalation-deintercalation) of a metal, such as lithium, on at least one electrode. In particular, in a lithium-ion accumulator, the Li+ cations thus go back and forth between the electrodes, respectively positive and negative, at each charge and discharge of the accumulator. The active material of the positive electrode is capable of releasing lithium ions at the time of charging and incorporating lithium ions at the time of discharging.

[0006] Supercapacitors are energy storage devices that provide power density and energy density that are intermediate between those provided by electrochemical batteries and conventional electrolytic capacitors. In addition, they release energy more quickly than an electrochemical battery. In standard supercapacitors, also known as "symmetrical" supercapacitors, the two electrodes (positive electrode and negative electrode) are both carbon-based. activated and the electrolyte is classically based on an ammonium salt, typically tetraethylammonium tetrafluoroborate (Et4NBF4) in acetonitrile or propylene carbonate. Supercapacitors, called "hybrid" or "asymmetric" have also been developed, falling somewhere between the symmetrical supercapacitors classically used based on activated carbon and batteries. One of the electrodes, classically the positive electrode, is based on activated carbon; the other electrode, classically the negative electrode, is made from a rechargeable battery material. Charge storage in a hybrid supercapacitor occurs at the negative electrode through a redox reaction, while charge storage at the positive electrode occurs through the formation of an electrochemical double layer.

[0007] It is important, for electrochemical storage systems, whether batteries or supercapacitors, to be able to know and monitor the evolution of the potential of each electrode independently and in relation to an electrode of known potential, during the operation of the electrochemical cell. In particular, for cells using electrolytes based on organic solvent(s), it is important that the cell of the electrochemical system does not operate outside the electrochemical stability range of said organic solvent(s), so as not to cause degradation and in particular the production of gas.Similarly, it is important to control, via potential limitation, the lithiation state of certain compounds such as the lamellar oxides LixM02 whose structure is unstable for x<0.5 (case of LiCoO2) or x<0.2 (LixM02 with M=Ni, Co, Mn with low Co content), or to avoid the electrodeposition of lithium in the metallic state at the negative electrode at potentials lower than 0V vs Lr7Li. In order to counter the appearance of any undesirable phenomenon, it is important to be able to know and follow the evolution of the potential of each electrode, during the operation of the electrochemical cell.

[0008] For this, it is desirable to be able to introduce a so-called reference electrode into the electrochemical system. A reference electrode must have a stable and known potential, making it possible to know at any time the potential of each of the positive and negative electrodes of the system.

[0009] Generally speaking, as listed for example in the document Cengiz et al. [1] or proposed in the documents US 2009 / 0104510 and US 9,379,418, the reference electrodes proposed for electrochemical storage systems, in particular for metal-ion batteries, such as lithium-ion, sodium-ion, calcium-ion, lithium-ion, potassium-ion, magnesium-ion batteries, or for supercapacitors, are based on the implementation, as active electrode material: - pure metals, such as lithium, sodium or potassium, or lithium-based alloys (e.g., LixSn, LixBi, LixAu, LixIn, LixSi, LixAl, etc.), respec- primarily based on sodium, potassium, etc.; and - intercalation / insertion materials of the LiMO2 type with M representing Co, Ni or Mn; LiM'2O4 with M' representing Ni or Mn; LiM”PO4 with M” representing Fe, Co, Mn or Ni; or of the Li4Ti50i2 type.

[0010] To be suitable for use as a reference electrode, the compounds must exhibit, during the galvanostatic charge / discharge process, a potential plateau (associated redox reaction exhibiting a constant electrochemical potential), and their thermodynamic potential must be independent of the salt concentration of the electrolyte.

[0011] As for metals and alloys, they can be used directly in a three-electrode system, but exhibit potential instability over time / cycling, particularly due to their very low thermodynamic potential. Indeed, their reducing power induces a strong reactivity with the electrolytes and their potential, lower than that of the H2O / H2 or O2 / H2O pair (which allows spontaneous reactions with H2O and O2), requires control of the atmosphere during the assembly steps. In addition, the thermodynamic potential of an electrode based on pure metals is likely to vary depending on the variations in the concentration of lithium ions in the electrolyte. Certainly, it can be considered that the average concentration of lithium ions in the inter-electrode space does not change.However, concentration gradients can be established, which are greater when the applied currents are high, and are therefore likely to vary the thermodynamic potential of the reference electrode. In addition, there is also a risk of variation in the potential of the reference electrode during aging, because the concentration of lithium ions in the electrolyte is likely to vary due to degradation mechanisms (drying of the liquid phase).

[0012] These disadvantages do not concern the other aforementioned electrode materials, for which the thermodynamic potential of the electrode is fixed by the number of active / inactive sites or the lithium concentration (respectively, in sodium, potassium) in the alloy. Materials having a wide potential plateau as a function of the insertion rate are particularly advantageous, since they allow stability of the potential, independently of changes in the lithiation state of the material likely to occur during aging.

[0013] Insertion materials, for example Li4Ti50i2 or LiFePO4, characterized by biphasic behavior, are therefore favored. In particular, the chemical, thermal and electrochemical stability of LiFePO4 (or LFP), as well as its average oxidation-reduction potential higher than the potential of the other aforementioned redox materials and constant over a wide window of lithiation rates, make LFP a material of interest for reference electrodes.

[0014] More recently, the CEA has also proposed the use, for the preparation of reference electrodes, of electroactive organic materials of type P (electron donor) or type N (electron acceptor).

[0015] However, electrodes based on the aforementioned materials (lithium insertion / intercalation materials, or P or N type materials) require prior "activation" in order to be operational as reference electrodes. Indeed, these electroactive materials only have a fixed electrochemical potential for intermediate charge states (oxidation / reduction states, or lithiation states) (from approximately 5% to 95% state of charge (SoC)), which implies prior conditioning of the electrodes in order to reach the potential plateau (generally, at approximately 50% SoC).

[0016] This step complicates the preparation and implementation of these reference electrodes. Currently, the activation of the reference electrodes is carried out exclusively electrochemically. Several methods for this electrochemical activation of the electrodes exist, the most common consisting of: (1) partially charge or discharge the reference electrode in a first two-electrode device (for example, in a half-cell with a lithium counter-electrode), then disassemble the assembly in order to recover the “activated” reference electrode, possibly rinsing it, before inserting it into the electrochemical cell for which it is intended. However, in this variant, the step of transposing the electrode from one cell to another induces a risk of deterioration of the reference electrode. (2) carrying out a partial charge or discharge of the reference electrode at the level of a three-electrode assembly, directly within the electrochemical cell for which the reference electrode is intended, by connecting, in a first stage, the reference electrode as a working electrode then, after activation, by carrying out the three-electrode connection with the reference electrode.

[0017] This approach, although it makes it possible to avoid the disassembly step required in the first variant mentioned above, is nevertheless not without its drawbacks. In fact, the activation step has the consequence of consuming part of the cell's capacity, and controlling the state of charge of the reference electrode is delicate. In addition, this approach requires knowing the exact quantity of active material deposited or carrying out an electrochemical quantification step (charge / discharge cycle) to know the effective capacity of the electrode. Finally, since this activation step is carried out after the formation step (first charge / discharge cycle(s)) of the cells, it is not possible to have access to the information on the potentials during the formation, although this is an interesting piece of data. Statement of the invention

[0018] The present invention aims precisely to propose a new method for preparing reference electrodes, making it possible to overcome the drawbacks, discussed previously, linked to the need to activate the electrode beforehand electrochemically.

[0019] More particularly, the invention proposes to prepare a reference electrode, directly functional (or usable) within a cell of an electrochemical storage system, from a mixture, in particular in the form of a powder, of the oxidized and reduced forms of an electroactive compound whose associated redox reaction has a constant electrochemical potential at intermediate charge states.

[0020] Thus, the invention relates, according to a first of its aspects, to a method for preparing an active material for a reference electrode, comprising at least the steps consisting of: (1) have an electroactive compound in its reduced form, and whose associated redox reaction presents a constant electrochemical potential (or potential plateau) for intermediate charge states; (2) having said electroactive compound in its oxidized form; and (3) mixing said reduced and oxidized forms of said electroactive compound in adjusted proportions to obtain a material whose chemical composition corresponds to an intermediate charge state.

[0021] Preferably, said electroactive compounds in reduced and oxidized form are in the form of powders, the material in step (3) being obtained by mixing said powders.

[0022] Thus, the invention relates more particularly to a method for preparing an active material powder for a reference electrode, said method comprising at least the steps consisting of: (1) have a powder of electroactive compound in its reduced form, and whose associated redox reaction presents a potential plateau for intermediate charge states; (2) having a powder of said electroactive compound in its oxidized form; and (3) mixing said powders (1) and (2) in adjusted proportions to obtain a powder whose composition corresponds to an intermediate charge state.

[0023] The invention also relates to the active material for a reference electrode, preferably in the form of a powder, obtained according to the method described above.

[0024] More particularly, the invention relates to a powder, useful as an active material of a reference electrode, comprising: (1) particles of an electroactive compound in its reduced form, and whose associated redox reaction exhibits a potential plateau for intermediate charge states; (2) particles of said electroactive compound in its oxidized form; the proportions of particles (1) and (2) being such that the chemical composition of said powder corresponds to an intermediate charge state.

[0025] The "potential plateau", associated with an electroactive compound or material, can be identified by a cycling test of an electrode based on said electroactive compound or material, assembled in a half-cell or in a complete cell, for example in the form of a button cell, by carrying out a charge or a discharge in galvanostatic mode. The constant electrochemical potential corresponds to the potential plateau observed at the level of the charge / discharge curves at constant current (for example, curves of variation of the potential E as a function of time or as a function of the absolute or specific mass capacity during cycling in galvanostatic mode). Note that in a complete cell (this is always the case in a half-cell with a lithium counter-electrode), this potential plateau is only observable if the counter-electrode used (which then plays the role of reference) also has a potential plateau during its charge / discharge at constant current.

[0026] The potential plateau is thus reached for intermediate charge states, which can generally range from 5% to 95% of the full charge.

[0027] Of course, the value of the potential plateau depends on the nature of the electroactive compound used.

[0028] By "intermediate charge state", we mean according to the invention a state of partial oxidation / partial reduction (or even partial lithiation) of said electroactive compound or material or a mixture of the oxidized and reduced forms, corresponding to the fixed potential (potential plateau) of the redox reaction associated with the electroactive compound.

[0029] In the case of LiFePO4, the material in an intermediate charge state is for example characterized by a two-phase behavior, with the coexistence of two different crystallographic phases (totally lithiated and totally delithiated phases) identifiable for example by X-ray diffraction analysis (XRD). In the case of a material of the LiM02 type, this intermediate charge state results in a solid solution phenomenon, with the metal M present in mixed oxidation states (M(III) and M(IV)) in the crystals.

[0030] The intermediate state of charge of the material (or mixture) obtained according to the invention can be verified by a cycling test (charge / discharge) of an electrode based on said material, assembled in a half-cell or complete cell, for example in a half-battery, as illustrated in the examples.

[0031] Advantageously, the chemical composition of the active material obtained according to the invention can be adjusted very precisely by controlling the proportions of compounds in reduced and oxidized form during mixing, in order to achieve the desired intermediate charge state.

[0032] Advantageously, the active material according to the invention is obtained from an electroactive compound in reduced or conjugated form, said compound in the oxidized or reduced conjugated form being obtained, prior to mixing in step (3), by total chemical oxidation or reduction.

[0033] In a particularly preferred embodiment, as detailed in the remainder of the text, the electroactive compound in its oxidized form (2) is previously obtained from said electroactive compound in its reduced form, by total chemical oxidation using at least one oxidizing agent.

[0034] Advantageously, the active material obtained according to the invention, in particular in the form of a powder, can be used for the preparation of a directly operational reference electrode when it is integrated within a cell of an electrochemical storage system.

[0035] The invention thus relates, according to another of its aspects, to the use of an active material obtained according to the invention, in particular in powder form, for the preparation of a reference electrode.

[0036] It also relates to a method for preparing a reference electrode, directly functional within a cell of an electrochemical storage system, said method comprising at least the steps consisting of: (i) preparation of a dispersion, called “ink”, comprising, in one or more aqueous and / or organic solvents, said active material, in particular in the form of a powder, obtained according to the process described above; one or more binders and optionally one or more electronically conductive additives; (ii) deposition of the ink on the surface of a current collector; (iii) evaporating said solvent(s) from the ink to form the electrode film; and preferably

[0037] (iv) calendering of the electrode.

[0038] As mentioned previously, a reference electrode is defined as being an electrode having a stable and known electrochemical potential, advantageously making it possible to serve as a reference point for independently measuring the potential of the positive and negative electrodes in an electrochemical cell.

[0039] By "directly functional" is meant that the electrode prepared according to the invention is directly operational / usable as a reference electrode. It can thus be inserted into an electrochemical cell to form a three-electrode cell and be used as a reference electrode from the first charge / discharge cycle of the cell.

[0040] In particular, the reference electrode obtained at the end of step (iii) or (iv) does not require any electrochemical activation step before it can be used as a reference electrode.

[0041] Advantageously, the method for preparing a reference electrode according to the invention thus makes it possible to dispense with any electrochemical step, prior to the use of the electrode as a reference electrode, which would be likely, as discussed previously, to damage the reference electrode or to disrupt the operation of the electrochemical system.

[0042] Thus, the method for preparing a reference electrode according to the invention does not include any electrochemical activation step, prior to the operation of the reference electrode within a three-electrode cell, whether within an electrochemical cell distinct from that in which the reference electrode is intended to be implemented or within the cell of the electrochemical system in which it is used.

[0043] Furthermore, the method for preparing a reference electrode according to the invention advantageously allows, by adjusting the composition of the active material formed according to the invention, very good control of the state of charge of the reference electrode material.

[0044] The functional reference electrode prepared according to the method of the invention can be implemented for various electrochemical storage systems or electrochemical generators, and in particular for metal-ion batteries, for example sodium-ion, lithium-ion, potassium-ion, magnesium-ion batteries; lithium-sulfur or lithium-air batteries, and supercapacitors, for example potassium hybrid supercapacitors.

[0045] It can thus be directly integrated into a cell of such an electrochemical storage system or electrochemical generator, to form a cell with three electrodes: a positive electrode, a negative electrode and the reference electrode prepared according to the invention.

[0046] The integration of a reference electrode obtained according to the method of the invention within at least one of the cells of an electrochemical storage system or electrochemical generator, advantageously makes it possible to access the potentials of each of the positive and negative electrodes, and this from the first charge / discharge cycle of the cell. Knowledge and monitoring of the potentials of the positive and negative electrodes makes it possible to detect, where appropriate, potential defects in their operation.

[0047] Monitoring of the potentials of the positive and negative electrodes during operation of the electrochemical storage system, by means of the reference electrode prepared according to the invention, advantageously allows use optimized use of said electrochemical system, in particular use at its maximum capacity, without exposing itself to risks of malfunction, and thus allows improved safety in the use of said electrochemical system.

[0048] Other characteristics, variants and advantages of the preparation of the active material according to the invention and of its implementation for the preparation of a reference electrode according to the invention will emerge more clearly upon reading the description, examples and figures which follow, given by way of illustration and not limitation of the invention. Brief description of the drawings

[0049] [Fig.l] shows, schematically, in cross-section, an example of assembly 100 of the different elements, including the reference electrode prepared according to the invention, within a cell of an electrochemical storage system;

[0050] [Fig.2] shows the diffractogram obtained by DRX analysis (ray diffraction) X) powder prepared in example 1.1. by total delithiation of LiFePO4;

[0051] [Fig.3] shows the evolution of the current (in A) and the potential (in V) during the charge / discharge cycles carried out according to example 1.3., with the reference electrode assembled in half-cell;

[0052] [Fig.4] shows the evolution of the potential (in V vs Li+ / Li) as a function of the capacity specific mass (in mAh / g) during the two charge / discharge cycles carried out according to example 1.3., with the reference electrode assembled in half-cell;

[0053] [Fig.5] shows, schematically, the welding of an electrode on a aluminum connecting tab as made in example 1;

[0054] [Fig.6] shows, schematically, the lithium-ion cell, in pouch format- cell, prepared in example 2, comprising a reference electrode prepared according to the invention;

[0055] [Fig.7] shows the evolution curves of the cell voltage of the lithium- ion, and potentials of the positive and negative electrodes, as a function of the capacity, obtained using a reference electrode prepared according to the invention, and using a conventional reference electrode (requiring activation) for a first discharge at C / 10, a first charge at C / 2 and a discharge at C / 2, as described in example 2;

[0056] [Fig.8] shows the evolution curves of the cell voltage of the lithium- ion, in galvanostatic charge and discharge, at different regimes, obtained using a reference electrode prepared according to the invention and a conventional reference electrode, as described in example 2.

[0057] It should be noted that, for reasons of clarity, the various elements in Figures 1, 5 and 6 are shown in free scale, the actual dimensions of the various parts not being respected.

[0058] In the rest of the text, the expressions “between ... and ...”, “ranging from ... to ...” and “varying from ... to ...” are equivalent and are intended to mean that the limits are included, unless otherwise stated. Detailed description Active material

[0059] As indicated previously, the development of a reference electrode according to the invention is based on the preparation of a material, a mixture of the conjugated forms of at least one electroactive compound whose electrochemical oxidation-reduction reaction, also called a redox reaction, associated with the reversible passage from the reduced state of the electroactive material to the oxidized state, has the particularity of presenting a constant electrochemical potential or electrochemical potential plateau.

[0060] Electroactive compound in reduced form and in oxidized form (1) and (2)

[0061] The electroactive compound(s) from which the active material is prepared for the reference electrode according to the invention can be chosen from the electroactive compounds usually used to form electrochemically activated reference electrodes, as mentioned previously.

[0062] As indicated previously, the active material according to the invention is more particularly obtained from an electroactive compound in its oxidized or reduced form, said compound in the reduced or oxidized conjugated form being obtained by total chemical oxidation or total chemical reduction.

[0063] According to a first variant embodiment, the active reference electrode material according to the invention is obtained from at least one electroactive compound in its reduced form, and therefore capable of being oxidized.

[0064] Electroactive compounds capable of being oxidized, which can be used for the preparation of an active material according to the invention, can be chosen for example from intercalation (or insertion) compounds of the LiM02 type with M representing Co, Ni or Mn; LiM'2O4 with M' representing Ni or Mn; and LiM”PO4 with M” representing Fe, Co, Mn or Ni.

[0065] It may also be electroactive organic compounds of type P whose associated redox reaction has a constant electrochemical potential.

[0066] By “organic” compound is meant a compound, in particular of a molecular, oligomeric or polymeric nature, of carbon chemistry, comprising carbon and hydrogen atoms, and one or more heteroatoms chosen from oxygen, nitrogen, sulfur, phosphorus, these atoms and heteroatoms being linked only by covalent bonds, this compound possibly being in the form of a salt, for example a metal salt.

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[0071] By "P-type" or "electron donor" electroactive compound is meant an organic compound which can undergo, due to the nature of its electroactive functions or redox functions, a reversible oxidation from the neutral state to a positively charged state, with anionic charge compensation. The electrochemical mechanism associated with a P-type compound can be schematized as follows: [Chem.l] The P-type electroactive organic compounds considered according to the invention can be chosen from organic compounds, molecules, oligomers or organic polymers, already proposed as active materials for organic electrodes, for example for batteries. Such materials are documented in the literature [3]. In particular, such electroactive organic compounds of type P may be chosen from compounds, in particular molecules, oligomers or polymers, carrying redox functions chosen from: - stable radicals O', N' or NO', in particular nitroxide (NO') or phenoxy (PhO') functions; Such polymers may be chosen, for example, from polymers having a linear skeleton chosen from poly(meth)acrylates, polythiophenes, polyfluorenes, polycarbazoles, polyanilines, polyphenylenes, polyisothionaphthenes, polyacetylenes, polyphenylenevinylenes, and their copolymers; and bearing at least side groups having at least one stable radical of type O', N' or NO', in particular having at least one nitroxide and / or phenoxy function, in particular at least one nitroxide function. These may be, for example, polymers carrying side groups of 2,2,6,6-tetramethylpiperidinyl-N-oxy (TEMPO) structure, such as a poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate), denoted PTMA. - conjugated ether (-O-) or thioether (-S-) functions; such as thianthrene, phenothiazine, and their oligomeric or polymeric derivatives; ir-conjugated conductive polymers of polythiophene (PT) type, for example poly(3,4-ethylenedioxythiophene), better known as “PEDOT”; - conjugated amine functions, in particular aromatic or heteroaryl amine groups, such as dianiline, triphenylamine, polyaniline or polypyrrole, molecules of the viologen or carbazole type, polyviologens and poly- carbazoles; and other ir-conjugated conductive polymers, such as polythiophenes, polyanilines and polypyrroles, e.g. polyacetylenes.

[0072] The P-type organic compound may be in different forms, provided that they allow the chemical oxidation of said P-type organic compound in the presence of an oxidizing agent to access the compound in its conjugated form (oxidized form). In particular, the P-type compound may be in a form such that it does not degrade, in particular does not dissolve in the electrolyte of the electrochemical cell in which the reference electrode will be implemented.

[0073] Several strategies are known to insolubilize redox organic compounds.

[0074] As mentioned above, one of the most commonly used strategies is to prepare the organic redox compound in the form of polymers or copolymers, crosslinked or not, with pendant or intramolecular redox functions.

[0075] Another strategy for insolubilizing electroactive organic compounds consists of producing composite structures by physisorption, with an inorganic substrate, for example silica, alumina, carbon, within which weak bonds are created (hydrogen, ji-ji interactions, Van der Waals), making it possible to limit, or even prevent, the dissolution of the redox organic compound.

[0076] It is also possible to graft the electroactive organic compound onto an inorganic solid (silica, alumina, carbon, etc.) via ionic or covalent bonds, for example by acid-base reaction with the hydroxyls of the surface of the silica or by reaction of diazonium salts with the surface of the carbon, in order to form an insoluble organic / inorganic hybrid compound.

[0077] Another strategy consists of adding ionic functions to the electroactive organic compound, for example of the carboxylate, sulfonate, alcoholate, imide, enolate, etc. type, making it possible to limit the dissolution of the compound in an organic medium.

[0078] Preferably, the reference electrode active material according to the invention is prepared from an electroactive intercalation compound of the LiM02 type with M representing Co, Ni or Mn; LiM'2O4 with M' representing Ni or Mn; and LiM”PO4 with M” representing Fe, Co, Mn or Ni.

[0079] As indicated previously, the electroactive compound in oxidized form (2) can be previously obtained by total chemical oxidation of said starting electroactive compound in its reduced form, using at least one oxidizing agent.

[0080] Preferably, said electroactive compounds are in the form of powders.

[0081] Thus, according to a particular embodiment of the invention, the method for preparing an active reference electrode material according to the invention comprises at least the following steps: - have a powder of electro-active compound in its reduced form, suitable for being oxidized, as described above, in particular chosen from intercalation compounds, for example LiFePO4; - obtaining a powder of said electroactive compound in its oxidized form, by total chemical oxidation of a powder of said electroactive compound using at least one oxidizing agent; - mixing the electroactive compound powders in reduced form and in oxidized form, in adjusted proportions to obtain an active material powder whose composition corresponds to an intermediate charge state.

[0082] Of course, the nature of the oxidizing agent, as well as the conditions of the oxidation reaction, are adjusted with regard to the nature of the electroactive compound that one seeks to oxidize.

[0083] Examples of oxidizing agents include: nitronium tetrafluoroborate (NO2BF4), nitrosyl tetrafluoroborate (NOBF4), potassium persulfate (K2S2O8), sodium nitrate (NaNO3), nitrile trifluoromethanesulfonate (NO2CF3SO3), potassium permanganate (KMnO4), nitric acid (HNO3), sulfuric acid (H2SO4), peroxydisulfuric acid (H2S2O8), peroxymonosulfuric acid (H2SO5), “Magic Blue” (Tris(4-bromophenyl)aminium hexachloridoantimonate), hydrogen peroxide (H2O2), lead dioxide (PbO2), organic peracids such as metachloroperbenzoic acid (mCPBA), metal cations such as Ce4+, Ag+, Cu2+, gases such as O2, O3, N2O, etc.

[0084] The oxidation reaction can be carried out in bulk (in the absence of solvent) or by wet method (in the presence of one or more solvents).

[0085] It is understood that the reaction of said electroactive compound with said oxidizing agent(s) may be followed by one or more steps of removing the solvent(s) and / or washing the compound obtained.

[0086] In a particularly preferred embodiment, the electroactive compound used for the preparation of an active reference electrode material according to the invention is chosen from intercalation compounds of the LiM”PO4 type with M” representing Fe, Co, Mn or Ni.

[0087] Advantageously, it may be LiFePO4 (or LFP) whose associated redox reaction LiFePO4 / FePO4 has a potential plateau at approximately 3.424 V. The electroactive compound in its oxidized form, FePO4, may for example be obtained by oxidation using NO2BF4, in particular in a solvent medium, preferably in acetonitrile. Such an oxidation reaction is for example described in publication [2].

[0088] Thus, according to a particular embodiment, the electroactive compound (1) is LiFePO4 and the electroactive compound (2) is FePO4, in particular said electroactive compound (2) being previously obtained by total chemical oxidation of LiFePO4 using an oxidizing agent, in particular by reaction with NO2BF4 in acetonitrile.

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[0097] In a particular embodiment, the preparation of an active material for a reference electrode according to the invention proceeds according to the following steps: - have LiFePO4 powder; - preparing a FePO4 powder, by total chemical oxidation of a LiFePO4 powder using at least one oxidizing agent, for example using NO2BF4 in acetonitrile; and - mixing said LiFePO4 and FePO4 powders in adjusted proportions to obtain said active material powder. According to another embodiment variant, the active reference electrode material according to the invention is obtained from an electroactive compound in its oxidized form and therefore capable of being reduced. Electroactive compounds capable of being reduced, which can be used for the preparation of an active material according to the invention, are for example chosen from intercalation / insertion compounds of the LqTisOn type. These may also be electroactive organic compounds of type N whose associated redox reaction has a constant electrochemical potential. By "N-type" or "electron acceptor" electroactive compound is meant an organic compound which can undergo, due to the nature of its electroactive functions, a reversible reduction from the neutral state to a negatively charged state, with cationic charge compensation. The electrochemical mechanism associated with an N-type material can be schematized as follows: [Chem. 2] The N-type electroactive organic compounds considered according to the invention can be chosen from organic compounds, molecules, oligomers or organic polymers, already proposed as active materials for organic electrodes, for example for batteries, as described in publication [3]. In particular, such electroactive organic compounds of type N can be chosen from compounds, in particular molecules, oligomers or polymers, carrying redox functions chosen from: - conjugated azo functions (-N=N-) such as azobenzene or one of its derivatives, - disulfide functions (-SS-) such as diphenyl disulfides, tetrathionap-thalene, poly(2,5-dimercapto-1,3,4-thiadiazole), poly(5,8-dihydro-1H,4H-2,3,6,7-tetrathia-anthracene); And - conjugated carbonyl functions, in particular contained within quinonoid structures, such as anthraquinone; anhydride structures such as pyromellitic anhydride, dimide structures such as perylene dimide; or carboxylate structures such as perylene tetracarboxylate.

[0098] For example, an electroactive organic compound of type N may be based, for example, on perylene tetracarboxylate, also noted PTCLi4.

[0099] As mentioned previously concerning the P-type organic compounds, said N-type organic compound(s), carrying redox units as considered above, may be in different forms, as indicated above, provided that they allow the total chemical reduction of said N-type organic compound in the presence of a reducing agent to access the compound in its conjugated (reduced) form.

[0100] Preferably, the active reference electrode material according to the invention is prepared from an electroactive intercalation compound of the Li4Ti50i2 (or LTO) type, the associated redox reaction of which has a potential plateau at approximately 1.5 V.

[0101] As indicated previously, the electroactive compound in reduced form (1) can be previously obtained by total chemical reduction of said starting electroactive compound in its oxidized form, using at least one reducing agent.

[0102] Thus, according to a particular embodiment, the method for preparing an active reference electrode material according to the invention comprises at least the following steps: - having a powder of electroactive compound in its oxidized form, capable of being reduced, as described previously, in particular chosen from intercalation compounds, for example Li4Ti50i2; - obtaining a powder of said electroactive compound in its reduced form, by total chemical reduction of a powder of said electroactive compound using at least one reducing agent; - mixing the electroactive compound powders in oxidized form and in reduced form in adequate proportions to obtain an active material powder whose composition corresponds to an intermediate charge state.

[0103] Of course, the nature of the reducing agent, as well as the conditions of the reduction reaction, are adjusted with regard to the nature of the electroactive compound that one seeks to reduce.

[0104] Examples of reducing agents include: alkali metals such as lithium, sodium, potassium; hydrides such as lithium hydride (LiH) and sodium borohydride (NaBH4), hydrogen, carbon, salts such as sodium sulfite; sodium (Na2SO3), sodium hydrosulfite (Na2S2O4), sodium thiosulfate (Na2S2O3), hydrazine (N2H4), lithium iodide (LiI).

[0105] Mixing to obtain the active material

[0106] As indicated previously, the proportions of electroactive compound in reduced form and electroactive compound in oxidized form in the mixture in step (3) are adjusted so as to obtain a material in an intermediate charge state, corresponding to the potential plateau of the redox reaction.

[0107] Advantageously, the method of the invention makes it possible to precisely control the state of charge of the active electrode material via the adjustment of the proportions of each of the conjugated forms of the electroactive compound.

[0108] The proportions of electroactive compound in reduced form and in oxidized form to be used to achieve the desired intermediate charge state can be determined by preliminary tests, by varying the proportions of the conjugated forms.

[0109] The intermediate state of charge of the material obtained according to the invention can be verified by a cycling test (charge / discharge) of an electrode based on said material, assembled in a half-cell or complete cell, for example in a half-battery, as illustrated in the examples.

[0110] As indicated previously, the mixing is preferably carried out by mixing a powder of said electroactive compound in reduced form and a powder of said electroactive compound in oxidized form.

[0111] Advantageously, said powders have a similar particle size, preferably with an average particle size of between 50 nm and 20 pm, in particular between 1 pm and 10 pm.

[0112] The average particle size can be evaluated for example by laser granulometry, scanning electron microscopy or light scattering.

[0113] Preferably, the active material powder obtained according to the invention, in particular by mixing the powders of said electroactive compound in oxidized and reduced form, has a particle size suitable for easy implementation during the formulation of the ink based on said electroactive material for the preparation of the reference electrode.

[0114] In particular, the active material powder may have an average particle size of between 50 nm and 20 pm, in particular between 1 pm and 10 pm.

[0115] In the particular case of the preparation of an active material from an electroactive intercalation compound of the LiM02; LiM'2O4 or LiM”PO4 type, for example of the LiFePO4 type, the proportions of electroactive compound in reduced form (LiM02; LiM'2O4 or LiM”PO4) and of electroactive compound in oxidized or delithiated form (MO2; M'2O4 or M”P04), preferably in the form of powders, in the mixture in step (3) are adjusted to obtain a material, in particular in powder form, whose chemical composition is Lii XMO2; Lii XM'2O4 or Lii XM”PO4, for example Li i xFePO4, with 0.30 < x < 0.70, in particular 0.50 < x < 0.60.

[0116] In particular, the preparation of an active material of composition Lii xFePO4, with 0.30 < x < 0.70, preferably involves the mixing of a LiFePO4 powder and a FePO4 powder in a LiFePO4 / FePO4 mass ratio of between 0.05 and 20, in particular between 0.5 and 2.

[0117] Implementation of the active material for the preparation of a reference electrode

[0118] As indicated previously, the active material formed by the mixture of the reduced and oxidized forms of said electroactive compound, preferably in the form of a powder, is used for the preparation of a reference electrode.

[0119] The integration of the active material at the level of an electrode can be carried out by any conventional method of preparing electrodes.

[0120] The formulation of said active material at the level of an electrode can thus comprise at least the following steps: (i) preparation of a dispersion, more commonly called “ink”, comprising, in one or more aqueous and / or organic solvents, said active material in an intermediate charge state; one or more binders and optionally one or more electronically conductive additives; (ii) deposition of the ink on the surface of a current collector; (iii) evaporating said solvent(s) from the ink to form the electrode film; and preferably (iv) calendering of the electrode.

[0121] The ink solvent may comprise water and / or one or more organic solvents, for example N-methyl-2-pyrrolidone.

[0122] The electronically conductive additives are implemented to improve the electronic conductivity of the electrode. They can be chosen for example from carbon fibers, carbon black, carbon nanotubes and their mixtures.

[0123] Said electronically conductive additive(s), when present, may preferably be implemented in a content ranging from 1 to 10% by mass, in particular from 2 to 8% by mass, relative to the mass of the electrode. It is understood that the total mass of the electrode does not include the mass of the current collector.

[0124] The binders are used to ensure the mechanical strength of the electrode on the current collector, the cohesion of the different components of the electrode or even its flexibility properties. They can be chosen from polymer binders, in particular chosen from fluorinated polymers, for example from polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polymers derived from carboxymethylcellulose (CMC), polysaccharides and latexes, particularly of the styrene-butadiene rubber (BR) type.

[0125] A particularly preferred binder is poly(vinylidene fluoride) (PVDF).

[0126] Said binder(s) may be present in an amount less than or equal to 20% by mass, relative to the total mass of the electrode, in particular less than or equal to 10% by mass, in particular between 1 and 10% by mass, relative to the total mass of the electrode. It is understood that the total mass of the electrode does not include the mass of the current collector.

[0127] The ink may be homogenized before spreading.

[0128] The deposition of the ink on the surface of the current collector can be carried out by any technique known to those skilled in the art, for example by coating, by a printing technique, by extrusion or by co-lamination.

[0129] The current collector may be solid, for example in the form of a strip or in the form of a grid.

[0130] The current collector may be made of a metallic material chosen from copper, aluminum, nickel or stainless steel potentially coated with carbon.

[0131] The evaporation of the solvent(s) from the ink may be carried out by drying, for example in an oven, at a temperature of between 20 and 150°C, in particular between 50 and 80°C, in particular for a period of between 1 and 15 hours.

[0132] Preferably, the ink drying step is followed by a calendering step.

[0133] Said active material may represent from 5 to 98% by mass, in particular at least 30% by mass, in particular at least 50% by mass and more particularly from 70 to 96% by mass, relative to the total mass of the electrode, excluding the mass of the current collector.

[0134] The grammage of the electrode, in other words the mass of active material formed according to the invention, per unit area, can be adjusted by controlling the rate of active material and the thickness of deposition, in particular coating, of the ink formulated on the current collector. Preferably, the grammage of the electrode is between 0.1 mg / cm2 and 50 mg / cm2, in particular between 1 mg / cm2 and 30 mg / cm2, and more particularly between 3 mg / cm2 and 10 mg / cm2. ELECTROCHEMICAL STORAGE SYSTEM

[0135] As indicated above, the electrode prepared from the active material obtained according to the invention is directly functional as a reference electrode. In other words, it can be used as a reference electrode, without a prior step, within a cell of an electrochemical storage system or electrochemical generator.

[0136] In particular, the method for preparing a reference electrode according to the invention does not include any step of electrochemical activation of the electrode obtained at the end of step (iii) or (iv), prior to its operation as a reference electrode.

[0137] The electrochemical storage systems, in which a reference electrode obtained according to the invention can be incorporated, can be diverse. It can be a metal-ion battery, in particular sodium, lithium, potassium, magnesium batteries; a lithium-sulfur or lithium-air battery.

[0138] It may also be a supercapacitor, in particular a “standard” supercapacitor or a so-called “hybrid” supercapacitor, in particular a potassium hybrid supercapacitor.

[0139] The cell of the electrochemical storage system may typically comprise a positive electrode, a negative electrode and an electrolyte, in particular impregnating a porous membrane separating said electrodes.

[0140] It may also be a cell based on a ceramic electrolyte membrane, or based on a solid electrolyte, for example a solid polymer electrolyte or hybrid solid electrolyte.

[0141] The reference electrode prepared according to the method of the invention is thus integrated as a third electrode at the cell level of the electrochemical storage system.

[0142] The invention thus relates, according to another of its aspects, to a method for preparing a cell of an electrochemical storage system, for example a lithium battery, comprising a step of preparing a reference electrode according to the method as described previously, followed by a step of integrating said reference electrode at the level of a cell of the electrochemical storage system to obtain a cell with three electrodes.

[0143] A cell of an electrochemical storage system, within which a reference electrode obtained according to the invention is used, is thus a three-electrode system, comprising more particularly: - a positive electrode; - a negative electrode; and - said reference electrode prepared according to the method of the invention as described previously.

[0144] The nature of the elements constituting the cell of the electrochemical storage system, in particular the positive and negative electrodes and the electrolyte, of course depends on the type of device considered, depending on whether it is a metal-ion battery, a lithium-sulfur battery, a lithium-air battery, a supercapacitor, etc.

[0145] The said active material(s) of the negative electrode may in particular be chosen from activated carbon, carbon materials for intercalating an element alkali such as graphite; silicon; conversion materials of the MO2 type with M representing for example Sn (SnO2), Si (SiO2), or of the M3O4 type with M representing for example Co (Co3O4), Mn (Mn3O4) etc., metals (Li, Na, K, Mg, Ca) and alloys based on these metals, titanium oxides of the Li4Ti50i2 type.

[0146] Said active material(s) of the positive electrode may in particular be chosen from activated carbon, LiM02 or LiM2O4 with M representing Mn, Co, Ni or one of their combinations, for example LiNiOj6MnOj2Cooj2O2; LiMPO4 with M representing Fe, Mn or one of their combinations.

[0147] The following example, with reference to the implementation of a reference electrode prepared according to the method of the invention at the level of a cell of a lithium-ion battery, is thus given solely as an illustration of the invention, it being understood that the use of a reference electrode obtained according to the invention is in no way limited to this particular mode of implementation.

[0148] Thus, as an example of implementation, a reference electrode prepared according to the method of the invention is introduced into a lithium battery, in particular a lithium-ion battery.

[0149] A reference electrode prepared according to the invention can for example be implemented at the level of a cell of a lithium-ion battery further comprising: - a positive electrode comprising a positive active material, for example LiNio.6 Mn0j2Coo>202; - a negative electrode comprising a negative active material, for example based on graphite; and - an electrolyte comprising at least one lithium salt, for example LiPF6, in one or more organic solvents, for example chosen from carbonate solvents, such as ethylene carbonate (EC), diethyl carbonate (DEC) and / or dimethyl carbonate (DMC).

[0150] It is understood that the different elements are assembled within the cell of the electrochemical storage system, for example the battery or the supercapacitor, so as, on the one hand, to allow efficient use of the reference electrode, for example for the purpose of knowing the potentials of the positive and negative electrodes and, on the other hand, so that the presence of this third electrode does not interfere negatively with the operation of the cell of said electrochemical storage system.

[0151] In particular, the reference electrode can be used when the cell of said electrochemical storage system is subjected to cycling for currents ranging from C / 100 to 100C, in particular from C / 50 to 100C, and more particularly from C / 50 to 5C, in particular from C / 2 to 5C, at temperatures ranging from -40°C to 100°C.

[0152] Generally speaking, the different electrodes of the cell of the system of electrochemical storage cells are separated from each other by a porous membrane, also called a “separator”, impregnated with the electrolyte.

[0153] Such porous separators may be made, for example, of polyethylene or polypropylene.

[0154] The cell of the electrochemical storage system may advantageously have a configuration of superimposed layers. In other words, the assembly of the cell may be carried out by stacking the different elements in the form of layers.

[0155] Each of the electrodes of a cell of the electrochemical storage system may be in the form of a layer of the electrode material, deposited on at least one of the faces or on both faces of a current collector in the form of a thin film, for example in the form of a metal strip.

[0156] According to an alternative embodiment, in the case of an architecture of the cell of the electrochemical storage system in the form of a stack of the different elements, the reference electrode can be positioned between the positive and negative electrodes.

[0157] [Fig.l] schematically represents, in cross-section, an example of assembly 100 of the different elements, integrating a reference electrode prepared according to the invention, within a cell of an electrochemical storage system, for example like the cell of a metal-ion battery.

[0158] As shown in this figure, in an alternative embodiment, a cell of an electrochemical storage system, for example a metal-ion battery, in which a reference electrode prepared according to the invention is incorporated, may comprise the assembly 100 of the following different elements, in this stacking order: - a positive electrode 22, in particular formed from a layer of active material 220 on a current collector 221, for example made of aluminum; - a porous separator 24, for example made of polypropylene, intended to be impregnated with the electrolyte; - said reference electrode 21, prepared as described previously, formed of a layer 211 of the electrode material incorporating said active material prepared according to the invention in an intermediate charge state, at the level of a current collector 212, for example made of aluminum;

[0159] - a porous separator 24, for example made of polypropylene, intended to be impregnated by the electrolyte; and - a negative electrode 23, in particular formed from a layer of active material 231 on a current collector 232, for example made of aluminum.

[0160] Within the cell of the electrochemical system, the assembly is quenched by an electrolyte comprising at least one alkali or alkaline-earth metal salt, for example for example a lithium salt in the case of a lithium battery, in one or more aqueous or organic solvents.

[0161] Porous separators provide electronic insulation between the electrodes, while allowing easy passage of ions from the electrolyte. Of course, other configurations are possible. For example, it is possible to enclose the reference electrode in a separator folded back on itself.

[0162] The reference electrode 21 prepared according to the invention, interposed within the assembly between the positive 22 and negative 23 electrodes, advantageously has a surface area in a plane orthogonal to the stacking direction of the different elements (electrodes and separators), reduced compared to the surfaces of the positive and negative electrodes in this same plane. Preferably, the projected area of ​​the reference electrode in a plane orthogonal to the stacking direction does not exceed 10%, in particular 5%, of the projected area in this same plane of the positive and negative electrodes.

[0163] The invention is not limited to the arrangement shown in [Fig.l]. Other assemblies may be considered, provided that they allow both the operation of the cell of the electrochemical storage system and the use of the reference electrode. Examples of configuration of cells with three electrodes are for example described in document US 9,379,418.

[0164] A reference electrode prepared according to the invention can advantageously be used in the manufacture of all-solid-state batteries (“all-solid-state” batteries).

[0165] Advantageously, a reference electrode prepared according to the invention can be implemented in cells of various formats: wound or stacked cells, in rigid or flexible packaging, in prismatic or cylindrical format, etc.

[0166] In a particular embodiment, a reference electrode prepared according to the invention can be integrated into a cell of an electrochemical storage system packaged in a prismatic format, and in particular in the form of a “pouch cell” as illustrated in the examples. Such a “pouch cell” notably has flexible aluminum-plastic packaging.

[0167] Advantageously, the integration of a reference electrode at the level of at least one of the cells of the electrochemical storage system makes it possible to monitor the potentials of the positive and negative electrodes, simultaneously or not, continuously, periodically, randomly or at predetermined times, during the operation of the electrochemical storage system.

[0168] Thus, the potential difference between the positive electrode (respectively, the negative electrode) and the reference electrode makes it possible to measure and monitor the potential of the positive electrode (respectively, the negative electrode) relative to a value known potential, during operation of the supercapacitor.

[0169] The connections and electrical circuits external to the assembly are not shown in the figures.

[0170] The external circuit between the positive electrode (respectively, the negative electrode) and the reference electrode may be electrically connected to a voltage monitor to enable the potential of the positive electrode (respectively, the negative electrode) to be indicated or recorded.

[0171] The invention will now be described by means of the following figures and examples, given of course for illustrative and non-limiting purposes of the invention. EXAMPLE 1

[0172] Preparation of a reference electrode according to the method of the invention

[0173] 1.1. Total chemical delithiation of LiFePO4

[0174] 2.5 g of LiFePO4 material are dispersed under argon atmosphere in 100 mL of acetonitrile. 2.53 g of NO2BF4 (1.2 eq) is added, then the reaction medium is stirred for 48 hours, at room temperature.

[0175] The suspension is then filtered, then the powder is washed with 4 x 100 mL of acetonitrile before being dried at 60°C for 12 hours. 1.9 g of black powder are obtained.

[0176] The powder thus prepared was characterized by DRX ([Fig.2]).

[0177] The diffractogram obtained shows the sole presence of the FePO4 phase.

[0178] 1.2. Preparation of the active material, mixing of powders 75% by mass LiFePO4 / 25% by mass FePO4

[0179] 1.94 g of FePO4 powder prepared in step 1.1 and 5.82 g of LiFePO4 powder have were mixed in a mortar to obtain the electrode active material powder.

[0180] 1.3. Preparation of a reference electrode deposited on an aluminum collector

[0181] An ink composed of 90.5% of the material, mixture of powders, prepared in step 1.2. ; 5% VGCF (vapor grown carbon fiber) and 4.5% polyvinylidene fluoride (PVDF) binder was prepared in N-methylpyrrolidone and coated onto an aluminum collector.

[0182] After drying and pressing at 10 tons, the electrode was assembled into a half-cell with a metallic lithium counter-electrode. Two charge / discharge cycles were carried out at a C / 10 regime.

[0183] [Fig.3] shows the evolution of voltage and current over time. [Fig.4] shows the evolution of potential (V vs Li+ / Li) with specific capacity (mAh / g), during the first two cycles.

[0184] This test shows that the starting potential of the electrode is 3.45V vs Lr7Li, and therefore well on the potential plateau, and that the state of charge of the electrode is 25%.

[0185] The overall composition of the electrode material is therefore Li0j25FePO4.

[0186] 1.4. Preparation of a reference electrode on an aluminum grid

[0187] An ink composed of 90.5% of the material, mixture of powders, prepared in step 1.2.; 5% of VGCF carbon fibers and 4.5% of PVDF binder was prepared in N-methylpyrrolidone and coated on an aluminum grid.

[0188] After drying, the coated grids are calendered until they reach a thickness of approximately 50 μm.

[0189] After calendering the coated aluminum grids, the reference electrodes are then cut.

[0190] The cut electrodes are ultrasonically welded onto aluminum connecting tabs, as shown schematically in [Fig.5]. EXAMPLE 2

[0191] Implementation of the reference electrode at the level of a battery cell

[0192] The reference electrode is introduced at the level of a lithium-ion battery cell.

[0193] The assembly is carried out in “pouch cell” format, from the following electrodes: - the reference electrode prepared according to example 1.4 above, ready for use; - a positive electrode based on LiNio^Mno^Coo^CL (2.9 mAh / cm2); - a graphite-based negative electrode (3.2 mAh / cm2).

[0194] The electrodes are separated by two Celgard® separators, and the assembly is impregnated with an electrolyte based on ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (EC:EMC 3:7 (Vol)), LiPF6 salt (1 M) and 2% by mass of vinylene carbonate (VC).

[0195] [Fig.6] schematically represents the pouch-cell format cell thus obtained.

[0196] The performance of the cell is compared with that of similar cells incorporating a conventional LiFePO4-based reference electrode and requiring prior activation.

[0197] This reference electrode is an LFP reference electrode with the same ink and coating parameters as that of the invention, the only difference being that the LFP is incorporated in its initial state, and does not come from a mixture of lithiated material + delithiated material according to the invention. After manufacture, the cell was charged to C / 10, then the reference electrode was electrochemically activated.

[0198] The cell integrating a reference electrode formed according to the invention and the cell integrating a conventional reference electrode are subjected to the same electrochemical tests at 25°C, namely: - Training step (for the non-activated comparative electrode) - Galvanostatic discharge at different regimes - Galvanostatic charge at different speeds.

[0199] Figures 7 and 8 show the curves of the evolution of the potential (V) with the capacity (mAh.h) for a first discharge at C / 10, a first charge at C / 2 and a final discharge at C / 2, as well as for different charge and discharge rates.

[0200] These tests show that the results obtained are comparable between the cells integrating the conventional reference electrodes and the cell integrating the reference electrode prepared according to the invention.

[0201] On the other hand, the reference electrode does not need to be activated before being usable, unlike conventional reference electrodes.

[0202] Thus, the data can be obtained from the first charge called the “formation step” in the case of conventional electrodes. On the other hand, in the case of a conventional reference electrode, requiring prior activation within the cell in which it is implemented, the cell first undergoes a first charge or “formation step”, during which the reference electrode cannot be used. References

[0203] [1] Cengiz et al., J. Electrochem. Soc. 2021;

[0204] [2] Yamada et al. Electrochem. Solid State Lett., 2005;

[0205] [3] Poizot et al., Chem. Rev. https: / / dx.doi.org / 10.1021 / acs.chemrev.9b00482.

Claims

Claims

1. A method of preparing an active material for a reference electrode, comprising at least the steps of: (1) providing at least one electroactive compound in its reduced form, and the associated redox reaction of which has a constant electrochemical potential for intermediate charge states; (2) providing said electroactive compound in its oxidized form; and (3) mixing the reduced and oxidized forms of said electroactive compound in adjusted proportions to obtain a material whose composition corresponds to an intermediate charge state.

2. Method according to the preceding claim, in which said electroactive compound in its reduced form is chosen from: • intercalation compounds of the LiMO2 type with M representing Co, Ni or Mn; LiM'2O4 with M' representing Ni or Mn; LiM”PO4 with M” representing Fe, Co, Mn or Ni; and • electroactive organic compounds of the P type whose associated redox reaction has a constant electrochemical potential, in particular chosen from compounds, in particular molecules, oligomers or polymers, carrying redox functions chosen from: - stable radicals 0', N' or NO', in particular nitroxide (NO') or phenoxy (PhO') functions; such as for example polymers carrying side groups of 2,2,6,6-tetramethylpiperidinyl-N-oxy (TEMPO) structure; - conjugated ether (-O-) or thioether (-S-) functions; such as thianthrene, phenothiazine, and their oligomeric or polymeric derivatives;ir-conjugated conductive polymers of poly-thiophene (PT) type, for example poly(3,4-ethylenedioxythiophene); - conjugated amine functions, in particular aromatic or heteroaryl amine groups, such as dianiline, triphenylamine, polyaniline or polypyrrole, molecules of viologen or carbazole type, polyviologens and polycarbazoles; and other ir-conjugated conductive polymers, such as poly-acetylenes.;

3. Method according to claim 1 or 2, in which the electroactive compound in its oxidized form (2) is previously obtained from said electroactive compound in its reduced form, by total chemical oxidation using at least one oxidizing agent, for example chosen from nitronium tetrafluoroborate (NO2BF4), nitrosyl tetrafluoroborate (NOBF4), potassium persulfate (K2S2O8), sodium nitrate (NaNO3), nitrile trifluoromethanesulfonate (NO2CF3SO3), potassium permanganate (KMnO4), nitric acid (HNO3), sulfuric acid (H2SO4), peroxydisulfuric acid (H2S2O8), peroxymonosulfuric acid (H2SO5), "Magic Blue" (Tris(4-bromophenyl)aminium hexachloridoantimonate), hydrogen peroxide (H2O2), lead dioxide (PbO2), organic peracids such as metachloroperbenzoic acid (mCPBA), metal cations such as Ce4+, Ag+, Cu2+, gases such as O2, O3, N2 O.

4. A method according to claim 1, wherein said electroactive compound in its oxidized form is selected from: • intercalation compounds of the Li4Ti50i2 type; and • electroactive organic compounds of type N whose associated redox reaction has a constant electrochemical potential, notably chosen from compounds, in particular molecules, oligomers or polymers, carrying redox functions chosen from: - conjugated azo functions (-N=N-) such as azobenzene or one of its derivatives, - disulfide functions (-SS-) such as diphenyl disulfides, tetrathionapphthalene, poly(2,5-dimercapto-1,3,4-thiadiazole), poly(5,8-dihydro-1H,4H-2,3,6,7-tetrathia-anthracene); and - conjugated carbonyl functions, in particular contained within quinonoid structures, such as for example anthraquinone; anhydride structures such as for example pyromellitic anhydride, dimide structures such as for example perylene dimide; or carboxylate structures such as for example perylene tetracarboxylate.

5. Method according to claim 1 or 4, wherein the electroactive compound in its reduced form (1) is previously obtained from said electroactive compound in its oxidized form, by total chemical reduction using at least one reducing agent, in particular chosen from alkali metals such as lithium, sodium, potassium; hydrides such as lithium hydride (LiH) and sodium borohydride (NaBH4), hydrogen, carbon, salts such as sodium sulfite (Na2SO3), sodium hydrosulfite (Na2S2O4), sodium thiosulfate (Na2S2O3), hydrazine (N2H4), lithium iodide (LiI).

6. A method according to any one of the preceding claims, wherein the electroactive compound (1) is LiFePO4 and the electroactive compound (2) is FePO4, in particular said electroactive compound (2) being previously obtained by total chemical oxidation of LiFePO4 using an oxidizing agent, in particular by reaction with NO2BF4 in acetonitrile.

7. Method according to the preceding claim, in which the proportions of electroactive compound in oxidized form and of electroactive compound in reduced form, preferably in the form of powders, are adjusted to obtain a material, in particular in the form of powder, of chemical composition Lii xFePO4, with 0.30 < x < 0.70, in particular 0.50 < x < 0.

60.

8. A method according to any one of the preceding claims, wherein said electroactive compounds are in the form of powders, the active material in step (3) being obtained by mixing said powders.

9. Active material for reference electrode, obtained according to the method of any one of claims 1 to 8.

10. An active material according to claim 9, in the form of a powder, comprising: (1) particles of said electroactive compound in its reduced form; (2) particles of said electroactive compound in its oxidized form; the proportions of particles (1) and (2) being such that the chemical composition of said powder corresponds to an intermediate charge state.

11. Use of an active material obtained according to the process of any one of claims 1 to 8, for the preparation of a reference electrode.

12. A method for preparing a reference electrode, directly functional within a cell of an electrochemical storage system, said method comprising at least the steps of: (i) preparing a dispersion, called "ink", comprising, in one or more aqueous and / or organic solvents, an active material, in particular in the form of a powder, obtained according to the method of any one of claims 1 to 8; one or more binders and optionally one or more electronically conductive additives; (ii) depositing the ink on the surface of a current collector; (iii) evaporating said solvent(s) from the ink to form the electrode film; and preferably (iv) calendering of the electrode.

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