REFERENCE ELECTRODE FOR A SUPERCAPACATOR

Description

technical field

[0001] The present invention relates to the field of electrochemical storage systems, more specifically to the field of electrochemical devices of the supercapacitor type, and in particular to that of conventional and hybrid potassium supercapacitors.

[0002] It aims more specifically at the implementation of a Li 1-x FePO 4 based electrode, as a reference electrode in a non-aqueous electrolyte supercapacitor cell, for a conventional or hybrid potassium supercapacitor. Previous technique

[0003] Electrochemical devices such as supercapacitors find application in many fields requiring the rapid supply of low energy densities, including powering thin embedded systems such as credit cards, smart tags, mobile phones, and electric vehicles.

[0004] From an operational point of view, supercapacitors operate on the principle of the electrochemical double layer, hence the Anglo-Saxon name sometimes encountered of "Electrochemical double layer capacitor" (also known by the abbreviation EDLC), or, in other words, on the principle of energy storage by distribution, within at least one cell, of ions from an electrolyte in the vicinity of the surface of two porous electrodes (respectively, a positive electrode and a negative electrode) impregnated with an ionic electrolyte, separated by a porous membrane allowing electronic isolation between the electrodes, while allowing easy passage of the electrolyte ions.

[0005] In conventional, or "symmetrical," supercapacitors, both electrodes (positive and negative) are made of activated carbon, and the electrolyte is based on alkali ions. These symmetrical supercapacitors exhibit high power density and high cycle life, but low energy density related to the average capacitance of the supercapacitor and the operating voltage.

[0006] More recently, so-called "hybrid" or "asymmetric" supercapacitors have also been developed, falling somewhere between the conventionally used symmetrical supercapacitors based on activated carbon and batteries. One electrode, typically the positive electrode, is made of activated carbon, while the other, typically the negative electrode, is made from rechargeable battery material. Charge storage in a hybrid supercapacitor occurs at the negative electrode via a redox reaction, while charge storage at the positive electrode occurs through the formation of an electrochemical double layer.

[0007] Hybrid supercapacitors offer the advantage of achieving higher energy densities than conventional supercapacitors due to the increased operating voltage of the system.

[0008] Application FR 3 005 199 describes an example of a hybrid potassium supercapacitor, comprising a negative electrode comprising graphite, a positive electrode comprising activated carbon and a non-aqueous electrolyte comprising at least one potassium salt.

[0009] It is important that the operation of supercapacitor systems, whether conventional or hybrid, remains within the electrochemical stability range of the organic solvent(s) used in the electrolyte, to avoid degradation and, in particular, the production of gas. To determine if this undesirable phenomenon occurs, it is essential to be able to know and monitor the potential evolution of each electrode during the operation of the supercapacitor's electrochemical cell. For this purpose, it would be advisable to introduce a reference electrode into the supercapacitor system. A reference electrode must have a stable and known potential, allowing the potential of each of the system's positive and negative electrodes to be determined at any given time.

[0010] Since these supercapacitor systems, conventional or hybrid, generally employ an electrolytic medium based on one or more organic solvents, such as carbonate and nitrile solvents, for example acetronitrile, we are faced with the absence of a known reference in this type of medium.

[0011] The realization of three-electrode systems, particularly for "pouch cell" format cells, has already been described in the context of metal-ion batteries in US document 9,379,418.

[0012] US documents 2019 / 044097 A1, WO 2012 / 049201 and WO 2017 / 156757 describe the implementation, in lithium batteries, of a reference electrode based on a non-metallic lithium compound, in particular based on LiFePO4 (LFP) or Li4Ti5O12 (LTO).

[0013] These documents do not relate to supercapacitor systems as defined by the invention, such as hybrid potassium supercapacitors.

[0014] In the field of supercapacitors, US patent 2011 / 0043968 proposes using a saturated calomel (SCC) reference electrode to evaluate the electrochemical characteristics of conventional and hybrid lithium supercapacitor systems. However, the use of such a reference electrode is not compatible with the desired cylindrical or prismatic cell packaging, particularly for pouch cells. US patent 2019 / 318882 discloses a hybrid supercapacitor cell comprising a non-aqueous electrolyte including a lithium salt, said cell having at least one positive electrode, one negative electrode, and one reference electrode, for example, in the form of a lithium foil.

[0015] Therefore, there remains a need for a reference electrode for supercapacitor cells, particularly in the context of conventional or hybrid potassium-based supercapacitors. Description of the invention

[0016] The present invention aims precisely to provide a useful electrode as a reference electrode in a supercapacitor cell, for example, for a hybrid potassium supercapacitor. The use of the reference electrode according to the invention is defined by the features of claim 1. The cell for a supercapacitor according to the invention is defined by the features of claim 14. The supercapacitor according to the invention is defined by the features of claim 17.

[0017] The inventors have thus discovered that it is possible to implement an electrode formed from a material, LiFePO4 (known as LFP), known in the context of the production of positive electrodes of Li-ion batteries, as a reference electrode in a supercapacitor cell, in particular for a potassium supercapacitor.

[0018] More particularly, the invention relates, according to a first of its aspects, to the use, as a reference electrode in a cell of a supercapacitor with non-aqueous electrolyte, the supercapacitor being conventional potassium or hybrid potassium, of an electrode based on Li 1-x FePO 4 , with 0.30 ≤ x ≤ 0.70, in particular 0.50 ≤ x ≤ 0.60.

[0019] An electrode based on Li 1-x FePO 4 implemented as a reference electrode within the framework of the present invention is referred to more simply in the following text as the "reference electrode according to the invention".

[0020] As mentioned previously, a reference electrode is defined as an electrode with a stable and known electrochemical potential, allowing it to serve as a reference point for independently measuring the potential of electrodes in an electrochemical cell.

[0021] A reference electrode is thus characterized by a stable potential, in particular of approximately 3.4 V vs Li / Li +< .

[0022] As detailed later in the text, a reference electrode according to the invention can be obtained, prior to its implementation in said supercapacitor cell, from a LiFePO4-based electrode, by delithiation (deintercalation of lithium ions), allowing a stable potential (potential plateau) to be reached.

[0023] A reference electrode according to the invention is implemented in a cell of a conventional potassium or hybrid potassium supercapacitor, in other words a cell of a supercapacitor operating on potassium (more simply called a "potassium supercapacitor"), that is to say, whose non-aqueous electrolyte includes at least one potassium salt.

[0024] The invention further relates, according to another aspect, to a cell of a conventional or hybrid supercapacitor comprising a non-aqueous electrolyte including at least one potassium salt, said cell comprising at least: a positive electrode, in particular based on activated carbon; a negative electrode, in particular based on activated carbon in the case of a conventional supercapacitor, or based on a carbon intercalation material of said alkali element, in particular based on graphite, in the case of a hybrid supercapacitor; and a reference electrode according to the invention based on Li 1-x FePO 4.

[0025] Thus, the invention implements a three-electrode system, the positive and negative electrodes of the supercapacitor cell and, in addition, a reference electrode according to the invention based on Li 1-x FePO 4.

[0026] The invention further relates to a supercapacitor, conventional or hybrid, comprising at least one cell as defined above, integrating a reference electrode based on Li 1-x FePO 4.

[0027] According to a first embodiment, a reference electrode according to the invention is implemented in at least one of the cells of a conventional potassium supercapacitor.

[0028] A cell of a conventional supercapacitor according to the invention can thus combine positive and negative electrodes based on activated carbon and said reference electrode according to the invention.

[0029] According to another particularly advantageous embodiment, a reference electrode according to the invention is implemented in at least one of the cells of a potassium hybrid supercapacitor (denoted "KIC" for "potassium-ion capacitor").

[0030] More particularly, a hybrid supercapacitor cell can combine a positive electrode based on activated carbon, a negative electrode based on a carbon intercalation material of at least potassium and more particularly based on graphite, and said reference electrode according to the invention.

[0031] Such variants of supercapacitor systems according to the invention, incorporating a reference electrode according to the invention, are detailed more precisely in the following text.

[0032] Advantageously, a reference electrode according to the invention is compatible with packaging cells in any desired format, particularly in a prismatic format, advantageously in the form of a flexible pouch cell, as illustrated in the following examples. Integrating a reference electrode according to the invention within at least one of the cells of a supercapacitor advantageously allows access to the potentials of each of the positive and negative electrodes. Knowing and monitoring the potentials of the positive and negative electrodes makes it possible to detect any potential faults in their operation.

[0033] Thus, the invention relates, according to another of its aspects, to the use of a reference electrode based on Li 1-x FePO 4, with 0.30 ≤ x ≤ 0.70, in particular 0.40 ≤ x ≤ 0.60, in a supercapacitor cell comprising a non-aqueous electrolyte, in particular a conventional or hybrid potassium supercapacitor cell, to know the potentials of at least one, preferably of each of the positive and negative electrodes, of said supercapacitor cell, in particular to follow the evolution of the potentials of at least one of the positive and negative electrodes, during the operation of said supercapacitor.

[0034] Monitoring the potentials of the positive and negative electrodes during the operation of the supercapacitor, via the reference electrode according to the invention, advantageously allows for optimized use of the supercapacitor, in particular use at its maximum capacity, without exposing oneself to risks of malfunction, and thus allows for improved safety in the use of the supercapacitor.

[0035] Other features, variants and advantages of a reference electrode according to the invention and of its implementation at the level of a supercapacitor cell according to the invention will become clearer from reading the description, examples and figures which follow, given by way of illustration and not limitation of the invention. Brief description of the drawings

[0036] [ Fig 1] presents, schematically, in cross-section, an example of an assembly 100 of the various elements, including the reference electrode 21 according to the invention, within a supercapacitor cell according to the invention; [ Fig 2 ] schematically presents the two-electrode pouch-cell implemented according to Example 1 (1.1) to prepare a reference electrode according to the invention, from an LFP electrode; [ Fig 3 ] presents the voltage profile of the graphite / LFP cell prepared according to example 1 (1.1.) during and after a half charge at C / 10; [ Fig 4 ] schematically presents a pouch-cell supercapacitor cell, as prepared in Example 1 (1.2), comprising a positive electrode, the reference electrode according to the invention, and a negative electrode, the electrodes being separated by porous polypropylene membranes impregnated with the electrolyte; Fig 5] presents the curves obtained according to example 2 of cell voltage and the potentials of the negative and positive electrodes for the hybrid supercapacitor cell prepared in example 1: during a cycle at 5C ( figure 5 (a) ), during a C / 2 cycle ( figure 5 (b) ) and during the different stages of formation of a hybrid potassium supercapacitor according to the process described in the application filed in France under number FR2109574: charging at C / 2 up to 3.2 V, holding for 24 hours at a voltage of 3.2 V and discharging at C / 2 down to 0.5 V ( figure 5 (c) ). [ Fig 6 ] presents the evolution of the capacitance of the conventional supercapacitor cell prepared in example 3, during cycles alternating between a C / 2 and 100C regime; [ Fig 7] presents the evolution of the potentials of the positive and negative electrodes at different stages of the cycling according to example 4, obtained for a cell of a conventional supercapacitor integrating a reference electrode according to the invention.

[0037] It should be noted that, for the sake of clarity, the various elements on the figures 1, 2 And 4 are represented to a free scale, the actual dimensions of the different parts not being respected.

[0038] In the following text, the expressions "between ... and ...", "ranging from ... to ..." and "varying from ... to ..." are equivalent and are meant to mean that the boundaries are included, unless otherwise stated. Detailed description Reference electrode

[0039] As previously stated, the invention implements a reference electrode formed from partially delithiated LiFePO4 material, and more specifically a Li 1-x FePO4 based electrode, with 0.30 ≤ x ≤ 0.70.

[0040] The Li 1-x FePO4-based reference electrode is such that it exhibits a stable electrochemical potential, in other words, fixed at a potential plateau, specifically at a value of approximately 3.4 V vs Li / Li +< .

[0041] The reference electrode according to the invention is more particularly based on a Li 1-x FePO 4 material, where x is between 0.40 and 0.60, in particular between 0.50 and 0.60 and more particularly about 0.50 (i.e. 0.50 ± 0.01).

[0042] A Li 1-x FePO 4 based electrode according to the invention can be obtained, as described in more detail below, prior to its implementation in said cell, from a LiFePO 4 based electrode, by partial delithiation allowing a stable electrochemical potential to be obtained, in particular stabilized at a value of 3.4 V vs Li / Li +< .

[0043] LiFePO4-based electrodes, more simply called LFP electrodes, are known as positive electrodes for lithium batteries.

[0044] The LFP electrode, from which a reference electrode according to the invention is obtained, can be prepared by any method known to a person skilled in the art.

[0045] The preparation of an LFP electrode can generally include at least the following steps: preparation of a dispersion, more commonly called "ink", comprising, in one or more aqueous and / or organic solvents, at least LiFePO4; one or more binders and possibly one or more electronically conductive additives; deposition of the ink on the surface of a current collector; and evaporation of said solvent(s) of the ink to form the electrode film.

[0046] The ink solvent may include water and / or one or more organic solvents, for example N-methyl-2-pyrrolidone.

[0047] Electronic conductive additives are used to improve the electronic conductivity of the electrode. Examples include carbon fibers, carbon black, carbon nanotubes, and mixtures thereof.

[0048] The aforementioned electronic conductive additive(s), when present, may preferably be implemented at a concentration of 1 to 10% by mass, and in particular 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.

[0049] Binders are used to ensure the mechanical stability of the electrode on the current collector, the cohesion of the different components of the electrode, and its flexibility properties. They can be chosen from polymer binders, particularly fluoropolymers, for example polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), carboxymethylcellulose (CMC) derivatives, polysaccharides, and latexes, especially styrene-butadiene rubber (SBR).

[0050] A particularly preferred binder is poly(vinylidene fluoride) (PVDF).

[0051] The binder(s) may be present in an amount less than or equal to 20% by mass, relative to the total mass of the electrode, and in particular less than or equal to 10% by mass, and 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.

[0052] The ink can be homogenized before spreading.

[0053] The deposition of ink on the surface of the current collector can be carried out by any technique known to a person skilled in the art, for example by coating, by a printing technique, by extrusion or by co-lamination.

[0054] The current collector can be solid, for example in the form of a strip or in the form of a grid.

[0055] The current collector can be made of a metallic material chosen from copper, aluminum, nickel or stainless steel.

[0056] Preferably, particularly when the electrolyte of the supercapacitor cell in which the reference electrode is used includes a nitrile-type organic solvent, especially acetonitrile, the current collector is advantageously aluminum-based. For example, it could be a carbon-coated aluminum foil.

[0057] Preferably, when the current collector of the reference electrode is made of copper, the electrolyte of the supercapacitor cell in which the reference electrode is implemented does not include acetonitrile, but may include at least one carbonate solvent.

[0058] The evaporation of the ink solvent(s) can be achieved by drying, for example in an oven, at a temperature between 20 and 150 °C, in particular between 50 and 80 °C, especially for a period of between 1 and 15 hours.

[0059] The active material, LiFePO4, of the LFP electrode preferably represents 60 to 97% by mass, in particular 70 to 96% by mass, relative to the total mass of the electrode, excluding the mass of the current collector.

[0060] Preferably, the LFP electrode weight (defined as the mass of LFP active material in grams per unit area in cm²) is between 1 mg / cm² and 30 mg / cm², particularly between 3 mg / cm² and 10 mg / cm². The LFP electrode weight, in other words, the mass of LiFePO₄ material per unit area, can be adjusted by controlling the LiFePO₄ active material content and the deposition thickness, particularly the coating thickness, of the ink formulated on the current collector. Preparation of the reference electrode

[0061] As previously stated, a reference electrode according to the invention is obtained by partial delithiation of an LFP electrode.

[0062] The partial delithiation (or lithium disintercalation) process of the LFP electrode is carried out more specifically electrochemically, so as to achieve a stable electrochemical potential.

[0063] More specifically, obtaining a partially delithiated LFP reference electrode involves the following steps: (i) supplying an electrochemical cell, comprising said LFP electrode as the positive electrode, and a negative counter electrode, in particular based on graphite, lithium or silicon dioxide (SiO₂) where y is between 1 and 2; (ii) charging said cell under suitable conditions to reach a potential plateau, in particular to a value of about 3.4 V vs Li / Li +< .

[0064] The electrochemical cell for preparing the partially delithiated LFP electrode is specifically a separate electrochemical cell from the supercapacitor cell in which the reference electrode is intended to be used. The negative counter electrode can be, for example, graphite-based, lithium-based, or silicon dioxide (SiO₂), where y is between 1 and 2.

[0065] The electrolyte of the electrochemical cell for the partial delithiation of the LFP electrode comprises more particularly one or more lithium salts in one or more organic solvents.

[0066] The organic solvent(s) may be, for example, carbonate solvents, such as ethylene carbonate (EC), diethyl carbonate (DEC) and / or dimethyl carbonate (DMC).

[0067] Examples of lithium salts include LiPF 6, LiClO 4, LiBF 4, LiAsF 6, LiCF 3 SO 3, LiN(CF 3 SO 2 ) 3, LiN(C 2 F 5 SO 2 ), lithium bistrifluoromethylsulfonylimide LiN[SO 2 CF 3 ] 2 (known by the abbreviation LiTFSI), lithium bis(fluorosulfonyl)amide (known by the abbreviation LiFSI) LiN[SO 2 F] 2 , and mixtures thereof.

[0068] Preferably, the electrolyte comprises, as a lithium salt, LiPF 6 or LiTFSI, preferably LiPF 6.

[0069] The said salt(s), for example lithium salt, may be present in the electrolyte in a concentration ranging from 0.3 M to 3 M.

[0070] According to a first variant of the preparation of the reference electrode based on the Li 1-x FePO 4 material, the cell can be partially charged (partial delithiation) to reach the potential plateau. The charging in step (ii) is more specifically a "half charge", thus enabling the reference electrode based on the Li 1-x FePO 4 material to be charged.

[0071] It can be carried out at a rate ranging from C / 2 to C / 100, particularly C / 10, until reaching the potential plateau, specifically a stable potential of approximately 3.4 V vs Li / Li+< . The charging time can, for example, be between 1 and 50 hours, in particular between 5 and 7 hours.

[0072] For example, as illustrated in example 1, in the context of delithiation against a graphite counter electrode, charging can be carried out for approximately 6 hours at a C / 10 regime, in order to reach a cell voltage of 3.3 V, corresponding to a potential plateau of approximately 3.4 V vs Li / Li +< .

[0073] During charging, Li+< ions are dislodged from the LFP electrode and deposited on the negative electrode, so that a material of formula Li1-xFePO4 with 0.30 ≤ x ≤ 0.70, in particular 0.50 ≤ x ≤ 0.60, is obtained.

[0074] According to another variant of the preparation of the reference electrode based on the material Li 1-x FePO 4, the cell can be fully charged (total delithiation), followed by a partial discharge until the target lithiation state (potential plateau) is reached.

[0075] The surface of the Li 1-x FePO 4 electrode can be cleaned prior to its use as a reference electrode in the supercapacitor cell. For example, it can be rinsed with dimethyl carbonate to remove residual salts from the electrode's pores.

[0076] The Li 1-x FePO 4 type material preferably represents 60 to 97% by mass, in particular 70 to 96% by mass, relative to the total mass of the reference electrode, excluding the mass of the current collector.

[0077] As previously stated, the reference electrode according to the invention preferably comprises, in addition to said Li 1-x FePO 4 material, one or more binders, in particular as defined previously, in particular chosen from polymeric binders, for example of PVDF type, and optionally one or more electronically conductive additives, in particular as defined previously, in particular chosen from carbon fibers, carbon black, carbon nanotubes and mixtures thereof.

[0078] The binder(s) may represent less than 20% by mass of the total mass of the electrode, in particular less than 10% by mass, especially between 1 and 10% by mass, of the total mass of the electrode, excluding the mass of the current collector.

[0079] The said electronic conductive additive(s), when present, may represent from 1 to 10% by mass, in particular from 2 to 8% by mass, of the total mass of the electrode, excluding the mass of the current collector.

[0080] Preferably, the weight of the Li 1-x FePO 4 based reference electrode (defined as the mass of Li 1-x FePO 4 material in grams per unit area in cm 2< ) is between 2 mg / cm 2< and 30 mg / cm 2< , in particular between 5 mg / cm 2< and 25 mg / cm 2< . Supercapacitor cell

[0081] As previously stated, the Li 1-x FePO 4 based electrode is implemented according to the invention as a reference electrode within at least one cell of a supercapacitor system comprising a non-aqueous electrolyte.

[0082] A basic cell of a supercapacitor typically comprises a positive electrode, a negative electrode and a porous membrane separating said electrodes and impregnated by said electrolyte.

[0083] The reference electrode according to the invention is thus integrated as a third electrode in the supercapacitor cell.

[0084] As previously stated, the reference electrode according to the invention can be implemented at the level of a "conventional" (also called "standard") supercapacitor or a so-called "hybrid" supercapacitor.

[0085] A supercapacitor cell according to the invention is thus a three-electrode system, comprising more particularly: a positive electrode, in particular based on activated carbon; a negative electrode, in particular based on activated carbon in the case of a conventional supercapacitor, or based on a carbon material intercalating an alkali element, in particular based on graphite, in the case of a hybrid supercapacitor; and said reference electrode according to the invention based on Li 1-x FePO 4, in particular prepared as described above.

[0086] Particular embodiments of supercapacitor cells according to the invention are described in more detail later in the text. Conventional supercapacitor

[0087] According to a first embodiment, a reference electrode according to the invention is integrated into a cell of a conventional supercapacitor, preferably a conventional potassium supercapacitor.

[0088] A conventional supercapacitor cell typically comprises positive and negative electrodes made of activated carbon.

[0089] An activated carbon-based electrode may more specifically comprise a content of at least 60% by mass of activated carbon, relative to the total mass of the electrode, it being understood that the total mass of the electrode does not include the mass of the current collector.

[0090] In particular, activated carbon can be present in a content ranging from 60% by mass to 95% by mass relative to the total mass of the electrode, in particular from 85% to 95% by mass, relative to the total mass of the electrode.

[0091] In addition to the presence of activated carbon, the activated carbon-based electrode may include one or more organic binders, which will contribute to ensuring the mechanical cohesion of said electrode.

[0092] These organic binders can be, in particular, polymeric binders comprising one or more polymers selected from: Fluorinated polymers, such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropene) copolymer (PVDF-HFP), ethylene-propylene fluorinated copolymer (FEP), copolymer from the copolymerization of tetrafluoroethylene and perfluoroalkoxyvinyl ether (PFA); polyimides; polyacrylonitriles; and mixtures thereof.

[0093] Advantageously, the binder(s) are present in a concentration chosen to be as low as possible without compromising mechanical strength during cycling across the entire operating temperature range, generally between -40°C and the boiling point of the organic solvent(s) in the non-aqueous electrolyte. For example, the binder(s) may be present in a concentration less than or equal to 15% by mass, relative to the total mass of the electrode, in particular from 1 to 15% by mass and more particularly from 2 to 7% by mass, relative to the total mass of the electrode; it being understood that the total mass of the electrode does not include the mass of the current collector.

[0094] An activated carbon-based electrode may also include at least one electrically conductive carbon additive other than activated carbon, in particular selected from carbon blacks, acetylene blacks, graphite, carbon nanotubes, carbon fibers and mixtures thereof, for example vapor-phase carbon fibers (known by the abbreviation VGCF).

[0095] The said carbonaceous additive(s) may be present in a content of up to 15% by mass, relative to the total mass of said electrode, in particular from 1 to 15% by mass and more particularly from 2 to 10% by mass, relative to the total mass of the electrode, it being understood that the mass of the electrode does not include the mass of the current collector. Hybrid supercapacitor

[0096] According to another embodiment, a reference electrode according to the invention is integrated into a cell of a hybrid supercapacitor, preferably a potassium hybrid supercapacitor.

[0097] A hybrid supercapacitor cell typically comprises an electrode, in particular the positive electrode, made of activated carbon, and a counter electrode, typically the negative electrode, made from rechargeable battery material.

[0098] The activated carbon-based electrode, typically the positive electrode, can be as described previously in the context of the electrodes of a conventional supercapacitor cell.

[0099] The electrode based on a rechargeable battery material, typically the negative electrode, is more particularly based on a carbon intercalation material of at least one alkali element, such as lithium, sodium, potassium, rubidium and / or cesium; in particular based on a carbon intercalation material of at least one alkali element other than lithium, such as sodium, potassium, rubidium and / or cesium, and more particularly a potassium intercalation carbon material.

[0100] Advantageously, the negative electrode is based on a carbonaceous material of the graphite type, and more particularly on particulate graphite, the average particle size of which can range from 1 to 50 µm, this particle size being measured by D 50 laser granulometry.

[0101] In one particular embodiment, in the case of a potassium-based supercapacitor cell, the negative electrode is graphite-based. Potassium has the ability to intercalate into the graphite of the negative electrode at high charging stages.

[0102] As with the activated carbon electrode, the negative electrode may include one or more organic binders, which will advantageously contribute to ensuring the mechanical cohesion of said electrode.

[0103] The said organic binder(s) may be polymeric binders, such as previously described for an activated carbon-based electrode, in particular chosen from fluorinated polymers, polyimides, polyacrylonitril and mixtures thereof.

[0104] As previously described for an activated carbon-based electrode, the binder(s) are advantageously present in a content chosen so as to be as low as possible, without compromising the mechanical strength during cycling over the entire temperature range of operation of the supercapacitor cell.

[0105] For example, the binder(s) may be present in a content less than or equal to 15% by mass, relative to the total mass of the electrode, in particular from 1 to 15% by mass and more particularly from 2 to 7% by mass, relative to the total mass of the electrode, it being understood that the total mass of the electrode does not include the mass of the current collector.

[0106] The negative electrode may also include at least one electrically conductive carbon additive other than the aforementioned carbon intercalation material, in particular selected from carbon blacks, acetylene blacks, carbon nanotubes, carbon fibers and mixtures thereof, for example vapor-phase carbon fibers (known by the abbreviation VGCF).

[0107] The said carbonaceous additive(s) may be present in a content of up to 15% by mass, relative to the total mass of said electrode, in particular from 1 to 15% by mass and more particularly from 2 to 12% by mass, relative to the total mass of the electrode, it being understood that the mass of the electrode does not include the mass of the current collector.

[0108] Furthermore, the negative electrode may also include, in addition to the intercalation material of at least one alkali element (when this is graphite), activated carbon identical or different from that of the positive electrode, which can advantageously improve the power performance of the negative electrode. This type of electrode can therefore be described as a composite electrode.

[0109] The positive electrode and the negative electrode of a cell of a supercapacitor according to the invention, conventional or hybrid, can each be associated with a current collector that conducts electricity.

[0110] A current collector can take the form of a metal strip affixed to one of the faces of said electrodes.

[0111] A current collector can be made of a metallic material chosen from copper, aluminum, nickel, or stainless steel. Preferably, especially when the electrolyte includes a nitrile-type solvent, such as acetonitrile, current collectors are made of aluminum.

[0112] Preferably, when at least one of the current collectors is made of copper, the electrolyte of the supercapacitor cell in which said reference electrode is implemented does not include acetonitrile, but may include at least one carbonate solvent. Non-aqueous electrolyte

[0113] The non-aqueous electrolyte of a conventional or hybrid supercapacitor cell according to the invention comprises one or more alkali metal salts, preferably potassium, in a non-aqueous solvent medium formed more particularly of one or more organic solvents.

[0114] Preferably, the electrolyte comprises at least one alkali metal salt selected from sodium salts, potassium salts, and mixtures thereof.

[0115] Sodium salts can be chosen for example from NaClO 4 , NaBF 4 , NaPF 6 , sodium bis(trifluoromethanesulfonyl)imide (NaTFSI ), sodium bis(fluorosulfonyl)imide (NaFSI ), sodium bis(oxalato)borate (NaBOB ), NaSCN , NaSbF 6 , NaAsF 6 , NaAlCl 4 , NaSiF 6 , NaSO 3 CF 3 and mixtures thereof.

[0116] Potassium salts can be chosen for example from potassium perchlorate (KClO 4 ), potassium fluoroborate (KBF 4 ), potassium hexafluorophosphate (KPF 6 ), potassium bis(trifluoromethanesulfonyl)imide (KTFSI ), potassium bis(fluorosulfonyl)imide (KFSI ), potassium bis(oxalato)borate (KBOB ), KSCN , KSbF 6 , KAsF 6 , LAlCl 4 , KSiF 6 , KSO 3 CF 3 , and mixtures thereof.

[0117] In a particular embodiment, a reference electrode according to the invention is implemented for a potassium-operating supercapacitor, in particular for a hybrid potassium supercapacitor.

[0118] In other words, the electrolyte of the potassium supercapacitor cell according to the invention comprises at least one potassium salt, in particular as described above, in one or more organic solvents.

[0119] Preferably, the potassium salt(s) may be chosen from KClO 4, KBF 4, KPF 6 and mixtures thereof.

[0120] In one particular embodiment, the potassium salt is KPF 6.

[0121] The concentration of the said salt(s) in the solvent medium of the electrolyte may be at least 0.05 mol / L, in particular until saturation of the solvent medium at 25 °C, in particular between 0.5 mol / L and 2.5 mol / L, for example 0.8 M.

[0122] As previously stated, the electrolyte more specifically comprises the said salt(s), for example the said potassium salt, in one or more organic solvents.

[0123] The organic solvent(s) of the electrolyte may be chosen from among the following: Nitrile solvents, such as acetonitrile, 3-methoxypropionitrile (MPN), adiponitrile (ADP), glutaronitrile (GN); carbonate solvents, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC); lactone solvents, such as γ-butyrolactone (GBL), γ-valerolactone (GVL); sulfone solvents, such as dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), diethyl sulfone (DES), sulfolane (SL); lactam solvents, such as N-methylpyrrolidone (NMP); ketone solvents, such as acetone, methyl ethyl ketone (MEK); nitroalkane solvents, such as nitromethane (NM), nitroethane (NE); amine solvents, such as 1,3-diaminopropane (DAP), ethylenediamine (EDA); sulfoxide solvents, such as dimethyl sulfoxide (DMSO); ester solvents, such as ethyl acetate (EA), methyl acetate (MA), propyl acetate (AP);Linear ether solvents, such as dimethoxyethane (DME); cyclic ether solvents, such as dioxane, dioxolane (DIOX), tetrahydrofuran (THF); oxazolidone solvents, such as 3-methyl-2-oxazolidone; amide solvents, for example dimethylformamide; and mixtures thereof.

[0124] Preferably, the said organic solvent(s) are chosen from carbonate solvents, linear ether solvents, nitrile solvents, lactone solvents, amide solvents and mixtures thereof, preferably nitrile solvents, such as acetonitrile.

[0125] When the electrolyte comprises at least one potassium salt, it advantageously comprises said potassium salt(s), preferably selected from KClO 4, KPF 6, KBF 4, and mixtures thereof, in particular KPF 6, in solution in at least one organic solvent selected from carbonate solvents, linear ether solvents, nitrile solvents, lactone solvents, amide solvents and mixtures thereof.

[0126] According to a particular embodiment, the electrolyte comprises, or is formed from, at least one potassium salt, in particular selected from KClO 4, KPF 6, KBF 4 and mixtures thereof, in particular KPF 6, in solution in at least one solvent selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, dimethoxyethane, acetonitrile, γ-butyrolactone, dimethylformamide and mixtures thereof.

[0127] Even more preferably, the electrolyte used in a supercapacitor cell according to the invention comprises at least one potassium salt, in particular KClO 4, KPF 6 or KBF 4, in particular KPF 6, in solution in a solvent or mixture of organic solvents, selected from propylene carbonate alone; a 1:1 ethylene carbonate / diethyl carbonate mixture; a 1:1 ethylene carbonate / dimethyl carbonate mixture; a 1:2 dimethoxyethane / propylene carbonate mixture; acetonitrile alone; γ-butyrolactone alone; or dimethylformamide alone.

[0128] In a particular embodiment, the electrolyte used in a cell of a potassium supercapacitor according to the invention comprises, or is formed from, a potassium salt, in particular KPF 6, in acetonitrile.

[0129] It is understood that the various particular embodiments described above, relating to the different electrodes and the electrolyte, can be combined, as far as possible, to define particular variants of the composition of a supercapacitor cell according to the invention.

[0130] In a particular embodiment, a reference electrode according to the invention is implemented in at least one cell of a conventional potassium supercapacitor, comprising: a positive electrode based on activated carbon, in particular as described previously; a negative electrode based on activated carbon, in particular as described previously; and an electrolyte comprising at least one potassium salt, in particular KPF 6, in one or more nitrile-type organic solvents, in particular acetonitrile.

[0131] In another particular embodiment, a reference electrode according to the invention is implemented in at least one cell of a hybrid potassium supercapacitor, comprising: a positive electrode based on activated carbon, in particular as described previously; a negative electrode based on graphite, in particular as described previously; and an electrolyte comprising at least one potassium salt, in particular KPF 6, in one or more nitrile-type organic solvents, in particular acetonitrile. Assembly within the supercapacitor cell

[0132] It is understood that the various elements are assembled within the supercapacitor cell in such a way, on the one hand, as 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, as to ensure that the presence of this third electrode does not negatively interfere with the operation of the supercapacitor cell.

[0133] In particular, the reference electrode can be used when the supercapacitor cell is subjected to cycling for currents ranging from C / 100 to 20C, especially from C / 20 to 5C.

[0134] In general, the different electrodes of the supercapacitor cell are separated from each other by a porous membrane, also called a "separator", impregnated by the electrolyte.

[0135] Such porous separators can be made of polyethylene or polypropylene, for example.

[0136] The cell of a supercapacitor according to the invention may advantageously have a stacked layer configuration. In other words, the cell assembly can be achieved by stacking the different elements in the form of layers.

[0137] Each of the electrodes of a supercapacitor cell according to the invention can be in the form of a layer of the electrode material, deposited on at least one or both faces of a current collector in the form of a thin film, for example in the form of a metal strip.

[0138] According to one embodiment, in the case of a supercapacitor cell architecture in the form of a stack of different elements, the reference electrode can be positioned between the positive and negative electrodes.

[0139] There Figure 1represents, schematically, in cross-section, an example of assembly 100 of the different elements, integrating a reference electrode according to the invention, within a supercapacitor cell according to the invention.

[0140] As shown in this figure, in one embodiment, a cell of a supercapacitor according to the invention, in particular of a potassium supercapacitor, may comprise the assembly 100 of the following different elements, in this stacking order: a positive electrode 22, in particular based on activated carbon; in particular formed of a double layer 220 based on activated carbon on a current collector 221, for example made of aluminum; a porous separator 24, for example made of polypropylene, intended to be impregnated by the electrolyte; said reference electrode 21 based on Li 1-x FePO 4, in particular prepared as described above, formed of a layer 211 of the Li 1-x FePO 4 based electrode material at the level of a current collector 212, for example made of carbon-coated aluminum; a porous separator 24, for example made of polypropylene, intended to be impregnated by the electrolyte; and a negative electrode 23, in particular formed of a layer 231 based on activated carbon in the case of a conventional supercapacitor, or based on a carbon intercalation material of at least one alkali element, for example based on graphite, in the case of a hybrid supercapacitor, on a current collector 232, for example made of aluminum.

[0141] Within the supercapacitor cell, the assembly is soaked in a non-aqueous electrolyte, comprising at least one alkali metal salt, advantageously a potassium salt such as KPF 6, in one or more organic solvents, preferably of the nitrile type, for example acetonitrile.

[0142] Porous separators provide electronic isolation between the electrodes, while allowing easy passage of electrolyte ions.

[0143] The reference electrode 21, interposed within the assembly between the positive electrode 22 and the negative electrode 23, advantageously has a surface area in a plane orthogonal to the stacking direction of the various elements (electrodes and separators) that is smaller compared to the surface areas of the positive and negative electrodes in the same plane. Preferably, the projected area of ​​the reference electrode in a plane orthogonal to the stacking direction does not exceed 10%, and in particular 5%, of the projected area in the same plane of the positive and negative electrodes.

[0144] The invention is not limited to the arrangement shown in figure 1 Other assemblies can be considered, provided they allow both the operation of the supercapacitor cell and the use of the reference electrode. Examples of three-electrode cell configurations are described in US patent 9,379,418.

[0145] A supercapacitor cell according to the invention can be packaged in a prismatic or cylindrical format. In the prismatic format, the assembly is preferably packaged in a container capable of preventing contamination of the cell by air or water.

[0146] In one particular embodiment, a supercapacitor cell according to the invention can be 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 features a flexible aluminum-plastic packaging.

[0147] A supercapacitor, conventional or hybrid, according to the invention may comprise at least one cell according to the invention incorporating a reference electrode.

[0148] Advantageously, applying a small current between the positive electrode (respectively, the negative electrode) and the reference electrode allows the potential of the positive electrode (respectively, the negative electrode) to be measured and monitored during the operation of the supercapacitor.

[0149] External electrical connections and circuits to the assembly are not shown in the figures.

[0150] The external circuit between the positive electrode (respectively, the negative electrode) and the reference electrode can be electrically connected to a voltage monitor to allow the potential of the positive electrode (respectively, the negative electrode) to be indicated or recorded.

[0151] The potentials of the positive and negative electrodes can be monitored simultaneously or not, continuously, periodically, randomly or at predetermined times, during the operation of the supercapacitor.

[0152] It is understood that all the characteristics described for the reference electrode and for the various elements of a supercapacitor cell apply to the supercapacitor cell, the supercapacitor and the uses claimed according to the invention.

[0153] The invention will now be described by means of the following figures and examples, given of course by way of illustration and not limitation of the invention. EXAMPLE 1 Preparation of a reference electrode according to the invention and implementation at the cell level of a potassium hybrid supercapacitor 1.1. Preparation of the reference electrode

[0154] An LFP electrode is prepared by coating an ink comprising 95% wt. LiFePO4 (grade 2B, Prayon), 2.5% wt. PVDF (Solvay 5130) and 2.5% wt. SuperC65 reference carbon black (obtained from Timcal Co., Switzerland) onto the surface of a current collector (carbon-coated aluminum foil), and then drying.

[0155] As schematically represented in figure 2 The LFP electrode is then mounted in a pouch cell facing a negative graphite counter electrode. The electrolyte is a mixture of carbonates and LiPF6 salt (1M).

[0156] The LFP electrode is delithiated by charging the electrochemical cell (half-charge) for 6 hours at a C / 10 regime to reach a cell voltage of 3.3 V, corresponding to a potential plateau at 3.4 V vs Li / Li +< .

[0157] After reaching the potential plateau, the cell is kept in an open circuit for several hours to verify the stability of the electrode potential.

[0158] There figure 3 represents the voltage profile of the graphite / LFP cell during and after a half charge at C / 10.

[0159] In a second step, the cell is disassembled into the glove box to retrieve the partially delithiated LFP-based reference electrode. This is rinsed with dimethyl carbonate (DMC) to clean the electrode's porosity of any salts present.

[0160] The weight of the electrode, corresponding to the mass of active material Li 1-x FePO 4 in grams per unit area in cm 2< , is 24 mg / cm 2< . 1.2. Implementation of the partially delithiated LPF electrode at the cell level of a potassium hybrid supercapacitor

[0161] The three-electrode setup is performed in the "pouch cell" format, using the following electrodes: the reference electrode according to the invention, prepared as described in point 1.1. and cut to the format 5 mm × 10 mm. a positive electrode based on activated carbon.

[0162] The positive electrode is purchased from Samwha Capacitor Group and used as received. It is a coated electrode on a 20 µm unetched aluminum collector, and its composition is as follows: 91% activated carbon; 3% SuperC65 conductive additive and 6% PVDF binder. a graphite-based negative electrode

[0163] The negative electrode is prepared by coating, on an etched aluminum collector 30 µm thick, with a composition comprising: 75.5% reference graphite SMG-HE2 (obtained from Hitachi) 11.5% reference graphite SFG6 (obtained from Timcal Co., Switzerland); 10% reference carbon black superC65 (obtained from Timcal Co., Switzerland); 2% styrene-butadiene rubber (obtained from BASF, 2427); 1% carboxymethylcellulose (obtained from Ashland, 7HXF), the percentages being mass percentages expressed in relation to the total mass of the electrode excluding the current collector.

[0164] The positive and negative electrodes are dried at 55 °C and then cut to a 35 mm × 35 mm format.

[0165] The cell is assembled in a flexible aluminum-plastic package by stacking the elements in the following order: 1. the positive electrode made of activated carbon; 2. a Celgard 2320 polypropylene separator; 3. the reference electrode; 4. a polypropylene separator; 5. the negative electrode made of graphite.

[0166] After drying at 55°C in an oven, the cell activation is carried out in a glove box by adding 450 µL of acetonitrile-based electrolyte and KPF 6 salt (0.8 M).

[0167] After the electrolyte is added, the package is hermetically sealed under vacuum.

[0168] There figure 4 schematically represents the pouch-cell format of the potassium supercapacitor thus obtained. EXAMPLE 2 Use of the reference electrode for monitoring the potentials of the positive and negative electrodes during different stages of formation of a hybrid potassium supercapacitor

[0169] Implementing the three-electrode setup as described in Example 1 allows for measuring the voltage of the potassium hybrid supercapacitor cell viaa connection between the negative and positive electrodes; and to calculate the respective potentials of the positive and negative electrodes of the potassium hybrid supercapacitor cell via a connection between the reference electrode and the positive or negative electrode.

[0170] For example, the figure 5 represents the cell voltage and the potentials of the negative and positive electrodes for the hybrid potassium supercapacitor cell prepared in example 1: during a 5C cycle ( figure 5 (a) ), during a C / 2 cycle ( figure 5 (b) ) and during the different stages of formation of a hybrid potassium supercapacitor according to the process described in the application filed in France under number FR210957: charging at C / 2 up to 3.2 V, holding for 24 hours at a voltage of 3.2 V and discharging at C / 2 down to 0.5 V ( figure 5 (c) ). EXAMPLE 3 Implementation of the reference electrode at the level of a cell of a conventional potassium supercapacitor

[0171] The three-electrode setup is performed in the "pouch cell" format, using the following electrodes: the reference electrode according to the invention, prepared as described in point 1.1. and cut to the format 5 mm × 10 mm. a positive electrode based on activated carbon, as described in example 2; a negative electrode, identical to the positive electrode, based on activated carbon.

[0172] The cell is assembled in a flexible aluminum-plastic package by stacking the elements in the following order: 1. the positive electrode made of activated carbon; 2. a polypropylene separator; 3. the reference electrode; 4. a polypropylene separator; 5. the negative electrode made of activated carbon.

[0173] After drying at 55°C in an oven, the cell is activated in a glove box by adding 450 µL of acetonitrile-based electrolyte and KPF 6 salt.

[0174] After the electrolyte is added, the package is hermetically sealed under vacuum. EXAMPLE 4 Using the reference electrode to determine the working potential ranges of the electrodes of a conventional potassium supercapacitor

[0175] The electrode potentials are monitored over time by means of cycles carried out at a C / 2 regime between the cycling periods at a 100C regime.

[0176] There figure 6 presents the evolution of the capacitance of the conventional supercapacitor cell prepared in example 3, during cycles alternating between a cycle at a C / 2 regime and 100C.

[0177] There figure 7 represents the evolution of the potentials of the positive and negative electrodes during cycles at a C / 2 regime at different stages of cycling.

Claims

1. Use of an electrode based on Li1-xFePO4, with 0.30 ≤ x ≤ 0.70, as a reference electrode in a cell of a nonaqueous electrolytic supercapacitor, said supercapacitor being a conventional potassium-ion supercapacitor or a potassium-ion hybrid supercapacitor.

2. Use according to the preceding claim, wherein said reference electrode is based on Li1-xFePO4, in which x is between 0.40 and 0.60, in particular between 0.50 and 0.60.

3. Use according to Claim 1 or 2, said electrode having a stable electrochemical potential, in particular of 3.4 V vs Li / Li+.

4. Use according to any one of the preceding claims, wherein said reference electrode is prepared, prior to being used in said cell, from an electrode based on LiFePO4, known as LFP electrode, by partial delithiation making it possible to obtain a stable electrochemical potential, in particular stabilized at a value of 3.4 V vs Li / Li+.

5. Use according to the preceding claim, wherein said reference electrode is obtained via at least the following steps: (i) providing an electrochemical cell, comprising said LFP electrode as a positive electrode, and a negative counter electrode, in particular based on graphite, lithium or silicon oxide SiOy where y is between 1 and 2; (ii) charging said cell under conditions suitable for reaching a potential plateau, in particular at a value of 3.4 V vs Li / Li+, said charging preferably being carried out at a regime ranging from C / 2 to C / 100, in particular C / 10, for a duration of between 1 and 50 hours, in particular between 5 and 7 hours.

6. Use according to any one of the preceding claims, wherein the Li1-xFePO4 material represents from 60% to 97% by mass, in particular from 70% to 96% by mass, relative to the total mass of the electrode, excluding the mass of the current collector of the electrode.

7. Use according to any one of the preceding claims, wherein said reference electrode comprises, in addition to said Li1-xFePO4 material, one or more binders, in particular selected from polymeric binders, and optionally one or more electronically conductive additives, in particular selected from carbon fibres, carbon black, carbon nanotubes and mixtures thereof.

8. Use according to any one of the preceding claims, wherein said reference electrode exhibits a grammage, defined as the mass of Li1-xFePO4 material in grams per unit of surface area in cm2, of between 2 mg / cm2 and 30 mg / cm2, in particular between 5 mg / cm2 and 25 mg / cm2.

9. Use according to any one of the preceding claims, in at least one of the cells of a conventional potassium-ion supercapacitor, said cell comprising positive and negative electrodes based on activated carbon.

10. Use according to any one of Claims 1 to 8, in at least one of the cells of a potassium-ion hybrid supercapacitor, said cell comprising a positive electrode based on activated carbon, a negative electrode based on a carbon-based material for intercalating at least potassium and more particularly based on graphite.

11. Use according to any one of the preceding claims, wherein the electrolyte of said cell comprises at least one potassium salt, preferably selected from potassium perchlorate (KClO4), potassium fluoroborate (KBF4), potassium hexafluorophosphate (KPF6), and mixtures thereof, in particular KPF6, in one or more organic solvents, said organic solvent (s) being in particular selected from carbonate solvents, linear ether solvents, nitrile solvents, lactone solvents, amide solvents and mixtures thereof, preferably nitrile solvents, such as acetonitrile.

12. Use according to any one of the preceding claims, said cell being in a prismatic format, in particular in the form of a pouch cell.

13. Use according to any one of the preceding claims, for determining the potentials of at least one of the positive and negative electrodes, preferably of each of the positive and negative electrodes of said supercapacitor cell, in particular for monitoring the evolution of the potentials of at least one of the positive and negative electrodes, during the operation of said supercapacitor.

14. Cell for a conventional or hybrid supercapacitor, comprising a non-aqueous electrolyte comprising at least one potassium salt, said cell comprising at least: - a positive electrode (22), in particular based on activated carbon; - a negative electrode (23), in particular based on activated carbon in the case of a conventional supercapacitor, or based on a carbon-based potassium intercalation material, in particular based on graphite, in the case of a hybrid supercapacitor; - a reference electrode (21) based on Li1-xFePO4 with 0.30 ≤ x ≤ 0.70, in particular 0.50 ≤ x ≤ 0.60.

15. Cell according to Claim 14, wherein said reference electrode is as defined in any one of Claims 2 to 8 and / or the nonaqueous electrolyte is as defined in Claim 11.

16. Cell according to Claim 14 or 15, said cell comprising the assembly (100), in this stacking order, of the following elements: - said positive electrode (22), in particular based on activated carbon; in particular formed of a double layer (220) based on activated carbon on a current collector (221), for example made of aluminium; - a porous separator (24), for example made of polypropylene, intended to be impregnated with the electrolyte; - said reference electrode (21), in particular as defined in any one of Claims 2 to 8, formed of a layer (211) of the electrode material based on Li1-xFePO4 at a current collector (212), for example made of carbon-coated aluminium; - a porous separator (24), for example made of polypropylene, intended to be impregnated with the electrolyte; and - said negative electrode (23), in particular formed of a layer (231) based on activated carbon in the case of a conventional supercapacitor, or based on a carbon-based material for intercalating at least potassium, for example based on graphite, in the case of a hybrid supercapacitor, on a current collector (232), for example made of aluminium.

17. Conventional or hybrid supercapacitor, comprising at least one cell as defined in any one of Claims 14 to 16.