Cathode composition

Abstract

The present invention relates to NVPF-based compositions and their use in the field of batteries as electrochemically active materials. The present invention also relates to conductive compositions comprising the compositions and to processes for obtaining the compositions.

Description

[Technical Field]

[0001] Cross-reference of related applications This application claims priority to European Patent Application No. 23306118.3 filed on 30 June 2023, and the entire contents of this application are incorporated herein by reference for any purpose.

[0002] The present invention relates to NVPF-based compositions and their use in the field of batteries as electrochemically active materials. The present invention also relates to conductive compositions comprising the compositions and to processes for obtaining the compositions. [Background technology]

[0003] Demand for lithium-ion batteries has increased in recent years due to their application in a wide variety of electronic devices, such as mobile phones and electric vehicles. In reality, lithium-based compounds are relatively expensive, and natural lithium sources are not readily available because they are unevenly distributed across the globe and concentrated in a few countries. Therefore, alternatives to this element have been sought. For this purpose, sodium-ion batteries have been developed. This is because sodium is very abundant, uniformly distributed, advantageously non-toxic, and more economically advantageous.

[0004] However, regarding the 3x molar mass, Na + The redox potential of the / Na couple is (-2.71V relative to SHE), and thus Li + This is greater than that of the Li couple (-3.05V relative to SHE). These specifications make the selection of host materials difficult. In this regard, the material Na3V2(PO4)2F3 (or NVPF) has proven to be an electrochemically active material that is particularly advantageous in terms of its electrochemical performance quality. In fact, NVPF typically exhibits a reversible capacity of over 120mAh / g and good cycling performance at room temperature.

[0005] To be used as a cathode active material (CAM), an NVPF composition must possess not only high ionic conductivity but also good electronic conductivity in order to efficiently discharge electrons toward the current collector during battery operation. When both criteria are met, high power density can be achieved.

[0006] For this purpose, NVPF particles are generally coated with a conductive material. The conductive material can have different properties and should not be involved in the electrochemical processes that contribute to the battery's cycling. Furthermore, the conductive material must be present in a moderate amount to maximize the content of the pure cathode active material, and therefore maximize the energy density of the electrode.

[0007] In general, carbon is preferred because it acts as an electron conductor without altering the electrochemical properties of NVPF.

[0008] U.S. Patent Application Publication No. 2018 / 0297847 describes NVPF particles obtained by exposing a mixture of a VPO4 material and an effective amount of sodium fluoride (NaF) and at least one hydrocarbon-containing and oxygen-containing compound that is an elemental carbon source to temperature conditions favorable for calcining the mixture to form a Na3V2(PO4)2F3 compound, for example, 800°C for 1 hour, under an inert atmosphere. Cellulose is used as the carbon source. The carbon coating represents 0.5% to 5.0% by weight of the total weight of the material.

[0009] U.S. Patent Application Publication No. 2021 / 0305549 relates to an NVPF-based composition containing NVPF particles and carbon in graphitized form. This composition is obtained by calcining a homogeneous mixture of VPO4 and cellulose as a carbon source with NaF at 800°C. The carbon content is expressed as 1.0% to 3.5% by weight of the total mass of the composition.

[0010] The applicant has experienced that an NVPF-based composition prepared from the calcination of a homogeneous mixture of VPO4, NaF, and at least one hydrocarbon-containing and oxygen-containing compound (i.e., cellulose) as an elemental carbon source yields a coated NVPF with only moderate electronic conductivity, provided that a reasonable amount of carbon is maintained to preserve the energy density.

[0011] Chemical vapor deposition (CVD), generally recognized as an efficient route for preparing highly conductive carbon, is sometimes used to deposit carbon materials onto electrochemically active materials for electrode applications. For example, Y. Fang et al. disclose the chemical deposition of conductive carbon onto Na3V2(PO4)3 (NVP) in Advanced Materials, 2015, 27, 5895-5900. For this purpose, the NVP material is introduced into a CVD furnace, where acetylene gas pyrolysis is performed to create a conductive carbon structure. The acetylene is introduced into the furnace in a mixture with argon, while increasing the furnace temperature to 690°C. The decomposition of acetylene results in carbon deposition on the surface of the NVP.

[0012] Similarly, Y. Zhang et al. disclose the chemical deposition of conductive carbon onto Na3V2(PO4)2F3 (or NVPF) in the Journal of Materials Chemistry, 2018, 6, 4525-4534. For this purpose, the NVPF material is introduced into a CVD furnace, where the acetylene gas is thermally decomposed to create a conductive carbon structure. The acetylene flow is a mixture with argon and is introduced into the furnace while increasing the furnace temperature to 500°C. The decomposition of acetylene results in carbon deposition on the surface of the NVPF. In certain cases of NVPF, operation under gas flow can lead to fluorine detachment associated with partial oxidation of the NVPF, which leads to a decrease in electrochemical performance. Therefore, a relatively low temperature (500°C) is set for thermal decomposition, with the risk of reduced graphitization of the carbon deposits and ultimately a decrease in electronic conductivity.

[0013] Furthermore, because acetylene is a somewhat inherently unstable, highly flammable gas, handling it remains difficult. Therefore, the use of acetylene presents difficulties related to its gaseous nature, safety issues, and consequently, additional costs, compared to the use of hydrocarbon-containing and oxygen-containing compounds such as cellulose, which are readily available, stable, and easy to handle.

[0014] [Problems the invention aims to solve] Cathode active materials that exhibit a good trade-off between power density and energy density are needed for sodium-ion battery applications. Na3V2(PO4)2F3 (or NVPF) is considered a prime material for constructing such cathode active materials, either due to its high ionic conductivity or its high gravimetric energy density, i.e., the amount of energy it can store per unit weight.

[0015] A composition is needed that contains NVPF particles coated with a conductive material suitable for efficiently discharging electrons generated during battery operation toward the current collector.

[0016] A composition containing NVPF particles coated with conductive carbon exhibiting improved electronic conductivity is needed. Such a composition having a tap density of at least 0.9 g / ml is also desirable.

[0017] A composition is needed that comprises NVPF particles coated with conductive carbon having all of the desired characteristics described above, wherein the carbon coating has improved resistance to oxidation under demanding thermal conditions.

[0018] A suitable process for preparing the above composition is also required. A process involving CVD is needed to ensure a thin, pure, and highly conductive carbon coating on the NVPF particles. Such a process involving CVD should be carried out under conditions that do not require very sophisticated equipment. Ideally, this process should involve raw materials that are readily available and easy to handle.

[0019] Electrodes are generally composed of an active material that guarantees the extraction and reinsertion of sodium ions and a suitable proportion of electrochemically inert materials such as a binder or a conductive additive. The conductive additive ensures electron transmission through the electrode. The binder ensures adhesion to the current collector and the mechanical strength of the composite electrode. Porosity is necessary, in part, for ion transmission between the electrolyte and the active material. Attempts have been made to limit the amount of electrochemically inert materials in the composite material in order to maximize the weight capacity (expressed in mAh / g in this patent application), but without impairing the extraction and reinsertion of sodium ions by the active material.

[0020] Therefore, it is required that a composition containing NVPF particles coated with conductive carbon having all of the desired characteristics described above can be easily formulated with electrochemically inert materials, and that the resulting conductive composition can be easily coated to form an electrode thin film.

[0021] Finally, a sodium ion battery containing the above-described electrodes that exhibits good cycle life durability is required.

[0022] All of these needs and more are met by the composition according to the invention obtained by the process of the invention.

Brief Description of the Drawings

[0023] [Figure 1]The graph shows the electronic conductivity values ​​in mS / cm, measured at 22°C by a DC method using a two-electrode setup, as a function of the carbon content (black solid line ●) in the NVPF-based composition according to the present invention and (white square □) in a comparative NVPF-based composition, as measured by weight percent.

[0024] This invention The present invention relates to a process for preparing particles of an NVPF material of the formula Na3V2(PO4)2F3, which is optionally partially oxidized, and a composition containing carbon, comprising the following steps: a) A step of providing a mixture of VPO4 and sodium fluoride; b) The mixture from step a) is introduced into a container, next to the oxygenated hydrocarbon compound that will decompose thermally to yield carbon, and the container is closed to obtain a confined reaction medium; c) To simultaneously form NVPF and carbon and reach the desired composition, the reaction medium from step b) is heated to a temperature in the range of 700°C to 900°C and maintained at this temperature for a time in the range of 0.5h to 6h (wherein the carbon is introduced into the composition by chemical vapor deposition). This includes processes related to the process.

[0025] In step a), both VPO4, which is in solid form, for example as a powder, are mixed with a stoichiometric amount of sodium fluoride (NaF). It is advantageous to use a mixture in which the solids are homogeneously mixed. To obtain a completely homogeneous mixture, it is possible to use VPO4 that has been pre-ground and / or sieved. For example, ground and sieved VPO4 exhibiting a Dv50 of 5-40 μm and a Dv90 of 40-100 μm (the distribution is measured by itself from a suspension in anhydrous ethanol) may be used. The mixture can be prepared by any means suitable for mixing solid materials, which are well known to those skilled in the art.

[0026] For example, a suitable VPO4 can be prepared starting from V2O5 and NH4H2PO4 as raw materials, according to the method described in U.S. Patent Application Publication No. 2021 / 0305549. High-purity commercial NaF can be used. The NaF can also be ground and sieved before being mixed with VPO4.

[0027] In step b), the oxygen-containing hydrocarbon compound may be a sugar, such as glucose, fructose, galactose, sucrose, or lactose, or a carbohydrate, such as starch, agar, or cellulose derivatives, or lignin. Preferably, it is a cellulose derivative, and more particularly, microcrystalline cellulose. The oxygen-containing hydrocarbon compound yields carbon present in the composition, particularly in graphitized or partially graphitized form, through thermal decomposition. Therefore, the oxygen-containing hydrocarbon compound is considered a carbon source in the process according to the present invention.

[0028] In step b), the stoichiometric mixture of VPO4 and NaF is not mixed with the oxygenated hydrocarbon compound. The mixture of VPO4 and NaF is introduced next to the oxygenated hydrocarbon compound in the container. Next to means that the mixture of VPO4 and NaF does not come into direct contact with the oxygenated hydrocarbon compound, despite the fact that they are in the same container. There are no restrictions on the form and structure of the container. For example, on a laboratory scale, the container can be a parallelepiped rectangular crucible made of alumina that can be introduced into a tubular furnace. The mixture of VPO4 and NaF on one side and the oxygenated hydrocarbon compound on the other can be placed in two different small crucibles. The latter can be further placed in a larger parallelepiped rectangular crucible, which is then closed with a similarly alumina lid to obtain a confined reaction medium and introduced into a tubular furnace. Furthermore, for example, the mixture of VPO4 and NaF can be placed on one side of a container separated into two parts by a wall that is permeable to gases but not to solids, while the oxygenated hydrocarbon compound is placed on the other side.

[0029] In step c), the reaction medium from step b) is heated to a temperature in the range of 700°C to 900°C. In some embodiments, the temperature is in the range of 750°C to 850°C. Good results were obtained at a temperature of 800°C. Generally, heating is carried out using a heating rate of 4°C / min to 50°C / min. In some embodiments, the heating rate is 5°C / min to 20°C / min. Good results were obtained at a heating rate of 10°C / min. This temperature is generally maintained for a time in the range of 0.5h to 6h to simultaneously form NVPF by calcination with carbon by thermal decomposition of VPO4, NaF, and oxygen-containing hydrocarbon compounds to reach the desired composition. Preferably, the temperature is maintained for 0.5h to 3h, more preferably for 1h. Good results were obtained when the temperature was maintained at 800°C for 1h. For example, step c) can be carried out by placing the container from step b), which is closed to obtain the confined reaction medium, into any furnace such as a tubular furnace for heating.

[0030] Surprisingly, the proportion of oxygenated hydrocarbon compounds in the container, based on VPO4 and NaF, was found to be related to the final carbon content in the resulting composition. This proportion could range from 0.5% to 30.0% by weight, and even from 5.0% to 25.0%, and this proportion is calculated for combinations of VPO4 and NaF mixtures.

[0031] Without being bound by any theory, the inventors assume that carbon is introduced into the composition by chemical vapor deposition (CVD) during the process. Chemical vapor deposition (CVD) may be performed on NVPF particles or on VPO4 before NVPF formation. Chemical vapor deposition (CVD) is generally defined as a controlled, gas-phase, chemical process for depositing thin film layers of various materials onto a substrate. According to KESpear in Pure & Appl. Chem., Vol. 54, No. 7, pp. 1297-1311, 1982, "A chemical vapor deposition process is one in which a gas phase chemically reacts to produce one or more condensed phases (deposits) plus gas product species."

[0032] During steps a), b), or c), it is advisable to minimize contact between the reaction medium and oxygen by carrying out these steps under an inert gas, such as nitrogen or argon. Since calcination is carried out in a confined environment, the presence of oxygen can be limited by introducing an inert gas, such as nitrogen or argon, into the vessel in which calcination is performed. The presence of oxygen can be limited, for example, by introducing an inert gas, such as nitrogen or argon, as a flow into the furnace in which the vessel in step b) is placed. However, since the vessel in step b) is closed to obtain a confined reaction medium, contact between the reaction medium and oxygen is considerably limited, and the use of an inert gas is not always necessary.

[0033] The present invention also relates to a process for preparing particles of NVPF material of formula Na3V2(PO4)2F3, which are optionally partially oxidized, and a composition containing carbon, the following steps: a') The step of introducing NVPF next to the oxygenated hydrocarbon compound in the container and closing the container to obtain the confined reaction medium; b') Heat the reaction medium from step a') to a temperature in the range of 700°C to 900°C and maintain this temperature for a time in the range of 0.5h to 6h to reach the desired composition (where carbon is introduced into the composition by chemical vapor deposition). Regarding processes that include this.

[0034] In step a'), the NVPF is in solid form, for example, as a powder. It is possible to use pre-ground and / or sieved NVPF. For example, ground and sieved NVPF exhibiting a Dv50 of 1-30 μm and a Dv90 of 5-100 μm (the distribution is measured from a suspension in anhydrous ethanol) may be used.

[0035] For example, a suitable NVPF can be prepared starting from VPO4 and NaF as raw materials without the addition of microcrystalline cellulose, according to the method described in Example 1 of U.S. Patent Application Publication No. 2021 / 0305549. "Next to" means that the NVPF does not come into direct contact with the oxygenated hydrocarbon compound, despite the fact that the NVPF and the oxygenated hydrocarbon compound are in the same container. A suitable setup for operating the process previously described for a mixture of VPO4 and NaF can also be used for NVPF.

[0036] In step b'), the reaction medium from step a') is heated to a temperature in the range of 400°C to 900°C. In some embodiments, the temperature is in the range of 500°C to 850°C. Good results were obtained at a temperature of 800°C. Generally, heating is carried out using a heating rate of 4°C / min to 50°C / min. In some embodiments, the heating rate is 5°C / min to 20°C / min. Good results were obtained at a heating rate of 10°C / min. This temperature is generally maintained for a time in the range of 0.5h to 6h to form carbon by thermal decomposition of the oxygen-containing hydrocarbon compound and reach the desired composition. Preferably, the temperature is maintained for 0.5h to 3h, more preferably for 1h. Good results were obtained when the temperature was maintained at 800°C for 1h.

[0037] Surprisingly, the proportion of oxygen-containing hydrocarbon compounds in the container, based on NVPF, was found to be related to the final carbon content in the resulting composition. This proportion can range from 0.5% to 30.0% by weight, and in fact even from 5.0% to 25.0% by weight, and this proportion is calculated relative to NVPF.

[0038] Without being bound by any theory, the inventors assume that carbon is introduced into the composition during the process by chemical vapor deposition (CVD) onto NVPF particles.

[0039] During step a') or b'), it is prudent to minimize contact between the reaction medium and oxygen by performing these steps under an inert gas, such as nitrogen or argon. Since calcination is carried out in a confined environment, the presence of oxygen can be limited by introducing an inert gas, such as nitrogen or argon, into the vessel in which calcination is performed. The presence of oxygen can be limited, for example, by introducing an inert gas, such as nitrogen or argon, as a flow into the furnace in which the vessel in step b) is placed. However, since the vessel in step b) is closed to obtain a confined reaction medium, contact between the reaction medium and oxygen is considerably limited, and the use of an inert gas is not always necessary.

[0040] The inventors have surprisingly found that by performing the process according to the present invention, it is possible to prepare compositions containing particles of NVPF material of the formula Na3V2(PO4)2F3 that are optionally partially oxidized, with a controllable carbon content. The inventors have also found that these compositions have improved electronic conductivity, measured at 4.75 MPa by a direct current (DC) method, even for carbon content as low as 0.2%, where this content is expressed as the weight of elemental carbon relative to the total weight of the composition. This electronic conductivity is generally higher than that of conductive NVPF-based compositions obtained by processes disclosed in the prior art for the same carbon content.

[0041] Furthermore, the inventors have surprisingly found that compositions prepared by the process according to the present invention enable the preparation of sodium-ion batteries having an improved first discharge capacity, expressed in mAh / g units measured at different C rates such as C / 10.

[0042] Considering the higher electronic conductivity of the products obtained by the process according to the present invention, higher capacity retention at high charging speeds (such as 1C, 2C, or 5C) is expected in sodium or Na ion electrochemical cells compared to electrodes prepared with materials of the latest technology.

[0043] Considering the higher electronic conductivity of the products obtained by the process according to the present invention, a higher capacity retention rate at high discharge rates (such as 2C, 5C, or 10C) can be expected in sodium or Na ion electrochemical cells compared to electrodes prepared with materials of the latest technology.

[0044] Therefore, the present invention also relates to particles of NVPF material of formula Na3V2(PO4)2F3 that are optionally partially oxidized, and to compositions comprising carbon. (1) Carbon content of 0.17% to 3.0% (this content is expressed as the weight of elemental carbon relative to the total weight of the composition), (2) Electronic conductivity σ expressed in mS / cm units, such as σ > 4.3717x + 0.2186 (where x is the carbon content expressed as a weight % relative to the total weight of the composition) Characterized by; Here, the electronic conductivity σ is measured with respect to a sample of the composition pressed at 4.75 MPa by the direct current (DC) method.

[0045] The compositions of the present invention are characterized by their carbon content, which ranges from 0.17% to 3.0%, and this content is expressed as the weight of elemental carbon relative to the total weight of the composition. The carbon content is measured by trace analysis.

[0046] Generally, the carbon content is between 0.17% and 3.0%. In some preferred embodiments, the carbon content is between 0.17% and 1.0%, more preferably between 0.20% and 0.9%, and even more preferably between 0.25% and 0.8%, where this content is expressed as the weight of elemental carbon relative to the total mass of the composition.

[0047] The compositions of the present invention are also characterized by their electronic conductivity. The electronic conductivity of the NVPF-based composition powder can be measured by a DC method. A sample of the powder is introduced into a PEEK cell (10 mm in diameter) equipped with a stainless steel plunger (10 mm in diameter). The cell is placed in a mechanical fixture equipped with a force sensor to control the applied pressure. The thickness of the powder bed is measured with a digital dial indicator (Mitutoyo). At room temperature (22°C ± 2°C), a voltage of 1 V is applied to the pressed sample using a VMP-3 potentiostat available from BioLogic at a pressure of 4.75 MPa, the thickness of the powder bed is measured at each step, and the current value is recorded 30 seconds after equilibrium is reached.

[0048] Electronic conductivity is given by the following formula: σ = I * t / (S * V) (where I is the measurement current expressed in amperes, t is the thickness of the powder bed expressed in centimeters, V is the voltage expressed in volts (1 V in this case), and S is the thickness of the powder bed expressed in centimeters). 2 It is obtained from a surface (expressed in units).

[0049] Generally, the electronic conductivity σ, expressed in mS / cm units, is such that σ > 4.3717x + 0.2186, where x is the carbon content expressed as a weight percent relative to the total weight of the composition. In some preferred embodiments, the electronic conductivity is such that σ > 6.0730x - 0.0607; or σ > 8.9879x - 0.5393; or σ > 14.5900x - 1.4590; or σ > 23.9010x - 2.9876.

[0050] The composition of the present invention comprises particles of NVPF that are optionally partially oxidized. Optionally partially oxidized NVPF is the dominant element of the composition. Its weight percentage is generally 92% or more, generally 95% or more, and generally 97% or more, and this percentage is expressed relative to the total weight of the composition. This percentage may be 92.0% to 99.83% by weight, preferably 95.0% to 99.83% by weight, and more preferably 97.0% to 99.83% by weight.

[0051] In the NVPF compound with the molecular formula Na3V2(PO4)2F3, vanadium exists in the +III oxidation state. NVPF can be partially oxidized. In this case, the product is also characterized by the presence of vanadium in the +IV oxidation state and the partial replacement of fluorine atoms by oxygen atoms. The partially oxidized NVPF has the formula Na3V2(PO4)2F 3-x O x (where x is an integer from 0 to 1.0).

[0052] Optionally partially oxidized NVPF crystallizes in an orthorhombic unit cell of the Amam space group. The unit cell parameter c can be 10.686 angstroms or more, indeed even 10.750 angstroms or more. It can be substantially equal to 10.750 angstroms. The unit cell parameter a can be 9.027 - 9.036 angstroms for a part of it, and preferably can be substantially equal to 9.029 angstroms. The unit cell parameter b can be 9.038 - 9.045 angstroms for a part of it, and preferably can be substantially equal to 9.044 angstroms. The unit cell volume V can be 872.604 - 878.390 angstroms 3 and preferably is substantially equal to 878.000 angstroms 3 .

[0053] For NVPF, the unit cell parameter c is 10.741 - 10.756 angstroms. The unit cell parameter a is 9.028 - 9.031 angstroms. The unit cell parameter b is 9.043 - 9.045 angstroms. The unit cell volume V can be 877.335 - 878.390 angstroms 3 and preferably is substantially equal to 878.000 angstroms 3 .

[0054] The composition contains carbon in graphitized form. This graphitized carbon contributes to the electronic conductivity of the NVPF particle surface. The presence of graphitized carbon in the composition can be demonstrated using Raman spectroscopy. More specifically, the graphitized carbon is present at 1580–1600 cm⁻¹. -1 It is located at approximately 1590cm. -1 The presence of vibrational bands centered at a certain point can be demonstrated by Raman spectroscopy. Carbon in graphitized form is obtained by high-temperature thermal decomposition of oxygen-containing hydrocarbon compounds, as described above. Thermal decomposition also leads to the formation of amorphous carbon.

[0055] The compositions of the present invention may exhibit a ratio R of 1.1 or less, preferably 1.0 or less, and indeed even 0.9 or less, where: - R is the ratio I calculated over at least six measurements performed at various points in the composition sample. D / I G It means the arithmetic mean; - I D It is approximately 1340cm -1 This refers to the intensity of the Raman vibration band centered at [location]; - I G It is approximately 1590cm -1 This refers to the intensity of the Raman vibration band centered at [a specific point].

[0056] Approximately 1340cm -1 The vibrational band is attributed to amorphous (or disordered) carbon. This band is generally 1330–1360 cm⁻¹. -1 It is located at approximately 1590cm. -1 The vibrational band is attributed to graphitized carbon. This band is generally 1580-1600 cm². -1 It is located in [location].

[0057] The compositions of the present invention further exhibit a tap density TD of 0.9 g / ml or more, even 1.0 g / ml or more, and even 1.1 g / ml or 1.2 g / ml or more. The tap density is measured by a known method using a powder tapping device. The tap density is generally 2.0 g / ml or less, even 1.8 g / ml or less, and even 1.5 g / ml or less.

[0058] The composition of the present invention can be used as an electrochemically active material for electrodes in sodium-ion batteries. The present invention also relates to an electrode comprising the composition of the present invention and a conductive composition (CC) comprising at least one conductive material and optionally a binder. The proportion of the composition of the present invention in the conductive composition (CC) is generally greater than 70.0% by weight, and this proportion is relative to the total weight of the conductive composition. This proportion may be between 70.0% and 98.0%. The proportion of the conductive material is generally less than 30.0% by weight, and this proportion is relative to the total weight of the conductive composition (CC). This proportion may be between 1.0% and 20.0%. More particularly, the conductive composition (CC) may comprise 80.0% to 98.0% by weight of the composition of the present invention, 1.0% to 15.0% by weight of the conductive material, and 1.0% to 15.0% by weight of the binder.

[0059] The conductive material may be selected from carbon fibers, carbon black, carbon nanotubes, graphene, and their analogues. An example of a conductive material is Super C45 Carbon Black Conductive Additive for Battery Cathode and Anode, available from MSE Supplies®. The binder may be a polymer, which is advantageous. The binder may be selected from copolymers of polytetrafluoroethylene, polyvinylidene fluoride, or vinylidene fluoride and at least one comonomer, such as hexafluoropropylene, polymers derived from carboxymethylcellulose, polysaccharides, and latex, particularly of the styrene / butadiene rubber type. The binder is preferably a copolymer of vinylidene fluoride and at least one comonomer, such as hexafluoropropylene. It may be, for example, the Solef 5130 brand sold by Solvay.

[0060] Conductive compositions can be prepared by mixing their constituent materials together in the presence of a polar solvent, such as N-methylpyrrolidone. If the viscosity of the mixture is high, a kneader suitable for high viscosity can be used. In the case of polymer binders, for example, the binder can first be dissolved in NMP, and then the conductive material and the composition according to the present invention can be added while stirring. The mixture can then be deposited on an aluminum sheet, and the NMP can then be evaporated, for example, by heating.

[0061] The electrode of the present invention can be used as the positive electrode of a sodium generator. Advantageously, it is suitable for use as the positive electrode for sodium or sodium-ion batteries.

[0062] If any disclosure of a patent, patent application, or publication incorporated herein by reference conflicts with any description in this application to such an extent that it obscures certain terminology, the description herein shall prevail. [Examples]

[0063] Measurement of carbon content The carbon content is measured by trace analysis using a Horiba EMIA 320 V2 brand carbon / sulfur analyzer. Measurements are performed on 80–120 mg of sample. The sampled powder is coated with tin, tungsten, and iron combustion accelerators.

[0064] Measurement of electronic conductivity The electronic conductivity of NVPF powder is measured by a DC method using a two-electrode setup. A fixed amount of powder (444 mg) is introduced into a PEEK cell (10 mm in diameter) equipped with a stainless steel plunger (10 mm in diameter). The cell is placed in a mechanical fixture equipped with a force sensor to control the applied pressure. The thickness of the powder bed is measured using a digital dial indicator (Mitutoyo). At room temperature (22°C ± 2°C), a voltage of 1 V is applied to the sample at a pressure of 4.75 MPa using a VMP-3 potentiostat available from BioLogic, the thickness of the powder bed is measured at each step, and the current value is recorded 30 seconds after equilibrium is reached. The electronic conductivity is given by the following formula: σ = I * t / (S * V) (where I is the measured current in amperes, t is the thickness of the powder bed in centimeters, V is the voltage in volts (1 V here), and S is the thickness of the powder bed in centimeters). 2 It is obtained from a surface (expressed in units).

[0065] Measurement of particle size distribution The particle size distribution is measured by laser diffraction of the suspension of particles in ethanol. A Malvern Mastersizer 3000 instrument equipped with a Hydro SV module is used. The instrument's tank is filled with ethanol (refractive index for ethanol at 1.360) and stirred at 3500 rpm. Then, 2-3 milligrams of powder are directly introduced into the tank to have an obscuration of 8-12%. The optical model used is Fraunhofer.

[0066] Measurement of tap density Using a 10 ml (±0.1 ml at 20°C) Class A+ graduated cylinder, fill it with approximately 7 ml of the loose composition. After applying 2500 blows, read the final volume occupied by the tapped powder. Then, the tap density is calculated using the following formula: TD (in g / ml) = Weight of composition / Measured final volume It is determined by [method].

[0067] Measurement of powder X-ray diffraction Powder X-ray diffractograms were obtained in a fixed-slit, Bragg-Brentano structure. Acquisition was performed at a voltage of 45kV and a current of 40mA using a Malvern Panalytical X Pert Pro MPD assembly with a copper-to-cathode X-ray source, at 2θ=5° to 2θ=90°. The detector was an X'Celerator linear detector with an effective length of 2.122°. The PHD spacing was set to the default of 37-80%. Copper K-beta emission was filtered using the Bragg-Brentano HD module. Exposure time was typically 40 seconds in 0.017° steps. Phase analysis was performed using High Score Plus software with the latest version of the ICDD PDF4+ database. Goniometer linearity was checked periodically using a polycrystalline silicon standard.

[0068] The cell parameters of the NVPF phase are obtained in the Amam space group by Full Profile Matching using FullProf software.

[0069] Recording and processing of Raman spectroscopy spectra The spectrum was obtained using a 532nm laser (150mW nominal power), 10× objective lens, 600rpm diffraction grating, 10% filter, 100μm confocal hole, and 30×5s capture time at 50 and 2000cm². -1The data was recorded using a Horiba HR800 spectrometer. The software used for acquisition and processing was Horiba Labspec version 6.6.1.11. To perform the acquisition, the sample was deposited in powder form on a calcium fluoride window, which was itself deposited on an aluminum-covered glass slide. Focusing was performed on the sample using a 10× objective lens. After acquisition, the spectrum was flattened, and then the two Gaussian curve contributions were obtained: approximately 1340 cm⁻¹. -1 Those with a center and approximately 1580cm -1 It deconvolutes to another object with its center at 700cm. -1 up to 2000cm -1 Measurements are taken from a baseline drawn between two points on the spectrum.

[0070] Preparation of a positive electrode from the composition of the present invention The conductive composition (i.e., electrode ink) is prepared by mixing an NVPF-based composition with carbon black (Super C45 Carbon Black Conductive Additive for Battery Cathode and Anode, available from MSE Suppliers®) and a fluoropolymer (PVDF Solef 5130) in a 92:4:4 weight ratio in N-methyl-2-pyrrolidone solvent to obtain a viscous ink. The mixing is carried out in an ST20 tube using an Ultra-Turrax® Tube Drive, operating at 2000 rpm for 30 minutes, then at 4000 rpm for 1.5 hours. This ink, containing 45 wt% solids, is then deposited on a 15 μm thick aluminum sheet to a wet thickness of 275 μm using a film applicator, and then dried at 90°C until the solvent is completely evaporated. The dried electrodes are calendered at 60°C using an MSK-HRP-01 Electric Hot Rolling Cylinder Press, available from MTI Corporation, to 100 μm and then 70 μm. They are then cut into 12 mm diameter discs, dried at 120°C for 12 hours under primary vacuum, and then transferred to a glove box under an argon atmosphere.

[0071] Coin cell type electrochemical cell assembly The NVPF electrode is assembled into a half-cell configuration with a 2032 (20 mm diameter x 3.2 mm thickness) button cell structure, facing the metallic sodium anode. The electrolyte used consists of an equal mass mixture of ethylene carbonate and dimethyl carbonate containing 1 mole of dissolved sodium hexafluorophosphate per liter, to which 1% by weight of monofluoroethylene carbonate is added.

[0072] The coin cell consists of an NVPF positive electrode, a metallic sodium negative electrode, 100 μl of electrolyte, a 1 mm thick stainless steel current collector, a 1.4 mm thick ring-shaped spring, a 16 mm diameter fiberglass separator, and a rigid casing of the cell (two hollow pieces connected with a seal). A thin, flat layer of metallic sodium is deposited on the current collector, and the weight of the sodium is sufficient but not limiting in the system. The separator is placed on top of the cathode and impregnated with the electrolyte, and then the metallic sodium is added facing the cathode. These elements are kept under spring pressure inside the rigid casing, and the casing is wrinkled to ensure no leakage of the system.

[0073] Electrochemical test The cell assembled from NVPF electrodes starts with charging (positive current), Na + The batteries were electrochemically tested under constant current conditions at 2.0V to 4.3V for / Na. The current used for charging and discharging is expressed in units of C-rate. The C-rate is a measure of the speed at which the battery is charged or discharged. It is defined as the applied current divided by the theoretical current required to deliver the battery's theoretical capacity in one hour. In this case, it corresponds to the exchange of two sodium ions per NVPF. The electrochemical test was performed at a C-rate of C / 10, which corresponds to theoretical charge or theoretical discharge in 10 hours.

[0074] Electrochemical testing allows for the measurement of the reversible charging capacity of NVPF electrodes. This capacity is reported by weight of the composition (active substance) and expressed in mAh / g units. The test is performed at room temperature (22°C ± 2°C).

[0075] C-rate performance evaluation The electrochemical test is performed according to a three-cycle program in which C / 10 is used for charging and C / 10 for discharging. Capacitance and polarization are extracted for each cycle. For some products, these three cycles at C / 10 are followed by 3 to 5 cycles in which C / 5 is used for charging and C / 5 for discharging, C / 2 is used for charging and C / 2 for discharging, C / 2 is used for charging and 1C for discharging, C / 2 is used for charging and 2C for discharging, C / 2 is used for charging and 5C for discharging, and C / 10 is used for charging and C / 10 for discharging.

[0076] Example 1: Preparation of VPO4 Stoichiometric amounts of V2O5 and NH4H2PO4 are mixed in a Controlab L0031.2 type kneader in the presence of 100 wt% water. The proportion of water is calculated by weight relative to the combination of the two reactants, V2O5 and NH4H2PO4. At the end of approximately 2 hours, the mixture thickens, yielding a yellow paste formed from NH4VO2HPO4 (its presence is confirmed by XRD) and water. This wet paste is placed in a well-confined environment, i.e., the paste is poured into a SiC crucible closed with a SiC lid. The paste is then calcined at 800°C for 3 hours with a heating gradient of 5.5°C / min.

[0077] The VPO4 produced from calcination is brittle and consists of pieces ranging from 1 mm to 5 cm in size. This product is ground using a jar mill at a rotational speed of 27 rpm for approximately 2.7 hours. A 10 cm diameter polyethylene jar is used, which allows the product to be loaded with 10 mm diameter yttria-stabilized zirconia balls. The loading ratio can be 1 kg of VPO4 for every 4 kg of balls loaded. The thus ground VPO4 is removed from the jar, separated from the balls, and sieved to a 400 μm diameter using a vibrating sieve with an amplitude of 0.5 to 1.6. The VPO4 sieved to 400 μm represents 95% to 98% of the total weight of the ground VPO4. The thus ground and sieved product exhibits a particle size with a Dv50 of 5 to 40 μm and a Dv90 of 40 to 100 μm.

[0078] Example 2: Preparation of NVPF The VPO4 obtained in Example 1 was mixed with a stoichiometric amount of NaF, the solid mixture was homogenized, and the mixture was pre-ground in a polyethylene jar filled with yttria-stabilized zirconia balls (10 mm balls at a weight ratio of 1:7 [VPO4+NaF] mixture to balls) by stirring the filling jar in a Turbula® type 3D mixer. The mixture was then sieved to remove the zirconia balls.

[0079] The homogeneous mixture of powders is then calcined at 800°C for 1 hour with a heating gradient of 10°C / min. The powder should be well contained throughout the calcination. NVPF is obtained with a unit cell parameter c equal to 10.738 angstroms according to XRD.

[0080] The resulting NVPF is finally deaggregated to obtain the desired particle size distribution. For example, ball milling or air jet milling can be performed. For air jet milling, an air mill based on the AFG-100 sold by Hosokawa was used. The NVPF is introduced into the grinding chamber using a metering feed screw. The feed rate of the grinding chamber is adjusted to bring the fluidized bed thus formed into a "steady" state. Pressurized air is introduced into the grinding chamber using a 2 mm diameter nozzle at a pressure of 5.5 bar. The finest particles rise to the top of the grinding chamber. A selector with a rotation speed of 3000-5000 rpm allows for the collection of the deaggregated product.

[0081] Examples 3-5: Preparation of NVPF-based compositions according to the present invention (Process A) The VPO4 obtained in Example 1 was mixed with a stoichiometric amount of NaF, the solid mixture was homogenized, and the mixture was pre-ground in a polyethylene jar filled with yttria-stabilized zirconia balls (10 mm balls at a weight ratio of 1:7 [VPO4+NaF] mixture to balls) by stirring the filling jar in a Turbula® type 3D mixer. The mixture was then sieved to remove the zirconia balls.

[0082] A homogeneous mixture of powders is then introduced into an alumina crucible next to 5%, 10%, or 20% by weight of microcrystalline cellulose relative to the total weight of VPO4+NaF. Prior to this introduction, the microcrystalline cellulose is ground in a polyethylene jar filled with yttria-stabilized zirconia balls (10 mm balls at a cellulose-to-ball weight ratio of 1:7) by agitating the filling jar in a Turbula® type 3D mixer. The crucible is then covered with an alumina lid and placed in a tubular furnace under a nitrogen stream (30 L / h). The furnace is heated to 800°C with a heating gradient of 10°C / min, maintained at this temperature for 1 hour, and then allowed to cool naturally.

[0083] The powder is kept well contained throughout the calcination process. The NVPF-based composition is finally obtained according to XRD. The unit cell parameter c of the NVPF is equal to 10.753, 10.754, and 10.754 angstroms, respectively. The carbon content represents 0.52 wt%, 0.65 wt%, and 0.81 wt%, respectively, of the total mass of the composition. The electronic conductivity measured at 22°C for samples pressed at 4.75 MPa is 17.0 mS / cm, 27.0 mS / cm, and 30.0 mS / cm, respectively.

[0084] Example 6: Preparation of an NVPF-based composition according to the present invention (Process B) The NVPF powder of Example 2 is introduced into an alumina crucible next to 13% by weight of microcrystalline cellulose relative to the total weight of the NVPF powder. Prior to this introduction, the microcrystalline cellulose is ground in a polyethylene jar filled with yttria-stabilized zirconia balls (10 mm balls with a cellulose-to-ball weight ratio of 1:7) by agitating the filling jar in a Turbula® type 3D mixer. The crucible is then covered with an alumina lid and placed in a tubular furnace under a nitrogen stream (30 L / h). The furnace is heated to 800°C with a heating gradient of 10°C / min, maintained at this temperature for 1 hour, and then allowed to cool naturally.

[0085] The powder is kept well contained throughout the calcination process. The NVPF-based composition is finally obtained according to XRD with a unit cell parameter c equal to 10.737 angstroms. The carbon content represents 0.23 wt% of the total mass of the composition. The electronic conductivity measured at 22°C for a sample pressed at 4.75 MPa is 3.70 mS / cm.

[0086] Example 7: Preparation of an NVPF-based composition according to the present invention (Process A) Example 7 is carried out under the same conditions as Example 5, except that a homogeneous mixture of powders is introduced into an alumina crucible next to two different portions of microcrystalline cellulose, each corresponding to 10% by weight relative to the total weight of VPO4NaF+NaF. Example 7 yields an NVPF-based composition confirmed by XRD. The unit cell parameter c of NVPF is equal to 10.755 angstroms. The carbon content represents 0.56% of the total mass of the composition. The electronic conductivity measured at 22°C for a sample pressed at 4.75 MPa is 34.0 mS / cm.

[0087] Comparative Example 1: Open Crucible Comparative Example 1 was carried out under the same conditions as Example 6, except that the crucible was not covered with a lid. The NVPF-based composition was finally obtained according to XRD. The carbon content represents 0.001% by weight of the total mass of the composition. The electronic conductivity measured at 22°C for a sample pressed at 4.75 MPa was 1.56.10.-5 It is mS / cm.

[0088] Comparative Examples 2-4: Comparative Examples 2-4 were carried out under the same conditions as Examples 3-5, except that VPO4 was mixed with a stoichiometric amount of NaF and 5% by weight, 10% by weight, and 20% by weight of microcrystalline cellulose relative to the total weight of VPO4+NaF. The solid mixture was homogenized and pulverized in a polyethylene jar filled with yttria-stabilized zirconia balls (10 mm balls at a weight ratio of 1:7 [VPO4+NaF] mixture to balls) by stirring the filled jar in a Turbula® type 3D mixer. The mixture was sieved to remove the zirconia balls. The mixture was then introduced into a crucible, the crucible was covered with an alumina lid and placed in a tubular furnace under a nitrogen stream (30 L / h). The furnace was heated to 800°C with a heating gradient of 10°C / min and maintained at this temperature for 1 hour, after which it was allowed to cool naturally.

[0089] The powder is kept well contained throughout the calcination process. The NVPF base composition is obtained according to XRD. The unit cell parameter c of the NVPF is equal to 10.755, 10.755, and 10.756 angstroms, respectively. The carbon content represents 0.93 wt%, 2.12 wt%, and 4.39 wt%, respectively, of the total mass of the composition. The electronic conductivity measured at 22°C for samples pressed at 4.75 MPa is 1.9 mS / cm, 7.6 mS / cm, and 11.4 mS / cm, respectively.

[0090] Table 1 reports some of the characteristics of the compositions obtained in Examples 3-6 and Comparative Examples 1-4.

[0091] The composition obtained in Comparative Example 1, which used an open crucible, contained a very small amount of carbon, thus highlighting the importance of operating in a confined reaction medium.

[0092] The results reported in Table 1 demonstrate that it is possible to monitor the final carbon content in the composition by setting the amount of cellulose involved in the process according to the present invention (see Examples 3-5 for comparison).

[0093] It is clear that compositions obtained by the process according to the present invention, with similar carbon content, have a much higher electronic conductivity at 22°C than compositions obtained as disclosed in the prior art (for example, compare Example 5: 0.81 wt% C and σ = 30 mS / cm with Comparative Example 2: 0.93 wt% C and σ = 1.9 mS / cm). This statement is even clearer when considering the results reported in Figure 1.

[0094] Furthermore, by comparing Example 5 with Comparative Example 2, it is clear that electrodes prepared from compositions obtained by the process according to the present invention, with similar carbon content, have a higher first discharge capacity expressed in mAh / g units than electrodes prepared from compositions obtained by the prior art process.

[0095] Considering the higher electronic conductivity of the products obtained by the process according to the present invention (see Figure 1), higher capacity retention at high charging speeds (such as 1C, 2C, or 5C) can be expected in sodium or Na ion electrochemical cells compared to electrodes prepared with materials of the latest technology.

[0096] Considering the higher electronic conductivity of the products obtained by the process according to the present invention (see Figure 1), higher capacity retention at high discharge rates (such as 2C, 5C, or 10C) can be expected in sodium or Na ion electrochemical cells compared to electrodes prepared with materials of the latest technology.

[0097] The results reported in Table II concern the discharge capacity, expressed in mAh / g units, for Example 7 and Comparative Example 2 of the present invention for different C rates. These results demonstrate that the products of the present invention have a more advantageously higher discharge capacity in sodium or Na ion electrochemical cells compared to electrodes prepared with state-of-the-art materials for each C rate.

[0098] [Table 1]

[0099] [Table 2]

Claims

1. The formula Na is selectively partially oxidized. ３ V ２ (PO ４ ) ２ F ３ A process for preparing NVPF material particles and a carbon-containing composition, comprising the following steps: a) VPO ４ A step of providing a mixture of sodium fluoride; b) The step of introducing the mixture from step a) into a container next to an oxygen-containing hydrocarbon compound that will be thermally decomposed to yield carbon, and closing the container to obtain a confined reaction medium; c) To simultaneously form NVPF and carbon and reach the desired composition, the reaction medium from step b) is heated to a temperature in the range of 700°C to 900°C and maintained at this temperature for a time in the range of 0.5 h to 6 h (wherein the carbon is introduced into the composition by chemical vapor deposition). A process that includes this.

2. Partially oxidized optionally, the formula Na ３ V ２ (PO ４ ) ２ F ３ Particles of the NVPF material, and a process for preparing a composition containing carbon, comprising the following steps: a') Introducing NVPF next to the oxygen-containing hydrocarbon compound in the container and closing the container to obtain a confined reaction medium; b') Heat the reaction medium from step a') to a temperature in the range of 700°C to 900°C to reach the desired composition, and maintain this temperature for a time in the range of 0.5 h to 6 h (wherein carbon is introduced into the composition by chemical vapor deposition). A process that includes this.

3. The formula Na is selectively partially oxidized. ３ V ２ (PO ４ ) ２ F ３ Particles of NVPF material, and a composition containing carbon, (1) Carbon content of 0.17% to 3.0% (this content is expressed as the weight of elemental carbon relative to the total weight of the composition), (2) Electronic conductivity σ at 22°C, expressed in mS / cm units, such as σ > 4.3717x + 0.2186 (where x is the carbon content expressed as a weight % relative to the total weight of the composition) Characterized by; Herein, the electronic conductivity σ is measured for a sample of the composition pressed at 4.75 MPa by a direct current (DC) method.

4. The composition according to claim 3, wherein the weight percentage of NVPF that is selectively partially oxidized is 92.0% by weight to 99.83% by weight, and this percentage is expressed in relation to the total weight of the composition.

5. The composition according to any one of claims 3 to 4, wherein the unit cell parameter c is 10.686 angstroms or more, and even 10.750 angstroms or more.

6. Unit cell volume V is 872.604–878.390 angstroms ３ The composition according to any one of claims 3 to 5.

7. A composition according to any one of claims 3 to 6, exhibiting a ratio R of 1.1 or less, preferably 1.0 or less, and even 0.9 or less, - R means the arithmetic mean of the ratio ID / IG calculated over at least six measurements taken at various points in a sample of the composition; - ID is approximately 1340 cm －１ This refers to the intensity of the Raman vibration band centered at; - IG is approximately 1590cm －１ This refers to the intensity of the Raman vibration band whose center is located at [location]. composition.

8. The composition according to any one of claims 3 to 7, wherein the tap density is 0.9 g / ml or more and 2.0 g / ml or less.

9. Use of the composition according to any one of claims 3 to 8 as an electrochemically active material for electrodes for sodium-ion batteries.

10. A conductive composition comprising the composition according to any one of claims 3 to 8, at least one conductive material, and optionally a binder.

11. The conductive composition according to claim 10, wherein the conductive material is selected from carbon fibers, carbon black, carbon nanotubes, graphene, and analogs thereof.

12. The conductive composition according to claim 10 or 11, wherein the binder is selected from polytetrafluoroethylene, polyvinylidene fluoride or vinylidene fluoride, copolymers of at least one comonomer such as hexafluoropropylene, polymers derived from carboxymethylcellulose, polysaccharides, and latex, particularly of the styrene / butadiene rubber type.

13. A positive electrode comprising the conductive composition according to any one of claims 10 to 12 or the composition according to any one of claims 3 to 8.

14. A sodium-ion battery comprising the positive electrode described in claim 13.

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