Positive electrode material, power storage device, and method for producing lithium vanadium phosphate carbon composite

Abstract

The present invention relates to a method for producing a lithium vanadium phosphate carbon composite which is useful as a positive electrode material, and addresses the technical problem of providing a method for industrially advantageously producing a lithium vanadium phosphate carbon composite.　The present invention provides a method for producing a composite of carbon and lithium vanadium phosphate, which has a nasicon structure, wherein: vanadium pentoxide, phosphoric acid, and a first carboxylic acid are added to a water solvent, and a reduction reaction of vanadium pentoxide is performed, thereby preparing a reduction reaction slurry; a second carboxylic acid is added to the reduction reaction slurry, thereby preparing a reduction reaction preparation liquid; a lithium source is added to the reduction reaction preparation liquid, thereby preparing a starting material mixed solution in the form of a solution; the starting material mixed solution is spray-dried, thereby obtaining a reaction precursor; and the reaction precursor is fired at 500-1300°C in an inert gas atmosphere or a reducing atmosphere so as to obtain a lithium vanadium phosphate carbon composite.

Description

Positive electrode material, power storage device, and method for producing lithium vanadium phosphate carbon composite

[0001] The present invention relates to a method for producing a vanadium phosphate lithium carbon composite useful as a positive electrode material for lithium secondary batteries and electrochemical capacitors, and to a positive electrode material and an electricity storage device using the same.

[0002] Lithium-ion batteries are used in portable devices, laptop computers, electric vehicles, and hybrid vehicles. Lithium-ion batteries are generally considered to have excellent capacity and energy density, and currently LiCoO2 is the main material used for the positive electrode. However, due to the resource issue of Co, LiMnO2, LiNiO2, Li-Ni-Mn-Co systems, and other materials are also being actively developed.

[0003] Currently, LiFePO4 is attracting attention as an alternative material, and research and development is underway at various institutions. Fe is an excellent resource, and LiFePO4 made from it has a slightly lower energy density, but its excellent high-temperature properties make it a promising positive electrode material for lithium-ion batteries for electric vehicles.

[0004] However, LiFePO4 has a rather low operating voltage, and attention has been drawn to lithium vanadium phosphate (Li3V2(PO4)3) having a NASICON (Na Super Ionic Conductor) structure in which V is used instead of Fe.

[0005] The present applicants previously proposed in Patent Document 1 listed below a method for producing a lithium vanadium phosphate carbon composite, which comprises: a first step of preparing a raw material mixture by mixing, in an aqueous solvent, a lithium source, a pentavalent or tetravalent vanadium compound, a phosphorus source, and a conductive carbon material source that generates carbon upon thermal decomposition; a second step of heating the raw material mixture to carry out a precipitation reaction to obtain a reaction solution containing a precipitate product; a third step of wet-pulverizing the reaction solution containing the precipitate product using a media mill to obtain a slurry containing a pulverized product; a fourth step of spray-drying the slurry containing the pulverized product to obtain a reaction precursor; and a fourth step of firing the reaction precursor at 600 to 1300°C in an inert gas atmosphere or a reducing atmosphere. Furthermore, in Patent Document 2 listed below, the present applicant has proposed a method for producing lithium vanadium phosphate by heat-treating a vanadium compound, a phosphorus source, and a conductive carbon material source that generates carbon upon thermal decomposition in an aqueous solvent, preferably at 60 to 100°C, to carry out a reaction, adding a lithium source to the heat-treated liquid, carrying out a reaction, spray-drying the resulting reaction liquid to obtain a reaction precursor, and calcining the reaction precursor in an inert gas atmosphere or a reducing atmosphere.

[0006] Furthermore, Patent Document 3 listed below proposes a method in which citric acid is used as a vanadium chelating agent and as a conductive carbon source for coating lithium vanadium phosphate, vanadium pentoxide, phosphoric acid, and citric acid are mixed in an aqueous solvent, and the mixture is heat-treated at 85°C to remove the aqueous solvent to obtain a dried product, which is then calcined at 850°C to obtain VPO4 / C, and the VPO4 is then reacted with a lithium source. Patent Document 4 listed below also proposes a method in which lithium hydroxide, vanadium pentoxide, citric acid, and ammonium dihydrogen phosphate are dissolved in an aqueous solvent in this order, the solution is evaporated to dryness, the dried precursor is pulverized in an automatic mortar, and then calcined in a nitrogen gas atmosphere.

[0007] Furthermore, an electricity storage device has been proposed that uses a vanadium phosphate carbon composite containing nanoparticles of lithium vanadium phosphate as an electrode material and has excellent high discharge characteristics, cycle characteristics, etc. (see Patent Document 5 and Non-Patent Document 1, etc.).

[0008] International Publication No. 2012 / 043367 Pamphlet Japanese Patent Application Laid-Open No. 2017-160107 International Publication No. 2018 / 142082 Pamphlet, 0121 paragraph Japanese Patent Application Laid-Open No. 2011-198657, 0044 paragraph Japanese Patent Application Laid-Open No. 2014-229830

[0009] Scientific Report, 4, 4047 (2014)

[0010] Lithium vanadium phosphate is highly safe even at high temperatures, and is therefore attracting attention as a positive electrode material for lithium secondary batteries for automotive applications, all-solid-state batteries, electrochemical capacitors, and the like. To commercialize this compound, it is desirable to develop a method for producing a lithium vanadium phosphate carbon composite that can further reduce costs by, for example, simplifying the process.

[0011] Specifically, an object of the present invention is to provide a method for industrially advantageously producing a lithium vanadium phosphate carbon composite that can produce lithium vanadium phosphate nanoparticles and contains lithium vanadium phosphate of high purity as measured by X-ray diffraction. Another object of the present invention is to provide a positive electrode material using the lithium vanadium phosphate carbon composite that can impart excellent cycle characteristics to an electricity storage device, and an electricity storage device having excellent cycle characteristics.

[0012] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors have found the following, and have completed the present invention based on the following findings. In an investigation into simplifying the steps of the methods for producing lithium vanadium phosphate described in Patent Documents 1 and 2, they discovered that by using a carboxylic acid such as citric acid instead of a reducing sugar in the preparation step of a reaction precursor, it is possible to carry out the reduction reaction of vanadium pentoxide without actively performing a heat treatment. Furthermore, if a carboxylic acid such as citric acid is added to an aqueous solvent containing vanadium pentoxide and phosphoric acid in an amount greater than that required for the reduction of vanadium pentoxide, in the hope of also acting as a vanadium chelating agent, the reduction reaction solution obtained by adding a lithium source after the reduction reaction of vanadium pentoxide is unstable at room temperature and the amount of precipitate gradually increases over time, making it difficult to handle industrially. In contrast, a reduction reaction slurry is intentionally prepared by first adding a carboxylic acid such as citric acid in an amount necessary for the reduction of vanadium pentoxide, and then a second carboxylic acid is added to this reduction reaction slurry to chelate the mixture, thereby obtaining a reduction reaction preparation solution. A lithium source is added to the reduction reaction preparation solution to form a solution, which remains stable and easy to handle even after 24 hours at room temperature without forming any precipitates. The second carboxylic acid also serves as a conductive carbon source for lithium vanadium phosphate. A reaction precursor with excellent reactivity can be obtained by spray-drying the reaction raw material solution, and a vanadium phosphate-carbon composite containing lithium vanadium phosphate of high purity as measured by X-ray diffraction can be obtained by calcining the reaction precursor. Furthermore, the addition of the second carboxylic acid also enables the production of lithium vanadium phosphate nanoparticles. Furthermore, the vanadium phosphate-carbon composite obtained by this production method has a carbon content within a specific range and contains lithium vanadium phosphate nanoparticles with depressions on the particle surface, and this vanadium phosphate-carbon composite particle as a positive electrode material exhibits excellent cycle characteristics.Furthermore, the vanadium lithium phosphate carbon composite obtained by this manufacturing method has a carbon content within a specific range, and the carbon content is in the form of irregular pulverized vanadium lithium phosphate carbon composite particles containing nanoparticles of vanadium lithium phosphate, which can be used as a positive electrode material in an electricity storage device that exhibits excellent cycle characteristics.

[0013] That is, the present invention (1) provides a method for producing a composite of lithium vanadium phosphate having a NASICON structure and carbon, comprising: a first step of adding vanadium pentoxide, phosphoric acid, and a first carboxylic acid to an aqueous solvent to carry out a reduction reaction of the vanadium pentoxide and prepare a reduction reaction slurry; a second step of adding a second carboxylic acid to the reduction reaction slurry to prepare a reduction reaction preparation liquid; a third step of adding a lithium source to the reduction reaction preparation liquid to prepare a raw material mixed solution in the form of a liquid; a fourth step of spray-drying the raw material mixed solution to obtain a reaction precursor; and a fifth step of firing the reaction precursor at 500 to 1300°C in an inert gas atmosphere or a reducing atmosphere to obtain the vanadium phosphate lithium carbon composite.

[0014] The present invention (2) also provides the method for producing the lithium vanadium phosphate carbon composite according to (1), characterized in that the reduction reaction in the first step is carried out at a temperature lower than 60°C.

[0015] The present invention (3) also provides the method for producing the lithium vanadium phosphate carbon composite according to (1) or (2), wherein the second carboxylic acid is a hydroxycarboxylic acid from which carbon is isolated by heating.

[0016] The present invention (4) provides a method for producing the lithium vanadium phosphate carbon composite according to any one of (1) to (3), wherein the first carboxylic acid is citric acid.

[0017] The present invention (5) also provides a method for producing a lithium vanadium phosphate carbon composite according to (4), characterized in that the second carboxylic acid is one or two selected from gluconic acid and malic acid.

[0018] The present invention (6) provides a method for producing the lithium vanadium phosphate carbon composite according to any one of (1) to (5), wherein the first carboxylic acid is gluconic acid.

[0019] The present invention (7) also provides the method for producing the lithium vanadium phosphate carbon composite according to (6), characterized in that the second carboxylic acid is malic acid.

[0020] The present invention (8) provides the method for producing a lithium vanadium phosphate carbon composite according to any one of (1) to (7), characterized in that the amount of the first carboxylic acid added is such that the molar ratio (C / V) of C atoms in the first carboxylic acid to V atoms in vanadium pentoxide is 2.0 to 6.0.

[0021] The present invention (9) provides the method for producing a lithium vanadium phosphate carbon composite according to any one of (1) to (8), characterized in that the amount of the second carboxylic acid added is such that the molar ratio (C / V) of C atoms in the second carboxylic acid to V atoms in vanadium pentoxide is 0.50 to 4.0.

[0022] The present invention (10) also provides a method for producing a lithium vanadium phosphate carbon composite according to any one of (1) to (9), characterized in that a Me source (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element) is further contained in the reduction reaction slurry in the first step and / or the liquid-like raw material solution in the third step.

[0023] The present invention (11) also provides a method for producing a lithium vanadium phosphate carbon composite according to (10), characterized in that the Me source contains at least one source selected from a Ti source and an Al source.

[0024] The present invention (12) also provides a method for producing a lithium vanadium phosphate carbon composite according to any one of (1) to (11), further comprising a sixth step of pulverizing the lithium vanadium phosphate carbon composite obtained after the fifth step.

[0025] The present invention (13) also provides a positive electrode material comprising lithium vanadium phosphate carbon composite particles, wherein the lithium vanadium phosphate carbon composite particles include lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface, the average particle size of the lithium vanadium phosphate carbon composite particles (A) being 5 μm or more and 40 μm or less, and the carbon content of the positive electrode material being 7.6 to 20 mass% in terms of C atoms.

[0026] The present invention (14) also provides a positive electrode material according to (13), characterized in that, when observed with a scanning electron microscope, the proportion by number of lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface among particles having a size of 5 μm or more and 40 μm or less in the positive electrode material is 10% or more.

[0027] The present invention (15) also provides the positive electrode material of (13) or (14), characterized in that the lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface contain doped Me (Me represents a metal element or transition metal element having an atomic number of 11 or more other than V).

[0028] The present invention (16) also provides a positive electrode material comprising irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing nanoparticles of lithium vanadium phosphate, wherein the average particle size of the lithium vanadium phosphate carbon composite particles (B) is 4 μm or more and 20 μm or less, and the carbon content of the positive electrode material is 7.6 to 20 mass% in terms of C atoms.

[0029] The present invention (17) also provides a positive electrode material according to (16), characterized in that, when observed with a scanning electron microscope, the proportion by number of the irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing lithium vanadium phosphate nanoparticles is 30% or more of particles having a size of 4 μm or more and 20 μm or less in the positive electrode material.

[0030] The present invention (18) also provides the positive electrode material of any one of (16) to (17), characterized in that the irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing nanoparticles of lithium vanadium phosphate contain doped Me (Me represents a metal element or transition metal element having an atomic number of 11 or more other than V).

[0031] In addition, the present invention (19) is a method for producing a granular material having an average particle diameter (D 50 BET specific surface area (m 2 / g) ratio (BET / D 50 ) is 2 or more.

[0032] In addition, the present invention (20) is a method for producing a granular material having an average particle diameter (D 50 BET specific surface area (m 2 / g) ratio (BET / D 50 ) is 2 or more.

[0033] The present invention (21) also provides a positive electrode material according to (15) or (18), characterized in that the linear expansion coefficient in the temperature range of 25 to 70°C is 5 ppm / K or less.

[0034] The present invention (22) also provides an electricity storage device characterized by using the positive electrode material according to any one of the present inventions (13) to (21).

[0035] According to the present invention, a reduction reaction of vanadium pentoxide can be carried out without actively performing a heat treatment, and a reaction precursor is prepared using a stable and easy-to-handle raw material mixed solution, thereby making it possible to industrially advantageously produce a vanadium phosphate lithium carbon composite containing vanadium phosphate lithium of high purity as measured by X-ray diffraction. Furthermore, according to the present invention, a composite in which nanoparticles of vanadium phosphate of high purity as measured by X-ray diffraction are composited with carbon can be produced. Furthermore, according to the present invention, a positive electrode material using a vanadium phosphate lithium carbon composite that can impart excellent cycle characteristics to an electricity storage device, and an electricity storage device having excellent cycle characteristics can be provided.

[0036] X-ray diffraction pattern of the reaction precursor obtained in the fourth step of Example 1. SEM photograph of the reaction precursor obtained in the fourth step of Example 1. X-ray diffraction pattern of the lithium vanadium phosphate carbon composite sample obtained in Example 1. SEM photograph of the lithium vanadium phosphate carbon composite sample obtained in Example 1. (a): Magnification 10,000 times, (b): Magnification 100,000 times. X-ray diffraction pattern of the lithium vanadium phosphate carbon composite sample obtained in Comparative Example 1. SEM photograph of the lithium vanadium phosphate carbon composite sample obtained in Comparative Example 1. SEM photograph of the lithium vanadium phosphate carbon composite sample obtained in Example 1 (magnification: 1,000 times). SEM photographs of the lithium vanadium phosphate carbon composite sample obtained in Example 5 (magnification: 2,000 times (top), 20,000 times (bottom)).

[0037] The present invention will be described below based on preferred embodiments. The method for producing lithium vanadium phosphate of the present invention is a method for producing a composite of lithium vanadium phosphate having a NASICON structure and carbon, comprising the following steps: a first step of adding vanadium pentoxide, phosphoric acid, and a first carboxylic acid to an aqueous solvent to carry out a reduction reaction of the vanadium pentoxide and prepare a reduction reaction slurry; a second step of adding a second carboxylic acid to the reduction reaction slurry to prepare a reduction reaction preparation liquid; a third step of adding a lithium source to the reduction reaction preparation liquid to prepare a raw material mixed solution; a fourth step of spray-drying the raw material mixed solution to obtain a reaction precursor; and a fifth step of calcining the reaction precursor at 500 to 1300°C in an inert gas atmosphere or a reducing atmosphere to obtain the lithium vanadium phosphate carbon composite.

[0038] The method for producing a lithium vanadium phosphate carbon composite of the present invention is a method for producing a lithium vanadium phosphate carbon composite having a NASICON structure (hereinafter simply referred to as "lithium vanadium phosphate carbon composite").

[0039] The vanadium phosphate-carbon composite obtained by the method for producing a vanadium phosphate-carbon composite of the present invention is a vanadium phosphate-carbon composite of high purity as measured by X-ray diffraction, and contains conductive carbon. In the present invention, "vanadium phosphate-carbon composite of high purity as measured by X-ray diffraction" means that the vanadium phosphate-carbon composite is detected as a single-phase lithium vanadium phosphate when subjected to X-ray diffraction analysis. The vanadium phosphate-carbon composite obtained by the method for producing a vanadium phosphate-carbon composite of the present invention is detected as a single-phase lithium vanadium phosphate when subjected to X-ray diffraction analysis, and contains 1.0 to 20.0 mass %, preferably 2.0 to 15.0 mass %, of carbon atoms that is not detected by X-ray diffraction analysis.

[0040] The lithium vanadium phosphate in the lithium vanadium phosphate carbon composite obtained by the method for producing a lithium vanadium phosphate carbon composite of the present invention is represented by the following general formula (1): Li x V y (PO4)3 (1) (wherein x is 2.5 or more and 3.5 or less, and y is 1.8 or more and 2.2 or less), or lithium vanadium phosphate in which lithium vanadium phosphate represented by general formula (1) is doped with Me element (Me is a metal element other than V having an atomic number of 11 or more or a transition metal element) as necessary.

[0041] In general formula (1), x is 2.5 or more and 3.5 or less, preferably 2.8 or more and 3.2 or less. y is 1.8 or more and 2.2 or less, preferably 1.9 or more and 2.1 or less. When lithium vanadium phosphate contains an Me element, the doped Me element may be one or more selected from Sr, Ba, Sc, Y, Hf, Ta, W, Ru, Os, Ag, Zn, Si, Ga, Ge, Sn, Bi, Se, Te, Na, K, Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo, and Cu. Among these, the Me element preferably contains Ti and / or Al. As the material containing Ti and / or Al, those selected from Ti, Al, Ti and Al, Ti and Y, Ti and Mg, Al and Y, and Al and Mg are particularly preferred.

[0042] The doping amount of the Me element is preferably 0.5 to 40 mol %, and more preferably 0.7 to 30 mol %, in terms of mole % of Me relative to V in the lithium vanadium phosphate (Me / V) in atomic terms, from the viewpoint of providing an electricity storage device using the lithium vanadium phosphate carbon composite as a positive electrode material with a high capacity retention rate.

[0043] The method for producing a lithium vanadium phosphate carbon composite of the present invention includes a first step, a second step, a third step, a fourth step, and a fifth step.

[0044] The first step is a step of adding vanadium pentoxide, phosphoric acid, and a first carboxylic acid to an aqueous solvent to carry out a reduction reaction of vanadium pentoxide, thereby preparing a reduction reaction slurry.

[0045] In conventional methods using reducing sugars as reducing agents, the reduction reaction of vanadium pentoxide usually needs to be carried out at 60 to 100°C. However, by using a carboxylic acid as the reducing agent, the reduction reaction of vanadium pentoxide can be carried out at temperatures below 60°C.

[0046] In the conventional method using a carboxylic acid such as citric acid, the chelating effect of the carboxylic acid such as citric acid on vanadium is utilized, and the carboxylic acid is used as a conductive carbon source that coats lithium vanadium phosphate. Therefore, compared to the present invention, the carboxylic acid such as citric acid is added in large excess to the amount necessary for reduction relative to the vanadium pentoxide raw material, and in many cases, a heat treatment is performed to temporarily prepare an aqueous solution in which the raw materials are dissolved. This point makes the conventional method using a carboxylic acid such as citric acid different from the first step of the present invention.

[0047] The present inventors have found that in the reduction reaction slurry obtained in the first step, vanadium pentoxide and phosphoric acid react in the presence of a first carboxylic acid to produce VOHPO4 or a hydrate thereof, and the reduction reaction slurry obtained in the first step contains VOHPO4 or a hydrate thereof as a solid component, and the liquid component of the reduction reaction slurry obtained in the first step contains dihydrogen phosphate ions (H2PO4 - It is believed that the first carboxylic acid is ions derived from phosphoric acid, such as phosphate ions, dissolved in the aqueous solvent. Furthermore, the reduction reaction slurry of the first step may contain unreacted first carboxylic acid remaining in the slurry as long as it remains in a slurry state. Such unreacted first carboxylic acid remaining in the reduction reaction slurry is ultimately converted to carbon and contained in the resulting lithium vanadium phosphate.

[0048] As the first carboxylic acid in the first step, a hydroxycarboxylic acid composed of the elements C, O, and H is preferred from the viewpoints of environmental considerations and the ability to produce a high-purity lithium vanadium phosphate carbon composite by leaving only carbon as a residual component. Examples of hydroxycarboxylic acids used as the first carboxylic acid include citric acid, malic acid, and gluconic acid. Among these, citric acid and gluconic acid are preferred because they can easily reduce vanadium pentoxide and have a high chelating effect on vanadium. Furthermore, as the hydroxycarboxylic acid used as the first carboxylic acid, citric acid is particularly preferred because it is industrially available at a lower cost than other carboxylic acids and is therefore industrially advantageous. The first carboxylic acid added may be a hydrate or an anhydride.

[0049] In the first step, the amount of the first carboxylic acid added is an amount such that the molar ratio (C / V) of C atoms in the first carboxylic acid to V atoms in vanadium pentoxide is, in atomic terms, preferably 2.5 to 6.0, particularly preferably 2.8 to 5.8, and even more preferably 3.0 to 5.5. If the amount of the first carboxylic acid added is less than the above range, the reduction reaction of vanadium pentoxide and the chelating effect on vanadium become insufficient, whereas if the amount of the first carboxylic acid added exceeds the above range, the carbon content of the finally obtained lithium vanadium phosphate tends to be too high, which is undesirable.

[0050] In the first step, the amounts of vanadium pentoxide and phosphoric acid added are preferably such that the molar ratio (V / P) of V atoms in vanadium pentoxide to P atoms in phosphoric acid is 0.50 to 0.80, preferably 0.55 to 0.75, in terms of atom conversion, since this makes it easier to obtain a single-phase lithium vanadium phosphate as a final product in X-ray diffraction analysis.

[0051] The vanadium pentoxide, phosphoric acid, and first carboxylic acid in the first step may have any production history, but in order to produce high-purity lithium vanadium phosphate, it is preferable that the impurity content be as low as possible.

[0052] The aqueous solvent used in the first step may be water, or a mixed solvent of water and a hydrophilic organic solvent.

[0053] The amount of the aqueous solvent used in the first step is preferably adjusted to an amount such that the solid content in the reduction reaction slurry becomes 10 to 50 mass %, and more preferably adjusted to an amount such that the solid content in the reduction reaction slurry becomes 20 to 40 mass %.

[0054] Furthermore, in order to highly disperse the Me source described below when it is added in powder form, a dispersant may be added to the aqueous solvent containing vanadium pentoxide, phosphoric acid, and the first carboxylic acid, if necessary.

[0055] As the dispersant, at least one anionic surfactant selected from carboxylates, sulfates, sulfonates, and phosphates is preferred, as it reduces the viscosity of the reduction reaction slurry and also provides a reaction precursor with excellent reactivity. As the anionic surfactant, polycarboxylic acid surfactants or polyacrylic acid surfactants are preferred, and polycarboxylic acid surfactants are particularly preferred. As the polycarboxylic acid surfactant, ammonium salts of polycarboxylic acids are preferred.

[0056] The anionic surfactant may be a commercially available one, and examples of commercially available polycarboxylic acid surfactants include SN Dispersant 5020, SN Dispersant 5023, SN Dispersant 5027, SN Dispersant 5468, and Nopcosperse 5600, manufactured by San Nopco Ltd., and Poise 532A, manufactured by Kao Corporation.

[0057] The concentration of the dispersant in the aqueous solvent is preferably 0.1 to 5.0% by mass, more preferably 0.3 to 3.0% by mass, in order to obtain a sufficient dispersion effect.

[0058] In the first step, the order in which vanadium pentoxide, phosphoric acid, the first carboxylic acid, and the dispersant added as needed are added to the aqueous solvent, and the means for mixing are not particularly limited.

[0059] In the first step, for example, vanadium pentoxide, phosphoric acid, and a first carboxylic acid are added to an aqueous solvent, and then these are mixed by stirring to carry out a reduction reaction of vanadium pentoxide.

[0060] The temperature at which the reduction reaction of vanadium pentoxide in the first step is carried out is not particularly limited, and the reduction reaction of vanadium pentoxide in the first step can be carried out at 80°C or lower. Since this reduction reaction proceeds sufficiently at temperatures below 60°C, which makes industrial production of a lithium vanadium phosphate carbon composite more advantageous, the temperature at which the reduction reaction of vanadium pentoxide in the first step is carried out is preferably below 60°C, more preferably 20 to 50°C, and even more preferably 25 to 45°C. The reduction reaction of vanadium pentoxide is an exothermic reaction, which causes a slight rise in the temperature of the reaction system, but the reduction reaction and the next step can be carried out as is.

[0061] The completion of the reduction reaction in the first step can be confirmed by visually confirming that the reduction reaction slurry turns greenish-blue.

[0062] The reaction time for the reduction reaction in the first step is not particularly limited, and is generally 0.5 hours or more, preferably 0.5 to 3.0 hours. If the reduction reaction is carried out for a reaction time within this range, a satisfactory reduction reaction slurry can be obtained.

[0063] The second step is a step of adding a second carboxylic acid to the reduction reaction slurry obtained in the first step to prepare a reduction reaction preparation solution. That is, the second step is a step necessary for obtaining a raw material mixture solution that is dissolved in the third step described below by adding the second carboxylic acid to the reduction reaction slurry. The reduction reaction preparation solution obtained in the second step may be in the form of a solution or a slurry, as long as the amount of the second carboxylic acid added is within the range described below. Note that when the reduction reaction preparation solution is in the form of a slurry, the resulting raw material mixture solution can be converted into a solution by adding a lithium source in the next step, the third step.

[0064] In the production method of the present invention, the second carboxylic acid is not only a component necessary for obtaining a liquid-like raw material mixed solution in the third step, but also a component that stabilizes the raw material mixed solution obtained in the third step so that no precipitates are formed at room temperature (25° C.) for at least 24 hours. Furthermore, the second carboxylic acid serves as a conductive carbon source for the obtained lithium vanadium phosphate, and also as a component that prevents oxidation of vanadium during the calcination in the fifth step.

[0065] The second carboxylic acid in the second step is a hydroxycarboxylic acid composed of the elements C, O, and H, from which carbon is isolated by calcination in the fifth step, and is preferably a hydroxycarboxylic acid having 4 to 6 carbon atoms, from the viewpoint that it can be used as a conductive carbon source for lithium vanadium phosphate to produce a high-purity lithium vanadium phosphate carbon composite.

[0066] As the hydroxycarboxylic acid used as the second carboxylic acid, gluconic acid and malic acid are preferred, from the viewpoints of having a high chelating effect on the solid components contained in the reduction reaction slurry obtained in the second step and being able to efficiently isolate carbon by the calcination in the fifth step and successfully leave it as conductive carbon.

[0067] In the method for producing a lithium vanadium phosphate carbon composite of the present invention, the following combinations of a first carboxylic acid and a second carboxylic acid are preferred in terms of industrially advantageously producing a lithium vanadium phosphate carbon composite containing conductive carbon and lithium vanadium phosphate of high purity as measured by X-ray diffraction: (1) the first carboxylic acid is citric acid and the second carboxylic acid is gluconic acid; (2) the first carboxylic acid is citric acid and the second carboxylic acid is malic acid; or (3) the first carboxylic acid is gluconic acid and the second carboxylic acid is malic acid.

[0068] In the second step, the amount of the second carboxylic acid added is such that the molar ratio (C / V) of C atoms in the second carboxylic acid to V atoms in the vanadium pentoxide in the first step is preferably 0.50 to 4.0, more preferably 0.60 to 3.5, in terms of atoms. When the amount of the second carboxylic acid added is within the above range, the resulting lithium vanadium phosphate carbon composite is likely to contain 1.0 to 20.0 mass %, preferably 2.0 to 15.0 mass %, of carbon, which is preferable.

[0069] In the second step, the second carboxylic acid is preferably added in such an amount that the molar ratio (C / V) of the sum of C atoms in the first carboxylic acid and C atoms in the second carboxylic acid to V atoms in the vanadium pentoxide in the first step is 0.30 to 4.3, more preferably 0.50 to 4.1, in atomic terms. When the molar ratio (C / V) of the sum of C atoms in the first carboxylic acid and C atoms in the second carboxylic acid to V atoms in the vanadium pentoxide in the first step is within the above range, the resulting lithium vanadium phosphate carbon composite is likely to contain 1.0 to 20.0 mass %, preferably 2.0 to 15.0 mass %, of carbon, which is preferable.

[0070] In the second step, the temperature at which the second carboxylic acid is added and the reduction reaction preparation solution is prepared is not particularly limited, and is 80°C or lower, preferably less than 60°C, more preferably 20 to 50°C, and even more preferably 25 to 45°C.

[0071] In the second step, for example, a second carboxylic acid is added to the reduction reaction slurry obtained by carrying out the first step, and a mixing treatment such as stirring is carried out at 80°C or lower, preferably less than 60°C, more preferably 20 to 50°C, and even more preferably 25 to 45°C, for 0.5 minutes or more, preferably 60 minutes to 2 hours, to obtain a reduction reaction preparation liquid having satisfactory performance.

[0072] The third step is a step of adding a lithium source to the reduction reaction preparation solution obtained in the second step to prepare a raw material mixture solution in the form of a solution.

[0073] Examples of the lithium source for the third step include lithium hydroxide, lithium carbonate, etc. The lithium source is preferably added to the reduction reaction preparation solution obtained in the second step in the form of a solution in which it is dissolved in water or a suspension in which water is used as a dispersion solvent.

[0074] In the third step, the amount of the lithium source added is an amount such that the molar ratio (Li / P) of Li atoms in the lithium source to P atoms in the phosphoric acid in the first step is 0.70 to 1.3, preferably 0.80 to 1.2, in terms of atoms. Having the amount of the lithium source added within the above range is preferred from the viewpoint that a vanadium lithium phosphate carbon composite containing vanadium lithium phosphate that is highly pure as measured by X-ray diffraction can be easily obtained as the final product.

[0075] In the third step, the temperature at which the lithium source is added to the reduction reaction preparation solution is not particularly limited, but is 80°C or lower, preferably less than 60°C, more preferably 15 to 50°C, and even more preferably 20 to 45°C.

[0076] In the third step, for example, a lithium source is added to the raw material mixed solution obtained by carrying out the second step, and a mixing treatment such as stirring is carried out at 80°C or lower, preferably less than 60°C, more preferably 15 to 50°C, and even more preferably 20 to 45°C, for 30 minutes or more, preferably 60 minutes to 2 hours, to obtain a raw material mixed solution.

[0077] The raw material mixed solution obtained by carrying out the third step is a stabilized solution in which tetravalent vanadium, phosphorus, and lithium are present in a dissolved state, and no precipitate is visually observed for at least 24 hours at room temperature (25°C) under stirring. The raw material mixed solution obtained by carrying out the third step has, for example, a turbidity of 20 NTU or less, preferably 15 NTU or less, both at room temperature (25°C) immediately after preparation and after standing at room temperature (25°C) for 24 hours. In the present invention, the turbidity of the solution is a value measured by diluting the raw material mixed solution 100 times and using a turbidimeter (TB250WL manufactured by Tintometer).

[0078] The fourth step is a step of spray-drying the raw material mixture solution obtained by carrying out the third step to obtain a reaction precursor.

[0079] Although methods other than spray drying are known for drying the liquid, the present invention employs this drying method based on the finding that selecting spray drying is advantageous. Specifically, when spray drying is used, a granular material in which each component is uniformly dispersed at the molecular level and densely packed is obtained, and therefore, this granular material is used as a reaction precursor in the method for producing a lithium vanadium phosphate carbon composite of the present invention, and by firing this reaction precursor in the fifth step described below, a lithium vanadium phosphate carbon composite containing lithium vanadium phosphate of high purity as measured by X-ray diffraction can be obtained.

[0080] In the spray drying method, a liquid is atomized by a predetermined means, and the resulting fine droplets are dried to obtain granules. For example, the liquid can be atomized using a rotating disk or a pressure nozzle. Either method can be used in the fourth step.

[0081] In spray drying, the relationship between the size of the atomized slurry droplets and the size of the pulverized material particles contained therein affects stable drying and the properties of the resulting dried powder. Specifically, if the size of the pulverized material raw material particles is too small compared to the droplet size, the droplets become unstable, making it difficult to successfully dry. From this perspective, the size of the atomized droplets is preferably 5 to 100 μm, and particularly preferably 10 to 50 μm. It is desirable to determine the amount of slurry supplied to the spray drying apparatus taking this into consideration.

[0082] The drying temperature in the spray dryer is preferably adjusted so that the hot air inlet temperature is 180 to 250°C, preferably 200 to 240°C, and the powder temperature is 90 to 150°C, preferably 100 to 130°C, since this prevents moisture absorption by the powder and makes it easier to recover the powder.

[0083] The reaction precursor obtained by carrying out the fourth step is amorphous because each element is uniformly dispersed at the molecular level. The amorphous nature of the reaction precursor can be confirmed by X-ray diffraction analysis.

[0084] The fifth step is a step of calcining the reaction precursor obtained by carrying out the fourth step at 500 to 1300°C to obtain a lithium vanadium phosphate carbon composite containing lithium vanadium phosphate of high purity as determined by X-ray diffraction.

[0085] The firing temperature in the fifth step is 500 to 1300° C., preferably 600 to 1100° C. If the firing temperature in the fifth step is below the above range, the firing time required to produce lithium vanadium phosphate becomes long, and if the firing temperature exceeds the above range, the lithium vanadium phosphate melts.

[0086] The firing atmosphere in the fifth step is an inert gas atmosphere or a reducing atmosphere to prevent oxidation of vanadium and melting. The inert gas used in the fifth step is not particularly limited, and examples thereof include nitrogen gas, helium gas, and argon gas.

[0087] In the fifth step, the calcination time is not particularly limited, and calcination for generally 2 hours or more, particularly 3 to 24 hours, can provide a lithium vanadium phosphate carbon composite containing lithium vanadium phosphate of high purity as determined by X-ray diffraction.

[0088] In the fifth step, the lithium vanadium phosphate carbon composite obtained by firing may be subjected to firing multiple times, if necessary.

[0089] In the method for producing a lithium vanadium phosphate carbon composite of the present invention, for the purposes of stabilizing the crystal structure of lithium vanadium phosphate and further improving battery performance, an Me source (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element) is added to the reduction reaction slurry in the first step and / or the liquid-like raw material solution in the third step, and then the steps related to the method for producing lithium vanadium phosphate of the present invention are carried out, thereby obtaining a lithium vanadium phosphate carbon composite in which lithium vanadium phosphate is doped with Me.

[0090] The Me element is present as a substitute at the Li site and / or V site of the lithium vanadium phosphate represented by the general formula (1).

[0091] The Me in the Me source is a metal element or transition metal element having an atomic number of 11 or greater other than V. Preferred Me elements include Sr, Ba, Sc, Y, Hf, Ta, W, Ru, Os, Ag, Zn, Si, Ga, Ge, Sn, Bi, Se, Te, Na, K, Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo, and Cu, and these may be used alone or in combination of two or more. In the present invention, the Me in the Me source preferably contains Ti and / or Al. Furthermore, as the Me containing Ti and / or Al, those selected from Ti, Al, Ti and Al, Ti and Y, Ti and Mg, Al and Y, and Al and Mg are particularly preferred.

[0092] Examples of Me sources include oxides, hydroxides, halides, carbonates, nitrates, phosphates, biphosphates, and organic acid salts containing Me. The Me source can be added to the reduction reaction slurry in the first step by adding the Me source during the first step or before adding the second carboxylic acid in the second step. The Me source can be present in the reduction reaction slurry as a dissolved substance or as a solid. When the Me source is present in the slurry as a solid, it is preferable to use a Me source having an average particle size of 100 μm or less, preferably 0.1 to 50 μm, in order to obtain a reaction precursor with excellent reactivity. Me sources containing a Ti source and / or an Al source are preferred in terms of further improving battery performance. The Me source can be added to the liquid raw material solution in the third step by adding the Me source during the third step or before performing the spray drying treatment in the fourth step. In the third step, the Me source can be added to a solution in which a lithium source is dissolved in water or a suspension of a lithium source in which water is used as a dispersion solvent, and then added to the reduction reaction preparation solution. In this production method, the method of adding a Me source to the reduction reaction slurry in the first step and the method of adding a Me source to the liquid raw material solution in the third step can be used in combination. In this case, the Me element in the Me source added to the reduction reaction slurry in the first step and the Me element in the Me source added to the liquid raw material solution in the third step may be the same type or different types.

[0093] Furthermore, when a Me source is mixed, the amount of the Me source to be mixed varies depending on the type of Me element to be doped, but in many cases, the amount is such that the molar ratio of the total of V atoms and Me atoms to P atoms in the reduction reaction slurry ((Me+V) / P) is 0.50 to 0.80, preferably 0.60 to 0.73, in atomic terms, and the molar ratio of Me atoms to V atoms (Me / V) is greater than 0 and less than 0.45, preferably greater than 0 and less than 0.1.

[0094] When a phosphate or biphosphate is used as the Me source, the phosphorus atoms in the phosphate or biphosphate also serve as a phosphorus source in the production method of the present invention, similar to phosphoric acid. Therefore, when a phosphate or biphosphate is used as the Me source, the amount of the phosphate or biphosphate mixed is preferably adjusted so that the total molar ratio of P atoms derived from the phosphate or biphosphate to P atoms derived from the phosphoric acid in the first step falls within the range of the molar ratio of V atoms and Me atoms to P atoms in the reaction precursor ((Me+V) / P).

[0095] In the method for producing the lithium vanadium phosphate carbon composite of the present invention, the lithium vanadium phosphate obtained may be subjected to a crushing treatment or a pulverization treatment, and may further be classified, as necessary.

[0096] The vanadium phosphate-carbon composite obtained by the method for producing a lithium vanadium phosphate-carbon composite of the present invention is a lithium vanadium phosphate-carbon composite containing lithium vanadium phosphate of high purity as measured by X-ray diffraction. The carbon contained in the lithium vanadium phosphate-carbon composite is conductive carbon. Therefore, the amount of carbon contained in the lithium vanadium phosphate-carbon composite obtained by the method for producing a lithium vanadium phosphate of the present invention is preferably 1.0 to 20.0 mass %, and more preferably 2.0 to 15.0 mass %, calculated as C atoms, from the viewpoint of being usable as a positive electrode material for an electricity storage device.

[0097] Furthermore, lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention, in which the carbon content has been further reduced, can be used as a positive electrode material for all-solid-state batteries. Examples of methods for reducing the carbon content in lithium vanadium phosphate include a method in which the lithium vanadium phosphate carbon composite obtained in the fifth step is heat-treated at 250 to 450°C, preferably 300 to 400°C, in an oxygen-containing atmosphere having an oxygen concentration of 5% by volume or more, preferably 10 to 30% by volume, to reduce the carbon content.

[0098] Furthermore, the lithium vanadium phosphate carbon composite obtained by the method for producing the lithium vanadium phosphate carbon composite of the present invention has a BET specific surface area of ​​15 m 2 / g or more, preferably 20 to 70m 2 / g, more preferably 40 to 70 m 2 The vanadium phosphate lithium carbon composite obtained by the method for producing a vanadium phosphate lithium carbon composite of the present invention has an average particle size measured by a laser diffraction scattering method of 1 to 30 μm, preferably 2 to 25 μm. The average particle size is determined by measuring the cumulative 50% (D 50 The vanadium phosphate lithium carbon composite obtained by the method for producing the vanadium phosphate lithium carbon composite of the present invention has a particle size of 90% cumulative (D ) determined by volume frequency particle size distribution measurement using a laser diffraction scattering method. 90 ) particle size and D 50 The particle size ratio (D 90 / D 50 ) is 3.0 or less, preferably 1.2 to 2.8.

[0099] The vanadium phosphate-carbon composite obtained by the method for producing a lithium vanadium phosphate-carbon composite of the present invention preferably has a particle appearance observed by scanning electron microscope (SEM) such that "lithium vanadium phosphate nanoparticles are dispersed in carbon." While the internal structure of the vanadium phosphate-carbon composite obtained by the method for producing a lithium vanadium phosphate-carbon composite of the present invention is unclear, the inventors believe that lithium vanadium phosphate nanoparticles are dispersed in carbon even within the particles, or that primary particles of lithium vanadium phosphate aggregate to form secondary particles, i.e., primary particles of lithium vanadium phosphate aggregate to form secondary particles, the surfaces of which are coated with carbon, and the primary particles of lithium vanadium phosphate are present as nanoparticles. In the method for producing a lithium vanadium phosphate-carbon composite of the present invention, a second step is performed in which a second carboxylic acid is added to the reduction reaction slurry obtained by the first step, thereby making the raw material mixture solution obtained by the third step a stable solution to the extent that no precipitate is visually observable at room temperature (25°C) for at least 24 hours. Therefore, the raw material mixed solution can be subjected to the fourth step in a state where there are no precipitates observable with the naked eye, i.e., the lithium vanadium phosphate raw material is dissolved in the solvent, and therefore the reaction precursor obtained by carrying out the fourth step contains the lithium vanadium phosphate raw material in a fine state. Then, by calcining the reaction precursor in which the lithium vanadium phosphate raw material is in a fine state in the fifth step, the lithium vanadium phosphate raw material in a fine state becomes lithium vanadium phosphate nanoparticles. Therefore, in the method for producing a lithium vanadium phosphate carbon composite of the present invention, a lithium vanadium phosphate carbon composite can be obtained whose particle appearance, observed with a scanning electron microscope (SEM), is such that lithium vanadium phosphate nanoparticles are dispersed in the carbon.The average primary particle diameter of the lithium vanadium phosphate nanoparticles in the lithium vanadium phosphate carbon composite obtained by the method for producing a lithium vanadium phosphate carbon composite of the present invention is preferably 5 to 200 nm, more preferably 7 to 150 nm, as determined by SEM.

[0100] The lithium vanadium phosphate nanoparticles dispersed in the carbon facilitate lithium desorption and insertion during charge and discharge when used as a positive electrode material, and therefore the lithium vanadium phosphate carbon composite obtained by the method for producing a lithium vanadium phosphate carbon composite of the present invention, in which lithium vanadium phosphate nanoparticles are present, has excellent battery performance.

[0101] On the other hand, if a precipitate of the lithium vanadium phosphate raw material is present in the raw material mixture liquid to be subjected to the spray drying treatment, the lithium vanadium phosphate carbon composite obtained by the spray drying treatment and firing will be a composite of large plate-like crystals of lithium vanadium phosphate and carbon.

[0102] The vanadium phosphate lithium carbon composite obtained by the method for producing a vanadium phosphate lithium carbon composite of the present invention is used as a positive electrode material for lithium secondary batteries, electrochemical capacitors, hybrid capacitors that utilize the advantages of both lithium secondary batteries and electric double layer capacitors, and the like.

[0103] In the method for producing a lithium vanadium phosphate carbon composite of the present invention, a first carboxylic acid such as citric acid is first added to vanadium pentoxide and phosphoric acid in an amount necessary for the reduction of vanadium pentoxide to intentionally prepare a reduction reaction slurry, and then a second carboxylic acid is added to this reduction reaction slurry to chelate the mixture, thereby obtaining a reduction reaction preparation solution. This process allows the addition of a lithium source to the reduction reaction preparation solution, resulting in a raw material mixture solution that is stable and easy to handle, free of precipitates, even after 24 hours at room temperature. Therefore, the method for producing a lithium vanadium phosphate carbon composite of the present invention is a method for producing nanoparticles using simple steps, and is therefore industrially advantageous.

[0104] On the other hand, when a carboxylic acid such as citric acid is added to vanadium pentoxide and phosphoric acid in one step to carry out a reduction reaction of vanadium pentoxide, the reduction reaction solution obtained by adding a lithium source is unstable at room temperature and the amount of precipitates gradually increases over time, making it difficult to handle industrially. Furthermore, when a carboxylic acid is added to vanadium pentoxide and phosphoric acid in one step, one method for suppressing the formation of precipitates from the resulting reduction reaction solution is to increase the amount of carboxylic acid added, but this would result in the content of lithium vanadium phosphate in the resulting lithium vanadium phosphate carbon composite being too low, below 80 mass%, and would result in a low battery capacity.

[0105] The lithium vanadium phosphate-carbon composite obtained by the method for producing a lithium vanadium phosphate-carbon composite of the present invention is suitable for use as a positive electrode material. That is, the method for producing a lithium vanadium phosphate-carbon composite of the present invention can produce a positive electrode material containing the lithium vanadium phosphate-carbon composite (A) or the lithium vanadium phosphate-carbon composite (B) described below.

[0106] A positive electrode material containing a lithium vanadium phosphate carbon composite obtained by the method for producing a lithium vanadium phosphate carbon composite of the present invention is particularly useful as a positive electrode material for an electricity storage device. The positive electrode material contains lithium vanadium phosphate carbon composite particles, and the lithium vanadium phosphate carbon composite particles include lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface, the lithium vanadium phosphate carbon composite particles (A) having an average particle diameter of 5 μm or more and 40 μm or less, and the carbon content of the positive electrode material is 7.6 to 20 mass %, preferably 8 to 15 mass %, calculated as C atoms (hereinafter also referred to as a positive electrode material of the first embodiment of the present invention). Hereinafter, the lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface may be simply referred to as "lithium vanadium phosphate carbon composite particles (A)."

[0107] Examples of the electricity storage device according to the present invention include a lithium secondary battery, an electrochemical capacitor, and a hybrid capacitor that utilizes the advantages of both a lithium secondary battery and an electric double layer capacitor.

[0108] In the present invention, the term "pits" refers to pits observed in SEM images obtained by observation with a scanning electron microscope (SEM observation) at 400 to 2000 magnifications, and having a diameter (d) of 500 to 7000 nm (for example, the circled areas in Figure 7). In SEM images obtained by SEM observation of lithium vanadium phosphate carbon composite particles, if pits (dents) are present on the particle surface, the image will appear darker in color than areas without pits. Therefore, in the present invention, areas on the particle surface in the SEM image that are darker in color than other surface areas are recognized as pits, and their diameters are defined as the pit diameters. Regarding the pit diameter (d), the maximum diameter of the dark-colored areas on the particle surface is defined as the pit diameter (d). In the present invention, the shape of the pits may be circular, elliptical, rectangular, or irregular.

[0109] The positive electrode material of the first embodiment of the present invention contains a plurality of lithium vanadium phosphate nanoparticles. The positive electrode material of the first embodiment of the present invention includes lithium vanadium phosphate nanoparticles having two or more depressions with a diameter (d) of 500 to 7,000 nm on the particle surface. Among such lithium vanadium phosphate-carbon composites, lithium vanadium phosphate-carbon composite particles containing nanoparticles having two or more depressions with a diameter (d) of 500 to 7,000 nm on the particle surface are referred to as lithium vanadium phosphate-carbon composite particles (A). That is, the positive electrode material of the first embodiment of the present invention contains lithium vanadium phosphate-carbon composite particles, and the lithium vanadium phosphate-carbon composite particles (A) are contained as part of the lithium vanadium phosphate-carbon composite particles. The number of depressions with a diameter (d) of 500 to 7,000 nm formed on the particle surface of the lithium vanadium phosphate nanoparticles contained in the lithium vanadium phosphate-carbon composite particles (A) is two or more, preferably three or more, and particularly preferably four to six.

[0110] In the positive electrode material of the first embodiment of the present invention, the average particle size of the lithium vanadium phosphate carbon composite particles (A) is 5 to 40 μm, preferably 7 to 35 μm, and more preferably 9 to 30 μm. Note that the average particle size of the lithium vanadium phosphate carbon composite particles (A) in the present invention is a value determined for 200 lithium vanadium phosphate carbon composite particles (A) having a plurality of depressions on the particle surface, randomly selected from an SEM image obtained by scanning electron microscope observation.

[0111] In the positive electrode material of the first embodiment of the present invention, when observed with a scanning electron microscope, the number ratio ((t2 / t1) x 100) of lithium vanadium phosphate carbon composite particles (A) (t2) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface among particles (t1) having a size of 5 μm to 40 μm in the positive electrode material is preferably 10% or more, more preferably 12% or more, and even more preferably 15 to 50%. Note that the number ratio ((t2 / t1) x 100) refers to the number ratio of lithium vanadium phosphate carbon composite particles (A), i.e., particles (t2) having two or more depressions with a diameter (d) of 500 to 7000 nm on the particle surface, among 200 particles (t1) arbitrarily selected from 200 particles having a size of 5 μm to 40 μm in SEM images obtained with a scanning electron microscope. In the positive electrode material of the first embodiment of the present invention, when the number ratio ((t2 / t1)×100) is in the above range, the cycle characteristics of an electricity storage device using the positive electrode material of the present invention can be further improved.

[0112] The carbon content of the positive electrode material of the first embodiment of the present invention is 7.6 to 20 mass %, preferably 8 to 15 mass %, calculated as C atoms. In the method for producing the lithium vanadium phosphate carbon composite of the present invention described above, the second carboxylic acid is converted to elemental carbon by the firing in the fifth step and remains as conductive carbon in the lithium vanadium phosphate carbon composite. Therefore, the positive electrode material of the first embodiment of the present invention can be produced by adjusting the amount of the second carboxylic acid so that the carbon remaining in the lithium vanadium phosphate carbon composite that produces the composite is contained in an amount of 7.6 to 20 mass %, calculated as C atoms.

[0113] The method for producing a lithium vanadium phosphate carbon composite of the present invention produces lithium vanadium phosphate carbon composite particles (A) and other lithium vanadium phosphate carbon composite particles (A'), i.e., particles having no depressions with a diameter (d) of 500 to 7,000 nm, particles having only one depression with a diameter (d) of 500 to 7,000 nm, or particles having depressions with a diameter (d) of less than 500 nm or more than 7,000 nm. In other words, the positive electrode material of the first embodiment of the present invention contains lithium vanadium phosphate carbon composite particles (A) and other lithium vanadium phosphate carbon composite particles (A'). Examples of the lithium vanadium phosphate carbon composite particles (A') include reactive precursor particles with depressions on their surface that have undergone a change in particle shape or are broken fragments, reactive precursor particles without depressions on their surface that have undergone a change in particle shape or are broken fragments, or particles that retain the particle shape of the reactive precursor particles.

[0114] In the present invention, the mechanism for producing the vanadium phosphate lithium carbon composite particles (A) is unclear, but many of the amorphous reactive precursor particles obtained in the fourth step have depressions on their particle surfaces. When these reactive precursor particles with depressions on their surface are fired under an inert atmosphere in the fifth step, those with depressions on their surface become vanadium phosphate lithium carbon composite particles (A') in a partially deformed or destroyed state upon firing, while those that maintain their original particle shape and have depressions on their surface become vanadium phosphate lithium carbon composite particles (A). The vanadium phosphate lithium carbon composite particles (A') in a deformed or destroyed state upon firing have many voids and other voids within the reactive precursor particles. Furthermore, although a large amount of gas is generated within the reactive precursor particles during firing due to the reaction, the reactive precursor particles lack the durability to maintain their particle shape against this large amount of gas generated. This is thought to be why the particle shape is deformed or destroyed upon firing. On the other hand, the vanadium phosphate lithium carbon composite particles (A) that have retained their particle shape by calcination are reactive precursor particles in which each component is uniformly dispersed at the molecular level, are more densely packed, have fewer voids, and are durable granules that retain their particle shape against the large amount of gas generated inside the reactive precursor particles during calcination due to the reaction, and the particle shape of the reactive precursor particles is maintained even when calcined. Therefore, the inventors speculate that the vanadium phosphate lithium carbon composite particles (A) are denser composites of carbon and lithium vanadium phosphate nanoparticles than the vanadium phosphate lithium carbon composite particles (A').

[0115] The positive electrode material of the first embodiment of the present invention, which contains the lithium vanadium phosphate carbon composite particles (A), has excellent cycle characteristics when used as a positive electrode material for an electricity storage device, despite having a higher carbon content than conventional materials. The reason for this is unclear, but it is believed to be because, while conventional positive electrode materials using lithium vanadium phosphate carbon composites contain 3 mass % or less of carbon in terms of C atoms, the positive electrode material of the first embodiment of the present invention, which contains the lithium vanadium phosphate carbon composite particles (A), has a higher carbon content than conventional materials, and the lithium vanadium phosphate nanoparticles are contained in dense carbon, so that the particle surfaces of the lithium vanadium phosphate nanoparticles are covered with dense carbon, facilitating lithium desorption and insertion during charge and discharge.

[0116] Furthermore, the vanadium phosphate lithium carbon composite particles (A) have lithium vanadium phosphate nanoparticles and a hard coating layer made of dense carbon from the particle surface toward the interior. Therefore, even if the vanadium phosphate lithium carbon composite particles (A) are subjected to a pulverization treatment, part of the skeleton of the hard coating layer remains as irregularly crushed particles. The vanadium phosphate lithium carbon composite particles (A) crushed contain irregularly crushed lithium vanadium phosphate carbon composite particles (B) having a particle diameter of 4 μm or more and 20 μm or less. Therefore, the positive electrode material obtained by pulverizing the positive electrode material of the first embodiment of the present invention containing lithium vanadium phosphate carbon composite particles (A) is a positive electrode material containing irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing lithium vanadium phosphate nanoparticles, the average particle diameter of the lithium vanadium phosphate lithium carbon composite particles (B) being 4 μm or more and 20 μm or less, and the carbon content of the positive electrode material being 7.6 to 20 mass % in terms of C atoms (hereinafter also referred to as the positive electrode material of the second embodiment of the present invention). In other words, the positive electrode material of the second embodiment of the present invention contains irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing lithium vanadium phosphate nanoparticles and having a particle diameter of 4 μm or more and 20 μm or less (hereinafter also referred to as "lithium vanadium phosphate carbon composite particles (B)"). In the present invention, "irregularly pulverized" means that the particles are pulverized and do not have a uniform shape.

[0117] The average particle size of the lithium vanadium phosphate carbon composite particles (B) is 4 to 20 μm, preferably 5 to 18 μm, and more preferably 6 to 16 μm. In the present invention, the average particle size of the lithium vanadium phosphate carbon composite particles (B) is a value determined by observing 200 randomly selected lithium vanadium phosphate carbon composite particles (B) with a scanning electron microscope.

[0118] In the positive electrode material of the second embodiment of the present invention, the number ratio ((t4 / t3) x 100) of irregularly pulverized lithium vanadium phosphate carbon composite particles (B) (t4) among particles (t3) having a size of 4 μm or more and 20 μm or less in the positive electrode material when observed with a scanning electron microscope is preferably 30% or more, preferably 40% or more, and more preferably 50 to 80%. Note that the number ratio ((t4 / t3) x 100) refers to the number ratio of irregularly pulverized lithium vanadium phosphate carbon composite particles (B), i.e., irregularly pulverized lithium vanadium phosphate carbon composite particles (t2), among 200 particles (t3) having a size of 4 μm or more and 20 μm or less observed in an SEM image obtained with a scanning electron microscope. In the positive electrode material of the second embodiment of the present invention, when the number ratio ((t4 / t3)×100) is within the above range, the cycle characteristics of an electricity storage device using the positive electrode material of the present invention can be further improved.

[0119] The carbon content of the positive electrode material of the second embodiment of the present invention is 7.6 to 20 mass %, preferably 8 to 15 mass %, calculated as C atoms. In the method for producing the lithium vanadium phosphate carbon composite of the present invention described above, the second carboxylic acid is converted to elemental carbon by the firing in the fifth step, and remains as conductive carbon in the lithium vanadium phosphate carbon composite. Therefore, the positive electrode material of the second embodiment of the present invention can be produced by carrying out steps 1 to 5, adjusting the amount of second carboxylic acid so that the carbon remaining in the lithium vanadium phosphate carbon composite to be produced is 7.6 to 20 mass %, calculated as C atoms, and then carrying out a pulverization treatment in step 6.

[0120] The pulverization treatment in the sixth step can be carried out by dry mechanical means. Examples of the pulverization device include known dry pulverization devices such as a high-speed mixer, a super mixer, a turbosphere mixer, an Eirich mixer, a Henschel mixer, a Nauta mixer, a ribbon blender, a V-type mixer, a conical blender, a jet mill, a cosmomizer, a paint shaker, a bead mill, a jet mill, and a ball mill. After the sixth step is completed, pulverization, classification, etc. can be carried out as necessary.

[0121] The vanadium phosphate lithium carbon composite obtained after the sixth step contains the vanadium phosphate lithium carbon composite particles (B) and other non-irregularly pulverized vanadium phosphate lithium carbon composite particles (B'). Examples of the vanadium phosphate lithium carbon composite particles (B') include aggregated vanadium phosphate lithium carbon composite particles formed by aggregation of non-irregularly pulverized primary particles, and spherical vanadium phosphate lithium carbon composite particles (B').

[0122] In the positive electrode material of the second embodiment of the present invention, those containing lithium vanadium phosphate nanoparticles originating from the lithium vanadium phosphate carbon composite particles (A) and a portion of the skeleton of the hard coating layer made of dense carbon, i.e., the lithium vanadium phosphate carbon composite (B), can be confirmed, for example, by observing the positive electrode material at 2,000 to 100,000 magnifications using a scanning electron microscope. Those containing a portion of the skeleton of the hard coating layer can be confirmed by the presence of dense plate-like portions on the particle surface where grain boundaries and pores are not observed. Note that, because this observation using a scanning electron microscope is a two-dimensional observation, dense plate-like portions cannot be confirmed in all lithium vanadium phosphate carbon composite particles (B).

[0123] In addition, the positive electrode material of the first embodiment of the present invention and the positive electrode material of the second embodiment of the present invention have an average particle diameter (D 50 BET specific surface area (m 2 / g) ratio (BET / D 50) is preferably 2 or more, more preferably 2 or more and 18 or less. In particular, the positive electrode material of the first embodiment of the present invention has an average particle diameter (D 50 BET specific surface area (m 2 / g) ratio (BET / D 50 ) is more preferably 2 or more and 5 or less, and the positive electrode material of the second embodiment of the present invention is characterized in that the positive electrode material has an average particle diameter (D 50 BET specific surface area (m 2 / g) ratio (BET / D 50 ) is more preferably greater than 5 and less than or equal to 18.

[0124] Furthermore, the positive electrode material of the first embodiment of the present invention and the positive electrode material of the first embodiment of the present invention contain a vanadium lithium phosphate carbon composite doped with Me, and the positive electrode material has a linear expansion coefficient of 5 ppm / K or less, preferably -5 to 5 ppm / K, in the temperature range of 25 to 70°C, thereby enabling an electricity storage device using the positive electrode material to have improved cycle characteristics in a high-temperature environment. Note that "a linear expansion coefficient of 5 ppm / K or less" means that the linear expansion coefficient is 0 to 5 ppm / K or that the linear thermal expansion coefficient is a negative value.

[0125] The thermal expansion coefficient between 25 and 70°C for the positive electrode material of the present invention is determined by the following procedure. First, 1.00 g of sample is pulverized and mixed in a mortar for 3 minutes, and then 0.15 g is weighed out and the entire amount is filled into a φ6 mm mold. Next, a powder compact is produced by molding at a pressure of 10 MPa using a hand press. The thermal expansion coefficient of the produced powder compact is then measured using a thermomechanical measuring device (e.g., TMA4000SE manufactured by NETZSCH JAPAN). The measurement conditions are a nitrogen atmosphere, a load of 10 g, and a temperature range of 0°C to 100°C. The measurement is repeated twice, and the thermal expansion coefficient between 25 and 70°C of the second measurement is taken as the thermal expansion coefficient of the positive electrode material.

[0126] The element Me represents one or more metal elements selected from Sr, Ba, Sc, Y, Hf, Ta, W, Ru, Os, Ag, Zn, Si, Ga, Ge, Sn, Bi, Se, Te, Na, K, Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Cr, Nb, Mo, and Cu, and among these, Al is particularly preferred as Me from the viewpoint of being able to improve cycle characteristics, particularly in high-temperature environments.

[0127] The doping amount of the Me element is preferably 0.5 to 40 mol %, and more preferably 0.7 to 30 mol %, in terms of mole % of Me relative to V in the lithium vanadium phosphate (Me / V) in atomic terms, from the viewpoint of providing an electricity storage device using the lithium vanadium phosphate carbon composite as a positive electrode material with a high capacity retention rate.

[0128] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0129] Example 1 <First Step> 2.8 L of ion-exchanged water was placed in a 10 L container, and 500 g of vanadium pentoxide, 635.5 g of citric acid monohydrate, and 947.2 g of 85 wt % phosphoric acid were added thereto at room temperature (25°C), in that order, and a reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, yielding a greenish-blue reduction reaction slurry. Note that the reduction reaction was exothermic, and the temperature of the reaction system rose from 25°C to 45°C.

[0130] <Second Step> Next, 341.5 g of a 50 wt % aqueous gluconic acid solution was added to the reduction reaction slurry obtained in the first step at 45° C., and the mixture was stirred for 60 minutes to obtain a deep blue solution-like reduction reaction preparation liquid.

[0131] <Third Step> A lithium carbonate-containing suspension was prepared by adding 304.7 g of lithium carbonate to 1.5 L of ion-exchanged water. Next, the entire amount of the lithium carbonate-containing suspension was added to the reduction reaction preparation solution over 30 minutes at 40°C, and stirring was continued for 60 minutes to obtain a dark blue raw material mixture solution. Furthermore, when the raw material mixture solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. The raw material mixture solution after 24 hours was diluted 100-fold and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), and was found to be 0.01 NTU.

[0132] <Fourth Step> Next, 24 hours after the completion of the third step, the raw material mixed solution was supplied to a spray dryer with the outlet temperature set to 120°C to obtain a reaction precursor. X-ray diffraction analysis of the reaction precursor confirmed that it was amorphous (see Figure 1). An electron microscope photograph (SEM image) of the reaction precursor is shown in Figure 2.

[0133] <Fifth Step> The obtained reaction precursor was placed in a mullite sagger and fired at 750°C for 4 hours in a nitrogen atmosphere. X-ray diffraction analysis of the obtained lithium vanadium phosphate carbon composite revealed that it was a single-phase lithium vanadium phosphate (see Figure 3). This was used as a lithium vanadium phosphate carbon composite sample. Figure 4 shows an SEM photograph of the obtained lithium vanadium phosphate carbon composite sample. It was also confirmed from Figure 4 that the lithium vanadium phosphate carbon composite (LVP carbon composite) sample obtained was composed of lithium vanadium phosphate (LVP) nanoparticles with an average primary particle size of 30 nm dispersed in carbon.

[0134] The average primary particle diameter was determined by observing the lithium vanadium phosphate particles at 100,000 magnifications under a scanning electron microscope and calculating the average value of 100 randomly selected lithium vanadium phosphate particles. The average particle diameter (D 50 ) is 15.4 μm and D 90 The BET specific surface area was 50.3 m 2 / g. 50 The ratio of the BET specific surface area to the 50The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the carbon atom content, and the residual carbon content was found to be 10.2 mass%.

[0135] Comparative Example 1: 2.8 L of ion-exchanged water was placed in a 10 L beaker, and 500 g of vanadium pentoxide, 818.4 g of citric acid monohydrate, and 947.2 g of 85 wt % phosphoric acid were added thereto in this order at room temperature (25°C). A reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, yielding a greenish-blue reduction reaction slurry. Because the reduction reaction was exothermic, the temperature of the reaction system rose from 25°C to 45°C. Next, 304.7 g of lithium carbonate was added to 1.5 L of ion-exchanged water to prepare a lithium carbonate-containing suspension. The entire lithium carbonate-containing suspension was then added to the reduction reaction slurry over 30 minutes at 40°C, and stirring was continued for 60 minutes to obtain a dark blue raw material mixture solution. When the raw material mixture solution was maintained with stirring at 25°C for 24 hours, it became a slurry and a green precipitate was observed.

[0136] The raw material mixed solution in which this slurry-like green precipitate was observed was diluted 100-fold, and the turbidity was measured using a turbidity meter (TB250WL, manufactured by Tintometer) and found to be 189.6 NTU. X-ray diffraction analysis of the precipitate confirmed the presence of LiVOPO4·2H2O. Scanning electron microscopy revealed that the precipitate consisted of agglomerated plate-like primary particles forming secondary particles. The raw material mixed solution in which this slurry-like green precipitate was observed was spray-dried in the same manner as in Example 1, and the resulting spray-dried product was placed in a mullite sagger and fired at 750°C for 4 hours in a nitrogen atmosphere. X-ray diffraction analysis of the resulting lithium vanadium phosphate carbon composite revealed that it was a single-phase lithium vanadium phosphate (see Figure 5). This was designated the lithium vanadium phosphate carbon composite sample. An SEM photograph of the resulting lithium vanadium phosphate carbon composite sample is shown in Figure 6. Furthermore, the lithium vanadium phosphate carbon composite sample obtained from FIG. 6 was one in which carbon was present on the particle surfaces of plate-like lithium vanadium phosphate particles having an average primary particle size of 0.25 μm.

[0137] The average primary particle diameter was determined by observing the lithium vanadium phosphate particles at 100,000 magnifications under a scanning electron microscope and calculating the average value of 100 randomly selected lithium vanadium phosphate particles. The average particle diameter (D 50 ) is 18.4 μm and D 90 The BET specific surface area was 21.8 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured in the same manner as in Example 1 to determine the content of C atoms, and the residual carbon content was found to be 8.9 mass %.

[0138] Comparative Example 2 A vanadium phosphate-lithium carbon composite was produced in the same manner as in Example 1, except that the second step was not performed. This was used as a vanadium phosphate-lithium carbon composite sample. A lithium carbonate-containing suspension was added to the reduction reaction slurry to obtain a dark blue solution of the raw material mixture. After the raw material mixture was stirred at 25°C for 24 hours, the mixture became slurry-like and a green precipitate was observed. The raw material mixture containing the slurry-like green precipitate was diluted 100 times and its turbidity was measured using a turbidity meter (TB250WL, manufactured by Tintometer Co., Ltd.) to find that it was 192.6 NTU. X-ray diffraction analysis of the precipitate confirmed the presence of LiVOPO4·2H2O. Scanning electron microscope observation revealed that the precipitate consisted of agglomerated plate-like primary particles forming secondary particles. X-ray diffraction analysis of the obtained lithium vanadium phosphate carbon composite revealed that it was a single-phase lithium vanadium phosphate, with carbon present on the particle surfaces of plate-like lithium vanadium phosphate particles having an average primary particle size of 0.25 μm.

[0139] The average primary particle diameter was determined by observing the lithium vanadium phosphate particles at 100,000 magnifications under a scanning electron microscope and averaging the diameters of 100 randomly selected lithium vanadium phosphate particles. The average particle diameter (D 50 ) is 20.1 μm and D90 The BET specific surface area was 17.9 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured in the same manner as in Example 1 to determine the content of C atoms, and the residual carbon content was found to be 3.2 mass %.

[0140]

[0141] In Table 1, the amount of the first carboxylic acid added indicates the molar ratio (C / V) of C atoms in the first carboxylic acid to V atoms in vanadium pentoxide, in atomic terms. The amount of the second carboxylic acid added indicates the molar ratio (C / V) of C atoms in the second carboxylic acid to V atoms in vanadium pentoxide, in atomic terms. The stability of the raw material mixed solution was evaluated as "Good" when no precipitate was observed visually after the raw material mixed solution was kept under stirring at 25°C for 24 hours, and evaluated as "Poor" when precipitate was observed and the solution became a slurry.

[0142]

[0143] Example 2 <First Step> 2.8 L of ion-exchanged water was placed in a 10 L container, and 500 g of vanadium pentoxide, 635.5 g of citric acid monohydrate, 173.6 g of 50 wt % aluminum phosphate monohydrate, and 898.8 g of 85 wt % phosphoric acid were added thereto at room temperature (25°C), in that order, and the reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, yielding a greenish-blue reduction reaction slurry. Note that the reduction reaction was exothermic, and the temperature of the reaction system rose from 25°C to 45°C.

[0144] <Second Step> Next, 341.5 g of a 50 wt % aqueous gluconic acid solution was added to the reduction reaction slurry obtained in the first step at 45° C., and the mixture was stirred for 60 minutes to obtain a deep blue solution-like reduction reaction preparation liquid.

[0145] <Third Step> A lithium carbonate-containing suspension was prepared by adding 320.7 g of lithium carbonate to 1.5 L of ion-exchanged water. Next, the entire amount of the lithium carbonate-containing suspension was added to the reduction reaction preparation solution over 30 minutes at 40°C, and stirring was continued for 60 minutes to obtain a dark blue raw material mixture solution. Furthermore, when the raw material mixture solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. The raw material mixture solution after 24 hours was diluted 100-fold and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), and was found to be 0.1 NTU.

[0146] <Fourth and fifth steps> The fourth and subsequent steps were carried out in the same manner as in Example 1. The obtained lithium vanadium phosphate carbon composite was subjected to X-ray diffraction analysis. The diffraction peak pattern matched that of the lithium vanadium phosphate carbon composite of Example 1, indicating that the composite was a single-phase lithium vanadium phosphate (LiV 1.9 Al 0.1 The resultant composite was confirmed to be lithium vanadium phosphate (LVP) nanoparticles dispersed in carbon. This composite was designated as a lithium vanadium phosphate carbon composite sample. The average primary particle size was measured using a scanning electron microscope in the same manner as in Example 1, and it was confirmed to be lithium vanadium phosphate (LVP) nanoparticles with an average primary particle size of 30 nm dispersed in carbon.

[0147] The average particle diameter (D 50 ) is 14.4 μm and D 90 The BET specific surface area was 45.2 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the carbon atom content, and the residual carbon content was found to be 8.9 mass%.

[0148]

[0149] In Table 2, the amount of the first carboxylic acid added indicates the molar ratio (C / V) of C atoms in the first carboxylic acid to V atoms in vanadium pentoxide, in atomic terms. The amount of the second carboxylic acid added indicates the molar ratio (C / V) of C atoms in the second carboxylic acid to V atoms in vanadium pentoxide, in atomic terms. The stability of the raw material mixed solution was evaluated as "Good" when no precipitate was observed visually after the raw material mixed solution was kept under stirring at 25°C for 24 hours, and evaluated as "Poor" when precipitate was observed and the solution became a slurry.

[0150]

[0151] (Example 3) <First Step> 2.8 L of ion-exchanged water was placed in a 10 L container, and 500 g of vanadium pentoxide, 635.5 g of citric acid monohydrate, 177.3 g of 50 wt % aluminum phosphate monohydrate, and 918.1 g of 85 wt % phosphoric acid were added thereto at room temperature (25°C), in that order, and the reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, yielding a greenish-blue reduction reaction slurry. Note that the reduction reaction was exothermic, and the temperature of the reaction system rose from 25°C to 45°C.

[0152] <Second Step> Next, 341.5 g of a 50 wt % aqueous gluconic acid solution was added to the reduction reaction slurry obtained in the first step at 45° C., and the mixture was stirred for 60 minutes to obtain a deep blue solution-like reduction reaction preparation liquid.

[0153] <Third Step> 327.6 g of lithium carbonate and 10.3 g of magnesium hydroxide were added to 1.5 L of ion-exchanged water to prepare a suspension containing lithium carbonate and magnesium hydroxide. Next, the entire amount of the lithium carbonate and magnesium hydroxide suspension was added to the reduction reaction preparation solution over 30 minutes at 40°C, and stirring was continued for 60 minutes to obtain a dark blue raw material mixture solution. Furthermore, when the raw material mixture solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. The raw material mixture solution after 24 hours was diluted 100-fold and the turbidity was measured using a turbidity meter (TB250WL, manufactured by Tintometer Co.), and the turbidity was 0.1 NTU.

[0154] <Fourth and fifth steps> The fourth and subsequent steps were carried out in the same manner as in Example 1. The obtained lithium vanadium phosphate carbon composite was subjected to X-ray diffraction analysis. The diffraction peak pattern matched that of the lithium vanadium phosphate carbon composite of Example 1, indicating that the composite was a single-phase lithium vanadium phosphate (LiV 1.86 Al 0.10 Mg 0.06 The resultant composite was confirmed to be lithium vanadium phosphate (LVP) nanoparticles dispersed in carbon. This was designated as a lithium vanadium phosphate carbon composite sample. Furthermore, the average primary particle size was measured using a scanning electron microscope in the same manner as in Example 1. It was confirmed that the composite was lithium vanadium phosphate (LVP) nanoparticles with an average primary particle size of 30 nm dispersed in carbon.

[0155] The average particle diameter (D 50 ) is 15.3 μm and D 90 The BET specific surface area was 49.9 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the carbon atom content, and the residual carbon content was found to be 9.2% by mass.

[0156] Example 4 <First Step> 2.8 L of ion-exchanged water was placed in a 10 L container, and 500 g of vanadium pentoxide, 635.5 g of citric acid monohydrate, 178.3 g of 50 wt % aluminum phosphate monohydrate, and 914.5 g of 85 wt % phosphoric acid were added thereto at room temperature (25°C), in that order, and the reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, yielding a greenish-blue reduction reaction slurry. Note that the reduction reaction was exothermic, and the temperature of the reaction system rose from 25°C to 45°C.

[0157] <Second Step> Next, 341.5 g of a 50 wt % aqueous gluconic acid solution was added to the reduction reaction slurry obtained in the first step at 45° C., and the mixture was stirred for 60 minutes to obtain a deep blue solution-like reduction reaction preparation liquid.

[0158] <Third Step> A suspension containing lithium carbonate and yttrium acetate was prepared by adding 326.7 g of lithium carbonate and 50.2 g of yttrium acetate tetrahydrate to 1.5 L of ion-exchanged water. The entire amount of the lithium carbonate and yttrium acetate suspension was then added to the reduction reaction preparation solution over 30 minutes at 40°C, and stirring was continued for 60 minutes to obtain a dark blue raw material mixture solution. Furthermore, when the raw material mixture solution was kept under stirring at 25°C for 24 hours, no precipitate was observed, and the solution was stable. The raw material mixture solution after 24 hours was diluted 100-fold and the turbidity was measured using a turbidity meter (TB250WL, manufactured by Tintometer Co., Ltd.), and the turbidity was 0.1 NTU.

[0159] <Fourth and fifth steps> The fourth and subsequent steps were carried out in the same manner as in Example 1. The obtained lithium vanadium phosphate carbon composite was subjected to X-ray diffraction analysis. The diffraction peak pattern matched that of the lithium vanadium phosphate carbon composite of Example 1, indicating that the composite was a single-phase lithium vanadium phosphate (LiV 1.85 Al 0.10 Y 0.05 The resultant composite was confirmed to be lithium vanadium phosphate (LVP) nanoparticles dispersed in carbon. This composite was designated as a lithium vanadium phosphate carbon composite sample. The average primary particle size was measured using a scanning electron microscope in the same manner as in Example 1, and it was confirmed to be lithium vanadium phosphate (LVP) nanoparticles with an average primary particle size of 30 nm dispersed in carbon.

[0160] The average particle diameter (D 50 ) is 16.5 μm and D 90 The BET specific surface area was 51.1 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the carbon atom content, and the residual carbon content was found to be 9.8% by mass.

[0161] Example 5 <First Step> 2.8 L of ion-exchanged water was placed in a 10 L container, and 500 g of vanadium pentoxide, 635.5 g of citric acid monohydrate, 581.9 g of 50 wt % aluminum phosphate monohydrate, and 784.7 g of 85 wt % phosphoric acid were added thereto at room temperature (25°C), in that order, and the reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, yielding a greenish-blue reduction reaction slurry. Note that the reduction reaction was exothermic, and the temperature of the reaction system rose from 25°C to 45°C.

[0162] <Second Step> Next, 341.5 g of a 50 wt % aqueous gluconic acid solution was added to the reduction reaction slurry obtained in the first step at 45° C., and the mixture was stirred for 60 minutes to obtain a deep blue solution-like reduction reaction preparation liquid.

[0163] <Third Step> A lithium carbonate-containing suspension was prepared by adding 358.5 g of lithium carbonate to 1.5 L of ion-exchanged water. Next, the entire amount of the lithium carbonate-containing suspension was added to the reduction reaction preparation solution over 30 minutes at 40°C, and stirring was continued for 60 minutes to obtain a dark blue solution-like raw material mixture solution. Furthermore, when the raw material mixture solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. The raw material mixture solution after 24 hours was diluted 100-fold and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), and was found to be 0.1 NTU.

[0164] <Fourth and fifth steps> The fourth and subsequent steps were carried out in the same manner as in Example 1. The obtained lithium vanadium phosphate carbon composite was subjected to X-ray diffraction analysis. The diffraction peak pattern matched that of the lithium vanadium phosphate carbon composite of Example 1, indicating that the composite was a single-phase lithium vanadium phosphate (LiV 1.7 Al 0.3 The resultant composite was confirmed to be lithium vanadium phosphate (LVP) nanoparticles dispersed in carbon. This composite was designated as a lithium vanadium phosphate carbon composite sample. The average primary particle size was measured using a scanning electron microscope in the same manner as in Example 1, and it was confirmed to be lithium vanadium phosphate (LVP) nanoparticles with an average primary particle size of 30 nm dispersed in carbon.

[0165] The average particle diameter (D50 ) is 14.8 μm and D 90 The BET specific surface area was 58.5 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the carbon atom content, and the residual carbon content was found to be 10.5% by mass.

[0166]

[0167] In Table 5, the amount of the first carboxylic acid added represents the molar ratio (C / V) of C atoms in the first carboxylic acid to V atoms in vanadium pentoxide, in atomic terms. The amount of the second carboxylic acid added represents the molar ratio (C / V) of C atoms in the second carboxylic acid to V atoms in vanadium pentoxide, in atomic terms. The stability of the raw material mixed solution was evaluated by "Good" when no precipitate was observed visually after the raw material mixed solution was kept under stirring at 25°C for 24 hours, and by "Poor" when precipitate was observed and the solution became a slurry.

[0168]

[0169] <Evaluation of Physical Properties of Lithium Vanadium Phosphate Carbon Composite Samples 2> The content of lithium vanadium phosphate carbon composite particles (A), average particle diameter (average secondary particle diameter), and coefficient of linear expansion of the lithium vanadium phosphate carbon composite samples obtained in the Examples were measured as follows. An SEM photograph (1000x magnification) of the lithium vanadium phosphate carbon composite sample obtained in Example 1 is shown in FIG.

[0170] (Evaluation of Average Particle Size of Lithium Vanadium Phosphate Carbon Composite Particles (A)) The particles were observed at 1000x magnification using a scanning electron microscope, and the average particle size of 200 randomly selected lithium vanadium phosphate carbon composite particles having two or more depressions on the particle surface, each having an opening diameter (d) of 500 to 7000 nm, was taken as the average particle size.

[0171] (Evaluation of Content of Lithium Vanadium Phosphate Carbon Composite Particles (A)) The particles were observed at 1000x magnification using a scanning electron microscope, and 200 particles (secondary particles) (t1) randomly selected from the group had a particle diameter of 5 μm or more and 40 μm or less. The number (t2) of lithium vanadium phosphate carbon composite particles (A) having two or more depressions on the particle surface, each having an opening diameter (d) of 500 to 7000 nm, was determined to calculate the content ((t2 / t1)×100) converted into the number.

[0172] (Measurement of Linear Expansion Coefficient) The linear expansion coefficients of the lithium vanadium phosphate carbon composite samples obtained in Examples 1, 2 and 5 were measured at 25 to 70° C. as follows.

[0173] (Preparation of powder compact) 1.00 g of sample was pulverized and mixed in a mortar for 3 minutes, and then 0.15 g was weighed out and the entire amount was filled into a φ6 mm mold. Next, a powder compact was prepared by molding at a pressure of 10 MPa using a hand press. The thermal expansion coefficient of the prepared powder compact between 25 and 70°C was evaluated as follows.

[0174] (Measurement of thermal expansion coefficient between 25 and 70°C) The thermal expansion coefficient of the produced powder compact was measured using a thermomechanical measuring device (TMA4000SE manufactured by NETZSCH JAPAN). The measurement conditions were a nitrogen atmosphere, a load of 10 g, and a temperature range of 0°C to 100°C, and the measurement was repeated twice. The thermal expansion coefficient between 25 and 70°C in the second repeated measurement was taken as the thermal expansion coefficient of the sample.

[0175]

[0176] Note: "-" in the table indicates that the measurement was not performed.

[0177] (Comparative Example 3) 2 L of ion-exchanged water was placed in a 5 L beaker, and 252 g of lithium hydroxide monohydrate was added and dissolved therein. 364 g of vanadium pentoxide was added to this solution and stirred for 1 hour. 72 g of glucose (grape sugar) and 692 g of 85% phosphoric acid were added to this solution and stirred for 1 hour to obtain a raw material mixture. Next, the raw material mixture was supplied to a spray dryer with the outlet temperature set to 120°C to obtain a reaction precursor. The obtained reaction precursor was placed in a mullite sagger and fired at 900°C for 12 hours in a nitrogen atmosphere. The fired product was crushed using a jet mill to obtain a lithium vanadium phosphate sample. X-ray diffraction analysis of the obtained lithium vanadium phosphate sample confirmed that it was a single-phase lithium vanadium phosphate (average particle diameter (D 50 ): 2.3 μm). The amount of residual carbon in the obtained lithium vanadium phosphate sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the content of C atoms, and the amount of residual carbon was found to be 0.1 mass%.

[0178] <Battery Performance Test 1> (I) Preparation of Lithium Secondary Battery: 91% by mass of each of the samples of Example 1, Example 2, and Comparative Example 3 prepared as described above, 6% by mass of graphite powder, and 3% by mass of polyvinylidene fluoride were mixed to prepare a cathode agent, which was then dispersed in N-methyl-2-pyrrolidinone to prepare a kneaded paste. The resulting kneaded paste was applied to aluminum foil, dried, pressed, and punched into a 15 mm diameter disk to obtain a cathode plate. A lithium secondary battery was fabricated using this cathode plate and various components, including a separator, anode, cathode, current collector, mounting hardware, external terminals, and electrolyte. Metallic lithium foil was used for the anode, and 1 mol of LiPF6 was dissolved in 1 liter of a 1:1 kneaded solution of ethylene carbonate and methyl ethyl carbonate, as the electrolyte.

[0179] (2) Battery Performance Evaluation The fabricated lithium secondary batteries were operated under the following conditions to evaluate their battery performance. <Evaluation of Cycle Characteristics> The batteries were charged to 4.2 V at 0.5 C, followed by constant-current, constant-voltage (CCCV) charging, maintaining the battery at 4.2 V for a total charge time of 5 hours, followed by constant-current (CCCV) discharging to 2.0 V at 0.1 C. These operations constituted one cycle, and the discharge capacity was measured for each cycle. This cycle was repeated 20 times at 25°C, and the capacity retention rate was calculated from the discharge capacities at the first and 20th cycles using the following formula. The discharge capacity at the first cycle was defined as the initial discharge capacity.

[0180] Capacity retention rate (%)=((discharge capacity at 20th cycle) / (discharge capacity at 1st cycle))×100

[0181]

[0182] (Example 6) (Sixth Step) The vanadium phosphate lithium carbon composite sample obtained in Example 1 was pulverized using a jet mill to obtain a vanadium phosphate lithium carbon composite sample. The average particle diameter (D 50 ) is 4.5 μm and D 90 The BET specific surface area was 51.2 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the carbon atom content, and the residual carbon content was found to be 10.3 mass%.

[0183] (Example 7) (Sixth Step) The vanadium phosphate lithium carbon composite sample obtained in Example 5 was pulverized using a jet mill to obtain a vanadium phosphate lithium carbon composite sample. The average particle diameter (D 50 ) is 4.1 μm and D 90The BET specific surface area was 58.1 m 2 / g. 50 The ratio of the BET specific surface area to the 50 The residual carbon content of the obtained lithium vanadium phosphate carbon composite sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the carbon atom content, and the residual carbon content was found to be 10.6 mass%.

[0184]

[0185] <Evaluation of Physical Properties of Lithium Vanadium Phosphate Carbon Composite Samples 3> Furthermore, the lithium vanadium phosphate carbon composite samples obtained in Examples 6 and 7 were measured for the content of lithium vanadium phosphate carbon composite particles (B), average particle diameter (average secondary particle diameter), and linear expansion coefficient as described below. The results are shown in Table 10. Furthermore, SEM photographs (2,000x and 20,000x magnification) of the lithium vanadium phosphate carbon composite sample obtained in Example 6 are shown in Figure 8.

[0186] (Evaluation of Average Particle Size of Lithium Vanadium Phosphate Carbon Composite Particles (B)) The particles were observed under a scanning electron microscope at 2000x magnification, and the average size of 200 randomly selected lithium vanadium phosphate carbon composite particles (B) was calculated.

[0187] (Evaluation of Content of Lithium Vanadium Phosphate Carbon Composite Particles (B)) The sample was observed under a scanning electron microscope at 2000x magnification, and 200 particles (secondary particles) (t3) having a particle diameter of 4 μm or more and 20 μm or less were randomly selected. The number (t4) of lithium vanadium phosphate carbon composite particles (B) was determined, and the content ((t4 / t3)×100) converted to the number of particles was calculated.

[0188] (Measurement of Linear Expansion Coefficient) The linear expansion coefficient was measured in the same manner as in Examples 1, 2 and 5.

[0189]

[0190] <Battery Performance Test 2> Lithium secondary batteries were fabricated using the sample obtained in Example 6 in the same manner as in Examples 1, 2, and Comparative Example 3, and the capacity retention rate was calculated from the discharge capacities at the first and 20th cycles at 25° C. The results are shown in Table 11.

[0191]

[0192] <Battery Performance Test 3> Lithium secondary batteries were prepared for the samples of Examples 1, 2, 5, 6, and 7 in the same manner as in Examples 1 and 2. The batteries were charged to 4 V at 2 C, followed by constant-current, constant-voltage (CCCV) charging, in which the batteries were held at 4 V for a total charge time of 3.5 hours. They were then discharged to 2 V at 2 C by constant-current (CC) discharging. These operations constituted one cycle, and the discharge capacity was measured for each cycle. This cycle was repeated 20 times at 25°C, 40°C, or 55°C, and the capacity retention rate was calculated from the discharge capacities at the first and 20th cycles using the following formula. The results are shown in Table 12. The discharge capacity at the first cycle was defined as the initial discharge capacity. Capacity retention rate (%) = ((discharge capacity at the 20th cycle) / (discharge capacity at the first cycle)) × 100

[0193]

[0194] From Table 12, when Example 1 is compared with Example 2 and Example 5, it is seen that in Example 2 and Example 5, which are doped with Al, the cycle characteristics in a high-temperature environment (40°C, 55°C) are improved compared to Example 1, which is not doped with Al. Furthermore, when Example 6 is compared with Example 7, it is seen that in Example 7, which is doped with Al, the cycle characteristics in a high-temperature environment (40°C, 55°C) are improved compared to Example 6, which is not doped with Al.

Claims

1. A method for producing a composite of lithium vanadium phosphate having a NASICON structure and carbon, comprising: a first step of adding vanadium pentoxide, phosphoric acid, and a first carboxylic acid to an aqueous solvent to carry out a reduction reaction of the vanadium pentoxide and prepare a reduction reaction slurry; a second step of adding a second carboxylic acid to the reduction reaction slurry to prepare a reduction reaction preparation liquid; a third step of adding a lithium source to the reduction reaction preparation liquid to prepare a raw material mixed solution in liquid form; a fourth step of spray-drying the raw material mixed solution to obtain a reaction precursor; and a fifth step of firing the reaction precursor at 500 to 1300°C in an inert gas atmosphere or a reducing atmosphere to obtain a vanadium phosphate lithium carbon composite.

2. The method for producing a lithium vanadium phosphate carbon composite according to claim 1, wherein the reduction reaction in the first step is carried out at a temperature below 60°C.

3. The method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, wherein the second carboxylic acid is a hydroxycarboxylic acid from which carbon is isolated by heating.

4. The method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, characterized in that the first carboxylic acid is citric acid.

5. The method for producing a lithium vanadium phosphate carbon composite according to claim 4, wherein the second carboxylic acid is one or two selected from the group consisting of gluconic acid and malic acid.

6. The method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, wherein the first carboxylic acid is gluconic acid.

7. The method for producing a lithium vanadium phosphate carbon composite according to claim 6, wherein the second carboxylic acid is malic acid.

8. A method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, characterized in that the amount of the first carboxylic acid added is such that the molar ratio (C / V) of C atoms in the first carboxylic acid to V atoms in the vanadium pentoxide is 2.0 to 6.

0.

9. A method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, characterized in that the amount of the second carboxylic acid added is such that the molar ratio (C / V) of C atoms in the second carboxylic acid to V atoms in the vanadium pentoxide is 0.50 to 4.

0.

10. A method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, characterized in that a Me source (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element) is further contained in the reduction reaction slurry in the first step and / or the liquid-like raw material solution in the third step.

11. The method for producing a lithium vanadium phosphate carbon composite according to claim 10, wherein the Me source contains at least one selected from a Ti source and an Al source.

12. A method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 10, further comprising a sixth step of pulverizing the lithium vanadium phosphate carbon composite obtained after the fifth step.

13. A positive electrode material comprising lithium vanadium phosphate carbon composite particles, wherein the lithium vanadium phosphate carbon composite particles comprise lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface, the average particle diameter of the lithium vanadium phosphate carbon composite particles (A) being 5 μm or more and 40 μm or less, and the carbon content of the positive electrode material being 7.6 to 20 mass % in terms of C atoms.

14. The positive electrode material according to claim 13, characterized in that, when observed with a scanning electron microscope, the proportion of lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface among particles of 5 μm or more and 40 μm or less in size in the positive electrode material is 10% or more.

15. The positive electrode material according to claim 13, characterized in that the lithium vanadium phosphate carbon composite particles (A), which contain lithium vanadium phosphate nanoparticles having a plurality of depressions on the particle surface, contain doped Me (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element).

16. A positive electrode material comprising irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing nanoparticles of lithium vanadium phosphate, wherein the average particle size of the lithium vanadium phosphate carbon composite particles (B) is 4 μm or more and 20 μm or less, and the carbon content of the positive electrode material is 7.6 to 20 mass% in terms of C atoms.

17. The positive electrode material according to claim 16, characterized in that, when observed under a scanning electron microscope, the proportion of irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing nanoparticles of lithium vanadium phosphate among particles of 4 μm or more and 20 μm or less in the positive electrode material is 30% or more.

18. The positive electrode material according to claim 16, characterized in that the irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing nanoparticles of lithium vanadium phosphate contain doped Me (Me represents a metal element or transition metal element other than V having an atomic number of 11 or more).

19. Average particle diameter (D) measured by laser diffraction scattering method 50 BET specific surface area (m 2 / g) ratio (BET / D 50 16. The positive electrode material according to claim 13 or 15, wherein the number of atoms is 2 or more.

20. Average particle diameter (D) measured by laser diffraction scattering method 50 BET specific surface area (m 2 / g) ratio (BET / D 50 19. The positive electrode material according to claim 16 or 18, wherein the number of atoms is 2 or more.

21. A positive electrode material according to claim 15 or 18, characterized in that the linear expansion coefficient in the temperature range of 25 to 70°C is 5 ppm / K or less.

22. An electricity storage device characterized by using the positive electrode material according to any one of claims 13 to 21.

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