WO2025173660A1 - Positive electrode material, power storage device, and method for producing lithium vanadium phosphate carbon composite - Google Patents

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

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, as well as a positive electrode material and an electricity storage device using the same.

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.

Currently, _LiFePO4 is attracting attention as a further alternative material, and research and development into it 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.

However, LiFePO ₄ has a rather low operating voltage, and attention has been drawn to lithium vanadium phosphate (Li ₃ V ₂ (PO ₄ ) ₃ ) having a NASICON (Na Super Ionic Conductor) structure in which V is used instead of Fe.

In Patent Document 1 listed below, the present applicants previously proposed a method for producing a lithium vanadium phosphate carbon composite, which comprises a first step of preparing a raw material mixture by mixing a lithium source, a pentavalent or tetravalent vanadium compound, a phosphorus source, and a conductive carbon material source that generates carbon upon thermal decomposition in an aqueous solvent; a second step of heating the raw material mixture to cause a precipitation reaction and obtain a reaction solution containing a precipitate product; a third step of wet-pulverizing the reaction solution containing the precipitation 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 applicant 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.

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 heated at 85°C to remove the aqueous solvent to obtain a dried product, which is then fired 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 fired in a nitrogen gas atmosphere.

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

International Publication No. 2012/043367 JP 2017-160107 A WO 2018/142082, paragraph 0121 JP 2011-198657 A, paragraph 0044 JP 2014-229830 A

Scientific Report, 4, 4047 (2014)

Because lithium vanadium phosphate is highly safe even at high temperatures, it is attracting attention as a positive electrode material for lithium secondary batteries, all-solid-state batteries, electrochemical capacitors, and other applications in automobiles. To commercialize this compound, it is desirable to develop a method for producing lithium vanadium phosphate carbon composites that can further reduce costs by simplifying the process, etc.

That is, an object of the present invention is to provide a method for industrially advantageously producing a lithium vanadium phosphate carbon composite that can produce nanoparticles of lithium vanadium phosphate 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 a lithium vanadium phosphate carbon composite that can impart excellent cycle characteristics to an electricity storage device, and to provide an electricity storage device with excellent cycle characteristics.

As a result of extensive research to solve the above problems, the present inventors have discovered the following and have completed the present invention based on the following findings.
In the course of studying ways to simplify the steps in the methods for producing lithium vanadium phosphate described in Patent Documents 1 and 2, it was discovered that by using a carboxylic acid such as citric acid instead of a reducing sugar in the step of preparing a reaction precursor, the reduction reaction of vanadium pentoxide can be carried out without actively carrying out a heat treatment.
Furthermore, when 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 an effect 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 to this, 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 it, thereby obtaining a reduction reaction preparation liquid. When a lithium source is added to this reduction reaction preparation liquid to form a solution, the solution remains stable and easy to handle, free from precipitates, even after 24 hours at room temperature.
In addition, the second carboxylic acid serves as a conductive carbon source for lithium vanadium phosphate.
Furthermore, by spray-drying the reaction raw material solution, a reaction precursor having excellent reactivity can be obtained, and by calcining the reaction precursor, a lithium vanadium phosphate carbon composite containing lithium vanadium phosphate of high purity as measured by X-ray diffraction is obtained.
Furthermore, the addition of a second carboxylic acid can produce nanoparticles of lithium vanadium phosphate.
Furthermore, the lithium vanadium phosphate carbon composite obtained by this manufacturing method has a carbon content within a specific range, and the carbon content is in a specific range. The lithium vanadium phosphate carbon composite particles contain lithium vanadium phosphate nanoparticles having depressions on the particle surface. This results in an electricity storage device having excellent cycle characteristics as a positive electrode material.
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.

That is, the present invention (1) is a method for producing a composite of lithium vanadium phosphate having a NASICON structure and carbon,
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 vanadium pentoxide to 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 solution;
a third step of adding a lithium source to the reduction reaction preparation solution to prepare a raw material mixture solution in a liquid state;
a fourth step of spray-drying the raw material mixture solution to obtain a reaction precursor;
a fifth step of calcining the reaction precursor at 500 to 1300°C in an inert gas atmosphere or a reducing atmosphere to obtain a lithium vanadium phosphate carbon composite;
The present invention provides a method for producing a lithium vanadium phosphate carbon composite, which is characterized by having the following:

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

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

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

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

The present invention (6) also 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.

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

The present invention (8) also provides a 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 the vanadium pentoxide is 2.0 to 6.0.

The present invention (9) also provides a 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 0.50 to 4.0 in terms of the molar ratio (C/V) of C atoms in the second carboxylic acid to V atoms in the vanadium pentoxide.

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 an Me source (Me represents a metal element other than V with an atomic number of 11 or greater or a transition metal element) is further contained in the reduction reaction slurry of the first step and/or the liquid-like raw material solution of the third step.

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 selected from a Ti source and an Al source.

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.

The present invention (13) is a positive electrode material containing lithium vanadium phosphate carbon composite particles,
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) is 5 μm or more and 40 μm or less,
the carbon content of the positive electrode material is 7.6 to 20 mass% in terms of C atoms;
The present invention provides a positive electrode material characterized by the following:

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

The present invention (15) also provides a positive electrode material according to (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 other than V with an atomic number of 11 or greater or a transition metal element).

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,
The lithium vanadium phosphate carbon composite particles (B) have an average particle size of 4 μm or more and 20 μm or less,
the carbon content of the positive electrode material is 7.6 to 20 mass% in terms of C atoms;
The present invention provides a positive electrode material characterized by the above.

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 of irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing lithium vanadium phosphate nanoparticles is 30% or more of the particles in the positive electrode material having a size of 4 μm or more and 20 μm or less.

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

The present invention (19) also provides a positive electrode material according to (13) or ( ₁₅ ), characterized in that the ratio (BET/ _D50 ) of the BET specific surface area ( ^m2 /g) to the average particle diameter (D50: μm) measured by a laser diffraction scattering method is 2 or more.

The present invention (20) also provides a positive electrode material according to (16) or ( ₁₈ ), characterized in that the ratio (BET/ _D50 ) of the BET specific surface area ( ^m2 /g) to the average particle diameter (D50: μm) measured by a laser diffraction scattering method is 2 or more.

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.

Furthermore, the present invention (22) provides an electricity storage device characterized by using the positive electrode material described in the present inventions (13) to (21).

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

1 is an X-ray diffraction pattern of the reaction precursor obtained in the fourth step of Example 1. 1 is an SEM photograph of the reaction precursor obtained in the fourth step of Example 1. 1 is an X-ray diffraction diagram of the lithium vanadium phosphate carbon composite sample obtained in Example 1. SEM photographs of the lithium vanadium phosphate carbon composite sample obtained in Example 1. (a) Magnification: 10,000 times, (b) Magnification: 100,000 times. 1 is an X-ray diffraction diagram of the lithium vanadium phosphate carbon composite sample obtained in Comparative Example 1. 1 is a SEM photograph of a lithium vanadium phosphate carbon composite sample obtained in Comparative Example 1. 1 is an SEM photograph (magnification: 1000 times) of the lithium vanadium phosphate carbon composite sample obtained in Example 1. 10 is a SEM photograph of the lithium vanadium phosphate carbon composite sample obtained in Example 5 (magnification: 2000 times (top), 20000 times (bottom)).

The present invention will be described below based on preferred embodiments thereof.
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,
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 vanadium pentoxide to 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 solution;
a third step of adding a lithium source to the reduction reaction preparation solution to prepare a raw material mixed solution;
a fourth step of spray-drying the raw material mixture solution to obtain a reaction precursor;
a fifth step of calcining the reaction precursor at 500 to 1300°C in an inert gas atmosphere or a reducing atmosphere to obtain a lithium vanadium phosphate carbon composite;
The method for producing a lithium vanadium phosphate carbon composite is characterized by comprising the steps of:

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").

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 single-phase lithium vanadium phosphate when subjected to X-ray diffraction analysis. Furthermore, the vanadium phosphate-carbon composite obtained by the method for producing a vanadium phosphate-carbon composite of the present invention is detected as 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.

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 (PO ₄ ) ₃ (1)
(In the formula, 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 represented by general formula (1) doped with Me element (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element), as required.

In the general formula (1), x is 2.5 or more and 3.5 or less, preferably 2.8 or more and 3.2 or less, and 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. Furthermore, as the element containing Ti and/or Al, particularly preferred are those selected from Ti, Al, Ti and Al, Ti and Y, Ti and Mg, Al and Y, and Al and Mg.

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 percent (Me/V) of Me relative to V in the lithium vanadium phosphate, from the viewpoint of achieving a high capacity retention rate in an energy storage device using the lithium vanadium phosphate carbon composite as a positive electrode material.

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

The first step is to add vanadium pentoxide, phosphoric acid, and a first carboxylic acid to an aqueous solvent, carry out a reduction reaction of the vanadium pentoxide, and prepare a reduction reaction slurry.

In conventional methods that use reducing sugars as reducing agents, the reduction reaction of vanadium pentoxide typically needs to be carried out at temperatures between 60 and 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.

Incidentally, conventional methods using carboxylic acids such as citric acid utilize the chelating effect of carboxylic acids such as citric acid on vanadium, and use the carboxylic acid as a conductive carbon source to coat lithium vanadium phosphate. Therefore, compared to the present invention, the carboxylic acid such as citric acid is added in large excess to the amount required 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 is why conventional methods using carboxylic acids such as citric acid differ from the first step of the present invention.

The inventors believe 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, that the reduction reaction slurry obtained in the first step contains _VOHPO4 or a hydrate thereof as a solid component, and that the liquid component of the reduction reaction slurry obtained in the first step is composed of ions derived from phosphoric acid, such as dihydrogen phosphate _ions ⁽ _H2PO4- ), 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.

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 fact that only carbon remains, allowing a high-purity lithium vanadium phosphate carbon composite to be produced.
Examples of hydroxycarboxylic acids used as the first carboxylic acid include citric acid, malic acid, gluconic acid, etc. 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 to be added may be a hydrate or an anhydride.

In the first step, 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, 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 this range, the reduction reaction of vanadium pentoxide and the chelating effect on vanadium will be insufficient. On the other hand, if the amount of the first carboxylic acid added exceeds this range, the carbon content of the final lithium vanadium phosphate tends to be too high, which is undesirable.

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 atomic terms, as this makes it easier to obtain a single-phase lithium vanadium phosphate as the final product in X-ray diffraction analysis.

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

The aqueous solvent used in the first step may be water, or a mixture of water and a hydrophilic organic solvent.

The amount of water solvent used in the first step is preferably adjusted so that the solids content in the reduction reaction slurry is 10 to 50% by mass, and more preferably so that the solids content in the reduction reaction slurry is 20 to 40% by mass.

Furthermore, when the Me source described below is added in powder form, a dispersant can be added to the aqueous solvent containing vanadium pentoxide, phosphoric acid, and the first carboxylic acid, if necessary, to ensure high dispersion.

The dispersant is preferably at least one anionic surfactant selected from carboxylates, sulfates, sulfonates, and phosphates, as this reduces the viscosity of the reduction reaction slurry and allows for the production of a highly reactive reaction precursor. As anionic surfactants, polycarboxylic acid surfactants or polyacrylic acid surfactants are preferred, with polycarboxylic acid surfactants being particularly preferred. As polycarboxylic acid surfactants, ammonium salts of polycarboxylic acids are preferred.

The anionic surfactant may be commercially available. 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, and Poise 532A, manufactured by Kao Corporation.

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

In the first step, the order in which vanadium pentoxide, phosphoric acid, first carboxylic acid, and optionally a dispersant are added to the aqueous solvent, and the mixing method are not particularly limited.

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 stirred and mixed to carry out a reduction reaction of the vanadium pentoxide.

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 the 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.

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

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

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, so long as the amount of the second carboxylic acid added is within the range described below.
When the reduction reaction preparation liquid is in a slurry state, the resulting raw material mixture solution can be made into a solution state by adding a lithium source in the next step, the third step.

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

The second carboxylic acid used 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. Hydroxycarboxylic acid having 4 to 6 carbon atoms is preferred, as it can be used as a conductive carbon source for lithium vanadium phosphate to produce a high-purity lithium vanadium phosphate carbon composite.

Gluconic acid and malic acid are preferred as hydroxycarboxylic acids used as the second carboxylic acid, as they have a strong chelating effect on the solid components contained in the reduction reaction slurry obtained in the second step, and can be used to efficiently isolate carbon by the calcination in the fifth step, successfully leaving it as conductive carbon.

In the method for producing a lithium vanadium phosphate carbon composite of the present invention, the following combinations of the first carboxylic acid and the second carboxylic acid are preferred in that they enable industrially advantageous production of a lithium vanadium phosphate carbon composite that is highly pure as measured by X-ray diffraction and contains conductive carbon:
(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.
(3) The first carboxylic acid is gluconic acid and the second carboxylic acid is malic acid.

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, in atomic terms, preferably 0.50 to 4.0, and more preferably 0.60 to 3.5. Having the amount of the second carboxylic acid added within this range is advantageous in that the resulting lithium vanadium phosphate carbon composite is likely to contain 1.0 to 20.0% by mass, preferably 2.0 to 15.0% by mass, of carbon.

In the second step, the amount of the second carboxylic acid added is preferably such that the molar ratio (C/V) of the sum of the C atoms in the first carboxylic acid and the C atoms in the second carboxylic acid to the V atoms in the vanadium pentoxide in the first step is 0.30 to 4.3, and more preferably 0.50 to 4.1, in atomic terms. Having the molar ratio (C/V) of the sum of the C atoms in the first carboxylic acid and the C atoms in the second carboxylic acid to the V atoms in the vanadium pentoxide in the first step within the above range is preferred because it makes it easier for the resulting lithium vanadium phosphate carbon composite to contain 1.0 to 20.0% by mass, and preferably 2.0 to 15.0% by mass, of carbon.

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, but is preferably 80°C or lower, preferably less than 60°C, more preferably 20 to 50°C, and even more preferably 25 to 45°C.

In the second step, for example, a second carboxylic acid is added to the reduction reaction slurry obtained in the first step, and a mixing treatment such as stirring is carried out at a temperature of 80°C or less, 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 with satisfactory performance.

The third step is to add a lithium source to the reduction reaction preparation solution obtained in the second step to prepare a raw material mixture solution in liquid form.

Examples of the lithium source in the third step include lithium hydroxide and lithium carbonate.
The lithium source is preferably added to the reduction reaction preparation liquid obtained in the second step as a solution in which it is dissolved in water or as a suspension in which water is used as a dispersion solvent.

In the third step, the amount of lithium source added is 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 atomic terms. Having the amount of lithium source added within this range is preferred from the viewpoint that it makes it easier to obtain a vanadium lithium phosphate carbon composite that contains vanadium lithium phosphate that is highly pure as measured by X-ray diffraction.

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 preferably 80°C or lower, preferably less than 60°C, more preferably 15 to 50°C, and even more preferably 20 to 45°C.

In the third step, for example, a lithium source is added to the raw material mixture solution obtained by carrying out the second step, and a mixing treatment such as stirring is carried out at 80°C or less, 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 mixture solution.

The raw material mixed solution obtained by performing 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 performing the third step has, for example, a turbidity of 20 NTU or less at room temperature (25°C) immediately after preparation and a turbidity of 15 NTU or less after standing at room temperature (25°C) for 24 hours. In the present invention, the turbidity of the solution is the value measured using a turbidity meter (TB250WL, manufactured by Tintometer) after diluting the raw material mixed solution 100 times.

The fourth step is to spray-dry the raw material mixture solution obtained in the third step to obtain a reaction precursor.

Although methods other than spray drying are known for drying liquids, spray drying is used in the present invention based on the knowledge that it is advantageous to select this drying method. In particular, when spray drying is used, densely packed granules are obtained in which each component is uniformly dispersed at the molecular level. Therefore, these granules are 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.

In the spray drying method, a liquid is atomized using a specified means, and the resulting fine droplets are dried to obtain granules. Liquid atomization can be achieved, for example, by using a rotating disk or a pressure nozzle. Either method can be used in the fourth step.

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 raw material particles is too small compared to the droplet size, the droplets will become unstable, making successful drying difficult. From this perspective, the size of the atomized droplets is preferably 5 to 100 μm, with 10 to 50 μm being particularly preferred. It is desirable to determine the amount of slurry supplied to the spray drying device taking this into consideration.

The drying temperature in the spray dryer is preferably adjusted so that the hot air inlet temperature is 180-250°C, preferably 200-240°C, and the powder temperature is 90-150°C, preferably 100-130°C, as this prevents the powder from absorbing moisture and makes it easier to recover the powder.

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.

The fifth step is to calcinate the reaction precursor obtained in 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.

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 this range, the firing time until lithium vanadium phosphate is produced will be long, and if the temperature exceeds this range, the lithium vanadium phosphate will melt.

The firing atmosphere in the fifth step is an inert gas atmosphere or a reducing atmosphere to prevent oxidation of vanadium and melting. There are no particular restrictions on the inert gas used in the fifth step, and examples include nitrogen gas, helium gas, and argon gas.

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

In the fifth step, the vanadium phosphate lithium carbon composite obtained by calcination may be subjected to multiple calcinations, if necessary.

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, if necessary, an Me source (Me represents a metal element other than V with an atomic number of 11 or greater 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 the Me element.

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

Me in the Me source is a metal element or transition metal element with an atomic number of 11 or higher 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, Me in the Me source preferably contains Ti and/or Al. Furthermore, particularly preferred elements containing Ti and/or Al are selected from Ti, Al, Ti and Al, Ti and Y, Ti and Mg, Al and Y, and Al and Mg.

Examples of Me sources include oxides, hydroxides, halides, carbonates, nitrates, phosphates, biphosphates, and organic acid salts containing Me. The Me source can be incorporated into the reduction reaction slurry in the first step by adding the Me source during the first step or before the addition of the second carboxylic acid in the second step. The Me source may be present in the reduction reaction slurry in a dissolved form 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. As the Me source, one containing a Ti source and/or an Al source is preferable in terms of further improving battery performance.
In addition, the method of incorporating the Me source into the liquid-like raw material solution in the third step may involve adding the Me source during the third step or before the spray drying treatment in the fourth step. Alternatively, the Me source may be incorporated into a solution in which the lithium source is dissolved in water or a suspension of the lithium source in which water is used as a dispersion solvent, and the resulting solution may be added to the reduction reaction preparation solution in the third step.
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.

Furthermore, when a Me source is mixed, the amount of Me source mixed will depend on the type of Me element to be doped, but in most cases, the amount mixed is such that the molar ratio of the sum 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.

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, just like phosphoric acid. Therefore, when a phosphate or biphosphate is used as the Me source, it is preferable to prepare the amount of phosphate or biphosphate mixed so that the molar ratio of the total P atoms resulting from the phosphate and biphosphate and the P atoms resulting from the phosphoric acid in the first step falls within the range of the molar ratio of the total V atoms and Me atoms to the P atoms in the reaction precursor ((Me + V)/P).

In the method for producing the lithium vanadium phosphate carbon composite of the present invention, the resulting lithium vanadium phosphate may be crushed or pulverized as needed, and may also be classified.

The vanadium phosphate lithium carbon composite obtained by the method for producing a vanadium phosphate lithium carbon composite of the present invention is a vanadium phosphate lithium carbon composite containing lithium vanadium phosphate of high purity as measured by X-ray diffraction. The carbon contained in the vanadium phosphate lithium carbon composite is conductive carbon. For this reason, the amount of carbon contained in the vanadium phosphate lithium carbon composite obtained by the method for producing a vanadium phosphate lithium carbon composite of the present invention is preferably 1.0 to 20.0 mass %, and 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 as is.

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

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

The vanadium phosphate lithium carbon composite obtained by the method for producing a vanadium phosphate lithium carbon composite of the present invention preferably has a particle appearance observed with a scanning electron microscope (SEM) such that "lithium vanadium phosphate nanoparticles are dispersed in carbon."
Although the internal structure of the particles of the lithium vanadium phosphate carbon composite obtained by the production method of the present invention is not clear, the present inventors believe that either nanoparticles of lithium vanadium phosphate are present dispersed in the carbon even within the particles, or that primary particles of lithium vanadium phosphate are aggregated to form secondary particles, i.e., primary particles of lithium vanadium phosphate are aggregated 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 in the first step. This allows the raw material mixed solution obtained in the third step to be a stable solution to the extent that no precipitates are 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 free of visually observable precipitates, i.e., in a state in which the lithium vanadium phosphate raw material is dissolved in a solvent. Therefore, the reaction precursor obtained in the fourth step contains the lithium vanadium phosphate raw material in a fine state. Then, by calcining the reaction precursor containing the lithium vanadium phosphate raw material in a fine state in the fifth step, the finely divided lithium vanadium phosphate raw material is converted into 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, as observed by 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.

The presence of lithium vanadium phosphate nanoparticles dispersed within the carbon facilitates lithium desorption and insertion during charging and discharging when used as a positive electrode material. Therefore, the lithium vanadium phosphate nanoparticles present in the carbon composite obtained by the method for producing a lithium vanadium phosphate carbon composite of the present invention have excellent battery performance.

On the other hand, if precipitates of the lithium vanadium phosphate raw materials are present in the raw material mixture subjected to the spray drying process, the lithium vanadium phosphate carbon composite obtained by the spray drying process and firing will be a composite of large plate-like crystals of lithium vanadium phosphate and carbon.

The vanadium phosphate lithium carbon composite obtained by the method for producing a vanadium phosphate lithium carbon composite of the present invention can be used as a positive electrode material in lithium secondary batteries, electrochemical capacitors, and hybrid capacitors that combine the advantages of both lithium secondary batteries and electric double-layer capacitors.

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 the amount required for the reduction of vanadium pentoxide to intentionally prepare a reduction reaction slurry. A second carboxylic acid is then added to this reduction reaction slurry to chelate the mixture, yielding a reduction reaction preparation solution. This process allows for 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 industrially advantageous because it is a method for producing nanoparticles using simple steps.

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 the 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, increasing the amount of carboxylic acid added is considered as a method for suppressing the formation of precipitates in the resulting reduction reaction solution. However, 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.

The vanadium phosphate lithium carbon composite obtained by the method for producing a vanadium phosphate lithium carbon composite of the present invention is suitable for use as a positive electrode material. In other words, the method for producing a vanadium phosphate lithium carbon composite of the present invention can be used to produce a positive electrode material containing the vanadium phosphate lithium carbon composite (A) or the vanadium phosphate lithium carbon composite (B) described below.

Positive electrode materials containing the lithium vanadium phosphate carbon composite obtained by the method for producing the lithium vanadium phosphate carbon composite of the present invention include:
a cathode material comprising lithium vanadium phosphate carbon composite particles,
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) is 5 μm or more and 40 μm or less,
A positive electrode material having a carbon content of 7.6 to 20 mass %, preferably 8 to 15 mass %, calculated as C atoms (hereinafter also referred to as the positive electrode material of the first embodiment of the present invention) is particularly useful as a positive electrode material for an electricity storage device.
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)."

Examples of the power storage device according to the present invention include lithium secondary batteries, electrochemical capacitors, and hybrid capacitors that utilize the advantages of both lithium secondary batteries and electric double-layer capacitors.

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

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 also 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). In other words, 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.

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

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.

The carbon content of the positive electrode material of the first embodiment of the present invention is 7.6 to 20 mass %, and 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 calcination in the fifth step, remaining 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 second carboxylic acid so that the carbon remaining in the lithium vanadium phosphate carbon composite that produces it is 7.6 to 20 mass %, calculated as C atoms.

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 with no depressions having a diameter (d) of 500 to 7000 nm, particles with only one depression having a diameter (d) of 500 to 7000 nm, and particles with depressions, but with a diameter (d) of less than 500 nm or more than 7000 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 lithium vanadium phosphate carbon composite particles (A') include reactive precursor particles with depressions on the particle surface whose particle shape has changed or are broken fragments due to firing, reactive precursor particles without depressions on the particle surface whose particle shape has changed or are broken fragments due to firing, or particles that retain the particle shape of the reactive precursor particles.

In the present invention, the mechanism for producing the lithium vanadium phosphate carbon composite particles (A) is not clear, but many of the amorphous reaction precursor particles obtained in the fourth step have pits on their particle surfaces. When a mixture containing these reaction precursor particles with pits on their surface is fired in an inert atmosphere in the fifth step, those with pits on their surface will become lithium vanadium phosphate carbon composite particles (A') in a state where their shape is partially deformed or destroyed by firing, but those that maintain their particle shape and have pits on their surface will become lithium vanadium phosphate carbon composite particles (A).
Lithium vanadium phosphate carbon composite particles (A') that have been deformed or destroyed by calcination have many voids and other imperfections within the reactive precursor particles. Furthermore, while a large amount of gas is generated within the reactive precursor particles during calcination, the particles lack the durability to maintain their shape against the large amount of gas generated. This is thought to be why the particle shape is deformed or destroyed by calcination. On the other hand, lithium vanadium phosphate carbon composite particles (A) that have retained their shape after calcination have the components uniformly dispersed at the molecular level as reactive precursor particles, are more densely packed, have fewer voids, and are durable enough to maintain their shape against the large amount of gas generated within the reactive precursor particles during calcination. Even after calcination, the reactive precursor particle shape is maintained. Therefore, the inventors speculate that the lithium vanadium phosphate carbon composite particles (A) are denser composites of carbon and lithium vanadium phosphate nanoparticles than the lithium vanadium phosphate carbon composite particles (A').

The positive electrode material of the first embodiment of the present invention, which contains 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 that, 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 lithium vanadium phosphate carbon composite particles (A), has a higher carbon content than conventional materials, and because the lithium vanadium phosphate nanoparticles are contained in dense carbon, the particle surfaces of the lithium vanadium phosphate nanoparticles are covered with dense carbon, facilitating lithium desorption and insertion during charge and discharge.

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 the lithium vanadium phosphate carbon composite particles (A) has the following properties:
A positive electrode material comprising irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing nanoparticles of lithium vanadium phosphate,
The lithium vanadium phosphate carbon composite particles (B) have an average particle size of 4 μm or more and 20 μm or less,
The carbon content of the positive electrode material is 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).
That is, the positive electrode material of the second embodiment of the present invention contains lithium vanadium phosphate nanoparticles and contains irregularly pulverized lithium vanadium phosphate carbon composite particles (B) having a particle diameter of 4 μm or more and 20 μm or less (hereinafter, may be referred to as "lithium vanadium phosphate carbon composite particles (B)").
In the present invention, the term "irregularly pulverized" means particles that are pulverized and do not have a uniform shape.

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) under a scanning electron microscope.

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.

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 positive electrode material of the second embodiment of the present invention, in the above-described method for producing a lithium vanadium phosphate carbon composite of the present invention, 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 amount of the second carboxylic acid is adjusted so that the carbon remaining in the lithium vanadium phosphate carbon composite to be produced is 7.6 to 20 mass % in terms of C atoms, and steps 1 to 5 are carried out, followed by a pulverization treatment in step 6, whereby the positive electrode material of the second embodiment of the present invention can be produced.

The grinding treatment in the sixth step can be carried out dry by mechanical means. Examples of grinding equipment include known dry grinding equipment such as high-speed mixers, super mixers, turbosphere mixers, Eirich mixers, Henschel mixers, Nauta mixers, ribbon blenders, V-type mixers, conical blenders, jet mills, cosmomizers, paint shakers, bead mills, jet mills, and ball mills. After completion of the sixth step, crushing, classification, etc. can be carried out as necessary.

The vanadium phosphate lithium carbon composite obtained after the sixth step contains vanadium phosphate lithium carbon composite particles (B) and other non-irregularly pulverized vanadium phosphate lithium carbon composite particles (B'). Examples of 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').

In the positive electrode material of the second embodiment of the present invention, those containing lithium vanadium phosphate nanoparticles originating from the vanadium phosphate lithium carbon composite particles (A) and part of the skeleton of a hard coating layer made of dense carbon, i.e., the vanadium phosphate lithium carbon composite (B), can be confirmed, for example, by observing the positive electrode material with a scanning electron microscope at 2,000 to 100,000 magnifications. Those containing part of the skeleton of the hard coating layer can be confirmed by the presence of dense plate-like portions on the particle surface where no grain boundaries or pores are observed. Note that, because this observation with a scanning electron microscope is two-dimensional, dense plate-like portions cannot be confirmed in all vanadium phosphate lithium carbon composite particles (B).

Furthermore, in 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, from the viewpoint of improving the rate characteristics of an electricity storage device using the positive electrode material, it is preferable that the ratio (BET/ _D50 ) of the BET specific surface area ( ^m2 /g) to the average particle diameter ( _D50 : μm) measured by a laser diffraction scattering method is 2 or more, preferably 2 or more and 18 or less.
In particular, it is more preferable that the positive electrode material of the first embodiment of the present invention has a ratio (BET/ _D50 ) of the BET specific surface area ( ^m2 /g) to the average particle diameter ( _D50 : μm) measured by laser diffraction scattering method of 2 or more and 5 or less, and it is more preferable that the positive electrode material of the second embodiment of the present invention has a ratio (BET/ _D50 ) of the BET specific surface area ( ^m2 /g) to the average particle diameter ( _D50 : μm) measured by laser diffraction scattering method of more than 5 and 18 or less.

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 phosphate lithium carbon composite doped with the element 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 improving the cycle characteristics of an electricity storage device using the positive electrode material in a high-temperature environment. Note that "a linear expansion coefficient of 5 ppm/K or less" refers to a linear expansion coefficient of 0 to 5 ppm/K or a negative linear thermal expansion coefficient.

In addition, 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 ground 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 compacting 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 from the second measurement is taken as the thermal expansion coefficient of the positive electrode material.

The Me element 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. Of these, Al is particularly preferred as Me, as it can improve cycle characteristics, especially in high-temperature environments.

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 percent (Me/V) of Me relative to V in the lithium vanadium phosphate, from the viewpoint of achieving a high capacity retention rate in an energy storage device using the lithium vanadium phosphate carbon composite as a positive electrode material.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

Example 1
<First step>
A 10 L container was charged with 2.8 L of ion-exchanged water, 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, to obtain a greenish-blue reduction reaction slurry.
Since the reduction reaction was exothermic, the temperature of the reaction system rose from 25°C to 45°C.

<Second process>
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.

<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 liquid over 30 minutes at 40° C., and stirring was continued for 60 minutes to obtain a raw material mixture solution in the form of a deep blue solution.
Furthermore, when the raw material mixed solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. After 24 hours, the raw material mixed solution was diluted 100 times and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), which was 0.01 NTU.

<Fourth step>
Next, 24 hours after the completion of the third step, the raw material mixture 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.

<Fifth process>
The resulting 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 resulting lithium vanadium phosphate carbon composite revealed that it was a single-phase lithium vanadium phosphate (see Figure 3). This was designated the lithium vanadium phosphate carbon composite sample. Figure 4 shows an SEM photograph of the resulting 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 contained lithium vanadium phosphate (LVP) nanoparticles with an average primary particle size of 30 nm dispersed in carbon.

The average primary particle size was determined by observing the particles with a scanning electron microscope at 100,000 magnifications and calculating the average value of 100 arbitrarily selected lithium vanadium phosphate particles.
The obtained lithium vanadium phosphate carbon composite sample had an average particle size ( _D50 ) of 15.4 μm and a _D90 of 28.9 μm as measured by a laser diffraction scattering method. The BET specific surface area was 50.3 ^m2 /g. The ratio of the BET specific surface area to the _D50 (BET/ _D50 ) was 3.3.
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 10.2 mass%.

(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), 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, so 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.
Next, the entire amount of the lithium carbonate-containing suspension was 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 mixed solution. When the raw material mixed solution was held under stirring at 25°C for 24 hours, it became a slurry and a green precipitate was observed.

The raw material mixed solution in which this slurry-like green precipitate was observed was diluted 100 times and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.) to find that it was _189.6 NTU. X-ray diffraction analysis of the precipitate confirmed that it was _LiVOPO4.2H2O , and observation with a scanning electron microscope revealed that the precipitate was formed by aggregation of plate-like primary particles to form secondary particles.
The raw material mixture 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 then 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 as a lithium vanadium phosphate carbon composite sample. An SEM photograph of the resulting lithium vanadium phosphate carbon composite sample is shown in Figure 6. The lithium vanadium phosphate carbon composite sample obtained from Figure 6 contained carbon present on the particle surfaces of plate-like lithium vanadium phosphate particles with an average primary particle size of 0.25 µm.

The average primary particle size was determined by observing the particles with a scanning electron microscope at 100,000 magnifications and calculating the average value of 100 arbitrarily selected lithium vanadium phosphate particles.
The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 18.4 μm, and the _D90 was 30.5 μm. The BET specific surface area was 21.8 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 1.2.
Furthermore, 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 %.

(Comparative Example 2)
A lithium vanadium phosphate carbon composite was produced in the same manner as in Example 1 except that the second step was not carried out, and this was used as a lithium vanadium phosphate 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 solution became slurry-like and a green precipitate was observed. The raw material mixture solution in which the slurry-like green precipitate was observed was diluted 100 times and the 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 that it was _{LiVOPO4.2H2O .} Observation with a scanning electron microscope revealed that the precipitate was composed of agglomerates of 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.

The average primary particle size was determined by observing the particles with a scanning electron microscope at 100,000 magnifications and averaging the diameter of 100 arbitrarily selected lithium vanadium phosphate particles.
The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 20.1 μm, and the _D90 was 33.5 μm. The BET specific surface area was 17.9 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 0.9.
Furthermore, 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 %.

In Table 1, the amount of the first carboxylic acid added is expressed as 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 is expressed as 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 holding the raw material mixed solution at 25°C for 24 hours under stirring, and the case where no precipitate was observed with the naked eye was evaluated as "Good", and the case where precipitate was observed and the solution became a slurry was evaluated as "Poor".

Example 2
<First step>
A 10 L container was charged with 2.8 L of ion-exchanged water, and to this was added, in this order, at room temperature (25°C), 500 g of vanadium pentoxide, 635.5 g of citric acid monohydrate, 173.6 g of 50 wt % aluminum phosphate monobasic solution, and 898.8 g of 85 wt % phosphoric acid. A reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, to obtain a greenish-blue reduction reaction slurry.
The reduction reaction was exothermic, and the temperature of the reaction system rose from 25°C to 45°C.

<Second process>
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.

<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 liquid over 30 minutes at 40° C., and stirring was continued for 60 minutes to obtain a raw material mixture solution in the form of a deep blue solution.
Furthermore, when the raw material mixed solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. After 24 hours, the raw material mixed solution was diluted 100 times and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), which was 0.1 NTU.

<Fourth to fifth steps>
The fourth step and subsequent steps were carried out in the same manner as in Example 1. X-ray diffraction analysis of the obtained lithium vanadium phosphate carbon composite revealed that the diffraction peak pattern matched that of the lithium _vanadium phosphate carbon composite of Example 1, confirming that the composite was a single-phase lithium _{vanadium phosphate} (Li3V1.9Al0.1( _PO4 ) ₃ ). This was used as a lithium vanadium phosphate carbon composite sample. Furthermore, measurement of the average primary particle size using a scanning electron microscope in the same manner as in Example 1 confirmed that the composite was lithium vanadium phosphate (LVP) with an average primary particle size of 30 nm, with nanoparticles dispersed in carbon.

The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 14.4 μm, and the _D90 was 26.6 μm. The BET specific surface area was 45.2 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 3.1.
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% by mass.

In Table 2, the amount of the first carboxylic acid added is expressed as 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 is expressed as 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 holding the raw material mixed solution at 25°C for 24 hours under stirring, and the case where no precipitate was observed with the naked eye was evaluated as "Good", and the case where precipitate was observed and the solution became a slurry was evaluated as "Poor".

Example 3
<First step>
A 10 L container was charged with 2.8 L of ion-exchanged water, 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 a reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, to obtain a greenish-blue reduction reaction slurry.
Since the reduction reaction was exothermic, the temperature of the reaction system rose from 25°C to 45°C.

<Second process>
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.

<Third step>
A suspension containing lithium carbonate and magnesium hydroxide was prepared by adding 327.6 g of lithium carbonate and 10.3 g of magnesium hydroxide to 1.5 L of ion-exchanged water.
Next, the entire amount of the lithium carbonate and magnesium hydroxide-containing suspension was added to the reduction reaction preparation liquid over 30 minutes at 40° C., and stirring was continued for 60 minutes to obtain a raw material mixture solution in the form of a deep blue solution.
Furthermore, when the raw material mixed solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. After 24 hours, the raw material mixed solution was diluted 100 times and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), which was 0.1 NTU.

<Fourth to fifth steps>
The fourth and subsequent steps were carried out in the same manner as in Example 1. X-ray diffraction analysis of the obtained lithium vanadium phosphate carbon composite confirmed that it was a single-phase lithium vanadium phosphate ( _{Li3V1.86Al0.10Mg0.06} (PO4) ₃ ) because the _diffraction peak pattern matched that of _{the lithium} vanadium phosphate carbon composite of Example _1. This was used as a lithium vanadium phosphate carbon composite sample. Furthermore, measurement of the average primary particle size using a scanning electron microscope in the same manner as in Example 1 confirmed that it was lithium vanadium phosphate (LVP) with an average primary particle size of 30 nm, with nanoparticles dispersed in carbon.

The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 15.3 μm, and the _D90 was 29.1 μm. The BET specific surface area was 49.9 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 3.3.
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.

Example 4
<First step>
A 10 L container was charged with 2.8 L of ion-exchanged water, 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 a reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, to obtain a greenish-blue reduction reaction slurry.
Since the reduction reaction was exothermic, the temperature of the reaction system rose from 25°C to 45°C.

<Second process>
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.

<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.
Next, the entire amount of the lithium carbonate and yttrium acetate-containing suspension was added to the reduction reaction preparation liquid over 30 minutes at 40° C., and stirring was continued for 60 minutes to obtain a raw material mixture solution in the form of a deep blue solution.
Furthermore, when the raw material mixed solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. After 24 hours, the raw material mixed solution was diluted 100 times and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), which was 0.1 NTU.

<Fourth to 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, and the diffraction peak pattern matched that of the lithium _vanadium phosphate carbon composite of Example 1, confirming that the composite was a single-phase _{lithium vanadium} phosphate ( _{Li3V1.85Al0.10Y0.05} ( _{PO4)3 )} .
This was used as a lithium vanadium phosphate carbon composite sample. When 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) with an average primary particle size of 30 nm, and that nanoparticles were dispersed in carbon.

The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 16.5 μm, and the _D90 was 30.2 μm. The BET specific surface area was 51.1 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 3.1.
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.

Example 5
<First step>
A 10 L container was charged with 2.8 L of ion-exchanged water, 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 a reduction reaction was carried out at 25 to 45°C for 40 minutes with stirring, to obtain a greenish-blue reduction reaction slurry.
Since the reduction reaction was exothermic, the temperature of the reaction system rose from 25°C to 45°C.

<Second process>
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.

<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 liquid over 30 minutes at 40° C., and stirring was continued for 60 minutes to obtain a raw material mixture solution in the form of a deep blue solution.
Furthermore, when the raw material mixed solution was kept under stirring at 25°C for 24 hours, no precipitate was observed and the solution was stable. After 24 hours, the raw material mixed solution was diluted 100 times and the turbidity was measured using a turbidity meter (TB250WL manufactured by Tintometer Co., Ltd.), which was 0.1 NTU.

<Fourth to fifth steps>
The fourth step and subsequent steps were carried out in the same manner as in Example 1. X-ray diffraction analysis of the obtained lithium vanadium phosphate carbon composite revealed that the diffraction peak pattern matched that of the lithium _vanadium phosphate carbon composite of Example 1, confirming that it was single-phase lithium _{vanadium phosphate} (Li3V1.7Al0.3( _PO4 ) ₃ ). This was used as a lithium vanadium phosphate carbon composite sample. Furthermore, measurement of the average primary particle size using a scanning electron microscope in the same manner as in Example 1 confirmed that it was lithium vanadium phosphate (LVP) with an average primary particle size of 30 nm, with nanoparticles dispersed in carbon.

The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 14.8 μm, and the _D90 was 25.9 μm. The BET specific surface area was 58.5 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 4.0.
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.

In Table 5, the amount of the first carboxylic acid added is expressed as 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 is expressed as 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 holding the raw material mixed solution at 25°C for 24 hours under stirring, and the case where no precipitate was observed with the naked eye was evaluated as "Good", and the case where precipitate was observed and the solution became a slurry was evaluated as "Poor".

<Evaluation of physical properties of lithium vanadium phosphate carbon composite sample 2>
For the lithium vanadium phosphate carbon composite samples obtained in the examples, the content of lithium vanadium phosphate carbon composite particles (A), the average particle diameter (average secondary particle diameter), and the linear expansion coefficient were measured as follows.
FIG. 7 shows an SEM photograph (1000x magnification) of the lithium vanadium phosphate carbon composite sample obtained in Example 1.

(Evaluation of Average Particle Size of Lithium Vanadium Phosphate Carbon Composite Particles (A))
Observation was performed under a scanning electron microscope at 1000x magnification, and the average value 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 used.

(Evaluation of the content of lithium vanadium phosphate carbon composite particles (A))
Observation was performed under a scanning electron microscope at 1000x magnification, and 200 particles (secondary particles) (t1) randomly selected from the sample 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 on their particle surfaces two or more depressions whose opening diameters (d) were 500 to 7000 nm was determined, and the content ((t2/t1)×100) converted into the number of particles was calculated.

(Measurement of linear expansion coefficient)
The lithium vanadium phosphate carbon composite samples obtained in Examples 1, 2 and 5 were measured for linear expansion coefficient at 25 to 70° C. as follows.

(Production of powder compact)
1.00 g of the sample was pulverized and mixed in a mortar for 3 minutes, and then 0.15 g was weighed out and filled into a φ6 mm mold. The mixture was then molded at a pressure of 10 MPa using a hand press to produce a powder compact.
The thermal expansion coefficient of the produced powder compacts between 25 and 70° C. was evaluated as follows.

(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 of the second repeated measurement between 25°C and 70°C was taken as the thermal expansion coefficient of the sample.

Note: "-" in the table indicates that the measurement has not been carried out.

(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 fed into a spray dryer with the outlet temperature set to 120°C.
A reaction precursor was obtained.
The resulting reaction precursor was placed in a mullite sagger and calcined at 900°C for 12 hours in a nitrogen atmosphere. The calcined product was pulverized using a jet mill to obtain a lithium vanadium phosphate sample. X-ray diffraction analysis of the resulting lithium vanadium phosphate sample confirmed that it was a single-phase lithium vanadium phosphate (average particle size (D ₅₀ ): 2.3 μm). The residual carbon content of the resulting lithium vanadium phosphate 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 0.1% by mass.

<Battery performance test 1>
(I) Preparation of a lithium secondary battery;
A positive electrode agent was prepared by mixing 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, and this was dispersed in N-methyl-2-pyrrolidinone to prepare a kneaded paste. The obtained kneaded paste was applied to aluminum foil, dried, pressed, and punched into a disk with a diameter of 15 mm to obtain a positive electrode plate.
A lithium secondary battery was fabricated using this positive electrode plate, a separator, a negative electrode, a positive electrode, a current collector, mounting hardware, external terminals, an electrolyte, etc. Metallic lithium foil was used for the negative electrode, and 1 mol of _LiPF6 dissolved in 1 liter of a 1:1 kneaded solution of ethylene carbonate and methyl ethyl carbonate was used as the electrolyte.

(2) Evaluation of Battery Performance The fabricated lithium secondary battery was operated under the following conditions, and the battery performance was evaluated.
<Evaluation of cycle characteristics>
The battery was charged to 4.2 V at 0.5 C, followed by constant-current constant-voltage (CCCV) charging for a total charge time of 5 hours, during which the battery was held at 4.2 V. This was followed by constant-current (CCCV) discharging at 0.1 C to 2.0 V. This cycle 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.

Capacity retention rate (%) = ((20th cycle discharge capacity) / (1st cycle discharge capacity)) x 100

Example 6
(Sixth step)
The lithium vanadium phosphate carbon composite sample obtained in Example 1 was pulverized in a jet mill to obtain a lithium vanadium phosphate carbon composite sample.
The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 4.5 μm, and _D90 was 10.1 μm. The BET specific surface area was 51.2 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 11.4.
The residual carbon amount in 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 amount was found to be 10.3 mass%.

Example 7
(Sixth step)
The lithium vanadium phosphate carbon composite sample obtained in Example 5 was pulverized in a jet mill to obtain a lithium vanadium phosphate carbon composite sample.
The average particle size ( _D50 ) of the obtained lithium vanadium phosphate carbon composite sample measured by a laser diffraction scattering method was 4.1 μm, and _D90 was 10.4 μm. The BET specific surface area was 58.1 ^m2 /g. The ratio of the BET specific surface area to _{the D50} (BET/ _D50 ) was 14.2.
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%.

<Evaluation of physical properties of lithium vanadium phosphate carbon composite sample 3>
The lithium vanadium phosphate carbon composite particles (B) content, average particle diameter (average secondary particle diameter), and linear expansion coefficient were measured as follows for the lithium vanadium phosphate carbon composite samples obtained in Examples 6 and 7. The results are shown in Table 10.
FIG. 8 shows SEM photographs (2000x and 20000x) of the lithium vanadium phosphate carbon composite sample obtained in Example 6.

(Evaluation of Average Particle Size of Lithium Vanadium Phosphate Carbon Composite Particles (B))
Observation was performed under a scanning electron microscope at 2000 magnifications, and the average value of 200 randomly selected lithium vanadium phosphate carbon composite particles (B) was calculated.

(Evaluation of the content of lithium vanadium phosphate carbon composite particles (B))
Observation was performed 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 sampled. The number (t4) of lithium vanadium phosphate carbon composite particles (B) was determined, and the content ((t4/t3)×100) converted into the number was calculated.

(Measurement of linear expansion coefficient)
The linear expansion coefficient was measured in the same manner as in Examples 1, 2 and 5.

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

<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, and subsequently charged by constant current constant voltage (CCCV) charging in which the battery was held at 4 V for a total charging time of 3.5 hours. Thereafter, the batteries were 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 20th cycle)/(discharge capacity at 1st cycle))×100

From Table 12, it can be seen that when Example 1 is compared with Example 2 and Example 5, the cycle characteristics in high temperature environments (40°C, 55°C) are improved in Example 2 and Example 5, which are doped with Al, compared with Example 1, which is not doped with Al.
Furthermore, when Example 6 and Example 7 are compared, it is found that Example 7, which is doped with Al, has improved cycle characteristics in high temperature environments (40°C, 55°C) compared to Example 6, which is not doped with Al.

Claims (22)

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 vanadium pentoxide to 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 solution;
a third step of adding a lithium source to the reduction reaction preparation solution to prepare a raw material mixture solution in a liquid state;
a fourth step of spray-drying the raw material mixture solution to obtain a reaction precursor;
a fifth step of calcining the reaction precursor at 500 to 1300°C in an inert gas atmosphere or a reducing atmosphere to obtain a lithium vanadium phosphate carbon composite;
1. A method for producing a lithium vanadium phosphate carbon composite, comprising: The method for producing lithium vanadium phosphate carbon composite according to claim 1, characterized in that in the first step, the reduction reaction is carried out at a temperature below 60°C. The method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, characterized in that the second carboxylic acid is a hydroxycarboxylic acid from which carbon is isolated by heating. 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. The method for producing a lithium vanadium phosphate carbon composite according to claim 4, characterized in that the second carboxylic acid is one or two selected from gluconic acid and malic acid. The method for producing a lithium vanadium phosphate carbon composite according to claim 1 or 2, characterized in that the first carboxylic acid is gluconic acid. The method for producing a lithium vanadium phosphate carbon composite according to claim 6, characterized in that the second carboxylic acid is malic acid. The 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. The 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. The method for producing lithium vanadium phosphate carbon composite according to claim 1 or 2, further comprising adding an Me source (Me represents a metal element other than V with an atomic number of 11 or higher or a transition metal element) to the reduction reaction slurry in the first step and/or the liquid-like raw material solution in the third step. The method for producing a lithium vanadium phosphate carbon composite according to claim 10, characterized in that the Me source contains at least one selected from a Ti source and an Al source. The 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. a cathode material comprising lithium vanadium phosphate carbon composite particles,
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) is 5 μm or more and 40 μm or less,
the carbon content of the positive electrode material is 7.6 to 20 mass% in terms of C atoms;
A cathode material characterized by: 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 multiple depressions on the particle surface among particles of 5 μm or more and 40 μm or less in the positive electrode material is 10% or more. The positive electrode material described in claim 13, characterized in that the lithium vanadium phosphate carbon composite particles (A) containing lithium vanadium phosphate nanoparticles having multiple depressions on the particle surface contain doped Me (Me represents a metal element or transition metal element other than V having an atomic number of 11 or higher). A positive electrode material comprising irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing nanoparticles of lithium vanadium phosphate,
The lithium vanadium phosphate carbon composite particles (B) have an average particle size of 4 μm or more and 20 μm or less,
the carbon content of the positive electrode material is 7.6 to 20 mass% in terms of C atoms;
A cathode material characterized by: The positive electrode material according to claim 16, characterized in that, when observed with a scanning electron microscope, the proportion of irregularly pulverized lithium vanadium phosphate carbon composite particles (B) containing lithium vanadium phosphate nanoparticles is 30% or more of the particles in the positive electrode material having a size of 4 μm or more and 20 μm or less. The positive electrode material described in 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 higher). The BET specific surface area (m) relative to the average particle diameter (D ₅₀ : μm) measured by the laser diffraction scattering method
20. The positive electrode material according to claim 13, wherein the ratio ( ^BET /D ₅₀ ) of the total carbon content of the positive electrode material to the total carbon content of the positive electrode material (g) is 2 or more. 19. The positive electrode material according to claim 16, wherein the ratio (BET/ _D50 ) of the BET specific surface area ( ^m2 /g) to the average particle diameter ( _D50 : μm) measured by a laser diffraction scattering method is 2 or more. The positive electrode material described in 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. An electricity storage device using the positive electrode material according to any one of claims 13 to 21.