Production method of lithium vanadium phosphate

Abstract

To provide a method for industrially advantageous production of lithium vanadium phosphate which is highly pure from an X-ray diffractometric point of view.SOLUTION: A method for producing lithium vanadium phosphate having a NASICON structure includes: a first step of adding vanadium pentoxide, phosphoric acid, and citric acid to an aqueous solvent and performing a reduction reaction of the vanadium pentoxide at less than 60°C to prepare a reduction reaction slurry; then, a second step of adding a lithium source to the reduction reaction slurry to prepare a raw material mixed slurry including a lithium-vanadium-phosphorus complex oxide; then, a third step of performing wet grinding treatment of the raw material mixed slurry with a media mill to prepare a wet grinding treatment slurry; then, a fourth step of performing spray-drying treatment of the wet grinding-treated slurry to obtain a reaction precursor; and then, a fifth step of firing the reaction precursor in an inert gas atmosphere or a reduction atmosphere at 500 to 1300°C to obtain lithium vanadium phosphate.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing lithium vanadium phosphate, which is useful as a cathode material for lithium secondary batteries, all-solid-state batteries, electrochemical capacitors, and the like. [Background technology]

[0002] Lithium-ion batteries are used in mobile 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 resource issues with Co, LiMnO2 and LiNiO 2、 Development of Li-Ni-Mn-Co systems is also underway.

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

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

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

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

[0007] [Patent Document 1] International Publication No. 2012 / 043367 Brochure [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-160107 [Patent Document 3] WO 2018 / 142082, paragraph 0121 [Patent Document 4] JP 2011-198657 A, paragraph 0044 Summary of the Invention [Problem to be solved by the invention]

[0008] Lithium vanadium phosphate has attracted attention as a positive electrode active material for lithium secondary batteries for automotive applications, all-solid-state batteries, and the like, due to its high safety even at high temperatures. To commercialize this compound, it is desirable to develop a method for producing lithium vanadium phosphate at even lower cost by, for example, simplifying the process.

[0009] The present inventors have been investigating ways to simplify the steps of the methods for producing lithium vanadium phosphate described in Patent Documents 1 and 2, and have found that by performing a specific step using citric acid instead of a reducing sugar in the preparation of a reaction precursor, a reaction precursor having excellent reactivity can be obtained without actively carrying out a heat treatment, and that by calcining the reaction precursor, lithium vanadium phosphate having high purity as measured by X-ray diffraction can be obtained, which has led to the completion of the present invention.

[0010] That is, an object of the present invention is to provide an industrially advantageous method for producing lithium vanadium phosphate having high purity as measured by X-ray diffraction. [Means for solving the problem]

[0011] The above problems are solved by the present invention described below. That is, the present invention (1) is a method for producing lithium vanadium phosphate having a NASICON structure, a first step of adding vanadium pentoxide, phosphoric acid, and citric acid to an aqueous solvent and carrying out a reduction reaction of vanadium pentoxide to prepare a reduction reaction slurry; Next, a lithium source is added to the reduction reaction slurry to form a compound represented by the following general formula (1): LiVOPO4·xH2O (1) (wherein x is 0 to 2) A second step of preparing a raw material mixed slurry containing a lithium vanadium phosphorus composite oxide represented by the formula: A third step of wet-pulverizing the raw material mixed slurry in a media mill to prepare a wet-pulverized slurry; A fourth step of spray-drying the wet-milled slurry 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 lithium vanadium phosphate; The method for producing lithium vanadium phosphate is characterized by comprising the steps of:

[0012] The present invention (2) is the method for producing lithium vanadium phosphate according to (1), characterized in that the amount of citric acid added in the first step is 0.9 to 6.0 in terms of the molar ratio (C / V) of V atoms in vanadium pentoxide to C atoms in citric acid.

[0013] The present invention (3) is the method for producing lithium vanadium phosphate according to (1) or (2), characterized in that the average particle size of the solid content in the wet-pulverized slurry in the third step is 2.0 μm or less.

[0014] The present invention (4) is a method for producing lithium vanadium phosphate according to (1) or (2), characterized in that in the first step, a Me source (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element) is further contained in the reduction reaction slurry.

[0015] The present invention (5) is a method for producing lithium vanadium phosphate according to (4), characterized in that the Me source is selected from a Ti source and an Al source. [Effects of the Invention]

[0016] According to the method for producing lithium vanadium phosphate of the present invention, lithium vanadium phosphate having high purity as measured by X-ray diffraction can be obtained in an industrially advantageous manner. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is an X-ray diffraction pattern of the reaction precursor obtained in Example 1. [Figure 2] 1 is an SEM photograph of the reaction precursor obtained in Example 1. [Figure 3] 1 is an X-ray diffraction diagram of lithium vanadium phosphate obtained in Example 1. [Figure 4] SEM photographs of the lithium vanadium phosphate sample obtained in Example 1. (a) Magnification: 400x, (b) Magnification: 50,000x DETAILED DESCRIPTION OF THE INVENTION

[0018] 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 lithium vanadium phosphate having a NASICON structure, A first step is to add vanadium pentoxide, phosphoric acid, and citric acid to an aqueous solvent and carry out a reduction reaction of vanadium pentoxide at a temperature of less than 60°C to prepare a reduction reaction slurry; a second step is to add a lithium source to the reduction reaction slurry to produce a compound represented by the following general formula (1): LiVOPO4·xH2O (1) (wherein x is 0 to 2) a third step of wet-pulverizing the raw material mixed slurry using a media mill to prepare a wet-pulverized slurry; a fourth step of spray-drying the wet-pulverized slurry to obtain a reaction precursor; and a fifth step of firing the reaction precursor at 500 to 1300°C in an inert gas atmosphere or a reducing atmosphere to obtain lithium vanadium phosphate.

[0019] The method for producing lithium vanadium phosphate of the present invention is a method for producing lithium vanadium phosphate having a NASICON structure (hereinafter simply referred to as "lithium vanadium phosphate").

[0020] The lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention is lithium vanadium phosphate of high purity as measured by X-ray diffraction. In the present invention, lithium vanadium phosphate of high purity as measured by X-ray diffraction means that the lithium vanadium phosphate is detected as single-phase lithium vanadium phosphate when subjected to X-ray diffraction analysis, and contains carbon not detected by X-ray diffraction analysis, or does not contain carbon.

[0021] The lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention is represented by the following general formula (2): Li x V y (PO4)3(2) (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 (2) 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 needed.

[0022] In the general formula (2), 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 Me element to be doped may be one or more selected from Sr, Ba, Sc, Y, Hf, Ta, W, Ru, Os, Ag, Zn, Si, Ga, Ge, Sn, Bi, S, Se, Te, Cl, Br, I, Na, K, Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo, and Cu. Among these, the Me element is preferably Al and / or Ti.

[0023] The method for producing lithium vanadium phosphate of the present invention includes a first step, a second step, a third step, a fourth step, and a fifth step.

[0024] The first step is a step of adding vanadium pentoxide, phosphoric acid, and citric acid to an aqueous solvent, and carrying out a reduction reaction of vanadium pentoxide at a temperature of less than 60°C to prepare a reduction reaction slurry.

[0025] In conventional methods using reducing sugars as reducing agents, the reduction reaction of vanadium pentoxide usually needs to be carried out at temperatures between 60 and 100°C. However, by using citric acid as a reducing agent, the reduction reaction of vanadium pentoxide can be carried out at temperatures below 60°C.

[0026] In the present invention, the temperature of the reduction reaction can be 60°C or higher, but the temperature of the reduction reaction in the first step is specified to be less than 60°C from the viewpoint of industrial advantages over conventional methods in which the reduction reaction is carried out by heat treatment.

[0027] In the conventional method using citric acid, the chelating effect of citric acid on vanadium is utilized, and citric acid is used as a conductive carbon source for coating lithium vanadium phosphate. Therefore, compared to the present invention, citric acid is added in large excess over the amount necessary for reduction relative to the vanadium pentoxide raw material, and if necessary, heat treatment is carried out to temporarily prepare an aqueous solution in which the raw materials are dissolved, which differs from the first step of the present invention.

[0028] The present inventors have found that the reduction reaction slurry obtained in the first step is a mixture of vanadium pentoxide and phosphoric acid, which produce VOHPO4 or its hydrate in the presence of citric acid. Therefore, the reduction reaction slurry obtained in the first step contains VOHPO4 or its hydrate as solids, as well as citric acid and dihydrogen phosphate ions (H2PO4) which remain unconsumed in the reduction reaction. ― It is thought that these ions originating from phosphoric acid, such as phosphate ions, are dissolved in the aqueous solvent.

[0029] In the first step, the mixing amounts of vanadium pentoxide and phosphoric acid are preferably 0.50 to 0.80, and more preferably 0.55 to 0.75, in terms of the molar ratio (V / P) of V atoms in vanadium pentoxide to P atoms in phosphoric acid, since this makes it easier to obtain a final product that is a single-phase lithium vanadium phosphate in X-ray diffraction analysis.

[0030] The citric acid used in the first step promotes the reduction reaction of vanadium pentoxide and also makes the reduction reaction slurry obtained in the first step a slurry with a favorable viscosity that allows stirring. In addition, the citric acid remaining without being consumed in the reduction reaction is thermally decomposed in an inert or reducing atmosphere during the firing in the fifth step to isolate carbon, which serves as a component necessary for preventing the oxidation of vanadium. Furthermore, in this production method, the citric acid can also function as a component of a conductive carbon source that imparts conductivity to lithium vanadium phosphate.

[0031] In the first step, the amount of citric acid mixed is preferably 0.9 to 6.0, particularly preferably 1.0 to 5.0, and even more preferably 1.5 to 4.5, in terms of the molar ratio of carbon atoms converted to V atoms in vanadium pentoxide (C / V). The reason for this is that if the amount of citric acid mixed is less than the above range, the reduction of vanadium pentoxide will be insufficient, whereas if the amount of citric acid mixed exceeds the above range, the amount of carbon will be excessive relative to lithium vanadium phosphate, which will tend to result in insufficient capacity as a positive electrode material, and this is undesirable.

[0032] The vanadium pentoxide, phosphoric acid, and citric acid used in the first step may have any production history, but in order to produce high-purity lithium vanadium phosphate, it is preferable that the impurity content be as low as possible. The citric acid used may be either hydrous or anhydrous.

[0033] In the first step, from the viewpoint of appropriately adjusting the viscosity of the slurry, a dispersant may be further added to the aqueous solvent containing vanadium pentoxide, phosphoric acid, and citric acid, if necessary.

[0034] As the dispersant, at least one anionic surfactant selected from carboxylates, sulfates, sulfonates, and phosphates is preferred, as it reduces the viscosity of the reduction reaction slurry and provides a reaction precursor with excellent reactivity. As the anionic surfactant, polycarboxylic acid surfactants or polyacrylic acid surfactants are preferred, and polycarboxylic acid surfactants are particularly preferred. As the polycarboxylic acid surfactant, ammonium salts of polycarboxylic acids are preferred.

[0035] The anionic surfactant may be a commercially available one, and examples of commercially available polycarboxylic acid surfactants include SN Dispersant 5020, SN Dispersant 5023, SN Dispersant 5027, SN Dispersant 5468, and Nopcosperse 5600 manufactured by San Nopco, and Poise 532A manufactured by KAO.

[0036] The concentration of the dispersant in the aqueous solvent is preferably 0.5 to 10% by mass, and more preferably 1 to 10% by mass, in order to obtain a sufficient dispersion effect.

[0037] The aqueous solvent used in the first step may be water, or a mixed solvent of water and a hydrophilic organic solvent.

[0038] In the first step, the order in which vanadium pentoxide, phosphoric acid, citric acid and a dispersing agent are added to the aqueous solvent and the means for mixing are not particularly limited.

[0039] The temperature at which the reduction reaction of vanadium pentoxide is carried out in the first step is not particularly limited, but since the reduction reaction proceeds sufficiently at less than 60° C., in consideration of the advantages of the present invention, the temperature is less than 60° C., preferably 20 to 50° C., and more preferably 25 to 45° C. In the present invention, the reduction reaction is carried out within the above temperature range, and furthermore, by carrying out the reduction reaction with the amount of citric acid mixed within the above range, a reduction reaction slurry containing solids can be obtained. 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.

[0040] In the first step, the completion of the reduction reaction can be confirmed by visually confirming that the reduction reaction slurry has turned deep green.

[0041] The reaction time for the reduction reaction in the first step is not particularly limited, and is generally 0.5 hours or more, preferably 1 to 3 hours. If the reduction reaction is carried out for a reaction time within this range, a satisfactory reduction reaction solution can be obtained.

[0042] The second step is a step of adding a lithium source to the reduction reaction slurry obtained in the first step to prepare a raw material mixture slurry containing the lithium vanadium phosphorus composite oxide represented by the general formula (1).

[0043] The mixed raw material slurry obtained in the second step contains the lithium vanadium phosphorus composite oxide represented by the general formula (1), lithium dihydrogen phosphate (LiH2PO4), and citric acid that remains unconsumed in the reduction reaction.

[0044] The lithium vanadium phosphorus composite oxide obtained in the second step is represented by the following general formula (1). LiVOPO4·xH2O (1) (wherein x is 0 to 2) The lithium vanadium phosphorus composite oxide represented by general formula (1) is composed of plate-like primary particles that form an aggregate of secondary particles, and the average thickness of the plate-like primary particles determined by SEMM observation is 1 to 100 nm, preferably 5 to 80 nm, and the average particle diameter of the secondary particles determined by SEM observation is 0.2 to 40 μm, preferably 0.5 to 30 μm.

[0045] The lithium vanadium phosphorus composite oxide represented by general formula (1) produced in the second step is wet-pulverized in a media mill in the third step, whereby it is easily pulverized into fine lithium vanadium phosphorus composite oxide represented by general formula (1), thereby obtaining a raw material mixture slurry containing the fine lithium vanadium phosphorus composite oxide represented by general formula (1).

[0046] Examples of the lithium source for the second step include lithium hydroxide and lithium carbonate. These lithium sources are preferably used as a lithium source-containing liquid in which they are dissolved or dispersed in an aqueous solvent, and then added to the reduction reaction slurry.

[0047] The amount of the 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 0.50 to 0.80, preferably 0.55 to 0.75. When the amount of the lithium source added is within the above range, it is preferred from the viewpoint that the final product is easily obtained as lithium vanadium phosphate having high purity as measured by X-ray diffraction.

[0048] In the second step, the lithium source may have any manufacturing history, but it is preferable that the impurity content be as low as possible in order to produce high-purity lithium vanadium phosphate.

[0049] The temperature at which the lithium source is added is not particularly limited, but from the viewpoint of carrying out this step subsequent to the first step, it is preferable to add the lithium source at a temperature of less than 60°C, preferably 20 to 50°C, and more preferably 25 to 45°C.

[0050] In the second step, the lithium source-containing solution in which the lithium source is dissolved or dispersed in an aqueous solvent is preferably added at a constant rate, in order to obtain a product of stable quality.

[0051] In the second step, after the lithium source is added, an aging reaction can be carried out subsequently, if necessary, to complete the reaction between VOHPO or its hydrate in the reduction reaction slurry and the lithium source and promote the precipitation of the lithium vanadium phosphorus composite oxide represented by general formula (1).

[0052] The temperature at which the aging reaction is carried out is not particularly limited, but is preferably 10 to 40° C., and more preferably 15 to 35° C., from the viewpoint of properly completing the reaction. The aging reaction time is also not particularly limited, but a satisfactory raw material mixed slurry can be obtained by carrying out the aging reaction for usually 1 hour or more, preferably 3 to 30 hours, and more preferably 10 to 30 hours.

[0053] The third step is a step of wet-pulverizing the raw material mixed slurry obtained by carrying out the second step using a media mill to prepare a wet-pulverized slurry.

[0054] By carrying out the third step, it is possible to obtain a wet-milled slurry containing a fine lithium vanadium phosphorus composite oxide represented by general formula (1). The reaction precursor obtained from this fine lithium vanadium phosphorus composite oxide represented by general formula (1) has excellent reactivity, and by calcining in the fifth step, it is easy to produce lithium vanadium phosphate that is highly pure as measured by X-ray diffraction.

[0055] In the third step, the solids concentration of the raw material mixed slurry to be wet-pulverized by the media mill is preferably 10 to 40% by mass, and more preferably 15 to 30% by mass, from the viewpoints of good operability and efficient pulverization. Therefore, after the second step, it is desirable to adjust the solids concentration of the raw material mixed slurry, if necessary, so that the concentration falls within the above range, and then wet-pulverize the raw material mixed slurry in the third step.

[0056] In the third step, the mixed raw material slurry is wet-pulverized using a media mill. By using this method, the lithium vanadium phosphorus composite oxide represented by general formula (1) can be pulverized more finely, thereby obtaining a reaction precursor with even better reactivity.

[0057] Examples of media mills include bead mills, ball mills, paint shakers, attritors, sand mills, etc., with bead mills being preferred. When using a bead mill, the operating conditions and the type and size of the beads may be appropriately selected depending on the size of the apparatus and the processing amount.

[0058] In the third step, it is preferable to carry out the wet grinding treatment using a media mill until the average particle size of the solid content reaches 2.0 μm or less, preferably 0.1 to 1.5 μm, particularly preferably 0.2 to 1.0 μm, as measured by a laser scattering diffraction method, in order to obtain a reaction precursor having excellent reactivity. The average particle size is determined by measuring the volume frequency particle size distribution using the laser diffraction scattering method. 50 ) refers to the particle size.

[0059] The fourth step is a step of spray-drying the wet-milled slurry obtained by carrying out the third step to obtain a reaction precursor.

[0060] The reaction precursor obtained in the fourth step is a particle surface of the lithium vanadium phosphorus composite oxide represented by the general formula (1) above, to which lithium dihydrogen phosphate (LiH2PO4) and citric acid remaining without being consumed in the reduction reaction of the first step are attached.

[0061] The average particle size of the primary particles of the lithium vanadium phosphorus composite oxide represented by general formula (1) constituting the reaction precursor is approximately the same as the average particle size of the primary particles of the solid content in the wet-milled slurry obtained by carrying out the third step.

[0062] Although methods other than spray drying are known for drying the liquid, the present invention employs spray drying based on the finding that it is advantageous to select this drying method. Specifically, by using spray drying in the fourth step, citric acid and lithium dihydrogen phosphate (LiH2PO4) are uniformly attached to the particle surfaces of the lithium vanadium phosphorus composite oxide represented by general formula (1), and a granulated product in which the particles of the lithium vanadium phosphorus composite oxide represented by general formula (1) are densely packed is obtained, which makes it easier to produce lithium vanadium phosphate of high purity as measured by X-ray diffraction in the firing step in the fifth step.

[0063] In the spray drying method, a liquid is atomized by a predetermined means, and the resulting fine droplets are dried to obtain granules. For example, the liquid can be atomized using a rotating disk or a pressure nozzle. Either method can be used in the fourth step.

[0064] In spray drying, the relationship between the size of the atomized slurry droplets and the size of the pulverized material particles contained therein affects stable drying and the properties of the resulting dried powder. Specifically, if the size of the pulverized material raw material particles is too small compared to the size of the droplets, the droplets become unstable, making it difficult to successfully dry. From this perspective, the size of the atomized droplets is preferably 5 to 100 μm, and particularly preferably 10 to 50 μm. It is desirable to determine the amount of slurry to be supplied to the spray drying apparatus taking this into consideration.

[0065] The drying temperature in the spray dryer is preferably adjusted so that the hot air inlet temperature is 180 to 250°C, preferably 200 to 240°C, and the powder temperature is 90 to 150°C, preferably 100 to 130°C, as this prevents the powder from absorbing moisture and makes it easier to recover the powder.

[0066] The reaction precursor obtained by carrying out the fourth step may be dried to produce a mixture of a lithium vanadium phosphorus composite oxide represented by general formula (1) in which x is 2 and a lithium vanadium phosphorus composite oxide in which the water of crystallization in the formula has been partially removed, as determined by X-ray diffraction analysis. However, such a mixture is also preferably used in the present invention.

[0067] The fifth step is a step in which the reaction precursor obtained by carrying out the fourth step is fired at 500 to 1300° C. to obtain lithium vanadium phosphate that is highly pure as measured by X-ray diffraction.

[0068] The firing temperature in the fifth step is 500 to 1300° C., preferably 600 to 1100° C. If the firing temperature in the fifth step is below the above range, the firing time until lithium vanadium phosphate is produced becomes long, and if the firing temperature exceeds the above range, lithium vanadium phosphate melts.

[0069] The firing atmosphere in the fifth step is an inert gas atmosphere or a reducing atmosphere to prevent oxidation of vanadium and melting. The inert gas used in the fifth step is not particularly limited, and examples thereof include nitrogen gas, helium gas, and argon gas.

[0070] In the fifth step, the calcination time is not particularly limited, and calcination for generally 2 hours or more, particularly 3 to 24 hours, will yield lithium vanadium phosphate of high purity as measured by X-ray diffraction.

[0071] In the fifth step, the lithium vanadium phosphate obtained by calcination may be subjected to multiple calcinations, if necessary. When multiple calcinations are performed, the calcined product may be crushed or disintegrated and then calcined.

[0072] In the method for producing a lithium vanadium phosphate carbon composite of the present invention, for the purpose of stabilizing the crystal structure of lithium vanadium phosphate and further improving battery performance, an Me source (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element) is added to the reduction reaction slurry in the first step as needed, and then steps two to five of the method for producing lithium vanadium phosphate of the present invention are carried out, thereby obtaining lithium vanadium phosphate doped with Me element.

[0073] The Me element is present as a substitute at the Li site and / or V site of the lithium vanadium phosphate represented by the general formula (2).

[0074] Me in the Me source is a metal element or transition metal element having an atomic number of 11 or more other than V. Preferred Me elements include Sr, Ba, Sc, Y, Hf, Ta, W, Ru, Os, Ag, Zn, Si, Ga, Ge, Sn, Bi, S, Se, Te, Cl, Br, I, Na, K, Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo, and Cu, which may be used alone or in combination of two or more. In the present invention, Me in the Me source is preferably Ti and / or Al, and particularly preferably Al.

[0075] 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 adding the lithium source in the second step. The Me source may be present in the reduction reaction slurry in the form of a solution or 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.

[0076] Furthermore, when a Me source is mixed, the amount of the Me source to be mixed depends on the type of Me element to be doped, but in many cases, it is preferable that the mixed amount be such that the molar ratio of the total of V atoms and Me atoms to P atoms in the reduction reaction slurry ((Me+V) / P) is 0.50 to 0.80, preferably 0.60 to 0.73, and the molar ratio of Me atoms to V atoms (Me / V) is greater than 0 and 0.45 or less, preferably greater than 0 and 0.1 or less.

[0077] When a phosphate or biphosphate is used as the Me source, the phosphorus atoms in the phosphate or biphosphate also serve as a phosphorus source in the production method of the present invention, similar to phosphoric acid. Therefore, when a phosphate or biphosphate is used as the Me source, the amount of the phosphate or biphosphate mixed is preferably adjusted so that the total molar ratio of P atoms derived from the phosphate or biphosphate to P atoms derived from the phosphoric acid in the first step falls within the range of the molar ratio of V atoms and Me atoms to P atoms in the reaction precursor ((Me+V) / P).

[0078] In the method for producing lithium vanadium phosphate of the present invention, the obtained lithium vanadium phosphate may be subjected to a crushing treatment or a pulverization treatment, and may further be classified, if necessary.

[0079] The lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention is highly pure as measured by X-ray diffraction. The carbon contained in the lithium vanadium phosphate is conductive carbon. Therefore, the amount of carbon contained in the lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention is preferably 1 to 20 mass %, and more preferably 2 to 15 mass %, calculated as C atoms, from the viewpoint of being usable as a positive electrode material for lithium secondary batteries, electrochemical capacitors, and the like.

[0080] Furthermore, lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention, which has a further reduced carbon content, can be used as a positive electrode material for all-solid-state batteries. Examples of methods for reducing the carbon content of lithium vanadium phosphate include a method in which the reaction precursor obtained in the fourth step of the present invention is heated in an oxygen-containing atmosphere having an oxygen concentration of 5 vol% or more, preferably 10 to 30 vol%, at 270 to 370°C, preferably 290 to 360°C, until the molar ratio of carbon atoms to vanadium atoms in the reaction precursor reaches 0.3 to 0.6, preferably 0.35 to 0.55, to reduce the carbon content, followed by the fifth step; or a method in which the lithium vanadium phosphate obtained in the fifth step is heated in an oxygen-containing atmosphere having an oxygen concentration of 5 vol% or more, preferably 10 to 30 vol%, at 250 to 450°C, preferably 300 to 400°C, to reduce the carbon content.

[0081] Furthermore, the lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention has a BET specific surface area of ​​15 m 2 / g or more, preferably 20 to 70m 2 / g. The lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention has an average particle size of 1 to 30 μm, preferably 2 to 25 μm, as measured by laser diffraction scattering. The average particle size is determined by measuring the volume frequency particle size distribution using the laser diffraction scattering method. 50 ) refers to the particle size. Furthermore, the lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention has a cumulative particle size distribution of 90% (D 90 ) particle size and D 50 The particle size ratio (D 90 / D 50 ) is 3.0 or less, preferably 1.2 to 2.8.

[0082] The lithium vanadium phosphate obtained by the method for producing lithium vanadium phosphate of the present invention is used as a cathode material for lithium secondary batteries, all-solid-state batteries, and electrochemical capacitors. [Example]

[0083] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Example 1 <First step> 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 in that order at room temperature (25°C). The reduction reaction was carried out at 25-45°C for 40 minutes with stirring, yielding a greenish-blue (dark green) reduction reaction slurry. The reduction reaction was exothermic, and the temperature of the reaction system rose from 25°C to 45°C. The amount of citric acid monohydrate added was 4.25 in terms of the molar ratio (C / V) of C atoms in citric acid monohydrate to V atoms in vanadium pentoxide. <Second process> 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 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 dissolved raw material mixture solution. After continuing the aging reaction of the raw material mixture solution with stirring at 25°C for 24 hours, the raw material mixture solution became a slurry, and a raw material mixture slurry in which a green precipitate was observed was obtained. Furthermore, X-ray diffraction analysis of the precipitate confirmed the presence of LiVOPO4·2H2O, and observation with a scanning electron microscope revealed that the precipitate was composed of plate-like primary particles agglomerated to form secondary particles. <Third step> The raw material mixture slurry was then stirred and fed to a media-agitated bead mill containing zirconia beads with a diameter of 0.5 mm, and wet-pulverized to prepare a wet-pulverized slurry. The average particle size of the solid content in the wet-pulverized slurry was determined by laser diffraction scattering to be 0.8 μm. <Fourth process> The wet-milled slurry was fed into a spray dryer with the outlet temperature set to 120°C to obtain a reaction precursor. X-ray diffraction analysis of the reaction precursor showed that it contained LiVOPO4·2H2O and LiH2PO4 (see Figure 1). Figure 2 shows an electron microscope (SEM) image of the reaction precursor. <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 revealed that it was a single-phase lithium vanadium phosphate (see Figure 3). This was designated the lithium vanadium phosphate sample. An SEM photograph of the resulting lithium vanadium phosphate sample is shown in Figure 4. The average particle diameter (D 50 ) is 20.1 μm and D 90 The BET specific surface area was 35.3 m 2 / g. Furthermore, the amount of residual carbon in the obtained lithium vanadium phosphate sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation) to determine the content of C atoms, and the amount of residual carbon was found to be 8.1% by mass.

Claims

1. A method for producing lithium vanadium phosphate having a NASICON structure, comprising: a first step of adding vanadium pentoxide, phosphoric acid, and citric acid to an aqueous solvent and carrying out a reduction reaction of vanadium pentoxide at a temperature of less than 60°C to prepare a reduction reaction slurry; Next, a lithium source is added to the reduction reaction slurry to form a compound represented by the following general formula (1): 2000 ４ ・xH ２ O (1) (wherein x is 0 to 2) A second step of preparing a raw material mixed slurry containing a lithium vanadium phosphorus composite oxide represented by the formula: A third step of wet-pulverizing the raw material mixed slurry in a media mill to prepare a wet-pulverized slurry; A fourth step of spray-drying the wet-milled slurry 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 lithium vanadium phosphate; 1. A method for producing lithium vanadium phosphate, comprising:

2. 2. The method for producing lithium vanadium phosphate according to claim 1, wherein the amount of citric acid added in the first step is such that the molar ratio (C / V) of V atoms in vanadium pentoxide to C atoms in citric acid is 0.9 to 6.

0.

3. 3. The method for producing lithium vanadium phosphate according to claim 1, wherein the average particle size of the solid content in the wet-milled slurry in the third step is 2 μm or less.

4. 3. The method for producing lithium vanadium phosphate according to claim 1, wherein in the first step, a Me source (Me represents a metal element other than V having an atomic number of 11 or more or a transition metal element) is further contained in the reduction reaction slurry.

5. 5. The method for producing lithium vanadium phosphate according to claim 4, wherein the Me source is selected from a Ti source and an Al source.

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