Method for producing lithium vanadium phosphate

Abstract

The present invention provides lithium vanadium phosphate which is a single phase as seen in X-ray diffraction, particularly useful as a positive electrode active material for a lithium secondary battery or the like, by an industrially advantageous method. The present invention is a method for producing lithium vanadium phosphate having a sodium superion conductor (NASICON) structure, the method being characterized by comprising: a first step for preparing a mixed slurry by mixing a tetravalent or pentavalent vanadium compound, a phosphorus source, and a reducing sugar in an aqueous solvent; a second step in which the mixed slurry is heated and solubilized; a third step of adding a lithium source to the solution to prepare a raw material mixed solution; a fourth step for obtaining a reaction precursor by spray-drying the raw material mixed solution; and a fifth step in which the reaction precursor is subsequently fired at 500-1300 DEG C in an inert gas atmosphere or a reducing atmosphere.

Description

[0001] This application is a divisional application of the same name filed on March 1, 2017, Application No. 201780016103.9. TECHNICAL FIELD

[0002] The present application relates to a manufacturing method of lithium vanadium phosphate which is particularly useful as a positive electrode active material for lithium secondary batteries. BACKGROUND

[0003] As a battery for portable devices, notebook computers, electric vehicles, and hybrid vehicles, lithium ion batteries are widely used. It is generally considered that lithium ion batteries have excellent capacity and energy density, and LiCoO2 is mainly used as a positive electrode thereof, but from the viewpoint of Co resources, LiMnO2, LiNiO2, and the like are also widely developed.

[0004] Currently, as a further alternative material, research focusing on LiFePO4 is being conducted in various institutions. LiFePO4 using Fe, which is abundant in resources, has slightly lower energy density, but has excellent high-temperature characteristics, and thus is expected as a positive electrode material for lithium ion batteries for electric vehicles.

[0005] However, LiFePO4 has a slightly low operating voltage, and thus lithium vanadium phosphate (Li3V2(PO4)3) having a sodium superionic conductor (NASICON) structure in which V is used instead of Fe is attracting attention.

[0006] As a manufacturing method of lithium vanadium phosphate, for example, a method in which a lithium source, a vanadium compound, and a phosphorus source are pulverized and mixed, the obtained uniform mixture is molded into a pellet shape, and then the molded product is fired (for example, refer to Patent Documents 1 and 2) is proposed. In addition, in Patent Document 3, a method in which vanadium (V) oxide is dissolved in an aqueous solution containing lithium hydroxide, a phosphorus source, and carbon and / or a non-volatile organic compound are further added, the obtained raw material mixed solution is dried to obtain a precursor, and the precursor is heat-treated in a non-active atmosphere to obtain a composite of Li3V2(PO4)3 and a conductive carbon material is proposed.

[0007] Further, the present applicant has previously proposed in Patent Literature 4 below a method for producing a lithium vanadium phosphate carbon composite, which includes: a first step of preparing a raw material mixed solution by mixing a lithium source, a vanadium compound of valence 5 or 4, a phosphorus source, and a source of an electrically conductive carbon material that generates carbon by thermal decomposition in an aqueous solvent; a second step of performing a precipitation generation reaction on the raw material mixed solution by heating to obtain a reaction solution containing a precipitate; a third step of performing a wet pulverization treatment on the reaction solution containing the precipitate using a media mill to obtain a slurry containing a pulverization treatment product; a fourth step of performing a spray drying treatment on the slurry containing the pulverization treatment product to obtain a reaction precursor; and a step of firing the reaction precursor at 600 to 1300°C in a non-reactive gas atmosphere or a reducing gas atmosphere.

[0008] Prior Art Documents

[0009] Patent Literature

[0010] Patent Literature 1: Japanese Patent Application Laid-Open (kokai) No. 2001-500665

[0011] Patent Literature 2: Japanese Patent Application Laid-Open (kokai) No. 2002-530835

[0012] Patent Literature 3: Japanese Patent Application Laid-Open (kokai) No. 2008-052970

[0013] Patent Literature 4: International Publication No. 2012 / 043367 SUMMARY

[0014] Problems to be Solved by the Invention

[0015] It is known that the theoretical capacity of Li3V2(PO4)3 is as high as 197 mAh g -1 .

[0016] However, the discharge capacity of a lithium secondary battery using Li3V2(PO4)3 as a positive electrode active material is low, and in addition, according to the method for producing a lithium vanadium phosphate of Patent Literature 4, although a product with a high discharge capacity can be obtained, in order to obtain a reaction precursor with excellent reactivity, it is necessary to perform a precipitation generation reaction and a pulverization treatment using a media mill, and the manufacturing process becomes complicated, which is industrially disadvantageous.

[0017] Therefore, the object of the present application is to provide a lithium vanadium phosphate that is particularly useful as a positive electrode active material or the like for a lithium secondary battery and is a single phase from the viewpoint of X-ray diffraction, in an industrially advantageous method.

[0018] Method for Solving the Problem

[0019] The inventors of the present application have intensively studied the above problem and found that, by subjecting a mixed slurry containing a vanadium compound of valence 4 or 5, a phosphorus source and a reducing sugar to a heating treatment to perform a reduction reaction of the vanadium compound, adding a lithium source to the resulting solution to prepare a raw material mixed solution, and subjecting the raw material mixed solution to spray drying to obtain a reaction precursor, the reactivity of the reaction precursor is excellent, and a single-phase, highly crystalline lithium vanadium phosphate can be obtained even at a low temperature of about 600°C. Furthermore, a lithium secondary battery using the lithium vanadium phosphate obtained using the reaction precursor as a positive electrode active material has excellent battery performance, thereby completing the present application.

[0020] That is, the present application provides a method for producing lithium vanadium phosphate having a sodium super ion conductor (NASICON) structure, which includes: a first step of mixing a vanadium compound of valence 4 or 5, a phosphorus source and a reducing sugar in an aqueous solvent to prepare a mixed slurry; a second step of subjecting the mixed slurry to a heating treatment to solubilize it; a third step of adding a lithium source to the solution to prepare a raw material mixed solution; a fourth step of subjecting the raw material mixed solution to a spray drying treatment to obtain a reaction precursor; and a fifth step of subjecting the reaction precursor to a firing at 500 to 1300°C in a non-reactive gas atmosphere or a reducing gas atmosphere.

[0021] Effects of the Invention

[0022] According to the present application, a single-phase lithium vanadium phosphate, which is particularly useful as a positive electrode active material of a lithium secondary battery and the like, can be provided in an industrially advantageous manner, and a lithium secondary battery using the lithium vanadium phosphate as a positive electrode active material has excellent battery performance. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 X-ray diffraction patterns of the reaction precursors obtained in Example 1, Comparative Example 1 and Comparative Example 2.

[0024] Figure 2 SEM photograph of the reaction precursor obtained in Example 1.

[0025] Figure 3 X-ray diffraction patterns of the lithium vanadium phosphate samples obtained in Example 1 and Comparative Example 1.

[0026] Figure 4 SEM photograph of the lithium vanadium phosphate obtained in Example 1.

[0027] Figure 5 SEM photograph of the reaction precursor obtained in Example 5.

[0028] Figure 6 X-ray diffraction pattern of the lithium vanadium phosphate sample obtained in Example 5.

[0029] Figure 7 SEM photograph of the lithium vanadium phosphate sample obtained in Example 5.

[0030] Figure 8 X-ray diffraction pattern of the lithium vanadium phosphate sample obtained in Example 6.

[0031] Figure 9 SEM photograph of the lithium vanadium phosphate sample obtained in Example 6. DETAILED DESCRIPTION

[0032] Hereinafter, the present application will be described based on preferred embodiments.

[0033] The lithium vanadium phosphate obtained by the production method of the present application is lithium vanadium phosphate having a sodium super ion conductor (NASICON) structure (hereinafter referred to as "lithium vanadium phosphate").

[0034] In the present application, the above-mentioned lithium vanadium phosphate is lithium vanadium phosphate represented by the following general formula (1) or lithium vanadium phosphate doped with a Me element (Me represents a metal element having an atomic number of 11 or more other than V or a transition metal element) as necessary in lithium vanadium phosphate represented by general formula (1).

[0035] Li x V y (PO4)3(1)

[0036] (In the formula, x represents 2.5 or more and 3.5 or less, and y represents 1.8 or more and 2.2 or less.)

[0037] x in formula (1) is preferably 2.5 or more and 3.5 or less, and particularly preferably 2.8 or more and 3.2 or less. y is preferably 1.8 or more and 2.2 or less, and particularly preferably 1.9 or more and 2.1 or less.

[0038] The doped Me element is preferably exemplified by one or two or more selected from the group consisting of Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo and Cu.

[0039] The production method of the above-mentioned lithium vanadium phosphate of the present application is characterized by comprising: a first step of mixing a vanadium compound (hereinafter sometimes referred to as "vanadium compound") having a valence of 4 or 5, a phosphorus source and a reducing sugar in an aqueous solvent to prepare a mixed slurry; a second step of subjecting the obtained mixed slurry to a heat treatment to solubilize it; a third step of adding a lithium source to the obtained solution to prepare a raw material mixed solution; a fourth step of subjecting the obtained raw material mixed solution to a spray drying treatment to obtain a reaction precursor; and a fifth step of subjecting the obtained reaction precursor to a firing at 500 to 1300°C in a non-active gas atmosphere or a reducing gas atmosphere.

[0040] The first process of the present application is a process of mixing and treating a vanadium compound, a phosphorus source, and a reducing sugar in an aqueous solvent to obtain a mixed slurry in which each of the raw materials is mixed.

[0041] As the vanadium compound, vanadium pentoxide, vanadium trioxide, ammonium vanadate, vanadyl oxalate, and the like can be used. Among these, from the viewpoint of being able to be obtained at low cost in industry and having excellent reactivity, vanadium pentoxide is preferred.

[0042] As the phosphorus source, phosphoric acid, polyphosphoric acid, phosphoric anhydride, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and the like can be used. Among these, from the viewpoint of being able to be obtained at low cost in industry and being able to obtain a reaction precursor having excellent reactivity, phosphoric acid is preferred.

[0043] Regarding the addition amount of the vanadium compound and the phosphorus source, from the viewpoint of easily obtaining a single phase of lithium vanadium phosphate as a final product, the addition amount of the vanadium compound is preferably such that the molar ratio (V / P) of V atoms in the vanadium compound to P atoms in the phosphorus source is 0.50 to 0.80, and more preferably 0.60 to 0.73.

[0044] The reducing sugar is a substance that is separated out as carbon by heating decomposition in a non-active or reducing atmosphere at least by the firing of the fifth process. The reducing sugar promotes the reduction reaction of the vanadium compound in the second process, and can make the obtained solution a reaction solution having good viscosity that can be stirred, and also an essential component for preventing oxidation of vanadium in the fifth process. The remaining reducing sugar is converted into conductive carbon by the firing of the fifth process, and therefore in the present production method, the reducing sugar can be added in excess to function as a component that imparts conductivity to the lithium vanadium phosphate.

[0045] As the reducing sugar that can be used, for example, glucose, fructose, lactose, maltose, sucrose, and the like can be exemplified, and among these, from the viewpoint of being able to obtain a reaction precursor having excellent reactivity, lactose and sucrose are preferred.

[0046] Regarding the addition amount of the reducing sugar, it is preferably added in an amount of 0 to 20 mass% with respect to the lithium vanadium phosphate produced, in terms of C atoms.

[0047] There is a tendency that the amount of C atoms contained in the reducing sugar after firing decreases compared to that before firing. Therefore, in the first step, when the amount of the reducing sugar added with respect to 100 parts by mass of the generated lithium vanadium phosphate is 0.3 to 40 parts by mass in terms of C atoms with respect to 100 parts by mass of the lithium vanadium phosphate, the amount of the conductive carbon converted from the reducing sugar with respect to 100 parts by mass of the lithium vanadium phosphate easily reaches 0 to 20 parts by mass in terms of C atoms. By making the amount of the reducing sugar added with respect to 100 parts by mass of the generated lithium vanadium phosphate within the above range, when the lithium vanadium phosphate is used as the positive active material of the lithium secondary battery, the lithium secondary battery can be given excellent performance. In particular, by adding the reducing sugar in an amount of 0.5 to 40 parts by mass, preferably 5 to 30 parts by mass in terms of C atoms with respect to 100 parts by mass of the lithium vanadium phosphate, the amount of the conductive carbon converted from the reducing sugar with respect to 100 parts by mass of the lithium vanadium phosphate reaches 0.1 to 20 parts by mass, preferably 0.5 to 15 parts by mass, and sufficient conductivity can be given using the conductive carbon, so that the internal resistance of the lithium secondary battery can be reduced, and the discharge capacity per unit mass or volume is increased. On the other hand, when the amount of the reducing sugar added with respect to 100 parts by mass of the generated lithium vanadium phosphate is less than the above range, there is a tendency that it is difficult to obtain a single-phase lithium vanadium phosphate, and thus it is not preferable. In addition, when the amount of the reducing sugar added exceeds the above range, the discharge capacity per unit mass or volume easily decreases.

[0048] In the first step according to the present application, the manufacturing process of the above vanadium compound, the phosphorus source, and the reducing sugar is not limited, but in order to manufacture a lithium vanadium phosphate having high purity, a substance having as little impurities as possible is preferable.

[0049] As the aqueous solvent used in the first step according to the present application, not only water but also a mixed solvent of water and a hydrophilic organic solvent can be used.

[0050] The order of adding the vanadium compound, the phosphorus source, and the reducing sugar to the aqueous solvent, and the mixing means are not particularly limited, and they are performed in a manner that a mixed slurry in which each of the above raw materials is uniformly dispersed can be obtained.

[0051] The second step according to the present application is a step of performing a heating treatment on the mixed slurry obtained in the above first step, and at least performing a reduction reaction of the vanadium compound, and converting the mixed slurry into a solution in which each component is dissolved. The temperature of the heating treatment according to the second step is 60 to 100°C, preferably 80 to 100°C. The reason for this is that when the temperature of the heating treatment is lower than 60°C, the reaction time is prolonged, and thus it is industrially disadvantageous; and in order to make the temperature of the heating treatment higher than 100°C, a pressurized container must be used, and thus it is industrially disadvantageous. Therefore, the second step can be performed under atmospheric pressure.

[0052] The time for the reduction reaction is not critical in the present production method. A satisfactory solution can be obtained when the heat treatment is performed for 0.2 hours or more, particularly for 0.5 to 2 hours.

[0053] The third step of the present application is a step of adding a lithium source to the solution obtained in the above-mentioned second step to obtain a raw material mixture.

[0054] The raw material mixture can be a solution or a slurry.

[0055] As the lithium source, lithium carbonate, lithium hydroxide, lithium oxide, lithium nitrate, or lithium oxalate, or the like can be used. These can be hydrous or anhydrous. Of these, lithium carbonate and lithium hydroxide are preferred from the viewpoint of being able to be obtained inexpensively in industry and being able to obtain a reaction precursor having excellent reactivity.

[0056] In addition, these lithium sources can also be added to the solution obtained in the second step in the form of a solution dissolved in an aqueous solvent.

[0057] The amount of the lithium source added is preferably such that the molar ratio of Li atoms in the lithium source to P atoms in the phosphorus source (Li / P) is 0.70 to 1.30, preferably 0.83 to 1.17, from the viewpoint of easily obtaining a single-phase lithium vanadium phosphate as the final product.

[0058] The manufacturing process of the lithium source in the third step of the present application is not limited, but in order to manufacture a high-purity lithium vanadium phosphate, a substance having as little impurities as possible is preferred.

[0059] The temperature at which the lithium source is added is not particularly limited, but precipitates are observed when it exceeds 35°C, although the slurry-like raw material mixture containing the precipitates can also be used in the fourth step described later in the present production method, a solution state of the raw material mixture obtained in the third step is preferred from the viewpoint of obtaining a reaction precursor that is more uniform and has excellent reactivity. From this viewpoint, the temperature at which the lithium source is added is less than 35°C, preferably 30°C or less, and particularly preferably 0 to 30°C.

[0060] After the lithium source is added, a ripening reaction can be performed as needed in order to obtain a reaction precursor having further improved properties such as particle size distribution and the like.

[0061] The raw material mixture after the ripening reaction is obtained as a slurry in which the components are uniformly mixed, but unlike Patent Document 4, it is a slurry containing finer particles, so that pulverization using a media mill is not necessary, and in the present production method, spray drying can be directly performed in the fourth step of the subsequent step to obtain a reaction precursor having excellent reactivity.

[0062] The temperature of the ripening reaction is 60 to 100°C, preferably 80 to 100°C. Also, the ripening time is 0.5 hours or more, preferably 0.5 to 10 hours.

[0063] The fourth process according to the present application is a process of spray-drying the raw material mixture obtained in the third process to obtain a reaction precursor.

[0064] Methods of drying a liquid other than spray-drying are also known, but based on the insight that it is advantageous to select spray-drying in the present production method, this drying method is employed.

[0065] Specifically, when spray-drying is used, a granular substance in which each component is uniformly dispersed and tightly packed can be obtained, and therefore, in the present application, this granular substance is used as a reaction precursor, and by using this reaction precursor to perform firing in the fifth process described later, a lithium vanadium phosphate that is a single phase from the viewpoint of X-ray diffraction can be obtained.

[0066] In spray-drying, the liquid is atomized by a prescribed means, and the fine droplets thus produced are dried, thereby obtaining a reaction precursor. Atomization of a liquid is performed, for example, using a rotating disk or using a pressure nozzle. Any method can be used in the present process.

[0067] In spray-drying, the size of the droplets of the atomized liquid affects stable drying and the properties of the dried powder obtained. From this viewpoint, the size of the atomized droplets is preferably 5 to 100 μm, particularly preferably 10 to 50 μm. The amount of liquid supplied to the spray-drying apparatus can be determined taking this viewpoint into consideration.

[0068] The reaction precursor obtained by spray-drying is fired in the subsequent process, and the powder properties of the secondary particles of the lithium vanadium phosphate obtained are basically inherited from the properties of the reaction precursor. Therefore, from the viewpoint of controlling the particle diameter of the target lithium vanadium phosphate, spray-drying is performed so that the particle diameter of the secondary particles of the reaction precursor, as observed using a scanning electron microscope (SEM), is 5 to 100 μm, particularly 10 to 50 μm.

[0069] Here, regarding the drying temperature of the spray-drying apparatus, from the viewpoint of preventing the powder from absorbing moisture and making the powder easy to recover, the hot air inlet temperature is preferably adjusted to 180 to 250°C, more preferably 200 to 240°C, and the temperature of the powder is adjusted to 90 to 150°C, more preferably 100 to 130°C.

[0070] The reaction precursor used in Patent Literature 4 contains a crystalline substance, and it is also a feature that the reaction precursor used in the present application is amorphous. The inventors of the present application have surmised that because the reaction precursor is amorphous, the reactivity is more excellent than that of the reaction precursor of Patent Literature 4, and firing can be performed at a low temperature, and the crystallinity is also improved.

[0071] The reaction precursor thus obtained is then subjected to a fifth step of firing at 500 to 1300°C in a non-active gas atmosphere or a reducing gas atmosphere, to obtain the target lithium vanadium phosphate.

[0072] The fifth step of the present application is a step of firing the reaction precursor obtained in the fourth step described above at 500 to 1300°C, to obtain a lithium vanadium phosphate that is a single phase from the viewpoint of X-ray diffraction.

[0073] The firing temperature in the fifth step is 500 to 1300°C, and is preferably 600 to 1000°C. The reason for this is that when the firing temperature is less than 500°C, the firing time required to achieve a single phase is prolonged, and when the firing temperature is greater than 1300°C, the lithium vanadium phosphate is dissolved.

[0074] In order to prevent oxidation and melting of vanadium, the firing atmosphere is in a non-active gas atmosphere or a reducing gas atmosphere.

[0075] As the non-active gas that can be used in the fifth step, there is no particular limitation, and examples that can be given include nitrogen, helium, argon, and the like.

[0076] The firing time is not critical in the present production method. When firing is performed for 2 hours or more, and particularly 3 to 24 hours, a lithium vanadium phosphate that is a single phase from the viewpoint of X-ray diffraction can be obtained.

[0077] The lithium vanadium phosphate thus obtained can be subjected to a plurality of firing steps as necessary.

[0078] In the present application, in order to stabilize the crystal structure of the lithium vanadium phosphate and further improve the battery performance such as the cycle characteristics, a Me source (Me represents a metal element or a transition metal element having an atomic number of 11 or more other than V) can be mixed in the mixed slurry in the first step of the production method of the present application as necessary, and then the second step to the fifth step of the production method of the present application are performed, whereby a substance containing the Me element can be doped in the lithium vanadium phosphate represented by the general formula (1) described above. In the present application, the Me element exists by substituting the Li site or / and the V site of the lithium vanadium phosphate represented by the general formula (1) described above.

[0079] The Me in the Me source is a metal element or a transition metal element having an atomic number of 11 or more other than V, and as the preferable Me element, examples that can be given include Mg, Ca, Al, Mn, Co, Ni, Fe, Ti, Zr, Bi, Cr, Nb, Mo, Cu, and the like, and it can be a single one of these or a combination of two or more of these.

[0080] As the Me source, oxides, hydroxides, halides, carbonates, nitrates, organic acid salts, and the like containing the Me element can be exemplified. Among them, the Me source can be present dissolved in the mixed slurry of the first step, or can be present in the form of a solid substance. In the case where the Me source is present in the form of a solid substance in the mixed slurry, from the viewpoint of obtaining a reaction precursor having excellent reactivity, it is preferable to use a substance having an average particle diameter of 100 μm or less, preferably 0.1 to 50 μm.

[0081] In addition, in the case where the Me source is mixed, the mixed amount of the Me source varies depending on the kind of the doped Me element, and in most cases, it is preferable that the molar ratio of the total of V atoms of the vanadium compound and Me atoms in the Me source (V + Me = M) to P atoms in the phosphorus source (M / P) is 0.5 to 0.80, preferably 0.60 to 0.73, and the molar ratio of Me / V is greater than 0 and 0.45 or less, preferably greater than 0 and 0.1 or less.

[0082] In addition, in the present production method, the lithium vanadium phosphate obtained in the fifth step can be further subjected to a heating treatment, and a sixth step of adjusting the amount of the electrically conductive carbon contained in the lithium vanadium phosphate can be performed.

[0083] The operation of the specific sixth step is performed by subjecting the lithium vanadium phosphate containing the electrically conductive carbon obtained in the fifth step to a heating treatment, and oxidizing the electrically conductive carbon.

[0084] The heating treatment involved in the sixth step is preferably performed in an oxygen-containing atmosphere. From the viewpoint of efficiently oxidizing the electrically conductive carbon, it is preferable that the above oxygen concentration is 5 Vol% or more, preferably 10 to 30 Vol%.

[0085] From the viewpoint of efficiently oxidizing the electrically conductive carbon, it is preferable that the temperature of the heating treatment involved in the sixth step is 250 to 450°C, preferably 300 to 400°C.

[0086] The time of the heating treatment involved in the sixth step is not critical in the present production method. The longer the time of the heating treatment, the lower the content of the electrically conductive carbon contained in the lithium vanadium phosphate. It is preferable to perform the treatment under appropriate conditions in advance to achieve the desired content of the electrically conductive carbon.

[0087] The lithium vanadium phosphate thus obtained is a single-phase lithium vanadium phosphate from the viewpoint of X-ray diffraction, and as preferable physical properties, it is preferable that a plurality of primary particles having an average primary particle diameter of 10 μm or less, preferably 0.01 to 5 μm are aggregated to form secondary particles having an average particle diameter of 5 to 100 μm, preferably 10 to 50 μm.

[0088] In the present production method, the obtained lithium vanadium phosphate may be further subjected to a disintegration treatment or a pulverization treatment, or may be further subjected to classification, as needed.

[0089] In addition, the lithium vanadium phosphate obtained by this production method can be a lithium vanadium phosphate carbon complex whose particles are coated with conductive carbon derived from reducing sugar. By using this lithium vanadium phosphate carbon complex as a positive electrode active material, a lithium secondary battery with higher discharge capacity can be obtained.

[0090] Furthermore, the lithium vanadium phosphate obtained by the present production method can also be used as a solid electrolyte.

[0091] Example

[0092] Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to these Examples.

[0093] {Example 1}

[0094] <First Step>

[0095] 2 L of ion-exchanged water was added to a 5 L beaker, and 605 g of 85% phosphoric acid, 320 g of vanadium pentoxide, and 170 g of sucrose were added thereto. The mixture was stirred at room temperature (25° C.) to obtain an ochre-colored mixed slurry.

[0096] <Second step>

[0097] The obtained mixed slurry was heated and stirred at 95° C. for 1 hour to perform a reduction reaction, thereby obtaining a dark blue reaction solution.

[0098] <Third step>

[0099] The reaction solution was cooled to room temperature (25° C.) Next, a solution of 220 g of lithium hydroxide monohydrate dissolved in 1.5 L of ion-exchanged water was prepared and added to the reaction solution at room temperature to obtain a dark blue raw material mixed liquid solution.

[0100] <Fourth step>

[0101] Next, the liquid was supplied to a spray drying apparatus set at an outlet temperature of 120° C. to obtain a reaction precursor. The average secondary particle size of the reaction precursor determined by SEM observation was 12 μm.

[0102] The obtained reaction precursor was subjected to X-ray diffraction measurement using CuKα radiation as a radiation source, and it was confirmed that the reaction precursor was amorphous. Figure 1 In addition, the electron microscope photo (SEM image) of the reaction precursor is shown in Figure 2 .

[0103] <Fifth step>

[0104] The obtained reaction precursor was put in a mullite-made crucible and fired at 600°C for 10 hours under a nitrogen atmosphere. The obtained lithium vanadium phosphate sample was subjected to X-ray diffraction analysis, and as a result, it was confirmed to be a single-phase lithium vanadium phosphate. The X-ray diffraction pattern of the obtained lithium vanadium phosphate sample is shown in Figure 3 . In addition, the electron microscope photograph (SEM image) of the obtained lithium vanadium phosphate sample is shown in Figure 4 .

[0105] In addition, the residual carbon amount of the obtained lithium vanadium phosphate sample was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation), and the content of C atoms was obtained.

[0106] {Examples 2 to 4}

[0107] The reaction was carried out in the same manner as in Example 1 except that the firing temperature in the fifth step was set to 700 to 900°C, and a lithium vanadium phosphate sample was obtained.

[0108] In addition, the obtained lithium vanadium phosphate sample was subjected to X-ray diffraction analysis, and as a result, it was confirmed to be a single-phase lithium vanadium phosphate. In addition, the residual carbon amount was obtained in the same manner as in Example 1.

[0109] [Table 1]

[0110]

[0111] Note)

[0112] 1) The "molar ratio of V / P" of the first step indicates the molar ratio of V atoms in the added vanadium pentoxide with respect to P atoms in the phosphoric acid.

[0113] 2) The "molar ratio of Li / P" of the third step indicates the molar ratio of Li atoms in the added lithium hydroxide in the third step with respect to P atoms in the added phosphoric acid in the first step.

[0114] {Comparative Example 1}

[0115] Ion exchange water 3.5 L was added to a 5 L beaker, and lithium hydroxide monohydrate 220 g, vanadium pentoxide 320 g, 85% phosphoric acid 605 g, and sucrose 170 g were added thereto, after which, heating and stirring were carried out at 95°C for 1 hour, and a green slurry was obtained. Next, the slurry was supplied to a spray-drying device with the outlet temperature set to 120°C, and a reaction precursor was obtained. The obtained reaction precursor was subjected to X-ray diffraction analysis, and it was confirmed that the reaction precursor had clear diffraction peaks. In addition, the X-ray diffraction pattern of the reaction precursor is shown in Figure 1 and drawn in Figure 1 .

[0116] The obtained reaction precursor was put into a mullite-made crucible and fired at 600°C for 10 hours under a nitrogen atmosphere to obtain a lithium vanadium phosphate sample.

[0117] The obtained lithium vanadium phosphate sample was subjected to X-ray diffraction analysis Figure 1 and plotted Figure 3 . From the results of the X-ray diffraction analysis, it was confirmed that there was another phase other than lithium vanadium phosphate. Further, the residual carbon amount was calculated in the same manner as in Example 1, and the residual carbon amount was 1.9 mass%.

[0118] {Comparative Example 2}

[0119] In a 5 L beaker, ion exchange water 2 L was added, and lithium hydroxide monohydrate 252 g was added thereto and dissolved. After vanadium pentoxide 364 g was added to the solution and stirred for 1 hour, glucose 72 g and 85% phosphoric acid 692 g were added to the liquid and stirred for 1 hour to obtain a raw material mixture solution. Subsequently, the raw material mixture solution was supplied to a spray-drying device in which the temperature of the hot air inlet was set to 230°C and the temperature of the outlet was set to 120°C to obtain a reaction precursor. Further, the reaction precursor was subjected to X-ray diffraction Figure 1 and plotted Figure 5 .

[0120] The obtained reaction precursor was put into a mullite-made crucible and fired at 900°C for 12 hours under a nitrogen atmosphere. The fired product was pulverized by a jet mill to obtain a lithium vanadium phosphate sample. The obtained lithium vanadium phosphate sample was subjected to X-ray diffraction analysis, and it was confirmed that it was a single-phase lithium vanadium phosphate. Further, the residual carbon amount was calculated in the same manner as in Example 1, and the residual carbon amount was 0.1 mass%.

[0121] {Example 5}

[0122] <First Step>

[0123] To a 5 L beaker, ion exchange water 2.5 L was added, and 85% phosphoric acid 864.7 g, vanadium pentoxide 457.7 g, and lactose monohydrate 156.6 g were added thereto and stirred at room temperature (25°C) to obtain a yellowish brown mixed slurry.

[0124] <Second Step>

[0125] The obtained mixed slurry was heated and stirred at 95°C for 1 hour to perform a reduction reaction, and a dark blue reaction solution was obtained.

[0126] <Third Step>

[0127] The reaction solution was cooled to room temperature (25°C). Subsequently, a solution in which lithium hydroxide monohydrate 314.7 g was dissolved in ion exchange water 1.5 L was prepared, and the solution was added to the reaction solution at room temperature to obtain a dark blue solution of a raw material mixture solution.

[0128] <Fourth Step>

[0129] Next, the liquid was supplied to a spray drying device set at an outlet temperature of 120°C to obtain a reaction precursor. The obtained reaction precursor was subjected to X-ray diffraction measurement using Cu Kα rays as a ray source, and it was confirmed that the reaction precursor was amorphous. In addition, an electron microscope photograph (SEM image) of the reaction precursor is shown in Figure 6 .

[0130] <Fifth Step>

[0131] The obtained reaction precursor was put in a mullite-made sagger and fired at 800°C for 10 hours under a nitrogen atmosphere. The obtained lithium vanadium phosphate sample was subjected to X-ray diffraction analysis, and as a result, it was confirmed to be a single-phase lithium vanadium phosphate. In addition, an X-ray diffraction chart of the lithium vanadium phosphate sample is shown in Figure 7 . An electron microscope photograph (SEM image) of the obtained lithium vanadium phosphate sample is shown in Figure 8 .

[0132] In addition, the amount of residual carbon in the obtained lithium vanadium phosphate sample was measured to be 1.5 mass% using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation). In addition, the BET specific surface area was 9.8 m 2 / g.

[0133] {Example 6}

[0134] <First Step>

[0135] Into a 5L beaker, 2.5L of ion exchange water was added, and 864.7g of 85% phosphoric acid, 457.7g of vanadium pentoxide, and 156.6g of lactose monohydrate were put therein, and stirring was performed at room temperature (25°C), whereby a yellowish brown mixed slurry was obtained.

[0136] <Second Step>

[0137] The obtained mixed slurry was heated and stirred at 95°C for 1 hour to perform a reduction reaction, and a dark blue reaction solution was obtained.

[0138] <Third Step>

[0139] The reaction solution was cooled to room temperature (25°C). Next, a solution in which 277.1g of lithium carbonate was dissolved in 1.5L of ion exchange water was prepared, and the solution was added to the reaction solution at room temperature, and a dark blue raw material mixed solution was obtained.

[0140] <Fourth Step>

[0141] Next, the liquid was supplied to a spray drying device set at an outlet temperature of 120°C to obtain a reaction precursor. The obtained reaction precursor was subjected to X-ray diffraction measurement using CuKα rays as a ray source, and it was confirmed that the reaction precursor was amorphous.

[0142] <Fourth Step>

[0143] The obtained reaction precursor was put in a mullite-made sagger and fired at 800°C for 10 hours under a nitrogen atmosphere. The obtained lithium vanadium phosphate sample was subjected to X-ray diffraction analysis, and as a result, it was confirmed to be single-phase lithium vanadium phosphate. In addition, the X-ray diffraction pattern of the lithium vanadium phosphate sample is shown in Figure 9 . An electron microscope photograph (SEM image) of the obtained lithium vanadium phosphate sample is shown in Capacity maintenance rate (%) .

[0144] In addition, the amount of residual carbon of the obtained lithium vanadium phosphate sample was measured to be 1.2 mass% using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation). In addition, the BET specific surface area was 9.2 m 2 / g.

[0145] <Evaluation of battery performance>

[0146] <Battery performance test>

[0147] (1) Production of lithium secondary battery

[0148] The lithium vanadium phosphate sample of Example 4, Example 5, and Comparative Example 2, each produced in the above-described manner, was mixed at 91 mass%, graphite powder at 6 mass%, and polyvinylidene fluoride at 3 mass% to produce a positive electrode agent, which was dispersed in N-methyl-2-pyrrolidone to produce a kneaded paste. The obtained kneaded paste was coated on an aluminum foil, and then dried, and punched into a disc of 15 mm in diameter to produce a positive electrode plate.

[0149] Using the positive electrode plate, a lithium secondary battery was produced using each of a separator, a negative electrode, a positive electrode, a current collector plate, a mounting member, an external terminal, an electrolyte, and the like. Among these, the negative electrode used a metal lithium foil, and the electrolyte used an electrolyte in which 1 mole of LiPF6 was dissolved in 1 liter of a 1:1 mixed solution of ethylene carbonate and methyl ethylene carbonate.

[0150] (2) Evaluation of battery performance

[0151] The produced lithium secondary battery was operated under the following conditions, and the battery performance was evaluated.

[0152] <Evaluation of cycle characteristics>

[0153] After the charge after charging by charging at 0.5 C to 4.2 V, then constant current constant voltage (CCCV) charging of 5 hours of full charge time at 4.2 V, discharging at 0.1 C to 2.0 V was performed, and these operations were taken as 1 cycle, the discharge capacity per 1 cycle was measured. This cycle was repeated 20 times, and from the respective discharge capacities of the 1st cycle and the 20th cycle, the capacity retention rate was calculated by the following equation. Here, the discharge capacity of the 1st cycle was taken as the initial discharge capacity.

[0154] Capacity retention rate (%) = ((discharge capacity of the 20th cycle) / (discharge capacity of the 1st cycle)) x 100

[0155]

[0156] [Table 2]

[0157] Initial discharge capacity (mAh g -1 )]]> Example 4 Example 5 127.6 95.8 Comparative Example 2 131.8 97.6 ​ 102.2 76.6

[0158] {Example 7}

[0159] <Sixth Step>

[0160] The lithium vanadium phosphate test material obtained in Example 3 (residual carbon amount 1.6 mass%) was heat-treated at 350°C for 15 hours under an atmospheric atmosphere (oxygen concentration 20 Vol%) using an electric furnace.

[0161] X-ray diffraction analysis was performed on the obtained lithium vanadium phosphate test material, and as a result, single-phase lithium vanadium phosphate was confirmed. Further, the residual carbon amount of the obtained lithium vanadium phosphate test material was measured using a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation), and was 0.1 mass%. Further, the BET specific surface area was 7.1 m 2 / g.​

Claims

1. A method for producing lithium vanadium phosphate having a sodium superion conductor (NASICON) structure, characterized in that: include: The first step is to mix a tetravalent or pentavalent vanadium compound, a phosphorus source, and a reducing sugar in an aqueous solvent to prepare a mixed slurry; Then, the mixed slurry is subjected to a second step of heating to dissolve it; Then, a lithium source is dissolved in an aqueous solvent to prepare a solution, and the solution is added to the solution obtained in the second step at 0-30° C. to prepare a dark blue raw material mixed solution; Then, the solution of the raw material mixture is spray-dried to obtain a reaction precursor in the fourth step; Then, a fifth step of firing the reaction precursor at 500-1300° C. in an inert gas atmosphere or a reducing atmosphere; and The sixth step is to heat-treat the lithium vanadium phosphate obtained after the fifth step at a temperature of 250 to 450° C. in an oxygen-containing atmosphere with an oxygen concentration of 5 Vol% or more. In the first step, the molar ratio V / P of the V atoms in the vanadium compound to the P atoms in the phosphorus source is 0.50 to 0.80, and the amount of the reducing sugar added is 0.3 to 40 parts by mass in terms of carbon atoms per 100 parts by mass of the produced lithium vanadium phosphate. In the third step, a molar ratio Li / P of Li atoms in the lithium source to P atoms in the phosphorus source is 0.70 to 1.

30.

2. The method for producing lithium vanadium phosphate according to claim 1, wherein: The temperature of the heating treatment in the second step is 60 to 100°C.

3. The method for producing lithium vanadium phosphate according to claim 1, wherein: The lithium sources are lithium hydroxide and lithium carbonate.

4. The method for producing lithium vanadium phosphate according to claim 1, wherein: The vanadium compound is vanadium pentoxide.

5. The method for producing lithium vanadium phosphate according to claim 1, wherein: The phosphorus source is phosphoric acid.

6. The method for producing lithium vanadium phosphate according to claim 1, wherein: The reducing sugar is selected from sucrose and lactose.

7. The method for producing lithium vanadium phosphate according to claim 1, wherein: In the first step, a Me source is further mixed into the mixed slurry, wherein Me represents a metal element having an atomic number of 11 or greater, excluding V, or a transition metal element.

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