Aqueous solution containing niobium polyacid ions, lithium ions, and phosphate ions, method for producing same, and method for producing active material for lithium secondary battery

Abstract

[Problem] To provide: an aqueous solution containing a polyacid ion of niobium, a lithium ion, and a phosphate ion; and a method for producing the aqueous solution. When the surface of a positive electrode active material particle of a lithium ion secondary battery is covered with a covering layer containing niobium, lithium, and phosphorus as a solid electrolyte, the positive electrode active material has excellent storage stability while suppressing an increase in the specific surface area of the covered positive electrode active material. [Solution] The present invention provides an aqueous solution containing niobium polyacid ions, lithium ions, and phosphate ions, the value of the ratio P / (Nb + Li + P) of the number of moles of phosphorus to the total number of moles of niobium, lithium, and phosphorus contained in the aqueous solution being 0.04 or more and less than 0.5, and the molar ratio Li / Nb of lithium to niobium being more than 0.6 and 2.0 or less, it is preferable that hydrogen peroxide is contained in an amount of 0.01-10% by mass (inclusive).

Description

Technical Field

[0001] The present invention relates to an aqueous solution containing niobium polyacid anions, lithium ions and phosphate ions, a method for producing the aqueous solution containing niobium polyacid anions, lithium ions and phosphate ions, and a method for producing an active material for a lithium secondary battery having a coating layer containing lithium niobate, and the aqueous solution containing niobium polyacid anions, lithium ions and phosphate ions is used to coat the surface of the positive electrode active material particles of a lithium ion secondary battery with lithium niobate as a solid electrolyte. Background Art

[0002] The positive electrode active material of a lithium ion secondary battery has generally been composed of a composite oxide of lithium and a transition metal. Among them, a composite oxide having Co in its composition, that is, lithium cobaltate (LiCoO2), has been widely used. In addition, recently, the use of lithium nickelate (LiNiO2), lithium manganate (LiMn2O4) or a ternary system (LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, etc.) or a composite thereof has also been increasing.

[0003] As the electrolyte of a lithium ion secondary battery, an electrolyte obtained by dissolving a lithium salt such as LiPF6 or LiBF4 in a mixed solvent of a cyclic carbonate such as PC (propylene carbonate) or EC (ethylene carbonate) and a chain ester such as DMC (dimethyl carbonate), EMC (ethyl methyl carbonate) or DEC (diethyl carbonate) has mainly been used. Such an organic solvent is not resistant to an oxidative atmosphere, and particularly easily undergoes an oxidative decomposition reaction when contacting a transition metal such as Co, Ni or Mn on the positive electrode surface. As the main reason, it is considered that the positive electrode surface has a high potential and the transition metal in a high oxidation state plays a catalytic role, etc. Therefore, it is effective to prevent the electrolyte from contacting the transition metal (for example, one or more of Co, Ni, Mn) constituting the positive electrode active material as much as possible in order to maintain the performance of the electrolyte.

[0004] In addition, as a method for fundamentally solving the problems of the above-mentioned organic solvent-based electrolyte, a all-solid-state lithium ion secondary battery in which the electrolyte is replaced with a non-flammable solid electrolyte has been proposed. Generally, the electrode reaction of a battery occurs at the interface between the electrode active material and the electrolyte. Here, when a liquid electrolyte is used as the electrolyte, the electrolyte penetrates the surface of the electrode active material present on the electrode to form a reaction interface for charge transfer. In the case of an all-solid-state battery, a solid electrolyte having ionic conductivity plays the role of the electrolyte, but the solid electrolyte itself does not have the fluidity of a liquid. Therefore, before forming the secondary battery, it is necessary to mix the powder that becomes the electrode active material with the solid electrolyte, or coat the powder that becomes the electrode active material with the solid electrolyte and perform pre-compositeization.

[0005] However, in the case of all-solid-state lithium-ion secondary batteries, there is a problem that the resistance generated when lithium ions move at the interface between the positive electrode active material and the solid electrolyte (hereinafter sometimes referred to as "interface resistance") increases, and the performance such as the battery capacity of the all-solid-state lithium-ion secondary battery is likely to decrease. It is known that the increase in this interface resistance is due to the formation of a high-resistance portion on the surface of the positive electrode active material due to the reaction between the positive electrode active material and the solid electrolyte, and the interface resistance can be reduced by covering the surface of lithium cobaltate, which is the positive electrode active material, with lithium niobate.

[0006] For example, in Patent Document 1, in order to improve the interface resistance between the positive electrode active material and the solid electrolyte, a technique of wet-covering the surface of the positive electrode active material with a lithium-ion conductive oxide containing niobium or tantalum and lithium by a wet method is disclosed. However, in this coating technique, since their alkoxides are used as the niobium or tantalum source and volatile organic compound absolute alcohol is used as the solvent, there is a concern about the deterioration of the working environment. In addition, in the case of this covering technique, explosion-proof measures and the like are required for the apparatus used in the covering, and there are also cost problems.

[0007] To solve these problems, for example, a method of covering with lithium niobate using water as a solvent is disclosed in Patent Document 2. The lithium niobate covering solution disclosed in Patent Document 2 makes a niobium oxide that is hardly soluble in water into a peroxy complex or an oxalic acid complex to become water-soluble. Therefore, in the case of the lithium niobate covering solution using a peroxy complex disclosed in Patent Document 2, more than 8 moles of hydrogen peroxide need to be added relative to 1 mole of niobic acid. Since niobic acid is insoluble in the hydrogen peroxide solution, more than 1 mole of ammonia also needs to be added relative to 1 mole of niobic acid. However, in the case of this covering solution, since the peroxy complex is chemically unstable, there is a problem of poor storage stability. In addition, in the case of this covering solution, there is also a problem that the metal components of the positive electrode active material, which is the covering object, are eluted due to the ammonia added to the covering solution.

[0008] As a technique for improving the chemical stability of the niobium peroxy complex, for example, a technique of adding a phosphoric acid-based ion to the lithium niobate covering solution is disclosed in Patent Document 3. In this case, although not explicitly described in Patent Document 3, it is considered that the phosphoric acid-based ion acts as a negative catalyst for the decomposition reaction of hydrogen peroxide. However, in the case of this covering solution, since a large amount of hydrogen peroxide and ammonia are required to make niobium into a peroxy complex, the chemical stability is insufficient, and the problem of elution of the metal components of the positive electrode active material cannot be solved.

[0009] As an aqueous niobate lithium covering solution with excellent chemical stability, a precursor aqueous solution of niobate lithium using a polyacid root ion of niobium and a manufacturing method thereof are disclosed in Patent Document 4. In the case of this precursor aqueous solution, niobium is solubilized in water in the form of a polyacid root ion, and since ammonia is not used, there is no problem of elution of the metal component of the positive electrode active material either. Prior Art Documents

[0010] Patent Documents Patent Document 1: International Publication No. 2007 / 004590 Patent Document 2: Japanese Patent Application Laid-Open No. 2012-074240 Patent Document 3: Japanese Patent Application Laid-Open No. 2017-191667 Patent Document 4: Japanese Patent Application Laid-Open No. 2020-066570 Summary of the Invention Problems to be Solved by the Invention

[0011] The precursor aqueous solution of niobate lithium disclosed in Patent Document 4 is a covering solution having excellent characteristics of excellent storage stability of the solution and suppression of dissolution of the positive electrode active material during covering. However, when using this precursor aqueous solution to cover niobate lithium on the surface of the positive electrode active material, there is a problem that the specific surface area of the positive electrode active material covered with niobate lithium is larger than that of the positive electrode active material before covering.

[0012] It is speculated that the increase in the specific surface area after covering is due to the formation of extremely fine pores in the niobate lithium covering layer. If the covering layer becomes porous, moisture is adsorbed on the surface of the positive electrode active material through these pores, which may lead to deterioration of the positive electrode active material. In addition, if the covering layer becomes porous, the interfacial bonding area between the covering layer as a solid electrolyte and the positive electrode active material decreases, which is not preferable.

[0013] An object of the present invention is to provide a precursor aqueous solution of niobate lithium and a manufacturing method thereof, such that when covering niobate lithium as a solid electrolyte on the surface of positive electrode active material particles of a lithium ion secondary battery, an increase in the specific surface area of the positive electrode active material covered with niobate lithium is suppressed, and the storage stability is excellent. Means for Solving the Problems

[0014] [1] To achieve the above object, in the present invention, an aqueous solution containing a polyacid root ion of niobium, a lithium ion, and a phosphate ion is provided, and the ratio P / (Nb + Li + P) of the number of moles of phosphorus contained in the aqueous solution to the total number of moles of niobium, lithium, and phosphorus is 0.04 or more and less than 0.5, and the molar ratio Li / Nb of lithium to niobium is greater than 0.6 and 2.0 or less. [2] The aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion described in item [1] above may further contain 0.01% by mass or more and 10% by mass or less of hydrogen peroxide. [3] The molar ratio of hydrogen peroxide to niobium (PO C / Nb) in the aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion described in item [2] above is preferably 0.01 or more and 1 or less. [4] The absorbance of the aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion described in item [1] above at a wavelength of 660 nm of the aqueous solution is preferably 0.1 or less. [5] Further, in the present invention, as a method for producing an aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion, there is provided a method for producing an aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion, which includes the following steps: a step of dissolving niobium hydrous oxide and lithium-containing salt in water to obtain an aqueous solution containing niobium polyoxoanion and lithium ion, and a step of adding phosphate ion to the aqueous solution containing niobium polyoxoanion and lithium ion. [6] Further, in the present invention, as a method for producing an aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion, there is provided a method for producing an aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion, which includes the following steps: a step of dissolving niobium hydrous oxide and lithium-containing salt in water to obtain an aqueous solution containing niobium polyoxoanion and lithium ion, and a step of adding phosphate ion and hydrogen peroxide to the aqueous solution containing niobium polyoxoanion and lithium ion. [7] In the method for producing an aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion described in item [5] or [6] above, preferably, the value of the molar ratio of phosphorus P / (Nb + Li + P) to the total molar number of niobium, lithium and phosphorus is 0.04 or more and less than 0.5, and the molar ratio of lithium to niobium Li / Nb is greater than 0.6 and 2.0 or less. [8] In the method for producing an aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion described in item [6] above, the concentration of hydrogen peroxide in the aqueous solution is preferably 0.01% by mass or more and 10% by mass or less. [9] In the method for producing an aqueous solution containing niobium polyoxoanion, lithium ion and phosphate ion described in item [6] above, the value of the molar ratio of the added amount of hydrogen peroxide to the molar number of niobium (PO A / Nb) is preferably 0.1 or more and 1.0 or less.

[10] In the method for manufacturing an aqueous solution containing a niobium-containing polyoxoanion, lithium ions, and phosphate ions described in the above item [5] or [6], relative to 1 mole of lithium in the aqueous solution containing the niobium-containing polyoxoanion and lithium ions, phosphate ions are preferably added at a rate of 0.07 mol / min or more and 3.07 mol / min or less.

[11] In the present invention, there is also provided a method for manufacturing a lithium secondary battery having a lithium niobate-containing coating layer, which includes the following steps: a step of covering the surface of the active material for a lithium secondary battery with the aqueous solution containing a niobium-containing polyoxoanion, lithium ions, and phosphate ions described in any one of the above items [1] to [4], and a step of heat-treating the active material for a lithium secondary battery that has been subjected to the above covering treatment. Advantages of the Invention

[0015] The storage stability of the aqueous solution containing a niobium-containing polyoxoanion, lithium ions, and phosphate ions obtained by the manufacturing method of the present invention is excellent. Even when using this aqueous solution to form a coating layer on the surface of the positive electrode active material for a lithium ion secondary battery, an increase in specific surface area can be suppressed. Therefore, the present invention can contribute to improving the performance of all-solid-state lithium ion secondary batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is the FT-IR spectrum of the aqueous solutions obtained in Comparative Example 1 and Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0017] [Positive Electrode Active Material] The positive electrode active material covered (coated) with the aqueous solution containing a niobium-containing polyoxoanion, lithium ions, and phosphate ions of the present invention is not included in the scope of the present invention, but as an example, the following substances can be mentioned. The positive electrode active material is composed of a composite oxide of Li and a transition metal M, and substances conventionally used in lithium ion secondary batteries, such as lithium cobaltate (Li 1+X CoO2, -0.1 ≤ X ≤ 0.3), Li 1+X NiO2, Li 1+X Mn2O4, Li 1+X Ni 1 / 2 Mn 1 / 2 O2, Li 1+X Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (all -0.1 ≤ X ≤ 0.3), Li 1+X [Ni Y Li 1 / 3-2Y / 3 Mn 2 / 3-Y / 3O2 (0 ≤ X ≤ 1, 0 < Y < 1 / 2), etc., or a lithium multi-transition metal oxide in which a part of Li or a transition metal element thereof is replaced by another element such as Al, Li 1+X FePO4, Li 1+X MnPO4 (both -0.1 ≤ X ≤ 0.3), etc., phosphates having an olivine structure, lithium manganate (LiMnO4) as a spinel-type compound, or a compound in which a part of Mn is replaced by Al or Ti, Cr, Fe, Zr, Y, W, Ta, Nb, Ni, Co, Fe, etc. (LiAl 0.1 Mn 0.9 O4, LiNi 0.5 Mn 1.5 O4, etc.).

[0018] [Nb source] Anhydrous niobium oxide (Nb2O5) is hardly soluble in water. Therefore, in the manufacturing method of the present invention, amorphous and water-soluble hydrated niobium oxide is used as the Nb source. The hydrated niobium oxide is a substance represented by the general formula Nb2O5·nH2O (n is not 0, for example, 3 ≤ n ≤ 16).

[0019] [Polyoxoanion] Polyoxoacid is an anionic species formed by condensation of oxoacids. The polyoxoacid of a transition metal element can be regarded as a molecular ionic species of a metal oxide. It should be noted that when the metal element is one kind, it is called heteropolyoxoacid, and when the metal element is multiple kinds, it is called heteropolyoxoacid.

[0020] At present, a potential-pH diagram for niobium has not been established. However, since niobium hydroxide precipitates in the neutral pH region, it is considered that in this pH region, solid phases such as Nb(OH)5 or HNbO3 are stable chemical substances. If an alkali is added to an aqueous solution in which these solid phases exist to raise the pH of the system, due to the presence of excessive OH - ions, niobium starts to dissolve in the form of, for example, Nb(OH)6 - or NbO3 - and so on. When the pH of the system further increases, it is considered that the soluble niobium oxide takes various condensation states according to the pH of the system. However, in this specification, it is considered that the niobium oxide dissolved on the alkali side together forms polyoxoanions of niobium. It should be noted that the polyoxoanions of niobium dissolved at once on the alkali side maintain the polyoxoacid form even if phosphoric acid is added to lower the pH of the system to about 4. The aqueous solution of the present invention containing polyoxoanions of niobium, lithium ions, and phosphate ions contains niobium in the form of polyoxoacid, and thus has excellent storage stability. The existence form of niobium only needs to be soluble polyoxoanions, and its structure is not particularly limited. It can be confirmed by FT-IR measurement that the niobium in the aqueous solution is polyoxoanions. When the niobium in the aqueous solution adopts the form of polyoxoanions, at a wave number of 850 cm-1 ±20 cm -1 Absorption peaks caused by Nb-O bonds were observed nearby.

[0021] [Li source] In the manufacturing method of the present invention, lithium hydroxide (LiOH) is used as the Li source. LiOH can be anhydrous or hydrated, either is acceptable. The LiOH added to the aqueous solution dissociates into Li + and OH - , showing strong alkalinity. In the manufacturing methods of the precursor aqueous solutions of lithium niobate described in Patent Document 2 and Patent Document 3, first, niobic acid is reacted with hydrogen peroxide, then ammonia water is added to raise the pH, and finally, an Li salt is added to adjust the precursor aqueous solution. In contrast, the manufacturing method of the aqueous solution containing polyoxoanions of niobium, lithium ions, and phosphate ions in the present invention is characterized in that first, LiOH is added to the aqueous solution containing niobic acid to dissolve the niobic acid.

[0022] LiOH itself is a strong base. Therefore, when LiOH is added to the aqueous solution containing niobic acid, the pH of the system rises, niobium oxide dissolves, and polyoxoanions of niobium are stably formed. After that, hydrogen peroxide is added to the alkaline aqueous solution containing polyoxoanions of niobium and lithium ions. The effect of hydrogen peroxide will be described later.

[0023] Since a covering layer of a solid electrolyte having ionic conductivity is finally formed, the molar ratio Li / Nb of the amount of lithium added to 1 mole of niobium contained in the polyoxoanions of niobium is greater than 0.6 and 2.0 or less. If Li / Nb is 0.6 or less, due to lithium deficiency, the lithium ion conductivity becomes poor, so it is not preferred. In addition, if Li / Nb exceeds 2.0, the excessive lithium hydroxide will cause the pH to rise, resulting in poor storage stability, so it is not preferred. From the viewpoint of ionic conductivity, Li / Nb is preferably 0.9 or more, more preferably 0.95 or more. In addition, from the viewpoint of ionic conductivity, Li / Nb is preferably 1.5 or less, more preferably 1.4 or less.

[0024] The reason why the storage stability of the aqueous solution containing polyoxoanions of niobium, lithium ions, and phosphate ions in the present invention is better than that of the precursor aqueous solutions obtained by the techniques described in Patent Document 2 and Patent Document 3 is presumed as follows. In the techniques described in Patent Document 2 and Patent Document 3, it is known that niobium exists in the form of a peroxo complex in the aqueous solution, and the peroxo ions coordinated to niobium are likely to dissociate and are unstable. In contrast, in the aqueous solution of the present invention containing niobium polyoxoanion, lithium ion and phosphate ion, niobium mainly takes a relatively stable polyoxo form. When hydrogen peroxide is added to this aqueous solution, the storage stability is further improved. Since the aqueous solution of the present invention does not contain organic substances like the covering liquid described in Non-Patent Document 1, a covering layer with a low volume resistivity can finally be obtained.

[0025] [Phosphate ion] The greatest technical feature of the present invention is the coexistence of phosphate ions in an aqueous solution containing niobium polyoxoanion and lithium ion. As a supply source of phosphate ions, orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid or their water-soluble phosphoric (PO4 3- ) salts such as sodium salts and ammonium salts can be used. Here, phosphoric acid is a ternary acid and dissociates in three stages in an aqueous solution, so it can take various existing forms such as phosphate ions, hydrogen phosphate ions, dihydrogen phosphate ions, pyrophosphate ions, etc. in the aqueous solution. However, its existing form is not the type of drug used as a supply source of phosphate ions, but is determined by the pH of the aqueous solution. Therefore, the ions containing the above-mentioned phosphate groups are collectively referred to as phosphate ions. It should be noted that in the case of the aqueous solution of the present invention containing niobium polyoxoanion, lithium ion and phosphate ion, an absorption peak caused by phosphate ions is observed at 1000 - 1150 cm -1 in the FT-IR measurement.

[0026] When phosphate ions coexist in an aqueous solution containing niobium polyoxoanion and lithium ion, the reason for suppressing the increase in the specific surface area of the covered positive electrode active material is not yet clear at present, but the inventors of the present invention presume as follows. That is, it is considered that there is the following mechanism: the presence of phosphate ions contained in the aqueous solution of the present invention containing niobium polyoxoanion, lithium ion and phosphate ion improves the wettability of this aqueous solution to the surface of the positive electrode active material. Through the action of the phosphate ions contained in the aqueous solution, the double-layer structure at the interface changes, the structure of the covering layer changes, or the trace phosphate ions contained in the covering layer inhibit the generation of pores, etc. in the covering layer during drying.

[0027] In the method for manufacturing an aqueous solution containing a niobium-containing polyoxoanion, lithium ions, and phosphate ions according to the present invention, by adding the above-mentioned aqueous phosphate ion solution to an aqueous solution of a polyacid containing lithium and niobium that stably forms a niobium-containing polyoxoanion, precipitation of niobic acid in the aqueous solution containing the niobium-containing polyoxoanion, lithium ions, and phosphate ions can be suppressed. At this time, since the concentration of phosphate ions rises sharply, precipitates (colloids) mainly composed of Li3PO4 and the like may be generated in large amounts. If the concentration of colloidal particles in the aqueous solution containing the niobium-containing polyoxoanion, lithium ions, and phosphate ions is high, it becomes the main cause of the increase in the specific surface area of the covered positive electrode active material, which is not preferable.

[0028] In order to prevent the above-mentioned colloid generation, it is preferable to adjust the addition rate of the aqueous phosphate ion solution to the aqueous solution containing the niobium-containing polyoxoanion and lithium ions. In order to prevent colloid generation, the addition rate of the aqueous phosphate ion solution is preferably 3.07 mol / min or less, more preferably 2 mol / min or less, and further preferably 1 mol / min or less or 0.6 mol / min or less with respect to 1 mol of lithium in the aqueous solution containing the niobium-containing polyoxoanion and lithium ions. The lower limit value of the addition rate is not particularly limited, but from the viewpoint of the manufacturing time, for example, it may be 0.07 mol / min or more.

[0029] The presence or absence of the above-mentioned colloid generation can be confirmed by the absorbance value at a wavelength of 660 nm in the aqueous solution containing the niobium-containing polyoxoanion, lithium ions, and phosphate ions. This is because the absorbance at a wavelength of 660 nm indicates the intensity of scattered light caused by colloidal particles present in the aqueous solution containing the niobium-containing polyoxoanion, lithium ions, and phosphate ions. Therefore, a high absorbance for the precursor solution of the present invention indicates a high concentration of fine particles present in the aqueous solution (for example, refer to JIS-K0101).

[0030] Moreover, if the positive electrode active material is covered with the aqueous solution having a high concentration of these fine particles, the fine particles adhere to the surface of the positive electrode active material, the covering layer becomes uneven, and the thickness of the covering layer becomes uneven, which may become a factor for increasing the specific surface area. According to the research by the present inventors, if the absorbance value at a wavelength of 660 nm of the aqueous solution containing the niobium-containing polyoxoanion, lithium ions, and phosphate ions is 0.1 or less, even if colloidal particles are present in the aqueous solution, the amount thereof is small, the unevenness of the thickness of the covering layer is suppressed, and the increase in the specific surface area is also suppressed. More preferably, the absorbance value is 0.08 or less, and further preferably 0.07 or less. In the present invention, the lower limit value of the absorbance is not particularly specified, and for example, it may be 0.001 or more.

[0031] Regarding the content of phosphate ions in an aqueous solution containing niobium-containing polyoxoanion ions, lithium ions, and phosphate ions, the ratio P / (Nb + Li + P) of the number of moles of phosphorus contained in the aqueous solution to the total number of moles of niobium, lithium, and phosphorus is preferably 0.04 or more and less than 0.5. When the above ratio is less than 0.04, the addition effect of phosphate ions is insufficient. Additionally, from the perspective of ionic conductivity, P / (Nb + Li + P) is preferably 0.08 or more. When the value of P / (Nb + Li + P) is 0.5 or more, the addition effect of phosphate ions saturates. From the perspective of ionic conductivity, P / (Nb + Li + P) is preferably 0.35 or less. The range of P / (Nb + Li + P) is more preferably 0.1 or more and 0.4 or less, and even more preferably 0.2 or more and 0.4 or less.

[0032] The total content of lithium, niobium, and phosphorus in the aqueous solution containing niobium-containing polyoxoanion ions, lithium ions, and phosphate ions is preferably 1% by mass or more and 20% by mass or less, more preferably 3% by mass or more and 10% by mass or less, and even more preferably 5% by mass or more and 8% by mass or less. When the total content of lithium, niobium, and phosphorus is less than 1% by mass, in order to obtain the necessary film thickness of the coating layer, it is necessary to increase the amount of the aqueous solution containing niobium-containing polyoxoanion ions, lithium ions, and phosphate ions, and the amount of water increases, which is disadvantageous in terms of cost. When the concentration is low, in order to achieve the necessary coating film thickness, the amount of water increases, which is uneconomical. When the total content of lithium, niobium, and phosphorus exceeds 20% by mass, the viscosity of the aqueous solution containing niobium-containing polyoxoanion ions, lithium ions, and phosphate ions increases, and there is a possibility of clogging the fine pipes of the device used for the coating treatment.

[0033] In the aqueous solution containing niobium-containing polyoxoanion ions, lithium ions, and phosphate ions, the amount of impurity elements other than lithium, niobium, phosphorus, oxygen, and hydrogen is preferably 10% by mass or less. More preferably 1% by mass or less. As inevitable impurities, carbonate ions, etc. are considered. Here, impurities refer to the impurities contained in the evaporation residue when the precursor aqueous solution is evaporated to dryness at 200°C. It should be noted that when evaporated to dryness at 200°C, carbonate ions combine with the cations as counterions and precipitate as carbonates.

[0034] [Hydrogen peroxide] In order to improve the storage stability of the aqueous solution of the present invention, hydrogen peroxide can be further added. At this time, the concentration of hydrogen peroxide in the aqueous solution is preferably added so as to be 0.01% by mass or more and 10% by mass or less. When the concentration of hydrogen peroxide is less than 0.01% by mass, the formed niobium polyacid reacts with hydroxide ions and is liable to decompose. In addition, when it exceeds 10% by mass, sometimes the formed niobium polyacid reacts with hydrogen peroxide to form an unstable peroxo complex. The concentration of hydrogen peroxide in the aqueous solution is preferably added so as to be 0.05% by mass or more and 5% by mass or less, and more preferably so as to be 0.1% by mass or more and 1% by mass or less.

[0035] It should be noted that the molar ratio of hydrogen peroxide to the added amount of niobium (PO A / Nb) is preferably in the range of 0.1 or more and 1 or less. By setting it to 0.1 or more, it is beneficial to avoid the decomposition of the niobium polyacid by reaction with hydroxide ions. In addition, by setting it to 1 or less, it is beneficial to avoid the formation of an unstable peroxo complex due to the reaction of the formed niobium polyacid with hydrogen peroxide. The molar ratio of hydrogen peroxide to the added amount of niobium (PO A / Nb) is preferably 0.05 or more and 0.5 or less, and more preferably 0.1 or more and 0.35 or less. From the viewpoint of reactivity, the addition of the above-mentioned hydrogen peroxide is preferably carried out after dissolving hydrated niobium oxide and LiOH together to form stable niobium polyacid anions. It should be noted that either the addition of the above-mentioned phosphate ions or the addition of hydrogen peroxide in this item can be carried out first.

[0036] Here, the added hydrogen peroxide decomposes over time, but it is preferably contained in the form of hydrogen peroxide of 0.01% by mass or more and 6.7% by mass or less in the aqueous solution. When hydrogen peroxide is less than 0.01% by mass, the effect of maintaining the stability of the precursor aqueous solution is small. In addition, when hydrogen peroxide exceeds 6.7% by mass, the effect of adding hydrogen peroxide is saturated. The concentration of hydrogen peroxide in the aqueous solution is preferably 0.05% by mass or more and 5% by mass or less, and more preferably 0.1% by mass or more and 1% by mass or less.

[0037] In addition, the molar ratio of hydrogen peroxide to 1 mole of niobium (PO C / Nb) calculated from the concentration of hydrogen peroxide and niobium present in the aqueous solution is preferably in the range of 0.01 or more and 1 or less. If it exceeds the above range, not only is the addition effect of hydrogen peroxide saturated, but also when used as a coating solution, it may dissolve the positive electrode active material. If hydrogen peroxide is too little, the niobium polyacid reacts with hydroxide ions and is liable to decompose. PO C / Nb is more preferably in the range of 0.05 or more and 0.5 or less, and further preferably 0.1 or more and 0.35 or less.

[0038] [pH] In the manufacturing method of the present invention, the pH of the aqueous solution when forming the polyoxo anion of niobium is basically automatically determined by the addition amounts of strong base LiOH, phosphoric acid, and hydrogen peroxide. Therefore, no special pH adjustment is required, but on the alkaline side, it is 7.5 to 12.5. For stable formation of the polyoxo anion of niobium, the pH is more preferably 9.0 to 12.0. When phosphoric acid is added to the solution containing the polyoxo acid of niobium and lithium, the pH of the system decreases, but it is confirmed that the polyoxo acid of niobium stably exists when the pH is 3 or more. The pH of the solution after adding phosphoric acid is preferably 12.5 or less, and more preferably 12 or less. It should be noted that here, the pH value is corrected based on JIS Z8802 using an appropriate buffer solution according to the pH region to be measured, and is measured using a glass electrode with a pH meter equipped with a temperature compensation electrode.

[0039] [Manufacturing method] In the manufacturing method of the aqueous solution containing polyoxo anion of niobium, lithium ion, and phosphate ion of the present invention, as described above, hydrous niobium oxide is used as the starting material of niobium. After dissolving hydrous niobium oxide and LiOH in water to obtain an aqueous solution containing polyoxo anion of niobium and lithium, phosphate ions are added to this aqueous solution to obtain an aqueous solution containing polyoxo anion of niobium, lithium ion, and phosphate ion. At this time, the ratio P / (Nb + Li + P) of the number of moles of phosphorus to the total number of moles of niobium, lithium, and phosphorus is 0.04 or more and less than 0.5, and the molar ratio Li / Nb of lithium to niobium is greater than 0.6 and 2.0 or less. In addition, the total concentration of niobium is preferably 1% by mass or more and 20% by mass or less in terms of niobium. When adding hydrogen peroxide, it is preferably added in such a way that the concentration of hydrogen peroxide in the aqueous solution becomes 0.01% by mass or more and 10% by mass or less.

[0040] [Coating treatment] As an embodiment of coating the positive electrode active material for a lithium ion secondary battery with the aqueous solution containing polyoxo anion of niobium, lithium ion, and phosphate ion of the present invention, the following can be cited. As the positive electrode active material to be the object of the coating treatment, a positive electrode active material with a specific surface area of about 0.1 to 2 m 2 / g can be used. The specific surface area of the positive electrode active material is inversely proportional to its particle size, so the specific surface area of the positive electrode active material is appropriately selected according to the use of the lithium ion secondary battery. The thickness of the coating layer covering the positive electrode active material is preferably 3 to 20 nm, and more preferably 5 to 15 nm. In the coating treatment, using a known method such as a rotary fluidized bed coating method or a collision shearing method of high-speed air flow, after the positive electrode active material is brought into contact with the aqueous solution containing niobium polyacid root ions, lithium ions and phosphate ions of the present invention, by performing heat treatment such as drying, a positive electrode active material having a coating layer can be obtained. Example

[0041] [Qualitative evaluation of aqueous solution] In the qualitative evaluation of the types of dissolved chemical substances in the aqueous solution, it is carried out by a Fourier transform infrared spectrometer (FT-IR) using NICOLET7600 manufactured by Thermo Fisher Scientific. In order to remove the peaks on the low wavenumber side of water as the solvent, water is used for the blank measurement, and the measurement is carried out by the attenuated total reflection (ATR) method. A germanium prism is used in the measurement, and the incident angle is 45°. By performing this FT-IR measurement, it can be confirmed that in the aqueous solution containing niobium polyacid root ions, lithium ions and phosphate ions of the present invention, niobium exists in the form of polyacid root ions and phosphorus exists in the form of phosphate ions.

[0042] [Quantification of Li, Nb and P in aqueous solution] The composition of the aqueous solution is analyzed by an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Agilent Technology 700 series). Weigh 0.1 g of the sample from the precursor solution, add 15 mL of pure water and 5 mL of hydrochloric acid thereto, heat and then cool. Then add 2 mL of hydrogen peroxide solution, cool, and then make the liquid volume up to 100 mL to perform ICP-AES analysis. The mole numbers of the present invention are calculated from the contents of niobium, lithium and phosphorus obtained by the quantitative analysis of this ICP.

[0043] [Quantitative method for hydrogen peroxide in aqueous solution] Before the quantitative operation, a Ti-PAR solution is prepared in advance. The Ti-PAR solution is prepared by mixing "20 mL of a Ti solution with a titanium concentration of 1 mmol / L prepared using a commercially available titanium standard solution" with "15 mL of a PAR solution in which 11 mg of PAR (4-(2-pyridylazo)-resorcinol) is dissolved in a 1 mass% sodium hydroxide aqueous solution and made up to 50 mL with ultrapure water". In a plastic container pre-filled with 0.05 mL of reagent grade nitric acid and 20 mL of reagent grade methanol, add 0.1 mL of the aqueous solution as the sample, stir gently and mix, let the precipitate stand, and then filter using a 0.45 μm filter. It should be noted that at this time, it is preferably about 1 minute from the addition of the solution to the start of filtration. Take 0.25 mL of the filtrate, and successively add 5 mL of methanol, 1 mL of Ti-PAR solution, 5 mL of a mixed 1 mol / L ammonia water, and 1.2 mL of a buffer solution with a pH of 8.6 prepared by adding 40 mL of 1 mol / L ammonium chloride. After allowing the measurement solution made up to 10 mL with methanol to stand at 40 - 45 °C for 30 minutes, use a spectrophotometer U-2800 manufactured by Hitachi High-Technologies Corporation to measure the absorbance at a wavelength of 520 nm. It should be noted that for the quantification of hydrogen peroxide concentration, the standard addition method is used, and it is measured by the relative intensity of the absorbance of hydrogen peroxide and the standard solution. The molar number of the present invention is calculated from the content of hydrogen peroxide obtained by this quantitative analysis.

[0044] [Measurement of absorbance] Take 3.5 mL of the aqueous solution in a quartz cell (10 mm × 10 mm × 45 mm), and use a spectrophotometer U-2800 manufactured by Hitachi High-Technologies Corporation to measure the absorbance at wavelengths from 400 to 700 nm. The temperature of the precursor solution during the measurement is 25 °C. At this time, the zero point of the absorbance is measured using the absorbance when ultrapure water with a conductivity of 17 MΩ·cm or more at 25 °C is filled in the measurement cell. It should be noted that the absorbance at a wavelength of 660 nm indicates the intensity of the scattered light generated by the fine particles in the precursor aqueous solution, and a higher absorbance indicates a higher fine particle concentration (refer to JIS-K0101).

[0045] [Measurement of BET specific surface area] The BET specific surface area of the positive electrode active material before and after coating is determined by the BET single-point method using a 4-Sorb US manufactured by Yuasa Ionics Corporation, Japan. The increase rate of the BET specific surface area before and after coating is defined by the following formula (1). BET specific surface area increase rate = 100 × (BET A - BET B ) / BET B …(1) Here, BET A is the BET specific surface area after coating, and BET B is the BET specific surface area before coating. In the present invention, when the BET specific surface area increase rate is 100% or less, it is determined that the increase in the specific surface area is suppressed.

[0046] [Evaluation of ionic conductivity] The ionic conductivity of the coating obtained from the aqueous solution of the present invention was evaluated using the bulk powder formed from the aqueous solution. The aqueous solution was evaporated to dryness in a dryer set at 100 °C, and the resulting powder was further fired in the atmosphere at 300 °C for 12 hours. 0.5 g of the fired powder was placed in a mold with a diameter of 10 mm and pressed by a press at 20 MPa to obtain a pressed powder. For the obtained pressed powder, in an Ar atmosphere at a temperature of 25 °C, an impedance measurement was performed in the frequency range of 100 Hz to 4 MHz using a potentiostat / galvanostat (1470E manufactured by Solartron) and a frequency response analyzer (1255B manufactured by Solartron). The resistance value of the oxide powder sample was obtained from the intercept of the Cole-Cole curve (complex impedance plane diagram) of the measured value, and the ionic conductivity was calculated from the obtained resistance value.

[0047] [Calculation of the thickness of the coating] The thickness of the coating obtained by the above coating treatment was calculated assuming that all the aqueous solution used in the coating treatment was used to form the coating. At this time, it was considered that lithium, niobium, and phosphorus contained in the aqueous solution were changed to Li2O, Nb2O5, and P2O5, respectively, and the volume of their mixture was calculated. The thickness of the coating is defined by the following formula (2). [Thickness of the coating (nm)] = 10 3 × [Volume of the mixture of Li2O, Nb2O5, and P2O5 (cm 3 )] / [Amount of positive electrode active material × Specific surface area of positive electrode active material (m 2 / g)]…(2) Here, the volume of the above mixture was obtained as follows. When the Li content in the aqueous solution was set as a (mass %), the Nb content was set as b (mass %), and the P content was set as c (mass %), it was obtained from the atomic weights of Li, Nb, and P and the molecular weights of Li2O, Nb2O5, and P2O5: Li2O content (mass %) a′ = a × Molecular weight of Li2O / (Atomic weight of Li × 2) Nb2O5 content (mass %) b′ = b × Molecular weight of Nb2O5 / (Atomic weight of Nb × 2) P2O5 content (mass %) c′ = c × Molecular weight of P2O5 / (Atomic weight of P × 2) When the amount of the aqueous solution used in the coating treatment was set as x (g), the density of Li2O was set as 2.0 g / cm 3 , the density of Nb2O5 was set as 4.6 g / cm 3 , and the density of P2O5 was set as 2.4 g / cm 3 , Volume (cm of the mixture of Li2O, Nb2O5 and P2O5 3 ) = (a' / 2.0 + b' / 4.6 + c' / 2.4) × x / 100

[0048] [Comparative Example 1] Put 81.6 g of pure water and 4.195 g of lithium hydroxide monohydrate (LiOH·H2O) into a 0.5 L beaker. After stirring to dissolve lithium hydroxide, mix 13.84 g of hydrated niobium oxide (Nb2O5·nH2O, Nb concentration: 67.4 mass%) dried at 300°C. Stir at 70°C for 8 hours to dissolve the hydrated niobium oxide, and obtain a clear aqueous solution containing niobium polyacid ions and lithium ions. The pH of the obtained aqueous solution is 11.8. At this time, the molar ratio Li / Nb of the lithium addition amount to 1 mole of niobium contained in the niobium polyacid ions is 1.0. Keep the aqueous solution containing niobium and lithium at 20°C, and at the same time add 3.2 g of a 30% aqueous solution of hydrogen peroxide. For final concentration adjustment, further add 71 g of pure water to obtain an aqueous solution containing niobium polyacid ions and lithium ions. At this time, the hydrogen peroxide concentration in the aqueous solution is 0.49 mass%. No turbidity was visually observed in this aqueous solution. As a result of measuring the absorbance of the aqueous solution of Comparative Example 1, the absorbance value at a wavelength of 660 nm is 0.01. The preparation conditions of the aqueous solution of this comparative example are shown in Table 1, and the calculated values of the concentrations of each element in the aqueous solution and the hydrogen peroxide concentration are shown in Table 2. As a result of FT-IR measurement of the aqueous solution of this comparative example, an absorption peak caused by the Nb-O bond was observed near a wavenumber of 850 cm -1 ±20 cm -1 It can be considered that the dissolved hydrated niobium oxide exists in the form of niobium polyacid ions. The FT-IR spectrum of the aqueous solution obtained by this comparative example is as Figure 1 shown.

[0049] Using a rotary fluidized bed coating device (manufactured by MP-micro Powrex), while spraying 25.2 g of the aqueous solution obtained in this comparative example onto the positive electrode active material (LiNi 2 / g, manufactured by MTI Corporation, EQ-Lib-LNCM111) with a BET specific surface area of 0.636 m 1 / 3 Mn 1 / 3 Co 1 / 3 O2, dry the aqueous solution in parallel, thereby covering the surface of the positive electrode active material with the aqueous solution. It should be noted that the operating conditions of the rotary fluidized bed coating device are set as follows: intake air: atmosphere, intake air temperature: 100°C, intake air volume: 0.1 m 3 / h, rotor speed: 1000 revolutions per minute, spraying speed: 0.5 g / minute. The amount of the above aqueous solution is the amount equivalent to a 10 nm thickness of the coating layer. The BET specific surface area after coating is 1.91 m 2 / g. When the coating treatment is carried out using the aqueous solution of this comparative example, the BET specific surface area increase rate is 200%. The coating treatment conditions and the BET specific surface area increase rate are shown in Table 3.

[0050] [Example 1] Before adding hydrogen peroxide, following the same procedure as in Comparative Example 1 above, after further adding 1.0 g of phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd., 80% by mass), 75 g of pure water for concentration adjustment was added to obtain an aqueous solution containing niobium polyacid ions, lithium ions, and phosphate ions of Example 1. It should be noted that when adding phosphoric acid, while stirring the solution, phosphate ions were added at a rate of 0.25 mol / minute relative to 1 mol of lithium in the aqueous solution containing niobium polyacid ions and lithium ions. At this time, P / (Nb + Li + P) was 0.04, and no formation of white colloid was observed. As a result of measuring the absorbance of the aqueous solution of Example 1, the absorbance value at a wavelength of 660 nm was 0.01.

[0051] As a result of FT-IR measurement of the aqueous solution obtained in this example, an absorption peak caused by the Nb-O bond was also observed near a wavenumber of 850 cm -1 ±20 cm -1 It can be considered that the dissolved hydrated niobium oxide exists in the form of niobium polyacid ions. In addition, an absorption peak caused by phosphate ions was observed at a wavenumber of 1000 - 1150 cm -1 . The ionic conductivity of the powder obtained by evaporating and drying the aqueous solution obtained in this example at 300 °C was 1.225×10 -8 S / cm. Using 25.2 g of the aqueous solution obtained in this example, the coating treatment was carried out under the same conditions as in Comparative Example 1. As a result, the thickness of the coating layer was 10 nm, and the BET specific surface area increase rate was 90%. Compared with the above Comparative Example 1, an effect of suppressing the increase in BET specific surface area was observed. The preparation conditions of the aqueous solution of this example are shown in Table 1, the calculated values of the concentrations of each element in the aqueous solution and the hydrogen peroxide concentration are shown in Table 2, the coating treatment conditions and the BET specific surface area increase rate are shown in Table 3, and the ionic conductivity of the powder obtained by evaporation and drying is shown in Table 4.

[0052] [Examples 2 - 5] Before adding hydrogen peroxide, in the same steps as in Example 1 above, the addition amounts of phosphoric acid and pure water for concentration adjustment and the addition rate of phosphate ions were varied as shown in Table 1, and aqueous solutions containing niobium polyacid ions, lithium ions, and phosphate ions of Examples 2 to 5 were obtained. At this time, P / (Nb + Li + P) was 0.10 to 0.40, but the formation of white colloid was not observed in any of the examples. In addition, for each example, the results obtained by performing absorbance measurement in the same manner as in Example 1 are shown in Table 2 together. As a result of performing FT-IR measurement on any of the examples, an absorption peak caused by Nb-O bond was observed near the wave number of 850 cm -1 ±20 cm -1 It can be considered that the dissolved hydrated niobium oxide exists in the form of niobium polyacid ions. In addition, an absorption peak caused by phosphate ions was observed at the wave number of 1000 to 1150 cm -1 . The FT-IR spectrum of the aqueous solution obtained in this example is shown in Fig. 3 together. Using the aqueous solutions obtained in Examples 2 to 5, a coating treatment was performed under the same conditions as in Comparative Example 1. The amount of the aqueous solution used was adjusted so that the thickness of the coating layer was 10 nm in each case. As a result, the BET specific surface area increase rate was -1% to 27%. The preparation conditions of the aqueous solutions of Examples 2 to 5 are shown in Table 1, the calculated values of the concentrations of each element in the aqueous solution and the hydrogen peroxide concentration are shown in Table 2, the conditions of the coating treatment and the BET specific surface area increase rate are shown in Table 3, and the ionic conductivity of the powder obtained by evaporation to dryness is shown in Table 4. From the results of Table 4, it can be said that from the viewpoint of ionic conductivity, the P / (Li + Nb + P) ratio is preferably 0.35 or less, and more preferably 0.08 or more.

[0053] [Examples 6 to 9] In the same steps as in Example 1, the addition amounts of lithium hydroxide, phosphoric acid, and pure water for concentration adjustment and the addition rate of phosphate ions were varied as shown in Table 1, respectively, and thus aqueous solutions containing niobium polyacid ions, lithium ions, and phosphate ions of Examples 6 to 9 in which the values of Li / Nb and P / (Nb + Li + P) were changed were obtained. At this time, Li / Nb was 0.78 to 2.00, and P / (Nb + Li + P) was 0.20 to 0.40. The formation of white colloid was not observed in any of the examples. In addition, for each example, the results obtained by performing absorbance measurement in the same manner as in Example 1 are shown in Table 2 together. As a result of performing FT-IR measurement on any of the examples, an absorption peak caused by Nb-O bond was observed near the wave number of 850 cm -1 ±20 cm -1 It can be considered that the dissolved hydrated niobium oxide exists in the form of niobium polyacid ions. In addition, an absorption peak caused by phosphate ions was observed at the wave number of 1000 to 1150 cm -1Absorption peaks caused by phosphate ions were observed at this position. Using the aqueous solutions obtained in Examples 6 to 9, coating treatments were carried out under the same conditions as in Comparative Example 1. The amount of the aqueous solution used was adjusted so that the thickness of the coating layer was all 10 nm. As a result, the BET specific surface area increase rate was -2% to -3%. The preparation conditions of the aqueous solutions of Examples 6 to 9 are shown in Table 1, the calculated values of the concentrations of various elements in the aqueous solution and the hydrogen peroxide concentration are shown in Table 2, the coating treatment conditions and the BET specific surface area increase rate are shown in Table 3, and the ionic conductivities of the powders obtained by evaporation to dryness of Examples 6 and 7 are shown in Table 5. From the results in Table 5, it can be said that from the viewpoint of ionic conductivity, the Li / Nb ratio is preferably 0.9 or more, more preferably 0.95 or more.

[0054] [Example 10] 36.3 g of the aqueous solution containing polyoxoniobate ions, lithium ions, and phosphate ions obtained in Example 4 was used to coat the positive electrode active material with a BET specific surface area of 1.085 m 2 / g. As a result, the BET specific surface area increase rate was -1%.

[0055] [Example 11] The addition rate of phosphate ions was set to 3.38 mol / min. Otherwise, an aqueous solution containing polyoxoniobate ions, lithium ions, and phosphate ions was obtained according to the same procedure as in Example 2. In this example, as a result of adding phosphoric acid, a small amount of white colloid was generated, but no turbidity of the aqueous solution was visually observed. The absorbance obtained by measuring the absorbance of this example is shown in Table 2. Using the aqueous solution obtained in this example, a coating treatment was carried out under the same conditions as in Comparative Example 1. The amount of the aqueous solution used was adjusted so that the thickness of the coating layer was all 10 nm. As a result, the increase rate of the BET specific surface area was 73%. Compared with the above Comparative Example 1, an effect of suppressing the increase in the BET specific surface area was observed. From the comparison results of Example 2 and Example 11, for the addition rate of phosphate ions, by setting the addition rate of phosphate ions to 3.07 mol / min or less relative to 1 mol of lithium in the aqueous solution containing polyoxoniobate ions and lithium ions, the generation of colloidal particles can be suppressed, and further the increase in the BET specific surface area caused by coating can be suppressed.

[0056] [Comparative Example 2] Before adding hydrogen peroxide, following the same procedure as in Comparative Example 1 above, 24.5 g of phosphoric acid was further added at a rate of 0.24 mol / min of phosphate ions relative to 1 mol of lithium in the aqueous solution containing niobium-containing polyoxoanions and lithium ions. As a result, a large amount of white colloid was produced. At this time, P / (Nb + Li + P) was 0.50. This white colloid did not dissolve even when diluted with pure water for concentration adjustment, and the aqueous solution was observed to be turbid by visual inspection. Therefore, the coating treatment using this comparative example was not carried out.

[0057] [Comparative Example 3] The amount of lithium hydroxide added was set to 2.517 g and Li / Nb was set to 0.50. Otherwise, following the same procedure as in Comparative Example 1 above, 4.9 g of phosphoric acid was added after adding hydrogen peroxide. 4.9 g of phosphoric acid was further added at a rate of 0.24 mol / min of phosphate ions relative to 1 mol of lithium in the aqueous solution containing niobium-containing polyoxoanions and lithium ions. As a result, a large amount of white colloid was produced. This white colloid did not dissolve even when diluted with pure water for concentration adjustment, and the aqueous solution was observed to be turbid by visual inspection. Therefore, the coating treatment using this comparative example was not carried out.

[0058] As described above, by using the aqueous solution containing niobium-containing polyoxoanions, lithium ions, and phosphate ions specified in the present invention to coat the positive electrode active material of the lithium ion battery positive electrode, an increase in the BET specific surface area caused by coating can be suppressed. [Table 2] *1: Wavelength 660 nm [Table 3] [Table 4] [Table 5]

Claims

1. An aqueous solution containing niobium-containing polyoxoanion ions, lithium ions, and phosphate ions, wherein, The ratio P / (Nb + Li + P) of the number of moles of phosphorus contained in the aqueous solution to the total number of moles of niobium, lithium, and phosphorus is 0.04 or more and less than 0.5, and the molar ratio Li / Nb of lithium to niobium is greater than 0.6 and 2.0 or less.

2. The aqueous solution containing polyacid anions containing niobium, lithium ions, and phosphate ions according to claim 1, which further contains hydrogen peroxide at a concentration of 0.01% by mass or more and 10% by mass or less.

3. The aqueous solution containing polyoxoacid anions containing niobium, lithium ions and phosphate ions as described in claim 2, wherein, The ratio PO of the number of moles of hydrogen peroxide present in the aqueous solution to the number of moles of niobium C / Nb is 0.01 or more and 1 or less.

4. An aqueous solution containing polyoxoacid anions containing niobium, lithium ions and phosphate ions as described in claim 1, wherein, The absorbance of the aqueous solution at a wavelength of 660 nm is 0.1 or less.

5. A method for producing an aqueous solution containing polyacid anions containing niobium, lithium ions, and phosphate ions, which includes the following steps: A step of dissolving a niobium-containing hydrous oxide and a lithium-containing salt in water to obtain an aqueous solution containing polyacid anions containing niobium and lithium ions, and A step of adding phosphate ions to the aqueous solution containing polyacid anions containing niobium and lithium ions.

6. A method for producing an aqueous solution containing polyacid anions containing niobium, lithium ions, and phosphate ions, which includes the following steps: A step of dissolving a niobium-containing hydrous oxide and a lithium-containing salt in water to obtain an aqueous solution containing polyacid anions containing niobium and lithium ions, and A step of adding phosphate ions and hydrogen peroxide to the aqueous solution containing polyacid anions containing niobium and lithium ions.

7. A method for manufacturing an aqueous solution containing polyoxoanions containing niobium, lithium ions, and phosphate ions according to claim 5 or 6, wherein, The ratio P / (Nb + Li + P) of the number of moles of phosphorus to the total number of moles of niobium, lithium, and phosphorus is 0.04 or more and less than 0.5, and the molar ratio Li / Nb of lithium to niobium is greater than 0.6 and 2.0 or less.

8. A method for manufacturing an aqueous solution containing a niobium-containing polyoxoanion, a lithium ion, and a phosphate ion, as described in claim 6, wherein, The concentration of hydrogen peroxide in the aqueous solution is 0.01% by mass or more and 10% by mass or less.

9. The method for manufacturing an aqueous solution containing polyoxoanions containing niobium, lithium ions, and phosphate ions according to claim 6, wherein, The ratio PO of the number of moles of hydrogen peroxide added to the number of moles of niobium A / Nb is 0.1 or more and 1.0 or less.

10. The method for manufacturing an aqueous solution containing polyoxoanions containing niobium, lithium ions, and phosphate ions according to claim 5 or 6, wherein, Phosphate ions are added at a rate of 0.07 mol / min or more and 3.07 mol / min or less per 1 mol of lithium in the aqueous solution containing polyacid anions containing niobium and lithium ions.

11. Method for manufacturing an active material for a lithium secondary battery, wherein, The active material has a coating layer containing lithium niobate, The manufacturing method includes the following steps: A step of covering the surface of the active material for a lithium secondary battery with the aqueous solution containing polyacid anions containing niobium, lithium ions, and phosphate ions according to any one of claims 1 to 4, and A step of heat-treating the covered active material for a lithium secondary battery.

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