Powder containing niobium complex and lithium and a method for manufacturing thereof, and a method for manufacturing a positive electrode active material for lithium secondary batteries having a coating containing lithium niobate.

Abstract

A lithium niobate precursor solution containing niobium complex and lithium, suitable for use as a solid electrolyte in the preparation of lithium niobate coating on the surface of positive electrode active material particles of lithium-ion secondary batteries, is provided. The powder contains niobium complex and lithium and has high solubility in water. A niobium compound, a lithium compound, an alkali, hydrogen peroxide, and water are mixed to obtain an aqueous solution containing niobium complex and lithium. This aqueous solution is then dried below the decomposition temperature of the niobium complex to obtain a powder containing niobium complex and lithium, comprising 25% by mass and 75% by mass of niobium. The proportion of niobium in the metal elements contained in the powder, expressed as a mass ratio, is 0.775 or more and 0.950 or less. When the powder is dissolved in 8 times its mass of water at 25°C, the niobium content in the filtrate is 80% by mass or more of the amount of niobium contained in the powder before dissolution.

Description

Technical Field

[0001] This invention relates to a powder containing lithium niobate complexes suitable for preparing an aqueous solution of lithium niobate used as a solid electrolyte for wet coating of positive electrode active material particles for lithium-ion secondary batteries, and a method for manufacturing the same, as well as a method for manufacturing a positive electrode active material for lithium secondary batteries having a coating layer containing lithium niobate. Background Technology

[0002] The positive electrode active material of lithium-ion secondary batteries has traditionally been composed of composite oxides of lithium and transition metals. Lithium cobalt oxide (LiCoO2), a composite oxide containing Co, is commonly used. Recently, lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), or ternary systems (LiNiO2, LiNiO2, LiMn2O4, LiNiO2 ... 1 / 3 Mn 1 / 3 Co 1 / 3 The use of O2 and other compounds is also increasing.

[0003] As an electrolyte for lithium-ion secondary batteries, the main type used is the product of dissolving lithium salts such as LiPF6 and LiBF4 in a mixed solvent of cyclic carbonates such as PC (propylene carbonate) and EC (ethylene carbonate) and chain esters such as DMC (dimethyl carbonate), EMC (ethyl methyl carbonate), and DEC (diethyl carbonate). Such organic solvents are not resistant to oxidizing atmospheres, especially when the cathode surface comes into contact with transition metals such as Co, Ni, and Mn, where oxidative decomposition reactions easily occur. This is primarily due to the high potential and highly oxidized state of the cathode surface, where transition metals catalyze their action. Therefore, to maintain electrolyte performance, it is effective to minimize contact between the electrolyte and the transition metals (e.g., one or more of Co, Ni, and Mn) that constitute the positive electrode active material.

[0004] In addition, as a method to fundamentally solve the problem of the above-mentioned organic solvent-based electrolyte, an all-solid-state lithium-ion secondary battery is proposed, which replaces the electrolyte with a non-flammable solid electrolyte.

[0005] Generally, the electrode reaction in a battery occurs at the interface between the electrode active material and the electrolyte. When a liquid electrolyte is used, the electrolyte permeates the surface of the electrode active material present on the electrode, forming a reaction interface for charge movement. In the case of an all-solid-state battery, a solid electrolyte with ion conductivity functions as the electrolyte. Since the solid electrolyte itself does not have the fluidity of a liquid, it is necessary to pre-complicate the powder that will become the electrode active material by mixing it with the solid electrolyte or by coating the powder with the solid electrolyte before constructing the secondary battery.

[0006] However, in the case of all-solid-state lithium-ion secondary batteries, the resistance (hereinafter sometimes referred to as "interface resistance") generated when lithium ions move at the interface between the positive electrode active material and the solid electrolyte increases, leading to a potential decrease in battery capacity and other performance characteristics. Regarding this increase in interface resistance, it is believed that the reaction between the positive electrode active material and the solid electrolyte, forming high-resistivity regions on the surface of the positive electrode active material, is the cause. It is known that coating the surface of lithium cobalt oxide, which serves as the positive electrode active material, with lithium niobate can reduce the interface resistance. Various methods can be used for lithium niobate coating; for example, patent documents 1-4 disclose a processing solution for forming a lithium niobate coating layer on the surface of lithium cobalt oxide, which serves as the positive electrode active material, using a wet process.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2014-210701

[0010] Patent Document 2: Japanese Patent Application Publication No. 2020-066570

[0011] Patent Document 3: Japanese Patent Application Publication No. 2020-164401

[0012] Patent Document 4: Japanese Patent Application Publication No. 2020-035607 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] Patent Document 1 discloses an aqueous solution containing a lithium and niobium complex with a pH value of 8.0 or higher and 10.5 or lower, and a method for manufacturing the same. Patent Document 2 discloses a precursor solution of lithium niobate containing niobium polybasic acid ions, Li ions, and hydrogen peroxide, and a method for manufacturing the same. Furthermore, Patent Document 3 discloses a solution containing lithium, a niobium complex, and nitrous acid, a method for manufacturing the same, and a method for manufacturing an active material for lithium secondary batteries using the solution. The aqueous solutions disclosed in these patent documents are all precursors to lithium niobate, and the storage stability of these aqueous solutions is listed as a problem to be solved in each patent document. However, according to the researchers of the present invention, the precursor solutions described in Patent Documents 1 to 3 may not necessarily meet the requirements for storage stability. In addition, regarding these precursor aqueous solutions, the proportion of solvent mass relative to the total mass is relatively large, resulting in high transportation costs.

[0015] Therefore, it is considered to dry and solidify the precursor solution of lithium niobate containing niobium and lithium, store it in a solid state, and then dissolve it in water to prepare an aqueous solution when needed. However, powders containing niobium and lithium manufactured using conventional techniques have the problem of low water solubility. In particular, niobium has a low solubility in water, so it is not possible to obtain powders containing niobium and lithium with high water solubility.

[0016] For example, Patent Document 4 discloses a technique for obtaining lithium niobate by heating a lithium niobate precursor obtained by drying a solution containing niobium ions and lithium ions at a temperature of 250°C to 300°C. The lithium niobate obtained by heating at this temperature is poorly soluble in water.

[0017] In view of the above-mentioned problems, the present invention aims to provide a powder containing niobium complex and lithium and having high solubility in water, suitable for preparing a lithium niobate precursor solution for coating the surface of positive electrode active material particles of lithium-ion secondary batteries with lithium niobate as a solid electrolyte, and a method thereof.

[0018] It should be noted that the term "niobium complex contained in the powder" refers to the solidified product of niobium in a state of coordination with a ligand (complexing agent). Therefore, the term "niobium complex" in this specification is used to refer to both complexes formed in aqueous solution and products formed by solidifying them, the difference of which is evident from the context.

[0019] Methods for solving problems

[0020] To achieve the above objectives, the present invention provides

[0021] [1] A powder containing niobium complex and lithium, wherein the powder contains 25% by mass and 75% by mass of niobium, and the proportion of niobium in the metal elements contained in the powder, expressed as a mass ratio, is 0.775 or more and 0.950 or less, and the niobium solubility of the powder containing niobium complex and lithium, as defined by the following formula (1), is 80% or more.

[0022] Niobium solubility (%) = Nb w ×100 / Nb HF …(1)

[0023] Among them, Nb w and Nb HF The mass of niobium was calculated using the following steps.

[0024] A powder containing niobium complex and lithium was weighed and dissolved in 8 times its mass of water at 25°C. The solution was then filtered through a 0.20 μm mesh membrane filter. The concentration of niobium in the filtrate was determined using inductively coupled plasma optical emission spectrometry (ICP-AES). The mass of niobium dissolved from each unit mass of the weighed powder into the filtrate, calculated from the niobium concentration and the mass of the filtrate, was defined as Nb. w ,

[0025] A powder containing niobium complex and lithium was weighed, and hydrofluoric acid was added to the weighed powder and dissolved. After cooling, the concentration of niobium in the resulting solution was determined using an inductively coupled plasma optical emission spectrophotometer (ICP-AES). The mass of niobium per unit mass of the weighed powder, calculated from the niobium concentration in the resulting solution and the volume of the solution, was defined as Nb. HF .

[0026] [2] The powder containing niobium complex and lithium in the above [1] item preferably has a niobium content of 40% by mass or more and 50% by mass or less.

[0027] [3] The powder containing niobium complex and lithium in item [1] or [2] above preferably contains 2.5% by mass and 6.0% by mass of lithium, wherein the total proportion of niobium and lithium in the metal elements contained in the powder is 0.80 or more and 1.00 or less by mass, and the molar ratio of Li / Nb is 0.7 or more and 1.5 or less.

[0028] [4] The lithium content of the powder containing niobium complex and lithium in items [1] to [3] above is preferably 3.0% by mass or more and 4.5% by mass or less.

[0029] [5] The powder containing niobium complex and lithium in items [1] to [4] above preferably has a lithium solubility of 80% or more as defined by the following formula (2).

[0030] Lithium solubility (%) = Li w ×100 / Li HF …(2)

[0031] Among them, Li w and Li HF The mass of lithium is calculated using the following steps.

[0032] A powder containing niobium complex and lithium was weighed and dissolved in 8 times its mass of water at 25°C. The solution was then filtered through a 0.20 μm mesh membrane filter. The lithium concentration in the filtrate was determined using inductively coupled plasma optical emission spectrometry (ICP-AES). The mass of lithium dissolved from each unit mass of the weighed powder into the filtrate, calculated from the lithium concentration and the mass of the filtrate, was denoted as Li. w ,

[0033] A powder containing niobium complex and lithium was weighed. Hydrofluoric acid was added to the weighed powder and dissolved. After cooling, the lithium concentration in the resulting solution was determined using an inductively coupled plasma optical emission spectrophotometer (ICP-AES). The mass of lithium per unit mass of the weighed powder, calculated from the lithium concentration in the resulting solution and the volume of the solution, was denoted as Li. HF .

[0034] [6] The powder containing niobium complex and lithium in items [1] to [5] above preferably contains oxygen, and the total content of niobium, lithium and oxygen is 85% by mass or more.

[0035] [7] Preferably, the powder containing niobium complex and lithium described in items [1] to [6] above is dissolved in 8 times its mass of water at 25°C, and then filtered through a membrane filter with a mesh size of 0.20 μm, so that the content of ammonium ions in the filtrate is less than 0.5% by mass.

[0036] [8] Preferably, the powder containing niobium complex and lithium described in items [1] to [7] above is dissolved in 8 times its mass of water at 25°C and then filtered through a membrane filter with a mesh size of 0.20 μm, so that the absorbance of the filtrate at a wavelength of 660 nm is less than 1.0.

[0037] [9] The cumulative 50% particle size D of the powder containing niobium complex and lithium in items [1] to [8] above is preferably based on volume. 50 It is less than 1mm.

[0038] This invention also provides

[0039]

[10] A method for manufacturing a powder containing niobium complex and lithium, comprising: a step of mixing a niobium compound, a lithium compound, an alkali, hydrogen peroxide, and water to obtain an aqueous solution containing niobium complex and lithium; and a step of drying the aqueous solution containing niobium complex and lithium below the decomposition temperature of the niobium complex.

[0040]

[11] In the method for manufacturing powder containing niobium complex and lithium in the above

[10] item, it is preferable that the pressure during drying of the aqueous solution is below the saturated vapor pressure of water at the drying temperature of the aqueous solution.

[0041] The present invention further provides

[0042]

[12] A method for manufacturing an active material for a lithium secondary battery having a coating containing lithium niobate, comprising: a step of dissolving a powder containing niobium complex and lithium obtained by the manufacturing method of the powder containing niobium complex and lithium using the above-described

[10] or

[11] method in water to obtain an aqueous solution containing niobium complex and lithium; a step of coating the surface of a positive electrode active material for a lithium secondary battery with the aqueous solution containing niobium complex and lithium; and a step of performing heat treatment on the positive electrode active material for a lithium secondary battery coated with the aqueous solution containing niobium complex and lithium.

[0043] Invention Effects

[0044] A precursor solution of lithium niobate can be readily obtained by dissolving a powder containing niobium complex and lithium, manufactured using the method of the present invention, in water. Using this precursor solution, no high-resistivity layer is formed in the lithium niobate coating layer formed on the surface of the positive electrode active material for lithium-ion secondary batteries. Therefore, the present invention can contribute to the improvement of the performance of all-solid-state lithium-ion secondary batteries. Detailed Implementation

[0045] [Positive electrode active material]

[0046] Regarding the positive electrode active material prepared from the powder containing niobium complex and lithium of the present invention and coated with a precursor solution of lithium niobate, the following examples can be cited as examples. The positive electrode active material is composed of a composite oxide of Li and a transition metal M; therefore, materials conventionally used in lithium-ion secondary batteries can be cited, such as cobalt lithium oxide (Li₂O₃). 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 (both -0.1≤X≤0.3), Li 1+X [Ni Y Li 1 / 3-2Y / 3 Mn 2 / 3-Y / 3 O2 (0≤X≤1, 0<Y<1 / 2), etc., lithium transition metal oxides in which a portion of Li or transition metal elements are replaced by elements such as Al, Li 1+X FePO4, Li 1+X Phosphates with olivine structure, such as MnPO4 (all of which are -0.1≤X≤0.3).

[0047] [Powder containing niobium complex and lithium]

[0048] This invention provides a powder comprising a niobium complex and lithium for preparing a precursor solution of lithium niobate. The powder contains niobium, lithium, and unavoidable impurities resulting from the manufacturing process as metallic elements. The niobium content in the powder is preferably 25% by mass or more and 75% by mass or less.

[0049] The niobium content in the lithium niobate precursor solution is set according to the design of the battery cell, such as the positive electrode active material. As a powder, a high niobium content makes concentration adjustment easier and wider-ranging due to dilution from subsequent water dissolution. Therefore, the niobium content in this powder is preferably 25% by mass or more, more preferably 40% by mass or more. Furthermore, since the powder contains lithium, oxygen, and complexing agents as constituent components, the upper limit of the niobium content in the powder is no more than 75% by mass. More preferably, the niobium content in the powder is 50% by mass or less.

[0050] The proportion of niobium in the metal elements of this powder, expressed as a mass ratio, is preferably 0.775 or higher and 0.950 or lower. If the proportion of niobium in the metal elements is less than 0.775, the lithium ratio becomes excessive, or the content of metal elements other than niobium and lithium increases. If the lithium ratio becomes excessive, when using this powder to prepare a lithium niobate precursor solution and forming a coating on the positive electrode active material, components such as lithium hydroxide, which lack lithium-ion conductivity, are generated, resulting in poor battery performance, which is therefore undesirable. When the content of metal elements other than niobium and lithium increases, when using this powder to prepare a lithium niobate precursor solution and forming a coating on the positive electrode active material, components lacking lithium-ion conductivity are also generated, resulting in poor battery performance, which is also undesirable. Furthermore, if the proportion of niobium in the metal elements exceeds 0.950, the Li / Nb molar ratio described later will be less than 0.7, which is also undesirable.

[0051] The niobium contained in the powder preferably forms a complex. This complex formation increases the solubility of niobium in water. Furthermore, the presence or absence of the niobium complex formation was determined using Fourier transform infrared spectroscopy (FT-IR), based on the 845±20 cm⁻¹ region of the complex. -1 The presence of the absorption peak was used to confirm this.

[0052] The niobium content in the powder will be discussed later.

[0053] [Water-soluble]

[0054] As described above, the powder containing niobium complex and lithium obtained according to the present invention has high solubility in water, i.e., high water solubility. In particular, it is a powder with high niobium solubility (niobium originally has low solubility in water). In this specification, water solubility is evaluated based on the niobium solubility of the powder containing niobium complex and lithium as defined by the following formula (1).

[0055] Niobium solubility (%) = Nb w ×100 / Nb HF …(1)

[0056] Among them, Nb w and Nb HF The mass of niobium is calculated using the following steps.

[0057] A powder containing niobium complex and lithium was weighed and dissolved in 8 times its mass of water at 25°C. The solution was then filtered through a 0.20 μm mesh membrane filter. The concentration of niobium in the filtrate was determined using inductively coupled plasma optical emission spectrometry (ICP-AES). The mass of niobium dissolved from each unit mass of the weighed powder into the filtrate, calculated from the niobium concentration in the filtrate and the mass of the filtrate, was defined as Nb. w .

[0058] A powder containing niobium complex and lithium was weighed, and hydrofluoric acid was added to the powder and dissolved. After cooling, the concentration of niobium in the resulting solution was determined using an inductively coupled plasma optical emission spectrophotometer (ICP-AES). The mass of niobium per unit mass of the weighed powder, calculated from the niobium concentration in the resulting solution and the volume of the solution, was denoted as Nb. HF .

[0059] The niobium solubility (Nb) can be calculated from these two values. w ×100 / Nb HF ), which serves as an indicator of the water solubility of powders containing niobium complexes and lithium.

[0060] In the powder containing niobium complex and lithium of the present invention, the niobium solubility, as defined by formula (1), is 80% or higher. If the niobium solubility is less than 80%, the prepared lithium niobate precursor solution contains insoluble solid components containing niobium. If the aforementioned solid components are mixed into the lithium niobate coating layer ultimately formed on the surface of the positive electrode active material, the coating layer thickness becomes uneven, and sometimes the process of forming high-resistivity areas from the thin film portion progresses, deteriorating battery performance, which is therefore undesirable.

[0061] The powder containing niobium complex and lithium of the present invention is used to ultimately form a lithium niobate coating on the surface of the positive electrode active material, therefore the amount of lithium and niobium added needs to be close to equimolar.

[0062] The powder comprising niobium complex and lithium of the present invention preferably contains 2.5% by mass or more and 6.0% by mass or less of lithium, more preferably 3.0% by mass or more and 4.5% by mass or less. If the lithium content in the powder is less than 2.5% by mass, the proportion of unwanted components in the mass increases as a raw material for lithium niobate precursor solution, leading to higher transportation costs, which is therefore undesirable. Furthermore, since the powder contains niobium, oxygen, complexing agents, etc., as constituent components, the upper limit of the lithium content in the powder is no more than 6.0% by mass.

[0063] Furthermore, the combined proportion of niobium and lithium in the powder, expressed as a mass ratio, is preferably 0.80 or more and 1.00 or less. If the combined proportion of niobium and lithium in the powder is less than 0.80 by mass, the proportion of unwanted components in the mass increases as a raw material for the lithium niobate precursor solution, leading to higher transportation costs, which is undesirable. The molar ratio of niobium to lithium, Li / Nb, is preferably 0.7 or more and 1.5 or less. If Li / Nb is less than 0.7, lithium is deficient relative to lithium niobate represented by LiNbO3, resulting in deterioration of the lithium-ion conductivity in the final lithium niobate film, which is also undesirable. Additionally, if Li / Nb exceeds 1.5, the excess lithium hydroxide during the preparation of the precursor solution causes a pH increase, deteriorating the storage stability of the liquid, which is also undesirable.

[0064] Furthermore, the niobium and lithium contents mentioned above can be determined using the method described later in the section on [Determination of Niobium and Lithium Content].

[0065] The water solubility of the powder containing niobium complex and lithium of the present invention can also be defined by the lithium solubility of the powder containing niobium complex and lithium as defined by the following formula (2). In the powder containing niobium complex and lithium of the present invention, the lithium solubility is preferably 80% or more.

[0066] Lithium solubility (%) = Li w ×100 / Li HF …(2)

[0067] Among them, Li w and Li HF The mass of lithium is calculated using the following steps.

[0068] A powder containing niobium complex and lithium was weighed and dissolved in 8 times its mass of water at 25°C. The solution was then filtered through a 0.20 μm mesh membrane filter, and the lithium concentration in the filtrate was determined using inductively coupled plasma optical emission spectrometry (ICP-AES). The mass of lithium dissolved from each unit mass of the weighed powder into the filtrate, calculated from the lithium concentration in the filtrate and the mass of the filtrate, is denoted as Li. w .

[0069] A powder containing niobium complex and lithium was weighed, and hydrofluoric acid was added to the powder and dissolved. After cooling, the lithium concentration in the resulting solution was determined using an inductively coupled plasma optical emission spectrophotometer (ICP-AES). The mass of lithium per unit mass of the weighed powder, calculated from the lithium concentration in the resulting solution and the volume of the solution, is denoted as Li. HF .

[0070] If the lithium dissolution rate is less than 80%, the prepared lithium niobate precursor solution contains insoluble solid components including niobium or lithium. If these solid components are mixed into the lithium niobate coating layer that is finally formed on the surface of the positive electrode active material, the coating layer thickness becomes uneven, and sometimes the process of forming high-resistivity areas from the thin film portion is advanced, resulting in deterioration of battery performance, which is therefore undesirable.

[0071] The powder of the present invention, containing niobium complex and lithium, also contains oxygen in addition to niobium and lithium, preferably with a total content of niobium, lithium, and oxygen of 85% by mass or more. The powder further contains other hydrogens derived from hydrogen peroxide; since the mass of hydrogen is negligible, the upper limit of the total content of niobium, lithium, and oxygen can be approximately 100% by mass. Furthermore, regarding the oxygen content of the powder containing niobium complex and lithium, the oxygen concentration described in the [Determination of Carbon and Oxygen Content] section of the later embodiments uses the value of oxygen concentration (by mass%) measured using an oxygen-nitrogen analyzer.

[0072] The carbon content of the powder containing niobium complex and lithium of the present invention is preferably 0.5% by mass or less. If the carbon content is greater than 0.5% by mass, the possibility of lithium carbonate formation increases when a lithium niobate coating is finally formed on the surface of the positive electrode active material. Since lithium carbonate does not have lithium-ion conductivity, it sometimes degrades battery performance and is therefore not preferred. Furthermore, the carbon concentration in the dried powder sample is determined using the trace carbon and sulfur analysis apparatus described in the [Determination of Carbon and Oxygen Content] section of the following embodiments.

[0073] Furthermore, after dissolving the powder containing niobium complex and lithium of the present invention in 8 times its mass of water at 25°C, and filtering it through a membrane filter with a mesh size of 0.20 μm, the ammonium ion content in the resulting filtrate is preferably 0.5% by mass or less. If the ammonium ion content in the filtrate is 0.5% by mass or less, the niobium complex in the lithium niobate precursor solution, which is an aqueous solution containing niobium complex and lithium, becomes stable, and the decomposition of the niobium complex and the precipitation of insoluble solid components in water can be suppressed, which is therefore preferable. Furthermore, the ammonium ion content in the filtrate solution is determined using ion chromatography as described in the [Determination of Ammonium Ion Content in Filtrate] example described later.

[0074] After dissolving the powder containing niobium complex and lithium of the present invention in 8 times its mass of water at 25°C, the solution was filtered through a membrane filter with a mesh size of 0.20 μm. The resulting filtrate contained insoluble particles that had passed through the 0.20 μm membrane filter, and the amount of these particles could be qualitatively evaluated by the absorbance at a wavelength of 660 nm. Preferably, the absorbance of the resulting filtrate at a wavelength of 660 nm is 1.0 or less after dissolving the powder containing niobium complex and lithium of the present invention in 8 times its mass of water at 25°C and filtering it through a membrane filter with a mesh size of 0.20 μm. Furthermore, the insoluble particles that passed through this membrane filter are included in the amount dissolved as defined by water solubility.

[0075] Furthermore, the powder of the present invention comprising niobium complex and lithium preferably has a cumulative particle size D of 50% based on volume after pulverization. 50 Less than 1mm. If D 50 If the particle size exceeds 1 mm, its solubility in water decreases, increasing the likelihood of dissolved powder residue as a solid component in the prepared lithium niobate precursor solution, thus making it undesirable. For D 50 There is no particular lower limit to the preferred size; typically, a D of around 10 μm or larger is obtained. 50 Therefore, D 50 The preferred range is 10μm or larger and 1mm or smaller. Furthermore, D... 50 The particle size distribution was determined using a laser diffraction scattering particle size distribution measuring device.

[0076] The dried powder sample obtained by drying the powder containing niobium complex and lithium described above contains moisture. The amount of moisture can be determined by electrostatic titration based on the Karl Fischer method, and is typically below 20% by mass. Furthermore, based on TG-DTA measurements, this moisture is considered to be water of crystallization rather than adhering water.

[0077] The powder containing niobium complex and lithium of the present invention preferably contains amorphous materials. Herein, "containing amorphous materials" means that the powder containing niobium complex and lithium does not have a defined crystalline structure. If the powder containing niobium complex and lithium contains water as a constituent component, the powder becomes more easily amorphized. If the powder contains amorphous materials, its solubility in water is increased, and the possibility of dissolved residues of the powder as a solid component in the prepared lithium niobate precursor solution is reduced, which is therefore preferred. Furthermore, the powder containing niobium complex and lithium of the present invention does not necessarily have to be entirely amorphous; it is sufficient if a portion of it is amorphous.

[0078] Whether a powder containing niobium complexes and lithium contains amorphous materials can be determined by observing a halo pattern in the region of 2θ: 20° to 60° in the diffraction pattern obtained by X-ray diffraction (when a Cu X-ray tube is used for the measurement). It should be noted that a "halo" refers to a pattern observed as a broad bulge in the diffraction pattern instead of a clear peak. In this invention, a broad bulge with a full width at half maximum (FWHM) of 2θ: 2° or greater is referred to as a halo.

[0079] [Method for manufacturing powders containing niobium complexes and lithium]

[0080] [Raw Material Solution]

[0081] In the method for manufacturing powder containing niobium complex and lithium according to the present invention, niobium compound, lithium compound, hydrogen peroxide (as a complexing agent (ligand) for making niobium complex soluble in aqueous solution) and alkali are mixed in water to obtain a raw material solution (starting material) containing niobium complex and lithium in aqueous solution.

[0082] Regarding the ligands for the aforementioned niobium complex, there are no particular limitations as long as the niobium complex is water-soluble. In this embodiment, hydrogen peroxide is used as the ligand to form a peroxide complex of niobic acid ([Nb(O2)4)). 3- The peroxide complex of niobate does not contain carbon in its chemical structure, and therefore does not leave carbon residue in the final lithium niobate coating, which is particularly preferred in the method of manufacturing the powder comprising niobate complex and lithium of the present invention.

[0083] The peroxide complex of this niobic acid can be obtained by, for example, the following methods.

[0084] Aqueous niobium oxide (Nb₂O₅·nH₂O) is added to hydrogen peroxide water and mixed. In this mixture, niobic acid does not dissolve in hydrogen peroxide water, resulting in a milky white suspension.

[0085] By adding an alkali such as ammonia to a suspension of niobic acid peroxide complex and mixing, a transparent niobic acid peroxide complex can be obtained.

[0086] The amount of alkali added to the suspension, as described below, is sufficient to ensure that the pH of the final solution containing the niobium complex and lithium is above 8.0 and below 11.5.

[0087] Furthermore, when ammonia water is added as a base to the suspension, the amount of ammonia water added can be determined by considering the volatilization of ammonia during the reaction.

[0088] Alternatively, an alkaline solution can be added instead of ammonia. An aqueous solution of lithium hydroxide can also be used as this alkaline solution.

[0089] [Niobium compounds]

[0090] Anhydrous niobium oxide (Nb₂O₅) is poorly soluble in water; therefore, in the manufacturing method of the present invention, amorphous, water-soluble hydrated niobium oxide is used as the Nb source. Hydrated niobium oxide is a substance represented by the general formula Nb₂O₅·nH₂O (where n is not 0, for example, 3 ≤ n ≤ 16).

[0091] The concentration of niobium in the raw material solution is not specifically specified in this invention. From a manufacturing point of view, the concentration of niobium is preferably 0.1 mol / L to 10.0 mol / L.

[0092] [Lithium compounds]

[0093] In the manufacturing method of this invention, lithium hydroxide (LiOH) is used as the Li compound. LiOH can be anhydrous or hydrated, either is acceptable. The LiOH added to the aqueous solution dissociates into Li... + and OH - The system exhibits strong alkalinity. Since LiOH is itself a strong base, adding LiOH to an aqueous solution containing niobic acid causes the pH of the system to rise, dissolving niobium oxide. Then, adding hydrogen peroxide to the alkaline aqueous solution containing niobium and Li ions forms a niobium peroxide complex.

[0094] The amount of Li added to the raw material solution is expressed as the Li / Nb molar ratio, and is added in a manner that is greater than 0.7 and less than 1.5. The rationale for this is as described above.

[0095] Hydrogen peroxide

[0096] In the method for manufacturing the powder containing niobium complex and lithium according to the present invention, hydrogen peroxide is added to the raw material solution to ensure the water solubility of the powder and its stability when dissolved in water, thereby complexing the niobium to form a peroxide complex. Regarding the concentration of added hydrogen peroxide, it is preferably 2 mol or more and 25 mol or less relative to 1 mol of niobium. More preferably, it is 4 mol or more and 20 mol or less. If the concentration of hydrogen peroxide is less than 3 mol relative to 1 mol of niobium, the niobium cannot form a peroxide complex, which is therefore undesirable. Furthermore, if it exceeds 25 mol, unreacted hydrogen peroxide remains in the raw material solution, deteriorating the stability of the niobium complex, which is also undesirable.

[0097] [pH]

[0098] As mentioned above, LiOH is a strong base, thus the raw material solution becomes alkaline. Regarding the pH of the raw material solution, 8.0 to 11.5 is preferred to prevent instability of the niobium complex. To adjust the pH of the raw material solution to this range, a base other than LiOH can be added. In this case, ammonia or ammonium carbonate, which are less likely to remain as impurities in powders containing niobium complexes and lithium, are preferred. Furthermore, the pH value is measured using a glass electrode with a pH meter including a temperature-compensated electrode, based on JIS Z8802, corrected using an appropriate buffer solution corresponding to the measured pH range.

[0099] Furthermore, when the powder containing niobium complex and lithium obtained by the manufacturing method of the present invention is dissolved in 8 times its mass of water at 25°C, the content of ammonium ions in the filtrate is preferably 0.5% by mass or less.

[0100] [Drying Process]

[0101] In the method for manufacturing the powder containing niobium complex and lithium according to the present invention, the raw material solution prepared under the above conditions is dried below the decomposition temperature of the niobium complex to obtain the powder. Regarding the decomposition temperature of the niobium complex, for each preparation condition of the raw material solution, a preliminary experiment is performed using thermogravimetric-differential calorimetry (TG-DTA) to detect the temperature of the endothermic peak, which is then taken as the decomposition temperature.

[0102] If drying is carried out at atmospheric pressure below the decomposition temperature of the niobium complex, the solvent evaporates slowly, resulting in decreased productivity. Therefore, from a productivity point of view, the drying of the feed solution is preferably carried out below the saturated vapor pressure of water at the drying temperature of the feed solution.

[0103] Furthermore, when the decomposition temperature of the niobium complex is less than 100°C, water of crystallization may sometimes remain in the dried powder containing the niobium complex and lithium. The amount of this water of crystallization can be determined by electrostatic titration based on the Karl Fischer method. An amount of this water of crystallization of less than 20% by mass is acceptable.

[0104] The resulting dry powder can also be crushed. There are no particular limitations on the crushing method; for example, it can be crushed using a mortar and pestle.

[0105] [Manufacturing method of positive electrode active material for lithium secondary batteries]

[0106] By dissolving the niobium-containing powder and lithium powder obtained by the manufacturing method of the present invention in water to obtain a lithium niobate precursor solution as an aqueous solution containing niobium complex and lithium; coating the surface of the lithium secondary battery positive electrode active material with the aqueous solution of the niobium-containing powder and lithium precursor solution; and performing heat treatment on the lithium secondary battery positive electrode active material coated with the aqueous solution containing niobium complex and lithium, it is possible to manufacture a lithium secondary battery positive electrode active material coated with lithium niobate. For specific manufacturing conditions, the methods described in Patent Documents 1 to 4 or known methods can be used.

[0107] Example

[0108] [Qualitative Evaluation of Niobium Complexes]

[0109] The presence or absence of niobium complexes in the powder was determined using Fourier transform infrared spectroscopy (FT-IR) with single-reflection ATR. Specifically, for the dried powder samples obtained in the examples, measurements were taken using a NICOLET 6700 FT-IR apparatus manufactured by Surmofit Co., Ltd., and a Smart OMNI-Sampler single-reflection ATR accessory (germanium crystal incident angle 45°), with 16 scans and a resolution of 4. Background was measured using ion-exchanged water.

[0110] For the measured spectra, automatic baseline correction was performed using the analytical software (OMNICSpecta) included with the FT-IR device NICOLET 6700, followed by one automatic smoothing. Peak detection was performed at a sensitivity of 50 and a wavenumber of 845 cm⁻¹. -1 ±20cm -1 When a peak is observed, it is determined that the powder being measured contains a niobium complex.

[0111] [Determination of niobium and lithium content]

[0112] For powdered samples, such as the dried powder sample obtained in the examples, approximately 0.1 g of the dried powder sample was accurately weighed. A small amount of pure water and 1 mL of 46% hydrofluoric acid were added to the powder sample, and the solution was dissolved by heating with a hot plate at approximately 140–170°C. After cooling the solution to room temperature at approximately 20–30°C, it was diluted to volume using a graduated cylinder. After appropriate dilution, the niobium and lithium concentrations of the diluted solution were determined using an inductively coupled plasma optical emission spectrophotometer (ICP-AES, manufactured by Aerotech Co., Ltd., CP-720), and the niobium and lithium content of the dried powder sample was calculated.

[0113] In addition, for samples in solution state, such as raw material solutions, 0.1g of the solution sample was taken, 15ml of pure water and 5ml of hydrochloric acid were added, and then 2ml of hydrogen peroxide water was added to obtain a solution in which niobium and lithium were dissolved. After adjusting the volume, the operation was carried out.

[0114] [Evaluation of water solubility]

[0115] The mass of niobium and lithium dissolved in the filtrate obtained by filtering an aqueous solution containing a unit mass of the dried powder obtained in the examples dissolved in water at 25°C (Nb in equations (1) and (2) below) w and Li w ), and the mass of niobium and lithium contained in the dried powder obtained in the examples (Nb in equations (1) and (2) below) HF and Li HF The niobium and lithium dissolution rates, as defined by equations (1) and (2) below, were calculated. Specifically, the niobium and lithium dissolution rates were calculated using the following steps.

[0116] <1> Accurately weigh 1.0 g or 2.0 g of the dry powder sample obtained in the examples, dissolve it in 8 times (8.0 g, 16.0 g) of its mass in water at 25°C, and filter the solution using a membrane filter (DISMIC-25HP, Toyo Filter Paper Co., Ltd., mesh size 0.20 μm).

[0117] <2> The concentrations of niobium and lithium in the filtrate obtained in <1> were determined using the above-described method for determining the concentrations of samples in solution state. Based on the measured values ​​and the mass of the filtrate obtained above, the mass of niobium and lithium dissolved from a unit mass of powder dissolved in water at 25°C into the above filtrate was calculated (Nb in equations (1) and (2) below). w and Li w ).

[0118] <3> The mass of niobium and lithium contained in the dried powder sample obtained in the examples was determined using the above-described method for determining the powder state of the sample. Based on the measured values ​​and the mass of the weighed dried powder sample, the mass of niobium and lithium contained per unit mass of dried powder (Nb in equations (1) and (2) below) was calculated. HF and Li HF ).

[0119] <4> Nb obtained from <2> and <3> above w Li w 、Nb HF and Li HF The values ​​are used to calculate the niobium solubility and lithium solubility as defined by the following equations (1) and (2).

[0120] Niobium solubility (%) = Nbw ×100 / Nb HF …(1)

[0121] Lithium solubility (%) = Li w ×100 / Li HF …(2)

[0122] [Determination of carbon and oxygen content]

[0123] The carbon concentration in the dried powder sample was determined using a trace carbon and sulfur analyzer (ETMA-U510 manufactured by Horiba Corporation), and the oxygen concentration was determined using an oxygen and nitrogen analyzer (EMGA-920 manufactured by Horiba Corporation).

[0124] Determination of Ammonium Ion Content in Filtrate

[0125] Accurately weigh 1.0 g or 2.0 g of the dried powder sample obtained in the examples, dissolve it in 8 times its mass (8.0 g, 16.0 g) of water at 25°C, and filter the solution using a membrane filter (DISMIC-25HP, Toyo Filter Paper Co., Ltd., 0.20 μm mesh). The ammonium ion content in the filtrate was determined using ion chromatography (ICS-300, Dionex Corporation). For the determination, an IonPac CS14 column was used as the cation exchange column, and 10 mmol / L methanesulfonic acid (both manufactured by Dionex Corporation) was used as the dissolution solution.

[0126] [Determination of absorbance of the filtrate]

[0127] Accurately weigh 1.0 g or 2.0 g of the dried powder sample obtained in the examples, dissolve it in 8 times its mass (8.0 g, 16.0 g) of water at 25°C, and filter the solution using a membrane filter (DISMIC-25HP, Toyo Filter Paper Co., Ltd., 0.20 μm mesh). The amount of insoluble particles transferred to the filtrate through the membrane filter is qualitatively evaluated by measuring the absorbance at 660 nm using a UV-Vis spectrophotometer (SHIMADZU Co., Ltd. UV-1800). Furthermore, a quartz cell (10 mm × 10 mm × 45 mm) is used in the determination, and the measurements are performed at 25°C.

[0128] [Cumulative 50% particle size based on volume]

[0129] For dry powder samples, a laser diffraction scattering particle size distribution measuring device (SYMPATEC Helos particle size distribution measuring device (HELOS & RODOS (airflow dispersion module))) was used to measure the particle size distribution of the volume reference at a dispersion pressure of 5 bar, and the cumulative 50% particle size (D50) of the volume reference was determined.

[0130] [Moisture content]

[0131] For the dry powder samples, the moisture content was determined by electrostatic titration using a Karl Fischer moisture meter (HIRANUMA Co., Ltd., AQS-225010S). The moisture vaporization temperature was set to 100°C, and nitrogen was used as the carrier gas. The generating liquid was an Access Light RS-A (HIRANUMA Co., Ltd.). Additionally, the polarizing liquid was an Access Light CN (HIRANUMA Co., Ltd.).

[0132] [X-ray diffraction measurement]

[0133] For the dry powder sample, X-ray diffraction measurements were performed using an X-ray diffraction apparatus (Shimadzu Corporation, XRD-6100) under the following measurement conditions.

[0134] X-ray tube: Cu

[0135] Tube voltage: 40kV

[0136] Tube current: 30mA

[0137] Diverging slit: 1.0°

[0138] Scattering slit: 1.0°

[0139] Light-receiving slit: 0.3mm

[0140] Scanning speed: 2.0° / min

[0141] Step width: 0.02°

[0142] [Example 1]

[0143] In a 5L beaker, 82.0g of ion-exchanged water (resistivity: 17.2 MΩ·cm), 288.6g of 28% (w / w) ammonia solution, 1424.0g of 35% (w / w) hydrogen peroxide solution, 33.8g of lithium hydroxide monohydrate (LiOH·H₂O), and 166.7g of hydrated niobium oxide (Nb₂O₅·nH₂O, n = 4.6, Nb₂O₅ content: 76.4% (w / w)) were added and mechanically stirred at 60°C for 10 hours. The resulting solution was filtered through 0.5μm filter paper to obtain an aqueous solution containing niobium complex and lithium, which was used to prepare a lithium niobate feedstock solution. The concentrations of niobium and lithium in the obtained lithium niobate feedstock solution were 4.41% (w / w) and 0.38% (w / w), respectively, and the Li / Nb molar ratio was 1.15.

[0144] Furthermore, the concentration of ammonium ions in the above-mentioned raw material solution was measured, and the result was 960 ppm. In addition, the absorbance of the precursor solution at a wavelength of 660 nm was measured, and the result was 0.001.

[0145] 114.2 g of the raw material solution prepared under the above conditions was aliquoted into a container, which was then placed in a vacuum dryer and maintained at 25°C and a gauge pressure below -0.1 MPa for 12 hours to obtain the dried product. The gauge pressure is the pressure difference between the vacuum dryer and atmospheric pressure. Next, the dried product was crushed in a mortar to obtain 9.9 g of dried powder. The volumetric particle size distribution of the crushed dried powder was measured using a laser diffraction particle size distribution analyzer, and the cumulative 50% particle size D... 50 It is 140μm.

[0146] X-ray diffraction was performed on the crushed and dried powder. The results showed that a halo pattern was observed in the 2θ region of 20° to 60°. Almost no peaks from lithium niobate (LiNbO3) crystals were observed. Therefore, it can be concluded that the dried powder obtained in this embodiment is a powder with an amorphous structure as the main component.

[0147] In addition, 2.0 g of the dried powder was placed in a muffle furnace and heat-treated at 600 °C in an atmospheric atmosphere for 2 hours. Then, it was crushed in a mortar to obtain 1.3 g of calcined powder. X-ray diffraction analysis of the obtained calcined powder revealed peaks originating from lithium niobate (LiNbO3) crystals, thus confirming that the dried powder was a precursor of lithium niobate.

[0148] A 6.4 mg sample was taken from the dried powder and subjected to TG-DTA analysis in a N2 atmosphere at a flow rate of 200 mL / min, with a heating rate of 10 °C / min from 30 °C to 120 °C. The results showed an endothermic peak at 70 °C, presumed to be the decomposition temperature of the niobium complex. Therefore, the aforementioned drying temperature is below the decomposition temperature of the niobium complex. FT-IR analysis of the dried powder confirmed the presence of a peak at 845 cm⁻¹ belonging to the niobium complex. -1 ±20cm -1 The peaks indicated that the dried powder contained niobium complexes. Following the above [Determination of Niobium and Lithium Content], the metal elements contained in the dried powder dissolved in an aqueous solution of hydrofluoric acid were determined by ICP-AES. The results showed that the niobium and lithium contents in the dried powder were 47.20% by mass and 3.47% by mass, respectively.

[0149] Furthermore, the Li / Nb molar ratio is 0.98. Based on the above measurements, it can be confirmed that the dried powder contains niobium complex and lithium, and the proportion of niobium among the metal elements contained in the dried powder, expressed as a mass ratio, is 0.932. Additionally, based on the niobium and lithium content in the aforementioned dried powder, the Nb content can be calculated. HF and Li HF The dried powder was dissolved in water according to the above [evaluation of water solubility], and the metal elements contained in the filtrate obtained by filtration were determined by ICP-AES. Nb was also determined. W and Li W The solubility of niobium (%) and lithium (%) was calculated, and the results showed that 86.2% of Nb and 99.9% of Li dissolved in water.

[0150] Table 1 shows the various measurement results for the dried powder obtained in this embodiment. If the dried powder obtained in this embodiment is dissolved again in water, an aqueous solution containing niobium complex and lithium with a composition substantially the same as the precursor solution before solidification is obtained. Therefore, the powder containing niobium complex and lithium obtained by the manufacturing method of the present invention is a highly water-soluble powder suitable for preparing a lithium niobate precursor solution in which the surface of the positive electrode active material particles of lithium-ion secondary batteries is coated with lithium niobate as a solid electrolyte. Furthermore, it is known that by storing the powder containing niobium complex and lithium obtained by the manufacturing method of the present invention, problems related to the storage stability of the aqueous solution containing lithium niobate precursor and the reduction of transportation costs can be eliminated.

[0151] [Comparative Example 1]

[0152] 505.7 g of the aqueous solution prepared under the same conditions as in Example 1 was aliquoted into a container, which was then placed in a rack dryer and dried at 120°C under atmospheric conditions for 12 hours. The dried product was then crushed in a mortar to obtain 43.1 g of dried powder. FT-IR analysis of the dried powder showed no 845 cm⁻¹ region belonging to the niobium complex. -1 ±20cm -1 The peak indicates that the dried powder does not contain niobium complexes.

[0153] The various measurement results for the dried powder obtained in this comparative example are shown in Table 1. It can be seen that the dried powder of this comparative example, which does not contain niobium complex, has poor water solubility compared with the examples.

[0154] [Table 1]

[0155]

Claims

1. A powder comprising a niobium complex and lithium, wherein, The powder contains 25 mass% or more and 75 mass% or less of niobium, the proportion of niobium in the metal elements contained in the powder is 0.775 or more and 0.950 or less in terms of mass ratio, the niobium solubility of the powder containing a niobium complex and lithium defined by the following (1) is 80% or more, Dissolution rate of niobium (%) = Nb w x 100 / Nb HF ... (1) wherein Nb w and Nb HF is the mass of niobium calculated using the following steps, A powder containing a niobium complex and lithium was weighed, the weighed powder was dissolved in 8 times the mass of the powder of water at 25°C, then filtered with a membrane filter having a mesh size of 0.20 μm, the concentration of niobium in the obtained filtrate was measured using an inductively coupled plasma atomic emission spectrometry device (ICP-AES), and the mass of niobium dissolved from the weighed powder per unit mass into the filtrate calculated from the niobium concentration in the obtained filtrate and the mass of the filtrate was set as Nb w , A powder containing a niobium complex and lithium is weighed, hydrofluoric acid is added to the weighed powder and dissolved, and after cooling, the concentration of niobium in the obtained dissolved solution is measured using an inductively coupled plasma atomic emission spectrometry device (ICP-AES). The mass of niobium contained in the weighed powder per unit mass calculated from the concentration of niobium in the obtained dissolved solution and the volume of the dissolved solution is set as Nb HF .

2. The powder comprising a niobium complex and lithium according to claim 1, wherein, The powder contains 40 mass% or more and 50 mass% or less of niobium.

3. The powder comprising a niobium complex and lithium according to claim 1, wherein, The powder contains 2.5 mass% or more and 6.0 mass% or less of lithium, the total proportion of niobium and lithium in the metal elements contained in the powder is 0.80 or more and 1.00 or less in terms of mass ratio, and the molar ratio of Li / Nb is 0.7 or more and 1.5 or less.

4. The powder comprising a niobium complex and lithium according to claim 1, wherein, The powder contains 3.0 mass% or more and 4.5 mass% or less of lithium.

5. The powder comprising a niobium complex and lithium according to claim 1, wherein, When the powder is dissolved in 8 times the mass of water at 25°C, the lithium solubility of the powder containing a niobium complex and lithium defined by the following (2) is 80% or more, Lithium solubility rate (%) = Li w x 100 / Li HF … (2) wherein Li w and Li HF is the mass of lithium calculated using the following steps, A powder containing a niobium complex and lithium was weighed, the weighed powder was dissolved in 8 times the mass of the powder of water at 25°C, then filtered with a membrane filter having a mesh size of 0.20 μm, the concentration of lithium in the obtained filtrate was measured using an inductively coupled plasma atomic emission spectrometry device (ICP-AES), and the mass of lithium dissolved from the weighed powder per unit mass into the filtrate calculated from the lithium concentration in the obtained filtrate and the mass of the filtrate was set as Li w , A powder containing a niobium complex and lithium is weighed, hydrofluoric acid is added to the weighed powder and dissolved, and after cooling, the concentration of lithium in the obtained dissolved solution is measured using an inductively coupled plasma atomic emission spectrometry device (ICP-AES). The mass of lithium contained in the weighed powder per unit mass calculated from the lithium concentration in the obtained dissolved solution and the volume of the dissolved solution is set as Li HF .

6. The powder comprising a niobium complex and lithium according to claim 1, wherein, The powder contains oxygen, and the total content of niobium, lithium, and oxygen is 85 mass% or more.

7. The powder comprising a niobium complex and lithium according to claim 1, wherein, After the powder is dissolved in 8 times the mass of water at 25°C, the content of ammonium ions contained in the filtrate obtained by filtering with a membrane filter having a mesh size of 0.20 μm is 0.5 mass% or less.

8. The powder comprising a niobium complex and lithium according to claim 1, wherein, After the powder is dissolved in 8 times the mass of water at 25°C, the absorbance at a wavelength of 660 nm of the filtrate obtained by filtering with a membrane filter having a mesh size of 0.20 μm is 1.0 or less.

9. The powder comprising a niobium complex and lithium according to claim 1, wherein, Volume-based cumulative 50% particle size Dv50 50 is 1 mm or less.

10. A method of making a powder comprising a niobium complex and lithium, comprising: a step of mixing a niobium compound, a lithium compound, a base, hydrogen peroxide, and water to obtain an aqueous solution containing a niobium complex and lithium; and a step of drying the aqueous solution containing a niobium complex and lithium at a temperature below the decomposition temperature of the niobium complex.

11. The method of manufacturing a powder comprising a niobium complex and lithium according to claim 10, wherein, The pressure at the time of drying the aqueous solution is below the saturated vapor pressure of water at the drying temperature of the aqueous solution.

12. A method for producing an active material for a lithium secondary battery having a coated layer containing lithium niobate, the method comprising: a step of dissolving a powder containing a niobium complex and lithium obtained by the production method according to claim 10 in water to obtain an aqueous solution containing a niobium complex and lithium; a step of coating the surface of a lithium secondary battery positive electrode active material with the aqueous solution containing a niobium complex and lithium; and a step of subjecting the lithium secondary battery positive electrode active material coated with the aqueous solution containing a niobium complex and lithium to heat treatment.

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