Positive-electrode active material for lithium-ion secondary battery, lithium-ion secondary battery, and method for producing positive-electrode active material for lithium-ion secondary battery

Abstract

The present invention addresses the problem of providing a positive-electrode active material for a lithium-ion secondary battery, the positive-electrode active material having excellent cycling stability. The present invention further addresses the problem of providing a lithium-ion secondary battery and a method for producing the positive-electrode active material for a lithium-ion secondary battery. This positive-electrode active material for a lithium-ion secondary battery includes active-material particles, and the active-material particles comprise composite oxide particles and an adherent layer covering at least some of the surfaces of the composite oxide particles and including lithium aluminosilicate. In a pore distribution examination, the volume of pores having pore diameters of 6-30 nm is 0.00030-0.00100 cm3 / g. The ratio of an Si content to a total content of transition metal elements, which are determined by X-ray photoelectron spectroscopy, is 0.15-2.00 in terms of molar ratio.

Description

Positive electrode active material for lithium ion secondary battery, lithium ion secondary battery, and method for producing positive electrode active material for lithium ion secondary battery

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery and a method for producing the same. The present invention also relates to a lithium ion secondary battery.

[0002] In recent years, due to the worldwide increase in awareness of global environmental protection, reduction of fossil fuel use and reduction of CO ２ emissions are required. As one of the means, power generation using renewable energy is becoming widespread. On the other hand, power generation using renewable energy such as solar power generation and wind power generation is likely to have its output fluctuate depending on the time of day, weather conditions, seasons, etc. Therefore, by combining a power storage system (Energy Storage System, hereinafter also referred to as ESS) for leveling the output fluctuation with power generation using renewable energy, the stabilization of power supply can be achieved. As a means of power storage used in the power storage system, for example, a secondary battery can be mentioned. Since the secondary battery used in the power storage system needs to have a large capacity, a lithium ion secondary battery (LIB) having a high energy density per unit volume and mass and being capable of being miniaturized has attracted attention.

[0003] In an LIB, lithium ions are transferred through an electrolyte between a positive electrode active material capable of occluding and releasing lithium ions and a negative electrode active material capable of occluding and releasing lithium ions to perform discharging and charging. The transfer of lithium ions is performed through an electrolyte (for example, a non-aqueous liquid and a solid electrolyte) disposed between the positive electrode active material and the negative electrode active material. An LIB using a solid electrolyte has attracted attention because the solid electrolyte is non-flammable.

[0004] Here, in a LIB using a solid electrolyte, lithium ions may have difficulty moving when moving lithium ions between the positive electrode active material and the solid electrolyte. Hereinafter, the difficulty of lithium ion movement between the positive electrode active material and the electrolyte is also referred to as "interface resistance of lithium ions". As a technique for reducing the interface resistance of lithium ions as described above, for example, there is a disclosure in Patent Document 1. Specifically, in Patent Document 1, a coating layer containing LiNbO ３ or LiTi ５ O １２ is formed on the surface of the positive electrode active material, and a technique for reducing the interface resistance is disclosed. It is also disclosed that the above coating layer can suppress the reaction between the positive electrode active material and the solid electrolyte.

[0005] Japanese Patent Application Laid-Open No. 2009-193940

[0006] In recent years, further increase in the energy density of LIBs has been demanded. As one of the methods for increasing the energy density of LIBs, the use of a positive electrode active material with a high potential for lithium ion intercalation and deintercalation can be cited. Examples of the positive electrode active material as described above include a positive electrode active material having an operating voltage of about 2.00 to 3.58 V based on the Li-In counter electrode. When the present inventors applied the coating layer described in Patent Document 1 to the above positive electrode active material, although the effect of reducing the interface resistance was recognized, it was revealed that the interface resistance tended to increase when repeated charge and discharge were performed, and the cycle characteristics were inferior. Since LIBs are assumed to be used for repeated charge and discharge, a positive electrode active material with excellent cycle stability has been demanded.

[0007] Therefore, an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery having excellent cycle stability. Another object of the present invention is to provide a lithium ion secondary battery using the above positive electrode active material for a lithium ion secondary battery. Another problem of the present invention is to provide a method for manufacturing an active material for a lithium ion secondary battery.

[0008] As a result of intensive studies to solve the above problems, the present inventor has completed the present invention. That is, it has been found that the above problems are solved by the following configuration.

[0009] 〔1〕 A positive electrode active material for a lithium-ion secondary battery, comprising active material particles, wherein the active material particles have composite oxide particles and an adhesion layer covering at least a part of the surface of the composite oxide particles and containing lithium aluminosilicate, and in the pore size distribution measurement, the pore volume of pores with a pore diameter of 6 to 30 nm is 0.00030 to 0.00100 cm ３ / g, and the ratio of the Si content to the total content of transition metal elements measured by X-ray photoelectron spectroscopy is 0.15 to 2.00 in molar ratio, a positive electrode active material for a lithium-ion secondary battery. 〔2〕 The positive electrode active material for a lithium-ion secondary battery according to 〔1〕, wherein the composite oxide particles have a composition represented by the following formula (1). Li ａ Ni ｂ Co ｃ Mn ｄ X ｅ O ｆ (1) In formula (1), X is at least one element selected from the group consisting of B, Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Zr, Nb, Mo, Ba, La and W. In formula (1), a, b, c, d, e and f are each real numbers and satisfy the following relationships. 0.2 ≦ a ≦ 2.1 b + c + d + e = 1 1.7 ≦ f ≦ 4.1 〔3〕 A lithium-ion secondary battery comprising a positive electrode including the positive electrode active material for a lithium-ion secondary battery according to 〔1〕 or 〔2〕. 〔4〕 A positive electrode including the positive electrode active material for a lithium-ion secondary battery according to 〔1〕 or 〔2〕, a negative electrode including a negative electrode active material, and a solid electrolyte interposed between the positive electrode and the negative electrode to conduct lithium ions, wherein the solid electrolyte is a sulfide, a lithium-ion secondary battery. 〔5〕 A method for manufacturing a positive electrode active material for a lithium-ion secondary battery, which manufactures the positive electrode active material for a lithium-ion secondary battery according to 〔1〕 or 〔2〕, and includes a step of forming a precursor layer of lithium aluminosilicate on the surface of the composite oxide particles and performing heat treatment at 140 to 350 °C, a method for manufacturing a positive electrode active material for a lithium-ion secondary battery.

[0010] According to the present invention, a positive electrode active material for lithium-ion secondary batteries with excellent cycle stability can be provided. Furthermore, according to the present invention, a lithium-ion secondary battery can also be provided. Furthermore, according to the present invention, a method for producing a positive electrode active material for lithium-ion secondary batteries can also be provided.

[0011] This is a cross-sectional view showing an example of the configuration of the lithium-ion secondary battery of the present invention.

[0012] The present invention will now be described in detail. The following descriptions of constituent elements may be based on typical embodiments of the present invention, but the present invention is not limited to such embodiments.

[0013] The following definitions of each description in this specification are as follows. The embodiments of the present invention will be described in detail below. However, the embodiments described below are examples only, and the present invention is not limited to the embodiments described below. In this specification, numerical ranges represented using "~" mean a range that includes the numbers before and after "~" as the lower and upper limits. In this specification, elements may be represented by their corresponding element symbols. For example, Li, B, O, Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Zr, Nb, Mo, Ba, La, and W represent lithium, boron, oxygen, sodium, magnesium, aluminum, silicon, phosphorus, sulfur, potassium, calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, strontium, zirconium, niobium, molybdenum, barium, lanthanum, and tungsten, respectively.

[0014] <Positive Electrode Active Material for Lithium-Ion Secondary Batteries> The positive electrode active material for lithium-ion secondary batteries of the present invention comprises active material particles, the active material particles comprising composite oxide particles and an adhering layer containing lithium aluminosilicate covering at least a portion of the surface of the composite oxide particles. Furthermore, in pore distribution measurements, the positive electrode active material for lithium-ion secondary batteries of the present invention has a pore volume of 0.00030 to 0.00100 cm³ for pores with a pore diameter of 6 to 30 nm. ３The ratio of Si content to the total content of transition metal elements, as measured by X-ray photoelectron spectroscopy, is 0.15 to 2.00 in molar ratio.

[0015] The mechanism by which the positive electrode active material for lithium-ion secondary batteries of the present invention (hereinafter also simply referred to as "the positive electrode active material of the present invention") exhibits low interfacial resistance (difficulty in the movement of lithium ions between the positive electrode active material and the electrolyte) and excellent cycle stability is not entirely clear, but the inventors speculate as follows: The active material particles contained in the positive electrode active material of the present invention have composite oxide particles and an attached layer containing lithium aluminosilicate. Since lithium aluminosilicate has lithium ion conductivity, it does not easily increase interfacial resistance. Furthermore, it is thought that the attached layer containing lithium aluminosilicate suppresses direct contact between the composite oxide particles and the electrolyte, thereby suppressing the reaction between the composite oxide particles and the electrolyte and making it easier to lower the interfacial resistance. In addition, the positive electrode active material of the present invention has a predetermined pore volume in pore distribution measurements, and the ratio of Si content to the total sum of transition metal elements measured by X-ray photoelectron spectroscopy is within a predetermined range. In other words, it is thought that having a predetermined pore volume makes it easier for lithium ions to pass through the attached layer and access the composite oxide particles, further reducing interfacial resistance. Furthermore, because lithium aluminosilicate contained in the adhering layer has high chemical stability, even when high-voltage charge-discharge cycles are repeated, for example, the reaction between the active material particles and the electrolyte (especially the solid electrolyte which is a sulfide) can be suppressed, maintaining a low interfacial resistance and resulting in excellent cycle stability.

[0016] [Active Material Particles] The positive electrode active material of the present invention includes active material particles. The active material particles have composite oxide particles and an attached layer, as described above. The characteristics of the positive electrode active material of the present invention and the active material particles will be described below.

[0017] [Morphology of Active Material Particles] The active material particles contained in the positive electrode active material of the present invention may consist only of primary particles, or may consist of secondary particles formed by aggregation of primary particles, or may contain both primary and secondary particles. Primary particles are not particularly limited, but often have a polyhedral shape. The morphology of the active material particles contained in the positive electrode active material can be observed, for example, by a scanning electron microscope (SEM). When the positive electrode active material of the present invention contains secondary particles, observation by SEM often reveals secondary particles formed by aggregation of polyhedral primary particles into a substantially spherical shape.

[0018] The average particle diameter of the positive electrode active material of the present invention is not particularly limited, but is 10 μm or more. Furthermore, the average particle diameter of the positive electrode active material of the present invention is not particularly limited, but is 20 μm or less. In this specification, the average particle diameter of the positive electrode active material is determined, for example, as the median diameter of the volume-based particle size distribution obtained by laser diffraction particle size distribution measurement.

[0019] In the positive electrode active material of the present invention, the content of active material particles is preferably 95% by mass or more, and more preferably 98% by mass or more, relative to the total mass of the positive electrode active material. The content of active material particles may also be 100% by mass, relative to the total mass of the positive electrode active material.

[0020] [Composition of Composite Oxide Particles] The active material particles contained in the positive electrode active material of the present invention are not particularly limited as long as they can intercept and release lithium ions. In particular, the composition of the composite oxide particles in the active material particles is preferably represented by the following formula (1). Li ａ Ni ｂ Co ｃ Mn ｄ X ｅ O ｆ(1) In equation (1), X is at least one element selected from the group consisting of B, Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Zr, Nb, Mo, Ba, La, and W. In equation (1), a, b, c, d, e, and f are real numbers and satisfy the following relationships: 0.2 ≤ a ≤ 2.1 b + c + d + e = 1 1.7 ≤ f ≤ 4.1 If X contains two or more elements, e is the sum of the amounts of the two or more elements contained in X.

[0021] In formula (1) above, a is preferably 0.3 or more, more preferably 0.5 or more. Also, a is preferably 1.5 or less, more preferably 1.2 or less. In formula (1) above, if b is not 0, b is preferably 0.1 or more, more preferably 0.2 or more. If b is not 0, b is 1.0 or less. b may be 0. In formula (1) above, if c is not 0, c is preferably 0.1 or more, more preferably 0.2 or more. If c is not 0, c is 1.0 or less. c may be 0. In formula (1) above, if d is not 0, d is preferably 0.1 or more, more preferably 0.2 or more. If d is not 0, d may be 0. In formula (1) above, if e is not 0, e is preferably 0.01 or more, may be 0.1 or more, and may be 0.3 or more. If e is not 0, then e is 1.0 or less, preferably 0.5 or less. e may also be 0.

[0022] Examples of crystal structures for the composition represented by formula (1) above include layered rock salt structure, spinel structure, and olivine structure. Specific examples of compounds satisfying the composition represented by formula (1) above include, for example, LiCoO ２ LiNiO ２ LiNi ０．５ Mn ０．５ O ２ LiNi ０．５ Mn １．５ O ４ LiFePO ４ LiNiPO ４ LiMnPO ４ LiCoPO４ , and Li ２ MnO ３ Examples include the above. In specific examples of the above compounds, some of the constituent elements (for example, Ni, Co, and Mn) may be substituted with other elements. Examples of the above other elements include one or more elements selected from the group consisting of B, Na, Mg, Al, Si, S, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Zr, Nb, Mo, Ba, La, and W.

[0023] [Adhesion Layer] The active material particles contained in the positive electrode active material of the present invention have an adhesion layer containing lithium aluminosilicate that covers at least a portion of the surface of the composite oxide particles. In this specification, lithium aluminosilicate refers to an oxide containing Li, Al, and Si. There are no particular limitations on lithium aluminosilicate, but for example, LiAlSiO ４ The compound LiAlSi has the following composition: ２ O ６ The compound LiAlSi has the following composition: ４ O １０ The compound has the composition Li ２ Al ２ SiO ６ The compound has the composition Li ３ AlSiO ５ A compound having the composition of Li ５ AlSi ２ O ８ Examples of compounds with the above composition include the attached layer, which may contain lithium aluminosilicate or consist solely of lithium aluminosilicate. It is not particularly limited as long as the above-mentioned requirements regarding pore volume and elemental content ratios measured by XPS are met, but the attached layer may be formed on a part of the surface of the composite oxide particles or on the entire surface of the composite oxide particles. Furthermore, if the active material particles are secondary particles, the attached layer may be formed only on the outer edge of the secondary particles, or the attached layer may be formed on the surface of the primary particles constituting the secondary particles.

[0024] The thickness of the attached layer is, for example, 0.5 nm or more, preferably 2 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more, in terms of superior cycle stability. In addition, the thickness of the attached layer is often 50 nm or less, preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 10 nm or less, in terms of lower interfacial resistance. The thickness of the attached layer can be measured by a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM). Specifically, a thin section sample with the cross-section of the active material particles exposed is prepared, observed, and the thickness of the attached layer is measured at five arbitrary points on 10 active material particles. The arithmetic mean of the thicknesses of the 50 measured points is taken as the thickness of the attached layer.

[0025] [Pore Volume] In pore distribution measurements, the positive electrode active material of the present invention has a pore volume of 0.00030 cm³ for pores with a diameter of 6 to 30 nm. ３ It is greater than or equal to 0.00040 cm². ３ Preferably 0.00045 cm² or more, and 0.00045 cm². ３ It is more preferable that the pore volume be greater than or equal to the above preferred range. It is believed that when the above pore volume is within the above preferred range, lithium ion conduction paths are more easily formed in the attached layer, and the interfacial resistance is lower. Furthermore, in pore distribution measurements, the positive electrode active material of the present invention has a pore volume of 0.00100 cm³ for pores with a pore diameter of 6 to 30 nm. ３ It is less than / g and 0.00080 cm ３ Preferably less than or equal to 0.00060 cm². ３ Less than or equal to 0.00050 cm² is more preferable. ３ A value of less than or equal to / g is even more preferable. It is believed that when the pore volume is within the above preferred range, the contact area between the active material particles and the electrolyte increases more easily, and the interfacial resistance becomes lower.

[0026] In this specification, the pore volume of pores in the positive electrode active material of the present invention having a pore diameter of 6 to 30 nm is obtained by measuring the pore distribution using the following procedure and then analyzing it using the following procedure. First, gas adsorption measurement is performed using the positive electrode active material as a sample. The gas adsorption measurement is performed under the following conditions to obtain the adsorption isotherm. • Equipment used: autosorbiQ (manufactured by Anton Paar) • Adsorbed gas: Nitrogen (N) ２ ) • Adsorption temperature: 77K (liquid nitrogen atmosphere) • Isotherm measurement range: 10 －７ ≤ P / P ０ ≤0.999

[0027] Next, the obtained adsorption isotherms are analyzed using the Density Functional Theory (DFT) method to obtain a pore distribution curve. The log differential pore volume distribution curve is used as the pore distribution curve. In the obtained pore distribution curve, the horizontal axis represents the pore diameter (in nm), and the vertical axis represents the integrated pore volume (in cm). ３ The result is ( / g). From the above pore distribution curve, the pore volume of pores with a pore diameter of 6 to 30 nm is calculated. In the analysis by the DFT method, the following conditions are assumed: Adsorbed gas: Nitrogen (N) ２ ) • Adsorption temperature: 77K • Pore shape: Cylinder • Adsorption surface material: Silicon dioxide

[0028] The specific surface area of ​​the positive electrode active material of the present invention is 0.30 m². ２ Preferably 0.35 m ２ More preferably 0.40 m ２ A value of 1 / g or more is even more preferable. It is believed that when the specific surface area is within the above preferred range, lithium ion conduction paths are more easily formed in the attached layer, and the interfacial resistance becomes lower. Furthermore, the specific surface area of ​​the positive electrode active material of the present invention is 0.60 m². ２ Preferably less than or equal to 0.55 m ２ More preferably less than or equal to 0.50 m ２ A value of less than or equal to / g is even more preferable. It is believed that when the pore volume is within the above preferred range, the contact area between the active material particles and the electrolyte is more easily increased, and the interfacial resistance is lowered. In this specification, the specific surface area is determined by the optimal multipoint BET method.

[0029] [Surface Element Content Ratio] The positive electrode active material of the present invention has a molar ratio of Si content to the total content of transition metal elements measured by X-ray photoelectron spectroscopy (XPS) of 0.15 to 2.00. Hereinafter, the ratio of Si content to the total content of transition metal elements will also be referred to as the "Si / TM ratio". In this specification, the XPS measurement method will be carried out by the following procedure. First, the positive electrode active material will be fixed onto a carbon tape to be used as a measurement sample. The measurement sample will be attached to a measurement holder and measured under the following conditions. • Equipment: Quantera SXM (ULVAC-PHI) • X-ray source: Monochromatic Al-Kα rays (voltage: 15kV, output: 14.9W) • X-ray beam diameter: 100μmφ • Measurement area: 100μmφ • Narrow-field photoelectron spectrum measurement Pass Energy: 112eV • Narrow-field photoelectron spectrum measurement Energy Step: 0.1eV • Broad-field photoelectron spectrum measurement Pass Energy: 280eV • Broad-field photoelectron spectrum measurement Energy Step: 1.0eV • Charge neutralization: Electron beam and Ar ＋

[0030] Following the procedure described above, narrow-field photoelectron spectra corresponding to the binding energy range of photoelectrons for each element are obtained for each element. The Si / TM ratio is calculated from the area of ​​the narrow-field photoelectron spectra for each element and the relative sensitivity coefficients between elements. More specifically, the Si / TM ratio is calculated using the area of ​​the peaks corresponding to the 2s orbital of Si and each orbital of the transition metal, as well as their relative sensitivity coefficients. The background used when calculating the area of ​​the narrow-field photoelectron spectra for each element is determined based on the Shirley method.

[0031] The Si / TM ratio is 0.15 or higher, preferably 0.50 or higher, and more preferably 0.80 or higher. When the Si / TM ratio is within the above preferred range, the area on which the surface of the composite oxide particles in the active material particles is covered by the adhesive layer becomes larger, and it is thought that the interfacial resistance tends to be lower. Alternatively, the Si / TM ratio is 2.00 or lower, preferably 1.80 or lower, and more preferably 1.50 or lower. When the Si / TM ratio is within the above preferred range, it is thought that the conductivity of lithium ions is less likely to decrease due to the adhesive layer, and the interfacial resistance tends to be lower.

[0032] [Manufacturing Method] The manufacturing method for the positive electrode active material of the present invention is not particularly limited. An example of a manufacturing method for the positive electrode active material of the present invention is a manufacturing method comprising a preparation step of preparing the above-mentioned composite oxide particles and an adhesion layer formation step of forming an adhesion layer on the surface of the prepared composite oxide particles. An example of the above manufacturing method will be described below.

[0033] (Preparation Step) In the preparation step, composite oxide particles are prepared. The preferred composition of the composite oxide particles is as described above. The composite oxide particles may be commercially available or synthesized. There are no particular limitations on the method of synthesizing the composite oxide particles, but one example is to obtain a precursor containing the elements contained in the composite oxide particles and calcine it at a predetermined temperature. As a method for obtaining the above precursor, for example, a salt of the metal element contained in the composite oxide particles is made into an aqueous solution, the pH of the aqueous solution is adjusted to produce a precipitate, and the precipitate is used as the precursor. Alternatively, the salt of the metal element contained in the composite oxide particles may be mixed to obtain the precursor. The preparation step may include a procedure for adjusting the composite oxide particles to a desired particle size.

[0034] (Adhesion layer formation step) In the adhesion layer formation step, an adhesion layer is formed on the surface of the composite oxide particles prepared in the preparation step. The method for forming the adhesion layer is not particularly limited as long as the resulting active material satisfies the requirements of the positive electrode active material of the present invention as described above, but a method of forming the adhesion layer by bringing the composite oxide particles into contact with an adhesion layer precursor solution containing a lithium aluminosilicate precursor is preferred.

[0035] The method of contacting the composite oxide particles with the above-mentioned adhesion layer precursor solution is not particularly limited. For example, the composite oxide particles may be added to the adhesion layer precursor solution and stirred, the adhesion layer precursor solution may be added dropwise to the composite oxide particles and stirred, or fine droplets of the adhesion layer precursor solution may be supplied to the composite oxide particles. Among these, the method of supplying fine droplets of the adhesion layer precursor solution to the composite oxide particles is preferred. As a method of supplying fine droplets of the adhesion layer precursor solution to the composite oxide particles, for example, the method of supplying fine droplets of the adhesion layer precursor solution to the composite oxide particles contained in a fluidized bed or a jet bed is preferred. Apparatus capable of carrying out the above method includes the MP series (e.g., FD-MP-01 and MP-10), SFP series (e.g., SFP-01 and SFP-10), GPCG series and SPC series (e.g., GPCG-5SPC) manufactured by Pawrec. It is believed that when composite oxide particles are brought into contact with the above-mentioned adhesion layer precursor solution, a precursor layer (a lithium aluminosilicate precursor layer) is formed.

[0036] Examples of the above-mentioned adhesion layer precursor solution include a solution containing a Li source, an Al source, and a Si source. Examples of solvents included in the adhesion layer precursor solution include alcohol solvents such as methanol and ethanol, and water. It is also preferable that the solvent does not contain water. Examples of Li sources include lithium nitrate, lithium sulfate, lithium carbonate, lithium acetate, and lithium hydroxide. Examples of Al sources include aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum hydroxide, aluminum ethoxide, and aluminum isopropoxide. Examples of Si sources include tetraethoxysilane, tetramethoxysilane, and silicon chloride. The adhesion layer precursor solution may also contain a compound containing two or more elements selected from the group consisting of Li, Si, and Al. Furthermore, for example, a first adhesion layer precursor solution containing only Li, a second adhesion layer precursor solution containing only Si, and a third adhesion layer precursor solution containing only Al may be used and each brought into contact with the composite oxide particles.

[0037] The thickness of the formed adhesion layer can be adjusted by adjusting the supply amount of the above-mentioned adhesion layer precursor solution. It is also preferable to adjust the supply amount of the adhesion layer precursor solution so that it is a predetermined amount relative to the surface area of ​​the composite oxide particles. For example, the molar amount of Li contained in the adhesion layer precursor solution is 0.05 to 2.5 mmol / m² relative to the surface area of ​​the composite oxide particles used in the adhesion layer formation process. ２ It is preferable to adjust the settings so that the following conditions are met. Furthermore, the molar amount of Si contained in the adhesion layer precursor solution should be 0.05 to 1 mmol / m² relative to the surface area of ​​the composite oxide particles subjected to the adhesion layer formation process. ２ It is preferable to adjust the settings so that the molar amount of Al contained in the adhesion layer precursor solution is 0.05 to 1 mmol / m² relative to the surface area of ​​the composite oxide particles subjected to the adhesion layer formation process. ２ It is preferable to adjust the solution to achieve this result. Furthermore, the elemental content in the adhesion layer precursor solution can be adjusted according to the desired composition of the adhesion layer.

[0038] Furthermore, in the adhesion layer formation process, it is also preferable to dry the composite oxide particles after bringing them into contact with the adhesion layer precursor solution. Drying removes solvent components contained in the adhesion layer precursor solution. Drying also facilitates the formation of a precursor layer (lithium aluminosilicate precursor layer) on the surface of the composite oxide particles. Drying may also be performed simultaneously in the apparatus that brings the composite oxide particles and the adhesion layer precursor solution into contact. The drying method is not particularly limited, but examples include contact with a heating medium and reduced pressure, with reduced pressure being preferred.

[0039] In the adhesion layer formation process, heat treatment is also preferable. The heat treatment is preferably performed after bringing the composite oxide particles into contact with the above-mentioned adhesion layer precursor solution and drying. That is, it is also preferable to form a precursor layer (a lithium aluminosilicate precursor layer) on the surface of the composite oxide particles and then heat treat the precursor layer. The atmosphere for heat treatment is preferably an oxygen-containing atmosphere. An example of a heat treatment atmosphere is an air atmosphere. The heat treatment temperature is preferably 140°C or higher, more preferably 180°C or higher, even more preferably 200°C or higher, and particularly preferably 220°C or higher. Furthermore, the heat treatment temperature is preferably 350°C or lower, more preferably 300°C or lower, and even more preferably 250°C or lower. By setting the heat treatment temperature within the above preferred range, it is easier to obtain the positive electrode active material of the present invention that satisfies the above requirements. The heat treatment time when performing the above heat treatment is preferably 10 minutes or more, more preferably 20 minutes or more, and even more preferably 30 minutes or more. The heat treatment time is preferably 3 hours or less, more preferably 2 hours or less, and may also be 1 hour or less.

[0040] <Lithium-ion secondary battery> The lithium-ion secondary battery of the present invention comprises a positive electrode containing the positive electrode active material of the present invention described above. Other components of the lithium-ion secondary battery of the present invention can be conventionally known configurations. For example, the lithium-ion secondary battery of the present invention preferably comprises a positive electrode containing the positive electrode active material of the present invention described above, a negative electrode containing a negative electrode active material, and an electrolyte interposed between the positive electrode and the negative electrode to conduct lithium ions. The lithium-ion secondary battery of the present invention may also include a separator. The positive electrode active material contained in the positive electrode and the negative electrode active material contained in the negative electrode are capable of intercalating and releasing lithium. Here, the electrolyte is preferably a solid electrolyte, and it is also preferable that the solid electrolyte is a sulfide. In other words, the lithium-ion secondary battery of the present invention is preferably an all-solid-state lithium-ion secondary battery.

[0041] The form of the lithium-ion secondary battery of the present invention can be arbitrarily selected from cylindrical, prismatic, coin-type, sheet-type, laminate-type, and button-type batteries, depending on the application, the device it is installed in, and the required charge / discharge capacity.

[0042] The lithium-ion secondary battery of the present invention will be described below with reference to the drawings. In the following description, a preferred embodiment of the lithium-ion secondary battery of the present invention (all-solid-state lithium-ion secondary battery) will be described, but the lithium-ion secondary battery of the present invention is not limited to the following embodiment and various modifications are possible. Figure 1 is a cross-sectional view showing an example of the configuration of the lithium-ion secondary battery of the present invention. The lithium-ion secondary battery 10 shown in Figure 1 comprises a positive electrode 12, a negative electrode 14, and a solid electrolyte layer 16 disposed between the positive electrode 12 and the negative electrode 14. The positive electrode 12 comprises a positive electrode active material layer 24 and a positive electrode current collector 22. The negative electrode 14 comprises a negative electrode active material layer 26 and a negative electrode current collector 28. The positive electrode active material layer 24 contains the positive electrode active material and solid electrolyte of the present invention. The negative electrode active material layer 26 contains the negative electrode active material and solid electrolyte.

[0043] When the positive electrode current collector 22 and the negative electrode current collector 28 are connected to an external circuit (not shown in Figure 1), the lithium-ion secondary battery 10 can be charged and discharged. During charging of the lithium-ion secondary battery 10, a positive potential is applied to the positive electrode current collector 22 by the external circuit, with the negative electrode current collector 28 as the reference, and electrons move to the negative electrode side through the external circuit. When a potential is applied, lithium contained in the positive electrode active material of the positive electrode active material layer 24 is released from the positive electrode active material as lithium ions, and the lithium ions conduct through the solid electrolyte layer 16 to the negative electrode active material layer 26 and are incorporated into the negative electrode active material of the negative electrode active material layer 26. On the other hand, during discharge of the lithium-ion secondary battery 10, a load is connected through the external circuit, and electrons move from the negative electrode current collector 28 to the positive electrode current collector 22. During discharge, lithium ions incorporated into the negative electrode active material of the negative electrode active material layer 26 conduct through the solid electrolyte layer 16 to the positive electrode active material layer 24 and are incorporated into the positive electrode active material of the positive electrode active material layer 24. In the positive electrode 12 (positive electrode active material layer 24) containing the positive electrode active material of the present invention, there is an adhesive layer containing lithium aluminosilicate at the interface between the positive electrode active material and the solid electrolyte, so as described above, the interfacial resistance is low. Furthermore, because the chemical stability of the adhesive layer is high, even when charging and discharging are performed at high operating voltages, the reaction between the active material particles and the solid electrolyte is suppressed, resulting in excellent cycle stability.

[0044] The following describes preferred examples of the configurations of the lithium-ion secondary battery of the present invention and methods for manufacturing them.

[0045] [Positive Electrode] The positive electrode includes a positive electrode active material layer. The positive electrode active material layer includes the positive electrode active material of the present invention as described above. The positive electrode active material of the present invention is as described above. The positive electrode active material layer may also include positive electrode active materials other than the positive electrode active material of the present invention. When the electrolyte used in the lithium-ion secondary battery of the present invention is a solid electrolyte, it is preferable that the positive electrode active material layer includes a solid electrolyte. The solid electrolyte can be the same material as the solid electrolyte included in the solid electrolyte layer described later. When the electrolyte used in the lithium-ion secondary battery of the present invention is a solid electrolyte, for example, the positive electrode active material layer can be formed by applying a paste-like positive electrode mixture coating consisting of a positive electrode active material, a solid electrolyte, a binder, and a conductive agent to one or both sides of the positive electrode current collector. As a binder, polyvinylidene fluoride, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), etc. can be used. Examples of conductive materials that can be used include fine carbon materials, fibrous carbon materials, graphite, carbon black, and VGCF (vapor-grown carbon fiber). The shape of the positive electrode current collector is not particularly limited and can be, for example, foil-shaped; mesh-shaped; expanded metal or the like; etc. Typically, aluminum, nickel, stainless steel foil, etc., can be used as the material for the positive electrode current collector. If the positive electrode active material layer itself is conductive, the positive electrode current collector may be omitted.

[0046] When the electrolyte used in the lithium-ion secondary battery of the present invention is liquid, the positive electrode active material layer can be formed by, for example, applying a paste-like positive electrode mixture coating consisting of a positive electrode active material, a binder, and a conductive agent to one or both sides of the positive electrode current collector. A carbonate such as lithium carbonate may also be added to the positive electrode active material layer.

[0047] [Negative Electrode] The negative electrode includes a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material is not particularly limited as long as it is capable of intercalating and releasing lithium, but examples include carbon-based materials, metallic lithium, lithium-containing alloys, silicon, and silicon-containing materials. When the electrolyte used in the lithium-ion secondary battery of the present invention is a solid electrolyte, it is preferable that the negative electrode active material layer includes a solid electrolyte. The solid electrolyte can be the same material as the solid electrolyte contained in the solid electrolyte layer described later. When the electrolyte used in the lithium-ion secondary battery of the present invention is a solid electrolyte, the negative electrode active material layer can be formed by, for example, applying a paste-like negative electrode mixture coating consisting of a negative electrode active material, a solid electrolyte, a binder, and a conductive agent to one or both sides of the negative electrode current collector. The binder can be the same as the one used for the positive electrode. The conductive agent may include, for example, carbon black or carbon fiber. The shape of the negative electrode current collector is not particularly limited and may be the same as that of the positive electrode current collector. Examples of materials for the negative electrode current collector include copper, stainless steel, and nickel. However, if the negative electrode active material layer itself is conductive, the negative electrode current collector may be omitted.

[0048] When the electrolyte used in the lithium-ion secondary battery of the present invention is liquid, the positive electrode active material layer can be formed by, for example, applying a paste-like positive electrode mixture coating consisting of a positive electrode active material, a binder, and a conductive agent to one or both sides of the positive electrode current collector. A carbonate such as lithium carbonate may also be added to the positive electrode active material layer.

[0049] When forming the negative and positive electrodes described above, various additives such as conventionally known conductive agents and binders can be used as appropriate.

[0050] [Electrolyte] The electrolyte is not particularly limited as long as lithium ions can move, and may be a liquid non-aqueous electrolyte, a polymer electrolyte such as a gel electrolyte, or a solid electrolyte such as an oxide or sulfide. As mentioned above, a solid electrolyte is preferred, and a solid sulfide electrolyte is more preferred.

[0051] Liquid non-aqueous electrolytes include LiPF ６ , and LiBF ４ Examples include conventional non-aqueous electrolytes containing lithium salts as electrolyte salts. When using a liquid non-aqueous electrolyte solution, aprotic organic solvents such as ethylene carbonate, propylene carbonate, and dimethyl carbonate can be used as the non-aqueous solvent.

[0052] When a polymer electrolyte is used, it includes a matrix polymer gelled with a plasticizer (non-aqueous electrolyte). This matrix polymer can be ether-based polymers such as polyethylene oxide and its crosslinked products, polymethacrylate-based polymers, polyacrylate-based polymers, fluorine-based polymers such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer, etc., either alone or in combination. Among these, fluorine-based polymers are preferred from the viewpoint of oxidation-reduction stability, etc. The electrolyte salt and non-aqueous solvent constituting the plasticizer (non-aqueous electrolyte) contained in the polymer electrolyte can be those that can be used in liquid electrolytes.

[0053] In the lithium-ion secondary battery of the present invention, when a liquid electrolyte is used, a separator such as a microporous material made of polypropylene or polyethylene, or a layered structure thereof; or a nonwoven fabric is typically used.

[0054] Examples of solid electrolytes include solid electrolytes of oxides containing Li and solid electrolytes of sulfides containing Li. While the solid electrolyte is not particularly limited, solid electrolytes of sulfides are preferred. The above-mentioned oxide solid electrolytes and sulfide solid electrolytes are not particularly limited, and conventionally known solid electrolytes can be used. In the lithium-ion secondary battery of the present invention, even when the solid electrolyte is a sulfide, the cycle stability of the positive electrode active material of the present invention provides excellent cycle characteristics.

[0055] The present invention will be described in more detail below based on the following examples. The materials, amounts used, proportions, processing content, and processing procedures shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the following examples.

[0056] <Example 1> [Preparation of positive electrode active material] First, composite oxide particles (manufactured by JFE Mineral Co., Ltd.) were prepared. The composition of the composite oxide particles was Li １ . ０３ Ni ０ . ７８ Co ０ . １９ Al ０ . ０３ O ｘ (x was a real number). The above composition satisfies the composition represented by formula (1) above and satisfies the requirements described above. An adhesion layer was formed on the above composite oxide particles. A rolling fluidized bed coating apparatus (Powrec Co., Ltd., FD-MP-01D) was used to form the adhesion layer.

[0057] First, lithium nitrate, aluminum nitrate nonahydrate, and tetraethoxysilane were dissolved in super-dehydrated ethanol so that the molar amounts of lithium, aluminum, and silicon in each compound were 1:1:1, to obtain an adhesion layer precursor solution. Next, a solution of 0.122 mmol / m² was applied relative to the surface area of ​​the weighed composite oxide particles. ２ A deposition layer precursor solution containing the required amount of lithium was weighed. Next, the weighed composite oxide particles were made into a fluid state using the coating apparatus described above, and minute droplets of the weighed deposition layer precursor solution were supplied to the fluid composite oxide particles. The above process was carried out in a dry room with a dew point of -40°C or lower. After the above process, heat treatment was performed at 230°C for 30 minutes in an atmospheric environment to obtain composite oxide particles with an deposition layer formed on them (the positive electrode active material of Example 1).

[0058] <Comparative Example 1> The positive electrode active material of Comparative Example 1 was obtained in the same manner as in Example 1, except that the heat treatment temperature was set to 400°C in the procedure for obtaining the positive electrode active material of Example 1.

[0059] <Comparative Example 2> The composite oxide particles used in the procedure for obtaining the positive electrode active material of Example 1 were used as the positive electrode active material of Comparative Example 2.

[0060] <Comparative Example 3> A positive electrode active material for Comparative Example 3 was obtained in the same manner as in Example 1, except that a solution of lithium niobethoxide dissolved in super-dehydrated ethanol was used as the deposition layer precursor solution and the heat treatment temperature was 350°C. In the positive electrode active material of Comparative Example 3, lithium niobate (LiNbO) was applied to the surface of the composite oxide particles. ３ A layer of ) was formed.

[0061] In Example 1, Comparative Example 1, and Comparative Example 3, the entire amount of the components contained in the adhesion layer precursor solution was used to form the adhesion layer, and the density of the adhesion layer was 2.56 g / cm³. ３ If that is the case, the surface area of ​​the composite oxide particles is 0.27 m². ２ Since the concentration is per gram, it is thought that an adhesive layer with a thickness of approximately 6 nm will be formed.

[0062] <Measurement> For each positive electrode active material obtained, the pore volume of pores with a diameter of 6 to 30 nm and the ratio of Si content to the total content of transition metal elements measured by XPS (Si / TM ratio) were measured using the method described above. In addition, the specific surface area was measured using the method described above.

[0063] <Evaluation> Using each positive electrode active material obtained by the procedure described above, all-solid-state lithium-ion secondary batteries were fabricated and their charge-discharge characteristics, interfacial resistance, and cycle characteristics were evaluated. First, the positive electrode active material, a solid electrolyte which is a sulfide, and acetylene black were mixed in a mortar to obtain a positive electrode composite material. The composition (molar ratio) of the above solid electrolyte was 75Li ２ S・25P ２ S ５ Next, a solid electrolyte was placed in a cylindrical container with a diameter of 10 mm and pressurized to form a solid electrolyte layer. Then, a positive electrode composite material was added to the cylindrical container and pressurized to form a positive electrode active material layer. Furthermore, indium foil and lithium foil were placed in the cylindrical container on the side of the solid electrolyte layer opposite to the positive electrode active material layer and pressurized to form a negative electrode active material layer. After forming the positive electrode active material layer and the negative electrode active material layer relative to the solid electrolyte layer, the all-solid-state battery formed by the powder compaction method was restrained with a predetermined load, sealed in a glass container, and then removed from the argon glove box to obtain an all-solid-state lithium-ion secondary battery.

[0064] The obtained all-solid-state lithium-ion secondary battery was used to perform impedance measurements under the following conditions to evaluate the interfacial resistance: • Frequency: 10 mHz to 10 kHz • Applied voltage: 10 mV • SOC: 100% An impedance spectrum was obtained by performing impedance measurements under the above conditions. The interfacial resistance was then estimated from the diameter of the semicircular component in the real axis direction in the obtained impedance spectrum. In the table below, the following classifications are used based on the ratio of each interfacial resistance, with the interfacial resistance in Comparative Example 2 set to 1. The following classification A is preferred for the interfacial resistance ratio: • A: Interfacial resistance ratio of 0.1 or less • B: Interfacial resistance ratio greater than 0.1

[0065] Next, charge-discharge tests were conducted under the following conditions to evaluate the discharge capacity of each positive electrode active material. • Charging conditions: Upper limit potential: 3.58V (Li-In counter electrode reference) Current: 0.05mA • Discharge conditions: Lower limit potential: 1.88V (Li-In counter electrode reference) Current: 0.05mA • Rest period between charge and discharge: 10 minutes A charge-discharge test was conducted under the above conditions, and the discharge capacity of the first cycle was evaluated. In the table shown later, the discharge capacity is described according to the following classifications. The discharge capacity is preferably in the following A category. • A: Discharge capacity of 150mAh / g or more • B: Discharge capacity of less than 150mAh / g

[0066] Furthermore, using the obtained all-solid-state lithium-ion secondary batteries, cycle tests were conducted under the following conditions to evaluate the cycle stability of each positive electrode active material. • Charging conditions: Upper limit potential: 3.88V (Li-In counter electrode reference) Current: 0.05mA • Discharge conditions: Lower limit potential: 1.88V (Li-In counter electrode reference) Current: 0.05mA • Rest time between charge and discharge: 10 minutes • Number of cycles: 30 Cycle tests were conducted under the above conditions, and the ratio of the discharge capacity at cycle 30 to the discharge capacity at cycle 1 was defined as the capacity retention rate, which was used as an indicator of the cycle stability of the positive electrode active material. A higher capacity retention rate indicates superior cycle characteristics of the all-solid-state lithium-ion secondary battery. In the table shown later, the capacity retention rate is described according to the following classifications. Note that classification A is preferable in terms of superior cycle stability of the positive electrode active material. • A: Capacity retention rate of 50% or more • B: Capacity retention rate of less than 50%

[0067] <Results> The preparation conditions for the positive electrode active material in each example, the measurement results, and the evaluation results are shown in Table 1.

[0068]

[0069] From the results shown in Table 1, the pore volume of pores containing lithium aluminosilicate with a pore diameter of 6 to 30 nm is 0.00030 to 0.00100 cm³. ３ The capacity retention rate of the all-solid-state lithium-ion secondary battery using the positive electrode active material of Example 1, which has a Si / TM ratio of 0.15 to 2.00, was confirmed to be higher than that of the all-solid-state lithium-ion secondary batteries using the positive electrode active materials of Comparative Examples 1 to 3. In other words, the cycle stability of the positive electrode active material of Example 1 was confirmed to be superior. In addition, the interfacial resistance of the positive electrode active material of Example 1 was confirmed to be lower than that of the positive electrode active materials of Comparative Examples 1 and 3. Furthermore, the discharge capacity of the all-solid-state lithium-ion secondary battery using the positive electrode active material of Example 1 was confirmed to be larger than that of the all-solid-state lithium-ion secondary batteries using the positive electrode active materials of Comparative Examples 1 to 3.

[0070] 10 Lithium-ion secondary battery 12 Positive electrode 14 Negative electrode 16 Solid electrolyte layer 22 Positive electrode current collector 24 Positive electrode active material layer 26 Negative electrode active material layer 28 Negative electrode current collector

Claims

1. A positive electrode active material for a lithium-ion secondary battery, comprising active material particles, wherein the active material particles comprise composite oxide particles and an adhering layer containing lithium aluminosilicate covering at least a portion of the surface of the composite oxide particles, and in pore distribution measurement, the pore volume of pores with a pore diameter of 6 to 30 nm is 0.00030 to 0.00100 cm³. ３ A positive electrode active material for lithium-ion secondary batteries, wherein the content is 0.15 to 2.00 in molar terms, and the ratio of Si content to the total content of transition metal elements, as measured by X-ray photoelectron spectroscopy, is 0.15 / g.

2. The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the composite oxide particles have a composition represented by the following formula (1). ａ Ni ｂ Co ｃ Mn ｄ X ｅ O ｆ (1) In equation (1), X is at least one element selected from the group consisting of B, Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Zr, Nb, Mo, Ba, La, and W. In equation (1), a, b, c, d, e, and f are real numbers and satisfy the following relationships: 0.2 ≤ a ≤ 2.1 b + c + d + e = 1 1.7 ≤ f ≤ 4.1 3. A lithium-ion secondary battery comprising a positive electrode containing the positive electrode active material for lithium-ion secondary batteries described in claim 1 or claim 2.

4. A lithium-ion secondary battery comprising: a positive electrode containing the positive electrode active material for a lithium-ion secondary battery described in claim 1 or claim 2; a negative electrode containing the negative electrode active material; and a solid electrolyte interposed between the positive electrode and the negative electrode to conduct lithium ions, wherein the solid electrolyte is a sulfide.

5. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, comprising the steps of forming a precursor layer of lithium aluminosilicate on the surface of the composite oxide particles and performing heat treatment at 140 to 350°C.

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