Cathode active material for lithium ion secondary battery, and all-solid lithium ion secondary battery

Abstract

To provide a cathode active material for a lithium ion secondary battery, which is a cathode active material having a lithium nickel composite oxide particle and a coating layer for coating a surface of the particle, which has a higher charge and discharge capacity, and which has proper cycle characteristics.SOLUTION: A cathode active material for a lithium ion secondary battery comprises a lithium nickel composite oxide particle, and a coating layer for coating at least a part of a surface of the lithium nickel composite oxide particle. The lithium nickel composite oxide particle contains Li, Ni, Co and oxygen. The coating layer contains a compound including Li, Nb and Ti.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery and an all-solid-state lithium ion secondary battery using the same. [Background technology]

[0002] In recent years, with the widespread use of electric vehicles, there has been a strong demand for the development of small, lightweight secondary batteries with high energy density. One such secondary battery is the lithium-ion secondary battery. Currently, typical lithium-ion secondary batteries use oxides such as LiCoO2, LiNiO2, or LiMn2O4 as the positive electrode active material, lithium metal, lithium alloys, metal oxides, or carbon as the negative electrode active material, and an electrolyte solution in which a Li salt such as LiClO4 or LiPF6 is dissolved as a supporting salt in an organic solvent such as ethylene carbonate, dimethyl carbonate, or ethyl methyl carbonate.

[0003] Among the components of lithium-ion secondary batteries, the electrolyte in particular is a factor that limits battery performance such as high-speed charging, safety, and lifespan due to its chemical properties such as heat resistance and potential window. Therefore, research and development is currently being actively conducted on all-solid-state lithium-ion secondary batteries (hereinafter also referred to as all-solid-state batteries), which improve battery performance by using a solid electrolyte instead of the electrolyte.

[0004] In the course of this research and development, it has been proposed, for example, in Patent Document 1, that sulfide solid electrolytes have high lithium ion conductivity and are suitable for use in all-solid-state batteries. However, as disclosed in Non-Patent Document 1, for example, when a sulfide electrolyte comes into contact with an oxide positive electrode active material, a reaction occurs at the interface between the electrolyte and the positive electrode active material during charge and discharge, and a high-resistance phase is formed, which can impede the operation of the battery.

[0005] Therefore, for example, Patent Document 2 proposes providing a coating layer made of LiNbO3 on the surface of the positive electrode active material in order to suppress the formation of a high resistance phase. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-56661 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-170715 [Non-patent literature]

[0007] [Non-Patent Document 1] Narumi Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications 9 (2007) 1486-1490 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the present inventors have found that if a coating of LiNbO3 is applied in an amount sufficient to inhibit the reaction, the LiNbO3 coating layer itself tends to become a resistive layer, resulting in a decrease in capacitance.

[0009] The present invention has been made in view of the above problems, and aims to provide a positive electrode active material for a lithium ion secondary battery, which has a lithium nickel composite oxide particle and a coating layer that coats the particle surface, and which has a higher charge / discharge capacity and good cycle characteristics. [Means for solving the problem]

[0010] According to a first aspect of the present invention, there is provided a positive electrode active material for a lithium ion secondary battery, which comprises particles of a lithium nickel composite oxide and a coating layer that coats at least a part of the surface of the particles of the lithium nickel composite oxide. The particles of the lithium nickel composite oxide contain Li, Ni, Co, oxygen, and optionally at least one element M selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo, and W. The molar ratio of the elements other than oxygen contained in the particles of the lithium nickel composite oxide is represented by Li:Ni:Co:M = t:(1 - x - y):x:y (0.95 ≤ t ≤ 1.20, 0 < x ≤ 0.22, 0 ≤ y ≤ 0.15). The coating layer contains a compound containing Li, Nb, and Ti.

[0011] Further, it is preferable that the molar ratio of the elements contained in the coating layer is represented by Li:Nb:Ti = (a + 1):(1 - a):a (0.2 ≤ a ≤ 0.8). Further, the total molar amount of Nb and Ti contained in the coating layer is preferably 27 μmol or more and 404 μmol or less per 1 m of the surface area of the particles of the lithium nickel composite oxide. Further, the ratio ((Nb + Ti) / (Ni + Co + M + Nb + Ti)) of the total molar amount of Nb and Ti present on the surface of the positive electrode active material to the total molar amount of Ni, Co, M, Nb, and Ti present on the surface of the positive electrode active material is preferably 83% or more and 100% or less.

[0012] Further, it is preferable that the carbon content is 0.5 mass% or less with respect to the entire positive electrode active material, and the moisture content is 0.2 mass% or less with respect to the entire positive electrode active material. Further, the specific surface area of the particles of the lithium nickel composite oxide is preferably 0.1 m 2 / g or more and 1.5 m 2 / g or less. Further, the particles of the lithium nickel composite oxide are preferably a layered rock salt type compound having a crystal structure belonging to the space group R-3m.

[0013] According to a second aspect of the present invention, there is provided an all-solid-state lithium ion secondary battery comprising at least a positive electrode, a negative electrode, and a solid electrolyte, wherein the positive electrode contains the above-described positive electrode active material. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a cathode active material for an all-solid-state lithium-ion secondary battery, which comprises lithium nickel composite oxide particles and a coating layer that coats the surfaces of the particles, and which has a higher charge / discharge capacity and good cycle characteristics. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing an example of a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to this embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view of a solid secondary battery used for battery evaluation. DETAILED DESCRIPTION OF THE INVENTION

[0016] A positive electrode active material for a lithium ion secondary battery (hereinafter also simply referred to as a positive electrode active material) according to one embodiment of the present invention, a lithium ion secondary battery, and a method for producing the positive electrode active material for a lithium ion secondary battery will be described below.

[0017] 1. Positive electrode active material for lithium-ion secondary batteries The positive electrode active material according to this embodiment includes lithium nickel composite oxide particles and a coating layer that covers at least a portion of the surface of the particles. The coating layer contains a compound containing lithium (Li), niobium (Nb), and titanium (Ti) (hereinafter also referred to as a "lithium-niobium-titanium compound").

[0018] As a result of intensive studies, the present inventors have found that by providing a coating layer containing a specific amount of a lithium-niobium-titanium compound on the surface of particles of a lithium nickel composite oxide, it is possible to obtain a positive electrode active material for an all-solid-state battery that has a high charge-discharge capacity and good cycle characteristics while providing a sufficient amount of coating for reaction suppression. Hereinafter, each component of the positive electrode active material will be described.

[0019] 1-1. Particles of Lithium Nickel Composite Oxide The particles of the lithium nickel composite oxide contain at least Li, Ni, Co, and oxygen. Further, the particles of the lithium nickel composite oxide may optionally contain an element M. The element M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Cu, Cr, Mn, Zn, Zr, Nb, Mo, and W.

[0020] The molar ratio (1) of each of the above elements other than oxygen contained in the particles of the lithium nickel composite oxide is represented by Li:Ni:Co:M = t:(1-x-y):x:y (0.95 ≦ t ≦ 1.20, 0 < x ≦ 0.22, 0 ≦ y ≦ 0.15). Hereinafter, each element contained in the particles of the lithium nickel composite oxide will be described.

[0021] (Lithium) In the molar ratio (1), the value of t indicating the molar ratio of lithium (Li) is 0.95 or more and 1.20 or less, preferably 0.98 or more and 1.10 or less, and more preferably 1.00 or more and 1.10 or less.

[0022] When the value of t in the molar ratio (1) is 0.95 or more, the internal resistance of the secondary battery can be suppressed and the output characteristics can be improved. When the value of t is 1.20 or less, the initial discharge capacity of the secondary battery can be increased. That is, by setting the value of t within the above range, the output characteristics and capacity characteristics of a secondary battery using a positive electrode active material containing particles of a lithium nickel composite oxide can be improved.

[0023] [[ID= Cobalt (Co) is an element that contributes to reducing irreversible capacity in secondary batteries using a positive electrode active material containing lithium-nickel composite oxide particles. In the substance amount ratio (1), the value of x, which indicates the content of cobalt (Co), is greater than 0 and not more than 0.22, preferably 0.10 to 0.22, and more preferably 0.10 to 0.20.

[0024] In the substance amount ratio (1), when the value of x is greater than 0, the irreversible capacity, which is the difference between the charge capacity and the discharge capacity, of the secondary battery can be reduced. Also, when the value of x is 0.22 or less, a high battery capacity can be obtained.

[0025] (Element M) The lithium nickel composite oxide particles may contain, in addition to the above metal elements, an additive element, element M. The element M may be at least one element selected from magnesium (Mg), aluminum (Al), calcium (Ca), silicon (Si), scandium (Sc), titanium (Ti), vanadium (V), iron (Fe), copper (Cu), chromium (Cr), Mn (manganese), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), and tungsten (W). The element M may be appropriately selected depending on the application and required performance of the secondary battery. The element M itself may be an element that does not contribute to the redox reaction.

[0026] In the substance amount ratio (1), the value of y indicating the content of element M is 0.15 or less, preferably 0.10 or less, and more preferably 0.05 or less. Furthermore, the lithium nickel composite oxide particles may not contain element M, and the lower limit of y may be 0. Furthermore, element M may contain Al. When Al is contained, the value of y indicating the content of element M may be 0.01 or more and 0.1 or less.

[0027] (nickel) In the lithium nickel composite oxide particles, nickel (Ni) is an element that contributes to increasing the capacity of a secondary battery using a positive electrode active material containing lithium nickel composite oxide. In the substance amount ratio (1), the value of the substance amount ratio of nickel (Ni) is expressed as 1-xy, where the sum of the nickel (Ni) content (1-xy), the cobalt (Co) content (x), and the element M content (y) is 1. That is, in the substance amount ratio (1), the value of the substance amount ratio of nickel (Ni) (1-xy) is 0.63 or more and less than 1.00, preferably 0.70 or more and 0.98 or less, and more preferably 0.80 or more and 0.95 or less. Within the above range, the higher the nickel (Ni) content, the higher the capacity of a secondary battery that can be obtained.

[0028] (crystal structure) It is preferable that the lithium nickel composite oxide particles have a diffraction pattern obtained by X-ray diffraction (XRD) measurement in which peaks attributable to the layered rock salt crystal structure of the "R-3m" structure are detected. In particular, it is more preferable that only peaks attributable to the layered rock salt crystal structure of the "R-3m" structure are detected from the diffraction pattern. This is because the layered rock salt oxide of the "R-3m" structure can particularly suppress internal resistance when a positive electrode active material containing the lithium nickel composite oxide particles is used in a secondary battery.

[0029] In addition, peaks attributable to impurity phases other than the peaks attributable to the layered rock-salt crystal structure of the "R-3m" structure may be detected in the lithium nickel composite oxide particles. For example, a lithium nickel composite oxide having a layered rock-salt crystal structure may not be obtained in a single phase, and impurity phases may be mixed in. Even in such cases where impurity phases are mixed in, it is preferable that the intensities of the peaks attributable to these impurity phases other than the layered rock-salt structure of the "R-3m" structure do not exceed the peak intensity attributable to the layered rock-salt structure of the "R-3m" structure.

[0030] (particle structure) The lithium nickel composite oxide particles are preferably single primary particles, secondary particles formed by agglomeration of multiple primary particles, or a mixture thereof. The primary particles and secondary particles can be observed using an electron microscope such as a SEM or a TEM. Furthermore, the secondary particles may contain spaces or voids surrounded by one or more primary particles.

[0031] (specific surface area) The lithium nickel composite oxide particles have a specific surface area of ​​0.1 m 2 / g or more 1.5m 2 / g or less, and 2 / g or more 1.0m 2 / g or less is more preferable. The release / intercalation of lithium ions in the lithium nickel composite oxide during charge / discharge of the secondary battery occurs at the interface between the lithium nickel composite oxide particles and the solid electrolyte, i.e., through the surface of the lithium nickel composite oxide. Therefore, by making the specific surface area of ​​the lithium nickel composite oxide contained in the positive electrode active material 0.1 m2 / g or more, the release / intercalation of lithium ions is sufficiently promoted, and the internal resistance of the secondary battery during charge / discharge can be reduced, which is preferable. Furthermore, as mentioned above, the electrolyte may decompose due to a side reaction occurring at the interface between the positive electrode active material and the solid electrolyte, and a coating layer that serves as a buffer layer is required. However, it is possible to make the specific surface area of ​​the lithium nickel composite oxide particles 1.0 m2 / g or more. 2 By setting the content to be not more than 1 / g, the amount of electrochemically inactive coating layer components can be kept small.

[0032] The specific surface area of ​​the lithium nickel composite oxide particles can be measured, for example, by the BET method using nitrogen adsorption.

[0033] (Volume average particle size) The volume average particle size of the lithium nickel composite oxide particles is not particularly limited, but is preferably 2 μm to 20 μm, more preferably 2 μm to 15 μm, and even more preferably 3 μm to 15 μm. This is because when the volume average particle size of the lithium nickel composite oxide particles is 2 μm to 20 μm, a secondary battery using a positive electrode active material containing the lithium nickel composite oxide particles in the positive electrode can have a sufficiently large battery capacity per capacity and can obtain excellent battery characteristics such as high safety and high output. Here, the volume average particle size can be measured using a laser light diffraction / scattering particle size distribution analyzer or the like.

[0034] 1-2.Coating layer The coating layer contains a compound containing Li, Nb, and Ti (lithium-niobium-titanium compound), and coats at least a portion of the surface of the lithium nickel composite oxide particles.

[0035] (Coating layer composition) The Li, Nb, and Ti contained in the coating layer are preferably expressed by the substance ratio (2) Li:Nb:Ti=(a+1):(1-a):a, where a, which indicates the Ti content, is in the range of 0.2≦a≦0.8.

[0036] When a in the substance amount ratio (2) is within the above range, the coexistence of Nb and Ti in the coating layer in a suitable ratio improves Li-ion conductivity, allowing a secondary battery to obtain a high charge / discharge capacity. In particular, in the coating layer according to this embodiment, niobium and titanium are arranged in the same compound as the lithium-niobium-titanium compound, which suppresses an increase in positive electrode resistance and allows a higher charge / discharge capacity to be obtained. On the other hand, when a is outside the above range, the high Li-ion conductivity of the coating layer due to the coexistence of niobium and titanium is impaired, resulting in a decrease in the charge / discharge capacity of the secondary battery. The substance amount ratio of each metal element contained in the coating layer can be adjusted to the above range, for example, by coating the surface of the lithium nickel composite oxide particles with a solution (coating agent) containing Li, Nb, and Ti in the substance amount ratio within the above range.

[0037] The lithium-niobium-titanium compound may be a lithium-niobium-titanium oxide. The lithium-niobium-titanium oxide may have a composition such as Li a+1 Ti a Nb 1-a O3 (0.2≦a≦0.8). The coating layer preferably contains lithium-niobium-titanium oxide as a main component, and may be composed of lithium-niobium-titanium oxide. The presence of lithium-niobium-titanium oxide can be confirmed, for example, by powder XRD to confirm that a single-phase compound of Li and Ti and a single-phase compound of Li and Nb are not detected, and by EDX measurement using a scanning transmission electron microscope (S-TEM) to confirm that there is no difference in the distribution of Ti and Nb on the particle surface and that both elements are present in the same location.

[0038] The coating layer and the lithium nickel composite oxide particles may or may not have a clear boundary. In this specification, the coating layer refers to a portion (region) on the surface side of the positive electrode active material of this embodiment, where the concentrations of niobium and titanium are higher than in the central region of the lithium nickel composite oxide particles coated with the coating layer (e.g., a region within 50% of the radius from the center of the particle).

[0039] The niobium and titanium in the coating layer may be partially dissolved in the lithium nickel composite oxide particles. By partially dissolving the niobium and titanium in the lithium nickel composite oxide, it may be possible to suppress the reaction at the interface between the lithium nickel composite oxide and the solid electrolyte. Furthermore, the coating layer may contain a compound other than lithium-niobium-titanium oxide.

[0040] (Coating metal content per surface area) The contents of niobium (Nb) and titanium (Ti) in the coating layer are preferably adjusted according to the specific surface area of ​​the lithium nickel composite oxide particles to be coated. Specifically, the total amount of Nb and Ti contained in the coating layer is adjusted according to the specific surface area of ​​the lithium nickel composite oxide particles per m2 It is preferable that the concentration is 27 μmol or more and 404 μmol or less per unit area.

[0041] The total amount of Nb and Ti contained in the coating layer is 1 m2 of the surface area of ​​the lithium nickel composite oxide particle. 2 If the amount of Nb and Ti contained in the coating layer is 27 μmol or more per square meter of surface area, it indicates that the coating layer is uniformly distributed over the entire surface of the lithium nickel composite oxide particle. 2 per m2 of surface area. 2 per m2 of surface area. 2 It may be 50 μmol or more per unit area.

[0042] In conventional positive electrode active materials, the addition of a coating layer containing niobium can improve cycle characteristics and suppress battery swelling, but at the same time, there is a risk of increasing internal resistance. Therefore, the total amount of Nb and Ti contained in the coating layer was adjusted to 1 m2 of the surface area of ​​the lithium nickel composite oxide particles. 2 By adjusting the content to 404 μmol or less per unit area, it is possible to prevent the coating layer from interfering with the lithium desorption / sorption reaction of the lithium nickel composite oxide, thereby reducing the internal resistance.

[0043] The methods for evaluating and calculating the amounts of Nb and Ti contained in the coating layer are not particularly limited, but for example, they can be measured by the following method.

[0044] First, the contents (μmol / g) of niobium (Nb) and titanium (Ti) in 1 g of the obtained positive electrode active material are measured by a method such as chemical analysis. As a method for chemical analysis, for example, ICP (Inductively Coupled Plasma) can be used.

[0045] Next, the specific surface area (m 2 / g) is measured by the BET method using nitrogen adsorption.

[0046] The niobium and titanium contents (μmol / g) in 1 g of the obtained positive electrode active material were calculated based on the specific surface area (m 2 / g), the surface area of ​​the lithium nickel composite oxide particles is calculated as 1m 2 Niobium and titanium content per unit mass (μmol / m 2 ) can be calculated.

[0047] When the lithium nickel composite oxide particles (base material) before the coating treatment contain niobium and titanium, it is preferable to use the difference in the niobium and titanium contents before and after the coating treatment as the amount of niobium and titanium used in the coating.

[0048] (Metal coating ratio on particle surface) The ratio of the total amount of Nb and Ti present on the surface of the positive electrode active material to the total amount of Ni, Co, M, Nb, and Ti present on the surface of the positive electrode active material ((Nb+Ti) / (Ni+Co+M+Nb+Z): hereinafter, also simply referred to as the "coating metal ratio") is preferably 83% or more and 100% or less.

[0049] When the coating metal ratio of the positive electrode active material is within the above range, an all-solid-state lithium ion secondary battery with high capacity and excellent cycle characteristics can be obtained. Furthermore, when the coating metal ratio is 83% or more, the niobium and titanium present as a coating layer, i.e., not diffused into the lithium nickel composite oxide particles but present as a coating layer, are sufficiently secured, meaning that a coating layer sufficiently uniform to suppress reaction at the interface between the positive electrode active material and the electrolyte is formed. Furthermore, within the above range, the coating metal ratio is preferably 95% or less from the viewpoint of higher charge / discharge capacity. Furthermore, from the viewpoint of higher cycle characteristics, the coating metal ratio is preferably 85% or more.

[0050] The abundance ratio (mol %) of each element on the surface of the lithium nickel composite oxide can be confirmed by semi-quantitative analysis using X-ray photoelectron spectroscopy (XPS). The metal coverage ratio can be determined from the intensity (area) ratio of the peaks attributable to each metal element in the XPS spectrum obtained by measuring the surface of the lithium nickel composite oxide particles by X-ray photoelectron spectroscopy (XPS) using an X-ray photoelectron spectrometer (ULVAC-PHI, Versa Probe II) with a monochromated Al-Kα X-ray source, an X-ray beam width of 100 μm, and an output of 25 W.

[0051] XPS has the ability to selectively obtain information from a depth of 1 nm to 5 nm from the surface of the object being measured, making it possible to determine the composition ratio of the surface layer of the sample. Furthermore, the upper limit of the metal coverage ratio is 100% or less due to the nature of the value.

[0052] 1-3. Characteristics of positive electrode active material According to the positive electrode active material for a lithium ion secondary battery according to this embodiment (hereinafter also simply referred to as "positive electrode active material"), it is possible to provide a positive electrode active material for an all-solid-state lithium ion secondary battery having high capacity and excellent cycle characteristics. Preferred examples of the characteristics of the positive electrode active material according to this embodiment will be described below.

[0053] The positive electrode active material according to this embodiment is preferably composed only of the lithium nickel composite oxide particles and the coating layer, but impurities may be mixed in during production. In particular, moisture and carbon may increase due to the coating process. The mixing of moisture and carbon into the positive electrode active material may affect cycle characteristics, so it is preferable that the amounts are controlled within a specific range.

[0054] (carbon content) The carbon content of the positive electrode active material is preferably 0.5% by mass or less, more preferably 0.2% by mass or less, based on the total amount of the positive electrode active material, and the range including the lower limit of the carbon content is preferably 0.05% by mass or more.

[0055] When the carbon content is 0.5% by mass or less, the generation of carbon dioxide due to, for example, the decomposition of organic components contained in the coating layer during charging and discharging of the secondary battery can be sufficiently suppressed, and the occurrence of battery expansion, etc. can be suppressed.

[0056] Since carbon is almost inevitably mixed into the positive electrode active material due to carbon dioxide in the air, etc., it is difficult to set the carbon content to less than 0.05% by mass, and therefore the lower limit of the carbon content is preferably 0.05% by mass.

[0057] The carbon content in the positive electrode active material can be evaluated by, for example, infrared absorption spectroscopy.

[0058] (moisture content) The amount of water contained in the positive electrode active material (water content) is preferably 0.2 mass % or less based on the total amount of the positive electrode active material. When the water content is 0.2 mass % or less, the hydrolysis reaction of the solid electrolyte in the secondary battery can be more reliably suppressed.

[0059] In a secondary battery, when the solid electrolyte undergoes hydrolysis, hydrogen sulfide gas is generated and a phosphate-based resistance layer is formed. However, when the water content of the positive electrode active material is within the above range, the hydrolysis reaction of the electrolyte can be more reliably suppressed, thereby suppressing deterioration.

[0060] The amount of water in the positive electrode active material can be evaluated by the Karl Fischer method at a heating temperature of 300°C.

[0061] 2. Manufacturing method of positive electrode active material for lithium-ion secondary batteries Next, a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention will be described with reference to the drawings.

[0062] 1 is a process diagram showing an example of a schematic method for producing a positive electrode active material for a lithium ion secondary battery according to the present invention (hereinafter also simply referred to as "the method for producing a positive electrode active material"). The method for producing a positive electrode active material according to the present invention can highly productively produce the above-mentioned positive electrode active material for a lithium ion secondary battery, which comprises particles of lithium nickel composite oxide and a coating layer that coats at least a portion of the surface of the lithium nickel composite oxide particles.

[0063] 1, the method for producing a positive electrode active material includes, for example, a precursor crystallization step S1, a lithium-nickel composite oxide synthesis step S3, and a coating step S4. Alternatively, an oxidizing roasting step S2 may be included after the precursor crystallization step S1 and before the lithium-nickel composite oxide synthesis step S3. Each step will be described in detail below. Note that the following description is merely an example of the production method and is not intended to limit the scope of the production method.

[0064] <Precursor crystallization step S1> The precursor crystallization step S1 is a step of preparing a nickel composite hydroxide by a crystallization reaction.

[0065] In the precursor crystallization step S1, for example, a mixed aqueous solution (raw material mixed aqueous solution) prepared using water-soluble raw materials containing each metal element and an alkali metal aqueous solution are subjected to a crystallization reaction together in a reaction tank to obtain a nickel composite hydroxide.

[0066] The substance amount ratio of each metal element in the mixed aqueous solution can be adjusted to Ni:Co:M = (1 - xy):x:y. The range of x and y in the substance amount ratio can be the same as the range of x and y in the substance amount ratio (1) described above for the lithium nickel composite oxide particles. An example of the precursor crystallization step S1 will be described below.

[0067] (Preparation of raw material mixed aqueous solution) The raw material mixed aqueous solution contains a metal compound containing nickel, a metal compound containing cobalt, and optionally a metal compound containing element M in predetermined proportions.

[0068] The substance amount ratio of each metal in the raw material mixed aqueous solution is almost the same as the substance amount ratio of the nickel composite hydroxide finally obtained. Therefore, the substance amount ratio of each metal in the raw material mixed aqueous solution can be adjusted by the proportion of the amount of metal compound dissolved in water so that the substance amount ratio becomes the same as the substance amount ratio of each metal in the target nickel composite hydroxide particles.

[0069] The metal compound may be water-soluble. For example, sulfates, chlorides, nitrates, etc. can be used as the metal compound, but sulfates are preferred from the viewpoint of cost. The raw material mixed aqueous solution may be prepared by dissolving each metal compound in water, or by preparing a plurality of aqueous solutions in advance, each containing one or more metal element compounds dissolved in water, and then mixing these aqueous solutions.

[0070] The metal compound containing element M may be added in the oxidizing roasting step S2 or the lithium nickel composite oxide synthesis step S3, which will be described later, without being added to the raw material mixed aqueous solution. After obtaining a nickel composite hydroxide by a crystallization reaction, the obtained nickel composite hydroxide may be coated with the metal compound containing element M by, for example, crystallization, spray drying, impregnation, or the like.

[0071] (Preparation of initial aqueous solution) Before the crystallization reaction, water may be charged into a reaction tank, and an alkaline substance and an appropriate amount of an ammonium ion donor may be added to prepare an initial aqueous solution. In this case, it is preferable to adjust the pH value of the initial aqueous solution to 10 to 13 at a liquid temperature of 25°C and the ammonia concentration to 2 g / L to 20 g / L. The pH value and ammonia concentration of the initial aqueous solution may be adjusted appropriately within the above ranges in accordance with the pH value and ammonia concentration of the reaction aqueous solution described below.

[0072] When a crystallization reaction is carried out to prepare a nickel composite hydroxide, impurities resulting from anions constituting metal compounds contained in the raw material mixed aqueous solution used may be mixed into the nickel composite hydroxide. When the pH value of the initial aqueous solution is 10 or higher, it is possible to prevent the mixing of impurities resulting from anions constituting metal compounds of the raw materials. Furthermore, when the pH value of the initial aqueous solution is 13 or lower, it is possible to prevent the particles of the obtained nickel composite hydroxide from becoming finer, and to achieve an optimal size.

[0073] When the ammonia concentration of the initial aqueous solution is 2 g / L or more, the particles of the obtained nickel composite hydroxide can be made to be spherical. Also, when the ammonia concentration of the initial aqueous solution is 20 g / L or less, an excessive increase in the solubility of nickel that forms an ammonia complex can be prevented, and the substance amount ratio of the obtained nickel composite hydroxide can be made to be the target substance amount ratio more reliably.

[0074] The alkaline substance used in preparing the initial aqueous solution is not particularly limited, but is preferably one or more selected from sodium carbonate, sodium bicarbonate, potassium carbonate, sodium hydroxide, and potassium hydroxide. From the viewpoint of easily adjusting the amount added, the alkaline substance is preferably added in the form of an aqueous solution.

[0075] The ammonium ion donor is not particularly limited, but one or more selected from an aqueous ammonium carbonate solution, aqueous ammonia, an aqueous ammonium chloride solution, and an aqueous ammonium sulfate solution can be preferably used.

[0076] (crystallization reaction) The raw material mixed aqueous solution is added dropwise to the initial aqueous solution to form a reaction aqueous solution, and the crystallization reaction proceeds. More specifically, for example, the initial aqueous solution is placed in a reaction tank, and after adjusting the pH, ammonia concentration, temperature, etc., the raw material mixed aqueous solution is continuously added dropwise to the reaction tank at a constant rate to cause the reaction, thereby proceeding with the crystallization reaction of nickel composite hydroxide particles, which are the precursor.

[0077] Furthermore, when the mixed aqueous solution is dropped into the reaction aqueous solution, it is preferable to also dropwise add the ammonium ion donor and the alkaline substance to the reaction aqueous solution at a constant rate to adjust the conditions to achieve the desired crystallization conditions. Preferred conditions for the crystallization reaction are described below.

[0078] (Atmosphere inside the reactor) The atmosphere in the reaction vessel is preferably a non-oxidizing atmosphere, for example, an atmosphere with an oxygen concentration of 1% by volume or less. This is because the non-oxidizing atmosphere can prevent the metals contained in the metal compound from being oxidized. For example, it can prevent cobalt in the reaction aqueous solution from being oxidized and precipitating as fine particles.

[0079] (Temperature inside the reactor) The temperature inside the reaction tank is preferably maintained at 40° C. or higher and 60° C. or lower, and more preferably at 45° C. or higher and 55° C. or lower. The initial aqueous solution and the reaction aqueous solution placed in the reaction tank are also preferably maintained within the same temperature range as the temperature inside the reaction tank.

[0080] The temperature of the reaction vessel naturally rises due to the heat of reaction and the energy of stirring. Therefore, it is preferable to keep the temperature inside the reaction vessel at 40°C or higher, as this does not require additional energy consumption for cooling. In addition, it is preferable to keep the temperature inside the reaction vessel at 60°C or lower, as this can suppress the evaporation of ammonia from the initial aqueous solution and the reaction aqueous solution, making it easier to maintain the target ammonia concentration.

[0081] (pH and ammonia concentration) As with the initial reaction aqueous solution, the reaction aqueous solution is preferably prepared so that the pH value is maintained in the range of 10 to 13 at a liquid temperature of 25°C and the ammonia concentration is maintained in the range of 2 g / L to 20 g / L. It is more preferable to include a step of controlling the pH value of the reaction aqueous solution during the crystallization reaction so that the pH value is maintained in the range of 11.2 to 12.2 at a liquid temperature of 25°C and the ammonia concentration is maintained in the range of 2 g / L to 15 g / L. Maintaining the pH of the reaction aqueous solution at 11.2 to 12 appropriately reduces the rate at which the nickel-cobalt-aluminum composite hydroxide precipitates from the liquid phase to the solid phase, thereby improving the uniformity of the particle size distribution and the sphericity of the resulting particles. The pH fluctuation range is preferably controlled within ±0.2.

[0082] The conditions in the crystallization reaction (e.g., the atmosphere in the reaction tank, pH, etc.) may be maintained the same throughout the entire crystallization reaction, or a multi-stage crystallization process may be carried out by setting a plurality of different crystallization conditions within the above ranges during the crystallization reaction (e.g., switching the atmosphere in the reaction tank, changing the pH, etc.). For example, if the pH at the end of the crystallization reaction is set to a value higher than that of the previous process (e.g., pH greater than 12.2), nickel ions that are complexed with ammonia and dissolved in the liquid phase are precipitated on nickel composite hydroxide particles, and deviation of the actually obtained chemical composition from the target chemical composition can be suppressed.

[0083] After the crystallization reaction, the slurry containing the nickel composite hydroxide particles is recovered from an overflow port provided in the reaction tank, and is then filtered and dried, thereby obtaining powdered nickel composite hydroxide particles as a precursor.

[0084] <Oxidation roasting process S2> Furthermore, an oxidizing roasting step S2 may be provided after the precursor crystallization step S1. The oxidizing roasting step S2 is a step in which the nickel composite hydroxide obtained in the precursor crystallization step S1 is oxidizing roasted to obtain a nickel composite oxide. In the oxidizing roasting step S2, the nickel composite oxide is roasted (heat treated) in an oxygen-containing atmosphere, and then cooled to room temperature, thereby obtaining the nickel composite oxide.

[0085] The roasting conditions in the oxidation roasting step S2 are not particularly limited as long as at least a part of the nickel composite hydroxide is converted into a nickel composite oxide, and it is preferable that all of the nickel composite hydroxide is converted into a nickel composite oxide. As roasting conditions, it is preferable to perform heat treatment in an oxygen-containing atmosphere, for example, an air atmosphere, at a temperature of 500°C or higher and 700°C or lower for 1 hour or higher and 12 hours or lower.

[0086] When the roasting temperature is 500°C or higher, the nickel composite hydroxide particles can be easily converted completely into nickel composite oxide. When the roasting temperature is 700°C or lower, the specific surface area of ​​the nickel composite oxide can be prevented from becoming excessively small.

[0087] When the roasting time is 1 hour or longer, the temperature in the roasting vessel can be made uniform, and the reaction can proceed uniformly. Furthermore, even if roasting is performed for a time longer than 12 hours, no significant changes are observed in the obtained nickel composite oxide. Therefore, from the viewpoint of energy efficiency, the roasting time is preferably 12 hours or shorter.

[0088] The oxygen concentration in the oxygen-containing atmosphere during roasting is preferably equal to or higher than the oxygen concentration in the air atmosphere, i.e., 20% by volume or higher. Since an oxygen atmosphere can also be used, the upper limit of the oxygen concentration in the oxygen-containing atmosphere can be 100% by volume.

[0089] Note that, for example, when a compound containing the element M is not added in the precursor crystallization step S1, for example, a compound containing the element M may be added to the nickel composite hydroxide to be subjected to the oxidizing roasting step S2 in an amount that becomes the same as the target composition ratio, and then oxidizing roasting may be performed. The compound containing the element M is not particularly limited, and for example, an oxide, a hydroxide, a carbonate, or a mixture thereof may be used.

[0090] If slight sintering is observed in the nickel composite oxide particles after the oxidizing roasting step, a crushing treatment may be added.

[0091] <Lithium nickel composite oxide synthesis step S3> The lithium nickel composite oxide synthesis step S3 includes a step of mixing the nickel composite hydroxide and / or nickel composite oxide obtained in the precursor crystallization step S1 and / or the oxidizing roasting step S2 with a lithium compound (lithium mixture preparation step: S3-2), and a step of calcining the resulting mixture (calcination step: S3-1). Each step will be described below.

[0092] (Lithium mixture preparation process: S3-1) First, a lithium compound is added to and mixed with the nickel composite oxide obtained in the oxidizing roasting step S2 to obtain a lithium mixture (preparation of lithium mixture: S3-1).

[0093] The mixing ratio of the lithium compound is, for example, such that the amount of substance of lithium (Li) is 95% or more and 120% or less relative to the total amount of substance of the component metal elements (Ni, Co, element M) contained in the nickel composite oxide.

[0094] The lithium compound is not particularly limited, and for example, lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture thereof can be used. As the lithium compound, it is preferable to use lithium hydroxide, which has a low melting point and high reactivity.

[0095] <Firing process S3-2> Next, the resulting lithium mixture is calcined in an oxygen-containing atmosphere and then cooled to room temperature to obtain a lithium nickel composite oxide (calcination step: S3-2). The calcination conditions are not particularly limited, but it is preferable to calcinate at a temperature of 700°C or higher and 800°C or lower for 1 hour or longer and 24 hours or shorter, for example.

[0096] The oxygen-containing atmosphere is preferably an atmosphere containing 80% by volume or more of oxygen. This is because, by setting the oxygen concentration in the atmosphere to 80% by volume or more, cation mixing, in which Ni atoms mix with the Li sites in the resulting lithium nickel composite oxide, can be particularly suppressed, which is preferable. Since an oxygen atmosphere can also be used, the upper limit of the oxygen concentration of the oxygen-containing atmosphere can be set to 100% by volume.

[0097] A firing temperature of 700°C or higher is preferable because the crystal structure of the lithium nickel composite oxide can be sufficiently grown, while a firing temperature of 800°C or lower is preferable because the above-mentioned cation mixing can be suppressed.

[0098] A calcination time of 1 hour or more is preferable because it makes it possible to uniformize the temperature inside the calcination vessel and to uniformly proceed with the reaction. Furthermore, even if calcination is performed for a time longer than 24 hours, no significant changes are observed in the resulting lithium nickel composite oxide, so from the viewpoint of energy efficiency, it is preferable that the calcination time be 24 hours or less.

[0099] After the lithium nickel composite oxide synthesis step S3, if slight sintering is observed in the resulting lithium nickel composite oxide, a crushing treatment may be added.

[0100] <Coating process S4> In the coating step S4, a coating layer containing a compound containing lithium (Li), niobium (Nb), and titanium (Ti) (hereinafter also referred to as "lithium-niobium-titanium compound") is formed on at least a portion of the surface of the obtained lithium nickel composite oxide particles.

[0101] In the coating step S4, the total amount of niobium (Nb) and titanium (Ti) contained in the coating layer is adjusted to a value equal to or larger than the surface area of ​​the lithium nickel composite oxide particles per 1 m 2 A raw material containing niobium and titanium is added so that the amount of niobium and titanium is 27 μmol or more and 404 μmol or less per 10 ...

[0102] (Adjustment of coating agent) First, the specific surface area of ​​the lithium nickel composite oxide obtained in the lithium nickel composite oxide synthesis step S3 is measured, and a coating agent is prepared according to the target amounts of niobium and titanium to be contained in the coating layer per unit surface area of ​​the particles. The coating agent is preferably liquid at room temperature.

[0103] The coating agent is not particularly limited as long as it contains lithium, niobium, and titanium. From the viewpoint of uniform coating, for example, a solution in which a lithium compound, a niobium compound, and a titanium compound are dissolved or dispersed in a solvent can be preferably used as the coating agent. When dispersing the above compounds in a solvent, it is preferable to use a low-melting-point compound that can be melted by low-temperature heat treatment, or a sol-based material in which nanoparticles of lithium, niobium, and titanium are dispersed in a solvent.

[0104] Examples of niobium compounds include one or more selected from alkoxides such as niobic acid, niobium oxide, niobium nitrate, niobium pentachloride, and niobium pentaethoxide, chelates such as niobium peroxo complexes, and dispersions of solvent-insoluble fine particles of 50 nm or less.

[0105] Examples of titanium compounds include one or more selected from alkoxides such as titanium tetraisopropoxide, chelates such as titanium peroxo complexes, and dispersions of solvent-insoluble fine particles of 50 nm or less. Titanium sponge (metallic titanium) may also be used as the titanium raw material.

[0106] Examples of lithium compounds include alkoxides such as lithium ethoxide, and water-soluble salts such as lithium hydroxide.

[0107] As the coating agent, from the viewpoint of ease of preparation, it is preferable to use a solution in which a lithium compound, a niobium compound, and a titanium compound (titanium raw material) are dissolved in a solvent. Furthermore, from the viewpoint of suppressing the inclusion of impurities, for example, a solution prepared by mixing an aqueous solution in which niobium oxide and sponge titanium are mixed and dissolved in hydrogen peroxide water and ammonia water, and then mixing this with an aqueous lithium hydroxide solution can be preferably used as the coating agent. Note that by setting the mass ratio of Li, Nb, and Ti contained in the coating agent to, for example, Li:Nb:Ti = (a + 1):(1 - a):a (0.2 ≦ a ≦ 0.8), the mass ratio of Li, Nb, and Ti in the coating layer can also be set to a similar range.

[0108] (Adhesion of coating agent to particle surface) Next, a coating agent is applied to the surface of the lithium nickel composite oxide particles. As a result, at least a portion of the particle surface is coated with the coating agent. As the application method, any known method can be used as long as it can apply the coating liquid to the particle surface so as to satisfy the above-mentioned properties.

[0109] For example, the coating agent may be attached by mixing the lithium nickel composite oxide particles with the coating agent and then drying the mixture. A general mixer can be used to mix the lithium nickel composite oxide particles with the coating agent. Furthermore, from the viewpoint of attaching the coating agent more uniformly, the coating agent may be sprayed into droplets while stirring and mixing the lithium nickel composite oxide particles, and then attached to the surface of the particles.

[0110] After the coating agent is attached to the particle surface, drying may be performed. Drying can be performed at a temperature sufficient to reduce (remove) the solvent and moisture in the coating agent. For example, drying can be performed at a temperature of 80°C or higher and lower than 300°C. The drying step may be performed simultaneously with the step of attaching the coating agent to the particle surface, or may be performed separately.

[0111] (Heat treatment) After the coating agent is attached to the particle surface, a heat treatment may be further performed as necessary. By performing the heat treatment, the lithium-niobium-titanium compound can be fixed to the particle surface as a coating layer. After the heat treatment, the mixture is cooled to room temperature to obtain the positive electrode active material.

[0112] The conditions for the heat treatment are not particularly limited, but it is preferable to perform the heat treatment in an oxygen-containing atmosphere at a temperature (maximum temperature) of 200° C. to 600° C. for 1 hour to 5 hours.

[0113] The heat treatment atmosphere can be an oxygen-containing atmosphere, for example, an air atmosphere. The oxygen concentration in the oxygen-containing atmosphere is preferably equal to or higher than the oxygen concentration in the air atmosphere, i.e., 20% by volume or higher. By setting the heat treatment atmosphere to an oxygen concentration equal to or higher than the oxygen concentration in the air atmosphere, it is possible to particularly suppress the occurrence of oxygen defects in the resulting positive electrode active material. The heat treatment atmosphere can also be an oxygen atmosphere, and the upper limit of the oxygen concentration in the oxygen-containing atmosphere can be 100% by volume.

[0114] When the heat treatment temperature (maximum temperature) is 200°C or higher, it is possible to particularly prevent impurities contained in the coating agent from remaining in the positive electrode active material. Furthermore, when the heat treatment temperature is 600°C or lower, it is possible to prevent the components of the coating layer from excessively diffusing into the positive electrode active material, thereby maintaining the shape of the coating layer. The heat treatment temperature can be appropriately selected depending on the target amounts of niobium and titanium (amounts per unit surface area) in the coating layer, etc., so that the coating layer can maintain its sufficient thickness.

[0115] When the heat treatment time is 1 hour or longer, it is possible to particularly prevent impurities contained in the coating agent from remaining in the positive electrode active material. Furthermore, even if the heat treatment is performed for a time longer than 5 hours, no significant changes are observed in the resulting positive electrode active material. Therefore, from the viewpoint of energy efficiency, it is preferable that the heat treatment time be 5 hours or shorter.

[0116] It should be noted that heat treatment does not have to be performed. This is because, even without heat treatment, a uniform and strong coating layer can be formed on the surface of the lithium nickel composite oxide particles by using, for example, a liquid coating agent. Furthermore, when heat treatment is not performed, it is preferable to perform drying as necessary to reduce and remove the solvent, moisture, etc. of the coating agent.

[0117] When a portion of the lithium, niobium, and titanium in the coating solution is dissolved in the particles of the lithium-nickel composite oxide, the degree of dissolution can be adjusted by controlling the above-mentioned heat treatment conditions so that the positive electrode active material can also fully exhibit the effect of improving the cycle characteristics.

[0118] If slight sintering is observed in the positive electrode active material obtained after the coating step S4, a crushing treatment may be further carried out.

[0119] 3. All-solid-state lithium-ion secondary battery A secondary battery according to one embodiment of the present invention may have a configuration including a positive electrode using the above-described positive electrode active material, a negative electrode, and a solid electrolyte. Note that the embodiments described below are merely illustrative, and the secondary battery may be implemented in various forms, including the following embodiments, with various modifications and improvements based on the knowledge of those skilled in the art. Furthermore, the use of the secondary battery is not particularly limited.

[0120] 3-1. Positive electrode The positive electrode contains the above-mentioned positive electrode active material. The positive electrode can be formed, for example, by molding the positive electrode mixture. The positive electrode may be appropriately processed according to the battery to be used. For example, a pressure compression treatment using a press or the like can be performed to increase the electrode density.

[0121] The positive electrode mixture can be formed by mixing, for example, the powdered positive electrode active material described above with a solid electrolyte.

[0122] The solid electrolyte is added to the electrode to provide it with suitable ionic conductivity. The solid electrolyte is not particularly limited, but examples thereof include Li3PS4 and Li7P3S. 11 , Li 10 GeP2S 12 Sulfide-based solid electrolytes such as Li7La3Zr2O 12 , Li 0.34 La 0.51 TiO 2.94 and polymer electrolytes such as PEO.

[0123] The positive electrode mixture may contain a binder and a conductive additive.

[0124] The binder serves to bind the positive electrode active material together. The binder used in the positive electrode mixture is not particularly limited, but may be one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc.

[0125] The conductive additive is added to the electrode to provide it with appropriate conductivity. The material of the conductive additive is not particularly limited, but examples thereof include graphite such as natural graphite, artificial graphite, and expanded graphite, and carbon black materials such as acetylene black and Ketjenblack (registered trademark).

[0126] The mixing ratio of each material in the positive electrode mixture is not particularly limited. For example, the content of the positive electrode active material in the positive electrode mixture may be 50 parts by mass or more and 90 parts by mass or less, and the content of the solid electrolyte may be 10 parts by mass or more and 50 parts by mass or less. The method for producing the positive electrode is not limited to the above-mentioned example, and other methods may also be used.

[0127] 3-2. Negative electrode The negative electrode may be formed, for example, by molding a negative electrode mixture. Alternatively, the negative electrode may be a sheet-like member made of a material containing a metal that alloys with lithium, such as metallic lithium or indium.

[0128] The negative electrode mixture can be prepared by mixing a negative electrode active material with a solid electrolyte. The negative electrode active material can be, for example, an occlusion material capable of occluding and desorbing lithium ions.

[0129] The storage material is not particularly limited, and can be one or more selected from, for example, natural graphite, artificial graphite, baked organic compounds such as phenolic resin, and powdered carbon materials such as coke. When such a storage material is used as the negative electrode active material, a sulfide electrolyte such as Li3PS4 can be used as the solid electrolyte, as in the positive electrode.

[0130] The negative electrode obtained by molding the negative electrode mixture is formed by substantially the same method as the positive electrode described above, although the components constituting the negative electrode mixture and the blending thereof are different, and may be subjected to various treatments as necessary, similar to the positive electrode.

[0131] 3-3.Solid electrolyte The solid electrolyte is Li + A solid electrolyte is an ion-conducting material. The solid electrolyte can be selected from sulfides, oxides, polymers, etc., and can be used alone or in combination of two or more.

[0132] The sulfide-based solid electrolyte is not particularly limited, and any sulfur (S)-containing solid electrolyte having lithium ion conductivity and electronic insulation can be used. Examples of sulfide-based solid electrolytes include Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2S-B2S3, Li3PO4-Li2S-Si2S, Li3PO4-Li2S-SiS2, LiPO4-Li2S-SiS, LiI-Li2S-P2O5, and LiI-Li3PO4-P2S5.

[0133] The oxide-based solid electrolyte is not particularly limited, and any oxide-based solid electrolyte containing oxygen (O) and having lithium ion conductivity and electronic insulation can be used. Examples of oxide-based solid electrolytes include lithium phosphate (Li3PO4), Li3PO4NX, LiBO2NX, LiNbO3, LiTaO3, Li2SiO3, Li4SiO4-Li3PO4, Li4SiO4-Li3VO4, Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3-ZnO, Li 1+X Al X Ti 2-X (PO4)3(0≦X≦1), Li 1+X Al X Ge 2-X (PO4)3(0≦X≦1), LiTi2(PO4)3, Li3XLa 2 / 3-X TiO3(0≦X≦2 / 3), Li5La3Ta2O 12 , Li7La3Zr2O 12 , Li6BaLa2Ta2O 12 , Li 3.6 Si 0.6 P 0.4 Examples include O4.

[0134] As the inorganic solid electrolyte, materials other than those mentioned above may be used, for example, Li3N, LiI, Li3N-LiI-LiOH, etc.

[0135] The polymer solid electrolyte is not particularly limited as long as it is a polymer compound that exhibits ion conductivity, and examples thereof include polyethylene oxide, polypropylene oxide, and copolymers thereof. The organic solid electrolyte may also contain a supporting salt (lithium salt). When using a solid electrolyte, the solid electrolyte may also be mixed into the positive electrode material to ensure contact between the electrolyte and the positive electrode active material.

[0136] 3-4. Shape and structure of secondary batteries The shape of the secondary battery of this embodiment is not particularly limited, and various shapes such as a coin type or a laminate type can be used. Regardless of the shape of the secondary battery, the positive electrode and the negative electrode can be laminated with a solid electrolyte interposed therebetween. Then, the positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the negative electrode terminal connected to the outside can be connected using a current collecting lead or the like, and the secondary battery can be sealed in a battery case to form a secondary battery.

[0137] 3-5.Characteristics of secondary batteries A secondary battery according to one embodiment of the present invention using the above-described positive electrode active material can exhibit high capacity. For example, a test battery shown in FIG. 2 was fabricated using the positive electrode active material according to this embodiment in the positive electrode, and a current density of 0.2 mA / cm was obtained in a 25° C. environment. 2 When the battery is charged to a cutoff voltage of 4.3 V (vs. Li), allowed to rest for one hour, and then discharged to a cutoff voltage of 2.5 V (vs. Li), the discharge capacity (initial discharge capacity) is preferably 140 mAh / g or more, and more preferably 145 mAh / g or more.

[0138] The cycle characteristics are evaluated by the ratio of the discharge capacity at the 20th cycle to the discharge capacity at the 1st cycle (cycle capacity retention rate) after 20 cycles of charge and discharge of the battery under the same conditions as those used to measure the initial discharge capacity in a 60°C environment. The cycle capacity retention rate is preferably 60% or more, and more preferably 70% or more. [Example]

[0139] Next, the positive electrode active material for a non-aqueous electrolyte secondary battery, the method for producing the positive electrode active material for a non-aqueous electrolyte secondary battery, and the non-aqueous electrolyte secondary battery according to one embodiment of the present invention will be described in detail with reference to examples. Note that the present invention is not limited to these examples.

[0140] The methods for analyzing the lithium nickel composite oxide contained in the positive electrode active material in the examples and comparative examples, and the methods for evaluating the positive electrode active material and secondary batteries are as follows.

[0141] [Evaluation of lithium nickel composite oxide particles] (a) Chemical composition The composition of the lithium nickel composite oxide was confirmed by quantitative analysis using an ICP optical emission spectrometer (Varian, 725ES).

[0142] (b) Crystal structure The crystal structure of the lithium nickel composite oxide particles was measured using an XRD (X'Pert, PROMRD, manufactured by PANALYTICAL).

[0143] (c) Specific surface area The BET specific surface area of ​​this lithium nickel composite oxide was measured using a fully automatic BET specific surface area measuring device (Macsorb, manufactured by Mountec Co., Ltd.).

[0144] (d) Volume average particle size The volume average particle size of this lithium nickel composite oxide was measured using a laser diffraction scattering particle size distribution measuring device (Microtrac HRA, manufactured by Nikkiso Co., Ltd.).

[0145] [Evaluation of positive electrode active material] The positive electrode active material thus obtained was evaluated as follows.

[0146] (a) Composition The composition of the positive electrode active material was analyzed using an ICP optical emission spectrometer (725ES, manufactured by VARIAN).

[0147] (b) Surface analysis The surface of the positive electrode active material was measured using XPS (ULVAC-PHI, Versa Probe II), and the Ni2p 3 / 2 Spectrum, Co2p 3 / 2 Spectrum, Nb3d 5 / 2 Spectrum, Ti2p 3 / 2 The mass ratio of (Nb+Ti) / (Ni+Co+Al+Nb+Ti) was determined from the semi-quantitative value calculated from the peak area of ​​the spectrum.

[0148] (c) Carbon content The carbon content of the positive electrode active material was measured by a high-frequency combustion infrared absorption method using a carbon analyzer (manufactured by LECO, model: CS-600).

[0149] (d) Moisture content The moisture content of the positive electrode active material was measured using a Karl Fischer moisture meter (manufactured by Kyoto Electronics Manufacturing Co., Ltd., Model: MKC210) at a vaporization temperature of 300°C.

[0150] [Evaluation of battery characteristics] (1) Fabrication of secondary batteries To evaluate the battery characteristics of the obtained positive electrode active material, a battery (hereinafter referred to as "test battery SBA") having a structure as shown in Fig. 2 was used. The test battery SBA included a case having a negative electrode can NC and a positive electrode can PC, and a compacted cell C housed in the case.

[0151] The case has a hollow negative can NC with one open end and a positive can PC placed in the opening of the negative can NC. A space for accommodating the compacted cell C is formed between the positive can PC and the negative can NC. The positive can PC is fixed to the negative can NC with, for example, a thumbscrew SW and a nut N.

[0152] The negative can NC is equipped with a negative terminal, and the positive can PC is equipped with a positive terminal. The case also has an insulating sleeve ISV, which secures the negative can NC and the positive can PC together so that they do not come into contact with each other.

[0153] A pressure screw PSW is provided at one closed end of the negative can NC, and after fixing the positive can PC to the negative can NC, the pressure screw PSW is tightened toward the storage space for the compacted cell C, thereby maintaining the compacted cell C in a pressurized state through a hemispherical washer W. In addition, a screw-in plug P is provided at the end of the negative can NC where the pressure screw PSW is located. O-rings OL are provided between the negative can NC and positive can PC, and between the negative can NC and the plug P, which seal the gaps between the negative can NC and positive can PC and maintain an airtight seal inside the case.

[0154] The compact cell C is a pellet in which the positive electrode layer PL, solid electrolyte layer SEL, and negative electrode layer NL are stacked in this order. The positive electrode layer PL contacts the inner surface of the positive electrode can PC through the lower current collector LCC. The negative electrode layer NL contacts the inner surface of the negative electrode can NC through the upper current collector UCC, washer W, and pressure screw PSW. The lower current collector LCC, compact cell C, and upper current collector UCC are protected by a sleeve SV to prevent electrical contact between the positive electrode layer PL and negative electrode layer NL.

[0155] (2) Preparation of evaluation battery A test battery SBA was prepared as follows.

[0156] First, 60 mg of the synthesized solid electrolyte (0.75Li2S-0.25P2S5) was pressurized at 25 MPa in a pelletizer to obtain a solid electrolyte pellet. Next, 100 mg of mesocarbon microbeads (MCMB), which are the negative electrode active material, and 100 mg of the solid electrolyte were mixed in a mortar to obtain a negative electrode mixture. Next, the obtained solid electrolyte pellet and 22 mg of the negative electrode mixture were placed in a pelletizer and pressurized at 25 MPa to form a negative electrode layer NL on the solid electrolyte pellet.

[0157] Next, 120 mg of the positive electrode active material and 80 mg of the solid electrolyte were mixed in a mortar to obtain a positive electrode mixture. The negative electrode layer NL on the solid electrolyte pellet and 15 mg of the positive electrode mixture were placed in a pellet former and pressurized at 360 MPa to form a positive electrode layer PL on the surface of the solid electrolyte pellet (solid electrolyte layer SEL) opposite to the negative electrode layer NL, thereby obtaining an electrode (compressed powder cell C).

[0158] The electrodes were sealed in a case, and the pressure screws were tightened with a torque of 6–7 N·m. The test battery was fabricated in a glove box with an Ar atmosphere and a controlled dew point of −80°C.

[0159] (3) Evaluation method The performance of the prepared test battery SBA was evaluated as follows.

[0160] (a) Initial discharge capacity and cycle capacity retention rate The initial discharge capacity was measured by placing the test battery in a thermostatic chamber at 25°C, and after the open circuit voltage (OCV) had stabilized, applying a current density of 0.2 mA / cm to the positive electrode. 2 The battery was charged to a cutoff voltage of 4.3 V (vs. Li+ / Li), and after a one-hour rest, discharged to a cutoff voltage of 2.5 V (vs. Li+ / Li), and the discharge capacity (initial discharge capacity) was measured. The temperature of the thermostatic chamber was then raised to 60°C, and charge / discharge was repeated under the same conditions up to 20 cycles. The discharge capacity after 20 cycles was measured, and the cycle capacity retention rate (%) relative to the initial discharge capacity was evaluated.

[0161] Example 1 1. Preparation of Lithium Nickel Composite Oxide Particles The positive electrode active material was manufactured by carrying out the following steps.

[0162] (a) Precursor crystallization step First, 10 L of ion-exchanged water was placed in a reaction tank (60 L) and the temperature inside the tank was set to 50°C while stirring. At this time, the reaction tank was filled with a nitrogen atmosphere with an oxygen concentration of 1% by volume or less. Appropriate amounts of 25% by mass aqueous sodium hydroxide solution and 25% by mass aqueous ammonia were added to the water in the reaction tank to prepare an initial aqueous solution with a pH value of 12.8 at a liquid temperature of 25°C and an ammonia concentration of 15 g / L.

[0163] Nickel sulfate and cobalt sulfate were simultaneously dissolved in pure water so that the ratio of nickel to cobalt was Ni:Co = 0.842:0.158, to prepare 25 L of a 2.0 mol / L mixed nickel-cobalt aqueous solution. Also, 7 L of a 0.37 mol / L aluminum sulfate aqueous solution was prepared.

[0164] Then, 200 mL of the nickel-cobalt mixed aqueous solution was added dropwise to the initial aqueous solution in the reaction vessel at a constant rate to form a reaction aqueous solution. At this time, 25% by mass ammonia water and 25% by mass sodium hydroxide aqueous solution were also added dropwise to the initial aqueous solution at a constant rate, and the pH value of the reaction aqueous solution was controlled to be maintained at 12.8 at a liquid temperature of 25°C.

[0165] Thereafter, sulfuric acid was added dropwise to the reaction tank to adjust the pH of the reaction solution to 11.5. This operation was intended to lower the pH value of the reaction solution, thereby slowing down the rate at which the nickel-cobalt-aluminum composite hydroxide precipitates from the liquid phase to the solid phase in the subsequent precursor crystallization step, and to improve the uniformity of the particle size distribution and the sphericity of the resulting particles.

[0166] After adjusting the pH, 24.8 L of a nickel-cobalt mixed aqueous solution was added dropwise to the reaction aqueous solution in the reaction tank at 103 mL / min, and simultaneously 7 L of an aluminum sulfate aqueous solution was added dropwise at 29.3 mL / min. At this time, 25 mass% ammonia water and 25 mass% sodium hydroxide aqueous solution were also added dropwise to the initial aqueous solution at a constant rate, and the pH value of the reaction aqueous solution was controlled to be 11.5 at a liquid temperature of 25°C, and the ammonia concentration was maintained at 15 g / L.

[0167] After the entire amount of the nickel-cobalt mixed aqueous solution and the aluminum sulfate aqueous solution was added dropwise, the pH of the aqueous solution in the reaction tank was increased until it reached 13.0 at a liquid temperature of 25°C. This operation was intended to precipitate the nickel ions, which had been complexed with ammonia and dissolved in the liquid phase, onto the hydroxide, thereby obtaining the target chemical composition.

[0168] The reaction solution was then subjected to solid-liquid separation using a Buchner funnel, a filter, and a vacuum pump vacuum filter. Furthermore, the resulting solid phase was washed by repeating the procedure of dispersing it in 20 L of pure water at 40°C and performing solid-liquid separation twice. By washing, at least a portion of the water-soluble impurities contained in the nickel composite hydroxide, such as sodium sulfate, was removed.

[0169] After washing, the cake-like solid phase obtained after solid-liquid separation was dried in an air atmosphere in a stationary dryer at 120°C for 24 hours, and then passed through a sieve with 100 μm openings to obtain a powdery nickel composite hydroxide.

[0170] (b) Oxidation roasting process The obtained nickel composite hydroxide was calcined in an atmospheric calcination furnace (BM-50100M, manufactured by Siliconit Co., Ltd.) in an air atmosphere with an oxygen concentration of 20% by volume at 600°C for 2 hours, and then cooled to room temperature to obtain nickel composite oxide particles.

[0171] (c) Lithium nickel composite oxide synthesis process Lithium hydroxide monohydrate was weighed out so that the ratio of the substance amount of lithium to the total substance amount of nickel, cobalt, and aluminum contained in the nickel composite oxide was 1.03, and the mixture was mixed using a Turbula shaker mixer (T2F, manufactured by Dalton Co., Ltd.) to obtain a lithium mixture (lithium mixture preparation step).

[0172] The lithium mixture was placed in an alumina sagger and fired in an atmosphere firing furnace (BM-50100M, manufactured by Siliconit Co., Ltd.) in an oxygen-nitrogen mixed gas atmosphere with an oxygen concentration of 90% by volume at 750°C for 10 hours, and then cooled to room temperature (firing step). As a result, lithium nickel composite oxide particles were obtained.

[0173] (Evaluation of lithium nickel composite oxide) It was confirmed that the mass ratio of the metal elements in the obtained lithium nickel composite oxide particles was Li:Ni:Co:Al=1.02:0.80:0.15:0.05.

[0174] It was also confirmed that the crystalline structure of the lithium nickel composite oxide particles was a layered rock salt type crystalline structure, with peaks attributed to the R-3m structure detected in the diffraction pattern.

[0175] In addition, the BET specific surface area of ​​the lithium nickel composite oxide particles is 0.60 m2 / g, and it was confirmed that the volume average particle size of the lithium nickel composite oxide particles was 6.1 μm.

[0176] 2. Coating process The resulting lithium nickel composite oxide particles were subjected to the following coating step.

[0177] First, the total surface area of ​​the lithium nickel composite oxide was calculated from the mass and specific surface area of ​​the lithium nickel composite oxide to be coated. Then, in the reaction vessel, a volume of 1 m2 was added to the total surface area of ​​the calculated lithium nickel composite oxide. 2 Niobium oxide trihydrate was weighed out so that the amount of niobium per gram was 218 μmol, and was added to 20 mL of 30 mass% aqueous hydrogen peroxide per gram of niobium oxide trihydrate. Then, while cooling the reaction vessel in a water bath so that the temperature was 10°C or less, 23 mL of 25 mass% aqueous ammonia per gram of niobium oxide trihydrate was added dropwise into the reaction vessel while stirring the contents of the reaction vessel, to obtain a solution in which niobium oxide trihydrate was dissolved (niobium solution).

[0178] The obtained niobium solution was added with 1 m of lithium nickel composite oxide in an amount of 1 m relative to the total surface area of ​​the lithium nickel composite oxide. 2 A titanium sponge weighed out so that the amount of titanium per unit mass was 54 μmol was added to obtain a solution in which titanium was dissolved (niobium-titanium solution). Furthermore, a 1.6 mol / L lithium hydroxide aqueous solution was added to the niobium-titanium solution in an amount such that the mass ratio Li / (Nb+Ti) = 1.2 to prepare a coating solution (lithium-niobium-titanium solution) (coating agent preparation). The coating solution was prepared by setting the target composition of the coating layer to Li 1.2 Ti 0.2 Nb 0.8 It was created as O3.

[0179] (Covering) Using the above coating solution, 500 g of lithium nickel composite oxide was coated using a tumbling fluidized coating device (MP-01, manufactured by Powrex Corporation).

[0180] 500g of lithium nickel composite oxide was heated to 120℃ and the flow rate was 0.3m 3 Air was flowed through the chamber at a rate of 1.0 ml / min, and the coating liquid was sprayed onto the lithium nickel composite oxide at a rate of 1.0 ml / min.

[0181] After the entire amount of the coating liquid was sprayed, the lithium nickel composite oxide was recovered from the chamber and heat-treated at 300°C for 10 hours in an oxygen stream using an atmospheric firing furnace (BM-50100M, manufactured by Siliconit Co., Ltd.). Thereafter, the mixture was cooled to room temperature to obtain a positive electrode active material having a coating layer on the surfaces of the lithium nickel composite oxide particles.

[0182] Analysis using an ICP optical emission spectrometer revealed that this positive electrode active material contained 1.2 mass% Nb and 0.29 mass% Ti, and the amount of niobium in the coating layer, calculated from the specific surface area of ​​the lithium nickel composite oxide particles before coating, was 215 μmol / m 2 , titanium content is 53 μmol / m 2 It was.

[0183] Example 2 When carrying out the coating process, the coating agent was prepared to set the target niobium content of the coating layer to 135 μmol / m 2 , titanium content 135 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0184] Example 3 When carrying out the coating process, the coating agent was prepared to achieve a target niobium content of 53 μmol / m 2 , titanium content 213 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0185] Example 4 When carrying out the coating process, the coating agent was prepared to set the target niobium content of the coating layer to 27 μmol / m2 , titanium content 27 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0186] Example 5 When carrying out the coating process, the coating agent was prepared to set the target niobium content of the coating layer to 202 μmol / m 2 , titanium content 202 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0187] Example 6 When carrying out the coating process, the coating agent was prepared to set the target niobium content of the coating layer to 13 μmol / m 2 , titanium content 13 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0188] Example 7 When carrying out the coating process, the coating agent was prepared to set the target niobium content of the coating layer at 269 μmol / m 2 , titanium content 269 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0189] (Comparative Example 1) When carrying out the coating process, the amount of niobium in the coating layer is set to 0 μmol / m 2 , titanium content 265 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0190] (Comparative Example 2) When carrying out the coating process, the coating agent was prepared to set the target niobium content of the coating layer at 274 μmol / m 2 , titanium content 0 μmol / m 2 Except for this, a positive electrode active material was obtained under the same conditions as in Example 1. Table 1 shows the evaluation results of the obtained lithium nickel composite oxide particles, the positive electrode active material, and the battery characteristics.

[0191] [Table 1]

[0192] (Evaluation results) As shown in Table 1, the positive electrode active materials of Examples 1 to 5 maintain high cycle characteristics and have very high discharge capacities compared to the positive electrode active materials of Comparative Examples 1 and 2, which have coating layers containing lithium titanate or lithium niobate alone.

[0193] The positive electrode active material of Example 6 has a low coating amount, and therefore has a low cycle characteristic compared to the other Examples, but has a high discharge capacity. The positive electrode active material of Example 7 maintains its cycle characteristic and tends to show a small decrease in discharge capacity despite the high coating amount.

[0194] The above-described embodiments do not unduly limit the content of the present invention as defined in the claims, and modifications are possible without departing from the spirit of the present invention. Furthermore, not all of the configurations described in the above embodiments are necessarily essential as the means for solving the problems of the present invention. [Explanation of symbols]

[0195] SBA...Test battery PC: Positive electrode can NC: Negative electrode can ISV...insulating sleeve C: Compacted powder cell PL…Positive electrode layer NL…Negative electrode layer SEL…Solid electrolyte layer LCC: Lower current collector UCC: Upper current collector P...plug PSW...Pressure screw W...Washer OL...O-ring SV...Sleeve SW...Screw N...Nut

Claims

1. A positive electrode active material for a lithium ion secondary battery, comprising: particles of a lithium nickel composite oxide; and a coating layer that coats at least a portion of the surface of the particles of the lithium nickel composite oxide, The lithium nickel composite oxide particles contain Li, Ni, Co, oxygen, and optionally, an element M which is at least one element selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Cu, Cr, Mn, Zn, Zr, Nb, Mo, and W, The substance amount ratio of the elements other than oxygen contained in the particles of the lithium nickel composite oxide is Li:Ni:Co:M=t:(1-x-y):x:y (0.95≦t≦1.20, 0<x≦0.22, 0≦y≦0.15), The coating layer contains a compound containing Li, Nb, and Ti. Positive electrode active material for lithium-ion secondary batteries.

2. The substance amount ratio of the elements contained in the coating layer is 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material is represented by Li:Nb:Ti=(a+1):(1-a):a (0.2≦a≦0.8).

3. The total amount of Nb and Ti contained in the coating layer is less than 1 m of the surface area of ​​the lithium nickel composite oxide particles. ２ 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material has a molecular weight of 27 μmol or more and 404 μmol or less per mole.

4. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein a ratio ((Nb+Ti) / (Ni+Co+M+Nb+Ti)) of a total amount of Nb and Ti present on the surface of the positive electrode active material to a total amount of Ni, Co, M, Nb, and Ti present on the surface of the positive electrode active material is 83% or more and 100% or less.

5. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the carbon content is 0.5% by mass or less, based on the total amount of the positive electrode active material, and the water content is 0.2% by mass or less, based on the total amount of the positive electrode active material.

6. The specific surface area of ​​the lithium nickel composite oxide particles is 0.1 m ２ / g or more 1.5m ２ The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the SiO2 content is 0.15 / g or less.

7. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the lithium nickel composite oxide particles are a layered rock salt type compound having a crystal structure belonging to the space group R-3m.

8. An all-solid-state lithium ion secondary battery comprising at least a positive electrode, a negative electrode, and a solid electrolyte, wherein the positive electrode contains the positive electrode active material according to any one of claims 1 to 7.

Images