Positive electrode active material for lithium ion secondary battery, and lithium ion secondary battery

Abstract

The present invention provides a positive electrode active material for a lithium ion secondary battery, the positive electrode active material having resistance against high voltage, being sufficient for practical use in a lithium ion secondary battery, and having a high battery capacity.　The positive electrode active material for a lithium ion secondary battery has particles of a lithium-nickel-manganese composite oxide and a coating layer for coating at least a part of the surfaces of the particles of the lithium-nickel-manganese composite oxide. As elements other than oxygen, the lithium-nickel-manganese composite oxide contains Li, Ni, Mn, and optionally an element M. The ratio of the amounts of substance of these elements is expressed as Li:Ni:Mn:M = t:1-x-y:x:y (where 0.47 ≤ t ≤ 0.6, 0.7 ≤ x ≤ 0.8, and 0 ≤ y ≤ 0.15). The coating layer contains a compound containing Li, P, and an element X (where X is at least one element selected from the group consisting of elements forming a hexavalent cation).

Description

Positive electrode active material for lithium ion secondary battery, and lithium ion secondary battery

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

[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. Lithium-ion secondary batteries are one such type of secondary battery. Among these, lithium-ion secondary batteries using layered or spinel-type lithium metal composite oxides as the positive electrode active material are gaining in practical use as batteries with high energy density due to their high voltage capability.

[0003] Currently, typical lithium ion secondary batteries use, for example, LiCoO as the positive electrode active material. ２ , LiNiO ２ , LiMn ２ O ４ The negative electrode active material is lithium metal, a lithium alloy, a metal oxide, or carbon.

[0004] The electrolyte solution is an organic solvent such as ethylene carbonate, dimethyl carbonate, or ethyl methyl carbonate, and LiClO ４ , LiPF ６ An electrolyte solution in which a Li salt such as the above is dissolved as a supporting salt is used.

[0005] 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.

[0006] In the course of 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, in all-solid-state batteries, 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, generating a high-resistance phase that impairs the operation of the battery. In order to suppress the generation of this high-resistance phase, it is necessary to form a LiNbO ３ For example, Patent Document 2 and the like propose providing a coating layer made of

[0007] JP 2014-056661 A JP 2010-170715 A International Publication No. 2017 / 094416 JP 2020-068181 A

[0008] N. Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries”, Electrochemistry Communications 9 (2007) 1486-1490

[0009] The energy density of a lithium ion secondary battery increases as the charge / discharge voltage increases. The inventors have conducted extensive research into lithium ion secondary batteries that operate at high voltages of 4.5 V or higher, and have found that LiNbO ３ It has been found that a positive electrode active material having a coating layer made of the above is not stable under high voltage.

[0010] On the other hand, for example, Patent Document 3 describes a positive electrode active material for a lithium secondary battery, which includes a coated positive electrode active material in which the surface of the positive electrode active material is directly coated with lithium metaphosphate. This positive electrode active material is said to be able to suppress oxidation of the solvent at the positive electrode and suppress gas generation even when the operating voltage of the lithium secondary battery is set to a high potential.

[0011] In addition, Patent Document 4 discloses a positive electrode active material and (100-x)LiPO ３ xLiPO ３ ・xLiVO ３The document describes a positive electrode characterized by comprising a positive electrode active material layer containing LiPO (0<x≦60) and a current collector. ３ -LiVO ３ It is said that by including such a glass in the positive electrode active material layer, the layer has excellent moldability and excellent interface formation with the oxide-type solid electrolyte.

[0012] Therefore, the present inventors have developed a method for producing lithium metaphosphate (LiPO ) as described in, for example, Patent Documents 3 and 4. ３ ) was used to coat the positive electrode active material, but ３ It was found that the positive electrode active material coated with LiPO has high resistance and is not suitable for practical use. ３ This is thought to be due to the low lithium ion conductivity of the material itself.

[0013] On the other hand, as the positive electrode active material, lithium manganese composite oxide particles having a spinel-type crystal structure, in particular, lithium nickel manganese composite oxide (LiNi) in which part of the manganese is substituted with Ni, are used. ０．５ Mn １．５ O ４ ) particles, Li ＋ Lithium nickel manganese composite oxides are expected to be high-energy density materials capable of achieving operating voltages of 4.5 V or higher relative to the potential of Li / Li. However, even in such lithium nickel manganese composite oxides, further improvement in their high-voltage resistance is required.

[0014] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that has high voltage resistance and a high battery capacity sufficient for practical use in a lithium ion secondary battery.

[0015] In a first aspect of the present invention, there is provided a positive electrode active material for a lithium ion secondary battery, comprising particles of lithium nickel manganese composite oxide and a coating layer that coats at least a portion of the surface of the particles of the lithium nickel manganese composite oxide, wherein the lithium nickel manganese composite oxide contains, as elements other than oxygen, Li, Ni, Mn, and 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, Co, Cu, Cr, Zn, Zr, Nb, Mo, and W), and the amount of substance ratio of these elements is Li:Ni:Mn:M=t:1-x-y :x:y (0.47≦t≦0.6, 0.7≦x≦0.8, 0≦y≦0.15), and the coating layer contains a compound containing Li, P, and an element X (element X is at least one element selected from the group consisting of elements that form hexavalent cations).

[0016] In addition, the composition ratio of elements other than oxygen in the above compound is preferably expressed as Li:P:X=1-a:1-a:a (0.01≦a≦0.1). ２ The total amount of P and X derived from the coating layer per 1000 .mu.m is preferably 50 μmol or more and 410 μmol or less. Furthermore, X in the coating layer is preferably Mo, W, or both. Furthermore, it is preferable that the carbon content of the positive electrode active material for a lithium ion secondary battery is 0.5% by mass or less and the water content is 0.2% by mass or less. Furthermore, it is preferable that the specific surface area of ​​the lithium nickel manganese composite oxide is 0.1 m. ２ / g or more 2.0m ２ The lithium nickel manganese composite oxide preferably has a spinel crystal structure belonging to the space group Fd-3m.

[0017] In a second aspect of the present invention, there is provided a lithium ion secondary battery comprising a positive electrode containing the above-mentioned positive electrode active material for a lithium ion secondary battery, a negative electrode, and an electrolyte.

[0018] According to the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery that has high voltage resistance and high battery capacity.

[0019] Fig. 1 is a diagram showing an example of a method for producing a positive electrode active material according to this embodiment. Fig. 2 is a diagram showing an example of a method for producing a lithium nickel manganese composite oxide according to this embodiment. Fig. 3 is an explanatory diagram of a cross-sectional structure of an evaluation battery used for battery evaluation.

[0020] As a result of intensive research to solve the above problems, the present inventors have discovered that by coating particles of lithium nickel manganese composite oxide and at least a portion of the surface of the lithium nickel manganese composite oxide particles with a compound containing lithium, phosphorus, and a hexavalent cation element compound, it is possible to achieve both high voltage resistance and high capacity at a high level, and have completed the present invention. An example of an embodiment of the present invention will now be described.

[0021] It should be noted that the present embodiment described below does not unduly limit the content of the present invention described in the claims, and modifications are possible within the scope of the present invention. Furthermore, not all of the configurations described in the present embodiment are necessarily essential as the solution of the present invention.

[0022] 1. Positive Electrode Active Material for Lithium-Ion Secondary Batteries A positive electrode active material for lithium-ion secondary batteries according to one embodiment of the present invention (hereinafter referred to as "positive electrode active material") has particles of lithium nickel manganese composite oxide and a coating layer that coats at least a portion of the surface of the lithium nickel manganese composite oxide particles. Hereinafter, the positive electrode active material for lithium-ion secondary batteries according to this embodiment (hereinafter also simply referred to as "positive electrode active material") will be specifically described.

[0023] <1-1. Lithium nickel manganese composite oxide particles> The lithium nickel manganese composite oxide particles are a composite oxide containing at least Li, Ni, Mn, and optionally an element M (M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Co, Cu, Cr, Zn, Zr, Nb, Mo, and W).

[0024] (Composition) The lithium nickel manganese composite oxide particles can contain the above elements other than oxygen, i.e., lithium (Li), nickel (Ni), manganese (Mn), and optionally element M (M), in a substance amount ratio (molar ratio) of Li:Ni:Mn:M=t:1-x-y:x:y (0.47≦t≦0.6, 0.7≦x≦0.8, 0≦y≦0.15).

[0025] In the above substance amount ratios, the value of t representing the substance amount ratio of lithium (Li) can be set to 0.47 or more and 0.60 or less.

[0026] By setting the value of t to 0.47 or more, the internal resistance of a secondary battery using a positive electrode active material containing the lithium nickel manganese composite oxide can be suppressed, and the output characteristics can be improved. Furthermore, by setting the value of t to 0.60 or less, the initial discharge capacity of a secondary battery using a positive electrode active material containing the lithium nickel manganese composite oxide can be increased. In other words, 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 the lithium nickel manganese composite oxide can be improved.

[0027] Nickel (Ni) and manganese (Mn) in the lithium nickel manganese composite oxide are elements that, depending on their content ratio, contribute to the capacity development of a secondary battery using a positive electrode active material containing the lithium nickel manganese composite oxide. Among the substance ratios, the value of (1-x-y), which indicates the substance ratio of nickel (Ni), is 0.05 or more and 0.3 or less, and this value varies depending on the proportions of manganese (Mn) and element M, which will be described later. The value of (1-x-y) may also be 0.2 or more.

[0028] In the above substance ratio, x, which indicates the manganese content, is 0.7 or more and 0.8 or less. By setting the value of x to 0.7 or more, it is possible to suppress the generation of a heterogeneous phase of nickel oxide. On the other hand, by setting x to 0.8 or less, it is possible to relatively increase the nickel content, and ＋ / Li) or more can be obtained.

[0029] Furthermore, the lithium nickel manganese composite oxide may contain, in addition to the above metal elements, an additive element, element M. As the element M, at least one element selected from magnesium (Mg), aluminum (Al), calcium (Ca), silicon (Si), scandium (Sc), titanium (Ti), vanadium (V), iron (Fe), cobalt (Co), copper (Cu), chromium (Cr), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), and tungsten (W) can be used. The element M is appropriately selected depending on the application and required performance of the secondary battery constructed using the positive electrode active material.

[0030] Since some element M itself does not contribute to the redox reaction, the value of y, which indicates the content of element M in the substance ratio of each element in the lithium nickel manganese composite oxide, can be 0.15 or less, preferably 0.10 or less, and more preferably 0.05 or less. The value of y may be 0.01 or less, 0.005 or less, or 0.001 or less. Since the lithium nickel manganese composite oxide does not need to contain element M, the lower limit of y, which indicates the content of element M, can be 0.

[0031] The lithium nickel manganese composite oxide may contain elements other than the above-described Li, Ni, Mn, element M, and oxygen, provided that the effects of the present invention are not impaired. Alternatively, the lithium nickel manganese composite oxide may be composed of the above-described Li, Ni, Mn, element M, and oxygen.

[0032] (Crystal Structure) When the lithium nickel manganese composite oxide particles are subjected to X-ray diffraction (XRD) measurement, it is preferable that a peak attributable to a spinel crystal structure of the "Fd-3m" structure is detected in the diffraction pattern obtained. In particular, it is more preferable that only peaks attributable to the spinel crystal structure of the "Fd-3m" structure are detected in the diffraction pattern. This is because the spinel oxide of the "Fd-3m" structure is preferable because it can particularly suppress internal resistance when used as a positive electrode active material for a secondary battery.

[0033] However, a lithium nickel manganese composite oxide having a spinel crystal structure cannot be obtained in a single phase, and impurity phases may be mixed in. Even when impurity phases are mixed in, it is preferable that the intensity of the peaks attributable to these impurity phases other than the spinel crystal structure of the "Fd-3m" structure does not exceed the intensity of the peaks attributable to the spinel crystal structure of the "Fd-3m" structure.

[0034] (Particle Structure) The lithium nickel manganese composite oxide particles may be single primary particles or secondary particles formed by aggregation of multiple primary particles. Furthermore, the lithium nickel manganese composite oxide particles are preferably a mixture of single primary particles and secondary particles. Here, the primary particles and secondary particles can be observed using an electron microscope such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Furthermore, each secondary particle may have a space or void surrounded by one or more primary particles.

[0035] (Specific surface area) The lithium nickel manganese composite oxide has a specific surface area of ​​0.1 m ２ / g or more 2.0m ２ / g or less, and ２ / g or more 1.0m ２ The release / absorption of lithium ions in the lithium nickel manganese composite oxide during charge / discharge of the secondary battery occurs through the interface between the particles of the lithium nickel manganese composite oxide and the electrolyte, i.e., through the surface of the lithium nickel manganese composite oxide. Therefore, it is preferable that the specific surface area of ​​the lithium nickel manganese composite oxide contained in the positive electrode active material is 0.1 m ２ / 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. Also, as described above, the electrolyte may be decomposed due to a side reaction occurring at the interface between the positive electrode active material and the electrolyte, and a coating layer serving as a buffer layer is required. ２ By setting the content to be not more than 1 / g, the amount of electrochemically inactive coating layer components can be kept small.

[0036] The specific surface area of ​​the lithium nickel manganese composite oxide can be measured, for example, by the BET method using nitrogen adsorption.

[0037] (Volume Average Particle Size) The volume average particle size of the lithium nickel manganese composite oxide according to one embodiment of the present invention is preferably 2 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less, and even more preferably 3 μm or more and 15 μm or less. This is because, when the volume average particle size of the lithium nickel manganese composite oxide particles is within the above range, a secondary battery using a positive electrode active material containing the lithium nickel manganese composite oxide particles in a positive electrode can sufficiently increase the battery capacity per capacity and 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. The volume average particle size of the lithium nickel manganese composite oxide may also be 10 μm or less, or may be 8 μm or less.

[0038] <1-2. Coating Layer> The coating layer contains a compound containing lithium, phosphorus, and element X (X is an element that forms a hexavalent cation), and coats at least a portion of the lithium nickel manganese composite oxide particles. By containing element X in addition to lithium and phosphorus, the coating layer can improve high-voltage resistance.

[0039] In the compound containing lithium, phosphorus, and a hexavalent cation element, the composition ratio (molar ratio) of phosphorus (P) to the hexavalent cation element (X) is preferably expressed as P:X = 1 - a:a (0.01 ≦ a ≦ 0.1). When a is in the above range, high voltage resistance is improved. Furthermore, the ratio of lithium (Li) to phosphorus (P) (Li:P, molar ratio) may be 1 to 3:1, or 1 to 2:1, and is preferably 1:1. Furthermore, the compound may be an oxide.

[0040] Furthermore, in the compound containing lithium, phosphorus, and a hexavalent cation element, the composition ratio of elements other than oxygen is preferably expressed as Li:P:X=1-a:1-a:a (0.01≦a≦0.1). When a in the composition ratio is within the above range, the battery has a high capacity and improves high voltage resistance. Furthermore, the coating layer may be composed of an oxide having a composition expressed as Li:P:X=1-a:1-a:a (0.01≦a≦0.1). When a is less than 0.01, LiPO ３ The Li holes in the skeleton may disappear, forming a highly resistant coating layer. On the other hand, if a exceeds 0.1, the high voltage resistance may be impaired and the charge / discharge capacity may decrease. The composition ratio of the above compounds contained in the coating layer can be adjusted to the above range, for example, by adjusting the composition ratio in the coating agent to the above range.

[0041] The element X contained in the compound is not particularly limited as long as it is an element that forms a hexavalent cation, and examples thereof include Mo, W, S, Cr, Te, and Se. Among these, Mo, W, or both are particularly preferred, and may be Mo or W.

[0042] The coating layer and the lithium nickel manganese 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 concentration of phosphorus and element X (e.g., Mo, W, etc.) is higher than that of the lithium nickel manganese composite oxide particles, i.e., the central region, which are the coated material. Furthermore, the elements constituting the coating layer may be partially dissolved in the lithium nickel manganese composite oxide.

[0043] The contents of phosphorus and element X in the positive electrode active material are not particularly limited, but it is preferable to adjust the contents depending on the specific surface area of ​​the lithium nickel manganese composite oxide to be coated.

[0044] For example, the coating layer is formed on the surface of the lithium nickel manganese composite oxide particles with a surface area of ​​1 m ２ It is preferable that the lithium nickel manganese composite oxide particles contain phosphorus and element X in a ratio of 50 μmol to 410 μmol per m.２ By making the total content of phosphorus and element X per particle 50 μmol or more, the coating layer can be more uniformly disposed over the entire surface of the lithium nickel manganese composite oxide particle.

[0045] Furthermore, if the positive electrode active material has a coating layer, the cycle characteristics can be improved and swelling of the battery can be suppressed, but at the same time, there is a risk of the internal resistance increasing. In the positive electrode active material according to this embodiment, the surface area of ​​the lithium nickel manganese composite oxide particles is 1 m ２ By setting the total content of phosphorus and element X per unit area to 410 μmol or less, the coating layer is prevented from interfering with the lithium desorption / sorption reaction of the lithium nickel manganese composite oxide, and the internal resistance can be reduced, which is preferable.

[0046] The method for evaluating and calculating the total content of phosphorus and element X in the coating layer is not particularly limited. For example, the content (μmol / g) of phosphorus and element M in 1 g of the obtained positive electrode active material is first measured by a method such as chemical analysis. For example, this can be measured by ICP (Inductively Coupled Plasma) or the like. In addition, the specific surface area (m ２ The content (μmol / g) of phosphorus and element X in 1 g of the obtained positive electrode active material is measured by the BET method using nitrogen adsorption or the like. The specific surface area (m ２ / g), the surface area of ​​the lithium nickel manganese composite oxide particles is calculated as 1 m ２ The content of phosphorus and element X per unit area (μmol / m ２ ) can be calculated.

[0047] When the lithium nickel manganese composite oxide before the coating treatment contains the same element as the element X contained in the coating layer (i.e., when the element M and the element X are the same element), it is preferable to use the difference between the contents of lithium and the element M (or element X) before and after the coating treatment as the amount of element X used in the coating.

[0048] The compound containing lithium, phosphorus, and element X in the coating layer may react with the lithium nickel manganese composite oxide, so that part of the phosphorus and element X used in the coating treatment are dissolved in the lithium nickel manganese composite oxide. For example, by performing a heat treatment after the coating treatment and adjusting the conditions at that time, the lithium, phosphorus, and element X in the coating layer can be dissolved in the lithium nickel manganese composite oxide.

[0049] The formation of a solid solution of some of the elements constituting the compound containing lithium, phosphorus, and element X in the coating layer in the lithium nickel manganese composite oxide prevents particles of the lithium nickel manganese composite oxide, which does not have a lithium phosphorus molybdenum compound dissolved therein, from coming into direct contact with the solid electrolyte, thereby not only reducing the opportunity for an interfacial reaction but also having the effect of suppressing the reaction itself between the lithium nickel manganese composite oxide and the solid electrolyte. However, it is preferable to adjust the degree of solid solution so that the positive electrode active material can also fully exhibit the effect of improving the cycle characteristics.

[0050] <1-3. Characteristics of Positive Electrode Active Material> The positive electrode active material according to this embodiment preferably has the following characteristics.

[0051] The positive electrode active material according to this embodiment may be composed only of the lithium nickel manganese composite oxide particles and the coating layer. However, small amounts of impurities may be mixed in during the production of the positive electrode active material. Among these impurities, moisture and carbon are particularly likely to increase due to the coating process used to form the coating layer. Because moisture and carbon may affect cycle characteristics, they are preferably controlled within a predetermined range.

[0052] (Carbon Content) The carbon content of the positive electrode active material according to this embodiment is, for example, 0.5% by mass or less, preferably 0.05% by mass or more and 0.5% by mass or less, and more preferably 0.2% by mass or less. By setting the carbon content within the above range, the generation of carbon dioxide due to, for example, decomposition of organic components contained in the coating layer during charging and discharging of a secondary battery using the positive electrode active material can be sufficiently suppressed, and battery expansion and the like can be suppressed. The lower limit of the carbon content is, for example, 0.05% by mass. The carbon content can be evaluated, for example, by infrared absorption method or the like.

[0053] (Water Content) The cathode active material according to one embodiment of the present invention preferably has a water content (water content) of 0.2% by mass or less, and may be 0.1% by mass or less. By setting the water content within the above range, deterioration of battery characteristics can be further suppressed in a secondary battery using the cathode active material. The lower limit of the water content is not limited, and may be, for example, more than 0% by mass or 0.01% by mass or more.

[0054] For example, in the case of an all-solid-state battery, hydrolysis of the solid electrolyte generates hydrogen sulfide gas and a phosphate-based resistance layer. However, by setting the water content of the positive electrode active material to 0.2 mass% or less, the hydrolysis reaction of the electrolyte in the electrolytic solution can be more reliably suppressed, thereby suppressing such deterioration. The water content of the positive electrode active material is evaluated by the Karl Fischer method at a heating temperature of 300°C.

[0055] 2. Manufacturing Method of Positive Electrode Active Material for Lithium-Ion Secondary Battery Next, an example of a manufacturing method of the positive electrode active material for a lithium-ion secondary battery will be described with reference to the drawings. Fig. 1 is a diagram showing an example of a manufacturing method of a positive electrode active material for a lithium-ion secondary battery, and Fig. 2 is a process diagram (schematic diagram) showing an example of a manufacturing method of a lithium-nickel-manganese composite oxide. As shown in Fig. 1, the manufacturing method of a positive electrode active material for a lithium-ion secondary battery according to this embodiment includes at least a coating step S4 in which a coating layer containing lithium, phosphorus, and a compound containing element X is formed on at least a portion of the surface of a particle of lithium-nickel-manganese composite oxide.

[0056] 2, the lithium-ion nickel composite oxide can be obtained, for example, by a method including a precursor crystallization step S1, an oxidizing roasting step S2, and a lithium-nickel-manganese composite oxide synthesis step S3. Each step will be described in detail below. Note that the following description is an example of the production method and is not intended to limit the production method.

[0057] <2-1. Precursor Crystallization Step S1> In the precursor crystallization step S1 for obtaining a nickel composite hydroxide, a nickel manganese composite hydroxide, which is a precursor of a lithium nickel manganese composite oxide, is prepared by a crystallization reaction.

[0058] Specifically, for example, a mixed aqueous solution is prepared using water-soluble raw materials of each element so that the substance amount ratio of each element is Ni:Mn:M=1-x-y:x:y, and the mixed aqueous solution is reacted in a reaction tank together with an alkali metal aqueous solution or the like, to obtain a nickel-manganese composite hydroxide.

[0059] In addition, x and y in the above formula can be set to the same suitable ranges as x and y explained in the lithium nickel manganese composite oxide particles.

[0060] An example of the precursor crystallization step S1 will be described below. First, a metal compound containing nickel, a metal compound containing manganese, and optionally a metal compound containing element M are dissolved in water in predetermined proportions to prepare a raw material mixed aqueous solution (raw material mixed aqueous solution preparation step).

[0061] The ratio of the amounts of substances of the metals in the raw material mixed aqueous solution will be the same as the ratio of the amounts of substances in the nickel-manganese composite hydroxide finally obtained. Therefore, it is preferable to prepare the raw material mixed aqueous solution by adjusting the ratio of the metal compounds dissolved in water so that the ratio of the amounts of substances of the metals in the raw material mixed aqueous solution will be the same as the ratio of the amounts of substances of the metals in the target nickel-manganese composite hydroxide particles. Any water-soluble metal compound can be used, such as sulfates, chlorides, and nitrates, but sulfates are preferred from the viewpoint of cost. Note that if no suitable water-soluble metal compound can be found for element M, for example, it may be added in the oxidizing roasting step S2 or the lithium-nickel-manganese composite oxide synthesis step S3 described below, rather than being added to the raw material mixed aqueous solution.

[0062] Next, water is charged into a reaction tank, and an alkaline substance and an ammonium ion donor are added in appropriate amounts to prepare an initial aqueous solution (initial aqueous solution preparation step). At this time, it is preferable to prepare the initial aqueous solution so that the pH value is 11.2 or more and 13.0 or less at a liquid temperature of 25°C, and the ammonia concentration is 2 g / L or more and 20 g / L or less.

[0063] When the precursor crystallization step S1 is performed to prepare a nickel-manganese 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. However, by setting the pH value of the initial aqueous solution to 11.2 or higher, it is possible to suppress the mixing of impurities resulting from anions constituting the metal compounds of the raw materials, which is preferable. Furthermore, by setting the pH value of the initial aqueous solution to 13.0 or lower, it is possible to suppress the formation of fine particles in the obtained nickel-manganese composite hydroxide particles and achieve an optimal size.

[0064] Furthermore, by setting the ammonia concentration of the initial aqueous solution to 2 g / L or more, particles of the obtained nickel-manganese composite hydroxide can be made more likely to become spherical, which is preferable. And, by setting the ammonia concentration of the initial aqueous solution to 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-manganese composite hydroxide can be made to be the target substance amount ratio more reliably, which is preferable.

[0065] 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. It is preferably added in the form of an aqueous solution, since the amount added can be easily adjusted. The ammonium ion donor is not particularly limited, but is preferably one or more selected from an aqueous ammonium carbonate solution, ammonia water, an aqueous ammonium chloride solution, and an aqueous ammonium sulfate solution.

[0066] In the precursor crystallization step S1, the raw material mixed aqueous solution can be added dropwise to the initial aqueous solution to form a reaction aqueous solution.

[0067] 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 by making the atmosphere in the reaction vessel a non-oxidizing atmosphere, oxidation of the raw material compounds and the like can be suppressed, which is preferable. For example, precipitation of oxidized cobalt as fine particles can be prevented.

[0068] The temperature inside the reaction tank in the precursor crystallization step S1 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. In order to maintain the reaction tank within this temperature range, it is preferable that the initial aqueous solution and the reaction aqueous solution placed in the reaction tank are also maintained within the same temperature range.

[0069] Since the temperature of the reaction tank naturally rises due to the heat of reaction and the energy of stirring, by keeping the temperature inside the reaction tank at 40°C or higher, it is possible to avoid consuming extra energy for cooling. Furthermore, by keeping the temperature of the reaction tank at 60°C or lower, it is possible to suppress the evaporation of ammonia from the initial aqueous solution and the reaction aqueous solution, making it easier to maintain the target ammonia concentration.

[0070] Next, as described above, the initial aqueous solution is placed in the reaction tank, and after adjusting the temperature and the like, the raw material mixed aqueous solution is continuously added dropwise to the reaction tank at a constant rate to cause a reaction, thereby allowing the crystallization reaction of nickel composite hydroxide particles, which are precursors, to proceed (crystallization).

[0071] The pH value and ammonia concentration of the reaction aqueous solution are also preferably within the same suitable ranges as those described for the initial aqueous solution. Therefore, when adding the mixed aqueous solution dropwise to the initial aqueous solution or the reaction aqueous solution, it is also preferable to add an ammonium ion donor or an alkaline substance dropwise to the initial aqueous solution or the reaction aqueous solution at a constant rate. The pH value of the reaction aqueous solution is preferably controlled to be 11.2 or more and 13.0 or less at a liquid temperature of 25°C, and the ammonia concentration is preferably maintained at 2 g / L or more and 20 g / L or less. The crystallization process may also include a two-stage crystallization process, clearly distinguishing between a nucleation process and a particle growth process. In this case, the process may include a nucleation process in which the pH value of the reaction aqueous solution at a liquid temperature of 25°C is adjusted to be 12.0 or more and 14.0 or less to perform nucleation, and a particle growth process in which the pH value of the reaction aqueous solution containing the nuclei obtained in the nucleation process is controlled to be lower than the pH value of the nucleation process and to be 10.5 or more and 12.0 or less at a liquid temperature of 25°C to grow the nuclei. During the crystallization reaction, it is preferable that the pH and ammonia concentration are maintained at constant values ​​within the above ranges.

[0072] After the crystallization reaction is completed, the slurry containing the nickel composite hydroxide particles in the reaction tank is filtered and dried, thereby obtaining powdery nickel composite hydroxide particles as a precursor. Furthermore, the nickel composite hydroxide particles obtained after filtration may be washed with water to remove at least a part of the impurities.

[0073] <2-2. Oxidizing Roasting Step S2> The precursor crystallization step S1 may be followed by an oxidizing roasting step S2. In the oxidizing roasting step S2, 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 calcined in an oxygen-containing atmosphere and then cooled to room temperature, thereby obtaining a nickel composite oxide.

[0074] The roasting conditions in the oxidation roasting step S2 are not particularly limited, but it is preferable to perform the roasting in an oxygen-containing atmosphere, for example, an air atmosphere, at a temperature of 500°C to 700°C for 1 hour to 12 hours. This is because a roasting temperature of 500°C or higher can completely convert the nickel composite hydroxide particles into nickel composite oxide, which is preferable. Also, a roasting temperature of 700°C or lower can prevent the specific surface area of ​​the nickel composite oxide from becoming excessively small, which is preferable.

[0075] A firing time of 1 hour or more is preferable because it is possible to make the temperature in the firing container uniform and to allow the reaction to proceed uniformly. Furthermore, even if firing is performed for a time longer than 12 hours, no significant changes are observed in the obtained nickel composite oxide, so from the viewpoint of energy efficiency, it is preferable that the firing time be 12 hours or less.

[0076] The oxygen concentration in the oxygen-containing atmosphere during the heat treatment 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.

[0077] In addition, for example, when a compound containing element M is not coprecipitated in the precursor crystallization step S1, for example, a compound containing element M may be added to the nickel composite hydroxide and / or nickel composite oxide to be subjected to the lithium mixture preparation step in the oxidizing roasting step S2 or the lithium nickel manganese composite oxide synthesis step S3 so as to have the same composition ratio as the target, and then heat treatment or calcination may be performed. The compound containing element M to be added is not particularly limited, and for example, an oxide, hydroxide, carbonate, or a mixture thereof may be used.

[0078] After the oxidizing roasting step is completed, if slight sintering is observed in the nickel composite oxide particles, a crushing treatment may be added.

[0079] <2-3. Lithium nickel manganese composite oxide synthesis step S3> In the lithium nickel manganese composite oxide synthesis step S3, the nickel composite hydroxide and / or nickel composite oxide obtained in the precursor crystallization step S1 and / or the oxidizing roasting step S2 is mixed with a lithium compound and calcined to obtain a lithium nickel manganese composite oxide. An example of the lithium nickel manganese composite oxide synthesis step S3 will be described below.

[0080] In the lithium nickel manganese composite oxide synthesis step S3, a lithium compound is first added to and mixed with the nickel composite hydroxide and / or nickel composite oxide obtained in the precursor crystallization step S1 and / or the oxidizing roasting step S2 so that the amount of substance of lithium is 95% or more and 120% or less of the total amount of substance of the component metal elements contained in this composite oxide, thereby obtaining a lithium mixture (lithium mixture preparation step). Also, in the lithium mixture preparation step, a compound containing element M may be mixed in the same composition ratio as the target, and the calcination step described below may be carried out.

[0081] The lithium compound to be added is not particularly limited, and examples thereof include lithium hydroxide, lithium nitrate, lithium carbonate, and mixtures thereof. As the lithium compound, it is particularly preferable to use lithium hydroxide, which has a low melting point and high reactivity.

[0082] Next, the resulting lithium mixture is calcined in an oxygen-containing atmosphere and then cooled to room temperature to obtain a lithium nickel manganese composite oxide (calcination step). The calcination conditions are not particularly limited, but it is preferable to calcinate at a temperature of 700°C or higher and 1000°C or lower for 1 hour or longer and 24 hours or shorter, for example.

[0083] 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 are mixed into the Li sites in the resulting lithium nickel manganese 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 100% by volume.

[0084] By setting the firing temperature to 700° C. or higher, the crystal structure of the lithium nickel manganese composite oxide can be sufficiently grown, and by setting the firing temperature to 1000° C. or lower, the above-mentioned cation mixing can be suppressed.

[0085] A calcination time of 1 hour or more is preferable because it makes it possible to make the temperature in the calcination vessel uniform and to allow the reaction to proceed uniformly. Furthermore, even if calcination is performed for a time longer than 24 hours, no significant changes are observed in the obtained lithium nickel manganese composite oxide, so from the viewpoint of energy efficiency, it is preferable that the calcination time be 24 hours or less.

[0086] After the lithium nickel manganese composite oxide synthesis step S3, if slight sintering is observed in the resulting lithium nickel manganese composite oxide, a crushing treatment may be added.

[0087] <2-4. Coating step S4> In the coating step S4, a coating layer containing a compound containing lithium (Li), phosphorus (P), and an element X (X is at least one element selected from the group consisting of elements that form hexavalent cations) is formed on at least a portion of the surface of the lithium nickel manganese composite oxide particles obtained in the lithium nickel manganese composite oxide synthesis step S3.

[0088] In the coating step S4, the total amount of phosphorus and element X in the compound is equal to or less than 1 m of the surface area of ​​the lithium nickel manganese composite oxide. ２ It is preferable to add the compound containing phosphorus and the element X so that the amount is 50 μmol to 410 μmol per mol. An example of the coating step S4 will be described below.

[0089] First, the specific surface area of ​​the lithium nickel manganese composite oxide obtained in the lithium nickel manganese composite oxide synthesis step S3 is measured, and a liquid coating agent is prepared according to the target surface amounts of phosphorus and element X in the coating layer (coating agent preparation step S40).

[0090] The coating agent is not particularly limited as long as it contains lithium, phosphorus, and element X, or a compound containing lithium, phosphorus, and element X. For uniform coating, the coating agent that can be preferably used is one in which lithium, phosphorus, and element X compound are dissolved in a solvent, a compound containing lithium, phosphorus, and molybdenum with a low melting point that is liquid at room temperature or melts by low-temperature heat treatment, or a sol-based material in which nanoparticles are dispersed in a solvent.

[0091] Examples of compounds containing phosphorus and lithium include lithium metaphosphate (LiO ３ P), lithium dihydrogen phosphate (LiH ２ P.O. ４ ), Li ２ HPO ４ , Li ４ P ２ O ７ , Li ３ P.O. ４ Examples of compounds containing element X include ammonium molybdate and ammonium tungstate. For example, by adjusting the molar ratio of lithium to phosphorus in a compound containing phosphorus and lithium, it is possible to adjust the molar ratio of lithium to phosphorus in the compound contained in the coating layer.

[0092] As the coating agent, a solution of lithium metaphosphate, ammonium molybdate, and / or ammonium tungstate in water is particularly preferred, from the viewpoints of ease of preparation and ability to suppress the inclusion of impurities.

[0093] Next, the lithium nickel manganese composite oxide particles and the coating agent can be mixed using a general mixer (mixture preparation step S41a), and then dried (drying step S41b).

[0094] The drying can be carried out at a temperature sufficient to remove the solvent of the coating agent, for example, at 80°C or higher and lower than 300°C.

[0095] The mixture preparation step S41a and the drying step S41b may be carried out in parallel. In this case, for example, a tumbling fluidized bed coating device can be used. When a tumbling fluidized bed coating device is used, the coating liquid is sprayed onto the lithium nickel manganese composite oxide particles that are being fluidized by the heated airflow in the device, so the mixture preparation step and the drying step are repeated in parallel, and a more uniform coating layer with fewer gaps can be obtained.

[0096] Next, a heat treatment may be performed as needed (heat treatment step S42). The heat treatment allows the compound containing lithium, phosphorus, and element X to be fixed as a coating layer. The heat treatment conditions are not particularly limited, but it is preferable to perform the heat treatment in an oxygen-containing atmosphere, for example, an air atmosphere, at a temperature of 200°C to 600°C for 1 hour to 10 hours. After the heat treatment, the mixture is cooled to room temperature, and the final product, a positive electrode active material, is obtained, which is lithium nickel manganese composite oxide particles having a coating layer.

[0097] The oxygen concentration in the oxygen-containing atmosphere during the heat treatment is preferably equal to or higher than the oxygen concentration in the air atmosphere, i.e., the oxygen concentration is preferably 20% by volume or higher. By setting the oxygen concentration in the oxygen-containing atmosphere during the heat treatment to be equal to or higher than the oxygen concentration in the air atmosphere, the occurrence of oxygen defects in the obtained positive electrode active material can be particularly suppressed, which is preferable. 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.

[0098] The maximum temperature during the heat treatment is preferably 200°C or higher, since this particularly prevents impurities contained in the coating agent from remaining in the positive electrode active material. Furthermore, the maximum temperature is preferably 600°C or lower, since this prevents the components of the coating layer from excessively diffusing into the positive electrode active material and allows the shape of the coating layer to be maintained. The maximum temperature during the heat treatment is preferably selected so that the coating layer can maintain its thickness sufficiently, depending on the target surface amounts of phosphorus and element X in the coating layer, etc.

[0099] A heat treatment time of 1 hour or more is preferable because it can particularly prevent impurities contained in the coating agent from remaining in the positive electrode active material. Furthermore, even if the calcination is performed for a time longer than 10 hours, no significant change is observed in the resulting positive electrode active material, so from the viewpoint of energy efficiency, a treatment time of 10 hours or less is preferable.

[0100] If slight sintering is observed in the positive electrode active material obtained after the coating step S4, a crushing treatment may be added.

[0101] It should be noted that the heat treatment does not have to be carried out. That is, the process up to the preparation of the mixture can be carried out to produce a positive electrode active material. Even if the heat treatment is not carried out, by using a liquid coating agent, a uniform and strong coating layer can be formed on the surface of the lithium nickel manganese composite oxide particles. Even if the heat treatment is not carried out, it is preferable to dry the coating agent as needed to reduce and remove the solvent, moisture, etc. of the coating agent.

[0102] 3. 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 an electrolyte. The secondary battery may be any secondary battery that charges and discharges by desorption and insertion of lithium ions. For example, the secondary battery may be a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, or an all-solid-state lithium secondary battery including a positive electrode, a negative electrode, and a solid electrolyte. The following mainly describes an example of an all-solid-state battery. 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. The secondary battery is not particularly limited in its intended use.

[0103] <3-1. Positive Electrode> The positive electrode can be formed by molding the positive electrode mixture. The positive electrode is processed appropriately depending on the battery to be used. For example, a pressure compression process using a press or the like can be performed to increase the electrode density.

[0104] The positive electrode mixture can be formed by mixing, for example, the above-mentioned positive electrode active material in powder form with a solid electrolyte, which is added to impart appropriate ionic conductivity to the electrode.

[0105] The material of the solid electrolyte is not particularly limited, but for example, Li ３ P.S. ４ , Li ７ P ３ S １１ , Li １０ GeP ２ S １２ Sulfide-based solid electrolytes such as Li ７ La ３ Zr ２ O １２ , Li ０．３４ La ０．５１ TiO ２．９４ An oxide-based solid electrolyte such as SiO 2 or a polymer-based electrolyte such as PEO can be used.

[0106] In addition to the above components, a binder and a conductive additive may also be added to the positive electrode mixture.

[0107] 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, and the like.

[0108] The conductive material is added to the electrode to provide it with appropriate conductivity. The material of the conductive material 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).

[0109] 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 can be 50 parts by mass or more and 90 parts by mass or less, and the content of the solid electrolyte can be 10 parts by mass or more and 50 parts by mass or less.

[0110] However, the method for producing the positive electrode is not limited to the above-mentioned example, and other methods may be used.

[0111] <3-2. Negative Electrode> The negative electrode can be formed by molding the negative electrode mixture.

[0112] Although the components constituting the negative electrode mixture and their blending are different, the negative electrode is formed in substantially the same manner as the positive electrode described above, and is subjected to various treatments as necessary in the same manner as the positive electrode.

[0113] 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.

[0114] The storage material is not particularly limited, and may be one or more selected from, for example, natural graphite, artificial graphite, sintered 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 solid electrolyte containing Li, ３ P.S. ４ Sulfide electrolytes such as the above can be used.

[0115] The negative electrode may also be a sheet-like member made of a material containing a metal that alloys with lithium, such as metallic lithium or indium.

[0116] <3-3. Electrolyte>

[0117] (Solid electrolyte) The solid electrolyte is Li ＋ The solid electrolyte is a solid having ion conductivity. As the solid electrolyte, one selected from sulfides, oxides, polymers, etc. can be used alone or in combination of two or more.

[0118] The sulfide-based solid electrolyte is not particularly limited, and any sulfide-based solid electrolyte containing sulfur (S) and having lithium ion conductivity and electronic insulation can be used. ２ S-P ２ S ５ , Li ２ S-SiS ２ , LiI-Li ２S-SiS ２ , LiI-Li ２ S-P ２ S ５ , LiI-Li ２ S-B ２ S ３ , Li ３ P.O. ４ -Li ２ S-Si ２ S., Li. ３ P.O. ４ -Li ２ S-SiS ２ , LiPO ４ -Li ２ S-SiS, LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ etc.

[0119] 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 properties can be used.

[0120] Examples of oxide-based solid electrolytes include lithium phosphate (Li ３ P.O. ４ ), Li ３ P.O. ４ NX, LiBO ２ NX, LiNbO ３ , LiTaO ３ , Li ２ SiO ３ , Li ４ SiO ４ -Li ３ P.O. ４ , Li ４ SiO ４ -Li ３ VO ４ , Li ２ Alumni ２ O ３ -P ２ O ５ , Li ２ O—SiO ２ , Li ２ Alumni ２ O ３-ZnO, Li １＋Ｘ Al Ｘ Ti ２－Ｘ (P.O. ４ ) ３ (0≦X≦1), Li １＋Ｘ Al Ｘ Ge ２－Ｘ (P.O. ４ ) ３ (0≦X≦1), LiTi ２ (P.O. ４ ) ３ , Li ３ XLa ２／３－Ｘ TiO ３ (0≦X≦2 / 3), Li ５ La ３ Ta ２ O １２ , Li ７ La ３ Zr ２ O １２ , Li ６ BaLa ２ Ta ２ O １２ , Li ３．６ Si ０．６ P ０．４ O ４ etc.

[0121] In addition, inorganic solid electrolytes other than those mentioned above may be used. For example, Li ３ N, LiI, Li ３ N-LiI-LiOH or the like may also be used.

[0122] 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.

[0123] (Non-aqueous Electrolyte) The non-aqueous electrolyte may be prepared by dissolving a lithium salt as a supporting salt in an organic solvent.

[0124] The organic solvent may be one selected from the group consisting of cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate, and may be used alone or in combination of two or more.

[0125] The supporting salt is LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiN(CF ３ SO ２ ) ２ and complex salts thereof. The non-aqueous electrolyte may contain, in addition to the above, a radical scavenger, a surfactant, a flame retardant, and the like.

[0126] <3-4. Separator> The secondary battery may also include a separator. The separator is disposed between the positive electrode and the negative electrode, and serves to separate the positive electrode from the negative electrode and retain the electrolyte. As such a separator, for example, a thin membrane made of polyethylene, polypropylene, or the like, having many fine pores, can be used, but there is no particular limitation as long as it has the above-mentioned function.

[0127] <3-5. Shape and Configuration of Secondary Battery> Next, an example of the arrangement and configuration of the components of the secondary battery of this embodiment will be described.

[0128] The secondary battery of this embodiment, which is composed of the positive electrode, negative electrode, and solid electrolyte described above, can be formed into various shapes, such as a coin shape or a laminated shape. Regardless of the shape, the positive electrode and negative electrode can be laminated with the solid electrolyte interposed therebetween. The positive electrode current collector and a positive electrode terminal connected to the outside, and the negative electrode current collector and a negative electrode terminal connected to the outside, can then be connected using a current collecting lead or the like, and the battery can be sealed in a battery case to form a secondary battery.

[0129] <3-6. Characteristics of Lithium-Ion Secondary Battery> According to the positive electrode active material for lithium-ion secondary batteries according to this embodiment, it is possible to provide a positive electrode active material for lithium-ion secondary batteries that has a high capacity and is excellent in high voltage resistance.

[0130] For example, when the positive electrode active material of this embodiment is used in the positive electrode of a test battery shown in Fig. 3, the initial discharge capacity measured under the following conditions is preferably 100 mAh / g or more, and more preferably 105 mAh / g or more. That is, when the test battery shown in Fig. 3 is constructed and a current density of 0.2 mA / cm is applied in a 25°C environment, ２ The battery is charged to a cutoff voltage of 5.0 V (vs. Li) (initial charge capacity), and after a one-hour rest, is discharged to a cutoff voltage of 3.0 V (vs. Li) to measure the initial discharge capacity.

[0131] The ratio of the initial discharge capacity to the initial charge capacity is called the Coulomb efficiency, and serves as an index of the voltage endurance characteristics of the positive electrode active material. For example, when the positive electrode active material of this embodiment is used in the positive electrode of the test battery shown in FIG. 3, the Coulomb efficiency is preferably 78% or more, more preferably 81% or more, and may even be 85% or more. The excess capacity relative to discharge during the initial charge contains components resulting from side reactions related to the alteration of the coating layer, and therefore, the closer the Coulomb efficiency is to 100%, the higher the voltage endurance characteristics of the positive electrode active material can be said to be.

[0132] Next, the active material for a non-aqueous electrolyte secondary battery, the method for producing a positive electrode active material for a non-aqueous secondary battery, and the non-aqueous electrolyte secondary battery according to one embodiment of the present invention will be described in detail using examples. Note that the present invention is not limited to these examples. Note that the analysis methods for metals contained in the positive electrode active material and the various evaluation methods for the positive electrode active material in the examples and comparative examples are as follows.

[0133] Example 1 1. Production of Particles of Lithium Nickel Manganese Composite Oxide A positive electrode active material was produced by carrying out the following steps.

[0134] (a) Precursor Crystallization Step First, 10 L of ion-exchanged water was placed in a reaction tank (60 L) and stirred while the temperature inside the tank was set to 50° C. At this time, the inside of the reaction tank was filled with a nitrogen atmosphere having an oxygen concentration of 1% by volume or less.

[0135] Appropriate amounts of 25% by mass aqueous sodium hydroxide solution and 25% by mass aqueous ammonia were added to the water in this 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.

[0136] At the same time, nickel sulfate and manganese sulfate were dissolved in pure water so that the mole ratio of nickel to manganese was Ni:Mn=0.25:0.75, to prepare 25 L of a 2.0 mol / L mixed aqueous solution.

[0137] 200 mL of the nickel-cobalt-manganese mixed aqueous solution was added dropwise to the initial aqueous solution in the reaction vessel at a constant rate to prepare a reaction aqueous solution. At this time, 25% by mass of ammonia water and 25% by mass of 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.

[0138] Subsequently, sulfuric acid was added dropwise to the reaction tank to adjust the pH of the reaction aqueous solution to 11.5. This operation was intended to lower the pH value, thereby slowing down the rate at which the composite hydroxide of nickel, cobalt, and manganese precipitates from the liquid phase to the solid phase in the subsequent precursor crystallization step, thereby improving the uniformity of the particle size distribution and the sphericity of the resulting particles.

[0139] After the pH control, 24.8 L of a nickel-cobalt-manganese mixed aqueous solution was added dropwise to the reaction aqueous solution in the reaction tank at 103 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.

[0140] After the entire amount of the nickel-cobalt-manganese mixed aqueous solution was added dropwise, the pH value of the reaction aqueous solution 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 targeted chemical composition.

[0141] Thereafter, the reaction aqueous solution was subjected to solid-liquid separation using a Buchner funnel, a filter can, and a vacuum pump vacuum filter. Furthermore, the operation of dispersing the obtained solid phase in 20 L of pure water at 40°C and performing solid-liquid separation was repeated twice, thereby removing water-soluble impurities such as sodium sulfate from the nickel composite hydroxide.

[0142] After the washing, the cake-like solid phase obtained after solid-liquid separation was dried in an air atmosphere at 120° C. for 24 hours in a stationary dryer, and then passed through a sieve with 100 μm openings to obtain a powdery nickel composite hydroxide.

[0143] (b) Oxidation Roasting Step The composite hydroxide prepared above 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-manganese composite oxide particles.

[0144] (c) Lithium-nickel-manganese composite oxide synthesis step: Lithium carbonate weighed out so that the ratio of the substance amount of lithium to the total substance amount of nickel and manganese contained in this composite oxide was 0.5 was added to the nickel composite oxide, and the mixture was mixed using a Turbler shaker mixer (T2F, manufactured by Dalton Co., Ltd.) to obtain a lithium mixture (lithium mixture preparation step).

[0145] 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 900°C for 10 hours, and then cooled to room temperature (firing step). This produced lithium nickel manganese composite oxide particles.

[0146] 2. Evaluation of Lithium Nickel Manganese Composite Oxide Particles (a) Chemical Composition Quantitative analysis using an ICP optical emission spectrometer (725ES, manufactured by VARIAN) confirmed that the lithium nickel manganese composite oxide had a mass ratio of the metal elements expressed as Li:Ni:Mn=0.5:0.25:0.75.

[0147] (b) Crystal structure The crystal structure of the particles of this lithium nickel manganese composite oxide was measured using XRD (X'Pert, PROMRD, manufactured by PANALYTICAL), and it was confirmed that the crystal structure was a spinel type, with peaks attributed to the Fd-3m structure being detected in the diffraction pattern.

[0148] (c) Specific Surface Area The BET specific surface area of ​​this lithium nickel manganese composite oxide was measured using a fully automatic BET specific surface area measuring device (Macsorb, manufactured by Mountec Co., Ltd.). As a result, the specific surface area was 1.0 m ２ / g.

[0149] (d) Volume average particle size: The volume average particle size of this lithium nickel manganese composite oxide was measured using a laser diffraction scattering particle size distribution measuring device (Microtrac HRA, manufactured by Nikkiso Co., Ltd.), and was found to be 8.1 μm.

[0150] 3. Production of Positive Electrode Active Material (d) Coating Step The lithium nickel manganese composite oxide was subjected to the following coating step to produce a positive electrode active material. The total surface area of ​​the lithium nickel manganese composite oxide to be coated was calculated from the mass and specific surface area of ​​the lithium nickel manganese composite oxide. ２ Lithium metaphosphate was weighed so that the amount of phosphorus per unit area (surface amount of phosphorus) was 283 μmol.２ Ammonium molybdate was weighed out so that the amount of molybdenum per particle (surface amount of molybdenum) was 3 μmol, and dissolved in ion-exchanged water (coating agent preparation step).

[0151] Using the coating solution, 500 g of lithium nickel manganese composite oxide was coated using a rolling fluidized coating device (MP-01, manufactured by Powrex Corporation).

[0152] 500 g of lithium nickel manganese composite oxide was heated to 120°C and the flow rate was 0.3 m 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 manganese composite oxide.

[0153] After the entire amount of the coating liquid was sprayed, the lithium nickel manganese 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 lithium nickel manganese composite oxide particles having a coating layer as the positive electrode active material.

[0154] 4. Evaluation of Positive Electrode Active Material The positive electrode active material obtained in this manner was evaluated as follows.

[0155] (a) Composition Analysis using an ICP optical emission spectrometer (725ES, manufactured by VARIAN) revealed that this positive electrode active material contained 0.85 mass % of P and 0.03 mass % of Mo, and the amount of phosphorus on the surface of the coating layer, calculated from the specific surface area before coating treatment, was 270 μmol / m ２ , the molybdenum surface amount is 3 μmol / m ２ It was found that...

[0156] (b) Carbon Content The carbon content of the obtained positive electrode active material was measured by high-frequency combustion infrared absorption method using a carbon analyzer (model: CS-600 manufactured by LECO Corporation) and was found to be 0.08% by mass.

[0157] (c) Water Content The water content of the obtained 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, and was found to be 0.06% by mass.

[0158] 5. Fabrication of Secondary Battery To evaluate the capacity of the obtained positive electrode active material, a battery (hereinafter referred to as "test battery") having the structure shown in Fig. 3 was used. As shown in Fig. 3, the test battery SBA includes a case having a negative electrode can NC and a positive electrode can PC, and a compacted cell C housed in the case.

[0159] The case has a hollow anode can NC with one open end and a cathode can PC that is placed in the opening of the anode can NC, and is configured so that when the cathode can PC is placed in the opening of the anode can NC, a space is formed between the cathode can and the anode can to accommodate the compacted cell. The cathode can PC is fixed to the anode can NC with a thumbscrew SW and a nut N.

[0160] The negative electrode can NC has a negative terminal, and the positive electrode can PC has a positive terminal. The case is provided with an insulating sleeve ISV, which fixes the negative electrode can NC and the positive electrode can PC so that they are kept out of contact with each other.

[0161] A pressure screw PSW is provided at one closed end of the negative electrode can NC, and after the positive electrode can PC is fixed to the negative electrode can NC, the pressure screw PSW is tightened toward the storage space of the compacted cell C, thereby maintaining the compacted cell C in a pressurized state through a hemispherical washer W. A screw-in plug P is provided at the end of the negative electrode can NC where the pressure screw PSW is located. O-rings OL are provided between the negative electrode can NC and the positive electrode can PC, and between the negative electrode can NC and the plug P, which seal the gap between the negative electrode can NC and the positive electrode can PC and maintain an airtight seal inside the case.

[0162] The compact cell C is a pellet composed of a positive electrode layer PL, a solid electrolyte layer SEL, and a negative electrode layer NL, stacked in this order. The compact cell C is housed in a case such that the positive electrode layer PL contacts the inner surface of the positive electrode can PC through the lower current collector LCC, and the negative electrode layer NL contacts the inner surface of the negative electrode can NC through the upper current collector UCC, a hemispherical washer W, and a 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 the negative electrode layer NL.

[0163] Such a test battery SBA was fabricated as follows.

[0164] First, 60 mg of the synthesized solid electrolyte (0.75Li2S-0.25P2S5) was pressed at 25 MPa in a pelletizer to obtain solid electrolyte pellets.

[0165] Next, 100 mg of mesocarbon microbeads (MCMB) as the negative electrode active material and 100 mg of the solid electrolyte were mixed in a mortar. The solid electrolyte pellets and 22 mg of the MCMB + solid electrolyte mixture were placed in a pellet former and pressurized at 25 MPa to form a negative electrode layer (NL) on the solid electrolyte (SEL) pellets.

[0166] Next, 120 mg of the positive electrode active material and 80 mg of the solid electrolyte were mixed in a mortar.

[0167] A 15 mg mixture of a solid electrolyte (SEL) + anode layer (NL) pellet and a positive electrode active material + solid electrolyte was placed in a pellet former and pressurized at 360 MPa to form a cathode layer (PL) on the side of the solid electrolyte (SEL) pellet opposite the anode layer (NL). The electrodes were sealed in a case, and the pressure screw (PSW) was tightened with a torque of 6 to 7 N m to form a compacted powder cell (C). The test battery (SBA) was fabricated in a glove box with an Ar atmosphere and a dew point controlled at -80°C.

[0168] 6. Evaluation of Secondary Battery The charge / discharge capacity and coulomb efficiency, which indicate the performance of the prepared test battery, were evaluated as follows.

[0169] (a) Initial Discharge Capacity and Coulomb Efficiency The initial discharge capacity was measured by placing a 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 cutoff voltage is 5.0 V (vs. Li ＋ / Li) (initial charge capacity), and after a 1-hour rest, the cut-off voltage was 3.0 V (vs. Li ＋ The discharge capacity (initial discharge capacity) was measured when the positive electrode active material was discharged to a maximum capacity (Li / Li). The measurement result was 105 mAh / g, and the coulomb efficiency (initial discharge capacity / initial charge capacity) was 85%. Table 1 shows the preparation conditions of the positive electrode active material, its morphology characteristics, and the evaluation results of the secondary battery.

[0170] (Example 2) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 260 μmol / m ２ , molybdenum surface amount 14 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The evaluation results are shown in Table 1.

[0171] (Example 3) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 233 μmol / m ２ , molybdenum surface amount 26 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The evaluation results are shown in Table 1.

[0172] (Example 4) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 52 μmol / m ２ , the molybdenum surface amount is 3 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The evaluation results are shown in Table 1.

[0173] (Example 5) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 389 μmol / m ２, molybdenum surface amount 20 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The evaluation results are shown in Table 1.

[0174] (Example 6) In the coating agent preparation step, the total surface area of ​​the lithium nickel manganese composite oxide to be coated was calculated from the mass and specific surface area of ​​the lithium nickel manganese composite oxide. ２ Lithium metaphosphate was weighed so that the amount of phosphorus per unit area (surface amount of phosphorus) was 280 μmol. ２ A lithium nickel manganese composite oxide particle, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1, except that ammonium tungstate weighed out so that the amount of tungsten per particle (amount of tungsten on both sides) was 3 μmol was dissolved in ion-exchanged water. The evaluation results are shown in Table 1.

[0175] Analysis using an ICP optical emission spectrometer (725ES, manufactured by VARIAN) revealed that this positive electrode active material contained 0.85 mass % of P and 0.05 mass % of W, and the amount of phosphorus on the surface of the coating layer, calculated from the specific surface area before coating treatment, was 270 μmol / m ２ , tungsten surface amount is 3 μmol / m ２ It was.

[0176] (Example 7) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 247 μmol / m ２ , tungsten surface amount 13 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 6. The evaluation results are shown in Table 1.

[0177] (Example 8) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 213 μmol / m ２ , tungsten surface amount 24 μmol / m ２Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 6. The evaluation results are shown in Table 1.

[0178] (Example 9) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 49 μmol / m ２ , tungsten surface amount 3 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 6. The evaluation results are shown in Table 1.

[0179] (Example 10) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 371 μmol / m ２ , tungsten surface amount 20 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 6. The evaluation results are shown in Table 1.

[0180] (Comparative Example 1) In the coating agent preparation step, the target phosphorus surface amount of the coating layer was 289 μmol / m ２ No compound containing element X (molybdenum or tungsten) was added, and the amount of element X was 0 μmol / m ２ Except for this, particles of lithium nickel manganese composite oxide, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1. The evaluation results are shown in Table 1.

[0181]

[0182] (Evaluation Results) The positive electrode active material of the example had a cutoff voltage of 5 V (vs. Li) compared to the positive electrode active material of Comparative Example 1, which did not contain the element X in the coating layer. ＋ When charged and discharged at a current of 1000 kJ / L, the battery had a high discharge capacity and a high coulombic efficiency, indicating that it had improved high voltage resistance.

[0183] The technical scope of the present invention is not limited to the aspects described in the above-mentioned embodiments. One or more of the requirements described in the above-mentioned embodiments may be omitted. Furthermore, the requirements described in the above-mentioned embodiments may be combined as appropriate. Furthermore, to the extent permitted by law, the disclosures of all documents cited in the above-mentioned embodiments are incorporated by reference into this description. Furthermore, to the extent permitted by law, the contents of Japanese Patent Application No. 2024-150213 are incorporated by reference into this description.

[0184] SBA...Test battery PC...Positive electrode can NC...Anode can ISV...Insulating sleeve C...Powder compact cell PL...Positive electrode layer NL...Anode layer SEL...Solid electrolyte layer LCC...Lower current collector UCC...Upper current collector P...Plug PSW...Pressure screw W...Hemispherical 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 lithium nickel manganese composite oxide and a coating layer covering at least a portion of the surface of the lithium nickel manganese composite oxide particles, wherein the lithium nickel manganese composite oxide contains, as elements other than oxygen, Li, Ni, Mn, and optionally an element M (element M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Sc, Ti, V, Fe, Co, Cu, Cr, Zn, Zr, Nb, Mo, and W), and the ratio of the amounts of substance of these elements is expressed as Li:Ni:Mn:M=t:1-x-y:x:y (0.47≦t≦0.6, 0.7≦x≦0.8, 0≦y≦0.15), and the coating layer contains a compound containing Li, P, and an element X (element X is at least one element selected from the group consisting of elements that form hexavalent cations). Positive electrode active material for lithium-ion secondary batteries.

2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the compound has a composition ratio of elements other than oxygen expressed as Li:P:X=1-a:1-a:a (0.01≦a≦0.1).

3. The surface area of ​​the lithium nickel manganese composite oxide is 1 m ２ 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the total amount of substances of P and element X derived from the coating layer per unit area is 50 μmol or more and 410 μmol or less.

4. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the element X is Mo, W, or both.

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

6. The specific surface area of ​​the lithium nickel manganese composite oxide is 0.1 m ２ / g or more 2.0m ２ The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the SiO2 content is 0.15 / g or less.

7. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the lithium nickel manganese composite oxide has a spinel crystal structure belonging to the space group Fd-3m.

8. A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, a negative electrode, and an electrolyte.

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