Positive electrode active material, method for producing the same, and non-aqueous electrolyte secondary battery including the positive electrode active material

Abstract

To provide a positive electrode active material capable of realizing a non-aqueous electrolyte secondary battery having both high levels of input / output characteristics and cycle characteristics.SOLUTION: A positive electrode active material 1 disclosed herein includes a particulate Ni-containing lithium transition metal composite oxide 2 and a coating layer 4 attached to at least a portion of the surface of the Ni-containing lithium transition metal composite oxide 2, the coating layer 4 includes an oxide containing a first element selected from Co and Mn and a second element belonging to Group 6, and has an average thickness of 50 nm or more and 200 nm or less based on the cross-sectional observation with a transmission electron microscope.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material, a method for producing the same, and a non-aqueous electrolyte secondary battery including the positive electrode active material. [Background technology]

[0002] Lithium transition metal composite oxides are widely used as positive electrode active materials for non-aqueous electrolyte secondary batteries. Recently, in order to improve battery characteristics, the surface of particulate lithium transition metal composite oxides has been coated with a tungsten-containing compound (see Patent Documents 1 to 4). In this regard, Patent Document 1, for example, discloses a method for producing a positive electrode active material, which includes the steps of mixing a Ni-containing lithium transition metal composite oxide with nickel tungstate to obtain a mixture and drying the mixture at, for example, 150°C. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-133839 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-040383 [Patent Document 3] International Publication No. 2013 / 015069 [Patent Document 4] International Publication No. 2015 / 141194 Summary of the Invention [Problem to be solved by the invention]

[0004] As non-aqueous electrolyte secondary batteries become more widespread, further improvements in their performance are being demanded. In particular, high-input / output type electricity storage devices mounted on moving objects such as vehicles are being required to have both high-level input / output characteristics and high-level cycle characteristics.

[0005] The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide a positive electrode active material that can realize a nonaqueous electrolyte secondary battery that combines high levels of input / output characteristics and cycle characteristics. [Means for solving the problem]

[0006] The present invention provides a positive electrode active material comprising a particulate Ni-containing lithium transition metal composite oxide and a coating layer adhered to at least a portion of the surface of the Ni-containing lithium transition metal composite oxide, wherein the coating layer contains a composite oxide containing a first element selected from Co and Mn and a second element belonging to Group 6, and the coating layer has an average thickness of 50 nm or more and 200 nm or less based on cross-sectional observation with a transmission electron microscope.

[0007] By providing the particulate Ni-containing lithium transition metal composite oxide with the coating layer, reaction with the electrolyte can be suppressed, resulting in excellent cycle characteristics. Furthermore, by setting the thickness of the coating layer to a predetermined value or less, Li migration is less likely to be inhibited, resulting in excellent input / output characteristics. Therefore, the above configuration makes it possible to suitably realize a nonaqueous electrolyte secondary battery that combines high levels of input / output characteristics and cycle characteristics.

[0008] The present invention also provides a method for producing a positive electrode active material, which includes a mixing step of mixing a particulate Ni-containing lithium transition metal composite oxide with a compound containing a first element selected from Co and Mn and a second element belonging to Group 6 to obtain a mixture, and a firing step of firing the mixture in the presence of oxygen at a temperature of 500° C. to 800° C. This allows the above-mentioned positive electrode active material to be suitably produced. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a positive electrode active material according to one embodiment. [Figure 2] FIG. 2 is a cross-sectional view schematically showing the internal structure of a nonaqueous electrolyte secondary battery according to one embodiment. [Figure 3]FIG. 3 is a cross-sectional SEM image of the positive electrode active material of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, some preferred embodiments of the present invention will be described with reference to the drawings as appropriate. Matters necessary for carrying out the present invention other than those specifically mentioned in this specification (for example, the general configuration and manufacturing process of a non-aqueous electrolyte secondary battery that do not characterize the present invention) can be understood as design matters for those skilled in the art based on prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this specification and the technical common sense in the relevant field.

[0011] In the following drawings, components and parts that perform the same function are denoted by the same reference numerals, and redundant explanations may be omitted or simplified. In this specification, the term "lithium ion secondary battery" refers to any secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes. In this specification, the expression "A to B" indicating a range includes not only the meaning of A or more and B or less, but also the meanings of "preferably larger than A" and "preferably smaller than B."

[0012] [Cathode active material] FIG. 1 is a cross-sectional view schematically showing a positive electrode active material 1 according to one embodiment. The positive electrode active material 1 includes a Ni-containing lithium transition metal composite oxide 2 as a base material, and a coating layer 4 attached to at least a portion of the surface of the Ni-containing lithium transition metal composite oxide 2. The coating layer 4 is typically attached to the Ni-containing lithium transition metal composite oxide 2 by physical and / or chemical bonding. The coating layer 4 is preferably fused to the Ni-containing lithium transition metal composite oxide 2. Note that FIG. 1 is an example and is not limited to what is shown in the figure.

[0013] The Ni-containing lithium transition metal composite oxide 2 is a composite oxide containing Li and Ni as essential elements. Specific examples of the Ni-containing lithium transition metal composite oxide 2 include lithium nickel composite oxide, lithium nickel cobalt composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These may be used alone or in combination of two or more. The Ni-containing lithium transition metal composite oxide 2 preferably further contains at least one of Co and Mn in addition to Ni. Among these, lithium nickel cobalt manganese composite oxides containing Ni, Co, and Mn are preferred because of their excellent battery properties such as low initial resistance.

[0014] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ba, Sr, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, K, Fe, Cu, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium nickel cobalt composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide described above.

[0015] From the perspective of improving the energy density of the battery, the lithium nickel cobalt manganese composite oxide preferably contains a high-Ni-containing lithium transition metal composite oxide in which the Ni content is set high at 70 mol% or more with respect to the total of metal elements other than lithium. The Ni content is preferably 75 mol% or more, for example 80 mol% or more. According to the findings of the present inventors, the high-Ni-containing lithium transition metal composite oxide is relatively likely to have deteriorated cycle characteristics compared to a lithium composite oxide having a low Ni content. Therefore, it is particularly effective to apply the technology disclosed herein.

[0016] The lithium nickel cobalt manganese composite oxide preferably has a composition represented by the following formula (I). Li α Ni x Mn y Co z M t O2 (I) In the above formula (I), α, x, y, z, and t each satisfy 1.00 ≤ α ≤ 1.30, 0.25 < x < 0.90, 0 < y < 0.60, 0 < z < 0.60, 0 ≤ t ≤ 0.10, and x + y + z + t = 1. When 0 < t, M is at least one element selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, and W.

[0017] α preferably satisfies α ≤ 1.20, more preferably satisfies α ≤ 1.10. From the perspective of improving battery characteristics (for example, input / output characteristics and cycle characteristics), x preferably satisfies x ≥ 0.5, more preferably satisfies x ≥ 0.7, for example 0.8 ≤ x ≤ 0.9, y preferably satisfies y ≥ 0.01, more preferably satisfies 0.03 ≤ y ≤ 0.20, for example 0.05 ≤ y ≤ 0.1, z preferably satisfies 0.01 ≤ z ≤ 0.50, more preferably satisfies 0.05 ≤ z ≤ 0.30, for example 0.10 ≤ z ≤ 0.20. t preferably satisfies 0 ≤ t ≤ 0.05, more preferably 0.

[0018] The Ni-containing lithium transition metal composite oxide 2 preferably has a layered rock salt type crystal structure. Examples of lithium composite oxides having such a crystal structure include lithium nickel cobalt manganese composite oxides and lithium nickel cobalt aluminum composite oxides. However, the crystal structure of the Ni-containing lithium transition metal composite oxide 2 may be a spinel structure or the like. The crystal structure can be confirmed by X-ray diffraction or the like.

[0019] In Fig. 1, the Ni-containing lithium transition metal composite oxide 2 is in the form of particles. The shape of the Ni-containing lithium transition metal composite oxide 2 is not particularly limited, but is generally spherical in this example. However, it may have an irregular shape, etc. In this specification, the term "approximately spherical" refers to a shape that can be regarded as roughly a sphere overall, and an average aspect ratio (ratio of major axis / minor axis) based on a cross-sectional observation image taken with an electron microscope is generally 1 to 2, for example, 1 to 1.5.

[0020] In some embodiments, the Ni-containing lithium transition metal composite oxide 2 is preferably in the form of secondary particles formed by agglomeration of multiple primary particles through physical or chemical bonding forces. The Ni-containing lithium transition metal composite oxide 2 (i.e., secondary particles) is preferably an aggregate of primary particles, in which a single particle is formed by the aggregation of multiple primary particles. In this specification, the term "primary particle" refers to the smallest particle unit constituting the Ni-containing lithium transition metal composite oxide 2, specifically the smallest unit determined from the geometric shape of its appearance. The number of primary particles constituting one secondary particle is typically 10 or more, for example, 20 or more, preferably 100 or more, and more preferably several hundred or more. The number of primary particles in one secondary particle can be confirmed by observing the secondary particle using an electron microscope (e.g., a scanning electron microscope) at a magnification of 10,000 to 30,000 times.

[0021] Although not particularly limited, when the Ni-containing lithium transition metal composite oxide 2 is in the form of secondary particles, the average particle diameter (D50) of the secondary particles is preferably 1 to 50 μm, more preferably 3 to 30 μm, from the viewpoint of improving battery characteristics (e.g., input / output characteristics and cycle characteristics). In this specification, the term "average particle diameter (D50)" refers to the median diameter (D50), and means the particle diameter corresponding to a cumulative frequency of 50% by volume from the side of fine particles with smaller particle diameters in a volume-based particle size distribution based on a laser diffraction / scattering method.

[0022] The average primary particle diameter of the primary particles constituting the secondary particles is preferably 0.1 to 2 μm or more, more preferably 0.2 to 1 μm, and particularly preferably 0.4 to 0.5 μm, from the viewpoint of suitably enhancing the mechanical strength of the secondary particles and the cycle characteristics of the battery. In this specification, the term "average primary particle diameter" refers to the average value of the major axes of a plurality of (e.g., 10 or more) primary particles grasped from the cross-sectional observation image and arbitrarily selected. The average primary particle diameter can be determined, for example, by using image analysis particle size distribution measurement software (e.g., "Mac-View") to determine the major axes of a plurality of primary particles arbitrarily selected from the cross-sectional observation image and then calculating the average value.

[0023] In some embodiments, the Ni-containing lithium transition metal composite oxide 2 may be in the form of a single particle. In this specification, the term "single particle" refers to a particle formed by the growth of a single crystal nucleus, and therefore a single crystalline particle that does not include a grain boundary. The number of primary particles constituting one single particle is typically less than 10, for example, about 2 to 5. The fact that the particle is a single crystal can be confirmed, for example, by analyzing an electron beam diffraction image using an electron microscope (e.g., a scanning electron microscope).

[0024] The coating layer 4 is attached to at least a portion of the surface of the Ni-containing lithium transition metal composite oxide 2, and constitutes at least a portion of the outer surface of the positive electrode active material 1. The coverage of the Ni-containing lithium transition metal composite oxide 2 with the coating layer 4 is preferably 50% or more, more preferably 80% or more, and particularly preferably 95% or more (i.e., substantially covering the entire Ni-containing lithium transition metal composite oxide 2). The fact that the coating layer 4 is attached to the surface of the secondary particles and the proportion of the area covered with the coating layer 4 (coverage) can be confirmed by, for example, subjecting the positive electrode active material 1 to X-ray photoelectron spectroscopy (XPS) analysis, which will be described later.

[0025] The coating layer 4 contains a composite oxide (hereinafter also referred to as "coating oxide") containing a first element and a second element. The first element is cobalt (Co) and / or manganese (Mn). The first element preferably contains Co, and more preferably is Co. This allows the input / output characteristics and cycle characteristics to be balanced at a higher level, thereby enabling the effects of the technology disclosed herein to be exerted at a higher level. The second element contains at least one element belonging to Group 6 (chromium group), such as chromium (Cr), molybdenum (Mo), and tungsten (W). Among these, it preferably contains W, and more preferably is W. This allows the input / output characteristics and cycle characteristics to be balanced at a higher level, thereby enabling the effects of the technology disclosed herein to be exerted at a higher level.

[0026] The coating oxide preferably has a composition represented by the following formula (II): Ae 1 Ae 2 O4(II) Ae in the above formula (II) 1 is the first element, and Ae 2 is the second element. Specific examples of the coating oxide include CoWO4, MnWO4, CoCrO4, MnCrO4, CoMoO4, and MnMoO4.

[0027] The coating layer 4 may be composed of the coating oxide described above, or, for example, a portion of the coating layer 4 close to the Ni-containing lithium transition metal composite oxide 2 may contain a compound (e.g., a Li compound) containing an element derived from the Ni-containing lithium transition metal composite oxide 2. The proportion of the coating oxide in the entire coating layer 4 is, on a mass basis, preferably 80 mass % or more, more preferably 90 mass % or more, and particularly preferably 95 mass % or more (i.e., the coating layer 4 is substantially composed of the coating oxide).

[0028] The coating layer 4 has an average thickness of 50 to 200 nm based on cross-sectional observation using a transmission electron microscope. By setting the average thickness to a predetermined value or more, reaction with the electrolyte can be suitably suppressed. As a result, growth of an SEI (Solid Electrolyte Interface) film is suppressed even during repeated charge and discharge, and the cycle characteristics of the battery can be suitably improved. From this perspective, the average thickness of the coating layer 4 is preferably 60 nm or more, more preferably 80 nm or more, and particularly preferably 100 nm or more. Furthermore, by setting the average thickness to a predetermined value or less, Li migration in the Ni-containing lithium transition metal composite oxide 2 is less likely to be inhibited, thereby reducing the resistance of the positive electrode and suitably improving the input / output characteristics of the battery. From this perspective, the average thickness of the coating layer 4 is preferably 190 nm or less, more preferably 170 nm or less, and particularly preferably 150 nm or less.

[0029] The coating layer 4 preferably does not substantially contain WO3. This makes it difficult for Li migration in the Ni-containing lithium transition metal composite oxide 2 to be inhibited, thereby improving battery characteristics (for example, energy density and input / output characteristics). The coating layer 4 preferably does not substantially contain LiWO4. This makes it possible to suppress elution of Li from the Ni-containing lithium transition metal composite oxide 2, thereby improving battery characteristics (for example, cycle characteristics).

[0030] Preferably, the coating layer 4 is substantially free of Ni element. Preferably, the coating layer 4 is substantially free of NiWO4 as described in Patent Document 1. According to the inventors' investigations, if NiWO4 is contained, a reaction between Ni and Li may occur in the firing step (2) of the manufacturing method described below, resulting in Ni migrating to the Li layer, a phenomenon known as cation mixing. By being substantially free of NiWO4, battery characteristics (e.g., energy density and cycle characteristics) can be further improved. In this specification, the term "substantially free" is used to mean that the coating layer 4 contains a trace amount of the target component, provided that it does not significantly impair the effects of the technology disclosed herein. For example, it means that the proportion of the target component in the coating layer 4 is less than 1% by mass.

[0031] When the coating layer 4 is substantially free of Ni, the positive electrode active material 1 is analyzed by XPS analysis in a region from the outermost surface to a depth of 10 nm. In this region, when the sum of the transition metal element contained in the Ni-containing lithium transition metal composite oxide 2 and the first and second elements contained in the coating layer 4 is taken as 100 mol%, the proportion of the Ni element is preferably approximately 3 mol% or less, and more preferably 1 mol% or less. In the XPS analysis, the Ni element is more preferably below the lower limit of detection. The proportion of the Ni element serves as an index representing the thickness of the coating layer 4. The proportion of the Ni element being a predetermined value or less (preferably below the lower limit of detection) indicates that the thickness of the coating layer 4 is 10 nm or more. The proportion of the Ni element being below the predetermined value further improves the cycle characteristics of the battery. Note that detailed measurement conditions for the XPS analysis are shown in the Examples section below.

[0032] When the coating layer 4 is substantially free of Ni, a 3 μm × 3 μm area on the surface of the positive electrode active material 1 is observed using a scanning electron microscope (SEM) and elemental mapping is performed using energy dispersive X-ray (EDX) spectrometry. Within this area, the proportion of Ni is preferably approximately 10 mol % or less, and more preferably 5 mol % or less, where the sum of the transition metal elements contained in the Ni-containing lithium transition metal composite oxide 2 and the first and second elements contained in the coating layer 4 is taken as 100 mol %. The proportion of Ni is an index of the proportion of Ni-containing lithium transition metal composite oxide 2 exposed on the outermost surface of the positive electrode active material 1, in other words, the variation in the coating of the coating layer 4. In other words, a proportion of Ni equal to or less than a predetermined value indicates a high coverage of the Ni-containing lithium transition metal composite oxide 2 with the coating layer 4, and that the coating layer 4 is evenly coated on the Ni-containing lithium transition metal composite oxide 2. By keeping the proportion of Ni element at or below the predetermined value, the cycle characteristics of the battery can be further improved. Note that detailed measurement conditions for the SEM-EDX analysis will be shown in the Examples section below.

[0033] [Method for producing positive electrode active material] The above-described positive electrode active material 1 can be suitably produced by a production method including, for example, (1) a mixing step and (2) a firing step in this order. The production method disclosed herein may further include other steps at any stage.

[0034] (1) The mixing step is a step of mixing a particulate Ni-containing lithium transition metal composite oxide 2 as a base material with a coating oxide source to obtain a mixture. The mixing method is not particularly limited, and conventionally known dry mixing methods or wet mixing methods can be employed. From the viewpoints of simplicity and cost-effectiveness, the dry mixing method is preferred. Dry mixing can be performed using conventionally known methods, such as a jet mill, a ball mill, a planetary mixer, a disperser, or a mortar. During dry mixing, ethanol may be added as a dispersant to more uniformly coat the coating oxide source. The particulate Ni-containing lithium transition metal composite oxide 2 can be produced by conventionally known methods (e.g., crystallization) or may be purchased as a commercially available product. The coating oxide source is a compound containing a first element selected from Co and Mn and a second element belonging to Group 6. The coating oxide source may be an oxide. Examples of the coating oxide source include CoWO4 and MnWO4.

[0035] The mixing ratio of the Ni-containing lithium transition metal composite oxide 2 and the coating oxide source may be determined so that the thickness of the coating layer 4 is a desired value, for example, 50 to 200 nm. Although this may vary depending on factors such as the average particle size of the Ni-containing lithium transition metal composite oxide 2, for example, to achieve a thickness of about 10 nm, it is preferable to add approximately 0.01 mass % (100 ppm) of the coating oxide source to the Ni-containing lithium transition metal composite oxide 2. To achieve a thickness of about 100 to 150 nm, it is preferable to add approximately 0.1 mass % (1000 ppm) of the coating oxide source to the Ni-containing lithium transition metal composite oxide 2. To achieve a thickness of about 1000 nm, it is preferable to add approximately 1 mass % (10000 ppm) of the coating oxide source to the Ni-containing lithium transition metal composite oxide 2.

[0036] (2) The calcination step is a step of calcining the mixture obtained in the mixing step in the presence of oxygen at a temperature of 500 to 800°C. According to the inventors' findings, a calcination temperature of 500°C or higher allows the coating oxide source to be suitably melted and fused to the surface of the Ni-containing lithium transition metal composite oxide 2. Therefore, compared to low-temperature coating at 150°C, as described in Patent Document 1, for example, unevenness in the formation of the coating layer 4 can be relatively reduced, increasing the coverage. Furthermore, a coating layer 4 with less variation can be formed. From this perspective, the calcination temperature is preferably 600°C or higher, more preferably 700°C or higher. The upper limit of the calcination temperature is preferably about 900°C or lower, for example, 850°C or lower, from the viewpoints of lithium reduction due to lithium volatilization during high-temperature calcination and secondary particle growth. The calcination time is preferably about 1 to 24 hours, preferably 5 to 12 hours. The temperature rise rate is preferably, for example, 5 to 40°C / min. The firing atmosphere is preferably an oxygen-containing atmosphere, for example, an oxygen atmosphere or an air atmosphere. In this manner, the coating oxide is adhered to the surface of the Ni-containing lithium transition metal composite oxide 2 as the base material, and the cathode active material 1 disclosed herein can be produced.

[0037] [Nonaqueous electrolyte secondary battery] Fig. 2 is a cross-sectional view schematically showing the internal structure of a nonaqueous electrolyte secondary battery 100 according to one embodiment. The nonaqueous electrolyte secondary battery 100 shown in Fig. 2 is a prismatic battery in which a flat electrode assembly 20 and a nonaqueous electrolyte 80 are housed and sealed in a flat, prismatic battery case 30. Note that Fig. 2 is an example and is not limited to what is shown. In other embodiments, the nonaqueous electrolyte secondary battery may be a coin type, a button type, a cylindrical type, a laminate case type, or the like.

[0038] The battery case 30 is an outer container that houses the electrode assembly 20 and the non-aqueous electrolyte 80. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum. On the outer surface of the battery case 30, a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, as well as a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level, are provided. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a, and the negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a.

[0039] The electrode assembly 20 here is a wound electrode assembly in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped with two strip-shaped separator sheets 70 interposed therebetween and wound in the longitudinal direction. However, in other embodiments, the electrode assembly may be a laminated electrode assembly in which a rectangular positive electrode and a rectangular negative electrode are stacked with a rectangular separator interposed therebetween. As shown in a partially cutaway view in FIG. 2, the positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a strip-shaped positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a strip-shaped negative electrode current collector 62.

[0040] At both ends of the electrode assembly 20 in the winding axis direction (i.e., the width direction perpendicular to the longitudinal direction), there are formed protruding outwardly: a positive electrode active material layer-free portion 52a where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed; and a negative electrode active material layer-free portion 62a where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a function as current collectors. A positive electrode current collector 42a and a negative electrode current collector 44a are provided on the positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a, respectively. The shapes of the positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are not limited to those shown in the drawings. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a may be formed as current collecting tabs processed into a predetermined shape.

[0041] The positive electrode current collector 52 is strip-shaped. The positive electrode current collector 52 is preferably made of metal, and more preferably made of metal foil. In this example, the positive electrode current collector 52 is aluminum foil. The positive electrode active material layer 54 contains at least the above-described positive electrode active material 1. The positive electrode active material layer 54 may contain a type of positive electrode active material other than the above-described positive electrode active material 1. When the total positive electrode active material contained in the positive electrode active material layer 54 is taken as 100% by mass, the proportion of the above-described positive electrode active material 1 is approximately 50% by mass or more, preferably 60% by mass or more, more preferably 80% by mass or more, for example, 85 to 100% by mass. This allows the effects of the technology disclosed herein to be exerted at a high level.

[0042] The positive electrode active material layer 54 may further contain additive components other than the positive electrode active material. Examples of additive components include a conductive material, a binder, and trilithium phosphate. Examples of conductive materials include carbon black such as acetylene black (AB) and carbon materials such as graphite. Examples of binders include fluorine-based resins such as polyvinylidene fluoride (PVdF).

[0043] Although not particularly limited, when the entire positive electrode active material layer 54 is taken as 100% by mass, the proportion of the positive electrode active material is preferably 70% by mass or more, more preferably 80 to 99% by mass, and even more preferably 85 to 98% by mass. The proportion of the conductive material is preferably 0.5 to 15% by mass, for example, 1 to 10% by mass, and even more preferably 1 to 5% by mass. The proportion of the binder is preferably 0.5 to 15% by mass, for example, 0.8 to 10% by mass, and even more preferably 1 to 5% by mass.

[0044] The negative electrode current collector 62 is strip-shaped. The negative electrode current collector 62 is preferably made of metal, and more preferably made of metal foil. In this example, the negative electrode current collector 62 is copper foil. The negative electrode active material layer 64 contains a negative electrode active material. Examples of the negative electrode active material that can be used include Si-containing materials such as Si, SiO (silicon oxide), and SiC (silicon carbide), as well as carbon materials such as graphite, hard carbon, and soft carbon. The negative electrode active material layer 64 may contain additional components other than the negative electrode active material. Examples of the additional components include a binder and a thickener. Examples of the binder include rubbers such as styrene butadiene rubber (SBR) and fluorine-based resins such as polyvinylidene fluoride (PVdF). Examples of the thickener include celluloses such as carboxymethyl cellulose (CMC).

[0045] The separator sheet 70 is strip-shaped. Examples of the separator sheet 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), and polyester. Such porous sheets may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator sheet 70.

[0046] The nonaqueous electrolyte 80 is typically a nonaqueous electrolytic solution containing a nonaqueous solvent and a supporting salt (electrolyte salt). However, in other embodiments, a polymer electrolyte may be used. As the nonaqueous solvent, various organic solvents such as carbonates, ethers, and esters that are used in the electrolytic solutions of general nonaqueous electrolyte secondary batteries can be used alone or in appropriate combinations of two or more. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). As the supporting salt, for example, lithium salts such as LiPF6 and LiBF4 can be used.

[0047] The nonaqueous electrolyte secondary battery 100 can be used for a variety of purposes, and because it combines high levels of input / output characteristics and cycle characteristics, it can be suitably used, for example, as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car, truck, etc. The type of vehicle is not particularly limited, and examples thereof include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and a battery electric vehicle (BEV).

[0048] Examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples.

[0049] <Preparation of positive electrode active material> [Examples 1 to 7] In Examples 1 to 7, a Ni-containing lithium transition metal composite oxide with a coating layer formed thereon was prepared as a positive electrode active material. That is, first, a Ni-containing lithium transition metal composite oxide having the following formula: Li 1.04 Ni 0.83 Co 0.06 Mn 0.1 A particulate lithium nickel cobalt manganese composite oxide (NCM) represented by the formula (1) was prepared. Next, the Ni-containing lithium transition metal composite oxide and the coated oxide source shown in Table 1 were dry-mixed (mixing step). The mixing ratio was adjusted so that the coating layer after firing had a thickness shown in Table 1. For example, in Example 3, the mixing ratio of the Ni-containing lithium transition metal composite oxide to the coated oxide source was 1.00 mol:0.0005 mol. Then, firing was performed at 790°C for 8 hours in an oxygen atmosphere to melt the coated oxide source and fuse the coating layer onto the surface of the Ni-containing lithium transition metal composite oxide (firing step). In this manner, a positive electrode active material was obtained.

[0050] Reference Example In the reference example, the lithium nickel cobalt manganese composite oxide (NCM) having the above composition was used as it was as the positive electrode active material.

[0051] <Evaluation of positive electrode active material 1 (XPS)> XPS analysis was performed under the following conditions for the positive electrode active materials of Examples 1 to 3, and 7 and the Reference Example. Then, composition analysis of the measured elements was performed, and the amount (mol) of each element present in the region from the outermost surface to a depth of 10 nm was calculated. Using this value, the proportion (mol%) of Ni element was calculated from the following formula: {Ni / (W+Ni+Co+Mn)}×100. The results are shown in Table 1. In Table 1, "ND" indicates that the Ni element was below the lower limit of detection. Equipment: PHI 5000 VersaProbe II (manufactured by ULVAC-PHI) X-ray source: AlKα monochromatic light Irradiation range: φ100μm HP (1400×200) Current / Voltage: 100W, 20kV

[0052] <Evaluation of positive electrode active material 2 (SEM-EDX)> SEM-EDX analysis was performed on the positive electrode active materials of Examples 1 to 3, and 7 and the Reference Example. Specifically, a designated area (3 μm long × 3 μm wide) on the surface of the positive electrode active material was first observed under the following conditions with an SEM, and an SEM image was obtained. As an example, FIG. 3 shows a cross-sectional SEM image of the positive electrode active material of Example 1. Equipment: SU1000 (Hitachi Electron Microscope Systems) Acceleration voltage: 3 to 5 kV Spot Intensity: 90 Focus distance: 10mm

[0053] Next, distances A and B were measured from the obtained SEM observation image using image analysis software according to the following procedure, and the thickness of the coating layer (= distance A - distance B) was calculated. The results are shown in Table 1. Distance A: The distance from the center of the positive electrode active material to the surface of the coating layer. Distance B: The distance from the center of the positive electrode active material to the surface of the lithium nickel cobalt manganese composite oxide.

[0054] Next, a designated area of ​​the obtained SEM observation image was subjected to EDX analysis under the following conditions, and elemental mapping was performed for each of the elements Ni, Co, Mn, and W, and the amount of each element present was calculated. Using this value, the proportion of Ni element (mol%) was calculated using the following formula: {Ni / (W+Ni+Co+Mn)}×100. The results are shown in Table 1. Note that "ND" in Table 1 indicates that the Ni element was below the lower limit of detection. ·Device: JSM-7800 (manufactured by JEOL) Number of times accumulated: 3 times

[0055] <Preparation of Lithium-ion Secondary Batteries for Evaluation> The positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed in a mass ratio of positive electrode active material:AB:PVdF = 100:1:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode active material layer-forming slurry. This positive electrode active material layer-forming slurry was applied to an aluminum foil (positive electrode current collector) and dried to form a positive electrode active material layer. The positive electrode active material layer was then roll-pressed with a rolling roller and cut to a predetermined size to prepare a positive electrode sheet.

[0056] In addition, a Si-containing material (SiO) and graphite (C) as negative electrode active materials, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in a mass ratio of (SiO + C):SBR:CMC = 100:1:1, and an appropriate amount of ion-exchanged water was added to prepare a slurry for forming a negative electrode active material layer. This slurry for forming a negative electrode active material layer was applied to a copper foil (negative electrode current collector) and dried to form a negative electrode active material layer. The negative electrode active material layer was then roll-pressed with a rolling roller and cut to the specified dimensions to produce a negative electrode sheet.

[0057] A porous polyolefin sheet with a three-layer structure of PP / PE / PP was prepared as a separator. Next, the positive electrode sheet and the negative electrode sheet were stacked with the separator interposed between them to prepare an electrode assembly. Next, an electrode terminal was attached to the electrode assembly, which was then inserted into a battery case made of aluminum laminate film, and a non-aqueous electrolyte was poured into it. The non-aqueous electrolyte used was a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC = 1:3, with LiPF6 dissolved as a supporting electrolyte at a concentration of 1 mol / L. The battery case was then sealed to obtain a lithium-ion secondary battery for evaluation.

[0058] <Evaluation of initial characteristics (initial capacitance and initial resistance)> Each lithium-ion secondary battery for evaluation was charged at a constant current of 0.1 C to 4.2 V at a charge rate of 0.1 C under a 25°C environment, and then discharged at a constant current of 0.1 C to 3.0 V. The discharge capacity during constant current discharge was then divided by the weight of the positive electrode active material to determine the initial discharge capacity (Ah / g). The results are shown in Table 1. In addition to the discharge capacity values, Table 1 also indicates the initial discharge capacity as follows: "◎" if the initial discharge capacity was 190 mAh / g or greater, "◯" if the initial discharge capacity was 180 mAh / g or greater but less than 190 mAh / g, and "×" if the initial discharge capacity was less than 180 mAh / g.

[0059] Next, the battery was adjusted to a SOC of 50% in a 25°C environment, and after resting for 1 hour, it was discharged at a constant current of 5 C for 10 seconds. The initial resistance was calculated from the open circuit voltage (V0) immediately before discharge and the closed circuit voltage (V1) after 10 seconds of discharge using the following formula: resistance = (V0 - V1) / current value at 5 C.

[0060] <Evaluation of cycle characteristics> Each lithium-ion secondary battery for evaluation was subjected to 100 high-rate charge-discharge cycles, where one cycle consisted of constant-current charging at a charge rate of 0.5 C to 4.3 V, followed by constant-current discharging at a discharge rate of 0.5 C to 2.8 V, in an environment of 25°C. The discharge capacity after the cycle test was determined in the same manner as for the initial discharge capacity. The results are shown in Table 1. The capacity retention rate (%) was calculated using the following formula: capacity retention rate = (discharge capacity after cycle test / initial discharge capacity) × 100. The results are shown in Table 1. In addition to the capacity retention rate, Table 1 also indicates the following: "◎" for a value of 90% or more, "◯" for a value of 85% or more but less than 90%, and "×" for a value of less than 85%.

[0061] Next, the resistance after the cycle test was determined in the same manner as the initial resistance, and the resistance increase rate (%) was calculated using the following formula: resistance increase rate = [(resistance after cycle test - initial resistance) / initial resistance] x 100. The results are shown in Table 1. In addition to the resistance increase rate value, Table 1 also shows the following: "x" if it is 50% or more, "o" if it is 31% or more but less than 50%, and "◎" if it is 31% or less.

[0062] [Table 1]

[0063] The results in Table 1 show that the Reference Example and Example 1 showed a significant decrease in battery characteristics after cycling. Example 7 showed a low initial discharge capacity. In contrast, Examples 2 to 6 showed a relatively high initial discharge capacity and suppressed the decrease in battery characteristics after cycling. These results demonstrate the technical significance of the invention disclosed herein.

[0064] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0065] For example, in the above-described embodiment, the Ni-containing lithium transition metal composite oxide and the coated oxide source are mixed by a dry mixing method, but this is not limiting. In another embodiment, the Ni-containing lithium transition metal composite oxide and the coated oxide source may be mixed by a wet mixing method. In this case, the (1) mixing step may include (1-1) a first mixing step, (1-2) a second mixing step, and (1-3) a drying step, in this order. Furthermore, other steps may be included at any stage. For example, the (1-3) drying step may be followed by (1-4) a particle size adjusting step.

[0066] (1-1) In the first mixing step, an aqueous solution containing a first element source (typically a water-soluble ionic compound) is prepared. The aqueous solution can be prepared by dissolving the first element source (Co source and / or Mn source) in an aqueous solvent. The aqueous solvent is typically water, but it may also be a mixed solvent mainly composed of water. The solvent other than water that constitutes the mixed solvent can be an organic solvent that is uniformly miscible with water, such as a lower alcohol or a lower ketone. The anion of the first element source can be appropriately selected so that the first element source is water-soluble, and can be, for example, acetate ion, sulfate ion, nitrate ion, carbonate ion, hydroxide ion, chloride ion, etc. Among these, acetate ion is preferred. The concentration of the aqueous solution is preferably prepared so that the total concentration of the first metal element (Co and / or Mn) is approximately 1 to 3 mol / L.

[0067] (1-2) In the second mixing step, a hydroxide precursor is formed on the surface of a particulate Ni-containing lithium transition metal composite oxide by a conventionally known crystallization method. Specifically, a reaction solution is prepared by adding a particulate Ni-containing lithium transition metal composite oxide, the aqueous solution containing the first element source prepared above, and an alkaline compound of the second element to a reaction vessel. The alkaline compound preferably contains a strong base (such as a hydroxide of an alkali metal) and / or a weak base (such as ammonia) that does not inhibit the formation of the hydroxide. A specific example is an ammonium salt of the second element. Among these, ammonium tungstate is preferred. The ammonium ion concentration in the reaction solution is preferably adjusted to approximately 0 to 30 g / L, preferably 5 to 25 g / L, e.g., 5 to 20 g / L.

[0068] Next, the prepared reaction solution is stirred and mixed under alkaline conditions (pH > 7). Stirring and mixing can be performed, for example, by ultrasonic irradiation or using a magnetic stirrer. The pH of the reaction solution is generally 10≦pH≦14, preferably 11≦pH≦14, for example, 11≦pH≦12. The stirring speed is generally 400 rpm or higher, preferably 600 rpm or higher, more preferably 800 rpm or higher, for example, 1000 to 1500 rpm. This causes a hydroxide containing the first element and the second element to precipitate (crystallize) on the surface of the particulate Ni-containing lithium transition metal composite oxide, thereby obtaining a precursor.

[0069] (1-3) In the drying step, after the hydroxide precipitation is complete, the precursor is isolated, washed, and then dried. Isolation can be performed by a conventionally known method, for example, a solid-liquid separation method such as centrifugation, filtration, or decantation, or a spray drying method. The isolated precursor is then washed with water or the like to remove unreacted raw material compounds and impurities. Drying can be performed by a conventionally known method, for example, natural drying, heat drying, air drying, vacuum drying, etc. The drying temperature is typically above the boiling point of the aqueous solvent, specifically, approximately 100°C or less, for example, 60 to 100°C is preferred.

[0070] In the (1-4) particle size adjustment step, the particle size of the dried precursor is adjusted. In one example, precursor agglomerations are broken down by a conventionally known method. In another example, precursors of a predetermined particle size are selected through a sieve (a mesh member). The mixture obtained in this manner can be subjected to the (2) firing step of the above-described embodiment.

[0071] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A positive electrode active material comprising: a particulate Ni-containing lithium transition metal composite oxide; and a coating layer attached to at least a part of the surface of the Ni-containing lithium transition metal composite oxide, wherein the coating layer contains a composite oxide containing a first element selected from Co and Mn and a second element belonging to Group 6, and has an average thickness of 50 nm or more and 200 nm or less based on cross-sectional observation with a transmission electron microscope. Item 2: The positive electrode active material according to Item 1, wherein the coating layer is substantially free of Ni, and when a region from the outermost surface of the positive electrode active material to a depth of 10 nm is measured by X-ray photoelectron spectroscopy, the proportion of Ni element in the region is 1 mol % or less when the total of the transition metal element contained in the Ni-containing lithium transition metal composite oxide and the first element and the second element contained in the coating layer is taken as 100 mol %. Item 3: The positive electrode active material according to Item 1 or 2, wherein the coating layer is substantially free of Ni, and when an area of ​​3 μm lengthwise and 3 μm widthwise on the surface of the positive electrode active material is observed with a scanning electron microscope and elemental mapping is performed with energy dispersive X-ray analysis, it is found that, in the area, when the total of the transition metal element contained in the Ni-containing lithium transition metal composite oxide and the first element and the second element contained in the coating layer is taken as 100 mol %, a proportion of Ni element is 5 mol % or less. Item 4: The positive electrode active material according to any one of Items 1 to 3, wherein the Ni-containing lithium transition metal composite oxide is a lithium nickel cobalt manganese composite oxide. Item 5: The positive electrode active material according to any one of Items 1 to 4, wherein the first element is Co. Item 6: The positive electrode active material according to any one of Items 1 to 5, wherein the second element is W. Item 7: A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode contains the positive electrode active material according to any one of items 1 to 6. Item 8: A method for producing a positive electrode active material, comprising: a mixing step of mixing a particulate Ni-containing lithium transition metal composite oxide with a compound containing a first element selected from Co and Mn and a second element belonging to Group 6 to obtain a mixture; and a firing step of firing the mixture in the presence of oxygen at a temperature of 500°C or higher and 800°C or lower. [Explanation of symbols]

[0072] 1 Cathode active material 2. Ni-containing lithium transition metal composite oxide 4 Covering layer 20 Electrode body 30 Battery case 50 Positive electrode sheet (positive electrode) 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 64 Negative electrode active material layer 70 Separator Sheet 80 Non-aqueous electrolyte 100 Nonaqueous electrolyte secondary battery

Claims

1. A positive electrode active material comprising a particulate Ni-containing lithium transition metal composite oxide and a coating layer attached to at least a portion of the surface of the Ni-containing lithium transition metal composite oxide, The coating layer is The present invention relates to an oxide containing a first element selected from Co and Mn, and tungsten (W) as a second element belonging to Group 6, and represented by the following formula: Ae 1 WO 4 (wherein Ae 1 is the first element); the positive electrode active material is substantially free of Ni, and when a region from the outermost surface to a depth of 10 nm of the positive electrode active material is measured by X-ray photoelectron spectroscopy, the proportion of Ni element in the region is 1 mol % or less when the total of the transition metal element contained in the Ni-containing lithium transition metal composite oxide and the first element and the second element contained in the coating layer is taken as 100 mol %, A positive electrode active material having an average thickness of 50 nm or more and 200 nm or less, as determined by cross-sectional observation using a transmission electron microscope.

2. the coating layer is substantially free of Ni, an area of ​​3 μm lengthwise × 3 μm widthwise on the surface of the positive electrode active material is observed with a scanning electron microscope, and element mapping is performed by energy dispersive X-ray analysis, and it is found that, in the area, when the total of the transition metal element contained in the Ni-containing lithium transition metal composite oxide and the first element and the second element contained in the coating layer is taken as 100 mol %, the proportion of Ni element is 5 mol % or less; The positive electrode active material according to claim 1 .

3. The Ni-containing lithium transition metal composite oxide is a lithium nickel cobalt manganese composite oxide. The positive electrode active material according to claim 1 or 2.

4. The first element is Co. The positive electrode active material according to claim 1 or 2.

5. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode comprises the positive electrode active material according to claim 1 or 2. Nonaqueous electrolyte secondary battery.

6. A method for producing the positive electrode active material according to claim 1 or 2, comprising: a mixing step of mixing a particulate Ni-containing lithium transition metal composite oxide with a compound containing a first element selected from Co and Mn and W to obtain a mixture; a firing step of firing the mixture at a temperature of 500°C or higher and 800°C or lower in the presence of oxygen; A method for producing a positive electrode active material, comprising:

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