Positive electrode active material, positive electrode plate, and nonaqueous electrolyte secondary battery

Abstract

To provide a positive electrode active material capable of obtaining a nonaqueous electrolyte secondary battery having excellent discharge capacity and excellent cycle characteristics.SOLUTION: A positive electrode active material includes a secondary particle 1 in which a primary particle 10 is aggregated. The secondary particle is lithium transition metal composite oxides having a layered crystal structure. When a direction from a center to an outer periphery of the secondary particle is defined as a first direction and a direction orthogonal to the first direction is defined as a second direction, a ratio (L11 / L12) of a maximum length L11 in the first direction to a maximum length L12 in the second direction in the first particle 11 is 0.5 to 1.10. A ratio (L21 / L22) of a maximum length L21 in the first direction to a maximum length L22 in the second direction in the second particle 12 is 1.40 or more. The second particle includes oriented particles whose major axis direction is radially oriented in the first direction and whose maximum length L21 is 640 nm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Secondary particles formed by aggregation of primary particles are sometimes used as a positive electrode active material. Patent Document 1 discloses the use of a positive electrode active material in which primary particles with a high aspect ratio and primary particles with a low aspect ratio are mixed within the same secondary particle in order to improve the cycle characteristics of a nonaqueous electrolyte secondary battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2016 / 002158 Summary of the Invention [Problem to be solved by the invention]

[0004] When a positive electrode active material in which two types of primary particles with different aspect ratios are mixed within a secondary particle is used, the discharge capacity and cycle characteristics of the non-aqueous electrolyte secondary battery may be reduced.

[0005] An object of the present disclosure is to provide a positive electrode active material that can provide a nonaqueous electrolyte secondary battery having excellent discharge capacity and excellent cycle characteristics, a positive electrode plate using the same, and a nonaqueous electrolyte secondary battery including the positive electrode plate. [Means for solving the problem]

[0006] [1] A positive electrode active material containing secondary particles formed by aggregation of primary particles, the secondary particles are lithium transition metal composite oxides having a layered crystal structure, The lithium transition metal composite oxide is Li, Ni, Mn, Co, and M [M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, and W.] are included, and the molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M = a:x:y:z:t [a, x, y, z, and t satisfy 1.0 ≦ a ≦ 1.3, x + y + z + t = 1, 0.25 ≦ x ≦ 0.9, 0 < y ≦ 0.6, 0 < z ≦ 0.6, and 0 ≦ t ≦ 0.1]. In the cross-sectional image obtained by observing the cross-section of the secondary particles, when the primary particles are classified into the first particles constituting the outer periphery of the secondary particles and the second particles other than the first particles, and the direction from the center to the outer periphery of the secondary particles is defined as the first direction and the direction orthogonal to the first direction is defined as the second direction, the ratio (L11 / L12) of the maximum length L11 in the first direction to the maximum length L12 in the second direction in the first particles is 0.5 to 1.10. the ratio (L21 / L22) of the maximum length L21 in the first direction to the maximum length L22 in the second direction in the second particles is 1.40 or more. The second particles include oriented particles whose long axis direction is radially oriented in the first direction and whose maximum length L21 is 640 nm or less, and it is a positive electrode active material. 〔2〕 The positive electrode active material according to 〔1〕, wherein the maximum length L21 of the oriented particles is 600 nm or less. 〔3〕 The positive electrode active material according to 〔1〕 or 〔2〕, wherein the ratio (L21 / L22) in the second particles is 1.50 to 3.00. 〔4〕 The positive electrode active material according to any one of 〔1〕 to 〔3〕, wherein the ferret diameter of the first particles is 270 nm or less. 〔5〕 The positive electrode active material according to any one of 〔1〕 to 〔4〕, wherein the maximum length L21 of the second particles is 640 nm or less. 〔6〕 The positive electrode active material according to any one of 〔1〕 to 〔5〕, wherein 50 or more of the primary particles are aggregated in the secondary particles. 〔7〕 A positive electrode plate having a positive electrode active material layer containing the positive electrode active material according to any one of 〔1〕 to 〔6〕. [8] A non-aqueous electrolyte secondary battery comprising the positive electrode plate according to [7]. [Effects of the Invention]

[0007] A non-aqueous electrolyte secondary battery obtained using the positive electrode active material of the present disclosure has excellent discharge capacity and cycle characteristics. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a schematic diagram of secondary particles contained in a positive electrode active material according to an embodiment. [Figure 2] FIG. 2 is an explanatory diagram for explaining directions determined for secondary particles. DETAILED DESCRIPTION OF THE INVENTION

[0009] In this specification, unless otherwise specified, a numerical range such as "m to n" includes both the upper and lower limits. That is, "m to n" represents a numerical range of "m or more and n or less." A numerical value arbitrarily selected from within the numerical range may be set as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described elsewhere in this specification, in a table, or in a figure.

[0010] In the drawings, in order to facilitate understanding of the invention, some parts of each component in the drawings are shown emphasized or simplified, and the structure, shape, scale, etc. of each component in the drawings may be changed from the actual configuration.

[0011] (Cathode active material) Fig. 1 is a schematic diagram of secondary particles contained in a positive electrode active material according to an embodiment. Fig. 2 is an explanatory diagram for explaining the direction determined for the secondary particles. The positive electrode active material according to this embodiment is used in a positive electrode plate of a non-aqueous electrolyte secondary battery (hereinafter also referred to as a "secondary battery") such as a lithium ion battery.

[0012] As shown in FIG. 1, the positive electrode active material contains secondary particles 1 formed by aggregation of primary particles 10. Preferably, 50 or more primary particles 10 are aggregated in the secondary particles 1. The number of aggregated primary particles 10 in the secondary particles 1 may be 100 or more, may be 1000 or more, and is usually 5×10 6 or less, and may be 5×10 5 or less. The number of aggregated primary particles 10 can be adjusted by the firing conditions (firing temperature, number of firings, firing time, etc.) when manufacturing the secondary particles 1. The number of aggregated primary particles 10 contained in the secondary particles 1 can be confirmed by, for example, a SEM image obtained by observation with a scanning electron microscope (hereinafter also referred to as "SEM").

[0013] The secondary particles 1 are a lithium transition metal composite oxide having a layered crystal structure (hereinafter also referred to as "composite oxide"). That the composite oxide has a layered crystal structure can be confirmed by, for example, measurement by X-ray diffraction method (XRD). Examples of the layered crystal structure of the composite oxide include a hexagonal crystal structure (layered rock salt type) or a monoclinic crystal structure. The composite oxide having a layered crystal structure easily allows lithium ions to be smoothly inserted and released.

[0014] The composite oxide contains Li, Ni, Mn, Co, and M [M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, and W.], and the molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M = a:x:y:z:t [a, x, y, z, and t satisfy 1.0≦a≦1.3, x + y + z + t = 1, 0.25≦x≦0.9, 0 < y≦0.6, 0 < z≦0.6, and 0≦t≦0.1].

[0015] The metal element M contained in the composite oxide only needs to contain one or more of the above-described metal elements, and preferably contains at least W (tungsten). When the metal element M contains W, it becomes easier to obtain a positive electrode active material containing secondary particles 1 having the first particles 11 and the second particles 12 described later.

[0016] The molar ratio of Li is 1.0 ≤ a ≤ 1.3, and it may be 1.0 ≤ a ≤ 1.25, 1.01 ≤ a ≤ 1.2, 1.03 ≤ a ≤ 1.15, or 1.04 ≤ a ≤ 1.1. The molar ratio of Ni is 0.25 ≤ x ≤ 0.9, and it may be 0.3 ≤ x ≤ 0.9, 0.4 ≤ x ≤ 0.88, or 0.5 ≤ x ≤ 0.85. The molar ratio of Mn is 0 < y ≤ 0.6, and it may be 0.05 ≤ y ≤ 0.5, 0.08 ≤ y ≤ 0.3, or 0.10 ≤ y ≤ 0.2. The molar ratio of Co is 0 < z ≤ 0.6, and it may be 0 < z ≤ 0.5, 0.01 ≤ z ≤ 0.3, or 0.02 ≤ z ≤ 0.1. The molar ratio of M is 0 ≤ t ≤ 0.1, and it may be 0 < t ≤ 0.08, 0.001 ≤ t ≤ 0.05, or 0.002 ≤ t ≤ 0.01. When the positive electrode active material contains two or more metal elements M, the molar ratio of M refers to the total amount of the two or more metal elements.

[0017] The composition of the composite oxide can be adjusted by the types of raw materials used and the blending amounts of the raw materials when manufacturing the composite oxide. The composition of the composite oxide can be determined by ICP (Inductively Coupled Plasma) optical emission spectrometry (ICP-AES). More specifically, it can be measured in accordance with the general rules for optical emission spectrometry in JIS K 0116:2014. For example, using a high-resolution ICP optical emission spectrometer (PS3500DDII manufactured by Hitachi High-Technologies Corporation), the composite oxide is dissolved by an alkali fusion method and diluted to a predetermined amount with ultrapure water, tartaric acid, or hydrochloric acid for analysis. The measurement wavelengths of each element by ICP-AES can be Li: 670.784 nm, Co: 238.892 nm, Mn: 257.61 nm, and Ni: 231.604 nm.

[0018] As shown in Fig. 1, secondary particles 1 contained in the positive electrode active material include the following primary particles 10. In a cross-sectional image obtained by observing a cross section of secondary particle 1, [i] primary particles 10 are classified into first particles 11 that form the periphery of secondary particle 1 and second particles 12 other than first particles 11 (Fig. 1), and [ii] when the direction from center c of secondary particle 1 toward the periphery is defined as first direction d1 and the direction perpendicular to first direction d1 is defined as second direction d2 (Fig. 2), first particles 11 and second particles 12 satisfy the following relationship: 1st particle 11: The ratio (L11 / L12) of the maximum length L11 in the first direction d1 to the maximum length L12 in the second direction d2 of the first particle 11 is 0.50 to 1.10. 2nd particle 12: the ratio (L21 / L22) of the maximum length L21 in the first direction d1 to the maximum length L22 in the second direction d2 of the second particle 12 is 1.40 or more; The second particles 12 include oriented particles whose major axis directions are aligned radially in the first direction d1 and whose maximum length L21 is 640 nm or less.

[0019] In this specification, the cross-sectional image of the secondary particle 1 is an SEM image obtained by SEM observation.

[0020] The first particle 11 refers to a primary particle 10 in which at least a portion of the periphery of the primary particle 10 forms part of the periphery of the secondary particle 1 in a cross-sectional image of the secondary particle 1. The second particle 12 refers to a primary particle 10 other than the first particle 11. In other words, the second particle 12 is a primary particle 10 in which the periphery of the primary particle 10 does not form the periphery of the secondary particle 1 in a cross-sectional image. The arrangement of the first particle 11 and the second particle 12 within the secondary particle 1 can be adjusted by the firing conditions (firing temperature, number of firings, firing time, etc.) when producing the secondary particle 1.

[0021] As shown in FIG. 2 , the first direction d1 is the direction from the center c of the secondary particle 1 toward the outer periphery in the cross-sectional image of the secondary particle 1, i.e., the radial direction of the secondary particle 1. The first particle 11 and the second particle 12 may have multiple lengths in the first direction d1 and the second direction d2, and the maximum lengths L11, L12, L21, and L22 refer to the maximum lengths among the multiple lengths. That is, the maximum length L11 of the first particle 11 in the first direction d1 refers to the maximum length among the lengths of the first particle 11 in the first direction d1 in the cross-sectional image. The maximum length L12 of the first particle 11 in the second direction d2 refers to the maximum length among the lengths of the first particle 11 in the second direction d2 perpendicular to the first direction d1 that defines the maximum length L11 in the cross-sectional image. Similarly, the maximum length L21 of the second particle 12 in the first direction d1 refers to the maximum length among the lengths of the second particle 12 in the first direction d1 in the cross-sectional image. The maximum length L22 of the second particle 12 in the second direction d2 refers to the maximum length of the second particle 12 in the cross-sectional image in the second direction d2 that is perpendicular to the first direction d1 in which the maximum length L21 is determined. As will be described in the examples below, the maximum lengths L11, L12, L21, and L22 are each calculated as the average value of 20 or more target primary particles 10 (first particles 11 or second particles 12) selected from the cross-sectional image.

[0022] The ratio (L11 / L12) of the primary particles 11 is 0.50 to 1.10, and may be 0.55 to 1.08, 0.60 to 1.05, or 0.70 to 1.00. Since the ratio (L11 / L12) of the primary particles 11 is within the above range, it can be said that the primary particles 11 have an isotropic shape, that is, a shape close to a sphere. The ratio (L11 / L12) of the primary particles 11 can be adjusted by the firing conditions (firing temperature, number of firings, firing time, etc.) when producing the secondary particles 1.

[0023] The ratio (L21 / L22) of the second particles 12 is 1.40 or more, and may be 1.40 to 3.00, preferably 1.50 to 3.00, 1.70 to 2.90, 2.00 to 2.80, or 2.20 to 2.70. Since the ratio (L21 / L22) of the second particles 12 is within the above range, it can be said that the second particles 12 have a highly anisotropic shape, that is, a shape that is different from a sphere such as an oval sphere. The ratio (L21 / L22) of the second particles 12 can be adjusted by the firing conditions (firing temperature, number of firings, firing time, etc.) when producing the secondary particles 1.

[0024] The long axis direction d3 of the second particle 12 refers to the longest length among the lengths of straight lines connecting any two points on the periphery of the second particle 12 in the cross-sectional image of the secondary particle 1. "The long axis direction is radially oriented in the first direction d1" means that the second particles 12 are arranged within the secondary particle 1 so that the smaller angle between the long axis direction d3 of the second particle 12 and the first direction d1 is within 45°. The angle is preferably within 35°, more preferably within 30°, and may be within 20°, or may be within 10°.

[0025] The second particles 12 include oriented particles. The oriented particles are particles of the second particles 12 whose long axis direction d3 is radially oriented in the first direction d1 in a cross-sectional image of the secondary particles 1, and whose maximum length L21 is 640 nm or less. The maximum length L21 of the oriented particles is preferably 600 nm or less, and may be 550 nm or less, 500 nm or less, or 450 nm or less. The maximum length L21 may be 200 to 640 nm, 250 to 600 nm, 300 to 550 nm, or 350 to 500 nm. The oriented particles can be formed by adjusting the firing conditions (firing temperature, number of firings, firing time, etc.) when producing the secondary particles 1.

[0026] When the area occupied by the second particles 12 in the cross-sectional image of the secondary particle 1 is taken as 100%, the proportion of the area occupied by the oriented particles in the second particle 12 is preferably 80% or more, may be 85% or more, may be 90% or more, may be 80 to 100%, may be 85 to 98%, or may be 90 to 95%.

[0027] The maximum length L21 of the secondary particles 12 is preferably 640 nm or less, preferably 600 nm or less, and may be 550 nm or less, 500 nm or less, or 450 nm or less. The secondary particles 12 include oriented particles as well as particles other than oriented particles. The maximum length L21 may be 200 to 640 nm, 250 to 600 nm, 300 to 550 nm, or 350 to 500 nm. The maximum length L21 of the secondary particles 12 can be adjusted by the firing conditions (firing temperature, number of firings, firing time, etc.) when producing the secondary particles 1.

[0028] As described above, the secondary particle 1 has the second particles 12 arranged on its inner side, and the second particles 12 have radially oriented particles. Because the ratio (L21 / L22) is within the above-described range, the second particles 12 have a highly anisotropic shape and tend to expand and contract significantly in the minor axis direction during charging and discharging of the secondary battery. Because the second particles 12 are arranged on the inner side of the secondary particle 1 and have radially oriented particles, the direction of expansion and contraction of the second particles 12 tends to be aligned circumferentially. Therefore, stresses generated within the secondary particle 1 during expansion and contraction of the second particles 12 are easily alleviated by being canceled out by other stresses. Therefore, even if the second particles 12 are arranged on the inner side of the secondary particle 1, the stresses generated by the expansion and contraction of the second particles 12 can be alleviated. This improves the cycle characteristics of the secondary battery.

[0029] Since the ratio (L11 / L12) is within the above-mentioned range, the first particles 11 have an isotropic shape. The expansion and contraction of the first particles 11 during charging and discharging of the secondary battery is less likely to be anisotropic and more likely to be isotropic. This means that stress generated within the secondary particles 1 during expansion and contraction of the first particles 11 is less likely to be alleviated. The primary particles 10 constituting the outer surface of the secondary particles 1 are less likely to be in contact with each other than the primary particles 10 arranged inside the secondary particles 1. Therefore, on the outer surface side of the secondary particles 1, the expansion and contraction of the primary particles 10 toward the outer surface is less restricted than on the inner side, thereby reducing the influence of stress associated with expansion and contraction. Therefore, in the secondary particles 1, the first particles 11, which tend to expand and contract isotropically, are arranged to form the outer surface of the secondary particles 1, thereby reducing the influence of stress associated with expansion and contraction of the first particles. This improves the cycle characteristics of the secondary battery.

[0030] In the secondary particle 1, the first particles 11 having an isotropic shape are arranged so as to form the outer peripheral surface, which makes it easy to increase the specific surface area of ​​the secondary particle 1. This makes it possible to suppress a decrease in the discharge capacity of the secondary battery. It also makes it easy to suppress a deterioration in the resistance of the secondary battery.

[0031] The particle diameter of the first particles 11 is not particularly limited, but is preferably small from the viewpoint of increasing the specific surface area of ​​the secondary particles 1. This can suppress a decrease in the discharge capacity of the secondary battery and also suppress a deterioration in the resistance of the secondary battery. In a surface image obtained by observing the surface of the secondary particles, the Feret diameter of the first particles 11 may be 340 nm or less, or may be 300 nm or less, preferably 270 nm or less, more preferably 250 nm or less, or may be 230 nm or less. The Feret diameter of the first particles 11 is, for example, 150 to 340 nm, or may be 150 to 270 nm, or may be 180 to 250 nm, or may be 200 to 230 nm. When the Feret diameter of the first particles 11 is within the above range, a decrease in the discharge capacity of the secondary battery and a deterioration in the resistance of the secondary battery are easily suppressed.

[0032] In this specification, the surface image of the secondary particle 1 is an SEM image obtained by observation with an SEM. In this specification, the Feret diameter refers to the distance between parallel tangents to the outline of the first particle 11 in the surface image of the secondary particle 1, where the shortest distance between the tangents is the longest, and is calculated as the average value of 200 or more first particles 11 selected from the surface image. The number of first particles 11 selected from the surface image may be 400 or more. The Feret diameter of the first particles 11 can be adjusted by the firing conditions (firing temperature, number of firings, firing time, etc.) when producing the secondary particle 1.

[0033] The positive electrode active material containing the secondary particles 1 can be obtained, for example, by mixing a compound containing Ni, Mn, and Co (hereinafter also referred to as a "NiMnCo-containing precursor"), a lithium compound, and, if necessary, an M-containing compound containing the metal element M to obtain a mixture, and then firing this mixture. Alternatively, the positive electrode active material may be obtained by mixing a Ni-containing compound containing Ni, a Mn-containing compound containing Mn, a Co-containing compound containing Co, a lithium compound, and an M-containing compound containing the metal element M to obtain a mixture, and then firing this mixture. The Ni-containing compound, the Mn-containing compound, and the Co-containing compound may contain the metal element M.

[0034] Examples of NiMnCo precursors include composite oxides or composite hydroxides containing Ni, Mn, and Co. Examples of lithium compounds include lithium hydroxide and lithium carbonate. Examples of M-containing compounds include ammonium compounds containing the metal element M.

[0035] (positive electrode plate) The positive electrode plate of this embodiment is formed using the above-described positive electrode active material. The positive electrode plate of this embodiment can provide a nonaqueous electrolyte secondary battery that is excellent in capacity retention rate during charge-discharge cycles and can suppress a decrease in discharge capacity.

[0036] The positive electrode plate can have a positive electrode current collector foil and a positive electrode active material formed on one or both sides of the positive electrode current collector foil. The positive electrode active material is contained in a positive electrode active material layer, and the positive electrode active material layer can further include at least one of a binder and a conductive additive. The positive electrode active material layer can be formed by applying a positive electrode mixture slurry to the positive electrode current collector foil, drying, and compressing the slurry.

[0037] The positive electrode current collector foil is, for example, a metal foil made of an aluminum material such as aluminum or an aluminum alloy.

[0038] Examples of binders include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; cellulose-based resins such as carboxymethyl cellulose (CMC), methyl cellulose (MC), and hydroxypropyl cellulose; and styrene butadiene rubber (SBR), and one or more of these can be used.

[0039] Examples of the conductive additive include carbon materials. Examples of the carbon materials include graphite and fibrous carbon. Examples of the graphite include one or more types selected from the group consisting of carbon black (acetylene black, ketjen black, etc.), coke, and activated carbon. Examples of the fibrous carbon include carbon nanotubes (CNTs). The carbon nanotubes may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes such as double-walled carbon nanotubes (DWCNTs).

[0040] (Nonaqueous electrolyte secondary battery) The nonaqueous electrolyte secondary battery (secondary battery) of this embodiment has the above-mentioned positive electrode plate. The secondary battery can include an electrode assembly including the positive electrode plate and a nonaqueous electrolyte, and may have a battery case that houses the electrode assembly and the nonaqueous electrolyte.

[0041] The battery case may include an exterior body having an opening and a sealing plate that seals the opening. The exterior body and the sealing plate are preferably made of metal and can be formed using aluminum, an aluminum alloy, iron, an iron alloy, or the like. A resin sheet serving as an electrode holder may be disposed between the electrode body and the exterior body. The battery case may also be a laminate film. The laminate film has a layered structure in which, for example, a metal layer and a resin layer are stacked. A pouch-shaped battery case can be formed by overlapping and welding the edges of the laminate film.

[0042] The electrode assembly may include a positive electrode plate, a negative electrode plate, and a separator. In the electrode assembly, the active material layer of the positive electrode plate and the negative electrode active material layer of the negative electrode plate face each other with the separator interposed therebetween. The electrode assembly may be a laminated type in which the positive electrode plate, the negative electrode plate, and the separator are laminated, or a wound type in which a laminate in which the positive electrode plate, the negative electrode plate, and the separator are laminated is wound. The wound type electrode assembly may have a flat shape that is pressed after the laminate is wound.

[0043] A negative electrode plate typically includes a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector is, for example, a metal foil made of a copper material such as copper or a copper alloy. The negative electrode active material layer includes a negative electrode active material and may further include a conductive additive, a binder, etc. The negative electrode active material layer can be formed by applying a negative electrode mixture slurry to a negative electrode current collector foil, drying, and compressing the slurry. The negative electrode active material layer can be formed by adding a solvent such as water to materials for forming the negative electrode active material layer, such as the negative electrode active material, the binder, and the conductive additive.

[0044] Examples of negative electrode active materials include carbon-based active materials and metal-based active materials. Examples of carbon-based active materials include particles of one or more types selected from the group consisting of graphite (e.g., natural graphite and artificial graphite), hard carbon, soft carbon, and carbon (C) such as amorphous coated graphite. Examples of metal-based active materials include particles of metal elements such as simple metals or metal oxides containing elements selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). Examples of metal-based active materials include particles of one or more types selected from the group consisting of Si, SiOx (x=0.5 to 1.5), a composite of Si and C, and Sn.

[0045] Examples of the conductive additive include the conductive additives described above as the conductive additives that may be contained in the positive electrode active material layer. The conductive additive may include one or more of the conductive additives described above. Examples of the binder include cellulose-based resins, polyacrylic acid, styrene butadiene rubber (SBR), and the like, which are described above as the binders that may be contained in the positive electrode active material layer. The binder may include one or more of the binders described above.

[0046] The separator may have a substrate and a functional layer on at least one side of the substrate. The substrate may be a film or a porous sheet such as a nonwoven fabric made of a resin such as a polyolefin (e.g., polyethylene or polypropylene), polyester, cellulose, or polyamide. The substrate may have a single-layer structure or a multi-layer structure. Examples of the functional layer include an adhesive layer and a heat-resistant layer, and the separator may have one or both of these. The adhesive layer may be formed, for example, with an adhesive. The heat-resistant layer may contain, for example, a filler and a binder.

[0047] The non-aqueous electrolyte preferably contains an electrolyte in a non-aqueous solvent such as an organic solvent. Examples of the electrolyte include LiPF6, LiBF4, LiClO4, LiFSO3, and LiBOB (lithium bis(oxalato)borate). The non-aqueous electrolyte may contain one or more of these electrolytes. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate (PC), butylene carbonate (BC), and diethyl carbonate (DEC). The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. The non-aqueous electrolyte may further contain an additive such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), or fluoroethylene carbonate. [Example]

[0048] Hereinafter, the present disclosure will be described more specifically with reference to examples and comparative examples.

[0049] [Examples 1 to 4, Comparative Examples 1 to 3] (Preparation of positive electrode active material) A mixture was obtained by mixing a NiMnCo-containing precursor containing Ni, Mn, and Co in a molar ratio of Ni:Mn:Co = 83:12:5 with lithium hydroxide monohydrate and, if necessary, ammonium paratungstate. The lithium hydroxide monohydrate and ammonium paratungstate were mixed so that the moles of Li and W were as shown in Table 1 per mole of the total of Ni, Mn, and Co in the NiMnCo-containing precursor (i.e., Ni: 0.83 mol, Mn: 0.12 mol, Co: 0.05 mol). When the moles of W were 0 mol, no ammonium paratungstate was added to the mixture.

[0050] The mixture was filled into an alumina crucible and then placed in an electric furnace to be fired, yielding a lithium transition metal composite oxide as a cathode active material. The mixture was fired by first heating the mixture to 500°C at a temperature ramp rate of 5°C / min with an oxygen flow rate of 4 L / min, while measuring the temperature with an alumina-coated K-type thermocouple inserted into the crucible. The mixture was then held at 500°C for 3 hours, and then heated at a rate of 5°C / min to the Tmax temperature shown in Table 1, and held at this temperature for 10 hours. All of the lithium transition metal composite oxides were secondary particles with a primary particle aggregation number of 50 or more.

[0051] [Table 1]

[0052] (Preparation of positive electrode plate) A positive electrode plate was fabricated using the positive electrode active material obtained above. The positive electrode active material, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were prepared in a mass ratio of positive electrode active material:AB:PVdF = 100:1:1. These were then mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum foil serving as a positive electrode current collector, dried, compressed, and then cut to the specified size to obtain a positive electrode plate.

[0053] (Preparation of negative electrode plate) A negative electrode active material was prepared as a mixture of graphite and SiO. Styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were used as binders. The negative electrode active material, SBR, and CMC were prepared in a mass ratio of negative electrode active material:SBR:CMC = 100:1:1, and these were mixed with water to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to copper foil as a negative electrode current collector foil, dried, compressed, and then cut to a specified size to obtain a negative electrode plate.

[0054] (Fabrication of non-aqueous electrolyte secondary battery) A separator with a three-layer structure of polypropylene / polyethylene / polypropylene was prepared. A positive electrode plate and a negative electrode plate were stacked with the separator interposed therebetween to obtain an electrode assembly. A positive electrode tab formed of aluminum foil in the region of the positive electrode current collector foil where the positive electrode active material layer was not formed, and a negative electrode tab formed of copper foil in the region of the negative electrode current collector foil where the negative electrode active material layer was not formed, were exposed at both ends of the electrode assembly. The positive electrode tab was welded to an aluminum plate serving as an external positive electrode current collector, and the negative electrode tab was welded to a copper plate serving as an external negative electrode current collector. The electrode assembly was then inserted into an exterior body of an aluminum laminate film, and the film was welded to form a liquid injection port. A nonaqueous electrolyte was injected through the liquid injection port, and the liquid injection port was sealed to obtain a secondary battery.

[0055] The non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF) as an electrolyte at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of EC:EMC = 1:3.

[0056] [Cross-sectional image acquisition of lithium transition metal complex oxide compounds] The cross sections of the lithium transition metal composite oxides (composite oxides) obtained in the examples and comparative examples were observed with a scanning electron microscope (SEM) to obtain cross-sectional images. The primary particles among the secondary particles in the cross-sectional images were classified into first particles that constitute the outer periphery of the secondary particles and second particles other than the first particles. In all of the composite oxides, the second particles included particles whose major axes were radially oriented in the first direction d1.

[0057] Twenty or more first particles were selected from the cross-sectional image, and the maximum lengths in the first direction d1 and the second direction d2 were measured for each first particle. The average values ​​were taken as the maximum lengths L11 and L12 of the first particles. The ratio (L11 / L12) was calculated based on the maximum lengths L11 and L12. The results are shown in Table 2.

[0058] Twenty or more second particles were selected from the cross-sectional image, and the maximum lengths in the first direction d1 and the second direction d2 were measured for each second particle. The average values ​​were taken as the maximum lengths L21 and L22 of the second particles. The ratio (L21 / L22) was calculated based on the maximum lengths L21 and L22. The results are shown in Table 2.

[0059] [Calculation of Feret diameter of first particle] The surfaces of the lithium transition metal composite oxides obtained in the examples and comparative examples were observed with a scanning electron microscope (SEM) to obtain surface images. More than 200 first particles were selected from the surface images, and the distance between the tangent lines that were parallel to the outline of each first particle and that were the shortest distance between the tangent lines was measured. The average value was taken as the Feret diameter of the first particle. The results are shown in Table 2.

[0060] [Measurement of discharge capacity and evaluation of cycle characteristics] As an activation charge treatment for the secondary batteries obtained in the examples and comparative examples, they were charged to 4.2 V at a current rate of 0.1 C. Thereafter, they were discharged to 3.0 V at 0.1 C, and the discharge capacity [mAh / g] at this time was measured. The results are shown in Table 2.

[0061] Subsequently, the battery was charged at a current rate of 1C to 4.2V at a temperature of 25°C, then discharged at 1C to 2.5V, followed by a 10-minute rest period. This cycle was repeated 20 times. The discharge capacities obtained in the first and 20th charge / discharge cycles were measured, and the capacity retention rate [%] was calculated using the following formula. The results are shown in Table 2. Capacity retention rate [%] = (discharge capacity at 20th cycle / discharge capacity at 1st cycle) x 100

[0062] [Measurement of resistance increase rate] After the activation charge process, the secondary battery was charged at a temperature of 25°C until the state of charge (SOC) reached 50%, then rested for 1 hour, and discharged for 10 seconds at a current of 5 C. This cycle was repeated 20 times. The OCV (open circuit voltage) immediately before discharge was defined as V0 [V], and the voltage 10 seconds after discharge was defined as V1 [V]. The resistance R was calculated according to the following formula. Resistance R [Ω] = (V0-V1) [V] / 1C current value [A]

[0063] The resistance increase rate was calculated according to the following formula, where the resistance at the first cycle was R1 and the resistance at the 20th cycle was R20. The results are shown in Table 2. Resistance increase rate [%] = (R20-R1) / R1 x 100

[0064] [Table 2] [Explanation of symbols]

[0065] 1 secondary particle, 10 primary particle, 11 first particle, 12 second particle, d1 first direction, d2 second direction, d3 major axis direction.

Claims

1. A positive electrode active material containing secondary particles formed by aggregation of primary particles, the secondary particles are lithium transition metal composite oxides having a layered crystal structure, The lithium transition metal composite oxide is Li, Ni, Mn, Co, and M (M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, and W), and the molar ratios of Li, Ni, Mn, Co, and M are Li:Ni:Mn:Co:M=a:x:y:z:t, where a, x, y, z, and t are 1.0≦a≦1.3, x+y+z+t=1, 0.25≦x≦0.9, 0<y≦0.6, 0<z≦0.6, and 0≦t≦0.1; In a cross-sectional image obtained by observing a cross section of the secondary particle, the primary particle is classified into a first particle that constitutes the periphery of the secondary particle and a second particle other than the first particle, and a direction from the center of the secondary particle toward the periphery is defined as a first direction, and a direction perpendicular to the first direction is defined as a second direction. a ratio (L11 / L12) of a maximum length L11 in the first direction to a maximum length L12 in the second direction of the first particle is 0.50 to 1.10; a ratio (L21 / L22) of a maximum length L21 in the first direction to a maximum length L22 in the second direction of the second particle is 1.40 or more; the second particles include oriented particles whose major axes are radially oriented in the first direction and whose maximum length L21 is 640 nm or less.

2. The positive electrode active material according to claim 1 , wherein the maximum length L21 of the oriented particles is 600 nm or less.

3. 2. The positive electrode active material according to claim 1, wherein the ratio (L21 / L22) in the second particles is 1.50 to 3.

00.

4. The positive electrode active material according to claim 1 , wherein the first particles have a Feret diameter of 270 nm or less.

5. The positive electrode active material according to claim 1 , wherein the maximum length L21 of the second particle is 640 nm or less.

6. The positive electrode active material according to claim 1 , wherein the secondary particles are aggregates of 50 or more of the primary particles.

7. A positive electrode plate having a positive electrode active material layer containing the positive electrode active material according to any one of claims 1 to 6.

8. A non-aqueous electrolyte secondary battery comprising the positive electrode plate according to claim 7.

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