Positive electrode active material, positive electrode plate, and non-aqueous electrolyte secondary battery

Abstract

To provide a positive electrode active material that can suppress gas generation during storage of a non-aqueous electrolyte secondary battery and can suppress a decrease in the capacity retention rate when the non-aqueous electrolyte secondary battery is repeatedly used under high-temperature conditions.SOLUTION: A positive electrode active material includes a first active material having an average particle diameter (D50) of 2 to 10 μm and a second active material having an average particle diameter (D50) of 12 to 20 μm. The first active material includes first particles that are at least one of single particles and secondary particles formed by agglomeration of 2 to 10 primary particles, and are made of a lithium transition metal composite oxide. The second active material includes second particles that are secondary particles formed by agglomeration of 50 or more primary particles, and are made of a lithium transition metal composite oxide. The sphericity of the second active material is 0.770 to 0.810.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] It is known that two types of lithium transition metal composite oxides with different average particle sizes (D50) are used as positive electrode active materials. Patent Document 1 discloses a means for suppressing gas generation during storage of a non-aqueous electrolyte secondary battery and achieving good cycle characteristics when such a positive electrode active material is used. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-091566 Summary of the Invention [Problem to be solved by the invention]

[0004] Since non-aqueous electrolyte secondary batteries are sometimes exposed to high temperature conditions, they are required to have excellent cycle characteristics even under high temperature conditions.

[0005] The present disclosure aims to provide a positive electrode active material that can suppress gas generation during storage of a nonaqueous electrolyte secondary battery and can suppress a decrease in capacity retention rate when the nonaqueous electrolyte secondary battery is repeatedly used under high-temperature conditions. [Means for solving the problem]

[0006] [1] A first active material having an average particle diameter (D50) of 2 to 10 μm and a second active material having an average particle diameter (D50) of 12 to 20 μm, the first active material includes first particles that are at least one of single particles and secondary particles formed by aggregation of 2 to 10 primary particles and are lithium transition metal composite oxides; the second active material is a secondary particle formed by agglomerating 50 or more primary particles, and includes a second particle that is a lithium transition metal composite oxide; The positive electrode active material, wherein the second active material has a sphericity of 0.770 to 0.810. [2] The positive electrode active material according to [1], wherein the first particles and the second particles each contain 50 mol % or more of Ni relative to the total number of moles of metal elements excluding Li. [3] The cathode active material according to [1] or [2], wherein an average particle size obtained by averaging particle sizes of the single particles contained in the first particles and the primary particles constituting the secondary particles is 2 to 6 μm. [4] The positive electrode active material according to any one of [1] to [3], wherein the primary particles constituting the secondary particles of the second particles have an average primary particle diameter of 1.2 to 2.0 μm. [5] The second active material further includes a coating layer that coats at least a portion of the surface of the second particle, The positive electrode active material according to any one of [1] to [4], wherein the coating layer contains one or more elements selected from the group consisting of B, Al, Co, Mo, W, Ga, In, and Ti. [6] The coating layer contains B, The positive electrode active material according to [5], wherein the content of B in the coating layer is 600 to 1400 ppm relative to the mass of the second particles. [7] A positive electrode plate having an active material layer containing the positive electrode active material according to any one of [1] to [6]. [8] The density of the active material layer is 3.0 to 4.0 g / cm 3 The positive electrode plate according to [7], [9] A non-aqueous electrolyte secondary battery having the positive electrode plate according to [7]. [Effects of the Invention]

[0007] The positive electrode active material of the present disclosure can provide a nonaqueous electrolyte secondary battery that can suppress gas generation during storage and can suppress a decrease in capacity retention rate when repeatedly used under high-temperature conditions. DETAILED DESCRIPTION OF THE INVENTION

[0008] In this specification, unless otherwise specified, a numerical range such as "x to y" includes both the upper and lower limits. That is, "x to y" represents a numerical range of "greater than or equal to x and less than or equal to y." 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.

[0009] (Cathode active material) The positive electrode active material of this embodiment (hereinafter also referred to as "the present positive electrode active material") is used in the positive electrode plate of a non-aqueous electrolyte secondary battery (hereinafter also referred to as "secondary battery") such as a lithium ion battery.

[0010] This positive electrode active material includes a first active material having an average particle diameter (D50) of 2 to 10 μm and a second active material having an average particle diameter (D50) of 12 to 20 μm. The first active material includes first particles that are at least one of single particles and secondary particles (hereinafter also referred to as "first secondary particles") formed by agglomeration of 2 to 10 primary particles, and are a lithium transition metal composite oxide (hereinafter also referred to as "composite oxide"). The second active material includes second particles (hereinafter also referred to as "second secondary particles") formed by agglomeration of 50 or more primary particles, and are a composite oxide. The sphericity of the second active material is 0.770 to 0.810.

[0011] The active material layer of the positive electrode plate, which will be described later, is formed by forming a coating layer on the positive electrode current collector using a positive electrode mixture slurry containing the positive electrode active material on a positive electrode current collector foil, and then compressing this coating layer, etc. This active material contains a first active material and a second active material that have different average particle diameters (D50), so it is possible to form an active material layer in which the positive electrode active material is densely packed, and it is possible to improve the volumetric energy density of the secondary battery.

[0012] Since the second active material has a sphericity within the above range, it can be said that it has a moderately anisotropic shape rather than an isotropic shape. Therefore, when forming an active material layer using this positive electrode active material, the compressive force received by the second active material is appropriately distributed, making it easier to apply the compressive force evenly to the entire coating layer, and therefore easier to compress the coating layer uniformly. As a result, the conductive paths are less likely to break even when the secondary battery is repeatedly charged and discharged under high-temperature conditions, and a decrease in capacity retention rate can be suppressed. On the other hand, an active material with an isotropic shape makes it difficult to distribute the compressive force when forming the active material layer, making it harder to uniformly compress the coating layer, making the conductive paths more likely to break when the secondary battery is repeatedly charged and discharged under high-temperature conditions.

[0013] Appropriately dispersing the compressive force during active material layer formation as described above is believed to reduce the compressive force required to obtain an active material layer with a specific density, thereby suppressing the stress applied to the second active material. This suppresses the occurrence of cracks in the second secondary particles of the second active material due to the compressive force during active material layer formation, thereby suppressing the amount of gas generation during storage of the secondary battery. In contrast, an active material with low sphericity and high anisotropy is likely to disperse the compressive force during active material layer formation, but is believed to require a larger compressive force to obtain an active material layer with a specific density. This increases the stress applied to the active material, making it more susceptible to damage such as cracks in the active material, and therefore more likely to generate gas during storage of the secondary battery.

[0014] The first active material has an average particle diameter (D50) smaller than the average particle diameter (D50) of the second active material. The average particle diameter (D50) of the first active material is 2 to 10 μm, and may be 2.5 to 8 μm, or 3 to 6 μm. In this specification, the average particle diameter (D50) is the particle diameter at which the cumulative frequency of the smaller particle diameters in the volume-based particle size distribution is 50%. The volume-based particle size distribution can be measured using a laser diffraction particle size distribution analyzer.

[0015] The first active material may contain first particles, and may include first particles and a first coating layer that coats at least a portion of the surface of the first particles. The first coating layer may contain, for example, elements that may be contained in the second coating layer described below. The presence of the first coating layer in the first active material can be confirmed by analysis using X-ray photoelectron spectroscopy (XPS). The first active material is preferably first particles, but may also be first particles having a first coating layer.

[0016] The first particles are at least one of single particles and first secondary particles. The first particles may be single particles, first secondary particles, or a mixture of single particles and second secondary particles. The aggregation number of primary particles in the first secondary particles may be 2 to 8, 2 to 5, or 3 to 5. In this specification, the aggregation number of primary particles can be confirmed, for example, by a scanning electron microscope (hereinafter also referred to as "SEM") image obtained by SEM.

[0017] The average particle diameter (hereinafter also referred to as "average particle diameter") obtained by averaging the particle diameters of the single particles and primary particles of the first secondary particles contained in the first particles is preferably 2 to 6 μm, and may be 2.3 to 5.8 μm, 2.5 to 5.5 μm, or 3 to 5 μm. In this specification, the average particle diameter is a value obtained by observing the particle surfaces of the first active material with a scanning electron microscope (SEM) and analyzing the SEM images obtained, determining the major axes (longest diameters) of the single particles and primary particles of the first secondary particles in the SEM images, and averaging the determined major axes.

[0018] The first active material is at least one of single particles and second secondary particles, and has a smaller average particle diameter than the second active material. Therefore, even when the secondary battery is repeatedly charged and discharged, the conductive path is less likely to be broken, and particle cracking of the first active material is less likely to occur. For this reason, the sphericity of the first active material is not particularly limited. This is because the sphericity of the first active material is thought to have little effect on the capacity retention rate when the secondary battery is repeatedly charged and discharged under high-temperature conditions, and on the amount of gas generation during storage of the secondary battery.

[0019] The sphericity of the first active material may be the same as or different from the sphericity of the second active material, but is preferably close to the sphericity of the second active material. The sphericity of the first active material is, for example, 0.650 to 0.950, preferably 0.700 to 0.850. It is believed that when the sphericity of the first active material and the sphericity of the second active material are close to each other, it becomes easier to disperse the compressive force during the formation of the active material layer. In this specification, the sphericity of the first active material is calculated by measuring the area S1 and the perimeter L1 of the first active material through image analysis of SEM images obtained by observing the particle surface of the first active material, and then calculating the ratio of the radius when the area S1 is assumed to be the area of ​​a perfect circle to the radius when the perimeter L1 is assumed to be the circumference of the perfect circle. Specifically, the sphericity of the first active material is calculated using the following formula (i): Sphericity = 4πS1 / L1 2 (i)

[0020] Theoretically, the second active material is considered to be a perfect sphere when the sphericity is 1. The closer the sphericity is to 1, the more isotropic the shape is, and the further the sphericity is from 1, the more anisotropic the shape is.

[0021] The second active material has an average particle diameter (D50) larger than that of the first active material. The average particle diameter (D50) of the second active material is 12 to 20 μm, or may be 13 to 19 μm, 14 to 18 μm, or 15 to 18 μm. The average particle diameter (D50) of the first active material and the average particle diameter (D50) of the second active material can be combined in any manner within the above-mentioned range.

[0022] The second active material may contain second particles. As described later, the second active material may contain second particles and a second coating layer (coating layer) that covers at least a portion of the surface of the second particles. The second active material may be second particles, but is preferably second particles having a second coating layer.

[0023] The secondary particles are secondary particles formed by agglomeration of 50 or more primary particles. The agglomeration number of primary particles in the secondary particles may be 100 or more, or may be 1000 or more, and is usually 5×10 6 5 x 10 or less 5 It may be less than one.

[0024] The average primary particle size of the primary particles constituting the second secondary particles of the second active material is preferably smaller than the average particle size (described above) of the single particles and first secondary particles constituting the first active material. The average primary particle size of the primary particles constituting the second secondary particles is preferably 1.2 to 2.0 μm, but may be 1.25 to 1.8 μm, or 1.3 to 1.7 μm. In this specification, the average primary particle size is determined by analyzing SEM images obtained by observing the particle surfaces of the second active material with a scanning electron microscope (SEM), determining the major axis (longest diameter) of each primary particle of the second secondary particles in the SEM image, and averaging the determined major axes. The average particle size and the average primary particle size can be combined in any manner within the above-mentioned ranges.

[0025] The second active material has a sphericity of 0.770 to 0.810. The sphericity of the second active material is preferably 0.770 to 0.800, and more preferably 0.780 to 0.800. In this specification, the sphericity of the second active material is calculated by measuring the area S2 and the perimeter L2 of the second active material through image analysis of SEM images obtained by observing the particle surfaces of the second active material, and then calculating the ratio of the radius when the area S2 is assumed to be the area of ​​a perfect circle to the radius when the perimeter L2 is assumed to be the circumference of the perfect circle. Specifically, the sphericity of the second active material is calculated using the following formula (ii): Sphericity = 4πS2 / L2 2 (ii)

[0026] As described above, a second active material having a sphericity within the above range can appropriately distribute the compressive force during the formation of the active material layer and suppress the stress applied to the second active material, thereby suppressing the decrease in capacity retention even when the secondary battery is repeatedly charged and discharged under high-temperature conditions, and suppressing the amount of gas generation during storage of the secondary battery.

[0027] The sphericity of the second active material can be adjusted by the manufacturing conditions when manufacturing the second active material, for example, by the firing conditions such as the firing temperature, firing time, and number of firings when manufacturing the second particles described below.

[0028] The second active material may include a second coating layer (coating layer) that coats at least a portion of the surface of the second particle. The coverage of the second coating layer on the second active material is 50% or more, and may be 50 to 100%, 70 to 98%, 80 to 95%, or 90 to 95% of the entire surface of the second particle, i.e., the entire surface of the second secondary particle. The components constituting the second coating layer may penetrate into the interior of the second particle, i.e., into the gaps formed between the primary particles constituting the second secondary particle. The presence of the second coating layer in the second active material and the coverage of the second coating layer can be confirmed by analysis using X-ray photoelectron spectroscopy (XPS) or inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0029] The second coating layer may contain one or more elements selected from the group consisting of B, Al, Co, Mo, W, Ga, In, and Ti. The element contained in the second coating layer is preferably one or more elements selected from the group consisting of B, Al, and W, and more preferably B.

[0030] When the second coating layer contains B, the B content in the second coating layer is preferably 600 to 1400 ppm, or alternatively 700 to 1300 ppm, 800 to 1200 ppm, or 900 to 1100 ppm, relative to the mass of the second particles. The B content in the second coating layer can be determined by X-ray photoelectron spectroscopy (XPS) or inductively coupled plasma atomic emission spectroscopy (ICP-AES). The B content in the second coating layer can be adjusted, for example, by adjusting the amount of boron source added during production of the second active material.

[0031] It is believed that a second active material having a sphericity within the above range is more likely to form a uniform second coating layer on the surface of the second particles than an active material having a sphericity below this range, and therefore, by using a second active material having a sphericity within the above range, it is possible to further reduce the amount of gas generated during storage of the secondary battery.

[0032] The composite oxides constituting the first and second particles may have the same composition or different compositions. The first and second particles are each independently a composite oxide preferably containing Ni, more preferably containing 50 mol% or more of Ni relative to the total number of moles of metal elements excluding Li. The first and second particles are each independently a nickel-cobalt-manganese oxide preferably containing Ni, Co, and Mn.

[0033] The Ni content in the first particles and the second particles is independently preferably 50 mol% or more, and may be 50 to 88 mol%, 52 to 85 mol%, 55 to 80 mol%, 57 to 75 mol%, or 60 to 70 mol%, based on the total number of moles of metal elements excluding Li. The Ni content in the first particles may be the same as or different from the Ni content in the second particles. The Ni content in the first particles is preferably greater than the Ni content in the second particles, but may be smaller than the Ni content in the second particles.

[0034] When the Ni content in the first particles and the second particles is within the above range, a secondary battery with a large capacity is easily obtained.

[0035] The first particle and the second particle may each independently be, for example, a compound represented by formula (I). Li 1-a Ni x Me 1-x O2(I) [In formula (I), -0.3≦a≦0.2, and 0.5≦x≦0.88; Me may include one or more elements selected from the group consisting of Co, Mn, Al, B, Zr, Ti, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, and Si.

[0036] In the above formula (I), a may be -0.25≦a≦0.15 or -0.20≦a≦0.10. x may be 0.52≦x≦0.85, 0.55≦x≦0.80, 0.57≦x≦0.75, or 0.60≦x≦0.70. Me may contain one or more elements selected from the group consisting of Co, Mn, Al, B, Zr, Ti, Mg, Mo, and Nb, and preferably contains at least one of Co and Mn, and more preferably contains Co and Mn. The ranges of a, x, and Me can be set by any combination from the above ranges.

[0037] The compositions of the first particles and the second particles can be determined by, for example, ICP (inductively coupled plasma) emission spectrometry.

[0038] The composite oxide constituting the first and second particles can be produced by mixing and firing a lithium compound and a transition metal compound. For example, a first mixture containing a lithium compound and a transition metal compound is fired to obtain a fired product, and a second mixture containing the fired product and a transition metal compound is fired to obtain a composite oxide. Examples of lithium compounds include lithium hydroxide and lithium carbonate. Examples of transition metal compounds include Ni-containing compounds and NiCoMn-containing compounds containing Ni, Co, and Mn. The transition metal compound may be an oxide or a hydroxide. The transition metal compounds contained in the first and second mixtures may have the same composition or different compositions. By adjusting the firing conditions (firing temperature, firing time, number of firings), etc., when producing the composite oxide, the average particle size, particle aggregation morphology, and sphericity of the first and second active materials can be adjusted.

[0039] The first particles and second particles obtained by the above-described manufacturing method may be used as the first active material and the second active material, respectively. When the first active material includes a first coating layer, the first particles having the first coating layer may be produced, for example, by mixing the first particles with an element source for forming the first coating layer and heat-treating the mixture. When the second active material includes a second coating layer, the second particles having the second coating layer may be produced, for example, by mixing the second particles with an element source for forming the second coating layer and heat-treating the mixture. When the second coating layer includes B, for example, boric acid may be used as the boron source.

[0040] The mass ratio of the first active material to the second active material in the present positive electrode active material may be first active material:second active material=7:3 to 3:7, or 6:4 to 4:6, or 5.5:4.5=4.5 to 5.5. When the mass ratio is within the above range, an active material layer (described below) formed using the present positive electrode active material can be easily formed at a high density, and the capacity of the secondary battery can be easily increased.

[0041] The present positive electrode active material may contain only the first active material and the second active material, or may further contain an active material other than the first active material and the second active material. The total content of the first active material and the second active material in the present positive electrode active material may be 85 to 100 mass%, 90 to 100 mass%, 92 to 99 mass%, or 95 to 98 mass% of the total amount of the present positive electrode active material.

[0042] (positive electrode plate) The positive electrode plate of this embodiment has an active material layer (hereinafter also referred to as "positive electrode active material layer") containing the present positive electrode active material. Because the positive electrode plate of this embodiment contains the present positive electrode active material, gas generation during storage in a secondary battery can be suppressed, and a decrease in capacity retention rate when repeatedly used under high-temperature conditions can be suppressed.

[0043] The positive electrode plate can have a positive electrode active material layer on one or both sides of a positive electrode current collector foil. The positive electrode current collector foil is a metal foil made of an aluminum material such as aluminum or an aluminum alloy. In addition to the positive electrode active material, the positive electrode active material layer can further contain one or more selected from the group consisting of a conductive additive, a binder, and a thickener.

[0044] Examples of binders include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE); polyacrylonitrile (PAN); polyimide; acrylic resin; polyolefin; cellulose-based resins such as carboxymethyl cellulose (CMC), methyl cellulose (MC), and hydroxypropyl cellulose; and polyethylene oxide (POE). Carboxymethyl cellulose (CMC) can also be used as a thickener. The binder may contain one or more of the above-mentioned binders.

[0045] Examples of the conductive additive include carbon materials. Examples of the carbon material include one or more materials selected from the group consisting of fibrous carbon, carbon black (acetylene black, ketjen black, etc.), coke, and activated carbon. Examples of the fibrous carbon include carbon nanotubes (CNT). The CNT may be a single-walled carbon nanotube (SWCNT) or a multi-walled carbon nanotube such as a double-walled carbon tube (DWCNT). The conductive additive may include one or more of the conductive additives described above.

[0046] The density of the positive electrode active material layer is preferably 3.0 to 4.0 g / cm 3 and 3.2 to 3.8 g / cm 3 and 3.3 to 3.7 g / cm 3 When the density of the positive electrode active material layer is within the above range, a secondary battery with improved volumetric energy density can be easily obtained.

[0047] The density of the positive electrode active material layer can be adjusted by the degree of compression of the coating layer (described later) when forming the positive electrode active material layer, the average particle diameters (D50) of the first and second active materials, the mixing ratio of the first and second active materials, etc. The density of the positive electrode active material layer can be calculated by measuring the mass of a punched piece obtained by punching out a positive electrode plate with predetermined planar dimensions, and subtracting the mass of the positive electrode current collector foil (measured in advance) from the mass of the positive electrode active material layer calculated based on the thickness of the positive electrode active material layer in the punched piece.

[0048] The positive electrode plate can be obtained, for example, by forming a positive electrode active material layer on a positive electrode current collector foil. For example, a positive electrode mixture slurry containing the present positive electrode active material is applied to the positive electrode current collector foil and dried to form a coating layer, and the coating layer is then compressed to form a positive electrode active material layer, thereby obtaining a positive electrode plate. In addition to the present positive electrode active material, the positive electrode mixture slurry can contain a binder, a conductive additive, and a solvent such as N-methyl-2-pyrrolidone (NMP).

[0049] (Nonaqueous electrolyte secondary battery) The nonaqueous electrolyte secondary battery of this embodiment (hereinafter also referred to as "the battery") has the above-described positive electrode plate. Because the battery contains the positive electrode active material, gas generation during storage can be suppressed and a decrease in capacity retention rate can be suppressed when the battery is repeatedly used under high-temperature conditions.

[0050] The battery may include an electrode assembly including a positive electrode plate and a non-aqueous electrolyte, and may also include a battery case that accommodates the electrode assembly and the non-aqueous electrolyte. 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 assembly 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.

[0051] The electrode body may include a negative electrode plate and a separator in addition to the positive electrode plate. In the electrode body, the positive electrode active material layer of the positive electrode plate and the negative electrode active material layer of the negative electrode plate face each other via the separator. The electrode body may be a laminate type in which a positive electrode plate, a negative electrode plate, and a separator are laminated, or a wound type in which a long laminate in which a positive electrode plate, a negative electrode plate, and a separator are laminated is wound. The wound type electrode body may have a flat shape obtained by pressing a long laminate after winding it.

[0052] A negative electrode plate typically includes a negative electrode current collector foil and a negative electrode active material layer. The negative electrode plate may include a negative electrode active material layer on one or both sides of the negative electrode current collector foil. The negative electrode current collector foil 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 one or more selected from the group consisting of a conductive additive, a thickener, and a binder.

[0053] Examples of negative electrode active materials include carbon-based active material particles and metal-based active material particles. Examples of carbon-based active material particles include one or more particles 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 material particles 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 material particles include one or more particles selected from the group consisting of Si, SiOx (x=0.5 to 1.5), a composite of Si and C (hereinafter also referred to as "SiC composite"), and Sn.

[0054] Examples of binders include cellulose-based resins such as carboxymethyl cellulose (CMC), methyl cellulose (MC), and hydroxypropyl cellulose; polyacrylic acid; and styrene-butadiene rubber. The binder may contain one or more of these. Carboxymethyl cellulose (CMC) may also be used as a thickener. Examples of conductive aids include those listed above, and the conductive aid may contain one or more of the conductive aids listed above.

[0055] The negative electrode plate can be obtained, for example, by forming a negative electrode active material layer on a negative electrode current collector foil. For example, a negative electrode mixture slurry containing the negative electrode active material is applied to the negative electrode current collector foil, dried, and compressed to form a negative electrode active material layer, thereby obtaining a negative electrode plate. The negative electrode mixture slurry can contain, in addition to the negative electrode active material, a conductive additive, a binder, and a solvent such as water.

[0056] 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.

[0057] 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]

[0058] Hereinafter, the present disclosure will be described more specifically with reference to examples and comparative examples.

[0059] [Examples 1-2, Comparative Examples 1-4] (Preparation of positive electrode plate) As the first active material, first particles were prepared, which were lithium transition metal composite oxides containing Ni, Co, and Mn in a molar ratio of Ni:Co:Mn=60:20:20. The first particles were at least one of single particles and secondary particles formed by agglomeration of 2 to 10 primary particles. The average particle diameter (average particle diameter) of the single particles and primary particles of the first secondary particles contained in the first particles was within the range of 2 to 6 μm.

[0060] A second active material having the sphericity shown in Table 1 was prepared. The second active material is a second particle having a second coating layer. The second particle is a lithium transition metal composite oxide containing Ni, Co, and Mn in a molar ratio of Ni:Co:Mn=55:20:25. The second particle is a secondary particle formed by agglomeration of 50 or more primary particles. The second coating layer coats the surface of the second particle and contains 1000 ppm of B (boron) relative to the mass of the second particle.

[0061] A positive electrode active material containing a first active material and a second active material in a 1:1 (mass ratio), carbon black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were mixed in a mass ratio of 97.5:1.5:1.0 (positive electrode active material:conductive additive:binder), and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added 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 foil and dried to form a coating layer. The coating layer was compressed to a predetermined thickness using a rolling roller to form a positive electrode active material layer, which was then cut to a predetermined size and attached with an aluminum tab to obtain a positive electrode plate.

[0062] The positive electrode plate was punched out to a predetermined planar dimension, and its mass was measured. The thickness of the positive electrode active material layer of the punched piece was measured. The mass of the positive electrode current collector foil, which was measured in advance, was subtracted to calculate the mass of the positive electrode active material layer contained in the punched piece. The density of the positive electrode active material layer was calculated based on the thickness and mass of the positive electrode active material layer, and was found to be 3.50 g / cm. 3 It was.

[0063] (Preparation of negative electrode plate) Graphite as the negative electrode active material, carboxymethyl cellulose (CMC) as the thickener, and styrene butadiene rubber (SBR) as the binder were mixed in a mass ratio of 98:1:1 (negative electrode active material:thickener:binder), and the mixture was dispersed in water to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to a copper foil as a negative electrode current collector foil, dried, and compressed to a predetermined thickness using a rolling roller to form a negative electrode active material layer. After cutting to a predetermined size, a nickel tab was attached to obtain a negative electrode plate.

[0064] (Preparation of test cell) A laminated electrode assembly was obtained by stacking a positive electrode plate and a negative electrode plate with a polyolefin separator interposed therebetween. The electrode assembly was inserted into an aluminum laminate film exterior body, and a non-aqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a test cell. The non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) as an electrolyte at a concentration of 1.15 mol / L in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC = 30:30:40, and adding vinylene carbonate (VC) as an additive in an amount of 1.0 mass% relative to the total amount of the mixed solvent.

[0065] [Measurement of sphericity of second active material] A scanning electron microscope (SEM) was used to obtain SEM images of the particle surfaces of the second active materials used in each Example and Comparative Example. The area S2 and perimeter L2 of the projection of the second active material in the SEM image were measured using the free software ImageJ. The sphericity of the second active material was calculated according to the above-mentioned formula (ii) as the ratio of the radius when the area S2 is assumed to be a perfect circle to the radius when the perimeter L2 is assumed to be the circumference of the perfect circle. The sphericities calculated for 20 or more second active materials in the SEM image were averaged to determine the sphericity of the second active material. The results are shown in Table 1.

[0066] [Measurement of average particle diameter (D50) of first active material and second active material] The average particle diameter (D50) of the first active material and the second active material used in each example and each comparative example was measured using a particle size distribution analyzer (Malvern Panalvtical, "Mastersizer-3000"). The results are shown in Table 1.

[0067] [Measurement of average primary particle diameter of second active material] Using a scanning electron microscope (SEM), SEM images of the particle surfaces of the second active materials used in each Example and Comparative Example were obtained. Using image analysis software ("Mac-View" manufactured by MOUNTECH), 20 or more primary particles were selected from the SEM images of the second active material, and the major axis (longest diameter) of each was determined. The average value was taken as the average primary particle diameter. The results are shown in Table 1.

[0068] [Evaluation of gas generation during storage] The test cells prepared in each example and comparative example were subjected to a current of 0.2 mA / cm under a temperature condition of 25°C. 2 The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 4.25 V. 2 After a 10-minute rest period, the battery was charged at a constant voltage of 0.2 mA / cm 2 The battery was discharged at a constant current density of 1000 V until the voltage reached 3.0 V. The amount of gas generated at this time was determined and used as the amount of gas generated before storage.

[0069] Subsequently, the test cell was charged at 0.2 mA / cm under a temperature condition of 25°C. 2 The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 4.25 V. 2 After storing the battery in a thermostatic chamber at 60°C for 20 days, the battery was charged at a constant voltage of 0.2 mA / cm 2 The battery was discharged at a constant current density of 100 V until the voltage reached 3.0 V. The amount of gas generated at this time was determined and used as the amount of gas generated after storage.

[0070] The amount of gas generated during storage was calculated by subtracting the amount of gas generated before storage from the amount of gas generated after storage. The amount of gas generated during storage in Example 1 was set to 100, and the amounts of gas generated during storage in Example 2 and Comparative Examples 1 to 4 were calculated as relative values ​​to the amount of gas generated during storage in Example 1. The results are shown in Table 1.

[0071] [Evaluation of capacity retention rate in cycle tests] A cycle test was performed on the test cells prepared in each Example and Comparative Example under the following procedure under temperature conditions of 25°C and 40°C. The capacity retention rate calculated in the cycle test at 25°C was subtracted from the capacity retention rate calculated in the cycle test at 40°C to calculate the difference [%] in the capacity retention rates. The results are shown in Table 1.

[0072] (Cycle test) The test cell was charged at 0.2 mA / cm 2 The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 4.25 V. 2 After a 10-minute rest period, the battery was charged at a constant voltage of 0.2 mA / cm 2 The test cell was then discharged at a constant current density of 0.2 mA / cm until the voltage reached 3.0 V. 2 The battery was charged at a constant current density of 0.04 mA / cm until the voltage reached 4.18 V. 2 The battery was charged at a constant voltage of 0.2 mA / cm 2 The battery was discharged at a constant current density of 1000 V until the voltage reached 3.48 V, and the discharge capacity at this point was determined and recorded as the discharge capacity of the first cycle. This charge-discharge cycle was counted as one cycle, and 400 cycles were repeated to determine the discharge capacity at the 400th cycle. The capacity retention rate [%] was calculated according to the following formula. Capacity maintenance rate [%] = (discharge capacity at 400th cycle / discharge capacity at 1st cycle) x 100

[0073] [Table 1]

Claims

1. a first active material having an average particle diameter (D50) of 2 to 10 μm and a second active material having an average particle diameter (D50) of 12 to 20 μm; the first active material includes first particles that are at least one of single particles and secondary particles formed by agglomeration of 2 to 10 primary particles and are lithium transition metal composite oxides; the second active material is a secondary particle formed by agglomerating 50 or more primary particles, and includes a second particle that is a lithium transition metal composite oxide; The positive electrode active material, wherein the second active material has a sphericity of 0.770 to 0.

810.

2. The positive electrode active material according to claim 1 , wherein each of the first particles and the second particles contains 50 mol % or more of Ni relative to the total number of moles of metal elements excluding Li.

3. 2. The positive electrode active material according to claim 1, wherein an average particle diameter of the single particles contained in the first particles and the primary particles constituting the secondary particles is 2 to 6 μm.

4. 2. The positive electrode active material according to claim 1, wherein the primary particles constituting the secondary particles of the second particles have an average primary particle diameter of 1.2 to 2.0 μm.

5. the second active material further includes a coating layer that coats at least a portion of the surface of the second particle, 2. The positive electrode active material according to claim 1, wherein the coating layer contains one or more elements selected from the group consisting of B, Al, Co, Mo, W, Ga, In, and Ti.

6. The coating layer contains B, 6. The positive electrode active material according to claim 5, wherein the content of B in the coating layer is 600 to 1400 ppm relative to the mass of the second particles.

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

8. The density of the active material layer is 3.0 to 4.0 g / cm ３ The positive electrode plate according to claim 7,

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

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