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

Abstract

To efficiently suppress the increase in resistance accompanied by formation of a skin film of boron oxide.SOLUTION: A positive electrode material comprises a plurality of composite particles 5. Each composite particle 5 includes a base particle 1, a coating film 2, and a carbon nanotube 3. The coating film 2 covers at least part of the surface of the base particle 1. The base particle 1 contains a positive electrode active material. The coating film 2 contains boron oxide. The carbon nanotube 3 includes a first portion and a second portion. The first portion is buried in the coating film 2. The second portion is exposed from the surface of the coating film 2.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] Japanese Patent Laid-Open Publication No. 2014-127235 (Patent Document 1) discloses an amorphous boron oxide coating layer that covers at least a portion of the surface of positive electrode active material particles. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-127235 A Summary of the Invention [Problem to be solved by the invention]

[0004] In a non-aqueous electrolyte secondary battery (hereinafter, may be abbreviated as "battery"), the positive electrode active material particles have a high potential. Therefore, the electrolyte may be oxidized and decomposed on the surface of the positive electrode active material particles. The oxidative decomposition of the electrolyte is considered to be one of the causes of the decrease in cycle life.

[0005] For example, a boron oxide film may be formed on the surface of the positive electrode active material particles. The boron oxide is expected to prevent direct contact between the positive electrode active material particles and the electrolyte, which is expected to improve the cycle life, for example.

[0006] However, boron oxide can inhibit electronic conduction. The formation of a film of boron oxide can increase battery resistance. In order to suppress the increase in battery resistance, it is possible to increase the amount of conductive material (e.g., carbon black) in the electrode (positive electrode). However, the conductive material does not contribute to the battery capacity. It is thought that an increase in the amount of conductive material reduces the energy density of the battery.

[0007] An object of the present disclosure is to efficiently suppress the increase in resistance that accompanies the formation of a boron oxide film. [Means for solving the problem]

[0008] The technical configuration and effects of the present disclosure are described below. However, the mechanism of action of the present disclosure includes assumptions. The validity of the mechanism of action does not limit the scope of the claims.

[0009] [1] The positive electrode material is used in non-aqueous electrolyte secondary batteries. The positive electrode material is composed of a plurality of composite particles. Each of the plurality of composite particles includes a substrate particle, a coating, and a carbon nanotube. The coating covers at least a portion of a surface of the substrate particle. The substrate particle includes a positive electrode active material. The coating includes boron oxide. The carbon nanotube includes a first portion and a second portion. The first portion is embedded in the coating. The second portion is exposed at a surface of the coating.

[0010] FIG. 1 is a first conceptual diagram showing electronic conduction in an electrode. The composite particle 5 is disposed on the surface of the positive electrode current collector 11. The composite particle 5 includes a base particle 1 and a coating 2. The base particle 1 includes a positive electrode active material. The coating 2 covers the surface of the base particle 1. The coating 2 includes boron oxide. Electrons (e - ) move between the base particles 1. Furthermore, electrons move between the base particles 1 and the positive electrode current collector 11. The coating 2 can inhibit the movement of electrons. As a result, it is believed that the battery resistance increases.

[0011] It is also conceivable to increase the amount of conductive material 4 in order to form an electronic conductive path. However, even if conductive material 4 is added after film 2 is formed on the surface of base particle 1, it is considered difficult to form an electronic conductive path connecting base particles 1 with each other. Furthermore, it is considered that an increase in the amount of conductive material 4 reduces the energy density of the battery. It is considered inefficient to suppress the increase in resistance accompanying film formation by conductive material 4.

[0012] FIG. 2 is a second conceptual diagram showing electronic conduction in an electrode. The composite particle 5 in the present disclosure includes a carbon nanotube (CNT) 3 in addition to a base particle 1 and a coating 2. The CNT 3 can have high electronic conductivity. The CNT 3 includes a first portion and a second portion. The first portion is embedded in the coating 2. The second portion is exposed on the surface of the coating 2. The CNT 3 can form an electronic conduction path connecting the base particles 1 to each other. It is therefore believed that an increase in resistance due to the formation of the coating can be efficiently suppressed.

[0013] [2] The percentage of the substance amount of boron relative to the total substance amount of metal elements other than lithium (Li) and boron contained in the positive electrode active material, as measured by X-ray photoelectron spectroscopy (XPS), may be, for example, 80% or more.

[0014] Hereinafter, "the percentage of the substance amount of boron relative to the total substance amount of boron and metal elements excluding Li contained in the positive electrode active material, as measured by XPS" is also referred to as "boron ratio." The boron ratio is an index of the coverage rate of the base particles. It is considered that the higher the boron ratio, the higher the coverage rate of the base particles. With a boron ratio of 80% or more, an improvement in cycle life is expected.

[0015] [3] The percentage of the mass of the carbon nanotubes relative to the mass of the positive electrode active material may be, for example, 0.02% to 0.06%.

[0016] Hereinafter, "the percentage of the mass of carbon nanotubes relative to the mass of the positive electrode active material" is also referred to as "CNT ratio." A CNT ratio of 0.02% or more is expected to reduce battery resistance. A CNT ratio of 0.06% or less is expected to improve cycle life.

[0017] [4] A non-aqueous electrolyte secondary battery includes the positive electrode material according to any one of [1] to [3] above. The battery according to the present disclosure is expected to be excellent in energy density, battery resistance, and cycle life.

[0018] [5] The positive electrode material is used in a non-aqueous electrolyte secondary battery. The method for producing the positive electrode material includes the following steps (A) to (C). (A) A first precursor is prepared by mixing substrate particles, boric acid, carbon nanotubes and a solvent. (B) A second precursor is prepared by drying the first precursor. (C) Preparing composite particles by heating the second precursor. The positive electrode material comprises a plurality of composite particles. Each of the plurality of composite particles includes a substrate particle, a coating, and a carbon nanotube. The coating covers at least a portion of a surface of the substrate particle. The substrate particle includes a positive electrode active material. The coating includes boron oxide. The coating is formed such that the carbon nanotube includes a first portion and a second portion. The first portion is embedded in the coating. The second portion is exposed at a surface of the coating.

[0019] By mixing the CNTs during the formation of the coating, some of the CNTs may be incorporated into the coating, which may result in some of the CNTs being embedded in the coating and the remainder of the CNTs being exposed on the surface of the coating. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a first conceptual diagram showing electronic conduction in an electrode. [Diagram 2] FIG. 2 is a second conceptual diagram showing electronic conduction in an electrode. [Diagram 3] FIG. 3 is a schematic diagram showing an example of the nonaqueous electrolyte secondary battery of this embodiment. [Figure 4] FIG. 4 is a schematic flow chart of the method for producing the positive electrode material of this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, an embodiment of the present disclosure (hereinafter also referred to as "the present embodiment") will be described. However, the following description does not limit the scope of the claims. For example, in the present embodiment, a lithium ion battery will be described. However, a lithium ion battery is merely one example of a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery may be any battery system as long as it contains a non-aqueous electrolyte. For example, the non-aqueous electrolyte secondary battery may be a sodium ion battery or the like.

[0022] In this embodiment, unless otherwise specified, a description such as "0.02% to 0.06%" indicates a range including a boundary value. For example, "0.02% to 0.06%" indicates a range of "0.02% or more and 0.06% or less."

[0023] <Cathode materials> As shown in FIG. 2, the positive electrode material of this embodiment is composed of a plurality of composite particles 5. That is, the positive electrode material is a particle group (powder). The positive electrode material may have a median diameter of, for example, 1 μm to 30 μm. The positive electrode material may have a median diameter of, for example, 5 μm to 20 μm. The "median diameter" refers to the particle diameter at which the cumulative particle volume from the small particle diameter side is 50% of the total particle volume in the volume-based particle size distribution. The median diameter can be measured by a laser diffraction particle size distribution measuring device. Each of the composite particles 5 includes a base particle 1, a coating 2, and a CNT 3.

[0024] 《Base material particles》 The base particle 1 is the core of the composite particle 5. The base particle 1 may have a median diameter of, for example, 1 μm to 30 μm. The base particle 1 may have a median diameter of, for example, 5 μm to 20 μm.

[0025] The base particle 1 includes a positive electrode active material. The base particle 1 may be substantially composed of a positive electrode active material. The positive electrode active material is a so-called host-guest compound. The guest is lithium (Li). The host structure may include, for example, a transition metal element. The host structure may include, for example, a layered rock salt structure, a spinel structure, an olivine structure, or the like.

[0026] The positive electrode active material may contain at least one selected from the group consisting of, for example, lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, and lithium iron phosphate.

[0027] The positive electrode active material may be doped with various elements. The positive electrode active material may contain impurity elements. The composition of the positive electrode active material can be specified by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0028] The positive electrode active material may have, for example, the following formula (I): LiMeO2 (I) and may have a composition represented thereby.

[0029] In the above formula (I), "Me" represents a metal element. "Me" may contain, for example, at least one selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al).

[0030] The positive electrode active material may have, for example, the following formula (II): LiNi a Co b Mn c O2 (II) and may have a composition represented thereby.

[0031] In the above formula (II), for example, the relationship of "0 < a < 1, 0 < b < 1, 0 < c < 1, a + b + c = 1" may be satisfied. In the above formula (II), for example, the relationship of "0.5 < a < 1, 0 < b < 0.5, 0 < c < 0.5, a + b + c = 1" may be satisfied. In the above formula (II), for example, the relationship of "0.7 < a < 1, 0 < b < 0.3, 0 < c < 0.3, a + b + c = 1" may be satisfied. In the above formula (II), for example, the relationship of "0.8 ≤ a < 1, 0 < b < 0.2, 0 < c < 0.2, a + b + c = 1" may be satisfied.

[0032] The positive electrode active material may have, for example, the composition represented by the following formula (III): LiNi a Co b Al c O2(III) and may have a composition represented by

[0033] In the above formula (III), for example, the relationship of "0.7 < a < 1, 0 < b < 0.3, 0 < c < 0.3, a + b + c = 1" may be satisfied.

[0034] 《Coating》 The coating 2 covers at least a part of the surface of the base material particles 1. The coating 2 may cover only a part of the surface of the base material particles 1. The coating 2 may substantially cover the entire surface of the base material particles 1. The coating 2 is expected to inhibit direct contact between the base material particles 1 and the electrolyte. That is, the coating 2 is expected to inhibit oxidative decomposition of the electrolyte.

[0035] The composition of the coating 2 can be specified by, for example, XPS. The coating 2 contains boron oxide. The coating 2 may consist essentially of boron oxide. Boron oxide refers to a compound containing boron (B) and oxygen (O). Boron oxide may consist essentially of B and O. Boron oxide may be crystalline. Boron oxide may be amorphous. As long as boron oxide contains B and O, it may further contain other elements. For example, boron oxide may further contain Li. Boron oxide may consist essentially of B, O and Li. Boron oxide can have any composition ratio. For example, boron oxide may contain lithium borate (LiBO2, Li3BO3, Li2B4O7, etc.).

[0036] The percentage of the amount of boron with respect to the total amount of metal elements excluding Li contained in the positive electrode active material and boron (the "boron ratio") may be, for example, 56% or more. The boron ratio is an indicator of the coverage rate of the base material particles 1. It is considered that the higher the boron ratio, the higher the coverage rate of the base material particles 1. The boron ratio may be, for example, 80% or more. By the boron ratio being 80% or more, an improvement in cycle life is expected. The boron ratio may be, for example, 81% or more. The boron ratio may be, for example, 82% or more. The boron ratio may be, for example, 91% or more. The boron ratio may be, for example, 100% or less. The boron ratio may be, for example, 91% or less.

[0037] The boron ratio is measured by XPS. By XPS, the concentration of boron in the positive electrode material is measured. By XPS, the total concentration of metal elements excluding Li contained in the positive electrode active material and boron in the positive electrode material is measured. For example, when the composition of the positive electrode active material is represented by the above formula (II) "LiNi a Co b Mn c O2(0 < a < 1, 0 < b < 1, 0 < c < 1, a + b + c = 1)", the total concentration of metal elements excluding Li contained in the positive electrode active material and boron is equal to the total concentration of Ni, Co, Mn and B. The percentage of the value obtained by dividing the concentration of boron by the total concentration of metal elements excluding Li contained in the positive electrode active material and boron is regarded as the "boron ratio". That is, the boron ratio is calculated by the formula "boron ratio [%] = {B / (Ni + Co + Mn + B)} × 100". The boron ratio is measured 3 or more times. The arithmetic mean value of the measurement results of 3 or more times is adopted.

[0038] The measurement conditions of XPS are, for example, as follows. Apparatus: "PHI 5000 VersaProbe II" manufactured by ULVAC-PHI (equivalent products are acceptable) X-ray source: Monochromatized Al Kα ray (1486.6 eV) Acceleration voltage: 20 kV Power: 100 W Measurement mode: Wide scan Detection angle: 45°

[0039] Carbon nanotubes The CNT 3 includes a first portion and a second portion. The first portion is embedded in the coating 2. The first portion may be in contact with the base particle 1. The second portion refers to the remainder excluding the first portion. The second portion is exposed on the surface of the coating 2. For example, by observing the composite particle 5 with a scanning transmission electron microscope (STEM) or the like, it can be confirmed that the CNT 3 includes the first portion and the second portion.

[0040] The CNT3 may have high electronic conductivity. The CNT3 may form an electronic conduction path connecting the base particles 1 with each other. The CNT3 may be, for example, a single-walled carbon nanotube (SWCNT). The CNT3 may be, for example, a double-walled carbon nanotube (DWCNT). The CNT3 may be, for example, a multi-walled carbon nanotube (MWCNT).

[0041] The CNT3 may have a diameter of, for example, 1 nm to 100 nm. The CNT3 may have a diameter of, for example, 2 nm to 4 nm. The diameter of the CNT3 may be measured by a high-resolution transmission electron microscope (HRTEM). The diameter is measured for 10 or more CNTs3. The arithmetic average value of 10 or more CNTs is adopted.

[0042] The length of the CNT3 may be, for example, shorter than the median diameter of the base particle 1. The length of the CNT3 may be, for example, 0.1 to 0.5 times the median diameter of the base particle 1. The CNT3 may have a length of, for example, 1 μm to 10 μm. The CNT3 may have a length of, for example, 1 μm to 5 μm. The CNT3 may have a length of, for example, 2 μm to 4 μm. The length of the CNT3 may be measured by TEM or SEM (scanning electron microscope). The length is measured for 10 or more CNT3. The arithmetic average value of 10 or more CNTs is adopted.

[0043] The percentage of the mass of carbon nanotubes relative to the mass of the positive electrode active material ("CNT ratio") may be, for example, 0.01% to 0.07%. The CNT ratio may be, for example, 0.02% to 0.06%. A CNT ratio of 0.02% or more is expected to reduce battery resistance. A CNT ratio of 0.06% or less is expected to improve cycle life. The CNT ratio may be, for example, 0.04% or more. The CNT ratio may be, for example, 0.04% or less.

[0044] <Nonaqueous electrolyte secondary battery> FIG. 3 is a schematic diagram showing an example of the nonaqueous electrolyte secondary battery of this embodiment. The battery 100 includes a housing 90. The housing 90 has a cylindrical outer shape. However, the cylindrical shape is merely one example. The housing 90 may have a rectangular outer shape, for example. The housing 90 may be made of metal, for example. The housing 90 may be made of resin, for example. The housing 90 may be a pouch made of aluminum laminate film, for example.

[0045] The housing 90 houses the electricity storage element 50 and the electrolyte. The housing 90 may be sealed. The electricity storage element 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. The electricity storage element 50 is of a wound type. The electricity storage element 50 is formed by winding a strip-shaped electrode in a spiral shape. The electricity storage element 50 may be of a stack type. The electricity storage element 50 may be formed by stacking three or more sheets of leaf-shaped electrodes.

[0046] 《Positive electrode》 The positive electrode 10 may be in the form of a sheet. The positive electrode 10 may have a thickness of, for example, 10 μm to 200 μm. The positive electrode 10 includes a positive electrode material. That is, the battery 100 includes a positive electrode material. The positive electrode 10 may include a conductive material, a binder, a positive electrode current collector, and the like in addition to the positive electrode material. The positive electrode 10 may be formed, for example, by coating a slurry. That is, a positive electrode material, a conductive material, a binder, a dispersion medium, and the like are mixed to prepare a positive electrode slurry. The positive electrode 10 may be formed by coating the positive electrode slurry on the surface of a positive electrode current collector and drying it.

[0047] The positive electrode current collector may include, for example, an Al foil, etc. The positive electrode current collector may have a thickness of, for example, 5 μm to 50 μm.

[0048] The conductive material may include any component. The conductive material may include at least one selected from the group consisting of carbon black and graphene flakes. The carbon black may include acetylene black, for example. The amount of the conductive material may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode material.

[0049] The binder may contain any component. For example, the binder may contain at least one selected from the group consisting of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) and tetrafluoroethylene (PTFE). The amount of the binder may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode material.

[0050] 《Negative electrode》 The negative electrode 20 may be in the form of a sheet. The negative electrode 20 may have a thickness of, for example, 10 μm to 200 μm. The negative electrode 20 includes a negative electrode active material. The negative electrode 20 may include a conductive material, a binder, a negative electrode current collector, and the like in addition to the negative electrode active material. The negative electrode 20 may be formed, for example, by coating a slurry. That is, a negative electrode active material, a conductive material, a binder, a dispersion medium, and the like are mixed to prepare a negative electrode slurry. The negative electrode 20 may be formed by coating the negative electrode slurry on the surface of a negative electrode current collector and drying it.

[0051] The negative electrode active material may include any component. For example, the negative electrode active material may include at least one selected from the group consisting of graphite, soft carbon, hard carbon, silicon (Si), silicon oxide (SiO), silicon-based alloy, tin (Sn), tin oxide (SnO), tin-based alloy, and lithium titanate.

[0052] The negative electrode current collector may include, for example, copper (Cu) foil, etc. The negative electrode current collector may have a thickness of, for example, 5 μm to 50 μm.

[0053] The conductive material may include any component. For example, the conductive material may include carbon black. The amount of the conductive material may be, for example, 0.1 parts by mass to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material.

[0054] The binder may contain any component. For example, the binder may contain at least one selected from the group consisting of carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and polyacrylic acid (PAA). The amount of the binder may be, for example, 0.1 parts by mass to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material.

[0055] <Separator> The separator 30 is in the form of a film. The separator 30 may have a thickness of, for example, 5 μm to 30 μm. The separator 30 is interposed between the positive electrode 10 and the negative electrode 20. The separator 30 physically separates the positive electrode 10 and the negative electrode 20. The separator 30 is electrically insulating. The separator 30 may be made of, for example, polyolefin. The separator 30 is permeable to Li ions. The separator 30 is porous. The separator 30 may have a porosity of, for example, 30% to 60%. The porosity can be measured by mercury intrusion porosimetry.

[0056] The separator 30 may have a single-layer structure. For example, the separator 30 may be composed of only a polyethylene (PE) layer. The separator 30 may have a multi-layer structure. For example, the separator 30 may have a three-layer structure. The separator 30 may be formed by laminating a polypropylene (PP) layer, a PE layer, and a PP layer in this order.

[0057] 《Electrolytes》 The electrolyte is a Li-ion conductor. The electrolyte may be a liquid, a gel, or a solid. The solid electrolyte may also function as a separator.

[0058] The electrolyte may include, for example, a solvent and a supporting salt. The solvent is aprotic. The solvent can dissolve the supporting salt. The solvent may include, for example, at least one selected from the group consisting of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0059] The supporting salt may include, for example, at least one selected from the group consisting of LiPF6, LiBF4, and Li(FSO2)2N. The concentration of the supporting salt may be, for example, 0.5 mol / L to 2 mol / L.

[0060] The electrolyte may further contain various additives in addition to the solvent and the supporting salt, such as at least one selected from the group consisting of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), 1,3-propane sultone (PS), cyclohexylbenzene (CHB), tert-amylbenzene (TAB), and lithium bis(oxalato)borate (LiBOB).

[0061] <Method of manufacturing positive electrode material> FIG. 4 is a schematic flow chart of the method for producing the positive electrode material of this embodiment. The manufacturing method of the positive electrode material includes "(A) wet mixing", "(B) drying" and "(C) heating".

[0062] 《(A) Wet mixing》 The method for producing the positive electrode material includes preparing a first precursor by mixing substrate particles 1, boric acid, CNTs 3 and a solvent.

[0063] A powder of base particles 1 is prepared. Details of the base particles 1 are as described above. The base particles 1 contain a positive electrode active material.

[0064] CNT3 is prepared. A powder of CNT3 may be prepared. For example, a dispersion of CNT3 may be prepared. The dispersion medium may include, for example, water. The dispersion may include a dispersant. The dispersant may include, for example, polyvinylpyrrolidone (PVP). A plurality of CNT3 may aggregate to form a bundle. The dispersion may be prepared, for example, by a bead mill. For example, the bundle may be crushed by a bead mill in the presence of PVP. It is expected that the bundle is loosened and the CNT3 is chopped. The CNT3 before chopping may have a length of, for example, 10 μm to 100 μm. The CNT3 before chopping may have a length of, for example, 10 μm to 30 μm. The CNT3 after chopping may have a length of, for example, 1 μm to 10 μm. The CNT3 after chopping may have a length of, for example, 1 μm to 5 μm. The chopped CNTs 3 may have a length of, for example, 2 μm to 4 μm. The blending amount (mass) of the CNTs 3 may be, for example, 0.01% to 0.07% with respect to the mass of the positive electrode active material.

[0065] Boric acid is a raw material of the coating 2 (boron oxide). The boric acid may contain, for example, orthoboric acid (H3BO3) or the like. The blending amount (substance amount) of the boric acid may be, for example, 0.1% to 10% with respect to the substance amount of the positive electrode active material. The blending amount of the boric acid may be, for example, 0.5% to 2% with respect to the substance amount of the positive electrode active material.

[0066] A solvent is prepared. In this embodiment, a solvent capable of dissolving boric acid is selected. The solvent may include, for example, water, ethanol, glycerin, etc. The solvent may consist essentially of water.

[0067] The base particle 1, boric acid, CNTs 3, and a solvent are mixed by an arbitrary stirrer or the like. The mixing conditions can be adjusted so that the liquid component and CNTs 3 are uniformly attached to the surface of the base particle 1. By mixing, a first precursor (wet mixture) is prepared.

[0068] 《(B) Drying》 A method for making a positive electrode material includes preparing a second precursor by drying a first precursor.

[0069] The first precursor may be dried by any method. For example, the first precursor may be naturally dried. For example, a dryer or the like may be used. Drying conditions are adjusted so that the solvent contained in the first precursor is substantially entirely evaporated. A second precursor (dried mixture) is prepared by drying.

[0070] 《(C)Heating》 The method for producing the positive electrode material includes preparing the composite particles 5 by heating the second precursor.

[0071] For example, an electric furnace or the like may be used. The heating atmosphere may be, for example, air or the like. The second precursor is fired by heating. As a result, boron oxide is generated on the surface of the base particle 1. That is, a coating 2 is formed on the surface of the base particle 1. The heating temperature may be, for example, 160°C to 300°C.

[0072] After heating, for example, agglomerates of the composite particles 5 may be crushed. A positive electrode material is formed from a plurality of composite particles 5. That is, the positive electrode material is made of a plurality of composite particles.

[0073] In this embodiment, the CNTs 3 are mixed with the raw material of the coating 2. Therefore, the coating 2 can be formed such that the CNTs 3 include a first portion and a second portion. The first portion is embedded in the coating 2. The second portion is exposed on the surface of the coating 2. EXAMPLES

[0074] Hereinafter, an embodiment of the present disclosure (hereinafter also referred to as "the present embodiment") will be described. However, the following description does not limit the scope of the claims.

[0075] <Example 1> <Production of cathode materials> 《(A) Wet mixing》 The following materials were prepared: Base material particles: Positive electrode active material (LiNi 0.8 Co 0.1 Mn 0.1 O2) Boric Acid: H3BO3 CNT: SWCNT (diameter 2nm to 4nm, length 10μm to 30μm) Solvent: Water

[0076] The base particles, boric acid, carbon nanotubes, and a solvent were mixed together to prepare a first precursor. The amount (substance amount) of boric acid was 1% relative to the substance amount of the positive electrode active material.

[0077] 《(B) Drying》 The first precursor was dried in a dryer, thereby preparing the second precursor.

[0078] 《(C)Heating》 The second precursor was heated in an electric furnace. The heating atmosphere was air. The heating temperature was 250°C. As a result, composite particles 5 were formed. The aggregates of the composite particles 5 were crushed. As a result, a plurality of composite particles 5 were formed. That is, a positive electrode material was produced. The timing of mixing the CNTs in Example 1 was "when the coating was formed".

[0079] <Boron ratio measurement> The boron ratio of the positive electrode material was measured by XPS. In this example, the percentage of the concentration of B relative to the total concentration of Ni, Co, Mn, and B was considered as the "boron ratio."

[0080] <Manufacture of non-aqueous electrolyte secondary battery> <Production of positive electrodes> The following materials were prepared: Conductive material: Acetylene black Binder: PVdF Dispersion medium: N-methyl-2-pyrrolidone Positive electrode collector: Aluminum foil (thickness 15 μm)

[0081] The positive electrode material, conductive material, binder, and dispersion medium obtained above were mixed to prepare a positive electrode slurry. The positive electrode slurry was applied to the surface of a positive electrode current collector and dried to manufacture a positive electrode. The material composition was "positive electrode material / conductive material / binder = 96.3 / 2.5 / 1.2 (mass ratio)".

[0082] <Production of negative electrodes> The following materials were prepared: Negative electrode active material: graphite Binder: "CMC / SBR=1 / 1 (mass ratio)" Dispersion medium: water Negative electrode current collector: Cu foil (thickness 10 μm)

[0083] The negative electrode active material, the binder, and the dispersion medium were mixed to prepare a negative electrode slurry. The negative electrode slurry was applied to the surface of the negative electrode current collector and dried to produce a negative electrode. The material composition was "negative electrode active material / binder = 100 / 2 (mass ratio)".

[0084] "assembly" A separator was prepared. The separator had a three-layer structure. The separator was formed by laminating a PP layer, a PE layer, and a PP layer in this order.

[0085] An electric storage element was assembled. The electric storage element was of a wound type. The electric storage element included a positive electrode, a negative electrode, and a separator. A cylindrical housing was prepared. The electric storage element was housed in the housing.

[0086] An electrolyte was prepared, which contained the following components: Solvent: "EC / DMC / EMC=3 / 4 / 3 (volume ratio)" Supporting salt: LiPF6 (concentration 1mol / L) Additive: VC (concentration 2% by mass)

[0087] The electrolyte was injected into the housing. After the electrolyte was injected, the housing was sealed. Thus, the battery was manufactured. The battery was a cylindrical 18650 size (diameter 18 mm, height 65 mm). The design capacity of the battery was 1 Ah.

[0088] <Examples 2 to 7> As shown in Table 1 below, a positive electrode material and a nonaqueous electrolyte secondary battery were manufactured in the same manner as in Example 1, except that the "boron ratio" and "CNT ratio" were changed.

[0089] <Comparative Example 1> In Comparative Example 1, untreated substrate particles were used as the positive electrode material as they were. Except for this, a battery was manufactured in the same manner as in Example 1.

[0090] <Comparative Example 2> A cathode material and battery were fabricated similarly to Example 1, except that no CNTs were used.

[0091] <Comparative Example 3> As in Comparative Example 2, a positive electrode material not containing CNT was produced. A positive electrode slurry was prepared by mixing a positive electrode material, CNT, acetylene black, a binder, and a dispersion medium. A positive electrode was produced by applying the positive electrode slurry to a positive electrode current collector and drying it. A battery was produced in the same manner as in Comparative Example 2, except for these. The timing of mixing CNT in Comparative Example 3 was "when preparing the positive electrode slurry."

[0092] <Comparative Example 4> Untreated base particles were used as the positive electrode material as in Comparative Example 1. Except for this, a battery was manufactured in the same manner as in Comparative Example 3. The timing of mixing CNTs in Comparative Example 4 was "when preparing the positive electrode slurry."

[0093] In Comparative Examples 3 and 4, the material composition was "positive electrode material / (CNT+acetylene black) / binder=96.3 / 2.5 / 1.2 (mass ratio)."

[0094] <Evaluation> Battery Resistance In this embodiment, "1C" indicates the amount of current at which the fully charged capacity is discharged in one hour. The battery's SOC (state of charge) was adjusted to 50%. After adjusting the SOC, the battery was discharged for 10 seconds with a discharge current of 10C in a temperature environment of 25°C. The voltage drop 10 seconds after the start of discharge was measured. The battery resistance was calculated by dividing the voltage drop by the discharge current.

[0095] Cycle Test The battery was charged and discharged 100 times at a temperature of 25°C. One cycle is a cycle of constant current-constant voltage (CCCV) charging and constant current (CC) discharging. The conditions for CCCV charging and CC discharging are as follows:

[0096] CCCV charging: charging current 0.5C, end voltage 4.2V, end current 0.05C CC discharge: Discharge current 0.5C, final voltage 3.0V

[0097] The capacity retention rate was determined. The capacity retention rate is the percentage of the discharge capacity at 100 cycles to the discharge capacity at 1 cycle. The capacity retention rate is shown in Table 1 below. It is considered that the higher the capacity retention rate, the better the cycle life.

[0098] [Table 1]

[0099] <Result> In the above Table 1, for example, in Comparative Examples 1 and 2, the formation of the film tends to improve the cycle life. At the same time, the formation of the film also tends to increase the battery resistance.

[0100] For example, in Comparative Examples 1 and 4, the addition of CNTs tends to reduce the battery resistance. Even when CNTs are added alone, the cycle life tends not to change substantially.

[0101] In Comparative Example 3, CNTs are mixed in when preparing the positive electrode slurry. Therefore, it is considered that some of the CNTs are not embedded in the coating of Comparative Example 3. The boron ratio in Comparative Example 3 is 80%. The CNT ratio in Comparative Example 3 is 0.02%. The battery resistance in Comparative Example 3 is 44 mΩ.

[0102] In Example 4, CNT is mixed in when the coating is formed. Therefore, in Example 4, it is considered that a part of the CNT is embedded in the coating, and the remaining part of the CNT is exposed on the surface of the coating. The boron ratio in Example 4 is 82%. The CNT ratio in Example 4 is 0.01%. The battery resistance in Example 4 is 44 mΩ. Example 4 has a higher boron ratio and a lower CNT ratio than Comparative Example 3. Therefore, it is considered that the battery resistance of Example 4 is higher than that of Comparative Example 3. However, Example 4 shows a battery resistance equivalent to that of Comparative Example 3. It is considered that the increase in resistance due to the coating formation is efficiently suppressed in Example 4 compared to Comparative Example 3.

[0103] In Examples 1 to 7, when the boron ratio is 80% or more, there is a tendency for the cycle life (capacity retention rate) to improve.

[0104] In Examples 1 to 7, when the CNT ratio is 0.02% or more, the battery resistance tends to be low. When the CNT ratio is 0.06% or less, the capacity retention rate tends to be high. This embodiment and this example are illustrative in all respects. This embodiment and this example are not limiting. The technical scope defined based on the claims includes all modifications in the sense equivalent to the claims. [Explanation of symbols]

[0105] 1 base particle, 2 coating, 3 carbon nanotube (CNT), 4 conductive material, 5 composite particle, 10 positive electrode, 11 positive electrode current collector, 20 negative electrode, 30 separator, 50 storage element, 90 housing, 100 battery (nonaqueous electrolyte secondary battery).

Claims

1. A positive electrode material for use in a nonaqueous electrolyte secondary battery containing a liquid electrolyte, comprising: Consists of a plurality of composite particles, each of the plurality of composite particles includes a substrate particle, a coating, and a carbon nanotube; The coating covers at least a portion of the surface of the base particle, The substrate particles include a positive electrode active material, the coating comprises boron oxide; the carbon nanotube includes a first portion and a second portion; the first portion is embedded in the coating; the second portion is exposed at a surface of the coating, a percentage of the substance amount of boron relative to a total substance amount of metal elements excluding lithium and boron contained in the positive electrode active material, as measured by X-ray photoelectron spectroscopy, is 56% or more and 82% or less; The percentage of the mass of the carbon nanotubes relative to the mass of the positive electrode active material is 0.01% to 0.04%. Positive electrode material.

2. The length of the carbon nanotubes is 0.1 to 0.5 times the median diameter of the substrate particle. The positive electrode material according to claim 1 .

3. The length of the carbon nanotubes is 2 μm to 4 μm. The positive electrode material according to claim 1 or 2.

4. The positive electrode material according to any one of claims 1 to 3, Nonaqueous electrolyte secondary battery.

5. A method for producing a positive electrode material according to any one of claims 1 to 3, comprising the steps of: preparing a first precursor by mixing the substrate particles, boric acid, the carbon nanotubes and a solvent; preparing a second precursor by drying the first precursor; and preparing said composite particles by heating said second precursor; Including, A method for producing a positive electrode material.

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