Positive electrode active material, electrode, and battery

Abstract

To improve rate characteristics while maintaining energy density. [Solution] The positive electrode active material contains a plurality of secondary particles. The cross-section of the secondary particles consists of a central part and an outer periphery. The central part contains a plurality of first primary particles having a first average particle size. The outer periphery contains a plurality of second primary particles having a second average particle size. The first primary particles and the second primary particles contain an olivine-type phosphate compound. d2

Description

[Technical Field]

[0001] This disclosure relates to positive electrode active materials, electrodes, and batteries. [Background technology]

[0002] Japanese Patent Publication No. 2019-40854 (Patent Document 1) discloses a lithium manganese iron phosphate particle comprising a core portion containing lithium manganese iron phosphate nanoparticles having a first average particle diameter and a shell portion containing lithium manganese iron phosphate nanoparticles having a second average particle diameter larger than the first average particle diameter. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-40854 [Overview of the project] [Problems that the invention aims to solve]

[0004] Olivine-type phosphate compounds are being considered as positive electrode active materials. Reducing the size of the primary particles of olivine-type phosphate compounds improves the rate characteristics, but may decrease the energy density. There is room for improvement in balancing energy density and rate characteristics (e.g., discharge capacity and high-current discharge performance).

[0005] The purpose of this disclosure is to improve rate characteristics while maintaining energy density. [Means for solving the problem]

[0006] The technical configuration and effects of this disclosure are described below. However, the mechanism of action of this disclosure is based on assumptions. The validity of the mechanism of action does not limit the scope of the claims.

[0007] [1] A positive electrode active material, The positive electrode active material includes a plurality of secondary particles, The cross-section of the secondary particle consists of a central portion and an outer peripheral portion, The central portion includes a plurality of first primary particles having a first average particle size, The outer peripheral portion includes a plurality of second primary particles having a second average particle size, The first primary particles and the second primary particles contain an olivine-type phosphate compound, The relationship of d2 < d1 is satisfied, The d1 represents the first average particle size, The d2 represents the second average particle size, a positive electrode active material.

[0008] In a high-density electrode or a thick-film electrode, it becomes more difficult to secure a lithium (Li) ion movement path to the inside of the electrode, and the Li ion supply in the electrode thickness direction and the particle radius direction is insufficient, which may lead to performance degradation. In particular, when a positive electrode active material with a large particle size is applied in a thick-film electrode, the Li ion movement distance becomes longer, so it is more prominent. By reducing the size of the primary particles, the diffusibility of Li ions is improved, but the amount of pores in the secondary particles increases, so the energy density may decrease. According to the configuration of [1], by arranging particles with a large average particle size in the central portion of the secondary particle and particles with a small average particle size in the outer peripheral portion, improvement in rate characteristics is expected while maintaining the energy density.

[0009] 〔2〕The positive electrode active material according to 〔1〕, in which the relationship of d2 ≤ 50 nm and 100 nm ≤ d1 is satisfied.

[0010] 〔3〕The D50 of the secondary particle is 5 μm or more, The pore volume of the secondary particle is 0.10 cm 3 / g or more and 0.15 cm 3 / g or less, the positive electrode active material according to 〔1〕 or 〔2〕.

[0011] 〔4〕Including a positive electrode layer, The positive electrode layer contains the positive electrode active material according to any one of 〔1〕 to 〔3〕, The basis weight of the positive electrode layer is 28 mg / cm³. 2 That's all for the electrodes.

[0012] A battery including the electrodes described in [5] and [4]. [Brief explanation of the drawing]

[0013] [Figure 1] This is a conceptual diagram showing secondary particles in this embodiment. [Figure 2] This is a schematic flowchart illustrating the method for producing the positive electrode active material in this embodiment. [Figure 3] This is a schematic perspective view of the battery in this embodiment. [Figure 4] This is a schematic cross-sectional view along the line IV-IV in Figure 3. [Figure 5] This is a table showing the experimental results. [Modes for carrying out the invention]

[0014] Embodiments of the present disclosure (hereinafter abbreviated as "Embodiments") and examples of the present disclosure (hereinafter abbreviated as "Examples") are described below. However, these embodiments and examples do not limit the technical scope of the present disclosure.

[0015] <Cathode active material> The positive electrode active material in this embodiment includes a plurality of secondary particles 2. The positive electrode active material may also be an aggregate of a plurality of secondary particles 2. That is, the positive electrode active material may be a powder. The D50 of the positive electrode active material may be, for example, 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The D50 of the positive electrode active material may be, for example, 50 μm or less, 45 μm or less, 40 μm or less, 35 μm or less, 30 μm or less, or 25 μm or less. "D50" indicates the particle size at which the cumulative value in the volume-based particle size distribution (cumulative distribution) becomes 50%. D50 is measured by a laser diffraction particle size distribution analyzer.

[0016] Figure 1 is a conceptual diagram showing a secondary particle in this embodiment. A minimum circumscribed circle is fitted to the secondary particle 2. The radius of the minimum circumscribed circle is D. That is, the diameter of the minimum circumscribed circle is 2D. The circular portion that includes the center of the minimum circumscribed circle and has a radius less than D is considered to be the "center 2a". The radius of the center 2a may be, for example, 0.5D. In the cross-section of the secondary particle 2, the remainder excluding the center 2a is considered to be the "outer periphery 2b". That is, the cross-section of the secondary particle 2 consists of the center 2a and the outer periphery 2b. The outer periphery 2b surrounds the center 2a.

[0017] The central region 2a contains a plurality of first primary particles 1a having a first average particle size. The central region 2a may consist of a plurality of first primary particles 1a. d1 indicates the first average particle size of the first primary particles 1a. d1 may be, for example, 100 nm or more, 120 nm or more, 140 nm or more, or 160 nm or more. d1 may also be, for example, 200 nm or less, or 180 nm or less. In this specification, the average particle size of the primary particles may be the average value of the maximum Ferret diameter. "Maximum Ferret diameter" indicates the length of the long side of the minimum circumscribing rectangle (rectangle or square) of the particle. If the minimum circumscribing rectangle is a square, the length of the long side indicates the length of one side. The maximum Ferret diameter of the primary particles can be measured, for example, in a TEM (Transmission Electron Microscopy) image. The average particle size of the first primary particles 1a indicates the arithmetic mean of 10 first primary particles 1a. Here, the measurement area for the average particle size of the first primary particle 1a is the area inside the circular portion that includes the center of the smallest circumscribed circle of the secondary particle 2 and has a radius of 0.25D.

[0018] The outer peripheral portion 2b contains a plurality of second primary particles 1b having a second average particle size. The outer peripheral portion 2b may be composed of a plurality of second primary particles 1b. d2 represents the second average particle size of the second primary particles 1b. d2 may be, for example, 80 nm or less, 60 nm or less, 50 nm or less, or 40 nm or less. d2 may be, for example, 10 nm or more, 20 nm or more, or 30 nm or more. The average particle size of the second primary particles 1b represents the arithmetic mean of 10 second primary particles 1b. Here, the measurement region of the average particle size of the second primary particles 1b is the remaining portion excluding a circular portion having a radius of 0.75D and including the center of the minimum circumscribed circle of the secondary particle 2 in the cross-section of the secondary particle 2.

[0019] In the present embodiment, the relationship d2 < d1 is satisfied. That is, d1 - d2 is greater than 0 nm. By satisfying the relationship d2 < d1, an improvement in rate characteristics is expected while maintaining the energy density. d1 - d2 may be, for example, 50 nm or more, 100 nm or more, or 120 nm or more. d1 - d2 may be, for example, 200 nm or less, 180 nm or less, or 150 nm or less. Further, d1 and d2 may satisfy, for example, the relationship d2 ≤ 50 nm and 100 nm ≤ d1.

[0020] The D50 of the secondary particle 2 may be, for example, 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The D50 of the secondary particle 2 may be, for example, 50 μm or less, 45 μm or less, 40 μm or less, 35 μm or less, 30 μm or less, or 25 μm or less.

[0021] The pore volume of the secondary particle 2 is, for example, 0.10 cm 3 / g or more, 0.11 cm 3 / g or more, 0.12 cm 3 / g or more, 0.13 cm 3 / g or more, 0.14 cm 3 / g or more, or 0.15 cm 3 / g or more. The pore volume of the secondary particle 2 is, for example, 0.20 cm 3 / g or less, 0.19 cm 3 / g or less, 0.18cm 3 / g or less, 0.17cm 3 / g or less, 0.16cm 3 Less than / g, or 0.15cm 3 The concentration may be less than / g. The pore size of secondary particle 2 can be measured by the BJH (Barret-Joyner-Halenda) multipoint method.

[0022] Carbon may be attached to at least a portion of the surface of the first primary particle 1a and the second primary particle 1b. The carbon may form a carbon layer. Hereinafter, "first primary particle 1a and second primary particle 1b" may be abbreviated as "primary particle". The amount of attached carbon may be, for example, 0.1% or more, 0.5% or more, 1% or more, 2% or more, 3% or more, or 4% or more by mass fraction relative to the primary particle. The amount of attached carbon may be, for example, 5% or less, 4% or less, or 3% or less by mass fraction relative to the primary particle.

[0023] The primary particles contain an olivine-type phosphate compound. "Olivine-type" refers to a crystalline structure belonging to the space group Pnma. The space group is identified by powder X-ray diffraction (XRD) measurement. The primary particles may be, for example, a single-phase compound. The primary particles may further contain phases belonging to other space groups, as long as they contain an olivine-type crystalline phase. The primary particles may further contain, for example, an amorphous phase.

[0024] The olivine-type phosphate compound may contain, for example, at least one selected from the group consisting of lithium iron phosphate (LFP), lithium manganese phosphate (LMP), and lithium iron manganese phosphate (LMFP). The olivine-type phosphate compound may have, for example, a composition represented by the following general formula. Li a Mn 1-x Fe x PO4 For example, the relationship "0.5 ≤ a ≤ 1.5" may be satisfied. x may be, for example, 0 or greater, 0.05 or greater, 0.1 or greater, 0.2 or greater, 0.3 or greater, 0.4 or greater, 0.5 or greater, 0.6 or greater, 0.7 or greater, 0.8 or greater, or 0.9 or greater. x may be, for example, 1 or less, 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less.

[0025] In olivine-type phosphate compounds, elements other than Li, manganese (Mn), iron (Fe), phosphorus (P), and oxygen (O) may be doped (dopants). The amount of doping (molecular fraction relative to the amount of Li) may be, for example, 0.01 to 0.1.

[0026] The olivine-type phosphate compounds contained in the first primary particle 1a and the second primary particle 1b may have the same composition or different compositions. For example, the Mn ratios of the first primary particle 1a and the second primary particle 1b may be different. In this case, the Mn content of the first primary particle 1a may be greater than that of the second primary particle 1b, or vice versa.

[0027] The positive electrode active material may further contain other components, as long as it contains an olivine-type phosphate compound. These other components may include, for example, lithium nickel composite oxide (LNO), lithium cobalt composite oxide (LCO), lithium manganese composite oxide (LMO), etc. The mixing ratio (mass ratio) of the olivine-type phosphate compound to the other components may be, for example, "olivine-type phosphate compound / other components = 9 / 1 to 1 / 9", "olivine-type phosphate compound / other components = 8 / 2 to 2 / 8", "olivine-type phosphate compound / other components = 7 / 3 to 3 / 7", or "olivine-type phosphate compound / other components = 6 / 4 to 4 / 6". The positive electrode active material may also be, for example, a mixture of powdered olivine-type phosphate compound and powdered other components.

[0028] LNO may have, for example, a crystal structure belonging to the space group R-3m. LNO may have, for example, a composition represented by the following general formula. Li 1-a Ni x M 1-x O2 In the formula, the relationships -0.5 ≤ a ≤ 0.5 and 0 ≤ x ≤ 1 are satisfied. M may contain, for example, at least one selected from the group consisting of Co, Mn, and Al.

[0029] LNO may be represented, for example, by the following general formula. The compound represented by the following general formula may also be referred to as "NCM". Li 1-a Ni x Co y Mn z O2 In the formula, the relationships -0.5 ≤ a ≤ 0.5, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1 are satisfied.

[0030] LNO may be represented, for example, by the following general formula. The compound represented by the following general formula may also be referred to as "NCA". Li 1-a Ni x Co y Al z O2 In the formula, the relationships -0.5 ≤ a ≤ 0.5, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1 are satisfied.

[0031] <Method for manufacturing a positive electrode active material> FIG. 2 is a schematic flowchart of the method for manufacturing a positive electrode active material in the present embodiment. Hereinafter, "the method for manufacturing a positive electrode active material in the present embodiment" may be abbreviated as "this method". This method may include, for example, "(a) formation of a slurry", "(b) granulation", and "(c) firing", etc.

[0032] (a) Formation of a slurry This method may include forming a slurry by mixing a lithium compound, a manganese compound, an iron compound, a phosphate compound, and a solvent. For example, a compound with the composition formula "Li a Mn 1-x Fe x Lithium compounds, manganese compounds, phosphate compounds, and iron compounds may be weighed out to achieve the composition ratio (mole ratio) shown in PO4 (0.5 ≤ a ≤ 1.5, 0 ≤ x < 1). The lithium compound may include, for example, lithium hydroxide. The manganese compound may include, for example, manganese carbonate. The phosphate compound may include, for example, lithium dihydrogen phosphate. The iron compound may include, for example, ferric phosphate.

[0033] When carbon is attached to the surface of primary particles, a carbon source is added to the raw material mixture. The carbon source may include, for example, sugars, organic acids, etc. The carbon source may also include, for example, glucose, sucrose, fructose, citric acid, etc. The amount of carbon source added may be, for example, 1 to 20% by mass fraction relative to the raw material mixture.

[0034] The solvent may include, for example, water. The solid content concentration of the slurry may be, for example, 20 to 40% by mass fraction.

[0035] The particle size in the slurry may be adjusted by wet grinding. For example, wet grinding may be performed so that D50 is between 0.10 and 1 μm.

[0036] (b) Granulation This method may include granulation of secondary particles (precursors) by drying the slurry. For example, secondary particles may be granulated by spray drying. There may be two or more types of slurry; for example, slurries with different particle sizes or solid content concentrations may be used. The secondary particles formed by the granulation operation are also called "granulated bodies." In other words, secondary particles may be referred to as granulated bodies. Spray drying may be performed two or more times. For example, the central part of the granulated body may be formed by the first spray drying, and the outer part of the granulated body may be formed by the second spray drying. In this case, by using slurries with different particle sizes, granulated bodies with different average particle sizes in the central and outer parts may be formed.

[0037] The size of secondary particles tends to change depending on, for example, the gas-liquid ratio of the atomizing gas to the slurry during spray drying. For instance, the size of secondary particles tends to decrease as the nozzle pressure increases.

[0038] (c) Firing This method may include generating an olivine-type phosphate compound by heat-treating secondary particles (precursors). Any heat treatment furnace (e.g., electric furnace, muffle furnace, etc.) can be used. The heat treatment atmosphere may be, for example, a nitrogen atmosphere. The heat treatment temperature may be, for example, 400 to 700°C. The heat treatment time may be, for example, 4 to 6 hours. If spray drying is performed two or more times in the granulation process, a calcination process may be performed after each spray drying.

[0039] <Battery> In some embodiments of this invention, the battery has a monopolar structure. In some embodiments of this invention, the battery has a bipolar structure. As an example, a battery having a bipolar structure (a bipolar battery) will be described.

[0040] Figure 3 is a schematic perspective view of the battery in this embodiment. Figure 4 is a schematic cross-sectional view along the line IV-IV in Figure 3. Hereinafter, "orthoplane direction" refers to the direction normal to the surface of a sheet-like member (e.g., foil, electrode, etc.). "In-plane direction" refers to any direction perpendicular to the orthoplane direction. In the figures of this embodiment, the Z-axis direction corresponds to the orthoplane direction. The X-axis and Y-axis directions are examples of in-plane directions.

[0041] The battery 100 includes an outer casing 90 and a power generation element 50. The outer casing 90 houses the power generation element 50. The outer casing 90 may include, for example, a first current collector plate 91, a first laminate film 92, a second laminate film 93, and a second current collector plate 94. The first laminate film 92 and the second laminate film 93 are joined to each other at their in-plane edges. At the joint between the first laminate film 92 and the second laminate film 93, a sealing material (not shown) may be interposed between the first laminate film 92 and the second laminate film 93.

[0042] The first current collector plate 91 and the second current collector plate 94 are joined to the power generation element 50 at their ends in the stacking direction (Z-axis direction). The first laminate film 92 is joined to the first current collector plate 91. The second laminate film 93 is joined to the second current collector plate 94. A sealing material (not shown) may be interposed between the current collector plate and the laminate film at the joint between the current collector plate and the laminate film.

[0043] The power generation element 50 includes a plurality of bipolar electrodes 10. The plurality of bipolar electrodes 10 are stacked in the direction perpendicular to the plane (Z-axis direction). Each of the plurality of bipolar electrodes 10 includes, in the direction perpendicular to the plane, a positive electrode layer 11, a current collector foil 13, and a negative electrode layer 12 in this order. In the in-plane direction (for example, in the X-axis direction), the current collector foil 13 extends outward relative to the positive electrode layer 11 and the negative electrode layer 12. For example, the current collector foil 13 may extend outward relative to the positive electrode layer 11 and the negative electrode layer 12 over the entire circumference in the in-plane direction.

[0044] The current collector foil 13 is a conductor. The current collector foil 13 may include, for example, a metal foil, a conductive resin layer, etc. For example, the current collector foil 13 may be formed by bonding an Al foil and a Cu foil together. A carbon material may be coated on the surface of the current collector foil 13. The carbon material may include, for example, carbon black.

[0045] The power generation element 50 includes a sealing material 30. At its in-plane end, the sealing material 30 is joined to the current collector foil 13. The sealing material 30 may, for example, be heat-welded to the current collector foil 13. For example, the sealing material 30 may be arranged around the entire circumference of the in-plane periphery. The sealing material 30 may include, for example, a resin material. The sealing material 30 seals between adjacent current collector foils 13 in the direction perpendicular to the plane. The sealing material 30 between the current collector foils 13 partitions the cells 40. A cell 40 is the smallest unit of the power generation element 50. The battery 100 includes a plurality of cells 40 and may therefore also be called a "bipolar module". Each of the plurality of cells 40 is sealed. The plurality of cells 40 are isolated from each other. Each of the plurality of cells 40 includes a positive electrode layer 11, a separator 20, a negative electrode layer 12, and an electrolyte.

[0046] (Positive electrode layer) The positive electrode layer 11 is attached to one side of the current collector foil 13. The positive electrode layer 11 contains a positive electrode active material. Details of the positive electrode active material are as described above.

[0047] The positive electrode layer 11 may further contain, in addition to the positive electrode active material, a conductive material and a binder, for example. The amount of conductive material may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of positive electrode active material. The conductive material may be, for example, acetylene black (AB). The amount of binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of positive electrode active material. The binder may be, for example, polyvinylidene fluoride (PVdF).

[0048] The positive electrode layer 11 may further contain, for example, inorganic fillers, organic fillers, solid electrolytes, surface modifiers, dispersants, lubricants, flame retardants, protective agents, fluxes, coupling agents, adsorbents, and the like.

[0049] The electrode density of the positive electrode layer 11 is set to 1.9 g / cm³ from the viewpoint of increasing the energy density. 3 The above is 2.0 g / cm³. 3 The above values ​​may also be used. The electrode density of the positive electrode layer 11 is, for example, 2.2 g / cm³. 3 The following, or 2.1 g / cm³ 3 The following may also apply. The electrode density can be calculated, for example, from a cross-sectional SEM image of the positive electrode layer 11.

[0050] The basis weight of the positive electrode layer 11 is 20 mg / cm³. 2 More than 25mg / cm 2 Above, or 28 mg / cm³ 2 The above may also be acceptable. The basis weight of the positive electrode layer 11 is, for example, 50 mg / cm³. 2 The following, or 40 mg / cm³ 2 The following is also acceptable.

[0051] (Negative electrode layer) The negative electrode layer 12 is attached to one side of the current collector foil 13. The negative electrode layer 12 is located on the back side of the positive electrode layer 11. The negative electrode layer 12 may have a larger area than the positive electrode layer 11. The negative electrode layer 12 contains a negative electrode active material.

[0052] The negative electrode active material may include, for example, at least one selected from the group consisting of carbon-based active materials, alloy-based active materials, Si-C composite materials, Li metal, Li-based alloys, and lithium titanate.

[0053] The carbon-based active material may include, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. "Graphite" is a general term for natural graphite and artificial graphite. Graphite may also be a mixture of natural graphite and artificial graphite.

[0054] The negative electrode layer 12 may further contain, in addition to the negative electrode active material, a thickener and a binder, for example. The amount of thickener may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of negative electrode active material. The thickener may be, for example, carboxymethylcellulose (CMC). The amount of binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of negative electrode active material. The binder may be, for example, styrene-butadiene rubber (SBR).

[0055] (Separator) The separator 20 can separate the positive electrode layer 11 from the negative electrode layer 12. The separator 20 has electrical insulating properties. The separator 20 may include, for example, at least one selected from the group consisting of a resin film (polymer film), an inorganic particle layer, and an organic particle layer.

[0056] The resin film is porous. The resin film may include, for example, a microporous membrane, a nonwoven fabric, etc. The resin film includes a resin skeleton. The resin skeleton may be continuous, for example, in a mesh-like structure. Pores are formed in the gaps of the resin skeleton. The resin film can permeate the electrolyte. The resin film may have, for example, an average pore diameter of 1 μm or less. The average pore diameter of the resin film may be, for example, 0.01 to 1 μm, or 0.1 to 0.5 μm. The "average pore diameter" can be measured by the mercury intrusion method. The Gaurle value of the resin film is, for example, 50 to 250 s / 100 cm. 3 It may also be the case that the "Gehré value" can be measured by the Gehré test method.

[0057] The resin film may be, for example, an olefin resin. The resin film may be, for example, polyethylene (PE) or polypropylene (PP). The thickness of the resin film may be, for example, 5 to 50 μm or 10 to 25 μm.

[0058] The resin film may have, for example, a single-layer structure. The resin film may consist of, for example, a PE layer. The framework of the PE layer is formed of PE. The PE layer may have a shutdown function. The resin film may have, for example, a multilayer structure. The resin film may include, for example, a PP layer and a PE layer. The framework of the PP layer is formed of PP. The resin film may have, for example, a three-layer structure. The resin film may be formed by laminating a PP layer, a PE layer and a PP layer in this order. The thickness of the PE layer may be, for example, 5 to 20 μm. The thickness of the PP layer may be, for example, 3 to 10 μm.

[0059] (electrolyte) The electrolyte is a liquid electrolyte. The electrolyte contains a solute and a solvent. The concentration of the solute may be, for example, 0.5 to 3 mol / L. The solute contains a supporting salt (Li salt). The solute may also contain, for example, inorganic salts, imide salts, oxalate complexes, halides, etc.

[0060] The electrolyte may contain, for example, a carbonate-based solvent (carbonate ester-based solvent). The solvent may be, for example, ethylene carbonate (EC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC).

[0061] The electrolyte may contain any additives. The amount of additive (mass fraction of the total electrolyte) may be, for example, 0.01-5%, 0.05-3%, or 0.1-1%. The additives may include, for example, SEI (Solid Electrolyte Interphase) formation promoters, SEI formation inhibitors, gas generators, overcharge inhibitors, flame retardants, antioxidants, electrode protectants, surfactants, etc. An example of an additive is vinylene carbonate (VC).

[0062] The electrolyte may contain an ionic liquid. The ionic liquid may contain, for example, at least one selected from the group consisting of sulfonium salts, ammonium salts, pyridinium salts, piperidinium salts, pyrrolidinium salts, morpholinium salts, phosphonium salts, imidazolium salts, and derivatives thereof.

[0063] In some embodiments of this invention, the battery may include a gel electrolyte; that is, the battery may be a polymer battery. The gel electrolyte may include an electrolyte solution and a polymer material. The polymer material may form a polymer matrix. The polymer material may include, for example, at least one selected from the group consisting of PVdF, PVdF-HFP, polyacrylonitrile (PAN), PVdF-PAN, polyethylene oxide (PEO), polyethylene glycol (PEG), and derivatives thereof. [Examples]

[0064] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0065] <Manufacturing of positive electrode active material> Following the manufacturing method described above, positive electrode active materials (LMFPs) No. 1 to No. 5 were produced. The positive electrode active materials No. 1 to No. 5 contain primary particles with the average particle size shown in Figure 5. No. 3 is a positive electrode active material containing secondary particles formed only from primary particles with an average particle size of 160 nm, No. 4 is a positive electrode active material containing secondary particles formed only from primary particles with an average particle size of 40 nm, and No. 5 is a positive electrode active material containing secondary particles in which primary particles with an average particle size of 40 nm or 160 nm are randomly arranged.

[0066] <Manufacturing of evaluation cells> A slurry was prepared by mixing LMFP as the positive electrode active material, AB as the conductive material, PVdF as the binder, and N-methylpyrrolidone (NMP) as the dispersion medium. The solid content was set to "positive electrode active material / conductive material / binder = 97.8 / 0.8 / 1.4 (mass ratio)". The slurry was coated onto an Al foil (thickness: 30 μm) to form a positive electrode layer. The positive electrode layer was dried. The positive electrode was manufactured by compressing the positive electrode layer. The positive electrode layer had the electrode density shown in Figure 5.

[0067] A slurry was prepared by mixing graphite as the negative electrode active material, SBR as the binder, CMC as the thickener, and water as the dispersion medium. The solid content was "negative electrode active material / thickener / binder = 97 / 0.6 / 2.4 (mass ratio)". The slurry was coated onto a Cu foil (thickness: 15 μm) to form the negative electrode layer. The negative electrode layer was dried. The negative electrode was manufactured by compressing the negative electrode layer.

[0068] The following materials were prepared. Separator: Porous sheet made of PE (polyethylene) Electrolyte: LiPF6 (concentration: 1.0mol / L), EC+DEC+EMC Outer packaging: Pouch made of aluminum laminate film

[0069] The power generation element was formed by stacking the positive electrode, separator, and negative electrode in this order. The evaluation cell was manufactured by sealing the power generation element and electrolyte in an outer casing. The rated capacity of the evaluation cell is 150 mAh.

[0070] [1C discharge rate] Under room temperature conditions, the 0.1C and 1C discharge rates were measured in the voltage range from 4.25V to 3.0V. "C" is the symbol representing the discharge rate. At a 1C discharge rate, the rated capacity is discharged over one hour. The ratio of the 1C discharge capacity to the 0.1C discharge capacity (1C discharge capacity / 0.1C discharge capacity) was calculated. A higher 1C discharge rate indicates better discharge characteristics.

[0071] As shown in Figure 5, when the conditions of this disclosure are met, there was a tendency for the rate characteristics to improve while maintaining electrode density (i.e., energy density). However, as shown in No. 3, when the secondary particles consist only of primary particles with a large average particle size, the ion diffusion tends to be poor. Also, as shown in No. 4, when the secondary particles consist only of primary particles with a small average particle size, the amount of pores in the secondary particles increases, making them prone to cracking during compression when manufacturing the positive electrode, and thus preventing an increase in electrode density.

[0072] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims and not by the foregoing description, and all modifications are intended to be in the sense and scope equivalent to the claims. [Explanation of Symbols]

[0073] 1a First primary particle, 1b Second primary particle, 2 Secondary particle, 2a Center, 2b Outer periphery, 10 Bipolar electrode, 11 Positive electrode layer, 12 Negative electrode layer, 13 Current collector foil, 20 Separator, 30 Sealing material, 40 Cell, 50 Power generation element, 90 Outer casing, 91 First current collector plate, 92 First laminate film, 93 Second laminate film, 94 Second current collector plate, 100 Battery.

Claims

1. It is a positive electrode active material, The positive electrode active material includes a plurality of secondary particles, The cross-section of the aforementioned secondary particle consists of a central part and an outer peripheral part. The central part comprises a plurality of first primary particles having a first average particle size, The outer periphery includes a plurality of second primary particles having a second average particle size, The first primary particles and the second primary particles contain an olivine-type phosphate compound. The relationship d2 < d1 is satisfied, The aforementioned d1 represents the first average particle size, d2 is the positive electrode active material, which represents the second average particle size.

2. The positive electrode active material according to claim 1, wherein the relationship d2 ≤ 50 nm and 100 nm ≤ d1 is satisfied.

3. The D50 of the aforementioned secondary particles is 5 μm or larger. The pore size of the secondary particles is 0.10 cm. ３ / g or more 0.15cm ３ The positive electrode active material according to claim 1, wherein the amount is less than or equal to / g.

4. Including a positive electrode layer, The positive electrode layer comprises the positive electrode active material described in any one of claims 1 to 3. The basis weight of the positive electrode layer is 28 mg / cm². ２ That's all for the electrodes.

5. A battery comprising the electrode described in claim 4.

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