Positive electrode and battery

Abstract

To improve rate characteristics while maintaining energy density. [Solution] The positive electrode includes a positive electrode layer and a current collector foil. The cross-section of the positive electrode layer includes a first region and a second region. The first region is located between the current collector foil and the second region. The first region includes a first positive electrode active material. The second region includes a second positive electrode active material. The D50 of the first positive electrode active material is greater than the D50 of the second positive electrode active material. The first positive electrode active material includes a plurality of granules consisting of a central part and an outer peripheral part. The central part includes a plurality of first primary particles having a first average particle size. The outer peripheral part includes a plurality of second primary particles having a second average particle size. d2

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode and a battery. [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] When electrodes are thick, the supply of Li ions in the electrode thickness direction can become insufficient, especially during high-current, long-duration charging and discharging, potentially leading to a decrease in performance. Reducing the particle size of the active material improves the diffusion of Li ions, but this can sometimes lead to a decrease in energy density. There is room for improvement in balancing energy density and rate characteristics (especially 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 comprising a positive electrode layer and a current collector foil, The cross-section of the positive electrode layer includes a first region and a second region, The first region is disposed between the current collector foil and the second region, The first region contains a first positive electrode active material, The second region contains a second positive electrode active material, The D50 of the first positive electrode active material is larger than the D50 of the second positive electrode active material, The first positive electrode active material includes a plurality of granulated bodies composed of a central portion and an outer peripheral portion, The central portion includes a plurality of first primary particles having a first average particle diameter, The outer peripheral portion includes a plurality of second primary particles having a second average particle diameter, The relationship of d2 < d1 is satisfied, The d1 represents the first average particle diameter, The d2 represents the second average particle diameter, positive electrode.

[0008] In a high-density electrode or a thick-film electrode, it becomes more difficult to secure a lithium (Li) ion migration path to the inside of the electrode, and the supply of Li ions 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 migration distance of Li ions becomes long, so it is more prominent. Although the diffusibility of Li ions is improved by reducing the size of the primary particles, the amount of pores in the secondary particles increases, so the energy density may decrease. According to the configuration of [1], a granulated body composed of a central portion containing particles with a large average particle diameter and an outer peripheral portion containing particles with a small average particle diameter is disposed on the current collector foil side, and particles with a smaller average particle diameter than the granulated body are disposed on the electrode surface side, so that it is expected to improve the rate characteristics while maintaining the energy density.

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

[0010] 〔3〕The positive electrode according to [1] or [2], wherein the D50 of the granulated body is 5 μm or more.

[0011] [4] The basis weight of the positive electrode layer is 50 mg / cm³. 2 The above is the positive electrode described in any one of items [1] to [3].

[0012] A battery including the positive electrode described in any one of items [5], [1] through [4]. [Brief explanation of the drawing]

[0013] [Figure 1] This is a conceptual diagram showing the positive electrode in this embodiment. [Figure 2] This is a conceptual diagram showing the granulated material in this embodiment. [Figure 3] This is a schematic flowchart illustrating the method for producing the positive electrode active material in this embodiment. [Figure 4] This is a schematic flowchart illustrating the manufacturing method of the positive electrode in this embodiment. [Figure 5] This is a schematic perspective view of the battery in this embodiment. [Figure 6] This is a schematic cross-sectional view along the line IV-IV in Figure 5. [Figure 7] 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] <Positive electrode> Figure 1 is a conceptual diagram showing the positive electrode in this embodiment. The positive electrode includes a positive electrode layer 11 and a current collector foil 13. The positive electrode layer 11 includes a first positive electrode active material 3 and a second positive electrode active material 4. Hereinafter, "first positive electrode active material 3 and second positive electrode active material 4" may be abbreviated as "positive electrode active material".

[0016] The positive electrode layer 11 may further contain, for example, a conductive material and a binder in addition to the positive electrode active material. The blending 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 active material. The conductive material may be, for example, acetylene black (AB). The blending 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 active material. The binder may be, for example, polyvinylidene fluoride (PVdF).

[0017] The positive electrode layer 11 may further contain, for example, an inorganic filler, an organic filler, a solid electrolyte, a surface modifier, a dispersant, a lubricant, a flame retardant, a protective agent, a flux, a coupling agent, an adsorbent, etc.

[0018] From the viewpoint of increasing the energy density, the electrode density of the positive electrode layer 11 is 1.9 g / cm 3 or more, and may be 2.0 g / cm 3 or more. The electrode density of the positive electrode layer 11 may be, for example, 2.2 g / cm 3 or less, or 2.1 g / cm 3 or less. The electrode density can be calculated, for example, from a cross-sectional SEM image of the positive electrode layer 11.

[0019] The basis weight of the positive electrode layer 11 may be, for example, 20 mg / cm 2 or more, 30 mg / cm 2 or more, 40 mg / cm 2 or more, or 50 mg / cm 2 or more. The basis weight of the positive electrode layer 11 may be, for example, 60 mg / cm 2 or less, or 55 mg / cm 2 or less.

[0020] The positive electrode layer 11 includes a first region 11a and a second region 11b. The positive electrode layer 11 may consist of the first region 11a and the second region 11b. Each region may form a layer. The first region 11a is located between the current collector foil 13 and the second region 11b. The first region 11a includes the interface between the positive electrode layer 11 and the current collector foil 13. The second region 11b includes the surface of the positive electrode layer 11. The thicknesses of the first region 11a and the second region 11b may be the same or different. In addition to the first region 11a and the second region 11b, the positive electrode layer 11 may further include an additional region. The additional region may be located between the first region 11a and the second region 11b.

[0021] The first region 11a contains the first positive electrode active material 3. The first region 11a may contain a relatively large amount of the first positive electrode active material 3. The first region 11a may consist of the first positive electrode active material 3. The first positive electrode active material 3 contains a plurality of granules 2 consisting of a central part and an outer peripheral part. The first positive electrode active material 3 may be an aggregate of the plurality of granules 2. That is, the first positive electrode active material 3 may be a powder. The D50 of the first positive electrode active material 3 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 first positive electrode active material 3 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. In this specification, "D50" indicates the particle size at which the cumulative value in the volume-based particle size distribution (cumulative distribution) becomes 50%. The D50 of the first positive electrode active material 3 is measured in the cross-sectional SEM image of the first region 11a.

[0022] Figure 2 is a conceptual diagram showing the granules in this embodiment. The granules 2 contain first primary particles 1a and second primary particles 1b. That is, the granules 2 are secondary particles. A minimum circumscribed circle is fitted to the granules 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 the "center 2a". The radius of the center 2a may be, for example, 0.5D. In the cross-section of the granules 2, the remainder excluding the center 2a is considered the "outer periphery 2b". That is, the cross-section of the granules 2 consists of the center 2a and the outer periphery 2b. The outer periphery 2b surrounds the center 2a.

[0023] 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 particles. 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 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 granule 2 and has a radius of 0.25D.

[0024] The outer peripheral portion 2b contains a plurality of second primary particles 1b having a second average particle diameter. The outer peripheral portion 2b may be composed of a plurality of second primary particles 1b. d2 represents the second average particle diameter 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 diameter of the second primary particles 1b represents the arithmetic mean of 10 second primary particles 1b. Here, the measurement region of the average particle diameter 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 granule 2 in the cross section of the granule 2.

[0025] In the present embodiment, the relationship d2 < d1 is satisfied. That is, d1 - d2 is more 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.

[0026] The D50 of the granule 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 granule 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. The D50 of the granule 2 is measured by a laser diffraction particle size distribution measuring device.

[0027] The pore volume of the granule 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 granule 2 is, for example, 0.20 cm 3 / g or less, 0.19cm 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 amount may be less than / g. The pore size of granule 2 can be measured by the BJH multipoint method.

[0028] The second region 11b contains the second positive electrode active material 4. The second region 11b may contain a relatively large amount of the second positive electrode active material 4. The second region 11b may consist of the second positive electrode active material 4. The second positive electrode active material 4 may contain a plurality of primary particles. The second positive electrode active material 4 may contain a plurality of secondary particles. The second positive electrode active material 4 may be an aggregate of a plurality of secondary particles. That is, the second positive electrode active material 4 may be a powder. The D50 of the second positive electrode active material 4 may be, for example, 0.1 μm or more, 0.5 μm or more, 1 μm or more, or 5 μm or more. The D50 of the second positive electrode active material 4 may be, for example, 40 μm or less, 30 μm or less, 20 μm or less, or 10 μm or less. The D50 of the second positive electrode active material 4 is measured in the cross-sectional SEM image of the second region 11b.

[0029] In this embodiment, the D50 of the first positive electrode active material 3 is greater than the D50 of the second positive electrode active material 4. This is expected to improve the rate characteristics while maintaining the energy density. The D50 of the first positive electrode active material 3 may be, for example, 1.1 times or more, 1.5 times or more, 2 times or more, 5 times or more, 10 times or more, or 50 times or less than the D50 of the second positive electrode active material 4.

[0030] Carbon may be attached to at least a portion of the surface of primary particles (including the first primary particle 1a and the second primary particle 1b; the same applies hereinafter). The carbon may form a carbon layer. The amount of carbon attached 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 particles. The amount of carbon attached may be, for example, 5% or less, 4% or less, or 3% or less by mass fraction relative to the primary particles.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] In olivine-type phosphate compounds, elements other than lithium (Li), manganese (Mn), Fe, phosphorus (P), and oxygen (O) may be doped (dopants). The amount of doping (molecule fraction relative to the amount of Li) may be, for example, 0.01 to 0.1.

[0035] The positive electrode active material may further contain other components as long as it contains an olivine-type phosphate compound. The 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 and 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 be, for example, a mixture of a powder of an olivine-type phosphate compound and a powder of other components.

[0036] 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 relationship of -0.5 ≦ a ≦ 0.5, 0 ≦ x ≦ 1 is satisfied. M may include, for example, at least one selected from the group consisting of Co, Mn, and Al.

[0037] 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 of -0.5 ≦ a ≦ 0.5, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1 are satisfied.

[0038] <00002​​​​​​​​​​​ In the formula, the relationships of -0.5 ≤ a ≤ 0.5, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1 are satisfied.

[0039] FIG. 3 is a schematic flowchart of a 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 "the present method". The present method may include, for example, (a) formation of a slurry, (b) granulation, and (c) firing.

[0040] (a) Formation of a slurry The present 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, the lithium compound, the manganese compound, the phosphate compound, and the iron compound may be weighed so as to have a composition ratio (molar ratio) shown in the composition formula "Li a Mn 1-x Fe x 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.

[0041] When carbon is attached to the surface of the primary particles, a carbon source is added to the raw material mixture. The carbon source may include, for example, saccharides, organic acids, etc. The carbon source may include, for example, glucose, sucrose, fructose, citric acid, etc. The addition amount of the carbon source may be, for example, 1 to 20% by mass fraction with respect to the raw material mixture.

[0042] The solvent may include, for example, water, etc. The solid content concentration of the slurry may be, for example, 20 to 40% by mass fraction.

[0043] The particle size in the slurry may be adjusted by performing wet grinding. For example, wet grinding may be performed so that D50 is 0.10 to 1 μm.

[0044] (b) Granulation This method may include granulation of secondary particles (precursors) by drying the slurry. For example, the 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. Spray drying may be performed two or more times. The granulated body 2 of this embodiment can be formed by performing spray drying two or more times. For example, the central part may be formed by the first spray drying and the outer part by the second spray drying. By using slurries with different particle sizes, a granulated body with different average particle sizes in the central part and the outer part can be formed.

[0045] 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.

[0046] (c) Firing This method may include generating olivine-type phosphate compounds by heat-treating secondary particles (precursors). Any heat treatment furnace (e.g., electric furnace, muffle furnace) 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, calcination may be performed after each spray drying.

[0047] Figure 4 is a schematic flowchart of the method for manufacturing the positive electrode in this embodiment. The method for manufacturing the positive electrode in this embodiment may include, for example, "a step of preparing a positive electrode mixture," "a step of forming a positive electrode layer," and "a step of compressing the positive electrode layer."

[0048] In the "process of preparing the positive electrode mixture," a positive electrode mixture is prepared by mixing at least the positive electrode active material and the binder in a solvent. The positive electrode active material is prepared by the manufacturing method described above.

[0049] The "process of forming the positive electrode layer" may include a coating process, an orientation process, and a drying process. In the positive electrode manufacturing method of this embodiment, the "coating process" of applying the positive electrode mixture to the surface of the current collector foil is performed at least twice. For example, a first region may be formed in the first coating process, and a second region may be formed in the second coating process. In this case, the type of positive electrode mixture may be changed for each coating process. A drying process may be performed for each coating process.

[0050] In the "process of compressing the positive electrode layer," the positive electrode layer is pressed at a predetermined pressure, and the positive electrode is manufactured.

[0051] <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.

[0052] Figure 5 is a schematic perspective view of the battery in this embodiment. Figure 6 is a schematic cross-sectional view along the line IV-IV in Figure 5. 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] The positive electrode layer 11 is attached to one side of the current collector foil 13. Details of the positive electrode layer 11 are as described above.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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).

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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).

[0069] 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 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).

[0070] 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.

[0071] 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]

[0072] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0073] <Manufacturing of positive electrodes> (No.1, No.4 through No.6) First and second slurries were 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 first region was formed by coating the first slurry onto an Al foil (thickness: 30 μm). The second region was formed by coating the second slurry on top of the first slurry. The positive electrode layer was formed by drying the first and second regions. The positive electrode was manufactured by compressing the positive electrode layer. The positive electrode layer had the electrode density shown in Figure 7. Note that No. 6 was prone to particle cracking during compression, and it was not possible to increase the electrode density.

[0074] (No.2, No.3) The first region was formed in the same manner as described above. The positive electrode layer was formed by drying the first region. The positive electrode was manufactured by compressing the positive electrode layer. The positive electrode layer had the electrode density shown in Figure 7. Note that No. 2 was prone to particle cracking during compression, making it impossible to increase the electrode density.

[0075] Figure 7 shows the positive electrode active material contained in the first and second regions of each positive electrode. "Granulated material" refers to particles with an average particle size of 10 μm, where the particle size of the primary particles differs between the center and the outer periphery, while "non-granulated material" refers to particles with an average particle size of 1 μm.

[0076] <Manufacturing of evaluation cells> 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.

[0077] The power generation element was formed by stacking the positive electrode, separator (porous sheet made of PE), and negative electrode in this order. The evaluation cell was manufactured by sealing the power generation element and electrolyte (LiPF6 (concentration: 1.0 mol / L), EC+DEC+EMC) in an outer casing (a pouch made of aluminum laminate film). The rated capacity of the evaluation cell is 150 mAh.

[0078] [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.

[0079] As shown in Figure 7, when the conditions of this disclosure are met, the rate characteristics tend to improve while maintaining electrode density (i.e., energy density). For example, in No. 1, it was possible to manufacture electrodes with high electrode density and good rate characteristics. It is thought that the Li ion diffusion from the inside of the electrode to the surface was good because the current collector foil side (first region) is easily pressed during the manufacturing of the positive electrode, and the surface side (second region) has a structure with many voids. In addition, it is thought that the Li ion diffusion toward the current collector foil side was also good because the granules consisting of the central part and the outer periphery are arranged in the first region.

[0080] The embodiments and examples disclosed herein should be considered in all respects as illustrative and not restrictive. The scope of this disclosure is indicated by the claims rather than the foregoing description, and all modifications are intended to be in the sense and scope equivalent to the claims. [Explanation of Symbols]

[0081] 1a First primary particles, 1b Second primary particles, 2 Granules, 2a Center, 2b Outer periphery, 3 First positive electrode active material, 4 Second positive electrode active material, 10 Bipolar electrode, 11 Positive electrode layer, 11a First region, 11b Second region, 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. A positive electrode comprising a positive electrode layer and a current collector foil, The cross-section of the positive electrode layer includes a first region and a second region. The first region is located between the current collector foil and the second region. The first region includes the first positive electrode active material, The aforementioned second region includes the second positive electrode active material, The D50 of the first positive electrode active material is greater than the D50 of the second positive electrode active material. The first positive electrode active material includes a plurality of granules consisting 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 relationship d2 < d1 is satisfied, The aforementioned d1 represents the first average particle size, The aforementioned d2 represents the positive electrode, which exhibits a second average particle size.

2. The positive electrode according to claim 1, wherein the relationships d2 ≤ 50 nm and 100 nm ≤ d1 are satisfied.

3. The positive electrode according to claim 1, wherein the D50 of the granules is 5 μm or larger.

4. The basis weight of the positive electrode layer is 50 mg / cm². ２ The positive electrode described in claim 1 is as described above.

5. A battery comprising the positive electrode described in any one of claims 1 to 4.

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