electrode

Abstract

Improved adhesion between the current collector foil and the positive electrode layer. [Solution] An electrode comprising a current collector foil and a positive electrode layer, wherein the positive electrode layer is disposed on the surface of the current collector foil, the positive electrode layer comprises a positive electrode active material, the positive electrode active material comprises non-spherical particles, the non-spherical particles comprise an olivine-type phosphate compound, and the non-spherical particles have a circularity of 0.50 or less.

Description

[Technical Field]

[0001] This disclosure relates to electrodes. [Background technology]

[0002] Japanese Patent Publication No. 2016-524307 discloses the use of lithium iron manganese phosphate (LMFP) as a positive electrode active material for lithium-ion batteries, and that the LMFP is substantially spherical in shape. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Special Publication No. 2016-524307 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] The purpose of this disclosure is to improve the adhesion between the current collector foil and the positive electrode layer. [Means for solving the problem]

[0005] The technical configuration and effects of this disclosure are described below. However, the mechanism of action includes assumptions. The mechanism of action does not limit the technical scope of this disclosure.

[0006] [1] comprising current collector foil and positive electrode layer, The positive electrode layer is arranged on the surface of the current collector foil, The positive electrode layer contains a positive electrode active material, The positive electrode active material includes non-spherical particles, The non-spherical particles contain an olivine-type phosphate compound. The non-spherical particles are electrodes having a circularity of 0.50 or less.

[0007] Conventionally, olivine-type phosphate compounds have been synthesized to approximate a spherical shape. This is thought to be because the spherical particle shape of olivine-type phosphate compounds is expected to improve packing efficiency, and thus improve energy density.

[0008] The electrode of this disclosure includes non-spherical particles. In this disclosure, the non-spherical particles have a circularity of 0.50 or less. The non-spherical particles have an irregular shape. For example, the non-spherical particles may have angular portions. In the electrode (positive electrode layer), the angular portions of the non-spherical particles tend to be oriented along the thickness direction of the electrode. The inclusion of non-spherical particles in the electrode causes the non-spherical particles to penetrate the current collector foil (anchor effect). As a result, improved adhesion between the positive electrode layer and the current collector foil is expected.

[0009] [2] The non-spherical particles are present at the interface between the current collector foil and the positive electrode layer, The electrode according to [1], wherein the contact ratio of the non-spherical particles at the interface is 50% or more.

[0010] When these conditions are met, a greater improvement in the adhesion between the positive electrode layer and the current collector foil can be expected.

[0011] [3] The positive electrode active material further comprises spherical particles, The non-spherical particles contain an olivine-type phosphate compound. The electrode according to [1] or [2], wherein the spherical particle has a circularity of 0.80 or more.

[0012] By including spherical and non-spherical particles in the positive electrode active material, it is expected that, for example, the balance between adhesion and packing properties will be improved.

[0013] [4] The electrode according to [3], wherein the ratio of non-spherical particles to the total of non-spherical particles and spherical particles in the positive electrode active material is greater than 5% by mass fraction.

[0014] The inclusion of a certain amount or more of non-spherical particles in the positive electrode active material is expected to improve the adhesion between the positive electrode layer and the current collector foil.

[0015] [5] The olivine-type phosphate compound contains at least one selected from the group consisting of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate, and the electrode according to any one of [1] to [4].

[0016] Hereinafter, one embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment"), and one example of the present disclosure (hereinafter may be abbreviated as "this example") will be described. However, this embodiment and this example do not limit the technical scope of the present disclosure. This embodiment and this example are illustrative in all respects. This embodiment and this example are non-limiting. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configuration is extracted from this embodiment and their arbitrary combinations are also initially planned.

Brief Description of Drawings

[0017] [Figure 1] It is a schematic cross-sectional view of a part of the electrode in this embodiment. [Figure 2] It is a diagram for explaining a method of measuring the contact ratio in this embodiment. [Figure 3] [[ID=2 ]] [Figure 4] It is a schematic diagram showing the lithium ion secondary battery of this embodiment. [Figure 5] It is a table showing experimental results.

Modes for Carrying Out the Invention

[0018] <Terms and Phrases> "Comprising", "including", "having", and their modifications are open-ended expressions. A configuration expressed in an open-ended manner may further include additional elements in addition to essential elements, or may not include them.

[0019] ​The shapes of non-spherical and spherical particles are evaluated by the following procedure. For example, a dispersion is formed by dispersing 1 g of positive electrode active material (powder) in 10 g of epoxy resin (product name "EPOTEX JP", manufactured by Nisshin EM Co., Ltd.). The dispersion is stirred and mixed for 1 minute using a mixer (product name "Awatori Rentaro", manufactured by Shinky Co., Ltd.). The stirring speed may be, for example, around 2000 rpm. The dispersion is degassed under vacuum. After degassing under vacuum, the epoxy resin hardens inside a silicone mold, yielding a plate-shaped cured product. A cross-sectional sample with a smooth cross-section is prepared by cross-section processing of the plate-shaped cured product using a cross-section polisher (registered trademark). A cross-sectional SEM (Scanning Electron Microscope) image is obtained by observing the smooth cross-section.

[0020] In cross-sectional SEM images, the circularity of particles whose entire circumference is observable is measured. "Circularity" is calculated using the following formula. ψ = 4πS / L 2 ψ: Circularity π: Pi S: Particle cross-section (area of ​​the region enclosed by the particle's outline) L: Particle circumference (length of the particle's outline)

[0021] Particles with a circularity of 0.80 or greater are classified as spherical particles. The average circularity of a set of spherical particles is called the "average circularity of spherical particles." Particles with a circularity of 0.50 or less are classified as non-spherical particles. The average circularity of a set of non-spherical particles is called the "average circularity of non-spherical particles." Note that the average circularity is the average of 50 individual particles.

[0022] Referring to FIG. 2, the contact ratio of non-spherical particles at the interface between the current collector foil and the positive electrode layer is evaluated by the following procedure. A cross-sectional sample of the electrode is prepared. By performing SEM observation of the cross-section, a cross-sectional SEM image is obtained. The cross-sectional SEM image is adjusted so that the interface 13 is included in a range of about 50 μm in the width direction W. In the cross-sectional SEM image, non-spherical particles 1a and spherical particles 1b that contact the interface 13 are confirmed. In the non-spherical particle 1a, the length of the line segment connecting the two points farthest in the width direction W so as to be parallel to the interface is "c n ", and in the spherical particle 1b, the length of the line segment connecting the two points farthest in the width direction W so as to be parallel to the interface is "d n ". Let the sum of c n be "S c ", and the sum of d n be "S d ". When "100×S c / (S c +S d )", the contact ratio (%) is obtained.

[0023] Various dimensional measurements and shape analyses in the cross-sectional SEM image can be performed using, for example, image analysis software "ImageJ" or the like.

[0024] The chemical composition of the compound can be measured by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy). A 0.1 g sample (for example, the positive electrode active material) is dissolved in a mixed acid (10 ml) of hydrochloric acid and sulfuric acid to prepare a sample solution. The sample solution is diluted to an appropriate concentration using a volumetric flask. After dilution, composition analysis is performed using an ICP-AES apparatus.

[0025] The stoichiometric composition formula shows a representative example of a compound. The compound may have a non-stoichiometric composition. For example, "Al2O3" is not limited to compounds with a molar ratio of "Al / O = 2 / 3". Unless otherwise specified, "Al2O3" refers to a compound containing Al and O in any molar ratio. For example, the compound may be doped with trace elements. Some of the Al and O may be substituted with other elements.

[0026] "D50" indicates the particle size at which the cumulative value in the volume-based particle size distribution (cumulative distribution) reaches 50%. The volume-based particle size distribution is measured using a laser diffraction particle size analyzer.

[0027] "Maximum Ferret diameter" refers to the length of the longer side of the minimum bounding rectangle (MBR) relative to the particle's contour line in a cross-sectional SEM image.

[0028] <Electrode> Figure 1 is a schematic cross-sectional view of a part of the electrode in this embodiment. The electrode 10 may be, for example, the positive electrode of a monopolar battery. The electrode 10 includes a current collector foil 12 and a positive electrode layer 11.

[0029] The current collector foil 12 is a conductor. The current collector foil 12 supports the positive electrode layer 11. The current collector foil 12 may be, for example, in the form of a sheet. The thickness of the current collector foil 12 may be, for example, 1 to 50 μm, 3 to 30 μm, or 5 to 15 μm. The current collector foil 12 is conductive. The current collector foil 12 may contain, for example, a metal foil. The current collector foil 12 may contain, for example, at least one selected from the group consisting of Cu, Ni, Zn, Pb, Al, Ti, Fe, Ag, Au, and conductive resins. The current collector foil 12 may contain, for example, an Al foil, an Al alloy foil, etc. The current collector foil 12 may have, for example, a multilayer structure. For example, the current collector foil 12 may be formed by laminating an Al foil and a Cu foil.

[0030] The positive electrode layer 11 is located on the surface of the current collector foil 12. The positive electrode layer 11 may be located on only one side of the current collector foil 12. The positive electrode layer 11 may be located on both sides of the current collector foil 12. If the electrode 10 is a bipolar battery, the positive electrode layer 11 may be located on one side (front side) of the current collector foil 12, and the negative electrode layer (not shown) may be located on the other side (back side). The thickness of the positive electrode layer 11 may be, for example, 10 μm or more, 100 μm or more, 200 μm or more, 400 μm or more, 600 μm or more, 800 μm or more, or 1 mm or more. The thickness of the positive electrode layer 11 may be, for example, 1.2 mm or less, 1 mm or less, or 800 μm or less. In a bipolar structure, a thicker positive electrode layer 11 of 200 μm or more may be required.

[0031] The positive electrode layer 11 contains the positive electrode active material 1. That is, the electrode 10 contains the positive electrode active material 1.

[0032] The positive electrode active material 1 may, for example, be in the form of a powder. The D50 of the positive electrode active material 1 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 1 may be, for example, 30 μm or less, 25 μm or less, 20 μm or less, 15 μm or less, or 10 μm or less.

[0033] Figure 3 is a conceptual diagram of non-spherical particles in this embodiment. The positive electrode active material 1 contains non-spherical particles 1a. The non-spherical particles 1a have a circularity of 0.50 or less.

[0034] The circularity (average circularity) of the non-spherical particle 1a may be, for example, 0.45 or less, 0.42 or less, or 0.40 or less. The circularity of the non-spherical particle 1a may be 0.30 or more, 0.35 or more, 0.38 or more, or 0.40 or more. The circularity of the non-spherical particle 1a may be, for example, 0.30 or more and 0.50 or less, or 0.35 or more and 0.45 or less.

[0035] The non-spherical particle 1a may be, for example, an angular particle. An "angular particle" refers to a particle having a sharpest angle "θ" of 60° or less. The "sharpest angle" refers to the smallest sharpest angle among the interior angles formed by the particle's contour line in a cross-sectional SEM image. The sharpest angle may be, for example, 50° or less, 40° or less, 30° or less, or 20° or less. The sharpest angle may be, for example, 10° or more, 20° or more, 30° or more, 40° or more, or 50° or more. An angular particle may include, for example, multiple sharp corners, including the sharpest angle.

[0036] The non-spherical particles 1a may be, for example, flake-like particles. The "flake-like particles" are plate-like particles and angular particles. The "plate-like particles" have a planar direction (plate surface direction) and a thickness direction. The "first aspect ratio" represents the ratio of the maximum Ferret diameter "d1" in the plate surface direction to the thickness "t" of the plate-like particles. The first aspect ratio "d1 / t" may be, for example, 2 or more, 3 or more, 5 or more, 7 or more, or 9 or more. The first aspect ratio "d1 / t" may be, for example, 10 or less, 9 or less, 7 or less, 5 or less, or 3 or less.

[0037] In plate-like particles, the plate surface can have any shape. "Plate surface" refers to the surface that has the largest apparent area. The plate surface may be, for example, a polygon, a quadrilateral, a parallelogram, a rectangle, etc. "Second aspect ratio" refers to the ratio of the long side "d1" of the MBR to the short side "d2" of the plate surface of the plate-like particle. The second aspect ratio "d1 / d2" may be, for example, 1.2 or more, 1.5 or more, 2.0 or more, 3.0 or more, 4.0 or more, or 5.0 or more. The second aspect ratio "d1 / d2" may also be, for example, 10.0 or less, 5.0 or less, 4.0 or less, 3.0 or less, or 2.0 or less.

[0038] The positive electrode active material 1 may further contain spherical particles 1b in addition to non-spherical particles 1a. The spherical particles 1b have a circularity of 0.80 or more. The average circularity of the spherical particles 1b may be, for example, 0.85 or more, 0.90 or more, or 0.91 or more. The average circularity of the spherical particles 1b may be, for example, 0.95 or less, 0.93 or less, or 0.91 or less. The average circularity of the spherical particles 1b may be, for example, 0.80 or more and 0.95 or less, or 0.90 or more and 0.93 or less.

[0039] The positive electrode active material 1 may contain non-spherical particles 1a in a specific ratio. The ratio of non-spherical particles 1a to the total of non-spherical particles 1a and spherical particles 1b in the positive electrode active material 1 (hereinafter also referred to as the "mass ratio of non-spherical particles") may be greater than 5% in mass fraction. The mass ratio of non-spherical particles may be, for example, 8% or more, 10% or more, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, or 45% or more. The mass ratio of non-spherical particles may be, for example, 50% or less, 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, 20% or less, 15% or less, or 10% or less. The mass ratio of non-spherical particles may be, for example, greater than 5% and 50% or less, or 10% or more and 30% or less.

[0040] Carbon may coat the surface of the positive electrode active material 1 (non-spherical particles 1a and spherical particles 1b). That is, a carbon layer may be formed on the surface of the positive electrode active material 1. The carbon may cover a part of the surface of the positive electrode active material 1, or it may cover the entire surface of the positive electrode active material 1. The carbon may be derived from, for example, sugars. The amount of carbon attached may be, for example, 0.1% or more, 0.5% or more, 1% or more, 5% or more, or 10% or more by mass fraction relative to the positive electrode active material 1. The amount of carbon attached may be, for example, 20% or less, 15% or less, 10% or less, 5% or less, or 3% or less by mass fraction relative to the positive electrode active material 1.

[0041] Non-spherical particles 1a contain an olivine-type phosphate compound. That is, the positive electrode active material 1 contains an olivine-type phosphate compound. Spherical particles 1b may also contain an olivine-type phosphate compound. Hereinafter, the olivine-type phosphate compound may be abbreviated as "olivine". Olivine has a crystal structure that belongs to the space group Pnma. The space group to which the crystal structure belongs can be determined by the XRD (X-Ray Diffraction) pattern.

[0042] Olivine may further contain, for example, a glass phase, a crystallized glass phase, etc., in addition to the crystalline phase. For example, non-spherical particles 1a may be formed by appropriately grinding a glassy material. For example, a crystallized glass phase, etc., may be formed by vitrification and then crystallization of the raw materials during synthesis.

[0043] Olivine may include, for example, at least one selected from the group consisting of lithium iron phosphate (LFP), lithium manganese phosphate (LMP), and LMFP.

[0044] Olivine, for example, has the general formula "Li a Mn 1-x Fe x It may have a composition represented by "PO4". In the general formula, for example, the relationship "0.5 ≤ a ≤ 1.5" may be satisfied. In the general formula, for example, the relationship "0.2 ≤ x ≤ 0.5" may be satisfied.

[0045] The positive electrode active material 1 may further contain other components as long as it contains olivine. The mixing ratio (mass ratio) of olivine to other components may be, for example, "olivine / other components = 9 / 1 to 1 / 9" or "olivine / other components = 7 / 3 to 3 / 7". The positive electrode active material 1 may be, for example, a mixture of olivine powder and other component powder. The other components may include, for example, at least one selected from the group consisting of Li[NiCoMn]O2 (layered structure), Li[NiCoAl]O2 (layered structure), LiMnO2 (rock salt structure), and Li[NiMn]2O4 (spinel structure). Note that the notation "[NiCoMn]", etc., indicates that the sum of the composition ratios in parentheses is 1. As long as the sum is 1, each component in parentheses can take on any composition ratio.

[0046] Non-spherical particles 1a may be present at the interface 13 between the current collector foil 12 and the positive electrode layer 11. The contact ratio of non-spherical particles 1a at the interface 13 (hereinafter also simply referred to as "contact ratio") may be 50% or more. The contact ratio may be, for example, 55% or more, 60% or more, 63% or more, or 65% or more. The contact ratio may be, for example, 80% or less, 75% or less, 70% or less, or 69% or less. The contact ratio may be, for example, 50% or more and 80% or less, or 60% or more and 70% or less.

[0047] The positive electrode layer 11 may further contain, in addition to the positive electrode active material 1, a conductive material, a binder, a thickening agent, etc. 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 1. The conductive material may contain any components. For example, the conductive material may contain at least one selected from the group consisting of acetylene black (AB), Ketjenblack (registered trademark), vapor-grown carbon fiber (VGCF), carbon nanotubes (CNT), and graphene flakes (GF).

[0048] 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 contain any components. For example, the binder may contain at least one selected from the group consisting of polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP), and derivatives thereof. SBR, CMC, PAA, and PVP, etc., can also function as thickeners.

[0049] In this embodiment, the current collector foil 12 may have indentations. These indentations may be caused by non-spherical particles 1a. Indentations can be confirmed by the following procedure: The electrode is separated from the current collector foil and the positive electrode layer using an organic solvent and water. After separation, it is observed whether there are 10 or more indentations on the surface of the current collector foil (10 mm × 10 mm). The observation may be performed, for example, using a microscope or a laser microscope.

[0050] The electrode in this embodiment contains non-spherical particles. This is expected to improve the adhesion between the current collector foil and the positive electrode layer. Improved adhesion is expected to lead to, for example, a reduction in binder volume and improved rate characteristics. The orientation of the non-spherical particles along the thickness direction of the electrode is expected to straighten the electrolyte penetration path in the thickness direction of the electrode. Improved electrolyte diffusion in the thickness direction of the electrode is expected to lead to improved rate characteristics.

[0051] <Lithium-ion rechargeable battery> Figure 4 is a schematic diagram showing a lithium-ion secondary battery (hereinafter abbreviated as "battery") in this embodiment. The battery 100 includes a power generation element 50 and an electrolyte (not shown). The battery 100 may include an outer casing. The outer casing may house the power generation element 50 and the electrolyte. The outer casing may be, for example, a metal case or a pouch made of Al laminate film.

[0052] The power generation element 50 may have any form. For example, the power generation element 50 may be a wound type, a laminated type, etc. The power generation element 50 may have a monopolar structure or a bipolar structure. The power generation element 50 includes a positive electrode 10, a negative electrode 20, and a separator 30. The separator 30 is placed between the positive electrode 10 and the negative electrode 20. Electrolyte permeates the gaps between each component and the gaps within each component. Each component may be, for example, in the form of a sheet.

[0053] (Negative electrode) The negative electrode 20 may include a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector may include, for example, Cu foil. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. The negative electrode active material layer may further include, for example, a conductive material, a binder, and a thickening agent.

[0054] The conductive material may include, for example, CNTs. The binder may include, for example, CMC, SBR, etc. The amount of conductive material and binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of negative electrode active material.

[0055] (Separator) The separator 30 is porous. The separator 30 is permeable to the electrolyte. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 is electrically insulating. The separator 30 may contain, for example, a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator 30 may have, for example, a single-layer structure or a multi-layer structure. The separator 30 may consist substantially of a PE layer, or it may be formed by laminating a PP layer, a PE layer, and a PP layer in that order. A heat-resistant layer may be formed on the surface of the separator 30.

[0056] (electrolyte) The electrolyte contains a solvent and a lithium salt. The solvent is aprotic. The solvent may contain any components. For example, the solvent may contain at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0057] The lithium salt is a supporting electrolyte. The lithium salt is dissolved in the solvent. The lithium salt may contain, for example, at least one selected from the group consisting of LiPF6 and LiBF4. The lithium salt may have a molar concentration of, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0058] The electrolyte may further contain any additives. For example, the electrolyte may contain additives in an amount of 0.01% to 5% by mass. The additives may include at least one selected from the group consisting of vinylene carbonate (VC) and vinylethylene carbonate (VEC), for example.

[0059] A solid electrolyte may be used instead of the electrolyte and separator 30. In other words, in this embodiment, the battery may be an all-solid-state battery. [Examples]

[0060] <Manufacturing of positive electrode active material> (No.1) Compositional formula “Li1Mn 0.8 Fe 0.2The raw materials, lithium carbonate, manganese carbonate, iron oxalate, and phosphoric acid, were weighed to match the composition ratio shown in "PO4". A mixture was formed by mixing the powder materials, except for phosphoric acid. The mixture was then ground and mixed in a mortar while phosphoric acid was added until the reaction stopped, forming a powder. At this time, the gradual addition of phosphoric acid can suppress gas generation. The powder mixture was placed in a graphite crucible. The graphite crucible was placed in a firing furnace. Firing was carried out in an inert atmosphere using the following procedure. First, the furnace temperature was increased at a rate of 5°C / min until it reached 1100°C. The furnace temperature of 1100°C was maintained for 1 hour. After 1 hour, the furnace temperature was cooled at a rate of 5°C / min until it reached 600°C. Then, the furnace temperature was cooled at a rate of 15°C / min until it reached room temperature. This yielded a glassy active material mass. The active material mass is crushed by a mechanical crushing device, thereby forming a precursor.

[0061] A precursor mixture was formed by adding 10% fructose by mass fraction to the precursor. A first slurry was formed by grinding the precursor mixture in an aqueous solvent using a bead mill. Beads (grinding media) with a diameter (φ) of 0.1 mm were used during grinding. The peripheral speed was 10 m / s. The grinding time was 60 min.

[0062] The first slurry was dried to obtain a dried product. The dried product was placed in a graphite crucible. The graphite crucible was placed in a firing furnace. Under an inert atmosphere, heat treatment (firing) was carried out at 650°C to produce cathode active material No. 1 (LMFP).

[0063] (No.2) The precursor obtained in No. 1 was dispersed in water, and the second slurry was formed by grinding it using a colloid mill (magic LAB®) with the target gap adjusted to zero. The rotor speed during grinding was 16,000 rpm. The grinding time was 10 min.

[0064] The first slurry and the second slurry were mixed so that the mass fraction of the positive electrode active material contained in the slurry was 95:5 to obtain the third slurry. 10% fructose by mass fraction was added to the third slurry, and the mixture was dried to obtain a dried product. The dried product was placed in a graphite crucible. The graphite crucible was placed in a firing furnace. The positive electrode active material No. 2 was produced by heat treatment (firing) at 650°C under an inert atmosphere.

[0065] (No.3) The positive electrode active material was manufactured in the same manner as in No. 2, except that the first slurry and the second slurry were modified so that the mass fraction of the positive electrode active material contained in the slurry was 90:10.

[0066] (No.4) The positive electrode active material was manufactured in the same manner as in No. 3, except that the grinding time was changed to 30 minutes using a colloidal mill.

[0067] <Fabrication of the positive electrode> A mixture was formed by mixing the positive electrode active material, conductive material (acetylene black), and binder (PVdF). The mixing ratio (mass ratio) was "positive electrode active material / conductive material / binder = 92 / 5 / 3". A paste was formed by dispersing the mixture in a solvent (N-methyl-2-pyrrolidone). The solid content concentration of the paste was 50% by mass fraction. The positive electrode layer was formed by applying the paste to the surface of an Al foil and drying it. The density of the positive electrode layer was 1.8 g / cm³ by roll pressing. 3 By adjusting it, the positive electrode was formed.

[0068] <Rating> (Circularity) Using the method described above, cross-sectional SEM images of the positive electrode active material were obtained. The circularity of non-spherical and spherical particles was measured in the cross-sectional SEM images. The results are shown in Figure 5.

[0069] (contact ratio) Using the method described above, a cross-sectional SEM image of the electrode (positive electrode) was obtained. In the cross-sectional SEM image, the contact ratio of non-spherical particles at the interface between the Al foil and the positive electrode layer was measured. The results are shown in Figure 5.

[0070] (Adhesion strength) The positive electrode was cut into a 20mm x 30mm shape. After cutting, it was attached to a peel test apparatus using a 10mm wide tape. The peel load (N) was measured when peeling the positive electrode layer from the Al foil at a speed of 1mm / sec at a 90° angle. Considering the effects of tape elongation, the average of the rough peel loads over 5mm before and after the start of isolation was used as the adhesion strength. The results are shown in Figure 5. Note that the adhesion strength values ​​in Figure 5 are relative values ​​with the adhesion strength of No. 1 set to 100.

[0071] <Result> When the conditions of this disclosure are met, there is a tendency for the adhesion between the current collector foil and the positive electrode layer to improve. There is also a tendency for the adhesion to improve when non-spherical particles and spherical particles are included. [Explanation of Symbols]

[0072] 1 Positive electrode active material, 1a Non-spherical particles, 1b Spherical particles, 10 Electrode (positive electrode), 11 Positive electrode layer, 12 Current collector foil, 13 Interface, 20 Negative electrode, 30 Separator, 50 Power generation element, 100 Lithium-ion secondary battery.

Claims

1. Including current collector foil and positive electrode layer, The positive electrode layer is arranged on the surface of the current collector foil, The positive electrode layer contains a positive electrode active material, The positive electrode active material includes non-spherical particles, The non-spherical particles contain an olivine-type phosphate compound. The non-spherical particles are electrodes having a circularity of 0.50 or less.

2. The non-spherical particles are present at the interface between the current collector foil and the positive electrode layer. The electrode according to claim 1, wherein the contact ratio of the non-spherical particles at the interface is 50% or more.

3. The positive electrode active material further comprises spherical particles, The spherical particles contain an olivine-type phosphate compound. The electrode according to claim 1 or claim 2, wherein the spherical particles have a circularity of 0.80 or more.

4. The electrode according to claim 3, wherein the ratio of non-spherical particles to the total of non-spherical particles and spherical particles in the positive electrode active material is greater than 5% by mass fraction.

5. The electrode according to claim 1 or claim 2, wherein the olivine-type phosphate compound comprises at least one selected from the group consisting of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate.

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