Positive electrode material for lithium ion secondary battery, positive electrode for lithium ion secondary battery, and method for producing positive electrode material for lithium ion secondary battery

Abstract

A positive electrode material for a lithium ion secondary battery, the positive electrode material comprising a granulated body (10) having: a core (11) comprising olivine-type lithium iron manganese phosphate represented by the general formula LiMnxFeyPO4 (wherein x and y are numerical values satisfying x + y = 1, 0 < x < 1, and 0 < y < 1); and a carbon film (12) formed on the surface of the core (11). The core (11) has a structure in which primary particles of olivine-type lithium iron manganese phosphate are aggregated. The particle diameter of the primary particles is 100 nm or less. The granulated body (10) has a pore volume of 0.2 cm3 / g or less in the range of pore diameters of 2 nm to 300 nm inclusive, and a carbon content of 1.8 mass% to 3.0 mass% inclusive.

Description

Technical Field

[0001] This disclosure relates to a cathode material for lithium-ion secondary batteries, a cathode for lithium-ion secondary batteries, and a method for manufacturing the cathode material for lithium-ion secondary batteries. Background Technology

[0002] Patent document 1 discloses an energy storage device that uses olivine-type lithium iron phosphate (LiFePO4) as the positive electrode active material. Olivine-type positive electrode active materials, represented by olivine-type lithium iron phosphate, are positive electrode active materials with excellent thermal stability. Existing technical documents Patent documents

[0003] Patent document 1: Japanese Patent Application Publication No. 2019-185920. Summary of the Invention The problem the invention aims to solve

[0004] The positive electrode of a lithium-ion secondary battery is manufactured as follows: First, a slurry-like positive electrode composite material containing a positive electrode active material, a binder, and a solvent is coated onto the surface of a positive electrode current collector. Next, the coating film is dried, thereby forming a positive electrode active material layer on the surface of the positive electrode current collector. Here, as a method to increase the capacity of the lithium-ion secondary battery, it is possible to increase the weight per unit area of ​​the positive electrode active material layer by applying a thicker coating of the positive electrode composite material in the above manufacturing process. However, when using an olivine-type positive electrode active material, there is a problem that cracks easily form in the resulting positive electrode active material layer due to shrinkage of the coating film during drying. In particular, this problem easily occurs when using olivine-type lithium manganese iron phosphate (LiMnFePO4), which is an olivine-type positive electrode active material. Solution for solving the problem

[0005] In one embodiment of this disclosure, the cathode material for a lithium-ion secondary battery comprises a granulator having a core composed of the general formula LiMn0. x Fe y The core is composed of olivine-type lithium manganese iron phosphate (PO4, where x and y are numerical values ​​satisfying x + y = 1, 0 < x < 1, and 0 < y < 1); and a carbon coating formed on the surface of the core. The core has a structure formed by the aggregation of primary particles of the olivine-type lithium manganese iron phosphate. The particle size of the primary particles is less than 100 nm. The pore volume in the granulated body, within a pore diameter range of 2 nm to 300 nm, is 0.2 cm³. 3 / g or less. The carbon content of the granules is 1.8% by mass or more and 3.0% by mass or less. Attached Figure Description

[0006] Figure 1 This is a microscope image of the cross-section of the granules. Figure 2 This is a schematic diagram showing the structure of primary particles in a cross-section of the granulator. Figure 3 This is a cross-sectional view of the positive electrode. Figure 4 (a) is a photograph of the positive electrode active material layer used in Example 1. Figure 4 (b) is a photograph of the positive electrode active material layer used in Comparative Example 1. Detailed Implementation

[0007] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Hereinafter, the positive electrode material for a lithium-ion secondary battery and the positive electrode of a lithium-ion secondary battery will be referred to as positive electrode material and positive electrode, respectively.

[0008] <Cathode Materials> like Figure 1 As shown, the cathode material of this embodiment includes a granulator 10, which has a core 11 composed of olivine-type lithium manganese iron phosphate and a carbon coating 12 formed on the surface of the core 11. The cathode material may consist only of the granulator 10, or may include components other than the granulator 10 as needed.

[0009] The olivine-type lithium manganese iron phosphate constituting core 11 is a lithium iron phosphate with the general formula LiMn x Fe y PO4 represents a polyanionic compound with an olivine-type structure. In the general formula LiMn x Fe y In PO4, x and y are values ​​that satisfy x + y = 1, 0 < x < 1, and 0 < y < 1. Specific examples of the ranges of x and y include 0.5 ≤ x ≤ 0.9, 0.1 ≤ y ≤ 0.5, and 0.6 ≤ x ≤ 0.8, 0.2 ≤ y ≤ 0.4. The olivine-type lithium manganese iron phosphate constituting core 11 can be of one type or more.

[0010] Figure 2 schematically shown Figure 1 The structure of the range indicated by arrow A. For example... Figure 2 As shown, the core 11 has a structure formed by the aggregation of primary particles 11a of olivine-type lithium manganese iron phosphate. In addition, the core 11 has fine pores 11b formed between multiple primary particles 11a.

[0011] The particle size of the primary particle 11a is 100 nm or less, preferably 80 nm or less. Alternatively, the particle size of the primary particle 11a is, for example, 20 nm or more. The particle size of the primary particle 11a is determined by small-angle X-ray scattering (SAXS). In detail, the particle size of the primary particle 11a is defined as the average particle size (D50) obtained from the particle size distribution after determining the particle size distribution assuming the primary particle 11a to be spherical.

[0012] In core 11, the pore volume is 0.2 cm³ in the range of pore diameters between 2 nm and 300 nm. 3 / g or less, preferably 0.15cm 3 / g or less. Furthermore, the aforementioned pore volume is, for example, 0.05 cm³. 3 / g or more, preferably 0.1cm 3 / g or more. The above-mentioned pore volume can be determined by nitrogen adsorption and desorption measurement of granules 10 using the BJH (Barrett Joyner Halenda) method. Furthermore, the pore volume described below refers to the pore volume within the range of pore diameters of 2 nm or more and 300 nm or less.

[0013] The carbon content in the granules 10 is 1.8% by mass or more, preferably 1.9% by mass or more, and more preferably 2.0% by mass or more. When the carbon content is less than 1.8%, the carbon coating 12 cannot cover the entire surface of the core 11. In this case, the film-forming properties deteriorate when forming the active material layer described later, as the surface of the core 11 is exposed. Furthermore, the carbon content is 3.0% by mass or less, preferably 2.7% by mass or less. The carbon content can be measured using a carbon-sulfur analyzer (CS meter).

[0014] The content of olivine-type lithium manganese iron phosphate in the granulated body 10 is, for example, 95% by mass or more, preferably 96% by mass or more. Alternatively, the above content is 99% by mass or less, preferably 98% by mass or less.

[0015] In addition, the granules 10 may also contain other components besides olivine-type lithium manganese iron phosphate and carbon. In this case, the content of other components is, for example, less than 2% by mass.

[0016] The average particle size (D50) of the granules 10 is, for example, 3.0 μm or more, preferably 5.0 μm or more. Furthermore, the average particle size (D50) of the granules 10 is, for example, 30 μm or less, preferably 20 μm or less. The average particle size of the granules 10 can be determined, for example, using a laser diffraction particle size analyzer.

[0017] <Manufacturing Method of Cathode Material> Next, an example of a method for manufacturing cathode materials will be explained. An example of a method for manufacturing cathode materials includes a mixing process, a pulverizing process, a drying process, and a firing process. Cathode materials are manufactured by sequentially performing the mixing, pulverizing, drying, and firing processes.

[0018] [Mixed Process] The mixing process is a process of preparing the first slurry by mixing olivine-type lithium manganese iron phosphate particles, aqueous solvents, carbon sources, and carboxylic acids.

[0019] Olivine-type lithium manganese iron phosphate particles are those with the general formula LiMn x Fe y PO4 represents particles of polyanionic compounds with an olivine-type structure. In the general formula LiMn... x Fe y In PO4, x and y are values ​​that satisfy x + y = 1, 0 < x < 1, and 0 < y < 1. Specific examples of the ranges of x and y include 0.5 ≤ x ≤ 0.9, 0.1 ≤ y ≤ 0.5, and 0.6 ≤ x ≤ 0.8, 0.2 ≤ y ≤ 0.4. The olivine-type lithium manganese iron phosphate particles can consist of one or more types of olivine-type lithium manganese iron phosphate.

[0020] The average particle size (D50) of the above-mentioned particles is, for example, 3.0 μm or more, preferably 5.0 μm or more. Furthermore, the average particle size (D50) of the above-mentioned particles is, for example, 30 μm or less, preferably 20 μm or less.

[0021] Olivine-type lithium manganese iron phosphate particles can be manufactured, for example, as follows: First, a first precursor slurry is prepared by mixing a lithium (Li) source such as lithium hydroxide, a manganese (Mn) source such as a manganese compound, an iron (Fe) source such as an iron compound, a PO4 source such as orthophosphoric acid, and water. Next, the lithium, manganese, and iron sources contained in the first precursor slurry are pulverized using a pulverizer such as a bead mill. This yields a second precursor slurry containing pulverized lithium, manganese, and iron sources. Next, the second precursor slurry is dried using a spray drying method or the like to obtain a particle-shaped dried precursor body. The obtained dried precursor body is then calcined, for example, at a temperature of about 500°C, to obtain the aforementioned particles.

[0022] Aqueous solvents are water, or mixtures of water and non-aqueous solvents. Water is not particularly limited, but is preferably, for example, ion-exchanged water treated with an ion-exchange resin, or ultrapure water treated with a reverse osmosis membrane purification system. Examples of non-aqueous solvents constituting the mixture include lower alcohols, acetone, tetrahydrofuran, ethylene glycol, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, acetonitrile, dimethyl sulfoxide, and other solvents mixed with water. One non-aqueous solvent may be used alone, or two or more may be used in combination. The volume percentage of water in the mixture is preferably 50% by volume or more and 99.9% by volume or less, more preferably 60% by volume or more and 99% by volume or less.

[0023] Organic compounds can be used as carbon sources, for example. Examples of organic compounds include glucose, fructose, galactose, mannose, maltose, sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin, hyaluronic acid, chondroitin, agarose, polyethers, polyols, polyvinyl alcohol, polyvinylpyrrolidone, cellulose, starch, gelatin, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, polyacrylic acid, polystyrene sulfonic acid, polyacrylamide, and polyvinyl acetate. Examples of polyols include polyethylene glycol, polypropylene glycol, polyglycerol, and glycerol. A single carbon source can be used, or a combination of two or more can be used.

[0024] Examples of carboxylic acids include citric acid, formic acid, acetic acid, propionic acid, butyric acid, valeric acid, malic acid, fumaric acid, tartaric acid, ascorbic acid, gluconic acid, and polyacrylic acid. A single carboxylic acid can be used alone, or in combination of two or more. Among these carboxylic acids, citric acid is preferred. Details on this will be discussed later.

[0025] Next, the mixing amounts of olivine-type lithium manganese iron phosphate particles, aqueous solvents, carbon sources, and carboxylic acids in the first slurry will be explained. The amount of aqueous solvent mixed in is, for example, 40 parts by mass or more, preferably 50 parts by mass or more, relative to 100 parts by mass of olivine-type lithium manganese iron phosphate particles. Alternatively, the amount of aqueous solvent mixed in is, for example, 80 parts by mass or less, preferably 70 parts by mass or less, relative to 100 parts by mass of olivine-type lithium manganese iron phosphate particles.

[0026] The amount of carboxylic acid mixed in is, for example, 2 parts by mass or more, preferably 3 parts by mass or more, relative to 100 parts by mass of olivine-type lithium manganese iron phosphate particles. Furthermore, the amount of carboxylic acid mixed in is, for example, 15 parts by mass or less, preferably 12 parts by mass or less, relative to 100 parts by mass of olivine-type lithium manganese iron phosphate particles. Moreover, within a specific range below the mixing amount, as the amount of carboxylic acid mixed in increases, the pore volume of the core 11 of the granulated body 10 decreases. Additionally, as the amount of carboxylic acid mixed in increases, the pore diameter distribution of the core 11 of the granulated body 10 shifts towards the smaller diameter side. Therefore, by adjusting the amount of carboxylic acid mixed in, the pore volume of the core 11 of the granulated body 10 can be controlled.

[0027] In the mixing process, the carbon source is mixed with the olivine-type lithium manganese iron phosphate particles in such a way that the total carbon content, including the carbon in the carbon source and the carbon in the carboxylic acid, is a specific amount. Therefore, the mixing amount of the carbon source is, for example, an amount that makes the total carbon content 1.8 parts by mass or more relative to 100 parts by mass of the olivine-type lithium manganese iron phosphate particles, preferably an amount that makes the total carbon content 1.9 parts by mass or more relative to 100 parts by mass of the olivine-type lithium manganese iron phosphate particles. Alternatively, the mixing amount of the carbon source is, for example, an amount that makes the total carbon content 5.0 parts by mass or less relative to 100 parts by mass of the olivine-type lithium manganese iron phosphate particles, preferably an amount that makes the total carbon content 3.0 parts by mass or less relative to 100 parts by mass of the olivine-type lithium manganese iron phosphate particles. By adjusting the total carbon content based on the mixing amount of the carbon source, the carbon content of the granulated body 10 can be controlled.

[0028] In addition, the first slurry may also contain olivine-type lithium manganese iron phosphate particles, an aqueous solvent, a carbon source, and other components besides carboxylic acids. Other components include, for example, dispersants.

[0029] [Grinding Process] The pulverization process is a process of producing a second slurry containing pulverized olivine-type lithium manganese iron phosphate particles from the first slurry. The pulverization process is equivalent to producing the second slurry by pulverizing olivine-type lithium manganese iron phosphate particles in the presence of an aqueous solvent, a carbon source, and a carboxylic acid.

[0030] The average particle size (D50) of the pulverized particles is, for example, 20 nm or more, preferably 30 nm or more. Furthermore, the average particle size (D50) of the pulverized particles is, for example, 100 nm or less, preferably 90 nm or less. By reducing the average particle size (D50) of the pulverized particles, the particle size of the primary particles 11a of the granulator 10 can be reduced.

[0031] The pulverizing method used in the pulverizing process is not particularly limited as long as it can pulverize olivine-type lithium manganese iron phosphate particles in the first slurry. Examples of such pulverizing methods include those using bead mills, hammer mills, stirred mills, jet mills, and ball mills. Furthermore, the pulverizing temperature is, for example, 10°C or higher and 50°C or lower.

[0032] Here, during the pulverization of olivine-type lithium manganese iron phosphate particles, some of the metals (lithium, manganese, and iron) contained in the particles dissolve in the slurry. In the subsequent firing process, the dissolved metal causes necking, with the metal forming the neck portion, between the pulverized particles, i.e., between the primary particles 11a constituting the core 11 of the granulation body 10. When a positive electrode material containing numerous such necked granules 10 is used as the positive electrode of a lithium-ion secondary battery, the battery performance of the lithium-ion secondary battery degrades.

[0033] In this embodiment, the aforementioned necking is suppressed by including carboxylic acid in the first slurry. That is, in the slurry, the dissolved metal is captured by the carboxylic acid, thereby suppressing contact between the dissolved metal and the surface of the pulverized olivine-type lithium manganese iron phosphate particles, particularly the newly formed surface created by pulverization. As a result, necking with the dissolved metal forming the neck portion between the pulverized particles is suppressed.

[0034] Therefore, from the viewpoint of suppressing necking, the carboxylic acid used in the first slurry is preferably a carboxylic acid with a high ability to capture lithium, manganese, and iron, which are dissolved metals. Table 1 below shows the binding energies of various carboxylic acids relative to lithium, manganese, and iron. The binding energy values ​​shown in Table 1 are calculated as the energy difference before and after the carboxylic acid forms a complex with each dissolved metal; the smaller the value, the higher the ability to capture the corresponding dissolved metal. Furthermore, the binding energy values ​​shown in Table 1 below were calculated using CAE (Computer Aided Engineering). As shown in Table 1, citric acid has the lowest binding energy relative to any of the dissolved metals, including lithium, manganese, and iron. From the viewpoint of suppressing necking, citric acid is preferred as the carboxylic acid.

[0035] [Table 1]

[0036] [Drying Process] The drying process is a process of obtaining the aggregate of the above-mentioned pulverized material by spray drying the second slurry. Examples of spraying methods used in spray drying include those using disc nozzles, pressurized nozzles, pressurized two-fluid nozzles, and pressurized four-fluid nozzles. The spray temperature in spray drying is, for example, 180°C or higher and 300°C or lower.

[0037] [Firing process] The firing process is a process of carbonizing the carbon source contained in the condensate obtained in the drying process by firing it. There are no particular limitations on the firing conditions in the firing process, as long as the conditions are sufficient to carbonize the carbon source. The firing temperature in the firing process is, for example, 500°C or higher and 750°C or lower. The firing time in the firing process is, for example, 1 hour or higher and 12 hours or lower. The atmosphere in the firing process is, for example, a non-oxidizing atmosphere. Examples of non-oxidizing atmospheres include inert atmospheres such as nitrogen (N2) and argon (Ar), and reducing atmospheres containing reducing gases such as hydrogen (H2).

[0038] <Positive electrode> Next, the positive electrode 100 containing the positive electrode material of this embodiment will be described. like Figure 3 As shown, the positive electrode 100 includes: a positive current collector 101 having a first surface 101a; and a positive active material layer 102 formed on the first surface 101a of the positive current collector 101.

[0039] [Positive current collector] The positive current collector 101 is a chemically inert conductor used to continuously allow current to flow through the positive electrode active material layer 102 during the discharge or charging of the lithium-ion secondary battery. The positive current collector 101 is, for example, in the form of a foil. The thickness of the foil-shaped positive current collector 101 is, for example, 1 μm or more, preferably 10 μm or more. The thickness of the foil-shaped positive current collector 101 is, for example, 100 μm or less, preferably 60 μm or less.

[0040] Examples of materials constituting the positive current collector 101 include metallic materials, conductive resin materials, and conductive inorganic materials. Examples of metallic materials include copper, aluminum, nickel, titanium, and stainless steel. Examples of conductive resin materials include conductive polymer materials or resins in which conductive fillers are added to non-conductive polymer materials as needed. The positive current collector 101 may also have multiple layers including one or more layers containing the aforementioned metallic material or conductive resin material.

[0041] An example of the positive current collector 101 is an aluminum current collector. The aluminum current collector can be made of elemental aluminum or an aluminum alloy. Examples of aluminum alloys include aluminum (Al)-manganese alloys, aluminum-magnesium (Mg) alloys, and aluminum-magnesium-silicon (Si) alloys.

[0042] The first surface 101a of the positive current collector 101 can also be covered by a known protective layer such as a carbon coating. The first surface 101a of the positive current collector 101 can also be treated by known methods such as plating.

[0043] [Positive electrode active material layer] A positive electrode active material layer 102 is disposed on the first surface 101a of the positive electrode current collector 101. The area weight of the positive electrode active material layer 102 is, for example, 20 mg / cm³. 2 The above is preferably 30 mg / cm³. 2 The above. The weight per unit area of ​​the positive electrode active material layer 102 is, for example, 150 mg / cm². 2 The preferred value is 100 mg / cm³. 2 The thickness of the positive electrode active material layer 102 is, for example, 80 μm or more, preferably 100 μm or more. The thickness of the positive electrode active material layer 102 is, for example, 1000 μm or less, preferably 800 μm or less.

[0044] The positive electrode active material layer 102 contains a positive electrode active material capable of absorbing and releasing charge carriers such as lithium ions. The positive electrode active material layer 102 includes the aforementioned granules 10 as the positive electrode active material. The positive electrode active material layer 102 may also contain other positive electrode active materials besides the granules 10. Examples of other positive electrode active materials include lithium composite metal oxides with a layered rock salt structure, metal oxides with a spinel structure, and polyanionic compounds other than olivine-type lithium manganese iron phosphate. One or more other positive electrode active materials may be used alone or in combination. Furthermore, when other positive electrode active materials are included, the mass proportion of the granules 10 in the total positive electrode active material is preferably 50% or more, more preferably 90% or more.

[0045] The content of the positive electrode active material in the positive electrode active material layer 102 is not particularly limited. The content of the positive electrode active material in the positive electrode active material layer 102 is, for example, 95% by mass or more, preferably 96% by mass or more. The content of the positive electrode active material in the positive electrode active material layer 102 is, for example, 99.5% by mass or less, preferably 99% by mass or less.

[0046] The positive electrode active material layer 102 may also include conductive additives, binders, electrolytes (polymer matrix, ion-conducting polymer, liquid electrolyte, etc.) for improving electrical conductivity, and electrolyte supporting salts (lithium salts) for improving ion conductivity, as needed.

[0047] Conductive additives are added to improve the conductivity of the positive electrode 100. Examples of conductive additives include acetylene black, carbon black, graphite, and carbon nanotubes.

[0048] Examples of adhesives include fluorinated resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber; thermoplastic resins such as polypropylene and polyethylene; imide resins such as polyimide and polyamide-imide; resins containing alkoxysilyl groups; acrylic resins such as polyacrylic acid or polymethacrylic acid; styrene-butadiene rubber; carboxymethyl cellulose; alginates such as sodium alginate and ammonium alginate; water-soluble cellulose ester crosslinkers; and starch-acrylic acid graft polymers. These adhesives can be used alone or in combination. Solvents or dispersion media include, for example, water and N-methyl-2-pyrrolidone.

[0049] <Method for manufacturing the positive electrode> Next, an example of the manufacturing method of the positive electrode 100 will be explained. One example of a method for manufacturing the positive electrode 100 includes a preparation step of preparing a positive electrode composite material and an active material layer forming step of forming an active material layer using the positive electrode composite material. The positive electrode 100 is manufactured by sequentially performing the preparation step and the active material layer forming step.

[0050] [Preparation Process] The preparation process involves curing the positive electrode active material layer 102 into a positive electrode composite material. The positive electrode composite material is a slurry containing positive electrode active material, an aqueous binder, and an aqueous solvent.

[0051] The positive electrode active material mixed in the positive electrode composite material is the same as the positive electrode active material described in the description of positive electrode 100. The content of the positive electrode active material in the positive electrode composite material is, for example, 70 parts by mass and 99 parts by mass and less when the total mass of the solid components contained in the positive electrode composite material, that is, the mass of the positive electrode composite material excluding the aqueous solvent (hereinafter referred to as the mass of solid components), is set to 100 parts by mass, preferably 90 parts by mass and 98 parts by mass and less.

[0052] Aqueous binders are binders that can be dissolved or dispersed in aqueous solvents. They are binders used in combination with positive electrode active materials while dispersed or dissolved in an aqueous solvent. There are no particular limitations on aqueous binders; conventionally known materials can be used as aqueous binders included in the positive electrode active material layer of a lithium-ion secondary battery.

[0053] Examples of aqueous binders include fluorinated resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber; thermoplastic resins such as polypropylene and polyethylene; imide resins such as polyimide and polyamide-imide; resins containing alkoxysilyl groups; acrylic resins such as poly(meth)acrylic acid; styrene-butadiene rubber; carboxymethyl cellulose; alginates such as sodium alginate and ammonium alginate; water-soluble cellulose ester crosslinkers; and starch-acrylic acid graft polymers. The aqueous binder contained in the positive electrode active material layer 102 can be one type or two or more types.

[0054] The content of the aqueous binder in the positive electrode composite material is, for example, more than 1 part by mass and less than 30 parts by mass when the mass of the solid component is set to 100 parts by mass, preferably more than 1 part by mass and less than 5 parts by mass. As an aqueous solvent contained in the cathode composite material, the same aqueous solvent used in the cathode material manufacturing method can be used. The content of the aqueous solvent in the cathode composite material is not particularly limited, for example, it is a content that makes the solid content ratio of the cathode composite material 30 to 60% by mass.

[0055] The cathode composite material may also contain any components, such as conductive additives to improve electrical conductivity, electrolytes (polymer matrix, ion-conducting polymer, electrolyte, etc.), and electrolyte supporting salts (lithium salts) to improve ion conductivity, as needed.

[0056] [Active Substance Layer Formation Process] The active material layer formation process is a process of forming a positive electrode active material layer 102 on the first surface 101a of the positive electrode current collector 101 using the above-mentioned positive electrode composite material.

[0057] In the active material layer formation process, there is no particular limitation on the method of forming the positive electrode active material layer 102 on the first surface 101a of the positive electrode current collector 101 using a slurry-like positive electrode composite material, and known methods that are used in the formation of the positive electrode 100 having the positive electrode current collector 101 and the positive electrode active material layer 102 can be used.

[0058] For example, a positive electrode composite material is applied to the first surface 101a of the positive electrode current collector 101 to a specified thickness. Next, a process is performed to dry and cure the coating film formed by the application, thereby forming a positive electrode active material layer 102. Conventionally known methods such as roller coating, dip coating, doctor blade coating, spray coating, and curtain coating can be used as the coating method for the positive electrode composite material.

[0059] <Effect> Next, the effects of this embodiment will be explained. (1) The positive electrode material for lithium-ion secondary batteries includes a granulator 10, the granulator 10 having: a core 11, which is composed of the general formula LiMn x Fe y The core 11 is composed of olivine-type lithium manganese iron phosphate (PO4, where x and y are numerical values ​​satisfying x + y = 1, 0 < x < 1, and 0 < y < 1); and a carbon coating 12 is formed on the surface of the core 11. The core 11 has a structure formed by the aggregation of primary particles 11a of olivine-type lithium manganese iron phosphate. The particle size of the primary particles 11a is less than 100 nm. The pore volume in the granulation body 10, within the range of pore diameters of 2 nm to 300 nm, is 0.2 cm³. 3 The carbon content is between 1.8% and 3.0% by mass, and the carbon content is below 1.8% by mass.

[0060] The positive electrode material described above is used as the positive electrode active material contained in the positive electrode active material layer 102 of the positive electrode 100. The positive electrode active material layer 102 of the positive electrode 100 is manufactured by coating a slurry-like positive electrode composite material containing the positive electrode active material onto the first surface 101a of the positive electrode current collector 101 and drying the coating film formed by the coating. By using the positive electrode material described above as the positive electrode active material contained in the positive electrode composite material, even when the coating film is formed to increase the unit area weight of the positive electrode active material layer 102, cracks generated in the positive electrode active material layer 102 due to shrinkage of the coating film during drying can be suppressed. That is, by using the positive electrode material described above, a high unit area weight, for example, 30 mg / cm², can be manufactured. 2 The film-forming properties of the positive electrode active material layer 102 are improved in the above-described case. Furthermore, it is possible to achieve improved battery performance, such as increased capacity of lithium-ion secondary batteries, by increasing the area weight of the positive electrode active material layer 102. Therefore, the positive electrode material configured as described above can achieve both improved film-forming properties and improved battery characteristics while increasing the area weight of the positive electrode active material layer containing olivine-type lithium manganese iron phosphate.

[0061] (2) The manufacturing method of the positive electrode material for lithium-ion secondary batteries includes: processing LiMn in the presence of an aqueous solvent, a carbon source and a carboxylic acid. x Fe yThe process of pulverizing olivine-type lithium manganese iron phosphate particles represented by PO4 (x and y are values ​​that satisfy x + y = 1, 0 < x < 1, 0 < y < 1) to produce a slurry containing pulverized olivine-type lithium manganese iron phosphate (equivalent to the second slurry) is as follows (equivalent to the pulverizing process); the process of spray drying the slurry (equivalent to the second slurry) to obtain agglomerates of the pulverized material is as follows (equivalent to the drying process); and the process of carbonizing the carbon source contained in the agglomerates by firing the agglomerates is as follows (equivalent to the firing process).

[0062] According to the above configuration, since the particle pulverization is carried out in an aqueous solvent and in the presence of carboxylic acid, the dissolved metal released during pulverization is captured by the carboxylic acid. By capturing the dissolved metal with carboxylic acid, contact between the dissolved metal and the surface of the pulverized olivine-type lithium manganese iron phosphate particles, especially the newly formed surface created by pulverization, is suppressed. As a result, necking with the dissolved metal as the neck portion between the pulverized particles is suppressed. Furthermore, the suppression of necking results in the suppression of battery performance degradation in lithium-ion secondary batteries caused by necking. In addition, compared with the case where carboxylic acid is added after pulverizing olivine-type lithium manganese iron phosphate particles, the pore volume in the granulated body 10 within the range of pore diameters of 2 nm to 300 nm can be reduced.

[0063] (3) The carboxylic acid is citric acid. Citric acid has a high ability to capture lithium, manganese and iron, which are dissolved metals that dissolve during pulverization. Therefore, the effect of (2) above can be obtained more significantly.

[0064] (4) The content of carboxylic acid in the slurry is less than 10 parts by mass relative to 100 parts by mass of olivine-type lithium manganese iron phosphate. In this case, it is possible to manufacture granules 10 with small pore volume in the range of pore diameter of 2 nm or more and 300 nm or less.

[0065] <Example of Change> Furthermore, this embodiment can be implemented with modifications as follows. This embodiment and the following modifications can be combined and implemented within a technically compatible scope.

[0066] ○ In the manufacturing method of cathode materials, the timing of adding carboxylic acid can also be changed. For example, in the mixing process, a first slurry containing olivine-type lithium manganese iron phosphate particles, an aqueous solvent, and a carbon source is prepared. Then, in the pulverizing process, the olivine-type lithium manganese iron phosphate particles contained in the first slurry are pulverized while adding carboxylic acid or a solution of carboxylic acid. In this case, since the pulverization of the particles is carried out in an aqueous solvent and in the presence of carboxylic acid, an effect of suppressing necking based on the capture of dissolved metal by carboxylic acid can be obtained. Example

[0067] The following describes embodiments that further specify the above-described implementation methods. <Preparation of olivine-type lithium manganese iron phosphate> A first precursor slurry was obtained by mixing pure water (12000 ml), LiOH (245 g) as a lithium source, MnCO3 (840 g) as a manganese source, Fe2C2O4 (401 g) as an iron source, and H3PO4 (1125 g) as a PO4 source. After adding 0.3 mm diameter beads (medium particles) to the first precursor slurry, the lithium, manganese, and iron sources contained in the first precursor slurry were pulverized using a bead mill. This yielded a second precursor slurry containing pulverized lithium, manganese, and iron sources. The bead milling process was performed to ensure that the average particle size (D50) of the lithium, manganese, and iron sources was between 50 nm and 1000 nm. The second precursor slurry was then dried and granulated using a spray dryer (drying outlet temperature: 200 °C) to obtain the dried precursor. The obtained precursor was dried and heated at 500°C for 6 hours under a N2 atmosphere to obtain the product with the general formula LiMn. 0.75 Fe 0.25 PO4 represents olivine-type lithium manganese iron phosphate.

[0068] <Preparation of Granules> [Example 1] The first slurry was obtained by mixing 45g of the prepared olivine-type lithium manganese iron phosphate, fructose as a carbon source, and 2g of citric acid as a carboxylic acid in pure water (405ml). The amount of fructose mixed was set such that the total carbon content, including the carbon in fructose and the carbon in citric acid, was 2.5 parts by mass relative to 100 parts by mass of olivine-type lithium manganese iron phosphate.

[0069] After adding 0.1 mm diameter beads (medium particles) to the obtained first slurry, the olivine-type lithium manganese iron phosphate contained in the first slurry was pulverized using a bead mill. This yielded a second slurry containing pulverized olivine-type lithium manganese iron phosphate. The bead milling process was performed to ensure that the average particle size (D50) of the olivine-type lithium manganese iron phosphate was between 60 nm and 100 nm.

[0070] Next, the second slurry was dried and granulated using a spray dryer (drying outlet temperature: 200°C) to obtain agglomerates of the above-mentioned pulverized material. The obtained agglomerates were heated at 650°C for 6 hours under a N2 atmosphere to carbonize the fructose and citric acid contained in the agglomerates. Thus, the granulated body of Example 1, having a core composed of olivine-type lithium manganese iron phosphate and a carbon coating formed on the surface of the core, was obtained.

[0071] [Examples 2-10 and Comparative Examples 1-4] Examples 2-10 and Comparative Examples 1-4 are examples in which the amount of citric acid was varied. Except for the variation in the amounts of citric acid and fructose in the first slurry, the granules of Examples 2-10 and Comparative Examples 1-4 were obtained using the same method as in Example 1. The amount of citric acid in each example is shown in Table 2 below. The amount of fructose in each example was adjusted to match the amount of citric acid, such that the total carbon content was 2.5 parts by mass relative to 100 parts by mass of olivine-type lithium manganese iron phosphate.

[0072] [Comparative Examples 5-7] Comparative Examples 5-7 are mainly examples of coarser grinding processes using a bead mill. Except that the amount of citric acid mixed was set to 0 and the grinding process using a bead mill was carried out so that the average particle size (D50) of olivine-type lithium manganese iron phosphate was in the range of 60 nm or more and 150 nm or less, the granules of Comparative Example 5 were obtained by the same method as in Example 1.

[0073] Except that the grinding process using a bead mill was carried out so that the average particle size (D50) of olivine-type lithium manganese iron phosphate was in the range of 100 nm or more and 120 nm or less, the granules of Comparative Example 6 were obtained by the same method as in Example 5.

[0074] Except that the grinding process using a bead mill was carried out so that the average particle size (D50) of olivine-type lithium manganese iron phosphate was in the range of 140 nm or more and 160 nm or less, the granules of Comparative Example 7 were obtained by the same method as in Example 5.

[0075] [Comparative Examples 8-9] Comparative Examples 8 and 9 are examples in which the amount of fructose as a carbon source was increased. Except for adjusting the total carbon content to be 3.2 parts by mass relative to 100 parts by mass of olivine-type lithium manganese iron phosphate, the granules of Comparative Example 8 were obtained by the same method as in Example 5.

[0076] Except for adjusting the total carbon content to be 5.1 parts by mass relative to 100 parts by mass of olivine-type lithium manganese iron phosphate, the granules of Comparative Example 9 were obtained by the same method as in Example 5.

[0077] [Comparative Example 10] Comparative Example 10 is an example in which the timing of citric acid mixing was changed. Except that citric acid (2g) was not mixed in the first slurry but was mixed in the second slurry after pulverization using a bead mill, the granules of Comparative Example 10 were obtained by the same method as in Example 1.

[0078] <Analysis of Granulations> For the granules of each embodiment and comparative example, the particle size of the primary particles, the pore volume in the range of pore diameters of 2 nm to 300 nm, and the carbon content were measured. The results are shown in Table 2.

[0079] The particle size of the primary particles is determined as follows: The particle size distribution of the primary particles, assuming they are spherical, is obtained using the SAXS method. The average particle size (D50) obtained from the determined particle size distribution is calculated and taken as the particle size of the primary particles.

[0080] The pore volume was calculated as follows: The pore size of the positive electrode active material was measured using a specific surface area / pore distribution measuring device. The cumulative pore distribution of the granules was determined by analysis using the BJH method in the region with pore diameters greater than 2 nm and less than 300 nm. The pore diameter was plotted on the horizontal axis, and the cumulative pore volume was plotted on the vertical axis. The pore volume within the range of pore diameters greater than 2 nm and less than 300 nm was calculated from the difference between the cumulative pore volumes for pore diameters of 2 nm and 300 nm.

[0081] The carbon content was determined using a carbon-sulfur analysis device. <The Making of Positive Electrode> A positive electrode composite material was coated onto one side of an aluminum foil with a thickness of 15 μm. By drying the coated positive electrode composite material, a positive electrode with a positive electrode active material layer formed on one side of the positive electrode current collector was fabricated. As the positive electrode composite material, a slurry containing granules (positive electrode active material), carbon nanotubes (conductive additive), carboxymethyl cellulose (binder), and styrene-butadiene rubber (binder) of each example and comparative example in a solid component mass ratio of 98.25:0.05:0.4:1.3, and using water as a solvent, was used. The weight per unit area of ​​the positive electrode active material layer in each fabricated positive electrode was measured. The results showed that the measured values ​​were all approximately 65–75 mg / cm³. 2 .

[0082] <Evaluation of film-forming properties> The surface condition of the positive electrode active material layer in each fabricated positive electrode was visually evaluated. The results are shown in Table 2. The evaluation criteria are as follows. For reference, in Figure 4 In (a), a photograph is shown of the positive electrode active material layer of Example 1, which was rated as "○" for film-forming properties, and in Figure 4 (b) shows a photograph of the positive electrode active material layer of Comparative Example 1, which was rated as “×” for film-forming properties.

[0083] “○”: There are no cracks on the surface of the positive electrode active material layer. "×": There are cracks on the surface of the positive electrode active material layer. <Electrochemical Experiment> A positive electrode half-cell was fabricated using a positive electrode whose film-forming properties were rated as "○". An electrode body cell was fabricated by sandwiching a separator between a positive electrode (evaluation electrode) cut to 25 mm square and a negative electrode made of a 200 μm thick lithium metal foil cut to 27 mm square. A half-cell for electrochemical testing was obtained by housing the electrode body cell within a laminate as an outer casing, injecting a non-aqueous electrolyte, and sealing the outer casing. A glass filter manufactured by Hoechst Celanese was used as the separator. A non-aqueous electrolyte was used, prepared by dissolving lithium hexafluorophosphate at a concentration of 1 M in a mixed solvent of ethylene carbonate and diethyl carbonate in a 1:1 volume ratio.

[0084] The fabricated positive half-cell was charged to 4.3V at a fixed current of 0.05C at 25°C and then discharged to 3.0V. The discharge capacity was measured. The results are shown in Table 2.

[0085] [Table 2]

[0086] As shown in Table 2, the granules of Examples 1 to 10 satisfy all three conditions described below.

[0087] Condition 1: The particle size of the primary particles is less than 100 nm. Condition 2: The pore volume is 0.2 cm³. 3 / g or less. Condition 3: Carbon content is below 3.0% by mass. No cracks were found on the surface of the positive electrode active material layer formed using the granules of Examples 1-10 as the positive electrode active material. The discharge capacity when using the granules of Examples 1-10 as the positive electrode active material was 147-149.1 mAh / g.

[0088] In contrast, while the granules of Comparative Examples 1-4 satisfied conditions 1 and 3, they did not satisfy condition 2, which is related to pore volume, due to their large pore volume. Cracks were observed on the surface of the positive electrode active material layer formed using the granules of Comparative Examples 1-4 as the positive electrode active material. This result indicates that, to ensure good film formation even when forming a positive electrode active material layer with a large weight per unit area, a pore volume of 0.2 cm⁻² is recommended. 3 The following values ​​are valid: / g

[0089] On the other hand, although the granules of Comparative Examples 5-7 satisfied conditions 2 and 3, they did not satisfy condition 1 due to the large particle size of the primary particles. No cracks were observed on the surface of the positive electrode active material layer formed using the granules of Comparative Examples 5-7 as the positive electrode active material. However, the discharge capacity when using the granules of Comparative Examples 5-7 as the positive electrode active material was 132-141 mAh / g, which is significantly lower than the discharge capacity when using the granules of Examples 1-10 as the positive electrode active material.

[0090] Although the granules of Comparative Examples 8-9 met conditions 1 and 2, they did not meet condition 3 due to their high carbon content. No cracks were observed on the surface of the positive electrode active material layer formed using the granules of Comparative Examples 8-9 as the positive electrode active material. However, the discharge capacity when using the granules of Comparative Examples 8-9 as the positive electrode active material was 133.5-139.9 mAh / g, which is significantly lower than the discharge capacity when using the granules of Examples 1-10 as the positive electrode active material.

[0091] The above results, obtained using the granules of Comparative Examples 5-9, demonstrate that simply forming a high-weight positive electrode active material layer per unit area is insufficient to increase the capacity of the lithium-ion secondary battery. Furthermore, it is evident that to achieve this capacity increase, in addition to setting the pore volume to 0.2 cm³, [further details are needed]. 3 For particles smaller than 100 nm, the particle size of the primary particles must be less than 100 nm, and the carbon content must be less than 3.0% by mass.

[0092] Next, the relationship between the amount of citric acid mixed and the pore volume of the obtained granules was investigated. Referring to the results of Examples 1-10 and Comparative Examples 1-3, within the range of 10 parts by mass or less relative to 100 parts by mass of olivine-type lithium manganese iron phosphate, the pore volume of the obtained granules decreased as the amount of citric acid mixed increased. On the other hand, the pore volume of the granules in Comparative Example 4, with a citric acid mixing amount of 15 parts by mass, increased to more than twice that of the granules in Examples 8-10, with a citric acid mixing amount of 10 parts by mass. From this result, it can be seen that although increasing the mixing amount of carboxylic acids such as citric acid can reduce the pore volume of the granules, when the mixing amount of carboxylic acids is increased to a certain level or more, the pore volume of the granules actually increases.

[0093] Next, the timing of citric acid mixing was examined. Comparative Example 10 was an example in which citric acid was not mixed in the first slurry but was mixed in the second slurry after pulverization. The pore volume of the granules from Comparative Example 10 was 1.5 times larger than that of the granules from Example 1, which differed only in the timing of citric acid mixing. This result indicates that pulverization is necessary in the presence of carboxylic acids such as citric acid to reduce pore volume.

Claims

1. A positive electrode material for lithium-ion secondary batteries, characterized in that, The granulation body comprises a core having the general formula LiMn x Fe y The core is composed of olivine-type lithium manganese iron phosphate represented by PO4 (where x and y are values ​​satisfying x + y = 1, 0 < x < 1, and 0 < y < 1); and a carbon coating formed on the surface of the core. The core has a structure formed by the primary particle aggregation of the olivine-type lithium manganese iron phosphate. The particle size of the primary particles is less than 100 nm. The pore volume in the granulated material, within the range of pore diameters between 2 nm and 300 nm, is 0.2 cm³. 3 / g or less The carbon content of the granules is above 1.8% by mass and below 3.0% by mass.

2. A positive electrode for a lithium-ion secondary battery, characterized in that, It comprises: a positive current collector having a first surface; and A positive electrode active material layer is formed on the first surface of the positive electrode current collector. The positive electrode active material layer contains the positive electrode material for lithium-ion secondary batteries as described in claim 1.

3. The positive electrode for a lithium-ion secondary battery according to claim 2, wherein, The unit area weight of the positive electrode active material layer is 30 mg / cm³. 2 Above and 100mg / cm 2 the following.

4. A method for manufacturing a positive electrode material for a lithium-ion secondary battery, characterized in that, Include: By using the general formula LiMn x Fe y The process of pulverizing olivine-type lithium manganese iron phosphate particles (represented by PO4, where x and y are numerical values ​​satisfying x + y = 1, 0 < x < 1, and 0 < y < 1) in the presence of an aqueous solvent, a carbon source, and a carboxylic acid to prepare a slurry containing the pulverized olivine-type lithium manganese iron phosphate. The process of obtaining the agglomerate of the pulverized material by spray drying the slurry; and The process of carbonizing the carbon source contained in the condensate by firing it.

5. The method for manufacturing the positive electrode material for a lithium-ion secondary battery according to claim 4, wherein, The carboxylic acid is citric acid.

6. The method for manufacturing the positive electrode material for a lithium-ion secondary battery according to claim 4 or 5, wherein, The content of the carboxylic acid in the slurry is more than 2 parts by mass and less than 10 parts by mass relative to 100 parts by mass of the olivine-type lithium manganese iron phosphate.

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