Positive electrode active material for lithium-ion secondary batteries

Abstract

To provide a cathode active material for a lithium-ion secondary battery which enables the energy density per unit volume and rate characteristics of the lithium-ion secondary battery to be effectively increased.SOLUTION: The cathode active material for a lithium-ion secondary battery is formed by covering the surfaces of granulated bodies A with particles B. Each of the granulated bodies A is represented by the following formula (a): LifMngFehM1xPO4 ... (a) and has an average particle diameter of 8 μm to 50 μm, and each of the particles B is represented by following formula (b): LiM2aMnbO4 ... (b), wherein the mass ratio (A: B) of the granulated bodies A to the particles B is 95:5 to 55:45.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery that increases the energy density per unit volume of the lithium ion secondary battery and improves the rate characteristics. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are used in a wide range of applications, including mobile phones, digital cameras, laptop computers, hybrid vehicles, and electric vehicles. LiMn is a popular cathode material for these batteries due to its high safety and large capacity. x Fe 1-x Particles having an olivine structure, such as lithium manganese phosphate and lithium iron phosphate, are considered promising, and various developments utilizing such particles are being carried out.

[0003] For example, Patent Document 1 discloses a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer is arranged, the positive electrode active material layer being formed of a first layer containing an active material such as lithium manganese oxide and a second layer containing an active material such as LiFePO4, and attempts to improve the discharge rate characteristics. Furthermore, Patent Document 2 discloses an electrode active material having a core formed from a lithium-containing transition metal oxide such as LiMnO2, and a shell containing lithium metal oxide particles such as lithium manganese iron phosphate and a polymer, thereby improving safety and stability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-147790 [Patent Document 2] Special Publication No. 2015-503196 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the case of the positive electrode active material having a structure such as Patent Documents 1 and 2, in the lithium ion secondary battery obtained, the energy density per unit volume has not yet been sufficiently increased, and there is still room for improvement in order to exhibit excellent rate characteristics.

[0006] Therefore, an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery capable of effectively increasing the energy density per unit volume and the rate characteristics of the lithium ion secondary battery.

Means for Solving the Problems

[0007] Therefore, as a result of intensive studies to solve the above problems, the present inventor has found that particles represented by a specific formula such as LiMn2O4 are coated on the surface of lithium manganese iron phosphate granulated bodies having a specific average particle size, and these particles have a specific mass ratio, whereby in the obtained lithium ion secondary battery, it is possible to effectively increase the energy density per unit volume and also improve the rate characteristics, and positive electrode active material particles for a lithium ion secondary battery can be provided.

[0008] That is, the present invention provides the following formula (a): Li f Mn g Fe h M 1 x PO4···(a) (In formula (a), M 1 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd and Gd. f, g, h and x satisfy 0 < f ≤ 1.2, 0.3 ≤ g ≤ 1.2, ①0.2 ≤ h ≤ 1.2, 0 ≤ x ≤ 0.3, and 3 / 17 ≤ g / h ≤ 13 / 7, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 1 ) × x = 3. ) It is represented by , and on the surface of granule A having an average particle size of 8 μm to 50 μm, the following formula (b): LiM 2 a Mn b O4···(b) (In formula (b), M 2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b are numbers satisfying 0 ≦ a ≦ 0.1, 0 < b ≦ 2, and (valence of M 2 ) × a + (valence of Mn) × b = 7.) is coated with particles B represented by , and provides a positive electrode active material for a lithium-ion secondary battery in which the mass ratio (A:B) of granule A to particles B is 95:5 to 55:45.

Effects of the Invention

[0009] According to the positive electrode active material for a lithium-ion secondary battery of the present invention, it is possible to realize a lithium-ion secondary battery in which both the energy density per unit volume and the rate characteristics are effectively enhanced.

Modes for Carrying Out the Invention

[0010] Hereinafter, the present invention will be described in detail. The positive electrode active material for a lithium-ion secondary battery of the present invention is represented by the following formula (a): Li f Mn g Fe h M 1 <00000!9>PO4···(a) (In formula (a), M<00!00020>represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x are numbers satisfying 0 < f ≦ 1.2, 0.3 ≦ g ≦ 1.2, 0.2 ≦ h ≦ 1.2, 0 ≦ x ≦ 0.3, and 3 / 17 ≦ g / h ≦ 13 / 7, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 1 ) × x = 3.) It is represented by, and on the surface of granule A having an average particle size of 8 μm to 50 μm, the following formula (b): LiM 2 a Mn b O4···(b) (In formula (b), M 2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b satisfy 0 ≦ a ≦ 0.1, 0 < b ≦ 2, and (valence of M 2 ) × a + (valence of Mn) × b = 7.) It is coated with particles B represented by, The mass ratio (A:B) of granule A to particles B is 95:5 to 55:45.

[0011] Thus, the positive electrode active material for a lithium ion secondary battery of the present invention is particles composed of granule A represented by a specific formula and having a specific average particle size, and particles B represented by a specific formula, and these granule A and particles B exhibit a specific particle structure in which particles B (shell part) are coated on the surface of granule A (core part) while maintaining a limited mass ratio, thereby improving the electrode density and the electron conductivity between the particles and the electrolyte, and in the obtained lithium ion secondary battery, it is possible to effectively increase the energy density per unit volume and the rate characteristics.

[0012] Granule A constituting the positive electrode active material for a lithium ion secondary battery of the present invention has the following formula (a): Li f Mn g Fe h M 1 x PO4···(a) (In formula (a), M 1represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0 < f ≤ 1.2, 0.3 ≤ g ≤ 1.2, 0.2 ≤ h ≤ 1.2, 0 ≤ x ≤ 0.3, and 3 / 17 ≤ g / h ≤ 13 / 7, and satisfy f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 1 (valence of) × x = 3.) It is a granule represented by and having an average particle size of 8 μm to 50 μm.

[0013] The granule A represented by the above formula (a) is an olivine-type lithium transition metal phosphate compound containing at least both manganese (Mn) and iron (Fe) as transition metals, and is an aggregate formed by granulating and aggregating the preliminary particles a (corresponding to primary particles, so-called LMFP particles) represented by the above formula (a). Since the surface of the granule A is coated with particles B that maintain a specific mass ratio with the granule A, it contributes to the improvement of rate characteristics while effectively increasing the energy density per unit volume.

[0014] Regarding the granule A, from the viewpoint of the average discharge voltage, for f, 0.6 ≤ f ≤ 1.2 is preferable, 0.65 ≤ f ≤ 1.15 is more preferable, and 0.7 ≤ f ≤ 1.1 is even more preferable. For g, 0.15 ≤ g ≤ 0.65 is preferable, 0.2 ≤ g ≤ 0.6 is more preferable, and 0.25 ≤ g ≤ 0.55 is even more preferable. For h, 0.35 ≤ h ≤ 0.85 is preferable, 0.4 ≤ h ≤ 0.8 is more preferable, and 0.45 ≤ h ≤ 0.75 is even more preferable. For x, 0 ≤ x ≤ 0.2 is preferable, 0 ≤ x ≤ 0.15 is more preferable, and 0 ≤ x ≤  0.1 is even more preferable. And g / h is the molar ratio of Mn and Fe constituting the so-called granule A, and 1 / 4 ≤ g / h ≤ 3 / 2 is preferable, 1 / 3 ≤ g / h ≤ 11 / 9 is more preferable, and 3 / 7 ≤ g / h ≤ 1 is even more preferable.

[0015] Specifically, for example, LiMn 0.2 Fe 0.8 PO4, LiMn 0.3Fe 0.7 PO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.75 Fe 0.15 Mg 0.1 PO4, LiMn 0.6 Fe 0.4 PO4, LiMn 0.5 Fe 0.5 PO4, etc. Among them, LiMn 0.2 Fe 0.8 PO4, LiMn 0.3 Fe 0.7 PO4 or LiMn 0.6 Fe 0.4 PO4 is preferred.

[0016] The average particle size of the granules (A) represented by the formula (a) is 8 μm to 50 μm, preferably 9 μm to 40 μm, more preferably 9.5 μm to 30 μm, and even more preferably 10 μm to 20 μm, from the viewpoints of being able to suppress the amount of expansion and contraction of the primary particles accompanying the insertion and desorption of lithium ions, effectively preventing particle cracking, and ease of handling. Here, the "average particle size" of granule A is the D obtained from the volume-based particle size distribution based on the laser diffraction / scattering method. 50 The values ​​are the particle diameters (median diameters) at 50% of the cumulative particle size.

[0017] Granules A, which are aggregates of preliminary particles a obtained by granulating preliminary particles a, may be particles in which the surfaces of the preliminary particles a are supported with carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material, from the viewpoint of ensuring excellent discharge capacity and further improving rate characteristics. Cellulose nanofibers are the skeletal component that accounts for approximately 50% of all plant cell walls. They are lightweight, high-strength fibers that can be obtained by defibrating the plant fibers that make up these cell walls to nano-size. The fiber diameter of these cellulose nanofibers is 1 nm to 1,000 nm, and they also have good dispersibility in water. Furthermore, the cellulose molecular chains that make up the cellulose nanofibers form a periodic structure of carbon. Therefore, when these cellulose nanofibers are carbonized to form carbon, which is firmly supported on the surface of preliminary particles a, carbon is also supported on the surface of granules A made up of preliminary particles a. Furthermore, particles B, which will be described later, densely coat the surface of granules A, thereby effectively suppressing the deterioration of the electronic conductive path and effectively increasing the energy density per unit volume, thereby ensuring the expression of excellent rate characteristics in the resulting battery.

[0018] When granule A is an aggregate of preliminary particles a having cellulose nanofiber-derived carbon supported on the surface thereof, the atomic amount of carbon derived from the carbonized cellulose nanofiber, i.e., the amount of carbon derived from the cellulose nanofiber, is preferably 0.1 mass% to 5.0 mass%, more preferably 0.3 mass% to 4.0 mass%, and even more preferably 0.5 mass% to 3.0 mass% in 100 mass% of granule A.

[0019] The water-soluble carbon material, like cellulose nanofibers, is carbonized to form carbon, and when the granules A are aggregates of preliminary particles a with this carbon supported on the surface, like cellulose nanofibers, the deterioration of the electronic conductive path is effectively suppressed, the energy density per unit volume is effectively increased, and the resulting battery can ensure excellent rate characteristics. Examples of such water-soluble carbonaceous materials include one or more selected from sugars, polyols, polyethers, and organic acids. More specifically, examples include monosaccharides such as glucose, fructose, galactose, and mannose; disaccharides such as maltose, sucrose, and cellobiose; polysaccharides such as starch and dextrin; polyols and polyethers such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, butanediol, propanediol, polyvinyl alcohol, and glycerin; and organic acids such as citric acid, tartaric acid, and ascorbic acid. Among these, from the viewpoint of increasing solubility and dispersibility in a solvent and effectively functioning as a carbonaceous material, glucose, fructose, sucrose, and dextrin are preferred, and glucose is more preferred.

[0020] When the granules A are aggregates of preliminary particles a having carbon derived from a water-soluble carbon material supported on their surfaces, the atomic equivalent amount of carbon derived from the water-soluble carbon material, i.e., the amount of carbon derived from the water-soluble carbon material supported, in 100% by mass of the granules A, is preferably 4.0% by mass or less, more preferably 3.0% by mass or less, and even more preferably 2.0% by mass or less.

[0021] Regarding the carbon derived from cellulose nanofibers and the carbon derived from water-soluble carbon materials, only carbon derived from cellulose nanofibers, only carbon derived from water-soluble carbon materials, or both carbon derived from cellulose nanofibers and carbon derived from water-soluble carbon materials may be supported on the surfaces of the preliminary particles a. Of these, it is preferable to support carbon derived from cellulose nanofibers on the surfaces of the granules A, which are aggregates of the preliminary particles a, so as to cover only a portion of the surfaces and not hinder the deformation of the granules A.

[0022] When granule A is an aggregate of preliminary particles a having cellulose nanofiber-derived carbon supported on the surface thereof, the sum of the atomic equivalent amount of carbon derived from the cellulose nanofiber and the atomic equivalent amount of carbon derived from the water-soluble carbon material, i.e., the total amount of carbon derived from the cellulose nanofiber and the amount of carbon derived from the water-soluble carbon material, is preferably 0.1% by mass to 5.0% by mass, more preferably 0.3% by mass to 4.0% by mass, and even more preferably 0.5% by mass to 3.0% by mass, in 100% by mass of granule A.

[0023] The atomic equivalent amount (supported amount) of carbon derived from cellulose nanofibers and the atomic equivalent amount (supported amount) of carbon derived from water-soluble carbon materials present in granule A refer to values ​​determined by measurements using a carbon / sulfur analyzer. Furthermore, when the granules A are aggregates of preliminary particles a having cellulose nanofiber-derived carbon supported on their surfaces, the amount of the granules A described below includes the amount of the carbon supported.

[0024] The granules A can be obtained, for example, by the following production method. Specifically, the granules A include the following steps (Ia) to (IVa): (Ia) A step of adding a metal compound including a lithium compound, a manganese compound, and an iron compound, a phosphate compound, and optionally cellulose nanofibers and / or a water-soluble carbon material, and water to obtain slurry water i, and then subjecting the slurry to a hydrothermal reaction to obtain preliminary particles a. (IIa) A step of adding the obtained preliminary particles a and water to obtain slurry water ii. (IIIa) A step of subjecting the slurry water ii to spray drying to obtain pre-granules x (IVa) A step of calcining the obtained pre-granules x in a reducing atmosphere or an inert atmosphere to obtain granules A. The manufacturing method includes the steps of:

[0025] The above step (Ia) is a step in which a metal compound including a lithium compound, a manganese compound, and an iron compound, a phosphate compound, and optionally cellulose nanofibers and / or a water-soluble carbon material, and water are added to obtain a slurry water i, which is then subjected to a hydrothermal reaction to obtain preliminary particles a. The preliminary particles a are LMFP particles represented by the above formula (a) and correspond to the primary particles of granules A. By undergoing subsequent steps, these preliminary particles a are appropriately aggregated to form granules A, which are so-called aggregates of preliminary particles a. Examples of the lithium compound to be used include hydroxides (for example, LiOH·H2O, LiOH), carbonates, acetates, and nitrates. Of these, hydroxides are preferred. Examples of manganese compounds include manganese acetate, manganese nitrate, and manganese oxide. These may be used alone or in combination of two or more. Among these, manganese oxide is preferred from the viewpoint of improving battery characteristics. In addition to these lithium compounds and manganese compounds, metal compounds other than manganese compounds and iron compounds (M 1 ) compounds may also be used.

[0026] Examples of phosphoric acid compounds include orthophosphoric acid (H3PO4, phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, etc. Among these, phosphoric acid is preferably used, and is preferably used as an aqueous solution with a concentration of 70% by mass to 90% by mass. When carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material is to be supported on the surface of the granules xA, the above-mentioned cellulose nanofibers and / or water-soluble carbon material may be further used.

[0027] The slurry water i may be prepared according to a conventional method by determining the amounts of lithium compounds, metal compounds including manganese compounds and iron compounds, phosphate compounds, etc., depending on the composition of the target granules A.

[0028] Next, the obtained slurry water i is subjected to a hydrothermal reaction to obtain preliminary particles a. The amount of water used in the hydrothermal reaction is preferably 10 mol to 50 mol, and more preferably 12.5 mol to 45 mol, per mol of phosphate ions contained in the slurry water i, from the viewpoints of the solubility of the metal compound, ease of stirring, synthesis efficiency, and the like.

[0029] The hydrothermal reaction may be carried out at a temperature of 100°C or higher, preferably 130°C to 200°C. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel, and when the reaction is carried out at 130°C to 200°C, the pressure is preferably 0.3MPa to 1.6MPa, and when the reaction is carried out at 140°C to 160°C, the pressure is preferably 0.3MPa to 0.6MPa. The hydrothermal reaction time is preferably 0.1 hours to 48 hours, more preferably 0.2 hours to 24 hours. The obtained preliminary particles a are isolated by filtering, washing with water, and drying, which may be performed by freeze drying or vacuum drying. The step (IIa) is a step of adding the obtained preliminary particles a and water to obtain slurry water ii. The solid content concentration of the obtained slurry water ii is preferably 5% by mass to 30% by mass, more preferably 5% by mass to 20% by mass, and even more preferably 5% by mass to 15% by mass.

[0030] After adding water, it is preferable to pre-stir the slurry water (ii) before proceeding to step (IIIa). The stirring time of the slurry water (ii) is preferably 3 to 60 minutes, more preferably 5 to 30 minutes. The temperature of the slurry water (ii) is preferably 10 to 60°C, more preferably 20 to 40°C.

[0031] The above step (IIIa) is a step of subjecting the slurry water ii to spray drying to obtain pre-granules x. In the spray drying, the operating conditions may be appropriately set depending on the apparatus used. For example, as the processing conditions in a micro mist dryer (MDL-050M manufactured by Fujisaki Electric Co., Ltd.) equipped with a 4-fluid nozzle, the hot air temperature is preferably 110°C to 300°C, and more preferably 150°C to 250°C. Also, the volume ratio of the supply amount of hot air to the supply amount of slurry water (supply amount of hot air / supply amount of slurry water) is preferably 500 to 10000, and more preferably 1000 to 9000.

[0032] The above step (IVa) is a step of firing the obtained preliminary granule x in a reducing atmosphere or an inert atmosphere to obtain granule A. The firing temperature is preferably 500°C to 750°C, more preferably 520°C to 720°C, the firing time is preferably 0.3 hours to 12 hours, and more preferably 0.5 hours to 6 hours.

[0033] The particles B constituting the positive electrode active material for the lithium ion secondary battery of the present invention are represented by the following formula (b): LiM 2 a Mn b O4···(b) (In formula (b), M 2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si. a and b satisfy 0 ≦ a ≦ 0.1, 0 < b ≦ 2, and (valence of M 2 ) × a + (valence of Mn) × b = 7. ) It is represented by.

[0034] The particles B represented by the above formula (b) are particles having a spinel structure. By coating the above granule A as a core part with such fine particles B while maintaining a specific mass ratio, the energy density per unit volume can be dramatically increased and the rate characteristics can be improved.

[0035] Specific examples of the particles B represented by the above formula (b) include LiMn2O4, LiNi 0.5 Mn 1.5O4, LiCoMnO4, LiCrMnO4, LiFeMnO4, LiAlMnO4, LiCu 0.5 Mn 1.5 Among them, LiMn2O4 is preferred.

[0036] From the viewpoint of ensuring excellent rate characteristics and ease of handling, the average particle size of particles B is preferably 50 nm to 200 nm, more preferably 60 nm to 190 nm, even more preferably 70 nm to 180 nm, and still more preferably 80 nm to 170 nm. Here, the "average particle size" of particles B means the average particle size of 100 particles observed under SEM.

[0037] The particles B can be obtained, for example, by the following production method. Specifically, the following steps (Ib) to (IIb): (Ib) A step of adding a lithium compound, a manganese compound, and water to obtain slurry water i', and then subjecting the slurry water to a hydrothermal reaction to obtain preliminary particles b. (IIb) A step of drying the obtained preliminary particles b in an atmospheric atmosphere to obtain particles B. The manufacturing method includes the steps of:

[0038] The above step (Ib) is a step in which a lithium compound, a manganese compound, and water are added to obtain a slurry water i', which is then subjected to a hydrothermal reaction to obtain preliminary particles b.

[0039] As the lithium compound, the same compounds as those used for the granules A can be used, but hydroxides are preferred. Examples of manganese compounds include manganese acetate, manganese nitrate, manganese oxide, and potassium permanganate. These may be used alone or in combination of two or more. Among these, manganese oxide and potassium permanganate are preferred from the viewpoint of reactivity. In addition to these lithium compounds and manganese compounds, metals other than these compounds (M 2 ) compounds may also be used.

[0040] The slurry water i' may be prepared by a conventional method, with the amounts of lithium compound, manganese compound, etc. appropriately determined depending on the composition of the desired granules A.

[0041] Next, the resulting slurry water i' is subjected to a hydrothermal reaction to obtain preliminary particles b. The hydrothermal reaction is preferably carried out at 160°C to 250°C, more preferably 170°C to 240°C. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel, and when the reaction is carried out at 160°C to 250°C, the pressure is preferably 0.6MPa to 4.0MPa, and when the reaction is carried out at 170°C to 240°C, the pressure is preferably 0.8MPa to 3.3MPa. The hydrothermal reaction time is preferably 12 hours to 168 hours, more preferably 24 hours to 96 hours. The obtained preliminary particles b are preferably filtered and then washed with water.

[0042] The step (IIb) is a step of obtaining particles B by drying the preliminary particles b obtained through the step (Ib) in the air atmosphere. The temperature during drying is preferably 50° C. to 150° C., more preferably 60° C. to 130° C. For such drying, it is advisable to use a general hot air dryer, but there is no particular limitation.

[0043] In the positive electrode active material for a lithium ion secondary battery of the present invention, particles B are coated on the surface of granules A. The coverage of particles B on the surface of granules A is preferably 95% to 100%, more preferably 96% to 100%, even more preferably 97% to 100%, and even more preferably 98% to 100%, from the viewpoints of densely coating particles B, effectively increasing the energy density per unit volume, and effectively improving rate characteristics.

[0044] The coverage (%) of particles B on the surface of granule A means a value determined by the following method. First, the cross section of the obtained particle of the positive electrode active material for a lithium ion secondary battery is observed by SEM electron microscope, the length of the periphery of granule A that is covered with particle B is measured along the entire periphery, and the coverage (%) is calculated using the following formula (x): Next, the average of the coverage (%) calculated for 100 particles of the positive electrode active material for a lithium ion secondary battery is calculated, and this is defined as the coverage (%) of particle B on the surface of granule A. Coverage (%) = {(length of the periphery covered by particle B) / (total length of the periphery of granule A)} × 100 (x)

[0045] In the positive electrode active material for a lithium ion secondary battery of the present invention, the mass ratio (A:B) of granules A to particles B is 95:5 to 55:45, preferably 90:10 to 55:45, more preferably 85:15 to 55:45, and even more preferably 80:20 to 55:45, from the viewpoint of effectively preventing unnecessary self-aggregation of particles B while densely coating the surfaces of granules A with such particles B and effectively enhancing rate characteristics.

[0046] The average particle size of the positive electrode active material for a lithium ion secondary battery of the present invention is preferably 10 μm to 45 μm, more preferably 12 μm to 34 μm, even more preferably 13 μm to 23 μm, and still more preferably 15 μm to 20 μm, from the viewpoint of forming particles in which particles B densely coat the surfaces of granules A and effectively improving rate characteristics. Here, the "average particle size" in the positive electrode active material for a lithium ion secondary battery of the present invention is the D 50 The values ​​are the particle diameters (median diameters) at 50% of the cumulative particle size.

[0047] The positive electrode active material for a lithium ion secondary battery of the present invention may be prepared by adjusting the amounts of the granules A and the particles B so as to satisfy the above mass ratio (A:B), and then dry-mixing these. For such dry mixing, a device that mixes while applying compressive and shearing forces, such as an MP Mixer (manufactured by Nippon Coke & Engineering Co., Ltd.) or an NOB-130 (manufactured by Hosokawa Micron Corporation), may be used. From the viewpoint of increasing the coverage of the particles B on the surface of the granules A, it is preferable to mix so that the cumulative energy load is 0.15 kJ / g to 0.45 kJ / g. The cumulative energy applied by the dry mixer can be calculated using the following formula (1). Accumulative energy (kJ / g) = Load applied to powder (kW) × Processing time (s) ÷ Processing amount (g) Equation (1)

[0048] The positive electrode active material for a lithium ion secondary battery of the present invention can be used as a positive electrode material to construct a lithium ion secondary battery essentially consisting of a positive electrode, a negative electrode, an electrolyte, and a separator, or a positive electrode, a negative electrode, and a solid electrolyte. Specifically, for example, the positive electrode active material for a lithium ion secondary battery of the present invention is mixed with acetylene black, ketjen black, polyvinylidene fluoride, N-methyl-2-pyrrolidone, or the like to prepare a positive electrode slurry, which is then coated on a current collector and press-molded to form a positive electrode. The positive electrode active material for a lithium ion secondary battery of the present invention exhibits a unique particle structure in which the particles B densely coat the surfaces of the granules A while maintaining a limited mass ratio between the granules A and the particles B. This effectively increases the electrode density, thereby providing a highly useful positive electrode that can effectively improve the rate characteristics.

[0049] The negative electrode is not particularly limited in terms of material composition, and any known material composition can be used as long as it can absorb lithium ions during charging and release them during discharging. For example, lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or carbon materials such as amorphous carbon can be used. It is preferable to use an electrode formed of an intercalating material capable of electrochemically absorbing and releasing lithium ions, particularly a carbon material. Furthermore, two or more of the above negative electrode materials may be used in combination, such as a combination of graphite and silicon-based materials.

[0050] The electrolyte solution is prepared by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent typically used in electrolyte solutions for lithium ion secondary batteries, and examples thereof include carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, and oxolane compounds.

[0051] The supporting salt is not particularly limited in type, but is preferably at least one of inorganic salts selected from LiPF6, LiBF4, LiClO4, and LiAsF6, derivatives of these inorganic salts, organic salts selected from LiSO3CF3, LiC(SO3CF3)2, LiN(SO3CF3)2, LiN(SO2C2F5)2, and LiN(SO2CF3)(SO2C4F9), and derivatives of these organic salts.

[0052] The separator serves to electrically insulate the positive and negative electrodes and retain the electrolyte solution, and may be, for example, a porous synthetic resin film, particularly a porous film of a polyolefin polymer (polyethylene, polypropylene).

[0053] The solid electrolyte electrically insulates the positive and negative electrodes and exhibits high lithium ion conductivity. 0.51 Li 0.34 TiO 2.94 , Li 1.3 Al 0.3 Ti 1.7(PO4)3, Li7La3Zr2O 12 , 50Li4SiO4·50Li3BO3, Li 2.9 PO 3.3 N 0.46 , Li 3.6 Si 0.6 P 0.4 O4, Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, Li 10 GeP2S 12 , Li 3.25 Ge 0.25 P 0.75 S4, 30Li2S 26B2S3 44LiI, 63Li2S 36SiS2 1Li3PO4, 57Li2S 38SiS2 5Li4SiO4, 70Li2S 30P2S5, 50Li2S 50GeS2, Li7P3S 11 , Li 3.25 P 0.95 Just use S4.

[0054] The shape of the lithium ion secondary battery having the above-described configuration is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, a square shape, or an irregular shape enclosed in a laminate outer casing. [Example]

[0055] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0056] Granules and particles were produced according to the procedures described in each production example. The physical properties of the resulting granules and particles were then measured and calculated according to the following methods. The results are shown in Table 1.

[0057] <Average particle size of granules A> The volumetric particle size distribution was determined by laser diffraction and scattering (Microtrac MT3000II, manufactured by MicrotracBEL). 50The value (particle size (median size) at 50% of the cumulative size) was taken as the average particle size. The measurement conditions were particle transmittance: transmission, particle shape: non-spherical, particle refractive index: 1.52, and ethanol was used as the solvent, with the solvent refractive index being 1.36.

[0058] 《Average particle size of particle B》 The average particle size of 100 particles observed under an SEM (JSM-7001F, manufactured by JEOL Ltd.) was taken as the average particle size.

[0059] Carbon loading The amount of carbon carried in the resulting particles of positive electrode active material for lithium ion secondary batteries was measured using a carbon / sulfur analyzer (EMIA-220V2, manufactured by Horiba, Ltd.).

[0060] <<Coverage of particle B on the surface of granule A>> The coverage of particles B on the surface of granule A was determined by the following method. First, as a pretreatment, the obtained positive electrode active material was kneaded with an epoxy resin and cured, and then the resin surface was polished with a cross-section polisher (manufactured by JEOL Ltd., IB-19530CP) to expose the particle cross-sections of the positive electrode active material. The polishing was performed at an acceleration voltage of 4 kV for 3 hours. The exposed particle cross section of the positive electrode active material was observed by SEM, and the total length of the outer periphery of granule A and the length of the outer periphery covered by particle B were measured. The coverage (%) was calculated using the above formula (x), and the average value for 100 particles of the positive electrode active material was determined, which was defined as the coverage (%) of particle B on the surface of granule A.

[0061] [Production Example 1: Production of Granules A-1] Slurry water i1 was obtained by mixing 1272 g of LiOH·H2O and 4 L of water. Next, 1153 g of 85% aqueous phosphoric acid solution was added dropwise at 35 mL / min to the resulting slurry water i1 while stirring for 3 minutes at 25 °C. Subsequently, 5892 g of cellulose nanofibers (Wma-10002, manufactured by Sugino Machine, fiber diameter 4-20 nm) were added and stirred at 400 rpm for 12 hours to obtain slurry water i2 containing Li3PO4. The resulting slurry water i2 was purged with nitrogen to adjust the dissolved oxygen concentration of the slurry water i2 to 0.5 mg / L. Then, 482 g of MnSO4·5H2O and 2224 g of FeSO4·7H2O were added to the total amount of slurry water i2 to obtain slurry water i3. The molar ratio of added MnSO4 to FeSO4 (manganese compound:iron compound) was 20:80. The resulting slurry water i3 was then placed in an autoclave and subjected to a hydrothermal reaction at 170°C for 1 hour. The pressure inside the autoclave was 0.8 MPa. After the hydrothermal reaction, the resulting crystals were filtered and then washed with 12 parts by mass of water per part by mass of the crystals. The washed crystals were freeze-dried at -50°C for 12 hours to obtain preliminary particles a1. 1500 g of the resulting preliminary particles a1 were taken and 1 L of water was added thereto to obtain slurry water ii1. The resulting slurry water ii1 was dispersed for 1 minute using an ultrasonic agitator (T25, manufactured by IKA) to uniformly color the entire mixture, and then spray-dried using a spray dryer (MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) (nozzle air flow rate 15 L / min, inlet air temperature 190°C) to obtain preliminary granules x1. The obtained pre-granules x1 were fired at 700°C for 1 hour in an argon-hydrogen atmosphere (hydrogen concentration 3%) to obtain granules A-1 (LiMn 0.2 Fe 0.8 PO4, carbon loading: 1.0 mass%, average particle size: 40 μm) was obtained.

[0062] [Production Example 2: Production of Granules A-2] Granules A-2 (LiMn) were prepared in the same manner as in Production Example 2, except that 7070 g of cellulose nanofibers, 964 g of MnSO4·5H2O, and 1668 g of FeSO4·7H2O were added, and the nozzle air flow rate, which was a spray-drying condition, was set to 40 L / min.0.4 Fe 0.6 PO4, carbon loading: 1.2 mass%, average particle size: 15 μm) was obtained.

[0063] [Production Example 3: Production of Granules A-3] Granules A-3 (LiMn) were prepared in the same manner as in Production Example 2, except that 7660 g of cellulose nanofibers, 1446 g of MnSO4·5H2O, and 2502 g of FeSO4·7H2O were added, and the nozzle air flow rate, which was a spray-drying condition, was set to 45 L / min. 0.6 Fe 0.4 PO4, carbon loading: 1.3 mass%, average particle size: 9 μm) was obtained.

[0064] [Production Example 4: Production of Granules A-4] Granules A-4 (LiMn) were prepared in the same manner as in Production Example 2, except that 5892 g of cellulose nanofibers, 964 g of MnSO4·5H2O, and 1668 g of FeSO4·7H2O were added, and the nozzle air flow rate, which was a spray-drying condition, was set to 50 L / min. 0.4 Fe 0.6 PO4, carbon loading: 1.0 mass%, average particle size: 5 μm) was obtained.

[0065] [Production Example 5: Production of Granules A-5] Granules A-5 (LiMn) were prepared in the same manner as in Production Example 2, except that 4714 g of cellulose nanofibers, 241 g of MnSO4·5H2O, and 2502 g of FeSO4·7H2O were added, and the nozzle air flow rate, which was a spray-drying condition, was set to 40 L / min. 0.1 Fe 0.9 PO4, carbon loading: 0.8 mass%, average particle size: 14 μm) was obtained.

[0066] [Production Example 6: Production of Granules A-6] Granules A-6 (LiMn) were prepared in the same manner as in Production Example 2, except that 7070 g of cellulose nanofibers, 1687 g of MnSO4·5H2O, and 834 g of FeSO4·7H2O were added, and the nozzle air flow rate, which was a spray-drying condition, was set to 40 L / min. 0.7 Fe0.3 PO4, carbon loading: 1.2 mass%, average particle size: 15 μm) was obtained.

[0067] [Production Example 7: Production of Particles B-1] 3160 g of potassium permanganate and 420 g of lithium hydroxide monohydrate were mixed with 10 L of water so that the molar ratio of Mn:Li was 2:1, to obtain slurry B. The obtained slurry water B was then placed in an autoclave and subjected to a hydrothermal reaction at 200 °C for 24 hours. The pressure inside the autoclave was 1.6 MPa. After the hydrothermal reaction, the produced crystals were filtered and then washed with 12 parts by mass of water per part by mass of the crystals. The washed crystals were dried in the air at 100 °C for 5 hours to obtain particles B-1 (LiMn2O4, average particle size 120 nm).

[0068] [Table 1]

[0069] [Examples 1, 3, 5, Comparative Examples 2 to 4] Granules A and particles B were blended according to the formulation shown in Table 2 to obtain a powder. Next, 300 g of this powder was sampled and mixed using an MP mixer (manufactured by Nippon Coke Company) while applying compressive and shearing forces (mixing for 3 minutes at a load of 0.4 kW on the powder, cumulative applied energy 0.24 kJ / g), to obtain a positive electrode active material in which the surfaces of granules A were coated with particles B.

[0070] [Example 2, Comparative Example 1] Granules A and particles B were blended to obtain a powder according to the formulation shown in Table 2. Next, 300 g of this powder was sampled and mixed using an MP mixer (manufactured by Nippon Coke Company) while applying compressive and shearing forces (mixing for 2 minutes at a load of 0.4 kW on the powder, cumulative applied energy 0.16 kJ / g), to obtain a positive electrode active material in which the surfaces of granules A were coated with particles B.

[0071] [Example 4] Granules A and particles B were blended to obtain a powder according to the formulation shown in Table 2. Next, 300 g of this powder was sampled and mixed using an MP mixer (manufactured by Nippon Coke Company) while applying compressive and shearing forces (mixing for 5.5 minutes at a load of 0.4 kW applied to the powder, cumulative applied energy: 0.44 kJ / g), to obtain a positive electrode active material in which the surfaces of granules A were coated with particles B.

[0072] <Evaluation of battery characteristics (rate characteristics)> The resulting positive electrode active materials were used as positive electrode materials to fabricate positive electrodes for lithium-ion secondary batteries. Specifically, the resulting positive electrode active materials, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added to the mixture and thoroughly kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a 20 μm-thick aluminum foil current collector using a coating machine and vacuum dried at 80°C for 12 hours. The resulting mixture was then punched into a φ14 mm disk and pressed at 16 MPa for 2 minutes using a hand press to form a positive electrode.

[0073] Next, a coin-type secondary battery was constructed using the above positive electrode. A lithium foil punched to a diameter of 15 mm was used as the negative electrode. The electrolyte was a 1 mol / L solution of LiPF6 in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7. A polymer porous film was used as the separator. These battery components were assembled and housed in an atmosphere with a dew point of -50°C or lower using standard methods to obtain a coin-type secondary battery (CR-2032).

[0074] Next, using the obtained coin-type secondary battery, the discharge capacity (mAh / g) at 0.2 C (34 mA / g) and 10 C (1.7 A / g) was measured in an environment of 30°C using a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko Corporation), and the rate characteristic value (capacity ratio (%)) was calculated using the following formula (y). Rate Characteristics = [(discharge capacity at 10 C) / (discharge capacity at 0.2 C)] × 100 (y)

[0075] Furthermore, the electrode density was calculated by the following formula (z1), and this was introduced into the following formula (z2) to calculate the energy density per unit volume. Electrode density (g / cm 3 )= Mass of positive electrode active material in the positive electrode (g) / electrode volume (cm 3 )(φ14mm×thickness(μm)) (z1) Energy density per unit volume of positive electrode at 30°C (Wh / L) = Discharge capacity at 30°C (mAh / g) x average voltage (V) x electrode density (g / cm 3 ) (z2) The results are shown in Table 2.

[0076] [Table 2]

Claims

1. The following formula (a): Li f Mn g Fe h M 1 x 2O 4 ・・・(a) (In formula (a), M 1 represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. f, g, h, and x satisfy 0<f≦1.2, 0.3≦g≦1.2, 0.2≦h≦1.2, 0≦x≦0.3, and 3 / 17≦g / h≦13 / 7, and the formula f+(Mn valence)×g+(Fe valence)×h+(Mn valence)×g+(Mn ... 1 (valence of x) × x = 3. and having an average particle size of 8 μm to 50 μm, the surface of which is coated with a compound represented by the following formula (b): LiM 2 a Mn b O 4 ・・・(A) (In formula (b), M 2 represents one or more elements selected from Ni, Co, Al, Mg, Ti, V, Cr, Fe, Zr, Ga, Cu, and Si; a and b are each a=0, 0<b≦2, and (M 2 (valence of Mn) × a + (valence of Mn) × b = 7. and the particle B represented by A positive electrode active material for a lithium ion secondary battery, in which the mass ratio (A:B) of granules A to particles B is 95:5 to 55:

45.

2. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein a coverage of the surface of the granules A with the particles B is 95% to 100%.

3. 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the average particle size of the particles B is 50 nm to 200 nm.

4. 4. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the granules A are particles represented by formula (a), and the granules A are aggregates of preliminary particles a, each of which has cellulose nanofiber-derived carbon and / or water-soluble carbon material-derived carbon supported on its surface.

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