Positive electrode active material for lithium-ion secondary batteries

Abstract

To provide a cathode active material for a lithium-ion secondary battery, enabling the effective improvement of the cycle characteristics of the lithium-ion secondary battery.SOLUTION: The cathode active material for a lithium-ion secondary battery is formed by covering the surfaces of particles A with particles B. Each of the particles A is represented by the following formula (a) LiM1aMnbO4 ... (a), and each of the particles B is represented by the following formula (b) LifMngFehM2xPO4 ... (b) and has an average particle diameter of 50 nm to 200 nm, wherein the mass ratio (B:A) of the particles B to the particles A is 10:90 to 45:55.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery that improves the cycle characteristics of the lithium ion secondary battery. [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 0.5 to 5.0 parts by weight of a shell containing lithium metal oxide particles such as lithium manganese iron phosphate and a polymer, based on 100 parts by weight of a core formed from a lithium-containing transition metal oxide such as LiMnO2, 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 a positive electrode active material having a layer structure such as that of Patent Document 1, in the obtained lithium ion secondary battery, the cycle characteristics have not yet been sufficiently improved. Also, even in the case of a positive electrode active material having a core-shell structure such as that of Patent Document 2, there is still room for improvement in terms of the components and quantitative relationships constituting the core part and the shell part when attempting to improve the cycle characteristics.

[0006] Therefore, an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that can effectively improve the cycle characteristics of the lithium ion secondary battery.

Means for Solving the Problems

[0007] Thus, as a result of intensive studies to solve the above problems, the present inventors have found that lithium iron manganese phosphate particles having a specific average particle size are coated on the surface of particles represented by a specific formula such as LiMn2O4, and by having these particles have a specific mass ratio, positive electrode active material particles for a lithium ion secondary battery that can exhibit excellent cycle characteristics in the obtained lithium ion secondary battery can be provided.

[0008] That is, the present invention provides the following formula (a): LiM 1 a Mn b O4···(a) (In formula (a), M 1 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 1 ) × a + (valence of Mn) × b = 7.) On the surface of particles A represented by the following formula (b): Li f Mn g Fe h M 2 x PO4···(b) (In formula (b), M 2 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 7 / 13 ≤ g / h ≤ 17 / 3, and f + (valence of Mn) × g + (valence of Fe) × h + (M 2 valence) × x = 3.) and is coated with particles B represented by and having an average particle size of 50 nm to 200 nm, and provides a positive electrode active material for a lithium-ion secondary battery in which the mass ratio (B:A) of particles B to particles A is 10:90 to 45:55.)

Advantages of the Invention

[0009] According to the positive electrode active material for a lithium-ion secondary battery of the present invention, a lithium-ion secondary battery with effectively improved cycle characteristics can be realized.)

Embodiments 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 has the following formula (a): LiM 1 a Mn b O4 ··· (a) (In formula (a), M 1 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 (M 1 valence) × a + (Mn valence) × b = 7.) On the surface of particles A represented by , the following formula (b): Li f Mn g Fe h M 2 x PO4 ··· (b) (In formula (b), M2 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 7 / 13 ≤ g / h ≤ 17 / 3, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 ) × x = 3. It is composed of particles B represented by and having an average particle size of 50 nm to 200 nm, and the mass ratio (B:A) of particles B to particles A is 10:90 to 45:55.

[0011] Thus, the positive electrode active material for a lithium-ion secondary battery of the present invention is particles composed of particles A represented by a specific formula and fine particles B. While these particles A and particles B maintain a limited mass ratio, a unique particle structure is formed in which the other particle B (shell part) covers the surface of one particle A (core part). By presenting this structure, it effectively suppresses the elution of the transition metal component of particle A into the electrolyte, and in the obtained lithium-ion secondary battery, it is possible to effectively improve the cycle characteristics.

[0012] The particles A constituting the positive electrode active material for a lithium-ion secondary battery of the present invention are represented by the following formula (a): LiM 1 a Mn b O4 ··· (a) (In formula (a), M 1 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 1 ) × a + (valence of Mn) × b = 7. are particles represented by.

[0013] The particles A represented by the formula (a) have a spinel structure and are secondary particles formed by aggregation of primary particles. The particles A serve as cores, and the fine particles B described below coat the cores while maintaining a specific mass ratio, thereby dramatically improving cycle characteristics.

[0014] Specific examples of the particles A represented by the formula (a) include LiMn2O4, LiNi 0.5 Mn 1.5 O4, LiCoMnO4, LiCrMnO4, LiFeMnO4, LiAlMnO4, LiCu 0.5 Mn 1.5 Among them, LiMn2O4 is preferred.

[0015] From the viewpoint of ensuring excellent cycle characteristics and ease of handling, the average particle size of particles A is preferably 8 μm to 50 μm, more preferably 9 μm to 40 μm, even more preferably 9.5 μm to 30 μm, and even more preferably 10 μm to 20 μm. Here, the "average particle size" of particle 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.

[0016] The particles A can be obtained, for example, by a production method including steps of adding a lithium compound and a manganese compound, pulverizing and mixing them in a ball mill, calcining, and then calcining. Examples of the lithium compound to be used include hydroxides (e.g., LiOH·H2O, LiOH), carbonates, acetates, and nitrates. Of these, carbonates 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, metals other than these compounds (M 1Compounds may be used. When firing, first pre-fire at 250°C to 700°C, preferably 300°C to 600°C for 1 hour to 15 hours, preferably 6 hours to 12 hours, and then perform the main firing at 500°C to 1000°C, preferably 600°C to 900°C for 3 hours to 30 hours, preferably 12 hours to 24 hours. Also, it is preferable to crush after pre-firing and then perform the main firing.

[0017] 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): Li f Mn g Fe h M 2 x PO4···(b) (In formula (b), M 2 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 7 / 13 ≤ g / h ≤ 17 / 3, and f + (valence of Mn) × g + (valence of Fe) × h + (valence of M 2 ) × x = 3. ) It is represented by and has an average particle size of 50 nm to 200 nm.

[0018] The particles B represented by the above formula (b) are fine particles and are olivine-type lithium transition metal phosphate compounds (so-called LMFP particles) containing at least both manganese (Mn) and iron (Fe) as transition metals. Such particles B are fine particles and densely coat the surface of the particles A while maintaining a specific mass ratio, thus effectively suppressing the elution of the metal component from the particles A and greatly contributing to the improvement of the cycle characteristics.

[0019] Regarding the particle B, from the viewpoint of the average discharge voltage, f is preferably 0.6≦f≦1.2, more preferably 0.65≦f≦1.15, and even more preferably 0.7≦f≦1.1. g is preferably 0.4≦g≦0.8, more preferably 0.5≦g≦0.8, and even more preferably 0.7≦g≦0.8. h is preferably 0.2≦h≦0.6, more preferably 0.2≦h≦0.5, and even more preferably 0.2≦h≦0.3. x is preferably 0≦x≦0.2, more preferably 0≦x≦0.15, and even more preferably 0≦x≦0.1. g / h is the molar ratio of Mn and Fe constituting the so-called particle B, and is preferably 2 / 3≦g / h≦4, more preferably 9 / 11≦g / h≦3, and even more preferably 1≦g / h≦7 / 3.

[0020] Specifically, for example, LiMn 0.4 Fe 0.6 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.75 Fe 0.15 Mg 0.1 PO4, LiMn 0.75 Fe 0.19 Zr 0.03 PO4, LiMn 0.6 Fe 0.4 PO4, LiMn 0.5 Fe 0.5 PO4, Li 1.2 Mn 0.63 Fe 0.27 PO4, Li 0.6 Mn 0.84 Fe 0.36 PO4, etc. Among them, LiMn 0.4 Fe 0.6 PO4, LiMn 0.7 Fe 0.3 PO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.6 Fe 0.4 PO4, Li 1.2 Mn 0.63 Fe 0.27 PO4 or Li 0.6 Mn 0.84 Fe0.36 PO4 is preferred.

[0021] The average particle size of particles B represented by the above formula (b) can suppress the amount of expansion and contraction of the primary particles accompanying the insertion and desorption of lithium ions, and from the viewpoints of effectively preventing particle cracking, handling, and coating on particles A, is 50 nm to 200 nm, preferably 60 nm to 190 nm, more preferably 70 nm to 180 nm, and even 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.

[0022] From the viewpoint of ensuring excellent discharge capacity and further improving cycle characteristics, particles B may be particles having cellulose nanofiber-derived carbon and / or water-soluble carbon material-derived carbon supported on their surfaces. Cellulose nanofibers are the skeletal components that account 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 the above-mentioned particle B, the particle B has an appropriate strength despite being very fine. By densely coating the surface of the particle A with the particle B, the deterioration of the electronic conductive path is effectively suppressed, the degree of compaction is effectively increased, and the resulting battery can exhibit excellent cycle characteristics.

[0023] When carbon derived from cellulose nanofibers is supported on the surface of particle B, the atomic equivalent amount of carbon derived from the carbonized cellulose nanofibers, i.e., the amount of carbon derived from cellulose nanofibers supported, 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, based on 100% by mass of particle B.

[0024] Like cellulose nanofibers, water-soluble carbon materials are carbonized to form carbon, and when this is supported on the surface of particle B, like cellulose nanofibers, it effectively suppresses the deterioration of the electronic conductive path and effectively increases the degree of compaction, thereby ensuring that the resulting battery exhibits excellent cycle 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.

[0025] When carbon derived from a water-soluble carbon material is supported on the surface of particles B, 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, is preferably 4.0 mass% or less, more preferably 3.0 mass% or less, and even more preferably 2.0 mass% or less, in 100 mass% of particles B.

[0026] The cellulose nanofiber-derived carbon and the water-soluble carbon material-derived carbon may be supported in the form of only cellulose nanofiber-derived carbon, only water-soluble carbon material-derived carbon, or both cellulose nanofiber-derived carbon and water-soluble carbon material-derived carbon. Of these, it is preferable to support cellulose nanofiber-derived carbon, from the viewpoint of covering only a portion of the surface of particle B and not interfering with the deformation of particle B.

[0027] When carbon derived from cellulose nanofibers and carbon derived from a water-soluble carbon material are supported on the surface of particle B, the sum of the atomic equivalent amount of carbon derived from cellulose nanofibers and the atomic equivalent amount of carbon derived from the water-soluble carbon material, i.e., the total amount of carbon derived from cellulose nanofibers and the total 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, based on 100% by mass of particle B.

[0028] The atomic equivalent amount (loading amount) of carbon derived from cellulose nanofibers and the atomic equivalent amount (loading amount) of carbon derived from water-soluble carbon materials present in particle B refer to values ​​determined by measurements using a carbon / sulfur analyzer. Furthermore, when particles B support carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material on their surfaces, the amount of particles B includes the amount of these carbons supported.

[0029] 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 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', followed by subjecting the slurry to a hydrothermal reaction to obtain preliminary particles b. (IIb) A step of firing the obtained preliminary particles b in a reducing atmosphere or an inert atmosphere to obtain particles B. The manufacturing method includes the steps of:

[0030] The above-mentioned step (Ib) 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 slurry water i', which is then subjected to a hydrothermal reaction to obtain preliminary particles b.

[0031] As the lithium compound, the same compounds as those used for the particles A can be used, but hydroxides are particularly preferred. As the manganese compound, the same one as that used for the particles A can be used. Examples of iron compounds include iron acetate, iron nitrate, and iron sulfate. These may be used alone or in combination of two or more. Among these, iron sulfate is preferred from the viewpoint of improving battery characteristics. In addition to these manganese compounds and iron compounds, metal compounds other than manganese compounds and iron compounds (M 2 ) compounds may also be used.

[0032] 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 the surface of the particles B is to be loaded with carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material, the above-mentioned cellulose nanofibers and / or water-soluble carbon material may be further used.

[0033] 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 desired particles B.

[0034] Next, the resulting slurry water i' is subjected to a hydrothermal reaction to obtain preliminary particles b. The amount of water used in the hydrothermal reaction is preferably 10 mol to 50 mol, 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, and synthesis efficiency.

[0035] The hydrothermal reaction is preferably carried out at 130°C to 200°C, more preferably 150°C to 180°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 0.3 MPa to 1.6 MPa, and when the reaction is carried out at 150°C to 180°C, the pressure is preferably 0.5 MPa to 1.0 MPa. The hydrothermal reaction time is preferably 0.1 hours to 48 hours, more preferably 0.2 hours to 24 hours. The obtained preliminary particles b are isolated by filtering, washing with water, and drying, which may be performed by freeze drying or vacuum drying.

[0036] The step (IIb) is a step of obtaining particles B by firing the preliminary particles b obtained through the step (Ib) in a reducing atmosphere or an inert atmosphere. The firing temperature is preferably 200°C to 750°C, more preferably 250°C to 700°C, and the firing time is preferably 0.3 hours to 3 hours, more preferably 0.5 hours to 2 hours.

[0037] In the positive electrode active material for a lithium ion secondary battery of the present invention, the surface of particle A is coated with particle B. The coverage of particle B on the surface of particle 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 viewpoint of densely coating particle B and effectively improving cycle characteristics.

[0038] The coverage (%) of particle B on the surface of particle 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 particle 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 particle A. Coverage (%) = {(length of the periphery covered by particle B) / (total length of the periphery of particle A)} × 100 (x)

[0039] In the positive electrode active material for a lithium ion secondary battery of the present invention, the mass ratio of particles B to particles A (B:A) is 10:90 to 45:55, preferably 12:88 to 45:55, more preferably 17:83 to 45:55, and even more preferably 20:80 to 45:55, from the viewpoint of effectively avoiding unnecessary self-aggregation of particles B, allowing particles B to densely cover the surfaces of particles A, and effectively improving cycle characteristics.

[0040] The average particle size of the positive electrode active material for a lithium ion secondary battery of the present invention is preferably 12 μm to 45 μm, more preferably 12.5 μm to 34 μm, and even more preferably 13 μm to 23 μm, from the viewpoint of forming particles in which particles B densely coat the surfaces of particles A and effectively improving cycle 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.

[0041] The positive electrode active material for a lithium ion secondary battery of the present invention may be prepared by adjusting the amounts of the particles A and B to satisfy the above mass ratio (B:A) and then dry-mixing them. 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 surfaces of the particles A with the particles B, it is preferable to mix them so that the cumulative energy applied 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)

[0042] 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 is composed of the above-mentioned particles A and fine particles B, and exhibits a unique particle structure in which particles B densely coat the surfaces of particles A while maintaining a limited mass ratio between particles A and particles B. This effectively increases the electrode density and effectively suppresses the elution of the transition metal component of particles A into the electrolyte, thereby making it possible to obtain a highly useful positive electrode that can effectively improve the cycle characteristics.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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).

[0047] 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.

[0048] 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]

[0049] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0050] Each particle was produced according to the description of each production example. Then, the physical properties of each obtained particle were measured and calculated according to the following methods. The results are shown in Table 1.

[0051] 《Average particle size of particle 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.

[0052] 《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.

[0053] 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.).

[0054] <<Coverage of particle B on the surface of particle A>> The coverage of particle B on the surface of particle 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-section of the positive electrode active material. The polishing was performed at an acceleration voltage of 4 kV for 3 hours. The cross section of the exposed particle of the positive electrode active material was observed by SEM, and the total length of the outer periphery of particle A and the length of the outer periphery covered by particle B were measured. The coverage rate (%) was calculated using the above formula (x), and the average value for 100 particles of the positive electrode active material was found, which was defined as the coverage rate (%) of particle B on the surface of particle A.

[0055] [Production Example 1: Production of Particles A-1] 348 g of manganese dioxide and 73.9 g of lithium carbonate were mixed and crushed in a ball mill so that the molar ratio of Mn:Li was 2:1, and then the mixture was calcined at 500°C for 12 hours in an air atmosphere and crushed. Then, the mixture was calcined at 800°C for 24 hours in an air atmosphere to obtain particles A-1 (LiMn2O4, average particle size 15 μm).

[0056] [Production Example 2: Production of Particles B-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 slurry water i2' to 0.5 mg / L. After that, 964 g of MnSO4·5H2O and 1668 g of FeSO4·7H2O were added to the total amount of slurry water i2' to obtain slurry water i3'. The molar ratio of the added MnSO4 to FeSO4 (manganese compound:iron compound) was 40:60. The resulting slurry water i3' was then placed in an autoclave and subjected to a hydrothermal reaction at 200°C for 5 hours. The pressure inside the autoclave was 1.6 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 b1. The obtained preliminary particles b1 were fired at 700°C for 1 hour in an argon-hydrogen atmosphere (hydrogen concentration 3%) to give particles B-1 (LiMn 0.4 Fe 0.6 PO4, carbon loading: 1.0 mass%, average particle size: 189 nm) was obtained.

[0057] [Production Example 3: Production of Particles B-2] Particles B-2 (LiMn 0.6 Fe 0.4 PO4, carbon loading: 1.3 mass%, average particle size: 121 nm) was obtained.

[0058] [Production Example 4: Production of Particles B-3] Particles B-3 (LiMn 0.8 Fe 0.2 PO4, carbon loading: 1.2 mass%, average particle size: 62 nm) was obtained.

[0059] [Production Example 5: Production of Particles B-4] Particles B-4 (LiMn 0.6 Fe 0.4 PO4, carbon loading: 0.9 mass%, average particle size: 43 nm) was obtained.

[0060] [Production Example 6: Production of Particles B-5] Particles B-2 (LiMn 0.3 Fe 0.7 PO4, carbon loading: 1.0 mass%, average particle size: 125 nm) was obtained.

[0061] [Production Example 7: Production of Particles B-6] Particles B-2 (LiMn 0.9 Fe 0.1 PO4, carbon loading: 1.0 mass%, average particle size: 123 nm) was obtained.

[0062] [Table 1]

[0063] [Examples 1, 3, 5, Comparative Examples 2 to 4] A powder was obtained by blending particles A and particles B according to Table 2. Next, 300 g of this powder was sampled and mixed using an MP mixer (manufactured by Nippon Coke Corporation) while applying compressive and shearing forces (mixing for 3 minutes at a load of 0.4 kW applied to the powder, cumulative applied energy 0.24 kJ / g), to obtain a positive electrode active material in which particles B coated the surfaces of particles A.

[0064] [Example 2, Comparative Example 1] A powder was obtained by blending particles A and particles B 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 Corporation) 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 particles A were coated with particles B.

[0065] [Example 4] A powder was obtained by blending particles A and particles B 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 Corporation) while applying compressive force and shear (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 particles A were coated with particles B.

[0066] <Evaluation of battery characteristics (cycle 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.

[0067] 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).

[0068] Using the obtained coin-type secondary battery, charging and discharging was repeated 100 times at 1C in an environment of 30°C using a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko Corporation), and the discharge capacity (mAh / g) was determined, and the cycle characteristic value (capacity retention rate (%)) was calculated using the following formula (y). Cycle characteristics = (discharge capacity after 100 cycles) / (discharge capacity after 1 cycle) × 100 (y) The results are shown in Table 2.

[0069] [Table 2]

Claims

1. The following formula (a): LiM 1 a Mn b O 4 ・・・(a) (In formula (a), M 1 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 1 (valence of Mn) × a + (valence of Mn) × b = 7. The surface of particle A represented by the following formula (b): Li f Mn g Fe h M 2 x 2O 4 ・・・(b) (In formula (b), M 2 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 7 / 13≦g / h≦17 / 3, and the formula f+(Mn valence)×g+(Fe valence)×h+(M 2 (valence of x) × x = 3. and is coated with particles B having an average particle size of 50 nm to 200 nm, A positive electrode active material for a lithium ion secondary battery, in which the mass ratio of particles B to particles A (B:A) is 10:90 to 45:

55.

2. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the coverage of the surface of the particle A with the particle 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 A is 8 μm to 50 μm.

4. 4. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the surface of the particles B is supported with carbon derived from cellulose nanofibers and / or carbon derived from a water-soluble carbon material.

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