Coated lithium-transition metal oxide particles and method for producing the same, positive electrode active material for lithium ion batteries, and lithium ion batteries

Abstract

To provide coated lithium-transition metal oxide particles, a positive electrode active material for a lithium ion battery containing the particles, and a method for producing the particles, which improve input / output characteristics of a lithium ion battery.SOLUTION: In the coated lithium-transition metal oxide particles, surfaces of lithium-transition metal oxide particles are at least partially coated with a copolymer which is represented by formula (1) and which has a weight-average molecular weight of 200,000-600,000. The copolymer is cross-linked by ring opening of an oxetane ring.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to coated lithium-transition metal oxide particles and a method for producing the same. The present invention also relates to a positive electrode active material for lithium ion batteries that includes the coated lithium-transition metal oxide particles, and a lithium ion battery that uses the positive electrode active material. [Background technology]

[0002] There is a demand for lithium ion batteries to have improved battery characteristics such as input / output characteristics. For example, Patent Document 1 discloses that, in order to improve output characteristics and cycle characteristics, a non-aqueous electrolyte for lithium ion batteries is used, which is a blend of a copolymer of methyl methacrylate and (3-ethyloxetan-3-yl)methyl methacrylate, a supporting salt, an aprotic solvent, and vinylene carbonate, and then crosslinked.

[0003] On the other hand, it is known that positive electrode active material particles of lithium ion batteries are coated with a polymer. For example, Patent Document 2 discloses the use of a positive electrode active material coated with a nitrile group-containing acrylic polymer in order to improve cycle characteristics at high potential. However, polymers containing nitrile groups have not been put to practical use due to their large environmental impact. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7047181 [Patent Document 2] International Publication No. 2014 / 088070 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of an embodiment of the present invention is to provide coated lithium-transition metal oxide particles that can be used as a positive electrode active material suitable for improving the input / output characteristics of lithium ion batteries. [Means for solving the problem]

[0006] The present invention includes the embodiments shown below. [1] At least a portion of the surface of a lithium-transition metal oxide particle is coated with a copolymer represented by the following general formula (1) and having a weight average molecular weight of 200,000 to 600,000: [ka] In formula (1), m and n each independently represent a number of 1 or more, and R 1 and R 2 each independently represents a hydrogen atom or a methyl group, and R 3 represents an alkyl group having 1 to 5 carbon atoms, and R 4 represents an alkanediyl group having 1 to 5 carbon atoms, and R 5 represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, The coated lithium-transition metal oxide particles are crosslinked by ring-opening of the oxetane rings. [2] The coated lithium-transition metal oxide particles according to [1], wherein the amount of the copolymer is 0.7 to 5 mass %. [3] A positive electrode active material for a lithium ion battery, comprising the coated lithium-transition metal oxide particles according to [1] or [2]. [4] A lithium ion battery comprising a positive electrode containing the positive electrode active material for lithium ion batteries according to [3]. [5] A method for producing coated lithium-transition metal oxide particles, comprising a step of heating a mixture containing lithium-transition metal oxide particles, a copolymer represented by the above general formula (1) and having a weight-average molecular weight of 200,000 to 600,000, and a polymerization initiator. [6] The method for producing coated lithium-transition metal oxide particles according to [5], wherein the amount of the polymerization initiator in the mixture is 1 to 5 parts by mass per 100 parts by mass of the total of the copolymer and the polymerization initiator. [7] The method for producing coated lithium-transition metal oxide particles according to [5] or [6], wherein the amount of the copolymer is 0.7 to 5 mass % relative to 100 mass % of the coated lithium-transition metal oxide particles. [Effects of the Invention]

[0007] According to an embodiment of the present invention, it is possible to provide coated lithium-transition metal oxide particles that can be used as a positive electrode active material suitable for improving the input / output characteristics of lithium ion batteries. DETAILED DESCRIPTION OF THE INVENTION

[0008] The coated lithium-transition metal oxide particles according to the embodiment are lithium-transition metal oxide particles whose particle surfaces are coated with an acrylic polymer having an oxetane ring, and the acrylic polymer is crosslinked by ring-opening of the oxetane ring.

[0009] [Lithium-transition metal oxide particles] The lithium-transition metal oxide particles may be particles of a composite oxide of lithium and a transition metal, i.e., particles of a lithium-containing composite metal oxide. For example, a lithium-containing composite metal oxide powder that is commonly used as a positive electrode active material for lithium ion batteries may be used. Here, the powder refers to an aggregate of particles.

[0010] Examples of transition metals constituting the lithium-transition metal oxide particles include Ni, Co, Mn, Ti, V, Cr, Fe, Cu, and Mo. Among these, at least one transition metal selected from the group consisting of Ni, Co, and Mn is preferred. Therefore, the lithium-transition metal oxide particles according to a preferred embodiment are lithium-containing composite metal oxide particles containing at least one selected from the group consisting of Ni, Co, and Mn.

[0011] Specific examples of lithium-transition metal oxide particles include lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing manganese oxide, lithium-containing nickel cobalt oxide, and lithium-containing nickel cobalt manganese oxide. These oxides may be doped with Al, Mg, Ca, Ba, F (fluorine), B (boron), etc. (e.g., lithium composite oxide of nickel cobalt aluminum), or may be undoped. These oxides may be used alone or in combination of two or more.

[0012] In one embodiment, the lithium-transition metal oxide particles are Li x Ni a Co b Mn c The lithium-containing nickel-cobalt-manganese oxide particles may be represented by O2, where x is 0.5 to 1.2, a, b, and c satisfy a>0, b>0, c>0, and a+b+c=1.

[0013] The average particle size of the lithium-transition metal oxide particles is not particularly limited and may be, for example, 1 to 30 μm, or 3 to 15 μm. In this specification, the "average particle size" refers to the "number-average particle size." This average particle size can be measured using a laser diffraction / scattering particle size distribution analyzer or the like.

[0014] [Acrylic polymer having an oxetane ring] As the acrylic polymer having an oxetane ring that coats the lithium-transition metal oxide particles, a copolymer represented by the following general formula (1) (hereinafter referred to as "copolymer (1)") is used. [ka] In formula (1), m and n each independently represent a number of 1 or more. 1 and R 2 R each independently represents a hydrogen atom or a methyl group. 3 represents an alkyl group having 1 to 5 carbon atoms. 4 represents an alkanediyl group having 1 to 5 carbon atoms.5 represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms.

[0015] Copolymer (1) is a copolymer of alkyl (meth)acrylate and a (meth)acrylic acid ester having an oxetane ring. Here, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid. In formula (1), the structural unit on the left side of "-co-" is derived from alkyl (meth)acrylate. The structural unit on the right side of "-co-" is derived from a (meth)acrylic acid ester having an oxetane ring (a four-membered cyclic ether structure). "-co-" means that the arrangement of these structural units is not specified. Therefore, copolymer (1) may be a random copolymer or a block copolymer.

[0016] R 1 and R 2 Preferably, both of are methyl groups. Therefore, the copolymer (1) is preferably a copolymer of an alkyl methacrylate and a methacrylic acid ester having an oxetane ring.

[0017] R 3 As mentioned above, R represents an alkyl group having 1 to 5 carbon atoms, and may be a linear or branched alkyl group. 3 is preferably an alkyl group having 1 to 3 carbon atoms, and more preferably a methyl group.

[0018] R 4 R represents an alkanediyl group having 1 to 5 carbon atoms (i.e., a divalent group obtained by removing two hydrogen atoms from an alkane), as described above, and may be linear or branched. 4 is preferably an alkanediyl group having 1 to 3 carbon atoms, and more preferably a methylene group.

[0019] R 5 As mentioned above, R represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and the alkyl group may be linear or branched. 5 is preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms.5 is a methyl group or an ethyl group.

[0020] In one embodiment, the copolymer (1) is preferably a copolymer of methyl methacrylate (MMA) and (3-ethyloxetan-3-yl)methyl methacrylate (OXMA) represented by the following formula (2). [ka] (m and n in formula (2) are the same as m and n in formula (1).)

[0021] The copolymer (1) has a weight average molecular weight (Mw) of 200,000 to 600,000, preferably 300,000 to 550,000, and may be 400,000 to 500,000. The number average molecular weight (Mn) of the copolymer (1) is not particularly limited and may be, for example, 50,000 to 200,000, 80,000 to 180,000, or 100,000 to 150,000.

[0022] In formula (1), n ​​represents the number (degree of polymerization) of structural units derived from alkyl (meth)acrylate (hereinafter referred to as structural unit 1) and is a number of 1 or greater. In formula (1), m represents the number (degree of polymerization) of structural units derived from a (meth)acrylate ester having an oxetane ring (hereinafter referred to as structural unit 2) and is a number of 1 or greater. The ratio m / n, the number of structural units 2 (m) to the number of structural units 1 (n), is not particularly limited, but is preferably 0.1 to 3.0, more preferably 0.1 to 1.0, even more preferably 0.1 to 0.5, and may even be 0.12 to 0.33, or may even be 0.15 to 0.25. m and n are not particularly limited as long as the weight-average molecular weight and m / n satisfy the above-mentioned ranges; for example, n may be 100 to 1600 or 500 to 1300. m may be 40 to 900 or 60 to 500.

[0023] Here, the weight average molecular weight Mw and number average molecular weight Mn of the copolymer (1) are measured in terms of polystyrene by gel permeation chromatography (GPC). m / n is 1The molar ratio (m / n) of structural unit 2 to structural unit 1 is determined by H-NMR measurement. n and m are calculated from the ratio of the number average molecular weight Mn of copolymer (1) to m / n. Therefore, n and m represent average values ​​and may not be integers.

[0024] Copolymer (1) preferably consists essentially of structural unit 1 and structural unit 2 as monomer-derived structural units, but may contain structural units derived from other monomers as long as the effects of the copolymer are not impaired. While not particularly limited, the total content of structural unit 1 and structural unit 2 is preferably 90 mol% or more, more preferably 95 mol% or more, and even more preferably 99 mol% or more, or may be 100 mol%, based on 100 mol% of all monomer-derived structural units. From the standpoint of environmental impact, it is preferable that the structural units derived from other monomers do not contain structural units derived from nitrile group-containing monomers.

[0025] The copolymer (1) can be obtained by radical copolymerization of an alkyl (meth)acrylate and a (meth)acrylic acid ester having an oxetane ring.

[0026] [Coated lithium-transition metal oxide particles] In the coated lithium-transition metal oxide particles, the particle surfaces of the lithium-transition metal oxide particles are coated with copolymer (1), and the copolymer (1) is crosslinked by ring-opening of the oxetane ring. Therefore, a coating layer containing a crosslinked copolymer (1) is formed on the particle surfaces of the lithium-transition metal oxide particles, and the coating layer is provided on a part or the entire particle surface. It is preferable that the coating layer is provided so as to cover the entire particle surface. By coating the particle surfaces of the lithium-transition metal oxide particles with a crosslinked copolymer (1), when used as a positive electrode active material in a lithium ion battery, the internal resistance is reduced, thereby improving the input / output characteristics of the lithium ion battery.

[0027] In copolymer (1), the oxetane ring in the structural unit 2 is opened by crosslinking. Therefore, in the coating layer, the crosslinked product does not need to contain any structural unit having the structure of the structural unit 2 shown in formula (1) as is. Alternatively, the oxetane ring of some structural units 2 may remain unopened.

[0028] In the coated lithium-transition metal oxide particles, the amount of copolymer (1) is preferably 0.7 to 5 mass%. That is, the content of copolymer (1) in 100 mass% of the coated lithium-transition metal oxide particles is preferably 0.7 to 5 mass%. By setting the amount of copolymer (1) within the above range, the effect of improving input / output characteristics can be enhanced. The amount of copolymer (1) is preferably 0.8 to 3 mass%, more preferably 0.9 to 2 mass%, and even more preferably 0.9 to 1.5 mass%.

[0029] The coating layer formed on the particle surface of the coated lithium-transition metal oxide particles may be composed only of the crosslinked copolymer (1), but may also contain other polymers, additives, etc., within a range that does not impair the effects thereof.

[0030] The average particle size of the coated lithium-transition metal oxide particles is not particularly limited, and may be, for example, 1 to 30 μm, or 3 to 15 μm.

[0031] [Method of manufacturing coated lithium-transition metal oxide particles] The method for producing the coated lithium-transition metal oxide particles, i.e., the method for coating the lithium-transition metal oxide particles with the crosslinked body of copolymer (1), is not particularly limited. In one embodiment, the method for producing the coated lithium-transition metal oxide particles preferably includes a step of heating a mixture containing lithium-transition metal oxide particles, copolymer (1), and a polymerization initiator.

[0032] Specifically, a polymer solution in which uncrosslinked copolymer (1) is dissolved in an organic solvent is mixed with lithium-transition metal oxide particles (specifically, a powder made of these particles) and a polymerization initiator to prepare a slurry. The resulting slurry is filtered under reduced pressure to remove the organic solvent. The mixture after reduced pressure filtration is heated. Upon heating, the oxetane rings of copolymer (1) are opened by the action of the polymerization initiator, and copolymer (1) is crosslinked. As a result, a coating layer made of crosslinked copolymer (1) is formed on the particle surface of the lithium-transition metal oxide particles.

[0033] After the heating, the resulting coated lithium-transition metal oxide particles may be washed with an organic solvent. For example, after the heating, the coated lithium-transition metal oxide particles may be mixed with an organic solvent to wash off the excess polymerization initiator, followed by filtering under reduced pressure to remove the organic solvent, and then drying by heating.

[0034] As the polymerization initiator, a cationic polymerization initiator can be used. As the cationic polymerization initiator, an initiator capable of generating an acid (cationic active species) by heat can be used, and examples thereof include sulfonium salts, phosphonium salts, quaternary ammonium salts, diazonium salts, and iodonium salts. Any one of these may be used alone, or two or more may be used in combination. Among these, it is preferable to use a sulfonium salt.

[0035] Examples of sulfonium salts include aromatic sulfonium salt-based cationic polymerization initiators such as triphenylsulfonium tetrafluoroborate, triphenylsulfonium hexafluoroantimonate, triphenylsulfonium hexafluoroarsenate, bis[4-(diphenylsulfonio)phenyl]sulfide bis(hexafluorophosphate), bis[4-(diphenylsulfonio)phenyl]sulfide bis(hexafluoroantimonate), diphenyl-4-(phenylthio)phenylsulfonium hexafluorophosphate, and diphenyl-4-(phenylthio)phenylsulfonium hexafluoroantimonate.

[0036] Examples of phosphonium salts include ethyltriphenylphosphonium hexafluoroantimonate and tetrabutylphosphonium hexafluoroantimonate.

[0037] Examples of quaternary ammonium salts include N,N-dimethyl-N-benzylanilinium hexafluoroantimonate, N,N-diethyl-N-benzylanilinium tetrafluoroborate, N,N-dimethyl-N-benzylpyridinium hexafluoroantimonate, N,N-diethyl-N-benzylpyridinium trifluoromethanesulfonate, N,N-dimethyl-N-(4-methoxybenzyl)pyridinium hexafluoroantimonate, and N,N-diethyl-N-(4-methoxybenzyl)pyridinium hexafluoroantimonate.

[0038] Examples of the diazonium salt include aromatic diazonium salt-based cationic polymerization initiators such as phenyldiazonium hexafluorophosphate, phenyldiazonium hexafluoroantimonate, and phenyldiazonium tetrafluoroborate.

[0039] Examples of iodonium salts include aromatic iodonium salt-based cationic polymerization initiators such as phenyliodonium hexafluorophosphate, diphenyliodonium hexafluoroantimonate, and diphenyliodonium tetrafluoroborate.

[0040] Commercially available polymerization initiators are not particularly limited, but examples thereof include San-Aid SI-45L, SI-60L, SI-80L, SI-100L, SI-110L, SI-150L, ​​and SI-300 (manufactured by Sanshin Chemical Industry Co., Ltd.).

[0041] The amount of the polymerization initiator used in the mixture is preferably 1 to 5 parts by mass per 100 parts by mass of the total of the copolymer (1) and the polymerization initiator. By using an amount of the polymerization initiator of 1 part by mass or more, the crosslinking density can be increased, thereby enhancing the effect of improving input / output characteristics. The amount of the polymerization initiator is more preferably 1.5 to 4 parts by mass.

[0042] The organic solvent for dissolving the uncrosslinked copolymer (1) is not particularly limited, and examples thereof include cyclic carbonates such as ethylene carbonate (ethylene carbonate, EC), propylene carbonate (PC), and butylene carbonate (BC); chain carbonates such as diethyl carbonate (diethyl carbonate, DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate, and ethyl propionate; γ-lactones such as γ-butyrolactone; chain ethers such as 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME); tetrahydrofuran, Examples of suitable aprotic solvents include cyclic ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran; fluorine derivatives thereof; and aprotic solvents such as dimethyl sulfoxide (DMSO), 1,3-dioxolane, dimethylformamide (DMF), acetonitrile, propylnitrile, nitromethane, phosphoric acid triesters, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, and fluorinated carboxylic acid esters. These aprotic solvents may be used alone or in combination of two or more.

[0043] In the above slurry containing the uncrosslinked copolymer (1), lithium-transition metal oxide particles, a polymerization initiator, and an organic solvent, the amount of the organic solvent is not particularly limited, and may be, for example, 10 to 50 mass % or 20 to 40 mass % relative to 100 mass % of the slurry.

[0044] [Positive electrode active material for lithium-ion batteries] The positive electrode active material for a lithium ion battery according to the embodiment includes the coated lithium-transition metal oxide particles. The positive electrode active material for a lithium ion battery is a powder containing the coated lithium-transition metal oxide particles, and may be composed of only the coated lithium-transition metal oxide particles, but may also include other particles as long as the effects of the coated lithium-transition metal oxide particles are not impaired.

[0045] [Lithium-ion battery] The lithium ion battery (more precisely, a lithium ion secondary battery) according to the embodiment includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode includes the above-described positive electrode active material for lithium ion batteries (hereinafter simply referred to as the positive electrode active material). A separator may be disposed between the positive electrode and the negative electrode.

[0046] The positive electrode contains at least the above-mentioned positive electrode active material, and may be, for example, a positive electrode mixture layer containing the above-mentioned positive electrode active material formed on one or both sides of a current collector made of a metal such as aluminum foil.

[0047] The positive electrode mixture layer can be formed by applying a positive electrode mixture-containing paste to a current collector, drying the paste, and compressing and molding it. The positive electrode mixture-containing paste can be obtained, for example, by dispersing and kneading the above-mentioned positive electrode active material together with a conductive additive such as carbon black or graphite and a binder such as polyvinylidene fluoride (PVDF) in a dispersion medium such as N-methyl-2-pyrrolidone (NMP).

[0048] The negative electrode, electrolyte, and separator may have any known configuration and are not particularly limited.

[0049] The lithium ion battery according to the embodiment can be formed into any shape, such as a cylindrical shape, a coin shape, a rectangular shape, or any other shape. The basic configuration of the battery is the same regardless of the shape, and the design can be modified depending on the purpose. For example, in the case of a cylindrical shape, a negative electrode formed by applying a negative electrode active material to a negative electrode current collector and a positive electrode formed by applying a positive electrode active material to a positive electrode current collector are wound with a separator interposed between them, and the wound body is housed in a battery can, and a non-aqueous electrolyte is poured into the battery can, and insulating plates are placed on the top and bottom and sealed. In addition, when applied to a coin-type lithium secondary battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and housed in a coin-type battery can, and a non-aqueous electrolyte is poured into the battery can, and sealed. [Example]

[0050] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0051] [Measurement and evaluation method] (Mw and Mn of copolymer (1)) Copolymer (1) was dissolved in tetrahydrofuran and analyzed by gel permeation chromatography (GPC) using four columns (Shodex GPC columns KF-601, KF-602, KF-603, and KF-604, Showa Denko) packed with polystyrene gel. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were measured using a Shimadzu Prominence GPC system. The column oven temperature was 40°C, the THF flow rate was 0.6 mL / min, the sample concentration was 0.1% by mass, and the sample injection volume was 100 μL. A differential refractive index detector (Shodex RI-504, Showa Denko) was used.

[0052] (m / n of copolymer (1)) Copolymer (1) was dissolved in deuterated chloroform and analyzed by a nuclear magnetic resonance spectrometer (JEOL). 1 H-NMR measurement was carried out to determine the molar ratio (m / n) of structural unit 2 to structural unit 1.

[0053] (Content of copolymer (1)) The particles (coated lithium-transition metal oxide particles) of each positive electrode active material of the Examples and Comparative Examples were heated in air from 25°C or lower to 800°C at a heating rate of 10°C / min using a simultaneous differential thermal and thermogravimetric analyzer, and the TG-DTA curve was measured to determine the content of copolymer (1) in the particles of each positive electrode active material.

[0054] (Discharge characteristic test) The fabricated lithium-ion batteries were CC (Constant Current) charged at a current value of 0.5C, and then CC discharged at a current value of 2C or 5C, and the discharge capacity retention rate was calculated. The charge / discharge voltage range was set to 2.8V to 4.3V. The discharge capacity retention rate [%] is the ratio of the 2C or 5C discharge capacity to the 0.2C discharge capacity obtained in the capacity confirmation test described below (2C or 5C discharge capacity / 0.2C discharge capacity).

[0055] The capacity confirmation test was performed by CC charging the lithium-ion battery at a current value of 0.2 C, followed by CC discharging at a current value of 0.2 C, to determine the 0.2 C discharge capacity. The 0.2 C current value refers to a current value that is 0.2 times the current value of 1 C, which is the current value that can discharge the cell capacity in 1 hour.

[0056] [Synthesis Example 1 (Synthesis of Copolymer (1)] A thoroughly dried 3000 mL separable flask was charged with 165.0 g of methyl methacrylate (MMA), 55.2 g of (3-ethyl-3-oxetanyl)methyl methacrylate (OXMA), and 880.8 g of propylene carbonate. The mixture was stirred for 90 minutes at 70 °C with nitrogen bubbling, and then 0.364 g of 2,2'-azobis(isobutyronitrile) (AIBN) was added to initiate the reaction. After 3 and 6 hours of reaction, 0.108 g of AIBN was added. The mixture was heated and stirred for a total of 9 hours, and then diluted with 365.9 g of propylene carbonate. The solution was dried over molecular sieves to obtain a 13% by mass solution of copolymer (1) represented by formula (2).

[0057] The resulting copolymer (1) was a random copolymer of MMA and OXMA, and had a weight-average molecular weight Mw of 480,000, a number-average molecular weight Mn of 132,000, and m / n of 0.19.

[0058] [Example 1] Lithium-transition metal oxide particles: LiNi 0.8 Co 0.1 Mn 0.1 A powder made of lithium-containing nickel-cobalt-manganese oxide particles (hereinafter referred to as NCM811 powder) consisting of O2 was used. As a polymerization initiator, "San-Aid SI-300" manufactured by Sanshin Chemical Industry Co., Ltd. was used.

[0059] A glass bottle was charged with 0.39 g of a 13% by mass propylene carbonate solution of copolymer (1) obtained in Synthesis Example 1, 0.4 g of a 0.26% by mass propylene carbonate solution of a polymerization initiator, 5 g of NCM811 powder, and 1.25 g of propylene carbonate, and the mixture was stirred and mixed at 400 rpm for 12 minutes and 30 seconds to prepare a slurry. The mass ratio of copolymer (1) to polymerization initiator was 98 / 2. The resulting slurry was filtered under reduced pressure to remove the solvent and then heated at 80°C for 24 hours. The resulting powder was added to 3 g of propylene carbonate and mixed to wash away excess polymerization initiator, followed by vacuum filtration. The mixture was then dried at 80°C for 24 hours to obtain a cathode active material composed of coated lithium-transition metal oxide particles of Example 1. The resulting cathode active material particles had an average particle size of 11 μm and a copolymer (1) content of 1% by mass.

[0060] [Example 2] The amount of 13 mass% propylene carbonate solution of copolymer (1) was 0.78 g, the amount of 0.26 mass% propylene carbonate solution of polymerization initiator was 0.8 g, the amount of NCM811 powder was 5 g, and the amount of propylene carbonate was 0.8 g, and the rest was the same as in Example 1, to obtain a positive electrode active material consisting of coated lithium-transition metal oxide particles of Example 2. In the positive electrode active material of Example 2, the content of copolymer (1) was 2 mass%.

[0061] [Comparative Example 1] The NCM811 powder was used as it was as the positive electrode active material.

[0062] Comparative Example 2 A positive electrode active material of Comparative Example 2 was obtained in the same manner as in Example 1, except that the 0.26 mass % propylene carbonate solution of the polymerization initiator was not added. In the positive electrode active material of Comparative Example 2, the content of copolymer (1) was 0 mass %.

[0063] [Making a lithium-ion battery] (Preparation of positive electrode) Using each of the positive electrode active materials of the Examples and Comparative Examples, 93 parts by mass of the positive electrode active material, 3 parts by mass of acetylene black (Li-400, manufactured by Denka Co., Ltd.) as a conductive agent, 3 parts by mass of PVDF (KF Polymer, manufactured by Kureha Corporation) as a binder, and 67 parts by mass of N-methyl-2-pyrrolidone as a dispersion medium were mixed in a planetary mixer to prepare a positive electrode mixture-containing paste with a solids content of 60% by mass.

[0064] The obtained paste containing the positive electrode mixture was applied onto an aluminum foil (thickness: 15 μm) as a current collector using a coating machine, and then vacuum dried at 130°C for 8 hours, followed by a roll press treatment to obtain a positive electrode.

[0065] (Battery assembly) A lithium ion battery for evaluation was fabricated using the positive electrode obtained above, a polyolefin-based single layer separator sandwiched therebetween, and a lithium foil as the counter electrode.

[0066] The lithium-ion battery was subjected to a discharge characteristic test using an electrolyte of 1M LiPF6 dissolved in EC / DMC (volume ratio 3:7) to measure the 2C discharge capacity retention rate and 5C discharge capacity retention rate as input / output characteristics. The charge / discharge test was performed at 25°C, and the voltage range was set to 2.8V-4.3V.

[0067] [Table 1]

[0068] The results are shown in Table 1. Comparative Example 1 is a control example in which NCM811 powder was used as is as the positive electrode active material. In Comparative Example 2, NCM811 powder was treated with copolymer (1), but because no polymerization initiator was added, copolymer (1) was not crosslinked, and copolymer (1) was removed from the particle surface by washing after the heat treatment. Therefore, no improvement in input / output characteristics was obtained compared to Comparative Example 1. In contrast, the positive electrode active materials of Examples 1 and 2, in which a coating layer made of a crosslinked copolymer (1) was formed on the particle surface, showed an improved 5C discharge capacity retention rate and improved input / output characteristics compared to Comparative Example 1.

[0069] [Example 3] Lithium-transition metal oxide particles: LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 A powder (hereinafter referred to as NCM111 powder) made of lithium-containing nickel-cobalt-manganese oxide particles composed of O2 was used, and the other procedures were the same as in Example 1 to obtain a cathode active material made of coated lithium-transition metal oxide particles of Example 3. The particles of the cathode active material of Example 3 had an average particle size of 7 μm and a content of copolymer (1) of 1 mass %. A lithium ion battery was fabricated using the cathode active material of Example 3 as the cathode active material, and the other procedures were the same as in Example 1.

[0070] Comparative Example 3 A lithium ion battery was fabricated in the same manner as in Example 1, except that the NCM111 powder was used as it was as the positive electrode active material.

[0071] The resulting lithium ion batteries of Example 3 and Comparative Example 3 were subjected to a discharge characteristic test, and the 2C discharge capacity retention rate and 5C discharge capacity retention rate were measured as input / output characteristics.

[0072] [Table 2]

[0073] The results are shown in Table 2. Compared with Comparative Example 3, which is a control, the positive electrode active material of Example 3, in which a coating layer made of a crosslinked body of copolymer (1) was formed on the particle surface, showed improvements in both the 2C discharge capacity and the 5C discharge capacity retention rate, and the input / output characteristics were improved.

[0074] [Example 4] A lithium ion battery was fabricated in the same manner as in Example 1, except that the positive electrode active material of Example 3 was used as the positive electrode active material and 1M-LiPF6 dissolved in EC / DEC (volume ratio 3:7) was used as the electrolyte.

[0075] [Example 5] The amount of 13 mass% propylene carbonate solution of copolymer (1) was 1.17 g, the amount of 0.26 mass% propylene carbonate solution of polymerization initiator was 1.2 g, and the amount of NCM111 powder was 5 g. The same procedures as in Example 3 were repeated to obtain a cathode active material consisting of coated lithium-transition metal oxide particles of Example 5. In the cathode active material of Example 5, the content of copolymer (1) was 3 mass%. A lithium ion battery was fabricated using the cathode active material of Example 5 as the cathode active material, and the same procedures as in Example 4 were repeated to fabricate a lithium ion battery.

[0076] Comparative Example 4 A lithium ion battery was fabricated in the same manner as in Example 4, except that the NCM111 powder was used as it was as the positive electrode active material.

[0077] The resulting lithium ion batteries of Examples 4 and 5 and Comparative Example 4 were subjected to a discharge characteristic test, and the 2C discharge capacity retention rate and 5C discharge capacity retention rate were measured as input / output characteristics.

[0078] [Table 3]

[0079] The results are shown in Table 3. Compared to Comparative Example 4, which is the control, the positive electrode active materials of Examples 4 and 5, in which a coating layer made of a crosslinked body of copolymer (1) was formed on the particle surface, showed an improved 5C discharge capacity retention rate and improved input / output characteristics.

[0080] The various numerical ranges described in this specification can be arbitrarily combined with their upper and lower limits, and all such combinations are considered to be preferred numerical ranges described in this specification. Furthermore, a numerical range described as "X to Y" means from X to Y.

[0081] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

Claims

1. At least a portion of the surface of the lithium-transition metal oxide particles is coated with a copolymer represented by the following general formula (1) and having a weight average molecular weight of 200,000 to 600,000: 【Chemistry 1】 In formula (1), m and n each independently represent a number of 1 or more, and R １ and R ２ each independently represents a hydrogen atom or a methyl group, R ３ represents an alkyl group having 1 to 5 carbon atoms, and R ４ represents an alkanediyl group having 1 to 5 carbon atoms, and R ５ represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, The copolymer is crosslinked by ring-opening of the oxetane rings, and the coated lithium-transition metal oxide particles.

2. 2. The coated lithium-transition metal oxide particles according to claim 1, wherein the amount of the copolymer is 0.7 to 5% by mass.

3. A positive electrode active material for a lithium ion battery, comprising the coated lithium-transition metal oxide particles according to claim 1 or 2.

4. A lithium ion battery comprising a positive electrode containing the positive electrode active material for lithium ion batteries according to claim 3.

5. The method includes a step of heating a mixture containing lithium-transition metal oxide particles, a copolymer represented by the following general formula (1) and having a weight average molecular weight of 200,000 to 600,000, and a polymerization initiator, 【Chemistry 2】 In formula (1), m and n each independently represent a number of 1 or more, and R １ and R ２ each independently represents a hydrogen atom or a methyl group, R ３ represents an alkyl group having 1 to 5 carbon atoms, and R ４ represents an alkanediyl group having 1 to 5 carbon atoms, and R ５ represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms; Method for producing coated lithium-transition metal oxide particles.

6. 6. The method for producing coated lithium-transition metal oxide particles according to claim 5, wherein the amount of the polymerization initiator in the mixture is 1 to 5 parts by mass per 100 parts by mass of the total of the copolymer and the polymerization initiator.

7. 7. The method for producing coated lithium-transition metal oxide particles according to claim 5, wherein the amount of the copolymer is 0.7 to 5 mass % based on 100 mass % of the coated lithium-transition metal oxide particles.

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