Positive electrode active material for lithium-ion secondary battery, manufacturing method for the same, and lithium-ion secondary battery using the same

Abstract

To provide a positive electrode active material for a lithium-ion secondary battery with high electrical capacity and high resource sustainability, a manufacturing method for the same, and a lithium-ion secondary battery using the same.SOLUTION: The manufacturing method for a positive electrode active material for a lithium-ion secondary battery includes hydrothermally treating a NaMnTi-containing oxide, which contains sodium, manganese, and titanium, has a tunnel-type structure, and has an average particle size in the range of 0.50 μm or more and 3.00 μm or less, in a lithium aqueous solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery using the same. [Background technology]

[0002] In recent years, research and development has been conducted on secondary batteries that contribute to energy efficiency in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. As a positive electrode active material for lithium-ion secondary batteries, LiMnTi-containing oxides, which are oxides containing lithium, manganese, and titanium and have a rock salt structure, are known (Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-96974 [Patent Document 2] International Publication No. 2017 / 122663 [Patent Document 3] International Publication No. 2019 / 087717 Summary of the Invention [Problem to be solved by the invention]

[0004] In secondary battery technology, improving electrical capacity and ensuring resource sustainability are key issues. Rock-salt LiMnTi-containing oxides are attracting attention from the perspective of resource sustainability because they do not contain rare metals such as cobalt and nickel, which are used as raw materials for manufacturing positive electrode active materials. Therefore, further improvements in the electrical capacity of rock-salt LiMnTi-containing oxides are desired.

[0005] The present invention has been made in view of the above-mentioned problems, and aims to provide a positive electrode active material for a lithium ion secondary battery that has a high electric capacity and a high resource sustainability, a method for producing the same, and a lithium ion secondary battery using the same. [Means for solving the problem]

[0006] The present inventors have found that a fine LiMnTi-containing oxide having a rock-salt structure can be obtained by hydrothermally treating a fine NaMnTi-containing oxide having a tunnel structure in a lithium aqueous solution. They have also confirmed that the fine LiMnTi-containing oxide having a rock-salt structure has a high discharge capacity, leading to the completion of the present invention. Therefore, the present invention provides the following:

[0007] (1) A positive electrode active material for a lithium ion secondary battery, which is an oxide containing lithium, manganese, and titanium, and where the total content of lithium, manganese, and titanium is taken as 100 mol %, the lithium content is in the range of 51 to 56 mol %, the manganese content is in the range of 22 to 39 mol %, the titanium content is in the range of 10 to 23 mol %, and the sodium content is 0.12 mol % or less, the oxide has a rock salt structure, and the average particle size is in the range of 0.55 μm or more and 1.65 μm or less.

[0008] According to the positive electrode active material for a lithium ion secondary battery (1), the contents of lithium, manganese, and titanium are within the above ranges, and the average particle size is within the above ranges. Therefore, the electric capacity is high while using materials with high resource sustainability.

[0009] (2) The positive electrode active material for a lithium ion secondary battery according to (1), wherein the lattice constant of the a-axis is in the range of 4.1030 Å or more and 4.1210 Å or less.

[0010] According to the positive electrode active material for a lithium ion secondary battery (2), when the average particle size is within the above range and the lattice constant of the a-axis is within the above range, the content of fine rock salt structures is high, resulting in a higher electrical capacity.

[0011] (3) The positive electrode active material for a lithium ion secondary battery according to (1) or (2), wherein in an X-ray diffraction pattern measured using CuKα as an X-ray source, the half-width of a diffraction peak at a diffraction angle 2θ in the range of 43 degrees to 45 degrees is in the range of 0.360 degrees to 0.530 degrees.

[0012] According to the positive electrode active material for a lithium ion secondary battery of (3), the half-value width is within the above range, and the crystallinity of the rock salt structure, which is the main phase, is even higher, so that the electric capacity is even higher.

[0013] (4) The positive electrode active material for a lithium ion secondary battery according to any one of (1) to (3), wherein the titanium content is in the range of 15 to 20 mol %.

[0014] According to the positive electrode active material for lithium ion secondary batteries of (4), the amounts of Mn and Ti are well balanced, so that the electric capacity is further improved.

[0015] (5) A method for producing a positive electrode active material for a lithium ion secondary battery, the method comprising: hydrothermally treating, in an aqueous lithium solution, an NaMnTi-containing oxide that contains sodium, manganese, and titanium, has a tunnel structure, and has an average particle size of 0.50 μm or more and 3.00 μm or less.

[0016] According to the method for producing a positive electrode active material for a lithium ion secondary battery (5), the average particle size of the NaMnTi-containing oxide is within the above range, and therefore the particle size is small and the specific surface area is large, which facilitates ion exchange between sodium and lithium during hydrothermal treatment and facilitates changes in the crystal structure. Therefore, according to the method for producing a positive electrode active material for a lithium ion secondary battery (5), it is possible to industrially advantageously produce a positive electrode active material for a lithium ion secondary battery having a fine rock salt structure as the main phase and high electrical capacity.

[0017] (6) The method for producing a positive electrode active material for a lithium ion secondary battery according to (5), wherein the NaMnTi-containing oxide has a diffraction peak having a half-width of 0.110 degrees or more and 0.190 degrees or less at a diffraction angle 2θ of 62 degrees or more and 63 degrees or less in an X-ray diffraction pattern measured using CuKα as an X-ray source.

[0018] According to the manufacturing method (6), the NaMnTi-containing oxide has a large half-value width of the diffraction peak and a small particle size, so that ion exchange between sodium and lithium and a change in the crystal structure are likely to occur during hydrothermal treatment.

[0019] (7) A lithium ion secondary battery comprising a positive electrode mixture layer containing the positive electrode active material for a lithium ion secondary battery according to any one of (1) to (4).

[0020] According to the lithium ion secondary battery of (7), since it contains the above-mentioned positive electrode active material for lithium ion secondary batteries, it has a high electric capacity while using materials with a high resource sustainability. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery that has a high electric capacity and is highly sustainable in terms of resources, a method for producing the same, and a lithium ion secondary battery using the same. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a flow diagram showing a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention. [Figure 2] 1 shows X-ray diffraction patterns of the NaMnTi-containing oxide powders (pulverized) prepared in Examples 1 to 6. [Figure 3] 1 shows X-ray diffraction patterns of the NaMnTi-containing oxide powders (no pulverization treatment) prepared in Comparative Examples 1 to 6. [Figure 4] 1 shows X-ray diffraction patterns of the LiMnTi-containing oxide powders obtained in Examples 1 to 6. [Figure 5] 1 shows X-ray diffraction patterns of the LiMnTi-containing oxide powders obtained in Comparative Examples 1 to 6. [Figure 6] 1 shows an X-ray diffraction pattern of the LiMnTi-containing oxide powder obtained in Comparative Example 7. [Figure 7] 1 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Example 1. [Figure 8] 1 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Example 2. [Figure 9] 1 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Example 3. [Figure 10] 1 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Example 4. [Figure 11] 10 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Example 5. [Figure 12] 1 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Example 6. [Figure 13] 1 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Comparative Example 1. [Figure 14] 1 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Comparative Example 2. [Figure 15] 10 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Comparative Example 3. [Figure 16] 10 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Comparative Example 4. [Figure 17] 10 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Comparative Example 5. [Figure 18]10 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Comparative Example 6. [Figure 19] 10 is a graph showing the initial charge / discharge curve of a two-electrode cell using the LiMnTi-containing oxide powder obtained in Comparative Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present invention will be described. However, the embodiments described below are merely examples of the present invention, and the present invention is not limited to the following.

[0024] The positive electrode active material for a secondary battery according to this embodiment contains a LiMnTi-containing oxide containing lithium (Li), manganese (Mn), and titanium (Ti). The positive electrode active material for a secondary battery may contain only a LiMnTi-containing oxide.

[0025] In the LiMnTi-containing oxide, when the total content of Li, Mn, and Ti is taken as 100 mol%, the Li content is in the range of 51 to 56 mol%, the Mn content is in the range of 22 to 39 mol%, the Ti content is in the range of 10 to 23 mol%, and the Na content is 0.12 mol% or less. The Ti content may be in the range of 15 to 20 mol%. When the Ti content is in this range, the balance between the amounts of Mn and Ti is good, thereby further improving the electrical capacity.

[0026] The LiMnTi-containing oxide is represented by the following general formula (I).

[0027] Li a Na b Mn x Ti y O2(I)

[0028] In the above general formula (I), a+x+y is 2, a satisfies the relationship 1.02≦a≦1.12, b satisfies the relationship 0≦b≦0.0024, x satisfies the relationship 0.44≦x≦0.78, and y satisfies the relationship 0.20≦y≦0.46. y may also satisfy the relationship 0.30≦y≦0.40.

[0029] The LiMnTi-containing oxide has a rock salt structure. The LiMnTi-containing oxide may have a single phase of the rock salt structure. The a-axis lattice constant of the LiMnTi-containing oxide may be in the range of 4.1030 Å to 4.1210 Å. When the average particle size is in the above range and the a-axis lattice constant is in this range, the content of fine rock salt structures is high, resulting in a higher electric capacity. In the X-ray diffraction pattern measured using a CuKα X-ray source, the LiMnTi-containing oxide may have a diffraction peak at a diffraction angle 2θ of 43 degrees to 45 degrees, with a full width at half maximum (FWHM) of 0.360 degrees to 0.530 degrees. The LiMnTi-containing oxide having this diffraction peak in this range has higher crystallinity of the rock salt structure, which is the main phase, and therefore has a higher electric capacity.

[0030] The LiMnTi-containing oxide has an average particle size of 0.55 μm to 1.65 μm. The average particle size can be measured by a laser diffraction scattering method. The particle shape of the LiMnTi-containing oxide is not particularly limited, and may be, for example, spherical, columnar, or amorphous.

[0031] The positive electrode active material for lithium ion secondary batteries of this embodiment can be produced by a method including the steps of obtaining a precursor of an LiMnTi-containing oxide and generating a LiMnTi-containing oxide from the obtained precursor. The precursor is an NaMnTi-containing oxide containing sodium, manganese, and titanium. A method for generating a LiMnTi-containing oxide from the NaMnTi-containing oxide can be a method of hydrothermally treating the NaMnTi-containing oxide in a lithium aqueous solution to replace sodium with lithium. The method for producing a positive electrode active material for lithium ion secondary batteries of this embodiment will be described with reference to FIG. 1.

[0032] As shown in FIG. 1, the process for obtaining the NaMnTi-containing oxide (precursor) includes a mixing step S1, a firing step S2, and a pulverization step S3.

[0033] In the mixing step S1, a sodium source, a manganese source, and a titanium source are mixed to obtain a raw material mixture. There are no particular limitations on the sodium source, manganese source, and titanium source, and various compounds such as oxides, carbonates, hydroxides, and chlorides can be used. In this embodiment, Na2CO3 is used as the sodium source, Mn2O3 is used as the manganese source, and TiO2 is used as the titanium source.

[0034] The mixing ratio of the sodium source, manganese source, and titanium source is, for example, such that the amount of sodium is 0.44 moles when the total amount of manganese and titanium is 1 mole. The mixing ratio of the manganese source and titanium source is such that the amount of manganese is 0.50 moles or more and 0.80 moles when the total amount of manganese and titanium is 1 mole. The method for mixing the sodium source, manganese source, and titanium source is not particularly limited, and may be a dry method or a wet method.

[0035] In the firing step S2, the raw material mixture obtained in the mixing step S1 is fired to produce an NaMnTi-containing oxide having a tunnel structure. The firing conditions for the raw material mixture can be, for example, in air at a firing temperature of 900 to 1200°C. The firing time varies depending on conditions such as the composition of the raw material mixture and the firing temperature, but is, for example, within the range of 1 to 30 hours.

[0036] In the pulverization step S3, the NaMnTi-containing oxide obtained in the firing step S2 is pulverized to a fine powder. The pulverization may be performed dry or wet. There are no particular limitations on the pulverization method, and various pulverization devices used for pulverizing inorganic materials, such as a ball mill, a bead mill, a jet mill, or a mortar and pestle, can be used. The average particle size of the pulverized NaMnTi-containing oxide is in the range of 0.50 μm to 3.00 μm.

[0037] The NaMnTi-containing oxide obtained as described above is represented by, for example, the following general formula (II).

[0038] Na 0.44 Mn q Ti r O2(II)

[0039] However, in the above general formula (I), q and r satisfy q+r=1. q may, for example, satisfy 0.50≦q≦0.80.

[0040] The NaMnTi-containing oxide may have a diffraction peak with a full width at half maximum (FWHM) of 0.110 degrees or more and 0.190 degrees or less at a diffraction angle 2θ of 62 degrees or more and 63 degrees or less in an X-ray diffraction pattern measured using a CuKα X-ray source. Since the full width at half maximum (FWHM) of this diffraction peak is within this range and the particle size is small, ion exchange between sodium and lithium is likely to occur during hydrothermal treatment.

[0041] As shown in FIG. 1, the process of producing a LiMnTi-containing oxide from a NaMnTi-containing oxide includes a mixing step S4, a hydrothermal synthesis step S5, a water-washing step S6, a drying step S7, and a pulverization step S8.

[0042] In the mixing step S4, the NaMnTi-containing oxide, a lithium source, and water are mixed to obtain a dispersion in which the NaMnTi-containing oxide is dispersed in an aqueous lithium solution. There are no particular limitations on the lithium source, and various compounds such as oxides, carbonates, hydroxides, and chlorides can be used. In this embodiment, LiOH·H2O is used as the lithium source. The lithium content in the dispersion is, for example, in the range of 2.0 to 30.0 moles, where the total molar amount of manganese and titanium contained in the NaMnTi-containing oxide in the dispersion is 1 mole.

[0043] In the hydrothermal synthesis step S5, the NaMnTi-containing oxide in the dispersion obtained in the mixing step S4 is hydrothermally treated to hydrothermally synthesize a LiMnTi-containing oxide. By hydrothermally treating the NaMnTi-containing oxide in a lithium aqueous solution, sodium in the NaMnTi-containing oxide is replaced with lithium, and the crystal structure of the NaMnTi-containing oxide changes to produce a LiMnTi-containing oxide with a rock salt structure. The hydrothermal treatment can be performed, for example, by placing the dispersion in a sealed container and heating the sealed container in air at a temperature of 150 to 230°C. The treatment time for the hydrothermal treatment varies depending on conditions such as the ratio of the NaMnTi-containing oxide to the lithium source in the dispersion and the capacity of the sealed container, but is, for example, within a range of 1 to 30 hours.

[0044] In the water-washing step S6, the LiMnTi-containing oxide produced in the hydrothermal synthesis step S5 is recovered and washed with water. By washing the LiMnTi-containing oxide with water, sodium substituted for lithium and unreacted lithium source are removed.

[0045] In the drying step S7, the LiMnTi-containing oxide washed in the water-washing step S6 is dried. The drying method is not particularly limited, and various methods used as a drying method for inorganic materials, such as heat drying, vacuum drying, and spray drying, can be used.

[0046] In the pulverization step S8, the LiMnTi-containing oxide dried in the drying step S7 is pulverized to a particle size suitable for use as a positive electrode active material for a secondary battery. The pulverization may be performed in a dry or wet manner. There are no particular limitations on the pulverization method, and the pulverization may be performed using various pulverizers commonly used for pulverizing inorganic materials, such as a ball mill, a bead mill, a jet mill, or a mortar and pestle.

[0047] The positive electrode active material for a lithium ion secondary battery according to the present embodiment can be used as the positive electrode active material for a lithium ion secondary battery. The lithium ion secondary battery includes, for example, a positive electrode, a negative electrode, an electrolyte, a separator disposed between the positive electrode and the negative electrode, and an exterior body that houses these components. A solid electrolyte may be used instead of the electrolyte.

[0048] The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector. The positive electrode active material layer includes the positive electrode active material for lithium ion secondary batteries of this embodiment. The positive electrode active material layer may include a conductive additive and a binder. Because the positive electrode active material for secondary batteries of this embodiment is chemically stable, the conductive additive and binder are not particularly limited, and known conductive additives and binders used in positive electrode active material layers of lithium ion secondary batteries can be used. In addition, the positive electrode current collector is not particularly limited, and known conductive additives and binders used in positive electrode current collectors of lithium ion secondary batteries, such as aluminum foil, can be used.

[0049] The negative electrode may be a laminate including a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. Examples of the negative electrode active material include metallic lithium, a material capable of absorbing and desorbing lithium, and a metal or semimetal that forms an alloy with lithium. Examples of materials capable of absorbing and desorbing lithium include lithium transition metal oxides such as lithium titanate, transition metal oxides such as TiO2, Nb2O3, and WO3, SiO, metal sulfides, metal nitrides, and carbon materials such as artificial graphite, natural graphite, graphite, soft carbon, and hard carbon. Examples of metals or semimetals that form an alloy with lithium include Mg, Si, Au, Ag, In, Ge, Sn, Pb, Al, and Zn. When the negative electrode active material is in powder form, the negative electrode active material layer may contain a conductive additive and a binder. There are no particular limitations on the conductive additive and binder, and known conductive additives and binders used in negative electrode active material layers of lithium-ion secondary batteries can be used. The negative electrode current collector is not particularly limited, and any known negative electrode current collector used in lithium ion secondary batteries, such as copper foil, can be used.

[0050] The electrolytic solution contains an organic solvent and an electrolyte. Examples of the organic solvent that can be used include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, hydrofluoroethers, aromatic ethers, sulfones, cyclic esters, chain carboxylic acid esters, and nitriles. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, vinylene carbonate, and fluoroethylene carbonate. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, and 4-methyl-1,3-dioxolane. Examples of chain ethers include 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, and diethyl ether. Examples of hydrofluoroethers include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl)ether, and 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane. Examples of aromatic ethers include anisole. Examples of sulfones include sulfolane and methylsulfolane. Examples of cyclic esters include γ-butyrolactone. Examples of chain carboxylic acid esters include acetate esters, butyrate esters, and propionate esters. Examples of nitriles include acetonitrile and propionitrile. The organic solvents may be used alone or in combination of two or more.

[0051] The electrolyte is a source of lithium ions, which are a charge transfer medium, and contains a lithium salt. Examples of lithium salts include LiPF, LiBF, LiClO, LiAsF, LiCFSO, LiC(CFSO), LiN(CFSO) (LiTFSI), LiN(FSO) (LiFSI), and LiBCO. The lithium salts may be used alone or in combination.

[0052] As the solid electrolyte, for example, a sulfide solid electrolyte, an oxide solid electrolyte, a nitride solid electrolyte, a halide solid electrolyte, or the like can be used. Examples of sulfide solid electrolytes include Li2S-P2S5 and Li2S-P2S5-LiI. Examples of oxide solid electrolytes include NASICON-type oxides, garnet-type oxides, and perovskite-type oxides. Examples of NASICON-type oxides include oxides containing Li, Al, Ti, P, and O (e.g., Li 1.5 Al 0.5 Ti 1.5 Examples of garnet-type oxides include oxides containing Li, La, Zr, and O (e.g., LiLaZrO 12 Examples of perovskite oxides include oxides containing Li, La, Ti, and O (for example, LiLaTiO3).

[0053] The separator is not particularly limited, and may be, for example, a porous sheet or a nonwoven fabric sheet. Examples of materials for the porous sheet include polyolefins such as polyethylene and polypropylene, aramid, polyimide, and fluororesin. Examples of materials for the nonwoven fabric sheet include glass fiber and cellulose fiber.

[0054] The exterior body is not particularly limited, and known exterior bodies used in lithium ion secondary batteries, such as a metal container or a container made of laminated film, can be used.

[0055] According to the positive electrode active material for a lithium ion secondary battery of the present embodiment configured as described above, the contents of lithium, manganese, and titanium are within the above ranges, and the average particle size is within the above ranges. Therefore, the positive electrode active material uses materials with high resource sustainability, while still having high reactivity.

[0056] According to the method for producing a positive electrode active material for a lithium ion secondary battery of this embodiment, the average particle size of the NaMnTi-containing oxide is within the above range, and therefore the particle size is small and the specific surface area is large. Therefore, ion exchange between sodium and lithium is likely to proceed during hydrothermal treatment, and a change in the crystal structure is likely to occur. Therefore, according to the method for producing a positive electrode active material for a lithium ion secondary battery of this embodiment, a positive electrode active material for a lithium ion secondary battery that is fine, has a high content of rock salt structures, and has a high electrical capacity can be produced in an industrially advantageous manner.

[0057] The lithium ion secondary battery of this embodiment contains the above-described positive electrode active material for lithium ion secondary batteries, and therefore has a high electric capacity while using materials with a high resource sustainability. [Example]

[0058] [Example 1] (Preparation of NaMnTi-containing oxide powder) Na2CO3, Mn2O3, and TiO2 were weighed out to a total mass of 3.0 g in a molar ratio of Na, Mn, and Ti of 0.440:0.500:0.500. The weighed Na2CO3, Mn2O3, and TiO2 were mixed using a mortar and pestle. The resulting raw material mixture was placed in an alumina crucible and fired in air at 1000°C for 12 hours. 2 g of the resulting fired material was placed in a 45 mL zirconia ball mill. 60 g of zirconia balls (diameter: 5 mm) and 15 mL of ethanol were then added to the ball mill, followed by grinding at 350 rpm for 12 hours. After grinding, the powder was collected and dried at 60°C under vacuum for 12 hours. This ball mill grinding process was repeated three times. From the X-ray diffraction pattern of the fired product after pulverization, it was confirmed that the fired product obtained was an NaMnTi-containing oxide powder having a tunnel structure.

[0059] (Preparation of LiMnTi-containing oxide powder) A hydrothermal reaction vessel was charged with 2.00 g of LiOH·HO and 50 mL of water and stirred to dissolve the LiOH·HO, preparing a LiOH solution. Next, 0.5 g of the NaMnTi oxide powder obtained above was added to the LiOH solution and stirred to obtain a dispersion of the NaMnTi oxide powder in the LiOH solution. The hydrothermal reaction vessel was then sealed and placed in a thermostatic chamber. The NaMnTi oxide powder was hydrothermally treated by heating in air at 180°C for 24 hours. After heating, the hydrothermally treated powder (LiMnTi-containing oxide powder) was recovered from the hydrothermal reaction vessel. The recovered LiMnTi-containing oxide powder was washed with water and then centrifuged three times to remove moisture. The dehydrated LiMnTi-containing oxide powder was placed in a petri dish and vacuum-dried at 100°C for 6 hours. The dried LiMnTi-containing oxide powder was crushed using a mortar and pestle.

[0060] [Examples 2 to 6] LiMnTi-containing oxide powder was prepared in the same manner as in Example 1, except that the blending amounts of Na2CO3, Mn2O3, and TiO2 were set to the amounts shown in Table 1 below.

[0061] [Comparative Example 1] An LiMnTi-containing oxide powder was produced in the same manner as in Example 1, except that the pulverization treatment using a ball mill was not carried out in the production of the NaMnTi-containing oxide powder.

[0062] Comparative Example 2 An LiMnTi-containing oxide powder was produced in the same manner as in Example 2, except that the pulverization treatment using a ball mill was not carried out in the production of the NaMnTi-containing oxide powder.

[0063] Comparative Example 3 An LiMnTi-containing oxide powder was produced in the same manner as in Example 3, except that the pulverization treatment using a ball mill was not carried out in the production of the NaMnTi-containing oxide powder.

[0064] Comparative Example 4 An LiMnTi-containing oxide powder was produced in the same manner as in Example 4, except that the pulverization treatment using a ball mill was not carried out in the production of the NaMnTi-containing oxide powder.

[0065] Comparative Example 5 An LiMnTi-containing oxide powder was produced in the same manner as in Example 5, except that the pulverization treatment using a ball mill was not carried out in the production of the NaMnTi-containing oxide powder.

[0066] Comparative Example 6 An LiMnTi-containing oxide powder was produced in the same manner as in Example 6, except that the pulverization treatment using a ball mill was not carried out in the production of the NaMnTi-containing oxide powder.

[0067] Comparative Example 7 The LiMnTi-containing oxide powder was prepared by the following method. (Preparation of Li2TiO3 powder) Li2CO3 and TiO2 were weighed out to a total mass of 2.0g with a molar ratio of Li to Ti of 2.00:1.00. The weighed Li2CO3 and TiO2 were mixed using a mortar and pestle. The resulting raw material mixture was placed in an alumina crucible and fired in air at 950°C for 12 hours. The fired product was then pulverized using a mortar and pestle to produce Li2TiO3 powder.

[0068] (Preparation of LiMnO2 powder) Li2CO3 and Mn2O3 were weighed out to a total mass of 2.0 g with a molar ratio of Li to Mn of 1.00:1.00. The weighed Li2CO3 and Mn2O3 were mixed using a mortar and pestle. The resulting raw material mixture was placed in an alumina crucible and fired in an argon atmosphere at 800°C for 12 hours. The fired product was then pulverized using a mortar and pestle to produce LiMnO2 powder.

[0069] (Preparation of LiMnTi-containing oxide powder) 0.66 g of the obtained Li2TiO3 powder and 0.85 g of LiMnO2 powder were placed in a 45 mL zirconia ball mill, and then 13.5 g of 4 mm diameter zirconia balls and 2.0 g of 2 mm diameter zirconia balls were placed in the ball mill, followed by pulverization at 600 rpm for 12 hours. The pulverized product was further pulverized using a mortar and pestle to produce a LiMnTi-containing oxide powder.

[0070] [Evaluation of NaMnTi-containing oxide powder] The average particle size, X-ray diffraction pattern, lattice constant, and full width at half maximum (FWHM) of the NaMnTi-containing oxide powders produced in Examples 1 to 6 and Comparative Examples 1 to 6 were measured by the following methods.

[0071] (Average particle size) Measurement was carried out using a laser diffraction scattering particle size distribution analyzer, and the results are shown in Table 1 below.

[0072] (X-ray diffraction pattern) X-ray diffraction patterns were measured under the following conditions: The X-ray diffraction patterns of the NaMnTi-containing oxide fine powders (pulverized) prepared in Examples 1 to 6 are shown in Figure 2, and the X-ray diffraction patterns of the NaMnTi-containing oxide fine powders (not pulverized) prepared in Comparative Examples 1 to 6 are shown in Figure 3.

[0073] Measurement equipment: RINT-2550V, manufactured by Rigaku Corporation X-ray source:CuKα X-ray output: 40kV, 200mA Measurement conditions: 1.0 s, 0.03 deg intervals

[0074] (lattice constant) The lattice constants of the a-axis, b-axis, and c-axis were measured using the X-ray diffraction pattern. The lattice constants were calculated by the least squares method using the indices of the diffraction peaks due to the tunnel structure extracted from the X-ray diffraction pattern and their interplanar spacings. The results are shown in Table 2 below.

[0075] (FWHM) From the X-ray diffraction pattern, diffraction peaks with a diffraction angle 2θ in the range of 62 degrees or more and 63 degrees or less were extracted. The full width at half maximum (FWHM) of the extracted peaks was measured. The results are shown in Table 1 below.

[0076] [Table 1]

[0077] 2 and 3 and the results in Table 1, it can be seen that the NaMnTi-containing oxide powders of Examples 1 to 6, which were subjected to a pulverization treatment, have a smaller average particle size and therefore a larger specific surface area, and the X-ray diffraction patterns are broad with a larger full width at half maximum (FWHM), and the crystallite size is smaller than the NaMnTi-containing oxide powders of Comparative Examples 1 to 6, which were not subjected to a pulverization treatment. Therefore, the NaMnTi-containing oxide powders of Examples 1 to 6 are more likely to undergo ion exchange between sodium and lithium during hydrothermal treatment than the NaMnTi-containing oxide powders of Comparative Examples 1 to 6.

[0078] [Evaluation of LiMnTi-containing oxide powder] The chemical composition, average particle size, X-ray diffraction pattern, a-axis lattice constant, full width at half maximum (FWHM), and discharge capacity of the LaNaMnTi-containing oxide powders produced in Examples 1 to 6 and Comparative Examples 1 to 7 were measured by the following methods.

[0079] (chemical composition) The sample was dissolved in acid. The contents of Li, Na, Mn, and Ti in the resulting solution were measured using an ICP emission spectrometer. The resulting contents were converted to molar contents, with the total content of Li and Mn taken as 100 mol %. The results are shown in Table 2 below.

[0080] (X-ray diffraction pattern) The X-ray diffraction patterns were measured under the same conditions as for the NaMnTi-containing oxide powder. The X-ray diffraction patterns of the LiMnTi-containing oxide powders obtained in Examples 1 to 6 are shown in Fig. 4, the X-ray diffraction patterns of the LiMnTi-containing oxide powders obtained in Comparative Examples 1 to 6 are shown in Fig. 5, and the X-ray diffraction pattern of the LiMnTi-containing oxide powder obtained in Comparative Example 7 is shown in Fig. 6.

[0081] (a-axis lattice constant) The a-axis lattice constant was measured using the X-ray diffraction pattern. The lattice constant was calculated by the least squares method using the indices of the diffraction peaks due to the tunnel structure extracted from the X-ray diffraction pattern and their interplanar spacing. The results are shown in Table 2 below.

[0082] (FWHM) Diffraction peaks with a diffraction angle 2θ in the range of 43 degrees or more and 45 degrees or less were extracted from the above X-ray diffraction pattern. The full width at half maximum (FWHM) of the extracted peaks was measured. The results are shown in Table 3 below.

[0083] (Charge / discharge characteristics) 5 mg of sample was mixed with 5 mg of acetylene black as a conductive material and 1 mg of PTFE as a binder. The resulting mixture was formed into a sheet and pressed onto an Al mesh to form the working electrode, and a lithium foil counter electrode. The working electrode and counter electrode were immersed in an electrolyte solution of LiPF6 dissolved in an EC+DMC solvent to prepare a two-electrode cell.

[0084] A charge-discharge test was carried out using a two-electrode cell. The conditions for the charge-discharge test were a current density (per sample) of 10 mA / g, a potential range of 2.0-4.8 V, and constant current-constant voltage charging (until 2 hours had elapsed). The charge-discharge test was carried out in an environment of 25°C. The initial (first cycle) discharge capacity is shown in Table 2 below. The initial charge-discharge curves for the two-electrode cells using the LiMnTi-containing oxide powders obtained in Examples 1 to 6 are shown in Figures 7 to 12, and the initial charge-discharge curves for the two-electrode cells using the LiMnTi-containing oxide powders obtained in Comparative Examples 1 to 7 are shown in Figures 13 to 19.

[0085] [Table 2]

[0086] 4 to 7, it was confirmed that the rock salt structure was the main phase of the LiMnTi-containing oxide powders obtained in Examples 1 to 6 and Comparative Examples 1 to 6. Furthermore, it was confirmed from the results in Table 2 that the LiMnTi-containing oxide powders of Examples 1 to 6 had smaller average particle sizes, smaller a-axis crystal constants, finer rock salt structures as the main phase, excellent crystallinity, and higher discharge capacities than the LiMnTi-containing oxide powders of Comparative Examples 1 to 6.

Claims

1. An oxide containing lithium, manganese, and titanium, When the total content of lithium, manganese, and titanium is 100 mol%, The lithium content is in the range of 51 to 56 mol %, the manganese content is in the range of 22 to 39 mol %, The titanium content is in the range of 10 to 23 mol %, The sodium content is 0.12 mol% or less, A positive electrode active material for a lithium ion secondary battery, which has a rock salt structure and an average particle size in the range of 0.55 μm to 1.65 μm.

2. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the lattice constant of the a-axis is in the range of 4.1030 Å or more and 4.1210 Å or less.

3. 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein in an X-ray diffraction pattern measured using CuKα as an X-ray source, a half-width of a diffraction peak at a diffraction angle 2θ in the range of 43 degrees or more and 45 degrees or less is in the range of 0.360 degrees or more and 0.530 degrees or less.

4. 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the titanium content is in the range of 15 to 20 mol%.

5. A method for producing a positive electrode active material for a lithium ion secondary battery, the method comprising: hydrothermally treating, in an aqueous lithium solution, an NaMnTi-containing oxide that contains sodium, manganese, and titanium, has a tunnel structure, and has an average particle size in the range of 0.50 μm to 3.00 μm.

6. 6. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 5, wherein in an X-ray diffraction pattern measured using CuKα as an X-ray source, the NaMnTi-containing oxide has a diffraction peak having a half-width at a diffraction angle 2θ in the range of 62 degrees to 63 degrees inclusive, the half-width being in the range of 0.110 degrees to 0.190 degrees inclusive.

7. A lithium ion secondary battery comprising a positive electrode mixture layer containing the positive electrode active material for lithium ion secondary batteries according to claim 1 or 2.

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