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

Abstract

To provide a positive electrode active material for a lithium ion secondary battery that can provide a lithium ion secondary battery with improved discharge capacity and capacity retention rate, a lithium ion secondary battery using the positive electrode active material, and a method for producing the positive electrode active material for a lithium ion secondary battery.SOLUTION: A positive electrode active material for a lithium-ion secondary battery is primarily composed of a lithium transition metal composite oxide, and the lithium transition metal composite oxide is in the form of particles having an outer layer on the surface. The lithium transition metal composite oxide is represented by the following formula (1). LimNiwMnxMgyTizO2 (1). (In formula (1), m is 1.00≤m≤1.04, w is 0.47<w<0.59, x is 0.40≤x<0.50, y is 0<y≤0.04, z is 0≤z<0.04, x≤w, and m+w+x+y+z=2.) The ratio of the number of Mn atoms to the number of Ni atoms in the outer layer (Mn / Ni ratio) is 1.0 or more and 1.5 or less.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 lithium ion secondary battery, and a method for producing a positive electrode active material for a lithium ion secondary battery. [Background technology]

[0002] In recent years, research and development has been conducted on secondary batteries that contribute to energy efficiency. In particular, lithium-ion secondary batteries are becoming increasingly important as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0003] Positive electrode active materials have attracted attention as important components that determine the capacity of lithium ion secondary batteries, and their development is underway. For example, composite oxides in which part of the nickel and manganese in lithium nickel manganese composite oxide is replaced with titanium or magnesium have been reported as positive electrode active materials for use in lithium ion secondary batteries (e.g., Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] S.-H.Kang, et al., “Comparative study of Li(Ni0.5-xMn0.5-xM2x')O2(M'=Mg,Al,Co,Ni,Ti;x= 0,0.025) cathode materials for rechargeable lithium batteries” Journal of Power Sources. 119-121, 150-155 (2003) Summary of the Invention [Problem to be solved by the invention]

[0005] In Non-Patent Document 1, the discharge capacity at 4.3 to 2.8 V is 130 mAhg -1The degree is such that there is room for improvement. In addition, for a lithium-ion secondary battery, it is required that the capacity does not decrease even when charge and discharge are repeated (to increase the capacity retention rate).

[0006] The present invention has been made to solve the above problems, and an object thereof is to provide a positive electrode active material for a lithium-ion secondary battery that can obtain a lithium-ion secondary battery with a higher discharge capacity and capacity retention rate, a lithium-ion secondary battery using the positive electrode active material, and a method for manufacturing the positive electrode active material for a lithium-ion secondary battery. And, by extension, it contributes to the improvement of energy efficiency.

Means for Solving the Problems

[0007] In order to achieve the above object, the present invention provides the following means. [1] A positive electrode active material for a lithium-ion secondary battery mainly composed of a lithium transition metal composite oxide, The lithium transition metal composite oxide is in the form of particles having an outer layer on the surface, The lithium transition metal composite oxide is represented by the following formula (1), Li m Ni w Mn x Mg y Ti z O2(1) (In formula (1), m is 1.00 ≦ m ≦ 1.04, w is 0.47 < w < 0.59, x is 0.40 ≦ x < 0.50, y is 0 < y ≦ 0.04, z is 0 ≦ z < 0.04, and x is in the range of x ≦ w, and m +w +x + y + z = 2) A positive electrode active material for a lithium-ion secondary battery, wherein the ratio of the number of Mn atoms to the number of Ni atoms (Mn / Ni ratio) in the outer layer is 1.0 or more and 1.5 or less.

[0008] The positive electrode active material for a lithium-ion secondary battery according to [1] (hereinafter also simply referred to as "positive electrode active material") contains magnesium (Mg), which has a smaller atomic weight than nickel (Ni) or manganese (Mn). Therefore, even if the Ni content is reduced, the decrease in discharge capacity can be reduced or kept the same. In addition, the positive electrode active material according to [1] has an Mn / Ni ratio in the outer layer that satisfies a specific numerical range. Therefore, the capacity retention rate of a lithium-ion secondary battery (hereinafter also simply referred to as "secondary battery") using this positive electrode active material can be further improved. Therefore, in a secondary battery, the number of batteries required can be reduced, contributing to extending the battery life. In other words, it can contribute to energy efficiency.

[0009] [2] The positive electrode active material for a lithium ion secondary battery according to [1], wherein a ratio of the number of Mg atoms to the number of Ni atoms in the outer layer (Mg / Ni ratio) is 0.02 or more and 0.15 or less, and a ratio of the ratio of the number of Mg atoms to the number of Ni atoms in the outer layer (Mg / Ni ratio) to the ratio of the number of Mg atoms to the number of Ni atoms in the entire particle (chemical composition of the particle) (Mg / Ni ratio) is 1.0 or more and 5.0 or less.

[0010] In the positive electrode active material according to [2], the Mg / Ni ratio in the outer layer and the ratio between the Mg / Ni ratio in the outer layer and the Mg / Ni ratio in the entire particle satisfy specific numerical ranges. This suppresses oxygen release from the positive electrode active material during initial charging, further increasing discharge capacity. In addition, the capacity retention rate is further improved. This contributes to further energy efficiency.

[0011] [3] The positive electrode active material for a lithium ion secondary battery according to [1] or [2], wherein the positive electrode active material contains Ti, the ratio of the number of Ti atoms to the number of Ni atoms in the outer layer (Ti / Ni ratio) is 0.02 or more and 0.25 or less, and the ratio of the number of Ti atoms to the number of Ni atoms in the outer layer (Ti / Ni ratio) to the number of Ti atoms to the number of Ni atoms in the entire particle (chemical composition of the particle) (Ti / Ni ratio) is 1.0 or more and 20.0 or less.

[0012] The positive electrode active material according to [3] contains titanium (Ti), which has a smaller atomic weight than Ni and Mn. Therefore, even if the Ni content is reduced, the decrease in discharge capacity can be reduced or maintained at the same level, and the capacity retention rate can be further improved. This contributes to further energy efficiency.

[0013] [4] The positive electrode active material for a lithium ion secondary battery according to any one of [1] to [3], wherein in an X-ray diffraction pattern obtained using a Cu radiation source, the diffraction peaks of the 108 plane and the 110 plane in the space group R-3m are split, and the half-width of the diffraction peak of the 110 plane is 0.10° or more and 0.21° or less.

[0014] The positive electrode active material according to [4] has a split diffraction peak between the 108th and 110th planes, and the half-width of the 110th diffraction peak is 0.10° or more and 0.21° or less. This indicates that Ni, Mn, and Mg, or Ni, Mn, Mg, and Ti, are uniformly dispersed without phase separation. In other words, Ni, Mn, and Mg, or Ni, Mn, Mg, and Ti, are solid-dissolved in the lithium transition metal composite oxide without phase separation. This can contribute to further energy efficiency improvement.

[0015] [5] The positive electrode active material for a lithium ion secondary battery according to any one of [1] to [4], wherein the lithium transition metal composite oxide has, among lattice constants in the space group R-3m, an a-axis length of 2.881 Å to 2.893 Å, a c-axis length of 14.28 Å to 14.31 Å, and c / a of 4.948 to 4.958.

[0016] The positive electrode active material according to [5] has a lattice constant that satisfies a specific range. Therefore, the lithium transition metal composite oxide has low resistance and allows lithium ions to easily diffuse within the particles. This contributes to further energy efficiency.

[0017] [6] A lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode active material for lithium ion secondary batteries according to any one of [1] to [5].

[0018] In the lithium ion secondary battery according to [6], the positive electrode contains the above-mentioned positive electrode active material for lithium ion secondary batteries. This can further increase the discharge capacity and capacity retention rate, reduce the number of batteries required, and contribute to extending the battery life. In other words, it can contribute to energy efficiency.

[0019] [7] A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of [1] to [5], A method for producing a positive electrode active material for a lithium ion secondary battery, comprising a step of calcining a raw material mixture of a lithium compound, a magnesium compound, and a nickel manganese compound, or a raw material mixture of a lithium compound and a nickel manganese magnesium compound, at 650°C or higher and 950°C or lower for 10 minutes or longer and 6 hours or shorter.

[0020] By providing the above-mentioned pre-baking step, lithium can be sufficiently diffused into the particles of the metal composite hydroxide or metal composite oxide, and a more uniform lithium transition metal composite oxide can be obtained. As a result, the discharge capacity and capacity retention rate can be further increased, which can contribute to further energy efficiency.

[0021] [8] A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of [1] to [5], A method for producing a positive electrode active material for a lithium ion secondary battery, comprising a step of calcining a raw material mixture of a lithium compound, a magnesium compound, a titanium compound, and a nickel manganese compound, or a raw material mixture of a lithium compound and a nickel manganese magnesium titanium compound, at 650°C or higher and 950°C or lower for 10 minutes or longer and 6 hours or shorter.

[0022] By providing the above-mentioned pre-baking step, lithium can be sufficiently diffused into the particles of the metal composite hydroxide or metal composite oxide, and a more uniform lithium transition metal composite oxide can be obtained. As a result, the discharge capacity and capacity retention rate can be further increased, which can contribute to further energy efficiency.

[0023] [9] The method for producing a positive electrode active material for a lithium ion secondary battery according to [7] or [8], further comprising, after the pre-baking step, a step of main-baking the pre-baked raw material mixture at 1020°C or higher and 1120°C or lower for 10 minutes or longer and 4 hours or shorter.

[0024] By using a method including the above-mentioned main calcination step, it is possible to produce a positive electrode active material for a lithium ion secondary battery that satisfies the range of the chemical composition of the lithium transition metal composite oxide represented by formula (1) and the Mn / Ni ratio in the outer layer.

[0025]

[10] The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of [7] to [9], further comprising, after the main-calcination step, subsequently holding the obtained lithium transition metal composite oxide at 500°C or higher and 900°C or lower for 1 hour or higher and 20 hours or lower.

[0026] By further providing the above-mentioned holding step (slow cooling step), it is possible to suppress the decrease in the valence of Mn in the lithium transition metal composite oxide and to obtain a more stable structure. This further increases the discharge capacity and capacity retention rate, thereby contributing to further energy efficiency. [Effects of the Invention]

[0027] According to the positive electrode active material for a lithium ion secondary battery, the lithium ion secondary battery, and the method for producing the positive electrode active material for a lithium ion secondary battery of the present invention, the discharge capacity and the capacity retention rate can be further increased. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a cross-sectional view schematically showing a lithium ion secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing powder X-ray diffraction patterns of the lithium transition metal composite oxides of Examples 1 to 3 and Comparative Example 1. [Figure 3]1 is a graph showing charge / discharge curves of lithium ion secondary batteries using the lithium transition metal composite oxides of Examples 1 to 3 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0029] Preferred embodiments of the present invention will now be described in detail.

[0030] [Cathode active material] The positive electrode active material of this embodiment is primarily composed of a lithium transition metal composite oxide and is used in the positive electrode of a lithium ion secondary battery. The phrase "primarily composed of" a lithium transition metal composite oxide means that the content of the lithium transition metal composite oxide is 75% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, relative to the total mass of the positive electrode active material, and may even be 100% by mass. The positive electrode active material may contain components other than the primary component, as long as the functionality of the present invention is not impaired.

[0031] The positive electrode active material of this embodiment may contain only one type of lithium transition metal composite oxide, or may contain two or more types of lithium transition metal composite oxide, so long as the lithium transition metal composite oxide is the main component.

[0032] When a positive electrode active material is produced using a lithium transition metal composite oxide as the main component, the composition ratio of the entire lithium transition metal composite oxide (Li:Ni:Mn:Mg:Ti) is maintained in the resulting positive electrode active material. When a positive electrode active material obtained using a lithium transition metal composite oxide having such a composition as the main component is used in a secondary battery, a high capacity can be achieved. Furthermore, the composition ratio of the lithium transition metal composite oxide is adjusted to be the same as the composition ratio required for the positive electrode active material to be obtained.

[0033] <Lithium transition metal composite oxide> The lithium transition metal composite oxide of this embodiment has a layered rock salt structure and is in the form of particles having an outer layer on the surface.

[0034] In this specification, the average particle diameter of the particles of the lithium transition metal composite oxide (hereinafter, also simply referred to as "average particle diameter") is not particularly limited, but for example, 0.25 to 10 μm is preferable, 0.25 to 5.0 μm is more preferable, and 0.50 to 2.5 μm is even more preferable. When the average particle diameter is not less than the above lower limit value, the productivity of the positive electrode active material can be further enhanced. When the average particle diameter is not more than the above upper limit value, the electrochemical characteristics of the secondary battery can be made better. The average particle diameter means, for example, D50 measured by a laser diffraction particle size distribution measuring device or the like.

[0035] (Chemical composition) In a conventional lithium transition metal composite oxide (for example, LiNi 0.5 Mn 0.5 O2), when Li is dissolved in the transition element, Ni 2+ involved in the redox reaction is converted to Ni 3+ . Therefore, in order to increase the discharge capacity as the positive electrode active material, it is necessary to increase the amount of Ni used. The present invention is based on the finding that by adding Mg or Mg and Ti as constituent elements to LiNi 0.5 Mn 0.5 O2 and replacing a part of Ni 0.5 Mn 0.5 with Mg and Ti, it is possible to suppress an increase in the valence of Ni ions to 3 when replaced with Li, and to reduce the atomic weight of the transition metal element in the lithium transition metal composite oxide. Thereby, in the present invention, it is possible to reduce the amount of Ni used while maintaining good electrochemical characteristics of the positive electrode active material.

[0036] The lithium transition metal composite oxide of the present embodiment is represented by the following formula (1). Li m Ni w Mn x Mg y Ti z O2(1) In formula (1), m is 1.00 ≤ m ≤ 1.04, w is 0.47 < w < 0.59, x is 0.40 ≤ x < 0.50, y is (0 < y ≤ 0.04), z is 0 ≤ z < 0.04, and x ≤ w. m +w+x+y+z=2.

[0037] The lithium transition metal composite oxide of this embodiment is more preferably such that in formula (1), m is in the range of 1.01≦m≦1.04, w is in the range of 0.475≦w≦0.56, x is in the range of 0.40≦x≦0.48, y is in the range of 0.005≦y≦0.03, z is in the range of 0.005≦z≦0.03, and x≦w; +w +x+y+z=2.

[0038] The chemical composition of the lithium transition metal composite oxide of this embodiment can be determined by inductively coupled plasma (ICP) emission spectroscopy.

[0039] (Surface composition) The lithium transition metal composite oxide particles have an outer layer on the surface. In this specification, the "outer layer" refers to the region extending from the surface of the particle to a depth of 25 nm toward the interior of the particle. Note that when the particle diameter is less than 50 nm, the particle is considered to have a single-layer structure consisting of only the outer layer.

[0040] In the lithium transition metal composite oxide particles of this embodiment, the composition of the outer layer has a higher Mn content than the composition of the entire particle (chemical composition of the particle). The ratio of the number of Mn atoms to the number of Ni atoms in the outer layer of the lithium transition metal composite oxide of this embodiment (Mn / Ni ratio) is 1.0 to 1.5, preferably 1.0 to 1.4, and more preferably 1.0 to 1.3. When the Mn / Ni ratio is within the above range, the movement of lithium ions is not inhibited, and when used as a positive electrode active material, the charge / discharge capacity of the secondary battery is high.

[0041] The Mn / Ni ratio can be determined by quantitative analysis using X-ray photoelectron spectroscopy (XPS). XPS allows for the analysis of the composition of transition metal elements on the surface of the entire particle. In other words, the analysis results obtained by XPS do not indicate the local composition of the entire surface of a single particle, but rather the composition of the entire surface of the particle.

[0042] The ratio of the number of Mn atoms to the number of Ni atoms in the outer layer of the lithium transition metal composite oxide of this embodiment (Mn / Ni ratio) to the number of Mn atoms to the number of Ni atoms in the entire particle (chemical composition of the particle) (hereinafter also referred to as the "outer layer / total particle ratio of Mn / Ni") is preferably 1.0 to 2.0, more preferably 1.0 to 1.8, and even more preferably 1.0 to 1.5. When the outer layer / total particle ratio of Mn / Ni is within the above range, the surface becomes manganese-rich, and the discharge capacity can be further increased even if the Ni content is reduced. The ratio of Mn / Ni in the outer layer to the entire particle can be determined by quantitative analysis using XPS.

[0043] The ratio of the number of Mg atoms to the number of Ni atoms (Mg / Ni ratio) in the outer layer of the lithium transition metal composite oxide of this embodiment is preferably 0.02 or more and 0.15 or less, more preferably 0.02 or more and 0.12 or less, and even more preferably 0.02 or more and 0.11 or less. When the Mg / Ni ratio is within the above range, oxygen release from the positive electrode active material during initial charging can be suppressed. This further increases the discharge capacity of the secondary battery. The Mg / Ni ratio can be determined by quantitative analysis using XPS.

[0044] The ratio of the number of Mg atoms to the number of Ni atoms in the outer layer of the lithium transition metal composite oxide of this embodiment (Mg / Ni ratio) to the number of Mg atoms to the number of Ni atoms in the entire particle (chemical composition of the particle) (Mg / Ni ratio) (hereinafter also referred to as the "outer layer / total particle ratio of Mg / Ni") is preferably 1.0 to 5.0, more preferably 1.0 to 4.5, and even more preferably 1.0 to 4.0. When the outer layer / total particle ratio of Mg / Ni is within the above range, the surface becomes magnesium-rich, which can suppress an increase in the Ni valence and further increase the discharge capacity. The ratio of Mg / Ni in the outer layer to the entire particle can be determined by quantitative analysis using XPS.

[0045] The lithium transition metal composite oxide of this embodiment preferably contains Ti. By containing titanium (Ti), which has a smaller atomic weight than Ni and Mn, the atomic weight of the transition metal element in the lithium transition metal composite oxide can be further reduced. When the lithium transition metal composite oxide of this embodiment contains Ti, the ratio of the number of Ti atoms to the number of Ni atoms in the outer layer of the lithium transition metal composite oxide (Ti / Ni ratio) is preferably 0.02 to 0.25, more preferably 0.02 to 0.20, and even more preferably 0.02 to 0.18. When the Ti / Ni ratio is within the above range, an increase in the Ni valence can be suppressed, and the atomic weight of the transition metal element in the lithium transition metal composite oxide can be reduced. As a result, the discharge capacity of the secondary battery can be further increased. The Ti / Ni ratio can be determined by quantitative analysis using XPS.

[0046] When the lithium transition metal composite oxide of this embodiment contains Ti, the ratio of the number of Ti atoms to the number of Ni atoms in the outer layer of the lithium transition metal composite oxide (Ti / Ni ratio) to the number of Ti atoms to the number of Ni atoms in the entire particle (chemical composition of the particle) (hereinafter also referred to as the "outer layer / total particle ratio of Ti / Ni") is preferably 1.0 to 20.0, more preferably 1.0 to 18.0, and even more preferably 1.0 to 15.0. When the outer layer / total particle ratio of Ti / Ni is within the above numerical range, the surface becomes titanium-rich, which can suppress an increase in the Ni valence and further increase the discharge capacity. The outer layer / total particle ratio of Ti / Ni can be determined by quantitative analysis using XPS.

[0047] The lithium transition metal composite oxide particles of this embodiment may be primary particles or secondary particles, but are preferably secondary particles formed by agglomeration of a plurality of primary particles, as this results in relatively dense particles.

[0048] (lattice constant) The lithium transition metal composite oxide of this embodiment is a rhombohedral layered compound and has a crystal structure of space group R-3m. Of the lattice constants of the lithium transition metal composite oxide, the a-axis length is preferably 2.881 Å to 2.893 Å. The c-axis length is preferably 14.28 Å to 14.31 Å. The ratio expressed as c-axis length / a-axis length (hereinafter also referred to as "c / a") is preferably 4.948 to 4.958. With the lattice constants in the above ranges, the lithium transition metal composite oxide allows lithium ions to easily diffuse within the primary particles, resulting in low resistance. The lattice constant of the crystal can be determined by measuring the X-ray diffraction pattern of the lithium transition metal composite oxide and then using each index and its interplanar spacing by the least squares method.

[0049] (X-ray diffraction (XRD) pattern) In the X-ray diffraction (XRD) pattern of the positive electrode active material of this embodiment, the diffraction peaks of the 108th and 110th planes in the space group R-3m are split, and the half-width of the 110th plane diffraction peak is preferably 0.10° or more and 0.21° or less. The splitting of the diffraction peaks of the 108th and 110th planes in the XRD pattern indicates that Ni, Mn, and Mg, or Ni, Mn, Mg, and Ti, in the lithium transition metal composite oxide are uniformly dispersed without phase separation. This means that the lithium transition metal composite oxide of this embodiment certainly has the chemical composition represented by formula (1). In this specification, the term "a split diffraction peak" means that the diffraction peak has two or more peaks. The XRD pattern of the positive electrode active material of this embodiment can be determined by the method shown in the Examples, for example, using Cu (copper) as a target for electron beam irradiation and Kα rays as characteristic X-rays.

[0050] In an X-ray diffraction pattern using a Cu radiation source, the half-width of the diffraction peak of the 110 plane in the space group R-3m is preferably 0.10° to 0.21°, more preferably 0.10° to 0.18°. The fact that the half-width of the diffraction peak of the 110 plane in the space group R-3m is within the above numerical range means that the diffraction peak of the 110 plane in the space group R-3m is not tailing. The half-width of the diffraction peak can be determined by analyzing the XRD pattern.

[0051] [Method of manufacturing positive electrode active material] The positive electrode active material of this embodiment contains the above-mentioned lithium transition metal composite oxide as a main component. The lithium source for the lithium transition metal composite oxide can be any known compound, including hydroxides such as lithium hydroxide monohydrate (LiOH·H2O), carbonates such as lithium carbonate (Li2CO3), and acetates such as lithium acetate (CHClCOOLi) and lithium acetate dihydrate (CHClCOOLi·2H2O), and is not particularly limited. The transition metals nickel, manganese, magnesium, and titanium can be selected from a wide variety of known oxides, hydroxides, or metal salts of nickel, manganese, magnesium, and titanium, and are not particularly limited. For example, nickel compounds that can be used include, but are not limited to, nickel hydroxide (Ni(OH)2), nickel(II) chloride (NiCl2), nickel(II) chloride hexahydrate (NiCl2·6H2O), and nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O). Examples of manganese compounds that can be used include, but are not limited to, manganese(II) chloride (MnCl2), manganese(II) chloride tetrahydrate (MnCl2·4H2O), manganese carbonate hexahydrate (MnCO3·6H2O), and manganese(II) nitrate hexahydrate (Mn(NO3)2·6H2O). Examples of magnesium compounds that can be used include, but are not limited to, magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium chloride hexahydrate (MgCl2·6H2O), magnesium carbonate (MgCO3), and magnesium sulfate heptahydrate (MgSO4·7H2O). Examples of titanium compounds that can be used include, but are not limited to, titanium oxide (IV) (TiO2), titanium sulfate (IV) (Ti(SO4)2), and the like. The above transition metal compounds can be used alone or in the form of composite hydroxides (for example, nickel-manganese-magnesium composite hydroxide) obtained by coprecipitation or the like.

[0052] The lithium transition metal composite oxide of this embodiment can be synthesized using a known method. For example, a composite hydroxide or composite oxide of a nickel compound and a manganese compound is prepared as an intermediate compound, and this intermediate compound is mixed with a magnesium compound and a lithium compound to form a raw material mixture, and this raw material mixture is heat-treated (e.g., calcined) in a predetermined atmosphere at a predetermined temperature for a predetermined time. Alternatively, the lithium transition metal composite oxide can be synthesized by preparing a composite hydroxide or composite oxide of a nickel compound, a manganese compound, and a magnesium compound as an intermediate compound, and this intermediate compound is mixed with a lithium compound to form a raw material mixture, and this raw material mixture is heat-treated (e.g., calcined) in a predetermined atmosphere at a predetermined temperature for a predetermined time. Alternatively, a titanium compound may be mixed with these intermediate compounds, and a composite hydroxide or composite oxide of a nickel compound, a manganese compound, a magnesium compound, and a titanium compound may be prepared as the intermediate compound.

[0053] The raw material mixture is preferably pre-calcined (hereinafter also referred to as "calcination") at a temperature lower than that of the heat treatment before the heat treatment. By performing the calcination, lithium is sufficiently diffused into the particles of the metal composite hydroxide or metal composite oxide, and a more uniform lithium transition metal composite oxide can be obtained.

[0054] The lithium transition metal composite oxide obtained by the heat treatment is preferably further subjected to a slow cooling step, in which the oxide is maintained at a predetermined temperature range for a predetermined time. The slow cooling conditions vary depending on the treatment conditions (e.g., the heat treatment atmosphere, such as an oxygen atmosphere or an air atmosphere), and are therefore preferably adjusted appropriately. The inventors have discovered that by appropriately selecting the heat treatment conditions and the slow cooling conditions, the transition metals in the lithium transition metal composite oxide can be uniformly dispersed, thereby increasing the Mn / Ni ratio on the particle surface. The method for producing the positive electrode active material of this embodiment will be described in more detail below with reference to examples.

[0055] <Final firing process> The firing step is a step of firing the raw material mixture at 1020° C. or higher and 1120° C. or lower for 10 minutes or longer and 4 hours or shorter. In the firing step, first, a predetermined amount of magnesium compound and a predetermined amount of lithium compound are added to the intermediate nickel-manganese compound, and the mixture is dispersed in a solvent such as ethanol. It should be noted that a predetermined amount of the intermediate compound, a predetermined amount of magnesium compound, and a predetermined amount of lithium compound may be mixed by dry mixing without using a solvent, in addition to wet mixing using a solvent. For example, magnesium oxide (MgO) is used as the magnesium compound, and lithium carbonate (Li2CO3) is used as the lithium compound, and Li 1.02 Ni 0.48 Mn 0.48 Mg 0.020 When synthesizing O2, Li2CO3 is preferably weighed out in an amount 1 mass % to 5 mass %, for example, 4 mass % more than the stoichiometric ratio.

[0056] The nickel-manganese compound can be synthesized by a known method. When the nickel-manganese compound is a hydroxide, for example, nickel sulfate hexahydrate (NiSO4·6H2O) and manganese sulfate pentahydrate (MnSO4·5H2O) are weighed out so that the molar ratio of Ni:Mn is 1:1, and pure water is added to dissolve the mixture. Then, an alkaline aqueous solution is added dropwise to the resulting sulfate solution, thereby coprecipitating the nickel-manganese composite hydroxide.

[0057] Instead of using the nickel manganese compound as the intermediate compound, a nickel manganese magnesium compound may be used. For example, a predetermined amount of a lithium compound is added to the nickel manganese magnesium compound, and the mixture is dispersed in a solvent such as ethanol and mixed. In addition to wet mixing using a solvent, a predetermined amount of the nickel manganese magnesium compound and a predetermined amount of a lithium compound may be mixed by dry mixing without using a solvent. For example, lithium carbonate (Li2CO3) is used as the lithium compound, and Li 1.02 Ni 0.48 Mn 0.48 Mg 0.020 When synthesizing O2, Li2CO3 is preferably weighed out in an amount 1 mass % to 5 mass %, for example, 4 mass % more than the stoichiometric ratio.

[0058] The nickel manganese magnesium compound can be synthesized by a known method. When the nickel manganese magnesium compound is a hydroxide, for example, nickel sulfate hexahydrate (NiSO4·6H2O), manganese sulfate pentahydrate (MnSO4·5H2O), and magnesium sulfate heptahydrate (MgSO4·7H2O) are weighed out so that the molar ratio of Ni:Mn:Mg is 48:48:2, and pure water is added to dissolve the mixture. Then, an alkaline aqueous solution is added dropwise to the resulting sulfate solution, whereby the nickel manganese magnesium composite hydroxide can be coprecipitated.

[0059] In addition to the lithium compound and magnesium compound, a titanium compound may also be used as the intermediate compound. Alternatively, a nickel manganese magnesium titanium compound may be used in place of the nickel manganese magnesium compound. The nickel manganese magnesium titanium compound can be synthesized in the same manner as the nickel manganese magnesium compound, for example, by using titanium (IV) sulfate (Ti(SO4)2) as the titanium compound.

[0060] The precursors, such as a raw material mixture of a lithium compound, a magnesium compound, and a nickel-manganese compound, a raw material mixture of a lithium compound and a nickel-manganese-magnesium compound, a raw material mixture of a lithium compound, a magnesium compound, a titanium compound, and a nickel-manganese compound, or a raw material mixture of a lithium compound and a nickel-manganese-magnesium-titanium compound, are crushed to a desired size, mixed, and then filled into a crucible, for example, and the raw material mixture is fired (hereinafter also referred to as "main firing"). Examples of crucibles that can be used include an alumina rectangular sagger, an alumina crucible, a platinum crucible, and a gold crucible. For the main firing of the raw material mixture, a firing furnace or a roller hearth kiln is used.

[0061] The raw material mixture placed in a sagger or crucible is heated at a temperature increase rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min, until the firing temperature is reached. The firing atmosphere is not particularly limited, and examples include air (air atmosphere) and oxygen flow. The firing atmosphere is preferably oxygen flow.

[0062] The firing temperature is preferably from 1020° C. to 1120° C., more preferably from 1045° C. to 1095° C. The firing time is preferably from 10 minutes to 4 hours, more preferably from 20 minutes to 3 hours, even more preferably from 30 minutes to 2 hours, and particularly preferably from 30 minutes to 1 hour. The firing time can be appropriately set depending on the firing temperature. The firing time means the time during which the firing temperature is maintained.

[0063] <Pre-firing process> The pre-firing step is a step of firing the raw material mixture at a temperature lower than the firing temperature, ie, 650° C. to 950° C., for 10 minutes to 6 hours. In the pre-firing step, the same firing furnace or roller hearth kiln as in the main firing step can be used. The firing atmosphere in the pre-firing step is not particularly limited, and may be the same as the firing atmosphere in the main firing step.

[0064] The firing temperature in the pre-firing step is preferably 650°C or higher and 950°C or lower, and more preferably 750°C or higher and 850°C or lower. The calcination time in the pre-calcination step is preferably 10 minutes to 6 hours, more preferably 20 minutes to 4 hours, even more preferably 30 minutes to 2 hours, and particularly preferably 30 minutes to 1 hour.

[0065] <Slow cooling process> The slow cooling step is a step following the main firing step in which the obtained lithium transition metal composite oxide is held at 500° C. or higher and 900° C. or lower for 1 hour or higher and 20 hours or lower. In the slow cooling step, the powder obtained after the firing step is cooled to 500 to 900°C, for example, at a temperature drop rate of 5 to 25°C / min, preferably 10 to 25°C / min, and then the powder is held at 500 to 900°C for 1 to 20 hours. The atmosphere in which the powder is held at 500 to 900°C is not particularly limited, and examples include the atmosphere (air atmosphere), air flow, and oxygen flow.

[0066] The temperature at which the powder is maintained in the slow cooling step is preferably 500°C or higher and 900°C or lower, and more preferably 600°C or higher and 800°C or lower. The time for which the powder is held in the slow cooling step is preferably 1 hour or more and 20 hours or less, more preferably 2 hours or more and 15 hours or less, and even more preferably 3 hours or more and 10 hours or less.

[0067] The slow cooling step may be performed two or more times as long as the temperature is gradually decreased. By appropriately selecting the slow cooling conditions, the positive electrode active material of this embodiment can further increase the Mn / Ni ratio on the particle surfaces of the lithium transition metal composite oxide.

[0068] [Lithium-ion secondary battery] The lithium ion secondary battery (hereinafter simply referred to as "secondary battery") of this embodiment includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains a positive electrode active material mainly composed of the above-mentioned lithium transition metal composite oxide. The secondary battery of this embodiment may include other battery components as necessary.

[0069] The secondary battery of this embodiment can use the same battery elements as known secondary batteries, except that the positive electrode contains a positive electrode active material mainly composed of the lithium transition metal composite oxide described above. The secondary battery of this embodiment may be of any type, such as a coin type, button type, cylindrical type, prismatic type, or laminate type. Furthermore, the secondary battery of this embodiment can be used in a wide range of applications, such as mobile devices such as mobile phones and laptops, and in-vehicle applications.

[0070] Hereinafter, the secondary battery of this embodiment will be described as a secondary battery using an electrolytic solution (coin-type secondary battery). Each battery element described below can be similarly applied to an all-solid-state secondary battery that does not use an electrolytic solution.

[0071] Fig. 1 is a cross-sectional view schematically showing a secondary battery according to this embodiment. Fig. 1 shows an example in which the secondary battery according to this embodiment is a coin-type secondary battery. As shown in Fig. 1, the secondary battery 1 according to this embodiment includes a negative electrode can (negative electrode terminal) 20, a negative electrode 3, a separator 4 impregnated with an electrolyte, an insulating packing (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0072] The positive electrode can 10 is disposed below the separator 4, and the negative electrode can 20 is disposed above the separator 4, with the positive electrode can 10 and the negative electrode can 20 forming the outer shape of the secondary battery 1. A positive electrode 2 and a negative electrode 3 are provided between the positive electrode can 10 and the negative electrode can 20 via a separator 4 impregnated with an electrolyte, and the positive electrode 2 and the negative electrode 3 are separated by the separator 4. The positive electrode can 10 and the negative electrode can 20 are electrically insulated by an insulating packing 5.

[0073] The secondary battery 1 can be manufactured by preparing a positive electrode mixture by blending a conductive agent, a binder, etc. with the positive electrode active material of this embodiment as needed, and then pressing this onto a current collector (not shown) to produce a positive electrode 2. The current collector can preferably be a stainless steel mesh, aluminum foil, etc. The conductive agent can preferably be acetylene black, Ketchen black, etc. The binder can preferably be tetrafluoroethylene, polyvinylidene fluoride, etc.

[0074] The blending ratio of the positive electrode active material, conductive agent, and binder in the positive electrode mixture is not particularly limited. The content of the conductive agent in the positive electrode mixture is preferably 1% by mass to 15% by mass, more preferably 0.1% by mass to 5% by mass. The content of the binder in the positive electrode mixture is preferably 0.1% by mass to 10% by mass, more preferably 0.1% by mass to 5% by mass. The positive electrode active material, conductive agent, and binder are preferably blended so that the remainder of the positive electrode mixture (the portion other than the positive electrode active material and conductive agent) becomes the positive electrode active material.

[0075] In the secondary battery 1, the negative electrode 3 for the positive electrode 2 may be made of any known material that functions as a negative electrode active material and can absorb and release lithium, such as metal-based materials such as metallic lithium and lithium alloys, carbon-based materials such as graphite and MCMB (mesocarbon microbeads), and silicon-based materials such as silicon (Si), Si alloys, and silicon oxide.

[0076] The separator 4 and the battery container (positive electrode can 10, negative electrode can 20) can be made of known battery elements.

[0077] The electrolyte may be a known electrolytic solution, solid electrolyte, etc. For example, the electrolytic solution may be one obtained by dissolving an electrolyte such as lithium perchlorate or lithium hexafluorophosphate in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC).

[0078] Furthermore, the all-solid-state secondary battery can also have the same structure as a known all-solid-state lithium ion secondary battery, except that it uses a positive electrode active material containing the above-mentioned lithium transition metal composite oxide as a main component.

[0079] In the case of an all-solid-state secondary battery, the electrolyte may be, for example, a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, a sulfide-based solid electrolyte, or an oxide-based solid electrolyte.

[0080] For the positive electrode of the all-solid-state secondary battery, for example, a positive electrode mixture containing the above-mentioned positive electrode active material, conductive agent, and binder as well as a solid electrolyte can be supported on a positive electrode current collector made of aluminum, nickel, stainless steel, or the like.

[0081] In the secondary battery 1 of this embodiment, the positive electrode 2 contains a positive electrode active material containing the above-mentioned lithium transition metal composite oxide as a main component, and therefore the discharge capacity and capacity retention rate can be further increased. [Example]

[0082] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0083] [Example 1] (Synthesis of lithium nickel manganese magnesium titanium composite oxide: Li 1.00 Ni 0.48 Mn 0.48 Mg 0.027 Ti 0.013 O2) Li2CO3 (Kojundo Kagaku), Ni 0.5 Mn 0.5(OH)2, MgO (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and TiO2 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were weighed out so that the molar ratio of Li:Ni:Mn:Mg:Ti was 1.00:0.48:0.48:0.027:0.013, and taking into account the evaporation of Li, Li2CO3 was weighed out so that there was 4% more by mass than the stoichiometric ratio. 0.5 Mn 0.5 The total mass of (OH)2, MgO, and TiO2 was 50 g. These were dispersed in ethanol in a mortar and mixed. Then, the mixture was filled into an alumina rectangular sagger. Using a firing furnace, the raw material mixture filled into a platinum crucible was heated in air at a heating rate of 10°C / min and pre-fired at 775°C for 60 minutes. The resulting powder was then left to cool to room temperature (25°C). The resulting powder was then heated at a heating rate of 10°C / min and pre-fired at 1070°C for 30 minutes. The powder after the main firing was cooled at a temperature decreasing rate of 10°C / min to reach 750°C, and then the powder was held in air at 750°C for 5 hours. Next, the powder was cooled at a temperature decreasing rate of 10°C / min to reach 650°C, and then the cooled powder was held in air at 550°C for 5 hours. Thereafter, the powder was left to stand until its temperature reached room temperature (25°C), and the lithium nickel manganese magnesium titanium composite oxide of Example 1 was obtained.

[0084] [Example 2] (Synthesis of lithium nickel manganese magnesium titanium composite oxide: Li 1.02 Ni 0.56 Mn 0.40 Mg 0.010 Ti 0.010 O2) The lithium nickel manganese magnesium titanium composite oxide of Example 2 was obtained in the same manner as in Example 1, except that the compounds were weighed out so that the molar ratios thereof were as shown in Table 1 and the firing conditions were as shown in Table 1.

[0085] [Example 3] (Synthesis of lithium nickel manganese magnesium composite oxide: Li 1.02 Ni 0.48 Mn 0.48 Mg0.020 O2) A lithium nickel manganese magnesium composite oxide of Example 3 was obtained in the same manner as in Example 1, except that the compounds were weighed out so that the molar ratios shown in Table 1 were achieved.

[0086] [Comparative Example 1] (Synthesis of lithium nickel manganese titanium composite oxide: Li 1.02 Ni 0.48 Mn 0.48 Ti 0.020 O2) The lithium nickel manganese titanium composite oxide of Comparative Example 1 was obtained in the same manner as in Example 1, except that the compounds were weighed out so that the molar ratios thereof were as shown in Table 1 and the firing conditions were as shown in Table 1.

[0087] (analysis) The chemical compositions of the samples obtained in Examples 1 to 3 and Comparative Example 1 were analyzed using an ICP optical emission spectrometer (trade name: Agilent 5110 VDV, manufactured by Agilent Technologies). The results are shown in Table 1. In the table, "-" indicates that the element in question was not contained.

[0088] The X-ray diffraction (XRD) patterns of the samples obtained in Examples 1 to 3 and Comparative Example 1 were measured using a powder X-ray diffractometer (trade name: SmartLab, manufactured by Rigaku). Cu (copper) was used as the target for electron beam irradiation, and Kα rays were used as the characteristic X-rays. The lattice constants were calculated by the least squares method using the indices and interplanar spacings of the obtained XRD patterns. The space group of the obtained samples was set to R-3m, and the lattice constants were calculated. The powder X-ray diffraction patterns are shown in Figure 2. The values ​​of the lattice constants are shown in Table 1.

[0089] X-ray photoelectron spectroscopy (XPS) analyzer (product name: K-Alpha +The compositions of the outer layers of the samples obtained in Examples 1 to 3 and Comparative Example 1 were analyzed by quantitative analysis using a Thermo Fisher Scientific (manufactured by Thermo Fisher Scientific). The Mn / Ni ratio, Mg / Ni ratio, and Ti / Ni ratio for the entire particle (center and outer layer), as well as the Mn / Ni ratio, Mg / Ni ratio, and Ti / Ni ratio for the outer layer, are shown in Table 1. In the table, "-" indicates that Mg or Ti is not contained. The measurement conditions for XPS measurement are shown below. <XPS measurement conditions> Model used: Thermo Fisher Scientific, K-Alpha + (Product name) Irradiation X-ray: Single crystal spectrometer AlKα (12 keV, 72 W) X-ray spot diameter: 400 μm Neutralization electron gun: used Reference spectrum: CC, CH 284.6 eV Detection depth: 6 to 7 nm

[0090] [Table 1]

[0091] 2, it was confirmed that the two diffraction peaks of the 108 and 110 planes in the space group R-3m were both split in the samples obtained in Examples 1 to 3 and Comparative Example 1. This indicates that Ni, Mn, and Mg, or Ni, Mn, Mg, and Ti in the lithium transition metal composite oxides were uniformly dispersed without phase separation in the samples obtained in Examples 1 to 3 and Comparative Example 1.

[0092] [Fabrication of lithium-ion secondary batteries] The lithium nickel manganese magnesium titanium composite oxides of Examples 1 and 2, the lithium nickel manganese magnesium composite oxide of Example 3, and the lithium nickel manganese titanium composite oxide of Comparative Example 1 were used as positive electrode active materials, and acetylene black (AB) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 96:2:2 using NMP (N-methyl-2-pyrrolidone) as a solvent to prepare a slurry. The slurry was then coated onto a 15 μm thick aluminum foil and dried to prepare a 16.5φ positive electrode. The coating area density was 10.0 mg / cm. 2 , volume density is 2.8g / cm 3 A 200 μm-thick, 18-diameter lithium metal counter electrode and a 20 μm-thick, 19.4-diameter polyethylene microporous membrane were used as the separator. The electrolyte was a 1.2 mol / L solution of lithium hexafluorophosphate (LiPF6) dissolved in a mixed solvent (volume ratio 3:4:3) of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). A lithium-ion secondary battery (2032 coin-type cell) with the structure shown in Figure 1 was fabricated. The battery was fabricated according to known cell construction and assembly methods.

[0093] [Charge / discharge test] For each of the fabricated lithium-ion secondary batteries, a charge-discharge test was performed at a temperature of 25°C, a constant current of 0.05C (1C: 250 mA / g), a current density of 12.5 mA / g, and a cutoff potential of 4.6 V to 2.75 V, to evaluate the initial discharge capacity. The charge-discharge test began with charging. The initial discharge capacity values ​​are shown in Table 1, and the charge-discharge curves are shown in Figure 3.

[0094] FIG. 3 shows the voltage change during discharge, where the cell voltage decreases as the capacity increases, and the voltage change during charge, where the cell voltage increases as the capacity increases. As shown in Figure 3, the lithium-ion secondary batteries of each example had the initial discharge capacity shown in Table 1, and were found to have higher capacities than conventional lithium-ion secondary batteries. This is presumably because adjusting the composition of the lithium transition metal composite oxide and the firing conditions in the firing step makes the structure of the lithium transition metal composite oxide more stable, suppresses the elution of Ni into the electrolyte, and increases the Mn / Ni ratio in the outer layer of the lithium transition metal composite oxide particles.

[0095] [Cycle test] After evaluating the initial discharge capacity, each lithium-ion secondary battery was subjected to a cycle test 50 times at a temperature of 25°C, a current density of 50.0 mA / g, and a cutoff potential of 4.3 V to 2.75 V, and the capacity retention rate (discharge capacity at the 50th cycle / discharge capacity at the 1st cycle) was evaluated. The charge / discharge test began with charging. The results are shown in Table 1. The higher the capacity retention rate, the better the cycle characteristics.

[0096] As shown in Table 1, the lithium ion secondary batteries of Examples 1 to 3 to which the present invention was applied exhibited a high capacity retention rate of 90% or more. In contrast, the capacity retention rate of the lithium ion secondary battery of Comparative Example 1, which did not contain magnesium in its chemical composition and did not contain the lithium transition metal composite oxide represented by formula (1), was 83%.

[0097] From the above results, it was found that the present invention can provide a positive electrode active material for a lithium ion secondary battery that can further increase the discharge capacity and capacity retention rate, and a lithium ion secondary battery that includes the positive electrode active material. [Explanation of symbols]

[0098] 1...Lithium-ion secondary battery (secondary battery) 2...Positive electrode 3...Negative electrode 4...Separator 5...Insulating packing (gasket) 10...Positive electrode can 20...Negative electrode can (negative electrode terminal)

Claims

1. A positive electrode active material for a lithium ion secondary battery containing a lithium transition metal composite oxide as a main component, the lithium transition metal composite oxide is in the form of particles having an outer layer on the surface thereof, The lithium transition metal composite oxide is represented by the following formula (1): From ｍ Yes ｗ MỎ ｘ Mẽ ｙ Till ｚ Oh ２ (1) (In formula (1), m is in the range of 1.00≦m≦1.04, w is in the range of 0.47<w<0.59, x is in the range of 0.40≦x<0.50, y is in the range of 0<y≦0.04, z is in the range of 0≦z<0.04, x≦w, and m+w+x+y+z=2.) a ratio of the number of Mn atoms to the number of Ni atoms in the outer layer (Mn / Ni ratio) of 1.0 or more and 1.5 or less; a ratio of the number of Mn atoms to the number of Ni atoms in the outer layer to the number of Mn atoms to the number of Ni atoms in the entire particle (outer layer / entire particle ratio of Mn / Ni ratio) of 1.03 or more and 2.0 or less.

2. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein a ratio of the number of Mg atoms to the number of Ni atoms in the outer layer (Mg / Ni ratio) is 0.02 or more and 0.15 or less, and a ratio of the ratio of the number of Mg atoms to the number of Ni atoms in the outer layer (Mg / Ni ratio) to the ratio of the number of Mg atoms to the number of Ni atoms in the entire particle (Mg / Ni ratio) is 1.0 or more and 5.0 or less.

3. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material contains Ti, and the ratio of the number of Ti atoms to the number of Ni atoms in the outer layer (Ti / Ni ratio) is 0.02 or more and 0.25 or less, and the ratio of the number of Ti atoms to the number of Ni atoms in the outer layer (Ti / Ni ratio) to the number of Ti atoms to the number of Ni atoms in the entire particle (Ti / Ni ratio) is 1.0 or more and 20.0 or less.

4. 2. The positive electrode active material for lithium ion secondary batteries according to claim 1, wherein in an X-ray diffraction pattern using a Cu radiation source, the diffraction peaks of the 108 plane and the 110 plane in the space group R-3m are split, and the half width of the diffraction peak of the 110 plane is 0.10° or more and 0.21° or less.

5. The lithium transition metal composite oxide has a lattice constant in the space group R-3m of an a-axis length of 2.881 Å to 2.893 Å, a c-axis length of 14.28 Å to 14.31 Å, and c / a of 4.948 to 4.

958. The positive electrode active material for a lithium ion secondary battery according to claim 1.

6. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode active material for lithium ion secondary batteries according to any one of claims 1 to 4.

7. A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, A method for producing a positive electrode active material for a lithium ion secondary battery, the method comprising: calcining a raw material mixture of a lithium compound, a magnesium compound, and a nickel-manganese compound, or a raw material mixture of a lithium compound and a nickel-manganese-magnesium compound, at 650°C or higher and 950°C or lower for 10 minutes to 6 hours.

8. A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, A method for producing a positive electrode active material for a lithium ion secondary battery, the method comprising the step of calcining a raw material mixture of a lithium compound, a magnesium compound, a titanium compound, and a nickel manganese compound, or a raw material mixture of a lithium compound and a nickel manganese magnesium titanium compound, at 650°C or higher and 950°C or lower for 10 minutes to 6 hours.

9. 8. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 7, further comprising, after the pre-baking step, subsequently performing a main baking step of the pre-baked raw material mixture at 1020°C or higher and 1120°C or lower for 10 minutes or higher and 4 hours or shorter.

10. 9. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 8, further comprising, after the pre-baking step, subsequently performing a main baking step of the pre-baked raw material mixture at 1020°C or higher and 1120°C or lower for 10 minutes or higher and 4 hours or shorter.

11. 10. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9, further comprising, after the main firing step, subsequently holding the obtained lithium transition metal composite oxide at 500°C or higher and 900°C or lower for 1 hour or higher and 20 hours or lower.

12. 11. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 10, further comprising, after the main-baking step, subsequently holding the obtained lithium transition metal composite oxide at 500°C or higher and 900°C or lower for 1 hour or higher and 20 hours or lower.

13. A positive electrode active material for a lithium ion secondary battery, comprising a lithium transition metal composite oxide as a main component, the lithium transition metal composite oxide is in the form of particles having an outer layer on the surface thereof, The lithium transition metal composite oxide is represented by the following formula (1): Li m Ni w Mn x Mg y Tiz O 2 (1) (In formula (1), m is in the range of 1.00≦m≦1.04, w is in the range of 0.47<w<0.59, x is in the range of 0.40≦x<0.50, y is in the range of 0<y≦0.04, z is in the range of 0≦z<0.04, x≦w, and m+w+x+y+z=2.) a ratio of the number of Mn atoms to the number of Ni atoms in the outer layer (Mn / Ni ratio) of 1.0 or more and 1.5 or less; a ratio of the number of Ti atoms to the number of Ni atoms in the outer layer (Ti / Ni ratio) of 0.02 or more and 0.25 or less, and a ratio of the number of Ti atoms to the number of Ni atoms in the outer layer (Ti / Ni ratio) to the number of Ti atoms to the number of Ni atoms in the entire particle (Ti / Ni ratio) of 1.0 or more and 20.0 or less.

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