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

The purpose of the present invention is to provide a positive electrode active material for a lithium ion secondary battery, 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, with which it is possible to obtain a lithium ion secondary battery having a further improved discharge capacity and capacity retention rate. A positive electrode active material for a lithium ion secondary battery, the positive electrode active material having a lithium transition metal composite oxide as a main component, the lithium transition metal composite oxide being in the form of particles having an outer layer on the surface, the lithium transition metal composite oxide being represented by formula (1): LimNiwMnxMgyTizO2 (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, and x is greater than or equal to 1 and less than or equal to 1). In the outer layer, 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 and in the range of x < = w, m + w + x + y + z = 2, and the ratio of the number of Mn atoms to the number of Ni atoms (Mn / Ni ratio) in the outer layer is 1.0-1.5.

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 the positive electrode active material for a lithium ion secondary battery. Background Art

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

[0003] Positive electrode active materials are attracting attention as a key component that determines the capacity of lithium-ion secondary batteries, and their development is progressing. For example, lithium nickel-manganese composite oxides have been reported as positive electrode active materials for use in lithium-ion secondary batteries, where the nickel and manganese in the composite oxide are partially replaced with titanium or magnesium (e.g., Non-Patent Document 1).

[0004] Prior art literature Non-patent literature Non-patent literature 1: S.-H. Kang, et al., "Comparative study of Li (Ni 0.5-x Mn 0.5-x M 2x ') O2 (M'=Mg, Al, Co, Ni, Ti; Summary of the Invention

[0005] Technical problem to be solved by the invention In Non-Patent Document 1, the discharge capacity at 4.3V to 2.8V is 130mAhg -1 In addition, lithium-ion secondary batteries are required to maintain a constant capacity even after repeated charge and discharge (improve capacity retention).

[0006] The present invention was completed to solve the above-mentioned problems, and its purpose is to obtain a positive electrode active material for a lithium ion secondary battery that can further improve the discharge capacity and capacity retention rate of a lithium ion secondary battery, 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. Moreover, it further contributes to the efficiency of energy.

[0007] Technical solutions to technical problems In order to achieve the above-mentioned object, the present invention provides the following means.

[0008] [1] A positive electrode active material for a lithium ion secondary battery, comprising a lithium transition metal composite oxide as a main component, wherein: 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 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, and is in the range of x≤w, m+w+x+y+z=2, 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.

[0009] The positive electrode active material for lithium-ion secondary batteries (hereinafter also referred to as "positive electrode active material") mentioned in [1] contains magnesium (Mg) whose atomic weight is smaller than that of nickel (Ni) and manganese (Mn). Therefore, even if the Ni content is reduced, the reduction in discharge capacity can be reduced or made equivalent. In addition, the Mn / Ni ratio in the outer layer of the positive electrode active material mentioned in [1] satisfies a specific numerical range. Therefore, the capacity retention rate of the lithium-ion secondary battery (hereinafter also referred to as "secondary battery") using the positive electrode active material can be further improved. Therefore, in the secondary battery, the number of batteries required can be reduced, which helps to extend the battery life. In other words, it can contribute to the efficiency of energy.

[0010] [2] The positive electrode active material for a lithium ion secondary battery according to [1], wherein the ratio of the number of Mg atoms to the number of Ni atoms (Mg / Ni ratio) in the outer layer is greater than or equal to 0.02 and less than or equal to 0.15, and the ratio of the ratio of the number of Mg atoms to the number of Ni atoms (Mg / Ni ratio) in the outer layer 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 greater than or equal to 1.0 and less than or equal to 5.0.

[0011] [2] The Mg / Ni ratio in the outer layer of the positive electrode active material, and the ratio of the Mg / Ni ratio in the outer layer to the Mg / Ni ratio of the entire particle, satisfy specific numerical ranges. Therefore, the release of oxygen from the positive electrode active material during initial charge can be suppressed, and the discharge capacity can be further improved. Furthermore, the capacity retention rate can be further improved. Therefore, it can contribute to further energy efficiency.

[0012] [3] A positive electrode active material for a lithium ion secondary battery according to [1] or [2], wherein the lithium transition metal composite oxide contains Ti, the ratio of the number of Ti atoms in the outer layer to the number of Ni atoms (Ti / Ni ratio) is greater than or equal to 0.02 and less than or equal to 0.25, and the ratio of the ratio of the number of Ti atoms in the outer layer to the number of Ni atoms (Ti / Ni ratio) to the ratio of the number of Ti atoms in the entire particle (chemical composition of the particle) to the number of Ni atoms (Ti / Ni ratio) is greater than or equal to 1.0 and less than or equal to 20.0.

[0013] [3] The positive electrode active material 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, and the capacity retention rate can be further improved. This can contribute to further energy efficiency.

[0014] [4] A 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 using a Cu ray source, the diffraction peaks of the 108 plane and the 110 plane in the space group R-3m are broken, and the half width of the diffraction peak of the above-mentioned 110 plane is greater than or equal to 0.10° and less than or equal to 0.21°.

[0015] [4] The diffraction peaks of the positive electrode active material in the 108 plane and the 110 plane are broken, and the half-width of the diffraction peak in the 110 plane is greater than 0.10° and less than 0.21°. This shows that Ni, Mn and Mg, or Ni, Mn, Mg and Ti are not phase-separated but are uniformly dispersed. In other words, it is shown that in the lithium transition metal composite oxide, Ni, Mn and Mg, or Ni, Mn, Mg and Ti are not phase-separated but are solid-dissolved. Therefore, it can contribute to further energy efficiency.

[0016] [5] The positive electrode active material for a lithium ion secondary battery according to any one of [1] to [4], wherein the lattice constants of the lithium transition metal composite oxide in the space group R-3m are 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.

[0017] [5] The lattice constant of the positive electrode active material satisfies a specific numerical range. Therefore, lithium ions easily diffuse within the particles of the lithium transition metal composite oxide, resulting in low resistance. This can contribute to further energy efficiency.

[0018] [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 a lithium ion secondary battery according to any one of [1] to [5].

[0019] [6] The positive electrode of the lithium-ion secondary battery contains the above-mentioned positive electrode active material for lithium-ion secondary batteries. Therefore, the discharge capacity and capacity retention rate can be further improved, the number of required batteries can be reduced, and the battery life can be extended. In other words, it can contribute to the efficiency of energy use.

[0020] [7] A method for producing a positive electrode active material for a lithium ion secondary battery, which is the method for producing a positive electrode active material for a lithium ion secondary battery according to any one of [1] to [5], wherein the method for producing a positive electrode active material for a lithium ion secondary battery comprises: A step of pre-firing a raw material mixture composed of a lithium compound, a magnesium compound, and a nickel-manganese compound, or a raw material mixture composed of a lithium compound and a nickel-manganese-magnesium compound at 650° C. to 950° C. for 10 minutes to 6 hours.

[0021] By providing the above-mentioned pre-calcination step, lithium is fully 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. Therefore, the discharge capacity and capacity retention rate can be further improved. Therefore, it can contribute to further energy efficiency.

[0022] [8] A method for producing a positive electrode active material for a lithium ion secondary battery, which is the method for producing a positive electrode active material for a lithium ion secondary battery according to any one of [1] to [5], wherein the method for producing a positive electrode active material for a lithium ion secondary battery comprises: A step of pre-calcining a raw material mixture composed of a lithium compound, a magnesium compound, a titanium compound, and a nickel-manganese compound, or a raw material mixture composed of a lithium compound and a nickel-manganese-magnesium-titanium compound at 650° C. to 950° C. for 10 minutes to 6 hours.

[0023] By providing the above-mentioned pre-calcination step, lithium is fully 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. Therefore, the discharge capacity and capacity retention rate can be further improved. Therefore, it can contribute to further energy efficiency.

[0024] [9] A method for producing a positive electrode active material for a lithium-ion secondary battery according to [7] or [8], wherein, after the above-mentioned pre-calcination step, it further includes: a step of formally calcining the above-mentioned raw material mixture after pre-calcination at a temperature of not less than 1020°C and not more than 1120°C for not less than 10 minutes and not more than 4 hours.

[0025] By using the method including the main calcination step, a positive electrode active material for a lithium ion secondary battery can be produced that satisfies the range of the chemical composition of the lithium transition metal composite oxide and the Mn / Ni ratio in the outer layer represented by Formula (1).

[0026]

[10] A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of [7] to [9], wherein, after the above-mentioned formal calcination step, the method further includes: maintaining the obtained lithium transition metal composite oxide at a temperature of not less than 500°C and not more than 900°C for not less than 1 hour and not more than 20 hours.

[0027] By further providing the aforementioned holding step (slow cooling step), the decrease in the valence of Mn in the lithium-transition metal composite oxide can be suppressed, resulting in a more stable structure. Consequently, the discharge capacity and capacity retention rate can be further improved, thereby contributing to further energy efficiency.

[0028] Effects of the Invention According to the positive electrode active material for lithium ion secondary batteries, the lithium ion secondary batteries, and the method for producing the positive electrode active material for lithium ion secondary batteries of the present invention, the discharge capacity and the capacity retention rate can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a cross-sectional view schematically showing a lithium-ion secondary battery according to one embodiment of the present invention.

[0030] Figure 2 Graphs showing powder X-ray diffraction patterns of the lithium-transition metal composite oxides of Examples 1 to 3 and Comparative Example 1.

[0031] Figure 3 This is a graph showing charge and 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

[0032] Hereinafter, preferred embodiments of the present invention will be described in detail.

[0033] [Positive electrode active material] The positive electrode active material of this embodiment is mainly composed of a lithium transition metal composite oxide and is used for the positive electrode of a lithium ion secondary battery. The term "main component" refers to a lithium transition metal composite oxide having a content of 75% by mass or greater, preferably 80% by mass or greater, more preferably 90% by mass or greater, further preferably 95% by mass or greater, and may be 100% by mass, relative to the total mass of the positive electrode active material. The positive electrode active material may also contain components other than the main component, as long as they do not impair the functions of the present invention.

[0034] The positive electrode active material of the present 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, as long as the main component is the lithium transition metal composite oxide.

[0035] When a positive electrode active material is produced using a lithium-transition metal composite oxide as its primary component, the overall compositional ratio (Li:Ni:Mn:Mg:Ti) of the lithium-transition metal composite oxide is maintained in the resulting positive electrode active material. When a positive electrode active material using a lithium-transition metal composite oxide having such a composition as its primary component is used in a secondary battery, a high capacity can be achieved. Furthermore, the compositional ratio of the lithium-transition metal composite oxide is adjusted to match the desired compositional ratio of the desired positive electrode active material.

[0036] <Lithium Transition Metal Composite Oxide> The lithium transition metal composite oxide of the present embodiment has a layered rock salt structure and is in the form of particles having an outer layer on the surface.

[0037] In this specification, the average particle size of the particles of the lithium transition metal composite oxide (hereinafter also referred to simply as “average particle size”) is not particularly limited, but is preferably 0.25 μ m~10 μ m, more preferably 0.25 μ m~5.0 μ m, more preferably 0.50 μ m~2.5 μ If the average particle size is greater than or equal to the above lower limit, the productivity of the positive electrode active material can be further improved. If the average particle size is less than or equal to the above upper limit, the electrochemical characteristics of the secondary battery can be further improved.

[0038] The average particle size refers to D50 measured by, for example, a laser diffraction particle size distribution analyzer.

[0039] (Chemical composition) In the past, lithium transition metal composite oxides (such as LiNi 0.5 Mn 0.5O2), if Li is dissolved in the transition element, Ni 2+ Converted to Ni 3+ Therefore, in order to increase the discharge capacity of the positive electrode active material, it is necessary to increase the amount of Ni used. The present invention is based on the following discovery: 0.5 Mn 0.5 O2 adds Mg or Mg and Ti as constituent elements, and replaces Ni with Mg and Ti. 0.5 Mn 0.5 Compared with the case of replacing with Li, the valence of Ni ions can be suppressed from increasing to 3, and the atomic weight of the transition metal element in the lithium transition metal composite oxide can be reduced. Therefore, in the present invention, the amount of Ni used can be reduced while maintaining the electrochemical properties of the positive electrode active material.

[0040] The lithium transition metal composite oxide of this embodiment is represented by the following formula (1).

[0041] Li m Ni w Mn x Mg y Ti z O2 (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, and is in the range of x≤w, and m+w+x+y+z=2.

[0042] The lithium transition metal composite oxide of this embodiment is more preferably such that m in formula (1) 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, and m+w+x+y+z=2.

[0043] The chemical composition of the lithium transition metal composite oxide of the present embodiment can be determined by inductively coupled plasma (ICP) emission spectrometry.

[0044] (Surface composition) The particles of the lithium-transition metal composite oxide have an outer layer on the surface.

[0045] In this specification, the "outer layer" refers to the region extending from the surface of the particle to within 25 nm of the particle interior. When the particle diameter is less than 50 nm, the particle has a single-layer structure consisting solely of the outer layer.

[0046] The Mn content in the outer layer of the lithium-transition metal composite oxide particles of this embodiment is higher than the Mn content in the overall particle composition (particle chemical composition). 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 this numerical range, the movement of lithium ions is not impeded, and when used as a positive electrode active material, the charge and discharge capacity of a secondary battery is high.

[0047] The Mn / Ni ratio can be determined through quantitative analysis using X-ray photoelectron spectroscopy (XPS). XPS allows analysis of the transition metal element composition across the entire particle surface. Specifically, XPS analysis results represent the composition of the entire particle surface, not just the local composition of the entire particle surface.

[0048] 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 (particle chemical composition) (hereinafter also referred to as the "Mn / Ni ratio outer layer / total particle ratio") 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 Mn / Ni ratio outer layer / total particle ratio is within this numerical range, the surface becomes manganese-rich, and the discharge capacity can be further improved even if the Ni content is reduced.

[0049] The outer layer / whole particle ratio of Mn / Ni can be determined by quantitative analysis using XPS.

[0050] 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 to 0.15, more preferably 0.02 to 0.12, and even more preferably 0.02 to 0.11. When the Mg / Ni ratio is within this numerical range, oxygen release from the positive electrode active material during initial charge can be suppressed. Consequently, the discharge capacity of the secondary battery can be further increased.

[0051] The Mg / Ni ratio can be determined by quantitative analysis using XPS.

[0052] The ratio of 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 to the ratio of the number of Mg atoms to the number of Ni atoms (Mg / Ni ratio) in the entire particle (particle chemical composition) (hereinafter also referred to as the "Mg / Ni ratio outer layer / total particle ratio") is preferably 1.0 or more and 5.0 or less, more preferably 1.0 or more and 4.5 or less, and even more preferably 1.0 or more and 4.0 or less. When the Mg / Ni ratio outer layer / total particle ratio is within this numerical range, the surface becomes magnesium-rich, which can suppress an increase in Ni valence and further improve the discharge capacity.

[0053] The ratio of the outer layer to the entire particle of Mg / Ni can be determined by quantitative analysis using XPS.

[0054] 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.

[0055] 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. If the Ti / Ni ratio is within the above numerical range, the increase in Ni valence can be suppressed, and the atomic weight of the transition metal element in the lithium transition metal composite oxide can be reduced. Therefore, the discharge capacity of the secondary battery can be further improved.

[0056] The Ti / Ni ratio can be determined by quantitative analysis using XPS.

[0057] When the lithium transition metal composite oxide of this embodiment contains Ti, the ratio of the number of Ti atoms in the outer layer of the lithium transition metal composite oxide to the number of Ni atoms (Ti / Ni ratio) to the number of Ti atoms in the entire particle (chemical composition of the particle) (hereinafter also referred to as the "Ti / Ni ratio outer layer / total particle ratio") is preferably 1.0 or more and 20.0 or less, more preferably 1.0 or more and 18.0 or less, and even more preferably 1.0 or more and 15.0 or less. When the Ti / Ni ratio outer layer / total particle ratio is within the above numerical range, the surface becomes rich in titanium, which can suppress the increase in Ni valence and further improve the discharge capacity.

[0058] The outer layer / whole particle ratio of Ti / Ni can be determined by quantitative analysis using XPS.

[0059] The lithium transition metal composite oxide particles of this embodiment may be primary particles or secondary particles. In order to obtain relatively dense particles, the lithium transition metal composite oxide particles are preferably secondary particles formed by agglomeration of a plurality of primary particles.

[0060] (lattice constant) The lithium-transition metal composite oxide of this embodiment is a rhombohedral layered compound having a crystal structure of space group R-3m. The lattice constants of the lithium-transition metal composite oxide preferably have an a-axis length of 2.881 to 2.893 Å, a c-axis length of 14.28 to 14.31 Å, and a ratio of c-axis length to a-axis length (hereinafter also referred to as "c / a") of 4.948 to 4.958. By maintaining the lattice constant within this range, lithium ions easily diffuse within the primary particles of the lithium-transition metal composite oxide, resulting in low electrical resistance.

[0061] The lattice constant of the crystal can be determined by measuring the X-ray diffraction pattern of the lithium transition metal composite oxide and using the respective indices and their interplanar spacings by the least square method.

[0062] (X-ray diffraction (XRD) pattern) The positive electrode active material of this embodiment preferably exhibits an X-ray diffraction (XRD) pattern using a Cu-ray source, in which the diffraction peaks for the 108 and 110 planes in the space group R-3m are broken, and the half-width of the diffraction peak for the 110 plane is 0.10° or more and 0.21° or less. The broken diffraction peaks for the 108 and 110 planes in the XRD pattern indicate that Ni, Mn, and Mg, or Ni, Mn, Mg, and Ti, in the lithium transition metal composite oxide are not phase-separated but are uniformly dispersed. This indicates that the lithium transition metal composite oxide of this embodiment truly has the chemical composition represented by formula (1).

[0063] In this specification, "a diffraction peak is broken" means that a diffraction peak has two or more apexes.

[0064] The XRD pattern of the positive electrode active material of this embodiment can be obtained by using, for example, Cu (copper) as a target for electron beam irradiation and K (K) as a target for electron beam irradiation. α The rays are obtained as characteristic X-rays by the method described in the examples.

[0065] In the X-ray diffraction pattern using a Cu ray 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 half-width of the diffraction peak of the 110 plane within the above numerical range means that the diffraction peak of the 110 plane in the space group R-3m is not tailed.

[0066] The half width of the diffraction peak is determined by analyzing the XRD pattern.

[0067] [Method for producing positive electrode active material] The positive electrode active material of this embodiment contains the aforementioned lithium transition metal composite oxide as a main component. As the lithium source for the lithium transition metal composite oxide, known compounds such as hydroxides such as lithium hydroxide monohydrate (LiOH·H2O), carbonates such as lithium carbonate (Li2CO3), and acetates such as lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O) can be used without particular limitation. As the transition metal source compounds for nickel, manganese, magnesium, and titanium, a wide range of known oxides, hydroxides, or metal salts of nickel, manganese, magnesium, and titanium can also be used without particular limitation.

[0068] For example, nickel hydroxide (Ni(OH)2), nickel(II) chloride (NiCl2), nickel(II) chloride hexahydrate (NiCl2·6H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), etc. can be used as nickel compounds, but are not limited to these.

[0069] As the manganese compound, manganese (II) chloride (MnCl2), manganese (II) chloride tetrahydrate (MnCl2·4H2O), manganese carbonate hexahydrate (MnCO3·6H2O), manganese (II) nitrate hexahydrate (Mn(NO3)2·6H2O) and the like can be used, but are not limited thereto.

[0070] As the magnesium compound, magnesium oxide (MgO), magnesium hydroxide (Mg(OH) 2 ), magnesium chloride hexahydrate (MgCl 2 ·6H 2 O), magnesium carbonate (MgCO 3 ), magnesium sulfate heptahydrate (MgSO 4 ·7H 2 O) and the like can be used, but are not limited thereto.

[0071] As the titanium compound, titanium (IV) oxide (TiO 2 ), titanium (IV) sulfate (Ti(SO 4 ) 2 ), and the like can be used, but the present invention is not limited thereto.

[0072] The above-mentioned transition metal compounds can be used individually or as composite hydroxides (eg, nickel-manganese-magnesium composite hydroxide) by a coprecipitation method or the like.

[0073] The lithium transition metal composite oxide of this embodiment can be synthesized using a known method. For example, it can be synthesized by preparing a composite hydroxide or composite oxide of a nickel compound and a manganese compound as an intermediate compound, mixing the intermediate compound, a magnesium compound, and a lithium compound as a raw material mixture, and subjecting the raw material mixture to a heat treatment (e.g., calcination) in a given atmosphere and at a given temperature for a given time. Alternatively, it 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, mixing the intermediate compound and a lithium compound as a raw material mixture, and subjecting the raw material mixture to a heat treatment (e.g., calcination) in a given atmosphere and at a given temperature for a given time.

[0074] Alternatively, a titanium compound may be mixed with these intermediate compounds to prepare a composite hydroxide or composite oxide of a nickel compound, a manganese compound, a magnesium compound, and a titanium compound as the intermediate compound.

[0075] Prior to the heat treatment, the raw material mixture is preferably pre-calcined at a lower treatment temperature than the heat treatment (hereinafter also referred to as "pre-calcination"). Pre-calcination allows lithium to diffuse fully into the particles of the metal composite hydroxide or metal composite oxide, thereby obtaining a more uniform lithium transition metal composite oxide.

[0076] The lithium transition metal composite oxide obtained in the above-mentioned heat treatment is preferably further maintained for a given time within a given temperature range through a slow cooling process. The slow cooling conditions vary depending on the treatment conditions (e.g., heat treatment atmospheres such as oxygen atmosphere and air atmosphere), and are therefore preferably adjusted appropriately. The present inventors have found that by appropriately selecting the above-mentioned heat treatment conditions and the above-mentioned slow cooling conditions, the transition metals of the lithium transition metal composite oxide can be uniformly dispersed and the Mn / Ni ratio on the particle surface can be increased. Below, the method for manufacturing the positive electrode active material of this embodiment is further described in detail through the embodiment.

[0077] <Main firing process> The main 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 higher and 4 hours or lower.

[0078] In the main firing process, first, a predetermined amount of magnesium compound and a predetermined amount of lithium compound are added to the intermediate nickel-manganese compound, dispersed in a solvent such as ethanol, and mixed. It should be noted that the predetermined amount of the intermediate compound, the predetermined amount of magnesium compound, and the predetermined amount of lithium compound can be mixed not only by wet mixing using a solvent but also by dry mixing without a solvent. For example, when using magnesium oxide (MgO) as the magnesium compound and lithium carbonate (Li2CO3) as the lithium compound to synthesize Li1.02 Ni 0.48 Mn 0.48 Mg 0.020 In the case of O 2 , it is preferable to weigh 1 to 5% by mass more than the stoichiometric ratio, for example, 4% by mass more Li 2 CO 3 .

[0079] The nickel-manganese compound can be synthesized by known methods. When the nickel-manganese compound is a hydroxide, for example, nickel sulfate hexahydrate (NiSO4·6H2O) and manganese sulfate pentahydrate (MnSO4·5H2O) are weighed to achieve a Ni:Mn molar ratio of 1:1. Pure water is added to dissolve the mixture, and then an alkaline aqueous solution is added dropwise to the sulfate aqueous solution to coprecipitate the nickel-manganese composite hydroxide.

[0080] As an intermediate compound, a nickel-manganese-magnesium compound can be used instead of the above-mentioned nickel-manganese compound. For example, a predetermined amount of a lithium compound is added to the nickel-manganese-magnesium compound, dispersed in a solvent such as ethanol, and mixed. It should be noted that the predetermined amount of the above-mentioned nickel-manganese-magnesium compound and a predetermined amount of a lithium compound can be mixed not only by wet mixing using a solvent but also by dry mixing without using a solvent. For example, when lithium carbonate (Li2CO3) is used as the lithium compound to synthesize Li 1.02 Ni 0.48 Mn 0.48 Mg 0.020 In the case of O 2 , it is preferable to weigh 1% by mass to 5% by mass more Li 2 CO 3 than the stoichiometric ratio, for example, 4% by mass more Li 2 CO 3 .

[0081] The nickel-manganese-magnesium compound can be synthesized using known methods. 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 to achieve a molar ratio of Ni:Mn:Mg of 48:48:2. Pure water is then added to dissolve the mixture, and an alkaline aqueous solution is then added dropwise to the sulfate aqueous solution to coprecipitate the nickel-manganese-magnesium composite hydroxide.

[0082] Among the above-mentioned intermediate compounds, titanium compounds can be used in addition to lithium compounds and magnesium compounds.

[0083] Alternatively, a nickel-manganese-magnesium-titanium compound may be used instead of the nickel-manganese-magnesium compound. The nickel-manganese-magnesium-titanium compound can be synthesized using, for example, titanium (IV) sulfate (Ti(SO 4 ) 2 ) as the titanium compound by the same method as the nickel-manganese-magnesium compound.

[0084] As a precursor, a raw material mixture consisting of a lithium compound, a magnesium compound and a nickel-manganese compound, a raw material mixture consisting of a lithium compound and a nickel-manganese-magnesium compound, a raw material mixture consisting of a lithium compound, a magnesium compound, a titanium compound and a nickel-manganese compound, or a raw material mixture consisting of a lithium compound and a nickel-manganese-magnesium-titanium compound is pulverized into a preferred size and mixed, and then, for example, filled in a crucible and the raw material mixture is fired (hereinafter also referred to as "formal firing"). As a crucible, an alumina square sagger, an alumina crucible, a platinum crucible, a gold crucible, etc. are used. The formal firing of the raw material mixture uses, for example, a firing furnace or a roller kiln.

[0085] 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, to reach the firing temperature. The firing atmosphere is not particularly limited, and examples include air (in an air atmosphere) and an oxygen flow. The firing atmosphere is preferably an oxygen flow.

[0086] The calcination temperature is preferably 1020° C. to 1120° C., more preferably 1045° C. to 1095° C. The calcination time is preferably 10 minutes to 4 hours, more preferably 20 minutes to 3 hours, further preferably 30 minutes to 2 hours, and particularly preferably 30 minutes to 1 hour.

[0087] The firing time can be appropriately set according to the firing temperature. Note that the firing time refers to the time during which the firing temperature is maintained.

[0088] <Pre-firing process> The preliminary firing step is a step of firing the raw material mixture at a temperature lower than the above-mentioned firing temperature of 650° C. to 950° C. for 10 minutes to 6 hours.

[0089] In the preliminary firing step, the same firing furnace and roller kiln as those used in the main firing step can be used.

[0090] The firing atmosphere in the preliminary firing step is not particularly limited, and can be the same firing atmosphere as that in the main firing step.

[0091] The firing temperature in the preliminary 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.

[0092] The firing time in the preliminary firing step is preferably 10 minutes to 6 hours, more preferably 20 minutes to 4 hours, further preferably 30 minutes to 2 hours, and particularly preferably 30 minutes to 1 hour.

[0093] <Slow Cooling Process> The slow cooling step is a step of maintaining the obtained lithium transition metal composite oxide at 500° C. to 900° C. for 1 hour to 20 hours, following the main calcination step.

[0094] In the slow cooling step, the powder obtained after the calcining step is cooled to 500°C to 900°C at a cooling rate of, for example, 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min, and then the powder is held at 500°C to 900°C for 1 to 20 hours. The atmosphere in which the powder is held at 500°C to 900°C is not particularly limited, and examples thereof include air (in an air atmosphere), an air flow, and an oxygen flow.

[0095] 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.

[0096] The time for holding the powder in the slow cooling step is preferably 1 hour to 20 hours, more preferably 2 hours to 15 hours, and even more preferably 3 hours to 10 hours.

[0097] The slow cooling step may be performed twice or more as long as the maintained temperature is lowered stepwise. The positive electrode active material of this embodiment can further increase the Mn / Ni ratio on the surface of the lithium transition metal composite oxide particles by appropriately selecting the slow cooling conditions.

[0098] [Lithium-ion secondary battery] The lithium-ion secondary battery (hereinafter referred to simply as "secondary battery") of this embodiment includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains a positive electrode active material primarily composed of the lithium-transition metal composite oxide described above. The secondary battery of this embodiment may include other battery elements as needed.

[0099] The secondary battery of this embodiment can directly adopt the battery elements of a known secondary battery except that the positive electrode contains a positive electrode active material with the above-mentioned lithium transition metal composite oxide as the main component. The secondary battery of this embodiment can be any structure among coin type, button type, cylindrical type, square type, and laminated type. In addition, the secondary battery of this embodiment can be applied to a wide range of uses such as mobile devices such as mobile phones and laptop computers, and in-vehicle use.

[0100] The following describes a secondary battery according to this embodiment that uses an electrolyte solution (a coin-type secondary battery). The battery elements described below are also applicable to an all-solid-state secondary battery that does not use an electrolyte solution.

[0101] Figure 1: is a cross-sectional view schematically showing a secondary battery according to this embodiment. Figure 1 In FIG. 1 , an example is shown in which the secondary battery of this embodiment is a coin-type secondary battery. Figure 1 As shown, the secondary battery 1 of this embodiment includes a negative electrode can (negative electrode terminal) 20 , a negative electrode 3 , a separator 4 impregnated with an electrolyte, an insulating spacer (gasket) 5 , a positive electrode 2 , and a positive electrode can 10 .

[0102] The positive electrode can 10 is positioned below the separator 4, and the negative electrode can 20 is positioned above the separator 4. The positive electrode can 10 and the negative electrode can 20 together form the outer shape of the secondary battery 1. A positive electrode 2 and a negative electrode 3 are disposed between the positive electrode can 10 and the negative electrode can 20, with the separator 4 impregnated with electrolyte solution interposed therebetween. 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 spacer 5.

[0103] The secondary battery 1 can prepare a positive electrode mixture by mixing a conductive agent, a binder, etc. with the positive electrode active material of this embodiment as needed, and press-bonding the mixture to a current collector (not shown) to produce the positive electrode 2 .

[0104] As the current collector, a stainless steel mesh, aluminum foil, etc. can be preferably used. As the conductive agent, acetylene black, Ketjen black, etc. can be preferably used. As the binder, tetrafluoroethylene, polyvinylidene fluoride, etc. can be preferably used.

[0105] The composition of the positive electrode active material, conductive agent, and binder in the positive electrode mixture is not particularly limited. The conductive agent content in the positive electrode mixture is preferably 1% to 15% by mass, more preferably 0.1% to 5% by mass. The binder content in the positive electrode mixture is preferably 0.1% to 10% by mass, more preferably 0.1% to 5% by mass. The remainder of the positive electrode mixture (excluding the positive electrode active material and conductive agent) is preferably a combination of the positive electrode active material, conductive agent, and binder to form the positive electrode active material.

[0106] In the secondary battery 1, as the negative electrode 3 opposite the positive electrode 2, for example, metal materials such as metallic lithium and lithium alloys, carbon materials such as graphite and MCMB (mesocarbon microbeads), silicon materials such as silicon (Si), Si alloys, and silicon oxide can be used, which are known materials that function as negative electrode active materials and can absorb and release lithium.

[0107] The separator 4 and the battery container (the positive electrode can 10 and the negative electrode can 20 ) can employ known battery elements.

[0108] The electrolyte may be a known electrolyte solution, solid electrolyte, or the like. Examples of the electrolyte solution include a solution prepared 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).

[0109] The all-solid-state secondary battery can have the same structure as a known all-solid-state lithium ion secondary battery except for using a positive electrode active material containing the above-mentioned lithium transition metal composite oxide as a main component.

[0110] In the case of an all-solid-state secondary battery, as the electrolyte, 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, an oxide-based solid electrolyte, or the like can be used.

[0111] The positive electrode of the all-solid-state secondary battery can be made of, for example, a positive electrode mixed material containing a solid electrolyte in addition to the above-mentioned positive electrode active material, conductive agent, and binder, supported on a positive electrode current collector such as aluminum, nickel, or stainless steel.

[0112] Since the positive electrode 2 of the secondary battery 1 of this embodiment contains a positive electrode active material mainly composed of the above-mentioned lithium transition metal composite oxide, the discharge capacity and capacity retention rate can be further improved.

[0113] Example Hereinafter, examples of the present invention will be described, but the present invention is not limited to the following examples.

[0114] [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 (high-purity chemical production), 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 in a molar ratio of Li:Ni:Mn:Mg:Ti=1.00:0.48:0.48:0.027:0.013. Considering the evaporation of Li, the amount was weighed based on the stoichiometric ratio so that Li2CO3 increased by 4% by mass. Li2CO3, Ni 0.5 Mn 0.5The total mass of (OH)2, MgO, and TiO2 is 50g. Disperse and mix them in ethanol using a mortar. Then, fill the mixture into an alumina square crucible. Using a sintering furnace, heat the raw material mixture in air at a temperature increase rate of 10°C / minute in a platinum crucible. Pre-sinter at 775°C for 60 minutes, then allow the resulting powder to stand until the temperature reaches room temperature (25°C). Then, heat the resulting powder at a temperature increase rate of 10°C / minute and sinter at 1070°C for 30 minutes.

[0115] The calcined powder was cooled at a rate of 10°C / minute to 750°C, then held in air at 750°C for 5 hours. Next, it was cooled at a rate of 10°C / minute to 650°C, then held in air at 550°C for 5 hours. The powder was then allowed to stand until it reached room temperature (25°C), yielding the lithium nickel manganese magnesium titanium composite oxide of Example 1.

[0116] [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 molar ratio of each compound was weighed to obtain the molar ratio shown in Table 1 and the main calcination conditions were set to those shown in Table 1. The lithium nickel manganese magnesium titanium composite oxide of Example 2 was obtained in the same manner as in Example 1.

[0117] [Example 3] (Synthesis of lithium nickel manganese magnesium composite oxide: Li 1.02 Ni 0.48 Mn 0.48 Mg 0.020 O2) The lithium nickel manganese magnesium composite oxide of Example 3 was obtained in the same manner as in Example 1 except that the molar ratio of each compound was measured so as to obtain the molar ratio described in Table 1.

[0118] [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 molar ratio of each compound was weighed to obtain the molar ratio shown in Table 1 and the main calcination conditions were set to those shown in Table 1. The lithium nickel manganese titanium composite oxide of Comparative Example 1 was obtained in the same manner as in Example 1.

[0119] (analyze) The chemical compositions of the samples obtained in Examples 1 to 3 and Comparative Example 1 were analyzed using an ICP 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 was not contained.

[0120] 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 K was used as the characteristic X-ray. α The lattice constant was calculated by the least square method using the indices and interplanar spacing of the obtained XRD pattern. The space group of the obtained sample was set to R-3m and the lattice constant was calculated. The powder X-ray diffraction pattern is shown in Figure 2 Table 1 shows the values ​​of lattice constants.

[0121] The X-ray photoelectron spectroscopy (XPS) analysis device (trade name: K-Alpha + Quantitative analysis of the outer layer of the samples obtained in Examples 1 to 3 and Comparative Example 1 was performed using XPS (manufactured by Thermo Fisher Scientific). The Mn / Ni, Mg / Ni, and Ti / Ni ratios of the entire particle (center and outer layer), as well as the Mn / Ni, Mg / Ni, and Ti / Ni ratios of the outer layer, are shown in Table 1. "-" in the table indicates the absence of Mg or Ti. The XPS measurement conditions are as follows.

[0122] XPS measurement conditions Machine type: Made by Thermo Fisher Scientific, K-Alpha + (Product Name) X-ray irradiation: single crystal spectroscopy of AlK α (12keV, 72W) X-ray spot diameter: 400 μ m Neutralizing electron gun: Use Reference spectrum: CC, CH 284.6eV Detection depth: 6nm~7nm [Table 1]

[0123] according to Figure 2The XRD patterns shown here confirm that both the diffraction peaks of the 108 plane and the 110 plane in the space group R-3m of the samples obtained in Examples 1 to 3 and Comparative Example 1 are broken. This indicates that Ni, Mn, and Mg, or Ni, Mn, Mg, and Ti, in the lithium transition metal composite oxides obtained in the samples of Examples 1 to 3 and Comparative Example 1 are uniformly dispersed without phase separation.

[0124] [Manufacturing 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 respectively used as positive electrode active materials, and mixed with acetylene black (AB) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder at a weight ratio of 96:2:2 in NMP (N-methyl-2-pyrrolidone) as a solvent to prepare a paste. Then, the paste was applied to a 15 mm thick layer. μ m aluminum foil, dry it and make 16.5 φ The coating area density is 10.0 mg / cm 2 , the volume density is 2.8g / cm 3 For this positive electrode, the thickness is 200 μ m, 18 φ The lithium metal is used as the counter electrode, and the thickness is 20 μ m, 19.4 φ The polyethylene microporous membrane is used as the separator. The electrolyte is a 1.2 mol / L solution obtained by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (volume ratio 3:4:3). Figure 1 The lithium-ion secondary battery (2032 coin-shaped battery) of the structure shown is manufactured according to the known battery structure and assembly method.

[0125] [Charge and discharge test] For each lithium-ion secondary battery produced, a charge-discharge test was conducted at a temperature of 25°C, at a rate of 0.05C (1C: 250mA / g), at a constant current density of 12.5mA / g, and a cutoff potential of 4.6V to 2.75V to evaluate the initial discharge capacity. The charge-discharge test started with charging. The initial discharge capacity values ​​are shown in Table 1, and the charge-discharge curves are shown in Figure 3 .

[0126] exist Figure 3 , the voltage change during discharge, in which the battery voltage decreases as the capacity increases, and the voltage change during charge, in which the battery voltage increases as the capacity increases, are shown.

[0127] like Figure 3 As shown in Table 1, the lithium-ion secondary batteries of each example have an initial discharge capacity, which is higher than that of conventional lithium-ion secondary batteries. This is presumably because by adjusting the composition of the lithium-transition metal composite oxide and the calcination conditions in the calcination step, the structure of the lithium-transition metal composite oxide becomes more stable, the dissolution of Ni into the electrolyte can be suppressed, and the Mn / Ni ratio in the outer layer of the lithium-transition metal composite oxide particles can be further increased.

[0128] [Cycle test] After evaluating the initial discharge capacity of each produced lithium-ion secondary battery, a 50-cycle test was performed at 25°C, a current density of 50.0 mA / g, and a cutoff potential of 4.3 V to 2.75 V. The capacity retention (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. A higher capacity retention indicates better cycle characteristics.

[0129] As shown in Table 1, the lithium ion secondary batteries of Examples 1 to 3 to which the present invention is applied exhibited a high capacity retention rate of 90% or more.

[0130] In contrast, the capacity retention rate of the lithium ion secondary battery of Comparative Example 1, which does not contain magnesium and the lithium transition metal composite oxide represented by formula (1) in its chemical composition, is 83%.

[0131] From the above results, it is clear that the present invention can provide a positive electrode active material for a lithium ion secondary battery capable of further improving the discharge capacity and capacity retention rate, and a lithium ion secondary battery including the positive electrode active material.

[0132] Description of Reference Numerals 1…Lithium-ion secondary battery (secondary battery) 2…Positive electrode 3…Negative electrode 4…Isolators 5…Insulation pad (gasket) 10…Positive electrode tank 20…Negative tank (negative terminal).

Claims

1. A positive electrode active material for a lithium ion secondary battery, comprising a lithium transition metal composite oxide as a main component, wherein: 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: Formula 1: Li m Nor w Mn x Mg y You z O2 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, and is in the range of x≤w, m+w+x+y+z=2, The ratio of the number of Mn atoms to the number of Ni atoms in the outer layer, that is, the Mn / Ni ratio, is 1.0 or more and 1.5 or less.

2. The positive electrode active material for lithium ion secondary batteries according to claim 1, wherein The ratio of the number of Mg atoms in the outer layer to the number of Ni atoms, i.e., the Mg / Ni ratio, is greater than 0.02 and less than 0.15, and the ratio of the number of Mg atoms in the outer layer to the number of Ni atoms, i.e., the Mg / Ni ratio, to the ratio of the number of Mg atoms in the entire particle to the number of Ni atoms, i.e., the Mg / Ni ratio, is greater than 1.0 and less than 5.

0.

3. The positive electrode active material for lithium ion secondary batteries according to claim 1, wherein The lithium transition metal composite oxide contains Ti, and the ratio of the number of Ti atoms in the outer layer to the number of Ni atoms, i.e., the Ti / Ni ratio, is greater than 0.02 and less than 0.25, and the ratio of the number of Ti atoms in the outer layer to the number of Ni atoms, i.e., the Ti / Ni ratio, to the ratio of the number of Ti atoms in the entire particle to the number of Ni atoms, i.e., the Ti / Ni ratio, is greater than 1.0 and less than 20.

0.

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

5. The positive electrode active material for lithium ion secondary batteries according to claim 1, wherein The lithium transition metal composite oxide has a lattice constant in the space group R-3m, wherein the a-axis length is 2.881 Å to 2.893 Å, the c-axis length is 14.28 Å to 14.31 Å, and c / a is 4.948 to 4.

958.

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 a lithium ion secondary battery 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, the method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein: The method for producing the positive electrode active material for a lithium ion secondary battery comprises: A step of pre-firing a raw material mixture composed of a lithium compound, a magnesium compound, and a nickel-manganese compound, or a raw material mixture composed of a lithium compound and a nickel-manganese-magnesium compound at 650° C. to 950° C. for 10 minutes to 6 hours.

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

9. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 7, wherein: After the preliminary firing step, the method further includes the step of firing the preliminary fired raw material mixture at a temperature of 1020° C. to 1120° C. for 10 minutes to 4 hours.

10. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 8, wherein: After the preliminary firing step, the method further includes the step of firing the preliminary fired raw material mixture at a temperature of 1020° C. to 1120° C. for 10 minutes to 4 hours.

11. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9, wherein: After the main calcination step, the method further includes the step of maintaining the obtained lithium transition metal composite oxide at 500° C. to 900° C. for 1 hour to 20 hours.

12. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 10, wherein: After the main calcination step, the method further includes the step of maintaining the obtained lithium transition metal composite oxide at 500° C. to 900° C. for 1 hour to 20 hours.

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