Positive electrode active material for non-aqueous electrolyte secondary battery

Abstract

To provide a positive electrode active material for a nonaqueous electrolyte secondary battery capable of imparting cycle characteristics, especially excellent cycle characteristics under high voltage to the nonaqueous electrolyte secondary battery.SOLUTION: An average particle size (D50) of the primary particles is from 50nm to 220nm, a particle size of the primary particles is from 80nm to 300nm, a length of an a-axis of a crystal lattice is from 2.840 Å to 2.890 Å, a length of a c-axis of the crystal lattice is from 14.160 Å to 14.220 Å, and a cation mixing amount, which is an amount of metals moved from a metallic site to a lithium site and substituted for lithium in the lithium site, is from 2.3% to 6.0%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention provides a positive electrode active material for a non-aqueous electrolyte secondary battery that can impart excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, to the non-aqueous electrolyte secondary battery, and can also impart excellent thermal stability. Quality Regarding. [Background technology]

[0002] Small, lightweight, and high-energy-density non-aqueous secondary batteries are used as power sources for mobile phones, laptops, etc. Among these, lithium-ion secondary batteries, which use materials such as lithium cobalt oxide or lithium nickel oxide for the positive electrode and have a large charge / discharge capacity, are widely used.

[0003] As a positive electrode active material for lithium-ion secondary batteries, for example, there is an NCM-based positive electrode active material in which part of the nickel is replaced with cobalt and manganese is introduced. However, research into NCA-based positive electrode active materials in which part of the nickel is replaced with cobalt and aluminum is introduced is being actively conducted, as they are expected to provide high capacity and high energy density to lithium-ion secondary batteries.

[0004] The positive electrode active materials for lithium-ion secondary batteries, including the NCA-based positive electrode active materials, require a certain amount of oxygen during the reaction with the lithium source, but a high oxygen partial pressure is particularly required for positive electrode active materials with a high nickel content, which are expected to provide high capacity.If this oxygen partial pressure is insufficient, it becomes difficult to achieve the expected effects.

[0005] Furthermore, although lithium ion secondary batteries using the positive electrode active material with a high nickel content as a positive electrode have a high energy density, they generally have poor cycle characteristics, particularly at high voltages, due to the tendency for oxygen to be released from the crystalline structure. In particular, it is known that this oxygen release can cause thermal runaway in lithium ion secondary batteries, making it necessary to stabilize the crystalline structure.

[0006] Therefore, various methods have been proposed to solve the above problems in cathode active materials having a particularly high nickel content, such as NCA-based cathode active materials and NCM-based cathode active materials.

[0007] For example, in the production method described in Patent Document 1, a compound of the formula: Li y Ni 1-x Co x1 M x2 In the composite oxides of O2 (M is at least one selected from Al, Fe, Mn, and B), especially in the case of NCA-based composite oxides, Co is 3+ The present inventors have focused on the fact that a different phase is generated due to the fact that the different phase becomes an impurity during lithiation, and the effect of imparting high capacity is impaired. 2+ 1-x (Co 2+ ,Co 3+ ) x1 M 3+ x2 (OH) 2-nz (A n- z )·mH2O, a lithium compound is added to the basic metal salt in an aqueous medium, and the mixture is spray-dried or freeze-dried. 3+ To prevent this, preliminary firing is carried out in a non-oxidizing atmosphere at about 300 to 500°C, and then firing is carried out in an oxidizing atmosphere at about 600 to 900°C.

[0008] The manufacturing method described in Patent Document 2 focuses on the fact that, in a lithium transition metal composite oxide having a basic composition of LiNiO2 or LiCoO2, in which the proportion of lithium in the particle surface layer composition is greater than the proportion of lithium in the average composition of the entire particle, by increasing the proportion of lithium in the particle surface layer and uniformly dispersing the composite oxide, and then firing the composite oxide in an oxygen atmosphere, an increase in internal resistance can be suppressed even when stored in a charged state for a long period of time. That is, in the manufacturing method of the lithium transition metal composite oxide, nickel hydroxide particles or cobalt hydroxide particles are suspended in an aqueous solution of a lithium compound to prepare a suspension, which is spray-dried, and the composite oxide in which lithium is attached to the surfaces of the nickel hydroxide particles or cobalt hydroxide particles is pre-fired at approximately 300 to 600°C in a nitrogen atmosphere, and then fired in an oxygen atmosphere.

[0009] The manufacturing method described in Patent Document 3 focuses on the fact that the charge-discharge characteristics can be improved by drying a mixture of lithium hydroxide and nickel composite oxide in a reduced pressure atmosphere before calcining. That is, in this method for manufacturing a lithium nickel composite oxide, the mixture is dried at 40°C to 200°C in an atmosphere with a carbon dioxide partial pressure of 10 Pa or less before calcining at 650°C to 800°C in an atmosphere with an oxygen content of 60% by volume or more. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 10-316431 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-184403 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-052988 Summary of the Invention [Problem to be solved by the invention]

[0011] However, the manufacturing method described in Patent Document 1 is complicated because it requires adding a lithium compound to a basic metal salt in an aqueous medium and then spray-drying or freeze-drying. Furthermore, although the NCA-based composite oxide obtained by this manufacturing method certainly does not impair the effect of imparting high capacity to lithium-ion secondary batteries, it is unable to improve cycle characteristics. Furthermore, Patent Document 1 does not describe the stability of the crystal structure of the NCA-based composite oxide.

[0012] The manufacturing method described in Patent Document 2 is cumbersome because it requires spray drying of a suspension prepared by suspending nickel hydroxide particles or cobalt hydroxide particles in an aqueous solution of a lithium compound. Moreover, although a lithium ion secondary battery using a lithium transition metal composite oxide obtained by this manufacturing method as a positive electrode can suppress an increase in internal resistance even when stored in a charged state for a long period of time, it still has poor cycle characteristics.

[0013] In the manufacturing method described in Patent Document 3, a mixture of lithium hydroxide and nickel composite oxide is dried at a low temperature in a reduced pressure atmosphere before calcination, and therefore, a lithium ion secondary battery using the obtained lithium nickel composite oxide in the positive electrode has improved charge and discharge characteristics. However, a lithium nickel composite oxide dried at such a low temperature cannot improve the cycle characteristics of a lithium ion secondary battery, particularly the cycle characteristics under high voltage.

[0014] The present invention has been made in view of the above-mentioned problems in the prior art, and its object is to provide a non-aqueous electrolyte secondary battery that can provide excellent cycle characteristics, particularly excellent cycle characteristics under high voltage. For non-aqueous electrolyte secondary batteries positive electrode active material Quality The purpose is to provide. [Means for solving the problem]

[0015] In order to achieve the above object, in the present invention, a mixture of a precursor compound and a lithium compound is subjected to two-stage firing in different atmospheres, that is, firing in a non-oxygen atmosphere at a specific temperature range and firing in an oxygen atmosphere in that order, thereby forming a positive electrode active material. Quality Configured.

[0016] Positive electrode active material for non-aqueous electrolyte secondary battery according to the present invention Quality is , A method for producing a positive electrode active material containing at least lithium (Li) and nickel (Ni), A step (1) of preparing a mixture by mixing a precursor compound containing at least Ni and a lithium compound in a non-solvent system; (2) pre-firing the mixture at 450°C to 700°C in an oxygen-free atmosphere; a step (3) of subjecting the mixture after the pre-baking to a main baking in an oxygen atmosphere; The method is characterized in that at least the following are performed in order. [Effects of the Invention]

[0017] According to the present invention, a positive electrode active material for a non-aqueous electrolyte secondary battery has a stable crystal structure despite its high nickel content, and therefore can impart excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, to the non-aqueous electrolyte secondary battery, and can also impart excellent thermal stability. Offer It can be provided. [Brief explanation of the drawings]

[0018] [Figure 1] Electron microscope photographs of primary particles of the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2, where (a) is an electron microscope photograph of the positive electrode active material of Example 1, (b) is an electron microscope photograph of the positive electrode active material of Comparative Example 1, and (c) is an electron microscope photograph of the positive electrode active material of Comparative Example 2. [Figure 2] 1 shows particle size distribution curves of particle sizes and frequencies of the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2. [Figure 3]1 is a graph showing the relationship between temperature (T) and the value (dW / dT) obtained by differentiating the weight change (W) with respect to temperature (T), based on the results of thermogravimetric differential thermal analysis of the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2. [Figure 4] 1 is a graph plotting the cycle retention rate at each cycle in a 100-cycle charge-discharge test in which the upper limit voltage was set to 4.30 V for nonaqueous electrolyte secondary batteries using the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2 for the positive electrode. [Figure 5] 1 is a graph plotting cycle retention rates at each cycle in a 100-cycle charge-discharge test in which the upper limit voltage was set to 4.40 V for nonaqueous electrolyte secondary batteries using the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2 for the positive electrode. [Figure 6] 1 is a graph plotting the cycle retention rate at each cycle in a 100-cycle charge-discharge test in which the upper limit voltage was set to 4.50 V for nonaqueous electrolyte secondary batteries using the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2 for the positive electrode. DETAILED DESCRIPTION OF THE INVENTION

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description of preferred embodiments is merely exemplary in nature and is not intended to limit the present invention, its application, or uses.

[0020] <Method of manufacturing a positive electrode active material for a non-aqueous electrolyte secondary battery> The present invention Cathode active material for non-aqueous electrolyte secondary batteries The positive electrode active material contains at least Li and Ni. With materials can be, the Made Construction At least the following steps are performed in order: This is done by . (1) A step of mixing a precursor compound containing at least Ni with a lithium compound in a non-solvent system to prepare a mixture. (2) Pre-firing the mixture at 450°C to 700°C in an oxygen-free atmosphere (3) A step of firing the mixture after the pre-firing in an oxygen atmosphere.

[0021] [Process (1)] In step (1), a precursor compound containing at least Ni and a lithium compound are mixed to prepare a mixture.

[0022] The method for synthesizing the precursor compound is not particularly limited, and for example, a method can be employed in which an aqueous solution containing an aqueous solution of a nickel compound and various aqueous solutions of compounds containing other elements according to the composition of the target positive electrode active material is dropped into a reaction tank in which an aqueous alkaline solution such as an aqueous sodium hydroxide solution or an ammonia solution is stirred as a mother liquid, and while also dropping sodium hydroxide or the like, the pH is monitored and controlled to be within an appropriate range, and the mixture is coprecipitated mainly as hydroxide by a wet reaction.

[0023] In the synthesis reaction, after preparing the aqueous alkaline solution to serve as the mother liquid, it is preferable to create a nitrogen atmosphere in the reaction tank using an inert gas, or industrially preferably nitrogen gas, to reduce the oxygen concentration in the reaction tank system and in the solution as much as possible. If the oxygen concentration is too high, there is a risk that the coprecipitated hydroxide may be oxidized by a predetermined amount or more of remaining oxygen, or that the formation of aggregates by crystallization may be hindered.

[0024] The nickel compound is not particularly limited, but examples thereof include nickel sulfate, nickel oxide, nickel hydroxide, nickel nitrate, nickel carbonate, nickel chloride, nickel iodide, and metallic nickel.

[0025] The other elements constituting the positive electrode active material are not particularly limited, but include, for example, cobalt (Co), aluminum (Al), manganese (Mn), titanium (Ti), magnesium (Mg), zinc (Zn), niobium (Nb), tungsten (W), molybdenum (Mo), vanadium (V), chromium (Cr), calcium (Ca), iron (Fe), gallium (Ga), strontium (Sr), yttrium (Y), ruthenium (Ru), indium (In), tin (Sn), tantalum (Ta), bismuth (Bi), zirconium (Zr), boron (B), and the like.

[0026] The compound containing another element is not particularly limited, but includes, for example, a cobalt compound, an aluminum compound, a manganese compound, a titanium compound, a magnesium compound, a zinc compound, a niobium compound, a tungsten compound, and the like.

[0027] The cobalt compound is not particularly limited, but examples thereof include cobalt sulfate, cobalt oxide, cobalt hydroxide, cobalt nitrate, cobalt carbonate, cobalt chloride, cobalt iodide, and metallic cobalt.

[0028] The aluminum compound is not particularly limited, but examples thereof include aluminum sulfate, aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum carbonate, aluminum chloride, aluminum iodide, sodium aluminate, and metallic aluminum.

[0029] The manganese compound is not particularly limited, but examples thereof include manganese sulfate, manganese oxide, manganese hydroxide, manganese nitrate, manganese carbonate, manganese chloride, manganese iodide, and manganese metal.

[0030] The titanium compound is not particularly limited, but examples thereof include titanyl sulfate, titanium oxide, titanium hydroxide, titanium nitrate, titanium carbonate, titanium chloride, titanium iodide, and metallic titanium.

[0031] The magnesium compound is not particularly limited, but examples thereof include magnesium sulfate, magnesium oxide, magnesium hydroxide, magnesium nitrate, magnesium carbonate, magnesium chloride, magnesium iodide, and metallic magnesium.

[0032] The zinc compound is not particularly limited, but examples thereof include zinc sulfate, zinc oxide, zinc hydroxide, zinc nitrate, zinc carbonate, zinc chloride, zinc iodide, and metallic zinc.

[0033] The niobium compound is not particularly limited, but examples thereof include niobium oxide, niobium chloride, lithium niobate, and niobium iodide.

[0034] The tungsten compound is not particularly limited, but examples thereof include tungsten oxide, sodium tungstate, ammonium paratungstate, tungsten hexacarbonyl, and tungsten sulfide.

[0035] The compounding ratio of the nickel compound to various compounds containing other elements may be appropriately adjusted in consideration of the composition of the target positive electrode active material so that the amount of Ni and the amount of various other elements are in a desired ratio.

[0036] The target positive electrode active material is, for example, a compound represented by the following formula (I): Li a Ni b M 1-b O2(I) (wherein M is an element other than Li, Ni, and O, 0.95≦a≦1.15, 0.80≦b<1) In the case where the composition is represented by the formula (I), the ratio of the amount of Ni to the amounts of various other elements, i.e., b in the formula (I), is preferably 0.80≦b<1, more preferably 0.82≦b≦0.98, and particularly preferably 0.82≦b≦0.96.

[0037] In addition, M in the formula (I) is, for example, Co, Al, Mn, Ti, Mg, Zn, Nb, W, Mo, V, Cr, Ca, Fe, Ga, Sr, Y, Ru, In, Sn, Ta, Bi, Zr, B, etc., which are exemplified as the other elements, and among these, it is particularly preferable that Co and / or Al is included.

[0038] The appropriate pH range for synthesizing the precursor compound is preferably 11.0 to 13.5. By controlling the pH during the reaction within this range, a high pH allows for the synthesis of aggregated particles with a small average secondary particle size, and a low pH allows for the synthesis of aggregated particles with a large average secondary particle size.

[0039] As described above, it is preferable to subject the precursor compound obtained by the wet reaction to a washing treatment, dehydration, and then drying treatment.

[0040] By carrying out the washing treatment, impurities such as sulfate groups, carbonate groups, and sodium that are incorporated into the aggregated particles during the reaction or that adhere to the surface can be washed away. For small amounts of the washing treatment, Nutsche washing using a Buchner funnel or a method of sending the suspension after the reaction through a press filter, washing with water, and dehydration can be employed. For the washing treatment, pure water, an aqueous sodium hydroxide solution, an aqueous sodium carbonate solution, etc. can be used, but pure water is preferably used industrially. However, if there is a large amount of residual sulfate groups, it is preferable to carry out washing treatment using an aqueous sodium hydroxide solution whose pH is controlled according to the amount of residual sulfate groups.

[0041] Next, the precursor compound thus synthesized and a lithium compound are mixed in a predetermined ratio to prepare a mixture, and the mixing is carried out in a non-solvent system. By mixing the precursor compound and the lithium compound in a non-solvent system, it is possible to easily mix, in addition to the precursor compound and various lithium compounds, for example, a compound of the element represented by M in formula (I) above, or an additive compound containing an element not substituted by the precursor.

[0042] In addition, this Clearly oh And, The term "mixing in a non-solvent system" refers to mixing without preparing each compound into a solution such as an aqueous solution and without using a solvent such as water, in other words, by weighing out the powder of the precursor compound and the powder of the lithium compound to have a predetermined mixing ratio and mixing them in a dry state.

[0043] The lithium compound is not particularly limited, and various lithium salts can be used. Examples of the lithium compound include anhydrous lithium hydroxide, lithium hydroxide monohydrate, lithium nitrate, lithium carbonate, lithium acetate, lithium bromide, lithium chloride, lithium citrate, lithium fluoride, lithium iodide, lithium lactate, lithium oxalate, lithium phosphate, lithium pyruvate, lithium sulfate, and lithium oxide. Among these, anhydrous lithium hydroxide and lithium hydroxide monohydrate are particularly preferred.

[0044] In the present invention, it is considered that the important point in selecting the lithium compound is related to ensuring that the precursor compound and the lithium compound react with each other without generating a different phase in the pre-baking step (2). In addition, it is known that the reaction between the precursor compound and the lithium compound starts at a temperature lower than the melting point of the lithium compound.

[0045] Therefore, in the present invention, when lithium hydroxide is used as the lithium compound, for example, the pre-baking temperature is set to be higher than 450°C, which is about 20°C lower than the melting point of lithium hydroxide, 462°C, thereby ensuring a reaction between the precursor compound and lithium hydroxide and preventing the generation of a different phase, and enabling the desired positive electrode active material to be obtained.

[0046] The blending ratio of the lithium compound and the precursor compound may be appropriately adjusted in consideration of the composition of the target positive electrode active material so that the total amount of the amount of Li, the amount of Ni, and optionally the amounts of various other elements is in a desired ratio.

[0047] The target positive electrode active material may be, for example, as described above, represented by the following formula (I): Li a Ni b M 1-b O2(I) (wherein M is an element other than Li, Ni, and O, 0.95≦a≦1.15, 0.80≦b<1) In the case where the composition is represented by the formula (I), the ratio of the amount of Li to the total amount of Ni and optionally various other elements, i.e., a in the formula (I), is preferably 0.95≦a≦1.15, and more preferably 0.96≦a≦1.10.

[0048] [Process (2)] In step (2), the mixture prepared in step (1) is pre-fired in an oxygen-free atmosphere. Clearly In this case, performing step (2) is one of the major features.

[0049] As described above, a positive electrode active material, such as a positive electrode active material that is a nickel-based layered compound having a crystal structure, for example, belonging to the space group R-3m, generally requires a certain amount of oxygen during reaction with a lithium source. In particular, to obtain a positive electrode active material with a high Ni content, a high oxygen partial pressure is required. However, it is difficult to expect a high capacity from a nonaqueous electrolyte secondary battery that uses, in its positive electrode, a positive electrode active material that has been produced by firing under conditions where the oxygen partial pressure is insufficient.

[0050] However, when the partial pressure of oxygen is insufficient, i.e., when the battery is not only fired in an oxygen-free atmosphere but also fired in an oxygen atmosphere after the non-oxygen atmosphere, it is possible to specifically reduce the initial discharge capacity while maintaining the initial charge capacity. Furthermore, it has been found that the cycle characteristics, particularly those under high voltage, are significantly improved, and the thermal stability is also improved, leading to the completion of the present invention.

[0051] The reason why the sequential pre-firing in an oxygen-free atmosphere and the main firing in an oxygen atmosphere significantly improve the cycle characteristics and thermal stability is not clear, but it can be considered, for example, as follows.

[0052] Generally, when attempting to obtain a positive electrode active material containing Ni, especially one with a high Ni content in the metal sites, Ni is highly reductive, i.e., Ni is easily reduced to a divalent state. This causes the divalent Ni in the metal sites to remain at the metal sites or migrate to the Li sites, resulting in an unstable crystal structure. Furthermore, it is known that firing sinters the primary particles, resulting in particle size variations. As a result, nonaqueous electrolyte secondary batteries using positive electrodes with a positive electrode active material with an unstable crystal structure are known to have poor cycle characteristics and thermal stability.

[0053] However, by mixing a precursor compound containing Ni with a lithium compound and pre-baking it in an oxygen-free atmosphere, it is possible to obtain divalent Ni (Ni 2+ ) are uniformly located in the crystal structure of the primary particles, and part of the Li that should be included in the crystal lattice of the final positive electrode active material is located in this Ni 2+ In this way, part of the Ni present in the metal site is replaced by Ni 2+ As a result, Ni 2+ As a result, the cation mixing of Ni at the Li site can be saturated. 2+ This increases the electrostatic bonding force between the Li site and oxygen in the crystal structure, and a kind of pillar effect is manifested at the Li site, which is thought to stabilize the crystal structure even when the Ni content is high.

[0054] Therefore, when used in a non-aqueous electrolyte secondary battery, it is thought that the expansion and contraction of the crystal structure when Li is desorbed and inserted during charging and discharging can be reduced, and the crystal structure is stabilized, which is thought to have improved cycle characteristics and thermal stability. 2+ It is believed that the electrostatic bonding strength between the carbon and oxygen is improved, which stabilizes the crystal structure during charging and discharging, suppresses oxygen release, and provides excellent thermal stability.

[0055] The oxygen-free atmosphere in step (2) may be any atmosphere substantially free of oxygen, for example, an atmosphere having an oxygen concentration of 1 vol% or less, preferably 0.8 vol% or less, and particularly preferably 0.1 vol% or less. The atmosphere having an oxygen concentration of 1 vol% or less may be, for example, at least one of a rare gas such as argon and nitrogen, and is preferably nitrogen.

[0056] The temperature of the pre-baking carried out in an oxygen-free atmosphere is 450° C. to 700° C., preferably 450° C. to 650° C., and more preferably 460° C. to 630° C. If the pre-baking temperature is below the lower limit, Ni present at the metal site may be easily removed. 2+ In this case, a portion of the Li atoms is not sufficiently substituted for the Li site, making it difficult to stabilize the crystal structure, and the effects of improving cycle characteristics and thermal stability cannot be obtained. If the temperature of the pre-firing exceeds the upper limit, the temperature difference with the main firing in the next step (3) performed in an oxygen atmosphere becomes too small, causing the pre-firing product to be over-sintered, which may result in unexpected crystal growth or require a crushing treatment before the next step (3), which may impair productivity. Furthermore, over-sintering may result in insufficient conversion by the main firing, making it difficult to expect improvements in cycle characteristics and thermal stability.

[0057] The pre-baking temperature is preferably determined taking into consideration the composition of the precursor compound and the melting point of the lithium compound used. For example, as described above, when lithium hydroxide is used as in the examples of the present invention, the pre-baking temperature can be set to be higher than 450°C, which is about 20°C lower than the melting point of 462°C, to sufficiently react the precursor compound with lithium hydroxide, preventing unexpected crystal growth and sintering.

[0058] There is no particular limitation on the time for pre-baking in an oxygen-free atmosphere, and the Ni present in the metal site is the same as in the precursor compound. 2+ The time may be such that the cations are retained as such and a portion of them migrates sufficiently to the Li site, but it is preferably 1 to 10 hours, more preferably 2 to 8 hours, for example.

[0059] The pre-firing in the present invention may be carried out in an oxygen-free atmosphere, and may be carried out in a stationary furnace or a roller hearth kiln, for example, in which the mixture is filled into a sagger and pre-firing is carried out. Alternatively, a rotary kiln or the like may be used in which the mixture is pre-firing while being fluidized, allowing the pre-firing to proceed more uniformly. When carrying out pre-firing, it is necessary to ensure that the reaction of the mixture proceeds uniformly in an oxygen-free atmosphere, so it is preferable to select equipment according to the amount of the mixture to be pre-fired.

[0060] [Process (3)] In step (3), the mixture preliminarily fired in step (2) is fired in an oxygen atmosphere.

[0061] The oxygen atmosphere in step (3) may be any atmosphere in which essentially only oxygen is present, and for example, an atmosphere with an oxygen concentration of 80 vol % or more, preferably 90 vol % or more.

[0062] The temperature of the main firing carried out in an oxygen atmosphere is preferably 700°C to 880°C, more preferably 710°C to 830°C. If the main firing temperature is below the lower limit, a cathode active material having the desired crystal structure cannot be obtained, and a large amount of unreacted components may remain, impairing battery characteristics. If the main firing temperature is above the upper limit, crystal growth may proceed too far, resulting in a decrease in the battery characteristics of a nonaqueous electrolyte secondary battery using the resulting cathode active material in the cathode.

[0063] The time for the main baking carried out in an oxygen atmosphere is not particularly limited as long as it is a time sufficient to obtain a positive electrode active material having a desired crystal structure, but it is preferably, for example, 1 hour to 15 hours, and more preferably 2 hours to 10 hours.

[0064] In addition, this BrightIn step (3), the firing temperature is lowered (cooled down) in an oxygen atmosphere, for example, at the temperature and for the time described above, and then the cathode active material is cooled to the desired temperature. The atmosphere during the cooling is not particularly limited, but a low-oxygen concentration atmosphere is preferred, as this can further improve the cycle characteristics of a nonaqueous electrolyte secondary battery using the resulting cathode active material in a cathode. The low-oxygen concentration atmosphere is an atmosphere with an oxygen concentration lower than that of the air atmosphere. For example, an oxygen concentration of 20 vol% or less is sufficient, but an oxygen concentration of 5 vol% or less, or even 1 vol% or less, is preferred. Examples of atmospheres with an oxygen concentration of 1 vol% or less include at least one of rare gases such as argon and nitrogen, with nitrogen being particularly preferred.

[0065] When the positive electrode active material obtained by sequentially performing steps (1) to (3) has a high Ni content, such as a positive electrode active material having a composition represented by formula (I), the amount of remaining Li compounds (hereinafter referred to as "residual Li compounds"), which is the sum of unreacted lithium compounds and Li compounds that are released from the crystalline structure to the particle surface during the firing step, may be larger than that of a positive electrode active material with a low Ni content. The amount of remaining Li compounds can be reduced, for example, by washing the positive electrode active material with water or by performing a surface treatment on the surfaces of the primary particles and / or secondary particles of the positive electrode active material.

[0066] The surface treatment method is not particularly limited, and examples of methods that can be used include a method in which fine particles of aluminum oxide are applied to the particle surface layer of the positive electrode active material in a dry manner while applying shear force, followed by heat treatment at about 300° C. to 700° C., or a method in which a predetermined amount of the positive electrode active material is brought into contact with an aqueous solution in which a predetermined amount of sodium aluminate has been dissolved, stirred for about 5 to 10 minutes, dehydrated, dried, and then heat treated at about 300° C. to 700° C., thereby coating the particle surface layer with an aluminum compound. In addition to aluminum compounds, boron compounds and tungsten compounds can also be used for the surface treatment, and can be selected depending on the application.

[0067] <Cathode active material for non-aqueous electrolyte secondary batteries> Original Ming The properties of a positive electrode active material for a non-aqueous electrolyte secondary battery cannot be determined in general terms because they vary depending on the composition. However, for example, it is preferable that the average particle size (D50) and particle size distribution (full width at half maximum (FWHM)) of the primary particles, the lengths of the a-axis and c-axis of the crystal lattice, the crystallite size, and the amount of cation mixing each fall within the ranges shown below.

[0068] The average particle size (D50) of the primary particles is preferably about 50 nm to about 220 nm, and the particle size distribution (full width at half maximum (FWHM)) of the primary particles is preferably about 80 nm to about 300 nm, more preferably about 80 nm to about 250 nm.

[0069] In this specification, the average particle diameter (D50) is a value obtained based on an electron micrograph (SEM photograph) of primary particles of the positive electrode active material taken using a scanning electron microscope SEM-EDX [S-4300, manufactured by Hitachi High-Technologies Corporation] under the following conditions. The scale displayed in the electron micrograph is used as the reference scale. At least 100 primary particles were observed. (conditions) Accelerating voltage: 10 kV WD:8mm Magnification: 20000x

[0070] In this specification, the particle size distribution (full width at half maximum (FWHM)) is a value determined from a particle size distribution curve of particle sizes and frequencies when the average particle size (D50) is determined.

[0071] The lengths of the a-axis and c-axis of the crystal lattice are preferably 2.840 Å to 2.890 Å, more preferably 2.845 Å to 2.885 Å, and the c-axis is preferably 14.160 Å to 14.220 Å, more preferably 14.170 Å to 14.210 Å.

[0072] The crystallite size is preferably 50 nm to 250 nm, more preferably 60 nm to 230 nm. If the crystallite size is below the lower limit, the crystalline structure of the positive electrode active material may become unstable. If the crystallite size is above the upper limit, the battery characteristics of a nonaqueous electrolyte secondary battery using the positive electrode active material in a positive electrode may be degraded.

[0073] The cation mixing amount generally refers to the following amount: Normally, the site occupancy rate of Li at the Li site is 100%, but during the firing process when producing a positive electrode active material containing Ni, Ni contained mainly in the metal site becomes Ni. 2+ and migrates to the Li site. The amount of metal that migrates to the Li site and replaces Li in this way is called the cation mixing amount, which is known to be normally about 0.1% to 6.0%, but since a smaller cation mixing amount is generally preferred, it is adjusted to about 0.1% to 2.0%.

[0074] Therefore, the present invention Positive In the electrode active material, the amount of cation mixing is also determined by the amount of Ni substituted at the Li site. 2+ The amount of Ni 2+ When a divalent element other than Li is present, the content of the divalent element also varies depending on the amount of the divalent element substituting the Li site, the molar ratio of Li to Ni and any other element, and the like, and is preferably 2.3% to 6.0%, more preferably 2.8% to 6.0%, and particularly preferably 3.2% to 6.0%.

[0075] In this specification, the lengths of the a-axis and c-axis of the crystal lattice, the crystallite size, and the amount of cation mixing are values ​​that are determined by obtaining XRD diffraction data of the positive electrode active material by the following method and then performing Rietveld analysis.

[0076] Using an X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation), XRD diffraction data of the positive electrode active material was obtained under the following X-ray diffraction conditions. The XRD diffraction data was then subjected to Rietveld analysis with reference to R.A. Young, ed., "The Rietvelt Method," Oxford University Press (1992). The analysis was performed so that the S value in the fitting was in the range of 1.20 to 1.45. (X-ray diffraction conditions) Source: Cu-Kα Acceleration voltage and current: 45 kV and 200 mA Sampling width: 0.02 deg. Scanning width: 15°~122° Scan speed: 1.2° / min step Divergence slit width: 0.65 deg. Receiving slit width: 0.2 mm Scattering slit: 0.65 deg.

[0077] The present invention Positive In addition to the above physical properties, the electrode active material also has characteristics in terms of the results of thermal analysis when used in the positive electrode of a non-aqueous electrolyte secondary battery.

[0078] The present invention Positive In the case of electrode active materials, Same as Compared to a positive electrode active material of the same composition, cation mixing, i.e., a state in which a large amount of Ni element is present at the Li site, and even in a state in which Li is extracted (charged state), a certain amount of Ni is present at the Li site, which is thought to slow structural collapse upon heating, resulting in a lower peak height at the peak top temperature. The peak top temperature cannot be determined in general because it varies depending on the composition of the positive electrode active material, but the peak top temperature is preferably 220°C to 280°C, and the peak height at the peak top temperature is preferably 0.25% / °C to 0.45% / °C, or even more preferably 0.25% / °C to 0.40% / °C.

[0079] In this specification, the peak top temperature and the peak height at the peak top temperature are values ​​that can be determined by performing a thermogravimetric differential thermal analysis (TG-DTA measurement) of a positive electrode active material by the following method, and then, based on the obtained results, creating a graph (see FIG. 3 ) with temperature (T) on the horizontal axis and the value (dW / dT) obtained by differentiating the weight change (W) with respect to temperature (T) on the vertical axis.

[0080] A thermogravimetric differential thermal analysis (TG-DTA) instrument [DTG-60H, manufactured by Shimadzu Corporation] was used to perform TG-DTA measurements under the following conditions. A coin cell was fabricated using the positive electrode active material according to the method described below. The positive electrode was charged (cc-cv) to 4.30 V at a current density of 20 mA / g in a 25°C environment. The charged coin cell was then disassembled in an argon-atmosphere glove box, and the positive electrode was removed. The removed positive electrode was washed in dimethyl carbonate for 10 minutes and dried under vacuum. The positive electrode material powder was then scraped off from the aluminum foil in the glove box. 15 mg of the resulting positive electrode material powder was then filled into a platinum container and placed on the measurement balance of the TG-DTA instrument. (TG-DTA measurement conditions) Reference: Platinum container filled with 15 mg of Al2O3 Maximum temperature: 500℃ Heating rate: 10℃ / min Measurement environment: Nitrogen atmosphere (flow rate: 200 mL / min)

[0081] <Nonaqueous electrolyte secondary battery> The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention can be suitably used for the positive electrode of a non-aqueous electrolyte secondary battery.

[0082] The nonaqueous electrolyte secondary battery is composed of the positive electrode, the negative electrode, and an electrolytic solution containing an electrolyte.

[0083] When producing the positive electrode, a conductive agent and a binder are added to and mixed with the positive electrode active material of the present invention according to a conventional method. Examples of the conductive agent include acetylene black, carbon black, and graphite. Examples of the binder include polytetrafluoroethylene and polyvinylidene fluoride.

[0084] For the negative electrode, not only negative electrode active materials such as lithium metal, graphite, and low-crystalline carbon materials, but also at least one nonmetallic or metallic element selected from the group consisting of Si, Al, Sn, Pb, Zn, Bi, and Cd, an alloy containing the same, or a chalcogen compound containing the same can be used.

[0085] As the solvent for the electrolytic solution, for example, an organic solvent containing at least one of carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, and ethers such as dimethoxyethane can be used.

[0086] As the electrolyte, in addition to lithium hexafluorophosphate (LiPF6), at least one kind of lithium salt such as lithium perchlorate or lithium tetrafluoroborate can be dissolved in the solvent and used.

[0087] As mentioned above, negative electrode active materials such as lithium metal, graphite, and low-crystalline carbon materials are often used for the negative electrodes of non-aqueous electrolyte secondary batteries, but there has been an increasing trend to use alloy materials containing nonmetals or metals such as Si and Sn as negative electrode active materials. However, negative electrodes using such alloy materials have a very large initial irreversible capacity (very low initial charge / discharge efficiency) compared to negative electrodes using lithium metal, graphite, low-crystalline carbon materials, and the like, and therefore must be used after consuming excess lithium in advance (Li pre-doping) (after performing a separate charging process).

[0088] Here, the present invention Positive Conventionally, non-aqueous electrolyte secondary batteries using electrode active materials for the positive electrode have PositiveCompared with the case where the electrode active material is used, the initial charge capacity is almost the same, but the initial discharge capacity is low and the initial charge / discharge efficiency tends to be low, that is, the initial irreversible capacity tends to be large. As a result, the excess lithium in the positive electrode during the initial charge can be reacted in the negative electrode, and it is thought that the same operation as Li pre-doping can be performed. That is, the negative electrode made of the alloy material and the present invention Positive When used in combination with a positive electrode made of a negative electrode active material, the Li pre-doping can be easily performed in the positive electrode instead of the negative electrode, which has the advantage of increasing the utility value of the alloy material as a negative electrode active material.

[0089] <effect> The present invention Positive The electrode active material can impart excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, to the non-aqueous electrolyte secondary battery, and can also impart excellent thermal stability. [Example]

[0090] The present invention will be specifically described below with reference to representative examples and comparative examples of the present invention, but the present invention is not limited to these examples.

[0091] <Production of precursor compounds> [Precursor compound 1] A mixed aqueous solution was obtained by mixing an aqueous nickel sulfate solution, an aqueous cobalt sulfate solution, and an aqueous aluminum sulfate solution so that the molar ratio of Ni to Co to Al was Ni:Co:Al = 85:10:5. 10 L of pure water containing 300 g of sodium hydroxide solution and 500 g of ammonia water was prepared in advance in the reaction vessel as a mother liquor, and a nitrogen atmosphere was created in the reaction vessel by adding nitrogen gas at a flow rate of 0.7 L / min. The reaction was also carried out in a nitrogen atmosphere.

[0092] Thereafter, while rotating the stirring blade at 1000 rpm, the mixed aqueous solution, sodium hydroxide aqueous solution, and ammonia water were simultaneously added dropwise at a predetermined rate, and the amount of the alkaline solution added was adjusted so that the pH was 12.5. Through a crystallization reaction, Ni, Co, and Al were crystallized and coprecipitated to form aggregated particles, thereby obtaining a coprecipitate.

[0093] The slurry in the reactor was then subjected to solid-liquid separation and further washed with pure water to reduce residual impurities. The coprecipitate cake was then dried at 110°C for 12 hours in an air environment to obtain precursor compound 1.

[0094] [Precursor compound 2] Precursor compound 2 was obtained in the same manner as precursor compound 1, except that an aqueous nickel sulfate solution, an aqueous cobalt sulfate solution, and an aqueous aluminum sulfate solution were mixed so that the ratio (molar ratio) of Ni, Co, and Al was Ni:Co:Al=90:5:5.

[0095] <Composition of precursor compound and positive electrode active material> The compositions of the precursor compound and the positive electrode active material were determined as follows. A 0.2 g sample of the precursor compound or the positive electrode active material was heated and dissolved in 25 mL of 20% hydrochloric acid solution. After cooling, the sample was transferred to a 100 mL measuring flask and purified water was added to prepare a solution. The elements in the solution were quantified using ICP-AES [Optima 8300, manufactured by PerkinElmer Co., Ltd.].

[0096] <Coin cell using positive electrode active material> A 2032-type coin cell using the positive electrode active material was manufactured using a positive electrode, a negative electrode, and an electrolyte solution prepared by the following methods. (positive electrode) Acetylene black and graphite were used as conductive agents in a weight ratio of 1:1, and polyvinylidene fluoride was used as a binder. The positive electrode active material, conductive agent, and binder were mixed in a weight ratio of 90:6:4, and these were mixed in N-methylpyrrolidone to form a slurry, which was then applied to aluminum foil. This was dried at 110°C to produce a sheet, which was then punched out to a diameter of 15 mm, and the density of the composite was 3.0 g / cm. 3 The resultant was rolled to form a positive electrode. (Negative electrode) A lithium foil with a thickness of 500 μm and punched to 16 mm diameter was used as the negative electrode. (electrolyte) A mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) was prepared so that the volume ratio of EC:DMC was 1:2, and this was mixed with 1M LiPF6, which was the electrolyte, to prepare an electrolytic solution.

[0097] <Initial Charge Capacity, Initial Discharge Capacity, and Initial Charge / Discharge Efficiency of Nonaqueous Electrolyte Secondary Battery> The coin cell manufactured by the above method was charged at a constant current density of 20 mA / g up to 4.30 V (upper voltage limit) in an environment of 25°C, and then charged at a constant voltage until the current reached 2 mA / g. The capacity at this time was defined as the initial charge capacity (mAh / g).

[0098] Next, after a 5-minute pause, constant-current discharge was performed at a current density of 20 mA / g up to 2.80 V under the same environment, and after a 5-minute pause, the initial discharge capacity (mAh / g) was measured. The series of steps up to the measurement of the initial discharge capacity was defined as the charge-discharge cycle under condition A.

[0099] The initial charge / discharge efficiency was calculated using the measured values ​​of the initial charge capacity and the initial discharge capacity according to the following formula. Initial charge / discharge efficiency (%) = (initial discharge capacity / initial charge capacity) x 100

[0100] <Cycle characteristics of non-aqueous electrolyte secondary batteries> (1) 4.3V cycle retention rate After measuring the initial discharge capacity, 100 charge / discharge cycles were performed under the condition A. Using the measured value of the discharge capacity at the first cycle and the measured value of the discharge capacity at the 100th cycle, the 4.3V cycle retention rate was calculated according to the following formula. 4.3V cycle retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0101] (2) 4.4V cycle retention rate Under the same conditions as in Condition A, except that the upper limit voltage was changed from 4.30 V to 4.40 V, 100 charge / discharge cycles were performed, and the 4.4 V cycle retention rate was calculated according to the following formula. 4.4V cycle retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0102] (3) 4.5V cycle retention rate Under the same conditions as in Condition A, except that the upper limit voltage was changed from 4.30 V to 4.50 V, 100 charge / discharge cycles were performed, and the 4.5 V cycle retention rate was calculated according to the following formula. 4.5V cycle retention rate (%) = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0103] Example 1 The precursor compound 1 and anhydrous lithium hydroxide were weighed out so that the ratio (molar ratio) of Li to the total amount of Ni, Co, and Al was Li / (Ni+Co+Al) = 1.03, and these were mixed in a non-solvent system using a mixer to prepare a mixture.

[0104] Next, the mixture was pre-fired in an electric furnace under a nitrogen atmosphere (oxygen concentration: 1 vol % or less) at 500° C. for 6 hours.

[0105] Next, the pre-baked mixture was baked in an oxygen atmosphere (oxygen concentration: 97 vol%) at a maximum temperature of 750°C for 3 hours, and then cooled in a nitrogen atmosphere (oxygen concentration: 1 vol% or less) to obtain a positive electrode active material.

[0106] <Comparative Example 1> A positive electrode active material was obtained in the same manner as in Example 1, except that the pre-baking was not carried out and the mixture was baked under the conditions for the main baking.

[0107] <Comparative Example 2> A positive electrode active material was obtained in the same manner as in Example 1, except that the temperature of the pre-baking was changed to 400°C.

[0108] The firing conditions in Example 1 and Comparative Examples 1 and 2 are summarized in Table 1.

[0109] The properties of the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2 were determined according to the above-mentioned methods, including the average particle size (D50) and particle size distribution (FWHM) of the primary particles, the lengths of the a-axis and c-axis of the crystal lattice, the crystallite size, the amount of cation mixing, the peak top temperature, and the peak height at the peak top temperature. These results are shown in Table 2.

[0110] Furthermore, the battery characteristics of the nonaqueous electrolyte secondary batteries using the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2 in the positive electrode were determined according to the above-mentioned methods, including the initial charge capacity, initial discharge capacity, initial charge / discharge efficiency, 4.3 V cycle retention rate, 4.4 V cycle retention rate, and 4.5 V cycle retention rate. These results are shown in Table 3.

[0111] Furthermore, FIG. 1 shows electron microscope photographs (SEM photographs) of primary particles of the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2, and FIG. 2 shows particle size distribution curves of particle sizes and frequencies of the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2.

[0112] FIG. 3 shows a graph illustrating the relationship between temperature (T) and the value (dW / dT) obtained by differentiating the weight change (W) with respect to temperature (T), based on the results of thermogravimetric differential thermal analysis (TG-DTA) of the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2.

[0113] 4, 5, and 6 are graphs plotting the cycle retention ratios at each cycle in a 100-cycle charge-discharge test when determining the "(1) 4.3 V cycle retention ratio," "(2) 4.4 V cycle retention ratio," and "(3) 4.5 V cycle retention ratio" for nonaqueous electrolyte secondary batteries using the positive electrode active materials obtained in Example 1 and Comparative Examples 1 and 2. The cycle retention ratios at each cycle were calculated according to the following formula (n=1 to 100): Cycle maintenance rate (%) = (discharge capacity at nth cycle / discharge capacity at 1st cycle) × 100

[0114] [Table 1]

[0115] [Table 2]

[0116] [Table 3]

[0117] Example 1 of The positive electrode active materials obtained according to the manufacturing method are Comparative Examples 1 and 2. Nino Compared with the positive electrode active material obtained by the manufacturing method, the crystallite size is smaller and the amount of cation mixing is much larger. In addition, in the thermal analysis, although the peak top temperature is about the same, the peak height at the peak top temperature is about 60%. From these facts, it can be seen that in the positive electrode active material of Example 1, Ni present in the metal site 2+It is thought that a portion of the cations is sufficiently substituted and fixed at the Li site, saturating the cation mixing, increasing the electrostatic bonding force with oxygen, resulting in a pillar effect and stabilizing the crystal structure.

[0118] The nonaqueous electrolyte secondary battery using the positive electrode active material of Example 1 having such characteristics in the positive electrode has excellent cycle characteristics compared to the nonaqueous electrolyte secondary batteries using the positive electrode active materials of Comparative Examples 1 and 2 in the positive electrode, and exhibits particularly excellent cycle characteristics even at high voltages of 4.4 V or 4.5 V.

[0119] <Example 2> The precursor compound 2 and anhydrous lithium hydroxide were weighed out so that the ratio (molar ratio) of Li to the total amount of Ni, Co, and Al was Li / (Ni+Co+Al) = 1.03, and these were mixed in a non-solvent system using a mixer to prepare a mixture.

[0120] Next, the mixture was pre-fired in an electric furnace under a nitrogen atmosphere (oxygen concentration: 1 vol % or less) at 500° C. for 6 hours.

[0121] Next, the pre-baked mixture was baked at a maximum temperature of 750°C for 2 hours in an oxygen atmosphere (oxygen concentration: 97 vol%), and then cooled in a nitrogen atmosphere (oxygen concentration: 1 vol% or less) to obtain a positive electrode active material.

[0122] Example 3 A positive electrode active material was obtained in the same manner as in Example 2, except that after the main baking, the temperature was lowered in an oxygen atmosphere (oxygen concentration: 97 vol%).

[0123] The firing conditions in Examples 2 and 3 are summarized in Table 4.

[0124] The properties of the positive electrode active materials obtained in Examples 2 and 3 were determined according to the above-mentioned methods, including the lengths of the a-axis and c-axis of the crystal lattice, the crystallite size, and the amount of cation mixing. These results are shown in Table 5.

[0125] Furthermore, the initial charge capacity, initial discharge capacity, and initial charge / discharge efficiency were determined according to the above-mentioned methods as battery characteristics for the nonaqueous electrolyte secondary batteries using the positive electrode active materials obtained in Examples 2 and 3 as positive electrodes. These results are shown in Table 6.

[0126] [Table 4]

[0127] [Table 5]

[0128] [Table 6]

[0129] In Examples 2 and 3, the present invention Positive The positive electrode active material has a relatively small crystallite size. In particular, in the positive electrode active material of Example 2 obtained by lowering the temperature in a low-oxygen concentration atmosphere after the main firing, Ni was substituted from the metal site to the Li site and fixed. 2+ It is believed that the amount of cations mixed is greater than that of the cations mixed in the saturation state.

[0130] It is clear that the nonaqueous electrolyte secondary batteries using the positive electrode active materials of Examples 2 and 3 having such properties in the positive electrode have small initial charge-discharge efficiency values ​​and can achieve excellent cycle characteristics. Furthermore, when Example 2 and Example 3 are compared, it is clear that the positive electrode active material of Example 2 obtained by calcining and then lowering the temperature in a low-oxygen concentration atmosphere has a smaller initial charge-discharge efficiency value and can achieve even more excellent cycle characteristics. [Industrial Applicability]

[0131] The present invention Ru The electrode active material can provide excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, and can also provide excellent thermal stability, making it suitable for the positive electrode of a non-aqueous electrolyte secondary battery.

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery containing at least lithium and nickel, The average particle diameter (D50) of the primary particles is 50 nm to 220 nm, The particle size distribution of the primary particles is 80 nm to 300 nm, The length of the a-axis of the crystal lattice is 2.840 Å to 2.890 Å, and The length of the c-axis of the crystal lattice is 14.160 Å to 14.220 Å; A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the cation mixing amount, which is the amount of metal that has migrated from a metal site to a lithium site and substituted for lithium at the lithium site, is 2.3% to 6.0%.

2. The following formula (I): Li ａ Ni ｂ M １－ｂ O ２ (I) (In the formula, M is at least one element other than Li, Ni, and O, a is 0.95≦a≦1.15, and b is 0.80≦b<1) The positive electrode active material according to claim 1 , having a composition represented by the formula:

3. The positive electrode active material according to claim 2 , wherein in formula (I), b satisfies 0.82≦b≦0.

98.

4. The positive electrode active material according to claim 2 or 3, wherein in formula (I), M contains Co and / or Al.

5. A positive electrode active material for a non-aqueous electrolyte secondary battery containing at least lithium and nickel, The peak height at the peak top temperature is 0.25% / °C to 0.45% / °C, The peak top temperature and the peak height at the peak top temperature are values ​​that can be determined by performing a thermogravimetric differential thermal analysis of the positive electrode active material under predetermined measurement conditions using a thermogravimetric differential thermal analyzer, and then plotting a graph based on the obtained results, with the horizontal axis representing temperature (T) and the vertical axis representing a value (dW / dT) obtained by differentiating the weight change (W) with respect to the temperature (T).

6. The following formula (I): Li ａ Ni ｂ M １－ｂ O ２ (I) (In the formula, M is at least one element other than Li, Ni, and O, a is 0.95≦a≦1.15, and b is 0.80≦b<1) The positive electrode active material according to claim 5 , having a composition represented by the formula:

7. The positive electrode active material according to claim 6 , wherein in formula (I), b satisfies 0.82≦b≦0.

98.

8. The positive electrode active material according to claim 6 or 7, wherein in formula (I), M contains Co and / or Al.

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