Positive electrode active material for non-aqueous electrolyte secondary battery and method for producing same

Abstract

This method is characterized by performing, at least in sequence: a step (1) for preparing a mixture by mixing, in a non-solvent system, a precursor compound containing at least nickel and a lithium compound; a step (2) for performing preliminary-baking on the mixture at 450-700 °C in a non-oxygen atmosphere; and a step (3) for performing, in an oxygen atmosphere, main-baking on the mixture which has been subjected to the preliminary-baking. According to this method, it is possible to easily produce a positive electrode active material which contains at least lithium and nickel, and can impart, to a non-aqueous electrolyte secondary battery, cycle characteristics, in particular, excellent cycle characteristics under high voltage.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which can impart excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, to a non-aqueous electrolyte secondary battery, and can also impart excellent thermal stability. [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 firing is then 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 conventional problems, and an object of the present invention is to provide a method for easily producing a positive electrode active material that can impart excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, to a nonaqueous electrolyte secondary battery. [Means for solving the problem]

[0015] In order to achieve the above object, the present invention provides a method for producing a positive electrode active material by subjecting a mixture of a precursor compound and a lithium compound to two-stage firing in different atmospheres, i.e., firing in a non-oxygen atmosphere within a specific temperature range and firing in an oxygen atmosphere, in that order.

[0016] The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention includes the steps of: 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 method can be provided for easily producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which has a stable crystal structure despite a high nickel content and can therefore impart excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, to a non-aqueous electrolyte secondary battery, and can also impart excellent thermal stability. [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 the cycle retention rate 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 its uses.

[0020] <Method of manufacturing a positive electrode active material for a non-aqueous electrolyte secondary battery> The production method according to the present invention is a method for producing a positive electrode active material containing at least Li and Ni, and in this production method, at least the following steps are carried out in order. (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 the production method of the present invention, "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-baked in an oxygen-free atmosphere. One of the major features of the production method of the present invention is the inclusion of step (2).

[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 the manufacturing method of the present invention, in step (3), after performing the main calcination in an oxygen atmosphere, for example, at the temperature and for the time described above, the calcination temperature is lowered (cooled down) to cool the positive electrode active material to the desired temperature. The atmosphere during the temperature drop is not particularly limited, but a low-oxygen concentration atmosphere is preferred because it can further improve the cycle characteristics of a non-aqueous electrolyte secondary battery using the resulting positive electrode active material in a positive electrode. 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 nonaqueous electrolyte secondary batteries> The properties of the positive electrode active material for a non-aqueous electrolyte secondary battery obtained by the production method of the present invention cannot be determined in general 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 deteriorate.

[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, in the positive electrode active material obtained by the manufacturing method of the present invention, the amount of cation mixing is also 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] In addition to the above-mentioned physical properties, the positive electrode active material obtained by the manufacturing method of the present invention 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] In the case of the cathode active material obtained by the manufacturing method of the present invention, compared to a cathode active material of the same composition obtained by a conventional manufacturing method, 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, and structural collapse upon heating is gradual, so it is thought that this results 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 cathode 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, a non-aqueous electrolyte secondary battery using a positive electrode active material obtained by the manufacturing method of the present invention as a positive electrode has a substantially equivalent initial charge capacity compared to a battery using a positive electrode active material obtained by a conventional manufacturing method, but a lower initial discharge capacity and a lower initial charge / discharge efficiency, i.e., a tendency for the initial irreversible capacity to be large. As a result, the excess lithium in the positive electrode during initial charging can be reacted at the negative electrode, which is thought to enable the same operation as Li pre-doping. That is, when a negative electrode made of the alloy material and a positive electrode made of the positive electrode active material obtained by the manufacturing method of the present invention are used in combination, the Li pre-doping can be easily performed at 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 positive electrode active material easily obtained by the production method of the present invention can impart excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, to a nonaqueous 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 non-aqueous electrolyte secondary batteries> 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 based on the measured initial charge capacity and the measured 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] The positive electrode active material obtained in Example 1 according to the manufacturing method of the present invention has a smaller crystallite size and a much larger amount of cation mixing than the positive electrode active materials obtained in Comparative Examples 1 and 2 by conventional manufacturing methods. Furthermore, in thermal analysis, the peak top temperatures are similar, but the peak height at the peak top temperature is about 60%. From these findings, it can be concluded that the positive electrode active material of Example 1 has Ni present in the metal sites. 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] The positive electrode active materials obtained according to the manufacturing method of the present invention in Examples 2 and 3 have relatively small crystallite sizes. In particular, the positive electrode active material of Example 2, which was obtained by lowering the temperature in a low-oxygen concentration atmosphere after the main firing, had Ni ions substituted from the metal site to the Li site and fixed thereon. 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 positive electrode active material obtained by the production method according to the present invention can be imparted with excellent cycle characteristics, particularly excellent cycle characteristics under high voltage, and can also be imparted with excellent thermal stability, and is therefore suitable for use as a positive electrode in a non-aqueous electrolyte secondary battery.

Claims

1. A method for producing a positive electrode active material containing at least lithium and nickel, comprising: Step (1) of preparing a mixture by mixing a precursor compound containing at least nickel 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; and the oxygen atmosphere has an oxygen concentration of 80 vol % or more.

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

3. The method according to claim 2, wherein in formula (I), b satisfies 0.82≦b≦0.

98.

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

5. 3. The method according to claim 1, wherein lithium hydroxide is used as the lithium compound.

6. 3. The production method according to claim 1 or 2, wherein when the precursor compound and the lithium compound are mixed in a non-solvent system, a compound of an element other than lithium, nickel, and oxygen, and / or an additive compound containing an element that is not substituted by the precursor compound is further mixed.

7. 3. The manufacturing method according to claim 1, wherein the oxygen-free atmosphere has an oxygen concentration of 1 vol % or less.

8. The manufacturing method according to claim 7 , wherein the atmosphere having an oxygen concentration of 1 vol % or less is a nitrogen atmosphere.

9. The method according to claim 1 or 2, wherein the temperature of the main firing is 700°C to 880°C.

10. The method according to claim 1 or 2, wherein in the step (3), after the main firing, the firing temperature is lowered to cool the positive electrode active material to a desired temperature.

11. The method according to claim 10, wherein the atmosphere in which the temperature is lowered during the firing is a low-oxygen atmosphere, and the low-oxygen atmosphere has an oxygen concentration lower than that of the air atmosphere.

12. The manufacturing method according to claim 11 , wherein the low oxygen concentration atmosphere is an atmosphere having an oxygen concentration of 5 vol % or less.

13. The manufacturing method according to claim 11 , wherein the low oxygen concentration atmosphere is an atmosphere having an oxygen concentration of 1 vol % or less.

14. The manufacturing method according to claim 13 , wherein the atmosphere having an oxygen concentration of 1 vol % or less is a nitrogen atmosphere.

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