JP2015088343A - Method for manufacturing positive electrode active material for nonaqueous electrolyte secondary batteries - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium nickelate-based positive electrode active material arranged so that the generation of gas can be suppressed, and enables the achievement of a high charge and discharge capacities and good cycle characteristics.SOLUTION: A method for manufacturing a positive electrode active material for nonaqueous electrolyte secondary batteries comprises: a cleaning step of preparing cleaned particles by rinsing, by deionized water, core particles including a lithium transition metal complex oxide expressed by the general formula, LiNiMO(where 0.95≤a≤1.05; 0≤x≤0.25; M is at least one element selected from Co, Mn and Al); a mixing step of preparing mix particles by mixing the cleaned particles with a boron compound containing oxygen; and a thermal treatment step of performing a thermal treatment on the mix particles at a thermal treatment temperature of 100-400°C, thereby preparing the positive electrode active material.

Description

  The present invention relates to a method for producing a positive electrode active material. In particular, the present invention relates to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery using a lithium nickel oxide-based lithium transition metal composite oxide.

  In recent years, portable devices such as VTRs, cellular phones, and notebook personal computers have become widespread and miniaturized, and non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been used for the power supply. Furthermore, it has been attracting attention as a power battery for electric vehicles and the like due to recent environmental problems.

  As a positive electrode active material of a lithium ion secondary battery, lithium cobaltate having a layered structure is typically put into practical use. However, since valuable cobalt is used in terms of resources, there is a cost disadvantage. In view of these circumstances, lithium nickelate having a layered structure using nickel as a transition metal has been studied. Lithium nickelate has the advantage that it has a higher charge / discharge capacity per unit weight than lithium cobaltate, but also has the disadvantage that it is difficult to synthesize and unreacted raw materials are likely to be generated. When lithium nickelate in which unreacted raw material remains is used as the positive electrode active material, the unreacted raw material is decomposed during charging, causing gas generation. Since the pressure is increased inside the battery due to the generation of gas, when the shape of the battery container is other than the cylindrical shape, the shape of the battery container changes in most cases because it cannot withstand the pressure change. Even if the structure can withstand pressure changes, there is a risk of the battery container bursting, so some countermeasures are necessary. Other problems are different from lithium cobaltate.

  In a lithium transition metal composite oxide (so-called lithium nickelate type) lithium oxide using nickel as a main component of a transition metal, such as lithium nickelate, there is a technique of cleaning with pure water or the like according to the purpose.

  Patent Document 1 discloses a technique of washing or pickling a positive electrode active material for the purpose of reducing impurities in the positive electrode and improving discharge capacity in a secondary battery using lithium nickelate as the positive electrode active material. Yes.

  Patent Document 2 discloses a technique in which a lithium transition metal composite oxide containing nickel as an essential component is washed with water by a specific method to remove a substance that causes gas generation such as lithium carbonate.

  Patent Document 3 discloses a technique for washing the positive electrode active material with pure water while adjusting the pH range of the supernatant to a specific range in order to suppress gelation of the positive electrode slurry.

  According to Patent Document 4, when lithium nickel composite oxide is randomly washed with water, elution of lithium ions and structural change of the positive electrode active material due to high temperature are caused. For this reason, in patent document 4, in order to improve the thermal stability at the time of high temperature, the washing time is prescribed | regulated on the specific conditions.

  On the other hand, depending on various purposes, there is a technique in which a boron compound such as boric acid and a lithium transition metal composite oxide are mixed, or a boron compound is present on the surface of the lithium transition metal composite oxide.

  In Patent Document 5, in a positive electrode using lithium manganate, a boron compound soluble in an electrolyte such as boron oxide, orthoboric acid, metaboric acid, tetraboric acid, or the like, and a halogenated spinel lithium manganate are included. A technique for improving the cycle characteristics by suppressing the reaction with hydroacid has been disclosed.

  In Patent Document 6, on the surface of the lithium transition metal composite oxide, a surface treatment layer having excellent ion conductivity including a hydroxide, an oxyhydroxide, or the like of a coating element such as boron is formed to increase an average discharge potential, A technique for improving the life characteristics is disclosed. Specifically, the coating technique disclosed is formed by depositing a coating element dissolved in a solvent on the surface of the lithium transition metal composite oxide and removing the solvent.

  Patent Document 7 discloses a technique for preventing gelation of an electrode paste by containing boric acid or the like as an inorganic acid in an electrode using a lithium transition metal composite oxide or the like. As a specific lithium transition metal composite oxide, lithium nickelate is disclosed.

  In Patent Document 8, boric acid compounds such as ammonium borate and lithium borate are deposited on the surface of lithium transition metal composite oxide particles essentially containing nickel or cobalt and heat-treated in an oxidizing atmosphere. Thus, a technique for increasing the capacity of the secondary battery and improving the charge / discharge efficiency of the secondary battery is disclosed. The lithium transition metal complex oxide specifically disclosed is lithium nickelate in which a part of nickel is substituted with cobalt and aluminum.

JP-A-6-111820 JP 2003-017054 A JP 2003-031222 A JP 2003-273108 A JP 2001-257003 A JP 2002-124262 A JP-A-10-079244 JP 2009-146739 A

  According to Patent Document 1, it is said that the charge / discharge capacity is improved by removing impurities by washing with water. However, according to the study by the present inventors, it has been confirmed that the charge / discharge capacity tends to decrease. Moreover, the tendency for the cycle characteristics to deteriorate was also confirmed. As also in Patent Document 2, washing the lithium nickel oxide-based lithium transition metal composite oxide with water is effective in suppressing gas generation, but cannot be compatible with charge / discharge capacity and cycle characteristics, A solution was sought.

  The present invention has been made in view of these circumstances. An object of the present invention is to suppress gas generation and realize high charge / discharge capacity and good cycle characteristics in a lithium nickelate-based positive electrode active material.

  In order to achieve the above object, the present inventors have conducted intensive studies and have completed the present invention. The present inventors washed lithium nickel oxide-based lithium transition metal composite oxide, mixed with a specific boron compound, and heat treated at a specific heat treatment temperature to suppress gas generation, charge / discharge capacity, and cycle characteristics. It was found that can be satisfied.

The method for producing a positive electrode active material according to the present invention has a general formula Li _a Ni _1-x M _x O ₂ (0.95 ≦ a ≦ 1.05, 0 ≦ x ≦ 0.25, where M is Co, Mn, and Al. The core particles containing a lithium transition metal composite oxide represented by at least one element selected from the above are washed to obtain washing particles, and the washing particles and a boron compound containing oxygen are mixed and mixed A mixing step of obtaining particles, and a heat treatment step of heat-treating the mixed particles at a heat treatment temperature of 100 ° C. or higher and 400 ° C. or lower.

  Since the method for producing a positive electrode active material of the present invention has the above-described features, in a non-aqueous electrolyte secondary battery using a lithium nickelate-based lithium transition metal composite oxide as a positive electrode active material, gas generation is suppressed. In addition, a high charge / discharge capacity and good cycle characteristics can be realized.

  Hereinafter, the manufacturing method of the positive electrode active material of this invention is demonstrated in detail using embodiment and an Example. However, the present invention is not limited to these embodiments and examples.

  The production method of the present invention includes a cleaning step, a mixing step, and a heat treatment step. Hereinafter, these steps will be mainly described.

[Core particles]
As the core particle, a particle whose main component is a lithium transition metal composite oxide (so-called lithium nickelate) containing nickel as an essential transition metal is used. In order to take full advantage of the charge / discharge capacity of the lithium nickelate system, the ratio of nickel in the transition metal is set to 75 mol% or more. As the metal replacing nickel, at least one element composed of cobalt, manganese and aluminum can be selected. On the other hand, the ratio of lithium to transition metal is around 1. The stoichiometric ratio may be about 5%. In summary, the main component of the core particles is the general formula Li _a Ni _1-x M _x O ₂ (0.95 ≦ a ≦ 1.05, 0 ≦ x ≦ 0.25, where M is Co, Mn, Al At least one element selected from If the core particles are left as they are, unreacted raw materials and the like are present and gas generation occurs, so a cleaning process described later is performed. In addition, what is necessary is just to manufacture core particle itself using a well-known method.

[Washing process]
The core particles are washed to obtain washed particles. The type of the liquid phase used for washing is not particularly limited as long as unreacted raw materials can be removed. Usually, pure water is sufficient. The fine conditions of the washing process are not particularly limited. By this step, components other than the main component such as unreacted raw materials are washed from the core particles. There are no impurities in the cleaning particles, or there is only a degree that cannot be detected even if they exist. However, the vicinity of the surface of the cleaning particle has lithium deficiency, and lithium ion desorption / insertion is inhibited in a region having lithium deficiency. Therefore, when the cleaning particles are used as the positive electrode active material, charge / discharge characteristics and cycle characteristics are deteriorated. Based on these circumstances, the steps described below are performed.

[Mixing process]
The resulting cleaning particles are mixed with a boron compound containing oxygen to obtain mixed particles. If the amount of the boron compound to be mixed is too small relative to the cleaning particles, the effect is not sufficiently exhibited. If the amount is too large, the charge / discharge capacity decreases, so the amount is adjusted as appropriate. A preferable range is 0.4 mol% or more and 1.5 mol% or less as boron with respect to the cleaning particles. Since the effect of improving the charge / discharge characteristics and the cycle characteristics is not yet exhibited with the mixed particles, the following heat treatment process is performed. The boron compound containing oxygen can be selected from at least one selected from the group consisting of boron oxide, boron oxoacids, and boron oxoacid salts. More specific examples include lithium tetraborate (Li ₂ B ₄ O ₇ ), ammonium pentaborate (NH ₄ B ₅ O ₈ ), orthoboric acid (H ₃ BO ₃ ; so-called ordinary boric acid), metabora Examples include lithium acid (LiBO ₂ ) and boron oxide (B ₂ O ₃ ).

[Heat treatment process]
The obtained mixed particles are subjected to a heat treatment to obtain a positive electrode active material in which core layers are formed with a coating layer containing boron and oxygen. The coating layer is obtained as a result of a reaction between at least part of the boron compound mixed in the mixing step and part of the elements constituting the core particle. The presence or absence of the heat treatment step or the difference in the process used for forming the coating layer is reflected in the spectrum of X-ray fluorescence analysis (XPS) from the outermost surface to about 20 nm around the positive electrode active material surface. When the heat treatment temperature is lower than 100 ° C., a coating layer is not formed. When the heat treatment temperature is higher than 400 ° C., lithium elutes from the core particles to the particle surface and changes to a gas generation source such as lithium carbonate. To do.

  Hereinafter, it demonstrates more concretely using an Example.

To obtain a core particle composed mainly of general formula _LiNi 0.829 _Co 0.155 lithium transition metal composite oxide represented by _{Al 0.016} O ₂ using a known technique.

  The obtained core particles were transferred to a washing container, and 10 times as much pure water as mass ratio was added to the washing container, and allowed to stand until the boundary between the solid phase and the liquid phase became clear. After standing, the electric conductivity of the liquid phase was measured and decanted. The addition of pure water, standing and decantation were repeated until the electric conductivity of the liquid phase was 0.5 mS / cm or less. After the final decantation, the solid phase was dehydrated and further dried at 120 ° C. for 10 hours to obtain washed particles.

  0.5 mol% boric acid as boron was added to the obtained washed particles and mixed using a stirrer to obtain mixed particles. The obtained mixed particles were heat-treated at 250 ° C. for 10 hours, and passed through a dry sieve having an opening of 75 μm to obtain a target positive electrode active material.

  A target positive electrode active material was obtained in the same manner as in Example 1 except that the boric acid to be added was 1.0 mol% as boron with respect to the cleaning particles.

  The target positive electrode active material was obtained in the same manner as in Example 1 except that the boric acid to be added was 0.3 mol% as boron with respect to the cleaning particles.

A target positive electrode active material was obtained in the same manner as in Example 1 except that the core particles having a composition of the lithium transition metal composite oxide having the general formula LiNi _0.8 Co _0.1 Mn _0.1 O ₂ were used.

[Comparative Example 1]
In Example 1, the steps after the cleaning step were omitted, and the target positive electrode active material was obtained.

[Comparative Example 2]
In Example 1, the steps after the mixing step were omitted to obtain the target positive electrode active material.

[Comparative Example 3]
In Example 1, the heat treatment step was omitted, and the target positive electrode active material was obtained.

[Comparative Example 4]
The target positive electrode active material was obtained in the same manner as in Example 1 except that the heat treatment temperature in the heat treatment step was 600 ° C.

[Comparative Example 5]
In Example 4, the steps after the washing step were omitted to obtain the target positive electrode active material.

[Comparative Example 6]
In Example 4, the steps after the mixing step were omitted to obtain the target positive electrode active material.

[Cycle characteristics evaluation]
About Examples 1-3 and Comparative Examples 1-6, a cycle characteristic is evaluated as follows.

[1. Preparation of positive electrode]
90 parts by weight of a positive electrode active material, 2.5 parts by weight of acetylene black, 2.5 parts by weight of graphite carbon, and 5 parts by weight of PVDF (polyfucavinylidene) are dispersed and dissolved in NMP (normal methyl-2-pyrrolidone) to obtain a positive electrode slurry Adjusted. The obtained positive electrode slurry was applied to a current collector plate made of aluminum foil and dried to obtain a positive electrode.

[2. Production of negative electrode]
An anode slurry was prepared by dispersing 97.5 parts by weight of artificial graphite, 1.5 parts by weight of CMC (carboxymethylcellulose) and 1.0 part by weight of SBR (styrene butadiene rubber) in water. The obtained negative electrode slurry was applied to a copper foil, dried, and further compression molded to obtain a negative electrode.

[3. Preparation of non-aqueous electrolyte]
EC (ethylene carbonate) and MEC (methyl ethyl carbonate) were mixed at a volume ratio of 3: 7 to obtain a solvent. Lithium hexafluorophosphate (LiPF ₆ ) was dissolved in the obtained mixed solvent so as to have a concentration of 1 mol / l to obtain a nonaqueous electrolytic solution.

[4. Assembly of evaluation battery]
After the lead electrodes were attached to the positive and negative electrode current collectors, vacuum drying was performed at 120 ° C. Next, a separator made of porous polyethylene was disposed between the positive electrode and the negative electrode, and these were stored in a bag-like laminate pack. After storage, the moisture adsorbed on each member was removed by vacuum drying at 60 ° C. After the vacuum drying, the above-described non-aqueous electrolyte was poured into the laminate pack and sealed to obtain a laminate-type non-aqueous electrolyte secondary battery for evaluation.

[5. aging]
The obtained battery for evaluation was charged at a constant voltage and a constant current at a charging voltage of 4.2 V and a charging current of 0.1 C (current that discharges after 1 C≡1 hour), a discharging voltage of 2.75 V and a discharging current of 0.2 C Charging / discharging consisting of constant current discharge was performed twice. Thereafter, the charge current was changed to 0.2C, and charge / discharge was performed once, and the nonaqueous electrolyte was applied to the positive electrode and the negative electrode.

[6. Discharge capacity maintenance rate measurement]
After aging, a constant voltage and constant current charge at a charge voltage of 4.2 V and a charge current of 1 C and a constant current discharge at a discharge voltage of 2.75 V and a discharge current of 1 C are taken as one cycle, and the discharge capacity after each cycle is measured. . The ratio (≡Ed (n) / Ed (1)) of the discharge capacity Ed (n) after n cycles to the discharge capacity Ed (1) after one cycle is expressed as the discharge capacity retention ratio Rs (n) after n cycles. And

[Charge / discharge characteristics evaluation]
For Examples 1 and 2 and Comparative Examples 1 to 3, charge / discharge characteristics were evaluated as follows.

  A secondary battery similar to that for cycle characteristic evaluation was produced and aged. After aging, constant voltage and constant current charging was performed at a charging voltage of 4.3 V and a charging current of 0.2 C, and the charging capacity Ec was measured. After the measurement, a constant current discharge was performed at a discharge voltage of 2.75 V and a discharge current of 0.2 C, and the discharge capacity Ed was measured.

[Gas generation evaluation]
About Examples 1-4 and Comparative Examples 1-6, the gas generation amount was evaluated as follows.

  A secondary battery similar to that for cycle characteristic evaluation was produced and aged. After aging, the last constant voltage and constant current charging was performed at a charging voltage of 4.2 V and a charging current of 0.2 C. After the last charge, the evaluation battery was disassembled and the positive electrode and the negative electrode were taken out. The taken out positive electrode plate, negative electrode plate, and 500 μL of a new non-aqueous electrolyte solution were sealed in a new laminate pack, and left in a constant temperature bath at 80 ° C. for 24 hours to generate gas. The volume change of the laminate pack before and after standing was measured using Archimedes' principle, and the amount of gas generated was Vg.

  The production conditions of Examples 1 to 4 and Comparative Examples 1 to 6 are shown in Table 1, and the gas generation amount Vg, the charge capacity Ec, the discharge capacity Ed, and the discharge capacity maintenance rate Rs (100) after 100 cycles are shown in Table 2.

  Table 1 and Table 2 show the following.

  From the results of Comparative Example 1 and Comparative Example 2 or the results of Comparative Example 5 and Comparative Example 6, a cleaning step is necessary to reduce the amount of gas generated. On the other hand, from the results of Comparative Example 2 and Examples 1 to 3, or the results of Comparative Example 6 and Example 4, it is necessary to form a coating layer after the mixing step in order to satisfy the charge / discharge capacity and cycle characteristics. There is. From the results of Comparative Examples 3 and 4 and Examples 1 to 3, or the results of Comparative Examples 6 and 4, a heat treatment step at an appropriate heat treatment temperature is required for forming the coating layer.

  When the positive electrode active material obtained by the production method of the present invention is used, a non-aqueous electrolyte secondary battery that suppresses gas generation and is excellent in charge / discharge capacity and cycle characteristics can be obtained. Since the secondary battery obtained in this way suppresses gas generation, it is possible to use the power supply space effectively by making the shape of the battery container square. For this reason, the secondary battery can be particularly suitably used as a power source for large-scale equipment that has a high energy density per volume such as an electric vehicle and requires high output and long life.

Claims (2)

General formula Li _a Ni _1-x M _x O ₂ (0.95 ≦ a ≦ 1.05, 0 ≦ x ≦ 0.25, M is at least one element selected from Co, Mn, and Al)
Washing the core particles containing the lithium transition metal composite oxide represented by
A mixing step of mixing the cleaning particles and a boron compound containing oxygen to obtain mixed particles;
A heat treatment step of heat treating the mixed particles at a heat treatment temperature of 100 ° C. or higher and 400 ° C. or lower;
A method for producing a positive electrode active material, comprising:   The manufacturing method according to claim 1, wherein the boron compound in the mixed particles is 0.4 mol% or more and 1.5% or less as boron with respect to the cleaning particles.