JP2016009583A - Method for manufacturing positive electrode active material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a positive electrode active material purified by extracting impurities from a positive electrode active material for lithium ion secondary batteries.SOLUTION: A method for manufacturing a purified positive electrode active material (B) comprises the steps of: bringing a positive electrode active material (B0) for lithium ion secondary batteries into contact with a supercritical or subcritical fluid (A), thereby making a mixture (C); and removing, from the mixture, the supercritical or subcritical fluid (A). The supercritical or subcritical fluid (A) is a carbon dioxide. The mixture (C) is a mixture (C1) arranged by putting the positive electrode active material (B0) into contact with the supercritical or subcritical fluid (A) and an organic solvent (D). It is preferable that the positive electrode active material (B0) for lithium ion secondary batteries is a lithium-containing complex oxide which includes Li element, and at least one transition metal element selected from Ni, Co and Mn, and in which the mole quantity of the Li element is over 1.2 times the total mole quantity of the transition metal element.

Description

  The present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery.

Lithium ion secondary batteries are already widely used in portable electronic devices such as mobile phones and notebook computers, but in recent years they have become smaller and lighter as portable electronic devices and in-vehicle lithium ion secondary batteries. Therefore, further improvement in performance such as discharge capacity per unit mass, cycle characteristics, and stability as a battery is desired.

For the purpose of improving the performance, a composite oxide (hereinafter sometimes referred to as “Li-rich positive electrode material”) in which the ratio of transition metal elements such as Ni, Co, and Mn to the Li element is increased has been proposed. Yes. As an example of the Li-rich positive electrode material, a solid solution of LiMO ₂ (M is at least one transition metal element selected from Ni, Co, and Mn) and Li ₂ MnO ₃ has been proposed. In order to use a lithium ion secondary battery having the solid solution as a positive electrode active material at a high capacity, it is necessary to charge at a high voltage of 4.5 V or more at the first charge.

However, since the Li-rich positive electrode material has excessive Li, Li that has not been taken into the crystal tends to remain as free Li on the surface of the positive electrode material. Free Li is considered to exist in the form of LiOH or Li ₂ CO ₃ , but if the amount of free Li is large, the electrolytic solution is likely to be decomposed during charge and discharge. The decomposed electrolytic solution comes into contact with the transition metal in the positive electrode material and gradually elutes the transition metal in the positive electrode material into the electrolytic solution. When the transition metal in the positive electrode material is eluted in the electrolyte solution, the crystal structure of the positive electrode material becomes unstable, and the charge / discharge capacity decreases. For this reason, even in a lithium ion secondary battery using a Li-rich positive electrode material as a positive electrode, sufficiently high cycle characteristics could not be obtained.

In order to solve this problem, a method for producing a positive electrode active material for a lithium ion secondary battery (Patent Document 1) characterized by bringing a lithium-containing composite oxide into contact with a fluorine gas has been proposed. Cycle characteristics are not obtained.

JP 2014-75177 A

  The subject of this invention is providing the manufacturing method of the refined positive electrode active material which extracts an impurity from the positive electrode active material for lithium ion secondary batteries.

As a result of intensive studies to achieve the above object, the present inventors have reached the present invention. That is, the present invention removes the supercritical fluid or subcritical fluid (A) from the material (C) in which the supercritical fluid or subcritical fluid (A) is contacted with the positive electrode active material (B0) for the lithium ion secondary battery. It is a manufacturing method of the refined positive electrode active material (B) including the process to make.

  ADVANTAGE OF THE INVENTION According to this invention, the refined | purified positive electrode active material (B) for lithium ion secondary batteries excellent in a capacity | capacitance and cycling characteristics can be obtained.

The present invention is described in detail below.
The positive electrode active material (B0) for a lithium ion secondary battery used in the present invention contains Li element and at least one transition metal element selected from Ni, Co, and Mn, and the molar amount of Li element is the transition metal. The lithium-containing composite oxide is a lithium-containing composite oxide that is more than 1.2 times the total molar amount of elements. The transition metal element contained is more preferably made of Mn, and particularly preferably containing Ni, Co, and Mn. Further, the transition metal element contained in the lithium-containing composite oxide may consist of only Ni, Co, and Mn, and if necessary, a metal element other than Ni, Co, Mn, and Li (hereinafter, other metals) May be included). Examples of other metal elements include elements such as Cr, Fe, Al, Ti, Zr, Mo, Nb, V, and Mg. The proportion of the other metal element is preferably 0.001 to 0.50 mol, and more preferably 0.005 to 0.05 mol in the total amount (1 mol) of the transition metal element.

The supercritical fluid (A1) used in the present invention exists as a non-aggregating high-density fluid in a temperature / pressure region exceeding the limit (critical point) at which gas and liquid can coexist, and does not aggregate even when compressed. As long as the fluid is in a state of a critical temperature or higher and a critical pressure or higher, there is no particular limitation, and the fluid can be appropriately selected according to the purpose. This supercritical fluid includes, for example, carbon monoxide, carbon dioxide, ammonia, nitrogen, water, methanol, ethanol, ercol solvents such as n-butanol, ethane, propane, 2,3-dimethylbutane, benzene, toluene and the like. Hydrocarbon solvents, halogen solvents such as methylene chloride and chlorotrifluoromethane, and ether solvents such as dimethyl ether are preferred. Among these, carbon dioxide is particularly preferable because it has a critical pressure of 7.3 MPa and a critical temperature of 31 ° C., so that it can easily create a supercritical state and is nonflammable and easy to handle. These fluids may be used alone or in combination of two or more.

The subcritical fluid (A2) used in the present invention is not particularly limited as long as it exists as a high-pressure liquid in the temperature and pressure regions near the critical point, and can be appropriately selected according to the purpose. The compound mentioned as the supercritical fluid (A1) described above can be suitably used as a subcritical fluid.

  The critical temperature and critical pressure of the supercritical fluid are not particularly limited and can be appropriately selected according to the purpose. However, the critical temperature is preferably −273 ° C. or more and 300 ° C. or less, particularly 0 ° C. or more and 200 ° C. or less. preferable.

  Furthermore, in addition to the supercritical fluid (A1) and subcritical fluid (A2) described above, an organic solvent (D) can also be added. By adding the organic solvent (D), the solubility in the supercritical fluid can be adjusted more easily. Such an organic solvent (D) is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, ethyl formate, ethyl acetate, Alternatively, ester solvents such as n-butyl acetate, ether solvents such as diisopropyl ether, dimethoxyethane, tetrahydrofuran, dioxolane, or dioxane, N, N-dimethylformamide, N, N-dimethylacetamide, or N-methyl-2- Amide solvents such as pyrrolidone, dichloromethane, chloroform, bromoform, methyl iodide, dichloroethane, trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene, fluorobenzene, bromobenzene, iodobenzene Alternatively, halogenated hydrocarbon solvents such as 1-chloronaphthalene, n-pentane, n-hexane, n-octane, 1,5-hexadiene, cyclohexane, methylcyclohexane, cyclohexadiene, benzene, toluene, o-xylene, m- Examples thereof include hydrocarbon solvents such as xylene, p-xylene, ethylbenzene, and cumene.

Examples of the impurity (E) in the present invention include Li salt and Na salt. Examples of the Li salt include lithium hydroxide, lithium carbonate, and lithium sulfate. Examples of the Na salt include sodium hydroxide, sodium carbonate, sodium sulfate and the like.

Hereinafter, the process will be described in detail.
In this supercritical carbon dioxide treatment step, a low boiling point solvent may be used in combination as necessary.
The present invention includes (A) a supercritical carbon dioxide (CO ₂ ) treatment step for removing impurities remaining in the positive electrode active material with carbon dioxide in a supercritical state.
When a low-boiling solvent is used in combination, (B) a supercritical carbon dioxide (CO ₂ ) treatment step in which the positive electrode active material is dispersed in the low-boiling solvent and the dispersion is removed with carbon dioxide in a supercritical state, (C ) In order to remove impurities remaining in the positive electrode active material, in the state where the low boiling point solvent is attached to the coating film, the solvent is attached to the supercritical carbon dioxide-supercritical carbon dioxide (CO ₂ ) treatment step, or (D) Solvent immersion-supercritical carbon dioxide (CO ₂ ) in which the coating film is immersed in a low-boiling solvent in order to remove impurities remaining in the positive electrode active material, and is treated with supercritical carbon dioxide. A processing process is mentioned.
Among these, the method (A) or (B) is preferably used from the viewpoint of impurity removal efficiency.

  EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited thereto. In the following description, “parts” indicates parts by weight.

<Production Example 1>
<Lithium-containing composite oxide (synthesis of Ni-Co-Mn ternary system>
Nickel sulfate (II) hexahydrate (140.6 parts), cobalt sulfate (II) heptahydrate (131.4 parts), manganese sulfate (II) pentahydrate (482.2 parts) and distilled water (1245.9 parts) was added and uniformly dissolved to obtain a raw material solution. Distilled water (320.8 parts) was added to ammonium sulfate (79.2 parts) and dissolved uniformly to obtain an ammonia source solution. Distilled water (1920.8 parts) was added to ammonium sulfate (79.2 parts) and dissolved uniformly to obtain a mother liquor. Distilled water (600 parts) was added to sodium hydroxide (400 parts) and dissolved uniformly to obtain a pH adjusting solution.
The mother liquor was placed in a 2 L baffled glass reaction vessel, heated to 50 ° C. with a mantle heater, and a pH adjusting solution was added so that the pH was 11.0. While stirring the solution in the reaction vessel with an anchor-type stirring blade, the raw material solution was added at a rate of 5.0 parts / minute, and the ammonia source solution was added at a rate of 1.0 part / minute, resulting in composite hydroxylation of nickel, cobalt, and manganese. The product was precipitated. During the addition of the raw material solution, the pH adjusting solution was added so as to keep the pH in the reaction vessel at 11.0. Moreover, nitrogen gas was flowed at a flow rate of 0.5 L / min in the reaction tank so that the precipitated hydroxide was not oxidized. Further, the liquid was continuously extracted so that the amount of the liquid in the reaction tank did not exceed 2 L.
In order to remove impurity ions from the obtained composite hydroxide of nickel, cobalt, and manganese, washing was repeated by pressure filtration and dispersion in distilled water. When the electrical conductivity of the filtrate reached 25 μS / cm, the washing was finished and dried at 120 ° C. for 15 hours to obtain an untreated positive electrode active material (B0-1). When the contents of the precursors nickel, cobalt, and manganese were measured by ICP (high frequency inductively coupled plasma), they were 11.6 mass%, 10.5 mass%, and 42.3 mass%, respectively.

<Production Example 2>
<Synthesis of lithium-containing composite oxide (LiCoO ₂ )>
Instead of nickel (II) sulfate hexahydrate (140.6 parts), cobalt (II) sulfate heptahydrate (131.4 parts), manganese sulfate (II) pentahydrate (482.2 parts) An untreated positive electrode active material (B0-2) was obtained in the same manner as in Production Example 1 except that cobalt (II) sulfate heptahydrate (843 parts) was used.

<Production Example 3>
<Synthesis of lithium-containing composite oxide (LiNiO ₂ )>
Instead of nickel (II) sulfate hexahydrate (140.6 parts), cobalt (II) sulfate heptahydrate (131.4 parts), manganese sulfate (II) pentahydrate (482.2 parts) An untreated positive electrode active material (B0-3) was obtained in the same manner as in Production Example 1 except that nickel (II) sulfate hexahydrate (780 parts) was used.

<Production Example 4>
<Synthesis of lithium-containing composite oxide (LiMnO ₂ )>
Instead of nickel (II) sulfate hexahydrate (140.6 parts), cobalt (II) sulfate heptahydrate (131.4 parts), manganese sulfate (II) pentahydrate (482.2 parts) An untreated positive electrode active material (B0-4) was obtained in the same manner as in Production Example 1 except that manganese (II) sulfate pentahydrate (724 parts) was used.

<Example 1>
100 parts of the untreated positive electrode active material (B0-1) produced in Production Example 1 was dispersed in acetone and charged into a pressure vessel. The temperature in the kettle was raised to 40 ° C. After raising the temperature, carbon dioxide was supplied to 10 MPa and stirred for 10 minutes. Then, the content was taken out from the take-out nozzle to return to normal pressure to remove carbon dioxide, and acetone was removed by filtration. Furthermore, the positive electrode active material (B-1) was obtained by drying under reduced pressure drying (50 ° C., 0.5 kPa) conditions.

<Example 2>
A positive electrode active material (B-2) was obtained in the same manner as in Example 1 except that (B0-2) was used instead of (B0-1) as an untreated positive electrode active material.

<Example 3>
A positive electrode active material (B-3) was obtained in the same manner as in Example 1 except that (B0-3) was used instead of (B0-1) as an untreated positive electrode active material.

<Example 4>
A positive electrode active material (B-4) was obtained in the same manner as in Example 1 except that (B0-4) was used instead of (B0-1) as the untreated positive electrode active material.

<Comparative Example 1>
20 parts of the untreated positive electrode active material (B0-1) produced in Production Example 1 was placed in a state where it was allowed to stand in a tubular furnace having an inner diameter of 2.15 cm, and a flow rate of N ₂ gas containing 20 mol% of F ₂ gas in the tubular furnace. By continuously supplying at 0.1 L / min for 3 minutes, the lithium-containing composite oxide and the fluorine gas were brought into contact with each other (fluorine treatment) to obtain a positive electrode active material (B′-1) of Comparative Example 2.

<Comparative Example 2>
A positive electrode active material (B′-2) was obtained in the same manner as in Comparative Example 1 except that (B0-2) was used instead of (B0-1) as the untreated positive electrode active material.

<Comparative Example 3>
A positive electrode active material (B′-3) was obtained in the same manner as in Comparative Example 1 except that (B0-3) was used instead of (B0-1) as the untreated positive electrode active material.

<Comparative Example 4>
A positive electrode active material (B′-4) was obtained in the same manner as in Comparative Example 1 except that (B0-4) was used instead of (B0-1) as the untreated positive electrode active material.

[Evaluation of positive electrode active material]
The positive electrode active materials obtained in Examples 1 to 4 and Comparative Examples 1 to 4 were evaluated by producing lithium ion secondary batteries.

[Manufacture of lithium-ion batteries]
<Examples 5-8, Comparative Examples 5-12>
A lithium ion battery was produced by the following method. Table 2 shows the results of evaluating the charge / discharge cycle characteristics and low-temperature charge / discharge cycle characteristics of the obtained lithium ion battery.

[Production of positive electrode for lithium ion battery]
About the positive electrode active material manufactured in the said Examples 1-4, Comparative Examples 1-4, and Manufacturing Examples 1-4, the positive electrode was produced with the following method.
After 90.0 parts of the positive electrode active material, 5 parts of Ketjen black [Sigma-Aldrich Co., Ltd.] and 5 parts of polyvinylidene fluoride [Sigma-Aldrich Co., Ltd.] were thoroughly mixed in a mortar, 1-methyl-2- 70.0 parts of pyrrolidone [manufactured by Tokyo Chemical Industry Co., Ltd.] was added and further mixed well in a mortar to obtain slurry. The obtained slurry was applied to one side of an aluminum electrolytic foil having a thickness of 20 μm using a wire bar in the air, dried at 80 ° C. for 1 hour, and further under reduced pressure (1.3 kPa) at 80 ° C. It was dried for 2 hours and punched out to 15.95 mmφ to produce a positive electrode for a lithium ion battery.

[Production of negative electrode for lithium ion battery]
92.5 parts of graphite powder having an average particle diameter of about 8 to 12 μm, 7.5 parts of polyvinylidene fluoride, and 200 parts of 1-methyl-2-pyrrolidone [manufactured by Tokyo Chemical Industry Co., Ltd.] are thoroughly mixed in a mortar to obtain a slurry. Obtained. The obtained slurry was applied to one side of a 20 μm-thick copper foil in the air using a wire bar, dried at 80 ° C. for 1 hour, and further under reduced pressure (1.3 kPa) at 80 ° C. for 2 hours. It was dried, punched out to 16.15 mmφ, and made into a thickness of 30 μm with a press machine to obtain a negative electrode for a lithium ion battery.

The positive electrode and the negative electrode prepared by the above method were arranged at both ends in the 2032 type coin cell so that the coated surfaces face each other, and a separator (polypropylene nonwoven fabric) was inserted between the electrodes to prepare a cell for a lithium ion battery. . The solution was poured and sealed in a cell in which an electrolytic solution was prepared by dissolving LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1: 1) at a ratio of 12 wt%. The charge / discharge cycle characteristics and output characteristics were evaluated by the following methods.

<Evaluation of charge / discharge cycle characteristics>
Using a charge / discharge measuring device “Battery Analyzer 1470” [manufactured by Toyo Technica Co., Ltd.] at room temperature, the battery was charged to a voltage of 4.3 V with a current of 0.1 C, and after a pause of 10 minutes, 0.1 C The battery voltage was discharged to 3.0 V with the current of and this charge / discharge was repeated. At this time, the battery capacity at the first charge and the battery capacity at the 50th cycle charge were measured, and the charge / discharge cycle characteristics were calculated from the following formula. It shows that charging / discharging cycling characteristics are so favorable that a numerical value is large.
Charging / discharging cycle characteristics (%) = (battery capacity at the 50th cycle charge / battery capacity at the first charge) × 100

<Evaluation of output characteristics>
Using a charge / discharge measuring device “Battery Analyzer 1470” [manufactured by Toyo Technica Co., Ltd.] at room temperature, the battery was charged to a voltage of 4.3 V with a current of 0.1 C, and after a pause of 10 minutes, 0.1 C The voltage was discharged to 3.0 V at a current of, and the discharge capacity (hereinafter referred to as 0.1 C discharge capacity) was measured. Next, the battery is charged to a voltage of 4.3 V with a current of 0.1 C, and after a pause of 10 minutes, the voltage is discharged to 3.0 V with a current of 1 C, and the capacity (hereinafter referred to as 1 C discharge capacity) is measured. The capacity maintenance rate at the time of 1C discharge is calculated. The larger the value, the better the output characteristics.
Capacity maintenance rate during 1 C discharge (%) = (1 C discharge capacity / 0.1 C discharge capacity) × 100

  As shown in Examples 5-8 and Comparative Examples 5-12, the lithium ion batteries made of the positive electrode active material according to the production method of the present invention showed excellent charge / discharge cycle characteristics and output characteristics. This is probably because impurities such as lithium hydroxide remaining in the positive electrode could be removed by treating the positive electrode active material with a supercritical fluid. It is worthy of special mention that the battery characteristics are greatly superior to those of Comparative Examples 5 to 8 purified using a dangerous gas such as fluorine gas.

Since the production method of the present invention uses a cheap and safe supercritical fluid or subcritical fluid, it contributes to the provision of an inexpensive and high-performance positive electrode active material. Moreover, since the lithium ion battery using the positive electrode active material manufactured in this way is excellent in charge / discharge cycle characteristics and output characteristics, it can be suitably used as an all-solid-state lithium ion battery or a vehicle-mounted lithium ion battery.

Claims (4)

Purification including a step of removing the supercritical fluid or subcritical fluid (A) from the contact (C) of the positive electrode active material (B0) for the lithium ion secondary battery with the supercritical fluid or subcritical fluid (A). For producing the positive electrode active material (B). The production method according to claim 1, wherein the supercritical fluid or subcritical fluid (A) is carbon dioxide. The contact (C) is a contact (C1) obtained by bringing a supercritical fluid or subcritical fluid (A) and an organic solvent (D) into contact with a positive electrode active material (B0) for a lithium ion secondary battery. 3. The production method according to 1 or 2. The positive electrode active material (B0) for a lithium ion secondary battery contains Li element and at least one transition metal element selected from Ni, Co, and Mn, and the molar amount of Li element is the total mole of the transition metal element The method according to any one of claims 1 to 3, wherein the lithium-containing composite oxide is more than 1.2 times the amount.