JP2014022294A - Positive electrode active material for nonaqueous electrolyte secondary battery and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery good in charge/discharge cycle characteristics and high in safety, under a high-temperature environment.SOLUTION: There are provided: a positive electrode active material for a nonaqueous electrolyte secondary battery, which is a particle of a lithium-manganese composite oxide represented by general formula: LiMnMO(where 0≤x≤1 and 0≤y≤0.5 are satisfied, and M represents one or more selected from the group consisting of Fe, Ni, Co, Al, Mg, Ti and Zr.) and produced in a method including a step of covering the surface of the composite oxide particle with a phosphate film and carbon, in this order; and a method for producing the positive electrode active material for a nonaqueous electrolyte secondary battery.

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery having good charge / discharge cycle characteristics and high safety in a high temperature environment, and a method for producing the same.

In recent years, with the widespread use of portable electronic devices such as mobile phones and notebook personal computers, development of non-aqueous electrolyte secondary batteries having high energy density, small size and light weight is strongly desired. Furthermore, in the automobile industry, development of large-sized lithium secondary batteries to be mounted on electric vehicles (EV) and hybrid electric vehicles (HEV), which are expected to reduce greenhouse gas emissions, has been actively conducted.
Examples of such secondary batteries include lithium ion secondary batteries. The lithium ion secondary battery includes a negative electrode, a positive electrode, an electrolytic solution, and the like, and a material capable of desorbing and inserting lithium is used as an active material for the negative electrode and the positive electrode.

  This lithium ion secondary battery is currently under active research and development. Among these, lithium ion secondary batteries using a layered lithium metal composite oxide or a spinel type lithium metal composite oxide as the positive electrode material are among them. Since a high voltage of 4V class can be obtained, practical use is progressing as a secondary battery having a high energy density.

Here, as materials proposed as the positive electrode active material, lithium-cobalt composite oxide (LiCoO ₂ ) that is relatively easy to synthesize, lithium-nickel composite oxide (LiNiO ₂ ) using nickel that is cheaper than cobalt. ) And lithium / manganese composite oxide (LiMn ₂ O ₄ ) using manganese.

Lithium / manganese composite oxide is not only richer in resources than lithium / cobalt composite oxide and lithium / nickel composite oxide, but also lower in raw material cost, and also has thermal safety when a secondary battery is manufactured. Therefore, it has been put to practical use as a secondary battery for automobiles because it has excellent characteristics such as high voltage and high voltage.
However, the lithium-manganese composite oxide has a problem that manganese is likely to be eluted during charging in a high temperature environment, and the charge / discharge cycle characteristics of the secondary battery are likely to deteriorate.

Thus, various studies have been made so far on methods for suppressing elution of manganese from lithium-manganese composite oxides and improving charge / discharge cycle characteristics.
For example, lithium manganese oxide (LiMn ₂ O ₄ ) having a spinel structure is mixed with lithium hydroxide (LiOH) and heat-treated at 375 ° C., whereby lithium manganese oxide having a spinel structure (Li _{1 -x Mn 2 O 4: 0 ≦} x ≦ 1) on the surface of the particles to form a _Li 2 MnO ₃ layer, techniques of improving the cycle characteristics has been proposed (e.g., see Patent Document 1).
However, if the ratio of lithium hydroxide to be mixed is increased, the cycle characteristics are certainly improved, but there is a new problem that the discharge capacity is lowered.

The primary particle size is 1 μm or more and the average particle size (D ₅₀ ) of the behavior particles is 1 μm or more and 10 μm or less, and substantially single crystal particles are formed. The chemical formula is Li _{1 + x} Mn _2-xy Y _y O. ₄ (Y = Al, Mg, Co, 0.03 ≦ x ≦ 0.15, 0.05 ≦ y ≦ 0.20), the Y element is uniformly dispersed inside the particles, and I ( 400) / I (111) is 33% or more and I / 440 / I (111) is 16% or more.
This is because a heterogeneous metal such as Al, Co or Mg is uniformly present inside the particles, and since the crystallinity is high, a positive electrode active material for a secondary battery having high output characteristics and excellent high-temperature storage characteristics is obtained. (For example, refer to Patent Document 2).
However, when x is small, the capacity is increased, but the high temperature characteristics are remarkably deteriorated. On the other hand, when x is large, the high temperature characteristics are improved, but the capacity is remarkably reduced, the Li-rich phase is generated and the resistance is increased. Is insufficient.

On the other hand, in the olivine type lithium phosphate compound represented by the general formula: LiFePO ₄ or the like (see, for example, Patent Documents 3 and 4), a strong (PO ₄ ) ^3- polyanion is formed by a covalent bond between phosphorus and oxygen. Therefore, the volume change due to charging / discharging is small and oxygen is not easily released.
For this reason, it is strong against overdischarge and overcharge that cause problems in safety, and has high stability at high temperatures. Furthermore, there is an advantage in cycle life and rapid charge / discharge.

However, since the electrical conductivity is as low as 10 ⁻⁸ S / cm 2 or less, a device on the process is required. At present, in most cases, the surface of LiFePO ₄ particles is coated with conductive carbon.
Furthermore, since the diffusibility of Li is low and sufficient performance cannot be obtained as it is, fine particles are formed to a diameter of several tens of nm, the diffusion distance is shortened and the surface area is increased. On the other hand, the finely divided LiFePO ₄ particles are difficult to be filled at high density, and the discharge capacity cannot be sufficiently improved.

As another method, there is a non-aqueous electrolyte secondary battery in which the negative electrode is made of a carbonaceous material, the positive electrode active material is made of a lithium / manganese composite oxide, and the electrolyte contains 0.01 to 1% by weight of Li ₂ S. Proposed.
That is, high-temperature cycle deterioration is caused by the formation of a malignant film on the negative electrode surface by Mn, which is a positive electrode component eluted in a high-temperature environment, and lithium sulfide (Li ₂ S) is contained in the non-aqueous electrolyte. By doing so, a good organic film due to the sulfur component of the dissolved lithium sulfide is preferentially formed on the surface of the negative electrode, and high temperature cycle deterioration is solved (see Patent Document 5).

  However, when the lithium sulfide content is low, a good organic film is not formed, and the cycle improvement effect cannot be obtained. On the other hand, if the content is large, the coating film is formed thick, so that there is a problem that it becomes a resistance component and load discharge characteristics deteriorate. Further, since elution of Mn is not suppressed and the fundamental problem cannot be solved, further improvement is required for improving the cycle characteristics.

JP-A-6-1111819 JP 2008-251390 A JP-A-9-134725 WO2005 / 041327 JP 2003-257482 A

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery having good charge / discharge cycle characteristics and high safety under a high temperature environment.

  In view of the above situation, the present inventor has found that the elution of Mn is suppressed by treating the surface of the lithium-manganese composite oxide particles with phosphoric acid and coating with a phosphate film. Furthermore, by covering the outside of the phosphate film with carbon, the conductivity between the particles is ensured, leading to a high-capacity and high-safety positive electrode active material for non-aqueous electrolyte secondary batteries. As a result, it was found that a non-aqueous electrolyte secondary battery constituting the positive electrode plate was obtained, and the present invention was completed.

  The first invention of the present invention is a lithium-manganese composite oxide particle, wherein the particle surface of the composite oxide particle is coated in the order of a phosphate film and carbon. It is a positive electrode active material for secondary batteries.

The second aspect of the present invention, a lithium-manganese composite oxide particles in the first invention, the general formula _{Li x Mn 2-y M y} O 4 ( where, 0 ≦ x ≦ 1,0 ≦ y ≦ 0 .5, M is one or more selected from the group consisting of Fe, Ni, Co, Al, Mg, Ti and Zr.) It is a positive electrode active material for water electrolyte secondary batteries.

  According to a third aspect of the present invention, the surface of the lithium-manganese composite oxide particles is coated with a phosphate coating by phosphoric acid treatment, and the surface of the particles coated with the phosphate coating is coated with carbon. It is a manufacturing method of the positive electrode active material for nonaqueous electrolyte secondary batteries characterized by including a process.

In a fourth aspect of the present invention, the lithium metal composite oxide particles in the third aspect are Li _x Mn _{2 -y} M _y O ₄ (wherein 0 ≦ x ≦ 1, 0 ≦ y ≦ 0.5, M is one or more selected from the group consisting of Fe, Ni, Co, Al, Mg, Ti, and Zr.) A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery.

  According to the present invention, the surface of the lithium / manganese composite oxide particles is coated with a phosphate film, and the surface thereof is further coated with carbon. A positive electrode active material for a non-aqueous electrolyte secondary battery having the characteristics of suppressing Mn elution and maintaining the conductivity between particles is obtained. By using this positive electrode active material, charge / discharge cycle characteristics are excellent in a high-temperature environment. Thus, a highly safe non-aqueous electrolyte secondary battery can be obtained.

It is sectional drawing of the positive electrode active material for nonaqueous electrolyte secondary batteries which concerns on this invention. It is sectional drawing of a 2032 type coin battery produced for evaluation of a positive electrode active material.

The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention has a structure in which the surface of lithium-manganese composite oxide particles is treated with phosphoric acid and coated with a phosphate film, and further coated with carbon. .
Here, the technical feature of the positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention is that the surface of the lithium-manganese composite oxide particles is treated with phosphoric acid and coated with a phosphate film, and further, carbon. The structure is covered.

The positive electrode active material for a nonaqueous electrolyte secondary battery of the present invention and the method for producing the same will be described in detail.
<1. Cathode active material>
The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention will be specifically described with reference to the drawings.
FIG. 1 is a cross-sectional view specifically showing an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention.
As shown in FIG. 1, the positive electrode active material 11 for a nonaqueous electrolyte secondary battery according to the present invention has a phosphate coating 2 formed by phosphoric acid treatment on the surface of a lithium / manganese composite oxide particle 3, and The lithium-manganese composite oxide particles having the carbon film 1 formed on the surface of the phosphate film.

[Lithium / manganese composite oxide particles]
The lithium-manganese composite oxide particles 3 mainly constituting the positive electrode active material of the present invention have a general formula Li _x Mn _{2 -y} M _y O ₄ (where 0 ≦ x ≦ 1 and 0 ≦ y ≦ And M is one or more selected from the group consisting of Fe, Ni, Co, Al, Mg, Ti, and Zr).

[Coating with phosphate coating]
The phosphate coating 2 of the present invention may be any coating that covers the surface of the lithium manganese oxide particles 3 and suppresses the elution of Mn when the temperature rises.
That is, when lithium-manganese composite oxide particles are used as the positive electrode, Li is extracted from the lithium-manganese composite oxide particles during charging, resulting in an unstable structure and an interface between the lithium-manganese composite oxide particles and the electrolyte. Mn elution occurs.
Therefore, the elution of Mn is suppressed by coating the surface of the lithium / manganese composite oxide particles with a strong (PO ₄ ) ^3- polyanion by covalent bonding of phosphorus and oxygen.
Furthermore, it is only necessary that the lithium-manganese composite oxide particles are continuously connected to the surface, and the lithium ions can be easily diffused with a structure that does not easily peel off.

[Coating with carbon film]
The coating with the carbon film 1 covers the surface of the lithium / nickel composite oxide particles coated with the phosphate film 2 to improve the electron conductivity.
As the carbon source, organic acids such as acetic acid and citric acid, sugars such as cellulose, glucose, sucrose, lactose and maltose, polymers such as polyvinyl alcohol, and the like can be used. Although the mass ratio of the carbon material to the lithium / manganese composite oxide particles coated with the phosphate film is not limited, the lower the mass ratio of the carbon material, the more the positive electrode active material amount can be reduced.

<2. Method for producing positive electrode active material>
[Method for producing lithium-manganese composite oxide particles]
The method for producing lithium-manganese composite oxide particles is not particularly limited as long as it is a method in which Mn is dissolved in Li.
For example, lithium hydroxide (LiOH), which is a starting material, and manganese dioxide are mixed so as to have a molar ratio of Li: Mn = 1: 2 to obtain a mixture, which is calcined in air at 850 ° C. for 20 hours, and LiMn ₂ O _And a method of preparing lithium / manganese composite oxide particles comprising ₄ .

[Method of coating with phosphate film]
As a coating method using a phosphate film, a method of grinding in an organic solvent in the presence of phosphoric acid can be used.
According to this method, by adding phosphoric acid when pulverizing lithium-manganese composite oxide particles by an attritor or the like, even if a new surface is generated on the aggregated particles by pulverization, it reacts instantaneously with phosphoric acid in the solvent, A stable phosphate film is formed on the particle surface. Thereafter, even if the pulverized lithium-manganese composite oxide particles are aggregated, the contact surface is already stabilized, and an incomplete region of coating does not occur due to crushing.

  Furthermore, the thickness of the phosphate film necessary for protecting the lithium / manganese composite oxide particle surface is usually 5 to 100 nm on average. When the average thickness of this phosphate film is less than 5 nm, sufficient suppression of Mn elution cannot be obtained, and when it exceeds 100 nm, battery characteristics deteriorate.

There is no restriction | limiting in particular as phosphoric acid used for formation of such a phosphate membrane | film | coat, Commercially available normal phosphoric acid, for example, 85% concentration phosphoric acid aqueous solution can be used.
The method for adding the phosphoric acid is not particularly limited. For example, when the lithium-manganese composite oxide particles are pulverized with an attritor or the like, phosphoric acid is added to an organic solvent used as a solvent. Phosphoric acid only needs to finally have a desired phosphoric acid concentration, and may be added all at once before the start of pulverization or gradually during pulverization.
The organic solvent is not particularly limited, and usually alcohols such as ethanol or isopropyl alcohol, ketones, lower hydrocarbons, aromatics, or a mixture thereof is used.

The amount of phosphoric acid added is generally unrelated because it is related to the particle size, surface area, and the like of the pulverized lithium / manganese composite oxide particles. More than 0.1 mol / kg and less than 2 mol / kg, More preferably, it is 0.15-1.5 mol / kg, More preferably, it is 0.2-0.4 mol / kg.
That is, if it is less than 0.1 mol / kg, the surface treatment of the lithium / nickel composite oxide particles is not sufficiently performed, so that elution suppression of Mn is not sufficient.

  Furthermore, it is preferable to heat-treat the lithium / manganese composite oxide particles coated with the phosphate film obtained as described above at 100 ° C. or higher in an inert gas or in a vacuum. When the heat treatment is performed at less than 100 ° C., drying does not proceed sufficiently and formation of a stable surface film is inhibited.

[Method of coating with carbon film]
For example, a cellulose acetate solution dissolved in acetone is impregnated with lithium / nickel composite oxide particles coated with a phosphate film. After drying, the temperature is raised and held in an argon atmosphere in a heating furnace. Subsequently, it cools in steps in argon atmosphere. Through this step, the surface of the lithium / nickel composite oxide particles coated with the phosphate film can be further coated with carbon.

<3. Production of secondary battery>
Using the positive electrode active material for a nonaqueous electrolyte secondary battery, for example, a positive electrode is produced as follows.
First, a powdered positive electrode active material, a conductive material, and a binder are mixed, and, if necessary, a target solvent such as activated carbon and viscosity adjustment is added and kneaded to prepare a positive electrode mixture paste.
Each mixing ratio in the positive electrode mixture paste is also an important factor that determines the performance of the secondary battery. When the total mass of the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, the content of the positive electrode active material is 50 to 95 parts by mass as in the case of the positive electrode of a general non-aqueous electrolyte secondary battery, and the conductive material It is desirable to set the content of 1 to 30 parts by mass and the content of the binder to 1 to 20 parts by mass.

The obtained positive electrode mixture paste is applied to the surface of a current collector made of, for example, an aluminum foil and dried to disperse the solvent. If necessary, pressurization may be performed by a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode can be produced.
The produced sheet-like positive electrode can be cut into an appropriate size or the like according to the intended battery and used for battery production. However, the method for manufacturing the positive electrode is not limited to the above-described examples, and other methods may be used.

Examples of the conductive agent to be used include graphite (natural graphite, artificial graphite, expanded graphite, etc.), carbon black materials such as acetylene black and ketjen black.
The binder plays a role of anchoring the active material particles. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine rubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, polyacrylic, and the like. An acid or the like can be used.

  Furthermore, if necessary, a positive electrode active material, a conductive material, and activated carbon are dispersed, and a solvent that dissolves the binder is added to the positive electrode mixture. Specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent. In addition, activated carbon can be added to the positive electrode mixture in order to increase the electric double layer capacity.

Next, components other than the positive electrode used in the secondary battery of the present invention will be described.
However, the secondary battery of the present invention is characterized in that the positive electrode active material is used, and other components are not particularly limited.

As the negative electrode, for example, metallic lithium, lithium alloy, or the like, a negative electrode mixture made by mixing a binder with a negative electrode active material capable of occluding and releasing lithium ions, and adding a suitable solvent to form a paste, For example, a metal foil current collector such as a metal foil is applied and dried, and is compressed to increase the electrode density as necessary.
As the negative electrode active material constituting this negative electrode, for example, natural graphite, artificial graphite, a fired organic compound such as phenol resin, and a powdery carbon material such as coke can be used.
As the negative electrode binder, as in the positive electrode, fluorine-containing resins such as polyvinylidene fluoride can be used. As the solvent for dispersing these active materials and the binder, organic solvents such as N-methyl-2-pyrrolidone are used. Can be used.

The separator to be used is disposed by being sandwiched between the positive electrode and the negative electrode.
This separator separates a positive electrode and a negative electrode and retains an electrolyte. For example, a thin film of polyethylene, polypropylene or the like and a film having many minute holes can be used.

  The non-aqueous electrolytic solution used is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate, tetrahydrofuran, and 2-methyltetrahydrofuran. At least one selected from ether compounds such as dimethoxyethane, sulfur compounds such as ethyl methyl sulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate can be used.

As the supporting salt, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , or a composite salt thereof can be used. Furthermore, the non-aqueous electrolyte solution may contain a radical scavenger, a surfactant, a flame retardant, and the like.

The shape of the lithium secondary battery according to the present invention composed of the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte can be various, such as a cylindrical type and a laminated type.
In any case, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, and the electrode body is impregnated with the non-aqueous electrolyte solution. A current collector lead or the like is connected between the positive electrode current collector and the positive electrode terminal that communicates with the outside, and between the negative electrode current collector and the negative electrode terminal that communicates with the outside. The battery having the above structure can be sealed in a battery case to complete the battery.

  The surface of the lithium / manganese composite oxide particles of the present invention is treated with phosphoric acid, coated with a phosphate film, and further coated with carbon to form a lithium / manganese positive electrode active material for a non-aqueous electrolyte secondary battery. A high capacity can be obtained by the composite oxide particles, and the phosphate film can suppress the elution of Mn at the electrolyte interface at high temperatures, resulting in a highly safe nonaqueous electrolyte secondary battery.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example at all.

<Preparation of positive electrode active material>
(1). Preparation of lithium-manganese composite oxide particles First, lithium hydroxide (LiOH), which is a starting material, and manganese dioxide are mixed at a molar ratio of Li: Mn = 1: 2 to obtain a mixture, which is 850 ° C. in air. Was fired for 20 hours to prepare lithium-manganese composite oxide particles made of LiMn ₂ O ₄ .

(2). Next, using an attritor in which the inside of the container is replaced with nitrogen, 1 kg of lithium-manganese composite oxide particles are pulverized in 1.5 kg of isopropanol for 2 hours at an average speed of 3 μm. Lithium / manganese composite oxide particles were prepared. During or after the grinding, a predetermined amount of 85% orthophosphoric acid aqueous solution was added to and mixed with the lithium-manganese composite oxide particles according to the description in Table 1.
Thereafter, the lithium / manganese composite oxide particles were dried in vacuum at 120 ° C. for 4 hours to obtain lithium / manganese composite oxide particles coated with a phosphate film.

(3). Coating with Carbon Lithium / manganese composite oxide particles coated with a phosphate film are impregnated in a solution of cellulose acetate (acetyl group content: 39.7 mass%, weight average molecular weight Mw 50000) dissolved in acetone. The amount of cellulose acetate added is 5% by mass of the lithium / manganese composite oxide particles coated with the treated phosphate film.

  Utilization of the carbon precursor as a solution enables complete distribution onto the lithium-manganese composite oxide particles coated with the phosphate coating. After drying, the temperature is raised to 700 ° C. at 6 ° C./min in an argon atmosphere in a heating furnace and held for 1 hr. Subsequently, it cools in steps in argon atmosphere.

Through the steps 1 to 3, the surface of the lithium / manganese composite oxide particles coated with the phosphate film could be further coated with carbon.
FIG. 1 shows a cross-sectional view of a lithium / manganese composite oxide particle 11a which is a positive electrode active material for a nonaqueous electrolyte secondary battery according to Example 1 coated with the carbon film 1 and the phosphate film 2. 3a is a lithium-manganese composite oxide particle before coating with phosphate and carbon.
This sample contains 1% by weight of carbon, which corresponds to a carbonization efficiency of cellulose acetate of 20%.

[Measurement of phosphate film thickness]
As for the phosphate film thickness of the obtained lithium / manganese composite oxide particles, the P and O spectra were monitored by XPS.
From the P profile of the film, the position where the maximum strength was reduced to 50% was defined as the interface position between the film and the base, and the sputtering time L (sec) from the surface to the interface position was read. This L was multiplied by a sputtering rate of 5 nm / min in the standard sample SiO ₂ to obtain a SiO ₂ equivalent film thickness.
Table 1 shows the results.

[Battery fabrication]
60 parts by mass of the positive electrode active material powder was mixed with 30 parts by mass of acetylene black (manufactured by Denki Kogyo Co., Ltd.) and 10 parts by mass of PTFE (manufactured by Daikin Kogyo Co., Ltd.). A pellet was prepared and used as a positive electrode.
Lithium metal was used as the negative electrode, and an equivalent mixed solution of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) using 1M LiPF ₆ as a supporting salt (manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the electrolyte.
Using these, a 2032 type coin battery was manufactured in a glove box of Ar atmosphere in which the dew point was controlled at −80 ° C.

  As shown in FIG. 2, the produced 2032 type coin battery has a separator 5 impregnated with an electrolyte disposed between a positive electrode 6 as an evaluation electrode and a negative electrode 4 made of lithium metal, The whole is covered with a negative electrode can 8 from the negative electrode side, and a positive electrode can 9 from the positive electrode side. A gasket 7 is disposed between the positive electrode can 9 and the negative electrode can 8 to prevent the positive electrode can 9 and the negative electrode can 8 from being short-circuited and to shield the inside of the 2032 type coin battery 10 from the outside.

[Discharge capacity evaluation]
The produced battery was allowed to stand for about 24 hours, and after the OCV was stabilized, the discharge capacity was measured in an environment of 60 ° C.
The discharge capacity was evaluated by conducting a charge / discharge test with a current density of 0.5 mA for the positive electrode and a cut-off voltage of 4.3 to 3.0 V.
Table 1 shows the initial discharge capacity, the measurement results of the discharge capacity at the 20th cycle, and the capacity retention rate.

[Thermal stability evaluation]
As evaluation of thermal stability, the heat generation behavior of the charged positive electrode mixture was examined using DSC (Differential Scanning Calorimeter) (manufactured by Rigaku Corporation: DSC-10A).
Specifically, the exothermic peak intensity, the initial total calorific value, which is the total amount of heat generated by the positive electrode mixture after the first charge, and the total amount of heat generated by the positive electrode mixture at the 20th charge / discharge cycle. The total calorific value at the cycle was measured.
The measurement results are shown in Table 2.

Details of the measurement method are as follows.
First, the 2032 type coin battery shown in FIG. 2 was allowed to stand for about 24 hours to stabilize the OCV. After that, the current density with respect to the positive electrode is set to 0.5 mA / cm ² and charged to a voltage of 4.3 V. When the current value becomes 0.01 mA or less according to the voltage regulation, the charging is terminated by the constant current constant voltage (CCCV) method. Went.

  Thereafter, the charged coin battery was disassembled, the internal positive electrode mixture was taken out, and the adhered electrolyte solution was removed as much as possible until the deposited electrolyte solution was 0.05 mg or less. Then, 3 mg of this positive electrode mixture and 1.3 mg of the electrolyte used for the coin battery were placed in an Al pan for DSC measurement, and the Al pan was caulked and sealed. After sealing, a small sample was made in the surface of the Al pan for degassing to complete a measurement sample. Further, 3 mg of alumina powder was sampled and placed in an Al pan, and the caulked product was completed as a measurement sample in the same manner as described above, and used as a reference sample.

  And about the produced sample, the range from room temperature to 305 degreeC was scanned with the temperature increase rate of 10 degree-C / min using DSC, the exothermic behavior was measured, and the exothermic peak intensity | strength and the initial total calorific value were measured.

Regarding the total calorific value after 20 cycles, up to the 19th cycle, the current density with respect to the positive electrode was set to 0.5 mA / cm ² and charge / discharge was performed at a cut-off voltage of 4.3 to 3.0 V. After charging to 3 V, CCCV was performed to complete charging when the current value was 0.01 mA or less by voltage regulation, and the coin battery was disassembled in the same manner as described above, and then evaluated by DSC.

  Except for the thickness of the phosphate film being 22 nm, lithium-manganese composite oxide particles were prepared in the same manner as in Example 1, and a 2032 type coin provided with a positive electrode plate using the lithium-manganese composite oxide particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured. The results are shown in Tables 1 and 2.

  Except that the thickness of the phosphate coating was 69 nm, lithium-manganese composite oxide particles were prepared in the same manner as in Example 1, and a 2032 type coin provided with a positive electrode plate using the lithium-manganese composite oxide particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured. The results are shown in Tables 1 and 2.

(Comparative Example 1)
A coin battery was manufactured in the same manner as in Example 1 except that the coating step with the phosphate film was omitted.
The initial discharge capacity of the produced battery was measured by the same method as in Example 1, and the thermal stability was also evaluated by using a DSC (differential scanning calorimeter) for the charged positive electrode mixture as in Example 1. This was done by examining the heat generation behavior.
Table 1 shows the measurement results of the initial discharge capacity, and Table 2 shows the measurement results of the exothermic peak intensity, the initial total calorific value, and the total calorific value at the 20th cycle.

Li ₂ CO ₃ , Mn ₂ O ₃ and Fe ₂ O ₃ were mixed at a molar ratio of 1: 1.5: 0.5, calcined in air at 750 ° C. for 8 hours, and LiMn _1.5 Fe _0. Lithium / manganese composite oxide particles coated with a phosphate coating and a carbon coating were prepared in the same manner as in Example 1 except that lithium / manganese composite oxide particles composed of ₅ O ₄ were used.
FIG. 1 shows a cross-sectional view of a lithium / manganese composite oxide particle 11b which is a positive electrode active material for a non-aqueous electrolyte secondary battery according to Example 4 coated with the carbon film 1 and the phosphate film 2. 3b is a lithium-manganese composite oxide particle before coating with phosphate and carbon.
This sample contains 1% by weight of carbon, which corresponds to a carbonization efficiency of cellulose acetate of 20%.
A coin battery using these as a positive electrode active material was produced. The initial discharge capacity of the produced battery was measured by the same method as in Example 1 except that the cut-off voltage was 5.0 to 3.5 V, and the thermal stability was evaluated in the same manner as in Example 1. The charged positive electrode mixture was examined by examining the heat generation behavior using a DSC (differential scanning calorimeter).
Table 3 shows the measurement results of the initial discharge capacity, and Table 4 shows the measurement results of the exothermic peak intensity, initial total heat generation, and total heat generation at the 20th cycle.

  Except that the thickness of the phosphate coating was 22 nm, lithium-manganese composite oxide particles were produced in the same manner as in Example 4, and a 2032 type coin provided with a positive electrode plate using the lithium-manganese composite oxide particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured. The results are shown in Tables 3 and 4.

  Except for the thickness of the phosphate coating being 69 nm, lithium-manganese composite oxide particles were produced in the same manner as in Example 4, and a 2032 type coin provided with a positive electrode plate using the lithium-manganese composite oxide particles A battery was prepared, and its phosphate film thickness and battery characteristics were measured. The results are shown in Tables 3 and 4.

(Comparative Example 2)
A coin battery was manufactured in the same manner as in Example 4 except that the coating step with the phosphate film was omitted.
The initial discharge capacity of the produced battery was measured by the same method as in Example 4, and the thermal stability was also evaluated using a DSC (differential scanning calorimeter) for the charged positive electrode mixture as in Example 4. This was done by examining the heat generation behavior.
Table 3 shows the measurement results of the initial discharge capacity, and Table 4 shows the measurement results of the exothermic peak intensity, initial total heat generation, and total heat generation at the 20th cycle.

  As is clear from Tables 1, 2 and 3, and Table 4, by forming a phosphate film and a carbon film on the surface of the lithium / manganese composite oxide particles in order, the lithium / manganese composite oxide particles It can be seen that it is possible to improve charge / discharge cycle characteristics in a high temperature environment while maintaining safety.

1 Carbon film 2 Phosphate film 3 Lithium / manganese composite oxide particles 3a Lithium / manganese composite oxide particles 3b according to Example 1 Lithium / manganese composite oxide particles 4 according to Example 4 Lithium metal negative electrode 5 Separator (electrolysis Liquid impregnation)
6 Positive electrode (Evaluation electrode)
7 Gasket 8 Negative electrode can 9 Positive electrode can 10 2032 type coin battery 11 Positive electrode active material for non-aqueous electrolyte secondary battery according to the present invention (lithium / manganese composite oxide particles coated in the order of phosphate film and carbon film)
11a Lithium / manganese composite oxide particles coated in the order of phosphate film and carbon film according to Example 1 11b Lithium / manganese composite oxide particles coated in the order of phosphate film and carbon film according to Example 4

Claims (4)

Lithium-manganese composite oxide particles,
A cathode active material for a non-aqueous electrolyte secondary battery, wherein the particle surface of the composite oxide particles is coated in the order of a phosphate film and carbon. The lithium-manganese composite oxide particles have a general formula Li _x Mn _2-y M _y O ₄ (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 0.5, M is Fe, Ni, Co, Al, The positive electrode active for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium-manganese composite oxide particles are selected from the group consisting of Mg, Ti and Zr. material.   The method comprises a step of coating a particle surface of a lithium / manganese composite oxide particle with a phosphate coating by phosphoric acid treatment, and a step of coating the particle surface coated with the phosphate coating with carbon. A method for producing a positive electrode active material for a water electrolyte secondary battery. The lithium metal composite oxide particles are Li _x Mn _2-y M _y O ₄ (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 0.5, M is Fe, Ni, Co, Al, Mg, Ti And at least one selected from the group consisting of Zr.) The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 3.

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