JP7722232B2 - Positive electrode active material, method for producing positive electrode active material, positive electrode mixture, non-aqueous lithium ion secondary battery, and all-solid-state lithium ion secondary battery - Google Patents

Description

This disclosure relates to a positive electrode active material, a method for manufacturing a positive electrode active material, a positive electrode mixture, a nonaqueous lithium ion secondary battery, and an all-solid-state lithium ion secondary battery.

Patent Document 1 discloses a lithium ferrite-based oxide represented by the composition formula Li _2-x Ti _1-z Fe _z O _3-y (0≦x<2, 0≦y≦1, 0.05≦z≦0.95) and having a cubic rock salt structure.

Patent Document 2 discloses a positive electrode for a secondary battery, which includes a current collector and a positive electrode active material layer provided on the current collector, the positive electrode active material layer containing a positive electrode active material and a binder, wherein the positive electrode active material is a lithium ferrite composite oxide represented by the composition formula Li2 _-x ( _{Mn1-m- nFenNim )} O3 _-y (where 0≦x<2, 0≦y≦1, 0.01≦m≦0.30, 0.05≦n≦0.75, 0.06≦m+n<1) and having a layered rock salt structure, and the binder is a polymer containing polymerization units of a (meth)acrylic acid ester monomer, polymerization units of a vinyl monomer having an acid component, and polymerization units of an α,β-unsaturated nitrile monomer, and the content of the polymerization units of the vinyl monomer having an acid component is 1.0 to 3.0 wt % of all polymerization units of the polymer.

Patent Document 3 discloses a method for producing LiFeO 2 having a layered rock salt structure, which is characterized by hydrothermally treating at least one of water-soluble iron salt, iron hydroxide, iron oxide hydroxide, and metallic iron in an aqueous solution containing _lithium hydroxide and sodium hydroxide and/or potassium hydroxide at 130 to 300°C.

Patent Document 4 discloses a method for producing a layered rock salt type lithium iron oxide represented by the composition formula LiFe 1-x Ni x O 2 (0<x<0.5), which comprises the steps of (a) alkali treating a mixed solution containing iron ions _{(III) and} Ni ions (II ₎ , (b) air-oxidizing the produced coprecipitate, (c) aging the produced oxide at 100°C or less, and (d) hydrothermally treating the aged product in an alkaline solution containing a Li compound at ₄₀₀ °C or less.

Japanese Patent Application Laid-Open No. 2003-48717 International Publication No. 2011/078212 Japanese Patent Application Publication No. 10-139593 Japanese Patent Application Laid-Open No. 2002-338251

There is a need to improve the reversible capacity of lithium-ion secondary batteries.

The purpose of this disclosure is to provide a positive electrode active material that can improve the reversible capacity of lithium-ion secondary batteries, and a method for producing the same.

The present inventors have found that the above object can be achieved by the following means:
<<Aspect 1>>
A positive electrode active material having a composition formula of Li _x Fe _y O ₂ (wherein 0<x<2 and 0<y<2) and a crystallite diameter of 200 Å or less.
Aspect 2
2. The cathode active material of embodiment 1, wherein 0.95≦x≦1.05 and 0.95≦y≦1.05.
Aspect 3
3. The positive electrode active material of claim 1 or 2, having a cubic rock salt structure.
Aspect 4
The positive electrode active material according to any one of Aspects 1 to 3, wherein the crystallite diameter is 170 Å or less.
Aspect 5
Mixing _Fe2O3 and _Li2CO3 by a first ball mill method to obtain a _mixed powder _;
calcining the mixture powder; and pulverizing the calcined mixture powder by a second ball mill method.
A method for producing the positive electrode active material according to any one of aspects 1 to 3, comprising:
Aspect 6
A positive electrode mixture for a lithium ion secondary battery, comprising the positive electrode active material according to any one of aspects 1 to 4.
Aspect 7
a positive electrode assembly, a separator, and a negative electrode assembly are housed in this order in an exterior body filled with a non-aqueous electrolyte solution, and the positive electrode assembly contains the positive electrode active material according to any one of aspects 1 to 4.
Non-aqueous lithium-ion secondary battery.
Aspect 8
a positive electrode layer, a solid electrolyte layer, and a negative electrode layer stacked in this order;
The positive electrode layer contains the positive electrode active material according to any one of aspects 1 to 4.
All-solid-state lithium-ion secondary battery.

This disclosure provides a positive electrode active material that can improve the reversible capacity of lithium-ion secondary batteries, as well as a method for producing the same.

FIG. 1 is a schematic diagram of a non-aqueous lithium-ion secondary battery 100 according to a first embodiment of the present disclosure. FIG. 2 is a schematic diagram of an all-solid-state lithium-ion secondary battery 200 according to a first embodiment of the present disclosure. FIG. 3 is a graph showing the X-ray crystal diffraction spectrum of each example of the positive electrode active material. FIG. 4 is a graph showing the relationship between the crystallite size of the positive electrode active material of each example and the discharge capacity of the nonaqueous lithium ion secondary battery of each example. FIG. 5 is a graph showing the results of a charge-discharge test of the nonaqueous lithium ion secondary battery of Comparative Example 1. FIG. 6 is a graph showing the results of a charge-discharge test of the nonaqueous lithium ion secondary battery of Example 1.

Embodiments of the present disclosure are described in detail below. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the disclosure.

《Cathode active material》
The positive electrode active material of the present disclosure is a positive electrode active material represented by the composition formula Li _x Fe _y O ₂ (wherein 0<x<2 and 0<y<2) and has a crystallite diameter of 200 Å or less.

In order to improve the reversible capacity of a positive electrode active material represented by the composition formula Li _x Fe _y O ₂ (wherein 0<x<2 and 0<y<2), attempts have been made to substitute with different elements or to change the crystal structure.

In contrast, the positive electrode active material of the present disclosure is expressed by the composition formula Li _x Fe _y O ₂ (wherein 0<x<2 and 0<y<2) and has a crystallite diameter of 200 Å or less, thereby improving the reversible capacity. This is thought to be because the diffusion distance of lithium ions becomes shorter as the crystallite diameter becomes smaller.

In general, it is believed that the larger the crystallite size of a positive electrode active material, which is an intercalation compound, the greater the reversible capacity, from the perspective of intercalating lithium ions between the layers.

In the composition formula Li _x Fe _y O ₂ , 0<x<2 and 0<y<2. Here, x may be greater than 0, 0.05 or more, 0.10 or more, 0.50 or more, or 0.95 or more, and may be less than 2.00, 1.95 or less, 1.50 or less, or 1.05 or less. Preferably, 0.95≦x≦1.05. Similarly, y may be greater than 0, 0.05 or more, 0.10 or more, 0.50 or more, or 0.95 or more, and may be less than 2.00, 1.95 or less, 1.50 or less, or 1.05 or less. Preferably, 0.95≦y≦1.05.

Furthermore, the positive electrode active material of the present disclosure may have a cubic rock salt structure.

The crystallite diameter of the positive electrode active material disclosed herein is 200 Å or less.

When the crystallite size of the positive electrode active material represented by the composition formula Li _x Fe _y O ₂ (wherein 0<x<2 and 0<y<2) is 200 Å or less, a high discharge capacity can be obtained.

The crystallite diameter of the positive electrode active material may be 200 Å or less, 170 Å or less, 150 Å or less, 120 Å or less, or 100 Å or less, or may be 10 Å or more, 30 Å or more, 40 Å or more, or 50 Å or more.

<<Method for producing positive electrode active material>>
The method for producing a positive electrode active material according to the present disclosure includes the following steps (A) to (C).
(A) Mixing _Fe2O3 and _Li2CO3 by a first ball _mill method to obtain a mixed powder _;
(B) calcining the mixture powder; and (C) pulverizing the calcined mixture powder by a second ball milling method.

<Process (A)>
Step (A) is to mix _Fe2O3 and _Li2CO3 by a first ball _mill method to obtain a mixed _powder .

The first ball milling method may be either a wet ball milling method or a dry ball milling method, but a wet ball milling method is preferred from the viewpoint of preventing the mixed powder from adhering to the container.

The rotation speed, rotation time, and size and number of balls in the first ball mill method are not particularly limited.

The rotation speed may be, for example, 200 rpm to 2000 rpm. The rotation speed may be 200 rpm or more, 300 rpm or more, or 500 rpm or more, and may be 2000 rpm or less, 1500 rpm or less, 1000 rpm or less, or 500 rpm or less.

The rotation time may be between 5 minutes and 10 hours. The rotation time may be 5 minutes or more, 10 minutes or more, 15 minutes or more, 30 minutes or more, or 1 hour or more, and may be 10 hours or less, 5 hours or less, 2 hours or less, 1 hour or less, or 30 minutes or less.

The size of the balls can be, for example, 0.1 mm to 50 mm in diameter. The size of the balls may be 0.1 mm or more, 1 mm or more, or 5 mm or more, and may be 50 mm or less, 25 mm or less, or 10 mm or less.

Balls of different sizes may be used in combination. For example, balls with diameters of 10 mm, 5 mm, and 1 mm may be used in combination.

<Process (B)>
Step (B) is to calcinate the mixture powder.

The firing may be carried out in an atmosphere inert to the powder mixture, such as a nitrogen atmosphere or a rare gas atmosphere, or more specifically, an Ar atmosphere.

The firing temperature may be between 600°C and 1200°C. The firing temperature may be 600°C or higher, 750°C or higher, or 900°C or higher, and may be 1200°C or lower, 1000°C or lower, or 900°C or lower.

<Positive electrode mixture>
The positive electrode mixture of the present disclosure is a positive electrode mixture for a lithium ion secondary battery that contains the positive electrode active material of the present disclosure.

The positive electrode mixture of the present disclosure may optionally contain a solid electrolyte, a conductive additive, and a binder. The positive electrode mixture of the present disclosure may be obtained, for example, by mixing the positive electrode active material of the present disclosure with these optional materials.

<Solid electrolyte>
The solid electrolyte is preferably an inorganic solid electrolyte, such as a sulfide solid electrolyte, an oxide solid electrolyte, or a nitride solid electrolyte.

Examples of sulfide solid electrolytes include, but are not limited to, sulfide-based amorphous solid electrolytes, sulfide-based crystalline solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide solid electrolytes include Li ₂ S—P ₂ S ₅ systems (Li ₇ P ₃ S ₁₁ , Li ₃ PS ₄ , Li ₈ P ₂ S ₉ , etc.), Li ₂ S—SiS ₂ , LiI—Li ₂ S—SiS ₂ , LiI—Li ₂ S—P ₂ S ₅ , LiI—LiBr—Li ₂ S—P ₂ S ₅ , Li ₂ S—P ₂ S ₅ —GeS ₂ (Li ₁₃ GeP ₃ S ₁₆ , Li ₁₀ GeP ₂ S ₁₂ , etc.), LiI—Li ₂ S—P ₂ O ₅ , LiI—Li ₃ PO ₄ —P ₂ S ₅ , Li _7-x PS _6-x Cl _x , etc.; or combinations thereof.

Examples of oxide solid electrolytes include, but are not limited to, Li ₇ La ₃ Zr ₂ O _12, Li _7-x La ₃ Zr _1-x Nb _x O _12, Li _7-3x La ₃ Zr ₂ Al _x O ₁₂ , Li _3x La _2/3-x TiO ₃ , Li 1 _+x Al _x Ti _2-x (PO ₄ ) ₃ , Li _{1 +x} Al _x Ge _2-x (PO ₄ ) ₃ , Li 3 PO ₄ , or Li _3+x PO _4-x N _x (LiPON).

The solid electrolyte may be glass, glass ceramics, or a crystalline material. Glass can be obtained by amorphous processing of a raw material composition (e.g., _a mixture of _Li2S and _P2S5 ). Examples of amorphous processing include mechanical milling. Mechanical milling may be dry mechanical milling or wet mechanical milling, with the latter being preferred. This is because it can prevent the raw material composition from adhering to the wall surface of a container, etc. Glass ceramics can be obtained by heat treating glass. Crystalline materials can be obtained, for example, by solid-phase reaction processing of the raw material composition.

The solid electrolyte is preferably in the form of particles. The solid electrolyte has an average particle size (D50) of, for example, 0.01 μm or more. On the other hand, the solid electrolyte has an average particle size (D50) of, for example, 10 μm or less, or may be 5 μm or less. The lithium ion conductivity of the solid electrolyte at 25° C. is, for example, 1×10 ⁻⁴ S/cm or more, and preferably 1×10 ⁻³ S/cm or more.

<Conductive additive>
The conductive additive may be a known material, such as a carbon material or metal particles. Examples of the carbon material include at least one material selected from the group consisting of carbon black (e.g., acetylene black or furnace black), vapor-grown carbon fiber (VGCF), carbon nanotubes, and carbon nanofibers. From the viewpoint of electron conductivity, at least one material selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers may be used. Examples of the metal particles include nickel, copper, iron, and stainless steel particles.

<Binder>
The binder may be, for example, but is not limited to, materials such as polyvinylidene fluoride (PVdF), carboxymethyl cellulose (CMC), butadiene rubber (BR), or styrene butadiene rubber (SBR), or combinations thereof.

<<Non-aqueous lithium-ion secondary battery>>
The nonaqueous lithium ion secondary battery of the present disclosure is a nonaqueous lithium ion secondary battery in which a positive electrode body, a separator, and a negative electrode body are housed in this order in an exterior body filled with a nonaqueous electrolyte solution, and the positive electrode body contains the positive electrode active material of the present disclosure.

Figure 1 is a schematic diagram of a nonaqueous lithium-ion secondary battery 100 according to a first embodiment of the present disclosure.

As shown in FIG. 1, a nonaqueous lithium-ion secondary battery 100 according to a first embodiment of the present disclosure includes a positive electrode assembly 110, a separator 120, and a negative electrode assembly 130, arranged in this order, housed in an exterior case 150 filled with a nonaqueous electrolyte solution 140. The positive electrode assembly 110 has a positive electrode current collector layer 111 coated on both sides with a positive electrode active material layer 112. The negative electrode assembly 130 has a negative electrode current collector layer 131 coated on both sides with a negative electrode active material layer 132. The positive electrode active material layer 112 contains the positive electrode active material of the present disclosure.

<Positive electrode body>
The positive electrode body includes a positive electrode current collector and a positive electrode active material. The positive electrode body may be, for example, a layer or rod-shaped positive electrode body, the surface of which is partially or entirely covered with a layer containing the positive electrode active material, i.e., a positive electrode active material layer.

The positive electrode body contains the positive electrode active material of the present disclosure. More specifically, the positive electrode active material of the present disclosure can be contained in the positive electrode active material layer that constitutes the positive electrode body.

(Positive electrode current collector layer)
The material used for the positive electrode current collector layer may be, but is not limited to, stainless steel (SUS), aluminum, copper, nickel, iron, titanium, carbon, etc. Among these, the material for the positive electrode current collector layer is preferably aluminum.

The shape of the positive electrode current collector layer is not particularly limited, and examples include rod, foil, plate, and mesh shapes. Of these, foil shapes are preferred.

(Positive electrode active material layer)
The positive electrode active material layer may contain the positive electrode active material of the present disclosure. The positive electrode active material layer may be formed from the positive electrode mixture of the present disclosure and optionally other materials.

<Separator layer>
The separator layer may be a porous resin layer, for example a polypropylene or polyethylene layer.

The separator layer can be a porous polymer membrane such as a porous polyethylene membrane (PE), a porous polypropylene membrane (PP), a porous polyolefin membrane, or a porous polyvinyl chloride membrane. A lithium ion conductive polymer electrolyte membrane can also be used as the separator layer. These separator layers can be used alone or in combination. From the perspective of increasing battery output, it is preferable to use a triple-layer coated separator layer in which a porous polyethylene membrane (PE) is sandwiched between two upper and lower layers of porous polypropylene membrane (PP).

<Negative electrode body>
The negative electrode body has a negative electrode current collector and a negative electrode active material. The negative electrode body may have a shape in which the surface of a layered or rod-shaped negative electrode body is partially or entirely covered with a layer containing the negative electrode active material, i.e., a negative electrode active material layer.

(Negative electrode current collector layer)
The material used for the negative electrode current collector layer may be the same as the material used for the positive electrode current collector layer, but is preferably copper.

(Negative electrode active material layer)
The negative electrode active material layer contains a negative electrode active material and, optionally, a solid electrolyte, and may further contain additives used in negative electrode active material layers of lithium ion batteries, such as a conductive additive or a binder, depending on the intended use or purpose.

The negative electrode active material may be, for example, metallic lithium, or a material capable of absorbing and releasing metal ions such as lithium ions. Examples of materials capable of absorbing and releasing metal ions such as lithium ions include, but are not limited to, alloy-based negative electrode active materials or carbon materials.

The alloy-based negative electrode active material is not particularly limited and may, for example, be a Si alloy-based negative electrode active material or a Sn alloy-based negative electrode active material. Examples of Si alloy-based negative electrode active materials include silicon, silicon oxide, silicon carbide, silicon nitride, or solid solutions thereof. Si alloy-based negative electrode active materials may also contain elements other than silicon, such as Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, or Ti. Sn alloy-based negative electrode active materials include tin, tin oxide, tin nitride, or solid solutions thereof. Sn alloy-based negative electrode active materials may also contain elements other than tin, such as Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Ti, or Si. Among these, Si alloy-based negative electrode active materials are preferred.

The carbon material is not particularly limited, and examples include hard carbon, soft carbon, and graphite.

The conductive additives and binders described above for the positive electrode mixture can be used as appropriate.

(Non-aqueous electrolyte)
The non-aqueous electrolyte may be a composition containing a supporting salt in a non-aqueous solvent. The non-aqueous solvent may be a material selected from the group consisting of an organic electrolyte, a fluorine-based solvent, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and a combination of two or more thereof.

Preferred non-aqueous solvents are fluorinated solvents, such as fluorinated carbonates. Specific examples of fluorinated carbonates include methyl 2,2,2-trifluoroethyl carbonate (MFEC; Carbonic acid, methyl 2,2,2-trifluoroethyl ester; CAS 156783-95-8) and/or difluorodimethyl carbonate (DFDMC), with a 50:50 volumetric mixture of these being particularly preferred.

Examples of the supporting salt include _materials selected from _the group _consisting of LiPF6, _LiBF4 , LiClO4 _{, LiAsF6} , _LiCF3SO3 , _LiC4F9SO3 , LiN( _CF3SO2 ) ₂ , LiC( _CF3SO2 ) ₃ , lithium compounds ( _{lithium salts} ) of LiI, and combinations of _two or more thereof. From the viewpoints of improving battery voltage and durability, _LiPF6 is preferred as the supporting salt.

(Exterior body)
The exterior body can be made of a material that is inactive to the non-aqueous electrolyte solution, such as a resin.

<<All-solid-state lithium-ion secondary battery>>
The all-solid-state lithium ion secondary battery according to the present disclosure is an all-solid-state lithium ion secondary battery in which a positive electrode layer, a solid electrolyte layer, and a negative electrode layer are laminated in this order, and the positive electrode layer contains the positive electrode active material according to the present disclosure.

Figure 2 is a schematic diagram of an all-solid-state lithium-ion secondary battery 200 according to a first embodiment of the present disclosure.

As shown in FIG. 2 , the all-solid-state lithium-ion secondary battery 200 according to the first embodiment of the present disclosure comprises a positive electrode current collector layer 210, a positive electrode active material layer 220, a solid electrolyte layer 230, a negative electrode active material layer 240, and a negative electrode current collector layer 250 stacked in this order, and the positive electrode active material layer 220 contains the positive electrode active material of the present disclosure. Here, the positive electrode active material layer 220 and the positive electrode current collector layer 210 are collectively referred to as the positive electrode layer, and the negative electrode active material layer 240 and the negative electrode current collector layer 250 are collectively referred to as the negative electrode layer.

For the positive electrode layer and negative electrode layer of the all-solid-state lithium-ion secondary battery disclosed herein, please refer to the description of the "Non-aqueous Lithium-ion Secondary Battery" above.

The solid electrolyte layer may also contain the solid electrolyte described above in the "Positive Electrode Composite" section. The solid electrolyte layer may also optionally contain a binder.

Examples 1 to 3 and Comparative Examples 1 and 2
<Preparation of Positive Electrode Active Material>
For Examples 1 to 3 and Comparative Example 1, Fe ₂ O ₃ and Li ₂ CO ₃ were mixed in ethanol by a wet ball mill method under the conditions shown in Table 1. The resulting mixed powder was molded into pellets (step (A)).

Next, the boat-shaped aluminum boat containing the molded pellets was wrapped in Cu foil and fired in an Ar atmosphere at 900°C for 12 hours (step (B)).

Finally, the obtained powder was pulverized by dry ball milling under the conditions shown in Table 1 (step (C)). The crystallite size was controlled by adjusting the ball milling time. Specifically, as shown in Table 1, the number of sets used in the dry ball milling method varied depending on the example.

In addition, in Comparative Example 2, ball milled _LiCoO2 was used as the positive electrode active material.

X-ray diffraction test
For each example of the positive electrode active material, an X-ray diffraction test was performed using a non-reflective sample plate, and the X-ray diffraction spectrum was analyzed. The results are shown in Figure 3. The crystallite diameter was calculated from the half-width of the peak representing the (200) plane in the obtained X-ray diffraction spectrum.

More specifically, the crystallite diameter (nm) was defined as D, the Scherrer constant as K, the wavelength of the X-ray as λ, the half-width as B, and the Bragg angle as θ, and the calculation was performed using the following formula:
D = Kλ/BCosθ

The calculation results are shown in Figure 4.

Note that the half-width is the width of the diffraction line at half the height of the diffraction line intensity.

<Evaluation of charge/discharge characteristics>
A coin-type lithium-ion battery cell (CR2032) was prepared using the positive electrode active material of each example. Specifically, the positive electrode active material of each example was weighed out in a weight ratio of 85/15/5 of AB/PVdF, and dispersed and mixed in N-methyl-2-pyrrolidone to form a slurry. The slurry was applied to an aluminum current collector foil serving as a positive electrode current collector layer and dried overnight in a vacuum at 120°C to form a positive electrode active material layer on the positive electrode current collector layer. This was used as a positive electrode body.

TDDK-217 (Daikin) was used as the electrolyte, and metallic Li foil was used as the negative electrode.

Charge-discharge characteristics were evaluated in a thermostatic chamber maintained at 60°C, with a voltage range of 1.5-5V and a 0.1C rate (1C = 185mA/g). The evaluation results for Comparative Example 1 and Example 1 are shown in Figures 5 and 6, respectively.

<result>
Table 2 shows the relationship between the crystallite size (Å) and the discharge capacity (mAh/g) for each example.

As shown in Table 2, the lithium ion batteries using the positive electrode active materials of Examples 1 to 3, which had a composition of Li _x Fe _y O ₂ and crystallite diameters of 55 Å, 109 Å, and 167 Å, respectively, had discharge capacities of 180 mAh/g, 98 mAh/g, and 81 mAh/g, respectively. In contrast, the lithium ion battery using the positive electrode active material of Comparative Example 1, which had a composition of Li _x Fe _y O ₂ and a crystallite diameter of 830 Å, had a discharge capacity of 5 mAh/g, which was much lower than the lithium ion batteries using the positive electrode active materials of Examples 1 to 3.

In addition, in the lithium ion battery using the positive electrode active material of Comparative Example 2, which had a composition of Li _x Co _y O ₂ and a crystallite diameter of 200 or less, the discharge capacity was 5 mAh/g, which was much lower than that of the lithium ion batteries using the positive electrode active materials of Examples 1 to 3.

REFERENCE SIGNS LIST 100 Non-aqueous lithium ion secondary battery 110 Positive electrode body 111 Positive electrode current collector layer 112 Positive electrode active material layer 120 Separator 130 Negative electrode body 131 Negative electrode current collector layer 132 Negative electrode active material layer 140 Non-aqueous electrolyte 150 Exterior body 200 All-solid-state lithium ion secondary battery 210 Positive electrode current collector layer 220 Positive electrode active material layer 230 Solid electrolyte layer 240 Negative electrode active material layer 250 Negative electrode current collector layer

Claims (1)

A method for producing a positive electrode active material represented by the composition formula Li _x Fe _y O ₂ (wherein 0<x<2 and 0<y<2) and having a crystallite diameter of 50 Å or more and 100 Å or less , comprising the steps of:
Mixing Fe2O3 and Li2CO3 by a first ball mill method to obtain _{a mixed powder ;}
Calcining the powder mixture; and
pulverizing the fired mixture powder by a second ball mill method;
A method for producing a positive electrode active material, comprising: