Method for preparing positive electrode active material for non-aqueous secondary battery

Abstract

There is provided a simple and easy method of preparation of a positive electrode active material for a non-aqueous secondary battery which comprises a compound comprising lithium, nickel and manganese and having a layered structure. Said method comprises firing a mixture of (1) at least one member selected from the group consisting of dinickel trioxide and boron compounds and (2) one or more metal compounds comprising lithium, nickel and manganese as their metal elements.

Description

This is a continuation of application Ser. No. 10/394,049 filed Mar. 24, 2003
wherein 0<x≦0.5, 0≦y≦⅙, x>y. It is preferable that, in the composition formula (I), y is greater than 0, i.e., Co is contained because then the discharge capacity and the cycle characteristic at room temperature is improved. Further, it is preferable that, in the composition formula (I), x is smaller than 0.5, i.e., Ni content is smaller than Mn content and Li is contained in the transition metal site, because then the cycle characteristic at high temperature is improved. When x decreases, discharge capacity tends to decrease, therefore 0.4<x<0.5 is preferable as the range of x, more preferably y>0 at the same time. The respective sites of lithium, nickel, manganese and cobalt may be replaced with Na, K, Mg, Ca, Sr, Ba, B, Al, Ga, In, Si, Zr, Sn, Ti, V, Cr, Fe, Cu, Ag, Zn, etc. within the range of not more than 50% by mole of the respective sites. Further, as to oxygen, it may be replaced with a halogen, sulfur and nitrogen within the range of not more than 5% by mole so long as the crystal structure does not change and the resulting product is a compound which is identified in X-ray diffraction as a compound represented by the composition formula (I). The replacing method is not particularly limited and conventional known methods can be used.

Hereunder, with reference to a case where the positive electrode material for a non-aqueous secondary battery of this invention is used for the positive electrode of a lithium secondary battery, a suitable constitution for making a battery will be explained.

A positive electrode for a lithium secondary battery which is one embodiment of this invention can be produced by supporting on a positive electrode current collector a positive electrode mix containing the active material for a non-aqueous secondary battery of this invention and additionally a carbonaceous material as a conductive material, a binder, or the like.

The carbonaceous material may be, for example, natural graphite, artificial graphite, cokes, and carbon black. These can be used as a conductive material each alone or as a mixture of, for example, artificial graphite with carbon black.

As the binder is usually employed thermoplastic resins. Specific examples of the resins used include poly(vinylidene fluoride)(hereinafter sometimes referred to as “PVDF”), polytetrafluoroethylene (hereinafter sometimes referred to as “PTFE”), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer, hexafluoropropylene-vinylidene fluoride copolymer, and tetrafluoroethylene-perfluorovinyl ether copolymer. They may be used each alone or in admixture of two or more thereof.

When a fluororesin and a polyolefin are used together as binders in combination with the positive electrode active material of this invention such that the proportion of the fluororesin in the positive electrode mix is 1-10% by weight and that of the polyolefine is 0.1-2% by weight, adhesion to the current collector is excellent and safety from external heating is further improved.

Al, Ni, stainless steel, etc. can be used for the positive electrode current collector; Al is preferred because it can be easily worked into a thin sheet and is inexpensive. The methods used for supporting the positive electrode mix on the positive electrode current collector include a method of compression molding, and a method of making the positive electrode mix into paste by using a solvent or the like, coating the paste on the positive electrode current collector, drying the coat and then pressing the collector to adhere the coat.

As to the negative electrode material of a lithium secondary battery which is one embodiment of this invention, there may be used, for example, lithium metal, lithium alloys or materials capable of being doped/undoped with lithium ions. As examples of the materials capable of being doped/undoped with lithium ions, mention may be made of carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds; chalcogen compounds such as oxides and sulfides capable of being doped/undoped with lithium ions at a potential lower than that of positive electrode; etc.

The form of the carbonaceous materials may be any of, for example, the flaky form such as of natural graphite, spherical form such as of mesocarbon microbeads, fibrous form such as of graphitized carbon fibers, or fine powder aggregates, and, if necessary, a thermoplastic resin as a binder may be added thereto. The thermoplastic resin may be, for example, PVDF, polyethylene and polypropylene.

The chalcogen compounds, such as oxides and sulfides, used as the negative electrode include, for example, oxides of the elements of groups 13, 14 and 15 of the periodic table. These compounds may also be incorporated according to necessity, with carbonaceous materials as conductive materials and thermoplastic resins as binders.

For a negative electrode current collector, Cu, Ni, stainless steel and the like can be used; particularly in lithium secondary batteries, Cu is preferred because it can hardly form alloys with lithium and, moreover, can be easily worked into a thin sheet. As to the method for supporting a mix containing a negative electrode active material on the negative electrode current collector, mention may be made of a method of compression molding and a method of making the negative electrode active material into a paste by using a solvent, etc., coating the paste on the current collector, drying the coat and then pressing the collector to adhere the coat.

For a separator used in a lithium secondary battery which is one embodiment of this invention, there may be used materials in the form of porous membrane, nonwoven fabrics and woven fabrics of, for example, polyolefine, such as polyethylene and polypropylene, fluororesin, nylon, aromatic aramid, etc. The thickness of the separator is preferably as thin as possible so far as necessary mechanical strength is kept, from the viewpoint of increasing the volume energy density of the battery and decreasing internal resistance, and is preferably about 10-200 μm.

The electrolyte used in the lithium secondary battery which is one embodiment of this invention may be known ones, for example, an electrolyte selected from either a non-aqueous electrolyte solution containing a lithium salt dissolved in an organic solvent or a solid electrolyte. The lithium salt may be LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN (CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, a lower aliphatic carboxylic acid lithium salt, LiAlCl₄, and the like, used each alone or as a mixture of two or more thereof.

The organic solvent used in the lithium secondary battery which is one embodiment of this invention may be, for example, carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one and 1,2-di(methoxycarbonyloxy)ethane; ethers such as 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoro-propyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran and 2-methyl-tetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; nitrites such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propanesultone; and further, the above-mentioned organic solvents having a fluorine substituent further introduced therein; generally, they are used as a mixture of two or more thereof. Among them, mixed solvents containing carbonates are preferred, and mixed solvents of cyclic carbonates with acyclic carbonates or mixed solvents of cyclic carbonates with ethers are more preferred.

Among the mixed solvents of cyclic carbonates with acyclic carbonates, those which contain ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate are preferred because they have a wide operating temperature range, are excellent in loading characteristics and are hardly decomposed even when graphite materials, such as natural graphite and artificial graphite, are used as the active material of negative electrode.

Further, in point of giving a particularly excellent safety improving effect, the use of those electrolytes is preferred which contain a fluorine-containing lithium salt, such as LiPF₆, and/or an organic solvent containing a fluorine substituent. Mixed solvents containing ethers having a fluorine substituent, such as pentafluoropropyl methyl ether and 2,2,3,3-tetrafluoropropyl difluoromethyl ether, and dimethyl carbonate are more preferred because they are excellent also in high-current discharging characteristics.

As to solid electrolytes, there may be used such polymer electrolytes as high molecular compounds of polyethylene oxide type and high molecular compounds containing at least one of the polyorganosiloxane chain and polyoxyalkylene chain. There may also be used so-called gel type electrolytes comprising a polymer and a non-aqueous electrolyte solution held therein. In some cases, safety can be improved when sulfide type electrolytes, such as Li₂S—SiS₂, Li₂S—GeS₂, Li₂S—P₂S₅ and Li₂S—B₂S₃, or inorganic compound type electrolytes containing sulfides, such as Li₂S—SiS₂—Li₃PO₄ and Li₂S—SiS₂—Li₂SO₄, are used.

The shape of the non-aqueous secondary battery of this invention is not particularly limited, and may of the paper type, coin type, cylindrical type, rectangular type, etc.

As the outer casing of the battery, there may be used, beside a metal hard case which doubles as the negative electrode or positive electrode terminal, a bag-formed package formed of a laminate sheet comprising aluminum, or the like.

EXAMPLES

This invention is described in more detail with reference to Examples, but the invention is in no way limited thereto. Unless stated otherwise, the electrodes and flat plate type batteries for charging-discharging tests were prepared by the following method.

(1) Preparation of Electrodes and Flat Plate Type Batteries for Charging-Discharging Test

To a mixture of a positive electrode active material and acetylene black of a conductive material was added as a binder a solution of PVDF in 1-methyl-2-pyrrolidone (hereinafter sometimes referred to as “NMP”) so as to give a proportion of active material conductive material:binder of 86:10:4 (weight ratio), the resulting mixture was kneaded to form a paste, the paste was coated on #100 stainless steel mesh, which was to constitute a positive electrode current collector, and dried under reduced pressure at 150° C. for 8 hours, to obtain a positive electrode.

The positive electrode obtained above was combined with a solution obtained by dissolving LiPF₆ in a liquid mixture of ethylene carbonate (hereinafter sometimes referred to as EC), dimethyl carbonate (hereinafter sometimes referred to as “DMC”) and ethyl methyl carbonate (hereinafter sometimes referred to as “EMC”) of a volume ratio of 30:35:35 to give a LiPF₆ concentration of 1 mole/l, as an electrolyte (hereinafter sometimes referred to as “LiPF₆/EC+DMC+EMC”), polypropylene porous membrane as a separator and metallic lithium as a negative electrode, whereby a flat plate type battery was prepared.

(2) Powder X-Ray Diffraction Measurement

(2-1) Measurement Conditions in Examples 1, 2, 3 and 4 and Comparative Example 1

Measurement was made with RU200 system (a trade name, mfd. by Rigaku Corporation, Japan) under the following conditions.

• • X-ray: CuK_α
  • Voltage-Current: 40 kV-30 mA
  • Measurement angle range: 2θ=10-90°
  • Slit: DS-1°, RS-0.3 mm, SS-1°
  • Step: 0.02°
    (2-2) Measurement Conditions in Examples 5, 6 and 7

Measurement was made with Type RINT (a trade name, mfd. by Rigaku Corporation, Japan)

• • X-ray: CuK_α
  • Voltage-Current: 40 kV-140 mA
  • Measurement angle range: 2θ=10-90°
  • Slit: DS-1°, RS-0.3 mm, SS-1°
  • Step: 0.02°

Example 1

(1) Synthesis of Positive Electrode Active Material

Dinickel trioxide (mfd. by Hayashi Pure Chemical Industries Ltd., Japan, nickel content 73.4% by weight; BET specific surface area 134 m²/g; powder X-ray diffraction measurement result is shown in FIG. 1), manganese carbonate (mfd. by Wako Pure Chemical Industries Ltd., Japan, guaranteed reagent, manganese content 46.4% by weight) and lithium hydroxide (mfd. by The Honjo Chemical Corporation, Japan) were weighed out in a molar ratio of respective elements, Li:Ni:Mn, of 1.0:0.5:0.5, and were then thoroughly mixed in a mortar. The powder mixture thus obtained was placed in a box type furnace and was fired by keeping it in the air at 1000° C. for 15 hours, to obtain a positive electrode active material E1 for non-aqueous secondary battery (a material corresponding, in the composition formula (I), to a case wherein x=0.5 and y=0, namely Li[Ni_0.5Mn_0.5]O₂). The result of powder X-ray diffraction measurement of the electrode material E1 is shown in FIG. 2. It was confirmed that the material E1 had a layered structure identical with that reported by Ohzuku et al. (Chemistry Letters, 744 (2001)).

(2) Charging-Discharging Performance Evaluation in a Case where the Active Material E1 was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the compound particles E1 obtained above, and subjected to a charging-discharging test based on constant-current constant-voltage charging and constant-current discharging under the following conditions.

Maximum charging voltage 4.3V, Charing time 8 hours, Charging current 0.5 mA/cm²,

Minimum discharging voltage 3.0V, Discharging current 0.5 mA/cm², and

Charging-discharging temperature 25° C.

The change of discharging capacity is shown in FIG. 3. The discharging capacities at the 10th cycle and the 20th cycle were respectively 123 and 117 mAh/g, which are higher than the corresponding capacities obtained by using spinel type lithium manganese oxide, thus showing good cycle characteristics.

Example 2

(1) Synthesis of Positive Electrode Active Material

The same procedures as in Example 1 were followed except for using dimanganese trioxide (mfd. by Kojundo Chemical Laboratory Co., Ltd., Japan, purity 99.9% by weight) as the manganese starting material, to obtain a positive electrode material E2 for non-aqueous secondary battery. The result of powder X-ray diffraction measurement of the material E2 is shown in FIG. 2. It was confirmed that the material E2 also had a layered structure identical with that reported by Ohzuku et al.

(2) Charging-Discharging performance Evaluation in a Case where the Active Material E2 was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the compound particles E2 obtained above, and subjected to a charging-discharging test in the same manner as in Example 1.

The change of discharging capacity is shown in FIG. 3. The discharging capacities at the 10th cycle and the 20th cycle were respectively 115 and 108 mAh/g.

Example 3

(1) Synthesis of Positive Electrode Active Material

Dinickel trioxide (mfd. by Hayashi Pure Chemical Industries Ltd., Japan, nickel content 73.4% by weight; BET specific surface area 134 m²/g; powder X-ray diffraction measurement result is shown in FIG. 1), tricobalt tetroxide (mfd. by Nihon Kagaku Sangyo Co., Ltd., Japan, Product name of PRM-73; cobalt content 72.8% by weight), manganese dioxide (mfd. by Kojundo Chemical Laboratory Co., Ltd., Japan, 2N grade reagent) and lithium hydroxide (mfd. by The Honjo Chemical Corporation, Japan) were weighed out in a molar ratio of respective elements, Li:Ni:Mn:Co, of 1.04:0.34:0.42:0.20, and were then thoroughly mixed in a mortar. The powder mixture thus obtained was placed in a box type furnace and was fired by keeping it in the air at 1000° C. for 15 hours, to obtain a positive electrode active material E3 for non-aqueous secondary battery (a material corresponding, in the composition formula (I), to a case wherein x=0.44 and y=0.10, namely Li[Ni_0.34Li_0.04Mn_0.42Co_0.20]O₂). The result of powder X-ray diffraction measurement of the electrode material E3 is shown in FIG. 4. It was confirmed that the material E3 had a layered structure identical with that reported by Ohzuku et al.

(2) Charging-Discharging Performance Evaluation in a Case where the Active Material E3 was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the compound particles E3 obtained above, and subjected to a charging-discharging test in the same manner as in Example 1.

The change of discharging capacity is shown in FIG. 5. The discharging capacities at the 10th cycle and the 20th cycle were respectively 143 and 142 mAh/g.

Example 4

(1) Synthesis of Positive Electrode Active Material

The same procedures as in Example 3 were followed except that the powder mixture was fired at 950° C., to obtain a positive electrode material E4 for non-aqueous secondary battery. The result of powder X-ray diffraction measurement of the material E4 is shown in FIG. 4. It was confirmed that the material E4 also had a layered structure identical with that reported by Ohzuku et al.

(2) Charging-Discharging Performance Evaluation in a Case where the Active Material E4 was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the compound particles E4 obtained above, and subjected to a charging-discharging test in the same manner as in Example 1.

The change of discharging capacity is shown in FIG. 5. The discharging capacities at the 10th cycle and the 20th cycle were respectively 135 and 134 mAh/g.

Comparative Example 1

(1) Synthesis of Positive Electrode Active Material

The same procedures as in Example 1 were followed except for using nickel hydroxide (mfd. by Tanaka Chemical Corporation, Japan, nickel content 61.8% by weight) as the nickel starting material, to obtain a positive electrode material Cl for non-aqueous secondary battery. The result of powder X-ray diffraction measurement of the material Cl is shown in FIG. 2. It was observed that the material Cl showed, beside the layered structure identical with that reported by Ohzuke et al., diffraction peaks due to NiO and Li₂MnO₃.

(2) Charging-Discharging Performance Evaluation in a Case where the Active Material Cl was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the compound particles Cl obtained above, and subjected to a charging-discharging test in the same manner as in Example 1.

The change of discharging capacity is shown in FIG. 3. The discharging capacities at the 10th cycle and the 20th cycle were low, respectively 84 and 83 mAh/g.

Example 5

(1) Synthesis of Positive Electrode Active Material

Lithium hydroxide (mfd. by The Honjo Chemical Corporation, Japan), nickel hydroxide (mfd. by Tanaka Chemical Corporation, Japan, nickel content 61.8% by weight), manganese carbonate (mfd. by Wako Pure Chemical Industries Ltd., Japan, guaranteed reagent, manganese content 46.4% by weight), and boric acid (H₃BO₃, mfd. by Wako Pure Chemical Industries Ltd., Japan), were weighed out in a molar ratio of respective elements, Li:Ni:Mn:B, of 1.0:0.5:0.5:0.02, and were then thoroughly mixed in a mortar. The powder mixture thus obtained was placed in a box type furnace and was fired by keeping it in the air at 1000° C. for 15 hours, to obtain a positive electrode active material E5 for non-aqueous secondary battery (a material corresponding, in the composition formula (I), to a case wherein x=0.5 and y=0, namely a compound represented by the composition formula Li[Ni_0.5Mn_0.5]O₂. The combination of the starting materials of this example corresponds to that of Comparative Example 1 except that the boric acid was combined. The result of powder X-ray diffraction measurement of the material E5 is shown in FIG. 6. It was confirmed that the material E₅ had a layered structure identical with that reported by Ohzuku et al.

(2) Charging-Discharging Performance Evaluation in a Case where the Active Material E5 was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the active material E5 and subjected to a charging-discharging test in the same manner as in Example 1.

The change of discharging capacity is shown in FIG. 7. The discharging capacities at the 10th cycle and the 20th cycle were respectively 127 and 124 mAh/g, which are higher than the corresponding capacities obtained by using spinel type lithium manganese oxide, thus showing good cycle characteristics.

Example 6

(1) Synthesis of Positive Electrode Active Material

The same procedures as in Example 3 were followed except that a molar ratio of respective elements were weighed out, Li:Ni:Mn:Co, of 1.00:0.40:0.40:0.20, and obtained a positive electrode active material E6 for non-aqueous secondary battery (a material corresponding, in the composition formula (I), to a case wherein x=0.50 and y=0.10, namely Li[Ni_0.40Mn_0.40Co_0.20]O₂.

The result of powder X-ray diffraction measurement of the material E6 is shown in FIG. 8. It was confirmed that the material E6 had a layered structure identical with that reported by Ohzuku et al.

(2) Charging-Discharging Performance Evaluation in a Case where the Active Material E6 was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the active material E6 and subjected to a charging-discharging test in the same manner as in Example 1.

The change of discharging capacity is shown in FIG. 9. The discharging capacities at the 10th cycle and the 20th cycle were respectively 147 and 144 mAh/g.

Example 7

(1) Synthesis of Positive Electrode Active Material

The same procedures as in Example 3 were followed except that a molar ratio of respective elements were weighed out, Li:Ni:Mn:Co, of 1.06:0.31:0.43:0.20, and obtained a positive electrode active material E7 for non-aqueous secondary battery (a material corresponding, in the composition formula (I), to a case wherein x=0.41 and y=0.10, namely Li[Ni_0.31Li_0.06Mn_0.43Co_0.20]O₂.

The result of powder X-ray diffraction measurement of the material E7 is shown in FIG. 8. It was confirmed that the material E7 had a layered structure identical with that reported by Ohzuku et al.

(2) Charging-Discharging Performance Evaluation in a Case where the Active Material E7 was Used as the Positive Electrode Active Material of Lithium Secondary Battery

A flat plate type battery was prepared by using the active material E7 and subjected to a charging-discharging test in the same manner as in Example 1.

The change of discharging capacity is shown in FIG. 9. The discharging capacities at the 10th cycle and the 20th cycle were respectively 137 and 137 mAh/g.

Next, charging-discharging behavior of the compound particles of E3, E6 and E7 were investigated at 60° C. A flat type battery was prepared with a mixture solution of EC and EMC (1:1 volume ratio) in which LiPF₆ was dissolved to be 1 mol/l as an electrolyte instead of LiPF₆/EC+DMC+EMC, and subjected to a charging-discharging test in a thermostat maintained at 60° C.

The change of discharging capacity is shown in FIG. 10. The discharging capacities at the 10th cycle and the 20th cycle were respectively 154 and 151 mAh/g (E3); 155 and 147 mAh/g (E6) and 148 and 145 mAh/g (E7), thus showing good cycle characteristics of high capacities. E3 and E7, in which x<0.5, i.e., Ni content is smaller than Mn content was superior in cycle characteristics compared with E6, in which x=0.5, i.e., Ni content is equal to Mn content.

According to the method of this invention, a non-aqueous secondary battery positive electrode active material comprising lithium, nickel and manganese and having a layered structure can be prepared easily and simply, and a non-aqueous secondary battery using the active material has a high capacity. Accordingly, this invention is of great industrial value.

Claims

1. A method for preparing a positive electrode active material for a non-aqueous secondary battery comprising a step of firing a mixture of (1) at least one member selected from the group consisting of dinickel trioxide and boron compounds and (2) one or more metal compounds comprising lithium, nickel and manganese as their metal elements and a step of obtaining a compound comprising lithium, nickel and manganese and having a layered structure, wherein the compound comprising lithium, nickel and manganese and having a layered structure is a compound which is identified in x-ray diffraction as a compound represented by the composition formula Li[Ni_(x-y)Li_(1/3-2x/3)Mn_(2/3-x/3-y)Co_2y]O₂, wherein 0<x≦0.5, 0≦y≦⅙ and x>y, and having a layered structure.

2. A positive electrode active material for a non-aqueous secondary battery obtained by the method of preparation according to claim 1.

3. A positive electrode active material for a non-aqueous secondary battery comprising a compound which comprises lithium, nickel, manganese and cobalt and being represented by the composition formula Li[Ni_0.34Li_0.04Mn_0.42Co_0.02]O₂ or Li[Ni_0.31Li_0.06Mn_0.43Co_0.02]O₂ and having a layered structure identified in x-ray diffraction.

4. A non-aqueous secondary battery obtained by using the positive electrode active material for a non-aqueous secondary battery according to claim 2.

5. A non-aqueous secondary battery obtained by using the positive electrode active material for a non-aqueous secondary battery according to claim 3.

6. A method of preparing a positive electrode active material for a non-aqueous secondary battery comprising a step of firing a mixture of (1) at least one member selected from the group consisting of dinickel trioxide and boron compounds and (2) one or more metal compounds selected from the group consisting of a lithium compound, a nickel compound and a manganese compound and a step of obtaining a compound comprising lithium, nickel and manganese and having a layered structure, wherein the compound comprising lithium, nickel and manganese and having a layered structure is a compound which is identified in x-ray diffraction as a compound represented by the composition formula Li[Ni_(x-y)Li_(1/3-2x/3)Mn_(2/3-x/3-y)Co_2y]O₂, wherein 0<x≦0.5, 0≦y≦⅙ and x>y, and having a layered structure.

7. A positive electrode active material for a non-aqueous secondary battery obtained by the method of preparation according to claim 6.

8. A non-aqueous secondary battery obtained by using the positive electrode active material for a non-aqueous secondary battery according to claim 7.

9. A method of preparing a positive electrode active material for a non-aqueous secondary battery comprising a step of firing a mixture of (1) at least one member selected from the group consisting of dinickel trioxide and boron compounds and (2) a lithium compound, a nickel compound, a manganese compound and optionally a cobalt compound and a step of obtaining a compound comprising lithium, nickel and manganese and having a layered structure, wherein the compound comprising lithium, nickel and manganese and having a layered structure is a compound which is identified in x-ray diffraction as a compound represented by the composition formula Li[Ni_(x-y)Li_(1/3-2x/3)Mn_(2/3-x/3-y)Co_2y]O₂, wherein 0<x≦0.5, 0≦y≦⅙ and x>y, and having a layered structure.

10. A positive electrode active material for a non-aqueous secondary battery obtained by the method of preparation according to claim 9.

11. A non-aqueous secondary battery obtained by using the positive electrode active material for a non-aqueous secondary battery according to claim 10.