Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery including the same

1. A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium-containing transition metal oxide having a closest-packed cubic structure of oxygen, wherein:

said lithium-containing transition metal oxide has a composition represented by the formula (1):

Li[Li p (Ni x Mn y Co z ) 1-p ]O 2 ,

where x, y, and z represent element contents of nickel, manganese, and cobalt, respectively, and satisfy

0.2 +y≦x≦ 0.7,

0.25 ≦y,

0.05 <z,

x+y+z= 1, and

0 ≦p≦ 0.1,

Ni, Mn and Co are evenly dispersed at atomic level in said lithium-containing transition metal oxide, and

in a triangular phase diagram showing the element contents of nickel, manganese, and cobalt in terms of molar ratio, the composition represented by said formula (1) falls within a pentagonal region defined by five vertices satisfying (x:y:z)=(6:2:2), (5:2.5:2.5), (5:3.3:1.7), (6.36:2.73:0.091), and (6.25:2.5:1.25).

2. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein said lithium-containing transition metal oxide includes a domain where nickel and manganese are contained in a molar ratio of 1:1 or 2:1 and a domain where nickel and cobalt are contained in a molar ratio of 1:1.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein in performing charge and discharge using said positive electrode active material formed into a sheet, at a current density of 0.33 mA/cm 2 or less per unit area of said sheet and 8 mA/g or less per unit weight of said positive electrode active material and at a potential ranging from 4.3 V to 3.0 V versus lithium metal potential, the difference between a charge capacity at the first cycle and a discharge capacity at the third cycle (irreversible capacity) is 20 mAh/g or less, and the reversible capacity at the third cycle is 165 mAh/g or more.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein said lithium-containing transition metal oxide includes a secondary particle having an average particle size (D50) of 11 μm or less, and said secondary particle is a sintered body of primary particles having an average particle size (D50) of 2 μm or less.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein the tap density of said lithium-containing transition metal oxide is 2.0 g/cm 3 or more.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein the tap density of said lithium-containing transition metal oxide is 2.2 g/cm 3 or more.

7. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein the potential at which potential V sharply drops in a final stage of discharge (the potential that gives a maximum d 3 Q/dV 3 by triple differentiation when capacity is denoted by Q) is higher than 3.45 V versus lithium metal.

8. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein in performing a differential scanning calorimetry (DSC) at a temperature elevation rate of 10° C./min after charging up to 4.4 V as a potential versus lithium metal, no substantial exothermic peak appears at temperatures of 200° C. or lower.

9. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 4 , wherein the interior or the surface of said primary particles is doped with at least one different element selected from the group consisting of aluminum, magnesium, zirconium, manganese, and tin.

10. A non-aqueous electrolyte secondary battery comprising a positive electrode including the positive electrode active material in accordance with claim 1 , a negative electrode, and a non-aqueous electrolyte.

11. The non-aqueous electrolyte secondary battery in accordance with claim 10 , wherein said negative electrode includes a carbonaceous material capable of absorbing and desorbing.

12. A method for producing the positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 comprising the steps of:

preparing a precursor by a coprecipitation method performed by adding dropwise an aqueous alkaline solution into an aqueous acidic solution containing nickel ion, manganese ion, and cobalt ion; and

mixing said precursor and lithium carbonate and baking the resultant mixture, thereby to prepare a particle of lithium-containing transition metal oxide with nickel, manganese, and cobalt evenly dispersed at atomic level therein.

13. The method for producing the positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 12 , wherein

the temperature in said baking is higher than 800° C.

14. The method for producing the positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 12 , wherein

the temperature in said baking is 900° C. or higher.

15. The method for producing the positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 12 , wherein, in the step of preparing a precursor by a coprecipitation method, manganese is prevented from being oxidized from bivalent to trivalent.

16. The method for producing the positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 15 , wherein manganese is prevented from being oxidized from bivalent to trivalent by purging dissolved oxygen by bubbling an inert gas into the aqueous solution or adding a reducing agent into the aqueous acidic solution.

17. The positive electrode active material for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein x satisfies 0.55≦x≦0.65.