Lithium ion secondary battery with improved electrode stability and safety

1. A lithium ion secondary battery comprising: a positive electrode capable of charging and discharging; a negative electrode capable of charging and discharging; and a non-aqueous electrolyte,

wherein said positive electrode includes active material particles, said active material particles include secondary particles of a lithium composite oxide, having a surface layer portion, said lithium composite oxide is represented by Li x M 1-y L y O 2 , where 0.85≦x≦1.25; 0.005≦y≦0.35; element M includes Ni and Co; element L is at least one selected from the group consisting of an alkaline earth element, a transition metal element other than Ni and Co, a rare-earth element, a IIIb group element, and a IVb group element,

wherein, an atomic ratio a of Co relative to the total of said element M and said element L is 0.05≦a≦0.25,

wherein, said secondary particles include a secondary particle with a crack which reaches an inner portion of the secondary particle from the surface layer portion and a secondary particle without a crack,

wherein, a ratio R of the number of said secondary particles with a crack to the number of said secondary particles without a crack is 2% or more and 5% or less,

wherein, said secondary particles being an aggregated plurality of primary particles, the average particle size of said primary particles is 0.1 to 3.0 μm, the average particle size of said active material particles is 10 to 30 μm, at least said surface layer portion of said active material particles includes element Me of at least one selected from the group consisting of Mn, Al, Mg, Ca, Zr, B, W, Nb, Ta, In, Mo, and Sn, said element Me is distributed more in said surface layer portion of said active material particles compared with an inner portion of said secondary particle, and said element me is distributed more in said surface layer portion compared with a second formed by said crack.

2. The lithium ion secondary battery in accordance with claim 1 , wherein element L includes at least one selected from the group consisting of Al, Mn, Ti, Mg, Zr, Nb, Mo, W, Y, Ca, B, Ta, In, and Sn.

3. The lithium ion secondary battery in accordance with claim 1 , wherein said element Me and said element L form crystal structures different from each other.

4. The lithium ion secondary battery in accordance with claim 1 , wherein in said surface layer portion, said element Me forms an oxide having a crystal structure different from said lithium composite oxide.

5. The lithium ion secondary battery in accordance with claim 1 , wherein the amount of said element Me is 2 mol % or less relative to the lithium composite oxide.

6. The lithium ion secondary battery in accordance with claim 1 , wherein said ratio R of the number of said secondary particles with a crack is 3.5% or more and 5% or less.

7. The lithium ion secondary battery in accordance with claim 1 , wherein 0.01≦y≦0.1.

8. The lithium ion secondary battery in accordance with claim 1 , wherein when said element L includes Al, atomic ratio b of Al relative to the total of said element M and said element L is 0.01≦b≦0.08.

9. The lithium ion secondary battery in accordance with claim 1 , wherein when said element L includes Mn, atomic ratio c of Mn relative to the total of said element M and said element L is 0.01≦c≦0.35.

10. The lithium ion secondary battery in accordance with claim 1 , wherein when said element L includes Ti, atomic ratio d of Ti relative to the total of said element M and said element L is 0.01≦d≦0.1.

11. The lithium ion secondary battery in accordance with claim 1 , wherein element Me is at least one selected from the group consisting of Mn, Al, Mg, Ca, B, W, Nb, Ta, In, Mo, and Sn.

12. A method for producing a lithium ion secondary battery comprising the steps of:

(i) applying a positive electrode material mixture including active material particles including secondary particles of a lithium composite oxide on a positive electrode core material, at least a surface layer portion of said active material particles including element Me of at least one selected from the group consisting of Mn, Al, Mg, Ca, Zr, B, W, Nb, Ta, In, Mo, and Sn, said element Me being distributed more in said surface layer portion of said active material particles compared with the inner portion,

(ii) rolling said positive electrode material mixture with said positive electrode core material to obtain a positive electrode where said secondary particles include a secondary particle with a crack which reaches an inner portion of the secondary particle from the surface layer portion and a secondary particle without a crack, and ratio R of the number of said secondary particles with a crack to the number of said secondary particles without a crack is 2% or more and 5% or less,

(iii) obtaining a negative electrode, and

(iv) interposing a separator between said positive electrode and said negative electrode

wherein: said lithium composite oxide is represented by Li x M 1-y L y O 2 , where 0.85≦x≦1.25; 0.005≦y≦0.35; element M includes Ni and Co; element L is at least one selected from the group consisting of an alkaline earth element, a transition metal element other than Ni and Co, a rare-earth element, a IIIb group element, and a IVb group element, an atomic ratio a of Co relative to the total of said element M and said element L is 0.05≦a≦0.25, said secondary particles being an aggregated plurality of primary particles, the average particle size of said primary particles is 0.1 to 3.0 μm, the average particle size of said active material particles is 10 to 30 μm, and said element me is distributed more in said surface layer portion compared with a second formed by said crack.