Positive electrode active material, positive electrode active material composition and lithium ion secondary battery

A lithium ion secondary battery comprising

• (i) a positive electrode active material comprising a particulate Li-transition metal composite oxide having an average particle size of not less than 10 μm, wherein [20/(average particle size×specific surface area)]=7-9;
• (ii) a conductive material which is a mixture of a particulate conductive material having a particle size of not less than 3 μm and a particulate conductive material having a particle size of not more than 2 μm, or a mixture of a particulate conductive material having a particle size of not less than 3 μm and a fibrous conductive material having an aspect ratio of 3 or above and a fiber diameter of not more than 2 μm;
• (iii) a negative electrode active material comprising graphitized carbon having a specific surface area of not more than 2.0 m2/g, a spacing of lattice planes of not more than 0.3380 nm and a crystallite size in the c-axis direction of not less than 30 nm;
• and
• (iv) a mixed solvent for an electrolyte, which comprises ethylene carbonate, propylene carbonate, dimethyl carbonate and at least one member selected from the group consisting of diethyl carbonate and ethylmethyl carbonate.