Cathode active material made of cobalt-oxide particles for non-aqueous electrolyte secondary cell and process for producing the same, and non-aqueous electrolyte secondary cell

Cobalt oxide particles, useful as a precursor of a cathode active material for a non-aqueous electrolyte secondary cell, have a composition represented by the formula: (Co(1-x)Mgx)3O4 wherein x is 0.001 to 0.15, a BET specific surface area value of 0.5 to 50 m/g, and an average particle diameter of not more than 0.2 mu m.