Patent ID: 8808920

Claim:
A positive electrode active material prepared by: mixing a lithium-containing compound, a compound containing a transition metal M1 to be involved in a solid solution, and a compound containing a metallic element M2 different from the transition metal M1, and firing the mixture to form composite oxide particles having a laminar rock salt structure and an average composition represented by Chemical formula 1: Li a A b M 1-b O c Chemical formula 1 where A represents at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn) and iron (Fe); M represents at least one element selected from the group consisting of manganese (Mn), magnesium (Mg), aluminum (Al), nickel (Ni), boron (B), titanium (Ti), cobalt (Co) and iron (Fe); and a, b and c are numbers in the ranges of 0.2≦a≦1.4, 0≦b≦1.0, and 1.8≦c≦2.2; depositing a compound containing at least one element selected from among sulfur (S), phosphorus (P) and fluorine (F) on surfaces of the composite oxide particles; and firing the composite oxide particles with the compound containing at least one element selected from among sulfur (S), phosphorus (P) and fluorine (F) deposited thereon; whereby each of the composite oxide particles is provided with a concentration gradient such that the concentration of the metallic element M2 increases from the center toward the surface of the composite oxide particle, and at least one element selected from among sulfur (S), phosphorus (P) and fluorine (F) is made present in the form of being aggregated at the surfaces of the composite oxide particles.