NOVELTY - A positive electrode active material consists of solid solution particles of lithium-transition metal composite oxide (320) and lithium manganese oxide (310). The solid solution particles have concentration gradient in which concentration of lithium manganese oxide is more than concentration of lithium-transition metal composite oxide such that lithium manganese oxide is close to center portion of particles, and concentration of lithium-transition metal composite oxide is more than concentration of lithium manganese oxide such that composite oxide is close to outer surface of particles.    USE - Positive electrode active material for lithium secondary battery used as power supply for vehicles (both claimed). Uses include but are not limited to electric motor mounted in motor vehicles, hybrid vehicles, electric vehicles, fuel cell electric vehicles, personal computer, etc.    ADVANTAGE - The positive electrode active material provides lithium secondary battery with excellent cycle characteristics.    DETAILED DESCRIPTION - A positive electrode active material consists of solid solution particles of lithium-transition metal composite oxide of formula: LiMO2, where M is transition metal element chosen from cobalt, nickel and manganese, and lithium manganese oxide of formula: Li2MnO3. The solid solution particles have concentration gradient in which concentration of lithium manganese oxide is more than concentration of lithium-transition metal composite oxide such that lithium manganese oxide is close to center portion of particles, and concentration of lithium-transition metal composite oxide is more than concentration of lithium manganese oxide such that composite oxide is close to outer surface of particles. INDEPENDENT CLAIMS are included for the following:    (1) manufacture of positive electrode active material; and    (2) lithium secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of concentration gradient of solid solution particles.    Lithium manganese oxide (310)    Lithium-transition metal composite oxide (320)