NOVELTY - A lithium secondary battery comprises a positive electrode (21); a negative electrode (22); and an electrolyte solution, where the positive electrode contains a positive electrode active material.    USE - As a lithium secondary battery used in an electric tool, an electric vehicle, a power storage system (claimed), mobile electronic apparatus such as video camera, digital still camera, mobile phone, laptop PC, wireless phone, headphone stereo, mobile radio, mobile TV, or mobile information terminal (PDA: personal digital assistant), home appliance such as an electric shaver, storage device such as a backup power supply and a memory card, an electric drill and an electric cutter, medical electronic apparatus such as a pacemaker and a hearing aid, hybrid vehicle, and in home battery system storing power for an emergency.    ADVANTAGE - The lithium secondary battery has high durability, improved structural stability and thermal stability; and exhibits excellent battery capacity characteristics, life characteristics, cycle characteristics, preservation characteristics, and swell characteristics.    DETAILED DESCRIPTION - A lithium secondary battery comprises a positive electrode; a negative electrode; and an electrolyte solution, where the positive electrode contains a positive electrode active material including element M2 incorporated in a crystal structure in a surface layer area of a complex oxide, where the complex oxide includes element M1 and is represented by formula: Li1+a(MnbCocNi1-b-c)1-a(M1)dO2-e (1) (where M1 is at least one of aluminum, magnesium, zirconium, titanium, barium, boron, silicon, and iron, a is 0-0.25, b is 0.5-0.7, c is 0-(1-b), d is 0.01-0.2, and e is 0-1), and M2 is different from M1, and where the element M2 is at least one selected from Mg, Ca, Ti, Zr, S, F, Fe, Cu, B, Al, P, C, Mn, Ni, and Co. INDEPENDENT CLAIMS are included for the following:    (1) a positive electrode active material including element M2 incorporated in a crystal structure in a surface layer area of the complex oxide; and    (2) a positive electrode comprising the positive electrode active material.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view illustrating a configuration of a lithium ion secondary battery (cylindrical type) using a positive electrode active material.    Thermal resistance element (16)    Electrode body (20)    Positive electrode (21)    Negative electrode (22)    Separator (23)