NOVELTY - A secondary battery consists of an electrolyte solution, a positive electrode (21), and a negative electrode (22). The positive electrode contains a lithium-containing compound in which magnesium, calcium, titanium, zirconium, sulfur, fluorine, iron, copper, boron, aluminum, phosphorus, carbon, manganese, nickel and/or cobalt, different from element (M1) is incorporated in crystal structure of surface layer region of a complex oxide of lithium, manganese, cobalt, nickel, and aluminum, magnesium, zirconium, titanium, barium, boron, silicon and/or iron.    USE - Secondary battery e.g. lithium secondary battery, is used in battery packs, electric vehicles, electric power storage systems, electric tools, and electronic devices (all claimed) e.g. mobile telephones and portable information terminals, such as personal digital assistants.    ADVANTAGE - The secondary battery has excellent cell properties, such as energy density, charging/discharging characteristics, capacity, storage property, and swelling resistance.    DETAILED DESCRIPTION - A secondary battery consists of an electrolyte solution, a positive electrode, and a negative electrode. The positive electrode contains a lithium-containing compound in which element (M2) different from element (M1) is incorporated in the crystal structure of the surface layer region of a complex oxide of formula: Li1+a(MnbCocNi1-b-c)1-aM1dO2-e, where M2 is magnesium, calcium, titanium, zirconium, sulfur, fluorine, iron, copper, boron, aluminum, phosphorus, carbon, manganese, nickel and/or cobalt, M1 is aluminum, magnesium, zirconium, titanium, barium, boron, silicon and/or iron, a is greater than 0 and less than 0.25, b is 0.5 or more and less than 0.7, c is 0 or more and less than 1-b, and d and e are 0-1. The molar fraction given by, ((total content of element (M2))/(total content of cobalt, nickel and manganese of element (M2)))x 100 is smaller in the center than in the surface layer side of lithium-containing compound. INDEPENDENT CLAIMS are included for the following:    (1) positive electrode active material;    (2) positive electrode;    (3) battery pack;    (4) electric vehicle;    (5) electric power storage system;    (6) electric tool; and    (7) electronic device.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of a cylindrical secondary battery.    Battery cover (14)    Positive electrode (21)    Negative electrode (22)    Positive electrode lead (25)    Negative electrode lead (26)