NOVELTY - A lithium ion secondary battery comprises positive electrode (21); negative electrode (22); and electrolytic solution. The positive electrode includes first lithium composite oxide and second lithium composite oxide as positive electrode active material. The negative electrode includes negative electrode active material having at least one of silicon and tin as a constituent element.    USE - As lithium ion secondary battery which is useful in electric tool, electric vehicle, and power storage system (claimed).    ADVANTAGE - The lithium ion secondary battery has excellent battery capacity characteristic and gas emission characteristic.    DETAILED DESCRIPTION - A lithium ion secondary battery comprises positive electrode (21); negative electrode (22); and electrolytic solution. The positive electrode includes first lithium composite oxide and second lithium composite oxide as positive electrode active material. The second lithium composite oxide is expressed by Li1+a(Nib(M1)c(M2)1-b-c)1.5-0.5aO2, where the M1 is at least one selected from group 13 to 15 in an extended periodic table of elements excluding boron, carbon or nitrogen N (preferably aluminum, silicon, indium, and tin), M2 is at least one selected from group 3 to 12 (preferably copper, cobalt, manganese, iron, zinc, yttrium, titanium and molybdenum), and a, b, and c satisfy relationships of 0.95-1.05, 0-0.99, and 0-0.15, and where the second lithium oxide has change capacity greater than the first lithium capacity oxide. The negative electrode includes negative electrode active material at least one of silicon and tin as a constituent element. INDEPENDENT CLAIMS are included for the following:    (1) positive electrode active material comprising first lithium composite oxide and the second lithium composite oxide; and    (2) positive electrode comprising the positive electrode active material.    DESCRIPTION OF DRAWING(S) - The figure shows cross-sectional view of lithium ion secondary battery.    Hollow columnar battery (11)    Gasket (17)    Positive electrode (21)    Negative electrode (22)    Separator (23)    Center pin (24)