NOVELTY - A secondary battery comprises electrolyte solution, positive electrode (21), and negative electrode (22). Positive electrode contains 1st lithium-containing compound and 2nd lithium-containing compound. Crystalline diameter D1 (nm) of primary particle of 1st lithium-containing compound is 1-1900nm at D of 1200 nm. Crystalline diameter D2 (nm) of primary particle of 2nd lithium-containing compound is 2-900 nm at D of 350 nm. Weight ratio 1st lithium-containing compound:2nd lithium-containing compound is 80:20-70:30.    USE - A secondary battery for battery pack used for electric vehicle, electric power storage system, electric tool, and electronic device (all claimed).    ADVANTAGE - Outstanding battery characteristics can be acquired.    DETAILED DESCRIPTION - A secondary battery comprises electrolyte solution, positive electrode (21), and negative electrode (22). Positive electrode contains 1st lithium-containing compound and 2nd lithium-containing compound. Lithium-containing compounds are represented by formula Li1+a(MnbCocNi1-b-c)1-aMdO2-e (1). Crystalline diameter D1 (nm) of primary particle of 1st lithium-containing compound is 1-1900nm at D of 1200 nm. Crystalline diameter D2 (nm) of primary particle of 2nd lithium-containing compound is 2-900 nm at D of 350 nm. Weight ratio 1st lithium-containing compound:2nd lithium-containing compound is 80:20-70:30.    M=Al, Mg, Zr, Ti, Ba, B, Si, and/or Fe;    a=0.1-0.25;    b=0.5-0.7;    c=0-1-b; and    d, e=0-1.    INDEPENDENT CLAIMS are included for:    (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 diagram of the secondary battery. (Drawing includes non-English language text).    Battery can (11)    Winding electrode body (20)    Positive electrode (21)    Negative electrode (22)    Separator (23)