NOVELTY - A positive-electrode active material comprises a solid solution lithium-containing transition-metal oxide (A) and solid solution lithium-containing transition metal oxide (B). The oxides have a layer structure which changes to spinel structure and a layer structure which does not change, during charging/discharging at an electric potential of 4.3-4.8 V. The oxides have a spinel structural change ratio of 0.25-1.    USE - Active material is used for positive electrodes of electrical devices e.g. lithium ion secondary batteries (all claimed) used in electric vehicles, fuel-cell electric vehicles, hybrid electric vehicles. Can also be used for lithium ion capacitors.    ADVANTAGE - The active material has excellent discharge operating voltage and initial-stage rate characteristics, and maintains high discharge capacity.    DETAILED DESCRIPTION - A positive-electrode active material comprises a solid solution lithium-containing transition-metal oxide (A) and solid solution lithium-containing transition metal oxide (B). The oxides have a layer structure which changes to spinel structure and a layer structure which does not change, during charging/discharging at an electric potential of 4.3-4.8 V. Oxide (A) is of formula: Li1.5(NiaCobMnc(Li)d)O3 and oxide (B) is of formula: Li1.5(NieCofMng(Li)h)O3, where a is 0.1 or more and less than 0.5, b is 0 or more and less than 1.15, c is greater than 0 and less than 1.15, d is greater than 0.25 and 0.4 or less, a+b+c+d is 1.5, a+b+c is 1.1 or more and less than 1.25, e is 0.25-0.7, f and g are greater than 0 and less than 1.1, h is 0.15-0.25, e+f+g+h is 1.5, and e+f+g is 1.25-1.35. When the spinel structural change ratio of Li2MnO3 to LiMn2O4 of spinel structure is set to 1, the spinel structural change ratio is 0.25-1.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of a lithium ion secondary battery.    Lithium ion secondary battery (1)    Battery element (10)    Positive electrode (11)    Negative electrode (12)    Electrolyte layer (13)