Positive electrode active material, lithium ion storage device using the same, and manufacturing method thereof

Provided are a positive electrode active material (31a), a lithium ion electric storage device (30) using the same, and a manufacturing method thereof. The positive electrode active material (31a) contains (1) lithium nickel cobalt manganese oxide, and at least one type of a lithium ion acceptance capacity adjustment compound selected from (2a) lithium vanadium composite oxide, vanadium oxide, lithium vanadium phosphate, and lithium vanadium fluorophosphate and (2b) NbO, TiO, LiTiO, WO, MoO, and FeO. A lithium ion secondary battery (30) contains this positive electrode active material (31a).