NOVELTY - A positive electrode (1) material comprises a positive electrode active material which occludes lithium ion and discharge. The positive electrode active material includes a primary positive electrode active material containing lithium-nickel-manganese-based composite oxide and a secondary positive electrode active material containing lithium-iron-phosphorus-based oxide. The content of secondary positive electrode active material is 5-20 %mass with respect to the positive electrode material.    USE - Material is used for positive electrode of lithium ion secondary battery for lithium ion battery system (all claimed) for electric vehicle and electric power storage system.    ADVANTAGE - The positive electrode material provides lithium ion secondary battery having high energy density, and improved state of charge (SOC) detection precision from a voltage.    DETAILED DESCRIPTION - A positive electrode material comprises a positive electrode active material which occludes lithium ion and discharge. The positive electrode active material includes a primary positive electrode active material containing lithium-nickel-manganese-based composite oxide of formula: LixNiaMnbMcO2 (where x is more than 1 and 1.2 or less, a is more than 0.2 and 0.4 or less, b is 0.4 or more and less than 0.6, c is 0.02 or more, a+b+c is 0.8 and M is element chosen from vanadium, molybdenum, tungsten, zirconium, niobium, titanium, aluminum, iron, magnesium and copper), and a secondary positive electrode active material containing lithium-iron-phosphorus-based oxide of formula: LiFe1-yM'yPO4 (where y is 0-0.2 and M' is element chosen from manganese, cobalt, nickel, vanadium, magnesium, molybdenum, tungsten, aluminum, niobium, titanium and copper). The content of secondary positive electrode active material is 5-20 %mass with respect to the positive electrode material. INDEPENDENT CLAIMS are included for the following:    (1) lithium ion secondary battery (10); and    (2) lithium ion battery system.    DESCRIPTION OF DRAWING(S) - The drawing shows the (Drawing includes non-English language text)    Positive electrode (1)    Negative electrode (2)    Separator (3)    Battery can (4)    Lithium ion secondary battery (10)