NOVELTY - Solid-solution lithium comprises complex oxide of lithium, nickel, cobalt and manganese, and has a layered structure portion and a portion which changes to spinel structure due to charge/discharge performed in predetermined potential range. The solid-solution lithium satisfies preset relation with respect to k", which is spinel structure change ratio of the solid-solution lithium when the layered structure of dilithium dioxido(oxo)manganese in the solid-solution lithium completely changes to the spinel structure lithium manganese oxide in the process.    USE - Solid-solution lithium containing transition metal oxide is used as positive electrode active material in nonaqueous electrolyte secondary batteries (claimed). Uses include but are not limited to motor vehicles, electric vehicles, fuel cell electric vehicles, and hybrid electric vehicles.    ADVANTAGE - The transition metal oxide-containing solid-solution lithium provides secondary batteries with high initial discharge capacity, capacity retention rate, and average voltage.    DETAILED DESCRIPTION - Solid-solution lithium containing transition metal oxide is of formula: Li1.5(NiaCobMnc(Li)d)O3, and has a layered structure portion and a portion which changes to spinel structure due to charge or charge/discharge performed in predetermined potential range, where d is 0.1-0.4, a+b+c+d is 1.5, a+b+c is 1.1-1.4 and a is (-2/3x d+0.4) to (-2.5x d+1.15), (excluding (-2/3x d+0.4) and (-2.5x d+1.15)). The solid-solution lithium satisfies the relation: (4/3x d'+1/15) less than or equal to k' less than or equal to 1, where d' is 0.1-0.25 (excluding 0.1 and 0.25), and k' is 0.4-1, which is an average value of k" calculated as an average value at the time of charging/discharging up to each of maximum voltages of 4.5V, 4.6V, 4.7V, and 4.8V (relative to lithium metal), k" is spinel structure change ratio of the transition metal oxide-containing solid-solution lithium relative to the spinel structure change ratio when the layered structure of dilithium dioxido(oxo)manganese in the solid-solution lithium completely changes to the spinel structure lithium manganese oxide in the process.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the outline of lithium ion secondary battery.    Lithium ion secondary battery (1)    Battery element (10)    Positive electrode (11)    Positive electrode electrical power collector (11A)    Positive electrode active material layer (11B)