NOVELTY - A solid solution comprises lithium-containing transition metal oxide (I), and structure having layer structure portion and portion (lithium-manganese oxide) which changes to spinel structure by performing charge or charging/discharging in a predetermined electric potential range. The spinel structural change ratio (k) is 0.25 to less than 1, with spinel structural change ratio (k) is 1 being the fraction when the portion changing into spinel structure having entirely changed into spinel structure of lithium-manganese oxide.    USE - Solid solution is used for positive electrode of nonaqueous electrolyte secondary battery (claimed) e.g. lithium-ion secondary battery for electric vehicle, fuel cell electric vehicle and hybrid electric vehicle.    ADVANTAGE - The nonaqueous electrolyte secondary battery containing the solid solution has high initial-stage discharge capacity, capacity retention ratio, and charging and discharging efficiency.    DETAILED DESCRIPTION - A solid solution comprises lithium-containing transition metal oxide of formula: Li1.5(NiaMbMnc(Li)d)O3 (I), and structure having layer structure portion and portion (lithium-manganese oxide (Li2MnO3)) which changes to spinel structure by performing charge or charging/discharging in a predetermined electric potential range. The spinel structural change ratio (k) is 0.25 to less than 1, with spinel structural change ratio (k) is 1 being the fraction when the portion changing into spinel structure having entirely changed into spinel structure of lithium-manganese oxide (Li2MnO3).    M=silicon, phosphorus, or metallic element (excluding nickel, manganese and lithium);    a,c=more than 0 and less than 1.4;    b=more than 0 to 0.2;    d=more than 0.1 to 0.4;    a+b+c+d=1.5;    a+b+c=1.1 to less than 1.4;and    n=average valence of each element of M (when M consists of two or more elements), preferably 3-6.    An INDEPENDENT CLAIM is included for nonaqueous electrolyte secondary battery (1), which has positive electrode (11) containing solid solution comprising lithium-containing transition metal oxide (I). The highest electric potential of battery in predetermined electric potential range converts into lithium metal counter electrode, and charging/discharging process of 4.8 V is carried out to obtain predetermined spinel structural change ratio (k) of solid solution comprising lithium-containing transition metal oxide.    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of the lithium-ion secondary battery.    Lithium-ion secondary battery as nonaqueous electrolyte secondary battery (1)    Positive electrode (11)    Positive-electrode active material layer (11B)    Negative electrode (12)    Electrolyte layer (13)