NOVELTY - A lithium ion secondary battery (10) has non-aqueous ionic-conductive medium (20) which conducts lithium ion provided between positive electrode and negative electrode. The positive electrode contains positive electrode active material (12) containing lithium and transition metal. The negative electrode contains negative electrode active material (17) containing oxide of element of group 6. The non-aqueous ionic-conductive medium includes electrolyte compound including cation and anion compound (I), and supporting electrolyte.    USE - Lithium ion secondary battery is used for electric vehicle.    ADVANTAGE - The lithium ion secondary battery has excellent cycle characteristics and internal resistance.    DETAILED DESCRIPTION - A lithium ion secondary battery has non-aqueous ionic-conductive medium which conducts lithium ion provided between positive electrode and negative electrode. The positive electrode contains positive electrode active material containing lithium and transition metal. The negative electrode contains negative electrode active material containing oxide of element of group 6. The non-aqueous ionic-conductive medium includes electrolyte compound including cation and anion compound of formula (I), and supporting electrolyte. The molar ratio of electrolyte compound with respect to the total content of electrolyte compound and supporting electrolyte is 2-80.    M=transition element of groups 13-15;    b=1-3;    m=1-4;    n=0-8;    q=0 or 1;    R1=1-10C alkylene, 1-10C haloalkylene, 6-20C arylene, or 6-20C haloarylene (all optionally substituted by substituent and heteroatom);    R2=1-10C alkyl, 1-10C haloalkyl, 6-20C aryl, or 6-20C haloaryl (all optionally substituted by substituent and heteroatom), halo or -X3R3;    X1-X3=O, S, or NR4;    R3,R4=1-10C alkyl, 1-10C haloalkyl, 6-20C aryl or 6-20C haloaryl (optionally substituted by substituent and heteroatom), or H;or    two or more R3 or R4=ring.    (i) When q is 1 and m is 2-4, mR1 are coupled together; and (ii) When n is 2-8, each nR2 form a ring.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of lithium ion secondary battery.    Lithium ion secondary battery (10)    Collector (11)    Positive electrode active material (12)    Negative electrode active material (17)    Non-aqueous ionic-conductive medium (20)