NOVELTY - A secondary battery consists of electrolyte solution, and an electrode element consisting of a positive electrode (c) and a negative electrode (a) which are arranged facing each other. The negative electrode contains negative electrode active material containing a metal which is alloyed with lithium and/or metal oxide which can occlude and discharge lithium ions. The electrolyte solution is a nonaqueous electrolyte formed by dissolving electrolytic salt in nonaqueous solvent. The electrolyte solution contains a conjugated carbonyl compound (I).    USE - Secondary battery e.g. lithium-ion secondary battery is used for assembled battery used as motor driving power supply for vehicle (all claimed). Uses include but are not limited to mobile telephone, notebook personal computer, electric train, satellite, submarine, electric vehicles, hybrid car, electric motorcycle and electric-power-assisted bicycle.    ADVANTAGE - The secondary battery has excellent cycle characteristics in high-temperature environment.    DETAILED DESCRIPTION - A secondary battery consists of electrolyte solution, and an electrode element consisting of a positive electrode and a negative electrode which are arranged facing each other. The negative electrode contains negative electrode active material containing a metal which is alloyed with lithium and/or metal oxide which can occlude and discharge lithium ions. The electrolyte solution is a nonaqueous electrolyte formed by dissolving electrolytic salt in nonaqueous solvent. The electrolyte solution contains a conjugated carbonyl compound of formula: R1-CH=CH-CH=CH-C(=O)-R2 (I).    R1=R2a or -CO-R2a;    R2=R2a;    R2a=H, (un)substituted alkyl, cycloalkyl, alkenyl, alkynyl, (un)substituted aryl, oxyalkylene of formula: -(R21O)n-R22 (i), (un)substituted alkoxy, cycloalkyloxy, alkenyloxy, alkynyloxy, (un)substituted aromatic oxy, or oxyalkylene oxy of formula: -O-(R21O)n-R22 (ii);    R21=1-6C alkylene;    R22=1-12C hydrocarbyl;and    n=1-10.    INDEPENDENT CLAIMS are included for the following:    (1) assembled battery, which has two or more secondary batteries; and    (2) vehicle, which is equipped with the secondary battery or the assembled battery as motor driving power supply.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the electrode element of secondary battery.    Negative electrode (a)    Separator (b)    Positive electrode (c)    Negative electrode electrical power collector (d)    Positive electrode electrical power collector (e)