NOVELTY - Secondary battery comprises: a cathode (21); an anode (22); and an electrolytic solution including unsaturated six-membered ring ester carbonate compounds (Q).    USE - The secondary battery, which is a lithium ion secondary battery, is useful as an electric power supply source in an electronic device, a battery pack, an electric vehicle, an electric power storage system and an electric power tool (all claimed), where the electronic device includes mobile phone or a personal digital assistant, the electric vehicle includes an electric automobile, the electric power storage system includes a home electric power server, and an electric power tool includes an electric drill.    ADVANTAGE - The electrolytic solution has superior charge and discharge characteristics and stored in a severe temperature environment such as a high-temperature environment and a low temperature environment. The secondary battery reduces the size and the weight of the electronic devices and has long life and high energy density.    DETAILED DESCRIPTION - Secondary battery comprises: a cathode (21); an anode (22); and an electrolytic solution including unsaturated six-membered ring ester carbonate compounds (Q) of formula (I) and/or (II).    W1, X, Y1 = C-C(R1)(R2) or C(R3)(R4), where one of W1, X and Y1 are C-C(R1)(R2);    Z = C-C(R7)(R8) or C(R9)(R10); and    R1-R10 = H, halo, a monovalent hydrocarbyl (optionally halogenated) or a monovalent silicon-containing hydrocarbyl, where arbitrary two or more of R1-R10 are allowed to be bonded with each other.    INDEPENDENT CLAIMS are included for:    (1) an electrolytic solution comprising (Q);    (2) a battery pack comprising the secondary battery, a control section controlling a usage state of the secondary battery, and a switch section switching the usage state of the secondary battery according to an instruction of the control section;    (3) an electric vehicle comprising the secondary battery, a conversion section converting electric power supplied from the secondary battery to drive power, a drive section operating according to the drive power, and a control section controlling a usage state of the secondary battery;    (4) an electric power storage system comprising the secondary battery, one, or two or more electric devices supplied with electric power from the secondary battery, and a control section controlling the supplying of the electric power from the secondary battery to the electric device; and    (5) an electric power tool comprising the secondary battery, and a movable section that is supplied with electric power from the secondary battery.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional configuration of a secondary battery.    Battery can (11)    Insulating plates (12, 13)    Spirally wound electrode body (20)    Cathode (21)    Anode (22)    Separator. (23)