NOVELTY - An electrolyte solution contains a positive electrode (21), a negative electrode (22), a non-aqueous solvent and electrolytic salt. The electrolytic solution comprises ester compounds (I and II), sulfonyl compound (III), lithium monofluorophosphate (IV), lithium difluorophosphate (V) and/or cyclic carbonate compound (VI).    USE - Electrolyte solution for a lithium secondary battery of an electric power storage system in an electric tool e.g. electrically-driven drill and electric saw, and in an electric vehicle (all claimed) i.e. hybrid motor vehicle. Can also be used for portable electronic devices such as video camera, digital still camera, mobile telephone, notebook personal computer, cordless telephone apparatus, headphone stereo cassette tape recorder, portable radio, portable television, or portable information terminal i.e. personal digital assistant.    ADVANTAGE - The electrolyte solution has improved chemical stability, solubility and compatibility. The electrolyte solution provides high charging and discharging capacity and energy density, and improved cycling characteristics, storing characteristics and load characteristics, to the battery.    DETAILED DESCRIPTION - An electrolyte solution contains a positive electrode, a negative electrode, a non-aqueous solvent and electrolytic salt. The electrolytic solution comprises ester compounds of formulae (R1-O-C(=O)-O-R2-O-C(=O)-O-R3) (I) and (R4C(=O)-O-R5-O-C(=O)-R6) (II), sulfonyl compound of formula (R7-S(=O)2-O-R8-O-S(=O)2-R9) (III), lithium monofluorophosphate (Li2PFO3) (IV), lithium difluorophosphate (LiPF2O2) (V) and/or cyclic carbonate compound of formula (VI).    R1,R3,R4,R6,R7,R9=alkyl, alkenyl, alkynyl (all substituted by aromatic hydrocarbon or alicyclic hydrocarbon), aryl or heterocyclic group;    R2,R5,R8=bivalent group containing linear or branched alkylene, arylene, 2C bivalent group containing an ether bond (-O-) or 12C alkylene;    R11-R20=H, halo, 1-12C alkyl, 2-12C alkenyl, 2-12C alkynyl, 6-18C aryl or 1-12C alkoxy;and    R11+R12+R13+R14+R15+R16+R17+R18+R19+R20=ring.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of a cylindrical shaped lithium secondary battery. '(Drawing includes non-English language text)'    Battery can (11)    Insulation boards (12, 13)    Battery cover (14)    Safety valve mechanism (15)    Thermal-sensitive resistive element (16)    Gasket (17)    Positive electrode (21)    Negative electrode (22)