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), lithium fluorophosphate compound (VI), lithium fluoroborate compound (VII) and/or lithium bis(fluorosulfonyl)imide (VIII).    USE - An electrolyte solution for a lithium secondary battery of an electric power storage system in an electric tool and an electric vehicle (all claimed).    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), lithium fluorophosphate compound of formula (Li(PFmR106-m)) (VI), lithium fluoroborate compound of formula (Li(BFnR114-n)) (VII) and/or lithium bis(fluorosulfonyl)imide (Li(N(SO2F)2)) (VIII).    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;    R10,R11=perfluoroalkyl or perfluoroaryl;    m=0-5;and    n=0-3.    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)