NOVELTY - An electrolyte composition comprises aprotic organic solvent(s), conducting salt(s), oxirane compound (I), and optionally additive(s).    USE - Electrolyte composition is used for electrochemical cell e.g. lithium ion secondary battery (all claimed).    ADVANTAGE - The lithium ion secondary battery comprising electrolyte composition has high energy density, and durability, and low operating voltage.    DETAILED DESCRIPTION - An electrolyte composition comprises aprotic organic solvent(s), conducting salt(s), oxirane compound of formula (I), and optionally additive(s).    R1=C(O)OR7, C(O)C(O)OR7, S(O)2R7, S(O)2OR7, (CH2)sSO2(CH2)tR7, P(O)(OR7)R7, or P(O)(OR7)2;    R2=1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 3-6C (hetero)cycloalkenyl, 2-6C alkynyl, 5-7C (hetero)aryl or 6-13C aralkyl (all optionally substituted by fluoro, CN, 1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 5-7C (hetero)aryl, OR8, C(O)R8, C(O)OR8, OC(O)R8, OC(O)OR8, OC(O)C(O)OR8, S(O)20R8 or OS(O)2R8), H or fluoro;    R3,R4=1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 3-6C (hetero)cycloalkenyl, 2-6C alkynyl, 5-7C (hetero)aryl or 6-13C aralkyl (all optionally substituted by fluoro, CN or optionally fluorinated oxiranyl), H or fluoro;    R5,R6=1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 3-6C (hetero)cycloalkenyl, 2-6C alkynyl, 5-7C (hetero)aryl or 6-13C aralkyl (all optionally substituted by fluoro, CN or optionally fluorinated oxiranyl), H or fluoro;    R7=1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 3-6C (hetero)cycloalkenyl, 2-6C alkynyl, 5-7C (hetero)aryl or 6-13C aralkyl (all optionally substituted by fluoro, CN, 1-6C alkyl, 2-6C alkenyl, 5-7C (hetero)aryl, OR9, OC(O)R9, C(O)R9, C(O)OR9, OC(O)OR9, OC(O)C(O)OR9, S(O)2OR9, OS(O)2R9, or 3-6C (hetero)cycloalkyl), H or fluoro;    R8,R9=H, 1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 3-6C (hetero)cycloalkenyl, 2-6C alkynyl, 5-7C (hetero)aryl or 6-13C aralkyl (all optionally substituted by fluoro, CN or optionally fluorinated oxiranyl);and    s,t=1, 2 or 3.    INDEPENDENT CLAIMS are included for the following:    (1) use of compound (I), which involves using compound (I) as additive for electrolyte; and    (2) electrochemical cell, which comprises electrolyte composition, cathode(s) including cathode active material(s), and anode(s) including anode active material(s).