NOVELTY - An electrolyte composition comprises greater than or equal to 1 aprotic organic solvent, greater than or equal to 1 conducting salt, greater than or equal to 1 oxiranyl derivative, and optionally greater than or equal to 1 additive.    USE - An electrolyte composition for electrochemical cell which is a secondary lithium ion battery (claimed) used for mobile devices, such as vehicles e.g. automobiles, bicycles, aircraft, or boats or ships, portable devices e.g. laptops, telephones, or electrical power tools, and stationary energy storage.    ADVANTAGE - The electrolyte additive prolongs lifetime and cycle stability of secondary electrochemical cells. The electrochemical cell has high energy density and/or high operating voltage and good performance characteristics and long lifetime.    DETAILED DESCRIPTION - An electrolyte composition comprises greater than or equal to 1 aprotic organic solvent, greater than or equal to 1 conducting salt, greater than or equal to 1 oxiranyl derivative of formula (I), and optionally greater than or equal to 1 additive.    X=C, S(O), or P(OR8);    n=0 or 1;    R1=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 which are optionally substituted by F, CN, 1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 5-7C (hetero)aryl, OR5, C(O)R5, C(O)OR5, OC(O)R5, OC(O)OR5, OC(O)C(O)OR5, OS(O)2R5, or S(O)2OR5;    R2-R4=H, C(O)R6, C(O)OR6, S(O)2OR6, or R1 provided that alkyl, (hetero)cycloalkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, (hetero)aryl, or aralkyl in R1 are optionally substituted by F, CN, 1-6C alkyl, 3-6C (hetero)cycloalkyl, 2-6C alkenyl, 5-7C (hetero)aryl, OR7, C(O)R7, C(O)OR7, OC(O)R7, OC(O)OR7, OC(O)C(O)OR7, OS(O)2R7, or S(O)2OR7 in which R2 and R3, together with C-C single bond of oxirane cycle, form 5-7-membered hydrocarbon cycle substituted by (O) and/or oxiranyl, and/or 2 adjacent C-atoms of hydrocarbon carbon cycle, together with O-atom, form an additional oxirane cycle; and    R5-R8=H or R1 provided that alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, or (hetero)aryl of R1 is optionally substituted by F, CN, and/or oxiranyl.    INDEPENDENT CLAIMS are included for:    (1) use of oxiranyl derivative as additive for electrolytes in electrochemical cells; and    (2) electrochemical cell comprising electrolyte composition, greater than or equal to 1 cathode having greater than or equal to 1 cathode active material, and greater than or equal to 1 anode having greater than or equal to 1 anode active material.