An electrolyte composition containing (i) at least one aprotic organic solvent; (ii) at least one conducting salt; (iii) at least one silyl ester phosphonate containing the structure of formula wherein T is selected from p is an integer from 0 to 6 and one or more CH 2 groups of (CH 2)p may be replaced by O and one or more H of (CH 2)p may be replaced by C 1 -C 4 alkyl; R 1 is selected independently at each occurrence from H, F, Cl, R 4, OR 4, OSi(R 5) 3, OSi(OR 4) 3, and OP (OR 4)R 5; R 4 is independently at each occurrence selected from C 1 -C 10 alkyl, C 3 -C 7 (hetero)cycloalkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 5 -C 7 (hetero)aryl, and C 6 -C 13 (hetero)aralkyl which may be substituted by one or more substituents selected from CN and F and wherein one or more CH 2 -groups of alkyl, alkenyl and alkynyl which are not directly bound to the Si-atom or the O-atom may be replaced by O; R 3 and R 5 are selected independently at each occurrence from H, F, C 1 -C 10 alkyl, C 3 -C 7 (hetero)cycloalkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 5 -C 7 (hetero)aryl, and C 6 -C 13 (hetero)aralkyl which may be substituted by one or more substituents selected from CN and F and wherein one or more CH 2 -groups of alkyl, alkenyl and alkynyl which are not directly bound to the P-atom may be replaced by O. (i) at least one aprotic organic solvent; (ii) at least one conducting salt; (iii) at least one silyl ester phosphonate containing the structure of formula wherein T is selected from p is an integer from 0 to 6 and one or more CH 2 groups of (CH 2)p may be replaced by O and one or more H of (CH 2)p may be replaced by C 1 -C 4 alkyl; R 1 is selected independently at each occurrence from H, F, Cl, R 4, OR 4, OSi(R 5) 3, OSi(OR 4) 3, and OP (OR 4)R 5; R 4 is independently at each occurrence selected from C 1 -C 10 alkyl, C 3 -C 7 (hetero)cycloalkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 5 -C 7 (hetero)aryl, and C 6 -C 13 (hetero)aralkyl which may be substituted by one or more substituents selected from CN and F and wherein one or more CH 2 -groups of alkyl, alkenyl and alkynyl which are not directly bound to the Si-atom or the O-atom may be replaced by O; R 3 and R 5 are selected independently at each occurrence from H, F, C 1 -C 10 alkyl, C 3 -C 7 (hetero)cycloalkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 5 -C 7 (hetero)aryl, and C 6 -C 13 (hetero)aralkyl which may be substituted by one or more substituents selected from CN and F and wherein one or more CH 2 -groups of alkyl, alkenyl and alkynyl which are not directly bound to the P-atom may be replaced by O. T is selected from p is an integer from 0 to 6 and one or more CH 2 groups of (CH 2)p may be replaced by O and one or more H of (CH 2)p may be replaced by C 1 -C 4 alkyl; R 1 is selected independently at each occurrence from H, F, Cl, R 4, OR 4, OSi(R 5) 3, OSi(OR 4) 3, and OP (OR 4)R 5; R 4 is independently at each occurrence selected from C 1 -C 10 alkyl, C 3 -C 7 (hetero)cycloalkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 5 -C 7 (hetero)aryl, and C 6 -C 13 (hetero)aralkyl which may be substituted by one or more substituents selected from CN and F and wherein one or more CH 2 -groups of alkyl, alkenyl and alkynyl which are not directly bound to the Si-atom or the O-atom may be replaced by O; R 3 and R 5 are selected independently at each occurrence from H, F, C 1 -C 10 alkyl, C 3 -C 7 (hetero)cycloalkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 5 -C 7 (hetero)aryl, and C 6 -C 13 (hetero)aralkyl which may be substituted by one or more substituents selected from CN and F and wherein one or more CH 2 -groups of alkyl, alkenyl and alkynyl which are not directly bound to the P-atom may be replaced by O.