te and diethyl carbonate; and d) a phosphonate additive having the formula: (R1O)P(=O) (OR2) (R3) provided in the electrolyte, wherein R3 is a hydrogen atom and wherein at least one, but not both of R1 and R2 is a hydrogen atom or at least one of R1 and R2 is a saturated organic group containing 1 to 13 carbon atoms. 19.