An electrolyte for Li-ion and other secondary electrochemical cells includes an FSI anion and at least one of methyltriethylphosphonium; trimethylisobutylphosphonium; methyltributylphosphonium; and trihexyltetradecylphosphonium. The electrolyte uniquely enables stable cell cycling even when water is present in the electrolyte at levels as high as 5000 ppm. Methyltriethylphosphonium and trimethylisobutylphosphonium-containing electrolytes are particularly effective in this water-stabilizing capacity.