Non-aqueous electrolyte solutions capable of protecting lithium metal and lithium-inserted carbonaceous electrodes include an electrolyte salt, preferably LiPF₆, and a non-aqueous electrolyte solvent mixture comprising at least one of trialkyl phosphites, one or more cyclic or/and linear carbonates, and optionally other additives, such as, gelling agents, ionically conductive solid polymers, and other additives. The trialkyl phosphites have the following general formula: ##STR1##

wherein the oxidation number of the phosphorus atom is III (three), R¹, R², and R³ are the same or different, independently selected from linear or branched alkyl groups having 1 to 4 carbon atoms, optionally but not limited to, with one or more of the alkyl substituents being substituted by one or more halogen atoms, preferably fluorine atoms.