Disclosed are non-flammable electrolyte compositions of the present invention that comprise a mixed solvent comprising carbonate-, thiocarbonate-, and phosphate-based solvents and inorganic additives such as metal compounds or metal halogen compounds. A mixed ratio of carbonate:thiocarbonate:phosphate-based solvent is in the range of 20˜75:5˜30:15˜50. The metal compounds or metal halogen compounds are exemplified by triphenylbismuth carbonate, triphenylbismuth, bismuth subnitrate, bismuth subcarbonate, dimethyl pyrocabonate, diethyl pyrocarbonate, bismuth fluoride, and the like. The inorganic additives are contained in the electrolytes of the present invention in the amounts of about 0.3 to about 5 weight percent of the mixed solvent.