Ternary salt mixtures, consisting of triethylmethylammonium benzoate (TEMAB), lithium acetate (LiOAc) and lithium bis(trifluoromethylsulfonyl) imide (LiTFSI), form stable molten salts at moderate temperatures at certain compositions. A typical composition of the molten salts is TEMAB/LiOAc/LiTFSI = 7/2/1 in molar ratio, and the glass transition temperature, the crystallization temperature and the melting temperature are -52 degrees C, 30 degrees C and 55 degrees C, respectively. Quenched salt mixture having the above composition forms a stable supercooled liquid at room temperature, and the ionic conductivity is 10(-4) Scm(-1) at 30 degrees C and 10(-3) Scm(-1) at 60 degrees C. Some commercially available polymers such as polyacrylonitrile and poly(vinyl butyral) are compatible with the salt mixture to result in him forming polymer electrolytes with 1-2 order(s) of magnitude of loss of the molten salt conductivity.