Thermal stability of lithium-ion battery electrolytes composed of LiPF6 in a mixture of organic carbonates was probed. Generation of PF5 via the thermal dissociation of LiPF6 is the primary source of thermal decomposition. It was found that the addition of low concentrations (3-12%) of Lewis basic additives provides a dramatic increase in the thermal stability of the electrolyte. The increased stability is linked to the formation of base: PF5 complexes which reversibly sequester PF5, preventing the decomposition of the electrolyte. Lewis bases investigated include pyridine, hexamethoxycyclotriphosphazene, and hexamethylphosphoramide. Our results can serve as a valuable model for stabilizing organic electrolytes against thermal decomposition initiated by Lewis acids. (c) 2005 The Electrochemical Society. All rights reserved.