The electrochemical and thermal properties of nonaqueous electrolytes containing a flame-retardant additive hexamethoxycyclotriphosphazene ([NP(OCH3)(2)](3)) were measured using cyclic voltammetry, electrochemical cycling, differential scanning calorimetery, and accelerating rate calorimetery. The flame-retardant additive was synthesized by reacting sodium methoxide (NaOCH3) and hexachlorocyclotriphosphazene (NPCl2)(3). The electrochemical stability of the electrolyte was determined using cyclic voltammetry. A mixture of 1 M lithium hexafluorophosphate and ethylene carbonate-dimethyl carbonate (50:50 wt %) was used as electrolyte. Li/LiNi0.8Co0.2O2 cells were fabricated with and without the flame-retardant additive to study their electrochemical performance.