We report that tris(2,2,2- trifluoroethyl) phosphite (TTFP), in which the oxidization number of phosphor is three (III), can effectively improve thermal stability of LiPF6-based electrolytes of Li-ion cells. Effect of TTFP on the performance of Li/graphite, Li/cathode, and Li-ion cells is estimated using cyclic voltammetry and galvanostatic cycling. Results show that TTFP is electrochemically stable in Li-ion chemistry. We also found that TTFP can prevent propylene carbonate (PC) decomposition and graphite exfoliation in Li/graphite half-cell. We propose a mechanism of TTFP protecting LiPF6 and discuss the effect of TTFP on cell performance. (C) 2002 The Electrochemical Society.