Thermal reactions between 1.0 M LiPF6 in 1:1:1 ethylene carbonate/dimethyl carbonate/diethyl carbonate and metal-oxide cathode particles were investigated by analyzing both the liquid electrolyte and solid cathode particles through the combined use of nuclear magnetic resonance spectroscopy, gas chromatography with mass selective detection, scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The reactions between the electrolyte and cathode particles inhibit the thermal decomposition of the electrolyte and modify the surface of the cathode particles. The Li2CO3 on the surface of the metal oxides is removed and replaced by a complex mixture including poly(ethylene oxide), polycarbonate, ROCO2Li, LiF, and LixPOyFz. Higher surface concentration of Li2CO3 on LiNi0.8Co0.2O2 allows a fragile temperature-dependent equilibrium to be established, accounting for the thermal stability of the electrolyte. Higher temperature leads to more LixPOyFz and less Li2CO3 on the surface. With LiCoO2, no equilibrium is established due to lower surface concentration of Li2CO3. Consequently, thermal reactions of the electrolyte with LiCoO2 generate decomposition products, including LixPOyFz and cobalt fluorides on the surface and bulk electrolyte decomposition. Independent addition of Li2CO3 enhances the thermal stability of the LiPF6-based electrolyte, confirming the thermal-stabilizing properties of Li2CO3 surface films. The addition of a carbonate solution of PF5 to LiNi0.8Co0.2O2 generates LiPF6 in the solution and LixPOyFz on the surface. (c) 2006 The Electrochemical Society.