On the stability of LiFePO4 olivine cathodes under various conditions (electrolyte solutions, temperatures)

LiFePO4 is one of the most important cathode materials for Li-ion batteries studied over the past few years. Impressive work has revealed important structural aspects and the correlations between structure and composition and electrochemical properties. Fewer efforts have been devoted to the surface chemical aspects of this material. We report herein on a study of the stability aspects of LiFePO4 at two temperatures, 30 and 60 degrees C. Three types of solutions were used based on EC-DMC 1:1 solvent mixtures those involving no acidic contamination (using LiClO4 as the electrolyte), those contaminated by HF (using LiPF6 as the Li salt), and LiPF6 solutions deliberately contaminated with H2O. Iron dissolution from LiFePO4 in these electrolytes, as well as the electrochemical response as a function of solution composition and aging, were studied at the two temperatures. The effect of additives that neutralize acidic species in solution was also studied. In general, LiFePO4 develops a unique surface chemistry. Highly stable behavior of LiFePO4 cathodes, without any substantial iron dissolution at elevated temperatures, was observed and measured when the solution contains no acidic or protic contaminants. (c) 2006 The Electrochemical Society.