Nature of LiFePO4 aging process: Roles of impurity phases

LiFePO4 has been extensively studied in recent years because of superior thermal stability for the next generation of lithium-ion batteries. Nevertheless, LiFePO4 still undergo iron dissolution at high temperature or moisture-contaminated electrolyte, and the detailed mechanism is still not clear. Few efforts have been devoted to the correlations between surface chemistry and aging mechanisms. Here, we present a direct visual observation of surface corrosion process at olivine LiFePO4, and found the direct relationship between impurity phases and LiFePO4 corrosion. By using the LiFePO4 ingot sample with a flat surface as model materials, two types of impurity phase (iron-rich and phosphorus-rich) can be clearly observed and their influences on LiFePO4 corrosion were investigated in detail by SEM, Tof-SIMS, and electrochemical Tafel analysis. Similar to the electrochemical cell mechanism in a common metal corrosion process, an oxidation–reduction mechanism was suggested at the impurity phases-relevant corrosion behavior. Iron-rich impurity phases are seriously corroded due to the lower corrosion potentials, which inhibit the corrosion of the adjacent LiFePO4 bulk. On the contrary, phosphorus-rich impurity phase is stable due to higher corrosion potentials, which evokes the serious corrosion occurring at the adjacent LiFePO4 bulk. These findings provide the deep understanding the underlying mechanism in the LiFePO4 aging.