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 LiFePO ₄ aging.