Elucidating the LiFePO4 air aging mechanism to predict its electrochemical performance

Although LiFePO4-based chemistry has been extensively studied and developed in the last decade due to its promise for the next generation of Li-ion battery applications, the impact of ambient air exposure and the concomitant aging mechanism is still a controversial matter. In the present study, we describe quantitatively the aging process in terms of the successive phase formation, distribution and iron local environments. Formation and growth of a disordered ferric phosphate phase are directly observed for the first time from the surface of particles toward the core, preceding the crystallization of tavorite LiFePO4(OH) as observed through the combination of amorphous phase quantification by XRD, and by Mossbauer and Electron Energy Loss spectroscopies. Structural and electrochemical characterization prove that the amorphous ferric phosphate formed in the early aging stage exhibits already tavorite composition and structure at the local scale and shed light on a crucial step of the aging mechanism. Based on the correlation we establish here between the amounts of Fe(III) and hydroxyls groups present in aged samples and specific capacity of the corresponding electrodes, we show that the degradation of active material upon storage and the electrochemical performance can be predicted easily through simple TGA measurements.