Rate-Dependent, Li-Ion Insertion/Deinsertion Behavior of LiFePO<sub>4</sub> Cathodes in Commercial 18650 LiFePO<sub>4</sub> Cells

We have performed operando synchrotron high-energy X-ray diffraction (XRD) to obtain nonintrusive, real-time monitoring of the dynamic chemical and structural changes in commercial 18650 LiFePO<sub>4</sub>/C cells under realistic cycling conditions. The results indicate a nonequilibrium lithium insertion and extraction in the LiFePO<sub>4</sub> cathode, with neither the LiFePO<sub>4</sub> phase nor the FePO<sub>4</sub> phase maintaining a static composition during lithium insertion/extraction. On the basis of our observations, we propose that the LiFePO<sub>4</sub> cathode simultaneously experiences both a two-phase reaction mechanism and a dual-phase solid-solution reaction mechanism over the entire range of the flat voltage plateau, with this dual-phase solid-solution behavior being strongly dependent on charge/discharge rates. The proposed dual-phase solid-solution mechanism may explain the remarkable rate capability of LiFePO<sub>4</sub> in commercial cells.