Intermediate Phase-Assisted Li-Intercalation/Extraction Behavior for LiFePO4 Cathode Materials

Understanding the basic mechanism of Li-insertion/extraction in LiFePO4 batteries will be greatly beneficial to enhance their high-power capability or low-temperature performance, which is vitally important for the electric vehicle industry. Here, we present a viewpoint on the Li-insertion/extraction mechanism for our hierarchical LiFePO4/C samples. Instead of a traditional direct FePO4/LiFePO4 reaction, Fe2PO5 and (LiPO3)4·2H2O phases emerge, assisted by electrolytes as intermediate phases, which make the Li-ion (de)intercalation process most likely a nucleation-limited reaction. During this process, the rate of enablement of the phase transition is the lowest, and once a phase has initiated at the moment of nucleation, the subsequent phase transformation is much faster. The Fe2PO5 phase, which contains both Fe2+ and Fe3+ ions, would result in the delocalization of electrons and enable electrons to conduct from nanocrystalline grains to conductive carbon-coated films nearby, accelerating electron conduction near the lithium site. This work provides a possibility for improving the high rate or low-temperature performance of LiFePO4 batteries by appropriate electrolyte additives that increase supersaturation of the (LiPO3)4·2H2O or Fe2PO5 intermediate phase based on the understanding of intermediate phase-assisted Li-intercalation/extraction behavior.