The decisive role of electrostatic interactions in transport mode and phase segregation of lithium ions in LiFePO 4 .

Understanding the mechanism of slow lithium ion (Li + ) transport kinetics in LiFePO 4 is not only practically important for high power density batteries but also fundamentally significant as a prototypical ion-coupled electron transfer process. Substantial evidence has shown that the slow ion transport kinetics originates from the coupled transfer between electrons and ions and the phase segregation of Li + . Combining a model Hamiltonian analysis and DFT calculations, we reveal that electrostatic interactions play a decisive role in coupled charge transfer and Li + segregation. The obtained potential energy surfaces prove that ion-electron coupled transfer is the optimal reaction pathway due to electrostatic attractions between Li + and e - (Fe 2+ ), while prohibitively large energy barriers are required for separate electron tunneling or ion hopping to overcome the electrostatic energy between the Li + -e - (Fe 2+ ) pair. The model reveals that Li + -Li + repulsive interaction in the [010] transport channels together with Li + -e - (Fe 2+ )-Li + attractive interaction along the [100] direction cause the phase segregation of Li + . It explains why the thermodynamically stable phase interface between Li-rich and Li-poor phases in LiFePO 4 is perpendicular to [010] channels.