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

Understanding the mechanism of slow lithium ion (Li) transport kinetics in LiFePO 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), while prohibitively large energy barriers are required for separate electron tunneling or ion hopping to overcome the electrostatic energy between the Li-e (Fe) pair. The model reveals that Li-Li repulsive interaction in the [010] transport channels together with Li-e (Fe)-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 is perpendicular to [010] channels.