The decisive role of electrostatic interactions in transport mode and phase segregation of lithium ions in LiFePO4

Abstract

Understanding the mechanism of slow lithium ion (Li⁺) transport kinetics in LiFePO₄ 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⁻ (Fe²⁺), while prohibitively large energy barriers are required for separate electron tunneling or ion hopping to overcome the electrostatic energy between the Li⁺–e⁻ (Fe²⁺) pair. The model reveals that Li⁺–Li⁺ repulsive interaction in the [010] transport channels together with Li⁺–e⁻ (Fe²⁺)–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 LiFePO₄ is perpendicular to [010] channels.