Kinetically Controlled Lithium-Staging in Delithiated LiFePO4 Driven by the Fe Center Mediated Interlayer Li-Li Interactions

Employing density functional theory (DFT) calculations, we demonstrate that the stage-II configuration in delithiated LiFePO4 is a thermodynamically metastable but kinetically controlled state, distinct from the thermodynamically favorable stages in graphite intercalation compounds (GICs). Based on the computational results, we propose a dual-interface model to describe the delithiation mechanism of LiFePO4 upon charging. Accordingly, the experimentally observed LiFePO4/stage-II/FePO4 three-phase coexistence could be successfully reproduced. Formation of lithium-staging configuration is mainly attributed to the Fe center mediated interlayer Li-Li interactions, which is an essential indirect electrostatic force. The indirect interaction originates from the localized nature of Fe 3d electrons, for which the effective oxidation state of Fe redox is determined by the Li ion arrangement and, in turn, has an impact on the behavior of Li ion diffusion. Besides a better understanding of the microscopic lithium diffusion mechanism in LiFePO4, our results also shed light on the interactions between electron and ion and further emphasize the importance of studying the Li diffusion kinetics at phase boundary in phase separation materials.