Phase transformation and electronic structure characterization of Li$_{x}$FePO$_{4}$ by ab-initio calculations and soft x-ray spectroscopy

Olivine-structured Li$_{x}$FePO$_{4}$ with appropriate surface treatment is a battery cathode material with promising capacity, cost and safety specifications. Fe-L and O-K edge soft x-ray absorption and emission spectra directly probe the unoccupied and occupied electronic states in the vicinity the Fermi energy. We present a first principles calculation and a comparison with the spectra to investigate the electronic states of Li$_{x}$FePO$_{4}$. Upon fully (de)lithiation, the redistributed unoccupied Fe-3d and O-2p states indicate the fingerprints of the two-phase transformation. The redox couple is pinned such that a single electron injection into the valence states is well separated from the top of O-2p valence states due to Coulomb repulsion. We further explore the surface properties and discuss their implications on the performance and optimization of Li$_{x}$FePO$_{4}$. Work supported by the US DOE.