The Spin-Polarized Electronic Structure of LiFePO4 and FepO4 Evidenced by in-Lab XPS

LiFePO4 has an interesting spin-polarized electronic structure showing a (3d dagger)(5)(3d down arrow)(1) electron configuration of the Fe2+ ion. In this work, we have experimentally evidenced the valence electronic structure of LiFePO4 and of its delithiated compound FePO4 by X-ray photoelectron spectroscopy (XPS), which allows a visualization or the occupied densities of slates (DOS) in the valence band. XPS valence spectra were compared with the DOS obtained from DFT calculations by considering GGA and GGA + U approaches. Thanks to electrochemical extraction/insertion of Li ions in LiFePO4/LiFePO4 it was possible to display the Fe 3d spindown electron of LiFePO4, which is not present in the valence spectrum of FePO4. We show that the study of XPS valence spectra is an efficient way to access the lithium insertion rate in LixFePOx positive electrode materials for lithium-ion batteries. Besides the contribution to the Li-ion battery field, this paper is also a rare example of experimental evidence of a spin-resolved electronic structure from in-lab XPS experiments.