Re-evaluation of experimental measurements for the validation of electronic band structure calculations for LiFePO₄ and FePO₄

Abstract

Experimental measurements used to validate previous electronic band structure calculations for olivine LiFePO₄ and its delithiated phase, FePO₄, have been re-investigated in this study. Experimental band gaps of LiFePO₄ and FePO₄ have been determined to be 6.34 eV and 3.2 eV by electron energy loss spectroscopy (EELS) and UV-Vis-NIR diffusion reflectance spectroscopy, respectively. X-ray photoemission (XPS) and Raman spectroscopy show that the surfaces of very carefully synthesized LiFePO₄ display Li-depletion, which affects optical reflectance determinations. Based on these experimental measurements, functionals for density functional theory (DFT) calculations of the electronic properties have been revisited. Overall, electronic structures of LiFePO₄ and FePO₄ calculated using sX-LDA show the best self-consistent match to combined experimentally determined parameters. Furthermore, the open-circuit voltages of the LiFePO₄ half-cell have been interpreted in terms of both Fermi levels and Gibbs free energies, which provides additional support for the electronic band structures determined by this research.