Revisiting lithium K and iron M 2,3 edge superimposition: The case of lithium battery material LiFePO 4

Abstract

Experimental electron energy-loss spectra are presented for FePO 4, LiFePO 4 and NaFePO 4 from 0 to 80 eV. With the help of the NaFePO 4 spectrum in the 50–80 eV range, the double peak observed in LiFePO 4 could be ascribed to the presence of Fe II and not to the Li K edge, contrary to what was thought previously. Crystal field multiplet calculations confirm this attribution. Using VASP programme based on density functional theory, dielectric response calculations including local field effects in the Hartree approximation are then proven to properly simulate the fine structures due to the lithium K edge. By comparing absolute spectrum intensities, it is shown that the lithium K edge cannot be used to quantify lithium in such compounds. This detailed comparison between theoretical calculations and experimental spectra helps defining the relevant parameters governing intensities in the 50–80 energy range.