Electronic structure changes upon lithium intercalation into graphite – Insights from ex situ and operando x-ray Raman spectroscopy

This study probes the electrochemical intercalation mechanism of Li into graphitic carbon using x-ray Raman spectroscopy (XRS), an inelastic x-ray scattering technique. Operando and high resolution spectra of electrochemically lithiated composite graphitic electrodes at discrete states of Li uptake (stages III, II and I) show gradually changing spectral features with Li intercalation, the end state agreeing well with chemically fully lithiated LiC6 highly oriented pyrolytic graphite (HOPG). The two most dominant changes in the XRS spectrum of the C-K edge are the reduced intensity of the π* peak and shift the onset of the σ* states to lower energies upon full lithiation. The excellent instrumental energy resolution uncovered novel spectral features and, thus, electronic changes with varying lithium content. The general spectral changes with progressing Li intercalation agree well with our accompanying DFT calculations.