Understanding the High Capacity of Li2FeSiO4: In Situ XRD/XANES Study Combined with First-Principles Calculations

The electrochemical mechanism of the cathode material Li2FeSiO4 with reversible extraction/insertion of more than one Li+ from/into the structure has been studied by techniques of in situ synchrotron X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD). These advanced techniques provide effective solutions to address the limitations of characterization by traditional ex situ methods. The study of in situ Fe Kedge XANES indicates that the Fe ion in the Li2FeSiO4 is oxidized continuously to high valence during the charging process from open circuit potential to 4.8 V, which contributes to the high reversible capacities of the materials. In situ XRD and theoretical study from first-principles calculations have been employed to reveal the structural evolution of the Li2FeSiO4 underlying the high capacity during the charge/discharge process. The results of both experimental and theoretical studies are consistent and indicate that Li2FeSiO4 undergoes two two-phase reactions when the electrode is charged to a high voltage of 4.8 V.