Crystal structural changes and charge compensation mechanism during two lithium extraction/insertion between Li2FeSiO4 and FeSiO4

Li2FeSiO4 is a promising cathode material for lithium ion batteries because of its theoretically high capacity if two lithium ions can be extracted/inserted per formula unit; however, the extraction/insertion of two lithium ions from Li2FeSiO4 remains a challenge. Herein, we successfully synthesized carbon-coated Li2FeSiO4 nanoparticles which exhibit a capacity commensurate to a reversible two-lithium extraction/insertion at elevated temperature. This study investigates the mechanism underlying a two lithium ion extraction/insertion in Li2FeSiO4 using synchrotron X-ray absorption spectroscopy and X-ray diffraction. Our results reveal that the contribution of the Fe-3d band is dominant for the first lithium extraction process from Li2FeSiO4 to LiFeSiO4. During the second lithium extraction process from LiFeSiO4 to FeSiO4, however, ligand holes are formed in the O-2p band rather than further oxidation of Fe3+. Structural analyses further reveal a phase transformation between Li2FeSiO4 and LiFeSiO4, while a single-phase behavior is observed for Li2-xFeSiO4 (1.0 ≤x≤ 2.0). Together with a tentatively refined crystal structure of the FeSiO4 phase (x = 2.0), we discuss the charge compensation mechanism during two lithium extraction/insertion in Li2FeSiO4.