Crystal Structural Changes and Charge Compensation Mechanism during Two Lithium Extraction/Insertion between Li2FeSiO4 and FeSiO4

Li 2 FeSiO 4 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 Li 2 FeSiO 4 remains a challenge. Herein, we successfully synthesized carbon-coated Li 2 FeSiO 4 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 Li 2 FeSiO 4 using synchrotron X-ray absorption spectroscopy and X-ray diffraction. Our results reveal that the contribution of the Fe-3 d band is dominant for the first lithium extraction process from Li 2 FeSiO 4 to LiFeSiO 4 . During the second lithium extraction process from LiFeSiO 4 to FeSiO 4, however, ligand holes are formed in the O-2 p band rather than further oxidation of Fe 3+ . Structural analyses further reveal a phase transformation between Li 2 FeSiO 4 and LiFeSiO 4, while a single-phase behavior is observed for Li 2– x FeSiO 4 (1.0 ≤ x ≤ 2.0). Together with a tentatively refined crystal structure of the FeSiO 4 phase ( x = 2.0), we discuss the charge compensation mechanism during two lithium extraction/insertion in Li 2 FeSiO 4 .