Relationship between Phase Transition Involving Cationic Exchange and Charge–Discharge Rate in Li₂FeSiO₄

Li 2 FeSiO 4 is considered a promising cathode material for the next-generation Li-ion battery systems owing to its high theoretical capacity and low cost. Li 2 FeSiO 4 exhibits complex polymorphism and undergoes significant phase transformations during charge and discharge reaction. To elucidate the phase transformation mechanism, crystal structural changes during charge and discharge processes of Li 2 FeSiO 4 at different rates were investigated by X-ray diffraction measurements. The C/50 rate of lithium extraction upon initial cycling leads to a complete transformation from a monoclinic Li 2 FeSiO 4 to a thermodynamically stable orthorhombic LiFeSiO 4, concomitant with the occurrence of significant Li/Fe antisite mixing. The C/10 rate of lithium extraction and insertion, however, leads to retention of the parent Li 2 FeSiO 4 (with the monoclinic structure as a metastable phase) with little cationic mixing. Here, we experimentally show the presence of metastable and stable LiFeSiO 4 polymorphic phases caused by lithium extraction and insertion.