Li2FeSiO4 is considered a promising cathode material for the next-generation Li-ion battery systems owing to its high theoretical capacity and low cost. Li2FeSiO4 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 Li2FeSiO4 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 Li2FeSiO4 to a thermodynamically stable orthorhombic LiFeSiO4, 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 Li2FeSiO4 (with the monoclinic structure as a metastable phase) with little cationic mixing. Here, we experimentally show the presence of metastable and stable LiFeSiO4 polymorphic phases caused by lithium extraction and insertion. Li2FeSiO4 is considered a promising cathode material for the next-generation Li-ion battery systems owing to its high theoretical capacity and low cost. Li2FeSiO4 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 Li2FeSiO4 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 Li2FeSiO4 to a thermodynamically stable orthorhombic LiFeSiO4, 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 Li2FeSiO4 (with the monoclinic structure as a metastable phase) with little cationic mixing. Here, we experimentally show the presence of metastable and stable LiFeSiO4 polymorphic phases caused by lithium extraction and insertion.