First-principles calculations have been performed on the polymorphs of Li2FeSiO4 and the corresponding delithiated products. The calculated results indicate that a structural transformation from the pristine (P2 1 /n) structure to cycled (P2 1 /n-cycl or Pmn2 1 -cycl) polymorph would occur in the first charge cycle. Then, Li ions can be reversibly intercalated/deintercalated into/from the cycled structures in the subsequent cycles. More importantly, our results show that ∼1.5 Li ions can be extracted from the P2 1 /n-cycl Li2FeSiO4, corresponding to two redox reactions of Li2FeSiO4/LiFeSiO4 and LiFeSiO4/Li0.5FeSiO4, which can be used to understand the experimentally obtained high capacity of over 200mAh/g, as well as the different charge–discharge behaviors observed in different experiments. Moreover, the oxidation of Fe2+ to Fe3+ ions is observed upon the first Li removal, while charge balance upon further delithiation is achieved mainly by the oxidation of oxygen ions and partly by iron. Finally, the structural evolution during the charge–discharge process, as well as the Li diffusion mechanism in the P2 1 /n-cycl structure, is also investigated.