Insights into electrochemical performance of Li2FeSiO4 from first-principles calculations

• Different charge–discharge behaviors observed in different experiments are explained.
• Different structural transformation methods could occur during first charge cycle.
• Average voltages, structural evolution and Li diffusion mechanism are studied.
• Experimentally obtained high capacity and the multi-electron process are analyzed.
• We investigate the redox mechanism of Li2FeSiO4 upon lithium removal.

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 (P21/n) structure to cycled (P21/n-cycl or Pmn21-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 P21/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 200 mAh/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 P21/n-cycl structure, is also investigated.

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