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Robust diamond-like Fe-Si network in the zero-strain NaxFeSiO4 Cathode

Sodium orthosilicates Na2MSiO4 (M denotes transition metals) have attracted much attention due to the possibility of exchanging two electrons per formula unit. In this work, we report a group of sodium iron orthosilicates Na2FeSiO4, the crystal structures of which are characterized by a diamond-like Fe-Si network. The Fe-Si network is quite robust against the charge/discharge process, which explains the high structural stability observed in experiment. Using the density functional theory within the GGA+U framework and X-ray diffraction studies, the crystal structures and structural stabilities during the sodium insertion/deinsertion process are systematically investigated. The calculated average deintercalation voltages are in good agreement with the experimental result.