Sodium Iron(II) Pyrosilicate Na2Fe2Si2O7: A Potential Cathode Material in the Na2O‑FeO-SiO2 System

As lithium-ion battery technology becomes widely popular with increasing demand for efficient energy-storage devices for a wide range of applications, the scarcity of lithium resources poses a concern for increasing costs. Replacing lithium with much more abundant sodium in combination with abundant transition metals such as iron (instead of traditionally used cobalt or nickel) as the charge compensation center in the cathode materials is expected to make large-scale battery technology a reality. To activate iron as a reversible redox center, oxyanions (XO4)n− have been introduced to stabilize the structures and raise the redox potentials, and silicates (X = Si, n = 4) form the best candidate group in terms of abundance and cost. In this regard, we explored the Na2O-FeO-SiO2 pseudoternary system and identified a new phase, Na2Fe2Si2O7, with an efficient chemical composition for charge accumulation (Na/Fe = 1), providing a large one-electron theoretical capacity of 164.5 mAhg–1 as a sodium-ion battery cathode.