New Class of 3.7 V Fe-Based Positive Electrode Materials for Na-Ion Battery Based on Cation-Disordered Polyanion Framework

We report a new triplite-type iron fluoro-sulfate compound, a cation-disordered NaFeSO4F that has redox potential of ∼3.7 V versus Na+/Na0 and can have 138 mA·h/g of theoretical capacity. This compound shows practical energy density (∼430 W·h/kg) comparable to that of several Li-ion battery positive electrode materials such as LiMn2O4 (430 W·h/kg). Therefore, triplite NaFeSO4F is a candidate positive electrode material which can meet the requirements for high energy density Na-ion batteries. Furthermore, this study contributes to expanding the spectrum of Na electrode materials to cation-disordered polyanion compounds and opens up new opportunities for the design of electrode materials for advanced Na-ion batteries.