A Low‐Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium‐Ion Storage

Abstract Sodium‐ion batteries (NIBs) are the most promising alternatives to lithium‐ion batteries in the development of renewable energy sources. The advancement of NIBs depends on the exploration of new electrode materials and fundamental understanding of working mechanisms. Herein, via experimental and simulation methods, we develop a mixed polyanionic compound, Na 2 Fe(C 2 O 4 )SO 4 ⋅H 2 O, as a cathode for NIBs. Thanks to its rigid three dimensional framework and the combined inductive effects from oxalate and sulfate, it delivered reversible Na insertion/desertion at average discharging voltages of 3.5 and 3.1 V for 500 cycles with Coulombic efficiencies of ca. 99 %. In situ synchrotron X‐ray measurements and DFT calculations demonstrate the Fe 2+ /Fe 3+ redox reactions contribute to electron compensation during Na + desertion/insertion. The study suggests mixed polyanionic frameworks may provide promising materials for Na ion storage with the merits of low cost and environmental friendliness.