Na2Fe(SO4)2: an anhydrous 3.6 V, low cost and good safety cathode for rechargeable sodium-ion battery

Iron-based sulfate cathode materials are promising for rechargeable batteries due to their elevated operating voltages and earth-abundant elemental composition. However, the inherent unstable SO42- units in those sulfate materials result in their low-temperature decomposition (<450 °C) and lead to SO2 gas evolution, which would hinder sulfate electrodes from outputting high voltage in safety. Herein, a new alluaudite-type sulfate cathode Na2Fe(SO4)2 for sodium ion battery is reported, which displays a high operating voltage at ~3.6 V based on Fe2+/Fe3+ redox couple as well as superior thermal stability (~580 °C). In both air and inert ambient, its SO42-units demonstrates high thermal stability, assuring good safety for battery application. Furthermore, the Na2Fe(SO4)2 cathode material shows superior stability toward moisture for ease handling. The cathode exhibits a reversible capacity of 82 mAh g-1 at 0.1 C under nonoptimal carbon coating and maintains over 60% capacity retention at 2 C. The excellent sodium storage ability tested at 0 °C and 55 °C further demonstrates the advantages of Na2Fe(SO4)2 for future energy storage applications in a wide temperature range. The present exploration on Na2Fe(SO4)2 for sodium ion battery can pave the way for developing low cost sulfate cathodes combining high voltage and good safety.