Na2Fe(SO4)2: an anhydrous 3.6 V, low-cost and good-safety cathode for a 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 inherently unstable SO42− units in these sulfate materials result in their low-temperature decomposition (<450 °C) and lead to SO2 gas evolution, which can hinder the sulfate electrodes from producing high voltage safely. Herein, a new alluaudite-type sulfate cathode Na2Fe(SO4)2 for sodium-ion batteries is reported, which displayed a high operating voltage of ∼3.6 V based on the Fe2+/Fe3+ redox couple as well as superior thermal stability (∼580 °C). In both air and inert ambient conditions, its SO42− units demonstrated high thermal stability, assuring good safety for battery applications. Furthermore, the Na2Fe(SO4)2 cathode material showed superior stability towards moisture for easy handling. The cathode exhibited reversible capacity of 82 mA h g−1 at 0.1C under a nonoptimal carbon coating and maintained over 60% capacity retention at 2C. The excellent sodium storage abilities at 0 °C and 55 °C further demonstrated the advantages of Na2Fe(SO4)2 for future energy storage applications in a wide temperature range. The present exploration of Na2Fe(SO4)2 for sodium-ion batteries can pave the way for developing low-cost sulfate cathodes that combine high voltage and good safety.