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

Abstract

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.