Al-substituted Na5FeS4 as earth-abundant positive electrode active materials for all-solid-state sodium batteries

Abstract

Abstract All-solid-state sodium batteries are attracting attention as next-generation energy devices because of their high safety, low cost, and high energy density. Na 5 FeS 4 , an earth-abundant sodium iron sulfide, is a promising candidate positive electrode active material with high capacity and reversibility. In this study, Al-substituted Na 5 FeS 4 positive electrode active materials were synthesized, and the effect of Al substitution on the electrode properties was investigated. It was found that Al substitution increases the apparent ionic conductivity of Na 5 FeS 4 . Moreover, all-solid-state cells using Na 5 Fe 0.95 Al 0.05 S 4 positive electrode active material exhibited an initial capacity of approximately 500 mAh g − 1 , exceeding that of Na 5 FeS 4 , and operated reversibly for more than 70 cycles. Na 5 Fe 1− x Al x S 4 active materials with x ≤ 0.5 maintained sufficient apparent ionic diffusivity throughout the charge process, contributing to their high capacities. This study revealed three key findings: the formation of a solid solution between Na 5 FeS 4 and Na 5 AlS 4 over the entire composition range, the enhancement of apparent ionic diffusivity and capacity of Na 5 FeS 4 through slight Al substitution, and unexpectedly high electrode performance even up to 50% Al substitution. These findings contribute to the development of environmentally friendly, high-capacity sodium iron sulfide-based active materials for all-solid-state sodium batteries.