Sulfide electrolyte additive enables multi-ionic transfer pathways in alkaline iron redox

Abstract

Traditional alkaline iron (Fe) batteries rely on the conversion reaction between Fe and Fe(OH)₂ but suffer from hydrogen generation upon Fe formation on charging. Fe²⁺/Fe³⁺ redox is a promising anode reaction for alkaline Fe batteries, as it alleviates the formation of hydrogen gas during charging. However, achieving complete Fe²⁺/Fe³⁺ redox with a one-electron transfer reaction is challenging due to the formation of electrochemically inert Fe₃O₄ materials. Here, we demonstrate that an alkaline sulfide-containing electrolyte facilitates the reversible multi-ion transfer and transport pathways within (and beyond) the Fe²⁺/Fe³⁺ redox system, including Fe(OH)₂/Fe₃O₄ conversion, intercalation of hydrosulfide into layered Fe(OH)₂, and hydrogen deposition, mediated by hydroxyl ions, hydrosulfide anions, and protons, respectively. This multi-ionic charge storage mechanism delivers a compelling discharge capacity of up to 330 mA h g⁻¹, as determined by chronopotentiometry measurements at a current density of 0.1 A g⁻¹, exceeding the theoretical iron redox capacity solely relying on Fe²⁺/Fe³⁺ redox (∼299 mA h g⁻¹). Our study will highlight the potential of alkaline iron redox as a green anode reaction for various aqueous energy storage systems by utilizing a scalable and low-concentration hydrosulfide electrolyte additive.