Depth‐of‐Discharge‐Dependent Chemical Evolution in Sulfurized Polyacrylonitrile Cathodes for Ether‐Based Room‐Temperature Sodium–Sulfur Batteries

Room-temperature sodium–sulfur (RT Na–S) batteries are emerging as promising next-generation energy storage systems owing to their high theoretical capacity and environmental friendliness. Nevertheless, the intrinsic insulating nature of elemental sulfur and the polysulfide shuttle effect significantly limit the widespread practical and large-scale applications of RT Na–S batteries. Sulfurized polyacrylonitrile (SPAN) is a potential cathode candidate for Na–S batteries, which provides efficient charge transfer due to the conjugated structure of SPAN and avoids the formation of long-chain polysulfide by its structure that only has a short sulfur–sulfur chain. However, the decay mechanism of the SPAN positive electrode in ether-based electrolyte RT Na–S batteries remains poorly understood. In this study, SPAN was studied as a cathode material in ether-based RT Na–S cells. The focus was on the structural evolution of the material during the first few cycles and on variations at different depths of discharge (DOD). This work reveals that deep discharge below 1.0 V affects the structure of SPAN, the equilibrium of polysulfides in the electrolyte, and the growth of sodium dendrites at the negative electrode. On this basis, to enhance the cycling stability and rate performance, carbon-coated functionalized separators are incorporated in the cell.