Conversion mechanism of sulfur in room-temperature sodium-sulfur battery with carbonate-based electrolyte

Room temperature sodium-sulfur batteries have attracted considerable interest due to their remarkable cost-effectiveness and specific capacity. However, due to the limited comprehension of its conversion mechanism, the decrease in sulfur cathode capacity in carbonate electrolytes is usually loosely attributed to the shuttle effect, which is well known in lithium-sulfur batteries that work in ether-based electrolytes. This work proposes a complete sulfur reaction mechanism in which the confined space is very important by combining the results from the theoretical calculations and electrochemical characterization. Specifically, crystal sulfur outside the pores is reduced to polysulfides, leading to irreversible reactions with carbonate solvents. Meanwhile, amorphous sulfur within the narrow pores undergoes an activation process during the first discharge and experiences a reversible conversion in subsequent cycles through a two-step solid-state reaction. Furthermore, the discharge/charge processes unveil divergent dynamics that can be clarified through the lens of chemomechanical stress in a confined environment. The increased comprehension of the sulfur conversion process in electrolytes composed of carbonate highlights the importance of confined space and electrolytes. This newly acquired knowledge holds the potential to offer theoretical insights guiding the design of high-performance sulfur cathodes.