Na₂S Cathodes for Safer and High-Energy Sodium-Ion Storage

Room-temperature sodium-sulfur (RT-Na/S) batteries have been recognized as promising next-generation batteries with high energy density and low material price, but their practical implementation is severely obstructed by fatal safety issues owing to the direct application of Na metal. To address this concern, sodium sulfide (Na₂S) has been emerging as a promising cathode material which could enable the utilization of sodium-free anodes whilst additionally mitigating the volume expansion of S. However, Na₂S-based RT-Na/S batteries are still in their infancy, facing significant challenges related to a high activation energy barrier required to convert Na₂S to polysulfides during the initial charge and sluggish reaction kinetics for sulfur species. In this review, the three most promising types of Na₂S-based sodium-free RT-Na/S full-cells are first comprehensively discussed to establish the strengths and obstacles being researched for each system. Subsequently, the working mechanism and inherent advantages and weaknesses of Na₂S are methodically detailed. Utilizing this knowledge, strategies to enhance its performance are reviewed from three perspectives: synthesis methods, structural regulation, and catalytic optimization. Finally, this review summarizes these developments and offers an outlook of future research directions into developing superior Na₂S-based cathodes toward safer and high-energy Na₂S-based sodium-free RT-Na/S batteries.