Recent advances in sodium-sulfur batteries: cyclability and stability through electrolyte and separator engineering

Metal-sulfur (M-S) batteries are emerging as promising solutions for future energy storage, thanks to their high theoretical energy density and the plentiful supply of sulfur. Among these, sodium-sulfur (Na-S) batteries are gaining popularity as affordable alternatives to lithium-sulfur (Li-S) systems because sodium is widely available and low-cost. However, developing effective room-temperature Na-S batteries for practical use remains challenging because sluggish reaction kinetics, sodium polysulfide dissolution, and shuttle effects together drive rapid capacity degradation. This comprehensive review examines the electrochemical mechanisms governing room-temperature Na-S batteries, with particular emphasis on the solubility and dissolution kinetics of sulfur species across diverse electrolyte systems. It points to the need to synergistically optimise electrolyte chemistry and calls for high-energy (high-concentration liquid, solid/quasi-solid and high-entropy) electrolytes, separator design, and solid-electrolyte interlayer architectures to prevent polysulfide migration and improve electrochemical stability, as well as for advanced experimental approaches (in situ and operando characterisation) and theoretical simulations and machine learning techniques. The review also highlights recent advances in Na-S battery prototypes to evaluate their practical performance and potential for technological adoption. Concluding with a perspective on Na-S batteries within the broader context of metal-sulfur energy storage systems, the review outlines critical directions for future research essential to realising their full practical potential.