Chemical Processes in All‐Solid‐State Li─S and Na─S Batteries: A Perspective

ABSTRACT All‐solid‐state Li–S and Na–S batteries (ASSSBs) employing highly conductive sulfide solid electrolytes combine high energy density with improved safety by enforcing solid–solid sulfur conversion and eliminating polysulfide shuttling. However, sulfide electrolytes possess narrow electrochemical stability windows, while sulfur's low conductivity, sluggish redox kinetics, and ∼80% volume change further challenge high‐loading (>30 wt.% S, or > 3 mg cm −2 ) operation under practical stack pressures. This perspective adopts an intrinsic chemical framework to dissect sulfur conversion in ASSSBs, emphasizing the coupled chemistry of sulfur, conductive carbon, and sulfide electrolytes during both fabrication and cycling. Sulfur redox pathways in liquid versus solid‐state systems are compared, followed by an analysis of electrolyte stability, S/electrolyte interactions, and interphase evolution. The differences in redox pathways for Li–S vs Na–S are discussed. Strategies to enhance sulfur reactivity and to tailor electrolyte behavior are then evaluated, culminating in design principles for high‐loading, low‐pressure ASSSBs capable of meeting practical performance targets.