In Situ Observation of Na ₂ S Growth: A Step Toward High‐Energy and Safer Room Temperature Sodium Sulfur Batteries

The Na 2 S cathode presents a promising metal‐free sodium configuration for high‐energy room‐temperature sodium sulfur (RT Na─S) batteries. However, the rational design of Na 2 S cathodes to overcome their poor electronic conductivity and sluggish conversion kinetics remains a major challenge. Here, an in situ carbothermic reduction strategy is reported to fabricate atomic Mo anchored on Na 2 S/C (including single Mo atom and Mo clusters) as cathode materials for RT Na─S batteries. In situ transmission electron microscopy and X‐ray diffraction technique reveal that atomic Mo could form a low‐temperature eutectic phase, which catalytically lowers the formation temperature of Na 2 S. This Mo‐Na 2 S/C exhibits remarkable cyclic stability, achieving an initial capacity of 1617 mAh g −1 at 0.1 A g −1 and a low activation voltage of 1.89 V. Notably, when paired with hard carbon, the safe sodium full cell delivers an impressive initial reversible capacity of 952 mAh g −1 . Experimental results and theoretical calculations reveal that the atomic Mo facilitates the interfacial electron transfer between Mo and Na 2 S, which modulates the bandgap of Na 2 S and reduces its reaction barriers to polysulfide, thereby enhancing the reaction kinetics. These findings offer an effective strategy for developing high‐performance Na 2 S cathodes and provide deeper insights into electrode preparation mechanisms.