Electron‐Rich Niobium Oxide Sub‐Nanoclusters Boosting Charge Transfer for Highly Reversible Sodium–Sulfur Batteries

Abstract Ether‐based room‐temperature sodium–sulfur (RT Na─S) batteries are a promising energy‐storage system, yet hindered by the unregulated sulfur redox pathway, severe polysulfide shuttling and rapid capacity fading. Herein, highly unsaturated niobium‐oxide sub‐nanoclusters (≈0.7 nm) anchored on defective carbon black (NbO x ‐DCB) as a dynamic sulfur‐conversion catalyst are introduced. The delocalized Nb d‐electrons in the sub‐nanocluster configuration create a mixed Nb 4+ /Nb 5+ valence state that functions as a bidirectional electron reservoir, thereby enabling a distinct d ‐band‐center self‐regulation mechanism. The strong d – p orbital coupling enabled by a Nb 4+ ‐rich surface effectively captures sodium polysulfides and accelerates sulfur conversion kinetics during discharge, while a Nb 5+ ‐rich surface promotes facile solid‐polysulfide decomposition during charging. Consequently, the NbO x ‐DCB/S cathode delivers a reversible capacity of 1184 mAh g S −1 at 0.1 A g −1 after 100 cycles and retains 547 mAh g S −1 after 3000 cycles at 2 A g −1 , corresponding to a decay rate of 0.0027% per cycle. The general applicability of this approach is validated by high‐performance tungsten and vanadium oxide sub‐nanocluster‐based sulfur cathodes. These findings highlight sub‐nanoscale metal‐oxide engineering as a versatile route to high‐performance RT Na–S batteries.