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

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 Nb⁴⁺/Nb⁵⁺ 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 Nb⁴⁺-rich surface effectively captures sodium polysulfides and accelerates sulfur conversion kinetics during discharge, while a Nb⁵⁺-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.