Vanadium Nitride Quantum-Dot Bidirectional Catalysis for Accelerated Polysulfide Redox in Room-Temperature Na-S Batteries.

The formation of soluble polysulfide intermediates is crucial for the multistep redox reactions of sulfur-based batteries; however, their irreversibility and uncontrollable shuttling through the electrolyte lead to suboptimal capacity, self-discharge, and rapid capacity fading. Here, we report a vanadium nitride quantum dot-graphene (VNQ-G) composite bidirectional catalyst that accelerates the reversible conversion of polysulfides during both discharging and charging for room-temperature sodium-sulfur (Na-S) batteries. The VNQ-G composite with a quantum-confined VNQ catalyst and a conductive graphene scaffold enables strong d-p hybridization with sodium polysulfide, accelerating redox kinetics and ensuring effective polysulfide trapping and utilization during discharge, and accelerating Na2S oxidation to soluble polysulfides during charging. As a result, the Na-S battery employing VNQ-G effectively mitigates the shuttle effect, delivering a maximum capacity of 302 mAh g-1 at 1C, maintaining 70% of its capacity over 400 cycles, and achieving a specific capacity of approximately 30 Ah per dollar of electrolyte. Our study demonstrates that the bidirectional catalyst is a critical factor in the reversibility of Na-S, and establishes that VNQ-G is a practical interfacial design strategy for realizing high-performance, cost-effective Na-S batteries.