Dual-Functional NbN Ultrafine Nanocrystals Enabling Kinetically Boosted Lithium-Sulfur Batteries

Herein, NbN nanocrystals immobilized on N-doped carbon nanosheets to functionalize a polypropylene (PP) membrane (NbN@NC/PP) with a thin coating of only 4 mu m are designed and synthesized. The functional modifier layer allows for sulfur-involved transformations and also lithium plating behaviors. On the one hand, the sulfur cell with NbN@NC/PP separator exhibits excellent cycling stability and rate capacity. The good electrochemical performance partially results from the strong chemical interactions between NbN and lithium polysulfides via the formation of Nb-S and N-Li bonds, which is proven by the first-principles calculations and X-ray photoelectron spectroscopy analyses. The formation of tiny nanoscrystals (<2 nm) and clusters tends to maximize the surface of NbN to interact with polysulfides and enable the effective catalysis over the sulfur-involved reactions. The higher exchange current density and Li+ diffusion coefficient of NbN@NC/PP cells experimentally verify that the introduction of NbN indeed catalytically accelerates the reaction kinetics. On the other hand, the performance of Li//Li symmetric cells demonstrates that the NbN@NC modifier layer can well induce homogeneous lithium deposition. This work confirms the application potential of NbN in lithium-sulfur batteries and encourages the exploration of prospective nitrides to engineer high performance next-generation batteries.