Revealing the Origin of Highly Efficient Polysulfide Anchoring and Transformation on Anion-Substituted Vanadium Nitride Host

Metal nitrides and quasi-metallic compounds have been extensively employed as sulfur hosts for confining polysulfide shuttling and improving the electronic conductivity. Their electronic structures and surface chemical bonds significantly determine the adsorption and catalytic abilities for polysulfide. However, the surface compositions of the reported metal nitrides and their sulfur anchoring mechanisms are still controversial. Herein, the authors demonstrate the anion-substituted mechanism from vanadium oxide, oxynitride to nitride during ammonia-annealing process and systematically unravel the long-range disorder rock-salt structure of vanadium oxynitride with abundant vanadium (V) and nitrogen (N) vacancies by synchrotron X-ray absorption spectra, atomic pair distribution function, and density functional theory calculation. The defect-rich vanadium oxynitride that is previously considered as vanadium nitride possesses the enhanced electron delocalization of V, N, and oxygen (O) atoms. It strengthens the polar Li N/O and V S bonds, especially near V vacancy, resulting in a strong polar adsorption for polysulfide. Meanwhile, the vanadium oxynitride effectively catalyzes the breaking and conversion of polysulfide, improving the reduction kinetics during discharge process. The bifunctional effects render the excellent cycling and rate performances. This work deeply understands the sulfur redox mechanisms on vanadium oxynitride and nitride and promotes the developments of the quasi-metallic compounds/sulfur cathodes in Lithium-sulfur battery.