Regulating the d band in WS₂@NC hierarchical nanospheres for efficient lithium polysulfide conversion in lithium-sulfur batteries

The performance of lithium-sulfur batteries is deteriorated by the inferior conductivity of sulfur, the shuttle effect of lithium polysulfides (LiPSs), sluggish redox kinetics of polysulfide intermediates and serious volumetric expansion of sulfur. To overcome these challenges, we report a versatile route to prepare multi-functional nanocomposites with tuable hierarchical structure via ammonium hydroxide (NH₃·H₂O) induced self-assembly. The versatility of the system has been demonstrated that the organization of the hierarchical structure can be regulated by adding different amounts of NH₃·H₂O, and WS₂ and Co₉S₈ with nitrogen-doped carbon coating (denoted as WS₂@NC and Co₉S₈@NC) can be prepared by adding different precursor salts. When the as-prepared materials are applied for Li-S batteries, the WS₂@NC composite exhibits a reversible capacity of 1107.4 mAh g⁻¹ at 0.1C after 500 cycles and even 728.9 mAh g⁻¹ at 2C for 1000 cycles, which is significantly better than the Co₉S₈ counterpart and other reported WS₂ sulfur hosts. Experimentally, the advantageous performance of WS₂ could be attributed to its higher surface area and total pore volume, giving rise to the easier access to electrolyte and better ability to buffer the volume change during the charge/discharge process. Theoretically, the density function theory (DFT) calculation reveals that the as-prepared WS₂ has a higher binding energy towards LiPSs as well as a lower energy barrier for Li⁺ diffusion on the surface than Co₉S₈. More significantly, the density of states (DOS) analysis further confirms that the superior performance is mainly ascribed to the more prominent shifting and the more charge compensation from d band of W than Co, which increase electronic concentration and give more hybridization of d-p orbitals in the Fermi level of the adsorbed Li₂S₄ to accelerate the lithium polysulfide interfacial redox and conversion dynamics in WS₂. By proposing this mechanism, this work sheds new light on the understanding of catalytic conversion of lithium polysulfides at the atomic level and the strategy to develop advanced cathode materials for high-performance lithium-sulfur batteries.