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 center dot 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 center dot 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. (C) 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Automated summary

A versatile route to prepare multi-functional nanocomposites with tuable hierarchical structure via ammonium hydroxide induced self-assembly was reported for improving the performance of lithium-sulfur batteries. The as-prepared WS2@NC composite exhibits superior performance due to higher surface area and total pore volume, easier access to electrolyte, better volume change buffering ability, and more prominent shifting and charge compensation from d band of W compared to Co. The density function theory calculation reveals the higher binding energy towards LiPSs and lower energy barrier for Li+ diffusion on the surface of WS2, contributing to enhanced electronic concentration and more hybridization of d-p orbitals in the Fermi level for improved lithium polysulfide interfacial redox and conversion dynamics.