Active Sulfur-Host Material VS 4 with Surface Defect Engineering: Intercalation-Conversion Hybrid Cathode Boosting Electrochemical Performance of Li-S Batteries.

Transition-metal sulfides as late-model electrocatalysts usually remain inactive in lithium-sulfur (Li-S) batteries in spite of their advantages to accelerate the rapid conversion of lithium polysulfides (LiPSs). Herein, a series of cobalt-doped vanadium tetrasulfide/reduced graphene oxide ( x %Co-VS 4 /rGO) composites with an ultrathin layered structure as an active sulfur-host material are prepared by a one-pot hydrothermal method. The well-designed two-dimensional ultrathin 3%Co-VS 4 /rGO with heteroatom architecture defects (defect of Co-doping and defect of S-vacancies) can significantly improve the adsorption ability on LiPSs, the electrocatalytic activity in the Li 2 S potentiostatic deposition, and the active sulfur reduction/oxidation conversion reactions and greatly boost the electrochemical performances of Li-S batteries. On the one hand, the ultrathin 3%Co-VS 4 /rGO possesses good conductivity inheriting from rGO which contributes to the capacity of internal redox reactions on lithiation from VS 4 . On the other hand, the hybrid architectures provide strong adsorption and excellent electrocatalytic ability on LiPSs, which benefit from the surface defects caused by heteroatom doping. The S@3%Co-VS 4 /rGO cathode displays a high specific capacity of 1332.6 mA h g -1 at 0.2 C and a low-capacity decay of only 0.05% per cycle over 1000 cycles at 3 C with a primary capacity of 633.1 mA h g -1 . Furthermore, when the sulfur loading (single-side coating) reaches 4.48 mg cm -2 , it still can deliver 756.2 mA h g -1 after the 100th cycle at 0.2 C with 89.5% capacity retention. In addition, the in situ X-ray diffraction test reveals that the sulfur conversion mechanism is the processes of α-S 8 → Li 2 S → β-S 8 (first cycle) and then β-S 8 ↔ Li 2 S during the subsequent cycles. The designing strategy with heteroatom doping and self-intercalation capacity adopted in this work would provide novel inspiration for fabricating advanced sulfur-host materials to achieve excellent electrochemical capability in Li-S batteries.