The role of selenium vacancies functionalized mediator of bimetal (Co, Fe) selenide for high-energy-density lithium-sulfur batteries.

Lithium-sulfur (Li-S) batteries are currently only in the basic research stage and have not been commercialized, which is mainly affected by the poor conductivity of sulfur/lithium sulfide (S/LiS), volume expansion effect of sulfur and the shuttle effect of lithium polysulfides (LiPSs). Herein, a three dimensional (3D) carbon nanotubes (CNTs) decorated cubic CoSe/FeSe (0 ＜ x ＜ 8, 0 ＜ y ＜ 2) composite (CoSe/FeSe@CNTs) is developed, and used as the functionalized mediator on polypropylene (PP) in Li-S batteries. Benefiting from the good electrical conductivity, large number of Se vacancies and the triple block/adsorption/catalytic effects of CoSe/FeSe@CNTs, the cell with CoSe/FeSe@CNTs//PP modified separator delivers a high reversible capacity (1103.5 mA h g) at 1C after three cycles activation at 0.5C and remains 446 mA g h after 750 cycles with a 0.08% capacity decay rate each cycle. Moreover, at 0.2C, a high areal capacity of 3.63 mA h cm after 100 cycles with a high sulfur loading of 4.1 mg cm is obtained. The in-situ XRD tests revealing the transition path of α-S → LiS → β-S during the first charge-discharge process, then β-S → LiS → β-S conversion reaction in the next cycles, and firstly determine the sulfur-selenide active intermediates (SeS) during cycles. The work provides a new insight into the development of bimetallic selenide composites by defect engineering with highly adsorptive and catalytic properties for Li-S batteries.