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/Li 2 S), volume expansion effect of sulfur and the shuttle effect of lithium polysulfides (LiPSs). Herein, a three dimensional (3D) carbon nanotubes (CNTs) decorated cubic Co 9 Se 8−x /FeSe 2−y (0 ＜ x ＜ 8, 0 ＜ y ＜ 2) composite (Co 9 Se 8−x /FeSe 2−y @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 Co 9 Se 8−x /FeSe 2−y @CNTs, the cell with Co 9 Se 8−x /FeSe 2−y @CNTs//PP modified separator delivers a high reversible capacity (1103.5 mA h g −1 ) at 1C after three cycles activation at 0.5C and remains 446 mA g h −1 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 −2 after 100 cycles with a high sulfur loading of 4.1 mg cm −2 is obtained. The in-situ XRD tests revealing the transition path of α -S 8  → Li 2 S →  β -S 8 during the first charge–discharge process, then β -S 8  → Li 2 S →  β -S 8 conversion reaction in the next cycles, and firstly determine the sulfur-selenide active intermediates (Se 1.1 S 6.9 ) 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.