Carbon@titanium nitride dual shell nanospheres as multi-functional hosts for lithium sulfur batteries

Lithium-sulfur (Li-S) cells have received particular attention as a post lithium ion energy storage system. However, low sulfur utilization and poor redox kinetics are still key challenges to improving cycling efficiency. Herein, we develop a multi-functional polysulfide mediator based on carbon hollow nanospheres supported titanium nitride (C@TiN) dual-shell hollow nanospheres, in which the physical confinement, chemical adsorption, and catalysis for sulfur species conversion were successfully achieved simultaneously. As a result, C@TiN-S composites (approximately 70 wt% sulfur content) exhibit faster reaction kinetics and a higher polysulfide trap capability than that of C@TiO2-S composites when used as cathode materials in Li-S batteries. The C@TiN-S electrode delivers a reversible capacity of 453 mA h g(-1), coupled with a high average Coulombic Efficiency (similar to 99.0%). There is also limited capacitance decay (only 0.0033% per cycle), at a current density of 3 C, over 300 cycles. In particular, when the sulfur loading is increased to 4.2 mg cm(-2), the C@TiN-S electrode can provide a high capacity of 820 mA h g(-1) over 150 cycles at 0.2 C. DFT calculations reveal that the long-chain Li2S8 tend to break down into two shorter chain segments due to the strong interaction of TiN and LiPSs. Electrochemical analysis techniques indicate that TiN can effectively catalyze the reduction of polysulfide and the oxidation of Li2S during discharge and charge processes, respectively. Our work offers a new strategy to develop high-performance Li-S batteries based on multi-functional mediators.