Phosphorous-Based Heterostructure for the Effective Catalysis of Polysulfide Reactions with Phase Changes in High-Sulfur-Loading Lithium-Sulfur Batteries

High sulfur loading and long cycle life are the design targets of commercializable lithium-sulfur (Li-S) batteries. The sulfur electrochemical reactions from Li2S4 to Li2S, which account for 75% of the battery's theoretical capacity, involve liquid-to-solid and solid-to-solid phase changes in all Li-S battery electrolytes in use today. These are kinetically hindered processes that are exacerbated by a high sulfur loading. In this study, it is observed that an in situ grown bimetallic phosphide/black phosphorus (NiCoP/BP) heterostructure can effectively catalyze the Li2S4 to Li2S reactions to increase the sulfur utilization at high sulfur loadings. The NiCoP/BP heterostructure is a good polysulfide adsorber, and the electric field prevailing at the Mott-Schottky junction of the heterostructure can facilitate charge transfer in the Li2S4 to Li2S2 liquid-to-solid reaction and Li+ diffusion in the Li2S2 to Li2S solid-state reaction. Consequently, a sulfur cathode with the NiCoP/BP catalyst can deliver a specific capacity of 830 mAh g-1 at the sulfur loading of 6 mg cm-2 for 500 cycles at the 0.5 C rate. High sulfur utilization is also possible at a higher sulfur loading of 8 mg cm-2 for 440 cycles at the 1 C rate. The NiCoP@BP heterostructure is used as electrocatalyst to enhance the sluggish redox kinetics of sulfur cathode. Black phosphorus combined with NiCoP match energy band positions for forming a Mott-Schottky heterojunction. The built-in electric field formed as such facilitates the phase changes conversion of Li2S4(liquid)-> Li2S2(solid)-> Li2S(solid) during cycling. High sulfur utilization is achieved in high sulfur loading Li-S batteries.image