In-situ constructing hetero-structured Mo2C-Mo2N embedded in carbon nanosheet as an efficient separator modifier for high-performance lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) are promising electrochemical energy storage devices. However, the shuttle effects and slow conversion kinetics of lithium polysulfides (LiPSs) still impede the practical application of LSBs. Herein, with the assistance of g-C3N4, a hetero-structured Mo2C-Mo2N embedded in the nitrogen-doped carbon sheets composite (Mo2C-Mo2N/C) is facilely in-situ synthesized. The g-C3N4 not only acts as the nitriding reagent for the heterostructure formation, but also is the structure-directing reagent for the formation of two-dimensional structures. More interestingly, Mo2C-Mo2N/C is also employed as a modifier of separators for LSBs. As evidenced experimentally and theoretically, the hetero-structured Mo2C-Mo2N exhibits a strong adsorption ability to LiPSs and concurrently offers a built-in electric field inside the heterostructure that ensures the fast conversion of LiPSs, thus significantly suppressing the shuttle effect and accelerating the electrochemical kinetics of LSBs. Therefore, the prepared LSBs with the Mo2C-Mo2N/C modified separator deliver a discharge specific capacity of 1404 mAh/g at 0.1 C and 660 mAh/g at 4 C, and achieve 1000 stable cycles at 1 C, with a capacity decay rate of only 0.028% per cycle. This study not only provides an efficient strategy for synthesizing hetero-structured materials but also sheds new light on the design of functional separators for high-performance and long-life cycle LSBs.