Electron delocalization-enhanced sulfur reduction kinetics on an MXene-derived heterostructured electrocatalyst

Lithium-sulfur (Li-S) batteries mainly rely on the reversible electrochemical reaction of between lithium ions (Li+) and sulfur species to achieve energy storage and conversion, therefore, increasing the number of free Li+ and improving the Li+ diffusion kinetics will effectively enhance the cell performance. Here, Mo-based MXene heterostructure (MoS2@Mo2C) was developed by partial vulcanization of Mo2C MXene, in which the introduction of similar valence S into Mo-based MXene (Mo2C) can create an electron delocalization effect. Through theoretical simulations and electrochemical characterisation, it is demonstrated that the MoS2@Mo2C heterojunction can effectively promote ion desolvation, increase the amount of free Li+, and accelerate Li+ transport for more efficient polysulfide conversion. In addition, the MoS2@Mo2C material is also capable of accelerating the oxidation and reduction of polysulfides through its sufficient defects and vacancies to further enhance the catalytic efficiency. Consequently, the Li-S battery with the designed MoS2@Mo2C electrocatalyst performed for 500 cycles at 1 C and still maintained the ideal capacity (664.7 mAh·g−1), and excellent rate performance (567.6 mAh·g−1 at 5 C). Under the extreme conditions of high loading, the cell maintained an excellent capacity of 775.6 mAh·g−1 after 100 cycles. It also retained 838.4 mAh·g−1 for 70 cycles at a low temperature of 0 °C, and demonstrated a low decay rate (0.063%). These results indicate that the delocalized electrons effectively accelerate the catalytic conversion of lithium polysulfide, which is more practical for enhancing the behaviour of Li-S batteries.