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.