The commercialization of lithium–sulfur (Li–S) batteries has been struggling due to the uncontrollable shuttle effect and slow polysulfides redox kinetics. Single atom alloys (SAAs) with the advantages of single-atom and nanoparticle catalysts are still rarely studied in the field of Li–S batteries. Herein, a NiCo SAA was supported on MXene (NiCo_SAA-MXene), which served as the cascade electrocatalyst for improving the Li⁺ desolvation and polysulfides conversion kinetics. In this design, the special structure of the single atom alloy can fully inhibit the mutual stacking of MXene and achieve good stability, while the MXene nanosheet serves as support to enable the uniform dispersion of single atom alloy with ultra-small particle size, facilitating maximum atom utilization. Moreover, the electron cloud of Co was redistributed with the assistance of Ni, resulting in an enhanced electrocatalytic performance, as confirmed by theoretical calculations. At the cascade catalysis of NiCo_SAA-MXene, more free Li⁺ was released, and the diffusion of Li⁺ was enhanced to participate in the polysulfides redox reaction, effectively inhibiting the shuttle effect of polysulfides, as proved by in-situ/ex-situ Raman and electrochemical characterization. As a result, the Li–S battery with NiCo_SAA-MXene modified separator achieved a reversible capacity of 992 mAh·g⁻¹ at 0.2 C after 100 cycles and a superior rate capability of 746 mAh·g⁻¹ at 4 C. At a high sulfur loading of 4.7 mg·cm⁻², the Li–S battery also maintains an excellent cycling stability (100 cycles, 3.1 mAh·cm⁻²), demonstrating significant promise for commercial applications.

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.