Engineering NiCo single atom alloy on MXene as cascade catalyst for high-performance Li–S batteries

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.