N-doped carbon-coated CoSe₂ nanocrystals anchored on two-dimensional MXene nanosheets for efficient electrochemical sodium- and potassium-ion storage

Sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) have received much attention as next-generation energy storage systems owing to the abundance. Nevertheless, they face great challenges in the design of optimum electrode materials for practical applications. Herein, we prepared a unique two-dimensional structured composite consisting of N-doped carbon-coated CoSe₂ nanocrystals and MXene nanosheets (CoSe₂@NC/MX) for efficient electrochemical Na- and K-ion storage. In this strategy, a Co-based zeolitic imidazole framework (ZIF-67) was deposited on the surface of MXene nanosheets, and the subsequent selenization process resulted in the transformation of the ZIF-67 into CoSe₂@NCs. The unique structure can shorten the transport pathways for electrons/ions and provide sufficient space to accommodate the volume change of the active materials. Furthermore, the N-doped carbon matrix and MXene can enhance the robustness of the electrode materials. Accordingly, the composites exhibited enhanced electrochemical performance in terms of cycle stability (317 mA h g⁻¹ after 200 cycles at 0.5 A g⁻¹) and rate capability (343 mA h g⁻¹ at 7.0 A g⁻¹) for SIBs. For KIBs, they exhibited a high reversible capacity of 358 mA h g⁻¹ at 0.5 A g⁻¹ after 100 cycles and 276 mA h g⁻¹ at a high rate of 2.0 A g⁻¹.