Optimized Co–S bonds energy and confinement effect of hollow MXene@CoS₂/NC for enhanced sodium storage kinetics and stability

The sluggish kinetics and severe volume expansion are two major drawbacks that limiting the application of transition metal sulfides electrode materials for sodium ion batteries (SIBs). Herein, we assembled Ti₃C₂T_x MXene nanosheets into thin-walled hollow spheres with PMMA spheres as sacrificial template, on which MOF-derived CoS₂ nanoparticles embedded in N-doped carbon were grown (MXene@CoS₂/NC). The MXene@CoS₂/NC exhibits state-of-the-art sodium ion storage properties, including a high reversible capacity of 620 mAh g⁻¹ at 0.2 A g⁻¹, superior rate capability (394 mAh g⁻¹ at 5 A g⁻¹), and excellent cycling stability (355 mAh g⁻¹ after 5000 cycles). The corresponding electrochemical tests prove that the MXene@CoS₂/NC has fast Na⁺ ion diffusion and electron transfer compared with its counterpart without MXene interfaces. XPS and XANES characterizations disclose the introduced MXene and increased pyrrolic N can weaken the Co-S bonds of CoS₂, which facilitates the conversion reaction between CoS₂ and Na₂S, and thus improves the sodium storage kinetics. The DFT calculations also demonstrate the MXene can improve the conductivity of electrode materials by fast interfacial electron transfer. In addition, the MXene hollow spheres and MOF-derived NC provide host structures for CoS₂, which can increase the contact between electrolyte and active materials, buffer the volume expansion of CoS₂, and thus enhancing the electrochemical stability. This work provides a feasible strategy to construct anode materials for SIBs with improved sodium storage kinetics and cycling stability.