In situ fragmented and confined CoP nanocrystals into sandwich-structure MXene@CoP@NPC heterostructure for superior sodium-ion storage

The investigation of suitable anode materials for sodium-ion batteries (SIBs) is highly appealing in order to tackle the obstacles of large electrode volume variation and sluggish charge kinetic caused by the larger radius of Na+ than that of Li+. Recently, cobalt phosphides (CoP) have attracted extensive attention for anode materials because of their relatively high theoretical capacity and electric conductivity. In light of that, we propose a new fabrication approach for the synthesis of layered zeolitic imidazolate framework-67 (ZIF-L)-derived CoP nanoparticles encapsulated into N, P co-doped carbon layers and conductive MXene substrate to form sandwich-structure MXene@CoP@NPC hybrids. Specifically, the MXene and wafer-like ZIF-L hybrids are used as precursor, which is treated with subsequent carbonation with low temperature (435 °C) for a long time of 8 h, and then coupled with phosphidation reaction. The MXene substrate not only provides abundant growing sites for ZIF-L, for avoiding aggregation of CoP nanoparticles, but also enhances the electric conductivity of sandwich-structure hybrid. Furthermore, the in situ formed carbon layer can effectively ensure stable structural integrity by buffering volume expansion and improving overall conductivity. Concurrently, CoP nanocrystals, outer carbon layer, and MXene substrate form stable interface by strong chemical interaction, inducing promoted surface charge transfer kinetic. Consequently, the MXene@CoP@NPC anode exhibits outstanding rate capability with a highly reversible capacity of 198 mAh/g at 5.0 A/g and long-time cycling performance with 155 mAh/g at 1.0 A/g after 1000 cycles (0.023% capacity loss per cycle).