MXene-Confined Cobalt-Rich Co2P Nanoparticles Enable Highly Reversible Conversion Reaction for Stable and Fast Sodium-Ion Batteries.

Transition metal phosphides have attracted considerable attention as alternative conversion-type anodes for SIBs because of their high specific capacities. However, their sodium-storage mechanism typically shifts to an alloying reaction between P and Na+ after the initial sodiation process, leading to rapid capacity decay. Herein, we report a highly reversible conversion reaction of Co2P in SIBs by confining ultrasmall Co2P nanoparticles within Ti3C2Tx MXene interlayers. The resulting composites exhibit greatly improved sodium-storage performance, including a high reversible capacity (562 mAh g-1 at 0.2 A g-1 after 200 cycles), excellent rate capability (288 mAh g-1 at 10 A g-1), outstanding energy density (291 Wh kg-1), and cycling stability (84% capacity retention over 1000 cycles at 2 A g-1) in full cells, representing the best performance reported for Co2P anodes to date. In situ and ex situ analyses reveal that MXene confinement enables highly reversible conversion reactions in Co2P anodes.