MXene‐Confined Cobalt‐Rich Co 2 P 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 Co 2 P in SIBs by confining ultrasmall Co 2 P nanoparticles within Ti 3 C 2 T x 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 Co 2 P anodes to date. In situ and ex situ analyses reveal that MXene confinement enables highly reversible conversion reactions in Co 2 P anodes.