Cu‐Driven Active Cu 2 Se@MXene Heterointerface Reconstruction and Co Electron Reservoir Toward Superior Sodium Storage

Abstract Heterostructure engineering and active component reconstruction are effective strategies for efficient and rapid charge storage in advanced sodium‐ion batteries (SIBs). Herein, sandwich‐type CoSe 2 @MXene composites are used as a model to reconstruct new active Cu 2 Se@MXene heterostructures by in situ electrochemical driving. The MXene core provides interconnected pathways for electron and ion conduction, while also buffering volumetric expansion to stabilize the structure. This reconstructed Cu 2 Se@MXene heterointerface features abundant sodium storage active sites, enhanced Na + adsorption, and diffusion kinetics, thus increasing sodium storage capacity. Moreover, the elevated Co valence state during the discharge process allows it to act as an electron reservoir to provide additional electron supply for Cu 2 Se conversion and accelerate the sodium storage kinetics. When employed as an anode in SIBs, the CoSe 2 @MXene electrode exhibits high capacity (694 mAh g −1 at 0.1 A g −1 ), excellent rate performance (425 mAh g −1 at 20 A g −1 ), and exceptional durability (437 mAh g −1 after 10 000 cycles at 5 A g −1 with a 0.0014% capacity decay per cycle). The electrochemical reconstruction and sodium storage mechanism of Cu 2 Se@MXene anode is further revealed through ex situ characterization and theoretical calculations. This work provides a new approach for designing advanced conversion‐type anodes for SIBs.