Mixed‐Valence Copper Selenide as an Anode for Ultralong Lifespan Rocking‐Chair Zn‐Ion Batteries: An Insight into its Intercalation/Extraction Kinetics and Charge Storage Mechanism

Abstract Environment‐friendly and low‐cost aqueous zinc‐ion batteries (ZIBs) have received considerable attention for large‐scale energy storage. However, the low coulombic efficiency and potential safety hazards of Zn‐metal anodes severely hinder their practical implementations. Herein, for the first time, mixed‐valence Cu 2− x Se is proposed as a new intercalation anode to construct Zn‐metal‐free rocking‐chair ZIBs with a long lifespan. It is found that the introduction of low‐valence Cu not only modify active sites for Zn 2+ ion storage, but also optimizes the electronic interaction between the active sites and the intercalated Zn 2+ ion, leading to a favorable intercalation formation energy (−0.68 eV) and reduced diffusion barrier, as demonstrated by first‐principles calculation. Ex situ X‐ray diffraction, ex situ transmission electron microscopy and galvanostatic intermittent titration technique measurements reveal the reversible insertion/extraction of Zn 2+ in Cu 2− x Se via an intercalation reaction mechanism. Owing to the rigid host structure and facile Zn 2+ diffusion kinetics, the Cu 2− x Se nanorod anode shows an enhanced coulombic efficiency (above 99.5%), outstanding rate capability and excellent cycling stability. The as‐fabricated Zn x MnO 2 ||Cu 2− x Se Zn‐ion full battery exhibits an impressive electrochemical performance, particularly an ultralong cycle life of over 20 000 cycles at 2 A g −1 . This study is expected to provide new opportunities for developing high‐performance rechargeable aqueous ZIBs.