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

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.