Four‐Electron Transfer Reaction Endows High Capacity for Aqueous Cu–Se Battery

Abstract Selenium as a potential alternative cathode material for storing metal ions exhibits a series of advantages such as favorable electronic conductivity, attractive volumetric specific capacity, and mass density. However, due to the shortage of feasible redox couples for conversion reactions, the development of Se‐based batteries has been seriously impeded. Herein, based on the quantitative analysis of chemical thermodynamic properties, a novel conversion‐type Cu–Se battery is constructed with Cu 2+ as the variable‐valence charge carrier. Benefitting from the synergism between the variable‐valence charge carrier and the chemical stability of copper selenides, the Cu–Se battery delivers a high initial reversible capacity of 1045.4 mAh g −1 at 0.1 A g −1 , exceptional long‐span cycling stability with 89.7% capacity retention over 1500 cycles at 2 A g −1 , and outstanding rate performance. Under a joint theoretical and experimental study from density functional theory calculations, X‐ray diffraction, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy, it is demonstrated that the energy storage mechanism is a stepwise four‐electron conversion process of Se↔Cu II Se↔Cu I 2 Se. Thus, this work establishes an available aqueous Cu–Se battery with excellent performance and paves the way for exploring more promising metal–Se batteries by screening thermodynamic parameters.