Self‐Confined Semi‐Conversion Reaction Endowing an Ultra‐Long Life Aqueous Cu‐Cu ₂ Se Battery

Aqueous rechargeable metal-selenium batteries possess remarkable advantages including high theoretical capacity, safety, and reliability, endowing them as a new generation of large-scale electrochemical energy storage devices. Nevertheless, Se-based materials typically suffer from huge volume expansion and limited electrical conductivity during cycling, leading to subpar cycling performance. To address these scientific challenges, this paper creatively proposes a self-confined semi-conversion reaction mechanism (Cu₂Se↔CuSe) verified through in situ X-ray diffraction and ex situ transmission electron microscopy, where the charging and discharging products exhibit high crystallinity and reversible stress changes, facilitating its electrochemical performance. Hence, this battery exhibits high initial reversible capacity (230.5 mAh g^-1 at 0.5 A g^-1) and unimaginable cycling stability (a capacity retention of 100% after 15 000 cycles at 2.6 A g^-1). Through a multi-technique analysis combining in situ electrochemical impedance spectroscopy, relaxation time distribution measurements, and cyclic voltammetry, the superior performance originates from substantial pseudocapacitive contribution and minimized charge transfer resistance. This study provides new design principles and theoretical foundations for developing high-performance aqueous copper-based batteries.