Achieving a Highly Reversible Four‐Electron Redox of S/Cu 2 S for Aqueous Zn/S─Cu Battery

Abstract The combination of sulfur (S) cathode with Cu 2+ /Cu + redox carriers has been considered as a promising cathode for the next‐generation aqueous energy storage device due to the high specific capacity (two‐step four‐electron conversion), intrinsic safety, and low cost. Nevertheless, the unsatisfactory cycling stability of a sulfur–copper (S–Cu) cathode hinders its practical application. Herein, the two‐step four‐electron conversions are first decoupled in our study, identifying the inferior conversion reversibility between the intermedia CuS and the final product S as the primary cause for deteriorating cycling capability. Concerning the different hydrated dimensions of the Cu(H 2 O) 6 2+ and Cu(H 2 O) 2 + , the strategy of “spatial confinement” is proposed to alter the oxidation path of Cu 2 S and prevent the formation of the intermedia CuS. To ensure efficient S incorporation into the spatially confined carbon matrix, selenium (Se) is employed to “cut” the S 8 into short‐chain molecules. Consequently, the well‐designed S–Cu cathode achieves a one‐step four‐electron reaction and exhibits superior electrochemical stability with an average capacity attenuation of 0.034% during 700 cycles. Our study provides an in‐depth understanding of the conversion mechanism for the S–Cu cathode within the spatial‐confinement environment and renders valuable insights for developing advanced conversion‐type cathodes in aqueous energy‐storage device.