Achieving a Highly Reversible Four-Electron Redox of S/Cu₂S for Aqueous Zn/S─Cu Battery

The combination of sulfur (S) cathode with Cu²⁺/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(H₂O)₆ ²⁺ and Cu(H₂O)₂ ⁺, the strategy of "spatial confinement" is proposed to alter the oxidation path of Cu₂S 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 S₈ 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.