Asymmetric Anion Zinc Salt Derived Solid Electrolyte Interphase Enabled Long‐Lifespan Aqueous Zinc Bromine Batteries

Abstract Organic additives with high‐reduction potentials are generally applied in aqueous electrolytes to stabilize the Zn anode, while compromise safety and environmental compatibility. Highly concentrated water‐in‐salt electrolytes have been proposed to realize the high reversibility of Zn plating/stripping; however, their high cost and viscosity hinder their practical applications. Therefore, exploring low‐concentration Zn salts, that can be used directly to stabilize Zn anodes, is of primary importance. Herein, we developed an asymmetric anion group, bi(difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (DFTFSI ‐ )‐based novel zinc salt, Zn(DFTFSI) 2 , to obtain a high ionic conductivity and a highly stable dendrite‐free Zn anode. Experimental tests and theoretical calculations verified that DFTFSI − in the Zn 2+ solvation sheath and inner Helmholtz plane would be preferentially reduced to construct layer‐structured SEI films, inhibiting hydrogen evolution and side reactions. Consequently, the Zn Zn symmetric cell with 1M Zn(DFTFSI) 2 aqueous electrolyte delivers an ultralong cycle life for >2500 h outperforming many other conventional Zn salt electrolytes. The Zn Br 2 battery also exhibits a long lifespan over 1200 cycles at ~99.8 % Coulombic efficiency with a high capacity retention of 92.5 %. Furthermore, this outstanding performance translates well to a high‐areal‐capacity Zn Br 2 battery (~5.6 mAh ⋅ cm ‐2 ), cycling over 320 cycles with 95.3 % initial capacity retained.