Design of a Solid Electrolyte Interphase for Aqueous Zn Batteries

Aqueous Zn batteries are challenged by water decomposition and dendrite growth due to the absence of a dense Zn‐ion conductive solid electrolyte interphase (SEI) to inhibit the hydrogen evolution reaction (HER). Here, we design a low‐concentration aqueous Zn(OTF)2‐Zn(NO3)2 electrolyte to in situ form a robust inorganic ZnF2‐Zn5(CO3)2(OH)6‐organic bilayer SEI, where the inorganic inner layer promotes Zn‐ion diffusion while the organic outer layer suppresses water penetration. We found that the insulating Zn5(OH)8(NO3)2⋅2 H2O layer is first formed on the Zn anode surface by the self‐terminated chemical reaction of NO3− with Zn2+ and OH− generated via HER, and then it transforms into Zn‐ion conducting Zn5(CO3)2(OH)6, which in turn promotes the formation of ZnF2 as the inner layer. The organic‐dominated outer layer is formed by the reduction of OTF−. The in situ formed SEI enables a high Coulombic efficiency (CE) of 99.8 % for 200 h in Ti∥Zn cells, and a high energy density (168 Wh kg−1) with 96.5 % retention for 700 cycles in Zn∥MnO2 cells with a low Zn/MnO2 capacity ratio of 2:1.