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(NO 3 ) 2 electrolyte to in situ form a robust inorganic ZnF 2 ‐Zn 5 (CO 3 ) 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 Zn 5 (OH) 8 (NO 3 ) 2 ⋅2 H 2 O layer is first formed on the Zn anode surface by the self‐terminated chemical reaction of NO 3 − with Zn 2+ and OH − generated via HER, and then it transforms into Zn‐ion conducting Zn 5 (CO 3 ) 2 (OH) 6 , which in turn promotes the formation of ZnF 2 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∥MnO 2 cells with a low Zn/MnO 2 capacity ratio of 2:1.