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 H2 O 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.