Solid Electrolyte Interphase Design 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 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 bi-layer SEI where the inorganic inner layer promotes Zn-ion diffusion while the organic outer layer suppresses water penetration. Comprehensive characterization reveals that insulating Zn5(OH)8(NO3)2·2H2O layer is first formed on Zn anode surface by 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 promots ZnF2 formation 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.