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.