Tailored Polymer-Inorganic Bilayer SEI with Proton Holder Feature for Aqueous Zn Metal Batteries

Conventional solid-electrolyte interface (SEI) in aqueous Zn-ion batteries mainly acts as a physical barrier to prevent hydrogen evolution reaction (HER), while such SEI is prone to structural deterioration stemming from uneven Zn deposition at high current densities. Herein, we propose an in situ structural design of polymer-inorganic bilayer SEI with a proton holder feature by aniline-modulated electrolytes. The Zn(OTF)₂ exhibits a lower LUMO energy level in comparison to aniline, resulting in the formation of a bilayer structure characterized by an inner ZnF₂ layer and an outer polyaniline (PANI) layer. The ZnF₂> with high stiffness and strength effectively suppresses Zn dendrites. Meanwhile, the PANI regulates the current distribution, minimizing the concentration gradient, and delays the Sand's time of dendrites growth. Furthermore, the =N− in PANI is capable of reversible proton holder, thereby inhibiting HER. With this bilayer SEI, the Zn anode achieves an impressive cycle life of 126 h under 40 mA cm⁻² & 40 mAh cm⁻² (depth of discharge, DOD=70.8 %), solving the bottleneck of single-layer inorganic SEI that could not be cycled under these conditions. The Zn || NaVO pouch battery with bilayer SEI exhibits a high capacity of 1.2 Ah and a cycle life of 350 h with 78 % capacity retention. At −30 °C, the same battery delivers a capacity of 335 mAh and a cycle life of 507 h with 72 % capacity retention, attributed to the modulation mechanism of the hydrogen bonding in the electrolyte. Our findings offer profound insights into the design of SEI with tailored structure and functionality, paving the way for the next generation of advanced high-performance batteries. © 2025 Wiley-VCH GmbH.