Cation–Anion Synergistic Chemistry Enables Localized Lean-Water and High-Potential-Difference Interfacial Engineering for Stable Hydrogen-Evolution-Free Zn-Metal Anodes

The hydrogen evolution reaction (HER) and uncontrolled deposition dynamics during Zn plating severely undermine the reversibility of Zn-metal anodes. The inner Helmholtz plane and resulting solid electrolyte interphase (SEI) are crucial in profoundly regulating the HER. We report a localized lean-water, high-potential-gradient interfacial design using thulium trifluoroacetate as an electrolyte additive. Trifluoroacetate anions form an anion-enriched layer on the Zn surface, serving as a proton-blocking barrier and inducing a locally water-deficient interface. This layer, together with an in situ ZnF₂-rich SEI, suppresses HER from H₂O decomposition. Concurrently, Tm³⁺-mediated electric double layers enhance Zn²⁺ transport and direct the growth of hexagonally arranged Zn with dominant (002) orientation. As a result, Zn∥Zn symmetric cells exhibit negligible H₂ generation and >1000 h stability at 20 mA cm^-2/5 mAh cm^-2, while Zn∥polyaniline pouch cells retain 82.3% capacity without swelling after 1000 cycles. This cation-anion synergistic strategy offers a route to hydrogen-evolution-free Zn-metal anodes.