Aqueous zinc-ion batteries are promising energy storage systems owing to the abundance of zinc resources and the safety of aqueous electrolytes. However, direct Zn–electrolyte contact induces dendritic growth and side reactions, compromising cycle life. Herein, a mixed ionic-electronic conducting interphase is fabricated via a layer-by-layer (LBL) assembly of poly(diallyldimethylammonium chloride) (PDDA) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). This interphase i) directs Zn2⁺ flux, ii) maintains electronic conductivity for uniform field distribution, and iii) mitigates water-induced side reactions. Electrochemical and spectroscopic analyses confirm reduced Zn2⁺ desolvation energy and suppressed hydrogen evolution and corrosion. Consequently, Zn/Zn symmetric cells exhibit stable cycling over 1800 h at 1 mA cm−2, while five bilayers of (PDDA/PEDOT:PSS)-coated Zn/MnO2 full cells retain 3.92 mAh cm−2 after 350 cycles at 0.25 A g−1. This ecofriendly and cost-effective LBL strategy significantly enhances the energy density and cycle life of aqueous zinc-metal batteries, facilitating their industrial application.