Taming Zn Electrochemistry with Carbon Nitride: Atomically Gradient Interphase for Highly Reversible Aqueous Zn Batteries

The irreversibility issuesof metallic zinc (Zn) anode of low Coulombic efficiency, persistent parasitic reactions, and severe dendrite growth remain a fundamental, century‐old challenge hindering the practical applications in rechargeable aqueous batteries. Herein, a promising atomically gradient solid electrolyte interphase (SEI) strategy is demmonstrated, in which the bottom sublayer of atomic Cu dispersed carbon nitride tightly anchors the whole SEI layer onto Zn anode, whereas the top carbon nitride uniformizes Zn2+ flux, facilitates Zn2+ diffusion, and detaches the reactive water molecules. Theoretical simulations and structural analysis confirm the strong interactions of this SEI with Zn2+ ions that launch an ion‐sieving effect to enable single Zn2+ ion conduction, and the porous and stiff feature accommodates the deposition stress and volume change under plating/stripping, ensuring consistent conformal contact on the substrate meanwhile suppressing the generation of Zn protuberant tips. Representative X‐ray computed tomography study demonstrates the failure mode of the Zn anodes under aqueous electrolyte and verifies the homogeneous Zn electrodeposition behavior and spatially compact metallic structure in the presence of this hydrophobic‐zincophilic SEI. Consequently, dendrite‐free Zn plating/stripping at ≈99.2% Coulombic efficiency for 200 cycles, steady charge–discharge for 2000 h, and impressive full cell cyclability are achieved. Metallic zinc anode in aqueous electrolyte suffers from parasitic water decomposition and zinc dendrite growth; here, a gradient‐functionalized g‐C3N4 solid electrolyte interphase (SEI) is developed, on zinc surface by incorporating atomically dispersed Cu atoms on g‐C3N4, which assures the consistent conformal contact of the SEI with zinc anode and endows single zinc ion conduction pathway upon electrochemical cycling.