Unlocking the Facet‐Governed Zn Homoepitaxy Growth Induced by Dynamic <scp>l</scp>‐Theanine Evolution Process Realized Highly Reversible Zn Anodes

Abstract Achieving highly ordered and compact assembly of zinc (Zn) deposits, side‐reaction‐free, and no by‐product deposition manner of Zn anodes is crucial to avoid the premature failure for Zn‐ion batteries, which is jointly determined by the state of polycrystalline Zn substrates, interfacial microenvironment, epitaxial growth, crystal orientation, and their interaction. Herein, the progressive electrochemical behavior of Zn anodes, regulated by the converted species of l ‐theanine (THE) molecules from the hydrogel matrix, is uncovered. THE molecules are capable of accommodating the electrolyte environment and provide a weakly acidic condition at the infancy stage of electrodeposition, which induces an in situ acid etching process for releasing the residual stress of polycrystalline Zn substrates. This in situ acid etching process achieves lattice match between the substrate and overgrowth crystals, which renders the overgrowth crystals seamlessly amalgamate with the substrate to form the highly oriented and densely packed Zn deposit via the homoepitaxial growth combined with the converted THE + cations. Concomitantly, the interfacial adsorbed THE + cations facilitate the formation of the inorganic–organic hybrid solid electrolyte interphase layer. Consequently, the Zn||Zn cell with the THE‐filled hydrogel electrolyte achieves a significantly long cycling stability of 2000 cycles, with an ultrahigh average Coulombic efficiency of 99.0% over 1000 cycles.