Optimizing the Imidazolium Cation Alkyl Chain Length in Water‐in‐Ionic Liquid Electrolytes for Enhanced Interfacial Stability in Aqueous Zinc‐Ion Batteries

Abstract Aqueous Zn‐ion batteries (AZIBs) show promise for large‐scale energy‐storage applications due to Zn being Earth‐abundant and their stability. Nevertheless, the translational viability of these systems is curtailed by nonuniform Zn electrodeposition, particularly dendritic growth, and by multiple parasitic reactions occurring at the Zn | electrolyte interphase. To resolve these issues, hybrid electrolytes are prepared by first making a 2 m Zn(OTf) ₂ aqueous solution, which is subsequently mixed with imidazolium‐based ionic liquids—1‐ethyl‐3‐methylimidazolium (EMIm), 1‐butyl‐3‐methylimidazolium (BMIm), and 1‐hexyl‐3‐methylimidazolium (HMIm)—at a 1:1 (v/v) ratio. After mixing, the final hybrid electrolyte contained 1 m Zn(OTf) ₂ . Short alkyl chain length EMIm⁺ is preferentially adsorbed onto the Zn(100) and Zn(101) facets, thereby suppressing needle‐like dendrite formation and promoting uniform Zn deposition, whereas an OTf‐derived inorganic‐organic composite solid electrolyte interphase mitigates parasitic interfacial reactions. Consequently, an ammonium vanadate nanofiber∥Zn half‐cell retains 82% of its initial capacity after 3600 cycles at 2 A g ⁻¹ , while an EMIm‐based Zn∥Zn symmetric cell operates stably for more than 830 h at 1 mA cm ⁻² , thereby corroborating the electrolyte's interfacial robustness. This study reveals that choosing the appropriate alkyl chain length of the imidazole cation in a hybrid electrolyte enables highly reversible Zn chemistry and long‐term cycling stability in AZIBs.