Optimizing the Imidazolium Cation Alkyl Chain Length in Water-in-Ionic Liquid Electrolytes for Enhanced Interfacial...

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.