Microstructural Evolution of Zinc-Ion Species from Aqueous to Hydrated Eutectic Electrolyte for Zn-Ion Batteries

Despite their intrinsic safety and environmental friendliness, typical aqueous Zn-ion rechargeable batteries have been struggling with poor reversibility and electrochemical stability. Hydrated eutectic electrolytes (HEEs) have been attracting extensive attention due to their appealing features of high designability and superior performances over typical aqueous electrolytes. However, an in-depth understanding of unique microstructure in HEEs and the ensuing superior performances remains obscure, limiting the development of enhanced electrolytes. Herein, we demonstrate a distinct evolution path of Zn-ion species from aqueous to superior hydrated eutectic electrolytes, which experience a special transition state enriched with H-bonds between eutectic molecules. Complementary with the well-studied reorganized solvation structure induced by short-ranged salt-solvent interaction, long-range solvent-solvent interactions arising from the H-bond reorganizes the extended electrolyte microstructure, which in turn influences the cation diffusion mechanisms and interfacial reaction kinetics. Overall, we highlight the importance of ion species microstructural evolution in the rational design of superior aqueous electrolytes.