Tailoring zinc-ion deep eutectic electrolytes through dual regulation of solvation shell volume and solvent polarity

Abstract

The tunability of Lewis acid–base and hydrogen-bonding (HB) interactions in deep eutectic electrolytes (DEEs) provides an effective strategy for addressing vital challenges in zinc-ion batteries. However, in aqueous DEEs, undesirable intermolecular forces severely obstruct ion hopping, as reflected in high solute viscosity and transport behavior. The tunability of Lewis acid–base and HB interactions remains effective only over short distances and becomes ineffective over longer ranges. The Stokes–Einstein formula and Debye–Hückel theory, based on Kohlrausch's law, describe the transport mechanism of cations in non-ideal fluids, where a small solvation shell enables faster ion migration under an electric field. The Et(30) parameter, which captures the trade-off between specific HB interactions and solvent polarity, serves as a key descriptor for tailoring the desolvation kinetics of the solvation sheath. Based on systematic screening of solvation shell volumes and solvent polarity, acetonitrile (AN), with a small solvation shell volume and moderate polarity, is the optimal choice. Furthermore, the maximal decoupling of the system from the environment validates the concept of long-range efficient ion transport. Ultimately, the DEE system achieves a fourfold enhancement in ionic conductivity (8.3 mS cm⁻¹) and exceptional cycling durability in a Zn–I₂ pouch cell, retaining a reversible capacity of 95.4 mAh g⁻¹ after 1500 cycles at 1 A g⁻¹. This work significantly narrows the theoretical gap between aqueous and non-aqueous battery systems.