Anion Engineering Toward Regulated Solvation Structure and Enhanced Performance in Hydrated Eutectic Electrolytes for Aluminum‐Ion Batteries

ABSTRACT The practical deployment of rechargeable aluminum‐ion batteries (AIBs) is hindered by the absence of electrolytes that concurrently offer low cost, efficient ion transport, and stable interfacial electrochemistry. While hydrated deep eutectic electrolytes (HEEs) present a promising avenue, their advancement has been limited by high viscosity and poorly controlled interfacial reactions. This work establishing anion engineering, rather than solely focusing on ligand modulation, as a critical yet previously underappreciated design strategy. We systematically elucidate how the identity of the anion governs the coordination architecture within an ethylene glycol (EG)–based HEE. Crucially, the NO 3 − anion induces the formation of a homogeneous, inner‑sphere Al 3+ solvation complex, which simultaneously reduces the dynamic viscosity, improves interfacial wettability, and elevates the cathodic stability limit. This tailored solvation structure lowers the desolvation energy barrier. As a result, the NO 3 − ‑HEE enables exceptional aluminum anode reversibility, evidenced by a high exchange current density and stable, dendrite‑free plating/stripping over 400 h. Full cells coupled with a CuHCF cathode deliver a specific capacity of ∼123 mAh·g −1 and demonstrate outstanding capacity retention over 500 cycles. This study provides a foundational anion‑centric design principle for next‑generation, high‑performance eutectic electrolytes.