Anion Engineering Toward Regulated Solvation Structure and Enhanced Performance in Hydrated Eutectic Electrolytes for...

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 NO₃ ^- anion induces the formation of a homogeneous, inner‑sphere Al³⁺ 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 NO₃ ^-‑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.