Binary Solvation Engineering Unlocks Ultra-Stable Cycling of Alloy Foil Anodes in Sodium-Ion Batteries

Alloy anodes offer high theoretical capacities for sodium-ion batteries (SIBs) but are plagued by severe volume fluctuations and unstable solid electrolyte interphases (SEIs). Here, a low-salt binary electrolyte composed of dimethoxyethane (DME) and tetrahydrofuran (THF) is reported, in which the cyclic ether effectively regulates the solvation structure to enhance both interfacial stability and Na⁺ transport. Molecular dynamics simulations and spectroscopic analyses reveal that THF incorporation promotes contact ion pair-dominated coordination, even at low salt concentrations. This tailored solvation environment drives the formation of a thin, NaF-rich SEI with high mechanical robustness and ionic conductivity, suppressing Sn foil expansion and enabling homogeneous Na⁺ infusion into the bulk. As a result, Sn||Na half-cells achieve exceptional cycling stability-up to 8 920 cycles at 1 mAh cm^-2 with 99.95% Coulombic efficiency, alongside robust performance at 3 and 10 mAh cm^-2. Full cells with Na₃V₂(PO₄)₃ cathodes further demonstrate long-term stability and high retention. This study establishes solvation engineering as a powerful strategy for electrolyte design, enabling durable alloy foil anodes and advancing the practical deployment of high-performance SIBs.