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 Na 3 V 2 (PO 4 ) 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.