Bulky Molecules Tailor Solvent Packing Fraction of Electrolytes for High‐Voltage Sodium‐Ion Battery

ABSTRACT High‐voltage sodium‐ion batteries (SIBs) suffer from rapid degradation caused by electrolyte oxidation and interfacial parasitic reactions, which are strongly influenced by the local solvation environment near the electrode surface. However, conventional electrolyte design focuses on tuning Na + –solvent interactions while overlooking solvent packing within the solvation shell. Herein, we reveal that a bulky molecular solvent mediates restricted solvent packing around Na + , with its reduced packing efficiency quantitatively described by the fractal dimension ( D f ). The enlarged molecular volume creates a more open solvation environment, facilitating anion participation and promoting ion clusters dominated by contact ion pairs (CIPs) and small aggregates (SAGGs). The resulting inorganic‐rich interphase suppresses electrolyte oxidation and interfacial parasitic reactions, enabling stable high‐voltage operation. Sodium iron sulfate (NFS) paired with the bulky‐solvent electrolyte retains 80% capacity over 1407 cycles at 1 C. An Ah‐level HC||NFS pouch cell retains 81.53% capacity after 500 cycles. A practical pouch cell further achieves 115.05 Wh kg −1 and retains 80.87% of its capacity after 150 cycles. This work demonstrates that restricted solvent packing can modulate ion association and interfacial chemistry, providing a molecular design consideration for high‐energy SIBs.