Binary Solvent Induced Stable Interphase Layer for Ultra‐Long Life Sodium Metal Batteries

Abstract Sodium foil, promising for high‐energy‐density batteries, faces reversibility challenges due to its inherent reactivity and unstable solid electrolyte interphase (SEI) layer. In this study, a stable sodium metal battery (SMB) is achieved by tuning the electrolyte solvation structure through the addition of co‐solvent 2‐methyl tetrahydrofuran (MTHF) to diglyme (Dig). The introduction of cyclic ether‐based MTHF results in increased anion incorporation in the solvation structure, even at lower salt concentrations. Specifically, the anion stabilization capabilities of the environmentally sustainable MTHF co‐solvent lead to a contact‐ion pair‐based solvation structure. Time‐of‐flight mass spectroscopy analysis reveals that a shift toward an anion‐dominated solvation structure promotes the formation of a thin and uniform SEI layer. Consequently, employing a NaPF 6 ‐based electrolyte with a Dig:MTHF ratio of 50% (v/v) binary solvent yields an average Coulombic efficiency of 99.72% for 300 cycles in Cu||Na cell cycling. Remarkably, at a C/2 cycling rate, Na||Na symmetric cell cycling demonstrates ultra‐long‐term stability exceeding 7000 h, and full cells with Na 0.44 MnO 2 as a cathode retain 80% of their capacity after 500 cycles. This study systematically examines solvation structure, SEI layer composition, and electrochemical cycling, emphasizing the significance of MTHF‐based binary solvent mixtures for high‐performance SMBs.