Quantifying the Dissolution of Organic and Inorganic Components in Solid Electrolyte Interphase for Sodium‐Ion Batteries

ABSTRACT Anode interphase dissolution is a major cause of instability in sodium‐ion batteries (SIBs) because Na‐based solid electrolyte interphase (SEI) compounds are more soluble than their Li analogues. Here, glyme electrolytes with varied chain lengths are used to regulate Na + solvation structure and clarify its effect on SEI dissolution. A component‐resolved framework combining ion chromatography, mass spectrometry, electrochemical quartz crystal microbalance, and X‐ray photoelectron spectroscopy is developed to quantify the dissolution and retention of organic and inorganic SEI species. This approach determines absolute SEI retention rather than relying on aggregate interphase loss. Compared with the glyme, tetraglyme reduces total SEI dissolution from 75% to 53% and increases the retention of inorganic F‐containing species from 18% to 30%. This improvement arises from an anion‐enriched Na + solvation shell, which promotes the formation of a thinner, denser, and more inorganic‐rich SEI. Consequently, 1.25 Ah NaNi 1/3 Fe 1/3 Mn 1/3 O 2 ||hard carbon pouch cells using the tetraglyme‐based electrolyte retain 91.2% capacity after 500 cycles and exhibit stable voltage retention during 21‐day aging at 60°C. These results establish a quantitative methodology for distinguishing the dissolution behavior of organic and inorganic SEI components, reveal the role of ether chain length in interphase stability, and provide practical guidance for designing durable SIBs.