1 nm‐Level Solid Electrolyte Interphase on Coal‐Based Hard Carbon Enables Superior Sodium Storage

Solid electrolyte interphase (SEI) stands as a pivotal determinant of battery performance, governing ion transport and storage behavior, yet precise control over its thickness remains a formidable challenge. Here, we construct a 1 nm‐level SEI on coal‐based hard carbon through developing a synergistic regulation strategy toward surface chemistry and microstructure. Oxygen‐lean surface chemistry and typical micropore structure are created via phosphate‐directed oxygen and carbon etching in a confined microenvironment established by pitch light component surface coating. The surface oxygen content is remarkably reduced from 6.80 to 1.73 at.%, while the pore volume is enlarged by four times. The surface chemistry and structure properties of hard carbon contribute to the construction of 1 nm‐level SEI featured by an organic outer layer and an inner layer rich in Na 2 O and Na 2 CO 3 , which represents the lowest value in current hard carbon anodes of sodium‐ion batteries. Consequently, the as‐designed coal‐based hard carbon achieves superior initial coulombic efficiency (92.18%), reversible capacity (363 mAh g −1 ) and rate capability (231.2 mAh g −1 at 3 A g −1 ). This study provides a new material design approach to precise SEI thickness control, promising to inspire extensive research across diverse battery chemistries.