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 Na₂O and Na₂CO₃, 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.