Engineering Solid Electrolyte Interface at Nano‐Scale for High‐Performance Hard Carbon in Sodium‐Ion Batteries

Abstract Engineering the structure and chemistry of solid electrolyte interface (SEI) on electrode materials is crucial for rechargeable batteries. Using hard carbon (HC) as a platform material, a correlation between Na + storage performance, and the properties of SEI is comprehensively explored. It is found that a “good” SEI layer on HC may not be directly associated with certain kinds of SEI components, such as NaF and Na 2 O. Whereas, arranging nano SEI components with refined structures constructs the foundation of “good” SEI that enables fast Na + storage and interface stability of HC in Na‐ion batteries. A layer‐by‐layer SEI on HC with inorganic‐rich inner layer and tolerant organic‐rich outer flexible layer can facilitate excellent rate and cycling life. Besides, SEI layer as the gate for Na + from electrolyte to HC electrode can modulate interfacial crystallographic structures of HC with pillar‐solvent that function as “pseudo‐SEI” for fast and stable Na + storage in optimal 1 m NaPF 6 ‐TEGDME electrolytes. Such a layer‐by‐layer SEI combined with a “pseudo‐SEI” layer for HC enables an outstanding rate of 192 mAh g −1 at 2 C and stable cycling over 1100 cycles at 0.5 C. This study provides valuable guidance to improve the electrochemical performance of electrode materials through regulation of SEI in optimal electrolytes.