High‐Performance Na‐Storage and Sodiation Behavior Identification in Cellulose‐Derived Hard Carbon by High‐Resolution Element Mapping Microscopy

Nowadays, hard carbon is one of the most promising anode materials for sodium‐ion batteries. However, the Na‐storage performance of hard carbon varies for different precursors and synthetic processes due to disparities in the dopant content, dimensions, and categories of defects, graphitic domains, which lead to controversial explanations for the energy storage mechanism. Herein, self‐freestanding membrane of cellulose‐derived carbon fibers is presented, delivering a reversible Na‐storage capacity of 330 mAh g −1 at 50 mA g −1 due to abundant ordered graphitic zones with enlarged interlayer spacings around 0.39 ± 0.03 nm. Benefiting from a robust solid electrolyte interphase layer rich in NaF nanocrystals, the membrane also holds a superior rate capability of 100 mAh g −1 at 2 A g −1 and long‐term cycling stability with capacity retention of 82.6% at 1.5 A g −1 for 4000 cycles. A clear sodiation behavior of Na + intercalation into enlarged carbon graphitic layers at the slope step above 0.10 V and Na cluster evolution in the micropores at the plateau step around 0.01–0.10 V is disclosed by high‐resolution element mapping microscopy, different from the accepted mechanism of Na + intercalation into graphitic layers at the plateau step.