Beyond Passive Conductor: Conductive Carbon Black Dictates Interfacial Chemistry in Hard Carbon Anodes for Sodium‐Ion Batteries

Conductive carbon blacks (CCBs) are indispensable for electron‐conducting networks in sodium‐ion batteries (SIBs), yet their intrinsic electrochemical activities and impact on the solid electrolyte interphase (SEI) remain critically overlooked. This work decouples the microstructures of three commercial CCBs—Acetylene Black (AB), Super P (SP), and Ketjen Black (KB)—and their dynamic interfacial chemistries. Advanced cryo‐electron microscopy visualizes that CCBs act beyond passive conductors, actively dictating local SEI formation. Defective SP and highly porous KB trigger massive electrolyte decomposition, generating thick organic layers and Na 2 O‐rich byproducts, respectively. In contrast, highly crystalline AB selectively induces an ultrathin, incomplete Na 2 CO 3 ‐based SEI, which preserves intrinsic particle conductivity and minimizes interfacial impedance. As standalone anodes, AB delivers a superior reversible capacity of 194 mAh g −1 , significantly outperforming SP (152 mAh g −1 ) and KB (130 mAh g −1 ). Translating this advantage into practice, a hard carbon anode formulated with 10 wt% AB achieves an exceptional initial capacity of 324 mAh g −1 with 72% initial Coulombic efficiency and robust cycling stability (321 mAh g −1 after 100 cycles). This stands in stark contrast to the inferior efficiencies (∼64%) and severe capacity fading induced by SP and KB additives. This work provides a critical paradigm for formulating high‐efficiency sodium‐ion batteries.