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 Na2O-rich byproducts, respectively. In contrast, highly crystalline AB selectively induces an ultrathin, incomplete Na2CO3-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.