Cross‐Scale Self‐Supporting Carbon Anode for Fast and Reversible Potassium Storage at Low Voltage

The trade‐off between low‐voltage activity and reversibility remains a central obstacle to achieving high‐performance carbon anodes in potassium‐ion batteries (PIBs). Here, we employ a cross‐scale structural modulation strategy in a flexible, self‐supporting hard carbon (SSHC) anode to realize fast and reversible potassium storage at low voltages. The SSHC consists of microscale carbon fibers (∼9 µm in diameter) that form a binder‐ and additive‐free framework, thereby limits surface side reactions, together with nanoscale stacked nanographtic layers (d 002 = 0.367 nm) that provide enlarged K + diffusion channels and stabilize low‐voltage insertion. Benefiting from this cross‐scale architecture, SSHC‐1100 electrode delivers 236.8 mAh g −1 at 30 mA g −1 , with 179.9 mAh g −1 below 0.5 V, outperforming graphite electrode (216 mAh g −1 in total, 150.4 mAh g −1 below 0.5 V). Despite its large specific surface area of 642.4 m 2 g −1 , the binder‐free self‐supporting fiber framework effectively suppresses irreversible side reactions and affords a high initial coulombic efficiency of 76.3%, higher than that of graphite (73.8%). These results indicate that coupling a microscale self‐supporting architecture with nanoscale diffusion pathways is a practical route to simultaneously achieve low‐voltage capacity and reversibility, and provide guidance for the design of advanced PIB anodes.