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 (d₀₀₂ = 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 m² 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.