Exfoliated Coal-Derived Hard Carbon Enabling the Trade-Off Between Plateau Capacity and Fast Na+ Kinetics for Sodium-Ion Batteries

Coal is an attractive precursor for hard carbon anodes in sodium-ion batteries. However, these carbons often face the challenge of achieving high capacity and fast Na* kinetics simultaneously. Herein, we propose an oxidizedcoal precursor-derived hard carbon that exhibits a sheet-like structure and a tunable interlayer spacing, which addresses the aforementioned problem. A H2O2/H2SO4 chemical oxidation-exfoliation produces an oxidized-coal precursor with nanosheet morphology enriched in -COOH/-OH. -OH groups. These functional groups induce premature crosslinking of organic macromolecules, constructing a turbostratic carbon framework that suppresses ordered layer growth and expands the interlayer spacing. As the carbonization temperature further increases, polycondensation and structural reorganization are enhanced, driving more compact stacking of carbon layers. This enables a controllable decrease in interlayer spacing accompanied by the evolution of closed pores. The result small microcrystallite size with expanded interlayer spacing reduces Na* intercalation/diffusion resistance. The optimized sample exhibits a capacity of 327 mAh g-1, including a high plateau capacity of 191 mAh g-1. Note that the capacity of 214 mAh g-1 at an ultra-high current density of 10 A g-1 is retained, much higher than the previous reports. This work provides a new insight into the preparation of high-power, high-energy coal-based hard carbon for advanced sodium ion batteries.