Regulating the Interlayer Spacings of Hard Carbon Nanofibers Enables Enhanced Pore Filling Sodium Storage

Hard carbon (HC) represents an attractive anode material for sodium-ion batteries. However, most HC materials deliver limited capacity and the sodium storage mechanisms in the slope and plateau regions are controversial. Herein, a series of hard carbon nanofibers (HCNFs) with tunable interlayer spacings are designed to understand the sodium storage manners in HC. The optimized HCNFs featuring short-range graphitic layers with sufficient interlayer spacings (0.37-0.40 nm) for Na⁺ intercalation deliver a high reversible capacity (388 mAh g^-1 at 30 mA g^-1 ) and good rate capability. In-situ X-ray diffraction and Raman characterizations reveal a revised adsorption/insertion-filling sodium storage mechanism. Combined with the density functional theory (DFT) calculation, the detailed relationship between pore-filling plateau capacity and interlayer spacing is disclosed. It is found that sufficient interlayer spacings (>0.37 nm) provide diffusion channels for Na⁺ to reach the pores for further filling. Additionally, the reason for plateau-region capacity degradation of the HCNFs is completely demonstrated. This contribution provides insights into the sodium storage mechanism and rational construction of high-performance HC anode materials.