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

Abstract 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.