Additive- and binder-free hard carbon nanofibers for sodium-ion batteries

Hard carbon (HC) has been the most promising negative electrode candidate for sodium-ion batteries (SIBs). However, its microstructure still needs to be optimized and better understood to improve sodium-ion storage and thus achieve widespread commercialization. In this study, self-supporting HC nanofibers as negative electrodes for SIBs were thoroughly investigated. This research focused on understanding the influence of the carbonization temperature (1000–1600 °C) on the microstructure and electrochemical performance. Higher carbonization temperatures result in more organized microstructures with fewer defects but do not necessarily result in the best specific capacity. Therefore, the HC nanofibers obtained at 1400 °C showed the best balance between the slope (adsorption) and plateau (intercalation) capacities, suggesting an optimized microstructure for sodium-ion storage. Additionally, operando Raman spectroscopy, GITT and ex situ HRTEM analysis were used to elucidate the Na⁺ storage mechanism, and the results suggested a multistage process involving adsorption, intercalation, and pore filling. In a practical comparison, self-supporting HC electrodes outperformed traditional ink-based HC electrodes; the corresponding reversible capacity and initial Coulombic efficiency (ICE) was 363 mAh g⁻¹ (78 %) for the HC14-nanofiber but only 98 mAh g⁻¹ (36 %) for the HC14-ink. This drastic reduction in the electrochemical performance of the ink-based electrode is due to microstructural modifications imposed by the conventional processing steps and the loss of microporosity caused by binder infiltration. This work contributes to the understanding of the Na⁺ storage mechanisms in HCs and highlights the potential of self-supporting HC nanofibrous electrodes for high-performance SIBs.