Free-Standing, Self-Doped Porous Hard Carbon: Na-Ion Storage with Enhanced Initial Coulombic Efficiency

The use of porous hard carbons (PHCs) as electrode materials in sodium-ion batteries has great potential; however, the exposure of large surface areas to electrolyte flow results in irregular and irreversible solid electrolyte interfaces (SEIs), leading to deteriorated ionic and electronic mobility and inferior initial Coulombic efficiency (ICE). These issues can be addressed through suitable structural modifications of PHC materials. Herein, the integration of high-surface-area PHCs with carbon nanofibers (CNFs) was accomplished by a simple electrospinning technique, which resulted in a uniform and reversible SEI layer. In the meantime, the CNFs' mesh provided connectivity and conductivity in the as-integrated electrodes, whereas PHCs offered fast diffusion kinetics and high Na⁺ ion storage capacity. Additionally, PHC integration with CNFs demonstrated an excellent ICE of 77% and a specific capacity of 505 mAh/g at 25 mA/g. Furthermore, the conjugated microstructure also provided flexibility and stability to the electrode (260 mAh/g after 500 cycles). This remarkable synergy may promote the development of free-standing, flexible, and highly porous properties in a single material for advanced energy storage applications.