Salt and sugar derived high power carbon microspheres anode with excellent low-potential capacity

Herein, a salt-assisted hydrothermal carbonization (HTC) strategy is applied to fabricate low-surface-area carbon microspheres (as low as 5.5 m² g⁻¹) for sodium ion batteries (SIBs) by using water containing eutectic salt melt (e.g. NaCl) and sugar (e.g. glucose) as reaction media. The small amount of salt increases the carbon conversion efficiency from 15.0 to 58.3%, and microsphere size from the nanoscale to the microscale. Meanwhile, the specific surface area of carbon microsphere is minimized and the microstructure is optimized. Ex-situ X-ray diffraction (XRD) and kinetic analysis revealed that the narrower lateral width of pseudographitic domains and lower micropore volume are the key factors to promote sodium storage ability and Na ion diffusion. The carbon microsphere anode delivers a capacity of 350 mAh g⁻¹ with 73.0% from the low potential (0–0.2 V) at 100 mA g⁻¹, a high initial Coulombic efficiency (ICE) of 86.1% (excluding conductive carbon black), and an excellent rate capability with capacity of 261 mAh g⁻¹ even at 500 mA g⁻¹. This research highlights a salt-assisted HTC method to synthesize low-surface-area carbon microspheres with superior ICE and energy/power density.