Hard carbon with embedded graphitic nanofibers for fast-charge sodium-ion batteries

The development of fast-charging sodium-ion batteries need the anode to have a high rate capacity with a long and reversible charging plateau at low voltage (<0.1 V). Hard carbons are extensively investigated as the anodes for sodium-ion batteries, but slow charge-transfer kinetics and low reversible capacity at low potential region are still major obstacles for their practical applications. Herein, we develop a facile strategy via in-situ carbonization of the cross-linking network of bacterial cellulose and resin to synthesize hard carbons with intrinsically embedded graphitic nanofibers. The nanostructured hard carbons anodes have a reversible capacity of 345 mAh g-1 and an ultra-large low-voltage plateau capacity of 165.5 mAh g-1 (72.7% of the reversible capacity) at 2 A g-1. Kinetics and mechanism studies reveal that the embedded graphitic nanofibers greatly boost the charge transfer and ionic diffusion, i.e. the Na+ diffusion coefficient at the plateau region can be readily improved from ≈10-10.2 to ≈10-9.0 cm2 s-1. Evidence from in/ex-situ transmission electron microscopy (TEM) demonstrates that the graphic nanofibers, with an expanded interlayer spacing, provide sufficient diffusion channels for Na+ ions' migration and storage. Full-cell sodium-ion batteries using the nanostructured hard carbon as anodes achieve superior fast-charge capability, showing great potential applications of the nanostructured hard carbon in the low-cost and environmentally friendly energy storage devices.