A surface-induced assembly strategy to fabricate flexible carbon nanofiber/coal-based carbon dots films as free-standing anodes for high-performance sodium-ion batteries

Flexible electrodes with superior stability, high performance, and low cost have drawn numerous considerations in energy storage devices. Here, we developed a facile and robust strategy to prepare coal-based carbon dots (i.e., CDs) with high yields derived from lignite by microwave-assisted mild oxidation and further fabricate a carbon nanofiber film decorated with CDs (i.e., CNF@CDs) by electrospinning water-soluble CDs with polyacrylonitrile (i.e., PAN) through thermal treatment and carbonization and applied as flexible electrodes for sodium-ion batteries. The as-prepared CNF@CDs as binder-free anode presents a reversible capacity of 223 mAh/g at 20 mA g−1 and obtains a high capacity retention of 96 % over 400 cycles. The outstanding electrochemical properties were attributed to the unique structure of CNF@CDs with the optimal ratio of carbon nanofiber and coal-based carbon dots, in which carbon fiber with the 3D framework facilitates the speed of ion diffusion and electron transport as well as the surface-induced assembly of CDs on the surface of the PAN fibers brings in strong structure stability and electrode integrity. The current study supplies a novel and effective approach to constructing flexible electrodes with superior stability and high performances from low-cost materials, which has potential application prospects in flexible energy storage devices.