Free-standing hard carbon anode based on cellulose nanocrystal-reinforced chitosan substrate for eco-friendly sodium-ion batteries

Hard carbon (HC) is recognized as a promising anode material for sodium-ion batteries due to its high capacity and low operating voltage. However, the usage of polyvinylidene fluoride (PVDF) binder leads to a shortened HC cycle life because of the high reactivity of PVDF toward the carbonate ester-based electrolyte. Herein, we have reported the synthesis of a cellulose nanocrystal (CNC) reinforced chitosan (Ch)-derived free-standing HC anode, which features hydroxyl surface chemistry on HC and flexible framework. The CNC-Ch not only serves as a binder and a substrate but also increases the Na-ions adsorption and diffusion. The CNC content induced the pseudocapacitive behavior and enhanced the cycle stability of the HC anode. The CNC of 6 % exhibits an initial discharge capacity of 285 mAh g−1, an initial Coulombic efficiency (ICE) of 82 %, and a reversible capacity of 244 mAh g−1 at a current density of 25 mA g−1 for 5 cycles. For 100-cycle charge-discharge, it showed excellent cycle stability, retaining 67 % of its capacity in the first 50 cycles at 25 mA g−1 and 82 % in the subsequent 50 cycles at 50 mA g−1. This finding suggests that the free-standing HC anode in CNC-Ch presents a new opportunity for the development of a low-cost, stable, flexible, and environmentally sustainable anode for SIB application.