Hard carbon/graphene microfibers as a superior anode material for sodium-ion batteries

Hard carbon is one of the most promising anode materials for sodium-ion batteries (SIBs). However, it still suffers from low specific capacity and poor rate capability, which impede its practical application. Herein, one-dimensional hard carbon/graphene microfibers are synthesized by encapsulating hard carbon particles in graphene nanosheet using a wet-spinning technique followed by pyrolysis treatment. Benefiting from the unique structure, when used as the anode material for SIBs, the as-obtained hard carbon/graphene microfibers not only exhibit a high reversible capacity of 321 mAh g⁻¹ at 0.1 C but also deliver long cycling stability and excellent rate capability, maintaining its capacity as 203 mAh g⁻¹ even after 350 cycles at 1 C. In addition, the sodium storage mechanism of the as-prepared hard carbon/graphene microfibers is investigated by in-situ Raman technique. Moreover, a full cell adopting the hybrid microfiber as the anode and Na3V2(PO4)3 as the cathode exhibits high specific capacities of 291 mAh g⁻¹ and 204 mAh g⁻¹ at 0.2 C and 4 C (vs. anode), respectively. This work provides a sustainable and cost-effective strategy for the synthesis of high-capacity and stable hard carbon anode materials.