Polyvinyl pyrrolidone coated hard carbon spheres as a high performance anode for sodium-ion batteries

Hard carbon (HC) has broad prospects as anode material for sodium-ion batteries (SIBs). However, the low initial coulombic efficiency (ICE) and poor cycle stability limit its further development, due to the formation of unstable solid electrolyte interphase (SEI) layer during the charge and discharge process. Herein, a simple coating strategy was used to modify sucrose-based hard carbon spheres, which effectively improved the ICE and cycle stability of HC. Sucrose is used as the carbon source, and low-cost and low-toxicity polyvinyl pyrrolidone (PVP) is utilized as the raw material for the coating layer. The PVP coating layer can effectively reduce the contact between HC spheres and the electrolyte, thereby inhibiting the decomposition of the electrolyte and the formation of the SEI layer, and thus obtaining enhanced ICE. In addition, the N element in the coating layer can improve the conductivity of the HC material, thereby improving the electrochemical performance. Specifically, the optimal HC@NC-7 anode delivers a reversible capacity of 297.5 mAh g-1, an ICE of 67.8%, and excellent cycling stability (88.8% capacity retention after 200 cycles). Therefore, this work provides a simple and low-cost strategy to develop high-performance hard carbon anode materials for SIBs.