Hard carbon with tailored microstructure via thermal regulation for high-efficiency sodium-ion batteries

Hard carbon (HC) is the most promising commercially available anode material for sodium-ion batteries (SIBs) due to the high specific capacity, low operation voltage and low cost. However, HC faces the problems of poor initial Coulombic efficiency (ICE) and ambiguous sodium storage mechanism. Furthermore, the relationship between solid electrolyte interphase (SEI) characteristics, particularly its chemical composition and microstructural features, and electrochemical performance remains poorly understood. To this end, HC with abundant closed pores and adjustable defect concentration was prepared in this study using cheap pine bark as raw material through precise thermal regulation. At high pyrolysis temperature, the carbon layer in HC fully grows and promotes the growth of closed pore. The optimized PHC-1300 exhibits a high ICE of 89.6% and an outstanding specific capacity of 347.62 mAh g^-1 at 0.1C. Moreover, the PHC-1300//Na₃V₂ (PO₄)₃ full-cells also exhibit excellent cycling performance. Based on the electrochemical performance and microstructure of the pine bark-based HC, it is proposed that the sodium storage mechanism is "adsorption-intercalation-filling". Notably, it is found that the HC surface with suitable defect concentration can induce the formation of fluorine-rich organic SEI phase, which is beneficial to maintain the interfacial stable to improve the cycling stability.