Regulating the microstructure of cross-linked starch-derived hard carbon for highly reversible sodium-ion storage

Optimizing the interlayer spacing and closed porosity of hard carbon (HC) is crucial for enhancing the reversible capacity of sodium-ion batteries (SIBs). Nevertheless, the unclear structure-performance relationship and the sodium storage mechanism significantly hinder the precise tuning of HC structures. Herein, this paper reports an esterified cross-linked starch-derived HC material and investigates its evolution mechanisms of microcrystalline carbon layers and closed porosity at various carbonization temperatures. By adjusting the spacing of the carbon layers and the distribution of closed pores, effective sodium ion storage is achieved. The optimized HC exhibits an initial Coulombic efficiency (ICE) of 90 % and a high reversible capacity of 376 mAh g- 1 , including a plateau capacity of 246 mAh g- 1 . After 100 cycles at 0.5 C, it retains a high capacity retention rate of 99.6 %. The sodium ion storage mechanism is investigated in depth using galvanostatic intermittent titration technique (GITT) and in-situ Raman, revealing that the plateau capacity is related to the insertion of sodium ions between carbon layers and the filling in pores, providing new evidence for the "adsorption-intercalation/filling" mechanism. Moreover, the assembled full-cell achieves a high energy density of 212 Wh kg- 1 .