Starch-Based Hard Carbon Microspheres: Insights from Precursor Structure to Closed Pore Formation for Sodium Storage

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The structure of biomass precursors plays an important role in determining the structure of hard carbon. However, the effects of biomass structures on the closed pore sodium storage performance of the resultant hard carbon have yet to be explored comprehensively. As this limitation has hindered the progress of high-capacity hard carbon anodes, three types of starch samples with varying amylopectin contents─namely, amylopectin, corn starch, and high-amylose starch─were selected in this study to prepare hard carbon microspheres as anodes in sodium-ion batteries. Amylopectin, composed of both α-1,6 and α-1,4 glycosidic bonds, contains more hydroxyl groups than amylose. Starch with a high amylopectin content generates more C═O and C═C bonds through dehydration during stabilization, facilitating chain cross-linking. This cross-linking effect creates narrow and long graphite microcrystalline structures, leading to the formation of large closed pores during carbonization. The hard carbon microspheres derived from the precursor with a high amylopectin content exhibit a higher closed-pore area of 519 m 2 g –1, resulting in a higher reversible specific capacity of 333.2 mAh g –1 and remarkable cycling stability. This study reveals the structure–function relationship between the amylopectin content and closed pore formation in hard carbon, providing valuable insights into the future application of starch-based carbon in sodium-ion batteries.