Crosslinked starch-derived hard carbon with abundant closed pores for enhanced plateau capacity in sodium-ion storage

Starch is a widely available and low-cost biomass precursor of hard carbon anode for sodium-ion batteries (SIBs). However, hard carbons derived from direct starch carbonization often exhibit low plateau capacity and initial coulombic efficiency due to structural instability and suboptimal pore architecture. Herein, we propose a cross-linking pretreatment strategy using polyphosphoric acid to promote dehydrogenation and aromatization, suppress foaming and expansion during annealing. This approach enables the formation of a coral reef-like starch-derived hard carbon (CR-SHC) with uniformly distributed micropores and abundant closed-pore structures during carbonization, which significantly enhance sodium storage in the low-voltage plateau region. Accordingly, CR-SHC delivers a high capacity of 332 mAh g⁻¹ and an initial coulombic efficiency of 78 % at 0.1 A g⁻¹. Even at a high current density of 2 A g⁻¹, it maintains a capacity of 165 mAh g⁻¹, with the plateau region contributing up to 81 % of the total capacity. Furthermore, full cells assembled with NaNi_1/3Fe_1/3Mn_1/3O₂ cathodes exhibit excellent cycling stability, retaining 77 % of capacity after 150 cycles at 0.05 A g⁻¹. This work presents a simple and effective strategy for constructing advanced starch-based hard carbon anodes, offering new opportunities for the development of high-performance SIBs.