Molecular Stitching in Polysaccharide Precursor for Fabricating Hard Carbon with Ultra‐High Plateau Capacity of Sodium Storage

High energy density of sodium-ion batteries (SIBs) requires high low-voltage capacity and initial Coulombic efficiency for hard carbon. However, simultaneously achieving both characteristics is a substantial challenge. Herein, a unique molecular stitching strategy is proposed to edit the polymeric structure of common starch for synthesizing cost-effective hard carbon (STHC-MS). A mild air-heating treatment toward starch is employed to trigger the esterification reaction between carboxyl and hydroxy groups, which can effectively connect the branched polysaccharide chains thereby constructing a highly cross-linked polymeric network. In contrast with the pristine branched-chain starch, the cross-linking structured precursor evolves into highly twisted graphitic lattices creating a large population of closed ultramicro-pores (<0.3 nm) enabling the storage of massive sodium clusters. Resultantly, STHC-MS delivers a reversible capacity of 348 mAh g-1 with a remarkable low-voltage (below 0.1 V) capacity of 294 mAh g-1, which becomes more attractive by combining the high initial Coulombic efficiency of 93.3%. Moreover, STHC-MS exhibits outstanding stability of 0.008% decay per cycle over 4800 cycles at 1 A g-1. STHC-MS||Na3V2(PO3)4 full cells achieve an energy density of 266 Wh kg-1, largely surpassing the commercial hard carbon-based counterpart. This work opens the avenue of molecular-level modulation in organic precursors for developing high-performance hard carbon in SIBs.