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

Abstract 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||Na 3 V 2 (PO 3 ) 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.