Cross‐Linking Polycondensation‐Driven Topology Structure Modulation of Pitch for Fabricating Optically Isotropic Hard Carbon

ABSTRACT To address the technical bottleneck of low soft point pitches being difficult to fabricate high‐performance hard carbon materials via conventional oxidative cross‐linking methods, this study proposes a novel cross‐linking strategy based on Friedel–Crafts alkylation. Medium–low temperature pitch as the precursor, a three‐dimensional topological network interconnected by methylene‐bridged, was successfully constructed by introducing p‐xylene glycol as a cross‐linker and boric acid catalytic system. The cross‐linking conditions precisely control the molecular cross‐linking degree of the pitch precursor, while the methylene‐bridged topology substantially improves its thermal stability, resulting in a threefold increase in residual carbon yield relative to the untreated pitch. This distinctive architectural configuration critically inhibits the linear molecular alignment of pitch during carbonization, thereby facilitating the formation of an optically isotropic hard carbon material. This material exhibits a distinct structural organization featuring short‐range order while maintaining long‐range disorder, accompanied by significantly reduced stacking layers ( R = 3.53) and notably expanded interlayer spacing (d 002 = 0.377 nm). The defect engineering arising from the self‐doping effects of oxygen (originating from the pitch precursor) and boron (derived from the boric acid catalyst) during carbonization, synergistically coupled with the abundant closed‐pore structures and pseudo‐graphitic domains observed via HRTEM, is anticipated to collaboratively enhance the sodium‐ion storage performance of the electrode material. This study provides a pioneering approach to the synthesis of optically isotropic hard carbons, and the resulting materials are expected to show significant potential for use in alkaline‐ion battery anode materials.