Synergistic deep eutectic solvent swelling and in-situ activation for engineering defect-rich coal-based porous carbon toward high-performance zinc-ion hybrid capacitors

Coal possesses a dense, cross-linked structure and strong π-π stacking interactions that hinder activator penetration and cause excessive graphitization, which limits its application in zinc-ion hybrid capacitors (ZIHCs). Herein, we propose a synergistic deep eutectic solvent (DES) swelling and in-situ air carbonization strategy to fabricate defect-rich, heteroatom-doped porous carbon from raw coal. Utilizing the ZnCl 2 -H 3 PO 4 DES, the non-covalent interactions of coal are disrupted, which significantly increases its fraction of free volume from 16.29% to 31.45% to facilitate deep activator penetration. During the subsequent air carbonization, the DES functions as a condensed-phase flame retardant to prevent over-burning, alongside serving as an in-situ activator and P dopant. The in-situ formation of thermally stable C-O-C bridge bonds introduces spatial steric hindrance that is proposed to promote a local hybridization shift from sp 2 toward sp 3 carbon. This effectively inhibits the tight stacking of aromatic sheets and suppresses excessive graphitization. The optimized carbon is characterized by a greatly disordered structure, a high specific surface area, rich sp 3 -C defective sites, a hierarchical pore structure, and P/N co-doping. Benefiting from these structural composition merits, the assembled flexible ZIHC with a ZnCl 2 -PVA gel electrolyte achieves a specific capacity of 248 mAh g −1 alongside an energy density of 174 Wh kg −1 . The synergy of DES swelling and in-situ activation offers a sustainable, efficient route for coal-based porous carbon, with potential to extend to diverse precursors for structurally and compositionally rich carbon materials.