ABSTRACT The scalable synthesis of hard carbon (HC) with optimized pore structure and interfacial properties remains a challenge for alkali‐ion battery anodes. Herein, we propose a steam‐assisted pyrolysis (SAP) strategy that mitigates the low efficiency and safety concerns of conventional solvothermal methods, enabling cost‐effective and scalable control of carbon nanostructures. Glycerol (boiling point ≈ 290 °C) is introduced into phenolic resin to achieve controlled and synchronized steam release during thermal decomposition. The resulting internal steam pressure facilitates precursor decomposition at lower temperatures, suppresses graphitization, and promotes the formation of highly defective carbon with abundant closed nanopores. The optimized high‐boiling‐point‐derived HC (HB‐HC) thereby delivers high Na + and K + storage capacities of 413.1 and 396.5 mAh g −1 , respectively. Na‐ion full cells retain 93.6% capacity after 250 cycles at 0.1 A g −1 , with 68.8% capacity retention at 0.5 A g −1 . Ah‐level pouch cells achieve 80.2% retention after 300 cycles at 0.5 C and exhibit excellent low‐temperature performance. Simultaneously enhanced kinetics and cycling stability arise from defect‐mediated surface adsorption, stable NaF‐rich solid electrolyte interphase (SEI) and the formation of quasi‐metallic Na/K clusters within closed nanopores. This work presents a scalable and industrially viable approach to high‐performance HC, offering new insights into microstructure design for next‐generation alkali‐ion batteries.
Zhou et al. (2026) studied this question.