Spatiotemporal Evolution in Hard Carbon Synthesis via Electrothermal Coupling Strategy for High‐Performance Sodium‐Ion Batteries

Abstract Conventional hard carbon synthesis through prolonged sintering suffers from structural degradation, including amorphous‐to‐graphitic transitions and pore collapse, critically impairing sodium storage performance. Here, a spatiotemporally controlled electrothermal coupling strategy is proposed that revolutionizes carbonization via in situ joule heating, achieving ultrafast synthesis (30 s) with preserved structural integrity. By precisely regulating current density distribution, this method enables defect‐selective graphitization while maintaining abundant micropores and expanded interlayer spacing (0.39 nm). The optimized hard carbon synthesized at 1000 °C demonstrates exceptional sodium storage capacity (306.83 mAh g⁻¹) and record‐high initial Coulombic efficiency (91.99%), outperforming furnace sample by 16.7% in capacity. During the spatiotemporal evolution process, localized electric field can induce directional charge redistribution and lower C─C bond dissociation barriers, enabling rapid formation of microporous structure with enhanced Na⁺ diffusion kinetics and stable interfacial properties. Temporal superiority of this method is evidenced by 79.45% capacity retention after 1000 cycles. This work establishes a paradigm for energy‐efficient carbon material synthesis via spatiotemporal electrothermal control, providing fundamental insights into field‐assisted reaction kinetics for next‐generation battery manufacturing.