Self‐Sealing Molten‐Salt Microreactors Enable Controllable High‐N Doping in Air for Durable and Fast Sodium Storage

ABSTRACT Achieving high‐level nitrogen doping in carbon materials is crucial for advanced energy applications. However, conventional synthesis relies on costly inert atmospheres, limiting practical deployment. Controllable nitrogen incorporation in air, especially at high contents with tunable configurations, remains a major unsolved challenge. Here we overcome this barrier by developing a self‐sealing molten‐salt microreactor strategy that enables the controllable synthesis of N‐rich carbon directly in air. A halide salt that densifies and melts dynamically forms confined microreactors that retain nitrogenous species, elevate their local pressure in accordance with Le Châtelier's principle, and thereby suppress nitrogen loss, yielding up to 25.07 wt.% N. The resulting material features a dominant pyridinic‐N fraction and hierarchical porosity while retaining a near‐graphitic interlayer spacing. As a sodium‐ion battery anode, the optimized material delivers high reversible capacity (393.5 mAh g −1 after 300 cycles at 0.1 A g −1 ) and exceptional long‐term stability (217 mAh g −1 after 2000 cycles at 1 A g −1 ). Ex situ characterization combined with density functional theory reveals a three‐stage sodium‐storage process, in which high‐affinity N‐6/O and vacancy sites provide capacity, whereas low‐barrier N‐Q/graphitic domains enable rapid ion migration. This work presents a paradigm for designing advanced heteroatom‐doped carbons for energy storage and beyond.