Rapid Closed Pore Regulation of Biomass‐derived Hard Carbons Based on Flash Joule Heating for Enhanced Sodium Ion Storage

Abstract Closed pores are essential for enhancing the low‐potential (<0.1 V) plateau capacity and initial Coulombic efficiency of hard carbon (HC) anode materials for energy‐dense sodium‐ion batteries. However, the lack of simple and effective closed‐pore construction strategies has severely hindered their future commercialization. Herein, a rapid closed‐pore regulation strategy for biomass‐derived HCs is proposed through pre‐heat treatment followed by flash Joule heating (FJH). The pre‐heat treatment is critical for transforming vulnerable biomass into high‐carbonizability frameworks that are resistant to over‐graphitization. FJH treatment helps to generate enriched closed pores surrounded by the resulting carbon walls with expanded interlayer spacing as accessible Na + channels. This strategy shows remarkable universality and applicability for biomass feedstocks, enabling rapid conversion of various carbonization‐vulnerable precursors to high‐yield (e.g. HC600‐J‐1500 compared with HC25‐J‐1500, ≈14‐fold yield increase) and closed‐pore enriched HCs. The optimized sample demonstrates an outstanding reversible capacity of 377 mAh g −1 with a superior initial Coulombic efficiency of 93.3%, which stands in a record value prepared with FJH and is even competitive via conventional carbonization. Comprehensive tests reveal that the efficient Na storage originates from the pore‐filling mechanism in the closed nanopores. This work suggests a facile and universal closed‐pore regulation approach for the rational design of high‐performance HCs.