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

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.