Harnessing Native Al/Si Impurities in Bamboo Black Liquor for Self‐Templated Closed‐Pore Hard Carbon Anodes

ABSTRACT Biomass‐derived hard carbon is considered a promising anode material for sodium‐ion batteries (SIBs). However, the efficient formation of abundant, effective closed pores for sodium storage remains a major challenge. Herein, an innovative strategy has been proposed using naturally occurring alumina (Al ₂ O ₃ ) and silica (SiO ₂ ) from industrial bamboo lignin black liquor as self‐sacrificial templates to formulate high‐performance hard carbon anodes with well‐developed closed‐pore structures. Alkali treatment after carbonization (CW process) can remove residual Al ₂ O ₃ and SiO ₂ impurities effectively, and subsequent high‐temperature treatment at 1500°C induces the in‐situ formation of numerous closed‐pore. Of the resulting samples, the 600‐CW‐15 exhibits exceptional electrochemical performance, with an initial discharge capacity of 382.4 mAh g ^{− 1} while maintaining 264.8 mAh g ^{− 1} at a high current density of 800 mA g ^{− 1} . Furthermore, after the 100 cycles at 300 mA g ^{− 1} , it retains 83.4% of its capacity. Structural analyses confirm that the 600‐CW‐15 sample possesses a substantial closed‐pore volume of 0.223 cm ³ g ^{− 1} , which provides abundant active sites for sodium storage. Both the in situ and ex situ characterizations further reveal that the low‐potential plateau capacity originates primarily from the formation of Na clusters within the closed pore. This strategy will enable papermaking waste black liquor to be utilized for hard carbon anodes with excellent rate capability and cycling stability. This study underscores the potential application of black liquor waste to high‐performance, long‐life SIBs.