Molecular Engineering of Lignin for Mediating the Formation of Closed Pores in Hard Carbon to Enhance Sodium Ion Storage

ABSTRACT Lignin‐derived hard carbon has shown considerable promise as an advanced anode material for sodium‐ion batteries, owing to its renewable nature and cost‐effectiveness. However, conventional carbonization yields hard carbons with unsatisfactory initial coulombic efficiency (ICE) and limited rate capability because of poorly controlled pore architecture. Herein, we present a molecular engineering strategy for effective regulation of the closed‐pore structure of lignin‐based hard carbon to enhance sodium ion storage. The cross‐linking of lignin plays a pivotal role in directing microstructure evolution, which subsequently facilitates the formation of closed pores during high‐temperature treatment. The optimized porous architecture significantly improves the Na ⁺ storage performances, with a remarkable reversible capacity of 361.4 mAh g ⁻¹ at 0.02 A g ⁻¹ and even 167.4 mAh g ⁻¹ at a high current density of 4.0 A g ⁻¹ , along with a high ICE of 90.8%. The full cell achieves an energy density of 257.8 Wh kg ⁻¹ . This work provides a fundamental insight into the molecular‐level effect of lignin on pore formation and establishes a practical pathway for transforming lignin into high‐performance energy storage materials through rational structural design.