A Cross-Linking Strategy To Regulate the Surface Defects and Pore Structures of Lignin-Derived Hard Carbon as High-Performance Anode for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have emerged as a viable alternative to lithium-ion batteries due to abundant sodium resources, low cost, and high safety. While hard carbon stands out as a promising anode material for SIBs, its practical implementation remains constrained by the challenge of simultaneously achieving a high specific capacity and superior initial Coulombic efficiency (ICE). To address this critical need, we propose a microstructure engineering strategy through molecular configuration reconstruction of lignin precursors. By strategically cross-linking oxygen-containing active groups in lignin with polyaromatic ring-enriched phenolic resins, we develop a class of hard carbon materials with an optimized sodium storage architecture. The resultant carbon matrix demonstrates four synergistic structural advantages: (1) enhanced proportion of ordered pseudographitic domains for efficient ion intercalation, (2) expanded interlayer spacing (0.40 nm) facilitating rapid Na⁺ diffusion, (3) minimized surface defect density to suppress parasitic reactions, and (4) abundant closed micropores ensuring structural integrity during cycling. These characteristics work synergistically, enabling the efficient intercalation and transport of sodium ions and endowing the materials with an excellent initial Coulombic efficiency (ICE). In this work, when the lignin/phenolic resin mass ratio is 7:3, the reaction product (LP73) reaches the highest degree of cross-linking. The corresponding hard carbon material CLP73 obtained after carbonization exhibits the optimal electrochemical performance with a specific capacity of 321 mAh g^–1 at 30 mA g^–1 and an ICE as high as 92.8%. The CLP73 can maintain a specific capacity of 257 mAh g^–1 after 600 cycles at a high current of 1000 mA g^–1, with a retention rate of 89.6%, showing good cycle stability. The high capacity and ICE achieved in this work, along with the simple synthesis process, make it suitable for large-scale industrial development.