Designing the Precursor Structure Through Composite Engineering for Achieving High‐Capacity Lignin‐Derived Hard Carbon Anodes

The hard carbon anode material for sodium-ion batteries consists of curved graphene sheets and cross-linked structures with abundant sp³-hybridized carbon. These randomly stacked graphene sheets form numerous closed pores. Both these closed pores and the randomly stacked graphene sheets provide abundant sodium-ion storage sites. To further enhance the sodium-ion storage performance of hard carbon, it is essential to rationally design the precursor structure of hard carbon. These precursors should endow hard carbon with larger interlayer spacing and more closed-pore structures. Alkali lignin (AL), with its high carbon content and innate 3D network molecular structure, is considered one of the most promising biomass precursors for hard carbon. However, the aggregate structure in lignin hinders the fabrication of hard carbons with large interlayer spacing and a closed-pore-rich structure during carbonization. In this study, polyaniline (PANI) was combined with AL through intermolecular interactions to form lignin-polyaniline composite (AL/PANI) as a precursor. This composite was then subjected to one-step high-temperature carbonization to prepare a lignin-polyaniline-derived hard carbon (LPHC). PANI decomposes and disrupts the inherent aromatic aggregate structure of the lignin macromolecules. Ultimately, this process results in the formation of hard carbon with a rich closed-pore structure and enlarged interlayer spacing after carbonization. LPHC delivers a high specific capacity of 360 mAh g^-1 at a current density of 0.1 A g^-1, with a plateau-potential capacity as high as 250 mAh g^-1. This work proposes a strategy of utilizing guest molecules to regulate the composite structure of lignin via structure engineering, thereby tailoring the microstructure of lignin-derived hard carbon materials.