Unraveling the Microcrystalline Carbon Evolution Mechanism of Biomass-Derived Hard Carbon for Sodium-Ion Batteries

Microcrystalline carbon is the essential constituent unit that constitutes the hard carbon material for sodium-ion batteries. However, the evolution mechanism of microcrystalline carbon remains controversial, on account of the diversity of biomass composition. Here, we conducted a systematic study of the evolutionary mechanism of microcrystalline carbon using lignin and cellulose as models. It was found that lignin is more readily converted into microcrystalline carbon structures than cellulose. Owing to the differences in pyrolysis processes, lignin-derived microcrystalline carbon exhibits isotropic arrangement properties and evolves into long-range ordered graphite-like structures with increasing pyrolysis temperatures. In contrast, the anisotropic arrangement of cellulose-derived microcrystalline carbon allows them to maintain long-range disordered structures under high-temperature pyrolysis. Upon further analysis using four forestry biomass wastes with different compositional ratios to prepare hard carbon, we found that proper ratios of lignin and cellulose ensure a sufficient amount of microcrystalline carbon while avoiding overgrowth of microcrystalline carbon, where the tightness of the microcrystalline carbon stacking structure was positively correlated with lignin content. Besides, coconut-shell-derived hard carbon has a long-range disordered and short-range ordered microcrystalline stacking structure and exhibits a high capacity of 329.3 mAh g–1.