On the Structural Evolution Mechanism of Lignin‐Derived Hard Carbon for Refining Sodium‐Ion Storage

ABSTRACT Understanding the structural formation mechanism of hard carbon and the relationship between its microstructure and sodium‐ion storage performance is critical for the precise fabrication of hard carbon. Herein, lignin‐derived hard carbons with tunable microstructures were prepared via one‐step carbonization by tuning carbonization temperatures (600°C−1600°C) and time (0−6 h). The graphitic microcrystalline structures in hard carbons become more ordered with increasing carbonization temperature and time, which in turn leads to the reduced interlayer spacing of graphene layers, decreased defect concentration, and increased size and volume of the closed pores. The pseudo‐graphitic structures with expanded interlayer spacing and abundant defect structures contribute to enhanced slope‐potential capacity, whereas the sodium‐ion accessible closed pores with large pore volume contribute to improved plateau‐potential capacity. A small closed‐pore size is beneficial for enhancing the rate performance. The hard carbons displayed an adsorption/intercalation‐dominated to a pore‐filling‐dominated mechanism with increasing carbonization temperature and time. The optimized hard carbon exhibited a high reversible capacity (322 mAh g −1 at 0.05 A g −1 ) with a plateau‐potential capacity of 246 mAh g −1 , and good rate performance. These findings provide fundamental insights for the structural revolution and sodium‐ion storage mechanism of hard carbon anodes, which could pave the way for understanding how the structure evolves and how we precisely design high‐performance hard carbon anodes.