Spherical Lignin‐Derived Hard Carbon Anode for Sodium‐Ion Batteries

Biomass‐derived hard carbon is a promising anode for high‐performance sodium‐ion batteries (SIBs) because of its low cost and structural tunability. Here, spherical lignin‐derived hard carbon was developed, and the carbonization temperature was tuned from 900 to 1400 °C to regulate surface chemistry, turbostratic structure, and pore texture, enabling a systematic correlation between structure evolution and electrochemical behavior. Increasing the carbonization temperature progressively reduces oxygen‐containing functional groups and shifts the initial capacity distribution toward the low‐voltage plateau region, accompanied by an increase in initial Coulombic efficiency. Consequently, the spherical hard carbon synthesized at 1300 °C exhibits a high reversible discharge capacity of 335.9 mAh g −1 at 50 mA g −1 and a long cycle life over 1600 cycles with 80.45% capacity retention at 500 mA g −1 . Moreover, high‐temperature battery tests from 20 to 80 °C with different NaPF 6 concentrations in the electrolytes show that operating temperature and NaPF 6 concentration jointly regulate the capacity. The hard carbon anode with 2 M NaPF 6 in diglyme (DEGDME) electrolyte demonstrates the increased capacity at high temperature, indicating improved high‐temperature performance. This work provides a practical strategy to develop spherical lignin‐derived hard carbon anodes for high‐temperature SIBs by coordinating carbonization temperature and electrolyte formulation.