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 NaPF₆ concentrations in the electrolytes show that operating temperature and NaPF₆ concentration jointly regulate the capacity. The hard carbon anode with 2 M NaPF₆ 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.