Lignin as an Inexpensive, High-Yield Precursor to Hard Carbon Active Materials for Sodium-Ion Batteries

Currently, hard carbon (HC) is the most promising negative electrode material for sodium-ion batteries due to its low redox potential, high-rate performance, and good reversible capacity. However, the synthesis procedure and choice of precursors are known to strongly impact the resulting HC performance. Herein, we focus on lignin as an abundant, high-yield precursor for the synthesis of HC. Using a three-step procedure involving preheating, washing, and postheating steps, we observed major improvement in the performance (ca. 340 mAh g^-1) and with high HC yields of ∼40%. We found that the preheating temperature (200-800 °C) impacted the reversible capacities and the plateau potential during sodiation, located around ∼40-70 mV vs Na⁺/Na. Based on small-angle X-ray scattering, we could correlate the size and content of the micropores with the observed plateau potential. The washing step was found to be key to removing impurities within the lignin structure, which tended to activate the carbon and reduce the reversible capacity. For the postheating step, HCs prepared using 1200-1300 °C showed the highest performance. Overall, lignin is a very promising, cost-effective precursor for preparing HC, and by optimizing its synthesis, major improvements to the negative electrode performance can be realized, which may also be relevant for HC synthesis from other precursors.