Free radicals trigger the closure of open pores in lignin-derived hard carbons toward improved sodium-storage capacity

The chemical activation of various precursors is effective for creating additional closed pores in hard carbons for sodium storage. However, the formation mechanism of closed pores under the influence of pore-forming agents is not well understood. Herein, an effective chemical activation followed by a high-temperature self-healing strategy is employed to generate interconnected closed pores in lignin-derived hard carbon (HCs). By systematic experimental design combined with electron paramagnetic resonance spectroscopy, it can be found that the content of free radicals in the carbon matrix influences the closure of open pores at high temperatures. Excessively high activation temperature (>700 °C) leads to a low free radical concentration, making it difficult to achieve self-healing of open pores at high temperatures. By activation at 700 °C, a balance between pore making and self-healing is achieved in the final hard carbon. A large number of free radicals triggers rapid growth and aggregation of carbon microcrystals, blocking pre-formed open micropores and creating additional interconnected closed pores in as-obtained hard carbons. As a result, the optimized carbon anode (LK-700-1300) delivers a high reversible capacity of 330.8 mA h g−1 at 0.03 A g−1, which is an increase of 86 mA h g−1 compared to the pristine lignin-derived carbon anode (L-700-1300), and exhibits a good rate performance (202.1 mA h g−1 at 1 A g−1). This work provides a universal and effective guidance for tuning closed pores of hard carbons from other precursors.