Releasing Free Radicals in Precursor Triggers the Formation of Closed Pores in Hard Carbon for Sodium‐Ion Batteries

Abstract Increasing closed pore volume in hard carbon is considered to be the most effective way to enhance the electrochemical performance in sodium‐ion batteries. However, there is a lack of systematic insights into the formation mechanisms of closed pores at molecular level. In this study, a regulation strategy of closed pores via adjustment of the content of free radicals is reported. Sufficient free radicals are exposed by part delignification of bamboo, which is related to the formation of well‐developed carbon layers and rich closed pores. In addition, excessive free radicals from nearly total delignification lead to more reactive sites during pyrolysis, which competes for limited precursor debris to form smaller microcrystals and therefore compact the material. The optimal sample delivers a large closed pore volume of 0.203 cm 3 g −1 , which leads to a high reversible capacity of 350 mAh g −1 at 20 mA g −1 and enhanced Na + transfer kinetics. This work provides insights into the formation mechanisms of closed pores at molecular level, enabling rational design of hard carbon pore structures.