Lowering Energy Barriers of Free Radicals Facilitates Defect‐Suppressed Carbon Layers of Hard Carbon

Abstract Construction of closed pores by optimizing the length of the carbon layer is considered the most effective way to improve the electrochemical performance of hard carbon in sodium‐ion batteries. However, the lack of understanding of the carbon layer growth mechanism limits the development of hard carbon. Here, Mn ²⁺ is introduced during the pyrolysis process to control the length of the carbon layer of hard carbon. It is demonstrated that the introduction of the transition metal facilitates electron transfer to C─O bonds and thereby promotes homolytic cleavage of chemical bonds to generate free radicals. The simple aromatic radicals generated from oxygen‐centered radicals, coupled with those directly derived from biomass, synergistically accelerate the formation of well‐developed carbon layers, which improve the initial coulombic efficiency and plateau capacity. The optimal sample has a reversible capacity of 325 mAh g ⁻¹ and a competitive ICE of 92.7%. Pouch cell batteries exhibit a capacity retention of 280 mAh g ⁻¹ after 200 cycles and a capacity retention rate of 93.3% at 100 mA g ⁻¹ . This work enables rational design of carbon structure at molecular level.