Defects and Oxygen Functional Groups Modulated Soft Carbon with Abundant Closed Pores and Robust Interface for High‐Performance Na ⁺ ‐Ion Batteries

Abstract Soft carbons with inherently high graphitization degree typically suffer from limited specific capacity and low initial Coulombic efficiency (ICE) for Na + ‐ion batteries (NIBs), hindering their development and practical application. Herein, defect and oxygen functional group engineering strategy by incorporating resin‐derived hard carbon and cost‐effective iron‐based substances is proposed enables the design of a unique layered soft carbon with enhanced efficiency and stable interfaces with the electrolytes. The increased C═O functional groups and Fe─O─C bond contributed by Fe‐based species significantly improve reversible Na + ‐ adsorption in the inclined region and enhance the structural stability of the electrode. Additionally, phenolic resin‐derived carbon increases the micropores and thus reduces the graphitization degree. The increased closed pores induced by the reconstruction of the carbon structure at high temperature enhance the Na + ‐storage ability. Consequently, the material achieves a high Na + ‐storage capacity of 311.0 mAh g −1 with an improved ICE of 93.89% at 0.1 A g −1 . Even cycled at 1 A g −1 , it can deliver a high capacity of 269.3 mAh g −1 with excellent stability over 800 cycles, outperforming previously reported soft carbon anodes. This innovative strategy provides a practical and cost‐effective pathway for designing advanced carbon anodes for NIBs.