Soft and hard carbon composite with regulated closed-pore structure for enhanced low-potential plateau sodium storage...

Developing anode materials that simultaneously deliver high reversible capacity, high initial Coulombic efficiency (ICE), and sufficient low-potential plateau capacity is critical for the practical application of sodium-ion batteries (SIBs). Herein, a soft carbon-hard carbon composite was fabricated by chemically cross-linking pre-oxidized phenolic resin with pitch, followed by carbonization. This synthetic strategy effectively regulated the pore structure, resulting in a high closed-pore volume of 0.224 cm³ g^-1 and a low specific surface area of 2.37 m² g^-1. Structural analysis revealed that pre-oxidation drove the formation of chemical linkages between phenolic resin and pitch, thereby enabling the conversion of open pores into closed pores and suppressing excessive electrolyte decomposition. Owing to these structural features, the composite delivered a high ICE of 91.8% and a reversible capacity of 350 mAh g^-1. Significantly, a substantial plateau capacity of 226 mAh g^-1 was maintained at a low potential (≤0.1 V). Additionally, the material demonstrated impressive rate capability and cycling stability, achieving a capacity retention of 93.2% after completing 100 cycles. This study illustrates that developing a closed-pore configuration via the design of a soft carbon-hard carbon composite is an effective method for enhancing sodium storage at low potentials.