Tuning microstructure and surface chemistry of hard carbon for high-rate sodium-ion batteries

The closed-pore structure and surface properties are crucial for enhancing the low-voltage platform capacity (<0.1 V) of hard carbon (HC) anodes for sodium-ion batteries (SIBs). Nevertheless, the lack of simple yet facile strategies for the fabrication of HC with fast sodium ion (Na⁺) storage kinetics has severely impeded the development of high-performance SIBs. Herein, a straightforward strategy is proposed to fabricate HC anodes with high specific capacity and superior rate performance by employing magnesium nitrate (MN)-assisted oxidation coupled with a surface coating of pitch-derived carbon layer. The oxidizing properties of MN are leveraged to introduce oxygen-containing functional groups into the porous carbon frameworks, thereby impeding the formation of graphitic structures during the high-temperature carbonization process. Simultaneously, the metal oxide nanocrystals generated by the decomposition of MN and the surface coating of the carbon layer are capable of tuning the microstructure and surface properties of the HC anodes. The optimized HC sample (PC-0.5-1300) exhibits a high reversible capacity of 277.6 mAh g^-1 at 0.1 A g^-1 with an infusive platform capacity of 181.2 mAh g^-1 and superior rate capability of 185.0 mAh g^-1 at a high output current density of 5 A g^-1, highlighting a great potential of Na⁺ storage. Impressively, the PC-0.5-1300 anode also delivers good long-term cycling stability with a capacity retention rate of 74.9 % after 2000 cycles at 1 A g^-1. This work provides a simple oxidation coupled with a surface coating method for regulating the electrochemical performance of HC anode for SIBs.