Enhanced sodium storage performance of asphalt-derived hard carbon through intramolecular oxidation for high-performance sodium-ion batteries

The development of high-performance and low-cost hard carbon plays a crucial role in the commercialization of sodium-ion batteries (SIBs). Asphalt is considered a suitable hard carbon precursor due to its wide distribution, abundance, and cost-effectiveness. However, its low capacity and poor electrochemical reaction kinetics limit its further application. Herein, we have successfully synthesized asphalt-based hard carbon nanosheets through a process of intramolecular oxidation, facilitated by the synergistic action of mixed acids. The introduction of sulfuric acid plays a crucial role in expanding the tightly packed asphalt molecules, which in turn allows for the intramolecular oxidation of asphalt molecules by nitric acid. This oxidation process effectively introduces oxygen-containing functional groups (OFGs), leading to an increase in interlayer spacing and the formation of a more nanoporous structure, resulting in both enhanced capacity and improved rate performance. The optimized asphalt-based hard carbon boosts reversible capacity from 115.0 to 304.4 mAh∙g⁻¹ at 0.03 A g⁻¹, and the plateau capacity is increased by 5.5 times. This work provides a profound understanding of the impact of liquid-phase acid oxidation on the structure and composition of sodium-storage hard carbon, and further unveils an effective method for obtaining low-cost and high-performance asphalt-based hard carbon.