Orbital Rehybridization Enabling Abundant Pentagonal-Heptagonal Rings in Hard Carbon for Optimal Sodium-Ion Batteries

Curvature structure modulation through the introduction of abundant non-hexagonal (pentagonal and/or heptagonal) rings results in structural mutation and tuning of the electronic structure of hard carbon. This functional utilization of hard carbon is promising yet very challenging. For instance, in an electrochemical storage system, the curvature effect leads to the formation of structurally defective carbon materials with higher reactivity than pure graphite. Here, we reveal that the introduction of pentagonal-heptagonal rings results in partial structural distortion by distorting C═C bonds in pyrolyzed topologically defective hard carbon (TDC). Theoretical and experimental evidence indicates that hard carbon with abundant pentagonal-heptagonal rings forms localized curvature structures. The corresponding changes in electronic properties from the sp² hybridization state are mainly due to the rehybridization of σ and π orbitals. The resulting charge-rich structure has high reactivity for sodium ion adsorption. Samples with abundant pentagonal-heptagonal rings exhibited excellent rate performance and high cycling stability, as indicated by a capacity of 411 mAh g^-1 (TDC-2) after 400 cycles at a current density of 2 A g^-1, in contrast to 212 mAh g^-1 for TDC-1.