Unveiling the Microscopic Origin of Irreversible Capacity Loss of Hard Carbon for Sodium-Ion Batteries

The primary bottleneck hindering the application of hard carbon in sodium-ion batteries (SIBs) anodes lies in its inadequate initial Coulombic efficiency (ICE). Unclear causes of capacity loss at the microscopic level restrict the improvement of hard carbon anodes. Here, two pivotal stages that influence the structure and composition of hard carbon, namely synthesis, and storage are evaluated; subsequently identifying crucial determinants contributing to irreversible capacity loss. The results suggest that undergrown carbon layers allowing the intrusion of solvent molecules into the interior of the hard carbon is a key factor during the synthesis stage, while the gradual formation of oxygen-containing functional groups on the surface of the hard carbon is another factor leading to irreversible loss of capacity during storage stage. This research microscopically clarifies the irreversible capacity loss mechanism on hard carbon and provides guidelines for designing and applying high ICE hard carbon for SIBs. Carbon materials with tunable microstructures and low cost are considered very promising anode materials for sodium-ion batteries, but their electrochemical performance is still limited by their low initial coulombic efficiency. These results reveal the key factors responsible for the irreversible capacity loss in the synthesis of hard carbon materials as well as in the application phase. image