Mechanical Stress Reveals Asymmetry of Sodiation and Desodiation of Hard Carbon

Sodium-ion batteries (SIBs) are an important and environmentally friendly drop-in technology for widespread lithium-ion batteries. Currently, hard carbon (HC) composite electrodes are considered to be the most promising anodes for commercial SIBs. Still, research is required to improve hard carbons to reduce irreversible losses during the first cycle and to gain a better understanding of how their structure correlates with their electrochemical performance. Herein, a HC composite electrode is investigated by operando substrate curvature measurements to determine mechanical stresses and infer volume changes during operation. These mechanical data in conjunction with electrochemistry are used to compare different reaction pathways proposed in the literature. It is observed that sodium loss in the first cycle does not lead to large changes in volume. The mechanical data agree with the model based on intercalation and pore filling. A strong asymmetry in the volume of the HC between sodiation and desodation is identified and explained by modifications of the model: intercalation and pore filling are mostly sequential, but both mechanisms interact so that sodium deposition into and dissolution from pores occur at different levels of intercalation.