Revealing sodium storage mechanism of hard carbon anodes through in-situ investigation of mechano-electrochemical coupling behavior

Hard carbon (HC) is considered a promising anode material for sodium-ion batteries due to its relatively low price and high specific capacity. However, HC still suffers from unclear reaction mechanisms and unsatisfactory cycling stability. The study of mechano-electrochemical coupling behavior by in-situ measurement techniques is expected to understand the sodium storage and degradation mechanisms. In this paper, the strain and stress evolution of HC anodes at different sodiation/desodiation depths and cycles are investigated by combining electrochemical methods, digital image correlation, and theoretical equations. The observation by monitoring the in-situ strain evolution during the redox process supports the "adsorption-intercalation/filling" mechanism in reduction and the "de-filling/de-intercalation-desorption" mechanism in oxidation. Further studies have demonstrated that the strain and stress of the electrode show periodic changes accompanied by a continuous accumulation of residual stress during cycles, explaining the capacity degradation mechanism of HC from a mechanical perspective. In addition, when the higher current density is applied, the electrodes experience greater strain and stress associated with the Na+ insertion rate. This work clarifies the Na-storage mechanism and the mechano-electrochemical coupling mechanism of HC anodes by in-situ strain measurement, which helps optimize and design the anode materials of sodium-ion batteries from the perspective of interface microstructure and multi-field coupling, such as in situ integrated interface structure design.