“EES” Zhailing Dengyun and Kang Feiyu’s Team: Degradation of Intercalation Compounds and Evolution of Ether-Based Interfacial Phases in Graphite Anodes During Calendar Aging

Calendar aging is crucial for long-life batteries, referring to capacity loss caused by storage under open-circuit conditions. Extensive research on calendar aging has been conducted on commercial graphite anodes in lithium-ion batteries, concluding that capacity fade due to storage is caused by lithium inventory loss in carbonate electrolytes and the growth of the solid electrolyte interphase (SEI). Additionally, this calendar aging is highly dependent on the state of charge (SOC), with aging typically minimized at partial lithiation states.In sodium-ion batteries, the compatibility of graphite anodes with low-concentration ether electrolytes enables highly reversible co-intercalation involving the solvent, offering excellent rate capabilities due to fast ion diffusion and partial desolvation. However, high-rate performance is often accompanied by severe aging processes, and studies on the calendar life of graphite based on co-intercalation electrochemistry are almost non-existent. Therefore, understanding the calendar aging of graphite anodes in low-concentration ether electrolytes, especially regarding SOC dependence, remains a critical issue.

Capacity loss caused by calendar aging is inevitably related to the degradation of intercalation compounds in charged graphite anodes. In low-concentration ether systems, the ion de/intercalation process of graphite anodes involves significant volume changes (over 300%), which are detrimental to interfacial stability. Additionally, there is ongoing debate about the existence of SEI on graphite surfaces in low-concentration ether electrolytes, as conflicting results have been reported in the literature, such as the absence of SEI in high-resolution transmission electron microscopy (HRTEM) and its capture in atomic force microscopy (AFM). This has limited the study of graphite interfacial stability in ether systems.Today, whether it is SEI dissolution on hard carbon in carbonate-based electrolytes or continuous thickening of the interfacial layer on lithium anodes, the evolution of SEI during calendar aging is directly reflected in capacity or Coulombic efficiency loss. Even in highly efficient reported electrolytes, SEI formation after aging can still be observed with lithium metal growth. Therefore, calendar aging amplifies interphase evolution, making it an effective tool to investigate the presence of SEI on graphite and assess interfacial stability.