Mechanism for the Fluctuation in Coulombic Efficiency of Lithium Metal Anodes After Calendar Aging

Abstract Calendar aging significantly affects lithium metal battery performance, with even short rest periods causing measurable drops in Coulombic efficiency (CE). However, most studies focus only on CE losses during rest, and the impact on subsequent cycles remains unclear. Here, calendar aging behavior is systematically examined in various carbonate and ether electrolytes, showing that aging influences not only CE after rest but also the following cycles. While all electrolytes show CE loss during rest, distinct CE patterns appear in the two post‐rest cycles. To clarify the mechanism behind these fluctuations, dead lithium formation during and after aging is tracked using titration gas chromatography. This also enables quantification of CE loss associated with solid electrolyte interphase (SEI) evolution. The results indicate that the interplay between SEI evolution and dead lithium recovery governs the electrolyte‐dependent CE response during post‐rest cycling. Resting frequency in full cells is further varied, and it is found that increased resting frequency does not necessarily accelerate capacity loss, suggesting that rest‐induced degradation is largely reversible. These findings provide insight into post‐calendar aging mechanisms in lithium metal batteries and highlight the dynamic nature of rest‐induced degradation.