Thermal Behavior of Solid Electrolyte Interphase Films Deposited on Graphite Electrodes with Different States-of-Charge

The thermal degradation of surface films (solid electrolyte interphase, SEI) and their repairing behaviors are examined on graphite electrodes that have different states-of-charge (SOC). In detail, a fully passivating SEI layer is generated in advance and then stored at 85°C for a prolonged time. The surface films (SEI) are thermally degraded upon high-temperature exposure. The damaged films are repaired by electrolyte decomposition and concomitant film deposition, during which Li+ ions/electrons are supplied from the graphite electrodes, which appears as an increase in the open-circuit voltage (OCV) of the graphite electrodes. The repaired surface films show a comparable passivating ability along with a similar morphology and chemical composition to that of the initial SEI layers. The degradation/repairing continues until the graphite electrodes are fully de-lithiated (OCV = 3.0 V vs. Li/Li+). Once the graphite electrodes are fully de-lithiated, the damaged surface films cannot be repaired because the Li+ ions/electrons are exhausted in the graphite electrodes. Because of the incomplete repairing, the graphite surface becomes poorly covered by surface films, which leads to a loss of passivating ability.