Deciphering the Calendar Aging Degradation Mechanism of LiFePO₄-Graphite Pouch Cells

Lithium iron phosphate-graphite (LFP-C) lithium-ion batteries are highly favored in electric vehicles and energy storage systems due to their extended cycle life and low cost. However, in actual use, batteries will be stored for a long time, which will lead to battery capacity decay and shorten the service life. In this work, the calendar aging performance of the cells under different states of charge (50% SOC and 100% SOC) and storage temperatures (25 and 50 °C) has been investigated. It has been revealed that an increase in storage temperature and SOC results in a notable decline of cell capacity, with temperature exerting a more pronounced influence than SOC, which is in line with the continuous decomposition of the liquid electrolyte during aging, resulting in an increase of the LiF content in the cathode–electrolyte interphase (CEI) and solid electrolyte interphase (SEI) layers. In addition, the batteries bulge at different SOCs at 50 °C, producing a large amount of H2 and a small amount of CO2 and CH4 gases, which is consistent with the decrease in the relative content of Li2CO3 in the CEI layers due to decomposition at high temperature. This work provides a deeper understanding of the capacity decay mechanism of pouch cells under different calendar aging conditions by exploring the evolution of CEI/SEI composition via systematic characterizations, especially gas production analysis.