Calendar aging of a graphite/LiFePO4 cell

Graphite/LFP commercial cells are stored under 3 different conditions of temperature (30 °C, 45 °C, and 60 °C) and SOC (30%, 65%, and 100%) during up to 8 months. Several non-destructive electrochemical tests are performed at different storage times in order to understand calendar aging phenomena. After storage, all the cells except those stored at 30 °C exhibited capacity fade. The extent of capacity fade strongly increases with storage temperature and to a lesser extent with the state of charge. From in-depth data analysis, cyclable lithium loss was identified as the main source of capacity fade. This loss arises from side reactions taking place at the anode, e.g. solvent decomposition leading to the growth of the solid electrolyte interphase. However, the existence of reversible capacity loss also suggests the presence of side reactions occurring at the cathode, which are less prominent than those at the anode. The analyses do not show any evidence about active-material loss in the electrodes. The cells do not suffer substantial change in internal resistance. According to EIS analysis, the overall impedance increase is 70% or less.