Calendar aging and post mortem analysis of Graphite/LiFePO4 cell
Résumé
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. The calendar aging phenomena was evaluated by employing a various set of non-destructive electrochemical tests at different storage times. After storage, the cells stored at 45°C and 60°C exhibit capacity fade while those stored at 30°C do not. The extent of capacity fade strongly increases with storage temperature and to a lesser extent with the state of charge. The data analysis identifies the loss of cyclable lithium as the main source of capacity fade. This loss mostly 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 differential voltage analysis does not show any evidence about active-material loss in the electrodes and the EIS analysis shows an overall impedance increase of 70% or less. In addition, XRD (X-ray diffraction), SEM (scanning electron microscopy), and half-cell studies were carried out on the fresh and used electrodes (LFP and graphite electrodes) harvested from the dismantled cells. The contribution of each individual electrode to the overall degradation of the cells is investigated. The results obtained are in accordance with those suggested from the nondestructive analysis, i.e., (i) loss of cyclable lithium, and (ii) no loss of active material in the electrodes.