Degradation Mechanisms of C6/LiFePO4 Batteries: Experimental Analyses of Calendar Aging

The capacity loss and material decay of C6/LiFePO4 (LFP) batteries have been investigated under various storage conditions in dependence of State-of-Charge (SoC) and temperature. The electromotive force (EMF) curves, which are regularly determined by mathematical extrapolation of the measured voltage discharge curves, are used to investigate the aging mechanisms during storage. The irreversible capacity loss, which is accurately determined on the basis of the maximum storage capacity estimated from the EMF curves, increases as a function of temperature and SoC. The cyclable Li-ion loss during storage is considered to be the main source of the irreversible capacity loss. Strikingly, the inaccessibility of graphite is observed during storage at 60 °C. The graphite capacity decay has been quantitatively determined by non-destructive analyses on the basis of dVEMF/dQ curves. Deposition of Fe on the graphite electrode has experimentally been confirmed by X-ray photoelectron spectroscopy (XPS). The increasing graphite inaccessibility is shown to be the consequence of Fe dissolution from the cathode and the subsequent deposition onto the anode.