Degradation Mechanisms of C6/LiFePO4 Batteries: Experimental Analyses of Cycling-induced Aging

Electromotive force (EMF) voltage curves are regularly determined to facilitate in-depth understanding of aging mechanisms of C6/LiFePO4 batteries during cycling. The irreversible capacity losses under various cycling conditions and temperatures are accurately obtained from the extrapolated EMF curves and are found to increase with cycle number and time. A new mathematical extrapolation method is proposed to distinguish between calendar ageing and cycling-induced ageing. The capacity losses due to calendar aging are obtained by extrapolating the total irreversible capacity losses to zero cycle number. It is found that calendar ageing increases logarithmically in time. On the other hand, cycling-induced ageing is accurately determined by extrapolating the capacity losses to zero time. In this case the capacity losses are found to increase linearly with cycle number. It is furthermore found that iron dissolution from the cathode at 60 °C and the subsequent deposition onto the anode enhances significantly the SEI formation on the graphite electrode and, consequently, battery ageing. Interestingly, the graphite electrode decay has been quantified in much more detail, by analyzing the dVEMF/dQ curves. The analyses show that the electrode decay can be related to both the structural deterioration and the inter-layer surface blockage of the graphite electrode, as has also been experimentally confirmed by Raman and XPS spectroscopy.