Overcharge cycling effect on the surface layers and crystalline structure of LiFePO₄ cathodes of Li-ion batteries

Electrochemical cells using LiFePO₄ cathode material are considered one of the safest and most resistant to overcharging among Li-ion batteries. However, if LiFePO₄-based electrodes are exposed to high potentials, surface and structural changes may occur in the electrode material. In this study Li/LiFePO₄ half-cells were overcharged under different modes with variable cut-off voltages and charge currents. The change in voltage profile, discharge capacity, surface layers composition, and crystalline structure were characterized after overcharge cycles. It was demonstrated that the cathode material is resistant to short-term overcharging up to 5 V, but undergoes irreversible changes with increasing overcharge time or potential. Thus, despite the well-known tolerance of LiFePO₄-based batteries to overcharge, a long overcharge time or high cut-off voltage leads to destructive changes in the cathode and should be avoided.