Electrochemical Impedance Spectroscopy Study of Failure of Commercial LiFePO4 Cells Under Overcharge Conditions

One of the major problems remaining to be solved for the lithium-ion batteries (LIB) is the safety issue, especially, the failure of LIB cells under extreme conditions (overcharge, over-discharge, high temperature, low temperature, etc.). A Li-ion cell experiences overcharge/over-discharge when it is practically used in a battery pack for electric vehicles (EVs) hybrid electric vehicles (HEVs) because the capacity variation of different cells in the pack and difficulty to control the state of charge (SOC) of each single LIB cell/module in a battery pack exactly the same^1,2.

Electrochemical impedance spectroscopy (EIS) was employed to investigate the failure phenomena of commercial 18650 Li-ion batteries at different states of charge (SOCs) under overcharge conditions. The change of voltage profile, discharge capacity, charge acceptance, temperature, and impedance were characterized and analyzed during overcharge cycles. During overcharge cycling, the charge acceptance, charging efficiency, cell temperature, and cycle life change dramatically with cycle numbers. The EIS results at different cycle numbers were characterized and analyzed (Fig. 1). Further study of the EIS for LiFePO₄ cells at different SOCs (0, 50, and 100%) revealed that these de-convoluted parameters show similar patterns of changing with cycle numbers; and some of them, the Ohmic resistance, resistance of SEI, charge transfer resistance and Warburg coefficient, are associated with failure and can be used as indicators of incoming failures.  EIS results from both the two-electrode system and the three-electrode system were compared, demonstrating that the direct application of EIS to the practical 18650 LiFePO₄ cells can lead to results as reliable as those obtained from the three-electrode system. The three-electrode result showed that impedance of the cathode is much higher than the anode, and both the impedance of anode and cathode increase significantly when the cell is failed caused by overcharge, which indicates that both the SEI on the anode surface and CEI on the surface of the cathode are changed a lot during overcharge process (Fig. 2). In short, the EIS can be an effective tool to monitor the state of health (SOH) of the battery or pack and issue an early warning signal before the failure happens.

Reference

1. P. G. Balakrishnan, R. Ramesh and T. Prem Kumar, Journal of Power Sources, 155, 401 (2006).

2. J. Chen, C. Buhrmester and J. R. Dahn, Electrochemical and Solid-State Letters, 8, A59 (2005).