Failure Study of Commercial LiFePO4 Cells in Overcharge Conditions Using Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was employed to investigate the failure phenomena of commercial 18650 LiFePO4-based Li-ion batteries at different states of charge (SOCs) in overcharge conditions. The change of voltage profile, discharge capacity, charge acceptance, temperature, and impedance were characterized and analyzed during overcharge cycles. Results show that increases in the solid electrolyte interphase (SEI)/cathode electrolyte interphase (CEI) resistance, the charge transfer resistance, and the Warburg coefficient are the main parameter changes during overcharge cycles, indicating SEI/CEI breakdown and reformation on the electrode surface, faradaic reaction rate, and diffusion coefficient changes, respectively. These parameters can be used as indicators of incipient failure. The EIS results from a three-electrode system separate the impedance changes of each electrode, and the results show that the impedance of the anode is much lower than that of the cathode, which indicates that the CEI on the cathode surface contributes more to whole-cell impedance. In short, 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 failure happens. (C) 2015 The Electrochemical Society. All rights reserved.