Failure Mechanism Investigation of Commercial LiFePO4 Cells in Different Operating Conditions
In this work, the capacity fade/failure processes of commercial 18650 LiFePO4 cell during the long term normal charge/discharge, overcharge, and over-discharge cycling was systematically investigated respectively. The commercial 18650 LiFePO4cells (A123 Systems) were chosen to be cycled for all cycling conditions until these cells reach 80% of initial capacity (typical criteria for EV application) or cannot be charged/discharged.
The cells were in situ characterized and the microstructure of the electrodes was investigated using synchrotron high-energy X-ray diffraction during long term normal cycling and overcharge/over-discharge cycling conditions. Synchrotron high-energy X-rays with photon energy of 115 KeV are capable of penetrate through thick samples, which allows us to probe commercial 18650 LiFePO4 cells without any cell modification and the results are presented herein. The result of 18650 cell after 2500 cycles is shown in Fig. 1. Our in situ experiment result show that loss of the active lithium source in the system is the primary cause of the capacity fade and the appearance of the inactive FePO4phase is proportional to the decrease of available lithium source during cycling.
Electrochemical Impedance Spectroscopy (EIS) is a powerful and quick analytical and diagnostic tool that can be used to study the internal resistance of Li-ion batteries during long-term normal cycling or overcharge/ over-discharge cycling3. The EIS results during overcharge and over-discharge process are shown in Fig. 2 and 3 respectively, and the de-convoluted Ohmic resistance, solid electrolyte interphase (SEI) resistance, and Warburg Coefficient, change with cycle number in some patterns, indicating the occurrence of corrosion of the current collector, SEI breakdown/decomposition and reformation, and the development of diffusion barriers of Li+ in the electrode, respectively. These parameters are associated with failure and can be used as indicators of incoming failure. Overall, EIS can be used as an effective and reliable tool to monitor the state of health, predict incoming failure of the LIB cells, and issue a warning before failure without disturbing the operation of the cells.