Failure Mechanism Investigation of Commercial LiFePO4 Cells in Different Operating Conditions

The rapid growth of the Li-ion battery (LIB) market has greatly driven research on LIBs, particularly on LIB failure mechanisms under different operating conditions (long-term cycling, overcharge, over-discharge, high temperatures, low temperatures, etc.) due to safety issues¹. Up to now, there are only very few of reports on the capacity fade mechanisms in the commercial batteries². It is paramount of scientific and engineering importance to understand the microstructure evolution during normal cycle in the commercial cells. To our knowledge, in situ monitoring of microstructure evolution during the capacity fade process in either a commercial battery or lab-made batteries has never been reported before.

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.