Electrochemical impedance spectroscopy (EIS) studies were carried out on commercial 18650 LiFePO4 cells at different States of Charge (SOCs) to investigate failure in over-discharge conditions. The charge/discharge curves, capacity, charge acceptance, temperature, and impedance were characterized and analyzed. The EIS results show that the de-convoluted Ohmic resistance, R-0, solid electrolyte interphase (SEI) resistance, R-SEI, and Warburg Coefficient, sigma, 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, R-0, R-SEI, and sigma are associated with failure and can be used as indicators of incoming failure. The EIS results from the three-electrode system verify that the EIS results from the two-electrode system (practical 18650 cells) are reliable, which lays the foundation for the use of electrochemical impedance on practical applications of LIB cells. Overall, electrochemical impedance spectroscopy can be used as an effective and reliable tool to monitor the state of health, predict incoming failure of the cell, and issue a warning before failure without disturbing the operation of the cell. (C) 2014 The Electrochemical Society. All rights reserved.