Failure Investigation of LiFePO₄ Cells in Over-Discharge Conditions

The failure mechanism of LiFePO₄ cells in over-discharge conditions has been systematically studied using commercial A123 18650 cells at a 1.0 C rate and different conditions - from 5% to 20% over-discharge (DOD = 105% to 120%). SEM/EDAX, high-energy synchrotron XRD (HESXRD), and cyclic voltammetry (CV) were used to characterize the morphology, structure, and electrode potentials of cell components both in situ and ex situ. The failure behaviors of A123 18650 cells experiencing different degrees of over-discharge were found to be similar, and the 20% over-discharge process was analyzed as the representative example. The Cu electrochemical potentials in the 1.2 M LiPF₆ EC/EMC electrolyte were measured during the charge/over-discharge process using CV, proving that Cu oxidation and reduction in the cell during the charge/over-discharge cycle were theoretically possible to proceed. A possible failure mechanism is proposed: during the over-discharging process, Cu foil oxidized first to Cu⁺, then to Cu²⁺ cations; next, these Cu⁺ and Cu²⁺ cations diffused to the cathode side from the anode side; and finally, these Cu²⁺ cations reduced to Cu ⁺ cations, and then reduced further, back to metallic Cu. During charge/over-discharge cycling, Cu dendrites continued growing from the cathode side, penetrating through the separator and forming a copper bridge between the anode and cathode. The copper bridge caused micro-shorting and eventually led to the failure of the cell. During the charge/over-discharge cycles, the continued cell temperature increase at the end of over-discharge is evidence of the micro-shorting. © 2013 The Electrochemical Society.