Failure Investigation of LiFePO4 Cells under Overcharge Conditions

Abstract

The failure mechanism of LiFePO4 cells during overcharge conditions has been systematically studied using commercial A123 18650 cells at a 1C rate and different conditions – from 5% to 20% overcharge (SOC = 105% to 120%). SEM/EDX, 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 for A123 18650 cells experiencing different degrees of overcharges were found to be similar, and the 10% overcharge process was analyzed as the representative example. The Fe redox potentials in the 1.2 M LiPF6 EC/EMC electrolyte were measured during the overcharge/discharge process using CV, proving that Fe oxidation and reduction in the cell during the overcharge/discharge cycle is theoretically possible. A possible failure mechanism is proposed: during the overcharging process, metallic Fe oxidized first to Fe2+, then to Fe3+ cations; next, these Fe2+ and Fe3+ cations diffused to the anode side from the cathode side; and finally, these Fe3+ cations reduced first to Fe2+ cations, and then reduced further, back to metallic Fe. During overcharge/discharge cycling, Fe dendrites continued growing from both the anode and the cathode sides simultaneously, penetrating through the separator and forming an iron bridge between the anode and cathode. The iron bridge caused micro-shorting and eventually led to the failure of the cell. During the overcharge/discharge cycles, the continued cell temperature increase at the end of overcharge is evidence of the micro-shorting.