Failure Investigation of LiFePO₄Cells in Over-Discharge Conditions

Abstract

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.

Enhanced Overdischarge Stability of LiCoO2by a Solution Deposited AlPO4Coating

“…As shown, the charge voltage profile is nearly identical for all cycles shown, with a ∼10 mV increase in the initial charge polarization of the cell is observed on cycle 19, which is about 40 mV less than in the as-received LiCoO 2 . This result is consistent with the AlPO 4 coating improving the reversibility of LiCoO 2 to over-insertion of lithium which lessens the initial cathode polarization in the AlPO 4 coated sample. 35 Figure 5a shows the X-ray diffraction (XRD) patterns for pristine as-received LiCoO 2 and as-received LiCoO 2 after overdischarge testing described in Figure 3a.…”

“…35 After the potential of the electrode decreases to about 1. 4 II-III O 2-y (0 < x, 0 ≤ y). 35 As shown for cycles 13, 16 and 19 in Figure 3c, the features of the voltage profile change significantly compared to cycle 10 as the LiCoO 2 is repeatedly overinserted.…”

“…AlPO 4 coatings on the surface of cathode active particles have been shown to improve the stability of several cathode active materials. [21][22][23][24][25] In particular several studies have shown that AlPO 4 coatings of LiCoO 2 particles stabilizes the LiCoO 2 against charge to high potentials >4.2 V vs. Li/Li + .…”

“…In the present work, a solution deposited coating of AlPO 4 onto LiCoO 2 particles is tested for its utility in improving the tolerance of LiCoO 2 to over-insertion of lithium at potentials <3.0 V vs. Li/Li + . Half-cell testing in coin cells is used to study the effect of an AlPO 4 coating to stabilize a LiCoO 2 cathode discharge performance against a 5% (of the 140 mAh/g nominal capacity of LiCoO 2 ) over-insertion of lithium by fixed resistive load.…”

A Metal–Organic Compound as Cathode Material with Superhigh Capacity Achieved by Reversible Cationic and Anionic Redox Chemistry for High‐Energy Sodium‐Ion Batteries

“…The co‐existence of Cu I and Cu II signals suggests partial conversion from Cu I to Cu II at 4.1 V. After being discharged to 3.7 V, the ratio of Cu II decreases as the result of reduction to Cu I . This is the first time to find that redox reaction happens at such a high potential for Cu I /Cu II couple, which is even higher than that of Cu II /Cu III in a layered oxide . The capacity delivered at this voltage range is less than the theoretical capacity from one electron transfer reaction (Figure a).…”