Oxidative Electrolyte Breakdown Causing Rapid Anode Potential Decrease and Cell Voltage Recovery during Fixed Resistive Load Overdischarge of LiFePO4/Graphite Li-Ion Cell
Authors: Crompton Kyle Richard 
Published: December 2020

Recently published work1 reported a rapid anode potential decrease leading to a cell voltage increase during fixed resistive load overdischarge of 0.7 mAh LiFePO4/graphite Li-ion pouch cells. An initial test of a 0.7 mAh LiFePO4/graphite Li-ion pouch cell with a 1.0 M LiPF6 1:1 Ethylene Carbonate:Ethyl Methyl Carbonate v/v electrolyte was overdischarged by a fixed 35.6 kOhm resistive load applied continuously for 2 weeks. The resistive load was applied after the 5th conditioning cycle discharge by constant 0.07 mA current to 2.5 V cell voltage. 3-electrode measurements show that the anode increases to 3.4 V vs. Li/Li+ after about an hour after application of the fixed resistive load as the cell voltage decreases to <200 mV. At about the 6th hour of the fixed resistive load overdischarge step, the cell voltage rapidly (<5 minutes) increases from 200 mV to about 1.5 V, plateaus for about 4 hours then decreases during the next ~4 hours to 0.0 V. The 3-electrode results show that the cell voltage increase is driven by the anode potential rapidly decreasing from 3.4 V vs. Li/Li+ to about 1.5 V vs. Li/Li+ at the 6th hour for about 4 hours. The cathode potential rapidly decreases from 3.5 to ~3.0 V vs. Li/Li+ at the 6th hour and plateaus for about 4 hours. The cathode and anode potentials then decrease to ~1.0 and ~0.5 V vs. Li/Li+, respectively, during hours 10-14 of the fixed resistive load step until the electrode potentials asymptote together and increase to about 2.6 V vs. Li/Li+ during the remaining ~322 hours of the fixed resistive load step. When the cell was recharged after the 2 week fixed resistive load overdischarge step the cell charge/discharge capacity increased by about 0.08 mAh. The electrode potential characteristics also changed consistent with an increase in lithium inventory of the cell compared to before the fixed resistive load overdischarge.
Visual inspection of the anode after testing and disassembly shows darkening in some areas of the composite. Scanning electron microscopy (SEM)/energy dispersive x-ray (EDX) and x-ray photoelectron spectroscopy (XPS) results show an increased presence of fluorine, phosphorous, copper and lithium on the surface compared to a control cell that was not overdischarged. The increased presence of the phosphorous, lithium and fluorine was more substantial in the visually darkened areas. Hi-resolution XPS scans of the P 2p, F 1s, and O 1s show that the fluorine, phosphorous, and lithium on the surface forms LixPFy, LiF, LixPOyFz and phosphates compounds. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) of dried electrolyte samples shows an increase in copper content of the electrolyte in an overdischarged cell compared to a cell that was not overdischarged.
These results are consistent with copper dissolution as well as an oxidative reaction of the electrolyte occurring at the anode surface during overdischarge. An additional LiFePO4/graphite Li-ion cell was overdischarged after conditioning with the same fixed resistive load but the resistive load was removed after 5 hours, prior to the anode potential rapidly decreasing. No visually darkened areas or F, O or P depositions were observed on the anode in that case after cell recharge, cycling and disassembly. Another LiFePO4/graphite Li-ion cell was overdischarged after conditioning with the same fixed resistive load but the resistive load was removed after 48 hours and the cell was not recharged or cycled before cell disassembly. Depositions of F, P and O were found on the anode of the cell, similar to the cell overdischarged for 2 weeks then recharged and cycled. Additionally, a cell modified with added lithium inventory such that the anode does not increase to the copper oxidation potential during fixed resistive load overdischarge did not exhibit a delayed rapid cell voltage increase. Increased presence of oxygen was found on the anode of this cell after recharge, cycling and disassembly, but no increase in F or P.
Overall, while a precise reaction mechanism cannot be determined the present results indicate that the anode potential decrease, and subsequent cell voltage increase during fixed resistive load of these 0.7 mAh LiFePO4/graphite Li-ion pouch cells is caused by an oxidative breakdown of the electrolyte onto the anode surface that is initiated by copper oxidation. This cell voltage increase after cell overdischarge to <200 mV can have substantial implications for cells that overdischarge in storage, as periodic voltage checks may miss a cell overdischarge. Additionally, the observed increase in lithium inventory could be utilized for end-of-life cell capacity recovery with controlled cell overdischarges.
References

K. R. Crompton, J. Electrochem. Soc., 167, 090518 (2020).

Figure 1