Cu Dissolution during Over-Discharge of Li-Ion Cells to 0 V: A Post-Mortem Study

Abstract

Cu dissolution in Li-ion cells during over-discharge to 0 V was investigated by Post-Mortem analysis. Commercial 18650 type cells with graphite anode and NMC/LMO cathodes as well as pilot-scale pouch full cells with graphite/NMC chemistry with reference electrode were investigated. The effects of discharge time at 0 V in the range of 100 h to 1000 h for fresh cells as well as the effect of cells cycled under Li deposition conditions were considered. For comparison, electrodes from cells discharged to the end-of-discharge voltage (2.0 V) were examined. By extensive Post-mortem analysis using inductively coupled plasma (ICP-OES), scanning electron microscopy (SEM) with BSE and SE detectors, energy dispersive X-ray analysis (EDX), and glow discharge optical emission spectroscopy (GD-OES), we show that Cu compounds are present on the anode surface and on the cathode from cells, which were over-discharged. Cross-sections show that the Cu originates from pitting corrosion of the negative current collector. Combined electrochemical/ICP-OES measurements in commercial cells as well as reference electrode measurements in 3-electrode pouch full cells suggest that Cu is dissolved as Cu+ ions.

Reviewing the Safe Shipping of Lithium-Ion and Sodium-Ion Cells: A Materials Chemistry Perspective

“…Thus, if not managed correctly, the cells might slip in time to overdischarge conditions, especially at elevated temperatures [23,32,54]. This situation might also occur in electric vehicles that are parked at low SOCs for long durations especially at high temperatures above 40 °C-they can actually overdischarge during this time (and thus significantly affect lifetime of the battery as already outlined in the previous section) [31].…”

“…Perhaps the most common cause might be when Li-ion cells are connected in series to form strings of cells, as occurs in most high voltage Li-ion battery packs used for applications such as different types of electric vehicles or stationary energy storage. As each cell in a series string would experience the same amount of current flow, if the string has even one underperforming cell with lower capacity compared with its sister cells in the string, that underperforming cell might have the potential to experience overdischarge or overcharge conditions; healthy cells with their rated capacity would not experience this [31,37,38].…”

“…As noted by the authors, Cu deposition on the cathode occurred due to the fact that the cathode potential was lower than the anode potential in the overdischarged states; Cu deposition was identified by SEM-EDX (scanning electron microscopy-energy dispersive X-ray spectroscopy) [29]. While there might be some uncertainty regarding whether Cu oxidizes as Cu + or Cu 2+ , results from several studies indicate that the preferred oxidation of the copper current collector in typical nonaqueous Li-ion electrolytes appears to proceed via the Cu/Cu + redox couple (this cannot, however, be stated definitively currently due to conflicting reports in the literature), and that Cu deposition onto the cathode can be metallic in nature as well as Cu compounds such as Cu 2 O or Cu(OH) 2 [26,[29][30][31][32]. There are numerous examples in the literature on the pernicious effects of overdischarging commercial Li-ion cells.…”

Effects of Over-Discharging Cycling on Battery Degradation at Low Temperatures

“…In practical applications, cells are connected in series in a battery pack, and their capacities are not identical. As the cells with higher capacities are fully discharged, those with lower capacities will probably be discharged beyond their rated capacities, leading their voltages lower than the standard cut-off voltage, namely overdischarging [3]. Over-discharging is more likely to happen at low temperatures because it is easier to drop below the discharging cut-off voltage due to the large polarization effect, that is, the difference between the electrode potential and the equilibrium potential under the experimental condition is large.…”

“…However, most investigations, including the above of the degradation characteristics during over-discharge have been conducted at room temperatures with few studies of low-temperature effects. Different time of constant voltage at a certain over-discharge voltage may affect the cell because of the large polarization [3], which should be studied further.…”