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

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.