Cu Dissolution and re-deposition during Over-Discharge of Li-Ion Cells – A Post-Mortem Study

Summary:

Over-discharge of Li-ion cells, i.e. discharging below the end-of-discharge voltage can lead to dissolution of Cu from the current collector.
In order to learn more about the underlying mechanisms, we conducted a systematic study on re-deposited Cu in commercial Li-ion cells by Post-Mortem analysis. Our study focusses on the questions (i) where is Cu it re-deposited, (ii) what is the effect of time, (iii) what is the difference in Cu deposition for fresh and aged cells and (iv) which combination of analytical methods is suitable to detect Cu deposition in Post-Mortem analysis.
Cu dissolution and re-deposition in the commercial cells was forced by discharge to 0 V. A baseline cell, which was discharged to the specified end-of-discharge voltage, served as comparison. Cu is found in both the anodes and cathodes of over-discharged anodes with Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) and Energy Dispersive X-Ray Spectroscopy (EDX). Scanning Electron Microscopy (SEM) was used to investigate the morphology of the Cu re-depositions. A new method based on Glow Discharge – Optical Emission Spectroscopy (GD-OES) was developed to quantify the amount of Cu in the anodes. GD-OES and EDX of cross-sectioned anodes reveal that Cu is mainly deposited on the anode surface (i.e. near the separator). The amount of re-deposited Cu determined by ICP-OES and GD-OES in the anodes range from 1.3 wt.-% to 4.7 wt.-%. The measurements show an increased Cu dissolution for previously aged cells. SEM images of the anode cross-section indicate that the Cu originates from pitting corrosion of the Cu current collector of the negative electrode.
The anode potential, at which Cu dissolution occurs, was investigated with 3-electrode pouch cells. During Discharge from 0 V to -0.5 V the anode potential stayed mostly constant at 3.58 V vs. Li/Li+, indicating the ongoing Cu dissolution during over-discharge.