In Situ Observation of the Electrochemical Dissolution and Deposition of Copper Contaminations in Li-Ion Batteries
Authors: Kuwabara Junji , Sato Kenji 
Published: February 2020

Background and Purpose
In the Li ion battery(LIB) manufacturing process, unintentional metallic contamination is a major problem.
Among various metallic species, copper contamination on the positive electrode can be electrochemically dissolved at the positive electrode potential and can be dendritically deposited on the negative electrode. As such a dissolution-deposition reaction may cause the micro-short-circuit between positive and negative electrodes, strict quality control is required to prevent the metallic contaminations into LIB.
Despite the industrial importance, little is known about the electrochemical dissolution- deposition reactions of metallic contaminations is LIB. We should reveal the phenomenon at the moment of the micro-short-circuit formation, the phase change of the dendrite at the copper-dissolving potential, and so on. In this study we aim to clarify these behaviors by an electrochemical confocal laser scanning microscope.

Method
The test cell is composed of a positive electrode, a negative electrode and a separator. The observation window is installed so that the cell cross-section can be observed.
A small fragment of copper foil is located on the positive electrode surface during cell assembly imitating the contamination.
In situ cross-sectional observations with a confocal laser scanning microscope (Lasertec Corp.) were carried out to investigate the dissolution of contamination and the deposition and the re-dissolution of dendritic deposition during charging and discharging.

Result
Dissolution of Cu contamination was observed immediately after charging started. After that, Cu dendrite was deposited on the surface of negative electrode and gradually grew. Continuing the charging, the dendrite-like deposition penetrated the separators and a micro-short-circuit between the positive electrode and the negative electrode was formed.
In the micro-short-circuited state, the cell voltage couldn’t rise by a low charging current and the voltage dropped down to 0V when the charging ended.
By increasing the charging current, a part of the dendrite in contact with the positive electrode surface began to dissolve. Continuing a constant voltage charging at high voltage, almost all the positive electrode contact portion of the dendrite was observed to dissolve.
The micro-short-circuit has been eliminated after the dendrite re-dissolution.
Fig.1 shows a cross-sectional view of the micro-short-circuited state and the partially dissolved dendrite.
These experimental results suggest that constant voltage charging made the potential of the positive electrode near the dendrite, which was reduced by a micro-short-circuit, increase up to Cu dissolution potential.

Conclusion
In situ Observation using a Confocal Laser Scanning Microscope, we found that the micro-short-circuit is eliminated by re-dissolution of Cu dendrites in constant voltage charging.
By applying this discovery to battery manufacturing inspection or vehicle control, the reliability of battery can be improved furthermore.

Figure 1