Case Study #3: Severe Cathode Contamination

Several low voltage self-discharging cells were found in a single batch of cells. When opened they contained large, mm size SD spots.


Large SD spot found on separator


Self-discharging cells are an unavoidable fact of life for Li-ion chemistries but most are culled by the cell manufacturer before they are shipped. Nevertheless, self-discharging cells are a common cause of returns, especially for multi-cell battery packs where they cause imbalance. Though self-discharge is usually a benign failure mode it is worth checking such returns because as the following example shows they can provide early warning of serious safety issues.

SD spots are produced by metal impurity particles in the cathode. After softening the cathode coating with solvent, macroscopic magnetic particles could be extracted in this case.



SEM EDX shows that these metal shavings are steel (primarily Fe with a few % Cr and Mn). The most alarming feature of these particles is that they are larger than the coating thickness. After the cathode was cleaned off the aluminum foil, the impressions of these shavings are clearly visible on the foil.


Impression of a metal shaving on the Al foil after stripping the cathode coating.


The danger posed by these large impurity particles is that when they penetrate the separator, either via corrosion or puncture, the cell will short through these metal shavings to the Al foil and they will act as glowing igniters in the thermally unstable cathode.