Case Studies in Failure Analysis: Cell Quality Assessment and Root Cause Investigations

Summary:

With the increasing number of battery powered devices on the market, battery safety is more important now than ever. Exponent works with battery and device manufacturers around the world to help make their products safer and more reliable. Through the use of both non-destructive and destructive techniques, device and battery manufacturers are able to determine the quality of their battery cells and inspect them for manufacturing or design related issues.

Computed tomography X-ray scanning is a powerful non-destructive technique that allows users to see inside their device and battery cells and inspect for design or manufacturing related issues. Destructive analysis such as cell teardowns are also useful to assess the quality of battery cells. Chemical analysis of contaminants, quality and height of tab welds, and numerous other features can be inspected for during these teardowns.

More specialized testing can also be done to qualify the safety of a product. Exponent has designed mechanical test rigs for numerous different applications. Here we used an aluminum t-slot frame and an indenter with an attached load cell to apply a specific force to the trackpad of a laptop and inspect the effects of applied force and cyclic loading on the battery pack underneath.

Exponent also performs cell cycling studies. Here, cells were cycled to various voltages and their capacity as a function of cycle number was monitored. You can see the dramatic effects the upper and lower cutoff voltages have on long term performance of lithium-ion cells.

While cell and device manufacturers can take numerous steps to ensure their products are safe for the market, issues may still arise. One notorious example was the Samsung Galaxy Note 7 which Exponent helped perform failure analysis investigations for. In this case Samsung had two different issues, one related to the pouch volume and one related to the tab welds.

In order to ensure failures such as these do not occur, techniques exist for analysis of production and abused cells. Mechanical cross sections of cells are useful to inspect for contaminant particles or damage to the current collectors such as pitting corrosion or dissolution and deposition of metallic copper. Submerging cells in liquid nitrogen and performing cryo HiPot testing allows for non-destructive examination to identify cells with soft shorts after cell assembly. Opening cells in an argon glovebox and inserting a lithium metal reference electrode allows for the independent monitoring of the positive and negative electrode potentials during operation. The data from one such cell is shown in the bottom right which demonstrates that the average negative electrode potential was about 500 mV too high even though the overall cell voltage stayed below the 4.2 V cut-off.