Battery Abuse Tolerance 141 continually updated as the test results begin to come to completion. It is typical to create a table such as this one for each phase of the design and testing process. Failure Modes of Lithium-Ion Batteries Perhaps at this point it would be valuable to briefly discuss the manners in which lithium-ion batteries could fail before we begin discussing the actual types of testing that may be done. We can generally break the failure modes down into two different categories: internal failure modes and external failure modes. An internal failure mode can occur from a manufacturing defect, such as manufacturing debris getting inside the jellyroll.
Another internal failure that may occur is an increase in internal resistance, which generally may occur toward the end of the batteries life. An exter- nal failure mode can occur when the battery is either operated outside of its safety zone or due to a failure of the controls or thermal systems. The worst case external failure mode occurs during a vehicular collision or when one or more cells go into thermal runaway. One of the biggest lithium-ion battery recalls that has occurred in the past 20 years was at least in part the result of an internal failure mode.
In the early 2000s, Sony recalled hundreds of thousands of lithium-ion batteries that were used in laptop computers due to the risk of fire. While there may have been multiple potential causes for the failures of these cells, one item that was identified was a flaw in the manufacturing process when the lids of the 18650-type cells were crimped in place. Upon investigation it was found that the nickel plating on the cans was being damaged and flaking off micro-sized particles during the crimping process. This resulted in material entering the jellyroll and ultimately with an internal short circuit (Sony, 2006).
Subsequently to this, Sony among many other manufacturers changed their assembly processes to move away from crimping and toward laser welding the lids in place. Another, and more common, internal failure mode is the increase in internal resistance of the cell. By measuring the rate at which the internal impedance (resistance) grows, it is possible to determine the life of the cell. However, this is not as easy as it sounds. The internal imped- ance is essentially measuring the growth of the solid electrolyte interphase (SEI) layer on the anode. As the cell cycles, lithium-ions tend to get trapped in the SEI layer over time.
And as more and more lithium-ions get trapped inside the SEI layer, it becomes more difficult for the free lithium-ions to pass through to the cathode (Voelker, 2014). Other internal failure modes may include a chemical breakdown of the electrolyte, which could include generation of gasses within the cell. This generally occurs when the lithium-ion battery is being used at higher temperatures. In fact, many cell manufacturers include special additives in their electrolytes in order to prevent the cell from going into premature thermal runaway. These additives will generally begin to create gasses within the cell at certain temperatures.