142 Chapter 12 The separator may also begin to break down at high temperatures, depending on the type of separator, which causes the flow of lithium-ions to slow and eventually stop as the “pores” in the separator become melted or clogged. Again this tends to happen at high temperatures and is in fact designed to begin failure modes at certain temperatures in order to prevent thermal runaway events from occurring. Yet one more internal failure mode that can occur is called lithium plating, or in some areas it is called “white out.” This condition occurs when the cell goes into overvoltage or undervolt- age operating conditions.
In these conditions, the lithium-ions will get stuck, typically in the anode, and will no longer pass back and forth in the cell. The more lithium that gets stuck the more plating occurs and the more internal resistance grows and capacity fades. External failure modes range from operating the batteries at continuously high loads or high temperatures and less frequently from impacts and vehicle crash scenarios. MIT’s Impact Crashworthiness Laboratory has been working to understand the causes and pos- sible solutions and remedies for lithium-ion battery design under impact conditions (Sah- raie, Meier, & Wierzbicki, 2014; Xia, Wierzbicki, Sahraei, & Zhang, 2014).
When lithium-ion cells are punctured during an impact an external short-circuit event is the most likely scenario. This type of external failure mode causes a rapid loss of electrolyte, mas- sive heat generation, and under most conditions a thermal runaway event. The MIT group has proposed the use of increased strength metals to “armor plate” the bottom of the pack in order to reduce the potential for external debris to penetrate into the pack. This of course is assuming that the battery pack is mounted external to the vehicle, underneath it rather than inside the vehicle. However, even with a battery that is mounted internally there is a risk of penetration during a crash event.
Many automotive original equipment manufactur- ers (OEMs) will conduct crash simulations to ensure that the battery is located out of the “crush zone” of the vehicle. Some are even going as far as designing the battery to become a structural member of the vehicle. Characterization and Performance Testing Battery performance testing typically occurs in several phases, beginning with cell character- ization. The purpose of cell characterization is to determine how the cell will perform under a certain set of operating criteria. This allows the system engineer to design a system that ensures that the cells do not operate outside of this range.
Characterization is usually a series of charge/discharge cycles that are performed at multiple temperature ranges under identical test cycles. Characterization testing is generally conducted using a “cycler” or “channel” that provides current and power to the cell, module, or pack, which is controlled with a separate unit which provides the programmable characteristics ( Figure 1 ) and finally is done in a thermal chamber to manage the temperatures the cell will experience during the testing. The thermal chamber