heating and fire that, while very rare, have put these batteries in the public spotlight.5 In many cases, the battery failures were linked to ISCs that led to thermal runaway, resulting in the explosive release of energy along with fire. These incidents have provided an impetus for research aimed at understanding the causes of lithium-ion battery failures and guiding safer battery cell designs. The number of lithium-ion batteries in use, the complexity of the lithium-ion battery cells and the numerous usage conditions make the design of safe cells and the development of tests for battery safety standards extremely challenging.6 These challenges underscore the need for a reliable ISC simulation method that helps improve product safety by ensuring that consensus-based battery safety standards effectively accommodate the rapidly changing state of lithium-ion cell design and applications.
WHAT DID UL DO?
UL invested research resources and collaborated with other organizations with the goal of developing a reliable and repeatable testing methodology that met two key criteria. First, the test needed to be able to generate a localized ISC within a closed cell that would simulate the conditions similar to those found in the field failures of lithium-ion batteries. Second, the new test needed to be acceptable for battery safety standards.
Our research resulted in the development of the innovative Indentation Induced ISC test. After demonstrating the potential of this testing method, we partnered with NASA and Oak Ridge National Laboratories (ORNL) to further develop the test approach. NASA already had its own ISC test method, but seeing the advances made in the UL test method, it adopted and fine-tuned the Indentation Induced ISC approach. This method is now part of NASA’s battery qualification process for space applications. Next, UL collaborated with ORNL to extend the test setup to cover a variety of form factors.
The Indentation Induced ISC test is appropriate for cylindrical cells and other form factors, such as pouch and prismatic cells, with some variations in setup. In the test setup, the cell is placed in a holder that prevents its rotation or translation. An indenter with a smooth profile presses from above against the cell casing at a constant speed (0.01 – 0.1 mm/s). Test measurements include temperature of the casing surface at a point near the indentation site, distance traveled by the indenter (amount of cell casing deflection), applied force through the indenter and open circuit voltage. The cells can be at different states of charge (SOC) or stages of aging. The entire setup is placed in a chamber that allows for control of ambient temperature.7
As the indenter presses against the casing, layers of separator, anode and cathode immediately below the indentation region are deformed due to localized high curvature (Figure 1). The resulting high stress/strain will lead to a mechanical failure of the separator (with failure of the casing), allowing for direct contact between electrodes at a distance only a few layers below the casing surface (Figure 2). The effect of the separator failure is a sudden alternate pathway for charge flow and a subsequent drop in the open circuit voltage (OCV) (Figure 3). For some cells, seconds after a measured drop in the open circuit voltage (100mV), there is a rapid increase in cell surface temperature (as high as 700°C) with an outcome involving explosive release of gases and generation of flames (Figure 4).8
Risk is typically measured in terms of the severity of failure multiplied by the probability of failure. In forcing a failure, the Indentation Induced ISC test is basically measuring the severity of cell failure. As noted above, NASA uses this technique in its screening of commercial off-the-shelf (COTS) rechargeable batteries for space applications. Cells that do not perform well under this type of test would then be subjected to a more stringent secondary testing schedule to help establish the probability of ISC cell failure.9
Today, UL is developing tests and standards for app