h 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 applications involving cell safety through battery system safety. The focus is on refining large-format lithium-ion battery standards (UL 2580 for electric vehicles, UL 2271 for light electric vehicles and UL 1973 for light electric rail and stationary applications — for more information, please refer to the third article in this journal, “Advancing Lithium-ion Battery Standards”), revising cell requirements to address specific applications, verifying cell operating regions, ensuring that battery systems maintain safe cell operating regions, and exploring system failure mode effects analysis (FMEA) and functional safety.10
Research at UL, along with collaborations with well-known battery safety research laboratories, has resulted in the development of the Indentation Induced ISC test. This testing approach shows promise as a candidate for battery safety standards, most likely as a screening test. To date, analysis of results from cells subjected to the Indentation Induced ISC test shows a correlation between test performance (observed severity of failure) and a variety of cell parameters, including energy density, thermal stability of active materials and cell chemistry.11
With recalls and other safety issues related to lithium-ion batteries still making headlines, there is a heightened need for the kind of open and cooperative dialogue UL and other key stakeholders are engaging in to share information on the failure modes of lithium-ion cells and to help develop and refine ISC tests for consensus-based safety standards.12 We are committed to evolving standards to help drive the safe use of lithium-ion batteries as their applications expand to more and more industries and products. As the leading organization for lithium-ion battery safety testing, UL is focused on the full range of battery chemistries and designs, including different materials, component-level characterization at the cell level and highly integrated battery systems. We will continue to dedicate significant resources to battery safety research and w