Indentation Induced ISC Test
An innovative testing method that was developed to help understand the potential severity of internal short circuits (ISCs).
WHY THE INDENTATION INDUCED ISC TEST MATTERS
Driven largely by their long cycle lives, low self-discharge rates and high energy/power densities,1 lithium-ion batteries are becoming an important sustainable energy technology. When considering reported incidents involving lithium-ion batteries, many cite internal short circuits (ISCs) as a possible intermediate cause for the overheating of the cell. Though other test methods exist to simulate ISCs in lithium-ion cells, the Indentation Induced ISC test was developed based on best-practice principles to provide a practical and simple method that is very suitable for battery safety standards. This test gives UL the ability to simulate how a lithium-ion cell behaves when subjected to an ISC condition, which will help mitigate the hazards of ISCs and support the safe commercialization of lithium-ion batteries.
The performance characteristics of lithium-ion batteries, coupled with the projected one-third decrease in their costs by 2017,2 make them increasingly popular in a broad range of applications. For example, lithium-ion batteries now comprise in excess of 95 percent of mobile phone batteries worldwide.3 Lithium-ion batteries are also used in a variety of consumer electrical and electronic devices (e.g., laptop computers, tablet computers and digital cameras), medical devices (e.g., patient monitors, handheld surgical tools and portable diagnostic equipment), industrial equipment (e.g., cordless power tools, wireless security systems and outdoor portable electronic equipment), automotive applications (e.g., electric vehicles), aerospace applications (e.g., aircraft and spacecraft), and energy (e.g., grid-connected electricity storage).4
Although lithium-ion batteries are designed with integrated passive safeguards and active safeguards for pack designs, these batteries have been involved in incidents involving overheating 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 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 will continue to actively improve existing standards and develop new ones.