UL scientists and researchers are helping define and develop safety tests that assess the propensity of a battery to experience a short circuit under certain abuse conditions.
With both high-energy and high-power density, lithium-ion cells have become the chemistry of choice for rechargeable batteries that are increasingly used in portable consumer electronics, power tools, medical equipment, aerospace applications and electric and hybrid electric vehicles. The worldwide market for lithium batteries is projected to reach nearly $10 billion in annual sales by 2014, with the market for lithium-ion batteries representing almost 86 percent of those sales ($8.6 billion).1
At the same time, however, several highly publicized incidents involving fire and the explosion of devices powered by lithium-ion cells have raised concerns about the safety of such small yet powerful products. Reports of battery defects that lead to internal short circuits and thermal runaways (where a sharp increase in heat sets off a self-sustaining and potentially intensifying heat or exothermic reaction) have caused thousands of products to be recalled for a singular product failure.2
WHAT DID UL DO?
Some of the highly publicized field failures of lithium-ion batteries have been linked to an internal short circuit (ISC) within the battery. Notably, most lithium-ion battery safety standards and testing protocols do not specifically include testing for ISCs.
Over the past two years, UL has partnered with key battery research facilities such as Argonne National Laboratories and the National Aeronautics and Space Administration to better understand the root causes of ISCs. The goal of UL’s innovative research is to define and develop safety tests that assess the propensity of a battery to experience a short circuit under certain abuse conditions.
Although an ISC may have many causes, it is essentially a pathway between the cathode and the anode that allows for efficient but unintended charge flow. This highly localized charge flow results in Joule heating due to internal resistance, with subsequent heating of the active materials within the lithium-ion battery, such as the electrolytes, separator and electrodes. The increased heat may destabilize the active materials, in turn starting a self-sustaining exothermic reaction. The subsequent heat and pressure buildup within the cell may lead to catastrophic structural failure of the battery casing and the risk of additional combustion as a result of exposure to outside air.3
Lithium-ion batteries are designed with integrated safety devices that interrupt the external electrical load in the event of an overcurrent condition or relieve excessive pressure buildup in the cell. However, these safety devices are unable to mitigate all internal cell fault situations, such as an ISC. For products like electric vehicles, the presence of hundreds (or likely thousands) of these battery cells requires more sophisticated safeguards such as battery management systems.
Clearly, the desired goal is a test portfolio (simulating a wide variety of abuse conditions) that can assess the likelihood of a battery to manifest a short circuit. Importantly, in designing a test for a specific failure, the root causes and failure pathways must be known. These causes may include a large internal defect or a severe external force that deforms the inner layers of the battery sufficiently to compromise the separator. In many failure incidents, only partial root cause and failure information is available. Lithium-ion battery designers and researchers are working to create new battery designs that mitigate the impact of these causes.
The variety of root causes for ISCs makes it difficult to design a single safety test that can assess the robustness of a lithium-ion battery. To date, only JIS C8714 specifies an ISC test, known as the forced internal short circuit test. (Note that IEEE 1625, Annex D references the FISC test found in JIS C8714.) This test creates an ISC by carefully disassembling a charged cell sample casing and placing a specified nickel particle under the cell-winding construction to simulate an internal defect. The cell sample, minus the casing, is then subjected to a specified crushing action at an elevated temperature. However, best practices in safety test design preclude disassembly of a product.4
UL researchers have developed a first-of-its-kind test that induces ISCs by subjecting intact lithium-ion battery cells to a localized indentation under elevated temperature conditions. During this test, the open circuit voltage, cell surface temperature force and position of the indenter probe are measured in real time. The test is currently under development for possible inclusion in the UL 1642 and UL Subject 2580 standards. By understanding and analyzing the specific battery chemistries, battery components (separators, anodes and cathodes) and cell designs (cylindrical, prismatic and pouch), UL is able to understand the behavior of the newly commercialized lithium-ion battery designs from the inside out. Correlating these design attributes to safety behavior and performance is a major factor in UL’s approach to New Science.5
WHY IT MATTERS
The increased usage and demand for lithium-ion batteries has resulted in some unanticipated usage conditions, poor design executions and unconventional use or abuse of a product — all of which create hazards because of the potential for lithium- ion batteries to fault with ISCs and thermal runaway, leading to fires or explosions. It is important that UL help mitigate risk by innovating tests to keep consumers safe and reduce costly manufacturer and retailer product recalls.
Because the development of lithium-ion batteries is an active area in fundamental research and product development, knowledge regarding the use and abuse of these products and their possible failure modes is still growing. Therefore, it is important that safety standards evolve to help drive the safe commercial use of these energy storage devices as they power more and more products. UL will continue dedicating significant resources to translating battery safety research into safety standards. This focus will cover the wide range of chemistries and battery designs. The work covers the multiscale continuum — from material and component-level characterization at the cell level to highly integrated battery systems and beyond.