Few technologies are paving the way for innovation and catalyzing product advancements like lithium-ion batteries. The market is projected to double in size within the next three years as the batteries continue to be adopted in industries such as automotive, aerospace, renewable energy, utilities and telecom.
The interest in lithium-ion batteries is attributed to the evolution of secondary (or rechargeable) technology. As a result, these batteries provide greater energy density, longer life cycle, higher efficiency and improved reliability and performance standards. Given the critical importance of lithium-ion batteries to electric cars, advanced commercial jets and electric grids, it is imperative to conduct research and advance battery standards, maximizing safety as well as safeguarding the adoption of new applications and uses.
The continuing advancements in high power and energy applications require a sustained effort in analyzing cell and battery safety and performance. In addition, the expected demands on the high power and energy cells and batteries (i.e., “duty cycle”) puts greater emphasis on understanding the limits of the electrochemical reactions. Advancing safety is critical to facilitating progress, as innovation is not possible without assessing and mitigating risk.
While the rate of failure is small for lithium-ion batteries, there have been incidents that call attention to their potential risks. These concerns for safety must be addressed, and standards have been enacted to mandate a number of tests designed to assess specific risks associated with their use, thus helping to ensure minimal risk and maximum efficiency.
Challenges and testing
The widespread commercial use of lithium-ion batteries began in the 1990s, and designs were developed to meet an array of product demands. Normally, the choice of battery is driven by considerations such as application requirements for power and energy, the anticipated environment in which the battery-powered product will be used, and cost. With lithium-ion batteries, the selection means choosing something generally more expensive than alternatives, but offering significant advantages including higher energy and energy-to-volume or energy-to-weight ratios.
Challenges in performance and safety still exist for all batteries in various applications. Users increasingly expect lithium-ion batteries to last for longer periods of time, with some applications expected to have a battery life of five to 20 years. Also, some lithium-ion cells used in large format applications such as electric vehicles may be considered for reuse in energy storage applications since they often still have as much as 80 percent of their usable capacity left at the end of their life in a vehicle. These long-term usages are important to consider when developing safety requirements, because failures may be dependent on how the state of the lithium-ion cell changes over time. Current safety standards either do not address the potential impact of battery aging or do not go far enough in the aging process to understand long-term usage effects. Given the trend toward lithium-ion battery long-term usage and potential for reuse, the effects of aging need to be studied to understand how it affects battery failure.
[pagebreak]At UL, a first-stage research study was developed to understand the potential role aging played in lithium-ion battery field failures. The research identified two critical safety concerns: the polarization effect on aged batteries (which can have a significant effect on battery efficiency and safety due to increased heating and the accelerated aging process), and the lower thermal stability of active materials in aged batteries.
Thermal stability is the stability of the cell at higher temperatures. A “hot box” test on aged and fresh samples demonstrates the effect of lower thermal stability of active materials in aged batteries. Research identified this as a critical safety concern, with data from a differential scanning calorimeter suggesting that heat-generating reactions within the cells occur earlier for an aged cell.
New risks, new science
Research is still in the early stages for assessing the effects of aging on lithium-ion batteries, and it will need to go beyond the single chemistry studied thus far, moving into other common cell chemistries. Additionally, research will be extended over more cycles and conducted on large-format lithium-ion batteries such as those in stationary energy storage applications. Discovering the full impact of aging on lithium-ion battery safety will allow independent standards organizations to update standards and help to ensure the safe use of lithium-ion batteries over time.
Innovation allows greater efficiency and productivity, but along with innovation comes risk. With the continuing development and proliferation of lithium-ion batteries, there is a need for research that informs rigorous safety standards. Devices powered by lithium-ion batteries offer tremendous opportunities, and as manufacturers respond with new technologies, there is a need to put safeguards in place that help to ensure the safety of lithium-ion batteries, affirming smooth adoption by transportation, energy, healthcare and other industries.
Laurie Florence is the principal engineer at UL for batteries, among other areas. With more than 20 years of experience in testing and certification, she has responsibility for technical competency criteria for UL staff and supports UL certification programs. Alvin Wu is research engineer, corporate research at UL, focusing on battery technology in industrial settings, including advancing battery safety research and updating battery safety. For more information on UL research, visit UL.com/newscience.
Photo by Brookhaven National Laboratory.