SMA Actuated Safety Vent for Lithium-ion Battery Pack: Project Background and Introduction
September 9, 2023 - By Hedda Grelz, Andres Calderon, Oselumenosen Ekhuemelo, and Adolfo Diaz
Electric Vehicles (EVs) are gaining popularity as a greener alternative to traditional gasoline-powered vehicles. In 2023, it is expected that 18% of total car sales will be electric vehicles [1]. According to the US Department of Energy, most of today's all-electric vehicles use lithium-ion batteries (LIBs) for power due to their high energy density [2]. One of the main development areas of LIBs is addressing safety concerns with regard to overheating, specifically thermal runaway.
Thermal runaway is an uncontrollable chain reaction of heat generation within a battery cell, potentially causing life-threatening fires and explosions in the absence of heat release [3]. Figure 1 illustrates this process. The risk of thermal runaway is low for a single-cell battery ( 0.0001%), but the probability of a cell initiating thermal runaway increases significantly in large installations (such as car batteries) with thousands of cells (0.1%). Additionally, if a single cell in these large battery packs goes into thermal runaway, adjacent cells may ignite and start a cascading thermal runaway event which can result in catastrophic explosions, property damage, bodily injuries, and death [4].
Figure 1: Sequence of events from initial damage to final thermal runaway in LIBs [5]
Some of the most significant safety concerns of LIBs that cause thermal runaway are flammable electrolytes, combustible separators, and temperature-sensitive electrode materials. Because of these traits, LIBs need to be in an enclosed space during operation. However, if the pressure gets too high, the protection itself becomes dangerous. A state-of-the-art solution to reduce the effects of thermal runaway, and the ensuing pressure build up, is to introduce a safety vent [3]. Through releasing pressure and lowering the system temperature, the safety vent will lessen the results of thermal runaway (see Figure 2).
There are numerous vent designs present in industry and explored in literature, but there are still shortcomings that create opportunities for innovative safety solutions. One common design solution includes a spike and a gasket with a puncture film that deforms under high pressure to allow the spike to puncture the film and release the pressure [6]. The issue with the vent is that it cannot be closed upon opening, leaving the flammable components of the battery exposed to oxygen which can result in larger fires. To tackle this issue, we can introduce actively actuated vents that can be closed after ventilation. However, the key drawback of these vents is that they cannot open if the control system fails - which is likely under extreme temperature or pressure conditions.
Figure 2: Typical thermal runaway phenomena in two different batteries where only the top one has a safety vent [3].
For the UH Capstone project, Team Navy will construct a safety vent for lithium-ion battery packs using a Shape Memory Alloy (SMA). This approach focuses on the critical problem of thermal runaway in LIBs by addressing shortcomings of current vent designs. Figure 3 shows the basic working principle of an SMA. Our specific focus revolves around simplifying current vent designs and creating a vent that can be both passively and actively actuated. Unlike other actively actuated vents, the SMA can act as both a temperature sensor and a vent opening actuator. This reduces the number of components required for the design. Active opening is crucial since it allows the SMA to trigger vent opening even at small temperature increases, upon heating. This action adds an increased level of safety through focusing on preventing potential damage. However, it is also crucial that the vent can open passively, in case of control system failure which is likely in a collision or extreme temperature conditions. Because of the SMA's ability to memorize its shape at a higher temperature, the vent will open passively (without interaction from controls) by deforming once the temperature in the battery reaches a certain threshold.
The following physical constraints have been identified for our design. In the process of thermal runaway, most LIB cells ignite at 120-130°C [3]. Therefore, the SMA transition temperature (when shape change happens) needs to be slightly below this, around 110°C. The vent will be designed for an electric car battery. For dimensional references, our team intends to use Tesla’s small battery which has the size of 68.5 x 30 x 75 cm (length x width x height) [7]. Another important constraint is the budget which the team has set to 500 USD. The time from initial battery defect to ignition in the event of thermal runaway has been reported to be 100 seconds, which means that this is the maximum reaction time a control system can have. However, the team will set goals for significantly faster operation times from earlier prevention of thermal runaway. The opening and closing time of the vent, as well as the maximum deformation required, is yet to be quantified as this is dependent on the detailed design of the vent. Our team plans to set performance goals based on control system response time, SMA deformation based on control input and passive actuation, and minimizing material used and weight to keep the solution cost-effective.
Figure 3: Basic working principle of shape memory alloy (SMA) [8]
The main obstacles that our team expects to face are the team’s limited knowledge of FEA modeling of SMAs and the development of control systems using microcontrollers, both critical aspects for a successful solution. To address this, we will collaborate with Dr. Gangbing Song in the UH Department of Mechanical Engineering, who specializes in SMA and other smart materials and controls. Additionally, a budget constraint poses another challenge, as the team wants to develop a budget-friendly solution. This issue is critical since the main drawback of including lithium-ion batteries during project testing is their high price. The team also needs to do extensive market research to avoid ordering a custom-made SMA, since these are expensive.
Except for the microcontroller, other components should not add significant cost. Our team also needs to address the high risk of testing and validation of the device as it is meant to operate at very high temperatures and around hazardous material. To ensure the safety of all people involved, we plan to test the vent using a chamber with a hot air blower rather than an actual Li-ion battery. The air blower will be used to raise the temperature in the chamber to simulate the event of thermal runaway in a battery without risking property damage or bodily injury.
By addressing these challenges and focusing on the main goals and constraints of the problem, our team envisions designing an SMA safety vent that can be both passively and actively actuated to increase the safety of lithium-ion batteries in electric vehicles. At the end of this capstone project, a working prototype could be used as a starting point to create a design that can be tested and implemented on a real lithium-ion battery pack.
References
[1] Guo, J., Li, Y., Pedersen, K., and Stroe, D.-I., 2021, “Lithium-Ion Battery Operation, Degradation, and Aging Mechanism in Electric Vehicles: An Overview,” Energies, 14(17), p. 5220.
[2] “Batteries for Electric Vehicles.” Alternative Fuels Data Center: Batteries for Electric Vehicles, afdc.energy.gov/vehicles/electric_batteries.html
[3] Ouyang, D., Weng, J., Chen, M., and Wang, J., 2022, “What a Role Does the Safety Vent Play in the Safety of 18650-Size Lithium-Ion Batteries?,” Process Safety and Environmental Protection, 159, pp. 433–441.
[4] Shurtz, Randy, Shurtz, Randy, Shurtz, Randy, Shurtz, Randy, Kurzawski, Andrew, Kurzawski, Andrew, Hewson, John C., Hewson, John C., Preger, Yuliya, Preger, Yuliya, Torres-Castro, Loraine, Torres-Castro, Loraine, Lamb, Joshua, and Lamb, Joshua. Evaluating Safety Characteristics of Lithium-Ion Battery Systems Through Cascading Thermal Runaway Experiments and Modeling.. United States: N. p., 2019. Web.
[5] Feng, X., Ouyang, M., Liu, X., Lu, L., Xia, Y., and He, X., 2018, “Thermal Runaway Mechanism of Lithium Ion Battery for Electric Vehicles: A Review,” Energy Storage Materials, 10, pp. 246–267.
[6] Zhu, J., Zhang, X., Sahraei, E., and Wierzbicki, T., 2016, “Deformation and Failure Mechanisms of 18650 Battery Cells under Axial Compression” Journal of Power Sources, 336, pp. 332–340.
[7] “Electric Vehicle Conversion Using Tesla Batteries: Power Battery Blog: Power Battery.” Power Battery Blog | Power Battery, www.powerbattery.nl/resources/blog/electric-vehicle-conversion-using-tesla-batteries
[8] Dr. Gangbing Song, Chapter 5: Shape Memory Alloy Materials Lecture, University of Houston, MECE 6387: Intelligent Structural Systems, 06-2023
Comments