Center for Advanced Life
Cycle Engineering

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CALCE  Battery  Research  Group


Safety incidents plague the lithium-ion battery industry, with widely publicized recalls and hazardous events regularly appearing in the news. Lithium-ion batteries are used as power sources for many types of systems; however, despite technological improvements in battery chemistry and design, safety is still a concern. When improperly fabricated or used, lithium-ion batteries are prone to escalating chemical reactions known as thermal runaway. CALCE is analyzing the root causes of battery thermal runaway. Based on the root cause analysis, strategies to improve battery safety and prevent thermal runaway failures are being recommended.

Although lithium-ion batteries are reliable and popular as energy storage devices, the thermal and electrochemical instability of the electrodes and the flammability of the electrolyte make lithium-ion batteries prone to catastrophic failures. Failures involving fires and explosions have resulted in numerous recalls in the consumer electronics industry involving millions of batteries. Lithium-ion batteries used in electric vehicles (EVs) are susceptible to the same types of failure. However, due to different life cycle load, the batteries used in EVs may experience additional failure modes and mechanisms. Additionally, EVs often use hundreds or thousands of high-energy batteries, and battery safety remains a major concern. Figure 1 shows the latest accident that electric vehicle battery caught fire in the charging station on April 26th, 2015 in Shenzhen, China. Figure 2 shows a Chevy Volt that was destroyed by a battery three weeks after a side-impact collision test that was conducted by the U.S. National Highway Traffic Safety Administration (NHTSA).

Figure 1. Wuzhoulong EV bus with LiFePO4 power batteries caught fire in the charging station on Apr. 26th, 2015 in Shenzhen, China
Figure 2. Chevy Volt battery fire three weeks after side-impact test by the U.S. NHTSA
Figure 3. (a) UPS cargo plane fire (b) lithium-ion battery pack in Boeing 787

The transportation or use of lithium-ion batteries in aerospace applications have also resulted in catastrophic failure. Figure 3 (a) below shows a damaged United Parcel Service (UPS) cargo plane that was carrying a large number of lithium-ion batteries as cargo when a fire broke out. Figure 3 (b) below shows the failed battery pack in a Boeing 787. It was suspected that the problems originated from malfunctioning lithium-ion batteries: the batteries showed signs of short circuit and thermal runaway, which led to overheating and fire. Figure 4 shows major EV battery incidents since 2008.

Figure 4. Major EV battery fire accidents since 2008

One of the most significant cell components to ensure cell safety is the separator. Its main function is to prevent physical contact between the anode and cathode and at the same time facilitate ion transport in the cell. The challenging issue on designing a battery separator is how to balance between mechanical robustness and transport properties. Now, the separator has been designed with multi-layers and a shutdown feature. The idea behind it is that different multi-layers have different phase transition temperatures. The shutdown feature is expected to work when there is an increase in cell temperature. The lower melting component melts and fills the pores of one of the solid layers. Hence, the ion transport will stop and the current drain of the cell will become zero. Figure 5 shows how the shutdown separator functions. The related work can be found in [1]. Through root cause analysis for design becoming an effective way to improve battery safety.

Figure 5. How separator shutdown functions [1]

[1] C. J. Orendorff, "The role of separators in lithium-ion cell safety," Electrochemical Society Interface, vol. 21, pp. 61-65, 2012.