Runaway battery improves safety

In your pocket, your bag or on your desk at work, there's probably a gadget with a lithium-ion battery. Small but mighty, these rechargeable powerhouses have become a mainstay for electronics, from pacemakers and laptops to electric vehicles. However, as the safety checklist at the airport will tell you, they can be a hazard. News stories abound of lithium-ion batteries overheating, smoking or even exploding. This makes safety testing a top priority for both manufacturers and consumers.

When batteries are subjected to unusual conditions, such as high temperatures, thermal shock, penetration, crushing, dropping, or vibration, chemical chain reactions can be triggered, causing the battery to heat up at an alarming rate (potentially up to several thousand degrees per minute). This phenomenon, known as thermal runaway, can ultimately lead to a catastrophic fire or explosion. To minimize the risk of thermal runaway, various testing methods have been suggested. Among them, the accelerating rate calorimetry (ARC) test provides quantitative data, including the onset temperature of battery self-heating and thermal runaway, and the related heat generation. However, as you might imagine, this testing is dangerous and costly.

"Current safety testing methods depend on large-capacity, commercial-scale batteries, which require substantial material resources, complex manufacturing processes and stringent explosion-proof standards," explained Professor Atsuo Yamada from the Graduate School of Engineering. "This renders thermal runaway testing inaccessible for most academic and research institutions, significantly limiting the development of safer and more advanced next-generation batteries."

To overcome this limitation, a team from the University of Tokyo and Japan's National Institute for Materials Science has developed an innovative method to evaluate thermal runaway by designing a mini battery intentionally more prone to thermal runaway (and therefore more dangerous). They also created a simple equation, which incorporates data on battery heat accumulation and dissipation, so they could calculate what they termed the thermal runaway factor (TRF).

"Developing a battery that was intentionally dangerous was the key. According to TRF, batteries generate more heat with higher energy, higher volume-to-surface area (V/S) ratios and lower specific heat capacities (the amount of heat required to raise the temperature of one gram of the battery by one degree Celsius) of their materials," explained Yamada. "Since increasing battery capacity wasn't feasible for small-scale tests, we focused on reducing the heat-release capability by increasing the V/S ratio, minimizing the use of high specific heat capacity materials and removing nonheat-generating components like the external battery case."

At just one-fiftieth the size of conventional batteries, this strategic design significantly reduces the raw materials needed for ARC testing, while enhancing the detection sensitivity of thermal runaway. Its compact size allows for controlled, small-scale testing in a lab, minimizing risk even in the event of thermal runaway.

"We found that applying our method makes it possible to quickly and precisely screen the effects of various factors related to battery safety, such as the battery's constituent materials, design factors, storage conditions and degree of deterioration," said Yamada. "This enables rapid safety screening and early-stage feedback for battery design, and can be used by researchers and manufacturers striving to enhance battery safety. Ultimately, we hope it will accelerate the transition to a carbon-neutral society."