FACES: Tokyo Tech Researchers, Issue 38
Department of Mechanical Engineering, School of Engineering
Annual CO2 emissions in Japan total about 1.3 billion tons. In liquid form, this is enough to fill 1,000 Tokyo Domes. To reduce CO2 emissions, Japan is currently promoting the development and use of electric vehicles (EVs) and fuel-cell vehicles (FCVs). Increasing the prevalence of EVs, however, requires improved performance. Professor Shuichiro Hirai has been working toward this goal through development of real-time visualization technology for a wide range of battery cells installed on EVs and FCVs.
EVs today principally employ lithium-ion batteries. Akira Yoshino, an honorary fellow with Asahi Kasei Corporation, and two others were awarded the 2019 Nobel Prize in Chemistry for the development of the lithium-ion battery. Although lithium-ion batteries have found broad application around the world, much of their underlying mechanisms remain unclear. Take, for example, the power indicator on a smartphone. After charging, the icon will initially show a full charge. But if you wait a short while, the indicator will suddenly drop. Hirai explains, "Battery power usually decreases gradually, so it is very strange that the remaining power suddenly drops. This illustrates that we are not yet able to make sensors that can accurately measure remaining battery power. Not yet understanding what occurs in lithium-ion batteries makes them a black box to us."
In addition, FCVs have, as the name implies, fuel cells onboard. Fuel cells extract electrical energy generated from a reaction between hydrogen and oxygen in the air. Because FCVs only emit water, they are an outstanding candidate for next-generation "green" automobiles, with expectations for broad adoption. However, there are many things about fuel cells that are still not understood.
Hirai says, "To improve battery performance, it is necessary to visualize in real time and at the micrometer level what is happening inside batteries. Our laboratory is working to improve battery performance through research and development of visualization devices and technology that can only be found here."
Hirai is now working on visualization of all-solid-state lithium-ion batteries ("all-solid-state batteries"). Instead of conventional liquid electrolytes, all-solid-state batteries employ solid electrolytes, which reduce the risk of fire and enable quick recharge. Development towards practical application has been actively pursued both domestically and abroad.
Because the material used is solid, however, lithium ion mobility is not as readily achieved as with liquid. "This prompted me to try using an x-ray CT unit to image the interior of the all-solid-state battery under development by our own Ryoji Kanno, a frontrunner in the field," says Hirai.
Extremely small voids running vertically, horizontally, and diagonally exist in electrodes and solid electrolytes, and these interfere with the mobility of lithium ions. We must reduce these voids as much as possible to allow lithium ions to move at high speeds. Hirai, however, sought to identify the optimum void size through real-time visualization. To do so, he applied pressure to the electrode and solid electrolyte materials.
"To observe the interior of the battery using x-rays, it was necessary to distinguish between electrode material, solid electrolyte material, and void space. Distinguishing between the electrolyte material and void space was quite difficult, but the staff and students in my lab rose to the challenge and, by tuning the apparatus, succeeded in achieving high-resolution visualization," explains Hirai.
This enabled the team to see that many microscale cracks occurred in the materials as a result of the way in which pressure was applied. They learned that these cracks limited lithium ion mobility and led to deterioration of performance. They also found that controlling pressurization conditions prevented cracks from occurring. "This breakthrough will certainly contribute to performance improvement and practical application of all-solid-state batteries," states Hirai.
Fuel-cell batteries contain thin films less than 1 mm thick. These films form "cells" that include a catalytic layer coated with platinum catalyst. Fuel cells employ catalyst to cause hydrogen passed one side of the cell's thin film to react with oxygen in the air passed on the other side and produce water. The energy generated is then converted to electricity. The water produced in the reaction remains on the surface of the cells, however, which hinders the hydrogen-oxygen reaction and decreases power generation performance. Therefore, the water being generated needs to be controlled. Unfortunately, the behavior of water produced in this reaction, including where in the cell the water is generated and where it moves to, were not well understood.
Hirai and the Fuel Cell Cutting-Edge Research Center Technology Research Association (FC-Cubic TRA) jointly developed an x-ray CT unit capable of visualizing the behavior of water produced inside fuel cells in real time. The development visualizes behavior of water by irradiating cells with x-rays and detecting those x-rays with a detector. "X-rays used in standard radiography cannot detect water, so we tuned the wavelength to maximize sensitivity to water. In addition, conventional x-rays, which are emitted radially, were adjusted to enable parallel emission and thus detection of water. We then introduced hydrogen and oxygen into the visualization unit to observe their behavior under reaction. We succeeded, for the first time in the world, in visualizing water produced inside operating fuel cells at high resolution (micrometer-level) in real time and over an extended period," explains Hirai.
Hirai has also been collaborating with the private sector on research and development of adsorption heat pumps. This heat pump uses water-absorbing silica gel, which cools air using the heat of vaporization generated when the silica gel adsorbs water. This makes it possible to cool air without using electricity. However, in order to do so, it is necessary to provide heat externally and rapidly vaporize the water adsorbed to the silica gel. Currently, researchers are considering the use of waste heat discharged from plants as a heat source. They also needed to clarify the behavior of the water absorbed in the silica gel to identify practical applications. Hirai's x-ray CT unit was key to this research for visualizing the behavior of water in silica gel.
"Visualization equipment as well as technology that are suitable for the characteristics of target objects need to be developed as a set rather than independently. It is also necessary for us to understand the fundamental principles. For batteries in particular, the equipment and technology must allow for visualizing the movement and reaction of substances as well as the specific phenomena occurring. In this regard, I can make full use of my specialties in mechanical engineering — thermal engineering and hydromechanics," says Hirai.
Hirai is seeking to further improve the visualization equipment and technology. He also would like to expand the range of application from lithium-ion, all-solid-state, and fuel-cell batteries to lithium-air batteries and adsorption heat pumps, thereby contributing to the reduction of CO2 emissions.
At the end of the interview, Hirai had a message for young people considering a career in research. "I was in elementary school at the time of the oil crisis. Since then, I have been very interested in energy. Recently in particular, I have contemplated a lot about what constitutes the prowess of a nation. Although Japan lacks energy resources, the nation has a capacity in science and technology unrelated to its total area or large population. Utilizing this ability is the best strategy for continued economic growth, and I hope more young people become interested in careers in this area. Having the scientific curiosity to ask "how?" and "why?" is key. Rather than wholly accepting classroom knowledge without question, we should search deeper. For example, by asking why the person who first thought of entropy came up with the concept in the first place, we can delve into this history from our individual perspectives, and I think that process can lead to discovering something important.
Even if we achieve great results, our research will not receive recognition if it is not original. After completing my master's degree, I told the professor of the lab I belonged to that I wanted to do research on a theme that I had set, not the theme that the professor had set. This was instrumental in my achieving good results. The professor always told us not to follow him, but to surpass him. He was a great researcher and educator.
Many people start with what others before them have done. However, if you start this method of mimicry, you will ultimately hit a dead end. If you set your own research theme from the start, you will blaze your own trail. I tell my students to think differently from the way I do, to set their own methods, and accept challenges that truly excite them."
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Published: December 2019