I was born in Suita City, Osaka Prefecture, in 1948. My father, Sojiro, was an electrical engineer who worked at a power company. My mother worked at a bank until she married, after which she became a housewife. Suita is located about 10 km north of the center of Osaka City. My home was surrounded by bamboo groves. As a child, I enjoyed playing in this nature-filled environment.

My path to chemistry began in the fourth grade of elementary school, when my teacher recommended that I read The Chemical History of a Candle by Michael Faraday. The book made it easy to understand why a candle burns, what happens when it burns, and why it has a wick. This stimulated my curiosity, and I became fascinated with chemistry. In 1966, I entered Kyoto University as a student of the Department of Petrochemistry, Faculty of Engineering. My major was quantum organic chemistry. At the time, the Department of Petrochemistry was a very prestigious one at Kyoto University. The faculty included Professor Kenichi Fukui, who would go on to receive the 1981 Nobel Prize in Chemistry. My university research involved quantum chemistry theory and observation of experimental results. I focused on organic photochemistry in particular. Professor Fukui was the mentor of my mentor at the university. I entered graduate school at the same university and studied photochemistry of charge transfer complexes. By irradiating ultraviolet to charge transfer complexes of 1,2,4,5-tetracyanobenzene as electron acceptor with electron donors such as toluene, I discovered that a previously unknown reaction occurred.

After receiving a master’s degree from Kyoto University, I joined Asahi Kasei Corp. in 1972. I was assigned to a laboratory in Kawasaki City, Kanagawa Prefecture, and began my career as a corporate researcher. My work was basic exploratory research, which meant I was expected to find the seeds of new technology. I had to decide what subjects to research, and I performed experiments myself. My first idea was to develop a new interlayer film for the laminated safety glass of automobiles. At the time, polyvinyl butyral film was used. I worked eagerly find a new material to replace it, but without success. The performance requirements for safety glass are extremely demanding, and I was unable to obtain suitable characteristics. Polyvinyl butyral is still used as interlayer film in laminated safety glass today. I was up against a formidable opponent.

In 1974, I married my wife Kumiko. We have been blessed with two daughters, Miho and Yuko, and a son, Satoshi. My next project at work was to develop nonflammable thermal insulation material. Saving energy was a hot topic at the time, and high-performance insulation was considered important. Polyurethane foam and polystyrene foam were available, but they are both flammable. I tried to develop inorganic polymer of phosphate as a nonflammable alternative, but this too was unsuccessful. Although I was able to reach fairly high insulation performance, the material had insufficient mechanical strength. My third project was for something like what we now call photocatalysts. I wanted to develop a product with a sterilizing and deodorizing effect by activating the oxygen in the air using sunlight. The foundation of this research was the photochemistry I had studied in college, specifically photosensitized oxidation. I kept at it for four years and achieved reasonably good results, but once again I could not succeed. The market was too immature. Photocatalyst technology would not be commercialized until thirty years later.

In 1981 I began looking for my next line of research, and this would turn out to be a fateful year for me. There was a new material that everyone was excited about: electroconductive polyacetylene. Although it was a plastic, surprisingly it could conduct electricity. It was discovered by Professor Hideki Shirakawa, who would go on to receive the 2000 Nobel Prize in Chemistry along with Professor Alan Heeger and Professor Alan MacDiarmid. At the time, Professor Shirakawa was doing joint research with Kyoto University. I visited Kyoto University, the first time for me to return to my alma mater in a long time. I was amazed when he showed me a sample of polyacetylene in a test tube. It had such a metallic luster that I could scarcely believe it was plastic. I decided that this amazing material would be my next subject of research. I visited Kyoto University many times after that, and he taught me all about it, including how to polymerize it. The year 1981 turned out to have another surprise in store as well: Professor Fukui’s Nobel Prize in Chemistry. The frontier molecular orbital theory for which Professor Fukui received the Nobel Prize was the very theory that explained why polyacetylene was electroconductive.

I synthesized polyacetylene at the Asahi Kasei laboratory in Kawasaki. Next, I began thinking about what kind of product to use it for. Among polyacetylene’s many unique properties, I was most interested in its electrochemical properties. This meant that polyacetylene could be used as a battery material. What’s more, since its electrochemical reactions were reversible, it could be used as a rechargeable battery material. I surveyed the battery research being done at the time and learned of Professor Stanley Whittingham’s then novel concept of intercalation. He first applied this concept to battery cathode material in 1976, and after that there was a groundswell of work to develop a small and lightweight rechargeable battery utilizing the concept of intercalation. Successful commercialization, however, proved extremely difficult due to issues with the anode material. I was thrilled to find that polyacetylene would work as an anode material which could be doped with cations such as Li ions. I confirmed this idea and concluded that the ideal use for polyacetylene was as anode material for a rechargeable battery.

I was not an expert on battery technology, but from that point forward I became deeply involved in battery technology. I began tests to evaluate the polyacetylene I synthesized as rechargeable battery anode material. The results were encouraging. The material had large charge and discharge capacity, and it didn’t deteriorate even after repeated charge/discharge cycles. The next step was to decide what cathode material to pair with it. But this turned out to be a bigger problem than I had anticipated. There weren’t any cathode materials that were suitable. Since polyacetylene doesn’t contain Li ions, I needed a cathode material that contained Li ions. I struggled in vain to find one until I at last encountered Professor John Goodenough’s 1980 paper on lithium cobalt oxide (LiCoO₂) as a new cathode material. Without hesitation, I assembled a test cell using LiCoO₂ cathode and polyacetylene anode. This was the first instance of the new rechargeable battery system that would later become known as the lithium-ion battery. It was 1983. While the weight of the new rechargeable battery was reduced to about one-third that of a nickel-cadmium battery, the volume was unfortunately unchanged. This was a disappointment because the objective was to achieve both lighter weight and smaller size. Nevertheless, I began to show my new battery system to potential customers. I didn’t know whether they considered smaller size to be more important than lighter weight or not. Unfortunately, I was told that smaller size was indeed more important.

Once again, I found myself at a crossroads. The reason I couldn’t reduce the size of my new battery system was that the specific gravity of polyacetylene is 1.2, relatively low. While the low specific gravity was advantageous for achieving lighter weight, it turned out to be disadvantageous for achieving smaller size. My calculations indicated that I needed a material with a specific gravity of at least 2 in order to reach the desired size. It immediately occurred to me that carbonaceous material would have a specific gravity of 2 or more while having conjugated double bonds similarly to polyacetylene. I evaluated the many carbonaceous materials then available but could find none that would function as anode material. When I was beginning to lose hope, I learned of a new kind of carbon fiber called VGCF that was being developed at another laboratory of Asahi Kasei. I obtained a sample and found that it performed very well as an anode material. VGCF is made using a very special process that gives it a very special crystalline structure. This special crystalline structure is what makes it work well as an anode material. By replacing polyacetylene with carbonaceous material, I thus completed the basic configuration of a practical new rechargeable battery. After further development of technology required for commercialization, the first lithium-ion batteries made their debut in the world in the early 1990s.

On October 9, 2019, I received a surprising notification from Stockholm. I had been chosen for the Nobel Prize in Chemistry along with Professor Goodenough and Professor Whittingham for the development of lithium-ion batteries. Two reasons were given for the award. The first reason was the huge impact that lithium-ion batteries had for the achievement of today’s mobile IT society. The second reason was the vital contribution that lithium-ion batteries are expected to make for the achievement of a sustainable society moving forward. While the latter of the two is still a work in progress, emboldened by my receipt of the Nobel Prize I am determined to do my part to make it happen.

This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/ Nobel Lectures/The Nobel Prizes. The information is sometimes updated with an addendum submitted by the Laureate.