Yoshino spent his entire non-academic career at Asahi Kasei Corporation. Immediately after graduating with his master's degree in 1972, Yoshino began working at Asahi Kasei. He began work in the Kawasaki Laboratory in 1982 and was promoted to manager of product development for ion batteries in 1992. In 1994, he became manager of technical development for the LIB manufacturer A&T Battery Corp., a joint venture company of Asahi Kasei and Toshiba. Asahi Kasei made him a fellow in 2003 and, in 2005, general manager of his own laboratory. Since 2017, he has been a professor at Meijo University and his status at Asahi Kasei has changed to honorary fellow.
Research
In 1981 Yoshino began research on rechargeable batteries using polyacetylene. Polyacetylene is the electroconductive polymer discovered by Hideki Shirakawa, who later would be awarded the Nobel Prize in Chemistry for its discovery. In 1983 Yoshino fabricated a prototype rechargeable battery using lithium cobalt oxide as cathode and polyacetylene as anode. This prototype, in which the anode material itself contains no lithium, and lithium ions migrate from the LiCoO cathode into the anode during charging, was the direct precursor to the modern lithium-ion battery. Polyacetylene had low real density which meant high capacity required large battery volume, and also had problems with instability, so Yoshino switched to carbonaceous material as anode and in 1985 fabricated the first prototype of the LIB and received the basic patent. This was the birth of the current lithium-ion battery. The LIB in this configuration was commercialized by Sony in 1991 and by A&T Battery in 1992. Yoshino described challenges and history of the invention process in a book chapter from 2014. Yoshino discovered that carbonaceous material with a certain crystalline structure was suitable as anode material, and this is the anode material that was used in the first generation of commercial LIBs. Yoshino developed the aluminum foil current collector which formed a passivation layer to enable high cell voltage at low cost, and developed the functional separator membrane and the use of a positive temperature coefficient device for additional safety. The LIB's coil-wound structure was conceived by Yoshino to provide large electrode surface area and enable high current discharge despite the low conductivity of the organic electrolyte. In 1986 Yoshino commissioned the manufacture of a batch of LIB prototypes. Based on safety test data from those prototypes, the United States Department of Transportation issued a letter stating that the batteries were different from the metallic lithium battery.
