2011 Recipients of C&C Prize
Dr. Akira Yoshino
Fellow, Asahi Kasei Corporation
For Pioneering Contribution to the Development and Commercialization of the Lithium-Ion Battery
In the early 1980s, many researchers and organizations began working hard for over a decade to develop and commercialize the re-chargeable (secondary) lithium-ion battery (LIB). These efforts resulted in the high-power, small LIBs used today in mobile and personal information devices and audio-video equipment. Among the many researchers who worked on the LIB, Dr. Akira Yoshino played a central role in the development and bringing them into commercial use.
Dr. Yoshino started his research on LIBs in 1981. The most important development at that time was finding suitable materials for both positive and negative electrodes. Dr. Yoshino started to develop the negative electrode first.
Although attempts had been made to convert the primary metallic lithium battery into a secondary battery, even the best efforts did not succeed for three main reasons. While charging, lithium tended to leave deposits on the negative electrode in the form of dendrites, which easily caused short-circuiting. And the high chemical reactivity of metallic lithium resulted in poor battery characteristics, including inadequate cycle durability due to side reactions. Moreover, that chemical reactivity posed an insurmountable problem in terms of safety due to the inherent risk of a thermal runaway reaction.
Dr. Yoshino conceived the idea of a new secondary battery using electroconductive polyacetylene as the negative electrode and LiCoO2 as the positive one. The electroconductive polyacetylene had been developed by Prof. Hideki Shirakawa at Tsukuba University, a Nobel prize winner in 2000. The LiCoO2 was first reported by Dr. J.B. Goodenough in 1980, and was found to be solely applicable to positive electrode material at that time. Dr. Yoshino confirmed the principle of this new secondary battery with an operational model in a sealed glass test tube in a non-aqueous electrolyte. His test-tube cell functioned with the same cell reaction and operating principles of the LIB as it exists today. It was the world’s first use of an LIB in a non-aqueous electrolyte showing high electromotive force of around 4 V. Although this cell was functional, the low real density of the polyacetylene placed limitations on the available capacity, and the chemical stability of polyacetylene proved to be limited.
Dr. Yoshino thus searched for a new carbonaceous material to use as the negative electrode. Although graphite had been studied as a negative electrode material, it was known at that time that propylene carbonate, which was then a common organic electrolyte, would decompose during charging when graphite was used. Moreover, the use of a solid electrolyte resulted in electrical resistance that was too high to enable practical charging and discharging. Dr. Yoshino therefore studied the suitability of several carbonaceous materials as the negative electrode. He found that carbonaceous material with a certain crystalline structure provided greater capacity without causing the decomposition of the propylene carbonate electrolyte solvent that the graphite did. The secondary battery that he successfully fabricated in 1985 based on this new combination of component materials enabled stable charging and discharging over many cycles for a long period for the first time in the world.
His combination of electrode materials and cell reaction principles gave the LIB the following characteristics.
a) The avoidance of problems stemming from the high chemical reactivity of metallic lithium, which had inhibited the practical development of a non-aqueous electrolyte secondary battery using metallic lithium for the negative electrode.
b) The supply of lithium ions from the LiCoO2 of the positive electrode to the carbonaceous material of the negative electrode, which marked a new concept for a secondary battery based on the transfer of lithium ions.
c) The achievement of an electromotive force of 4 V or more and a substantial improvement in energy density with the use of a non-aqueous electrolyte, which enabled a significant reduction in the size and weight of the secondary battery.
d) The utilization of a cell reaction without chemical transformation, which provided stable battery characteristics over a long service life, including excellent cycle durability with little degradation due to side reactions and excellent storage characteristics.
Dr. Yoshino also devised the other constituent technologies essential for achieving a practical LIB. His selection of aluminum as the positive electrode’s current collector material was one of the most important aspects. Previously, only precious metals such as gold and platinum were considered able to withstand high voltages of 4 V or more. However, Dr. Yoshino found that aluminum foil was suitable for use as the positive electrode’s current collector material because a passivation layer forms on the aluminum surface.
Dr. Yoshino’s invention of a highly functional membrane separator was also a particularly important factor in achieving the safety required for successful LIB commercialization. The use of a micro-porous polyethylene membrane 20 to 30 microns thick for use as the separator provided a “fuse” function in which the material of the separator melted to close the micro-pores and turn off battery operation in the case of abnormal heat generation.
As stated above, Dr. Yoshino played an extremely important role in developing and commercializing LIBs in the 1980s. His inventions are still used in LIBs today. And, to put it another way, LIBs could not exist today without Dr. Yoshino’s inventions. The NEC C&C Foundation thus highly praises Dr. Yoshino for his contributions to the advancement of the information technology industry through the development and commercialization of the lithium-ion battery.