2019 年 87 巻 6 号 p. 312-320
Rechargeable batteries are capable of storing electric energy on the basis of pairing electrochemical redox reactions to realize sustainable energy society in our future. Since lithium-ion batteries with the highest specific energy among all the practical batteries were commercialized in 1991, many studies on lithium insertion materials and their electrochemical characterization have been reported to achieve even higher energy density, longer cycle life, and safer lithium-ion battery technologies. It is quite fortunate that the author had an opportunity to contribute to the research and development of lithium battery materials since 1997. In particular, studies on the influence of dissolved metallic ions like Mn2+, Co2+, Ni2+, Na+, and K+ ions in electrolyte solution on graphite negative electrodes in lithium-ion batteries motivated the author to extend the research scope to electrochemical sodium insertion chemistry. Furthermore, the author’s research experiences as a postdoctoral fellow in Dr. Delmas’ group in FY 2003 and a remarkable oral presentation on alpha-NaFeO2 electrode properties given by Professor Okada’s group in 2004 provided motivations and opened up new avenue toward the successful demonstration of non-aqueous sodium-ion batteries later in the career. Since 2009, the author’s research group has successfully demonstrated 3-volt class charge and discharge of a sodium-ion battery of a NaNi1/2Mn1/2O2 // hard carbon cell and a brand-new potassium-ion battery of a K2Mn[Fe(CN)6] // graphite cell. The systematic studies of three different alkali-metal insertion systems synergistically induce deeper understanding and faster development of new materials for the next-generation rechargeable batteries.