The electrochemical redox reactions of organic polymers bearing robust unpaired electrons were investigated to determine the applicability of these polymers to rechargeable batteries. Such an "organic radical battery" would be environmentally friendly and have high-power characteristics. This highlight review describes the performance of a battery using a nitroxyl radical polymer as the cathode active material. The electrontransfer mechanism and recent developments that should lead to the practical application of the organic radical battery are also described.
Ç 1. IntroductionLithium-ion batteries are widely used power sources for portable electric devices such as cellular phones and laptop computers because of their high-energy density and long life. Application of these batteries is expanding to electric vehicles and domestic energy storage. 15 In these batteries, a lithium transition-metal oxide cathode and a graphite anode are used as energy storage electrodes. Since the lithium transition-metal oxide cathode comprises the main part of a lithium ion battery, the lithium transition-metal oxide mainly determines the battery's energy density and capacity. The lithium transition-metal oxide is charged/discharged electrochemically in accordance with deintercalation/intercalation of the lithium ions and oxidation/ reduction of the transition-metal ions. Although toxicity, safety, and resource availability are known problems with lithium transition-metal oxide, alternatives have been hardly reported except for conducting polymers and disulfide compounds. 6,7 Radical polymers are candidates for replacing lithium transition-metal oxide. 811 We call a polymer which has unpaired electrons a radical polymer. The disappearance of the unpaired electrons due to chemical bond formation can be suppressed by a precisely designed chemical structure through a combination of the resonance effect of the electrons and the sterically hindered effect of the substituent groups. Chemical groups such as nitroxyl, phenoxyl, and hydrazyl are robust structures with lessreactive unpaired electrons in the uncharged state. Radical polymers with alicyclic nitroxyl such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 2,2,5,5-tetramethylpyrrolidine-1-oxyl (PROXYL) in particular have been studied in detail.