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Radical polymers are aliphatic or nonconjugated polymers bearing organic robust radicals as pendant groups per repeating unit. A large population of the radical redox sites allows the efficient redox gradient‐driven electron transport through the polymer layer by outer‐sphere self‐exchange reactions in electrolyte solutions. The radical polymers are emerging as a new class of electroactive materials useful for various kinds of wet‐type energy storage, transport, and conversion devices. Electric‐field‐driven charge transport by hopping between the densely populated radical sites is also a remarkable aspect of the radical polymers in the solid state, which leads to many dry‐type devices such as organic memories, diodes, and switches.

A n‐type and redox‐active radical polymer bearing galvinoxyl radicals, poly(galvinoxylstyrene), is utilized as an anode‐active material, which enabled, for the first time, the fabrication of a totally organic polymer‐ based rechargeable battery in conjunction with p‐type redox‐active radical polymer. This battery was characterized by its remarkably high power rate capability.

Charge transport processes in nonconjugated redox-active polymers with electrolytes were studied using a diffusion-cooperative model. For the first time, we quantitatively rationalized that the limited Brownian motion of the redox centers bound to the polymers resulted in the 10-fold decline of the bimolecular and heterogeneous charge transfer rate constants, which had been unexplained for half a century. As a next-generation design, a redox-active supramolecular system with high physical mobility was proposed to achieve the rate constant as high as in free solution system (>10 M s) and populated site density (>1 mol/L).

Redox polymer layers with 2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl (TEMPO) groups showed nernstian adsorbate-like electrochemical behaviors up to submicrometer thicknesses, based on a fast charge propagation within the bulk layer and persistency in electrolyte solutions.

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.

p-and n-Type bipolar organic polymers have attracted remarkable interest in the development of organic-based devices, such as organic light-emitting diodes, organic thin-fi lm transistors, and photovoltaics, because they allow the proper balance of hole-and electron-conduction and simplifi cation of device structure. [ 1 ] Bipolarity is mostly carried on separate donor and acceptor sites and a few exceptions are p-and n-doped polythiophenes. [ 2 ] Insuffi cient stability of the n-doped state has limited the exploration of bipolar redox-active polymers, which are even accompanied by counterion migration. Stoichiometric bipolar redox activity for charge storage (long term) is challenging, but here we achieved three redox states (n-doped, neutral, and p-doped states) via judicious molecular design of the organic polymers.Recently, we successfully utilized redox polymers bearing robust, redox-active radical pendant groups, such as 2,2,6,6-tetramethylpiperidinyl-oxy (TEMPO) (p-type) [ 3 ] and galvinoxyl (n-type), [ 4 ] as cathode-and anode-active materials, respectively, and demonstrated high power rate capability in a totally organic-based rechargeable battery. We also reported either p-or n-type redox activity of poly(nitroxylstyrene) switched with substituent electronic effects, [ 5 ] however, these radical polymers did not show any bipolar redox activity, which is even more challenging than n-type redox activity. [ 6 ] Here we focus on redox reactions of nitronylnitroxide ( Figure 1 a ), [ 7 ] stabilized by the conjugated structure of two NO sites and by tuning the electrolyte conditions and report for the fi rst time poly(nitronylnitroxylstyrene) as the bipolar (p-and n-dopable) electrode-active material. In this report, we construct two unprecedented battery confi gurations: a) a symmetric confi guration (poleless battery) composed of poly(nitronylnitroxylstyrene) 1 for both electrodes and b) a both n-type electrode confi guration ("rocking-chair-type") utilizing poly(nitronylnitroxylstyrene) 1 and poly(galvinoxylstyrene) 2 [ 4 ] as the anode-and the cathode-active materials, respectively. A poleless battery confi guration is curious in the interest of simplifying the battery confi guration, and a "rocking-chair-type" confi guration would be favored to tremendously reduce the electrolyte solution. We demonstrate the versatility and signifi cance of bipolar redoxactive radical polymers.Poly[4-(nitronylnitroxyl)styrene] 1 was synthesized via radical polymerization of the silyl-protected precursor monomer, followed by deprotection and chemical oxidation to generate the corresponding radical polymer ( M n = 52 000, M w / M n = 3.2, 0.96 unpaired electrons per monomer unit. The theoretical redox capacity, Q , of 1 (103 mAh g − 1 ) was calculated from the formula weight ( F w ) of the monomer repeating unit and the number of radicals in the repeating unit ( a ) using the following equation: Q = 96 485 × a /( F w × 3 600) (mAh g − 1 ). See Experimental Section and Supporting Information for details). The polymer soluti...

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