An organic radical battery (ORB) is a type of battery first developed in 2005. As of 2011, this type of battery was generally not available for the consumer, although their development at that time was considered to be approaching practical use. ORBs are potentially more environmentally friendly than conventional metal-based batteries, because they use organic radical polymers (flexible plastics) to provide electrical power instead of metals. ORBs are considered to be a high-power alternative to the Li-ion battery. Functional prototypes of the battery have been researched and developed by different research groups and corporations including the Japanese corporation NEC. The organic radical polymers used in ORBs are examples of stable radicals, which are stabilized by steric and/or resonance effects. For example, the nitroxide radical in (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), the most common subunit used in ORBs, is a stable oxygen-centered molecular radical. Here, the radical is stabilized by delocalization of electrons from the nitrogen onto the oxygen. TEMPO radicals can be attached to polymer backbones to form poly(2,2,6,6-tetramethyl- piperidenyloxyl-4-yl methacrylate) (PTMA). PTMA-based ORBs have a charge-density slightly higher than that of conventional Li-ion batteries, which should theoretically make it possible for an ORB to provide more charge than a Li-ion battery of similar size and weight. As of 2007, ORB research was being directed mostly towards Hybrid ORB/Li-ion batteries because organic radical polymers with appropriate electrical properties for the anode are difficult to synthesize.
Applications As of 2015, ORBs were still under development and not in commercial use. Theoretically, ORBs could replace Li-ion batteries as more environmentally friendly batteries of similar or higher charge capacity and similar or shorter charge time. This would make ORBs well-suited for handheld electronic devices. Organic radical batteries were first researched and developed by NEC in 2005 with the intent of being widely used to power tiny gadgets in the near future. They began with a size of 0.3 mm and an extremely quick charge time. Since the beginning of development, smart cards and RFID tags were the main targets for ORB usage. NEC has also worked on a larger 0.7 mm battery which is thicker, but also has a high charge capacity of 5 mAh. Given the fast redox chemistry of nitroxide radicals, ORBs have been shown useful in keeping a computer running momentarily following a power outage. Although the amount of additional time provided is short, it is adequate to allow a computer to backup any crucial data before completely shutting down.
Function Radical polymer batteries rely on a redox reaction of an organic radical to generate an electrochemical potential. The most studied example of such an organic radical redox reaction is that of nitroxide radicals, such as the one found on a molecule called (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, also known as TEMPO. A nitroxide radical can be oxidized to an oxammonium cation or reduced to a hydroxylamine anion.
The positive electrode uses the nitroxide - oxammonium cation redox pair to create an electrochemical potential, i.e. when the battery discharges the nitroxide radical is oxidized to the oxammonium cation and when the battery charges the oxammonium cation is reduced back to the nitroxide. The redox potentials for nitroxide show some variation and for the TEMPO nitroxide for this redox pair has an oxidation potential of +0.87 V. The positive electrode often takes the shape of a gel made of organic radical solids and graphite, permeated with electrolytes. Graphite is mixed with the polymer to increase the conductivity. The negative electrode uses the nitroxide - hydroxylamine anion redox pair to create an electrochemical potential, i.e. when the battery discharges the nitroxide radical is reduced to the hydroxylamine anion and when the battery charges the hydroxylamine anion is oxidized back to the nitroxide. This half-reaction has an oxidation potential of -0.11 V. Since this half-reaction is not readily reversible as the half-reaction at the positive electrode, several research groups have steered away from using pure organic radical batteries and instead use metal/ORB hybrid batteries usually consist of a radical polymer cathode and the same anode found in rechargeable Li-ion batteries. Much like a traditional battery such as a Li-ion battery, an organic radical battery consists of a cathode and an anode that are separated by a porous film and submerged in an electrolyte. In a pure organic radical battery, both terminals are made of organic radical polymers (a p-type and an n-type polymer), while a metal/ORB hybrid battery usually has a radical polymer cathode and a Li-ion/graphite anode.
Synthesis of radical polymers Several synthetic approaches have been utilized in the synthesis of polyradical species for use in organic radical batteries. The following methods have been used to synthesize poly(2,2,6,6- tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA) and other nitroxide polymers.
Free-radical polymerization Initial attempts to synthesize PTMA involved synthesizing the polymer without radical functionality via free radical polymerization. Once the polymer is synthesized, the nitroxide function can be introduced by oxidation. Several groups have described synthesis of PTMA (4) using free radical polymerization of 2,2,6,6-tetramethylpiperidine methacrylate (2) with 2,2'-azobisiobutryonitrile (AIBN) as a radical initiator. The monomer was prepared via 2,2,6,6-tetramethyl-4-piperidinol (1) and methacryloyl chloride. The precursor neutral polymer (3) was oxidized to the stable radical polymer (4) by 3-chloroperoxybenzoic acid (mCPBA). Similar synthetic approaches have been proposed using 4-methacryloyloxy-N-hydroxy-2,2,6,6-tetramethylpiperidine as a monomer rather than 2,2,6,6- tetramethylpiperidine methacrylate.
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