Preparation and Investigation of a Novel Organic Polymer Consisting of 2,2,6,6-Tetramethylpiperidine-N-oxy as a Cathode Active Material in Li-Ion Batteries

In the present study, a novel organic polymer consisting of 2,2,6,6-tetramethylpiperidine-N-oxyl group as an electroactive center is employed by synthesizing it from a commercially ready polymer. An investigation on electrochemical and battery properties of this material as a cathode active material in different electrolyte salts was conducted. A coin cell shows a discharge capacity of 40？mAh？g？1 at 1？C which is 76% of its theoretical capacity. It is observed that there is no significant decrease in capacity value even at 2？C and 5？C which indicates that it is applicable for the high-power applications. Besides, a good cycle stability is obtained with the organic radical battery. 1. Introduction The use of conjugated polymers as electroactive material alternative to the inorganic insertion compounds has been extensively reported in the literature. However, no satisfactory results were obtained [1]. Organic radical batteries bearing nitroxide group as an electroactive center appeared as a new concept in Li-ion batteries. Electrochemical oxidation and reduction properties of the nitroxide group enable it to act as an active center of the organic radical battery. In addition, the stability of TEMPO (2,2,6,6-tetramethylpiperidine-N-oxy) containing nitroxide group at ambient conditions makes the compound a perfect electroactive material. The nitroxide radical oxidized to oxyammonium cation by losing an electron, whereas it is reduced to aminoxy anion by gaining an electron (Figure 1). Electrochemical reactions occur by the single electron transfer caused by the radical active center of TEMPO molecule. TEMPO and PROXYL (2,2,5,5-tetramethyl-1-pyrrolidinyloxy) groups bearing nitroxide group substituted to polymer backbone as a repeating unit are synthesized as cathode active materials. Figure 1: Redox couple of the nitroxide radical. The first organic radical battery with poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) polymerization of 2,2,6,6-tetramethylpiperidine-N-oxyl methacrylate containing a nitroxide group as an active center was reported by Nakahara and coworkers [2, 3]. In following years, different polymers substituted with TEMPO and PROXYL moieties were employed as cathode active materials in Li-ion batteries such as polyethers [4–6], polyacetylene [7], polynorbornene [8, 9], poly(7-oxanorbornene) [10], cellulose [11], DNA complexes [12], polythiophenes [13], and polyalkenes [14] illustrated in Figure 2. Figure 2: Various TEMPO substituted cathode active polymers ( A) PTMA, ( B) polyethers, ( C) polyacetylenes, ( D) polynorbornene, and (