Crosslinkable Nitroxide Radical Polymer for Energy Storage Applications

Abstract

Due to the amount of waste generated by lithium ion batteries (LIB) disposal, a more environmentally friendly option such as organic batteries, is desired to fulfill the need of this large market. For many years, alternatives to LIBs have been researched, including the use of conductive polymers for completely organic batteries. Redox-active polymers, such as conjugated polyaniline and organic radical poly(2,2,6,6-tetramethylpiperidinyloxy-4-ylmethacrylate) (PTMA), are being studied as electrode materials for organic batteries. As linear PTMA may dissolve in the electrolyte solution and thus degrade battery capacity, crosslinked PTMA is gaining interest as a cathode material. A balance must be struck, however, as diffusion of the electrolyte solution into the polymer may become limited at sufficiently high crosslink density. To balance these competing mechanisms, a random copolymer consisting of PTMA monomer and glycidyl methacrylate (GMA), a UV and thermally crosslinkable monomer, is synthesized (P(TMA-GMA)) and the mechanism of redox activity is studied. This new PTMA copolymer is synthesized using free radical chain-growth polymerization and is then oxidized to produce the characteristic organic radical. Linear P(TMA-GMA) is characterized using UV-VIS, NMR spectroscopy, and gel permeation chromatography. The effect of crosslinking on the thermal properties and the temperature for thermally induced crosslinking is analyzed using differential scanning calorimetry. The crosslink density is calculated for each composition using the linear P(TMA-GMA) molecular weight and solvent uptake of the crosslinked P(TMA-GMA). Determining the most favored crosslink density is necessary for balancing electrolyte diffusion with the dissolution of the polymer. The electrochemical properties of the crosslinked P(TMA-GMA) is studied using cyclic voltammetry (CV) to determine if crosslinking impacts the redox properties. Preliminary results have found that the crosslinked P(TMA-GMA) exhibits a change in the shape of the CV curve and peak heights, associated with some change in the crosslinked polymers electroactivity. In addition, the crosslink density of the P(TMA-GMA) with 5 mol% GMA is quantified and is similar to previous studies of poly(ethylene glycol). Finally, the thermal properties are represented by a glass transition temperature that did not significantly change with varying GMA content. This new P(TMA-GMA) shows promise for application in organic batteries by providing a processable yet crosslinkable alternative to PTMA.