(402b) Crosslinkable Nitroxide Radical Polymer for Energy Storage Applications

Due to the amount of waste generated by lithium ion batteries disposal, a more environmentally friendly option such as organic batteries is desired to fulfill the need of this large market. Redox-active radical polymers such as poly(2,2,6,6-tetramethylpiperidinyloxy-4-ylmethacrylate) (PTMA) are popularly studied as electrode materials for organic batteries. However, linear PTMA may dissolve in the electrolyte and thus degrade the battery‘s capacity over time. Therefore, crosslinked PTMA is gaining interest as one solution to this challenge. Previous work focused upon crosslinking PTMA during polymerization, but the resulting crosslinked polymer is insoluble in organic solvents, which prevents direct solution processing of the polymer electrode. Further, electrodes made from post-synthetic crosslinking of PTMA suffer from low capacity due to charge diffusion limitations.

Here, we present a simple, post-synthetic route to crosslink PTMA while maintaining a relatively high discharge capacity. PTMA monomer is randomly copolymerized with glycidyl methacrylate (GMA). The product (P(TMA-GMA)) is a linear random copolymer that dissolves in organic solvents and thus allows characterization using UV-Vis spectroscopy, NMR spectroscopy, and gel permeation chromatography. Upon thermal treatment or UV radiation, the epoxide ring forms a network structure and immobilizes the PTMA. The radical concentration before and after crosslinking is characterized using solid-state electron paramagnetic resonance (EPR). P(TMA-GMA) copolymers with various mole percent GMA monomer in the feed are evaluated. The crosslink density and swelling ratio are calculated by measuring the thickness of dry and swelled electrode using electrochemical quartz crystal microbalance with dissipation (EQCM-D). The electrochemical properties of the P(TMA-GMA) electrodes are obtained using cyclic voltammetry and galvanostatic cycling to evaluate whether crosslink density affects cell performance. Our general finding is that PTMA dissolution was prevented by this crosslinking approach.