Bridging the Gap between Small Molecular π-Interactions and Their Effect on Phenothiazine-Based Redox Polymers in Organic Batteries

Abstract

Organic redox polymers are considered a “greener” alternative as battery electrode materials compared to transition metal oxides. Among these, phenothiazine-based polymers have attracted significant attention due to their high redox potential of 3.5 V vs Li/Li+ and reversible electrochemistry. In addition, phenothiazine units can exhibit mutual π-interactions, which stabilize their oxidized states. In poly­(3-vinyl-N-methylphenothiazine) (PVMPT), such π-interactions led to a unique charge/discharge mechanism, involving the dissolution and redeposition of the polymer during cycling, and resulted in an ultrahigh cycling stability. Herein, we investigate these π-interactions in more detail and what effect their suppression by molecular design has on battery performance. Our study includes a dimeric reference compound for PVMPT, polymers with bulky tolyl or mesityl substituents on the phenothiazine units to inhibit π-interactions and alternating copolymers with maleimide groups to increase spatial distancing between phenothiazine groups. UV/vis- and electron paramagnetic resonance (EPR)-spectroscopic as well as electrochemical measurements in composite electrodes demonstrate how the unique structure of PVMPT is instrumental in obtaining a high cycling stability in poly­(vinylene) derivatives of phenothiazine.

Evaluating the Polymer Backbone – Vinylene versus Styrene – of Anisyl‐substituted Phenothiazines as Battery Electrode Materials

“…In previously investigated PT-based polymers this behavior was caused by the (partial) dissolution of the polymer in the electrolyte in its oxidized form. [31,33,35] This also happened for PVAPT, as the photograph of the separator taken from a cell after CV measurement in Figure 3A shows, which is pink/purple-colored due to the presence of oxidized PVAPT. Cross-linked X-PVAPT as well as PSAPT and PSAPT-X-DVB, on the other hand, remained insoluble in the electrolyte even in their oxidized form (see photographs of separators in the respective CVs in Figure 3A).…”

“…A change in polymer backbone to poly(norbornenes) diminished both solubility and π-interactions and furnished attractive battery polymers, [34] while attaching sterically demanding groups (e. g., mesityl) to the phenothiazine units in PVMPT resulted in a low cycling stability due to a complete lack of π-interactions combined with a high electrolyte solubility of the oxidized polymers. [35] Most of these polymers had in common that the nitrogen atom of the phenothiazine groups was substituted with methyl. Changing this substituent alters the redox properties of phenothiazine, where aryl groups can enhance the stability of the phenothiazine radical cation and thereby the reversibility of the redox process.…”

p‐Type Redox‐Active Organic Electrode Materials for Next‐Generation Rechargeable Batteries

“…One is changing the polymer backbone from vinyl to bulkier, such as norbornene (PNMPT) [ 165 ] and methylmaleimide (P(VMPT‐alt‐MM)) (see the chemical structures in Figure 11d). [ 166 ] The other is introducing a bulky substituent into the N ‐position of the PTZ unit (PV‐p‐TPT, PV‐o‐TPT, PVMesPT, and PVAnthPT in Figure 11e). [ 166 ] Such a series of studies clearly showed that the small changes in the chemical structure of ROMs could considerably influence the electrochemical performance by modulating the supramolecular interactions.…”

“…[ 166 ] The other is introducing a bulky substituent into the N ‐position of the PTZ unit (PV‐p‐TPT, PV‐o‐TPT, PVMesPT, and PVAnthPT in Figure 11e). [ 166 ] Such a series of studies clearly showed that the small changes in the chemical structure of ROMs could considerably influence the electrochemical performance by modulating the supramolecular interactions. Meanwhile, several other papers reporting PTZ‐based electrode materials have also been published.…”

Conjugated Copolymer Design in Phenothiazine-Based Battery Materials Enables High Mass Loading Electrodes

“…We have intensively investigated phenothiazine-based redox polymers as positive electrode materials in recent years. ,− Due to the high reversibility and fast kinetics of the oxidation of the phenothiazine group, such polymers can show outstanding cycling stabilities and rate capabilities . Using aliphatic polymer backbones, however, the intrinsic conductivity in the polymer is low (transport through hopping processes), and thick electrodes with high mass loadings of active polymer are challenging to obtain.…”