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

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.