Organic electrode materials offer unique opportunities to utilize ion-electrode interactions to develop diverse, versatile, and high-performing secondary batteries. Their tunability and flexibility furthermore make them appealing as heterogeneous electrocatalysts and high-power application candidates. However, a lack of well-defined structure-property relationships for redox-active organic materials restricts the advancement of the field. Herein, we investigate a family of diimide-based polymer materials with several charge-compensating ions (Li⁺, Na⁺, K⁺) in order to systematically probe how redox-active moiety, ion, and polymer flexibility dictate their thermodynamic and kinetic properties. When favorable ion-electrode interactions are employed (e.g., soft K⁺ anions with soft perylene diimide dianions), the resulting batteries demonstrate increased working potentials and improved cycling stabilities. Further, we generally demonstrate that K⁺ accesses the highest percentage of redox-active groups due to its small solvation shell/energy, particularly at faster charge/discharge rates. Through crown ether experiments, cyclic voltammetry, and activation energy measurements, we provide insights into the charge compensation mechanisms of nine different PDI-based polymers and rationalize these findings in terms of the differing degrees of improvements observed when cycling with K⁺. Critically, we find that imparting some degree of flexible polymer enables access to a higher fraction of active sites due to the small activation energy barrier during charge/discharge compared to the completely rigid counterparts. However, overly increasing polymer flexibility increases diffusion-limited processes and reduces the high-power capability of the polymer. Furthermore, we hypothesize that the side reaction of trace formation of iminoimide groups at polymer ends with diamines of specific backbone lengths terminates the polymerization, leading to shorter and more soluble polymer strands. Overall, our in-depth structure-activity investigations demonstrate how simple tuning handles such as polymer structure and cation can be used to optimize redox-activity and stability of these materials.

Cara Gannett

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