Solvation-Controlled Accessibility and Irreversible Finalization of Aminoxy Intermediates in TEMPO/Benzothiadiazole Nonaqueous Redox Flow Batteries

Abstract

Understanding how reactive intermediates form and evolve during electrochemical operation is essential for improving the stability of molecular energy-storage systems. Here, we investigate the degradation chemistry of an all-organic nonaqueous redox flow battery based on a nitroxide catholyte and a benzothiadiazole anolyte. By combining electrochemical analysis, full-cell cycling experiments, and molecular dynamics simulations, we reveal how the solvation environment governs both the electrochemical accessibility and the chemical fate of transient reduced intermediates. To rationalize these effects, we introduce a chemically informed framework that decouples electrochemical exposure of parasitic pathways from the probability that such events culminate in irreversible product formation. This minimal model quantitatively captures the relationship between coulombic efficiency and capacity loss across a range of ionic liquid electrolytes, providing a unified description of degradation behavior under realistic operating conditions. Molecular-level analysis further shows that electrolyte-dependent solvation and local coordination environments modulate the stabilization and reactivity of key intermediates, thereby controlling the balance between reversible redox processes and permanent inventory loss. These findings establish a direct link between electrolyte solvation structure and electrochemical degradation pathways, offering a general strategy to diagnose and mitigate failure mechanisms in molecular electrochemical systems. More broadly, the approach provides a conceptual framework for connecting reactive intermediate chemistry with macroscopic device performance in organic energy-storage technologies.