N-(2-(2-methoxyethoxy)ethyl) phenothiazine (MEEPT) is considered a benchmark redox-active catholyte molecule for non-aqueous redox-flow batteries. Recent observations show that the MEEPT solubility limit with the counter-anion hexafluoroborate can be dramatically improved by switching the counter-anion. Though the physiochemical basis of the phenomenon was unclear, we hypothesized that the solubility of MEEPT was based on the flexibility and the size of the anion. Here we combine experimental observations with classical molecular dynamics (MD) simulations to explore the hypothesis through an in-depth study of eight different MEEPT/counter-anion (X) systems (Figure). This approach reveals that categorization of MEEPT-X systems based on low, moderate, and high solubility facilitates understanding certain trends in solubility. These categories exhibit specific characteristics in terms of radial distributions, contact pair distances, clustering trends, and physiochemical properties such as conductivity, viscosity, and diffusivity. Moreover, analyses of entropic and solvent-accessible surface area contributions reasonably align with the initial hypothesis. The intuitions gained through this study will assist materials chemists understand and improve solubility of similar redox-active systems by optimizing the choice of counterions.

Title: in-silico investigation of the effect of anion identity on the solubility of N-(2-(2-methoxyethoxy)ethyl) phenothiazine (MEEPT)

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