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The development of redox-active organic molecules (ROM) with large solubilities in all states of charge in organic electrolytes is imperative to the continued development of non-aqueous redox flow batteries. The capability to a priori predict ROM solubility would be a game changer, allowing for a move away from time and resource consuming trial-and-error approaches to materials design and deployment. However, it is not presently clear that such predictions are generally possible, even for chemically related ROM, given the large number of physicochemical factors in play. Here we use quantitative structure–property relationships (QSPR) to examine solubility trends for a set of thirty phenothiazine derivatives. The solubility in all states of charge (neutral and charged forms) of these molecules were obtained experimentally, and multiple linear regression models were used to correlate these properties with a large set (>100) of molecular descriptors. Minimal QSPR models rationalizing these data include four-to-six molecular descriptors, and cannot be further reduced. However, even such relatively complex models show limited ability to predict solubility of an unknown homologous compound. Thus, even in the controlled experimental environment, “predicting” the solubility may not be easy, suggesting the need for high-throughput measurements to develop the large data sets required for machine-informed materials design. The NMR method presented in this study is promising in this regard as it lends itself to automation. 

N -Ethyl-3,7-bis(trifluoromethyl)phenothiazine is a highly soluble redox shuttle for overcharge protection in lithium-ion batteries with an oxidation potential of ca. 3.8 V vs. Li +/0 in carbonate solvents. This compound has enabled extensive overcharge protection of LiFePO 4 /graphite cells and does so at high charging rates at high concentrations. Our initial synthesis of this compound suffered from low yields and difficult purifications. Here we report a cleaner, higher-yielding synthesis and additional characterization of the product and its stable radical cation salt.