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The demand for a stable and compatible redox shuttles for use in lithium-ion batteries has prompted us to explore strategies to tune and improve the properties of redox shuttles. We have studied over 50 new diarylamine derivatives synthesized in our laboratory including one compound in which we introduced trifluoromethyl groups (–CF3) at the positions para to the nitrogen atom in N-ethylphenothiazine (EPT). The high electronegativity of the CF3 group raises the oxidation potential, and its incorporation also significantly increases solubility in battery electrolyte. Here we report 3,7-bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) as a new redox shuttle, which we have observed to have the highest reported solubility in battery electrolyte of all redox shuttles that maintain extended overcharge performance. We have compared its performance with 1,3-di-tert-butyl-2,5-dimethoxybenzene (DBB), EPT, and other robust redox shuttles. In our hands, overcharge cycling of BCF3EPT far surpasses any reported redox shuttle, and – because it can be dissolved at higher concentrations – it tolerates faster charging rates than both DBB and EPT.

The performance of aromatic compounds as redox shuttles for overcharge protection in lithium-ion batteries is quite variable and is often difficult to predict. Redox shuttles may decompose in battery electrolyte in their neutral and radical cation forms, both of which are present during overcharge protection. While hundreds of compounds have been evaluated as redox shuttle candidates and a few have stood out as top performers, the reasons for increased stability over similar candidates with slightly different structures is often unclear, and the exploration of decomposition of redox shuttles has been severely limited, restricting our ability to design improved versions of redox shuttles that do not suffer from the same reactions in lithium-ion batteries. To better understand the stability and reactivity of redox shuttles (also relevant to the improvement of positive electrode materials in non-aqueous redox flow batteries) our research has focused on measuring the stability of neutral and oxidized forms of redox shuttle candidates as well as using a variety of spectroscopic methods to analyze the byproducts of decomposition, both from radical cations generated in model solvents and electrolytes from postmortem analysis of failed batteries.