A phenothiazine derivative with high solubility in carbonate solvents containing lithium salts showed extensive overcharge protection and, as a result, has been evaluated as a catholyte for non‐aqueous redox flow batteries. We report the testing of 3,7‐bis(trifluoromethyl)‐N‐ethylphenothiazine as a catholyte and 2,3,6‐trimethylquinoxaline as the anolyte in redox flow batteries containing 0.05, 0.15, and 0.35 M active material and found the longest capacity retention over about 60 cycles at 0.15 M. To our knowledge, this is the most soluble catholyte candidate with a robust radical cation.

Phenothiazine and five N-substituted derivatives were evaluated as electrolyte additives for overcharge protection in LiFePO4 /synthetic graphite lithium-ion batteries. We report on the stability and reactivity of both the neutral and radical-cation forms of these six compounds. While three of the compounds show extensive overcharge protection, the remaining three last for only one to a few cycles. UV/Vis studies of redox shuttle stability in the radical cation form are consistent with the overcharge performance: redox shuttles with spectra that show little change over time exhibit extensive overcharge performance, whereas those with changing spectra have limited overcharge protection. In one case, we determined that a C-N bond cleaves upon oxidation, forming the phenothiazine radical cation and leading to premature overcharge protection failure; in another case, poor solubility appears to limit protection.

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3,7-Disubstituted N-ethylphenothiazine derivatives were synthesized as redox shuttle candidates for lithium-ion batteries. Battery cycling results show that three derivatives prevent overcharge.

Constant overcharging of LiFePO4/synthetic graphite lithium-ion batteries in which the electrolyte additives 1,4-di-tert-butyl-2,5-dimethoxybenzene (DBB), N-ethylphenothiazine (EPT), and 3,7-bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) limit battery voltage.