Improved synthesis of N-ethyl-3,7-bis(trifluoromethyl)phenothiazine

Abstract

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 LiFePO₄/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.