(In)stability of 10-Methylphenothiazine Cations in Common Electrolytes

10-Methylphenothiazine (MPT) is used in energy devices, often to prevent overcharging. Surprisingly, there is a lack of fundamental knowledge on the stability of MPT dication, which is transiently produced during operation. Herein, we used spectroelectrochemistry and cyclic voltammetry to investigate the effect of different Li-based electrolytes (LiPF6, LiClO4, LiSbF6) and temperatures on the chemical reactivity of the MPT dication (MPT2+). This work shows that while the radical cation is extremely stable, MPT2+ is a superacid that can react with most common battery electrolytes, specifically LiClO4. Experiments conducted with 2,4,6-trimethylpyridine (TMP) and density functional theory (DFT) calculations suggest that a possible degradation pathway involves the deprotonation of MPT2+ to form a reactive singly charged iminium ion (ImPT+), with the electrolyte acting as a proton acceptor. This work offers insights into the reactivity of MPT2+ under battery-relevant conditions, providing an opportunity for the rational design of phenothiazine-based devices with improved stability.