N ‐Substituted Phenothiazine Derivatives: How the Stability of the Neutral and Radical Cation Forms Affects Overcharge Performance in Lithium‐Ion Batteries

Phenothiazine and five N ‐substituted derivatives were evaluated as electrolyte additives for overcharge protection in LiFePO 4 /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 CN 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.