First principles theory of unanticipated electron transfers in new organic energy-storage materials: application to phenothiazine-based polymer cathodes.

Organic polymer cathodes have promise as low-cost, environmentally clean, lightweight alternatives to traditional inorganic battery cathodes, but are limited by their instability. To explain the observed limits to the charge capacity of PT-DMPD, a phenothiazine-based organic material, we utilize recently developed Joint Density Functional Theory (JDFT) methods to study electron transfer (redox) events. Comparing to experimental cyclic voltammetry (CV) data, we find that electron transfer from the primary PT-DMPD redox sites actually occurs at lower voltages than anticipated. As a consequence, batteries using this and related materials have been operated at voltages where some of the primary redox reactions have actually already occurred, so that, during operation, unexpected redox centers are accessed. These new centers weaken important structural bonds, perhaps explaining the observed loss of capacity of PT-DMPD cathode batteries with cycling. These findings suggest a pathway for design of more power dense, stable energy storage materials.