Triphenylamines as a Class of Redox Shuttle Molecules for the Overcharge Protection of Lithium-Ion Cells

The number of molecules that have been proposed as shuttle molecules has been steadily growing as research in the area continues. However, the number of molecules that provide lengthy overcharge protection remains small. A class of molecules that provides better than average overcharge protection in lithium-ion coin cells is the triphenylamine class. With the charge spread throughout the three aromatic rings and the central nitrogen, these molecules are stable and have a tuneable oxidation potential, as shown when electron-withdrawing bromines are added to the rings. The oxidation potential of triphenylamine is raised by approximately per bromine added. Also, the addition of bromine can increase the electrochemical stability of triphenylamine, as seen by an increased number of overcharge cycles in coin cells containing tris(4-bromophenyl)amine instead of triphenylamine as the redox shuttle. This stability comes from the bromine preventing the formation of the dimer of triphenylamine, tetraphenylbenzidine.