Numerous organic and inorganic compounds have been investigated as shuttle candidates, but finding materials that possess both a sufficiently high oxidation potential and adequate durability has proven extremely challenging. The ideal shuttle should be compatible with all electrolyte and cell components, have an oxidation potential near or slightly higher than the maximum cell voltage (i.e., 4.2 V vs. Li/Li+for NMC cells), and be highly stable in both reduced and oxidized forms. It should also be inexpensive, highly soluble (even at low temperature), display a reasonably large diffusion coefficient, and exhibit favorable electrode kinetics.
One common approach is to begin with a redox-active heterocyclic skeleton and add appropriate electron withdrawing substituents until the desired oxidation potential is reached. However, because of the resultant lack of electron density in the ring system, this strategy often leads to systems that are susceptible to dimerization, unimolecular decomposition, or nucleophilic attack in their oxidized form. We have developed an alternative approach that exploits the change in molecular geometry that often accompanies the oxidation of heterocyclic compounds. Specifically, by introducing bulky groups to sterically impede this reorganization, computer modeling suggests that oxidation potentials can be shifted to more positive values without sacrificing stability. Our preliminary results employing this strategy on various substituted heterocycles have borne out these predictions, as we have observed increases in both oxidation potential and chemical reversibility.