The inclusion of a “redox shuttle” in a battery electrolyte can provide passive cell balancing as well as overcharge protection by functioning as a molecular shunt when the desired maximum charge voltage is reached.  There are many requirements for effective shuttle operation; chief among these are: 1) compatibility with other electrolyte components; 2) high oxidation potential (generally > 4.0 V vs Li/Li⁺); and 3) high stability in both reduced and oxidized forms (leading to long cycle life).

The most common traditional strategy for designing organic compounds that possess such high oxidation potentials involves the incorporation of electron withdrawing substituents; however, the resultant lack of electron density in the ring system often leads to undesirable side reactions and/or decomposition.  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 models suggest that oxidation potentials can be shifted to more positive values without sacrificing stability.  Preliminary results employing this strategy on various substituted carbazoles and phenothiazines will be presented.