Toward Improved Performance of All-Organic Nitroxide Radical Batteries with Ionic Liquids: A Theoretical Perspective

Nitroxide radicals have previously been successfully used as electrodes in all-organic radical batteries. However, one drawback of these batteries is significantly reduced redox potentials, in comparison to that of widely used lithium-ion batteries, making their energy-producing capacity rather small for use as a primary battery. In addition, strong propensity of nitroxide radicals to engage in side reactions with traditional electrolytes based on molecular solvents give rise to a series of undesirable and irreversible byproducts, thus significantly reducing the life of nitroxide batteries. Ionic liquids (ILs) have previously demonstrated their ability to reduce the reactivity of radicals through strong intermolecular interactions. In this study, we investigate the use of ILs as electrolytes with the view of increasing redox potentials of nitroxide radicals. A series of imidazolium, phosphonium, and pyrrolidinium-based ILs coupled with widely used anions were chosen to predict redox potentials of the 2,2,6,6-tetramethyl-1-piperidinyloxy nitroxide (TEMPO) radical using state-of-the-art quantum chemical calculations using one and two ion pairs to describe ILs. Some ILs showed a significant increase in the redox potential of this radical to reach as much as 5.5 eV, compared to the previously measured value of 2.2 eV in aqueous media. In particular, ILs were shown to stabilize the aminoxy anion, the reduced form of the nitroxide radical, which has not been achieved previously in traditional solvents. Although a simple model consisting of one and two ion pairs was used in the current study, these findings clearly demonstrate that ILs have a huge potential in improving redox potentials of nitroxide radicals.