Nitroxide radicals are considered as ideal redox species in all-organic redox flow batteries due to their redox potential of ∼2 V. These radicals are predominantly used in their polymerized form as cathode materials to a high efficiency. Attempts to use poly(nitroxide)s as anode materials have been unsuccessful due to irreversibility of the reduction process as the reduced form of a nitroxide undergoes a fast, irreversible proton transfer with an electrolyte. In this study, reduction of the nitroxide radical, TEMPO, was shown to become reversible in an ionic liquid. A redox potential of 2.5 V was achieved, with the reduction reversibility being maintained after 200 cycles. A fabricated symmetric electrochemical cell demonstrated a high coulombic efficiency of 60% over an extended period of time. This is the first report demonstrating a high degree of reversibility of nitroxide reduction, thus leading to a paradigm shift in the future design of redox flow batteries.

Accurate prediction of intermolecular interactions plays a pivotal role in many areas of chemistry and biology including (but not limited to) the design of pharmaceuticals, solid electrolytes and food additives. Here we present the application of the recently developed spin-ratio scaled MP2 method (termed SRS-MP2) to six different datasets covering a wide range of interaction types from strong hydrogen bonding to van der Waals dispersion and π-π stacking. The method achieves a remarkably low mean absolute error of 1.6 kJ mol across all interaction types including semi-Coulombic systems such as organic ionic salts. The new SRS-MP2 method offers high level of accuracy for studying intermolecular interactions commonly found in molecular systems of chemical and biological relevance without the need for including additional terms in the formulation. This finding represents a new paradigm in the development of wavefunction-based methods for intermolecular interactions.

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.