Investigation of The Dynamics of Extreme Redox-Active Molecule and Electrolyte Concentrations Relevant for Nonaqueous Redox-Flow Batteries

Organic redox-active molecules have been explored for many uses including, acting as the active material in energy-storage systems for redox flow batteries (RFBs) and providing overcharge protection in lithium-ion batteries (LIBs). The concentration of the redox active species and the supporting electrolyte in an RFB play a significant role in determining the energy density of a battery. Nevertheless, at very high concentrations, the physicochemical relationship between the redox active molecule, electrolyte salt, solvent, and the electrochemical performance of an RFB has not yet been well studied. Herein we present a molecular-level understanding of the effect of concentration on physical properties of the redox-active solution to complement experimental observations using molecular dynamic (MD) simulations. To examine this relationship, we explored the redox-active molecule 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and tetrabutylammonium hexafluorophosphate (TBAPF₆) electrolyte salts varied across concentrations of 1 mM to over 1000 mM in acetonitrile. We observed relationships between the transport properties of these solutions that were primarily based on solvation and ion-pairing effects. Furthermore, we also provide suggestions on obtaining optimum performance in such systems based on our theoretical insight.