SE984: Paths forward for non-aqueous electrolytes for redox flow batteries

Redox flow batteries (RFBs) have been considered as alternatives to Li-batteries for application on the grid but have not made substantial impact. There are numerous redox-active ions or compounds that have been considered for use in these systems. One area of interest has been compounds soluble in non-aqueous (NA) solvents. NA solvents offer wider voltage range of operation but these are partly offset by various disadvantageous properties.One issue that must be confronted for all types of RFBs is that of permeation of active components through the membrane separator. This has negative consequences ranging from self-discharge of the battery to catastrophic chemical reactions between the separated materials. Generally speaking, there is a direct correlation between the conductivity of the membrane, which should be maximized for highest performance, and the cross-over rate (permeability) which should be minimized.In the course of exploring the limits of NARFBs, we discovered a simple approach that breaks this correlation. Large organic cations interacting with membrane functional groups show surprising trends with respect to ion mobility while also modulating membrane swelling, a key factor in permeation. By manipulating this trade-off via design of the cation, we can achieve major decreases in cross-over while largely maintaining conductivity. We will present a detailed study of the underlying intermolecular interactions and ion mobility rates for these cases together with a discussion of the implications for NARFBs.