Accelerating Development of a Nonaqueous Flow Battery Membrane Separator Material

"To facilitate increased adoption of renewable energy, increasing amounts of stationary energy storage will be needed to accommodate the intermittency of wind and solar power. One promising option for grid-scale energy storage is flow batteries, which offer the flexibility of modular and scalable design while separating the energy and storage components. Commercial flow batteries rely on aqueous electrolytes; however, the use of aqueous electrolytes limits the operating voltage and energy density of these devices. Lower operating voltages also means that more cells are required to reach a desired output voltage, increasing the size, cost, and power stack complexity. An alternative is to develop flow batteries with nonaqueous electrolytes, where higher voltages are possible due to the wider voltage stability window. The higher voltage per cell increases the energy density and reduces the battery footprint, a consideration that can be important depending on the cost and footprint required at the installation site. Nonaqueous organic redox flow batteries enable new chemistry for the electrochemically active species that store energy, and energy density could be increased, for example, through increases in solubility of these compounds relative to aqueous counterparts. While nonaqueous flow batteries have the advantages described above, a critical limitation is that membrane separators have not been developed for successful operation of this technology. New ion-selective membranes are needed with 1) a high conductivity for the ion that facilitates charge/discharge, 2) selective exclusion of the organic redox species that participate in electrochemical reactions to reduce crossover and capacity fade, and 3) dimensional, mechanical, and chemical stability. There are very limited materials and process-structureproperty relationships available for these membranes, as development of nonaqueous flow batteries has not received as substantive research intensity as its aqueous counterparts. To meet this need, Luna Innovations and the University of Virginia (UVA) will continue development of a membrane based on poly(phenylene oxide) functionalized with phenoxyaniline trisulfonate. This UVAdeveloped material has among the best reported conductivity, selectivity, and stability properties in organic electrolytes while possessing the commercial potential to be applied to nonaqueous flow batteries. In Phase I, the membrane material will be improved with side chain modifications towards improved conductivity and crosslinking to improve dimensional stability and toughness. Luna and UVA have partnered to accelerate membrane testing, refinement and scale-up in Phase I and transition the technology to nonaqueous flow battery collaborators in Phase II. Luna also has accelerated test capabilities and process scaling experience to transition the membrane from the laboratory to pilot-scale (toll manufacturing) demonstrations in Phase II. It is expected that the knowledge gained from this project will both impact the commercialization of this promising membrane material and more broadly nonaqueous selective ion conducting polymers."