Abstract
Medium to long duration energy storage devices, such as redox flow batteries, will be imperative for implementing smart grids and accommodating the increasing use of renewable energy sources. Sodium (Na) based electrochemical energy storage is of interest for large-scale applications due to sodium’s low cost and high abundance compared to lithium and vanadium. The membrane plays a critical role in a flow battery, separating the catholyte and anolyte by preventing transport of the redox active species through the membrane. Thus, it is critical to develop mechanically robust, chemically resistant membranes. Considering that the membrane is a significant source of internal resistance and capacity fade in an operating battery, understanding the effect of electrolyte type and concentration on membrane transport properties is imperative for optimizing the performance of non-aqueous flow batteries. This study aims to unravel relationships between membrane transport properties and electrolyte concentration in a polar and non-polar organic solvent for a Na-based non-aqueous flow battery. A pentablock copolymer with a Na-transporting trifluoromethanesulfonimide (TFSI) center block was successfully synthesized. The fabricated membranes were equilibrated in tetraethylene glycol dimethyl ether (TEGDME) and propylene carbonate (PC) solutions with varying concentrations of sodium triflate (NaTf). Membrane properties such as uptake, conductivity, transport number, and salt absorption were measured as a function of electrolyte concentration. The membranes achieve an ionic conductivity of 7.2 x 10-5 S/cm in TEGDME and 1.4 x 10-4 S/cm in PC at moderate electrolyte concentrations. A high cation transport number (>0.75) was maintained across the concertation range for PC and up to 0.5M NaTf in TEGDME. This study suggests new design parameters for Na-conducting membranes for non-aqueous flow battery, where optimum membrane performance; a balance between uptake, conductivity, and charge carrier selectivity; can be achieved at moderate electrolyte concentrations of 0.5 M to 0.75 M NaTf.
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