In a range of electrical energy storage technologies, redox flow batteries (RFBs), which reversibly convert chemical energy to electrical energy, have shown a favorable balance of cost, safety, and performance for stationary applications. Shifting from aqueous to non-aqueous electrolytes may promise a higher cell voltage due to the extended electrochemically stable window (~ 4 V) and an enriched selection of redox materials due to the broader variety of organic solvents. As a key component in RFBs, an ideal membrane should have negligible electronic conductivity, high ionic conductivity, good chemical stability and mechanical strength, and low active species crossover. However, to date, no non-aqueous membrane has been identified that simultaneously fulfills these requirements, especially high ionic conductivity and low crossover rate.
To meet aggressive established cost targets ($120/kWh), our recent work suggests the maximum area-specific resistance for non-aqueous RFBs is 2.3 Ω∙cm2, which corresponds to the ionic conductivity of 1.1 mS/cm and 7.7 mS/cm for a membrane with a thickness of 25.4 µm (such as Nafion® 211) and 177.8 µm (such as Nafion® 117), respectively. However, there is little data on the effectiveness of successful aqueous membranes under non-aqueous conditions. To this end, we explore the performance of ion-selective lithiated Nafion 117 (Li N117) and non-ion selective nanoporous separators with a range of well-defined pore sizes in non-aqueous electrolytes with a focus on the relationships between ionic conductivity, species selectivity, and membrane microstructure (pore size). Key membrane properties and performance metrics are quantified in electrolytes consisting of different salt and solvent combinations, and compared between ion-selective membranes and non-selective separators. Results indicate a critical relationship between pore size, ionic conductivity and species selectivity and shed light on the future research directions in non-aqueous separations.
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