Non-aqueous redox flow batteries (RFBs) are potential alternatives to aqueous redox flow systems for large scale energy storage. The advantages of non-aqueous RFB’s over aqueous systems are a larger voltage operating window, wider temperature range and the potential for increased energy density. A sodium-based system is of interest due to sodium’s low cost and natural abundance. One major challenge for non-aqueous RFB’s is the lack of high performance membranes that can withstand their operation conditions. A membrane for an RFB needs to possess sufficient mechanical strength, high conductivity, ion selectivity and excellent chemical resistance. In this project, mechanically robust, chemically resistant, crosslinked membranes have been fabricated via thermal or UV curing. In one system, crosslinked poly(ethylene oxide)-based membranes containing 10 wt% to 40 wt% sodium triflate have been studied as dry and plasticized samples. Careful tailoring of the crosslinking chemistry allows the membranes to be free standing, flexible and robust with and without plasticizer. The membranes are stable over a wide temperature range, with a constant storage modulus of ~1 MPa from -20 °C to 180 °C even with the addition of plasticizer. The mechanical strength of the membranes is enhanced by 3 orders of magnitude to above 1 GPa with the addition of an inorganic filler, without any significant adverse effect on ionic conductivity. Specific ionic conductivities up to 2.0 x 10^-4 S/cm at 20 °C and 7.1 x 10^-4 S/cm at 70 °C are achieved for the plasticized membrane, while ionic conductivities for the dry membrane are 3.0 x 10 ^-6 S/cm at 20°C and 6.4 x 10^-5 S/cm at 70 °C. Furthermore, our efforts on the synthesis and characterization of sodium sulfonate functionalized cation exchange membranes will be discussed. This work is funded by Dr. Imre Gyuk, Office of Electricity Delivery and Reliability, Department of Energy and the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC.”