Developing long-duration energy storage (LDES) systems is key to promoting the penetration of intermittent energy storage, such as solar and wind into the electric grid. DOE’s Long Duration Storage Shot establishes a target to reduce the cost of grid-scale energy storage by 90% for systems with durations of >10 hours. Non-aqueous RFBs (non-aqueous RFB) using earth-abundant materials are promising to fulfill this target. A key bottleneck to keeping RFB from market penetration is the lack of high-performance and cost-effective membranes. Membranes represent the most critical component in non-aqueous RFB, and they are decisive to the cell stack performance and cost. For example, the ionic conductivity of the membrane determines the power density of the RFBs, while ion selectivity and solvent uptake govern non-aqueous RFB cycle life. Herein, we will present recent progress in our team on the membrane development for the non-aqueous flow batteries. The ionic conductivity - mechanical strength tradeoff is a long-standing challenge for all polymer electrolyte development. In a redox flow battery, solvent uptake promotes membrane ionic conductivity but decreases its storage modulus due to the increased polymer chain relaxation. Two strategies are found effective to alleviate such a tradeoff i) selective plasticization of the ion-conductive block such as poly(ethylene oxide) (PEO) in a polystyrene (PS)− PEO−PS block copolymer (SEO) electroconductivity by glyme-type of solvent, and ii) enhancement of the membrane mechanical strength by creating hydrogen and ionic bonds between the polymer matrix and the inorganic scaffold. Mitigating redox-active species crossover poses another challenge in membrane design. We show that the cation exchanged single ion conducting membrane can effectively decrease the crossover of the polysulfide species in a Na metal – polysulfide hybrid flow battery, which promotes the RFB capacity retention, Columbia efficiency, and cycle life benchmarked to a commercial porous membrane. Acknowledgment This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the U.S. Department of Energy through the Energy Storage Program led by Dr. Imre Gyuk, in the Office of Electricity.