Design of Mechanically Robust Polymer Membranes for Non-Aqueous Flow Battery

Large scale grid storage is imperative for an efficient use of renewable energies. While aqueous redox flow batteries (RFBs) such as a vanadium-based aqueous RFB are relatively mature technologies for a grid storage, non-aqueous RFBs present an attractive alternative strategy for potentially providing much higher energy density. One of the key enablers for non-aqueous RFBs is mechanically robust and highly ion-conductive membranes. In aqueous RFBs, membranes can achieve high proton or anion conductivity due to water mediating their ion conduction, thus there are various high performance membranes for aqueous RFBs. On the other hand, for membranes in non-aqueous RFBs, achieving high ion conductivity while maintaining mechanical and chemical stability is a major challenge. The design principle for such membranes is poorly understood due to the complexity of the system. Our membranes are developed for a non-aqueous RFB with an alkali alloy anode technology that has the potential to achieve high energy density, high safety, low cost and long cycle life. This presentation will discuss the fundamental design principle in polymer membranes for non-aqueous RFBs and our recent efforts on the development of novel plasticized polymer membranes that simultaneously provide high conductivity and tailored mechanical modulus. In this study, mechanically robust crosslinked poly(ethylene oxide) membranes were synthesized and doped with sodium triflate and tetraglyme. The relationships between ion conductivity (reached up to 10-4-10-3 S/cm at r.t.), salt/gel content, glass transition temperature (Tg) and mechanical properties are investigated. The synthesized membranes are mechanically robust with storage modulus maintaining at ~1 MPa from -20 °C to 180 °C even saturated with tetraglyme. Furthermore, mechanically tailored novel single-ion conducting polymer electrolytes (SICPEs); block and graft copolymers of poly[(4-styrenesulfonyl) (trifluromethane-sulfonyl)imide] (poly(STF)) and poly[1-[3-(methacryloyloxy)-propylsulfonyl]-1-(trifluoromethanesulfonyl)imide] (poly(MPA)), were investigated. The SICPEs were synthesized by covalently attaching anionic moieties to the polymer backbones that only allows specific cations, such as lithium or sodium ions, to move freely and provide ionic conductivity (i.e. the transference number is close to 1) which is imperative for RFBs. These novel SCPE membranes include a membrane with a flexible polymer backbone to afford mechanical robustness and stretchability, and poly(STF) or poly(MPA) as side chains to provide single-ion conductivity. The effect of monomer type, degree of polymerization of the side chain, and different cations are also studied. The obtained polymer membrane showed 100% elongation and around 10-4 S/cm single-ion conductivity (no salt) at 30 °C after being doped with propylene carbonate. 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.