Development of High-Efficient Ion Selective Membrane Separator for Lithium Polysulfide Redox Flow Batteries

Abstract

As our society demands greater utilization of intermittent renewable energies from solar and wind, the development of a low cost, reliable energy storage technology with high energy density is of great interest. In particular, highly efficient large-scale electrochemical energy storage has been an important issue to enhance the efficiency and quality of electrical grid. Among many energy storage systems, lithium-polysulfide (Li-PS) redox flow batteries (RFBs) have been considered one of the most promising electrochemical storage systems because they can offer the high energy density of the Li-S batteries and the general features of RFBs (e.g. flexible system design from decoupled energy storage, a power generation, safer operation, long cycle life) simultaneously. To successfully adopt Li-PS RFBs for a large scale electrochemical storage technology, it is imperative that the membrane separator should prevent the crossover of redox-active materials between the electrodes which would otherwise result in low columbic efficiency, rapid capacity fading and poor cycle life. Recently, Bae group demonstrated that aromatic polymers based on biphenyl backbones have excellent chemical and mechanical stability and prevent crossover of redox-active materials for RFBs. In this work, we will present new, multifunctional, highly ion selective, biphenyl polymer (BPSA) with high ionic transport properties (Figure 1). The unique design of this polymer involves (1) a lithiated sulfonate group, (2) tunable hydrophobic polymer backbone, and (3) a hydrocarbon linker between them. The covalent connection of these two distinctively different moieties would lead to nanometer-scale phase separation between hydrophilic ion conducting channels (via aggregation of sulfonated groups) and hydrophobic polymer backbone domains. The BPSA membranes demonstrated resistance in swelling in the commonly used Li-S RFB solvent solution dioxolane/dimethoxyethane (15% in x-y direction) and achieved good mechanical properties (tensile strength ≥20 MPa, percentage elongation ≥50). In an effort to decrease the areal resistance and increase lithium conductivity of the polymer membranes, a variety of reinforcement materials such as poly(ethylene) (PE), polytetrafluoroethylene (PTFE), and Celgard were coated with a thin layer of BPSA. The low-cost, chemically inert, reinforcing materials provide additional mechanical strength and reduce the swelling ratio of BPSA, lowering the permeation rate of the polysulfide anions. Performance of these hydrocarbon cation exchange membranes in Li-S RFB with respect to the separation of polysulfides and conductivity are compared to the state-of-the-art cation exchange membrane Nafion®, and will be discussed. A brief description of battery performance carried out by UIC will follow. Figure 1