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

A growth of renewable energy technologies will help to accommodate the growing demand for electricity, provided that energy storage technologies address the intermittency of renewable energies such as solar and wind power. Lithium polysulfide (Li-PS) redox flow batteries (RFBs) have been considered as one of the most promising electrochemical energy storage technologies because of their high energy density, design flexibility, and safe large-scale operation. However, Li-PS RFBs suffer from polysulfide crossover between two working electrodes which decreases the batteries’ columbic efficiency and shortens their cycle stabilities. Unfortunately, commercial battery separators (e.g. Celgard) and existing ion exchange membranes (e.g. Nafion) cannot be adopted because of their poor lithium ion selectivity over polysulfide ions. By integrating highly ion-selective polymers and nanoengineered pore-filled ion exchange membrane (IEM), our proposed solution will satisfy the desired characteristics of ion selective membranes for Li-PS RFBs, such as high lithium ion selectivity/conductivity, chemical/mechanical stability, low cost. The overall objectives of the Phase I project are to develop novel multifunctional IEMs, investigate fundamental transport properties, evaluate the electrochemical performances, and establish relationship between molecular structures and ion transport properties of the IEMs. During the Phase I, we will focus on (1) molecular-level design of high performance ion exchange polymers; (2) fabrication of nanoengineered pore-filled IEMs; (3) quantitative study of the molecular structure of IEMs and their correlation with ion transport properties (lithium and polysulfide ion) and electrochemical performance using various characterization tools; (4) numerical analysis to systematically investigate ion crossover, electrical potential distribution, and ion concentration distribution during charging/discharging. By the end of Phase I, we will demonstrate a new class of nanoengineered IEMs for Li-PS RFBs and other non-aqueous RFBs. Molecular structure design of ion exchange polymers and their transport properties characterizations will establish structure-property relationships as well as design criteria for high performance IEMs in energy storage applications. Energy storage can bring benefits to all functions of the grid, such as the restoration of quality parameters of energy storage for later use of excess generation, etc. Although all vanadium-based RFB has been widely developed for the power grid, their low energy density (20-60 Wh/L) and high price of vanadium have hindered their practical applications in many energy storages. Thus, the development of Li-PS RFB with high energy density has great potential to address all critical challenges of energy storage systems. Upon success of this program over Phase II or Phase III, our multifunctional IEM technology is expected to mature into a technology which can significantly lower the cost per cycle and enhance the cost-effectiveness of RFB modules. In addition, the results will be used to enhance the fundamental understanding of ion transports through ion exchange polymers for energy storage applications.