Diagnostics for Failure Modes in Non-Aqueous Flow Batteries

Abstract

Expansion of promising, non-aqueous flow battery chemistries may increase energy densities, lower costs, and ultimately increase penetration of energy storage technologies into electric grids throughout the world. However, significant research efforts are required to understand their potential relative to the well-studied aqueous vanadium and organic systems. Among the most promising non-aqueous chemistries are those based on the bipyridine metal coordination complexes. Like their aqueous counterparts, they are very susceptible to crossover. Having a key mechanistic understanding of the role that the electrolytes, solvents, and membranes play in crossover is critical to designing longer lasting batteries. Recently, we examined the role of the membrane in crossover using the aqueous organic methylviologen and TEMPO molecules and five commercially available anion exchange membranes. Capacities between the batteries were found to differ by >50% after 100 cycles with the different membranes. The capacity loss was attributed to crossover across the membrane and was dominated by mechanisms including diffusion, migration, and electroosmotic drag. For this study, we have investigated the role of not just the membrane on crossover, but the effects that both the electrolyte and solvents have on battery performance as well. Conditions were chosen to prevent crossover through both physical and chemical variabilities. This included using bulkier cations and anions in the electrolyte as well varying the dielectric constant of the solvents. Membranes were chosen based on chemical compatibility, tortuosity, and physical robustness. Collectively, the results have informed us on how to effectively design membranes within the parameters of electrolyte and solvent selection. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.