(Digital Presentation) Evaluating Stability and Performance of Nasicon Membranes for Crossover Mitigation in Aqueous Redox-Flow Batteries

Abstract

Most polymer-based membranes that are used in prototypical aqueous redox-flow batteries (RFBs) do not adequately prevent crossover of small-molecule reactants, causing high rates of capacity fade. Ceramic superionic conductor membranes are an attractive alternative due to their superior abilities to mitigate crossover;1 they can thus enable the deployment of electrolytes containing earth-abundant, small-molecule reactants.2 3 We test the performance and stability of a von Alpen sodium superionic conductor Na3.1Zr1.55Si2.3P0.7O11 (NaSICON) as an RFB membrane by examining its resistance, permeability and interfacial morphology as a function of electrolyte composition and temperature. The resistance of NaSICON is stable for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes, and its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. The glassy phase of the NaSICON microstructure at the membrane-electrolyte interface undergoes small amounts of etching while in contact with aqueous electrolytes containing sodium ions, which becomes more extensive when potassium ions are present in the electrolyte, leading in certain instances to complete disintegration of the membrane. We report a ferrocyanide-permanganate flow cell at a pH of 14.5 with a ~ 700 μm-thin NaSICON membrane supporting weeks of cycling with apparently negligible reactant crossover and very low capacity fade (< 0.04 %/day). Area-specific resistance of NaSICON falls dramatically with increasing temperature and decreasing membrane thickness, and a membrane that is 100 µm thick or thinner can enable power densities at above-ambient temperatures that are comparable to power densities of polymer membrane-containing flow cells.(1) Yu, X.; Gross, M. M.; Wang, S.; Manthiram, A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte. Advanced Energy Materials 2017, 7 (11), 1602454, https://doi.org/10.1002/aenm.201602454. DOI: https://doi.org/10.1002/aenm.201602454 (acccessed 2021/03/12).(2) Wei, X.; Xia, G.-G.; Kirby, B.; Thomsen, E.; Li, B.; Nie, Z.; Graff, G. G.; Liu, J.; Sprenkle, V.; Wang, W. An aqueous redox flow battery based on neutral alkali metal ferri/ferrocyanide and polysulfide electrolytes. J. Electrochem. Soc. 2016, 163 (1), A5150-A5153. DOI: 10.1149/2.0221601jes].(3) Colli, A. N.; Peljo, P.; Girault, H. H. High energy density MnO4-/MnO42- redox couple for alkaline redox flow batteries. Chem Commun (Camb) 2016. DOI: 10.1039/c6cc08070g.