Redox-Mediated Phenomena in Thin-Film Model Systems

The energy density of conventional redox flow battery (RFB) systems is limited by solubility of the charge-storing species in the mobile liquid electrolyte. To enable dramatic increases in energy density, a redox-mediated approach, where redox species transport charge between an “inert” electrode and a spatially-separated, high-capacity material (substrate), can been considered.^1–3 A fundamental aspect of this design is a heterogeneous chemical reaction between a soluble redox-active species with a material of a different phase (solid) that is submerged in the liquid electrolyte.⁴ Such reactions are analogous to, and of interest in, electrochemical deposition, wet-etching, and material processing. Despite potential energetic benefits for the RFB, the introduction of spatially and temporally distinct chemical reactions complicates system design and operation, requiring careful consideration of solid-solution reactant combinations, conditions, and reaction kinetics.⁴ With the intention of more broadly understanding redox-mediated reactions, we here focus on characterizing the reaction dynamics of model systems. Specifically, elemental metallic thin-films and both powder and single-crystal lithium-ion intercalation compounds are chosen as learning platforms from which important findings about reaction mechanisms can be gleaned, and comparisons made.<br/>In this presentation, we will describe several redox-mediated reactions of interest for energy dense RFB systems, and electrochemical processing of materials. Specifically, we focus on lithium-ion intercalation materials (e.g. LFP), and metals (e.g. zinc, copper) in combination with different soluble redox couples, in both aqueous and non-aqueous environments. Metal substrates evolve as reaction proceeds, causing surface passivation phenomena and growth of a corroded interface, while LFP is instead controlled by solid-state phase transformation. For all considered systems, surface and bulk composition, reaction products and rate, and morphology are expected to heavily depend upon choice of mediator and electrolyte conditions. Reactions will be characterized using <i>operando </i>optical microscopy and <i>in-situ </i>voltammetry, supported by analytical measurements (e.g., SEM, FTIR, XRD) performed before and after testing. The surface morphology and chemical evolution of both substrates and mediators will be discussed. We will also present theoretical considerations for the choice of reactants and solid-state materials for energy storage.<br/> <br/><b>Acknowledgements</b><br/>CTM would like to gratefully acknowledge support from the National Defense Science and Engineering Graduate fellowship (2020-2023), under the advisement of the Office of Naval Research.<br/> <br/><b>References</b><br/>(1) Huang, Q.; Wang, Q. Next-Generation, High-Energy-Density Redox Flow Batteries. <i>ChemPlusChem</i> <b>2015</b>, <i>80</i> (2), 312–322. https://doi.org/10.1002/cplu.201402099.<br/>(2) Wang, Q.; Zakeeruddin, S. M.; Wang, D.; Exnar, I.; Grätzel, M. Redox Targeting of Insulating Electrode Materials: A New Approach to High-Energy-Density Batteries. <i>Angew. Chem. Int. Ed.</i> <b>2006</b>, <i>45</i> (48), 8197–8200. https://doi.org/10.1002/anie.200602891.<br/>(3) Gupta, D.; Cai, C.; Koenig, G. M. Comparative Analysis of Chemical Redox between Redox Shuttles and a Lithium-Ion Cathode Material via Electrochemical Analysis of Redox Shuttle Conversion. <i>J. Electrochem. Soc.</i> <b>2021</b>, <i>168</i> (5), 050546. https://doi.org/10.1149/1945-7111/ac0068.<br/>(4) Matteucci, N. J.; Mallia, C. T.; Neyhouse, B. J.; Majji, M. V.; Brushett, F. R. Toward Electrochemical Design Principles of Redox-Mediated Flow Batteries. <i>Current Opinion in Electrochemistry</i> <b>2023</b>, <i>42</i>, 101380. https://doi.org/10.1016/j.coelec.2023.101380.