Exploring Heterogeneous Kinetics in Redox-Mediated Energy Storage
Authors: Mallia Christopher T. , Matteucci Nicholas J. , Neyhouse Bertrand J. , Brushett Fikile R. 
Published: September 2023

Electrochemical energy storage technologies typically rely on heterogenous electron transfer reactions at current collectors and concurrent chemical reactions that generate or consume ions at the electrode in order to balance charge. These reactions are often volumetrically-limited, at the device level, by the available surface area for electron transfer reactions and associated mass transport of reactants or by the form factor of the chemical compounds, frustrating development and/or limiting utility. Flow batteries partially address this limitation by using soluble charge-redox couples paired with “inert” electrodes rather than a composite electrode packed with solid charge-storage materials.1 The use of dissolved redox active species, combined with flow, imparts greater design flexibility albeit at the expense of energy density. To this end, a combination of these approaches can be considered, where redox species transport charge between an “inert” electrode and a spatially-separated, high-capacity material by leveraging redox-mediated processes in a flow battery architecture.2

Redox-mediated energy storage systems leverage the high energy density of solid materials in combination with the modularity of solution-based flow systems. They operate by sacrificing a portion of the cell voltage towards a reaction between the soluble redox species and the solid material, which allows the cell capacity to be primarily determined by the amount of solid material used.3,4 By leveraging electrochemical reaction engineering principles, optimization of redox-mediated systems can enable flow batteries with energy densities that approach those of closed-form rechargeable batteries. Despite potential benefits, the introduction of spatially and temporally distinct chemical reactions also complicates battery design and operation, requiring careful consideration of solid-solution reactant combinations, reaction conditions, and system architecture.
Herein, we investigate several redox-mediated reactions geared toward energy-dense flow battery systems, specifically focusing on lithium-storing positive electrode materials in combination with different soluble redox couples, in both aqueous and non-aqueous systems. Using in-situ voltammetry complemented by supporting measurements (thermogravimetric analysis, XRD), we characterize reactant materials during and after use in a redox-mediated system. We will also present theoretical considerations for the choice of reactants and solid-state materials for energy storage. Finally, experimental methods for characterizing such systems both in situ and operando will be discussed. The principles applied in experimental characterization of these systems are more broadly applicable to the mediated energy storage design space, and will inform future workflows for their study.

Acknowledgements

This work was funded by the Skoltech – MIT Next Generation Program. CTM would like to gratefully acknowledge support from the National Defense Science and Engineering Graduate fellowship, under the advisement of the Office of Naval Research. N.J.M and B.J.N gratefully acknowledge the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. We also gratefully acknowledge support from the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

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