Evaluation of High-Concentration TEMPO-Based Electrolytes for Use in Nonaqueous Redox Flow Batteries
Authors: Barton John Leonard , Carney Thomas , Brushett Fikile R. 
Published: February 2020

The rise of global greenhouse gas emissions necessitates the integration of sustainable energy technologies into the current energy economy spurring global research and development efforts. Unfortunately, the prevalent renewable sources, solar and wind, are highly intermittent often leading to mismatches between the demand for energy and supply. Energy storage technology is needed to smooth and meter the delivery of electricity from variable resources as well as offset congestion issues in the transmission & distribution infrastructure. Redox flow batteries (RFBs) are electrochemical systems well-suited for multi-hour energy storage which offer several key advantages over enclosed batteries (e.g., Li-ion) including independent scaling of power and energy, long service life, improved safety, and simplified manufacturing.1,2 To date, the majority of RFBs are based on aqueous chemistries and are thus limited by the electrochemical stability window of water. This low voltage is one of the key contributors to the high system costs. Transitioning to nonaqueous (NAq) electrolytes enable RFBs to operate at higher voltages (> 3 V), increasing the feasible energy density, as compared to their aqueous counterparts, and thus offering an alternative pathway to low cost operation. Though promising, NAq RFBs are a nascent concept and, to date, reported systems have been limited by the solubility of redox active organics in NAq electrolytes and by low energy efficiencies, particularly at higher active species concentrations.3

2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) is a promising redox active organic molecule because of its high solubility (> 1 M) in relevant NAq electrolytes and its chemical stability.1,4 However, when high concentration redox electrolytes are employed in a NAq flow cell, poor performance is observed which remains largely unexplained.1 Recent work in the area of Li-ion batteries has shown that at high concentrations (e.g., solvent-in-salt systems) significant changes in electrochemical and physicochemical behavior of NAq electrolytes is observed due to changes in solvation structure.2,5,6 At the high concentrations of redox species required for economic viability, similar solution-phase effects may be expected for NAq redox electrolytes which convolutes data analysis and has deleterious (or synergistic) effects on cell performance. Here we investigate physicochemical and electrochemical properties of NAq redox electrolytes containing high concentration (>1 M) TEMPO (figure 1) in supporting electrolytes (e.g., LiTFSI / MeCN). To investigate property changes as a function of state of charge, we develop a method for chemical oxidation of TEMPO via a disproportionation by sulfuric acid, followed by a double displacement reaction with a Li-ion salt.7 As compared to prior methods, this approach allows for anion selection without the use of particularly hazardous or costly synthesis reagents such as hexafluorophosphoric acid or bis(trifluoromethylsulfonyl)amine.8 Several TEMPO salts (TEMPOTFSI, TEMPOBF4) are synthesized and characterized, at high concentration, using a suite of analytical and electrochemical methods to determine the critical fundamental properties that underlie performance. Building on this knowledge, we then evaluate the performance characteristics of high concentration TEMPO-based redox electrolytes within a NAq RFB with a focus on understanding and overcoming performance limiting factors.

Acknowledgments

The authors acknowledge the financial support of the Joint Center for Energy Storage Research, which was formed under the Office of Basic Energy Sciences within the Department of Energy. This research was conducted with Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. We would like to thank J. D. Milshtein for his technical assistance, especially with regards to flow cell design, as well as A. Helal and G. H. McKinley for rheological guidance and use of the rheometer.

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Figure 1