(232g) Electrochemical and Physicochemical Properties of Highly Concentrated Acetate-Based Electrolytes for High-Voltage Aqueous Zinc Batteries

Zinc-based aqueous battery systems are attractive for grid-storage applications due to the inherent electrolyte safety, material non-toxicity, low cost, high volumetric capacity of zinc and the low polarizability of the zinc anode in aqueous solutions. Despite these key features, their relatively low energy density is limited by the working potential range for aqueous electrolytes. In addition, zinc hosts a slew of problems such as inhomogeneous material distribution upon cycling and the formation of inactive zinc species and dendrites, all of which contribute to capacity loss and ultimately, cell failure. Water-in-salt electrolytes (WiSE) have been found to be an effective strategy to expand the electrochemical window of aqueous electrolytes for high energy density aqueous-based batteries, including zinc batteries. These classes of electrolytes have also been recognized in literature to provide uniform and reversible metallic deposition. There is a growing amount of literature focusing on highly concentrated acetate-based electrolytes as low-cost and non-toxic WiSE for a multitude of di- and monovalent battery systems. However, work is scarce on the nature of zinc electrochemistry, morphology and reversibility in these acetate-based concentrated electrolytes at realistic current densities and higher zinc utilization. Understanding the electrochemical behavior of zinc in acetate-based WiSE will significantly aid in the development of a high-voltage zinc aqueous battery.

In this work, the physicochemical and electrochemical properties of bi-salt potassium-zinc acetate-based WiSE were studied and compared with those of traditional alkaline potassium hydroxide (KOH) electrolytes, such as the density, viscosity, equilibrium vapor pressure, ionic conductivity, and electrochemical stability window. The impact of the high salt concentration on the electrochemical working window of the electrolytes was measured using constant-current techniques, with the highest electrochemical window observed for 27 m potassium acetate solution at 3.13 V. Zinc symmetric cells were cycled for the first time at current densities at industry-relevant current densities of >10 mA/cm ² to examine zinc reversibility with respect to zinc depth-of-discharge (DOD), which were compared with zinc cycling in traditional 25% potassium hydroxide (KOH) electrolytes. The zinc surface morphology and roughness in these WiSE and KOH solutions were also examined using profilometry studies on cycled zinc foil anodes. Rotating disk electrode (RDE) experiments were used to quantify hydrogen evolution at 100 and 300 mV overpotentials (vs. Zn/Zn^II) to give us insights on the competition between mass transfer and electrochemical kinetics. Single-pulse ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy were conducted in conjugation with molecular dynamic simulations (MD) to elucidate solvation structures as a function of potassium and zinc acetate concentration, while pulsed field gradient (PFG)-NMR was performed to quantify the self-diffusion coefficients of water and acetate. These molecular and transport properties were used to better understand and explain the physicochemical and electrochemical properties of the WiSE electrolytes. Overall, these studies lay the scientific groundwork for understanding the use of concentrated acetate-based electrolytes with zinc electrodes and for the technological pursuit of a practical high-voltage aqueous zinc battery.

This work was supported by the U.S. Department of Energy Office of Electricity. Dr. Imre Gyuk, Director of Energy Storage Research at the U.S. Department of Energy Office of Electricity, is thanked for his financial support of this project. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. This work was also supported by the United States Nuclear Regulatory Commission under grant number NRC- HQ-60-17-G-0030. The views expressed in this article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. This article has been co-authored by an employee of National Technology & Engineering Solutions of Sandia, LLC under Contract No. DE-NA0003525 with the U.S. Department of Energy (DOE). The employee owns all right, title and interest in their contribution to the article and is solely responsible for its contents. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this article or allow others to do so, for United States Government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan https://www.energy.gov/downloads/doe-public-access-plan.