Creating water-in-salt-like environment using coordinating anions in non-concentrated aqueous electrolytes for efficient Zn batteries

Creating water-in-salt-like environment using coordinating anions in non-concentrated aqueous electrolytes for efficient Zn batteries†

Dario Gomez Vazquez,

^a Travis P. Pollard,

^b Julian Mars,^c Ji Mun Yoo,^a Hans-Georg Steinrück,

^d Sharon E. Bone,^e Olga V. Safonova,

^f Michael F. Toney,

^cg Oleg Borodin^b and Maria R. Lukatskaya

*^a

Author affiliations

* Corresponding authors

^a Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland
E-mail: mlukatskaya@ethz.ch

^b Battery Science Branch, DEVCOM Army Research Laboratory, Adelphi, MD, USA

^c Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO, USA

^d Department Chemie, Universität Paderborn, Paderborn, Germany

^e Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

^f Paul Scherrer Institute, Villigen PSI, Switzerland

^g Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, CO, USA

Abstract

Rechargeable aqueous Zn metal batteries are promising systems for grid storage because of their high energy density, low cost, and non-flammability. However, Zn metal anodes have major limitations due to dendrite formation and concurrent water splitting during charge–discharge cycling. Both processes negatively affect coulombic efficiency (CE) and long-term cycling stability. Water-in-salt (WIS) electrolytes were previously proposed to address these challenges, yielding improvements in the cycling stability of Zn metal anodes. While WIS electrolytes help increase CE, they require high amounts of salt (often toxic) and have dramatically increased viscosity, which in turn limit their transport properties, charge–discharge rates, and usability in advanced Zn batteries. In this manuscript, we propose a strategy for simultaneously achieving high CE (>99%), high rate, low cost and reduced environmental footprint. Specifically, we show that by using coordinating anions like acetate a WIS-like Zn coordination environment can be achieved even in relatively dilute conditions, enabling prolonged cycling of Zn metal anodes. Such electrolytes have an order of magnitude higher conductivity and lower viscosity than traditional WIS electrolytes, thus enabling lower overpotentials and higher rate of Zn plating/stripping.