Delving into the Properties of Non-Aqueous Zn Electrolytes and Reversible Intercalation Chemistry for Zn Metal Batteries

A non-aqueous multivalent (e.g., Mg²⁺, Ca²⁺ and Zn²⁺) metal cell is one of the potential candidates for a post-lithium-ion battery. The theoretical volumetric capacity of a metal anode coupled with the lack of dendrite formation at a multivalent metal anode provide an attractive opportunity in energy storage.¹ However, the development of multivalent cathodes and electrolytes is a significant challenge. For example, with Ca metal cells there is no known electrolyte compatible with reversible chemistry at a metal anode. In the Mg systems only diglyme-Mg(TFSI)₂ and various halide and pseudohalide electrolytes are known to be compatible with Mg metal where an unknown passivation layer on a Mg metal anode induces low Columbic efficiency in the cell chemistry. However, it appears that Zn²⁺ maybe an exception in the multivalent metals. Zn metal anodes coupled with a reversible intercalation cathodes chemistry have a number of promising features: 1) high volumetric capacity (much higher than Mg and Ca);¹ 2) similar ionic radius compared with Li⁺ and Mg²⁺ ions;² 3) relatively lower activation barrier energy for diffusion in cathode materials (e.g., FePO₄, CoO₂ and V₂O₅)³ and 4) reversible Zn deposition and higher anodic stability in non-aqueous Zn electrolytes. Most of the previous rechargeable Zn cell studies have been performed using an aqueous Zn electrolyte system,^4-6and very little information is available about non-aqueous Zn battery, in particular non-aqueous Zn electrolytes.

In this study, the general electrochemical and transport properties of non-aqueous Zn electrolytes—consisting of mixtures of Zn salts (e.g., Zn(PF₆)₂, Zn(TFSI)₂, Zn(BF₄)₂ and Zn(CF₃SO₃)₂) and organic solvents (e.g., diglyme, propylene carbonate, acetonitrile and dimethylformamide)—have been examined in detail. The ionic conductivity of the electrolytes was determined as a function of salt concentration and temperature, and their diffusion coefficient was estimated by the chronocoulometry technique.⁷ The anodic stability of each electrolyte was studied by linear sweep voltammetry, while reversible Zn deposition behavior of the electrolytes on a Zn metal anode was investigated by cyclic voltammetry. Classical molecular dynamics and DFT calculations have been utilized to complement the experimental work and to provide insights into the molecular-level solvation structure (ion solvation and ionic association behavior) and dynamics of the bulk electrolytes. In particular, a prediction of the electrochemical stability window via adiabatic ionization potential/electron affinity calculations was undertaken. Based on our experimental analysis and the simulation studies of the range of different electrolytes, we have selected promising electrolytes and performed electrochemical testing with cells consisting of a Zn metal anode and a variety of cathodes materials. Among them, the Zn metal cell consisting of an acetonitrile-Zn(TFSI)₂ electrolyte and an electrochemically synthesized nanostructured V₂O₅⁸ demonstrates good reversibility and stability for 120 cycles with nearly 100% coulombic efficiency and ~180 mAhg^-1of gravimetric capacity, albeit operating at a relatively low cell voltage of 0.9 V.

Acknowledgments

This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract no. DE-AC02-06CH11357.

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