Understanding Zinc Metal Electrodeposition for Aqueous Batteries

Aqueous zinc metal batteries (ZMBs) could enable safe and cost-effective electrochemical energy storage needed to transition to intermittent renewable energy sources. Zinc has an electrochemical potential of -0.76 V vs. SHE, allowing non-flammable aqueous electrolytes. Zinc also has a high volumetric specific capacity of 5854 mA per cubic centimeter which makes ZMBs a strong candidate for grid-scale energy storage applications. While promising, ZMBs face reversibility issues which prevent long-term rechargeability. This irreversibility is caused by 1) undesirable side reactions of zinc with the electrolyte, including hydrogen evolution and 2) non-homogeneous plating morphology. Non-uniform plating morphologies can lead to electrochemically inactive zinc or dendrites which short-circuit the battery. Understanding the mechanisms that influence homogeneity of zinc electrodeposition is of interest to design highly reversible ZMBs.

In this work, we characterize electrode morphology under different electroplating conditions in 1 molal zinc sulfate (ZnSO4). We also investigate the effects of the electrolyte additive dimethylformamidium triflurosulfonate (DOTf), which is designed to suppress the hydrogen evolution reaction (HER). To understand morphology, heterogeneity of zinc plating, and electrode surface composition, we used characterization techniques such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Electrochemical techniques including cyclic voltammetry and galvanostatic cycling are performed to support morphological findings and understand the conditions under which side reactions occur. Long term galvanostatic cycling is used to determine performance metrics of different electrolyte systems.

We find that certain electroplating parameters, such as increased current density at low capacity, allow for more compact and homogeneous zinc deposits. Low current densities show electrochemical behavior above the zinc plating potential that could be attributed to side reactions, alloying with the copper electrode, or underpotential deposition. This electrochemical behavior impacts morphology throughout cycling. DOTf as an additive leads to more compact zinc plating and shows reduced side products at the surface. However, NMR shows the DOTf additive electrolyte is not chemically stable and degrades over short periods of time, impacting reversibility. High concentrations of DOTf in the electrolyte also worsen long-term cycling. More stable additives that lead to similarly compact deposition should be considered. This work investigates the path towards smooth zinc electrodeposition for viable ZMBs.