2, Oak Ridge National Laboratory, Knoxville, Tennessee, United States
3, University of Tennessee, Knoxville, Knoxville, Tennessee, United States
4, Oak Ridge National Laboratory, Knoxville, Tennessee, United States
Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost, have a low toxicity and high chemical stability associated with them. To make Zn-air batteries viable for large scale use, issues at the Zn electrode need to be addressed that lead to a loss of capacity during cycling. Allowance must be made to prevent dendrite growth that eventually leads to cell shorting during charging. During the oxidation of Zn in the battery, a passive layer of ZnO is formed on the electrode. Additionally, electrode morphology changes between each cycle, producing inconsistent performance. Here we describe studies of electrode structures and processes targeting improvement of these issues. Porous carbon electrodes similar that used in air electrodes provide a conductive and permanent structure for Zn deposition and removal during battery operation. We show that flowing electrolyte through such a porous electrode during electrochemical cycling allows for significant mitigation the mass transport issues at the Zn electrode. Carbon felts were studied for their use as the negative electrode in rechargeable Zinc-based batteries. Polarization studies showed that flow improved the performance of the both deposition and dissolution reactions. A current density of 100mA/cm2 was observed with only 20mV of overpotential during deposition and dissolution at a flow rate of 15ml/min of electrolyte through the electrode. Cycling shows a long stable charging step with the overpotential decreasing as charge is passed. Due to the Zn area in the electrode increasing, which provides more sites for Zinc deposition to occur. During discharge overpotential remains stable over a long period of time, eventually increasing as most of the Zn has been removed indicating that the electrode is fully discharged. Cycling remains stable over long periods of time: a symmetric cell was cycled for 250 hours at 50mA/cm2 without any problems with cell shorting or major deviation of the cell performance. Behavior of the electrode is specific to the conditions of the electrolyte used in the cell and will be discussed further. Zinc deposition in the carbon felt varies based on the flow and potential applied. Generally, Zinc deposits appear to form a needle like coating around the carbon fibers in the felt. Zinc then fills in the gaps between fibers as more metal is deposited. Constant potential deposition at higher applied overpotential leads to more deposition away from the current collector, while lower overpotentials during deposition produce deposits throughout the electrode volume. Reasoning for why this occurs and implications for the electrode performance will be presented.
We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk) and by the ARPA-e IDEAS program.