Processes at the Negative Electrode of Alkaline Zinc-Air Batteries

Abstract

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 (Zinc is abundant in the earth’s crust and alkaline aqueous solutions are cheap and well understood) and have a low toxicity associated with them. To make Zn-air batteries viable for large-scale use, issues at the Zn electrode need to be addressed. Allowance must be made to prevent dendrite growth that eventually leads to cell shorting during charging and electrode passivation during discharge. Additionally electrode morphology changes between each cycle produce inconsistent performance. During the oxidation of Zn occurring in the discharge process in the battery, a sequence of chemical steps occur resulting in eventual production of ZnO on the electrode. Here we report detailed studies of the Zn plating and stripping processes in alkaline environments. These processes are investigated by in situ electrochemical STEM (in situ ec-STEM) measurements that allow us to directly observe nucleation and growth processes as well as the behavior of Zn structures during stripping. These are correlated with electrochemical QCM measurements (EQCM). These methods give us insight into the relative rates of different processes. In situ ec-STEM experiments show that deposition and dissolution processes occurred at different rates and directionality. During deposition, deposits are initially observed grow quickly in all directions consuming available Zinc ions near the electrode surface. In the experiment whose video frames are shown in figure 1, 10,000 µm2 of observed Zinc deposit area was added in the first 30 seconds of deposition and was half of the total observed deposit area for a 3 minute, 250mV overpotential deposition experiment. After this, growth follows the concentration gradient and grows at a linear rate of 3,500 µm2 of observed deposit area per minute. Additionally, the overpotential employed during deposition appeared to impact deposit morphology dramatically, as 250mV deposition produced large boulder type deposits while a 300mV deposition test produced a mossy structure that covered the electrode surface. Dissolution processes observed via in situ ec-STEM with a 300mV underpotential step to anodically strip deposits showed only half of the material area was removed over a 3 min experiment with only a third reduction of the max particle height. Figure 2 shows the video frames from this experiment which began immediately after the experiment from figure 1. Further material removal appears to occur universally along the surface of the deposit, leading to the possibility of the deposit being detached from the electrode surface. EQCM was used to observe the formation and removal of the ZnO passive layer described above. The formation of the passivation layer was observed as a deviation from the rate of Zinc removal in the massograms generated by the QCM as a ZnO layer forms on the electrode surface. EQCM confirmed that the passive layer was the result of a precipitation reaction occurring in the solution near the interface. Further EQCM indicated that during dissolution multiple pathways of removing Zn were possibly occurring at once when looking at the charge transfer with actual mass removed, while deposition steps appeared to have only one reaction path of Zinc deposition occurring. This indicates that the forward and reverse pathways of the Zn reaction are not the same. Motional resistance to the crystal oscillation was collected as well to determine relative roughness of the deposits formed during deposition to correlate to the variations of in deposition seen in the in situ ec-STEM work. To overcome macroscopic issues associated with the Zn electrode, we are exploring using porous carbon negative electrodes similar to those used as air catalyst support materials. These provide a conductive and permanent structure in which Zn can be deposited and removed for battery operation. Operation of this electrode under various circumstances will be discussed. 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). A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User facility. Figure 1