In situ x-ray computed tomography of zinc–air primary cells during discharge: correlating discharge rate to anode morphology

Abstract

Zinc-air batteries are gaining attention as safe battery alternatives, with high theoretical energy densities and a high abundance of their constituent materials. However, barriers to their widespread adoption include the need to improve their cycling lifetime, as well as stability and avoiding degradation mechanisms such as zinc dendrite growth and hydrogen-producing side reactions. X-ray computed tomography (CT) is a widely used technique for the study of batteries. In-situ/operando X-ray CT has been increasingly used to study the zinc anode of zinc-air batteries to evaluate the interesting morphological changes occurring during the reaction from Zn to ZnO during discharge (vice versa during charge). However, several studies have been carried out using synchrotron X-ray sources, which have limited availability for users. In this work, we present a comprehensive study of the discharge of commercial, primary zinc-air batteries using a laboratory based X-ray source for in-situ X-ray CT measurements. Four different discharge rates are investigated (C/30, C/60, C/90 and C/150), with tomograms collected at various stages throughout each discharge. Results confirm that with decreasing C-rate (i.e., decreasing discharge current) a greater volume of zinc is reacted, with average mass utilisations of 17%, 76%, 81% and 87% for C/30, C/60, C/90 and C/150, respectively. Furthermore, quantification using X-ray CT datasets showed that there is a direct correlation between the volume of zinc remaining in the cell and the state of charge (SoC) of the cell, which deviated from linearity for the longer C-rates. Finally, a potential new mechanism for shape change is discussed, where a zinc particle is replaced with a pore of a similar volume. As well as improvements in statistical relevance gained from multiple repeats for each C-rate, the results presented here could be used in both modelling of battery performance, as well as consideration for future anode design concepts.

Time- and Space-Resolved In Situ X-ray Diffraction Study of Phase Transformations and Redistribution in Zn–Air Battery Anodes

“…4 This has recently been demonstrated in greater detail by Hack et al, who used operando XCMT to quantify the volume expansion of the anode during morphology changes linked to the migration of Zn particles away from the separator and air electrode, as well as the formation of pores in the anode during slower discharges. 5 Schmitt et al used multidimensional simulations to model the morphology changes in ZAB anodes, and confirmed the model's validity using XCMT. 6 XCMT was used as early as 2007 by Manke et al to investigate alkaline Zn−MnO 2 cells, with Manke et al tracking the dissolution of Zn, the nucleation of ZnO and the morphology changes in the anode.…”

“…All these previous results − point toward the importance of morphology changes in the anode as a degradation mechanism and the need for good methods to study these phenomena in real time. The main limitation with XCMT is the poor time resolution, making real-time tracking of morphology changes difficult without resorting to making measurements at OCV with the cell held at equilibrium, where the measurements are not representative of actual electrochemical cycling.…”

“…The majority of in situ characterization was initially done to understand anode degradation in commercial button-type primary cells, as a way of understanding similar processes happening in secondary cells. 4,5 XCMT has been used extensively for the last decade, with Arlt et al using operando X-ray tomography in 2014 as a way to accurately correlate the state of charge of a commercial primary ZAB with the amount of metallic Zn in the anode. 4 This has recently been demonstrated in greater detail by Hack et al, who used operando XCMT to quantify the volume expansion of the anode during morphology changes linked to the migration of Zn particles away from the separator and air electrode, as well as the formation of pores in the anode during slower discharges.…”

“…Chief among these has been X-ray computed micro tomography (XCMT). The majority of in situ characterization was initially done to understand anode degradation in commercial button-type primary cells, as a way of understanding similar processes happening in secondary cells. , …”

“…using operando X-ray tomography in 2014 as a way to accurately correlate the state of charge of a commercial primary ZAB with the amount of metallic Zn in the anode . This has recently been demonstrated in greater detail by Hack et al, who used operando XCMT to quantify the volume expansion of the anode during morphology changes linked to the migration of Zn particles away from the separator and air electrode, as well as the formation of pores in the anode during slower discharges . Schmitt et al used multidimensional simulations to model the morphology changes in ZAB anodes, and confirmed the model’s validity using XCMT …”