Density and Volume Fraction Distribution of ZnO Discharge Products in Cylindrical Alkaline Battery Anodes after Intermittent Use

Abstract

The discharge of an alkaline Zn anode involves the conversion of active material to aqueous zincate ions and solid ZnO. With the mobility of zincate in the electrolyte, complex ZnO morphologies and radial distributions have been observed to occur when using different discharge protocols. An apparent continuum of ZnO densities can be seen in an intermittently discharged anode, and can be appropriately binned into two forms, namely type I and type II, which differ in density and passivating character.1 Not only are the relative amounts of these two types impacted by the discharge protocol, but their morphologies and volume distributions are also affected. This has implications for both primary cells, where undesirable ZnO formation near the separator can lead to cell failure, and secondary cells, where redistribution of active material inhibits cyclability. Previously reported models of Zn anodes poorly describe the behavior seen when using an intermittent discharge protocol meant to simulate typical real-world applications, and therefore require further development.2 Additionally, prior literature reported that ZnO forms passivating shells around undischarged Zn cores, restricting hydroxide transport, and thus resulting in reduced cell performance.3 In this work, we characterize the ZnO morphology and radial distribution for an array of discharge conditions that simulate both intermittent and continuous use. We then use these results to identify the impact of discharge protocol on ZnO formation within alkaline Zn anodes.This work paired high resolution synchrotron tomography with a novel segmentation algorithm to observe phase morphology and quantify radial distributions in situ. The results obtained when using a continuous discharge protocol matched that of previous literature.3 However, the results when using an intermittent discharge protocol were vastly different than that seen with a continuous protocol. The discharge products in a cell simulating intermittent use formed two distinct phases of varying densities, both of which preferentially formed in the inner portion of the anode. This is radically different than ZnO formation in continuously discharged cells, wherein an apparent single ZnO phase is concentrated at the outer portions of the anode near the separator, creating a dense crust around the remaining Zn. This outer crust can be detrimental to cell performance due to poor hydroxide transport from the cathode to the remaining undischarged Zn, suggesting superior cell performance with intermittent use. Acknowledgements This research was supported by funding from Energizer Holdings, Inc. This research also used resources of the Advanced Photon Source beamline 6-BM, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.