Morphology and Spatial Distribution of ZnO Formed in Discharged Alkaline Zn/MnO[sub 2] AA Cells

Abstract

A novel experimental technique was developed to extract the KOH solution from the porous zinc electrode in an inert environment. This technique permits the ZnO/electrolyte interface, the ZnO/Zn particle interface, and the spatial distribution of ZnO in the porous anode in commercial alkaline Zn/MnO 2 AA batteries to be characterized for the first time by optical and scanning electron microscopy. Both type I and type II ZnO, previously observed in oxidation of planar zinc electrodes, were found in each completely discharged zinc particle in the porous electrode. The morphology, size, and location of these two ZnO forms strongly suggested that they were produced by a solution-precipitation route. The drain rate had a significant impact on the spatial distribution of ZnO in the anode. The segregation of ZnO near the separator became dominant at drain rates higher than 500 mA. The mechanisms by which the morphology and segregation of ZnO within the porous electrode could result in the reduced battery runtime are discussed.Growth and advances in the portable electronic industry require commercial alkaline Zn/MnO 2 AA batteries to function under high power and high drain conditions. Unfortunately, AA batteries operate inefficiently at high drain rates. 1 For example, the runtime of a AA battery at a 1000 mA continuous drain rate is typically less than 20% of that discharged at 10 mA. This inefficiency is a result of severe polarization across the Zn ͑anode͒ and MnO 2 ͑cathode͒ electrodes at high drain rates. The Zn/MnO 2 AA cell has a bobbin-type construction, where both electrodes are porous, as shown in Fig. 1. The anode consists of zinc powder that is usually suspended in a gelled electrolyte of concentrated KOH in water, and the cathode includes a physical mixture of electrolytic manganese dioxide and graphite, respectively. 1 It is accepted that the oxidation of zinc proceeds by a dissolution-precipitation process, while MnO 2 is reduced by a solid-state intercalation of H ϩ into the MnO 2 lattice, as followsRecent efforts in analyzing the AA battery discharge performance by mathematical modeling have suggested that the spatial distribution of the discharge product, ZnO, within the anode can have a significant effect on the electrode polarization and the battery performance under high drain rates. 2 In addition, the morphology and spatial distribution of ZnO may have a significant impact on the rechargeability of secondary alkaline systems that utilize a metallic zinc anode. Although the morphology and reaction mechanism of ZnO formation on planar zinc electrodes in concentrated KOH electrolytes is well studied, 3-12 limited information is available on porous zinc electrodes. Our study aims to understand the nucleation and growth process of ZnO and to investigate the effect of drain rates on the spatial distribution of ZnO in discharged porous zinc anodes.The morphology and formation mechanism of ZnO on the surface of polished zinc-plate electrodes in concentrated KOH electrolytes was first studied by Powers et al...

Coupling Phenomena between Zinc Surface Morphological Variations and Ionic Mass Transfer Rate in Alkaline Solution

“…10 Morphological variations of ZnO precipitated in a solution-phase or gas-phase have further been reported in various fields of applications. 13,[19][20][21] …”

A Review of Model-Based Design Tools for Metal-Air Batteries

“…The porous BAE substrate contains carbon fibers and binder with mixed hydrophilic and hydrophobic properties to promote the formation of the so-called three phase boundary, while hindering the electrolyte from flooding out. The Zn electrode is often a paste consisting of zinc metal powder, electrolyte, and binder [54]. The capacity of the cell is determined by the Zn electrode, which is designed in such a way as to include as much active material as possible while minimizing the effects of shape-change and electrolyte concentration gradients.…”

“…When this ZnO barrier becomes too large, the electrochemical reaction can no longer proceed and the electrode is passivated [62,79]. ZnO passivation can take on two forms [54,80]. Type I ZnO has a porous white morphology.…”

Investigation of Energy‐Relevant Materials with Synchrotron X‐Rays and Neutrons

“…[135][136][137][138][139] Alkaline batteries consist of a Zn powder anode and MnO 2 at the cathode. [140,141] During discharge, the anode zinc is oxidized and the MnO 2 is reduced by solid-state intercalation of H into the MnO 2 lattice.…”

“…[135,142] It is very difficult to open a cell and to investigate a section through the battery by optical or electron microscopy, or other techniques. The material is unstable in air.…”