In Situ EC-TEM Studies of Hyper-Dendritic Zn Growth and Potential Cycling for Secondary Battery Applications

Abstract

Low cost, durable, and relatively safe electrode materials are highly desirable for secondary batteries, but the implementation of many materials, including zinc, is limited by morphological transformations that take place during potential cycling [1]. The use of zinc in the form of a nanoporous foam fabricated by electrodeposition, known as hyper-dendritic zinc (HD-Zn), can overcome some of the limitations of bulk zinc and act as a useful anode material [1,2]. The HD-Zn structure is composed of sub-hundred nanometer-size Zn crystallites on secondary dendrites in a three dimensional network of highly oriented crystals with high surface area. This structure shows improved capacity retention and more rapid kinetics in alkaline media compared to conventional zinc anodes [1,2]. At present, the growth mechanism and morphological changes taking place in HD-Zn during cycling, which presumably form the basis for its improved performance, are not well understood. Recent advances in in situ electrochemical transmission electron microscopy (EC-TEM), with its unique ability to provide simultaneous temporally and spatially resolved information as well as electrochemical parameters, enable exploration of the underlying physics of the electrochemical reactions that occur at the surface of this structure during cycling [3,4]. In this work, we therefore apply in situ EC-TEM to investigate the electrodeposition process that forms and takes place on HD-Zn. By employing a liquid cell with three electrodes and the capability for liquid flow, we can introduce electrolytes with varying composition of zinc ions and pH, and apply different current and voltage parameters to investigate their effects on the deposited morphology. We track the growth and the potential cycling of HD-Zn, and compare with the deposition of Zn in classical dendrite form onto surfaces. We discuss the effects of additives to the electrolyte, which can change the onset of dendrite formation during Zn deposition. We also compare these flow cell results to static liquid cell experiments in which we have recorded HD-Zn morphology after galvanostatic deposition under the optical microscope, and we will discuss the benefits and pitfalls of flow capabilities for liquid cell electron microscopy for the study of processes during battery operation.