Directing the Complex Behavior of Metal Anodes Using Two Dimensional Materials

Abstract

This thesis presents new understanding on mechanisms and mitigation strategies to suppress metallic lithium and zinc dendritic electrodeposition utilizing two dimensional carbon-based nanomaterials and the mechanisms governing the improved performance. In-situ imaging techniques utilized for understanding the mechanism of undesirable dendritic behavior of Li is an important aspect in the battery research, which was discussed in this thesis, as well. Li metal anode is the “holy grail” of metal anodes because of its ultrahigh capacity, low electrochemical potential and low density. However, the dendritic growth of lithium (Li) has severely impeded the practical application of Li-metal batteries. Basic understandings about the intrinsic Li depositing/stripping behavior during cycling is the first and major step towards tackling the Li dendrite growth and its commercialization challenge. The highly reactive nature of Li and its moisture/air sensitivity has inclined researchers to investigate the Li dendrite behavior mainly under in-situ and operando imaging conditions to obtain a realistic understanding of the Li dendrites growth mechanism. The mechanistic understanding of Li dendrites growth by in-situ/operando imaging techniques is comprehensively reviewed in this thesis. The imaging techniques include optical imaging, electron microscopy, scanning probe microscopy, X-ray imaging, neutron microscopy and resonance-based imaging techniques. It is generally understood that the Li dendrite formation is rooted in non-uniform ion distribution on the electrode surface and mechanically unstable electrodeposition interface. Therefore, by considering the governing mechanisms of this issue, we propose the use of a light, cost effective and scalable coating, which allows for Li-ions transport, is mechanically stable to suppress Li dendrites, and provides uniform charge distribution on the electrode surface. A 3D conformal graphene oxide nanosheet (GOn) coating, confined into the woven structure of a glass fiber (GF) separator, is reported, for suppression of Li dendrites. Utilizing various microscopy (scanning electron microscopy, focused ion beam, and optical imaging) and modeling (Ab initio molecular dynamics and phase-field modeling, in collaboration with Prof. Mashayek’s group from UIC and Prof. Balbuena from Texas A&M) techniques we showed that our coating provides a synergistic effect in regulating the Li deposition behavior. Spray coated GOn on the GF separator can physically block the anisotropic growth of Li and meanwhile regulate the Li-ion transport and reduction on the electrode surface, resulting in an improved electrochemical performance and stability of the Li-metal anode. The recent urge for green, safe and high energy density storage systems has shed light on the importance of aqueous batteries. Replacing organic electrolytes with aqueous ones diminish the risk of fire hazards due to the exothermic reactions and thermal runaway in short-circuited batteries. Zn-metal batteries based on mild aqueous electrolytes has recently emerged into the spotlight due to its stability in aqueous media, environmental benignity, non-toxicity, multi-electron redox capacity, high abundance and low cost. However, rechargeable zinc (Zn) batteries suffer from poor cycling performance that can be attributed to dendrite growth and surface-originated side reactions. In this thesis we report an epitaxial and non-dendritic behavior of Zn nucleation and growth utilizing monolayer graphene (Gr) as the electrodeposition substrate. The Gr layer, due to its high lattice compatibility with Zn, provides low nucleation overpotential sites for Zn electrodeposition and locks the planar crystallographic orientation growth. Atomistic calculations (in collaboration with Prof. Mashayek’s group from UIC) indicate that Gr has strong affinity to Zn, leading to uniform distribution of Zinc adatoms all over the Gr surface. This synergistic compatibility between Gr and Zn promotes subsequent homogeneous and planar Zn deposits conformal with the current collector surface. In addition, we identified a controversy between Li and Zn nucleation and growth behavior in the presence of graphene. We realized that due to the different affinity of Li and Zn to graphene, the deposition mechanism is different. Li ions diffuse through the inherent defect sites of the Gr layer and deposit on the Cu substrate where free electrons are available for reduction of Li-ions, while Zn-ions prefer to deposit directly on the surface of graphene and form layered hexagons with low interfacial energy parallel to the electrode surface.