(Electrodeposition Division Early Career Investigator Award Address) Regulating Electrochemical Deposition of Metals at Rechargeable Battery Electrodes

Abstract

Scalable approaches for precisely manipulating the growth of crystals are of broad-based science and technological interest, and among the most extensively studied fields of research. New research interests have re-emerged in a subgroup of these phenomena—electrochemical deposition of metals in battery electrodes, such as zinc, an anode material that underpins several opportunities for developing cost-competitive technologies for large-scale electrical energy storage. Quantitative assessment shows that a high efficiency/reversibility of the electrochemical plating/stripping process (ie Coulombic efficiency > 99%), is necessary for the successful development practical rechargeable electrochemical energy storage systems that utilize a Zn metal anode. In practice, however, such efficiency values are usually below 80~90% in conventional metal plating/stripping systems.I will show that the inefficiency/irreversibility observed in metal plating/stripping is predominantly attributable to the metals’ susceptibility to parasitic chemical reactions and propensity for producing so-called “orphaned” fragments, which have lost electrical access to the electrode substrate termed the current collector. Both of these mechanisms are found to share the same origin— the non-uniform, rough electrodeposition morphology of metals. This finding motivates the overarching science question in this thesis: can the electrochemical growth of metals be strictly regulated in electrochemical cells? Despite the large volume of recent literature devoted to answering this question, the fundamental crystalline nature of the metal electrodeposits has been largely overlooked. Removing this artifice allows us to borrow knowledge from a rich body of literature on regulating the deposition process of crystalline materials to pursue fresh answers and approaches for achieving highly reversible electrodeposition of metals.Such approaches enable development of multiple strategies that are unprecedentedly effective in regulating metal deposition morphology and in achieving high-enough plating/stripping efficiency to meet the reversibility requirements as set forth. Specifically, I report that heteroepitaxy can be used to direct the shape and orientation of the metal crystallites formed on a rationally designed substrate. This strategy leverages crystals’ inclination for mimicking the atomic arrangement of the substrate surface, especially when the lattice misfit is below a critical value. Secondly, in I consider the fundamental limitation of heteroepitaxy—i.e. the correlation length is finite, meaning that the initial regulation of shape and crystallography of a metal deposit are only preserved for a finite electrodeposits thickness/electrode capacity. For as fundamental reasons, it is shown that this limitation can be overcome by imposing a convective flow normal to the electrode surface. The artificial convective flow serves as an additional solution-state mass transport mechanism and has a heretofore unexplored effect in suppressing chemotaxis-like reorientation of the metal crystallites at high areal capacities. I then demonstrate that solid-state metallurgy strategies for eliminating the built-in crystallographic heterogeneity of commercial foil-type metal electrodes, opens a fruitful path to powerful homoepitaxy regulation processes in which the metal itself is able to reversibly regulate its own electrodeposition morphology. In particular, subjecting a metal electrode to severe plastic deformation in the electrode foil-forming process is used to fabricate strongly-textured metal electrodes where such crystallographic heterogeneities are eliminated.In addition to these physical aspects, it is found that the chemical interaction at the heterointerface between the deposited metal crystallites and the substrate—which is actually a crystal of a different chemistry—also shows nontrivial influences on the electrodeposition process. I will assess consider these effects from two types of perspectives — oxygen- mediated bonding between the metal electrodeposit and substrate and alloying, respectively. The results suggest that the chemical affinity at the heterointerface is necessary to promote a uniform nucleation/growth landscape and to suppress the formation of “orphaned” metal deposits.