Directing Chemistry at Electrodes Using Precisely-Defined Electrochemical Interfaces

Molecular-level understanding of chemistry occurring at electrode-electrolyte interfaces (EEIs) during charge transfer processes as a function of electric field strength and concentration gradient is necessary to enable the rational design of more efficient electrochemical technologies for energy storage. The scientific challenge is to unravel convoluted interfacial interactions (i.e., adsorption, coordination, transport) and control the reaction pathways that lead to formation of desired products over unwanted byproducts. Ultimately the objective is to modulate the evolution of the solid-electrolyte interphase (SEI) in batteries for enhanced performance and durability. Soft landing of mass-selected ions, a versatile approach to surface modification, is ideally suited to preparation of well-defined interfaces with predetermined electrochemically active ions. The “atom-by-atom” precision of ion soft landing enables experiments that unravel the complexity associated with the multitude of interfacial processes occurring simultaneously at electrified interfaces. We present evidence for a substantial advancement in understanding such complex interfacial reactions on Mg battery electrodes employing user-defined layer-by-layer assembly of electrochemically active cations and solvated counter-ions with precisely-selected stoichiometry and concentration. As an example, ion soft landing combined with operando infrared reflection-absorption spectroscopy (IRRAS) and in-situ x-ray photoelectron spectroscopy (XPS) was used to characterize the decomposition of counter electrolyte anions and solvent molecules on Mg electrodes. Furthermore, we explored using an aliovalent doped metal (e.g., Al) on the Mg surface to control the decomposition and stabilize the SEI layer formation as a function of both concentration and potential. Our in-situ multimodal analysis enabled the capture of spectroscopic signatures from key chemical transformations of electrolyte anions and solvent molecules on reactive metal surfaces. These experimental results were correlated with the evolution of the SEI layer as predicted by theory to provide molecular-level insight into the reaction mechanism. In summary, our unique approach allowed us to acquire more delineated understanding of electrochemical transformations across length scales in operating EEIs and aids in the rational design of improved sustainable electrochemical energy technologies.