Chemical transformations of electrolyte constituents, such as solvent and solvated electroactive ions, at the Mg-metal electrode determine the evolution of the solid-electrolyte interphase (SEI) and define the performance of Mg batteries. The ability to determine the fundamental chemistry occurring at electrode-electrolyte interfaces (EEIs) during charge transfer processes as a function of both electric field strength and concentration gradient is necessary to enable the rational design of more efficient and robust electrochemical technologies for energy storage. The scientific challenge is to predict and delineate myriad of interfacial interactions (i.e., adsorption, coordination, reactivity and transport) and control reaction pathways that lead to formation of desired products over unwanted byproducts that can result in decreasing battery performance. 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. Herein, we achieved a substantial advancement in preditive understanding of such complex interfacial reactions on Mg electrodes, employing precisely-defined layer-by-layer assembly of electrochemically active cations and solvated counter-ions with selected stoichiometry and concentration. As an example, ion soft landing combined with operando infrared reflection-absorption spectroscopy (IRRAS), Raman spectroscopy, and in-situ x-ray photoelectron spectroscopy (XPS) was used to characterize the decomposition of softlanded bis(trifluoromethanesulfonyl) imide (TFSI) ions and solvent molecules on Mg anode and MgV₂O₄ cathode material. Our in-situ multimodal analysis enabled the capture of spectroscopic signatures from key chemical transformations of electrolyte anions and solvent molecules on reactive Mg and MgV₂O₄ surface. These experimental results were correlated with the reaction pathways predicted and validated futher by theory to provide molecular-level insight into the reaction mechanisms. In summary, our joint experimental and theoretical approach allowed us to acquire a more delineated understanding of electrochemical transformations occurring across length scales in operating EEIs and will aid the rational design of improved and sustainable electrochemical energy technologies.

Jason Howard

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