Decoupling the Role of Anion and Solvent on Mg Electrodeposition Efficiency through Interfacial Speciation Determination

Abstract

Chemistries based on divalent working cations, such as Mg, are one class where significant effort is required to develop reductively and oxidatively stable electrolytes to support high efficiency metal deposition and cation insertion for practical energy storage systems beyond Li ion batteries. A growing body of literature exists describing bulk Mg(II) speciation over a range of electrolyte types providing insight into the origin of parasitic reactions that limit cell efficiency and overall performance. Bulk Mg(II) speciation can be argued to be an approximate descriptor of electrolyte stability at best, as the interfacial cation-anion-solvent coordination complex is what actually determines the consequences and efficiency of electron transfer. Limited knowledge is reported for interfacial Mg(II) coordination complexes, in part, due to limited available operando probes capable of discriminating between the coordination complexes at the electrode surface and within the bulk of the electrolyte. In this paper, we demonstrate the use of fluorescent and electron yield modes of operando x-ray absorption spectroscopy (XAS) to determine interfacial composition as a function of potential for select Mg(II)-anion-ether systems. We further conduct systematic variation in the coordination strength (e.g., donicity) and coordination diversity (e.g., mono- vs multi-dentate) of anions and ethers with respect to the divalent cation as a means of decoupling the role of anion and solvent in defining stability. Comparison of spectroscopic responses of Mg(II) with Zn(II) allow for additional modulation in anion/solvent coordination enthalpy further clarifying the roles of anion and solvent. The ability of XAS to discriminate early stage interphase formation at the anode providing chemical signatures of parasitic reaction will also be discussed. The methodology presented is equally applicable to electrolyte oxidative stability at the cathode, laying the foundation for design of electrolytes with suitable stability for practical, high energy density Mg ion batteries. This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.