Fast and efficient magnesium electrodeposition/dissolution is needed for practical energy storage and is dependent on the composition and structure of the Mg:electrolyte interface, including the nature of the solvated cation. Mg-chloro complex forming electrolytes are the demonstrated approach to achieving high efficiency at the expense of low rate and limited electrochemical window. Knowledge of how anion and solvent coordinate with the Mg(II) cation and facilitate the deposition process is necessary to develop the next generation of electrolytes. Decoupling roles of anion and solvent requires a framework with which to conduct systematic changes in anion and solvent to explore effects. We provide a demonstration of displacement of bis(trfiluoromethylsulfonyl)imide anion, which is electrochemically unstable as a contact ion pair with Mg(II), by chloride across a spectrum of monodentate to multidentate (poly)ethers. Displacement leads to Mg-chloro complexes that restore the efficiency and crystallography but reduce the kinetics, highlighted the trade-off working with electrolytes that are anion-dominated in their activity. The equally important role of the solvent must be explored directly in order to determine how solvent coordination of the Mg(II) cation limits activity and structure. New developments in synthesizing more weakly coordinating, stable anions, including carboranes, fluoroalkoxyborates, and fluoroalkoxyaluminates offer an opportunity to explore solvent effects across this same ether series, allowing separation of solvent and anion effects. A combination of NMR, Raman, and ab initio molecular dynamic simulations are used to determine Mg(II) coordination with ethers in the presence of these anions. The crystallography and structure of the evolving cycled interface is characterized using analytical transmission electron microscopy, while electroanalytical methods provide a clearer picture of solvent impact on efficiency and kinetics. Electrolyte design principles emerge from these studies to enable fast and efficient metal deposition and dissolution.

Decoupling electrolyte anion and solvent effects in magnesium electrodeposition for rechargeable batteries

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.