Our group has been involved in studies of the complexation and speciation of Mg²⁺ in ionic liquids as a means to understand the influence of coordination environment on Mg anode electrochemical behavior.^1,2 As part of those efforts, we showed the beneficial effects of borohydride and glyme electrolytes for improved magnesium anode behavior. In this presentation we will focus on the complexation of Mg²⁺ by glyme components in electrolytes and chemical reactivity of borohydride anion on the reversibility of Mg electrodeposition/dissolution. We show that the lessons learned from such studies of Mg²⁺ complexation allow the design of new chelating ionic liquids that show markedly improved Mg²⁺/Mg⁰ electrochemical behavior compared to the parent TFSI-based ionic liquids. Specifically, we describe reasonable coulombic efficiency and current densities that are achievable through judicious choice of the complexation environment and other chemical features of the electrolyte system.

We also discuss more generally the factors that act to suppress facile Mg electrodeposition/dissolution. We provide experimental verification of some of the means by which these various factors influence both chemical and electrochemical reversibility of Mg anode behavior.

Acknowledgment We acknowledge support from the Army Research Office grant number W911NF-11-1-0432.

References:

1. “Determination of Mg²⁺ Speciation in a TFSI^--Based Ionic Liquid With and Without Chelating Ethers Using Raman Spectroscopy”, Watkins, T., Buttry, D. A.; J. Phys. Chem. B, 2015, 119(23), 7003-7014. (DOI: 10.1021/acs.jpcb.5b00339)
2. “Designer Ionic Liquids for Reversible Electrochemical Deposition/Dissolution of Magnesium”, Watkins, T.; Kumar, A.; Buttry, D.A.; J. Am. Chem. Soc., 2016, 138(2), 641-650 (DOI: 10.1021/jacs.5b11031)