Electrochemical Stability of Closo-Carborane Anions and Their Impact on Cathode Interfacial Properties

Abstract

Electrochemically stable, charge delocalized anions are essential electrolyte constituents for rechargeable, high voltage magnesium batteries. Within this class of anions, magnesium closo-carborane salts were demonstrated to support reversible magnesium electrodeposition/dissolution in tetraethylene glycol dimethyl ether (tetraglyme, G4) highlighting their cathodic stability.1,2 The corresponding anodic stability of these anions remains undetermined due to the lower anodic stability of the glyme solvents preferred for increased salt solubility, ionic conductivity, and stability against magnesium metal. In this paper, we explore the anodic stability of the carba-closo-dodecaborate anion (CB11H12 -) and select functionalized derivatives thereof relative to the more typical battery electrolyte anions bis(trifluoromethylsulfonyl)imide (TFSI) and tetrafluoroborate (BF4). In solvents of higher anodic stability than the glymes, including 3-methylsulfolane and 1,1,3,3-hexafluoroisopropanol, we demonstrate an oxidation potential of 4.5 V vs. Mg/Mg(II) for the base CB11H12 - anion, which is surprisingly lower than that measured for TFSI or BF4. Computational screening of the adiabatic ionization potential and electron affinity of RCB11H11 - (where R is a functional group that replaces the H-C proton) indicates that substitution of the proton at the carbon vertex with an electron withdrawing group can yield a several hundred millivolt increase in oxidative stability without decreasing the cathodic stability. Synthesis and electrochemical evaluation of the fluoro-carba-closo-dodecaborate validates this prediction and offers a rational design strategy for improved electrolyte stability. Oxidation of either the parent or substituted carba-closo-dodecaborate anion is irreversible, resulting in a neutral radical that incorporates within a surface film. The surface film formed inhibits further electron transfer leading to a varying degree of passivation toward further electrolyte decomposition depending on the solvent employed. The composition and structure of this metastable interface and its Mg cation transport properties will be discussed. O. Tutusaus, R. Mohtadi, T.S. Arthur, F. Mizuno, E.G. Nelson, Y.V. Sevryugina, “An Efficient Halogen-Free Electrolyte for Use in Rechargeable Magnesium Batteries,” Angew. Chem. Int. Ed. 2015, 54, 7900.S.G. McArthur, R. Jay, L. Geng, J. Guo, V. Lavallo, “Below the 12-vertex: 10-vertex carborane anions as non-corrosive, halide free, electrolytes for rechargeable Mg batteries,” Chem. Comm. 2017, 53, 4453.