Comparative Studies on [B(HFIP)4]-Based Electrolytes with Mono- and Divalent Cations

The development of electrolyte materials that are compatible with reductive metals is an urgent requirement for realizing high-energy-density rechargeable batteries utilizing metallic negative electrodes. Due to successive changes and regeneration of the morphology and fresh metals, respectively, upon repeated cycling of the metallic electrodes, the electrolytes should possess sufficient (electro)chemical stabilities against such electrodes. Weakly coordinating anion (WCA)-based electrolytes, which were first proposed for lithium-based battery applications in 1995, have attracted significant attention, especially in recent years, owing to their successful application in magnesium and calcium metal batteries. Inspired by these studies, WCA-based electrolytes have been reimported into lithium- and sodium-ion battery chemistry. In this study, we conducted comprehensive comparative studies on the representative WCA-based electrolytes incorporating tetrakis(hexafluoro-iso-propoxyl)borate ([B(HFIP)4]−) anions as a model system to understand the effect of valency of paired cation species on transport properties and electrochemical characteristics. As revealed by X-ray crystallography, the monovalent lithium and sodium salts were obtained as adducts, where the anion participated in cation coordination along with a single solvent molecule, whereas divalent magnesium, calcium, and zinc salts formed fully isolated solvates with the divalent cations being coordinated by solvents alone. Such valency-dependent differences in the dissociation states would affect the solution properties, as the divalent electrolytes exhibited greater conductivities than their monovalent counterparts, even though the same number of charged species was present in the respective solutions. The electrochemical metal deposition/dissolution studies combined with morphological and subsequent elemental analysis on the deposits suggested the specific favorable combination of magnesium cations and [B(HFIP)4]− anion in ethereal solutions. The modest surface reactivity of the deposited macrocrystalline magnesium, moderate reductive nature of the magnesium metal, and well-balanced mutual interactions among the components may have jointly contributed to such outstanding performance.