Understanding Magnesium-ion Electrochemistry through Modification of Lewis Acid/Base Pairs.

Abstract

Lithium-ion batteries have long attracted attention for cultivation of portable energy storage devices. For larger portable applications such as hybrid electric or plug in vehicles the current Li-ion technology does not currently provide the power and energy required to meet the demands of these applications. Of the proposed post-Li ion technology, magnesium shows a good balance between redox potential, volumetric capacity, safety and abundance. This thesis focuses on electrolytes exhibiting high Mg-ion conductivity and analysis of how Lewis acid/ base pairings contribute to the physical and electrochemical properties of these electrolyte systems. Early electrolytes capable of reversible Mg deposition and stripping contained Grignard components. Later, it was discovered the incorporation of a Lewis acid, commonly AlCl3, improved Mg deposition and stripping and oxidative stability. The presence of the Grignard in these electrolyte systems created safety concerns due to their flammability. Incorporating phenols in the place of the Grignard component has been shown to decrease unwanted reactivity towards air and moisture. To understand how para- substituted phenols contribute to the performance of the electrolyte, a series of non-Grignard electrolytes were synthesized. The increasing electron withdrawing ability of the para-substituent resulted in a shift of the anodic stability of the electrolyte by ~600 mV with para-CF3 substituted phenolate exhibiting the highest stability of 2.9 V vs Mg. This stability is close to the oxidative stability of the common Grignard based electrolytes. The speciation of the electrolyte was examined using a number of spectroscopic techniques in an effort to infer relationships between the composition of the electrolyte and its electrochemical performance. The commercialization of Mg battery technology is hindered by the presence of parasitic current on non-noble metal current collector. This electrochemical corrosion of the current collector by the high concentration of chloride ions. Removing the chloride from the Lewis acid resulted in an electrolyte with decreased detrimental corrosion on stainless steel. This electrolyte also exhibits a surprisingly wide window of electrochemical stability, with electrolyte oxidation not occurring until ~ 5 V vs Mg2+/0. This stability is attributed to the formation of an electrochemical quasi-passivation layer primarily composed of phenyl groups.