Alkoxide-Based Electrolytes for Magnesium-Ion Batteries

Abstract

Battery chemistries based on multivalent ions have many theoretical advantages to current state-of-the-art lithium-ion batteries. Namely, batteries that utilize metallic magnesium anodes possess theoretical energy densities an order of magnitude larger than lithiated graphite due to the divalent charge of magnesium ions. Additionally, magnesium-based systems are inherently safer than lithium metal or lithium-ion batteries because of the tendency of magnesium to electroplate uniformly, rather than dendritically like lithium. However, the development of practical Mg batteries has proved challenging due the to the lack of stable electrolytes capable of supporting reversible Mg electrodeposition with low overpotentials that are also highly resistant to oxidation and have high solution conductivities.

This thesis explores numerous approaches to developing practical Mg-ion electrolytes. The first methodology uses magnesium dialkoxides and diaryloxides (Mg(OR)2) of increasing steric bulk in combination with aluminum chloride (AlCl3) in THF to synthesize electroactive Mg solutions. It was found that increasing the steric bulk has positive effects on electrochemical figures of merit. The second approach adopts a transalcoholysis pathway from magnesium methoxide to synthesize the fluorinated magnesium dialkoxide Mg(HFIP)2 (HFIP = hexafluoroisopropanol). Magnesium electrolytes of the type Mg(HFIP)2:AlCl3 in dimethoxyethane (DME) demonstrate improvements in most electrochemical figures of merit compared to the non-fluorinated dialkoxides and other state-of-the-art Mg electrolytes.

Using these approaches as a foundation, magnesium electrolytes based on weakly-coordinating fluorinated alkoxyaluminate anions of the type [Al(ORF)4]- are synthesized by reactions between fluorinated magnesium and aluminum alkoxides. Solutions of Mg[Al(HFIP)4]2 in DME possess high thermodynamic oxidative stabilities, high conductivities (>6.5 mS cm-1), and Coulombic efficiencies of up to 99.3%. This composition represents one of the best performing of the few chloride-free Mg-electrolyte compositions reported to date. Raman spectroscopy and NMR data show mobile Mg2+ cations are effectively encaged by DME molecules in the Mg[Al(HFIP)4]2 solution, resulting in the favorable electrochemical properties. Surface analysis shows that these solutions prevent rapid corrosion of aluminum current collectors via the formation of passivating AlF3 layers, thus enabling the study of high voltage cathode materials in standard battery testing cells. The excellent electrochemical properties make these solutions promising candidates for incorporation in practical Mg-ion batteries.