Application of Glyme Ether Additives to Ionic Liquid Based Electrolytes for Use With Magnesium-Ion Secondary Battery Systems

Abstract

Facing technological forcings from anthropogenic climate emissions mitigation objectives, the onus for developing robust, commercial scale advanced energy storage technologies is paramount. The current leading secondary battery technologies, lithium-ion based systems, suffer safety concerns from dendrite formation from the battery anode surface across the separator membrane to the cell cathode, causing circuit shorting and thermal runaway. Magnesium-ion secondary systems do not form dendrites and so offer potential superior safety performance to lithium-ion batteries, but cathode and electrolyte for these cells is underdeveloped and in need of further research. The following survey investigated development of new electrolytes for magnesium-ion secondary systems. The survey builds on the successfully Shanghai University Zhao group electrolyte, which uses an ionic liquid (ILs) solvent (BMImBF\(_{4}\)) mixed with a magnesium ion source (Mg(Tf)\(_{2}\)). The survey seeks to use a series of ‘organic ether molecules of the ‘glme’ family (glyme, diglyme, and tetraglyme) as additives to improve electrolyte safety and conductivity, building on past demonstration of PEO with ILs as electrolyte for magnesium battery applications. The experimental electrolytes were not successfully demonstrated as viable in magnesium-ion secondary systems. For cyclic voltammetry conducted within the IL electrochemical stability window the electrolyte displayed no appreciable redox chemistry, passing negligible currents (on the order to 10\(^{-6}\) to 10\(^{-5}\) A), with no noticeable oxidation or reduction peaks. Scanning Electron Microscopy imaging indicated that during cycling some species featuring carbon and fluorine had passivated on the magnesium working electrode surface, impeding electrochemistry; FTIR analysis suggests that a chemical reaction between the BMIm\(^{+}\) and BF\(_{4}\)\(^{-}\) ions in the Zhao electrolyte that inhibits the formation of surface passive films is somehow prevented by the glyme ether molecules in the experimental solution. The passivated species is not immediately attributable, as FTIR and NMR analysis of the various electrolyte materials before voltammetry indicated that no chemical reaction occurred between various constituent molecules except for the coordination of the glyme ethers about magnesium cations. Fluorine was only present in the electrolyte in BF\(_{4}\)\(^{-}\) and SO\(_{3}\)CF\(_{3}\)\(^{-}\), neither of which are thought to be directly passivated on the basis of the absence of elemental boron and sulfur in the high-fluorine surface regions, respectively. The absence of any fluorinated glyme derivatives present in the electrolyte solution before cycling, as identified via FTIR and NMR, as well as the absence of oxygen in the high-fluorine magnesium electrode surface regions, preclude the idea that some substituted glyme molecule is the source of passivation issues. Our best explanation for the passivation is some surface phenomenon between elemental magnesium and the electrolyte solution before electrochemical cycling (since no redox peaks were found during any of the early cycle sweeps), but without further experimentation, the nature of this reaction is unknown.