25Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes. (April 2018)

• Record Type: Journal Article
• Authors: Hu, Jian Zhi; Rajput, Nav Nidhi; Wan, Chuan; Shao, Yuyan; Deng, Xuchu; Jaegers, Nicholas R.; Hu, Mary; Chen, Yingwen; Shin, Yongwoo; Monk, Joshua; Chen, Zhong; Qin, Zhaohai; Mueller, Karl Todd; Liu, Jun; Persson, Kristin A.
• Is Part Of: Nano energy. Volume 46(2018)
• Page Start: 436
• Page End: 446
• Publication Date: 2018-04
• Subjects: Magnesium battery -- Mg(BH4)2 -- Mg(TFSI)2 -- Solvation structures -- 25Mg NMR -- Classical molecular dynamics calculations; Nanoscience -- Periodicals; Nanotechnology -- Periodicals; Nanostructured materials -- Periodicals; Power resources -- Technological innovations -- Periodicals; Nanoscience; Nanostructured materials; Nanotechnology; Power resources -- Technological innovations; Periodicals; 621.042
• DOI: 10.1016/j.nanoen.2018.01.051
• Languages: English
• ISSNs: 2211-2855

Abstract

Abstract: There is increasing evidence that the solvation structure of the active components in a liquid electrolyte solution strongly impacts the performance in electrochemical applications. In this work, the nanoscale solvation structures and dynamics of Mg(BH4 )2 and Mg(TFSI)2 dissolved in diglyme (DGM) at various concentrations and ratios of Mg(BH4 )2 /Mg(TFSI)2 were investigated using a combination of natural abundance 25 Mg NMR, quantum chemistry calculations of 25 Mg NMR chemical shifts, classical molecular dynamics (MD) calculations, and electrochemical performance tests. By mixing two competing Mg salts, we were able to reduce the strong covalent interactions between Mg 2+ and BH4 – anions. A small increase is observed in the coordination number of Mg-TFSI and a significant increase in the interaction of Mg 2+ ions with glymes. Through a combination of NMR, DFT and MD simulations, various stable species around 1 nm in size were detected in the mixed salt solution, which play key roles in the enhanced electrochemical performance of the mixed electrolyte. It is established that for the neat Mg(TFSI)2 in DGM electrolyte at dilute concentrations the TFSI - is fully dissociated from Mg 2+ . At higher concentrations, Mg 2+ and TFSI - are only partially dissociated as contact ion pairs are formed. In contrast, at 0.01 M Mg(BH4 )2 (saturated concentration) in DGM, the first solvation shell of a Mg 2+ ion contains two BH4 - anions and one DGM molecule, while the second solvation shell consists of five to six DGM molecules. An exchange mechanism between the solvation structures in the combined electrolyte containing both Mg(BH4 )2 and Mg(TFSI)2 in DGM was found to result in the observation of a single 25 Mg NMR peak. This exchange is responsible for an increase in uncoordinated anions, as well as improved stability and ionic conductivity as compared to single anion solution. Solvent molecule rearrangement and direct Mg-ion exchange between the basic solvation structures are hypothesized as likely reasons for the exchange. We elucidate that the solvent rearrangement is energetically much more favorable than direct Mg-ion hopping and is thus suggested as the dominant exchange mechanism. Graphical abstract: fx1 Highlights: Mixing 2 competing salts reduces the strong covalent interactions between Mg 2 + and BH4 - ions that hinder the application of Mg battery technology. High field NMR, DFT, MD, and electrochemical evaluations were used to identify specific nanoscale solvation structures of mixed salts in diglyme. Stable structures and an interconversion mechanism were verified with NMR and theory showing that solvent reordering prevails over Mg 2+ hopping. Solvent reordering permits increased uncoordinated anions, improved stability, and enhanced conductivity relative to a single anion solution.