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Title: 25 Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes

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 25Mg NMR, quantum chemistry calculations of 25Mg 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 Mg2+ and BH4– anions. A small increase is observed in the coordination number of Mg-TFSI and a significant increase in the interaction of Mg2+ 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 Mg2+. At higher concentrations, Mg2+ and TFSI- are only partially dissociated as contact ion pairs are formed. In contrast, at 0.01 M Mg(BH4)2 (saturated concentration)more » in DGM, the first solvation shell of a Mg2+ 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 25Mg 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.« less

Authors:
; ; ; ; ; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1455272
Report Number(s):
PNNL-SA-130126
Journal ID: ISSN 2211-2855; KC0208010
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Nano Energy; Journal Volume: 46; Journal Issue: C
Country of Publication:
United States
Language:
English
Subject:
Magnesium battery; electrolytes; Mg(BH4)2; Mg(TFSI)2; solvation structures; dynamics; 25Mg NMR; classical molecular molecular dynamics calculations

Citation Formats

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, and Persson, Kristin A. 25 Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes. United States: N. p., 2018. Web. doi:10.1016/j.nanoen.2018.01.051.
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. 25 Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes. United States. doi:10.1016/j.nanoen.2018.01.051.
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, and Persson, Kristin A. Sun . "25 Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes". United States. doi:10.1016/j.nanoen.2018.01.051.
@article{osti_1455272,
title = {25 Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes},
author = {Hu, Jian Zhi and Rajput, Nav Nidhi and Wan, Chuan and Shao, Yuyan and Deng, Xuchu and Jaegers, Nicholas R. and Hu, Mary and Chen, Yingwen and Shin, Yongwoo and Monk, Joshua and Chen, Zhong and Qin, Zhaohai and Mueller, Karl Todd and Liu, Jun and Persson, Kristin A.},
abstractNote = {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 25Mg NMR, quantum chemistry calculations of 25Mg 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 Mg2+ and BH4– anions. A small increase is observed in the coordination number of Mg-TFSI and a significant increase in the interaction of Mg2+ 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 Mg2+. At higher concentrations, Mg2+ 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 Mg2+ 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 25Mg 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.},
doi = {10.1016/j.nanoen.2018.01.051},
journal = {Nano Energy},
number = C,
volume = 46,
place = {United States},
year = {Sun Apr 01 00:00:00 EDT 2018},
month = {Sun Apr 01 00:00:00 EDT 2018}
}