Thermal and Electrochemical Stability of Tetraglyme–Magnesium Bis(trifluoromethanesulfonyl)amide Complex: Electric Field Effect of Divalent Cation on Solvate Stability

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• 作者：Shoshi Terada ; Toshihiko Mandai ; Soma Suzuki ; Seiji Tsuzuki ; Katsuya Watanabe ; Yutaro Kamei ; Kazuhide Ueno ; Kaoru Dokko ; Masayoshi Watanabe
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：January 28, 2016
• 年：2016
• 卷：120
• 期：3
• 页码：1353-1365
• 全文大小：609K
• ISSN：1932-7455

文摘

Phase behavior of binary mixtures of tetraglyme (G4) and Mg[TFSA]₂ (TFSA: bis(trifluoromethanesulfonyl)amide) was investigated. In a 1:1 molar ratio, G4 and Mg[TFSA]₂ formed a stable complex with a melting point of 137 °C. X-ray crystallography of a single crystal of the complex grown from a G4-Mg[TFSA]₂ binary mixture revealed that the G4 molecule wraps around Mg²⁺ to form a complex [Mg(G4)]²⁺ cation, and the two [TFSA]^? anions also participate in the Mg²⁺ coordination in the crystal. The thermal stability of [Mg(G4)][TFSA]₂ was examined by thermogravimetry, and it was found that the complex is stable up to 250 °C. Above 250 °C, desolvation of the Mg²⁺ ion takes place and G4 evaporates. On the other hand, the weight loss starts at around 140 °C in solutions containing excess G4 (n > 1 in Mg[TFSA]₂:G4 = 1:n) due to the evaporation of free (uncoordinated) G4. The suppression of G4 volatility in the [Mg(G4)][TFSA]₂ complex is attributed to strong electrostatic and induction interactions between divalent Mg²⁺ and G4. In addition, complexation of G4 with Mg²⁺ is effective in enhancing the oxidative stability of G4. Linear sweep voltammetry revealed that the oxidative decomposition of [Mg(G4)][TFSA]₂ occurs at electrode potentials >5 V vs Li/Li⁺, while the oxidation of uncoordinated G4 occurs at around 4.0 V. This oxidative stability enhancement occurs because the HOMO energy level of G4 is reduced by complexation with Mg²⁺, which is supported by the ab initio calculations.