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

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 Mg 2+ to form a complex [Mg(G4)] 2+ cation, and the two [TFSA] − anions also participate in the Mg 2+ 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 Mg 2+ 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 Mg 2+ and G4. In addition, complexation of G4 with Mg 2+ 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 Mg 2+, which is supported by the ab initio calculations.