Understanding the Unique Thermodynamic Behavior of MgTFSI2/DME Solutions. Part 1: Phase Diagram, Partial Volumes, and Densities

A unique thermodynamic behavior was observed for solutionsof magnesiumbis(trifluoromethanesulfonyl) imide in 1,2-dimethoxyethane (DME),highly promising solutions for rechargeable Mg batteries: between287 and 373 K, the solution exists as two immiscible phases, eachwith a different salt concentration, volume, and density. These characteristicsdepend strongly on temperature. To study this dependence, a phasediagram was constructed. In addition, partial phase volumes and densitieswere measured as a function of temperature. We observed that the temperature-molarfraction phase diagram exhibits closed-loop behavior, which is circumscribedby 287 and 373 K, with 333 K as an inversion locus. Below 287 K,the solution exists as a single homogeneous phase. At 287 K and above,the solution separates into two immiscible phases: a concentratedand dense lower phase and a dilute upper phase. As the temperatureincreases to 333 K, the lower phase becomes more concentrated anddenser, and its partial volume decreases. The reverse trends are observedfor the upper phase. From 333 K and above, the trends reverse: theupper phase becomes more concentrated but its partial volume continuesto grow at the expense of the lower phase, while the lower phase concentrationdecreases. At 373 K, the two phases merge into a single homogeneousphase.

Automated summary

A study on the behavior of magnesium bis(trifluoromethanesulfonyl) imide solution in 1,2-dimethoxyethane revealed that at temperatures between 287 and 373 K, the solution exists as two immiscible phases with different properties. A temperature-molar fraction phase diagram was constructed, showing a closed-loop behavior. At 287 K and above, the solution separates into a concentrated lower phase and a dilute upper phase. As the temperature increases to 333 K, the lower phase becomes more concentrated and denser, while the upper phase shows opposite trends. From 333 K and above, the upper phase becomes more concentrated but its partial volume continues to grow, while the lower phase concentration decreases. At 373 K, the two phases merge into a single homogeneous phase.