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

A unique thermodynamic behavior was observed for solutions of magnesium bis(trifluoromethanesulfonyl) imide in 1,2-dimethoxyethane (DME), highly promising solutions for rechargeable Mg batteries: between 287 and 373 K, the solution exists as two immiscible phases, each with a different salt concentration, volume, and density. These characteristics depend strongly on temperature. To study this dependence, a phase diagram was constructed. In addition, partial phase volumes and densities were measured as a function of temperature. We observed that the temperature-molar fraction phase diagram exhibits closed-loop behavior, which is circumscribed by 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 concentrated and dense lower phase and a dilute upper phase. As the temperature increases to 333 K, the lower phase becomes more concentrated and denser, and its partial volume decreases. The reverse trends are observed for the upper phase. From 333 K and above, the trends reverse: the upper phase becomes more concentrated but its partial volume continues to grow at the expense of the lower phase, while the lower phase concentration decreases. At 373 K, the two phases merge into a single homogeneous phase.