Ion Solvation-Driven Liquid-Liquid Phase Separation in Divalent Electrolytes with Miscible Organic Solvents

Liquid-liquid phase separation(LLPS) is a commonphenomenon,but LLPS of electrolytes prepared in miscible organic solvents israrely documented. In this study, we report four new cases of LLPSthat occur in MgTFSI2 or ZnTFSI2 electrolytesin mixed organic solvents, and we use multimodal NMR methods to investigatethe speciation differences between the upper and lower phases of theseelectrolytes. The conditions for the formation of this LLPS sharefour common features: cations with high charge density; anions withlow charge density; a strongly coordinating solvent (indicated byits Gutmann donor number) with high compressibility; and a relativelyweak coordinating solvent with low relative permittivity and highmobility. With these conditions, the cations tend to draw the stronglycoordinating solvents together to form a densely packed, energeticallyfavorable ion solvation region, while the cosolvents with low relativepermittivity tend to form an ion-depleted dilute phase. Therefore,the driving force for LLPS with miscible solvents is the preferredinteraction between cations and the strongly coordinating solventsthat overcomes entropy loss due to phase separation. Furthermore,contact ion pairs and large ion aggregates are clearly evidenced inthe dilute upper layer due to the incomplete screening of electrostaticinteractions by the weaker cosolvent. This LLPS driven by cation solvationmay be more common in multivalent electrolytes and is a design considerationthat should not be overlooked in this context.