Liquid Phase Boundaries, Dielectric Constant, and Viscosity of PC-DEC and PC-EC Binary Carbonates

Among the most important properties of a binary carbonate for its application in nonaqueous electrolytes for lithium batteries are its liquid phase boundaries, dielectric constant ɛ, and viscosity η. These properties were therefore experimentally evaluated in this work for two typical binary carbonates: propylene carbonate-diethyl carbonate (PC-DEC) and propylene carbonate-ethylene carbonate (PC-EC). Forming the upper temperature boundary of the liquid phase, the bubble point θ b was found to change monotonically with solvent composition, falling with a large curvature from the boiling point of PC to that of DEC in PC-DEC, but rising to that of EC in PC-EC. Giving the lowest temperature of the liquid phase, the eutectic point of PC-DEC was found to be about 10° below the melting point of DEC, and that of PC-EC, only a few degrees below PC. The ɛ changed with solvent composition monotonically, falling with the weight fraction w of DEC in PC-DEC but rising with w of EC in PC-EC, and fell with rising temperature θ in both solvents. Bjerrum critical distance for ion pairing was evaluated from the measured ɛ data and was found to depend weakly on θ but strongly on w . Glass transition temperature T g was measured for the two solvents for the purpose of evaluating their η and was found to fall with w of DEC in PC-DEC but rise slowly with w of EC in PC-EC. The apparent parallelism in the changes of θ b , ɛ, and T g was attributed to their common connection to the internal cohesion of a solvent. As η is also largely determined by this cohesion, it was argued and demonstrated that T g could be used to qualitatively evaluate η of the carbonates and their mixtures. η of the two solvents was thus seen to fall with w of DEC in PC-DEC and rise slowly with w of EC in PC-EC.