A Thermoconductometric Study of Transient Behavior of Liquid Electrolytes at Phase Transition

Abstract

As aqueous electrolytes have been gaining renewed interest in battery research, understanding how the properties of these electrolytes change at icing and other phase transitions becomes imperative for both monitoring such occurrences in extreme environments and identifying the service temperature limits for the battery devices containing such electrolytes. In this study, we devised a coupled thermoconductometric measurement system in which the impedance and temperature of an electrolyte sample, along with the temperature of a reference, were concurrently, continuously, and speedily measured while the sample was subjected to an external temperature ramp across its phase transitions. The synchronized curves of conductivity and temperature differential thus obtained were used effectively to study and understand the different processes in the phase transitions and their impact on the apparent conductivity of the electrolytes. The solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in water and in ethylene carbonate (EC) were chosen as the representative aqueous and nonaqueous electrolytes, respectively. Results showed that while the differential temperature signal at a phase transition was always pronounced, a change in apparent conductivity could be deceptively unremarkable and easily escape notice, which may lead to erroneous estimation for the temperature limits of the electrolytes. It was also revealed that the nature of the solvent, the degree of supercooling, and the electrolyte composition were factors in determining the magnitude of the apparent conductivity drop at a phase transition. Furthermore, the apparent conductivity curves of a precipitated hypoeutectic LiTFSI + EC electrolyte were accurately calculated from the properties and proportion of its remaining liquid, using several functional relationships made available by an experimentally mapped phase diagram and a measurement-based conductivity function plus a form of mixing rule with an adjustable factor to account for the spatial configuration of the precipitates. Such in-depth understanding of ion-transport behavior at phase transition sets the foundation for quantitative prediction of electrolyte properties at extreme temperatures.