Article pubs.acs.org/JPCC
Effects of Solvent Composition on Liquid Range, Glass Transition, and Conductivity of Electrolytes of a (Li, Cs)PF6 Salt in EC-PC-EMC Solvents Michael S. Ding,*,† Qiuyan Li,‡ Xing Li,‡,§ Wu Xu,‡ and Kang Xu† †
Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China ‡
ABSTRACT: Electrolytes of 1 M LiPF6 (lithium hexafluorophosphate) and 0.05 M CsPF6 (cesium hexafluorophosphate) in EC-PC-EMC (ethylene carbonate-propylene carbonateethyl methyl carbonate) solvents of varying solvent compositions were studied for the effects of solvent composition on the lower limit of liquid range, glass transition temperature (as a reflection of viscosity), and electrolytic conductivity. In addition, a ternary phase diagram of EC-PC-EMC was constructed, and crystallization temperatures of EC and EMC were calculated to assist the interpretation and understanding of the change of liquid range with solvent composition. A function based on the Vogel−Fulcher−Tammann equation was fitted to the conductivity data in their entirety and was plotted as conductivity surfaces in solvent composition space for more direct and clear comparisons and discussions. Changes of viscosity and dielectric constant of the solvents with their composition, in relation to those of the solvent components, were found to be underlying many of the processes studied.
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INTRODUCTION Electrolytes based on the LiPF6 (lithium hexafluorophosphate) salt in EC-PC-EMC (ethylene carbonate-propylene carbonateethyl methyl carbonate) solvent, with CsPF6 (cesium hexafluorophosphate) as the additive salt, have recently been shown to have significant benefits to lithium ion batteries using these electrolytes while mitigating the usual detriments of PC-cointercalation with Li+ into graphite anodes.1−3 In the present study, we focus on the effects of solvent composition on liquid range, viscosity (as measured by glass-transition temperature, Tg), and electrolytic conductivity of such an electrolyte system: 1 M (mol L−1) LiPF6 and 0.05 M CsPF6 salts in EC-PC-EMC solvents of compositions from 0.11 to 0.54 for xEC and 0.09 to 0.50 for xPC mole fraction. We systematically evaluatedin both heating and coolingthe lower limits of the liquid range of these electrolytes, determined their Tg for their viscosity and for some of their liquid ranges, and measured their conductivity in the temperature range of −70 to 80 °C. We also fitted a function based on the VFT (Vogel−Fulcher−Tammann) equation to the measured conductivity data in their entirety, plotted the function as conductivity surfaces in the composition space at different temperatures, and compared and discussed the results for an understanding of the underlying mechanisms. In addition, we constructed a ternary phase diagram of EC-PC-EMC on the basis of a thermodynamic regular solution model and on our previous work on binary and multicomponent carbonate solvents4−7 and calculated the crystallization temperatures of EC and EMC using the same model to assist in the interpretation, understanding, and predication for the change of liquid range with the change in solvent composition. © 2017 American Chemical Society
EXPERIMENTAL SECTION Sample Preparation. The solvent components EC, PC, and EMC, and the salt LiPF6, all battery grade (≥99.9% in purity), were purchased from BASF and were used as received. CsPF6 (≥99.0% in purity) was purchased from SynQuest Laboratories (Alachua, FL) and was dried at 65 °C under vacuum for 4 days before use. The ternary solvent compositions were prepared by weighing and the salt solutions by adding a solvent composition to the salts of specific amounts to reach a final volume in which the concentrations of LiPF6 and CsPF6 were 1.00 and 0.05 M, respectively. All of these steps were taken inside an argon-filled glovebox with the levels of both oxygen and moisture below 1 ppm.3 Measurement of Glass-Transition Temperature. The Tg of the samples was determined using a differential scanning calorimeter (DSC, model MDSC 2920, TA Instruments) cooled with liquid nitrogen. The DSC was calibrated for temperature with three phase transition standards: −87.06 °C of a solid−solid transition and 6.54 °C of melting in cyclohexane and 75.94 °C of melting in hexatriacontane. The DSC sample was prepared by crimp-sealing a small amount (∼10 mg) of liquid electrolyte in a pair of aluminum pan and lid (0219-0062, PerkinElmer Instruments). For the measurement, the DSC sample was first dip-quenched in liquid nitrogen to vitrify the enclosed electrolyte and then was quickly placed onto the DSC sample stage that had been kept at a temperature below the estimated Tg of the Received: April 7, 2017 Revised: May 8, 2017 Published: May 10, 2017 11178
DOI: 10.1021/acs.jpcc.7b03306 J. Phys. Chem. C 2017, 121, 11178−11183
Article
The Journal of Physical Chemistry C electrolyte. The sample was then heated through its Tg at 2 °C min−1, and the Tg was subsequently evaluated at the inflection point of the endothermic heat flow associated with the glass transition.8,9 Determination of the Lower Limit of Liquid Range. The lower limit of liquid range of each electrolyte was determined in two ways: in cooling and in heating, using the same instrument and setup as for Tg. To facilitate the crystallization of a solid phase from a liquid electrolyte, a very small amount (
