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Title: Characterisation of transport and thermodynamic properties for various binary electrolytes
Author: Hou, Tianhong
ISNI:       0000 0004 7966 3770
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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The electrolyte is an essential component in any electrochemical device. During the operation of a battery, an applied current introduces a concentration profile of salt within the electrolyte, whose shape is controlled by transport and thermodynamic properties of its constituent species. Concentration gradients are associated with overpotentials that can limit battery performance at high power. A better understanding of transport in battery electrolytes could help scientists optimize cell performance and design. To fully describe trans- port processes, techniques are needed to measure six key properties. We have accurately quantified some or all of the properties for binary electrolytes used in various energy storage systems, including redox flow, alkaline, lithium-ion, and magnesium-ion batteries. Density and ionic conductivity for aqueous VOSO4 solutions were quantified with respect to VOSO4 concentration up to 0.53 M at five temperatures: 15, 20, 25, 30, and 35 ◦C. The partial molar volumes of VOSO4 and water were calculated from the density/composition correlations. This was the first characterisation of both density and conductivity for aqueous VOSO4. The characterisation work was also used by a colleague to measure concentration dynamically during membrane cross-over experiments for all vanadium redox flow batteries. We further measured the density, ionic conductivity, and dynamic vis- cosity of aqueous KOH over concentrations ranging from 0.05 M to 12.00 M at three temperatures: 15, 25, and 35 ◦C.The partial molar volume of KOH rises with respect to concentration, while that of water stays relatively constant across the whole concentration range. The conductivity of aqueous KOH solutions reaches the maximum value around 6 M, which is the concentration used for commercial alkaline-battery electrolytes. We further attempted to quantify the diffusion coefficient using restricted-diffusion measurements. A MnO2- deposited nickel electrode was prepared and used to perform potentiometric restricted-diffusion measurements. The diffusion coefficient of 1 M KOH solution was determined to be 3.12 × 10−5 cm2/s. Although the diffusion coefficient obtained agreed with literature, we have noticed that the MnO2-plated electrode was consumed during the measurements, causing a variability in the open circuit potential before and after measurements. Thus, the accuracy of our characterisation method for aqueous KOH solution may be doubtful. Leveraging the experience gained from aqueous solutions, we performed the first complete characterisation of transport and thermodynamic properties for LiPF6 in PC, a non-aqueous electrolyte commonly used in commercial Li-ion batteries. Partial molar volumes of solute and solvent were calculated from a density-molarity correlation, which was established by densitometry. Ionic conductivity was quantified by AC conductometry within a customized, air-tight conductivity vial. A Hittorf cell was designed and fabricated to measure transference number. Thermodynamic factor was measured via a customized concentration cell. A potentiometric restricted-diffusion method was applied to determine Fickian diffusion coefficient, which was used in combination with other data to extract thermodynamic diffusivity. All six properties were char- acterised with respect to LiPF6 concentration up to 2 M. Density and ionic conductivity were measured at three temperatures: 20, 25, 30 ◦C, while trans- ference number, thermodynamic factor, and diffusion coefficient were determined at 25 ◦C only. Each property was fitted with a mathematical expression to describe its concentration dependence. Once the values of the six properties were obtained, we used them in a numerical model that simulated the OCP relaxation of our polarization measurements. The model fit new experimental results very well, verifying the measured property correlations. We further developed and characterised a new Mg2+-conductive elec- trolyte for use in Mg batteries with Mg metal anodes, which could be a potentially promising beyond-Li-ion technology. We started by producing previously reported conditioning-free Magnesium/Aluminum Chloride Complex (MACC) electrolytes, and discovered two alternative promoters that can be used to synthesise them: CuCl2 and ZnCl2 to synthesise the 'MaCC' electrolytes. Then, a novel conditioning-free, promoter-free Mg2+-conductive elec- trolyte was developed. It has been discovered that metal Mg can be sponta- neously oxidised from non-aqueous ZnCl2/THF, CuCl2/THF, or SnCl2/THF solutions, generating Mg2+-conductive electrolytes. Electrolytes synthesised from ZnCl2/THF solutions were completely studied. Various characterisation techniques, including electrochemical method, XRD, SEM-EDS, ICP-MS, 1H- NMR, 35Cl-NMR, and Raman spectroscopy were employed to explore the mechanism of electrolyte formation and the structure of the electrolytic solutions. The MZCC electrolyte (we synthesised from ZnCl2/THF solutions) can support reversible Mg plating and stripping with a Coulombic efficiency of essentially 100%, a deposition overpotential less than 150 mV, and a sta- bility window up to 3.2 V vs. Mg/Mg2+. We propose that the electrolytes are Mg-chloride-complex based, with a positive Mg dimer, [Mg2Cl3·6(THF)]+, as cation, and Cl as anion. Thus the MZCC electrolyte is a binary system. We further quantified density, ionic conductivity, and Fickian diffusion coefficient of 0.3 M MZCC in THF. The density is 991.4 g/L at 25 ◦C, ionic conductivity is 0.23 mS/cm at 25 ◦C, and diffusion coefficient is 8.93 × 10−7 cm2/s at 25 ◦C.
Supervisor: Monroe, Charles W. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available