Direct Electrochemical Determination of Thermodynamic Factors in Aprotic Binary Electrolytes

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In the literature, various numerical methods for the simulation of ion-transport in concentrated binary electrolytes for lithium ion batteries can be found, whereas the corresponding transport parameters are rarely discussed. In this contribution, a novel method for the determination of the transference number in non-aqueous electrolytes is proposed. The method is based on data from a concentration cell and on the value of the thermodynamic factor obtained from independent measurements based on quantifying the redox potential of ferrocene. The concentration dependent transference numbers obtained by this new method are compared to values obtained by the classical approach, which is based on experiments in a polarization cell and a concentration cell. For the latter, a set of commonly used and some newly proposed analysis methods as well as their theoretical justification are discussed. Using an exemplary electrolyte (lithium perchlorate in a mixture of ethylene carbonate and diethyl carbonate), we will demonstrate that our newly proposed method based on concentration cell experiments and a thermodynamic factor derived from independent measurements is a more accurate approach for obtaining concentration dependent transference numbers. At the end, the experimentally determined concentration dependent transference numbers are compared to data available in the literature.

Section: Resultsmentioning

confidence: 99%

Section: Short-term Relaxation From a Steady-state Concentration Profmentioning

confidence: 99%

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Determination of Transport Parameters in Liquid Binary Electrolytes: Part II. Transference Number

Ehrl

Landesfeind

Wall

et al. 2017

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“…9,[16][17][18][19] Traditional solvent blends for Li electrolytes have been made with mixtures of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and dimethyl carbonate (DMC). EC has a high dielectric constant, aiding the disassociation of the lithium salt in solution.…”

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confidence: 99%

A Study of the Physical Properties of Li-Ion Battery Electrolytes Containing Esters

Logan

Tonita

Gering

et al. 2018

188

192

Adding esters as co-solvents to Li-ion battery electrolytes can improve low-temperature performance and rate capability of cells. This work uses viscosity and electrolytic conductivity measurements to evaluate electrolytes containing various ester co-solvents, and their suitability for use in high-rate applications is probed. Among the esters studied, methyl acetate (MA) outperforms other esters in its impact on the conductivity and viscosity of the electrolyte. Therefore, viscosity and conductivity were measured as a function of temperature and LiPF 6 concentration for electrolytes ethylene carbonate (EC): linear carbonate: MA in the ratio 30:(70-x):x, where linear carbonate = {ethyl methyl carbonate (EMC), dimethyl carbonate (DMC)}, and x = {0, 10, 20, 30}. Adding MA leads to an increase in conductivity and decrease in viscosity over all conditions. Calculations of electrolyte properties from a model based on a statistical-mechanical framework, the Advanced Electrolyte Model (AEM), are compared to all measurements and excellent agreement is found. All electrolytes studied roughly agree with a Stokes' Law model of conductivity. A Walden analysis shows that the ionicity of the electrolyte is not significantly impacted by either MA content or LiPF 6 concentration. Li [Ni 0.5 Typical performance metrics of Li-ion batteries such as lifetime and power capabilities depend strongly on the electrolyte used. The ionic conductivity of the electrolyte is one transport property that helps to determine how fast a cell can be charged or discharged, and has been reported for a vast number of aqueous and non-aqueous electrolyte systems.1-14 While it does not give a full picture of ionic transport in an electrolyte, conductivity can be measured easily and accurately, giving a rapid evaluation of the electrolyte in question. In addition to conductivity, the dielectric constants and viscosities of the constituent solvents must be considered.5,15 For a more rigorous analysis of cell performance using physics-based models, other transport properties such as Li-ion transference number, diffusivity, and activity coefficient are required. 9,[16][17][18][19] Traditional solvent blends for Li electrolytes have been made with mixtures of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and dimethyl carbonate (DMC). EC has a high dielectric constant, aiding the disassociation of the lithium salt in solution. Traditionally, EC has also been required in the electrolyte to help form a passivating solid electrolyte interphase (SEI) on a graphite negative electrode. 20 DEC, EMC and DMC have lower viscosities and melting points than EC, and when mixed with EC result in an electrolyte with a good balance between desirable electrochemical properties, high dielectric constant, and low viscosity. [21][22][23][24] Aliphatic esters have lower melting points and viscosities than "low viscosity" linear carbonates such as EMC or DMC. 21,25,26 Many studies have investigated the impact of esters on the performance of Liion ...

“…2 While the conductivity κ(c) can be measured using turn-key conductivity sensors, the determination of the other three concentration dependent parameters is more elaborate. Experimental methods for the determination of the transference number and the thermodynamic factor are discussed, e.g., in Ehrl et al 3 and Landesfeind et al, 4 while the determination of a complete set of transport parameters can also be found in the literature. 5,6 An overview of the most popular experimental techniques for the determination of binary diffusion coefficients in lithium based electrolytes is given in the following.…”

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confidence: 99%

Determination of Transport Parameters in Liquid Binary Lithium Ion Battery Electrolytes

Ehrl

Landesfeind

Wall

et al. 2017

Self Cite

138

Various numerical methods for the simulation of ion-transport in concentrated binary electrolyte solutions can be found in the literature, whereas the corresponding transport parameters are rarely discussed. In this contribution, a polarization cell consisting of two electrodes separated by a porous separator is proposed to determine the concentration dependent binary diffusion coefficient of non-aqueous electrolyte solutions. Therefore, two different electrochemical methods are extended so that they can be applied to electrolyte solutions in a porous medium. Additionally, the different methods are compared with each other by means of numerical simulations. The proposed experimental setup is used to determine the concentration dependent binary diffusion coefficient of an exemplary electrolyte, lithium perchlorate dissolved in a mixture of ethylene carbonate and diethyl carbonate, and the data are compared to those available in the literature. It will be shown that the most reliable method to determine concentration dependent binary diffusion coefficients are long-term relaxation experiments in a two-electrode cell using a porous separator.