Untersuchung von Ionenkorrelationen und Transport in hochkonzentrierten Flüssigelektrolyten für Lithiumionen-Batterien
Loading...
Date
Authors
Publisher
Abstract
The aim of this dissertation was to expand and improve the understanding of ion correlations and transport in highly concentrated electrolytes. The results can be divided into three different projects. First, the development of a method to determine the transport quantities of highly concentrated electrolytes more accurately; second, the application of this method to various HCE systems; and third, the theoretical consideration of various parameters on ion correlations and transport mechanisms in HCEs via Monte Carlo simulations. Together with the working group of Prof. Dr. Monika Schönhoff (Univeristy of Münster)
a method for overdetermining the Onsager conductivity coefficients and the thermodynamic factor has been successfully developed. This method requires five experimental parameters from measurements using electrochemical impedance spectroscopy, concentration cells with transference and eNMR. Since four transport parameters are determined from these measurements, this implies an overdetermination. Compared to previous methods,[72], [73] which only consider four experimental quantities, the uncertainties of the Onsager coefficients and the thermodynamic factor can be significantly reduced. This allows for a more precise analysis of the Onsager coefficients and the thermodynamic factor. The overdetermination method was applied to three different electrolyte systems in this study. In the first electrolyte system, which serves as a reference system, sulfolane was used as the solvent with LiFSI as the conductive salt. The compositions 2.4/1 and 3.0/1 (molar ratio of solvent to salt) were investigated. For the second electrolyte, sulfolane was used as the solvent with the conductive salt LiTFSI in the compositions 2.0/1 and 3.0/1. In the third electrolyte, DMC was used as the solvent with LiFSI as the conductive salt in the compositions 1.1/1, 1.3/1 and 2.0/1. Using data from eNMR and partial volumes of the species studied, it was possible to show that the movements in the systems under consideration are subject to volume
conservation. This reference frame made it possible to calculate the solvent-related Onsager coefficients σ00, σ+0 and σ−0, thereby obtaining information on the correlations of the solvent molecules. A comparison of the anions FSI− and TFSI− shows that a stronger interaction between Li+ and anion, as is the case with Li+ and TFSI−, weakens the anticorrelation of cations and anions. In addition, the influence of the solvent on the transport properties was investigated. For this purpose, the parameter γ was introduced as a measure for the relevance of vehicular Li+-solvent transport. In the case of complete vehicular Li+-solvent transport, γ becomes unity, and in the case of no correlation, it becomes zero. It was shown that the sulfolane-based systems exhibit a clear tendency towards vehicular Li+-solvent transport, while transport in the DMC-based systems is classified as uncorrelated. However, the tendency towards vehicular Li+-solvent transport increases with increasing salt concentration. The different observations were attributed to the strength of the cation-solvent interaction compared to the cation-anion interaction. Sulfolane has a high permittivity, which weakens the cation-anion interaction. Therefore, for sulfolanebased
systems, a high γ value is obtained, which at the same time leads to a strong anticorrelation between cations and anions due to volume conservation. Although this is advantageous for charge transport in the system, it is also disadvantageous for mass transport. In DMC-based systems, the Li+-DMC and Li+-FSI− interactions are balanced due to the lower permittivity of DMC, resulting in uncorrelated transport. This is advantageous for neutral salt transport, while charge transport is slowed down. The investigation of real systems thus shows that volume conservation and the interactions between ions and between cations and solvents have a strong influence on ion correlations and transport processes. In order to analyse these effects separately, Monte Carlo simulations were performed with different salt concentrations and volume ratios. In systems with only hard-core interactions, positive correlations between all particles are observed due to the limited free space available. If the system is also subject to volume conservation, the positive correlations are weakened and anions and solvent molecules move in anticorrelation to each other. Due to additional interionic Coulomb interactions, cations and anions move in the same direction and charge transport in the system is restricted. The addition of attractive cation-solvent interactions slows down the overall dynamics of the system. The attractive cation-solvent interaction competes with the attractive cation-anion Coulomb interaction. If the cation-solvent interaction is stronger, the tendency towards vehicular Li+-solvent transport increases. This is accompanied by a stronger anticorrelation between cations and anions and thus also by favoured charge transport but also impaired mass transport in the system. However, at the same time, both charge and mass transport are greatly reduced by the slower dynamics in the system, which is a major disadvantage for the application of HCEs in battery systems. The electrolyte systems investigated here, LiFSI in sulfolane, LiTFSI in sulfolane and LiFSI in DMC, exhibit slow dynamics and thus slow charge and slow mass transport. Therefore, the electrolytes investigated here are not suitable for use in batteries with high power densities.
Review
Metadata
License
Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivatives 4.0 International