Quantification of Dynamic Ion Correlations in Highly Concentrated Liquid Electrolytes and of their Implications for Charge and Mass Transport

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Highly concentrated liquid electrolytes with molar ratios of salt: solvent of up to 1:1 exhibit promising properties for applications in the field of electrochemical energy storage in batteries and supercapacitors. Due to the high ion concentrations, strong directional correlations occur in the movement of cations and anions, which influence both charge and mass transport. A theoretical description of the transport properties can be done in the framework of the Onsager formalism in combination with linear response theory. For the experimental characterization of the transport parameters, typically just as many measured variables are combined as are absolutely necessary. This can lead to relatively large uncertainty intervals for the obtained transport coefficients. The present project goes beyond this approach, as by combining electrochemical measurements with NMR-based transport measurements in order to determine one more measured variable as absolutely necessary. In combination with a detailed uncertainty analysis, very precise values for the Onsager transport coefficients are to be obtained for selected electrolytes, yielding exact information about charge and mass transport properties. This approach is applied to three Li+- conducting liquid electrolyte systems: (i) Highly concentrated electrolytes based on organic carbonates; (ii) Sulfolane-based highly concentrated electrolytes; (iii) Localized high concentration electrolytes with a carbonate as a salt-coordinating solvent and a fluorinated ether as a non-coordinating solvent. Through the variation of lithium salts with anions of different Li+ coordination strength (FSI- weaker, BF4- stronger), cation-anion and cation-solvent interactions will be varied, and the influence of these interactions on charge and mass transport will be examined. Building on this, guidelines are to be drawn up on how the interactions in highly concentrated electrolytes can be balanced in such a way that the best possible charge and mass transport properties are achieved. In this way, the project will contribute to an improved fundamental understanding of the relationships between interactions in highly concentrated electrolytes and transport properties in energy storage cells.