Quantification of vehicular <i>versus</i> uncorrelated Li⁺–solvent transport in highly concentrated electrolytes <i>via</i> solvent-related Onsager coefficients

Highly concentrated salt solutions are promising electrolytes for battery applications due to their low flammability, their high thermal stability, and their good compatibility with electrode materials. Understanding transport processes in highly concentrated electrolytes is a challenging task, since strong ion-ion and ion-solvent interactions lead to highly correlated movements on the microscopic scale. Here, we use an experimental overdetermination method to obtain accurate Onsager transport coefficients for concentrated binary electrolytes composed of either sulfolane (SL) or dimethyl carbonate (DMC) as solvent and either LiTFSI or LiFSI as salt. NMR-based electrophoretic mobilities demonstrate that volume conservation applies as a governing constraint for the transport. This fact allows to calculate the Onsager coefficients <i>σ</i>₊₀, <i>σ</i>_-0 and <i>σ</i>₀₀ related to the solvent. A parameter <i>γ</i> is then defined, which is a measure for the relevance of a vehicular Li⁺-solvent transport mechanism. We analyze the influence of the salt anion and of the solvent on dynamic correlations and transport mechanisms. In the case of the sulfolane-based electrolytes, the <i>γ</i> parameter reaches values up to 0.38, indicating that Li⁺-sulfolane interactions are stronger than Li⁺-anion interactions and that vehicular Li⁺-sulfolane transport plays a significant role. In the case of DMC-based electrolytes, the <i>γ</i> parameter is close to zero, suggesting balanced Li⁺-DMC <i>vs.</i> Li⁺-anion interactions and virtually uncorrelated movements of Li⁺ ions and DMC molecules.