Overdetermination method for accurate dynamic ion correlations in highly concentrated electrolytes†

Highly concentrated battery electrolytes exhibit a low flammability as well as a high thermal and electrochemical stability, and they typically form stable solid electrolyte interphases in contact with electrode materials. The transport properties of these electrolytes in batteries are strongly influenced by correlated movements of the ions. In the case of a binary electrolyte, the transport properties can be described by three Onsager coefficients and a thermodynamic factor. In order to determine these four target quantities, at least four experimental quantities are needed. Overdetermination by measuring five or more experimental quantities is uncommon. Here we have combined electrochemical impedance spectroscopy, electrophoretic NMR measurements and concentration cell measurements for characterizing two highly concentrated sulfolane/LiFSI electrolytes. Two sets of four experimental quantities and one set of five experimental quantities were compared with regard to the uncertainties of the resulting four target quantities. We show that the methods employing only four experimental quantities either lead to large uncertainties of the Onsager coefficients or to large uncertainties of the thermodynamic factor, while only the overdetermination by five experimental quantities leads to acceptable uncertainties of all four target quantities. The results for the Onsager coefficients are discussed with regard to dynamic ion correlations and to transport limitations in battery cells.