Dataset supports: Raccichinia, Rinaldo (2018). Impedance Characterization of the Transport Properties of Electrolytes Contained within Porous Electrodes and Separators Useful for Li-S Batteries. Journal of the Electrochemical Society.Impedance spectroscopy is used to characterise the key transport properties (effective conductivity, MacMullin number, porosity and tortuosity) of electrolyte solutions confined in porous separators and carbon-sulfur composite electrodes useful for application in Li-S batteries. Three relevant electrolyte concentrations, ranging between 1 molal and 5 molal, are studied. Impedance measurements are carried out using symmetrical cell configurations, which significantly improve the accuracy of the results and avoids complications associated with the contributions of the counter-reference electrode in two-electrode cell measurements. The impedance response of the electrolyte-filled carbon-sulfur composite electrodes can be represented by an “open” Warburg element, modelling the finite-diffusion of ions through the pores coupled to the double-layer charging of the electrode-electrolyte interface. The as-prepared carbon-sulfur composite electrodes are at a high enough potential (ca. 3 V vs. Li+/Li) so that charge-transfer reactions of sulfur reduction to polysulfide species are absent during the impedance measurements, and hence capacitive-like behaviour (i.e., blocking behaviour) is observed at low frequencies. The analysis of the results shows that the rate of transport of ions through porous structures is markedly dependent on the electrode’s structure and composition as well as the electrolyte concentration. Synergistic effects, able to enhance the effective conductivity of the electrolyte inside porous composite electrodes, are observed for particular electrode/electrolyte combinations, which are correlated to enhanced performance in Li-S cells.