Understanding the Parameters Influencing the Impedance Response of Sulfur-Carbon Composite Cathode in Lithium-Sulfur Batteries

Abstract

Lithium-sulfur (Li-S) batteries, which in recent years have been aimed at a high gravimetric energy density of 500-600 Wh kg-1, indicate particular potential in aerospace applications such as aircraft, satellites and drones.1,2 In light of this, the German Aerospace Center (DLR) has been pursuing a strategy of developing and researching metal-sulfur batteries for a decade. Herewith, we present the results of a fundamental study on the investigation of structural and compositional parameters affecting the electrochemical performance of sulfur-carbon composite cathode.Highly porous carbon materials are widely employed in composite cathodes in Li-S batteries to compensate for the non-conductive property of the sulfur element. Indeed, this would have significant effects not only on the conductivity of the cathode, but also on the diffusion and charge transfer parameters. Herewith, we have systematically studied porous carbon-based electrodes employing electrochemical impedance spectroscopy. Using a suitable transmission-line model peculiar to the porous electrodes, the resistance of bulk and pore electrolyte, inter-particles, and charge transfer is defined and quantified. The dependence of the identified processes at the open circuit voltage on the fraction of the active material, the porosity of utilized carbon and thickness of the electrode is investigated elaborately. It is indicated that the electrolyte resistance in the pore increases on increasing the content of active material, electrode thickness and on reducing the pore size distribution of the C component.Additionally, an innovative symmetrical three-electrode cell configuration with an integrated Li-ring serving as counter electrode is implemented, enabling the investigation of the cathode impedance at different depths of discharge to shed light on the kinetics of the charge transfer depending on the infiltration method. To this end, various porous carbon materials such as Ketjenblack® (mesoporous) and carbon aerogel (microporous) in combination with different sulfur infiltration techniques including gas, melt and mechanical mixing have been employed as model systems3. The thermodynamic and kinetics of charge transfer mechanisms as a function of infiltration method as well as the variation of the charge transfer resistances upon discharge/charge are discussed in details4. A. Fotouhi, D. J. Auger, L. O’Neill, T. Cleaver and S. Walus, Energies, 2017, 10, 1937.Sripad S., Viswanathan V. (2017), ACS. Energy. Lett.2, 1669-1673M. Nojabaee, B. Sievert, M. Schwan, J. Schettler, F. Warth, N. Wagner, B. Milow, K. Andreas Friedrich, J. Mater. Chem. A, 2021,9, 6508-6519.Martina Gerle, Norbert Wagner, Joachim Häcker, Maryam Nojabaee, Andreas Friedrich J. Electrochem. Soc., 2022, 169, 030505.