Theoretical and Experimental Analysis of Practical Porous Electrodes for Lithium-Sulfur Batteries By Electrochemical Impedance Spectroscopy

摘要

We present in this talk a simplified model for a porous sulfur/carbon electrode based on a uniform cylindrical pores and a transmission line model (TLM) to explain the observations of Electrochemical Impedance Spectroscopy (EIS) measurements. We provide an understanding of the origin of the internal resistances of the porous sulfur electrode/electrolyte interface in Li-S batteries. The proposed model can serve as a diagnostic tool for determining the state of health of the cell and for quantifying the ionic resistances limiting the design of high-power systems. Nevertheless, the separation of impedance elements for a Li-S cell is not a straightforward task due to the multiple reaction steps and volume changes during the discharge and charge process involving the dissolution of the sulfur into polysulfides and their precipitation into the insulating low order polysulfides. EIS studies in combination with TLM have been presented for lithium-ion cells by several authors. Other researchers have characterized the resistances involved in the cell by fitting EIS data. In addition, there have been efforts using TLMs for estimating the impedance response of a Li-S cell, to provide an understanding about the internal resistances in the electrode and a description of its possible electrical circuit. Nevertheless, there is still no agreement on the parameters affecting the results nor neither their contribution, and thus, only trends have been postulated. Therefore, this work goes beyond the general scope of previous studies and attempts to rationalize the effect of the electrode architecture (geometry, thickness, surface area) and active material loading for the sulfur/carbon composite electrode in the initial performance of Li-S batteries. The use of a practical and simple electrode description captures the significant variables that have an effect on the initial discharge process of the Li-S cell. The geometrical simplification stands as an attractive and straightforward strategy for engineering new electrodes and predicting initial cell performance.