National Institute of Chemistry , Ljubljana SI-1000, Slovenia.
Faculty of Chemistry and Chemical Technology, University of Ljubljana , Ljubljana SI-1000, Slovenia.
ACS Appl Mater Interfaces. 2017 Sep 6;9(35):29760-29770. doi: 10.1021/acsami.7b08317. Epub 2017 Aug 22.
Polysulfides are central compounds in lithium-sulfur battery cells. However, the fundamental redox and diffusion properties of polysulfides are still poorly understood. We try to fill this gap by performing an accurate impedance spectroscopy investigation using symmetrical cells consisting of two planar glassy carbon electrodes separated with catholyte-soaked separator. The catholyte contains a mixture of selected polysulfides with predetermined nominal concentrations. Impedance measurements reveal textbook shapes of spectra for most polysulfide compounds or their mixtures. This allows reliable and accurate determination of the rate constant (exchange current density) for a given redox reaction as well as the diffusion coefficient and diffusion length for the rate-determining polysulfide species. Further, it is confirmed that polysulfides tend to disproportionate with time, which significantly changes the chemistry and electrochemistry of the system. Two approaches are proposed for identification of the prevailing redox mechanism in the resulting mixtures. The values of kinetic and transport parameters obtained for different cases of interest are commented on in significant detail. The study provides a solid basis for better understanding of the complex processes in polysulfide mixtures.
多硫化物是锂硫电池中的中心化合物。然而，多硫化物的基本氧化还原和扩散性质仍未被很好地理解。我们试图通过使用由两个平面玻璃碳电极组成的对称电池来进行精确的阻抗谱研究来填补这一空白，该电池用充满电解质的隔板隔开。电解质含有预定名义浓度的选定多硫化物混合物。阻抗测量显示大多数多硫化物化合物或其混合物的光谱具有教科书般的形状。这允许可靠且准确地确定给定氧化还原反应的速率常数（交换电流密度）以及速率决定的多硫化物物种的扩散系数和扩散长度。此外，证实多硫化物随着时间的推移趋于歧化，这会显著改变系统的化学和电化学性质。提出了两种方法来识别混合物中主要的氧化还原机制。详细讨论了不同情况下获得的动力学和传输参数的值。该研究为更好地理解多硫化物混合物中的复杂过程提供了坚实的基础。
