Operando Characterization of Lithium-Sulfur Battery Intermediates Using X-Ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) eliminates these restrictions and thus provides a powerful tool for monitoring compositional changes during operation of a battery. Different sulfur species exhibit characteristic features, dependent on their chemical composition, in the x-ray absorption near‑edge structure (XANES) at the S K‑edge, exploited in previous studies [5-8]. However, a drawback in these previous measurements was the data collection geometry in which the x-ray beam was penetrating the cell through the backside of the sulfur electrode.
In this work, we introduce a novel spectro‑electrochemical cell which allows direct observation of intermediates in the separator and use it to explore the influence of the electrolyte solvent’s dielectric constant on the conversion of polysulfides to Li2S by comparing solvents with a low-dielectric constant (DOL:DME, 1:1 v/v) and a high-dielectric constant (DMAC), the first discharge curves of which are shown in Figure 1.
Figure 1: Galvanostatic discharge curves measured in operando mode at a C-rate of 0.1 h-1 using either DOL-DME (1:1 v/v) or DMAC, each containing 1M LiClO4 and 0.5M LiNO3.
Our operando XAS measurements show that the formation of Li2S is faster in DOL:DME compared to DMAC, and also suggest that Li2S2 species is present at the end of discharge in DOL:DME. Furthermore, contrary to other reports, we do not detect any elemental sulfur at the end of discharge .
 N. A. Cañas, S. Wolf, N. Wagner and K. A. Friedrich, J. Power Sources, 226, 313 (2013).
 S. Walus, C. Barchasz, J.-F. Colin, J.-F. Martin, E. Elkaim, J.-C. Lepretre and F. Alloin, Chem. Commun., 49, 7899 (2013).
 R. Bonnaterre and G. Cauquis, J. Chem. Soc., Chem. Commun., 293 (1972).
 N. A. Cañas, D. N. Fronczek, N. Wagner, A. Latz and K. A. Friedrich, J. Phys. Chem. C, 118, 12106 (2014).
 M. Cuisinier, P.-E. Cabelguen, S. Evers, G. He, M. Kolbeck, A. Garsuch, T. Bolin, M. Balasubramanian and L. F. Nazar, J. Phys. Chem. Lett., 4, 3227 (2013).
 M. E. Fleet and X. Liu, Spectrochim. Acta B, 65, 75 (2010).
 T. A. Pascal, K. H. Wujcik, J. Velasco-Velez, C. Wu, A. A. Teran, M. Kapilashrami, J. Cabana, J. Guo, M. Salmeron, N. Balsara and D. Prendergast, J. Phys. Chem. Lett., 5, 1547 (2014).