In Situ X-Ray Absorption Spectroscopy of Lithium-Sulfur Battery Discharge/Charge Reaction Mechanisms

A necessity to the development of more efficient lithium-sulfur batteries is an understanding of the redox pathways through which soluble Li₂S_x reaction intermediates are formed.  Spectroelectrochemical studies of the charge/discharge reaction pathways have been complicated by difficulties faced in the assignment of Li₂S_x reaction intermediates to spectral peaks.  The development of spectroscopic techniques that can clearly differentiate Li₂S_xmolecules and be used to perform in-situ studies of Li-S cell chemistry has become an important focus of Li-S battery research. 

It has recently been shown that X-ray absorption spectroscopy (XAS) at the sulfur K-edge may be used to speciate Li₂S_x molecules of different ‘x’ values.^1-4  In this work, in situ XAS of Li-S cell cathodes will be used to probe cell cathodes during charge/discharge.  Collected spectra provide full speciation of sulfur throughout charge/discharge processes and reveal the pathway through which polysulfide species are formed.  Li-S cells consist of a PEO-based polymer electrolyte and thin film cathodes, employed to mitigate effects of X-ray overabsorption.  Interpretation of collected spectra is achieved using a principal component analysis.  Theoretical X-ray spectra obtained using first-principles molecular dynamics and spectral simulations of lithium polysulfides dissolved in tetraethylene glycol dimethyl ether (TEGDME) provide further support for spectral interpretation.  The molecular origins of observed spectral features are elucidated.

References

                (1)           Wujcik, K. V., J.; Wu, C.; Pascal, T.; Marcus, M.; Cabana, J.; Prendergast, D.; Guo, J.; Salmeron, M.; Balsara, N.P. Manuscript in Preparation 2014.

                (2)           Pascal, T. W., K.; Velasco-Velez, J.; Wu, C. H.; Teran, A.; Kapilashrami, M.; Cabana, J.; Guo, J.; Salmeron, M.; Balsara, N. P.; Prendergast, D. Manuscript in Preparation 2014.

                (3)           Cuisinier, M.; Cabelguen, P.-E.; Evers, S.; He, G.; Kolbeck, M.; Garsuch, A.; Bolin, T.; Balasubramanian, M.; Nazar, L. F. The Journal of Physical Chemistry Letters 2013, 4, 3227.

                (4)           Patel, M. U. M.; Arčon, I.; Aquilanti, G.; Stievano, L.; Mali, G.; Dominko, R. ChemPhysChem 2014, n/a.