A major barrier toward to the development of cost‑effective lithium‑sulfur (Li-S) batteries with sufficient energy densities to meet the requirements of battery electric vehicles is the lack of a mechanistic understanding of the processes occurring inside the battery. Motivated by improving the mechanistic understanding of lithium‑sulfur batteries, we focus on the charging process of the Li₂S cathode, a possible alternative to the conventional S₈ cathode, with a potential to enable batteries with silicon or tin rather than lithium anodes.(1) Li₂S presents an interesting system for mechanistic characterization, because the first charge of the cathode requires an application of a high overpotential, while each subsequent charge can be performed at a lower overpotential.(2) The presence of the high overpotential in the first charge has been hypothesized to relate to a charge transfer step that leads to a lithium‑deficient surface on the surface of large Li₂S particles.(2) This mechanism, however, does not provide a sufficient explanation for how an activation of less than 10% of the volume of the particle could lead to an extraction of the entire capacity.

To clarify the charging process of Li₂S cathode, we have performed X-ray absorption spectroscopy (XAS) characterization of Li₂S‑Li battery using a spectro-electrochemical cell designed in our laboratory (Fig. 1a),(3) which is capable of spatially resolved measurements (Fig. 1b). Our experiments demonstrate which solid and solution phase intermediates are produced in two locations of the cell, the Li₂S cathode and the separator, and thus generate a unique set of spatially resolved data, which we use to identify a plausible charging mechanism. Because the intermediates forming during the first charge of Li₂S cathode have so far only been characterized using X‑ray diffraction (XRD),(2, 4) a technique that is capable of detecting crystalline solids, but cannot provide information about amorphous species, our combination of XAS with spatial resolution facilitates a significantly improved mechanistic understanding of the operation of Li‑S batteries.

References

1. J. Hassoun and B. Scrosati, Angew. Chem., 49, 2371 (2010).

2. Y. Yang, G. Zheng, S. Misra, J. Nelson, M. F. Toney and Y. Cui, J. Am. Chem. Soc., 134, 15387 (2012).

3. Y. Gorlin, A. Siebel, M. Piana, T. Huthwelker, H. Jha, G. Monsch, F. Kraus, H. A. Gasteiger and M. Tromp, J. Electrochem. Soc., 162, A1146 (2015).

4. H. Jha, I. Buchberger, X. Cui, S. Meini and H. A. Gasteiger, J. Electrochem. Soc., 162, A1829 (2015).