Li2S electrodeposition poses a number of challenges due to slow redox kinetics, the formation of electronically disconnected material, and the fact that Li2S is an electronic insulator. This process is also responsible for the majority of the capacity in a Li-S battery, so it is therefore important to understand and improve its kinetics. Recently we have demonstrated that Li2S electrodeposition occurs by a surface-limited 2-D island nucleation and growth process.¹

Here, the kinetics of Li₂S deposition on carbon from polysulfide solutions of various concentrations (in DOL:DME 1:1) were characterized potentiostatically. In general, higher polysulfide concentrations were found to result in significantly slower electrodeposition. High polysulfide concentrations can be expected in Li-S batteries with low electrolyte/sulfur (E/S) ratios, as large amounts of electrolyte tend to dilute the polysulfide. We galvanostatically cycled Li-S cells with different E/S ratios and found that a low E/S ratio does indeed result in significantly higher polarization, especially at the beginning of the Li2S precipitation plateau when nucleation of Li2S occurs.

This has significant implications for the design of sulfur cathodes, as low E/S ratios are essential for high energy density Li-S batteries.² It is therefore necessary to improve electrodeposition kinetics at high polysulfide concentrations. Some strategies for doing so include redox mediators, as well as the use of alternative conducting substrates such as conducting oxides with higher affinities for Li2S.

 Figure: (a) Potentiostatic current vs. time curves for Li2S deposition from various concentrations (in molarity of sulfur) of polysulfide at 2.05V vs. Li/Li+. Wider peaks at higher concentrations indicate slower nucleation and growth. (b) Galvanostatic cycling of sulfur-carbon composite cathodes vs. Li at C/8 at various E/S ratios. 

References

1. F. Y. Fan, W. C. Carter, and Y.-M. Chiang, Adv. Mater., 27, 5203–5209 (2015) http://doi.wiley.com/10.1002/adma.201501559.

2. D. Eroglu, K. R. Zavadil, and K. G. Gallagher, J. Electrochem. Soc., 162, A982–A990 (2015).