Effect of Conductive Substrate Material on Li2S Electrodepositionkinetics in Li-S Batteries

Lithium-sulfur batteries are of interest because of their high theoretical energy density and the very low cost and high abundance of sulfur. As most of the theoretical capacity (~1670 mAh/g) of a lithium sulfur battery results from the electrodeposition of either Li2S (~1250 mAh/g) or sulfur (~210 mAh/g), it is critical to understand the kinetics of the nucleation and growth processes by which these insoluble species are formed. Moreover, the precipitation of these species is thought to have a negative impact on the cycle life of Li-S batteries due to the electronic isolation of some of the material.1
We have previously demonstrated, through electrodeposition from polysulfide solutions, that Li2S forms via a 2-dimensional island nucleation and growth process, where reduction occurs at the interface of the electrolyte and conducting substrate due to the high electronic resistivity of Li2S.2 Furthermore, we quantified the nucleation and growth rates of the precipitate on graphite fiber surfaces (to represent the conductive carbon additive used in the majority of Li-S batteries), and found them to be highly dependent on electrolyte solvent. However, even in the best case, electrodeposition is slow on carbon surfaces due to the relatively low affinity of Li2S with nonpolar carbon. Alternative surfaces, such as conducting metal oxides and polymer-modified carbon, have been proposed to lower the energy of the sulfide-substrate interface and thereby improve the electrodeposition kinetics.3–5
Here, we will discuss the effects of alternative surfaces, such as metal oxides, on the kinetics and morphology of Li2S precipitation in lithium-sulfur batteries. We will show that metal oxide surfaces such as Ti4O7 reduce the overpotential required (Figure 1) for nucleation by reducing the interface energy between nuclei and the substrate. We will also show that the enhancement in electrodeposition kinetics arises from this reduced barrier, rather than any increase in exchange current density for the reduction of polysulfides in solution.

Acknowledgements
This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.
References
(1) Elazari, R.; Salitra, G.; Talyosef, Y.; Grinblat, J.; Scordilis-Kelley, C.; Xiao, A.; Affinito, J.; Aurbach, D. J. Electrochem. Soc. 2010, 157 (10), A1131.
(2) Fan, F. Y.; Carter, W. C.; Chiang, Y.-M. Submitted 2015.
(3) Pang, Q.; Kundu, D.; Cuisinier, M.; Nazar, L. F. Nat. Commun. 2014, 5 (May), 4759.
(4) Yao, H.; Zheng, G.; Hsu, P.-C.; Kong, D.; Cha, J. J.; Li, W.; Seh, Z. W.; McDowell, M. T.; Yan, K.; Liang, Z.; Narasimhan, V. K.; Cui, Y. Nat. Commun. 2014, 5 (May), 3943.
(5) Zheng, G.; Zhang, Q.; Cha, J. J.; Yang, Y.; Li, W.; Seh, Z. W.; Cui, Y. Nano Lett. 2013, 13 (3), 1265–1270.

Figure Caption:
Figure 1. Potentiostatic intermittent titration technique (PITT) curves for the reduction of polysulfide solution (2.5 mol S/L as Li2S8 in triglyme) on carbon felt/Ti4O7 nanoparticle composite. The first current peak results from precipitation on the oxide surface, while the second results from precipitation on the carbon surface, which occurs at a ~100mV lower potential.