A Selective Ion Transport by Application of the Functionalized rGO as the Separator in Li-S Batteries

Lithium sulphur batteries (Li-S) are considered as a serious future candidate for portable energy storage systems, primarily due to their high energy density. Moreover, the cathode in Li-S batteries is based on the naturally abundant and cheap sulphur. However, in spite of attractive properties, the life time of Li-S batteries is still far from the optimum. One of the major reasons for the fast capacity degradation is connected to the polysulphide shuttle mechanism. Another reason for the low coloumbic efficiency and constant capacity fading are reactions of polysulphides with metallic lithium. An effective way to avoid the polysulphide shuttle is the use of an ion selective membrane between the sulphur cathode and the metallic lithium. For instance, the use of a lithium conductive ceramic membrane can lead to extended cycle life with a coloumbic efficiency over 99.9 % (Figure 1a – red symbols). This result highlights the importance of effective separation between the sulphur cathode and metallic lithium. To avoid problems related to the slow kinetics and increase the flexibility of the lithium ion conductive ceramics, we substituted the ceramic membrane with a chemically modified reduced graphene oxide (CMrGO). Reduced graphene oxide (rGO) offers numerous possibilities in terms of chemical and physical properties which can be tuned by a proper modification. In our approach we prepared a hydrophobic interlayer between the cathode and the anode with a function of stopping the polysulphide migration from the cathode side to the metallic lithium. A thin interlayer of rGO-R, where R=F or Ph-F or Ph-CF₃ was deposited on a fiberglass separator and tested in Li-S batteries as a separator. The use of hydrophobic interlayer significantly improved the cycling stability compared to the battery where only fiberglass separator was used (Figure 1a - blue and black symbols). Furthermore, the capacity decay was approximately about three times slower in the battery with the hydrophobic layer on the fibrous separator. On the other hand, the capacity stability of batteries with the ceramic membrane and those with the hydrophobic interlayer was very similar. Figure 1b shows the discharge-charge curve in the 10^th cycle. All three batteries showed a typical Li-S battery behaviour. The possible differences in the mechanism were checked with an analytical in operandomode technique recently developed in our laboratory. A detailed analysis of the results obtained by UV-Vis spectroscopy and a 4-electrode modified Swagelok cell suggested that the improved capacity retention could be attributed to the reduced migration of long chain polysulphides from the cathode to the metallic lithium and to the homogenous distribution of end discharge products during cycling.

Acknowledgement:

This research has received founding from the Slovenian Research Agency and the European Union Seventh Framework Programme under grant agreement No. 314515 (EUROLIS).

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