Solution-Based Preparation of Graphene-Li2s Composite Cathodes for Lithium/Sulfur and Lithium-Ion Batteries

Fully-lithiated lithium sulfide (Li₂S, theoretical capacity 1166 mAhg^-1) could be a more promising cathode material than sulfur (S), because it is compatible with safer and more stable Li-free anodes (such as graphite or silicon-based ones, to name a few). Unfortunately, Li₂S suffers from low electrical and low ionic conductivities as well as the dissolution and shuttling of lithium polysulfides during cycling, as S cathodes do ^[1]. Due to the high melting point of commercial Li₂S (950°C), the preparation of carbon-Li₂S composites by melt infiltration is difficult. The large particle size (30-50 µm) makes it challenging to achieve high capacity utilization of this cathode material. The ball milling has been the most common procedure utilized to reduce the size of commercial Li₂S powders to improve their rate performance and capacity utilization. Unfortunately, the ball milling procedure does not allow formation of uniform particles with controlled morphology, uniform carbon coating and good electrochemical performance ^{[2, 3]}.

In this research, we report for the first time a simple and versatile solution-based method to prepare the nano-sized Li₂S and graphene-Li₂S composites that does not induce release of any dangerous gases ^[4]. As shown in Figure 1, our finding of the lack of alcoholysis of Li₂S in ethanol combined with its relatively high solubility allowed the formation of nanostructured Li₂S by simple evaporating of ethanol from a Li₂S solution. The pure Li₂S powder demonstrates good uniformity of the homogeneously precipitated particles, their crystalline cuboid shape and an average size of ~200 nm. As expected, from SEM results, increasing graphene resulted in the reduction of both the average particle size and degree of the Li₂S nanoparticle agglomeration.

The addition of graphene and other nanostructured carbons in to the solution allows formation of Li₂S-containing composites with enhanced electrical conductivity and structural stability. Higher electrolyte molarity was found to enhance cycle stability of the prepared cathodes in half cells (vs. Li foil) due to the reduced polysulfide dissolution ^[5-7] and maintaining better mechanical properties of a polyvinylidene fluoride (PVDF) binder in a less swollen state ^[8]. The high rate performance and good cycle stability (less than 15 % degradation after 200 cycles in Figure 2) of the selected graphene-Li₂S nanocomposite cathodes suggest a great promise of the proposed method. The proposed methodology opens the door to solution-based fabrication of a variety of other Li₂S-containing composites. Further optimization of Li₂S-based composites and the formation of shells around the composite particles are explored to further enhance their performance in cells.

References

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[4] F. X. Wu, J. T. Lee, A. Magasinski, H. Kim and G. Yushin, 2013, in review.

[5]  J. T. Lee, Y. Zhao, S. Thieme, H. Kim, M. Oschatz, L. Borchardt, A. Magasinski, W. Cho, S. Kaskel and G. Yushin, Adv. Mater., 2013, DOI: 10.1002/adma.201301579.

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Acknowledgements

This work was partially supported by the Army Research Office (ARO grant W911NF-12-1-0259). Wu fellowship was supported by China Scholarship Council (No.201206370083).