Facile Synthesis of Lithium Sulfide-Graphene Composite As Cathode Material for Lithium Batteries
Authors: Li Zhe , Zhang Shiguo , Ueno Kazuhide , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

1. Introduction

Lithium sulfide (Li2S) with a high theoretical specific capacity of 1166 mAh/g has recently been adopted as a promising cathode-active material for lithium batteries, because of its conjunction with safer lithium-free anodes. However, Li2S is electronically and ionically insulating in nature, and has a high melting point, which are disadvantageous in term of the battery application. To overcome those main drawbacks in utilizing Li2S and enhance the electrochemical activity of Li2S, a variety of approaches have been made, with a focus on the Li2S-carbon composite. In this work, we propose a facile, low cost, scalable, and environmentally friendly strategy to prepare Li2S-graphene composite. Herein, the graphene material can partly supply the carbon source to reduce lithium sulfate at elevated temperature and form the Li2S in situ, while the remaining part can serve as conductive carbon matrix to immobilize the generated Li2S. When combined with a Li2S
x
(x = 8-4)-insoluble electrolyte developed by our group,1 the obtained Li2S-graphene composite demonstrates an excellent electrochemical performance.

2. 
Experimental

Graphene nanoplatelet aggregates (GNAs) and Li2SO4∙H2O were first dispersed into ultrapure water to form a suspension. The precipitant of Li2SO4 was then added dropwise to this suspension, which can allow the slow deposition of Li2SO4 onto the surface of GNAs. After filtering, washing, and drying, Li2SO4-GNAs composite was obtained. The Li2SO4-GNAs composite was then heated at high temperature, and ensured the sufficient reaction between Li2SO4 and the carbon (Li2SO4 + 2C→2CO2↑ + Li2S). Eventually, the resulting Li2S-graphene composite was ground into a fine powder for the cell preparation.

3. 
Results and discussion

Cycling performance of Li2S-graphene composite based electrode at 1/12 C is presented in Fig. 1, together with the result for a physical mixture of commercial Li2S and GNAs for comparison. This composite exhibits an initial discharge capacity of 693 mAh/g, which declines to 508 mAh/g after 40 cycles. When compared with the physical mixture, the Li2S-graphene composite based electrode exhibits much more excellent electrochemical properties. For the Li2S-graphene composite, the insulating Li2S is deposited uniformly on the graphene via this in-situ calcination method, providing an excellent electrochemical contact, which could benefit the improvement of reaction kinetics and significantly increase the utilization of active material. We also believe that this in-situ calcination approach is generally applicable, which may trigger research interests and provide a valuable reference for the low-cost preparation of other Li2S-C composites.

4. 
Acknowledgements

This research was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).

5. 
References

(1) K. Dokko, N. Tachikawa, K. Yamauchi, M. Tsuchiya, A. Yamazaki, E. Takashima, J. W. Park, K. Ueno, S. Seki, N. Serizawa, M. Watanabe, J. Electrochem. Soc.
160, A1304 (2013).

Figure 1