Facile Synthesis of Lithium Sulfide-Graphene Composite As Cathode Material for Lithium Batteries

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).