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日本語

Lithium-ion secondary batteries have been used as power sources in a wide variety of applications such as smartphones and electric vehicles. All-solid-state batteries have also attracted attention as a candidate for next-generation batteries in recent years because of the increase in the use of hybrid and electric vehicles. In particular, all-solid-state lithium-sulfur batteries have attracted attention because they have a five times higher energy density than conventional lithium-ion secondary batteries. However, sulfur is an insulator, which thus limits their application in battery devices. In order to solve this issue, sulfur must be provided with an ionic and electron-conductive path.

Therefore, the research group prepared a cathode composite of sulfur active material and carbon nanofibers (CNF) by a novel liquid phase synthesis process. All-solid-state lithium sulfur batteries using sulfur-CNF composites and electrochemically stable Li₂S-P₂S₅-LiI solid electrolytes synthesized by liquid phase process showed high discharge capacity equivalent to the theoretical capacity of sulfur and maintained high capacity after repeated charge-discharge cycles.

"In order to produce high-performance all-solid-state lithium sulfur batteries, the, the sulfur and carbon materials must be properly compounded. Conventionally, sulfur-carbon composites were synthesized by mechanical pulverization using a ball milling, or by a complex process of liquid mixing using a special organic solvent, in which sulfur is combined with a porous carbon material with a high specific surface area. However, few batteries have been reported that exhibit a high capacity and high cycling stability which approaches the theoretical capacity of sulfur. Therefore, we focused on finding a simple process for fabrication of carbonaceous materials and sulfur composite. In the sulfur-carbon complex obtained by the newly developed process, it was confirmed that sulfur is accumulated on the carbon in a thin sheet form, suggesting that sulfur is fully utilized as an active material. In addition, the process is easier to manufacture than the conventional process", explained the first author of the paper, Assistant Prof. Nguyen Huu Huy Phuc.

The composite was successfully synthesis by a – two – step – process in which sulfur nano sheet was nucleated and grown on CNF by a newly discovered chemical reaction.

This method is relatively simple for preparing sulfur-carbon composites, so it is suitable for mass production. The practical application of this method to high energy density all-solid-state lithium sulfur batteries is expected to lead to a dramatic increase in the popularity of storage batteries for large power sources, especially for electric vehicles and home and office applications.

This study was supported by the Advanced Low Carbon Technology Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING, JPMJAL1301) program of the Japan Science and Technology Agency (JST).