Researchers at Toyohashi University of Technology in Japan have developed an active sulfur material and carbon nanofiber (S-CNF) composite material for all-solid-state Li-sulfur batteries using a low-cost and straightforward liquid phase process.

In a paper published in the journal ACS Applied Energy Materials, they report that in an all-solid-state Li–S cell using the S-CNF composite, full sulfur capacity was obtained when the cell was cycled at 0.1C (0.177 mA cm^–2), and a sulfur capacity of ∼600 mA h g^–1_S was maintained at 1C (1.77 mA cm^–2).

Schematic images and electron microscope photograph of sulfur-carbon composites (top). Schematic images and cycle characteristics of all-solid-state sulfur battery (bottom). Copyright Toyohashi University Of Technology

All-solid-state lithium-sulfur batteries have attracted attention because of their potential 5x higher energy density than conventional lithium-ion secondary batteries. However, sulfur, which is an insulator, must be provided with an ionic and electron-conductive path. Another problem that Li–S batteries typically face is capacity degradation during repeated cell cycling owing to the formation of polysulfide species, which migrate to the negative electrode and result in weight loss in the positive electrode.

Alternatively, the use of sulfide-based solid electrolytes offers a potential route to suppress the migration of polysulfide species to the negative electrode. Sulfide-based solid electrolytes (SEs) possess excellent ionic conductivity at RT, a unity lithium ion transport number, and unique mechanical properties when compared with polymer- or oxide-based solid electrolytes. Reports have shown that sulfide-based SEs, with a decent high ionic conductivity at RT, can be simply prepared by liquid-phase synthesis. In addition, the development of liquid-phase synthesis has led to improvements in the performance of ASSB (all-solid-state batteries) owing to the formation of a solid–solid interface.

In this study, we report the synthesis of solid electrolytes 0.67Li₃PS₄–0.33LiI (LPSI) and sulfur–carbon nano fiber (CNF)–LPSI composite for all-solid-state Li–S batteries.

—Phuc et al.

The team assembled a solid-state Li-S battery using LPsI and an In-Li alloy as separator and negative electrode, respectively. LPSI was pelletized at 200MPa and a S-CNF-SE composite powder was pressed onto one side of LPSI at 330MPa followed by attaching indium foil onto the other side of the LPSI by pressing at 200 MPa. Two stainless steel rods served as current collectors.

It is required that a sulfur active material and a carbon material are appropriately combined for making high-performance all-solid-state lithium sulfur batteries. Conventionally, sulfur-carbon composites were synthesized by mechanical mixing, liquid mixing using a special organic solvent and complicated methods, in which sulfur is combined with a porous carbon material with a high specific surface area. However, there were few reports that all-solid-state lithium sulfur batteries showed high capacity almost equivalent to the theoretical capacity of sulfur and high cycle stability. Therefore, we focused on making a sulfur-carbon composite using a low-cost and simple electrostatic adsorption method which can uniformly combine nanomaterials.

It was confirmed that sulfur at the sulfur-carbon composite synthesized by electrostatic adsorption method was accumulated on carbon nanofiber in the form of sheets. Besides, we constructed all-solid-state lithium sulfur batteries and found that sulfur was fully utilized as an active material. The other merit is that this sulfur-carbon composite can be produced by lower cost than conventional processes.

—first author, Assistant Prof. Nguyen Huu Huy Phuc

This method is a low-cost and relatively simple method for preparing sulfur-carbon composites, so it is suitable for mass production.

Resources

• Nguyen Huu Huy Phuc, Maeda Takaki, Hiroyuki Muto, Matsuda Reiko, Hikima Kazuhiro, and Atsunori Matsuda (2020). “Sulfur-Carbon Nano Fiber Composite Solid Electrolyte for all-solid-state Li-S Batteries,” ACS Applied Energy Materials doi: 10.1021/acsaem.9b02062