Schematic of the coating process for the Li2S@C spheres. Click to enlarge.

Lithium/sulfur cells are seen as a promising next-generation “beyond Li-ion” technology. With a high theoretical specific capacity of 1675 mAh g⁻¹ as well as very low cost, high abundance, and low environmental impact, sulfur is a promising cathode candidate. The downside has been that Li/S batteries suffer from poor cyclability. (Earlier post.)

Researchers have been making significant progress to improve the utilization of sulfur and alleviate the capacity fading via a number of approaches, including size control of the sulfur particles; coatings on sulfur particles; use of sulfur−carbon composites; trapping of polysulfides; and electrolyte modification.

However, use of a sulfur cathode requires use of a lithium metal or a lightweight lithiated anode for high specific energy. Lithium metal forms dendrites in conventional organic solvent-based electrolytes, causing shorting and safety concerns. In addition, some of the protective coatings on sulfur particles may be easily destroyed because sulfur undergoes a volume expansion of ∼76% when totally converted to Li₂S.

As a result, several research groups have been exploring the use of lithium sulfide (Li₂S)—i.e., lithiated sulfur—as an attractive cathode material for lithium/sulfur (Li/S) cells (e.g., earlier post).

[Li₂S] can be paired with different kinds of lithium metal free materials, such as the high capacity silicon anode. Moreover, compared with sulfur, Li₂S has a higher melting point and is in the maximum volume state, so modifications on Li₂S materials can be performed at a higher temperature and the surface coating can be more stable.

Nevertheless, the problems of low electronic conductivity, and the solubility of polysulfides in many electrolytes still exist for the Li₂S cathode. Thus, it is also essential to use carbon-containing composites, control particle size, and provide protection for the Li₂S active material. A protective and conductive shell on the surface of small Li₂S particles, which will not be broken by expansion during discharge, seems to be a candidate solution for using Li₂S material in Li/S cells. Recently, some exciting results concerning Li₂S cathodes were reported. However, most of the reports are based on commercial Li₂S powder or commercial Li₂S subjected to ball-milling, resulting in random particle sizes and morphologies, not ideal for surface coatings. There are almost no reports of synthesized Li₂S with uniform particle size and morphology so far.

Herein, we report for the first time an easy strategy to synthesize Li₂S spheres with size control.

—Nan et al.

The research team synthesized three different sizes—2 μm, 1 μm, and 500 nm—of Li₂S sphere to serve as the core material for a variety of coating methods. In the work reported in JACS, they coated the spheres with carbon by chemical vapor deposition (CVD) method to form stable carbon-coated Li₂S core−shell (Li₂S@C) structures.

In addition to the high initial discharge capacity of 972 mAh g⁻¹ Li₂S, the spheres showed good capacity retention after 100 cycles.

Resources

• Caiyun Nan, Zhan Lin, Honggang Liao, Min-Kyu Song, Yadong Li, and Elton J. Cairns (2014) “Durable Carbon-Coated Li₂S Core–Shell Spheres for High Performance Lithium/Sulfur Cells,” Journal of the American Chemical Society 136 (12), 4659-4663 doi: 10.1021/ja412943h