Researchers from UC Berkeley, Lawrence Berkeley National Laboratory, and Tsinghua University have synthesized lithium sulfide (Li2S) spheres that, with protective and conductive carbon shells, show promising specific capacities and cycling performance as a lithium/sulfur cell cathode material, with a high initial discharge capacity of 972 mAh g–1 Li2S (1,394 mAh g–1 S) at the 0.2C rate.
When no carbon black was added to the electrode mixture, a very high Li2S content (88 wt % Li2S) electrode composed of 98 wt % 1 μm Li2S@C spheres and 2 wt % binder showed rather stable cycling performance, and little morphology change after 400 cycles at the 0.5C rate. A paper on their work is published in the Journal of the American Chemical Society.
|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.
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