Dual Functional Sulfur-Confined Cathode Material Derived from Organic Metal Sulfide
Authors: Wang Qian 
Published: February 2020

Li-S batteries have been intensively investigated for years, owing to their higher theoretical specific capacity (1672 mAh g-1), low cost and environmental benignity. However, they are still suffering from three innate problems of electronic insulation, volumetric expansion and shuttle effect, seriously hindering their practical applications.[1-2] Carbon-based materials have been reported as one of excellent hosts,[3] since their superior electron conductivity and various Microscopic pore structure, demonstrating space confinement effect on sulfur by physical capillary adsorption force. With further research, the nonpolar carbon-based hosts are discovered showing less affinity with the dissolved intermediates of lithium poly-sulfides (LPS), for the reason of big difference in polarity between the carbon host and LPS. Hence, a great deal of polar organic functional groups or inorganic compounds are introduced to combine with carbon materials, enduing the host with dual functions of physical limitation and chemical adsorption, which indeed display superior capacity retention.[4] Nevertheless, the complex synthesis process and costly polar compounds increase preparation difficulties and the cost of the sulfur cathode materials, which is not conducive to scale up.
In our previous work,[5-6] a kind of organic metal sulfide, containing the elements of carbon, nitrogen, metal and sulfur, was skillfully employed as the single source to prepare the nitrogen-doped carbon coated sulfur composite, through a single-precursor-decomposition and an in-situ oxidation. During the high-temperature decomposition, the elements of metal and sulfur were suposed to form metal sulfide molecules, and encapsulated inside the nitrogen-doped carbon that was originated from the elements of carbon and nitrogen. The metal sulfide (MS) in the nitrogen-doped carbon coated metal sulfide (MS@NC) precursor was oxidized into element sulfur inside the nitrogen-doped carbon shell by the oxidation agent of iodine. Wherein, the MS played the role of sulfur source and self-sacrificed template, so the obtained nitrogen-doped carbon coated sulfur (S@NC) maintained the outward appearance and the unique microscopic structure. Although the S@NC composite displayed quite good electrochemical performance, with space limitation provided with the carbon frame and the chemical adsorption of the well dispersed nitrogen/oxygen-containing functional groups. The chemical bonding capabilities of organic functional groups are limited. A great improvement of this approach could be made to further enhance the sulfur confinement effect.
Herein, the specific organic dual-metal sulfide is adopted as the source material to firstly synthesize the nitrogen-doped carbon coated ternary metal sulfide (ZnCuS@NC), as shown in Figure 1a and b. The cracked particle in Figure 1a demonstrates that the compound of ZnCuS is coated by a thin layer of carbon. And it shows the appearance of spheres stacked together (Figure 1b). During the following in-situ oxidized process, the ZnS is transformed into element sulfur. While the CuS, introduced as the chemical adsorption agent, is retained for its stable existence under the oxidation environment. The obtained S-CuS@NC composite exhibits unique structure that sulfur is well combined with CuS, and intactly coated with nitrogen-doped carbon. The generated LPS is not only limited by the outer carbon shell, but also chemically anchored by the distributed CuS and organic functional groups. Thereby, better performance will achieved for the preferable sulfur restriction effect.
Figure 1. SEM image of (a) the cracked particle and (b) the outline of ZnCuS@NC composite. (c) TEM image and (d) SEM iamge of the obtained S-CuS@NC composite.

References

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[6] Wang Q, Wang Z B, Li R R, et al. Flower-like Nitrogen-Oxygen-doped Carbon Encapsulating Sulfur Composite Synthesized via In-situ oxidation Approach, Chem. Eng. J. 2018, 345, 271-279.

Figure 1