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Generation of innovative Li2S@C hybrid materials via plasma processes
Active materials for Lithium metal/Sulfur/Air

Lithiumsulfid (Li2S) as battery material has attracted significant attention owing to low cost, natural abundance, eco-friendliness and the high theoretical energy density in comparison to lithium ion batteries. Since Li2S suffers from low ionic and electric conductivity as well as from volume changes during cell operation, extensive efforts have been made to develop conductive Li2S composite materials. One approach to enhance the electrical conductivity is the encapsulation of Li2S with carbon [1] or with electrical conducting polymers like polypyrrole [2]. A promising technique for the polymer encapsulation is the application of low-pressure plasma-polymerization, since it enables an environmentally friendly and solvent free synthesis route to generate various polymer coatings for high-performance cathode materials.
Here we present a novel combined method for the synthesis of Li2S@C nano hybrid material coated with a plasma polymer. The method is based on a sputtering process to integrate nano scaled carbon into the Li2S powder, generating the Li2S@C, followed by the polymerization and deposition of the thiophene monomer in the liquid phase on Li2S@C by a low-pressure plasma. It should be underlined that this method eliminates the use of cross linkers, solvents or other chemicals. The coatings, as well as the active material were characterized in terms of elemental composition (X-ray photoelectron spectroscopy (XPS), infrared spectroscopy), morphology (scanning electron microscopy) and their electrochemical properties. It turned out that the active material utilization as well as the capacity retention was significantly enhanced by carbon and polymer modification of Li2S. Compared to pristine Li2S, the capacity at 1 C was increased by 10 % and the capacity retention after 200 cycles was improved by 13 %.
The results show that the sputtering of carbon on Li2S combined with the plasma process for generating a polymer encapsulation is a powerful method for improving Li2S as cathode material for lithium-sulfur batteries. The electrical particle connection is enhanced by the carbon modification and the volume change during cell operation can be compensated by the cross-linked plasma polymer at simultaneous support of the charge transport through the polymer encapsulation of Li2S.

[1] C. Nan, Z. Lin, H. Liao, M.-K. Song, Y. Li, E.J. Cairns, Durable carbon-coated Li2S core–shell spheres for high performance lithium/sulfur cells, Journal of the American Chemical Society 136 (2014) 4659-4663.
[2] Z.W. Seh, H. Wang, P.-C. Hsu, Q. Zhang, W. Li, G. Zheng, H. Yao, Y. Cui, Facile synthesis of Li 2 S–polypyrrole composite structures for high-performance Li 2 S cathodes, Energy & Environmental Science 7 (2014) 672-676.

(attached file: ML-plasma source, active material)
Fig.1: The plasma source (left) and the plasma-polymerized thiophene coating deposited on a silicon wafer (middel top) with a microscopic image of the film (middle bottom, 400x magnification) and an SEM picture (right) of the active Material Li2S with nano-carbon and the plasma-polymerized thiophene.

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Sebastian Hirt, Sebastian Wennig, Bernd Oberschachtsiek, Angela Kruth

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