Coating of sulfur particles for Li-S batteries

Lithium sulfur battery (Li-S) is presently the most promising challenger for the next generation of commercial secondary (i.e. rechargeable) batteries with a theoretical energy density superior to 2500 Wh/kg. The practical performance is today limited to 350 Wh/kg (Sion Power Company). However, this is still higher than current Li-ion batteries which offer a maximum of 264 Wh/kg (US18650 from Sony Corporation). In addition, sulfur is abundant, low cost and usually considered as non-toxic.

The Li-S batteries suffer from several issues which hinder their large scale commercialization. Among them are limitations due to the sulfur in the positive electrode:
• Slow charge/discharge, characteristics of Li-S batteries attributed to the poor electronic conductivity of S8 and Li2S (final reduction product).
• Short lifespan, low coulombic efficiency and self-discharge due to polysulfides intermediates (Li2S8 and Li2S6) which are highly soluble in the electrolytic solution. They diffuse in the cell and react on the surface of the Li-metal negative electrode (polysulfide shuttle effect).

Conductive coatings on sulfur particles can increase their charge/discharge rate capability and mitigate the energetic density fading of Li-S batteries. The function of the coating is to enhance the electron transport in the positive electrode and to act as physical barrier for the polysulfides. However, currently state-of-the-art core/shell coatings on sulfur powders are obtained by slow and complex procedures which do not appear to be commercially up-scalable.

This PhD project, which is a collaboration between VITO and the Hasselt University, aims to develop new and improved processes for direct coating on sulfur powders (micron sized and sub-micron sized), either by wet solution based coating (Hasselt University) or dry coating routes using plasma technology (VITO). Thin (nm range) and homogeneous coatings are targeted based on conducting polymers and metal oxides.

The developed materials will be characterized by various techniques to determine the characteristics and effectiveness of the coatings. The electrochemical properties will be investigated using coin-cells assembled and tested in house. The most promising coatings will be applied on Li2S particles and corresponding Li-Li2S cells will be tested. All results of the coated materials will be compared with the pristine materials, S8 and Li2S.

Depending on the evolution of the PhD project, possible collaborations with the Paul Scherrer Institut (Switzerland) and/or the University of Padua (Italy) are foreseen.

We are looking for a motivated candidate for this PhD project holding a Master Degree in chemistry. A background in material chemistry, electrochemistry and/or plasma technology is preferred. Fluent English is mandatory, knowledge of Dutch is an asset.

For further information, please contact Dr. Sébastien Sallard ([Email Address Removed], tel: +32 14 33 51 75). Applications are registered online: http://wwwb.vito.be/VITODoctoraat/inschrijven/inschrijven.aspx?ID=522&Lang=EN.

Documents (CV, application letter, recommendation letter…) must be sent separatly by e-mail to the contact person.