A novel manufacturing approach to graphene-encapsulated sulfur and silicon nanoparticles for battery applications via freeze-drying micro-emulsion

• Applicant: Dr. Chuyen Van Pham, Ph.D.
• Subject Area: Synthesis and Properties of Functional Materials ; Chemical and Thermal Process Engineering
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 389154849

Sulfur and silicon have recently been considered as the most promising alternative cathodic and anodic materials, respectively for the next generation of Li-batteries. However, currently required performances of the known S/Si based Li cells are not sufficient. For S-based cells: this is due to: (i) the loss of sulfur during cycling due to the solubility of sulfur intermediate in the electrolytes; (ii) the 76% volume expansion/contraction which leads to disintegration of the electrodes; (iii) the low electrical conductivity (10^(-30) S/cm) of S. For the Si: Si anodes face material intrinsic challenges such as the large volume expansion (400%) and the chemical reaction of Si with electrolyte, leading to material pulverization and surface passivation, respectively. This project introduces a novel manufacturing approach to encapsulate both S and Si-nanoparticles (NPs) in reduced graphene oxide (rGO) nanostructures for Li battery applications via three steps: (i) fabrication of a micro-emulsion of S/Si dissolved in organic solvents as dispersed phase (S/Si-micelles) and GO aqueous solution as continuous phase (S/Si@GO micro-emulsion); (ii) freeze-drying of the S/Si@GO micro-emulsion; (iii) reduction of GO to rGO. During the micro-emulsion formation (step i), by using a cationic surfactant, the surface of S/Si-micelles becomes positive. Therefore, negatively charged GO flakes will attach to the positively charged surface of the spherical micelles, forming spherical GO-shells wrapping S/Si-micelles in a self-assembly manner. The freeze-drying (step ii) will evaporate both organic solvents in the micelles forming S/Si-NPs attached inside on GO-shells and frozen water in the continuous phase to create GO-nanoscaffolds. As a result, S/Si-NPs encapsulated in GO-shells integrated within GO-nanoscaffolds (S/Si@GO) are obtained. In step (iii), GO will be reduced to rGO to form S/Si@rGO. The reduction degree that determines the conductivity of rGO is controllable by reaction temperature and time.Scientific target is to demonstrate: (i) the formation of novel micro-emulsions comprising of S or Si-containing micelles encapsulated in GO-shells; (ii) the formation of GO-shells, on the basis of the static electrical attraction of negatively charged GO nanoflakes on positively charged surface of the micelles is obtained by using cationic surfactants.Technological significance: This project aims on achieving size control from molecular scale to nanoscale for efficiently encapsulating S/Si NPs in rGO shells, integrated within rGO-nanoscalfolds (S/Si@rGO). The S-loading and the rGO-nanostructured pores is controlled for an efficient sulfur utilization and confinement when used as cathodes in lithium/sulfur (Li-S) batteries (LSBs) . The final target is to overcome the challenges in LSB Technology. Also, this novel approach targets to achieve highly conformal rGO-encapsulated Si-NPs as anodic materials for Li-ion batteries.

• DFG Programme: Research Grants
• Major Instrumentation: Dynamic laser scattering and zeta potential
• Instrumentation Group: 1950 Partikelzählgeräte und -klassiergeräte (optisch, elektronisch, außer 35