Projects

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. For this purpose, the project focuses on the following objectives which correspond to working packages (WPs): WP 1. Development of S@GO micro-emulsion formula. The formation of a new micro-emulsion system is a challenge since it will be influenced by many factors such as surfactants and co-surfactant properties, present ions, and PH of the phases [51,52]. Here, GO and sulfur are introduced into the water and oil phase, respectively, which also affects the formation of the micro-emulsion. Aim of the WP is to establish a standard manufacturing approach, to find the appropriate organic solvents for S-containing oil phase formation and the suitable surfactants and co-surfactants as emulsifiers for this novel micro-emulsion. Moreover, this part will investigate how to control the attachment of GO nanosheets to the surface of the S-micelles for the formation of the wrapping GO-shells by engineering the surface charge of the micelles (Fig.1 b and e). The final target is to achieve the right S@GO micro-emulsion. WP 2. Freeze-drying of the S@GO micro-emulsion. In this part, a process is developed to achieve the encapsulation of S-NPs in GO-shells, integrated in GO-nanoscaffolds, S@GO. The as-received S@GO micro-emulsion will be freeze-dried to obtain the microporous structures. In this process, the organic solvents in the micelles evaporate and consequently S-NPs are formed. Also, frozen water will be removed to form GO-shells integrated within GO-nanoscaffolds. WP 3. Control of S-loading, GO-shell size and pore-sizes of GO-nanoscaffolds. Based on the results of WP 1 and WP 2, the range of morphologic tuning based on control paramenters in the S@GO micro-emulsion formula is determined.It is anticipated that the sizes of the S-NPs, GO- shells, and the pore size of GO-nanoscaffolds can be controlled via tuning the micelle sizes, the concentration of sulfur in oil phase and GO in water phase. WP 4. Reduction of GO to rGO to enhance its conductivity. This step is performed to overcome the low conductivity problem of sulfur. To preserve the nanostructures of S@GO, GO will be reduced to rGO in reductant vapor such as hydrazine, so that its conductivity increases. The conductivity will be controlled by adjusting the reduction degree of rGO via reaction time and temperature. WP 5. Battery assembly and electrochemical characterization of the produced materials. The novel materials are assembled into battery coin cells and are subsequently investigated in terms of charge discharge behavior and working stability. WP 6. Extension of the micro-emulsion approach to the synthesis of Si@rGO as anode materials for Li-batteries. Si-NPs will be synthesized by thermal annealing SiOx NPs. After surface modification, the Si-NPs can be dispersed in organic solvents to form the dispersed oil phase. Consequent steps are the same as those for the synthesis of S@rGO, except the GO reduction step for Si@rGO which can be performed by a thermal reduction.