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

硫和硅最近被认为是最有前途的替代阴极和阳极材料，分别用于下一代锂电池。然而，已知的S/Si基Li电池目前所需的性能是不够的。对于基于S的细胞：这是由于：（i）由于硫中间体在电解质中的溶解度，在循环过程中硫的损失;（ii）76%的体积膨胀/收缩，这导致电极的分解;（iii）S的低电导率（10^（-30）S/cm）。对于Si：硅阳极面临材料固有的挑战，如大体积膨胀（400%）和硅与电解质的化学反应，分别导致材料粉碎和表面钝化。该项目介绍了一种新的制造方法，通过三个步骤将S和Si纳米颗粒（NPs）封装在用于锂电池应用的还原氧化石墨烯（rGO）纳米结构中：（i）制造溶解在有机溶剂中的S/Si微乳液作为分散相（ii）将S/Si @ GO微乳液冷冻干燥;（iii）将GO还原成rGO。在微乳液形成（步骤i）期间，通过使用阳离子表面活性剂，S/Si-胶束的表面变为正性。因此，带负电荷的GO薄片将附着到球形胶束的带正电荷的表面，以自组装方式形成包裹S/Si胶束的球形GO壳。冷冻干燥（步骤ii）将蒸发胶束中的有机溶剂和连续相中的冷冻水，以形成附着在GO-壳上的S/Si-NP，从而产生GO-纳米支架。结果，获得了封装在集成在GO-纳米支架内的GO-壳中的S/Si-NP（S/Si@GO）。在步骤（iii）中，GO将被还原成rGO以形成S/Si@rGO。通过反应温度和反应时间可以控制还原程度，从而决定rGO的导电性。科学目标是证明：（i）形成由GO壳中包裹的含S或Si胶束组成的新型微乳液;（ii）GO壳的形成，基于带负电荷的GO纳米片在胶束的带正电荷的表面上的静电吸引，通过使用技术意义：该项目旨在实现从分子尺度到纳米尺度的尺寸控制，以有效地将S/Si NP包封在rGO壳中，整合在rGO纳米折叠（S/Si@rGO）内。当用作锂/硫（Li-S）电池（LSB）中的阴极时，控制S负载和rGO纳米结构孔以实现有效的硫利用和限制。最终目标是克服LSB技术的挑战。此外，这种新方法的目标是实现高度共形的rGO封装的Si-NP作为锂离子电池的阳极材料。

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.