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，目前已知的S/Si基锂电池的性能要求是不够的。对于基于基的电池：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/收缩导致电极解体；（iii） S的低电导率（10^(-30)S/cm）。对于Si: Si阳极面临材料固有的挑战，如体积膨胀（400%）和Si与电解质的化学反应，分别导致材料粉碎化和表面钝化。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）封装在还原氧化石墨烯（rGO）纳米结构中，用于锂电池的应用：(i)制造S/Si微乳液，溶解在有机溶剂中作为分散相（S/Si胶束），氧化石墨烯水溶液作为连续相（S/Si@GO微乳液）；（ii） S/Si@GO微乳液的冷冻干燥；（iii）将氧化石墨烯还原为氧化石墨烯。在微乳液形成过程（步骤1）中，通过使用阳离子表面活性剂，S/ si胶束表面变为正电荷。因此，带负电荷的氧化石墨烯薄片会附着在带正电荷的球形胶束表面，以自组装的方式形成包裹S/ si胶束的球形氧化石墨烯壳。冷冻干燥（第二步）将蒸发形成附着在氧化石墨烯外壳内部的S/Si-NPs胶束中的有机溶剂和连续相中的冷冻水，从而形成氧化石墨烯纳米支架。结果，得到了封装在氧化石墨烯壳中的S/Si-NPs，并集成在氧化石墨烯纳米支架内（S/Si@GO）。在步骤（iii）中，氧化石墨烯将被还原为rGO，形成S/Si@rGO。还原程度决定还原氧化石墨烯的电导率，反应温度和反应时间是可控的。科学目标是证明：(i)形成由氧化石墨烯壳包裹的含S或si胶束组成的新型微乳液；（ii）使用阳离子表面活性剂，使带负电荷的氧化石墨烯纳米片在带正电荷的胶束表面产生静电吸引，从而形成氧化石墨烯的壳层。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 NPs封装在还原氧化石墨烯壳中，并集成在还原氧化石墨烯纳米层中（S/Si@rGO）。在锂/硫电池（Li-S）中用作阴极时，硫负载和氧化石墨烯纳米结构孔隙被控制为有效的硫利用和限制。最终目标是克服LSB技术中的挑战。此外，这种新方法的目标是获得高度适形的rgo封装Si-NPs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.