While lithium-ion batteries (LIBs) have found wide applications in customer electronics, such as cellular phones and lap-top computers, the performance of the currently LIBs does not meet the market requirements for massive energy storage, such as electric vehicles and smart grids. One of the critical issues is the low energy capacity of the electrode materials. Graphite has been a common anode for LIBs, but the maximum capacity (372 mAh·g-1) of graphite has hindered its application in advanced LIBs. Substituting graphite with materials of high capacity has become an active research area. Graphene can store more Li+ than graphite because Li+ can not only be stored on both sides of graphene sheets, but also on the edges and covalent sites. Being electric conducting and mechanically strong, graphene is also an attractive support for other high capacity materials, such as silicon (Si). Si possesses the highest known theoretical capacity (4200 mAh·g-1, fully lithiated). However, Si suffers from a critical problem, namely severe volume change during charging/discharging, resulting in poor reversibility and rapid capacity decay. This PhD project aims to improve the stability and suitability of Si nanoparticles (SiNPs) for LIBs.
The first aspect in this thesis is to demonstrate a novel approach to wrapping SiNPs in a reduced graphene oxide (RGO) aerogel framework. The aerogel-typed RGO architecture not only provided a porous network to accommodate the volume change of entrapping SiNPs, but also facilitated electrolyte transport and electron transfer.
The second aspect in this thesis is to report a simple, green and scalable method to prepare RGO using gallic acid (GA) as a chemical reducing reagent. RGO samples reduced by GA showed a superior Li+ storage capacity compared with graphite and other reported graphene materials as anode for LIBs.
The third aspect in this thesis is to report on the preparation of RGO-stabilized SiNPs, a sandwich-typed composite material (Si@RGO). As a result, good battery performance of Si@RGO was obtained, 1074 mAh·g-1 at the 5th cycle and 900 mAh·g-1 at the 100th cycle with a capacity retention of about 84% as well as excellent rate capability.