Structure Design And Optimization Of Si/TiO₂Composite As Anode Material For Lithium Ion Batteries

Silicon has been considered the most promising anode material for lithium ion batteries (LIBs) since ithas much higher capacity (4200mA h g-1) in comparison with that of the commercialized graphite (372mAh g-1). In addition, Si has low insertion potential (below0.5V versus Li/Li+) and relative high abundance(26.3wt.%) in earth’s crust. Furthermore, Si is inexpensive and non-hazardous. However, the large volumechange during repeated alloying and de-alloying in cycling and the resulting short cycle life greatly hinderits practical application.In order to suppress the rapid capacity decay of Si, many attempts have been carried out to overcomeits large volume change through the preparation of nanostructured and low-dimensional Si and Si-basedcomposites. Chemically synthesized Si nanostructures, including nanowires, nano-crystals, core-shellnanofibers, nanotubes, nanospheres, nanoporous materials, Si-based composites such as Si-C, Si-N, Si-Ag,Si-TiN, Si-Ti and Si-Al2O3have demonstrated superior performances compared with their pristine Sinanoparticles.Another important factor which hinders its commercialization is whether the SEI film formed betweenelectrolyte and active material is stable or not. As well accepted, there will be tremendous side reactions ifstable SEI can not be formed, which leads to poor electrochemical performances. The method of coatinghas been widely employed to stabilize the SEI film.In our previous work, Si/TiO2composite was prepared by a hydrothermal method, which exhibitsimproved electrochemical performances. However, the coverage of TiO2outside the Si core is incompleteand inhomogeneous, and the volume change of Si can not be effectively constrained, which leads to fastcapacity decay in cycling (only395mAh g1remained after50cycles).In the present study, Si/TiO2composites were prepared by sol-gel method and Atomic layer deposition(ALD) technique. Physical properties and electrochemical performances were extensively compared. Thisthesis mainly comprises the following two aspects:1. Pineapple structured Si/TiO2composites were synthesized by a simple sol-gel method. X-raydiffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanningelectron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) are utilizedto characterize the structure, component, chemical environment and morphology of the composite. Theinvestigation in cycling performances demonstrates that Si/TiO2with molar ratio of1:4exhibits the bestcycling stability, with specific capacity of593mAh g1after50cycles at0.1C, much higher than those ofthe other composites and the pristine material. Cyclic voltammetry (CV) profiles are also measured andcompared. It is believed that the outside TiO2particles act as buffer against the huge volume change of Siduring repeated alloying and de-alloying, which explains the improved electrochemical performances.2. Si/TiO2composite was synthesized by atomic layer deposition (ALD). X-ray diffraction (XRD),Raman spectroscopy, and high resolution transmission electron microscopy (HRTEM) are utilized to characterize the structure, component and morphology of the composite. The investigation inelectrochemical performances shows that Si covered with the2nm TiO2layer made by ALD exhibits thebest performance, with specific capacity of776mAh g1after50cycles at0.1C, much higher than thepristine material, which is attributed to the desirable buffering effect of TiO2coating layer.