Si@TiO₂ Yolk-Shell Composite Materials For Li-Ion Battery Anodes

With the development of science and technology, the development of rechargeable lithium ion batteries whit high energy capacity, long cycle life, and safety, is very important for lithium ion batteries. Si and TiO2 are the new generation of lithium ion battery anode materials with much focus and attention, silicon presents a maximum theoretical capacity (4200 mAh/g) and abundant source in earth, but there is as high as 400% of the volume expansion; TiO2 has higher intercalation potential, small volume change and good cycling stability, but the theoretical capacity is very low. This paper presents a new ideas of Si@TiO2 yolk -shell structure composite, obtained anode materials with excellent performance, try to overcome the shortcomings of Si and TiO2 through the synergistic effect of the material and the new design of structure.First, this paper used HPC-assist method prepared Si@TiO2 yolk -shell structure composite, determined TEOS hydrolysis method is a better of method for forming cavity. By adjusting the amount of TEOS and TBOT of HPC-assist method, can adjust structure parameters of the cavity size and the shell thickness of Si@TiO2 yolk -shell structure composite, obtain multiple Si@TiO2 yolk -shell structure composite with different structure parameters. In the design range of cavity size and the shell thickness, increase the size of cavity is conducive to the first cycle efficiency and cycle stability improvement; on the contrary, reduce the shell thickness, the capacity and cycle performancewill increase. The HPC-assist method of Si@TiO2 yolk -shell structure composite sample Ms3, initial discharge capacity is 2459 mAh/g, the first time cycle efficiency is 58%, a hundred times cycle capacity is 288 mAh/g, capacity retention rate is 11.7%.Second, this paper prepared Si@TiO2 yolk -shell structure composite withPVP-assist method at room temperature, can obtained larger specific surface area and porosity with different surfactant and titanium source. With similar shell thickness, specific surface area of MR1 is 112.3539 m2/g, specific surface area of Ms3 prerared by HPC-assist only 53.3337 m2/g. The sample MR1 prepared by PVP-assist method, initial discharge capacity is 3095mAh/g, the first time cycle efficiency is 53.73%, a hundred times cycle capacity is 245mAh/g, capacity retention rate is 7.92%. Higher specific surface area and porosity of the samples can enhance first charge discharge capacity of sample, but will reduce the first time cycle efficiency and a hundred times cycle capacity retention rate.Finally, in order to obtain better materials, Si@TiO2 yolk -shell structure composite was prepared by direct hydrolysis method, this paper studied the impact of surfactant on the structure and properties of the product. Although, introduct surfactant in the process is conducive to make the shell more uniform, but the surfactant is a disadvantage for the performance of lithium ion battery. With the same shell thickness, the sample performance of direct hydrolysis method was far superior to HPC-assist method and PVP-assist method, even compared with the sample prepared by HPC-assist method and PVP-assist method with thinner shell thickness, direct hydrolysis method samples MD1 also has a batter lithium ion battery performance, it has a initial discharge capacity of 3123mAh/g, a hundred times cycle capacity is 480 mAh/g, capacity retention rate is 15.37%.