Preparation Of Multi-shelled TiO₂ Hollow Spheres And Their Application As Anode Materials For Lithium-ion Batteries

As anode material for lithium ion batteries (LIBs). TiO2 anode is of high safety since unstable solid electrolyte interface on the electrode will not be formed because the discharge voltage of TiO2 is relatively high. Moreover, the volumetric expansion during the lithiation/delithiation processes is extremely small resulting in long cycle life. Additionally, the rate capability of TiO2 is excellent, which is very important for high power devices, for example, electric vehicles. However, the reversible capacity is kind of low. and the cycling performance and rate capability remains poor. Therefore, it’s very meaningful to synthesize TiO2 as anode materials with higher capacity, better cycling performance and high rate performance for LIBs. In this thesis, multi-shelled TiO2 hollow spheres have been synthesized successfully, and the LIB performance of the hollow spheres with various morphologies have been studied. The key results are shown below.Firstly, by using carbonaceous microspheres (CMS) as templates to adsorb cheap TiO2 aqueous solution followed by calcination to remove CMS templates, traditional multi-shelled from 1-3 shelled TiO2 hollow microspheres (1S-to 3S-TiO2-HMS) and multi-shelled from 2-4 shelled TiO2 hollow microspheres with close outer double shells (2S-to 4S-TiO2-HMS-CDS) have been prepared via tuning the absorbance and calcination process for the first time. These hierarchical micor-/nano-structures could take advantages of the composed nanoparticles which could shorten the charge transfer and lithium ion diffusion pathways, and the stable micro hollow structures which could release the volumetric expansion during the lithiation and delithiation processes guaranteeing the cycle life. Compared to MS-TiO2-HMS-CDS. the MS-TiO2-HMS show higher reversible capacities because of the thinner shells which could contact better with electrolyte. And the LIB performance is enhanced with the shell number increasing resulting from larger specific surface area which could increase the interfacial lithium storage. The 3S-TiO2-HMS shows a high specific capacity of 237 mAh g1 after 100 cycles at a current rate of 1C (167.5 mA g-1) and a high capacity of 129 mAh g-1 is achieved at a high current rate of 10 C. After 1200 long-term cycles at 5 C and 10 C, discharge specific capacities of 159 and 119 mAh g-1 are remained.Secondly, by changing the CMS to quasi-nanosized carbonaceous spheres (CNS) as templates and reducing the calcination temperature. nanosized TiO2 hollow spheres (TiO2-HNS) and mesoporous spheres (TiO2-PNS) composed of about 7 nm sized nanoparticles have been fabricated. Core-shell TiO2 nanospheres. mesoporous nanospheres composed of relatively loose nanoparticles. hollow nanospheres and mesoporous nanospheres composed of relatively larger nanoparticles have also been synthesized by tuning the adsorbing and calcination processes. The shell thickness of TiO2-HNS is as thin as single layerd nanoparticles (-7 nm) and the specific surface areas of these nanospheres are apparently improved which is beneficial for high rate performance due to short lithium ion diffusion length. The TiO2-HNS and TiO2-PNS show high capacities of 211.9 and 196.0 mAh g-1 after 100 cycles at a current rate of 1 C (167.5 mA g-1) and high capacities of 125.9 and 113.4 mAh g-1 at high current rate of 20 C (only 3 min for a charge process). After 3000 long-term cycles at high current rate of 20 C, high capacities of 103.0 and 110.2 mAh g-1 are achieved.Thirdly. TiO2-B is introduced to multi-shelled TiO2 hollow microspheres including single-shelled, core-shell, double-shelled anatase/TiO2-B hollow microspheres via CMS template method under hydrothermal conditions for the first time. The specific surface areas are greatly increased, especially double-shelled anatase/TiO2-B hollow microspheres (2S-AB-TiO2-HMS) with a high specific surface area of 209.4 m2 g-1 which could make the electrode/electrolyte contact thoroughly and provide more interfacial lithium storage as well as additional lithium storage in the two-phase boundaries. The 2S-AB-TiO2-HMS show a high reversible specific capacity of 215.4 mAh g-1 after 100 cycles at a current rate of 1 C (335 mA g-1) and a superior specific capacity of 148.0 mAh g"1 at a high current rate of 20 C. Stable specific capacities of 141.6 and 125.7 mAh g’1 are achieved after 1000 long-term cycles at high current rates of 10 C and 20 C, with only 0.03% of capacity loss per cycle at 20 C.Finally, applications of multi-shelled TiO2 hollow spheres in dye-sensitized solar cells (DSSCs) and photocatalysis areas have been explored. The power conversion efficiency of DSSCs could be enhanced by using multi-shelled TiO2 hollow microspheres as scattering layer on P25 TiO2, or by using TiO2-HNS as photoanode. Anatase/TiO2-B hollow microspheres could thoroughly degrade Rhodamine B.