| Rechargeable lithium-ion batteries (LIBs) with high energy density and long lifetime have been regarded as promising energy storage and conversion devices for applications in electric vehicles (EVs) and smart grids. But the commercial LIBs based on the graphite anode material can hardly meet the requirements of those large-scale applications due to the limitations on power density and safety characteristics. Therefore, researchers paid more attention to novel anode candidates, among which lithium titanate (Li4Ti5O12, LTO) has been considered as the best alternative to graphite because of its excellent safety characteristics and ultra long lifetime. In this thesis, in order to improve the electrochemical performance of Li4Ti5O12, particular lithium titanate and its composites were prepared via different methods.In the first chapter, a general introduction is given on follow aspects:the history of LIBs’development, the structure and working mechanism of LIBs and characteristics of LIBs and electrode materials used in LIBs. Especially, the current survey on lithium titanate was reviewed carefully, and the importance of this thesis is also clarified.In the second chapter, a brief introduction to the chemicals and equipments used in the synthetic process was given. And a detailed description on structural analysis instruments, the process of making a coin cell and electrochemical measuring methods is presented.In the third chapter, homogeneous Li4Ti5O12/graphene composite is prepared via an in-situ solid state reaction, after carbon pre-coating has been carried out. Its microstructure is compared with the materials prepared by a similar way, but without carbon coating. The results reveal that the carbon coating not only effectively confines aggregation and agglomeration of the Li4Ti5O12particles, but also enhances the combination between Li4Ti5O12particles and graphene sheets. The Li4Ti5O12/graphene composite presents excellent rate capability and low-temperature performance. Even at120C, it still delivers a quite high capacity of about136mAh/g. When the charge-discharge tests are performed at-10℃and-20℃, its specific capacities are as high as149and102mAh/g, respectively.In the fourth chapter, effects of carbon morphologies and structures on the electrochemical performance of Li4Ti5O12/C composites were investigated carefully. Li4Ti5O12/C composites (LTO/C) with diverse carbon morphologies and structures were synthesized via a similar solid state method, along with carbon coating performed at different period. The carbon morphologies were characterized by means of electron microscopy, and the structures were further studied by Raman analysis. Effects of the coated carbon on lithium storage, especially high rate capability of LTO/C composites were investigated. The results revealed that the porous and homogeneous carbon layer contributed to both the good electronic conductivity and Li+insertion/extraction kinetics, and thus the corresponding LTO/C composite exhibited excellent cell performance. At low current rate of0.2C, its reversible capacity was as high as171mAh/g. At5C, the high capacity of150mAh g-1had no loss after300cycles. Especially at the high charge/discharge rate of30C, the reversible capacity still kept a high level of82mAh/g.Lithium titanate nanosheets with corrugated morphology were synthesized by a polyether surfactant assisted hydrothermal process. The Li4Ti5O12has an excellent rate capability and cycling performance:it delivers a quite high capacity of about130mAh/g at64C, and the capacity retention ratio is95.6%after400cycles at8C.In the last chapter, we give an overview on the achievement and the deficiency in this dissertation, and some suggestions and possible directions of future work are also pointed out. |
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