| Rechargeable lithium-ion batteries (LIBs) are considered as the most important energy storage and conversion equipment, which have been widely applied in communication facilities, portable electronics, stationary energy storage systems and electric vehicle and electrical/hybrid vehicles. Currently, graphite-based materials have been used as the most common anode material for commercial LIBs due to its low cost and high abundance. However, graphite-based anode materials suffer the safety issue derived from the Li dendrites formed on the surface when the potential approaches almost 0 V (vs. Li/Li+) at the end of Li insertion, and the capacity decay resulting from the formation of solid electrolyte interface (SEI) layer. Ti-based oxides (such as Li4Ti5O12 and TiO2) have been regarded as promising alternative anode materials to graphite because of better safety and cycling stability, which deliver higher operating voltage and small volume changes during repeated charge-discharge processes. Despite of these advantages, the practical application of Ti-based oxides is still largely restrained by the inherently kinetic problem derived from the low electronic conductivity and sluggish lithium ion diffusion. Thus, improving electronic conduction and lithium ion transport has been one of the main research direction for Ti-based oxide materials.In this dissertation, TiO2 and Li4Ti5O12 are selected as research objects based on the research status of lithium ion battery anode materials, especially, Ti-based lithium ion battery anode materials. Basing on the design of hierarchical porous, micro-/nanostructure, and multiphase complex structure, Hierarchically porous composite, Ellipsoid-like micro-nano structure Li4Ti5O12-TiO2 composite, Li4Ti5O12/TiO2/C nanocrystallines composite and porous TiO2 aerogels have been successfully synthesized through Sol-gel method. The Sol-gel preparation mechanisms, the relationships between the microstructure (size、morphology and multiphase composite) and electrochemical performances have been investigated. The functional mechanisms of nano-sizes, porous structure, micro/nano structure and multiphase composite have been revealed. The rate capability and cycle stability of Ti-based oxides have been improved by morphology tailoring and microstructure controlling. Then, a solution for the practical application for Ti-based oxides has been provided in the dissertation. The main results are as follows:(1) Preparation and lithium storage performance of hierarchically porous TiO2/C composite. TiO2/C composites were synthesized by a facile sol-gel process followed by post-calcination. TiOSO4·H2O is utilized as the titanium source, PVP acts as a phase separation inducer as well as a carbon source. The as-prepared TiO2 based composites possess an interesting hierarchically porous structure constructed by cocontinuous macropores (1-3 μm in size) and mesoporous skeletons consisting of interconnected nanocrystals (10-30 nm in size) and in situ distributed carbon derived from the pyrolysis of PVP. The as-prepared hierarchically porous TiO2/C composite shows excellent electrochemical performance, which delivers a remarkable discharge capacity of 132 mAh g-1 at 1 C after 100 cycles, and the specific capacity could keep over 96 mAh g-1 even at a high rate of 30 C. Such unique hierarchically porous structure, high specific surface area and incorporation of conductive carbon materials result in excellen cycling stability and rate capability.(2) Preparation and lithium storage performance of ellipsoid-like micro/nano structured Li4Ti5O12-TiO2 composite. The Li4Ti5012-Ti02 composite was successfully synthesized via a facile sol-gel route with tetrabutyl titanate as the titanium source, lithium acetate dihydrate as the lithium source and glacial acetic acid as the catalyst for gelation. The as-prepared products present an ellipsoid-like morphology with a typical size in the range of 3-5μm, which are constructed by numerous nanocrystals with sizes of about 30-80 nm and consisted of lithium titanate, anatase and rutile. Meanwhile, abundant phase interfaces are detected from the Li4Ti5O12-TiO2 composite. As an anode material for lithium ion batteries, Li4Ti5O12-TiO2 presents excellent electrochemical performance, which delivers a reversible capacity over 92 mAh g-1 at 10 C after 100 cycles as well as an acceptable rate capacity of 89 mAh g-1 at a high rate of 30 C. The micro/nano porous structure and multiphase complex are demonstrated an effective strategy to improve the electrochemical performance of Li4Ti5O12.(3) Preparation and lithium storage performance of Li4Ti5O12/TiO2/C nanocrystallines composite. A facile sol-gel strategy is developed for the fabrication of Li4Ti5O12/TiO2/C composite with tetrabutyl titanate as the titanium source, lithium acetate dihydrate as the lithium source and glacial acetic acid as the catalyst for gelation. The Li4Ti5O12/TiO2/C composite presents a dual phase component consisting of Li4Ti5O12 and anatase TiO2, and composed of uniform nanocrystalline particles of in the range of 50-100 nm. The Li4Ti5O12/TiO2/C composite shows enhanced rate capability and cycle stability in comparison with pure Li4Ti5O12 and composite. After 100 cycles, the reversible capacity of the composite is measured to be 102 mAh g-1 with a high cycle stability, which is about 80% of the initial capacity, and a high capacity of 88 mAh g-1 can be achieved at 30 C. The superior electrochemical performance of the Li4Ti5O12/TiO2/C nanocrystalline composite can be attributed to the synergistic effects of nanocrystalline structure, abundant grain boundaries and phase interfaces, and in situ formed carbon.(4) Preparation and lithium storage performance of porous TiO2 aerogel. Tetrabutyl titanate and triblock copolymer F127 were used as the titanium source and surface modifier, respectively. Porous TiO2 aerogels were successfully synthesized by a facile sol-gel process followed by ambient pressure drying. The electrochemical properties of the resultant TiO2 aerogels were firstly evaluated in detail. The as-obtained aerogels exhibit high porosity with a high BET surface area of 431 m2·g-1,which are converted to anatase TiO2 with crystallinity after the heat treatment at 900℃, and the specific surface area decreases to 131 m2·g-1 The TiO2 aerogel electrode calcined at 800℃ presents the best cycle performance, which delivers an initial discharge capacity of 180 mAh g-1 and charge capacity of 135 mAh g-1 with a coulombic efficiency of 75%, and the reversible capacity can still remain above 114 mAh g-1 after 100 cycles. The synergistic effect of uniform pore structure, high specific surface area and good crystallinity contributed to the better electrochemical performance. |
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