Preparation And Modification Of Titanium Based Anode Materials For Lithium Ion Batteries

With the depletion of traditional energy and the growing environmental problems,energy crisis and environmental pollution have become two key issues affecting human social life.Developing efficient energy storage systems is an effective way to resolve these two problems.Lithium-ion batteries have become the most popular energy storage system because of their high energy density,good safety,low self-discharge rate,wide application temperature range,and lightweight.Lithium titanate(Li₄Ti₅O₁₂)material not only has the advantages of high safety,long cycle life,but also an ultrahigh power density.However,the poor electrical and ionic conductivity of Li₄Ti₅O₁₂ limit its rate capability,which need to be improved by modified.In addition,as a new kind of titanium-based material,two-dimensional layered Ti₃C₂T_x has broad application prospects in electrochemical storage because of its excellent conductivity,and good lithium storage performance.In this thesis,the agglomeration of Li₄Ti₅O₁₂ particles in common synthesis is dispersed by introducing a special structure of carbon material.The introduction of carbon can enhance the electrical and ionic conductivity of Li₄Ti₅O_12.Furthermore,a metal oxide-modified Li₄Ti₅O₁₂ is prepared by a simple and highly efficient method.Both the capacity and the electronic conductivity of modified Li₄Ti₅O₁₂ are significantly improved without the reduction of tap density.Finally,a new titanium-based material Ti₃C₂T_x is synthesized.Some preliminary researches are carried out on the expanded and exfoliated Ti₃C₂T_x.The main results are as follows:1.Li₄Ti₅O₁₂ nanosheets（LTONS）embedded in three-dimensional amorphous carbon are fabricated by a facile hydrothermal method.X-ray diffraction（XRD）and Raman analysis confirm that Li₄Ti₅O₁₂ by adding 3DAC（LTONS@3DAC）effectively prevent the generation of Li₂TiO₃ phase,which obtains the pure phase Li₄Ti₅O₁₂.Scanning electron microscopy（SEM）and transmission electron microscopy（TEM）indicate that the three-dimensional porous structure of the 3DAC inhibits the stacking of Li₄Ti₅O₁₂nanosheets and enhances lithium ion diffusion coefficient.Compared with LTONS,LTONS@3DAC shows better eletrochemcial performance,which exhibits outstanding rate capability of 146,142,and 140 m Ah g^-1 at the current rates of 32 C,48C,and 64 C,respectively,and superior cycle performance of 140 m Ah g^-1 after 700 cycles at 8 C.2.MoO₂ nanoparticle-modified Li₄Ti₅O₁₂ materials are prepared via a facile solution-based method.XRD and HRTEM analyses reveal that the MoO₂ nanoparticles are anchored on the surface of spinel Li₄Ti₅O₁₂,and energy dispersive X-ray spectroscopy（EDX）mapping indicates the MoO₂ nanoparticles distributed uniformly.Compared with the pristine Li₄Ti₅O₁₂,the appropriate amount of MoO₂ on the surface of the Li₄Ti₅O₁₂ could not only effectively reduce the electrochemical polarization of Li₄Ti₅O₁₂,but also contribute to specific capacity for lithium storage.The 3%MoO₂modified Li₄Ti₅O₁₂ exhibits excellent rate capability of 116 m Ah g^-1 at 10 C,and superior cycling performance of 124 m Ah g^-1 after 450 cycles at 5 C.3.Expanded Ti₃C₂T_x（i-Ti₃C₂T_x）is prepared by intercalation of tetrabutylammonium hydroxide into layered Ti₃C₂T_x.XRD and BET analysis confirm that i-Ti₃C₂T_x has a larger interlayer spacing（1.983 nm）and a higher specific surface area(8.176 m² g^-1).Compared with Ti₃C₂T_x,i-Ti₃C₂T_x exhibits better electrochemical performance with a reversible capacity of 108 m Ah g^-1 at 3000 mA g^-1.In addition,by analyzing the discharge behavior of Ti₃C₂T_x electrode in three electrolyte systems and characterizing the Ti₃C₂T_x electrodes after discharge,we find out that some electrolyte begins to decompose when the solvent molecules intercalate between the Ti₃C₂T_x layers in the TMP-based electrolyte.The generated gas during this process exfoliates the Ti₃C₂T_x layer,producing a thin layer of exfoliated Ti₃C₂T_x.The voltage windows for exfoliating is also discussed.