In-situ Transmission Electron Microscopy Study On The Crystal Structure Transformation And Control Of Li₄Ti₅O₁₂ Electrode Material

Spinel structured Li₄Ti₅O₁₂is considered as one of the ideal electrode materials for commercial LIB due to excellent electrochemical properties.However,the key factor that limits the application of LTO is the low intrinsic electronic conductivity and ion diffusivity of LTO at room temperature.It is reported that external stimulus can lead to the migration of Li from the tetrahedral 8a sites to the octahedral 16c vacancies.The newly formed composite structure in which the rock salt structure LTO and spinel structure LTO coexist can significantly enhance the intrinsic electronic conductivity and ion diffusivity of LTO.However,the atomic-scale crystal structure transformation process and mechanism are still unclear.In addition,constructing heterojunctions or hetero-phase junctions can also effectively improve the intrinsic electronic conductivity and ion diffusivity of LTO.Transmission electron microscopy（TEM）is regarded as an important tool to study nanomaterials due to its ultra-high resolution.In-situ TEM technology is an important method in TEM research and the biggest feature is that the microstructure changes of nanomaterials under different external stimulus can be observed in real time at the atomic scale.Therefore,this article uses TEM as a research tool to study the dynamic evolution process of the LTO crystal structure through in-situ TEM technology and proposes the corresponding microscopic mechanism.The precise processing and control of the hetero-phase junction of LTO nanowires are realized,which provides a method for improving the electronic conductivity and ion diffusivity of LTO.The main research contents and results are as follows:（1）In this thesis,LTO nanosheets and LTO nanowires were synthesized by hydrothermal method and were analyzed and characterized by scanning electron microscopy（SEM）,X-ray diffraction（XRD）and transmission electron microscopy（TEM）.The results show that the synthesized LTO has uniform particle size and good crystallinity,and the corresponding crystal structure is spinel structure.（2）In this thesis,the dynamic evolution process of the crystal structure of LTO nanosheets was studied in real time at the atomic scale through in-situ electron beam irradiation.After detailed characterization,analysis and demonstration,the microscopic mechanism of the crystal structure transformation of LTO nanosheets is proposed.The results show that Li in the spinel structure LTO migrate from the tetrahedral 8a sites to octahedral 16c vacancies under the electron beam irradiation,forming a rock salt structure LTO.The phase transition rate is linear to the electron dose rate.The higher the electron beam dose rate,the faster the phase transition rate,and the shorter the time required to start the phase transition.The results provide theoretical guidance for improving the intrinsic electronic conductivity and ion diffusivity of LTO nanosheets.（3）In this thesis,the crystal structure transformation process of LTO nanowires was studied by in-situ TEM.We have realized the precise processing and control of the hetero-phase junction of LTO nanowires with different growth directions.The results provide a method for improving the intrinsic electronic conductivity and ion diffusivity of LTO.