Studies On Piezoelectric Materials LiTaO₃ And BaTiO₃ Surface Modification Of Li-rich Materials

Lithium-ion batteries,as a new energy material,have the advantages of green environmental protection,large specific energy,no memory,small volume and light weight compared with nickel-hydrogen battery.In recent years,the development of energy vehicles and electronic products requires the advancement of new energy batteries.High research value of lithium ion batteries due to the high energy density.However,the longevity of these high energy density lithium-ion batteries determines whether they can be used in a wide range of applications.This thesis mainly studies the surface modification of Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ with high energy density.Lithium-rich layered materials,with wide working voltage,high theoretical specific capacity and high energy density,have gradually become the research object of researchers.At the same time,the shortcomings of lithium-rich layered materials have also become an important cause of its application range,mainly including serious capacity degradation severe voltage decay and rate performance.The coulomb efficiency is low in the first week of the lithium-rich layered oxide due to the irreversible lithium ion,and the macroscopic phase transition in the cycle is characterized by voltage decay.The side effect of the material surface under high voltage affects the kinetic behavior of the lithium ion of the material,manifesting the poor rate performance.To solve the above problems,improve its electrochemical performance and structural stability is extremely urgent.Most researchers use surface coating and bulk doping to modify lithium-rich layered oxides.The above modification methods become common means to enhance the performance of lithium ion batteries.The electrochemical performance and thermal stability of lithium ion batteries are indeed improved by coating and doping in many reports,because these modification methods improve the structural stability of the materials.For the surface modification,as a protective layer,it can effectively inhibit the direct contact between material and electrolyte to suppress the growth of the CEI film,and improve lithium ion diffusion coefficient of the electrode and thermal safety of the battery to enhance the electrochemistry performance.In doping modification,lithium ions or excessive metal cations in the lithium-rich material are occupied by an element to suppress the formation of the heterophase,improve the structural stability of the material,and enhance the electrochemical performance of material.From this point of view,the coating means is more prominent for the structural integrity of the protective material.In this work,piezoelectric materials LiTaO₃ and BaTiO₃ were modified on the surface of Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ by sol-gel method to improve its electrochemical performances and the summary of specific works are as follows:1.LiTaO₃ was uniformly coated on the surface of the lithium-rich layered oxide Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ by sol-gel method,and the shape after coating was observed from the SEM.And the appearance has almost no effect.TEM proves that the uniform surface modification layer is coated on the surface of electrode material.XRD,XPS and Raman test results show that the coating does not have a great influence on the structure of the material.However,it can be testified from the electrochemical results that the long cycle,rate performance and thermal stability are greatly enhanced after modification an appropriate amount of LiTaO₃.In order to study the mechanism for the enhancement,in-situ XRD was tested.The results show that after the coating of the piezoelectric material LiTaO₃,the?003?peak shift upon charge and discharge is less obvious.It shows that the structural stability of the material is enhanced after modified LiTaO₃.Through the calculation of stress and strain,combined with the physical simulation software COMSOL,the voltages generated by the coating upon charge and discharge process are simulated,which facilitates lithium ion diffusion upon discharge process,resulting in more excellent electrochemical performance.2.BaTiO₃ was coated on the surface of Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ though sol-gel method.It can be seen from SEM and TEM that the coating has crystallized and does not affect the morphology of the original material.The XRD,XPS and Raman test results show that the coating have little effect on the structure of the pristine material.Electrochemical tests show that the cycle stability and rate performance are significantly improved after coating an appropriate amount of BaTiO₃.To further studying the mechanism of the enhancement,the in-situ XRD test is performed on the coated material.After the test,the coated material can be found,and the offset upon charge and discharge is weakened compared with the pristine,indicating that after coating the structural stability of the material has been improved.By using the calculation of stress and strain and the simulation software COMSOL,the voltages variation of the modification upon charge and discharge are obtained.The results show that the presence of the coating promotes the diffusion rate of lithium ions during discharge,resulting in more excellent electrochemical performance.