Study Of Lithium Expulsion From The Solid-state Electrolyte Li_6.4La₃Zr_1.4Ta_0.6O₁₂ By Controlled Electron Injection In A SEM

This dissertation works in advanced scanning electron microscopes,and uses different electron beam modes to observe the dynamic behavior of solid electrolyte Li6.4La3Zr1.4Ta0.6O12 under controllable electron beam irradiation,and analyzes and summarizes its variation law.Based on variation law,the root causes of this behavior changes were explained.The dissertation covers the following aspects:（1）Lithium metal precipitation induced by the electron beam and the effect of different irradiation modes.Using SEM-EDS for the first time through the increase of oxygen content,it was confirmed that the precipitated particles were metallic lithium after electron beam irradiation on the surface of solid electrolyte Li_6.4La₃Zr_1.4Ta_0.6O₁₂.Lithium metal particles will grow around the irradiation point in the form of a ring（single ring or concentric double ring）under stationary electron beam irradiation;With the irradiation time prolonged,the precipitation rate of lithium metal is gradually reduced,basically in line with the index growth curve.In the continuous scanning parallel electron beam irradiation mode,different precipitation behaviors of lithium metal are revealed.According to the final size and shape of the lithium metal particles,the precipitation process of lithium can be divided into four categories:the first type is started to precipitate later,but the growth rate is very fast,and the final size of the precipitates is large and can reach the micron level;the second and third types of precipitation begin earlier than the first type by nearly half the time,but the growth rate is slow,and the final size can reach submicron and nanometer levels respectively.The fourth type of growth rate is rapid and can be quickly precipitated into rods,and the faster precipitation rate sometimes exceeds the electron beam scanning rate.（2）Lithium metal precipitation is regulated by the electron irradiation dose.The time when the lithium metal starts to precipitate is not greatly related to the accelerating voltage of the electron beam and the beam current,and is only related to the electron dose,that is,after the electron dose irradiated to the sample reaches a certain amount（100 C）,the amount of electron dose can reach the requirement for lithium metal precipitation.（3）Analysis of phase components nonuniformity and the effect on lithium precipitation rate.Different from the XRD results,EDS-Mapping imaging showed that the composition of the solid electrolyte Li_6.4La₃Zr_1.4Ta_0.6O₁₂ is inhomogeneous,and according to the quantitative results,three different areas can be divided:zirconium-rich area,tantalum-rich area and matrix area.In the zirconium-rich area(R_Zr),the content of zirconium is the highest,reaching to 10.52%,and the content of tantalum is very low and almost free of tantalum.In the tantalum-rich area(R_Ta),the concentration of Ta is basically the same as that of Zr,and the content of tantalum is higher than that of the matrix(R_Mx).The matrix region is a uniform single cubic Li_6.4La₃Zr_1.4Ta_0.6O₁₂ phase.Under the electron beam irradiation,lithium metal is first precipitated in the zirconium-rich area.As the irradiation time increases,lithium metal begins to precipitate at the interface between the matrix region and tantalum-rich area or the matrix area,and lithium metal particles is then precipitated in the tantalum-rich area,in which substantially lithium precipitation may not occur.The zirconium-rich area precipitates lithium metal as early as possible,and the particle distribution is relatively dense,but its size is very small;there are fewer lithium metal particles precipitated in the rich tantalum region,and there is a tendency to precipitate at the intersection of the tantalum-rich area and the matrix area.The volume of lithium metal particles precipitated in the tantalum-rich area is very large;The growth of two kinds of growth in the zirconium-rich and tantalum-rich area is basically covered in the matrix region.When the current density is reduced,the difference in particle size of lithium metal precipitated in different regions decreases.（4）The internal driving principle of lithium ion precipitation.By a certain accelerating voltage,the incident electrons can penetrate into the bulk electrolyte,thus trapping themselves in the electrically insulating LLZTO lattice.As the irradiation time increases,a large number of electrons generate an internal electric field at depths of several hundred nanometers below the surface layer of the material,attracting lithium ions near the electric field to the electric field region.At the same time,lithium ions accumulated under the action of the electric field will be expelled to the surface layer,and combine with electrons to produces elemental lithium.As the accelerating voltage increases,the electron beam will gain more energy,which will increase the penetration depth of the electron beam,and form a larger depth and stronger field electric field in the electrolyte.and the driving force for lithium ion migration will be greater to motivate the ejection of lithium ion.The effect of composition non-uniformity on lithium precipitation may be related to the electron conductivity.There is the highest electron conductivity in the tantalum-rich area,and the electron conductivity in the zirconium-rich area is the lowest.The lower the electron conductivity,the more favorable the precipitation of lithium metal.