Preparation Of Doped Li₇La₃Zr₂O₁₂ Ceramics And The Correlation Between Their Composition,Structure And Performance

Garnet-type Li₇La₃Zr₂O₁₂（LLZO）solid electrolyte has good chemical and thermodynamic stability,and its ionic conductivity can reach10^-4 S cm^-1 at room temperature.However,the performances of existing LLZO ceramics are unstable,and the interfacial impedance between them and electrodes is too high,so they have not been successfully used in all-solid-state lithium-ion batteries.In this thesis,doped LLZO ceramics were selected as the research object.X-ray diffraction,scanning electron microscopy,electrochemical impedance spectroscopy,Galvanostatic charging/discharging test,and other characterizations were used to study the correlation between the composition,structure,and performance,which could help us to explore the best doping scheme and obtain the doped LLZO ceramics with good and stable performances.After the interfacial impedance of the doped LLZO ceramics was reduced by surface modification,they were tied to be applied to the all-solid-state lithium-ion batteries.The main research results are as follows:1.The effect of different contents of Ti⁴⁺,Nb⁵⁺,and Ta⁵⁺ions doping on the structure and properties of LLZO ceramic solid electrolyte was studied.Because Ti⁴⁺and Zr⁴⁺ions have the same valence and can not form a compensatory Li⁺ion vacancy after the integration,thus can not effectively promote the conversion of t-LLZO to c-LLZO.As a result,the Ti⁴⁺ions doped LLZO ceramics only have a relatively low ionic conductivity of about 10^-6 S cm^-1 at room temperature which is not ideal.However,when Nb⁵⁺and Ta⁵⁺replace Zr⁴⁺ions,compensatory Li⁺ion vacancies can be generated,so the room temperature ionic conductivity of the doped ceramics can reach the order of 10^-4 S cm^-1 which is relatively ideal.Among all Ti⁴⁺,Nb⁵⁺and Ta⁵⁺ion-doped LLZO ceramics prepared by the same process,the Ta⁵⁺ion-doped Li_6.4La₃Zr_1.4Ta_0.6O₁₂ has the highest ionic conductivity at room temperature of 2.34×10^-4 S cm^-1 and the lowest activation energy of 0.37 e V.Its relative density was 93.1%.It was also found that the ionic conductivity of the sample was further increased to 2.86×10^-4 S cm^-1 by the two-step heat treatment process of first high temperature（1280℃）and then low temperature（1180℃）.2.Y³⁺,Bi³⁺and Sr²⁺ions were doped into ceramic Li_6.4La₃Zr_1.4Ta_0.6O₁₂,respectively,to study their effect on the structure and properties of ceramic Li_6.4La₃Zr_1.4Ta_0.6O₁₂.The results show that the appropriate amount of Y³⁺ions can improve the sintering effect of ceramics,so as to further improve their properties.Bi³⁺ions can not be effectively doped into the LLZO lattice,but will produce impurity crystal phase,resulting in the decline of ceramic properties.The appropriate amount of Sr²⁺ions can significantly improve the occupancy of Li1（24d）points in LLZO ceramics,so that Li⁺ions can migrate along the shortest path.the room temperature ionic conductivity of double-doped ceramic samples can be as high as 8.54×10^-4 S cm^-1.After assembling the Sr²⁺and Ta⁵⁺ions co-doped ceramic samples into a symmetrical cell(Li|Li_6.6La_2.8Sr_0.2Zr_1.4Ta_0.6O₁₂|Li),it was found that the critical current density reached 0.4 m A cm^-2,which showed excellent electrochemical stability in charge-discharge cycles,experiencing 350quick charge-discharge cycles without failure.it shows potential for all-solid-state lithium-ion battery.3.The structure and performances of Li₂O-Ga₂O₃-Ge O₂-Gd₂O₃（LGGG）glass-doped LLZO ceramics were studied.The incorporation of LGGG glass greatly influences the crystal phase composition and microstructure of the LLZO ceramics.When the doping amount of LGGG was 3 wt.%,the crystalline phase of the LGGG-LLZO composite ceramics contained only c-LLZO,and the microstructure was most dense.The reason is that when the appropriate amount of LGGG glass is doped,Ga³⁺ions can promote the transformation of t-LLZO to c-LLZO in the LLZO crystal,while Gd³⁺and Ge⁴⁺ions can expand the bottleneck size of Li⁺ions migration,thus reducing the activation energy.The 3 wt.%LGGG-doped LLZO composite ceramic was assembled into a symmetric battery（Li|3wt.%LGGG-LLZO|Li）with a critical current density of 0.5 m A cm^-2;it also showed excellent electrochemical stability in the charging and discharging cycle and did not fail after 230 times.The results indicate that the composite electrolyte also has the potential to be used in all-solid-state lithium-ion batteries.4.Nano-silver ink was successfully prepared and used for the surface modification of Sr²⁺and Ta⁵⁺ions co-doped ceramic Li_6.6La_2.8Sr_0.2Zr_1.4Ta_0.6O₁₂.The wettability of the ceramics surface to lithium metal electrodes was improved obviously,and the interfacial impedance significantly decreased from 1300Ωto 320Ω.The symmetrical cell(Li-Ag|Li_6.6La_2.8Sr_0.2Zr_1.4Ta_0.6O₁₂|Li-Ag)assembled from this nano-silver ink surface-modified ceramic also showed excellent electrochemical stability.It remained stable even after 400 fast charge/discharge cycles at high current densities.The result shows that coating the surface of LLZO ceramic with special nano-silver ink is one of the effective methods to solve the solid-solid contact impedance problem between the solid electrolyte and electrode interface.The above results provide new ideas and theoretical guidance for the development and performance optimization of LLZO ceramic electrolytes and lithium-ion batteries.116 diagrams,17 tables,and 225 references...