Study On Synthesis And Interfacial Behavior Of Garnet-type Li₇La₃Zr₂O₁₂ Solid Electrolyte

In this paper,garnet-type Li7La3Zr2O12 solid electrolyte was prepared by sol-gel method.The effects of in-situ modification and interfacial doping modification on the structure of Li7La3Zr2O12 solid electrolyte were investigated.The structural information of the synthesized Li7La3Zr2O12 solid electrolyte was obtained by Retvield crystal structure refinement method.The structural models of Li7La3Zr2O12 solid electrolyte and related interface were established by first principle calculation based on the obtained structure information.The migration mechanisms of Li+ in the interior and interface of Li7La3Zr2O12 solid electrolyte were studied.The application possibility of interface modified Li7La3Zr2O12 solid electrolyte in new batteries was explored.The main contents are as follows:Li7La3Zr2O12 solid electrolyte was prepared by sol-gel method.The results show that the ionic conductivity of the prepared Li7La3Zr2O12 reached 5.36×10-4 S cm-1 and the activation energy was reduced to 0.31eV,when the Al doping amount was 0.1 mol%,the initial Li content excessed 10 mol%,and the sintering process was carried out at 1050 ? for 6 h in a flowing air atmosphere.The results of first-principles calculations show that there are three vacancies for Li+ in the crystal structure of Li7La3Zr2O12,which are 24d tetrahedral gaps and 48g,96h octahedral gaps.Combined with the theoretical and actual values of the ion migration barrier,it is known that the Li+ migrates in the Li7La3Zr2O12 crystal structure by transiting between the tetrahedral gap and the octahedral gap.The effects of heat treatment on the surface structure and electrochemical properties of Li7La3Zr2O12 solid electrolyte were investigated.The results show that the Li7La3Zr2O12 solid electrolyte was heat-treated at 900? from 0h to 3 h,the Li occupancy ratio in octahedral gap in Li7La3Zr2O12 crystal on the surface decreased from 0.606(2)to 0.517(2),and the ionic conductivity of Li7La3Zr2O12 solid electrolyte was reduced from 5.36×10-4 S cm-1 to 6.37×10-6 S cm-1.On the surface of the Li7La3Zr2O12 solid electrolyte,the Li-O bond length in the tetrahedral gap is 2.1(8)A,and the Li-O bond length in the octahedral gap is 2.1(9)-2.8(8)A.First-principles calculations indicate that the migration of Li+from the tetrahedral gap to the octahedral gap requires to overcome 0.24 eV energy barrier,which is much lower than value(0.49 eV)required to overcome the migration of Li+ between tetrahedrons.The grain boundaries of Li7La3Zr2O12 were modified by microwave sintering and spark plasma sintering.The effects of in-situ modification on the internal grain boundary structure and electrochemical properties of Li7La3Zr2O12 were studied.The results show that the migration barrier of Li at the grain boundary of the synthesized Li7La3Zr2O12 crystal is 0.21 eV,which is smaller than the migration barrier of Li+ inside the crystal(0.33eV).The in-situ modification technique of rapid sintering can inhibit the grain growth of Li7La3Zr2O12,increase the grain boundary specific surface area,and reduce the migration barrier of Li in Li7La3Zr2O12 structure.By controlling the parameters of rapid sintering process,the specific surface area of Li7La3Zr2O12 can be increased from 23077 cm 2 g-1 to 34178 cm2 g-1,and the ionic conductivity can be improved from 5.37×10-4 S cm-1 to 8.84×10-4 S cm-1.The effects of magnetron sputtering interface doping on the structure and electrochemical properties of the electrode-Li7La3Zr2O12 interface were investigated.The results show that the nano-sized LiCoO2 micro-crystal interface buffer layer can be sputtered on the electrode-Li7La3Zr2O12 interface by magnetron sputtering.The 10 nm LiCoO2 microcrystalline interface buffer layer reduces the contact resistance at the electrode-electrolyte interface by 1279 ? with the activation energy of 0.29 eV.The first principle calculation shows that the migration barrier of Li+ at the grain boundary of the LiCoO2 microcrystalline interface buffer layer(0.097 eV)is lower than the migration barrier inside the crystal(0.47 eV),indicating that Li+ tends to migrate in the interface of LiCoO2 microcrystalline buffer layer.The stability of interfacial modified Li7La3Zr2O12 solid electrolyte in aqueous solution was studied.The research shows that the in-situ modified Li7La3Zr2O12 solid electrolyte exhibits good stability in aqueous solution.The overall impedance of Li7La3Zr2O12 is reduced by an order of magnitude(from 1700 ? to 120 ?)because that the contact area between the Li7La3Zr2O12 surface and the aqueous-based solution can be improved by the good wettability.When the material reacted with the aqueous solution for 48h,the pH stabilized at about 11 and the ionic conductivity reached 7.33×10-3 S cm-1.Li7La3Zr2O12 solid electrolyte has great potential in the application ofa queous-based batteries.The performance of interfacial modification of Li7La3Zr2O12 solid electrolyte synthesized by sol-gel method in button-type all-solid lithium ion battery was studied.The results show that the initial capacity of all solid-state lithium-ion battery reaches 149.5mAh g-1 under charge and discharge conditions of 0.1 C,3.0-4.3V,and 60 ? by in-situ modification of Li7La3Zr2O12 and electrolyte-positive interface doping modification.As the thickness of the interface modified layer increased to 20 nm,the capacity of the all-solid-state lithium-ion battery was reduced to 92.3 mAh g-1 after 50 cycles,while the cycle performance was significantly improved to 84.1%.