Synthesis And Electrochemical Performance Study Of Ga-doped Li₇La₃Zr₂O₁₂ Solid Electrolytes

Solid-state Li-ion electrolytes?SSEs?are recently being widely investigated for application in next generation all-solid-state batteries.Among them,garnet-type Li₇La₃Zr₂O₁₂ SSEs are considered as a promising candidate due to its superior chemical and electrochemical stability against ambient air and metallic Li anode.In the LLZO family with various doping elements,Ga-doped LLZO?Ga-LLZO?delivers the highest Li-ion conductivity of higher than 1×10^-3 S cm^-1.However,Ga-LLZO severely suffered from impurity phase and overgrowth grain problems via conventional solid-state reaction route,resulting in poor mechanical strength and cell performance.Therefore,based on Ga-LLZO,this thesis carried out the research to solve the challenges in preparation and application.The major results are as follows:?1?Effects of Li excess on the ionic conductivity and microstructure of Li_6.4Ga_0.2La₃Zr₂O₁₂ ceramic.Usually the excess lithium salt is need to add into the starting materials to compensate the lithium loss during sintering.However,LiGaO₂and abnormal grain growth were found in Ga-LLZO pellets with 5 wt%Li excess.By decreasing the Li excess,the Li-ion conductivity of Ga-LLZO increases and the impurity phase is eliminated.The reason of this result is due to the competition between Li⁺and Ga³⁺in LLZO crystal lattice,and the Ga⁺will be replaced by excess lithium and produces LiGaO₂.?2?Inhibiting Ga-LLZO grain growth by adding MgO.After sintering at 1100°C for 320 min,the grain size of pristine Ga-LLZO was larger than 100?m while the Ga-LLZO with 6 wt%MgO added showed the uniform grain with the size of 5?m.Also,there is a significant increase of the relative density from 95%to 98%after MgO was added.However,MgO will reduce the conductivity of Ga-LLZO due to the side reaction and the formation of MgGa₂O₄.?3?Synthesis of high-quality Li_6.4Ga_0.2La₃Zr₂O₁₂ solid electrolyte by two-step sintering method.Pure cubic LLZO phase is obtained for Li_6.4Ga_0.2La₃Zr₂O₁₂ pellets after two-step sintering,and the pellets sintered at 1150°C for 1 min and 1000°C for 3h exhibit high Li-ion conductivity of 1.24×10^-3 S cm^-1 at 27°C,uniform grain size of5?m and high relative density of 97.3%.In addition,the Ga-LLZO pellets with homogeneous grain size exhibit improved stability against molten Li,and Li symmetric cells based on the Ga-LLZO pellets reach a critical current density of 0.7 mA cm^-2,realizing stable cycling of over 600 h at a high current density of 0.4 mA cm^-2 at 27°C.At the same time,the solid state battery with LiFePO₄ cathode deliver reversible capacity of 150 mA h g^-1 even after 50 cycles.?4?Study on the stability of Li/Ga-LLZO interface.Au coating layer and heat-treatment can effectively decrease the interfacial resistance from>1 M?·cm² of the pristine Li/Ga-LLZO to<10?·cm² of the heated Li/Au-coated Ga-LLZO.With sufficient contact with Li metal,the Li/Ga-LLZO/Li cell shows good plating/stripping performance.In addition,several important factors,such as the quality of Ga-LLZO,the cyclable areal capacity,that influence the critical current density of Ga-LLZO is summarized.Interestingly,a critical current density of>1.3 mA/cm² can be obtained via liquid electrolyte wetting the Li/Ga-LLZO interface.