Preparation And Properties Of Li_1.5Al_0.5Ge_1.5（PO₄）₃ Solid Electrolyte And Its Battery

Solid state battery with solid electrolyte has the advantages of high security and high energy density,which has become the main research direction of lithium-ion battery.Among all kinds of solid electrolytes（such as oxides,sulfides and polymer solid electrolytes）,oxide solid electrolytes have the advantages of good environmental stability,wide electrochemical window,and good electrochemical stability.Show broad application prospects.Among them,the NASICON solid electrolyte Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）is considered to be one of the most promising oxide solid electrolytes,but it has high grain boundary impedance,low room temperature ionic conductivity,and poor sintering performance,High interface impedance and other issues limit its practical application.In order to obtain LAGP solid electrolytes with good performance,the reported sol-gel preparation method of LAGP solid electrolytes was optimized in this thesis.In order to obtain the optimal performance of LAGP solid electrolyte,the effects of different heat treatment temperatures and different sintering aids（SiO₂,LiBF₄,B₂O₃）on the ionic conductivity,grain boundary resistance,relative density and microscopic morphology of LAGP solid electrolyte were studied,and An all-solid-state lithium-ion battery was assembled based on LAGP-LiBF₄ solid electrolyte,and its electrochemical performance was analyzed.In order to solve the interface problem between LAGP solid electrolyte and electrode materials,a composite polymer electrolyte buffer layer was prepared and studied,and the electrochemical performance of the buffer layer for all-solid-state batteries was analyzed.The specific research work is as follows:（1）In order to optimize the reported sol-gel method for preparing LAGP solid electrolyte powders and sheets,the use of organic germanium（Ge（OCH₃）₄,Ge（OC₂H₅）₄）as germanium source is expensive and inorganic germanium is difficult.problems such as dissolution.A novel citrate sol-gel-combustion method for preparing LAGP solid electrolyte powders and sheets was proposed.The effects of different heat treatment conditions on the microstructure and electrochemical performance of LAGP solid electrolytes were studied.The results show that this method can successfully prepare LAGP solid electrolyte,and the ionic conductivity can reach 1.14×10^-4S/cm under the condition of sintering temperature of 800°C.（2）In order to further improve the room temperature ionic conductivity and sintering performance of LAGP solid electrolyte,the effects of different sintering aids（SiO₂,LiBF₄,B₂O₃）on the ionic conductivity,grain boundary impedance,relative density and microscopic morphology of LAGP solid electrolyte were studied,and studied the electrochemical performance of C/LAGP-LiBF₄/LiFe PO₄ solid-state batteries assembled using LAGP-LiBF₄ solid-state electrolytes.The results show that the ionic conductivity of LAGP electrolyte with LiBF4 sintering additive can achieve up to 3.21×10^-4S/cm,and the interface impedance can be effectively reduced.（3）In order to solve the interface problem between the LAGP solid electrolyte and the electrode material,an innovative method of filling the interface with a composite polymer electrolyte solution and removing the solvent to form a film was proposed.The electrochemical performance and microscopic morphology of PEO-LAGP composite polymer electrolyte membrane were studied,and the performance of Li/PEO-LAGP/LAGP/LiFe PO₄ battery with buffer layer was studied.The results show that the buffer layer reduces the total interface impedance to 316Ωand achieves a capacity retention rate of 96.1%after 60 cycles.