Study On Preparation Process Of Al-doped Garnet Solid Electrolyte

Lithium-ion batteries（LIBs）were widely used in traditional portable electronic devices（such as mobile phones and laptops）,electric vehicles and electrochemical energy storage.However,a serial of safety issues occurred frequently for organic liquid electrolytes.All-solid-state batteries have been the best choice to replace traditional lithium-ion batteries due to their high energy density and excellent safety performance,and as key component of all-solid-state batteries,solid-state electrolytes have received widespread attention.Li7La3Zr2O12（LLZO）garnet solid electrolyte has a promising future in all-solid-state batteries because of its excellent performance,such as high ionic conductivity,stable contact against lithium metal anode and electrochemical window up to 6 V.Al-doped Li7La3Zr2O12（Al-LLZO）solid electrolyte was researched in this paper.Al-LLZO powder was synthesized by sol-gel method,and Al-LLZO garnet solid-state electrolyte was further prepared by different sintering processes.The effects of different process parameters on the phase composition,density,micromorphology,and ionic conductivity of Al-LLZO electrolyte were studied.The main research contents and results are as follows:Al-LLZO powder was synthesized by sol-gel method.The type of complexing agents would affect the phase composition of garnet solid electrolyte powder.The synthesized Li6.25Al0.25La3Zr2O12 electrolyte powder contains a small amount of La2Zr2O7 and La Al O3 impurity phases,which affect the electrolyte performance using water/ethanol mixed solution as complexing agent;when citric acid/ethylene glycol are used as the complexing agent,the pure cubic Li5.95Al0.35La3Zr2O12 electrolyte powder was obtained.Al-LLZO garnet solid electrolyte was prepared by atmospheric pressure sintering using two different electrolyte powders.The effect of sintering temperature on the phase composition and density of Al-LLZO electrolyte was studied.The Li6.25Al0.25La3Zr2O12 powder synthesized with water/ethanol as complexing agent was sintered at 1000℃for 10 h,and the density of electrolyte sample was 2.37 g/cm³;The Li5.95Al0.35La3Zr2O12 powder synthesized with citric acid/ethylene glycol as complexing agent was sintered at 1200℃for 2 h,and the density of electrolyte sample was 2.85 g/cm³.Al-LLZO garnet solid electrolyte was prepared by cold sintering process.The effects of cold sintering holding time,pressure,temperature and subsequent heat treatment on the properties of Li5.95Al0.35La3Zr2O12 electrolyte were studied.The results showed that density of Li5.95Al0.35La3Zr2O12 electrolyte,sintered at 680 MPa,250℃and 45 min holding time was lager,which was up to 2.99 g/cm³.The subsequent heat treatment after the cold sintering process had little effect on the phase composition of the sample,but it could improve ionic conduction properties;The crack was observed for too long cold sintering holding time.but the crack was observed for too long cold sintering holding time.It was effective to improve density and ionic conductivity by increasing cold sintering pressure.Li5.95Al0.35La3Zr2O12garnet solid electrolyte with higher ionic conductivity of 2.66×10^-6 S/cm could be obtained at200℃,510 MPa and 30 min,which exhibited lower grain boundary resistance.However,the density and ionic conductivity decreased when cold sintering pressure increased further,due to the decomposition of second phase and limitation of grain growth during heat treatment.Al-LLZO garnet solid electrolyte was prepared by spark plasma sintering,and the effects of sintering temperature and holding time on the density and ionic conductivity of Li5.95Al0.35La3Zr2O12 electrolyte were studied.The results showed that the density and ionic conductivity of the sample sintered at 1100℃was better.With the continuing increase of temperature,sample performance deteriorated due to lithium volatilization.The holding time had little effect on the density and ionic conductivity of Li5.95Al0.35La3Zr2O12 samples.