Preparation And Interface Modification Of LLZO Solid State Battery

Due to the excellent safety,good chemical stability,high thermal stability,wide electrochemical window,garnet solid electrolyte Li₇La₃Zr₂O₁₂?LLZO?was research by scholars all around the world.But as a solid-state electrolyte compared with liquid electrolyte needs further ascension,and in assembling the battery it still have large interfacial impedance,so we need to improve the preparation process and doping elements in order to enhance its ionic conductivity,we also needs to surface modification in order to reduce the interfacial impedance.This project adopts LLZO ceramics with excellent performance as the research object,and we studied the preparation process,electrochemical performance of LLZO and subsequent we studied the assembly and cycling performance of Li|LLZO|Li symmetrical battery.The main work is as follows:Sol-gel method can mix the elements at the molecular level to make the powder micro-uniform and reduce the calcination temperature.Sol-gel method of organic system adopts organic solvent with low surface tension,which has the above advantages and can meet the requirements of complex conditions such as spin-coated film.LLZO ceramic powder was prepared by sol-gel method of organic system.And the influences of raw materials,drying process and calcination process on the phase of LLZO were researched.The preparation process of pure phase was obtained as follows: LiNO₃,La?NO₃?₃ and ZrO?NO₃?were used as element sources.Solvent for ethylene glycol methyl ether: acetic acid = 4:1 mixed solvent;The dry gel was prepared by boiling mixing dry.The pure phase LLZO powder can be obtained by calcination at 850 ? for 5h,and the pure phase ceramics can be obtained by sintering the powder at 1060 ? to 1140 ?.LLZO ceramics were prepared by solid phase method.In order to stabilize cubed phase and improve its ionic conductivity,W element and Fe element were doped respectively to explore the influence of doping amount and sintering temperature on ceramic density and ionic conductivity.The doping of W element can stabilize the cubic phase of LLZO ceramic and improve its ionic conductivity.Fe doping can not only stabilize the cubic phase and improve the ionic conductivity of LLZO ceramics,but also reduce the sintering temperature so as to reduce Li volatilization in the sintering process.When W doped,the W1 component sintering at 1180 ? got the highest ionic conductivity,which is 2.54× 10⁴S·cm^-1.The highest density was got by W4 at 1200 ?,which is 99.18%.Through data fitting and calculation of its grain boundary resistance and grain internal resistance,it is found that the grain internal resistance of LLZO ceramics decreases to a certain extent with the doping of element W,but the grain boundary resistance increases slowly first and then sharply as the doping amount increases,thus increasing the total resistance.When Fe doped,the Fe1 component sintering at 1140 ? got the highest ionic conductivity,which is 5.87× 10⁴S·cm^-1.However,the ionic conductivity of Fe3 component is at a high level at various sintering temperatures,so this paper adopts Fe3 component ceramics for subsequent experiments.The Fe1 component sintering at 1200 ? got the highest density which is 93.83%.We tested the 1120 ? sintered Fe3-LLZO ceramics by variable temperature ac impedance spectroscopy and calculated the activation energy was Ea=0.28 e V.The data fitting of Fe-LLZO ceramics was used to calculate the grain boundary resistance and the grain internal resistance.It was found that the grain internal resistance was almost unchanged and the grain boundary resistance was greatly reduced after Fe element was added,so the total resistance was reduced.The Li|LLZO|Li symmetrical battery was assembled and the LLZO ceramics were interfaced by spraying gold and coating graphene oxide?GO?.Both gold spraying and coated GO can effectively improve wettability between the electrolyte and the Li metal electrode,thereby effectively reducing the interface resistance.When constant current cycle the spraying gold interface Li|LLZO|Li symmetric battery's resistance was down to about 20?,cycle as the increase of the number of battery internal resistance gradually reduced and tends to be stable,variable current loop cycle as the increase of the number of tending to the same direction,resistance and stability.The battery can circulate more than 100 cycles stably in the constant current mode and 100 cycles stably in the variable current mode.For GO interface Li|LLZO|Li battery,voltage curve is very smooth,and performance about 1700? interface resistance,constant current cycle as the increase of the number of battery resistance and the overpotential increases gradually and stable;As the number of cycles increases,the overpotential of the battery gradually increases,and the battery's resistance first decreases and then increases.During the variable current cycle,when the input current is too large,the battery is gradually destroyed,and the upper limit of the voltage of the stable cycle of the battery is about 4V.