Studies On The Microstructure And Sintering Properties Of Li_1.3Al_0.3Ti_1.7（PO₄）₃ Solid Electrolyte Prepared By Three Liquid Methods

Li_1.3Al_0.3Ti_1.7（PO₄）₃ solid electrolyte is favored by researchers due to its good electrical conductivity,wide electrochemical window,and excellent stability to air and humidity.At present,the main methods for preparing LATP materials include melt cooling method,solid method and sol-gel method,as one of the liquid-phase methods,the sol-gel method can overcome the shortcomings of the first two methods of high synthesis temperature and generation of secondary phases,so it got more attention.In view of this,this paper focuses on the liquid-phase synthesis of LATP,and adopts three liquid-phase synthesis methods—sol-gel method,hydrothermal method and precipitation method to prepare LATP solid electrolyte.X-ray diffraction（XRD）,scanning electron microscope（SEM）and thermogravimetry-differential scanning calorimetry（TG-DSC）were used to characterize the morphology and structure of the solid electrolyte,and using an electrochemical workstation to analyze the electrochemical performance of the solid electrolyte,aiming to explore the intrinsic relationship between preparation process,microstructure and performance in the process of synthesizing solid-state electrolytes by three liquid phase method,and revealing the key factors affecting the electrical conductivity of LATP materials.The main results of this paper are as follows:（1）The precursor powder of LATP solid electrolyte synthesied by sol-gel method is amorphous phase,and the pure-phase LATP is formed by high temperature sintering.The experimental results show that in the synthesis process,p H value of the sol can affect the crystal core and growth during crystal generation,under the neutral condition,the matching of crystal nucleation and growth rates can avoid the occurrence of pores and cracks caused by anisotropic tension in the sintering process;controlling the calcination temperature near the end point of the crystal phase transition can avoid the stress imbalance from phase transition,which can obtain the high compaction solid electrolyte;by optimizing the powder synthesis process and calcination temperature,the high conductivity is obtained at a low temperature of800℃;adding sintering aids can promote the sintering compaction,and Li BF₄ works the best.By systematic study,the optimal process conditions for LATP synthesied by sol-gel method is p H=7,lithium excess of 20 wt%,calcination temperature is 750℃,adding 3 wt%Li BF₄sintering aid,sintering at 800℃for 6 h,the electrical conductivity of solid electrolyte is8.5×10^-4 Scm^-1.（2）The precursor powder of LATP solid electrolyte synthesized by hydrothermal method was a mixture of crystal phase and amorphous phase,and the pure-phase LATP is formed by high temperature sintering.The experimental results show that the high temperature and pressure formed by the hydrothermal reaction are beneficial to promote lithium entry into the LATP lattice,reducing the volatilization of lithium at high-temperature sintering;forming high temperature and pressure environment by adjusting the alcohol-water ratio,hydrothermal temperature and time,the pure-phase LATP solid electrolyte is obtained without excess lithium;the powder synthesized by hydrothermal method is nano-spherical powder,which has a very small contact area,so making shape is very difficult;the presence of Li PO₄ in the precursor powder obtained by hydrothermal method can be used as sintering aid to promote the compaction of the solid electrolyte.By systematic study,the optimal process conditions for LATP synthesied by hydrothermal method is that the alcohol-water ratio is 2:8,the hydrothermal temperature is 200℃,hydrothermal time is 12 h,calcination temperature is 750℃,adding 3 wt%Li BF₄ sintering aid,sintering at 900℃for 6 h,the electrical conductivity of solid electrolyte is 4.58×10^-4 Scm^-1.（3）The precursor powder of LATP solid electrolyte synthesized by precipitation method is a mixture of crystal phase and amorphous phase,and LATP crystal phase was formed by high temperature sintering.The experimental results show that p H value of the solution affect the size and morphology of the powder,thus affecting the compaction of solid electrolyte,the powder obtained at a lower p H value is slightly reunited,with the increasing of p H,the phenomenon of powder agglomeration intensifies,even forms into irregular blocks,which is not good for compaction of solid electrolyte;adding excess lithium salt can eliminate Li P₂O₇secondary phase caused by lithium volatilization during high temperature sintering process.By systematic study,the optimal process conditions for LATP synthesied by cprecipitation method is p H=5,lithium excess of 15 wt%,calcination temperature is 750℃,sintering at900℃for 6 h,the electrical conductivity of solid electrolyte is 2.33×10^-4 Scm^-1.Compared with the previous methods,this method is much simpler,suitable for mass production,and the conductivity is higher without adding sintering aids.（4）Detailed analysis of sintering process shows that there are four key factors for obtaining a dense solid electrolyte,the first is the powder particle size and morphology,there is mainly volume diffusion during the early sintering period,the smaller the powder particle size is,the higher density of the solid electrolyte obtained,during the middle sintering period,in addition of volume diffusion,there is also crystal boundary diffusion,the crystal boundary velocity is mainly affected by the radius of curvature of the grains,the faster migration velocity of grain boundaries obtained under the smaller radius of curvature of the grains;the second is sintering temperature and time,appropriate sintering temperature and sintering time during the late sintering period help pore discharge and volume shrinkage;the third is sintering aids,the lower viscosity of the liquid phase produced by the addition of sintering aids,the more conducive to mass transfer,the formation of solid solvent with solid electrolyte lead to defects in the structure,which is conducive to lithium ion migration;the fourth is secondary phases,the existence of secondary phase will hinder the grain boundary and lithium ion migration.Li Ti PO₅ has less resistance to lithium ion due to its similar structure to Li Ti₂（PO₄）₃.