| An all-solid-state battery is a new type of secondary battery,which can effectively solve the safety problem of traditional battery due to electrolyte leakage,and has higher energy density,wider working voltage and working temperature.However,the low ionic conductivity of solid electrolyte and the large resistance between electrode material and solid electrolyte limit the application of solid electrolyte.One of the urgent problems is how to improve the ionic conductivity of solid electrolyte.Li7La7La3Zr2O12 possesses high ionic conductivity at room temperature,good chemical stability,good chemical compatibility with electrode materials and a wide electrochemical window,which is a promising material and has become a research hotspot.In this paper,Ta-doped LLZO(Ta-LLZO)and Ga-doped LLZO(Ga-LLZO)are synthesized by the traditional solid phase method,respectively.The particle size distribution,phase structure,microscopic morphology,ionic conductivity,electronic conductivity and bulk density of specimens are characterized by laser particle size analyzer,XRD,SEM,EIS,DC polarization and Archimedes method,respectively.We used ZrO2 crucibles as a sintering device to avoid the introduction of Al.Ultrafine powders were obtained by ball-milling.Using ultrafine powders is an effect way to solve the problem of relatively low density for Al-free Ta-LLZO.When the ball milling time was 10 h,the corresponding medium size(D50)was as lowest as 0.565 um,and the specific surface area was 8611m2/kg.The sintering temperature and sintering time have a great influence on the the density and Li-ion conductivity of Al-free Ta-LLZO.When the sintering temperature is not higher than 1180℃,the relative density is low and the grain size is small,leading to the decrease of ionic conductivity.But when the sintering temperature is as high as 1200℃,the pores become bigger due to the excessive lithium loss,leading to the decrease of ionic conductivity.When the sintering time is 6 h,the grain size is small,which casue higher grain boundary resistance.And the sintering time incrase to 18 h,the impurity phase La2Zr2O7 formated due to the excessive lithium loss,leading to the decrease of ionic conductivity.The optimum sintering process is 1180℃for 12 h.Ta doping amount has great influence on the density and ionic conductivity of the Al-free Ta-LLZO.It not only affects the content of lithium and lithium vacancies,but also affects its bulk density and grain size.Li7-xLa3Zr2-xTaxO12(x=0)belongs to the tetragonal phase,so its ionic conductivity is as low as 8.4×10-6 S/cm.As for Al-free Li7-xLa3Zr2-xTaxO12(x=0.2,0.3,0.4,0.5,0.6)are both belong to cubic phase,the increase of x(0.2≤x≤0.4)promotes the grain growth and sintering densification,but the increase of x(0.4<x≤0.6)has an adverse effect.Li6.7La3Zr1.7Ta0.3O12 sintered at 1180℃ for 12 h shows the relative density of 92%and the highest Li-ion conductivity of 1.03×10-3 S/cm at 30℃ with the activation energy of about 0.37 eV,while Li6.6La3Zr1.6Ta0.4O12 sintered at 1180℃ for 12 h shows the highest relative density of 96%and the Li-ion conductivity of 6.68×10-4 S/cm at 30℃ with the activation energy of about 0.46 eV.The electronic conductivity of Al-free Li7-xLa3Zr2-xTaxO12 is 10-9 S/cm orders of magnitude.Ga doped systems have much higher ionic conductivity than Al doped systems.The effects of sintering temperature and dopant content on Li-ion conductivity of Al-free Li7-3xGaxLa3Zr2O12 were investigated.Using finer powders is benefit for the formation of liquid phase.As for Al-free Li6.4Ga0.2La3Zr2O12,the density was low and some voids were observed when the sintering temperature is 1100℃.When the sintering temperature reaches 1150℃,the amorphous phase covering the grain boundary can be observred,resulting in the grain boundary cannot be observed.As the sintering temperature increases,the liquid phase gradually increases.The pores become larger at the sintering temperature of 1250℃.And the top of the pellet is caved.The Li-ion conductivity and bulk density improve with the sintering temperature rise.But when the sintering temperature reaches 1250℃,the Li-ion conductivity and bulk density decline.As for Al-free Li7-3xGa-xLa3Zr2O12,the tetragonal phase disappears completely when x>0.15.The Li-ion conductivity and bulk density of Al-free Li7-3xGaxLa3Zr2O12 improve with increase of x(x≤0.2).As for Al-free Li7-3xGaxLa3Zr2O12(x>0.2),the ionic conductivity declines with the increase of x.Li6.4Ga0.2La3Zr2O12 sintered at 1200 ℃ shows the highest Li-ion conductivity of 1.5×10-3 S/cm at 30℃ and the activation energy is about 0.28 eV. |
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