High densification and Li-ion conductivity of Al-free Li7-xLa3Zr2-xTaxO12 garnet solid electrolyte prepared by using ultrafine powders

Abstract Ta-doped Li 7 La 3 Zr 2 O 12 (Ta-LLZO) is considered as a promising solid electrolyte due to high Li-ion conductivity and good chemical stability against electrode materials. In this work, Ta-LLZO was prepared by a conventional solid-state reaction. Ultrafine powders were obtained by ball-milling to improve the surface activity. Ta-LLZO is sintered in ZrO 2 crucibles to avoid introducing Al into the samples. The particle size distribution, phase structure, morphology, ionic conductivity, electronic conductivity, density and electrochemical performance of semi-solid battery were characterized by laser diffraction particle size analyzer, X-ray diffraction, scanning electron microscope, AC-impedance, DC polarization, Archimedes method and a battery testing system, respectively. The results show that the ball milling to reduce the particle size is an effective way to solve the problem of relatively low density and Li-ion conductivity for Al-free Li 7- x La 3 Zr 2- x Ta x O 12 . For Al-free Li 7- x La 3 Zr 2- x Ta x O 12 , 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. Li 6.7 La 3 Zr 1.7 Ta 0.3 O 12 sintered at 1180 °C for 12 h shows the relative density of 92% and the highest Li-ion conductivity of 1.03 × 10 −3 S/cm at 3 0 °C with the activation energy of about 0.37 eV, while Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 sintered at 1180 °C for 12 h shows the highest relative density of 96% and the Li-ion conductivity of 6.68 × 10 −4 S/cm at 30 °C with the activation energy of about 0.46 eV. The electronic conductivity of Al-free Li 7- x La 3 Zr 2- x Ta x O 12 is 10 −9 S/cm orders of magnitude. The semi-solid battery shows the first discharge capacity of 104.6 mAh/g and 92.5% capacity retention after 20 cycles.