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

Abstract

Ta-doped Li7La3Zr2O12 (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 ZrO2 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 Li7-xLa3Zr2-xTaxO12. For Al-free Li7-xLa3Zr2-xTaxO12, 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 °C for 12 h shows the relative density of 92% and the highest Li-ion conductivity of 1.03 × 10−3 S/cm at 30 °C with the activation energy of about 0.37 eV, while Li6.6La3Zr1.6Ta0.4O12 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 Li7-xLa3Zr2-xTaxO12 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.