Synthesis of Garnet LLZO by Aliovalent Co-Doping, and Electrochemical Behavior of Composite Solid Electrolyte for All-Solid Lithium Batteries

Garnet-like Ga-M (M = Ta, Rb, Y) co-doped Li₇La₃Zr₂O₁₂ powders are prepared by the Taylor reaction, including Ga-LLZO, Ga-Ta LLZO, Ga-Rb LLZO, and Ga-Y LLZO. The lattice constant (12.97 Å) and large crystallite size (866 Å) of Ga-Rb LLZO powder enabled the highest ionic conductivity (2.03 × 10^-3 S cm⁻¹ at 25 °C) in the pellet form. Moreover, Ga-Rb LLZO powders are calcined in various temperature range and are utilized in composite solid electrolyte sheets consisting of Ga-Rb LLZO, polyethylene oxide, and salts. The ionic conductivity of the CSE sheets is increased in proportion to the calcination temperature in the range of 2.43 × 10⁻⁴ to 8.60 × 10⁻⁴ S cm⁻¹ at 70 °C. The LLZO particle-size and crystallite-size requirements are differ for the cathode and CSE sheet. Three types of all-solid-state lithium batteries are designed, among which all-solid lithium batteries-3 employing Ga-Rb LLZO powder calcined at 900 °C and 1100 °C for the cathode and CSE sheet, respectively, exhibits an initial capacity of ∼139 mAh g⁻¹ at 0.1C and 70 °C, with a capacity retention of ∼92% after 100 cycles. In particular, in terms of rate characteristics, 1C compared to 0.1C capacity shows excellent value, maintaining about 85%.