Achieving enhanced densification and superior ionic conductivity of garnet electrolytes via a co-doping strategy coupled with pressureless sintering

Lithium garnet oxides with 6.5 mol Li, such as Li 6.5 La 3 Zr 1.5 (Ta/Nb) 0.5 O 12 , typically crystallise in cubic structure and exhibit excellent room-temperature ionic conductivity close to 1 mS cm −1 . However, it is challenging to densify garnet oxides. In this work, we investigated how the co-doping of tantalum (Ta) and niobium (Nb) affects the densification of pressureless sintered garnet electrolytes with compositions of Li 6.5 La 3 Zr 1.5 Ta (0.5−x) Nb x O 12 , where x = 0–0.5. The highest densification (94.5% of relative density) was achieved in Li 6.5 La 3 Zr 1.5 Ta 0.1 Nb 0.4 O 12 (TN-LLZO) when it was sintered at 1150 °C for 6 h. This TN-LLZO garnet electrolyte delivers an ionic conductivity of 1.04 × 10 −3 S cm −1 (at 22 °C) with a low activation energy of 0.41 eV. Our findings demonstrate that the content of dopants (Ta and Nb) plays a critical role in enhancing the sintering performance of garnet ceramics at ambient pressure.