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 Li6.5La3Zr15(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 Li6.5La3Zr1.5Ta(0.5-x)NbxO12, where x = 0-0.5. The highest densification (94.5% of relative density) was achieved in Li6.5La3Zr1.5Ta0.1Nb0.4O12 (TN-LLZO) when it was sintered at 1150 degrees C for 6 h. This TN-LLZO garnet electrolyte delivers an ionic conductivity of 1.04 x 10(-3) S cm(-1) (at 22 degrees 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.