Experimental and computational study of Mg and Ta-doped Li₇La₃Zr₂O₁₂ garnet-type solid electrolytes for all-solid-state lithium batteries

Garnet-type Li₇La₃Zr₂O₁₂ electrolytes have garnered significant attention as promising solid-state electrolyte candidates in all-solid-state lithium batteries (ASSLBs). However, its susceptibility to forming Li₂CO₃ upon atmospheric exposure leads to performance degradation, limiting its application. This study introduces a co-doping strategy of Mg and Ta to enhance the properties of garnet electrolytes. Pure cubic Mg and Ta-doped LLZO electrolytes are successfully synthesized using the solid-state reaction method. Experimental results, coupled with density functional theory (DFT) calculation, reveal that Mg²⁺ doping occurs primarily at the La site (24c). This substitution, given the substantial disparity in ionic radii between Mg²⁺ and La³⁺, effectively narrows the transport bottleneck for Li-ions, resulting in a decreased ionic conductivity and an increased activation energy. Li₆.₆La₂.₈Mg₀.₂Zr₁.₄Ta₀.₆O₁₂ exhibits a relative density of ≈92.6%, demonstrating outstanding performance with a room temperature ionic conductivity of 4.31 × 10⁻⁴ S cm⁻¹ and low electronic conductivity of 2.48 × 10⁻⁸ S cm⁻¹. Notably, after 4 months of atmospheric exposure, its ionic conductivity decreased to ≈78% of the initial value, attributable to Li₂CO₃ formation. Furthermore, the material demonstrated exceptional long-term cycle stability over 1000 h at a current density of 0.1 mA cm⁻² at 25 °C, indicating effective suppression of Li dendrite formation.