Experimental and Computational Study of Mg and Ta-Doped Li7La3Zr2O12 Garnet-Type Solid Electrolytes for All-Solid-State Lithium Batteries

Abstract

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.