Fine and strengthened Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 high-entropy Li-garnet: A comparison study with Ta doped Li-garnet

In this paper, we demonstrate that Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 (LLZNTH) high-entropy Li-garnet has a fine microstructure and improved mechanical properties compared with Ta-doped Li-garnet (Li6.75La3Zr1.75Ta0.25O12, LLZT). The formation, sintering, and electrochemical properties are also studied. The results indicate that the LLZNTH sample has a finer particle size than LLZT after calcination and ball mill. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) regarding the solid-state reaction process indicate that the LLZNTH forms at a lower temperature than LLZT. Both samples can be densified to a relative density up to ∼93–94% at a temperature of 1100 °C, however, they show significantly different sintering and grain growth behaviors. The LLZNTH sample takes 16 h to reach the maximum relative density while the LLZT sample only needs 12 h. LLZNTH sample has a lower grain growth parameter due to the sluggish effects of high-entropy compounds so that it maintains fine microstructures (grain size ∼10 μm) than the LLZT sample (grain size over 100 μm). Due to the fine microstructures, the LLZNTH sample shows both higher flexural strength (84.8 ± 6.9 MPa compared with 47.9 ± 10.1 MPa) and hardness (8.5 ± 0.8 GPa compared with 7.7 ± 0.4 GPa) than the LLZT sample. Ionic conductivity characterizations indicate that the LLZNTH sample shows a moderate conductivity of 4.67 × 10-4 S/cm at room temperature and a low activation energy of 0.25eV.