First-principles study on structural, mechanical and electronic properties of Li7La3Zr2O12 solid electrolyte
Authors: Maphoto R , Masedi M , Ngoepe P , Ledwaba R 
Published: February 2022

The oxide garnet Li7La3Zr2O12 (LLZO) is a promising solid electrolyte for Li-based batteries due to its high Li-ion conductivity and chemical stability with respect to Li metal anode. However, at room temperature, it crystallizes into a poorly Li-ion conductive tetragonal phase. To this end, supervalent cation doping has been an effective way to stabilize the highly conductive cubic phase and enhance the ionic conductivity of the tetragonal phase at room temperature, through the creation of lithium vacancies. Yet, the fundamental aspects regarding this supervalent substitution remain poorly understood. In this study, we have employed the first-principle calculations to offer a better understanding of the stabilization of tetragonal Li7La3Zr2O12 phases by determining the structural, mechanical, and electronic properties for high-conductivity LLZO composition. We find that the structural properties calculated are in good agreement which is within a 2% error of the experimentally measured results. The negative energy of formation for t-LLZO shows that the material is thermodynamically stable. The calculated Young’s modulus is in good agreement with the experimental observations, which indicates that the material is mechanically stable. Owing to its wide electrochemical stability, the calculated band structure of t-LLZO shows that the material is a wide and indirect magnetic separator with a g-symmetry point band gap which is in good agreement with the experimental observations. Therefore, the structural, mechanical, and electronic stability of t-LLZO provides better insight about the stability of the material and this capacitates further investigations associated with ionic conductivity of the pure and supervalently doped LLZO.The oxide garnet Li7La3Zr2O12(LLZO) is a promising solid electrolyte for Li-based batteries due to its high Li-ion conductivity and chemical stability with respect to Li metal anode. However, at room temperature, it crystallizes into a poorly Li-ion conductive tetragonal phase. To this end, supervalent cation doping has been an effective way to stabilize the highly conductive cubic phase and enhance the ionic conductivity of the tetragonal phase at room temperature, through the creation of lithium vacancies. Yet, the fundamental aspects regarding this supervalent substitution remain poorly understood. In this study, we have employed the first-principle calculations to offer a better understanding of the stabilization of tetragonal Li7La3Zr2O12 phases by determining the structural, mechanical, and electronic properties for high-conductivity LLZO composition. We find that the structural properties calculated are in good agreement which is within a 2% error of the experimentally measured results. The negative energy of formation for t-LLZO shows that the material is thermodynamically stable. The calculated Young’s modulus is in good agreement with the experimental observations, which indicates that the material is mechanically stable. Owing to its wide electrochemical stability, the calculated band structure of t-LLZO shows that the material is a wide and indirect magnetic separator with a g-symmetry point band gap which is in good agreement with the experimental observations. Therefore, the structural, mechanical, and electronic stability of t LLZO provides better insight about the stability of the material and this capacitates further investigations associated with ionic conductivity of the pure and supervalently doped LLZO.