Multi-scale modelling of oxide-based solid state battery

Abstract

The oxide garnet Li7La3Zr2O12 (LLZO) exhibits high Li-ion conductivity and chemical stability when in contact with lithium metal anodes, making it a promising solid electrolyte for Li-based batteries. The tetragonal phase, however, is poorly Li-ion conductive at room temperature. It is essential for Tetragonal Li7La3Zr2O12 (t-LLZO) to be capable of enduring high temperatures with good phase stability in order to be used in practical devices. In this way, adding supervalent cations, such as Ta, to the Zr-site of t-LLZO stabilizes the low-temperature structure of the tetragonal crystals, and more importantly, creates vacancies in the tetragonal phase, thereby enhancing its ionic conductivity. However, the crystal structure behaviour at high temperatures remains unclear. Moreover, the fundamental aspect of supervalent substitution as well as which level of lattice expansion/contraction promotes ionic diffusion of this supervalent substitution remains poorly understood. The density functional theory calculations implemented in the Vienna ab initio simulation package were utilized to offer a better understanding of the stabilization of the tetragonal Li7La3Zr2O12 phase by determining the structural, mechanical, and electronic properties of the high-conductive LLZO structure. It was found that the structural properties calculated are in good agreement which is within a 2% error of the experimentally measured results from other studies. The t-LLZO structure has a negative energy of formation, which is consistent with experimental data. The calculated Young’s modulus is in good agreement with the experimental observations, and it satisfies the necessary stability constraints for the configuration. Owing to its large band gap for electrochemical stability, the calculated band structure of t-LLZO shows that the material is a magnetic separator with a wide and indirect band gap along the gsymmetry point, which is in good agreement with the experimental observations from other studies. In this study, first-principle calculations combined with cluster expansion simulation were performed on the t-LLZO to attain a fundamental understanding on the phase stability of Tadoped LLZO and generate new possible phases of Ta-doped LLZO. Furthermore, Monte Carlo simulation was utilised to gain an insight into the behaviour of the Ta-doped phase as a function of temperature under canonical ensemble. The cluster expansion generated 28 new multicomponent Ta-doped Li5La3Zr2-xTaxO12 structures, were all of the new structures are thermodynamically stable with a negative enthalpy of formation. The Monte-Carlo temperature profiles have a miscible gap with very small energy difference, indicating that there is no phase separation and the system mixes well at ~900K. Further density functional theory calculations iii were performed on the most stable generated Ta-doped LLZO structures to determine the structural, mechanical, and electronic properties of the structures for their application as active solid-state electrolytes. It is found that the generated structures exhibit good structural stability since the calculated lattice parameters of the Ta-doped LLZO structures are found to be smaller than that of pure t-LLZO. The results show that the distance between Li-Li in doped Ta-LLZO is smaller than in pure t-LLZO, which indicates that the smaller the difference between the dopant ionic radius and the critical dopant radius, the higher the conductivity. Therefore, the structural properties of Tantalum-doped structures are shown to improve due to the smooth decrease in calculated lattice parameters. Moreover, the Ta-doped structures show good elastic stability against deformation and exhibit magnetic separator behavior, which encourages their use as the next-generation solid electrolytes. Therefore, the findings provide a better understanding of the phase stability of the generated Ta-doped LLZO structures, which set a strong foundation for further analysis of the temperature effect on the rate of lithium-ion diffusion and the mobility of the lanthanum, zirconium, tantalum, and oxygen ions in the system at high temperatures, which is crucial for the development of these promising solidstate electrolytes for all-solid-state Li-ion batteries.