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TY  - THES
AU  - Schütt, Judith
TI  - The origins of high sodium ion conductivity in NaSICON-type materials: a computational investigation of structural and transport properties
PB  - RWTH Aachen University
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2024-10206
SP  - 1 Online-Ressource : Illustrationen
PY  - 2024
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
N1  - Dissertation, RWTH Aachen University, 2024
AB  - Sodium superionic conductors (NaSICONs), with general formula NaM2A3O12, have garnered significant attention as solid state electrolyte for all-solid-state Na+ ion batteries owing to their remarkable total ionic conductivity in the order of 10−3 S cm−1 at room temperature. Their flexible structural framework, facilitating the incorporation of various aliovalent cations, results in a vast compositional range affecting the Na+ ion transport. Consequently, the experimental data on ionic conductivities of NaSICONs available in the literature vary by several orders of magnitude, which are influenced not only by the composition but also by the sample preparation method. Hence, a comprehensive understanding of the Na+ transport in NaSICONs is still lacking. In this study, multi-cationic NaSICONs are investigated by combining state-of-the-art computational methods spanning large length and time scales. Density functional theory is employed to study the structure and Na+ ion migration at the atomistic level, while kinetic Monte Carlo and Molecular Dynamics simulations explore the macroscopic Na+ ion transport. The study initially focusses on the mono-substituted Na1+xM2SixP3−xO12 compounds to unravel and examine independently two key aspects impacting the Na+ ion transport in NaSICONs: structural factors determined by introduced M4+ cations and the substitution level x. Subsequently, the findings are extended to the co-substituted Na1+x+yScyZr2−ySixP3−xO12 compounds. The exploration of both the interstitial- and interstitialcy-migration mechanisms reveals their primarily dependence on the metastable transient states traversed during the Na+ ion migration. The stability of these transient states, in turn, depends on the spatial arrangement of the Na+ ions, the size of the M4+ cations, and the substitution level x. However, the associated migration barriers are not solely governed by the transient states. Other contributions, notably the size of the bottlenecks and the direction of the migration pathways, have additional impact. Additionally, the study reveals that the Na+ site energy strongly depends on the Na+–Na+ interactions resulting mainly from electrostatic effects and Na+–Si4+ interactions driven by both electrostatic and elastic effects. A numerical model is developed to generalize the findings and predict migration barriers for arbitrary NaSICON compositions in subsequent kinetic Monte Carlo simulations. The results of these simulations demonstrate that there is no straightforward correlation between NaSICON composition and the Na+ ion mobility due to competing influences such as framework alteration and stoichiometric changes of the substituents and thus the mobile Na+ ions. Rather, the interplay of the interactions between the Na+ ions and framework cations, migration barriers, and ratio of unoccupied and occupied charge carrier sites must be considered. The study shows that the Na+ ion transport can be modified through various cation substitution strategies on both the M and P sites within the host lattice of Na1+xM2SixP3−xO12. Especially, the co-substituted compounds Na1+x+yScyZr2−ySixP3−xO12 have emerged as promising electrolyte materials in recent years. Nevertheless, the comprehensive investigation of these structures is challenging due to their vast compositional space. This study addresses this challenge using low-cost force-field molecular dynamics simulations to investigate the influence of Sc3+ and Si4+ co-substitution on the Na+ ion transport behavior. For this purpose, force-field parameters are determined based on structural information obtained from short ab-initio molecular dynamics simulations. The findings demonstrate enhanced conductivities with simultaneous Sc3+ and Si4+ co-substitution. Overall, in this work, macroscopic measurable quantities, such as the ionic conductivity, are linked to atomistic parameters, such as the site occupancy and ionic jumps within the lattice, to provide a comprehensive understanding of the complex Na+ ion transport behavior in NaSICONs across variable compositions. Furthermore, the study showcases the efficacy of the applied methodologies enabling the exploration of extensive configurational spaces of NaSICONs.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2024-10206
UR  - https://publications.rwth-aachen.de/record/995864
ER  -