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@PHDTHESIS{Schtt:995864,
author = {Schütt, Judith},
othercontributors = {Neitzel-Grieshammer, Steffen and Lüchow, Arne and Martin,
Manfred},
title = {{T}he origins of high sodium ion conductivity in
{N}a{SICON}-type materials: a computational investigation of
structural and transport properties},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-10206},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2024},
abstract = {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.},
cin = {153420 / 150000},
ddc = {540},
cid = {$I:(DE-82)153420_20140620$ / $I:(DE-82)150000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-10206},
url = {https://publications.rwth-aachen.de/record/995864},
}
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