Solid-state
fast ionic conductors are of great interest due to
their application potential enabling the development of safer high-performance
energy and conversion systems ranging from batteries through supercapacitors
to fuel cells, electrolyzers, and novel neuromorphic devices. However,
identifying fast ion conductors has remained a slow trial-and-error
search process. High-throughput computational screening methods such
as our bond valence site energy method can significantly accelerate
this materials design, but their implementation not only needs to
be computationally efficient and dependable but also simple to be
used by experimentalists in order to find widespread usage for guiding
experimental efforts to promising classes of candidate materials.
To bridge the current gap between computational method developers
and application-oriented users, we combine the computationally low-cost
bond valence site energy calculations in our softBV software tool
using a new automated pathway analysis toolthe bond valence
pathway analyzer (BVPA). The integration of BVPA gives rapid comprehensive
access to and simplifies the visualization of the desired information
on the characteristics of ion transport properties in candidate materials.
Examples for the main application of identifying suitable structure
types for fast ion transport as solid electrolytes or mixed conducting
electrode materials with high-rate capability are given. A new dopant
predictor further simplifies defect engineering of the candidate systems
by automatically suggesting suitable substitutional dopants for each
site in the structure based on a new machine-learned approach.
LiTa2PO8 has recently been reported
as a
new fast Li-ion conducting structure type within the series of Li
x
(MO6/2)
m
(TO4/2)
n
polyanion oxides.
Here, we demonstrate the preparation of LiTa2PO8 by solid-state syntheses, clarify the temperature dependence of
lithium distribution and ionic conductivity, and study the structural
stability, densification, and achievable total conductivity as a function
of sintering conditions synergizing experimental neutron and X-ray
powder diffraction and electrochemical studies with computational
energy landscape analyses and molecular dynamics simulations. A total
room temperature conductivity of 0.7 mS cm–1 with
an activation energy of 0.27 eV is achieved after sintering at 1323
K for 10 h. Spark plasma sintering yields high densification >98%,
highly reproducible bulk conductivities of 2.8 mS cm–1, in agreement with our bond valence site energy-based pathway predictions,
and total conductivities of 0.6 mS cm–1 within minutes.
Powder diffraction studies from 3 to 1273 K reveal a reversible flipping
of the monoclinic angle from above to below 90° close to room
temperature as a consequence of rearrangements of the mobile ions
that change the detailed pathway topology. A consistent model of the
temperature-dependent Li redistribution, conductivity anisotropy,
and transport mechanism is derived from a synopsis of diffraction
experiments, experimental conductivity studies, and simulations. Due
to the limited electrochemical window of Li
x
(TaO6/2)2(PO4/2)1 (LTPO), a direct contact with Li metal or high voltage cathode materials
leads to degradation, but as demonstrated in this work, semi-solid-state
batteries, where LTPO is protected from direct contact with lithium
by organic buffer layers, achieve stable cycling.
To achieve higher energy density in safer energy storage systems, a transition to ceramic all‐solid‐state batteries is widely expected. Their performance and cycle‐life is largely controlled by processes at buried interfaces. While experimental operando probing of interfacial processes is under development, first‐principle computational methods are challenged by the complexity of the multiphase models and long simulation periods required to capture slow degradation processes. Thus, simpler empirical reactive forcefields have the potential to substantially accelerate the design and optimization of all‐solid‐state batteries, provided that parameters are available for a wide range of relevant atom types. The energy‐scaled bond valence‐based softBV forcefield has successfully enabled the design of new solid electrolytes or insertion‐type electrode materials and analyses of ion transport processes therein. As a two‐body forcefield, it enables fast simulations for complex structures over long periods, but inevitably shares the tendency of two‐body forcefields to maximize coordination numbers if free volume facilitates a reorganization of the atoms, which makes them less suitable for studying interfacial processes. Herein, this vulnerability of two‐body forcefields is overcome in a computationally efficient way by introducing an embedded‐atom‐method‐inspired bond‐valence‐sum‐based new class of transferable forcefields and its effective use for modeling of surfaces and interfaces is demonstrated.
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