8 research outputs found
Defining Diffusion Pathways in Intercalation Cathode Materials: Some Lessons from V<sub>2</sub>O<sub>5</sub> on Directing Cation Traffic
The invention of
rechargeable batteries has dramatically changed
our landscapes and lives, underpinning the explosive worldwide growth
of consumer electronics, ushering in an unprecedented era of electric
vehicles, and potentially paving the way for a much greener energy
future. Unfortunately, current battery technologies suffer from a
number of challenges, e.g., capacity loss and failure upon prolonged
cycling, limited ion diffusion kinetics, and a rather sparse palette
of high-performing electrode materials. Here, we discuss the origins
of diffusion limitations in oxide materials using V<sub>2</sub>O<sub>5</sub> as a model system. In particular, we discuss constrictions
in ionic conduction pathways, narrow energy dispersion of conduction
band states, and the stabilization and self-trapping of polarons as
local phenomena that have substantial implications for introducing
multiscale compositional and phase heterogeneities. Strategies for
mitigating such limitations are discussed such as reducing diffusion
path lengths and the design of metastable frameworks yielding frustrated
coordination and decreased barriers for migration of polarons
Topochemically De-Intercalated Phases of V<sub>2</sub>O<sub>5</sub> as Cathode Materials for Multivalent Intercalation Batteries: A First-Principles Evaluation
The
scarce inventory of compounds that allow for diffusion of multivalent
cations at reasonable rates poses a major impediment to the development
of multivalent intercalation batteries. Here, we contrast the thermodynamics
and kinetics of the insertion of Li, Na, Mg, and Al ions in two synthetically
accessible metastable phases of V<sub>2</sub>O<sub>5</sub>, ζ-
and Δ-V<sub>2</sub>O<sub>5</sub>, with the relevant parameters
for the thermodynamically stable α-phase of V<sub>2</sub>O<sub>5</sub> using density functional theory calculations. The metastability
of the frameworks results in a higher open circuit voltage for multivalent
ions, exceeding 3 V for Mg-ion intercalation. Multivalent ions inserted
within these structures encounter suboptimal coordination environments
and expanded transition states, which facilitate easier ion diffusion.
Specifically, a nudged elastic band examination of ion diffusion pathways
suggests that migration barriers are substantially diminished for
Na- and Mg-ion diffusion in the metastable polymorphs: the predicted
migration barriers for Mg ions in ζ-V<sub>2</sub>O<sub>5</sub> and Δ-V<sub>2</sub>O<sub>5</sub> are 0.62â0.86 and
0.21â0.24 eV, respectively. More generally, the results indicate
that topochemically derived metastable polymorphs represent an interesting
class of compounds for realizing multivalent cation diffusion because
many such compounds place cations in âfrustratedâ coordination
environments that are known to be useful for realizing low diffusion
barriers
Inside and Outside: Xâray Absorption Spectroscopy Mapping of Chemical Domains in Graphene Oxide
The oxidative chemistry of graphite has been investigated for over 150 years and has attracted renewed interest given the importance of exfoliated graphene oxide as a precursor to chemically derived graphene. However, the bond connectivities, steric orientations, and spatial distribution of functional groups remain to be unequivocally determined for this highly inhomogeneous nonstoichiometric material. Here, we demonstrate the application of principal component analysis to scanning transmission X-ray microscopy data for the construction of detailed real space chemical maps of graphene oxide. These chemical maps indicate very distinct functionalization motifs at the edges and interiors and, in conjunction with angle-resolved near-edge X-ray absorption fine structure spectroscopy, enable determination of the spatial location and orientations of functional groups. Chemical imaging of graphene oxide provides experimental validation of the modified LerfâKlinowski structural model. Specifically, we note increased contributions from carboxylic acid moieties at edge sites with epoxide and hydroxyl species dominant within the interior domains
Intercalation-Induced Exfoliation and Thickness-Modulated Electronic Structure of a Layered Ternary Vanadium Oxide
Solid-state compounds
wherein electrons cannot be described as
noninteracting particles and instead show strongly correlated behavior
are of interest both as systems manifesting novel quantum chemical
phenomena as well as for electronic device applications. In the absence
of predictive theoretical descriptors, modulation of the properties
of these compounds tends to be challenging, and generalizable strategies
for modulating closely coupled lattice, orbital, and spin degrees
of freedom are exceedingly sparse. Here, it is shown that exfoliation
mediated by cation intercalation can serve as a powerful means of
modulating the electronic structure of layered correlated materials.
Using a strongly correlated and charge-ordered layered compound, ÎŽ-Sr<sub>0.50</sub>V<sub>2</sub>O<sub>5</sub>, as a model system, it is shown
that the band gap can be drastically altered from ca. 1.07 to 2.32
eV and the electron correlation strength can be greatly modified by
intercalation-driven exfoliation to 2D nanosheets upon elimination
of structural coherence along one dimension. These findings suggest
that intercalation chemistry and solution-phase exfoliation provide
a versatile strategy for modulating the electronic structure of quantum
materials with potential for realizing Mott and neuromorphic circuitry
Vanadium KâEdge Xâray Absorption Spectroscopy as a Probe of the Heterogeneous Lithiation of V<sub>2</sub>O<sub>5</sub>: First-Principles Modeling and Principal Component Analysis
Understanding
the diffusion mechanisms of Li ions through host
materials and the resulting phase evolution of intercalated phases
is of paramount importance for designing electrode materials of rechargeable
batteries. The formation of lithiation gradients and discrete domains
during intercalation leads to the development of strain within the
host material and is responsible for the observed capacities of most
cathode materials being well below theoretically predicted values.
Such mesoscale heterogeneity has also been implicated in the loss
of capacity upon cycling. Due to their inherent complexity, the analysis
of such heterogeneity is rather complex and precise understanding
of the evolution of metal sites remains underexplored. In this work,
we use phase-pure, single-crystalline V<sub>2</sub>O<sub>5</sub> nanowires
with dimensions of 183 ± 50 nm and lengths spanning tens of microns
as a model cathode material and demonstrate that V K-edge X-ray absorption
near-edge structure can be used as an effective probe of the local
valence and geometry of vanadium sites upon lithiation. We demonstrate
that a highly lithiated phase is nucleated and grows at the expense
of a homogeneous low-lithium-content α-phase without mediation
of a solid-solution with intermediate lithium content. Density functional
theory calculations allow for assignment of the pre-edge feature to
dipolar transitions that are particularly sensitive to the V 3dâO
2p hybridization of the vanadyl bond and the local geometry of the
distorted [VO<sub>5</sub>] square pyramid. The quantitative analysis
of multiple vanadium sites and their evolution as a function of Li-ion
content provides insight into the mechanism of phase evolution and
the nature of lithiation gradients. The phase coexistence and segregation
is further observed in scanning transmission X-ray microscopy images
of individual lithiated V<sub>2</sub>O<sub>5</sub> nanowires. The
mechanisms and the dynamics of nucleation and growth unraveled here
are of great importance for the design and discovery of Li-ion cathode
materials
Mapping Catalytically Relevant Edge Electronic States of MoS<sub>2</sub>
Molybdenum
disulfide (MoS<sub>2</sub>) is a semiconducting transition
metal dichalcogenide that is known to be a catalyst for both the hydrogen
evolution reaction (HER) as well as for hydro-desulfurization (HDS)
of sulfur-rich hydrocarbon fuels. Specifically, the edges of MoS<sub>2</sub> nanostructures are known to be far more catalytically active
as compared to unmodified basal planes. However, in the absence of
the precise details of the geometric and electronic structure of the
active catalytic sites, a rational means of modulating edge reactivity
remain to be developed. Here we demonstrate using first-principles
calculations, X-ray absorption spectroscopy, as well as scanning transmission
X-ray microscopy (STXM) imaging that edge corrugations yield distinctive
spectroscopic signatures corresponding to increased localization of
hybrid Mo 4d states. Independent spectroscopic signatures of such
edge states are identified at both the S L<sub>2,3</sub> and S K-edges
with distinctive spatial localization of such states observed in S
L<sub>2,3</sub>-edge STXM imaging. The presence of such low-energy
hybrid states at the edge of the conduction band is seen to correlate
with substantially enhanced electrocatalytic activity in terms of
a lower Tafel slope and higher exchange current density. These results
elucidate the nature of the edge electronic structure and provide
a clear framework for its rational manipulation to enhance catalytic
activity
Mitigating Cation Diffusion Limitations and Intercalation-Induced Framework Transitions in a 1D Tunnel-Structured Polymorph of V<sub>2</sub>O<sub>5</sub>
The
design of cathodes for intercalation batteries requires consideration
of both atomistic and electronic structure to facilitate redox at
specific transition metal sites along with the concomitant diffusion
of cations and electrons. Cation intercalation often brings about
energy dissipative phase transformations that give rise to substantial
intercalation gradients as well as multiscale phase and strain inhomogeneities.
The layered α-V<sub>2</sub>O<sub>5</sub> phase is considered
to be a classical intercalation host but is plagued by sluggish diffusion
kinetics and a series of intercalation-induced phase transitions that
require considerable lattice distortion. Here, we demonstrate that
a 1D tunnel-structured ζ-phase polymorph of V<sub>2</sub>O<sub>5</sub> provides a stark study in contrast and can reversibly accommodate
Li-ions without a large distortion of the structural framework and
with substantial mitigation of polaronic confinement. Entirely homogeneous
lithiation is evidenced across multiple cathode particles (in contrast
to α-V<sub>2</sub>O<sub>5</sub> particles wherein lithiation-induced
phase transformations induce phase segregation). Barriers to Li-ion
as well as polaron diffusion are substantially diminished for metastable
ζ-V<sub>2</sub>O<sub>5</sub> in comparison to the thermodynamically
stable α-V<sub>2</sub>O<sub>5</sub> phase. The rigid tunnel
framework, relatively small changes in coordination environment of
intercalated Li-ions across the diffusion pathways defined by the
1D tunnels, and degeneracy of V 3d states at the bottom of the conduction
band reduce electron localization that is a major impediment to charge
transport in α-V<sub>2</sub>O<sub>5</sub>. The 1D ζ-phase
thus facilitates a continuous lithiation pathway that is markedly
different from the successive intercalation-induced phase transitions
observed in α-V<sub>2</sub>O<sub>5</sub>. The results here illustrate
the importance of electronic structure in mediating charge transport
in oxide cathode materials and demonstrates that a metastable polymorph
with higher energy bonding motifs that define frustrated coordination
environments can serve as an attractive intercalation host
Mitigating Cation Diffusion Limitations and Intercalation-Induced Framework Transitions in a 1D Tunnel-Structured Polymorph of V<sub>2</sub>O<sub>5</sub>
The
design of cathodes for intercalation batteries requires consideration
of both atomistic and electronic structure to facilitate redox at
specific transition metal sites along with the concomitant diffusion
of cations and electrons. Cation intercalation often brings about
energy dissipative phase transformations that give rise to substantial
intercalation gradients as well as multiscale phase and strain inhomogeneities.
The layered α-V<sub>2</sub>O<sub>5</sub> phase is considered
to be a classical intercalation host but is plagued by sluggish diffusion
kinetics and a series of intercalation-induced phase transitions that
require considerable lattice distortion. Here, we demonstrate that
a 1D tunnel-structured ζ-phase polymorph of V<sub>2</sub>O<sub>5</sub> provides a stark study in contrast and can reversibly accommodate
Li-ions without a large distortion of the structural framework and
with substantial mitigation of polaronic confinement. Entirely homogeneous
lithiation is evidenced across multiple cathode particles (in contrast
to α-V<sub>2</sub>O<sub>5</sub> particles wherein lithiation-induced
phase transformations induce phase segregation). Barriers to Li-ion
as well as polaron diffusion are substantially diminished for metastable
ζ-V<sub>2</sub>O<sub>5</sub> in comparison to the thermodynamically
stable α-V<sub>2</sub>O<sub>5</sub> phase. The rigid tunnel
framework, relatively small changes in coordination environment of
intercalated Li-ions across the diffusion pathways defined by the
1D tunnels, and degeneracy of V 3d states at the bottom of the conduction
band reduce electron localization that is a major impediment to charge
transport in α-V<sub>2</sub>O<sub>5</sub>. The 1D ζ-phase
thus facilitates a continuous lithiation pathway that is markedly
different from the successive intercalation-induced phase transitions
observed in α-V<sub>2</sub>O<sub>5</sub>. The results here illustrate
the importance of electronic structure in mediating charge transport
in oxide cathode materials and demonstrates that a metastable polymorph
with higher energy bonding motifs that define frustrated coordination
environments can serve as an attractive intercalation host