9 research outputs found
Temperature Extrapolation of Molecular Dynamics Simulations of Complex Chemistry to Microsecond Timescales Using Kinetic Models: Applications to Hydrocarbon Pyrolysis
We
develop a method to construct temperature-dependent kinetic
models of hydrocarbon pyrolysis, based on information from molecular
dynamics (MD) simulations of pyrolyzing systems in the high-temperature
regime. MD simulations are currently a key tool to understand the
mechanism of complex chemical processes such as pyrolysis and to observe
their outcomes in different conditions, but these simulations are
computationally expensive and typically limited to nanoseconds of
simulation time. This limitation is inconsequential at high temperatures,
where equilibrium is reached quickly, but at low temperatures, the
system may not equilibrate within a tractable simulation timescale.
In this work, we develop a method to construct kinetic models of hydrocarbon
pyrolysis using the information from the high-temperature high-reactivity
regime. We then extrapolate this model to low temperatures, which
enables microsecond-long simulations to be performed. We show that
this approach accurately predicts the time evolution of small molecules,
as well as the size and composition of long carbon chains across a
wide range of temperatures and compositions. Further, we show that
the range of suitable temperatures for extrapolation can easily be
improved by adding more simulations to the training data. Compared
to experimental results, our kinetic model leads to similar compositional
trends while allowing for more detailed kinetic and mechanistic insights
Data Mining for New Two- and One-Dimensional Weakly Bonded Solids and Lattice-Commensurate Heterostructures
Layered materials held together
by weak interactions including van der Waals forces, such as graphite,
have attracted interest for both technological applications and fundamental
physics in their layered form and as an isolated single-layer. Only
a few dozen single-layer van der Waals solids have been subject to
considerable research focus, although there are likely to be many
more that could have superior properties. To identify a broad spectrum
of layered materials, we present a novel data mining algorithm that
determines the dimensionality of weakly bonded subcomponents based
on the atomic positions of bulk, three-dimensional crystal structures.
By applying this algorithm to the Materials Project database of over
50,000 inorganic crystals, we identify 1173 two-dimensional layered
materials and 487 materials that consist of weakly bonded one-dimensional
molecular chains. This is an order of magnitude increase in the number
of identified materials with most materials not known as two- or one-dimensional
materials. Moreover, we discover 98 weakly bonded heterostructures
of two-dimensional and one-dimensional subcomponents that are found
within bulk materials, opening new possibilities for much-studied
assembly of van der Waals heterostructures. Chemical families of materials,
band gaps, and point groups for the materials identified in this work
are presented. Point group and piezoelectricity in layered materials
are also evaluated in single-layer forms. Three hundred and twenty-five
of these materials are expected to have piezoelectric monolayers with
a variety of forms of the piezoelectric tensor. This work significantly
extends the scope of potential low-dimensional weakly bonded solids
to be investigated
High-Performance Lithium Metal Negative Electrode with a Soft and Flowable Polymer Coating
The future development of low-cost, high-performance electric vehicles depends on the success of next-generation lithium-ion batteries with higher energy density. The lithium metal negative electrode is key to applying these new battery technologies. However, the problems of lithium dendrite growth and low Coulombic efficiency have proven to be difficult challenges to overcome. Fundamentally, these two issues stem from the instability of the solid electrolyte interphase (SEI) layer, which is easily damaged by the large volumetric changes during battery cycling. In this work, we show that when a highly viscoelastic polymer was applied to the lithium metal electrode, the morphology of the lithium deposition became significantly more uniform. At a high current density of 5 mA/cm2 we obtained a flat and dense lithium metal layer, and we observed stable cycling Coulombic efficiency of ???97% maintained for more than 180 cycles at a current density of 1 mA/cm2.clos
Atomic Layer Deposition of Stable LiAlF<sub>4</sub> Lithium Ion Conductive Interfacial Layer for Stable Cathode Cycling
Modern
lithium ion batteries are often desired to operate at a
wide electrochemical window to maximize energy densities. While pushing
the limit of cutoff potentials allows batteries to provide greater
energy densities with enhanced specific capacities and higher voltage
outputs, it raises key challenges with thermodynamic and kinetic stability
in the battery. This is especially true for layered lithium transition-metal
oxides, where capacities can improve but stabilities are compromised
as wider electrochemical windows are applied. To overcome the above-mentioned
challenges, we used atomic layer deposition to develop a LiAlF<sub>4</sub> solid thin film with robust stability and satisfactory ion
conductivity, which is superior to commonly used LiF and AlF<sub>3</sub>. With a predicted stable electrochemical window of approximately
2.0 ± 0.9 to 5.7 ± 0.7 V <i>vs</i> Li<sup>+</sup>/Li for LiAlF<sub>4</sub>, excellent stability was achieved for high
Ni content LiNi<sub>0.8</sub>Mn<sub>0.1</sub>Co<sub>0.1</sub>O<sub>2</sub> electrodes with LiAlF<sub>4</sub> interfacial layer at a
wide electrochemical window of 2.75–4.50 V <i>vs</i> Li<sup>+</sup>/Li