44 research outputs found
Particle-Level Modeling of the Charge-Discharge Behavior of Nanoparticulate Phase-Separating Li-Ion Battery Electrodes
In nanoparticulate phase-separating electrodes, phase separation inside the
particles can be hindered during their charge/discharge cycles even when a
thermodynamic driving force for phase separation exists. In such cases,
particles may (de)lithiate discretely in a process referred to as mosaic
instability. This instability could be the key to elucidating the complex
charge/discharge dynamics in nanoparticulate phase-separating electrodes. In
this paper, the dynamics of the mosaic instability is studied using Smoothed
Boundary Method simulations at the particle level, where the concentration and
electrostatic potential fields are spatially resolved around individual
particles. Two sets of configurations consisting of spherical particles with an
identical radius are employed to study the instability in detail. The effect of
an activity-dependent exchange current density on the mosaic instability, which
leads to asymmetric charge/discharge, is also studied. While we show that our
model reproduces the results of a porous-electrode model for the simple setup
studied here, it is a powerful framework with the capability to predict the
detailed dynamics in three-dimensional complex electrodes and provides further
insights into the complex dynamics that result from the coupling of
electrochemistry, thermodynamics, and transport kinetics
Simulation of coarsening in two-phase systems with dissimilar mobilities
In this work, we apply phase field simulations to examine the coarsening
behavior of morphologically complex two-phase microstructures in which the
phases have highly dissimilar mobilities, a condition approaching that found in
experimental solid-liquid systems. Specifically, we consider a two-phase system
at the critical composition ( volume fraction) in which the mobilities of
the two phases differ by a factor of 100. This system is simulated in two and
three dimensions using the Cahn-Hilliard model with a concentration-dependent
mobility, and results are compared to simulations with a constant mobility. A
morphological transition occurs during coarsening of the two-dimensional system
(corresponding to a thin film geometry) with dissimilar mobilities, resulting
in a system of nearly-circular particles of high-mobility phase embedded in a
low-mobility matrix. This morphological transition causes the coarsening rate
constant to decrease over time, which explains why a previous study found lack
of agreement with the theoretical power law. Three-dimensional
systems with dissimilar mobilities resulted in bicontinuous microstructures
that evolve self-similarly, as determined by quantitative analysis of the
interfacial shape distribution. Coarsening kinetics in three dimensions agreed
closely with the power law after the initial transient stage. A model
is derived to explain a nearly-linear relationship between the coarsening rate
constant and the variance of scaled mean curvature that is observed during this
transient stage.Comment: 25 pages, 12 figure
Architecture Dependence on the Dynamics of Nano-LiFePO 4 Electrodes
a b s t r a c t Elucidating the role of interparticle Li transport and multi-particle (de)lithiation kinetics in nanoparticulate two-phase electrode materials such as LiFePO 4 is a challenging task because of the small temporal and spatial scale associated with the process. Often, the relevant processes that determine the kinetics of (dis)charging an electrode are assumed to be exclusively those associated with Li transport to and from the counter-electrode, without a consideration of interactions between particles. However, the redistribution of Li between nanoparticles can have a strong influence on the overall cell rate performance. Using a continuum model to simulate the lithiation kinetics of a porous aggregate of LiFePO 4 nanoparticles, we demonstrate the impact of cell architecture (in terms of ionic and electronic connectivities between active particles) and cycling rate on the multi-particle (de)lithiation kinetics. Specifically, the connectivity between particles is shown to have a strong effect on "interparticle phase separation," a process by which active particles undergo additional cycling (charge during the overall discharge) and amplified reaction rates. We show that interparticle phase separation can be reduced or eliminated by improving ("homogenizing") the connectivity between particles. Extensive comparisons to experimental literature and insights toward improving the performance of nanoparticulate electrodes are also provided
Charge Attachment Induced Transport – Bulk and Grain Boundary Diffusion of Potassium in PrMnO3
The transport of potassium through praseodymium-manganese oxide (PrMnO3; PMO) has been investigated by means of the charge attachment induced transport (CAIT) technique. To this end, potassium ions have been attached to the front side of a 250 nm thick sample of PMO. The majority of the potassium ions become neutralized at the surface of the PMO, while some of the potassium ions diffuse through. Ex situ analysis of the sample by time-of-flight secondary ion mass spectrometry (ToF-SIMS) reveals pronounced concentration profiles of the potassium, which is indicative of diffusion. Two diffusion coefficients have been obtained, namely, the bulk diffusion coefficient and the diffusion coefficient associated with the grain boundaries. The latter conclusion is supported by transmission electron microscopy of thin lamella cut out from the sample, which reveals twin grain boundaries reaching throughout the entire sample as well as model calculations
Localized concentration reversal of lithium during intercalation into nanoparticles.
Nanoparticulate electrodes, such as Li x FePO4, have unique advantages over their microparticulate counterparts for the applications in Li-ion batteries because of the shortened diffusion path and access to nonequilibrium routes for fast Li incorporation, thus radically boosting power density of the electrodes. However, how Li intercalation occurs locally in a single nanoparticle of such materials remains unresolved because real-time observation at such a fine scale is still lacking. We report visualization of local Li intercalation via solid-solution transformation in individual Li x FePO4 nanoparticles, enabled by probing sub-angstrom changes in the lattice spacing in situ. The real-time observation reveals inhomogeneous intercalation, accompanied with an unexpected reversal of Li concentration at the nanometer scale. The origin of the reversal phenomenon is elucidated through phase-field simulations, and it is attributed to the presence of structurally different regions that have distinct chemical potential functions. The findings from this study provide a new perspective on the local intercalation dynamics in battery electrodes
A thermal‐gradient approach to variable‐temperature measurements resolved in space
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155923/1/jcr2te5056.pd