Modeling and Simulation of Nanoparticulate Lithium Iron Phosphate Battery Electrodes.

Elucidating the complex charge/discharge dynamics in nanoparticulate phase-separating electrode materials such as lithium iron phosphate, LiFePO4, is a challenging task because of the small temporal and spatial scale associated with the material and the process. During the charge/discharge cycles of nanoparticulate LiFePO4 electrodes, phase separation inside the particles can be hindered even when a thermodynamic driving force for phase separation exists. In such cases, particles may (de)lithiate via a process referred to as interparticle phase separation, which involves Li redistribution between particles. The role of interparticle Li transport and multi-particle (de)lithiation kinetics could be the key to understand these processes. In this thesis, the complex dynamics of lithium iron phosphate is investigated based on the particle-level electrochemical dynamics (PLED) and the porous electrode theory (PET). PLED combined with a phase field model and the Smoothed Boundary Method is utilized to study the kinetic processes of interparticle phase separation. Using this approach, simple two-particle systems are examined to elucidate the detailed dynamics of the lithiation/delithiation process. Additionally, more realistic structures consisting of many particles are utilized to analyze more complex cases of interparticle phase separation. The dependence of the electrochemical dynamics on (i) the exchange current density, (ii) the particle position, (iii) the presence of intraparticle phase separation, (iv) the particle size distribution, (v) the particle connectivity, and (vi) the equilibrium potential are elucidated. Simulations based on PET are employed to examine the overall behavior of the cell; these simulations elucidate the position dependence of the electrochemical dynamics on a coin-cell battery experimentally mapped. This thesis presents a comprehensive study on the interactions between LiFePO4 nanoparticles and their effect on battery performance.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/110427/1/orvanano_1.pd

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

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

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

Kinetics of Nanoparticle Interactions in Battery Electrodes

Nanoparticles with a tendency to phase separate interact with each other during a process of either intraparticle phase separation (occurring inside the particle) or interparticle phase separation (occurring between particles). In this paper, we examine a half-cell consisting of two particles to systematically analyze the particle interactions and their resulting voltage response at different insertion currents. The kinetics of the interactions between nanoparticles is studied for cases in which particles undergo interparticle and intraparticle phase separation. Our results indicate that the interactions between particles in a cell containing only particles that undergo intraparticle phase separation are similar to those in a cell containing only particles that undergo interparticle phase separation. In both cases, sequential transformation occurs at low currents, whereas a simultaneous transformation occurs at high currents. However, such transformation dynamics changes in cases where the cell contains a mixture of particles that undergo interparticle and intraparticle phase separations, which exhibits more complex dynamics.United States. Dept. of Energy. Office of Basic Energy Sciences. NorthEast Center for Chemical Energy Storage (Award DE-SC0001294)United States. Dept. of Energy. Office of Basic Energy Sciences. NorthEast Center for Chemical Energy Storage (Award DE-SC0012583

Localized concentration reversal of lithium during intercalation into nanoparticles

Nanoparticulate electrodes, such as LixFePO4, 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 LixFePO4 nano-particles, 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

Mapping the Inhomogeneous Electrochemical Reaction Through Porous LiFePO₄‑Electrodes in a Standard Coin Cell Battery

Nanosized, carbon-coated LiFePO₄ (LFP) is a promising cathode for Li-ion batteries. However, nano-particles are problematic for electrode design, optimized electrodes requiring high tap densities, good electronic wiring, and a low tortuosity for efficient Li diffusion in the electrolyte in between the solid particles, conditions that are difficult to achieve simultaneously. Using <i>in situ</i> energy-dispersive X-ray diffraction, we map the evolution of the inhomogeneous electrochemical reaction in LFP-electrodes. On the first cycle, the dynamics are limited by Li diffusion in the electrolyte at a cycle rate of C/7. On the second cycle, there appear to be two rate-limiting processes: Li diffusion in the electrolyte and electronic conductivity through the electrode. Three-dimensional modeling based on porous electrode theory shows that this change in dynamics can be reproduced by reducing the electronic conductivity of the composite electrode by a factor of 8 compared to the first cycle. The poorer electronic wiring could result from the expansion and contraction of the particles upon cycling and/or the formation of a solid-electrolyte interphase layer. A lag was also observed perpendicular to the direction of the current: the LFP particles at the edges of the cathode reacted preferentially to those in the middle, owing to the closer proximity to the electrolyte source. Simulations show that, at low charge rates, the reaction becomes more uniformly distributed across the electrode as the porosity or the width of the particle-size distribution is increased. However, at higher rates, the reaction becomes less uniform and independent of the particle-size distribution