Yue Qi

Melting and crystallization in Ni nanoclusters: The mesoscale regime

We studied melting and freezing of Ni nanoclusters with up to 8007 atoms ~5.7 nm! using molecular dynamics with the quantum-Sutten‐Chen many-body force field. We find a transition from cluster or molecular behavior below ;500 atoms to a mesoscale nanocrystal regime ~well-defined bulk and surface properties! above ;750 atoms ~2.7 nm!. We find that the mesoscale nanocrystals melt via surface processes, leading to Tm,N5Tm,bulk2aN 21/3 , dropping from Tm,bulk51760 K to Tm,336 5980 K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N.400 the supercooled clusters transform to FCC grains, but smaller values of N lead to a glassy structure with substantial icosahedral character.

Threefold Increase in the Young’s Modulus of Graphite Negative Electrode during Lithium Intercalation

Density functional theory (DFT) is used to reveal that the polycrystalline Youngs modulus (E) of graphite triples as it is lithiated to LiC 6 . This behavior is captured in a linear relationship between E and lithium concentration suitable for continuum-scale models aimed at predicting diffusion-induced deformation in battery electrode materials. Alternatively, Poissons ratio is concentration-independent. Charge-transfer analyses suggest simultaneous weakening of carbon-carbon bonds within graphite basal planes and strengthening of interlayer bonding during lithiation. The variation in bond strength is shown to be responsible for the differences between elasticity tensor components, C ij , of lithium-graphite intercalation (Li-GIC) phases. Strain accumulation during Li intercalation and deintercalation is examined with a core-shell model of a Li-GIC particle assuming two coexisting phases. The requisite force equilibrium uses different Youngs moduli computed with DFT. Lithium-poor phases develop tensile strains, whereas Li-rich phases develop compressive strains. Results from the core-shell model suggest that elastic strain should be defined relative to the newest phase that forms during lithiation of graphite, and Li concentration-dependent mechanical properties should be considered in continuum level models.

In Situ Observation of Strains during Lithiation of a Graphite Electrode

Electrodes in Li-ion batteries have complex microstructures. The polycrystalline particles of active materials are mixed with a binder and conductive carbon and then made into a porous composite. Dur- ing battery operation, Li diffuses into insertion and out of dein- sertion crystallites making up the active particles, usually causing the crystallites to expand or contract. This volume change may lead to stresses and stress-induced degradation that can be exhibited at several different length scales. 1 Intraparticle volume expansion of electrode crystallites has been observed with in situ X-ray diffrac- tion measurements 2-4 and has been predicted by theory. 5,6 Large volume expansions up to 400% corresponding to 60% strain if the expansion is isotropic occur in Si and Sn when they are lithiated, 7-11 which can lead to particle decrepitation. Graphite, the most commonly used negative electrode material, shows a volume expansion of up to 10%,

Mesoscale simulation of morphology in hydrated perfluorosulfonic acid membranes.

Current fuel cell proton exchange membranes rely on a random network of conducting hydrophilic domains to transport protons across the membrane. Despite extensive investigation, details of the structure of the hydrophilic domains in these membranes remain unresolved. In this study a dynamic self-consistent mean field theory has been applied to obtain the morphologies of hydrated perfluorosulfonic acid membranes (equivalent weight of 1100) as a model system for Nafion at several water contents. A coarse-grained mesoscale model was developed by dividing the system into three components: backbone, side chain, and water. The interaction parameters for this model were generated using classical molecular dynamics. The simulated morphology shows phase separated micelles filled with water, surrounded by side chains containing sulfonic groups, and embedded in the fluorocarbon matrix. The size distribution and connectivity of the hydrophilic domains were analyzed and the small angle neutron scattering (SANS) pattern was calculated. At low water content (lambda<6, where lambda is the number of water molecules per sulfonic group) the isolated domains obtained from simulation are nearly spherical with a domain size smaller than that fitted to experimental SANS data. At higher water content (lambda>8), the domains deform into elliptical and barbell shapes as they merge. The simulated morphology, hydrophilic domain size and shape are generally consistent with some experimental observations.

Using Atomic Layer Deposition to Hinder Solvent Decomposition in Lithium Ion Batteries: First-Principles Modeling and Experimental Studies

Passivating lithium ion (Li) battery electrode surfaces to prevent electrolyte decomposition is critical for battery operations. Recent work on conformal atomic layer deposition (ALD) coating of anodes and cathodes has shown significant technological promise. ALD further provides well-characterized model platforms for understanding electrolyte decomposition initiated by electron tunneling through a passivating layer. First-principles calculations reveal two regimes of electron transfer to adsorbed ethylene carbonate molecules (EC, a main component of commercial electrolyte), depending on whether the electrode is alumina coated. On bare Li metal electrode surfaces, EC accepts electrons and decomposes within picoseconds. In contrast, constrained density functional theory calculations in an ultrahigh vacuum setting show that, with the oxide coating, e(-) tunneling to the adsorbed EC falls within the nonadiabatic regime. Here the molecular reorganization energy, computed in the harmonic approximation, plays a key role in slowing down electron transfer. Ab initio molecular dynamics simulations conducted at liquid EC electrode interfaces are consistent with the view that reactions and electron transfer occur right at the interface. Microgravimetric measurements demonstrate that the ALD coating decreases electrolyte decomposition and corroborates the theoretical predictions.

Basal and prism dislocation cores in magnesium: comparison of first-principles and embedded-atom-potential methods predictions

A binary embedded-atom method (EAM) potential is optimized for Cu on Ag(111) by fitting to ab initio data. The fitting database consists of DFT calculations of Cu monomers and dimers on Ag(111), specifically their relative energies, adatom heights, and dimer separations. We start from the Mishin Cu-Ag EAM potential and first modify the Cu-Ag pair potential to match the FCC/HCP site energy difference then include Cu-Cu pair potential optimization for the entire database. The optimized EAM potential reproduce DFT monomer and dimer relative energies and geometries correctly. In trimer calculations, the potential produces the DFT relative energy between FCC and HCP trimers, though a different ground state is predicted. We use the optimized potential to calculate diffusion barriers for Cu monomers, dimers, and trimers. The predicted monomer barrier is the same as DFT, while experimental barriers for monomers and dimers are both lower than predicted here. We attribute the difference with experiment to the overestimation of surface adsorption energies by DFT and a simple correction is presented. Our results show that the optimized Cu-Ag EAM can be applied in the study of larger Cu islands on Ag(111).The core structures of screw and edge dislocations on the basal and prism planes in Mg, and the associated gamma surfaces, were studied using an ab initio method and the embedded-atom-method interatomic potentials developed by Sun et al and Liu et al. The ab initio calculations predict that the basal plane dislocations dissociate into partials split by 16.7 angstrom (edge) and 6.3 angstrom (screw), as compared with 14.3 angstrom and 12.7 angstrom (Sun and Liu edge), and 6.3 angstrom and 1.4 angstrom (Sun and Liu screw), with the Liu screw dislocation being metastable. In the prism plane, the screw and edge cores are compact and the edge core structures are all similar, while ab initio does not predict a stable prismatic screw in stress-free conditions. These results are qualitatively understood through an examination of the gamma surfaces for interplanar sliding on the basal and prism planes. The Peierls stresses at T = 0K for basal slip are a few megapascals for the Sun potential, in agreement with experiments, but are ten times larger for the Liu potential. The Peierls stresses for prism slip are 10-40MPa for both potentials. Overall, the dislocation core structures from ab initio are well represented by the Sun potential in all cases while the Liu potential shows some notable differences. These results suggest that the Sun potential is preferable for studying other dislocations in Mg, particularly the textless c + a textgreater dislocations, for which the core structures are much larger and not accessible by ab initio methods.

Effects of Concentration-Dependent Elastic Modulus on Diffusion-Induced Stresses for Battery Applications

Most lithium-ion battery electrodes experience large volume changes associated with Li concentration changes within the host particles during charging and discharging. Electrode failure, in the form of fracture or decrepitation, can occur as a result of repeated volume changes. It has been found recently that many electrode materials, such as graphite, Si, and LiFePO 4 , change their elastic properties upon lithiation. However, previous diffusion-induced stress (DIS) models have not considered this relationship. In this paper, we developed a mathematical model, with the assumption of a homogeneous isotropic cylindrical electrode particle, to describe the effect of concentration-dependent Youngs modulus on DIS in battery electrodes. The DIS model considers both increasing and decreasing Youngs modulus with concentration. The model shows that the concentration dependence of Youngs modulus has a significant effect on peak stress and stress evolution in the electrodes. Insertion and deinsertion are not symmetric in stress profiles. We conclude that Li stiffening is beneficial to avoid surface cracking during delithiation, and moderate Li softening is beneficial to avoid particle cracking from the center during lithiation.

The mixing mechanism during lithiation of Si negative electrode in Li-ion batteries: an ab initio molecular dynamics study.

In order to realize Si as a negative electrode material in commercial Li-ion batteries, it is important to understand the mixing mechanism of Li and Si, and stress evolution during lithiation in Si negative electrode of Li-ion batteries. Available experiments mainly provide the diffusivity of Li in Si as an averaged property, neglecting information regarding diffusivity of Si. However, if Si can diffuse as fast as Li, the stress generated during Li diffusion can be reduced. We, therefore, studied the diffusivity of Li as well as Si atoms in the Si-anode of Li-ion battery using an ab initio molecular dynamics-based methodology. The electrochemical insertion of Li into crystalline Si prompts a crystalline-to-amorphous phase transition. We considered this situation and thus examined the diffusion kinetics of Li and Si atoms in both crystalline and amorphous Si. We find that Li diffuses faster in amorphous Si as compared to crystalline Si, while Si remains relatively immobile in both cases and generates stresses during lithiation. To further understand the mixing mechanism and to relate the structure with electrochemical mixing, we analyzed the evolution of the structure during lithiation and studied the mechanism of breaking of Si-Si network by Li. We find that Li atoms break the Si rings and chains and create ephemeral structures such as stars and boomerangs, which eventually transform to Si-Si dumbbells and isolated Si atoms in the LiSi phase. Our results are found to be in agreement with the available experimental data and provide insights into the mixing mechanism of Li and Si in Si negative electrode of Li-ion batteries.

Calculation of Mechanical, Thermodynamic and Transport Properties of Metallic Glass Formers

Recently, we have parametrized Sutton-Chen type empirical many body force elds for FCC transition metals to study the thermodynamic, mechanical, transport and phase behavior of metals and their alloys. We have utilized these potentials in lattice dynamics calculations and molecular dynamics simulations to describe the structure, thermodynamic, mechanical and transport properties of pure metals and binary alloys in solid, liquid and glass phases. Here, we will describe these applications: mechanical properties of binary alloys (Pt ? Rh) and viscosity of a binary alloy, (Au ? Cu), as a function of composition , temperature, and shear rate, crystal-liquid, liquid-crystal phase transformation in (Ni ? Cu), liquid to glass transformation in a model glass former, (Ag ? Cu).

Visualizing the chemistry and structure dynamics in lithium-ion batteries by in-situ neutron diffraction

We report an in-situ neutron diffraction study of a large format pouch battery cell. The succession of Li-Graphite intercalation phases was fully captured under an 1C charge-discharge condition (i.e., charge to full capacity in 1 hour). However, the lithiation and dilithiation pathways are distinctively different and, unlike in slowing charging experiments with which the Li-Graphite phase diagram was established, no LiC24 phase was found during charge at 1C rate. Approximately 75 mol. % of the graphite converts to LiC6 at full charge, and a lattice dilation as large as 4% was observed during a charge-discharge cycle. Our work demonstrates the potential of in-situ, time and spatially resolved neutron diffraction study of the dynamic chemical and structural changes in “real-world” batteries under realistic cycling conditions, which should provide microscopic insights on degradation and the important role of diffusion kinetics in energy storage materials.