Eunseog Cho

Computational Screening for Design of Optimal Coating Materials to Suppress Gas Evolution in Li-Ion Battery Cathodes

Ni-rich layered oxides are attractive materials owing to their potentially high capacity for cathode applications. However, when used as cathodes in Li-ion batteries, they contain a large amount of Li residues, which degrade the electrochemical properties because they are the source of gas generation inside the battery. Here, we propose a computational approach to designing optimal coating materials that prevent gas evolution by removing residual Li from the surface of the battery cathode. To discover promising coating materials, the reactions of 16 metal phosphates (MPs) and 45 metal oxides (MOs) with the Li residues, LiOH, and Li2CO3 are examined within a thermodynamic framework. A materials database is constructed according to density functional theory using a hybrid functional, and the reaction products are obtained according to the phases in thermodynamic equilibrium in the phase diagram. In addition, the gravimetric efficiency is calculated to identify coating materials that can eliminate Li residues with a minimal weight of the coating material. Overall, more MP and MO materials react with LiOH than with Li2CO3. Specifically, MPs exhibit better reactivity to both Li residues, whereas MOs react more with LiOH. The reaction products, such as Li-containing phosphates or oxides, are also obtained to identify the phases on the surface of a cathode after coating. On the basis of the Pareto-front analysis, P2O5 could be an optimal material for the reaction with both Li residuals. Finally, the reactivity of the coating materials containing 3d/4d transition metal elements is better than that of materials containing other types of elements.

Charge transfer and nonadiabatic dynamics of diatomic anions in clusters

We have studied the photodissociation and recombination dynamics of diatomic anions in size-selected clusters by using simple model systems. The main purpose of the study is to provide a theoretical background for a better understanding of the salient features of the charge transfer and nonadiabatic transitions involved in the dynamics of solvated molecular ions. Calculations have been performed on the photodissociation and recombination of the model diatomic anion X2− embedded in N2O and CO2 clusters. The homonuclear diatomic anion is modeled as one-electron system consisting of two identical nuclei and an extra electron. The nuclear and electronic dynamics of X2− are treated quantum mechanically, while the motions of the solvent molecules are described by classical dynamics. Nonadiabatic theoretical calculations, in which the electronic and the nuclear dynamics are treated simultaneously, can reveal the importance of nonadiabatic effects by including intrinsically all electronic states. It is found that...

Structure and dynamics of I2−(N2O)n: Monte Carlo and molecular dynamics simulations

The structures and relaxation dynamics of I2− embedded in clusters of N2O molecules are studied by Monte Carlo and molecular dynamics simulations. The equilibrium structures of I2−(N2O)n clusters are obtained as a function of cluster size and the closing of the first solvation shell is found to occur at n=13, consistent with experimental observation. By comparing with the previous studies with different types of solvent molecules, it is found that differences in solvent polarity lead to noticeable changes in equilibrium structures and caging dynamics of clusters. N2O clusters tend to form more symmetric, spread-out solvent configurations, resulting in a weaker solvent electric field being exerted on the solute. The localization of the charge distribution for large internuclear separations happens for longer bond length and much more rapidly in I2−(N2O)16 than in I2−(CO2)16 clusters. Molecular dynamics simulations showed that I2− vibrational relaxation is very rapid, losing almost 90% of its internal energ...

Theoretical Prediction of Surface Stability and Morphology of LiNiO2 Cathode for Li Ion Batteries

Ni-rich layered oxides are considered to be a promising cathode material with high capacity, and their surface structure should be extensively explored to understand the complex associated phenomena. We investigated the surface stability and morphology of LiNiO2 as a representative of these materials by using density functional theory calculations. The results reveal that the Li-exposed surfaces have lower energies than the oxygen surfaces, irrespective of the facets, and the Ni-exposed ones are the least stable. The equilibrium morphology can vary from truncated trigonal bipyramid to truncated egg shape, according to the chemical potential, whose range is confined by the phase diagram. Moreover, the electrochemical window of stable facets is found to strongly depend on the surface elements rather than the facet directions. Contrary to the stable Li surfaces, oxygen exposure on the surface considerably lowers the Fermi level to the level of electrolyte, thereby accelerating oxidative decomposition of the electrolyte on the cathode surface.

Theoretical prediction of fracture conditions for delithiation in silicon anode of lithium ion battery

Structural instability such as fractures of a silicon anode in a lithium ion battery, intrinsically induced by the large variation of the ratio, Li/Si, upon lithiation and delithiation, limits its potential for commercial use. Here, we study mechanical properties during delithiation in lithiated silicon particles to identify the conditions under which fracture is preventing during delithiation in terms of Li contents and silicon particle sizes. We employed the first principles calculation within the density functional framework combined with the continuum based calculation for the macroscopic mechanical properties. The theoretical limit for the largest crystalline silicon particle size that can prevent fractures upon complete delithiation is ∼0.6 μm at the lithium flux per unit surface area of 5.657 × 10−2 s−1 nm out of amorphous Li3.75Si, much larger than the critical fracture size (0.15 μm) that occurs during the first lithiation of crystalline Si. Furthermore, fractures during delithiation are nearly u...

Machine learning assisted optimization of electrochemical properties for Ni-rich cathode materials

Optimizing synthesis parameters is the key to successfully design ideal Ni-rich cathode materials that satisfy principal electrochemical specifications. We herein implement machine learning algorithms using 330 experimental datasets, obtained from a controlled environment for reliability, to construct a predictive model. First, correlation values showed that the calcination temperature and the size of the particles are determining factors for achieving a long cycle life. Then, we compared the accuracy of seven different machine learning algorithms for predicting the initial capacity, capacity retention rate, and amount of residual Li. Remarkable predictive capability was obtained with the average value of coefficient of determinant, R2 = 0.833, from the extremely randomized tree with adaptive boosting algorithm. Furthermore, we propose a reverse engineering framework to search for experimental parameters that satisfy the target electrochemical specification. The proposed results were validated by experiments. The current results demonstrate that machine learning has great potential to accelerate the optimization process for the commercialization of cathode materials.

Characterization of Mechanical Degradation in Perfluoropolyether Film for Its Application to Antifingerprint Coatings

Enhancing the mechanical durability of antifingerprint films is critical for its industrial application on touch-screen devices to withstand friction damage from repeated rubbing in daily usage. Using reactive molecular dynamics simulations, we herein implement adhesion, mechanical, and deposition tests to investigate two durability-determining factors: intrachain and interchain strength, which affect the structural stability of the antifingerprint film (perfluoropolyether) on silica. From the intrachain perspective, it is found that the Si-C bond in the polymer chain is the weakest, and therefore prone to dissociation and potentially forming a C-O bond. This behavior is demonstrated consistently, regardless of the cross-linking density between polymer chains. For the interchain interaction, increasing the chain length enhances the mechanical properties of the film. Furthermore, the chain deposition test, mimicking the experimental coating process, demonstrates that placing shorter chains first to the surface of silica and then depositing longer chains is an ideal way to improve the interchain interaction in the film structure. The current study reveals a clear pathway to optimize the configuration of the polymer chain as well as its film structure to prolong the product life of the coated antifingerprint film.

Nonadiabatic dynamics of charge transfer in diatomic anion clusters

We have studied the photodissociation and recombination dynamics of the diatomic anions X(2)(-) and XY(-) designed to mimic I(2)(-) and ICl(-), respectively, by using a one-electron model in size-selected N(2)O clusters. The one-electron model is composed of two nuclei and an extra electron moving in a two-dimensional plane including the two nuclei. The main purpose of this study is to explain the salient features of various dynamical processes of molecular ions in clusters using a simple theoretical model. For heteronuclear diatomic anions, a mass disparity and asymmetric electron affinity between the X and Y atoms lead to different phenomena from the homonuclear case. The XY(-) anion shows efficient recombination for a smaller cluster size due to the effect of collision-mediated energy transfer and an inherent potential wall on excited state at asymptotic region, while the recombination for the X(2)(-) anion is due to rearrangement of solvent configuration and faster nonadiabatic transitions. The results of the present study illustrate the microscopic details of the electronically nonadiabatic processes which control the photodissociation dynamics of molecular ions in clusters.