arXiv subjects

Kevin Leung

Publications and source records attributed to Kevin Leung.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous Ion Confinement Penalties and Giant Ion-Screening Effects in One-Dimensional Nanopores

Nanoconfinement reduces the favorable hydration free energies of single ions, which is correlated with ion rejection and modified chemical reactivity in water-filled nanopores. Many factors contribute to the magnitude of the observed confinement effect. Here we use simple classical force fields and non-polarizable carbon nanotubes filled with water as minimal, "hydrogen atom"-like models to evaluate the single-ion intrinsic confinement hydration free energy penalty (Delta Delta G(hyd)). In tubes of radius R=7.5 Angstrom, we predict Delta Delta G(hyd)'s that are up to 7.8 kcal/mol, are much larger for Cl$^-$ than the smaller Na+ ion, and contradict the canonical Born Equation for ion solvation. Adding a 1.0~M background electrolyte reduces Delta Delta G(hyd) for the Na$^+$/Cl$^-$ pair by an amount exceeding the Debye-Huckel estimate in unconfined media by almost an order of magnitude. We identify concentration-dependent ion-screening of confinement effects as a major, unheralded consequence of electrolytes in cylindrical nanopores.

cond-mat.soft↗

arXiv2022

Adsorption at Nanoconfined Solid-Water Interfaces

Reactions at solid-water interfaces play a foundational role in water treatment systems, catalysis, chemical separations, and in predicting chemical fate and transport in the environment. Over the last century, experimental measurements and computational models have made tremendous progress in capturing reactions at solid surfaces. The interfacial reactivity of a solid surface, however, can change dramatically and unexpectedly when it is confined to the nanoscale. Nanoconfinement can arise in different geometries such as pores/cages (3-D confinement), channels (2-D confinement) and slits (1-D confinement). Therefore, measurements on unconfined surfaces, and molecular models parameterized based on these measurements, fail to capture chemical behaviors under nanoconfinement. This review evaluates recent experimental and theoretical advances, with a focus on adsorption at solid-water interfaces. We review how nanoconfinement alters the physico-chemical properties of water, and how the structure and dynamics of nanoconfined water dictate energetics, pathways, and products of adsorption in nanopores. The implications of these findings and future research directions are discussed.

physics.chem-ph↗

arXiv2022

First Principles Determination of the Potential-of-Zero-Charge in an Alumina-coated Aluminum/Water Interface Model for Corrosion Applications

The surfaces of most metals immersed in aqueous electrolytes have a several-nanometer-thick oxide/hydroxide surface layer. This gives rise to the existence of both metal|oxide and oxide|liquid electrotlyte interfaces, and makes it challenging to correlate atomic length-scale structures with electrochemical properties such the potential-of-zero-charge (PZC). The PZC value has been shown to be correlated the pitting onset potential for corrosion. In this work, we conduct large-scale Density Functional Theory and ab initio molecular dynamics to calculate the PZC of a Al(111)|gamma-Al(2)O(3)(110)|water double-interface model within the context of aluminum corrosion. By partitioning the multiple interfaces involved into binary components with additive contributions to the overall work function and voltage, we predict the PZC to be -1.53 V vs. SHE for this model. We also calculate the orbital energy levels of defects like oxygen vacancies in the oxide, which are critical parameters in theories associated with pitting corrosion. We predict that the Fermi level at the PZC lies above the impurity defect levels of the oxygen vacancies, which are therefore uncharged at the PZC. From the PZC estimate, we predict the voltage needed to create oxygen vacancies with net positive charges within a flat-band approximation.

cond-mat.mtrl-sci↗

arXiv2022

Galvanic Corrosion and Electric Field in Lithium Anode Passivation Films: Effects on Self-Discharge

Battery interfaces help govern rate capability, safety/stability, cycle life, and self-discharge, but significant gaps remain in our understanding at atomic length scales that can be exploited to improve interfacial properties. In particular, Li partially plated on copper current collectors, relevant to the anodeless, lithium metal cell which is a holy grail of high density energy battery research, has recently been reported to undergo galvanic corrosion and exhibit short shelf lives. We apply large scale Density Functional Theory (DFT) calculations and X-ray photoelectron spectroscopy to examine the reaction between the electrolyte and Li|Cu junctions coated with thin, uniform electrolyte interphase (SEI) passivating films at two applied voltages. These novel DFT galvanic corrosion simulations show that electrolyte degradation preferentially occurs on Li-plated regions and should lead to thicker SEI films. Our simulations reveal similarities but also fundamental differences between traditional metal localized pitting and Li-corrosion mechanisms. Furthermore, using the recently proposed, highly reactive lithium hydride (LiH) component SEI as example, we distinguish between electrochemical and chemical degradation pathways which are partially responsible for self-discharge, with the chemical pathway found to exhibit slow kinetics. We also predict that electric fields should in general exist across natural SEI components like LiH, and across artificial SEI films like LiI and LiAlO(2) often applied to improve battery cycling. Underlying and unifying these predictions is a framework of DFT voltage/overpotential definitions which we have derived from electrochemistry disciplines like structural metal corrosion studies; our analysis can only be made using the correct electronic voltage definitions.

cond-mat.mtrl-sci↗

arXiv2021

First Principles, Explicit Interface Studies of Oxygen Vacancy and Chloride in Alumina Films for Corrosion Applications

Pitting corrosion is a much-studied and technologically relevant subject. However, the fundamental mechanisms responsible for the breakdown of the passivating oxide layer are still subjects of debate. Chloride anions are known to accelerate corrosion; relevant hypotheses include Cl insertion into positively charged oxygen vacancies in the oxide film, and Cl adsorption on passivating oxide surfaces, substituting for surface hydroxyl groups. In this work, we conduct large-scale first principles modeling of explicit metal/Al(2)O(3) interfaces to investigate the energetics and electronic structures associated with these hypotheses. The explicit interface models allow electron transfer that mimics electrochemical events, and the establishment of the relation between atomic structures at different interfaces and the electronic band alignment. For multiple model interfaces, we find that doubly charged oxygen vacancies, which are key ingredients of the point defect model (PDM) often used to analyze corrosion data, can only occur in the presence of a potential gradient that raises the voltage. Cl-insertion into oxide films can be energetically favorable in some oxygen vacancy sites, depending on the voltage. We also discuss the challenges associated with explicit DFT modeling of these complex interfaces.

cond-mat.mtrl-sci↗

arXiv2021

Edge-Propagation Discharge Mechanism in CFx Batteries -- a First Principles and Experimental Study

Graphite fluoride (CFx) cathodes coupled with lithium anodes yield one of the highest theoretical energy densities (>860 Wh/g) among primary batteries. In practice, the observed discharge voltage (~2.5 V) is significantly lower than thermodynamic limits (>4.5 V), the discharge rate is low, and so far Li/CFx has only been used in primary batteries. Understanding the discharge mechanism at atomic length scales will improve practical CFx energy density, rate capability, and rechargeability. So far, purely experimental techniques have not identified the correct discharge mechanism or explained the discharge voltage. We apply Density Functional Theory calculations to demonstrate that a CFx-edge propagation discharge mechanism based on lithium insertion at the CF/C boundary in partially discharged CFx exhibits a voltage range of 2.5 to 2.9 V -- depending on whether solvent molecules are involved. The voltages and solvent dependence agrees with our discharge and galvanostatic intermittent titration technique measurements. The predicted discharge kinetics are consistent with CFx operations. Finally, we predict Li/CFx rechargeability under the application of high potentials, along a charging pathway different from that of discharge. Our work represents a general, quasi-kinetic framework to understand the discharge of conversion cathodes, circumventing the widely used phase diagram approach which most likely does not apply to Li/CFx because equilibrium conditions are not attained in this system.

cond-mat.mtrl-sci↗

arXiv2021

Interplay of Physically Different Properties Leading to Challenges in Separating Lanthanide Cations -- an Ab Initio Molecular Dynamics and Experimental Study

The lanthanide elements have well-documented similarities in their chemical behavior, which makes the valuable trivalent lanthanide cations (Ln(III)) particularly difficult to separate from each other in water. In this work, we apply ab initio molecular dynamics simulations to compare the free energies (Delta G(ads)) associated with the adsorption of lanthanide cations to silica surfaces at a pH condition where SiO- groups are present. The predicted Delta G(ads) for lutetium (Lu(III)) and europium (Eu(III)) are similar within statistical uncertainties; this is in qualitative agreement with our batch adsorption measurements on silica. This finding is remarkable because the two cations exhibit hydration free energies (Delta G(hyd}) that differ by >2 eV, different hydration numbers, and different hydrolysis behavior far from silica surfaces. We observe that the similarity in Lu(III) and Eu(III) Delta G(ads) is the result of a delicate cancellation between the difference in Eu(III) and Lu(III) hydration (Delta G(hyd})), and their difference in binding energies to silica. We propose that disrupting this cancellation at the two end points, either for adsorbed or completely desorbed lanthanides (e.g., via nanoconfinment or mixed solvents), will lead to effective Ln separation.

cond-mat.mtrl-sci↗

arXiv2020

DFT Modelling of Explicit Solid-Solid Interfaces in Batteries: Methods and Challenges

Density Functional Theory (DFT) calculations of electrode material properties in high energy density storage devices like lithium batteries have been standard practice for decades. In contrast, DFT modelling of explicit interfaces in batteries arguably lacks universally adopted methodology and needs further conceptual development. In this paper, we focus on solid-solid interfaces, which are ubiquitous not just in all-solid state batteries; liquid-electrolyte-based batteries often rely on thin, solid passivating films on electrode surfaces to function. We use metal anode calculations to illustrate that explicit interface models are critical for elucidating contact potentials, electric fields at interfaces, and kinetic stability with respect to parasitic reactions. The examples emphasize three key challenges: (1) the "dirty" nature of most battery electrode surfaces; (2) voltage calibration and control; and (3) the fact that interfacial structures are governed by kinetics, not thermodynamics. To meet these challenges, developing new computational techniques and importing insights from other electrochemical disciplines will be beneficial.

cond-mat.mtrl-sci↗

arXiv2020

Anodic Decomposition of Surface Films on High Voltage Spinel Surfaces -- Density Function Theory and Experimental Study

Oxidative decomposition of organic-solvent-based liquid electrolytes at cathode material interfaces has been identified as a main reason for rapid capacity fade in high-voltage lithium ion batteries. The evolution of "cathode electrolyte interphase: (CEI) films, partly or completely consisting of electrolyte decomposition products, has also recently been demonstrated to be correlated with battery cycling behavior at high potentials. Using Density Functional Theory (DFT) calculations, the hybrid PBE0 functional, and the (001) surfaces of spinel oxides as models, we examine these two interrelated processes. Consistent with previous calculations, ethylene carbonate (EC) solvent molecules are predicted to be readily oxidized on the Li(x)Mn(2)O(4) (001) surface at modest operational voltages, forming adsorbed organic fragments. Further oxidative decompostion of such CEI fragments to release CO2 gas is however predicted to require higher voltages consistent with Li(x)Ni(0.5)Mn(1.5)O(4) (LNMO) at smaller x values. We argue that multi-step reactions, involving first formation of CEI films and then further oxidization of CEI at higher potentials, are most relevant to capacity fade. Mechanisms associated with dissolution or oxidation of native Li2CO3 films, which is removed before the electrolyte is in contact with oxide surfaces, are also explored.

cond-mat.mtrl-sci↗

arXiv2018

Concerted Metal Cation Desorption and Proton Transfer on Deprotonated Silica Surfaces

The adsorption equilibrium constants of monovalent and divalent cations to material surfaces in aqueous media are central to many technological, natural, and geochemical processes. Cation adsorption/desorption is often proposed to occur in concert with proton-transfer on hydroxyl-covered mineral surfaces, but so far this cooperative effect has been inferred indirectly. This work applies Density Functional Theory (DFT)-based molecular dynamics simulations of explicit liquid water/mineral interfaces to calculate metal ion desorption free energies. Monodentate adsorption of Na(+), Mg(2+), and Cu(2+) on partially deprotonated silica surfaces are considered. Na(+) is predicted to be unbound, while Cu(2+) exhibits larger binding free energies to surface SiO(-) groups than Mg(2+). The predicted trends agree with competitive adsorption measurements on fumed silica surfaces. As desorption proceeds, Cu(2+) dissociates one of the H2O molecules in its first solvation shell, turning into Cu(2+)O(-)(H2O)(3), while Mg remains Mg(2+)(H2O)(6). The protonation state of the SiO(-) group at the initial binding site does not vary monotonically with cation desorption.

physics.chem-ph↗

arXiv2018

Kinetics-Controlled Degradation Reactions at Crystalline LiPON/Li(x)CoO(2) and Crystalline LiPON/Li-metal Interfaces

Detailed understanding of solid-solid interface structure-function relations is critical for the improvement and wide deployment of all solid-state batteries. The interfaces between lithium phosphorous oxynitride ("LiPON") solid electrolyte material and lithium metal anode, between LiPON and Li(x)CoO(2) cathode surfaces, have been reported to generate solid electrolyte interphase ("SEI")-like products and/or disordered regions. Using electronic structure calculations and crystalline LiPON models with atomic-layer-deposition-like stoichiometry, we predict LiPON models with purely P-N-P backbones are kinetically inert towards lithium at room temperature. In contrast, transfer of oxygen atoms from low-energy Li(x)CoO(2) (104) surfaces to LiPON is much faster under ambient conditions. The mechanisms of the primary reaction steps, LiPON motifs that readily react with lithium metal, experimental results on amorphous LiPON to partially corroborate these predictions, and possible mitigation strategies to reduce degradations are discussed. LiPON interfaces are found to be useful case studies for highlighting the importance of kinetics-controlled processes during battery assembly at moderate processing temperatures.

cond-mat.mtrl-sci↗

arXiv2017

Spatial Heterogeneities and Onset of Passivation Breakdown at Lithium Anode Interfaces

Effective passivation of lithium metal surfaces, and prevention of battery-shorting lithium dendrite growth, are critical for implementing lithium-metal-anodes for batteries with increased power densities. Nanoscale surface heterogeneities can be "hot spots" where anode passivation breaks down. Motivated by the observation of lithium dendrites in pores and grain boundaries in all-solid batteries, we examine lithium metal surfaces covered with Li(2)O and/or LiF thin films with grain boundaries in them. Electronic structure calculations show that, at >0.25 V computed equilibrium overpotential, L(2)O grain boundaries with sufficiently large pores can accommodate Li(0) atoms which aid electron leakage and passivation breakdown. Strain often accompanies Li-insertion, applying a ~1.7% strain already lowers the computed overpotential to 0.1 V. Lithium metal nanostructures as thin as 12 Angstroms are thermodynamically favored inside cracks in Li(2)O films, becoming "incipient lithium filaments." LiF films are more resistant to lithium metal growth. The models used herein should in turn inform passivating strategies in all-solid-state batteries.

cond-mat.mtrl-sci↗

arXiv2017

First Principles Modeling of Mn(II) Migration above and Dissolution from Li(x)Mn(2)O(4) (001) Surfaces

Density functional theory and ab initio molecular dynamics simulations are applied to investigate the migration of Mn(II) ions to above-surface sites on spinel Li(x)Mn(2)O(4) (001) surfaces, the subsequent Mn dissolution into the organic liquid electrolyte, and the detrimental effects on graphite anode solid electrolyte interphase (SEI) passivating films after Mn(II) ions diffuse through the separator. The dissolution mechanism proves complex, the much-quoted Hunter disproportionation of Mn(III) to form Mn(II) is far from sufficient. Key steps that facilitate Mn(II) loss include concerted liquid/solid-state motions, proton-induced weakening of Mn-O bonds forming mobile OH- surface groups, and chemical reactions of adsorbed decomposed organic fragments. Mn(II) lodged between the inorganic Li(2)CO(3) and organic lithium ethylene dicarbonate (LEDC) anode SEI components facilitates electrochemical reduction and decomposition of LEDC. These findings help inform future design of protective coatings, electrolytes, additives, and interfaces.

cond-mat.mtrl-sci↗

arXiv2016

Stability of Solid Electrolyte Interphase Components on Lithium Metal and Reactive Anode Material Surfaces

Lithium ion batteries (LIB) can feature reactive anodes that operate at low potentials, such as lithium metal or silicon, passivated by solid electrolyte interphase (SEI) films. SEI is known to evolve over time as cycling proceeds. In this modeling work, we focus on the stability of two main SEI components, lithium carbonate (Li2CO3) and lithium ethylene dicarbonate (LEDC). Both components are electrochemically stable but thermodynamically unstable near the equilibrium Li+/Li(s) potential. Interfacial reactions represent one way to trigger the intrinsic thermodynamic instability. Both Li(2)CO(3) and LEDC are predicted to exhibit exothermic reactions on lithium metal surfaces, and the barriers are sufficiently low to permit reactions on battery operation time scales. LEDC also readily decomposes on high Li-content Li(x)Si surfaces. Our studies suggest that the innermost SEI layer on lithium metal surfaces should be a thin layer of Li(2)O -- the only thermodynamically and kinetically stable component (in the absence of a fluoride source). This work should also be relevant to inadvertant lithium plating during battery cycling, and SEI evolution on Li(x)Si surfaces.

cond-mat.mtrl-sci↗

arXiv2015

How Voltage Drops are Manifested by Lithium Ion Configurations at Interfaces and in Thin Films on Battery Electrodes

Battery electrode surfaces are generally coated with electronically insulating solid films of thickness 1-50 nm. Both electrons and Li+ can move at the electrode-surface film interface in response to the voltage, which adds complexity to the "electric double layer" (EDL). We apply Density Functional Theory (DFT) to investigate how the applied voltage is manifested as changes in the EDL at atomic lengthscales, including charge separation and interfacial dipole moments. Illustrating examples include Li(3)PO(4), Li(2)CO(3), and Li(x)Mn(2)O(4) thin-films on Au(111) surfaces under ultrahigh vacuum conditions. Adsorbed organic solvent molecules can strongly reduce voltages predicted in vacuum. We propose that manipulating surface dipoles, seldom discussed in battery studies, may be a viable strategy to improve electrode passivation. We also distinguish the computed potential governing electrons, which is the actual or instantaneous voltage, and the "lithium cohesive energy" based voltage governing Li content widely reported in DFT calculations, which is a slower-responding self-consistency criterion at interfaces. This distinction is critical for a comprehensive description of electrochemical activities on electrode surfaces, including Li+ insertion dynamics, parasitic electrolyte decomposition, and electrodeposition at overpotentials.

cond-mat.mtrl-sci↗

arXiv2015

Predicting the Voltage Dependence of Interfacial Electrochemical Processes at Lithium-Intercalated Graphite Edge Planes

The applied potential governs lithium-intercalation and electrode passivation reactions in lithium ion batteries, but are challenging to calibrate in condensed phase DFT calculations. In this work, the "anode potential" of charge-neutral lithium-intercalated graphite (LiC(6)) with oxidized edge planes is computed as a function of Li-content n(Li)) at edge planes, using ab initio molecular dynamics (AIMD), a previously introduced Li+ transfer free energy method, and the experimental Li+/Li(s) value as reference. The voltage assignments are corroborated using explicit electron transfer from fluoroethylene carbonate radical anion markers. PF6- is shown to decompose electrochemically (i.e., not just thermally) at low potentials imposed by our voltage calibration technique. We demonstrate that excess electrons reside in localized states-in-the-gap in the organic carbonate liquid region, which is not semiconductor-like (band-state-like) as widely assumed in the literature.

cond-mat.mtrl-sci↗

arXiv2014

First Principles Study of Photo-oxidation Degradation Mechanisms in P3HT for Organic Solar Cells

We present a theoretical study of degradation mechanisms for photoinduced oxidation in organic polymers in the condensed phase, using poly(3-hexylthiophene)(P3HT) as an example. Applying density functional theory with a hybrid density functional and periodic boundary conditions that account for steric effects and permit the modeling of interchain chemical reactions, we investigate reaction pathways that may lead to the oxidation of thiophene backbone as a critical step toward disrupting the polymer conjugation. We calculate energy barriers for reactions of the P3HT backbone with oxidizing agents including hydroxyl radical (OH$\cdot$), hydroperoxide (ROOH), and peroxyl radical (ROO$\cdot$), following a UV-driven radical reaction starting at the $α$-carbon of the alkyl side chain as suggested by infrared (IR) and X-ray photoemission (XPS) spectrosocopy studies. The results strongly suggest that an attack of OH$\cdot$ on sulfur in P3HT is unlikely to be thermodynamically favored. On the other hand, an attack of a peroxyl radical on the side chain on the P3HT backbone may provide low barrier reaction pathways to photodegradation of P3HT and other polymers with side chains. The condensed phase setting is found to qualitatively affect predictions of degradation processes.

cond-mat.mtrl-sci↗

arXiv2014

Modeling Electrochemical Decomposition of Fluoroethylene Carbonate on Silicon Anode Surfaces in Lithium Ion Batteries

Fluoroethylene carbonate (FEC) shows promise as an electrolyte additive for improving passivating solid-electrolyte interphase (SEI) films on silicon anodes used in lithium ion batteries (LIB). We apply density functional theory (DFT), ab initio molecular dynamics (AIMD), and quantum chemistry techniques to examine excess-electron-induced FEC molecular decomposition mechanisms that lead to FEC-modified SEI. We consider one- and two-electron reactions using cluster models and explicit interfaces between liquid electrolyte and model Li(x)Si(y) surfaces, respectively. FEC is found to exhibit more varied reaction pathways than unsubstituted ethylene carbonate. The initial bond-breaking events and products of one- and two-electron reactions are qualitatively similar, with a fluoride ion detached in both cases. However, most one-electron products are charge-neutral, not anionic, and may not coalesce to form effective Li+-conducting SEI unless they are further reduced or take part in other reactions. The implications of these reactions to silicon-anode based LIB are discussed.

cond-mat.mtrl-sci↗