16 research outputs found
Non-covalent interactions in electrochemical reactions and implications in clean energy applications
Understanding and controlling non-covalent interactions associated with solvent molecules and redox-inactive ions provide new opportunities to enhance the reaction entropy changes and reaction kinetics of metal redox centers, which can increase the thermodynamic efficiency of energy conversion and storage devices. Here, we report systematic changes in the redox entropy of one-electron transfer reactions including [Fe(CN)6]3-/4-, [Fe(H2O)6]3+/2+and [Ag(H2O)4]+/0induced by the addition of redox inactive ions, where approximately twenty different known structure making/breaking ions were employed. The measured reaction entropy changes of these redox couples were found to increase linearly with higher concentration and greater structural entropy (having greater structure breaking tendency) for inactive ions with opposite charge to the redox centers. The trend could be attributed to the altered solvation shells of oxidized and reduced redox active species due to non-covalent interactions among redox centers, inactive ions and water molecules, which was supported by Raman spectroscopy. Not only were these non-covalent interactions shown to increase reaction entropy, but they were also found to systematically alter the redox kinetics, where increasing redox reaction energy changes associated with the presence of water structure breaking cations were correlated linearly with the greater exchange current density of [Fe(CN)6]3-/4-.United States. Department of Energy. Office of Basic Energy Science (Award Number DE-SC0001299/DE-FG02-09ER46577)Hong Kong (China). Innovation and Technology Commission (Project No. ITS/ 020/16FP)United States. Department of Energy (Contract No. DE-AC02-5CH11231
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Ion Transport at PolymerâArgyrodite Interfaces
Solid-state electrolytes, particularly polymer/ceramic composite electrolytes, are emerging as promising candidates for lithium-ion batteries due to their high ionic conductivity and mechanical flexibility. The interfaces that arise between the inorganic and organic materials in these composites play a crucial role in ion transport mechanisms. While lithium ions are proposed to diffuse across or parallel to the interface, few studies have directly examined the quantitative impact of these pathways on ion transport and little is known about how they affect the overall conductivity. Here, we present an atomistic study of lithium-ion (Li+) transport across well-defined polymerâargyrodite interfaces. We present a force field for polymerâargyrodite interfacial systems, and we carry out molecular dynamics and enhanced sampling simulations of several composite systems, including poly(ethylene oxide) (PEO)/Li6PS5Cl, hydrogenated nitrile butadiene rubber (HNBR)/Li6PS5Cl, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/Li6PS5Cl. For the materials considered here, Li-ion exhibits a preference for the ceramic material, as revealed by free energy differences for Li-ion between the inorganic and the organic polymer phase in excess of 13 kBT. The relative free energy profiles of Li-ion for different polymeric materials exhibit similar shapes, but their magnitude depends on the strength of interaction between the polymers and Li-ion: the greater the interaction between the polymer and Li-ions, the smaller the free energy difference between the inorganic and organic materials. The influence of the interface is felt over a range of approximately 1.5 nm, after which the behavior of Li-ion in the polymer is comparable to that in the bulk. Near the interface, Li-ion transport primarily occurs parallel to the interfacial plane, and ion mobility is considerably slower near the interface itself, consistent with the reduced segmental mobility of the polymer in the vicinity of the ceramic material. These findings provide insights into ionic complexation and transport mechanisms in composite systems, and will help improve design of improved solid electrolyte systems
Tuning mobility and stability of lithium ion conductors based on lattice dynamics
Lithium ion conductivity in many structural families can be tuned by many orders of magnitude, with some rivaling that of liquid electrolytes at room temperature. Unfortunately, fast lithium conductors exhibit poor stability against lithium battery electrodes. In this article, we report a fundamentally new approach to alter ion mobility and stability against oxidation of lithium ion conductors using lattice dynamics. By combining inelastic neutron scattering measurements with density functional theory, fast lithium conductors were shown to have low lithium vibration frequency or low center of lithium phonon density of states. On the other hand, lowering anion phonon densities of states reduces the stability against electrochemical oxidation. Olivines with low lithium band centers but high anion band centers are promising lithium ion conductors with high ion conductivity and stability. Such findings highlight new strategies in controlling lattice dynamics to discover new lithium ion conductors with enhanced conductivity and stability.United States. National Science Foundation. Graduate Research Fellowship Program (Grant 1122374)Taiwan. Ministry of Science and Technology (Grant 102-2917-I-564-006-A1)United States. National Science Foundation (Award DMR-0819762)United States. National Energy Research Scientific Computing Center (Contract DE-AC02-05CH11231)Extreme Science and Engineering Discovery Environment (Grant ACI-1548562
Influence of lattice dynamics on the ionic conductivity and stability of solid-state lithium-ion conductors
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 126-145).Electrochemical energy storage devices are clean and efficient, but their current cost and performance limit their use in many transportation and stationary applications. Lithium-ion batteries are one of the leading candidates for these large applications, however their current use of liquid electrolytes negatively effects their lifetime and safety. Furthermore, the liquid electrolyte's potential stability window, thermal stability, and volatility are of particular concern in these large-scale applications. Solid-state electrolytes are investigated as one of the best solutions to overcome these challenges. However, the ionic conductivity and especially (electro)chemical stability of many solid electrolytes are still problematic. The focus of this thesis is on ionic mobility and stability of solid-state Li-ion conductor and descriptors that correlates with these properties. We first provide a comprehensive review of several important families of Li-ion conductors that have been studied and published in the literature focusing on their and an overview of some descriptors that have been proposed to correlate with the ionic conductivity/activation energy, for instance, the volume of the diffusion pathway, high-frequency dielectric constants and frequencies of low-energy optical phonons. Build upon these previous understandings, we propose a new approach to understand ion mobility and stability against of lithium insertion/removal in ion conductors based on lattice dynamics. By combining inelastic neutron scattering measurements with density function theory computation, greater lithium ion mobility was correlated with decreasing lithium vibration frequency that was quantified using a newly proposed descriptor which we phonon band centers. Known superionic lithium conductors were shown to have not only low lithium phonon center but also low anion phonon band center, which unfortunately reduces stability against electrochemical oxidation. Therefore, the interplay between lattice dynamics and ion mobility and stability highlights the need and opportunities to search for fast lithium ion conductors having low lithium band center but high anion band center which exhibit high ion conductivity and high (electro)chemical stability in lithium ion batteries. We show and discuss that Olivines with low lithium band centers but high anion band centers are particularly promising to explore for lithium ion conductors with high ion conductivity and stability. With this new approach, we were able for the first time to account for the trend in ionic conductivity and electrochemical oxidation stability of lithium ion conductors from one common physical origin, their lattice dynamics. Such findings open new avenues for the discovery of new lithium ion conductors with enhanced conductivity and stability using lattice dynamics. Finally, to study the correlation between the actiation energy and the pre-exponential factor, the ionic conductivity and activation energy of lithium in the LiâPOâ-LiâVOâ-LiâGeOâ system was systematically investigated as model system. The sharp decrease in activation energy upon Ge substitution in LiâPOâ and LiâVOâ was attributed to the reduction in the enthalpy of defect formation while the variation in activation energy upon increasing Ge content was rationalized in term of the inductive effect. The series of compound with and without partial lithium occupancy were shown to fall into two distinct lines whose slope was related to the inverse of the energy scale associated with phonon in the systems according to multi-excitation entropy theory and the intercept to the Gibbs free energy of defect formation. Compiled data of pre-exponential factor and activation energy for commonly studied Li-ion conductors shows that this correlation is very general, implying an unfavorable trade-off between high pre-exponential factor and low activation energy needed to achieve high ionic conductivity.by Sokseiha Muy.Ph. D
Phonon-Ion Interactions: Designing Ion Mobility Based on Lattice Dynamics
This review is focused on the influence of lattice dynamics on the ionic mobility in superionic conductors in particular solid-state Li-ion conductors. After a succinct review of the static view of ionic conduction, the role of polarizability as the underlying cause of lattice softness is discussed in connection with the anharmonicity and the roles of lattice dynamics on ionic conductivity as proposed in early theories in the 70's and 80's by Mahan, Zeller, Rice and Roth are reviewed with the emphasis on various proposed correlations between Debye and Einstein frequency as well as other specific vibrational modes with the activation energy. The role of lattice dynamics on the correlation between the pre-exponential factor and activation energy, i.e. the Meyer-Neldel rule is also presented with emphasis on the entropy of migration and its dependence on the vibrational spectrum of the lattice. Moreover, a recent computational high-throughput screening based on the average vibrational frequency is also discussed to illustrate the application of lattice dynamics descriptors to design new lithium conductors. Finally, several open questions regarding the fundamental understanding of the role of lattice dynamics and new strategies to tune ionic conductivity based on these concepts are presented
Mechanochemical Synthesis: A Tool to Tune Cation Site Disorder and Ionic Transport Properties of Li 3
The lithium-conducting, rare-earth halides, LiâMXâ (M = Y, Er; X = Cl, Br), have garnered significantly rising interest recently, as they have been reported to have oxidative stability and high ionic conductivities. However, while a multitude of materials exhibit a superionic conductivity close to 1 mS cmâ»Âč, the exact design strategies to further improve the ionic transport properties have not been established yet. Here, the influence of the employed synthesis method of mechanochemical milling, compared to subsequent crystallization routines as well as classic solid-state syntheses on the structure and resulting transport behavior of LiâErClâ and LiâYClâ are explored. Using a combination of X-ray diffraction, pair distribution function analysis, density functional theory, and impedance spectroscopy, insights into the average and local structural features that influence the underlying transport are provided. The existence of a cation defect within the structure in which Er/Y are disordered to a new position strongly benefits the transport properties. A synthetically tuned, increasing degree of this disordering leads to a decreasing activation energy and increasing ionic conductivity. This work sheds light on the possible synthesis strategies and helps to systematically understand and further improve the properties of this class of materials
Mechanochemical Synthesis: A Tool to Tune Cation Site Disorder and Ionic Transport Properties of Li3MCl6(M = Y, Er) Superionic Conductors
The lithium-conducting, rare-earth halides, LiâMXâ (M = Y, Er; X = Cl, Br), have garnered significantly rising interest recently, as they have been reported to have oxidative stability and high ionic conductivities. However, while a multitude of materials exhibit a superionic conductivity close to 1 mS cmâ»Âč, the exact design strategies to further improve the ionic transport properties have not been established yet. Here, the influence of the employed synthesis method of mechanochemical milling, compared to subsequent crystallization routines as well as classic solid-state syntheses on the structure and resulting transport behavior of LiâErClâ and LiâYClâ are explored. Using a combination of X-ray diffraction, pair distribution function analysis, density functional theory, and impedance spectroscopy, insights into the average and local structural features that influence the underlying transport are provided. The existence of a cation defect within the structure in which Er/Y are disordered to a new position strongly benefits the transport properties. A synthetically tuned, increasing degree of this disordering leads to a decreasing activation energy and increasing ionic conductivity. This work sheds light on the possible synthesis strategies and helps to systematically understand and further improve the properties of this class of materials
High-Throughput Screening of Solid-State Li-Ion Conductors Using Lattice-Dynamics Descriptors
© 2019 The Authors Low lithium-ion migration barriers have recently been associated with low average vibrational frequencies or phonon band centers, further helping identify descriptors for superionic conduction. To further explore this correlation, here we present the computational screening of âŒ14,000 Li-containing compounds in the Materials Project database using a descriptor based on lattice dynamics reported recently to identify new promising Li-ion conductors. An efficient computational approach was optimized to compute the average vibrational frequency or phonon band center of âŒ1,200 compounds obtained after pre-screening based on structural stability, band gap, and their composition. Combining a low computed Li phonon band center with large computed electrochemical stability window and structural stability, 18 compounds were predicted to be promising Li-ion conductors, one of which, Li3ErCl6, has been synthesized and exhibits a reasonably high room-temperature conductivity of 0.05â0.3 mS/cm, which shows the promise of Li-ion conductor discovery based on lattice dynamics. Computational Method in Materials Science; Energy Materials; Solid State Physic
Ligand-Dependent Energetics for Dehydrogenation: Implications in Li-Ion Battery Electrolyte Stability and Selective Oxidation Catalysis of Hydrogen-Containing Molecules
The hydrogen adsorption energetics on the surface of inorganic compounds can be used to predict electrolyte stability in Li-ion batteries and catalytic activity for selective oxidation of small molecules such as Hâ and CHâ. Using first-principles density functional theory (DFT), the hydrogen adsorption was found to be unfavorable on high-band-gap insulators, which could be attributed to a lower energy level associated with adsorbed hydrogen relative to the bottom of the conduction band. In contrast, the hydrogen adsorption was shown to be the most favorable on metallic and semiconducting compounds, which results from an electron transfer from adsorbed hydrogen to the Fermi level or the bottom of the conduction band. Of significance, computed hydrogen adsorption energetics on insulating, semiconducting, and metallic oxides; phosphates; fluorides; and sulfides were decreased by lowering the ligand p band center, while the energy penalty for ligand vacancy formation was increased, indicative of decreased surface reducibility. A statistical regression analysis, where 16 structural and electronic parameters such as metalâligand distance, electronegativity difference, Bader charges, bulk and surface metal and ligand band centers, band gap, ligand band width, and work function were examined, further showed that the surface ligand p band center is the most accurate single descriptor that governs the hydrogen adsorption tendency, and additional considerations of the band gap and average metalâligand distance further reconcile the differences among compounds with different ligands/structures, whose ligand bands are different in shape and width. We discuss the implications of these findings for passivating coatings and design of catalysts and the need for novel theoretical methods to accurately estimate these quantities from first principles. These results establish a universal design principle for future high-throughput studies aiming to design electrode surfaces to minimize electrolyte oxidation by dehydrogenation in Li-ion batteries and enhance the HâH and CâH activation for selective oxidation catalysis