Session F61: Energy Research -- Energy Storage II / Electronics

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Li-air batteries are poised to revolutionize energy storage. With an extremely high theoretical energy density from the strong Li-O bonds in the cathode, Li-air cells outperform intercalation chemistries. Accurate cohesive and formation energies are crucial to understand reaction dynamics and stability of Li-air cathodes. Previous computational studies modeled the electronic and thermodynamic properties of Li₂O₂ using Density Functional Theory (DFT) with PBE, HSE and GW methods. However, reported values are not consistent due to functional dependences of DFT. In this study, we have used Quantum Monte Carlo (QMC), an approach to solving the many-body Schrödinger equation that is near chemical accuracy and less dependent on accurate DFT orbitals than GW. After carrying out convergence test for QMC simulation, we concluded that nonmagnetic Li₂O₂ is the mid-step in charging and discharging of LiO₂ (nonmagnetic) and Li₂O (antiferromagnetic). This mid-step has been highly debated as the energetic ordering depends on the DFT functional and U correction (Föppl and Féher phases). The QMC results provide not only a useful prediction related to the understanding of Li-air batteries, but also a crucial benchmarking data for the future use of DFT applied to these systems.   

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Lithium-oxygen batteries have a higher energy densities than traditional lithium-ion batteries. However, they are not yet commercially viable due to poor efficiency, high charging voltages, and low cycle lifetimes. These issues could be addressed with a deeper fundamental understanding of the atomistic behavior of these batteries. One tool to model such atomic scale behavior is ab initio molecular dynamics (AIMD) simulations. However, AIMD simulations are limited to timescales of tens of picoseconds. As a result, equilibration and sampling methodologies can have a significant effect on the behavior of AIMD simulations. We thus compared two equilibration methods for AIMD simulations of systems of common solvents and salts found in lithium-oxygen batteries: (1) using an AIMD temperature ramp and (2) using a classical MD simulation followed by a short AIMD simulation all at 300 K. I will discuss how the differences between our simulation results and experimental results for properties such as coordination number illustrate the importance of both equilibration method and independent sampling for extracting experimentally relevant quantities from AIMD simulations.   

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Molecular simulations play an important role in the development of new technologies for energy production and storage. There are, of course, various approaches to molecular simulation, each of which offers its own advantages and disadvantages. Classical molecular dynamics simulations, for example, are able to be applied to large, complex systems, yet electron transfer and changes of oxidation state during the simulations are not captured. Density functional theory molecular dynamics (DFTMD) simulations compute the electronic structure but are limited to smaller and less complex systems. Artificial neural networks (ANN) have surged in recent years as a means to relate various features of a system to a measurable quantity.

In this work, we propose an approach using an ANN trained to DFT calculations following classical molecular dynamics simulations to predict the so-called vertical energy gap, which can be used to quantify electrochemical properties. We investigate the electrochemical properties of solvated organic molecules proposed for use in supercapacitor devices. This approach reduces computational costs associated with predicting the electrochemical properties of complex systems and may also be used to improve classical force fields used for electrochemically active species.   

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The charge transport dynamics of a novel solid-like electrolyte material based on mixtures of the ionic liquid (IL) [BMIM] TFSI and various concentrations of lithium salt LiTFSI confined within a SiO₂ matrix has been investigated. The translational diffusion coefficients of BMIM⁺, TFSI^- and Li⁺ in ILs and confined ILs (ionogels, IGs) with different concentrations of lithium salt have been measured at variable temperatures (20-100^oC) using pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy. Additionally, the effect of confinement on IL rotational dynamics has been analyzed by measuring ¹H, ¹⁹F, and ⁷Li spin-lattice relaxation rate dispersions of IGs at different temperatures, using fast field-cycling NMR relaxometry. The analysis of the experimental data was performed assuming the existence of two fractions of the liquid: a bulk fraction (at least several ionic radii from the silica particles) and a surface fraction (close to the silica particles). The results show that the ion dynamics slowed only modestly under confinement, which evidences that IGs preserve IL transport properties, and this behavior is an encouraging indication for using IGs as a solid electrolyte for Li⁺ batteries.   

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Insights into the molecular mechanisms of transport and adsorption of electrolyte ions in porous materials under applied potentials are essential to develop next generation of materials for energy storage, water purification and desalination etc.. Here we present a small-angle neutron scattering (SANS) characterization of ion electrosorption in a conductive Metal-Organic Framework (MOF) electrode by taking advantage of neutron’s sensitivity on light elements and isotopes. The method tracks the arrangements of not only ions adsorbed in the micropores, also those adsorbed on the outer surface of cylindrical protrusions that the electrode is composed of under working conditions. The scattering results reveal that the solvent molecules can access most nanopores of the MOF electrode without applying potential. The changes in scattering intensity when potentials are applied suggests the ion rearrangement in the micropores following different mechanisms depending on the electrode polarization. These observations shed new insights on ion electrosorption in electrode materials.   

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Recently, layered transition metal (TM) oxide materials for cathodes have attracted attention in sodium ion batteries (SIBs). Despite appeal of SIBs for large-scale energy storage due to the relative abundance of sodium, its lower toxicity, and ease of production, the SIBs are yet to demonstrate sufficient cycle life.Understanding and imaging defect nucleation and evolution of strain in the layered cathode materials during the battery operation is crucial to overcome the SIB problems and improve material design. We extended operando Bragg coherent diffractive imaging to observe in 3D structural changes within nanoparticles of sodium ion battery cathode. We observe nucleation of defects perpendicular to the layers during the charging of the battery in layered SIB cathode materials. We also observe the differences in interlayer distance within the nanocrystal, implying difference in ionic diffusion. Operando observation of the defect formation and interaction opens ways to understanding and rational design of durable functional layered oxide materials.   

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In their most simple form, electrostatic capacitors consist of two conducting sheets separated by an insulator, with their ability to store charge proportional to the area of the sheets and inversely proportional to the thickness of the insulator. Spinodal decomposition in solution-processed composites has been shown to achieve large-area phase-separated morphologies, but have been developed primarily for improving the efficiencies of solar cells. In this talk, we will present models combining the morphological data from spinodally decomposed composites and polymer-nanoparticle interfaces with electronic properties of the components and heterojunctions to evaluate the fitness of solution-processed composites to improve electrostatic capacitive energy storage.   

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As we move toward a more renewable electricity grid, we need to find power electronics that can withstand high power and high voltage. Ultra-wide band gap (UWBG) materials are promising candidates. However, many UWBG materials either lack the thermal properties to dissipate heat (Ga₂O₃), are difficult to synthesize at scale (diamond, c-BN), or are difficult to dope (AlGaN). Using density function theory (DFT), we conduct a high-throughput computational search of >1,500 oxides, nitrides, sulfides, carbides, silicides, and borides. Generalized gradient approximation (GGA) calculations and semi-empirical models are used to evaluate high power electronic performance through the Baliga figure of merit (BFOM) and lattice thermal conductivity (κ_L). The top candidates are further assessed using low-throughput, higher-accuracy hybrid calculations. Our results find more than 28 promising candidates (65% oxides and 35% nitrides) that surpass current WBG materials (Ga₂O_3, SiC-4H, GaN). We also conduct a qualitative assessment of n-type dopability by evaluating the governing material properties for acceptor defects. This work allows confident selection of materials for experimental investigation. Ref: Energy Environ. Sci., 2019, 12, 3338.   

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Gallium nitride (GaN), a wide bandgap semiconductor, has been widely used in high-power electronic devices due to its ability to withstand high breakdown voltage and high temperature. Its relatively high thermal conductivity makes GaN a favorable material for such applications, where heat dissipation is a major concern for the long-term stability. However, in GaN-based transistors, where the active region can withstand extremely strong electric fields, the field effect on thermal transport properties has drawn little attention up to date. In this work, we apply first-principles method to investigate phonon properties of wurtzite GaN in the presence of a near breakdown electric field along different directions. We find that the electric field changes thermal conductivity significantly via impacting the phonon anharmonicity and scattering rates, although it has little effect on phonon dispersions. Our study provides insights into the effect of the extreme external electric field on phonon transport properties in wide-gap semiconductors.   

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Current InGaN/GaN structures are unable to emit efficiently at wavelengths longer than ~525 nm due to spontaneous polarization and phase separation reducing the radiative recombination rate, creating a need to expand the design space of nitride LEDs. ZnGeN₂/GaN quantum well structures are proposed as a solution to this green gap. ZnGeN₂ is almost lattice-matched to GaN, enabling integration into GaN-based systems for an expanded emission space with much less lattice mismatch than InGaN. ZnGeN₂/GaN QWs were grown using molecular beam epitaxy (MBE) demonstrating room-temperature and low-temperature photoluminescence (PL). Possible emission sources including defect emission, band-to-band (cation disorder), and spatially indirect excitons from band offsets will be discussed based on experiment and theory. Transmission electron microscopy (TEM) images show abrupt interfaces, which is promising for future device applications. Electronic structure calculations for ZnGeN₂/GaN show band gap reduction from disorder and a type II alignment, informing our understanding of the system.   

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III-V alloys incorporating Nitrogen and Bismuth are of significant interest for optoelectronic applications in the near- and mid-infrared. We report measurements of the photoconductivity lifetime and photocarrier mobility in a series of GaAs_1-x-yN_xBi_y samples grown on GaAs by MBE. We find short conductivity lifetimes of order 3ps to 5ps and carrier mobilities of order 30 to 80cm²/Vs. Time-resolved photoconductivity was obtained from optical pump, THz probe measurements, and steps were taken to avoid the influence of the GaAs substrate: A tunable optical pump permitted excitation below the GaAs bandgap, and transient THz reflection was used as a surface-specific probe of the conductivity. The short carrier lifetime and low carrier mobility likely arises due to rapid carrier trapping in these materials.   

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I will present our recent study of the quasiparticle excitations and band structures in organized donor-acceptor copolymers. We employ many-body perturbation theory, which accounts for the polarization effects among polymers, to calculate the quasiparticle energies in bulk polymers. Our computed results are in excellent agreements with the photoemission spectra data. We discover two types of states supporting band transport in bulk copolymers: the conjugated bands and impurity states. The non-local exchange interactions are found to enhance the band transport of hole along the polymer axis, but hinder the transport across the chain. The polarization interactions are found to stabilize charge carriers and hinder band transport. Further, we discover that depending on the molecular arrangement, bulk copolymers sustain electron and hole transport in two orthogonal directions; the holes are most efficiently transported along the polymer and the pi-pi stacking directions, while the electrons are transport along the edge-to-edge stacking direction.   

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TEMPO-substituted radical polymers represent a kind of high voltage and fast-charging active material for metal-free, aqueous organic batteries. However, the relationship between the structure and performance of TEMPO-substituted radical polymers remains unclear, and design principles of an electrode-electrolyte system to guide future designs are lacking. In this talk, the relationship between the structure and performance of a series of non-conjugated TEMPO-substituted radical polymers are elucidated in a water-based electrolyte containing organic salt. A series of these polymers with varying hydrophilicities are explored. It is shown that the polymer-water interactions influence the redox kinetics of the polymers. Moreover, the coupled mass and charge transfer behavior of these TEMPO-substituted radical polymers are monitored using electrochemical quartz crystal microbalance with dissipation monitoring, indicating a varying hydration degree of the transferred species during the anion doping/dedoping reaction. These results reveal the important role of designing the polymer structure and adjusting polymer-water interactions in driving the overall performance.   

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We develop a lattice Monte Carlo simulation based on the diffusion-limited aggregation model that accounts for the effect of the physical properties of ionic liquids on lithium dendrite growth. Our study focuses on the influence of IL properties, such as the size asymmetry between the cation and anion, dielectric constant, and the volume fraction of ILs, on the aspect ratio and average height of the dendrites. We show that these IL properties are critical to significantly suppress dendrite growth, primarily due to substantial changes in electric-field screening. Among others, the volume fraction of ILs has the optimal value. We also discuss that these key factors notably affect the effect of the applied voltage on the dendrite growth.   

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Scandium-doped aluminum nitride, Al_1-xSc_xN, represents a new class of ferroelectric materials with extraordinarily high polarization, moderate coercive fields, sharp hysteresis, high temperature resilience and facile synthesizability, even in its polycrystalline form. However, the role of Sc doping in transforming unswitchable piezoelectric AlN with strong covalent bonds into a switchable ferroelectric material is not known. Here, we use ab initio quantum molecular dynamics (QMD) simulations to quantify inhomogeneity of Sc distribution, phase segregation as a function of Sc doping and understand the fundamental physics of electronic structure and covalent bonding and the nature of the potential energy surface in these alloys that simultaneously possess deep potential wells required for large polarizations (>75 μC/cm²), while retaining moderate coercive fields (< 4 MV/m). We also perform direct-switching QMD simulations in the presence of an electric field to understand the mechanism of ferroelectric switching and the critical role of local Sc concentration.