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In situ grain fracture mechanics during uniaxial compaction of granular solids
Grain fracture and crushing are known to influence the macroscopic mechanical behavior of granular materials and be influenced by factors such as grain composition, morphology, and microstructure. In this paper, we investigate grain fracture and crushing by combining synchrotron x-ray computed tomography and three-dimensional x-ray diffraction to study two granular samples undergoing uniaxial compaction. Our measurements provide details of grain kinematics, contacts, average intra-granular stresses, inter-particle forces, and intra-grain crystal and fracture plane orientations. Our analyses elucidate the complex nature of fracture and crushing, showing that: (1) the average stress states of grains prior to fracture vary widely in their relation to global and local trends; (2) fractured grains experience inter-particle forces and stored energies that are statistically higher than intact grains prior to fracture; (3) fracture plane orientations are primarily controlled by average intra-granular stress and contact fabric rather than the orientation of the crystal lattice; (4) the creation of new surfaces during fracture accounts for a very small portion of the energy dissipated during compaction; (5) mixing brittle and ductile grain materials alters the grain-scale fracture response. The results highlight an application of combined x-ray measurements for non-destructive in situ analysis of granular solids and provide details about grain fracture that have important implications for theory and modeling.
A high-fidelity multiphysics model for the new solid oxide iron-air redox battery part I: Bridging mass transport and charge transfer with redox cycle kinetics
A high-fidelity two-dimensional axial symmetrical multi-physics model is described in this paper as an effort to simulate the cycle performance of a recently discovered solid oxide metal-air redox battery (SOMARB). The model collectively considers mass transport, charge transfer and chemical redox cycle kinetics occurring across the components of the battery, and is validated by experimental data obtained from independent research. In particular, the redox kinetics at the energy storage unit is well represented by Johnson-Mehl-Avrami-Kolmogorov (JIVIAK) and Shrinking Core models. The results explicitly show that the reduction of Fe3O4 during the charging cycle limits the overall performance. Distributions of electrode potential, overpotential, Nernst potential, and H-2/H2O-concentration across various components of the battery are also systematically investigated. (C) 2015 Elsevier B.V. All rights reserved.
Impact of Ionic Liquid on Lithium Ion Battery with a Solid Poly(Ionic Liquid) Pentablock Terpolymer as Electrolyte and Separator
In this work, a ternary blend solid polymer electrolyte (SPE) consisting of a poly(ionic liquid) (PIL) multiblock polymer, lithium salt, and ionic liquid (IL) was investigated with the goal of understanding the influence of IL concentration on ion transport mechanisms and electrochemical stability. The physical, transport, mechanical, morphological, and electrochemical properties of the ternary blend SPE were systematically investigated as a function of IL concentration (r = [IL]/[PIL] mole ratio between 0.1 and 0.7). The results indicate that with increasing IL concentration, the continuous conductive domain increases along with the polymer chain segmental mobility, as well as facilitate ionic conductivity, while the mechanical modulus exhibits a percolation threshold (from 52.17 MPa to 0.55 MPa at r = 0.2 to 0.3). Surprisingly, at higher IL concentrations, there is a reduction in the lithium cation mobility (i.e., 9.3 × 10-12 m2 s-1 versus 3.8 × 10-12 m2 s-1 for r = 0.2 versus r = 0.5, respectively) as evidenced by pulsed-field gradient nuclear magnetic resonance (PFG-NMR), which coincides with an increased overpotential (i.e., 50 mV versus 150 mV for r = 0.2 versus r = 0.5, respectively) evidenced by lithium metal stripping and plating. This work provides valuable insights into the impact of IL on lithium ion conducting PIL block polymers, which may enable the design of new SPEs with both high ionic conductivity and improved stability for the future lithium ion batteries.
Computational Exploration of the Li-Electrode|Electrolyte Interface in the Presence of a Nanometer Thick Solid-Electrolyte Interphase Layer [Computational exploration of the Li-electrode|electrolyte interface complicated by a nanometer thin solid-electrolyte interphase (SEI) layer]
A nanometer thick passivation layer will spontaneously form on Li-metal in battery applications due to electrolyte reduction reactions. This passivation layer in rechargeable batteries must have “selective” transport properties: blocking electrons from attacking the electrolytes, while allowing Li + ion to pass through so the electrochemical reactions can continue. The classical description of the electrochemical reaction, Li + + e → Li 0 , occurring at the Li-metal|electrolyte interface is now complicated by the passivation layer and will reply on the coupling of electronic and ionic degrees of freedom in the layer. We consider the passivation layer, called “solid electrolyte interphase (SEI)”, as “the most important but the least understood in rechargeable Li-ion batteries,” partly due to the lack of understanding of its structure–property relationship. In predictive modeling, starting from the ab initio level, we find that it is an important tool to understand the nanoscale processes and materials properties governing the interfacial charge transfer reaction at the Li-metal|SEI|electrolyte interface. Here, we demonstrate pristine Li-metal surfaces indeed dissolve in organic carbonate electrolytes without the SEI layer. Based on joint modeling and experimental results, we point out that the well-known two-layer structure of SEI also exhibits two different Li + ion transport mechanisms. The SEI has a porous (organic) outer layer permeable to both Li + and anions (dissolved in electrolyte), and a dense (inorganic) inner layer facilitate only Li + transport. This two-layer/two-mechanism diffusion model suggests only the dense inorganic layer is effective at protecting Li-metal in electrolytes. This model suggests a strategy to deconvolute the structure–property relationships of the SEI by analyzing an idealized SEI composed of major components, such as Li 2 CO 3 , LiF, Li 2 O, and their mixtures. After sorting out the Li+ ion diffusion carriers and their diffusion pathways, we design methods to accelerate the Li + ion conductivity by doping and by using heterogonous structure designs. We will predict the electron tunneling barriers and connect them with measurable first cycle irreversible capacity loss. We note that the SEI not only affects Li + and e – transport, but it can also impose a potential drop near the Li-metal|SEI interface. Our challenge is to fully describe the electrochemical reactions at the Li - metal|SEI|electrolyte interface. This will be the subject of ongoing efforts.
Enhancing the Chemical Energy Flux in a High-Temperature Tubular Counterflow Solid Fuel Synthesis Reactor Using a Bypass
Redox reactions of metal oxides offer a path towards using intermittent renewable resources for high-density thermochemical energy storage. Thermochemical energy storage often involves the flow of a particulate media. We describe a novel method to increase the throughput in a gravity-driven high-temperature thermochemical storage reactor flowing pelletized MgMnO. The moving bed reactor operates under counter-flow conditions and encounters particle flowability problems at temperatures of 1500 °C leading to sintering of the bed. Inertial forces of a counter-flowing gas can overcome the gravitational forces on the particles and limit the chemical energy storage rate of the reactor. We found that the insertion of a gas bypass (a slotted tube) into the reactor results in a 100% increase of the flow rates and achieved a 50% higher chemical energy storage flux compared to the operation without a bypass tube while mitigating the effects of sintering on the particles. As a result, the higher solid flow rates require a longer heated zone to reach a comparable residence time and extent of reduction compared to the lower flow rates.
Competing Singlet Fission and Excimer Formation in Solid Fluorinated 1,3-Diphenylisobenzofurans
Singlet fission (kSF) and excimer formation (kEXC) rate constants along with other photophysical properties of thin solid layers of 1,3-diphenylisobenzofuran and 11 of its fluorinated derivatives have been determined. The molecular properties of these compounds are similar, but their crystal packing varies widely. Most of them undergo singlet fission whereas excitation in others is trapped in excimers. The trend in rate constants kSF agrees qualitatively with results of calculations by a simplified version of the frontier orbital model for a molecular pair. The main shortcoming of the model is discussed.
Effect of Composition and Local Environment on CO 2 Adsorption on Nickel and Magnesium Oxide Solid Solutions
CO 2 adsorption energies on the (100) surfaces of a nickel oxide doped with Mg, magnesium oxide doped with Ni, and their 50:50 solid solution were calculated using density functional theory. The composition and atomic arrangement of the adsorption site was varied to understand how the local environment affects CO 2 adsorption and the basicity of the surfaces at the atomic level. The dispersive and electronic contributions to the adsorption energies were quantified, and the results indicate that the variation of the adsorption energy with adsorption site configuration and metal composition is dominated by electronic interactions. Interestingly, for magnesium oxide doped with nickel, a single substitution can create stronger CO 2 binding sites, which implies stronger basic sites, even though nickel oxide is less basic than magnesium oxide. The effect of double substitution at the binding site can be reasonably approximated by summing the effects of single substitutions. Furthermore, this work provides guidance for the preparation of metal oxides with tailored Lewis basicity.
Mesoscale Elucidation of Solid Electrolyte Interphase Layer Formation in Li-Ion Battery Anode
Capacity fade in lithium-ion batteries largely originates from the undesired electrolyte decomposition, which results in the formation of solid electrolyte interphase (SEI) and the anode surface passivation. In this work, a mesoscale interfacial modeling approach is developed to investigate the formation and growth of the SEI film on a typical graphite based anode over several cycles. It is found that lithium diffusion kinetics in the SEI film significantly affects the SEI growth rate. A lower lithium diffusion barrier leads to a higher growth rate. The present model demonstrates that the SEI thickness is a linear function of the square root of the charging time over long-time cycling. Growth of multicomponent SEI film is also elucidated. It is found that the heterogeneity of the SEI film may lead to instability in lithium ion concentration distribution.
Complex F,Cl Apatite Solid Solution Investigated Using Multinuclear Solid-State NMR Methods
In this paper, a series of synthetic F,Cl apatites with low hydroxyl content was investigated using 31 P, 19 F, and 35 Cl solid-state magic angle spinning (MAS) NMR spectroscopic methods. 31 P single-pulse (SP) NMR spectra show that for each composition the phosphate 31 P chemical shift depends on occupancy of the nearest anion site for each composition. Overall the average chemical shift deviates from linear across the F/Cl series. For intermediate compositions 19 F SP NMR spectra reveal complex, broadened spectral profiles that are not correlated to composition. 19 F{ 35 Cl} TRAPDOR results indicate that the spectral profiles do not reflect occupancy of F and Cl in adjacent column anion sites. We propose that the complex 19 F lineshapes are principally due to broad distributions of 19 F–Ca distances arising from displacement of F along the anion channel. Finally, approximate fluorine atomic positions are estimated based on 19 F chemical shifts of alkaline earth fluoride salts, and the results are in good accord with those proposed for intermediate composition F,Cl apatites from XRD data.
Investigation of Solid Electrolyte Interphase Layer Formation and Electrochemical Reversibility of Magnetite, Fe 3 O 4 , Electrodes: A Combined X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy Study
Magnetite (Fe 3 O 4 ) is a promising electrode material for the next generation of Li-ion batteries with multiple electron transfers per metal center and a theoretical capacity of 924 mAh/g. However, multiple phase conversions during (de)lithiation of Fe 3 O 4 and formation of a surface electrolyte interphase (SEI) contribute to capacity fade. In this study, X-ray Absorption Spectroscopy (XAS), and X-ray Photoelectron Spectroscopy (XPS) were used to determine the surface chemistry, redox chemistry, and the impact on the electrochemical reversibility in the presence and absence of fluoroethylene carbonate (FEC) solvent. With FEC improved capacity retention and enhanced reversibility are observed. In contrast, electrodes cycled with no FEC exhibit decreased reversibility where the active material remains as reduced Fe 0 . XPS results reveal LiF and lower quantities of oxygen containing species, especially carbonates at the electrode surface tested in FEC. The improvement in electrochemical reversibility with FEC is attributed to the formation of a solid electrolyte interphase which forms prior to initiation of the conversion reaction limiting SEI growth on the reduced products, Fe0 and Li 2 O. In contrast, EC-based carbonate electrolyte forms SEI at a potential where formation of Fe0 and Li 2 O has already initiated resulting in SEI formation on Fe 0 nanograins.
Assessing Melting and Solid–Solid Transition Properties of Choline Chloride via Molecular Dynamics Simulations
Choline chloride (ChCl) is used extensively as a hydrogen bond donor in deep eutectic solvents (DESs). However, determining its melting properties experimentally is challenging due to decomposition upon melting, leading to widely varying literature values. Accurate melting properties are crucial for understanding the solid–liquid phase behavior of ChCl-containing DESs. Here, we employ molecular dynamics simulations to compute the phase transitions of ChCl, testing a variety of atomistic force fields. We find that the results are sensitive to the choice of force field, but a melting temperature of 627 K and a melting enthalpy of 7.8 kJ/mol seem most reasonable, in good agreement with some literature values. Furthermore, we suggest these as the likely melting properties of ChCl, though the results are tentative due to limited experimental data for the liquid ChCl phase.
A Bottom-Up Formation Mechanism of Solid Electrolyte Interphase Revealed by Isotope-Assisted Time-of-Flight Secondary Ion Mass Spectrometry
Understanding the solid electrolyte interphase (SEI) formation mechanism is critically important for the performance and durability of lithium-ion batteries. However, the details of how SEI builds up into a nanometer-thick layer from molecular level reduction reactions on negative electrodes are missing. Here, isotope-assisted time-of-flight secondary ion mass spectrometry analyses were designed to answer this fundamental question. By investigating the isotope ratio profile in SEI during the initial SEI formation cycle, it is discovered that the topmost SEI near the electrolyte formed first and the SEI near the electrode formed later. Furthermore, this new “bottom-up” SEI growth mechanism was then correlated to the electrolyte one-electron and two-electron reduction reaction dynamics, which in turn explains the formation of the two-layered organic–inorganic SEI composite structure.
Design, Synthesis, and Characterization of Polymer Precursors to Li x PON and Li x SiPON Glasses: Materials That Enable All-Solid-State Batteries (ASBs)
LiPON-like glasses that form lithium dendrite impenetrable interfaces between lithium battery components are enabling materials that may replace liquid electrolytes permitting production of all-solid-state batteries (ASBs). Unfortunately, to date, such materials are introduced only via gas-phase deposition. Furthermore, we demonstrate the design and synthesis of easily scaled, lowtemperature, low-cost, solution-processable inorganic polymers containing LiPON/LiSiPON elements. OPCl 3 and hexachlorophosphazene [Cl 2 P=N] 3 provide starting points for elaboration using MNH 2 (M = Li/Na) or (Me 3 Si)NH followed by reaction with controlled amounts of LiNH 2 to produce oligomers/polymers with molecular weights (MWs) ≈1–2 kDa characterized by multinuclear NMR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and matrix-assisted laser desorption/ ionization (MALDI)-time-of-flight (ToF) offering stabilities to 150–200 °C and ceramic yields (800 °C) of 50–60%. 7 Li NMR suggests that precursor-bound Li + dissociates easily, beneficial for electrochemical applications. XPS shows higher N/P ratios (1–3) than via gas-phase methods (<1) correlating N/P ratios, 7 Li shifts, and Li + conductivities. Li 2 SiPHN offers the highest ambient conductivity of 3 × 10 –1 mS cm –1 at 400 °C/2 h/N 2 .
Interparticle spacing and structural ordering in superlattice PbS nanocrystal solids undergoing ligand exchange
Controlling the interparticle spacing in quantum dot (QD) thin films is the most readily accessible way to control transport rates between neighboring QDs and a critical component of device optimization. Here, we use X-ray scattering measurements to accurately measure the interparticle spacing in films of highly monodisperse lead sulfide (PbS) QDs that have undergone a variety of device-relevant ligand exchanges. We tabulate these values for use in simulations and data analysis. We find that monothiol and dithiol ligand species typically result in interparticle spacing values that are equal to the length of a single monothiol or dithiol ligand. Additionally, we find that spin-coating a thick film of QDs followed by a long-duration ligand exchange results in a more complete ligand exchange than spin-coating many thin layers with short-duration ligand exchanges in between. The former method also preserves a remarkable degree of the long-range ordering that was present in the film prior to ligand exchange. These results shed light on ways to produce highly-ordered QD solids with compact and functional ligands, which could lead to enhanced interdot coupling and transport phenomena.
Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes: Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy
Operando pair distribution function (PDF) analysis and ex situ Na-23 magic-angle spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy are used to gain insight into the alloying mechanism of high-capacity antimony anodes for sodium-ion batteries. Subtraction of the PDF of crystalline NaxSb phases from the total PDF, an approach constrained by chemical phase information gained from Na-23 ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electro-chemically; a-Na3-xSb (x approximate to 0.4-0.5), a structure locally similar to crystalline Na3Sb (c-Na3Sb) but with significant numbers of sodium vacancies and a limited correlation length, and a-Na1.7Sb, a highly amorphous structure featuring some Sb-Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na3-xSb and, finally, crystalline Na3Sb. Desodiation results in the formation of an electrode formed of a composite of crystalline and amorphous antimony networks. We link the different reactivity of these networks to a series of sequential sodiation reactions manifesting as a cascade of processes observed in the electrochemical profile of subsequent cycles. The amorphofis network reacts at higher voltages reforming a-Na1.7Sb, then a-Na3-xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na3-xSb without the formation of a-Na3-xSb. a-Na3-xSb is converted to crystalline Na3Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature Na-23 NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.