The metal-organic framework (MOF) MIL-53(Al) has a framework geometry with an unconstrained wine-rack mode that enables it to concertina between large-pore (lp) and narrow-pore (np) structures either under pressure or with the uptake of adsorbates. This article presents the results of equilibrium classical molecular dynamics simulations that show that, in these breathing MOFs, the thermal conductivity─an important property for gas sorption in high-surface-area materials─also changes between open and closed conformations. The observed conductivity change differs significantly from that of a network of resistors undergoing the same geometric change. The dispersion relations and phonon group velocities in both the lp and np configurations of MIL-53(Al) are computed using self-consistent-charge density functional-based tight binding (SCC-DFTB). These exhibit rattler-mode behavior and phonon-focusing effects. To provide a mechanistic understanding of these features, a reduced order model of the MOF architecture is presented that captures the lattice dynamics and the band avoidance of acoustic and rattler phonon bands. Moreover, it is shown that the observed phonon focusing is caused by band bending from the rattler modes.

What factors affect the electrochemical stability window of aqueous electrolytes? The primary challenges of aqueous electrolytes are water’s electrolytic reactions, namely hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). These parasitic reactions shorten the cycle life of batteries, limit the energy density of batteries, and cause safety concerns. There are generally two approaches to mitigate water’s electrolytic reactions: thermodynamic and kinetic. The thermodynamic route involves manipulating the chemical environment of water molecules in the bulk electrolyte so that water is more inert than its common behaviors. Water’s chemical environment is usually a hydrogen-bonding network for dilute electrolytes. However, upon increasing the concentration of the electrolyte salts or dissolving additives and co-solvents, the neighborhood of water molecules can be tailor-designed to be vastly different from an H-bonding structure. From the thermodynamic point of view, water’s electrolytic reactions can be suppressed by either strengthening the O-H bonds of water molecules, thus transforming the reactant of the electrolytic reactions more stable. Our results show that free water molecules that exist in weak-interaction environments are more stable. Alternatively, we found that frustrating the solvation of hydroxide and proton, which are the products of HER and OER, respectively, can help restrict water’s electrolysis. To date, the solvation energies of hydroxide and proton have received rather limited attention. Regarding the kinetic control over the electrolytic reactions, the most effective strategy is to grow a passivating solid electrolyte interphase (SEI) layer on top of the anode. However, a pure aqueous electrolyte usually cannot grow an effective SEI as such an SEI is purely inorganic and cannot weather through the repetitive ion insertion and de-insertion. Therefore, it is necessary to leverage electrolyte formulations with additives and co-solvents to induce the SEI formation on the electrodes. In this talk, I will give examples of the above strategies to widen the electrochemical stability window of electrolytes and to promote the reversibility of the metal anode materials.

The effect of 120 keV He++D+-ion irradiation on the thermal conductivity of ceramic tetragonal gamma- LiAlO 2 is studied with time-domain thermoreflectance (TDTR) at temperatures between 300 and 700 K. The thermal conductivity of single crystal gamma- LiAlO 2 is 13.5 W/(m & sdot;K) at 300 K, and scales with temperature like 1/T. The thermal conductivity of unirradiated polycrystalline gamma- LiAlO 2 is 7.4 W/(m & sdot;K). Irradiation at fluences of 1 x, 5 x, and 10 x 1016 ions/cm2 decreases the thermal conductivity by approximate to 30 %, 80 %, and 90 %. The effect of irradiation is saturated at ion fluences of 1017 ions/cm2. Irradiation decreases the temperature dependence of the thermal conductivity. For ion fluences larger than 1017 ions/cm2, the thermal conductivity reaches a minimum value of approximate to 1 W/(m & sdot;K) that is independent of temperature.

Predicting thermal conductivity from the transport of electrons is a complex challenge which must be addressed for the design and production of nano- and microscale devices. In this work, we describe our progress in simulating thermal electron transport in silicon for the purposes of thermal conductivity prediction. We describe a new approach to solve the Boltzmann transport equation for electrons, using the self-adjoint angular flux formulation coupled with a novel approach in computing electron temperature and Fermi energy which does not require the combination of linear “inner” and nonlinear “outer” iterations. The electron Boltzmann transport equation is discretized in space by the continuous finite element method, and in angle by discrete ordinates, using the Multiphysics Object Oriented Simulation Environment, implemented in the radiation transport code Griffin. We rely on density functional theory calculations to provide material properties such as electron density of states, energy, wavevector, mean free path, and more. Our method couples the discrete electron groups through temperature and Fermi energy, to simulate thermal electron transport (in the absence of electric fields) for the prediction of thermal conductivity, thermal and electrical flux, and heat capacity. We show effective thermal conductivity and temperature results for different sized 2D slabs of Si at 300 K, which agree with other studies in the literature. Additionally, we report Fermi energy and efficiency of this method using the generalized minimal residual method.

The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our findings indicate that ions with high ionic potentials, i.e., charge density, promote the formation of their respective hydration structures, enhancing electrolytic reactions via an inductive effect, particularly for small cations. Conversely, ions with lower ionic potentials increase the proportion of free water molecules-those not engaged in hydration shells or hydrogen-bonding networks-leading to greater electrolytic stability. Furthermore, we observe that the chemical environment created by bulky ions with lower ionic potentials impedes electrolytic reactions by frustrating the solvation of protons and hydroxide ions, the products of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. We found that the solvation of protons plays a more substantial role than that of hydroxide, which explains a greater shift for OER than for HER, a puzzle that cannot be rationalized by the notion of varying O-H bond strengths of water. These insights will help the design of aqueous systems.

In a typical semiconductor material, the majority of the heat is carried by long-wavelength, long-mean-free-path phonons. Nanostructuring strategies to reduce thermal conductivity, a promising direction in the field of thermoelectrics, place scattering centers of size and spatial separation comparable to the mean free paths of the dominant phonons to selectively scatter them. The resultant thermal conductivity is in most cases well predicted using Matthiessen’s rule. In general, however, long-wavelength phonons are not as effectively scattered as the rest of the phonon spectrum. In this work, using large-scale molecular-dynamics simulations, non-equilibrium Green’s function simulations, and Monte Carlo simulations, we show that specific nanoporous geometries that create narrow constrictions in the passage of phonons lead to anticorrelated heat currents in the phonon spectrum. This effect results in super-suppression of long-wavelength phonons due to heat trapping and reductions in the thermal conductivity to values well below those predicted by Matthiessen’s rule.

It remains a challenge to design aqueous electrolytes to secure the complete reversibility of zinc metal anodes. The concentrated water‐in‐salt electrolytes, e.g., 30 m ZnCl 2 , are promising candidates to address the challenges of the Zn metal anode. However, the pure 30 m ZnCl 2 electrolyte fails to deliver a smooth surface morphology and a practically relevant Coulombic efficiency. Herein, it is reported that a small concentration of vanillin, 5 mg mL water −1 , added to 30 m ZnCl 2 transforms the reversibility of Zn metal anode by eliminating dendrites, lowering the Hammett acidity, and forming an effective solid electrolyte interphase. The presence of vanillin in the electrolyte enables the Zn metal anode to exhibit a high Coulombic efficiency of 99.34% at a low current density of 0.2 mA cm −2 , at which the impacts of the hydrogen evolution reaction are allowed to play out. Using this new electrolyte, a full cell Zn metal battery with an anode/cathode capacity (N/P) ratio of 2:1 demonstrates no capacity fading over 800 cycles.

Rechargeable aqueous zinc batteries are finding their niche in stationary storage applications where safety, cost, scalability and carbon footprint matter most. However, harnessing this reversible two-electron redox chemistry is plagued by major technical issues, notably hydrogen evolution reaction (HER) at the zinc surface, whose impacts are often not revealed under typical measurement conditions. Here we report a concentrated electrolyte design that eliminates this parasitic reaction and enables a Coulombic efficiency (CE) of 99.95% for Zn plating/stripping measured at a low current density of 0.2 mA cm−2. With extra chloride salts and dimethyl carbonate in concentrated ZnCl2 electrolyte, the hybrid electrolyte with a unique chemical environment features low Hammett acidity and facilitates the in situ formation of a dual-layered solid electrolyte interphase, protecting zinc anodes from HER and dendrite growth. Benefiting from the near-unity CE, the pouch cell with a VOPO4·2H2O cathode sustains 500 deep cycles without swelling or leaking and delivers an energy density of 100 Wh kg−1 under practical conditions. Our work represents a critical step forward in accelerating the market adoption of zinc batteries as an energy storage system with higher sustainability. Rechargeable aqueous zinc batteries are heralded as a sustainable energy technology but still face technical challenges. The hybrid electrolyte here eliminates hydrogen evolution reaction, the most thorny issue, and allows for impressive battery performance even under harsh conditions.

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions—25 wt.% LiCl and 62 wt.% H 3 PO 4 —cooled to −78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li + ions become less hydrated and pair up with Cl − , ice-like water clusters form, and H⋅⋅⋅Cl − bonding strengthens. Surprisingly, this low-temperature solvation structure does not strengthen water molecules’ O−H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O−H covalent bond. We propose a more general mechanism for water's low temperature inertness in the electrolyte: less favorable solvation of OH − and H + , the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aqueous Li-ion battery using LiMn 2 O 4 cathode and CuSe anode with a high energy density of 109 Wh/kg. These results highlight the potential of aqueous batteries for polar and extraterrestrial missions.

Predicting nanoscale thermal transport in dielectrics requires models, such as the Boltzmann transport equation (BTE), that account for phonon boundary scattering in structures with complex geometries. Although the BTE has been validated against several key experiments, its computational expense limits its applicability. Here, we demonstrate the use of an analytic reduced-order model for predicting the thermal conductivity in dimensionally confined materials, i.e., monolithic and porous thin films, and rectangular and cylindrical nanowires. The approach uses the recently developed "Ballistic Correction Model," which accounts for materials' full distribution of phonon mean-free-paths. The model is validated against BTE simulations for a selection of base materials, obtaining excellent agreement. By furnishing a precise yet easy-to-use prediction of thermal transport in nanostructures, our work strives to accelerate the identification of materials for energy-conversion and thermal-management applications.

Zn Metal Batteries Designing effective aqueous electrolytes for zinc metal anodes remains a challenge. In article number 2301712, P. Alex Greaney, Xiulei Ji and co-workers show that adding a few ppm vanillin to 30 m ZnCl2 enhances the anode's reversibility, prevents dendrite formation, and boosts Coulombic efficiency. This electrolyte results in a Zn metal battery with an N/P ratio of 2 that retains stable cycling over 800 cycles.

We present a method for deterministically solving the frequency and temperature dependent phonon radiative transport (PRT) equation in the single-mode relaxation time (SMRT) approximation in the self-adjoint angular flux (SAAF) form. To handle the nonlinear coupling between the phonon intensities and the material temperature, we apply a linearization approach that is similar to one in thermal radiative transport. This procedure leads to the PRT equation with pseudo-scattering. The method presented includes acceleration of both the inner pseudo-scattering source iterations and outer temperature iteration with a gray diffusion synthetic acceleration (DSA) and Anderson acceleration, respectively. We use the finite-element method to discretize the PRT equation in space and the method of discrete ordinates (S-N) for angular discretization. The proposed method is verified by a gray method of manufactured solutions problem and demonstrated on a problem using temperature and direction dependent multigroup data from lithium aluminate (LiAlO2). The iterative performance of the acceleration method in each test is then compared to the unaccelerated method.

Nanostructured semiconducting alloys obtain ultra-low thermal conductivity as a result of the scattering of phonons with a wide range of mean-free-paths (MFPs). In these materials, long-MFP phonons are scattered at the nanoscale boundaries whereas short-MFP high-frequency phonons are impeded by disordered point defects introduced by alloying. While this trend has been validated by simplified analytical and numerical methods, an ab-initio space-resolved approach remains elusive. To fill this gap, we calculate the thermal conductivity reduction in porous alloys by solving the mode-resolved Boltzmann transport equation for phonons using the finite-volume approach. We analyze different alloys, length scales, concentrations, and temperatures, obtaining a very large reduction in the thermal conductivity over the entire configuration space. For example, a similar to 97% reduction is found for Al 0 . 8 In 0 . 2 As with 25% porosity. Furthermore, we employ these simulations to validate our recently introduced "Ballistic Correction Model" (BCM), an approach that estimates the effective thermal conductivity using the characteristic MFP of the bulk alloy and the length-scale of the material. The BCM is then used to provide guiding principles in designing alloy-based nanostructures. Notably, it elucidates how porous alloys such as Si x Ge 1 -x obtain larger thermal conductivity reduction compared to porous Si or Ge , while also explaining why we should not expect similar behavior in alloys such as Al x In 1 -x As . By taking into account the synergy from scattering at different scales, we provide a route for the design of materials with ultra-low thermal conductivity. (c) 2022 Published by Elsevier Ltd.

New acceptor‐type graphite intercalation compounds (GICs) offer candidates of cathode materials for dual‐ion batteries (DIBs), where superhalides represent the emerging anion charge carriers for such batteries. Here, the reversible insertion of [LiCl 2 ] − into graphite from an aqueous deep eutectic solvent electrolyte of 20 m LiCl + 20 m choline chloride is reported. [LiCl 2 ] − is the primary anion species in this electrolyte as revealed by the femtosecond stimulated Raman spectroscopy results, particularly through the rarely observed H–O–H bending mode. The insertion of Li–Cl anionic species is suggested by 7 Li magic angle spinning nuclear magnetic resonance results that describe a unique chemical environment of Li + ions with electron donors around. 2 H nuclear magnetic resonance results suggest that water molecules are co‐inserted into graphite. Density functional theory calculations reveal that the anionic insertion of hydrated [LiCl 2 ] − takes place at a lower potential, being more favorable. X‐ray diffraction and the Raman results show that the insertion of [LiCl 2 ] − creates turbostratic structure in graphite instead of forming long‐range ordered GICs. The storage of [LiCl 2 ] − in graphite as a cathode for DIBs offers a capacity of 114 mAh g −1 that is stable over 440 cycles.

Nanostructured materials enable high thermal transport tunability, holding promises for thermal management and heat harvesting applications. Predicting the effect that nanostructuring has on thermal conductivity requires models, such as the Boltzmann transport equation (BTE), that capture the non-diffusive transport of phonons. Although the BTE has been well validated against several key experiments, notably those on nanoporous materials, its applicability is computationally expensive. Several effective model theories have been put forward to estimate the effective thermal conductivity; however, most of them are either based on simple geometries, e.g., thin films, or simplified material descriptions such as the gray-model approximation. To fill this gap, we propose a model that takes into account the whole mean-free-path (MFP) distribution as well as the complexity of the material’s boundaries in infinitely thick films with extruded porosity using uniparameter logistic regression. We validate our approach, which is called the “Ballistic Correction Model” (BCM), against full BTE simulations of a selection of three base materials (GaAs, InAs, and Si) with nanoscale porosity, obtaining excellent agreement. While the key parameters of our method, associated with the geometry of the bulk material, are obtained from the BTE, they can be decoupled and used in arbitrary combinations and scales. We tabulated these parameters for a few cases, enabling the exploration of systems that are beyond those considered in this work. Providing a simple yet accurate estimation of thermal transport in nanostructures, our work sets out to accelerate the discovery of materials for thermal-related applications.

The ability to minimize the thermal conductivity of dielectrics with minimal structural intervention that could affect electrical properties is an important capability for engineering thermoelectric efficiency in low-cost materials such as Si. We recently reported the discovery of special arrangements for nanoscale pores in Si that produce a particularly large reduction in thermal conductivity accompanied by strongly anticorrelated heat current fluctuations [1] – a phenomenon that is missed by the diffuse adiabatic boundary conditions conventionally used in Boltzmann transport models. This manuscript presents the results of molecular dynamics simulations and a Monte Carlo ray tracing model that teases apart this phenomenon to reveal that special pore layouts elastically backscatter long-wavelength heat-carrying phonons. This means that heat carriage by a phonon before scattering is undone by the scattered phonon, resulting in an effective mean-free-path that is significantly shorter than the geometric line-of-sight due to the pores. This effect is particularly noticeable for the long-wavelength, long mean-free-path phonons whose transport is impeded drastically more than is expected purely from the usual considerations of scattering defined by the distance between defects. This “super-suppression” of the mean-free-path below the characteristic length scale of the nanostructuring offers a route for minimizing thermal conductivity with minimal structural impact, while the stronger impact on long wavelengths offers possibilities for the design of band-pass phonon filtering. Moreover, the ray tracing model developed in this paper shows that different forms of correlated scattering imprint a unique signature in the heat current autocorrelation function that could be used as a diagnostic in other nanostructured systems.

Sodium ion batteries (NIBs) are an attractive alternative to lithium‐ion batteries in applications that require large‐scale energy storage due to sodium's high natural abundance and low cost. Hard carbon (HC) is the most promising anode material for NIBs; however, there is a knowledge gap in the understanding of the sodium binding mechanism that prevents a rational design of HC. This study tunes sucrose‐derived HC via synthesis temperature then evaluates the structural, physical, and electrochemical properties. Neutron total scattering is used to generate structural models by fitting pair distribution functions (PDF) with a combination of molecular dynamics and reverse Monte Carlo methods. From this model, the number and type of structural features are identified, quantified, and correlated to the galvanostatic charge/discharge. A method of PDF “fingerprinting” binding sites using Na probe atoms is developed and analyzing these PDFs reveals an atomistic view of ion binding sites responsible for “defect” storage mechanisms. Combining these techniques results in an atomic‐level study that provides a big picture of the Na‐binding mechanism in NIBs, which allows for more precise tuning of the structure–property relationships in the future. The methodologies developed will also enable new strategies for the analysis of amorphous functional materials.

Two new rod-packing metal-organic frameworks (RPMOF) are constructed by regulating the in situ formation of the capping agent. In CPM-s7, carboxylate linkers extend 1D manganese-oxide chains in four additional directions, forming 3D RPMOF. The substitution of Mn2+ with a stronger Lewis acidic Co2+ , leads to an acceleration of the hydrolysis-prone sulfonate linker, resulting in presence of sulfate ions to reduce two out of the four carboxylate-extending directions, and thus forming a new 2D rod-packing CPM-s8. Density functional theory calculations and magnetization measurements reveal ferrimagnetic ordering of CPM-s8, signifying the potential of exploring 2D RPMOF for effective low-dimensional magnetic materials.

Silicon carbide nanoparticles with diameters around 8 nm and narrow size distribution have been finely mixed with doped silicon nanopowders and sintered into bulk samples to investigate the influence of nanoinclusions on electrical and thermal transport properties. We have compared the thermoelectric properties of samples ranging from 0%-5% volume fraction of silicon carbide. The silicon carbide nanoinclusions lead to a significant improvement in the thermoelectric figure of merit ZT largely due to an enhancement of the Seebeck coefficient. A semiclassical Boltzmann transport equation is used to model the electrical transport properties of the Seebeck coefficient and electrical conductivity. The theoretical analysis confirms that the enhancements in the thermoelectric properties are consistent with the energy selective scattering of electrons induced by the offset between the silicon Fermi level and the carbide conduction band edge. This study proves that careful engineering of the energy-dependent electron scattering rate can provide a route towards relaxing long-standing constraints in the design of thermoelectric materials.

The influence of strain rate across three orders of magnitude (1.70 x 10(-5)/s to 1.43 x 10(-2)/s) along with the effect of the plastic strain accumulation (up to 10%) on the serrated plastic flow were investigated in the nickel-chromium (Ni-Cr) solid solution alloy Nimonic 75 by performing constant-strain-rate tension testing at 600 degrees C. As the strain rate decreased, the critical strain for the onset of serrations transitioned from normal behavior to inverse behavior. The serrated flow was characterized as Type A+B serration at high strain rate (1.43 x 10(-2)/s). In the intermediate strain-rate regime (1.43 x 10(-3)/s and 1.45 x 10(-4)/s), Type B serrations were observed and followed by a transformation to Type C+B serrations. At the low strain rate (1.70 x 10(-)5/s), the plastic flow immediately displayed Type C serrations, which later evolved into Type C+B serrations. Regardless of the strain rate, plastic strain, or dislocation density, a critical dislocation velocity falling in the range of 1.2 x 10(-6) - 2.2 x 10(-6) m/s was identified to signify the onset of Type C serration, whereby the mobile dislocations break free from the solute cloud for short bursts of deformation. Finally, a novel model by solute rearrangement across dislocation cores was used to understand how the critical dislocation velocity is quantitatively determined by the rate at which solute atoms are able to hop across the glide plane as a partial dislocation core moves through the lattice.