Activation Energy For Transport Research Articles

Synthesis and electrochemical characterization of sulfur-doped Li3PO4 for all-solid-state battery applications

Isovalent and aliovalent cation-doping have been widely investigated to enhance ion transport in γ‑Li3PO4-type lithium superionic conductors (LISICONs). Anion doping, however, has been restricted to a full replacement of oxide ions by sulfide ions in some compounds (the so-called thio-LISICONs). The replacement of oxide ions by sulfide ions enhances both ion conductivity and deformability of the materials. Similar to other sulfide-type solid-electrolytes, thio-LISICONs, however, showed limited electrochemical stability against oxidation. In this report, a sonication-assisted liquid-phase synthesis approach was employed to achieve substantial sulfur‑oxygen mixing in γ-type Li3PO4. Sulfur-doped Li3PO4 possessed an ion conductivity four orders of magnitude higher than that of γ-Li3PO4, and a notably low activation energy of ion transport of 0.40(8) eV. Sulfur-oxygen mixing in sulfur-doped Li3PO4 enhanced electrochemical stability against oxidation (up to 4 V vs. Li+/Li) compared with sulfur-based Li3PS4. This study paves the way for developing mixed-anion phases in the γ-type Li3PO4 structure with enhanced electrochemical properties for all-solid-state battery applications.

Enriching Nano-Heterointerfaces in Proton Conducting TiO2-SrTiO3@TiO2 Yolk-Shell Electrolyte for Low-Temperature Solid Oxide Fuel Cells.

A challenging task in solid oxide fuel cells (SOFCs) is seeking for an alternative electrolyte, enabling high ionic conduction at relatively low operating temperatures, i.e., 300-600°C. Proton-conducting candidates, in particular, hold a significant promise due to their low transport activation energy to deliver protons. Here, a unique hierarchical TiO2-SrTiO3@TiO2 structure is developed inside an intercalated TiO2-SrTiO3 core as "yolk" decorating densely packed flake TiO2 as shell, creating plentiful nano-heterointerfaces with a continuous TiO2 and SrTiO3 "in-house" interfaces, as well the interfaces between TiO2-SrTiO3 yolk and TiO2 shell. It exhibits a reduced activation energy, down to 0.225eV, and an unexpectedly high proton conductivity at low temperature, e.g., 0.084 S cm-1 at 550°C, confirmed by experimentally H/D isotope method and proton-filtrating membrane measurement. Raman mapping technique identifies the presence of hydrogenated HO─Sr bonds, providing further evidence for proton conduction. And its interfacial conduction is comparatively analyzed with a directly-mixing TiO2-SrTiO3 composite electrolyte. Consequently, a single fuel cell based on the TiO2-SrTiO3@TiO2 heterogeneous electrolyte delivers a good peak power density of 799.7mW cm-2 at 550°C. These findings highlight a dexterous nano-heterointerface design strategy of highly proton-conductive electrolytes at reduced operating temperatures for SOFC technology.

Propylene carbonate activated electrochemical performances of in situ polymerized ionogel electrolytes

Ionogel electrolytes show great potential in solid-state batteries attributed to their outstanding characteristics. However, because of the strong ionic nature of ionic liquids, ionogel electrolytes generally exhibit low lithium-ion transference number and poor inferior interfacial contact, limiting their practical applications. In present research, propylene carbonate (PC) is added to form solvation separated ion pair with lithium ions in the conventional pyrrolidinium ionogel electrolytes, which comprehensively improves electrochemical properties of ionogels in lithium metal batteries. An astonished improvement is observed in an abrupt increase of the lithium-ion transference number from 0.1 to 0.41, thereby elevating ionic conductivities from 4.7 × 10−4 S/cm to 1.72 × 10−3 S/cm and reducing the activation energies of ion transport from 0.036eV to 0.018eV. As a result, PC highly activates electrochemical performances in assembled LiFePO4|Li batteries, exhibiting high discharging specific capacities: 134 mAh/g (1C), while that of PC free ionogel is 5 mAh/g (1C), respectively. The PC assisted LiFePO4|Li cell could retain 90 % of its initial specific capacity after 200 cycles and could work at a wide temperature range from 0 °C to 100 °C. The results demonstrate a new strategy to improve lithium-ion transference number in ionogel electrolytes for achieving improved performance in solid-state lithium metal batteries.

The Effect of Structural Disorder on Li-Ion Transport in the Inorganic Components of the Electrode-Electrolyte Interfaces of Li-Ion Batteries

Conventional organic electrolytes in Li-ion batteries have an electrochemical window that is too narrow, leading to a decomposition at the cathode and anode surfaces, especially at high voltages. The breakdown of the electrolyte leads to the formation of cathode-electrolyte interphase (CEI) and solid-electrolyte interface (SEI) at the anode. These interfaces are a multicomponent, 3-dimensional heterogeneous structures that span from a few nanometers to micrometers. To understand the atomic-scale impact of the interfaces’ structure and chemistry on function we have run molecular dynamics simulations of the Li-ion diffusivity in the most likely components: LiF, Li2CO3, and Li2O. Our work focuses on identifying the effect of local chemistry and lattice structure in disordered or amorphous component materials on Li-diffusivity. We extract Li-ion diffusivity and the activation energy for Li-ion transport from both machine learning force fields and ab-initio molecular dynamics from the individual components of the interface and explicit interfaces. Post analysis establishes the local correlation between atomic structure, chemistry, and Li-ion migration. If well designed, the SEI and CEI can protect the electrolyte and cathode to prevent further degradation during cycling and thus optimization of battery performance.This work was sponsored by the Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Interfaces Comprehension at Electrodes on Sulfide-Based Solid State Batteries

The increasing demand for safe, highly efficient, and cost-effective energy storage systems has accelerated the development of solid-state batteries (SSBs) with lithium metal (LiM) anodes. This technology offers remarkable advantages over conventional lithium-ion batteries with liquid electrolytes; from improved safety with non-flammable electrolyte to higher gravimetric and volumetric energy density enabled using LiM anode along with the multilayered bipolar stacking cell fabrication. The combination of solid electrolyte (SE) mechanical strength, flexibility, and safety against self-ignition allows for optimized battery design to meet the specific requirements.1–4 In recent years, soft, flexible, and easily deformable sulfide-based solid electrolytes have received significant attention due to their attractive mechanical properties, high conductivity, and feasibility of processing at ambient temperature, which makes them appealing candidates for upscaling the technology to large format devices.2,4,5 Despite the high ionic conductivity and attractive mechanical properties of sulfide-based SSBs, this chemistry still faces key challenges to encompass fast rate and long cycling performance, mainly arising from the dynamic and complex solid-solid interfaces. Solid-state materials are characterized by a significant impact of interface-related phenomena on their functional characteristics, such as mechanical properties, conductivity mechanisms, or electrochemical performance. For SSBs with ceramic or glass-ceramic electrolytes, the stacking of the composite cathode, the SE and the LiM anode leads to multiple interfaces from voids, cracks, secondary phases from chemical and electrochemical reactions and grain boundaries.6,7 As electrochemical kinetics is governed by the contact area at the interface, ensuring the adhesion between the electrolyte and active electrode materials is the key to obtain high performance devices, especially at fast rates.The goal of the present work is to provide understanding on the complex solid-solid interfaces regulating sulfide-based battery performance. As shown in Figure 1, cells under different configurations with LPSCl, Li or In metal anodes, and LiNi0.6Mn0.2Co0.2O2 (NMC622)-based composite cathodes have been systematically studied aiming to bring insights on the dynamic interfaces at component level dominating processes during cell operation. From a multi-configurational approach and an advanced deconvolution of electrochemical impedance signals into distribution of relaxation times, we disentangle intricate underlying interfacial processes taking place at the battery components and that play major role on the overall performance. For the Li metal solid-state batteries, the cycling performance is highly sensitive to the chemomechanical properties of the cathode active material, the formation of the SEI, and processes ascribed to Li diffusion in the cathode composite and in the space-charge layer. The outcomes of this work aim to facilitate the design of sulfide solid-state batteries and provide methodological inputs for battery ageing assessment.(1) Zheng, F.; Kotobuki, M.; Song, S.; Lai, M. O.; Lu, L. Review on Solid Electrolytes for All-Solid-State Lithium-Ion Batteries. Journal of Power Sources 2018, 389, 198–213. https://doi.org/10.1016/j.jpowsour.2018.04.022.(2) Randau, S.; Weber, D. A.; Kötz, O.; Koerver, R.; Braun, P.; Weber, A.; Ivers-Tiffée, E.; Adermann, T.; Kulisch, J.; Zeier, W. G.; Richter, F. H.; Janek, J. Benchmarking the Performance of All-Solid-State Lithium Batteries. Nat Energy 2020, 5 (3), 259–270. https://doi.org/10.1038/s41560-020-0565-1.(3) Schnell, J.; Günther, T.; Knoche, T.; Vieider, C.; Köhler, L.; Just, A.; Keller, M.; Passerini, S.; Reinhart, G. All-Solid-State Lithium-Ion and Lithium Metal Batteries – Paving the Way to Large-Scale Production. Journal of Power Sources 2018, 382, 160–175. https://doi.org/10.1016/j.jpowsour.2018.02.062.(4) Kim, K. J.; Balaish, M.; Wadaguchi, M.; Kong, L.; Rupp, J. L. M. Solid‐State Li–Metal Batteries: Challenges and Horizons of Oxide and Sulfide Solid Electrolytes and Their Interfaces. Advanced Energy Materials 2021, 11 (1), 2002689. https://doi.org/10.1002/aenm.202002689.(5) Liu, S.; Zhou, L.; Han, J.; Wen, K.; Guan, S.; Xue, C.; Zhang, Z.; Xu, B.; Lin, Y.; Shen, Y.; Li, L.; Nan, C. Super Long‐Cycling All‐Solid‐State Battery with Thin Li 6 PS 5 Cl‐Based Electrolyte. Advanced Energy Materials 2022, 12 (25), 2200660. https://doi.org/10.1002/aenm.202200660.(6) Pesci, F. M.; Bertei, A.; Brugge, R. H.; Emge, S. P.; Hekselman, A. K. O.; Marbella, L. E.; Grey, C. P.; Aguadero, A. Establishing Ultralow Activation Energies for Lithium Transport in Garnet Electrolytes. ACS Appl. Mater. Interfaces 2020, 12 (29), 32806–32816. https://doi.org/10.1021/acsami.0c08605.(7) Chen, R.; Li, Q.; Yu, X.; Chen, L.; Li, H. Approaching Practically Accessible Solid-State Batteries: Stability Issues Related to Solid Electrolytes and Interfaces. Chem. Rev. 2020, 120 (14), 6820–6877. https://doi.org/10.1021/acs.chemrev.9b00268. Figure 1

Tuning the electronic structure of monolayer MoS2 towards metal like via vanadium doping

Doping of two-dimensional layered semiconducting materials is becoming pivotal in tailoring their electronic properties, enabling the development of advanced electronic and optoelectronic devices, where the selection of dopant is important. Here, we demonstrate the potential of substitutional vanadium (V) doping in monolayer molybdenum disulfide (MoS2) lattice in different extents leading to tunable electronic and optoelectronic properties. We found that low-level V doping (∼1 at.%) induces p-type characteristics in otherwise n-type monolayer MoS2, whereas medium-level doping (∼5 at.%) leads to an ambipolar semiconductor. Degenerately doped MoS2 (∼9 at.%) facilitates a transition from semiconducting towards metallic (metal-like) with reduced electrical resistivity (∼4.5 Ωm of MoS2 to ∼2.2×10−5Ωm), low activation energy for transport (∼11 meV), and electric field independent drain current in field effect transistor–based transfer characteristics. A detailed temperature- and power-dependent photoluminescence study along with density functional theory–based calculations in support unravels the emergence of an excitonic transition at ∼850 nm with its intensity dependent on the amount of vanadium. This study shows the potential of V doping in MoS2 for generating multifunctional two-dimensional materials for next generation electronics, optoelectronics, and interconnects with systematic control over its electronic structure in a wide range. Published by the American Physical Society 2024

Operando Raman Gradient Analysis for Temperature-Dependent Electrolyte Characterization.

Transport and thermodynamic properties are integral parameters to understand, model, and optimize state-of-the-art and next-generation battery electrolytes. The accurate measurement of these properties is experimentally challenging as well as time- and resource-intensive, and consequently, reports are scarce. Their dependence on temperature is explored even less and is commonly limited to a few temperature points. Recently, we introduced an operando Raman gradient analysis (ORGA) tool to extract transport and thermodynamic properties. Here, we expand the capabilities of ORGA by incorporating a temperature-sensitive external reference into the design. With this enhancement, we are able to visualize the local concentration of any Raman-active species in the electrolyte and detect lithium filament nucleation. We demonstrate and validate this new functionality of ORGA via an examination of lithium bis(fluorosulfonyl)imide (LiFSI) in tetraethylene glycol dimethyl ether (G4) as a function of temperature. All transport properties and activation energies are reported, and the effect of temperature is discussed.

Marcus Theory and Long-Range Activationless Transport in Molecular Junctions.

Intrachain transport in molecular junctions (MJs) longer than 5 nm has been modeled within the theoretical framework of Marcus theory. We show that in oligo(bisthienylbenzene)-based MJs, electronic transport involves polarons, localized on three monomers that are close to 4 nm in length. They hop and tunnel between adjacent localized sites with reorganization energies λ close to 400-600 meV and electronic coupling parameters Hab close to λ/2. As a consequence, the activation energy for intrachain transport, given by the equation ΔG* = (λ/4)(1 - 2Hab/λ)2, is close to zero, and transport along the chain is activationless, in agreement with experimental observation. On the contrary, similar calculations on conjugated oligonaphthalenefluoreneimine wires show that Hab is much less than λ/2 and predict that the activation energies for intrachain hopping between adjacent sites, close to λ/4, are ∼115 meV. This work proposes a new perspective for understanding long-range activationless transport in MJs beyond the tunneling regime.

Renewing Fundamental Understanding of Ionic Transport in Inorganic Crystalline Solid‐State Electrolytes from the Perspective of Lattice Dynamics

AbstractLattice dynamics has been widely employed to study various properties of crystalline materials, by revealing the atomic vibration rules with phonon dispersion curves. Modulating lattice‐dynamics‐related factors is an emerging trend in the design of inorganic crystalline solid‐state electrolytes (ICSSEs) with promoted ionic conductivity. Nevertheless, due to complicated interplay of these enormous factors, fundamental understanding of lattice‐dynamics‐influenced ionic transport is still limited. In this review, all related factors are classified into lattice static and dynamic ones based on their variability in ionic transport process, facilitating a methodological evaluation of their individual and interrelated influences. The previous efforts are affirmed to design ICSSEs by constructing potential energy surfaces for ionic transport based on lattice static factors (e.g., ionic arrangement, time‐scale dependent ionic concentration, and lattice softening). As for lattice dynamic factors, the possibility to quantify ionic transport activation energy and to screen the type of migration ions by phonon frequency and amplitude, respectively is validated. Specifically, it is highlighted that the optimization of specific phonon mode and the coupling of both lattice static and dynamic factors are essential to achieve superior ionic transport in ICSSEs. Finally, challenges and opportunities are pointed out in this field, which can potentially guide the future development of ICSSEs.

Regulating ion affinity and dehydration of metal-organic framework sub-nanochannels for high-precision ion separation

Membrane consisting of ordered sub-nanochannels has been pursued in ion separation technology to achieve applications including desalination, environment management, and energy conversion. However, high-precision ion separation has not yet been achieved owing to the lack of deep understanding of ion transport mechanism in confined environments. Biological ion channels can conduct ions with ultrahigh permeability and selectivity, which is inseparable from the important role of channel size and “ion-channel” interaction. Here, inspired by the biological systems, we report the high-precision separation of monovalent and divalent cations in functionalized metal-organic framework (MOF) membranes (UiO-66-(X)2, X = NH2, SH, OH and OCH3). We find that the functional group (X) and size of the MOF sub-nanochannel synergistically regulate the ion binding affinity and dehydration process, which is the key in enlarging the transport activation energy difference between target and interference ions to improve the separation performance. The K+/Mg2+ selectivity of the UiO-66-(OCH3)2 membrane reaches as high as 1567.8. This work provides a gateway to the understanding of ion transport mechanism and development of high-precision ion separation membranes.

In-situ crosslinking of a novel conjugated dialkynyl-based polyimide for high performance carbon molecular sieve membrane

High-performance and reliable membranes for efficient gas separation and their design strategies are key points for membrane-based carbon capture. Here, we reported a high-performance carbon membrane design protocol from a microporous, highly rigid polyimide precursor (6FBD1-CR), which was produced by in-situ crosslinking a polyimide (6FBD1) containing conjugated dialkynyl group at 300 °C. The possible crosslinking mechanisms are the [2+2+2] trimerization of three ethynyl groups to form a benzene ring and the Diels-Alder reaction between the ethynyl bond and phenylacetylene to generate the fused naphthalene ring. After crosslinking, the membrane shows a ∼ 8-fold improved microporosity (37.1–298 m2 g−1). When pyrolyzed at 550 °C, the resulting 6FBD1-CMS demonstrates very high pure- and mixed-gas separation performance. E.g., the CO2 permeability reaches 7661 Barrer and CO2/CH4 selectivity of 44, additionally, even at the CO2/CH4 mixed-gas pressure of 30 bar, the PCO2 can still reach 4920 Barrer combined with a αCO2/CH4 of 28, which is well above the latest CO2/CH4 mixed-gas Upper Bound line. We consider this is due to the acetylene crosslinking induced a very stable microporous structure composed of rigid fused rings that alleviate the micropore collapse during the carbonization process, and thus, resulting in a relatively higher BET surface area, larger d-spacing, relatively higher concentration of continuous disordered phase (higher sp3/sp2) and small activation energy (Ep) for gas transport than the CMSM derived from polyimide without pre-crosslinking. This finding provides new opportunity for the structural design of high-performance CMSMs.

Ionic thermoelectric effect in Cu2-δSe during phase transition

The ionic Seebeck coefficient was studied in copper selenide with Cu1.99Se, Cu1.95Se and Cu1.8Se stoichiometry which was synthesized with a melt crystallization method. To measure the ionic Seebeck coefficient of copper ions, 0.15C6H12N4CH3I + 0.85CuI solid-state electrolyte was prepared. Electrolyte layers were pressed with copper selenide powder into a sandwich-like structure. At the temperature of 410 K, the materials have ionic Seebeck coefficient values close to each other, about 1100 μV/K. In the case of β-phase structure (Cu1.8Se material), changes in the measured Seebeck coefficient were observed—with decreasing temperature, the ionic thermopower firstly increased reaching about 1230 μV/K and then decreased to 950 μV/K at 355 K. In the Cu1.99Se material, a phase transition to the α-phase was observed during cooling. The ionic Seebeck coefficient values gradually increased from 1030 to 1220 μV/K at 370 K, when the material is in the low-temperature phase. The measured difference between the ionic thermopower of the two phases well matches calculations based on the entropy of the transition (presence part of the Seebeck coefficient) and different activation energies of ionic transport (transport part).Graphical abstract

Temperature controlled swelling of graphene oxide for switchable dehumidification membranes

Variation of water permeance and interlayer spacing of graphene oxide (GO) is traced with in-situ X-ray diffraction upon its heating/cooling in dry (∼1000 Pa), humid (∼100 % RH) air and liquid water. Reversible variation of GO interlayer distance ranging 8.3-7.2 Å in dry air, 11-7.5 Å in humid air and 13.9-12.4 Å in liquid water is revealed in the temperature range of 25–80 °C. Accounting for GO layer thickness of ∼6 Å it corresponds manifold alterations of slit width with temperature and water vapor pressure, resulting in over 3 orders of magnitude variation of membrane performance. A typical rise of the permeance of 300–500 % within the temperature range at constant humidity is contraposed to a miserable increase in the activation energy for H2O transport, revealing the decisive impact of slit sizes on GO performance. The effect is addressed to entropy-driven variation of water absorption heat with slit sizes in GO. Variation of interlayer spacing of ∼10 % was also exposed with thermal dissociation of GO groups and H+ migration to the interlayer space as supported by semi-empirical calculations. Reversible structural changes in GO are successfully exploited for fabrication thermally switched membranes for dehumidification with initial permeance of ∼1.1·10−6 mol m−2 Pa−1·s−1 and its variation over 50 % at supplied power of ∼500 W/m2.

Comparative study of moisture transfer of polyethersulfone-based membranes and kraft paper

The efficiency of membrane-based enthalpy exchangers primarily depends on the moisture permeability of the membranes used. While traditional kraft paper membranes exhibit favorable moisture adsorption properties, they lack durability and are prone to bacterial growth. To overcome these limitations, alternative membrane materials, including polyethersulfone (PES) film, asymmetric porous PES membrane (P-MEM), and composite membrane (C-MEM) comprising PES and polyether block amide (PEBAX), have been investigated. These membranes' physical properties and water adsorption affinity were compared with that of conventional kraft paper. Experimental measurements of moisture permeation and adsorption equilibrium were conducted to evaluate the moisture transfer properties of the membranes under different operating conditions. The experimental and modeling results showed that the moisture transfer resistance of the membranes varied depending on the material and temperature, with a lower degree of dependence on the surrounding fluid humidity. C-MEM exhibited the highest moisture resistance, followed by kraft paper, while P-MEM and PES had the lowest resistance. Additionally, the moisture diffusivity and activation energies for moisture transport were determined for each membrane.

Sodium titanate: A proton conduction material for ppb-level NO2 detection with near-zero power consumption

Constrained by the traditional charge transfer sensing mechanism, it is quite challenging to fabricate NO2 sensors that simultaneously exhibit high sensitivity, rapid response/recovery, and low power consumption. Herein, sodium titanate (NTO), a layered material with abundant surface-rooted OH groups (OHR), is demonstrated to be a promising NO2 sensing material. To understand the sensing behavior of NTO, the influences of operating temperature, applied voltage, and relative humidity are investigated, and a novel OHR-enabled proton conduction sensing mechanism is proposed. The sensing process mainly involves selective NO2 adsorption on OHR, thereby lowering the activation energy for proton transportation along the NTO surface. Meanwhile, the moderate intermolecular interaction makes NO2 both easily adsorbed and desorbed at room temperature. Hence, NTO exhibits a highly sensitive, rapid, and fully recoverable response (∼5.7–1 ppm NO2 within 3 s), wide detection range (1 ppb-20 ppm), good stability (>2 months), and near-zero power consumption (0.5 nW). Finally, we demonstrate that NTO has an excellent practical indoor/outdoor NO2 sensing ability. This work offers a new pathway to resolve the inherent conflicts in available NO2 sensors by using NTO via the OHR-enabled proton conduction sensing mechanism, which may also provide insight into designing high-performance sensors for other gases.

The Effect of Chlorides on the Performance of DME/Mg[B(HFIP)4]2 Solutions for Rechargeable Mg Batteries

One of the major issues in developing electrolyte solutions for rechargeable magnesium batteries is understanding the positive effect of chloride anions on Mg deposition-dissolution processes on the anode side, as well as intercalation-deintercalation of Mg2+ ions on the cathode side. Our previous results suggested that Cl− ions are adsorbed on the surface of Mg anodes and Chevrel phase MgxMo6S8 cathodes. This creates a surface add-layer that reduces the activation energy for the interfacial Mg ions transportation and related charge transfer, as well as promotes the transport of Mg2+ from the solution phase to the Mg anode surface and into the cathodes’ host materials. Here, this work further examines the effect of adding chlorides to the state-of-the-art Mg[B(HFIP)4]2/DME electrolyte solution, specifically focusing on reversible magnesium deposition, as well as the performance of Mg cells with benchmark Chevrel phase cathodes. It was observed that the presence of chlorides in these solutions facilitates both Mg deposition, and Mg2+ ions intercalation, whereby this effect is more pronounced as the purity level of the solution is lowered.

Simulated surface diffusion in nanoporous gold and its dependence on surface curvature

The morphological evolution of nanoporous gold is generally believed to be governed by surface diffusion. This work specifically explores the dependence of mass transport by surface diffusion on the curvature of a gold surface. The surface diffusivity is estimated by molecular dynamics simulations for a variety of surfaces of constant mean curvature, eliminating any chemical potential gradients and allowing the possible dependence of the surface diffusivity on mean curvature to be isolated. The apparent surface diffusivity is found to have an activation energy of ∼0.74 eV with a weak dependence on curvature, but is consistent with the values reported in the literature. The apparent concentration of mobile surface atoms is found to be highly variable, having an Arrhenius dependence on temperature with an activation energy that also has a weak curvature dependence. These activation energies depend on curvature in such a way that the rate of mass transport by surface diffusion is nearly independent of curvature, but with a higher activation energy of ∼1.01 eV. The curvature dependencies of the apparent surface diffusivity and concentration of mobile surface atoms is believed to be related to the expected lifetime of a mobile surface atom, and has the practical consequence that a simulation study that does not account for this finite lifetime could underestimate the activation energy for mass transport via surface diffusion by ∼0.27 eV.

Modeling oxygen diffusion in barium titanate using molecular dynamics: Comparison between Mg and Sc dopants

In this study, the diffusion coefficient of oxygen vacancies in barium titanate doped with 2.0% Sc was calculated by using molecular dynamics. The temperature was varied from 1273 K to 2500 K, and the simulation box consisted of 10 × 10 × 10 unit cells subject to periodic boundary conditions. The Sc dopants were incorporated into the B-sublattice and compensated for by using the randomly distributed oxygen vacancies on the oxygen sublattice. The diffusivity of the vacancies was determined from the slope of the mean-squared displacement of the oxygen ions over time. The Arrhenius plot of the diffusion coefficient showed a clear linear behavior, with an activation energy of 0.84 eV. The results were interpreted by computing radial pair distribution functions for various correlations (e.g., Ti–O and Sc–O) and by static lattice (nudged elastic band) calculations of energy barriers for the migration of oxygen. While Mg-doped BaTiO3 exhibited a strong trend of the formation of defect associates between the acceptor dopant and the oxygen vacancies that lead to a clear reduction in the observed activation energy for oxygen transport with increasing temperature (non-linear Arrhenius behavior), defect-induced interactions (associates) in case of Sc doping were nearly negligibly small, and gave rise to a linear Arrhenius plot with a single activation energy.

Charge transport and dielectric characteristics of Sm x Bi1−x FeO3 thin films from the perspective of grain and grain boundary properties

Samarium-substituted bismuth ferrite (Sm x Bi1−x FeO3) compositions comprise a system of important materials due to their combination of multiferroic properties. Several dielectric and charge transport reports in literature can be found in this system. However, as a typical polycrystalline electroceramic, their grains and grain boundaries (GBs) are expected to possess very different properties. To this date, these distinctions have not been determined for this system. In this work, through measurements via impedance spectroscopy on Sm x Bi1−x FeO3 thin films, we show that using a brick layer model allows the separation of the electrical properties of grains and GBs. Results indicate that grains have dielectric permittivity and electrical conductivity much higher than GBs. Their properties mostly control the characteristics observed in the studied thin films. The introduction of samarium reduces the electrical conductivity and increases the activation energies for charge transport in grains and GBs. In turn, dielectric permittivity is reduced in grains and subtly increased in GBs.

Broad Electronic Modulation of Two-Dimensional Metal-Organic Frameworks over Four Distinct Redox States.

Two-dimensional (2D) inorganic materials have emerged as exciting platforms for (opto)electronic, thermoelectric, magnetic, and energy storage applications. However, electronic redox tuning of these materials can be difficult. Instead, 2D metal-organic frameworks (MOFs) offer the possibility of electronic tuning through stoichiometric redox changes, with several examples featuring one to two redox events per formula unit. Here, we demonstrate that this principle can be extended over a far greater span with the isolation of four discrete redox states in the 2D MOFs LixFe3(THT)2 (x = 0-3, THT = triphenylenehexathiol). This redox modulation results in 10,000-fold greater conductivity, p- to n-type carrier switching, and modulation of antiferromagnetic coupling. Physical characterization suggests that changes in carrier density drive these trends with relatively constant charge transport activation energies and mobilities. This series illustrates that 2D MOFs are uniquely redox flexible, making them an ideal materials platform for tunable and switchable applications.