Effect of pre-intercalation on Li-ion diffusion mapped by topochemical single-crystal transformation and operando investigation.

Limitations in electrochemical performance as well as supply chain challenges have rendered positive electrode materials a critical bottleneck for Li-ion batteries. State-of-the-art Li-ion batteries fall short of accessing theoretical capacities. As such, there is intense interest in the design of strategies that enable the more effective utilization of active intercalation materials. Pre-intercalation with alkali-metal ions has attracted interest as a means of accessing higher reversible capacity and improved rate performance. However, the structural basis for improvements in electrochemical performance remains mostly unexplored. Here we use topochemical single-crystal-to-single-crystal transformations in a tunnel-structured ζ-V2O5 positive electrode to illustrate the effect of pre-intercalation in modifying the host lattice and altering diffusion pathways. Furthermore, operando synchrotron X-ray diffraction is used to map Li-ion site preferences and occupancies as a function of the depth of discharge in pre-intercalated materials. Na- and K-ion intercalation 'props open' the one-dimensional tunnel, reduces electrostatic repulsions between inserted Li ions and entirely modifies diffusion pathways, enabling orders of magnitude higher Li-ion diffusivities and accessing higher capacities. Deciphering the atomistic origins of improved performance in pre-intercalated materials on the basis of single-crystal-to-single-crystal topochemical transformation and operando diffraction studies paves the way to site-selective modification approaches for positive electrode design.

Cation reordering instead of phase transitions: Origins and implications of contrasting lithiation mechanisms in 1D ζ- and 2D α-V2O5.

Substantial improvements in cycle life, rate performance, accessible voltage, and reversible capacity are required to realize the promise of Li-ion batteries in full measure. Here, we have examined insertion electrodes of the same composition (V2O5) prepared according to the same electrode specifications and comprising particles with similar dimensions and geometries that differ only in terms of their atomic connectivity and crystal structure, specifically two-dimensional (2D) layered α-V2O5 that crystallizes in an orthorhombic space group and one-dimensional (1D) tunnel-structured ζ-V2O5 crystallized in a monoclinic space group. By using particles of similar dimensions, we have disentangled the role of specific structural motifs and atomistic diffusion pathways in affecting electrochemical performance by mapping the dynamical evolution of lithiation-induced structural modifications using ex situ scanning transmission X-ray microscopy, operando synchrotron X-ray diffraction measurements, and phase-field modeling. We find the operation of sharply divergent mechanisms to accommodate increasing concentrations of Li-ions: a series of distortive phase transformations that result in puckering and expansion of interlayer spacing in layered α-V2O5, as compared with cation reordering along interstitial sites in tunnel-structured ζ-V2O5 By alleviating distortive phase transformations, the ζ-V2O5 cathode shows reduced voltage hysteresis, increased Li-ion diffusivity, alleviation of stress gradients, and improved capacity retention. The findings demonstrate that alternative lithiation mechanisms can be accessed in metastable compounds by dint of their reconfigured atomic connectivity and can unlock substantially improved electrochemical performance not accessible in the thermodynamically stable phase.

Elucidating the Crystallite Size Dependence of the Thermochromic Properties of Nanocomposite VO2 Thin Films.

Fenestration elements that enable spectrally selective dynamic modulation of the near-infrared region of the electromagnetic spectrum are of great interest as a means of decreasing the energy consumption of buildings by adjusting solar heat gain in response to external temperature. The binary vanadium oxide VO2 exhibits a near-room-temperature insulator-metal electronic transition accompanied by a dramatic modulation of the near-infrared transmittance. The low-temperature insulating phase is infrared transparent but blocks infrared transmission upon metallization. There is considerable interest in harnessing the thermochromic modulation afforded by VO2 in nanocomposite thin films. However, to prepare a viable thermochromic film, the visible-light transmittance must be maintained as high as possible while maximizing thermochromic modulation in the near-infrared region of the electromagnetic spectrum, which necessitates the development of high-crystalline-quality VO2 nanocrystals of the optimal particle size embedded within the appropriate host matrix and refractive index matched to the host medium. Here, we demonstrate the preparation of acrylate-based nanocomposite thin films with varying sizes of embedded VO2 nanoparticles. The observed strong size dependence of visible-light transmittance and near-infrared modulation is explicable on the basis of optical simulations. In this article, we elucidate multiple scattering and absorption mechanisms, including Mie scattering, temperature-/phase-variant refractive-index mismatch between VO2 nanocrystals and the encapsulating matrix, and the appearance of a surface plasmon resonance using temperature-variant absorptance and diffuse transmittance spectroscopy measurements performed as a function of particle loading for the different sizes of VO2 nanocrystals. Nanocrystals with dimensions of 44 ± 30 nm show up to >32% near-infrared energy modulation across the near-infrared region of the electromagnetic spectrum while maintaining high visible-light transmission. The results presented here, providing mechanistic elucidation of the size dependence of the different scattering mechanisms, underscore the importance of nanocrystallite dimensions, refractive-index matching, and individualized dispersion of particles within the host matrix for the preparation of viable thermochromic thin films mitigating Mie scattering and differential refractive-index scattering.

Two-dimensional graphene as a matrix for MALDI imaging mass spectrometry.

Here, a matrix using two-dimensional (2D) graphene is demonstrated for the first time in the context of MALDI IMS using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Although graphene flakes have been used previously in MALDI, it is described here how a single 2D layer of graphene is applied directly on top of rat brain sections and soybean leaves. Several classes of molecules are desorbed and ionized off of the surface of the tissues examined using 2D graphene, with minimal background interference from the matrix. Moreover, no solvents are employed in application of 2D graphene, eliminating the potential for analyte diffusion in liquid droplets during matrix application. Because 2D graphene is an elemental form of carbon, an additional advantage is its high compatibility with the long duration needed for many IMS experiments. Graphical Abstract ᅟ.

An electronic structure perspective of graphene interfaces.

The unusual electronic structure of graphene characterized by linear energy dispersion of bands adjacent to the Fermi level underpins its remarkable transport properties. However, for practical device integration, graphene will need to be interfaced with other materials: 2D layered structures, metals (as ad-atoms, nanoparticles, extended surfaces, and patterned metamaterial geometries), dielectrics, organics, or hybrid structures that in turn are constituted from various inorganic or organic components. The structural complexity at these nanoscale interfaces holds much promise for manifestation of novel emergent phenomena and provides a means to modulate the electronic structure of graphene. In this feature article, we review the modifications to the electronic structure of graphene induced upon interfacing with disparate types of materials with an emphasis on iterative learnings from theoretical calculations and electronic spectroscopy (X-ray absorption fine structure (XAFS) spectroscopy, scanning transmission X-ray microscopy (STXM), angle-resolved photoemission spectroscopy (ARPES), and X-ray magnetic circular dichroism (XMCD)). We discuss approaches for engineering and modulating a bandgap in graphene through interfacial hybridization, outline experimental methods for examining electronic structure at interfaces, and overview device implications of engineered interfaces. A unified view of how geometric and electronic structure are correlated at interfaces will provide a rational means for designing heterostructures exhibiting emergent physical phenomena with implications for plasmonics, photonics, spintronics, and engineered polymer and metal matrix composites.

Interactions of aqueous Ag+ with fulvic acids: mechanisms of silver nanoparticle formation and investigation of stability.

This study investigated the possible natural formation of silver nanoparticles (AgNPs) in Ag(+)-fulvic acid (FA) solutions under various environmentally relevant conditions (temperature, pH, and UV light). Increase in temperature (24-90 °C) and pH (6.1-9.0) of Ag(+)-Suwannee River fulvic acid (SRFA) solutions accelerated the appearance of the characteristic surface plasmon resonance (SPR) of AgNPs. The rate of AgNP formation via reduction of Ag(+) in the presence of different FAs (SRFA, Pahokee Peat fulvic acid, PPFA, Nordic lake fulvic acid, NLFA) and Suwannee River humic acid (SRHA) followed the order NLFA > SRHA > PPFA > SRFA. This order was found to be related to the free radical content of the acids, which was consistent with the proposed mechanism. The same order of AgNP growth was seen upon UV light illumination of Ag(+)-FA and Ag(+)-HA mixtures in moderately hard reconstituted water (MHRW). Stability studies of AgNPs, formed from the interactions of Ag(+)-SRFA, over a period of several months showed that these AgNPs were highly stable with SPR peak reductions of only ~15%. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements revealed bimodal particle size distributions of aged AgNPs. The stable AgNPs formed through the reduction of Ag(+) by fulvic and humic acid fractions of natural organic matter in the environment may be transported over significant distances and might also influence the overall bioavailability and ecotoxicity of AgNPs.

Imaging local electronic corrugations and doped regions in graphene.

Electronic structure heterogeneities are ubiquitous in two-dimensional graphene and profoundly impact the transport properties of this material. Here we show the mapping of discrete electronic domains within a single graphene sheet using scanning transmission X-ray microscopy in conjunction with ab initio density functional theory calculations. Scanning transmission X-ray microscopy imaging provides a wealth of detail regarding the extent to which the unoccupied levels of graphene are modified by corrugation, doping and adventitious impurities, as a result of synthesis and processing. Local electronic corrugations, visualized as distortions of the π*cloud, have been imaged alongside inhomogeneously doped regions characterized by distinctive spectral signatures of altered unoccupied density of states. The combination of density functional theory calculations, scanning transmission X-ray microscopy imaging, and in situ near-edge X-ray absorption fine structure spectroscopy experiments also provide resolution of a longstanding debate in the literature regarding the spectral assignments of pre-edge and interlayer states.

Eccentric strengthening for chronic lateral epicondylosis: a prospective randomized study.

BACKGROUND: Effective treatments for chronic lateral epicondylosis have not been studied adequately. Eccentric overload exercises have been used with success for other chronic tendinopathy, mainly Achilles and patellar. HYPOTHESIS/PURPOSE: To compare a wrist extensor eccentric strengthening exercise program with a wrist extensor stretching/modality program for the treatment of chronic lateral epicondylosis. The authors hypothesized that the eccentric strengthening program would produce more favorable results than a stretching/modality program. STUDY DESIGN: Prospective randomized controlled trial. METHODS: Twenty-eight adults with lateral epicondylosis of greater than 4 weeks' duration were randomized to an eccentric strengthening group or a stretching group. Exercises were taught by a physical therapist, and participants performed most of the exercises on their own at home. Pain scores with visual analog scale from 0 to 100 were obtained at baseline and then at 4, 8, 12, 16, and 20 weeks after the start of the exercise program. RESULTS: Both groups improved their pain scores from baseline to the 4-week time point, followed by nonsignificant further decreases in pain scores thereafter. No statistically significant differences were found between the eccentric strengthening group and stretching groups at any follow-up time point. CONCLUSIONS: Despite previous reports documenting favorable results with eccentric exercises for other tendinopathy, the authors were unable to show any statistical advantage to eccentric exercises for lateral epicondylosis during these periods compared with local modalities and stretching exercises.