Gregory A. Horrocks

Finite size effects on the structural progression induced by lithiation of V2O5: a combined diffraction and Raman spectroscopy study

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the e-phase at 30 min. The e-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the e-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the e-phase nucleated within just 30 s and the Li-rich e′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.

Scalable Hydrothermal Synthesis of Free-Standing VO2 Nanowires in the M1 Phase

VO2 nanostructures derived from solution-phase methods are often plagued by broadened and relatively diminished metal-insulator transitions and adventitious doping due to imperfect control of stoichiometry. Here, we demonstrate a stepwise scalable hydrothermal and annealing route for obtaining VO2 nanowires exhibiting almost 4 orders of magnitude abrupt (within 1 °C) metal-insulator transitions. The prepared nanowires have been characterized across their structural and electronic phase transitions using single-nanowire Raman microprobe analysis, ensemble differential scanning calorimetry, and single-nanowire electrical transport measurements. The electrical band gap is determined to be 600 meV and is consistent with the optical band gap of VO2, and the narrowness of differential scanning calorimetry profiles indicates homogeneity of stoichiometry. The preparation of high-quality free-standing nanowires exhibiting pronounced metal-insulator transitions by a solution-phase process allows for scalability, further solution-phase processing, incorporation within nanocomposites, and integration onto arbitrary substrates.

Proliferation of metallic domains caused by inhomogeneous heating near the electrically driven transition inVO2nanobeams

We discuss the mechanisms behind the electrically driven insulator-metal transition in single crystalline VO

Lithiation across interconnected V2O5 nanoparticle networks

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Modeling of phase separation across interconnected electrode particles in lithium-ion batteries

nanobeams. Our DC and AC transport measurements and the versatile harmonic analysis method employed show that non-uniform Joule heating causes phase inhomogeneities to develop within the nanobeam and is responsible for driving the transition in VO

X-ray excited photoluminescence near the giant resonance in solid-solution Gd(1-x)Tb(x)OCl nanocrystals and their retention upon solvothermal topotactic transformation to Gd(1-x)Tb(x)F3.

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Transformers: the changing phases of low-dimensional vanadium oxide bronzes

. A Poole-Frenkel like purely electric field induced transition is found to be absent and the role of percolation near and away from the electrically driven transition in VO

Emptying and filling a tunnel bronze

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Vanadium K-Edge X-ray Absorption Spectroscopy as a Probe of the Heterogeneous Lithiation of V2O5: First-Principles Modeling and Principal Component Analysis

is also identified. The results and the harmonic analysis can be generalized to many strongly correlated materials that exhibit electrically driven transitions.