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1. Epitaxial stabilization versus interdiffusion: synthetic routes to metastable cubic HfO ₂ and HfV ₂ O ₇ from the core–shell arrangement of precursors

   https://doi.org/10.1039/c9nr07316g  

   Fleer, Nathan A.; Thomas, Melonie P.; Andrews, Justin L.; Waetzig, Gregory R.; Gonzalez, Oscar; Liu, Guan-Wen; Guiton, Beth S.; Banerjee, Sarbajit (November 2019, Nanoscale)

   Metastable materials that represent excursions from thermodynamic minima are characterized by distinctive structural motifs and electronic structure, which frequently underpins new function. The binary oxides of hafnium present a rich diversity of crystal structures and are of considerable technological importance given their high dielectric constants, refractory characteristics, radiation hardness, and anion conductivity; however, high-symmetry tetragonal and cubic polymorphs of HfO 2 are accessible only at substantially elevated temperatures (1720 and 2600 °C, respectively). Here, we demonstrate that the core–shell arrangement of VO 2 and amorphous HfO 2 promotes outwards oxygen diffusion along an electropositivity gradient and yields an epitaxially matched V 2 O 3 /HfO 2 interface that allows for the unprecedented stabilization of the metastable cubic polymorph of HfO 2 under ambient conditions. Free-standing cubic HfO 2 , otherwise accessible only above 2600 °C, is stabilized by acid etching of the vanadium oxide core. In contrast, interdiffusion under oxidative conditions yields the negative thermal expansion material HfV 2 O 7 . Variable temperature powder X-ray diffraction demonstrate that the prepared HfV 2 O 7 exhibits pronounced negative thermal expansion in the temperature range between 150 and 700 °C. The results demonstrate the potential of using epitaxial crystallographic relationships to facilitate preferential nucleation of otherwise inaccessible metastable compounds. 

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2. Lithium-based vertically aligned nancomposite films incorporating Li _x La _0.32 (Nb _0.7 Ti _0.32 )O ₃ electrolyte with high Li ⁺ ion conductivity

   https://doi.org/10.1063/5.0086844  

   Lovett, Adam J.; Kursumovic, Ahmed; Dutton, Siân; Qi, Zhimin; He, Zihao; Wang, Haiyan; MacManus-Driscoll, Judith L. (May 2022, APL Materials)

   Vertically aligned nanocomposite (VAN) thin films have shown strong potential in oxide nanoionics but are yet to be explored in detail in solid-state battery systems. Their 3D architectures are attractive because they may allow enhancements in capacity, current, and power densities. In addition, owing to their large interfacial surface areas, the VAN could serve as models to study interfaces and solid-electrolyte interphase formation. Here, we have deposited highly crystalline and epitaxial vertically aligned nanocomposite films composed of a Li x La 0.32±0.05 (Nb 0.7±0.1 Ti 0.32±0.05 )O 3±δ -Ti 0.8±0.1 Nb 0.17±0.03 O 2±δ -anatase [herein referred to as LL(Nb, Ti)O-(Ti, Nb)O 2 ] electrolyte/anode system, the first anode VAN battery system reported. This system has an order of magnitude increased Li + ionic conductivity over that in bulk Li 3x La 1/3−x NbO 3 and is comparable with the best available Li 3x La 2/3−x TiO 3 pulsed laser deposition films. Furthermore, the ionic conducting/electrically insulating LL(Nb, Ti)O and electrically conducting (Ti, Nb)O 2 phases are a prerequisite for an interdigitated electrolyte/anode system. This work opens up the possibility of incorporating VAN films into an all solid-state battery, either as electrodes or electrolytes, by the pairing of suitable materials. 

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3. Local structure elucidation of tungsten-substituted vanadium dioxide (V$$_{1-x}$$W$$_x$$O$$_2$$)

   https://doi.org/10.1038/s41598-022-18575-0  

   Wilson, Catrina E.; Gibson, Amanda E.; Cuillier, Paul M.; Li, Cheng-Han; Crosby, Patrice H.; Trigg, Edward B.; Najmr, Stan; Murray, Christopher B.; Jinschek, Joerg R.; Doan-Nguyen, Vicky (December 2022, Scientific Reports)

   Abstract Initially, vanadium dioxide seems to be an ideal first-order phase transition case study due to its deceptively simple structure and composition, but upon closer inspection there are nuances to the driving mechanism of the metal-insulator transition (MIT) that are still unexplained. In this study, a local structure analysis across a bulk powder tungsten-substitution series is utilized to tease out the nuances of this first-order phase transition. A comparison of the average structure to the local structure using synchrotron x-ray diffraction and total scattering pair-distribution function methods, respectively, is discussed as well as comparison to bright field transmission electron microscopy imaging through a similar temperature-series as the local structure characterization. Extended x-ray absorption fine structure fitting of thin film data across the substitution-series is also presented and compared to bulk. Machine learning technique, non-negative matrix factorization, is applied to analyze the total scattering data. The bulk MIT is probed through magnetic susceptibility as well as differential scanning calorimetry. The findings indicate the local transition temperature ( $$T_c$$ T c ) is less than the average $$T_c$$ T c supporting the Peierls-Mott MIT mechanism, and demonstrate that in bulk powder and thin-films, increasing tungsten-substitution instigates local V-oxidation through the phase pathway VO $$_2\, \rightarrow$$ 2 → V $$_6$$ 6 O $$_{13} \, \rightarrow$$ 13 → V $$_2$$ 2 O $$_5$$ 5 . 

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4. Cation reordering instead of phase transitions: Origins and implications of contrasting lithiation mechanisms in 1D ζ- and 2D α-V ₂ O ₅

   https://doi.org/10.1073/pnas.2115072119  

   Luo, Yuting; Rezaei, Shahed; Santos, David A.; Zhang, Yuwei; Handy, Joseph V.; Carrillo, Luis; Schultz, Brian J.; Gobbato, Leonardo; Pupucevski, Max; Wiaderek, Kamila; et al (January 2022, Proceedings of the National Academy of Sciences)

   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 (V 2 O 5 ) 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-V 2 O 5 that crystallizes in an orthorhombic space group and one-dimensional (1D) tunnel-structured ζ-V 2 O 5 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-V 2 O 5 , as compared with cation reordering along interstitial sites in tunnel-structured ζ-V 2 O 5 . By alleviating distortive phase transformations, the ζ-V 2 O 5 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. 

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5. Site-selective halogenation of mixed-valent vanadium oxide clusters

   https://doi.org/10.1039/D0DT01077D  

   Maiola, Michela L.; Petel, Brittney E.; Brennessel, William W.; Matson, Ellen M. (January 2020, Dalton Transactions)

   Here, we expand on the synthesis and characterization of chloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) clusters, to include the halogenation of mixed-valent vanadium oxide assemblies. These findings build on our previously disclosed results describing the preparation of a mono-anionic chloride-functionalized cluster, [V 6 O 6 Cl(OC 2 H 5 ) 12 ] 1− , by chlorination of [V 6 O 7 (OC 2 H 5 ) 12 ] 2− with AlCl 3 , aimed at understanding the electronic consequences of the introduction of halide-defects in bulk metal oxides ( e.g. VO 2 ). While chlorination of the mixed-valent POV-ethoxide clusters was not possible using AlCl 3 , we have found that the chloride-substituted oxidized derivatives of the Lindqvist vanadium-oxide clusters can be formed using TiCl 3 (thf) 3 with [V 6 O 7 (OC 2 H 5 ) 12 ] n ( n = 1−, 0) or WCl 6 with [V 6 O 7 (OC 2 H 5 ) 12 ] 0 . Characterization of the chloride-containing products, [V 6 O 6 Cl(OC 2 H 5 ) 12 ] n ( n = 0, 1+), was accomplished via 1 H NMR spectroscopy, X-ray crystallography, and elemental analysis. Electronic analysis of the redox series of Cl-doped POV-alkoxide clusters via infrared and electronic absorption spectroscopies revealed all redox events are localized to the vanadyl portion of the cluster, with the site differentiated V III –Cl moiety retaining its reduced oxidation state across a 1.9 V window. These results present new synthetic routes for accessing chloride-doped POV-alkoxide clusters from mixed-valent vanadium oxide precursors. 

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