Crystallographic Shear Structures Research Articles

Chemical and Structural Factors Affecting the Stability of Wadsley-Roth Block Phases.

Wadsley-Roth phases have emerged as highly promising anode materials for Li-ion batteries and are an important class of phases that can form as part of the oxide scales of refractory multiprinciple element alloys. An algorithmic approach is described to systematically enumerate two classes of Wadsley-Roth crystallographic shear structures. An analysis of algorithmically generated Wadsley-Roth phases reveals that a diverse set of oxide crystal structures belongs to the Wadsley-Roth family of phases. First-principles calculations enable the identification of crystallographic and chemical factors that affect Wadsley-Roth phase stability, pointing in particular to the importance of the number and nature of the edges shared by neighboring metal-oxygen octahedra. A systematic study of Wadsley-Roth phases in the Ti-Nb-O ternary system shows that the cations with the highest oxidation states segregate to octahedral sites that minimize the number of shared edges, while cations with the lowest oxidation state accumulate to edge-sharing octahedra at shear boundaries.

Crystal structures of five compounds in the aluminium-ruthenium-silicon system.

Single crystals of five compounds with approximate compositions ∼Ru16(Al0.78Si0.22)47, (I), ∼Ru9(Al0.70Si0.30)32, (II), ∼Ru10(Al0.67Si0.33)41, (III), ∼Ru(Al0.57Si0.43)5, (IV), and ∼Ru2(Al0.46Si0.54)9, (V), were obtained from polycrystalline lumps mainly composed of the target compounds, and their crystal structures were determined by means of single-crystal X-ray diffraction. The crystal structure of (I) can be related to that of a cubic rational crystalline approximant to an icosa-hedral quasicrystal through crystallographic shear and then unit-cell twinning. The crystal structure of (II) is isotypic with that of a phase with composition ∼Fe9(Al,Si)32. The crystal structure of (III) is comprised of edge-sharing Ru(Al,Si)9-11 polyhedra with disordered chains along edges of polyhedra. The crystal structure of (IV) is of the LiIrSn4 type. The crystal structure of (V) can be viewed as a crystallographic shear structure derived from that of (IV).

Regulating the local coordination model of homologous and heterogeneous niobium tungsten oxides toward ultrafast lithium storage

Intercalation-type niobium tungsten oxide anodes show potential for use in high-power-density lithium-ion batteries (LIBs). Lithium activation barriers are vital factors determining the rate performance of niobium tungsten oxides. Lithium activation barriers located in the crystallographic shear plane are the highest in the entire block structure, as the rate-limiting factor for Li+ diffusion in the Wadsley–Roth crystallographic shear structure of niobium tungsten oxides. Nb(W)O6 octahedrons are distorted by introducing Nb18W8O69 (Nb18) into Nb16W5O55 (Nb16) to form heterogeneous composite structures. This distortion induces metal–oxygen bond lengthening in Nb16/Nb18, increasing the interspace between the edge-sharing octahedrons and thereby reducing the highest lithium activation barriers in the Nb16/Nb18 crystallographic shear plane and mitigating volume expansion. Nb16/Nb18 has an extraordinary rate capacity of 85 mA h g−1 at 100 C and structural stability with 86.3% capacity retention of its highest value at 100 C after 1000 cycles, with a high volumetric energy density of 647 W h L−1 at 134930 W L−1. Nb16/Nb18//LiCoO2 have a high energy density of 158 W h kg−1 at 12480 W kg−1. Thus, a new approach is proposed to improve the rate performance of niobium-based oxides with Wadsley–Roth crystallographic shear structure by employing homologous and heterogeneous composites.

Modulated crystallographic shear structure in titanium–chromium oxides: their structure and phonon transport properties

Modulated crystallographic shear structure in titanium–chromium oxides: their structure and phonon transport properties

Defect-Mediated Growth of Crystallographic Shear Plane.

As representative extended planar defects, crystallographic shear (CS) planes, namely Wadsley defects, play an important role in modifying the physical and chemical properties of metal oxides. Although these special structures have been intensively investigated for high-rate anode materials and catalysts, it is still experimentally unclear how the CS planes form and propagate at the atomic scale. Here, the CS plane evolution in monoclinic WO3 is directly imaged via in situ scanning transmission electron microscope. It is found that the CS planes nucleate preferentially at the edge step defects and proceed by the cooperative migration of WO6 octahedrons along particular crystallographic orientations, passing through a series of intermediate states. The local reconstruction of atomic columns tends to form (102) CS planes featured with four edge-sharing octahedrons in preference to the (103) planes, which matches well with the theoretical calculations. Associated with the structure evolution, the sample undergoes a semiconductor-to-metal transition. In addition, the controlled growth of CS planes and V-shaped CS structures can be achieved by artificial defects for the first time. These findings enable an atomic-scale understanding of CS structure evolution dynamics.

Mechanistic aspects of the reduction of rutile titanium dioxide and its Re-oxidation. Development and destruction of crystallographic shear structures

A model is presented giving the mean dimensions of acicular octadecahedral microcrystallites of a rutile titanium dioxide powder. Reduction at 823 K, in conjunction with ESR, electrical conductivity and controlled re-oxidation has enabled the model to be applied to reduced microcrystallites. At 300 K they contain <0.1% of paramagnetic [Ti3+↑ VO:↑Ti3+] reduced edge sites and >99.9% of reduced spin-paired [Ti3+↑↓ Ti3+ VO:] sites. These sites are situated on the external crystal faces and on polygonal bulk crystallographic shear (CS) structures inclined to the microcrystal four-fold symmetry axis. CS structures are quantum-sized [Ti4O7VO:] environments which broaden the paramagnetic signals at 78 K. Temperature programmed reduction in H2(g) reveals atomic hydrogen as a precursor to CS structure formation via a lattice template formed on microcrystallite faces. Shear structures are oxidised on their polygonal perimeters at differing rates on the respective microcrystallite faces by anionic vacancy transfer from sub-surface regions.

Controlled synthesis of 3D urchinlike niobium tungsten oxide nanostructure for fast hydrogen ion storage

The shear crystal structure through metal doping can effectively promote the transport speed of ions and electrons in metal oxides, which has important dynamic significance for the design of high-performance energy storage materials. Herein, a 3D urchinlike niobium tungsten oxide (NWO) nanostructure as an efficient hydrogen ion storage material is reported for the first time, which exhibits a capacity of 88mAh g−1 at 20 °C (1 °C = 100 mA g−1). The large specific capacity of the 3D urchinlike NWO nanostructure is ascribed to the reversible reaction of a great quantity of W6+, W5+ and W4+ in the process of protonation and deprotonation processes. In addition, hydrogen ions can still be stored in large and stable quantities, even at rates as high as 100 °C (75 mAh g−1 at 100 °C). The improvement of hydrogen ion storage properties is arising from an optimized morphology of niobium tungsten oxide via tuning of the crystal structure. The high specific superficial area 3D urchinlike shape with rich one-dimensional nanostructures significantly shortens charge-carrier transport distances, ensuring rapid interfacial electronics movement to polish up ion storage kinetics. Consequently, this crystallographic shear structure strategy to boost hydrogen ion storage capacity may be universal and is likely to pave the way toward highly capacity hydrogen ion energy storage systems.

Novel Nonstoichiometric Niobium Oxide Anode Material with Rich Oxygen Vacancies for Advanced Lithium-Ion Capacitors

Given the inherent features of open tunnel-like structures, moderate lithiation potential (1.0-3.0 V vs Li/Li+), and reversible redox couples (Nb5+/Nb4+ and Nb4+/Nb3+ redox couples), niobium-based oxides with Wadsley-Roth crystallographic shear structure are promising anode materials. However, their practical rate capability and cycling stability are still hindered by low intrinsic electronic conductivity and structural stability. Herein, ultrathin carbon-confined Nb12O29 materials with rich oxygen vacancies (Nb12O29-x@C) were designed and synthesized to address above-mentioned challenges. Computational simulations combined with experiments reveal that the oxygen vacancies can regulate the electronic structure to increase intrinsic electronic conductivity and reduce the Li+ diffusion barrier. Meanwhile, the carbon coating can enhance structural stability and further improve the electronic conductivity of the Nb12O29 material. As a result, the as-prepared Nb12O29-x@C exhibits high reversible capacity (226 mAh g-1 at 0.1 A g-1), excellent high-rate performance (83 mAh g-1 at 5.0 A g-1), and durable cycling life (98.1% capacity retention at 1.0 A g-1 after 3000 cycles). The lithium storage mechanism and structural stability of Nb12O29-x@C were also revealed by in situ X-ray diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS), and ex situ Raman spectroscopy. When applied as the anode of lithium-ion capacitors (LICs), the as-built LIC achieves high energy density (72.4 Wh kg-1) within the voltage window of 0.01-3.5 V, demonstrating the practical application potential of the Nb12O29-x@C materials.

High Power Factor Nb-Doped TiO2 Thermoelectric Thick Films: Toward Atomic Scale Defect Engineering of Crystallographic Shear Structures.

Donor-doped TiO2-based materials are promising thermoelectrics (TEs) due to their low cost and high stability at elevated temperatures. Herein, high-performance Nb-doped TiO2 thick films are fabricated by facile and scalable screen-printing techniques. Enhanced TE performance has been achieved by forming high-density crystallographic shear (CS) structures. All films exhibit the same matrix rutile structure but contain different nano-sized defect structures. Typically, in films with low Nb content, high concentrations of oxygen-deficient {121} CS planes are formed, while in films with high Nb content, a high density of twin boundaries are found. Through the use of strongly reducing atmospheres, a novel Al-segregated {210} CS structure is formed in films with higher Nb content. By advanced aberration-corrected scanning transmission electron microscopy techniques, we reveal the nature of the {210} CS structure at the nano-scale. These CS structures contain abundant oxygen vacancies and are believed to enable energy-filtering effects, leading to simultaneous enhancement of both the electrical conductivity and Seebeck coefficients. The optimized films exhibit a maximum power factor of 4.3 × 10-4 W m-1 K-2 at 673 K, the highest value for TiO2-based TE films at elevated temperatures. Our modulation strategy based on microstructure modification provides a novel route for atomic-level defect engineering which should guide the development of other TE materials.

Recent developments in Nb‐based oxides with crystallographic shear structures as anode materials for high‐rate lithium‐ion energy storage

AbstractHigh‐power lithium‐ion batteries (LIBs) are required for a variety of technological applications, especially in the field of electric vehicles (EVs). Oxides based on niobium, titanium, and tungsten, and having crystallographic shear structures, are considered promising materials for high‐rate anodes of LIBs. The unique structures with open channels, multielectron redox processes, and a moderate potential window with a resulting solid electrolyte interface‐free interface provide them with rapid Li‐ion diffusion pathways, fairly high capacities, and high safety. In this review, the recent advancements in diverse crystallographic shear structure Nb‐based oxide anodes for fast Li‐ion energy storage are comprehensively presented, with a specific focus on the relationships between the crystal structures and electronic properties, lithiation mechanisms, kinetic properties, and electrochemical performance. The challenges in the design, optimization, and practical application of oxides with crystallographic shear structures are also discussed, together with strategies to overcome these challenges and prospects for the future.

Effect of Mechanical Activation and Carbon Coating on Electrochemistry of TiNb2O7 Anodes for Lithium-Ion Batteries

TiNb2O7 anode material with a Wadsley–Roth crystallographic shear structure was prepared by solid-state synthesis at a relatively low temperature (1000 °C) and a short calcination time (4 h) using preliminary mechanical activation of the reagent mixture. The as-prepared final product was then ball milled in a planetary mill with and without carbon black. The crystal structure and morphology of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical performance was studied in a galvanostatic mode in varied voltage intervals and at different cycling rates in combination with in situ electrochemical impedance spectroscopy (EIS) measurements. The resistance measured using in situ EIS had the highest values at the end of the discharge and the lowest when charging. The lithium diffusion coefficient, determined by galvanostatic intermittent titration technique (GITT), in samples milled with and without carbon black was an order of magnitude higher than that for the pristine sample. It was shown that improved electrochemical performance of the carbon composite TiNb2O7/C (reversible capacity of 250 mAh g−1 at C/10 with Coulomb efficiency of ~99%) was associated with improved conductivity due to the formation of a conductive carbon matrix and uniform distribution of submicron particles by size.

New Class of Titanium Niobium Oxide for a Li-Ion Host: TiNbO4 with Purely Single-Phase Lithium Intercalation

Entropy-stabilized titanium niobium oxides (TNOs) with crystallographic shear structures (e.g., TiNb2O7 and Ti2Nb10O29) are generally synthesized by high-temperature calcination in an air or an oxygen atmosphere to compensate for their positive enthalpies of formation. In this work, we demonstrate that changing the reaction atmosphere into a slightly reductive environment using in situ carbonization leads to the creation of a new class of TNO with a formula of TiNbO4. Unlike its predecessors, this new lithium reservoir is a rutile phase, and most strikingly, in situ X-ray diffraction analysis revealed that its lithium intercalation occurs via a purely solid-solution process. Since solid-electrolyte-interface-free, high capacity anode materials with long cyclic life are required to meet the stringent requirements of widespread lithium-ion battery utilization, this finding of a new electrode material with purely single-phase lithium intercalation is of great interest for the development of high-performance anode materials. Distinctive electrochemical behavior that is different from that of crystallographic shear structured TNO is revealed by in-depth electrochemical analyses, which is ascribed to the unique structural and electronic properties of TiNbO4. We believe this work opens a new avenue for the development of feasible oxide-based alternatives to graphite, which can be safer and suitable for high-power performance.

Optimization of hydrogen-ion storage performance of tungsten trioxide nanowires by niobium doping

The transport and storage of ions within solid state structures is a fundamental limitation for fabricate more advanced electrochemical energy storage, memristor, and electrochromic devices. Crystallographic shear structure can be induced in the tungsten bronze structures composed of corner-sharing WO6 octahedra by the addition of edge-sharing NbO6 octahedra, which might provide more storage sites and more convenient transport channels for external ions such as hydrogen ions and alkali metal ions. Here, we show that Nb2O5·15WO3 nanowires (Nb/W = 0.008) with long length-diameter ratio, smooth surface, and uniform diameter have been successfully synthesized by a simple hydrothermal method. The Nb2O5·15WO3 nanowires do exhibit more advantages over h-WO3 nanowires in electrochemical hydrogen ion storage such as smaller polarization, larger capacity (71 mAh g−1, at 10C, 1C = 100 mA g−1), better cycle performance (remain at 99% of the initial capacity after 200 cycles at 100C) and faster H+ ions diffusion kinetics. It might be the crystallographic shear structure induced by Nb doping that does result in the marked improvement in the hydrogen-ion storage performance of WO3. Therefore, complex niobium tungsten oxide nanowires might offer great promise for the next generation of electrochemical energy and information storage devices.

Crossover from incoherent to coherent thermal conduction in bulk titanium oxide natural superlattices

We have investigated thermal conduction in bulk titanium oxide natural superlattices with crystallographic shear (CS) structures, in which dense planar faults are introduced with different periodicities, prepared by reductive annealing of rutile TiO2 and crystal growth by the floating zone method. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) revealed that (132)rutile and (121)rutile CS planes with interspacings of 2.7 and 1.0 nm were introduced in the mother rutile structure. Time-domain thermoreflectance (TDTR) revealed that the thermal conductivity decreased by the introduction of CS planes, but that the decrease is not monotonic with increasing density of CS planes. Calculation of the thermal conductivity and the mean free path for phonons revealed that a crossover from incoherent to coherent thermal conduction took place, and coherent interfaces with nanoscale periodicity were formed as thermodynamically stable phases in bulk titanium oxide natural superlattices.

Three-Dimensional Cross-Linked Nb2O5 Polymorphs Derived from Cellulose Substances: Insights into the Mechanisms of Lithium Storage.

Niobium pentoxide (Nb2O5)-based materials have been regarded as promising anodic materials for lithium-ion batteries due to their abundant crystalline phases and stable and safe lithium storage performances. However, there is a lack of systematic studies of the relationship among the crystal structures, electrochemical characteristics, and lithium storage mechanisms for the various Nb2O5 polymorphs. Herein, pure pseudohexagonal Nb2O5 (TT-Nb2O5), orthorhombic Nb2O5 (T-Nb2O5), tetragonal Nb2O5 (M-Nb2O5), and monoclinic Nb2O5 (H-Nb2O5) with three-dimensional interconnected structures are reported, which were synthesized via a hydrothermal method using the commercial filter paper as the structural template followed by specific annealing processes. Impressively, the TT- and T-Nb2O5 species both possess bronze-like phases with "room and pillar" structures, while M- and H-Nb2O5 ones are both in the Wadsley-Roth phases with crystallographic shear structures. Among the pristine Nb2O5 materials, H-Nb2O5 exhibits the highest initial specific capacity (270 mA h g-1), while T-Nb2O5 performs with the lowest (197 mA h g-1) at 0.02 A g-1, meaning that crystallographic shear structures provide more lithium storage sites. TT-Nb2O5 realizes the best rate capability (207 mA h g-1 at 0.02 A g-1 and 103 mA h g-1 at 4.0 A g-1), indicating that the "room and pillar" crystal structures favor fast lithium storage. Electrochemical analyses reveal that the TT- and T-Nb2O5 phases with "room and pillar" crystal structures utilize a pseudocapacitive intercalation mechanism, while the M- and H-Nb2O5 phases with the Wadsley-Roth shear structures follow a typical battery-type intercalation mechanism. A fresh insight into the further understanding of the intercalation pseudocapacitance on the basis of the unit cells of the electrode materials and a meaningful guidance for crystalline structural design/construction of the electrode materials for the next-generation LIBs are thus provided.

Cation mixing in Wadsley-Roth phase anode of lithium-ion battery improves cycling stability and fast Li+ storage

Developing advanced electrode materials with high stability and high ion-diffusion rate is vital for the success of high-rate lithium-ion batteries (LIBs). However, the commonly used modification strategies such as carbon coating, nanoarchitecture engineering, and introducing oxygen vacancies are unavoidably meeting with the problems of high cost and complicated preparation process. Herein, we report cation-mixing effect enhanced fast Li+ storage in Wadsley-Roth phase Fe-Ti-Nb oxide (FTNO) materials by a facile solution combustion method. Co-existence of Fe3+ and Ti4+ in the crystallographic shear structure leads to enhanced cation-mixing effect with cations short-range order (SRO) in FTNO materials, thus resulting in outstanding capabilities of fast Li+ storage/diffusion, robust structure and low charge transfer resistance compared with the analogues of FeNb11O29 and Ti2Nb10O29. Consequently, a high-capacity retention of 71.8% is achieved upon 10 000 cycles at 10 C. Most importantly, the feasibilities of FTNO are also systematically verified in various practical electrochemical energy storage devices containing conventional lithium-ion full battery (FTNOǁLiFePO4), high-power lithium-ion hybrid capacitor [FTNOǁactive carbon (AC)], and novel dual-ion battery [FTNOǁmesocarbon microbeads (MCMB)]. It is worth noting that the FTNOǁMCMB with high output voltage of 3 V delivers a capacity of 105.7 mAh g−1, implying a great potential of FTNO applied in dual-ion batteries.

Shear Pleasure: The Structure, Formation, and Thermodynamics of Crystallographic Shear Phases

A renaissance of interest in crystallographic shear structures and our recent work in this remarkable class of materials inspired this review. We first summarize the geometrical aspects of shear plane formation and possible transformations in ReO3, rutile, and perovskite-based structures. Then we provide a mechanistic overview of crystallographic shear formation, plane ordering, and propagation. Next we describe the energetics of planar defect formation and interaction, equilibria between point and extended defect structures, and thermodynamic stability of shear compounds. Finally, we emphasize the remaining challenges and propose future directions in this exciting area.

Wadsley-Roth Crystallographic Shear Structure Niobium-Based Oxides: Promising Anode Materials for High-Safety Lithium-Ion Batteries.

Wadsley–Roth crystallographic shear structure niobium‐based oxides are of great interest in fast Li+ storage due to their unique 3D open tunnel structures that offer facile Li+ diffusion paths. Their moderate lithiation potential and reversible redox couples hold the great promise in the development of next‐generation lithium‐ion batteries (LIBs) that are characterized by high power density, long lifespan, and high safety. Despite these outstanding merits, there is still extensive advancement space for further enhancing their electrochemical kinetics. And the industrial feasibility of Wadsley–Roth crystallographic shear structure niobium‐based oxides as anode materials for LIBs requires more systematic research. In this review, recent progress in this field is summarized with the aim of realizing the practical applications of Wadsley–Roth phase anode materials in commercial LIBs. The review focuses on research toward the crystalline structure analyses, electrochemical reaction mechanisms, modification strategies, and full cell performance. In addition to highlighting the current research advances, the outlook and perspective on Wadsley–Roth anode materials is also concisely provided.

Lithium Diffusion in Niobium Tungsten Oxide Shear Structures.

Niobium tungsten oxides with crystallographic shear structures form a promising class of high-rate Li-ion anode materials. Lithium diffusion within these materials is studied in this work using density functional theory calculations, specifically nudged elastic band calculations and ab initio molecular dynamics simulations. Lithium diffusion is found to occur through jumps between 4-fold coordinated window sites with low activation barriers (80–300 meV) and is constrained to be effectively one-dimensional by the crystallographic shear planes of the structures. We identify a number of other processes, including rattling motions with barriers on the order of the thermal energy at room temperature, and intermediate barrier hops between 4-fold and 5-fold coordinated lithium sites. We demonstrate differences regarding diffusion pathways between different cavity types; within the ReO3-like block units of the structures, cavities at the corners and edges host more isolated diffusion tunnels than those in the interior. Diffusion coefficients are found to be in the range of 10–12 to 10–11 m2 s–1 for lithium concentrations of 0.5 Li/TM. Overall, the results provide a complete picture of the diffusion mechanism in niobium tungsten oxide shear structures, and the structure–property relationships identified in this work can be generalized to the entire family of crystallographic shear phases.

Superionic Lithium Intercalation through 2 × 2 nm2 Columns in the Crystallographic Shear Phase Nb18W8O69

Nb18W8O69 (9Nb2O5·8WO3) is the tungsten-rich end-member of the Wadsley–Roth crystallographic shear (cs) structures within the Nb2O5–WO3 series. It has the largest block size of any known, stable Wa...