Structural Stability from Crystallographic Shear in TiO2-Nb2O5 Phases: Cation Ordering and Lithiation Behavior of TiNb24O62

Insight into the Li-Storage Property of Surface-Modified Ti2Nb10O29 Anode Material for High-Rate Application

Ti-based anode materials are considered to be an alternative to graphite anodes to accomplish high-rate application requirements. Ti2Nb10O29 (TNO15) has attracted much attention due to its high lithium storage capacity through the utilization of multiple redox couples and a suitable operating voltage window of 1.0 to 2.0 V vs Li/Li+. However, poor intrinsic electronic conductivity has limited the futuristic applicability of this material to the battery anode. In this work, we report the modification of TNO15 by introducing oxygen vacancies and using few-layered carbon and copper coatings on the surface to improve its Li+ storage property. With the support of the galvanostatic intermittent titration technique (GITT), we found that the diffusion coefficient of carbon/copper coated TNO15 is 2 orders of magnitude higher than that of the uncoated sample. Here, highly conductive copper metal on the surface of the carbon-coated oxygen-vacancy-incorporated TNO15 increases the overall electronic and ionic conductivity. The prepared TNO15-800-C-Cu-700 half-cell shows a significant rate capability of 92% when there is a 10-fold increase in the current density. In addition, the interconnected TNO15 nanoparticles create a porous microsphere structure, which enables better Li-ion transportation during charge/discharge process, and experiences an enhancement after the carbon and copper coating on the surface of the primary TNO15 nanocrystallites.

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.

Rapid and Reversible Lithium Insertion in the Wadsley-Roth-Derived Phase NaNb13O33

The development of new high-performing battery materials is critical for meeting the energy storage requirements of portable electronics and electrified transportation applications. Owing to their exceptionally high rate capabilities, high volumetric capacities, and long cycle lives, Wadsley-Roth compounds are promising anode materials for fast-charging and high-power lithium-ion batteries. Here, we present a study of the Wadsley-Roth-derived NaNb13O33 phase and examine its structure and lithium insertion behavior. Structural insights from combined neutron and synchrotron diffraction as well as solid-state nuclear magnetic resonance (NMR) are presented. Solid-state NMR, in conjunction with neutron diffraction, reveals the presence of sodium ions in perovskite A-site-like block interior sites as well as square-planar block corner sites. Through combined experimental and computational studies, the high rate performance of this anode material is demonstrated and rationalized. A gravimetric capacity of 225 mA h g-1, indicating multielectron redox of Nb, is accessible at slow cycling rates. At a high rate, 100 mA h g-1 of capacity is accessible in 3 min for micrometer-scale particles. Bond-valence mapping suggests that this high-rate performance stems from fast multichannel lithium diffusion involving octahedral block interior sites. Differential capacity analysis is used to identify optimal cycling rates for long-term performance, and an 80% capacity retention is achieved over 600 cycles with 30 min charging and discharging intervals. These initial results place NaNb13O33 within the ranks of promising new high-rate lithium-ion battery anode materials that warrant further research.

Metal-Metal Bonding as an Electrode Design Principle in the Low-Strain Cluster Compound LiScMo3O8

Electrode materials for Li⁺-ion batteries require optimization along several disparate axes related to cost, performance, and sustainability. One of the important performance axes is the ability to retain structural integrity though cycles of charge/discharge. Metal-metal bonding is a distinct feature of some refractory metal oxides that has been largely underutilized in electrochemical energy storage, but that could potentially impact structural integrity. Here LiScMo₃O₈, a compound containing triangular clusters of metal-metal bonded Mo atoms, is studied as a potential anode material in Li⁺-ion batteries. Electrons inserted though lithiation are localized across rigid Mo₃ triangles (rather than on individual metal ions), resulting in minimal structural change as suggested by operando diffraction. The unusual chemical bonding allows this compound to be cycled with Mo atoms below a formally +4 valence state, resulting in an acceptable voltage regime that is appropriate for an anode material. Several characterization methods including potentiometric entropy measurements indicate two-phase regions, which are attributed through extensive first-principles modeling to Li⁺ ordering. This study of LiScMo₃O₈ provides valuable insights for design principles for structural motifs that stably and reversibly permit Li⁺ (de)insertion.

Porous Mixed-Metal Oxide Li-Ion Battery Electrodes by Shear-Induced Co-assembly of Precursors and Tailored Polymer Particles

Due to their various applications, metal oxides are of high interest for fundamental research and commercial usage. Per applications as catalysts or electrochemical devices, the tailored design of metal oxides featuring a high specific surface area and additional functionalities is of the utmost importance for the performance of the resulting materials. We report a new method for preparing free-standing films consisting of hierarchically porous metal oxides (titanium and niobium based) by combining emulsion polymerization and shear-induced monodisperse particle self-assembly in the presence of sol-gel precursors. After thermal treatment, the resulting porous materials can be used as electrodes in Li-ion batteries. The titanium and niobium sol-gel precursors were partially immobilized to the surface of organic core-interlayer particles featuring hydroxyl groups to obtain hybrid organic-inorganic particles through the melt-shear organization process. Free-standing particle-based films, in analogy to elastomeric opal films and colloidal crystals, can be prepared in a convenient one-step preparation process. After thermal treatment, ordered pores are obtained, while the pristine metal oxide precursor shell can be converted to the (mixed) metal oxide matrix. Heat treatment under CO₂ leads to mixed-TiNb oxide/carbon hybrid materials. The highly porous derivative structure enhances electrolyte permeation. When tested as Li-ion battery electrodes, it shows a specific capacity of 335 mAh·g^-1 at a rate of 10 mA·g^-1. After 1000 cycles at 250 mA·g^-1, the electrodes still provided a specific capacity of 191 mAh·g^-1.

A molecularly imprinted electrochemical biosensor based on hierarchical Ti2Nb10O29 (TNO) for glucose detection

A novel molecularly imprinted electrochemical biosensor for glucose detection is reported based on a hierarchical N-rich carbon conductive-coated TNO structure (TNO@NC). Firstly, TNO@NC was fabricated by a novel polypyrrole-chemical vapor deposition (PPy-CVD) method with minimal waste generation. Afterward, the electrode modification with TNO@NC was performed by dropping TNO@NC particles on glassy carbon electrode surfaces by infrared heat lamp. Finally, the glucose-imprinted electrochemical biosensor was developed in presence of 75.0 mM pyrrole and 25.0 mM glucose in a potential range from + 0.20 to + 1.20 V versus Ag/AgCl via cyclic voltammetry (CV). The physicochemical and electrochemical characterizations of the fabricated molecularly imprinted biosensor was conducted by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) method, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and CV techniques. The findings demonstrated that selective, sensitive, and stable electrochemical signals were proportional to different glucose concentrations, and the sensitivity of molecularly imprinted electrochemical biosensor for glucose detection was estimated to be 18.93 μA μM^-1 cm^-2 (R² = 0.99) at + 0.30 V with the limit of detection (LOD) of 1.0 × 10^-6 M. Hence, it can be speculated that the fabricated glucose-imprinted biosensor may be used in a multitude of areas, including public health and food quality.

Atomic-resolution STEM-EDS studies of cation ordering in Ti-Nb oxide crystals

Ternary metal oxide compounds, such as Ti-Nb and Nb-W oxides, have renewed research interest in energy storage materials because these oxides contain multivalent metal ions that may be able to control the ion transport in solid lithium batteries. One of these oxides is Ti₂Nb₁₀O₂₉, which is composed of metal-oxygen octahedra connected through corner-sharing and edge-sharing to form "block structures". In the early 1970s Von Dreele and Cheetham proposed a metal-atoms ordering in this oxide crystal using Rietveld refined neutron powder diffraction method. Most recent studies on these oxides, however, have not considered cation ordering in evaluating the battery electrode materials. In this paper, by utilizing the latest scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy imaging technology, the cation chemical ordering in those oxide crystals was directly revealed at atomic resolution.

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.

Niobium pentoxide based materials for high rate rechargeable electrochemical energy storage

The demand for high rate energy storage systems is continuously increasing driven by portable electronics, hybrid/electric vehicles and the need for balancing the smart grid. Accordingly, Nb₂O₅ based materials have gained great attention because of their fast cation intercalation faradaic charge storage that endows them with high rate energy storage performance. In this review, we describe the crystalline features of the five main phases of Nb₂O₅ and analyze their specific electrochemical characteristics with an emphasis on the intrinsic ion intercalation pseudocapacitive behavior of T-Nb₂O₅. The charge storage mechanisms, electrochemical performance and state-of-the-art characterization techniques for Nb₂O₅ anodes are summarized. Next, we review recent progress in developing various types of Nb₂O₅ based fast charging electrode materials, including Nb₂O₅ based mixed metal oxides and composites. Finally, we highlight the major challenges for Nb₂O₅ based materials in the realm of high rate rechargeable energy storage and provide perspectives for future research.

Ionic and Electronic Conduction in TiNb2O7

TiNb₂O₇ is a Wadsley-Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion mechanism and conduction in TiNb₂O₇, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb₂O₇. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li_0.60TiNb₂O₇. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in Li_xTiNb₂O₇) is rapid with low activation barriers from NMR and D_Li = 10^-11 m² s^-1 at the temperature of the observed T₁ minima of 525-650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100-200 meV down the tunnels but ca. 700-1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in Li_xTiNb₂O₇), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb₂O₇ and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na⁺, K⁺, Mg²⁺) in TiNb₂O₇ reveals high diffusion barriers of 1-3 eV, indicating that this structure is specifically suited to Li⁺ mobility.

Cation Disorder and Lithium Insertion Mechanism of Wadsley-Roth Crystallographic Shear Phases from First Principles

Wadsley-Roth crystallographic shear phases form a family of compounds that have attracted attention due to their excellent performance as lithium-ion battery electrodes. The complex crystallographic structure of these materials poses a challenge for first-principles computational modeling and hinders the understanding of their structural, electronic and dynamic properties. In this article, we study three different niobium-tungsten oxide crystallographic shear phases (Nb₁₂WO₃₃, Nb₁₄W₃O₄₄, Nb₁₆W₅O₅₅) using an enumeration-based approach and first-principles density-functional theory calculations. We report common principles governing the cation disorder, lithium insertion mechanism, and electronic structure of these materials. Tungsten preferentially occupies tetrahedral and block-central sites within the block-type crystal structures, and the local structure of the materials depends on the cation configuration. The lithium insertion proceeds via a three-step mechanism, associated with an anisotropic evolution of the host lattice. Our calculations reveal an important connection between long-range and local structural changes: in the second step of the mechanism, the removal of local structural distortions leads to the contraction of the lattice along specific crystallographic directions, buffering the volume expansion of the material. Niobium-tungsten oxide shear structures host small amounts of localized electrons during initial lithium insertion due to the confining effect of the blocks, but quickly become metallic upon further lithiation. We argue that the combination of local, long-range, and electronic structural evolution over the course of lithiation is beneficial to the performance of these materials as battery electrodes. The mechanistic principles we establish arise from the compound-independent crystallographic shear structure and are therefore likely to apply to niobium-titanium oxide or pure niobium oxide crystallographic shear phases.

Niobium-Based Oxides Toward Advanced Electrochemical Energy Storage: Recent Advances and Challenges

Niobium-based oxides including Nb₂ O₅ , TiNb_x O_2+2.5x compounds, M-Nb-O (M = Cr, Ga, Fe, Zr, Mg, etc.) family, etc., as the unique structural merit (e.g., quasi-2D network for Li-ion incorporation, open and stable Wadsley- Roth shear crystal structure), are of great interest for applications in energy storage systems such as Li/Na-ion batteries and hybrid supercapacitors. Most of these Nb-based oxides show high operating voltage (>1.0 V vs Li⁺ /Li) that can suppress the formation of solid electrolyte interface film and lithium dendrites, ensuring the safety of working batteries. Outstanding rate capability is impressive, which can be derived from their fast intercalation pseudocapacitive kinetics. However, the intrinsic poor electrical conductivity hinders their energy storage applications. Various strategies including structure optimization, surface engineering, and carbon modification are effectively used to overcome the issues. This review provides a comprehensive summary on the latest progress of Nb-based oxides for advanced electrochemical energy storage applications. Major impactful work is outlined, promising research directions, and various performance-optimizing strategies, as well as the energy storage mechanisms investigated by combining theoretical calculations and various electrochemical characterization techniques. In addition, challenges and perspectives for future research and commercial applications are also presented.

Observation of ZrNb14O37 Nanowires as a Lithium Container via In Situ and Ex Situ Techniques for High-Performance Lithium-Ion Batteries

Based on the high theoretical capacity and relatively high safety voltage, niobium-based oxides are regarded as promising intercalation-type electrode materials for advanced lithium-ion batteries (LIBs). Here, ZrNb₁₄O₃₇ nanowires are fabricated via a facile electrospinning method, presenting a nanoparticle-in-nanowire architecture. As an anode for LIBs, the as-fabricated ZrNb₁₄O₃₇ nanowires maintain a capacity of 244.9 mA h g^-1 at 100 mA g^-1 and present excellent cycling capability (0.026% of capacity fading per cycle during 1000 cycles) as well as outstanding rate performance. In situ X-ray diffraction measurement is conducted to understand the fundamental reaction mechanism during the lithiation/delithiation process. The ex situ observations, including X-ray photoelectron spectroscopy and transmission electron microscopy, are further performed to provide more lines of evidence of the reaction mechanism. Moreover, the excellent electrochemical performance of the full cell constructed using ZrNb₁₄O₃₇ nanowires and LiCoO₂ suggests that ZrNb₁₄O₃₇ nanowires are a promising anode material. This work sheds new light on understanding the lithium storage mechanism and may open new opportunities to develop new anode materials for LIBs.

Surface Characteristic Effect of Ag/TiO2 Nanoarray Composite Structure on Supercapacitor Electrode Properties

Ag-ion-modified titanium nanotube (Ag/TiO2-NT) arrays were designed and fabricated as the electrode material of supercapacitors for electrochemical energy storage. TiO2 nanotube (NT) arrays were prepared by electrochemical anodic oxidation and then treated by Ag metal vapor vacuum arc (MEVVA) implantation. The Ag amount was controlled via adjusting ion implantation parameters. The morphology, crystallinity, and electrochemistry properties of as-obtained Ag/TiO2-NT electrodes were distinguished based on various characterizations. Compared with different doses of Ag/TiO2-NTs, the electrode with the dose of 5.0 × 1017 ions·cm-2 exhibited much higher electrode capacity and greatly enhanced activity in comparison to the pure TiO2-NTs. The modified electrode showed a high capacitance of 9324.6 mF·cm-3 (86.9 mF·g, 1.2 mF·cm-2), energy density of 82.8 μWh·cm-3 (0.8 μWh·g, 0.0103 μWh·cm-2), and power density of 161.0 mW·cm-3 (150.4 μW·g, 2.00 μW·cm-2) at the current density of 0.05 mA. Therefore, Ag/TiO2-NTs could act as a feasible electrode material of supercapacitors.

High rate capability performance of ordered mesoporous TiNb6O17 microsphere anodes for lithium ion batteries

We report the synthesis and application of ordered mesoporous TiNb₆O₁₇ microspheres (M-TNO) using a one-step solvothermal method for the first time in lithium-ion batteries. The diameters of TiNb₆O₁₇ microspheres are in the range from 2.2 to 2.4 μm with a mesopore size of about 35 nm, which promotes the electron and ion migration in charge/discharge processes. M-TNO shows a high specific capacitance (307.2 mA h g^-1) at a low current density of 0.2 C and a long-term cycle life over 500 cycles as an electrode. The retentive capacity of the batteries is 77% of the initial cycle after 500 cycles. It is worth noting that M-TNO exhibits excellent rate capacity, which decreases slowly from 265.7 to 172.4 mA h g^-1 with the current density increasing from 1 C to 30 C. The retentive capacity at a current density of 30 C is 65% compared to that at 1 C.