Highly efficient lithium container based on non-Wadsley-Roth structure Nb18W16O93 nanowires for electrochemical energy storage

Redox Mechanisms upon the Lithiation of Wadsley-Roth Phases

The Wadsley-Roth family of transition metal oxide phases are a promising class of anode materials for Li-ion batteries due to their open crystal structures and their ability to intercalate Li at high rates. Unfortunately, most early transition metal oxides that adopt a Wadsley-Roth crystal structure intercalate Li at voltages that are too high for most battery applications. First-principles electronic structure calculations are performed to elucidate redox mechanisms in Wadsley-Roth phases with the aim of determining how they depend on crystal structure. A comparative study of two very distinct polymorphs of Nb2O5 reveal two redox mechanisms: (i) an atom-centered redox mechanism at early stages of Li intercalation and (ii) a redox mechanism at intermediate to high Li concentrations involving the bonding orbitals of metal-metal dimers formed by edge-sharing Nb cations. Our study motivates several design principles to guide the development of new Wadsley-Roth phases with superior electrochemical properties.

"Fast-Charging" Anode Materials for Lithium-Ion Batteries from Perspective of Ion Diffusion in Crystal Structure

"Fast-charging" lithium-ion batteries have gained a multitude of attention in recent years since they could be applied to energy storage areas like electric vehicles, grids, and subsea operations. Unfortunately, the excellent energy density could fail to sustain optimally while lithium-ion batteries are exposed to fast-charging conditions. In actuality, the crystal structure of electrode materials represents the critical factor for influencing the electrode performance. Accordingly, employing anode materials with low diffusion barrier could improve the "fast-charging" performance of the lithium-ion battery. In this Review, first, the "fast-charging" principle of lithium-ion battery and ion diffusion path in the crystal are briefly outlined. Next, the application prospects of "fast-charging" anode materials with various crystal structures are evaluated to search "fast-charging" anode materials with stable, safe, and long lifespan, solving the remaining challenges associated with high power and high safety. Finally, summarizing recent research advances for typical "fast-charging" anode materials, including preparation methods for advanced morphologies and the latest techniques for ameliorating performance. Furthermore, an outlook is given on the ongoing breakthroughs for "fast-charging" anode materials of lithium-ion batteries. Intercalated materials (niobium-based, carbon-based, titanium-based, vanadium-based) with favorable cycling stability are predominantly limited by undesired electronic conductivity and theoretical specific capacity. Accordingly, addressing the electrical conductivity of these materials constitutes an effective trend for realizing fast-charging. The conversion-type transition metal oxide and phosphorus-based materials with high theoretical specific capacity typically undergoes significant volume variation during charging and discharging. Consequently, alleviating the volume expansion could significantly fulfill the application of these materials in fast-charging batteries.

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.

Composite capillary carbon tube Nb18W16O93as advanced anode material for aqueous ion capacitors

Niobium-tungsten bimetal oxides have received wide attention due to their excellent lattice properties. In this work, Nb₁₈W₁₆O₉₃(NbWO) with a tetragonal tungsten bronze structure was synthesized by simple hydrothermal method. NbWO was modified to provide high specific surface area via combining with hollow carbon nanotubes. Meanwhile, NbWO grows along the tube wall of carbon nanotubes, thus buffering the volume effect of NbWO particles. Also, the migration distance of Li-ion is effectively shortened, as well as the improved ion transfer efficiency and the reaction kinetics. In addition, carbon tube can enhance conductivity of NbWO, contributing to outstanding charge storage capacity and rate energy. Precisely, NbWO@C as electrode possesses large specific capacity (249.6 F g^-1at 0.5 A g^-1) and good rate performance (55.9% capacity retention from 0.5 to 2 A g^-1). The aqueous Li-ion capacitor presents the advantages of high safety, low cost and good environmental friendliness. An asymmetric aqueous capacitor AC//NbWO@C, based on 'water-in-salt' electrolyte with high concentration lithium acetate, exhibits a large energy density of 43.2 Wh kg^-1and a power density of 9 kW kg^-1. Generally, NbWO@C as anode materials shows superior application perspective.

Reversible and rapid calcium intercalation into molybdenum vanadium oxides

Looming concerns regarding scarcity, high prices, and safety threaten the long-term use of lithium in energy storage devices. Calcium has been explored in batteries because of its abundance and low cost, but the larger size and higher charge density of calcium ions relative to lithium impairs diffusion kinetics and cyclic stability. In this work, an aqueous calcium-ion battery is demonstrated using orthorhombic, trigonal, and tetragonal polymorphs of molybdenum vanadium oxide (MoVO) as a host for calcium ions. Orthorhombic and trigonal MoVOs outperform the tetragonal structure because large hexagonal and heptagonal tunnels are ubiquitous in such crystals, providing facile pathways for calcium-ion diffusion. For trigonal MoVO, a specific capacity of ∼203 mAh g^-1 was obtained at 0.2C and at a 100 times faster rate of 20C, an ∼60 mAh g^-1 capacity was achieved. The open-tunnel trigonal and orthorhombic polymorphs also promoted cyclic stability and reversibility. A review of the literature indicates that MoVO provides one of the best performances reported to date for the storage of calcium ions.

Niobium Tungsten Oxides for Electrochromic Devices with Long-Term Stability

There is a keen interest in the use of electrochromic materials because they can regulate light and heat, thereby reducing the cooling and heating energy. However, the long response time, short cycle life, and high power consumption of an electrochromic film hinder its development. Here, we report an electrochromic material of complex niobium tungsten oxides. The Nb₁₈W₁₆O₉₃ thin films in the voltage range of 0 to -1.5 V show good redox kinetics with the coloration time of 4.7 s and bleaching time of 4.0 s, respectively. The electrochromic device based on the Nb₁₈W₁₆O₉₃ thin film has an optical modulation of 53.1% at a wavelength of 633 nm, with the coloration efficiency of ∼46.57 cm² C^-1. An excellent electrochemical stability of 78.1% retention after 8000 cycles is also achieved. These good performances are due to the fast and stable Li-ion intercalation/extraction in the open framework of Nb₁₈W₁₆O₉₃ with multiple ion positions. Our work provides a strategy for electrochromic materials with fast response time and good cycle stability.

High-Efficiency Hybrid Sulfur Cathode Based on Electroactive Niobium Tungsten Oxide and Conductive Carbon Nanotubes for All-Solid-State Lithium-Sulfur Batteries

All-solid-state lithium-sulfur batteries (ASSLSBs) have become a promising candidate because of their high energy density and safety. To ensure the high utilization and electrochemical capacity of sulfur in all-solid-state batteries, both the electronic and ionic conductivities of the sulfur cathode should be as high as possible. In this work, an intercalation-conversion hybrid cathode is proposed by distributing sulfur evenly on electroactive niobium tungsten oxide (Nb₁₈W₁₆O₉₃) and conductive carbon nanotubes (CNTs) for achieving high performance ASSLSBs. Herein, Nb₁₈W₁₆O₉₃ shows good electrochemical lithium storage in the hybrid cathode, which could serve as an effective Li-ion/electron conductor for the conversion of sulfur in the discharge/charge processes to achieve a high utilization of sulfur. However, CNTs could further increase the electronic conductivity of the hybrid cathode by constructing good conductive frameworks and suppress the volumetric fluctuation during the interconversion of sulfur and Li₂S. With this strategy, the S/Nb₁₈W₁₆O₉₃/CNT cathode achieves a high sulfur utilization of 91% after one cycle activation with a high gravimetric capacity of 1526 mA h g^-1. In addition, excellent rate performance is also obtained at 0.5 C with a reversible capacity of 1262 mA h g^-1 after 1000 cycles. This work offers a new perspective to develop ASSLSBs.

Copper niobate nanowires boosted by a N, S co-doped carbon coating for superior lithium storage

With the development of electric vehicles, more and more attention has been paid to the kinetic performance of batteries, which is related to rapid charge/discharge and safety issues. To improve this aspect, Cu₂Nb₃₄O₈₇ nanowires covered by nitrogen and sulfur co-doped carbon (Cu₂Nb₃₄O₈₇/NSC nanowires) are prepared by electrospinning combined with surface coating. As-prepared Cu₂Nb₃₄O₈₇/NSC nanowires present a high ion diffusion coefficient and electronic conductivity, showing good a kinetic performance. Specifically, they deliver a splendid capacity of 311.2 mA h g^-1 at 1 C and a superior cycling stability with a capacity fading of 0.031% per cycle upon 1000 cycles. At the same time, the electrochemical and structural reversibility is fully discussed and demonstrated by ex situ XRD, ex situ SEM and ex situ XPS based on the redox couples of Cu²⁺/Cu⁺, Nb⁵⁺/Nb⁴⁺, and Nb⁴⁺/Nb³⁺. Contributing to peculiar physico-chemical properties, Cu₂Nb₃₄O₈₇/NSC nanowires are expected to be a candidate material for the anode in lithium-ion batteries.

Optimization of the Electrospun Niobium-Tungsten Oxide Nanofibers Diameter Using Response Surface Methodology

The present research aimed to investigate the effect of working parameters on the electrospinning of niobium–tungsten oxide nanofibers and optimize the process using central composite design (CCD) based on the response surface methodology (RSM). An experiment was designed to assess the effects of five variables including the applied voltage (V), spinning distance (D), polymer concentration (P), flow rate (F), and addition of NaCl (N) on the resulting diameter of the nanofibers. Meanwhile, a second-order prediction model of nanofibers diameter was fitted and verified using analysis of variance (ANOVA). The results show that the diameter of the nanofibers was significantly influenced by all the variables except the flow rate. Some second-order and cross factor interactions such as VD, DP, PF, PN, and P² also have significant effects on the diameter of the nanofibers. The results of the ANOVA yielded R² and adjusted R² values of 0.96 and 0.93 respectively, this affirmed that the predictive model fitted well with the experimental data. Furthermore, the process parameters were optimized using the CCD method and a maximum desirability function of 226 nm was achieved for the diameter of the nanofibers. This is very close to the 233 nm diameter obtained from a confirmatory experiment using the optimum conditions. Therefore, the model is representative of the process, and it could be used for future studies for the reduction of the diameter of electrospun nanofibers.

Design of a niobium tungsten oxide/C micro-structured electrode for fast charging lithium-ion batteries

Secondary Nb18W16O93/C micro-grade particles are synthesized for use as a fast-charging anode with high stability and rate performance at 5C.

Characterization of K6Ta10.8O30 Microrods with Tetragonal Tungsten Bronze-Like Structure Obtained from Tailings from the Penouta Sn-Ta-Nb Deposit

In this work, a deep characterization of the properties of K6Ta10.8O30 microrods has been performed. The starting material used to grow the microrods has been recovered from mining tailings coming from the Penouta Sn-Ta-Nb deposit, located in the north of Spain. The recovered material has been submitted to a thermal treatment to grow the microrods. Then, they have been characterized by scanning electron microscopy, X-ray diffraction, micro-Raman and micro-photoluminescence. The results of our study confirm that the K6Ta10.8O30 microrods have a tetragonal tungsten bronze-like crystal structure, which can be useful for ion-batteries and photocatalysis.

Intergrowth of niobium tungsten oxides of the tetragonal tungsten bronze type

Since the 1970s, high-resolution transmission electron microscopy (HRTEM) is well established as the most appropriate method to explore the structural complexity of niobium tungsten oxides. Today, scanning transmission electron microscopy (STEM) represents an important alternative for performing the structural characterization of such oxides. STEM images recorded with a high-angle annular dark field (HAADF) detector provide not only information about the cation positions but also about the distribution of niobium and tungsten as the intensity is directly correlated to the local scattering potential. The applicability of this method is demonstrated here for the characterization of the real structure of Nb7W10O47.5. This sample contains well-ordered domains of Nb8W9O47 and Nb4W7O31 besides little ordered areas according to HRTEM results. Structural models for Nb4W7O31 and twinning occurring in this phase have been derived from the interpretation of HAADF-STEM images. A remarkable grain boundary between well-ordered domains of Nb4W7O31 and Nb8W9O47 has been found that contains one-dimensionally periodic features. Furthermore, short-range order observed in less ordered areas could be attributed to an intimate intergrowth of small sections of different tetragonal tungsten bronze (TTB) based structures.

Niobium Tungsten Oxide in a Green Water-in-Salt Electrolyte Enables Ultra-Stable Aqueous Lithium-Ion Capacitors

Aqueous hybrid supercapacitors are attracting increasing attention due to their potential low cost, high safety and eco-friendliness. However, the narrow operating potential window of aqueous electrolyte and the lack of suitable negative electrode materials seriously hinder its future applications. Here, we explore high concentrated lithium acetate with high ionic conductivity of 65.5 mS cm-1 as a green "water-in-salt" electrolyte, providing wide voltage window up to 2.8 V. It facilitates the reversible function of niobium tungsten oxide, Nb18W16O93, that otherwise only operations in organic electrolytes previously. The Nb18W16O93 with lithium-ion intercalation pseudocapacitive behavior exhibits excellent rate performance, high areal capacity, and ultra-long cycling stability. An aqueous lithium-ion hybrid capacitor is developed by using Nb18W16O93 as negative electrode combined with graphene as positive electrode in lithium acetate-based "water-in-salt" electrolyte, delivering a high energy density of 41.9 W kg-1, high power density of 20,000 W kg-1 and unexceptionable stability of 50,000 cycles.

Self-assembly and lithium storage performance of a nanoscale polyoxometalate based on the {MnTa18} cluster

A nanosized Ta/W mixed addendum polyoxometalate (Cs12K3H7[MnTa18Si6W54O231]·61H2O) based on the unprecedented {MnTa18} cluster was fabricated successfully under hydrothermal conditions. An excellent electrochemical performance of this compound was found in lithium-ion batteries (LIBs) as an anode material. The discharge capacity was 829.9 mA h g-1 at a current density of 100 mA g-1 in the first cycle and stable at 428.4 mA h g-1 after 100 cycles, which suggests the potential application of this new compound in LIBs.

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.

New Anode Material for Lithium-Ion Batteries: Aluminum Niobate (AlNb11O29)

This paper describes the syntheses and electrochemical properties of a new niobate compound, aluminum niobate (AlNb₁₁O₂₉), for Li⁺ storage. AlNb₁₁O₂₉-microsized particles and nanowires were synthesized based on the solid-state reaction and solvothermal methods, respectively. In situ X-ray diffraction results confirmed the intercalating mechanism of Li⁺ in AlNb₁₁O₂₉ and revealed its high structural stability against cycling. The AlNb₁₁O₂₉ nanowires with a novel bamboo-like morphology afforded a large interfacial area and short charge transport pathways, thus leading to the observed excellent electrochemical properties, including high reversible Li⁺-storage capacity (266 mA h g^-1), safe operating potential (around 1.68 V), and high initial Coulombic efficiency (93.3%) at 0.1 C. At a very high rate (10 C), the AlNb₁₁O₂₉ nanowires still exhibited a capacity as high as 192 mA h g^-1, indicating their good rate capability. In addition, at 10 C, 96.3% capacity was retained over 500 cycles, indicating superior cycling stability. A full cell fabricated with AlNb₁₁O₂₉ nanowires as the anode and LiNi_0.5Mn_1.5O₄ microparticles as the cathode delivered a high energy density of 390 W h kg^-1 at 0.1 C. This work suggests that the AlNb₁₁O₂₉ nanowires hold a great promise for the development of high-performance lithium-ion batteries for large-scale energy-storage applications.