Polymorphs of Nb2O5 Compound and Their Electrical Energy Storage Applications

Niobium pentoxide (Nb2O5), as an important dielectric and semiconductor material, has numerous crystal polymorphs, higher chemical stability than water and oxygen, and a higher melt point than most metal oxides. Nb2O5 materials have been extensively studied in electrochemistry, lithium batteries, catalysts, ionic liquid gating, and microelectronics. Nb2O5 polymorphs provide a model system for studying structure-property relationships. For example, the T-Nb2O5 polymorph has two-dimensional layers with very low steric hindrance, allowing for rapid Li-ion migration. With the ever-increasing energy crisis, the excellent electrical properties of Nb2O5 polymorphs have made them a research hotspot for potential applications in lithium-ion batteries (LIBs) and supercapacitors (SCs). The basic properties, crystal structures, synthesis methods, and applications of Nb2O5 polymorphs are reviewed in this article. Future research directions related to this material are also briefly discussed.

Copper niobate nanowires immobilized on reduced graphene oxide nanosheets as rate capability anode for lithium ion capacitor

Binary metal niobium oxides can offer a higher specific capacity compared to niobium pentoxide (Nb₂O₅) and thus are ideal anode candidates for lithium ion capacitors (LICs). However, their lower electronic conductivity limits their ability to achieve high energy and power densities. In this paper, one-dimensional (1D) copper niobate (CuNb₂O₆) nanowires are successfully prepared by electrospinning technology and then immobilized on two-dimensional (2D) reduced graphene oxide (rGO) nanosheets to form a unique 1D nanowire/2D nanosheet CuNb₂O₆/rGO structure. The 1D/2D CuNb₂O₆/rGO electrode exhibits a high specific capacity of 312.2 mAh g^-1 at 100 mA g^-1 as the anode of LICs. The proposed Li⁺ storage mechanism of the CuNb₂O₆ anode involves CuNb₂O₆ decomposition into lithium niobate (Li₃NbO₄) and copper (Cu) during the initial lithium insertion process. The intercalation-type Li₃NbO₄ will further serve as the host to Li⁺ and the inactive Cu phase will act as a conductive network for electron transportation. Furthermore, the energy density of the assembled CuNb₂O₆/rGO//activated carbon (CuNb₂O₆/rGO//AC) device could achieve a value as high as 92.1 Wh kg^-1 and could thus be considered as a possible alternative electrode material for high energy and power LICs.

Ultrafine antimony (Sb) nanoparticles encapsulated into a carbon microfiber framework as an excellent LIB anode with a superlong life of more than 5000 cycles

Antimony (Sb) anode has attracted increasing attention given its high theoretical capacity and suitable working potential. Nonetheless, its practical application is largely hindered by huge volume changes during the cyclic process, resulting in unsatisfactory long-term cycled stabilities at high current density. In this work, large-scale ultrafine Sb nanoparticles are functionally designed to encapsulate into a 3D carbon microfiber framework (CMF) via a scalable electrospinning approach followed by a thermal treatment process. This fabrication strategy effectively avoids the change in the volume of the Sb anode and provides a fast conductive network to serve as an efficient 3D e/Li⁺ transport pathway. Benefiting from this novel structural design, an ultrafine Sb nanoparticles@carbon microfiber framework (U-Sb-NPs@CMF) composite anode used for lithium-ion batteries (LIBs) delivers a high reversible capacity of 622 mAh g^-1 after 200 cycles at 0.5 A g^-1 and 507 mAh g^-1 after 2000 cycles at 2 Ag^-1 and a high-capacity retention of 350 mAh g^-1 even after 5000 long-term cycles. These outstanding charge-discharge performances suggest that the U-Sb-NPs@CMF composite is a promising candidate for an anode material in the application of LIBs.

Surfactant-Free Synthesis of Nb2O5 Nanoparticles Anchored Graphene Nanocomposites with Enhanced Electrochemical Performance for Supercapacitor Electrodes

Nb₂O₅/graphene nanocomposites without any surfactant are synthesized by an in situ microwave irradiation technique. Structural and morphological studies revealed that the prepared composites were composed of Nb₂O₅ nanoparticles intercalated into the graphene sheet. The thermal stability of graphene oxide, Nb₂O₅, and Nb₂O₅/graphene nanocomposite was studied by the TGA. The electrochemical properties are assessed by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy analyses. The specific capacitance of Nb₂O₅/graphene nanocomposites is greater (633 Fg⁻¹) than pure Nb₂O₅ nanoparticles (221 Fg⁻¹) and graphene (290 Fg⁻¹) at a current density of 1 Ag⁻¹. The long-term cyclic measurement confirms higher cyclic stability of the nanocomposite with capacitance retention of 99.3% after 5000 cycles without performance degradation. The composites exhibit higher electrochemical conductivity and allow effective ions and charge transport over the entire electrode surface with aqueous electrolyte. The electrochemical study suggests that Nb₂O₅/graphene nanocomposites have the potential to be an effective electrode for superior performance supercapacitor applications.

Urchin-like hierarchical H-Nb2O5 microspheres: synthesis, formation mechanism and their applications in lithium ion batteries

Urchin-like hierarchical Nb₂O₅ microspheres are successfully synthesized through a facile solvothermal method in glycerol-isopropanol mixed media followed by thermal treatment. The sample is characterized by XRD, FESEM, TEM, HRTEM, BET, and XPS, and the results reveal that the as-formed Nb₂O₅ microspheres have a pseudohexagonal structure and are composed of nanorods with an average diameter of ca. 20 nm. It is found that glycerol not only serves as a solvent but also acts as a reactant; furthermore, isopropanol plays an important part in the morphologies of the products. When used as anodic materials for lithium ion batteries, the Nb₂O₅ microspheres deliver initial discharge capacities of 201.7, 159.7, 148.5, 123.7, and 98.5 mA h g^-1 at the current densities of 0.5, 1, 2, 5, and 10C, respectively. Additionally, the discharge capacity of Nb₂O₅ remains at 105.5 mA h g^-1 even after 500 cycles at a high rate of 5C. The good electrochemical properties of the products may be ascribed to their large surface areas and hierarchical structures.

Free-anchored Nb2O5@graphene networks for ultrafast-stable lithium storage

Orthorhombic Nb₂O₅ (T-Nb₂O₅) has structural merit but poor electrical conductivity, limiting their applications in energy storage. Although graphene is frequently adopted to effectively improve its electrochemical properties, the ordinary modified methods cannot meet the growing demands for high-performance. Here, we demonstrate that different graphene modified routes play a vital role in affecting the electrochemical performances of T-Nb₂O₅. By only manual shaking within one minute, Nb₂O₅ nano-particles can be rapidly adsorbed onto graphene, then the free-anchored T-Nb₂O₅@graphene three-dimensional networks can be successfully prepared based on hydrogel method. As for the application in lithium-ion batteries, it performs outstanding rate character (129 mA h g^-1 (25C rate), 110 mA h g^-1 (50C rate) and 90 mA h g^-1 (100C rate), correspond to 79%, 67% and 55% capacity of 0.5C rate, respectively) and excellent long-term cycling feature (∼70% capacity retention after 20000 cycles). Moreover, it still maintains similar ultrafast-stable lithium storage performances when Cu foil is substituted by Al foil as current collector. In addition, relevant kinetics mechanisms are also expounded. This work provides a versatile strategy for the preparation of graphene modified Nb₂O₅ or other types of nanoparticles.

Carbon nanofibers (CNFs) supported cobalt- nickel sulfide (CoNi2S4) nanoparticles hybrid anode for high performance lithium ion capacitor

Lithium ion capacitors possess an ability to bridge the gap between lithium ion battery and supercapacitor. The main concern of fabricating lithium ion capacitors is poor rate capability and cyclic stability of the anode material which uses sluggish faradaic reactions to store an electric charge. Herein, we have fabricated high performance hybrid anode material based on carbon nanofibers (CNFs) and cobalt-nickel sulfide (CoNi₂S₄) nanoparticles via simple electrospinning and electrodeposition methods. Porous and high conducting CNF@CoNi₂S₄ electrode acts as an expressway network for electronic and ionic diffusion during charging-discharging processes. The effect of anode to cathode mass ratio on the performance has been studied by fabricating lithium ion capacitors with different mass ratios. The surface controlled contribution of CNF@CoNi₂S₄ electrode was 73% which demonstrates its excellent rate capability. Lithium ion capacitor fabricated with CNF@CoNi₂S₄ to AC mass ratio of 1:2.6 showed excellent energy density of 85.4 Wh kg⁻¹ with the power density of 150 W kg⁻¹. Also, even at the high power density of 15 kW kg⁻¹, the cell provided the energy density of 35 Wh kg⁻¹. This work offers a new strategy for designing high-performance hybrid anode with the combination of simple and cost effective approaches.

Formation of a Surficial Bifunctional Nanolayer on Nb2 O5 for Ultrastable Electrodes for Lithium-Ion Battery

Safe and long cycle life electrode materials for lithium-ion batteries are significantly important to meet the increasing demands of rechargeable batteries. Niobium pentoxide (Nb₂ O₅ ) is one of the highly promising candidates for stable electrodes due to its safety and minimal volume expansion. Nevertheless, pulverization and low conductivity of Nb₂ O₅ have remained as inherent challenges for its practical use as viable electrodes. A highly facile method is proposed to improve the overall cycle retention of Nb₂ O₅ microparticles by ammonia (NH₃ ) gas-driven nitridation. After nitridation, an ultrathin surficial layer (2 nm) is formed on the Nb₂ O₅ , acting as a bifunctional nanolayer that allows facile lithium (Li)-ion transport (10-100 times higher Li diffusivity compared with pristine Nb₂ O₅ microparticles) and further prevents the pulverization of Nb₂ O₅ . With the subsequent decoration of silver (Ag) nanoparticles (NPs), the low electric conductivity of nitridated Nb₂ O₅ is also significantly improved. Cycle retention is greatly improved for nitridated Nb₂ O₅ (96.7%) compared with Nb₂ O₅ (64.7%) for 500 cycles. Ag-decorated, nitridated Nb₂ O₅ microparticles and nitridated Nb₂ O₅ microparticles exhibit ultrastable cycling for 3000 cycles at high current density (3000 mA g^-1 ), which highlights the importance of the surficial nanolayer in improving overall electrochemical performances, in addition to conductive NPs.

Critical Insight into the Relentless Progression Toward Graphene and Graphene-Containing Materials for Lithium-Ion Battery Anodes

Used as a bare active material or component in hybrids, graphene has been the subject of numerous studies in recent years. Indeed, from the first report that appeared in late July 2008, almost 1600 papers were published as of the end 2015 that investigated the properties of graphene as an anode material for lithium-ion batteries. Although an impressive amount of data has been collected, a real advance in the field still seems to be missing. In this framework, attention is focused on the most prominent research efforts in this field with the aim of identifying the causes of such relentless progression through an insightful and critical evaluation of the lithium-ion storage performances (i.e., 1^st cycle irreversible capacity, specific gravimetric and volumetric capacities, average delithiation voltage profile, rate capability and stability upon cycling). The "graphene fever" has certainly provided a number of fundamental studies unveiling the electrochemical properties of this "wonder" material. However, analysis of the published literature also highlights a loss of focus from the final application. Hype-driven claims, not fully appropriate metrics, and negligence of key parameters are probably some of the factors still hindering the application of graphene in commercial batteries.

Boosted adsorption–photocatalytic activities and potential lithium intercalation applications of layered potassium hexaniobate nano-family

We demonstrated that the KN nano-family (including KN nanolaminas and nano hollow spheres) can be derived from the same Nb₂O₅-based hydrothermal reaction.

High-Rate Intercalation without Nanostructuring in Metastable Nb2O5 Bronze Phases

Nanostructuring and nanosizing have been widely employed to increase the rate capability in a variety of energy storage materials. While nanoprocessing is required for many materials, we show here that both the capacity and rate performance of low-temperature bronze-phase TT- and T-polymorphs of Nb2O5 are inherent properties of the bulk crystal structure. Their unique "room-and-pillar" NbO6/NbO7 framework structure provides a stable host for lithium intercalation; bond valence sum mapping exposes the degenerate diffusion pathways in the sites (rooms) surrounding the oxygen pillars of this complex structure. Electrochemical analysis of thick films of micrometer-sized, insulating niobia particles indicates that the capacity of the T-phase, measured over a fixed potential window, is limited only by the Ohmic drop up to at least 60C (12.1 A·g(-1)), while the higher temperature (Wadsley-Roth, crystallographic shear structure) H-phase shows high intercalation capacity (>200 mA·h·g(-1)) but only at moderate rates. High-resolution (6/7)Li solid-state nuclear magnetic resonance (NMR) spectroscopy of T-Nb2O5 revealed two distinct spin reservoirs, a small initial rigid population and a majority-component mobile distribution of lithium. Variable-temperature NMR showed lithium dynamics for the majority lithium characterized by very low activation energies of 58(2)-98(1) meV. The fast rate, high density, good gravimetric capacity, excellent capacity retention, and safety features of bulk, insulating Nb2O5 synthesized in a single step at relatively low temperatures suggest that this material not only is structurally and electronically exceptional but merits consideration for a range of further applications. In addition, the realization of high rate performance without nanostructuring in a complex insulating oxide expands the field for battery material exploration beyond conventional strategies and structural motifs.

Recent advances in nanostructured Nb-based oxides for electrochemical energy storage

For the past five years, nanostructured niobium-based oxides have emerged as one of the most prominent materials for batteries, supercapacitors, and fuel cell technologies, for instance, TiNb2O7 as an anode for lithium-ion batteries (LIBs), Nb2O5 as an electrode for supercapacitors (SCs), and niobium-based oxides as chemically stable electrochemical supports for fuel cells. Their high potential window can prevent the formation of lithium dendrites, and their rich redox chemistry (Nb(5+)/Nb(4+), Nb(4+)/Nb(3+)) makes them very promising electrode materials. Their unique chemical stability under acid conditions is favorable for practical fuel-cell operation. In this review, we summarized recent progress made concerning the use of niobium-based oxides as electrodes for batteries (LIBs, sodium-ion batteries (SIBs), and vanadium redox flow batteries (VRBs)), SCs, and fuel cell applications. Moreover, crystal structures, charge storage mechanisms in different crystal structures, and electrochemical performances in terms of the specific capacitance/capacity, rate capability, and cycling stability of niobium-based oxides are discussed. Insights into the future research and development of niobium-based oxide compounds for next-generation electrochemical devices are also presented. We believe that this review will be beneficial for research scientists and graduate students who are searching for promising electrode materials for batteries, SCs, and fuel cells.

Spectroscopic ellipsometry for compositionally induced bandgap tuning of combinatorial niobium–tantalum anodic oxides

Spectroscopic ellipsometry maps optical properties of anodic oxides grown on Nb–Ta thin film combinatorial library. The bandgap values deviate from the linear model based on mixed matter theory due to an in-depth compositional gradient of the oxides.

Free-Standing T-Nb₂O₅/Graphene Composite Papers with Ultrahigh Gravimetric/Volumetric Capacitance for Li-Ion Intercalation Pseudocapacitor

Free-standing electrodes with high gravimetric/volumetric capacitance will open up potential applications in miniaturized consumer electronics. Herein, we report a simple synthesis technology of free-standing orthorhombic Nb2O5 (T-Nb2O5)/graphene composite papers for Li-intercalating pseudocapacitive electrodes. Through a facile polyol-mediated solvothermal reaction, the Nb2O5 nanodots are homogeneously decorated onto the surface of reduced graphite oxide (rGO), which can form a homogeneous Nb2O5/rGO colloidal suspension that can be easily fabricated into flexible composite papers. The heat-treated T-Nb2O5/graphene composite papers exhibit a nanoporous layer-stacked structure with good ionic-electric conductive pathways, high T-Nb2O5 loading of 74.2%, and high bulk density of 1.55 g cm(-3). Such T-Nb2O5/graphene composite papers show a superior pseudocapacitor performance as free-standing electrodes, as evidenced by an ultrahigh gravimetric/volumetric capacitance (620.5 F g(-1) and 961.8 F cm(-3) at 1 mV s(-1)) and excellent rate capability. Furthermore, an organic electrolyte-based asymmetric supercapacitor is assembled based on T-Nb2O5/graphene composite papers, which can deliver a high energy density of 47 W h kg(-1) and power density of 18 kW kg(-1).