Unraveling the Nature of Anomalously Fast Energy Storage in T-Nb2O5

Sodium-Controlled Interfacial Resistive Switching in Thin Film Niobium Oxide for Neuromorphic Applications

A double layer 2-terminal device is employed to show Na-controlled interfacial resistive switching and neuromorphic behavior. The bilayer is based on interfacing biocompatible NaNbO3 and Nb2O5, which allows the reversible uptake of Na+ in the Nb2O5 layer. We demonstrate voltage-controlled interfacial barrier tuning via Na+ transfer, enabling conductivity modulation and spike-amplitude- and spike-timing-dependent plasticity. The neuromorphic behavior controlled by Na+ ion dynamics in biocompatible materials shows potential for future low-power sensing electronics and smart wearables with local processing.

Mechanochemically Enabled Metastable Niobium Tungsten Oxides

Metastable compounds have greatly expanded the synthesizable compositions of solid-state materials and have attracted enormous amounts of attention in recent years. Especially, mechanochemically enabled metastable materials synthesis has been very successful in realizing cation-disordered materials with highly simple crystal structures, such as rock salts. Application of the same strategy for other structural types, especially for non-close-packed structures, is peculiarly underexplored. Niobium tungsten oxides (NbWOs), a class of materials that have been under the spotlight because of their diverse structural varieties and promising electrochemical and thermoelectric properties, are ideally suited to fill such a knowledge gap. In this work, we develop a new series of metastable NbWOs and realize one with a fully cation-disordered structure. Furthermore, we find that metastable NbWOs transform to a cation-disordered cubic structure when applied as a Li-ion battery anode, highlighting an intriguing non-close-packed-close-packed conversion process, as evidenced in various physicochemical characterizations, in terms of diffraction, electronic, and vibrational structures. Finally, by comparing the cation-disordered NbWO with other trending cation-disordered oxides, we raise a few key structural features for cation disorder and suggest a few possible research opportunities for this field.

Solvent-free selective hydrogenation of nitroaromatics to azoxy compounds over Co single atoms decorated on Nb2O5 nanomeshes

The solvent-free selective hydrogenation of nitroaromatics to azoxy compounds is highly important, yet challenging. Herein, we report an efficient strategy to construct individually dispersed Co atoms decorated on niobium pentaoxide nanomeshes with unique geometric and electronic properties. The use of this supported Co single atom catalysts in the selective hydrogenation of nitrobenzene to azoxybenzene results in high catalytic activity and selectivity, with 99% selectivity and 99% conversion within 0.5 h. Remarkably, it delivers an exceptionally high turnover frequency of 40377 h-1, which is amongst similar state-of-the-art catalysts. In addition, it demonstrates remarkable recyclability, reaction scalability, and wide substrate scope. Density functional theory calculations reveal that the catalytic activity and selectivity are significantly promoted by the unique electronic properties and strong electronic metal-support interaction in Co1/Nb2O5. The absence of precious metals, toxic solvents, and reagents makes this catalyst more appealing for synthesizing azoxy compounds from nitroaromatics. Our findings suggest the great potential of this strategy to access single atom catalysts with boosted activity and selectivity, thus offering blueprints for the design of nanomaterials for organocatalysis.

Hybrid-Mechanism Synergistic Flexible Nb2O5@WS2@C Carbon Nanofiber Anode for Superior Sodium Storage

Sodium-ion batteries (SIBs) have demonstrated remarkable development potential and commercial prospects. However, in the current state of research, the development of high-energy-density, long-cycle-life, high-rate-performance anode materials for SIBs remains a huge challenge. Free-standing flexible electrodes, owing to their ability to achieve higher energy density without the need for current collectors, binders, and conductive additives, have garnered significant attention across various fields. In this work, we designed and fabricated a free-standing three-dimensional flexible Nb2O5@WS2@C carbon nanofiber (CNF) anode based on a hybrid adsorption-intercalation-conversion mechanism of sodium storage, using electrospinning and hydrothermal synthesis processes. The hybrid structure, aided by synergistic effects, releases the advantages of all materials, demonstrating a superior rate performance (288, 248, 211, 158, 90, and 48 mA h g-1 at the current density of 0.2, 0.5, 1, 2, 5, and 10 A g-1, respectively) and good cycling stability (160 mA h g-1 after 200 cycles at 1 A g-1). This work provides certain guiding significance for future research on hybrid and flexible anodes of SIBs.

Niobium-Based Oxide for Anode Materials for Lithium-Ion Batteries

Recently, it has become imperative to develop high energy density as well as high safety lithium-ion batteries (LIBS) to meet the growing energy demand. Among the anode materials used in LIBs, the currently used commercial graphite has low capacity and is a safety hazard due to the formation of lithium dendrites during the reaction. Among the transition metal oxide (TMO) anode materials, TMO based on the intercalation reaction mechanism has a more stable structure and is less prone to volume expansion than TMO based on the conversion reaction mechanism, especially the niobium-based oxide in it has attracted much attention. Niobium-based oxides have a high operating potential to inhibit the formation of lithium dendrites and lithium deposits to ensure safety, and have stable and fast lithium ion transport channels with excellent multiplicative performance. This review summarizes the recent developments of niobium-based oxides as anode materials for lithium-ion batteries, discusses the special structure and electrochemical reaction mechanism of the materials, the synthesis methods and morphology of nanostructures, deficiencies and improvement strategies, and looks into the future developments and challenges of niobium-based oxide anode materials.

Spatial Effect on the Performance of Carboxylate Anode Materials in Na-Ion Batteries

Developing low-voltage carboxylate anode materials is critical for achieving low-cost, high-performance, and sustainable Na-ion batteries (NIBs). However, the structure design rationale and structure-performance correlation for organic carboxylates in NIBs remains elusive. Herein, the spatial effect on the performance of carboxylate anode materials is studied by introducing heteroatoms in the conjugation structure and manipulating the positions of carboxylate groups in the aromatic rings. Planar and twisted organic carboxylates are designed and synthesized to gain insight into the impact of geometric structures to the electrochemical performance of carboxylate anodes in NIBs. Among the carboxylates, disodium 2,2'-bipyridine-5,5'-dicarboxylate (2255-Na) with a planar structure outperforms the others in terms of highest specific capacity (210 mAh g-1), longest cycle life (2000 cycles), and best rate capability (up to 5 A g-1). The cyclic stability and redox mechanism of 2255-Na in NIBs are exploited by various characterization techniques. Moreover, high-temperature (up to 100 °C) and all-organic batteries based on a 2255-Na anode, a polyaniline (PANI) cathode, and an ether-based electrolyte are achieved and exhibited exceptional electrochemical performance. Therefore, this work demonstrates that designing organic carboxylates with extended planar conjugation structures is an effective strategy to achieve high-performance and sustainable NIBs.

Amorphous Modulation of Atomic Nb-O/N Clusters with Asymmetric Coordination in Carbon Shells for Advanced Sodium-Ion Hybrid Capacitors

Anode materials with excellent properties have become the key to develop sodium-ion hybrid capacitors (SIHCs) that combine the advantages of both batteries and capacitors. Amorphous modulation is an effective strategy to realize high energy/power density in SIHCs. Herein, atomically amorphous Nb-O/N clusters with asymmetric coordination are in situ created in N-doped hollow carbon shells (Nb-O/N@C). The amorphous clusters with asymmetric Nb-O3/N1 configurations have abundant charge density and low diffusion energy barriers, which effectively modulate the charge transport paths and improve the reaction kinetics. The clusters are also enriched with unsaturated vacancy defects and isotropic ion-transport channels, and their atomic disordering exhibits high structural stress buffering, which are strong impetuses for realizing bulk-phase-indifferent ion storage and enhancing the storage properties of the composite. Based on these features, Nb-O/N@C achieves notably improved sodium-ion storage properties (reversible capacity of 240.1 mAh g-1 at 10.0 A g-1 after 8000 cycles), and has great potential for SIHCs (230 Wh Kg-1 at 4001.5 W Kg-1). This study sheds new light on developing high-performance electrodes for sodium-ion batteries and SIHCs by designing amorphous clusters and asymmetric coordination.

Fast-charging anodes for lithium ion batteries: progress and challenges

Slow charging speed has been a serious constraint to the promotion of electric vehicles (EVs), and therefore the development of advanced lithium-ion batteries (LIBs) with fast-charging capability has become an urgent task. Thanks to its low price and excellent overall electrochemical performance, graphite has dominated the anode market for the past 30 years. However, it is difficult to meet the development needs of fast-charging batteries using graphite anodes due to their fast capacity degradation and safety hazards under high-current charging processes. This feature article describes the failure mechanism of graphite anodes under fast charging, and then summarizes the basic principles, current research progress, advanced strategies and challenges of fast-charging anodes represented by graphite, lithium titanate (Li4Ti5O12) and niobium-based oxides. Moreover, we look forward to the development prospects of fast-charging anodes and provide some guidance for future research in the field of fast-charging batteries.

"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.

Hierarchically Structured Nb2O5 Microflowers with Enhanced Capacity and Fast-Charging Capability for Flexible Planar Sodium Ion Micro-Supercapacitors

Highlights

Hierarchically structured Nb2O5 microflowers consiste of porous and ultrathin nanosheets. Nb2O5 microflowers exhibit enhanced capacity and rate performance boosting Na ion storage. Planar NIMSCs with charge and kinetics matching show superior areal capacitance and lifespan.

Abstract

Planar Na ion micro-supercapacitors (NIMSCs) that offer both high energy density and power density are deemed to a promising class of miniaturized power sources for wearable and portable microelectronics. Nevertheless, the development of NIMSCs are hugely impeded by the low capacity and sluggish Na ion kinetics in the negative electrode. Herein, we demonstrate a novel carbon-coated Nb2O5 microflower with a hierarchical structure composed of vertically intercrossed and porous nanosheets, boosting Na ion storage performance. The unique structural merits, including uniform carbon coating, ultrathin nanosheets and abundant pores, endow the Nb2O5 microflower with highly reversible Na ion storage capacity of 245 mAh g−1 at 0.25 C and excellent rate capability. Benefiting from high capacity and fast charging of Nb2O5 microflower, the planar NIMSCs consisted of Nb2O5 negative electrode and activated carbon positive electrode deliver high areal energy density of 60.7 μWh cm−2, considerable voltage window of 3.5 V and extraordinary cyclability. Therefore, this work exploits a structural design strategy towards electrode materials for application in NIMSCs, holding great promise for flexible microelectronics.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40820-023-01281-5.

High Flux and Stability of Cationic Intercalation in Transition-Metal Oxides: Unleashing the Potential of Mn t2g Orbital via Enhanced π-Donation

Transition-metal oxides (TMOs) often struggle with challenges related to low electronic conductivity and unsatisfactory cyclic stability toward cationic intercalation. In this work, we tackle these issues by exploring an innovative strategy: leveraging heightened π-donation to activate the t2g orbital, thereby enhancing both electron/ion conductivity and structural stability of TMOs. We engineered Ni-doped layered manganese dioxide (Ni-MnO2), which is characterized by a distinctive Ni-O-Mn bridging configuration. Remarkably, Ni-MnO2 presents an impressive capacitance of 317 F g-1 and exhibits a robust cyclic stability, maintaining 81.58% of its original capacity even after 20,000 cycles. Mechanism investigations reveal that the incorporation of Ni-O-Mn configurations stimulates a heightened π-donation effect, which is beneficial to the π-type orbital hybridization involving the O 2p and the t2g orbital of Mn, thereby accelerating charge-transfer kinetics and activating the redox capacity of the t2g orbital. Additionally, the charge redistribution from Ni to the t2g orbital of Mn effectively elevates the low-energy orbital level of Mn, thus mitigating the undesirable Jahn-Teller distortion. This results in a subsequent decrease in the electron occupancy of the π*-antibonding orbital, which promotes an overall enhancement in structural stability. Our findings pave the way for an innovative paradigm in the development of fast and stable electrode materials for intercalation energy storage by activating the low orbitals of the TM center from a molecular orbital perspective.

Carbon-supported T-Nb2O5 nanospheres and MoS2 composites with a mosaic structure for insertion-conversion anode materials

Reasonably combining the strengths of insertion and conversion anode materials to create an advanced anode material remains a formidable challenge for rechargeable lithium-ion batteries (LIBs). In this work, bulk MoS2 embedded with T-Nb2O5 nanospheres was synthesized via a simple hydrothermal process and a polydopamine carbon source was introduced by heat treatment. The design strategy can effectively accelerate the charge transfer and reduce the volume expansion during electrochemical cycling, leading to an improvement in lithium storage performance. As a consequence, the coexistence of T-Nb2O5, MoS2 and C can achieve the best synergistic effect when the molar ratio of Nb and Mo sources was 1 : 1. Notably, the T-Nb2O5@MoS2@C-1-1 electrode not only delivered an excellent reversible capacity of 518 mA h g-1 at a current density of 0.1 A g-1 but also exhibited superb cycling stability. The specific capacity of this electrode maintained 187 mA h g-1 at 2 A g-1 after 1000 cycles with a negligible capacity fading rate of only 0.015% per cycle.

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.

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

Competitive Redox Chemistries in Vanadium Niobium Oxide for Ultrafast and Durable Lithium Storage

Niobium pentoxide (Nb2O5) anodes have gained increasing attentions for high-power lithium-ion batteries owing to the outstanding rate capability and high safety. However, Nb2O5 anode suffers poor cycle stability even after modified and the unrevealed mechanisms have restricted the practical applications. Herein, the over-reduction of Nb5+ has been demonstrated to be the critical reason for the capacity loss for the first time. Besides, an effective competitive redox strategy has been developed to solve the rapid capacity decay of Nb2O5, which can be achieved by the incorporation of vanadium to form a new rutile VNbO4 anode. The highly reversible V3+/V2+ redox couple in VNbO4 can effectively inhibit the over-reduction of Nb5+. Besides, the electron migration from V3+ to Nb5+ can greatly increase the intrinsic electronic conductivity for VNbO4. As a result, VNbO4 anode delivers a high capacity of 206.1 mAh g-1 at 0.1 A g-1, as well as remarkable cycle performance with a retention of 93.4% after 2000 cycles at 1.0 A g-1. In addition, the assembled lithium-ion capacitor demonstrates a high energy density of 44 Wh kg-1 at 5.8 kW kg-1. In summary, our work provides a new insight into the design of ultra-fast and durable anodes.

Graphite-Based Composite Anodes with C-O-Nb Heterointerfaces Enable Fast Lithium Storage

To better satisfy the increasing demands for electric vehicles, it is crucial to develop fast-charging lithium-ion batteries (LIBs). However, the fast-charging capability of commercial graphite anodes is limited by the sluggish Li⁺ insertion kinetics. Herein, we report a synergistic engineering of uniform nano-sized T-Nb₂ O₅ particles on graphite (Gr@Nb₂ O₅ ) with C-O-Nb heterointerfaces, which prevents the growth and aggregation of T-Nb₂ O₅ nanoparticles. Through detailed theoretical calculations and pair distribution function analysis, the stable existence of the heterointerfaces is proved, which can accelerate the electron/ion transport. These heterointerfaces endow Gr@Nb₂ O₅ anodes with high ionic conductivity and excellent structural stability. Consequently, Gr@10-Nb₂ O₅ anode, where the mass ratio of T-Nb₂ O₅ /graphite=10/100, exhibits excellent cyclic stability and incredible rate capabilities, with 100.5 mAh g^-1 after 10000 stable cycles at an ultrahigh rate of 20 C. In addition, the synergistic Li⁺ storage mechanism is revealed by systematic electrochemical characterizations and in situ X-ray diffraction. This work offers new insights to the reasonable design of fast-charging graphite-based anodes for the next generation of LIBs.

High-density frustrated Lewis pairs based on Lamellar Nb2O5 for photocatalytic non-oxidative methane coupling

Photocatalytic methane conversion requires a strong polarization environment composed of abundant activation sites with the robust stretching ability for C-H scissoring. High-density frustrated Lewis pairs consisting of low-valence Lewis acid Nb and Lewis base Nb-OH are fabricated on lamellar Nb₂O₅ through a thermal-reduction promoted phase-transition process. Benefitting from the planar atomic arrangement of lamellar Nb₂O₅, the frustrated Lewis pairs sites are highly exposed and accessible to reactants, which results in a superior methane conversion rate of 1456 μmol g^-1 h^-1 for photocatalytic non-oxidative methane coupling without the assistance of noble metals. The time-dependent DFT calculation demonstrates the photo-induced electron transfer from LA to LB sites enhances their intensities in a concerted way, promoting the C-H cleavage through the coupling of LA and LB. This work provides in-depth insight into designing and constructing a polarization micro-environment for photocatalytic C-H activation of methane without the assistance of noble metals.

Heterostructured and Mesoporous Nb2O5@TiO2 Core-Shell Spheres as the Negative Electrode in Li-Ion Batteries

Niobium pentoxides have received considerable attention and are promising anode materials for lithium-ion batteries (LIBs), due to their fast Li storage kinetics and high capacity. However, their cycling stability and rate performance are still limited owing to their intrinsic insulating properties and structural degradation during charging and discharging. Herein, a series of mesoporous Nb₂O₅@TiO₂ core-shell spherical heterostructures have been prepared for the first time by a sol-gel method and investigated as anode materials in LIBs. Mesoporosity can provide numerous open and short pathways for Li⁺ diffusion; meanwhile, heterostructures can simultaneously enhance the electronic conductivity and thus improve the rate capability. The TiO₂ coating layer shows robust crystalline skeletons during repeated lithium insertion and extraction processes, retaining high structural integrity and, thereby, enhancing cycling stability. The electrochemical behavior is strongly dependent on the thickness of the TiO₂ layer. After optimization, a mesoporous Nb₂O₅@TiO₂ core-shell structure with a ∼13 nm thick TiO₂ layer delivers a high specific capacity of 136 mA h g^-1 at 5 A g^-1 and exceptional cycling stability (88.3% retention over 1000 cycles at 0.5 A g^-1). This work provides a facile strategy to obtain mesoporous Nb₂O₅@TiO₂ core-shell spherical structures and underlines the importance of structural engineering for improving the performance of battery materials.

Single "Swiss-roll" microelectrode elucidates the critical role of iron substitution in conversion-type oxides

Advancing the lithium-ion battery technology requires the understanding of electrochemical processes in electrode materials with high resolution, accuracy, and sensitivity. However, most techniques today are limited by their inability to separate the complex signals from slurry-coated composite electrodes. Here, we use a three-dimensional "Swiss-roll" microtubular electrode that is incorporated into a micrometer-sized lithium battery. This on-chip platform combines various in situ characterization techniques and precisely probes the intrinsic electrochemical properties of each active material due to the removal of unnecessary binders and additives. As an example, it helps elucidate the critical role of Fe substitution in a conversion-type NiO electrode by monitoring the evolution of Fe₂O₃ and solid electrolyte interphase layer. The markedly enhanced electrode performances are therefore explained. Our approach exposes a hitherto unexplored route to tracking the phase, morphology, and electrochemical evolution of electrodes in real time, allowing us to reveal information that is not accessible with bulk-level characterization techniques.

Dense T-Nb2O5/Carbon Microspheres for Ultrafast-(Dis)charge and High-Loading Lithium-Ion Batteries

Orthorhombic niobium pentoxide (T-Nb2O5) is regarded as a potential anode material for lithium-ion batteries (LIBs) due to ultrafast charge/discharge and high safety. However, the poor electronic conductivity and low mass loading of nanostructured T-Nb2O5 limit its practical application in LIBs. Herein, we design and construct dense microspheres consisting of nanostructured T-Nb2O5 embedded in amorphous N-doped carbon (Nb2O5@NC) via a facile method to achieve fast ionic and electronic transport as well as a high mass loading. The dense micro-sized particles with an interconnected carbon network avoid the low mass loading and volumetric energy density of conventional nanostructures. Interconnected pores in the range of a few nanometers are also formed in the Nb2O5@NC microspheres. Notably, at a high mass loading of 12.8 mg cm-2, Nb2O5@NC can achieve a high specific capacity of 171.5 mAh g-1 and an areal capacity of 2.05 mAh cm-2, showing its high lithium storage capacity. The intercalation reaction mechanism with a small volume change during cycling at both crystal lattice and microsphere levels is confirmed by in situ X-ray diffraction and in situ high-resolution transmission electron microscopy. The elegant structure and the electrochemical reaction mechanism disclosed in the work is important for designing ultrafast-(dis)charge electrode materials.

Unraveling the Nature and Role of Layered Cation Ordering in Cation-Disordered Rock-Salt Cathodes

Cation-disordered rock salts (DRXs), a new class of cathode materials for Li-ion batteries, have attracted a great amount of attention in recent years due to their fascinatingly simple cubic structure, highly diverse composition, and great electrochemical performance. As cations in DRXs are randomly distributed in a long range, how the cations are spatially arranged is an intriguing question for the community of solid-state materials chemistry. In this work, we report the vibrational structure of a series of Mn- and Fe-based DRXs with well-controlled compositions and reveal significant layered-like cation ordering in the DRXs. A scheme is proposed to describe how the layered-like anisotropy could exist in rock salt structures with an overall cubic diffraction pattern. Furthermore, we raise a model of Li-ion transport based on the proposed scheme, which complements the theory of Li percolation in DRXs. The electrochemical behavior of the DRX cathodes used in the study supports the scheme and clearly demonstrates the role of layered anisotropy in the battery performance of DRXs.

Triple Conductive Wiring by Electron Doping, Chelation Coating and Electrochemical Conversion in Fluffy Nb2 O5 Anodes for Fast-Charging Li-Ion Batteries

High-rate anode material is the kernel of developing fast-charging lithium ion batteries (LIBs). T-Nb₂ O₅ , well-known for its "room and pillar" structure and bulk pseudocapacitive effect, is expected to enable the fast lithium (de)intercalation. But this property is still limited by the low electronic conductivity or insufficient wiring manner. Herein, a strategy of triple conductive wiring through electron doping, chelation coating, and electrochemical conversion inside the microsized porous spheres consisting of dendrite-like T-Nb₂ O₅ primary particles is proposed to achieve the fast-charging and durable anodes for LIBs. The penetrative implanting of conformal carbon coating (derivative from polydopamine chelate) and NbO domains (induced by excess discharging) reinforces the global supply of electronically conductive wires, apart from those from Co/Mn heteroatom or O vacancy doping. The polydopamine etching on T-Nb₂ O₅ spheres promotes their evolution into fluffy morphology with better electrolyte infiltration. The synergic electron and ion wiring at different scales endow the modified T-Nb₂ O₅ anode with ultralong cycling life (143 mAh g^-1 at 1 A g^-1 after 8500 cycles) and high-rate performance (144.1 mAh g^-1 at 10.0 A g^-1 ). The permeation of multiple electron wires also enables a high mass loading of T-Nb₂ O₅ (4.5 mg cm^-2 ) with a high areal capacity of 0.668 mAh cm^-2 even after 150 cycles.

Highly [001]-oriented N-doped orthorhombic Nb2O5 microflowers with intercalation pseudocapacitance for lithium-ion storage

Orthorhombic Nb₂O₅ (T-Nb₂O₅), a typical intercalation pseudocapacitor, is favorable for realizing high power and energy density for lithium-ion batteries; furthermore, the 2D layered channels perpendicular to the [001] direction facilitate fast Li⁺ intercalation in T-Nb₂O₅. Herein, N-doped T-Nb₂O₅ microflowers (N-Nb₂O₅) assembled from highly [001]-oriented nanoflakes are rationally synthesized using NH₄F as the nitrogen source and capping agent. It is found that NH₄⁺ can adsorb on the O-terminated (010) plane of T-Nb₂O₅via N-H⋯O hydrogen bonds, which is highly conducive to the generation of 1D nanorods and the subsequent fusion of the nanorods into highly [001]-oriented nanoflakes. The special growth orientation of the T-Nb₂O₅ nanoflakes endows them with abundant available Li⁺ intercalation channels; moreover, the bandgap of N-Nb₂O₅ is narrowed (∼2.91 eV) owing to the doping of N atoms, and the intrinsic electronic conductivity is improved. Accordingly, the intercalation pseudocapacitive behavior of N-Nb₂O₅ is notably promoted and N-Nb₂O₅ exhibits superior Li⁺ storage properties, including large discharge capacity (214.7 mA h g^-1 at 1C), excellent rate capability (203.7 and 174.6 mA h g^-1 at 1 and 20C), and superior cyclic stability (150.7 mA h g^-1 at 10C after 1000 cycles). In addition, the LiNi_0.5Mn_0.3Co_0.2O₂//N-Nb₂O₅ full cell delivers outstanding Li⁺ storage performance, especially in terms of long-term cycling (126.2 mA h g^-1 at 10C after 3500 cycles).

A Nonstoichiometric Niobium Oxide/Graphite Composite for Fast-Charge Lithium-Ion Batteries

Electrification of transportation has spurred the development of fast-charge energy storage devices. High-power lithium-ion batteries require electrode materials that can store lithium quickly and reversibly. Herein, the design and construction of a Nb₂ O_5-δ /graphite composite electrode that demonstrates remarkable rate capability and durability are reported. The presence of graphite enables the formation of a dominant Nb₁₂ O₂₉ phase and a minor T-Nb₂ O₅ phase. The high rate capability is attributed to the enhanced electronic conductivity and lower energy barriers for fast lithium diffusion in both Nb₁₂ O₂₉ and T-Nb₂ O₅ , as unraveled by density functional theory calculations. The excellent durability or long cycling life is originated from the coherent redox behavior of Nb ions and high reversibility of lithium intercalation/deintercalation, as revealed by operando X-ray absorption spectroscopy analysis. When tested in a half-cell at high cycling rates, the composite electrode delivers a specific capability of 120 mAh g^-1 at 80 C and retains over 150 mAh g^-1 after 2000 cycles at 30 C, implying that it is a highly promising anode material for fast-charging lithium-ion batteries.

In Situ Orthorhombic to Amorphous Phase Transition of Nb2O5 and Its Temperature Effect on Pseudocapacitive Behavior

Niobium pentoxide (Nb₂O₅) represents an exquisite class of negative electrode materials with unique pseudocapacitive kinetics that engender superior power and energy densities for advanced electrical energy storage devices. Practical energy devices are expected to maintain stable performance under real-world conditions such as temperature fluctuations. However, the intercalation pseudocapacitive behavior of Nb₂O₅ at elevated temperatures remains unexplored because of the scarcity of suitable electrolytes. Thus, in this study, we investigate the effect of temperature on the pseudocapacitive behavior of submicron-sized Nb₂O₅ in a wide potential window of 0.01-2.3 V. Furthermore, ex situ X-ray diffraction and X-ray photoelectron spectroscopy reveal the amorphization of Nb₂O₅ accompanied by the formation of NbO via a conversion reaction during the initial cycle. Subsequent cycles yield enhanced performance attributed to a series of reversible Nb^V, IV/Nb^III redox reactions in the amorphous Li_xNb₂O₅ phase. Through cyclic voltammetry and symmetric cell electrochemical impedance spectroscopy, temperature elevation is noted to increase the pseudocapacitive contribution of the Nb₂O₅ electrode, resulting in a high rate capability of 131 mAh g^-1 at 20,000 mA g^-1 at 90 °C. The electrode further exhibits long-term cycling over 2000 cycles and high Coulombic efficiency ascribed to the formation of a robust, [FSA]^--originated solid-electrolyte interphase during cycling.

Application of Advanced Vibrational Spectroscopy in Revealing Critical Chemical Processes and Phenomena of Electrochemical Energy Storage and Conversion

The future of the energy industry and green transportation critically relies on exploration of high-performance, reliable, low-cost, and environmentally friendly energy storage and conversion materials. Understanding the chemical processes and phenomena involved in electrochemical energy storage and conversion is the premise of a revolutionary materials discovery. In this article, we review the recent advancements of application of state-of-the-art vibrational spectroscopic techniques in unraveling the nature of electrochemical energy, including bulk energy storage, dynamics of liquid electrolytes, interfacial processes, etc. Technique-wise, the review covers a wide range of spectroscopic methods, including classic vibrational spectroscopy (direct infrared absorption and Raman scattering), external field enhanced spectroscopy (surface enhanced Raman and IR, tip enhanced Raman, and near-field IR), and two-photon techniques (2D infrared absorption, stimulated Raman, and vibrational sum frequency generation). Finally, we provide perspectives on future directions in refining vibrational spectroscopy to contribute to the research frontier of electrochemical energy storage and conversion.

All alginate-derived high-performance T-Nb2O5/C//seaweed carbon Li-ion capacitors

Benefitting from the well-matched kinetics and capacity between the counter electrodes and the ion transportability of sodium alginate, the all alginate-derived T-Nb2O5/C//seaweed carbon Li-ion capacitor has excellent electrochemical performance.

How Do Oxygen Vacancies Influence the Catalytic Performance of Two-Dimensional Nb2 O5 in Lithium- and Sodium-Oxygen Batteries?

Alkali metal-oxygen batteries possess a higher specific capacity than alkali-ion batteries and stand out as the most competitive next-generation energy source. The core reaction mechanism of the battery is mainly the formation of alkali metal oxide during the discharge process and the decomposition of these oxides during the charge process. A large number of researchers have devoted themselves to seeking promising catalysts for the reaction. Two-dimensional Nb₂ O₅ was discovered to be a highly potential catalyst that can promote the reaction of alkali-metal-oxygen batteries, but few studies focus on it. In this study, the catalytic performance of both pristine Nb₂ O₅ and oxygen-deficiency modified Nb₂ O₅ was investigated. Furthermore, the effect of oxygen defects on catalytic performance was analyzed from multiple angles, namely, the reaction mechanism, d-band center theory, and the diffusion behavior of alkali metals. The exploration revealed the microscopic mechanism of oxygen deficiency affecting the alkali-metal battery reaction and provided a theoretical basis for quantitatively changing the d-band center of the catalyst through oxygen deficiency to ultimately change the performance of the catalyst.

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.

Investigation of phase transition employing strain mapping in TT- and T-Nb2O5 obtained by HRTEM micrographs

This work aimed to study the crystalline structure of TT-,T-phases of Nb₂O₅ nanoparticles through XRD, Rietveld refinement, and HRTEM, using geometric phase analysis (GPA). The results show the presence of distorted NbO₆ and NbO₇ polyhedral, producing strain effects, mainly in the plane boundaries and along the b-c plane. XRD and HRTEM analyses show the TT→T transition at 700 °C, with increased particle size and increased strain in the boundaries between nanoparticles. The sample calcinated at 700 °C presents segregation of the TT-(001), (100), and T-(130) planes, where the strain effect is more relevant along the [100] zone axis and between phases.

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.

Stimulating the Reversibility of Sb2 S3 Anode for High-Performance Potassium-Ion Batteries

Conversion-alloy sulfide materials for potassium-ion batteries (KIBs) have attracted considerable attention because of their high capacities and suitable working potentials. However, the sluggish kinetics and sulfur loss result in their rapid capacity degeneration as well as inferior rate capability. Herein, a strategy that uses the confinement and catalyzed effect of Nb₂ O₅ layers to restrict the sulfur species and facilitate them to form sulfides reversibly is proposed. Taking Sb₂ S₃ anode as an example, Sb₂ S₃ and Nb₂ O₅ are dispersed in the core and shell layers of carbon nanofibers (C NFs), respectively, constructing core@shell structure Sb₂ S₃ -C@Nb₂ O₅ -C NFs. Benefiting from the bi-functional Nb₂ O₅ layers, the electrochemical reversibility of Sb₂ S₃ is stimulated. As a result, the Sb₂ S₃ -C@Nb₂ O₅ -C NFs electrode delivers the rapidest K-ion diffusion coefficient, longest cycling stability, and most excellent rate capability among the controlled electrodes (347.5 mAh g^-1 is kept at 0.1 A g^-1 after 100 cycles, and a negligible capacity degradation (0.03% per cycle) at 2.0 A g^-1 for 2200 cycles is delivered). The enhanced K-ion storage properties are also found in SnS₂ -C@Nb₂ O₅ -C NFs electrode. Encouraged by the stimulated reversibility of Sb₂ S₃ and SnS₂ anodes, other sulfides with high electrochemical performance also could be developed for KIBs.

"Water in salt/ionic liquid" electrolyte for 2.8 V aqueous lithium-ion capacitor

Development of high-voltage electrolytes with non-flammability is significantly important for future energy storage devices. Aqueous electrolytes are inherently non-flammable, easy to handle, and their electrochemical stability windows (ESWs) can be considerably expanded by increasing electrolyte concentrations. However, further breakthroughs of their ESWs encounter bottlenecks because of the limited salt solubility, leading to that most of the high-energy anode materials can hardly function reversibly in aqueous electrolytes. Here, by introducing a non-flammable ionic liquid as co-solvent in a lithium salt/water system, we develop a "water in salt/ionic liquid" (WiSIL) electrolyte with extremely low water content. In such WiSIL electrolyte, commercial niobium pentoxide (Nb₂O₅) material can operate at a low potential (-1.6 V versus Ag/AgCl) and contribute its full capacity. Consequently, the resultant Nb₂O₅-based aqueous lithium-ion capacitor is able to operate at a high voltage of 2.8 V along with long cycling stability over 3000 cycles, and displays comparable energy and power performance (51.9 Wh kg^-1 at 0.37 kW kg^-1 and 16.4 Wh kg^-1 at 4.9 kW kg^-1) to those using non-aqueous electrolytes but with improved safety performance and manufacturing efficiency.

Pseudocapacitive Ti-Doped Niobium Pentoxide Nanoflake Structure Design for a Fast Kinetics Anode toward a High-Performance Mg-Ion-Based Dual-Ion Battery

Magnesium-ion batteries (MIBs) have received increasing attention for next-generation energy storage recently because of the natural abundance, high capacity, and dendrite-free deposition of Mg. However, their applications are hindered by irreversible Mg anode plating in conventional electrolytes and the lack of cathode materials, demonstrating high working voltage, satisfactory Mg²⁺ diffusivity, and long cycling life. In this work, we first developed a novel magnesium-ion based dual-ion battery (Mg-DIB) by utilizing expanded graphite as the cathode and Ti-doped niobium pentoxide nanoflakes (Ti-Nb₂O₅ NFs) as the anode. The Ti-Nb₂O₅ NFs showed hierarchical structures of microspheres with diameters of 4-5 μm assembled by nanoflakes. For the first time, the Mg-ion storage mechanism in Ti-Nb₂O₅ NFs was investigated. Benefiting from the hierarchical structure design and pseudocapacitive intercalation behavior of Mg ions, the Ti-Nb₂O₅ NF anode exhibited fast Mg-ion diffusion. Consequently, the Mg-DIB exhibited a high discharge capacity of 93 mA h g^-1 at 1 C (1 C corresponding to 100 mA g^-1), along with good long-term cycling performance with a capacity retention of 79% at 3 C after 500 cycles. The Mg-DIB also demonstrated a capacity retention of 77% at 5C, indicating its good rate performance. Moreover, the Mg-DIB exhibited a high discharge medium voltage of ∼1.83 V, thus enabling a high energy density of 174 W h kg^-1 at 183 W kg^-1 and 122 W h kg^-1 at a high power density of 845 W kg^-1, among the best of the reported magnesium-ion full batteries. Our work provides a new strategy to improve the performance of MIBs and other rechargeable batteries.

Dual redox mediators accelerate the electrochemical kinetics of lithium-sulfur batteries

The sluggish electrochemical kinetics of sulfur species has impeded the wide adoption of lithium-sulfur battery, which is one of the most promising candidates for next-generation energy storage system. Here, we present the electronic and geometric structures of all possible sulfur species and construct an electronic energy diagram to unveil their reaction pathways in batteries, as well as the molecular origin of their sluggish kinetics. By decoupling the contradictory requirements of accelerating charging and discharging processes, we select two pseudocapacitive oxides as electron-ion source and drain to enable the efficient transport of electron/Li⁺ to and from sulfur intermediates respectively. After incorporating dual oxides, the electrochemical kinetics of sulfur cathode is significantly accelerated. This strategy, which couples a fast-electrochemical reaction with a spontaneous chemical reaction to bypass a slow-electrochemical reaction pathway, offers a solution to accelerate an electrochemical reaction, providing new perspectives for the development of high-energy battery systems.

The sluggish electrochemical kinetics of sulfur species remains a major hurdle for the broad adoption of lithium-sulfur batteries. Here, the authors construct an energy diagram of sulfur species to unveil their reaction pathways and propose a general strategy to accelerate electrochemical reactions.

Facile formation of tetragonal-Nb2O5 microspheres for high-rate and stable lithium storage with high areal capacity

Niobium pentoxide (Nb₂O₅) has attracted great attention as an anode for lithium-ion battery, which is attributed to the high-rate and good stability performances. In this work, TT-, T-, M-, and H-Nb₂O₅ microspheres were synthesized by a facile one-step thermal oxidation method. Ion and electron transport properties of Nb₂O₅ with different phases were investigated by both electrochemical analyses and density functional theoretical calculations. Without nanostructuring and carbon modification, the tetragonal Nb₂O₅ (M-Nb₂O₅) displays preferable rate capability (121 mAh g^-1 at 5 A g^-1), enhanced reversible capacity (163 mAh g^-1 at 0.2 A g^-1) and better cycling stability (82.3% capacity retention after 1000 cycles) when compared with TT-, T-, and H-Nb₂O₅. Electrochemical analyses further reveal the diffusion-controlled Li⁺ intercalation kinetics and in-situ X-ray diffraction analysis indicates superior structural stability upon Li⁺ intercalation/deintercalation. Benefiting from the intrinsic fast ion/electron transport, a high areal capacity of 2.24 mAh cm^-2 is obtained even at an ultrahigh mass loading of 22.51 mg cm^-2. This work can promote the development of Nb₂O₅ materials for high areal capacity and stable lithium storage towards practical applications.

Controlled growth and ion intercalation mechanism of monocrystalline niobium pentoxide nanotubes for advanced rechargeable aluminum-ion batteries

Rechargeable aluminum-ion batteries (RAIBs) have attracted increasing attention owing to their high theoretical volumetric capacity, high resource abundance, and good safety performance. However, the existing RAIB systems usually exhibit relatively low specific capacities limited by the cathode materials. In this study, we developed a one-step chemical vapor deposition method to prepare single-crystal orthogonal Nb2O5 nanotubes for serving as high-performance electrode materials for RAIBs, showing a high reversible capability of 556 mA h g-1 at 25 mA g-1 and good thermal endurability at elevated temperatures (50 °C). A combination of a series of detailed ex situ structural characterization studies verified the reversible intercalation/deintercalation of chloroaluminate anions (AlCl4-) into/from the (001) planes of monocrystalline Nb2O5 nanotubes. It also revealed that the nanoarchitecture of Nb2O5 nanotubes with thin tube walls, hollow inner space and a short ion transport distance is conducive to the rapid kinetics of the insertion/extraction process. This work provides a promising route to design high-performance electrode materials based on transition metal compounds for RAIBs via the rational modulation of their structure and morphology.

Ultrafast and Stable Li-(De)intercalation in a Large Single Crystal H-Nb2 O5 Anode via Optimizing the Homogeneity of Electron and Ion Transport

Exploring anode materials with fast, safe, and stable Li-(de)intercalation is of great significance for developing next-generation lithium-ion batteries. Monoclinic H-type niobium pentoxide possesses outstanding intrinsic fast Li-(de)intercalation kinetics, high specific capacity, and safety; however, its practical rate capability and cycling stability are still limited, ascribed to the asynchronism of phase change throughout the crystals. Herein this problem is addressed by homogenizing the electron and Li-ion conductivity surrounding the crystals. An amorphous N-doped carbon layer is introduced on the micrometer single-crystal H-Nb₂ O₅ particle to optimize the homogeneity of electron and Li-ion transport. As a result, the as-prepared H-Nb₂ O₅ exhibits high reversible capacity (>250 mAh g^-1 at 50 mA g^-1 ), unprecedented high-rate performance (≈120 mAh g^-1 at 16.0 A g^-1 ) and excellent cycling stability (≈170 mAh g^-1 at 2.0 A g^-1 after 1000 cycles), which is by far the highest performance among the H-Nb₂ O₅ materials. The inherent principle is further confirmed via operando transmission electron microscopy and X-ray diffraction. A novel insight into the further development of electrode materials forlithium-ion batteries is thus provided.

Interlayer engineering of Ti3C2Tx MXenes towards high capacitance supercapacitors

Electrochemical pseudocapacitors store energy via intercalation or electrosorption and faradaic charge transfer with redox reactions. MXenes represent the promising intercalation pseudocapacitive electrode materials for supercapacitors due to their ultrahigh theoretical capacitances. Achieving a high capacitance will greatly advance the large-scale applications as in power grids. However, a rational design concept has not been exploited to achieve the theoretical limit. Here, we show how interlayer engineering helps to achieve the limit. Interlayer engineering in this manner simultaneously creates a broadened yet uniform interlayer spacing - providing a "highway" for fast ion diffusion, and incorporates heteroatoms with lower electronegativity - offering "trucks" (redox active sites) on such a "highway" for speeding charge transfer, enabling high capacitance. Following the concept, through annealing the as-prepared Ti3C2Tx MXene under an ammonia atmosphere, the engineered MXene delivers much improved capacitance with excellent rate performance and cyclability. The overall performance of the engineered MXene outperforms that of all other pseudocapacitive electrode materials.

Nanoscale Phenomena in Lithium-Ion Batteries

The electrochemical properties and performances of lithium-ion batteries are primarily governed by their constituent electrode materials, whose intrinsic thermodynamic and kinetic properties are understood as the determining factor. As a part of complementing the intrinsic material properties, the strategy of nanosizing has been widely applied to electrodes to improve battery performance. It has been revealed that this not only improves the kinetics of the electrode materials but is also capable of regulating their thermodynamic properties, taking advantage of nanoscale phenomena regarding the changes in redox potential, solid-state solubility of the intercalation compounds, and reaction paths. In addition, the nanosizing of materials has recently enabled the discovery of new energy storage mechanisms, through which unexplored classes of electrodes could be introduced. Herein, we review the nanoscale phenomena discovered or exploited in lithium-ion battery chemistry thus far and discuss their potential implications, providing opportunities to further unveil uncharted electrode materials and chemistries. Finally, we discuss the limitations of the nanoscale phenomena presently employed in battery applications and suggest strategies to overcome these limitations.