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

Three-dimensional hierarchical urchin-like TiNb2O7 microspheres encapsulated in N-doped carbon for high-rate lithium storage

Wadsley-Roth phase TiNb2O7 has been known as a potential anode material in high-power lithium-ion batteries (LIBs) owing to its enhanced safety, reliable cycling performance, and substantial theoretical capacity. However, its commercial application is restricted by poor electronic conductivity and slow Li-ion migration kinetics. Herein, we have developed urchin-structured TiNb2O7 microspheres with an ultrathin N-doped carbon coating through a two-step hydrothermal and polydopamine carbonization process. When used as LIBs anode, the TiNb2O7@N-C composite demonstrates a remarkable rate capability of 216 mAh g-1 at 30 C and a durable lifespan with a capacity retention of 89 % after 500 cycles (10 C). In addition, by pairing with LiNi0.5Mn1.5O4 cathode, the full cell also reveals outstanding electrochemical performance even at high rates. The enhanced lithium storage capabilities of TiNb2O7@N-C can be credited to the integrated superiorities of the multilevel urchin-like architecture and the external conductive layer, which shortens the Li+/electron transfer paths, buffer the volume fluctuations of active substance, and improves the overall conductivity of the composite electrode.

In-depth insight into the effects of oxygen vacancies on the excellent Li+-storage performances of Cu2Nb34O87-x/N-doped carbon composite

Wadsley-Roth phase niobates demonstrate significant Li+-storage advantages. Even though their Nb5+ can fully transform into Nb4+ during lithiation process, however, only partial Nb4+ can further convert to Nb3+, leading to much lower practical capacities than the theoretical values according to the two-electron transfer per Nb5+. The specific mechanism to improve their conversion ratio of Nb4+ to Nb3+ during lithiation process has rarely been reported so far. Herein, the ultrafine oxygen-deficient Cu2Nb34O87-x nanoparticles are closely connected by the N-doped carbon-based 3D conductive framework to form a cloud-like Cu2Nb34O87-x/N-doped carbon composite (denoted as VU-CNO-NC) with nanoaggregate structure and porous structure. Based on density functional theory (DFT) calculations and ex situ X-ray photoelectron spectrometer (XPS), the oxygen vacancies in VU-CNO-NC can catalyze the conversion of Nb4+ to Nb3+ during lithiation process, which significantly enhance the conversion ratio of Nb4+ to Nb3+ to generate much higher capacity. This effect of oxygen vacancies has rarely been reported so far. Moreover, the oxygen vacancies, ultrafine primary nanoparticles, 3D conductive framework, porous structure, and nanoaggregate structure synergistically endow VU-CNO-NC with fast Li+-storage kinetics and highly stable structure. Consequently, VU-CNO-NC not only shows high capacity (287 mAh g-1 after 500 cycles at 1 C and 181 mAh g-1 after 1000 cycles at 10 C) and excellent rate performance as anode material of lithium-ion batteries, but also endows hybrid lithium-ion capacitor with high energy density (126 Wh kg-1 at 175 W kg-1) and remarkable capacity retention (87.3 % after 9000 cycles at 2 A g-1), demonstrating great application prospect.

In Situ TEM Studies of Tunnel-Structured Materials for Alkali Metal-Ion Batteries

Tunnel-structured materials have garnered significant attention as promising candidates for high-performance rechargeable batteries, owing to their unique structural characteristics that facilitate efficient ionic transport. However, understanding the dynamic processes of ionic transport within these tunnels is crucial for their further development and performance optimization. Analytical in situ transmission electron microscopy (TEM) has demonstrated its effectiveness as a powerful tool for visualizing the complex ionic transport processes in real time. In this review, we summarize the state-of-the-art in situ tracking of ionic transport processes in tunnel-structured materials for alkali metal-ion batteries (AMIBs) by TEM observation at the atomic scale, elucidating the fundamental issues pertaining to phase transformations, structural evolution, interfacial reactions and degradation mechanisms. This review covers a wide range of electrode and electrolyte materials used in AMIBs, highlighting the versatility and general applicability of in situ TEM as a powerful tool for elucidating the fundamental mechanisms underlying the performance of AMIBs. Furthermore, this work critically discusses current challenges and future research directions, offering perspectives on the development of next-generation battery materials through advanced in situ characterization techniques.

Li+ Diffusion in LinCoNb2O6 (0 < n ≤ 6) Anode with High Capacity Density: Fast Kinetics and Mechanistic Insights

The potential of high power/capacity density and Li+ solid diffusion mechanisms of niobium-based binary metal oxide (CoNb2O6) anode material are investigated by combining high-rate Nb2O5 with the redox-active 3d transition metal Co. CoNb2O6 exhibited exceptional rate capability and cycling stability, which is attributed to anisotropic expansion during cycling and dual diffusion mechanisms at high and low lithium concentrations. The anisotropic expansion of crystals ensures structural stability, whereas the organic combination of a direct-hopping diffusion mechanism in LinCoNb2O6 (0 ≤ n ≤ 3) and a knock-off diffusion mechanism in LinCoNb2O6 (3 < n ≤ 6) based on the nudged elastic band (NEB) calculations reveals rapid Li+ solid diffusion and excellent rate performance during lithiation/delithiation. The electrochemical performance of CoNb2O6 also depends on its morphology, where different structures modulate synergistic Nb and Co interactions, influencing Li+ diffusion in the Nb layers. Specifically, the micron-scale structure formed by secondary particle attachment (CoNb2O6-MP) provides space for anisotropic expansion, fully utilizing the dual ion diffusion mechanism, enhancing diffusion efficiency, and delivering both high-capacity density and excellent rate performance. This work not only introduces CoNb2O6 with superior electrochemical properties but also provides insights into the solid diffusion mechanisms under various lithium concentrations, offering a foundation for designing electrode materials with enhanced ion diffusion pathways.

Improving the fast-charging capability of NbWO-based Li-ion batteries

The discovery of Nb-W-O materials years ago marks the milestone of charging a lithium-ion battery in minutes. Nevertheless, for many applications, charging lithium-ion battery within one minute is urgently demanded, the bottleneck of which largely lies in the lack of fundamental understanding of Li+ storage mechanisms in these materials. Herein, by visualizing Li+ intercalated into representative Nb16W5O55, we find that the fast-charging nature of such material originates from an interesting rate-dependent lattice relaxation process associated with the Jahn-Teller effect. Furthermore, in situ electron microscopy further reveals a directional, [010]-preferred Li+ transport mechanism in Nb16W5O55 crystals being the "bottleneck" toward fast charging that deprives the entry of any desolvated Li+ through the prevailing non-(010) surfaces. Hence, we propose a machine learning-assisted interface engineering strategy to swiftly collect desolvated Li+ and relocate them to (010) surfaces for their fast intercalation. As a result, a capacity of ≈ 116 mAh g-1 (68.5% of the theoretical capacity) at 80 C (45 s) is achieved when coupled with a Li negative electrode.

Advances on Defect Engineering of Niobium Pentoxide for Electrochemical Energy Storage

The reasonable design of advanced anode materials for electrochemical energy storage (EES) devices is crucial in expediting the progress of renewable energy technologies. Nb2O5 has attracted increasing research attention as an anode candidate. Defect engineering is regarded as a feasible approach to modulate the local atomic configurations within Nb2O5. Therefore, introducing defects into Nb2O5 is considered to be a promising way to enhance electrochemical performance. However, there is no systematic review on the defect engineering of Nb2O5 for the energy storage process. This review systematically analyzes first the crystal structures and energy storage mechanisms of Nb2O5. Subsequently, a systematical summary of the latest advances in defect engineering of Nb2O5 for EES devices is presented, mainly focusing on vacancy modulation, ion doping, planar defects, introducing porosity, and amorphization. Of particular note is the effects of defect engineering on Nb2O5: improving electronic conductivity, accelerating ion diffusion, maintaining structural stability, increasing active storage sites. The review further summarizes diverse methodologies for inducing defects and the commonly used techniques for the defect characterization within Nb2O5. In conclusion, the article proposes current challenges and outlines future development prospects for defect engineering in Nb2O5 to achieve high-performance EES devices with both high energy and power densities.

Tungsten Bronze W3Nb2O14 Nanorod: The Low-Strain and Preferred Orientation Enables Long-Life for High-Rate Lithium-Ion Storage

Tungsten bronze oxides have emerged as attractive materials for energy storage owing to their fast charge-discharge property. However, the internal weakness of low capacity and short cycling performance impedes their development in wide application. In this work, the tungsten bronze W3Nb2O14 nanorods with preferred orientation (001) were prepared by hydrothermal method for the first time. It not only displays high-rate performance but also exhibits high capacity (201 mA h g-1 at 0.2 C) and long cycling property (80% capacity retention at 50 C after 3000 cycles). Notably, in situ XRD revealed the c evolution frustrates the V increase, and the maximum unit-cell volume expansion is only 2.9% during lithiation/delithiation, reflecting the long cycle performance is benefit from the low strain and preferred orientation. The ex-situ 6Li NMR spectra revealed that the lithiation and delithiation present preference based on the size of tunnels in the crystal framework.

Empowering lithium-ion storage: unveiling the superior performance of niobium-based oxide/perovskite heterojunction with built-in electric field

The increasing demand for high-performance electrode materials in lithium-ion batteries has driven significant attention towards Nb2O5 due to its high working voltage, large theoretical capacity, environmental friendliness, and cost-effectiveness. However, inherent drawbacks such as poor electrical conductivity and sluggish electrochemical reaction kinetics have hindered its lithium storage performance. In this study, we introduced KCa2Nb3O10 into Nb2O5 to form a heterojunction, creating a built-in electric field to enhance the migration and diffusion of Li+, effectively promoting electrochemical reaction kinetics. Under the regulation of the built-in electric field, the charge transfer resistance of the KCa2Nb3O10/Nb2O5 anode decreased by 3.4 times compared to pure Nb2O5, and the Li+ diffusion coefficient improved by two orders of magnitude. Specifically, the KCa2Nb3O10/Nb2O5 anode exhibited a high capacity of 276 mAh g-1 under 1 C, retaining a capacity of 128 mAh g-1 even at 100 C. After 3000 cycles at 25 C, the capacity degradation was only 0.012% per cycle. Through combined theoretical calculations and experimental validation, it was found that the built-in electric field induced by the heterojunction interface contributed to an asymmetric charge distribution, thereby improving the rates of charge and ion migration within the electrode, ultimately enhancing the electrochemical performance of the electrode material. This study provides an effective approach for the rational design of high-performance electrode materials.

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.

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.

Micron-Sized Cobalt Niobium Oxide with Multiscale Porous Sponge-Like Structure Boosting High-Rate and Long-Life Lithium Storage

Niobium-based oxides show great potential as intercalation-type anodes in lithium-ion batteries due to their relatively high theoretical specific capacity. Nevertheless, their electrochemical properties are unsatisfactorily restricted by the poor electronic conductivity. Here, micron-sized Co0.5Nb24.5O62 with multiscale sponge-like structure is synthesized and demonstrated to be a fast-charging anode material. It can deliver a remarkable capacity of 287 mA h g-1 with a safe average working potential of ≈1.55 V vs Li+/Li and a high initial Coulombic efficiency of 91.1% at 0.1C. Owing to the fast electronic/ionic transport derived from the multiscale porous sponge-like structure, Co0.5Nb24.5O62 exhibits a superior rate capability of 142 mA h g-1 even at 10C. In addition, its maximum volume change during the charge/discharge process is determined to be 9.18%, thus exhibiting excellent cycling stability with 75.3% capacity retention even after 3000 cycles at 10C. The LiFePO4//Co0.5Nb24.5O62 full cells also achieve good rate performance of 101 mA h g-1 at 10C, as well as an excellent cycling performance of 81% capacity retention after 1200 cycles at 5C, further proving the promising application prospect of Co0.5Nb24.5O62.

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.

FeNbO4 nanochains with a five-electron transfer reaction toward high capacity and fast Li storage

High capacity and outstanding rate performance of the FeNbO4 nanochain anode with both intercalation and conversion reactions for lithium-ion batteries are demonstrated. The unique one-dimensional structure and intercalation pseudocapacitive behavior of FeNbO4 accelerate the reaction kinetics. In situ X-ray diffractometer measurement confirms a five-electron transfer mechanism for Li storage.

Emerging Multiscale Porous Anodes toward Fast Charging Lithium-Ion Batteries

With the accelerated penetration of the global electric vehicle market, the demand for fast charging lithium-ion batteries (LIBs) that enable improvement of user driving efficiency and user experience is becoming increasingly significant. Robust ion/electron transport paths throughout the electrode have played a pivotal role in the progress of fast charging LIBs. Yet traditional graphite anodes lack fast ion transport channels, which suffer extremely elevated overpotential at ultrafast power outputs, resulting in lithium dendrite growth, capacity decay, and safety issues. In recent years, emergent multiscale porous anodes dedicated to building efficient ion transport channels on multiple scales offer opportunities for fast charging anodes. This review survey covers the recent advances of the emerging multiscale porous anodes for fast charging LIBs. It starts by clarifying how pore parameters such as porosity, tortuosity, and gradient affect the fast charging ability from an electrochemical kinetic perspective. We then present an overview of efforts to implement multiscale porous anodes at both material and electrode levels in diverse types of anode materials. Moreover, we critically evaluate the essential merits and limitations of several quintessential fast charging porous anodes from a practical viewpoint. Finally, we highlight the challenges and future prospects of multiscale porous fast charging anode design associated with materials and electrodes as well as crucial issues faced by the battery and management level.

Pt-Loaded Nb─W Metal Composite Oxide for Selective Cleavage of Secondary C─O Bonds

Selective hydrogenolysis of glycerol to 1,3-propanediol (1,3-PDO) is recognized as one of the most promising reactions for the valorization of biomass. Precise activation of C─O bonds of glycerol molecule is the key step to realize the high yield of catalytic conversion. Here, a Pt-loaded Nb-W composite oxides with crystallographic shear phase for the precise activation and cleavage of secondary C─O (C(2)─O) bonds are first reported. The developed Nb14 W3 O44 with uniform structure possesses arrays of W-O-Nb active sites that totally distinct from individual WOx or NbOx species, which is superior to the adsorption and activation of C(2)─O bonds. The Nb14 W3 O44 support with rich reversible redox couples also promotes the electron feedback ability of Pt and enhances its interaction with Pt nanoparticles, resulting in high activity for H2 dissociation and hydrogenation. All these favorable factors confer the Pt/Nb14 W3 O44 excellent performance for selective hydrogenolysis of glycerol to 1,3-PDO with the yield of 75.2% exceeding the record of 66%, paying the way for the commercial development of biomass conversion. The reported catalysts or approach can also be adopted to create a family of Nb-W metal composite oxides for other catalytic reactions requiring selective C─O bond activation and cleavage.

Micrometer-scale single crystalline particles of niobium titanium oxide enabling an Ah-level pouch cell with superior fast-charging capability

Wadsley-Roth phase niobium titanium oxide (TiNb2O7) is widely regarded as a promising anode candidate for fast-charging lithium-ion batteries due to its safe working potential and doubled capacity in comparison to the commercial fast-charging anode material (lithium titanium oxide, Li4Ti5O12). Although good fast charge/discharge performance was shown for nanostructured TiNb2O7, the small size would cause the low electrode compensation density and energy density of batteries, as well as parasitic reactions. Fundamental understanding of the electrochemical lithium insertion/extraction process and the structural evolution for the micrometer-scale single crystalline TiNb2O7 (MSC-TiNb2O7) could provide insights to understand its inherent properties and possibility for fast-charging application. Here, we revealed the highly reversible structural evolution of the MSC-TiNb2O7 during the lithiation/delithiation processes. Interestingly, an ion-conductive lithium niobate interphase was in situ formed on the MSC-TiNb2O7 surface during the formation cycle, which could facilitate fast ion diffusion on the material surface and support fast electrochemical reaction kinetics. Experimentally, the MSC-TiNb2O7 delivered a high reversible capacity of 291.9 mA h g-1 at 0.5C with a high initial Coulombic efficiency (>95%), and showed superb rate capability with a reasonable capacity of 55.6 mA h g-1 under a high current density of 40C. An Ah-level pouch cell with a lithium cobalt oxide (LiCoO2) cathode exhibited 91.5% capacity retention at 3C charging rate, which revealed the significant role of high crystallinity and in situ formation of an ion conductive nano-interphase in realizing fast charging capability of practical TiNb2O7-based lithium-ion 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.

Fast and Durable Lithium Storage Enabled by Tuning Entropy in Wadsley-Roth Phase Titanium Niobium Oxides

Wadsley-Roth phase titanium niobium oxides have received considerable interest as anodes for lithium ion batteries. However, the volume expansion and sluggish ion/electron transport kinetics retard its application in grid scale. Here, fast and durable lithium storage in entropy-stabilized Fe_0.4 Ti_1.6 Nb₁₀ O_28.8 (FTNO) is enabled by tuning entropy via Fe substitution. By increasing the entropy, a reduction of the calcination temperature to form a phase pure material is achieved, leading to a reduced grain size and, therefore, a shortening of Li⁺ pathway along the diffusion channels. Furthermore, in situ X-ray diffraction reveals that the increased entropy leads to the decreased expansion along a-axis, which stabilizes the lithium intercalation channel. Density functional theory modeling indicates the origin to be the more stable FeO bond as compared to TiO bond. As a result, the rate performance is significantly enhanced exhibiting a reversible capacity of 73.7 mAh g^-1 at 50 C for FTNO as compared to 37.9 mAh g^-1 for its TNO counterpart. Besides, durable cycling is achieved by FTNO, which delivers a discharge capacity of 130.0 mAh g^-1 after 6000 cycles at 10 C. Finally, the potential impact for practical application of FTNO anodes has been demonstrated by successfully constructing fast charging and stable LiFePO₄ ‖FTNO full cells.

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

Given the inherent features of open tunnel-like structures, moderate lithiation potential (1.0-3.0 V vs Li/Li⁺), and reversible redox couples (Nb⁵⁺/Nb⁴⁺ and Nb⁴⁺/Nb³⁺ redox couples), niobium-based oxides with Wadsley-Roth crystallographic shear structure are promising anode materials. However, their practical rate capability and cycling stability are still hindered by low intrinsic electronic conductivity and structural stability. Herein, ultrathin carbon-confined Nb₁₂O₂₉ materials with rich oxygen vacancies (Nb₁₂O_29-x@C) were designed and synthesized to address above-mentioned challenges. Computational simulations combined with experiments reveal that the oxygen vacancies can regulate the electronic structure to increase intrinsic electronic conductivity and reduce the Li⁺ diffusion barrier. Meanwhile, the carbon coating can enhance structural stability and further improve the electronic conductivity of the Nb₁₂O₂₉ material. As a result, the as-prepared Nb₁₂O_29-x@C exhibits high reversible capacity (226 mAh g^-1 at 0.1 A g^-1), excellent high-rate performance (83 mAh g^-1 at 5.0 A g^-1), and durable cycling life (98.1% capacity retention at 1.0 A g^-1 after 3000 cycles). The lithium storage mechanism and structural stability of Nb₁₂O_29-x@C were also revealed by in situ X-ray diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS), and ex situ Raman spectroscopy. When applied as the anode of lithium-ion capacitors (LICs), the as-built LIC achieves high energy density (72.4 Wh kg^-1) within the voltage window of 0.01-3.5 V, demonstrating the practical application potential of the Nb₁₂O_29-x@C materials.

Ion Flux Self-Regulation Strategy with a Volume-Responsive Separator for Lithium Metal Batteries

Lithium metal batteries (LMBs) are regarded as one of the most promising next-generation energy storage devices due to their high energy density. However, the conversion of LMBs from laboratory to factory is hindered by the formation of lithium dendrites and volume change during lithium stripping and deposition processes. In this work, a volume-responsive separator with core/shell structure thermoplastic polyurethane (TPU)/polyvinylidene fluoride (PVDF) fibers and SiO₂ coating layers is designed to restrict dendrite growth. The TPU/PVDF-SiO₂ separator can accommodate the volume change like an artificial lung and keep intimate contact with the electrodes, which leads to the formation of a uniform and high-density solid-electrolyte interphase. Meanwhile, the separator can regulate the transport channels and diffusion coefficients (D) of lithium ions with the change of porosity from both experimental and ab initio molecular dynamic analysis. The Li symmetric cells assembled with the TPU/PVDF-SiO₂ can run for 1000 h at the current of 1.0 mA cm^-2 without a short circuit. Moreover, the low melting point of PVDF can shut the ionic conduction down at 170 °C, guaranteeing the thermal safety of the batteries. With the above advantages, the TPU/PVDF-SiO₂ separator presents great potential to promote the commercial and industrial application of LMBs.

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.

Optimization of Electrode and Cell Design for Ultrafast-Charging Lithium-Ion Batteries Based on Molybdenum Niobium Oxide Anodes

Niobium oxides are an emerging class of anode materials for use in high-power lithium-ion batteries. Galvanostatic cycling and electrochemical impedance spectroscopy (EIS) were used in this study to investigate the influence of electrode porosity, electrode mass ratio, and cycling rate on the capacity, cycle life, and ionic conductivity of Li-ion battery cells based on a modified micron-sized MoNb₁₂O₃₃ (MNO) anode powder. Both electrode and cell designs were found to have a significant impact on the rate performance and cycle life of Li-ion half- and full cells. A higher specific capacity, improved rate performance, and a longer cycle life were obtained in both anode and cathode half-cells by lowering the electrode porosity through calendaring. MNO/Li half-coin cells displayed excellent cyclability, reaching 80% state of health (SOH) after 600 cycles at C/2 charge and 1C discharge. MNO/NMC622 full-coin cells displayed a high capacity of 179 mAh g^-1 at 100 mA g^-1 (0.5 mA cm^-2) and excellent cyclability at 25 °C, reaching 70% SOH after over 1000 cycles at 1 mA cm^-2 after optimizing their N/P ratio. Excellent cyclability was obtained at both 1C/1C and fast 2C/2C cycling, reaching 80% SOH after 700 and 470 cycles, respectively. Full-coin and small pouch cells had outstanding rate performance as they could be charged from 0 to 84% capacity in less than 5 min at 10 mA cm^-2 and to 70% SOC in 120 s at 20 mA cm^-2.

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.

Optimized Lithography-Free Fabrication of Sub-100 nm Nb2O5 Nanotube Films as Negative Supercapacitor Electrodes: Tuned Oxygen Vacancies and Cationic Intercalation

The direct growth of sub-100 nm thin-film metal oxides has witnessed a sustained interest as a superlative approach for the fabrication of smart energy storage platforms. Herein, sub-100 nm Zr-doped orthorhombic Nb₂O₅ nanotube films are synthesized directly on the Nb-Zr substrate and tested as negative supercapacitor electrode materials. To boost the pseudocapacitive performance of the fabricated films, supplement Nb⁴⁺ active sites (defects) are subtly induced into the metal oxide lattice, resulting in 13% improvement in the diffusion current at 100 m V/s over that of the defect-free counterpart. The defective sub-100 nm film (H-NbZr) exhibits areal and volumetric capacitances of 6.8 mF/cm² and 758.3 F/cm³, respectively. The presence of oxygen-deficient states enhances the intrinsic conductivity of the thin film, resulting in a reduction in the band gap energy from 3.25 to 2.5 eV. The assembled supercapacitor device made of nitrogen-doped activated carbon (N-AC) and H-NbZr (N-AC//H-NbZr) is able to retain 93, 83, 78, and 66% of its first cycle capacitance after 1000, 2000, 3000, and 4500 successive charge/discharge cycles, respectively. An eminent energy record of approximately 0.77 μW h/cm² at a power of 0.9 mW/cm² is achieved at 1 mA/cm² with superb capability.

Single-Crystal Nano-Subunits Assembled Accordion-Shape WNb2 O8 Framework with High Ionic/Electronic Conductivities towards Li-Ion Capacitors

Recently, Li-ion capacitors (LICs) have drawn tremendous attention due to their high energy/power density along with long cycle life. Nevertheless, the slow kinetics and stability of the involved anodes as bottleneck barriers always result in the modest properties of devices. The exploration of advanced anodes with both high ionic and electronic conductivities as well as structural stability thus becomes more significant for practical applications of LICs. Herein, a single-crystal nano-subunits assembled hierarchical accordion-shape WNb₂ O₈ micro-/nano framework is first designed via a one-step scalable strategy with the multi-layered Nb₂ CT_x as a precursor. The underlying solid solution Li-storage mechanism of the WNb₂ O₈ just with a volumetric expansion of ≈1.5% is proposed with in situ analysis. Benefiting from congenitally crystallographic merits, single-crystalline characteristic, and open accordion-like architecture, the resultant WNb₂ O₈ as a robust anode platform is endowed with fast electron/ion transport capability and multi-electron redox contributions from W/Nb, and accordingly, delivers a reversible capacity of ≈135.5 mAh g^-1 at a high rate of 2.0 A g^-1 . The WNb₂ O₈ assembled LICs exhibit an energy density of ≈33.0 Wh kg^-1 at 9 kW kg^-1 , coupled with remarkable electrochemical stability. The work provides meaningful insights into the rational design and construction of advanced bimetallic niobium oxides for next-generation LICs.

Superimposed Effect of La Doping and Structural Engineering to Achieve Oxygen-Deficient TiNb2O7 for Ultrafast Li-Ion Storage

TiNb₂O₇ (TNO) is a competitive candidate of a fast-charging anode due to its high specific capacity. However, the insulator nature seriously hinders its rate performance. Herein, the La³⁺-doped mesoporous TiNb₂O₇ materials (La-M-TNO) were first synthesized via a facile one-step solvothermal method with the assistance of polyvinyl pyrrolidone (PVP). The synergic effect of La³⁺ doping and the mesoporous structure enables a dual improvement on the electronic conductivity and ionic diffusion coefficient, which delivers an impressive specific capacity of 213 mAh g^-1 at 30 C. The capacity retention (@30C/@1C) increases from 33 to 53 and 74% for TNO, M-TNO, and La-M-TNO (0.03), respectively, demonstrating a step-by-step improvement of rate performance by making porous structures and intrinsic conductivity enhancement. DFT calculations verify that the enhancement in electronic conductivity due to La³⁺ doping and oxygen vacancy, which induce localized energy levels via slight hybridization of O 2p, Ti 3d, and Nb 4d orbits. Meanwhile, the GITT result indicates that PVP-induced self-assembly of TNO accelerates the lithium ion diffusion rate by shortening the Li⁺ diffusion path. This work verifies the effectiveness of the porous structure and highlights the significance of electronic conductivity to rate performance, especially at >30C. It provides a general approach to low-conductivity electrode materials for fast Li-ion storage.

Synchronous Manipulation of Ion and Electron Transfer in Wadsley-Roth Phase Ti-Nb Oxides for Fast-Charging Lithium-Ion Batteries

Implementing fast-charging lithium-ion batteries (LIBs) is severely hindered by the issues of Li plating and poor rate capability for conventional graphite anode. Wadsley-Roth phase TiNb2 O7 is regarded as a promising anode candidate to satisfy the requirements of fast-charging LIBs. However, the unsatisfactory electrochemical kinetics resulting from sluggish ion and electron transfer still limit its wide applications. Herein, an effective strategy is proposed to synchronously improve the ion and electron transfer of TiNb2 O7 by incorporation of oxygen vacancy and N-doped graphene matrix (TNO- x @N-G), which is designed by combination of solution-combustion and electrostatic self-assembly approach. Theoretical calculations demonstrate that Li+ intercalation gives rise to the semi-metallic characteristics of lithiated phases (Liy TNO- x ), leading to the self-accelerated electron transport. Moreover, in situ X-ray diffraction and Raman measurements reveal the highly reversible structural evolution of the TNO- x @N-G during cycling. Consequently, the TNO- x @N-G delivers a higher reversible capacity of 199.0 mAh g-1 and a higher capacity retention of 86.5% than those of pristine TNO (155.8 mAh g-1 , 59.4%) at 10 C after 2000 cycles. Importantly, various electrochemical devices including lithium-ion full battery and hybrid lithium-ion capacitor by using the TNO- x @N-G anode exhibit excellent rate capability and cycling stability, verifying its potential in practical applications.

High-pressure Bandgap Engineering and Amorphization in TiNb2O7 Single Crystals

Titanium niobate (TiNb2O7) possesses excellent photocatalytic, dielectric properties, and lithium-insertion capacity. And high-pressure (HP) is a powerful tool for bandgap engineering aiming at widening their applications. Herein, we report the...

Ten Thousand-Cycle Ultrafast Energy Storage of Wadsley-Roth Phase Fe-Nb Oxides with a Desolvation Promoting Interfacial Layer

Developing advanced electrode materials with enhanced charge-transfer kinetics is the key to realizing fast energy storage technologies. Commonly used modification strategies, such as nanoengineering and carbon coating, are mainly focused on electron transfer and bulk Li⁺ diffusion. Nonetheless, the desolvation behavior, which is considered as the rate-limiting process for charge-storage, is rarely studied. Herein, we designed a nitridation layer on the surface of Wadsley-Roth phase FeNb₁₁O₂₉ (FNO_-x@N) to act as a desolvation promoter. Theoretical calculations demonstrate that the adsorption and desolvation of solvated Li⁺ is efficiently improved at FNO_-x@N/electrolyte interphase, leading to the reduced desolvation energy barrier. Moreover, the nitridation layer can also help to prevent solvent cointercalation during Li⁺ insertion, leading to advantageous shrinkage of block area and reduced volume change of lattice cell during cycling. Consequently, FNO_-x@N exhibits a high-rate capacity of 129.7 mAh g^-1 with negligible capacity decay for 10 000 cycles.