Polymorphs of Nb₂O₅ Compound and Their Electrical Energy Storage Applications

Abstract

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.

1. Introduction

Recently, enthusiasm surrounding research on the structure, properties, and applications of Nb₂O₅ has grown. In particular, its potential use as catalysts, photocatalysts, supercapacitors (SCs), lithium-ion batteries (LIBs), etc., has been investigated. In nature, niobium (Nb) does not occur in a free state but rather is typically found in a mineral form, (Fe, Mn)Nb₂O₆, known as columbite [1,2,3]. Research on Nb₂O₅ commenced in the 1940s, with its crystal form the first to be evaluated [4]. The historical research progress on Nb₂O₅ is shown in Figure 1. Nb₂O₅ exhibits plentiful polymorphs linked to NbO₆ or NbO₇ polyhedra by corner- or edge-sharing forms, leading to exceptional properties and various applications [1,2]. As a white powder, Nb₂O₅ exhibits redox properties and is non-toxic and insoluble in water and acid but soluble in molten potassium bisulfate, alkali metal carbonates, and hydroxides [5].

Besides its structure, Nb₂O₅ has garnered significant attention due to its morphology, size, and corresponding properties. The nanostructured Nb₂O₅ often has distinct physical and chemical properties compared to its bulk forms due to its quantum effect and high specific surface area ratio [6]. Nb₂O₅ nanostructures and thin films have undergone thorough investigation as promising electrode materials for LIBs and SCs. Nb₂O₅ has charge/discharge plateaus (1–2 V vs. Li⁰/Li⁺) and two redox couples (Nb⁵⁺/Nb⁴⁺ and Nb⁴⁺/Nb³⁺), resulting in higher specific capacities [7]. Orthorhombic Nb₂O₅ possesses a two-dimensional Li-ion transport pathway without kinetics limitations, facilitating a rapid charge rate [8]. As such, research has focused considerably on controlling the crystal phase, nanostructured Nb₂O₅ synthesis, and composite formation [9].

This article provides a comprehensive overview of Nb₂O₅, encompassing its crystal structure, preparation techniques, fundamental properties, and diverse applications. It places a particular emphasis on the in-depth exploration of Nb₂O₅’s usage in LIBs and SCs. The final section discusses the potential for further research on Nb₂O₅ to expand its application scope.

2. Polymorphs of Nb₂O₅ and Synthesis Methods

The subsequent section provides a detailed description of the fundamental properties of Nb₂O₅, encompassing its synthesis methods, crystal structure, and product morphology. An overview of the physical and chemical properties of Nb₂O₅ is also provided, covering the fundamental aspects of this material.

2.1. Polymorphs of Nb₂O₅

Nb₂O₅ is a white crystal or powder with a wide band gap that boasts a relative density of 4.6 and an exceptionally high melting point of 1520 °C. Upon heating, it becomes yellow and exhibits solubility in hydrofluoric acid and sulfuric acid while remaining insoluble in water. Nb₂O₅ has diverse physical and chemical properties, making it highly valuable in various applications. Meanwhile, it remains stable in air, insoluble in water, complex in structure, and exhibits extensive polymorphism [1]. In recent decades, researchers have identified more than 15 distinct crystalline phases of Nb₂O₅ (as shown in

Table 1. Classification of Nb₂O₅ polymorphs.

Polymorphism	Designation of the Nb2O5 Form	Crystallization Temperature (K)	Ref.
TT-Nb2O5	Tief-tief	773–873	[11]
T-Nb2O5	Tief (γ)	873–1073	[12,13]
B-Nb2O5	Blätter (ζ)	1023–1123	[14]
M-Nb2O5	Medium (β)	1173–1223	[15]
H-Nb2O5	High (α)	1273	[16]
N-Nb2O5	Needles	1103	[17]
P-Nb2O5	Prisms (η)	1023	[11]
R-Nb2O5	neutral	-	[18]
ε-Nb2O5	-	1708	[11]
Nb2O5-I-high	-	1558	[11]
Nb2O5-II	-	1153–1223	[11]
The oxI to oxVI Forms of—Nb2O5	oxidation	1573	[11]

), including TT-, T-, B-, M-, N-, P-, R-, and H-Nb₂O₅. These diverse configurations have unique properties and can be utilized in a wide range of applications [10].

However, not all crystal phases of Nb₂O₅ are equally prevalent. The most common include TT-Nb₂O₅, T-Nb₂O₅, and H-Nb₂O₅; their corresponding lattice parameters are shown in

Table 2. Structure parameter of various Nb₂O₅ structures.

Materials	Crystal Structure	Cell Parameter	Space Group	Ref.
TT-Nb2O5	Pseudohexagonal	a = b = 3.607 Å/a = b = 3.600 Å c = 3.925 Å/c = 3.919 Å	P6/mmm (No. 191)	[12,20]
T-Nb2O5	Orthorhombic	a = 6.75 Å/a = 6.144 Å b = 29.175 Å/b = 29.194 Å c = 3.930 Å/c = 3.940 Å	Pbam (No. 55)	[12,13]
M-Nb2O5	Tetragonal	a = b = 20.44 Å c = 3.832 Å	I4/mmm (No. 139)	[21]
H-Nb2O5	Monoclinic	a = 21.153 Å/a = 21.163 Å b = 3.8233 Å/b = 3.824 Å c = 19.356 Å/c = 19.355 Å	P2/m (No. 10)	[13,21]

. These crystal structures possess distinct characteristics and can be utilized in various applications [1,19]. Notably, in certain articles, the crystal phases of Nb₂O₅ have been re-named as γ (i.e., T), β (i.e., M), and α (i.e., H) [1].

Amorphous Nb₂O₅ is typically obtained through various low-temperature synthesis methods and can be subsequently crystallized into either the TT or T phase at approximately 500 °C (Figure 2). At intermediate temperatures (~800 °C), the material can transform into the M phase (tetragonal), while the H phase forms at ≥1000 °C. It is important to note that, besides temperature, other factors can also influence the formation of Nb₂O₅ crystals. The phase stability is affected by environmental temperature and pressure, while the existence of polymorphs depends on the heating and preparation methods [22]. However, the nature of the starting material and the presence of impurities are also important factors affecting the formation of Nb₂O₅ crystals [11,23]. As a result, temperature should be considered an indicative factor rather than the sole determinant of the crystalline phase of Nb₂O₅.

The different polymorphs of Nb₂O₅ include distorted octahedra (NbO₆); the degree of distortion is influenced by the type of connection between the octahedra, whether through edges, angles or a combination of both [24]. Schäfer et al. [11] identified multiple modes of attachment between the octahedra while maintaining an O/Nb ratio of 2.5, confirming the existence of various arrangements of Nb₂O₅ and polycrystalline forms of niobium oxide. Nb atoms exhibit 6-fold (NbO₆) and 7-fold (NbO₇) coordination in the T and TT phases, along with distorted octahedral and pentagonal bipyramidal sites.

Undoubtedly, different crystalline phases of Nb₂O₅ exhibit different properties. For instance, the dielectric constant of the H phase can reach 100. The T-phase has applications in electrochemistry owing to its excellent electrochemical properties, including its resistance to reactions with other substances and cyclic stability. Extensive research has also been conducted to assess the application of the TT phase as an electrochromic material.

2.1.1. T- Nb₂O₅

The T-Nb₂O₅ phase possesses an orthorhombic crystal structure with a Pbam space group. In its conventional cell, it comprises 16.8 Nb and 42 O atoms. Among these, sixteen Nb ions occupy four Wyckoff positions 8i with half occupancy, while eleven O atoms are distributed across one 2b, four 4g, and six 4h positions. The remaining 0.8 Nb atoms exhibit random occupancies of 0.08, 0.08, and 0.04 across three 4g Wyckoff positions. The orthorhombic T-Nb₂O₅ phase comprises Nb atoms surrounded by six or seven oxygen atoms, forming twisted octahedral or pentagonal bipyramidal shapes [25] (Figure 3a). In the T and TT phases, Nb atoms have six-fold (NbO₆) and seven-fold (NbO₇) coordination, which can lead to the formation of distorted octahedrons and pentagonal bipyramids, respectively [24] (Figure 3a,d).

Serghiou et al. first discovered that the T phase of Nb₂O₅ becomes amorphous under certain conditions, including pressure and temperatures up to 19.2 GPa and 300 K, respectively. During this amorphous process, the oxide becomes amorphous and reduced [26].

In the 2010s, T-Nb₂O₅ gradually became a research hotspot. Li et al. reported a chemical method without catalyst topology, combined with molten salt synthesis (MSS), to achieve the large-scale synthesis of rod-like H-Nb₂O₅ and sheet-like T-Nb₂O₅ single crystals. Subsequently, Raman spectroscopy, SEM, X-ray diffraction, and TEM were employed to investigate the structural changes involved in the process [27].

In the mid-2010s, research on applying T-Nb₂O₅ intercalated pseudocapacitors gained popularity. Augustyn et al. recently demonstrated that mesoporous and nanocrystalline films of T-Nb₂O₅ (i.e., trapezoidal Nb₂O₅) exhibit behavior consistent with pseudocapacitance upon inserting lithium ions [28]. Meanwhile, Kong et al. employed a hydrothermal synthesis technique to anchor T-Nb₂O₅ nanocrystals onto conductive graphene sheets, fabricating asymmetric SCs. The T-Nb₂O₅/graphene nanocomposites and mesoporous carbon were used as negative/positive electrodes and exhibited high-rate responses, significantly enhancing the performance of SCs [29]. According to Lim et al., The Nb₂O₅@C NCs core-shell nanocrystals exhibit excellent electrochemical performance. More specifically, the core-shell structure provides a high surface area and efficient electron/ion transport pathways, with the carbon shell acting as a buffer to accommodate volume changes during cycling. The one-pot synthesis method offers a simple and scalable approach for producing high-performance anode materials for next-generation LIBs [30].

In the late 2010s, T-Nb₂O₅ gained attention as a promising anode material for sodium-ion batteries (NIBs) due to its unique surface frame and large interplanar lattice spacing [31]. In addition, research into applying T-Nb₂O₅ in potassium-ion batteries (KIBs) began. For the first time, Li et al. studied nanostructured T-Nb₂O₅ as a cathode material for KIBs [32].

During the late 2010s and early 2020s, most studies focused on modifying the T-Nb₂O₅ anode and cathode of hybrid supercapacitor (HSC) or optimizing synthesis methods to improve its defects. Additionally, the application of T-Nb₂O₅ in lithium-sulfur batteries was explored. Wang et al. [33] reported on a novel niobium oxide matrix that incorporates amorphous/crystalline hetero-conjunctions (A/T-Nb₂O₅), serving as a two-in-one host for a Li-S system. This matrix significantly improves the electrochemical performance of the Li-S system, with A-Nb₂O₅ providing a high surface area for sulfur adsorption and T-Nb₂O₅ offering a stable framework for sulfur confinement. The amorphous/crystalline hetero-conjunctions further enhance the conductivity of the matrix and facilitate the transport of lithium ions, presenting a promising strategy for developing high-performance electrode materials in next-generation energy storage systems.

2.1.2. TT-Nb₂O₅

At low temperatures, the two phases of Nb₂O₅, TT, and T, share many similarities. For example, the X-ray diffraction patterns of the TT and T phases of Nb₂O₅ exhibit remarkable resemblance (Figure 4a,b). However, a key distinguishing factor is that the XRD pattern of the TT-Nb₂O₅ phase includes a peak corresponding to a split reflection, which is not present in the T-Nb₂O₅ phase [1,23]. The emergence of the TT phase of Nb₂O₅ is not necessarily associated with using pure components as starting materials. Rather, TT may represent a less crystalline form of T, possibly stabilized by impurities [23]. In certain cases, oxygen atoms within Nb₂O₅ can be substituted by monovalent species like OH⁻ or Cl⁻ or even vacancies. This substitution process helps stabilize the TT phase of Nb₂O₅.

In TT-Nb₂O₅, the unit cell of Nb₂O₅ contains half of its formula equivalent, along with a constitutional defect involving the absence of one oxygen atom per unit cell [25] (Figure 3d). The Nb atom is positioned at the center of 4, 5, or 6 oxygen atoms within the ab plane. The crystal structure of Nb₂O₅ along the a-axis comprises two in-plane niobium layers interconnected by oxygen bonds, forming a relatively stacked structure. Meanwhile, the b-axis exhibits a tunnel-like structure with an approximate diameter of 4 Å, extending throughout the entire structure. This unique feature of Nb₂O₅ is responsible for its enhanced ion diffusion properties [35,36,37]. Along the c-axis, the crystal structure of Nb₂O₅ exhibits a disordered hexagon formed by oxygen atoms in six faces and an Nb-O-Nb-O chain structure. The absence of oxygen in the structure leads to the deformation of these polyhedra.

During the 1950s, Frevel and Rinn conducted studies on Nb₂O₅ and discovered a “low temperature” phase, now known as TT-Nb₂O₅. Their research utilized X-ray powder diffraction and identified a pseudo-hexagonal phase with lattice constants a = 3.607 Å and c = 3.925 Å. This phase transforms into T-Nb₂O₅ when heated to 973 K. Since its discovery, TT-Nb₂O₅ has been the subject of considerable scientific debate. Some believe the TT phase is merely a variant of T-Nb₂O₅, suggesting it is less crystalline than T-Nb₂O₅ [38]. Others say it is not even a separate phase [39]. Meanwhile, Terao was the first to identify the orthogonal structure of T-Nb₂O₅ and suggested that TT-Nb₂O₅ is a metal oxide with crystallization defects instead of a poorly crystallized T-phase [40]. Tamura suggested that TT-Nb₂O₅ possesses monoclinic cells and is stabilized by trace impurities in the sample [41]. Weissman et al. proposed that the structures of TT-Nb₂O₅ and T-Nb₂O₅ are highly similar, noting the absence of peak splitting and the presence of broad peaks in TT-Nb₂O₅, indicating that niobium resides in a more symmetrical 4h Wyckoff position rather than the 8i position found in T-Nb₂O₅ [42]. Their model suggests that TT-Nb₂O₅ can be indexed within the orthorhombic system of T-Nb₂O₅ and comprises subcell domains within a superlattice. Despite the distortions leading to unusual crystallization, TT-Nb₂O₅ displays pseudo-hexagonal subcell symmetry, while the superlattice follows an orthonormal crystal system [42]. Recently, Košutová et al. demonstrated an improved TT structure using hexagonal cells [43]. TT-Nb₂O₅ exhibits a complex structure with a disordered pseudohexagonal subcell of β = 120°, resulting in an orthogonal superlattice that resembles the T-phase. Niobium atoms occupy a more symmetrical Wyckoff position in this phase. The disordered polyhedron in TT-Nb₂O₅ forms a tunnel structure along the b-axis. Given its significance in the performance and applications of TT-Nb₂O₅, further investigation into its structure is warranted [44].

2.1.3. H-Nb₂O₅

H-Nb₂O₅ exhibits a high degree of order, with its structure divided into blocks [23]. In this monoclinic lattice, H-Nb₂O₅ comprises groups of ReO₃-type blocks, encompassing 3 × 4 and 3 × 5 blocks. These blocks are arranged in a layered structure, with the 3 × 4 blocks forming the first layer and the 3 × 5 blocks forming the second layer. It contains the NbO₆ octahedron [25] (Figure 3c). The ReO₃-type blocks in H-Nb₂O₅ are coupled by edge-sharing and move half-unit cell size along the c-axis. The NbO₆ units are connected through shared angles within a block [25]. For every 28 Nb atoms in the cell, one resides at the tetrahedral site, typically at certain block junctions.

This variant of Nb₂O₅ is readily available. Indeed, H-Nb₂O₅ can be produced in any other form when heated in air to ~1100 °C. If M-type single crystals are heated at 1100 °C, they retain their single-crystal properties during the transition to H-type, whereas all other Nb₂O₅ crystals convert to polycrystalline H-Nb₂O₅ [11].

H-Nb₂O₅ was initially identified as a high-temperature variant of Nb₂O₅ by Brauer in 1941. Later, in 1964, Gatehouse and Wadsley determined the crystal structure of H-Nb₂O₅ using Patterson and differential synthesis methods. Subsequently, Busing et al. used ORFLS to refine the structure by the full matrix least square method. Kato conducted additional refinements of the H-Nb₂O₅ structure in 1975 [16]. In the 21st century, extensive research has been conducted on the applications of H-Nb₂O₅.

2.1.4. M-Nb₂O₅

M-Nb₂O₅ shares a crystal structure similar to H-Nb₂O₅ and contains 4 × 4 ReO₃-type blocks. The close resemblance between M- and H-type X-ray powder patterns accounts for this similarity.

M-Nb₂O₅ belongs to the tetragonal system with lattice constants a = 20.44 Å and b = 3.832 Å. The space group is I4/mmm. Presumed metastable variants of Nb₂O₅ can be obtained through various pathways within the temperature range of ~900 °C. Single crystals are obtained using chemical transport techniques. In the structure of M-Nb₂O₅ (Figure 3b), Nb-O octahedrons are arranged in a blocky manner and can be considered part of the ReO₃ lattice. Formally, the structure of M-Nb₂O₅ is derived from that of N-Nb₂O₅. Perpendicular to the C-axis, the dimension of the block measures 4 × 4 octahedral diagonal units. In this plane, adjacent blocks are connected by edges. In the c direction, the octahedrons are connected by angles. In M-Nb₂O₅, four structures are interconnected to form a set of four edge-connected octahedrons (NbO_4/4O_2/2).

2.1.5. B-Nb₂O₅

B-Nb₂O₅ features a monoclinic crystal structure indexed in the space group C2/c, with four formula units in each cell. The structure comprises eight cations (Nb) located at the 8f Wyckoff position and 20 anionic oxygen ions (O) located at three Wyckoff locations: 4e (O₁) and two 8f (O₂ and O₃). The crystal structure of the B-Nb₂O₅ phase consists of twisted NbO₆ octahedral blocks arranged in a string of pairs of shared edge octahedra connected in a zigzag pattern of shared angle octahedrons (Figure 3e). According to the study of Pinto et al., DFT calculations and experimental evidence confirm that the B phase is the more stable phase at lower temperatures [14].

2.1.6. Other Nb₂O₅ Phases

In addition to the common crystal phases of Nb₂O₅, there are several less common or less studied phases, including N-Nb₂O₅, P-Nb₂O₅, and R-Nb₂O₅, among others. The presence of twin domains observed in previous studies likely results from the crystal structure and growth conditions during synthesis. The presence of coherent twins at 90° from each other suggests that the crystal structure of N-Nb₂O₅ possesses a high degree of symmetry. Regions containing blocks of different sizes may be attributed to defects or impurities within the crystal structure. Moreover, N-Nb₂O₅ and M-Nb₂O₅ differ only in structural arrangement. The synthesis of N-Nb₂O₅ was first investigated by Schafer et al. in 1964 [45], with its crystal structure reported by Andersson et al. in 1967 [17]. Meanwhile, Uyeda et al. (1984) [46] had earlier published HRTEM images of N-Nb₂O₅ and utilized the compound as a standard specimen; however, they did not describe the various types of twins in N-Nb₂O₅. Overall, these studies have provided insights into the microstructure of N-Nb₂O₅, emphasizing the importance of understanding crystal structure and defects in materials science.

RS-Nb₂O₅ is a nanostructured rock salt Nb₂O₅ electrode, reported by Barnes et al. [47]. This crystalline phase has excellent electrical properties through the amorphous to crystalline transition during the repeated electrochemical cycle with Li⁺.

2.2. Synthesis Methods

Researchers have taken a keen interest in Nb₂O₅, a non-toxic substance with ecological friendliness and a robust oxidation capacity. Due to its broad potential applications, extensive investigations have been performed to explore the various preparation methods for Nb₂O₅. The primary motivation for selecting the appropriate synthesis method lies in its capability to modify the properties of Nb₂O₅, particularly its crystal phase and morphology.

Significant attention has been devoted to developing niobium oxide films or particles, revealing their unique properties. Various techniques have been employed, including pulsed laser decomposition (PLD), electrodeposition, magnetron sputtering, plasma immersion ion implantation, and the sol-gel process. These methods have been utilized to prepare Nb₂O₅ nanostructures. Indeed, the characteristics of the surface nanostructures formed by these different synthesis methods are unique (Figure 5). For example, the hydrothermal method can be used to synthesize Nb₂O₅ nanorods and sea urchin nanostructures (Figure 5a,b). Meanwhile, the precipitation rule formed a nanosphere layer with a porous structure (Figure 5c). The samples prepared by the sol-gel method are coral-like nanostructures (Figure 5d), whereas longer Nb₂O₅ nanowires are synthesized by electrospinning (Figure 5e). Still further, filamentous nanostructures with chaotic surfaces can be formed via electrospinning (Figure 5f). Additionally, a self-organized microstructure of niobium oxide has been produced through potentiostatic anodization.

2.2.1. Hydrothermal and Solvothermal Methods

The hydrothermal and solvothermal methods are widely used due to their simplicity, low cost, and high yield. Typically, the reaction occurs within an autoclave or a Teflon-lined stainless steel vessel filled with water or organic solvents. The Nb ion arises from the reaction of niobium with acidic or basic solutions or by dissolving niobium salts; the solution is then heated to 100–600 °C for several hours or even days, during which Nb₂O₅ nanostructures grow [54,55,56]. Nb₂O₅ polycrystal hollow nanospheres and single-crystal nanotubes have been prepared by adjusting the molar ratio of Nb/Ti and the amount of F⁻ ions used [56]. Meanwhile, tree-like Nb₂O₅ nanotrees have been fabricated by secondary nucleation of Nb₂O₅ nanowires under hydrothermal conditions [57]. Additionally, monoclinic Nb₂O₅ nanotube arrays from pseudo-hexagonal Nb₂O₅ nanorod arrays have been reported. The phase transformation was due to energy differences between the pseudo-hexagonal and monoclinic Nb₂O₅ nanostructures [58]. Future developments may further optimize preparation methods and properties of Nb₂O₅ nanoparticles, potentially through doping with other elements or surface modifications with organic molecules to improve their electrochemical properties.

2.2.2. Anodization Method

The anodization method is a prevalent nanofabrication technique renowned for creating highly porous and well-ordered oxide structures. The anodization process is initiated by immersing the working and counter electrodes (usually a platinum sheet) in an electrolyte. Current and voltage are then applied, initiating chemical reactions at the interface between the electrolyte and electrode, forming a thin film structure. The shape and size of the resulting film are related to myriad factors, including the electrolyte composition, electrolyte temperature, amount of current or voltage applied, and the conduction time.

Anodized films are typically amorphous, however, they can be crystallized through annealing. Research on anodized Nb₂O₅ film dates back to the 1960s. Draper and colleagues studied the formation of oxide films on niobium tablets, investigating how the electrolyte’s composition affects the film material’s growth, particularly with regard to structural irregularities [57]. Subsequently, studies reported on the properties of anode Nb₂O₅ films (i.e., resistivity, dielectric constant). Other initial investigations into the surface morphology of anodized Nb₂O₅ and crystal analysis using electron microscopy electron beam crystallization have also been documented [58,59].

The anodization process of Nb has been extensively investigated using various electrolytes, including sulfuric acid, phosphoric acid, NaOH, Na₂CO₃, HF, glycerol, and phosphate-based solutions. These studies aimed to understand the electrochemical behavior and optimize the formation of oxide layers on the Nb surfaces for different applications. Most reported anodic Nb₂O₅ films possess a highly nanoporous structure on an Nb foil substrate [60]. One of the most significant studies in this area was conducted by Habazaki and colleagues, who discussed the influence of water content and elevated temperatures on the formation of porous Nb₂O₅ during the anodization process using a K₂HPO₄-glycerol electrolyte. They explored how these factors affect the morphology, structure, and properties of the resulting porous Nb₂O₅ films [60]. They reported that maintaining the water content of the electrolyte at 0.08 mass% at 160 °C significantly increased the film’s thickness. This was attributed to intensified field strength arising from the heightened concentration of phosphorus species in the electrolyte solution. Meanwhile, Ou and colleagues developed an Nb₂O₅ crisscross nanoporous network by high-temperature anodization [61].

Throughout these processes, ion diffusion during anodization is affected by several factors. Apart from nanoporous structures, microcones can also be produced through Nb anodization in an electrolyte solution with low hydrofluoric acid content in deionized water or a glycerol electrolyte solution containing K₂HPO₄ [62,63]. Wei and colleagues improved the anodic oxidation of Nb in NH₄F-based glycerol electrolytes and successfully fabricated Nb₂O₅ nanotubes up to 4 mm [64]. Lee et al. first reported the anodic oxidation process of highly ordered Nb₂O₅ nanochannels grown at 180 °C in a glycerol electrolyte solution containing K₂HPO₄ [65]. Lee et al. preheated the electrolyte to 200 °C before anodic oxidation to reduce the water content and successfully grew Nb₂O₅ thick nanochannel films. Similarly, Abdul Rani and colleagues reported the synthesis of Nb₂O₅ thin films on anodic nanochannels [66].

Other studies have reported on the production of nanoporous and nanochannel Nb₂O₅ thin films through Nb thin film anodization and deposition via RF sputtering on FTO glass substrates [67,68]. For the first time, Liu and colleagues successfully demonstrated the preparation of Nb₂O₅ nanotube powders by a simple electrochemical anodization method. In the anodizing process, the anodic oxide was released continuously and spontaneously into the electrolyte, which was collected to produce a white powder. The resulting powder primarily comprised nanotubes, with lengths ranging from ~50 to 100 nm and diameters of approximately 20 to 30 nm [69]. Meanwhile, Khairir et al. prepared a nanoporous thin film structure of Nb₂O₅ by the anodic oxidation method under different oxidation times; longer anodic oxidation times led to larger pore sizes [70].

Pligovka et al. conducted X-ray diffraction analysis on niobium oxide nanostructures, including defective non-uniform arrays and nanocolumns, fabricated via electrochemical anodization [71]. Gorokh et al. further studied the composition of columnar niobium oxide nanostructures (CNONS), synthesized initial samples by vacuum magnetron sputtering Nb/Al targets on a silicon substrate, and then synthesized samples by anodization and repeated anodization (reanodization) in different electrolytes. Finally, the growth mechanism of CNONS was proposed by analyzing the samples. The infrared spectroscopy analysis of CNONS identified three oxide phases in the NB-O system: NbO, NbO_2, and Nb₂O₅. Moreover, the top of the column was composed primarily of two Nb₂O₅ modifications: α-Nb₂O₅ and β-Nb₂O₅ [72]. In 2021, Pligovka et al. analyzed the morphology of niobium oxide nanocrystals formed by an electrochemical anodizing Al/Nb system with a similar synthesis method. They formed three different embryonic morphological types: skittle-, medusa- and goblet-like (Figure 6). Moreover, the morphological characteristics of the region between the top and bottom of the formed nanostructures were closely related to the electrolyte and the formation voltage of the anodic oxidation process [73].

The electrophysical properties of columnar niobium oxide nanostructures have also been studied. The measurement results showed that the current-voltage I-U curve is nonlinear and asymmetrical. An increase in temperature resulted in an increase in current. This behavior may represent a p-n junction or a metal-semiconductor junction [74]. Recently, its application in nano-optical biosensors has also been preliminarily studied [75].

2.2.3. Sol-Gel Methods

Since the sol-gel method was discovered in the 1970s, it has been rapidly developed and successfully applied in various material engineering fields [76]. Ulrich in 1988 [77] and Hench in 1990 [78] described the broad prospect of this method. To date, its application has been extensive, particularly in wet chemistry, as it allows for the precise control of composition and homogeneity during material synthesis at low temperatures. Furthermore, it offers a simple process with the potential for cost-effective large-scale production.

To generate sols, alcohol solutions are typically hydrolyzed with water, followed by polymerization or aggregation to produce dispersed fine particles. Depending on the desired sol composition and properties, these precursor compounds can be inorganic or metal-organic. The fundamental chemistry of sol-gel processes has been thoroughly covered in other reviews [79]. As per the summary of Bokov et al., during the sol-gel process, molecular precursors such as metallic alkoxides undergo hydrolysis or alcoholysis, resulting in gel formation from solutions in water or alcohol via heating and agitation. Since the gels resulting from these processes usually retain some water and solvent, it is necessary to dry them using appropriate methods that align with their intended properties and applications. For example, if an alcohol solution is utilized, drying can be accomplished by burning off the alcohol. Once dried, the resulting gel is often powdered and subjected to calcination [80]. In brief, the sol particles connect, forming a network of inorganic polymers known as gels, which retain some residual water and solvent. A dry gel is formed or coated in a transition window from sol to gel before removing the remaining water and solvent. The gel is then heat-treated to form the final dense product. Depositing oxide films using the sol-gel method is typically accomplished through dipping or spin coating techniques. However, the sol-gel method does have certain limitations, including the potential for uneven film thickness due to weak bonding. Moreover, it is difficult to control the reaction rate and porosity [81].

Alquier et al. first reported the preparation of Nb₂O₅ salt and its solvent by the sol-gel method in 1986 [82]. Post-annealing treatment is typically necessary to induce the crystallization of Nb₂O₅ films prepared via these processes. Schmitt et al. prepared these thin films by dissolving NbCl₅ powder in a mixture of butanol and acetic acid [83]. Following the preparation of the Nb₂O₅ film by dip-coating, the Nb₂O₅ coating was annealed at different temperatures from 400 to 600 °C, causing its structure to shift from amorphous to crystalline (i.e., a TT structure). Subsequently, others have demonstrated analogous synthetic procedures using alternative precursors, such as niobium ethanol and ammonium niobium oxalate. For instance, Melo et al. prepared Nb₂O₅ and Nb₂O₅: Li⁺ films by the sol-gel method with acoustic catalysis. They employed NbCl₅ as the precursor and butanol as the solvent, adding lithium salt LiCF₃SO₃ into the precursor solution to obtain thin films with different electrochemical properties [84]. In 2021, Xu et al. studied the impact of crystallinity on the optical properties of sol-gel-prepared Nb₂O₅. TG-DSC analysis revealed amorphous Nb₂O₅ up to 460 °C. Meanwhile, XRD results indicated the transformation of amorphous Nb₂O₅ into a pseudo-hexagonal phase, with higher temperatures resulting in improved crystallinity. UV-VIS and Raman spectra results demonstrated a progression in the arrangement of Nb₂O₅ atoms, starting with short-range order and progressing to medium-range, ultimately reaching a long-range order. That is, as the temperature increased, atoms became long-range ordered structures by connecting structural units [85].

2.2.4. Electrodeposition

Electrodeposition is widely used to manufacture nanostructured materials in various forms, such as powder, composite, and thin films. It is a well-established technique that produces a thin and uniform coating by applying an electric potential between two electrodes immersed in a solution, causing metal ions to deposit onto a substrate. In contrast to the anodization method described in Section 2.2.2, the metal oxide film formed by electrodeposition is at the cathode. Electroreduction is commonly used to produce low-cost (metal) materials, while electrooxidation is typically employed for high-value material production [86].

The electrodeposition process can be tailored by adjusting several parameters, including current density/voltage, temperature, and the addition of agents to the electrolyte solution. Additionally, modifying the substrate’s surface properties can achieve the desired electrodeposition characteristics. The current density/voltage pair is pivotal in controlling electrodeposition characteristics and determining the threshold for different reactions. A significant benefit of electrodeposition is the ability to regulate the reaction using an external circuit, enabling accurate and simple control over material deposition [86]. Niobium ions and hydrogen peroxide are typically required in the aqueous electrolytes for the electrodeposition of Nb₂O₅ [87].

Cathodic electrodeposition of niobium oxide (NbO_x) is hindered by Nb³⁺’s extremely negative reduction potential (−1.1 V vs. NHE), leading to the coevolution of destructive H₂ gas when the reaction occurs in aqueous solutions [88]. Crayston et al. [89] avoided this issue by using electrically generated OH⁻ to electrodeposit NbO_x, precipitating niobium ions, and immobilizing them in porous films prepared using the sol-gel technique. Meanwhile, in the late 1990s, Zhitomirsky electrodeposited Nb₂O₅ thin films on platinum-coated silicon or platinum substrates using peroxide as a precursor. They applied a stable 20 mA cm⁻² current during the 20-min electrodeposition process, which occurred at 1 °C. The electrolyte mixture comprised NbCl₅ and H₂O₂ [90,91]. The prepared Nb₂O₅ film had a uniform thickness and strong adhesion to the underlying material surface. Electrodeposition can also be performed using non-aqueous electrolytes, although a supporting electrolyte is typically required to facilitate current conduction in these systems.

In the study by Kamada et al., Nb served as the anode and platinum as the cathode. Electrodeposition was carried out in 0.01 M I²⁻ or Br²⁻ acetone as the solvent under a constant pressure of 50 V at room temperature and direct current. Introducing iodine to acetone led to anodization of the metal anode, however, electrochemical dissolution of the anode and its cathodic deposition did not occur. Conversely, including bromine in the solution promoted anodic dissolution and electrodeposition [92]. Meanwhile, Zhao et al. [93] applied the electrodeposition method to deposit T-Nb₂O₅ quantum dots onto a Ti nanorod array, creating a Ti@T-Nb₂O₅ core-shell array electrode with good electrical properties. The size of T-Nb₂O₅ particles can be manipulated by modifying the deposition current density. For example, with a deposition current of 6 mA cm⁻², quantum dots measuring several nanometers in diameter can be synthesized. Jha et al. improved Zhitomirsky’s electrodeposition method to obtain NbO_x colloids by rapidly injecting methanol-dissolved niobium salt into a hydrogen peroxide solution cooled to approximately 2 °C. This led to the production of a pure phase T-Nb₂O₅ film with a uniform and controllable thickness, high porosity, and good cycling efficiency [88].

2.2.5. Vapor Phase Deposition

Vapor deposition is a technique used to produce material layers by condensing vaporized source material under specific conditions. Vapor deposition can be classified into two primary categories: physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD involves evaporating solid or liquid material sources in a vacuum environment. This results in the formation of gaseous atoms, molecules, or partially ionized ions. These particles are then deposited onto the substrate surface as a gas (in a low-pressure state) or as plasma, producing films with different functions [94,95]. The PVD methods include vacuum evaporation, sputter coating [96], ion beam-assisted deposition [97,98], thermal evaporation [99], pulsed laser deposition (PLD) [100], etc. PVD technology has advanced to the point where it is possible to deposit various films, including those composed of metals, alloys, compounds, ceramics, semiconductors, and polymers, among other materials.

Sputtering is employed to prepare Nb₂O₅ thin films as it can use a direct current (DC) [95,101] or radio frequency (RF) [102,103,104,105] power supply in an oxygen (O₂) environment with metal niobium (Nb) [106] or Nb₂O₅ [102,103,105] as the target, using argon (Ar) and other carrier gas preparations. By manipulating the deposition parameters, such as pressure, target-substrate distance, substrate temperature, discharge voltage, and RF power, these methods can produce Nb₂O₅ thin films with precisely controlled size, crystallinity, and grain size [107]. Meanwhile, post-annealing treatment can improve the crystallinity of sputtered films. However, the deposition rate of this technique is low and time-consuming. However, further investigations have revealed that using pulsed magnetron sputtering and PLD technology, Nb₂O₅ thin films can be deposited at tens of microns per hour while maintaining the original stoichiometric ratio of the bulk target material.

CVD is a widely-used technique involving chemical reactions of gaseous or vapor-state substances to form solid deposits at the interface between gas and solid or between two gas phases. The main difference between CVD and PVD is that chemical reactions occur during CVD deposition. CVD is a gas-phase chemical growth process wherein various raw gas materials are introduced into a reaction chamber. Chemical reactions occur among the gases, generating new materials, subsequently deposited onto the substrate surface. As such, CVD is an effective method for preparing Nb₂O₅, allowing precise control over the thickness, morphology, shape, and composition of Nb₂O₅ films by regulating reaction conditions to meet diverse application requirements. In addition, CVD technology is widely applied in fabricating thin films, particularly conformal coatings and nanostructures [108]. Various CVD techniques include atmospheric pressure chemical vapor deposition (APCVD), low-pressure chemical vapor deposition (LPCVD) [109], vapor phase epitaxy (VPE) [110], ultra-high vacuum chemical vapor deposition (UHVCVD) [111], laser-induced chemical vapor deposition (LCVD) [112], among others.

Typically, Nb₂O₅ films are deposited using CVD by introducing precursor chemicals like NbCl₅ and pentaethoxy niobium [Nb(OC₂H₅)₅] into a reaction chamber via a carrier gas. They undergo thermal decomposition on the surface of a heated substrate to form the desired film. O’Neill et al. reported that the growth of Nb₂O₅ films via CVD is significantly influenced by the substrate’s temperature [113]. Other studies using the CVD method provide an excellent alternative to the production of layered Nb₂O₅, especially for fiber coating applications [114]. For example, Silveira et al. exposed glass fiber to air after reacting with NbCl₅, heat treating at 300 °C for 2 h, and heat treating and hydrolyzing in water to achieve a well-coated glass fiber with Nb₂O₅ [115].

Spray pyrolysis is another technique commonly employed to produce Nb₂O₅ films with various thicknesses through an aerosol-assisted CVD process [116,117]. This method features simple experimental equipment and high production efficiency. However, when depositing Nb₂O₅ films at low temperatures, post-annealing treatment is often required to induce crystallization and improve the structural quality of the films.

It is worth mentioning that HV-CVD is also an effective method for synthesizing Nb₂O₅ thin films [108]. According to Yury Kuzminykh’s research, HV-CVD technology offers unique advantages in Nb₂O₅ preparation, achieving deposition rates up to 500 nm/h, markedly higher than that of the MBE process. The film thickness of vegetation by this method is up to several microns. Moreover, HV-CVD requires lower substrate temperatures than MO-CVD, opening the possibility for in-situ high vacuum characterization techniques [108].

2.2.6. Thermal Oxidation

The preparation of Nb₂O₅ via the thermal oxidation process is typically a straightforward procedure involving using Nb metal/foil or powder as the starting material. The material is subsequently loaded into a furnace and heated in a high-oxygen or pure O₂ environment to temperatures up to 1000 °C to facilitate oxidation and the formation of Nb₂O₅ [118,119,120]. The thermal oxidation process relies on the diffusion of oxidizing agents like O₂ onto the substrate at elevated temperatures. These agents then react with the Nb material, leading to the direct creation of nanoscale Nb₂O₅ structures on the surface. This approach enables the fabrication of Nb₂O₅ nanowires, which can extend to lengths up to 20 mm [121]. The resulting nanostructures are influenced by factors such as temperature, time, metal catalysts and gas atmosphere.

3. Properties of Nb₂O₅

In the following section, other basic properties of Nb₂O₅ including electrical, optical and thermal properties will be discussed.

3.1. Electrical Properties

The properties of metal oxides play a critical role in developing electronic devices, with key parameters such as band structure, electrical conductivity, and permittivity holding particular importance. In electronic applications, niobium oxide, especially Nb₂O₅, the most thermodynamically stable niobium oxide, has garnered significant attention.

The dielectric constants for Nb₂O₅ have shown significant variation and inconsistency, ranging from 41 to 120. And its band gap spans from 3.4 to 5.3 eV [122], classifying Nb₂O₅ as an N-type semiconductor with a wide band gap. The conduction band is formed by the vacant 4d orbitals of Nb⁵⁺ and is approximately 0.2–0.4 eV greater than that of TiO₂. [123]. Nb₂O₅ is frequently employed in synthesizing alkali metal niobates like MNbO₃(M = Li, K, Na), commonly used in optoelectronic devices [124]. Due to its excellent dielectric properties, Ding et al. also applied it to Y₂Ti₂O₇ microwave dielectric ceramics to improve its dielectric properties [125].

Nb₂O₅ exhibits varying bandgap energies (Eg) ranging from 3.1 eV (semiconducting behavior) to 5.3 eV (insulating behavior). The bandgap energy can be adjusted by adding foreign ions and other methods. The nanostructured Nb₂O₅ can significantly affect its electrical properties, potentially causing a blue shift in the band gap. According to Clima et al., different structures in sol-gel deposited Nb₂O₅ correspond to different permittivities, such as ortho, δA hexagonal, and δB hexagonal [126]. Simultaneously, their examination of the permittivity tensor reveals that Nb₂O₅ exhibits anisotropic dielectric behavior.

When studying the dielectric constant (ε′) of Nb₂O₅, different structures yield different values [127]. For instance, amorphous Nb₂O₅ films formed via anodic oxidation can have dielectric constants ranging from 41 to 120. Meanwhile, sintered Nb₂O₅ pellets exhibit dielectric constants ranging from 38 to 165 depending on the processing routes [128]. Furthermore, Graca and colleagues [124] found that T-Nb₂O₅ and H-Nb₂O₅ powder compacts exhibit dielectric constant values of approximately 80 and 17, respectively, and a mixture of these phases reached ~600. Soares and colleagues [128] discovered that the dielectric constant of sintered Nb₂O₅ pellets differs based on the processing method. The electrical properties of bulk Nb₂O₅ are closely related to its crystal structure and material density, which are related to sintering conditions [124,128].

The electrical conductivity of niobium oxide is related to the formation of electrons in oxygen vacancies, with oxygen in the formation route determining conductivity [129]. Therefore, the electrical characteristics can significantly vary based on the experimental conditions. Under non-oxygen atmospheres, the conductivity of Nb₂O₅ increases owing to the creation of additional oxygen vacancies. When heated in air, however, the conductivity declines at ~550 K, attributed to molecular absorption reducing vacancies. Therefore, these alterations in transmission properties also relate to oxygen vacancies. This reliance on vacancies forms the foundation of the material’s technological applications.

In addition, electron mobility is an important parameter in the development of semiconductor devices, with studies indicating that the mobility of Nb₂O₅ increases with an increase in temperature. Indeed, the conductivity of Nb₂O₅ crystal exhibits an exponential dependence on temperature [130].

3.2. Optical Properties

Recently, there has been a growing interest and increasing scientific attention toward Nb₂O₅ due to its versatility as a multifaceted material. It is widely recognized as a transparent oxide semiconductor material [1,131,132], which exhibits transparency in the ultraviolet region owing to its wide band gap, insolubility in water, and stability in air. Due to the interesting characteristics of Nb₂O₅ films, including their wide band gap, high refractive index, excellent thermochemistry, and stability, they are currently used in several applications. These applications encompass, but are not limited to, photoelectric devices, catalysis, gas sensors, high-performance oxide glasses [133], and EC devices [106]. Nb₂O₅ has garnered attention due to its potential applications in solar cells, batteries, photodetectors, and other electronic devices [9,134].

Le and colleagues conducted studies to determine the band gap of crystalline Nb₂O₅ nanofibers, including H-Nb₂O₅, O-Nb₂O_5, and M-Nb₂O₅, which were sintered at three different temperatures (773 K, 1073 K, and 1373 K). The band gaps for H-Nb₂O₅, O-Nb₂O₅, and M-Nb₂O₅ were measured at 3.85 eV, 3.77 eV, and 3.79 eV, respectively [135]. In a separate study, Abe reported the band gap values of orthotropic Nb₂O₅, synthesized via powder, to be 3.4 eV, while that for monoclinic Nb₂O₅ was 3.1 eV [136]. Furthermore, Abe studied how incorporating varying concentrations of Ge into monoclinic Nb₂O₅ films affected their band gap. The optical absorption edge of H-Nb₂O₅, without Ge doping, was measured at 3.1 eV. However, increased Ge concentration shifted the absorption edge to 3.35 eV [136]. During the early 2010s, Liu and colleagues utilized a two-step solution method to fabricate single-crystal porous Nb₂O₅ nanotubes. They also prepared homogeneous single-crystal Nb₂O₅ nanorods. The band gap values for Nb₂O₅ nanotubes and nanorods were 3.97 eV and 3.72 eV, respectively. The observed variation in band gap values, specifically the 0.25 eV difference between porous nanotubes and solid nanorods, can be attributed to the blue shift of the absorption edges in the former, caused by the quantum confinement effect in the hollow Nb₂O₅ nanotubes [137]. Brayner and colleagues similarly noticed a blue shift [138]. Agarwal and colleagues also investigated how grain size influences the electronic properties of Nb₂O_5. They proposed that the energy of the absorption edge and local coordination can be affected by variations in grain size [139].

As previously mentioned, Nb₂O₅ possesses a band gap ranging from 3.1 to 4.0 eV. Its optical properties are highly dependent on its crystallinity and grain morphology, allowing it to effectively absorb light in the UV and near-UV spectra or serve as a transparent material for ultraviolet light. Niobium oxide films exhibit electrochromism, transitioning from transparency to shades of brown, gray, or blue when Li⁺ or H⁺ plasma is introduced. Nb₂O₅ is capable of modulating its optical transmission state, switching between high transmittance (T ~ 85%) in a quasi-transparent state and low transmittance (<T ~ 10%) in the UV, visible, or near-infrared (IR) ranges. Moreover, the material’s coloration can vary between blue and brown, depending on its crystallinity structure [140].

The study of Nb₂O₅’s electrochromic properties dates back to 1980, when Reichman and Bard reported its excellent chemical stability and resistance to acid and alkali corrosion. These characteristics have prompted extensive research into its application as an electrochromic material. This electrochromic phenomenon is related to the continuous and reversible optical changes produced by electrochemistry, which macroscopically manifest as color changes. However, electrochromic materials are typically evaluated based on color rendering efficiency, encompassing transmittance contrast, response time, and chemical stability when transitioning from colored to bleached states under an electric field and charge injection. Thus, the material properties of Nb₂O₅ films play a significant role in determining these characteristics. Nb₂O₅ thin sheets exhibit a low refractive index (reported value of ~2–2.3) [141], with the degree of crystallinity influencing the refractive index, which can decrease from 2.30 to 2.20 when the material is subjected to heat treatment at 700 °C under ambient conditions [97].

3.3. Mechanical Properties

In addition to its optical and electrical properties, the mechanical strength of Nb₂O₅ plays a pivotal role in producing electronic devices, particularly in manufacturing actuators and flexible mechanical components. Thin films often experience stresses and strains due to limitations imposed by the deposition method or substrate.

Sputtered Nb₂O₅ films typically exhibit an average hardness (H) ranging from 5.6 to 6.8 GPa, with a Young’s modulus (Er) reaching 117 to 268 GPa. These mechanical properties are influenced by the crystalline structure of the film [97,142]. The MIM capacitor, incorporating a sputtered Nb₂O₅ film, underwent the collapse radius test, a commonly employed bending test technique. The findings indicated that the device exhibited remarkable robustness and resilience, enduring repeated testing for up to 2500 times [143].

4. Application of Nb₂O₅

Over the past few decades, the excessive exploitation and utilization of fossil fuels by humans has led to resource depletion and significant environmental challenges. Therefore, new energy sources and technologies are needed to alleviate the pressure on energy sources and the environment. The replacement of conventional internal combustion engines with mobile energy storage systems is crucial for phasing out fossil fuels. For these applications, it is necessary to have high energy density energy storage materials [144].

Lithium-ion batteries (LIBs) and supercapacitors (SCs), the two main devices for storing electric energy, are vital to daily life. LIBs are widely used as the primary energy supply for handheld devices, such as mobile phones and laptops. Additionally, they have gained widespread adoption in electric vehicles, garnering significant global attention [145,146]. Moreover, due to the unique electrical properties of Nb₂O₅, extensive research has been conducted on its applications in LIBs and SCs. Researchers have achieved performance optimization in both LIBs and SCs by modifying the properties of Nb₂O₅ thin films. The various applications of Nb₂O₅ reported in the literature are described in the following sections.

4.1. Lithium-Ion Battery

Batteries and HSCs are crucial in electric vehicles, smartphones, and more, driving advancements in related industries. Developing sustainable and high-capacity anode materials is a key parameter of high-efficiency battery technology. Graphite (Gr) is the most widely utilized anode material in LIBs [147]. Carbon nanotubes and graphene also hold significance as anode materials [148]. However, these carbonaceous materials are limited by high production costs, complex, large-scale manufacturing processes, and unsustainable routes.

Consequently, it is imperative to explore the development of novel anode materials that are environmentally friendly and cost-effective [149]. Such advancements are essential to ensuring the sustainable growth of battery technology [150]. This push toward developing affordable and sustainable alternatives highlights the need for ongoing research and innovation in this field [151].

Recently, there has been significant interest in utilizing niobium oxide-based materials and their composites in LIBs. This is due to the unique properties that render them particularly suitable for this application, including their quasi-two-dimensional networks, ion insertion/extraction, abundant REDOX chemistry, and excellent chemical stability. Therefore, this discussion delves into various aspects of Nb₂O₅ application, including their classification, advantages, disadvantages, and research progress.

The anode materials for LIBs are assigned to three primary classifications: alloy, conversion, and insertion types [152]. While the specific capacity of insertion-type electrode material is theoretically lower than the other two, it possesses many advantageous properties that make it an ideal anode material for LIBs. In particular, it exhibits excellent cycling stability and the ability to maintain high capacity under high power conditions, critical factors for determining the performance of a battery.

The insertion lithium storage mechanism exhibited by niobium (Nb)-based oxides positions them as highly promising candidates for replacing high-rate negative electrode materials [153]. The crystal structure of T-Nb₂O₅ comprises two alternating atomic layers, namely the loosely arranged 4g layer and the densely arranged 4h layer (Figure 7a). Due to the large atomic space, the 4g layer is a preferred storage and transport site for lithium ions. Theoretical calculations reveal that the lithium ions occupying the 4g layer are relatively stable. Based on the neighboring Nb-O bond structure in T-Nb₂O₅, two distinct Li-ion diffusion pathways exist: Path A and Path B (Figure 7b). These diffusion pathways offer direct transport channels, ample transport space, and low-energy barriers, facilitating rapid Li-ion transportation [36].

First, most Nb-based oxides operate within a potential range from 1.0 to 2.0 V (relative to Li⁺/Li), with REDOX reactions involving Nb⁵⁺/Nb⁴⁺ and Nb⁴⁺/Nb³⁺ being the primary mechanisms. This operating potential range is selected to mitigate electrolyte deposition and the formation of lithium dendrites, critical for ensuring battery safety under high current densities and overcharging conditions. Second, compared to LTO, Nb oxides demonstrate a higher specific capacity, typically ranging from 200 to 400 mAh g⁻¹. This enhances the appeal of Nb-based oxides as negative electrode materials for LIBs. Third, Nb oxide has a high Li⁺ diffusion coefficient, attributable to its unique structure, making it an attractive option as a negative electrode material in LIBs, as it can improve the battery’s overall performance [99,154]. Fourth, the low volume expansion rate of Nb oxide reduces the risk of structural damage caused by the high levels of expansion and contraction during lithium insertion. This property increases the stability and longevity of the battery (Figure 8) [155].

In addition, even at micron sizes, monoclinic Nb oxide possesses a remarkable energy storage capacity [22,157]. The findings from these studies provide strong evidence that Nb-based oxide materials are highly advantageous as alternative anodes in high-performance LIB applications. In particular, they boast suitable operating potentials, high specific capacities, high Li⁺ diffusion coefficients, and low volume expansion rates. Indeed, Nb-based oxides have the potential to play a major role in improving the performance and safety of LIBs for future energy storage applications.

4.1.1. Lithium-Ion Battery Anode Performance

There has been a recent increased focus on Nb-based oxide materials, particularly Nb₂O₅, as candidates for various energy storage applications. These materials have been thoroughly investigated for their potential utilization in technologies such as LIBs, SIBs, and SCs [158,159]. This heightened interest can be attributed to the unique set of advantages that these materials offer, including a high specific capacity, high Li⁺ diffusion coefficient (Figure 9), low volume expansion rate, and stable cycling performance. As such, Nb-based oxides are widely recognized as a promising avenue for developing advanced energy storage systems. In addition, the high operating voltage of Nb oxides generally exceeds 1.0 V vs. Li⁺/Li, representing a critical advantage that mitigates the decomposition of organic electrolytes and the development of solid–electrolyte–interface (SEI) films in batteries. These complications can arise from the instability of certain materials at lower voltages, leading to unwanted reactions or side effects that limit the performance and lifespan of the battery. Thus, the high operating voltage of Nb-based oxides can enhance the safety and efficiency of energy storage systems. Rapid energy storage and release are widely regarded as fundamental technologies for developing next-generation battery systems. Achieving fast energy storage performance can be realized through the use of advanced electrode materials, novel electrolyte formulations, or innovative cell designs. In particular, the utilization of Nb-based oxides has shown promise in enhancing the energy storage performance of batteries, owing to their high specific capacity, low volume expansion rate, and excellent cycling stability (Figure 8). By incorporating these materials into next-generation batteries, it is possible to achieve faster charging times, longer lifespans, and higher energy densities, ultimately paving the way toward more efficient and sustainable energy storage technologies [160].

Electrode active materials and their properties are crucial in influencing the overall performance of LIBs as they are essential in facilitating the reversible flow of ions during charging and discharging cycles [162]. In the early 1980s, Nb₂O₅ was first studied as a LIB material [163,164]. A fundamental discovery was made by Reichman et al., who reported that Nb₂O₅ could intercalate with Li⁺ ions, suggesting the possibility of reversible charging and discharging [163]. Kumagai’s group further explored the use of Nb₂O₅ as a cathode material for LIBs, employing advanced techniques, including XPS, XRD, and XAFS. Their investigations focused on characterizing the structural changes in the Nb₂O₅ cathode during charging and discharging and elucidating the underlying mechanisms governing its electrochemical behavior [165]. Among different crystal structures, tetragonal-Nb₂O₅ exhibits the most favorable cycling performance. In fact, it can achieve a discharge capacity of up to 190 mAh g⁻¹ and sustain up to 30 cycles. XRD analysis revealed that the orthorhombic and tetragonal structures of Nb₂O₅ maintain their original lattice despite experiencing slight changes in cell volume following Li⁺ embedding. Meanwhile, the unique two-dimensional and layered structure of tetragonal Nb₂O₅ allows it to accommodate very high concentrations of embedded ions [165]. In 2011, researchers, led by Goodenough proposed TiNb₂O₇ as a notable material within the TiO₂-Nb system, demonstrating its potential as a negative electrode material for LIBs [166]. In 2013, Dunn’s group reported that orthorhombic Nb₂O₅ (T-Nb₂O₅) exhibits exceptional electrochemical energy storage capabilities at high rates, primarily due to the utilization of Li⁺ embedded pseudocapacitance [28]. Following this discovery, Nb₂O₅-related anode materials regained significant attention due to their promising application prospects in fast-charge energy storage devices.

Research on Nb₂O₅ LIBs has also included exploring various electrolytes and electrode compositions with varying amounts of graphite. Reducing the graphite content within the electrode composition leads to a decrease in discharge capacity. Furthermore, researchers have explored the application of sputtered Nb₂O₅ as a LIB electrode. For example, Nakazawa and colleagues reported a battery featuring a sputtered Nb₂O₅ negative electrode with a thickness of 100 nm, achieving highly favorable charging and discharging performance, maintaining a capacity range of 310–380 mA h cm⁻³ over 500 cycles [167].

4.1.2. Lithium Storage Mechanism

The electrochemical behavior of Nb₂O₅ electrode materials, which exhibit distinct crystal structures resulting from variations in temperature and pH value, demonstrates noticeable differences. These distinctions can be attributed to the specific material processing method employed and the underlying principles of the fuzzy reaction. Further understanding of the reaction mechanism can be obtained by exploring the storage of Li⁺. Bard et al. conducted the primary investigation on the electrochemical behavior of Li⁺ in Nb₂O₅ [101]. They found that the Nb₂O₅ electrode exhibited electrical conductivity in an acetonitrile solution containing 0.8 M LiClO₄, and an electrochemical redox process occurred between Nb₂O₅ and Li⁺. Hence, Li⁺ could undergo a reversible reaction with Nb₂O₅, resulting in the formation of Li_xNb₂O₅. The redox reaction mechanism can be represented by the following equation:

$${\mathrm{N}\mathrm{b}}_2{\mathrm{O}}_5+\mathrm{x}\mathrm{L}\mathrm{i}++\mathrm{x}{\mathrm{e}}^{-}\leftrightarrow \mathrm{L}{\mathrm{i}}_{\mathrm{x}}{\mathrm{N}\mathrm{b}}_2{\mathrm{O}}_5\left(\mathrm{x}=0–2\right)$$

(1)

When x = 2, it corresponds to a maximum theoretical capacity of 200 mA h g⁻¹.

Typically, the theoretical capacity of Nb₂O₅ is determined using Equation (2), which is derived from the two-electron redox reaction, as follows:

$${\mathrm{Q}}_{\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}}=\frac{\mathrm{n}\mathrm{F}}{{\mathrm{M}}_{\mathrm{W}}}=\frac{2\times 96485.3 \mathrm{C} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}}{265.8 \mathrm{g} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}}=725.9 \mathrm{C} {\mathrm{g}}^{-1}=201.6 \mathrm{m}\mathrm{A} \mathrm{h} {\mathrm{g}}^{-1}$$

(2)

where “n” represents the number of electrons involved in the reaction, “F” denotes the Faraday’s constant, “Mw” signifies the molecular weight of the active material, and 3.6 serves as the conversion factor between coulombs (C) and conventional milliampere-hours (mA h).

Among the various configurations of Nb₂O₅, the pseudo-hexagonal system TT-Nb₂O₅, orthorhombic system T/O-Nb₂O₅, and the monoclinic system H-Nb₂O₅ have undergone extensive investigation as anode materials for energy storage systems, each with distinct mechanisms for storing lithium. In 1999, Kumagai and colleagues [168] evaluated the electrochemical behavior of three different Nb₂O₅ configurations obtained at different heating temperatures. These configurations were designated the hexagonal, orthogonal, and monoclinic phases. The heating temperature during the synthesis process significantly impacted the crystal structure and properties of Nb₂O₅. They obtained Nb₂O₅ samples with different crystal structures by controlling the heating temperature. The Nb₂O₅ compounds exhibited comparable electrical activity, demonstrating discharge capacities ranging from 160 to 180 mA h g⁻¹ within the potential range of 1.2 to 3.0 V (vs Li⁺/Li).

Hexagonal and orthorhombic Nb₂O₅ electrodes do not exhibit distinct potential plateaus during charging and discharging. Instead, the potential varies linearly with the x value of Li_xNb₂O₅ (Figure 10a,b). On the contrary, the charge–discharge curve of the monoclinic Nb₂O₅ electrode displays multiple potential platforms (Figure 10c). Based on the research findings, the hexagonal and orthorhombic Nb₂O₅ undergo a single ternary phase formation of Li_xNb₂O₅ while embedding Li⁺. Meanwhile, the lithiation process of monoclinic Nb₂O₅ involves two-phase reactions, transitioning from x = 0.8 to x = 1.8 within Li_xNb₂O_5.

The crystal structure of T-Nb₂O₅, characterized by its NbO₆ and NbO₇ polyhedra and the irregular arrangement of Nb and O ions, significantly influences its properties [28], as discussed in Section 2.1.1. Chen et al. [36] investigated the rapid lithium storage mechanism of T-Nb₂O₅ by combining theoretical calculations and experimental measurements. The T-Nb₂O₅ structure consists of alternating atomic layers that exhibit two distinct arrangements. Approximately 40% of the O²⁻ ions occupy the 4g Wyckoff site, resulting in relatively loosely packed 4g layers. The remaining ions occupy the 4h Wyckoff site, forming more densely packed 4h layers. All niobium ions are situated in the relatively dense 4h layer, while lithium ions are positioned in the loosely packed 4g layer, providing space to accommodate Li⁺ ions, while the dense 4h layer facilitates the formation of a well-defined bridge coordination between oxygen and Li⁺ ions. This unique arrangement of bridging sites enables the formation of rapid Li⁺ diffusion paths with low migration barriers [36].

Kodama and colleagues demonstrated that a continuous transition from Nb⁵⁺ to Nb⁴⁺ occurs during the insertion of Li⁺ ions. Additionally, the cell volume of orthorhombic Nb₂O₅ undergoes only slight changes after the inclusion of Li⁺ ions [165]. According to Dunn et al., the preferential insertion of Li⁺ ions occurs through the (180) and (001) planes in T-Nb₂O₅. Furthermore, the redox reaction between Li⁺ ions and T-Nb₂O₅ occurs within a single-phase system [169], where the {001} group planes form a channel conducive to Li⁺ migration. Therefore, the orthorhombic T-Nb₂O₅ system boasts a unique crystal structure that features a two-dimensional Li⁺ channel unaffected by Li⁺ reincarceration behavior. This facilitates rapid Li removal in T-Nb₂O₅. Furthermore, theoretical calculations indicate that the (001) surface of T-Nb₂O₅ facilitates the transport of ions along degenerate paths with low energy barriers due to the large oxygen–oxygen distance of 3.9 Å [168,170]. In addition, the unique “room-and-pillar” NbO₆/NbO₇ framework of T-Nb₂O₅ offers a stable structure for embedding Li⁺ ions, ensuring that no phase transitions occur during the ion insertion process [22]. Nb₂O₅‘s special layered structure allows bulk T-Nb₂O₅ to facilitate rapid Li⁺ diffusion, exhibiting excellent intercalation pseudocapacitance [165].

T-Nb₂O₅ and TT-Nb₂O₅ are considered pseudocapacitor materials due to their similar crystal structure [155]. However, as TT-Nb₂O₅ possesses a more disordered crystal structure, it encounters more structural obstacles during Li⁺ embedding [171]. T-Nb₂O₅ exhibits a higher reversible specific capacity than TT-Nb₂O₅, leading to a noticeable difference in their performance.

The lithium storage mechanism of H-Nb₂O₅ differs from that of T-Nb₂O₅ and TT-Nb₂O₅, showcasing distinct characteristics in their respective behaviors. In contrast to T-Nb₂O₅ and TT-Nb₂O₅, monoclinic H-Nb₂O₅ features a densely packed oxygen array and exhibits a more significant repulsion intercalation effect. As previously mentioned, the extensive intercalation of lithium within the H-Nb₂O₅ structure can result in phase separation [168]. This phenomenon is generally recognized as the primary factor contributing to the inferior rate performance observed in numerous studies when comparing H-Nb₂O₅ to T-Nb₂O₅. However, Ding et al. [160] found that the performance failure of H-Nb₂O₅ was attributable to the anisotropy of electron and ion conduction within the H-Nb₂O₅ crystals. This asynchronous phase transition during lithium (de)intercalation is a direct consequence of the phase separation, further impacting the performance of H-Nb₂O₅ compared to T-Nb₂O₅ [172]. Researchers have employed a strategy involving the application of uniform amorphous carbon shells as a coating on the surface of micron-scale single-crystal H-Nb₂O₅ particles. This serves to homogenize electron and Li⁺ transport. This optimization technique significantly enhanced the fast lithium storage performance of the negative electrode composed of H-Nb₂O₅, surpassing the performance of many reported negative electrodes based on LTO and T-Nb₂O₅.

In conclusion, the inherent structural advantages of Nb₂O₅ have attracted considerable attention in advanced electrochemical energy storage applications. For example, the high operating voltage of Nb₂O₅ (>1.0 V vs. Li⁺/Li) can inhibit the decomposition of the electrolyte and the formation of SEI film and lithium dendrites, ensuring the safety of the battery. Continuous and high-demand research efforts will be focused on exploring more effective strategies for enhancing the conductivity of niobium-based oxides. Structural optimization, surface engineering, and carbon modification remain the primary avenues for investigating this field. Meanwhile, the composite utilization of anode electrode materials with high theoretical capacity and synergistic effects is significant.

4.1.3. Effect of Nb₂O₅ Nanostructures

Zero-dimensional (0D) Nb₂O₅ nanostructures, specifically large-sized nanoscale particles, have been the subject of recent studies. These investigations have primarily focused on exploring the unique properties and applications of such nanostructures. However, it is worth noting that the repetitive usage of these large-sized 0D Nb₂O₅ particles may lead to a decrease in performance or capacity due to potential degradation or other cycling-related factors [173,174,175,176]. The uniform distribution of particles within a battery is essential as it provides a large surface area and abundant reaction sites. This prevents particle agglomeration during the charging and discharging process, ultimately improving the electrochemical performance of the battery. The amorphous Nb₂O₅ precursors prepared in the air by the sol-gel method were calcined at different temperatures to obtain different crystalline phases of Nb₂O₅ (amorphous, T, TT). The grain size of these phases (hexagonal, orthorhombic, and monoclinic) increases with rising temperature. Extensive testing has demonstrated that Nb₂O₅ exhibits remarkable cyclic stability. In addition, Liu and colleagues [175] synthesized T-Nb₂O_5−x particles using a straightforward sol-gel process, followed by post-etching and calcination. However, a small portion of cell recombination occurred due to the removal of Sr and Ca atoms during the acid etching process. The significant stress associated with this process leads to the formation of high-density defects, contributing to the generation of more reaction sites so that the final sample has good magnification performance and cycle stability. Chen et al. conducted a solvothermal method to obtain Nb₂O₅ nanomaterials with varying graphene contents (1, 2, 3, 4, and 5 wt.%) and superphosphorus as conductors [176].

One-dimensional Nb₂O₅ nanostructure: Using pure Nb₂O₅ with one-dimensional nanostructures as an anode material for LIBs is not widespread. This may be attributed to the intricate preparation requirements and challenging control of the associated processes. Most research efforts in one-dimensional nanostructures have predominantly focused on nanofibers [52] and nanorods [48]. Despite the challenges associated with their preparation, the distinctive aspect ratio of pure Nb₂O₅ in one-dimensional nanostructures offers an effective pathway for transmitting Li⁺ ions. This unique characteristic continues to garner significant interest within the research community. In some instances, regulating heating rates during the heat treatment is considered an effective approach to fabricating nanofibers with exceptional properties.

Two-dimensional Nb₂O₅ nanostructure: Generally, two-dimensional Nb₂O₅ nanomaterials used for LIBs include nano-thin films [177], nano-sheets [178,179] and nanoribbons [180,181]. One advantage of controlling the heating rate during the treatment process is its ability to create a continuous framework that facilitates efficient ion and electron diffusion; more Li⁺ storage sites are on the open edge. Consequently, by optimizing reaction kinetics, controlling the heating rate can enhance the overall performance of these materials. Furthermore, the anisotropic growth of two-dimensional nanomaterials offers additional opportunities for REDOX reactions in batteries. Nano-thin film electrode materials can be prepared through physical or chemical deposition methods. Due to the large energy density area, two-dimensional flexible nanofilms grown directly on the substrate without additives and conductive agents can be used in micro-energy storage devices.

In the study conducted by Zhou et al. [182], metallic Nb powder served as a precursor for synthesizing Nb₂O₅ nanoribbons. Subsequent investigation primarily focused on evaluating the electrochemical characteristics of these nanoribbons when employed as anode materials in LIBs. Liu et al. successfully fabricated Nb₂O₅ nanosheets with dimensions of 50 nm in thickness and 500 nm in length through a hydrothermal reaction [178]. The excellent rapid energy storage capacity of these two-dimensional Nb₂O₅ nanostructures can be attributed to their large active surface area and short Li⁺ diffusion distance.

In addition to the zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) structures, Nb₂O₅ exhibits various heterogeneous multi-level complex structures. Notable examples include hollow porous spheres [183], sea urchin-shaped spheres [184,185,186], micro-structures composed of nanoparticles [187], 3D pore frames [188,189], and self-assembled nanosheets [190]. The casting process employed to generate complex structures, such as hollow porous spheres in Nb₂O₅, offers several advantages that compensate for the specific advantages that individual structures alone cannot achieve. The casting method allows for precise control over the structure’s shape, size, and composition, enabling enhanced properties such as improved structural stability, increased surface area, and optimized electrolyte penetration. The synergy created by incorporating various structures within a single material can lead to superior performance in various applications. Indeed, complex microsphere structures in Nb₂O₅ can be synthesized using relatively straightforward methods, such as precipitation, hydrothermal, and solvothermal techniques. Sun et al. [183] showed that a simple oil bath process can also generate nanostructures. They designed a technique for preparing hollow mesoporous Nb₂O₅ (HM-Nb₂O₅) nanospheres with a diameter of ~300 nm by calcination at 600 °C. The thickness of the nanosphere shell was achieved by extending the reaction time caused by the controlled hydrolysis of urea (Figure 11a). Liu et al. successfully synthesized sea urchin Nb₂O₅ microspheres with a 20 nm diameter in a glycerol-isopropyl alcohol mixture using a simple solvothermal method through the self-assembly of nanorods (Figure 11b) [187]. Lou et al. [188] achieved successful synthesis of three-dimensionally ordered macroporous (3DOM) T-Nb₂O₅ nanostructures using self-assembled polystyrene (PS) microsphere colloidal crystals as a hard template. The process involved immersing the template in a precursor solution to form the desired T-Nb₂O₅ nanostructures. This approach yielded a substantial quantity of tightly packed macropores measuring 200 nm in diameter. The surface of the synthesized material exhibited many additional macropores, ranging from 60 to 130 nm, along with mesoporous structures (as illustrated in Figure 11c,d).

Overall, the 0D nanostructure offers a significant specific surface area that enhances the contribution of pseudocapacitors. However, it is limited by particle agglomeration resulting from repeated high-rate charging and discharging, which negatively impacts electrode lifespan, contradicting its intended purpose. In the case of 1D nanomaterials grown along (001) surfaces, their unique aspect ratio is crucial in facilitating the rapid transmission of Li⁺ ions, significantly enhancing Li storage dynamics. Nevertheless, the inherent stable chemical properties of Nb₂O₅ contribute to the complexity and lack of control observed during the preparation process. Generally, 2D nanostructures with expandable anisotropic ion shuttle paths and open edges offer larger Li attachment sites, enabling higher capacity storage in LIBs. Researchers favor complex nanostructures, including heterogeneous multistage mesoporous structures, due to their inherent versatility and ability to address specific requirements that cannot be met by a single structure alone. This versatility extends to promoting solvent diffusion and fulfilling diverse application needs. However, complex nanostructures are also constrained by their structural durability deficiencies, resulting in a low volume packing density. Therefore, an increasingly popular approach to address these limitations involves optimizing structural engineering through the formation of composite materials (Figure 12).

Conclusion: Nb oxide plays a significant role in energy storage thanks to its distinctive crystal structure, similar to other important materials, and its exceptional chemical stability. Among its various forms, Nb₂O₅ offers clear advantages over commercial graphite anode materials. Nb₂O₅ boasts high power, enhanced safety, facilitation of abundant REDOX reactions, and minimal volume expansion. These advantages contribute to its potential for commercial applications, particularly in small batteries and the intercalated pseudocapacitor lithium storage mechanism. However, it is essential to acknowledge that while employing a high-voltage window can prevent the formation of lithium dendrites, it can also lead to a trade-off in terms of lower energy density. As a result, Nb₂O₅ faces many challenges in its electrochemistry application due to its low conductivity as a semiconductor material and harsh synthesis conditions.

4.2. Supercapacitor

Electric vehicles are witnessing a significant shift toward using batteries and HSCs, with applications in consumer electronics, large power stations, and hybrid electric vehicles (HEVs) [191,192]. SCs offer distinct advantages compared to batteries, including significantly higher power density, longer cycle life, and rapid charging capabilities. However, the primary disadvantage of SCs lies in their relatively low energy density due to limitations in electrode size, restricting their storage capacity. One potential strategy to address these challenges is to focus on developing high-energy-density electrodes.

The primary benefits of Nb₂O₅-based material being used in SCs and LIBs can be summarized as follows: (1) Nb₂O₅-based materials will not precipitate metal Li due to their high potential platform; (2) Nb₂O₅-based materials are non-toxic with higher specific capacity than Li₄Ti₅O₁₂; (3) Nb₂O₅-based material exhibit high thermodynamic stability, ensuring good safety performance of LIBs; (4) Nb₂O₅-based materials feature fast electrochemical reaction kinetics and long cycle life.

However, it has a drawback in its inherent poor electronic conductivity. In addition, Nb₂O₅-based materials lack a flat charge-discharge platform and possess a lower theoretical capacity than other alloy-type or converted compounds [153,193]. Furthermore, a power capacity imbalance exists between fast non-Faraday capacitive cathodes and slow Faraday battery-type anodes [30]. Despite these limitations, Nb₂O₅-based materials hold promise in EESC devices due to their unique structural advantages, strong rate performance, and cycle stability.

4.2.1. High Rate Electrode

Kim et al. conducted a study to assess the impact of crystallinity on the capacitance response of Nb₂O₅ for SCs [171]. Meanwhile, Augustyn and colleagues demonstrated the utilization of electrodes with a thickness of up to 40 mm to achieve high-speed charge storage devices through intercalated pseudocapacitors [28]. Lubimtsev et al. conducted an additional study to investigate the underlying factors contributing to the high-rate behavior of intercalated pseudocapacitors in Nb₂O₅ crystals [194]. An effective approach to address these challenges to developing high-energy-density electrodes is combining conductive carbon materials, doping techniques, and morphology control in the design of composite materials [195].

Lim and colleagues [30] employed a general process to prepare core-shell T-Nb₂O₅@carbon nanocrystals (T-Nb₂O₅/C NCs) and TT-Nb₂O₅/C composite materials. In a study by Lim et al. [196] the anode was fabricated using mesoporous T-Nb₂O₅@carbon nanocomposites. The resulting HSCs, assembled with activated carbon (AC) as the cathode, exhibited an impressive energy density of 74 Wh kg⁻¹ and a power density of 18.51 kW kg⁻¹ (Figure 13a). After subjecting the HSCs to 1000 cycles within the voltage range of 1–3.5 V at a current density of 1 A g⁻¹, the capacity retention rate remained approximately 90%. This indicates that the electrode material exhibited commendable cycling stability and retained a significant portion of its initial capacity even after prolonged use. In another study, Wang et al. [197] utilized thin carbon-coated T-Nb₂O₅ nanowires to assemble 3 V T-Nb₂O₅@C||AC HSCs. The HSCs exhibited a prominent energy density of ~43 Wh kg⁻¹ at 7.5 kW kg⁻¹ and significant cycle stability (Figure 13b). Wang and colleagues developed an SC electrode comprising a closely mixed network of carbon nanotubes (CNTs) and Nb₂O₅ nanocrystals. This innovative electrode design aimed to achieve SCs with high capacitance, excellent rate performance, and cycling capability [198]. Furthermore, Zhang and colleagues [199] successfully prepared T-Nb₂O₅@carbon (Nb₂O₅@C) and T-Nb₂O₅@mesoporous carbon (Nb₂O₅@MC). Both samples were coated with porous carbon shells, enhancing structural stability and electrochemical performance (Figure 13c,d). The improved electrochemical performance of Nb₂O₅@MC can be attributed to the Nb₂O₅ nanoparticles with a high specific surface area (SSA), enabling more charge storage capacity. The interlinked mesoporous carbon shells also facilitate efficient ion diffusion and enhance charge transfer kinetics within the electrode material.

Recently, researchers have been exploring using metal-organic frameworks (MOFs) as a promising platform for preparing porous carbon materials and porous tetramethyl orthosilicate (TMOs). MOFs are renowned for their high porosity, large specific surface area, and well-defined porous structures. These properties make MOFs an attractive choice for creating materials with desirable porosity and surface area, with applications in diverse fields, including energy storage and catalysis. For example, Liu et al. [200] prepared T-Nb₂O₅ quantum dots (QDs) encapsulated in N-doped porous carbon derived from ZIF-8, referred to as NQD-NC. The resulting T-Nb₂O₅ QD was a unique composite material with enhanced properties. Indeed, the design of carbon nanotube (CNT) composites has emerged as an effective strategy to enhance the electrochemical performance of electrode materials. CNTs possess high electron conductivity and a large specific surface area, making them ideal for improving the performance across various electrochemical reactions. The high electron conductivity of CNTs facilitates the rapid movement of electrons within the composite, enabling efficient transfer during electrochemical processes. This promotes faster reaction kinetics and can lead to enhanced electrochemical performance [201]. For instance, Wang [202] and colleagues prepared T-Nb₂O₅ nanocrystals grown in situ on carbon nanotubes. Graphene, known for its substantial specific surface area, impressive flexibility, superconductivity, and wide voltage window, is being extensively investigated as a potential electrode material for SCs [203]. The doping process can be utilized to improve the electronic conductivity of materials by reducing the band gap, a feature particularly crucial in the context of SCs [204,205].

In summary, the challenge of low electrical conductivity in Nb₂O₅ can be effectively addressed through two common strategies: fabricating nanostructures with diverse morphologies and integrating Nb₂O₅ with carbon-based materials. These approaches have demonstrated significant effectiveness in overcoming the conductivity limitations associated with Nb₂O₅.

4.2.2. Hybrid Supercapacitors

In contrast to rechargeable batteries, SCs exhibit an extended operational lifespan and capacity to facilitate rapid charge and discharge processes. These qualities contribute to their high power density and long cycle life. However, SCs typically possess a lower energy density than rechargeable batteries. Meanwhile, LIBs are known for their high energy density, providing significant stored energy. However, they are often limited in power density and cycle life. A hybrid supercapacitor (HSC) comprising a positive metal oxide electrode with pseudocapacitance behavior and a negative activated carbon electrode with EDLC behavior is an effective method to improve the energy density of EDLC.

The operation principle of this HSC involves reversible non-Faraday reactions, such as the adsorption of Li⁺ and Na⁺ ions on the surface of active and porous carbon materials at the positive electrode, facilitating charge storage. Simultaneously, reversible Faraday reactions take place at the metal oxide electrodes. SCs adopting this asymmetric configuration can accumulate charge utilizing the Faraday REDOX electrochemical reaction process. This enhances the specific capacitance and extends the operating voltage range, ultimately improving the energy density of the hybrid capacitor.

Previously, extensive research on RuO₂ as an electrode material for HSCs has been primarily driven by its excellent specific capacitance, cyclic stability, and conductivity [206]. Nonetheless, the adoption of RuO₂ as an electrode material for HSCs is hindered by its high cost, which restricts its widespread application.

Recent studies have revealed that bulk Nb₂O₅ has a higher capacity than the conventional Li₄Ti₅O₁₂ under high magnification conditions. This suggests that Nb₂O₅ exhibits pseudocapacitive electrochemical characteristics. This phenomenon is explicitly observed on the surface of Nb₂O₅ rather than throughout the entire bulk crystal.

DFT calculations have indicated that forming a solid solution by incorporating lithium atoms at specific sites enables the selective provision of electrons to adjacent atoms. This results in the reduction of niobium within that region. The observed high specific capacitance in the case of Nb₂O₅ may be attributed to its nanoparticle structure. The presence of nanoparticles provides a larger surface area for electrochemical reactions, resulting in enhanced pseudocapacitance and overall improved performance at high magnification [194]. These results also provide directions for improving the performance of nanoporous Nb₂O₅.

Among the various crystal structures of Nb₂O₅, orthogonal Nb₂O₅ exhibits the highest relative capacity. However, although the nanoparticle structure of Nb₂O₅ contributes to its exceptional performance at high magnifications, the formation of this structure at temperatures above 600 °C can potentially lead to nanoparticle aggregation. Nano-scale rhombic system Nb₂O₅ is considered a favorable material for enhancing anode system dynamics. Despite the challenges associated with synthesizing small-sized rhombic Nb₂O₅ nanoparticles, alternative approaches have been developed to incorporate Nb₂O₅ into nanocomposite structures. One example is synthesizing mesoporous Nb₂O₅/carbon (m-Nb₂O₅-C) nanocomposites using a one-pot method assisted by block copolymer self-assembly [196]. Nb₂O₅@C NCs with controllable crystal phases, including the rhombic system (T) and pseudohexagonal system (TT), were synthesized using a one-pot microemulsion method. The pH condition of the water-in-oil microemulsion system significantly influences the control of the crystalline phase of Nb₂O₅ [30]. Under acidic conditions, T-Nb₂O₅@C nanocrystals are formed, while under alkaline conditions, TT-Nb₂O₅@C nanocrystals are obtained.

Therefore, utilizing Nb₂O₅ as an SC electrode and combining it with conductive carbon materials to improve charge transfer has emerged as a new research direction. For example, materials such as T-Nb₂O₅/graphene and Nb₂O₅/CNTs have demonstrated remarkable power density and cycling performance [198,202,207,208].

In addition, selecting suitable electrolyte solutions presents a challenge when employing Nb₂O₅ for SC applications. Recent research has indicated that using an electrolyte consisting of 1 M lithium perchlorate in a mixture of ethyl carbonate and dimethyl carbonate leads to the attainment of the highest capacitance and Coulomb efficiencies [208]. Moreover, although the Nb₂O₅ electrode has excellent lithium storage performance, due to the lack of lithium resources and the low cost and abundant sodium resources, the application of sodium ion batteries (SIB) has been widely studied recently. Meanwhile, the application of its energy storage performance in SIB has declined sharply due to its larger Na-ion radius and sluggish Na-ion diffusion. Therefore, preparing new Nb₂O₅ electrodes using various modification strategies to improve the electrochemical performance of SIB is an important research direction.

5. Conclusions and Perspectives

This review provided an overview of the fundamental crystal structure and physical and chemical properties of Nb₂O₅. Various synthesis methods for fabricating Nb₂O₅ polycrystals and the general applications of Nb₂O₅ in lithium batteries and supercapacitors (SCs) were summarized.

Nb₂O₅ exhibits polymorphs, including pseudohexagonal (TT-Nb₂O₅), orthorhombic (T-Nb₂O₅), and monoclinic (H-Nb₂O₅) etc. The crystal structures of these different phases were presented, and the research progress pertaining to their crystal structures and applications was discussed. The TT phase Nb₂O₅ was given particular emphasis. Various nanostructured Nb₂O₅, including nanopores, nanosheets, nanorods, nanochannels, and nanowires, have been successfully synthesized using different methods. However, it’s worth noting that there are still unexplored and uncharacterized nanoforms.

Similar to many other transition metal materials, pure Nb₂O₅ primarily exhibits poor conductivity. To address this limitation, incorporating heteroatoms, carbon, metals, and conductive polymers has proven effective in enhancing the electrical conductivity of Nb₂O₅. Research on forming oxygen vacancies by doping non-metallic elements in Nb₂O₅ remains relatively limited. Further investigations in this area are necessary to improve the electrochemical performance of such materials.

Nb₂O₅ plays an essential role in energy storage due to its unique crystal structure for fast charging, rich REDOX reaction capability, intercalation-based storage mechanism, high chemical stability, ultra-small volume expansion, and high potential window, which often served as electrode material for Li-ion batteries or supercapacitors. However, the electrochemical performance of Nb₂O₅-based electrodes still faces two key challenges. The first challenge is that Nb₂O₅ exhibits low conductivity and a high synthesis temperature. The second challenge is that Nb₂O₅ has a relatively low theoretical capacity and energy density. The T-Nb₂O₅ has fast rate capability with limited space for cations intercalation, while H-Nb₂O₅ offers more cation intercalation sites with slow rate capability. Active particles are frequently reduced to nanoscale dimensions to overcome the relatively slow solid-state ion diffusion and achieve rapid charging and high power. Optimizing the crystal phase or nanostructure of Nb₂O₅ is a promising approach to enhancing both rate and capacity performances. Another effective strategy involves doping other materials with Nb₂O₅.

In addition, Nb₂O₅ has been utilized as a sensing material for sensors. The synergistic integration of Nb₂O₅ with biomedicine has been explored to develop innovative solutions for diagnostics, therapeutics, and medical devices. Recently, Nb₂O₅ has been investigated for application in transistors, memristors, and superconductors. A better understanding of the Nb₂O₅ structure–property relationship is needed for further research efforts. This will facilitate the full harnessing of its capabilities and optimize its performance.