Layered sodium vanadate (NaV₈O₂₀) nanobelts: a new high-performing pseudocapacitive material for sodium-ion storage applications

Abstract

Multifunctional layered nanostructures have attracted great attention for next-generation electrochemical supercapacitors and metal-ion batteries. Herein, we use a hydrothermal method to demonstrate the synthesis of 1D and layered sodium vanadate (NaV₈O₂₀) nanobelts architecture. These NaV₈O₂₀ nanobelts demonstrate outstanding electrochemical performance in supercapacitors (SCs) and sodium-ion batteries (SIBs). The possible formation mechanism of NaV₈O₂₀ nanobelts is briefly discussed. Benefiting from the 1D and layered nanostructure, pre-inserted cations, significantly enhanced electrochemical conductivity, and high electroactive surface area, the prepared NaV₈O₂₀ electrode material exhibited excellent charge storage capacity, favorable rate, and cyclic stability performance. The NaV₈O₂₀ nanobelts displayed outstanding electrochemical characteristics, including 676 F g⁻¹ of specific capacitance, 45 W h kg⁻¹ of energy density and 5224 W kg⁻¹ of power density. Additionally, on testing in Na-ion batteries, the NaV₈O₂₀ nanobelts exhibit a discharge capacity of 110 mA h g⁻¹ at 10 mA g⁻¹ and retain ∼52% capacity after 100 cycles. Along with this, the galvanostatic intermittent titration technique (GITT) measurements reveal a high diffusion coefficient for Na⁺ ions, highlighting the efficient Na⁺ ions transportation within the NaV₈O₂₀ structure. To our knowledge, this is the first report on the use of NaV₈O₂₀ nanobelts for both SIBs and SCs, marking a significant contribution to the development of multifunctional materials for energy storage applications.

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1. Introduction

The rapid advancement and proliferation of electrochemical energy storage (EES) technologies are reshaping modern life while laying the foundation for a sustainable energy future. The growing demand for portable/wearable electronics, electric vehicles, and stationary energy storage systems has sparked significant global interest in EES technologies, particularly supercapacitors (SCs) and metal-ion batteries (MIBs). These devices have emerged as critical components for addressing the energy challenges of modern technology due to their distinct capabilities in energy storage and power delivery.^1,2 However, to address the growing requirements for energy storage systems, the performance of these EES devices must be significantly improved. The working electrode material is one of the most essential parameters for EES applications.³ Therefore, enhancing the charge storage capacity of electrode materials has become a primary focus in recent EES research. One-dimensional (1D) and layered nanostructures have garnered significant scientific and technical attention for EES systems, attributed to their high electroactive surface area, structural stability, short charge carrier transport pathways, and favorable intercalation/de-intercalation processes.⁴ Various 1D and layered nanostructures, including carbon nanostructures, conducting polymers, metal phosphides and metal chalcogenides, have been investigated as potential electrode materials (both cathode and anode) for SC and MIB applications.^5–10 Among these, transition metal oxides (TMOs) stand out as highly favorable electrodes for EES due to their multifarious oxidation states, high theoretical specific capacitance, diverse 1D/2D nanostructured morphologies, high electroactive surface area, and cost-effective fabrication processes.^11–13

Transition metal vanadates, a class of bimetallic TMOs, have gathered great scientific attention as electrode materials for SCs and MIBs. Their appeal stems from their high surface area, low-dimensional and layered structures, excellent pseudocapacitive properties, enhanced electrical conductivity, and superior cyclic stability. These characteristics enable rapid and reversible surface redox reactions of electroactive species, making them highly suitable for advanced energy storage applications.^14,15 Currently, different binary transition metal vanadates such as Co₃V₂O₈, FeVO₄, MnV₂O₈, Ni₃V₂O₈, and Cu₃V₂O₈ have proven their ability in SCs and MIBs applications owing to their wide working potential window, high specific capacitance, and energy storage capacity compared to their pristine metal oxide counterparts.^16–20 For example, quasi-cuboidal CoV₂O₆ has been shown to exhibit a specific capacitance of 223 F g⁻¹ at 1 A g⁻¹, along with remarkable cyclic stability of 123% after 15 000 cycles, making it suitable for SC applications.²¹ Similarly, 3D flower-like Ni₃V₂O₈@CNTs have proven effective in enhancing energy storage capacity due to their high specific capacitance of 1054 F g⁻¹ at 1 A g⁻¹ and retain high cyclic stability of 89% after 10 000 cycles.²² Pseudocapacitive layered Fe₅V₁₅O₃₉(OH)₉·9H₂O nanosheets exhibit significant potential as a cathode material for lithium-ion batteries, achieving a high reversible capacity of 350 mA h g⁻¹ at 100 mA g⁻¹, indicating its capability for long-term cycling stability.²³ Similarly, Zr–NH₄V₄O₁₀ has been identified as a promising cathode material for sodium-ion batteries, achieving a high specific capacity of 342 mA h g⁻¹ at 100 mA g⁻¹, positioning it as an appealing option for large-scale energy storage systems.²⁴ However, despite these advancements, several challenges persist, including the complex synthesis procedures, use of toxic and hazardous materials and high cost of precursor materials, which hinder the feasibility of these materials for widespread industrial applications.¹⁵ Due to its numerous significant benefits, the hydrothermal method is the most frequently utilized for synthesizing metal vanadates. This approach enables accurate regulation of the morphology and particle size of resultant nanostructures, which is essential for enhancing the electrochemical performance of energy storage devices. This process is environmentally friendly and scalable, capable of being conducted at relatively low temperatures and pressures, contributing to its energy efficiency and cost-effectiveness. Furthermore, the hydrothermal approach facilitates the creation of well-crystallised nanostructures that exhibit high purity, thereby improving their applicability in energy storage applications. Considering these aspects, low-cost and environment-friendly alkali metals (Li, Na, K) can be appropriate alternatives to toxic and expensive metals for developing metal vanadate materials. However, alkali metal-inserted vanadate materials have been less explored for electrochemical SC and MIB applications.

In this study, we address these above-mentioned challenges by developing a simple, cost-effective hydrothermal route to prepare the 1D and layered structural sodium vanadate (NaV₈O₂₀) nanobelts without any post-annealing treatment. Sodium vanadate is particularly gaining attention due to its pre-inserted sodium ions, which enhances electrochemical performance and its suitability as a low-cost charge storage material for SCs and sodium-ion battery (SIB) applications. To the best of our knowledge, no prior publication has been reported that explored the electrochemical SCs and sodium-ion battery (SIB) applications of the 1D and layered NaV₈O₂₀ structures. Owing to the unique 1D layered structure and nanobelts morphology features, the NaV₈O₂₀ electrode displayed outstanding electrochemical features with high specific capacitance (C_sp) and cyclic stability. The NaV₈O₂₀ material was investigated using various electrochemical and physicochemical characterization techniques to evaluate its structural, morphological and electrochemical characteristics. We have also successfully demonstrated the practical applicability of NaV₈O₂₀ nanobelts in electrochemical applications, particularly in solid-state asymmetric SCs, highlighting their potential for advanced EES systems.

2. Experimental section

2.1 Synthesis of NaV₈O₂₀ nanobelts

The details of chemicals and reagents are provided in ESI, S1(a).† The chemicals and reagents used for all the experiments were of analytical grades. A hydrothermal method was used to synthesize NaV₈O₂₀ material (Fig. 1a). First, 0.75 g of V₂O₅ powder was dispersed in 40 ml of double distilled water and ultrasonicated for 90 min. Then, with continuous stirring, 1 M Na₂SO₄ (30 ml) aqueous solution was gradually added to the V₂O₅ suspension. After 30 min, the mixture was transferred into a 100 ml Teflon-lined stainless-steel autoclave and heated to 180 °C for 48 h. After hydrothermal treatment, the obtained greenish-colored product was washed several times with double distilled water. Finally, the obtained product was dried at 150 °C in a vacuum oven.

Fig. 1 Schematic representation of (a) hydrothermal synthesis of NaV₈O₂₀ nanobelts, (b) preparation of working electrode, (c) fabrication of ASC device and (d) fabrication of Na-ion battery.

2.2 Preparation of working electrode, AC//NaV₈O₂₀ asymmetric SC and Na-ion battery

The schematic representation of the working electrode fabrication via the slurry casting method is illustrated in Fig. 1b. Initially, a homogeneous slurry was prepared by mixing NaV₈O₂₀ nanobelts (active material), polyvinylidene difluoride (PVDF) as a binder, and activated carbon (AC) as a conductive additive in a weight ratio of 8 : 1 : 1, using N-methyl-pyrrolidone (NMP) as the solvent. This slurry was then uniformly coated onto a Toray carbon paper substrate. The coated electrode was subsequently dried under vacuum at 150 °C for 24 hours. The final electrode had an active surface area of 1 × 1 cm², with a mass loading of ∼1.2 mg.

Fig. 1c and d depicts the schematic for the device manufacture processes for SC and SIB coin cells of the NaV₈O₂₀ nanobelts material. As shown in Fig. 1c, the AC//NaV₈O₂₀ asymmetric SC (ASC) device was fabricated using NaV₈O₂₀ nanobelts as cathode (∼1.2 mg of mass loading) and AC as an anode (∼2 mg of mass loading) materials prepared onto a circular 16 mm carbon paper current collector. These electrodes were separated using a cellulose membrane. The PVA–NaClO₄ gel electrolyte was prepared by mixing 2 g NaClO₄ and 2 g PVA in 25 ml DDW and then heated at 80 °C for 3 h. Finally, the AC//NaV₈O₂₀ asymmetric SC device was fabricated with a CR2032 coin cell using the PVA–NaClO₄ gel electrolyte.

The sodium-ion storage performance of NaV₈O₂₀ nanobelts as the active cathode material was investigated. The electrode was fabricated by combining NaV₈O₂₀, PVDF, and AC with a weight ratio of 8 : 1 : 1 and thoroughly mixed in NMP solvent. The prepared homogeneous slurry was uniformly coated onto a current collector (aluminium foil) using the doctor blade method and subsequently dried under a vacuum oven at 100 °C for 12 hours. The dried film was then cut into 16 mm diameter discs with a mass loading of approximately 0.745 mg cm⁻². Sodium metal was utilized as the counter and reference electrode, while a 19 mm diameter glass-fiber Whatman separator was employed. The 2032-type coin cells were assembled in an argon-filled glove box, where oxygen and moisture levels were controlled at 0.1 ppm. A 1 M solution of NaClO₄ in ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 v/v), with the addition of 5% fluoroethylene carbonate (FEC), was used as the electrolyte. A schematic representation of the Na⁺ ion battery assembly process is provided in Fig. 1d.

2.3 Electrochemical measurements

All the electrochemical measurements of electrodes and fabricated devices, such as cyclic voltammetry (CV), galvanostatic charging–discharging (GCD), electrochemical impedance spectroscopy (EIS), rate capability, and cycling stability tests were conducted on the IVIUM-Vertex 1A electrochemical workstation (Ivium Technologies, Netherlands).

2.4 Physicochemical characterization of 1D and layered nanostructured NaV₈O₂₀ nanobelts

The physicochemical characteristics of the synthesized material were investigated using various techniques, including X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Further details regarding these methods are provided in the ESI, S1(b).†

3. Results and discussion

3.1 XRD studies

XRD characterization was carried out to identify the crystal structure and phase purity of the NaV₈O₂₀ material. Fig. 2a shows the comparative XRD pattern of bulk V₂O₅ and NaV₈O₂₀ powders. The diffraction pattern of bulk V₂O₅ powder displays sharp and intense diffraction peaks, which match well to α-V₂O₅ phase (JCPDS no. 77-2418) with an orthorhombic crystal structure. By comparing both the diffraction patterns, it is clear that, after hydrothermal treatment, the V₂O₅ phase is converted into the newly formed NaV₈O₂₀ phase. In the XRD pattern of NaV₈O₂₀ powder, all the observed peaks are consistent with the characteristic peaks of the NaV₈O₂₀ phase (JCPDS no. 45-1363) with a monoclinic crystal structure. Furthermore, no additional diffraction peaks related to V₂O₅ or any other sodium vanadate phases were observed, implying the high phase purity of the NaV₈O₂₀ phase. However, the NaV₈O₂₀ phase displays relatively low-intensity and broad diffraction peaks compared with bulk V₂O₅, corresponding to the small crystallite size and low crystallinity of the NaV₈O₂₀ nanobelts material. The intense diffraction from the (001) plane illustrates the preferential orientation of the NaV₈O₂₀ phase in the c direction, which possesses the layered crystal structure arrangement.²⁵ The interlayer spacing (d-value) estimated from the (001) diffraction peak was ∼10 Å. This large interlayer spacing is favorable for accommodating Na⁺ ions during electrochemical processes. Rietveld refinement of the NaV₈O₂₀ phase was conducted to identify the position of the sodium ions in the NaV₈O₂₀ phase. Fig. 2b depicts the Rietveld refinement analysis of the NaV₈O₂₀ nanobelts, performed using the MAUD software. The XRD pattern of the NaV₈O₂₀ sample was fitted using the standard crystallography information file (CIF) consisting of (Na,Ca)(V,Fe)₈O₂₀·nH₂O and JCPDS no. 45-1363.²⁶ By replacing the Ca and Fe atomic sites with corresponding amounts of Na and V, the refined analysis resulted in good agreement factors (R_wp = 3.03, R_exp = 0.62 and goodness of fit, GoF = 4.89). The parameters that are refined are (1) microstructure, (2) basic phase parameters, (3) background and scale parameters, and (4) crystal structure (lattice parameter, occupancy, fractional coordinate, and isotropic displacement factor, B_iso). The refined lattice parameters are: a = 11.74(2) Å, b = 3.6600(6) Å, and c = 11.117(5) Å with monoclinic phase, C2/m space group and β angle value = 102.4(1)°. The refinement data are mentioned in the ESI, Table S1.†

Fig. 2 (a) XRD patterns of V₂O₅ and NaV₈O₂₀ powder samples and (b) XRD refinement analysis of NaV₈O₂₀ powder sample.

3.2 FE-SEM, BET and TEM studies

The morphological properties of NaV₈O₂₀ material were elucidated by FESEM techniques (Fig. 3a and b). The low-resolution FESEM image (Fig. 3a) shows a network of uniformly distributed nanobelts adjoining each other to establish the self-assembled architecture of NaV₈O₂₀ nanobelts. This 1D and densely attached nanobelt morphology will be favorable for electron transportation and will help to enhancing the diffusion of Na⁺ ions by serving maximum electroactive sites during the electrochemical interactions.^27,28

Fig. 3 (a and b) FESEM images, (c) histogram for average width; (d) N₂ adsorption/desorption isotherms; (e and f) TEM images, (g–j) HR-TEM images, (k) SAED pattern and (l–o) elemental mapping of NaV₈O₂₀ nanobelts.

Fig. 3b represents the high-resolution FESEM image of the NaV₈O₂₀ nanobelts. The width of NaV₈O₂₀ nanobelts was measured using ImageJ software and the Gaussian function was used to calculate the average width of nanobelts. The calculated average width (Fig. 3c) of the NaV₈O₂₀ nanobelt was ∼49 nm. Additionally, to identify the surface area and pore size of the NaV₈O₂₀ nanobelts sample, N₂ adsorption/desorption isotherm analysis was used. Fig. 3d shows the mesopores nature of NaV₈O₂₀ nanobelts with a specific surface area of 12.41 m² g⁻¹ and pore size of ∼43.3 nm (inset of Fig. 3d). Furthermore, crystallinity, morphological and elemental analysis of the NaV₈O₂₀ nanobelts sample was investigated through TEM analysis. The low and high-magnification TEM images (Fig. 3e and f) display the nanobelts-like morphology of the NaV₈O₂₀ material. The corresponding HRTEM images (Fig. 3g–i) depict the well-observable lattice fringes. The measured d-spacings of 0.22, 0.45, and 0.36 nm correspond to the (204), (201), and (003) lattice planes of the NaV₈O₂₀ nanobelts. The selected area electron diffraction (SAED) pattern (Fig. 3k) further confirms the crystallinity of the NaV₈O₂₀ nanobelts, revealing their polycrystalline nature. Furthermore, the elemental mapping images (Fig. 3l–o) show a uniform spreading of Na, V, and O elements all over the NaV₈O₂₀ nanobelts surface.

The X-ray photoelectron spectroscopy analysis of the NaV₈O₂₀ nanobelts sample is provided in the ESI, S2.†

3.3 Formation mechanism of NaV₈O₂₀ nanobelts

The possible formation mechanism of the NaV₈O₂₀ nanobelts (Fig. 4) can be proposed based on their 1D nanobelt morphology and layered crystal structure, as confirmed by structural and morphological characterizations.

Fig. 4 Schematic representation for the possible formation mechanism of NaV₈O₂₀ nanobelts.

A hydrothermal synthesis route was used to prepare 1D and layered NaV₈O₂₀ nanobelts. In the typical synthesis procedure, bulk V₂O₅ powder (vanadium precursor) was pre-treated with ultrasonication in double distilled water to break bulk vanadium particles into smaller particles. This ultrasonication process promotes the dissociation and formation of free vanadium ionic species such as decavanadate ([V₁₀O₂₈]⁶⁻), polyoxovanadate (V₈O₂₀⁴⁻), vanadyl ions (VO²⁺), hydrated vanadium species etc., because of partial reduction and condensation of vanadium.^29,30

Here, the presence of water plays a vital role in the hydrolysis of V₂O₅. When Na₂SO₄ is added to the ionic solution of vanadium, the Na⁺ ions interact with V₈O₂₀⁴⁻ ions, forming hydrated sodium vanadate complexes. Here, the free sulfate ions (SO₄²⁻) do not participate directly in the formation of the final product and water molecules (crystal water) help to stabilize these complexes by maintaining charge balance, leading to the formation of the NaV₈O₂₀·2H₂O (complex), where water integrates sodium ions into the polyvanadate structure.

During the hydrothermal treatment, the system is subjected to high temperature and pressure, promoting dehydration and crystallization. The removal of crystal water during this step drives the organization of sodium vanadate species into well-defined and layered nanobelt structures. The dehydration process facilitates the reorganization of the vanadate layers, promoting anisotropic and preferentially growth NaV₈O₂₀ nanobelts along the c direction, confirmed by XRD with an intense peak at 8.8° with Miller indices (001).

Afterward, the FESEM image reveals that the NaV₈O₂₀ nanobelts adopt a self-assembled morphology. This may be due to the removal of crystal water during crystallization, facilitating the formation of smooth, well-defined nanobelts. The self-assembly process is further driven by the depreciation of structural energy during crystallization, with sodium ions playing a role in charge stabilization.³¹

4. Electrochemical investigation

4.1 Electrochemical performance of NaV₈O₂₀ nanobelts electrode

The electrochemical charge storage features of the NaV₈O₂₀ electrode material for supercapacitor studies were evaluated in 1 M Na₂SO₄ electrolyte within a 0 to 0.6 V voltage window. The CV curves of the NaV₈O₂₀ electrode were recorded for various scan rates ranging from 2.5 to 50 mV s⁻¹, as shown in Fig. 5a. In Fig. 5a, the CV curves of the NaV₈O₂₀ electrode exhibit a typical extrinsic pseudocapacitive nature consisting of a nearly rectangular shape with broad oxidation and reduction peaks. This extrinsic pseudocapacitive nature mainly arises from surface faradaic charge transfer reactions due to the intercalation or diffusion of charge-compensating electrolyte ions.³² However, this nature is well retained despite decreasing the scan rate up to 2.5 mV s⁻¹, implying outstanding rate performance of the NaV₈O₂₀ electrode in Na₂SO₄ aqueous solutions. From the CV curves, the C_sp values for different scan rates of the NaV₈O₂₀ electrode were calculated using eqn (E1) (ESI, S1(b)†) and are given in the ESI, Table S2.†

Fig. 5 (a) CV curves, (b) C_sp from CV curves versus square root of scan rate, (c) histogram for charge storage contribution; (d) GCD profile; (e) plot of C_spvs. current density; (f) Nyquist plots and (g) cyclic stability of NaV₈O₂₀ electrode.

The CV curves of the NaV₈O₂₀ electrode reveal a hybrid charge storage behaviour, combining pseudocapacitive and electric double-layer (EDLC) characteristics. The Trasatti method is employed to understand the charge storage mechanism of the NaV₈O₂₀ nanobelts. This method helps distinguish between the contributions of surface-capacitive and diffusion-controlled processes. According to Trasatti, the overall capacitance arises from a blend of these two mechanisms: surface capacitive reactions and diffusion-driven charge storage.^33,34Fig. 5b depicts the Trasatti plot, plotted using C_sp against the square root of the scan rate. The y-axis intercept of the linear fit represents the surface-controlled contribution. Based on this intercept, the proportion of surface capacitive and diffusion-controlled charge storage components was calculated across various scan rates. These results are displayed in Fig. 5c as a histogram. This histogram shows that the surface capacitive (EDLC) contribution has the dominant charge storage mechanism over the diffusion-controlled contribution. By increasing the scan rate from 2.5 to 50 mV s⁻¹, the surface capacitive contributions improved from 56 to 85% with no noticeable CV curve distortion detected, revealing a better rate performance of the NaV₈O₂₀ electrode. Furthermore, at lower scan rates (2.5 to 20 mV s⁻¹), broad redox peaks and quasi-rectangular CV curves were observed (ESI, Fig. S3(d)†). This is due to surface faradaic reactions, clarifying the extrinsic pseudocapacitive nature. During the cathodic CV scans (ESI, Fig. S3(d)†), two reduction peaks at ∼0.2 and ∼0.35 V vs. SCE arose because of the reduction of vanadium ionic species from V⁵⁺ → V⁴⁺ and V⁴⁺ → V³⁺. Whereas, in the anodic CV scans, two oxidation peaks at ∼0.1 and ∼0.24 V vs. SCE were detected due to the oxidation of vanadium ionic species from V³⁺ → V⁴⁺ and V⁴⁺ → V⁵⁺. Interestingly, by increasing the scan rates, the cathodic peaks gradually shifted towards the higher potentials, indicating that rapid and scan rate-dependent redox reactions occurred near the NaV₈O₂₀ electrode surface.³⁵Fig. 5d shows the GCD curves of the NaV₈O₂₀ electrode, performed 0.5 to 32 A g⁻¹ of current densities. All the GCD curves show an ideal triangular and symmetric nature with a slight hump at ∼0.4 V vs. SCE, which indicates fast faradaic reactions and high reversibility of Na⁺ ions on the NaV₈O₂₀ electrode surface. Additionally, the GCD curves show extended charging–discharging characteristics with a negligible iR drop, signifying the outstanding rate capability. The C_sp values were calculated using eqn (E2) (see ESI†) and summarized in the ESI, Table S3.† The NaV₈O₂₀ electrode displays an optimal C_sp of 676 F g⁻¹ at a current density of 0.5 A g⁻¹; afterwards, it decreases with an increase in current density (as shown in Fig. 5e). At a 32 A g⁻¹ current density, the NaV₈O₂₀ electrode still provides the admirable C_sp of 142 F g⁻¹. We believe these excellent C_sp values mainly arise from the pre-insertion of Na⁺ into the V₈O₂₀ structure, which offers a high electroactive surface area, enhances the overall electronic conductivity and simplifies the Na⁺ ions insertion/de-insertion process. The electrode material's low electric and electrochemical resistances are typically preferred for the supercapacitor's longer cyclic lifespan and versatile applicability. The electrochemical resistance characteristics of the NaV₈O₂₀ electrode material were evaluated through EIS measurements. The Nyquist plot for the NaV₈O₂₀ electrode is presented in Fig. 5f. In the high-frequency region of the Nyquist plot, two key components are observed: the equivalent series resistance (R_s) and the charge transfer resistance (R_ct). R_s represents the overall internal (ohmic) resistance, which encompasses the active electrode material's intrinsic resistance, the electrolyte's ionic resistance, and the contact resistance at the electrode/electrolyte interface.^36–38 Meanwhile, the diameter of the semicircle represents the charge transfer resistance, which is associated with faradaic reactions. The simulated circuit provided that the NaV₈O₂₀ electrode possesses R_s = 4.9 Ω and R_ct = 0.55 Ω. These low resistance values are attributed to the high electronic conductivity, large electroactive surface area, enhanced electron transport rate, and reduced charge transfer resistance of the NaV₈O₂₀ electrode, which collectively enable rapid faradaic reactions at the electrode/electrolyte interface. In the Nyquist plot, the Warburg impedance (W) is associated with the vertical line in the low-frequency region, corresponding to the intercalation or diffusion of electrolyte ions inside the active electrode material. The slope of the Warburg impedance approaches a 90° angle, signifying the capacitive behaviour of the NaV₈O₂₀ electrode.

Long cyclic life is one of the significant parameters for electrochemical supercapacitor applications. The electrochemical cycling stability over the NaV₈O₂₀ electrode (Fig. 5g) was performed at a scan rate of 50 mV s⁻¹ over 10 000 cycles. In the initial ∼1000 cycles, the C_sp gradually increases and then tends to stabilize for further CV cycling. This is because of the activation of the NaV₈O₂₀ electrode through the insertion/de-insertion of Na⁺ ions and complete infiltration of Na⁺ electrolyte ions inside the active electrode material surface.²¹ The NaV₈O₂₀ electrode retains 90% of its initial C_sp after 10 000 CV cycles. This outstanding cycling stability is mainly ascribed to the layered and 1D nanobelts structured assembly of the NaV₈O₂₀ electrode, which offers significant redox active sites and efficiently accommodates the possible volume deviations through Na⁺ ions insertion/de-insertion process, which ultimately enhances the charge storage capacity of the NaV₈O₂₀ electrode material.

4.2 Electrochemical performance of the AC//NaV₈O₂₀ asymmetric supercapacitor (ASC) device

To evaluate the practical applications of the NaV₈O₂₀ nanobelts for supercapacitors, the AC//NaV₈O₂₀ ASC coin cell device (Fig. 1d) was fabricated. Before the fabrication of the SC device, the NaV₈O₂₀ (cathode) and AC (anode) were electrochemically evaluated in a three-electrode system to assess the appropriate working voltage window.

Fig. 6a shows the CV profile for the AC and NaV₈O₂₀ electrodes carried out in a three-electrode system at a scan rate of 50 mV s⁻¹ in 1 M Na₂SO₄ electrolyte. The CV curves of NaV₈O₂₀ and AC electrodes are nearly rectangular, measured between the voltage windows of 0 to 0.6 V vs. SCE and −1 to 0 V vs. SCE, respectively. These results imply that integrating the cathode and anode can provide a 1.6 V working voltage. After the fabrication of the ASC device, first, CV measurements with different voltages from 1 to 1.6 V (Fig. 6b) were performed, which indicates the actual working voltage can be extended up to 1.6 V without showing any alteration in CV curves due to aqueous electrolyte redox processes. Fig. 6c displays the CV curves of the AC//NaV₈O₂₀ ASC device, measured at 10 to 100 mV s⁻¹ scan rates. These CV curves retain the quasi-rectangular shape observed for NaV₈O₂₀ nanobelts electrodes in a three-electrode setup, confirming the pseudocapacitive behaviour of NaV₈O₂₀ nanobelt material. Fig. 6d displays the GCD curves of the AC//NaV₈O₂₀ ASC device measured at 0.5 to 16 A g⁻¹ of current densities, demonstrating good symmetric and triangular GCD curves because of the high reversible charge/discharge processes. The C_sp of the AC//NaV₈O₂₀ ASC device (ESI, Table S3†) was calculated using eqn (E2) (ESI†). Fig. 6e shows the change in C_sp with current densities. The highest C_sp of 126 F g⁻¹ was achieved at 0.5 A g⁻¹ of current density. Energy (E_d) and power densities (P_d) of the AC//NaV₈O₂₀ ASC device were calculated using the following equations:³⁹

where, C_sp is the specific capacitance of the ASC device (F g⁻¹), ΔV is the voltage window, and Δt is the discharging time of the ASC device. Fig. 6f depicts the Ragone plot of the fabricated ASC coin cell device. The AC//NaV₈O₂₀ ASC device shows the highest E_d of 45 W h kg⁻¹ at P_d of 163.26 W kg⁻¹ and the highest P_d of 5224.48 W kg⁻¹ at E_d of 14.8 W h kg⁻¹ (ESI, Table S4†), which is higher than reported supercapacitor devices, as shown in Table 1. The resistive characteristics of the AC//NaV₈O₁₆ ASC device were evaluated by EIS measurement. The Nyquist plot and the equivalent circuit for the ASC device are shown in Fig. 6g. The AC//NaV₈O₂₀ ASC device displays R_s = 3.8 and R_ct = 2.9 Ω. The cyclic life of the SC is considered a key parameter for its long-term applicability in portable electronic devices. Therefore, the cyclic stability of the AC//NaV₈O₂₀ ASC device was evaluated at a current density of 6 A g⁻¹ for 10 000 cycles and presented in Fig. 6h. The AC//NaV₈O₂₀ ASC device displayed 89% of its initial capacitance value with ∼98% coulombic efficiency. This good cyclic stability can be accredited to (1) the proper combination of faradaic and pseudocapacitive electroactive material, (2) the appropriate interlayer spacing of the electrode materials, and (3) the large surface area of both electrode materials.

Fig. 6 (a) AC and NaV₈O₂₀ electrodes CV curves in the three-electrode system, (b) CV curves for different voltage windows and (c) for different scan rates; (d) GCD curves for different current densities; (e) C_spvs. current densities; (f) Ragone plot; (g) Nyquist Plot and (h) cyclic stability of AC//NaV₈O₂₀ ASC device.

Table 1 Comparative study of NaV₈O₂₀ electrode and ASC device with previously reported metal vanadate electrodes and devices

Vanadate material	Method	Morphology	Three-electrode system			Supercapacitor device				Ref.
			Electrolyte	C sp (F g^−1 @ A g^−1)	Electrode stability (% @ cycles)	Voltage window (V)	E d (W h kg^−1)	P d (W kg^−1)	Device stability (% @ cycles)
LiVO3	Chemical route	Nanorods	1 M Li2SO4	426 @ 0.5	80 @ 2000	1.6	25	3600	80 @ 5000	40
Li3VO4/carbon	Solid-state	Nanofibers	0.1 mol per L (NH4)2SO4	—	—	3.8	110	3870	86 @ 2400	41
β-Na0.33V2O5	Hydrothermal	Nanobelts	1 M LiCO4/PC	320 @ 5 mV s^−1	—	1	47	5000	66 @ 4000	42
β-Na0.33V2O5	Spin coating	Nanowires	1 M LiCO4/PC	498 @ 0.4	90 @ 1500	—	—	—	—	43
NaV3O8	Electrochemical conversion	Nanosheets	1 M Na2SO4	640.10 @ 0.5	87.7 @ 2000	1.6	62.6	7200	81 @ 5000	44
Na2V6O16	Chemical route	Nanobelts	1 M Na2SO4	455 @ 0.5	90 @ 5000	1.6	42	4300	80 @ 5000	27
Na6V10O28	Chemical route	Microrods	1 M LiClO4 + PC	354 @ 0.1	—	2.8	73	6238	70 @ 5000	45
KV3O8	Chemical route	Nanobelts	0.5 M K2SO4	677 @ 0.5	89 @ 5000	1.8	51	4127	90 @ 5000	46
Cu3V2O8	SILAR	Nanopebbles	1.5 M NaClO4	443 @ 5 mV s^−1	85 @ 2000	1.15	8.4	345	76 @ 4000	47
S–Co3V2O8	Hydrothermal	Nanosheets	6 M KOH	410 mA h g^−1 at 2A g^−1	94 @ 4000	1.5	36	11 600	98 @ 4000	48
Mn2V2O7	Hydrothermal	Nanoparticles	3 M KOH	327 C g^−1 at 1 A g^−1	93 @ 10 000	1.5	44.3	807.4	158 @ 10 000	49
Ni3−xCoxV2O8	Hydrothermal	Oval-shaped	3 M KOH	391 @ 0.5	98 @ 8000	1.3	25.5	7500	85 @ 10 000	50
Ni3V2O8/Ni	Hydrothermal	Nanosheets	3 M KOH	1300 @ 1	80 @ 7000	1.2	33.2	2400	73 @ 10 000	51
NiCo2V2O8	Microwave-assisted	Microspheres	2 M KOH	113.2 mA h g^−1 at 0.5 A g^−1	82 @ 10 000	1.6	34.8	4000	80 @ 10 000	52
NaV 8 O 20	Hydrothermal	Nanobelts	1 M Na 2 SO 4	676 @ 0.5	90 @ 10 000	1.6	45	5224	89 @ 10 000	This work

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A detailed comparative analysis of the NaV₈O₂₀ nanobelts developed in this study with the various metal vanadate electrodes for SC devices previously reported in the literature is presented in Table 1.

The comparative results suggest that, compared with reported metal vanadate electrode materials, the NaV₈O₂₀ nanobelts consistently demonstrate better electrochemical performance. The NaV₈O₂₀ nanobelts exhibit significantly higher capacitance with an operating window of 1.6 V, surpassing many vanadium-based alternatives. They offer excellent cyclic stability, retaining their electrochemical performance over 10 000 cycles, making them ideal for high-performance applications. Here, the key contributor to this exceptional electrochemical performance is the use of the Na₂SO₄ electrolyte, which provides numerous advantages, such as high ionic conductivity, high ionic mobility, and less corrosive compared to KOH. It also ensures compatibility with the Na⁺ pre-insertion in the electrode material, allowing for efficient ion transfer and enhanced electrochemical reactions. Moreover, the excellent electrochemical performance of the NaV₈O₂₀ nanobelts can be primarily ascribed to the pre-insertion of Na⁺ ions during synthesis. This pre-insertion enhances the redox active sites, provides an efficient diffusion path for Na⁺ ions, and ensures optimal interlayer spacing, facilitating effective Na⁺ ion intercalation. Additionally, the high electrical conductivity of the NaV₈O₂₀ nanobelts further improves their overall electrochemical behaviour, leading to enhanced charge storage and cyclic stability. These features collectively position NaV₈O₂₀ nanobelts as an auspicious material for next-generation energy storage technologies.

4.3 Practical application of the AC//NaV₈O₂₀ ASC coin cell device

Fig. 7 displays the AC//NaV₈O₂₀ ASC coin cell device for its practical application by glowing the 28-red LED-contained panel. Each red LED bulb requires 1.2 V to shine; therefore, two AC//NaV₈O₂₀ coin cell devices were connected in series for lighting. Then, fabricated ASC coin cell devices were first charged up to 1.6 V and discharged through an LED panel. These two devices can easily illuminate LED panels up to ∼120 s (ESI Media File†), demonstrating the successful practical applicability of NaV₈O₂₀ nanobelts material for SC application.

Fig. 7 Device demonstration of the AC//NaV₈O₂₀ ASC coin cell device.

4.4 Electrochemical performance of NaV₈O₂₀ nanobelts for Na⁺ ion battery

To identify the electrochemical features of the NaV₈O₂₀ nanobelts for the Na-ion battery, half-cell devices were assembled using Na metal foil as the reference/counter electrode. Fig. 8 shows the electrochemical measurements of a fabricated Na-ion battery performed in the voltage range of 1.5–4.0 V vs. Na/Na⁺ to investigate the electrochemical redox reaction kinetics of the NaV₈O₂₀ nanobelts. Fig. 8a shows the initial three CV curves recorded at a scan rate of 0.1 mV s⁻¹vs. Na/Na⁺. These CV curves show a significant degree of overlap, particularly during the initial three cycles, indicating excellent reversibility of Na⁺ ion insertion/extraction between the electrodes. The CV profiles exhibit two distinct intercalation peaks at 1.997 V and 3.631 V, along with two corresponding de-intercalation peaks at 3.528 V and 1.75 V. These peaks are associated with the reversible intercalation and de-intercalation of Na⁺ ions within the NaV₈O₂₀ structure, contributing to the material's high-capacity retention. Along with this, some minor oxidation peaks (3.075 and 3.204 V) and reduction (2.783 and 2.991 V) peaks were observed. These peaks may arise due to the multistep process of Na⁺ ions insertion and de-insertion.^53,54

Fig. 8 (a) Initial three CV profiles at 0.1 mV s⁻¹ between 1.5-4.0 V, (b) peak current vs. square root of the scan rates, (c) galvanostatic charge/discharge curves at 10 mA g⁻¹, (d) galvanostatic intermittent titration technique (GITT) profile and corresponding the diffusion coefficient of Na⁺ ions and corresponding the diffusion coefficient of sodium ions during 1^st, 2^nd and 3^rd charge at 100 mA g⁻¹ for 300 s followed by 2 h, (e) GCD curves with different current densities from 10 to 1000 mA g⁻¹, (f) rate capability plot, (g) cycling performance at a current density of 100 mA g⁻¹ and (h) Nyquist plot Na-ion battery device.

The CV measurements for 0.1 to 1 mV s⁻¹ scan rates were also performed (ESI, Fig. S3(e)†) to examine the rate performance and reaction kinetics. Fig. S3(e)† indicates that the peak positions slightly shift towards higher potentials, specifying that the Na⁺ ions' insertion and de-insertion kinetics are closely related at different scan rates. To evaluate the Na⁺ ions diffusion characteristics for the NaV₈O₂₀ electrode material, the diffusion coefficient (D) is determined by the Randles–Sevcik equation:⁵⁵

where i_p is the peak current, n is the electron density, C is the electrolyte molar concentration, F is Faraday's constant (C mol⁻¹), A is an electroactive area of an electrode (cm²), D is the diffusion coefficient of a redox mediator (cm² s⁻¹), θ is the scan rate (mV s⁻¹), R is the gas constant (J K⁻¹ mol⁻¹) and T is the temperature (K).

To calculate D from the CV measurements, the above formula can be modified as follows:

where, is the slope of the plot of peak current vs. the square root of the scan rate (Fig. 8b). Using this formula, the calculated Na⁺ ions diffusion coefficient value of the NaV₈O₂₀ cathode material was found to be 4.10 × 10⁻⁷ cm² s⁻¹, reflecting faster Na⁺ ion diffusion inside NaV₈O₂₀ cathode material. This excellent diffusion coefficient value results from (1) 1D nanobelts morphology and high surface area of NaV₈O₂₀ nanobelts material, which provides a long pathway for easier ion migration, and (2) high electrical conductivity, which is helpful for an efficient charge transfer process. Fig. 8c displays the GCD plot of the NaV₈O₂₀ material at a 10 mA g⁻¹ of current densities for the initial three cycles. Here, the plateaus are well corroborated with the CV curves and possess a maximum reversible discharge capacity of 110 mA h g⁻¹. These three GCD curves slightly overlapped, which showed excellent ionic diffusion features and structural stability, complementing the electronic conductivity of the NaV₈O₂₀ cathode material. The galvanostatic intermittent titration technique (GITT) was performed to analyze the diffusion kinetics of Na⁺ ions. During the measurement, closed-circuit voltage (CCV) was obtained by applying a current pulse of 100 mA g⁻¹ for 300 s, followed by a relaxation period of 2 h to measure the quasi-open circuit voltage (QOCV). The diffusion coefficient of sodium ions (D_Na⁺) of NaV₈O₂₀ was calculated from the obtained GITT data according to the following formula:where τ is the duration of the current pulse (s), n_mis the number of moles of active material (mol), V_m is the molar volume of the electrode (cm³ mol⁻¹), S is the electrolyte/electrode contact area (cm²), ΔE_s is the steady-state voltage change due to the current pulse, and ΔE_t is the voltage change during titration.⁵⁶ The obtained D_Na⁺ values range from 4.4 × 10⁻¹⁰ cm² s⁻¹ to 6.6 × 10⁻⁹ cm² s⁻¹, suggesting the effective diffusion of Na⁺ ions. For comparison, the diffusion coefficients of various sodium ion cathode materials are summarized in Table 2. The comparative table implies that the 1D and nanobelt morphology of the NaV₈O₂₀ electrode significantly enhances the diffusion of Na⁺ ions compared to the other reported materials, highlighting its potential in high-efficiency sodium-ion batteries.

Table 2 Summary of diffusion coefficient of various sodium-ion cathode materials reported in the literature with our work

Electrode material	Current density	GITT-pulse duration/relaxation time	Diffusion coefficient (DNa^+)	Ref.
NaVPO4F	14.3 mA g^−1	600 s/2 h	10^−11 to 10^−10 cm^2 s^−1	57
Na3MnTi(PO4)3	11.7 mA g^−1	10 min/30 min	10^−10 to 10^−15 m^2 s^−1	58
Fe-HCF (iron-based hexacyanoferrate)	—	600 s/2 h	∼10^−11 cm^2 s^−1	59
NaFePO4	3.85 mA g^−1	1 h/15 h	∼10^−17 cm^2 s^−1	60
Na0.66Ca0.05[Ni0.25Li0.11Mn0.64]O1.95F0.05	13 mA g^−1	30 min/5 h	10^−11 to 10^−12 cm^2 s^−1	61
Na8Fe5(SO4)9@rGO	—	—	10^−11 to 10^−12 cm^2 s^−1	62
NaV 8 O 20	100 mA g^− ^1	300 s/2 h	∼10^− ^9 cm ^2 s^− ^1	This work

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Fig. 8e displays the GCD behavior of second cycles for different rates from 10 mA g⁻¹ to 1 A g⁻¹. The NaV₈O₂₀ cathode provides a maximum discharge capacity of 105, 87, 70, 58, 48, 36, and 28 mA h g⁻¹ for the current densities of 10, 20, 50, 100, 200, 500, and 1000 mA g⁻¹. Additionally, the rate capability of the NaV₈O₂₀ cathode is shown in Fig. 8f; three cycles with each rate were tested, and the nature shows the electrochemical stability at a particular rate, but the capacity decreases with an increase in the rate. As the current density reverts to 10 mA g⁻¹, the NaV₈O₂₀ cathode recovers ∼83.2% (84 mA h g⁻¹) of its initial capacity. In addition, the cyclic stability measurement was performed over the fabricated Na⁺ ion battery. Fig. 8g displays the cyclic stability of the Na⁺ ion battery performed for 100 cycles at a current density of 100 mA g⁻¹. The Na⁺ ion battery device displays ∼100% coulombic efficiency while retaining ∼52.4% of the initial capacity. The observed capacity fading can be attributed to the forming of a passivation layer on the electrode surface and a reduction in the number of electroactive sites. Additionally, EIS was employed to analyze the reaction kinetics of the NaV₈O₂₀ cathode, both before and after cycling, as shown in Fig. 8h. By comparing both the EIS spectra, it can be found that the R_ct (quasi-semicircle) for the diffusion of sodium ions around ∼650 Ω, which after cycling gradually decreased to ∼562 Ω and shows an increase in ion and electron transport mobility as cycling is undertaken. This may be due to the decrease in the charge transfer characteristic in terms of lack of the electrolyte ions at the surface of the electrodes, activation, or reduction in active surface area during a prolonged cycling test.

5. Conclusions

In summary, we have successfully synthesized one-dimensional (1D) and layered sodium vanadate (NaV₈O₂₀) nanobelts using a simple, cost-effective hydrothermal method and demonstrated their dual application as high-performance electrode materials for SC and SIB devices. This study represents a significant advancement as it is the first to explore the electrochemical potential of NaV₈O₂₀ nanobelts in both SCs and SIBs. Compared to previously reported metal vanadate-based electrode materials, the NaV₈O₂₀ nanobelts exhibit remarkable electrochemical performance owing to their unique 1D and layered structure. This architecture significantly improves the electrochemical properties, leading to excellent cycling stability and superior rate performance for both SCs and SIBs. For SCs, the NaV₈O₂₀ nanobelts exhibit an impressive C_sp of 676 F g⁻¹ at 0.5 A g⁻¹ and remarkable cyclic stability, retaining ∼90% of the capacitance after 10 000 cycles. Moreover, the ASC coin cell device (AC//NaV₈O₂₀) delivers an optimal E_d of 45 W h kg⁻¹ and P_d of 5224.48 W kg⁻¹, with 89% cyclic stability after 10 000 cycles, demonstrating its suitability for high-performance energy storage systems. In addition to SCs, the NaV₈O₂₀ nanobelts were employed as a cathode material for SIBs, showing a high initial charge/discharge capacity of 110 mA h g⁻¹ and a retention of 52.4% capacity after 100 cycles at 100 mA g⁻¹. The GITT measurements reveal a high diffusion coefficient for Na⁺ ions, indicating that the NaV₈O₂₀ nanobelts enable efficient ion transport during cycling, which is essential for high-rate performance and long-term stability. These results highlight the potential of NaV₈O₂₀ nanobelts for dual electrochemical applications, with a simple synthesis process which is both practical and sustainable.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors express their gratitude to the Alexander von Humboldt Foundation for donating the potentiostat. ASV is thankful to Symbiosis International (Deemed University), Pune for the financial support provided through the Major Research Project (MJRP) grant (2024/4900). SSW acknowledges support from the SERB-TARE project (TAR/2020/000257) funded by the Department of Science and Technology (DST), India. SAS extends appreciation to the DST, Government of India, for the PhD fellowship provided under the DST-INSPIRE scheme. Also, this research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. RS-2019-NR040081).

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Footnote

† Electronic supplementary information (ESI) available: S1: (a) chemicals and reagents, (b) formulae for C_sp calculation of working electrode and ASC device, and (c) characterization techniques, S2: XPS studies of NaV₈O₂₀ nanobelts sample, S3: electrochemical measurements of carbon paper, bulk V₂O₅ and NaV₈O₂₀ electrode and Na ion battery, Table S1: crystallographic data of the NaV₈O₂₀ powder obtained from Rietveld refinement, Table S2: C_sp values of NaV₈O₂₀ electrode from CV curves, Table S3: C_sp values of NaV₈O₂₀ electrode from GCD curves, Table S4: C_sp, energy and power density values of AC//NaV₈O₂₀ ASC coin cell device. See DOI: https://doi.org/10.1039/d4ta08624d

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