Engineering the structural and electronic properties of VO₂ through Fe substitution for high-performance aqueous zinc-ion batteries

Abstract

Rechargeable aqueous zinc-ion batteries (AZIBs) are gaining attention for large-scale energy storage owing to safety and low cost. Among the numerous cathodes for AZIBs, VO₂ is a promising candidate due to its tunnel-like structure and high theoretical capacity; however, its practical application is limited by its low electronic conductivity. To address this issue, doping with several transition metals has been attempted; however, exploration of Fe doping is rare despite its cost-effectiveness and abundance. Herein, a series of Fe-substituted VO₂ cathodes (Fe_xV_1-xO₂: x = 0.15, 0.25, and 0.35) have been synthesized via a simple one-step hydrothermal method and systematically evaluated for AZIB applications. PXRD studies indicate a noticeable lattice expansion upon Fe substitution, providing relatively wider pathways for Zn²⁺ transport within the cathode framework. Among the investigated compositions, Fe_0.25V_0.75O₂ exhibited the best electrochemical performance, delivering a high specific capacity of 380.11 mAh g^-1 at 0.1 A g^-1, retaining 56% capacity at 5 A g^-1, and maintaining 82.75% capacity retention after 500 cycles at 2 A g^-1, significantly outperforming the pristine VO₂ cathode. Post-cycling characterization further revealed improved tolerance toward cycling-induced structural and morphological changes, while ICP-OES analysis showed approximately 92.3% lower vanadium dissolution for FeVO₂-25% than pristine VO₂. The Fe substitution effect is also rationalized by DFT calculations, which showed a modified electronic structure of VO₂, including band-gap narrowing through Fe 3d–V 3d/O 2p hybridization, consistent with the experimentally observed reduction in optical band gap. This work demonstrates that Fe, an earth-abundant and cost-effective dopant, provides an effective strategy for simultaneously tuning the structural and electronic properties of VO₂ and improving its Zn²⁺ storage performance in aqueous zinc-ion batteries.

Graphical Abstract

1 Introduction

The growing demand for sustainable energy technologies, driven by the depletion of fossil fuel resources and increasing environmental concerns, has intensified the search for safe, efficient, and long-lasting energy storage systems (Ran et al., 2026; Wan et al., 2025; Tao et al., 2025). Although lithium-ion batteries (LIBs) are currently ubiquitously employed as energy storage devices owing to their high energy density and mature technology, their large-scale deployment is challenging due to safety concerns associated with flammable organic electrolytes, and the limited and uneven distribution of lithium resources (Heng et al., 2020; Lolupima et al., 2024). Therefore, considerable attention is currently being given to alternative battery chemistries, including Na, K, Mg, Al, and Zn-based systems (Yuan et al., 2021; Tan and Paul, 2024). Among them, aqueous zinc-ion batteries (AZIBs) stand out because the suitable redox potential of Zn allows metallic Zn to be used directly as an anode in aqueous electrolytes, alleviating the need for host-type anodes, thereby enabling the utilization of its high theoretical capacity. Moreover, the high abundance and low cost of Zn and the safety advantages of aqueous electrolytes make AZIBs promising candidates for large-scale energy storage (Chen et al., 2023; Chen et al., 2025; Kou et al., 2024). However, the realization of high-performance AZIBs is bottlenecked by the development of efficient cathode materials that can rapidly accommodate reversible Zn²⁺ insertion and extraction and possess long-term structural stability during cycling application (Tang et al., 2025; He et al., 2021).

Various cathode materials have been explored for AZIBs, including Mn-based compounds (Guo et al., 2026), Prussian blue analogs (Wang J. et al., 2025; Gupta et al., 2025), organic materials (Cui et al., 2022), conductive polymers (Baghodrat et al., 2026), and V-based oxides (Zafar and Lochab, 2024; Gupta et al., 2026; Singh et al., 2025). Among the different vanadium oxide polymorphs, VO₂ has attracted considerable attention because of its open tunnel-like framework formed by corner-sharing and edge-sharing VO₆ octahedra (He et al., 2025; Xu et al., 2023). Besides providing accessible pathways for Zn2+ transport, such structural features also help accommodate the lattice strain during charge-discharge cycling (Zhang et al., 2023; Zhang et al., 2026). Despite these advantages, the practical application of VO₂ remains limited by several intrinsic drawbacks (Peng et al., 2026). The relatively poor electronic conductivity of VO₂, due to a larger energy difference between the V 3d and O 2p states, widening the O 2p-dominated conduction band and V 3d-dominated valence band, restricts charge transport and results in increased polarization, particularly at high current densities. Furthermore, the strong electrostatic interaction between highly polarising Zn2+ ions and the oxide framework often leads to sluggish diffusion kinetics, while structural degradation and dissolution of vanadium species during cycling contribute to gradual capacity fading. Therefore, improving both the electronic conductivity and ion-transport characteristics of VO₂ remains an important challenge (Qiu et al., 2023; Hu et al., 2025).

To address these limitations, several strategies have been explored, among which transition-metal doping has emerged as an effective approach for tuning both the crystal structure and electronic properties of VO₂ (Fu and Gao, 2021; Jiang et al., 2026; Wang et al., 2026a). For example, Wang et al., 2026b. reported Ni-doped VO₂ with improved electrochemical performance in AZIBs (Wang et al., 2026b), while Li et al. demonstrated enhanced cycling stability in W-doped VO₂ nanosheets (Li et al., 2021). These studies indicate that appropriate cation substitution can effectively regulate the local structure and electronic environment of VO₂, thereby improving Zn²⁺ storage behavior (Chen et al., 2026; Wang Y. et al., 2025).

Among the various transition-metal dopants, Fe is particularly attractive from both practical and scientific perspectives. Compared with many commonly investigated dopants, Fe is earth-abundant, inexpensive, environmentally benign, and readily available, making it highly attractive for large-scale energy-storage applications. In addition, the larger ionic radius of Fe³⁺ compared to V⁴⁺ is expected to induce lattice expansion within the VO₂ framework, which may facilitate Zn²⁺ transport by providing wider diffusion pathways. Furthermore, Fe³⁺ acts as an aliovalent dopant with respect to V⁴⁺. Such aliovalent substitution may introduce charge-compensation defects, including anion deficiency, which can modify the electronic structure and potentially enhance electronic conductivity. Despite these potential advantages, there are very few reports on improving VO₂ cathode performance in AZIBs through Fe doping.

Herein, we report the synthesis of a series of Fe_xV_1-xO₂ (x = 0.15, 0.25, and 0.35) via a simple one-step hydrothermal route and their evaluation as cathode materials for AZIBs. Structural characterization reveals that Fe incorporation preserves the parent VO₂ framework while inducing measurable lattice expansion. Density functional theory calculations further indicate that Fe substitution modifies the electronic structure and reduces the band gap of VO₂. As a result, the optimized FeVO₂-25% electrode (Fe_0.25V_0.75O₂) delivers a high specific capacity of 380.11 mAh g^-1 at 0.1 A g^-1 with 56% retention at 5 A g^-1 and exhibits 82.75% capacity retention after 500 cycles at 2 A g^-1. Compared with pristine VO₂, the Fe-substituted electrode exhibits significantly improved Zn²⁺ storage performance at all parameters, highlighting the beneficial role of Fe incorporation. This work demonstrates that Fe substitution provides a simple, scalable, and economically attractive strategy for improving the electrochemical performance of VO₂ cathodes for aqueous zinc-ion batteries.

2 Experimental section

2.1 Synthesis of VO₂ and Fe_xV_1-xO₂

Fe-substituted VO₂ samples were synthesized via a simple one-step hydrothermal method (Scheme 1). In a typical synthesis of Fe_0.25V_0.75O₂ (denoted as FeVO₂-25%), 0.682 g of V₂O₅ and 0.946 g of H₂C₂O₄·2H₂O were dissolved in 40 mL of deionized water. The resulting solution was heated at 75 °C for 30 min until a homogeneous solution was obtained. Subsequently, 20 mL of 30% H₂O₂ was added, and the mixture was stirred for 30 min. Thereafter, 1.010 g of Fe(NO₃)₃·9H₂O was introduced into the solution, followed by continuous stirring at room temperature for 30 min. The final reaction mixture was transferred into a Teflon-lined stainless-steel autoclave and heated at 180 °C for 24 h. After naturally cooling to room temperature, the blue precipitate was collected, washed three times with deionized water and three times with ethanol, and dried in a vacuum oven at 80 °C for 12 h.

SCHEME 1

To investigate the influence of Fe content, a series of Fe_xV_1-xO₂ samples (x = 0.15, 0.25, and 0.35) were synthesized under identical conditions using Fe(NO₃)₃·9H₂O precursor in a corresponding molar ratio. These samples have been denoted as FeVO₂-15%, FeVO₂-25%, and FeVO₂-35%, respectively. For comparison, pristine VO₂ was also synthesized following the same procedure without the addition of Fe(NO₃)₃·9H₂O.

2.2 Material characterization

The crystal structure of the synthesized samples was examined using X-ray diffraction (XRD) on an Empyrean diffractometer (Malvern Panalytical) with Cu Kα radiation (λ = 0.15418 nm) over a 2θ range of 10°–80°. Rietveld refinement of the XRD patterns was performed using the FullProf software package. The surface morphology and elemental distribution of the samples were investigated using a field-emission scanning electron microscope (FE-SEM, Carl Zeiss) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. The elemental composition of the samples was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5800 Technologies Instrument) and X-ray photoelectron spectroscopy (XPS) (Thermo Scientific United States of America Nexsa). Phase identification and bonding characteristics were further analyzed by Raman spectroscopy (Horiba LabRam) and Fourier-transform infrared spectroscopy (FTIR, PerkinElmer ATR spectrometer) in the spectral range of 400–4,000 cm^-1.

2.3 Electrochemical measurements

The electrochemical performance of the synthesized samples was evaluated using CR2032-type coin cells. For the preparation of the working electrode, the active material, carbon black, and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 7:2:1 using N-methyl-2-pyrrolidone (NMP) as the solvent to form a homogeneous slurry. The resulting slurry was coated onto a 12-mm-diameter stainless-steel current collector and dried in a vacuum oven at 80 °C for 12 h. The active cathode material loading was 1.7–1.9 mg per electrode, corresponding to an areal loading of 1.5–1.7 mg cm^-2. The coin cells were assembled under ambient conditions using 3.0 M Zn(CF₃SO₃)₂ aqueous solution as the electrolyte, Zn foil as the anode, and a Whatman GF/B glass microfiber filter as the separator. The electrochemical performance of the assembled cells was investigated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). GCD measurements were carried out using a multi-channel battery testing system (CT-ZWJ-4′S-T1U), while CV and EIS analyses were performed on an electrochemical workstation (Autolab PGSTAT204). CV measurements were conducted within a potential window of 0.1–1.6 V. EIS spectra were recorded over a frequency range of 0.1 Hz–100 kHz with an AC amplitude of 100 mV. All measurements were performed at room temperature.

2.4 DFT calculations

Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP). The projector augmented wave (PAW) method and the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) were employed to describe the electron–ion interactions and exchange-correlation effects, respectively. To account for the strong correlation effects of transition-metal d electrons, the DFT + U approach was adopted with U values of 3.3 and 5.3 eV for V and Fe, respectively. A Fe-substituted VO₂ model was constructed by replacing the desired fraction of V atoms in the pristine VO₂ structure with Fe atoms, followed by structural optimization. The optimized structures were further used for band-structure and differential charge-density calculations to examine the Fe-induced changes in the electronic structure. Spin-polarized calculations were carried out using a plane-wave cutoff energy of 520 eV and a 3 × 3 × 3 Monkhorst-Pack k-point mesh. The optimized structures were subsequently used to calculate the total and partial density of states (DOS/PDOS) to investigate the influence of Fe substitution on the electronic structure of VO₂.

3 Results and discussion

3.1 Characterization

The crystal structure and phase purity of the synthesized Fe_xV_1-xO₂ samples were first investigated by PXRD. As shown in Figure 1a, all diffraction peaks of pristine VO₂ and Fe-substituted samples can be indexed to triclinic VO₂, with no detectable impurity reflections observed. The PXRD patterns of FeVO₂-15% and FeVO₂-35% (Supplementary Figure S1) also exhibit diffraction features characteristic of the parent VO₂ phase, indicating that Fe substitution does not alter the fundamental crystal framework. The absence of secondary phases suggests that the VO₂ structure is retained throughout the investigated Fe-substitution range.

FIGURE 1

To obtain detailed structural information, Rietveld refinement was performed on the PXRD patterns of pristine VO₂ and the optimized FeVO₂-25% sample (Figures 1c,d). The refinement yielded χ² = 2.86, Rp = 7.35%, and Rwp = 5.25% for pristine VO₂, while the corresponding values for FeVO₂-25% were χ² = 3.81, Rp = 8.25%, and Rwp = 6.55%. These refinement parameters indicate a satisfactory agreement between the observed and calculated diffraction profiles. The refinement confirms that both samples crystallize in the triclinic P1 space group. For pristine VO₂, the refined lattice parameters were determined to be a = 12.0449Å, b = 3.6900Å, c = 6.4132Å, α = γ = 90°, β = 106.867°, whereas FeVO₂-25% exhibited lattice parameters of a = 12.0684 Å, b = 3.6796 Å, c = 6.4988 Å, α = γ = 90.00°, and β = 107.091°. Because of the slight extension of the crystal lattice due to Fe doping, the volume of the unit cell changed from 272.64ų in pure VO₂ to 275.854 Å³ in FeVO₂-25%, which corresponds to an increase of about 1.18%. The lattice expansion observed above is expected because the effective ionic radius of six-coordinate Fe³⁺ (0.645 Å) is larger than that of V⁴⁺ (0.58 Å) (Shannon, 1976); therefore, the Fe-induced lattice expansion can provide relatively wider pathways for Zn²⁺ (0.74 Å) transport and facilitate its insertion/extraction. The detailed crystallographic information of VO₂ and FeVO₂-25% obtained from the refinement is summarized in Supplementary Tables S1, S2. The crystal structure models of pristine VO₂ and FeVO₂-25% are shown in Figure 1b. In both cases, the structure consists of interconnected VO₆ octahedra that form the host framework for Zn²⁺ insertion and extraction. The preservation of this framework after Fe substitution, together with the observed lattice expansion, suggests that Fe incorporation can be achieved without disrupting the structural integrity of VO₂. Overall, the PXRD and Rietveld refinement results demonstrate that Fe substitution preserves the parent triclinic VO₂ structure while inducing measurable lattice expansion that may be beneficial for Zn²⁺ storage.

To further verify the chemical composition of the Fe-substituted sample, ICP-OES analysis was performed for FeVO₂-25%, and the results are summarized in Supplementary Table S3. The measured Fe/V atomic ratio was approximately 0.35, corresponding to an ICP-derived composition of Fe_0.25±0.01V_0.75O₂. The experimentally determined composition is in good agreement with the nominal stoichiometry, providing further evidence for the successful incorporation of Fe into the VO₂ framework.

The bonding in the synthesized samples was further investigated by FTIR spectroscopy. As shown in Figure 1e, the characteristic bands located at 501, 694, and 1,008 cm^-1 are assigned to V–O stretching, V–O–V symmetric stretching, and terminal V=O stretching vibrations, respectively, suggesting the formation of the VO₂ framework. In addition, the broad bands observed at 1,629 and 3,378 cm^-1 correspond to the bending and stretching vibrations of O-H groups, indicating the presence of adsorbed and/or lattice-associated water in the samples (Li et al., 2025).

Raman spectroscopy was employed to further probe the structural characteristics of pristine VO₂ and Fe-substituted VO₂ (Figure 1f). The Raman spectrum of pristine VO₂ exhibits several characteristic vibrational modes of the VO framework, including bands located at approximately 139, 190, 280, 404, 691, and 988 cm^-1, which are consistent with previously reported VO₂ structures (Guo et al., 2024). After Fe doping, the Raman characteristics have not changed much, suggesting that the parent structure of VO₂ has been retained, as confirmed by the PXRD analysis. There is, however, an extra peak observed at around 820 cm^-1 after Fe doping. The new peak may be due to the Fe-induced structural modification caused by V-O-Fe bond formation (Zhang et al., 2015; Xu et al., 2026).

To further investigate the chemical states and electronic environment associated with Fe substitution, XPS analysis was performed for FeVO₂-25%. The survey spectrum (Figure 2a) confirms the presence of V, Fe, and O in the sample. The high-resolution V 2p spectrum (Figure 2b) reveals the coexistence of V⁴⁺ and V⁵⁺ states. The V 2p₃/₂ components located at 515.18 and 516.66 eV are assigned to V⁴⁺ and V⁵⁺, respectively, while the corresponding V 2p₁/₂ components appear at 522.57 and 524.18 eV. The Fe 2p spectrum (Figure 2c) further shows the coexistence of Fe²⁺ and Fe³⁺ states. The Fe 2p₃/₂ components centered at approximately 709.51 and 711.65 eV are assigned to Fe²⁺ and Fe³⁺, respectively, while the corresponding Fe 2p₁/₂ components are observed at 722.60 and 724.85 eV. Additional features at approximately 714.87, 718.60, and 729.25 eV are attributed to satellite contributions associated with the Fe 2p multiplet structure. The O 1s spectrum (Supplementary Figure S2) can be deconvoluted into three components at approximately 530.15, 530.70, and 531.73 eV. The component at 530.15 eV is assigned to lattice O^2- associated with V–O/Fe–O bonding, whereas the component at 530.70 eV is attributed to oxygen in a defect-modified local environment. The higher-binding-energy component at 531.73 eV is associated with surface hydroxyl and/or adsorbed oxygen-containing species (Kumar et al., 2025; Luo et al., 2026; She and Zhang, 2017).

FIGURE 2

FE-SEM and EDS characterization were carried out on the synthesized samples to study their morphology and elemental composition. Figures 2d,e show that the morphology of both pristine VO₂ and FeVO₂-25% is similar to that of elongated flat belts. Both types of samples contain belt-shaped structures with widths around 100–125 nm and lengths around 0.75–1.0 μm, giving rise to a relatively high aspect ratio. Though the general morphology does not change after the introduction of Fe, the FeVO₂-25% sample contains more clearly defined and relatively homogeneous belt-like particles with well-defined edges compared to pristine VO₂. It can be stated that Fe content affects the growth of VO₂ without changing its morphology. EDS spectra presented in Figure 2f demonstrate the presence of V and O elements in pristine VO₂, while Fe signal is additionally detected in the FeVO₂-25% sample (Figure 2g). Furthermore, the elemental mapping images in Figures 2h,i reveal a homogeneous distribution of V, O, and Fe throughout the FeVO₂-25% sample, suggesting uniform incorporation of Fe within the VO₂ matrix. The absence of any Fe-rich regions further indicates that no detectable phase segregation occurs upon Fe substitution, which is consistent with the PXRD and Rietveld refinement results.

3.2 Electrochemical study

The electrochemical performance of pristine VO₂ and Fe-substituted VO₂ samples was evaluated by CV, GCD, EIS, rate capability, and cycling stability measurements. To investigate the influence of Fe substitution, CV measurements of pristine VO₂, FeVO₂-15%, FeVO₂-25%, and FeVO₂-35% were first recorded (Figure 3a); CV curves of the individual samples at different scan rates are presented in Supplementary Figure S3a-d. As shown in Figure 3a, all Fe-substituted samples exhibit electrochemical behavior similar to that of pristine VO₂, indicating that Fe substitution preserves the fundamental Zn²⁺ storage mechanism of the host material. The presence of two pairs of redox peaks suggests stepwise redox reactions associated with vanadium centers (V^3+/2+ and V^4+/3+) during Zn²⁺ insertion and extraction. It is also evident that Fe-doped VO₂ has a larger enclosed CV area and higher peak current compared to pristine VO₂, which means the charge storage capability is significantly improved. Among the different compositions examined, FeVO₂-25% exhibits the highest electrochemical activity, indicating that 25% Fe substitution is the optimal level.

FIGURE 3

To determine the charge storage capacity depending on Fe doping level, GCD measurement was performed at a constant current density of 2 A g^-1 (Figure 3b). The specific capacities of pristine VO₂, FeVO₂-15%, FeVO₂-25%, and FeVO₂-35% were determined to be 181.11, 201.04, 275.97, and 217.95 mAh g^-1, respectively. The progressive increase in capacity upon Fe substitution confirms the beneficial effect of Fe incorporation on Zn²⁺ storage performance; however, beyond 25% substitution, capacity starts to decline with increasing Fe content, suggesting that the optimal capacity is achieved at 25% Fe substitution. This behavior can be understood as a trade-off between the beneficial structural/electronic effects of Fe substitution and the available redox capacity. As discussed above, V centers can participate in sequential V⁴⁺/V³⁺ and V³⁺/V²⁺ redox processes, whereas increasing Fe substitution progressively decreases the fraction of such multielectron-active V centers. Thus, beyond the optimum Fe content, the gain from improved charge-transfer and ion-transport kinetics is increasingly offset by the reduced number of available V redox centers, resulting in the lower capacity of FeVO₂-35%. Therefore, FeVO₂-25% was selected as the optimized composition for detailed electrochemical investigation.

The comparative rate capabilities of pristine VO₂ and FeVO₂-25% were examined at current densities ranging from 0.1 to 5 A g^-1. As shown in Figures 3c,d, pristine VO₂ delivers specific capacities of 339.95, 301.49, 257.34, 221.67, 181.11, and 109.09 mAh g^-1 at 0.1, 0.2, 0.5, 1, 2, and 5 A g^-1, respectively. Under identical conditions, FeVO₂-25% exhibits significantly higher capacities of 380.11, 360.26, 336.55, 309.05, 275.97, and 212.65 mAh g^-1. Compared to pristine VO₂, FeVO₂-25% maintains substantially higher capacities across the entire current range. Notably, when the current density is increased from 0.1 to 5 A g^-1, FeVO₂-25% retains 56% of its initial capacity, whereas pristine VO₂ retains only 32%. More importantly, when the current density is returned to 0.1 A g^-1, FeVO₂-25% recovers 95.6% of its initial capacity, whereas pristine VO₂ recovers only 74.5% of its initial capacity (Figure 3e). The enhanced rate performance and excellent capacity recovery of FeVO₂-25% can be attributed to Fe substitution, which has a dual effect: firstly, lattice expansion facilitates Zn²⁺ transport, and secondly, electronic structure modification improves conductivity. Collectively, both effects result in more reversible Zn²⁺ insertion/extraction and improved structural stability during high-rate cycling.

To quantitatively evaluate the effect of Fe substitution on the interfacial charge-transfer and ion-transport processes, EIS measurements of both VO₂ and FeVO₂-25% were carried out, and the corresponding Nyquist plots are presented in Figure 3f. Both electrodes exhibit a depressed semicircle in the high-frequency region and an inclined line in the low-frequency region, corresponding to charge-transfer and ion-diffusion processes, respectively. The Nyquist plots were fitted using the equivalent circuit Rs-(Rct ∥ CPE)–W, where Rs represents the series resistance associated mainly with the electrolyte and electrode/contact contributions, Rct represents the interfacial charge-transfer resistance, the constant-phase element (CPE) accounts for the non-ideal double-layer response, and W represents the diffusion-related impedance in the low-frequency region. The fitted Rs values are comparable for VO₂ (3.507 Ω) and FeVO₂-25% (3.707 Ω), indicating similar series-resistance contributions in the two cells. Notably, the FeVO₂-25% electrode displays a significantly smaller semicircle diameter than pristine VO₂, with Rct decreasing from 58.67 Ω for VO₂ to 25.59 Ω for FeVO₂-25%, demonstrating substantially faster interfacial charge-transfer kinetics after Fe substitution. In addition, the Warburg coefficient decreases from 7.04 × 10⁴ Ω s^-1/2 for VO₂ to 2.04 × 10⁴ Ω s^-1/2 for FeVO₂-25%, indicating reduced diffusion impedance and more favorable ion transport in the Fe-substituted electrode. The improved charge-transfer characteristics of the Fe-doped material revealed by EIS are consistent with the structural and electronic modifications expected by Fe substitution. The aliovalent replacement of V⁴⁺ by Fe³⁺ can generate charge-compensating anion-deficiency defects, while the incorporation of Fe 3d states modify the electronic structure of VO₂ through Fe 3d–V 3d/O 2p hybridization and band-gap narrowing, both of which favor electronic charge transport and are consistent with the observed reduction in Rct. The reduced diffusion impedance is also consistent with the Fe-induced structural modification and lattice expansion, which can provide relatively more accessible pathways for ion transport.

The cycling stability of the electrodes was investigated to evaluate their structural durability during repeated Zn²⁺ insertion and extraction. Figure 3g shows the cycling performance of pristine VO₂ and FeVO₂-25% at a current density of 0.1 A g^-1 for 50 cycles. Pristine VO₂ retains only 28.85% of its initial capacity after 50 cycles, indicating substantial performance degradation. In contrast, FeVO₂-25% maintains 81.11% of its initial capacity under the same conditions. Furthermore, the coulombic efficiency remains close to 100% throughout cycling, indicating highly reversible charge storage behavior. The long-term durability of the optimized electrode under more demanding operating conditions was also evaluated at a higher current density of 2 A g^-1 for 500 cycles (Figure 3h). FeVO₂-25% exhibits excellent cycling stability, retaining 82.75% of its initial capacity after 500 cycles, whereas pristine VO₂ retains only 32.78% under identical conditions. The significantly improved capacity retention of FeVO₂-25% suggests that Fe substitution effectively enhances the structural stability of the VO₂ framework during repeated Zn²⁺ insertion and extraction.

Post-cycling structural and morphological analyses were performed on electrodes recovered after 170, 340, and 500 charge-discharge cycles in the discharged state. The ex situ PXRD patterns of pristine VO₂ and FeVO₂-25% after cycling (Supplementary Figures S4a-f) reveal considerable changes compared to the corresponding fresh materials, indicating that both electrodes undergo structural evolution during repeated Zn²⁺ insertion and extraction. The progressive changes observed with increasing cycle number suggest cycling-induced lattice distortion, loss of long-range structural order, and possible formation of surface/byproduct phases during repeated Zn²⁺/H⁺ storage. For PXRD recorded after 170, 340, and 500 cycles, the intense reflection around 43°–44° and some additional features may be attributed to the current collector and residual electrolyte-derived or surface species. Although the diffraction features of both samples show substantial evolution during prolonged cycling, the Fe-substituted electrode maintains substantially better electrochemical performance, suggesting an improved ability to accommodate cycling-induced structural changes (Zeng et al., 2026). Additional evidence is provided by the post-cycling FE-SEM images and elemental mapping analysis (Supplementary Figure S5). While the pristine VO₂ electrode demonstrates significant morphological degradation with fragmentation and coverage of surface deposits, the FeVO₂-25% electrode shows some resistance to morphological transformation, even though this compound also shows structural modification. In addition, the EDS spectra and elemental mapping demonstrate the presence of Zn in both electrodes after the cycling procedure due to insertion and extraction of Zn²⁺ ions during electrochemical processes. To quantitatively evaluate vanadium dissolution during cycling, ICP-OES analysis of the electrolyte was performed after 100 charge–discharge cycles at 0.5 A g^-1 (Supplementary Figure S6). The dissolved V concentration decreased from 0.78 ppm for pristine VO₂ to only 0.06 ppm for FeVO₂-25%, corresponding to an approximately 92.3% reduction in vanadium dissolution after Fe substitution. The suppressed V dissolution in FeVO₂ may be attributed to an Fe-induced inductive effect within the local V–O–Fe environment. Compared with V⁴⁺, the larger and lower-valent Fe³⁺ ion has a lower polarizing ability toward O^2-; therefore, partial substitution of V⁴⁺ by Fe³⁺ can redistribute the electron density within the V–O–Fe linkage and stabilize the neighboring V–O interactions, making vanadium less prone to dissolution during cycling. This interpretation is also consistent with the Fe-induced electron-density redistribution observed from the differential charge-density analysis (Supplementary Figure S7e). The photographs of the recovered electrolytes further provide qualitative visual support for this difference. Overall, based on the PXRD, FE-SEM, and ICP-OES analyses after cycling, one can say that Fe doping does not stop the structural transformation but increases tolerance of the VO₂ lattice to the cycling-induced changes in its structure and morphology while substantially suppressing vanadium dissolution.

For aqueous Zn-ion batteries, the storage of charge usually takes place via the combination of diffusional insertion of Zn-ions and pseudocapacitive charge-storage processes. Besides the insertion of Zn²⁺, H⁺ insertion has been observed due to the small ionic size and high transport kinetics of H⁺ ions (Chen et al., 2022). Consequently, the contribution of pseudocapacitance to the capacity is considerable, especially for cathodes based on vanadium. It may even be enhanced in tunnel structure cathodes like VO₂ owing to the possibility of rapid ion transport and easy accessibility of active sites for Zn²⁺ and H⁺ insertion. As Fe substitution considerably enlarged the crystal structure of parent VO₂ and improved its performance, in terms of both rate capability and cycling performance, it is necessary to study whether these enhancements occur alongside changes in the mechanisms of charge storage and reaction kinetics. Thus, the contribution of diffusion-controlled and capacitive processes to the overall charge storage process was estimated using Dunn’s method applied to CV data obtained at various scan rates by means of the following equation: i(V) = k₁v + k₂v^1/2, where i(V) is the current at a certain potential V; k₁v and k₂v^1/2 represent the capacitive and diffusion-controlled contributions, respectively; and v is the scan rate.

CV curves of pure VO₂ were recorded at different scan rates between 0.2 and 1.0 mV s^-1 as illustrated in Figure 4a. With the increase in scan rate, the current response rises steadily without any significant change in the shape of CV curves, thus indicating good reversibility of the redox reaction. Capacitive and diffusion contributions derived using Dunn analysis are given in Figure 4b, c. As anticipated, the capacitive contribution rises with an increase in scan rate while the diffusion contribution decreases. At a scan rate of 0.6 mV s^-1 (Figure 4c), approximately 75% of the current response is due to the capacitive contribution, while 25% is attributed to the diffusion-controlled contribution. Compared with pure VO₂, FeVO₂-25% shows larger CV areas with increased current responses at all the scan rates (Figure 4d). The capacitive contribution of FeVO₂-25% is consistently higher than that of pristine VO₂ at all investigated scan rates (Figures 4e,f). At 0.6 mV s^-1 (Figure 4f), the capacitive contribution reaches approximately 85%, while the diffusion-controlled contribution decreases to about 15%.

FIGURE 4

However, the capacitive contribution estimated by Dunn analysis should not be interpreted exclusively as surface-restricted Faradaic reactions or electrical double-layer charge storage. Rapid ion insertion into the bulk of a host material can also exhibit pseudocapacitive kinetics when the insertion process is sufficiently fast and is not strongly limited by solid-state diffusion, a behavior commonly described as intercalation pseudocapacitance (Augustyn et al., 2013). This consideration is particularly relevant for aqueous VO₂-based cathodes, where H⁺ can participate in the charge-storage process together with Zn²⁺. Owing to its much smaller size and rapid transport through the tunnelled VO₂ framework, H⁺ insertion can contribute to the capacitive component determined by Dunn analysis even though the underlying process remains Faradaic ion intercalation. Such pseudocapacitive proton insertion has previously been demonstrated in VO₂-based aqueous Zn-ion batteries (Zhang et al., 2020). Therefore, the increase in the capacitive contribution from approximately 75% for VO₂ to 85% for FeVO₂-25% should be understood primarily as an indication of faster and less diffusion-limited charge-storage kinetics rather than simply an increase in surface charge storage. This observation also aligns with the Fe-induced lattice expansion revealed by Rietveld refinement, which provides more accessible ion-transport pathways and can facilitate rapid ion insertion.

Supplementary Table S4 compares the performance of some previously reported doped VO₂-based cathodes for AZIBs with that of the present work. Unlike other reports, which use relatively expensive dopant metal ions, FeVO₂-25% exhibits competitive or better performance with Fe as an earth-abundant, cost-effective dopant. These results highlight Fe substitution as a practical and scalable strategy to improve the Zn²⁺ storage performance of VO₂ cathodes.

3.3 DFT analysis

To further rationalize the electronic effect of Fe substitution that boosts the electrochemical performance of FeVO₂-25%, we also performed density functional theory (DFT) calculations. The optimized structures of pristine and Fe-substituted VO₂ are shown in Supplementary Figures S7a,b and the calculated lattice parameters are reasonably comparable with the corresponding experimentally refined values obtained from PXRD. Corresponding TDOS of VO₂ and Fe-substituted VO₂ are presented in Figures 5a,b. The pristine VO₂ has a predicted band gap of around 1.6 eV, while that of the Fe-substituted VO₂ model is found to be considerably reduced, namely, 0.8 eV. The corresponding band structures (Supplementary Figures S7c,d) show the same band-gap narrowing upon Fe substitution, further supporting the modification of the electronic structure. To experimentally examine this predicted trend, the optical band gaps of VO₂ and FeVO₂-25% were estimated using Tauc analysis and found to be approximately 1.86 and 0.84 eV, respectively (Supplementary Figure S8). The experimentally observed decrease in the optical band gap upon Fe substitution is consistent with the band-gap narrowing predicted by the DFT calculations. It can be inferred from the narrowing of the band gap that Fe doping enhances electronic conductivity, thereby facilitating electron transport and improving charge-transfer efficiency. For further investigation of the contribution of the dopant to this modification of the electronic structure, PDOS of pristine VO₂ and Fe-substituted VO₂ were also explored Figure 5(c,d). In pristine VO₂, the electronic states near the band edges mainly consist of V 3d and O 2p states. After Fe doping, Fe 3d states appear and hybridize with the preexisting V 3d-O 2p states. This additional hybridization redistributes electronic states and increases the density of states near the Fermi level. The enhanced orbital hybridization reduces the band gap, improves charge-carrier mobility, and facilitates electron transport within the VO₂ framework. Furthermore, the differential charge-density map (Supplementary Figure S7e), obtained by subtracting the charge densities of the reference Fe and VO₂ components from that of the Fe-substituted VO₂ system, directly visualizes the electronic redistribution induced by Fe incorporation. The localized regions of electron accumulation and depletion around the Fe–O–V environment indicate appreciable rearrangement of the local electron density upon Fe substitution, consistent with the Fe 3d–V 3d/O 2p hybridization observed from the PDOS analysis. The improved electronic transport characteristics suggested by these electronic-structure calculations are consistent with the experimental observation of reduced charge-transfer resistance from EIS and enhanced rate capability from GCD. Together with the lattice expansion revealed by Rietveld refinement, these electronic structure modifications provide a plausible explanation for the improved Zn²⁺ storage kinetics and overall electrochemical performance of the Fe-substituted VO₂ cathode.

FIGURE 5

4 Conclusion

An array of Fe-substituted VO₂ cathodes was efficiently prepared via a simple hydrothermal method and extensively characterized for AZIB applications. Out of all the studied compositions, Fe₀.₂₅V₀.₇₅O₂ showed superior electrochemical characteristics, delivering a specific capacity of 380.11 mAh g^-1 at a current density of 0.1 A g^-1 and 56% of capacity retention even at 50 times higher charge-discharge rate (5 A g^-1), with 82.75% capacity retention after 500 cycles at 2 A g^-1. The values are much higher than those of the VO₂ cathode, suggesting that Fe substitution successfully enhances zinc-ion storage performance. Electrochemical kinetic studies showed an increased capacitive contribution together with substantially reduced charge-transfer and diffusion impedances, indicating faster and less diffusion-limited charge-storage kinetics after Fe substitution. Post-cycling analysis further demonstrated improved tolerance of FeVO₂-25% toward cycling-induced structural and morphological changes, while ICP-OES analysis revealed approximately 92.3% lower vanadium dissolution compared with pristine VO₂. As revealed by DFT calculations, Fe substitution leads to electronic structure modification of VO₂ through band-gap narrowing and redistribution of electronic states near the Fermi level, consistent with the experimentally observed reduction in optical band gap and charge-transfer resistance. In conclusion, Fe, being a cheap and earth-abundant element, is an efficient dopant for tuning the Zn²⁺ storage capability of VO₂ by regulating its electronic and structural features.

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