VN_xO_y@C nanowires: a high-performance cathode material for aqueous zinc-ion batteries with dual-redox reaction mechanisms

Bin Tang† ^a, Yongsheng Xiang†^a, Xinlu Li^a, Chaohe Xu*^ab and Ronghua Wang*^a
aSchool of Materials Science and Engineering, Chongqing University, Chongqing 400030, China. E-mail: wangrh@cqu.edu.cn
^bNational Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China. E-mail: xche@cqu.edu.cn

First published on 31st July 2025

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1. Introduction

The escalating challenges of ecological deterioration and diminishing fossil energy resources have necessitated a global shift toward eco-friendly and renewable energy systems.^1–8 In the current energy market, lithium-ion batteries (LIBs) dominate in industries such as automobiles, medical devices, and portable wearable devices due to their remarkable energy density and favorable environmental performance,^9–15 while confronting the critical challenges of intrinsic safety issues and shortage of lithium resources.^16,17 To address these systemic challenges, extensive efforts have been dedicated to developing alternative rechargeable systems beyond conventional lithium-ion architectures.¹⁸ Recent advancements include aqueous batteries utilizing monovalent charge carriers (Na⁺, K⁺)^19,20 and multivalent ion shuttling mechanisms (Zn²⁺, Mg²⁺, Al³⁺).^21–23 Particularly noteworthy are aqueous zinc-ion batteries (AZIBs), which have emerged as frontrunners for grid-scale applications owing to their unique merit combination: (i) exceptional theoretical capacities (820 mAh g⁻¹ gravimetrically; 5854 mAh cm⁻³ volumetrically), (ii) favorable redox potential (−0.763 V vs. SHE) enabling compatibility with aqueous media, (iii) intrinsic nonflammability and eco-friendliness, (iv) abundant terrestrial zinc reserves exceeding lithium availability by ≈300-fold, and (v) low cost.^24–32 Nevertheless, existing AZIB technologies face critical performance gaps in achieving ultrafast (dis)charge kinetics, enhanced capacity retention under high current densities, and extended cycling stability—key metrics for commercial viability. These challenges have led a large number of researchers to focus on the study of cathode materials.^33–35

Currently, the cathode materials of ZIBs are mainly classified as vanadium-based materials (including VN, V₂O₅, V₂O₃),^36–38 manganese-based materials (such as MnO₂),³⁹ Prussian blue analogues,⁴⁰ etc. Among them, vanadium oxides with multiple vanadium cation oxidation states and open frameworks in vanadium-based materials can undergo redox reactions with multi-electron transfer and thus have a high theoretical specific capacity (610 mAh g⁻¹), while vanadium nitride (VN) has a higher theoretical specific capacity (825 mAh g⁻¹) and higher electrical conductivity (106 S m⁻¹), thus attracting intensive interest in the field of ZIBs recently.^26,41–43 Although VN has the above appealing advantages, it also faces the same key problems as other cathode materials, such as slow reaction kinetics and unstable cycle performance that need to be solved. In this regard, one effective approach is to compound VN with conductive materials, such as carbon, graphene, Mxene etc.^44–46 For instance, Park et al.⁴⁷ constructed 3D VN-rGO composite cathodes exhibiting high initial capacities (809 mAh g⁻¹ at 0.1 A g⁻¹) and sustained cyclability (445 mAh g⁻¹ after 400 cycles at 1 A g⁻¹). Nevertheless, severe capacity fade (retention <40% after 50 cycles at 0.1 A g⁻¹) reveals persistent challenges in stability under low-rate cycling conditions.

On the other hand, the construction of vacancy structure has also been proved to boost the overall performance thanks to the abundant active sites, high conductivity and fast kinetics. Chen et al.⁴⁸ discovered and confirmed that oxygen-doped vanadium nitride (O-VN), with high-valence nitrogen anions being partially replaced by low-valence oxygen anions, could reach 705 mAh g⁻¹ at 0.2 A g⁻¹. Fang et al.⁴⁹ demonstrated oxygen-modified vanadium nitrides (VN_xO_y) enabling reversible V³⁺/V²⁺ cationic redox, achieving 200 mAh g⁻¹ at ultrahigh current density (30 A g⁻¹) through enhanced ion-transport kinetics. Despite the great progress reported so far, most of the produced VN-based materials generally display an irregular, dense and bulk structure. As demonstrated by Mai et al.,⁵⁰ it is extremely difficult for electrons and ions to maintain bicontinuous and rapid transportation simultaneously, and the electrochemical reaction is limited by the slower transporter. Therefore, the transport characteristics of electrons and ions differ greatly in such irregular, dense and bulk structured VN, and the electrochemical reaction rate will be greatly limited. On the other hand, the process of introducing a large number of oxygen atoms will lead to severe changes in the volume of the VN crystal or even structural collapse, resulting in poor cycle stability.⁵¹

It is generally regarded that electrons and ions are synchronously and cooperatively transported in the one-dimensional nanofiber structure,^52,53 which greatly improves the electrochemical reaction rate. Therefore, since the nanofiber is an ideal structure for realizing bicontinuous and rapid transportation of electrons and ions, designing and preparing VN nanofibers with abundant defects and stable microstructure will be highly desirable. Herein, we reported the preparation of VN_xO_y@C hybrid nanowires with abundant oxygen-defects and carbon coating (denoted as VNO@C) to overcome the limitations of the VN electrode. It is found that the VNO@C electrode undergoes a dual redox reaction involving both anions and cations (V³⁺ ↔ V²⁺/N³⁻ ↔ N²⁻), resulting in a marked improvement in specific capacity. Simultaneously, the carbon-coated conductive network and the nanofiber structure further mitigate the internal electron and ion transport impedance, effectively augmenting the electrochemical reaction kinetics of the electrode. Benefiting from the unique nanofiber microstructure, conductive surface carbon layer, and the optimized energy storage mechanism, our VNO@C electrode manifests discharge specific capacities of 433.2 mAh g⁻¹ (0.1 A g⁻¹) and 282.2 mAh g⁻¹ (8 A g⁻¹), and the remaining capacity reaches 262.4 mAh g⁻¹ after 1000 cycles at 5 A g⁻¹ with a high capacity retention rate of 83.1%. This underscores its dual advantages in energy density and cyclability for advanced battery systems.

2. Experimental section

2.1 Preparation of V₂O₅ nanowires

V₂O₅ nanowires were prepared via a hydrothermal method. 0.182 g of V₂O₅ powder was dissolved in 30 mL deionized water, followed by slow addition of 2.5 mL 30 wt% H₂O₂ under stirring. The mixture was stirred for 15 min, transferred into a 50 mL Teflon-lined autoclave, and reacted at 220 °C for 24 h. The resulting product was centrifugally washed with deionized water/ethanol and freeze-dried to obtain the final precursor.

2.2 Preparation of V₂O₅@PDA nanowires

V₂O₅@PDA is obtained by a self-polymerization reaction. Firstly, 50 mg of the prepared V₂O₅ nanowires were dispersed in 100 mL of DI (deionized water) containing Tris–HCl (99%) buffer solution (pH = 8.5). Then, 30 mg of C₈H₁₁NO₂ (DA) powder was added. After stirring for 60 minutes, DA underwent self-polymerization on the surface of the V₂O₅ nanowires to form polydopamine (PDA). The product was then vacuum-filtered, washed, and dried to obtain the PDA-coated V₂O₅ nanowires (V₂O₅@PDA).

2.3 Preparation of VN_xO_y@C (VNO@C) nanowires

The V₂O₅@PDA and melamine (C₃H₆N₆) powder were uniformly mixed in a mass ratio of 1:6, and then placed in an N₂ atmosphere. The temperature was increased to 800 °C and held for 2 hours, resulting in the formation of VNO@C nanowires. For comparison, the V₂O₅ nanowires without PDA coating were mixed with C₃H₆N₆ powder and subjected to the same heat treatment to obtain VN_xO_y (denoted as VNO) nanowires. Moreover, the V₂O₅ nanowires were heat treated in an N₂ atmosphere without C₃H₆N₆ powder, and the obtained samples were named VO.

2.4 Materials characterization

The crystalline structure of the samples was acquired using a Bruker D8 Advance powder diffractometer using Cu Kα radiation (λ = 1.5406 Å). The multiscale morphology was characterized via transmission electron microscopy (TEM, Talos F200S transmission electron microscope) and scanning electron microscopy (SEM, Quattro S), and EDS elemental mapping correlated spatial composition with morphological features. XPS (ESCALAB 250Xi) provided the chemical composition of samples.

2.5 Electrochemical measurements

To construct cathode electrodes, VNO@C (or VNO, VO), conductive acetylene black, and PVDF were well mixed with a ratio of 7:2:1 into NMP solvent. The slurry was homogenized by continuous grinding in an agate mortar for 30 min, ensuring uniform dispersion of active materials and binder. The ground slurry was then coated onto conductive carbon paper with a diameter of 12 mm. After that, it was placed in a vacuum drying oven and dried at 60 °C for 6 hours to obtain the final cathode.

The VNO@C (or VNO, VO) cathode, zinc metal foil anode, glass fiber separator, and aqueous 3M Zn(CF₃SO₃)₂ electrolyte were assembled in a CR2032-type coin cell. After standing for 4 hours to allow the electrolyte to fully infiltrate the active materials and the separator, they were used for electrochemical testing.

Galvanostatic charge–discharge (GCD) cycling (0.2 V to 1.5 V) was performed on a LAND CT3002A system to evaluate rate capability and cycling stability. Complementary electrochemical analyses—cyclic voltammetry (CV, 0.2 V to 1.5 V) and electrochemical impedance spectroscopy (EIS, with a frequency range from 100 kHz to 0.01 Hz) were carried out by means of a CHI760E electrochemical workstation. Zn²⁺ diffusion coefficients (D_Zn²⁺) were quantitatively assessed via the galvanostatic intermittent titration technique (GITT), applying 8 min charge/discharge pulses at 0.1 A g⁻¹ followed by 28 min voltage relaxation intervals.

3. Results and discussion

3.1 Preparation of VNO@C and structural characterization

The VN_xO_y@C (simplified as VNO@C) nanowires were engineered via a purpose-built cascade: the initial hydrothermal-synthesized V₂O₅ nanowires establish a continuous 1D pathway for rapid ionic/electronic transport. Subsequent polydopamine (PDA) encapsulation preserves structural integrity while serving as a carbon precursor for the conductive coating. Controlled co-annealing with melamine then synchronously triggers dual transformations under an inert atmosphere: (i) carbonization—PDA pyrolysis forms N-doped carbon shells enhancing conductivity; (ii) crystal reconfiguration—in situ nitridation converts V₂O₅ into vanadium oxynitride (VN_xO_y, simplified as VNO) while generating oxygen defects through lattice oxygen substitution. This integrated strategy combining morphological design, carbon coating, and oxygen defect formation concurrently addresses conductivity constraints, structural instability, and redox kinetics. The resultant architecture reconciles the long-standing stability-kinetics trade-off in vanadium-based cathodes, as validated below.

Fig. 1a illustrates the phase evolution of vanadium oxide during heat treatment. The initial V₂O₅ precursor exhibits a phase consistent with that of V₂O₅. In the absence of melamine, when V₂O₅ is subjected to heat treatment in a nitrogen atmosphere, the resulting spectra match precisely with the standard peaks of V₂O₃ (PDF # 85-0411). This indicates that, without melamine, high-temperature conditions induce crystalline transformation and phase change from V₂O₅ to V₂O₃ (denoted as VO). Conversely, in the presence of melamine, V₂O₅ undergoes complete nitridation and converts to the VN phase (PDF # 35-0768). To be specific, the nitridation process initiates with thermal decomposition of melamine (C₃H₆N₆) at elevated temperatures (typically >300 °C), which liberates reactive nitrogen species such as NH₃ under an inert atmosphere.⁴⁸ Next, the V₂O₅ nanowire undergoes a reaction with NH₃ under an inert atmosphere to trigger crystal reconfiguration, forming the new VN phase. Noteworthily, the mass ratio of V₂O₅:C₃H₆N₆ will have a significant effect on the phase transformation degree and the resultant zinc-storage performance. In this work, the optimal mass ratio of V₂O₅:C₃H₆N₆ was determined to be 1:6 (Fig. S1–S3 and Table S1).

The chemical composition of VNO@C was further characterized through X-ray photoelectron spectroscopy (XPS). In the XPS survey spectrum, the characteristic peaks of C 1s, N 1s, V 2p, and O 1s are detected at binding energies of 284.0, 399.8, 516.8, and 530.1 eV, respectively (Fig. S4). The presence of the N 1s and C 1s peaks provides additional evidence for the successful phase transformation from the V₂O₅@PDA precursor to VNO@C. Notably, the high-resolution spectra of V 2p_3/2 and V 2p_1/2 for both VNO and VNO@C samples (Fig. 1b) exhibit sub-peaks corresponding to V–N/V–N–O/V–O bonds (V³⁺/V⁴⁺/V⁵⁺),⁴⁹ whereas the VO sample displays only V³⁺ peaks. These findings indicate that the nitriding treatment significantly diversifies the valence states of vanadium in the sample, thereby facilitating multi-electron transfer reactions and accelerating the reaction kinetics. Similarly, the high-resolution N 1s spectrum of the VNO@C sample (Fig. 1c) reveals sub-peaks at binding energies of 399.2, 400.1, and 401.5 eV, attributed to N³⁻–V, pyridinic/pyrrolic N, and graphitic N, respectively.⁵⁴ This further confirms the successful synthesis of vanadium nitride and the effective coating with nitrogen-doped carbon. The EPR spectra of VNO@C (Fig. S5) exhibit a distinct signal at g = 2.003, which is a characteristic signature of oxygen defects.

SEM micrographs of the V₂O₅ nanowire precursor are displayed in Fig. 1d and e. As illustrated, the microstructure exhibits a well-defined nanowire morphology, with a diameter of approximately 100 nm and a length of several micrometers, resulting in a high aspect ratio. Likewise, the VNO sample displays a pronounced linear morphology (Fig. 1f and g), characterized by a larger diameter (approximately 200–300 nm) compared to the V₂O₅ nanowire precursor, which can be attributed to the crystal transformation experienced by the sample. Notably, the VNO nanowires exhibit a significant fracture phenomenon. However, the increased cross-sectional area at the fracture sites enhances the Zn²⁺ reactive active sites. Energy-dispersive X-ray spectroscopy (EDS) analysis (Fig. 1h) reveals the uniform distribution of elements V and N throughout the sample, with a minor presence of element O, confirming the material's chemical composition as VNO. By comparison, VNO@C retains a similar overall microstructure to VNO, albeit with a marginally larger nanowire diameter. The EDS image of the corresponding region in Fig. 1j and k clearly demonstrates the homogeneous distribution of the four elements (V, C, O, N) within the VNO@C, indicating that the polydopamine has transformed into a carbon layer coating on the surface of the VNO nanowires via heat treatment. This morphological configuration is expected to facilitate rapid electron transport in the longitudinal direction, while Zn²⁺ ions diffuse swiftly into the material's inner layers along a path significantly shorter than the longitudinal diameter, thereby preventing electrochemical polarization. Additionally, the high specific surface area of the nanowires and numerous Zn²⁺ reaction active sites would significantly enhance the reaction kinetics.⁵⁰ TEM and HR-TEM images of VNO@C are well consistent with the SEM results, and confirm that VNO nanowires are uniformly encapsulated by an amorphous carbon layer with an average thickness of ∼2 nm (Fig. S6).

3.2 Zinc-ion storage performance investigation

For the purpose of examining the effects of nitriding treatment and carbon coating on the electrochemical properties of V₂O₅ nanowires, the rate capability of VO, VNO and VNO@C electrodes was compared, as depicted in Fig. 2a. It can be observed that the capacity of the VNO electrode surpasses that of the VO electrode at any current density. This reaffirms that the nitriding process significantly enriches the valence state of V in the sample, facilitating multi-electron transfer reactions and thereby enhancing the zinc storage capacity. In contrast, when the current density is less than 1 A g⁻¹, the capacity of the VNO@C electrode is slightly lower than that of the VNO electrode. This is primarily due to the fact that the carbon material in the VNO@C electrode lacks the storage capacity for Zn²⁺ and occupies a certain mass, leading to a lower content of active components in the VNO@C electrode compared to the VNO electrode. However, at high current densities exceeding 1 A g⁻¹, the capacity of the VNO@C electrode is significantly higher than that of the VNO electrode. Notably, it reaches 305.7 and 282.2 mAh g⁻¹ at current densities of 5 A g⁻¹ and 8 A g⁻¹, respectively (Fig. 2b and S7). This enhancement can be ascribed to the carbon coating that improves the electrode conductivity and electrochemical reaction kinetics, enabling the electrode to store a higher capacity at high current densities. In addition, when the current density is restored to a small value of 0.1 A g⁻¹, the capacity of the VNO@C electrode is significantly higher than that of the other two electrodes, and it has the highest capacity recovery rate (54.1%, 77.8%, and 92.5% for VO, VNO, and VNO@C, respectively). This indicates that after nitriding treatment and carbon coating, the capacity of VNO@C to maintain stability and prevent polarization has been greatly improved. The GCD curves in Fig. 2c–e can further illustrate the above conclusions. As the current density continuously increases, the overall shape of the GCD curve for the VNO@C electrode is the most stable (Fig. 2e). The voltage plateau demonstrates remarkable stability during charge/discharge processes, with the plateau potential gap showing insignificant variations upon current density elevation. This behavior implies suppressed polarization effects and highlights the excellent electrochemical reversibility of the VNO@C electrode. Fig. S8 displays the rate performance comparison of VNO@C with other vanadium-based cathode materials reported in the literature. Clearly, our VNO@C electrode achieves a competitive performance level among similar vanadium-based cathode materials. Furthermore, the Ragone plot of VNO@C is presented in Fig. S9. VNO@C achieves a high energy density while maintaining a favorable power density. This performance positions our material advantageously in the trade-off between energy and power delivery, which is crucial for applications requiring both efficient energy storage and rapid power output.

Extended cycling stability assessments of the three electrodes were systematically conducted (Fig. 2g). After subjecting each electrode to 1000 charge/discharge cycles at a current density of 5 A g⁻¹, the VNO@C electrode exhibited a residual capacity of 262.4 mAh g⁻¹, with the capacity retention rate reaching 83.1%. Both the overall capacity and the capacity retention rate of VNO@C were substantially higher than those of the VO (11.9%) and VNO electrodes (70.5%). Notably, the coulombic efficiencies of the three parallel samples all remain approximately 100%. Specifically, the VNO@C electrode exhibits an initial cycle coulombic efficiency of 99.9% and retains 99.68% after 1000 cycles. This observation indicates that side reactions in the VNO@C electrode are negligible. In addition, the GCD profiles of the VNO@C electrode at the 1st, 400th, 700th, and 1000th cycles are presented in Fig. 2f. It can be observed that the shape of the GCD curve of the VNO@C electrode remained approximately consistent throughout the long cycle, and the potential difference between the charge and discharge voltage platforms did not increase (for instance, the values of ΔE_1st/ΔE_400th/ΔE_700th/ΔE_1000th were 0.77/0.61/0.65/0.65 V). This also implies that the electrode did not exhibit an obvious polarization phenomenon, which is one of the key factors contributing to its outstanding long-cycle performance.

3.3 Electrochemical reaction kinetics of VNO@C electrodes

To unravel the underlying mechanisms responsible for the performance augmentation, an extensive suite of electrochemical reaction kinetics characterizations was performed. Fig. 3a and S10–S11 are the cyclic voltammetry (CV) curves of the VO, VNO, and VNO@C electrodes at scan rates of 0.1, 0.3, 0.5, 0.8, and 1.0 mV s⁻¹, respectively. The two pairs of redox peaks manifested in the CV curves of the VO electrode are attributed to the two-step redox reaction:⁵⁵

V2O3 + xZn2+ + nH2O + 2x e− ⇌ ZnxV2O3·nH2O	(1)

Among the three electrodes under study, the peak voltage of the CV curve of the VNO@C electrode exhibits the least variation with the increment in sweep rate. This implies that the peak voltage remains relatively stable, signifying the absence of pronounced polarization.⁵⁶ Moreover, the shape of the CV curve of the VNO@C electrode can be effectively maintained. In the case of the VNO electrode, with the increase of the sweep rate, the peak current of peak 3 commences to decline at 1.0 mV s⁻¹. This phenomenon suggests that the electrochemical kinetics is limited by the imbalance between electron transport and ionic diffusion, resulting in pronounced polarization at high current densities.

As evidenced by cyclic voltammetry studies, the peak current (i) exhibits a characteristic dependency on scan rate (ν) that can be expressed as:^57,58

i = aνb	(2)

Or

log(i) = blog(ν) + log(a)	(3)

Among them, a and b represent variable parameters. Generally, the value of b spans from 0.5 to 1. When the value of b is close to 0.5, the charge–discharge process is dominated by ion diffusion, while when the value of b is 1, it suggests that the charge–discharge process is mainly controlled by surface capacitance.⁵⁹ Based on the linear relationship between log(i) and log(ν) in Fig. 3b, the b values of peak 2 and peak 4 of the VNO@C electrode are finally calculated to be 0.63 and 0.81, respectively. The combined contributions from ion diffusion (b ≈ 0.5) and surface capacitance (b approaching 1) suggest a dual kinetics regulation mechanism in the VNO@C electrode, where the nanofiber morphology facilitates ion transport while the carbon coating enhances charge transport. Additionally, the capacitance contribution rate can be quantitatively computed using the following formula:⁵⁹

i = k1ν + k2ν1/2	(4)

Or

i/ν1/2 = k1ν1/2 + k2	(5)

The k₁ and k₂ relate to the redox reactions controlled by capacitance and diffusion respectively under certain voltages, and through them, we can calculate the proportion of contributions from capacitive and diffusion-controlled processes (Fig. 3c). The capacitance contribution rates of the three electrodes are relatively small and all do not exceed 50% at small scan rates such as 0.1 and 0.3 mV s⁻¹. However, as the scan rate increases, the capacitance contribution rates of all three electrodes exceed 50% (Fig. 3d). This further indicates that the electrochemical kinetics of the electrodes is totally controlled by ion diffusion and surface capacitance, conforming to the research results about the b value. In addition, as the scan rate increases, the capacitance contribution rate of the VNO@C electrode begins to be higher than that of the VO and VNO electrodes and reaches 74.8% at 0.8 mV s⁻¹ (Fig. 3c), suggesting a larger capacitance contribution rate at high current densities that leads to an enhancement in the capacity and rate performance of VNO@C electrodes.

The galvanostatic intermittent titration technique (GITT) tests were performed on the three electrodes, and D_Zn²⁺ was calculated based on the results (Fig. 3f and S12). During this process, the battery underwent discharge at a steady current of 0.1 A g⁻¹ for a duration of 8 minutes, and then a relaxation step lasting for 28 minutes was performed to enable the voltage to recover and reach equilibrium. The diffusion coefficient of Zn²⁺ in the electrode can be calculated by applying the following formula:⁶⁰

	(6)

where τ represents the relaxation time of the current pulse, which is 28 minutes. L stands for the diffusion distance of Zn²⁺, which is half the electrode thickness (0.05 mm). ΔE_τ is the voltage change value during the constant current pulse, and ΔE_s represents the stable-state voltage variation of the corresponding step. Through calculation, the D_Zn²⁺ in the charging process of the VNO@C electrode (1.6 × 10⁻¹⁰–5.1 × 10⁻⁸ cm² s⁻¹) is generally significantly higher than that of the VNO electrode (1.2 × 10⁻¹⁰–2.1 × 10⁻⁸ cm² s⁻¹) and the VO electrode (5.0 × 10⁻¹²–1.5 × 10⁻¹⁰ cm² s⁻¹). The same regularity in the magnitude of D_Zn²⁺ also exists in the discharging process of the three electrodes (1.2 × 10⁻¹⁰–1.8 × 10⁻⁸ vs. 1.0 × 10⁻¹⁰–1.6 × 10⁻⁸ vs. 1.5 × 10⁻¹¹–8.9 × 10⁻¹⁰ cm² s⁻¹, Fig. S12).

Furthermore, electrochemical impedance spectroscopy (EIS) tests were conducted on the three electrodes. As shown in Fig. 3e and Table S2, the VNO@C electrode exhibited markedly reduced combined ohmic resistance (R_s = 3.607 Ω) and charge transfer resistance (R_ct = 821.18 Ω) compared to VNO (R_s = 6.879 Ω, R_ct = 1753.88 Ω) and VO (R_s = 8.302 Ω, R_ct = 1845.93 Ω). Meanwhile, the slope of its low-frequency linear region (k = 0.508) is higher than that of the other two materials (VNO: 0.472; VO: 0.404). The Constant Phase Element parameter (CPE-P) of VNO@C obtained from EIS fitting yields a value of 0.77946, which is between 1 (ideal capacitive behavior) and 0.5 (Warburg diffusion impedance). This EIS-derived CPE-P value confirms a strong capacitive characteristic of the electrode, validating the significant pseudocapacitive contribution observed in the Trasatti analysis. Such dual resistance reduction verifies enhanced charge-transfer kinetics and accelerated Zn²⁺ mobility in VNO@C, aligning with the elevated diffusion coefficient quantified via GITT methodology. The above results demonstrate that nitridation treatment and carbon coating effectively reduce the internal electron and ion transport impedance of the electrode, thus significantly promoting the internal reaction kinetics and further augmenting the capacity and rate performance. On the other hand, the improvement in the Zn²⁺ diffusion rate can effectively avert electrochemical polarization, thereby slowing down capacity attenuation and improving cycle stability.

3.4 Zinc-ion storage mechanism

The zinc storage mechanism of the VNO@C electrode was investigated through ex situ XPS testing. In the surface Zn 2p spectrum of the VNO@C electrode (Fig. 4a), sub-peaks with relatively weaker intensities were present at 1022.3 and 1045.4 eV in the initial state, which were mainly attributed to Zn²⁺ in the electrolyte that was adsorbed on the electrode surface.⁶¹ When the discharge of the electrode reached 0.2 V, the peak intensity of Zn 2p showed a distinct increase, suggesting that a large amount of Zn²⁺ was inserted into the positive electrode during the discharge process. When the electrode was charged to 1.5 V, the peak intensity of Zn 2p on the electrode surface decreased significantly and was close to the peak intensity level in the initial state. This indicates that a large quantity of Zn²⁺ was removed from the electrode and returned to the electrolyte, demonstrating excellent electrochemical reaction reversibility of the electrode.

Fig. 4b shows the in situ high-resolution V 2p spectrum. Compared with the initial state, the binding energies of covalent bonds such as V–O, V–N–O, and V–N (corresponding to V⁵⁺, V⁴⁺, and V³⁺, respectively) were reduced when the electrode was fully discharged, since the intercalation of Zn²⁺ was accompanied by the reduction of V ions.⁶² In particular, the binding energy of the V³⁺–N bond at 516.3 eV dropped to 515.6 eV, which corresponds to the V²⁺–N bond and means that V³⁺ was reduced to V²⁺ during the discharge process. When the electrode was fully charged to 1.5 V, the V²⁺–N bond at the binding energy of 515.6 eV was restored to the V³⁺–N bond at 516.3 eV, and the intensity was almost completely restored to the pristine state. The results imply that the valence state and content of element V returned to the initial state, which further demonstrates the high reversibility of the electrochemical reaction. Based on the above analysis, at the cationic level, the core energy storage mechanism can be briefly summed up as V³⁺ ↔ V²⁺. In addition, the high-resolution N 1s analysis of the pristine material (Fig. 4c) revealed three distinct contributions: V-bonded N³⁻ (399.0 eV), pyridinic/pyrrolic-N (400.5 eV), and graphitic-N (402.4 eV). As the electrode reached a full discharge state at 0.2 V, the peak of N³⁻–V shifted towards the binding energy of 399.4 eV, which was in line with the N²⁻–V peak, while the pyridinic/pyrrolic N and graphitic N peaks did not have any peak shift. This is because they were detection peaks from the carbon layer on the VNO@C electrode surface.⁴⁹ Similarly, when the electrode was fully charged, the above spectral peak returned to the initial level, and this redox reaction can be simply summarized as N³⁻ ↔ N²⁻.

Based on the aforementioned ex situ characterization and analysis, the energy storage mechanism of the VNO@C electrode can be concisely summarized as a collaborative redox reaction of V³⁺ ↔ V²⁺ and N³⁻ ↔ N²⁻. It can be deduced that, in contrast to the conventional vanadium-based electrode energy storage mechanism relying solely on cationic redox reactions, the VNO and VNO@C electrodes are capable of undergoing simultaneous anionic and cationic redox reactions. This characteristic accelerates the electrochemical reaction process, enhances the electrode reaction intensity, and consequently improves the electrode capacity and rate performance.

4. Conclusions

In summary, we have successfully fabricated VNO@C nanocomposite fibers via nitridation of polydopamine coated vanadium oxide, integrating the modification strategies of microstructure design, energy storage mechanism optimization, and carbon coating. An extensive suite of electrochemical reaction kinetics characterizations, including CV analysis, GITT, and EIS, demonstrate that the carbon-coated conductive network, the abundant oxygen defects and the nanofiber structure can effectively facilitate electron and ion transport, greatly augmenting the electrochemical reaction kinetics of the electrode. Moreover, the nitridation of vanadium oxide not only remarkably augments the valence states of V element (encompassing V³⁺, V⁴⁺, and V⁵⁺), but also transforms the single cationic redox reaction of vanadium oxide into a dual redox reaction involving both anions and cations (V³⁺ ↔ V²⁺/N³⁻ ↔ N²⁻), thus further accelerating the electrochemical reaction process and enhancing the specific capacity. Owing to the unique nanofiber microstructure, outstanding surface carbon layer with excellent electrical conductivity, and optimized energy storage mechanism, the VNO@C electrode exhibits outstanding rate capability, delivering capacities of 433.2 mAh g⁻¹ (0.1 A g⁻¹) and 282.2 mAh g⁻¹ (8 A g⁻¹). After 1000 cycles under 5 A g⁻¹, it retains 262.4 mAh g⁻¹ with a notable 83.1% capacity retention, underscoring its cycling robustness.

Author contributions

Bin Tang: investigation, data curation, writing the original draft. Yongsheng Xiang: investigation, data curation, methodology. Xinlu Li: project administration. Chaohe Xu: resources, supervision. Ronghua Wang: project administration, resources, supervision, writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

It consists of rate performance, EIS curves, XRD, XPS, EPR, TEM and HR-TEM, CV curves, and additional tables about EIS fitting results. See DOI: https://doi.org/10.1039/d5ta03224e.

Acknowledgements

This work was supported by the Natural Science Foundation of Chongqing, China (CSTB2024NSCQ-LZX0013, CSTB2024NSCQ-MSX1062), Chengdu Municipal Science and Technology Bureau (2023-YF11-00066-HZ), and Chongqing Overseas Chinese Entrepreneurship and Innovation Support Program (No. cx2022002).

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Footnote

† Bin Tang and Yongsheng Xiang contributed equally to the work.

This journal is © The Royal Society of Chemistry 2025