Inhibition of vanadium cathode dissolution in zinc-ion batteries via niobium pillaring

Abstract

Vanadium (V)-based oxides are considered promising cathodes for aqueous zinc-ion batteries (AZIBs) due to their multiple oxidation states and diverse crystal structures. However, their sluggish kinetics and severe V-dissolution result in rapid capacity fading and an unsatisfactory cycle life. Herein, a niobium (Nb) pillaring coupling with polyethylene glycol (PEG) assistance strategy has been proposed for the first time, which overcomes the limitation of separation inability of Nb-doped V oxides. The doped Nb acted as a structural pillar contributes to good structural stability, and the PEG-assitance Nb-doped V₃O₇·H₂O (PNVO) featured abundant oxygen vacancy and enhanced kinetics, leading to good rate performance. This approach results in high reversible capacities of 403 mAh g⁻¹ and 244 mAh g⁻¹ at 1 A g⁻¹ and 10 A g⁻¹, respectively, and remarkable cyclability with 68% capacity retention after 3000 cycles at 5 A g⁻¹. The quantitative comparison of V-dissolution suggests that PNVO-2 exhibits a low dissolution rate of 8.93%, while that for PVO and VO is 11.25% and 16.07%, respectively. These findings not only confirm the positive effect of Nb-pillaring on the inhibition of V-dissolution but also highlight the promising practical application of V-based cathodes for AZIBs.

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1. Introduction

Efficient and safe energy storage technologies are expected to promote the development of a renewable energy-based society; hence, they have attracted ever-increasing attention in recent years.¹ Aqueous rechargeable multivalent ion batteries (Ca²⁺, Mg²⁺, Al³⁺ and Zn²⁺) feature high capacities and inherent safety. Among these batteries, zinc-ion batteries (ZIBs), which use zinc metal as an anode, have attracted extensive research efforts due to their high energy density and low cost, and they are considered one of the most promising aqueous batteries for practical application in large-scale energy storage.^2–5

Exploration of feasible cathodes for ZIBs is currently focused on manganese-based oxides, vanadium (V)-based oxides, Prussian blue analogues and organic compounds.^6,7 Among them, the multiple valence states and crystalline structures of V-based oxides enable multielectron redox chemistry and accessible ion transfer channels, delivering high capacities. Nevertheless, V oxide-based cathodes suffer from sluggish kinetics and severe V-dissolution; the former leads to significant polarization, while the latter results in active mass loss, resulting in structural collapse and capacity fading and triggering parasitic reactions.^8,9

To address these challenges, various strategies have been proposed in recent years. For instance, engineering nanowires¹⁰ and nanosheets¹¹ help shorten ion-diffusion paths, and constructing V-based oxides with graphene¹² and nanotubes¹³ is favorable for improving their conductivity and mechanical properties. Concurrently, metal ion incorporation, including K⁺,¹⁴ Mn^2+ 15 and Al³⁺,¹⁶ has been adopted to enhance the electrochemical performance and structural stability, while organic molecule preinsertion has been reported to block electrostatic interactions between Zn²⁺ and host O-sites and reduce the binding energy.¹⁷ Among these approaches, metal-ion incorporation exhibits significant promise in comprehensively improving kinetics and inhibiting V-dissolution due to the structural pillaring effect and accelerated ion migration.

Notably, high-valence-state Nb⁵⁺ ions permit greater electron participation in redox reactions than monovalent ions (Na⁺ and K⁺) and multivalent ions (Mn²⁺, Ca²⁺ and Al³⁺), showing a key advantage in enhancing the electrochemical capacity.¹⁸ For instance, Nb⁵⁺ doping induces charge redistribution and crystal defects in sodium vanadium phosphate, creating abundant vacancies and facilitating charge transfer.¹⁹ Nb-doped P2-layered cathodes induce the formation of a rock salt-like phase on the surface, enhancing the stability of the electrode/electrolyte interface and enabling ultra-long-life sodium-ion batteries.²⁰ Despite the impressive advantage of Nb doping in Li/Na ion batteries, it has rarely been reported in V-based oxides for ZIBs. Our pre-experiment result answered the reason why Nb-doped V-based oxides (NVOs) are rarely reported. As shown in Fig. S1, the prepared NVO can not be separated out from the slurry, and the polyethylene glycol (PEG)-assisted NVO showed natural sedimentation, suggesting the product was able to be separated out.

Herein, a Nb-pillaring coupled PEG assistance strategy was proposed, for the first time, to synthesize Nb-doped V₃O₇·H₂O (PNVO) via a one-step hydrothermal method. A small amount of PEG was added to the solution before the hydrothermal treatment to promote the separation process, and Nb⁵⁺ ions were employed as interlayer pillars to enhance the structural stability. Furthermore, Nb⁵⁺ incorporation induced the formation of abundant oxygen vacancies, providing abundant active sites. Experimental results confirmed that the synergistic effect of Nb-pillaring and oxygen vacancies markedly improved the ion diffusion coefficient and reduced the V-dissolution rate, enabling PNVO's boosted kinetics and structural stability. The abundant active sites endowed PNVO with an enhanced reversible capacity and rate performance, delivering 403 mAh g⁻¹ and 244 mAh g⁻¹ at 1 A g⁻¹ and 10 A g⁻¹, respectively, and exhibiting remarkable cyclability with a 68% capacity retention after 3000 cycles at 5 A g⁻¹.

2. Results and discussion

2.1. Material synthesis and characterization

Fig. 1a illustrates the stepwise preparation of PNVO using V₂O₅, NbCl₅ and PEG-4000 as raw materials through a one-step hydrothermal method. It was found that the introduction of PEG changed the morphology of V-based oxides. As shown in Fig. 1b, the product without PEG-4000 (denoted as VO) showed a layered structure, and the samples with PEG (denoted as PVO) displayed nanowire-like structures (Fig. 1c). Furthermore, PNVO samples with different Nb doping concentrations were prepared to investigate the effect of Nb doping on their morphologies. Fig. 1d and Fig. S2 show the scanning electron microscopy (SEM) images of PNVO-1, PNVO-2 and PNVO-3, in which the Nb doping content gradually increased. The length of the nanowire decreased with an increase in the Nb doping content, ultimately enabling morphological transformation from nanowire-like to nanoribbon-like structures. The underlying mechanism for the morphological changes involves multiple synergistic effects. PEG acts as a structure-directing agent by interacting with vanadium species, transforming the morphology from a layered structure into nanowires.²¹ Meanwhile, the incorporation of Nb⁵⁺ induces lattice expansion through a pillaring effect (confirmed by the enlarged lattice spacing in HRTEM images and XRD patterns), which disrupts the ideal growth environment for elongated nanowires, thereby driving the morphological transition from nanowires to nanoribbons (with a shortened length) as the Nb content increases.

Fig. 1 (a) Schematic of the synthesis procedure for PNVO. SEM and TEM images of (b and e) VO, (c and f) PVO and (d and g) PNVO-2, (h) HRTEM, (i) HAADF TEM and (j) elemental mapping images of PNVO-2.

Transmission electron microscopy (TEM) was further used to characterize the crystal structures. Fig. 1e and Fig. S3 show a uniform sheet-like morphology without dark regions, confirming the layered crystal structure of VO devoid of impurities. Furthermore, Fig. 1f, g and Fig. S4 reveal the nanowire-like and nanoribbon-like structures, verifying the morphological transformation. Fig. 1h and Fig. S5 present the high-resolution TEM (HRTEM) image of PVO, showing a lattice spacing of 0.319 nm, which is indexed to the (510) facet of V₃O₇·H₂O. After Nb doping, the lattice spacing gradually increased from 0.322 nm for PNVO-1 to 0.327 nm and 0.332 nm for PNVO-2 and PNVO-3, respectively. This demonstrates that the introduction of Nb in the lattice of V₃O₇·H₂O, which acts as a structural pillar, not only enables morphological transformation but is also conducive to lattice expansion. Fig. S6 displays the high-angle annular dark-field (HAADF) TEM and elemental mapping images of VO, which show that the V and O elements are uniformly distributed. Fig. S7 shows the HAADF TEM and elemental mapping images of PVO, and the uniform distribution of C is ascribed to the introduction of PEG. Fig. 1i, j and Fig. S8, S9 exhibit the HAADF TEM and elemental mapping images of PNVO, respectively, and the uniform distribution of the Nb element confirmed that the doped Nb atom was individually pillared without aggregation.

Fig. 2a shows the X-ray diffraction (XRD) patterns of VO, PVO and PNVO. The diffraction peaks of PVO and PNVO-2 corresponded well with those of V₃O₇·H₂O (PDF#85-2401). Significantly, some diffraction peaks of VO were mismatched with the standard card, demonstrating that the introduction of PEG is able to not only change the morphology but also alter the crystal structure. Concurrently, as shown in Fig. S10, the diffraction peak of (200) gradually shifted to low angles with an increase in the Nb doping content, suggesting the expansion of the crystal lattice, consistent with the HRTEM images. Fig. 2b displays the thermogravimetric analysis (TGA) curves of VO, PVO and PNVO. The weight loss below 100 °C was indexed to the evaporation of the adsorbed water, and the weight losses below 260 °C and 400 °C corresponded to the evaporation of crystalline water and thermal decomposition of PEG, respectively. Accordingly, the crystalline water contents of VO, PVO and PNVO-2 were 8.09 wt%, 1.67 wt% and 2.40 wt%, respectively.²² VO displayed a much higher crystalline water content than PVO and PNVO-2, which is attributed to its bulk layered structure, triggering its structural instability. Compared to PVO, the high amount of crystalline water is attributed to the expanded interlayer spacing induced by Nb⁵⁺ pillaring, providing additional space for water molecules. This demonstrates that the expansion of the crystal lattice in PNVO is attributed to the pre-embedding of Nb⁵⁺ rather than crystalline water.

Fig. 2 (a) XRD patterns, (b) TGA curves and (c) Raman spectra of VO, PVO and PNVO-2. XPS spectra of (d) V 2p and (e) O 1s of VO, PVO, and PNVO-2. (f) EPR spectra of VO, PVO, and PNVO-2.

Fig. 2c displays the Raman spectra of VO, PVO and PNVO, and the peaks located at 284, 401, 519, 691, and 991 cm⁻¹ corresponded to V=O bending, H–O–H bending, V–O bending, O–V–O stretching and V=O stretching vibrations, respectively.²³ Compared to PVO, PNVO-2 exhibited a significant reduction in the peak intensity, which is attributed to the significant structural change that is caused by the incorporation of Nb⁵⁺ within the crystal framework. Meanwhile, the peaks of the V=O bond for PVO and PNVO-2 were right-shifted compared to VO, which is attributed to the symmetric environment variation of the local V=O bonds.²⁴ This suggests that the morphological change is driven by the V=O vibration mode variation after introducing PEG, confirming the structure-directing role of PEG. Additionally, the Fourier transform infrared (FTIR) spectra in Fig. S11 further confirmed the structural change because PNVO-2 showed reduced adsorption intensities for V=O (966 cm⁻¹) and V–O–V (755 cm⁻¹) than VO.²⁵

X-ray photoelectron spectroscopy (XPS) was carried out to further characterize the chemical state and surface elemental compositions. Fig. S12 shows the survey spectra of all samples, in which the C, O and V signals were detected, while the Nb signal (Fig. S13) was present in PNVO-1, PNVO-2 and PNVO-3. The peaks at 210.2 eV and 207.4 eV in the Nb 3d spectra were ascribed to Nb 3d_5/2 and Nb 3d_3/2, respectively, confirming the successful incorporation of Nb⁵⁺.¹⁹Fig. 2d displays the V 2p spectra, and the peaks at 517.8 eV and 525.6 eV corresponded to V⁵⁺, while the peaks at 516.7 eV and 524.2 eV were attributed to V⁴⁺.²⁶ Significantly, the V⁴⁺ ratio in PVO was considerably lower than that in VO, further verifying that the introduction of PEG is able to alter the valence distribution of V. Furthermore, PNVO-1, PNVO-2, and PNVO-3 exhibited significantly higher V⁴⁺ ratios than PVO (Fig. S14a) after Nb doping. Additionally, the percentage of V⁴⁺ in PNVO-2 was higher than that in VO and PVO (Fig. S14b), indicating that Nb incorporation results in spontaneous charge compensation, which is conducive to reconstructing the charge distribution and ultimately improving the kinetics.²⁷ Further, the mixed valence state endows V₃O₇·H₂O nanoribbons with boosted electrochemical activity, thus delivering a high reversible capacity.²⁸

Fig. 2e presents the O 1s spectra of VO, PVO and PNVO-2, and the peaks at 530.4 eV, 531.3 eV and 532.8 eV were indexed to lattice oxygen, oxygen vacancies and crystalline water, respectively. Compared to VO, PNVO-2 exhibited abundant oxygen vacancies, indicating that Nb doping is able to create oxygen vacancies and, therefore, is promising for reaction kinetics enhancement.²⁹ Additionally, a small amount of crystalline water was detected for PNVO-2, which is primarily attributed to the morphological and structural differences. PNVO-2 showed a nanoribbon-like structure with an enlarged lattice, enabling the signal detection of crystalline water. In comparison, the bulk layered structure of VO was not conducive to detecting crystalline water by XPS measurement, but it featured a high amount of crystalline water (Fig. 2b). The electron paramagnetic resonance (EPR) spectra of VO, PVO, PNVO-1, PNVO-2 and PNVO-3 are displayed in Fig. 2f and Fig. S15. PNVO-2 delivered an intensified resonance signal at a magnetic field of 3515 G (g = 2.0002), which is associated with surface-captured electrons from oxygen vacancies.²⁹ This further verifies that the Nb incorporation is conducive to introducing oxygen vacancies, consistent with the results of the O 1s spectral analysis.

2.2. Electrochemical performance

Comprehensive structural characterization confirmed that PNVO-2 featured abundant oxygen vacancies and active sites, as well as mixed valence states of vanadium, making it significantly promising for improved electrochemical performance. Fig. S16 displays the comparison of the rate performance and cycle stability of PNVO-1, PNVO-2 and PNVO-3 at different current densities, among which PNVO-2 delivered enhanced reversible capacity and cycle stability. Hence, PNVO-2 was employed to reveal the pivotal role of Nb doping in boosting V oxide-based cathodes' performance.

As shown in Fig. 3a, the capacity of PNVO-2 initially increased and then slightly decreased, achieving a maximum reversible capacity of 403 mAh g⁻¹ at 1 A g⁻¹. The capacities of VO and PVO decayed with cycling, which is attributed to their structural degradation and severe V-dissolution during cycling, resulting in an immediate decrease from the initial state. In comparison, Nb-pillaring afforded PNVO-2 with an expanded interlayer spacing, contributing to more active sites and the corresponding electrochemical activation process, thereby delivering an initially increased capacity during cycles.^5,30,31Fig. 3b displays the galvanostatic charge/discharge (GCD) curves of VO, PVO and PNVO-2 at 1 A g⁻¹. The two discharge plateaus correspond to the two-step reduction reaction from V⁵⁺ to V³⁺, and the corresponding charge plateaus suggest the good reversibility of V⁵⁺/V⁴⁺ and V⁴⁺/V³⁺ redox couples.³² Accordingly, PNVO-2 showed a reduced polarization voltage of ΔE = 539 mV, while those of VO and PVO were 725 mV and 709 mV, respectively. The cyclic voltammetry (CV) curves of VO, PVO and PNVO-2 at 0.2 mV s⁻¹ in Fig. 3c revealed that all samples exhibited two redox couples, and PNVO-2 showed intensified redox peak intensities, while the ones for PVO were the lowest, which indicates the highest reversible capacity of PNVO-2, consistent with their GCD curves.²⁹ The cathodic peaks of PNVO-2 were located at 0.96 V and 0.60 V, and the corresponding anodic peaks were located at 1.06 V and 0.69 V. In the cases of VO and PVO, the peaks for their two redox couples were located at 0.89 V/1.04 V and 0.58 V/0.78 V, 0.86 V/1.01 V and 0.61 V/0.73 V, respectively. The voltage gaps for the two redox peaks of PNVO-2 were 98 mV and 103 mV, lower than those of VO (205 mV/154 mV) and PVO (122 mV/144 mV), further verifying the rapid redox reaction kinetics and promising a good rate performance.³³

Fig. 3 (a) Cycle performance and (b) corresponding GCD curves of VO, PVO and PNVO-2 at 1 A g⁻¹. (c) CV curves of VO, PVO and PNVO-2 at 0.2 mV s⁻¹. (d) Rate performance of VO, PVO and PNVO-2 and the corresponding GCD curves of (e) PNVO-2 and (f) PVO. (g) Long-term cycle performance of VO, PVO and PNVO-2 at 5 A g⁻¹. (h) Comparison of the rate performances.

Fig. 3d displays the rate performance of VO, PVO and PNVO-2. At 1 A g⁻¹, the reversible capacity of PNVO-2 gradually increased, achieving 381 mAh g⁻¹ after 20 cycles. The initial capacities of VO and PVO were 347 mAh g⁻¹ and 364 mAh g⁻¹, respectively, and they decreased with cycling, consistent with the cycle performance at 1 A g⁻¹ in Fig. 3a. With an increase in the current density, PNVO-2 and VO delivered similar reversible capacities of 309 mAh g⁻¹, 285 mAh g⁻¹, 266 mAh g⁻¹, 244 mAh g⁻¹ and 298 mAh g⁻¹, 286 mAh g⁻¹, 270 mAh g⁻¹, 244 mAh g⁻¹ at 3 A g⁻¹, 5 A g⁻¹, 7 A g⁻¹ and 10 A g⁻¹, respectively, superior to the 223 mAh g⁻¹, 200 mAh g⁻¹, 187 mAh g⁻¹, and 175 mAh g⁻¹ for PVO, respectively. This demonstrates that the enhanced reversible capacity can be attributed to Nb doping, rather than the introduction of PEG. During the following cycles, PNVO-2 delivered high reversible capacities of 276 mAh g⁻¹, 303 mAh g⁻¹, 347 mAh g⁻¹, and 402 mAh g⁻¹ at 7 A g⁻¹, 5 A g⁻¹, 3 A g⁻¹ and 1 A g⁻¹, respectively, while the capacities of VO and PVO were 266/194 mAh g⁻¹, 279/214 mAh g⁻¹, 297/239 mAh g⁻¹, and 329/278 mAh g⁻¹, respectively. It was observed that when the current density returned to 3 A g⁻¹ and 1 A g⁻¹, the capacity was higher than that in the initial state. This is attributed to the electrochemical activation process, during which the electrolyte gradually and fully wets the electrode material in the initial high-rate cycles, and the ion transport channels are optimized.^34,35 It further confirms that Nb doping enables good structural stability. Accordingly, Fig. 3e shows the GCD curves of PNVO-2 at different current densities. The voltage plateaus were well-maintained, while the voltage gap between discharge and charge plateaus gradually increased with an increase in the current density.¹² In comparison, the voltage plateaus of PVO (Fig. 3f) were shortened, particularly for the first discharge plateau. The findings imply that the expanded lattice spacing due to Nb incorporation enables good reversibility and rapid kinetics of Zn²⁺ insertion/extraction, and the abundant oxygen vacancies endow it with a fast charge transfer ability and reduced polarization.

Fig. 3g displays the cycle performance of VO, PVO and PNVO-2 at 5 A g⁻¹. PNVO-2 delivered an initial capacity of 221 mAh g⁻¹, and the capacity then gradually increased, achieving a maximum capacity of 282 mAh g⁻¹ after 508 cycles. After 3000 cycles, the capacity was maintained at 192 mAh g⁻¹, with a capacity retention of 68%. The capacity of PVO showed a similar tendency to PNVO-2, delivering an initial capacity of 222 mAh g⁻¹ and a maximum capacity of 228 mAh g⁻¹ after 336 cycles. Furthermore, the capacity faded to 108 mAh g⁻¹ after 3000 cycles, with a low capacity retention of 48%. In the case of VO, its capacity decreased from 283 mAh g⁻¹ to 133 mAh g⁻¹ after 3000 cycles, and the capacity retention was only 47%. Additionally, the periodic fluctuations in the specific capacity are attributed to the ambient temperature variations over a 24-hour period.²⁶

The GCD curves of the PNVO-2, PVO and VO after different cycles are displayed in Fig. S17. The superior cycling stability of PNVO-2 was further confirmed, which showed a slower capacity fade and more stable charge–discharge plateaus even after 1000 cycles.³⁶ Additionally, the differential capacity (dQ/dV) analysis of PNVO-2 (Fig. S18) revealed that even after 1000 cycles, neither a significant decline nor a shift in the dQ/dV peak intensity was observed, indicating a well-preserved internal electrode structure compared to PVO and VO.³⁷ The enhanced cycle stability of PNVO-2 suggests that it features good structural stability, rapid kinetics and mitigated V-dissolution, which is attributed to the Nb-pillaring. Compared to previous reports, PNVO-2 exhibited an improved performance (Fig. 3h), further confirming its superiority.^{21,26,38–43}

2.3. Electrochemical kinetics analysis

Fig. 4a displays the Nyquist plots of VO, PVO and PNVO-2. In the high-frequency region, PNVO-2 exhibited the smallest semicircle, while PVO presented the largest one, which demonstrates that PNVO-2 features the lowest charge transfer resistance. This can be attributed to the Nb⁵⁺ doping, which promotes the excitation of charge carriers into the conduction band and facilitates effective electron transfer in redox reactions.²⁴ The galvanostatic intermittent titration technique (GITT) was used to quantitatively compare the ion diffusion ability. Fig. 4b shows the GITT curves of VO, PVO and PNVO-2 at 1 A g⁻¹, and the relaxation time was set to 30 min with a pulse time of 1 min (Fig. S19). As shown in Fig. 4c, the ion diffusion coefficient of PNVO-2 during the charge and discharge ranged from 10^−10.5 cm² s⁻¹ to 10^−8.5 cm² s⁻¹. In comparison, the ion diffusion coefficients of VO and PVO ranged from 10⁻¹¹ cm² s⁻¹ to 10⁻⁹ cm² s⁻¹, confirming the boosted electrochemical dynamics of PNVO-2. To further elucidate the interfacial kinetics and ion transport mechanisms, ex situ electrochemical impedance spectroscopy (EIS) coupled with distribution of relaxation times (DRT) analysis was conducted (Fig. S20–S23). The DRT spectra reveal four distinct resistive contributions, namely, the initial ion adsorption/desorption process (R_ads), Zn²⁺ migration across the interface (R_mig), interfacial charge transfer (R_ct), and bulk Zn²⁺ diffusion (R_diff).⁴⁴ Compared with the pristine VO and PVO, the PNVO-2 exhibited pronounced reductions in R_ads, R_mig, and R_ct, highlighting accelerated ion adsorption, interfacial migration, and bulk diffusion. This is attributed to the synergistic effect of Nb-pillaring, where the expanded interlayer reduces steric hindrance for ion diffusion, thereby lowering R_diff; the abundant oxygen vacancies provide additional active sites for ion adsorption, thus reducing R_ads; and the stabilized layered structure facilitates Zn²⁺ migration across the interface, consequently reducing R_mig.

Fig. 4 (a) Nyquist plots, (b) GITT curves and (c) calculated diffusion coefficients of VO, PVO and PNVO-2 cathodes. (d) CV curves of PNVO-2 at various scan rates and (e) corresponding log(i) versus log(ν) plots. (f) Pseudocapacitive contribution for PNVO-2 at 0.8 mV s⁻¹. (g) CV curves for PVO at various scan rates and (h) corresponding plots of log(i) versus log(ν). (i) Capacitive contributions for VO, PVO and PNVO-2 at various scan rates.

Fig. 4d displays the CV curves of PNVO-2 at different scan rates ranging from 0.1 to 0.8 mV s⁻¹. The intensities of the redox peaks increased with an increase in the scan rate.⁴⁵ The correlation between the peak current and scan rate is fitted in Fig. 4e, and the calculated b values were 0.69, 0.91, 0.77 and 0.83, suggesting that the electrochemical process is collaboratively controlled by diffusion and capacitive behaviors. Fig. 4g and h show the CV curves and corresponding correlation fitting results of PVO as well as those of VO in Fig. S24. The b values of PVO and VO ranged from 0.62 to 0.89. Fig. 4f illustrates the capacitive contribution of PNVO-2 at 0.8 mV s⁻¹, and the contribution ratio was calculated to be 76%. Fig. 4i exhibits the capacitive contribution ratios of VO, PVO and PNVO-2 at different scan rates. The pseudocapacitive contribution of PNVO-2 were 54%, 59%, 67%, 75% and 76% at 0.1 mV s⁻¹, 0.2 mV s⁻¹, 0.4 mV s⁻¹ and 0.8 mV s⁻¹, respectively, and the ones for PVO and VO were 44%, 51%, 58%, 63%, 69% and 45%, 51%, 59%, 64%, 70%, respectively. The complementary techniques employed herein collectively provide robust validation of the enhanced kinetics for PNVO-2. The consistent results obtained from EIS (the lowest R_ct), GITT (the highest ion diffusion coefficients), and CV pseudocapacitive analyses (the highest capacitive contribution) mutually corroborate the fast reaction kinetics and charge transfer capability of PNVO-2.^15,31,44,46 These multi-technique characterizations confirm that the Nb-pillaring strategy successfully optimizes the ion transport and electronic conductivity of the V₃O₇·H₂O cathode.

2.4. Inhibition of V-dissolution

Fig. 5a–c illustrate the CV curves of PNVO-2, PVO and VO at 0.1 mV s⁻¹, respectively. During the discharge process, the main peak around 0.6 V was attributed to the Zn²⁺ insertion, and the former subpeak at 0.76 V corresponded to Zn²⁺ ions, while the latter subpeak at 0.52 V was indexed to hydrated Zn²⁺ ions. It has been suggested that the insertion of hydrated Zn²⁺ ions primarily leads to structural collapse and V-dissolution.⁴⁰ Significantly, PNVO-2 displayed a greatly reduced peak at 0.52 V compared to VO and PVO, suggesting the blocked insertion of hydrated Zn²⁺ ions, and the peaks in the four curves of PNVO-2 overlapped well, whereas those of VO and PVO did not. During the initial four cycles, the peak at 0.76 V in the curves of PVO gradually diminished, whilst the peak at 0.52 V was maintained after the first discharge. In the case of VO, the subpeak related to the insertion of hydrated Zn²⁺ ions weakened compared to PVO, suggesting that the blocking effect against hydrated Zn²⁺ ion insertion in PNVO-2 mainly originates from Nb-pillaring rather than the introduction of PEG. Combining the shortened discharge plateaus and capacity attenuation in Fig. 3b and g, the differences in the two subpeaks (0.52 V and 0.76 V) for VO, PVO and PNVO-2 verifies that Nb doping is of significant ability in suppressing V-dissolution and improving the structural stability.

Fig. 5 CV curves of (a) PNVO-2, (b) PVO and (c) VO cathodes at 0.1 mV s⁻¹. 2-day OCP rest tests of (d) PNVO-2, (e) PVO and (f) VO at 1 A g⁻¹. SEM images of (g) PNVO-2, (h) PVO and (i) VO after 50 cycles at 1 A g⁻¹. (j) Photographs of VO, PVO and PNVO-2 soaked in a 2 M ZnSO₄ electrolyte for varying durations.

The open-circuit potential (OCP) rest test at 1 A g⁻¹ was conducted to quantitatively compare V-dissolution. As shown in Fig. 5d, the battery using the PNVO-2 cathode exhibited a capacity retention of 91.07% after a 2-day rest, implying 8.93% V-dissolution. In comparison, as shown in Fig. 5e and f, the capacity retentions of PVO and VO were 88.75% and 83.93%, respectively, and the corresponding V-dissolution ratios were 11.25% and 16.07%. Additionally, the self-discharge test was carried out to evaluate the V-dissolution.⁴⁷ As shown in Fig. S27, PNVO-2 exhibited a high coulombic efficiency of 76.08%, superior to both PVO (73.35%) and VO (69.40%). Fig. 5g–i illustrate the SEM images of the cycled electrodes of PNVO-2, PVO and VO, respectively. It was observed that the nanoribbon-like structure of PNVO-2 was maintained well, and the morphology was unchanged. However, the nanoribbon-like structure of PVO was shortened, and the layered structure of VO was disintegrated and fragmented. The significant difference in the structures and morphologies of PNVO-2 and PVO/VO confirms the suppressed V-dissolution in PNVO-2. Fig. 5j shows the optical images of VO, PVO and PNVO-2 electrodes after soaking in a 2 M ZnSO₄ electrolyte for 14 days. The most intense yellow color of PVO and the dark yellow color of VO suggested their severe V-dissolution. Fig. S28 illustrates the color variation in PVO, PNVO-1, PNVO-2 and PNVO-3, and the slight change for PNVO-1, PNVO-2 and PNVO-3 indicated that the inhibition of V-dissolution was attributed to Nb doping, rather than the introduction of PEG. Based on XPS analysis (Fig. 2d and Fig. S14) and the results of soaking test, it can be concluded that the predominant oxidation state of dissolved vanadium is V⁵⁺, with the proportion of V⁵⁺ in PNVO-2 being significantly lower than that in PVO. Additionally, the color change of the electrolyte to yellow (V⁵⁺ is present as VO₂⁺) further confirms that the main oxidation state of dissolved vanadium is V⁵⁺. This is consistent with recent reports indicating that high-valence vanadium species (V⁵⁺) are more prone to dissolution as soluble vanadyl species (VO₂⁺) in aqueous electrolytes.⁴⁸

2.5. Energy storage mechanism

Fig. 6a displays the ex situ XRD patterns during the second discharge and charge processes of PNVO-2. During the discharge process, the diffraction peak at ∼32° gradually shifted toward high degrees, indicating a decrease in the interlayer spacing, which is attributed to the strong electrostatic attraction between the V₃O₇ interlayers and the inserted Zn²⁺ ions.⁴⁹ Furthermore, the peak then shifted to the original position after charging, indicating good reversibility. Meanwhile, a new diffraction peak at ∼7.9°, indexed to the by-product Zn₄SO₄(OH)₆·5H₂O (PDF#78-0246), was observed below 0.5 V during the discharge process, and the peak disappeared above 0.98 V during the charge process. Fig. 6b–d show the SEM images of the PNVO-2 electrode in the initial charged state, fully discharged state and second charged state, respectively. The sheet-like species corresponded to the by-product, and the reversibility of the by-product formation was consistent with the ex situ XRD results. In general, the formation of Zn₄SO₄(OH)₆·5H₂O is caused by the accumulation of OH⁻, which is attributed to the H⁺ insertion.⁵⁰ This confirms that the high reversible capacity of PNVO-2 originates from the coinsertion of Zn²⁺ and H⁺ ions. Additionally, PVO exhibited similar phase transition and by-product formation (Fig. S29).

Fig. 6 (a) Ex situ XRD patterns of PNVO-2 at 2 A g⁻¹ in the second cycle. SEM images of the PNVO-2 electrode in the (b) 1^st fully charged, (c) 2^nd fully discharged and (d) 2^nd charged states. XPS spectra of (e) V 2p, (f) O 1s and (g) Zn 2p of PNVO-2 in the fully charged/discharged states. (h) Schematic of the energy storage mechanism of the PNVO-2.

Ex situ XPS analysis was conducted to investigate the changes in the elemental composition and valence state of PNVO-2 under different states. Fig. 6e displays the V 2p spectra in the fully discharged and charged states. Compared to the fully charged state, the peak intensity of V⁵⁺ was significantly reduced in the fully discharged state, while the peak intensity of V⁴⁺ was greatly intensified, indicating the reduction of V⁵⁺ species during Zn²⁺ and H⁺ coinsertion. In the O 1s spectra (Fig. 6f), the peak intensity of the V–O bond in the fully discharged state was weaker than that in the fully charged state, which results from the interaction between the inserted Zn²⁺/H⁺ and the lattice oxygen. Furthermore, the Zn 2p signals (Fig. 6g) did not completely disappear, showing a significantly reduced intensity that is caused by the partial residual Zn.

The charge storage mechanisms of PVO and PNVO-2 are summarized in Fig. 6h. Based on comprehensive structural and electrochemical analyses, it is revealed that Nb⁵⁺ doping plays a synergistic and progressive role in enhancing the overall performance of V₃O₇·H₂O cathodes. The introduced Nb⁵⁺ in the vanadium oxide acts as a structural pillar, leading to an expanded interlayer spacing. The enlarged spacing not only facilitates the diffusion of Zn²⁺ ions but also induces the formation of abundant oxygen vacancies. The combined effects of expanded interlayers and oxygen vacancies significantly enhance the reaction kinetics and provide additional active sites for ion storage, thereby contributing to the improved specific capacity and rate capability of PNVO-2. Furthermore, the stabilized framework effectively suppresses the insertion of hydrated Zn²⁺ ions, which is the primary cause of V-dissolution and structural degradation in PVO cathodes. As a result, PNVO-2 exhibits a markedly inhibited dissolution of VO₂⁺ species in the electrolyte. In contrast, PVO suffers from continuous structural deterioration and active material loss due to the persistent insertion of hydrated Zn²⁺ ions. In summary, the superior cycling stability and high reversible capacity of PNVO-2 originate from the synergistic effects of Nb-pillaring-enhanced structural integrity, oxygen vacancy-promoted kinetics, and effective suppression of V-dissolution, collectively affording a durable and high-performance cathode for aqueous zinc-ion batteries.

2.6. Promising practical applications

To further reveal the practical application prospects of PNVO-2, 2 × 2 cm pouch cells were assembled. Fig. 7a displays the rate performance at different current densities. With an increase in the current density from 1 A g⁻¹ to 5 A g⁻¹, the pouch cell delivered high reversible capacities of 255 mAh g⁻¹, 181 mAh g⁻¹, 135 mAh g⁻¹, 99 mAh g⁻¹ and 75 mAh g⁻¹, respectively. In the following cycles, the reversible capacity gradually reached 233 mAh g⁻¹, 167 mAh g⁻¹, 131 mAh g⁻¹ and 99 mAh g⁻¹, respectively. Accordingly, the corresponding GCD curves, shown in Fig. 7b, presented obvious redox plateaus, indicating its good rate performance. Fig. 7c exhibits the cycle performance at 1 A g⁻¹, and the initially increased capacity approached 313 mAh g⁻¹ after 130 cycles, and it was maintained at 266 mAh g⁻¹ after 300 cycles. Fig. 7d shows the long-term cycle performance at 5 A g⁻¹. After the initial activation, the pouch cell exhibited a reversible capacity of 76 mAh g⁻¹, and it was maintained at 64 mAh g⁻¹ after 2000 cycles, with a capacity retention of 84%, showing excellent cycle stability. Furthermore, Fig. 7e shows that the fan was driven by the one-pouch battery, exhibiting promising practicality.

Fig. 7 (a) Rate performance and (b) corresponding GCD curves of the Zn–V pouch cell using PNVO-2. Cycle performances of the pouch cells at (c) 1 A g⁻¹ and (d) 5 A g⁻¹. (e) Digital photograph of a fan driven by a pouch cell.

3. Conclusion

In summary, a Nb-pillaring coupled PEG assistance strategy has been proposed, for the first time, to synthesize Nb-doped V₃O₇·H₂O via a one-step hydrothermal method, which overcomes the limitation of separation inability of Nb-doped V oxides. Comprehensive structural characterization reveals that the introduction of Nb affords abundant oxygen vacancies and active sites, an expanded lattice spacing, boosted kinetics and reduced polarization, thus endowing PNVO-2 with an improved reversible capacity and structural stability and suppressed V-dissolution. Consequently, PNVO-2 delivers a high reversible capacity of 403 mAh g⁻¹ at 1 A g⁻¹, a good rate performance of 244 mAh g⁻¹ at 10 A g⁻¹, and excellent cycle stability with a capacity retention of 68% at 5 A g⁻¹ after 3000 cycles. This work not only provides a novel method to prepare Nb-doped V-based oxides, but also provides insights into the inhibition of V-dissolution, paving the way toward the design of durable V oxide-based cathodes for high-performance aqueous zinc batteries.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data will be available on request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6qi00190d.

Acknowledgements

This work was financially supported by the Doctoral Scientific Research Startup Foundation of Anhui Jianzhu University (Grant No. 2022QDZ01), the Open Project Program of Key Laboratory of Functional Molecular Solids of Ministry of Education (FMS2023004), the Anhui Province International Research Center on Advanced Building Materials (Grant No. JZCL2303ZR) and the Anhui Institute of Strategic Study on Carbon Dioxide Emissions Peak and Carbon Neutrality in Urban–Rural Development (Grant No. STY-2023-05).

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