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Stabilizing NH₄V₄O₁₀ by Al-doping: boosting kinetics and ammonium retention for practical zinc-ion batteries

Yuqi Peng^ab, Li'e Mo*^ab, Weilin Yan^ab, Lufan Liu^ab, Xuejun Zhu^ab, Yang Huang^ab, Zhaoqian Li^b, Hong Zhang*^c and Linhua Hu*^ab
aKey Laboratory of Photovoltaic and Energy Conservation Materials, CAS, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China. E-mail: lemo@rntek.cas.cn; lhhu@rntek.cas.cn
^bUniversity of Science and Technology of China, Hefei, Anhui 230026, P. R. China
^cHebei Computational Optical Imaging and Photoelectric Detection Technology Innovation Center, Hebei International Joint Research Center for Computational Optical Imaging and Intelligent Sensing, School of Mathematics and Physics Science and Engineering, Hebei University of Engineering, Handan, Hebei 056038, P. R. China. E-mail: zhanghong81@hebeu.edu.cn

First published on 8th August 2025

1. Introduction

Layered vanadium-based oxides have become one of the most promising materials for AZIB cathodes, attributed to their multiple valence states and diverse open frameworks.^13–17 Strategies such as introducing pillar ions (e.g., Li⁺, Na⁺, K⁺, Ca²⁺, Zn²⁺) or molecules (e.g., C₃N₄, imidazole) into the structures of these oxides can effectively expand the layer spacing.^18–25 This expansion facilitates ion diffusion kinetics and enhances cycle stability. In this context, the ammonium ion (NH₄⁺), characterized by its low molecular weight (18 g mol⁻¹) and large ion radius (1.43 Å), has drawn significant attention, exemplified by compounds such as NH₄V₃O₈, NH₄V₄O₁₀, (NH₄)₂V₁₀O₂₅, and (NH₄)₂V₆O₁₆.^26–29 Nevertheless, ammonium-based vanadium cathodes face two primary challenges for practical applications. First, the strong electrostatic interaction between Zn²⁺ and V–O bonds impedes the reversible de/intercalation of Zn²⁺. Second, the weak N–H⋯O hydrogen bonding and irreversible deammoniation during cycling lead to structural degradation.^30–34

To address these issues, the introduction of additional metal ions into the interlayer has been demonstrated to effectively stabilize the structure. Metal ions with lower electronegativity than vanadium can form stronger chemical bonds with oxygen, thereby reducing structural distortion.^35–37 Furthermore, the increased layer spacing provides extra ion diffusion pathways, which compensates for any potential decrease in specific capacity caused by metal ion doping.^38–40 Among various candidate metal ions (such as Ag⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ce³⁺ and Rb⁴⁺), Al³⁺ stands out with a high charge state, relatively large hydrated ion radius, moderate atomic weight, and strong Al–O bonds.^41–46 When Al³⁺ ions serve as interlayer pillar ions, they can effectively expand the interlayer spacing. Simultaneously, they suppress deammoniation through strengthened hydrogen bonding. The strong Al–O covalent bonds enhance structural stability by increasing shear resistance and restricting interlayer distortion.

In this study, we synthesized a nanobelt structured NH₄V₄O₁₀ material doped with Al³⁺ (denoted as Al-NVO) as a promising cathode for AZIBs. The nanobelt morphology of the cathode promotes increased contact between the active material and the electrolyte, thereby improving ion transport efficiency. The introduction of Al³⁺ ions enlarges the interlayer spacing, providing a smooth pathway for Zn²⁺ migration and ensuring favorable electrochemical kinetics. The pre-intercalated Al³⁺ ions act as pillars between layers, significantly enhancing structural stability and effectively suppressing irreversible deammoniation during cycling. The Al-NVO cathode delivers a high specific capacity of 460 mAh g⁻¹ at 0.2 A g⁻¹, remarkable cycling stability (retaining 94.6% capacity after 2000 cycles at 5 A g⁻¹), and excellent rate performance (309 mAh g⁻¹ at 10 A g⁻¹). The Al-NVO pouch cell achieves a capacity of 167 mAh g⁻¹ with stability over 250 cycles, while a 0.6 Ah prototype battery sustains operation for more than 300 hours under deep cycling conditions. This work represents a significant advancement in the development of high-performance cathode materials for AZIBs. By addressing key challenges related to material stability and electrochemical performance, it paves the way for the practical application of AZIBs in large-scale energy storage scenarios.

2. Results and discussion

	Fig. 1 Phase and structural analyses of Al-NVO and NVO (a) XRD patterns. (b) V 2p XPS spectra. (c) TG profiles under air atmosphere. (d) EDS of the Al-NVO. (e) Computational models and corresponding adsorption energies of Al atoms at three distinct lattice sites (S1–S3). (f and g) SEM, (h) TEM and (i) HR-TEM images of Al-NVO.

The thermogravimetric (TG) curves of NVO and Al-NVO can be divided into three characteristic stages, as shown in Fig. 1c. Stage I (30–150 °C) corresponds to the elimination of physically adsorbed water, followed by Stage II (150–400 °C) involving the sequential loss of crystalline water and ammonium ions. Stage III (400–600 °C) represents complete ammonium ion decomposition accompanied by structural crystallization.¹⁴ Although the total weight losses of Al-NVO and NVO are comparable (7.5% vs. 7.2%, respectively), indicating similar overall thermal decomposition behavior. The slightly higher total weight loss of Al-NVO is primarily attributed to the elimination of physically adsorbed water in the first stage (30–150 °C), which does not affect the intrinsic structural stability of the material. However, a critical distinction emerges in the second decomposition stage (150–400 °C), which corresponds to the loss of crystalline water and ammonium ions. Here, Al-NVO exhibits a 22% reduction in mass loss relative to NVO (4.2% vs. 5.4%). This significant difference directly demonstrates that Al doping stabilizes ammonium ions within the lattice framework by strengthening N–H⋯O hydrogen bonding, thereby suppressing irreversible deammoniation, a key factor for structural stability during cycling.

Energy-dispersive spectroscopy (EDS) analysis (Fig. 1d and Fig. S2) reveals the Al-doped sample maintains a V/Al atomic ratio of 1:0.14 while preserving the original N content (21.3% vs. 21.2% in pristine NVO). This, combined with TG data, confirms Al incorporation occurs without ammonium displacement, suggesting interlayer doping rather than cation substitution.

First-principles calculations were utilized to identify preferential Al³⁺ adsorption sites (Fig. 1e), demonstrating that site S1 exhibited the largest binding energy (−11.19 eV), indicating that Al³⁺ preferentially adsorbed at this site. For subsequent calculations of the energy barrier for zinc ion migration, we utilized a model where aluminum ions are adsorbed at site S1.

Elemental distribution mapping (Fig. S3 and S4) further confirms the successful incorporation of Al³⁺ ions. It also demonstrates the homogeneous dispersion of Al, N, V, and O throughout the Al-NVO architecture. Morphological characterization through scanning electron microscopy (SEM, Fig. 1f, g and Fig. S5a, b) and transmission electron microscopy (TEM, Fig. 1h and Fig. S5c) revealed preserved nanobelt morphology in both materials, confirming that Al³⁺ doping did not alter the morphology. High-resolution TEM analysis (Fig. 1i) showed an expanded interplanar spacing of 0.20 nm for the (004) plane in Al-NVO, compared to 0.19 nm in undoped NVO (Fig. S5d), which is consistent with XRD observations indicating lattice expansion.

To investigate the electrochemical enhancement induced by Al³⁺ doping, AZIBs were assembled using Al-NVO/NVO cathodes, Zn metal anodes, and 3 M ZnSO₄ electrolyte. Fig. 2a and b respectively present the Cyclic voltammetry (CV) profiles of Al-NVO and NVO cathodes, recorded at a scan rate of 0.1 mV s⁻¹ during the initial three cycles. The overlapped CV curves demonstrate the reversibility of the redox reactions, with characteristic peaks located at approximately 1.1 V (V⁵⁺/V⁴⁺) and 0.6 V (V⁴⁺/V³⁺), consistent with the reported vanadium-based cathodes.^41,42 A weak oxidation peak observed at around 1.45 V (highlighted by dashed black circles) can be attributed to irreversible deammoniation, in agreement with findings in prior studies.^31,47 Through comparative analysis, it is evident that the intensity of this peak in Al-NVO is significantly attenuated compared to that in NVO, indicating that Al³⁺ doping effectively suppresses irreversible deammoniation. Galvanostatic charge–discharge (GCD) profiles at 0.2 A g⁻¹ (Fig. 2c and d) exhibit dual voltage plateaus at ∼1.1 and ∼0.6 V, corresponding to the CV redox couples and confirming a two-step Zn²⁺ insertion mechanism. The highly overlapped charging and discharging profiles of Al-NVO indicate superior reversibility compared to NVO. Moreover, Al-NVO attains a discharge capacity of 460 mAh g⁻¹ at a current density of 0.2 A g⁻¹, which exceeds the 391 mAh g⁻¹ observed for NVO.

	Fig. 2 Electrochemical characterization of Al-NVO and NVO cathodes. (a and b) CV profiles during the initial three cycles. (c and d) GCD curves at 0.2 A g−1. (e) Rate performance across current densities (0.2–10 A g−1). (f) Nyquist plots. Cycling stability at (g) 0.2 and (h) 5 A g−1, respectively.

The rate capability is presented in Fig. 2e and Fig. S6. As the current density increases from 0.2 to 10 A g⁻¹, the Al-NVO cathode exhibits specific capacities of 460, 451, 437, 418, 402, 388, 375, and 309 mAh g⁻¹, respectively. In contrast, under the same test conditions, the undoped NVO cathode delivers lower capacities of 391, 380, 370, 352, 338, 326, 314, and 257 mAh g⁻¹ (Fig. S7). This concurrent improvement in capacity and rate performance underscores the effectiveness of Al³⁺ doping in enhancing the electrochemical performance of NVO.

Electrochemical impedance spectroscopy (EIS) analysis provides valuable mechanistic insights. The Al-NVO cathode displays a substantially reduced charge transfer resistance (R_ct), in contrast for the undoped NVO (Fig. 2f). The EIS spectra after 100 cycles (Fig. S8) show that Al-NVO retains a smaller charge transfer resistance compared to NVO. This indicates that Al³⁺ doping maintains efficient charge transfer even after prolonged cycling, reinforcing its role in enhancing dynamic stability. The decrease of R_ct after cycling may be attributed to the electrode electrochemical activation. Evidently, Al³⁺ doping effectively enhances bulk electronic conductivity and facilitates ion diffusion.

To comprehensively understand the Zn²⁺ storage mechanisms in the Al-NVO system, CV analysis was conducted across scanning rates ranging from 0.2 to 1.2 mV s⁻¹. The CV profiles maintained their consistent shapes, with only minor shifts in the redox peaks due to polarization effects (Fig. S9). This consistency indicates the stable electrochemical behavior of the Al-NVO. Quantitative analysis through b-value determination yielded an average value of 0.80 (Fig. S10), suggesting a mixed Zn²⁺ storage mechanism comprising both surface-controlled capacitive processes and diffusion-limited intercalation reactions. Further deconvolution of current contributions revealed that the capacitive-dominated portion increased progressively from 69.3% to 84.6% with scanning rate elevation (Fig. S11–S14). This value exceeded that of the NVO cathode, demonstrating the enhanced capacitive performance of Al-NVO. The predominance of pseudocapacitive behavior at higher scan rates is a key factor underlying the superior rate capability of Al-NVO. Surface-controlled processes enable faster charge storage kinetics, allowing the electrode to respond rapidly to changing current demands. The enhanced capacitive contribution in Al-NVO can be rationally associated with the introduction of Al³⁺ ions, which are likely to induce structural modifications. These modifications promote interfacial charge storage while maintaining efficient bulk diffusion pathways.

The cycling stability of Al-NVO and NVO was compared in Fig. 2g, h and Fig. S15–S17. The Al-NVO cathode delivered an initial discharge capacity of 458 mAh g⁻¹ at 0.2 A g⁻¹, retaining 422 mAh g⁻¹ (92.1% capacity retention) after 50 cycles with minimal degradation. In contrast, the NVO cathode experienced significant structural degradation after only 42 cycles under identical testing conditions. At a high current density of 5 A g⁻¹, the Al-NVO cathode maintained 94.6% of its initial capacity after 2000 cycles, outperforming the NVO, which failed after 1300 cycles. This improvement in cycling stability can be attributed to the stabilizing effect of Al³⁺ doping. The doping mitigates the structural degradation caused by the repeated insertion and extraction of Zn²⁺ during charge and discharge processes, ensuring long-term electrochemical stability.

To gain more understanding of the Zn²⁺ storage mechanism in the Al-NVO cathode, in situ XRD analysis was performed during the first two electrochemical cycles (Fig. 3a and b). No additional new peaks were observed at fully charged or discharged states compared to the pristine cathode, confirming the phase purity retention during Zn²⁺ intercalation/deintercalation. The magnified XRD pattern (Fig. 3c) reveals a slightly high-angle shift in the (001) reflection at full discharge, corresponding to an interlayer spacing contraction from 10.2 Å to 9.9 Å (Fig. 3k). This lattice compression can be attributed to enhanced electrostatic interactions within the van der Waals gap resulting from Zn²⁺ intercalation. Upon charging to 1.8 V, the (001) diffraction peak returns to a larger interlayer spacing of 10.4 Å. During subsequent electrochemical cycling, the interlayer spacing of the (001) plane exhibits reversible variations between 9.9 Å and 10.4 Å, directly mirroring the Zn²⁺ insertion/extraction processes. This demonstrates the structural reversibility of the Al-NVO cathode during Zn²⁺ intercalation/deintercalation. New diffraction peaks at 8.1°, 16.2°, 24.4°, and 33.0° emerge in low voltage range, corresponding to (Zn(OH)₂)₃(ZnSO₄)(H₂O)₅ (JCPDS 78-0246), formed through OH⁻-mediated reactions as a common byproduct in ZnSO₄ electrolyte. These secondary phase signatures completely disappear upon recharging to 1.8 V, with the diffraction profile recovering its initial configuration. This reversible phase evolution, coupled with the dynamic yet stable adjustments of the (001) spacing, provides compelling evidence for the highly reversible Zn²⁺ (de)intercalation mechanism in the Al-NVO cathode.

	Fig. 3 Structural evolution of the Al-NVO cathode during Zn2+ storage. (a) Discharge/charge curve of at 0.2 A g−1. (b) In situ XRD patterns collected during the first two cycles. (c) Ex situ XRD patterns of cycled electrodes. XPS spectra of (d) V 2p, (e) O 1s, and (f) N 1s at pristine, charged (1.8 V), and discharged (0.2 V) states. (g–j) SEM images obtained at pristine, discharged to 0.6 and 0.2 V, and charged to 1.8 V, respectively. (k) Schematic illustration of the reversible structural evolution of the Al-NVO cathode.

XPS tests reveal critical electronic structure evolutions, as presented in Fig. 3d–f and Fig. S18–S23. The emergence of Zn 2p_3/2 (1022 eV) and Zn 2p_1/2 (1045 eV) signals after discharge confirms the successful intercalation of Zn²⁺ (Fig. S18). The redox activity of vanadium is evident from the V 2p spectrum. A new V³⁺ component at 515.3 eV indicates deep reduction, which vanishes entirely upon recharging (Fig. 3d), demonstrating the reversible V⁴⁺/V³⁺ redox process. The O 1s spectral shows increased OH⁻ content (532.1 eV) during discharge (Fig. 3e), which is consistent with the formation of (Zn(OH)₂)₃(ZnSO₄)(H₂O)₅, as observed by XRD. NVO is similar to Al-NVO but has inferior reversibility and incomplete elimination of by-products (Fig. S19–S21). The Al 2p spectra (Fig. S22) maintain constant peak positions and oxidation state ratios throughout cycling, confirming Al³⁺ serves as a structural stabilizer rather than redox-active species. Comparative N 1s analysis (Fig. 3f vs. Fig. S23) reveals preserved N–H bonding in Al-NVO, demonstrating effective inhibition of deammoniation through Al³⁺ doping.

Morphological characterization (Fig. 3g–j and Fig. S24) corroborates the structural reversibility. Both the pristine and charged states maintain smooth surfaces, while discharged samples exhibit sheet-like by-products identified as (Zn(OH)₂)₃(ZnSO₄)(H₂O)₅. These secondary phases dissolve completely upon recharging, as evidenced by the restoration of the surface morphology. The combined multiscale evidence, including crystallographic reversibility (XRD), electronic state recovery (XPS), and morphological restoration (SEM), establishes a comprehensive reaction mechanism (Fig. 3k). The XRD (Fig. S25) and SEM (Fig. S26) of Al-NVO after 100 cycles reveal that the (001) diffraction peak position and nanobelt morphology remain consistent with the pristine state, with no significant lattice distortion or structural collapse. In contrast, NVO shows obvious peak shifts and surface degradation, confirming that Al³⁺ doping 1effectively stabilizes the layered structure during repeated Zn²⁺ intercalation/deintercalation. In this mechanism, Al³⁺ ions act as pillars to stabilize the host structure, while enabling reversible Zn²⁺ storage through synergistic vanadium redox and interlayer spacing adjustments.

Density functional theory (DFT) calculations provided atomic-scale insights into the mechanistic function of Al³⁺ doping in optimizing Zn²⁺ diffusion kinetics and structural stability. The incorporation of Al³⁺ led to lattice expansion, as manifested by an increase in interlayer spacing (Δd = 0.3 Å vs. NVO). This structural modification widened the atomic separation between adjacent layers. As a result, the charge density was redistributed, effectively alleviating the electrostatic interactions between migrating Zn²⁺ ions and the V–O frameworks. As shown in Fig. 4a and b, the Zn²⁺ migration pathway in Al-NVO was smoother compared to that in NVO. Notably, Al³⁺ ions located within the interlayers generate coulombic repulsive forces that partially counteract the Zn–O attraction, further facilitating cation mobility. The calculated migration barrier for Zn²⁺ decreased from 1.98 eV in NVO to 0.82 eV in Al-NVO (Fig. 4c), indicating a significant reduction in diffusion resistance. According to calculations of the Galvanostatic Intermittent Titration Technique (Fig. S27 and S28), the Zn²⁺ diffusion coefficient of the Al-NVO (10⁻⁸ cm² s⁻¹) is slightly higher than that of the NVO (10⁻⁹ cm² s⁻¹), suggesting the faster ion diffusion in the Al-NVO.

	Fig. 4 DFT calculations: theoretical migration pathways for Zn2+ in (a) NVO and (b) Al-NVO. (c) Calculated Zn2+ diffusion barriers in NVO and Al-NVO cathodes. (d) DOS analysis for NVO and Al-NVO cathodes. Schematic diagrams comparing structural stability evolution of (e) NVO and (f) Al-NVO cathodes during electrochemical cycling.

Electronic structure analysis through density of states (DOS) calculations (Fig. 4d) demonstrated enhanced metallicity in Al-NVO, with an increase in DOS near the Fermi level. This modification of the electronic structure corroborated the improved charge – transfer kinetics, which was consistent with the experimentally observed enhancements in rate capability.

Fig. 4e and f illustrated the stabilizing effect of Al³⁺ doping. During cycling, the NVO cathode underwent a progressive deamination process, with interlayer NH₄⁺ playing a critical role in maintaining structural stability. Al-NVO preserved its structural integrity via two stabilizing mechanisms: first, the strong Al–O covalent bond ensured the overall structural stability. Second, the coulombic repulsion force generated by Al³⁺ ions redistributed the charge density. This weakened the electrostatic interactions between O²⁻ and Zn²⁺, smoothing the migration pathway of Zn²⁺ and contributing to the long-term structural stability of Al-NVO.

To evaluate the practical application potential of the Al-NVO cathode, a pouch cell was assembled using Zn foil as the anode, 3M ZnSO₄ as the electrolyte, and Al-NVO as the cathode. Fig. 5a displays a photograph of the Zn anode, separator, and Al-NVO cathode. The electrode assembly had an active area of 80 cm², with a mass loading of ∼1 g for the Al-NVO material. The GCD profiles from the 1st to the 20th cycles at 0.2 A (Fig. S29) revealed minimal capacity fade, with voltage plateaus remaining stable throughout these cycles. As expected, the cell provides an impressive specific capacity of 167 mAh g⁻¹. Long-term cycling tests (Fig. 5b and Fig. S30) further demonstrated exceptional electrochemical stability, retaining a discharge capacity of 125 mAh g⁻¹ after 250 cycles at 0.2 A. When the mass loading of the Al-NVO active material was increased to 2 g, the discharge capacity of the pouch cell reached 350 mAh (Fig. 5c). This achievement is relatively rare in current reports (Table S1). To further demonstrate its practical applicability for grid-scale energy storage, an ampere-hour-level pouch cell was fabricated with a multi-layer architecture (3 cathode sheets paired with 2 anode sheets, as shown in Fig. 5d). This multi-layer stacked pouch cell provided an initial discharge capacity of 600 mAh (Fig. 5e), accompanied by an energy output of 492 mWh (Fig. S31) at 0.4 A, corresponding to a specific capacity of 136 mAh g⁻¹ based on the total cathode active material mass (4.4 g). Notably, the multi-layer pouch cell exhibited robust cycling stability, sustaining over 100 cycles at a high current of 0.4 A while maintaining continuous operation for more than 300 hours (Fig. 5f). Collectively, these results underscore the practical feasibility of Al-NVO for large-scale energy storage, achieving a balance between high capacity, long lifespan, and scalability.

	Fig. 5 Assembly process and electrochemical performance of Al-NVO//Zn pouch cells. (a) Photographic illustration of the essential components for pouch cell assembly, demonstrating the layered structure prior to encapsulation. (b) Cycling stability of the Al-NVO//Zn pouch cell operated at 0.2 A. (c) GCD curves of different mass loading at 0.2 A. (d) Schematic illustration of the configuration of a multi-layer Al-NVO//Zn pouch cell. (e) Voltage-capacity profiles of the multi-layer pouch cell with a substantial active material mass loading of 4.4 g and a current of 0.4 A. (f) Cycling performance of the multi-layer pouch cell.

3. Conclusion

Author contributions

Conflicts of interest

Data availability

Details about the experimental synthesis, the characterization methods, and the DFT method; XPS spectra of the Al-NVO and NVO materials; EDS of the NVO; Elemental distribution mapping images; SEM, TEM and HR-TEM images of the NVO; GCD profiles of the Al-NVO and NVO cathode at various current densities; CV curves at different scan rates of the Al-NVO and NVO cathode; GCD profiles of the single-layer pouch cell with Al-NVO cathode; Comparison of the electrochemical performance for our work and some representative cathode materials in AZIBs. See DOI: https://doi.org/10.1039/d5qi01280e.

Acknowledgements

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