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Divalent transition metal ion intercalated hydrated V₂O₅ nanosheet cathodes for ultra-long cycling aqueous zinc-ion batteries†

Jie Bai^a, Wenting Ji^b, Mengda Xue^a, De Li*^b, Huayu Wang^a and Lingyun Chen*^a
aDepartment of Applied Chemistry, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, China. E-mail: lychen@cqu.edu.cn
^bState Key Laboratory of Marine Resources Utilization in South China Sea, Key Laboratory of Research on Utilization of Si-Zr-Ti Resources of Hainan Province, School of Materials Science and Engineering, Hainan University, Haikou 570228, China. E-mail: lidehainu@hainanu.edu.cn

First published on 10th July 2025

Layered materials have been extensively recognized in the field of electrochemical energy storage due to their abundant and tunable intercalation chemistry, as well as their high active surface.^14–17 In addition, their two-dimensional (2D) nanosheets, with thicknesses ranging from monolayer to multilayer, offer broad opportunities for subsequent structural and functional modifications.¹⁸ Vanadium pentoxide (V₂O₅), as a representative layered vanadium oxide, possesses a high theoretical specific capacity of 589 mA h g⁻¹ due to its variable valence states (from 5+ to 3+), which renders it widely utilized as a cathode material in AZIBs.¹⁹ Research indicates that metal cations embedded into the layered structure of vanadium pentoxide serve as effective pillars within the structural framework, while also expanding the interlayer spacing and achieving exceptional zinc ion transport kinetics and storage capabilities.²⁰ Additionally, the incorporation of metal cations can effectively modulate the electronic structure near the Fermi level of pristine V₂O₅, enhancing its electrical conductivity, thereby improving electrochemical performance.^21,22 Notably, as intercalation ion candidates, transition metal cations have garnered significant attention due to their rich d-electron structures.^23,24 For instance, Bai et al. synthesized Cu²⁺ and Mn²⁺ co-intercalated hydrated V₂O₅ cathode materials, which exhibited superior Zn²⁺ diffusion kinetics and long cycling stability in AZIBs.²³ Herein, we demonstrate a hydrosol ion-mediated self-assembly strategy enabling the successful intercalation of three transition metal cations (Co²⁺, Ni²⁺, and Cu²⁺) into the layered architecture of a hydrated V₂O₅ nanosheet, while preserving their structural integrity and ion-transport functionality. Moreover, compared to hydrothermal/solvothermal synthesis approaches, this self-assembly synthesis strategy enables the formation of a uniform phase structure.²⁰

X-ray diffraction (XRD) patterns of the Co²⁺, Ni²⁺, and Cu²⁺ assembling products are given in Fig. 1a. The sharp peaks near 5–9° can be identified as the (001) characteristic peaks. XRD analyses reveal that, compared to pure V₂O₅ prepared by freeze-drying, the incorporation of Co²⁺ (13.2 Å), Ni²⁺ (13.4 Å), and Cu²⁺ (11.9 Å) ions induces a leftward shift of the (001) characteristic peak, indicating an expansion of the interlayer spacing. Fourier transform infrared (FTIR) and Raman spectra also confirmed the successful synthesis of the three products (Fig. S1 and S2, ESI†). X-ray photoelectron spectroscopy (XPS) analyses further confirmed the successful intercalation of Co²⁺, Ni²⁺, and Cu²⁺ (Fig. S3, ESI†). The field emission scanning electron microscopy (FESEM) and environmental scanning electron microscopy (ESEM) observations of the self-assembled samples reveal that their microscopic morphology exhibits large aggregates formed by the stacking of V₂O₅ nanosheets (Fig. 1b and Fig. S4, ESI†). The transmission electron microscopy (TEM) image of the Co²⁺ intercalated product (Fig. 1c) also reveals a two-dimensional nanosheet morphology. The high-resolution transmission electron microscopy (HRTEM) image captured from the surface of the nanosheets in Fig. 1d reveals well-defined 2D lattice fringes, with spacings of 0.187 and 0.196 nm corresponding to the interlayer spacings of the (006) planes of layered V₂O₅. The energy dispersive spectrum (EDS) mapping (Fig. 1e) demonstrates the uniform distribution of V, O, and Co elements within the sample, while the presence of Co distribution confirms the successful intercalation of Co²⁺ into the layered V₂O₅·nH₂O host. Atomic force microscopy (AFM) characterization revealed that the self-assembled Co²⁺ product exhibits a thickness ranging from 2.0 to 3.5 nm, confirming its two-dimensional stacked nanosheet structure, which is consistent with the findings from SEM analysis (Fig. 1f–h). The chemical formula of pristine V₂O₅·nH₂O and these samples intercalated by Co²⁺, Ni²⁺, and Cu²⁺ was determined by inductive coupled plasma (ICP) analysis (Table S1, ESI†) and thermogravimetric analysis (TGA) (Fig. S5, ESI†), which can be identified as V₂O₅·0.77H₂O, Co_0.14V₂O₅·0.95H₂O, Ni_0.15V₂O₅·0.63H₂O, and Cu_0.15V₂O₅·0.60H₂O, respectively.

	Fig. 1 (a) XRD patterns of the freeze dried V2O5 and Co2+, Ni2+, and Cu2+, cooperative self-assembly products, respectively. (b) The FESEM image, (c) TEM image and corresponding selected area electron diffraction (SAED) pattern, (d) HRTEM image, (e) EDS mapping images, (f) and (g) AFM images and (h) corresponding height profiles of the Co2+ self-assembly sample.

Subsequently, the self-assembled samples were employed as cathode materials to assemble AZIBs using a 2 M Zn(OTF)₂ electrolyte and a zinc foil. According to a previous study,²⁰ the specific capacity of self-assembled products is highly dependent on the aging time following the self-assembly process. Therefore, we systematically investigated the impact of aging time after the Co²⁺ self-assembly process and conducted electrochemical evaluations. Galvanostatic charge–discharge (GCD) testing at 0.05 A g⁻¹ revealed that the sample aged for 5 h exhibited the optimal capacity performance, further confirming the critical influence of aging time on the specific capacity (Fig. S6, ESI†). Consequently, an aging time of 5 h was ultimately adopted for all self-assembled products. Cyclic voltammetry (CV) tests demonstrated that Co_0.14V₂O₅·0.95H₂O, Ni_0.15V₂O₅·0.63H₂O and Cu_0.15V₂O₅·0.60H₂O all exhibited pronounced redox peaks, indicative of typical battery behavior (Fig. 2a, and Fig. S7, ESI†). Compared to the pristine V₂O₅ prepared by direct freeze-drying, the rate performance tests of Co²⁺, Ni²⁺, and Cu²⁺ self-assembled samples exhibit superior discharge specific capacities (Fig. 2b). This enhancement is likely attributed to the modulation of the electronic structure of the pristine V₂O₅ by the rich d-electron of the transition metals, leading to improved conductivity. The Co_0.14V₂O₅·0.95H₂O cathode exhibited the highest discharge-specific capacity, which achieved average specific capacities of 338.2, 298.3, 260.8, 229.5, and 208.8 mA h g⁻¹ at current densities of 0.1, 0.2, 0.4, 0.7, and 1.0 A g⁻¹, respectively. When the current density was reset to 0.1 A g⁻¹, the specific capacity recovered to 304.7 mA h g⁻¹. Notably, the Co_0.14V₂O₅·0.95H₂O cathode exhibited an outstanding discharge specific capacity of 373.2 mA h g⁻¹ at a current density of 0.05 A g⁻¹ (Fig. 2c). The electrochemical performance outlined above still presents certain advantages compared to previously published studies (Table S2, ESI†). The assembled Co_0.14V₂O₅·0.95H₂O, Ni_0.15V₂O₅·0.63H₂O, and Cu_0.15V₂O₅·0.60H₂O cathodes were subjected to high-current long-term cycling performance tests (Fig. 2d, and Fig. S8, ESI†). It is noteworthy that the Co²⁺ self-assembled sample demonstrates superior cycling performance. The Co_0.14V₂O₅·0.95H₂O cathode retains a discharge-specific capacity of 103.2 mA h g⁻¹ even after 5000 cycles at a current density of 5 A g⁻¹. The excellent cycling stability under high current density could be attributed to the appropriate interlayer spacing adjustment induced by Co²⁺ intercalation, as well as the pillaring effect between Co²⁺ and the host frame.²⁰ The GCD curves and low-current cycling performance profiles of the Co_0.14V₂O₅·0.95H₂O electrode further exemplify this characteristic (Fig. S9, ESI†).

	Fig. 2 (a) CV profile. (b) The corresponding rate performance profiles of Co2+, Ni2+ and Cu2+ cathodes. (c) The corresponding GCD profiles at various current densities of the Co0.14V2O5·0.95H2O cathode. (d) Long-term cycling stability at 5.0 A g−1 of Co0.14V2O5·0.95H2O and freeze-dried V2O5 cathodes.

A series of electrochemical evaluations confirmed that the Co_0.14V₂O₅·0.95H₂O cathode exhibits the optimal electrochemical performance among the self-assembled products derived from the three transition metal cations. Compared with Ni²⁺ and Cu²⁺, Co²⁺ demonstrates superior electrochemical activity and stability due to its unique d-electron configuration, appropriate electrochemical potential, and robust Co–O bonds. These attributes contribute to more efficient charge transfer processes, thereby affording enhanced electrochemical performance. To further investigate the zinc ion storage and diffusion mechanisms of this self-assembly series, kinetic analyses were conducted. The CV curves of the Zn/Co_0.14V₂O₅·0.95H₂O battery were measured at different scan rates (Fig. 3a and Notes S1, ESI†). As shown in Fig. 3b, the b values of peaks 1 to 4 are 0.792, 0.791, 0.790, and 0.935, respectively, indicating that the storage of Zn²⁺ is predominantly governed by capacitive contributions. CV measurements at various scan rates indicate that the contribution of capacitive behavior becomes increasingly dominant with the increase in scan rate (Fig. 3c and d and Fig. S10, Notes S2, ESI†). The above results demonstrate that the Zn²⁺ storage kinetics in the Zn/Co_0.14V₂O₅·0.95H₂O battery are predominantly governed by capacitive behavior. In addition, electrochemical impedance spectroscopy (EIS) measurements were conducted to elucidate the charge transfer resistance (R_ct). The R_ct values of Co_0.14V₂O₅·0.95H₂O, Ni_0.15V₂O₅·0.63H₂O, and Cu_0.15V₂O₅·0.60H₂O are 92.7, 111.3 and 74.3 Ω, respectively, all of which are notably lower than that of the freeze-dried pristine sample (V₂O₅·0.77H₂O, 225.1 Ω) (Fig. 3e). Notably, the average ion diffusion coefficient (_Zn²⁺) value of Co_0.14V₂O₅·0.95H₂O, as determined by the galvanostatic intermittent titration technique (GITT), is higher than those of Ni_0.15V₂O₅·0.63H₂O, Cu_0.15V₂O₅·0.60H₂O, and freeze-dried V₂O₅ (Fig. 3f and Fig. S11, Notes S3, ESI†). These results are consistent with the aforementioned unique electronic structure of Co²⁺.

	Fig. 3 (a) CV curves at various scan rates, (b) the as-calculated b value of four different peaks 1–4, (c) capacitive and diffusion contribution bar graph calculated from the dependence of peak current density and the scan rate, and (d) CV curves containing the schematic of capacitive contribution at a current density of 1.0 mV s−1, for the Co0.14V2O5·0.95H2O cathode. (e) EIS spectrum of freeze-dried V2O5, Co0.14V2O5·0.95H2O, Ni0.15V2O5·0.63H2O and Cu0.15V2O5·0.60H2O cathodes. (f) GITT analysis of the Co0.14V2O5·0.95H2O electrode.

The density functional theory (DFT) calculation results indicated that the insertion of Co²⁺ modulates the electronic structure of pristine V₂O₅, reducing its band gap and enhancing its electrical conductivity (Fig. S12, ESI†). This leads to superior charge transfer kinetics in Co²⁺-intercalated V₂O₅. The EIS measurements of V₂O₅·0.77H₂O and Co_0.14V₂O₅·0.95H₂O further corroborated these computational findings. Ex situ XRD characterization and XPS spectra further confirmed the typical Zn²⁺ intercalation/de-intercalation mechanism of the Co_0.14V₂O₅·0.95H₂O cathode, which is consistent with that of most previously reported vanadium-based cathodes (Fig. S13, ESI†). In summary, the electrochemical reaction mechanism of Co_0.14V₂O₅·0.95H₂O as a cathode for AZIBs can be delineated as shown in Fig. S14 (ESI†). The anodic reaction of the Zn/Co_0.14V₂O₅·0.95H₂O cell is a typical process of metallic zinc losing electrons (Zn → Zn²⁺ + xe⁻). We further performed ESEM analysis on different states of the first GCD cycle of the Co_0.14V₂O₅·0.95H₂O cathode at a current density of 0.1 A g⁻¹ (Fig. S15, ESI†). The formation of products distinct from the original cathode material at different discharge states is also consistent with the aforementioned ex situ XRD and XPS characterization results. Furthermore, the investigations of in situ optical microscopy (OM) unambiguously corroborated the exceptional structural stability of the Co_0.14V₂O₅·0.95H₂O electrode, as evidenced by its preserved dimensional integrity throughout prolonged electrochemical cycling under intercalation/de-intercalation conditions (Fig. 4, and Notes S4, ESI†).

	Fig. 4 The in situ OM characterization of the Co0.14V2O5·0.95H2O cathode. (a)–(j) correspond to discharge to 0.2 V, while (k)–(p) corresponds to charge to 1.6 V.

In conclusion, using a novel hydrosol ion-mediated self-assembly strategy, we successfully intercalated three transition metal cations (Co²⁺, Ni²⁺, and Cu²⁺) into the layered framework of hydrated vanadium pentoxide (V₂O₅·nH₂O) nanosheets. The self-assembled product of Co²⁺ demonstrates superior electrochemical performance, outperforming the self-assembled products of Ni²⁺ and Cu²⁺, which can be attributed to its high electrochemical activity and excellent structural stability. DFT calculation results indicated that Co²⁺-intercalated V₂O₅ exhibits an enhanced electronic DOS near the Fermi level and a reduced band gap value, enhancing the electrical conductivity of pristine V₂O₅. Electrochemical testing results demonstrated that the Co_0.14V₂O₅·0.95H₂O cathode retains a high discharge specific capacity of 103.2 mA h g⁻¹ even after 5,000 cycles at a high current density of 5 A g⁻¹. Our work provides valuable insights into the development of high-performance vanadium-based cathode materials for aqueous batteries.

Jie Bai: writing – original draft, software, methodology, investigation, formal analysis, and data curation. Wenting Ji: data curation, software, and formal analysis. Mengda Xue: data curation. De Li: data curation and software. Huayu Wang: data curation and analysis. Lingyun Chen: conceptualization, funding acquisition, supervision, validation, writing – review and editing.

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21676036), the Natural Science Foundation of Chongqing (No. CSTB2023NSCQ-MSX0580), and the Large-scale Equipment Sharing Fund of Chongqing University (No. 202403150240 and 202503150091). Thanks to Professor Xiaohua Chen (School of Chemistry and Chemical Engineering, Chongqing University) for his help in DFT calculations. And we would like to thank Dr. Wei Yin at Analytical and Testing Center of Chongqing University for her assistance with ICP analysis.

Conflicts of interest

Data availability

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