The facile and low-cost synthesis process is schematically illustrated in Fig. S1. Figure 1a compares the XRD patterns of V2O5 and V2O5-Cu samples. From the figure, it can be seen that the comparative results between the prepared samples of V2O5, V2O5-Cu and the V2O5 standard diffraction card (JCPDS NO.41-1426) show that the synthesized materials have identical peak positions to the standard card. And no obvious impurity peaks are observed, which indicates that the synthesized samples have high purity and good crystallinity. The diffraction peaks of V2O5 at approximately 15.4°, 20.3° and 26.2° correspond to the (200), (001) and (110) crystal planes, respectively. According to the Bragg equation, the calculated d-values for the (001) crystal plane of V2O5-Cu and V2O5 are 0.4384 nm and 0.4380 nm, respectively. This indicates that the introduction of Cu has a little change in the interlayer spacing of V2O5.
Figure 1b shows the Raman spectra of the V2O5 and V2O5-Cu samples, and Fig. S2 shows the crystal structure of V2O5. It can be seen from the figure that oxygen atoms occupy four positions in a single [VO5] layer. The Raman peak at 995 cm-1 is generated by the stretching vibration of the V = O1 bond, while the peaks at 407 and 284 cm-1 can be attributed to the bending vibration of the V = O1 bond [15–16]. The bending vibration peak of V-O2 is located at 473 cm-1, and the stretching vibration peak of V-O3 is located at 693 cm-1. The peaks at 524 and 308 cm-1 are the stretching and bending vibrations of the V-O4 bond, respectively [17–19]. Due to the weak van der Waals force between adjacent [VO5] layers, the external [VO5]-[VO5] vibration peaks appear at low wave numbers, specifically at 104, 147 and 199 cm-1 [14]. Compared with V2O5 samples, the redshift of V2O5-Cu peak value and Raman spectral can be explained by lattice expansion, which leads to increased interatomic spacing and a relatively relaxed lattice. As a result, the vibration frequency is reduced. Consistent with the results of XRD analysis.
As shown in Fig. 1c, X-ray photoelectron spectroscopy (XPS) was employed to further determine the valence states of V, O and Cu elements in V2O5-Cu and V2O5. As shown in Fig. 1d, the Cu 2p3/2 and Cu 2p1/2 peaks at 928.5 and 931.0 eV, respectively, are consistent with those observed in the literature for CuO [20], which can prove the existence of Cu2+ ions. Cu2+ has a ionic radius of 73 pm, which is larger than that of 54 pm for V5+. Copper ions dopped into the lattice of V2O5, which will cause an increase in the unit cell volume of V2O5 [10]. This is the same as the XRD analysis results. Peak analysis was performed on the V2O5 and V2O5-Cu samples (Fig. 1e). The V 2p3/2 peak of V2O5 is located at 516.3 eV (V4+) and 517.9 eV (V5+), with a V5+/V4+ ratio of 1:0.05. For V2O5-Cu, the ratio of V5+ to V4+ is 1:0.07. This indicates the presence of more low-valence vanadium in the V2O5-Cu samples. The increase in the peak area ratio of V4+ indicates that the oxidation state of vanadium is lower, which may be due to the introduction of oxygen vacancies in V2O5-Cu. In order to maintain its electrical neutrality, the proportion of low-valence vanadium (V4+) increases [21], which can enhance the electronic transition between V4+ and V5+. Meanwhile, the conductivity of the electrode material is improved. Figure 1f shows the O1s spectra of V2O5-Cu and V2O5 samples, with the OI peak at 530.3 eV and the OII peak at 531.8 eV [22]. By comparing the integrated area ratios, it can be determined that V2O5-Cu has 1.6% more oxygen vacancies than V2O5. Oxygen vacancies play a crucial role in facilitating the kinetics of ion diffusion. Their generation and movement create additional space for ion diffusion, thereby accelerating ion migration and enhancing ionic conductivity. Additionally, oxygen vacancies serve as favorable active sites for nucleation during phase transitions in energy storage processes, providing supplementary sites for the storage of zinc ions [23].
The SEM images of V2O5 and V2O5-Cu samples are shown in Fig. S3(a-b). It can be observed that undoped V2O5 exhibits a block-like aggregation with an average particle size of 200 nm. V2O5-Cu is composed of numerous small plate-like structures arranged in a regular flake structure, with the average grain size of 100 nm for the tiny plate-like structures, so that the grain is refined. Grain refinement can increase the specific surface area of the material and enhance its electrochemical activity sites, which is conductive to improving the energy storage performance of the material [24–26]. Fig. S3(c-d) show the TEM images of the V2O5 and V2O5-Cu samples, respectively. As measured by HRTEM in Fig. 2a, the lattice fringe of the (110) plane of V2O5 is 0.338 nm. In Fig. 2b, the lattice fringes of the V2O5-Cu sample shows an interplanar spacing of 0.342 nm, indicating that the introduction of Cu2+ increases its spacing, which is consistent with the above analysis results. In order to more intuitively display the distribution of Cu in the sample, the element distribution diagram of V2O5-Cu is shown in Fig. 2(c-f), from which it can be observed that the copper element is uniformly distributed throughout the sample.
As shown in Fig. 3a, the electrochemical reaction kinetics of V2O5-Cu was studied by measuring CV test at different scan rates. As the scan rates increased from 0.2 mV s-1 to 1.0 mV s-1, the oxidation and reduction peaks moved to higher and lower potential, respectively. According to the related b values of oxidation and reduction peaks are quantified according to the slope, as shown in Fig. 3b. (The specific algorithm for calculating the b-value is provided in the supplementary material). The average b value of the peaks is about 0.6, indicating the presence of a diffusion-controlled Faradaic process and a capacitive process in the reaction. The curves show similar morphology at different scan rates, and the broadening of peaks in CV curves reflects the pseudocapacitive behavior, which is conducive to the rapid electrochemical reactions. Figure 3c shows the second cycle CV curves of V2O5 and V2O5-Cu as electrode materials for zinc-ion batteries at a scan rate of 0.1 mV s-1, in the voltage range of 0.2–1.6 V. V2O5-Cu exhibits two major redox peaks around 1.11/0.84 V and 0.81/0.53 V, corresponding to the redox pairs of V5+/V4+ and V4+/V3+, respectively [27]. The redox of the shoulder is irreversible, indicating the occurrence of side reactions. In addition, as shown in the figure, between the positive and negative peaks of the V2O5-Cu, a higher peak current density is observed compared to V2O5, which indicates better redox activity in V2O5-Cu [28–29]. In the V2O5-Cu electrode, the enhancement of charge transfer kinetics and involvement of active sites in electrochemical reactions were attributed to the increased proportion of V4+, leading to improved conductivity. Moreover, the heightened interlayer spacing in V2O5-Cu facilitates increased accommodation of zinc ions during the intercalation process, effectively diminishing the energy barrier in the diffusion mechanism.
Ac impedance measurements of V2O5-Cu and V2O5 samples to investigate the influence of copper ions on the charge transfer process. The results of the EIS test analysis are shown in Fig. 3d. The illustration shows the simplified EIS fitting equivalent circuit, where Rct represents the charge transfer impedance, which mainly reflects the conductivity of electrons and ions; Rs represents the ohmic impedance; CPE1 represents the Warburg impedance, which reflects the diffusion of Zn2+ in the electrode material; and W1 represents the double-layer capacitance. The impedance test of the three-electrode system consists of a semicircle in the high frequency region followed by a diagonal line in the low-frequency region matched with the charge transfer resistance (Rct) and the ion diffusion impedance, respectively. The fitting results of the two samples show that the charge transfer resistances (Rct) of V2O5 and V2O5-Cu are 29.04 Ω and 9.18 Ω, respectively. The V2O5-Cu electrode exhibits a smaller charge transfer resistance, which indicates that electrons and ions transfer faster at the interface between the V2O5-Cu electrode and the electrolyte, mainly due to the addition of copper ions to improve the material' s conductivity and facilitate the transfer of electrons and ions at the liquid-solid interface [30].
The cycling performance of V2O5 and V2O5-Cu electrodes at current densities of 8 A g-1 show in Fig. S4. At 8 A g-1, the maximum discharge specific capacities of the two electrode materials are 125 and 232 mAh g-1 for V2O5 and V2O5-Cu, respectively. After 1000 cycles, the capacity retention rates are 32% and 86% for V2O5 and V2O5-Cu, respectively. Figure 3e shows the rate performance of V2O5-Cu and V2O5. The average discharge specific capacities of V2O5 at the current of 1, 2, 4 and 8 A g− 1 are 234, 219, 200 and 178 mAh g-1, respectively. V2O5-Cu exhibits higher rate performance, with average discharge specific capacities of 271, 247, 214 and 199 mAh g-1 at the current of 1, 2, 4 and 8 A g-1. Figure 3f demonstrates that the V2O5-Cu electrode and V2O5 electrode achieve efficiencies of over 100% in the initial several cycles. This can be attributed to the activation of the AZIBs at the beginning, which can be divided into the two steps listed below. As the charging and discharging progress, the electrolyte enters the interior of V2O5 [31]. Simultaneously, the insertion/extraction of Zn2+ and the phase transition of V2O5 generate additional active sites. It can also be observed that the V2O5-Cu electrode exhibits better cycling capability compared to the V2O5 electrode. The specific capacity of V2O5-Cu remains around 300 mAh g-1 at 4 A g-1. After 500 cycles, the discharge specific capacity is 285 mAh g-1 (95.0% of the highest capacity), and the Coulombic efficiency is close to 100%. However, the specific capacity of V2O5 remains around 180 mAh g-1 at 4 A g-1, and it decreases to 139 mAh g-1 (77.2% of the highest capacity) after 500 cycles at 4 A g-1. The enhanced discharge specific capacity and cycling stability of V2O5-Cu can be attributed to the increased interlayer spacing, improved electronic conductivity, and higher oxygen vacancy concentration.