A Method for Preparing Manganese-Doped V₂O₅ Films with Enhanced Cycling Stability

Figure 1 shows the XRD of pure and Mn-doped V₂O₅ films dried at room temperature for 12 h (Fig. 1a) and heated at 300°C for 30 min (Fig. 1b) separately. V₂O₅ has a typical layer structure, each layer being built up from VO₅ square pyramids sharing edges and corners. The peak centred around 2θ = 8° indicates a predominant amorphous nature similar to those of V₂O₅•nH₂O xerogels,³⁰ suggesting that pure V₂O₅ films and Mn/V₂O₅ films with low molar ratio are hydrous vanadium pentoxide. The (001) diffraction plane is related to the V₂O₅ interlayer structure. For the unheated films shown in Fig. 1a, they are amorphous and the interlayer distance are calculated to be 11.97 Å for Mn-doped V₂O₅ films (Mn/V₂O₅ = 0.94), larger than 11.11 Å for pure V₂O₅ films, which could be due to the doping of Mn ions into V₂O₅ interlayer space.³⁰ For the heated film shown in Fig. 1b, the diffraction peak of (001) plane gradually declines and then disappears with the increasing Mn content. The interlayer distance for different Mn/V₂O₅ molar ratio films (0:1, 0.27:1, 0.4:1 and 0.54:1) is 10.69 Å, 10.98 Å, 10.92 Å, and 10.75 Å, respectively. Compared with films dried at room temperature, the interlayer space shrinks for all heated films, because some water molecule intercalated between V₂O₅ layers is removed during the heating process. As the doping content increases, more interlayer water is squeezed out during the annealing process, causing the gradually decreased interlayer distance of the doped films. The XRD pattern also shows that the peak of (001) plane disappears at high doped Mn concentration, which illustrates that the layered structure of the doped films is disordered. In addition, the orthorhombic V₂O₅ (2θ = 21°) is found to coexist in all of the samples, indicating that the phase transition is not inhibited by the introduction of manganese ions into hydrous vanadium structure. The larger 2 theta window of XRD results are shown in Fig. 1c.

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To investigate the valence change of vanadium in doped films, and get more information about the doping process, all the films are examined by XPS. Figure 2a shows V2p spectra of pure V₂O₅ film and Mn-doped V₂O₅ film at room temperature with high resolution. For pure V₂O₅ films, the V2p_3/2 peak values are 517.4 eV,⁷ which is related to the states V⁵⁺. And one new peak at 516.1 eV (V⁴⁺) appeared near 517 eV (V⁵⁺) for Mn-doped V₂O₅ film and the intensity is similar, which means one new V = O bond appeared. The XPS spectra of Mn2p peaks for Mn-doped V₂O₅ film are also analyzed as shown in Fig. 2b. There are two main peaks which can be assigned to Mn⁴⁺ (642.1 eV) and Mn³⁺ (640.9 eV) with a 12.1 and 11.9 eV separation, respectively.⁴⁰ The doping process is illustrated in

During the doping process, negative voltage is applied, Mn ions and electrons are simultaneously doped into V₂O₅ films causing the low valence state of V (V⁴⁺), and the existence of Mn³⁺ and Mn⁴⁺ proves that the oxidation of Mn²⁺ occurred and that the ions do dope into the V₂O₅ interlayer space instead of depositing on its surface. When V⁵⁺ ions are partially replaced by Mn ions, [MnO₆] octahedral³⁰ may form in the framework of V₂O₅, introducing the three-dimensional character, which makes the structure stronger in dealing with the deformation of the material structure during Li-ion intercalation and de-intercalation cycling.³⁰

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Figure 3 shows the Raman spectra of V₂O₅ films by Mn doping. The Raman modes of V₂O₅ can be classified into external and internal vibrations. Most of the low wavenumber modes can be described in terms of external modes. The external modes can be considered as relative motions of the V₂O₅ units with respect to each other. The internal modes, on the other hand, which are observed in the medium and high wavenumber, can be described in terms of O-V-O and V-O-V bending modes and V-O stretching vibration modes.

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The Raman spectra is collected for different Mn-doped amorphous V₂O₅ films (Fig. 3). For pure V₂O₅ films, the peak at 155 cm⁻¹ describes the translational mode of V₂O₅ units.³⁸ The peaks between 250 and 750 cm⁻¹ correspond to the bending and stretching modes of V-O bond. The Raman spectra recorded for Mn-doped V₂O₅ films exhibits several changes, the wavenumber of the translational mode shifts from 154 to 147 cm⁻¹, and a new peak appears at 173 cm⁻¹. These changes in the translational mode illustrate the relative position changes of V₂O₅ unites, which influence the layer structure of V₂O₅ films. The peaks corresponding to V-O bond between 250 and 750 cm⁻¹ declines with an increasing concentration of Mn. The peak located at 514 cm⁻¹ disappear and new peaks appear between 800 and 1000 cm⁻¹ (805, 843, 870, and 932 cm⁻¹).^15,39 These changes prove the existence of different types of V = O bonds,³⁹ which are generated by the insertion of Mn ions into the V₂O₅ structure.

A scanning electron microscope (SEM) was used to study possible morphological changes for Mn-doped V₂O₅ films. Figure 4 shows the surface morphologies of Mn-doped V₂O₅ films with different Mn concentration. For pure V₂O₅ film, the surface is compact and contains many small particles (∼100 nm) with unclear boundaries (Fig. 4a). These particles are amorphous state confirmed by XRD results and are not crystalline grain. Compared with un-doped V₂O₅ films, there are many more small particles (∼50 nm) of Mn-doped films surface, and the boundaries become clearer and deeper. The situation appears obviously in the case of Mn/V₂O₅ = 0.64 and 0.94 is shown in Figs. 4e and 4f, which may be induced by the shrinkage of V₂O₅ structure with particles size decreasing, and similar results have been reported.³⁰ The cracks on the surface reveal the influence of Mn doping, which changes the microstructure of the films directly and which will allow more flexibility in volume changes because of ions intercalation/deintercalation. The clear boundaries are beneficial to a better cycling performance.

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To understand the oxidation and reduction behavior of the Mn-doped V₂O₅ films, cyclic voltammograms were recorded by a CHI 660D electrochemical workstation. The test was carried out using Pt sheet as the counter electrode and Ag wire as the reference electrode, and the electrolyte used is 0.1 M of LiClO₄ in PC.

Figure 5 demonstrates the reduction and oxidation process with 10 mVs⁻¹ scan speed for pure and Mn-doped V₂O₅ electrodes. Figure 5a reveals that the pure V₂O₅ film has an obvious anodic peak (0.42 V. vs. Ag/Ag⁺) and two cathodic peaks (−0.53 V, −0.97 V). For low doped content (Mn/V₂O₅ = 0.27), the intensity of the redox peaks increases with Mn content. In the doped V₂O₅ films with higher ratios, the intensity of both the cathodic and anodic peaks decrease as the increasing doped content, revealing a decrease in charge capacity. The anodic peak decreases to 0.33 V of the low doped content (Mn/V₂O₅ = 0.27), because of the enlarged interspace (10.98 Å). However, as more Mn is doped into V₂O₅ film, the layered structure is distorted and the interspace starts to decrease from 10.98 Å to 10.75 Å (Fig. 1), which makes the anodic peak increase to 0.67 V (Mn/V₂O₅ = 0.54). For the high doped content (Mn/V₂O₅ = 0.64/0.94), the anodic peak recovers back to 0.46 V because the layered structure is totally distorted (Fig. 1b) and more free space for the Li⁺ is provided. Figure 5b shows consistent results measured with the method of Chronocoulometry under ±1.2 V (vs. Ag/Ag⁺) for 20 s in a conventional three-electrode cell. Compared with pure V₂O₅ films, the charge capacity of the case (Mn/V₂O₅ = 0.27) slightly increases, mostly due to the enlarged interlayer space (Fig. 1b). However, the higher ratio also results in a decline in the charge capacity, which may be caused by the doped Mn occupying the efficient Li⁺ insertion sites and distorting parts of the original structure. The distorted structure will facilitate response time (the charge capacity reached 90% value of its max) that changes from 7.0 s (Mn/V₂O₅ = 0.27) to 5.0 s (Mn/V₂O₅ = 0.94), which is consistent with the result of anodic peak in Fig. 5a.

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To discuss the application of Mn-doped films in electrochromic devices, the optical transmittance spectra as a function of wavelength range from 300 to 1500 nm are shown in Fig. 6. This experiment was carried out under 1.2 V (vs. Ag/Ag⁺). In the visible spectra, it is observed that the maximum transmittance for doped films decreases from 88% to 80% as Mn content increases (Fig. 6a), which is caused by the doping of Mn inserted into the V₂O₅ interlayer structure and formed Mn-O oxide (brown), and parts of vanadium ions (V⁵⁺) are transferred to V⁴⁺ (Fig. 2) leading to a lower transmittance of Mn-doped film. The photos of V₂O₅ film and Mn doped V₂O₅ film are inset in Fig. 6a. Meanwhile, the peak position of maximum transmittance in both reduced and oxidized states express a gradual redshift. The transmittance of doped films is larger than 70% in the visible spectra shown in Fig. 6b, which is high enough that it can be used as a counter electrode in an electrochromic device.

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Finally, the Mn-doped V₂O₅ films were subjected to repeated insertion and release cycles to obtain cycling performance. Figure 7 describes the cycling characteristics of films, which is charged/discharged by applying constant voltage of 1.2 V. During the cycling process, the charge capacity of pure and low Mn-doped ratio (Mn/V₂O₅ = 0.64) V₂O₅ films decreases continually. The charge capacity of the pure films decreased to 38.3% of the initial capacity within 1000 cycles. In contrast to the pure films, the films with the molar ratio Mn/V₂O₅ = 0.64 exhibited palliatory charge fade, the charge capacity decreasing to 79.8% of the initial capacity. The addition of Mn in molar ratios Mn/V₂O₅ = 0.94 dramatically improved the cycling stability of the films, with almost no loss of the charge capacity. Such an excellent electrochemical stability of Mn-doped V₂O₅ films could be attributed to poor crystallinity or an amorphous-like nature and the presence of oxygen vacancies. The poor crystallinity makes it stronger in dealing with the deformation of the material structure during Li-ion intercalation/de-intercalation cycling. In addition, the presence of oxygen vacancies may also alleviate the stress or strain during Li-ion transference. The Mn content distorts the layer structure of V₂O₅ and provides more free space for the Li⁺ intercalation/deintercalation process. The cracks on the surface of films allow more flexibility in volume changes, which also leads to a better cycling performance.

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