Bio-Template Synthesis of V2O3@Carbonized Dictyophora Composites for Advanced Aqueous Zinc-Ion Batteries

In terms of new-generation energy-storing devices, aqueous zinc-ion batteries (AZIBs) are becoming the prime candidates because of their inexpensive nature, inherent safety, environmental benignity and abundant resources. Nevertheless, due to a restrained selection of cathodes, AZIBs often perform unsatisfactorily under long-life cycling and high-rate conditions. Consequently, we propose a facile evaporation-induced self-assembly technique for preparing V2O3@carbonized dictyophora (V2O3@CD) composites, utilizing economical and easily available biomass dictyophora as carbon sources and NH4VO3 as metal sources. When assembled in AZIBs, the V2O3@CD exhibits a high initial discharge capacity of 281.9 mAh g−1 at 50 mA g−1. The discharge capacity is still up to 151.9 mAh g−1 after 1000 cycles at 1 A g−1, showing excellent long-cycle durability. The extraordinary high electrochemical effectiveness of V2O3@CD could be mainly attributed to the formation of porous carbonized dictyophora frame. The formed porous carbon skeleton can ensure efficient electron transport and prevent V2O3 from losing electrical contact due to volume changes caused by Zn2+ intercalation/deintercalation. The strategy of metal-oxide-filled carbonized biomass material may provide insights into developing high-performance AZIBs and other potential energy storage devices, with a wide application range.


Introduction
Lithium-ion batteries (LIBs), with the advantages of mature preparation technology and high energy density, are viewed as the most alluring candidates for advanced portable and automotive electrical energy storage systems [1][2][3][4]. Nonetheless, the deficiency in lithium assets and unpredictable safety problems of flammable electrolytes have genuinely hindered the further development and wide application of LIBs [5,6]. Consequently, it is imperative to find alternatives to LIBs. Recently, aqueous metal-ion (Zn, Na, K, Mg, Ca, etc.) batteries have exhibited extraordinary prospects for applications in energy storage, owing to their fabulous quality of security [7]. Among them, rechargeable aqueous zincion batteries (AZIBs) are considered to be one of the foremost promising energy storage devices on account of their plenteous sources, nontoxicity and high theoretical capacity (820 mAh g −1 ) [8][9][10][11].
Within the past few decades, various types of host materials, including MnO 2 , Prussian blue analogues, organic compounds and V-based materials, have been considered narrow peaks of all products can be assigned to rhombohedral V 2 O 3 (space group: R-3c, ICSD-01-071-0342) with the lattice parameters of a = 4.95 Å, b = 4.95 Å, c = 14.00 Å, α = 90 • , β = 90 • and γ = 120 • [37,38]. It is worth mentioning that a low-intensity amorphous bump centered at approximately 24.5 • , which can be attributed to the (002) plane of amorphous C, and the intensity of this amorphous C peak, debilitates with the diminishing of the C atom substance [27]. No other noticeable impurity phases were detected in the pattern, suggesting that V 2 O 3 was successfully synthesized [39]. Due to the low content of V, the diffraction peaks of the VOCD-1 sample are the weakest among all the as-prepared composites. With a higher content of V, the VOCD-3 sample shows stronger diffraction peaks compared with the other two V 2 O 3 @CD composites, indicating better crystallinity, which is beneficial to the ion diffusion between the layers of the electrode. The FT-IR spectra of V 2 O 3 @CD composites are illustrated in Figure 1b. The bending vibration of O-H and the stretching vibration of H-O-H can be observed at 1624 and 3405 cm −1 , respectively, which is mainly because some water molecules are embedded between the layers and adsorbed on the surface of the composites [40]. The peaks at 805 and 533 cm −1 can be indexed to the symmetric and asymmetric stretching vibration of V-O-V [41]. Notably, the peak centered at 984 cm −1 is associated with the stretching vibration of the V 3+ =O, demonstrating the appearance of V 2 O 3 [42]. The peaks at 2855 and 2932 cm −1 are related to the stretching vibration of C-H bonds [43], while the peak located at 1421 cm −1 is characteristic of bending vibration of a C-H bond [44]. The asymmetrical stretching vibration of C-O is around 2356 cm −1 , aiming at CO 2 adsorption on KBr, which is negligible [45]. The above FT-IR spectra demonstrate that the as-obtained composites are composed of V 2 O 3 and amorphous carbon, which is in agreement with the XRD results.
XPS spectrum was employed to investigate the elemental valence and surface composition of V 2 O 3 /CD composites. As illustrated in Figure 1c-f, all the signal peaks can be assigned to C 1s, O 1s and V 2p in the survey spectra. The deconvoluted C 1s spectra of V 2 O 3 /CD composites revealed several components at 287.9, 286.3 and 284.8 eV, which corresponded to O-C=O − , C-O and C-C bond, respectively [46][47][48]. The O 1s spectrum of V 2 O 3 /CD presented three contributions, with binding energies of 533.2, 532.0 and 531.3 eV, which were associated with O=C-O − , C-OH and V-O, respectively [34,49]. In V 2p corelevel spectrum, the two peaks located at 524.6 and 517.5 eV were caused by the spin-orbit splitting of V 2p 1/2 and V 2p 3/2 , respectively, which is characteristic of vanadium in the +3 oxidation state [14,50]. Additionally, the energy difference between the binding energy of the O 1s and V 2p 3/2 level (∆E = E O 1s − E V 2p 3/2 ) could be employed to determine the oxidation states of the vanadium oxides. The ∆E value of as-obtained V 2 O 3 @CD in the present work is 14 eV, which is consistent with the values of V 3+ compounds reported in the literature, thus confirming the vanadium valence in +3 oxidation states [44]. Therefore, the XPS spectrum further illustrates the formation of V 2 O 3 in the composites.
TGA analyses were conducted to determine the mass content of each component in the V 2 O 3 @CD composites. Figure 1g-i reveal the TGA curves of three V 2 O 3 @CD composites. Obviously, three weight-loss stages during the decomposition of the composites are presented in the TGA curves. The first and second stages of weight loss appear at about 130 • C and 400 • C, which are correlated with the release of adsorptive and structural water, respectively. The third stage of weight loss occurs at 380-570 • C, which is mainly associated with the combustion of carbon. The weight loss of VOCD-1, VOCD-2 and VOCD-3 in the third stage is 18.94%, 20.14% and 11.11%, respectively. Based on the results of TGA analysis, the mass contents of biomass carbon and vanadium oxide in VOCD-1, VOCD-2 and VOCD-3 are 18.94% and 69.71%, 20.14% and 73.37%, and 11.11% and 75.86%, respectively.
The morphology and structure of as-obtained V 2 O 3 @CD composites were investigated using SEM, TEM and EDS, and the results are presented in Figure 2. It can be discerned that the VOCD-1 composites show an obvious petal shape (Figure 2a,b), which may be due to the low content of V 2 O 3 in the VOCD-1 composites. As displayed in Figure 3c,d, the structure of V 2 O 3 in the VOCD-2 composites is mainly spherical with a diameter of 0.5-1.3 µm and a small number of sheets. The VOCD-3 composites display a unique porous architecture, and the diameter of pores is 6-12 µm (Figure 2e), indicating that the incorporation of V 2 O 3 has little effect on the formation of the natural porous network of CD. As illustrated in Figure 2f,g, the structure of V 2 O 3 in the VOCD-3 composites is spherical, with almost no sheets, and these microspheres are evenly filled in the macropores of CD. It is noteworthy that the surfaces of the V 2 O 3 microspheres in the VOCD-2 and VOCD-3 composites are very coarse, indicating that both have a well-developed pore structure. The diffraction rings obtained from selected-area electron diffraction (SEAD) analysis (inset of Figure 2h

Discussion
To explore the electrochemical behavior of V 2 O 3 @CD electrodes, EIS tests were carried out and corresponding Nyquist plots and the equivalent circuit are gathered in Figure 3a. The curves of V 2 O 3 @CD electrodes consist of one sloped line at low frequencies, associated with Warburg impedance and one semi-circle at a high-to-medium frequency region, which could be regarded as the charge transfer resistance (R ct ) [52]. The VOCD-3 electrode exhibits the lowest R ct of 187.7 Ω compared to VOCD-1 (307.9 Ω) and VOCD-2 (220.4 Ω), demonstrating that the VOCD-3 electrode has the best electronic conductivity. Although V 2 O 3 has higher resistance and ion resistance than CD, the R ct value is also related to the pore structure and the bonding strength between CD [53]. Therefore, the R ct of VOCD-3 is the lowest among the composites. The CV curves were administered to evaluate the kinetics of the electrochemical process of V 2 O 3 @CD electrodes, and the initial three cycles are illustrated in Figure 3b-d. The CV curves exhibit similar shapes, with two coupled redox peaks located at approximately 0.62/0.81 V and 0.91/1.11 V, manifesting that a multi-step reaction associated with Zn 2+ ion (de)insertion process occurs in the V 2 O 3 @CD electrode [9,54]. In addition, VOCD-3 presents the widest redox peaks in the CV curve among the V 2 O 3 @CD composites, which is conducive to the kinetics of Zn 2+ (de)insertion, thus exhibiting the highest specific capacity [55]. It is worth noting that with an increase in cycle times, the area in the CV curve also increases, illustrating an increase in the capacity of V 2 O 3 @CD electrodes at the early stage of the electrochemical cycle process (see Figure 3g).
The galvanostatic charge/discharge (GCD) profiles of V 2 O 3 @CD electrodes cycled at 50 mA g −1 between 0.2 and 1.6 V versus Zn 2+ /Zn are displayed in Figure 3e. Multiple redox pairs of voltage plateaus at 0.9/1.1 and 0.6/0.8 V can be observed on each charge and discharge curve, which can be consistent with the CV analysis. Remarkably, VOCD-3 can obtain a distinguished reversible capacity of 281.94 mAh g −1 , which is higher than VOCD-1 of 264.47 mAh g −1 and VOCD-2 of 281.79 mAh g −1 at the initial cycle. The rate capabilities at different current densities, ranging from 0.05 C to 3 C for the V 2 O 3 @CD electrodes, are depicted in Figure 3f. In the three groups of rate capability, the specific capacity continues to decrease, regardless of the increase in current densities. VOCD-3 reveals significantly higher discharge capacities of 276.5, 231.60, 210.61, 193.60, 171.76, 155.81 and 151.38 mAh g −1 at 0.05, 0.1, 0.2, 0.5, 1, 2 and 3 C, respectively, compared to VOCD-1 and VOCD-2. When the current density is withdrawn to 0.05 C, VOCD-2 and VOCD-3 electrodes can recover to a high capacity, maintained at 257.33 and 270.24 mAh g −1 , corresponding to 98.82% and 97.74% of their initial capacity, respectively, while VOCD-1 can retain only 69.41% of its initial capacity (184.86 mAh g −1 ) (Figure 3f,g). The above results demonstrate that the VOCD-3 electrode possesses the best rate performance compared with VOCD-1 and VOCD-2, which can be mainly attributed to the unique porous architectures of CD and high content of V 2 O 3 . Figure 3h presents the long-cycle performance of V 2 O 3 @CD electrodes at 1 A g −1 . The capacity of the three V 2 O 3 @CD composites increases gradually in the initial cycles, which is probably correlated with a gradual electrochemical activation process widely existing in vanadium-based electrodes of AZIBs [6,13,46]. Remarkably, VOCD-3 exhibits the highest capacity, up to 170.19 mAh g −1 after 693 cycles, among the three V 2 O 3 @CD composites. After 1000 cycles, the capacity of VOCD-3 slightly decreases to 151.89 mAh g −1 , possessing capacity retention of 89.2%. In contrast, VOCD-1 and VOCD-2 show much lower capacities of 46.27 and 104.12 mAh g −1 , with inferior capacity retentions of 38.5% and 77.2%, respectively, mainly due to their higher content of CD, resulting in lower reversible capacity. It has been reported that low levels of V 2 O 3 may not take advantage of the benefits of V 2 O 3 , while high levels of V 2 O 3 may lead to low electrical conductivity and poor CD protection [34]. The excellent electrochemical performance of VOCD-3 can be ascribed to the high content of V 2 O 3 active and unique porous architectures in the CD matrix. The high content of V 2 O 3 is beneficial to achieving a high reversible capacity. The porous architecture of CD not only effectively promotes the entrance of the electrolyte but can also perform as a buffering effect to assuage the huge volume change of V 2 O 3 during the repeated (de)intercalation of Zn 2+ from/into V 2 O 3 process [15,34,56].
For a comparison, the electrochemical properties of V-based materials utilized as AZIB cathodes are collected in Table 1. The result shows that the V 2 O 3 /CD composites proposed in this work exhibit advantages. The excellent properties of V 2 O 3 /CD composites can probably be explained by the suitable ratio of V 2 O 3 and CD to maximize their individual benefits. To verify the structure stability, the morphology of the VOCD-3 electrode at diverse stages of pristine and after 300, 600 and 900 cycles were recorded by SEM, respectively, as shown in Figure 4a-d. In the initial state, nanoparticles are homogeneously distributed on the current collector without experiencing an agglomeration phenomenon in Figure 4a. After 300 cycles, the morphology of the VOCD-3 electrode remains almost the same as its original state, indicating good structural stability in the cycling process. After 900 cycles, apparent cracks emerged on the VOCD-3 electrode surface (Figure 4d), revealing that VOCD-3 is pulverizing during repetitive charge/discharge, thus resulting in capacity decline. From the element distribution mapping, in addition to the four elements of V, O, S and C, Zn is also detected from the electrode material, indicating that Zn 2+ is successfully embedded in VOCD-3. In particular, no obvious dendrites are observed in the VOCD-3 electrode at the 200, 500 and 900 cycles (see Figure 4a-d). Combined with the favorable properties of Zn 2+ storage, the protection mechanism of the porous structure can be intimated. Therefore, the as-obtained VOCD-3 composites employed as electrodes are conducive to improving the durability of the AZIBs.

Materials Synthesis
Urea (CH 4 N 2 O, ≥99.0%, AR), ethylene glycol (C 2 H 6 O 2 , AR) and ammonium metavanadate (NH 4 VO 3 , ≥99.0%, AR) were directly utilized without any purification. The V 2 O 3 @C composites were synthesized via evaporation-induced self-assembly technique. The detailed preparation process is schematically depicted in Figure 5. Firstly, 3 g CH 4 N 2 O and 2.2 g NH 4 VO 3 were dissolved in a beaker with 20 mL of distilled water. After continuous magnetic stirring at 60 • C for 0.5 h, 20 mL C 2 H 6 O 2 and 0.4 g dried dictyophora were added into the above-mentioned beaker and stirred for 0.5 h to form homogeneous liquid. Then, the obtained solution was heated completely in an oven at 100 • C for 10 h. The resultant powder sample was sintered first at 350 • C for 4 h in an argon-filled tube furnace to obtain VO 2 and carbonized dictyophora. Then, the obtained products were calcined at 800 • C for 8 h, with a heating rate of 10 • C min −1 . The chemical reactions occurring in this process and the reaction principle can be described by Gibbs free energy (G • ) as a function of temperature (T), as presented in Equation (1).
According to the thermodynamic data, T = (t + 273.5) K = 1073.5 K, ∆G • = 95300 − 158.68 T < 0, indicating that the reaction is positive. In this reaction, VO 2 can be deoxidized to V 2 O 3 using carbon as the reducing agent. The C acts as a reducing agent to reduce the high-valent VO 2 to low-valent V 2 O 3 .
The final synthesized sample was named VOCD-1. For comparison, VOCD-2 and VOCD-3 were also prepared in the same way by changing the mass ratio of dictyophora to NH 4 VO 3 . The dosage of raw materials for each sample is summarized in Table 2.

Materials Characterization
The crystal structure of the synthesized samples was characterized by a powder Bruker D8 Advance X-ray diffractometer (XRD, Rigaku Corporation, Japan)) equipped with Cu Kα radiation (λ = 0.152 nm). The FT-IR spectra with a wavenumber range of 400-4000 cm −1 were recorded on an FT-IR spectrometer (Vertex-70, Bruker, Heidelberg, Germany). X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd., Manchester, UK) analysis was conducted on a Thermo Escalab 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) employing a monochromatic Al-Kα X-ray source (Kα = 1486.6 eV). The measured binding energies were calibrated by using the containment carbon (C1s = 284.6 eV) as a reference. All XPS spectra analysis and curve fitting were carried out using PeakFit v4.12 software. The surface morphology and microstructure were determined by transmission electron microscopy equipped with Energy dispersive spectra (TEM-EDS, Tecnai F20, FEI Inc., Valley City, ND, USA) and scanning electron microscopic (SEM, Quanta 200S, FEI, Inc., Valley City, ND, USA) techniques. Thermogravimetric (TG) analyzer (TG, Netzsch, STA409C, Frankfurt, Germany) was applied to evaluate the thermal stability of the samples under nitrogen atmosphere.

Electrochemical Measurements
The cathode was composed of active materials, activated carbon and polyvinylidene fluoride (PVDF), with a mass ratio of 7:2:1, through mixing N-methyl-2-pyrrolidone (NMP) solvent to form slurry. After that, the slurry was pasted uniformly on a 316 L stainless-steel foil, subsequently heated at 100 • C overnight in oven and cut into Φ14 mm discs. The ZnSO 4 solution (3 mol·L −1 ) was applied as electrolyte, and zinc foil and glass-fiber film were applied as counter electrode and separator, respectively. The CR2025 coin batteries were assembled under normal air atmosphere conditions. The batteries were galvanostatically measured with various charge and discharge rates through an NEWARE testing instrument. The CHI660E electrochemical workstation is utilized to test the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Conclusions
In summary, a facile and cost-effective V 2 O 3 @CD composite as a cathode for AZIBs was successfully fabricated via an evaporation-induced self-assembly process. The resultant composite reveals a unique structure in which V 2 O 3 is homogeneously embedded in the macropores of bio-carbon with porous architectures. When V 2 O 3 @CD composites are employed as AZIB cathodes, the carbon skeleton can effectively enhance the conductivity and restrain the large volume change, guaranteeing the structural stability of the composites. As a consequence, the VOCD-3 composites exhibit the lowest R ct of 187.7 Ω as well as a superior rate capability of 151 mAh g −1 at 3 A g −1 among the as-prepared V 2 O 3 @CD composites. In addition, the maximum capacity of 270 mAh g −1 at 1 A g −1 after 693 cycles is obtained in the VOCD-3 composites. It is believed that this study on a V 2 O 3 /biomassderived carbon electrode could provide a new approach for the development of highperformance AZIBs using low-cost biomass as raw materials.