Fabrication of nanostructured V₂O₅ via urea combustion for high-performance Li-ion battery cathode

Abstract

Nanostructured vanadium pentoxide (V₂O₅) crusts were facilely synthesized via the combustion of a precursor by mixing commercial V₂O₅ with molten urea. The nanocrusts were transferred to nanorods during further annealing at 630 °C. Both the V₂O₅ nanocrusts and V₂O₅ nanorods were used preliminarily as a cathode material for Li-ion batteries. Their electrode performance was highly improved compared to commercial V₂O₅.

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Vanadium pentoxide (V₂O₅) has attracted considerable attention due to its great potential in a variety of applications, such as electrode materials in solid state batteries and supercapacitors.^1–6 In particular, V₂O₅ is one of the most promising cathode materials for high energy density Li-ion batteries owing to its high theoretical capacity (440 mA h g⁻¹ with three lithium insertions/extractions), which is much higher than those of more widely used cathode materials, such as LiCoO₂ (140 mA h g⁻¹), LiMnO₂ (148 mA h g⁻¹), and LiFePO₄ (170 mA h g⁻¹).^5–8 V₂O₅ also has the additional advantages of being inexpensive and abundant. On the other hand, the slow diffusion of lithium ions (D ∼ 10⁻¹² cm² s⁻¹) and low electronic conductivity (10⁻² to 10⁻³ S cm⁻¹) of commercial V₂O₅ crystals limit their use as high-performance electrode materials for Li-ion batteries in large scale production.^7–10 Recently, many studies indicate that nanostructures of electrode materials can improve the performance due to the dimension reduction of particles size, which reduces the distance of ions diffusion and electron transportation.^9–11

Up to now, various V₂O₅ materials with different nanostructures, such as nanorods, nanofibers, nanowires, and nanobelts have been prepared by chemical methods, such as hydrothermal growth, sol–gel synthesis and flame spray-pyrolyzed.^8–15 For instance, nanorod structured V₂O₅ synthesized by the thermal decomposition of vanadium precursors, exhibiting a good discharge capacity and cycle stability as a cathode for Li batteries.⁹ In addition, nanorods, nanowires and nanobelts have also been obtained by thermal evaporation, sputtering deposition and recrystallization, even though the products of these methods mainly are thin layers on substrates.^16,17 Glushenkov et al. fabricated V₂O₅ nanorods via long time ball milling and annealing, giving a stable electrochemical performance as a cathode for Li batteries.¹⁶ The main focus has been on the development of a convenient method that can be applied to produce V₂O₅ nanomaterials in large scale production. Therefore, there is a great need for a novel cost-effective method producing V₂O₅ nanomaterials with high performance for Li-ion battery.

In this study, we report a novel and convenient approach for producing nanostructured V₂O₅ crusts (nanocrusts), which is feasible for mass production. The V₂O₅ nanocrusts composed of nanoparticles were fabricated via the direct combustion of a black precursor at 450 °C in air for an hour. A black precursor was obtained by commercial V₂O₅ reacting with molten urea at a starting mass ratio of 1 : 4 at 140 °C for 30 min. Herein, urea is not only the combustion agent but also the reagent for the reaction of V₂O₅ in preprocessing.^18,19 V₂O₅ nanorods were obtained by a recrystallization process during further annealing at 630 °C. The electrochemical performance of the as-prepared V₂O₅ electrode materials for Li-ion batteries was demonstrated, exhibiting high-rate charge–discharge capacity and good cycle stability.

An image of the black precursor via the reaction of commercial V₂O₅ and molten urea is shown in Fig. S1a.† The colour of the melt mass changed gradually from yellow (commercial V₂O₅) to black, indicating that the V₂O₅ had been reacted with molten urea. The FT-IR spectrum of the black precursor is shown in Fig. S2.† The bands at about 3461, 3340, 2226, 1682, 1605, 1466, 1338, 1156, 981, and 574 cm⁻¹ were indexed to the black precursor. The bands at 1682 and 1466 cm⁻¹ were ascribed to stretching vibration of C=O and C–N, while those bands at 3461, 3340 and 1605 cm⁻¹ can be assigned to asymmetric, symmetric stretching and deformation vibration of N–H bonds, respectively. The band at 1156 cm⁻¹ can be attributed to the rocking vibration of NH₂. These can be attributed to the presence of unreacted urea in the black precursor. The absorption at 981 cm⁻¹ should be associated with the V=O band, which is related to vanadic complex after the reaction of V₂O₅ and molten urea.^20,21 All the three major absorption peaks of V₂O₅ at 617, 827, and 1022 cm⁻¹ were invisible in the spectrum (Fig. S1a†).²² Moreover, the bands at 2226 and 1338 cm⁻¹ can be ascribed to the absorptions of –N=C=O. These indicate that V₂O₅ has completely reacted with urea.^20–22

The thermal decomposition behaviour of the precursor was investigated by thermogravimetric (TG) analysis. Fig. S3† shows the TG results for the black precursor calcined in air. Two distinct stages were observed. The sharp weight loss around 200 °C in the first stage can be attributed to the condensation and deamination of urea in the mixed precursor.¹⁸ A steep slope indicating another weight loss at 405 °C was then observed, which demonstrated the successful thermal-decomposition of vanadate to vanadium oxide. The weight of the final product corresponds to the V₂O₅ in the initial mass ratio.¹⁹

The micro morphologies and structures of commercial V₂O₅ and the V₂O₅ nanocrusts are shown in Fig. 1a and b. As shown in Fig. 1a, the commercial V₂O₅ is micrometer-sized particles with irregular shapes. The yellow bread-like product with a foam structure (see Fig. S1b and c†) is obtained by calcinating the black precursor in air at 450 °C for 1 h. Fig. S1c† shows that the foam structure consists of nanostructured crusts (nanocrusts) during combustion. Fig. 1b indicates that the V₂O₅ nanocrusts are composed of nanoparticles. Furthermore, elongated nanorods have been formed by further annealing of the nanocrusts in air at 630 °C (Fig. 1d). The nanocrusts can also be crushed into individual nano-sized particles by grinding. The morphology and structure of the nanoparticles obtained by simply grinding the nanocrusts are shown in Fig. 1c and e; the size of the particles ranged from 50 nm to 300 nm. Fig. 1f shows a high resolution TEM image of a part of the nano-sized V₂O₅ particles, revealing a layer structure of V₂O₅. The lattice fringes with a spacing of 0.58 nm should correspond to the (200) planes of orthorhombic V₂O₅.²³

Fig. 1 Morphologies and structural information of V₂O₅ particles: (a) SEM image of commercial V₂O₅; (b) SEM image of the sample synthesized by thermal decomposition of the black precursor (the starting mass ratio of V₂O₅ to urea is 1 : 4) at 450 °C for 1 h; (c) SEM image of the sample after subsequent annealing in air at 630 °C for half an hour; (d) SEM image of the sample has been ground into individual particles; (e) TEM image of the sample has been ground into individual particles, and (f) high-resolution transmission electron microscopy (HRTEM) image of individual particles.

The phase and purity of the as-prepared nanocrusts and nanorods were determined by powder X-ray diffraction (XRD). The diffraction patterns are displayed in Fig. 2. Both of them can be indexed well to a monocline structure V₂O₅ space group with the space group: P2₁/c, a = 11.512 Å, b = 3.564 Å, c = 4.368 Å (JCPDS card no. 65-0131), showing the high phase purity of the nanocrusts and the nanorods. The peaks of the V₂O₅ nanocrusts were weaker and wider than those of the V₂O₅ nanorods, particularly the relative intensity of (001) peak, which suggests that the nanorods have better crystallinity and a larger particle size. The crystallite dimensions of nanocrusts and nanorods can be calculated from the (001) peak via the Scherrer equation. The V₂O₅ nanocrusts (36.7 nm) is much smaller than the V₂O₅ nanorods (78.5 nm).^9,16

Fig. 2 Powder X-ray diffraction (XRD) patterns of the nanocrusts synthesized via thermal decomposition of black precursor (the starting mass ratio of V₂O₅ to urea is 1 : 4) at 450 °C for an hour and the nanorods obtained by annealing the nanocrusts in air after subsequent at 630 °C for 30 min. The vertical lines indicate the peak positions expected for orthorhombic V₂O₅ from JCPDS card no. 065-0131.

The morphologies of the products obtained from different urea contents in the starting reagents are shown in Fig. 3. Fig. 3a and b show SEM images of the samples produced by thermal decomposition at 450 °C for 1 h with a mass ratio of V₂O₅ to urea of 1 : 5 and 1 : 4, respectively. The morphologies of the samples are nanocrusts assembled from nanoparticles. However, the nanostructured crusts cannot be obtained in the 1 : 3 V₂O₅ products (mass ratio of V₂O₅ to urea is 1 : 3). Instead, micro-aggregates were observed (Fig. 3c). The V₂O₅ bread could be formed eventually with excess urea decomposition during the calcination process. The nanocrusts are the bubble walls in V₂O₅ bread. Therefore, the thinner crust would be achieved in a higher urea mass ratio, because the formation of more gas from urea decomposition leads to larger bubbles and thinner walls in V₂O₅ bread.

Fig. 3 SEM images of the V₂O₅ nanocrusts synthesized by thermal decomposition of the black precursor with different starting molar ratios of V₂O₅ to urea: (a) 1 : 5; (b) 1 : 4; (c) 1 : 3.

The shape transformations of the V₂O₅ nanocrusts by further annealing progressing at 630 °C are shown in Fig. 4. The SEM image in Fig. 4a shows that the surface of the nanocrusts consists of nanoparticles with irregular shapes. The SEM images in Fig. 4b–d reveal the morphological changes in the nanocrusts after 5, 10 and 30 min of the annealing treatment. The shapes of the powders are transformed almost completely from nanocrusts to nanorods after 30 min annealing. These nanorods look like ice cream sticks, which have a round end and rectangular cross section. The width of the rods grown after 5 min was in the range of 300–800 nm. The thickness was between 100 and 300 nm. The length was up to several micrometers. According to Glushenkov et al.,¹⁶ the formation and growth of nanorods can be attributed to the surface energy driven recrystallization process. The morphology of the nanorods is a particular type of facet that is dominated by notable low energy {001} surfaces (by calculation).²⁴ As shown in Fig. 4b and c, new layers (shown with arrows) were formed on the {001} surfaces in the nanorod growth process.¹⁶ In addition, the nanocrusts provide enough space for particle growth to elongated nanorods, preventing the formation of larger crystals due to the coalescence on adjacent nanorods.

Fig. 4 Transformation of the nanocrusts into nanorods: (a) SEM image of the surface of the V₂O₅ nanocrusts synthesized by thermal decomposition of the black precursor (the starting mass ratio of V₂O₅ to urea is 1 : 4) at 450 °C for 1 h; V₂O₅ nanocrusts after subsequent annealing in air at 630 °C for 5, 10 and 30 min ((b), (c) and (d) respectively). Formation of new layers on existing {001} surfaces are shown with the arrows in (b) and (c).

This method involving a reaction between commercial V₂O₅ and molten urea and annealing in a muffle furnace is easy to produce real mass quantities of materials with a nanostructure. The products can be broken to nanocrusts, nanoparticles or nanorods by ball milling. If required, materials with various particle sizes can be separated by sieving. Both the roasting technology and mechanical milling can be easily capable of scaling up.

Considering the Li-ion storage performance of commercial V₂O₅, we investigated the applications in a Li ion battery (LIB) as a proof-of-concept demonstration of their potential use.

Galvanostatic charge–discharge measurements were performed on the V₂O₅ nanocrusts and the V₂O₅ nanorods at a current density of 30 mA g⁻¹ between 2.0 and 4.0 V. The initial five charge–discharge profiles for these measurements are illustrated in Fig. 5a and b. Both of the nanocrusts and nanorods showed three characteristic plateaus at the voltages of about 3.4, 3.2 and 2.3 V (vs. Li/Li⁺) in those discharge curves. These plateaus correspond to the phase transformation of crystalline α-V₂O₅ to α-Li_xV₂O₅ (x < 0.01), ε-Li_xV₂O₅ (0.35 < x < 0.7), δ-Li_xV₂O₅ (0.7 < x < 1), and γ-Li_xV₂O₅ (1 < x < 1.9), respectively.^2,25 Cyclic voltammetry (CV) of the nanocrusts (Fig. S6a†) between 1.5 and 4 V was measured at a scan rate of 1 mV s⁻¹. The pairs of redox peaks can be resolved clearly in the voltage window of 1.5–4.0 V, which is consistent with the plateaus of the discharge voltage/capacity profile. In previous studies, the electrochemical reaction between vanadium oxides and metal lithium can be described as follows:^2,25–28

V₂O₅ + 0.5Li⁺ + 0.5e⁻ ⇌ Li_0.5V₂O₅ (1)

Li_0.5V₂O₅ + 0.5Li⁺ + 0.5e⁻ ⇌ LiV₂O₅ (2)

LiV₂O₅ + Li⁺ + e⁻ ⇌ Li₂V₂O₅ (3)

Fig. 5 Electrochemical testing of the V₂O₅ materials as cathodes in a LiPF₆ electrolyte: The 1^st to 5^th charge–discharge profiles of (a) the V₂O₅ nanocrusts synthesized via thermal decomposition of black precursor (the starting mass ratio of V₂O₅ to urea is 1 : 4) at 450 °C for 1 h and (b) the V₂O₅ nanorods obtained by the V₂O₅ nanocrusts after subsequent annealing in air at 630 °C for half an hour in the voltage range of 2.0–4.0 V at a rate of 30 mA g⁻¹; (c) cycling performance of commercial V₂O₅, the V₂O₅ nanocrusts and the V₂O₅ nanorods in the voltage range of 2.0–4.0 V at a rate of 300 mA g⁻¹; (d) rate performance of the V₂O₅ nanocrusts and the V₂O₅ nanorods in the voltage range of 2.0–4.0 V.

The first discharge capacity of the V₂O₅ nanocrusts is more than 294 mA h⁻¹, which is more than the theoretical capacity for 2Li⁺ intercalation. The large capacity can be attributed to the nanocrusts, which have a large interfacial area with the electrolyte and defects in the V₂O₅ materials formed during thermal decomposition of the precursor. These can lead to more than two Li⁺ insertions in the voltage window of 2.0–4.0 V. As shown in Fig. S7,† the discharge capacity of the V₂O₅ nanocrusts and the V₂O₅ nanorods maintain a stable capacity around 220 and 180 mA h g⁻¹, respectively. Compared to that of the commercial V₂O₅ (the discharge capacity is ∼70 mA h g⁻¹), the discharge capacity of the V₂O₅ nanocrusts and the V₂O₅ nanorods are much higher.

Fig. 5c shows the specific capacity of the first 50 cycles at a current density of 300 mA g⁻¹ between 2.0 and 4.0 V (versus Li⁺/Li) for commercial V₂O₅, the V₂O₅ nanocrusts and the V₂O₅ nanorods, respectively. The V₂O₅ nanocrusts exhibit an initial capacity of 353 mA h g⁻¹ and 210 mA h g⁻¹ during the 1^st and 30^th cycle. The discharge capacity of the V₂O₅ nanorods only deliver about 160 mA h g⁻¹ after 30 cycles, which can be attributed to the growth of V₂O₅ crystals increasing the Li⁺ diffusion path length.²⁹ Although there is some loss of the capacity for the V₂O₅ nanocrusts and V₂O₅ nanorods, the capacity of V₂O₅ nanocrusts and V₂O₅ nanorods is 2–3 times of the capacity of commercial V₂O₅ after 50 cycles. The capacity loss could be attributed to the phase of a crystalline V₂O₅ irreversible transition during insertion/extraction of Li⁺ ions. These cyclic performances are better than the nano-sized V₂O₅ synthesized by a thermal treatment or flame spray pyrolysis.^13,30 The results of the V₂O₅ nanocrusts are also better than that of vanadium oxide nanobelts prepared by a hydrothermal method.^12,20,31,32 Compared to other transition metal oxides, such as MnO₂ and Co₃O₄, the discharge capacity of the as-obtained nanostructured V₂O₅ is still good as a cathode material owing to its high theoretical capacity.^1,33

The rate capability of the electrode materials for LIB is also an important factor for practical applications. To investigate the rate capability of the V₂O₅ nanocrusts and the V₂O₅ nanorods, galvanostatic cycling was carried out at various current densities. Fig. 5d shows the rate performance of the V₂O₅ nanocrusts and the V₂O₅ nanorods over the voltage range of 2.0–4.0 V (versus Li⁺/Li) at different current densities in the range of 30–3000 mA g⁻¹. The discharges of both samples decreased with increasing current density. Similar to the cycle performance, the nanocrusts V₂O₅ showed a relatively enhanced rate capability compared to the nanorods V₂O₅. An initial discharge capacity of 410 mA h g⁻¹ is attained at a low current density of 30 mA g⁻¹ for the nanocrusts V₂O₅, which stabilizes at above 300 mA h g⁻¹ after 6 cycles. The discharge capacities of the nanocrusts V₂O₅ are about 257, 237, 185, and 85 mA h g⁻¹, at 300, 600, 1000, and 3000 mA g⁻¹, respectively. Correspondingly, the nanorods V₂O₅ have values of 330, 178, 124, 89, and 21 mA h g⁻¹ at different rates of 30, 300, 600, 1000, and 3000 mA g⁻¹, respectively. Owing to the inherent low electronic conductivity of V₂O₅, the discharge capacities of both samples are greatly reduced at higher current densities.² Furthermore, the discharge capacities of both samples recovered to 250 and 205 mA h g⁻¹, upon returning to a current of 30 mA g⁻¹. This shows that both of the structural stability and reversibility for the two samples are quite good. The rate performance of the V₂O₅ nanocrusts is comparable to the previously reported carbon-coated vanadium oxide.^30,34 These results also indicate that the electrochemical property of the V₂O₅ nanocrusts and the V₂O₅ nanorods are close to the results of nano-structured vanadium oxide reported previously.^13,23–29 On the other hand, this method is more facile and capable of scaling up for broad application prospects.

Conclusions

A potential method for the fabrication of the V₂O₅ nanocrusts has been proposed via the combustion of precursor from mixing commercial V₂O₅ with molten urea. The V₂O₅ nanorods were further prepared through a controllable nanoscale growth originating from the nanocrusts during further annealing at 630 °C. The minimization of surface energy of V₂O₅ grain probably drives the growth of the nanorods dominated by the {001} surface. The discharge capacity of the as-obtained the V₂O₅ nanocrusts and the V₂O₅ nanorods are increased to 210 and 170 mA h g⁻¹, respectively, from 70 mA h g⁻¹ after 30 cycles compared to commercial V₂O₅.

Acknowledgements

This work was supported by the National High-tech R & D Program (863 Program, no. 2008AA03Z306), the National Natural Science Foundation of China (51003009, 21276046 and 21476045), the Ministry of Education Science and technology research projects, the Fundamental Research Funds for the Central Universities of China (DUT11LK16), (DUT13RC(3)043 and DUT14RC(3)040). We also thank Professor Zhang Xinbo (Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun) for electrochemical measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Supplemental data of Fourier transform infrared spectrum, thermal gravimetric analysis, electron microscopy images and cycle discharge performance. See DOI: 10.1039/c4ra11015c

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