Simple synthesis and electrochemical performance of V₆O₁₃ cathode materials as lithium-ion batteries

Abstract

V₆O₁₃ is synthesized by hydrothermal and solvothermal, respectively. The methods are compared to seek a simpler method for V₆O₁₃ synthesis. The results show that the best reaction time of hydrothermal-V₆O₁₃ is 3.5 h and the phase is pure. Compare with the solvothermal, the hydrothermal-V₆O₁₃ has smaller structure unit of about 100–200 nm, and there are also many pores, which is conducive to the transportation and storage of lithium ions. The results of charge-discharge test show that the electrochemical performance of hydrothermal-V₆O₁₃ is better than solvothermal-V₆O₁₃. The initial discharge capacity of hydrothermal-V₆O₁₃ is 319.2 mAh/g, and the retention rate is 50.5% after 100 cycles, which are 39.9 mAh/g and 12.6% higher than solvothermal-V₆O₁₃, respectively. The results of CV and EIS also confirm that hydrothermal-V₆O₁₃ has better electrochemical performance as lithium-ion batteries.

Introduction

With the development of society, the demand of human for energy is increasing. Developing new energy for sustainable development has become an inevitable trend. Energy storage device become a kind of device which can use energy effectively, and lithium-ion batteries are widely used as an energy storage device [1,2,3]. The cathode material has a great impact on the capacity of lithium-ion batteries [4].

The theoretical specific capacity of traditional cathode materials (LiCoO₄, LiFePO₂) are relatively low [5, 6]. Vanadium oxides (V₂O₅, V₆O₁₃, VO₂, V₂O₃) [7,8,9,10,11] as cathode materials have higher theoretical specific capacity. In which V₆O₁₃ has a kind of double cavity chain structure, which is conducive to intercalation and de-intercalation of lithium ions [12]. The theoretical capacity and theoretical specific energy of V₆O₁₃ are 420 mAh/g and 890 Wh/kg at a specific voltage, respectively [13, 14]. Compare with traditional cathode materials, V₆O₁₃ has the advantages of high discharge capacity, good thermal stability, and low cost. But V₆O₁₃ belongs to metastable state and the valence is between V⁴⁺ and V⁵⁺, so it is difficult to synthesize. The synthetic methods mainly include hydrothermal, solvothermal, sol-gel, and solid-phase method.

Prepared V₆O₁₃ by heat treatment of ammonium metavanadate (NH₄VO₃) [15] and it need a long reaction time. Zeng [16] synthesized V₆O₁₃ with the morphology of hollow flower by hydrothermal, blending V₂O₅ with H₂O₂ to form sol-gel and adding octylamine and acetone. Its synthesis method is more complicated. Microflower of V₆O₁₃ was synthesized by hydrothermal, using ammonium metavanadate (NH₄VO₃) reaction with oxalic acid and lithium nitrate [17]. Li [18] synthesized nanofibers-V₆O₁₃ by sol-gel, and it had higher initial discharge capacity, but the capacity decay has exceeded 20% after 20 cycles.

In this work, V₂O₅ and C₂H₂O₄·2H₂O were used as raw materials to study the effect of reaction time on the phase by one-step hydrothermal, and the optimum reaction time of V₆O₁₃ was determined. V₆O₁₃ was synthesized from V₂O₅ and C₂H₅OH by the solvothermal method. The effects of synthetic methods on the morphology and electrochemical performance of V₆O₁₃ were investigated. The best synthetic method was determined. Compared with V₆O₁₃ prepared by solvothermal method and related literature, it is found that the preparation time of hydrothermal-V₆O₁₃ is shorter and the cost is lower. It has a good electrochemical performance. The contribution of this paper is to optimize the synthesis method and it can provide a reference for the future synthesis of V₆O₁₃.

Experimental

Synthesis route

V₂O₅, C₂H₂O₄·2H₂O, and C₂H₅OH were used as raw materials. Hydrothermal synthesis of V₆O₁₃: 1 g V₂O₅ and 1.385 g C₂H₂O₄·2H₂O were mixed in the reactor lining, then added 50 ml deionized water and stirred at room temperature for 20 min. The product was transferred to the reaction kettle and kept at 170 °C for 1 h, 3.5 h, 6 h, and 8.5 h, respectively. and then the product was centrifuged three times with deionized water, and then freeze-dried at − 50 °C for 24 h. After freeze-dried, the final product was obtained by calcining the product in a nitrogen-protected tubular furnace at 350 °C for 1 h. Solvothermal synthesis of V₆O₁₃: 0.4 g V₂O₅, 25 ml C₂H₅OH, and 20 ml deionized water were stirred in the reactor lining at room temperature for 30 min, transferred to the reaction kettle, and kept at 160 °C for 24 h, repeat the above step of centrifugation and calcination, and obtain the final product.

Characterization

The phase of the products was determined by X-ray powder diffraction (Japan JSM-5610LV). The morphology of the products was characterized by field emission scanning electron microscopy (Hitachi S-4800).

Using X-ray photoelectron spectroscopy (ESCALAB 250Xi) to analyze the composition and valence state of elements. Transmission electron microscope (Philips FEI TECNALG2) was used to observe the nanotopography of samples.

Electrochemical performance measurements

The cathode material, acetylene black, and polyvinylidene fluoride (PVDF) were placed in a mortar at a ratio of 7:2:1, and an appropriate amount of N-methyl pyrrolidone (NMP) was added to make a mixture. It was coated on an aluminum foil and maintained at 90 °C for 12 h with vacuum. A button cell is assembled in the glove box. The charge-discharge tests were performed by a BTS cycler (Shenzhen, China). The cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) measurement were characterized by IM6ex electrochemical workstation (Zahner, Germany).

Results and discussion

Figure 1 a is an XRD pattern of hydrothermal-V₆O₁₃ and was synthesized at different reaction times. It is not difficult to see that when the reaction time is 1 h, the phases of V₂O₅ and V₄O₉ are present in the product, and the valence of V has a decreasing trend. When the reaction time is 3.5 h, and the diffraction peaks are basically the same as the V₆O₁₃ standard card (JCPDF NO.71-2235), which indicates that the synthesized phase is V₆O₁₃. As the reaction time increases, the phase and intensity of the diffraction peak of VO₂ in the product gradually increases. In the process of preparing V₆O₁₃ by hydrothermal, the change of the phase can be estimated roughly as V₂O₅ → V₄O₉ → V₆O₁₃ → VO₂. As the reaction time increases, the reaction time of V₂O₅ in C₂H₂O₄·2H₂O is increased and V⁵⁺ is completely reduced to V⁴⁺. Therefore, an appropriate reaction time is advantageous for the synthesis of V₆O₁₃. Fig. 1 b shows the XRD patterns of V₆O₁₃ prepared by hydrothermal and solvothermal. It can be seen that the diffraction peaks of the products prepared by the two methods are basically the same. Which are also the same as the V₆O₁₃ standard card (JCPDF NO.71-2235), indicating that the synthesized products are V₆O₁₃.

Fig. 1

a XRD patterns of V₆O₁₃ prepared by hydrothermal at different reaction times, b XRD patterns of V₆O₁₃ prepared by different methods

Figure 2 a shows the full spectra of V₆O₁₃ prepared by different methods. It can be seen that the composition of V₆O₁₃ prepared by different methods is the same, all of which are C 1 s, V 2p, and O 1 s. It indicates that there are elements of O, V, and C in the sample. Figure 2 b–c are narrow spectra in V 2p region of V₆O₁₃ prepared by hydrothermal and solvothermal. The peaks of both V 2p_3/2 and V 2p_1/2 were decomposed at 517.1, 516 eV and 524.4, 523.4 eV, corresponding to V⁵⁺ (524.4, 517.1 eV) and V⁴⁺ (523.4, 516 eV), respectively [14]. The ratio of V⁴⁺ and V⁵⁺ is 2:1, which is consistent with the ratio of V⁴⁺ and V⁵⁺ in V₆O₁₃.

Fig. 2

a The XPS survey spectra of V₆O₁₃ prepared by different methods, b XPS spectrum of V 2p region of hydrothermal-V₆O_13,c XPS spectrum of V 2p region of solvothermal-V₆O₁₃

Figure 3 a is a SEM image of V₆O₁₃ prepared by hydrothermal. It can be seen that the morphology of hydrothermal-V₆O₁₃ is composed of irregular thin rods. The thin rod has a width of about 100–200 nm and a length of about 2 μm, and there are many hollow holes in the middle. This structure facilitates the intercalation and de-intercalation of lithium ions, which can shorten the distance and reduce the resistance, and improve the electrochemical performance. Figure 3 b is a SEM image of the product prepared by solvothermal. It can be seen that the morphology is formed by stacking rods of different lengths. The width of the rod is about 300–400 nm and the length is about 1–4 μm. Figure 3 c–d are TEM images of hydrothermal-V₆O₁₃ and solvothermal-V₆O₁₃. It can be seen that the width of the hydrothermal-V₆O₁₃ is about 150 nm and the solvothermal-V₆O₁₃ is about 320 nm, which are consistent with the SEM test results. The increase in the width of the rod influence, the transportation distance of lithium ions, and more resistance need to be overcome during intercalation and de-intercalation. Different synthetic methods cause the morphologies of the product completely different and hydrothermal-V₆O₁₃ is more favorable for the electrochemical performance of the material.

Fig. 3

a SEM image of hydrothermal-V₆O₁₃, b SEM image of solvothermal-V₆O₁₃, c TEM image of hydrothermal-V₆O₁₃, d TEM image of solvothermal-V₆O₁₃

Figure 4 a is a charge-discharge diagram of hydrothermal-V₆O₁₃ and solvothermal-V₆O₁₃ at a discharge rate of 0.1 C (42 mA/g). The first discharge specific capacity of hydrothermal-V₆O₁₃ is 319.2 mAh/g, in which the specific capacity and the capacity retention rate are 161.3 mAh/g and 50.5% after 100 cycles, respectively. The first specific discharge capacity of solvothermal-V₆O₁₃ is 279.3 mAh/g. The capacity is 106 mAh/g after 100 cycles and the capacity retention rate is 37.9%. By comparison, the hydrothermal-V₆O₁₃ is superior to the solvothermal-V₆O₁₃ in the first discharge capacity and retention. We can also see that the coulombic efficiency of hydrothermal-V₆O₁₃ is 97%, which is higher than that of solvothermal-V₆O₁₃. The reason may be that the morphology of hydrothermal-V₆O₁₃ has a small particle size and a large number of hollow holes in the middle, which are advantageous for the intercalation and de-intercalation of lithium ions. And it can also make the material better contact with conductive agent. Figure 4 b is a graph showing the rate performance of hydrothermal-V₆O₁₃ and solvothermal-V₆O₁₃ at discharge rates of 0.1, 0.2, 0.5, and 1 C. It can be seen that with the increase of the discharge rate, the discharge specific capacity of V₆O₁₃ decreases, and when the discharge rate changes back to 0.1 C, the discharge specific capacity of the material is improved compared with 1 C. Indicating that the V₆O₁₃ shows poor electrochemical performance at large discharge rates. Comparing with the charge-discharge curves of V₆O₁₃ prepared by the two methods, the capacity of hydrothermal-V₆O₁₃ is always better than solvothermal-V₆O₁₃, which is also the same as the result of 100 charge-discharge cycle tests. The hydrothermal-V₆O₁₃ has excellent electrochemical performance than the solvothermal-V₆O₁₃.

Fig. 4

a The cyclic performance and coulombic efficiency of V₆O₁₃ prepared by different methods, b rate performance of V₆O₁₃ prepared by different methods, c first charge and discharge curve of V₆O₁₃ prepared by different methods

Figure 4 c is a graph of the first charge-discharge curves of V₆O₁₃ prepared by different methods at 0.1 C. It can be seen that each sample has two charging platforms and discharge platforms. The charge and discharge platform represent the phase transition in the material. The discharge platform is at the position of 2.53 V and 2.02 V, which the reduction reaction occurs in V₆O₁₃ (V⁵⁺ → V⁴⁺ → V³⁺). The charging platform is about 2.68 V and 2.21 V, and the oxidation reaction occurs (V³⁺ → V⁴⁺ → V⁵⁺). It indicates that the reaction is reversible. The voltage platform of hydrothermal-V₆O₁₃ is longer than solvothermal-V₆O₁₃, and the length of the voltage platform is related to the redox reaction kinetics and the reversibility of the system [19]. It indicates that hydrothermal-V₆O₁₃ is more reversible.

Figure 5 a is a CV diagram of V₆O₁₃ prepared by different methods after three charge-discharge cycles, the voltage range is 1.5–4.0 V, and the scanning speed is 0.1 mV/s. It can be seen from the figure that the CV curves of V₆O₁₃ prepared by the two methods are basically the same, and there are a relatively obvious reduction peak and oxidation peak, which reflects the intercalation and de-intercalation of lithium ions. The oxidation peak and reduction peak of hydrothermal-V₆O₁₃ are 2.47 V, 3.15 V, and 2.09 V, respectively. The oxidation peak and reduction peak of solvothermal-V₆O₁₃ are 2.43 V, 3.11 V, and 2.13 V, respectively. It matches the voltage platform of charge and discharge. The area of oxidation peak and reduction peak reflects the cyclic active substance in the sample. It is obvious that the area of CV curve of hydrothermal-V₆O₁₃ is much larger than solvothermal-V₆O₁₃, indicating that there are more cyclic active substances. Figure 5 b shows the EIS pattern of V₆O₁₃ prepared by different methods. The semicircle in the high-frequency region represents the charge shift resistance, and the oblique line in the low-frequency region reflects the diffusivity of lithium ions. After Zview software fitting, the charge transfer resistance of hydrothermal-V₆O₁₃ and solvothermal-V₆O₁₃ is 625 Ω and 1010 Ω, respectively. The charge shift resistance of hydrothermal-V₆O₁₃ is small, indicating that lithium ions are more easily remove from V₆O₁₃, which are also because of small size and hollow pores.

Fig. 5

a CV diagram of V₆O₁₃ prepared by different methods, b EIS diagram of V₆O₁₃ prepared by different methods

Conclusion

In this paper, V₆O₁₃ was synthesized by hydrothermal and solvothermal, respectively. The results show that the pure V₆O₁₃ is synthesized by hydrothermal with reaction time of 3.5 h. The electrochemical performance of hydrothermal-V₆O₁₃ is better than that of solvothermal-V₆O₁₃. The hydrothermal-V₆O₁₃ consists of thin rods with hollow holes in the middle, which is beneficial to the transportation and storage of lithium ions in V₆O₁₃. This morphology can also increase the contact between materials and conductive agents, and improving the electrochemical performance. The results of charge-discharge test show that the initial discharge capacity of hydrothermal-V₆O₁₃ is 319.2 mAh/g, the discharge capacity is 161.3 mAh/g after 100 cycles and the capacity retention rate is 50.5%, which are higher than that of solvothermal-V₆O₁₃. The curves of CV and EIS further proved that the hydrothermal-V₆O₁₃ has more active substances and less resistance. The hydrothermal-V₆O₁₃ is not only shorten in synthetic time but also has good electrochemical performance. The purpose of this paper is to research a simple method to synthesize V₆O₁₃ and provide a reference for the future synthesis of V₆O₁₃. In the future work, some metal ions will be doped on the hydrothermal-V₆O₁₃ and improve the electrochemical performance of V₆O₁₃.