Trans. Nonferrous Met. Soc. China 30(2020) 2200-2206

Synthesis and electrochemical performance of V₂O₅/NaV₆O₁₅ nanocomposites as cathode materials for sodium-ion batteries

Mu-lan QIN, Wan-min LIU, Yuan-jin XIANG, Wei-gang WANG, Bin SHEN

Hunan Provincial Key Laboratory of Environmental Catalysis & Waste Recycling, College of Materials and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China

Received 29 December 2019; accepted 24 June 2020

Abstract: V₂O₅/NaV₆O₁₅ nanocomposites were synthesized by a facile hydrothermal method using VO₂(B) nanoarrays as the precursor. X-ray diffraction, scanning electron microscopy and transmission electron microscopy, and galvanostatic charge-discharge test were used to evaluate the structures, morphologies and electrochemical performance of samples, respectively. The results show that the nanocomposites are composed of one-dimensional nanobelts, preserving the morphology of the precursor well, and the hydrothermal reaction time has a significant effect on the phase contents and electrochemical performance of the composites. Compared with pure V₂O₅, V₂O₅/NaV₆O₁₅ nanocomposites exhibit enhanced electrochemical performance as cathode for sodium-ion batteries. It should be ascribed to the synergistic effect between V₂O₅ with high capacity and NaV₆O₁₅ with good cycling performance, and the introduced massive interfacial areas which can provide additional ion storage sites and improve the electronic and ionic conductivities.

Key words: V₂O₅/NaV₆O₁₅; nanocomposite; cathode material; sodium-ion battery

1 Introduction

Lithium-ion batteries (LIBs) have been widely used in portable electronic products and electric vehicles (EVs) due to their long cycle life and high energy and power densities. However, the limited global reserves of lithium lead to high cost and will seriously hinder the large-scale application of LIBs. Compared with LIBs, sodium-ion batteries (SIBs) have many potential advantages such as abundant sodium resources and low cost, which are suitable for large-scale storage [1-5]. However, the large ionic size (1.02 ) and low standard electro- chemical potential (2.71 V) of Na result in low reversible specific capacity and energy density, hindering further developments of SIBs [6,7]. Thus, developing cathode materials with high capacity for SIBs is an urgent requirement [8].

Vanadium pentoxide (V₂O₅), which has been widely studied as a cathode material for  LIBs [9-14], shows great potential for SIBs owing to its high theoretical capacity (294 mA·h/g for 2 Na ions intercalation) [15]. However, due to larger radius of Na⁺, the intercalation of Na⁺ in V₂O₅ is more difficult, and V₂O₅ suffers from  poor electronic and ionic conductivities, thus resulting in lower reversible capacity and worse rate capability [16-19]. To resolve these issues, one of the most effective strategies is to design nanostructured V₂O₅, such as nanobelts [20], nanospheres [21] and nanosheets [22], which could decrease the transport pathways for sodium ions and enhance the contact area between the active material and electrolyte. On the other hand, NaV₆O₁₅ exhibits much stable cycling performance when used as a cathode for SIBs [23-26], although it has relatively low discharge capacity. So, V₂O₅/NaV₆O₁₅ composites with nanostructures should have good electrochemical performance as cathode for SIBs for the synergistic effect between V₂O₅ and NaV₆O₁₅. NIU et al [27] reported that three- dimensional V₂O₅/NaV₆O₁₅ hierarchical hetero- structure microspheres show better electrochemical performance than pure V₂O₅ as cathode for LIBs [27]. However, there seems to be less report about V₂O₅/NaV₆O₁₅ nanocomposites as cathode for SIBs.

Herein, a hydrothermal method was introduced to synthesize V₂O₅/NaV₆O₁₅ nanocomposites using VO₂(B) nanoarrays as the precursor, and the electrochemical properties of V₂O₅/NaV₆O₁₅ nanocomposites as cathode materials for SIBs were investigated.

2 Experimental

2.1 Synthesis of V₂O₅/NaV₆O₁₅ nanocomposites

All the reagents and solvent were of analytical grade and used without further purification. Firstly, VO₂(B) precursor with a thickness of 10-20 nm was synthesized according to a hydrothermal method reported in our previous work [28]. 0.5 g V₂O₅ powder was added with 20 mL distilled water and 10 mL ethyleneglycol (EG), which was then maintained at 180 °C for 18 h to obtain the VO₂(B) precursor. Secondly, 6 mmol VO₂(B) was added into 15 mL NaOH solution with concentration of 0.1 mol/L, which was then sealed in Teflon-lined stainless steel autoclaves and maintained at 180 °C for different durations of 2, 6 and 12 h. After being cooled down to room temperature, the precipitate was filtered off, washed with distilled water several times and then dried at 80 °C overnight. Finally, the collected powder was calcined at 350 °C for 4 h in air to obtain the composites. For comparison, pure V₂O₅ was synthesized by calcining VO₂(B) precursor at 350 °C for 4 h in air.

2.2 Characterization

The phase structures of obtained products  were determined by X-ray diffraction (XRD) (Rigaku D/max2500 XRD with Cu K_α radiation, λ=1.54178 ). A scanning electron microscope (SEM) (FEI Sirion200) and a field emission transmission electron microscope (FETEM) (JEOL JEM-2100F) were used to characterize the morphologies of obtained products.

The V₂O₅/NaV₆O₁₅ nanocomposites were assembled into coin-type cells to evaluate their electrochemical properties. V₂O₅/NaV₆O₁₅ nano- composites, acetylene black and polyvinylidene-fluoride (PVDF) binder in a mass ratio of 7:2:1 were dispersed in N-methyl-2-pyrrolidone (NMP) solution to make the slurry, which was then coated on an aluminum foil and dried in a vacuum oven at 120 °C for 12 h to make the cathode electrode. Coin-type cells (CR 2025) were assembled using glass fiber (Whitman GF/C) as the separator and 1.0 mol/L NaClO₄ dissolved in ethylene carbonate (EC)-diethyl carbonate (DEC) (1:1 in volume) as the electrolyte. The galvanostatic charge/discharge characteristics of the cells were recorded with a Land battery tester (Land CT2001A), and the electrochemical workstation (Multi Autolab M204, Metrohm) was applied to acquiring CV curves.

3 Results and discussion

The XRD patterns of V₂O₅/NaV₆O₁₅ composites obtained with different reaction time and the standard V₂O₅ and NaV₆O₁₅ patterns are shown in Fig. 1. It can be seen that the composites obtained with the hydrothermal reaction time of 2, 6 and 12 h, respectively, are composed of orthorhombic V₂O₅ phase and monoclinic NaV₆O₁₅ phase, with no evidence of other impurity phases, which indicates that V₂O₅/NaV₆O₁₅ composites can be easily synthesized within 2 h. Moreover, the sharp peaks in the patterns indicate that the composites are well crystallized.

Fig. 1 XRD patterns of V₂O₅/NaV₆O₁₅ composites obtained with different hydrothermal reaction time

The Rietveld refinement method (Jade 9.0 software, MDI, USA) was used to refine the XRD pattern of V₂O₅/NaV₆O₁₅ composites obtained with the hydrothermal reaction time of 6 h to analyze the crystal structures and phases. As shown in Fig. 2, all the diffraction peaks can be well indexed to orthorhombic V₂O₅ phase [space group: Pmmn (59), ICSD No. 1647638] and monoclinic layered NaV₆O₁₅ phase [space group: C2/m (12), ICSD No. 80813], and no other impurity phase is detected. Table 1 gives the refined cell parameters of V₂O₅ and NaV₆O₁₅ in V₂O₅/NaV₆O₁₅ composites. The observed and calculated patterns match well, and the reasonably small R factor (8.47%) suggests that the refinement results are convincible. The parameters of V₂O₅ and NaV₆O₁₅ present little distortion in V₂O₅/NaV₆O₁₅ composites compared with the pure individual phase, and the phase contents of V₂O₅ and NaV₆O₁₅ are 69 and 31 wt.%, respectively. By prolonging the hydrothermal reaction time to 12 h, the phase contents of V₂O₅ and NaV₆O₁₅ change to 40 and 60 wt.%, respectively, indicating that hydrothermal reaction time has a significant effect on the phase contents.

Fig. 2 XRD pattern with Rietveld refinement of V₂O₅/NaV₆O₁₅ composite obtained with hydrothermal reaction time of 6 h

The morphologies of V₂O₅/NaV₆O₁₅ composites obtained with different hydrothermal reaction time were studied by SEM and the results are shown in Figs. 3(a-c). It can be seen that V₂O₅/NaV₆O₁₅ composites obtained with 2, 6 and 12 h, respectively, are composed of one- dimensional nanobelts, preserving the morphology of the precursor well. Figure 3(d) shows the TEM image of V₂O₅/NaV₆O₁₅ nanocomposites obtained with the hydrothermal reaction time of 6 h. It is interesting that the composites display interconnected nanosheet structure, which is beneficial to shortening ion diffusion distance and providing large contact area for electrochemical reactions [15]. Moreover, the phase boundaries of V₂O₅ and NaV₆O₁₅ phases of V₂O₅/NaV₆O₁₅ nanocomposites and uniform distribution of elements Na, V and O can be detected by HRTEM and SEM-EDS mapping, respectively, as shown in earlier work [29]. The existence of substantial phase boundaries may introduce numerous crystal defects and active sites [30], which will promote the ion conductivity as well as supply vacancy for Na⁺ insertion.

Figure 4(a) shows the discharge-charge curves of the first cycles of V₂O₅/NaV₆O₁₅ nanocomposites obtained with different hydrothermal reaction time within the voltage range of 1.5-3.8 V at 20 mA/g. Three discharge plateaus located around 2.7, 2.2 and 2.0 V can be identified as the insertion of Na⁺ into V₂O₅/NaV₆O₁₅ nanocomposites. The multiple plateaus indicate multiple steps of sodium ions intercalation process [23,31]. In addition, high initial specific discharge capacities of 184, 209 and 133 mA·h/g are obtained for V₂O₅/NaV₆O₁₅ nanocomposites obtained with 2, 6 and 12 h, respectively. However, for pure V₂O₅, only two discharge plateaus located around 2.1 and 1.9 V and an initial specific discharge capacity of 110 mA·h/g are observed.

Table 1 Refined unit cell lattice parameters of V₂O₅ and NaV₆O₁₅ in V₂O₅/NaV₆O₁₅ composites and standard data of V₂O₅ (ICSD No. 1647638) and NaV₆O₁₅ (ICSD No. 80813)

Fig. 3 SEM images of V₂O₅/NaV₆O₁₅ nanocomposites obtained with hydrothermal reaction time of 2 h (a), 6 h (b) and 12 h (c), and TEM image of V₂O₅/NaV₆O₁₅ nanocomposite obtained with hydrothermal reaction time of 6 h (d)

Fig. 4 Galvanostatic discharge–charge curves (a) and cycling performance (b) of V₂O₅/NaV₆O₁₅ nanocomposites and pure V₂O₅ in voltage range of 1.5-3.8 V at current density of 20 mA/g

Figure 4(b) compares the cyclic performance of V₂O₅/NaV₆O₁₅ nanocomposites obtained with different hydrothermal reaction time at 20 mA/g. At the second cycle, V₂O₅/NaV₆O₁₅ nanocomposites obtained with 6 h deliver a specific discharge capacity of 164 mA·h/g and its capacity retains to be 100 mA·h/g after 30 cycles. V₂O₅/NaV₆O₁₅ nanocomposites obtained with 2 and 12 h deliver much lower discharge capacities of 84 and 70 mA·h/g at the second cycle, and the capacities then drop to 61 and 40 mA·h/g after 30 cycles, respectively. These results indicate that the hydrothermal reaction time has a significant effect on the electrochemical performance of V₂O₅/ NaV₆O₁₅ nanocomposites by affecting the phase contents of the nanocomposites. While for pure V₂O₅, the capacity can be nearly ignored after 10 cycles, just be 1 mA·h/g. Compared with pure V₂O₅, V₂O₅/NaV₆O₁₅ nanocomposites show enhanced electrochemical performance, which should be ascribed to the construction of nanocomposites. On one hand, the synergistic effect in V₂O₅/NaV₆O₁₅ nanocomposites is favorable to inherit the high capacity of V₂O₅ and good cycling performance of NaV₆O₁₅ [29,32]. On the other hand, introducing nanocomposites will increase massive interfacial areas, which can provide additional ion storage  sites and improve the electronic and ionic conductivities [33,34]. In addition, there is obvious capacity fading in initial cycles, which could be mainly due to the generation of solid electrolyte interphase (SEI) formed by the electrolyte decomposition [21,35].

Figure 5 presents the cyclic voltammetry (CV) curves of V₂O₅/NaV₆O₁₅ nanocomposites obtained with the hydrothermal reaction time of 6 h at a scan rate of 0.1 mV/s in the voltage range of 1.5-4.0 V. During the first reduction process, three obvious cathodic peaks at 2.7, 2.0 and 1.7 V are observed, which can be attributed to the multi-step insertion of Na⁺ ions into V₂O₅/NaV₆O₁₅ nanocomposites. For the corresponding oxidation process, two anodic peaks are observed at 3.1 and 3.6 V, indicating the multi-step Na⁺ ions extraction processes. The first CV curve possesses some difference compared with the latter ones, and this phenomenon should be ascribed to the structure rearrangement of the composites after Na⁺ ion insertion in the first cycle [15,23]. With the increase of cycle number, the CV curves are almost equal in size, suggesting a high reversibility of the insertion/ extraction reactions.

Fig. 5 CV curves of V₂O₅/NaV₆O₁₅ nanocomposites at scan rate of 0.1 mV/s

The second discharge-charge curves of V₂O₅/NaV₆O₁₅ nanocomposites obtained with 6 h in the voltage range of 1.5-4.0 V at different current densities are shown in Fig. 6(a). The charge- discharge plateaus correspond well with the peak positions shown on the CV curves (Fig. 5), and the capacities of 176, 126 and 101 mA·h/g are obtained at 20, 50 and 100 mA/g, respectively. Figure 6(b) shows the cycling performance of V₂O₅/NaV₆O₁₅ nanocomposites at different current densities. Initial specific discharge capacities of 231, 201 and 173 mA·h/g are obtained at current densities of 20, 50 and 100 mA/g, respectively. The capacities decrease to 101, 93 and 70 mA·h/g after 50 cycles, with the retention of 57.4%, 73.8% and 69.3% of capacities at the second cycle, respectively. It can be seen from Figs. 5 and 6 that the composites show higher capacity and better cycling performance in the voltage range of 1.5-4.0 V, which indicates that the voltage range has a significant effect on the electrochemical properties of V₂O₅/NaV₆O₁₅ nanocomposites. The synthesized nanocomposites in this work display better electrochemical performance compared with the pure V₂O₅ and NaV₆O₁₅ reported in earlier literatures [24,31], which can be ascribed to the construction of nanocomposites.

Fig. 6 Galvanostatic discharge–charge curves (a) and cycling performance (b) of V₂O₅/NaV₆O₁₅ nano- composites in voltage range of 1.5-4.0 V at different current densities

4 Conclusions

(1) V₂O₅/NaV₆O₁₅ nanocomposites were synthesized successfully by a hydrothermal method. The hydrothermal reaction time has a significant effect on the electrochemical performance of the composites by affecting the phase contents.

(2) V₂O₅/NaV₆O₁₅ nanocomposites show enhanced electrochemical performance compared with pure V₂O₅, which should be attributed to the synergistic effect between V₂O₅ and NaV₆O₁₅, and the introduced massive interfacial areas.

(3) V₂O₅/NaV₆O₁₅ nanocomposites deliver a high initial discharge capacity of 231 mA·h/g in the voltage range of 1.5-4.0 V, which indicates that V₂O₅/NaV₆O₁₅ nanocomposites are potential cathode materials for sodium-ion batteries.

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钠离子电池正极用V₂O₅/NaV₆O₁₅纳米复合材料的制备与电化学性能

秦牡兰，刘万民，向垣锦，王伟刚，申 斌

湖南工程学院 材料与化工学院 环境催化与废弃物再生化湖南省重点实验室，湘潭 411104

摘  要：采用 VO₂(B)纳米阵列作为前驱体，通过水热法制备 V₂O₅/NaV₆O₁₅ 纳米复合材料。通过X射线衍射、扫描电镜、透射电镜和恒流充放电测试对材料的结构、形貌和电化学性能进行表征。结果表明，纳米复合材料由一维纳米带组成，较好地保留了前驱体的形貌；水热反应时间对材料的相组成和电化学性能有显著的影响。V₂O₅/NaV₆O₁₅纳米复合材料比纯V₂O₅显示出更好的电化学性能，这归因于V₂O₅/NaV₆O₁₅纳米复合材料中具有高容量的V₂O₅和高循环稳定性的NaV₆O₁₅之间的协同效应，以及纳米复合材料中引入的大量相界面为钠离子嵌入提供了额外的位点并促进电子和离子的传输。

关键词：V₂O₅/NaV₆O₁₅；纳米复合材料；正极材料；钠离子电池

(Edited by Bing YANG)

Foundation item: Project (2020JJ5102) supported by the Natural Science Foundation of Hunan Province, China; Project (19A111) supported by the Scientific Research Fund of Hunan Provincial Education Department, China

Corresponding author: Wan-min LIU; Tel: +86-13787101796; E-mail: william@hnie.edu.cn

DOI: 10.1016/S1003-6326(20)65372-9