MoV₂O₈ nanostructures: controlled synthesis and lithium storage mechanism

Abstract

A facile two-step strategy involving a solvothermal method and a subsequent calcining treatment was successfully developed for the preparation of MoV₂O₈ nanorods in the absence of any surfactants. Acetic acid was chosen as the solvent to provide an acidic environment. The as-synthesized MoV₂O₈ nanorods were evaluated as an anode material in lithium ion batteries, which showed excellent lithium storage performance in terms of its specific capacity, rate performance, and cycling stability. It could deliver a specific capacity of over 1325 mA h g⁻¹ after 50 cycles at 0.2 A g⁻¹, which is much higher than that of bulk MoV₂O₈ (617 mA h g⁻¹). When the cell was cycled at a current density as high as 10.0 A g⁻¹, it still maintained a high specific capacity of around 570 mA h g⁻¹. The phase transformation, intercalation–deintercalation and partial redox processes are responsible for the lithium storage mechanism of MoV₂O₈ based on ex situ X-ray diffraction, X-ray photo electron spectroscopy and transmission electron microscopy studies, highlighting a new lithium storage mechanism for ternary metal oxides.

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1. Introduction

Recently, tremendous research effort has been triggered in investigating power sources with high specific energy density and long cycle life to meet the current demand in the expanding market of portable electronic products.^1–4 Lithium ion batteries (LIBs) are the most rapidly growing high-energy storage device due to their high volumetric and gravimetric capacity, enabling smaller and lighter battery packs.^5–7 Commercially available LIBs generally use layered transition metal oxides such as LiCoO₂ or Li₂Mn₂O₄ as cathode materials and graphitized carbon as anode materials.^8,9 As is well known, a graphite-based anode possesses a favorably low potential for lithium ion insertion/extraction, but its low theoretical capacity (372 mA h g⁻¹) and rate capacity make it difficult to meet the requirements for the large-scale applications of LIBs in the near future.¹⁰ Hence, developing high-performance anode materials for LIBs is highly desirable.

Although a lot of progress has been made in developing new anode materials for LIBs, these anode materials still suffer from severe capacity fading, low capacity and high cost, which have seriously hindered the development of LIBs. Among all of the reported materials, transition metal oxides have attracted much attention due to their low cost, environmental benignity, and higher theoretical capacity, resulting in them having been widely exploited as high-capacity anode materials for LIBs.^11,12 In particular, mixed metal oxides can synergistically improve the electrochemical properties including the reversible capacity, and mechanical stability.^13–15 Recently, several research groups have reported the synthesis of double metal oxide nano-/micro-meter materials and their lithium storage properties. It should be noted that their results were encouraging. For example, Christie et al. reported the fabrication of interconnected CoMoO₄ submicrometer particles, which could deliver a reversible capacity of 990 mA h g⁻¹ at a current density of 100 mA g⁻¹, with 100% capacity retention between the 5th and 50th cycles.¹⁶ AB₂O₄ (A = Fe, Co, Ni, Mn, B = Fe, Co, Ni, Zn, Mn, Fe, etc. A ≠ B) spinel compounds have emerged as promising anode materials and have been investigated extensively for use in next-generation LIBs.^17–25

Furthermore, a considerable number of vanadates, such as Co₃V₂O₈, Zn₃V₂O₈ and RVO₄ (R = In, Fe, Cu), have been investigated for lithium insertion materials for many years. For instance, Yang et al. have prepared multilayered Co₃V₂O₈ nanosheets, which showed high reversible capacity (1114 mA h g⁻¹ retained after 100 cycles) and excellent rate performance (361 mA h g⁻¹ at a high current density of 10 A g⁻¹) for lithium storage.²⁶ Gan et al. synthesized hexagonal Zn₃V₂O₈ nanosheets, which displayed an excellent lithium storage performance for LIBs with a reversible capacity as high as 1103 mA h g⁻¹ at a current density of 200 mA g⁻¹.²⁷ InVO₄ was tested by Reddy et al.²⁸ with negligible capacity fading between the 2nd and 50th cycles and a high capacity of 1241 mA h g⁻¹ at the end of the 20^th cycle, close to its theoretical capacity. Yang et al. investigated Fe₂VO₄ as an anode material, and the reversible specific capacity with 1 C was about 225 mA h g⁻¹, which was close to its theoretical specific capacity.²⁹ Zhang et al. synthesized hollow Cu₃V₂O₇(OH)₂·2H₂O nanostructures, which were used as a cathode material in primary LIBs. Electrochemical measurements showed that the sponge-like hollow structure exhibited a higher discharge capacity of 513 mA h g⁻¹ at the current density of 100 mA g⁻¹.³⁰

MoV₂O₈, a promising host for metal ion intercalation, has outstanding advantages due to its layered structure (the inset in Fig. 1b), which can improve its ability to intercalate ions in a wide range of sites. However, the complex nature of molybdates makes them difficult to synthesize, especially their nanostructures. Until now, there have been few reports on MoV₂O₈ nanostructures and their application as anode materials for LIBs. Therefore, it was of great significance to develop a simple yet effective method to prepare nanoscale MoV₂O₈. Herein we demonstrated a mild, facile route towards the synthesis of MoV₂O₈ nanorods through a solvothermal method followed by a subsequent calcining treatment in air. A Mo–V-based precursor was first prepared using the solvothermal process, and then the precursor was subjected to a calcining treatment to form the final product. The resultant MoV₂O₈ nanorods exhibited high capacity, excellent cycling stability and perfect rate capability as an anode material for LIBs. The lithium storage mechanism of the MoV₂O₈ nanorods was also investigated in detail, which was different from those of other vanadates. To the best of our knowledge, this is the first report on the lithium storage of MoV₂O₈ nanorods. It is believed that the MoV₂O₈ nanorods could be a promising candidate as an anode material because of their high capacity and rate performance.

Fig. 1 (a) TG curve of the Mo–V-based precursor. (b) XRD patterns of the samples obtained at different calcining temperatures. The inset is the crystal structure of MoV₂O₈, in which Mo–O octahedrons are green and V–O octahedrons are blue.

2. Experimental

Synthesis of MoV₂O₈ nanorods

All chemical regents used in this work were analytical grade and were used without further purification. Typically, 0.3 mmol of molybdenyl acetylacetonate [MoO₂(acac)₂] was added to 30 mL of acetic acid at room temperature to form a transparent solution. Meanwhile, 0.6 mmol of vanadyl acetylacetonate [VO(acac)₂] was added to another 30 mL of acetic acid. After being ultrasonically treated for several minutes, the resultant two solutions were mixed in one beaker under magnetic stirring followed by a heating process. Finally the formed clear solution was transferred into an 80 mL Teflon-lined stainless steel autoclave. The autoclave was sealed tightly and maintained at 200 °C for 36 h. Afterwards, the autoclave was cooled to room temperature naturally. The resultant precipitate was collected through centrifugation, washed with water and ethanol several times, and dried using the freeze drying method. Finally, the product was selectively calcined at different temperatures for 3 h under atmospheric air to obtain the final products.

For comparison, bulk MoV₂O₈ was also prepared (Fig. S1†). In a typical process, 0.143 mmol of (NH₄)₆Mo₇O₂₄·4H₂O, 2.0 mmol of NH₄VO₃ and 6.0 mmol of urea were added to 10 mL of deionized water at room temperature with magnetic stirring to form a transparent solution, which then was transferred to a 50 mL centrifuge tube and was rapidly frozen. The freeze drying was carried out for 24 h. Finally the powder was calcined at 600 °C in air for 12 h to yield the final product.

Characterizations

Powder X-ray diffraction (XRD) measurements were collected from 5° to 80° (2θ) with a scanning step of 5° min⁻¹ using Cu-Kα (λ = 1.54178 Å) incident radiation (40 kV, 50 mA) on a Shimadzu XRD-6000 instrument. The general size and morphology of the products were characterized using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were carried out on a H-8100 transmission electron microscope operating at a 200 kV accelerating voltage. X-ray photo electron spectra (XPS) were recorded on an ESCALAB 250 spectrometer (Perkin-Elmer) to characterize the surface composition. The Brunauer–Emmett–Teller (BET) surface area was estimated using a Belsorp-max surface area detecting instrument through N₂ physisorption at 77 K. The thermogravimetry (TG) analysis was performed from 25 to 700 °C in air with a heating rate of 10 °C min⁻¹ using a DTG-60AH instrument.

Electrochemical measurements

The electrochemical measurements of the as-prepared MoV₂O₈ sample were recorded by using standard CR2025 coin cells at room temperature. To prepare the working electrode, the as-prepared active material (80%), carbon black (10%) and carboxymethylcellulose sodium (10%) were mixed and ground in a mortar. Ultrapure water was used as the solvent and the resultant slurry was then uniformly pasted on a Cu foil current collector. The coated electrode was dried at 120 °C for 24 h under vacuum before being assembled into a coin cell in an argon-filled glove box. The mass loading of the pure active materials is about 0.8–1.04 mg. A 1 M solution of LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1 : 1 : 1, vol%) was used as the non-aqueous electrolyte. A pure lithium metal slice was used as both the counter and the reference electrodes. Galvanostatic charge–discharge tests of the assembled coin cell were performed on a LAND CT2001A with a cutoff voltage of 0.005–3.00 V versus Li⁺/Li. Cyclic voltammetry (CV) was performed using an electrochemical workstation (CHI660D) with a scanning rate of 0.5 mV s⁻¹ in the frequency range of 100 kHz–0.005 Hz at room temperature.

3. Results and discussion

As described in the Experimental section, the synthesis of the MoV₂O₈ nanorods involved two steps, a solvothermal process followed by a calcining treatment. The first step resulted in the formation of a Mo–V-based precursor, which was then calcined to form the target product via the second step. The calcination temperature was determined through TG analysis of the Mo–V-based precursor, which was performed from room temperature to 700 °C at a ramp rate of 10 °C min⁻¹ in air. As shown in Fig. 1a, the TG curve exhibits two main regions over the temperature ranges 25–200 °C and 200–450 °C, which involve a total weight loss of 27%. The weight loss of about 11% between 25 °C and 200 °C is ascribed to the release of adsorbed water and coordinated water,²⁷ while between 200 and 450 °C, there is a quick weight loss of 16%, which usually is regarded as the decomposition process of the precursor. Over this temperature range, the precursor is decomposed into a MoV₂O₈ phase, which will be proven through XRD measurements below. From 450 °C, no weight loss is observed, indicating that 450 °C is the lowest temperature for the formation of a pure MoV₂O₈ phase. Following the TG results, we tried different calcining temperatures. Fig. S2a† shows an XRD pattern of the precursor, which clearly indicates its amorphous structure. When the precursor is calcined at 400 °C, the resultant sample is a mixture of MoV₂O₈, MoO₃ and V₂O₅ (Fig. 2b). Interestingly, if the calcining temperature is increased to 500 °C or 600 °C, a pure MoV₂O₈ phase is obtained (JCPDS Card no. 74-0050). This result is consistent with the above TG analysis.

Fig. 2 XPS spectra of MoV₂O₈ nanorods. (a) Survey spectrum. High resolution spectra: (b) Mo 2p, (c) V 2p, and (d) O 1s. The red lines represent fitted curve, the blue lines represent base curve and the black lines represent original curve.

The XPS analysis was further used to elucidate the chemical composition and oxidation state of the resultant sample and here we use the sample obtained at 500 °C as an example. As shown in Fig. 2a, the survey spectrum further confirms that this sample consists of the elements Mo, V, and O. The high-resolution Mo 3d spectrum (Fig. 2b) shows two obvious signals at ca. 232.6 and 235.8 eV, which can be attributed to the Mo 3d_5/2 and Mo 3d_3/2 of Mo⁶⁺, in good agreement with previous reports.^31,32Fig. 2c displays the high-resolution V 2p spectrum, which shows two peaks with binding energies at 524.8, and 517.5 eV, which are associated with V 2p_1/2 and V 2p_3/2, respectively.^26,33 The high-resolution O 1s spectrum (Fig. 2d) can be resolved into two peaks at 529.5 and 531.6 eV. The former is ascribed to the lattice-oxygen and the latter is related to the adsorbed-oxygen.^34,35 Combining these results with that from XRD, it can be deduced that a pure MoV₂O₈ phase can be synthesized using the current approach.

Fig. 3 presents representative FE-SEM images of the resultant products. It can be clearly seen from Fig. 3a and b that the sample obtained at 500 °C is composed of relatively uniform nanorods and that some nanorods have twinned together to form nanorod bundles. The nanorods have lengths ranging from 150 to 300 nm, and diameters in the range of 20–30 nm. Although the MoV₂O₈ nanorods are obtained via the calcination treatment of the precursor at 500 °C, they almost maintains the morphology of the Mo–V-based precursor (Fig. S2a and b†). However, when the calcination temperature was increased to 600 °C, MoV₂O₈ nanobelts were formed instead of nanorods, as is clearly disclosed by Fig. 3c and d. The nanobelts typically have an average length of ca. 10 μm and thickness of ca. 100 nm. The morphology evolvement may be due to further growth of the nanorods at higher calcination temperature. From the above results, we can see that the calcined samples retain the one-dimensional structure of the precursor. In our experiments, acetic acid was used as the solvent to prepare the precursor. Patzke et al. reported previously that weak acetic acid can induce the formation of small-size MoO₃-based nanorods.^36,37 In fact, we also tried other solvents, such as ethylene glycol and N,N-dimethylformamide. As shown in Fig. S4,† the former results in the formation of larger irregular particles and the latter, similar to acetic acid, also leads to the formation of nanorods but with a much larger average diameter.

Fig. 3 (a and b) SEM images of MoV₂O₈ nanorods obtained at 500 °C. (c and d) SEM images of MoV₂O₈ nanobelts obtained at 600 °C.

The microstructure of the as-prepared MoV₂O₈ nanorods was further investigated through TEM measurements. The low-magnification TEM images show the nanorod morphology (Fig. 4a and b), which is consistent with the above SEM observations. A high-resolution TEM (HRTEM) image of an individual nanorod is shown in Fig. 4c, in which the lattice fringes are clearly visible. The distance between neighboring fringes is found to be 0.41 nm, which corresponds to the (201) planes of orthorhombic MoV₂O₈.³⁸ Surface-scanning element mapping clearly reveals that these elements are homogeneously distributed (Fig. 4d–g). In addition, the MoV₂O₈ nanorods have a large specific surface area of 18.26 m² g⁻¹, which was determined using N₂ adsorption–desorption isotherms (Fig. 4h). The value is far larger than those of the MoV₂O₈ nanobelts (2.48 m² g⁻¹) and bulk MoV₂O₈ (1.65 m² g⁻¹) (Fig. S3†). The larger specific surface area of the MoV₂O₈ nanorods may result from their small particle size, while the small particle size can dramatically reduce the diffusion time of lithium ions and improve the lithium insertion–extraction kinetics because of a shorter Li⁺ diffusion pathway. Therefore, this feature of the MoV₂O₈ nanorods may be expected to be beneficial for their electrochemical performance.

Fig. 4 (a and b) TEM images of the MoV₂O₈ nanorods. (c) HRTEM image of MoV₂O₈ nanorods. (d) The selected FE-SEM area for the element mapping images. (e–g) The corresponding element mapping images for Mo, V, and O elements, respectively. (h) N₂ adsorption–desorption isotherms of MoV₂O₈ nanorods.

As discussed above, although a higher calcining temperature was used, the MoV₂O₈ nanorods still maintained the morphology of the Mo–V-based precursor, which is very common in many reported works. In view of this fact, we performed time-dependent experiments to investigate the formation process of the precursor nanorods. As shown in Fig. S5a,† the sample obtained with a solvothermal reaction time as short as 3 h, was composed of small-size nanoparticles. When prolonging the reaction time to 12 h, it is clear that besides nanoparticles, some irregular nanorods started to form (Fig. S5b†). With further extending the reaction time to 24 h, more nanorods were formed (Fig. S5c†) and when the reaction time was as long as 36 h, uniform nanorods were obtained (Fig. S5d†). Based on the experimental results, it can be deduced that the formation process of the nanorods may involve the following several steps. In the initial stage, smaller nanoparticles were generated in the reaction system via a homogeneous nucleation process. In the following step, nanoparticles with larger sizes are able to form at the expense of smaller ones through Ostwald ripening. In the ripening process stage, larger particles can be directed to grow into nanorods with a uniform structure.^39–41

The Li-storage properties of the resultant samples as anode materials were evaluated with Li foil as a counter electrode. Fig. 5a shows the cycling performance of the MoV₂O₈ nanorods, MoV₂O₈ nanobelts and bulk MoV₂O₈ at a current density of 0.2 A g⁻¹ in the voltage range of 0.005–3 V vs. Li⁺/Li. It can be clearly seen that the cycling stability of the MoV₂O₈ nanorod electrode is much higher than those of the nanobelts and the bulk. The initial discharge capacity is as high as 1706 mA h g⁻¹. In the subsequent charge process, a charge capacity of 1327 mA h g⁻¹ is achieved, leading to a Coulombic efficiency of 77%. The irreversible capacity loss in the first discharge–charge process can be attributed to the inevitable decomposition of electrolyte and the formation of a solid electrolyte interphase (SEI) film on the surface of the electrode, which is commonly observed in metal oxide-based anode materials.⁴² However, for the MoV₂O₈ nanobelt and bulk MoV₂O₈ electrodes, their initial discharge capacities are 1046 and 780 mA h g⁻¹, respectively, which are far lower than that of the MoV₂O₈ nanorods. After 50 cycles, the MoV₂O₈ nanorods still delivered a high reversible capacity of 1325 m Ah g⁻¹, which is 77.7% of the initial discharge capacity. The delivered capacity of the MoV₂O₈ nanorods is much higher than those of the MoO₃ nanowire arrays (∼630 mA h g⁻¹) and V₂O₅ nanorods (∼460 mA h g⁻¹) at the same current density.^43,44 This capacity is also higher than other anode materials, such as ZnO (∼600 mA h g⁻¹), Fe₂O₃ (∼900 mA h g⁻¹), CuO (∼600 mA h g⁻¹) and carbonaceous materials (theoretical capacity of 372 mA h g⁻¹).^45–47 Considering that the MoV₂O₈ nanorods display high capacity and good cycling stability, the material could be a good candidate as an anode material for LIBs. As for the nanobelts and the bulk, their reversible capacities decrease to 854 mA h g⁻¹ and 617 mA h g⁻¹ after 50 cycles but with slightly higher Coulombic efficiencies of 81.6% and 79.1%, respectively. According to previous reports,^48–50 the anion (i.e. oxygen) plays an important role during the lithiation reaction and it can act as a redox center, thus leading to possible V–“O–Li” and Mo–“O–Li” interactions. Compared to the MoV₂O₈ nanobelts and the bulk, the MoV₂O₈ nanorods have a smaller size, and thus they have many more redox centers to interact with Li than the nanobelts and the bulk. These redox centers can result in an additional enhancement in the specific capacity. In addition, the average Coulombic efficiency of the MoV₂O₈ nanorods is calculated to be above 98%.

Fig. 5 (a) Cycling performance of the MoV₂O₈ nanorods, MoV₂O₈ nanobelts and bulk MoV₂O₈ at a current density of 0.2 A g⁻¹. (b) Rate performance of the MoV₂O₈ nanorods, MoV₂O₈ nanobelts and bulk MoV₂O₈. (c) Electrochemical impedance spectroscopy of the MoV₂O₈ nanorods, MoV₂O₈ nanobelts and bulk MoV₂O₈. (d) Electrochemical impedance spectroscopy of the MoV₂O₈ nanorods after certain cycles at 0.2 A g⁻¹ in a fully charged state in the frequency range from 0.1 MHz to 0.01 Hz. (e) Charge and discharge voltage profiles of the MoV₂O₈ nanorods for the first, second, 10th, 30th and 50th cycles at a current density of 0.2 A g⁻¹ between 0.005 and 3.0 V. (f) Cycling performance of the MoV₂O₈ nanorods at current densities of 0.2 and 0.8 A g⁻¹.

Besides the high capacity and good cycling stability, the MoV₂O₈ nanorod electrode also exhibited an excellent rate performance, as shown in Fig. 5b. The electrode was further tested at step-wise current densities from 0.2 to 10 A g⁻¹ and then reversed to 0.2 A g⁻¹. As the current densities were increased from 0.2 to 0.4, 0.8, 1.6, 3.2, 6.4, and 10.0 A g⁻¹, the reversible capacities of the MoV₂O₈ nanorods were 1360, 1270, 1130, 900, 800, 680 and 570 mA h g⁻¹, respectively, all of which are higher than those of the nanobelts and the bulk. The reversible capacity (570 mA h g⁻¹) obtained at a current density as high as 10.0 A g⁻¹ is even higher than the theoretical capacity of graphite. Furthermore, the rate capability of the as-prepared MoV₂O₈ nanorods is also comparable to those of other reported anode materials.^51–53 The excellent rate performance of the MoV₂O₈ nanorods can be attributed to their small-sized one-dimensional nanorod structure, which can result in fast kinetics for redox reactions involving lithium ions and electron diffusion to/from the electrolyte/particle interface. Meanwhile, they can also accommodate the local volume change during repeated Li⁺ insertion/extraction, and thus improve the cycling stability. As is well known, high rate capability materials are highly desirable for LIBs, which can greatly reduce the charge/discharge time in practical applications. The fast kinetics of the redox reactions of the MoV₂O₈ nanorods were further confirmed through electrochemical impedance spectroscopy (EIS) measurements, which were performed from 100 KHz to 0.01 Hz. As shown in Fig. 5c, the high frequency region of the semicircle represents the SEI resistance (R_SEI) and contact resistance (R_f), and the high-medium frequency region represents the charge transfer resistance (R_ct) on the electrode/electrolyte interface, whereas the inclined line at the low frequency after the semicircle corresponds to the diffusion barrier layer impedance (Z_T) and semi-infinite diffusion impedance (Z_w), respectively.^54–57 Obviously, the semicircle diameter of the MoV₂O₈ nanorods in the high-medium frequency is far smaller than those of the nanobelts and the bulk, implying their fast kinetics process. Furthermore, the EIS spectra of the MoV₂O₈ nanorods after different numbers of cycles at the discharge state at a current density of 0.2 A g⁻¹ were also performed (Fig. 5d). Different from most reports the cell impedance increases with the increase in the number of cycles. The diameter of the semicircle in the high-middle frequency range decreases obviously with the increasing number of cycles, indicating lower contact and charge-transfer impedances, which is consistent with some reports.^58–60 The possible reason for this result may be a gradual stabilization of the SEI layer during cycling.

The galvanostatic charge–discharge curves for the 1st, 2nd, 10th, 30th, and 50th cycles of the MoV₂O₈ nanorods at a current density of 0.2 A g⁻¹ are shown in Fig. 5e. The 1st discharge and charge capacities of the MoV₂O₈ nanorods are as high as 1706 and 1327 mA h g⁻¹, respectively. After 50 cycles, 77.7% of the initial discharge capacity and 99.8% of the initial charge capacity can be retained. Furthermore, the initial discharge curve shows a pronounced plateau at about 3.0–1.5 V, and the capacity fading is so rapid that only 200 mA h g⁻¹ of the specific capacity can be used. However most of the capacities could be used at a voltage range of 0.7–0.005 V. The first plateau at a higher voltage can be assigned to a Faradic capacitance of the material,⁶¹ and the capacity lower than 0.5 V is associated with a gradual lithium storage process. This phenomenon is very common in most metal oxides with higher theoretical specific capacity. Recently, Lian et al.⁶² demonstrated that high-rate pre-lithiation and reactivation is an effective strategy to restructure active materials and optimize the SEI layer ex situ to obtain high-performance electrodes. By pre-lithiation and pre-reactivation, electrode materials with an optimized microstructure can be packaged into battery cells for practical applications with exceptional rate performance and cyclic stability as well as high energy density. Therefore, in future work, we will use this prospective strategy to design other high-performance electrodes.

Moreover, the cycling performance of the MoV₂O₈ nanorod electrode with a current density of 0.8 A g⁻¹ was also tested (Fig. 5f). It is obvious from the results that even at higher current density, the electrode still exhibits good cycling stability and the capacity fading is only 18% after 50 cycles. The long cycling performance was also performed at the current density of 0.8 A g⁻¹. As shown in Fig. S6,† an excellent cycling stability with a highly stable discharge capacity of about 887 mA h g⁻¹ after 200 cycles, was observed. To investigate the structural stability of the MoV₂O₈ nanorods after cycling, SEM analysis was performed (Fig. S7†). The SEM image of the MoV₂O₈ nanorod electrode after 50 cycles at a current density of 0.2 A g⁻¹ shows that the morphology of MoV₂O₈ was well preserved after the Li⁺ insertion/extraction processes. Certainly, some residual organic electrolyte or related species are also observed on the electrode surface, which can form a protective film on the surface. This result further verifies that the small-sized one-dimensional nanorods can accommodate the local volume change during repeated Li⁺ insertion/extraction.

4. Electrochemical reaction mechanism

To gain insight into the lithium storage mechanism of the MoV₂O₈ nanorods, we first performed ex situ XRD analyses during the lithiation/delithiation processes. Fig. S8† shows ex situ XRD patterns of the MoV₂O₈ nanorod electrode with the specified discharge and charge voltages during the 10th cycle between 0.005 and 3 V, which exhibit notable differences under different charge and discharge voltages. When the discharge voltage is 2.2 V, the electrode material has changed from MoV₂O₈ to Li₂MoO₄. However, from the enlarged XRD patterns (Fig. 6a), it can be observed that the main peaks corresponding to the Li₂MoO₄ phase shift to lower angles decreasing the discharge voltage, and return to their initial scattering angles during the increase of the charge voltage.

Fig. 6 (a) Enlarged XRD patterns of the MoV₂O₈ nanorods under different charge and discharge voltages after 10 cycles. (b) High resolution Mo XPS spectra and (c) high resolution V XPS spectra of the lithiated electrode (discharged to 0.005 V) and delithiated electrode (charged to 3.0 V) at the current density of 0.2 A g⁻¹. (d) TEM and (e) HRTEM images of the lithiated electrode (discharged to 0.005 V). (f) TEM and (g) HRTEM images of the delithiated electrode (charged to 3.0 V). (h) CV curves of the MoV₂O₈ nanorods at a scan rate of 0.3 mV s⁻¹ in the voltage range of 0.005–3.0 V.

To further clarify the electrochemical processes during Li intercalation, XPS measurements were also performed at different stages. For the electrode discharged to 0.005 V (Fig. 6b) at 0.2 A g⁻¹ (after 10 cycles), two strong peaks, corresponding to Mo 3d_5/2 and Mo 3d_3/2, can be deconvoluted into three components. Besides Mo⁶⁺ (235.8 and 232.6 eV), other weak peaks with binding energies of 233.3 eV (Mo_5/2), 229.8 eV (Mo_3/2) and 232.5 eV (Mo_5/2), 228.9 eV (Mo_3/2) can be assigned to Mo⁴⁺ and metallic Mo,^32,63 respectively, indicating that Mo exists partially in a low oxidation state after the insertion of Li⁺. The peak at 55.2 eV in the Li 1s XPS pattern indicates the formation of Li₂O (Fig. S9†).⁶⁴ On the basis of the above discussions, it can be concluded that Li₂MoO₄ can partially undergo a redox reaction and then decompose into Mo and Li₂O during the discharge process. However, the characteristic peaks of metal Mo and Li₂O were not detected through XRD, which can be attributed to the amorphous nature of a small amount of metallic Mo and Li₂O.^65,66 When the electrode is charged to 3.0 V, low oxidation state Mo is partially re-oxidized to a high oxidation state again during the subsequent delithiation process. Furthermore, the high-resolution V 2p spectrum at the end of the first discharge state confirms the existence of two components (Fig. 6c). The peaks with the binding energies of 516.6 eV and 523.8 eV can be assigned to V³⁺, while the other peaks with binding energies of 515.5 eV and 522.7 eV can be indexed to V²⁺. This result implies that V⁵⁺ has been reduced into V³⁺ and V²⁺ during the discharge process. However, when the same electrode is charged to 3.0 V, V³⁺ and V²⁺ are not fully converted to V⁵⁺, as shown in Fig. 6c.^26,67–70 Based on the XRD and XPS results, we deduce that the electrochemical reaction mechanism of the MoV₂O₈ nanorods as an anode in LIBs should involve three processes, a phase transformation, a Li intercalation–deintercalation reaction and a partial redox reaction.

In order to get more in-depth information on the electrochemical performance of the MoV₂O₈ nanorods, the phase and microstructure evolution of the MoV₂O₈ nanorod electrode at the 10th discharged–charged state were also investigated using TEM. Fig. 6d presents a low-magnification TEM image of the lithiated electrode (discharged to 0.005 V), which clearly shows the rod-like morphology of the active material. The HRTEM image (Fig. 6e) shows that the interplanar spacing of 0.237 nm is larger than that of the (122) planes of Li₂MoO₄ (0.23 nm),⁷¹ which can be ascribed to the Li intercalation into Li₂MoO₄. Besides, as the electrode is charged to 3.0 V, the morphology of the nanorods can also be observed (Fig. 6f). The HRTEM image in Fig. 6g shows that the interplanar spacing of 0.233 nm is smaller than the interplanar spacing of 0.237 nm, but is larger than that of the pure Li₂MoO₄ phase. The TEM result further verifies the Li intercalation–deintercalation mechanism. Due to the amorphous characteristic of metallic Mo, LiV₂O₅ and Li_1+xV₂O₅, the related XRD and TEM parameters cannot be measured. This phenomenon of amorphization during the intercalation has also been reported previously in vanadates.⁷²

CV is a powerful technique for studying redox couples and structural transformations for both cathode and anode materials during the charge–discharge process. CV curves of the MoV₂O₈ nanorods were carried out at a scan rate of 0.3 mV s⁻¹ in the voltage window of 0.005–3.0 V vs. Li/Li⁺. As shown in Fig. 6h, it can be clearly seen that four cathodic peaks and two anodic peaks in the first cycle are observed at 0.20, 0.75, 1.75, 2.20, and 1.25, 2.50 V, respectively. On the basis of ex situ XRD, TEM, and XPS studies, it can be concluded that the first-discharge reaction is a phase transformation process, forming a mixture of amorphous “LiV₂O₅” and an electrochemically active “Li₂MoO₄” phase (Table 1, eqn (1)).⁷³ The peaks at 1.75 and 0.2 V can be assigned to the irreversible formation of an SEI layer and these peaks disappear in the following cycles. The peak at 0.75 V can be attributed to the consecutive intercalation of lithium ions to form lithium-rich Li_1+xV₂O₅ (Table 1, eqn (3)). The peak at 1.25 V is caused by the Li deintercalation from lithium-rich Li_1+xV₂O₅ to LiV₂O₅ (Table 1, eqn (4)).^26,27,68,69 The peak at 2.2 V is responsible for the Li intercalation into Li₂MoO₄ causing the formation of partially lithium-rich Li_2+yMoO₄ (including metallic Mo) (Table 1, eqn (2)). The peak at 2.5 V is associated with Li deintercalation from partially lithium-rich Li_2+yMoO₄ to Li₂MoO₄ (Table 1, eqn (5)).^74,75

Table 1 Equations for electrochemical reactions with lithium

Number	Reaction equations	Voltage
1	MoV2O8 → Li2MoO4 + LiV2O5
2	Li2MoO4 + yLi^+ + ye^− → Li2+yMoO4	Discharge to 2.2 V
3	LiV2O5 + xLi^+ + xe^− → Li1+xV2O5	Discharge to 0.75 V
4	Li1+xV2O5 → LiV2O5 + xLi^+ + xe^−	Charge to 1.25 V
5	Li2+yMoO4 → Li2MoO4 + yLi^+ + ye^−	Charge to 2.5 V

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5. Conclusions

In summary, we have reported the successful preparation of MoV₂O₈ nanorods via a simple solvothermal method followed by a calcining treatment in air at 500 °C for 3 h. The nanorods have lengths of about 150–300 nm and thicknesses of about 20–30 nm. The resultant MoV₂O₈ nanorods were first evaluated as an anode material for LIBs, and excellent lithium storage performance was achieved. At a current density of 0.2 A g⁻¹, the MoV₂O₈ nanorod electrode could deliver a specific capacity of over 1325 mA h g⁻¹ after 50 cycles, which was much higher than those of the nanobelts and bulk. When the current density was increased to 0.8 A g⁻¹, the specific capacity of 969 mA h g⁻¹ could be retained after 50 cycles. Even when cycled at 5 and 10 A g⁻¹, comparable capacities of 680 and 570 mA h g⁻¹ could still be achieved, indicating a superior rate capability. Using ex situ XRD, XPS and TEM measurements, one plausible mechanism has been proposed, which involves phase transformation, intercalation–deintercalation and partial redox processes. We believed that the findings in the current work represent a significant step forward in the development of high-performance LIBs for large-scale energy storage systems.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21471016 and 21271023) and the 111 Project (B07012).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr05602k

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