Coupling interconnected MoO₃/WO₃ nanosheets with a graphene framework as a highly efficient anode for lithium-ion batteries

Abstract

A rationally assembled three-dimensional graphene framework coupled with interconnected molybdenum/tungsten oxide nanosheets (MoO₃/WO₃-GF) has been developed via a one-step template-free strategy. With the unique nanostructure, the obtained anode material not only exhibits a high reversible capacity of about 1000 mA h g⁻¹, approaching the theoretical capacity of MoO₃ and WO₃ materials, but also shows excellent rate capability and cycling performance with negligible capacity attenuation after a long-time test. These features make it a promising candidate material for high-performance commercial lithium-ion batteries in the future.

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Introduction

Lithium-ion batteries (LIBs) with a high working voltage, long cycling life,¹ and low toxicity have attracted increasing attention in advanced electronic devices and electric vehicles.² As a commonly-used anode material in commercial LIBs, graphite possesses a low capacity of 372 mA h g⁻¹ which could not meet the ever-growing energy demands. Therefore, designing and developing favorable electrodes with high capacities and stabilities for LIBs is of great significance for practical applications.

Owing to their high theoretical reversible capacities, transition metal oxides, such as CuO,³ Co₃O₄,⁴ TiO₂,⁵ and Fe₃O₄,⁶ have been developed and applied in LIBs. Among them, molybdenum trioxide (MoO₃) has received much attention in recent years, which is regarded to be one of the promising candidates for LIBs because of its extremely high theoretical specific capacity of 1111 mA h g⁻¹,⁷ excellent chemical stability, environmental friendliness and safety.⁸ Nevertheless, MoO₃ has limited application in high-performance LIBs due to its inherent low electronic conductivity and poor structural integrity.⁹ A great effort has been devoted to improving the energy storage performance of MoO₃, including synthesizing MoO₃ materials with various nanostructures, such as nanoparticles,¹⁰ nanobelts,¹¹ nanowires,¹² nanosheets,¹³ and nanorods,¹⁴ and modulating the electron configuration of MoO₃ through the introduction of heteroatoms, metals or metal oxides into the matrix.^15–18 Tungsten (W), with a similar ionic radius to that of molybdenum, is regarded as an efficient candidate to incorporate into the molybdenum lattice¹⁹ and thus to promote the conductivity of the materials because of the relatively high conductivity of the tungsten species.²⁰ A lot of work has been carried out to introduce W into molybdenum oxide materials, and indeed the enhancement of LIB performances has been achieved.^21,22 However, these studies failed to finely control the morphology of the materials, which is believed to be one of the most important factors for anode materials.²³ Thus, it is still a challenge to achieve high-capacity electrode materials with a suitable nanostructure and satisfactory stability.

As a new emerging carbon material, graphene with a high specific surface area, outstanding electrical conductivity, and excellent mechanical/chemical stabilities^24–26 can be introduced into a system to solve this problem. On the one hand, graphene sheets are easily arranged into specific interconnected architectures, which greatly facilitate the electron transfer within electrode materials²⁷ and the ionic migration in the electrolyte.²⁸ On the other hand, graphene can suppress the volume change and agglomeration of the active materials during the charge–discharge process.²⁹ Although many efforts have been made to synthesize MoO₃ or WO₃/graphene hybrids,^30–34 little attention has been paid to combining and assembling these materials together into specific nanostructured architectures for a highly efficient LIB performance.

In this work, we have developed a highly efficient anode material for LIBs based on the rational assembly of interconnected MoO₃/WO₃ nanosheets on 2D graphene sheets to form a 3D hierarchical framework (MoO₃/WO₃-GF) via a straightforward one-step hydrothermal method. With the merits of the hierarchical nanostructure, the as-prepared anode shows a high reversible capacity of about 1000 mA h g⁻¹, good cycling stability and outstanding rate capability, demonstrating its great potential for advanced lithium-ion storage devices.

Results and discussion

The fabrication process of the MoO₃/WO₃-GF is presented in Scheme 1. We first produced a well-dispersed suspension by the ultrasonic mixing of (NH₄)₆Mo₇O₂₄·4H₂O, Na₂WO₄·2H₂O and GO in water. After adjusting the pH value of the solution to 1 with HCl, a brown suspension was obtained with continuous ultrasonic stirring for 30 min due to the hydrolysis of Mo₇O₂₄⁶⁻ and WO₄²⁻. It has been reported by Meng et al.³⁵ that metal ions can act as linkers for the pre-assembly of GO nanosheets through metal–oxygen covalent or electrostatic interactions between the metal ion and the hydrophilic groups (such as –OH and –COOH groups) of GO sheets, thus leading to the formation of a well-distributed metal-based nanostructure within a graphene hydrogel. Similarly, the intermediate hydrolyzate of Mo₇O₂₄⁶⁻ and WO₄²⁻ could coordinate to the oxygen groups of GO sheets because of the unsaturated coordinated d orbitals of Mo and W, promoting the formation of a coupling structure of metal species and the gelation process of GO. To form the MoO₃/WO₃-GF, the resulting suspension was then hydrothermally treated in a 40 mL Teflon-lined autoclave at 180 °C for 10 h. After the hydrothermal treatment, GO sheets were reduced to graphene and self-assembled into a 3D framework driven by the π–π stacking interactions of graphene sheets. Finally, the freeze-dried MoO₃/WO₃-GF was calcined at 400 °C in Ar for 2 h to remove unstable components and increase the conductivity of graphene.

Scheme 1 Schematic illustration of the fabrication process of MoO₃/WO₃-GF.

As shown in Fig. 1a, the as-prepared MoO₃/WO₃-GF exhibits a typical 3D interconnected macroscopic structure consistent with those commonly reported 3D graphene materials,³⁶ which provides favorable electron/mass transport pathways and prompts the electrochemical process. The enlarged scanning electron microscopy (SEM) image in Fig. 1b demonstrates that the MoO₃/WO₃-GF has a relatively smooth surface and loose structures without any visible aggregates, indicating that the interconnected MoO₃/WO₃ hybrids are uniformly distributed on graphene sheets. This is further confirmed by the following transmission electron microscopy (TEM). Fig. 1c shows that the unique hybrid structure consists of a large number of small MoO₃/WO₃ nanosheets which are randomly coated on the graphene basal plane. The high-resolution TEM (HRTEM) image shows that the MoO₃/WO₃ nanosheets with an average diameter of ∼15 nm are interconnected with each other on graphene sheets (Fig. 1d).

Fig. 1 The morphology characterization of the as-prepared MoO₃/WO₃-GF. (a, b) SEM images of MoO₃/WO₃-GF. (c) TEM image and (d) the typical HRTEM image of the MoO₃/WO₃-GF. The corresponding enlarged images in the inset of (d).

To investigate the formation of interconnected MoO₃/WO₃ nanosheets on graphene sheets, we performed a series of comparative experiments. In the absence of the tungsten species, only a 3D graphene framework with large MoO₃ microrods instead of nanosheets was obtained (Fig. S1a and 1b, ESI†), and if without the Mo species, irregular and disordered structures were observed which were randomly distributed on graphene sheets (Fig. S1c and d, ESI†). Besides, the interconnected MoO₃/WO₃ nanosheets were also not produced if the GO suspension was absent during the hydrothermal process. Instead, large thick bulk-like MoO₃/WO₃ composites were obtained (Fig. S1e and f, ESI†). Interestingly, the content ratio of MoO₃versus WO₃ is also important for the formation of interconnected MoO₃/WO₃ nanosheets. As shown in Fig. S2,† when a small amount of Na₂WO₄·2H₂O was added into the original suspension (the weight ratio of (NH₄)₆Mo₇O₂₄·4H₂O to Na₂WO₄·2H₂O is 2/1), large MoO₃/WO₃ sheets were obtained (named MoO₃/WO₃-GF-2/1). While with the continuous increase of the amount of Na₂WO₄·2H₂O (the weight ratio of (NH₄)₆Mo₇O₂₄·4H₂O to Na₂WO₄·2H₂O is 1/2), both the MoO₃/WO₃ aggregates and their randomly distributed nanosheets were formed on graphene sheets (named MoO₃/WO₃-GF-1/2).

Based on the above experimental results, the formation mechanism was proposed. In HCl solution (pH = 1), the hydrolytic WO₄²⁻ is more stable than Mo₇O₂₄⁶⁻, which could first nucleate at the residue oxygen groups, defects or folds of GO nanosheets (Scheme 1a and b) to effectively minimize the interface energy barrier between the solid surface and liquid solution. This indicates that WO₄²⁻ species dominate the amount of nucleation and growth of nanocrystals. Meanwhile, the free Mo₇O₂₄⁶⁻ could then be coupled to the WO₄²⁻ to form mixed crystal seeds on graphene sheets because of the similar radius of Mo⁶⁺ (0.059 nm) and W⁶⁺ (0.060 nm) during the hydrothermal treatment, thus leading to the efficient oriented growth of interconnected MoO₃/WO₃ nanosheets on graphene basal planes (Scheme 1c). The production of larger amounts of MoO₃/WO₃-GF is possible by scale up of the autoclave (Fig. S3a†) and the yield of MoO₃/WO₃-GF is 62% compared with the overall raw material.

The X-ray diffraction (XRD) pattern of the MoO₃/WO₃-GF is investigated in comparison with MoO₃/WO₃, MoO₃-GF and WO₃-GF. As shown in Fig. 2a and Fig. S3b,† the hydrothermally-formed MoO₃/WO₃ composite exhibits a series of typical diffraction peaks, in which the peaks at 22.6°, 26.2° and 32.9° correspond to ε-MoO₃ (JCPDS no. 09-0209) and the peaks at 22.8°, 24.0°, 28.2° and 46.5° belong to tetragonal WO₃ (JCPDS no. 05-0388), consistent with the previously reported work.³⁷ Similar to the diffraction peaks of the MoO₃/WO₃ composite, the as-prepared MoO₃/WO₃-GF exhibits weak broad and coarse peaks, indicating the formation of amorphous or mixed crystals. In addition, no obvious peak related to the graphene is observed in the MoO₃/WO₃-GF, which is probably attributed to the low restacking of graphene sheets³⁸ and the coverage of metal oxides.³⁹ However, unlike the reflections of MoO₃/WO₃ and MoO₃/WO₃-GF, the initial MoO₃-GF and WO₃-GF exhibit totally different narrow diffraction peaks (Fig. S4, ESI†), in which the peaks of MoO₃ in MoO₃-GF are indexed to orthorhombic MoO₃ (JCPDS no. 05-0508), while the peaks of WO₃ in WO₃-GF belong to the hexagonal WO₃ (JCPDS card no. 33-1387). These results imply that the co-existence of Mo and W has an impact on each other during the formation of metal oxides because of the similar ionic radius of Mo and W, and the high lattice matching of MoO₃ and WO₃ with similar crystalline structures.^19,40 The elemental mapping exhibits the well-distributed C, O, W and Mo elements over the MoO₃/WO₃-GF (Fig. 2b and c), indicating that the MoO₃/WO₃ hybrids are successfully formed and uniformly attached to graphene sheets. Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability and determine the composition of MoO₃/WO₃-GF (Fig. S5, ESI†). The weight loss of less than 1% at temperatures of up to 200 °C is attributed to the removal of physically adsorbed water. The fast mass loss of ∼23% from 300 to 550 °C can be ascribed to the combustion of the graphene nanosheets and a weight loss at a temperature above 700 °C corresponds to the evaporation of MoO₃ species. Therefore, the mass fraction of MoO₃/WO₃ is calculated to be ∼76%. Besides, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was also conducted, which reveals the molybdenum and tungsten elemental contents of 26.9% and 24.2%, respectively. Therefore, the corresponding mass fractions of MoO₃, WO₃ and graphene were determined to be 40.5%, 30.5% and 29%, respectively.

Fig. 2 (a) XRD patterns of MoO₃/WO₃-GF and MoO₃/WO₃ composites. (b) The SEM image of MoO₃/WO₃-GF and (c) the corresponding C-, O-, W-, and Mo-elemental mappings.

To further investigate the covalent state of the elements and the oxidation states of the samples, we conducted XPS measurement. As expected, the X-ray photoelectron spectroscopy (XPS) in Fig. 3a shows a series of pronounced Mo- and W-related peaks along with the obvious C 1s and O 1s peaks. The atomic ratio of Mo and W for the MoO₃/WO₃-GF is calculated to be 1/0.6. The two typical Mo peaks located at 232.5 and 235.8 eV are observed in the high-resolution Mo 3d spectrum, which belong to the binding energies of the Mo(VI) 3d_3/2 and 3d_5/2 electrons, respectively (Fig. 3b). The high-resolution W 4f spectrum (Fig. 3c) reveals two peaks at 35.8 and 37.9 eV, which could be ascribed to W 4f_7/2 and 4f_5/2 of the W(VI) oxide state in agreement with the previous work.¹⁹ Besides, the high-resolution C 1s spectrum (Fig. 3d) exhibits four typical peaks including C–C (284.75 eV), C–O–C (C–O–H) (286.26 eV), C=O (287.78 eV), and O=C–OH (289.34 eV). The nitrogen adsorption–desorption isotherm obtained from the Brunauer–Emmett–Teller (BET) analysis (Fig. S6, ESI†) reveals the mesoporous structure of MoO₃/WO₃-GF with a specific surface area of 46.25 m² g⁻¹, a pore volume of 0.061 cm³ g⁻¹, and an average pore diameter of 3.79 nm, which could facilitate the lithium ion and electron transportation.

Fig. 3 (a) XPS spectrum of the as-prepared MoO₃/WO₃-GF, and the corresponding high-resolution (b) Mo 3d, (c) W4f, and (d) C 1s peaks.

The MoO₃/WO₃-GF with unique interconnected nanostructures is expected to be an efficient anode material in LIBs. Fig. 4a shows the discharge/charge profiles of the MoO₃/WO₃-GF at 200 mA g⁻¹ over the potential range of 3.0–0.001 V (versus Li⁺/Li). As can be seen, an initial discharge capacity reaches ∼1262.9 mA h g⁻¹. It remains at a relatively high level of ∼999.6 mA h g⁻¹ over 50 cycles, which is far higher than their corresponding theoretical capacity of ∼860 mA h g⁻¹ (calculated based on the ICP-AES results), indicating the synergistic effect between MoO₃/WO₃ and graphene that would benefit the enhancement of electrochemical performance. A capacity loss of 24% for the first cycle is observed because of the trapping of some lithium ions in the MoO₃/WO₃ lattice and the formation of a solid electrolyte interphase (SEI) layer.^41,42 Nevertheless, the steady capacity of MoO₃/WO₃-GF is superior to that of a commercial graphite electrode (372 mA h g⁻¹). This is also further confirmed by its cycling stability and coulombic efficiency (Fig. 4b). As shown in Fig. 4b, the as-prepared MoO₃/WO₃-GF has a reversible capacity of 951.6 mA h g⁻¹ over 100 cycles with a coulombic efficiency of 99.2%. Except for the irreversible capacity loss in the initial cycle, no obvious capacity fading can be observed in the subsequent discharge/charge cycles, suggesting an excellent cycling performance for the MoO₃/WO₃-GF. More importantly, the MoO₃/WO₃-GF also exhibits an outstanding rate capability (Fig. 4c), which delivers a series of reversible discharge capacities of 1027.8, 972.3, and 889.1 mA h g⁻¹ at 100, 200, and 500 mA g⁻¹, respectively. Interestingly, the discharge capacity of MoO₃/WO₃-GF can reach a stable value quickly and maintain a high reversible capacity of 816.3 mA h g⁻¹ even at a high current density (1000 mA g⁻¹), indicating the good discharge/charge stability. The discharge capacity recovers to its initial value when the current density goes back to 100 mA g⁻¹. For comparison, the cycling performances of the MoO₃/WO₃-GF, MoO₃-GF, WO₃-GF, MoO₃/WO₃ and GF are also investigated at 200 mA g⁻¹ (Fig. 4d). Unlike the counterparts, for which the capacities of these LIBs have sharp declines in the first 5 cycles, the MoO₃/WO₃-GF exhibits a stable reversible capacity with no obvious decrease from the second cycle, indicating the remarkable retention of capacity.

Fig. 4 (a) The discharge/charge curves of MoO₃/WO₃-GF at 200 mA g⁻¹ over the potential range of 3.0–0.001 V (vs. Li⁺/Li). (b) Cycling performance and coulombic efficiency of MoO₃/WO₃-GF at 200 mA g⁻¹. (c) Rate performances of MoO₃/WO₃-GF at increasing current densities of 100, 200, 500 and 1000 mA g⁻¹ and then stepwise back to 100 mA g⁻¹, and (d) the cycling performance of the MoO₃/WO₃-GF, MoO₃-GF, WO₃-GF, MoO₃/WO₃ and GF, respectively.

Cyclic voltammetry (CV) of the MoO₃/WO₃-GF anode was conducted at a scan rate of 0.2 mV s⁻¹ over the potential range of 3.0–0 V (versus Li⁺/Li) to inspect the electrochemical Li-ion storage during the discharging/charging process (Fig. 5a). An inconspicuous redox peak at ∼0.7 V is observed in the first discharge cycle and disappeared in the following cycles, which can be attributed to the irreversible decomposition of the electrolyte and the formation of the SEI layer. In comparison with the graphene reported previously,⁴³ the reversible peaks between 1.2 and 1.7 V are probably due to the Li⁺ ion insertion/extraction process in the lattice of MoO₃/WO₃ nanosheets. These dominant peaks indicate the available active storage sites which contribute to the high capacity. In addition, the CV curves remain almost unchanged from the second cycle, indicating that the highly reversible Li ion insertion/extraction of the MoO₃/WO₃-GF could be achieved quickly. In fact, the different MoO₃/WO₃ ratios on graphene sheets (such as MoO₃/WO₃-GF-2/1 and MoO₃/WO₃-GF-1/2) can also be a factor to affect the battery performances (Fig. S7, ESI†). For example, both MoO₃/WO₃-GF-2/1 and MoO₃/WO₃-GF-1/2 electrodes show a sharp capacity loss of 31% and 43% for the first five cycles, respectively (see the detailed discussion in the ESI†). Moreover, the reversible discharge capacities of MoO₃/WO₃-GF-2/1 and MoO₃/WO₃-GF-1/2 are 619.8 and 393.1 mA h g⁻¹ at 200 mA g⁻¹ over 30 cycles, respectively, which are far lower than the as-prepared MoO₃/WO₃-GF material (Fig. 4b). This is probably ascribed to the large MoO₃/WO₃ crystal size and disordered structures, thus leading to decreased Li-ion storage sites.

Fig. 5 (a) CV curves of MoO₃/WO₃-GF at a scan rate of 0.2 mV s⁻¹, and (b) the EIS of MoO₃/WO₃-GF, MoO₃-GF, WO₃-GF and MoO₃/WO₃ before cycling.

The charge storage mechanism can be characterized by the sweep voltammetry method.⁴⁴ We assume that the relationship between the current (i) and scan rate (v) obeys the power law:^45,46

i = av^b (1)

where a and b are adjustable parameters. A b value of 0.5 indicates an ionic diffusion-controlled process, whereas if the b is equal to 1, the electrochemical system is controlled by pseudocapacitance. Consequently, the b value of MoO₃/WO₃-GF obtained by a plot of log (peak current, i) versus log (sweep rate, v) from 0.2 to 1 mV s⁻¹ is calculated to be 0.9 (Fig. S8a and b, ESI†). It suggests the existence of pseudocapacitive behavior in the redox processes of MoO₃/WO₃-GF, which leads to a fast Li⁺ intercalation/extraction process.

The overall pseudocapacitive contribution proportion can be calculated by the following equation^47–49

i(V) = k₁v + k₂v^1/2 (2)

For analytical purposes, eqn (2) can be changed into

i(V)/v^1/2 = k₁v^1/2 + k₂ (3)

where k₁v and k₂v^1/2 stand for the pseudocapacitive and inserted contributions, respectively. The pseudocapacitive contributions are 40% and 60.4% at a scan rate of 0.2 and 1 mV s⁻¹, respectively (Fig. S8c and d, ESI†). This result suggests that the pseudocapacitive charge-storage behavior exhibits an increasing advantage in the whole capacity with the increase of scan rate.

In order to further understand the kinetic process, the electrochemical impedance spectra (EIS) of MoO₃/WO₃-GF, MoO₃-GF, WO₃-GF and MoO₃/WO₃ were obtained in the frequency range from 10⁻² Hz to 10⁶ Hz before cycling. Fig. 5b shows the Nyquist plots of the samples. The MoO₃/WO₃-GF possesses the smallest semicircle diameter among all the compared samples in the high- and medium-frequency regions, indicating the low resistance of the electrolyte and the fast charge transfer reaction for the MoO₃/WO₃-GF anode. The slope lines in the low frequency region reflect the Warburg impedance of the Li-ion diffusion. Interestingly, a significantly decreased semicircle in the EIS spectrum can be observed over 100 cycles (Fig. S9, ESI†), indicating a decrement of charge transfer resistance during cycling. The improved electrochemical kinetics is probably attributed to an effective activation between the electrode material and electrolyte, and gradual conversion of MoO₃/WO₃ from the crystalline to the amorphous state during the Li ion insertion/extraction process,^16,50 which has been regarded to be beneficial for the battery performance.⁵¹ No changes in the morphologies of the MoO₃/WO₃ nanosheets on graphene sheets are observed after 100 cycles (Fig. S10, ESI†), demonstrating the good cycling performance of MoO₃/WO₃-GF with high structural stability. The good battery performance of MoO₃/WO₃-GF can be attributed to the following aspects: (1) the unique interconnected MoO₃/WO₃ nanosheet structures evenly anchored on the graphene basal plane provide plenty of Li-ion storage sites and increase the accessible reactive surface area, thus leading to an enhanced electrochemical process. (2) It is reported that the MoO₃/WO₃ sheets have a relatively higher conductivity than other types of transition-metal oxides,²⁰ which can serve as an efficient electron transport pathway. After coupling with graphene, the as-prepared anode material offers small internal resistances and a fast charge flow to achieve a high-rate discharging/charging performance. (3) The porous graphene framework with high chemical/mechanical stability buffers the volume variation during the lithiation/delithiation process which overcomes the usual structural collapse of bulk materials, thus demonstrating excellent Li-storage stability.

In summary, a well-designed MoO₃/WO₃-GF material has been successfully synthesized through a facile hydrothermal approach, in which the interconnected MoO₃/WO₃ nanosheets are evenly dispersed on graphene sheets. With the merits of the hierarchical nanostructure, the resulting material can be directly used as the anode for LIBs which exhibits high specific capacity, good cycling stability and excellent rate capability. The enhanced electrochemical performances can be ascribed to the synergistic effect of the nanostructured MoO₃/WO₃ hybrids and the highly conductive graphene framework.

Experimental

Synthesis of MoO₃/WO₃-GF

Graphite oxide (GO) was synthesized by a modified Hummers’ method.⁵² To synthesize the MoO₃/WO₃-GF, 0.25 g ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) and 0.23 g sodium tungstate dihydrate (Na₂WO₄·2H₂O) were firstly dissolved in 30 mL deionized water, followed by the addition of graphite oxide (120 mg). The pH of the resulting suspension was then regulated to be ∼1 by adding HCl solution, which was then transferred into a 40 mL Teflon-line autoclave and treated at 180 °C for 10 h after ultrasonic stirring for 30 min. After washing with deionized water several times, the MoO₃/WO₃-GF was finally obtained by annealing the freeze-dried sample at 400 °C with a ramp of 1 °C min⁻¹ for 2 h under an argon atmosphere. The yield of the as-prepared MoO₃/WO₃-GF determined turned out to be about 62% by calculating the weight ratio of the final product (∼0.37 g) to the raw material (∼0.6 g). Besides, MoO₃/WO₃-GF-2/1, MoO₃/WO₃-GF-1/2, MoO₃-GF, WO₃-GF, MoO₃/WO₃ and GF were also prepared under the same conditions for comparison.

Characterization

The morphology of the sample was detected by scanning (SEM, JSM-7500F) and transmission electron microscopies (TEM, JEM, 2100). The elemental mappings were carried out on a SEM unit with a high-angle annular dark-field (HAADF) detector (HITACHI S-5500) operating at 30 kV. X-ray diffraction (XRD) patterns were obtained by using a Netherlands 1710 diffractometer with a Cu Kα irradiation source (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) was performed with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. The base pressure was about 3 × 10⁻⁹ mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. The BET specific surface area was determined by nitrogen adsorption–desorption isotherm measurements at 77 K (NOVA 2200e). Before the adsorption measurements, the samples were degassed under vacuum at 573 K for 3 h. Thermogravimetric analysis (TGA) was carried out by using a thermobalance (TGA2050) from room temperature to 1000 °C at a rate of 10 °C min⁻¹ under a continuous air flow. The element proportion was determined based on Optima 8300 Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, PerkinElmer).

Electrochemical measurements

Electrochemical performance measurements were carried out using standard CR2025 coin cells at room temperature. Anode materials were prepared by mixing active materials (MoO₃/WO₃-GF, MoO₃-GF, WO₃-GF, MoO₃/WO₃ and GF), acetylene black (conducting agent) and polyvinylidene fluoride (PVDF, binder) in a weight ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidinone (NMP) to form a homogeneous slurry. Then, the slurry was uniformly pasted on a copper foil (current collector) and dried in a vacuum oven at 120 °C overnight. The loading density of the MoO₃/WO₃-GF on the current collector is calculated to be approximately 1.3 mg cm⁻². Testing cells were assembled in an Ar-filled glove box using Li foils as both counter electrodes and reference electrodes. Celgard 2400 membranes were used as separators and 1 M LiPF₆ solution in a mixture of ethylene carbonate and dimethyl carbonate (EC/DMC, 1 : 1 v/v) as the electrolyte. Galvanostatic charge–discharge cycling of all the assembled half-cells was carried out in the potential range of 0.001–3.0 V (vs. Li/Li⁺) using the Neware battery tester. Cyclic voltammetry (CV) at a scan rate of 0.2 mV s⁻¹ over the potential range of 0–3 V (vs. Li/Li⁺) and electrochemical impedance spectra (EIS) in a range of 0.01 Hz to 1 M Hz were measured on a 660E electrochemical workstation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are thankful for the financial support from the National Key R&D Program of China (2017YFB1104300), NSFC (21604003, 21325415 and 51673026), the Beijing Natural Science Foundation (2164070, 2152028), the Beijing Municipal Science and Technology Commission (Z161100002116022), and 111 Project 807012.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr07849h

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