Enhanced electrochemical performances of MoO₂ nanoparticles composited with carbon nanotubes for lithium-ion battery anodes

Abstract

The nanocomposites of carbon nanotubes (CNTs) with homogenously anchored molybdenum dioxide (MoO₂) nanoparticles of 20–50 nm have been successfully synthesized by a hydrothermal method. Glucose has dual functions, i.e., reducing agent and surfactant to prohibit the anisotropic growth, resulting in the direct deposition of MoO₂ nanoparticles on the CNT surfaces. As the anode of lithium-ion batteries (LIBs), the as-prepared MoO₂/CNT nanocomposites deliver a higher reversible capacity of 640 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles, compared to only 246 and 259 mA h g⁻¹ for MoO₃ nanobelts/CNT composites and MoO₂ nanoparticles, respectively. The superior electrochemical performances are attributed to the nanocomposite structure of MoO₂ nanoparticles anchored on CNTs, which have efficiently enhanced the electrical conductivity and lithium ion diffusion, and maintained the integrity of electrode during the charge/discharge processes.

---

1. Introduction

Recently, many efforts have been devoted to exploring novel electrode materials for the application of ever-growing high-power density lithium ion batteries (LIBs). As potential anode materials, transition metal oxides including Fe₃O₄,¹ Fe₂O₃,² Co₃O₄,³ NiO,⁴ Mn₃O₄,⁵ and MoO₃,⁶ have shown higher capacities than that of the commercially employed graphite anode (specific capacity of 372 mA h g⁻¹). Among them, molybdenum oxides such as MoO₃(ref. 6–8) and MoO₂ (ref. 9–11) have attracted extensive attention due to their high theoretical capacities (1117 mA h g⁻¹ for MoO₃, and 838 mA h g⁻¹ for MoO₂), high electrochemical activity, suitable lithium reaction voltage, and affordable cost. However, most of the transition metal oxides serving as anode materials for LIBs usually suffer from large volume expansion/shrinkage and pulverization during the lithiation/delithiation processes, which will lead to the destruction of the anode structure and thereby rapidly degrade the electrochemical performances.^9,12

To maintain the integrity of anode structure, the employment of nano-structured metal oxides is an efficient approach to relieve the stress–strain generated during the lithiation/delithiation processes. It also shortens the pathway for Li-ion diffusion and electron transport, and provides more contact area between anode material and electrolyte, leading to a higher energy density.^6,13,14 For example, Lee et al.⁷ have reported that MoO₃ nanocrystallites prepared by a chemical vapor deposition method exhibited a reversible capacity of 630 mA h g⁻¹ after 150 cycles at the current rate of 1/2C, while MoO₃ particles with the size of 5 μm displayed a rapid capacity loss with cycling. Core–shell MoO₂ hierarchical microcapsules have been synthesized by a template-free solvothermal method.⁹ The MoO₂ microcapsule anode delivered a reversible capacity of 623.8 mA h g⁻¹ after 50 cycles at a current rate of 1C, while the anode made of commercial MoO₂ powders showed a rapid capacity fading and only maintained the initial capacity of 35%, indicating that the nanostructured molybdenum oxides exhibited superior lithium storage capability.

The combination of metal oxides with various carbon materials, such as amorphous carbon,^15–19 graphene,^20,21 nanofibers,^22,23 and nanotubes,^24–26 is another strategy to improve the electric conductivity of anode and to accommodate the strain of volume change as a buffer, both are beneficial to enhance the electrochemical performances of LIBs. Among carbon materials, carbon nanotubes (CNTs) have attracted wide attentions as additives in constructing hybrid nanocomposites due to their superior electrical conductivity, high surface-to-volume ratio, ultrathin walls, and structural flexibility.^27–32 For example, the composites of CNTs with MoO₃ nanobelts showed a reversible capacity of 421 mA h g⁻¹ at the current density of 200 mA g⁻¹ after 100 cycles, and delivered 293 and 202 mA h g⁻¹ at current densities of 2 and 4 A g⁻¹, respectively.³¹ Inspired by the above researches, constructing the nanocomposites of MoO₂ nanoparticles anchored on CNTs will be more beneficial to its LIBs application since CNTs can be intertwined to form unique conductive networks, which provide continuous pathways for electron transport, and MoO₂ nanoparticles anchored on CNTs will supply high capacity. In addition, the CNTs with MoO₂ nanoparticles deposited on surface can serve as the buffer to prevent the aggregation and pulverization of MoO₂ nanoparticles during the lithiation/delithiation processes, and thereby maintain the integrity of electrode structure.

In this work, MoO₂/CNTs nanocomposites have been synthesized by a facile hydrothermal approach with the assistance of glucose, which was employed as the reducing agent and the structure directing surfactant. The electrochemical measurements demonstrate that the as-synthesized MoO₂/CNTs nanocomposite exhibits superior cycling and rate performances than the MoO₂ nanoparticles and the composite of CNTs with MoO₃ nanobelts.

2. Experimental

2.1. Materials

Sodium molybdate dihydrate (Na₂MoO₄·2H₂O), D-glucose (C₆H₁₂O₆), hydrochloric acid solution (HCl, 37 wt%), concentrated nitric acid (HNO₃, 68 wt%) and CNTs were purchased from Sinopharm Chemical Reagent Co., Ltd. Carbon black, Li foil and Celgard 2300 were provided by Hefei Kejing Material Technology Co., Ltd, China. Polyvinylidene fluoride (PVDF), LiPF₆ (dissolved in ethylene carbonate, dimethyl carbonate, and ethylene methyl carbonate with a volume ratio of 1 : 1 : 1) were purchased from Shenzhen Biyuan Technology Co., Ltd, China. All the chemicals were analytical grade and were used as-received without further purification.

2.2. Preparation of MoO₂/CNTs nanocomposite, MoO₃ nanobelts/CNTs composite and MoO₂ nanoparticles

Firstly, the purchased CNTs were treated by nitric acid at 140 °C for 6 h in a Teflon-lined stainless steel autoclave. After that, the acid-treated CNTs were centrifuged and washed with deionized water and ethanol several times until the pH value to about 7, and then dried under vacuum at 60 °C for 24 h. The MoO₂/CNTs nanocomposites were synthesized as follows. Typically, 0.02 g CNTs were dispersed in 10 mL deionized water under ultrasonic treatment for 1 h, and then 0.2 mmol Na₂MoO₄·2H₂O and 0.05 g glucose were added into the suspension, followed by the dropwise addition of 1 mL HCl (3 M) under magnetic stirring. Thirty minutes later, the mixture was transferred into a 30 mL Teflon-lined stainless steel autoclave, and subsequently sealed and heated at 180 °C for 12 h in an oven. The black precipitate was collected by centrifugation and washed with deionized water and ethanol several times, and then dried in vacuum overnight. The preparation procedures of the composites of CNTs with MoO₃ nanobelts and MoO₂ nanoparticles were the same as those of the MoO₂/CNTs nanocomposites only without adding glucose or CNTs in the hydrothermal process, respectively.

2.3. Characterizations

The structure of products was determined by X-ray powder diffraction (XRD) on a Rigaku D/Max-RC X-ray diffractometer with Ni filtered Cu Kα radiation (λ = 0.1542 nm, V = 40 kV, I = 40 mA) in the range of 10–80° at a scanning rate of 4° min⁻¹. A JSM-6700F field emission scanning electron microscope (FE-SEM) at an accelerating voltage of 20 kV and an electric current of 1.0 × 10⁻¹⁰ A; and A JEOL JEM-2100 high-resolution transmission electron microscopy (HR-TEM) with an accelerating voltage of 200 kV were utilized to examine the morphology and microstructure. X-ray photoelectron spectra (XPS) were recorded on a Kratos Analytical spectrometer, using Al Kα (hν = 1486.6 eV) radiation as the excitation source, under a condition of anode voltage of 12 kV and an emission current of 10 mA. Thermal gravimetric analysis (TGA) measurement was carried out in an SDT Q600 (TA Instruments Ltd., New Castle, DE) thermal-microbalance apparatus at a heating rate of 10 °C min⁻¹ in an air atmosphere from ambient temperature to 700 °C to evaluate carbon content in the products.

2.4. Electrochemical measurements

To prepare the working electrode, the homogenous slurry was formed by dispersing the as-prepared active material, carbon black, and polyvinylidene fluoride (PVDF) with a weight ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidinone (NMP). The slurry was coated on a copper foil substrate, followed by drying in a vacuum oven at 120 °C for 12 h. The CR2025-type cells were assembled in a glovebox filled with argon atmosphere using Li foil as counter electrode, Celgard 2300 as separator, and 1 M LiPF₆ (dissolved in ethylene carbonate, dimethyl carbonate, and ethylene methyl carbonate with a volume ratio of 1 : 1 : 1) as electrolyte. As the active material, the typical mass loading of MoO₂/CNTs, MoO₃ nanobelts/CNTs and MoO₂ on the working electrodes was 1.5–2.0 mg cm⁻². The performance of the cells was evaluated galvanostatically in the voltage range from 0.02 to 3 V at various current densities on a LAND CT2001A battery test system. Cyclic voltammogram (CV) was obtained by an IviumStat electrochemistry workstation at a scan rate of 0.3 mV s⁻¹ and the potential vs. Li/Li⁺ ranging from 0.01 to 3 V. The electrochemical impedance spectrum (EIS) was obtained on the same instrument with AC signal amplitude of 10 mV in the frequency range from 100 kHz to 0.01 Hz. The data were adopted to draw Nyquist plots using real part Z′ as X axis, and imaginary part Z′′ as Y axis.

3. Results and discussion

3.1. Characterizations of MoO₂/CNTs nanocomposites, MoO₃ nanobelts/CNTs composites and MoO₂ nanoparticles

From the XRD pattern of MoO₂/CNTs nanocomposite shown in Fig. 1a, all diffraction peaks can be assigned to monoclinic MoO₂ (JCPDS 65-5787), indicating the reduction of Mo⁶⁺ to Mo⁴⁺ by glucose during the hydrothermal process. For the composites of CNTs and MoO₃ nanobelts (Fig. 1b) fabricated without the addition of glucose, all the recorded peaks could be ascribed to those of orthorhombic MoO₃ (JCPDS 05-0508), and no diffraction peaks of MoO₂ or other molybdenum oxides were observed. The intensity of (020), (040) and (060) peaks is stronger than the standard diffraction data of MoO₃ (JCPDS 05-0508), indicating the anisotropic growth of MoO₃ nanobelts. Fig. 1c displays the XRD pattern of MoO₂ nanoparticles prepared without the addition of CNTs. All diffraction peaks match well with the monoclinic MoO₂ (JCPDS 65-5787), also confirming that Mo⁶⁺ has been reduced to Mo⁴⁺ by glucose in the hydrothermal reaction.

Fig. 1 XRD patterns of (a) MoO₂/CNTs nanocomposite, (b) MoO₃ nanobelts/CNTs composite, and (c) MoO₂ nanoparticles.

The FE-SEM image (Fig. 2a) of MoO₂/CNTs nanocomposites shows that the surface of CNTs is smooth and no aggregated MoO₂ particles are observed except for CNTs. However, from the high-magnification image (Fig. 2b), a lot of dispersed MoO₂ nanoparticles are observed to be attached on the surface of CNTs. For the sample prepared without the addition of glucose, FE-SEM images (Fig. 2c and d) reveal that CNTs are mixed with MoO₃ nanobelts with the width of ca. 500 nm and the length of ca. 10 μm to form composites. Thus, glucose is believed to play an important role as the surface modifier in controlling the morphology and size of molybdenum oxides. Glucose, a kind of surfactant, can physically absorb on the CNTs surface to link the subunits of MoO₂ and prohibit the anisotropic growth, resulting in the direct deposition of MoO₂ nanoparticles on CNTs surface. Fig. 2e and f show the SEM images of the sample fabricated without the addition of CNTs. The product is composed of nanoparticles with the size of 20–50 nm and a lot of nanoparticles are agglomerated together to form large clumps. No nanobelts are observed in the sample, further confirming that glucose prohibits the anisotropic growth of molybdenum oxides.

Fig. 2 FE-SEM images of (a and b) MoO₂/CNTs nanocomposites, (c and d) MoO₃ nanobelts/CNTs composites, and (e and f) MoO₂ nanoparticles.

Fig. 3 presents the HR-TEM images of MoO₂/CNTs nanocomposite. A lot of MoO₂ nanoparticles with the size of 20–50 nm are observed anchored on the surface of CNTs, Fig. 3a and b. The higher magnification image (Fig. 3c) reveals a fringe with a spacing of 0.34 nm, corresponding to the (002) crystalline plane of CNTs. The measured lattice distance of nanoparticle is around 0.34 nm, corresponding to the (011) plane of MoO₂.

Fig. 3 HR-TEM images of MoO₂/CNTs nanocomposites at different magnification (a–c).

To evaluate the carbon content, TGA tests have been carried out in the fabricated MoO₂/CNTs nanocomposites, MoO₃ nanobelts/CNTs composites, and MoO₂ nanoparticles. In the TGA and DSC curves of MoO₂/CNTs nanocomposites (Fig. 4a), one exothermic peak accompanying with the weight loss of about 2.5% is observed in the temperature range of 300–400 °C, corresponding to the oxidation process of glucose residue.³³ Another exothermic peak at 400–600 °C accompanying with an intensive 37.9% weight loss is attributed to the oxidation of CNTs to form volatile species such as CO and CO₂.²⁸ For the MoO₃ nanobelts/CNTs composites (Fig. 4b), only one exothermic peak observed at 400–600 °C in the DSC curve accompanying with 38.5% weight loss corresponds to the oxidation of CNTs. In Fig. 4c, an exothermic peak, appeared in the DSC curve of MoO₂ nanoparticles from 300 to 400 °C, indicates the oxidation of glucose residue. Meanwhile, a slight weight gain is observed in the TGA curve, suggesting the oxidation reaction from MoO₂ to MoO₃, which is also observed in previous works.^34,35

Fig. 4 TGA/DSC curves of (a) MoO₂/CNTs nanocomposites, (b) MoO₃ nanobelts/CNTs composites, and (c) MoO₂ nanoparticles.

The XPS measurement was carried out to determine the chemical composition of MoO₂/CNTs nanocomposites, Fig. 5. In the survey scan (Fig. 5a), the distinct peaks at 530.1, 415.5, 398.0, 284.6, and 233.2 eV are assigned to the O 1s, Mo 3p_1/2, Mo 3p_3/2, C 1s, and Mo 3d, respectively, indicating the presence of O, Mo and C elements in the nanocomposites. In the Mo 3d spectrum (Fig. 5b), two peaks at 233.3 and 230.2 eV correspond to Mo 3d_3/2 and Mo 3d_5/2, respectively, which are in agreement with the XPS results of MoO₂ in the previous reports.^36,37 The O 1s spectrum (Fig. 5c) is broad and deconvoluted into three peaks at 530.7, 532.3 and 533.6 eV, which are assigned to Mo–O–Mo, C–O–Mo and C=O bonds, respectively.³⁸ The C–O–Mo interfacial bonding introduced onto the CNTs surfaces has substantially improved the structural integrity and electrochemical cyclability of MoO₂/CNTs nanocomposites. Similar phenomena have been demonstrated in other CNTs composites.^39,40 The C 1s peak (Fig. 5d) can be deconvoluted into three peaks at 284.6, 285.7, and 286.6 eV, corresponding to the C–C, C–O and C=O groups.^41–43 On the base of above analysis, the proposed formation mechanism of the as-synthesized MoO₂/CNTs nanocomposite is given in Fig. 6.

Fig. 5 XPS spectra of MoO₂/CNTs nanocomposites: (a) survey scan, (b) Mo 3d, (c) O 1s, and (d) C 1s.

Fig. 6 Schematic diagrams illustrating the formation mechanism of MoO₂/CNTs nanocomposites.

3.2. Electrochemical performances of MoO₂/CNTs nanocomposite, MoO₃ nanobelts/CNTs composite and MoO₂ nanoparticles

To evaluate the applicability of MoO₂/CNTs nanocomposites as LIBs anode material, the electrochemical performances were investigated in the voltage window of 0.01–3.0 V. For comparison, the MoO₃ nanobelts/CNTs composites and MoO₂ nanoparticles were also employed as anode materials. Fig. 7a–c display the cyclic voltammetry (CV) curves of the MoO₂/CNTs, MoO₃ nanobelts/CNTs, and MoO₂ samples in the initial three cycles, respectively. For the MoO₂/CNTs nanocomposites (Fig. 7a), there are six cathodic peaks appeared at ca. 0.06, 0.5, 1.1, 1.4, 2.2 and 2.7 V in the first discharge process. According to the CV result and previous works, two cathodic peaks at 1.4 V (shifted to 1.5 V from the second cycle) and 1.1 V (shifted to 1.2 V from the second cycle) are the characteristics for Li⁺ insertion reactions in MoO₂ to form Li_x1MoO₂ and Li_x2MoO₂ (x₂ > x₁), respectively, accompanying with the phase transition.^32,44–47 One weak peak at 0.5 V disappeared from the second discharge process is related to the irreversible reduction of electrolyte and the formation of solid electrolyte interphase (SEI) layer.⁴⁸ The main cathodic peak at ca. 0.06 V (shifted to 0.01 V from the second cycle) can be ascribed to the Li conversion reaction, in which Mo metal and Li₂O are produced.^46,47 Compared with the CV curve of MoO₃ nanobelts/CNTs (Fig. 7b), two weak peaks at 2.2 and 2.7 V (Fig. 7a) disappeared from the second discharge process originate from the Li⁺ insertion reaction of MoO₃, suggesting that the slight oxidation of MoO₂ nanoparticles still existed in the MoO₂/CNTs nanocomposites due to the exposure to air although there is no diffraction peak of MoO₃ observed in XRD pattern (Fig. 1a). In the subsequent charge process, a wide and split anodic peak in 1.4–1.8 V is attributed to the oxidation of Mo⁰ to Mo⁴⁺ and the decomposition of Li₂O. After the second cycle, the CV curves tend to overlap very well, demonstrating that this electrode exhibits a gradually enhanced cycling stability for the insertion and extraction of lithium ions. Therefore, the reversible electrochemical reaction formula in lithium ion battery can be described as the lithium insertion/extraction in MoO₂ along with the phase transition and subsequent conversion reaction:^46,47

MoO₂ (monoclinic) + x₁Li⁺ + x₁e⁻ ↔ Li_x1MoO₂ (orthorhombic) (1)

Li_x1MoO₂ (orthorhombic) + (x₂ − x₁)Li⁺ + (x₂ − x₁)e⁻ ↔ Li_x2MoO₂(monoclinic) (2)

Li_x2MoO₂ + (4 − x₂)Li⁺ + (4 − x₂)e⁻ ↔ Mo + 2Li₂O (3)

Fig. 7 Cyclic voltammogram curves of (a) MoO₂/CNTs nanocomposites, (b) MoO₃ nanobelts/CNTs composites, (c) MoO₂ nanoparticles, and (d) Nyquist plots of the MoO₂/CNTs, MoO₃ nanobelts/CNTs and MoO₂ samples. The insets in (b) and (d) are the highlighted CV curve of MoO₃ nanobelts/CNTs composites and the equivalent circuit, respectively.

As to the MoO₃ nanobelts/CNTs composites, there are four reduction peaks at ca. 0.07, 0.5, 2.1 and 2.6 V observed in the first cathodic process, and three oxidation peaks at ca. 0.07, 1.3 and 1.7 V appeared in the counterpart anodic process (Fig. 7b). The cathodic peaks at 0.5, 1.3 and 1.7 V disappeared from the second discharge process, suggesting the irreversible decomposition of the electrolyte, the formation of SEI layer, and the Li⁺ insertion in the Mo–O octahedron layers and Mo–O octahedron intralayers with the formation of Li_xMoO₃.^49–52 The intense reduction peak at 0.07 V (shifted to 0.01 V from the second cycle) corresponds to the electrochemical reduction reaction of Li_xMoO₃ with Li to generate Mo⁰. From the second cycle, the CV spectra (inset of Fig. 7b) are similar to those of MoO₂/CNTs (Fig. 7a) and MoO₂ (Fig. 7c). A broad anodic peak in 1.2–1.8 V is attributed to the oxidation of Mo⁰ to Mo⁴⁺ and the decomposition of Li₂O. The above experimental results suggest that, the reduced Mo⁰ is mainly re-oxidized to Mo⁴⁺ rather than Mo⁶⁺ in the first charge process. That is to say, from the second cycle, the mainly reversible electrochemical reaction in cell is the lithium insertion/extraction reactions in MoO₂. Therefore, two cathodic peaks at ca. 1.5 and 1.2 V are the characteristics for Li⁺ insertion reactions in MoO₂ to form Li_xMoO₂ from the second cycle. For MoO₂ sample, the CV profile (Fig. 7c) is similar to that of MoO₂/CNTs. The two cathodic peaks at 1.3 and 1.0 V are attributed to the lithium insertion in MoO₂ to form Li_xMoO₂. The peak at 0.4 V disappeared from the second discharge process, corresponding to the irreversible reduction of electrolyte and the formation of SEI layer. The reduction peak at 0.04 V (shifted to 0.01 V from the second cycle) is ascribed to the reduction of lithiated Li_xMoO₂ to metallic Mo. A weak and broad peak at ca. 2.2 V disappeared from the second discharge process indicates the presence of Mo⁶⁺ due to the slight oxidation of MoO₂ nanoparticles exposed to air. In the following charge process, one broad and split anodic peak in 1.4–1.8 V is attributed to the oxidation of Mo⁰ to Mo⁴⁺ and the decomposition of Li₂O.

To further investigate the electrochemical performances of MoO₂/CNTs, MoO₃ nanobelts/CNTs, and MoO₂ samples, electrochemical impedance spectroscopy (EIS) measurements conducted on the cells at a current density of 100 mA g⁻¹ after 100 cycling are shown in Fig. 7d. The Nyquist plot of each cell is comprised of arc in the high- and medium-frequency region, and an inclined line in the low frequency region.^53,54 The diameter of the semicircle is in direct proportion to the impedance, which contains electrolyte resistance (R_e), surface film resistance (R_sf) and charge transfer resistance (R_ct).^55–57 The inclined line is related to the lithium-ion diffusion inside the electrode materials corresponding to the Warburg impedance (Z_w).^58,59 The impedance spectra can be fitted based on a reasonable equivalent circuit (inset of Fig. 7d), in accordance with the Li-ion insertion/extraction mechanism in electrode.^60,61 CPE in this model expresses the double layer capacitance. The (R_e + R_sf + R_ct) values for MoO₂/CNTs, MoO₃ nanobelts/CNTs, and MoO₂ samples are ca. 39, 83 and 112 Ω, respectively. The measured results indicate that the addition of CNTs into molybdenum oxides enhanced the electrical conductivity and charge transfer. Comparing with MoO₃ nanobelts/CNTs, the MoO₂/CNTs nanocomposites exhibited lower impedance, which could be attributed to the nanostructure of MoO₂ nanoparticles anchored on CNTs resulting in better electrical contact of CNTs network (Fig. 2). In the low frequency region, the slope of inclined line for MoO₂/CNTs nanocomposites and MoO₂ nanoparticles is larger than that of MoO₃ nanobelts/CNTs composites, suggesting that the lithium ion diffusion ability of these two samples is superior to MoO₃ nanobelts/CNTs composites. The reason could be ascribed to the size decrease of molybdenum oxides based on the SEM measurements (Fig. 2), which is beneficial to lithium ion diffusion. Therefore, the addition of CNTs in the MoO₂/CNTs nanocomposites can enhance the electrical conductivity, and the nanosized MoO₂ particles anchored on CNTs facilitate the lithium ion diffusion.

The galvanostatic discharge/charge curves of MoO₂/CNTs, MoO₃ nanobelts/CNTs and MoO₂ in the potential range from 0.02 to 3.0 V vs. Li⁺/Li reference electrode at the current density of 100 mA g⁻¹ are shown in Fig. 8. The MoO₂/CNTs electrode exhibits initial discharge–charge capacities of 782 and 459 mA h g⁻¹, respectively, with a coulombic efficiency (the ratio of charge capacity to discharge capacity) of 58.7% (Fig. 8a). The large initial irreversible loss is attributed to the irreversible reduction of MoO₂ to Mo and other possible irreversible processes (electrolyte decomposition and the formation of SEI layer).⁴⁷ The initial coulombic efficiency of MoO₃ nanobelts/CNTs is 57.9% (Fig. 8b), with initial discharge–charge capacities of 694 and 402 mA h g⁻¹, respectively. For the MoO₂ electrode, the first discharge and charge capacities are 475 and 299 mA h g⁻¹, respectively (Fig. 8c). The initial coulombic efficiency is 62.9%. The MoO₂ electrode shows the highest initial coulombic efficiency. Furthermore, the curves for MoO₂/CNTs and MoO₂ electrode of the second and third discharge/charge cycles coincide better than those of MoO₃ nanobelts/CNTs electrode. These results indicate the good reversibility of the electrochemical reaction of MoO₂ and MoO₂/CNTs.

Fig. 8 Galvanostatic discharge/charge curves of the 1^st, 2^nd, and 3^rd cycle for (a) MoO₂/CNTs nanocomposites, (b) MoO₃ nanobelts/CNTs composites and (c) MoO₂ nanoparticles.

Fig. 9a shows the cycling performance of MoO₂/CNTs nanocomposites at a constant current density of 100 mA g⁻¹. The nanocomposite electrode delivers the reversible capacity of 640 mA h g⁻¹ after 100 cycles. The reversible capacity of MoO₂/CNTs electrode exhibits an increasing tendency with the cycling onward within the initial 50 cycles, even reaches 805 mA h g⁻¹ after 50 cycles due to a gradual activation process of MoO₂/CNTs electrode during the discharge/charge processes. Similar phenomena have been also found in other anode nanomaterials.^5,30 As a comparison, the cycling performances of MoO₃ nanobelts/CNTs and MoO₂ nanoparticles at the current density of 100 mA g⁻¹ are also shown in Fig. 9a. In the initial 20 cycles, MoO₂ nanoparticles exhibit a similar capacity increase tendency as the MoO₂/CNTs electrode, however, the capacity decreases successively with further cycling and is only 259 mA h g⁻¹ after 100 cycles, suggesting that the addition of CNTs is beneficial to improve the cycling stability. For the MoO₃ nanobelts/CNTs composites, the capacity shows intensive decrease to 246 mA h g⁻¹ after 100 cycles probably due to the structural isolation between MoO₃ nanobelts and CNTs (Fig. 2), and the pulverization of large-sized MoO₃ nanobelts during the Li-ion insertion/extraction processes, which break down the integrity of electrode.

Fig. 9 (a) Cycling performances of MoO₂/CNTs, MoO₃ nanobelts/CNTs, and MoO₂ at the current density of 100 mA g⁻¹, and (b) the rate capabilities of MoO₂/CNTs and MoO₃ nanobelts/CNTs nanocomposites.

The higher reversible capacity of MoO₂/CNTs nanocomposites than those of MoO₃ nanobelts/CNTs and MoO₂ nanoparticles further demonstrates that the hybrid structure of MoO₂ nanoparticles anchored on CNTs significantly improves the cycling performance and leads to higher retention capacity. The cycling performance of MoO₂/CNTs nanocomposites is also much better than those of the reported MoO₂/graphite composites (720 mA h g⁻¹ after 30 cycles at the current density of 100 mA g⁻¹)⁶² and MoO₂/C hollow spheres (640 mA h g⁻¹ after 80 cycles at the current density of 50 mA g⁻¹)⁶³ in long cycling performance and reversible capacity. Moreover, the coulombic efficiency quickly reaches to 90% in the second cycle, even remains more than 96% from fifth to 100^th cycle. From the rate capability shown in Fig. 9b, the MoO₂/CNTs nanocomposite electrode achieves reversible capacities of ca. 560, 572, 539 and 356 mA h g⁻¹ at the current densities of 100, 200, 400 and 800 mA g⁻¹, respectively. When the current density is recovered to 100 mA g⁻¹ after the rate performance test, the discharge capacity reaches 692 mA h g⁻¹ higher than 560 mA h g⁻¹ acquired at the initial current density of 100 mA g⁻¹, suggesting that the high current charge/discharge processes not only did little to break down the integrity of the electrodes, but also leaded to a gradual activation of the electrode materials, which have also been found in other anode nanomaterials.^64–66 However, the MoO₃ nanobelts/CNTs composites only deliver reversible capacities of ca. 305, 232, 179, and 129 mA h g⁻¹ at the current densities of 100, 200, 400 and 800 mA g⁻¹, respectively. The MoO₃ nanobelts/CNTs composites show much lower rate capability at the same current density than MoO₂/CNTs nanocomposites, mainly attributing to the structural isolation between MoO₃ nanobelts and CNTs. On the base of the above results and discussion, the superior electrochemical performances of the MoO₂/CNTs nanocomposites can be attributed to the following reasons. Firstly, the presence of intertwined CNTs improves the electrical conductivity of the nanocomposites and accommodates the volume change during the lithiation/delithiation processes. The nanostructure of MoO₂ nanoparticles anchored on CNTs is beneficial to prevent the agglomeration of MoO₂ nanoparticles and maintain the electrode integrity. In addition, the small size of MoO₂ nanoparticles (ca. 20–50 nm) and the thin walls of CNTs facilitate the Li ions diffusion in the electrode and increase the contact surface between the active materials and electrolyte. Moreover, the good attachment between MoO₂ nanoparticles and CNTs can effectively avoid the electrical isolation of MoO₂/CNTs anode during the charge/discharge processes.

4. Conclusion

The nanocomposites of MoO₂ nanoparticles anchored on carbon nanotubes (MoO₂/CNTs) have been synthesized as anode material for Li-ion battery by a facile approach. Compared with the as-prepared MoO₃ nanobelts/CNTs and MoO₂ nanoparticles, the MoO₂/CNTs nanocomposites exhibit superior cycling and rate performances. The CNTs network in the nanocomposites not only provides a continuous long-distance pathway for electron transfer, but also acts as a favorable buffer to the aggregation of the anchored MoO₂ nanoparticles and the volume change, which maintains the integrity of electrode during the lithiation/delithiation processes, finally resulting in the enhanced electrochemical performances. The hybridization of MoO₂ nanoparticles with CNTs is proved to be an efficient way to improve the electrochemical performances of LIBs anode.

Acknowledgements

This work was supported by the Fundamental Research Funds of Shandong University (2015JC016) and Natural Science Fund for Distinguished Young Scholars of Shandong (JQ201312). The authors also acknowledge the financial supports from the Shandong Provincial Natural Science Foundation, China (No. ZR2009FM035).

References

1. D. Su, H. J. Ahn and G. Wang, J. Power Sources, 2013, 244, 742–746.
2. M. V. Reddy, T. Yu, C. H. Sow, Z. X. Shen, C. T. Lim, G. V. Subba Rao and B. V. R. Chowdari, Adv. Funct. Mater., 2007, 17, 2792–2799.
3. W. Y. Li, L. N. Xu and J. Chen, Adv. Funct. Mater., 2005, 15, 851–857.
4. D. Su, H. S. Kim, W. S. Kim and G. Wang, Chem.–Eur. J., 2012, 18, 8224–8229.
5. J. Gao, M. A. Lowe and H. D. Abruña, Chem. Mater., 2011, 23, 3223–3227.
6. L. A. Riley, S. H. Lee, L. Gedvilias and A. C. Dillon, J. Power Sources, 2010, 195, 588–592.
7. S. H. Lee, Y. H. Kim, R. Deshpande, P. A. Parilla, E. Whitney, D. T. Gillaspie, K. M. Jones, A. H. Mahan, S. Zhang and A. C. Dillon, Adv. Mater., 2008, 20, 3627–3632.
8. M. F. Hassan, Z. P. Guo, Z. Chen and H. K. Liu, J. Power Sources, 2010, 195, 2372–2376.
9. X. Zhao, M. Cao, B. Liu, Y. Tian and C. Hu, J. Mater. Chem., 2012, 22, 13334–13340.
10. L. C. Yang, Q. S. Gao, Y. Tang, Y. P. Wu and R. Holze, J. Power Sources, 2008, 179, 357–360.
11. J. H. Ku, J. H. Ryu, S. H. Kim, O. H. Han and S. M. Oh, Adv. Funct. Mater., 2012, 22, 3658–3664.
12. K. Sakaushi, J. Thomas, S. Kaskel and J. Eckert, Chem. Mater., 2013, 25, 2557–2563.
13. B. Guo, X. Fang, B. Li, Y. Shi, C. Ouyang, Y. S. Hu, Z. Wang, G. D. Stucky and L. Chen, Chem. Mater., 2012, 24, 457–463.
14. Z. Yuan and L. Si, J. Mater. Chem. A, 2013, 1, 15247–15251.
15. S. Qiu, X. Wang, G. Lu, J. Liu and C. He, Mater. Lett., 2014, 136, 289–291.
16. S. Guo, G. Lu, S. Qiu, J. Liu, X. Wang, C. He, H. Wei, X. Yan and Z. Guo, Nano Energy, 2014, 9, 41–49.
17. H. Qu, S. Wei and Z. Guo, J. Mater. Chem. A, 2013, 1, 11513–11528.
18. C. Hu, S. Guo, G. Lu, Y. Fu, J. Liu, H. Wei, X. Yan, Y. Wang and Z. Guo, Electrochim. Acta, 2014, 148, 118–126.
19. H. Cao, X. Wang, H. Gu, J. Liu, L. Luan, W. Liu, Y. Wang and Z. Guo, RSC Adv., 2015, 5, 34566–34571.
20. P. J. Lu, M. Lei and J. Liu, CrystEngComm, 2014, 16, 6745–6755.
21. D. Qiu, G. Bu, B. Zhao, Z. Lin, L. Pu, L. Pan and Y. Shi, Mater. Lett., 2014, 119, 12–15.
22. W. Luo, X. Hu, Y. Sun and Y. Huang, Phys. Chem. Chem. Phys., 2011, 13, 16735–16740.
23. H. Lv, S. Qiu, G. Lu, Y. Fu, X. Li, C. Hu and J. Liu, Electrochim. Acta, 2015, 151, 214–221.
24. C. Ban, Z. Wu, D. T. Gillaspie, L. Chen, Y. Yan, J. L. Blackburn and A. C. Dillon, Adv. Mater., 2010, 22, E145–E149.
25. X. Li, H. Gu, J. Liu, H. Wei, S. Qiu, Y. Fu, H. Lv, G. Lu, Y. Wang and Z. Guo, RSC Adv., 2015, 5, 7237–7244.
26. S. Qiu, H. Gu, G. Lu, J. Liu, X. Li, Y. Fu, X. Yan, C. Hu and Z. Guo, RSC Adv., 2015, 5, 46509–46516.
27. M. O. Guler, T. Cetinkaya, U. Tocoglu and H. Akbulut, Microelectron. Eng., 2014, 118, 54–60.
28. J. Wang, R. Ran, M. O. Tade and Z. Shao, J. Power Sources, 2014, 254, 18–28.
29. C. Xu, J. Sun and L. Gao, J. Power Sources, 2011, 196, 5138–5142.
30. Z. Wang, D. Luan, S. Madhavi, Y. Hu and X. W. D. Lou, Energy Environ. Sci., 2012, 5, 5252–5256.
31. J. Ren, J. Yang, A. Abouimrane, D. Wang and K. Amine, J. Power Sources, 2011, 196, 8701–8705.
32. J. P. Jegal, H. K. Kim, J. S. Kim and K. B. Kim, J. Electroceram., 2013, 31, 218–223.
33. X. Wu, H. Niu, S. Fu, J. Song, C. Mao, S. Zhang, D. Zhang and C. Chen, J. Mater. Chem. A, 2014, 2, 6790–6795.
34. Y. Sun, X. Hu, W. Luo and Y. Huang, ACS Nano, 2011, 5, 7100–7107.
35. K. H. Seng, G. D. Du, L. Li, Z. X. Chen, H. K. Liu and Z. P. Guo, J. Mater. Chem., 2012, 22, 16072–16077.
36. J. G. Choi and L. T. Thompson, Appl. Surf. Sci., 1996, 93, 143–149.
37. X. Chen, Z. Zhang, X. Li, C. Shi and X. Li, Chem. Phys. Lett., 2006, 418, 105–108.
38. A. Bhaskar, M. Deepa and T. N. Rao, ACS Appl. Mater. Interfaces, 2013, 5, 2555–2566.
39. C. Sun, K. Karki, Z. Jia, H. Liao, Y. Zhang, T. Li, Y. Qi, J. Cumings, G. W. Rubloff and Y. Wang, ACS Nano, 2013, 7, 2717–2724.
40. C. Sun, H. Zhu, M. Okada, K. Gaskell, Y. Inoue, L. Hu and Y. Wang, Nano Lett., 2015, 15, 703–708.
41. S. Kundu, Y. Wang, W. Xia and M. Muhler, J. Phys. Chem. C, 2008, 112, 16869–16878.
42. R. H. Bradley, K. Cassity, R. Andrews, M. Meier, S. Osbeck, A. Andreu, C. Johnston and A. Crossley, Appl. Surf. Sci., 2012, 258, 4835–4843.
43. F. Sun, J. Gao, Y. Zhu, G. Chen, S. Wu and Y. Qin, Adsorption, 2013, 19, 959–966.
44. B. Liu, X. Zhao, Y. Tian, D. Zhao, C. Hu and M. Cao, Phys. Chem. Chem. Phys., 2013, 15, 8831–8837.
45. L. Zhou, H. B. Wu, Z. Wang and X. W. D. Lou, ACS Appl. Mater. Interfaces, 2011, 3, 4853–4857.
46. J. H. Ku, Y. S. Jung, K. T. Lee, C. H. Kim and S. M. Oh, J. Electrochem. Soc., 2009, 156, A688–A693.
47. Y. Liu, H. Zhang, P. Ouyang, W. Chen and Z. Li, Mater. Res. Bull., 2014, 50, 95–102.
48. Y. Sun, X. Hu, J. C. Yu, Q. Li, W. Luo, L. Yuan, W. Zhang and Y. Huang, Energy Environ. Sci., 2011, 4, 2870–2877.
49. Y. Sun, J. Wang, B. Zhao, R. Cai, R. Ran and Z. Shao, J. Mater. Chem. A, 2013, 1, 4736–4746.
50. W. Li, F. Cheng, Z. Tao and J. Chen, J. Phys. Chem. B, 2006, 110, 119–124.
51. T. Tsumura and M. Inagaki, Solid State Ionics, 1997, 104, 183–189.
52. J. Ni, G. Wang, J. Yang, D. Gao, J. Chen, L. Gao and Y. Li, J. Power Sources, 2014, 247, 90–94.
53. G. Lu, S. Qiu, J. Liu, X. Wang, C. He and Y. Bai, Electrochim. Acta, 2014, 117, 230–238.
54. J. Kang, Y. Ko, J. Park and D. Kim, Nanoscale Res. Lett., 2008, 3, 390–394.
55. Y. Huang, Z. Dong, D. Jia, Z. Guo and W. I. Cho, Electrochim. Acta, 2011, 56, 9233–9239.
56. Q. Zhang, Z. Shi, Y. Deng, J. Zheng, G. Liu and G. Chen, J. Power Sources, 2012, 197, 305–309.
57. Z. Wu, W. Ren, L. Xu, F. Li and H. Cheng, ACS Nano, 2011, 5, 5463–5471.
58. Y. Deng, Q. Zhang, Z. Shi, L. Han, F. Peng and G. Chen, Electrochim. Acta, 2012, 76, 495–503.
59. V. Freger, Electrochem. Commun., 2005, 7, 957–961.
60. W. Yue, S. Tao, J. Fu, Z. Gao and Y. Ren, Carbon, 2013, 65, 97–104.
61. M. V. Reddy, T. Yu, C. H. Sow, Z. X. Shen, C. T. Lim, G. V. Subba Rao and B. V. R. Chowdari, Adv. Funct. Mater., 2007, 17, 2792–2799.
62. Y. Xu, R. Yi, B. Yuan, X. Wu, M. Dunwell, Q. Lin, L. Fei, S. Deng, P. Andersen, D. Wang and H. Luo, J. Phys. Chem. Lett., 2012, 3, 309–314.
63. H. Gao, C. Liu, Y. Liu, Z. Liu and W. Dong, Mater. Chem. Phys., 2014, 147, 218–224.
64. G. Lu, S. Qiu, H. Lv, Y. Fu, J. Liu, X. Li and Y. Bai, Electrochim. Acta, 2014, 146, 249–256.
65. S. Grugeon, S. Laruelle, L. Dupont and J. M. Tarascon, Solid State Sci., 2003, 5, 895–904.
66. C. Gong, Y. Bai, Y. Qi, N. Lun and J. Feng, Electrochim. Acta, 2013, 90, 119–127.

---