1. Introduction

Along with the serious pollution of car exhaust, the environment-friendly batteries such as lithium-ion batteries (LIBs) have received much concerns, and become one of the main power supplies. It is well-known that electrode materials are a key component, and exert major influences on the performance for LIBs. Graphite currently is universally acted as anode materials in commercial LIBs for the past few decades, which can potentially be replaced by transition metal oxides due to their high theoretical capacity and high volume energy densities in LIBs [1–10]. However, most of transitional metal oxides still suffer poor conductivity and imperfect cycling performances on account of large volume expansion/retraction within the lithium insertion and extraction process. In this context, exploring new materials with improved performance possessing high capacity and long service lifetime still face the great challenges.

Niobium oxide (Nb₂O₅) has been researched in a variety of fields such as hybrid capacitor, electrocatalyst, photocatalysis, particularly lithium ion battery and so on [2, 11–16]. Compared with other transition metal oxides, Nb₂O₅ possesses many excellent merits, such as its abundant resources, a relatively low average working voltage of 1.0–1.5 V (vs. Li⁺/Li) which figures out the possible safety problem connected with the electrolyte decomposition [17–19]. In particular, the crystal structure of orthorhombic Nb₂O₅ (T-Nb₂O₅) permits speedy ionic transport owing to the Li⁺ intercalation reaction occurring not only at the surface but also in the bulk crystals. It is ascribed to the existence of the mostly empty octahedral sites between (001) planes providing natural tunnels for lithium ion transport throughout the a-b plane [20, 21]. Nevertheless, its poor electric conductivity (σ~3 ×10^-6 S cm^-1) limits its practical application in LIBs [22]. Until now, numerous methods are explored to improve their performance, such as the fabrication of Nb₂O₅ with diverse crystal structures and morphologies, coating with conductive layers, and heating treatment to enhance its intrinsic conductivity[2, 15, 16, 23–28].However, itisstill a big challenge to develop high-performance Nb₂O₅ anode for LIBs [2, 15].

In this work, Nb₂O₅/C nanocomposites were simply prepared by annealing niobium chloride and oleic acid under Ar atmospheres at 600 ℃. Oleic acid was carbonized and simultaneously coated on Nb₂O₅ nanocrystals after heating treatment, which can efficiently reduce the volume change of Nb₂O₅ nanocomposites during the charging-discharging process, thus the cycling stability can be well improved. Furthermore, the addition of carbon can effectively enhance conductivity performance of electrode material, which effectively improves the rate performance. These results indicate that Nb₂O₅/C nanocomposites is a promising anode for LIBs.

2. Results and discussion

The morphology of the as-synthesized Nb₂O₅/C nanocomposites was characterized by SEM (as shown in Fig. 1a and b) and TEM (as shown in Fig. 1c) They indicate that the particles on the nanosheets could be confirmed to be from 2 nm to 10 nm. A high resolution TEM (HR-TEM) image of Nb₂O₅/C nanocomposites (Fig. 1d) shows the obvious lattice fringes with a lattice spacing of 0.3977 nm and 0.3148 nm, which are in accordance with the (001) and (180) planes of orthorhombic phase Nb₂O₅.

The X-ray diffraction (XRD) patterns of Nb₂O₅/C nanocomposites are shown in Fig. 2, All diffraction peaks are indexed to orthorhombic Nb₂O₅ (JCPDS Card No. 30-0873) with the typical diffraction peaks located at 22.7°, 28.5°, 36.7°, 46.2°, 50.6°, 55.1° and 71.0°, which are well consistent with observed from the HR-TEM image of the Nb₂O₅/C nanocomposite.

To further research the element distribution in the Nb₂O₅/C nanocomposite, A brief chemical analysis of the compound was executed through HAADF STEM and EDS elemental mapping. Fig. 3a shows a HAADF STEM image of Nb₂O₅/C nanocomposites. The elemental mappings of the constituting elements Nb, O, and C are tested as shown in Fig. 3b-d, which clearly indicates a welldefined composition profile of the Nb₂O₅/C nanocomposites, and the weight percentage of the carbon is 18.86%.

Fig. 4 exhibits the N₂ adsorption-desorption isotherms and pore size distribution of the synthesized Nb₂O₅/C nanocomposites. As clearly shown in Fig. 4a, Nb₂O₅/C nanocomposites have a Brunauer-Emmett-Teller (BET) surface area of 98.3316 m²g^-1 and a total pore volume of 0.6035 cm³ g^-1 (P/P₀ = 0.9903). The corresponding pore size obtained from the desorption branch of the isotherms is shown in Fig. 4b. It can be seen that there is a monomodal distribution (between 1.5 nm and 165 nm) for the synthesized Nb₂O₅/C nanocomposites. The average Barrett-JoynerHalenda (BJH) pore diameter of Nb₂O₅/C nanocomposites based on desorption curves is around 20.65 nm and the pores should be interparticle pores.

Fig. 5a and b show the typical discharge and charge profiles of the Nb₂O₅/C and pristine Nb₂O₅ at 50 mAh g^-1with the voltage window of 0.01–3 V at room temperature. In the first cycle of charge and discharge, Nb₂O₅/C shows discharge capacity of 878 mAh g^-1 which is far higher than pristine Nb₂O₅ with original capacity of 268 mAh g^-1, however, both the Nb₂O₅/C and pristine Nb₂O₅ have a great loss of capacity delivering an initial discharge/ charge capacity of 878/409 mAh g^-1 and 268/86 mAh g^-1, respectively, which may be result from the formation of SEI film on the electrode surface [4, 10]. In addition, the following curves of Nb₂O₅/C and pristine Nb₂O₅ after the first cycle have a high overlapping property, indicating well cyclic performance and good kinetics of the electrode [29, 30].

The cycling performance and Coulombic efficiency of the Nb₂O₅/C and Nb₂O₅ electrodes are shown in Fig. 5c, respectively. Obviously, compared with Nb₂O₅ electrodes, the Nb₂O₅/C electrodes shows a great storage capacity. The pristine Nb₂O₅ electrode shows a quite low capacity (~60 mAh g^-1) at a current of 50 mA g^-1. In contrast, the Nb₂O₅/C nanocomposites exhibit a high capacity of ~380 mAh g^-1 at the first several cycles, and then drop to 340 mAh g^-1 before 30 cycles. After that, the capacity can rise to ~400 mAh g^-1at 90 cycles. Generally, the coulombic efficiency can remains at 98%, indicating the stable cycling performance of Nb₂O₅/C. In addition, the Nb₂O₅/C nanocomposites still exhibit a specific capacity of ~150 mAh g^-1at a current density of 500 mA g^-1 after 100 cycles.

Fig. 5d shows Nb₂O₅/C and pristine Nb₂O₅ electrodes discharge rate capacity at different current densities from 50 mA g^-1 to 1000 mA g^-1, respectively. Clearly, Nb₂O₅/C nanocomposites have a considerably higher capacity in comparison with the pristine Nb₂O₅. at different current densities from 50 mA g^-1 to 1000 mA g^-1, For the Nb₂O₅/C nanocomposites, the discharge specific capacities are ~340, 290, 240, 200 and 160 mAh g^-1 at current densities of 100, 250, 500 and 1000 mA g^-1, respectively. However, the discharge capacity of the pristine Nb₂O₅ electrode, is only 60, 45, 30, 22 and 16 mAh g^-1 at current densities of 100, 250, 500 and 1000 mA g^-1. respectively. When the current densities returns back to 50 mA g^-1, the discharge capacity of the Nb₂O₅/C nanocomposites still keep in 320 mAh g^-1. These results illustrate that the Nb₂O₅/C electrode has better cycling performance and rate performance than the pristine Nb₂O₅ electrode. From the above, The enhanced electrochemical performance of the Nb₂O₅/C is potential to be used for anode material in high rate LIBs.

3. Conclusion

In summary, we have successfully synthesized Nb₂O₅/C nanocomposites with improved electrochemical performance through simple heat-treatment under Ar atmospheres. As expected, the carbon component can greatly improves the conductivity of the composite electrodes, meanwhile, as-synthesized Nb₂O₅/C nanocomposites exhibit a high and stable specific capacity of ~380 mAh g^-1 at the current density of 50 mA g^-1, as well as better rate capability, which is much higher than for the pristine Nb₂O₅.

4. Experimental

Synthesis of Nb₂O₅/C nanocomposites: In a typical synthesis, niobium chloride and oleic acid was uniformly mixed with a mole ratio of 1:5. After that, the obtained muddy composites are calcined at 600 ℃ for 1 h under Ar, then cooled down to room temperature. Finally, Nb₂O₅/C was obtained.

Characterization: The as-prepared samples were characterized and analyzed by various kinds techniques. The surface morphology Nb₂O₅@C nanocomposites was studied by using scanning electron microscopy (SEM: Nova NanoSEM 450 FE-SEM), and transmission electron microscopy (TEM: JEOL JEM 2100, (200 kV) with LaB6 electron gun). X-ray diffraction (XRD) experiments were used to investigate the material composition and crystal structure of the synthesized powders which were conducted on an X'Pert Pro X-ray diffractometer (Panalytical B.V.). The amount of carbon present in the nanocomposite was estimated using high-angle annular darkfield (HAADF) scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDS) elemental mapping.

Electrochemical measurements: Appropriate ratios of nanocomposites were dispersed in the mixed liquids of ethanol and deionized water and stired for 24 h to form homogeneous and stable dispersions, where carboxymethyl cellulose (CMC) and acetylene black were also added. The composites electrodes were prepared by coating the slurry containing 10 wt% acetylene black, 10 wt% CMC and 80 wt% of the composites onto copper foil collectors. The test electrodes were dried in a vacuum oven at 60 ℃ for 12 h. Galvanostatic charge/discharge cycling were carried out in VMP3 BioLogic electrochemical workstation.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Nos. 51402103 and 51302079).