Hong Zhanga,
Binqiang Tiana,
Jian Xuea,
Guoqing Dingb,
Xiaoming Ji*a and
Yang Cao*b
aCollege of Tobacco Science, Henan Agricultural University, Zhengzhou, Henan 450002, China. E-mail: xiaomingji@henau.edu.cn
bCollege of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China. E-mail: 2016055@zzuli.edu.cn
First published on 8th August 2019
Hierarchical non-woven fabric NiO/TiO2 film is prepared using a facile two-step synthesis by electrospinning and subsequent hydrothermal reaction to yield TiO2 film decorated with NiO nanosheets. As an anode material for Li-ion batteries, the NiO/TiO2 composite exhibits discharge and charge capacities of 1251.3 and 1284.3 mA h g−1, with good cycling performance and rate capability. Characterization shows that the NiO nanosheets are distributed on the surface of the TiO2 nanofibers. The total specific capacity of NiO/TiO2 is higher than the sum of pure TiO2 and NiO, indicating a positive synergistic effect of TiO2 and NiO on the improvement of electrochemical performance. The results suggest that non-woven fabric NiO/TiO2 film is a promising anode material for lithium ion batteries.
Unfortunately, the poor electrical and ionic conductivities of NiO hinder its electrochemical performances.8–10 Besides, the sluggish kinetics of the conversion reaction and the large volume expansion (95.69%) of the large-sized electrode materials severely restrict the development of NiO anodes.11 To address these shortcomings, Poizot et al. first proposed the nano-sized TMOs that undergo conversion reactions with lithium and provide two- or three-fold higher reversible capacity than graphitic anodes.12 After that, great efforts have been made to improving the performance of NiO as the anode material through fabricating various nanostructures, including nanoparticles,13 nanosheets,14 nanotubes,15,16 nanofibers,17 hollow microspheres,18,19 and sandwich-like films.20 The nanoscaled materials not only shorten the electron and Li+ diffusion pathways, but also increase the electrode–electrolyte interface, enhancing the cycling stability and high-rate capacity.21,22 Nevertheless, the issues for nanoscaled materials are that they can easily become aggregated and get more structural defects, resulting in poor cycling performance.23 Thus, sophisticated nanomaterials, particularly with designed unique structures, are highly needed.
Herein, a novel hierarchical non-woven fabric film composed of NiO nanosheets (NSs) grown on TiO2 nanofibers is prepared via an electrospinning method combined with a hydrothermal post-treatment and used as an efficient anode material for LIBs. The special three-dimensional (3D) nano-architecture possesses outstanding advantages. First, the direct growth of NiO nanosheets on TiO2 nanofibers improves the conductivity and separates active materials from each other, completely avoiding the aggregation and agglomeration. In addition, this 3D morphology helps to shorten the pathways for Li+ and electron transport and enlarges exposed surface for more lithium-exchange channels, leading to high rate capability and cycling stability.
The XRD patterns of the samples are shown in Fig. 1. The TiO2 film exhibits diffraction peaks that agree well with the characteristic peaks of anatase TiO2 crystal. Namely, the peaks at 25.4, 48.1 and 54.6° correspond to the (101), (200) and (105) crystal planes of TiO2 (JCPDS card no. 21-1272), respectively. All the reflections of NiO sample can be perfectly indexed as NiO (JCPDS card no. 65-2901) in terms of the relative intensity and position. The well-resolved diffraction peaks appearing at 37.08, 43.23, and 62.81° are correspondingly assigned to the (111), (200), and (220) planes, respectively. For the NiO/TiO2 film, all the diffraction peaks are in agreement with the standard patterns of TiO2 (JCPDS card no. 21-1272) and NiO (JCPDS card no. 65-2901), and no impurity diffraction peaks are observed.
The morphologies of the samples were examined by field-emission scanning electron microscope (SEM) and transmission electron microscope (TEM). Fig. 2 shows the SEM images of the TiO2 nanofibers (Fig. 2a and b) and the NiO/TiO2 composite (Fig. 2c and d). It can be seen that the electrospinning TiO2 nanofibers have a relatively smooth surface with diameters ranging from 350 to 700 nm, which are intact during the film fabrication process. The long fiber structure should be effective in facilitating electron transport. The nanofibers stack loosely with each other, leaving lots of voids, which facilitate the electrolyte penetration as well as Li+ transport between the electrode and the electrolyte. After the hydrothermal treatment, as shown in Fig. 2c and d, each nanofiber is coated with NiO nanosheets with the average thickness of ∼7 nm and height of ∼350 nm. The TEM analyses (Fig. 2e and f) also demonstrate the highly branched structure, in which NSs are disorderly grown on the surface. The high-resolution TEM (HR-TEM) image (Fig. 2g) of a single NS shows a typical lattice distance of 0.21 nm, which is indexed to the (200) plane of NiO. Compared with the pristine TiO2 nanofiber film, the large interspaces between fibers are filled with NSs, resulting in more active sites of the composite film. The corresponding element mapping of Ti, O, and Ni in Fig. 3 also confirms the uniform dispersion of NiO nanosheets on the TiO2 fiber surface. However, in the absence of TiO2 nanofibers, NiO NSs tend to self-assemble into nanoflowers with sizes ranging from 6 to 10 μm (Fig. S1†).
Fig. 3 (a) SEM image of NiO/TiO2 and (b–e) the corresponding EDX element mapping of (b) Ti, (c) O, (d) Ni, and (e) the reconstructed chemical map. |
Cyclic voltammetry (CV) tests and galvanostatic discharge/charge measurements were investigated in half-cell configurations to evaluate the lithium storage properties of the as-prepared NiO/TiO2 composite film as an anode electrode in LIBs. Fig. 4a shows the CV curves of the NiO/TiO2 film for the 1st and 2nd cycles at a scan rate of 0.01 mV s−1 over the voltage range of 0.01–3 V. In the first cathodic scan, a strong peak appears at ∼0.47 V, which corresponds to the formation of a solid electrolyte interphase (SEI) film and the reduction of NiO to Ni.27 In the following cycle, this cathodic peak significantly shifts to about 0.9 V, which agrees well with previous reports.28,29 The anodic scans show similar profiles, in which two oxidation peaks are found at 1.49 V and 2.1 V, indicating the decomposition of the SEI film and the oxidation of Ni to NiO, respectively.30
Fig. 4b and c show the discharge/charge curves of NiO and NiO/TiO2 electrodes at 0.1C and 0.2C (1C = 718 mA h g−1), respectively. The NiO electrode exhibits discharge and charge capacities of 615.8 and 662.6 mA h g−1 at 0.1C, and 626.4 and 579.6 mA h g−1 at 0.2C. Impressively, the NiO/TiO2 electrode exhibits discharge and charge capacities of 1251.3 and 1284.3 mA h g−1 at 0.1C, and 1075 and 1154.4 mA h g−1 at 0.2C, with the coulombic efficiency of 97% and 93%, respectively. Thus, the discharge and charge capacities of the NiO/TiO2 electrode are about twofold of those delivered by the NiO electrode.
The electrochemical performance is crucial for the potential anode materials in LIBs. The rate performance of the NiO/TiO2 electrode is first examined at different charge/discharge rates from 0.05C to 0.5C for 5 cycles, and then returns to 0.05C. As illustrated in Fig. 4d, the NiO/TiO2 electrode exhibits high discharge capacities at lower current densities, which is 1388.9 mA h g−1 at the rate of 0.05C and decreases to 509.5 mA h g−1 at 0.5C. When the current returns to 0.05C, the discharge capacity recovers to 1015 mA h g−1, showing good stability. Fig. 4e presents the cycling performance of the two electrodes. The initial discharge capacity of NiO/TiO2 electrode is 1285.6 mA h g−1, and the capacity maintains a high value of 487.1 mA h g−1 after 50 cycles at 0.1C. While for the NiO electrode, the initial discharge capacity is 655.2 mA h g−1, decreasing to only 198.9 mA h g−1 after 50 cycles.
To confirm the electrochemical kinetics related to Li+ diffusion, electrochemical impedance spectroscopy (EIS) measurements were carried out. Fig. 4f presents the Nyquist plots of the electrodes in the frequency range of 10−2 to 105 Hz at the open-circuit potential. The semicircle in the high-frequency region is attributed to the charge transfer resistance (Rct) at the electrode–electrolyte interface, and the inclined line in the low-frequency range (the Warburg impedance) corresponds to Li-ion diffusion in the electrode.31 The NiO/TiO2 electrode shows the lowest Rct and Warburg impedance among the testing electrodes. The low Rct is ascribed to the high number of electroactive sites due to the good dispersion of NiO nanosheets, while the tunneled TiO2 nanofibers shorten the Li+ diffusion pathways, reducing the Warburg impedance.
Cycling performance under high charging/discharging rate was also examined to explore the potential of the NiO/TiO2 electrode. Fig. 5 records its capacity variation under high current density of 6C (for NiO, 1C = 718 mA g−1) for 500 cycles. To start with, the discharge capacity of NiO/TiO2 decreases to 152.4 mA h g−1 in the beginning 100 cycles. Then excitingly, it increases steadily in the following cycles. Afterwards, it gradually rises to 243.9 mA h g−1 at the 500th cycle. Even though the capacity experiences some up and down movements, which may be related with the domination of the interfacial Li+ storage mechanism, the long life under such high power is still remarkable.32 In addition, high coulombic efficiencies (about 100% in average) are also found after the first cycle. The Fig. S2† reveal that after suffering intense cycling test, the NiO/TiO2 remains original structure with uniform solid electrolyte interface (SEI) layer coating on the surface, which prevents the aggregation of the active materials. In addition, the integrity of the electrode is well preserved.
Fig. 6 outlines the proposed electron transfer and Li+ diffusion mechanism during the lithiation–delithiation process. The reasons for the high electrochemical performance can be explained as follows. First, high conductive TiO2 non-woven fabric film as the current collector can enhance the electron transfer and effectively suppress the aggregation of NiO NSs, enabling the electrode material to achieve good rate capability. Second, the large interfacial areas can provide more sites for Li+ storage as well as facilitate the Li+ transport between the electrode and the electrolyte.33,34
Fig. 6 Schematic illustration of the lithiation–delithiation process of the NiO/TiO2 composite film. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra04947a |
This journal is © The Royal Society of Chemistry 2019 |