Accepted Manuscript Interconnected Si-coated NiO nanosheet arrays with enhanced electrochemical performance for lithium-ion batteries J.B. Wu, X.H. Huang, Y.Q. Cao, Y. Lin, R.Q. Guo PII: DOI: Reference:
S0167-577X(17)31894-3 https://doi.org/10.1016/j.matlet.2017.12.126 MLBLUE 23624
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Received Date: Revised Date: Accepted Date:
28 August 2017 14 December 2017 25 December 2017
Please cite this article as: J.B. Wu, X.H. Huang, Y.Q. Cao, Y. Lin, R.Q. Guo, Interconnected Si-coated NiO nanosheet arrays with enhanced electrochemical performance for lithium-ion batteries, Materials Letters (2017), doi: https:// doi.org/10.1016/j.matlet.2017.12.126
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Interconnected Si-coated NiO nanosheet arrays with enhanced electrochemical performance for lithium-ion batteries J.B. Wu, X.H. Huang*, Y.Q. Cao, Y. Lin, R.Q. Guo College of Physics & Electronic Engineering, Taizhou University, Taizhou 318000, China * Corresponding author. Tel.: +86 576 88661938. E-mail address: [email protected]
Abstract NiO/Si composite film is prepared by chemical bath deposition and magnetron sputtering techniques. The sample is characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The film is constructed by interconnected Si-coated NiO nanosheet arrays. As an anode material for lithium-ion batteries, the film is analyzed by galvanostatic discharge-charge and cyclic voltammetry (CV). NiO/Si nanosheet arrays deliver higher capacity than that of NiO nanosheet arrays, and shows better cycling stability that of Si film. The enhanced electrochemical performance is ascribed to the interconnected Si-coated NiO nanosheet array structure.
Keywords: Porous materials; Nanocomposites; Sputtering; Energy storage and conversion; Lithium ion battery
1. Introduction As a typical 3d transition-metal oxide, NiO is a widely-used material for energy storage and conversion. It has attracted much attention in the lithium-ion battery field because its theoretical capacity is almost twice as high as that of traditional graphite [1–4]. However, for most unmodified NiO, the actual capacity is much lower, and fades quickly, especially at high current densities. To improve the performance, fabricating nanostructured composites is frequently used. Some ductile conductive components, such as carbon  and metals , are introduced into active materials, and meanwhile, the composites are designed into porous [7, 8], hollow [9, 10], spherical [11, 12] and [email protected]
[13, 14] nanostructures. This approach can effectively reduce electrode polarization and enhance its stability. Nevertheless, the content of carbon or metals in the composites should be strictly controlled because they undoubtedly reduce the overall specific capacity due to their low-capacity or lithium-inactive nature, so forming composite with high-capacity components, such as Si, whose theoretical capacity is 4200 mAh g−1, can be used instead [15–21]. Although Si has some drawbacks, i.e., low conductivity and high brittleness, it is still possible to get an improved performance for NiO/Si nanocomposite due to the combined effects between two components. Therefore, in the present work, NiO is designed into interconnected nanosheet-arrays, and then coated with Si.
2. Experimental The first step is the preparation of NiO nanosheet arrays by chemical bath deposition. The bath solution contains 10.51 g nickel sulfate hexahydrate (NiSO4·6H2O), 2.03 g potassium persulfate (K2S2O8), 10 mL concentrated ammonia (NH3·H2O), and 90 mL distilled water. The deposition proceeded for 1 h at room temperature using nickel foil as the substrate. The precursor film was cleaned and calcined in a quartz-tube furnace at 350 °C for 1 h under flowing argon. The second step is the magnetron sputtering of Si (DE500, DE Technology Co., Ltd., Beijing) on NiO nanosheet arrays. A polycrystalline Si wafer, 6 cm in diameter, was used as the target. High purity argon (99.999%) was used as the working gas and its pressure was kept at 8×10−3 Torr. The sputtering proceeded for 2 h at the radio-frequency power of 60 W. The obtained composite film was finally annealed at 500 °C for 2 h inside the vacuum chamber. Meanwhile, Si film was deposited on bare nickel foil under the same conditions. The mass of the films were determined by a simultaneous thermal analyzer (Netzsch, STA 449 F3).
The films were characterized by X-ray diffraction (XRD, Bruker D8 advance), scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F20). Coin-type cells were assembled in a glove box using Li foil as counter electrode. The electrolyte was 1 mol L−1 LiPF6 in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 V/V). The cells were tested by a battery analyzer (LAND CT2001A) and an electrochemical workstation (AutoLab PGSTAT302N).
3. Results and discussion Fig. 1 presents characterization results of samples. NiO film (Fig. 1a) is porous, assembled by interconnected ultrathin nanosheet arrays. After Si sputtering (Fig. 1b), the interconnected nanosheet-array structure is well preserved, but the thickness increases to about 100 nm. The cross-sectional image show that the upright nanosheets are about 2 µm in height. According to the comparison of TEM images of the nanosheet before and after Si sputtering (Fig. 1c), it can be concluded that the nanoparticles uniformly distributed on the nanosheet are Si. Fig. 1d is the XRD pattern of the composite film. Three strong diffraction peaks are related to nickel substrate. The peaks at 36.8°, 43.2° and 62.5° are originated from (111), (200) and (220) atomic planes of NiO, respectively (JCPDS No. 47-1049). The peaks at 28.4°, 47.3° and 56.1° are ascribed to (111), (220) and (311) planes of Si, respectively (JCPDS No. 27-1402). This indicates that both NiO and Si in the composite film are crystalline. According to the weight increment after chemical bath deposition and magnetron sputtering, it is determined that the areal densities of NiO and Si are 0.24 and 0.26 mg cm−2, respectively. Fig. 2 compares discharge-charge curves of NiO/Si nanosheet arrays, NiO nanosheet arrays and Si film. It can be concluded that NiO/Si composite film shows discharge/charge characteristics of both NiO and Si. The plateau at 0.7 V in the first discharge curve is ascribed to the lithiation process of NiO, and it changes to slopes around 1.2 V in the following discharge curves. The plateaus below 0.3 V mainly correspond to the multi-step lithiation process of Si. In charge curves, the plateaus around 0.5 V mainly correspond to the multi-step delithiation process of Li-Si alloy, and the slopes at 2.1 V correspond to the conversion of Ni to NiO. The first discharge and charge capacities for NiO/Si electrode are 2300 and 1860 mAh g−1, respectively. For NiO electrode, these values are 1100 and 880 mAh g−1, respectively, and for Si electrode, they are 3490 and 2620 mAh g−1,
respectively. The initial coulombic efficiency of NiO/Si electrode is 81%, higher than that of NiO (80%) and Si (75%) electrodes. Fig. 3 compares their CV curves. NiO/Si electrode shows characteristic reduction/oxidation peaks of both NiO and Si. In cathodic curves, the peak at 0.51 V is related to the lithiation process of NiO, and it shifts to 1.10 V in the subsequent cycles. The peaks at 0.25 and 0.14 V are related to the lithiation process of Si. In anodic curves, the peaks at 0.37 and 0.52 V are ascribed to the delithiation process of Li-Si alloy, and the peak at 2.28 V is ascribed to that of NiO. There is another pair of peaks, at 1.55 and 1.51 V, correspond to the formation and decomposition of SEI layer, respectively . The cycling stability of NiO/Si electrode is shown in Fig. 4a. Compared with NiO, it delivers much higher capacities in the whole cycling process. Compared with Si, although the capacity is lower in the initial stages, it shows much better cycling performance. The charge capacity after 100 cycles is 1170 mAh g−1, 63% of the initial value, much higher than that of Si (only 23%). SEM images of electrodes after cycling are compared in Fig. 4b. Only NiO/Si electrode preserves some of its original structure, indicative of better structural stability. The performance of electrodes at different current densities is compared in Fig. 4c. NiO/Si electrode delivers high capacities at high current densities, showing good rate capability. The interconnected nanosheet-array structure plays important role in the enhancement of electrochemical performance. It can provide large interface for electrochemical reactions, short diffusion length of lithium-ions, strong contact between active particles and substrates, and good buffering ability against volume expansion. All of these advantages are very critical to reduce electrode polarization and enhance electrode stability. In addition, NiO and Si have combined actions on the enhancement of electrochemical performance. The Si coating layer can stabilize the interconnected-array structure of NiO nanosheet, and meanwhile, Ni nanoparticles, generated by the decomposition of NiO, which occurs firstly in the lithiation process, can enhance the electrode conductivity and promote the subsequent alloying and dealloying processes of Si.
4. Conclusions Interconnected Si-coated NiO nanosheet arrays have been successfully prepared using chemical bath deposition and magnetron sputtering techniques. The nanosheets are about 100 nm in thickness, and about 2 µm in height. As lithium-ion anode materials, the composite arrays show
electrochemical characteristics of both NiO and Si. The composite film delivers higher reversible capacity than that of NiO, and shows better cycling performance than that of Si. The enhanced performance can be attributed to the interconnected Si-coated NiO nanosheet array structure.
Acknowledgements This work is supported by Zhejiang Provincial Natural Science Foundation of China (Grant No. LY16E020004), and Science and Technology Project of Taizhou (Grant No. 14GY03). We would like to acknowledge them for financial support.
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Fig. 1. Characterization results of the samples. (a) SEM image of NiO film. (b) SEM image of NiO/Si film; the insert is the cross-sectional image. (c) TEM image of NiO/Si nanosheet; the insert is NiO nanosheet. (d) XRD pattern of NiO/Si film.
Fig. 2. Discharge-charge curves of (a) NiO/Si, (b) NiO and (c) Si electrodes at 0.05 A g−1 between 0.02 and 3 V. 7
Fig. 3. CV curves of (a) NiO/Si, (b) NiO and (c) Si electrodes at 0.1 mV s−1 between 0 and 3 V.
Fig. 4. (a) Cycling performances of the three electrodes. (b) SEM images of the three electrodes after cycling. (c) Rate capabilities of the three electrodes.
NiO/Si film is composed of interconnected Si-coated NiO nanosheet arrays.
The areal densities of NiO and Si are 0.24 and 0.26 mg cm−2, respectively.
NiO/Si film shows enhanced performance than those of NiO and Si films.
Interconnected nanosheet-array structure is critical for the enhanced performance.
NiO and Si have combined effect on the enhancement of performance.