Synthesis of polyaniline shell on nickel oxide nanoflake arrays for enhanced lithium ion storage

Accepted Manuscript Title: Synthesis of polyaniline shell on nickel oxide nanoflake arrays for enhanced lithium ion storage Authors: Hong Ma, Xiufeng Liu, Dong Zhang, Jiayuan Xiang PII: DOI: Reference:

S0025-5408(17)31059-0 http://dx.doi.org/doi:10.1016/j.materresbull.2017.04.033 MRB 9294

To appear in:

MRB

Received date: Revised date: Accepted date:

19-3-2017 31-3-2017 10-4-2017

Please cite this article as: Hong Ma, Xiufeng Liu, Dong Zhang, Jiayuan Xiang, Synthesis of polyaniline shell on nickel oxide nanoflake arrays for enhanced lithium ion storage, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.04.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of polyaniline shell on nickel oxide nanoflake arrays for enhanced lithium ion storage Hong Maa*, Xiufeng Liub, Dong Zhangc, Jiayuan Xiangd* a

Shandong Provincial Key Laboratory of Optics and Photonic device, School of Physics and Electronics,

Shandong Normal University, Jinan 250014, China b

Wenzhou Environmental Monitoring Center Station, Wenzhou, 35000, P. R. China

c

Hang Zhou Institute of Calibration and Testing for Quality and Technical Supervision,

Hangzhou 310019, China d Research

Institute of Narada Power Source Co., Ltd, Hangzhou 311305, China

* Corresponding author. Tel.: +86 531-86180066; Fax: +86 531-86180066

NiO/PANI NiO

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Self-supported NiO/PANI core/shell arrays are prepared by successive electrodeposition methods and exhibit good lithium ion storage with high capacity & stable cycling life.

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Highlights 

> Construct self-supported porous NiO/PANI core/shell arrays

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> Porous NiO/PANI composite arrays show high capacity and good cycling life

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> Hierarchical porous structure is favorable for fast ion/electron transfer

Abstract

Developing conducting polymer/oxide composites are of great importance to construct advanced electrochemical devices. In this work, we report a successive electrodeposition methods to fabricate polyaniline (PANI) on nickel oxide (NiO) nanoflake forming composite core-shell arrays. A thin layer of PANI of 10 nm is successfully coated on the surface of NiO nanoflake core. High porosity and enhanced conductivity are obtained in this composite electrode. As anode materials for lithium ion batteries, the NiO/PANI core-shell arrays exhibit weaker polarization and better cycling performance as compared to the bare NiO film. The second discharge capacity of NiO/PANI composite arrays is about 780 mAh g1 at 0.1 A g-1, higher than that of the bare NiO film (679 mAh g1). The improvements of the electrochemical properties are attributed to the PANI, which forms a uniform conductive network leading to improved electrochemical performance. Keywords: Porous materials; Thin films; Li ion batteries; Metal oxides; Energy storage and conversion

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1. Introduction Since the pioneering work by Tarascon team in 2000 [1], transition metal oxides have been widely studied as anodes for high-capacity lithium ion batteries (LIBs) because of high capacity and excellent redox reactivity [2-4]. Among them, NiO is considered as one of the most promising anodes due to the fact that it can deliver high capacity more than 700 mAh g1 at high current densities [5-9]. Nevertheless, its application has been hindered by fast capacity fading due to low electrical conductivity and severe volume change during cycles. To overcome this headwind, it is highly necessary to design porous composite structure to achieve high performance by using short diffusion path of ions/electrons, and stable structural stability [10-13]. Typically, fabrication of porous NiO based composites with other conductive components is an effective way to boost power/energy density and enhance cycling life [14-16]. Polyaniline (PANI) is one of the most popular conducting polymers because of its high conductivity and easy deposition. Previously, Huang et al. [17] prepared PANI on nickel oxides to enhance the rate capability with the help of chemical bath deposition and electrodeposition. But its cycling life is still not satisfactory. There is still great room left for improving the high-rate performance. It is accepted that electrodeposited NiO nanoflake arrays show good Li ion storage capacity, much better than those prepared by chemical bath deposition [18]. In recent years, anodic electrodeposition of NiO nanoflake arrays has been demonstrated and this deposition is general. In this present work, we have developed a successive electrodeposition method to coat PANI shell on the electrodeposited NiO nanoflake core. Uniform core/shell combination between PANI and electrodeposited NiO is

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realized in the integrated film electrode. The Li ion storage performance of NiO/PANI core/shell arrays is thoroughly characterized and proven with better capacity and superior cycles to the unmodified NiO nanoflake arrays. This enhancement is due to the introduction of conductive PANI network facilitating fast electron transfer and enhancing structural stability. 2. Experimental The NiO/PANI films were fabricated as follows. First, clean nickel foam was used as the deposition substrate for the growth of electrodeposited NiO nanoflake arrays, which was conducted in a three-electrode system. The electrolyte consisted of 0.5 M NiSO4 and 0.1 M sodium acetate. The anodic current was 1 mA cm-2. The electrodeposition was 1h. Pt plate was used as the counter electrode. Ag/AgCl electrode was used as the reference electrode. Then, the sample was annealed in argon for 2 h to obtain NiO nanoflake arrays. After that, PANI was coated on the NiO nanoflake arrays by another anodic electrodeposition. The electrolyte consisted of 10 mmol L1 of aniline and 100 mmol L1 of LiClO4. The anodic deposition of PANI was performed at a current density of 1 mA cm-2 for 600 s with a saturated calomel electrode as the reference electrode, and a Pt foil as the counter electrode. The morphology and microstructure of samples were characterized by a field emission scanning electron microscopy (FESEM, Hitachi S-4700) and X-ray diffraction (XRD, Philips PCAPD with Cu K radiation), and Fourier transform infrared spectrometry (FTIR, Thermo Nicolet-380). The loading weight for NiO and PANI was about 2.5 and 0.3 mg cm-2, respectively.

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Test cells were assembled in a glove box filled with argon using the NiO or NiO/PANI film as the working electrode, Li foil as the counter electrode, and polypropylene film as the separator. The electrolyte was a mixed solution containing ethylene carbonate and diethyl carbonate, in which dissolved 1 mol L1 of LiPF6. The galvanostatic charge-discharge tests were conducted on LAND battery program-control test system from 0.02 to 3.0 V (versus Li/Li+) at room temperature (25  1°C). Cyclic voltammetry (CV) tests were carried out using the CHI660C electrochemical workshop. 3. Results and discussion Fig. 1 shows the synthesis schematics of NiO/PANI core/shell arrays. Obviously, NiO nanoflake arrays are acted as backbone for the deposition of PANI shell forming the final NiO/PANI core/shell arrays. As shown in Fig. 1b-c, it is seen that the electrodeposited NiO nanoflake film is porous and composed of many interconnected NiO nanoflakes of 20-30 nm. It is noted that the electrodeposited NiO nanoflakes are perpendicular to the substrate and construct a highly porous net-like structure with pore diameters of 100-250 nm (Fig. 1b-c). After the following electrodeposition, the porous nanoflake morphology is still well kept and the NiO nanoflake is intimately wrapped by the electrodeposited PANI shell forming core/shell nanoflake arrays (Fig. 1d-e). The appearance becomes rougher and the porous channel does not be blocked. It can be seen that the nanoflakes become thicker up to 30-40 nm. The large pore diameters are about 60-230 nm (Fig. 1d-e). Obviously, this porous composite structure possesses a large electrode/electrolyte contact area, and short diffusion length of lithium ions. This is beneficial for the improvement of high-rate performance [7, 1922].

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Fig. 2 shows TEM images of NiO nanoflake and NiO/PANI composite nanoflake. Note that the pure NiO nanoflake shows smooth texture (Fig. 2a), while the NiO/PANI composite nanoflake shows much rougher appearance (Fig. 2b) due to the uniform coating of PANI shell. Selected area electron diffraction (SAED) of both samples show that only polycrystalline cubic NiO patterns are seen, indicating the PANI shell is amorphous. Meanwhile, it is demonstrated the deposition of PANI on the NiO core is very compatible and will not harm the NiO backbone. According to the Fig. 3a, it is seen that typical NiO diffraction peaks exist in both samples, and no peaks of PANI are noticed, indicating the amorphous nature of PANI. This result is consistent with that of SAED analysis above. The presence of NiO and PANI is further confirmed by FTIR spectrum of NiO/PANI composite nanoflake (Fig. 3b). The peak at 3430 cm1 is due to OH group of adsorbed water in the composite. Two characteristic peaks of PANI at 1585cm1 and 1502 cm1 corresponding to the quinoid ring (Q) and the benzene ring, are detected, respectively. The bands at 1138 and 1305 cm1 are the CN stretching band of an aromatic amine. The characteristic peak at 1138 cm1 is owing to the N=Q=N stretching. The peak at 412 cm1 is characteristic of Ni-O stretching. All these results above support each other demonstrating the successful formation of NiO/PANI core/shell arrays. The electrochemical comparison of NiO nanoflake arrays and NiO/PANI core/shell arrays is shown in Fig. 4. As shown in Fig. 4a, it is noted that, for the pure NiO nanoflake arrays, the reduction peak at 1.36 1.23 V associated with the formation of solid electrolyte interphase (SEI) layer. The main reduction peak at 0.72 V is due to the reaction between NiO and Li+ forming Ni and Li2O. The oxidation peaks at 1.55 and 2.32 V are due to the partial

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decomposition of the SEI layer, and decomposition of Li2O reforming NiO. Obviously, the NiO/PANI core/shell arrays show much higher peak current densities and larger enclosed CV loop, indicating better electrochemical reactivity. In addition, the NiO/PANI core/shell arrays show corresponding higher reduction peaks and lower oxidation peaks, implying lower polarization due to the introduction of PANI shell, which can enhance the electrical conductivity and facilitate the fast electron transfer. The enhanced electrochemical properties are also reflected by charge/discharge curves at 0.1 A g-1 at the second cycle (Fig. 4b). The discharge specific capacity of the NiO/PANI core/shell arrays and NiO nanoflake is 781 and 679 mAh g1, respectively. In addition, the NiO/PANI core/shell arrays exhibit a higher discharge plateau and a lower charge plateau than the NiO nanoflake arrays, indicating smaller potential hysteresis and lower internal resistance. The improved performance is mainly due to the enhancement of electrochemical activity and reaction kinetics via the PANI shell. Fig. 4c shows the rate capability of both arrays. In strong contrast to the NiO nanoflake arrays, the NiO/PANI core/shell arrays show superior high-rate capability than the unmodified NiO nanoflake arrays. Taking 0.5 and 2 A g -1 for example, the capacity for the NiO/PANI core/shell arrays is 723 and 627 mAh g1, respectively, higher than those of NiO nanoflake arrays (550 and 398 mAh g1). The PANI shell can greatly improve the cycling life (Fig. 4d). Set at 0.1 A g-1, after 50 cycles, the NiO/PANI core/shell arrays exhibit a specific capacity of 650 mAh g1 at 0.1 A g1, much higher than the bare NiO nanoflake arrays (401 mAh g1 at 0.1 A g1). These values are also much better than other NiO nanoflakes prepared by chemical bath deposition. The improved performance is due to the following factors. The PANI shell constructs an integrated conductive network for the metal oxides to facilitate a facile electron transport, leading to enhanced reaction kinetics [23-25]. 7

In addition, the PANI shell serves as an armor to protect the NiO nanoflake core from pulverization, resulting in improved cycling performance [26, 27].

4. Conclusions We have constructed a successive electrodeposition method for rational fabrication of porous NiO/PANI composite core/shell arrays. The deposition of PANI can enhance the fast electron transfer and improve the structural stability. As cathode of lithium ion batteries, the obtained NiO/PANI composite core/shell arrays show superior Li ion storage capacity than bare NiO counterpart. Due to the highly conductive network composed of PANI shell, higher capacity and lower polarization are demonstrated in the composite core/shell arrays. Our research method shows a new way for construction of advanced anodes for Li ion batteries.

5. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 11304186), Excellent Young Scholars Research Fund of Shandong Normal University.

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Figure captions Fig. 1 (a) Schematics of synthesis of NiO/PANI core/shell arrays. SEM images of (b-c) NiO nanoflake arrays and (d-e) NiO/PANI core/shell arrays. Fig. 2 TEM images (a) NiO nanoflake and (b) NiO/PANI composite nanoflake. Fig. 3 (a) XRD patterns of NiO nanoflake and NiO/PANI core/shell composite arrays. (b) FTIR spectrum of NiO/PANI core/shell composite arrays. Fig. 4 Electrochemical comparison of NiO nanoflake arrays and NiO/PANI core/shell arrays. (a) CV curves at a scan rate of 0.1 mV s-1 at the second cycle. (b) Charge/discharge curves at 0.1 A g-1 at the second cycle. (c) Rate capability and (d) Cycling life at 0.1 A g-1.

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