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Facile fabrication of binder-free NiO electrodes with high rate capacity for lithium-ion batteries
Applied Surface Science 368 (2016) 298–302
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Facile fabrication of binder-free NiO electrodes with high rate capacity for lithium-ion batteries Lili Gu, Wenhe Xie, Shuai Bai, Boli Liu, Song Xue, Qun Li, Deyan He ∗ School of Physical Science and Technology, Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 23 January 2016 Accepted 29 January 2016 Available online 2 February 2016 Keywords: NiO nanocones Lithium-ion batteries Hydrothermal synthesis Annealing SEI layer
a b s t r a c t NiO nanocone arrays are fabricated on nickel foam substrate by a simple hydrothermal synthesis and a subsequent annealing in air. The obtained architecture is directly used as an anode for lithium-ion batteries without any binder. It delivers a capacity of 969 mAh g−1 in the 120th cycle at a current density of 0.5 C. Even at 10 C, the electrode can still deliver a capacity as high as 605.9 mAh g−1 . The excellent electrochemical performance could be ascribed to the integrity and porosity of the cycled electrodes. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, more and more lithium-ion batteries (LIBs) have been applied in consumer electronics, medical devices, portable power tools, hybrid vehicles and electric vehicles due to their high energy density, high voltage, long-life cycling, environmentally friendly and good safety [1–3]. However, the anode material in most of commercial LIBs is graphite which shows a low specific capacity (the theoretical specific capacity is ∼372 mAh g−1 ) and a poor rate capability. In recent years, transition metal oxides [4–6] are widely investigated as anode materials of LIBs mainly owing to their higher lithium storage capacities. Among these oxides, NiO has been considered as one of the promising anode materials because of its high theoretical specific capacity (∼718 mAh g−1 ), low cost, non-toxicity and natural abundance [7]. However, its practical applications are hindered by huge volume changes during cycling, inherent low electrical conductivity and poor rate capacity [8]. Great efforts have been made to enhance the electrical conductivity and accommodate the volume changes for NiO materials. Especially, the architectures of three-dimensional (3D) nanostructured NiO materials grown directly on current-collecting substrates have been proved to be effective in improving the electrochemical performance of the NiO anodes [9]. The improvement profits from
∗ Corresponding author. E-mail address: [email protected] (D. He). http://dx.doi.org/10.1016/j.apsusc.2016.01.270 0169-4332/© 2016 Elsevier B.V. All rights reserved.
3D nanostructures which can offer easy accessibility for electrolyte, shorten path length for lithium ion transportation, accelerate phase transition and alleviate cracking and crumbling of active materials. Furthermore, the direct growth technique without binders and conductors could enhance the electrical contact between the substrates and the active materials and increase the overall specific capacity of the electrodes. For example, Sun et al. prepared multifunctional Ni/NiO hybrid nanomembranes by annealing assynthesized Ni nanomembranes in air, the resultant anode could tolerate a superhigh current density of 82.6 A g−1 [10]. Wang et al. fabricated NiO nanocone arrays by an electrodeposition and a subsequent thermal oxidation, which delivered a capacity up to 1058 mAh g−1 after 100 cycles at a rate of 0.4 C [11]. Kvasha et al. developed a Ni/NiO based 3D core–shell foam nanostructure by ionic liquid-based electrodeposition, which exhibited a reversible discharge capacity around 0.8 mAh cm−2 [12]. However, to make NiO nanostructures become an applicable anode material for high-power LIBs, further investigations are needed to be done for improving their rate and cyclic capabilities, and it is a growing trend to design nanostructures by a cheap and facile approach for large-scale production. In this work, NiO nanocone arrays were fabricated on nickel foam substrates via a facile hydrothermal synthesis and a subsequent thermal oxidation in air. The major merits of the hydrothermal synthesis are that the operation is simple and the product is of good uniformity and easily scaled-up [13], and the synthesized material is firmly adhered on the substrate [14–16]. The
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Fig. 1. (a) XRD patterns of the used Ni foam and the representative NiO nanocone arrays on Ni foam. (b) Typical Raman spectrum of the NiO nanocone arrays.
unique nanocone structure provides fast transportation for electrons and ions. The representative NiO nanocone array electrode shows a capacity of 969 mAh g−1 in the 120th cycle at a current density of 0.5 C. Even at 10 C, it can deliver a capacity as high as 605.9 mAh g−1 . In addition, such a binder-free nanostructured NiO electrode is expected to be used in other energy storage devices [17,18]. 2. Experimental 2.1. Synthesis of Ni nanocone arrays on nickel foam In a typical experiment, 2.6 mmol NiCl2 ·H2 O and 2.6 mmol cetyltrimethylammonium bromide (CTAB) were solved in 26 mL deionized water and then magnetically stirred for 10 min. The resulting solution, together with 26 mL aqua ammonia, 2.6 mL hydrazine hydrate and the cleared nickel foam piece were transferred into a 65 mL of Teflon-lined stainless steel autoclave. The autoclave was maintained at 180 ◦ C for 60 min. The obtained product was washed with deionized water and ethanol for several times. A microbalance (Mettler XS105DU, 0.01 mg resolution) was employed to weigh the mass of the product. 2.2. Fabrication of NiO nanocone arrays The NiO nanocone arrays were fabricated by annealing the obtained Ni nanocone arrays at 400 ◦ C for 30 min in air. The active mass was derived from mNiO = 4.668m according to the reaction of 2Ni + O2 = 2NiO, where m is the mass difference before and after the oxidization. 2.3. Structural characterization The structures and morphologies of the active materials were characterized by X-ray diffraction (XRD, Rigaku D/Max-2400, Cu K␣ radiation, = 0.15418 nm), high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2 F30), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800), Raman spectrometer (Jobin-Yvon Horiba HR800 with an excitation wavelength of 532 nm), and X-ray photoelectron spectroscopy (XPS, Kratos axis ultra DLD instrument with Al K␣ or Mg K␣ probe beam). 2.4. Electrochemical characterization The electrochemical characterization was carried out using CR-2032 coin cells. The cells were assembled in a high-purity
Fig. 2. SEM images of (a, b) the Ni nanocone arrays and (c, d) NiO nanocone arrays. (e) TEM and (f) HR-TEM images of a single NiO nanocone.
argon-filled glovebox (H2 O, O2 < 0.5 ppm, MBraun, Unilab). The fabricated NiO nanocone array was used as working electrode and lithium foil as counter and reference electrode. The separator membrane was Celgard 2320, and the electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. The galvanostatic discharge–charge tests were conducted with a multichannel battery tester (Neware
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Fig. 3. (a) Cycle voltammetry profiles in the initial three cycles, (b) cyclic performance and the corresponding coulombic efficiency, (c) rate cycling capability, and (d) Nyquist plots of the NiO nanocone array electrode. The inset in (b) shows the galvanostatic charge–discharge voltage profiles in the initial three cycles, the insets in (d) are an equivalent electrical circuit and a magnification of the Nyquist plots in the high frequency range, respectively.
BTS-610), and the cyclic voltammetry (CV) curves were measured with an electrochemical workstation (Chenhua CHI-660C) at room temperature. Electrochemical impedance spectra (EIS) were measured with a frequency range of 100 kHz to 0.01 Hz at a bias voltage of 5 mV. The pristine electrode was tested after aging for 12 h, and the cycled electrode was tested in a charged state after 50 cycles. 3. Results and discussion XRD patterns were measured to evaluate the phase composition of the prepared samples before and after the oxidization. As shown in Fig. 1a, three strong peaks at 2 = 44.4◦ , 51.7◦ and 76.2◦ correspond well to the diffractions from the (111), (200) and (220) faces of metallic Ni (JCPDS 04-0850). The other weak peaks at 2 = 37.4◦ , 43.6◦ and 63.0◦ can be respectively indexed to the diffractions from the (101), (012) and (110) planes of cubic NiO (JCPDS 44-1159) [19]. Raman spectrum measurement was performed on the samples. As shown in Fig. 1b, there are three broad characteristic peaks around at 156.5, 545.4, and 1068.8 cm−1 , which can be attributed to the first-order transverse optical (TO) phonon mode, longitudinal optical (LO) phonon mode, and the combination 2LO of NiO, respectively [20]. Fig. 2a and b shows SEM images of the as-synthesized Ni nanocone arrays on Ni foam. The bottom diameters of the nanocones range from 200 to 500 nm. After a treatment at 400 ◦ C for 30 min in air, as shown in Fig. 2c and d, the obtained NiO nanocone arrays basically maintain the morphology of the as-synthesized Ni nanocone arrays. It can be seen that some pores appear on the NiO
nanocone arrays. As an anode material of LIBs, the rough and porous morphology of the formed NiO nanocones is favorable for the insertion and extraction of lithium-ions during the charge–discharge cycles, as a result of the reduced diffusion paths of lithium-ions and enhanced specific surface area. Fig. 2e and f shows TEM and HR-TEM images of a single NiO nanocone, respectively. The shape and size are consistent with those of the SEM observation, and the marked interplanar spacings are 0.21 nm and 0.24 nm, which correspond to the (012) and (101) planes of cubic NiO, respectively. CV curves were measured in the voltage range between 0.02 and 3.0 V vs. Li+ /Li at a scan rate of 0.2 mV s−1 . As shown in Fig. 3a, a sharp peak appears at ∼0.45 V in the initial cathodic scan, which involves the reduction from NiO to Ni and the formation of solid electrolyte interface (SEI) layer on the electrode surface [21]. In the subsequent cycles, the cathodic peak shifts to ∼1.2 V. The broad peak located at ∼2.2 V in the anodic scans can be attributed to the oxidization process of Ni and partial decomposition of the formed SEI layer [22,23]. From the second scan, the peak intensities as well as the integral areas tend to be the same, which suggest that the electrode has high reversible capacity and improved stability. The electrochemical reaction can be described as follows: NiO + 2Li+ + 2e− ↔ Ni + Li2 O Fig. 3b shows the cyclability and coulombic efficiency of the NiO nanocone array electrode tested at a current density of 0.5 C (1 C is defined as 718 mA g−1 ). It shows a stable cyclability and
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delivers a high reversible capacity of 969 mAh g−1 after 120 cycles. The reversible capacity is much higher than the theoretical capacity of NiO, maybe due to the reversible forming of the polymeric gel-like film resulting from kinetically activated electrolyte degradation [24,25]. After the initial several cycles, the coulombic efficiency increases to nearly 100% with cycling. The inset in Fig. 3b shows the initial three charge–discharge profiles of the NiO nanocone array electrode at a current density of 0.5 C between 0.02 and 3.0 V. In the first discharge curve, it shows a long voltage plateau at ∼0.67 V, corresponding to the formation of SEI layer and the reduction of NiO [26]. The result is in good agreement with that of the CV measurement. The first discharge and charge capacities are 1284.4 mAh g−1 and 880.9 mAh g−1 , respectively. In the second cycle, the discharge plateau shifts to about 1.28 V, the charge plateau is enhanced from ∼2.15 to ∼2.24 V, and a reversible capacity of 840.8 mAh g−1 is attained. In the subsequent cycles, the reversible capacity tends to be stabilized at about 856.8 mAh g−1 . To further evaluate the electrochemical performance of the obtained NiO nanocone array electrode, the rate capability was tested as shown in Fig. 3c. The discharge capacities are 971.2 mAh g−1 , 938.7 mAh g−1 , 924.5 mAh g−1 , 860.1 mAh g−1 , and 741.6 mAh g−1 at current densities of 0.2 C, 0.5 C, 1 C, 2 C and 5 C, respectively. Even at 10 C, it can still deliver a capacity of 605.9 mAh g−1 , which is much higher than the theoretical capacity of graphite. As the current density returns to 0.2 C, the electrode can endow with its initial capacity before the rate test. EIS experiment was employed to investigate lithium ion migration through the formed SEI layer, charge transfer through the electrode–electrolyte interface, and lithium ion diffusion in NiO nanocones [27]. As shown in Fig. 3d, it is clearly seen that Nyquist plots of the typical electrode are composed of semicircles in the high- and medium-frequency regions and an inclined line at low frequencies. The impedance data were analyzed by fitting the equivalent electrical circuit shown in the inset of Fig. 3d. The elements in the equivalent circuit include ohmic electrolyte resistance R0 , SEI layer resistance RSEI and the corresponding capacitances CPE1, charge-transfer resistance Rct at the interface between the electrode and electrolyte and the corresponding capacitances CPE2, Warburg impedance Zw [28–30]. The single semicircle impedance can be attributed to a combination of RSEI and Rct . The fitting parameter Rct (66.8 ) for the 50 time-cycled electrode at 0.5 C is much smaller than that (600 ) for the pristine electrode, and the fitting parameter RSEI is only 22.5 , indicating the improved kinetics of the electrochemical reaction upon cycling [31]. SEM morphology was examined for the NiO nanocone array electrode after 100 cycles at 0.5 C. As shown in Fig. 4a, the electrode well retains its nanocone array morphology, indicating that the NiO nanocones still firmly bond on the current collector. The inset of Fig. 4a is the high-magnification SEM image, a colloidal layer can be obviously seen on the nanocone surfaces, which may be one of the reasons for the improved electrode capacity and reversibility [32]. For comparison, the cycled electrode was immersed in 1 M hydrochloric acid for 1 h. As shown in Fig. 4b, the colloidal layer on the nanocone surfaces disappears and the nanocones become sharper, which indicates that hydrochloric acid has partially eliminated the colloidal layer. Fig. 4c and d shows the TEM morphologies of a single NiO nanocone after cycles. It can be seen that the cycled nanocone still shows the rough and porous architecture. The result is in good agreement with that shown in Fig. 2d. Fig. 4e shows a survey scan XPS spectrum measured using Al K␣ probe beam for the formed colloidal layer on the NiO nanocone array after 100 cycles. A major C 1s peak and a strong O 1s peak can be seen at binding energies of 285 eV and 531.8 eV, respectively, indicating that the formed colloidal layer consists largely of carbon and oxygen. Li and F elements, the corresponding Li 1s and F 1s peaks located at 54.9 and 685.7 eV, are from the electrolyte. It
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Fig. 4. (a, b) SEM images, (c, d) TEM images, and (e) survey scan XPS spectrum of the NiO nanocone array electrode after 100 cycles at 0.5 C. XPS spectrum was measured using Al K␣ probe beam.
should be noted that the signals of Ni element were not detected probably due to a thick colloidal layer formed on the NiO nanocones [33] or an overlaping of Ni 2p signal with F KLL Auger peak. As the XPS measurement was carried out using Mg K␣ probe beam for the samples, we found that F KLL Auger peak locates at 599 eV, while Ni 2p exhibits two signals at 853 eV and 870 eV which can be attributed to the spin-orbit states of Ni 2p3/2 and Ni 2p1/2 in NiO, respectively. However, the Ni 2p signals are very weak, probably because the NiO phase has been buried under the formed SEI layer. The superior reversible capacity, remarkable rate capability and cyclability of the NiO nanocone array electrodes can be ascribed to the featured 3D architecture, which can increase contact area of the electrode and electrolyte [34], shorten transportation paths for lithium-ions and buffer the volume change during the lithiation and delithiation processes [35]. In addition, the direct synthesis of NiO nanocones on Ni foam promotes the electrical contact conductivity between the current collector and the active material.
4. Conclusion In summary, NiO nanocone arrays have been fabricated on nickel foam by a hydrothermal synthesis and a subsequent oxidization. As an anode for LIBs without any binder and conductor, it shows superior rate capacity and excellent cycling stability. The representative electrode delivers a high reversible capacity up to 969 mAh g−1 at a current density of 0.5 C after 120 cycles and a high capacity of 605.9 mAh g−1 at 10 C.
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Acknowledgements This project was financially supported by the National Natural Science Foundation of China with grant no. 11179038 and the Specialized Research Fund for the Doctoral Program of Higher Education with grant no. 20120211130005. References [1] X. Pu, L. Li, H. Song, C. Du, Z. Zhao, C. Jiang, G. Cao, W. Hu, Z.L. Wang, A self-charging power unit by integration of a textile triboelectric nanogenerator and a flexible lithium-ion battery for wearable electronics, Adv. Mater. 27 (2015) 2472–2478. [2] C. Zhang, J. Chen, Y. Zeng, X. Rui, J. Zhu, W. Zhang, C. Xu, T.M. Lim, H.H. Hng, Q. Yan, A facile approach toward transition metal oxide hierarchical structures and their lithium storage properties, Nanoscale 4 (2012) 3718–3724. [3] X. Sun, C. Yan, Y. Chen, W. Si, J. Deng, S. Oswald, L. Liu, O.G. Schmidt, Three-dimensionally “curved” NiO nanomembranes as ultrahigh rate capability anodes for Li-ion batteries with long cycle lifetimes, Adv. Eng. Mater. 4 (2014) 1300912. [4] Q. Hao, D. Zhao, H. Duan, C. Xu, Porous Co3 O4 /CuO composite assembled from nanosheets as high-performance anodes for lithium-ion batteries, ChemSusChem 8 (2015) 1435–1441. [5] T. Li, C. Guo, B. Sun, T. Li, Y. Li, L. Hou, Y. Wei, Well-shaped Mn3 O4 tetragonal bipyramids with good performance for lithium ion batteries, J. Mater. Chem. A 3 (2015) 7248–7254. [6] W. Xie, S. Li, S. Wang, S. Xue, Z. Liu, X. Jiang, D. He, N-doped amorphous carbon coated Fe3 O4 /SnO2 coaxial nanofibers as a binder-free self-supported electrode for lithium ion batteries, ACS Appl. Mater. Interfaces 6 (2014) 20334–20339. [7] N. Wang, L. Chen, X. Ma, J. Yue, F. Niu, H. Xu, J. Yang, Y. Qian, Facile synthesis of hierarchically porous NiO micro-tubes as advanced anode materials for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 16847–16850. [8] X. Wang, L. Qiao, X. Sun, X. Li, D. Hu, Q. Zhang, D. He, Mesoporous NiO nanosheet networks as high performance anodes for Li ion batteries, J. Mater. Chem. A 1 (2013) 4173–4176. [9] P. Lv, H. Zhao, Z. Zeng, C. Gao, X. Liu, T. Zhang, Self-assembled three-dimensional hierarchical NiO nano/microspheres as high-performance anode material for lithium ion batteries, Appl. Surf. Sci. 329 (2015) 301–305. [10] X. Sun, W. Si, X. Liu, J. Deng, L. Xi, L. Liu, C. Yan, O.G. Schmidt, Multifunctional Ni/NiO hybrid nanomembranes as anode materials for high-rate Li-ion batteries, Nano Energy 9 (2014) 168–175. [11] X. Wang, Z. Yang, X. Sun, X. Li, D. Wang, P. Wang, D. He, NiO nanocone array electrode with high capacity and rate capability for Li-ion batteries, J. Mater. Chem. 21 (2011) 9988–9990. [12] A. Kvasha, E. Azaceta, O. Leonet, M. Bengoechea, I. Boyano, R. Tena-Zaera, I.D. Meatza, O. Miguel, H.-J. Grande, J.A. Blazquez, Ni/NiO based 3D core–shell foam anode for lithium ion batteries, Electrochim. Acta 180 (2015) 16–21. [13] C. Zhao, S.-L. Chou, Y. Wang, C. Zhou, H.-K. Liu, S.-X. Dou, A facile route to synthesize transition metal oxide/reduced graphene oxide composites and their lithium storage performance, RSC Adv. 3 (2013) 16597–16603. [14] X. Li, A. Dhanabalan, K. Bechtold, C. Wang, Binder-free porous core–shell structured Ni/NiO configuration for application of high performance lithium ion batteries, Electrochem. Commun. 12 (2010) 1222–1225. [15] J. Zhang, M. Zhang, L. Zhang, Binder-free three-dimensional porous structured metal oxides as anode for high performance lithium-ion battery, Electrochim. Acta 105 (2013) 282–288. [16] L. Sun, M. Li, Y. Jiang, W. Kong, K. Jiang, J. Wang, S. Fan, Sulfur nanocrystals confined in carbon nanotube network as a binder-free electrode for
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Facile fabrication of binder-free NiO electrodes with high rate capacity for lithium-ion batteries Lili Gu, Wenhe Xie, Shuai Bai, Boli Liu, Song Xue, Qun Li, Deyan He ∗ School of Physical Science and Technology, Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 23 January 2016 Accepted 29 January 2016 Available online 2 February 2016 Keywords: NiO nanocones Lithium-ion batteries Hydrothermal synthesis Annealing SEI layer
a b s t r a c t NiO nanocone arrays are fabricated on nickel foam substrate by a simple hydrothermal synthesis and a subsequent annealing in air. The obtained architecture is directly used as an anode for lithium-ion batteries without any binder. It delivers a capacity of 969 mAh g−1 in the 120th cycle at a current density of 0.5 C. Even at 10 C, the electrode can still deliver a capacity as high as 605.9 mAh g−1 . The excellent electrochemical performance could be ascribed to the integrity and porosity of the cycled electrodes. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, more and more lithium-ion batteries (LIBs) have been applied in consumer electronics, medical devices, portable power tools, hybrid vehicles and electric vehicles due to their high energy density, high voltage, long-life cycling, environmentally friendly and good safety [1–3]. However, the anode material in most of commercial LIBs is graphite which shows a low specific capacity (the theoretical specific capacity is ∼372 mAh g−1 ) and a poor rate capability. In recent years, transition metal oxides [4–6] are widely investigated as anode materials of LIBs mainly owing to their higher lithium storage capacities. Among these oxides, NiO has been considered as one of the promising anode materials because of its high theoretical specific capacity (∼718 mAh g−1 ), low cost, non-toxicity and natural abundance [7]. However, its practical applications are hindered by huge volume changes during cycling, inherent low electrical conductivity and poor rate capacity [8]. Great efforts have been made to enhance the electrical conductivity and accommodate the volume changes for NiO materials. Especially, the architectures of three-dimensional (3D) nanostructured NiO materials grown directly on current-collecting substrates have been proved to be effective in improving the electrochemical performance of the NiO anodes [9]. The improvement profits from
∗ Corresponding author. E-mail address: [email protected] (D. He). http://dx.doi.org/10.1016/j.apsusc.2016.01.270 0169-4332/© 2016 Elsevier B.V. All rights reserved.
3D nanostructures which can offer easy accessibility for electrolyte, shorten path length for lithium ion transportation, accelerate phase transition and alleviate cracking and crumbling of active materials. Furthermore, the direct growth technique without binders and conductors could enhance the electrical contact between the substrates and the active materials and increase the overall specific capacity of the electrodes. For example, Sun et al. prepared multifunctional Ni/NiO hybrid nanomembranes by annealing assynthesized Ni nanomembranes in air, the resultant anode could tolerate a superhigh current density of 82.6 A g−1 [10]. Wang et al. fabricated NiO nanocone arrays by an electrodeposition and a subsequent thermal oxidation, which delivered a capacity up to 1058 mAh g−1 after 100 cycles at a rate of 0.4 C [11]. Kvasha et al. developed a Ni/NiO based 3D core–shell foam nanostructure by ionic liquid-based electrodeposition, which exhibited a reversible discharge capacity around 0.8 mAh cm−2 [12]. However, to make NiO nanostructures become an applicable anode material for high-power LIBs, further investigations are needed to be done for improving their rate and cyclic capabilities, and it is a growing trend to design nanostructures by a cheap and facile approach for large-scale production. In this work, NiO nanocone arrays were fabricated on nickel foam substrates via a facile hydrothermal synthesis and a subsequent thermal oxidation in air. The major merits of the hydrothermal synthesis are that the operation is simple and the product is of good uniformity and easily scaled-up [13], and the synthesized material is firmly adhered on the substrate [14–16]. The
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Fig. 1. (a) XRD patterns of the used Ni foam and the representative NiO nanocone arrays on Ni foam. (b) Typical Raman spectrum of the NiO nanocone arrays.
unique nanocone structure provides fast transportation for electrons and ions. The representative NiO nanocone array electrode shows a capacity of 969 mAh g−1 in the 120th cycle at a current density of 0.5 C. Even at 10 C, it can deliver a capacity as high as 605.9 mAh g−1 . In addition, such a binder-free nanostructured NiO electrode is expected to be used in other energy storage devices [17,18]. 2. Experimental 2.1. Synthesis of Ni nanocone arrays on nickel foam In a typical experiment, 2.6 mmol NiCl2 ·H2 O and 2.6 mmol cetyltrimethylammonium bromide (CTAB) were solved in 26 mL deionized water and then magnetically stirred for 10 min. The resulting solution, together with 26 mL aqua ammonia, 2.6 mL hydrazine hydrate and the cleared nickel foam piece were transferred into a 65 mL of Teflon-lined stainless steel autoclave. The autoclave was maintained at 180 ◦ C for 60 min. The obtained product was washed with deionized water and ethanol for several times. A microbalance (Mettler XS105DU, 0.01 mg resolution) was employed to weigh the mass of the product. 2.2. Fabrication of NiO nanocone arrays The NiO nanocone arrays were fabricated by annealing the obtained Ni nanocone arrays at 400 ◦ C for 30 min in air. The active mass was derived from mNiO = 4.668m according to the reaction of 2Ni + O2 = 2NiO, where m is the mass difference before and after the oxidization. 2.3. Structural characterization The structures and morphologies of the active materials were characterized by X-ray diffraction (XRD, Rigaku D/Max-2400, Cu K␣ radiation, = 0.15418 nm), high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2 F30), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800), Raman spectrometer (Jobin-Yvon Horiba HR800 with an excitation wavelength of 532 nm), and X-ray photoelectron spectroscopy (XPS, Kratos axis ultra DLD instrument with Al K␣ or Mg K␣ probe beam). 2.4. Electrochemical characterization The electrochemical characterization was carried out using CR-2032 coin cells. The cells were assembled in a high-purity
Fig. 2. SEM images of (a, b) the Ni nanocone arrays and (c, d) NiO nanocone arrays. (e) TEM and (f) HR-TEM images of a single NiO nanocone.
argon-filled glovebox (H2 O, O2 < 0.5 ppm, MBraun, Unilab). The fabricated NiO nanocone array was used as working electrode and lithium foil as counter and reference electrode. The separator membrane was Celgard 2320, and the electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. The galvanostatic discharge–charge tests were conducted with a multichannel battery tester (Neware
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Fig. 3. (a) Cycle voltammetry profiles in the initial three cycles, (b) cyclic performance and the corresponding coulombic efficiency, (c) rate cycling capability, and (d) Nyquist plots of the NiO nanocone array electrode. The inset in (b) shows the galvanostatic charge–discharge voltage profiles in the initial three cycles, the insets in (d) are an equivalent electrical circuit and a magnification of the Nyquist plots in the high frequency range, respectively.
BTS-610), and the cyclic voltammetry (CV) curves were measured with an electrochemical workstation (Chenhua CHI-660C) at room temperature. Electrochemical impedance spectra (EIS) were measured with a frequency range of 100 kHz to 0.01 Hz at a bias voltage of 5 mV. The pristine electrode was tested after aging for 12 h, and the cycled electrode was tested in a charged state after 50 cycles. 3. Results and discussion XRD patterns were measured to evaluate the phase composition of the prepared samples before and after the oxidization. As shown in Fig. 1a, three strong peaks at 2 = 44.4◦ , 51.7◦ and 76.2◦ correspond well to the diffractions from the (111), (200) and (220) faces of metallic Ni (JCPDS 04-0850). The other weak peaks at 2 = 37.4◦ , 43.6◦ and 63.0◦ can be respectively indexed to the diffractions from the (101), (012) and (110) planes of cubic NiO (JCPDS 44-1159) [19]. Raman spectrum measurement was performed on the samples. As shown in Fig. 1b, there are three broad characteristic peaks around at 156.5, 545.4, and 1068.8 cm−1 , which can be attributed to the first-order transverse optical (TO) phonon mode, longitudinal optical (LO) phonon mode, and the combination 2LO of NiO, respectively [20]. Fig. 2a and b shows SEM images of the as-synthesized Ni nanocone arrays on Ni foam. The bottom diameters of the nanocones range from 200 to 500 nm. After a treatment at 400 ◦ C for 30 min in air, as shown in Fig. 2c and d, the obtained NiO nanocone arrays basically maintain the morphology of the as-synthesized Ni nanocone arrays. It can be seen that some pores appear on the NiO
nanocone arrays. As an anode material of LIBs, the rough and porous morphology of the formed NiO nanocones is favorable for the insertion and extraction of lithium-ions during the charge–discharge cycles, as a result of the reduced diffusion paths of lithium-ions and enhanced specific surface area. Fig. 2e and f shows TEM and HR-TEM images of a single NiO nanocone, respectively. The shape and size are consistent with those of the SEM observation, and the marked interplanar spacings are 0.21 nm and 0.24 nm, which correspond to the (012) and (101) planes of cubic NiO, respectively. CV curves were measured in the voltage range between 0.02 and 3.0 V vs. Li+ /Li at a scan rate of 0.2 mV s−1 . As shown in Fig. 3a, a sharp peak appears at ∼0.45 V in the initial cathodic scan, which involves the reduction from NiO to Ni and the formation of solid electrolyte interface (SEI) layer on the electrode surface [21]. In the subsequent cycles, the cathodic peak shifts to ∼1.2 V. The broad peak located at ∼2.2 V in the anodic scans can be attributed to the oxidization process of Ni and partial decomposition of the formed SEI layer [22,23]. From the second scan, the peak intensities as well as the integral areas tend to be the same, which suggest that the electrode has high reversible capacity and improved stability. The electrochemical reaction can be described as follows: NiO + 2Li+ + 2e− ↔ Ni + Li2 O Fig. 3b shows the cyclability and coulombic efficiency of the NiO nanocone array electrode tested at a current density of 0.5 C (1 C is defined as 718 mA g−1 ). It shows a stable cyclability and
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delivers a high reversible capacity of 969 mAh g−1 after 120 cycles. The reversible capacity is much higher than the theoretical capacity of NiO, maybe due to the reversible forming of the polymeric gel-like film resulting from kinetically activated electrolyte degradation [24,25]. After the initial several cycles, the coulombic efficiency increases to nearly 100% with cycling. The inset in Fig. 3b shows the initial three charge–discharge profiles of the NiO nanocone array electrode at a current density of 0.5 C between 0.02 and 3.0 V. In the first discharge curve, it shows a long voltage plateau at ∼0.67 V, corresponding to the formation of SEI layer and the reduction of NiO [26]. The result is in good agreement with that of the CV measurement. The first discharge and charge capacities are 1284.4 mAh g−1 and 880.9 mAh g−1 , respectively. In the second cycle, the discharge plateau shifts to about 1.28 V, the charge plateau is enhanced from ∼2.15 to ∼2.24 V, and a reversible capacity of 840.8 mAh g−1 is attained. In the subsequent cycles, the reversible capacity tends to be stabilized at about 856.8 mAh g−1 . To further evaluate the electrochemical performance of the obtained NiO nanocone array electrode, the rate capability was tested as shown in Fig. 3c. The discharge capacities are 971.2 mAh g−1 , 938.7 mAh g−1 , 924.5 mAh g−1 , 860.1 mAh g−1 , and 741.6 mAh g−1 at current densities of 0.2 C, 0.5 C, 1 C, 2 C and 5 C, respectively. Even at 10 C, it can still deliver a capacity of 605.9 mAh g−1 , which is much higher than the theoretical capacity of graphite. As the current density returns to 0.2 C, the electrode can endow with its initial capacity before the rate test. EIS experiment was employed to investigate lithium ion migration through the formed SEI layer, charge transfer through the electrode–electrolyte interface, and lithium ion diffusion in NiO nanocones [27]. As shown in Fig. 3d, it is clearly seen that Nyquist plots of the typical electrode are composed of semicircles in the high- and medium-frequency regions and an inclined line at low frequencies. The impedance data were analyzed by fitting the equivalent electrical circuit shown in the inset of Fig. 3d. The elements in the equivalent circuit include ohmic electrolyte resistance R0 , SEI layer resistance RSEI and the corresponding capacitances CPE1, charge-transfer resistance Rct at the interface between the electrode and electrolyte and the corresponding capacitances CPE2, Warburg impedance Zw [28–30]. The single semicircle impedance can be attributed to a combination of RSEI and Rct . The fitting parameter Rct (66.8 ) for the 50 time-cycled electrode at 0.5 C is much smaller than that (600 ) for the pristine electrode, and the fitting parameter RSEI is only 22.5 , indicating the improved kinetics of the electrochemical reaction upon cycling [31]. SEM morphology was examined for the NiO nanocone array electrode after 100 cycles at 0.5 C. As shown in Fig. 4a, the electrode well retains its nanocone array morphology, indicating that the NiO nanocones still firmly bond on the current collector. The inset of Fig. 4a is the high-magnification SEM image, a colloidal layer can be obviously seen on the nanocone surfaces, which may be one of the reasons for the improved electrode capacity and reversibility [32]. For comparison, the cycled electrode was immersed in 1 M hydrochloric acid for 1 h. As shown in Fig. 4b, the colloidal layer on the nanocone surfaces disappears and the nanocones become sharper, which indicates that hydrochloric acid has partially eliminated the colloidal layer. Fig. 4c and d shows the TEM morphologies of a single NiO nanocone after cycles. It can be seen that the cycled nanocone still shows the rough and porous architecture. The result is in good agreement with that shown in Fig. 2d. Fig. 4e shows a survey scan XPS spectrum measured using Al K␣ probe beam for the formed colloidal layer on the NiO nanocone array after 100 cycles. A major C 1s peak and a strong O 1s peak can be seen at binding energies of 285 eV and 531.8 eV, respectively, indicating that the formed colloidal layer consists largely of carbon and oxygen. Li and F elements, the corresponding Li 1s and F 1s peaks located at 54.9 and 685.7 eV, are from the electrolyte. It
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Fig. 4. (a, b) SEM images, (c, d) TEM images, and (e) survey scan XPS spectrum of the NiO nanocone array electrode after 100 cycles at 0.5 C. XPS spectrum was measured using Al K␣ probe beam.
should be noted that the signals of Ni element were not detected probably due to a thick colloidal layer formed on the NiO nanocones [33] or an overlaping of Ni 2p signal with F KLL Auger peak. As the XPS measurement was carried out using Mg K␣ probe beam for the samples, we found that F KLL Auger peak locates at 599 eV, while Ni 2p exhibits two signals at 853 eV and 870 eV which can be attributed to the spin-orbit states of Ni 2p3/2 and Ni 2p1/2 in NiO, respectively. However, the Ni 2p signals are very weak, probably because the NiO phase has been buried under the formed SEI layer. The superior reversible capacity, remarkable rate capability and cyclability of the NiO nanocone array electrodes can be ascribed to the featured 3D architecture, which can increase contact area of the electrode and electrolyte [34], shorten transportation paths for lithium-ions and buffer the volume change during the lithiation and delithiation processes [35]. In addition, the direct synthesis of NiO nanocones on Ni foam promotes the electrical contact conductivity between the current collector and the active material.
4. Conclusion In summary, NiO nanocone arrays have been fabricated on nickel foam by a hydrothermal synthesis and a subsequent oxidization. As an anode for LIBs without any binder and conductor, it shows superior rate capacity and excellent cycling stability. The representative electrode delivers a high reversible capacity up to 969 mAh g−1 at a current density of 0.5 C after 120 cycles and a high capacity of 605.9 mAh g−1 at 10 C.
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