Synthesis of nickel sulfide monolayer hollow spheres arrays as cathode materials for alkaline batteries

Synthesis of nickel sulfide monolayer hollow spheres arrays as cathode materials for alkaline batteries

Materials Letters 178 (2016) 120–123 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet S...

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Materials Letters 178 (2016) 120–123

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Synthesis of nickel sulfide monolayer hollow spheres arrays as cathode materials for alkaline batteries Wen Zhang a, Xiaoyan Yan a,n, Xili Tong b,n, Jing Yang a, Ling Miao a, Yaoyao Sun a, Liyuan Peng a a b

Institute of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, PR China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan 030001, China

art ic l e i nf o

a b s t r a c t

Article history: Received 21 March 2016 Received in revised form 19 April 2016 Accepted 27 April 2016 Available online 27 April 2016

In this work, we report interconnected nickel sulfide (NiS) monolayer hollow spheres arrays via a facile template-assisted electrodepositionþ ion exchange method. The obtained NiS arrays show unique architecture composed of cross-linked monolayer hollow spheres with diameters of ∼500 nm. The NiS monolayer hollow sphere (MHSA) is composed of nanoparticles of 5–10 nm. The NiS MHSA electrode is investigated as cathode of high-rate alkaline batteries, and delivers a high capacity (68.5 mAh g  1 at 2 Ag  1) and good cycling life (65 mAh g  1 at 2 Ag  1 after 3000 cycles). The interconnected monolayer hollow spheres structure is responsible for the good electrochemical performances attributed to its fast transfer paths for ions/electrons, and large contact area between and electrolyte and active materials. & 2016 Elsevier B.V. All rights reserved.

Keywords: Nickel sulfides Porous materials Thin films Energy storage and conversion Alkaline battery

1. Introduction High-rate alkaline batteries with metal oxides/sulfides (such as CoO, NiO, CoS, NiS) [1–3] cathodes is one of the most promising secondary battery systems. It is typical that strong redox reactions take place at cathodes and deliver high capacity with obvious charge/discharge plateaus [4]. But in recent years, this kind of cathode, sometimes, is wrongly called as pseudo-capacitive cathode [5–7]. In 2014 and 2015, Brousse and Gogotsi groups not only pointed out their differences, but also raveled out the conceptual confusion and redefined these two terms [5,6]. According to their definitions, metal oxides/sulfides or hydroxides with obvious charge/discharge plateaus and redox behaviors, is classified as battery-type materials, not pseudo-capacitive materials. In view of their high-rate application, they are usually used as cathodes of high-rate alkaline batteries due to their fast recharge capability, high power density and long cycling life. Among the explored cathode systems, NiS has been regarded as one of the most competitive candidates due to its high redox reactivity and large capacity [7,8]. Nevertheless, the NiS bulk cathode still suffers from some problems, such as slow transfer of ions/ electrons resulting in poor electrochemical reaction kinetics, n

Corresponding authors. E-mail addresses: [email protected] (X. Yan), [email protected] (X. Tong). http://dx.doi.org/10.1016/j.matlet.2016.04.204 0167-577X/& 2016 Elsevier B.V. All rights reserved.

compromises of high-rate capability and reversibility, as well as poor cycling stability. To circumvent the problems, binder-free electrode design strategy, particularly, porous integrated arrays have been adopted to improve the transfer of ions/electrons leading to enhanced power/energy densities [9,10]. It is proven that, as compared with the bulk powder counterparts, the porous integrated arrays possess lots of merits as follows. 1) Large surface area and better electrical contact with the substrates are combined together in one electrode. 2) Nanoarrays architecture can effectively accommodate the strains caused by the redox reactions and keep electrode structure stable, leading to improved high-rate cycling stability. 3) No polymer binders and additives is used and this can reduce the inner resistance of electrode. 4) High porosity with different pore systems can provide short transfer paths of ions/electrons, resulting in better electrochemical reaction kinetics and higher utilization of active materials. Currently, this integrated arrays electrode is demonstrated reasonable and successful in metal oxides/sulfides (e.g., Co3O4 [11], CoO [12] ) arrays and impressive electrochemical enhancement (supercapacitors and Li ion batteries) has been demonstrated. Over recent years, integrated hollow nanosphere arrays have attracted great attention owing to their unique hollow configuration. To date, there is no report on the synthesis of NiS monolayer hollow spheres arrays (MHSA) and their application as cathode of high-rate alkaline batteries. In the present work, we report NiS MHSA by a facile template-assisted electro-deposition (ED) þ ion exchange (IE) method. Highly porous architecture composed of

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hollow cross-linked spheres is achieved in one electrode. As the cathodes of high-rate alkaline batteries, the interconnected NiS MHSA electrode exhibits a high capacity and good high-rate cycling life due to the unique monolayer hollow sphere architecture. The proposed method can be useful for fabrication of other porous metal sulfide hollow spheres arrays for applications electrochemical energy storage and conversion.

2. Experimental The NiS MHSA electrode was prepared by a polystyrene sphere (PS) template-assisted ED plus IE method. The multilayer PS (particle size of ∼500 nm) template was assembled on the nickel

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substrate described in detail in the previous work [13]. Then, an cathodic ED method was performed in a standard three-electrode cell at 25 °C, with the above PS spheres template electrode as the working electrode, saturated calomel electrode (SCE) as the reference electrode and a Pt foil as the counter-electrode. The electrolyte consisted of 0.25 M Ni(NO3)2 þ0.05 M NaNO3. The ED was conducted at a cathodic current density of 1 mA cm  2 for 15 min Then, the sample was immersed in toluene for 12 h to remove the PS spheres template, and followed by annealing at 350 °C in argon for 2 h. After that, the sample was immersed into 0.1 M Na2S and kept at 90 °C for 9 h, and finally annealed at 450 °C in argon for 1 h to form NiS MHSA electrode. The mass of NiS was about 0.65 mg cm  2, respectively. The inductively coupled plasma-optical emission spectroscopy (ICP-OES, Spectro Arcos) analysis and

Fig. 1. Structural and morphological characterizations of NiS monolayer hollow sphere arrays: (a, b) SEM images (profile image in inset); (c–e) TEM-HRTEM images and (f) XRD pattern.

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elemental analysis were used to determine the load weight of different components. The morphology and microstructure of samples were characterized by X-ray power diffraction (XRD, Rigaku D/max 2550 PC, Cu Kα), a scanning electron microscopy (SEM, Hitachi S-4700 and FESEM, FEI Sirion-100) and transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 200 kV). The electrochemical performances were tested in a threeelectrode electrochemical cell with 6 M KOH electrolyte. The NiS MHSA electrode was used as the working electrode, Hg/HgO as the reference electrode and a Pt foil as the counter-electrode. A series of electrochemical tests including cyclic voltammetry (CV), and galvanostatic charge/discharge measurements were performed on CHI660c electrochemical workstation and Neware battery program-control test system.

(300) planes of NiS phase (JCPDS 12–0041) (Fig. 1(e)). The phase of the samples are confirmed by the XRD pattern (Fig. 1(f)). Except for the peaks of nickel foil substrate, the other diffraction peaks are indexed to the NiS phase (JCPDS 12–0041). Taking the results above, it is justified that the porous monolayer NiS MHSA can be fabricated by the combination of template-assisted EDþ IE methods, and have impressive porous system favorable for the fast ion/ electron transportation. The electrochemical performance of the NiS MHSA are tested as cathode for high-rate alkaline batteries. Cyclic voltammetry (CV) curves of the NiS MHSA electrode at different scan rates are shown in Fig. 2(a). Only one redox couple is observed in the CV curves, indicating that the capacities are mainly governed by the faradaic redox reactions, which can be simply expressed as follows [8]. NiSþ OH  - e  2 NiSOH

3. Results and discussion Fig. 1(a) and (b) show SEM images of the NiS MHSA on nickel foil substrates. Notice that the NiS spheres are well organized into to close-packed arrays. The NiS spheres have diameters of ∼500 nm and are perpendicular to the substrate. As shown in the profile image, the monolayer hollow spheres structure is clearly observed. The wall thicknesses of hollow spheres are ∼25 nm. As shown in TEM images (Fig. 1c–d), the hollow spheres are composed of interconnected nanoparticles of 5–10 nm. The whole architecture is highly open and porously connected. This characteristic is beneficial for fast transfer of ions/electrons and provide large active sites for electrochemical reactions. Additionally, the lattice fringes with a lattice spacing of ∼0.28 nm correspond to the

(1)

Discharge curves and corresponding specific capacities of the NiS MHSA electrode at different current densities are presented in Fig. 2(b) and (c). Note that the NiS MHSA electrode exhibits good high-rate capability with a capacity of 68.5 mAh g  1 at 2 Ag  1, 62.5 mAh g  1 at 5 Ag  1, 55.3 mAh g  1 at 10 Ag  1, 49.0 mAh g  1 at 20 Ag  1, and 46.2 mAh g  1 at 30 Ag  1, respectively. 67% of capacity is retained when the current density increases from 2 Ag  1 to 30 Ag  1. The achieved capacity values are higher than those of CoO/TiO2 arrays [14], NiO films [15], and Co3O4 spheres powders [1], but a little lower than the CoS nanowire arrays [8]. Furthermore, the NiS MHSA electrode shows good high-rate cycling life (Fig. 2(d)). After 3000 cycles, a specific capacity of 65 mAh g  1 at 2 Ag  1 is obtained with a capacity retention of ∼94.8%. The good high capacity achieved at high rates implies that

Fig. 2. Electrochemical performances of NiS monolayer hollow sphere arrays: (a) CV curves at different scanning rates; (b) discharge profiles and (c) corresponding specific capacities at different current densities; (d) cycling performance at 2 Ag  1.

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this type of electrode can be a promising candidate for high-power alkaline batteries applications. The excellent capacity retention and high-rate capability are attributed to the hollow sphere arrays structure. First, it can provide large specific surface area to facilitate the transportation of ion and electron and good electric contact with the substrate. Second, the carbon layer can effectively improve the electrical conductivity of the whole electrode and improve reaction kinetics resulting in better rate capability [16– 18]. Third, hollow spheres are intimately connected with each other. This can ensure every spheres to participate in the electrochemical reaction, resulting in high utilization of active materials. In view of these positive characteristics, the NiS MHSA electrode exhibits impressive high-rate performance.

4. Conclusion In summary, we have demonstrated the fabrication of NiS monolayer hollow spheres arrays via an template-assisted electrodeposition plus ion exchange route. Hollow interconnected spheres network is achieved in the integrated arrays of NiS. Due to the unique hollow spheres array structure, the NiS MHSA electrode is demonstrated with high-capacity and good cycling performance when applied as cathode of alkaline batteries. The proposed design of hollow sphere arrays is proven as an effective way for construction of advanced metal oxides cathodes for high-rate batteries.

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