Introduction

Nickel sulfide, a transition metal chalcogenide, has recently attracted much attention due to its exceptional electrochemical properties, good electrical conductivity and low cost1,2,3. Interestingly, nickel sulfide can assume various thermodynamically stable crystal structures and stoichiometric forms including NiS, NiS2, Ni3S2, Ni3S4, Ni4S3, Ni7S6 and Ni9S84,5,6. NiS with the simplest stoichiometry exists in two possible phases: α-NiS (hexagonal, P63/mmc) and β-NiS (rhombohedral, R3m). The former is metalic while the latter is insulating. α-β phase transition can take place under certain conditions, accompanied by 4% volume change7,8.

The intrinsic properties of a nanomaterial are largely determined by its unique crystal structure9,10,11. As known, the crystal phase of nanocrystals is sensitive to the synthetic conditions including temperature, pressure and reactant composition12,13,14. Therefore, it is important to develop phase-controlled synthesis methods to obtain desired materials for specific applications. It has been shown that crystal structures may be dictated by size constraint via crystallization of materials in nanoporous media15,16,17,18. For instance, Hu et al. have obtained rhombohedral single-crystal nickel sulfide nanowires by confined synthesis in the gaps of silicon dioxide nanospheres19.

Taking advantages of their high electrochemical activities, both α-NiS and β-NiS have been utilized as the electrode materials20,21. α-NiS outperforms β-NiS due to its better conductivity and the fact β-NiS irreversibly transforms to Ni(OH)2 in alkaline solution22,23. Current nickel sulfide based supercapacitors and Li-ion batteries, however, are suffered from poor cycling stability due to the agglomeration and pulverization of nickel sulfide materials during repetitive charge/discharge processes24,25.

In view of these problems, we herein report a facile and phase-controlled synthesis of α-NiS nanoparticles (NPs) embedded in carbon nanorods (denoted as α-NiS/CRs) by in-situ sulfurating the preformed Ni/CRs. The electrode based on such hybrid nanocomposite material demonstrates high specific capacitance (1092 F g−1 at 1 A g−1 and 740 F g−1 at 10 A g−1) and outstanding cycling stability (no apparent capacitance decrease after 2000 cycles).

Results

Precursor characterizations

The Ni/CRs composite was prepared by carbonization of nickel dimethyglyoximate in argon at 350°C for 1 h. As shown in Fig. S1a (Supporting Information), numerous rectangular nanorods (500 nm - 1 μm long and ~100 nm wide) were obtained. The atomic ratio of Ni and C is 18.3:81.7 as revealed by energy dispersive spectroscopy (EDS) (Fig. S1b). The spectrum of X-ray diffraction (XRD) displays three prominent peaks which can be perfectly indexed to Ni (JCPDS card No.04-0850) and a weak and broad peak (at ~24°) originated from amorphous carbon (Fig. S1c)26. The Ni content was further evaluated by thermogravimetric analysis (TGA) in air. A net weight loss of 36.4% is observed due to oxidation of CRs (≥372°C, causing weight loss) and oxidation of Ni to NiO (≥393°C, causing weight gain) (Fig. S1d)27. The Ni content is calculated to be ~50.1 wt. %, which is in accordance with the EDS analysis. This high loading of Ni is superior to other reported Ni/Carbon composites28,29,30.

Microstructure characterizations

As illustrated in Fig. 1, α-NiS/CRs composites were exclusively synthesized when Ni/CRs were directly in-situ sulfurated by refluxing in glycerol for 1 h. In comparison, sulfuration of the mixture of Ni NPs and mesoporous CRs (MCRs) leads to formation of β-NiS/CRs composite. MCRs were produced by removing Ni NPs from Ni/CRs via overnight incubation with 3 M HCl. The microstructure of α-NiS/CRs composite was characterized by SEM and TEM which was show in Fig. 2a–2d As revealed by field-emission scanning electron microscopy (FESEM), CRs are decorated with numerous α-NiS NPs after in-situ sulfidation (Fig. 2a). EDS microanalysis indicates that the atomic ratio of Ni/S (12.8/13.1) is closed to the stoichiometry of NiS (Fig. S2). The unaltered C/Ni ratio before and after sulfidation suggests an in-situ nucleation process. Transmission electron microscopy (TEM) shows that α-NiS NPs (black dots) with uniform particle size (~5 nm) are homogenously and densely embedded in the CRs (gray matrix) (Fig. 2b). As reported by previous literatures, the CRs coating can not only partially act as a conductive binder to increase the contact between nanoparticles but also alleviate the aggregation of the active materials and lead to the excellent electrochemical cycling stability31,32. High-resolution transmission electron microscopy (HRTEM) in Fig. 2c shows that the d-spacing of the NPs is 0.198 nm, which is in agreement with the d102-spacing of α-NiS (highlighted by dash circles). Uniform dispersion of NiS NPs in the carbon matrix is further confirmed by EDS mapping (Fig. 2d).

Figure 1
figure 1

Schematic illustration for the synthesis of α and β-NiS/CRs composites.

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Figure 2
figure 2

Characterizations of α- and β-NiS/CRs composites.

(a) FESEM, (b) TEM, (c) HRTEM, (d) EDX mapping of α-NiS/CRs; (e) XRD patterns of α- and β-NiS/CRs and N2 adsorption-desorption isotherms and corresponding pore size distributions (inset) of mesoporous carbon (f) and α-NiS/CRs composite (g) calculated from BJH model.

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In contrast, sulfurating the mixture of Ni NPs and Ni-free MCRs leads to formation of randomly dispersed β-NiS aggregates as shown in SEM and TEM images (Fig. S3a and S3b). The HRTEM image in Fig. S3c reveals the lattice interplanar spacing of ~0.294 nm corresponding to the (101) plane distance of rhombohedral β-NiS. Evidently, the nanopore-confinement is critical to the formation of α-NiS and prevent its possible transformation to β-NiS, since there is a 4% volume expand from α to β phase. So it is believed that the size effect caused by the carbon nanorods may be benefit for the formation of α-NiS.

The identity of two different crystal forms of nickel sulfide obtained from disparate synthetic routes can be confirmed by XRD analysis (Fig. 2e). The two patterns can be unambiguously assigned to α-NiS (hexagonal, P63/mmc, JCPDS card No.02-1280) and β-NiS (rhombohedral, R3m, JCPDS card No.86-2281), separately. A broad peak located at 26° was attributed the amorphous carbon. Nitrogen physisorption measurements was adopted to further confirm that α-NiS NPs were confined into the CRs. Fig. 2e and 2f show the N2 adsorption-desorption isotherms of MCRs and α-NiS/CRs (inset are corresponding pore size distribution). The former are nanoporous (narrowly distributed around 5 nm) due to removal of originally embedded Ni NPs whereas the later are no longer nanoporous (the indicated pore size of ~60 nm is from the interstitial space between α-NiS/CRs). Besides, the BET surface area decreased from 200.1 m2 g−1 to 30.8 m2 g−1 after loaded with α-NiS NPs.

Electrochemical properties

We speculate that α-NiS/CRs composite could be a good electrode material for charge storage owing to its unique hierarchical structure, conductive nature and high electrochemical performance expected from α-NiS NPs. Fig. 3a shows the cyclic voltammetry (CV) analysis of bare CRs and the two composites in the potential range of 0–0.50 V (vs. standard calomel electrode). α-NiS/CRs shows a much larger CV area than that from β-NiS/CRs and bare CRs, indicating a higher capacitance (also see CV profiles at different scan rates shown in Fig. S4a and S4c). The redox peaks of α-NiS/CRs is originated from the following NiMH battery-like reaction:

while the redox peaks of β-NiS/CRs (which disappear after the first cycle) is due to the following irreversible reaction21:

Figure 3
figure 3

Electrochemical properties of α-NiS/CRs, β-NiS/CRs composites and bare CRs.

(a) CV curves of α-, β-NiS/CRs composites and bare CRs at 5 mV s−1. (b) Specific capacitance of α-, β-NiS/CRs composites and bare CRs at various current density. (c) Cycling performance and (d) Nyquist plots of the EIS of α and β-NiS/CRs composites.

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The excellent electrochemical reversibility and kinetics of α-NiS/CRs are evidenced by its symmetric galvanostatic charge/discharge curves and its higher capacity than that of β-NiS/CRs is indicated by its longer charge/discharge time (Fig. S4b and S4d). The specific capacitances calculated based on the galvanostatic charge/discharge curves at different current densities are plotted in Fig. 3b. The α-NiS/CRs electrode exhibits high capacitance of 1092 F g−1 at 1 A g−1. ~67.8% of the capacitance retains at the high current density of 10 A g−1. This electrode outperforms the β-NiS/CRs electrode (514 F g−1 at 1 A g−1) and the previously reported nickel sulfide based electrodes, such as NiS hollow spheres33, flower-like β-NiS21 and Ni3S2-graphene hybrid25. The α-NiS NPs well-dispersed into the CRs provide numerous electroactive sites in favor of the high specific capacitance34.

To evaluate the contribution of each of the components, CV and galvanostatic charge/discharge curves of bare mesopore CRs at various current densities were characterized, as shown in Figure S5. As expected, the bare carbon nanorods showed a typical double layer capacitor behavior with a weak current response and short discharge time compared with α-NiS/CRs composites. After combined with NiS NPs, this “supercapacitor-battery electrode” materials emerged as a hybrid behavior from supercapacitor (at high scan rate) to battery (at low scan rate). As reported by previous literatures35,36, this kind of hybrid electrode makes a synergistic use of capacitor and battery components rather than a merely additive composite, which gives a significant improvements for the rate performance, cycle lifetime and safety of electrodechemical charge storage devices.

Fig. 3c shows the specific capacitances versus the number of charge/discharge cycles for the two composites at a current density of 5 A g−1. As seen, the specific capacitance of α-NiS/CRs increases from 835 F g−1 to 925 F g−1 at the first 200 cycles due to activation of the electrode37. Afterwards, the capacitance sustains without apparent decrease till 2000 cycles. In contrast, the capacitance of β-NiS/CRs electrode fades away quickly (from 290 F g−1 to 144 F g−1, 50.3% loss after 2000 cycles). The high cycling stability is because α-NiS NPs are stably confined and protected within CRs without the issues of aggregation and volume expansion. Fig. 3d presents the electrochemical impedance spectra (EIS) of the two different NiS/CRs composites. The α-NiS/CRs electrode exhibits smaller charge transfer resistance (2.5 Ω indicated as the radius of the preceding semicircle of the Nyquist plot)38. This attributable to the high conductivity of carbon nanorodes and readily access of the electrolytes to the electrochemically active sites. Comparison between our α-NiS/CRs electrode and other nickle sulfide electrodes is provided in Table S1.

To demonstrate the electrochemical evolution of the α-NiS/CRs hybrid electrode, EIS and CV measurements were performed before and after 2000 charge/discharge cycles, as shown in Fig. 4a and Fig. 4b. Both EIS and CV characteristics are almost unaltered (the slightly stronger response of final CV curve in accordance with the ~10% increased capacitance after activation), confirming the excellent stability of the electrode and highly reversible NiS/NiSOH redox reaction. Furthermore, as revealed by TEM, the structure of α-NiS/CRs hybrid perfectly preserves after 2000 cycles (Fig. 4c and 4d), certifying that the unique structure can effectively prevent the exfoliation and alleviate the aggregation of the active NPs, thus leading to the excellent cycling stability.

Figure 4
figure 4

Electrochemical evolution of the α-NiS/CRs hybrid electrode before and after 2000 charge/discharge cycles.

(a) EIS spectra and (b) cyclic voltammogram curves of α-NiS/CRs composite before and after 2000 charge/discharge cycles. (c) TEM and (d) HRTEM of α-NiS/CRs composite after 2000 charge/discharge cycles.

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Discussion

We proposed a simple strategy to incorporate α-NiS nanocrystals into carbon nanorods (α-NiS/CRs) by in-situ sulfidation of a Ni-carbon nanorods composite (Ni/CRs) prepared by carbonization of nickel dimethyglyoximate. The nanopore confinement by the carbon matrix is essential for the formation of α-NiS and preventing its transition to β-phase. The hybrid “supercapacitor-battery electrode” materials based on α-NiS/CRs demonstrate high specific capacitance and excellent cycling stability. The outstanding performance can be attributed to 1) the high electrochemical activities and good conductivity of α-NiS; 2) conductive matrix provided by carbon nanorods; 3) dispersed and stable confinement of α-NiS NPs; 4) synergistic use of capacitor and battery components rather than a merely additive composite. This study not only suggests a general approach for phase-controlled synthesis of transition metal sulfides but also opens the door to the rational design and fabrication of novel nickel-based/carbon hybrid supercapacitor-battery electrode materials with enhanced properties.

Methods

Dimethylglyoximate, Nickel chloride (NiCl2), thiourea and glycerol were purchased and used without further purification. All aqueous solutions were freshly prepared with ultra pure water (18 MΩ).

Synthesis of Ni/CRs

As similar to the previous report39, typically, 0.278 g dimethylglyoxime was dispersed in 24 mL ethanol and the PH value was adjusted to 13.0 by dropping 0.5 mol/L NaOH in ethanol. 0.284 g NiCl2 was dispersed in 800 mL distilled water. Dimethylglyoxime was added into NiCl2 under ultrasonication. After dried, the cotton-like precipitate at 80°C overnight, the sample was carbonized in a tube furnace under argon at 350°C for 1 h.

Synthesis of α-NiS/CRs

α-NiS/CRs was prepared by a facile solvothermal synthesis method,5.0 mg Ni/CRs was dispersed in 50 mL glycerin containing 2.0 mmol thiourea. The solution was refluxed at 280°C for 1 h to ensure the synthesis of α-NiS/CRs. The product was collected by filtration, washed several times with deionized water and then dried at 80°C overnight.

Synthesis of β-NiS/CRs

For comparison, the Ni/CRs composite was immersed into 3.0 M aqueous HCl solution overnight to leave the free-standing MCRs. The nickel NPs were synthesis according to the literature40. The synthetic process of β-NiS/CRs composite was same as the α-NiS/CRs except the Ni NPs and MCRs were added separately as the mass ratio of Ni/CRs.

Microscopic Characterization

The crystallographic information of the samples were characterized by X-ray diffraction (XRD) on a Rigaku powder X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Morphologies of the samples were investigated on a Hitachi S-4800 field-emission scanning electron microscope (FESEM) at acceleration voltage of 5.0 kV. The morphologies and structures of the samples were further investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), operated on a JEOL JEM-2100 with acceleration voltage of 200 kV. The nitrogen adsorption-desorption isotherm was conducted using Brunauer-Emmett-Teller (BET) theory by a Mircomeritics ASAP 2020 surface area and porosity analyzer. The pore size distributions were evaluated using Barrett-Joyner-Halenda (BJH) model. The thermogravimetric analysis (TGA) was conducted by Shimadzu-60 thermo-analyzer under air flow at 10°C min-1 from 50°C to 750°C.

Electrochemical Performance measurements

The electrochemical performance was carried out in a CHI660C electrochemical workstation (Chenghua, Shanghai) with 2 M KOH aqueous as the electrolyte at room temperature. A Pt plate and standard calomel electrode (SCE) were used as counter and reference electrode, respectively. The electrochemical impedance spectroscopy (EIS) was performed using an amplitude of 5 mV and frequency range from 500 kHz to 5 mHz. The specific capacitance (C) was calculated according to equation (4):

Where I (mA) the discharge current and m(g), ΔV (V), Δt (s), the designated mass of active materials, potential drop during discharge, discharge time, respectively.