High-performance nickel cobalt sulfide materials via low-cost preparation for advanced asymmetric supercapacitors

Abstract

In this work, porous nickel cobalt sulfide electrode materials with different Ni–Co ratios were successfully synthesized through a simple and efficient coprecipitation method. Benefiting from the high electronic conductivity, loosely porous morphology and cooperative redox reactions of the as-prepared electrode materials, the nickel cobalt sulfide electrode materials with the Ni/Co ratio being 2 : 1 exhibit high specific capacitance (1259 F g⁻¹ at 1 A g⁻¹), excellent rate capability (945 F g⁻¹ at 50 A g⁻¹) and long cycling life (90.0% retention after 2000 cycles). Additionally, the asymmetric device, using the nickel cobalt sulfide electrode materials with the Ni/Co ratio being 2 : 1, fabricated in this work deliver a high energy density of 44.44 W h kg⁻¹ at a power density of 954.14 W kg⁻¹ and a high power density of 23.853 kW kg⁻¹ at an energy density of 25.6 W h kg⁻¹, and also show excellent cycling stability. Our results may pave the way for employing transition metal sulfides for advanced energy storage systems through a low-cost coprecipitation method.

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1 Introduction

The rapid development of industry and the economy has caused increasing energy demand.^1,2 In this context, energy storage and conversion devices with high comprehensive electrochemical performance (such as supercapacitors, lithium ion batteries, fuel cells, and so on) have become the research hotspots.^3,4 In particular, supercapacitors have been considered as one of the most promising energy storage systems because of their intriguing characteristics of high power density, long cycling life and rapid charging–discharging time in comparison with rechargeable batteries and dielectric capacitors.^5,6 However, the energy density of supercapacitors is still much lower than that of lithium ion batteries while they are several orders of magnitude higher than traditional capacitors.^7,8 Therefore, great efforts have been made to improve the performance of supercapacitors to solve this problem by the fabrication of various electrode materials.⁹

According to the equation the energy density (E) of supercapacitors increased as the increasing of the values of C_cell (specific capacitance) and/or V_cell (cell voltage). It is a promising strategy to improve the energy density by designing asymmetric supercapacitors for the enhanced C_cell. Furthermore, transition metal sulfides (such as NiS,¹⁰ CuS,¹¹ MnS,¹² MoS₂ ¹³ and their ternary sulfides^14,15) as positive electrode can greatly improve the energy density for the enhanced C_cell caused by the high specific capacitance of the pseudocapacitance behavior.¹⁶ In particular, the electronic conductivity of transition metal sulfides is higher than that of their oxide counterparts due to their narrow band gaps.¹⁷ Among them, nickel cobalt sulfides are of particular interest owing to their high theoretical specific capacitance, high redox activity, superior electronic conductivity and excellent electrochemical reversibility.¹⁸ In addition, nickel cobalt sulfides show more cooperative redox reactions compared with single sulfides.¹⁹ These prominent features indicate that nickel cobalt sulfides may possess high specific capacitance to meet the demands of the high energy density for supercapacitors applications.²⁰

Therefore, extensive researches have been carried out to improve the electrochemical performance of nickel cobalt sulfides electrode materials with different morphologies and composition by different synthesis methods, such as hydrothermal method,²¹ solvothermal method,^22,23 sacrificial template,^24,25 electrodeposition,^26,27 and so on. However, the above mentioned nickel cobalt sulfides were prepared by using multi-step methods or/and thermal reaction approaches, which are too complicated and time-consuming for practical applications.²⁸ Therefore, how to obtain transition metal sulfides with excellent electrochemical performance through a low-cost and efficient method remains as a big challenge for the practical applications.^29,30

In this work, the nickel cobalt sulfides electrode materials with different Ni/Co ratios were synthesized via a coprecipitation method. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses demonstrate that the as-prepared nickel cobalt sulfides with a porous microstructure are consisted of numerous nanoparticles. Electrochemical measurements show that the as-prepared electrode materials exhibit high specific capacitance (1259 F g⁻¹ at 1 A g⁻¹), excellent rate capability (945 F g⁻¹ at 50 A g⁻¹) and long cycling life (90.0% retention after 2000 cycles) when the Ni/Co ratio is 2 : 1. Furthermore, an asymmetric device was fabricated by using the NCS-4 and activated carbon (AC) as the positive and negative electrode, respectively. The asymmetric device shows a high energy density of 44.44 W h kg⁻¹ at a power density of 954.14 W kg⁻¹ and a high power density of 23.85 kW kg⁻¹ at an energy density of 25.6 W h kg⁻¹. It also exhibits excellent cycling stability. These results demonstrate that the as-prepared nickel cobalt sulfides are one of the most promising candidates for supercapacitor applications. To the best of our knowledge, there are no related researches in the literature till now. Last but not least, our work may provide a simple, efficient, and facile route for synthesizing transition metal sulfides with high electrochemical activity for energy storage devices.

2 Experimental

2.1 Preparation of NCS materials with different Ni/Co ratios

All the reagents used in this work were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd., and used without further purification. In a typical synthesis procedure, the total 3 mmol Co(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O, with the stoichiometric ratios of Ni and Co being 1 : 3, 1 : 2, 1 : 1, 2 : 1, and 3 : 1, were completely dissolved in 50 ml distilled water under mild stirring, and then 0.1 M sodium sulfide aqueous solution was dropwise added until all the Ni and Co ions were completely turned into black precipitation. Finally, the samples with different Ni and Co ratios were collected by centrifugation, rinsed with deionized water and ethanol for several times before they were dried in a vacuum oven at 60 °C for 24 h. The obtained nickel cobalt sulfides were named hereafter as NCS-1, NCS-2, NCS-3, NCS-4, and NCS-5, with the stoichiometric ratios of Ni and Co being 1 : 3, 1 : 2, 1 : 1, 2 : 1, and 3 : 1, respectively.

2.2 Materials characterization

The phases of the samples were examined by X-ray diffraction (XRD, SHIMADZU, XRD-7000) with Cu Kα radiation (voltage: 40.0 Kv, current: 30.0 mA, λ = 0.15406 nm). Field-Emission Scanning Electron Microscopy (FESEM, Hitachi, SU-8010), Transmission Electron Microscopy (TEM, Tecnai G² F20S-Twin), High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED) were used to characterize the morphology and crystal structure of the samples. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, VG ESCALAB220i-XL) was used to study the compositions and chemical valence states of the samples.

2.3 Fabrications of the asymmetric supercapacitor

The asymmetric supercapacitor was fabricated by using the NCS-4 materials and activated carbon (AC) as the positive electrode and negative electrode, respectively, separated by hydrophilic cellulose membrane. A 6 M KOH aqueous solution was used as the electrolyte for electrochemical measurements. For the asymmetric supercapacitors, the mass ratio of positive and negative electrode materials should comply with the following formula based on the principle of charge balance:^31–33

In which, C₊, ΔV₊ and m₊ represent the specific capacitance, the voltage window and the mass of the positive electrode materials, respectively, while C₋, ΔV₋ and m₋ are those for the negative electrode materials. Therefore, the optimum loading mass ratio of NCS-4 and AC were calculated to be 0.4 in asymmetric devices of this work.

2.4 Electrochemical measurements

Electrochemical measurements were carried out in both three-electrode cell and two-electrode cell by using an electrochemical workstation (CHI-660D) and battery testing system (NEWARE, CT-4008) in 6 mmol KOH aqueous solution. A conventional three-electrode cell was consisted of the as-synthesized sample (as the working electrode), platinum foil (as the counter electrode) and Ag/AgCl electrode (as the reference electrode). The loading mass of the NCS-4 electrode was 2.32 mg cm⁻².

For the asymmetric device measurement, the electrochemical performance of NCS-4//AC was tested by the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) techniques using an electrochemical workstation. The cycling stability was measured by using a battery testing system.

The specific capacitance of the electrode materials can be calculated from the galvanostatic discharge curve according to the following formula:

C = IΔt/mΔV (2)

C_cell = It/MV_cell (3)

where the specific capacitance (C) applies to the single NCS electrode and the specific capacitance (C_cell) applies to the asymmetric device. I is the discharge current. Δt and t is the discharge time of the three-electrode cell and the asymmetric device, respectively. ΔV stands for the voltage window of the three-electrode cell, V_cell stands for the voltage of the asymmetric device. m is the weight of the nickel cobalt sulfides on the single electrode and M is the total mass of the electroactive materials including both the positive electrode and negative electrode (M = m₊ + m₋).

The energy density (E) and power density (P) for the asymmetric device was calculated from the galvanostatic discharge curve according to the following formulas:³⁴

P = E/t (5)

In which C_cell, V_cell and t have the same meaning as the formulas (3).

3 Results and discussion

The as-synthesized nickel cobalt sulfides materials with different Ni/Co ratios were firstly characterized by XRD analysis, as shown in Fig. 1. For all the samples prepared in this work, the diffraction peaks gradually disappear with the decrease of the Ni content, especially for those with the Ni/Co ratios being 1 : 3, 1 : 2 and 1 : 1. In contrast, the diffraction peaks of the NCS-4 and NCS-5 samples are clear, and can be assigned to hexagonal NiS (JCPDS 00-02-1280) and cubic Co₃S₄ (JCPDS 00-011-0068), respectively. Our data don't exclude the possibility of other residual Co–S phases involved in the samples. In the periodic table of elements, due to the adjacent location of Ni with Co, the radii of Ni and Co atom are very close; Ni ions can partially replace Co ions in the Co₃S₄ phase while maintaining its original crystal structure, which results in the formation of the Ni_xCo_3−xS₄ phase.³⁵ As a result, both the NiS and Co₃S₄ (or Ni_xCo_3−xS₄) phase are formed in the NCS-4 and NCS-5 samples which can be clearly seen from the XRD results. All the above-mentioned results demonstrate that the as-prepared samples are a mixture of the Co–S phase, Ni_xCo_3−xS₄ phase and NiS phase.

Fig. 1 XRD patterns of the as-prepared NCS samples.

The morphology of electrode materials is another important factor affecting the electrochemical performance of supercapacitors. The SEM images in Fig. 2a and S1† reveal that all the nickel cobalt sulfides samples are comprised of numerous nanoparticles, and there is no obvious difference in the size of nanoparticles for the samples in spite of the different Ni/Co ratios. These results indicate that the effect of morphology on the electrochemical performance of the as-prepared materials is negligible in this work. The differences in electrochemical performance of the electrode materials with different Ni/Co ratios mainly originate from the different Ni and Co redox reactions and their cooperative redox reactions. To determine the distribution of each element in our sample, elemental mapping of the NCS-4 electrode material was measured, and the results are shown in Fig. 2b. The results of these maps demonstrate the coexistence and uniform distribution of Ni, Co and S elements in the samples. Besides, the NCS-4 sample was analyzed using TEM and the results are shown in Fig. 2c and d. The TEM image reveals the as-prepared electrode materials are consisted of numerous nanoparticles, forming agglomerates with porous morphology. The sizes of the nanoparticles are basically less than 40 nm and uniformly distributed, which is consistent with the SEM results. The HRTEM image (Fig. 2d) shows the lattice fringes with an interplanar distance of 0.258 nm and 0.197 nm, corresponding to the (101) and (102) plane of hexagonal NiS, respectively. The interplanar distance of 0.286 nm in HRTEM corresponds to the (311) plane of cubic Co₃S₄. The diffraction rings in the SAED image (Fig. 2e) indicates the polycrystalline nature of the NCS-4 and they could be indexed to the (101), (110) and (004) planes of the hexagonal NiS phase, respectively, which is in good agreement with the XRD analysis.²³

Fig. 2 SEM and TEM images of the as-prepared NCS-4 samples. (a) SEM images; (b) the elemental mapping; (c) TEM images of the NCS-4 materials; (d) HRTEM; (e) SAED.

The elemental composition and chemical valence state of the NCS-4 sample was measured by XPS and the results are shown in Fig. 3. It can be seen that all the XPS spectrum peaks in Fig. 3a can be attributed to the elements of Ni, Co, S, O, and C. The existence of C and O elements are ascribed to air exposure. Therefore, the XPS data reveal that the end product only contains Ni, Co, S, which is in agreement with the elemental mapping results shown previously. High-resolution XPS spectra of Ni 2p, Co 2p, and S 2p were measured and are shown in Fig. 3b–d. Fig. 3b shows the XPS spectrum of Ni 2p. The peaks at 857.0 eV and 874.4 eV correspond to the spin–orbit characteristic of Ni²⁺, while the peaks located at 863.0 and 880.0 eV correspond to the Ni 2p_3/2 and Ni 2p_1/2 shake-up satellite peaks (marked as “Sat.”), respectively.³⁶ The XPS spectrum of the Co 2p is also fitted with two spin–orbit doublets and shake-up satellite peaks. The peaks located at 782.1 eV for Co 2p_3/2 belonging to characteristic of Co²⁺, and at 797.6 eV for Co 2p_1/2 belonging to Co²⁺ and Co³⁺ (see Fig. 3c). The binding energies located at 803.7 eV and 787.5 eV are characteristic of shake-up satellite peaks (marked as “Sat.”).³⁷ According to previous studies on Ni or Co-based compounds, it was found that Ni²⁺ ions are the most stable ones in Ni-based compounds.^38,39 In contrast, some of the Co²⁺ ions could be oxidized into Co³⁺.^40–42 Consequently, our XPS analysis demonstrates that Co²⁺ and Co³⁺ ions co-exist in the samples, which is consistent with the XRD results. The S 2p_3/2 peak at 162.4 eV is the typical metal sulfur bond, while the peak at 169.3 eV could be attributed to the surface sulfur with high oxide state, such as sulfates.⁴³ According to the XPS analysis, the surface or near-surface of this sample consists of Co²⁺, Co³⁺, Ni²⁺, and S²⁻ ions. The results demonstrate that the sample is a mixture of Co–S phase, Ni_xCo_3−xS₄ phase, and NiS phase, which is consistent with the XRD analysis.

Fig. 3 XPS spectra of the NCS-4 materials. (a) Survey spectrum, (b) Ni 2p, (c) Co 2p, and (d) S 2p.

The associated chemical reactions involved in the preparation of the NCS-4 samples can be written as follows:³⁵

Ni² + S²⁻ = NiS (7)

where x = 0–3. Based on the above analysis, it is apparent that the nickel cobalt sulfides samples have been successfully prepared by using a simple coprecipitation method. The electrochemical performance of the samples with different Ni/Co ratios were measured in 6 M KOH aqueous electrolyte. Fig. 4a shows the CV curves of different nickel cobalt sulfides materials under a potential window ranging from −0.2 to 0.6 V at a scan rate of 10 mV s⁻¹. It is clearly that there is a distinct pair of redox peaks in the CV curves for all the as-prepared materials, revealing the reversible faradaic reactions at the surface of the electrode materials.⁴⁴ In addition, the area inside the redox peaks firstly increases as the Ni content increases, and it peaks for the sample with the Ni/Co ratio being 2 : 1, and then it decreases for the sample with the Ni/Co ratio being 3 : 1, which indicates the outstanding capacitive properties of the NCS-4 sample.

Fig. 4 Electrochemical evaluation of the as-prepared NCS samples: (a) the CV curves of the as-prepared NCS samples at 10 mV s⁻¹; (b) the GCD curves at 10 A g⁻¹; (c) the CV curves of NCS-4 at different scan rates; (d) the GCD curves of NCS-4 under different current densities; (e) the change in specific capacitance of the as-prepared NCS samples under different current densities; and (f) the cycling performance of the as-prepared NCS samples at 10 A g⁻¹ after 2000 cycles.

The GCD curves of all the NCS samples at a current density of 10 A g⁻¹ are shown in Fig. 4b. All the GCD curves exhibit symmetric charge and discharge processes without obvious internal resistance drop during one completed cycle, indicating excellent electrochemical reversibility of the samples. Interestingly, the NCS-4 sample exhibits the longest GCD process among the samples, which implies that this sample possesses the highest specific capacitance for advanced supercapacitors. The GCD results are consistent with the CV data shown in Fig. 4a.

To further explore the electrochemical properties of the NCS-4 sample, other electrochemical tests were performed, and the results are shown in Fig. 4c. The CV curves of the NCS-4 sample were recorded under the voltage window of −0.2 to 0.6 V with various scan rates ranging from 5 to 20 mV s⁻¹. A pair of redox peaks is symmetric and clearly visible, indicating typical pseudocapacitive behavior. The anodic peaks shift to more positive direction while the cathodic peaks shift to more negative direction as the scan rate increases due to the internal resistance drop within the electrode.⁴⁵ The formation of the redox peaks is attributed to the Faraday reaction between the electrode materials and the alkaline electrolyte. The detailed reversible reactions are summarized as follows:^46,47

Ni_xCo_3−xS₄ + 2OH⁻ ↔ Ni_xS_4−yOH + (3 − x)CoS_yOH + 2e⁻ (8)

CoS_yOH + OH⁻ ↔ CoS_yO + H₂O + e⁻ (9)

NiS + OH⁻ ↔ NiSOH + e⁻ (10)

where x = 0–3. In Fig. 4d, the GCD curves of the NCS-4 sample measured at different current densities is displayed. The GCD curves show that the charge and discharge processes are symmetric even at high current density of 50 A g⁻¹, implying excellent coulombic efficiency and lower polarization.⁴⁸ The specific capacitance of the NCS-4 sample is calculated according to eqn (2). Encouragingly, the capacitances are 1259, 1247, 1206, 1167, 1109 and 1004 F g⁻¹ for the current densities of 1, 2, 5, 10, 20, and 40 A g⁻¹, respectively. The specific capacitance is still up to 945 F g⁻¹ at a high current density of 50 A g⁻¹, which indicates that the NCS-4 sample can be charged/discharged within a few seconds and is of high utilization and reversibility. The GCD curves measured at low current densities clearly show two obvious symmetric voltage plateaus, which correspond to the electrochemical reactions described in eqn (8) and (9). Further, the electrochemical impedance spectroscopy (EIS) of the NCS-4 is performed in the frequency region of 100 kHz to 0.01 Hz in the three-electrode cell and the corresponding Nyquist plots can be seen is Fig. S2.† The small semicircle in the high frequency regions exhibits the small resistance of the electrochemical system and the fast charge transfer process at the electrode/electrolyte interface. The ideal straight line in low frequency regions indicate the fast ion diffusion and the ideal capacitive behavior.⁴⁹ The EIS results demonstrate the loosely porous NCS-4 enable fast pseudocapacitive reactions and rapid electron transports in the electrochemical process.

Moreover, the specific capacitances of all the NCS samples measured at different current densities are plotted in Fig. 5e. It is noteworthy that the specific capacitance of the NCS-4 samples is the highest among all the NCS samples under the same test conditions (647 F g⁻¹ for NCS-1948 F g⁻¹ for NCS-2, 1089 F g⁻¹ for NCS-3, 1157 F g⁻¹ for NCS-5), and it could be as high as 1259 F g⁻¹ at 1 A g⁻¹. The specific capacitances of these NCS samples are also higher than most of the nickel cobalt sulfides samples studied in the literature.⁵⁰ In addition, even under an extremely high current density of 50 A g⁻¹, the specific capacitance still retains 63.19%, 61.34%, 61.75%, 75.07% and 59.72% of those measured at 1 A g⁻¹ for NCS-1, NCS-2, NCS-3, NCS-4, NCS-5, respectively. The higher rate capability is of great significance in the practical application for supercapacitors. The reduction in the specific capacitance at high current densities is attributed to the low diffusion of electrolyte ions because of the electrochemical reaction time constraint during high rate charge–discharge process, in which only the outer surface active material could be effectively utilized.^51,52

Fig. 5 Electrochemical evaluation of the NCS-4//AC asymmetric supercapacitor: (a) the schematic illustration of the asymmetric supercapacitor fabrication; (b) the CV curves under different scan rates; (c) the GCD curves under different current densities; (d) specific capacitance versus different current densities; (e) the Ragone plot of the asymmetric device; (f) the cycling performance and coulombic efficiency of the NCS-4//AC asymmetric supercapacitor at 25 A g⁻¹ after 10 000 cycles.

Long cycling life is another key requirement for supercapacitors. The cycling stability of all the samples is studied by using GCD measurements. Fig. 4f shows the change in specific capacitances of all the samples with respect to the GCD cycling number at 10 A g⁻¹. It is found that all the specific capacitances firstly increase due to the activation of the electrode materials, and then decreases as the cycling number further increases due to the degradation of the active materials in the long cycling process.⁵³ Again, the NCS-4 sample demonstrates a high specific capacitance of 1048 F g⁻¹ after 2000 GCD cycles, corresponding to a 10.9% decrease in its initial specific capacitance. The retained capacitance of the NCS-4 sample is the highest among all the samples (e.g., 560 F g⁻¹ for NCS-1, 839 F g⁻¹ for NCS-2, 978 F g⁻¹ for NCS-3, 932 F g⁻¹ for NCS-5). Our work therefore suggests that the NCS-4 electrode materials deliver a high capacitance along with an excellent cycling stability.

From the electrochemical measurements shown above, it is clear that the NCS-4 sample prepared in this work exhibits superior electrochemical properties. The comparisons of electrochemical performance of the NCS samples in our work and those in other reports on nickel cobalt sulfides, prepared by different preparation methods, are listed in Table S1.†^{22,24,25,28,43,46,54–56} The outstanding electrochemical properties of NCS-4 is attributed to both the higher electrochemical activity and higher electronic conductivity of the electrode material.⁵⁷ Meanwhile, the synergistic effect in bimetallic sulfides plays a crucial role in enhancing electrochemical performance. Both Ni and Co ions in the redox reactions offer richer reaction sites compared with monometallic oxides/sulfides.^58,59 Furthermore, the loosely porous morphology of the active materials in this work can provides many “holes” in the structure, which increases the contact area between the electrode materials and the electrolyte. All these features lead to enhanced electrochemical reaction and higher specific capacitance, benefitting from the effective flow of the electrolyte ions throughout the structure of electrode material, the extensive contact of the electrode materials with the electrolyte ions, and the effective utilization of the active materials.⁶⁰

As mentioned in the XRD analysis previously, NiS and Co₃S₄ phases coexist in the NCS-4 and NCS-5 sample. The theoretical specific capacitance of the electrode materials is calculated according to the following formula:³⁵

here, n represents the number of electrons transferring in the electrochemical process, F stands for the Faraday constant, M is the molar mass of the electroactive material, and V refers to the voltage window. As both the molar mass and voltage window of Ni and Co are nearly the same, the theoretical specific capacitance of Ni and Co ions is mainly determined by the mean number of transferred electrons in the redox reaction.

During electrochemical measurements, the reaction between the electrode materials and the alkaline electrolyte could be summarized by eqn (8)–(10). It is obvious that the electrons involved in the redox reaction of the Co₃S₄ active materials are three times higher than those of the NiS phase, according to the chemical reactions of (8)–(10). Therefore, the Co₃S₄ electrode materials are supposed to exhibit higher specific capacitance under the same conditions. However, higher capacitive characteristics of the pure Co₃S₄ electrode materials were not found in this work (see Fig. S3† for the comparison of the CV curves of the pure Co–S phase and the NCS-4 sample at 5 mV s⁻¹ in three-electrode cell). As we mentioned previously, the specific capacitances of the samples gradually increase with the increasing of Ni/Co ratio, and then decrease when the Ni/Co ratio is 3 : 1. The differences in electrochemical performances of the nickel cobalt sulfides samples are attributed to the following reasons: (1) since the radii of Ni and Co ions are very close, Ni ions can replace some of the Co ions in the Co₃S₄ phase, which results in the formation of Ni_xCo_3−xS₄ phase while maintaining the original crystal structure of the Co₃S₄ phase. Ni_xCo_3−xS₄ has higher electrochemical activity than Co₃S₄, and therefore could deliver more electrons in the redox reaction, and consequently exhibit high capacitive characteristics. (2) The formation of the Ni_xCo_3−xS₄ phase is dependent strongly of the Ni/Co ratios. The total amount of the Ni_xCo_3−xS₄ phase gradually increases with increasing the Ni content. Meanwhile, the decrease in the Co content also restricted to the formation of the Ni_xCo_3−xS₄ phase. To conclude, the Ni_xCo_3−xS₄ phase is crucial for enhancing the electrochemical performance of the nickel cobalt sulfides materials. The electrochemical performance of the nickel cobalt sulfides electrode materials could be optimized by adjusting the Ni/Co ratio. In this work, it was found that the electrochemical performance of the NCS-4 sample is the best among all the as-prepared materials, according to our results measured in three-electrode cell configuration.

To further explore the practical applications, an asymmetric supercapacitor was fabricated by using the NCS-4 material as the positive electrode and AC as the negative electrode, separated by hydrophilic cellulose membrane (Fig. 5a). Firstly, The AC was characterized in three-electrode cell, as shown in Fig. S4a.† The CV curves of the AC were measured at different scan rates. The shapes of the CV curves are nearly rectangular, suggesting excellent electrochemical double layer capacitance of the AC electrode. The GCD curves measured at different current densities are symmetric and linear indicate the double layer capacitive behavior of the AC electrode, which is in agreement with the CV curves (Fig. S4b†). Furthermore, the specific capacitance of the AC electrode can reach up to 276.23 F g⁻¹ at the current density of 1 A g⁻¹ and still remains 179.0 F g⁻¹ at 20 A g⁻¹ (see Fig. S4c†). The excellent electrochemical properties of the AC electrode suggest that it be an ideal negative electrode for making the asymmetric supercapacitors. The mass ratio of NCS-4 and AC were calculated to be 0.4, based on the charge balance principle, according to the eqn (1).

The CV curves of the NCS-4 and the AC electrode measured at 10 mV s⁻¹ are plotted in Fig. S5a.† The potential window of the AC electrode and the NCS-4 active material is in a range of −1 to 0 V, and −0.2 to 0.6 V, respectively. From their individual potential windows, the voltage of the NCS-4//AC asymmetric supercapacitor can be determined to be less than or equal to 1.6 V. Fig. S5b† show the CV curves under different voltage windows, that is, 0–1.4 V, 0–1.55 V, and 0–1.6 V. For the voltage of 1.6 V, the curled-up tail of the CV curve implies that the positive electrode starts to release oxygen. This is detrimental for the packaged full cell in practical applications.⁶¹ On the other hand, the redox reaction is insufficient for the voltage window of 0–1.4 V. Therefore, the suitable voltage window of the asymmetric device is decided to be 0–1.55 V.

Fig. 5b shows the CV curves of the asymmetric device at different scan rates, measured under 0–1.55 V. The area of redox peak gradually increases as the scan rate increases from 10 to 100 mV s⁻¹. There is no apparent change or distortion in the shapes of the CV curves, even for a high scan rate of 100 mV s⁻¹, suggesting that the asymmetric supercapacitor shows excellent capacitive behavior and fast ion transfer.⁶² The nearly symmetric shapes of the GCD curves with small voltage drop at different current densities also indicate the rapid current–voltage response and good electrochemical reversibility (see Fig. 5c).^63,64 The specific capacitance was then calculated based on the total mass of the two electrodes according to eqn (3). It is noteworthy that the specific capacitance of the NCS-4//AC asymmetric device remains high and demonstrates excellent rate capability. For instance, it varies from 133.2 at 1.2 A g⁻¹ to 76.7 F g⁻¹ at 30 A g⁻¹.

Energy density and power density are two key parameters for evaluating the practical applications of supercapacitors. The energy density and power density were calculated according to eqn (4) and (5), respectively, and the Ragone plot of energy density and power density of the asymmetric device is shown in Fig. 5e. One can see that the asymmetric device reveals a high energy density of 44.44 W h kg⁻¹ at a power density of 954.14 W kg⁻¹ and a high power density of 23.85 kW kg⁻¹ at an energy density of 25.6 W h kg⁻¹. This suggests that the asymmetric device made in this work shows promise for energy storage applications. The high energy density of the asymmetric device is attributed to the porous morphology, high specific capacitance and wide voltage window of the as-prepared NCS-4 electrode materials. The electrochemical performance of the asymmetric supercapacitor fabricated in this work is superior to other nickel cobalt sulfides based supercapacitors, such as NiCo₂S₄//AC (25.5 W h kg⁻¹ at 334 W kg⁻¹, 41.4 W h kg⁻¹ at 414 W kg⁻¹),^65,66 NiCo₂S₄ symmetric supercapacitor (35.17 W h kg⁻¹ at 555.6 W kg⁻¹),⁶⁷ NiCo₂S₄/nitrogen-doped carbons foam//ordered mesoporous carbon (45.5 W h kg⁻¹ at 512 W kg⁻¹),⁶⁸ NiCo₂S₄//graphene/carbon spheres asymmetric supercapacitor (42.3 W h kg⁻¹ at 476 W kg⁻¹),⁶⁹ etc. The assembled NCS-4//AC asymmetric supercapacitor could readily power a red LED, as shown in Fig. S6,† which further demonstrates the potential practical applications of the NCS-4 electrode materials in energy storage devices.⁷⁰

Finally, the cycling stability and coulombic efficiency of the asymmetric device were measured at a high current density of 25 A g⁻¹, and the results are shown in Fig. 5f. The specific capacitance of the NCS-4//AC asymmetric supercapacitor increases from 67.7 F g⁻¹ to 83.2 F g⁻¹ during the first 3000 cycles and then becomes saturated. After the later 7000 cycles, the specific capacitance becomes almost constant and is stabilized at 87.8 F g⁻¹. The cycling stability significantly improved in comparison with that of the NCS-4 electrode materials in the three-electrode cell. This is ascribed to the incorporation of the AC electrode materials, and similar phenomena were found in some previous works.^71–73 Last but not least, the asymmetric supercapacitor made in this work exhibits a high coulombic efficiency of 99.5% during the entire cycling test, which further demonstrate high conversion efficiency of the as-fabricated asymmetric supercapacitor for practical applications.

4 Conclusions

In summary, the nickel cobalt sulfides electrode materials with different Ni/Co ratios were synthesized via a coprecipitation method. SEM and TEM results demonstrate that the as-prepared porous NCS electrode materials are consisted of numerous nanoparticles. Electrochemical measurements in three-electrode cell show that the NCS-4 electrode materials exhibit superior electrochemical performance, such as high specific capacitance (1259 F g⁻¹ at 1 A g⁻¹), excellent rate capability (945 F g⁻¹ at 50 A g⁻¹) and long cycling life (90.0% retention after 2000 cycles). Additionally, an asymmetric device was fabricated by employing the NCS-4 material and AC. The NCS-4//AC asymmetric supercapacitor demonstrates a high energy density of 44.44 W h kg⁻¹ at a power density of 954.14 W kg⁻¹ and a high power density of 23.853 kW kg⁻¹ at an energy density of 25.6 W h kg⁻¹, along with excellent cycling stability. Our work may pave the way for employing transition metal sulfides of high energy density prepared by using a low-cost coprecipitation method for advanced energy storage applications.

Acknowledgements

This work was supported by the National Science Foundation of China (NSFC No. 51372195), the CSS project (Grant No. YK2015-0602006), the Ministry of Science and Technology of China through a 973-Project (No. 2012CB619401), the Fundamental Research Funds for the Central Universities (2013JDGZ03), and Program for Innovative Research Team in University of Ministry of Education of China (IRT13034). X. J. Lou would like to thank the “One Thousand Youth Talents” program for support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04462j

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