One-step synthesis of nickel cobalt sulphides particles: tuning the composition for high performance supercapacitors

Abstract

NiS₂–CoS₂ composites with different Ni and Co molar ratios for supercapacitors (SCs) were synthesized by one-step hydrothermal co-deposition method using cheap Na₂S₂O₃·5H₂O as sulfur source. With the increase of Ni content, the composites particle size increases gradually and the hollow sphere structure becomes more obvious. The electrochemical measurements demonstrate that these composites possess a high specific capacitance (C_m) performance, good rate capability and long cycle stability. To be specific, the C_m of Ni/Co/S-1 composite is the largest, up to 954.3 F g⁻¹ at 1 A g⁻¹, and as high as 309.5 F g⁻¹ even at large current density of 20 A g⁻¹. Furthermore, the Ni/Co/S-1 maintains 99.9% of its initial C_m after 1000 cycles at 5 A g⁻¹. Moreover, the asymmetric supercapacitors with Ni/Co/S-1 as positive electrode and active carbon as negative electrode are of prominent energy density of 29.3 W h⁻¹ kg⁻¹ at the power density of 0.7 kW kg⁻¹, and superior cycling stability of 99.1% initial value retention after 1000 cycles.

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1. Introduction

Supercapacitors (SCs) have attracted increasing attention due to their prominent properties like high power density, long lifespan and fast charge and discharge rate, which make them great potential for applications in next generation energy storage devices. To date, SCs have been applied in various areas such as large-scale smart grids, hybrid electric vehicles, portable electronic devices and back-up power supplies.^1–5 Generally, SCs can be typically classified into two categories based on the energy storage mechanism, electric double layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy through electrostatic charge accumulating at the electrode/electrolyte interface. Pseudocapacitors depending on fast and reversible faradaic reactions of electrode materials could provide higher specific capacitance.^6,7 It is widely known that electrode material is one of the most important factors affecting SCs performance. For example, carbon-based materials are mainly used in EDLCs, while the materials used in pseudocapacitors are usually transition oxides or conducting polymers. Overall, according to the previous reports about SCs, improving electroactive materials capacitance performances (including energy density and power density etc.) has always been the research hotspot during the development process.^8–11

In recent years, transition metal sulfides with superior electrical, optical, magnetical and catalytic properties have been widely used in the fields of SCs, Li-ion batteries, solar cells, sensors and catalysts.^12–16 The mechanical strength, thermal stability and redox reaction activity of transition metal sulfides used as SCs electrodes are superior to that of the corresponding oxides.¹⁷ Among them, Co sulphides and Ni sulphides have aroused great interests due to their high redox activity, high theoretic C_m value, abundant raw materials and low toxicity.^18–20 The valence states of Ni and Co are various, which is conductive to redox reactions. Furthermore, the electronegativity of sulfur is lower than that of oxygen, making Co sulphides and Ni sulphides with more flexible crystal structure and better ductility. It is well known that crystal structure has great influence on the capacitance performance of active materials. Better ductility can release the shrinkage and swelling pressure on materials structure during continuous charge–discharge process, thus to increase the cycling stability.²¹ Meanwhile, the capacitance performance largely depended on the active materials morphology.²¹ For example, CoS₂ octahedras,²² Co₉S₈ nanotubes,²³ NiS₂ nanocubes²⁴ and Ni₃S₂ nanosheets²⁵ as electroactive materials for SCs exhibit high C_m value of 236.5 F g⁻¹ (1 A g⁻¹), 285 F g⁻¹ (0.5 A g⁻¹), 695 F g⁻¹ (1.25 A g⁻¹) and 717 F g⁻¹ (2 A g⁻¹), respectively. However, the conductivity of single Co sulphide or Ni sulphide is too low to support fast electron transport at high current density, which imposes restrictions on the application in high performance SCs.²⁶

However, the physical and chemical properties of Ni and Co are similar, and there exits various sulfide valence states of Ni and Co ion in bimetallic Ni–Co sulfides. When the two sulfides are composited, the redox reaction is easy to conduct.²² Particularly, it is noted that Ni–Co sulfides exhibit much higher conductivity and lower optical band gap energy than corresponding Ni–Co oxides.²⁷ So, Ni–Co sulfides showed higher C_m in KOH electrolyte. For instance, porous nanotubes NiCo₂S₄,²⁸ mesoporous nanoparticles NiCo₂S₄ (ref. 29) and hollow nanoprisms NiCo₂S₄,²⁷ have considerable high C_m of 933 F g⁻¹ (1 A g⁻¹), 1440 F g⁻¹ (3 A g⁻¹) and 895.2 F g⁻¹ (1 A g⁻¹), respectively.

Till now, the above mentioned Ni–Co sulfides were usually prepared by two-step reactions. Typically, the Ni–Co composite precursors were synthesized and then vulcanized to be transformed into Ni–Co sulfides. First, most of precursors were prepared in the presence of heating organic solvent. Without solvent recycling process, it will lead to raw material consumption. Secondly, the involvement of CH₃CSNH₂, Na₂S and CS(NH₂)₂ as high cost sulfur source can cause environment and health risks.^27–29

In this paper, Ni–Co sulfides have been successfully synthesized through a one-step hydrothermal co-deposition method, using cheap Na₂S₂O₃·5H₂O as the sulfur source and H₂O as the solvent. The capacitance performance of SCs can be controlled by adjusting the ratios of Ni and Co. Electrochemical tests indicated that the C_m of Ni/Co/S-1 electrode reached up to 954.3 F g⁻¹ at 1 A g⁻¹ and still maintained 99% of initial C_m after 1000 cycles at 5 A g⁻¹. In order to evaluate the practical application of Ni/Co/S-1, we prepared asymmetric SCs, using Ni/Co/S-1 as the positive electrode and activated carbon as the negative electrode. As the testing results show that the asymmetric SCs has large potential of 1.45 V, and high energy density of 29.3 W h⁻¹ kg⁻¹ at power density of 0.7 kW kg⁻¹, manifesting that the Ni–Co sulfides have great potential for applications in high performance SCs.

2. Experimental section

2.1 Synthesis of Ni–Co sulfides

All reagents Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O and Na₂S₂O₃·5H₂O are analytical grade purity The preparation processes of CoS₂ are as follows: 3 mmol Co(NO₃)₂·6H₂O was dissolved in 30 mL deionized water, and then 4 mmol Na₂S₂O₃·5H₂O was added in the above solution under continuous stirring. After 30 min, the mixed solution was transferred into 50 mL Teflon-lined stainless steel autoclave kept for 16 h at 160 °C, and then cooled down to room temperature. The black precipitates were collected by centrifugation, and washed with deionized water and absolute ethanol repeatedly. The products were dried in vacuum oven at 60 °C for 10 h. Subsequently, the Ni–Co sulfides were prepared under such condition: the total mole masses of Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O are 3 mmol, with Ni/Co moral ratios of 1 : 2, 1 : 1, 2 : 1 and 1 : 0, respectively. The synthetic conditions of NiS₂/CoS₂ composites and pure NiS₂ are the same as that of CoS₂ mentioned above. The as-prepared samples were denoted as Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂, correspondingly.

2.2 Characterization

The crystal structures and purity of the obtained samples were characterized by X-ray diffraction (XRD, Bruker, D8 Advance, Germany) with Cu K_α radiation (λ = 1.5406 Å). Energy dispersive X-ray spectrometer (EDS, OXFORD, INCA300, England) measurements were performed on the environment scanning electron microscopy (ESEM, Philips, XL-30, Holland) to confirm the atomic percentages of the samples. The chemical valence states and elemental compositions were conducted through X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi, USA). Morphologies, sizes, and parameters of the samples were characterized by scanning electron microscopy (FESEM, Hitachi, S-4800II, Japan), transmission electron microscopy (TEM, Philips, TECNAI12, Holland) and high resolution transmission electron microscope (HRTEM, FEI, TECNAI G2F30 S-TWIN, USA). Elemental maps were also confirmed by HRTEM. The surface areas and the pore-size distribution were studied by N₂ adsorption analyzer (Micromeritics, TriStar 3000, USA).

2.3 Electrodes preparation and electrochemical measurements

The electrochemical performance of the samples was evaluated using a conventional three-electrode system in 3 M KOH electrolyte, with Ag/AgCl electrode as the reference electrode and platinum plate as the counter electrode. The working electrodes were fabricated according to the following steps: 85% active material, 10% conductive graphite and 5% polytetrafluoroethylene (PTFE) were mixed in paste with proper amount of absolute ethanol as solvent. The pastes mixtures were smeared on Ni foam (1 × 1 cm²) and then dried at 60 °C for 12 h in vacuum oven. Subsequently, the electrodes were pressed into sheet under 10 MPa for 30 s.

Asymmetric SCs were assembled using the Ni–Co sulfides as positive electrodes and activated carbon (AC) as negative electrodes in 3 M KOH electrolyte. The charge storage between the negative electrode and positive electrode was determined based on the charge balance theory (Q₊ = Q₋). The specific charge storage in each electrode can be determined by the follow equation: Q = C_m × ΔE × m, where C_m represents the specific capacitance of single electrode (F g⁻¹), ΔE is the potential range of each electrode (V), and m is the weight of active material of each electrode (g). Mass balancing can be calculated by the following equation: (m₊/m₋ = C_m− × ΔE₋/C_m+ × ΔE₊). The fabrication of the negative electrode was the same as that of the positive electrode.

Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) experiments were carried out on CHI660E electrochemical workstation. Electrochemical impedance spectroscopy (EIS) measurements were performed on Autolab-PGSTAT30 in frequency range from 0.01 to 10⁵ Hz, and alternating current potential amplitude of 5 mV.

3. Results and discussion

3.1 Structure and morphology characterizations

Fig. 1A displays SEM images of pure CoS₂, octahedron structure with edge length around 180–220 nm,²² and pure NiS₂ is composed of spherical particles about 435–720 nm (Fig. 1E). Fig. 1B–D reveals that Ni/Co/S-n composites consist of octahedron and spherical particles with irregular size. The content of octahedral CoS₂ and spherical NiS₂ is associated with the ratios of Co and Ni. And the surface of Ni/Co/S-n is much rougher than those of pure CoS₂ and NiS₂. Combining with TEM images, the formation of Ni–Co sulphides can be ascribed to the growth and self-aggregation process, followed by Ostwald ripening mechanism.³⁰ The Ni/Co/S-n samples (Fig. 1F–J) are tetragonal and circular projected shapes, consistent with SEM results. However, the particles agglomerate together seriously in order to reduce high surface tension and surface energy.³¹ The TEM image (Fig. 1F) shows that the CoS₂ are mainly solid constructions, while the different brightness in Fig. 1J demonstrates that NiS₂ are likely to be hollow sphere structures. Rough surfaces and hollow spheres structure not only provide high specific surface areas, but also are conductive to improve the wettability of active material for better electrolyte access.²⁴

Fig. 1 SEM and TEM images of Ni–Co sulphides: (A and F) CoS₂, (B and G) Ni/Co/S-0.5, (C and H) Ni/Co/S-1, (D and I) Ni/Co/S-2, (E and J) NiS₂, (insets: the magnified SEM images).

Fig. 2A shows a HRTEM image of Ni/Co/S-1, octahedral and spherical structure coincided with SEM image. Fig. 2B and C presents the lattice spacing 0.283 and 0.254 nm, corresponding to (200) and (210) planes of NiS₂. And the lattice spacing from Fig. 2D and E are 0.248 and 0.167 nm, corresponding to (210) and (311) planes of CoS₂. Meanwhile, EDS mapping of Ni/Co/S-1 in Fig. 2F demonstrated the maldistribution of Ni and Co elements in the samples. But in general, the formation process of Ni/Co/S-1 belongs to intergrowth process. The distribution of S in the samples indicated that NiS₂ is of hollow sphere structure, matching well with TEM results.

Fig. 2 (A–E) HRTEM image of Ni/Co/S-1, (F) corresponding EDS mapping.

From XRD patterns in Fig. 3A, several diffraction peaks at 2θ values of 27.2°, 31.5°, 35.3°, 38.7°, 45.3°, 53.6°, 56.2°, 58.7° and 61.1° corresponding to (111), (200), (210), (211), (220), (311), (222), (230) and (321) planes of NiS₂ (JCPDS 11-0099). With the increase of Ni content, the peaks position have a slight shift toward higher angle and reach to 27.9°, 32.3°, 36.2°, 39.8°, 46.3°, 54.9°, 57.6°, 60.2° and 62.7°, indexed to CoS₂ (JCPDS 41-1471). A slight shift of peak position can be attributed to physical and chemical properties similarities of Ni and Co as well as structural compatibility.³² Meanwhile, no peaks of impurities can be detected, demonstrating high purity of samples. Several intense and sharp diffraction peaks from NiS₂, indicate that the as-prepared NiS₂ is of high crystallinity, while the crystallinity of other samples with broad and weak peaks are relatively low. The EDS analysis in Fig. S1A–C† demonstrate that the as-prepared samples are mainly composed of Ni, Co and S elements, which agree well with the XRD results. The Ni/Co atomic ratios of Ni/Co/S-0.5, Ni/Co/S-1 and Ni/Co/S-2 are about 1 : 1.93, 1 : 1.09 and 1.81 : 1 respectively, slight different with the initial ratios (1 : 2, 1 : 1 and 2 : 1), which is caused by different reaction activities of Ni and Co ions.³³

Fig. 3 (A) XRD patterns of Ni–Co sulphides, (B) N₂ adsorption–desorption isotherm and pore size distribution curve (the inset) of Ni–Co sulphides, (C–F) XPS spectrum of Ni/Co/S-1: (C) survey spectrum, (D) Ni 2p region, (E) Co 2p region and (F) S 2p region.

The specific surface area and pore size distribution have important influence on the capacitive performance in SCs. Fig. 3B shows N₂ adsorption–desorption isotherms, and pore size distribution (inset in Fig. 3B) calculated from adsorption isotherms. The samples (with the exception of CoS₂) present type IV isotherms with H3 hysteresis loops, suggesting the existence of mesopores. The hysteresis loop of Ni/Co/S-2 was obviously larger than other samples. It is because the Ni/Co/S-2 possesses plenty of NiS₂ and less amount of CoS₂, makes Ostwald ripening process happened easier and produced more hollow spheres, thus leading to more mesopores. The mesoporous structure of samples is mainly attributed to NiS₂ hollow spheres. However, the CoS₂ displays unconspicuous hysteresis loop at relative pressure (0.4 < P/P₀ < 1.0), indicating it is mainly consist of micropores and little amount of small mesopores. This result matches well with pore size distribution curve of CoS₂ (Fig. 3B inset).

The BET specific surface areas (S_BET) of the CoS₂, Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂ were 11.7, 9.2, 17.3, 19.1 and 18.3 m² g⁻¹, respectively. The pore sizes distribution centered at 6.0, 7.1, 4.4 and 5.9 nm for Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂, respectively (Fig. 3B inset), while the pore size of CoS₂ was mostly below 5 nm. Generally, the meosopore structure of samples is becoming more obviously with the increase of Ni content. As is known, mesoporous materials not only provide rich electroactive sites, but also offer more free volume changes during cycling charge/discharge process, resulting in excellent electrochemical properties of electroactive materials.³⁰

The surface composition and chemical state were further monitored by XPS. Fig. 3C shows the survey spectrum of the Ni/Co/S-1 sample and the binding energy ranges from 0–1300 eV. The peaks at 162.3, 778.5 and 853.5 eV are attributed to S 2p, Co 2p and Ni 2p, respectively, indicating Ni/Co/S-1 contains S, Co and Ni elements. The Ni 2p, Co 2p and S 2p spectra were fitted by the Gaussian fitting method. As illustrated in Fig. 3D, the main peaks of Ni 2p_3/2 and Ni 2p_1/2 located at 853.5 eV, 870.9 eV can be assigned to Ni²⁺.³⁴ And in Fig. 3E, the two main peaks at 778.5 eV, 793.6 eV for Co 2p_3/2 and Co 2p_1/2 are the character of Co²⁺.^35,36 Fig. 3F shows the spectrum of S 2p region, the two main peaks at 162.3 eV and 163.5 eV correspond to S 2p_3/2 and S 2p_1/2. The peak at 162.3 eV can be ascribed to the sulphur ion in low coordination on surface, and the peak at 163.5 eV is considered as the metal-sulphur bonds. Moreover, the peak at 168.7 eV for O, impurity is due to surface O absorption of the samples exposed to air during testing processing.^37–39

3.2 Electrochemical measurements

Fig. 4A shows the CV curves of Ni–Co sulphides at scan rate of 5 mV s⁻¹ in the potential window from −0.1 to 0.5 V. A pair of redox peaks can be observed for Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂ as well as two pairs for CoS₂, which result from the reversible faradic reaction occurring at the electroactive materials/electrolyte interface. The redox reactions for Ni–Co sulphides in KOH alkaline electrolyte are as follows:^22,40

CoS₂ + OH⁻ ↔ CoS₂OH + e⁻ (1)

CoS₂ + OH⁻ ↔ CoS₂O + H₂O + e⁻ (2)

NiS₂ + OH⁻ ↔ NiS₂OH + e⁻ (3)

Fig. 4 Electrochemical performance of CoS₂ (a), Ni/Co/S-0.5 (b), Ni/Co/S-1 (c), Ni/Co/S-2 (d) and NiS₂ (e): (A) CV curves at scan rate of 5 mV s⁻¹, (B) GCD curves at current density of 1 A g⁻¹, (C) C_m versus current density, (D) cycling performance at current density of 5 A g⁻¹ and (E) Nyquist plots, (F) partial magnified Nyquist plots.

From CV curves, the redox peaks gradually enhanced as Ni content in samples increased. The CV integrated area of Ni/Co/S-1 is larger than other samples, suggesting Ni/Co/S-1 has the best capacitive performance. As shown in Fig. S2,† the redox peaks of Ni/Co/S-1 are almost symmetric, demonstrating the excellent reversibility of the oxidation and reduction process. Furthermore, with the increase of the scan rate, the anodic peaks and cathodic peaks shift to the converse direction because of the polarization at high scan rate.³⁸

Meanwhile, the nonlinear GCD curves of samples in Fig. 4B are typical pseudocapacitive, much different from that of linear characteristic of EDLCs. Moreover, obvious plateaus in every GCD curves correspond to the redox peaks positions in the CV curves. The C_m of the samples was achieved by GCD measurement using the following equation:

C_m = I × Δt/(m × ΔV) (4)

where C_m is the specific capacitance (F g⁻¹), I is the discharge current (A), Δt is the discharge time (s), m is the weight of active materials (g) and ΔV is the potential range (V). From the discharge curves, the C_m of CoS₂, Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂ are 293.9, 682.8, 954.3, 840.8 and 790.4 F g⁻¹ at 1 A g⁻¹, respectively. Interestingly, it is obvious that the C_m of Ni–Co sulphides are increase first and then decrease with the increase of Ni amount because the electrochemical activity of the Ni ion is superior to that of the Co ion.³⁶ Generally, a certain amount of Ni ion is helpful to enhance capacitive performance of Ni/Co/S composites, but excessive Ni ion will be contrary, which agrees with the reported literature.²³ To be specific, Ni/Co/S-1 possesses the highest C_m value, consistent with the CV profiles. From the Fig. S3C,† GCD curves of the Ni/Co/S-1 at different current densities are all highly symmetrical, revealing that the Ni/Co/S-1 has excellent Coulombic efficiency, pseudocapacitive behavior and electrochemical reversibility. Such excellent capacitive performance of Ni/Co/S-1 is mainly attributed to the unique structure and synergetic effects of NiS₂ and CoS₂: (1) Ni/Co/S-1 with rough surface can utilizes active material and provide rich electroactive sites, beneficial to fast redox reactions;²⁴ (2) in the process of redox reaction, Co³⁺ will supply extra holes as p-type doping while Ni²⁺ will produce extra electrons by n-type doping. Thus, coexistence of more Ni³⁺ and Co³⁺ will possess higher conductivity and capacitive performance.⁴¹

According to the GCD curves (Fig. S3†), the C_m of Ni–Co sulphides can be obtained, as plotted in Fig. 4C. The C_m gradually decrease as the current density increase, which can be attributed to the increment of voltage and insufficient active material involved in redox reaction during the high rate GCD process.²³ The C_m of Ni/Co/S-1 is up to 954.3, 886.8, 836.4, 751.8, 578.6 and 309.5 F g⁻¹, corresponding to current densities 1, 2, 3, 5, 10 and 20 A g⁻¹, larger than other samples at the same current densities. Therefore, the electrochemical performance of Ni–Co sulphides could be improved by tuning Ni and Co content. Even at a high current density of 20 A g⁻¹, the C_m retention rate of Ni/Co/S-1 can still be 32.4%. These results indicate that Ni/Co/S-1 has good rate capability and the strongest synergistic effect. The C_m of CoS₂ is relative low, but the capacity retention is superior to other samples (about 41.5%) due to its good structural stability. Noticeably, at a current density of 20 A g⁻¹, the capacity retention of Ni/Co/S-0.5, Ni/Co/S-2 and NiS₂ is 27.6%, 12.9% and 5.9% at 1 A g⁻¹, respectively, suggesting their C_m decreased faster than other samples because the NiS₂ has a low capacitance retention as well as the C_m of CoS₂ is low. Cycle performance of samples under a current density of 5 A g⁻¹ for 1000 cycles was tested, as shown in Fig. 4D. The C_m of CoS₂, Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂ retain about 100.9%, 100.4%, 99.9%, 99.6% and 103.1% of the original one even after 1000 cycles. It is evident the C_m of samples increase firstly, dropped slightly and tend to be stable. This phenomenon can be explained as follows: the electroactivity of materials has been stimulated and enhanced at original GCD phase, while the active materials could also be degraded to some extent at alkaline condition at following GCD cycling.²¹

Fig. 4E shows the Nyquist plots of samples, including a semicircle in the high frequency region and an incline line in the low frequency region. The equivalent circuit model and the enlargement of Nyquist plots are shown in Fig. 4E and F respectively, where R_s is the equivalent series resistance, W is the Warburg impedance, R_ct is the charge–discharge resistance, C_d is the double-layer capacitance and C_F is the faradaic pseudocapacitor. Here, the R_s value of samples can be obtained by the semicircle intersect on the real axis. It can be observed that the R_s of CoS₂, Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂ are 0.21, 0.36, 0.18, 0.22 and 0.31 Ω respectively, indicating that Ni/Co/S-1 has the lowest intrinsic resistance and contact resistance.⁴⁰ As illustrated in the Fig. 4F, in high frequency region, the samples show clear semicircles.⁴² Moreover, the R_ct obtained by ZSimpWin software for CoS₂, Ni/Co/S-0.5, Ni/Co/S-1, Ni/Co/S-2 and NiS₂ were 3.76, 1.91, 0.71, 1.35 and 2.2 Ω, respectively. To recap, the R_ct of Ni–Co sulphides are decrease first and then increase with the increase of Ni content, suggesting that a certain amount of Ni is useful to improve the electrical conductivity of electroactive materials, but excessive Ni goes against electron transfer.³⁸ It can be known that low R_ct means fast electron transport, and also could provide higher C_m value especially for pseudocapacitor.³⁶ Therefore, Ni/Co/S-1 presents smallest R_ct among all the samples, suggesting its superior capacitive performance. The straight lines in the low frequency region correspond to diffusive resistance of the electrolyte for the redox material, and have not relation with charge storage.⁴³ As shown in Fig. 4E, the straight lines of Ni/Co/S-1 electrodes in the EIS spectra incline at an angle of nearly 90° to the Z′-axis, indicating the capacitive performance are not controlled by diffusion process.⁴⁴ The reason is Ni/Co/S-1 have more rough surfaces and hollow spheres structures, which could enhance the wettability of active materials for better electrolyte access.

Fig. 5 (A) CV curves and (B) Nyquist plots of Ni/Co/S-1 electrode before and after 1000 GCD cycles.

Fig. 6 SEM images of Ni/Co/S-1 electrode: (A) before cycling and (B) after 1000 GCD cycles.

Fig. 5A shows the CV curves of Ni/Co/S-1 electrode before and after 1000 GCD cycles. After 1000 cycles, two CV curves are nearly coincident, demonstrating the high cycling stability. The Nyquist plot of Ni/Co/S-1 electrode before and after 1000 cycles is shown in Fig. 5B. The R_s of Ni/Co/S-1 electrode is only increased from 0.18 Ω to 0.49 Ω. What's more, the diameter of the semicircle has a slight increase and the slope for straight line decreased after 1000 cycles. These results further clarify the Ni/Co/S-1 exhibits well cycling stability and reversible redox reaction.²²

Fig. 6 is the SEM images of Ni/Co/S-1 electrode before (A) and after (B) 1000 cycles. In comparison Fig. 6A with Fig. 6B, the surface of Ni/Co/S-1 electrode appeared cracks after 1000 cycles. This is explained by the fact that continuous redox reaction makes volume of active material shrinkage and expansion constantly, leading to shell layer fracture of Ni/Co/S-1 electrode.

In order to evaluate the practical application potential of Ni/Co/S-1, asymmetric SCs were fabricated using Ni/Co/S-1 and AC as the positive and negative electrodes, respectively. Based on the principle of charge balance, as well as the C_m of Ni/Co/S-1 and AC electrode from the CV curves at a scanning rate of 5 mV s⁻¹ (shown in Fig. 7A), the mass ratio of Ni/Co/S-1 and AC is determined to be 1 : 3.07. Due to the different working potential of the Ni/Co/S-1 and AC, the voltage of asymmetric SCs can be extended to 0–1.45 V. Fig. 7B shows the CV curves of asymmetric SCs at various scan rates own both electric double layer capacitance and pseudocapacitance characteristic. Moreover, there is no noticeable distortion of CV curves with increasing scan rate, indicating the fast charge–discharge quality of the device.⁴⁵ Fig. 7C illustrated a set of GCD curves under various current densities. The C_m of asymmetric SCs was calculated to be 100.4, 88.9, 80.6, 69.2 and 45.9 F g⁻¹ corresponding to the current density 1, 2, 3, 5 and 10 A g⁻¹, respectively. The Ragone plot derived from the GCD curves is listed in Fig. 7E. Energy density and power density of the device were calculated from discharge curves according to the following equations:

E = C_m × (ΔV)²/7.2 (5)

P = 3.6 × E/Δt (6)

where E represents energy density (W h⁻¹ kg⁻¹), C_m represents the specific capacitance (F g⁻¹), ΔV represents voltage interval (V), P represents power density (kW kg⁻¹) and Δt represents discharge time (s). The energy density of asymmetric SCs reaches 29.3 W h⁻¹ kg⁻¹ at a power density of 0.7 kW kg⁻¹, and still remains 13.4 W h⁻¹ kg⁻¹ at a higher power density of 7.4 kW kg⁻¹, indicating their advantage in the retention of energy density. Moreover, the cycle life of asymmetric SCs was tested and shown in Fig. 7F by repetitive GCD process at a current density of 5 A g⁻¹. The device exhibits good cycle performance, 99.2% of its initial capacitance can be retained after 1000 cycles. Furthermore, a superior electrochemical reversibility with almost 100% coulombic efficiency can be retained (Fig. 7F). All the results demonstrate that the asymmetric SCs are promising in the application for high-performance energy storage systems.

Fig. 7 (A) CV curves of Ni/Co/S-1 and AC at scan rate of 5 mV s⁻¹, electrochemical performance of Ni/Co/S-1//AC asymmetric SCs: (B) CV curves at different scan rates, (C) GCD curves at different current density, (D) C_m versus different current density, (E) Ragone plot and (F) cycling performance and coulombic efficiency at current density of 5 A g⁻¹.

4. Conclusions

In summary, we have successfully synthesized Ni–Co sulphides through one-step hydrothermal co-deposition, using Na₂S₂O₃·5H₂O as the sulfur source. The Ni–Co sulphides are mainly composed of octahedral CoS₂ and spherical NiS₂ based on the Ostwald ripening process. Electrochemical measurements results show that the capacitive performance of Ni–Co sulphides can be controlled via adjusting the ratios of Ni and Co content. Among them, Ni/Co/S-1 exhibited the maximum C_m 954.3 F g⁻¹ at a current density 1 A g⁻¹ as well as desirable cycling stability with capacitance retention of 99.9% at 5 A g⁻¹ after 1000 cycles. Meanwhile, EIS test results show that Ni/Co/S-1 possesses the lowest R_ct (0.71 Ω), indicating its ideal pseudocapacitance behavior. The remarkable capacitive performance can be ascribed to the unique structure and strong synergistic effect from Ni ion and Co ion, which provide short ion diffusion pathway and high electrical conductivity. Additionally, an asymmetric SCs based on Ni/Co/S-1 as positive electrode and AC as negative electrode delivers an energy density of 29.3 W h⁻¹ kg⁻¹ at a power density of 700 W kg⁻¹ and a high energy density of 13.4 W h⁻¹ kg⁻¹ at a very high power density of 7.4 kW kg⁻¹. The Ni–Co sulphides, particularly Ni/Co/S-1, have remarkable capacitive performance, rate capability and cycling stability, presenting great potential for practical application.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (No. 21106124 and No. 21375116) and Postdoctoral Science Foundation of China (2014M551668). The related measure and analysis instrument for this work was supported by the Testing Center of Yangzhou University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10048a

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