Formation of Micron-Sized Nickel Cobalt Sulfide Solid Spheres with

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Formation of micron-sized nickel cobalt sulfide solid spheres with high tap density for enhancing pseudocapacitive properties Li Su, Lijun Gao, Qinghua Du, Liyin Hou, Xucai Yin, Mengya Feng, Wang Yang, Zhipeng Ma, and Guangjie Shao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01906 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Sustainable Chemistry & Engineering

Formation of micron-sized nickel cobalt sulfide solid spheres with high tap density for enhancing pseudocapacitive properties Li Su†,‡, Lijun Gao‡, Qinghua Du‡, Liyin Hou‡, Xucai Yin‡, Mengya Feng‡, Wang Yang‡, Zhipeng Ma‡, Guangjie Shao*,†,‡

† State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

‡ Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

*Corresponding author at: College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

E-mail address: [email protected] (G.J. Shao)

Tel.: 0086-335-8061569; Fax: 0086-335-8059878.

ACS Paragon Plus Environment

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Abstract While the synthesis of nickel cobalt sulfide is well established owing to its well electrical conductivity and great structural flexibility with low cost. It is extremely challenging to fabricate unique structure with high tap density to improve their volumetric energy density and practical application. Here we report a simple one-step hydrothermal method to synthesize micron-sized Ni-Co mixed sulfides solid sphere (1~2 µm in diameter). Owing to the high tap density of more than 1.0 g cm-3 and the unique structure, the Ni-Co mixed sulfides demonstrate exceptional energy storage performance. When act as an electrode material for supercapacitors, these Ni-Co solid spheres can deliver a specific capacitance of 1492 F g-1 at a current density of 1.0 A g-1, and a retention of 76% at 10 A g-1 after 10000 cycles. An asymmetric supercapacitor based on these solid spheres exhibits a high energy density of 48.4 Wh kg-1 at a power density of 371.2 W kg-1 with excellent long-term cycling performance (91% retention of the initial specific capacitance at 5 A g-1 after 20000 cycles). All the experiment results illustrate that the micron-sized solid sphere Ni-Co mixed sulfides will to be a promising electrode material for high-performance supercapacitors. Keywords: NiCo2S4 solid sphere; Micron-sized; Tap density; Long-term cycling performance; Supercapacitors.


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Introduction Extensive research efforts have been taken to explore renewable and sustainable energy storage devices to adapt to the rapid ever-increasing demand for our daily life.1-3 Owing to the prominent properties such as high specific capacity, long cycle life and environmental friendliness, supercapacitors have attracted significant attention in recent years.4-7 Based on the energy storage mechanism, supercapacitors can be divided into two types, i.e., electrostatic double-layer capacitors (EDLCs) and pseudo-capacitors. EDLCs are composed of carbon based materials, which store energy from the electrostatic charge accumulation at the electrode/electrolyte interface.8-9 While electrode materials for pseudo-capacitors including transition metal oxides (TMOs) and conducting polymers (CPs) make use of fast and reversible Faraday reaction occured at the electrode surface, resulting in a higher specific capacitance than EDLCs.10-12 However, pseudo-capacitors suffer from poor stabilities and owing to the structure collapse during the charge-discharge process.13-14 As we all know, the properties of the electrode materials are the critical factors to the performance of the supercapacitors.15-17 For the past few years, single transitionmetal sulfides (e.g., CoS,18-19 MoS2,20 NiS,21 etc.), have been viewed as the promising

electrode

materials

for

supercapacitors

because

of

their

well

electrochemical active and good conductivity. However, they own some shortcomings such as low specific capacity at high current densities and poor cycling stabilities. To overcome these problems, it is necessary to prepare mixed metal sulfids with richer


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redox sites, higher capacitance, and improved electrical conductivity compared with single transitionmetal sulfides and transitionmetal oxides.22-24 Among the teranry metal sulfids, NiCo2S4 has drawn great attention owing to its excellent electrochemical performance.25-26 Recently, huge efforts have been devoted to developing the morphologies of NiCo2S4 electrodes mainly focus on the nanosheet and nanotube structures.27-28 However, when used as electrodes, these produced nanomaterials are easy to reunite and reflect poor tap density, which limit their practical application. On the other hand, the previous synthesis routes of NiCo2S4 are relatively complicated and require high-temperature thermal conditions. For example, Lou and co-workers synthesized hollow nanofibers, ball-in-ball hollow spheres, and onion-like NiCo2S4 particles via ion-exchange,29-31 also many other researches prepared NiCo2S4 materials by sulfurating the Ni-Co precursor with Na2S,32-34 which make the synthesis of NiCo2S4 time-consuming and costly. Therefore, it is essential to build a unique morphology with high tap density and splendid electrochemical performance through a cost-efficient method as a potential electrode material. Herein, we construct a micron-sized Ni-Co mixed sulfide solid sphere with a high tap density of more than 1.0 g cm-3 act as an outstanding electrode material through a simple one-step hydrothermal procedure. In the process of the reaction, the addition of the thiourea is so crucial for the morphology. The Ni-Co mixed sulfides synthesized with appropriate thiourea delivered good rate capacity and excellent long-term cycling stability, putting down to the micron-sized solid sphere structure


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with high tap density than other morphologies including nanowire, nanosheet, and hollow structures. One also note that NiCo2S4 owns nice electrical conductivity, richer redox sites, and with high theoretical capacitance, which achieves remarking specific capacitance and low impedance. So far as this point is concerned, Ni-Co mixed sulfides may be regard as a promising electrode material in supercapacitors fields.

Experimental section Synthesis of micron-sized Ni-Co mixed sulfides All the analytical reagents used without further purification. Firstly, 1 mmol Ni(NO3)2·6H2O, 2 mmol Co(NO3)2·6H2O, and 4 mmol thiourea were prepared carefully, and then dissolved them into 40 mL mixture of deionized water and ethylene glycol (volume ratio is 1:1) with vigorous stirring to form homogeneous solution. Then putting the mixed solution into a Teflonlined autoclave (80 mL), and kept it in an oven with the temperature of 220 °C for 12h. Waiting for some time until the autoclave cooled off, then took it out. By now, the sample was washed with deionized water and absolute ethanol for several times to remove excess impurities, finally dried at 80 °C overnight, the obtained sample was marked Ni-Co mixed sulfides-4. Moreover，the Ni-Co mixed sulfides-2 and Ni-Co mixed sulfides-8 were obtained by the same way with 2 mmol thiourea and 8 mmol thiourea, respectively. Materials characterization X-ray powder diffraction (XRD, RigakuSmart Lab, X-ray Diffractometer, Japan) was performed to study the crystalline structure of the products. X-ray photoelectron


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spectra (XPS, Thermal ESCALAB 250) was used to analyze the chemical binding energy of samples. The morphology and structural properties of the materials were characterized by field-emission scanning electron microscopy (FE-SEM, Carl Zeiss Super55 operated at 5 kV) and field-emission transmission electron microscope (TEM, Hitachi HT7700 operated at 120 kV), respectively. The tap density of the materials was measured by vibrating the glass cylinder until the surface of samples are not falling off, then read the scale of the glass cylinder, finally, the tap density is calculated by the formula: ρ=m/V

(1)

Where ρ is the tap density (g cm-3), m is the mass of the material (g), and V is the volume after the material is vibrated (cm3). Electrochemical study The main electrochemical tests as follows. Electrochemical tests were carried out in three-electrode system, a nickel foam coated with Ni-Co mixed sulfides was used as the working electrode, active carbon (AC) as the counter electrode, and Hg/HgO electrode as the reference electrode. Our working electrode was prepared with a mass ratio of 80:10:10 by mixing the Ni-Co mixed sulfides powder as active material, acetylene black as conductive additive, and polytetrafluoroethylene (PTFE) as binder, respectively. Firstly, PTFE was dropped into ethanol, where after 8 mg Ni-Co mixed sulfide powder and 1 mg acetylene black were added into above solution. Then ultrasonic vibration for 15 minutes to dispersed the mixed solution. Subsequently, the


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as-prepared mixed slurry was coated onto a piece of Ni foam (1 cm× 1 cm), following the electrode dried at 80 ℃ for 12 h, then pressed the electrode at a pressure of 10 MPa. The typical loading mass of the as-prepared electrode material is 2.0 mg cm-2. Just the same method for the preparation of the counter electrode, also mixing AC, acetylene black, and PTFE with a same weight ratio of 80:10:10 to smear onto a nickel foam (2.5 cm × 2.5 cm). All of the electrochemical measurements were performed in a 6 M KOH aqueous solution at room temperature. NEWARE (Shenzhen, China), as the galvanostatic charge−discharge testing system, was performed at different current density (0 to 0.55 V vs Hg/HgO). CHI 660E electrochemical workstation, was worked for the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) tests. Besides, CV measurements were carried out from 0 to 0.55 V at the different scanning rates and EIS test was within a frequency of 0.01 to 100000 Hz. The specific capacitance of electrode material was obtained according to the charge-discharge test, as following equation: Csp = I ∆t / ( m∆V)

(2)

Where Csp is the specific capacitance (F g−1), I is the charge−discharge current (A), ∆t is the discharge time (s), ∆V is the charge−discharge potential window (V), and m is the mass of the active material (g). Asymmetric supercapacitor (ASC) devices The ASC devices were fabricated with using the Ni-Co mixed sulfides-4 electrode as the cathode and an AC electrode as the anode in 6M KOH aqueous


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electrolyte, with one piece of cellulose paper as the separator. The mass ratio of negative electrode and positive electrode was determined by the charge balance as follows: q+ = q-

(3)

q = m C ∆V

(4)

m+ / m- = (C- ∆V-) / (C+ ∆V+)

(5)

Where q is the charge, m is the mass of the electrode material, C is the specific capacitance in the three-electrode system, and ∆V is the voltage window of positive and negative electrodes, respectively. The specific capacitances of Ni-Co mixed sulfides-4 and AC were calculated to be 1492 F g-1 and 204 F g-1, respectively. Therefore, the optimized mass loading of AC was estimated to be 15 mg after balancing charge capacity on 2 mg of Ni-Co mixed sulfides-4. The specific capacity (C), the energy density (E) and the power density (P) were calculated using the following equation: C = I∆t / (M∆V)

(6)

E = (1/2) CV2

(7)

P = E / ∆t

(8)

Where C is the total cell discharge specific capacitance (F g-1), and M is the total mass of active materials on both electrodes (g).

Results and discussions Morphological and structural characterization


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Figure 1 illustrates the schematic of our synthesis approach of Ni-Co mixed sulfides solid sphere. In the process of thiourea hydrolyzation, ammonia and sulfur source (S2-) were generated and the alkaline environment was developed.35 Afterwards, the Ni2+ and Co2+ are coprecipitated to form Ni-Co hydroxides, and then S2- in solution reacted with Ni-Co hydroxides which further converted to NiCo2S4.30, 36 The synthetic strategy is so simple that simplified the previous experimental procedure combined chemical coprecipitation and sulfidation two steps, which makes it time-saving and low cost. Furthermore, the morphology of NiCo2S4 can be well controlled by turning the amount of thiourea. Figure 2a presents the XRD pattern of Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8, which indicates that diffraction peaks of the as-derived product can be approximately indexed to the cubic NiCo2S4 (JCPDS card No. 20-0782, space group: Fd-3m(227), a=9.387, b=9.387, c=9.387) especially Ni-Co mixed sulfides-8. However, it can be seen that the peaks of Ni-Co mixed sulfides-2 and Ni-Co mixed sulfides-4 show some difference from the standard card. Obviously, all these extra diffraction peaks from Ni-Co mixed sulfides-2 can be successfully indentified tetragonal Ni3(NO3)2(OH)4 (JCPDS card No. 22-0752) marked by the olive star, which caused by the absence of thiourea and the combination of Ni(NO3)2·6H2O and ethylene glycol. As we know, Ni2+ precipitates more preferentially owing to the different solubility of metal hydroxides (the solubility of Ni hydroxide lower than that of Co hydroxide), the precipitation rates of Ni2+ and


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Co2+ under the same alkaline solution were also not equal.37 As a consequence, under the competitive co-precipitation, Ni3(NO3)2(OH)4 firstly formed. Meanwhile, the XRD pattern of Ni-Co mixed sulfides-4 with poor crystallinity shows a few extra peaks marked by violet five-pointed star, which can be approximately indexed to the tetragonal Co1-xS phase (JCPDS card No. 42-0826) although the peak intensity is relatively weak. Besides, the peak at about 2θ=47° is higher than at about 2θ=51°, which caused by the potentiation of Co1-xS and NiCo2S4. The corresponding XRD pattern of Ni-Co mixed sulfides-8 is readily indexed to cubic NiCo2S4 phase (JCPDS card no. 20-0782) with no residues or other phases detected. X-ray photoemission spectroscopy (XPS) results of the Ni-Co mixed sulfides-4 are also presented in Figure 2b-d. Besides, in order to compare with the pure NiCo2S4, the XPS of Ni-Co mixed sulfides-8 is also tested as shown in Figure S1. In the high-resolution XPS spectrum of Ni 2p region (Figure 2b), two spin-orbit doublets can be deconvoluted by using the Gaussian fitting method, the binding energy situated at 855.7 eV in Ni 2p3/2 and 872.7 eV in Ni 2p1/2 corresponds to the characteristics of Ni2+, however, peaks at 856.8 eV in Ni 2p3/2 and 875.3 eV in Ni 2p1/2 symbolize Ni3+,38 which is in accord with that of pure NiCo2S4 (Figure S1a). As regards the Co 2p region (Figure 2c), it shows two distinguished doublets located at a low energy band (Co 2p3/2) and a high energy band (Co 2p1/2), respectively. The binding energies at around 781.1 and 794.9 eV of the Co 2p peaks are assigned to Co2+ and those at 777.8 and 792.8 eV are characteristics of Co3+.39 Compared with Co 2p region of pure NiCo2S4 (Figure S1b), slight deviation


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happens in its Co 2p1/2 signature, it may due to the presence of Co1-xS. But Co 2p3/2 signature is close to that of NiCo2S4, suggesting similar valence states and ion distribution as in NiCo2S4.40 Figure 2d shows the S 2p region. In detail, the binding energy of 162.2 eV (S 2p1/2) is associated with sulfur-metal bonds (Ni-S and Co-S bonding). Besides, 161.0 eV (S 2p3/2) was also detected which implies the S2- at the surface of the material.36, 41-42 Besides, the high energy of 168.7eV is attributed to the S4+ species at the surface and/or edges of Ni-Co mixed sulfides-4 with highly oxidized state.43 Moreover, the spectrum of S 2p region is also similar with Ni-Co mixed sulfides-8 (Figure S1c), which indicates Co1-xS shows similar valence states as in pure NiCo2S4.44 According to the XPS analysis, we can estimate the near-surface constituent of the Ni-Co mixed sulfides-4 sample includes Ni2+/Ni3+, Co2+/Co3+, and S2-. Figure 3 presents the SEM images of samples prepared with different addition of thiourea by the hydrothermal process, and there are some differences among these morphologies. From the Figure 3a and 3b, we can see that the like-spheres consist of massive particles, and the surface of the agglomerate is fairly rough. Simultaneously, few nanorods or nanotubes exist in the Ni-Co mixed sulfides-2, which may caused by the absence of thiourea. Figure 3d and Figure 3e show the morphology of Ni-Co mixed sulfides-4. There are round spheres uniformed with an average size of about 2µm and independent of each other, which contributes to diffusion and transmission of the active materials in the electrolyte. In addition, close observation exhibits that there are some mild textures composed by nanoparticles on the surface of each round


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sphere as the inset of Figure 3e shows, which is critical to increase efficient redox reactions with abundant electrochemical active sites, enlarge the material-electrolyte contact area and decrease the electrolyte ion transportation path in the process of charging and discharging. Figure 3g and Figure 3h demonstrate the morphology of Ni-Co mixed sulfides-8. As we have seen, it remains the sphere structure, but the rough spheres occurs slightly adhesion each other, which not contributes to the ion diffusion in the electrolyte. Moreover, as illustrated in Figure 3h, the surface gets rough owing to the increasing amount of thiourea. For further indicating the uniform distribution of elements for Ni-Co mixed sulfides-4, the energy dispersive X-ray spectroscopy (EDS) mapping for the selected area of sample is shown in Figure 3j, which presents a uniform distribution of element Ni, Co, and S within the Ni-Co mixed sulfides-4. To further reveal the detailed morphology of the samples, the TEM of Ni-Co mixed sulfides was analyzed (in Figure 3c, 3f, and 3i). We can see that all of these spheres are solid, in contrast, it enhances the tap density and volumetric energy density of the electrode. Three-electrode electrochemical evaluation For measuring the electrochemical performance of these micron-sized NiCo2S4 solid

spheres

as

active

supercapacitor

electrode,

the

galvanostatic

charging-discharging, CV, and EIS tests were investigated in a standard three-electrode system with 6 M KOH aqueous electrolyte, which use Hg/HgO and AC as the reference and counter electrodes, respectively. At the same scan rate of 5


12

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mV/s (Figure 4a), all of the electrodes exhibit similar Faradaic charge-storage mechanism, and the Ni-Co mixed sulfides-4 electrode exhibits a dramatically expanded CV integrated area and higher current density with respect to those of Ni-Co mixed sulfides-2 electrode and Ni-Co mixed sulfides-8 electrode, which suggests that the Ni-Co mixed sulfides-4 electrode facilitates electron transport and enhanced the electrochemical utilization of NiCo2S4. Figure 4b illustrates a series of CV curves of the Ni-Co mixed sulfides-4 electrode at various sweep rates of 5, 10, 50, 100, and 200 mV s-1 with potential window of 0 to 0.55 V ( vs Hg/HgO). Obviously, all of the CV curves show a set of redox peaks. These peaks put down to typical redox reactions of Ni(II)/Ni(III) and Co(II)/Co(III) conversion in accordance with XPS, that stands for the faradaic behavior of the battery-type electrodes. Besides, when the current density is 1 A g-1, the charge-discharge curves of the three products display in Figure 4c. Obviously, Ni-Co mixed sulfides-4 is with the longest discharge time of 843 s when the current density is 1 A g-1 (within a voltage window of 0-0.55V), which represents the higher specific capacitance of 1492 F g-1 than that of Ni-Co mixed sulfides-2 (1016 F g-1) and Ni-Co mixed sulfides-8 (884 F g-1). Galvanostatic charging-discharging

tests

was

performed

to

investigated

the

excellent

electrochemical behavior of Ni-Co mixed sulfides-4 at a serious of current densities from 1 to 20 A g-1 as shown in Figure 4d. It is suggested that the distinct plateau regions in the curves can further confirm the intriguing pseudocapacitive properties of the Ni-Co mixed sulfides-4 once again. As shown in Figure 4e, the specific


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capacitance of the Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8 were calculated with different current densities. The specific capacitance of Ni-Co mixed sulfides-4 is 1492.36 F g-1, 1407.27 F g-1, 1240.36 F g-1, 1091.13 F g-1, and 909.16 F g-1 at current density of 1, 2, 5, 10, and 20 A g-1, respectively. About 61% of the capacitance can be retained with the current density increased, suggesting good rate performance of these solid spheres. Compared with Ni-Co mixed sulfides-4, the Ni-Co mixed sulfides-2 and Ni-Co mixed sulfides-8 deliver similar rate capability but lower specific capacitance. In general, the pseudocapacitive performance is mostly dependent on the surface chemistry and structure of the electrode materials, which results from the efficient redox reactions with abundant electrochemical active sites and high reactivity of the surface.45-47 The excellent capacitance of the Ni-Co mixed sulfides-4 than the other two is due to the independent solid spheres with more active sites are exposed, which enlarge the material-electrolyte contact area and provide efficient redox reactions in the process of charging and discharging. The higher pseudocpapcitance contributed from the mostly use of surface active materials promotes more excellent electrochemical performance (Figure S2). In addition, we analyzed the pseudocapacitance contribution with CV method from the perspective of electrochemistry.48-50 The capacitance of transition metal oxides mainly consists of electric double layer capacitance(EDLC), pseudocapacitance resulting from redox reaction on the material surface and capacitance of electrolyte ions insertion/extration.


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Besides, the EDLC so small that can be ignored. These capacitive contributions were obtained by analyzing the CV tests at a set of sweep rates as follows 51 i = aνb

(9)

where the i follows a power law relationship with the sweep rate ν. Both a and b are adjustable parameters, with b-values determined from the slope of the plot of log i vs log ν. For b = 0.5, the current is proportional to the square root of the scan rate ν, according to the following equation50 i = nFAC*D1/2ν1/2(αnF/RT)1/2π1/2χ(bt)

(10)

The current response is diffusion controlled, which is indicative of a faradaic intercalation process. For b = 1.0, the capacitive current is proportional to the sweep rate, according to the following equation51 i = νCdA

(11)

It is representative of a surface capacitive effect. A closer examination of the voltammetric sweep rate dependence uses for distinguish quantitatively the capacitive contribution to the current response. Through the theories above, we can combine two separate mechanisms, surface capacitive effects and diffusion-controlled insertion processes, to express the current response at a fixed potential. Thus, the relationship of response current(i) and scanning speed(ν) in the CV test is available under the said: i(ν) = k1ν1/2 + k2ν

(12)

In the formula, k1 and k2 are constant. k1ν1/2 and k2ν represent the current contributions from the diffusion process and the surface capacitive effects,


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respectively. The process of electrolyte ions insertion/extration is related to ν1/2, while redox reaction on the material surface is related to ν. Therefore, according to the formula (12), the curve of ν1/2 and discharge specific capacitance shows that electrolyte ions have plenty time to migrate when ν is infinitesimal, and the capacitance

results

from

both

pseudocapacitance

and

electrolyte

ions

insertion/extration (Figure S2a). Thus we can calculate from the inset of the Figure S2a that the maximum specific capacity of Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8 is 1960.78 F g-1, 2008.03 F g-1, and 1587.30 F g-1, respectively. On the other said, once ion diffusion is the control step when ν is infinite, the capacitance mainly derives from the surface pseudocapacitance (Figure S2b), which calculated from the inset of the Figure S2b that the minimum specific capacity of Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8 is 478.56 F g-1, 1210.26 F g-1, and 581.67 F g-1, respectively. As a consequence, the surface capacitance is occupied 60.27% of the total capacitance for Ni-Co mixed sulfides-4 electrode, which is higher than that of Ni-Co mixed sulfides-2 (24.41%) and Ni-Co mixed sulfides-8 (36.65%). It thus appears that the electrochemical reaction mostly occurs on the electrode surface, and the capacitance of Ni-Co mixed sulfides-4 mainly puts down to pseudocapacitance resulting from the redox reaction on the surface of the material. Hence, it displays outstanding specific capacitance and good rate performance.


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Additionally, Figure 4f shows the EIS measurement, in the low frequency area, the three products display similar impedance spectra with a straight line and also with a single semicircle in the high frequency. As we all know, the charge-transfer resistance (the semicircle at the real axis) stands for the resistance of the electrochemical reaction at the electrode and electrolyte interface. On the other hand, the Warburg impedance (the slope of the curves) reveals the electrolyte diffusion in the electrode.52 It can be seen that the charge transfer resistance of the Ni-Co mixed sulfides-2 and Ni-Co mixed sulfides-4 are similar and smaller than that of the Ni-Co mixed sulfides-8. Meanwhile, the Ni-Co mixed sulfides-4 shows a more vertical line than Ni-Co mixed sulfides-2 and Ni-Co mixed sulfides-8, implying the lower diffusion resistance. That because the appropriate size of these independent solid spheres contributes to diffusion and transmission of the active materials in the electrolyte. Moreover, repeated charging/discharging test at a current density of 10 A g-1 is conducted to study the long-term cycling stability of the Ni-Co mixed sulfides-4 electrode, as presented in Figure 5. The capacitance increases to 1073 F g-1 in the first two hundred cycles, which caused by the enlarged effective interfacial area at electrode and electrolyte when the electrode gradual activated.53 Impressively, the specific capacitance still reaches 800 F g-1 at 10 A g-1 and about 76% of the initial capacitance is preserved after 10000 cycles, revealing that the as-prepared Ni-Co mixed sulfides-4 electrode holds excellent long-term cyclability. At the same time, the columbic efficiency maintained nearly at 100%, showing an excellent reversibility of


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the Ni-Co mixed sulfides-4 electrode. Furthermore, the morphology and EIS of Ni-Co mixed sulfides-4 after stability test are shown in the inset of Figure 5. From Figure 5 (a), we can seen that some changes have taken place in morphology but still maintains the morphology of sphere. As the original SEM shown that there are some mild textures on the surface of each round sphere(in the inset of Figure 3e), it may occur the electrochemical recrystallization process owing to the repeated charge and discharge processes, which causes the surface of the material composed by crosslinked nanosheets. Besides, the material reveals large surface area and better electrochemical performance under long cycling. The EIS of the Ni-Co mixed sulfides-4 was measured before and after stability testing, as shown in Figure 5 (b), the EIS curve was similar before and after stability testing, indicating that no morphological defects were created in the Ni-Co mixed sulfides-4. Therefore, the Ni-Co mixed sulfides-4 reveals outstanding cycling stability. The electrochemical performance of the Ni-Co mixed sulfides-4 electrode in our work is still much superior to most Ni-Co based mixed sulfide electrodes reported previously (Table S1). Electrochemical characterization of the ASC To further evaluate the practical application of these NiCo2S4 solid spheres, ASC device is constructed by using the Ni-Co mixed sulfides-4 electrode as the cathode and an AC electrode as the anode in 6M KOH aqueous electrolyte, with one piece of cellulose paper as the separator. Additionally, to estimate the appropriate potential


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window of Ni-Co mixed sulfides-4 and AC electrodes, CV measurements were done with a standard three-electrode system in an aqueous solution of 6 M KOH. The good rate capability of pure AC is confirmed by CV curves as a function of scan rate and galvanostatic charge/discharge curves at various current densities (Figure S3). The Ni-Co mixed sulfides-4 electrode was measured at a scan rate of 10 mV s-1 from 0 to 0.55 V potential window (vs Hg/HgO), and within −1.0 to 0 V (vs Hg/HgO) for AC electrode. Therefore, the voltage window of the ASC was chosen to be 1.55V in 6M KOH from electrochemical behaviors of both electrodes (Figure 6a). Besides, for assembling the ASC device, the mass ratio of the two electrode materials needs to be obtained according to each specific capacitance. Figure 6b shows CV curves of the ASC at sweep rates from 5 to 100 mV s-1 between 0 and 1.55 V. A combination of both battery-like and electrical double-layer capacitive characteristics can be seen at all sweep rates. The galvanostatic charge/discharge curves of the Ni-Co mixed sulfides-4//AC ASC at different current densities of 1, 2, 5, 10, and 20 A g-1 are displayed in Figure 6c. We can see that the ASC delivers a specific capacitance of 304 F g-1 at a current density of 1 A g-1, and the capacitance retention is about 64% with a tenfold increase in the current density (Figure S4). Furthermore, Ragone plot, as shown in Figure 6d, was displayed to demonstration the relationship between the energy and power density of the ASC. An energy density of 48.4 Wh kg-1 can be achieved at a power density of 371.2 W kg-1, and an energy density of 26.3 Wh kg-1 can be still retained at a high power density of 6309.9 W kg-1. The performance also


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compared with the previous reports about Ni-Co-based ASC devices, such as HM-NCS//AC (28.9 Wh kg-1 at 188 W kg-1),54 NiCo2S4/CFP//AC (17.3 Wh kg-1 at 180 W kg-1),55 NiCo2S4//C (22.8 Wh kg-1 at 160 W kg-1),56 and NiCo2S4//RGO (31.5 Wh kg-1 at 156.6 W kg-1),57 Figure 6e shows the long-term cycling stability of the NiCo2S4//AC ASC at a current density of 5 A g-1. It still achieved 91% capacitance retention even endured 20000 long cycling, suggesting excellent stability of the ASC device. To further evaluate the practical application of the Ni-Co mixed sulfides-4//AC ASC, a green light-emitting-diode is lighted for 15 min with two cells in series connection, as presented in Figure 7. The ASC device assembled by unique Ni-Co mixed sulfides-4 and AC electrode proves great value for practical applications.

Conclusion In summary, micron-sized Ni-Co mixed sulfides solid spheres have been synthesized via a simple one-step hydrothermal method with the core reaction of the hydrolyzation

of

thiourea.

Outstanding

electrochemical

performances

for

supercapacitor were demonstrated, especially amazing discharge specific capacitance, nice rate performance, and splendid long-term cycling stability. Moreover, the ASC device made of Ni-Co mixed sulfides-4 and AC was constructed and showed a high energy density and power density as well as outstanding cycling performance. The superior energy storage performance of these Ni-Co solid spheres may benefit from the follows: (i) micron-sized solid structures with high tap density of more than 1.0 g


20

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cm-3 can effectively avoid the reunion phenomenon of nanomaterials and improve the volumetric energy density of the electrode; (ii) composed by nanoparticles of each round sphere can provide abundant electrochemical active sites for redox reaction on the material surface, also the high pseudocpapcitance contribution is beneficial to the outstanding electrochemical performance; (iii) independent of appropriate size of solid spheres contributes to diffusion and transmission of the active materials in the electrolyte. Thus, we believe that the construction of micron-sized solid sphere with high tap density as an advanced Ni-Co based mixed sulfide electrode material is a significative idea for high-performance energy storage device. ASSOCIATED CONTENT

Supporting Information Additional XPS and electrochemical performance test. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail

address:

[email protected].

Tel.:

0086-335-8061569;

fax:

0086-335-8059878. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We are grateful for the financial support from the Natural Science Foundation of


21


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China (51674221), the China Postdoctoral Science Foundation Funded Project (2016M591405), and Youth Scholars Research Fund of Yanshan University (16GA012), and Science and Technology Research and Development Program of Qinhuangdao (201602A004).


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List of Figure and Table Caption Figure 1 Fabrication process of the Ni-Co mixed sulfides solid sphere structure.


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Figure 2 (a) XRD patterns and (b~d) XPS spectra of (b) Ni 2p, (c) Co 2p, and (d) S 2p of the Ni-Co mixed sulfides solid sphere structure.



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Figure 3 SEM, TEM images of Ni-Co mixed sulfides solid sphere structures with different addition of thiourea, where (a, b, c) Ni-Co mixed sulfides-2, (d, e, f) Ni-Co mixed sulfides-4, and (g, h, i) Ni-Co mixed sulfides-8; (j) Energy dispersive X-ray spectroscopy (EDS) mapping image of Ni-Co mixed sulfides-4.


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Figure 4 CV of (a) Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8 at 5 mV s-1 and (b) Ni-Co mixed sulfides-4 under different scan rates in 6 M KOH; charge–discharge curves of (c) Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8 at 1 A g-1 and (d) Ni-Co mixed sulfides-4 at different current densities; (e) specific capacitance of Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8 at different current densities; (f) EIS curves of the Ni-Co mixed sulfides-2, Ni-Co mixed sulfides-4, and Ni-Co mixed sulfides-8.



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Figure 5 Long-term cycle performance of Ni-Co mixed sulfides-4 at 10 A g-1 [Inset shows SEM (a) and EIS (b) of Ni-Co mixed sulfides-4 after stability test].


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Figure 6 (a) Comparative CV curves of the AC and the Ni-Co mixed sulfides-4 electrodes performed in a three-electrode cell in 6 M KOH aqueous solution at a scan rate of 10 mV s-1; (b) CV curves of the Ni-Co mixed sulfides-4//AC ASC at various scan rates; (c) Galvanostatic charge-discharge curves of the asymmetric supercapacitor at different current densities; (d) Ragone plot related to energy and power densities of the Ni-Co mixed sulfides-4//AC ASC; (e) Long-term cycle performance of Ni-Co mixed sulfides-4//AC ASC at 5 A g-1.



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Figure 7 Digital image of a green-lightemitting diode (LED) lighted by two series connection cells ASC.


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The table of contents: An asymmetric supercapacitor based on Ni-Co solid spheres exhibits reveals superior cycling stability beyond 20000 cycles, and a green-lightemitting diode (LED) is lighted for more than 15 minutes with two cells in series connection.

ToC figure


Formation of Micron-Sized Nickel Cobalt Sulfide Solid Spheres with

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