Synthesis of ultrafine CoNi2S4 nanowire on carbon cloth as an efficient positive electrode material for high-performance hybrid supercapacitors

Journal Pre-proof Synthesis of ultrafine CoNi2S4 nanowire on carbon cloth as an efficient positive electrode material for high-performance hybrid supercapacitors Yuying Yang, Yan Zhang, Cuimei Zhu, Yandong Xie, Liwen Lv, Wenlian Chen, Yuanyuan He, Zhongai Hu PII:

S0925-8388(20)30248-6

DOI:

https://doi.org/10.1016/j.jallcom.2020.153885

Reference:

JALCOM 153885

To appear in:

Journal of Alloys and Compounds

Received Date: 22 November 2019 Revised Date:

13 January 2020

Accepted Date: 15 January 2020

Please cite this article as: Y. Yang, Y. Zhang, C. Zhu, Y. Xie, L. Lv, W. Chen, Y. He, Z. Hu, Synthesis of ultrafine CoNi2S4 nanowire on carbon cloth as an efficient positive electrode material for high-performance hybrid supercapacitors, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153885. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Dear Editor: The article is original and unpublished and is not being considered for publication elsewhere. The article has been written by the authors who are all aware of its content and approve its submission. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher. Thank you very much for your consideration, and we look forward to receiving comments from you soon. Yours Sincerely, Yuying Yang Department of Chemistry Northwest Normal University Lanzhou, 730070 P. R. China E-mail: [email protected]

Corresponding Author: Yuying Yang; E-mail: [email protected]; Tel.: +86 931 7973255; Fax: +86 931 8859764

Graphical abstract

Submitted to Journal of alloys and compounds

Synthesis of ultrafine CoNi2S4 nanowire on carbon cloth as an efficient positive electrode material for high-performance hybrid supercapacitors

Yuying Yang*, Yan Zhang, Cuimei Zhu, Yandong Xie, Liwen Lv, Wenlian Chen, Yuanyuan He, Zhongai Hu*

Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China *

Corresponding author. Tel.: +86 931 7970806 fax: +86 931 7971989 Email address: [email protected]

Abstract: Rational designing and preparing of electrode materials with desirable electrochemical performance by a simple, efficient and safe method is of great significance for electrochemical energy storage devices, but it is also challenging. In this study, ultrafine CoNi2S4 nanowire arrays directly grown on carbon cloth (CC) are synthesized by sulfuration of Co-Ni hydroxide precursor. Benefiting from the high conductivity, the synergistic effect between Co and Ni ions and open self-supporting three-dimensional (3D) nanoarchitectures constructed by 1D CoNi2S4 ultrafine nanowires, the as-obtained CoNi2S4@CC electrode exhibits excellent electrochemical capacitance performances: high specific capacitance of 1872 F g

-1

at 1 A g -1and 1565 F g

-1

at 5 A

g-1. Meanwhile, a battery-supercapacitor hybrid (BSH) device is fabricated using the as-obtained CoNi2S4@CC as the positive electrode and porous carbon nanosheets (PCNS) derived from wood fungus as the negative electrode. The constructed BSH exhibits a high energy density of 37.2 Wh kg

-1

at a power density of 0.75 kW kg-1, as well as a robust

long-term cycling stability (97.6% capacitance retention after 10 000 charge-discharge cycles at a constant current density of 1 A g−1). These outstanding results verify CoNi2S4 material has potential application in the energy storage field and it has been explored and expected to be the promising electrode materials for the supercapacitor.

Kerwords: Nickel cobalt sulfides, Pseudocapacitor, Electrochemical performance, Hydrothermal synthesis method, Battery-supercapacitor hybrid device.

1. Introduction With the development of the global economy and science technology, the development of energy storage devices with high energy density and power density, excellent cycling stability, and short charging and discharging time is urgently needed. As an emerging energy storage device, supercapacitors have been widely recognized as the promising energy storage devices for hybrid electric vehicles and portable equipment and electronics industry due to their advantages such as high power density (10 KW kg−1), superior cycling stability (over 10 0000 cycles), rapid charge/discharge ability and wide range of operating temperature. However, the lower energy density restricts their wide applications in practice [1-3]. Battery-supercapacitor hybrid (BSH) device can take full advantage of both batteries and supercapacitors. The energy density of the BSH device is higher than that of the conventional symmetric supercapacitors, and at the same time the power density is higher than that of the batteries, because high-capacity battery-type electrode and high-rate capacitive electrode are used as positive and negative electrodes in the BSH device, respectively [4,5]. Since the performances of BSH are closely related to the electrode material, the development of earth-abundant, low-cost and high activity electrode materials have become the primary task in commercial applications of BSH device in the future. Up to now, various electrode nanomaterials have been developed, like carbon materials (activated carbon, graphene, and carbon nanotube) [6-8], transition metal oxide/hydroxides (RuO2, NiO, MnO2 and Co(OH)2/Ni(OH)2) [9-11], conducting polymers [12], MOFs [13], organic molecule composites [14], and so on. In recent years, transition-metal sulfides (TMS) have been widely investigated as one of potential candidates for the supercapacitor electrode materials due to their good

conductivity, mechanical, thermal stability, multiple redox reactions and high theoretical capacitance [15]. Compared to transition-metal oxides, the replacing of oxygen with sulfur can promote electron transfer in the structure due to the lower electronegativity of sulfur than that of oxygen. Therefore, the transition-metal sulfides have better electrochemical performances than the transition-metal oxide [16-18]. For example, the electronic conductivity of binary metal nickel cobalt sulfides NiCo2S4 is nearly 100 times higher than that of NiCo2O4 [19,20]. On the other hand, the lower electronegativity of sulfur might bring about a more flexible structure and prevent structural expansion, which can increase the mechanical stability of the materials [21,22]. Furthermore, it has been reported recently that the conductivity of nickel cobalt ternary sulfides is better than that of binary sulfide (such as NiS and CoS), because both nickel and cobalt ions in the ternary sulfides can provide effective transport rate of ions and electrons. More importantly, based on the coexistence of two pseudocapacitive materials, the ternary transition metal sulfides can occur the multiple redox reactions, hence they possess excellent energy storage properties than that of binary nickel sulfides/cobalt sulfides [23]. Alshareef et al. prepared ternary nickel cobalt sulfides nanosheet arrays on conductive carbon cloth by a facile one-step electrochemical deposition method. The obtained ternary sulfide electrodes exhibited high capacitance (1418 F g-1 at a current density of 5 A g-1) and excellent rate capability in the three-electrode system. An asymmetric supercapacitor ternary sulfide//graphene showed a very high energy density of 60 Wh kg-1 at a power density of 1.8 kW kg-1 with robust long-term cycling stability up to 50 000 cycles [24]. Zhao and his collaborators synthesized honeycomb-like NiCo2S4 on activated carbon fiber (ACF). The optimized ACF/NCS-HL demonstrated high specific

capacitance of 1682 F g-1 at 1 A g-1 in KOH solution in the three-electrode measurement. They also fabricated an asymmetric supercapacitor with the as-obtained ACF/NCS-HL as the positive electrode and ACF as the negative electrode, and the asymmetric supercapacitor exhibited a high energy density of 49.38 Wh kg-1 at the power density of 0.8 kW kg-1, as well as a preeminent cycling stability (82.88% after 5000 cycles) [25]. Bao et al. in situ synthesized CoNi2S4 nanosheet arrays on nickel foams by a facile two-step hydrothermal method. The as-fabricated CoNi2S4 nanosheet arrays exhibited ultrahigh specific capacitance of 2906 F g−1 at a current density of 5 mA cm−2 as well as good rate capability and cycling stability. An aqueous asymmetric supercapacitor was also assembled with CoNi2S4 nanosheet arrays as the positive electrode and active carbon as the negative electrode, and the aqueous asymmetric supercapacitor cell delivered a high energy density of 33.9 Wh kg−1 at a power density of 409 W kg−1. Furthermore, four redlight-emitting diodes (LEDs) with a working voltage of 1.8 V could be easily lighted more than 30 min by two asymmetric supercapacitors in series [26]. Wang and his workers demonstrated an easy method for synthesizing CoxNi1-xS2 (0 < x < 1) by solid-state sulfurization at a lower temperature. This CoxNi1-xS2 also demonstrated enhanced electrochemical performances [27]. Another effective way to improve the performance of electrode materials is to combine the active material with high-conductive substrate, and to simultaneously increase the specific surface areas by controlling the morphology and structure of the active materials. Growing active materials on conductive substrate can reduce the agglomeration of materials so as to increase the specific surface areas. On the other hand, it can improve the conductivity of materials, so as to improve the electrochemical energy

storage performances of materials. The most commonly used conductive substrate include carbon nanotubes (CNTs) [28], graphene [29], nickel foam (NF) [30], carbon cloth (CC) [31], and so on. Amongst them, CC has been widely used due to its better corrosion resistance, desirable conductivity, good flexible and large surface area. Furthermore, the as-prepared CC-supported materials can be used directly as a binder-free electrode. Inspired by the previous researches, in this work, we present a facile two-steps hydrothermal method to fabricate CoNi2S4 nanowire arrays directly grown on CC as a self-supported and binder-free electrode. The two-steps involve the Co-Ni hydroxides were grown on the CC, and the conversion of hydroxides to sulfides. The features of CoNi2S4@CC can be summarized as follows: the 3D order nanowires array structure vertically to the surface of carbon cloth can increase the specific surface areas of material, high electrical conductivity due to the low resistance of CoNi2S4 and the compact bond between CoNi2S4 nanowires and carbon fibers. Benefiting from the above features, the optimized self-supported CoNi2S4@CC electrode exhibits high specific capacitance of 1872 F g

-1

at 1 A g-1 in KOH solution in the three-electrode measurement. A

battery-supercapacitor hybrid (BSH) device was successfully fabricated by using the obtained CoNi2S4@CC as a positive electrode and the porous interconnected carbon nanosheets (PCNS) as the negative electrodes (porous carbon nanosheets derived from wood fungus), respectively. The as-assembled BSH exhibits a high energy density of 37.2 Wh kg -1 at a power density of 0.75 kW kg-1 and a robust long-term cycling stability (97.6% capacitance retention after 10 000 charge-discharge cycles at a constant current density of 1 A g−1). Therefore, the as-prepared CoNi2S4@CC electrode indicates great prospects in

the field of energy storage and conversion. 2. Experimental 2.1 Materials All reagents in the experiment are of analytical grade, and were used as received without further treatment. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), urea (CO(NH2)2), sodium sulfide (Na2S), absolute ethyl alcohol (C2H5OH), acetone (C3H6O), hydrochloric acid (HCl), potassium hydroxide (KOH) and ultrapure water. 2.2 Preparation of CoNi2S4@CC composites 2.2.1 Treatment of carbon cloth Carbon cloth (CC) was purchased from the Hong Kong Physicochemical Limited Company and used as the substrate to support active materials. Being used before, CC was first washed with acetone, ultrapure water, and absolute ethyl alcohol several times to remove the surface impurities, and then dried in an oven at 70 °C for 6h. In order to improve the flexibility and enlarge the space interval between the fibers, the CC was heated at 900 °C for 1.5h at 8 °C min-1 in industry nitrogen (99% purity). Successively, the treated CC was immersed in HCl solution for 2.5h, and then washed with ultrapure water and vacuum-dried. 2.2.2 Synthesis of Co-Ni hydroxide@CC precursor In a typical preparation procedure, cobalt nitrate hexahydrate (0.5mmol), nickel nitrate hexahydrate (1mmol), and urea (6mmol) were dissolved in 15 mL ultrapure water with vigorous stirring for 30 min to form a clear solution. Then, the obtained clear solution was transferred into a 20 mL Teflon-lined stainless autoclave and a piece of

pretreated CC (2 cm × 2 cm) was immersed into the solution. Then, the sealed autoclave was transferred in an oven and maintained at 120 °C for 12h. After the autoclave slowly cooled to the ambient temperature, the precursor Ni(OH)2/Co(OH)2@CC was taken out and washed with ultrapure water and vacuum-dried at 60 °C for 6h. For comparison, Co-Ni hydroxide precursors with different Co contents were synthesized by adjusting the number of molar of Co2+, respectively. 2.2.3 Synthesis of CoNi2S4@CC composites To obtain the CoNi2S4@CC sample, a low-temperature hydrothermal sulfuration method was used in this system. In a typical preparation procedure, 90 mg of Na2S was dissolved into 15 mL ultrapure water under vigorous stirring to form a clear solution. Then, the obtained solution was transferred into a 20 mL Teflon-lined stainless autoclave and the prepared Co-Ni hydroxide@CC (2 cm × 2 cm) was placed in the autoclave. After that, the sealed autoclave was transferred in an oven and maintained at 90 °C for 8h. Then, the autoclave was slowly cooled, and the CoNi2S4@CC product was washed with ultrapure water and ethanol several times and vacuum-dried at 60 °C for 6h. The effect of Co:Ni molar ratios on the properties is also studied, and the product is denoted as CoNi-S@CC-1(Co:Ni molar ratio is 1:1 ), CoNi-S@CC-2(Co:Ni molar ratio is 2:1 ) and without addition of Co salts as NiS@CC, respectively. 2.3 Preparation of porous carbon nanosheets (PCNS) The PCNS used as the negative material for our asymmetric was prepared by carbonization of swelling agarics as reported in our earlier publication [32, 33]. 2.4 Materials characterizations 2.4.1 Physical characterizations

The crystal phase of the synthesized CoNi2S4@CC were investigated using a powder X-ray diffraction (XRD) equipped with Cu Ka radiation (l ¼ 0.15418 nm) operating at 40 kV, 60 mA. The morphologies of the CoNi2S4@CC were investigated by field-emission scanning electron microscope (FESEM; LTRA plus, Germany) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2100 system operating at 200 kV). The specific surface areas and pore size distributions of the CoNi2S4@CC were evaluated using an automatic adsorption instrument (ASAP 2020, Micromeritics). 2.4.2 Electrochemical measurement The electrochemical performances were evaluated by using three- and two-electrode configurations in 6 M KOH electrolyte solution, CoNi2S4@CC was directly used as the working electrode, Hg/HgO electrode and platinum foil were used as the reference electrode and the counter electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were tested to study the electrochemical performances of the working electrode. The cycling stability was performed on LANHE test system (Wuhan electronic co., LTD, China). The specific capacitance (C, F g-1), energy density (E, Wh kg-1) and power density (P, W kg-1) of the assembled BSH were calculated using the following formulas: Csp = I ⋅ t / m ⋅ ∆V , E = Csp ⋅ ∆V 2 / 2 , P = 3600E / t

Where Csp is the device specific capacitance (F g-1), i (A) is the current density, t (s)

is the discharging time, m (g) is the total mass of active materials, and ∆V (V) is the cell voltage. While the power density value (P) is obtained by dividing the E with time t for one complete discharging segment.

3. Results and discussion 3.1 Morphology and structure of samples The fabrication of CoNi2S4@CC is schematically depicted in Fig. 1. A typical preparation process includes two steps: (1) Co-Ni hydroxide@CC precursor arrays were grown on the CC fiber by hydrothermal process. (2) The target product of porous core-shell NiS@CoS@CC was obtained by a low temperature hydrothermal sulfuration of the resultant Co-Ni hydroxide@CC precursor. Notably, it is a critical step for the fabrication of Co-Ni hydroxide nanowires dispersively distributed on the surface of CC, which can efficiently prevent the aggregation of CoNi2S4, increasing the actual utilization of CoNi2S4 material and exposing abundant CoNi2S4 active sites.

Fig. 1. Schematic illustration of the formation of NiS@CoS@CC core-shell composites.

The crystal phase and structure information of the products were analyzed by XRD. Fig.2. shows XRD patterns of the Co-Ni hydroxide@CC precursor and the target product CoNi2S4@CC. Fig. 2b shows that the two crystalline phases of Co hydroxide (CoO(OH), PDF 02-0214) and Nickel oxide hydroxide (NiOOH, PDF 06-0141) coexist in the precursor composite, but the reflection peaks are not obvious. The result illustrates that the cobalt hydroxide is amorphous due to the low temperature of 120 °C. After

sulfuration, the diffraction peaks of Co-Ni hydroxide phase disappeared and the CoNi2S4 phase appeared (as shown in Fig. 2c). The diffraction peaks at 16.3°, 31.5°, 38.2°, 50.2° and 55.0° collected from can be perfectly matched the diffraction peaks of CoNi2S4 (PDF 24-0334), which correspond to (111), (311), (400), (511), and (440) planes, respectively.

Fig. 2. XRD pattern of CC, Co-Ni hydroxide@CC precursor and CoNi2S4@CC.

The morphologies and nanostructures of carbon cloth and the as-prepared CoNi2S4@CC were studied by field emission scanning electron microscopy (FESEM). As shown in Fig. 3a the fiber of carbon cloth has a smooth surface. In the CoNi2S4@CC composites numerous tiny CoNi2S4 nanowires are evenly and vertically grown on the surface of the individual fiber (Fig. 3b). A closer observation shows that the tiny CoNi2S4

Fig. 3. (a) SEM images of CC; (b) SEM images of Co-Ni hydroxide@CC; (c, d) SEM images of CoNi2S4@CC; (e) Elemental mapping of CoNi2S4@CC; (f, g) TEM images of CoNi2S4@CC.

nanowires intimately contacted with the carbon cloth fiber, and they are interconnected with each other (Fig. 3c) to form a good 3D porous network structure. This particular

structure can provide abundant electrochemical active sites of the CoNi2S4 materials, increase the effective liquid-solid contact area, supply a fast channel for the electrons and electrolyte ions and consequently exhibit excellent electrochemical capacitance performance. And as can be seen from the Fig. 3d the existence of carbon cloth can not only serve as the conductive support for the growth of CoNi2S4 nanowires, but also prevent the CoNi2S4 nanowires from aggregation. The corresponding elemental mapping reveals the uniform distribution of Ni, Co and S element. To further observe the morphology and microstructure of CoNi2S4, the CoNi2S4 nanowires are peeled off from the carbon cloths and further investigated by HRTEM. It can be seen from Fig. 3e, after peeling off from the carbon cloth, the CoNi2S4 nanowires have obvious agglomeration phenomenon, but the nanowires show a low contrast with the background at the edges, representing the ultrathin characteristics of these nanowires. These ultrafine nanowires can expose abundant active surfaces and full utilize of active materials, which is favorable to achieving a high specific capacitance. To compare the products with different ratio, additional SEM images are presented in Fig. S1.

Fig. 4. (a) Nitrogen adsorption-desorption isotherm and (b) Pore size distribution plot of CoNi2S4@CC

The surface area and pore size distribution of the CoNi2S4@CC are investigated by

the N2 adsorption desorption measurement. As shown in Fig. 4a, the curve can be classified as typical types of IV isotherm. The weak hysteresis hoop between 0.7 and 1.0 at relative pressure and gradually increasing positive slope are features of mesopores, indicating the existence of mesopore in the CoNi2S4@CC sample [34]. The specific surface area of CoNi2S4@CC is calculated by the Brunauer–Emmett–Teller (BET) method, and the value is relatively small only about 3.03 m2 g−1. The pore size distribution of the CoNi2S4@CC is also observed by using the Barrette-Joynere Halenda (BJH) method, the result displays the mesoporous diameter is around 10 nm. Such porosity can facilitate the transfer of ions and electrons at the electrode/electrolyte interface [35, 36].

Fig. 5. (a) XPS spectra survey spectrum of CoNi2S4@CC; (b) Ni 2p; (c) Co 2p and (d) S 2p.

XPS measurement was utilized to investigate the chemical states and elemental

bonding configuration of the materials. Fig. 5a shows the full survey XPS spectrum of the Ni, Co, and S elements. Fig. 5b exhibits the spectrum of Ni 2p region, which can be divided into four peaks. In detail, the peaks locating at 862.083 and 879.913 eV can be ascribed to Ni 2p3/2 and Ni 2p1/2 confirming the presence of Ni2+. The other two peaks at 857.202 eV and 874.907 eV are attributed to Ni 2p3/2 and Ni 2p1/2 both as typical characteristic of Ni3+, heralding the co-existence of Ni2+ and Ni3+. The appearance of Ni3+ is that NiS on the surface could convert to nickelic oxide because of its high reactivity toward oxygen. For the Co 2p spectrum (Fig. 5c), the peaks centered 775.408 and 782.062 eV can be assigned to Co 2p3/2 and Co 2p1/2 binding energies, respectively. The S 2p spectra of CoNi2S4@CC (Fig. 5d) can be deconstruct into two peaks. The component peak with a lower binding intensity centered 159.180 eV come from S2-. Meanwhile, the satellite peak centered 169.071 is closely related to SO42- species formed on surface of metal sulfides owing to inevitable contacting with air. Based on the above aforementioned analysis, all these results mentioned above unanimously confirm the formula of the CoNi2S4@CC in the literature, which agree well with previous XRD results. 3.2 Electrochemical performances 3.2.1 Three-electrode system The electrochemical performances of all the materials are investigated using CV, GCD and EIS measurements in 6 M KOH aqueous solution. Fig. 6a shows the CV curves of NiS@CC, CoNi-S@-1, CoNi-S@-2, and CoNi2S4@CC composites electrodes between a potential window of 0-0.6 V at a scan rate of 10 mV s -1. Obviously, the CV area of the CoNi2S4@CC electrode is much higher than that of others, indicating the CoNi2S4@CC

Fig. 6. (a) CV comparison curves of different CoNi-S@CC at a scan rate of 10 mV s-1, (b) CV curves of CoNi2S4@CC at different scan rates, (c) Galvanostatic charging-discharging comparison curves of different CoNi-S@CC, (d) Galvanostatic charge-discharge curves at various current densities of CoNi2S4@CC, and (e) Nyquist plot of CoNi2S4@CC.

electrode exhibits much larger charge storage capability. Furthermore, all curves display a pair of strong redox peaks, indicating reversible Faradaic redox reaction on the surface of the electrode. The redox peaks are attributed to the reversible Faradaic redox reaction of Co2+/Co3+/Co4+ and Ni2+/Ni3+. Fig. 6b presents the CV curves of CoNi2S4@CC

composites electrode at a scan rate of 5-30 mV s-1. It can be seen from the curves that the current density increases with the increase of scan rate, and the peak potential shifts about 60 mV for a 6 time increase in the scan rate, representing fast response under electrode processes. This is partly due to the excellent electric conductivity of carbon cloth in the CoNi2S4@CC composites electrode. In addition, CoNi2S4 nanowires grow on the carbon cloth surface, and these nanowires interlace with one another to form a 3D network “electrolyte reservoir”, which is benefical to store electrolyte to ensuring steady supply of electrolyte ions for redox reaction on the CoNi2S4@CC electrode even at a high specific current.Fig. 6c shows the GCD curves of all the materials at 1 A g-1. Apparently, the charging and discharging time of CoNi2S4@CC electrode are much longer than that of others, indicating the specific capacitance of the CoNi2S4@CC electrode is greatly enhanced. Fig. 6d gives the GCD curves of CoNi2S4@CC electrode at various current densities. The specific capacitance of the CoNi2S4@CC electrode at a current density of 1 A g-1 is as high as 1872 F g-1 (binary nickel sulfide shows inferior capacitance performance of 569 F g-1 at 1 A g-1), and still retains 1565 F g-1 when the current density increasing to 5 A g-1. The capacitance retention is 83.6% when the current density increases from 1 A g-1 to 5 A g-1 (the capacitance retention of binary nickel sulfide is only 60.6%), indicating good rate capability. The enhanced capacitance and good rate capability of CoNi2S4@CC electrode can be attributed to its 3D porous network nanoarchitecture and the synergistic effect nickel and cobalt ions in the ternary sulfides. First, the 3D porous network structure would provide abundant electrochemical active sites of the CoNi2S4 materials, increase the effective liquid-solid contact area, supply a rapid channel for the electrolyte ions and charge and consequently exhibit high specific

capacitance. Second, based on the coexistence of two pseudocapacitive materials, the ternary transition metal sulfides can occur the multiple redox reactions, hence they possess excellent energy storage performance. Third, the CoNi2S4 nanowires are directly grown on the carbon cloth, which not only increase the electrical conductivity but also greatly improve the stability of the material and consequently ensure the good rate capability. Last but not least, the CoNi2S4@CC is directly used as a binder-free and self-supported electrode, which can lower the resistance and facilitate electron transfer at the interface between electrolyte solution and electrode. The EIS analysis is one of the important factors to evaluate the fundamental behavior of electrode materials for supercapacitors. Fig. 6e shows the Nyquist plots of CoNi2S4@CC electrode, the frequency ranged from 0.1 Hz to 10 kHz. The intercept of the real axis in the range of high frequency represents the total internal resistance, which includes the ionic resistance of electrolyte, the intrinsic resistance of active materials, the contact resistance at interface between electrolyte solution and electrode, and the contact resistance between the active material and current collector [37]. As seen from Fig. 6e, the intercept is about 2.8, indicating that the CoNi2S4@CC material has good conductivity. The diameter of the semicircle in the high frequency region represents the faradic charge transfer resistance (Rct), where the smaller semicircle represents that the Rct is minimal. At the lower frequencies, the slope of the straight line represents the diffusive resistance (Warburg impedance). As is shown in Fig. 6e, the angle of the straight line is larger than 45°, representing that the capacitive behavior of the CoNi2S4@CC electrode is controlled by the surface process [38]. These attributed to the closely contact CoNi2S4 nanowires with carbon cloth and 3D porous network structure,

which can facilitate electrolyte ion diffusion and electron transfer, thereby resulting in ideal capacitive behavior. 3.2.2 Two-electrode system

Fig. 7. (a) Schematic diagram of as-assembled CoNi2S4@CC//PCNS; (b) CV curves of CoNi2S4@CC and HPN electrode in the three-electrode configuration.

To evaluate the practical application of the CoNi2S4@CC material, a battery-supercapacitor hybrid (BSH) device is assembled using CoNi2S4@CC and PCNS as positive and negative electrodes, 6 M KOH solution as the electrolyte. The cell voltage of the device is estimated by the CV curves of the negative and positive electrodes in the three-electrode system. As shown in Fig. 7b, the CV curve of PCNS shows nearly a rectangular shape with the potential window of -1.0-0 V, representing the electric double layer capacitive feature, and the CV curve of the CoNi2S4@CC exhibits an obvious redox peaks at the potential window of 0-0.6 V, representing the Faraday pseudocapacitance behavior. So we can deduce that the operation cell voltage of the CoNi2S4@CC//PCNS BSH can be extended up to at least 1.6 V. In the experiment, potential window can be expanded to 1.8 V. The charge (Q) stored at the positive materials (Q+) and negative materials (Q-) are balanced based on the following equation: Q=C×∆V×m, where C, ∆V and m are specific capacitance, operating potential window and the mass of the active

Fig. 8. (a) CV curves of CoNi2S4@CC//PCNS device at different scan rates, (b) Galvanostatic charge–discharge curves at various current densities of CoNi2S4@CC//PCNS device, (c) Cycling performance of CoNi2S4@CC//PCNS device at a current density of 3 A g-1, and (d) Ragone plot of CoNi2S4@CC//PCNS device, in comparison with other reported devices.

materials, respectively. In order to achieve a satisfactory electrochemical performance, it is important to realize Q+= Q-. The loading mass ratio of positive and negative active materials can be calculated by the equation m+/m-=C-×∆V-/C+×∆V+, and the optimal loading mass ratio between positive and negative ( m(CoNi2 S4 @ CC ) m（PCNS) ) is estimated to be

0.21 from the specific capacitance and potential window obtained from their galvanostatic charge-discharge curves. Fig. 8a shows the CV curves of the CoNi2S4@CC//PCNS BSH device at various scan rates. The CV curves show capacitances from both pseudo-capacitance of CoNi2S4@CC electrode and EDLC of PCNS electrode. The peaks reveal the existence of the reversible and fast Faraday redox reactions during the electrochemical process, and the peak current increases with the scanning rate increases from 5 to 30 mV s-1. Meanwhile, there is no obvious deformation in the CV curves, meaning a rapid current response. Fig. 8b shows the GCD curves at various current densities. The specific capacitance values of the device are calculated from the discharge curves based on the total mass of the active materials of the two electrodes. The specific capacitance reaches a maximum of 119 F g-1 at 1 A g-1. Fig. 8b also shows that the specific capacitance gradually decreases with the increase of current density, indicating the low utilization efficiency of active materials under a high current density. More importantly, as shown in Fig. 8c, the CoNi2S4@CC// PCNS BSH device shows an excellent cycling stability (97.6% of the initial after 10,000 cycles). In order to make the comparison, the SEM images of cycled electrodes after long-term cycling are shown in Fig. S4 in supporting information. Obviously, after long-term discharge/charge, it is able to hold its original nanowire morphology, no obvious morphological change is found, indicating the good structural stability of the electrode. The power density and energy density are two important indicators to evaluate the performance of energy storage devices. The GCD curves can be applied to estimate the power and energy densities of the CoNi2S4@CC//PCNS BSH device. With increasing specific current from 1 to 10 A g-1, the Ragone plot of the

capacitor is shown in Fig. 8d. The energy density reaches 37.2 Wh kg-1 along with power density of 0.75 KW kg-1, which are higher than the previous reports, such as CoNi2S4/CC//r-GO (33.20 Wh kg-1 at 0.800 KW kg-1) [39], CoNi2S4//AC (33.90 Wh kg-1 at 0.409 KW kg-1) [40], CoNi2S4//CoNi2S4 (16.74 Wh kg-1 at 0.498 KW kg-1) [41], CoNi2S4//YS-CS (35 Wh kg-1 at 0.640 KW kg-1) [42], Ni3S2/MWCNT-NC//AC (19.8 Wh kg-1 at 0.798 KW kg-1) [43], NiCo2S4//r-GO (16.6 Wh kg-1 at 2.348 KW kg-1) [44], Co9S8//AC (13.3 Wh kg-1 at 1.11 KW kg-1) [45], NiS/rGO//AC (18.7 Wh kg-1 at 0.124 KW kg-1) [46]. Furthermore, an LED indicator can be lighted by two ASC devices, as shown in Fig. 8d. These results further demonstrate that the as-fabricated CoNi2S4 on CC graphene is an effective strategy to achieve the high-efficient Energy storage device. 4. Conclusions In summary, in this paper we demonstrate a facile, convenient and controllable synthesis method for CoNi2S4@CC composites as a self-supported and binder-free electrode for high-performance battery-supercapacitor hybrid device. The synthesis method involves two-steps: the Co-Ni hydroxide precursors are grown on the CC, and the conversion of Co-Ni hydroxide to CoNi2S4@CC. The optimized nickel cobalt sulfides have a 3D porous network structure, which is constructed by interconnected ultrafine nanowires grown on a conductive carbon cloth substrate (CoNi2S4@CC), and the as-prepared CoNi2S4@CC as a self-supported electrode exhibits high specific capacitance (1872 F g−1 at 1 A g-1). Furthermore, a battery-supercapacitor hybrid device is fabricated by the optimized CoNi2S4@CC nanowires as the positive electrode and porous carbon nanosheets derived from wood fungus as the negative electrode. The as-fabricated CoNi2S4@CC//PCNS device also shows a high energy density 37.2 Wh kg−1 at a power

density of 750 W kg−1 and a superior cycling stability (the capacitance retained 97.6% of the initial after 10,000 cycles). This work shows a facile and convenient method for fabricating metal sulfides, and this synthesis strategy can easily be extended to prepare other composite materials for energy storage and conversion devices. Acknowledgements The authors gratefully acknowledge the financial support offered by the National Natural Science Foundation of China (21773187, 21563027, 21163017, and 20963009).

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Highlights of this work achieves as following: > CoNi2S4@CC with 3D porous structure is prepared by a facile hydrothermal method. > 3D structure is constructed by 1D ultrafine nanowires vertically to the surface of CC. > The as-fabricated CoNi2S4@CC exhibited ultrahigh specific capacitance of 1872 F g -1 at 1 A g -1. > CoNi2S4@CC//PCNS BSH device was constructed. > The CoNi2S4@CC//PCNS BSH device showed high energy and excellent cycling stability.

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