Effect of MgO and Ta2O5 co-coatings on electrochemical performance of high-voltage spinel LiNi0.5Mn1.5O4 cathode material

Journal Pre-proof Hydrothermal synthesis of transition metal sulfides/MWCNT nanocomposites for highperformance asymmetric electrochemical capacitors Yangyang Luo, Wenxiu Que, Chenhui Yang, Yapeng Tian, Xingtian Yin PII:

S0013-4686(19)31609-3

DOI:

https://doi.org/10.1016/j.electacta.2019.134738

Reference:

EA 134738

To appear in:

Electrochimica Acta

Received Date: 12 June 2019 Revised Date:

29 July 2019

Accepted Date: 19 August 2019

Please cite this article as: Y. Luo, W. Que, C. Yang, Y. Tian, X. Yin, Hydrothermal synthesis of transition metal sulfides/MWCNT nanocomposites for high-performance asymmetric electrochemical capacitors, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.134738. 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. © 2019 Published by Elsevier Ltd.

Graphical Abstract

The Ni-S/MWCNT and Ni-Fe-S/MWCNT composites were prepared by a facile hydrothermal method. Especially, the Ni-S/MWCNT composite as positive electrode shows a high specific capacity of 265.6 mAh g-1 at 1 A g-1 and an enhanced rate performance

due

to

an

integration

of

MWCNTs.

The

fabricated

Ni-S/MWCNT//Ni-Fe-S/MWCNT asymmetric electrochemical capacitor (AEC) delivers a high energy density of 42.2 Wh kg-1 at a high power density of 3.7 kW kg-1.

Hydrothermal synthesis of transition metal sulfides/MWCNT nanocomposites for high-performance asymmetric electrochemical capacitors Yangyang Luo, Wenxiu Que*, Chenhui Yang, Yapeng Tian and Xingtian Yin Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Shaanxi Engineering Research Center of Advanced Energy Materials and Devices, School of Electronic & Information Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China ∗

Prof. W. Que, Tel. & Fax No.: +86-29-83395679. E-mail address: [email protected]

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Abstract: Achieving both high energy and power densities for electrochemical capacitors is always one of the greatest scientific and societal challenges. Owing to the outstanding electrochemical performances of transition metal sulfides, especially high theoretical capacities and wide potential windows, they have drawn extensive attention as promising electrode materials for electrochemical capacitors recently. Herein, the Ni-S/MWCNT composite as positive electrode and the Ni-Fe-S/MWCNT composite as negative electrode have been prepared by a hydrothermal method. The Ni-S/MWCNT electrode shows a high specific capacity of 265.6 mAh g-1 at 1 A g-1 and an excellent rate performance, which are attributed to a larger BET surface area and faster charge transfer rate due to an introduction of MWCNTs compared to the pure Ni-S electrode. The Ni-Fe-S/MWCNT electrode also exhibits a good electrochemical performance due to its rich redox reaction and high total pore volume after the integration of MWCNTs. The fabricated Ni-S/MWCNT//Ni-Fe-S/MWCNT asymmetric electrochemical capacitor delivers a high cell potential of 1.7 V, a high energy density of 42.2 Wh kg-1 at a high power density of 3.7 kW kg-1 (even 19.1 Wh kg-1 at 11.9 kW kg-1), and a relatively good cycling stability. Our work may provide an alternative protocol for the fabrication of high-performance electrochemical capacitors.

Keywords: Transition metal sulfide; Multi-walled carbon nanotubes (MWCNTs); Hydrothermal method; Asymmetric electrochemical capacitors (AECs)

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1. Introduction With the exhaustion of traditional fossil fuels and the deterioration of global climate and environment, the development of clean and sustainable energy has become an urgent demand for human society. Researchers have made great efforts to develop wind turbines as well as solar energy harvesting and conversion technologies recently [1-4]. However, the intermittent and instable characteristics of them inevitably limit their extensive utilization. Hence, developing energy storage or energy supply technologies becomes more and more important. Among various energy storage devices, both the rechargeable batteries and electrochemical capacitors (ECs) are the two leading electrochemical energy storage (EES) ones [5-8]. Lithium-ion batteries have currently been widely used in consumer electronics due to their high energy densities, but the generation of heat and dendrite can restrict their wide applications when operated at a high power in some fields. By contrast, ECs can be charged in minutes, showing the much higher power densities, more excellent reversibility and longer cycle life [9, 10]. However, the lower energy densities of ECs have hindered their practical application scopes. Thus, the development of new electrode materials and novel EES systems that can realize the substantial improvements in the energy and power densities has been an important future direction for EES. It is acknowledged that the energy density (E) of ECs is determined by both the capacitance (C) and voltage window (V) of the devices based on the equation of E=0.5CV2. [11, 12] Thus, to improve the energy density without sacrificing the power density and cycling stability of ECs, either or both of the capacitance and voltage window should be increased. Importantly, the asymmetric electrochemical capacitors (AECs) have properly been designed and assembled by 3/36

combining positive and negative electrodes with separated potential windows in recent years [13-16], which is deemed to be one of the most promising strategies for enhancing the energy density of ECs. To date, many researchers have been devoted to investigating and optimizing cathode materials, while various carbon materials are generally adopted as anode materials, such as activated carbon [17], porous carbon [18] and graphene [19]. Whereas, these anode materials still show the relatively lower specific capacitances, and the corresponding ECs present the lower energy densities as compared to those by utilizing pseudocapacitive or battery-type anode materials [20, 21]. Transition metal sulfides (TMSs) such as nickel sulfides [22], cobalt sulfides [23], copper sulfides [24], iron sulfides [25] or the mixed metal sulfides [26] have attracted considerable attention as a new promising class of pseudocapacitive or battery-type electrode materials, due to their high electrical conductivity, excellent redox reversibility and low electronegativity compared to their corresponding oxides. Especially, nickel sulfides and iron sulfides have received considerable attention due to their high theoretical capacities, wide potential windows, and low costs [27-29]. Whereas, the low rate capability and/or poor electrochemical stability of them still restrict their potential applications. To overcome these problems, numerous efforts have been made, such as growing active materials onto carbonaceous materials (carbon nanotubes [30], graphene[31], carbon fiber papers [32], etc.), and regulating or modifying active materials directly [33]. For example, Dai and co-workers [19] demonstrated that the NixSy/rGO composite delivered a higher specific capacity and better rate capability because that the rGO could strengthen the mechanical integrity 4/36

and enhance the electrical conductivity of the whole electrode, and the NixSy/rGO//graphene hybrid supercapacitors showed a high energy density of 46 Wh kg-1 at a power density of 1.8 kW kg-1. Chen and co-workers[32] stated that the carbon fiber@Ni3S2 non-woven structured electrode had an ultrahigh volume/area capacitance (911 F cm-3/15.5 F cm-2), which were attributed to the conductive network of carbon fiber papers, the stable structure of Ni3S2 on carbon fibers and the pores and tunnels formed between fibers. Thus, anchoring pseudocapacitive or battery-type electroactive species on carbonaceous matrixes is currently regarded as one of the most promising and effective tactics for achieving the high-performance ECs. Carbon nanotubes (CNTs) have been deemed to be an ideal matrix for constructing nanocomposites due to the advantages of large surface area, high conductivity, excellent mechanical and electrochemical stability. Herein, we have successfully prepared the Ni-S/MWCNT and Ni-Fe-S/MWCNT nanocomposites by a facile and cost-effective hydrothermal method. The MWCNTs could not only increase the electronic conductivity of the nanocomposites, but also prevent the TMSs electroactive species from aggregation, thus, resulting in a superior redox reaction kinetics and specific capacity. Results indicated that the Ni-S/MWCNT composite as positive electrode delivered an improved specific capacity of 265.6 mAh g-1 at 1 A g-1 and a better rate capability. While the Ni-Fe-S/MWCNT composite as negative electrode also showed a good electrochemical performance. The fabricated Ni-S/MWCNT//Ni-Fe-S/MWCNT asymmetric electrochemical capacitor achieved a high cell potential window of 1.7 V, a high energy density of 42.2 Wh kg-1 at a high 5/36

power density of 3.7 kW kg-1 (even 19.1 Wh kg-1 at an ultrahigh power density of 11.9 kW kg-1), and a relatively good cycling stability. Our work may provide an alternative protocol for the fabrication of high-performance ECs.

2. Experimental Section 2.1. Preparation of the Ni-S/MWCNT composite The multi-walled carbon nanotubes (MWCNTs) (250 mg) were purified by refluxing them in concentrated nitric acid (50 mL) at 120 °C for 5 h. Then, the MWCNTs precipitate was filtered off, washed with ultrapure water several times, and dried in an oven at 60 °C. The Ni-S/MWCNT composite was fabricated by a one-step hydrothermal method. In a typical procedure, 0.75 M of NiCl2·6H2O solution was dropwise added into 20 mL of the acid-treated MWCNTs aqueous solution (2 mg mL-1) under continuous stirring, to form a homogeneous dispersion. Then, 0.5 mmol of Na3C3N3S3·xH2O (as sulfur source) was added into the mixture of diethanolamine (DEA, 5 mL) and ultrapure water (2 mL) under stirring for 1 h. After that the Na3C3N3S3-contained mixture was dropwise added into the above homogeneous Ni-contained MWCNTs dispersion with continuous stirring for 1 h again. Afterwards, the precursor mixture was transferred into a 50 mL stainless-steel autoclave, which was then sealed and maintained at 180 °C for 6 h. After natural cooling to room temperature, the precipitate was taken out and rinsed with ultrapure water and absolute ethyl alcohol several times. Finally, the product was dried under vacuum at 70 °C. For comparison, the Ni-S powders were also synthesized by using the similar procedure without the addition of MWCNTs. 2.2. Preparation of the Ni-Fe-S/MWCNT composite 6/36

The Ni-Fe-S/MWCNT composite was prepared by two-step hydrothermal processes. In brief, 291 mg of Ni(NO3)2·6H2O), 404 mg of Fe(NO3)3·9H2O, 300 mg of urea, and 74 mg of NH4F were dissolved in 50 mL of acid-treated MWCNTs aqueous solution (1 mg mL-1) in sequence, and stirred for 1 h. Then, the mixture was transferred to a 100 mL stainless-steel autoclave, which was then sealed and maintained at 120 °C for 12 h. After the reaction being completed, the precipitate was rinsed with ultrapure water several times and dried at 60 °C in vacuum oven overnight. Afterwards, the as-obtained Ni-Fe LDH/MWCNT precursor was added into 0.1 M of Na2S·9H2O aqueous solution (50 mL) in 100 mL Teflon container and stirred well. The autoclave was then sealed and maintained at 120 °C for 8 h, followed by being washed with ultrapure water several times and dried at 60 °C in vacuum oven. For comparison, the Ni-Fe-S powders were also prepared using the similar procedure without an addition of MWCNTs. The preparation processes of the Ni-S/MWCNT and Ni-Fe-S/MWCNT composites were clearly illustrated in Scheme 1.

Scheme 1 Schematic illustration for the preparation of the Ni-S/MWCNT and Ni-Fe-S/MWCNT composites and their application in an asymmetric electrochemical capacitor (AEC). 2.3. Materials characterization The crystalline phases of the as-prepared samples were analyzed by an X-ray 7/36

diffraction (XRD) using Rigaku D/max 2200 pc diffractometer (Cu Ka, λ = 1.5406 Å). The Raman spectra of the samples were collected using Renishaw inVia Raman Microscope with a He-Ne laser (λ = 633 nm). The Brunauer-Emmett-Teller (BET) surface area and pore size distribution were measured from a Micromeritics ASAP 2020 Hd88 analyzer. The elemental species and electronic states were obtained by X-ray photoelectron spectroscopy (XPS) on a VG Thermo ESCALAB 250 spectrometer. The surface morphologies and elemental compositions were investigated by field-emission scanning electron microscope (FE-SEM) on a Hitachi S-4800, equipped with energy-dispersive X-ray (EDS). The microstructures of the samples were observed by transmission electron microscopy (TEM) on FEI company Tecnai G220 S-twin at 200 kV. 2.4. Electrochemical measurements The electrochemical performances were carried out on a CHI 660E electrochemical workstation (Chenhua, Shanghai) at ambient temperature in 6.0 M KOH aqueous electrolyte in a three-electrode configuration for a single electrode and in a two-electrode system for the AEC. The working electrode was fabricated by mixing the as-prepared composite, acetylene black, and polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) in a mass ratio of 70:20:10. Then, the slurry was pressed onto nickel foams (coated area ≈ 1×1 cm2), and dried at 120 °C for 24 h under vacuum. Finally, the electrode plates were pressed under 20 MPa for one minute. In the three-electrode configuration, the platinum foil and Hg/HgO electrodes were used as the auxiliary and reference electrodes, respectively. For the battery-like electrodes, the specific capacity Cs (mAh g-1) values were calculated from galvanostatic discharge curves by the following equation [34]:

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 =

 · ∆ 3.6

where I is the discharge current (A), ∆t is the discharge time (s), and m is the mass of active material (g). The asymmetric electrochemical capacitor (AEC) was fabricated by using the Ni-S/MWCNT composite as positive electrode and the Ni-Fe-S/MWCNT composite as negative electrode. Prior to the assembly of the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC, the mass loading of the positive and negative electrodes was balanced on the basis of the following equation: 
 = 
 where m+ and m- are the masses of active materials (g), and Cs+ and Cs- are the specific capacities (mAh g-1) of the positive and negative electrodes, respectively. Consequently, the mass ratio of the Ni-Fe-S/MWCNT composite to the Ni-S/MWCNT composite was balanced to be about 2.3. The energy density (E, Wh Kg-1) and power density (P, W Kg-1) of the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC were calculated from the galvanostatic discharge curves based on the total mass of the active material on both electrodes by the following equations [35, 36]:

=

∫  · () 3.6

 = 3600

 ∆

Where I is the discharge current (A), V(t) is the discharge cell potential excluding the IR drop (V), dt is the time differential, M is the total mass of the active material on both electrodes (g), and ∆t is the discharge time (s). 9/36

3. Results and Discussion 3.1. Structural analysis of the Ni-S/MWCNT and Ni-Fe-S/MWCNT composites Fig. 1a shows the XRD patterns of the Ni-S, MWCNT and Ni-S/MWCNT composite. It can be seen that the diffraction peaks of the Ni-S/MWCNT composite are well-indexed to the hexagonal phase Ni3S2 (JCPDS no. 44-1418, space group: R32(155); a0 = b0 = 5.7454 Å, c0 = 7.1350 Å), hexagonal phase NiS (JCPDS no. 12-0041, space group: R3m(160); a0 = b0 = 9.62 Å, c0 = 3.149 Å), and cubic phase Ni3S4 (JCPDS no. 47-1739, space group: Fd-3m(227); a0 = b0 = c0 = 9.4761 Å). The results are similar to those of the Ni-S sample. In addition, the characteristic diffraction peak of MWCNTs that is assigned to the (002) plane can be obviously observed at 2θ = 26.1° in the Ni-S/MWCNT composite. These results indicate that the three phases of Ni3S2, NiS and Ni3S4 still can steadily coexist in the Ni-S/MWCNT composite after an introduction of MWCNTs. Furthermore, the structural feature of the Ni-S/MWCNT composite was also studied by Raman spectroscopy, as shown in Fig. 1b. The Raman spectrum of the Ni-S/MWCNT composite consists of the characteristic vibrational modes of the Ni-S and three prominent peaks corresponding to D, G and 2D bands of MWCNTs. The intensity ratio (ID/IG) of the D band to G band for the Ni-S/MWCNT composite (1.55) is higher than that of the pure MWCNTs (1.15), which is probably related to the defects and vacancies introduced by the acid treatment of the MWCNTs and the formation of the Ni-S nanoparticles. The results are well consistent with the above XRD results, further revealing the successful introduction of the MWCNTs into the Ni-S nanoparticles. The morphology of the 10/36

Ni-S/MWCNT composite is shown in Fig. 1c. It can be seen that the abundant Ni-S nanoparticles are well anchored on the dispersed MWCNTs surfaces. Obviously, the MWCNTs can prevent the Ni-S nanoparticles from aggregation and be conducive to the electron transfer rapidly. From the TEM image of the Ni-S/MWCNT composite shown in Fig. 1d, it can further be seen that the Ni-S nanoparticles are well distributed on the conductive substrate of the MWCNTs surfaces, and the sizes of the Ni-S nanoparticles are in a range of 50-115 nm. Fig. 1e further shows the HRTEM images of the Ni-S/MWCNT composite. The lattice fringes of d = 2.78 Å and 2.87 Å can be detected, which correspond to the (300) plane of NiS and (110) plane of Ni3S2, respectively, exhibiting a well-crystalline nature of the obtained Ni-S nanoparticles. The surface morphology and microstructure of the pure Ni-S powders were also characterized, as shown in Fig. S1 (Supplementary Information), indicating that the pure Ni-S sample is composed of abundant Ni-S nanoparticles and in good agreement with the above XRD results. As shown in Fig. 1f, the EDS mapping of the Ni-S/MWCNT composite and its corresponding color elemental mappings of Ni, S and C elements clearly demonstrate the homogeneous distribution of the Ni-S nanoparticles on the MWCNTs surfaces. Moreover, the chemical composition and electronic states of elements for the Ni-S/MWCNT composite were also examined by XPS analysis. As shown in Fig. 1g, the characteristic peak of Fe 2p can be distinctly observed for the Ni-Fe-S/MWCNT composite in comparison with the Ni-S/MWCNT composite. As shown in Fig. 1h, for the Ni-S/MWCNT composite, the Ni 2p3/2 and Ni 2p1/2 peaks can be fitted with two 11/36

spin-orbit doublet characteristics of Ni2+ and Ni3+, accompanied with two broad satellites, which indicates the presence of Ni2+ and Ni3+ states[18]. Fig. 1i shows the high-resolution S 2p spectrum, in which the S 2p3/2 and S 2p1/2 peaks accompanied with one broad satellite can be detected, indicating an existence of the S2- state and Ni-S bonds in the Ni-S/MWCNT composite [37].

Fig. 1 (a) XRD patterns and (b) Raman spectra of the Ni-S, MWCNT and Ni-S/MWCNT composite; (c) FE-SEM image, (d) TEM image, (e) HRTEM images 12/36

and (f) EDS mapping and its corresponding color elemental mappings of Ni, S and C elements for the Ni-S/MWCNT composite; (g) XPS survey spectra of the Ni-S/MWCNT and Ni-Fe-S/MWCNT composites; high-resolution XPS spectra of (h) Ni 2p and (i) S 2p for the Ni-S/MWCNT composite. The structure and composition of the Ni-Fe-S/MWCNT composite as negative electrode in the AEC were also characterized, as shown in Fig. 2. In addition to the characteristic diffraction peak corresponding to the (002) plane of MWCNTs, the other diffraction peaks can be assigned to the FeS2, FeS and Ni3S2 phases, respectively, suggesting that the multiphase coexists in the Ni-Fe-S/MWCNT composite, which is similar to the pure Ni-Fe-S sample (Fig. 2a). Raman spectroscopy can further provide the structural information of the Ni-Fe-S/MWCNT composite, as shown in Fig. 2b. The characteristic vibrational modes of the Ni-Fe-S species can clearly be observed in the Ni-Fe-S/MWCNT composite [21, 30], except for the characteristic D, G and 2D bands of the MWCNTs. Similarly, the intensity ratio (ID/IG) of D band to G band for the Ni-Fe-S/MWCNT composite (1.52) is also higher than that of pure MWCNTs (1.15), implying more defects due to the acid treatment of MWCNTs and the formation of Ni-Fe-S species in the Ni-Fe-S/MWCNT composite. The TEM image of the Ni-Fe-S/MWCNT composite (Fig. 2c) reveals the formation of the large Ni-Fe-S nanosheets that are tiled on or intertwined with MWCNTs. For comparison, the surface morphology of the pure Ni-Fe-S sample was further investigated, as shown in Fig. S2 (Supplementary Information), which contains numerous agglomerated nanoparticles and stacked nanosheets of Ni-Fe-S. The HRTEM images of the 13/36

Ni-Fe-S/MWCNT composite (Fig. 2d) clearly show the lattice fringes of d = 5.00 Å, 4.10 Å and 2.71 Å that correspond to the (001), (101) and (200) planes of the FeS, Ni3S2 and FeS2, respectively. The SAED pattern further confirms the polycrystalline feature [38] of the as-obtained Ni-Fe-S/MWCNT composite. EDS mapping was used to detect the composition distribution of specific elements such as Ni, Fe, S and C of the Ni-Fe-S/MWCNT composite, as seen in Fig. 2e. It is noted that the Ni-Fe-S species are well distributed on the MWCNTs surfaces, which are in a good agreement with the aforementioned XRD results. As shown in Fig. 2f, the Ni 2p spectrum can be well fitted into Ni2+ and Ni3+ accompanied with two shakeup satellites. Similarly, the Fe 2p spectrum in Fig. 2g can be fitted into Fe2+ and Fe3+ accompanied with two shakeup satellites. The S 2p spectrum in Fig. 2h can be fitted into S 2p3/2 that is attributed to the S2- in a low coordination at surface, and S 2p1/2 that is related to the metal-sulfur bonds accompanied with one strong shakeup satellite, which is assigned to the surface sulfur species at higher oxidation states [39]. These results demonstrate that the Ni2+, Ni3+, Fe2+, Fe3+ and S2- species appear near the surface of the Ni-Fe-S/MWCNT composite. Notably, the specific surface area and pore size distribution of electrode materials also play an important role in the electrochemical energy storage. Fig. 3 shows the N2 adsorption-desorption isotherms and their corresponding pore size distribution plots (the insets) of the Ni-S, Ni-S/MWCNT composite, Ni-Fe-S and Ni-Fe-S/MWCNT composite. The typical type IV hysteresis loops of isotherms prove their mesoporous features [40]. The BET specific surface areas, total pore volumes and average pore 14/36

diameters of these samples are contrastively listed in Table 1. Impressively, compared with pure Ni-S, the Ni-S/MWCNT composite shows a larger BET surface area and total pore volume, which can provide more active sites and facilitate the penetration of electrolyte ions into electrode, enabling the composite electrode a superior electrochemical

performance.

Although

the

BET

surface

area

of

the

Ni-Fe-S/MWCNT composite is slightly lower than that of the Ni-Fe-S sample, the total pore volume and average pore diameter of the Ni-Fe-S/MWCNT composite is nearly twice that of the Ni-Fe-S sample, which are beneficial for the fast diffusion of electrolyte ions, so as to improve its electrochemical performance.

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Fig. 2 (a) XRD patterns of the Ni-Fe-S and Ni-Fe-S/MWCNT composite; (b) Raman spectra of the Ni-Fe-S, MWCNT and Ni-Fe-S/MWCNT composite; (c) TEM images, (d) HRTEM images and SAED pattern, and (e) EDS mapping and its corresponding color elemental mappings of Ni, Fe, S and C elements for the Ni-Fe-S/MWCNT composite; high-resolution XPS spectra of (f) Ni 2p, (g) Fe 2p and (h) S 2p for the 16/36

Ni-Fe-S/MWCNT composite.

Fig. 3 N2 adsorption-desorption isotherms (the insets show the corresponding pore size distribution plots) of the Ni-S and Ni-S/MWCNT composite (a), the Ni-Fe-S and Ni-Fe-S/MWCNT composite (b). Table 1 The BET specific surface areas, total pore volumes and average pore diameters of the Ni-S, Ni-S/MWCNT composite, Ni-Fe-S and Ni-Fe-S/MWCNT composite. Sample

Specific surface

Total pore volume

Average pore

area (m2 g-1)

(m3 g-1)

diameter (nm)

Ni-S

25.2

0.11

17.39

Ni-S/MWCNT

38.3

0.22

22.55

Ni-Fe-S

11.9

0.06

21.65

Ni-Fe-S/MWCNT

10.4

0.12

46.92

3.2. Electrochemical performances of the Ni-S/MWCNT and Ni-Fe-S/MWCNT composites The electrochemical performances of the Ni-S/MWCNT and Ni-Fe-S/MWCNT composite electrodes were evaluated in a three-electrode configuration in 6.0 M KOH 17/36

aqueous solution. The CV curves of the Ni-S and Ni-S/MWCNT composite electrodes at 10 mV s-1 are shown in Fig. 4a. It can be seen that the Ni-S/MWCNT composite electrode shows a similar CV shape to that of the Ni-S electrode and exhibits an increased CV curve area and redox peak intensity, indicating an improved specific capacity and a faster redox reaction kinetics [41]. Fig. 4b shows the CV curves of the Ni-S/MWCNT composite electrode at different scan rates, in which the redox peaks can clearly be observed at various scan rates, implying that the capacity is mainly attributed to the electrochemical redox reactions of Ni2+/Ni3+ redox couples [18, 42]. The GCD curves of the Ni-S/MWCNT composite electrode at different current densities (Fig. 4c) reveal the almost symmetric potential-time curves, suggesting the good reversible redox reactions of the Ni-S/MWCNT composite electrode in charge-discharge processes [43]. The CV and GCD curves of the Ni-S electrode are also presented in Fig. S3 (Supplementary Information). As displayed in Fig. 4d, the specific capacity of the Ni-S/MWCNT composite electrode is calculated as high as 265.6 mAh g-1 at 1 A g-1, whereas only 170.5 mAh g-1 can be achieved at 1 A g-1 for the Ni-S electrode. Especially, the specific capacity of the Ni-S/MWCNT composite electrode still can maintain at 145.0 mAh g-1 with a high capacity retention rate of 54.6% when the current density increases to 30 A g-1. These results demonstrate that the introduction of MWCNTs into the Ni-S nanoparticles plays a significant role in the improvement of the capacity and redox reaction kinetics, thus delivering a superior rate capability. The specific capacitances/capacities of the Ni-S and Ni-S/MWCNT composite electrodes are also calculated in the form of F g-1 and C g-1, 18/36

respectively, as shown in Fig. S4 (Supplementary Information). EIS analysis can give evidence for the superior electrochemical performance of the Ni-S/MWCNT composite electrode. As shown in Fig. 4e, the Nyquist plots of both the Ni-S and Ni-S/MWCNT composite electrodes present a suppressed semicircle at high frequency range and a long sloped line at low frequency range. The intercept of the impedance arc with the real axis at high frequency range denotes the internal resistance, the diameter of the semicircle represents the interfacial charge transfer resistance, and the slope of the straight line in low frequency region is usually related to the diffusion resistance of electrolyte ions [40, 44]. The equivalent circuit diagram is simulated, as shown in the inset of Fig. 4e, where Rs represents the internal resistance, Rct represents the interfacial charge transfer resistance, CPE is the constant phase angle element, and W is the half-infinite diffusion impedance of the electrodes. As a result, the simulated results of the EIS data for the Ni-S and Ni-S/MWCNT composite electrodes are listed in Table S1 (Supplementary Information). It is clear that the Ni-S/MWCNT composite electrode exhibits the lower Rs and Rct than those of the Ni-S electrode, implying a lower bulk solution resistance and charge transfer resistance due to the introduction of MWCNTs, which thus gives rise to a favorable reaction kinetics and an excellent energy storage performance for the Ni-S/MWCNT composite. As presented in Fig. 4f, the battery-type Ni-S/MWCNT composite electrode shows a relatively good cycling performance.

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Fig. 4 (a) CV curves of the Ni-S and Ni-S/MWCNT composite electrodes at 10 mV s-1; (b) CV curves at different scan rates and (c) GCD curves at different current densities for the Ni-S/MWCNT composite electrode; (d) specific capacities of the Ni-S and Ni-S/MWCNT composite electrodes at different current densities; (e) Nyquist plots of the Ni-S and Ni-S/MWCNT composite electrodes (the inset shows the equivalent circuit diagram); (f) cycling stability and coulombic efficiency of the Ni-S/MWCNT composite electrode at the current density of 4 A g-1. Fig. 5a shows the CV curves of the Ni-Fe-S/MWCNT composite electrode at different scan rates with the potential window from -1.1 to 0 V. It can be seen that all CV curves of the Ni-Fe-S/MWCNT composite electrode exhibit a pair of irregular redox peaks due to the overlap of the Ni2+/Ni3+ and Fe2+/Fe3+ redox peaks, suggesting a fast redox reaction kinetics during charge-discharge process. As seen in Fig. 5b, the GCD curves of the Ni-Fe-S/MWCNT composite electrode at different current densities present the voltage plateaus at about -0.72 V and -0.93 V, which are in good accordance with the redox peaks detected in CV curves. For comparison, the CV and 20/36

GCD curves of the Ni-Fe-S electrode are also shown in Fig. S5 (Supplementary Information). The specific capacity of the Ni-Fe-S/MWCNT composite electrode (Fig. 5c) is calculated to be 90.1 mAh g-1 at 2 A g-1, which is much higher than that of the Ni-Fe-S electrode (47.1 mAh g-1 at 2 A g-1). Moreover, the specific capacity of 56.7 mAh g-1 can still be kept for the Ni-Fe-S/MWCNT composite electrode with a capacity retention of 62.9% when the current density increases to 20 A g-1, exhibiting a good rate capability due to the integration of MWCNTs. Similarly, the specific capacitances/capacities of the Ni-Fe-S and Ni-Fe-S/MWCNT composite electrodes are also calculated in the form of F g-1 and C g-1, respectively, as shown in Fig. S6 (Supplementary Information). Fig. 5d shows the Nyquist plots of the Ni-Fe-S and Ni-Fe-S/MWCNT composite electrodes with the equivalent circuit diagram inserted. From the simulated results of the EIS data in Table S1 (Supplementary Information), it can be seen that the Ni-Fe-S/MWCNT composite electrode possesses the smaller Rs and Rct compared to those of the Ni-Fe-S electrode, suggesting a smaller charge transfer resistance in the Ni-Fe-S/MWCNT composite electrode, which is mainly ascribed to the introduction of MWCNTs. The MWCNTs can act as the fast charge transfer channels to enhance the redox reaction kinetics of the Ni-Fe-S/MWCNT composite electrode, enabling a better electrochemical performance [45]. The Ni-Fe-S/MWCNT composite electrode also presents a relatively good cycling performance, as shown in Fig. 5e. Furthermore, the sweep voltammetry can provide a better understanding of the charge storage kinetics. In theory, the current (i) is dependent on the scan rate (v) 21/36

based on the equation of i = a vb, where b = 0.5 indicates that the current is controlled by semi-infinite diffusion, while b = 1 implies the capacitive behavior [6, 46]. The log(peak current) versus log(scan rate) plots of the cathodic current responses for the Ni-S/MWCNT and Ni-Fe-S/MWCNT composite electrodes are shown in Fig. 5f. It can be noted that the b value of the Ni-S/MWCNT composite electrode is about 0.5, indicating that the current is controlled by the near bulk diffusion of electrolyte ions and confirming its battery-type faradaic behavior [6, 19, 33]. While the Ni-Fe-S/MWCNT composite electrode exhibits the b value of approximately 0.7, revealing that the charge storage involves the capacitor-like and diffusion-controlled behaviors [6, 46, 47].

Fig. 5 (a) CV curves at different scan rates and (b) GCD curves at different current densities for the Ni-Fe-S/MWCNT composite electrode; (c) specific capacities of the Ni-Fe-S and Ni-Fe-S/MWCNT composite electrodes at different current densities; (d) Nyquist plots of the Ni-Fe-S and Ni-Fe-S/MWCNT composite electrodes (the inset shows the equivalent circuit diagram); (e) cycling stability and coulombic efficiency 22/36

of the Ni-Fe-S/MWCNT composite electrode at the current density of 4 A g-1; (f) the log(peak current) versus log(scan rate) plots of the cathodic current responses for the Ni-S/MWCNT and Ni-Fe-S/MWCNT composite electrodes. 3.3. Electrochemical performance of the Ni-S/MWCNT//Ni-Fe-S/MWCNT asymmetric electrochemical capacitor To evaluate the practical energy storage application of the as-prepared electrode materials, an asymmetric electrochemical capacitor (AEC) was fabricated by using the Ni-S/MWCNT composite as positive electrode and the Ni-Fe-S/MWCNT composite as negative electrode, as illustrated in Scheme 1. It can be seen from Fig. 6a that the Ni-Fe-S/MWCNT composite electrode exhibits the potential window from -1.1 to 0 V, and the Ni-S/MWCNT composite electrode possesses the positive window from 0 to 0.6 V. In addition, the CV curves of both electrodes present the superior faradaic redox reaction kinetics. To achieve high energy and power densities, the cell potential window of the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC should be optimized by CV analysis with different cell potential windows at 30 mV s-1, as shown in Fig. 6b. An incomplete faradaic redox reaction takes place when the cell potential increases from 1.2 to 1.5 V, whereas starting from 1.6 V, the CV curves exhibit the typical faradaic redox peaks. Especially, when the cell potential window reaches at 1.7 V, a sufficient redox reaction is ensured, suggesting a faster redox reaction kinetics in the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC. Nevertheless, an oxygen evolution reaction occurs when the cell potential window exceeds 1.7 V. Fig. 6c shows the CV curves of the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC at different scan rates, which clearly 23/36

presents

a

typical

faradaic

redox

behavior.

The

GCD

curves

of

the

Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC at different current densities are displayed in Fig. 6d, which exhibits the voltage plateaus for all GCD curves and is fully consistent with the CV results. The specific capacities of the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC at various current densities were calculated based on the total mass of the active material on both electrodes, as shown in Fig. 6e. The specific capacity of 46.0 mAh g-1 is obtained at 4 A g-1, and 24.2 mAh g-1 can be retained at 15 A g-1 with a high capacity retention of 52.6%. These results indicate that the fabricated Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC has a high capacity and good rate performance. In addition, the specific capacitances of the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC are also calculated in the form of F g-1, as shown in Fig. S7 (Supplementary Information). Fig. 6f shows the Ragone plots related to the energy and power densities of the as-fabricated Ni-S/MWCNT//Ni-Fe-S/MWCNT, Ni-S/MWCNT//AC and other recently reported AECs. It can be seen that an energy density of 42.2 Wh kg-1 can be obtained at a high power density of 3.7 kW kg-1 for the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC. Furthermore, an energy density of 19.1 Wh kg-1 can still be maintained at an ultrahigh power

density

of

11.9

Ni-S/MWCNT//Ni-Fe-S/MWCNT

kg-1.

kW AEC

is

The much

performance better

than

that

of of

the the

Ni-S/MWCNT//AC AEC (only 20.9 Wh kg-1 at 0.7 kW kg-1), and comparable to or even higher than those reported AECs related to the transition metal sulfides-based materials [18, 19, 33, 37, 48], as presented in Table S2 (Supplementary Information) 24/36

in detail. The cycling performance of the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC was also evaluated by GCD cycles at the current density of 4 A g-1, as shown in Fig. 6g. It can be seen that the capacity shows a slight increment before the early 100 cycles, which is mainly attributed to the activation of the electrodes due to the gradually improved permeation of electrolyte ions into the interior of the electrode materials [49, 50]. 72.1% of the initial specific capacity can be maintained after 3000 cycles, demonstrating a relatively good electrochemical cycling stability. It should be mentioned here that for the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC, in consideration of the typical faradaic redox behaviors and charge storage mechanisms of the cathode and anode, there inevitably exists the structural pulverization caused by the huge volume expansion from the faradaic redox reactions during charge-discharge processes [51, 52]. Furthermore, the comparison of the GCD curves before and after cycling indicates that the Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC still holds a relatively good charge-discharge performance after 3000 cycles.

25/36

Fig. 6 (a) CV curves of the Ni-Fe-S/MWCNT and Ni-S/MWCNT composite electrodes performed in a three-electrode system at 10 mV s-1; (b) CV curves of the 26/36

Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC operated with different cell potential windows at 30 mV s-1; (c) CV curves at different scan rates, (d) GCD curves at different current densities, and (e) specific capacities at various current densities for the

Ni-S/MWCNT//Ni-Fe-S/MWCNT

AEC;

(f)

Ragone

plots

of

the

Ni-S/MWCNT//Ni-Fe-S/MWCNT and Ni-S/MWCNT//AC AECs compared with other

recently

reported

AECs;

(g)

cycling

performance

of

the

Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC at the current density of 4 A g-1 (the inset shows the GCD curves before and after cycling).

4. Conclusion In summary, the Ni-S/MWCNT composite has been successfully prepared by a facile one-step hydrothermal method. The Ni-S/MWCNT composite as positive electrode exhibits a high specific capacity of 265.6 mAh g-1 at 1 A g-1 and an improved rate capability (capacity retention of 54.6% at 30 A g-1), which is ascribed to the high BET surface area and the integration of the conductive channels of MWCNTs in comparison with the Ni-S electrode. Furthermore, the Ni-Fe-S/MWCNT composite as negative electrode has also been prepared by combining a hydrothermal process with an anion exchange method. Results indicate that the Ni-Fe-S/MWCNT composite shows a larger total pore volume and the abundant redox reactions, thus, leading to a superior electrochemical performance than that of Ni-Fe-S electrode. Finally, the fabricated Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC demonstrates a high cell potential window of 1.7 V, a high energy density of 42.2 Wh kg-1 at a high power 27/36

density of 3.7 kW kg-1, and a relatively good electrochemical cycling stability.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was supported by the National Natural Science Foundation of China (61774122), the Science and Technology Developing Project of Shaanxi Province (2015KW-001) and the 111 Project of China (B14040). The SEM and TEM work was conducted at International Center for Dielectric Research, Xi’an Jiaotong University. Thanks Dr. Jiamei Liu at Instrument Analysis Center of Xi’an Jiaotong University for her assistance with X-ray photoelectron spectrometer (XPS, ESCALAB Xi+, Thermo Fisher Scientific, USA) analysis.

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Highlights


Ni-S/MWCNT and Ni-Fe-S/MWCNT composites have been prepared by hydrothermal method.




Ni-S/MWCNT electrode shows a high specific capacity of 265.6 mAh g-1 at 1 A g-1.




An integration of MWCNTs can accelerate charge transfer of composite electrodes.




Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC delivers a high cell potential of 1.7 V.




Ni-S/MWCNT//Ni-Fe-S/MWCNT AEC has a high energy density of 42.2 Wh kg-1 at 3.7 kW kg-1.