English

• 0.   Introduction

  Supercapacitors are electrochemical energy storage devices characterized by high power density and excellent cycling stability for various applications^[1-6]. However, their widespread commercial application is hindered by the low energy density. Consequently, hybrid supercapacitors (HSCs) have garnered significant attention due to their ability to combine the advantages of battery-type positive electrodes with double-layer capacitor negative electrodes, resulting in significantly higher energy densities while maintaining a high-rate capability^[7-10]. The rate performance of HSCs primarily relies on the battery-type electrode, leading to extensive investigations into metal sulfides, oxides, and hydroxides as potential candidates. Among these materials, sulfides exhibit exceptional conductivity, multiple oxidation states, and larger specific capacitance, making them highly promising for assembling high-energy HSCs^[11-14].

  The utilization rate of active materials is one of the important factors affecting the theoretical specific capacity. Previous research shows that the amorphous structure has many unsaturated dangling bonds, which is beneficial to the rapid transport of electrolytes in active materials^[15]. However, pure amorphous materials tend to exhibit lower electrochemical activity than crystalline materials due to low electrical conductivity. Inspired by this, crystalline sulfide can be combined with the amorphous state to build a unique crystalline-amorphous structure, combining the advantages of the high specific capacity and high conductivity of the crystalline phase and the disordered and stable structure of the amorphous phase to achieve excellent electrochemical performance^[16-19]. For instance, Wang et al. fabricated amorphous-crystalline MoO₃-Ni₃S₂ nanosheet arrays on Ni foam (NF) as battery-type electrodes and highlighted that the disordered structure is likely to alleviate the structural stress caused by repetitive ion deintercalation/intercalation behavior. The optimized amorphous-crystalline structure enhanced conductivity, leading to significant improvement in both structural stability and specific energy^[20]. Unfortunately, crystalline sulfides in the core have low active site utilization. Thus, creating defects and vacancies in the crystalline material can expose the active sites to further increase capacity. Specifically, the intentional creation of S vacancies in the transition metal sulfide lattice has been proven to be an effective method to improve the reactivity of electrochemical active sites and change the electrical properties. Zong et al. reported that the introduction of S vacancies into Bi₂S₃ resulted in excellent electrochemical performance, remarkable specific capacitance of 466 F·g^-1 at a discharge current density of 1 A·g^-1, and can achieve a high energy density of 22.2 Wh·kg^-1 at a power density of 677.3 W·kg^{-1 [21]}. Nonetheless, few studies have investigated the synthesis of crystalline-amorphous heterostructures accompanied by defect regulation^[22-24].

  Herein, we report a novel amorphous MoS₂ nanosheet grown on the surface of the NiCo₂S₄ array v-NiCo₂S₄@MoS₂ (v-NCS@MS) via an ethylene glycol (EG) induction strategy. The crystalline phase of NiCo₂S₄ must be rich in sulfur vacancies by changing the viscosity of the solvent, which effectively enhances the cycle life and specific capacity. Taking advantage of this unique structure, the v-NCS@MS electrode exhibited a high specific capacity of 1 034 mAh·g^-1 at 1 A·g^-1 in 2 mol·L^-1 KOH aqueous solution. Furthermore, by assembling the battery-type cathode with activated carbon anode, an HSC with a high specific energy of up to 111 Wh·kg^-1 at a specific power of 219 W·kg^-1 was demonstrated. It demonstrated long cycling stability with a remarkable capacity retention of 91.5% up to 10 000 cycles. This work provides creative insights for the development of hybrid energy storage devices with high specific energy and long cycle life.

  1.   Experimental

  1.1   Materials

  All the reagents were purchased from Aladdin Chemical Reagent Co., Ltd. All chemicals were analytical grade and used as obtained without any purification. NF (60 mg·cm^-2, 0.4 mm in thickness) was purchased from Changsha Liyuan New Materials Co., Ltd.

  1.2   Material synthesis

  The Ni-Co precursor was grown on NF by a facile hydrothermal process. After 0.4 mmol Ni(NO₃)₂·6H₂O, 0.8 mmol Co(NO₃)₂·6H₂O, and 4.8 mmol Co(NH₂)₂ were added to 40 mL of deionized water, NF (1 cm×1 cm) was placed in the solution and the mixture was heated at 120 ℃ for 8 h. The obtained sample was etched with 50 mL 0.6 mol·L^-1 Na₂S solution at 160 ℃ for 8 h to obtain NiCo₂S₄ nanowires on NF. For the synthesis of v-NCS@MS, the NiCo₂S₄ sample was immersed in 30 mL EG with 2 g·L^-1 NaMoO₄ and 4 g·L^-1 CH₄N₂S, and the mixture was heated at 170 ℃ for 6 h. The as-obtained sample was ultrasonic cleaned and dried at 60 ℃ for 12 h. The samples with different reaction times (2, 4, and 8 h) were prepared and denoted as v-NCS@MS-2h, v-NCS@MS-4h, and v-NCS@MS-8h, respectively. Similarly, the sample was synthesized by replacing the solvent EG with DI water under the same conditions (the reaction time was 6 h) and the product was denoted as NCS@MS. Moreover, v-NCS@MS-1 and v-NCS@MS-2 were also synthesized by halving and doubling the mass of NaMoO₄ and CH₄N₂S simultaneously. The average loading of all the above samples on NF was maintained at 1.5 mg·cm^-2. For comparison, the NiCo₂S₄ and MoS₂ samples were also prepared directly on NF under the same conditions.

  1.3   Material characterization

  The crystalline information of the as-prepared sample was established by powder X-ray diffraction (XRD, D/max TTR-Ⅲ, Cu Kα, maximum tube voltage: 40 kV, maximum tube current: 40 mA, 2θ=5°-168°, wavelength: 0.154 18 nm). The structural investigation of all samples was determined by a scanning electron microscope (SEM, SUPRA 40, Zeiss, German) and transmission electron microscopy (TEM, FEI, Tecnai G² F20, 200 kV, accelerating voltage: 20-200 kV). Elemental mapping of the samples was measured on an FEI Tecnai F20 under the high-angle annular dark-field-scanning TEM mode (HAADF-STEM). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific Esxalab 250Xi.

  1.4   Electrochemical measurements

  All electrochemical measurements were conducted with a CHI660E electrochemical workstation (Chenhua, Shanghai) in 2 mol·L^-1 KOH aqueous electrolyte. The platinum foil and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. And the as-prepared samples were served as the working electrode (1 cm×1 cm). The measurement method was performed for cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The specific capacity can be calculated by Eq.1:

  $ C_{\mathrm{s}}=\frac{2 i \int V \mathrm{d} t}{3.6 m V} $

  (1)

  where C_s (mAh·g^-1) is the specific capacity, i (A) is current during the discharge process, $ \int V \mathrm{d} t$ (V is a potential interval) is the area under the discharge plot, and m (g) is the quality of active substances.

  To achieve the superior electrochemical properties of the device, the load mass ratio between the positive and negative active materials was calculated by Eq.2:

  $ \frac{m_{+}}{m_{-}}=\frac{C_{\mathrm{s}-} \Delta V_{+}}{C_{\mathrm{s}+} \Delta V_{-}} $

  (2)

  where m₊ and m_- are the masses of the anodic and cathode electrodes; C_s+ and C_s- are the specific capacities of the anodic and cathode electrodes; ΔV₊ and ΔV_- are the potential windows of the anode and cathode electrodes. The quality loadings of active material in activated carbon (AC) and v-NCS@MS electrodes were 7.5 and 1.5 mg·cm^-2, respectively.

  Moreover, the corresponding specific energy (E) and specific power (P) were derived from the Eq.3 and 4:

  $ E=\frac{i \int V \mathrm{d} t}{3.6 m} $

  (3)

  $ P=\frac{3\;600 E}{\Delta t} $

  (4)

  where Δt is the discharge time (s) and m (g) is the total quality of active materials on both electrodes.

  2.   Results and discussion

  Fig. 1 schematically illustrates the preparation process of v-NCS@MS. Firstly, crystalline NiCo₂S₄ nanowire arrays were directly grown on NF via a one-step hydrothermal method. Then, MoS₂ nanostructures with 2D nanosheets were grown on the pre-synthesized NiCo₂S₄ with a reaction time of 6 h in EG to obtain the heterogeneous electrode. It is interesting to note that the defective NiCo₂S₄ was developed during the solvothermal process of epitaxial growth of MoS₂. v-NCS@MS-2h, v-NCS@MS-4h, v-NCS@MS-8h, v-NCS@MS-1, v-NCS@MS-2, and NiCo₂S₄ were obtained by a two-step hydrothermal synthesis and MoS₂ was grown directly on NF, the SEM images of the above samples are shown in Fig.S1 and S2 (Supporting information).

  Figure 1

  

  Figure 1.  Schematic illustration of the formation of the v-NCS@MS heterostructure on NF

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  In Fig.S1a, it can be observed that NiCo₂S₄ was composed of uniformly distributed nanowires. By further adjusting the precursor ratio of the heterogeneous samples, different morphologies can be obtained. When the concentration was low (1 g·L^-1 NaMoO₄ and 2 g·L^-1 CH₄N₂S), the coating shell was not uniform, and when the concentration was too high (4 g·L^-1 NaMoO₄ and 8 g·L^-1 CH₄N₂S) (Fig.S1b, S1c), the thicker MoS₂ formed a dense structure, which is not conducive to electrolyte penetration indicating that the morphology is greatly influenced by the reactant mass and the optimal amount of precursor was 2 g·L^-1 NaMoO₄ and 4 g·L^-1 CH₄N₂S. For comparison, by replacing EG solvents with DI water, the obtained NCS@MS almost presented an irregular and collapsed structure (Fig.S1d). Different solvents result in different morphologies. According to previous reports, EG can not only serve as a reducing agent but also can be utilized as a structure-directing agent to control the nucleation and crystallization of materials^[25], therefore, three-dimensional (3D) v-NCS@MS hybrid microstructure was obtained in EG. Moreover, different reaction time was explored for the synthesis of MoS₂ in EG using various solvents. When the reaction time was insufficient (2-4 h), it failed to form a complete and continuous core-shell heterostructure. However, when the reaction time was extended to 8 h, a thick MoS₂ layer covered all the NiCo₂S₄ nanowires, so the optimal reaction time was determined to be 6 h (Fig.S2a-S2c). In addition, the SEM image of MoS₂ showed that compared with v-NCS@MS, MoS₂ grown directly on NF presented serious accumulation, greatly reducing the specific surface area of the reaction, which further proves the superiority of the heterogeneous structure (Fig.S2d). Thus, under suitable reaction conditions, highly ordered and densely packed nanowire arrays with a size of 500 nm can be formed (Fig. 2a, 2b). The detailed structural information of the as-prepared v-NCS@MS was further obtained by the TEM and HAADF-STEM images. As shown in Fig. 2c, the MoS₂ nanosheet was tightly stacked on the outer layer of the nanowires with an average width of 100-200 nm. The edge region was selected to observe the HRTEM images, well-defined crystalline-amorphous boundaries can be seen (Fig. 2d). In the left region, the ultra-thin nanosheets were almost amorphous (Fig. 2e), and the inner region was NiCo₂S₄ suggested by the lattice fringe of 0.283 and 0.235 nm corresponding to the (400) and (311) crystal planes, respectively, which is consistent with the XRD results (Fig. 3a). Importantly, partial defects and dislocations can be observed on the crystal plane of NiCo₂S₄ (Fig. 2f), which may be due to the effect of solvation of EG on the crystallinity of the material and the creation of more active sites for reoxidation. An energy dispersive X-ray spectroscopy (EDS) mapping of v-NCS@MS showed that Mo (purple) and S (green) were evenly distributed along the needle structure, while Ni (red) and Co (yellow) were mainly located in the center of v-NCS@MS (Fig. 2g). Moreover, Fig.S3 shows the TEM images of v-NCS@MS and the core-shell structure is further proved.

  Figure 2

  

  Figure 2.  (a, b) SEM images, (c-f) TEM images, and (g) elemental mapping images of Ni, Co, S, and Mo elements of v-NCS@MS

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  Figure 3

  

  Figure 3.  (a) XRD patterns and (b) survey, (c) Ni2p, (d) Co2p, (e) S2p, and (f) Mo3d XPS spectra of v-NCS@MS and NCS@MS

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  Fig. 3a shows the XRD patterns of the as-prepared samples (v-NCS@MS, NCS@MS) grown on NF. The diffraction peaks at ca. 44.5°, 51.8°, 76.4°belong to the NF substrate, and the other peaks can be assigned to the cubic NiCo₂S₄ phase (PDF No.20-0782)^[26-27]. However, no diffraction peaks of MoS₂ can be observed, confirming its amorphous nature. In addition, the XRD patterns of NiCo₂S₄ and MoS₂ are shown in Fig.S4a, S4b.

  XPS measurements were carried out to further investigate the surface chemical composition of v-NCS@MS. The survey spectra (Fig. 3b) confirmed the presence of Ni, Co, Mo, and S elements in v-NCS@MS and NCS@MS. Fig. 3c and 3d show the typical Ni2p and Co2p XPS spectra of two samples. The fitted Ni2p peaks centered at 852.2 and 855.6 eV belong to Ni2p_3/2, and peaks at 869.3 and 873.1 eV belong to Ni2p_1/2. Similarly, the Co2p_3/2 and Co2p_1/2 peaks are assigned to the binding energies of 777.9, 782.9 eV, and 794.7, 799.1 eV, respectively, suggesting the existence of Co²⁺ and Co³⁺. Moreover, changes in the concentration of sulfur vacancy can also be identified by changes in the chemical valence states of Ni and Co in the sample^[28]. It is worth noting that the peak area of low valency Co²⁺ increased significantly, indicating that part of Co³⁺ was reduced to Co²⁺. Fig.S4c and S4d show the XPS spectra of NiCo₂S₄. In addition, the partial reduction of Co³⁺ indicates the formation of sulfur vacancies^[29]. Moreover, the shift of the peak indicates a change in the binding energy, which is considered to be the gain/loss of electrons. When sulfur vacancy is introduced, the Co³⁺ and Ni³⁺ peaks show a certain negative displacement. This peak shift is more pronounced in comparison to NiCo₂S₄ (Fig.S5), further confirming the fact that vacancies are generated in NiCo₂S₄ rather than MoS₂. This sulfur vacancy in NiCo₂S₄ is beneficial to the activation of electrochemically active sites in the core layer. The XPS spectra of S2p are shown in Fig. 3e, the lower binding energy peaks belong to the S2p_3/2 core level attributed to typical metal-sulfur bonds, meanwhile, the higher binding energy peaks belong to S2p_1/2 are assigned to sulfur with low coordination, which is generally related to sulfur vacancies^[30]. The relative area of S2p_1/2 in the S2p spectra of v-NCS@MS, NCS@MS, and NiCo₂S₄ was about 49.5%, 33.3%, and 49.3%, respectively, indicating a high concentration of sulfur vacancy in v-NCS@MS. The spectra of Mo2p are shown in Fig. 3f, the peaks located at 231.7 and 234.8 eV are related to Mo⁴⁺2p_5/2 and Mo⁴⁺2p_3/2, and the peak at 226.1 eV is regarded as S2s, which further proves that the amorphous MoS₂ was synthesized.

  The electrochemical performance of the as-prepared v-NCS@MS, NCS@MS, NiCo₂S₄, and MoS₂ electrodes was measured in a three-electrode system as the working electrode with 2 mol·L^-1 KOH solution as the electrolyte. Fig. 4a shows the CV curves of v-NCS@MS, NCS@MS, NiCo₂S₄, and MoS₂ electrodes at a scanning rate of 3 mV·s^-1, it is obvious that v-NCS@MS had the maximum current density and integrated area, suggesting that v-NCS@MS had the highest specific capacity. Among them, the GCD tests at a potential window of 0-0.5 V of the four electrodes at a current density of 1 A·g^-1 are shown in Fig. 4b, the specific discharge capacities of those samples were 1 034, 707, 517, and 255 mAh·g^-1, respectively. The redox peak comes from the joint contribution of NiCo₂S₄ and MoS₂. The CV curves showed no significant changes under the different scanning rates (Fig. 4c), indicating that the v-NCS@MS electrode facilitates rapid redox reactions. It is clear that v-NCS@MS had the highest capacity among others, it is further evidence of the synergistic effect of crystalline and amorphous heterostructures, which is consistent with the results of CV curves. The GCD curves of v-NCS@MS at different current densities from 1 to 10 A·g^-1 are shown in Fig. 4d. The voltage plateaus appeared in the charge-discharge curves, confirming the battery-type behavior of v-NCS@MS electrode. The v-NCS@MS electrode also had a better specific capacity retention of 49% when the current density increased by 10 times (Fig. 4e), further confirming the better electrochemical performance. In addition, the CV and GCD curves of the reference electrodes were also analyzed at the same scan rates and specific currents and the specific capacity retentions of NCS@MS, MoS₂, and NiCo₂S₄ were 32.2%, 39.1%, and 31.6%, respectively (Fig.S6-S8).

  Figure 4

  

  Figure 4.  (a) CV at 3 mV·s^-1 and (b) GCD curves at 1 A·g^-1 of v-NCS@MS, NCS@MS, NiCo₂S₄, and MoS₂ electrodes; (c) CV curves of the v-NCS@MS electrode at different scan rates; (d) GCD curves at different current densities; (e) Specific capacities at different specific currents; (f) EIS

  Inset: (f1) EIS in the high-frequency region and (f2) the equivalent circuit.

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  The electrical conductivity and ion transport ability of v-NCS@MS were evaluated by EIS, and the related Nyquist plots and the fitted equivalent circuit are shown in Fig. 4f. The equivalent circuit (Fig. 4f, Inset) can be marked according to the EIS data, which shows the equivalent internal resistance (R_s), the charge-transfer resistance (R_ct) at the electrode-electrolyte interface, the Warburg impedance (Z_W), the double-layer capacitance (C_dl), and the Faraday pseudocapacitance (C_F). The contact resistance is represented by the real intercept (R_s) in the high-frequency region. v-NCS@MS had a low R_s value of 1.26 Ω (supported by analog equivalent circuit results, Fig.S9 and Table S1), indicating enhanced conductivity and charge transfer kinetics. Among them, v-NCS@MS had the highest linear slope in the low-frequency region, suggesting the electrolyte ions migrate more easily on the amorphous-crystal structure.

  To further distinguish the charge storage mechanism of the v-NCS@MS electrode, the capacitive charge storage capacity and redox reaction contributed capacity were analyzed by the CV curves under different scan rates (Fig. 4c). The relationship between the measured current (i) and sweep rate (v) can be described through the following equation^[31-32]:

  $ i=a v^b $

  (5)

  where i is current and v is sweep rate, a and b are variable elements, respectively.

  Fig. 5a shows that the b-values of v-NCS@MS for anodic and cathode peaks in the KOH electrolyte were 0.59 and 0.56, respectively, which means a diffusion-controlled electrochemical behavior. The proportion of capacitive contribution at different scan rates can be quantified by the following equation^[33-34]:

  $ i_{\mathrm{p}}=k_1 v+k_2 v^{1 / 2} $

  (6)

  Figure 5

  

  Figure 5.  (a) lg i vs lg v plot and b-values (Inset) of v-NCS@MS for anodic peaks and cathode peaks in the KOH electrolyte; (b) Area comparison diagram of capacitance and diffusion-controlled current of the v-NCS@MS electrode at the scan rate of 5 mV·s^-1; (c) Capacitance and diffusion-controlled contribution from 0.1 to 18.0 mV·s^-1

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  where i_p (A) represents the peak current at the fixed voltage; k₁ and k₂ are suitable constants. The slope k₁ is obtained by the relationship between i_p and v^1/2, which can calculate the capacitive contribution at various voltages of 0-0.9 V and different scan rates ranging from 0.1 to 18.0 mV·s^-1. As shown in Fig. 5b, the contribution of capacitive behavior of the v-NCS@MS electrode was 37.38% at 1 mV·s^-1. It can be observed in Fig. 5c that the proportion of capacitive contribution increased with the increase in scanning rates, indicating that the capacitive process accounts for the main contribution in the response at a high scanning rate. This may be due to the reduction of electron transport distance and ion diffusion^[35].

  A hybrid supercapacitor (v-NCS@MS||AC) was assembled with 2 mol·L^-1 KOH as the electrolyte, the v-NCS@MS electrode as the positive electrode, and AC as the negative electrode, as shown in Fig. 6a. The electrochemical performance of the AC negative electrode was also studied in a three-electrode system (Fig.S10). At a scan rate of 3 mV·s^-1, the CV curves of the v-NCS@MS and AC electrodes exhibited a maximum potential window of 0 to 0.9 V and -1 to 0 V, respectively. Therefore, the expected total working voltage window of v-NCS@MS||AC can reach 1.9 V (Fig. 6b). Fig. 6c shows the GCD curves under different potential windows. When the potential window was extended from 1.3 to 1.6 V under 8 mA·cm^-2, the shape of the GCD curves remained a nearly symmetric shape. Additionally, when the potential window was increased from 1.3 to 1.6 V at a scanning rate of 8 mV·s^-1, the CV curves also maintained the same without any significant polarization phenomena (Fig. 6d). These observations indicate that a potential window of 0-1.6 V can be used for the v-NCS@MS||AC HSC devices. The CV curve of the v-NCS@MS device at different scan rates in Fig. 6e exhibited both double-layer and capacitive redox characteristics. The shape of the CV curve remained even under the high scanning rate, indicating excellent rate performance^[36-38]. The symmetric GCD curves at different current densities in Fig. 6f showed that HSC had significant Coulombic efficiency (98%-100%).

  Figure 6

  

  Figure 6.  (a) Schematic illustration of the v-NCS@MS||AC HSC device; (b) CV curves of v-NCS@MS and AC electrodes at 3 mV·s^-1; (c) GCD curves at various voltage windows, (d) CV curves at different operating voltages, (e) CV curves at different scan rates, and (f) GCD curves at different specific currents of HSC device

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  To further evaluate the performance of v-NCS@MS||AC, the specific energy and power of v-NCS@MS||AC based on the mass of active materials were calculated. As shown in Fig. 7a, the HSC device exhibited a high specific energy density of 110.9 Wh·kg^-1 at a specific power of 218.9 W·kg^-1, which is superior to the previously reported results, such as Co₉S₈@MoS₂||AC^[39], NiMoO₄@MoS₂||RPHPC-750^[40], NiCo₂S₄-C-MoS₂||AC^[41], Mn_xCo_yO₄@MoS₂||AC^[42], and FeCo₂O₄@MoS₂||AC^[43]. Furthermore, the cyclic stability of HSCs was tested under higher current densities, as shown in Fig. 7b. After 5 000 charge-discharge cycles at a specific current of 20 mA·cm^-2, the specific capacity increased by nearly two times. This may be attributed to the pore expansion of the material and the full utilization of the core-shell material. Even at a higher specific current of 40 mA·cm^-2, the capacity retention rate was 91.5%, and it remained at 80.5% even when the specific current increased to 60 mA·cm^-2 for 15 000 cycles. In addition, two connected coin cells could light up a red-light emitting diode (LED) bulb (Inset in Fig. 7b), demonstrating their potential for practical applications. The high specific energy density and long cycle life should be attributed to the following factors^[44-45]: (1) The solvent plays a dual role in the introduction of an amorphous shell and defective nucleus; (2) The synergistic effect between amorphous molybdenum sulfide and crystalline NiCo₂S₄ greatly promotes the complete contact between the electrode and electrolyte, resulting in more abundant redox reactions; (3) Defect-rich NiCo₂S₄ nanowires provide more active sites for redox reactions, promoting the rapid adsorption of electrolytes and efficient ion transfer during the charge-discharge process.

  Figure 7

  

  Figure 7.  (a) Ragone plot of energy and specific power of the HSC device; (b) Cycling performance of the HSC device at 20, 40, and 60 mA·cm^-2

  b1, b3: cycling curves of the last five cycles at 20 and 60 mA·cm^-2, b2: a red LED bulb lighted up by two connected coin cells.

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  3.   Conclusions

  In summary, v-NCS@MS electrode materials have extremely high specific energy and long cycle life, which are mainly attributed to the defect regulation of crystalline materials, and abundant surface-active sites. The SEM, XRD, and XPS were employed for the analysis of core-shell heterostructures, crystallographic patterns, and defects. The unique structure of the electrode materials resulted in exceptional specific capacity (1 034 mAh·g^-1 at 1 A·g^-1). When fabricated into HSCs using v-NCS@MS as a positive electrode and AC as a negative electrode, the device exhibited high specific energy of 111 Wh·kg^-1 at a specific power of 219 W·kg^-1 and excellent capacity retention (80.5% after 15 000 cycling at different current density). This work provides a creative insight into hybrid energy storage devices with long cycle life.

  Supporting information is available at http://www.wjhxxb.cn

  Declaration of competing interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  
  
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  Figure 1  Schematic illustration of the formation of the v-NCS@MS heterostructure on NF

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  Figure 2  (a, b) SEM images, (c-f) TEM images, and (g) elemental mapping images of Ni, Co, S, and Mo elements of v-NCS@MS

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  Figure 3  (a) XRD patterns and (b) survey, (c) Ni2p, (d) Co2p, (e) S2p, and (f) Mo3d XPS spectra of v-NCS@MS and NCS@MS

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  Figure 4  (a) CV at 3 mV·s^-1 and (b) GCD curves at 1 A·g^-1 of v-NCS@MS, NCS@MS, NiCo₂S₄, and MoS₂ electrodes; (c) CV curves of the v-NCS@MS electrode at different scan rates; (d) GCD curves at different current densities; (e) Specific capacities at different specific currents; (f) EIS

  Inset: (f1) EIS in the high-frequency region and (f2) the equivalent circuit.

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  Figure 5  (a) lg i vs lg v plot and b-values (Inset) of v-NCS@MS for anodic peaks and cathode peaks in the KOH electrolyte; (b) Area comparison diagram of capacitance and diffusion-controlled current of the v-NCS@MS electrode at the scan rate of 5 mV·s^-1; (c) Capacitance and diffusion-controlled contribution from 0.1 to 18.0 mV·s^-1

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  Figure 6  (a) Schematic illustration of the v-NCS@MS||AC HSC device; (b) CV curves of v-NCS@MS and AC electrodes at 3 mV·s^-1; (c) GCD curves at various voltage windows, (d) CV curves at different operating voltages, (e) CV curves at different scan rates, and (f) GCD curves at different specific currents of HSC device

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  Figure 7  (a) Ragone plot of energy and specific power of the HSC device; (b) Cycling performance of the HSC device at 20, 40, and 60 mA·cm^-2

  b1, b3: cycling curves of the last five cycles at 20 and 60 mA·cm^-2, b2: a red LED bulb lighted up by two connected coin cells.

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