Hierarchical WS2@NiCo2O4 Core-shell Heterostructure Arrays Supported on Carbon Cloth as High-Performance Electrodes for Symmetric Flexible Supercapacitors.

Nowadays, rationally preparing heterostructure materials can not only make up for the shortage of individual components, but also exert unexpected performance through synergistic interactions between the components. Herein, a core-shell of WS2@NiCo2O4 screw-like heterostructure arrays grown on carbon cloth (CC) was prepared by a two-step solvothermal method for supercapacitors. As a binder-free flexible electrode, a high areal capacitance of 2449.9 mF cm-2 can be achieved for WS2@NiCo2O4/CC at a current density of 1 mA cm-2. Benefiting from the core-shell of the WS2@NiCo2O4 heterostructure, the capacitive property of the flexible WS2@NiCo2O4/CC electrode is better than those of WS2/CC and NiCo2O4/CC electrodes. Based on WS2@NiCo2O4/CC electrodes, the assembled flexible solid-state symmetric supercapacitor (FSS) device shows a high energy density of ∼45.67 W h kg-1 at a power density of 992.83 W kg-1. Meantime, the WS2@NiCo2O4/CC-assembled FSS device also exhibits high cycling stability with an excellent capacity retention of ∼85.59% after 5000 cycles.

Introduction

As energy crises and environmental friendliness are the most important issues in modern society, in the past decade enormous research effort has been devoted to developing advanced energy storage devices such as lithium-ion batteries (LIBs) and supercapacitors (SCs). Compared to LIBs, SCs have a high power output, faster charge–discharge rate, and long cycle life so that it can be designed for wearable electronic devices (such as smart wristbands and portable devices) and other vital applications.[1,2] Hereof, the emergence of the flexible all-solid-state SC has aroused widespread interest among researchers. As the key to the flexible solid-state symmetric SCs (SSCs), electrode materials need to have not only good mechanical properties but, more importantly, have also excellent long cycle, power density, and energy density.[3] However, conventional carbon-based materials cannot meet the demand of high-performance energy storage because of its intrinsic electric double-layer capacitor (EDLC) mechanism (usually 70–250 F g–1, depending on the electrolyte used).[4] Therefore, combining carbon-based materials with pseudocapacitor materials is considered as one of the most effective ways for improving capacitor performance.[5]

As a kind of pseudocapacitor materials, nickel cobaltite (NiCo2O4) has been an extraordinary class of energy storage material for electrochemical SCs owing to its many advantages such as low cost, abundant resources, and environmental friendliness as well as its good electrochemical performance.[6−8] As is well known, the redox reactions provided by NiCo2O4, including contributions from both the nickel and cobalt ions, are richer than those of the monometallic nickel oxides and cobalt oxides.[9] Significant advances in this field have been achieved using several different types of NiCo2O4 nanostructures, including nanoparticles,[10] nanoplatelets,[11,12] nanosheets,[13] nanowires,[14] nanotubes,[15] microspheres,[16] and the formation of composites with carbon nanofibers[17] and graphene.[18,19] For example, Wang et al.[20] designed a NiCo2O4/reduced graphene oxide (RGO) composite based on electrostatic interactions. The initial specific capacitance of NiCo2O4/RGO composite can achieve 835 F g–1 at a current density of 1 A g–1 which is much higher than those of pure NiCo2O4 and RGO. Combining carbon nanotubes with NiCo2O4, Abouali et al.[21] prepared the NiCo2O4/CNT nanocomposites through a low-temperature, one-pot hydrothermal method. They reported a high Li-storage capacity of 1020 mA h g–1 at 300 mA g–1 after 100 cycles while pseudocapacitive performance of the electrode was equal with a capacitance of 680 F g–1 when discharged at 1 A g–1. Obviously, the combination of NiCo2O4 with electronically conducting carbon-based materials can significantly enhance the charge transport and lead to the improvement of capacitor performance.

For meeting the demand of SSCs, the molding and film formation performances of electrode materials are very vital. Generally, self-assembly, template-assembly, and supporting methods are widely used for fabrication of electrode materials for SSCs. Ni foam and carbon cloth (CC) are more convenient and effective materials as a substrate for flexible electrodes.[22,23] As a binder-free electrode for Li-ion batteries, the NiCo2O4/carbon composite textiles exhibited a high specific capacity of ∼1012 mA h g–1 at a current density of 0.5 A g–1, retaining 854 mA h g–1 after 100 cycles. An excellent specific capacitance of 1010 F g–1 (79% of the capacitance at 1 A g–1) at a current density of 20 A g–1 and remarkable cycling stability (without negligible specific capacitance lose after 5000 cycles) can be achieved as an electrochemical capacitor electrode.[24]

Although many signs of progression on NiCo2O4-based SC have been observed, the poor intrinsic conductivity and large volume change still limit the further applications of NiCo2O4 in high-performance SC. For the NiCo2O4/CC electrode material, a single NiCo2O4 loading typically exhibits limited kinetics in the redox reaction on the CC because they have a lower surface area and the short ion pathways, resulting in reduced electrochemical storage properties of SCs.[24] In addition, NiCo2O4-based electrodes suffer from low ion diffusion rate because of the Faradaic nature with limited diffusion length at the electrode/electrolyte interface, which make them show an unsatisfied rate capability and capacitance. The potential window of NiCo2O4 is still relatively low usually −0.1 to 0.6 V versus SCE or Ag/AgCl.[25−27] Therefore, the potential window needs to be extended and fits for the real application.

Recently, the development of interface/surface engineering provides an effective way to solve the mentioned problems. Rational designing of the core–shell heterostructures can be used as an ideal unit to enhance the electrochemical energy storage properties, caused by the active surface area and short ion pathway and more efficient ion-accessibility between the interface of the electrolyte and electrode.[28,29] He et al.[23] prepared hierarchical FeCo2O4@NiCo-LDH core–shell heterostructures on CC. The FeCo2O4@NiCo-LDH/CC electrode exhibits a remarkable specific capacitance of 2426 F g–1 at 1 A g–1 and ultrahigh rate capability (72.5% of the capacitance retention at 20 A g–1), as well as excellent cycling stability. They attributed the improved electrochemical behavior to judiciously engineer the interface with unique hierarchical core–shell heterostructures. The ZnCo2O4@MnO2 core–shell structure using a simple hydrothermal method was reported by Ma et al.[30] The hybrid ZnCo2O4@MnO2 electrode delivered a specific capacitance of 2380 mF cm–2 at a high current density of 5 A g–1 and has a good long-term cycling stability because of the synergistic effect between ZnCo2O4 and MnO2.

Tungsten disulfide (WS2), a kind of two-dimensional transition-metal dichalcogenides, has been considered as promising capacitive materials because of its unique chemical and physical properties.[28−30] It exhibits electrochemical charge storage behavior via both EDLC and pseudocapacitance reactions. Its unique layered structure and high capacitance especially makes WS2 easy to combine ions and enhancing ion adsorption and transportation.[31−34] Liu et al.[35] synthesized the WS2/CC composite as a negative electrode for flexible asymmetric SCs. They reported that the crystallinity of WS2 nanosheets boosted the durability and area capacitance of WS2 nanosheets (0.93 F cm–2 at 4 mA cm–2), thereby improving their cycle stability and capacitance retention of thes electrode (82% after 10,000 cycles). Shang et al.[31] fabricated interwoven WS2 nanoplates on CC (WS2/CC) by a facile solvothermal method.[32] The resulting electrodes achieved a high specific capacitance of 399 F g–1 at 1.0 A g–1 and ultra-high cycling stability with 99% retention of capacitance after 500 cycles, which can be attributed to the close combination between WS2 and CC.

In this work, we successfully fabricated a core–shell heterostructure of WS2@NiCo2O4 on CC (WS2@NiCo2O4/CC) by a simple two-step solvothermal method. The large number of active sites and proper strain regulation of the WS2@NiCo2O4 heterostructure boosted both the ion acceleration and the electron transfer, thereby improving the electrochemical performance of the electrode and also solving the problem of poor conductivity of WS2. The WS2@NiCo2O4/CC electrodes show a high area capacitance of 2449.9 mF cm–2 at a current density of 1 mA cm–2. The SSCs assembled with the WS2@NiCo2O4/CC electrode has a high energy density of 45.67 W h kg–1 at 992.83 W kg–1. The device also exhibited significant cycle stability even after 5000 cycles at a large current density of 2 mA cm–2, maintaining an initial capacitance of 85.59% while still having excellent reliability. The achieved results revealed the importance of the effects of hybrid design and the involved synergetic effect between the components of the WS2@NiCo2O4/CC composite electrode on electrochemical performance of symmetric SCs.

Results and Discussion

WS2@NiCo2O4/CC electrodes were prepared by a two-step solvothermal method (Figure a). In the first step of the solvothermal reaction, the needle-like NiCo2O4 arrays were deposited on the surfaces of the CC substrate (Figure b). It is clearly revealed in Figures b and S1 that the NiCo2O4 needle array was formed and arranged on the surface of carbon fibers of CC. From the inset of Figure b, the height and width of NiCo2O4 needles are found to be about 1.5 μm and 100 nm, respectively. In the second step of the solvothermal reaction, the WS2 nanoplates were further grown on the surface of the as-prepared NiCo2O4/CC hybrid (Figure c,d). The CC were uniformly covered by NiCo2O4@WS2 core–shell structures (Figure c). Obviously, the surface of NiCo2O4 became very rough with screw-like threads, which may offer growing WS2 nanoplates on NiCo2O4 needles (Figure c). As can be seen in Figure d, the WS2@NiCo2O4 structures exhibited a screw-like morphology and the heads of NiCo2O4 needles were linked together. We successfully prepared the core–shell WS2@NiCo2O4 screw-like arrays which chemically bonded to the conductive CC substrate via a two-step solvothermal approach. In fact, there are lots of space between adjacent NiCo2O4 needles which provide enough growth spaces for WS2 (Figure b).

Figure 1

(a) Schematic illustration of the preparation process of hierarchical WS2@NiCo2O4/CC, (b) SEM image of the NiCo2O4 needle-like arrays on CC, and (c,d) low- and high- magnification SEM images of core–shell of WS2@NiCo2O4 screw-like heterostructure arrays grown on CC.

Figure shows the WS2@NiCo2O4/CC hybrid electrodes obtained from different solvothermal reaction times. The surface of CC became more and more dense by changing the reaction time of the solvothermal synthesis at 200 °C for 4, 8, 12, and 16 h. As clearly observed in Figure e,f, there is no needle-like morphology which is almost totally coated by layered WS2 at the end of 16 h. It means that the increase in growing density of WS2 on the surface of NiCo2O4 needles lacks its stability specifically after the reaction time of 12 h. It does not mean that a high capacitive performance for the WS2@NiCo2O4/CC hybrid electrodes can be obtained with higher WS2 loading in practical SSCs. This was proved by the results of capacitive performance tests which will be further discussed in the following sections.

Figure 2

Low- and high-magnification SEM images of as-prepared WS2@NiCo2O4/CC after the second step solvothermal reaction with different times: (a,b) 4, (c,d) 8, and (e,f) 16 h.

The detail screw-like core–shell heterostructures arrays of WS2@NiCo2O4 hybrids on the surfaces of the CC substrate was further investigated by transmission electron microscopy (TEM). The sheet-like structures observed in the edge of WS2@NiCo2O4 structures (Figure a) revealed that the needle-like NiCo2O4 arrays were surrounded by WS2 nanosheets with a thickness of several nanometers. As shown in the energy-dispersive system (EDS) elemental mapping images (Figure b), the Ni, Co, W, O, and S elements are uniformly distributed on the surfaces of WS2@NiCo2O4 hybrid structures, confirming the successful combination of WS2 with NiCo2O4 and their core–shell hierarchical structure. The high-resolution TEM (HRTEM) images provide the direct information of the core–shell heterostructure of WS2@NiCo2O4 hybrids on the surfaces of the CC substrate (Figure c). The various crystal faces with polycrystalline properties exist, and there is an observable grain boundary between the NiCo2O4 (zone I) and WS2 (zone II) in the structure of WS2@NiCo2O4 hybrids. The interplanar spacing of 0.469 nm in zone I is indexed to the (111) lattice plane of the cubic NiCo2O4 phase. In zone II, the interplanar crystal spacing of 0.267 and 0.273 nm were ascribed to the (101) and (100) planes of the WS2 phase, respectively. It means that the core–shell heterostructure of the WS2@NiCo2O4 hybrid was successfully formed on the surfaces of the CC substrate. X-ray diffraction (XRD) patterns were also recorded to examine the composition and phase evolution of the WS2@NiCo2O4/CC hybrid structure (Figure d). The diffraction peaks are substantially identical to the characteristic peaks identified for the standard cubic NiCo2O4 and 2H-WS2. Two diffraction peaks observed around 2θ of 26 and 42° are attributed to the peaks of (002) and (100) planes of carbon fibers. The peaks detected at around 31.2, 36.7, 59.1, and 65.1° in pattern are corresponding to the (220), (311), (440), and (511) planes, respectively, which can be assigned to the cubic spinal NiCo2O4 phase (JCPDS no. 20-0781). Besides the diffraction peaks of carbon fibers and NiCo2O4, well-resolved peaks at around 14.3, 33.5, 43.9, and 58.4° are indexed to the (002), (101), (006), and (110) crystal planes, respectively, which can be attributed to the 2H-WS2 phase (JCPDS no. 08-0237). These results also demonstrated the effective interparticle interaction between NiCo2O4 needles and WS2 nanoplates on the surfaces of the CC substrate.

Figure 3

(a) TEM image of the core–shell WS2@NiCo2O4 heterostructure, (b) dark-field TEM image and EDS mapping image of the core–shell WS2@NiCo2O4 heterostructure, (c) HRTEM images of WS2@NiCo2O4, and (d) XRD patterns of NiCo2O4/CC and WS2@NiCo2O4/CC.

The composition and valence states of the WS2@NiCo2O4 core–shell heterostructure were identified by X-ray photoelectron spectroscopy (XPS). The distinct Ni 2p, Co 2p, W 4f, S 2p, and O 1s signals along with C 1s from CC were presented in the full scan survey spectrum of WS2@NiCo2O4 (Figure S2). In Figure a,b, the core-level XPS spectra of Ni 2p and Co 2p were optimally deconvoluted into four main peaks that can be assigned to two spin–orbit doublets and two shakeup satellites (marked as “Sat.”) by using the Gaussian fitting method. Two peaks centered at 856.5 and 874.8 eV were corresponding to the Ni2+, whereas the other two peaks appearing at 854.8 and 872.6 eV were ascribed to the Ni3+ (Figure a).[36,37]Figure b shows the Co 2p high-resolution spectrum where the fitted peaks at 780.2 and 796.1 eV were attributed to Co2+, and the fitted peaks at 781.6 and 797.6 eV were attributed to Co3+.[38] For the W 4f XPS spectrum, the peaks appearing at 35.2 and 37.3 eV were attributed to W 4f5/2 and W 4f7/2, which are originated from the W4+ oxidation state (Figure c).[39,40] The peaks detected at 36.0 and 38.0 eV agreed well with the characteristics of W 4f5/2 and W 4f7/2 for the higher state of W4+ denoted as W6+. The relative higher intensity of W6+ peaks further proved the higher content of tungsten trioxide oxide deposition on WS2, which may be attributed to the slight oxidation in air during thermal treatment. The S 2p pattern in Figure d can be fitted with three pairs of the peaks (S 2p): the first pair of 163.5 and 170.1 eV, the second pair of 162.4 and 169.6 eV, and the third pair of 161.3 and 168.6 eV. The peaks at 170.1, 169.6, and 168.6 eV are associated with the residual SO42– substance.[41−43] The other peaks which are centered at 163.5, 162.4, and 161.3 eV corresponded to the S2– ligand. The successful preparation of the WS2@NiCo2O4 core–shell structure was proved by the anchored mixed composition of Co2+, Co3+, Ni2+, Ni3+, W4+, and S2– on the surface of the WS2@NiCo2O4 heterostructure in addition to the various metal–sulfur and metal–oxide coexisting in the interior and surface of the structure. The above XPS results are in accordance with XRD and EDS measurements.

Figure 4

XPS spectra of WS2@NiCo2O4 heterostructure nanonail (12 h): (a) Ni 2p spectrum, (b) Co 2p spectrum, (c) W 4f spectrum, and (d) S 2p spectrum.

The electrochemical performance of WS2@NiCo2O4/CC core–shell hybrid electrodes was evaluated by a three-electrode method using 3 M KOH as the electrolyte. Figure a shows the cyclic voltammetry (CV) curves of WS2/CC, NiCo2O4/CC, and WS2@NiCo2O4/CC electrodes at a scan rate 10 mV s–1 between −0.4 and 0.6 V (vs Ag/AgCl). The CVs (with WS2/CC being an exception) exhibited obvious pseudocapacitive characteristics with two pairs of redox peaks which is likely due to the Faradaic reaction (Figure a). The redox reactions of the WS2@NiCo2O4/CC electrode are based on the following equations[44−46]

Figure 5

(a) CV curves of NiCo2O4/CC, WS2/CC, and WS2@NiCo2O4/CC electrodes at a scan rate of 10 mV s–1, respectively, (b) CV curves of the WS2@NiCo2O4/CC electrode at different scan rates, (c) GCD curves of the WS2@NiCo2O4/CC electrode at various current densities, (d) GCD curves of the WS2@NiCo2O4/CC electrode with different solvothermal reaction times at a current density of 5 mA cm–2, (e) ACs of NiCo2O4/CC, WS2/CC, and WS2@NiCo2O4/CC electrodes at different current densities, and (f) Nyquist plots of NiCo2O4/CC and WS2@NiCo2O4/CC electrodes.

The integrated area of the CV curve of WS2@NiCo2O4/CC is significantly larger than that of the individual NiCo2O4/CC and WS2/CC electrodes. Two redox peaks of the NiCo2O4/CC electrode located at ∼0.4 and ∼0.3 V could also be ascribed to the pseudocapacitance redox reaction. However, there are two pairs of redox peaks located at 0.16/0 and 0.35/0.30 V for the WS2@NiCo2O4/CC hybrid electrode, which could be probably caused by the redox transition between the WS2 nanoplates and NiCo2O4 nanoneedles. As expected, the WS2@NiCo2O4/CC hybrid electrode showed the highest current densities and integrated area, indicating the improvement in electrochemical capacitance because of synergistic effects between NiCo2O4 needles and WS2 nanoplates. Although WS2 is known to contribute capacitance, the CV curve of WS2/CC is minuscule compared to those of NiCo2O4/CC and WS2@NiCo2O4/CC, demonstrating that most of the capacitance comes from the pseudocapacitance of NiCo2O4. However, it should be pointed out here that the enhancement of the electrochemical activity of the WS2@NiCo2O4/CC electrode was triggered by the use of WS2 nanoplate coatings, resulting in the synergistic effect among the NiCo2O4, WS2, and CC substrate. It is mainly due to the fact that the combination of NiCo2O4 and conductive carbon-based materials can significantly enhance the charge transfer and improve the capacitance performance, while the special layered structure of WS2 makes it easy for ions to combine, which can promote the rapid adsorption and transport of ions. Through the core–shell heterostructure of WS2@NiCo2O4, WS2 grows on the NiCo2O4/CC hybrid array, and the original smooth NiCo2O4 needle becomes very rough, and the head of the NiCo2O4 needle is connected together, so that a large number of active sites and proper strain adjustment appear in the WS2@NiCo2O4 heterostructure, which promotes ion acceleration and electron transfer, thus improving the electrochemical performance of the electrode and solving the problem of poor conductivity of WS2. As a result, the electrochemical performance of the system was improved by the synergistic interaction of WS2 and NiCo2O4. Figure b shows the CVs of the WS2@NiCo2O4/CC electrode recorded at different scan rates from 10 to 100 mV s–1. All CVs exhibited a strong redox peak, reconfirming rapid redox reactions related to M–O/M–O–OH/W–S associated with OH– in the electrolyte, where M refers to Ni or Co.[7,36] The linear relation observed between the peak current at different scan rates confirmed the good interfacial kinetics and high rate performance.[47,48] The shift of the anode and cathode peaks to higher and lower potentials can be attributed to the polarization effect of the electrodes.[49] To further study the capacitive performances of WS2@NiCo2O4/CC hybrid electrodes, the galvanostatic charge–discharge (GCD) tests were conducted in the potential window between −0.6 and 0.3 V at different current densities (Figures c and S3). The observation of approximately symmetric curves at all current densities are believed to be the main reasons for the high charge–discharge Coulombic efficiency and low polarization of the electrode.

The WS2@NiCo2O4/CC electrode prolonged the discharge time as compared to the WS2/CC and NiCo2O4/CC electrodes. The discharge time obtained from these measurements showed a maximum for solvothermal reaction time at 12 h (Figure d). The minimum discharge time, measured at 16 h, is by a factor of around 4.0 lower than that found at 12 h, which could be attributed to the poor conductivity of WS2@NiCo2O4 caused by the high amount of WS2 in the structure. The excess solvothermal reaction time (>12 h) may act as a trigger for the reduction observed in the discharge time of the WS2@NiCo2O4 hybrid electrode. An approximate symmetric curve and voltage plateau regions can clearly be observed, indicating high Coulombic efficiency during charging and discharging. Two pairs of obvious charge and discharge platforms observed in charge and discharge curves could be ascribed to the redox potential window of NiCo2O4 and WS2. As is well known, the pseudocapacitance discharge curve has a voltage drop that is related to the internal resistance at the interface between the electrode and the electrolyte.[50] This resistance triggers the redox electrochemical reaction that occurs on the voltage platform. The rapid, continuous, and multimaterial synchronization of the redox reaction rate with the electron transfer rate offers the good electrochemical pseudocapacitance characteristics, which is corresponding to the large voltage platform (Figure d). The areal capacitances (ACs) of the WS2@NiCo2O4/CC electrode can be calculated by using the data given in Figure c and benchmarked against those of the NiCo2O4/CC and WS2/CC electrodes in Figure e. The ACs of the WS2@NiCo2O4/CC electrode were calculated to be 2449.9, 1878.4, 1325.6, and 928.9 mF cm–2 at current densities of 1, 2, 5, and 10 mA cm–2, respectively. After conversion, the WS2@NiCo2O4/CC electrode could deliver high mass-specific capacitances of 770.4, 555.7, 392.1, and 274 F g–1 at current densities of 1, 2, 5, and 10 mA cm–2, respectively, which are much higher as compared to that of NiCo2O4/CC and WS2/CC electrodes. The synergism originated from the combination of WS2 and NiCo2O4 particles are responsible for the improved ACs of the WS2@NiCo2O4/CC hybrid electrode. NiCo2O4 and WS2 appeared to have the open space between their needles and the mesopores existing in the nanosheets, which could not only serve as a robust reservoir for ions but also greatly enhance the diffusion kinetics within the WS2@NiCo2O4/CC electrode.

The outstanding capacitive performance observed in the case of the WS2@NiCo2O4/CC electrode is not just a simple superposition of the NiCo2O4/CC and WS2/CC electrodes. First, the highly ordered conductive needle-like NiCo2O4 were uniformly dispersed on the surfaces of the CC substrate. Then, a frame structure was formed, which is beneficial to the electron transfer and diffusion in the electrolyte. Afterward, WS2 coated on the surfaces of NiCo2O4 needles provided more efficient electron channels. Furthermore, the direct contact between the surfaces of nanosheets with the intrinsic conductivity to the CC substrate built up an express path for fast electron transport, thus avoiding the use of the polymer binder and a conductive additive which commonly added extra contact resistance.

In order to verify that the electroconductivity of the electrodes, the electrochemical impedance tests were performed in the frequency range of 0.01–100 Hz (Figure f). Nyquist plots of the three electrode samples confirmed that the WS2@NiCo2O4/CC hybrid structure exhibited the minimum charge-transfer resistance compared with WS2/CC and NiCo2O4 electrodes. The WS2/CC electrode has a large curve diameter and, thus, has a large charge-transfer resistance, shown in the inset of Figure f. The NiCo2O4/CC electrode has a small curve diameter and good conductivity, thereby having the capacity to improve the conductivity of the composite materials. A typical semicircle in the high-medium frequency region and an inclined line in low frequencies could correspond to the charge-transfer resistance at the NiCo2O4/CC electrode/electrolyte interface. Considering the Nyquist plot of the WS2@NiCo2O4/CC hybrid electrode, it could be said that the charge-transfer rate of the NiCo2O4/CC electrode became faster after incorporating with WS2 nanoplates. As confirmed by the impedance spectroscopy measurements, the electrolyte ion can easily diffuse throughout the WS2@NiCo2O4/CC hybrid electrode and enhance the utilization of active electrode materials.

Electrochemical processes in a flexible SSCs were evaluated for charge–discharge cycles (Figure a). A mixture of polyvinyl alcohol (PVA) and potassium hydroxide (KOH) was used as a gel electrolyte, while Cu foil served as collectors. The CVs of the WS2@NiCo2O4/CC hybrid SSC device at different scanning rates in a voltage range of 0–1 V (Figure b). Two pairs of redox peaks observed in Figure b are consistent with the two voltage plateaus in the GCD curves (Figure c). The ions in the electrolyte can fully react with the active material at a low scanning speed, greatly improving the utilization of the electrode material.[51] All the CVs exhibit a slightly sloping shape with the increase of scan rates, indicating the effective interaction between the electrode and the electrolyte (Figure b). Even at a higher scan rate of 100 mV s–1, the CV curve still has a clockwise response and pair of redox peaks. The reason for retaining the shape of CVs at high scan rates could probably be attributed to the lack of the restriction of mass-transfer and ionic transport of charges between the electrodes (Figure b). All the entire capacitance of the device benefits from the contribution of Faradaic pseudocapacitance. Figure c shows the GCD curves of the WS2@NiCo2O4/CC hybrid SSC from 1 to 10 mA cm–2. The almost symmetrical charging and discharging curves and small voltage drops observed immediately after the discharge started arises primarily from good capacitive behavior and high conductivity within the capacitor. The specific capacitance of the WS2@NiCo2O4/CC hybrid SSC device can be calculated by using the loading mass of the active materials. The ACs of the device are 196, 175.4, 125.5, and 82 mF cm–2 at 1, 2, 5, and 10 mA cm–2, respectively. From Figure d, the WS2@NiCo2O4/CC hybrid SSC also exhibited excellent cycling stability with 85.59% capacitance retention after 5000 charges–discharge cycles. It was found that there is almost no change in Coulombic efficiency after 5000 cycles, which could be attributed to the excellent reversibility and structural stability of an all-in-one configuration. The bottom inset of Figure d depicted nine charge–discharge profiles after 5000 cycles. The curves remained symmetric and unchangeable after a long period of the charge–discharge process. This result demonstrated that the WS2@NiCo2O4/CC hybrid SSC was a very reliable electrode material with high cycling stability. To further confirm this result, we performed the scanning electron microscopy (SEM) analysis of the used WS2@NiCo2O4/CC hybrid capacitor after the 5000 cycles (Figure S4). It was clear that the WS2@NiCo2O4/CC capacitor maintained its structural integrity without any separation and powdering. The SSC assembled with WS2@NiCo2O4/CC electrodes displayed a maximum energy density of 45.67 W h kg–1 at a power density of 992.83 W kg–1 (Figure e). Even at a high-power density of 10,000 W kg–1, the device still was able to deliver an energy density of 17.5 W h kg–1. Such high energy and power densities observed further confirmed that the WS2@NiCo2O4/CC hybrid was an excellent electrode material for SSCs. The WS2@NiCo2O4/CC hybrid SSC device showed much better performance than those in some previously reported SCs assembled with CC/Ni foam-supported electrode materials (Figure e, Tables S1, and S2).[52−64]

Figure 6

(a) Assembly structure of all-solid-state symmetric SCs. The illustration shows the digital photos of the FASS assembled for WS2@NiCo2O4/CC. (b) CV curves at different scan rates and (c) GCD curves for symmetrical solid-state devices in the 0–1.0 V potential range. (d) Capacitance retention and Coulombic efficiency in 5000 charge–discharge cycles at a current density of 2 mA cm–2, the inset shows the last nine cycles of the GCD curves. (e) Ragone diagram of the device and some other previous reported related SCs.

As is well known, such flexible electrodes are anticipated to properly maintain power supply upon repeated bending, folding, stretching, or twisting without sacrificing the capacitor performance. The CVs of the capacitor being bent at different angles at 10 mV s–1 nearly overlapped with each other and no apparent loss in calculated capacitance indicated the stable performance of the WS2@NiCo2O4/CC hybrid capacitor at these states (Figure a). By connecting two WS2@NiCo2O4/CC hybrid capacitors in series, it gave a 2.0× higher voltage window compared to a single capacitor with an output voltage of ∼1.0 V (Figure b). When two capacitors were connected in a series configuration, the discharge time of the assembly was found to be about half that of a single device discharged at the same current density of 5 mA cm–2 (Figures c and 6c). However, this configuration rendered a two times larger voltage window at the same current density. The active areas of the device can be adjusted when WS2@NiCo2O4/CC capacitors were connected in series. The capacitance of the device increases almost linearly as the area of series-connected WS2@NiCo2O4/CC capacitors increase from 2 to 10 cm2 (Figure d), indicating the possibility of integrating large-area WS2@NiCo2O4/CC-assembled SSC devices to enlarge the output energy requirements. A real-time demonstration exhibited that after charging at a current density of 2 mA cm–2, a three series-connected WS2@NiCo2O4/CC capacitor could light a commercial light-emitting diode (LED) (3 V, 4.5 W) up to 9 min (Figure e). The WS2@NiCo2O4/CC devices connecting in series configuration can also supply power to a calculator (3 V, 4.5 W) (Figure f). We represented a wristband made by connecting the WS2@NiCo2O4/CC capacitors which can serve as a wearable surgical and diagnostic implement in the promising area of healthcare when combined with wearable sensors (inset of Figure f).

Figure 7

(a) CV curves of the WS2@NiCo2O4/CC-assembled FSS device at different twist angles, (b) CV curve of two series-connected FSS devices at a scan rate of 10 mV s–1, (c) GCD curve of two series-connected FSS devices at a current density of 5 mA cm–2, (d) ACs of series-connected FSS devices with different areas at a discharge current of 1 mA cm–2, (e) photographs of LED light powered by three series-connected FSS devices at different durations, and (f) photographs of wristbands made by bended series-connected FSS devices and a calculator powered by series-connected FSS devices.

Conclusions

In summary, a core–shell heterostructure of WS2@NiCo2O4 on CC was successfully fabricated by the two-step solvothermal method. The structural and chemical synergy being existed between the WS2, NiCo2O4, and CC significantly improved the electronic conductivity of the WS2@NiCo2O4/CC electrode, thereby providing high ion-accessibility and low transport resistance between the electrode/electrolyte interfaces. The WS2@NiCo2O4/CC architecture possesses the mesopore structure and open space between neighboring NiCo2O4 nanoneedles which ensures fast transport of the electrolyte ions to the surfaces of the electrode, leading to rapid charge transfer reactions owing to the shortened ion diffusion paths. The WS2@NiCo2O4/CC-assembled SSCs provided a high energy density of ∼45.67 W h kg–1 at a power density of 992.83 W kg–1. Based on the stable internal structure of the hybrid electrode, WS2@NiCo2O4/CC SSCs also exhibits high cycling stability with an excellent capacity retention of ∼85.59% even after 5000 cycles. Thus, we have reasons to believe that the hierarchical core–shell WS2@NiCo2O4 heterostructure can be used as promising materials for practical applications of advanced energy storage.

Experimental Section

Chemicals and Materials

Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), urea (CH4N2O), ammonium thiotungstate (H8N2S4W), N,N-dimethylformamide (DMF), PVA, and KOH were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemical reagents were AR grade and used without further purification. CC (WOS1002 from CeTech) was obtained and treated with double distilled water and ethanol.

Synthesis of the NiCo2O4 Needle-like Arrays Grown on the CC Substrate

The NiCo2O4/CC electrodes with NiCo2O4 needle-like arrays grown on the CC substrate were synthesized by the solvothermal reaction and a thermal annealing process. First, Co(NO3)2·6H2O (1 mmol), Ni(NO3)2·6H2O (0.5 mmol), and carbamide (4.5 mmol) were added into deionized H2O (25 mL) under magnetic stirring. After stirring for 30 min, the as-prepared pink solution was transferred into a polytetrafluoroethylene autoclave. Then, a piece of cleaned and acid-treated activated CC substrate (2 × 2 cm2) was immersed into the reaction solution. Next, the CC substrate with Ni–Co precursors was sealed in an autoclave and heated at 120 °C for 6 h in an oven. After naturally cooling down to room temperature, the resultant CC substrate was removed and carefully washed with distilled water and ethanol several times to remove the impurities and dried at 60 °C for 12 h. Finally, the precursors were converted to NiCo2O4 needle-like arrays by calcination at 350 °C for 2 h at a ramping rate of 2 °C min–1 under an air atmosphere. The mass loading of the NiCo2O4 needle-like array on CC was found to be around 0.4 mg cm–2.

Preparation of the Core–shell WS2@NiCo2O4 Screw-like Heterostructure on the CC Substrate

First, the reaction solution was obtained by dissolving H8N2S4W (1 mmol) in DMF, and the resultant solution was then sealed in a polytetrafluoroethylene autoclave. Then, a piece of as-prepared NiCo2O4/CC electrode (2 × 2 cm2) was immersed into the reaction solution. Next, the autoclave was transferred into an electric furnace and maintained at 200 °C for 12 h. After naturally cooling down to room temperature, the WS2@NiCo2O4/CC hybrid electrode was removed and carefully washed with distilled water and ethanol several times to remove the impurities and dried at 60 °C for 12 h. Finally, the WS2@NiCo2O4/CC hybrid electrode was calcined at 350 °C for 2 h at a ramping rate of 1 °C min–1 under a nitrogen atmosphere. The mass loadings of WS2 and WS2@NiCo2O4 hybrids were found to be around 0.78 and 1.18 mg cm–2, respectively.

Instruments and Material Characterization

XRD characterization was carried out at ambient temperature using a Bruker D8 advance diffractometer. The XPS data were determined on a Thermo Scientific ESCALAB 250Xi using Al Kα (1486.6 eV) excitation. TEM observations were performed on a JEM-2100F microscope. The morphologies of the electrode samples were characterized by a field-emission SEM (JSM-7800F).

Electrochemical Measurements

All the electrochemical measurements were performed by using a CHI760d electrochemical workstation (Shanghai CHInstruments Co., China) at room temperature. CV and GCD tests were conducted by a three-electrode system in the 3 M KOH aqueous electrolyte. The prepared WS2@NiCo2O4/CC hybrid was used as the working electrode. A Pt plate and an Ag/AgCl served as a counter and a reference electrode, respectively. The following equation was used to calculate the areal specific capacitance (Ca) of the electrodes: Ca = It/sV, where I, V, t, and s are discharge current, discharge voltage, discharge time, and size of the working area of the electrode, respectively. In order to assemble the flexible SSCs, a solid-state electrolyte was prepared by dissolving PVA (2 g) into H2O (10 mL) and then mixed with 3 M KOH (10 mL) solution at 85 °C. Then, the as-prepared two WS2@NiCo2O4/CC hybrid electrodes were immersed into the electrolyte for 5 min, then dried at room temperature overnight to evaporate excess water. Hybrid electrodes were then assembled face-to-face, considering that the impregnated polymer electrolyte into the electrodes will be served as both the electrolyte and the ion-porous separator in the resultant WS2@NiCo2O4/CC hybrid SC device. As long as the electrolyte was solidified, a mechanically robust cell could be tested. The energy density (E) and power density (P) of the full cell were calculated by: E = 1/2CV2, P = E/t, where C, V, t, and m are the specific capacitance of the full cell, the discharge voltage, the discharge time, and the active mass of the electrodes, respectively.