A Self‐Branched Lamination of Hierarchical Patronite Nanoarchitectures on Carbon Fiber Cloth as Novel Electrode for Ionic Liquid Electrolyte‐Based High Energy Density Supercapacitors

The developments of rationally designed binder‐free metal chalcogenides decorated flexible electrodes are of paramount importance for advanced energy storage devices. Herein, binder‐free patronite (VS4) flower‐like nanostructures are facilely fabricated on a carbon cloth (CC) using a facile hydrothermal method for high‐performance supercapacitors. The growth density and morphology of VS4 nanostructures on CC are also controlled by varying the concentrations of vanadium and sulfur sources along with the complexing agent in the growth solution. The optimal electrode with an appropriate growth concentration (VS4‐CC@VS‐3) demonstrates a considerable pseudocapacitance performance in the ionic liquid (IL) electrolyte (1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate), with a high operating potential of 2 V. Utilizing VS4‐CC@VS‐3 as both positive and negative electrodes, the IL‐based symmetric supercapacitor is assembled, which demonstrates a high areal capacitance of 536 mF cm−2 (206 F g−1) and excellent cycling durability (93%) with superior energy and power densities of 74.4 µWh cm−2 (28.6 Wh kg−1) and 10154 µW cm−2 (9340 W kg−1), respectively. As for the high energy storage performance, the device stably energizes various portable electronic applications for a long time, which make the fabricated composite material open up news for the fabrication of fabrics supported binder‐free chalcogenides for high‐performance energy storage devices.


Introduction
Supercapacitors (SCs) are gaining greater attention in the recent energy storage technology due to their high-power density and long cycle life. [1,2] However, they require an enormous amount of power to work as an autonomous or complement distinctive vitality sources such as batteries and fuel cells. [3] Supercapacitors are commonly divided into two types namely porous carbon-based non-Faradic electrical double-layer capacitors and metal oxide/ sulfides-based reversible faradaic pseudocapacitors. [4] In recent decades, various types of transition metal sulfides (TMSs) have been examined beyond the state-ofart of materials, which showed exceptional energy storage performance compared to the former generation materials, including metal oxides and carbon-based materials. The materials such as MoS 2 , [5] WS 2 , [6] NiS, [7] VS 4 , [8] VS 2 , [9] and CoS [10] have been the subject of intense research in pseudocapacitors. [11] Particularly, the transition metal sulfide-like vanadium sulfides (VS 4 or VS 2 ) exhibits an immense interest among the researchers owing to its promising energy-storage activities. [12] Among the various vanadium sulfides, the VS 4 consisted of two S 2 2− dimers and unusual linear-chain structure where the oxidation state of vanadium is similar to VS 2 structure. Moreover, the structure of VS 4 varies to prevent the bundle formation of nanosheets and which effectively improves the surface area of the material for better electrochemical properties. [13] Recently, VS 4 nanostructures with various morphologies, such as urchin-like, octopus-like, sea grass-like, and nanodendrites have been synthesized and applied especially as the anode materials in lithium-ion batteries. [14] Generally, large scale synthesis of VS 4 with high purity is complicated due to the necessity of precise controlled partial pressure of sulfur during sulfurization and existence of various nonstoichiometric phases of vanadium sulfides with different oxidation states. [13] However, the pure form of patronite-VS 4 nanostructures can be synthesized via template-assisted growth method over graphene, carbon nanotubes (CNTs), carbon fibers etc. [12a,15] This reveals that the VS 4 nanostructures have better growth affinity on carbonaceous materials. Therefore, an attempt was made to grow pure phase of VS 4 on conducting carbon fiber cloth (CC), which is an excellent substrate for designing a lightweight and flexible supercapacitor electrode. Since 3D structure of CC can be anticipated to facilitate a fast charge transfer and enhances the accessibility of the electrolytic ions into the electrode material. [16] Apart from this, the significantly high capacity and capacity retention of the VS 4 [17] can act as a potential pseudocapacitance electrode material for the development of supercapacitors. Hence, for the first time, a binder-free strategy has been adopted to grow VS 4 as a pure phase on conducting carbon fiber cloth and utilized for the fabrication of symmetric supercapacitor in an ionic liquid electrolyte.
To enhance the energy density (E d ) of supercapacitors, currently, the researchers have adopted two major strategies. Namely, synthesize of self-assembled binder-free electrode materials on the conductive substrate (such as Ni-foam, carbon cloth, and stainless-steel mesh, etc.), and utilization of ionic liquid or organic electrolytes. The binder-less electrode materials on the current collector are favorable for affording more active sites to increase the specific capacitance (C sc ) of the SCs. [18] On the other hand, the ionic or organic electrolytes can increase the maximum operating potential window of devices, and which undoubtedly exalt the energy storage properties of supercapacitors. [19] Recently, ionic liquids (ILs) are considered as a major domain in the field of electrochemistry due to their high ionic mobility, the flexibility of ions and broad potentials. [20] The recent studies established two specific reasons of ILs as a potential electrolyte for supercapacitors, especially in double-layer charging. [21] The first reason is a primary task of the electrolyte to provide charge species at the electrode/electrolyte interface instead of diffusion of specific electroactive species and the other is the wide stable potential window of ILs which guarantees high energy densities even greater and safer than the organic electrolytes. [22][23][24] Moreover, the ILs have very promising strategies to be employed in supercapacitors due to their structural flexibility to functionalize them to present specific contacts with the surface or redox performance. [21] Besides, the ILs have better stability in high temperature due to its high melting point and high boiling point, which certifies a prosperous safety of the device. However, the pseudocapacitive performance of transition metal sulfides/oxides in ILs is infrequently reported. Subsequently, there is a huge interest to develop novel metal chalcogenides for high-performance supercapacitors due to its low cost, naturally abundant and environmental benignity. [16b] Thus, choosing appropriate IL electrolyte for these materials can effectively improve the operating voltage of the metal chalcogenide-based SCs with a considerable increase in the energy density.
In this work, a novel flower-like VS 4 (patronite) nanostructures were grown on carbon cloth (CC) using the onestep hydrothermal method without the aid of any polymeric binders. Further, the effect of various precursor concentrations on the growth, morphology and electrochemical performance was studied. The fabricated growth reagents/concentrationdepended binder-free VS 4 electrodes were initially tested in a three-electrode system using 1 m 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf ]), which demonstrates that the VS 4 electrode has a considerable affinity towards the IL with a pseudocapacitive nature of energy storage process. In addition, the symmetric supercapacitors were assembled using different concentration-dependent VS 4 nanostructure-based electrodes in [EMIM][OTf ] electrolyte with an operating voltage of 2 V. Among these, the VS 4 -CC@VS-3//VS 4 -CC@VS-3 symmetric device displays a high C ac of 536 mF cm −2 at 1 mA cm −2 . Furthermore, the device exhibited high energy density of 74.4 µWh cm −2 and maximum power density of 10154 µW cm −2 with better capacitance maintenance (≈93%) after 1000 cycles in [EMIM][OTf ] electrolyte. The electrochemical performances of the VS 4 -CC@VS-3//VS 4 -CC@VS-3 device were comparatively studied and discussed in detail.

Results and Discussion
The schematic representation in Figure 1 illustrates the onestep growth of hierarchical VS 4 nanostructures on CC by hydrothermal technique. As a well-known flexible electrode, CC has received significant attention in designing energy storage devices owing to its 3D architecture, great flexibility, good thermal resistivity, and high conductivity. Utilizing this feasible substrate, hierarchical and binder-free VS 4 nanostructures were densely grown and directly used as the supercapacitor electrode without any polymer binders or conductive additives. During the hydrothermal process, the growth solution comprising VOSO 4 .xH 2 O and C 2 H 5 NS with a complexing agent CH 3 COOH were reacted and precipitated together and forms a binder-free VS 4 nanostructures on CC with robust adhesion. The theoretical conjecture of chemical reaction takes place in the hydrothermal reactor during the growth of VS 4 nanostructures on CC at various concentration of growth solution are given in Equations (1-3) For the uniform growth of VS 4 nanostructures on CC, the placement of the growth substrate inside the hydrothermal reactor is crucial to control the optimum growth of electroactive material over CC. As represented in Figure S2a in the Supporting Information and Figure 1, the vertically mounted substrate in growth solution showed a uniform and controlled growth of flower-like VS 4 nanostructures with better adhesion on CC substrate. But, the horizontally placed substrate ( Figure S2b, Supporting Information) evidenced the vigorous deposition of agglomerated VS 4 nanostructures with poor adhesion on the CC fibers. To corroborate the above observation, the morphological aspects of vertically and horizontally grown substrates are displayed in Figure 3 and Figure S3 in the Supporting Information. Moreover, the CC substrate was supported with a glass slide ( Figure 1) for better vertical alignment and also determines the growth of VS 4 nanostructures on the exposed CC surface. Figure 2ai,ii displays the photographic images of growth solutions and the fabricated electrodes. The color of growth solution varies from light blue to dark blue, which gives a strong influence on the growth rate of VS 4 concerning the concentration of the precursor. Moreover, the increase in concentration slightly increases the acidic nature of the growth solution and possess an additional tendency to converts the hydrophobic CC into more hydrophilic. This evidently enables the better growth of VS 4 nanostructures on CC, as shown in Figure 2aii. (the CC is fully covered with darker deposition of black colored VS 4 ). Further, the mechanical durability of the VS 4 laminations on CC was ensured by subsequent bending and twisting experiments. But no visible dispersion of materials was observed from the electrode, which confirms better adhesion of VS 4 on CC and great mechanical durability of the fabricated electrode (Figure 2aiii). Figure 2b presents the X-ray diffraction (XRD) patterns of the fabricated VS 4 -CC@VS-1, VS 4 -CC@VS-2, VS 4 -CC@VS-3 samples and the bare CC substrate. The bare CC (black curve) shows two significant broad diffraction peaks at 26.3° and 44.4°, which coincide to (002) and (101) diffraction planes of the standard hexagonal phase of the crystalline carbon (JCPDS no. 75-1621). [25] The additional diffraction peaks observed in VS 4 -CC@VS-1 (blue), VS 4 -CC@VS-2 (orange), and VS 4 -CC@VS-3 (green) samples were indexed to the monoclinic VS 4 phase, corresponding to (110), (020), (123), (132), (114), (−215), and (332) planes (JCPDS No. 072-1294, space group: I 2/c). [26] The VS 4 -CC@VS-1 sample shows noisily and low intense peaks represent the poor crystallinity and less Adv. Funct. Mater. 2020, 30,1906586  distribution of VS 4 on CC, which is ascribed to the low concentration of growth solution in the growth process. Moreover, the VS 4 -CC@VS-2 sample displays two additional impurity peaks, which can be assigned to the V 2 S 3 (JCPDS No. 19-1407) and sulfur (JCPDS No. 65-1101) formed due to the improper concentration of growth solution. Further increasing the concentration of growth solution, the VS 4 -CC@VS-3 XRD profile shows the formation of pure crystalline phase of VS 4 with well-defined narrow and sharp diffraction peaks at 15.78° and 17.01°, corresponds to the (110) and (020) crystal planes, which discloses the highly crystalline and dense growth of VS 4 on CC. These substantial differences in XRD patterns of the VS 4 expose to the various concentration of growth solution have an essential effect on the structural and crystallinity of the samples. Furthermore, the precipitated powder samples were collected from the hydrothermal vessel of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS4-CC@VS-3 after reaction and studied for XRD as shown in Figure S4 in the Supporting Information. The diffraction peaks of the samples display the presence of sulfur and some unreacted precursor complexes without any trace of VS 4 diffractions. This results confirm that the CC substrate act as a potential catalyst for the growth of VS 4 nanostructures. The chemical oxidation states of V and S in the VS 4 nanostructure on CC surface was investigated using X-ray photoelectron spectroscopy (XPS), the corresponding results are presented in Figure 2c-g. Figure 2c displays the survey spectrum which represents the presence of carbon (C), vanadium (V), sulfide (S), and oxygen (O) atoms in the VS 4 -CC@VS-3 sample. The carbon peak detected in the spectrum is mainly originated from the CC substrate. The C 1s spectrum (Figure 2d) was deconvoluted into three different components at 284.4, 285.8, and 287.5 54 eV, corresponds to the overlapping CC bonds, COH bands, and CO (carbonyl) bonds, [27] respectively. The XPS peaks observed at 523.4 and 516.2 eV ( Figure 2e) are assigned to the spin-orbit splitting V 2p 3/2 and V 2p 1/2 of V 2p energy state. The slight shift in the binding energies indicates the reasonably high V 4+ state than V 5+ (since the V 2p peak of V 5+ is normally placed at ≈517.2 and ≈524.5 eV) [28] signifying the influence of V 4+ in VS 4 nanostructures. [26] In Figure 2f, S 2p spectrum reveals two peaks at 162.3 and 163.5 eV, which represents the spin-orbit splitting of sulfur 2p 3/2 and 2p 1/2 of the S2 −2 dimer in VS 4 . [12a] Meanwhile, the resultant O 1s peak observed in Figure 2g, can be deconvoluted into two peaks at 530.4 (C-O) and 531.6 (C-O-H) eV, which is due to the C functionalities from CC conductive medium associated with moisture oxygen molecules. [29] The morphology of the synthesized samples under different growth concentrations are examined using field-emission scanning electron microscopy (FE-SEM), as shown in Figure 3.  dense growth of VS 4 . Hence, the growth solution concentration was further doubled than that of VS 4 -CC@VS-2 solution and the resultant FESEM images of VS 4 -CC@VS-3 confirmed the appropriate mass of the precursor concentration in the growth solution to attain an enriched growth of the VS 4 nanoflowers as depicted in Figure 3ci-iii. The enriched VS 4 nanoflowers slightly resemble a datura ferox fruit-like morphology. This densely grown VS 4 flower-like morphology over the CC substrate can provide larger electroactive sites for the better charge-storage process. Further, investigate the adhesion of the VS 4 nanostructure on the CC substrate, the samples were subjected to strong ultrasonification (ultrasonic power 120 W) for 20 s and the FESEM were recorded and displayed in Figure S5a-c in the Supporting Information. The SEM images show the sample retains its original surface morphology with a very slight removal of unbounded particles confirming better structural distribution and well adhesion of VS 4 nanostructures on the CC substrate.
In order to examine the purity and proper distribution of the material on CC, the energy-dispersive X-ray spectroscopy (EDX) and elemental mapping images were carried out, as shown in Figure 4a,b. The EDX spectrum showed that the material consists of vanadium (V), sulfur (S), oxygen (O), and carbon (C), representing the successful growth of VS 4 on CC after the hydrothermal process. As shown in Figure 4bi,ii, the corresponding EDX discrete element mapping reveals the signals of V, S, and C elements in the VS 4 -CC@VS-3 sample, representing the presence of VS 4 and C from the CC substrate. Moreover, the V and S components are uniformly distributed throughout the CC substrate without any visible impurities. These EDX mapping results confirmed that the VS 4 nanostructures were strongly bonded on the surface of the CC. The morphology of the VS 4 flower-like nanostructures was further examined using a transmission electron microscope (TEM) analysis, as given in Figure 4ci,ii. The TEM (low and high magnification) images show the VS 4 sample showed nearly aggregated nanospikes introducing the form of closed pack flower-like morphology with size ranges from ≈200-300 nm. Furthermore, this kind of self-assembly vertically aggregated nanospikes arrays permit the electrolyte ions efficiently and activate the entire material. The selected area electron diffraction (SAED) pattern for VS 4 nanostructure was displayed in Figure S6 in the Supporting Information. The blended SAED pattern proposed the low crystalline nature of VS 4 nanostructure. The ring patterns can be determined to the (031) [30] Moreover, [EMIM][OTf ] electrolyte for VS 4 -CC@VS-3-based SCs can effectually improve the operating voltage (2 V) with a significant increase in energy density. Additionally, the pseudocapacitive nature of VS 4 -CC@ VS-3 electrode in an IL electrolyte depends upon the intercalation/deintercalation process of [EMIM] + cation. [16b,c] Initially, the cell voltage of the supercapacitor was optimized by conducting CV and GCD measurements for various operating voltage using the VS 4 -CC@VS-3//VS 4 -CC@VS-3 SSC. The CVs of VS 4 -CC@VS-3//VS 4 -CC@VS-3 SSC tested for various potential ranges from 1 to 2 V at a scan rate of 25 mV s −1 are shown in Figure S8a in the Supporting Information. Similarly, Figure S8b in the Supporting Information shows the GCD curves of the fabricated SSC under various potential ranges from 1 to 2 V at a constant current density of 4 mA cm −2 . Prominently, the curves verified that the SSC showed slightly tilted triangular profile with good capacitance performance without any evaluation. From the resultant CVs and GCD curves, the optimum operating voltage of the SSCs was fixed to 2 V for further electrochemical studies. Figure 5b shows the CVs of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 based SSC measured at a constant scan rate 5 mV s −1 . Comparing CVs of the three devices, the VS 4 -CC@VS-3//VS 4 -CC@VS-3 device reveals greater background current with a couple of broad redox peaks and displays a quasi-rectangular profile, which signifies good pseudocapacitance property of the electrode. Moreover, the CV of VS 4 -CC@VS-3 SSC displays two sets of redox peaks, where the oxidation peaks at +1.32 and +1.85 V attributes the oxidation of V 4+ to V 5+ and (S 2 ) 2− to S 2− and the corresponding reduction peaks observed at +0.62 and +1.15 V ascribed to the consecutive reduction of V 5+ to V 4+ and S 2− to (S 2 ) 2− respectively. [8] Meanwhile, the similar sets of redox peaks were not observed in the CVs of VS 4 -CC@VS-1, and VS 4 -CC@ VS-2 devices, but they exhibited broad redox peaks with the irregular CVs. Thus, the incomplete growths of VS 4 on CC electrodes have an insufficient electrochemical active site for performing redox reactions. The high peak current values of the corresponding VS 4 -CC@VS-3 device is mainly attributed to the dense growth of VS 4 flower-like nanostructures on the CC, which adequately increase the electroactive sites of the electrode for better redox reactions with electrolyte ions. [18c] The CVs of VS 4 -CC@VS-3, VS 4 -CC@VS-1, and VS 4 -CC@VS-2 full cells for different scan rates (5-100 mV s −1 ) are presented in Figure 5d and Figure S8c,d in the Supporting Information. From the CVs, the VS 4 -CC@VS-3 device showed high background current with better reversibility for all the measured scan rates compared to VS 4 -CC@VS-1 and VS 4 -CC@VS-2 devices. Furthermore, the electrochemical performance of the VS 4 -CC@ VS-3 SSCs samples synthesized under the two growth positions (i.e., vertical and horizontal) are presented in Figure S9 in the Supporting Information. Both CVs display excellent capacitive performance with quasi-rectangular profile with strong redox Adv. Funct. Mater. 2020, 30,1906586   binder-free electrode decreases the internal resistance of the electrode due to the strong adhesion and dense growth architectures, which delivers rapid electron transportation for better energy storage performance.
The electrochemical performance of the devices was further test by galvanostatic charge-discharge (GCD) measurements. Figure 5c compares the GCD curves of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3-based SSCs at a constant charge/discharge current density of 1 mA cm −2 . The GCD profiles of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3-based SSCs exhibited nearly symmetric with slightly distorted triangular shapes, which ensure the better reversibility and pseudocapacitive nature of the electrode materials. [31] In addition, the GCD curve of VS 4 -CC@VS-3 device exhibited the highest discharge period, which highlights the better electrochemical properties compared to the other two devices. From the GCD discharge curve, the areal capacitance (C ac ) of the devices was calculated using Equation (5). The devices showed a maximum C ac of 119 mF cm −2 (VS 4 -CC@VS-1), 277 mF cm −2 (VS 4 -CC@VS-2), and 536 mF cm −2 (VS 4 -CC@VS-3), respectively. Furthermore, the GCD curves of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 SSC at various chargedischarge current densities from 1 to 10 mA cm −2 also reveal excellent charge storage performance of the VS 4 -CC@VS-3 device (Figure 5e; Figure S10a,b, Supporting Information). Besides, the GCD curves show a decreasing trend of discharge time with increasing current density. Generally, under high current density, electrolyte ions have been stored on the outer surface of active material, which led to the poor redox process of the electrode. While at low current density, the charge storage ions utilize the bulk of the active material and provide a large number of electrolytic ions to enable the faster diffusion process. [32] Figure 5f depicts the variation of areal capacitance of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 SSCs for various current densities. Among these, the VS 4 -CC@VS-3 device shown high areal capacitances of 536, 452, 335, 254, 194, and 149 mF cm −2 (specific capacitances (C sc ) 206,173,135, 106, 82, and 68 F g −1 , calculated from Equation (4)) for 1, 2, 4, 6, 8, and 10 mA cm −2 , respectively. This again confirms the dense growth of VS 4 nanoflowers on CC surface can offer a large interaction area for the fast diffusion of electrolyte ions between the active material/electrolyte interfaces. Moreover, the fabricated SSC with VS 4 -CC@VS-3 electrodes replicates high areal/ specific capacitance values compared to previously reported metal sulfide/oxide nanostructures in various electrolytes and potential windows, as presented in Table 1. The table confirmed that the areal capacitance and specific capacitance values of the fabricated VS 4 -CC@VS-3 SSC device are comparable or even higher than that of previously reported SCs.
Electrochemical impedance spectroscopy (EIS) is one of the most crucial tools to determine the electrochemical performance of energy storage devices. The Nyquist impedance plots of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 SSCs and their corresponding equivalent circuit with fit curves (fitted using Z-view software) are displayed in Figure 5g and Figure S11 in the Supporting Information. The fitted electric circuit parameters are tabulated in Table 2. The impedance plots consist of an inclined straight line and a compressed semi-circle in the scanned frequency range. The compressed semicircle at the high-frequency region is due to the combination of internal (R s ) and charge transfer resistance (R ct ) whereas the inclined Adv. Funct. Mater. 2020, 30,1906586   spike at lower frequency region corresponded to the Warburg resistance (W R ), respectively. Along with these resistances, two constant phase elements (CPE1 and CPE2) also included in the electric circuit for their nonideal capacitive behavior. [33] The internal resistance R s of the device was ascribed to the combination of ionic resistance of the electrolyte and contact resistance between the active material and the conductive medium. The R s value of VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 SSC devices was found to be 4.629, 4.14, and 3.765 Ω cm 2 . The R s values start decreases with increase in reaction solution concentration because the electroactive material VS 4 is densely grown over CC substrate that readily enhances the electrical conductivity, which in turn leads to making better charge transfer reaction at the electrode/electrolyte interfaces. [34] To corroborate this, an attempt was made to investigate the wettability of bare CC, VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 electrodes. But, all the four electrodes showed better wettability in ILs even though the R s value decreases with increase in the density of VS 4 on CC. Therefore the study was extended further to examine the absorption capability of electrodes after 10 s immersion in IL electrolyte and determined the mass difference of IL absorbed electrodes (Table S1, Supporting Information). The electrodes mass increases bare CC (0.00801 g) < VS 4 -CC@ VS-1 (0.00916 g) < VS 4 -CC@VS-2 (0.00972 g) < VS 4 -CC@VS-3 (0.01055 g) electrodes, respectively. Thus, the appropriate grown of VS 4 on CC surface improved the wettability of the electrode and subsequently facilitate more penetration and the mobility of the electrolyte ions through the entire electrode. [35] The semicircle that denoted R ct in the equivalent circuit might be due to various factors such as electronic conductivity, arrangements of interparticle in electrode and electrode surface features etc. The R ct values for VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@ VS-3 SSC devices are 5.675, 3.8, and 3.25 Ω cm 2 , respectively. Consequently, the lower value of R ct designates the rapid charge transfer rate of ions in the electrolyte, which is indicating the better redox reaction properties of the electrode. [36] The capacitive elements of the VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 SSC devices are divided into CPEl and CPE2, which represents the double-layer capacitance and pseudocapacitance of the symmetric supercapacitors. [37] In addition, the Warburg resistance (W R ) is associated with ionic diffusion over the electrode/electrolyte interfaces. The W R value of the VS 4 -CC@VS-3 is smaller than VS 4 -CC@VS-1 and VS 4 -CC@ VS-2 SSCs owing to the higher growth of VS 4 on CC substrate, which in turn improve the diffusion and transport path of ions in the electrode surface. [38] Figure 5h shows the Bode plots of the VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 SSC devices and it reveals the phase angles values of −49°, −64°, and −72° at low-frequency region (10 mHz). The high phase angle value of the VS 4 -CC@VS-3 SSC again supports the excellent capacitance behavior than the other two devices. However, the phase angle value is comparatively lesser than that of an ideal capacitor (−90°) this deviation attributes the pseudocapacitive property of electrode materials. the fast diffusion of ions from electrolyte intensely interacts with the electrode and reaches the maximum capacitance with very fast recharging. [39] The most important parameters such as energy density (E d ) and power density (P d ) of the VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3-based SSCs were estimated and represented in Ragone plot (Figure 5i). The VS 4 -CC@VS-1 SSC device exhibits a maximum E d of 16.52 µWh cm −2 and P d of 9997 µW cm −2 , respectively. Correspondingly, VS 4 -CC@VS-2 SSC shows a maximum E d of 38.46 µWh cm −2 and P d of 10021 µW cm −2 . Meanwhile, the VS 4 -CC@VS-3 SSC device displays the highest E d of 74.4 µWh cm −2 (28.6 Wh kg −1 ) and P d of 10154 µW cm −2 (9340 W kg −1 ), respectively. The maximum energy and power densities of the VS 4 -CC@VS-3 SSC further reveal the excellent performance of the device. The energy and power density values of the present work were compared with previous reports and presented in Table 1. Figure 5j displays the electrochemical merits of the laminated VS 4 -CC@VS-3 nanoflower on CC for electrochemical energy storage. The compact growth of VS 4 nanoflower network on CC could be certified for the fast diffusion of electrolyte ions. From the result, the hierarchical nanostructure of VS 4 has shown nearly aggregated nanospikes, which can offer a short ionic interchange between the active material/electrolyte interface. Therefore, VS 4 -CC@ VS-3 allows a rapid motion of electrons through the charge/ discharge processes, which mainly improves areal capacitance (C ac ), high rate performance, and cycling stability, respectively.
Cycling stability is another factor, which determines the device practical suitability for real-time applications. Figure 6a displays the cyclic stability plot of the VS 4 -CC@VS-3 SSC device for the first consecutive 1000 GCD cycles at a current density of 10 mA cm −2 . As displayed in the figure that the SSC device represents better areal capacitance retention of ≈93% even after 1000 cycles. Furthermore, the stability of the VS 4 -CC@VS-3 SSC device had been examined through EIS of fresh and after cycling stability test. The corresponding Nyquist plots and fitted equivalent circuit are demonstrated in Figure 6b and Figure S11a in the Supporting Information. The corresponding fitted electric circuit parameters are listed in Table 3, and the results show no significant difference in the EIS parameters of the device before and after cyclic stability test. Although, the slight increase in the R s values (3.765 to 3.826 Ω cm 2 ) after cyclic stability test, is mainly attributed to the scant poisoning of the ionic liquid electrolyte by consecutive charge/discharge cycles. Moreover, the R ct values slightly increases (3.25 to 4.515 Ω cm 2 ) after stability test, which designates that the VS 4 -CC@VS-3 electrodes are slightly damaged during the 1000 consecutive GCD cycles in the wide potential window of 2 V. To further confirm the stability and surface morphology of the electrode after cycling test, FE-SEM analysis was examined, as shown in Figure S12ai-iii in the Supporting Information. The obtained FE-SEM images of the VS 4 -CC@VS-3 SSC electrode after stability indicates that the electrode retains its self-assembled nanoflower structure with some slight fissure even after 1000 constant GCD cycles. This confirms the electrode has excellent stability in ionic liquid electrolyte even after 1000 GCD cycles. The real-time applications of the fabricated SSCs device were tested and depicted in Figure 6c,d. The figure shows the photo graphic images of the powered conventional portable devices and the schematic diagram of the two coin cell-type SSCs connected in series to increase the cell potential. After charging to <3.2 V within 20 s, the devices successfully powered a digital clock up to 10 min, as shown in Figure 6ei,ii. Furthermore, the high energy and power density of the SSCs energetically lighten up a three-point configuration torch light and five blue (2.2 V, 20 mA) LEDs for long time, as shown in Figure 6f,gi,ii. The aforementioned electrochemical properties with practical demonstration signify that the uniform growth of VS 4 -CC@VS-3 flexible electrodes paves a way for the development of other metal chalcogenides for energy storage applications.

Conclusion
In conclusion, hierarchically structured binder-free VS 4 nanostructures were successfully grown on the flexible CC under a controlled growth concentration using a facile hydrothermal method. The optimized growth of the sample with beneficial properties of like high electrochemical activity, good redox performance and binder-free novel electrode material put a key role in elevating the electrochemical performance. Particularly, the fabricated VS 4 -CC@VS-3-based SSC revealed the maximum areal and specific capacitance of 536 mF cm −2 and 206 F g −1 in ionic liquid electrolyte with a wide potential Adv. Funct. Mater. 2020, 30,1906586   window 2 V. Furthermore, the assembled SSC device demonstrated better electrochemical properties in terms of maximum energy density 74.4 µWh cm −2 (28.6 Wh kg −1 ) and power density 10154 µW cm −2 (9340 W kg −1 ) along with excellent cyclic retention of 93% after 1000 charge/discharge. Utilizing the maximum energy and power, the fabricated SSC device powered electronic gadgets like a digital clock and lightening LEDs, which demonstrate its potential capability for real-time applications. The improved energy storage performance with binder-free growth of metal chalcogenide-based electrodes may reveal better prospects for the development of high-performance electrochemical energy storage devices. Moreover, the study confirms 1-ethyl-3-methylimidazolium trifluoro methane sulfonate ([EMIM] [OTf ]) ionic liquid as a potential electrolyte for the metal sulfidebased symmetric device with excellent electrochemical performances. Thus, this work considerably encourages the research and utilization of ionic liquid-based electrolyte in metal oxide or metal sulfide supercapacitors for achieving high energy/power density device for future energy storage systems.

Experimental Methods
Chemicals: The starting materials VOSO 4 .xH 2 O and thioacetamide (C 2 H 5 NS) were purchased from Aldrich Chemicals, the carbon fiber cloth (CC) was purchased from Foshan Energetic Film Co., Ltd., (China), nitric acid and ethanol were obtained from Sigma Aldrich Chemicals, the 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EmIm][OTf ]) ionic liquid (IL) was purchased from Tokyo Chemical Industry co., Ltd., (Japan). All the above chemicals were of analytical grade and used without further purifications.
Growth or Deposition of Patronite (VS 4 ) Nanostructures on Carbon Fiber Cloth (CC) Substrate: The binder-free growth of VS 4 -CC@VS-1, VS 4 -CC@ VS-2, and VS 4 -CC@VS-3 electrodes were performed via a facile one-step hydrothermal method. Initially, the carbon cloth substrates (CCs) were pretreated by dipping into the concentrated nitric acid at 70 °C and washed with deionized (DI) water several times to achieve neutral pH and dried at 90 °C for 12 h in a vacuum oven. The growth solution was prepared by dissolving calculated amounts of VOSO 4 .xH 2 O, C 2 H 5 NS, and CH 3 COOH (complexing agent) chemicals in DI water and stirred for 1 h at room temperature (RT). These reagents with suitable portions were mixed in a glass beaker having 50 mL of DI water under constant magnetic stirring until form a homogeneous blue solution at RT. Meanwhile, the pretreated CCs were horizontally attached on a glass slide using Teflon tape for better mechanical support and controlled growth over appropriate surface of the CC. [40] The active portion 1 × 1 cm 2 area of CC was left unmasked for material growth and the remaining area was tightly covered by Teflon tape as shown Figure S1 in the Supporting Information. Then the CCs attached glass slide is vertically immersed in the growth solution until complete absorption of the solution. Subsequently, the reaction solution with CC was transferred into a Teflon-lined stainless-steel autoclave and maintained at 160 °C in a muffle furnace for 24 h. After completion of the hydrothermal reaction, the autoclave was naturally cool down to RT. Finally, the obtained dark black VS 4 grown CCs substrates were washed several times with deionized (DI) water and dried for overnight at 80 °C in a vacuum oven. The samples synthesized with various growth solution concentration are labeled as VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 corresponding to the mixture proportions xVS, 2×VS, and 4×VS (x denote the quantity of precursors), respectively. The mass loading of VS4 on CC was determined to be ≈0.53, ≈0.90, and ≈1.30 mg cm −2 for VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 respectively.
Structural Characterization: X-ray diffraction patterns of the electrodes were collected (XRD, D/max-2400, Rigaku, Ultima IV) using Cu Kα source operated at 40 kV and 30 mA in the 2θ range 10-60°. XPS measurement was performed by the Thermo Electron MultiLab2000 and pattern was collected using a Al Kα radiation. The morphology of the synthesized nanostructures was observed using field emission scanning electron microscopy (FESEM) and elemental mapping of the fabricated electrodes was performed using Hitachi-S4800 at an accelerating voltage of 3 kV. Transmission electron microscopy (TEM) images of VS3@CC sample was recorded utilizing JEOL model JEM-2100F (Japan). For the TEM analysis, the VS 4 -CC@VS-3 sample was cut into small pieces and dispersed in distilled water under deep ultrasonication. The aqueous solution was then drop cast on the TEM grid, dried and recorded the images Electrochemical Measurements: The electrochemical properties of the synthesized VS 4 -CC@VS-1, VS 4 -CC@VS-2, and VS 4 -CC@VS-3 electrodes were investigated using ZIVE-SP2 electrochemical workstation (Korea) at RT. Before the experiment, the VS 4 -CC electrodes were submerged in the electrolyte solution for 12 h under vacuum. The cyclic voltammetry of the VS 4 -CC@VS-3 electrodes was initially studied in a conventional three-electrode system, with the VS 4 -CC@VS-3 as the working electrode, a platinum wire as the counter electrode and Ag/AgCl as the reference electrode in 1 m 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf ]) ionic liquid (IL) in acetonitrile solution. The symmetric supercapacitors were fabricated utilizing a stainless-steel split test cell (EQ-STC) from MTI Korea Ltd. The symmetric supercapacitor was designed by nearly similar weight of two as-prepared VS 4 -CC electrodes assembled face to face by sandwiching a filter paper separator socked in ILs electrolyte. Cyclic voltammetry (CV) was tested for different scan rates from 5 to 100 mV s −1 and the galvanostatic charge/discharge analysis was performed for various current densities (1, 2, 4, 6, 8, and 10 A g −1 ) at a prolonged potential window of 2 V. EIS was characterized in the frequency range between 0.01 Hz-100 kHz at 0 V bias condition with an AC perturbation of 10 mV.
The specific capacitance (C sc ) and areal capacitance (C ac ) of the symmetric device was calculated by using a charge/discharge curves using Equations (4) and (5) (5) where I is the constant discharge current (mA), Δt is the discharge time (s), m is the mass of the one electrode (g), a is the total active area of the electrodes (cm 2 ), and ΔV is the potential window (V). The specific energy and energy density (E d ) and the power density (P d ) of the SSC were obtained from the Equations (6-8)

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.