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Rational Design of Hierarchical Titanium Nitride@Vanadium Pentoxide Core-shell Heterostructure Fibrous Electrodes for High-performance 1.6 V Non-polarity Wearable Supercapacitors Jiabin Guo, Qichong Zhang, Qiulong Li, Juan Sun, Chaowei Li, Bing He, Zhenyu Zhou, Liyan Xie, Ming-Xing Li, and Yagang Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11997 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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Rational

ACS Applied Materials & Interfaces

Design

of

Hierarchical

Titanium

Nitride@Vanadium

Pentoxide

Core-shell

Heterostructure Fibrous Electrodes for High-performance 1.6 V Non-polarity Wearable Supercapacitors Jiabin Guo+a,b,c, Qichong Zhang+a,b, Qiulong Li+a,b, Juan Suna,b, Chaowei Lia,b, Bing Hea,b, Zhenyu Zhoua,b, Liyan Xiea,b, Mingxing Lic, Yagang Yaoa,b,* a. Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, Joint Key Laboratory of Functional Nanomaterials and Devices, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China b. Division of Nanomaterials, Suzhou Institute of Nano-Tech and Nano-Bionics, Nanchang, Chinese Academy of Sciences, Nanchang 330200, P. R. China c. College of Sciences, Shanghai University, Shanghai 200444, P. R. China [*] Email: [email protected] [+] These authors contribute equally to this work.

Keywords: Fiber-shaped asymmetric supercapacitors; Non-polarity; Core-shell structure; Titanium Nitride; Vanadium Pentoxide; Wearable electronics.

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ABSTRACT Extensive progress has been made in fiber-shaped asymmetric supercapacitors (FASCs) for portable and wearable electronics. However, positive and negative electrodes must be distinguished and low energy densities are crucial challenge and thus limit their practical applications. This paper reports an efficient method to directly grow TiN nanowire arrays@V2O5 nanosheets core-shell heterostructures on carbon nanotube fibers as non-polarity electrodes. Benefiting from their unique heterostructure, single electrodes possess high specific capacitances of 195.1 F cm-3 and 230.7 F cm-3 as positive and negative electrodes, respectively. Furthermore, all-solid-state non-polarity FASC devices with maximum voltage of 1.6 V were successfully fabricated. Our devices achieve outstanding specific capacitance of 74.25 F cm-3 and remarkable energy density of 26.42 mWh cm-3. More importantly, their electrochemical performance changed negligibly whatever charge-discharge process is in positive or negative directions, indicating excellent non-polarity. Therefore, this high-performance non-polarity FASCs paves the way to next-generation wearable energy storage devices.


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Introduction As one kind of emerging energy storage devices for portable and wearable electronics, fibershaped supercapacitors have aroused enormous attention in virtue of outstanding power density, remarkable weavability, light weight and long cycling life.1-13 However, their unsatisfactory energy density hindered their large-scale applications. In view of the formula E = 1/2 CV2, improving C (the specific capacitance) and expanding V (the operating voltage) are two feasible routes for enhancing E (the energy density). Therefore, to set up fiber-shaped asymmetric supercapacitors (FASCs) have been considered as an extremely expedient route to extend operating voltage and corresponding increase energy density via associating two different materials that possess nonoverlapping parts of potential windows in the same electrolyte.14-34 Despite this, there is still another major technical issue that limited FASCs’ practical applications. During use, FASCs should be indifferent whether it is charged in a positive or negative direction, but most reported FASCs are unable to realize this goal because of the distinct potential windows of positive and negative electrodes. If there is a kind of materials with both positive and negative potential windows but different mechanisms of charge storage, we can called the supercapacitors which have both positive and negative electrodes using this material the non-polarity asymmetric supercapacitors. Recently, studies of non-polarity planar asymmetric supercapacitors have been carried out. Jiang et al. reported a novel non-polarity electrode with nickel nanoparticle@carbon nanotube network films by CVD method and obtained a 1.8 V assembled device with the volumetric energy density of 1.39 mWh cm-3.35 By growing a V2O3@carbon nanosheet array directly on the current collector of 10 μm ultrathin Ti film, Zhu et al. fabricated an ultrathin flexible non-polarity 2 V supercapacitor device with volumetric energy density of 15.9 mWh cm-3.36 These works indicate non-polarity supercapacitors an excellent candidate for next-generation energy storage device. However, the low specific capacitance



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that results from the single-component electrode materials remains a barrier for the application of highenergy-density non-polarity FASCs. To construct the high-performance non-polarity configuration, the employed electrode materials should be simultaneously suitable for both positive and negative potential window ranges in the same electrolyte. As a kind of transition metal nitrides, titanium nitride (TiN), which has both positive and negative operating potential windows in the neutral electrolyte, possesses a remarkable electronic conductivity but its capacitance is relatively low. Vanadium pentoxide (V2O5) is one of the most promising electrode materials that holds great potential because of its inexpensive cost, multiple oxidation states and facile synthesis. It is also applicable for both positive and negative operating potential windows in the neutral electrolyte. However, the insufficient electronic conductivity makes charge transport difficult. To make full use of the advantages of these two materials, an efficient way to construct high-performance electrodes is to synthesize hierarchical heterostructures composed of superior electronic conducting substances served as the core and high-specific-capacitance materials as the shell. Compared with the single-component electrode of core materials, the core-shell heterostructures can sufficiently enhance capacitance. Simultaneously, for the shell materials, the coreshell heterostructure offers a scaffold to significantly increase the mass loading and possesses better electronic conductivity. In this work, we adopted the TiN@V2O5 core-shell heterostructure synthesized on carbon nanotube fibers (CNTFs) as both positive and negative fibrous electrode to fabricate a highperformance non-polarity FASC, where the aligned TiN nanowires arrays (NWAs) acted as core and V2O5 nanosheets (NSs) served as shell. On account of the bipolarity of as-fabricated TiN@V2O5 coreshell heterostructure, the non-polarity FASC was successfully assembled which had 1.6 V maximum working voltage. Benefiting from intriguing structure of electrode materials features, the assembled non-polarity FASC device possesses an outstanding specific capacitance of 74.25 F cm-3 (222.75 mF 4 ACS Paragon Plus Environment

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cm-2) and an excellent energy density of 26.42 mWh cm-3 (79.27 μWh cm-2). After bending 4000 times, 92.7% of capacitance retention can be still obtained due to its remarkable flexibility. In addition, whatever charged by positive or negative direction, the performance of the device exhibited almost no change, indicating the excellent non-polarity property. Results and Discussion The TiN NWAs@V2O5 NSs core-shell heterostructures grown on a CNTF were prepared by a facile two-step method consisting of a hydrothermal method and a solvothermal process both followed by an annealing treatment, which is schematically shown in Figure 1a. First, CNTFs treated in O2 plasma served as the current collector and substrate. Then, TiN NWAs was directly grown on the CNTFs through the hydrothermal method and heated in NH3 at 800 oC to form the TiN NWAs. Finally, V2O5 NSs were grown on the CNTFs directly by solvothermal procedure and subsequent annealing treatment in Ar at 320 oC to get the TiN NWAs@V2O5 NSs core-shell heterostructure. As exhibited by the scanning electron microscope (SEM) photographs (Figure 1b, c and S1), the TiN NWAs and the TiN NWAs@V2O5 NSs core-shell heterostructure are both synthesized into a uniform and well-aligned one-dimensional structure. Compared with the glossy surface of the TiN NWAs, the wrinkled V2O5 NSs in the core-shell heterostructure make the electrode possess more abundant surface area in contact with electrolyte, resulting in faster and easier ion diffusion. To further characterize the core-shell configuration, transmission electron microscope (TEM) test was performed, which is revealed in Figure 1d. It can be plainly observed that a stick of synthesized NW possesses the classic heterostructure composed of a TiN NW core scaffold and randomly oriented V2O5 NSs shell. Figure 1e provides the high-magnification TEM image of V2O5 NSs, where the fringes of crystalline lattice can be obviously viewed. The 0.29 nm inter-planar distance corresponds to the (301) crystallographic plane for V2O5, (JCPDS card No. 41-1426), verifying the successful preparation of crystallized V2O5 NSs. The energydispersive X-ray spectrometry (EDX) mapping images (Figure 1f) further displays the elemental 5 ACS Paragon Plus Environment


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distribution of the TiN NWAs@V2O5 NSs core-shell heterostructure. The definite core-shell heterostructure was again confirmed: the elements Ti and N are shown in core area, while the elements V and O distribute on shell potion.


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Figure 1. (a) The sketch map of the manufacturing procedure of the TiN NWAs@V2O5 NSs core-shell heterostructure on CNTF. (b) The SEM photograph of the TiN NWAs on the CNTF. (c) The SEM image of the TiN NWAs@V2O5 NSs core-shell heterostructure on a CNTF. (d) The low-magnification TEM photograph of a sole TiN@V2O5 nanowire. (e) The high-resolution TEM of V2O5 NSs. (f) The TEM photograph and corresponding EDX mappings of the elements Ti, N, V and O in the TiN NWAs@V2O5 NSs core-shell heterostructure. To further obtain the details of the core-shell structures and compositions, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) testings were performed. The XRD patterns of the TiN NWAs@V2O5 NSs, bare TiN NWAs and single V2O5 NSs were analyzed for comparison, which is displayed in Figure 2a. The bare TiN NWAs possess the clear diffraction peaks centered at 36.7o, 42.6o and 61.8o, respectively corresponding to the (111), (200) and (220) lattice planes of the osbornite phase (JCPDS card No. 38-1420). The diffraction peaks of single V2O5 NSs centered at 15.3o, 20.3o, 21.7o, 26.1o, 31.0o, 32.4o and 34.3o correspond to (200), (001), (101), (110), (301), (011) and (310) crystallographic planes of the shcherbinaite phase (JCPDS card No. 41-1426), respectively. Expectedly, the XRD pattern of the TiN NWAs@V2O5 NSs consists of the major diffraction peaks of both the TiN and V2O5 but not the others, proving that the synthesized core-shell heterostructures just contain the compositions of TiN and V2O5. In addition, the XPS spectrum of CNTF@TiN NWAs@V2O5 NSs core-shell indicates that the major present elements are Ti, N, V, O and C (Figure S2). In Figure 2b-f, the survey high-resolution XPS spectra of the TiN NWAs@V2O5 NSs core-shell heterostructure are provided. As the charts show, the elements Ti with peaks of binding energy centered at 455.9 eV (2p3/2) and 461.5 eV (2p1/2) and N with the peak of binding energy centered at 396.7 eV (1s) both owe to the TiN NWAs. Simultaneously, stemming from the V2O5 NSs, the element V possesses peaks of binding energy centered at 417.5 eV (2p3/2) and 424.7 eV (2p1/2), while the element O has a peak of binding energy centered at 530.7 eV (1s). These results further confirmed the compositions of the TiN 8 ACS Paragon Plus Environment

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NWAs@V2O5 NSs core-shell heterostructure. Incidentally, the C element with peaks of binding energy centered at 284.8 eV and 286.2 eV comes from the CNTF and partly oxidized CNTF, respectively.

Figure 2. (a) XRD patterns of TiN, V2O5 and the TiN NWAs@V2O5 NSs core-shell heterostructure samples. (b-f) The high-resolution XPS spectra of the TiN NWAs@V2O5 NSs core-shell. To evaluate the electrochemical properties of the as-prepared non-polarity TiN NWAs@V2O5 NSs core-shell heterostructure fiber electrode, single electrode measurements in the positive and negative potential windows were performed in 3 M LiCl aqueous electrolyte, using the TiN NWAs@V2O5 NSs core-shell heterostructure, the Ag/AgCl and the Pt wire respectively served as the working, reference and counter electrodes. Figure 3 gives the performance of the as-prepared TiN NWAs@V2O5 NSs core-shell heterostructure electrode in the positive potential range. As is shown in Figure 3a, the representative cyclic voltammetry (CV) profiles of the TiN, V2O5 and TiN NWAs@V2O5 NSs coreshell heterostructure electrodes are contrasted with the equal scan rate of 25 mV s-1 within potential window between 0 and 0.8 V. As expected, the area of CV profile of the TiN NWAs@V2O5 NSs coreshell heterostructure is apparently larger than those of both TiN and V2O5, further affirming the 9 ACS Paragon Plus Environment


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electrochemical advantages of this core-shell heterostructure. CV curves within potential window between 0 and 0.8 V at different scan rates from 0.5 to 25 mV s-1 of the TiN NWAs@V2O5 NSs coreshell heterostructure electrode are presented in Figure 3b, where obvious redox peaks from all of the CV curves prove the pseudocapacitive property of the as-prepared core-shell electrode. The possible reaction processes can be described by the following equation: V2O5 + xM+ + xe- ↔ MxV2O5 (M+ = Li+, H+). The emblematic galvanostatic charge-discharge (GCD) profiles of the TiN NWAs@V2O5 NSs core-shell heterostructure electrode collecting with changing current densities between 0 and 0.8 V are also provided (Figure 3c). These curves are symmetric quasi-rectangles with a pair of change-discharge platforms perfectly corresponding to the CV curves, which manifests the excellent reversibility and again illustrates the pseudocapacitive behavior of the as-prepared core-shell electrode. The specific capacitances calculated by area and volume are displayed in Figure 3d from their corresponding discharge profiles. The as-prepared TiN NWAs@V2O5 NSs core-shell heterostructure electrode in the positive potential range exhibits an excellent specific capacitance of 195.1 F cm-3 (585.25 mF cm-2) at the current density of 2 mA cm-2 and retains 129.2 F cm-3 (387.5 mF cm-2) even at the high current density of 20 mA cm-2, demonstrating the preeminent rate capability.


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Figure 3. Electrochemical performance of non-polarity TiN NWAs@V2O5 NSs core-shell heterostructure fiber electrode in the positive potential range. (a) Comparison of CV profiles of the TiN, V2O5 and TiN NWAs@V2O5 NSs core-shell heterostructure electrodes at the scan rate of 25 mV s-1. (b) CV profiles of the TiN NWAs@V2O5 NSs core-shell heterostructure electrodes at distrinct scan rates. (c) GCD profiles of the TiN NWAs@V2O5 NSs core-shell heterostructure electrodes at different current densities. (d) Specific capacitances of the TiN NWAs@V2O5 NSs core-shell heterostructure electrodes computed from the corresponding discharge profiles by area and volume. In addition, the results of electrochemical measurements of the TiN NWAs@V2O5 NSs core-shell heterostructure electrode in the negative potential range in 3 M LiCl aqueous electrolyte are also displayed in Figure 4. Similarly, as presented in Figure 4a, the typical CV profiles of the TiN, V2O5 and TiN NWAs@V2O5 NSs core-shell heterostructure electrodes are contrasted with the constant scan rate 11 ACS Paragon Plus Environment


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of 25 mV s-1 within potential window between -0.8 and 0 V. The area of CV curve exhibits the semblable results that the area of core-shell electrode is observably larger than the other two singlecomponent electrode materials, once more showing the superiority of the core-shell heterostructure. Figure 4b provides the CV profiles of the TiN NWAs@V2O5 NSs core-shell heterostructure electrode within potential window between -0.8 and 0 V at changing scan rates from 1 to 50 mV s-1. Due to the synergy of TiN and V2O5, no distinct redox peaks can be seen since the corporate effect of pseudocapacitive and electric double layer capacitance. GCD curves of TiN NWAs@V2O5 NSs coreshell heterostructure electrode with changing current densities from 2 to 20 mA cm-2 between -0.8 and 0 V are shown in Figure 4c. The triangle-like patterns also illustrate coefficient effects of electrode materials and the reversible charge-discharge course. Figure 4d gives the calculated specific capacitances by area and volume stemming from the relevant discharge profiles. An outstanding specific capacitance of 230.7 F cm-3 (692 mF cm-2) at the current density of 2 mA cm-2 and 118.3 F cm3

(355 mF cm-2) retention even at high current density of 20 mA cm-2 have been realized, which

indicates the good rate capability.


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Figure 4. Electrochemical performance of non-polarity TiN NWAs@V2O5 NSs core-shell heterostructure fiber electrode in the negative potential range. (a) Comparison of CV profiles of the TiN, V2O5 and TiN NWAs@V2O5 NSs core-shell heterostructure electrodes at the scan rate of 25 mV s-1. (b) CV profiles of the TiN NWAs@V2O5 NSs core-shell heterostructure electrodes at distinct scan rates. (c) GCD profiles of the TiN NWAs@V2O5 NSs core-shell heterostructure electrodes at different current densities. (d) Specific capacitances of the TiN NWAs@V2O5 NSs core-shell heterostructure electrodes computed from the relevant discharge profiles by area and volume. To further evaluate the practical applications of the as-prepared TiN NWAs@V2O5 NSs core-shell heterostructure electrode materials in high-performance FASCs, we fabricated an all-solid-state FASCs, which is schematically shown in Figure S3, by employing the same two TiN NWAs@V2O5 NSs coreshell heterostructure fibrous electrode coated by gel electrolyte of LiCl polyvinyl alcohol (PVA) 13 ACS Paragon Plus Environment


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through twisting and performed the electrochemical measurements in the two-electrode system. As Figure 5a shows, CV profiles of the as-assembled FASC device within varied working voltages from 0.8 to 1.6 V with the constant 25 mV s-1 scan rate all have similar rectangle-like shapes, illustrating that the working voltage of our FASC device can achieve 1.6 V. Moreover, Figure 5b gives GCD profiles that were obtained in different voltages with the same current density of 1 A cm-3. We can clearly observe that the charge-discharge curves are all asymmetrically triangular, which indicates the remarkable capacitive behavior and well corresponds to the CV curves. The calculated volumetric energy densities of the FASC device in varied working voltages based on corresponding discharge profiles are given by Figure 5c. With the operating voltage rising from 0.8 to 1.6 V, a huge leap of volumetric energy density was achieved with its increasing from 5.76 to 26.42 mWh cm -3. The CV profiles of the assembled FASC device at changing scan rates from 5 to 100 mV s-1 with 1.6 V operating voltage all exhibit quasi-rectangular shapes (Figure 5d). These patterns with no noticeable redox peaks demonstrate that the assembled FASC device possessed good reversibility and desirable capacitive behaviors. Furthermore, GCD test with 1.6 V working voltage was performed at changing current densitie from 1 to 10 A cm-3, which is displayed in Figure 5e. The symmetric and nearly linear curves again confirm the splendid capacitive properties of assembled FASC device. The specific capacitances by area and volume of assembled FASC device from the corresponding discharge curves were calculated and displayed in Figure 5f. A prominent specific capacitance of 74.25 F cm -3 (222.75 F cm-2) was achieved and 37.5 F cm-3 (112.5 F cm-2) was maintained at the current density of ten times, manifesting the good rate capability. Ragone plots of energy density versus power density computed by volume of the FASC devices are exhibited in Figure 5g. Taking advantages of this smart core-shell heterostructure, our as-assembled FASC device possesses a max energy density of 26.42 mWh cm-3 at the power density of 800 mW cm-3 computed by volume. It is extraordinarily superior to those of the anteriorly published high-performance FASC devices such as Co3O4/Ni wire//rGO/C fiber,21 14 ACS Paragon Plus Environment

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CuO@LDH NWAs/Cu//Cu/AC,28 graphene NSs@TiN/C fiber//graphene NSs@Fe2N C fiber,25 CuO@AuPd@MnO2/Cu

wire//Fe2O3@C/C

fiber

wire,26

Ni0.25Mn0.75O@C//AC32

and

MnO2/PEDOT:PSS/CNTF//ordered microporous C/CNTF37. In addition, 89.8% of the capacitance was retained after the GCD cycling process (Figure S4-S8) at 1 A cm-3 for 4000 cycles, illustrating that our FASC device has an outstanding cycling performance. The Nyquist plot of the prepared FASC device is also obtained with frequencies from 10-5 to 102 Hz that are provided in Figure S9 and S10. The equivalent series resistance of 79.9 Ω is noted in high-frequency region and nearly erect line at lowfrequency region further displays the capacitive properties of the assembled device. More importantly, the non-polarity test was performed by directly charging and discharging the assembled device within operating voltage from -1.6 to 1.6 V, which is given by Figure 5h. GCD curves in both positive and negative directions with negligible changes strongly demonstrate the outstanding practicability of this core-shell heterostructure for non-polarity FASC. Simultaneously, mechanical flexibility measurements were carried out due to the significance of flexibility for evaluating portable and wearable electronics (Figure 5i). No apparent varieties in CV profiles at 50 mV s-1 with changed bend angles and the capacitance retention of 92.7% maintained (Figure S11) after bending 90° for 4000 cycles indicate the noticeable mechanical flexibility and constancy of our non-polarity FASC device.



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Figure 5. (a) CV profiles of our FASC device at the immobile scan rate of 25 mV s-1 in varied working voltages. (b) GCD profiles of assembled FASC device obtained with the same current density of 1 A cm-3 within varied voltages from 0.8 to 1.6 V. (c) Energy densities computed from the corresponding discharge profiles by volume. (d) CV profiles of our FASC device with 1.6 V working voltage by the changed scan rates between 5 and 100 mV s-1. (e) GCD profiles with varied current densities in 1.6 V working voltage. (f) Specific capacitances of our FASC device computed from the corresponding discharge curves by volume and area. (g) The Ragone plots of volumetric energy densities versus power densities of the assembled and anteriorly published high-performance FASC devices. (h) The GCD curves in both positive and negative directions at 1 A cm-3. (i) CV profiles of our FASC device obtained with the 100 mV s-1 scan rate under changed bending angles. Conclusions 16 ACS Paragon Plus Environment

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In summary, the TiN NWAs@V2O5 NSs core-shell heterostructures on CNTF have been developed through a facile two-step method containing the hydrothermal method and the solvothermal procedure both followed by post annealing treatment as both positive and negative fibrous electrode for high-performance non-polarity FASCs. Attributed to the smart core-shell heterostructure, the asprepared TiN NWAs@V2O5 NSs core-shell heterostructure fibrous electrodes exhibit high specific capacitances of 195.1 F cm-3 (585.25 mF cm-2) in the positive and 230.7 F cm-3 (692 mF cm-2) in the negative potential range with 2 mA cm-2, respectively. After twisting the same two TiN NWAs@V2O5 NSs core-shell heterostructure fibrous electrodes coated by gel electrolyte of LiCl/PVA, the FASC device was successfully prepared with 1.6 V maximum working voltage. These assembled FASC devices possess an extraordinary specific capacitance of 74.25 F cm-3 (222.75 mF cm-2) and a prominent energy density of 26.42 mWh cm-3 (79.27 μWh cm-2). Significantly, it shows no visible changes whether charging and discharging in positive or negative directions, proving the adopted coreshell heterostructure a promising non-polarity electrode material. Meanwhile, the as-assembled FASC device has remarkable flexibility for capacitance retention of 92.7% when bending 90° for 4000 times. Thus, this paper have developed a feasible method to establish high-performance non-polarity wearable FASCs for practical applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51522211, 51602339, 51703241 and U1710122), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDB-SSW-SLH031), the Thousand Youth Talents Plan, the Postdoctoral Foundation of China (Nos. 2016M601905 and 2017M621855), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20160399), and the Science and Technology Project of Nanchang (2017-SJSYS-008).



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Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: . Materials and Characterizations; Experimental; Electrochemical Performance Calculations; Other measurements of the CNTF@TiN NWAs@V2O5 NSs core-shell electrode material. These materials are available free of charge via the internet at http://pubs.acs.org.

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