Facile Synthesis of Na-Doped MnO2 Nanosheets on Carbon

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Functional Inorganic Materials and Devices

Facile Synthesis of Na-Doped MnO2 Nanosheets on Carbon Nanotube Fibers for Ultrahigh-Energy-Density All-Solid-State Wearable Asymmetric Supercapacitors Qijun Zong, Qichong Zhang, Xue Mei, Qiulong Li, Zhenyu Zhou, Dong Li, Mingyuan Chen, Feiyang Shi, Juan Sun, Yagang Yao, and Zengxing Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12486 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Applied Materials & Interfaces

Facile Synthesis of Na-Doped MnO2 Nanosheets on Carbon Nanotube Fibers for Ultrahigh-Energy-Density All-Solid-State Wearable Asymmetric Supercapacitors Qijun Zong,a,§ Qichong Zhang,a,b,§ Xue Mei,a Qiulong Li,b Zhenyu Zhou,b Dong Li,a Mingyuan Chen,a Feiyang Shi,a Juan Sun,b Yagang Yaob* and Zengxing Zhanga,c* aShanghai

Key Laboratory of Special Artificial Microstructure Materials and Technology, School of

Physics Science and Engineering, Tongji University, Shanghai 200092, China bDivision

of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, CAS Center

for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China cState

Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai

200433, China §These

authors contributed equally to this work.

E-mail: [*] [email protected] [email protected]

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Abstract Flexible fiber-shaped supercapacitors hold promising potential in the area of portable and wearable electronics. Unfortunately, their general application is hindered by the restricted energy densities due to low operating voltage and small specific. Herein, an all-solid-state fiber-shaped asymmetric supercapacitor (FASC) possessing ultrahigh energy density is reported, in which the positive electrode was designed as Na-doped MnO2 nanosheets on carbon nanotube fibers (CNTFs) and the negative electrode as MoS2 nanosheets coated CNTFs. Owing to the excellent properties of the designed electrodes, our FASCs exhibit a large operating potential window (0-2.2 V), a remarkable specific capacitance (265.4 mF/cm2) as well as an ultrahigh energy density (178.4 µWh/cm2). Moreover, the devices are of outstanding mechanical flexibility.

KEYWORDS: wearable device, asymmetric supercapacitor, carbon nanotube fiber, Na-doped MnO2, MoS2

Introduction Fiber-shaped supercapacitors (FSCs) have attracted a huge amount of attention, and were utilized as effective energy storage devices because of their extended cycling life, high power density, rapid charge/discharge

capability

and

excellent

flexibility.1-6 Compared

with

the

conventional

supercapacitors (SCs), the FSCs hold promising applications in future portable electronics, this is due 2


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to the FSCs’ tiny volume, ultralight weight, excellent flexibility as well as easy wearability.7,8 Recently, carbon nanotube fibers (CNTFs) were selected as the fibrous electrodes to construct FSCs because of their excellent mechanical flexibility, exceptional conductivity and light weight.9-14 These CNTFs based FSCs exhibited outstanding properties and have become very valuable in this field. Although they have shown great potential, the major challenge of FSCs resides in their ability to enhance the energy density and specific capacitance whilst maintaining their life cycle and power density.15-17 In accordance with energy density E=1/2CV2, we can determine that the specific energy (E) of SCs may be enhanced through extending the specific capacitance (C) or operating voltage (V). Furthermore, besides the specific capacitance (C), elevating the operating voltage (V) is considered an alternative approach to enhance the energy density. Along this line, asymmetric supercapacitors (ASCs) are often employed to construct high-operating-voltage supercapacitances for that the operating windows could take full advantage of the positive and negative electrodes in the same electrolyte.18-21 However, the operating voltage of the previous published aq. ASCs is often in the range of 1.4-2.0 V.22-27 So far, it is still challenging to extend their operating voltage beyond 2 V, which is greatly dependent on the design of appropriate positive and negative electrodes with large oxygen evolution over potential and hydrogen evolution over potential, respectively. Whereas carbon-based materials depend on their electrical double layers (EDLs) for the purpose of store energy, pseudocapacitive active structures possess high energy density and specific capacitance due to interfacial reversible Faradaic reactions.28,29 During the hunt for high-performance pseudocapacitive active materials, CoO, Co3O4, NiO, Ni(OH)2, Co(OH)2, as well as other transition-metal oxides were thoroughly examined as positive electrodes for supercapacitors (SCs).30-35 3



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Among these positive electrode materials, MnO2 has attracted great attention due to their outstanding theoretical specific capacitance (1370 F g-1), high oxygen evolution potential, low cost, and its abundance in nature.36 However, the low conductivity of MnO2 commonly gives a slow electron transfer, which limits the electrochemical performance of the prepared SCs.37-40 Several groups found that the preinsertion of cations (such as Na+ and K+) into MnO2 can improve its conductivity and thus enhance the supercapacitive performance.41-44 So far, cation-preinserted MnO2 has been primarily used for planar electrodes.41-44 In comparison with the fiber-shaped electrodes, the planar electrodes are relatively large in size and not easily integrated into fabrics, making it difficult to exploit the full advantages of the flexible energy storage devices. Based on the above consideration, herein, an effective and facile procedure was developed to synthesize thin Na-doped MnO2 nanosheet arrays on CNTFs as a binder-free positive electrode. The thin nanosheets afford a large surface area for the electrode and the preinsertion of Na+cations enables the electrode conductive to deliver a large specific capacitance, leading to a significantly large potential window extended up to 0-1.2 V. Moreover, by choosing flower-like thin MoS2 nanosheet arrays on the CNTFs as the negative electrode,45 a high-performance fiber-shaped asymmetric supercapacitor (FASC) was accomplished possessing a max. operating voltage as high as 2.2 V, incredible 265.4 mF/cm2 specific capacitance, ultrahigh energy density (178.4 µWh/cm2) and outstanding flexibility (120o bending, over 3500 times). Furthermore, in order for high-performance fiber-shaped energy storage devices to be developed for future applications as portable and wearable electronics, the design described above is paramount to its success.

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Experimental section Preparation of the FASCs. Na-MnO2@CNTFs. Utilizing a hydrothermal method MnO2 nanosheets were produced on the CNTFs. The procedure used is as follows: KMnO4 (0.1579 g) and concentrated HCl (0.5 mL) were added to H2O (40 mL) with an additional 10 min of stirring, giving a homogeneous solution. A teflon-lined stainless steel autoclave (50 mL inner volume) was charged with the solution. For CNTFs (typical length: 5 cm, Figure S1a), they were first pretreated for 5 min using O2 plasma, followed by immersion in concentrated HCl for another 5 min. After being carefully washed with deionized (DI) water, they were placed in the above-prepared aqueous solution. After which it was placed in an autoclave for 30 min at 85 °C. The CNTFs were cooled to room temperature, washed with distilled water, which was repeated numerous times, and then dried in vacuum at 60 °C overnight. Finally, they were electrochemically oxidized using CV scans (0-1.2 V) (vs Ag/AgCl) in the saturated Na2SO4 electrolyte, this was conducted for 500 cycles. MoS2@CNTFs. The synthesis of MoS2 nanosheets on the CNTFs utilized a one-step approach, as previously reported by Zhang, et al.46 Typically, CNTFs (typical length: 5 cm, Figure S1a) were first treated by O2 plasma for 5 min at 150 W. Thiourea (0.4565 g) and ammonium molybdate tetrahydrate (0.519 g) were dissolved in DI water (45 mL) and stirred until a homogenous solution was obtained. The CNTFs as well as the solution were removed in an autoclave, and kept at 200 °C for a period of 9 h. The obtained compounds then cooled to room temperature and washed with DI water followed by ethanol, repeatedly. They were then dried overnight at 60 °C under vacuum. FASCs assembly. To assemble FASCs, Na2SO4/PVA gel was used as the electrolyte. The gel was 5



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synthesized using Na2SO4 (10 g) and PVA (polyvinyl alcohol) (10 g) in distilled water (100 mL), which was stirred vigorously at 95°C for 2 h resulting in a transparent solution. Both of the Na-MnO2@CNTF (5 cm in length) and MoS2@CNTF (5 cm in length) were placed into the Na2SO4/PVA gel electrolyte for 15 min and dried at 60 °C for 3 h. Both compounds were then twisted together and dried overnight until they were solidified. Typically, after the above procedure, FASCs were obtained in ca. 3 cm in length (Figure S1b).

Results and discussion Na-MnO2@CNTF-based positive electrodes

Figure 1. Na-MnO2@CNTFs. (a) and (b) SEM images of the Na-MnO2 nanosheets on the CNTFs. (c) TEM image of the Na-MnO2 nanosheets. (d) Typical HRTEM image of the Na-MnO2 nanosheets. (e) to (g) EDX mappings of Mn, Na and O of the Na-MnO2 nanosheets in the area as indicated in Figure (c).

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Generally, thin δ-MnO2 nanosheet arrays can be produced on the CNTFs utilizing the hydrothermal method as detailed above (Figure S2). The electrochemical oxidizing process can successfully insert the Na+ into the MnO2 without changing the morphology of the nanosheets. Figures 1a&b show scanning electron microscopy (SEM) images of the final products on the CNTFs, indicating that a layer of thin nanosheet arrays are coated on the CNTFs. Figure 1c shows a transmission electron microscopy (TEM) image, and Figure 1d is a typical high-resolution TEM (HRTEM) image. The HRTEM image shows lattice fringes, in which the interlunar spacing is 0.26 nm, which is consistent with the (-111) of δ-MnO2, indicating that the obtained nanosheets are crystalline. We utilized energy dispersive X-ray (EDX) spectrometry to characterize the component. As shown in Figure S3, the EDX spectrum shows that the products mainly include C, O, Mn and Na, in which the C should originate from the CNTFs. This also proves that the nanosheets are Na-MnO2. EDX mapping results of the area indicated in Figure 1c are shown in Figure 1e-g, showing that Mn, Na and O are uniformly dispersed in the products. Furthermore, X-ray photoelectron spectroscopy (XPS) was utilized for analysis of the products before and after electrochemical oxidation (Figure S4). O1s and Mn2p are clearly present in the XPS spectrum of the products before electrochemical oxidation. Their binding energy is ca. 529.9 eV and 642.4 eV, respectively, further proving that the nanosheets consist of MnO2. After the electrochemical oxidation, Na is introduced. Its binding energy is ca. 1071.4 eV (Figure S4b, Na1s). High resolution XPS spectra show that O (70.2%), Mn (22.3%) and Na (7.5%) in atoms (Figure S4c-e). Here part of the O should originate from the oxidized CNTFs. It suggests the successful intercalation of the Na+ into the MnO2 to obtain the Na-MnO2 via the simple electro-oxidation method.

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Figure 2. Electrochemical characterization of the Na-MnO2@CNTFs positive electrode. (a) The electrode’s CV curves at various potential windows of 0-0.8, 0-1.0, and 0-1.2 V at 20 mV/s. (b) GCD curves obtained for various voltages, between 0.8-1.2 V, current density 3 mA/cm2. CV curves at various (c) scanning rates and (d) GCD current densities. (e) Areal specific capacitance of the MnO2@CNTFs and the Na-MnO2@CNTF electrode as a function of the current density, respectively. (f) Na-MnO2@CNTFs Nyquist plots. Insert: Enlarged area in high frequencies and equivalent circuit of the Nyquist plots.

The electrochemical measurement of the synthesized Na-MnO2@CNTFs was conducted with a three-electrode system and 3M Na2SO4 aq. as the electrolyte. To evaluate Na-MnO2@CNTFs electrochemical behavior, cyclic voltammetry (CV) was first used to examine the materials; scanning rate 20 mV/s, using variable potential windows (0-0.8, 0-1.0 and 0-1.2 V). Figure 2a shows the obtained CV curves. As shown in the results, regardless of the potential windows, the CV curves 8


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showed a couple of distinctive redox peaks at ca. 0.45 V for the anodic scanning and 0.62 V for the cathodic scanning, respectively, which originates from the reversible redox reaction of the Mn3+/Mn4+ along with the insertion/extraction of the Na+.44 Notably, the potential windows maintain a good quadrilateral shape even with a broad scanning range (0-1.2 V), which is significantly greater than MnO2@CNTFs (0-0.8 V) (Figure S5).47-49 This indicates that the insertion of the Na+ can increase the charging voltage dramatically. It gives the possibility to assemble SCs possessing high energy density. The Na-MnO2@CNTFs galvanostatic charge-discharge (GCD) curves, possessing a 3 mA/cm2 current density, are shown in Figure 2b, and show a similar behavior with increasing and decreasing voltage. The symmetry is well maintained even with the operating voltage up to 1.2 V, indicating that our products exhibit ideal capacitance characteristics. Na-MnO2@CNTFs CV curves at different scanning rates are displayed in Figure 2c. The CV curves hold ideal rectangular shapes, indicating that the electrodes should have excellent reversibility and rate capability. Figure 2d shows Na-MnO2@CNTFs charge/discharge curves operated from 0-1.2 V with various current densities from 1-10 mA /cm2. Possessing a 1 mA/cm2 current density, Na-MnO2@CNTFs electrodes specific capacitance can reach up to 743.3 mF/cm2, which is far beyond that of MnO2@CNTFs produced in a similar method (590.7 mF/cm2, Figure S6). These results are also significantly higher than the previous reported ones for MnO2-based electrodes.50-53 Moreover, the Na-MnO2@CNTF electrodes also demonstrate a better rate performance than the MnO2@CNTF electrode (Figure 2e). Besides, the reasonable symmetry of the GCD curves in Figures 2b&d further indicates that the electrodes should be of excellent reversibility. While the current density varied (1-10 mA/cm2), the specific capacitance of the Na-MnO2@CNTFs remained above 70% (Figure 2e), also indicating that the electrodes should be 9



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of good rate capability. The charge-discharge cycling stability of the Na-MnO2@CNTFs and MnO2@CNTFs is shown in Figures S7-S9 at 2 mA/cm2 current density. After 5000 cycles, ca. 7.9% capacitance loss of the Na-MnO2@CNTFs was observed (Figure S7), which is higher than that of MnO2@CNTFs (Figure S8&9). In order to examine the conductibility, electrochemical impedance spectroscopy (EIS) was employed. Figure 2f shows the Nyquist plots of the Na-MnO2@CNTFs. The steep profile of the Na-MnO2@CNTFs represents an ideal SCs behavior. The insert shows the equivalent circuit fitted to the Nyquist plots, consisting of the double layer capacitance (CDL), series resistance (RS), Warburg behavior (W) and charge transfer resistance (RCT).54 RS is the intercept value, which is located on the real axis, and is attributed to the electrolyte’s resistance as well as the electrode materials. The enlarged image of the Nyquist plots (Figure 2f insert) indicates that the RS of the Na-MnO2@CNTFs is ca. 8.7 Ω . The small RS should be due to CNTFs excellent conductivity. In accordance with the GCD curves at different current densities (Figure 2d), we further calculated the internal resistance by plotting the values of initial voltage drop (IR) as a function of current density (Figure S10). The results show that the internal resistance is ca. 8.7 Ω. The small resistance further indicates that they can act as potential SCs in the application of positive electrodes.

MoS2@CNTF-based negative electrodes

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Figure 3. MoS2@CNTFs. (a) and (b) SEM images of the MoS2 nanosheets on the CNTFs. (c) TEM image with its EDX mappings of the S and Mo. (d) HRTEM image of the MoS2 nanosheets.

Figures 3a&b displays the SEM images of CNTFs containing MoS2 nanosheets. As shown in the images, the flower-like thin nanosheets are uniformly coated onto CNTFs surface. The EDX, XPS, Raman and XRD results confirm that the produced nanosheets are hexagonal-phase MoS2 (Figure S11-S14). MoS2 nanosheets TEM image, which are flower-like in appearance, is shown in Figure 3c. The inserted EDX mappings in Figure 3c clearly demonstrate the existence of the S and Mo elements that are evenly dispersed throughout the MoS2 nanosheets. Figure 3d shows a typical HRTEM image, in which the clear lattice fringes are present with an interlunar, with 0.27 nm spacing that correlates with the (100) face of MoS2 crystal, indicating that the prepared nanosheets are crystalline.

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Figure 4. Electrochemical study of MoS2@CNTFs negative electrode. (a) CV curves of the pristine CNTFs and MoS2@CNTFs with a 30 mV/s scanning rate. (b) MoS2@CNTFs CV curves at different scanning rates. (c) MoS2@CNTFs GCD curves at various current densities. (d) MoS2@CNTFs areal specific capacitance at various current densities.

The electrochemical behavior of the as-prepared MoS2@CNTFs was studied using the three-electrode system, which was conducted in a 3M Na2SO4 aqueous solution, where the MoS2@CNTFs are the working electrode, the Ag/AgCl acts as the reference electrode and Pt wire is the counter electrode. The CV curves of pure CNTFs and MoS2@CNTFs with a scanning rate of 30 mV/s and a voltage from -1 to 0 V are shown in Figure 4a. The results exhibit that the CV curve area of the MoS2@CNTFs is much greater than pure CNTFs, indicating that MoS2 coated CNTFs can significantly enhance the capacitive property of the CNTFs. Figure 4b shows MoS2@CNTFs CV 12


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curves at various scanning rates, which the potential window is -1-0 V. All of the windows display a quasi-rectangular shape in the absence of intense redox peaks, suggesting common pseudocapacitive performance. Figure 4c shows MoS2@CNTFs GCD curves and present nearly symmetric characteristic, further highlighting their pseudocapacitive properties. In the case of areal specific capacitances, they were derived from the GCD curves (Figure 4d). It can be clearly observed that MoS2@CNTFs displays a reasonable performance. Whereas, when 1 mA/cm2 current density is reached the observed specific capacitance can project up to 490 mF/cm2. Notably, MoS2@CNTFs capacitance still remains 93.3% of the initial capacitance even as the current density increases to 10 mA/cm2, indicating that MoS2@CNTFs possess good rate capability. Moreover, the charge-discharge cycling stability of the MoS2@CNTFs was also examined (Figure S15), showing somewhat good cycling stability with an ca. 7.8% capacitance loss post 5000 cycles.

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FASCs based on assembled Na-MnO2@CNTFs and MoS2@CNTFs

Figure 5. Electrochemical study of the FASCs. (a) A detailed illustration of the FASCs. (b) CV curves of the separated Na-MnO2@CNTFs and MoS2@CNTFs electrodes in different potential windows, respectively. Scanning rate: 20 mV/s. (c) CV curves of the FASCs possessing various operating voltages and a 25 mV/s constant scanning rate. (d) GCD curves of the FASCs with different charge/discharge voltages from 1.2 to 2.2 V at 2 mA/cm2 current density. (e) Derived energy density and areal specific capacitance generated from GCD curves at different potential windows with 2 mA/cm2.

Figure 5a shows a schematic illustration of the FASCs, which consists of three main parts; these include the positive (Na-MnO2@CNTFs) and negative (MoS2@CNTFs) electrode, and the 14


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Na2SO4/PVA gel electrolyte. After the simple process described above, the all-solid-state FASCs can be assembled for further characterization. In order to estimate the prepared FASCs total voltage, the CV curves were first measured for the Na-MnO2@CNTFs and the MoS2@CNTFs in a 3M Na2SO4 at a scanning rate of 20 mV/s, respectively. Figure 5b shows the potential windows of Na-MnO2@CNTFs and MoS2@CNTFs are 0-1.2 V and -1-0 V, respectively. Therefore, we anticipated that the max. operating voltage may be as high as 2.2 V for our as-assembled devices. Figure 5c shows the CV profiles of the fabricated devices, with a 25 mV/s constant scanning rate at various operating potential windows. The CV curves maintain reasonable quasi-rectangular shapes, devoid of redox peaks even with the operating potential window at 2.2 V. In addition, at 2 mA/cm2 current density, the GCD curves exhibit favorable symmetry even with the operating potential up to 2.2 V (Figure 5d). The results indicate that the FASCs have an ideal capacitive characteristic with low equivalent series resistance and rapid current-voltage response. We further calculated the energy densities and areal specific capacitances from the GCD (Figure 5e). Notably, the areal specific capacitance of our FASCs increases dramatically from 174.2 mF/cm2 to 205.6 mF/cm2 and the energy density is significantly enhanced from 34.8 µWh/cm2 to 138.2 µWh/cm2 with the potential window gradually changed from 1.2 to 2.2 V.

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Figure 6. Analysis of the FASCs. (a) FASCs CV curves at various scanning rates 0-2.2 V. (b) GCD curves with various current densities 1-10 mA/cm2. (c) Areal specific capacitance as a function of the current density. (d) FASCs power density and areal energy compared to previous FSCs. (e) CV curves of the FASCs under various bending angles. (f) Capacitance retention of the FASCs possessing bending angles of 120° for 3500 cycles.

Based on the above results, we further focused on examining FASCs possessing an operating potential window between 0-2.2 V. The CV curves with different scanning rate from 5-100 mV/s are presented in Figure 6a. The entire CV curves shows quasi-rectangular shapes devoid of redox peaks, exhibiting the FASCs are of fast charge/discharge properties and exceptional capacitive behaviors. Figure 6b shows the GCD curves with varied current density (1-10 mA/cm2). Even when the current density is as high as 10 mA/cm2, the GCD curve still keeps relatively good symmetry, further indicating that the FASCs have excellent capacitive properties and outstanding charge-discharge reversibility. The areal specific capacitance dependent on the current density is displayed in Figure 6c, 16


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which is calculated from the GCD curves in Figure 6b. It can be seen from Figure 6c that a large specific capacitance of 265.4 mF/cm2 is achieved at a 1 mA/cm2 current density. The capacitance remains at about 154.1 mF/cm2 with the current density up to 10 mA/cm2. This indicates that the rate capability of the assembled FASCs is not as excellent as the composed positive and negative electrode (Figure 2e and Figure 4d). Two reasons should cause the rate capability decrease of the assembled FASCs. The first is that all-solid electrolytes have poor electrical conductivity that leads to a high resistance of the device. The second is that our work uses a twisting structure that results in a relatively small effective contact area for the positive and negative electrodes. To resolve this problem, a coaxial structure is used to assemble the device, which can increase the effective contact area of the positive and negative electrodes. This requires further investigation. The Ragone plots of the prepared FASCs power density and areal energy and previously obtained results are shown in Figure 6d. Due to the prolonged 2.2 V operating voltage as well as high specific capacitance, the FASCs we prepared show an incredible energy density (178.4 µWh/cm2) at 1100.9 µW/cm2 power density. Whilst at 11032.5 µW/cm2 power density, the energy density still remains ca. 103.6 µWh/cm2. Values such as these are much higher compared with those previously published for FSCs, i.e. coaxial ultrathin MnO2 nanosheet/carbon fiber (1.428 µWh/cm2, 51.4 µW/cm2),55 MnO2/graphene yarn (8.2 µWh/cm2, 930 µW/cm2),56 MnO2/carbon nanotube (36.4 µWh/cm2, 780 µW/cm2),57 3DCS@P/3DCS (30.92 µWh/cm2, 1780 µW/cm2),58 all-carbon coaxial fiber (9.8 µWh/cm2, 189.4 µW/cm2),59 and MnO2-modified nanoporous gold wire with coaxial structure (5.4 µWh/cm2, 284 µW/cm2).60 FSCs mechanical property is an important characteristic. An examination of the FASC device’s electrochemical stability and mechanical flexibility was conducted by performing a bending test. 17



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Figure 6e demonstrates the CV curves at 25 mV/s scanning rate in which the FASCs are bent at various angles from 0° to 180°. It is clear that there are no obvious variations while the devices are bent in different angles, indicating that they are of excellent flexibility. Besides, the FASCs also exhibit a remarkable bending stability. As shown in Figure 6f, 92.4% of the capacitance still remained with the device bending at 120° for 3500 cycles. We also tested the charge-discharge cycling stability of the FASCs at 2 mA/cm2 current density and the result is presented in Figure S16. After 5000 cycles, ca. 10% of the capacitance is lost, exhibiting a good cycling stability. More interestingly, as shown in Figure S17, the assembled FASCs can be used to light a LED lamp, indicating that the devices should be potential in real applications. Conclusion In summary, through inserting Na+ into the MnO2 nanosheets, we have successfully produced a high-performance fiber-shaped Na-MnO2@CNTFs positive electrode with a 743.3 mF/cm2 specific capacitance and a broad potential window (0-1.2 V), which was further assembled with the excellent MoS2@CNTFs negative electrode (specific capacitance: 490 mF/cm2) for FASCs. Owing to the excellent properties of the assembled electrodes, our FASCs show an incredible 265.4 mF/cm2 specific capacitance and ultrahigh energy density of 178.4 µWh/cm2. Besides, FASCs are of excellent flexibility with the capacitance being remained 92.4% after bending 3500 times. And they can keep outstanding stability at various angles. Thus, our FASCs are promising high-performance energy storage devices applicable to the succeeding-generation of portable and wearable electronics.

■ASSOCIATED CONTENT 18


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Characterization method; FASC performance and surface area calculation method; additional SEM images, XRD, XPS, Raman and EDX spectra; charge-discharge cycling stability results; additional CV and GCD curves; photo images of the CNTFs and FASCs; photo image of the FASC-powered LED. ■AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Z.X.) [email protected] (Y. Y.) Author Contributions §These

authors contributed equally to this work

Notes The authors declare no competing financial interest. ■ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of Shanghai (16ZR1439400, 17ZR1447700), and National Natural Science Foundation of China (51522211, 51528203, 51602339).

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Facile Synthesis of Na-Doped MnO2 Nanosheets on Carbon

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