Achieving ultrahigh-energy-density in flexible and lightweight all-solid-state internal asymmetric tandem 6.6 V all-in-one supercapacitors

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Achieving ultrahigh-energy-density in flexible and lightweight all-solid-state internal asymmetric tandem 6.6 V all-in-one supercapacitors Zhenyu Zhou a, b, c, 1, Qiulong Li a, 1, Liqian Yuan b, c, 1, Lei Tang b, Xiaona Wang b, Bing He b, Ping Man b, Chaowei Li b, Liyan Xie b, Weibang Lu b, Lei Wei d, Qichong Zhang d, **, Yagang Yao a, b, * a National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Jiangsu Key Laboratory of Artificial Functional Materials, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, 210093, China b Division of Advanced Nanomaterials, Key Laboratory of Nano Devices 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, China c Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, 215123, China d School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

A R T I C L E I N F O

A B S T R A C T

Keywords: Core-shell nanostructure Psedocapacitive material High voltage Flexible Internal asymmetric tandem supercapacitors

Internal asymmetric tandem supercapacitors with wide working voltage have drawn an increasing attention to develop high-energy-density supercapacitors. However, the small specific capacitance and low working voltage of single-supercapacitor restrict further improvement of their energy density. A rational solution to this restriction would be to synthesize high-performance electrode materials. Accordingly, this work specifies a simple and costeffective method to directly grow manganese dioxide and vanadium nitrogen nanosheets on zeolitic imidazolate framework-67 derived N-doped carbon conductive skeletons. These well-designed core-shell pseudocapacitive materials integrate the features of large specific surface area, rich reaction sites, high mass loading, short electron/ion diffusion paths and remarkable conductivity, affording prominent electrochemical performance. Furthermore, a flexible all-solid-state internal asymmetric tandem 6.6 V all-in-one supercapacitor was successfully assembled by matching as-fabricated cathode and anode materials as well as using carbon nanotube film as a lightweight current collector. The resulting all-in-one devices exhibited a high specific capacitance of 336.7 mF/ cm2 (19.6 F/cm3) and an exceptional energy density of 2032.8 μWh/cm2 (118.2 mWh/cm3) and thus substantially outperform most previously reported state-of-the-art asymmetric supercapacitors. Our work provides a promising strategy for the rational construction of high-performance, inexpensive and safe all-in-one supercapacitors for next-generation portable and wearable electronic devices.

1. Introduction As efficient energy storage and supply devices for the flourishing portable and wearable electronic product, flexible all-solid-state supercapacitors have drawn an increasingly significant attention due to their high power density, fast charge-discharge rate, outstanding flexibility, long cycle life and superior safety [1–13]. Nevertheless, the inadequate energy density of these devices, arising from their low operating voltages and small specific capacitance, seriously hampers their further

application in various energy-consuming devices. Internal tandem supercapacitors exhibit extended voltage windows and thus increased energy density and thus may be a possible solution [14–17]. However, it should be noted that the overall electrochemical performance of internal tandem supercapacitors relies on that of a single supercapacitor. Moreover, to date, internal tandem supercapacitors have been fabricated only from metal foils, which leads to high mass and poor flexibility. Thus, it is imperative to develop high-voltage and lightweight all-solid-state single-supercapacitors with high specific capacitance and superior

* Corresponding author. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Jiangsu Key Laboratory of Artificial Functional Materials, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, 210093, China. ** Corresponding author. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Yao). 1 These authors contribute equally to this work. https://doi.org/10.1016/j.ensm.2019.09.002 Received 3 July 2019; Received in revised form 17 August 2019; Accepted 1 September 2019 Available online xxxx 2405-8297/© 2019 Published by Elsevier B.V.

Please cite this article as: Z. Zhou et al., Achieving ultrahigh-energy-density in flexible and lightweight all-solid-state internal asymmetric tandem 6.6 V all-in-one supercapacitors, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.09.002

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Fig. 1. (a) SEM image of N–C. (b) SEM image of N–C@MnO2. (c) Higher magnification TEM image of N–C@MnO2. (d) Low magnification TEM image and EDX mappings of different elements of Mn, O, C and N recorded from an individual N–C@MnO2. (e–h) XPS survey scans of (e) Mn2p, (f) C1s, (g) O1s, and (h) N1s regions for the N–C@MnO2.

flexibility for use in internal tandem supercapacitors with a high energy density. Extensive research has been devoted to the fabrication of hybrid or asymmetric supercapacitors to effectively improve single-supercapacitor energy density [17–23]. However, despite the improved energy density demonstrated by hybrid devices as a result of their different charge-storage mechanisms and enlarged operating voltage in aqueous electrolytes, the low specific capacitance of the carbon-based anode materials widely used in the construction of these devices has seriously limited further enhancement of their energy density. Efforts have therefore focused on the development of high-capacity pseudocapacitive materials such as transition metal oxides, sulfides, and nitrides [24–28]. These materials have shown higher specific capacitance and energy densities at least an order of magnitude higher than those of carbon-based materials [29–32]. However, a mass of dormant reactive sites on these transition metal materials, caused by inferior conductivity and insufficient ionic diffusion rates, results in a large difference between their specific capacitance and the level that they should theoretically be capable of attaining. To activate these potentially active sites, intensive efforts have been devoted to the construction of hierarchical core shell structures with abundant surface area and short ion-diffusion paths [27, 29,33,34]. Metal-organic frameworks (MOFs) have been used as sacrificial templates to derive N-doped porous nanocarbons that consist of millions of nitrogen to carbon bonds. Due to the different distribution of electron cloud density of N and C atoms as well as the inheriting porous structure of MOFs, this skeleton will performs high conductivity and

large specific surface area, which can serve as effective charge-transfer centers for loading sufficient pseudocapacitive materials [35–42]. Such charge-transfer centers not only bridge the gap between the actual and potentially achievable specific capacitance and superior rate capability of pseudocapacitive materials, they also balance the electrochemical performance and the maximal mass loading by generating a net “charge center-pseudocapacitive material” structure. In this work, a zeolitic imidazolate framework-67(ZIF-67)–derived Ndoped carbon skeleton (N–C) was used as a conductive center for rational synthesis of the core-shell structures N-doped carbon skeleton@manganese dioxide(N–C@MnO2) and N-doped carbon skeleton@vanadium nitrogen(N–C@VN). Due to their superior core-shell structures, carbon nanotube film supported N–C@MnO2 and carbon nanotube film/N-doped carbon skeleton@vanadium nitrogen (CTF/ N–C@VN) electrodes exhibit remarkable capacitances of 1483.5 mF/cm2 and 1478.4 mF/cm2 at a current density of 2 mA/cm2 and outstanding rate capability with capacitance retentions of 76.2% and 72.6% at a current density of 20 mA/cm2 and high mass loading of 3.39 and 2.24 mg/cm2. Furthermore, a high-performance flexible and lightweigh all-solid-state internal asymmetric tandem all-in-one supercapacitor with maximum operating potential of 6.6 V was assembled by matching CTF/ N–C@MnO2 cathode and CTF/N–C@VN anode. The resulting device achieves a high specific capacitance of 336.7 mF/cm2 (19.6 F/cm3) and an exceptional energy density of 2032.8 μWh/cm2 (118.2 mWh/cm3). Besides, our as-assembled device can be bended up to 6000 cycles with 97.2% retention of the initial specific capacitance.

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Fig. 2. (a) Comparison of CV curves of CTF/N–C, CTF/MnO2, CTF@MnO2 and CTF/N–C@MnO2 electrodes measured at a scan rate of 50 mV/s. (b) Comparison of GCD curves of CTF/N–C, CTF/MnO2, CTF@MnO2 and CTF/N–C@MnO2 electrodes obtained at a current density of 2 mA/cm2 (c) CV curves of the CTF/ N–C@MnO2electrode at different scan rates. (d) GCD curves of the CTF/N–C@MnO2 electrode at different current densities. (e) Areal specific capacitance of the CTF/ N–C@MnO2 electrode calculated from the GCD curves as a function of current density. (f) Nyquist plots of the CTF/MnO2, CTF@MnO2 and CTF/N–C@MnO2 electrodes. (g) Comparision of the details conferring areal capacitance (mF/cm2), gravimetric capacitance (F/g), mass loading (mg/cm2), coulombic efficiency (%) and ten folds rate capability ((C10/C0)%) of the CTF/MnO2, CTF@MnO2 and CTF/N–C@MnO2 electrodes.

2. Experimental section

acid solution and stirred for 5 h, the same procedures of sample collection and drying were again performed.

2.1. Synthesis of N–C framework 2.2. Synthesis of CTF/N–C@MnO2

To obtain ZIF-67, a typical process, 400 mL methanol solution containing 32 mmol Co(NO3)2⋅6H2O and 400 mL methanol solution containing 96 mmol 2-methylimidazole were mixed together under magnetic stirring. After 18 h of reaction at the room temperature, the resulting precipitates were collected by vacuum filtration, washed with ethanol several times, and finally dried in vacuum at 60  C for 10 h. Then the powers were annealed into N, Co-doped carbon skeleton(N–C–Co) at 800  C in Ar for 2 h with a heating rate of 2  C/min. Furthermore, to remove the Co, the N–C–Co powers were moved into 6 M hydrochloric

To obtain the N–C@MnO2, the N–C granules were wrapped in copper mesh (500 meshes), then immersed in a deionized water solution (45 mL) containing 0.23 g of KMnO4 and 0.75 mL of concentrated hydrochloric acid, which had been stirred for 10 min prior to transfer into a Teflonlined stainless steel autoclave (50 mL inner volume). The autoclave was then sealed and maintained at 85  C in a vacuum oven for 35 min. After cooing to room temperature, the N–C@MnO2 were collected and dried with same procedures of as-prepared ZIF-67. The N–C@MnO2 slurry was 3

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Fig. 3. (a–b) SEM image of N–C@VN and a single one. (c) Higher magnification TEM image of N–C@VN. (d) Low magnification TEM image and EDX mappings of different elements of V, N and C recorded from an individual N–C@VN. (e) XRD spectrum of the N–C and N–C@VN. (f–h) XPS survey scans of (f) V2p, (g) N1s, and (h) C1s regions for the N–C@VN.

3, 5 and 7 were spread with N–C@VN slurry. After being dried at 60  C for 3 h under vacuum, the six sides including 2–7 were coated with gel electrolyte and heaped together layer by layer showing in Fig. 5a. Finally, the integrated device was dried at 60  C under vacuum until it was solidified.

prepared by dispersing the synthesized N–C@MnO2 material (70 wt %), acetylene carbon black (20 wt %) and PVDF (10 wt %) in N-Methyl pyrrolidone, and further it was coated on the CTF. 2.3. Synthesis of CTF/N–C@VN

3. Results and discussion

Typically, 0.3 mL vanadium oxytriisopropoxide (VOT) was dissolved in 45 mL of isopropanol alcohol (IPA) and stirred for 10 min to form a homogeneous solution. The solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave, and the N–C particles wrapped in copper mesh (500 meshes) were immersed in the solution. After a hydrothermal reaction for 10 h at 200  C, the resulting N–C@VOx NSs were rinsed in ethanol and dried at 60  C overnight under vacuum. At last, the N–C@VOx wrere annealed in ammonia at 600  C for 2 h to obtain the N–C@VN. The preparation procedure of CTF/N–C@VN is same with CTF/N–C@MnO2.

Figs. S1 and 1 show the synthesis of N–C@MnO2 and its relevant chemical and structural characteristics. Specifically, ZIF-67 (Fig. S1a) is fabricated via a mild solution method, followed by being annealed under 800  C to form N–C–Co (Fig. S1b), and the N–C is finally gained after wiping out the Co impurity (Fig. S1c and Fig. 1a). A thick layer of MnO2 is then densely germinated on the surface of the N–C to form core-shell N–C@MnO2 (Fig. 1b). This structure imparts the required synergistic properties of high conductivity to the core and superior electrochemical capacity to the shell. The X-ray diffraction (XRD) spectra of N–C@MnO2 and N–C are displayed in Fig. S2; both (002) lattice plane peaks can be attributed to the N–C framework, and the former N–C@MnO2 diffraction peaks can be assigned to α-MnO2 (JCPDS File No. 44–0141). The highresolution transmission electron microscopy (TEM) image of N–C@MnO2 in Fig. 1c shows that the two types of lattice fringes have spacings of 0.27 and 0.208 nm, which correspond to the (411) and the (211) planes of α-MnO2. The energy-dispersive spectroscopy (EDS) mapping results of N–C@MnO2 in Fig. 1d confirm the uniform distribution of the elements Mn, O, C, and N in N–C@MnO2. The low-

2.4. Assembly of internal asymmetric tandem all-in-one supercapacitor The gel electrolyte was prepared by adding 12.7 g of LiCl and 10 g of polyvinyl alcohol (PVA) into 100 mL of distilled water under quick stirring at 95  C for 2 h until the gel became transparent. Four pieces of carbon nanotube film (CTF) with the size of 1.5 cm  3 cm were treated in O2 plasma for 5 min at 150 W. As shown in Fig. 5a, we defined eight sides of the four pieces of CTF as 1–8 respectively. We firstly evenly coated N–C@MnO2 slurry onto the surfaces of 2, 4 and 6, then the sides of 4

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Fig. 4. (a) Comparison of CV curves of CTF/N–C, CTF/VN, CTF@VN and CTF/N–C@VN electrodes measured at a scan rate of 50 mV/s. (b) Comparison of GCD curves of CTF/N–C, CTF/VN, CTF@VN and CTF/N–C@VN electrodes obtained at a current density of 2 mA/cm2 (c) CV curves of the CTF/N–C@VN electrode at different scan rates. (d) GCD curves of the CTF/N–C@VN electrode at different current densities. (e) Areal specific capacitance of the CTF/N–C@VN electrode calculated from the GCD curves as a function of current density. (f) Nyquist plots of the CTF/VN, CTF@VN and CTF/N–C@VN electrodes. (g) Comparision of the details conferring areal capacitance (mF/cm2), gravimetric capacitance (F/g), mass loading (mg/cm2), coulombic efficiency (%) and ten folds rate capability ((C10/C0)%) of the CTF/VN, CTF@VN and CTF/N–C@VN electrodes.

divided into two peaks centered at 530.1 and 531.6 eV. The low-bindingenergy component at 530.1 eV is attributed to the O2 ions in MnO2, – C–O. Fig. 1h shows the XPS whereas the latter peak is assigned to O– spectrum of N 1s, displaying one peak centered at 399.8 eV, which can be attributed to the N–C. To demonstrate the impressive electrochemical performance of CTF electrode coated with N–C@MnO2 (CTF/N–C@MnO2), Fig. 2a shows a comparison of cyclic voltammogram (CV) curves that were obtained for equal-area electrodes comprising a N–C coating on CTF (CTF/N–C), directly growing MnO2 on CTF (CTF@MnO2), an MnO2 coating on the CTF (CTF/MnO2) and CTF/N–C@MnO2 at a scan rate of 50 mV/s. A photograph and an SEM image of CTF are presented in Fig. S4a, S4b and S4c, and SEM images of CTF@MnO2 and MnO2 powder are shown in

magnification TEM images of N–C@MnO2 in Fig. S1d and the distribution of C (Fig. 1d) illustrate the sharp dividing line that existed between core and shell, which indicated the presence of large nanosheets of α-MnO2. The chemical compositions and valence states of the N–C@MnO2 samples were further characterized by X-ray photoelectron spectroscopy (XPS). The full survey XPS spectra (Fig. S3) suggestes the presence of Mn, C, O, and N. As shown in Fig. 1e, the XPS spectrum of Mn 2p, the peaks at 642.9 and 654.4 eV can be assigned to the Mn 2p3/2 and Mn 2p1/2 spin-orbit states, respectively. The spin energy separation between these two peaks is 11.5 eV, which matches well with that of standard MnO2. The deconvoluted peak of the C 1s spectrum in Fig. 1f is resolved into two component, centered at 284.7 and 285.7eV, which can be assigned to N–C and C–O. The O 1s spectrum presented in Fig. 1g is

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Fig. 5. (a) Schematic of theinternal asymmetric tandem all-in-one supercapacitor. (b) CV curves of the all-in-one supercapacitor measured at different operating voltages at a constant scan rate of 50 mV/s (c) GCD curves of the as-fabricated all-in-one supercapacitor collected over different voltages from 2.2 to 6.6 V at a current density of 2 mA/cm2.

rectangular, indicating the ideal capacitive behavior, excellent reversibility, and fast charge/discharge capability of our CTF/N–C@MnO2 electrode. The superior electrochemical performance of the CTF/ N–C@MnO2 electrode can be further verified by the nearly symmetric triangular GCD curves (Fig. 2d), which show that no obvious IR drop occurred, even at a current density of 20 mA/cm2. The rate capability curve of the CTF/N–C@MnO2 electrode is presented in Fig. 2e. The CTF/ N–C@MnO2 electrode achieved a maximum areal specific capacitance of 1483.5 mF/cm2 at a discharge current density of 2 mA/cm2 and maintained an outstanding rate capability of 76.2% even at a high discharge current of 20 mA/cm2. Figs. S5 and S6 present the CV and GCD curves of CTF/MnO2 and CTF@MnO2, respectively. Fig. S5a shows that the CV curve of the CTF/MnO2 electrode exhibited obvious polarization, indicating the poor reversibility and inferior ion-diffusion rate of this electrode. Furthermore, the inferior electrochemical behavior of the CTF/ MnO2 electrode was also demonstrated by the asymmetric triangular GCD curves in Fig. S5b. Although the CV curves of the CTF@MnO2 electrode in Fig. S6a were all rectangular and its GCD curves were triangular (Fig. S6b), the GCD curves also showed the very limited CV area and discharge time of this electrode, which reflect its low mass loading of MnO2. The impedances of these three electrodes were also measured, and the resulting electrochemical impedance spectroscopy

Figs. S4g and S4h. The CV area of the CTF/N–C electrode was much smaller than that of the other electrodes, which indicates that N–C contributed little to the specific capacitance of the CTF/N–C@MnO2 electrode. The CV areas of the CTF/MnO2 and CTF@MnO2 electrodes were not particularly different from each other, although the CV shape of CTF/MnO2 displayed evidence of polarization, which suggests that greater interface impedance was present in the CTF/MnO2 electrode. Impressively, the CTF/N–C@MnO2 electrode not only had a considerably larger CV area than those of CTF/MnO2 and CTF@MnO2, but the CV was also rectangular, illustrating the superior conductivity, impressive reversibility, and high mass loading of N–C@MnO2. Fig. 2b compares the galvanostatic charge-discharge (GCD) curves of the CTF/N–C, CTF/ MnO2, CTF@MnO2, and CTF/N–C@MnO2 electrodes at a current density of 2 mA/cm2. The CTF/N–C@MnO2 electrode had the longest discharge time, and the CTF/N–C had the shortest, which further demonstrates the low specific capacity of N–C. In terms of coulombic efficiency and internal resistance (IR), the GCD curves of CTF@MnO2 and CTF/ N–C@MnO2 showed good symmetry and little IR compared to that visible in the GCD curve of CTF/MnO2, which again indicates the superior conductivity of CTF/N–C@MnO2. Fig. 2c shows the CV curves of the CTF/N–C@MnO2 electrode in a potential window from 0 to 1 V at various scan rates between 10 and 100 mV/s. All CV curves were

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from coating N–C@VN on the CTF (CTF/N–C@VN) electrode compared with coating N–C on the CTF (CTF/N–C), directly growing VN on CTF (CTF@VN), and coating VN on the CTF (CTF/VN) electrodes at a scan rate of 50 mV/s. The CV curves of CTF/N–C, CTF@VN, CTF/VN, and CTF/N–C@VN with electrodes are given in Fig. 4a, and their corresponding GCD curves at a current density of 2 mA/cm2 are compared in Fig. 4b, SEM images of CTF@VOx, CTF@VN, and VN powder are presented in Fig. S4d-e, 4f, and 4i. Evidently, the CTF/N–C electrode CV area was the smallest of the electrodes, which shows the negligible contribution of the specific capacitance. The CV areas of the CTF/VN and CTF@VN electrodes were not distinctive, but the CV area of CTF/VN showed obvious polarization with a potential of approximately 1.2 V, demonstrating the larger interface impedance and inferior reversibility of the CTF/VN electrode. Encouragingly, the CV area of the CTF/N–C@VN electrode was much larger than those of the CTF/VN and CTF@VN electrodes and also had a clear rectangular shape with little polarization evident, even when the potential increased to 1.2 V. This demonstrated the excellent conductivity, superior reversibility, and high mass loading of the N–C@VN electrode. The GCD curves of the CTF/N–C, CTF/VN, CTF@VN, and CTF/N–C@VN electrodes at a current density of 2 mA/ cm2 (Fig. 4b) show that the CTF/N–C@VN electrode had the longest discharge time and the CTF/N–C electrode had the shortest, further indicating the negligible specific capacitance of N–C. Compared to the weak coulombic efficiency of the CTF/VN electrode, the GCD curves of both the CTF/@VN and CTF/N–C@VN electrode possessed good symmetry and showed little IR, which again indicated the superior conductivity and high areal specific capacitance of CTF/N–C@VN. Fig. 4c shows the CV curves of the CTF/N–C@VN electrode in a potential window from 0 to 1.2 V at various scan rates. All CV curves with rectangular indicate the ideal capacitive behavior and the excellent reversibility as well as fast charge/discharge capability of CTF/N–C@VN. The superior electrochemical performance of the CTF/N–C@VN electrode can be further supported by the nearly symmetric triangular GCD curves (Fig. 4d). The rate capability curve of the CTF/N–C@VN electrode is presented in Fig. 4e, which shows that the electrode achieved a maximum areal specific capacitance of 1478.4.5 mF/cm2 at a discharge current density of 2 mA/cm2 and still maintained an outstanding rate capability of 72.6% even at a high discharge current of 20 mA/cm2. The CV and GCD curves of CTF/VN and CTF@VN are presented in Fig. S8 and Fig. S9. As shown in Fig. S8a, the CV curves of the CTF/VN electrode indicated the existence of polarization, thus resulting in its poor reversibility and inferior ion diffusion rate. This inferior electrochemical behavior of the CTF/VN electrode was also demonstrated by the asymmetric triangular GCD curves in Fig. S8b. Although the CV curves of the CTF@VN electrode in Fig. S9a were all rectangular and the GCD curves of the CTF@VN electrode were all triangular (Fig. S9b), the CV surrounding area and the discharge time of GCD were significantly decreased, which demonstrated the low mass-loading of active materials on these electrodes. To analyze the impedances of the above three electrodes, their EIS were analyzed in Fig. 4f. The CTF/N–C@VN electrodes present the shortest intercepts of the Nyquist plots on the real axis, largest angles between spectral line and the real axis and smallest the radii of circular arcs with the comparison to CTF/VN and CTF@VN, revealing ideal capacitive behavior of CTF/ N–C@VN electrodes. It should be noted that the size nanosheets and mass loading of VN are constrained by the finite area, kinetics and thermodynamic for CTF@VN electrodes. To highlight the enhanced performance of our designed electrode, we compared the areal capacitance, gravimetic vapacitance, mass loading, coulombic efficiency and rate capability of CTF/N–C@VN electrode with CTF/VN and CTF@VN (Fig. 4g). Significantly, the CTF/N–C@VN electrode has most outstandingt electrochemical performance and highest mass loading. Thus, N–C supplied a highly conductive skeleton to support large layers of VN nanosheets, which enable them to regard as an attractive anode material for high-performance supercapacitors. To further estimate the electrochemical performance of various sizes of MnO2 and VN nanosheets, we obtained two sizes of both MnO2 and VN

(EIS) curves (Nyquist plots) are shown in Fig. 2f. In the high-frequency region, the intercepts of the Nyquist plots on the real axis were all at approximately the same starting point, a small value, which indicated both low electrolyte contact resistance (Re) and the existence of a uniform contact condition between the interface and the electrolyte. The medium-high frequency region of the Nyquist plots contain three circular arcs from which the radii of the three electrodes could be ranked as r1 < r2 < r3, corresponding to a charge transfer resistance trend of RctCTF@MnO2 < RctCTF/N–C@MnO2 < RctCTF/MnO2. Moreover, the order of magnitude of the angles (α) between the spectral line and the real axis of the three electrodes, α1 > α2 > α3, suggests that the ion diffusion ratesequence for the electrodes of VCTF@MnO2 > VCTF/N–C@MnO2 > VCTF/ MnO2. These analyses of the three EIS curves correspond well with the observed relative electrochemical performance of the electrodes. To further demonstrate the excellent electrochemical performance and high mass loading of the CTF/N–C@MnO2 electrode, the details of areal capacitance (mF/cm2), gravimetric capacitance (F/g), mass loading (mg/ cm2), coulombic efficiency (%), and tenfold rate capability ((C10/C0)%) of the CTF/MnO2, CTF@MnO2, and CTF/N–C@MnO2 electrodes are presented in Fig. 2g. Specifically, the CTF@MnO2 electrode demonstrated the highest gravimetric capacitance (455.7 F/g), the highest coulombic efficiency (98.2%), and the highest tenfold rate capability (78.5%), but showed the lowest areal capacitance (651.6 mF/cm2) and lowest mass loading (1.43 mg/cm2) attributed to the constrains of the finite base area, kinetics and thermodynamic. Impressively, the CTF/ N–C@MnO2 electrode not only displayed almost the same excellent performance, in terms of gravimetric capacitance (437.5 F/g), coulombic efficiency (96.4%), and rate capability (76.2%), but possessed almost twice the areal capacitance (1483.5 mF/cm2) and mass loading (3.39 mg/cm2) of the CTF@MnO2 electrode. It should be noted that the performance of the CTF/MnO2 electrode (which also had a mass loading of 3.39 mg/cm2) was far lower than that of CTF/N–C@MnO2. This underscores the importance of the unique core shell structure of N–C@MnO2 and the synergy it enables between the remarkable electrochemical property of MnO2 and the outstanding conductivity of N–C. Hence, N–C@MnO2 is a novel and promising candidate cathode material for high-energy-density supercapacitors. To synthesize the core-shell N–C@VN, the conductive framework of N–C was wrapped in copper mesh and transferred into a reactor. This mixture then underwent a solvothermal reaction and was finally annealed in ammonia. The details of this method are presented in the experimental section. Fig. 3a and b illustrate that N–C particle was uniformly coated with large VN nanosheets. The high-resolution TEM image (Fig. 3c) of the VN nanosheet presented well-defined lattice fringes with interplanar spacings of 0.24 and 0.21 nm, corresponding to the (111) and (200) planes of VN. The low-magnification TEM image and corresponding EDS mapping results (Fig. 3d) clearly showed the homogeneous distribution of the elements V, N, and C, and the mapping image of C clearly delineated the boundary of core and shell, thus confirming the core shell structure of N–C@VN. Furthermore, XPS was performed to investigate the chemical composition and valence states of N–C@VN. As shown in Fig. S7, the full spectrum of N–C@VN included the signals of V, N, and C, confirming their coexistence in the material. The XRD patterns of N–C@VN and N–C are presented in Fig. 3e, which showed that the peaks at the (002) lattice plane of the two compounds can be assigned to the N–C framework and the left diffraction peaks of N–C@VN can be attributed to VN (JCPDS File No. 35–0768). In Fig. 3f, the spectrum of V2p contains two groups of three distinct peaks at the binding energies of 514.2, 515.8, 517.2, 521.6, 523.4, and 524.7 eV, and each group of three peaks can be assigned to V–N, V–N–O, and V–O, respectively. The spectrum of N1s (Fig. 3g) contains three peaks centered at 397.2, 399.1, and 401.2 eV, which can be attributed to N–V, N–V, and N–C, respectively. The deconvoluted peak (Fig. 3h) of the C 1s spectrum is resolved into one component centered at 284.9 eV, which can be assigned to C in the N–C conductive framework. To highlight the extraordinary electrochemical performance resulting 7

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Fig. 6. (a) CV curves measured at different scan rates between 0 and 6.6 V. (b) GCD CV curves of the as-fabricated all-in-one supercapacitor measured at different current densities between 0 and 6.6 V. (c) Areal specific capacitances calculated from the charge-discharge curves as a function of the current density and the thickness of the device. (d) Areal energy and power densities of our all-in-one supercapacitor device in comparison with previously reported devices. (e) CV curves of the asprepared all-in-one supercapacitor measured at a current density of 4 mA cm 2 under different bending angles. (f) Normalized capacitances of the as-obtained all-inone supercapacitor with a bending angle of 90 for 6000 cycles. (g) The voltage of a fully charged device. (h–k) Two blue-light LEDs powered by the all-in-one supercapacitor (bending angle from 0 to 135 ). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

N–C@MnO2 and N–C@VN as the positive and negative electrode materials on both sides of the carbon nanotubes, and successfully assembled these three devices into tandem as an internal asymmetric all-solid-state 6.6 V all-in-one supercapacitor. Fig. 5a presents the schematic diagram of the device. To well demonstrare the fact of internal asymmertic tandem and the thickness of the device, Fig. S14 presents the SEM cross section image of the supercapatitor. As expected, the CV curves of the asassembled all-in-one supercapacitor collected at 60 mV/s maintained a rectangular shape even in a high window up to 6.6 V (Fig. 5b). Furthermore, the nearly symmetrical triangular GCD curves of our device in at a current density of 4 mA/cm2 further confirm its ideal capacitive characteristics (Fig. 5c), comprising a rapid I–V response and low equivalent series resistance. The detailed electrochemical performance of our all-in-one supercapacitor is depicted in Fig. 6. Fig. 6a shows the CV curves of the device at scan rates from 10 to 100 mV/s with a potential range of 0–6.6 V. These CV curves were quasi-rectangular with no notable redox peaks or polarization, which demonstrates that the as-assembled device has fast electron transfer, remarkably reversibility, and desirable capacitive behavior. This superior electrochemical performance is further demonstrated by the nearly triangular GCD curves (Fig. 6b) at current densities between 2 and 20 mA/cm2. On the basis of these GCD curves, the specific capacitances of

nanosheets via regulation of reaction time. The relevant experimental details, SEM images, and electrochemical data are shown in Figs. S10, S11, S12, and S13. Fig. S12b shows the ideal size of MnO2 nanosheets; a shorter reaction time afforded small MnO2 nanosheets (Fig. S12a), and a longer reaction time led to agglomeration of active materials (Fig. S12c) due to the limitations of reaction kinetics and reaction thermodynamics. This phenomenon was also apparent in the regulation of VN nanosheetsize (Fig. S13a shorter reaction time, Fig. S13b ideal reaction time, and Fig. S13c longer reaction). Furthermore, the GCD curves of different sizes of MnO2 and VN nanosheets (Figs. S12d, S12e, S12f and Figs. S13d, S13e, S13f) demonstrated that smaller nanosheets normally had a shorter discharge time, whereas the agglomeration-active substance often possessed worse coulomb efficiency. Figs. S12g and S13g illustrate the rate capabilities of these different-sized nanosheets. On the whole, the ideal-sized MnO2 and VN nanosheets not only had the maximum areal specific capacitance (MnO2: 1483.5 mF/cm2, VN: 1478.4 mF/cm2) but also tenfold rate capability (MnO2: 76.2%, VN: 72.6%) superior to the corresponding properties of smaller nanosheets (MnO2: 795.4 mF/cm2, 77%,VN: 635.7 mF/cm2, 73.2%) and agglomeration-active-substances (MnO2: 1265.6 mF/cm2, 55.2%,VN: 1440.3 mF/cm2, 56.4%). To demonstrate the practical application of these as-fabricated highperformance core-shell electrode materials, we uniformly coated 8

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Acknowledgements

the device were calculated and presented in Fig. 6c, from which it can be seen that the device displayed high specific capacitances (336.7 and 236.5 mF/cm2 at current densities of 2 and 20 mA/cm2, respectively), indicating an exceptional rate capability. Moreover, the thickness of the device was only 0.172 mm, as measured with a spiral micrometer (Fig. 6c). The Ragone plots of the device are shown in Fig. 6d; the device displayed an ultrahigh areal energy density of 2023.8 μWh/cm2 (118.2 mWh/cm3) at a power density of 6583.6 μW/cm2 (282.8 mW/cm3) and while maintaining a high energy density of 1379.4 μWh/cm2 (80.2 mWh/cm3) even at a high power density of 65737.9 μW/cm2 (3821.9 mW/cm3). These values are substantially superior those of previously reported devices, such as Li thin-film battery (4 V/500 μA) [43], commercial AC-SC (2.75 V/44 mF) [44], VOS nanowires//MnO2@graphene (0.87 mWh/cm3, 9 mW/cm3) [45], oxygen-deficient Fe2O3nanorods//MnO2 (0.41 mWh/cm3, 100 mW/cm3) [46], Ni@AC//Ni@MnO2 (0.78 mWh/cm3, 2.5 mW/cm3) [47], mesoporous carbon fiber//Ni(OH)2 nanowires (2.16 mWh/cm3, 1600 mW/cm3) [48], Paper LDH//Paper Fe2O3 (10.3 mWh/cm3, 103.2 mW/cm3) [49], Fe2O3@ACC//Fe2O3@ACC(9.2 mWh/cm3, 12 mW/cm3; 4.5 mWh/cm3, 204 mW/cm3) [50], PBpy-based MSC//PBpy-based MSC(12.1 mWh/cm3, 280 mW/cm3) [51]. We summarized and compared the electrochemical performance of this supercapacitor with previous reports of the performance of supercapacitors (Table S1). Our supercapacitor was clearly superior, with an energy density far higher than any of the others. It also compares well to other recent internal tandem supercapacitors (Table S2), again confirming the excellent electrochemical properties of the device generated in this study. It is also impressive that the GCD curves were almost unchanged when the bending angle of the device was varied from 0 to 180 at a current density of 4 mA/cm2 (Fig. 6e), indicating the outstanding flexibility of our all-in-one supercapacitor. Remarkably, after bending at 90 for more than 6000 cycles (Fig. 6f), the specific capacitance of the device still retained a rate value of 97.2%, further confirming its excellent mechanical stability. The Nyquist plot of the electrochemical impedance of the device is presented in Fig. S15. Fig. S16 shows that our device retained 95.1% capacitance after long-term charging/discharging cycling at 4 mA/cm2 for 15,000 cycles), illustrating its superior cycling stability. The voltage of our fully charged all-in-one supercapacitor tested by multimeter is 6.59 V  6.60 V (Fig. 6g); this difference of 0.01 V is attributed to the device’s small voltage drop. To further illustrate the practical application and the superior mechanical stability of our all-in-one supercapacitor, we used it to power two blue LEDs in series and found that the brightness of the small bulbs barely changed as the bending angle of the supercapacitor increased from 30 to 135 (Fig. 6h–k).

This work was supported by the National Natural Science Foundation of China (No. 51703241), the Fundamental Research Funds for the Central Universities (No. 020514380183), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDB-SSWSLH031), the Thousand Youth Talents Plan, and the Science and Technology Project of Nanchang (2017-SJSYS-008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.09.002. References [1] D.S. Yu, K.L. Goh, H. Wang, L. Wei, W.C. Jiang, Q. Zhang, L.M. Dai, Y. Chen, Scalable synthesis of hierarchically structured carbon nanotube–graphene fibres for capacitive energy storage, Nat. Nanotechnol. 9 (2014) 555–562. [2] Q.C. Zhang, X.N. Wang, Z.H. Pan, J. Sun, J.X. Zhao, J. Zhang, C.X. Zhang, L. Tang, J. Luo, B. Song, Z.X. Zhang, W.B. Lu, Q.W. Li, Y.G. Zhang, Y.G. 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4. Conclusion In summary, we used a ZIF-67–derived N-doped carbon skeleton (N–C) as a conductive center to rationally synthesize core-shell structures N–C@MnO2 and N–C@VN comprising large size nanosheets. Due to their superior structure, CTF/N–C@MnO2 and CTF/N–C@VN electrodes exhibited remarkable areal capacitances of 1483.5 and 1478.4 mF/cm2 at a current density of 2 mA/cm2 and superior rate capability with capacitance retentions of 76.2% and 72.6% at a current density of 20 mA/cm2, together with high mass loadings of 3.39 and 2.24 mg/cm2, respectively. Furthermore, a high-performance all-solid-state 6.6 V all-inone supercapacitor was successfully assembled by matching a CTF/ N–C@MnO2 cathode and a CTF/N–C@VN anode in an internal asymmetric tandem fashion. This as-assembled all-in-one supercapacitor exhibited a high specific capacitance of 336.7 mF/cm2 (19.6 F/cm3) and an impressive energy density of 2032.8 μWh/cm2 (118.2 mWh/cm3), which exceeded the corresponding properties of most previously reported state-of-the-art asymmetric supercapacitors. This work thus provided a cost-effective and scalable route to develop flexible ultrahigh–energy density supercapacitors to power next-generation portable and wearable electronics.

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