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Hierarchical Structure Formation and Effect Mechanism of Ni/ Mn Layered Double Hydroxides Microspheres with Largescale Production for Flexible Asymmetric Supercapacitors Tie Li, Jian Wang, Yan Xu, Yudong Cao, Hongzhen Lin, and Ting Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00321 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Energy Materials

Hierarchical Structure Formation and Effect Mechanism of Ni/Mn Layered Double Hydroxides Microspheres with Large-scale Production for Flexible Asymmetric Supercapacitors Tie Li, Jian Wang, Yan Xu, Yudong Cao, Hongzhen Lin, Ting Zhang* i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS), 398 Ruoshui Road, Suzhou, 215123, P. R. China *Corresponding author: Ting Zhang, E-mail: [email protected] (Phone: +86-0512-62872706 Fax: +86-0512-6260-3079)

Abstract: In this study, Ni/Mn layered double hydroxides (LDHs) microspheres with three-dimensional flower-like hierarchical structure are fabricated by a large-scale and self-assembled chemical co-precipitation strategy (at 55 oC under normal pressure). In each integrated microsphere, the petals consist of ultrathin two-dimensional nanosheets. The self-assembly formation mechanism of this flower-like construction is systematically investigated according to the analysis results from the regulation of various reaction factors. The Ni/Mn LDHs microspheres show optimal capacitance value as high as 1379 F·g−1 at 1 A·g−1. Sum Frequency Generation spectroscopy and Charge Storage Behavior mechanism analysis further approve that a favorable capacitive constitution is indeed depended on the well synergistic effect of active sites (different feed ratios) and surface area (different architectural features). Moreover, a novel flexible all-solid-state asymmetric supercapacitor is assembled by exploiting these microspheres and active carbon as positive and negative electrode materials respectively, which exhibits a superior capacitance value of 393 F·g−1 (1 A·g−1) and energy and power density of 131.17 Wh·kg-1 and 1.45 KW·kg-1 with favorable cyclic life (remains over 75.1% after 5000 cycles under 50 mV·s-1), respectively. Overall, this work provides a significant prospect and deep understanding for the development of next generation of flexible energy devices.

Keywords: Ni/Mn LDHs, self-assembling, hierarchical microspheres, supercapacitors, SFG spectroscopy

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1. Introduction Recently, due to the attractive potential applications in the next generation of electronic products, the wearable flexible electronic systems have received more and more attentions in the field of personal health management, human-machine interaction, bionic robots and so on.1, 2 However, their practical reliability, adaptability and service life are constricted by the corresponding lagging progress of flexible power supply components.3-5 Among various kinds of power sources, flexible all-solid-sate supercapacitors (FASs), derived from electrochemical capacitors and designed using solid electrolytes, have attracted tremendous research interest due to the dual virtues of high power density, long cycling lifetime, rapid charge-discharge efficiency inherited from the traditional supercapacitors and flexibility, lightweight, solid state gained from flexible devices.6-8 Unfortunately, regardless of the shapes it has been designed to match different flexible systems (fiber shape,9 sandwich construction10 and planar microcapacitors11), the energy density of FASs is still unsatisfactory and has seriously hampered their practical applied effect.12 The intensive exploration of novel active materials possessed supernal power and energy densities together is crucial for improving holistic electrochemical properties of the FASs.13 Currently most of capacitive materials, shuc as carbonaceous material,14-16 conducting polymers,17, 18 transition metal oxides/hydroxides,19-21 and transition metal nitrides,22, 23 are not extensively used for some drawbacks more or less. Developing electrode materials with high capacitance, stable cyclic performance and earth-abundant deposits are the most valuable strategy for enhancing the 2 ACS Paragon Plus Environment

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performances of the FASs. Among various reported candidates, the layered double hydroxides (LDHs), especially nickel-manganese LDHs (Ni/Mn LDHs), recently have attracted lots of interest in the fields of superior pseudocapacitive materials due to its desirable characters of high active surface area, fast redox activity and rapid ion transfer reaction, which is caused by the unique hydrotalcite-like structure consist of two-dimensional (2D) layered arrangement and charge compensating anions.24-26 And more, the eco-friendly and non-toxic nature of the rich LDHs mineral resources can dramatically reduce the cost and are suitable for large-scale industrialized application.27 However, the cyclic and rate reversibility of the LDHs based electrodes are often weak due to the fragile but swelling nature of their layered structure.28 In order to overcome this bottleneck, one feasible strategy is to construct controlled hierarchical architecture of electrode materials, especially self-assembly into special three-dimensional (3D) nanostructured configuration with large active contact area and effective protons and electrons diffusion, leading to the improved charge-transfer resistance and capacitive stability.29 Nonetheless, to the best of our knowledge, pure 3D nanoflakes-like Ni/Mn LDHs is still limited to the controlled preparation of high-quality uni-structural products. On the other hand, presently, the most popular synthesis method for special structural LDHs is the hydrothermal strategy, generally including an appropriate surfactant like CTAB and a long-term high pressure and temperature procedures, which is relatively complex and inappropriate for massive production, and also results in the impurity.30 Thus, it is urgently expected to develop 3 ACS Paragon Plus Environment


a facile and efficient energy-/time-saving way to prepare high purity of Ni/Mn LDHs with accurate adjustment of their 3D hierarchical constitutions for high energy density FASs. Besides, the electrochemical reaction usually occurs at the place of interfacial sites between electrolyte and active materials. In order to reveal the probable mechanism of the electrochemical properties, it is preferable to use in situ characterization techniques which can investigate the surface and interface variation characteristics under the actual electrochemical reaction with an alkaline electrolyte. Recently, Sum Frequency Generation (SFG) spectroscopy has been employed to investigated the structure of water molecules existed in various interfaces, which usually have prominent functions in many chemical and physical reaction processes. More importantly, it is possible with the utilization of SFG spectroscopy to reveal three important features about the electrode surface under the close to the actual redox conditions. And for all we know, the study about in situ observations of surface hydroxyl groups and the subsequent ions exchange during electrochemical process of Ni/Mn LDHs have not been reported. Hence, togerther with above analysis, in this study we have proposed a facile eco-friendly and energy-saving strategy, named hypothermal self-assembled chemical co-precipitation, to synthesis Ni/Mn LDHs microspheres with 3D flower-like hierarchical structure in a large scale, which displays high capacitance value of 1379 F·g−1 (1 A·g−1). In addition, the mechanism relation between the change tendency of C values and the morphology evolution of various Ni/Mn LDHs samples was studied innovatively by the SFG spectroscopy and 4 ACS Paragon Plus Environment

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Charge Storage Behavior (CSB) analysis. Furtherly, a flexible all-solid-state asymmetric supercapacitors (FAAS) was assembled by exploiting Ni/Mn LDHs and active carbon as positive and negative materials, respectively, showing high capacitance (393 F·g−1 at 1 A·g−1) and power and energy density (1.45 KW·kg-1 and 131.17 Wh·kg-1) with favorable cyclic life. Overall, these superior electrochemical performances can be attributed to the hierarchical 3D flower-like microsphere construction, which enhance the faradaic pseudocapacitive reaction to perfect the cyclic durability.

2. Experimental Sections 2.1 Preparation of 3D flower-like Ni/Mn LDHs microspheres Raw materials were all purchased from Sinopharm Chemical Reagent Co., Ltd with analytical purity. The Ni/Mn LDHs microspheres were prepared by employing facile hypothermal self-assembled chemical co-precipitation strategy. For instance, typical process for Ni:Mn = 4:6 is: 0.3 mmol MnCl2, 0.45 mmol NiCl2, 4 mmol NH4Cl and 1.375 mmol NaOH were added in 20 mL deionized water and stirred for 1 minutes to produce metal-ammine mixture. Then, this precursor was sealed and reacted at an oven under 55 oC for 15 h. Finally, the as-fabricated Ni/Mn LDHs sample was obtained by washed and dried. For comparison, the products with various reaction times (30 min to 30 h), feeding proportion (Ni:Mn of 0:10 to 10:0), and acid-base degree were obtained using the same process. And the detailed characterizations of the electrode materials are described in SW-1 of Supporting 5 ACS Paragon Plus Environment


Information. 2.2 Three-electrode electrochemical characterizations for electrode materials Before the test, Ni/Mn LDHs slurry was prepared by mixing Ni/Mn LDHs (5-8 mg) with conductive agents of carbon black and binder of polyvinylidene fluoride under ratio of 7:2:1. Here, the measurements of electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge-discharge (GC) are all characterized by ChenHua CHI 660B workstation (Shanghai ChenHua Instrument). A three-electrode system with Ni/Mn LDHs as working electrode, platinum and Ag/AgCl as counter and reference electrode respectively was constructed and executed in alkaline electrolyte (6 M KOH). The working electrode was formed by pressing the Ni/Mn LDHs slurry onto nickel foam current collector (4-8 mg, 1 cm2). The capacitance value of Ni/Mn LDHs electrodes was calculated from GC curves from the following formula: = × ∆⁄(∆ × ), where C (F/g) is capacitance value, I (A) is current value, ∆V (V) is potential window, ∆t (s) is discharge time and m (g) is the quality of active materials in the working electrode. 2.3 Assembly and electrochemical measurements for FAAS The polymer gels electrolyte was obtained as: the mixture solution (60 mL deionized water) with 6 g PVA and 3 g KOH was stirred continually at 85 oC for 12 h. The stacked FAAS was assembled by employing above-mentioned Ni/Mn LDHs slurry (4-8 mg) and equivalent active carbon (10-16 mg) tiled on the polydimethylsiloxane/copper foil substrates (about 1.5 cm × 1.5 cm) as positive and negative electrode respectively, and then two substrates carpeted with active materials 6 ACS Paragon Plus Environment

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were covered by PVA/KOH gels electrolyte and pressed face to face together after water evaporating. A two-electrode system was employed to measure the electrochemical properties of the FAAS, and the results of Cs value, energy (E) and average power (Pave) densities were computed from the following formula: = × ∆⁄(∆ × ) , E = 0.5 × × and = ⁄∆ , here ∆t(s) is discharge time, V (V) is voltage except resistance drop and m (g) is the quality of all active materials in whole electrodes.

3. Results and Discussions 3.1 Large-scale Preparation and Characterizations of Ni/Mn LDHs Microspheres

Figure 1. Typical SEM images of (a) 3D flower-like hierarchical product and (b) 7 ACS Paragon Plus Environment


single enlarged LDHs nanosheets based microsphere, (i) insert in (a) represents large-scale preparation of this LDHs electrode material; HRTEM images of (c) the Ni/Mn LDHs sample and (d) the hybrid nanosheets, the insert part shows the SAED pattern of the nanosheet; (e) The size distribution and (f) XRD pattern of the as-obtained microspheres. (Sample was prepared at Ni:Mn ratio of 4:6). The typical morphology and structure of the as-synthesized Ni/Mn LDHs samples were characterized by SEM as shown in Figure 1, demonstrating the formation of high-quality products of unbroken microspheres with 3D flower-like hierarchical architecture (Figure 1a-d), which possesses a uniform dimension distribution with average size of 1.97 µm (Figure 1e, over 74% diameters range from 1.7 to 2.1 µm). The enlarged details in Figure 1b reveal that a single microsphere is orderly self-assembled by numerous interconnected petal-like nanosheets with the thickness of about 25 nm and width of several hundred nanometers, which appear radial porous pattern and interlace into the spherical core. The smooth surface of the petal is probably contributed to the process of Ostwald ripening.31 This porous nanostructure can offer favorable specific surface area caused by the intersective reticular shape as shown in Table S1 (Supporting Information), displaying BET and Langmuir surface areas of 55.06 m2·g-1 and 88.02 m2·g-1 respectively with single point surface area (P/Po=0.3) of 55.19 m2·g-1. These well-defined 3D flower-like self-assembled hierarchical microspheres and the polyporous morphology effectively refrain from the aggregation of abundant active sites to reduce the “dead surface”,32 which is helpful for the unimpeded migrating paths of electrolyte ions and also makes the electrode materials sufficiently participate in pseudocapacitive reaction33, leading to high-rate cycling and enhanced energy density. The TEM and SAED were further employed to investigate more morphological 8 ACS Paragon Plus Environment

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properties of the Ni/Mn LDHs micro-flowers. In Figure 1c, it can be seen that the hierarchical microspheres are built from many secondary ultrathin and interconnected nanoflakes, which germinate from the center of those particles and then gradually assemble into the unabridged flower-like characters as a whole self-assembled configuration. Additionally, it implicitly reveals this self-assembled LDHs product possesses the remarkable structural stability even after long-time harsh ultrasonication. Figure 1d shows the details of an ultrathin petal with compact, flat and transparent nature, and the corresponding SAED pattern of (003) plane presents the symmetry and bright diffraction rings and spots, showing long-distance regular crystalline feature of the LDHs nanoflakes.34 The observations of TEM images are in well accordance with those of SEM images, which also confirm the self-assembly activity existing in the process of hypothermal chemical co-precipitation. Moreover, the hydrophilic nature of LDHs is highly conducive to the diffusion of ions in aqueous KOH/PVA electrolyte. As proven by previously reported references24-28, such laminar and ultrathin constitutions of hierarchical LDHs microspheres particularly favor access of electrolyte to active species and accelerate charging/discharging process of electron and ion transportation, which are highly beneficial to reduce the interfacial resistance between gel electrolyte and electrode materials to enhance the energy storage ability. Furthermore, the XRD results of the Ni/Mn LDHs microspheres were shown in Figure 1f, the observed peaks at 2θ of 11.5°, 23.3°, 33.5°, 38.4°, 45.3°, 59.5° and 61.1° are contributed to the lattice plane of (003), (006), (009), (015), (018), (110) and (113), 9 ACS Paragon Plus Environment


which can be well matched with hydrotalcite-like LDHs structure in accordance with standard α-Ni(OH)2-like phase (JCPDS 38-0715).35 The other impurity peaks are non-detectable, confirming the high purity of hierarchical Ni/Mn LDHs microspheres. Meanwhile, the high and narrow peak of (003) indicates the well crystallinity of this Ni/Mn hybrid product in line with the SAED analysis. Accordingly, the interplanar d-spacing of this LDHs crystal is calculated as 0.86 nm on the basis of the Bragg formula, which was broader than that of pure β-Mn(OH)2 (0.47 nm) and α-Ni(OH)2 (0.75 nm) phase. Lately, reference reported that, except the surface, the entire crystal structure of LDHs can take part in the faradaic redox process relying on deintercalation/intercalation of electrolyte ions.36 Hence, benefited from this enhanced interlayer spacing, more electrolyte anions served as electroactive species, such as alkali groups (OH−), acid radical (CO32−/NO3−) and organic electrolyte, can be accommodated to participate in the pseudocapacitive redox for an extra Faradaic capacitance, which is expected to possess superior capacitive properties of this flower-like Ni/Mn LDHs microspheres.37


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Figure 2. (a) EDX spectrum and (b) corresponding element mapping of the Ni, Mn and O elements existed in the LDHs microspheres; XPS analysis of (c) Ni 2p and (d) Mn 2p of the Ni/Mn LDHs. (Sample was prepared at Ni:Mn ratio of 4:6). The constitution of elements existed in the active materials is one of the most important factors to obtain desirable capacitive performances. For example, transition metals at a high valence state will conduct diverse and ampliative redox behaviors within electrochemical reactions, resulting in enhanced pseudocapacitive capacity.38 The EDX spectrum (Figure 2a) and corresponding element mapping (Figure 2b) were used to detect the elementary composition and distribution in the as-obtained LDHs microsphere, which unambiguously validates that the presence of Ni (36.10 At%) and Mn (11.46 At%) together with O elements is homogeneously incorporated into the petal-like nanosheets without segregation. No other impurities are observed and the molar ratio of Ni/Mn is calculated to be almost 3:1. The uniform distribution of inclusive elements guarantees the electron transmit through active species on an even keel to avoid the unnecessary performance degradation induced by the partial 11 ACS Paragon Plus Environment


impedance fluctuation, which can availably improve the rate capability and cyclic durability of the active species.39 Further, XPS was employed to analyze the element valence states of the Ni/Mn LDHs. From the Figure 2c and Figure 2d, the Ni 2p3/2 and Ni 2p1/2 peaks appeared at 853.6 eV and 871.2 eV with a spin-orbit splitting of 17.6 eV can be attributed to Ni2+ same as α-Ni(OH)2, and the peaks of Mn 2p3/2 and Mn 2p1/2 emerged at 641.7 and 652.8 eV are ascribed to the presence of Mn as Mn3+ status, respectively.40 Here, the hypothermal coprecipitation reaction was achieved under alkaline condition and ambient atmosphere, it is extremely favorable for Mn2+ to convert to Mn3+ (0.65 Å) and consequently replaces isomorphous Ni2+ (0.69 Å) presented in hydrotalcite-like phase for their similar ionic radius.41 In other words, all of the above results demonstrate the as-produced LDHs microspheres indeed have a hydrotalcite-like structure with a general formula of [Ni3Mn(OH)x]·nH2O, which further verify the XRD results on the successful preparation of Ni/Mn LDHs. As described in introduction section, the rich Ni and Mn based mineral resources can availably reduce the preparation cost, and the eco-friendly and non-toxic nature of the Ni/Mn LDHs are perfectly suitable for industrialized application. However, the LDHs based active materials did not realize their optimistic potential roles on the aspect of capacitive electrode until now, which was limited by two pivotal constraints: one is the cyclic stability degradation when LDHs used as the powder particle state directly, and another is the hydrothermal process with high energy consumption employed for the fabrication of special structural LDHs. Here, to prove the proposed hypothermal self-assembled chemical co-precipitation is a large-scale, facile and 12 ACS Paragon Plus Environment

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straightforward solution, the method adopted in the laboratory as ascribed in

“Preparation of 3D flower-like Ni/Mn LDHs microspheres” part was repeated to fabricate Ni/Mn LDHs samples except the quantity of those raw reagents were magnified 150 times. As shown in Figure 1a-(i) and Figure S1, a mass of reaction precipitate was generated after pouring 45 mmol MnCl2 and 67.5 mmol NiCl2 into 3 liters of NH4Cl/NaOH alkaline solution under the same hypothermal co-precipitation condition (55 oC and normal atmospheric following 15 h). The characterization of the product from Figure 3b proves that the precipitate also presents homogeneous 3D flower-like microspheres as same as the morphology of the laboratory obtained Ni/Mn LDHs. Even the calculated content (Ni:Mn = 37.33 At%:10.57 At%) and molar ratio (3:1) of Ni/Mn are almost entirely same with that of the sample in Figure 1. Hence, these results clearly reveal that this facile, low-cost but efficient strategy can be employed to realize large-scale preparation of 3D flower-like Ni/Mn LDHs microspheres.

3.2 The Formation Mechanism of the 3D Flower-like Structure



Figure 3. SEM results of the as-obtained samples under various Ni:Mn ratios of (a) 10:0, (b) 8:2, (c) 6:4, (d) 4:6, (e) 2:8, (f) 0:10. To explore the assembled mechanism of the flower-like Ni/Mn LDHs microspheres, a serial of key factors which can influence the self-assembled process of hypothermal co-coprecipitation were systematically investigated. Firstly, the products prepared under different mole ratios (R) of the Ni/Mn were conducted and shown in Figure 3. Clearly, in the absence of Mn3+ (R=10:0), the obtained sample presents a robust cluster composed of stubby and thick nanosheets (Figure 3a). Along with the quantity of Mn3+ increased to R=8:2 and 6:4 (Figure 3b and Figure 3c), the constitution of the micro-flowers gradually turns into visible and lightsome pattern accompanied by the formation of porous morphology, which is caused by the drastically reduction of the petals thickness and the increase of sheet-crossed degree. When the R is 4:6, the thickness and porous characters of the ultrathin nanoflakes in 14 ACS Paragon Plus Environment

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Ni/Mn LDHs microspheres are completely transformed to the optimized interconnected configuration, in which the ultrathin nanosheets even display transparent feature, resulting in the desired perfect hierarchical microspheres with 3D flower-like architecture (Figures 3d). The reason can be attributed to the phase transition evolving step by step from pure β-Ni(OH)2 phase (R=10:0) to hydrotalcite-like phase (R=4:6) induced by the phenomenon of Mn3+ isomorphically substitution of Ni2+ as described in XPS analysis,42 leading to the granular growth enhancing but the self-assembly weakening of LDHs nanosheets in the present of Mn3+. However, it can be seen that excess Mn3+ participation further reduces the thickness of nanosheets and leads the structure of LDHs microspheres to shrink and collapse (Figures 3e), until the product is only the dispersed petal-like shapes of pure Mn(OH)2 phase without specific feature at R reached 0:10 (Figures 3f). In addition, the actual photographs of the serial Ni/Mn LDHs are illustrated in Figure S2, showing that the color of as-prepared samples increasingly transforms from the reseda of Ni(OH)2 to the earthy yellow of Ni/Mn LDHs and last to the deep khaki of manganese oxides along with the increasing of feed ratios, which just prove the samples are consisted of Ni and Mn and the products directly hinges on the feed ratios. Shortly, the morphology changes of the resulting hybrids were accompanied by gradual phase transition as the increase of R, and the as-prepared flower-like Ni/Mn LDHs microspheres with R=4:6 show the optimal 3D porous architecture with interconnected nanosheets, which will take the electrode materials to the desired pseudocapacitive performances as narrated in the parts of SEM and TEM analyzing 15 ACS Paragon Plus Environment


results.24 Through the above-mentioned crystal structure (XRD) and element statue (XPS) results, it is found that the Ni/Mn LDHs possesses the hydrotalcite-like phase same with the standard α-Ni(OH)2-like phase. As we known, Ni(OH)2 is commonly prepared by controllable hydrolysis of Nickel based complex like [Ni(NH3)6]2+ in the alkaline solution,43 in this work, NH3·H2O generated from the reaction of NH4Cl/NaOH relaxedly decomposes to supply enough NH3 and OH- for combination with Ni2+/Mn3+ to produce Ni/Mn LDHs. Hence, it is found that the experimental alkali sources could address a pivotal effect of influencing the existing form of Ni/Mn LDHs. To verify the hypothesis, the comparative samples carried out at a serial of 1/2 (pH 8.33), 2/2 (pH 8.71), 2/3 (pH 8.99) and 5/2 (pH 9.59) times of original NaOH concentrations (1.375 mmol) were synthesized respectively as shown in Figure S3. The as-fabricated product at pH=8.71 exhibited the 3D flower-like morphology with petal-like interconnected nanosheets integrating into the center of microspheres (Figure S3b). However, higher or lower solubility of the NH4Cl/NaOH mixture did not achieve the favorable homogeneous LDHs micro-flower, in which thicker and discontinued nanosheets with spherical stacking were observed in product at pH=8.33 (Figure S3a), while samples at pH=8.99 and 9.59 showed disorderly stacked sheets (Figure S3c) and out-of-order structure (Figure S3d), respectively. The reason should be attributed to the releasing quantity and equilibrium of NH3 and OH- by pH regulation. At lower pH value, untense microspheres are formed due to lack of sufficient OH- continually participate in co-precipitation to crystallization, and the 16 ACS Paragon Plus Environment

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dissolution speed of α-Ni(OH)2 surpasses its crystallization speed.44 On the contrary, higher concentration of OH- accelerates the crystallization behavior of β-Ni(OH)2 but decelerates that of α-Ni(OH)2, leading to the gradually collapse of the 3D micro-flower and the formation of hexagonal nanoflakes and even particles without Ostwald ripening happening.30 These results illustrate that the self-assembled constitution of the obtained hybrids intensively relies on the content of initial NaOH concentrations. In order to thoroughly investigate the feature evolution of the 3D Ni/Mn LDHs microspheres, the time-dependent experiments were also conducted and the intermediate samples extracted at specific time intervals were characterized by SEM (Figure S4). At the beginning of 1 hour (Figure S4a), only flake-like individual units with numbered petals germinated from their almost smooth surface were obtained. After 5 hours (Figure S4b), an interlaced sheets network gradually ripened from the growth of previous rudimentary petals by the self-assembly behavior of newly generated LDHs nanoparticles. Along with the time increased to 10 hours (Figure S4c), the continuous morphology progress was also observed in samples, which formed more complex interconnected and porous hierarchical nanostructures leading to self-assembled microsphere. When the time reached 15 hours (Figure S4d), fully Ni/Mn LDHs microspheres with 3D flower-like hybrid architecture were created as similar to those shown in Figure 1, since the prolonged time was tremendously beneficial to NH4Cl/NaOH controllably release OH- slowly to react with Ni2+ to form narrow dimension and well crystallization of Mn3+ replaced [Ni3Mn(OH)x]·nH2O 17 ACS Paragon Plus Environment


resulting in the growth of nanosheets into microspheres.40, 41 Further prolonged time to 25-30 hours, except the spherical morphology of LDHs did not change too much, some underdeveloped nanosheets (Figure S4e) and nanocore (Figure S4f) also existed in the as-fabricated products, revealing that oriented attachment was still underway but completely distinguished from the crystallization of LDHs nanosheets. In short, the conversion of both nanosheet dimension and interlaced level was closely along with the time addition from 1 hour to 30 hours, revealing that a stepwise and regularly self-assemble into flower-like microspheres of nanoparticles and nanopetals. This great regularity structure composed with ultrathin nanosheets can take the satisfactory electrochemical action of electron transports as well as contact and inner resistance between electrode material and current collector, resulting in the enhanced capacitance.45 So, the duration of 15 hours was selected as the optimal reaction times.

Figure 4. Schematic of the Ni/Mn LDHs microspheres growth mechanism. Step I: the emergence of initial nanoparticles by absorption of Ni2+/Mn2+ on nuclei surface; Step II: the separation and growth of initial nucleation; Step III: the conversion and generation of primal Ni/Mn LDHs nano-compound; Step IV: the oriented development of LDHs based nanosheets; Step V: the presentation of interconnected architecture with porous structure; Step VI: the self-assembly and Ostwald ripening of 3D flower-like Ni/Mn LDHs. The sequential time-dependent experiments are illustrated as the morphological 18 ACS Paragon Plus Environment

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evolution from initial flakes, then nanosheet aggregates, and ultimately to the entire 3D flower-like microspheres. Hence, the growth mechanism of this hierarchical Ni/Mn LDHs structure could be summarized as precipitation-crystallization-oriented nanosheet growth-crossed assembly process,46 which is schematically demonstrated in Figure 4. At beginning, NH3 firstly combined with Mn2+/ Ni2+ to produce complex[Mn(NH3)6]2+/[Ni(NH3)6]2+, which is then resolved and released NH3 to offer OH- ions for the generation ofMn(OH)2/Ni(OH)2 monomer nuclei.26 Subsequently, more Ni2+/Mn2+ are absorbed on nuclei surface to emerge the initial nanoparticles (Figure 4-Step I). These fresh products are instability for the vast surface energy and tended to aggregate and form larger nanoparticles.29 Simultaneously, under the combined decreasing effects of free Ni2+/Mn2+ caused by the formation of NH3 based complex and OH− ions duo to the controlled release of NH4Cl/NaOH, the separation of nucleation and growth steps occur, which is a crucial factor for high-quality sheet-like crystallinity (Figure 4-Step II). As above mentioned, Mn2+ can convert to Mn3+ and replaces isomorphous Ni2+ presented in hydrotalcite-like phase under alkaline condition, so the continually aggregated hydroxide nanoparticles promptly begin to execute olation reactions and substitution process with each other to generate the primal Ni/Mn LDHs nano-compound as similar with the nanocore in Figure S4f (Figure 4-Step III).28, 29 As the procedure goes on deeper, more nano-compound gather at one nuclei center from various orientations and continue to incorporate vertically along c-axis and progress into a LDHs nanosheet step by step as the undeveloped nanosheets displayed in Figure S4e, in which the developing orientation 19 ACS Paragon Plus Environment


is mainly underwent by NH3 molecules preferably adsorbed on their particular (001) lattice plane due to the oriented attachment through hydrogen bond between nitrogen and hydroxyl (Figure 4-Step IV).33-35 In addition, it is worth mentioning that much more numbers of new LDHs nano-compound are also produced accompanied with the above Figure 4-Step IV, and some of those loading on the nanopetals gradually grow along various directions to form nanosheets in the same way, resulting in the favorable interconnected architecture with porous structure beginning to present as the described results derived from Figure S4a and Figure S4b (Figure 4-Step V). Finally, due to the self-assembly and Ostwald ripening, Ni/Mn LDHs hierarchical microspheres with 3D flower-like architecture are obtained through the further aggregation coordination of the nanosheets (Figure 4-Step VI). Accordingly, this facile, low-cost but efficient and controllably strategy, approved by the time-dependent experiments, can be expanded to fabricate other LDHs based functional electrode materials. 3.3 Electrochemical Properties and Mechanism analysis of Electrode Materials


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Figure 5. (a) The CV results of the Ni/Mn LDHs-, Ni(OH)2- and Co(OH)2- based electrode materials at a scan rate of 2 mV·s-1. Comparisons of (b) the GCD curves (1 A·g-1), (c) Cs and (d) Nyquist plots of the LDHs based electrodes prepared under various Ni:Mn feed ratios. The electrochemical performances of the 3D flower-like hierarchical Ni/Mn LDHs microspheres were examined through the CV, GCD and EIS, as displayed in Figure 5. From the contrastive CV curves (Figure 5a) with a potential window of 0-0.55 V under the scan rate of 2 mV·s-1, the Ni/Mn LDHs based electrode presents two distinct redox peaks existed in each sweep segment of the anodic and cathodic process, which can be attributed to the representative dual faradaic pseudocapacitive behaviors of Ni2+/Ni3+, Mn3+/Mn4+ and Mn2+/Mn3+ when LDHs reacting with OH- in alkaline electrolyte.40, 41 The position of the redox peaks of LDHs sample occur at a relatively lower potential, showing its well reversibility of rapid charge-discharge reaction. Additionally, the enclosed area of LDHs material (α-Ni(OH)2-like phase) dramatically surpass that of pure nickel hydroxide (β-Ni(OH)2 phase) and manganic 21 ACS Paragon Plus Environment


oxide due to the difference in their crystal structures (XRD in Figure 1), in which the layered structure possesses larger surface area and greater interlayer spacing than the granule structure, promoting the contact between electrolyte and active sites and the diffusion of electrolyte ions. And more, Figure S5a exhibits the CV curves of samples with serial feed ratios, it can be seen that the Ni/Mn LDHs with R=4:6 possess the largest enclosed area, indicating it have the best calculated capitative values, which is benefited from the increased redox sites caused by the structural and morphological characterizations as in Figure 4. The GCD curves were further used to research the electrochemical behaviors of the LDHs materials under various current densities, as shown in Figure 5b and Figure S5b. Matching with CV analysis, all electrodes display a classic pseudocapacitive feature with obviously nonlinear discharge platforms caused by the reduction processes. Accordingly, the capacitive results (C) of the products are calculated based on the discharge curves as shown in Figure 5c, which the Ni/Mn LDHs microspheres with R=4:6 yields the highest C values of 1379, 1252, 1177, 1039, 946 and 805 F·g-1 at current densities of 1, 2, 3, 5 7 and 10 A·g-1 respectively. An excellent retention rate of over 58 % is obtained even at 10 A·g-1 and the capacitance of 1379 F·g-1 (1 A·g-1) have been close to the theoretical capacitive value of Mn-based oxides electrode materials, which surpass the most of previously reported nickel/manganic oxides or hydroxides,47, 48 summarized as Table S2 in Supporting Information. Moreover, it must be pointed out that the change tendency of C values is in accordance with the morphology evolution of Ni/Mn LDHs obtained from sequential R as analyzed in 22 ACS Paragon Plus Environment

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Figure 3, which more perfect spherical structure with porous interconnected nanosheets can take more effective pseudocapacitive activity. The cycling performance of the LDHs electrodes with Ni:Mn=4:6 in Figure S5c shows that over 87% initial capacitor value is maintained after 400 cycles under 50 mV·s-1. In Addition, the C results of LDHs products synthesized with different times of alkali solubility show the optimal quantity of NaOH with 1.375 mmol (Figure S6). This well C can be certainly contributed to the 3D flower-like hierarchical constitution of LDHs microspheres as ascribed in Figure 1: the resultant favorable porous morphology can the accelerate diffusion rate and enlarge contact area between the electrolyte ions and the active species. The homogeneous distribution of element permits the rapid ion and electron transport, and the well crystal orientation enhances intercalation and deintercalation process of electron and ions. Subsequently, to approve the above merits, the EIS measurements was conducted (Figure 5d). All as-obtained samples display small equivalent series resistance caused by the low internal resistance, and the LDHs product of R=4:6 reveals the minimum bend diameter at high frequencies indicating the lowest intrinsic resistance of the active sites. Meanwhile, the slop of the LDHs product of R=4:6 is closer to Z" axis at low frequencies, showing its better reversibility and ultralow diffusion resistance for the various ions transport.



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Figure 6. (a) The fundamental principle and (b) the testing results of SFG spectroscopy used in this study under the KOH alkaline solution; (c) The schematic illustration of the whole capacitive source from (i) the pseudocapacitance and (ii) the non-faradaic capacitor; (d) Plots of i/ʋ1/2 vs. ʋ1/2 used to calculate k1 and k2 for Ni:Mn=4:6 at different potentials; (e) Double-layer charge storage contributions for Ni:Mn=2:8 and 6:4 (inset) at a scan rate of 2 mV·s-1; (f) Comparison of faradaic pseudocapacitor and double-layer capacitor contributions for different samples at rate of 2 mV·s-1. To investigate the reason why the change tendency of C values is in accordance with the morphology evolution of various Ni/Mn LDHs samples with different additive amounts of Mn element, the mechanism is further studied by the VSF spectroscopy and CSB analysis, as displayed in Figure 6. As above-mentioned redox current peaks in the CV curves, the typical pseudocapacitive behavior of Ni/Mn LDHs are corresponded to Ni2+/Ni3+, Mn3+/Mn4+ and Mn2+/Mn3+ under reacting with OH-, which can be expressed according to:27, 28 LDHs-Ni(OH)2 + OH-

LDHs-NiOOH + H2O + e-

(1)

LDHs-Mn(OH)2 + OH-

LDHs-MnOOH + H2O + e-

(2)

LDHs-MnOOH + OH-

LDHs-MnO2 + H2O + e-

(3)

It is well known that the adsorption mode of water molecules on solid surface is 24 ACS Paragon Plus Environment

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strongly dependent on the local environment of the surfaces such as surface charges and chemical groups.49 From the above equation, it can be seen that the hydroxyl group existed not only in the electrolyte but also on LDHs surface all participate in entire capacitive reaction, hence the status of OH can be chosen as a probe to in situ monitor the electrode surficial transformation process. The best tool to accomplish this task is SFG vibrational spectroscopy (SFG-VS). The fundamental principle of SFG-VS technique is demonstrated in Figure 6a, which is a second-order nonlinear optical process that oscillates at the sum of the two incoming laser frequencies:50 one visible (ω1, incident angle β1) and one IR (ω2, incident angle β2) laser beams overlap in an interfacial region of broken symmetry to generate a third beam (the SFG signal, ω=ω1+ω2, emergence angle β). In this study, the LDHs paste covering on Cu foil and Pt slice were chosen as the active electrodes respectively, and all components were sealed in a transparent crystal cover together with 6M KOH electrolyte or pure water. Figure S7a shows the SFG results of the sample reacting in pure water. Assigning the vibrational bands according to the previous reports,49-52 it can be seen that the broad 3000-3500 cm-1 band including ∼3200 cm-1 for “ice-like water” (tetrahedrally coordinated water molecules) and ∼3400 cm-1 for “liquid-like water” (a more random arrangement) observed in the R=8:2 sample is also presented in the R=4:6 case, but almost disappeared in the R=2:8 one. The ~3690 cm-1 peak can be readily assigned to “free OH” (without hydrogen bonding),50 while the assignment of the bands around 3520 cm-1, 3650 cm-1 and 3690 cm-1 is ambiguous, most likely originating from the small “water clusters” and surficial OH of LDHs (structural OH),51 respectively. 25 ACS Paragon Plus Environment


These peaks also display different statuses in the three samples, indicating that water species layers with different extents of hydrogen bonding exist at the LDH interfaces as varying the different Mn additive amount. The schematic is illustrated in Figure S7b, containing close Stern Layer (including free OH, water clusters and little liquid-like water) and Diffusion Layer (including ice-like water and liquid-like water). Contrastively, when the LDHs electrodes were immersed in KOH solution, the SFG signal of OH stretching region at ∼3000-3500 cm−1 disappears (“ice-like water” and “liquid-like water” layer) and only the spectra of ∼3500-3800 cm−1 is detected (Figure 6b). The disappearing of the ∼3000-3500 cm−1 bands in the presence of KOH reflects these bands are mainly contributed by the water molecules in the diffusion layer, which is polarized by the potential field generated near the charged surface and becomes very thin at high ionic strength due to the screening effect. Along with the increase of Mn elements contents, the SFG contribution from the diffusion layer in pure water becomes less and less, indicating that the negatively charged at R=8:2 gradually transforms to a relatively natural one upon the addition of Mn.52 That should be ascribed to the fact that the Ksp of Mn2+ with OH- to produce MnOOH/Mn(OH)2 is larger than that of Ni2+ (one fact is that MnCl2 will produce precipitation immediately when added to the NH4Cl2 solution, but NiCl2 will not), so that the LDH surface species at higher Mn loading is more difficult to ionize in water. A neutral surface is expected to exert less energy barrier for the OH- ions to approach than the negatively charged one. This feature is beneficial to decrease electrolyte ion diffusion resistance and expose the surficial active sites on LDHs nanosheets, leading 26 ACS Paragon Plus Environment

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to superior electrochemical behavior. Another factor besides the surface charge is the hydrogen bonding status of water molecules closely adjacent to the electrode surface (the Stern Layer as displayed in Figure S7b). In the presence of high concentration of KOH, the absence of free OH band and the maintenance of the clustering water band at ~3520 cm-1 for the higher Mn loading samples imply that there exists water hydrogen bonding network tightly associated with the LDH surfaces in such cases. This hydrogen bonding network will cost additional energy for the OH- ions to pass through the electrode/electrolyte interface, and therefore showing a contrary role to the surface neutralization effect. As a balance of these two opposite factors, the Ni/Mn LDHs microspheres with R=4:6 show the optimal electrochemical performances. This analysis indeed approves the change tendency of C values is in accordance with the Mn additive amounts of Ni/Mn LDHs. On the other side, the whole stored charges in this flower-like LDHs based electrode materials can be divided into two components as schematically illustrated in Figure 6c: (i) one is the pseudocapacitive contribution of the diffusion limited reaction from the faradaic redox process of transition metal elements with the electrolyte (OH-) as ascribed in equation (1), (2) and (3), which the charge-transfer behaviors occur with active atoms not only on the outside surface but also in the interlayer lattice planes; (ii) the other is the non-faradaic contribution from the surficial plate (double-layer) capacitor arising from the positive and negative charges gathering on the surface of adjacent nanoplates.53 Accordioning the CSB analysis reported by Bruce Dunn,54 the current (i) from the CV curves can be separated as together 27 ACS Paragon Plus Environment


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combination of the above-mentioned two mechanisms of double-layer (plate) capacitor (k1v) and diffusion limited

(faradaic) reaction (k2v1/2) as follows:

i = k1ʋ + k2ʋ1/2

(4)

where ʋ is the scan rate. The fraction of the above components can be distinguished quantitatively based on the calculation of the constant values of k1 and k2 that represent the double-layer and faradaic capacitor, respectively. Here, the cathodic (Figure 6d) and anodic (Figure S8a) voltammetric results for Ni/Mn LDHs with R=4:6 were used to analyze the linear behavior of i/ʋ 1/2 versus ʋ1/2 to obtain k1 and k2 values from the slope and the y-axis intercept point of a fitting straight line at each fixed potential varied from 0.5 to 0.04V under scan rate of 2 to 20 mV·s-1.55 As a result, though the relative contribution of plate process compared with faradaic reaction in a individual sample with R=4:6 is about 31.5% at 2 mV·s-1 (Figure 6e), which is lower than that for samples with R=6:4 (about 39.5%, inset of Figure 6e) and 2:8 (Figure S8b), its calculated absolute capacitor value is the maximum of various samples. Figure 6f summarizes the practical contribution values for the samples with various feed ratios, indicating that the LDHs microspheres (R=4:6) with the optimal 3D flower-like interconnected morphology possesses not only the highest pseudocapacitive values (944.75 F·g-1) but also the maximal plate capacitor value (434.25 F·g-1), which completely verify the direct correlation between the change tendency of C values and the gradually morphology evolution of various Ni/Mn LDHs samples in Figure 3. The above favorable capacitive constitution can be attributed to their well synergistic effect of plentiful active sites 28 ACS Paragon Plus Environment

(different feed ratios)

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and large surface area (different architectural features), which are the fatal factors for achieving excellent electrochemical property. 3.4 Electrochemical Performances of the Ni/Mn LDHs Based FAAS

Figure 7. (a) CV results of the FAAS measured at various voltage windows; (b) GCD results of the FAAS at various current densities; (c) Capacitance values obtained from the GCD; (d) the 5000-cyclic behavior measured at 50 mV·s-1, and the inset is a Red LED powered by the tandem FAAS; (e-f) Ragone plots of the FAAS, and the comparison with references. Flexible supercapacitors are required to possess broad working voltage widow (V), high energy density (E), and satisfactory cyclic stability for desired performance. Notably, according to the calculation method of energy density of E = 0.5×C×V2, the E can be improved by enhancing the C value together with the V range.56 Thus, a feasible strategy to increase E is to prepare the asymmetric FASs, which can take full advantage of two different electrode materials to offer the maximum V window 29 ACS Paragon Plus Environment


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integrated in a unit system. Here, an FAAS device was assembled by applying this Ni/Mn LDHs microspheres and active carbon as positive and negative materials respectively, and its constitution is illustrated in the inset of Figure S9a. From the CV results in Figure 7a, it can be seen that the working window is successfully extended from 0.55 to 1.55 V, resulting in the received E of this FAAS device enlarged by almost 8 times than its composed electrode materials with operating window of 0.55 V. Figure S9a exhibits the CV curves of FAAS scanned from 5 mV·s-1 to 200 mV·s-1, which appears almost symmetrical rectangular patterns for associated action of faradaic pseudocapacitors and parallel-plate capacitors,57 demonstrating its favorable capacitive feature in the overall charged-discharged process. The GCD results of FAAS device in Figure 7b show that the discharge activity is nearly symmetrical and different from the charge process, illustrating the linearity of GCD versus time in according with the CV results. The Cs is calculated to be 393, 347, 306, 226 and 148 F·g-1 under current densities of 1, 2, 3, 5 and 10 A·g-1 through the discharge curve respectively (Figure 7c), which surpass the previous reported flexible all-solid-state supercapacitors.8,

12, 16, 18, 19, 29, 38, 39 ,58

The EIS result was

conducted in Figure S9b, the Nyquist plots curve and its equivalent circuit of the FAAS device reveal its intrinsic chemical electrode resistance (RL) and charge transfer interfacial resistance (RP) are about 1.1 Ohm and 2.0 Ohm, respectively. The flexibility of the assembled device was also evaluated under various bending radius up to 120° and the result was displayed in Figure S9c, indicating its well mechanical flexibility without cracks producing caused by the ion transportation activity between 30 ACS Paragon Plus Environment

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electrode materials and electrolyte cannot be impacted. Cyclic stability of the FAAS is also a key parameter for practical application, which was measured by CV method for 5000 cycles under high speed of 50 mV·s-1, showing the Cs still reserves over 75.1% of the initial capacitive value (Figure 7d). These results demonstrate the great stability of the flexible supercapacitors. Benefited from the above measured properties, we employed this FAAS device to demonstrate powering light-emitting diodes (LEDs). It can be seen that two tandem devices can charge the LEDs with various color of red (Figure 7d, insert), green and blue (Figure S10) owning switching voltages of 1.74, 2.26 and 2.56 V, respectively. According to the Cs results, Figure 7e summarized the Ragone plot of FAAS device. Energy densities (E) of this flexible supercapacitor are calculated to be 131.17, 115.72, 102.21, 75.28 and 49.45 Wh·Kg-1 at Pave of 1.45, 2.85, 4.09, 6.78 and 17.80 KW·Kg-1, respectively. So, the largest values of 131.17 Wh·kg-1 and 17.80 kW·kg-1 for energy and power densities are obtained in this research. Especially, the received maximum E value of 165.5 Wh·kg-1 at 1.53 kW·kg-1 of this 3D flower-like Ni/Mn microspheres based FAAS device have surpassed most of previously reported Ni/Mn oxides and hydroxides electrodes based flexible supercapacitors (Figure 7f),19, 20, 25, 31-33, 35-37, 47, 59

such as flower-like Ni/Al LDHs grown on nickel foam-based FASs

(30.2 Wh·kg-1 at 0.8 kW·kg-1),19 3D-architectured Ni-Co-Mn LDHs/reduced graphene oxide composite-based FASs (92.8 Wh·kg-1 at 0.46 kW·kg-1)20 and Co/Mn LDHs-based FASs (23.7 Wh·kg-1 at 0.28 kW·kg-1).25 Though the energy density is 186 Wh·kg-1, the energy density of Ni(OH)2-MnO2//RGO-based FASs is as low as 0.78 31 ACS Paragon Plus Environment


kW·kg-1,35 and it is not an all-solid-state supercapacitor which uses aqueous KOH solution as electrolyte.

4. Conclusions In summary, we proposed a facile hypothermal self-assembled chemical co-precipitation strategy at mild conditions to synthesis Ni/Mn LDHs microspheres with 3D flower-like hierarchical structure in a large scale. The ultrathin 2D nanopetals of Ni/Mn LDHs microspheres can not only provide the sufficient reactive diffusion area but also plentiful reaction sites for enhanced faradaic redox. Benefiting from the favorable structural characters, the Ni/Mn LDHs microspheres showed capacitance value of 1379 F·g−1 at 1 A·g−1. The Sum Frequency Generation (SFG) spectroscopy and CSB analysis were employed to reveal the relation between the change tendency of C values and the morphology evolution of various Ni/Mn LDHs samples with different additive amounts of Mn element. Moreover, a novel Flexible All-solid-state Asymmetric Supercapacitors (FAAS) was assembled by exploiting Ni/Mn LDHs and active carbon as positive and negative materials respectively, displaying an excellent electrochemical performance with high capacitance value and energy/power densities. This work is expected to lay theoretical and experimental evidences for the development of LDHs based high-powered flexible energy devices in the future. Supporting Information: Supporting Information is available from the ACS Online Library or from the author. Surface area, Large-scale preparation strategy, Optical photographs, SEM images with 32 ACS Paragon Plus Environment

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different heating times, SEM images with different NaOH concentrations, Performances comparison of supercapacitors, SFG spectroscopy and CSB analysis, Performances of the solid-state FAAS, Different color LEDs were powered by the devices.

Acknowledgements: The authors acknowledge the funding support from the National Natural Science Foundation of China (51702354, 61574163), the Science Foundation for Distinguished Young Scholars of Jiangsu Province (BK20170008), the Science Foundation Research Project of Jiangsu Province (BK20150364), and the China Postdoctoral Science Foundation (2015M571837). And the NANO-X Workstation scientifically supported this research. This infrastructure would not be possible without the significant contributions of Chinese Academy of Sciences, Jiangsu Province, Suzhou City, Suzhou Industrial Park.

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Supercapacitors.

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Table of Contents Figure:


Mn

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