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Enhanced High Temperature Cyclic Stability of Aldoped Manganese Dioxide and Morphology Evolution Study through In-situ NMR under High Magnetic Field Shenggen Huang, Jian Sun, Jian Yan, Jia-Qin Liu, Weijie Wang, Qingqing Qin, Wenping Mao, Wei Xu, Yu Cheng Wu, and Junfeng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18762 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Applied Materials & Interfaces

Enhanced High Temperature Cyclic Stability of Al-doped Manganese Dioxide and Morphology Evolution Study Through In-situ NMR Under High Magnetic Field #, Shenggen Huang†, Jian Sun†, Jian Yan†, ∗, Jiaqin Liu†, Weijie Wang†, Qingqing Qin†, Wenping Mao‡,*,

Wei Xu§, Yucheng Wu†, ,* , Junfeng Wang‡ #

†School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China ‡High Magnetic Field Laboratory of Chinese Academy of Sciences, Hefei 230031, Anhui, China #Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei 230009, China §Key Lab of Material Physics, Institute of Solid State Physics, Hefei 230031, Anhui, China

KEY WORD:

cycling stability, morphology evolution, Al doped MnO2, supercapacitor, In-situ

NMR

∗

To whom correspondence should be addressed.

†School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China #Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei 230009, China Email: [email protected], [email protected] ‡ High Magnetic Field Laboratory of Chinese Academy of Sciences, Hefei 230031, Anhui, China Email: [email protected] 1

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ABSTRACT: In this work, Al-doped MnO2 (Al-MO) nanoparticles have been synthesized by a simple chemical method with the aim to enhance cycling stability. At room temperature and 50 °C, the specific capacitances of Al-MO are well maintained after 10000 cycles. Compared with pure MnO2 nanospheres (180.6 F g-1 at 1 A g-1), Al-MO also delivers an enhanced specific capacitance of 264.6 F g-1 at 1 A g-1. During cycling test, Al-MO exhibited relatively stable structure initially and transformed to needle-like structures finally both at room temperature and high temperature. In order to reveal the morphology evolution process, in-situ NMR under high magnetic field has been carried out to probe the dynamics of structural properties. The morphology

evolution

may

follow

23

Na spectra and the SEM observation suggest that the pulverization/reassembling

process.

The

Na+

intercalation/deintercalation induced pulverization leading to the formation of tiny MnO2 nanoparticles. After that, the pulverized tiny nanoparticles reassembled into new structures. In Al-MO electrodes, doping of Al3+ could slow down this structure evolution process, resulting in a better electrochemical stability. This work deepens the understanding on the structural changes in faradic reaction of pseudo-capacitive materials. It is also important for the practical applications of MnO2 based supercapacitors.

Specific Capacitance (F g-1)

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240 220 200

50 °C

180

25 °C

160 0

2000

4000

6000

Cycles

8000

10000

2


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INTRODUCTION: Recently, supercapacitors are particularly attractive due to their high power density, long cycle life and fast charge/discharge rate, good safety and non-pollution.1, 2 Cycle life is a very important factor for the practical applications of supercapacitors. The long cycle life is commonly the typical feature of carbon based electrical double layer capacitors (EDLCs).3, 4 The cycle number could be up to million times since the charge-discharge in EDLCs is a physical process. Different from that, charge-discharge process of pseudo-capacitors is based on the fast faradic reaction, leading to lower cycling stability but higher energy density.5-8 Among pseudo-capacitive materials, MnO2 shows the best cycling performance with the specific capacitance could remain stable up to ten thousand times at room temperature and low temperature.9 Recently, several groups have reported that the capacitance retention is above 95% after 10000 cycles test.10-12 Meanwhile, MnO2 also exhibit advantages of natural abundance, low cost, environment friendly and high theoretical specific capacitance (1370 F g-1), which make MnO2 a promising electrode material.13, 14 However, at elevated temperature, the capacitance degradation of MnO2 becomes severe.15 The specific capacitance might decrease very fast. For example, Kang’s group reported a fast capacitance loss up to 18.6% after 1000 cycles tested at 75 °C.16 During practical usage, supercapacitors are supposed to deliver high output, which may lead to temperature rising. In summer, the working environmental temperature might be up to 40 ~ 50 °C. In such severe cases, the degradation of devices may be accelerated leading to a short life. Therefore, the cycling stability at elevated temperature is necessary to be improved. Typically, “dissolution” is accepted as the main reason causing the capacitance loss of MnO2 based electrodes.17 However, recent reports indicate that the cycling stability is related to the morphology transformation.18 During the charge-discharge process, MnO2 nanostructures or composites would 3


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transform to flake-like or wire-like structures.18 As an example, Li et al. reported the structure change of carbon nanotubes/MnO2 composites during cycling test.19 Ivey’s group suggested that the morphology evolution might follow a dissolution/redeposition process.20 However, the absence of Mn2+ in the electrolyte and the clear color of electrolyte after cycling test indicated no “dissolution”.18 It is not clear what induces such structural evolution; it might be related to the intrinsic physiochemical properties of MnO2 species considering that the combination of MnO2 with other materials could not prevent the structure change.18, 19 Doping is an effective method to change the intrinsic properties of materials. Therefore, in this work we attempt to use doping method to tailoring the physiochemical properties of MnO2 with the aim to improve cycling stability. Since radius of Al3+ (53.5 pm) and Mn4+ (53 pm) are quite close, Al3+ might be a good dopant candidate.21 In addition, Al-doped α-MnO2 shows enhanced specific capacitance (213 F g-1 at A g-1) and good cycling performance with capacitance retention of 91% after 15,000 cycles at 2 A g-1.22 Huang’s and Yang’s works also suggested that Al doping is a promising method to improve the performance of MnO2.23, 24 On the other hand, morphology evolution brings up new questions on the charge-discharge process of MnO2. Generally, the charges are stored by fast faradic reactions, which happens on the surface or near-surface as reported by Toupin et al.5 It is thought to be reversible. However, the morphology evolution indicates that MnO2 could not maintain its structure. Meanwhile, capacitive behavior has been found in LiCoO2, typical battery type material, when the size is decreased to 6 nm.25 It blurs the gap between pseudo-capacitive behavior and the battery electrochemical reaction. Detailed study on the morphology transformation process of MnO2 will deepen the understanding on charge storage mechanism. This would be of benefit for achieving high performance. Among various characterization technologies, in-situ nuclear magnetic resonance (NMR) is a very powerful one. NMR chemical shift 4


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is very sensitive to the changes in the surrounding environment and reveal important information about the substance of interests.26 By tracing a characteristic ion, the NMR signal could provide the structural changes and dynamic information in real time during the charge-discharge process.27, 28 In situ or ex situ NMR have been employed to study the electrolyte issue, electrolyte ion diffusion and charge storage mechanisms in the field of EDLC type supercapacitors by Simon, Grey and other groups.28-31 In-situ NMR would be a good candidate to investigate the morphology evolution of MnO2 electrode during charge-discharge process. Note that the NMR spectra of nucleus in solid phase normally show low resolution with broad peaks.32 It is a powerful method in revealing the short range structure and dynamic information of the active materials where intercalation/deintercalation happens. It is also complementary to In-situ Raman and In-situ XRD technologies since they can only detect the overall long range order characteristics of the sample.33, 34 In this work, Al-doped MnO2 (Al-MO) nanoparticles have been synthesized by a simple chemical method. At 50 °C, the capacitance is well maintained after 10000 cycles indicating an excellent cycling stability. The Al-MO also shows enhanced specific capacitance of 264.6 F g-1 at 1 A g-1 compared with pure MnO2 nanospheres (MO) (180.6 F g-1 at 1 A g-1). During cycling test, Al-MO transformed to needle-like structure at room temperature and 50 °C, in contrast to the morphology evolution trend of MO electrodes. In-situ NMR under high magnetic field (14.1 T) has been employed to reveal the structural information during the evolution process. The results suggest that the MnO2 might endure pulverization/reassembling process during charge-discharge. A simple model is developed to illustrate this process. These findings would deepen the understanding on the long cycling charge storage mechanism of pseudo-capacitive behavior of MnO2 electrode. It also provides a useful method to enhance the cycling stability of MnO2. 5



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Results and Discussions 1. Material characterization The morphology of the as-synthesized samples characterized by field emission scanning electron microscopy (FE-SEM) is shown in Figure 1. Figure 1a and b depict that Al-MO are composed with a large number of solid nanoparticles with the diameter of 200-300 nm. The pure MO shows fluffy sphere-like morphology with hundred nanometers in diameter as indicated in Figure S1a. Figure S1b shows typical X-ray diffraction (XRD) patterns of Al-MO and MO. The main distinct diffraction peaks observed in MO could be indexed to the α-manganese dioxide (JCPDF No.44-0141).35 The broad and weak peaks indicate the poor crystallinity. As to Al-doped MnO2, no diffraction peaks from aluminum oxide have been observed, indicating that no crystallized aluminum oxide is formed.36 In Figure S1c and d, the element analyses indicate the presence of Al detected by the energy-dispersive X-ray spectroscopy (EDS) attached to the FE-SEM. The transmission electron microscopy (TEM) image (Figure 1c) and high resolution TEM (HRTEM, Figure 1d) indicate that the Al-MO is composed with many smaller particles (Figure S1e). In the HRTEM image (Figure 1d), the marked interplanar d-spacing of about 0.49 nm corresponds to that of the (200) lattice planes of α-MnO2 (JCPDF NO.44-0141). This is in agreement with the XRD result. The selected area electron diffraction (SAED) indicates the polycrystalline nature. The EDS attached to TEM was employed to detect the chemical composition with elements mapping shown in Figure 1e-g. The uniform dispersion of Al elements suggests the successful doping considering that no peaks from aluminum oxide was found in XRD patterns. In order to further investigate the chemical composition of the Al-MO, X-ray photoelectron spectra analyses (XPS) were carried out. Figure 2a is the survey spectrum of Al-MO with Al, Mn, O and C 6


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elements detected. Figure 2b shows the XPS spectrum of the Mn 2p. The peaks at 642.4 and 654.1 eV can be assigned to the signals of Mn 2p3/2 and Mn 2p1/2, respectively. The spin-energy separation of the two peaks is 11.7 eV, which indicates that the chemical state of Mn is 4+. This is in accordance with the value reported for MnO2.37,

38

The spectrum of O 1s in Figure 2c shows two types of

contributions from oxygen, which can be divided to two components at 529.8eV, 531.5eV, corresponding to the oxygen species in the MnO2 (Mn-O-Mn), hydroxyl groups (Mn-O-H), respectively.32,39, 40 A weak peak of Al 2p located at 74.1 eV is shown in Figure 2d.21, 41 The Al 2p peak exhibits a symmetry feature with a full width at half maxima of 1.93 eV, and in this spectrum did not find the metallic aluminum peak at 72.7eV.41, 42 This result shows one valence state of Al3+ indicates that Al elements have been successfully doped into the MnO2 nanoparticles lattice. 2. Electrochemical characterization Figure 3a shows the cyclic voltammetry (CV) curves of Al-MO in 1.0 M Na2SO4 electrolyte at scan rate from 5 to 100 mV s-1 in a potential range of -0.1-0.9 V (vs Ag/AgCl). The CV curves of Al-MO exhibit typical rectangular shape, indicating an ideal capacitive behavior. MO electrodes also show similar box-like CV curves as indicated in Figure S2a. The Al-MO electrode was then tested by galvanostatic charge-discharge (CD) at current densities from 1 A g-1 to 50 A g-1 at room temperature (25 °C). As shown in Figure 3b, the curves are in the triangle shape suggesting the good capacitive behavior. Similar CD curves of MO are displayed in Figure S2b. At high temperature (50 °C), CD measurements of Al-MO and MO electrodes were also tested with curves shown in Figure 3c and Figure S2c, respectively. The specific capacities calculated from the CD curves are displayed in Figure 3d. At 1 A g-1, the specific capacitance of Al-MO electrode is 264.6 F g-1, which is clear higher than180.6 F g-1 obtained from MO electrode at room temperature. The specific capacitance of Al-MO 7



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electrode could remain at 114 F g-1 at 50 A g-1, indicating a relative good rate capability. These results demonstrate that Al3+ doping could significantly improve the capacitive performance of MnO2 nanomaterial.22,

24

The increase in specific capacitance of Al-MO might be due to the enhanced

conductivity.21, 22 Better conductivity is important for achieving high electrochemical performance.43 At 50 °C, the specific capacitances of both electrodes exhibit higher value due to the higher ionic conductivity and accelerated faradic reaction.9 Long cycle life is one key parameter for the practical application of supercapacitors, especially at high temperatures. Figure 3e, f depict the cycling performance of Al-MO and MO at room temperature and high temperature (50 °C) tested at a current density of 10 A g-1, respectively. At room temperature, both Al-MO and MO show good cycle performance with 100% capacitance retention after 10 000 cycles. For both samples, no color change was observed. In some recent reports, such good cycling performance has also been achieved.10-12, 18 However, at high temperature the cycling stability is much poorer. Fast capacitance degradation has been reported by many groups.15, 16 For example, Hu’s group reported considerable capacitance loss of MnO2 nanobelts tested at 50 °C.15 The MO electrodes also show capacitance degradation above 10 percent after 10000 cycles, which is similar to our previous report.18, 44 The color of the electrolyte became yellow after cycling test, suggesting peeling off of active materials from the electrode.18 In contrast, the Al-MO electrodes exhibit very good cycling stability at 50 °C. After 10000 cycles test, the specific capacitance still remains about 100% of its original value. No color change was observed. It suggests that doping of Al3+ in MnO2 nanostructures could enhance the cycling stability effectively and prevent the “dissolution” of MnO2 into electrolytes. This finding is of importance to the practical application of manganese dioxides based supercapacitors at high temperature. 8


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3. Morphology Transformation Observation The Al-MO exhibits excellent electrochemical stability at high temperature. However, the reason is not clear. In previous works, it was found that the capacitance degradation of MnO2 is related to the morphological evolution.18-20 Therefore, the morphologies of Al-MO and MO were checked to disclose the effect of Al dopant on the microstructure change during charge-discharge process. FE-SEM observation was carried out to monitor the morphology of the samples collected at 100th, 500th, 2000th, 5000th and 10000th cycle at 10 A g-1. At room temperature, MO nanospheres tend to form flakes firstly and then transform to nanowires during cycling test as shown in Figure S3, which is in agreement with previous reports.18 However, Al-MO exhibits a different structure evolution trend as depicted in Figure 4. In the initial 500 cycles, the Al-MO could maintain its original morphology indicating good structure stability. At 2000th cycle, needle-like tip grown on the side of the Al-MO nanoparticle could be observed. In the following cycles, the needle-like structure became longer and more needles could be found. On each Al-MO nanoparticle, the needle-like structures exhibit preferred direction and are parallel to each other. After 10000 cycles, this feature could be still observed. The shrinking of the initial Al-MO particle suggests that the growth of the needle-like structure is based on the consumption of the particle itself. The structures of Al-MO after cycling test were further characterized by the TEM. Paralleled needle-like strips could be clearly observed as shown in Figure 4g. The inserted SAED pattern indicates that the needle-like strips have preferred grow direction. In Figure 4h, the HRTEM image was taken from a single needle-like strip. The marked interplanar d-spacing of about 0.23 nm could correspond to that of the (211) lattice planes of α-MnO2 (JCPDF NO44-0141), indicating that the crystal structure did not change. This is different from the flakes

9



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formed after cycling test of MO. These results clearly suggest that doping of Al3+ in MnO2 has affected the morphology evolution process. The morphology evolution of Al-MO electrode at high temperature has also been monitored by FE-SEM observation with the samples collected at 100th, 500th, 2000th, 5000th and 10000th cycle, as presented in Figure 5. It could maintain the structure initially. Then, needle-like structure started to grow on the side of the Al-MO nanoparticles. After that, the needle-like structure became more and longer. It shows the same trend of Al-MO tested at room temperature. The results suggest that the temperature shows limited effect on the morphology evolution process of Al-MO. For comparison, MO transformed to nanowires at high temperature as shown in Figure S4. This might be the apparent reason that the Al-MO exhibits better cycling stability at high temperature. Possibly, doping of Al3+ in the MnO2 may enhance the thermodynamic stability. 4. In-situ NMR investigation and structure evolution process In order to further investigate the relationship between the morphology transformation and the electrochemical

stability,

in-situ

NMR

experiments

were

carried

out

to

study

the

intercalation/deintercalation of Na+ into Al-MO and MO nanostructures during charge-discharge process. With the aim to place a single electrode in the NMR detection region, the positive electrode (Al-MO or MO) and negative electrode (activated carbon, AC) are shifted with a gap in between as shown in Figure 6a. It is similar to Grey’s cell design.26 The Al-MO was coated on the carbon fiber paper (CFP) with an area of 4mm * 8mm. The negative electrode was coated with AC electrode on the CFP with an area of 4mm * 8mm. The gap is about 4 mm long. After adding 1M Na2SO4 electrolyte, the cell was vacuum sealed using a plastic bag. Ag strips serve as the connecting wire. The photograph of the device is also shown in Figure 6a. 10


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The devices were tested by CD method at a current of 60 μA in the potential window of 0 ~ 1.2 V as shown in Figure S5a and b. During the charge-discharge process, the chemical shift signals were collected at a series of different voltages (0, 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 V). In this work, we focus on 23

Na NMR to study the structure change of active materials. Figure 6b depicts the 23Na NMR spectra

obtained for the MO (~1.5 mg) electrode during CD test with curves shown in Figure S5a. The resonance at about -2 ppm (Figure 6b, Figure S5c, d) could be assigned to the electrolyte cations since the peak position did not change in CD process. The broad peaks in the region from 10 to 2 ppm could be assigned to the Na+ intercalated/deintercalation in the MO. At different voltage, the position of the peak also shifts indicating the structure changes. It could be found that the peaks denoting the Na+ intercalation/deintercalation in MO still exhibit clear changes at each cycle. From 2nd to 20th cycle, the peak position shifts to higher frequency and the line shape also changes as indicated in Figure 6b and Figure S5c and d. It suggests that the chemical environment of Na+ in the MO varies a lot in a short period, and likely accompanied by fast structural change. Previous literatures reported that in the redox reaction, Mn2+ ions were dissolved into the electrolyte, and then redeposited on the surface of manganese dioxide, resulting in morphology evolution during cycles.45 In Figure 6b, the 23Na spectra of MO electrode suggests structural change that are repeatedly occurring (refer to the cyclic shift of the peak at 8ppm). It is rational to deduce that Na ions play an important role in the morphology evolution especially in the first cycle, after which the intercalation and deintercalation of Na ions can be traced with the repeated peak shift indicating the structural dissolution and precipitation are occurring in cyclic manner. As to the Al-MO, the chemical shift denoting the Na+ intercalation/deintercalation is highly repeatable during cycling, even in the first cycle in which no peak shift occurs (Figure 6c and Figure S5e and f). It indicates that the Al-MO could maintain its structure. Doping of Al3+ in the 11



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manganese oxides has enhanced the structural stability, especially in preserving the structural integrity during the first cycle. This result coincides with the FE-SEM observation mentioned above. Note that the active materials are in solid state. Therefore, the NMR spectra of 23Na exhibit broad features. It is hard to identify the exact structure of active material. But, they still suggest qualitatively that the Na+ intercalation/deintercalation in active materials plays an important role in the morphology evolution, which is of benefit to disclose the detailed charge-discharge process. Normally, the faradic reaction is recognized to be different from the battery type reactions since it only happens on the surface/near-surface of the electrode materials.5 It is thought to be highly reversible.25 In 2014, pseudo-capacitive behavior has been found in typical battery electrode material, namely, LiCoO2 with 6 nm in size.25 This result indicates that there is similarity between battery type reactions and faradic reactions. In Li-ion batteries, the electrode materials often suffer from pulverization due to the volume change.46 Recently, intercalation/deintercalation of Na+ in MnO2 nanowires have been investigated by in-situ TEM.47 Volume change and tunnel structure degradation has been observed due to the intercalation/deintercalation of Na ion. In typical charge-discharge process of MnO2, it is reasonable to deduce that the Na+ intercalation/deintercalation in MnO2 surface/near-surface could cause the volume change and structure change. In previous report, “dissolution/redeposition process” is employed to illustrate the morphology transformation. If this is true, there should be “dissolution” of MnO2 into electrolyte. Typically, “dissolution” of MnO2 would cause color change of electrolyte. However, the color of electrolyte remained clear during cycling test. Furthermore, in previous report, it has demonstrated that no Mn2+ was found in electrolyte.18 Therefore, the morphology transformation could not follow the “dissolution/redeposition process”. Based on FE-SEM observation and in-situ NMR analyses, we 12


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propose a model to illustrate the structure evolution process as indicated in Figure 7. During discharge, the Na ions intercalate in the MnO2 nanoparticles. It could induce the volume expansion of MnO2 surface layer. In the charge process, the Na ions deintercalate from MnO2 resulting in volume shrinking. Repeating of the charge/discharge process would induce the pulverization of the surface layer. Tiny manganese oxides particles may form in this stage. In our case, the Al-MO and MO are both composed of such tiny crystals as indicated in the TEM images (Figure 1c and Figure S1e). During the volume expansion/shrinking, the tiny manganese oxides particles may reassemble together in preferred crystal direction resulting in formation of new needle-like strips on the side of Al-MnO2 nanoparticles as indicated by the SEM images of Al-MO after cycling. Morphology after cycling might be also nanoflakes or nanowires in the case of MO, which might be determined by the thermodynamic features. Considering the structure degradation, the newly formed nanostructure might be in amorphous state as reported previously.18, 47 The bonding between the tiny manganese oxides particles might be strong or weak. In case of weak bonding, the tiny particles may be separated and detached into electrolyte by liquid flow agitation.48 With increase of the tiny particles suspended in the electrolyte, the electrolyte would turn into yellow color, which is indicated in Figure S6. Due to the loss of active materials, it commonly leads to capacitance degradation resulting in lower stability. It happens more frequently at elevated temperature, which is possibly due to the more intense destabilization. In our case, the MO electrodes present lower stability, which show a fast morphology transformation process. On the contrary, doping of Al3+ in manganese oxides (Al-MO) may enhance the bonding between these tiny particles resulting in a slow transformation process. Otherwise, color change of electrolyte may be observed during the cycling test of Al-MO electrodes. In Figure 1a and b, the Al-MO nanoparticles are solid, while the MO is in the form of fluffy nanosphere (Figure S1a). The 13



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difference in the structure features may support that the bonding between secondary nanoparticles in Al-MO is stronger than that in MO. In addition, a recent paper reported that Al doping could narrow the band gap and increase the active oxygen.21 High electron concentration is of benefit to trap Na+.21 Therefore, Al-MO exhibits better stability during cycling test, which might be derived from better thermodynamic stability. However, the volume change during Na+ intercalation/deintercalation in MnO2 is inevitable, Al-MO still exhibit structure evolution finally. It could be seen that the Na+ intercalation/deintercalation could induce pulverization of MnO2, which is similar to that in battery type reactions.46 Differently, pulverized MnO2 particles could reassemble into new structure. The above results are of benefit for the deep understanding on the pseudo-capacitive behavior of MnO2. Conclusions In this work, Al-doped MnO2 and MnO2 have been synthesized by a simple chemical precipitation method. Al-MO exhibits superior cycling stability at room temperature and 50 °C with capacitance well maintained after ten thousand cycles. It also delivers an enhanced specific capacitance of 264.6 F g-1 at 1 A g-1, which is clearly higher than that achieved in MO electrodes. After cycling test, Al-MO nanoparticles transformed to needle-like structures at room temperature and high temperature. In-situ NMR under high magnetic field is employed to disclose the structure evolution process. During charge-discharge process, Na+ intercalation/deintercalation could induce pulverization of MnO2. Then the pulverized tiny MnO2 nanoparticles reassemble into new structures, resulting in morphology transformation. Doping of Al3+ in the MnO2 could slow down this structure transformation process, leading to the good cycling stability. It could deepen the understanding on the charge storage mechanism of pseudo-capacitive supercapacitors. These findings are also important for practical

14


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applications of MnO2 based supercapacitors working under high temperature environment or high output condition. Experiment section Synthesis of Al-MO and MO Al-doped MnO2 was prepared by a simple precipitation method. Firstly, 5.7 mL 0.5 M MnSO4 solution stirred about 30 min followed by adding 15 mL 0.01 M AlCl3 solution. Then, the 0.5 M KMnO4 solution (4 mL) was added into solution under stirring. The obtained precipitation was filtered, washed with distilled water and absolute ethanol. Finally the products were freeze-dried. The synthesis of MO follows the same method without adding AlCl3 solution. Characterization The crystal structure of the product was investigated by XRD (X’Pert PRO MPD), using Cu Kα (λ = 0.15406 nm) radiation at 50 kV and 50 mA in a 2θ range from 10 to 80°. The morphology and structure of the samples were characterized by FE-SEM (SU8020) with EDS, TEM (JEOL 2010) with EDS, X-ray photoeletron spectrometer (XPS, Thermo ESCALAB 250). Electrochemical Measurement and NMR Characterization A mixture of 80 wt % active materials, 15 wt % super-p and 5 wt % polyvinylidenefluoride binder (PVDF) was dissolved in N-methylpyrrolidone and stirred for more than 10 hours. Then, the slurry was coated on graphene paper (GP, 1.0 × 2.0 cm) in an area of 1.0 × 1.0 cm. After drying at 100 °C for 10 hours, the Al-MO and MO electrodes were prepared. Electrochemical measurements were carried out by an electrochemical analyzer (Autolab Potentiostat, PGSTAT101). The three-electrode cell consisted of the as-synthesized sample as the working electrode, Ag/AgCl as the reference electrode and Pt as the counter electrode. Aqueous 15



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solution of 1 M Na2SO4 was served as electrolyte. CV was done at different scan rates of 5, 10, 20, 50, and 100 mV s-1. Galvanostatic charge/discharge curves were measured at 1, 2, 5, 10, 20, 50 A g-1. The potential window in the range of -0.1-0.9 V was used in three electrode measurements. The cycling stabilities tested at different temperatures were performed by LAND BT2013S supercapacitor test system in temperature controller box with accuracy ±1 °C (Shanghai Shuli, SPC-70). For in-Situ NMR test, Al-MO, MO and AC electrode coated on carbon fiber paper with size of 4 mm × 22 mm were prepared. The slurry is prepared using the above method. Asymmetric devices (Al-MO//AC and MO//AC) have been assembled. The positive electrode material is MO or Al-MO, while the negative electrode material is AC. Filter paper serves as the separator. Electrolyte is 1M Na2SO4 solution. Silver strips serves as the connecting wire. The devices are vacuum sealed using a plastic bag. The in-situ NMR experiments were performed on a Bruker AVANCE III 600 spectrometer equipped with a home-built static probe.

Supporting Information Available: Structural characterization of MO and Al-MO (Figure S1), electrochemical performance of MO (Figure S2), SEM images of MO during cycles at room temperature(Figure S3), SEM images of MO during cycles at 50 °C (Figure S4), CD curves and NMR spectra of Al-MO and MO (Figure S5), images of electrolytes before and after cycling test of Al-MO and MO at 50 °C (Figure S6).

ACKNOWLEDGMENTS The authors thank Prof. Lee PS for fruitful discussion. The authors acknowledge the financial support of the Natural Science Foundation of China (21503065, 51672065) and the General Financial Grant from the China Postdoctoral Science Foundation (2015M571924). The authors thank the staff in 16


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the Analytical and Testing Center of HFUT for their assistance in the materials characterization.

Author Contributions: SG Huang conducted material synthesis, XRD characterization and electrochemical measurements. J Sun performed SEM characterization. JQ Liu performed the XPS analysis. WJ Wang, QQ Qin and WP Mao performed the In-situ NMR characterization. W Xu performed TEM analysis. J Yan and YC Wu planed and supervised the research on material synthesis, characterization and electrochemical study. J Yan, WP Mao and JF Wang planed and supervised the research on In-situ NMR study. J Yan, WP Mao and YC Wu wrote the manuscript.

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Figures and Figure captions:

a

b

c

d

e

g

f

Figure 1. (a,b) FE-SEM images of Al-MO; (c) low-magnification TEM image and SAED pattern of the Al-MO; (d) HRTEM of one particle; (e-g) elements mapping images.

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b

1400 1200 1000

800

600

400

200

Mn IV

660

0

Mn 2p 3/2

655

540

Mn-O-H

536

534

532

645

640

635

Al 2p

Intensity (a.u.)

Mn-O-Mn

538

d

O 1s

650

：654.1Mn IV:642.4

Energy Binding (eV)

Binding Energy (eV)

c

Mn 2p

Mn 2p 1/2

Intensity (a.u.)

Al 2p

C 1s

Intensity (a.u.)

O 1s

Mn 2p

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


530

528

Binding Energy (eV)

526

70

72

74

76

Binding Energy (eV)

78

Figure 2. (a)XPS survey spectra of Al-doped MnO2, (b) Mn 2p spectra, (c) O 1s spectra and (d) Al 2p spectra.

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a

0.00

-0.02 0.0

0.2

0.4

0.6

5 10 20 50 100 Unit mV s-1 0.8 1.0

Voltage (V vs Ag/AgCl)

1.0

c

0.8

0.8 0.6 0.4

1 2 5 10 20 50 Unit: A g-1

0.2 0.0 0

100

350

200

300

Time (s)

400

500

d

300


-0.01

250

0.6

200

0.4

1 2 5 10 20 50 Unit: A g-1

0.2 0.0 0

220

100

200

300

400

Time (s)

150 100

500

e

200

160 -2

Al-MO #1 (0.70 mg cm ) Al-MO #2 (0.78 mg cm-2) MO #1 (0.72 mg cm-2) MO #2 (0.75 mg cm-2)

120 0

0

2000

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Cycles

8000

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20

30

Current Density (A g-1)

40

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f

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Al-MO(50 °C) Al-MO(25 °C) MO(50 °C) MO(25 °C)

50 0

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Current (A)

0.01


b

1.0


0.02


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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220 200 180 160

Al-MO #3 (0.68 mg cm-2) Al-MO #4 (0.72 mg cm-2) MO #3(0.725mg cm-2) MO #4 (0.73 mg cm-2)

140 120 0

2000

4000

6000

Cycles

Figure 3. (a) CV curves of Al-MO electrode (0.70 mg cm-2) obtained at scan rate from 5 mV s-1 to 100 mV s-1, (b,c) CD curves of Al-MO at different current density from 1 A g-1 to 50 A g-1 at room temperature (25 °C) and high temperature(50 °C), (d) specific capacitances of Al-MO and MO calculated at different current densities at different temperatures; (e,f) cycling performances of Al-MO and MO at a current density 10 A g-1 at room temperature and high temperature (50 °C), respectively. 22


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a

b

a c

d

e

f

g

h

Figure 4. FE-SEM images of Al-MO after 100th (a), 500th (b), 2000th (c), 5000th (d) and 10000th (e,f) cycles at 10 A g-1 at room temperature, (g) TEM images of Al-MO after 10000th cycles and the inserted image denoting the SAED pattern; (h) HRTEM of a single needle-like strip. 23



a

b

c

d

e

f

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Figure. 5. FE-SEM images of Al-MO after 100th (a), 500th (b), 2000th (c), 5000th (d) and 10000th (e,f) cycles at 10 A g-1 at high temperature (50 °C).

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aa

b

c

Figure 6. (a) Schematic diagram and the photograph image of the device for NMR measurements, (b) In-situ NMR images of 23Na spectra from the positive electrode of MO supercapacitor at 1st, 2nd, 5th, 10th and 20th cycle, (c) In-situ 23Na NMR spectra of Al-MO based supercapacitor during initial ten cycles. 25



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Figure 7. Schematic illustration of the morphology evolution process in Al-MO nanoparticles based electrode during charge-discharge cycling test.

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Enhanced High-Temperature Cyclic Stability of Al ... - ACS Publications

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