English

• 1. INTRODUCTION

  In recent years, due to the increased demand of energy, a great attention has been devoted into the energy storage devices, comprising supercapacitors^[1-3], lithium-sulfur batteries^{[4, 5]}, lithium-ion batteries^[6-8], and fuel cells. Electrochemical supercapacitors have become a research hotspot in power source because of their properties of high energy density, long cycle life and high power density^[9]. They can be used either by themselves as the primary power source or in combination with batteries or fuel cells, as they can repair the deficiency of rechargeable batteries in the energy storage field. There are two energy storage mechanisms for the supercapacitors: the double-layer capacitance and pseudocapacitance. It is well established that the energy densities of devices based on pseudocapacitance are higher than those on double-layer capacitance. Therefore, the asymmetric supercapacitors (ASCs), which typically consist of the electrical double layers material as the negative electrode, and the pseudocapacitive or battery alike materials as the positive electrode, have been explored^[10]. Heretofore, RuO₂ has been reported as a prospective positive electrode material as its high specific capacitance for ASCs^[11-13]. However, its application was limited due to its high cost and environmental toxicity. Transition mental oxides, like manganese oxide, cobalt oxide, etc., which have been widely used in the energy storage systems^[14-16], are promising candidates as their environmental friendliness, low cost and high specific capacitance, compared with RuO₂ material.

  Recently, the layered sodium manganese oxides (LSMOs) materials, which can provide two-dimensional channels to Na⁺ ions, have been investigated as the cathode materials for ASCs in aqueous electrolytes^[17-19]. Layered sodium manganese oxides materials are most widely used for sodium ion batteries (SIBs)^[20-23]. LSMOs for SIBs operate via the intercalation/deintercalation of Na⁺ and thus have been exploited in ASCs^[24-26]. The cell voltage of ASCs in aqueous electrolytes can be extended to ultrahigh stage, even beyond 2.0 V^[27], as LSMOs are used as the positive electrode materials. With the wide voltage window, the energy density can also be increased properly by the give equation of energy density (E = 1/2CU², where C and U are the specific capacitance and potential window, respectively). For example, it was demonstrated that the layered Na_0.21MnO₂ has excellent pseudocapacitive behavior in Na₂SO₄. This high-performance cathode and rGO anode were assembled as a practical cell, which exhibits a large operational voltage window between 0 and 2.7 V, without any water splitting for 1000 cycles with a capacitance retention of 86.7%^[28]. An ASC cell consisting of Na_xMnO₂@CNF nanocomposite and activated carbon as the positive and negative electrodes can be reversibly charged and discharged to a cell voltage of 2.0 V in 1 mol/L Na₂SO₄ and 4 mmol/L NaHCO₃ with the specific energy and power of 21 Wh/kg and 1 kW/kg, respectively^[29]. The ASCs with LSMOs positive materials can also be applied for capacitive deionization^{[30, 31]}.

  In general, the layered sodium manganese oxides materials have been prepared from the mixture of Na₂CO₃ and MnO_x by the high temperature annealing with mechanical force. However, this method is not only complicated, but also uncontrollable. In this paper, we report a facile one-step hydrothermal route to synthesize the layered Na_0.5Mn₂O₄·1.5H₂O for ASCs. Moreover, we assembled button asymmetric supercapacitors using Na_0.5Mn₂O₄·1.5H₂O as cathode, active carbon (AC) as anode and 1 mol/L Na₂SO₄ solution as electrolyte. As a result, the assembled asymmetric supercapacitor delivers a wide voltage window of 2.0 V with a specific energy density of 10.13 Wh/kg at a specific power density of 500 W/kg.

  2. EXPERIMENTAL

  2.1 Synthesis of the sodium manganese oxides

  All chemical reagents were analytical reagent grade and used without further purification. In our typical synthesis, 10 mL of 1 mol/L manganese nitrate (Mn(NO₃)₂) and 10 mL of 1~5 mol/L sodium hydroxide (NaOH) were mixed together and stirred for ten minutes. The molar ratio of Na and Mn atoms in the mixed solution were 1:1, 1:2, 1:3, 1:4 and 1:5, respectively. Then, 5 mL of 0.5 mol/L hydrogen peroxide (H₂O₂) was added into the above mixture. The homogeneous solution was dynamoelectro-nically stirred for 30 minutes while the original light pink color aqueous solution gradually turned into black suspension. Then, the suspension was aging for 1 h. Afterwards, the liquid was allowed to be filtrated. The samples were obtained by washing the asprepared precipitates several times with distilled water and then dried under oven for hours at 80 ℃.

  The as-prepared sample (0.65 g) was transferred to a 20 mL stainless-steel autoclave with 10 mL 1 mol/L NaOH, and then heated at 150 ℃ for 16 h. The products were thoroughly filtered and washed with deionized water for several times. Finally, the products were vacuum dried at 30 ℃ for 12 h to obtain the desired sodium manganese oxides.

  2.2 Preparation of working electrode and active carbon cathode

  The as-prepared sodium manganese oxide samples, carbon black and polyvinylidence difluoride (PVDF) binder with a mass ratio of 7:2:1 were mixed with constantly grinding for 30 minutes, and then added into N-methyl pyrrolidinone (NMP) solvent with constantly grinding, until the slurry was obtained. The slurry was applied on the Ni foam current collector, which was treated previously, and then dried at 120 ℃ for 12 h (the mass loading of active material, ~1.5 mg).

  The active carbon (AC) electrodes were prepared by mixing the YP17 activated carbon from Kuraray with 5 wt.% conductive acetylene black and 5 wt.% polytetrafluoroethylene emulsion to form a slurry. The slurry was filtered with Ti mesh, dried and roll-pressed to approximately 4.5 mg.

  2.3 Characterization

  The phase structure of the as-prepared sample was characterized by X-ray diffraction (XRD) using a Shimadzu XRD-6000 power X-ray diffractometer (Shimadzu, Japan) with CuKα radiation in the range of 5°~80° at a scan rate of 7 °/min. The surface morphologies of the as-prepared samples were characterized using field scanning electron microscopy (FESEM) (Carl Zeiss, Germany). The cyclic voltammetry (CV) curves and the galvanostatic charge/discharge measurements (GCD) were tested on an Autolab (AUT86742, Wantong, Switzerland) electrochemical workstation over a voltage range of –1.0~1.0 V with a rate of 2 mV/s. The galvanostatic charge/discharge cycling tests were tested on a LAND (CT2001A, Land, Wuhan) instrument with the current density of 5 A/g at room temperature. Electrochemical impedance spectroscopy (EIS) was carried out over the frequency ranging from 0.1 to 10⁵ Hz with an excitation voltage of 5 mV, which was performed on Autolab electrochemical workstation (AUT86742, Wantong, Switzerland). In all the three-electrode system tests, 1 mol/L Na₂SO₄ solution was the medium solution, an SCE was the reference electrode, a Pt plate electrode was the counter electrode, and the prepared composite electrode was the working electrode.

  For the two-electrode system, the working electrode was the as-obtained Na_0.5Mn₂O₄·1.5H₂O applied on Ni foam, and the counter electrode was activated carbon (AC) on Ni foam. These two electrodes were assembled in CR2032-type coin cells separated by NKK-MPF30AC-100 membrane (NKK, Japan) with 1 mol/L Na₂SO₄ solution as electrolyte. The fabricated cells, denoted Na_0.5Mn₂O₄·1.5H₂O//AC asymmetric supercapacitor, were subjected to further electrochemical studies.

  The cyclic voltammetry curves (CV) and the galvanostatic charge/discharge measurements (GCD) were tested on a CHI 660E electrochemical workstation (Chenhua, China) over a voltage range of 0~2.0 V with a rate of 5~100 mV/s and current density of 0.5~5 A/g.

  The specific capacitance (F/g), energy density (Wh/kg) and power density (kW/kg) were calculated according to the following equations:

  $C =\frac{I \cdot {\mathit{\Delta}} t}{m \cdot {\mathit{\Delta}} V}$

  (1)

  $E =\frac{1}{2}\left(C \Delta V^2\right)$

  (2)

  $P =\frac{E}{{\mathit{\Delta}} t}$

  (3)

  Where I (mA) is the discharge current, m (mg) the mass loading of active material, ΔV (V) the potential window during the discharge process, and Δt (s) the time of a discharge process. In three-electrode system, the specific capacitance and current density were calculated based on the mass of the as-obtained active material. For the two-electrode mode, the mass is the total of the mass of both positive and negative electrodes.

  3. RESULTS AND DISCUSSION

  Here, we report an approach to synthesize sodium manganese oxide through hydrothermal method, and obtain five samples (denoted as samples A, B, C, D and E) by adjusting the molar ratio of sodium and manganese atoms. The crystalline structures of all samples were investigated by XRD. As shown in Fig. 1, the XRD patterns of the as-prepared samples obtained at the ratios of 1:1 and 1:2 (samples A and B) present (111), (020), (002), (211) signal peaks, which are identical with the standard diffraction pattern of Mn³⁺O(OH) (JCPDS: 41-1379). With the increase of the amount of sodium atoms, another three sodium manganese oxide samples (samples C, D and E) characteristic diffraction peaks were observed at 12.38°, 24.907°, 35.661°, 36.977° and 49.986°, corresponding to the (001), (002), (200), (111) and (113) crystal planes of the monoclinic Na_0.5Mn₂O₄·1.5H₂O (JCPDS: 43-1456) indexed with the C2/m space group. Comparing these five samples, the crystalline of the samples changed from Mn³⁺O(OH) to Na_0.5Mn₂O₄·1.5H₂O as the amount of sodium increased. Samples C, D and E can be named Na_0.5Mn₂O₄·1.5H₂O (denoted as SMO-1, SMO-2 and SMO-3) when the molar ratios of sodium and manganese atoms are 1:3, 1:4 and 1:5. The general morphology of all samples is characterized by FESEM, and the results are shown in Fig. 1(a~e). The FESEM images clearly illustrate different morphology of the five samples with different amount of manganese nitrate solution. Every sample has a low magnification and a closer observation. From the FESEM image (Fig. 1a), sample A is very uniform, disordered and stacked with an acicular-like appearance about 20~50 nm wide. When the amount of NaOH and H₂O₂ mixed solution is increasing to double time, sample B shows flake-like irregular appearance with a little tiny acicular-like appearance at the edge (Fig. 1(b)). That can also be illustrated by the XRD patterns shown in Fig. 1(f) that samples A and B are the same material. In contrast with sample A, sample B has a lower crystallinity with a weaker and broader peak. The morphology of samples C, D and E show a layered performance with the ratio between 1:3 and 1:5, as presented in Fig. 1(c~e). With increasing the amount of sodium, it appears as a mixture of chunks, layers and a thicker layer, indicating that merely increasing the amount of sodium can not cause a structural change, but make the material stacking more serious, which corresponds to the XRD results.

  Figure 1

  

  Figure 1.  (a~e) SEM images of as-obtained samples with different molar ratios of sodium and manganese atoms. (f) XRD pattern of as-obtained samples in which the molar ratio of sodium and manganese atoms are different

  DownLoad: Full-Size Img PowerPoint

  To investigate the electrochemical performance of the as-prepared sodium manganese oxides synthesized by facile hydrothermal method, SMO-1, SMO-2 and SMO-3 were applied on the Ni foam and used as the working electrodes in three-electrode systems. Cyclic voltammetry (CV) measurements of samples C, D and E prepared by the hydrothermal method were tested in 1 mol/L Na₂SO₄ electrolyte within the range of –1~1 V at a low scan rate of 2 mV/s (Fig. 2(a~c)). SMO-1 and SMO-2 have a pair of sharp redox peaks (–0.25 and 0.15 V), corresponding to the repetitive Na⁺ intercalation and deintercalation. Contrast to the CV curve of SMO-1, SMO-2 has a narrow area owing to the thicker thickness. The CV curve of SMO-3 has numbers of redox peaks, which can be attributed to the repetitive Na⁺ intercalation and deintercalation and other side reactions, corresponding to the FESEM results we got.

  Figure 2

  

  Figure 2.  (a~c) Cyclic voltammetry (CV) curves of SMO-1, SMO-2 and SMO-3 at 2 mV/s. (d~e) Galvanostatic charge/discharge (GCD) curves of SMO-1, SMO-2 and SMO-3 at 0.5 A/g. (g) Column chart of specific capacity of SMO-1, SMO-2 and SMO-3. (h) Cycling performance of SMO-1 at 5 A/g

  DownLoad: Full-Size Img PowerPoint

  The results of galvanostatic charge and discharge tests are shown in Fig. 2(d~f). Each sample was tested in a three-electrode system with the current density of 0.5 A/g. The specific capacitance of nanocrystalline sodium manganese oxide materials was calculated according to the equation as mentioned before. Samples C and D have a plateau around 0.2 V in the charging time and –0.2 V in the discharging time, which agrees with the CV tests. Due to the side reactions of sample E, there are so many reaction mechanisms in it, causing no more researches to be done. As shown in Fig. 2(g), the specific capacity is calculated from the GCD curves. The cycle stability test of the electrode material is a critical factor to evaluate the application of supercapacitor. As SMO-1 has the best specific capacitance for all samples, the cycling performance was carried out on the SMO-1 electrode with 2000 cycles at a current density of 5 A/g. The result of the cycling performance of SMO-1 exhibited a high specific capacitance of 265 F/g after test.

  The Nyquist plots of SMO-1 electrode in 1 M Na₂SO₄ solutions are presented in Fig. 3. The plots are characterized by two distinct parts, a compressed arc in the high frequency range and a sloped line in the low-frequency region. An intercept at z´-axis in the high frequency region identifies the bulk solution resistance (R_s), while the radius of the semicircle at high frequency region on the z´-axis is related to the charge transfer resistance (R_ct). In the high-frequency region, the diameters of the semicircle on the real axis indicated a small interfacial charge-transfer resistance, reflecting the resistance of electrochemical reactions on the electrode and also can be called Faraday resistance. The Nyquist plot of SMO-1 exhibits a slope close to 90° along the imaginary axis in the low-frequency region. The vertical line represents the diffusive resistance of electrolyte in the electrode pores and the proton diffusion in host materials. The Nyquist plot of SMO-1 at low frequency corresponds to Warburg resistance (W₀), which is related to the diffusion of ions between the electrolyte and the electrode surface. The value of each device is presented in Table 1.

  Figure 3

  

  Figure 3.  Ac impedance spectra of SMO-1 electrode

  DownLoad: Full-Size Img PowerPoint

  Table 1

  

  Table 1.  Value of Each Device of the Equivalent Circuit Model

  DownLoad: CSV

  Sample name	Rs (Ω)	Rct (Ω)	CPE (mF/cm2)	n	W0 (Ω/s)
  SMO-1	1.943	2.595	0.001883	0.6751	0.04921

  To further evaluate the asymmetric supercapacitor application of SMO-1, we assembled a button hybrid supercapacitor using SMO-1 and activate carbon (AC) as the positive and negative electrodes, with a separator between two electrodes. The optimal mass ratio of negative active material (AC) to positive active material (Na_0.5Mn₂O₄·1.5H₂O) was 3:1, which was based on the specific capacitance, the potential window of negative and positive active materials. The CV curves and galvanostatic charge/discharge curves of active carbon are demonstrated in Fig. 3. The CV curves of active carbon showed a fair rectangular shape, suggesting a double-layer capacitance. Active carbon exhibited a capacitance of 183 F/g at the current density of 1 A/g.

  As shown in Fig. 4(a), the CV curve of the as-assembled button hybrid supercapacitor was tested at a scan rate of 50 mV/s at different potential windows. In this study, when the voltage window is not beyond 2.0 V, the CV curves maintain its rectangular shape and the peak position. But when the voltage window is up to 2.0 V or even higher, oxygen evolution reaction starts obviously with a slight deviation and unnecessary peak, as shown in the CV results. Fig. 4(b) shows the CV curves of SMO-1 hybrid supercapacitor devices at different scan rates of 5~100 mV/s. The shapes of CV curves maintain similar characteristics, even those obtained at higher scan rates. The galvanostatic charge/discharge measurement results of as-assembled button hybrid supercapacitor at different current density are shown in Fig. 4(c). The device delivered a specific capacitance of 18.375, 13.9, 13.6 and 11.5 F/g at a current density of 0.5, 1, 2 and 5 A/g, respectively. All tests are based on the total mass of both positive and negative electrodes, and show a good rate capability.

  Figure 4

  

  Figure 4.  (a) CV curves of active carbon at different scan rates; (b) Galvanostatic charge/discharge curves of active carbon at different current density

  DownLoad: Full-Size Img PowerPoint

  To illustrate the practical application of hybrid device, the ragone plot of SMO-1//AC hybrid supercapacitor is shown in Fig. 4(d). The as-assembled device delivers a maximum energy density of about 10.2 Wh/kg at a power density of 500 W/kg, which is higher than the asymmetric supercapacitors, such as ZnCo₂O₄/H: ZnO//AC (3.75677 Wh/kg at 653.34 W/kg)^[32], NiCo₂O₄-MnO₂//activated graphene asymmetric device (9.4 Wh/kg at 175 W/kg)^[33], CoMnLDH/Ni//AC asymmetric device (4.4 Wh/kg at 2500W/kg)^[34], and Co(OH)₂//Co(OH)₂ supercapacitors (3.96 Wh/kg)^[35]. Even at a high power density of 5 kW/kg, the energy density of SMO-1 is still up to 6.39 Wh/kg.

  Figure 5

  

  Figure 5.  CV curves of (a) SMO-1//AC ASC device at different voltage windows at a scan rate of 50 mV/s; CV curves of (b) the SMO-1//AC ASC device at different scan rates; Galvanostatic charge/discharge curves of (c) SMO-1//AC ASC device at different current density; (d) Ragone plot of the energy density and power density at different current densities of the SMO-1//AC ASC devices

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In this paper, we report a facile hydrothermal synthesis of layered Na_0.5Mn₂O₄·1.5H₂O and its application on the asymmetric supercapacitor. When the ratio of sodium to manganese is 3:1, Na_0.5Mn₂O₄·1.5H₂O has shown the best capacitance of 369 F/g by three electrodes test, and after 2000 deep cycles at the current density of 5 A/g, the specific capacitance remained 265 F/g. The asymmetric supercapacitor consists of the sodium manages oxides (SMO-1) as the positive electrode and AC as the negative electrode in 1 mol/L Na₂SO₄ solution. When the value of mass ratio of AC to SMO was 3:1, the fabricated hybrid supercapacitor demonstrated good power performance with an energy density conservation of 10.13 Wh/kg at the power density of 500 kW/kg, and has a wide voltage range of 0 to 2.0 V.

  
  
• 1. [1]

     Wang, Y. J.; Lu, Y.; Chen, K. Y.; Cui, S. Z.; Chen, W. H.; Mi, L. W. Synergistic effect of Co₃O₄@C@MnO₂ nanowire heterostructures for high-performance asymmetry supercapacitor with long cycle life. Electrochim. Acta 2018, 283, 1087–1094. doi: 10.1016/j.electacta.2018.06.163

  2. [2]

     Li, C.; Balamurugan, J.; Thanh, T. D.; Kim, N. H.; Lee, J. H. 3D hierarchical CoO@MnO₂ core-shell nanohybrid for high-energy solid state asymmetric supercapacitors. J. Mater. Chem. A. 2017, 5, 397–408. doi: 10.1039/C6TA08532F

  3. [3]

     Gong, Q. H.; Li, Y. J.; Huang, H.; Zhang, J.; Gao, T. T.; Zhou, G. W. Shape-controlled synthesis of Ni-CeO₂@PANI nanocomposites and their synergetic effects on supercapacitors. Chem. Eng. J. 2018, 344, 290–298. doi: 10.1016/j.cej.2018.03.079

  4. [4]

     Tang, C.; Li, B. Q.; Zhang, Q.; Zhu, L.; Wang, H. F.; Shi, J. L.; Wei, F. CaO-templated growth of hierarchical porous graphene for high-power lithium-sulfur battery applications. Adv. Funct. Mater. 2016, 26, 577–585. doi: 10.1002/adfm.201503726

  5. [5]

     Pei, F.; Lin, L. L.; Ou, D. H.; Zheng, Z. M.; Mo, S. G.; Fang, X. L.; Zheng, N. F. Self-supporting sulfur cathodes enabled by two dimensional carbon yolk-shell nanosheets for high energy-density lithium-sulfur batteries. Nat. Commun. 2017, 8, 482–10. doi: 10.1038/s41467-017-00575-8

  6. [6]

     Luo, D.; Deng, Y. P.; Wang, X. L.; Li, G. R.; Wu, J.; Fu, J.; Lei, W.; Liang, R. L.; Liu, Y. S.; Ding, Y. L.; Yu, A. P.; Chen, Z. W. Tuning shell numbers of transition metal oxide hollow microspheres towards durable and superior lithium storage. Acs Nano 2017, 11, 11521–11530. doi: 10.1021/acsnano.7b06296

  7. [7]

     Chen, B.; Meng, Y. H.; He, F.; Liu, E. Z.; Shi, C. S.; He, C. N.; Ma, L. Y.; Li, Q. Y.; Li, J. J.; Zhao, N. Q. Thermal decomposition-reduced layer-by-layer nitrogen-doped graphene/MoS₂/nitrogen-doped grapheme heterostructure for promising lithium-ion batteries. Nano. Energy 2017, 41, 154–163. doi: 10.1016/j.nanoen.2017.09.027

  8. [8]

     Fang, Y.; Lv, Y. Y.; Gong, F.; Elzatahry, A. A.; Zheng, G. F.; Zhao, D. Y. Synthesis of 2D-mesoporous-marbon/MoS₂ heterostructures with well-defined interfaces for high-performance lithium-ion batteries. Adv. Mater. 2016, 28, 9385–9390. doi: 10.1002/adma.201602210

  9. [9]

     Miller, J. R.; Simon, P. Electrochemical capacitors for energy management. Science 2008, 321, 651–652. doi: 10.1126/science.1158736

  10. [10]

      Tang, Y. F.; Li, Y. S.; Guo, W. F.; Wang, J.; Li, X. M.; Chen, S. J.; Mu, S. C.; Zhao, Y. F.; Gao, F. M. Highly ordered multi-layered hydrogenated TiO₂-Ⅱ phase nanowire arrays negative electrode for 2.4 V aqueous asymmetric supercapacitors with high energy density and long cycle life. J. Mater. Chem. A 2018, 6, 623–632. doi: 10.1039/C7TA09590B

  11. [11]

      Hu, C. C.; Huang, Y. H.; Chang, K. H. Annealing effects on the physicochemical characteristics of hydrous ruthenium and ruthenium-iridium oxides for electrochemical supercapacitors. J. Power Sources 2002, 108, 117–127. doi: 10.1016/S0378-7753(02)00011-3

  12. [12]

      Kim, B. C.; Wallace, G. G.; Yoon, Y. I.; Ko, J. M.; Too, C. O. Capacitive properties of RuO₂ and Ru-Comixed oxide deposited on single-walled carbon nanotubes for high-performance supercapacitors. Synth. Met. 2009, 159, 1389–1392. doi: 10.1016/j.synthmet.2009.02.037

  13. [13]

      Kim, I. H.; Kim, J. H.; Lee, Y. H.; Kim, K. B. Synthesis and characterization of electrochemically prepared ruthenium oxide on carbon nanotube film substrate for supercapacitor applications. J. Electrochem. Soc. 2005, 152, A2170–A2178. doi: 10.1149/1.2041147

  14. [14]

      Lv, Z. S.; Luo, Y. F.; Tang, Y. X.; Wei, J. Q.; Zhu, Z. Q.; Zhou, X. R.; Li, W. L.; Zeng, L.; Zhang, W.; Zhang, Y. Y.; Qi, D. P.; Pan, S. W.; Loh, X. J.; Chen, X. D. Editable supercapacitors with customizable stretchability based on mechanically strengthened ultralong MnO₂ nanowire composite. Adv. Mater. 2018, 30, 1704531. doi: 10.1002/adma.201704531

  15. [15]

      Liu, P. B.; Zhu, Y. D.; Gao, X. G.; Huang, Y.; Wang, Y.; Qin, S. Y.; Zhang, Y. Q. Rational construction of bowl-like MnO₂ nanosheets with excellent electrochemical performance for supercapacitor electrodes. Chem. Eng. J. 2018, 350, 79–88. doi: 10.1016/j.cej.2018.05.169

  16. [16]

      Zhai, T.; Wang, L. M.; Sun, S.; Chen, Q.; Sun, J.; Xia, Q. Y.; Xia, H. Phosphate ion functionalized Co₃O₄ ultrathin nanosheets with greatly improved surface reactivity for high performance pseudocapacitors. Adv. Mater. 2017, 29, 1604167. doi: 10.1002/adma.201604167

  17. [17]

      Wang, L.; Lu, Y. H.; Liu, J.; Xu, M. W.; Cheng, J. G.; Zhang, D. W.; Goodenough, J. B. A superior low-cost cathode for a Na-ion battery. Angew. Chem. Int. Ed. 2013, 52, 1964–1967. doi: 10.1002/anie.201206854

  18. [18]

      Pang, H.; Zhang, Y. Z.; Cheng, T.; Lai, W. Y.; Huang, W. Uniform manganese hexacyanoferrate hydrate nanocubes featuring superior performance for low-cost supercapacitors and nonenzymatic electrochemical sensors. Nano. Scale 2015, 7, 16012–16019.

  19. [19]

      Wessells, C. D.; McDowell, M. T.; Peddada, S. V.; Pasta, M.; Huggins, R. A.; Cui, Y. Tunable reaction potentials in open framework nanoparticle battery electrodes for grid-scale energy storage. Acs Nano. 2012, 6, 1688–1694. doi: 10.1021/nn204666v

  20. [20]

      Demirel, S.; Oz, E.; Altin, E.; Altin, S.; Bayri, A.; Kaya, P.; Turan, S.; Avci, S. Growth mechanism and magnetic and electrochemical properties of Na_0.44MnO₂ nanorods as cathode material for Na-ion batteries. Mater. Charact. 2015, 105, 104–112. doi: 10.1016/j.matchar.2015.05.005

  21. [21]

      Guo, S. H.; Yu, H. J.; Jian, Z. L.; Liu, P.; Zhu, Y. B.; Guo, X. W.; Chen, M. W.; Ishida, M.; Zhou, H. S. A high-capacity, low-cost layered sodium manganese oxide material as cathode for sodium-ion batteries. ChemSusChem. 2014, 7, 2115–2121. doi: 10.1002/cssc.201402138

  22. [22]

      Fu, B.; Zhou, X.; Wang, Y. P. High-rate performance electrospun Na_0.44MnO₂ nanofibers as cathode material for sodium-ion batteries. J. Power Sources 2016, 310, 102–108. doi: 10.1016/j.jpowsour.2016.01.101

  23. [23]

      Huang, J. J.; Luo, J. Composites of sodium manganese oxides with enhanced electrochemical performance for sodium-ion batteries: tailoring properties via controlling microstructure. Sci. China Technol. Sc. 2016, 59, 1042–1047. doi: 10.1007/s11431-016-6067-5

  24. [24]

      Smith, K. C.; Dmello, R. Na-ion desalination (NID) enabled by Na-blocking membranes and symmetric Na-intercalation: porous-electrode modeling. J. Electrochem. Soc. 2016, 163, A530–A539. doi: 10.1149/2.0761603jes

  25. [25]

      Smith, K. C. Theoretical evaluation of electrochemical cell architectures using cation intercalation electrodes for desalination. Electrochim. Acta 2017, 230, 333–341. doi: 10.1016/j.electacta.2017.02.006

  26. [26]

      Pasta, M.; Wessells, C. D.; Cui, Y.; Mantia, F. L. A desalination battery. Nano. Lett. 2012, 12, 839–843. doi: 10.1021/nl203889e

  27. [27]

      Shao, Y. L.; El-Kady, M. F.; Sun, J. Y.; Li, Y. G.; Zhang, Q. H.; Zhu, M. F.; Wang, H. Z.; Dunn, B.; Kaner, R. B. Design and mechanisms of asymmetric supercapacitors. Chem. Rev. 2019, 118, 9233–9280.

  28. [28]

      Karikalan, N.; Karuppiah, C.; Chen, S. M.; Velmurugan, M.; Gnanaprakasam, P. Three-dimensional fibrous network of Na_0.21MnO₂ for aqueous. Chem. Eur. J. 2017, 23, 2379–2386. doi: 10.1002/chem.201604878

  29. [29]

      Lin, S. C.; Lu, Y. T.; Chien, Y. A.; Wang, J. A.; Chen, P. Y.; Ma, C. C. M.; Hu, C. C. Asymmetric supercapacitors based on electrospun carbon nanofiber/sodium-pre-intercalated manganese oxide electrodes with high power and energy densities. J. Power Sources 2018, 393, 1–10. doi: 10.1016/j.jpowsour.2018.05.019

  30. [30]

      Kim, S.; Yoon, H.; Shin, D.; Lee, J.; Yoon, J. Electrochemical selective ion separation in capacitive deionization with sodium manganese oxide. J. Colloid Interf. Sci. 2017, 506, 644–648. doi: 10.1016/j.jcis.2017.07.054

  31. [31]

      Wallas, J. M.; Young, M. J.; Sun, H. X.; George, S. M. Efficient capacitive deionization using thin film sodium manganese oxide. J. Electrochem. Soc. 2018, 165, A2330–A2339. doi: 10.1149/2.0751810jes

  32. [32]

      Boruah, B. D.; Maji, A.; Misra, A. Synergistic effect in heterostructure of ZnCo₂O₄ and hydrogenated zinc oxide nanorods for high capacitive response. Nano Scale. 2017, 9, 9411–9420.

  33. [33]

      Kuang, M.; Wen, Z. Q.; Guo, X. L.; Zhang, S. Z.; Zhang, Y. X. Engineering firecracker-like beta-manganese dioxides@spinel nickel cobaltates nanostructures for high-performance supercapacitors. J. Power Sources 2014, 270, 426–433. doi: 10.1016/j.jpowsour.2014.07.144

  34. [34]

      Jagadale, A. D.; Guan, G. Q.; Li, X. M.; Du, X.; Ma, X. L.; Hao, X. G.; Abudula, A. Ultrathin nanoflakes of cobalt-manganese layered double hydroxide with high reversibility for asymmetric supercapacitor. J. Power Sources 2016, 306, 526–534. doi: 10.1016/j.jpowsour.2015.12.097

  35. [35]

      Jagadale, A. D.; Kumbhar, V. S.; Dhawale, D. S.; Lokhande, C. D. Performance evaluation of symmetric supercapacitor based on cobalt hydroxide [Co(OH)₂] thin film electrodes. Electrochim. Acta 2013, 98, 32–38. doi: 10.1016/j.electacta.2013.02.094

• 

  Figure 1  (a~e) SEM images of as-obtained samples with different molar ratios of sodium and manganese atoms. (f) XRD pattern of as-obtained samples in which the molar ratio of sodium and manganese atoms are different

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a~c) Cyclic voltammetry (CV) curves of SMO-1, SMO-2 and SMO-3 at 2 mV/s. (d~e) Galvanostatic charge/discharge (GCD) curves of SMO-1, SMO-2 and SMO-3 at 0.5 A/g. (g) Column chart of specific capacity of SMO-1, SMO-2 and SMO-3. (h) Cycling performance of SMO-1 at 5 A/g

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Ac impedance spectra of SMO-1 electrode

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) CV curves of active carbon at different scan rates; (b) Galvanostatic charge/discharge curves of active carbon at different current density

  下载: 全尺寸图片 幻灯片
  

  Figure 5  CV curves of (a) SMO-1//AC ASC device at different voltage windows at a scan rate of 50 mV/s; CV curves of (b) the SMO-1//AC ASC device at different scan rates; Galvanostatic charge/discharge curves of (c) SMO-1//AC ASC device at different current density; (d) Ragone plot of the energy density and power density at different current densities of the SMO-1//AC ASC devices

  下载: 全尺寸图片 幻灯片

  Table 1.  Value of Each Device of the Equivalent Circuit Model

  Sample name	Rs (Ω)	Rct (Ω)	CPE (mF/cm2)	n	W0 (Ω/s)
  SMO-1	1.943	2.595	0.001883	0.6751	0.04921

  下载: 导出CSV
•