Polypyrroleâcoated sodium manganate hollow microspheres as a

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Polypyrrole-coated sodium manganate hollow microspheres as a superior cathode for sodium ion batteries Di Lu, Zhujun Yao, Yu Zhong, Xiuli Wang, Xinhui Xia, Changdong Gu, Jianbo Wu, and Jiangping Tu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Materials & Interfaces

Polypyrrole−coated sodium manganate hollow microspheres as a superior cathode for sodium ion batteries

Di Lu,† Zhujun Yao,† Yu Zhong,† Xiuli Wang,*† Xinhui Xia,† Changdong Gu,† Jianbo Wu,†† and Jiangping Tu *†

†

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

††

Zhejiang Provincial Key Laboratory for Cutting Tools, School of Materials Science and Engineering, Taizhou University, Taizhou 318000, China

*Corresponding author. Tel: +86 571−87952856; Fax: +86 571−8792573 Email: [email protected]; [email protected]; [email protected]

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Abstract Advanced electrode materials play a very important role in the development of largescale production of sodium ion batteries. Herein, Na0.7MnO2.05 hollow microspheres with diameters of 2 μm and a shell thickness of 200 nm are prepared and then modified by polypyrrole (PPy) coating. As cathodes for sodium ion batteries, the designed PPy−coated sodium manganate hollow microspheres demonstrate enhanced electrochemical performances with an initial capacity of 165.1 mAh g1, capacity retention of 88.6% at 0.1 A g−1 after 100 cycles and improved rate capability. The excellent electrochemical properties are attributed to the improved electro-conductivity and high stability of hollow spherical structure of sodium manganate oxide particles due to the introduction of conductive polymer coating.

Keywords: Sodium ion battery; Sodium manganate; Hollow microsphere; Polypyrrole; Electrochemical property


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1. Introduction Lithium ion batteries (LIBs) have been widely used in today’s world, ranging from portable devices to electrical vehicles.1,2 However, the limited resources of lithium on earth will lead to continuously rising prices, hindering long−term and large−scale applications.3-6 Thus, exploring a new kind of alternative energy storage device with low price and relative comparable performance is crucial for the development of related technologies. In this context, sodium has stood out as a promising candidate with low cost and extensive storage.7-9 Up to now, on the basis of deep research on LIBs, sodium ion batteries (SIBs) have obtained great progress due to their similar chemistry to lithium. However, due to the relative large ion diameter of Na (Na 1.02 Å vs. Li 0.76 Å), it is necessary to find suitable materials as cathodes and anodes that can perfectly accommodate the intercalation/deintercalation of sodium ions.9-12 Recently, intensive attention has been focused on SIBs and great efforts have been made in developing cathode materials for SIBs. A series of cathode materials are investigated such as sodium transition metal oxides (NaxMO2, M = Mn, Co, Ni, Fe, etc.),13-15 mineral eldfellite (Na2−Fe2(SO4)3, NaFe(SO4)2),16 sodium vanadium phosphate (Na3V2(PO4)3)17 and Prussian blue with its analogues.18 Among them, layered sodium manganese oxides (NaxMnO2+y, y= 0.05−0.25)

19-22

have the

advantages of high theoretical capacity up to 243 mAh g−1 in contrast to other cathodes such as 120 mAh g−1 for Na2Fe2(SO4)3 and 117.6 mAh g−1 for Na3V2(PO4)3 23 . Layered structural materials are not only suitable for half batteries, but also work well in full sodium batteries, 24-27 which makes them more promising in the future. NaxMnO2+y has



P− and O− type structures (octahedral (O), orprismatic (P))28 and both the structures consist of edge−sharing MnO6 octahedra,29 and sodium ions exist in the interlayer sites in different coordinations.30-32 Although O3−NaxMnO2+y possesses a superior electrochemical activity, its capacity falls far behind that of P2−NaxMnO2+y (more than 200 mAh g−1 for P2 vs. 120 mAh g−1 for O3). Nevertheless, the potential merits of P2−NaxMnO2+y are blocked by some unfavorable factors, one of which is the low conductivity that makes a contradiction between the capacity and charge/discharge rate, resulting in poor rate performance.33-35 It is necessary to overcome this issue for the future development of SIBs.36-38 After a long period of exploration, it is widely believed that well designed nanostructures can greatly promote the reaction kinetics of SIB cathodes because they provide short transportation length for ions/electrons.38-41 Actually, many works have been carried on to research spherical-shaped materials due to their gorgeous dispersity and mobility as electrodes.42-47 For example, Sun’ group has reported that Co and Ni containing P2−Na0.66MnO2 with buckyball structure shows excellent electrochemical stability with 90% capacity retention after 150 cycles at 1 C.48 Additionally, hollow nanostructures have been of continuous concern over these years.44, 49-55 This kind of materials has high specific surface area which provides more sites for reaction contact with electrolyte, and shortens the transportation length for ions/electrons. Moreover, the hollow structure supplies enough space to accommodate the volume expansion during charge and discharge process. Tu’ group fabricated Na0.7MnO2.05 nanotubes achieving 71% capacity retention after 200 cycles in Na0.7MnO2.05/CNT// HCNF full SIB.3 Liu et al found that the hollow K0.27MnO2


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nanospheres exhibited a discharge capacity of 68.7 mAh g−1 after 100 cycles in aqueous SIBs.39 Although the above progress, the hollow nanostructures still cannot achieve fast ion/electron transportation which is necessary for high rate performance. Additionally, the dissolution of Mn in sodium manganese oxides during cycling has to be prevented for good cycling stability.30, 38, 56 In view of these results, surface modifications are needed to improve the electrochemical performance of these oxides. Carbon coating is one of the most common method to enhance the electronic conductivity of electrode materials.57-60 However, simple carbon coating is not suitable for sodium transition metal oxides due to their instability in hot solution and side effects with reducing gases during the preparation process at high temperature.35, 36 Conductive polymer coating seems to be a good choice for their low temperature preparation. Polypyrrole (PPy) is chosen here coated on the surface of NaxMnO2+y as its high electrical conductivity. As reported by Chou et al, PPy coated Na1+xMnFe(CN)6 exhibited improved electrochemical performance with 46% capacity at 40 C, and 67% capacity retention after 200 cycles, indicating that the PPy coating can not only enhance the electrical conductivity of electrode material but also prevent Mn from dissolution during cycling.18 In addition, the PPy−promoted NaxFe[Fe(CN)6] displayed superior cycling ability and rate capability with 79% retention after 500 circles and 75 mAh g−1 at 3000 mA g−1.61 Herein, we propose a simple method for fabricating homogeneous Na0.7MnO2.05 hollow microspheres (NMOHS) with MnO2 hollow microspheres as precursor.



Afterwards, the PPy−coated Na0.7MnO2.05 hollow microspheres (NMOHS@PPy) were synthesized through a chemical process at 0−5 °C. The resulting composite shows excellent cycling stability and enhanced rate capability, which indicates the advantages of hollow microsphere structure of sodium transition metal oxide and surface modification of PPy coating.

2. Experimental The self−assembly MnO2 hollow microspheres were prepared by a reported method.38, 62 First, MnCO3 solid microspheres were prepared by mixing NaHCO3 and MnSO4·H2O aqueous solutions with addition of ethanol at room temperature. Typically, 0.169 g MnSO4·H2O and 0.84 g NaHCO3 were separately dissolved in 70 mL deionized water. After adding 7 mL ethanol to the MnSO4·H2O aqueous solution, the NaHCO3 solution was mixed directly and maintained at room temperature for 3 h under stirring. Then, MnCO3 precipitate was obtained by centrifugation, washed with deionized water and ethanol. The MnO2 hollow microspheres were obtained through chemical reaction between the MnCO3 precursor and KMnO4 aqueous solution with the presence of HCl. In a typical procedure, 0.1 g MnCO3 powder was dispersed in 20 mL deionized water. 5 mL 0.032 M KMnO4 aqueous solution was mixed with the above mixture under stirring for 6 min, and then 5 mL 0.01 M HCl was added to the mixture. After stirring for 1 min, the MnO2 hollow microspheres were collected by centrifugation, washed with deionized water and ethanol for several times.


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The as−prepared MnO2 hollow microspheres were mixed with NaOH in the proportion of Mn and Na 1: 0.44 and then milled for 30 min. Finally, NMOHS was obtained after calcination at 650 °C for 3 h in air. For comparison, the Na0.7MnO2.05 solid microspheres (NMOSS) were prepared by the same process as above with the MnCO3 solid microspheres as precursor. NMOHS@PPy was synthesized via a chemical ice water bath process. Typically, 100 mg NMOHS was dispersed in 40 ml deionized water following by an ultraviolet ultrasound process for 20 min. As dopant, 10 mg NaClO4 was mixed with 20 ml 5 vol. % pyrrole aqueous solution. The mixture was added to the NMOHS suspension drop by drop under vigorous stirring. The whole polymerization process occurred at 0−5 °C for 6 h with continuous nitrogen flowing. After centrifugation with deionized water and ethanol, the NMOHS@PPy powder was dried at 80 °C for 6 h. As comparison, pure PPy was synthesized only using FeCl3 as oxidant. X−ray diffraction (XRD, Rigaku D/max 2550 PC, Cu Kα) was utilized to identify the crystalline structure of all samples. Scanning electron microscopy (SEM, Hitachi S−4800) and transmission electron microscopy (TEM, JEOL 2010F) coupled with energy dispersive X−ray spectroscopy (EDX) are applied to characterize their morphology and microstructure. To determine the proportion of PPy in NMOHS@PPy composite, thermogravimetry (TG) tests were carried out using a Netzsch STA 449C thermal analyzer from 50 to 600 °C at a heating rate of 10 °C min−1 in N2. Fourier Transform infrared spectroscopy (FTIR) tests were made for pure PPy, NMOHS and NMOHS@PPy via an IR spectrometer (Thermo IS50). To further make sure the PPy



coating on the surface of Na0.7MnO2.05 particles, NMOHS, NMOHS@PPy and pure PPy were analyzed by an X−ray photoelectron spectrometer (XPS, ESCALAB_250Xi) and a Raman microscope with laser excitation at 532 nm. The Brunauer– Emmett– Teller (BET, Autosorb−1−C) test was employed to measure the specific surface areas of solid and hollow microspheres. Inductively coupled plasma-atomic emission spectrometry (ICP-AES, Poly Scan 60 E, Hewlett Packard) was applied to measure the ratio of Na and Mn. The electrochemical measurements were conducted using coin−type half cells (CR2025) with the active materials (Na0.7MnO2.05 solid and hollow microspheres separately) as the working electrode and sodium metal as the counter electrode. The working electrode was prepared by mixing the active materials, carbon conductive agent (super−P) and carboxymethyl cellulose (CMC) with a mass ratio of 8: 1: 1 followed by blending into a homogeneous slurry in deionized water. The resulting slurry was then cast onto aluminum foil and dried in a vacuum oven at 80 °C for 8 h to remove the solvent completely. In this work, the active material loading is about 1 mg cm−2. The cells were assembled in a pure Ar−filled glovebox, which the water and oxygen contents were less than 1 ppm. 1 M NaClO4 was dissolved in ethylene carbonate and dimethyl carbonate with volume ratio 1: 1 to form electrolyte. The separator here is a glass microfiber membrane (GF/F 1825−025, WHATMAN). Galvanostatic charge/discharge tests were performed at room temperature using a Land Battery Test System between 1.8 and 4.4 V. Cyclic voltammetry (CV) tests were carried out on a CHI660D electrochemistry workstation (Chenhua Instrument) in the potential range of


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1.84.4 V (vs. Na/Na+) at a scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were performed on a Princeton Applied Research advanced electro−chemical system in the frequency range from 100 kHz to 0.01 Hz.

3. Results and discussion The preparation process of NMOHS@PPy is shown in Fig. 1a. Firstly, MnO2 hollow microspheres were synthesized in solution with MnCO3 precursor, determined by the XRD patterns shown in Fig. S2. NMOHS were obtained after a solid−state calcination process between the MnO2 and NaOH. Finally, the NMOHS@PPy composite was fabricated through a chemical bath process at 0−5 °C. The MnO2 hollow spheres illustrated in Fig. 1b show a uniform diameter of about 2 μm, which is the same as NMOHS and NMOHS@PPy (Fig. 1c, d). After PPy coating, the NMOHS@PPy shows no significant change in size while the surface of spheres becomes rough. The hollow structure of NMOHS can also be found by TEM in Fig. 2a, and the hollow microspheres have shell thickness of 200 nm. The high-resolution TEM image indicates the highly crystalline nature of NMOHS with a lattice distance of 0.55 nm corresponding to the (121) plane of Na0.7MnO2.05 (Fig. S1a). The hollow structure and layered Na0.7MnO2.05 phase of the composite does not change after PPy coating (Fig. 2b, 2d), and the amorphous PPy layer has a thickness of about 12 nm (Fig. 2c). Furthermore, the uniform distribution of Na, Mn, and O in NMOHS and NMOHS@PPy is further confirmed by the EDS elemental mapping (Fig. S1b-d, 2e-g). In addition, the N distribution in NMOHS@PPy also shows the successful and uniform coating of PPy on



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NMOHS surface (Fig. 2h). The phase and composition of the NMOHS and NMOHS@PPy are characterized by XRD, TG, FTIR, Raman and XPS analyses. All the characteristic diffraction peaks of NMOHS and NMOHS@PPy match for layered Na0.7MnO2.05 (PDF 27-0751) (Fig. 3a). The FTIR spectra of NMOHS, pure PPy and NMOHS@PPy are further determined the existence of PPy in NMOHS@PPy (Fig. 3b). The pure PPy and NMOHS@PPy show almost the same bands, since the band of Na0.7MnO2.05 is at around 912 cm−1, very close to the band of ClO4− (910 cm−1). As a result, it not only verifies the presence of PPy in the composite but also shows that the process of PPy coating does not result in any change of the Na0.7MnO2.05 crystal. The Raman spectrum is obtained to further characterize the NMOHS@PPy composite (Fig. S3a). The strong peak at around 660 cm−1 for the NMOHS is ascribed to Na0.7MnO2.05. For the NMOHS@PPy, other four strong peaks at around 925, 1050, 1323, and 1553 cm−1 are corresponding to PPy, further confirming the successful preparation of the composite. The XPS measurements were carried out to investigate the chemical state of the elements contained in NMOHS@PPy composite. Elements Na, Mn, O, C, N are displayed in the survey scan spectrum (Fig. 3c). In the Mn 2p spectrum (Fig. 3d), two peaks at 641.9 and 653.5 eV are found to represent Mn 2p3/2 and 2p1/2, respectively. Both the peaks can be deconvoluted into two components, assigning to the oxidation states of Mn3+ and Mn4+ respectively, and the fitting result is in consistent with the previous result.

3, 67

The binding energy difference between two peaks in the Mn 3s

spectrum is about 5 eV within the range of 4.75.3 eV.

67

According to the previous

report, the distance between two peaks in the Mn 3s spectrum is related to the proportion


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of Mn3+ and Mn4+ in accordance with the Mn 2p spectrum.12 In addition, the valence analysis shows that the proportion of Mn3+/Mn4+ is around 6.02/3.98 so that the stoichiometric ratio between Na and Mn is 0.7/1, which is consistent with the XRD and ICP results (Table S1). The peak at 1070.9 eV in the Na 1s spectrum corresponds to the metal−oxygen bonds, which ensures the presence of Na−O bond (Fig. 3e). The TG curves of NMOHS and NMOHS@PPy composite are shown in Fig. S3b. It can be simply calculated that the PPy content in the composite is around 4.7%. The specific surface area of NMOHS tested by BET is 19.3 m2 g1. To verify the advantage of the hollow structure of NMOHS@PPy, we also fabricated NMOSS for comparison. The detail characterization such as SEM, TEM, XRD and Raman tests for NMOSS are demonstrated in Figure S4-6. The CV curves of NMOHS@PPy, NMOSS and NMOHS are investigated from 1.8 to 4.4 V (Na/Na+) at a scan rate of 0.1 mV s−1 (Fig. 4a, Fig. S7a and S7b). The three electrodes demonstrate almost the same main redox peaks. Many previous work such as Na0.44MnO2 63 and Na0.7MnO2, 3 has confirmed that the electrode reaction of layered manganese oxides cathodes is continuous and multistep. There are two main pairs of redox peaks in the CV curves. The oxidation peaks at 2.3 V and 4.3 V and the reduction peaks at 2.1 V and 4.2 V are in good accordance with those previously prepared Mn−based

cathode

materials3,

63.

Corresponding

to

the

reported

P2−Na2/3Ni1/3Mn2/3O2,57 the pair of redox peaks at 4.2 V and 4.3 V are probably related to the phase transformation from P2 to O3. Na+ extraction/ insertion makes the oxygen layer internal the crystal structure of NMO dislocated slightly resulting in the phase



change. In addition, the pair of redox peaks at 2.1 V and 2.3 V and other broad peaks can be attributed to the single−phase transformation due to the Na+ intercalation/ deintercalation process.

Apparently, the PPy-coated Na0.7MnO2.05 cathode shows the

least polarization, confirming that the existence of PPy improves the electrochemical kinetics of Na0.7MnO2.05 cathode. The charge and discharge profiles of NMOHS@PPy, NMOSS and NMOHS at a current density of 0.1 A g−1 for the 2nd and 50th cycles are demonstrated in Fig. 4b and S7c. The plateaus in charge and discharge process are in conformity with the consequence from CV tests and the three electrodes display almost the same plateaus as CV curves implying the same Na+ insertion/extraction mechanism. It is obvious that the NMOHS@PPy cathode shows the best cycling performance after 50 cycles followed by NMOHS and then NMOSS. Moreover, the NMOHS@PPy cathode also shows the smallest polarization from the charge and discharge profiles in accordance with CV curves, indicating the excellent electrode kinetics of NMOHS@PPy. Fig. 4c shows the cycling performances of NMOSS, NMOHS and NMOHS@PPy cathodes at a current density of 0.1 A g−1. After 100 cycles, the NMOSS and NMOHS electrodes undergo a varying decrease in capacity where the hollow cathode demonstrates higher capacity retention of 72.7% than 35.6% for the solid microsphere. Note that the initial capacity (121.1 mAh g−1) of the solid microsphere cathode is much lower than that of the hollow one. In this work, the capacity of NMOHS@PPy is based on the total weight of the composite. As a result, the initial discharge capacity of the NMOHS@PPy composite is a little lower than that of NMOHS. Compared to the


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NMOHS (80.5 mAh g−1 after 100 cycles with 48.8% capacity retention), the NMOHS@PPy electrode shows better cycling stability obviously which exhibits a capacity of 142.6 mAh g−1 after 100 cycles with 88.6% capacity retention. The remarkable improvement of cycling performance after PPy coating is attributed to the fact that the presence of PPy enhances the conductivity of electrode material and weakens the dissolution of Mn2+ and prevents the hollow microsphere structure from collapsing. The rate properties of NMOSS, NMOHS and NMOHS@PPy cathodes are compared at current densities of 0.1, 0.2, 0.5, 1 and 2 A g−1. As shown in Fig. 4d, apparently, the PPy coating sharply improves the rate performance of Na0.7MnO2.05 cathode. The discharge capacity of NMOHS@PPy at the above rates reaches 165.1, 148.6, 134.2, 119.0 and 100.5 mAh g−1, while the NMOHS cathode delivers 164.6, 143.5, 124.6, 97.6 and 72.5 mAh g−1, respectively. For the NMOSS cathode, the discharge capacity only remains 29.6 mAh g−1 when the current density reaches to 2 A g−1. When the current density goes back to 0.1 A g−1, the discharge capacity of NMOHS@PPy cathode rebounds to around 150.6 mAh g−1 comparing to 134.0 mAh g−1 for NMOHS and 93.2 mAh g−1 for NMOSS. The NMOHS@PPy cathode with enhanced conductivity and ideal contact area with electrolyte exhibits superior rate capability. The EIS spectrum demonstrated in Fig. 5a also confirms the decrease in charge transfer resistance for the Na0.7MnO2.05 with hollow structure and PPy surface modification. The Nyquist plots of all three electrodes consist of one semicircle in the high-frequency region corresponding to the charge transfer process and one slanted



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straight line in the low-frequency region corresponding to semi−infinite Warburg diffusion process. To further study the impedance spectra, the equivalent circuit diagram is obtained inset in Fig. 5a. Rs represents the Ohmic resistance between electrolyte and cathode, Rct is the charge-transfer resistance, and CPE is the passivation film capacitance and double layer capacitance. The values for these parameters are listed in Table 1. The NMOHS@PPy electrode shows much lower Rct than NMOHS and NMOSS, which give reasonable explanation to the fact that the designed PPy coating and hollow structure effectively increase the electric conductivity of active material so that the PPy-modified hollow Na0.7MnO2.05 presents excellent rate capability. Moreover, the composite electrode shows better electrochemical performance than other sodium manganate materials (Table S2), such as Na4Mn9O18, Na0.67MnO2, Na0.44MnO2 and Na0.7MnO2.33, 64-66 The diffusion coefficient of sodium ions (DNa+) in the electrode materials can be calculated with the impedance parameters as follows. Z𝑟𝑒𝑎𝑙 = 𝑅𝑠 + 𝑅𝑐𝑡 + 𝜎𝜔 ―1/2 R2T2

DNa + = 2A2𝑛4𝐹4𝐶2𝜎2

(1) (2)

where R represents the gas constant, T signifies the absolute temperature, A means the surface area of the active materials, n is the quantity of transferred electrons, F denotes the Faraday constant, C is the concentration of sodium ions, σ is the Warburg coefficient, and ω is the angular frequency in the low frequency region. After linear fitting between Z' and ω−1/2, σ is obtained as the value of slope (Fig. 5b). The DNa+ value of NMOHS@PPy electrode is calculated as 2.07×10−12, much higher than that of NMOHS


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(4.73×10−13 cm2 s−1) and NMOSS (7.08×10−13 cm2 s−1). This result certifies that the PPy coating improves the Na+ ion diffusion in the hollow Na0.7MnO2.05 electrode. The morphology of NMOHS and NMOHS@PPy after 50 cycles at 0.1 A g−1 are evaluated by SEM to confirm the structure change during cycling (Fig. S8a, b). The microsphere structure of PPy-coated Na0.7MnO2.05 composite is well preserved, while the Na0.7MnO2.05 hollow microspheres partially break down. The PPy coating is helpful to weaken the dissolution of Mn2+ preventing the hollow microsphere structure from collapsing. Moreover, the high resolution TEM image of the NMOHS@PPy after 50 cycles at 0.1 A g−1 is also obtained in Fig. S8c to verify that the PPy layer and the crystal structure of NMOHS are stable during the successive sodiation/desodiation processes. It is obvious seen that the thickness of PPy layer remains about 12 nm after 50 cycles and the lattice distance of crystalline phase is still around 0.55 nm corresponding to the (121) plane of Na0.7MnO2.05, indicating that the crystalline structure of NMOHS keeps stable during cycling. This stable hollow structure ensures that the NMOHS@PPy electrode exhibits much improved cycling stability.

4. Conclusions PPy-coated Na0.7MnO2.05 hollow microspheres were fabricated through a high temperature calcination and a chemical ice water bath process. The designed hollow structure can provide large contact with electrolyte, short diffusion length for ion/electron transportation and enough space to accommodate volume expansion during cycling. Moreover, the conductive polymer coating can enhance the electrical



conductivity of electrode material, prevent Mn from dissolution during cycling and keep the hollow structure stability. As a result, the achieved cathode exhibits high capacity, cycling stability and rate capability. The PPy-coated Na0.7MnO2.05 hollow composite can be a promising cathode material for SIBs.

Acknowledgement This work is supported by the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

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FIGURE CAPTIONS Figure 1. (a) Schematic illustration for fabrication of NMOHS and NMOHS@PPy; SEM images of MnO2 hollow microspheres (b), NMOHS (c), NMOHS@PPy (d). Figure 2. TEM images of NMOHS (a), NMOHS@PPy (b), NMOHS@PPy (c); (d) high-resolution TEM image of NMOHS@PPy, (e−h) Element mapping images of Mn, Na, O, N in NMOHS@PPy. Figure 3. (a) XRD patterns of NMOHS and NMOHS@PPy, (b) FTIR spectra of NMOHS, PPy and NMOHS@PPy. XPS spectra of NMOHS@PPy: (c) survey, (d) Na 1s, (e) Mn 2p, and (f) Mn 3s. Figure 4. (a) CV curves of NMOHS@PPy for the first three cycles. (b) Discharge/charge profiles at 0.1 A g−1 for the 50th cycling, (c) cycling performances at 0.1 C, (d) rate capabilities at 0.1, 0.2, 0.5, 1 and 2 A g−1, Figure 5. (a) Nyquist plots of NMOS, NMOHS and NMOHS@PPy after the 5th cycle, (b) Z′ versus ω−1/2 scheme.


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



Page 30 of 31

Table 1. Impedance parameters of NMOHS@PPy, NMOHS and NMOSS cathodes obtained from equivalent Circuit Fitting Electrode

DNa+ (cm2 s−1)

Rs

Rct

σ

NMOHS@PPy

7.62

50.88

5.57

2.07×10−12

NMOHS

11.70

141.48

9.51

4.73×10−13

NMOSS

15.44

232.92

11.64

7.08×10−13


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TABLE OF CONTENTS (TOC)

31


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