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A high capacity, good safety and low cost Na2FeSiO4based cathode for rechargeable sodium-ion battery Wenhao Guan, Bin Pan, Peng Zhou, Jin-Xiao Mi, Dan Zhang, Jiacheng Xu, and Yinzhu Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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ACS Applied Materials & Interfaces

A High Capacity, Good Safety and Low Cost Na2FeSiO4-based Cathode for Rechargeable Sodium-ion Battery Wenhao Guan,† Bin Pan,† Peng Zhou,† Jinxiao Mi,‡ Dan Zhang,† Jiacheng Xu,† and Yinzhu Jiang*,†,ǁ †

State Key Laboratory of Silicon Materials, Key Laboratory of Novel Materials for

Information Technology of Zhejiang Province and School of Materials Science & Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China ‡

Department of Materials Science & Engineering, Xiamen University, Xiamen 361005, P. R.

China ǁ

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure

of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China *E-mail: Prof. Y. Jiang ([email protected])

Abstract: Rechargeable sodium-ion batteries (SIBs) are receiving intense interest because the resource abundance of sodium and its lithium-like chemistry make them low cost alternatives to the prevailing lithium-ion batteries in large-scale energy storage devices. Two typical classes of materials including transition metal oxides and polyanion compounds have been under intensive investigation as cathodes for SIBs, however they are still limited to poor stability or low capacity of the state-of-art. Herein, we report a low cost carbon-coated Na2FeSiO4 with simultaneous high capacity and good stability, owing to the highly pure Narich triclinic phase and the carbon-incorporated three-dimensional network morphology. The present carbon-coated Na2FeSiO4 demonstrates the highest reversible capacity of 181.0 mAh g-1 to date with multi-electron redox reaction occurred among various polyanion-based SIBs 1 ACS Paragon Plus Environment


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cathodes, which achieves a close-to-100% initial Coulombic efficiency and a stable cycling with 88% capacity retention up to 100 cycles. In addition, such electrode shows excellent stability either charged at a high voltage of 4.5 V or heated up to 800 oC. The present work might open up the possibility for developing high capacity, good safety and low cost polyanion-based cathodes for rechargeable SIBs. Keywords: sodium-ion battery, Na2FeSiO4-based cathode, low cost, high capacity, good safety.

Introduction There have been tremendous appeals for renewable energy in the fact of the finite fossil energy and the aggravated environmental pollution. However, the green transition of energy was slowed down due to the intermittent and unstable nature of renewable energy, where large-scale energy storage devices are critical for bridging the generation and the supply of electricity smoothly.1, 2 Rechargeable sodium-ion batteries (SIBs) have been widely expected to be a low cost and sustainable candidate for large-scale energy storage owing to the abundant sodium resource and its similar chemistry as that of lithium.3-6 High-performance cathode materials are the key to the development of SIBs considering their relatively low specific capacity and poor rate capability.7-13 Moreover, safety and cost issues are always accompanied with rechargeable batteries, particularly in the large-scale applications of SIBs, where cathode materials account for big deal in reducing the cost and diminishing the operation risk of battery.3, 4, 14 Therefore, it is highly desirable to develop a high capacity SIB cathode, as well as low cost and high safety. Two typical types of cathode materials have been developed for SIBs, including transition metal oxides (TMOs) and polyanion compounds. The available capacity and safety (the cycle stability and thermal decomposition or phase transition temperature have been taken into account) of cathodes materials are summarized in Figure 1. TMO cathodes that contain multi2 ACS Paragon Plus Environment

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metals in the framework can easily deliver high specific capacity owing to their relatively light weight. However, the layered structure of TMOs is usually unstable under a high operating voltage or an elevated temperature, resulting in poor cycling stability and short cycle life.15-24 Worse, TMO cathodes may react with organic electrolyte at a high-charged state and generate CO2/O2 in Na-cells, resulting in potential serious safety risks.25-27 On the other hand, polyanion compounds are confirmed as an alternative class of cathode materials to enhance the stability and safety of SIBs. Owing to the induced effect of polyanion groups, the stability of the structure of polyanion cathodes could be well kept during Na+ extraction/insertion processes, and in general, their thermal stability is also excellent.28-31 Unfortunately, for most reported polyanion cathodes, the large molar mass of polyanion groups and single-electron reaction during charge/discharge result in the low specific capacity, which seriously impedes the improvement of energy density of SIBs. 11, 29, 30, 32-40 Transition metal orthosilicates (Na2MSiO4, M=transition metal), which might possess a two-electron electrochemical reaction, open up the possibilities for high-capacity polyanion cathodes.41, 42 Na2FeSiO4 is currently attracting attention due to its high theoretical capacity of 276 mAh g-1 assuming a two-electron reaction occurred and its low cost nature due to the component earth-abundant elements.43 Ye et al. predicted that Na+ could be de-intercalated at ~1.9 and ~4.3 V for Na2FeSiO4, suggesting the possibility of a two-electron reaction in the voltage range of 1.5-4.5 V.44 However, Na2FeSiO4 with a structure of the monoclinic system showed an initial discharge capacity of 126 mAh g-1, and the major Na2FeSiO4 phase became amorphous that resulted in a large irreversible capacity and poor cycle performance.45 The cubic Na2FeSiO4 framework remained stable even at a charged state of 4.0 V, but this zerostrain cathode delivered a relatively low capacity of 106 mAh g-1.46 All these preliminary attempts have been failed to achieve a high reversible capacity, in the consideration of a twoelectron reaction of 276 mAh g-1 theoretically. Furthermore, the electrode stability of Na2FeSiO4 are urgently needed to be investigated for potential large-scale applications. 3 ACS Paragon Plus Environment


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Herein, we demonstrate a high capacity, low cost and high safety SIB cathode of the carbon-coated Na2FeSiO4, which possesses a relative pure triclinic Na2FeSiO4 phase and three-dimensional network morphology, exhibiting a high reversible capacity of 181.0 mAh g1

at C/10 (1 C=276 mA g-1) in the voltage range of 1.5-4.5 V with a ~100% first Coulombic

efficiency and 88% capacity retention after 100 stable cycles. Worthy of noting is that such a high reversible capacity of carbon-coated Na2FeSiO4 is firstly reported among all polyanion cathodes previously reported (Figure 1). Furthermore, XPS characterizations were conducted for the NFS/C cathodes at different charge states to reveal the redox mechanism of Fe during cycling. In the meantime, the Na2FeSiO4 phase remains stable under a high charge state (4.5 V) or an elevated temperature (800 oC), which is crucial as a high-safety and low-cost SIB cathode for large-scale battery applications.

Experimental Section Preparation of carbon-coated Na2FeSiO4 and pristine Na2FeSiO4 The citric assisted sol-gel method was used for preparing the carbon-coated Na2FeSiO4 and bare Na2FeSiO4 precursors. The stoichiometric amounts of FeC2O4·2H2O and CH3COONa with citric acid were first dissolved in ethanol (60 mL) under magnetic stirring at 50 oC. After 3 hours, 0.02 mol Si(OC2H5)4 (TEOS) were added to the solution as a silicon source. After stirring for 5 hours, the temperature was rose to 80 oC, then kept stirring until the solution yielded the gel precursor. The precursor was dried at 60 oC for 12 hours. For carbon-coated Na2FeSiO4, 5 wt% sucrose was mixed in the above precursor as carbon source followed by ball milling for 10 hours, then it was dried at 100 oC overnight. The precursors were sintered at 600 oC for 8 hours under argon flow for forming the NFS and NFS/C samples. Materials characterization The crystalline phase of Na2FeSiO4 was characterized by X-ray diffraction (XRD) on a PANalytical X’Pert Powder diffractometer. The sample was scanned at a range of 10-80 °

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with a scan rate of 1 °/min. To confirm the valence state of Fe in the samples, the X-ray photoelectron spectroscopic (XPS) narrow spectra of Fe 2p was performed with an Escalab250Xi X-ray photoelectron spectrometer, and the excitation energy is 1486.6 eV (Al Kα). The morphology and the microstructure was observed by scanning electron microscopy (SEM, Hitachi SU70) and transmission electron microscope (TEM, FEI Tecnai G2 F20). The thermogravimetric (TG) and the differential scanning calorimetry (DSC) profiles were conducted on a simultaneous differential scanning calorimetry and thermogravimetry (STA 449 F3) in a temperature range from room temperature to 800 oC. The carbon content of NFS/C was tested with a Vario MICRO cube Elementar Analysen system. Electrochemical measurements Electrodes for coin-type cells were prepared by mixing the active material, Super-P and carboxymethylcellulose sodium (CMC) at a weight ratio of 70:20:10, with an appropriate amount of N-methyl-2-pyrrolidone (NMP) as the solvent. The slurry was casted onto Al foil and dried at 120 oC in a vacuum oven for 20 hours. The mass loading of the electrode was around 3 mg cm-2 and the diameter was 17 mm. Coin-type cells (CR2025) were assembled in a glovebox filled with argon, where both moisture and oxygen levels were less than 1 ppm. Metallic sodium was used as the counter/reference electrode. The Galvanostatic charge/discharge profiles were obtained in a voltage range between 1.5 and 4.5 V vs. Na/Na+ with a Neware BTS-5 battery test system. Electrochemical impedance spectroscopy of Nacells were conducted in the frequency range of 0.1-100000 Hz with an amplitude of 5 mV on an electrochemistry workstation (CHI660E).

Results and Discussion Pristine sodium iron silicate (Na2FeSiO4) and carbon-coated sodium iron silicate (Na2FeSiO4/C), labeled as NFS and NFS/C respectively, were successfully synthesized via a citric acid assisted sol-gel method. After heating treatment at 600 oC for 8 hours, both samples



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possess high crystallinity as confirmed by the X-ray diffraction (XRD) analysis (Figure 2a). Although the XRD pattern of NFS/C in our work is similar to that of cubic phase,46 its reflections can be indexed to a triclinic cell and the step-scanning XRD pattern fitted by Pawley refinement method can converge to Rp = 10.296% and Rwp = 14.898 % (Figure S1), indicating a new polymorph of Na2FeSiO4. The indices and positions of the reflections, and the refined cell parameteres are listed in Table S1 and Table S2, respectively. Due to the similarity of major XRD peaks, the framework of this triclinic phase is likely to be composed of a Fe-Si diamond-like network which keeps stable during Na extraction/intercalation processes.44,46 The b and c axes of this new cell can be considered to be diagonals in the cubic cell and the a axis remain unchanged, thus the structure of our sample displays a lower triclinic symmetry. It is worthy to be noted that some reflections corresponding to the impurity phase of Na2SiO3 are observed in the XRD pattern of NFS, which is due to the partial oxidation of Fe2+ in the synthesis process. Upon introducing sucrose in the precursor, a relative phase-pure Na2FeSiO4/C with only minor impurities was successfully prepared owning to the reducing atmosphere generated by the pyrolysis of sucrose. Such pure phase of cathode materials is critically important for providing more active sites during sodium intercalation/de-intercalation. As shown in Figure 2a, it could be found that higher sodium content is demonstrated as characterized by the stronger peak at around 34.5° on the XRD pattern of NFS/C as compared to that in previous report,46 verified by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis (Na/Fe=1.923, Table 1). The Narich cathodes are favorable for improving the initial Coulombic efficiency and therefore enabling a non-sodium metal anode to assemble practical SIBs. The carbon content in NFS/C was 4.63 wt% as tested by elemental analyzer. X-ray photoelectron spectroscopy (XPS) analysis was conducted to further verify the valence state of Fe in NFS/C, as shown in Figure 2b. The narrow spectra of Fe 2p apparently illustrate that Fe 2p signal consists two components due to the spin-orbit interaction, corresponding to Fe 2p3/2 and Fe 2p1/2 with the 6 ACS Paragon Plus Environment

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binding energies located at 710.7 and 724.5 eV respectively. These peaks observed in our study are in agreement with the same core level peaks of iron in Li2FeSiO447, NaFe(SO4)2 at discharge state38 and iron sodium silicate glass48, indicating that the Fe exists in the form of Fe2+ in the NFS/C. The morphology of the NFS and NFS/C was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figure 3. Bare NFS displays irregular sphere-like particle morphology with a diameter of approximately 500 nm, as shown in Figure 3a and b. TEM image of bare NFS exhibits that the secondary submicron particle is composed of a large amount of primary particles with 20-50 nm in size, with a dspacing of 0.26 nm corresponding to the NFS (212) in good agreement with the XRD analysis (Figure 3c and d). By contrast, the secondary particle sizes of NFS/C are much smaller resulting from the restriction of carbon. Interestingly, a three-dimensional network morphology is observed for the NFS/C (Figure 3e and f), which might be caused by the release of gaseous carbon oxides during heat treatment. High resolution TEM (HR-TEM) image in Figure 3h shows that the primary nanoparticle in the NFS/C is uniformly coated by an amorphous carbon layer, which possesses good crystallinity with a d-spacing of 0.42 nm corresponding to the (110) planes of Na2FeSiO4. Such network morphology of NFS/C is believed in favor for the penetration of the electrolyte, enabling fast ion diffusion. Besides, the uniform carbon-coating on closely contacted particles could construct fast pathway across active particles for electron transfer. Electrochemical measurements were conducted to explore the sodium storage performance of the NFS and NFS/C. The galvanostatic charge/discharge profiles at a current density of 0.1 C (27.6 mA g-1) are shown in Figure 4a. Surprisingly, the NFS/C electrode delivers a high reveisible capacity of 181 mAh g-1, which is the highest value among all reported polyanion cathodes previously. In the meantime, the initial Coulombic efficiency is close to 100% due to the Na-rich nature of the NFS/C, which is very attractive for pracitical full cell applications. 7 ACS Paragon Plus Environment


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Apparently, two voltage plateaus are observed at around 2.0 and 4.3 V during the second charge process for both NFS and NFS/C, which could be assigned to the multi-step electrochemical reaction, and this behavior is in good agreement with the calculated results.44 The 2.0 V plateau is prolonged after introducing the carbon source, indicating the improved electrochemical activity of the NFS/C electrode. The galvanostatic charge/discharge profiles under different cutoff charge voltages were performed to further verify the reversible capacity at the high voltage plateau of 4.0-4.5 V of NFS/C. As Figure 4c shows, only 100 mAh g-1 discharge capacity is maintained when the cutoff potential is fixed at 4.0 V, suggesting a single electron electrochemical reaction occurred during this voltage range. Oppositely, while the cutoff potential is increased to be 4.5 V, around 1.4 Na+ per formula unit can be reversibly extracted/inserted in the NFS/C electrode, clearly demonstrating the possibility of multielectron electrochemical reaction for developing high capacity polyanion cathodes. XPS investigation were conducted for the NFS/C cathodes at different charge/discharge states to reveal the redox mechanism during cycling. As Figure 5b shows, upon charging to 4.3 V, the binding energies of Fe 2p1/2 and Fe 2p3/2 shifts to 726.3 and 712.6 eV respectively, which implies that Fe2+ in NFS/C has been oxidized to be Fe3+, delivering ~140.7 mAh g-1 in consistent with the single electron electrochemical reaction occurred. This result is consistent with the calculated study.43 In a further charging to 4.5 V (fully charged state), the Fe3+ in NFS/C was partially oxidized to be Fe4+ and thus multi-peaks have been separated by means of XPS-peak-differentiating analysis (Figure 5c),49-51 indicating the potential of achieving multi-electron reactions and high capacity by fully oxidation of iron ions for this material. Figure 5d reveals that the binding energies of Fe 2p shift back to 724.7 and 710.9 eV of Fe2+ when the electrode was discharged to 1.5 V, demonstrating that the full reversibility of redox reactions of Fe ions in the NFS/C electrode. It is worthy to note that the rigid Fe-Si framework remains stable during charge/discharge process (Figure 6). In the meantime, the sodium ions were randomly extracted or intercalated with the rearrangement of oxygen ions.46 8 ACS Paragon Plus Environment

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The cycling performance and rate capability of the NFS/C were further evaluated as shown in Figure 4b and d. Bare NFS electrodes were also included for comparison. The NFS/C electrode undergoes a stable cycling up to 100 cycles, with a high capacity retention of 88%. A high Coulombic efficiency of ~99% was recorded for most cycles, demonstrating the good reversibility of the electrochemical reactions. In contrast, the NFS electrode suffers a rapid capacity fading during the initial 10 cycles, which is considered to be the impact of impurity phases, and the reversible capacity is only stabilized around 120 mAh g-1. The rate performance up to 1 C of the NFS/C and NFS is shown in Figure 4d. As compared to the NFS electrode, the NFS/C counterpart delivers higher capacities at all current densities, suggesting the rate capability of NFS/C is greatly enhanced after the incorporation of carbon. To further investigate the electrode kinetics of both the NFS/C and NFS electrodes, electrochemical impedance spectroscopy (EIS) measurements were carried out as shown in Figure 7. The Nyquist plots reveal a semicircle in the high-frequency region and a declining line in the lowfrequency region, which correspond to the charge transfer process and the Warburg diffusion process, respectively. By fitting the plots with an equivalent circuit (as shown in the inset of Figure 7a), the charge-transfer resistance (Rct) of NFS/C (196.2 Ω) is calculated only around half of that of NFS (378.9 Ω), indicating faster charge transfer process of the NFS/C. Furthermore, the linear fitting at the low-frequency range was conducted for comparing the sodium ion diffusion coefficients (DNa+) of NFS/C and NFS. D can be obtained by the following equation : D=

R2 T 2 2A2 n4 F4 C2 σ2

Z' =RS +RCT + σω-1/2

(1) (2)

where R, T, n, and F are constant, A is the surface area of electrode (238.84 and 292.72 cm2 for NFS and NFS/C respectively, calculated by BET equation, Figure S2), C is the sodium ion 9 ACS Paragon Plus Environment


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concentration in electrode, σ (Warburg factor) is the slope of linear fitting of the relationship plot of Z’ and ω-1/2 (equation 2, Table S3), Rs is the solution resistance and Rct is the charge transfer resistance. D varies inversely with σ2 based on the materials with the same structure.52 The Warburg factor of NFS/C (32.23) is much lower than that of NFS (44.79). Based on the calculation, NFS/C possesses a higher sodium ion diffusion coefficient (1.92× 10-13 and 2.53×10-13 cm2 s-1 for NFS and NFS/C respectively). As mentioned above, the smaller particle size and the carbon-incorporated network morphology of NFS/C might account for the better reaction kinetics, characterized by the smaller charge transfer resistance and the higher ion diffusion coefficient, leading to the better rate capability of NFS/C. The structural stability of NFS/C at various charge/discharge states was examined by exsitu XRD. Figure 6 shows the XRD patterns of the electrodes at different charge/discharge states. The crystal framework remains triclinic without any phase transition upon Na+ intercalation/de-intercalation. Moreover, there is negligible shift of XRD peaks during the whole charge/discharge process, suggesting that there is effective accommodation of volume change. Such phenomenon is similar as the well-known zero-strain electrodes,46, 53-55 which is beneficial for long cycle life of batteries. It is worthy to note that the second major peak on the XRD pattern of NFS/C electrode becomes weak and broad, which might be resulted from the reduction of the order degree due to the perturbation in the structure during charge/discharge processes and the moisture sensitivity (Figure S3) of Na2FeSiO4-based materials.43-46 Thermogravimetric (TG) and differential scanning calorimetry (DSC) were performed from room temperature to 800 oC to further evaluate the thermal stability of NFS/C, which is one of the important safe characters for large-scale applications. Figure 8a shows two endothermal peaks at 91.5 oC and 184.5 oC on the DSC curve with a mass loss of 5.1%, which is due to the dehydration processes.45 Another endothermic reaction occurs at around 600 oC, which can be assigned to the oxidation of amorphous carbon.56 It is noticeable that the major 10 ACS Paragon Plus Environment

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phase remained stable even at an elevated temperature of 800 oC, indicating an excellent thermal stability of Na2FeSiO4. More attractively, even the completely desodiated NFS/C (charged to 4.5 V) is also thermally stable up to 800 oC, and the weight loss at temperature of 230-450 oC is due to the decomposition of the binder (Figure 8b). All these results clearly demonstrate the excellent stability of the NFS/C either at a high charged state or under elevated temperature, which assures safe operation of battery for practical applications.

Conclusion In summary, relative phase-pure NFS/C cathode with high reversible capacity and excellent stability was prepared by a facile citric acid assisted sol-gel method. The Na-rich NFS/C delivers a high reversible discharge capacity of 181.0 mAh g-1 at a current density of 27.6 mA/g with a ~100% first Coulombic efficiency and a good capacity retention of 88% after 100 cycles, which is owing to the unique carbon-incorporated network morphology and the outstanding phase stability. In addition, the multi-electron redox reaction of Fe is studied to reveal the origin of such a high reversible capacity. At the same time, the NFS/C remains stable under a high charge state (4.5 V) or an elevated temperature (800 oC). All these impressive results might trigger wide research interest of Na2FeSiO4-based materials as a high capacity, good safety and low cost cathode in rechargeable SIBs for large-scale energy storage.

Supporting Information Pawley refinement of Na2FeSiO4 with Rp = 10.296% and Rwp = 14.898%. The nitrogen adsorption and desorption plots of NFS and NFS/C. Comparison of XRD patterns of the as prepared material and the powder stored in the air for two months. Indexed XRD pattern lists. Refined Cell parameters.



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Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC21373184) and the Fundamental Research Funds for the Central Universities (2016QNA4007 & 2016XZZX005-07).

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Figure Captions Figure 1 A general comparison of reversible capacity and stability of various SIBs cathode materials. Figure 2 (a) XRD pattern of the NFS and the carbon-coated NFS/C prepared with sol-gel method. (b) XPS narrow spectra of the Fe 2p of NFS/C. Figure 3 (a-b) SEM images of the NFS particles and (c) TEM observation of the NFS (d) HRTEM image of the NFS particle (e-f) SEM images of the NFS/C 3D network structure (g) TEM image of the NFS/C (h) HR-TEM image of the NFS/C. Figure 4 Electrochemical performance of the NFS and NFS/C. (a) the initial two charge/discharge curves with operating potential from 1.5 to 4.5 V vs. Na/Na+ (b) cycling performance at 0.1 C (27.6 mA/g) (c) the initial two cycles of NFS/C between voltage ranges 1.5-4.0 V and 1.5-4.5 V vs. Na/Na+ at 0.1 C (d) rate performance. Figure 5 XPS narrow spectra of the Fe 2p of (a) as-prepared NFS/C, (b) cathode charged to 4.3 V, (c) cathode charged to 4.5 V, (d) cathode discharged to 1.5 V. Figure 6 ex-situ XRD patterns of the NFS/C at different charge/discharge states, pattern of Al foil is shown as a black line. The electrons-voltage plot recorded under a current density of 27.6 mA g-1 is presented on the right. Figure 7 (a) Nyquist plots of the NFS and the NFS/C in Na half-cells. The equivalent circuit is shown in the inset and fitting results are shown as red lines (b) linear fitting to Z’ versus ω1/2

plots in the low-frequency range of the NFS and NFS/C electrode.

Figure 8 (a) TG and DSC curves of the NFS/C. TG and DSC are shown as black and red profiles respectively (b) TG curves of the as-prepared and desodiated NFS/C and the binder.



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Figure 1


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Figure 2



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Table 1 Elemental ratios of NFS/C Na/Fe

Si/Fe

Value

Deviation (±)

Value

Deviation (±)

1.923

0.004

0.833

0.009


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Figure 7



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Figure 8


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