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A Green Route to a Na2FePO4F-based Cathode for Sodium-ion Batteries of High Rate and Long Cycling Life Xiang Deng, Wenxiang Shi, Jaka Sunarso, Meilin Liu, and Zongping Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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ACS Applied Materials & Interfaces

A Green Route to a Na2FePO4F-based Cathode for Sodium-ion Batteries of High Rate and Long Cycling Life Xiang Deng†, Wenxiang Shi†, Jaka Sunarso‡, Meilin Liu*, # and Zongping Shao*, †, §, //

†

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of

Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, P.R. China.

#

Center for Innovative Fuel Cell and Battery Technologies, School of Materials Science and Engineering,

Georgia Institute of Technology, Atlanta, GA 30332-0245, USA.

§

Department of Energy, Nanjing Tech University, Nanjing 210009, China.

//

Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia.

‡

Chemical Engineering Department, Faculty of Engineering, Computing and Science, Swinburne University of

Technology, Jalan Simpang Tiga, 93350 Kuching, Sarawak, Malaysia

KEYWORDS: Na2FePO4F, carbon coating, environmental friendly cathode, green electrode fabrication, sodium-ion batteries 1 ACS Paragon Plus Environment


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ABSTRACT: Sodium-ion batteries (SIBs) are considered one of the most promising alternatives for large-scale energy storage due largely to the abundance and low cost of sodium. However, the lack of high-performance cathode materials at low cost represents a major obstacle towards broad commercialization of SIB technology. In this work, we report a green route strategy that allows cost-effective fabricaiton of carbon-coated Na2FePO4F cathode for SIBs. By using vitamin C as a green organic carbon source and environment-friendly water-based polyacrylic latex as the binder, we have demonstrated that the Na2FePO4F phase in the as-derived Na2FePO4F/C electrode shows a high reversible capacity of 117 mAh g-1 at a cycling rate of 0.1 C. More attractively, excellent rate capability is achieved while retaining outstanding cycling stability (~85% capacity retention after 1,000 charge-discharge cycles at a rate of 4C). Further, in operando X-ray diffraction has been used to probe the evolution of phase structures during the charge-discharge process, confirming the structural robustness of the Na2FePO4F/C cathode (even when charged to 4.5V). Accordingly, the poor initial Coulombic efficiency of some anode materials may be compensated by extracting more sodium ions from Na2FePO4F/C cathode at higher potentials (up to 4.5V).

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Introduction Large-scale energy storage technology is vital to future energy system that is believed dependent mainly on renewable energies such as solar, wind, and geothermal powers. Electrochemical energy storage systems are particularly promising for large-scale applications because of their high energy and power density, long cycling lifetime, and flexibility in size and location.1, 2 To date, the most successful electrochemical energy systems are lithium-ion batteries (LIBs), which have been extensively used in many applications, notably in portable electronic devices. Unfortunately, the application of LIBs to large scale energy storage is limited by the low content of Li element in the earth’s crust (around 0.0065%) and by the poor accessibility of most lithium reserves in remote or in politically sensitive areas (e.g., 70% in South America).3 In contrast, sodium is one of the most abundant elements on the earth,4 implying that sodium-ion batteries (SIBs) have potential to be a cost-effective technology for large-scale energy-storage applications (e.g., power grid and stationary electrical storage). When size is not a critical factor, SIBs are highly desirable.2, 3, 5 Due to the larger size of Na+ than Li+, the electrochemical intercalation/de-intercalation of Na+ in electrode materials is usually more difficult than Li+. To make SIBs commercially competitive, considerable efforts have been devoted to the development of suitable electrodes with high Na+ storage capacity, high rate capability, and long cycling life, especially the cathode which largely determines the performance of a SIB. Up to now, many cathode materials have been developed with different theoretical capacity, cycling stability, and discharge potential. 3 ACS Paragon Plus Environment


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Among them, Ni and Co-free polyanionic compound Na2FePO4F, first reported by Nazar’s group6 in 2007, is of particular interest. Na2FePO4F has two-dimensional pathways for sodium ion transport, high structural stability and relatively high discharge potential due to the strong inductive effect of the anions, and favorable theoretical capacity of around 124 mA h g-1 as cathode for SIBs. However, the conductivity of Na2FePO4F was estimated to be relatively low, leading to unsatisfactory rate capacity and poor cycling performance, more so at higher cycling rates.6-8 Improving its rate and cycling capability is thus urgently needed to make Na2FePO4F a practical cathode of SIBs for large-scale energy storage, where a long cycling life is a necessity and fast charge/discharge rates are frequently required. Herein, we report our findings in the development of carbon-coated Na2FePO4F (denoted as NFPF-C) cathode for SIBs, demonstrating excellent rate capability and cycling stability. Vitamin C (VC) was used as a green organic carbon source for creating a carbon coating on NaFePO4F, which was synthesized using a facile and green mechano-chemical process (high energy ball milling). Further, a water-soluble polyacrylic latex (LA132) was used as the binder, aiming to replace the conventional polyvinylidene fluoride (PVDF) binder, together with the hazardous N-methyl-2-pyrrolidone (NMP) solvent needed for the preparation of electrode paste. The as-prepared Na2FePO4F/C electrode for SIBs, derived from a green and easily scalable water-based electrode manufacturing process, exhibited superior rate performance and cycling stability while maintaining a high specific capacity of 66.8 mAh g-1 at a high cycling rate of 4.0 C and superior capacity retention of 84.7% after 1000 cycles. The attractive performance 4 ACS Paragon Plus Environment

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achieved in this study, especially the rate capability and cycling stability, suggests that the polyanion-type Na2FePO4F/C has potential to be a highly promising SIB cathode for large-scale electrochemical energy storage. In addition, by charging up to 4.5V at the first cycle, the Na2FePO4F cathode could provide more Na+, allowing a simple way to compensate the low Coulombic efficiency of some newly-developed anode materials for SIBs.

Results and Discussion

Figure 1 a) FESEM, b) TEM, c) HR-TEM and d) corresponding STEM-EDX elemental distribution of NFPF-C sample. e) Nitrogen adsorption-desorption isotherms of NFPF-C and NFPF samples. The inset Figure is the corresponding BJH pore size distribution plots.

Figure 1 and Figure S2 of the Supporting Information shows some typical images of the samples as-prepared from the green mechano-chemical process (after calcination in argon 5 ACS Paragon Plus Environment


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atmosphere at 600 oC for 6 h) with and without the presence of VC as an organic carbon source. The as-prepared pure Na2FePO4F sample without the presence of VC during the ball milling process (denoted as NFPF) showed primary particle sizes of 200 to 400 nm (Figure S2a, b, Supporting Information). In contrast, when VC was added, aggregation was effectively suppressed, and the primary particle size was reduced to 50 - 150 nm (Figure 1a, b). A smaller particle size could create more active sites for electrochemical reactions and shorten the solid-state Na+ diffusion distance, thus improving the rate capability of the cathode material. In addition, a carbon coating (~5 nm thick) was observed on the surface of the particles (Figure 1c and Figure S3), suggesting that the Na2FePO4F particles are uniformly coated by VC-derived carbon. Such carbon coating has effectively suppressed the mass diffusion and grain growth during calcination. Moreover, the carbon coating has significantly improved the electrical conductivity of the composite electrode.9-11 The conductivity of the pristine and carbon-coated Na2FePO4F pellet samples at room temperature was ~3.2×10-6 and ~8.8×10-4 S cm-1, respectively, as determined from 4-probe DC measurements under the same conditions (described in the supporting information). The two orders of magnitude improvement in electrical conductivity is an indication of the enhancement effect of carbon coating under a similar experimental situation. Both the reduced crystalline size and increased conductivity are beneficial to the electrochemical performance of the material as a cathode in SIBs. Figure 1d shows a Scanning TEM (STEM) micrograph and the corresponding EDX mapping of the elements, indicating the successful formation of NFPF-C composite with uniform elemental 6 ACS Paragon Plus Environment

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distributions throughout the sample. The specific surface area and pore structure of the materials were analyzed using nitrogen adsorption-desorption measurements (Figure 1e). For the carbon-free NFPF sample, no obvious pores were detected. However, a typical hysteresis was observed for the NFPF-C sample in the medium relative pressure region, together with the risen adsorption volume at high relative pressure region, suggesting the presence of mesopores and macropores.12 The average pore sizes were peaked at approximately 3.4 nm and 17 nm (Figure 1e), suggesting that the carbon coating derived from VC led to not only the formation of smaller Na2FePO4F primary particles but also the creation of a hierarchical pore structure. The specific surface area of the NFPF-C was calculated to be ~31.9 m2 g-1 whereas that of the NFPF was ~5.4 m2 g-1. Clearly, the reduced particle size and increased pore volume contributed to the significant increase in specific surface area of the NFPF-C sample.

Figure 2 a) X-ray diffraction (XRD) patterns of the samples with different preparation conditions. b) X-ray photoelectron spectroscopy (XPS) high-resolution Fe 2p spectra of the samples with or without carbon coating. c) Raman spectra of NFPF-C and NFPF samples.



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To further confirm the formation of the targeted phase from the mechano-chemical synthesis, various samples prepared under different conditions were examined by X-ray diffraction (XRD). As shown in Figure 2a, the XRD pattern of the precursor sample prepared from the ball-milling process showed only three characteristic diffraction peaks of NaF-type phase at 2-theta of 38.8 o, 56.0 o and 70.3 o. After a pre-calcination of the solid precursor at 350 o

C, the Na2FePO4F phase began to form. The samples calcined at 600 oC with carbon coating

(NFPF-C) or without (NFPF) both showed a dominant orthrorhombic phase of Na2FePO4F (space group: Pbcn)6, except that a small peak was observed at 2-theta about 21o for the NFPF-C sample, which can be attributed to the carbon coating effect.8 The surface electronic structure of NFPF-C and NFPF samples was investigated by X-ray photoelectron spectroscopy (XPS). The high resolution Fe 2p spectrum of NFPF-C was similar to that of NFPF in Figure 2b, showing the Fe 2p3/2 and 2p1/2 main peaks at ∼710.8 eV and ∼725.0 eV, respectively, which are characteristic of Fe2+ ions, agreeing well with 2+ oxidation state of iron in Na2FePO4F. To further confirm the formation of carbon in NFPF-C, a Raman spectrum was taken in Figure 2c. Two typical peaks located at 1358 and 1587 cm-1 was observed, corresponding to the disorder-induced mode related to structural defects (D-band) and the in-plane vibrations of the sp2 carbon atoms (G-band).13, 14 The ID/IG ratio of 1.006 obtained from fitting using Lorentz method suggests a high degree of graphitization of the carbon layer in the sample.9, 15 However, the NFPF sample showed no sign of carbon peaks, with only small peaks of tetrahedral PO43- (point group symmetry of Td) with four internal modes (ν1, ν2, ν3, ν4) caused by the absence of carbon coating.16 The carbon content 8 ACS Paragon Plus Environment

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of NFPF-C was determined to be ~11.3 wt% from a thermogravimetric analysis (TGA) (Figure S4, Supporting Information).

Figure 3 Galvanostatic charge-discharge curves of a) NFPF-P and b) NFPF-CL at 0.1 C. The inset pictures are the corresponding differential capacity dQ/dV plots.c) Cyclic voltammetry (CV) profile of NFPF-CL with the scan rate of 0.1 mV s-1. d) Rate performance of the cells with NFPF-P, NFPF-L, NFPF-CP and NFPF-CL electrodes at different current densities. e) Cycling performance comparison of the as-synthesized samples with related Coulombic efficiencies at a current density of 1.0 C. f) Long cycling performance of NFPF-CL sample at 4.0 C.



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The electrochemical performances of the as-prepared samples were evaluated in coin-type sodium half cells (the detailed cathode preparation and cell assembly method is described in the Experimental section of Supporting Information). As shown in Figure 3a, the NFPF electrode with conventional PVDF binder (denoted as NFPF-P) has an initial charge capacity of 98.2 mA h g-1 at 0.1 C rate (1C is equal to 124 mA g-1). The corresponding dQ/dV plot reveals two redox peaks located at around 3.0/3.16 V and 2.82/2.95 V. In contrast, the NFPF-C electrode with LA132 binder (denoted as NFPF-CL) exhibited two characteristic voltage plateaus during charge/discharge (Figure 3b), corresponding to two reversible phase transformations of Na2FeIIPO4F ⇌ Na1.5FePO4F and Na1.5FePO4F ⇌ NaFeIIIPO4F centered at around 2.92 V and 3.07 V, respectively.8, 17 This two-voltage plateaus were associated with the activation energies for Na+ migration along the [100] and [001] directions.18 The NFPF-CL electrode showed a specific capacity of 103 mA h g-1 at 0.1 C rate with an initial Coulombic efficiency of 92.4%. Considering the negligible capacity of carbon in the NFPF-CL composite, the actual discharge capacity of Na2FePO4F in NFPF-CL electrode was calculated to be ~117 mA h g-1, which is very close to the theoretical capacity of 124 mA h g-1, suggesting that most Na2FePO4F in the NFPF-CL electrode is electrochemically active. From the second cycle on, both the NFPF-P and NFPF-CL electrodes showed nearly 100% Coulombic efficiency. The sodium-ion diffusion coefficients, as estimated from impedance measurements (Figure S6, Supporting Information), are 1.97×10-12 and 3.23×10-13 cm2 s-1 for NFPF-CL and NFPF-P, respectively. The porous structure of the NFPF-C electrode likely facilitated the sodium diffusion in the Na2FePO4F 10 ACS Paragon Plus Environment

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phase. Both the improved electronic conductivity and sodium ion diffusion rate contributed to the superior electrochemical performance of the NFPF-CL electrode. To demonstrate the advantage of aqueous LA132 binder for electrode performance, the NFPF-C electrode with conventional organic polyvinylidene difluoride binder (NFPF-CP) was also prepared and characterized under similar conditions. The charge-discharge curves of the NFPF-CP electrode are similar to those of the NFPF-CL electrode (Figure S7, Supporting Information) and both electrodes showed comparable specific capacity at low rates (103.2 mA h g-1 for 0.1 C and 95.5 mA h g-1 for 0.2 C). However, the NFPF-CL electrode demonstrated much better performance at higher charge-discharge rates (Figure 3e), providing reversible capacities of 90.3, 85.2, 75.4, 70.5, and 66.8 mA h g-1 at cycling rates of 0.5, 1.0, 2.0, 3.0 C and 4.0 C. In comparison, the NFPF-CP electrode showed capacities of 62.3, 45.1, 39.8, 31.2 and 25.1 mA h g-1 at those cycling rates. The better performance of NFPF-CL could be attributed to the fact that LA132 (0.54 N cm-1) had higher binding strength than that of PVDF (0.36 N cm-1),19 in addition, LA132 could provide better dispersion of active particles in electrode slurries.20 Both factors likely made the water-based binder perform much better than the organic binder in improving the current collection between NFPF-CL, conductive carbon, and current collector. It should also be mentioned that the NFPF-P electrode exhibited much lower rate capability, displaying a capacity of 97.1 mA h g-1 at 0.1 C and only 25.6 mA h g-1 at 4.0 C. And as seen in Figure 3d and Figure S8 (Supporting Information), the electrochemical performances of the controlled sample NFPF-L (bare NFPF electrode with aqueous LA132 binder) were improved only slightly, further 11 ACS Paragon Plus Environment


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confirming that the electronic conductivity is another crucial factor to electrode performance. Figure 3e shows that NFPF-CL has a stable cycling performance with a Coulombic efficiency of approximately 100 % after the first activation cycle. After 200 cycles at high current rate of 1.0 C, a reversible capacity of 78.2 mA h g-1 was still maintained (equivalent to a capacity retention of 91.8%), superior to both the NFPF-CP (61.0 mA h g-1) and the NFPF-P (40.4 mA h g-1) electrodes. The excellent cycling performance of the NFPF-CL electrode was further supported by high capacity retention of 84.7% after 1000 charge-discharge cycles at a high cycling rate of 4.0 C, which shows the longest cyclic stability for the Na2FePO4F-based material in the literatures.7, 8, 21

Figure 4 a) In operando XRD characterization of structure evolutions during electrochemical sodium insertion/extraction process. The patterns were collected during the first charge/discharge process of the NFPF-CL sample between 2.0 and 4.5 V under a current rate of 0.1 C. b) The schematic illustration of the phase structure of


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Na2FePO4F with the fully sodiation status (2.0 V), Na2 site fully desodiation status (4.0 V) and the assumption of a Na1-deficiency status when charged to 4.5V.

To

probe

the

phase

transition

of

the

cathode

material

during

sodium

intercalation/de-intercalation, we performed in operando XRD analysis during the first charge/discharge cycle at 0.1 C using a specially designed sample chamber from Rigaku (Figure S9, Supporting Information). As seen in Figure 4a, upon initial sodium de-intercalation, the intensity of the two peaks near 2-theta of ~30o decreased gradually as the cell voltage was varied from 2.7 V (Open Circuit Voltage, OCV) to 3.0 V and the triplet peaks near 2-theta of ~34.5o progressively

transferred

to

one

broad

peak

centered

at

~34.2o,

indicating

the

‘quasi-solid-solution’ behavior that represents the transition from Na2FeIIPO4F to Na1.5FePO4F.6 Then, the following charge process from 3.1 V to 4.0 V showed the further transition from the intermediate phase of Na1.5FePO4F to NaFeIIIPO4F, with the new peaks emerging at 2-theta of 31.2 o and 35.6 o, as well as the shift of the peak at ~34.2 o to higher angle. The cell potential was extended to 4.5 V to examine the structure stability of the cathode at high potentials. XRD patterns at 4.0-4.5 V (colored red) suggested that a stable sodium de-intercalation crystal structure was still maintained with characterized peak patterns of NaFeIIIPO4F. The subsequent discharge process was just identical to the reversal phase transition from NaFePO4F → Na1.5FePO4F→Na2FePO4F, showing the reversibility of the cathode material even charged to 4.5 V. The phase structure of Na2FePO4F during different electrochemical stages were then proposed 13 ACS Paragon Plus Environment


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and shown in Figure 4b. According to previous reports,18 the two Na cations (Na1 and Na2 atom, colored in green and blue, respectively) located in the interlayer space of the crystal structure possess facile two-dimensional migration pathways and the Na2 site was proved to be more mobile. When Na2FePO4F (2.0 V) was de-sodiated to form NaFePO4F (4.0 V), this could result in the de-intercalation of all sodium from Na2 site, while the Na1 site still remains fully occupied.6 Interestingly, further charge to a high potential of 4.5 V was supposed to cause the Na1 atom deficiency accompanied with partial de-intercalation of Na1 atoms to provide more capacity in the first cycle, while the crystal structure remained unchanged according to Figure 4a. According to previous reports,22, 18 it is likely caused by the active sodium ion exchange (or the Na1-Na2 site swap) under high oxidation state, although this ion exchange rate seems relatively slow. The origin of the high stability of Na2FePO4F/C at 4.5V is still under investigation; a series of carefully designed experiments must be performed to gain insight into the mechanism of performance enhancement.


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Figure 5 a) The Galvanostatic charge-discharge curves of NFPF-CL with a cutting off voltage of 4.5 V for the first charge. b) Nyquist plots of a NFPF-CL electrode acquired during the first cycle in a half-cell configuration. The

impedance data were collected after the cells were charged at a constant rate of 0.1 C until a pre-determined cell voltage (or potential) was reached (e.g., 2.5, 3.0, 3.5, 4.0, 4.5 V) and then charged at this potential in a potentiostatic mode for another 10 minutes. c) Schematic illustration of the amount of Na+ ions extracted from and re-inserted into the positive electrode in the battery and the corresponding Na+ efficiency (extracted/re-inserted) for different charge up-end potentials of the first cycle. d) The cycling performance of NFPF-CL with different cycle modes in a half-cell configuration.



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To gain more insight into the phenomenon, we performed galvanostatic charge-discharge cycling by increasing the cutting-off voltage to 4.5 V at 0.1 C (Figure 5a). The first charge capacity was ~187.4 mA h g-1, and the first discharge capacity was improved to ~129.3 mA h g-1. To address the concern of the possible side reactions between the cathode material and the electrolyte on the charge capacity at 4.5 V, we performed additional experiments. First, we evaluated the stability of the electrolyte (1 M NaClO4 in PC + 2 vol % FEC) in the potential window of 2.0-4.5 V. As shown in Figure S10a (Supporting Information), the PC-based electrolyte was quite stable in the voltage window of 2.0-4.5 V and no obvious side reaction or electrolyte decomposition was observed up to 4.5 V (Figure S10b, Supporting Information); this is consistent with several previous reports on the stability of PC-based electrolyte for sodium-ion batteries.23-25 Accordingly, the large charge capacity of the NFPF-C cathode at 4.5 V in the electrolyte is due not to the side reactions, but to the cathode material itself. Furthermore, we also tested the NFPF-CL material in another high-potential electrolyte reported previously26 (purchased from the Beijing Institute of Chemical reagents, SN 1610008) with 1 M NaClO4, and the observed electrochemical behavior (Figure S11, Supporting Information) was similar to that shown in Figure 5a. Based on these results, we believe that the large charge capacity of NFPF-C cathode at 4.5 V is not due to side reactions nor electrolyte decomposition. The results imply that, in addition to the sodium in Na2 site, partial Na1 site sodium of the Na2FePO4F was indeed moveable at potential higher than 4.0 V. Figure 5b shows some typical Nyquist plots of an NFPF-CL electrode acquired at different stages of charge. The diameter of the impedance loop 16 ACS Paragon Plus Environment

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represents the charge transfer resistance of the electrode material, which becomes smaller at higher potential and is an indication of improved sodium-ion transfer kinetics. The initial sodium-ion efficiency (SE), which is defined as the ratio of the amount of sodium ions extracted from the electrode material to that re-inserted back into the electrode material in the first charge-discharge cycle, is another important property for battery electrode materials. As shown in Figure 5c, the initial SE of the NFPF-CL cathode reached 144.8% at a charging cutting-off voltage of 4.5 V. As the cutting-off voltage for charging is reduced, so is the initial SEs, decreasing to 122.5%, 113.8%, 102.7% and 100.4%, respectively, for a cutting-off voltage of 4.4, 4.3, 4.2 and 4.1 V. It suggests that the initial sodium-ion efficiency can be tailored by controlling the charging cutting-off voltage. Considering the fact that some newly-developed anode materials for SIBs have a low initial Coulombic efficiency of around 60% or lower 13, 27-32, the active Na+ loss of anode materials may be compensated by matching with NFPF-CL cathode with a proper upper-end potential; consequently, no requirement of other complex or costly methods like pre-sodiation of anode or extra loading of cathode material is needed. To be specific, two ways are available for such compensation, the increase on the end-of-charge potential to over 4.0 V was applied for all the cycles, or just for the first cycle. First, Figure 5d compares the cycling performance of NFPF-CL in a half-cell configuration under three different modes: (I) cycling within the potential of 2.0-4.0 V, (II) cycling within the potential window of 2.0-4.5V, and (III) first charging the electrode at an upper potential of 4.5 V at 0.1 C, then discharge-charge cycling at 1.0 C within the potential window of 2.0-4.0 V. The capacity fading 17 ACS Paragon Plus Environment


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was observed for NFPF-CL sample progressively charged to 4.5 V after 70 cycles (cycle mode II),

suggesting

the

negative

effect

on

the

cycling

stability

of

repeated

intercalation/de-intercalation of Na1 site sodium. However, if only the first charge potential was elevated to 4.5 V, while the subsequent charge potential was cut-off at 4.0 V, the cycling stability was largely unaffected. To construct a full sodium-ion cell based on the NFPF-CL cathode, we used a sulfur-doped flexible graphene (SFG) film as a kind of hard carbon anode, which we developed in a previous study.13 The electrochemical performances of the cell are shown in Figure S12 (Supporting Information). First, the as-assembled full sodium-ion cell exhibited a good specific capacity of ~75 mA h g-1 (based on the total mass of the balanced cathode and anode) if we take the low Coulombic efficiency of SFG anode (~60%) into account. However, because of the high charge platform of the SFG hard carbon anode, the full cell displays a relatively low operating voltage (~1.0 V), which we need to investigate and improve further in the further work. Figure S12b (Supporting Information) shows the cycling performance of the full sodium-ion cell under different cycle modes: (I) cycling within the potential window of 0-4.0 V, (II) cycling within the potential window of 0-4.5 V, and (III) first charging the electrode to a voltage of 4.5 V at a constant rate of 10 mA g-1, then discharge-charge cycling at 100 mA g-1 within the potential window of 0-4.5 V. Interestingly, the full sodium-ion cell shows an improved cycling capacity following the cycle mode II, which is different from the result of NFPF-CL half-cell. We believe that it is because the low Coulombic efficiency of the hard carbon anode in the full-cell will consume a considerable amount of active Na+ inside the battery, 18 ACS Paragon Plus Environment

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as indicated in the inset schematic of Figure S12a. Therefore, repeated cycling to a cutting-off voltage of 4.5 V in cycle mode II could provide more active Na+ for the anode, resulting in improved overall capacity of the full cell by mitigating the shortcomings originated from the repeated intercalation/de-intercalation of Na1 site sodium. The extension of the charge potential to 4.5 V indeed provided more Na+ ions and improved the cell performance.

Conclusion In summary, a green synthesis route of carbon-coated Na2FePO4F composite cathode for sodium-ion batteries has been successfully developed with high rate capability and good cycling stability. It is also shown that Na2FePO4F has excellent tolerance against air and moisture. When coupled with the use of polyacrylic latex (LA132) as the binder (for an eco-friendly aqueous slurry-coating electrode manufacturing process), the Na2FePO4F/C cathode showed a high specific capacity of 78.2 mA h g-1 at 1.0 C, and an outstanding cycling stability with a capacity retention of 84.7 % after 1000 charge-discharge cycles at 4.0 C. Further, in operando XRD characterization during cycling revealed that the carbon coated Na2FePO4F has high structural reversibility even when charged to 4.5V. Extending the charging cutting-off voltage to 4.5 V may effectively compensate the problems associated with low initial Coulombic efficiency of some anode materials for SIBs.



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ASSOCIATED CONTENT

Supporting Information

Detailed experimental procedure, material characterization method, accelerated aging experiment, electrode preparation and electrochemical measurement method, in operando XRD characterization method and device photos, additional XRD, SEM, TEM, TG and EIS results of Na2FePO4F-based samples, Cyclic voltammetry of a blank sodium-ion cell in specific electrolyte, the performance of the NFPF-CL electrode tested in another high potential electrolyte and assembled full sodium-ion cell using NFPF-CL as the cathode and sulfur-doped flexible graphene as the anode. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Meilin Liu).

*E-mail: [email protected] (Zongping Shao). Tel.: +86 25 83172256; Fax: +86 25 83172242.

ACKNOWLEDGMENT


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This work was supported by the Chinese Academy of Sciences Innovative and Interdisciplinary Team Award, the Changjiang Scholars Program (T2011170) and the Major Project of Educational Commission of Jiangsu Province of China under contract No. 13KJA430004, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the National Nature Science Foundation of China under contract No. 21576135, the Six Talent Peaks Project of Jiangsu Province (Grant No. XNY-CXTD-001), the Program for Jiangsu Specially-Appointed Professors, the Youth Fund in Jiangsu Province under contract No. BK20150945, and the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200). The authors also acknowledge the scholarship from the doctoral candidate international training fund of Nanjing Tech University.

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Figure 1. a) FESEM, b) TEM, c) HR-TEM and d) corresponding STEM-EDX elemental distribution of NFPF-C sample. e) Nitrogen adsorption-desorption isotherms of NFPF-C and NFPF samples. The inset figure is the corresponding BJH pore size distribution plots. 265x160mm (300 x 300 DPI)

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Figure 2. a) X-ray diffraction (XRD) patterns of the samples with different preparation conditions. b) X-ray photoelectron spectroscopy (XPS) high-resolution Fe 2p spectra of the samples with or without carbon coating. c) Raman spectra of NFPF-C and NFPF samples. 266x79mm (300 x 300 DPI)



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Figure 3. Galvanostatic charge-discharge curves of a) NFPF-P and b) NFPF-CL at 0.1 C. The inset pictures are the corresponding differential capacity dQ/dV plots.c) Cyclic voltammetry (CV) profile of NFPF-CL with the scan rate of 0.1 mV s-1. d) Rate performance of the cells with NFPF-P, NFPF-L, NFPF-CP and NFPF-CL electrodes at different current densities. e) Cycling performance comparison of the as-synthesized samples with related Coulombic efficiencies at a current density of 1.0 C. f) Long cycling performance of NFPF-CL sample at 4.0 C. 220x202mm (300 x 300 DPI)


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Figure 4. a) In operado XRD characterization of structure evolutions during electrochemical sodium insertion/extraction process. The patterns were collected during the first charge/discharge process of the NFPF-CL sample between 2.0 and 4.5 V under a current rate of 0.1 C. b) The schematic illustration of the phase structure of Na2FePO4F with the fully sodiation status (2.0 V), Na2 site fully desodiation status (4.0 V) and the assumption of a Na1-deficiency status when charged to 4.5V. 238x201mm (300 x 300 DPI)



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Figure 5. a) The Galvanostatic charge-discharge curves of NFPF-CL with a cutting off voltage of 4.5 V for the first charge. b) Nyquist plots of a NFPF-CL electrode acquired during the first cycle in a half-cell configuration. The impedance data were collected after the cells were charged at a constant rate of 0.1 C until a pre-determined cell voltage (or potential) was reached (e.g., 2.5, 3.0, 3.5, 4.0, 4.5 V) and then charged at this potential in a potentiostatic mode for another 10 minutes. c) Schematic illustration of the amount of Na+ ions extracted from and re-inserted into the positive electrode in the battery and the corresponding Na+ efficiency (extracted/ re-inserted) for different charge up-end potentials of the first cycle. d) The cycling performance of NFPF-CL with different cycle modes in a half-cell configuration. 283x206mm (300 x 300 DPI)


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A Green Route to a Na2FePO4F-Based Cathode ... - ACS Publications

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