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Alluaudite-Na1.47Fe3(PO4)3: Structural and electrochemical properties of potential cathode material for Na-ion Batteries
Accepted Manuscript Alluaudite-Na1.47Fe3(PO4)3: structural and electrochemical properties of potential cathode material for Na-ion Batteries
Katarzyna Walczak, Andrzej Kulka, Janina Molenda PII:
S1293-2558(18)30851-3
DOI:
10.1016/j.solidstatesciences.2018.10.017
Reference:
SSSCIE 5782
To appear in:
Solid State Sciences
Received Date:
05 August 2018
Accepted Date:
28 October 2018
Please cite this article as: Katarzyna Walczak, Andrzej Kulka, Janina Molenda, Alluaudite-Na1.47Fe 3
(PO4)3: structural and electrochemical properties of potential cathode material for Na-ion
Batteries, Solid State Sciences (2018), doi: 10.1016/j.solidstatesciences.2018.10.017
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Alluaudite-Na1.47Fe3(PO4)3: structural and electrochemical properties of potential cathode material for Na-ion Batteries Katarzyna Walczak, Andrzej Kulka, Janina Molenda(a),(b) (a)
AGH University of Science and Technology, Faculty of Energy and Fuels, Krakow, Poland
(b)
AGH University of Science and Technology, Centre of Energy, Krakow, Poland [email protected]
Abstract The structural and electrochemical investigation of alluaudite-NaxFe3(PO4)3 obtained by middle temperature solid-state synthesis is presented. XRD analysis with Rietveld refinement revealed contaminations for samples with x = 2.0. Reduction of the initial sodium content to x=1.5 allowed to obtain single-phase Na1.47Fe3(PO4)3 with real x value determined by iodometric titration. SEM images and particle size distribution showed the fine-grained structure, which might have positive impact on kinetics of the charge/discharge processes. For the electrochemical properties investigation, the Na/Na+/Na1.47Fe3(PO4)3 cells were assembled. The cyclic charge/discharge tests were carried out under various current loads showing high discharge capacity values (near the theoretical value 108 mAh/g) under C/10. For further analysis the CV tests with sodium diffusion coefficient evaluation using Randles-Sevcik equation was provided. The 𝐷 value (~10-14 cm2/s) allowed to state that the intercalation/deintercalation processes in alluaudite-NaxFe3(PO4)3 are diffusion-limited. Keywords: Na-ion batteries, intercalation, electrochemistry, alluaudite, Na2Fe3(PO4)3, Na1.47Fe3(PO4)3, Randles-Sevcik equation
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1.
Introduction Taking into consideration extremely fast growth in demand for portable
electronics, hybrid or electric cars and other replaceable power sources, Lithium-ion Batteries (LiBs) became one of the most common energy storage technologies [1,2], what is the major reason of the world lithium sources contraction [3,4] and increase of lithium price [3–5]. It seems that the Sodium-ion Batteries (SiBs) might be the most suitable substitute for lithium-ion technology [6,7], however only in certain, but notable applications. Due to bigger mass and higher ionic radius of sodium comparing to lithium, both the capacity and charge/discharged kinetics of SiBs are limited and thus not suitable for mobile devices and electric vehicles. Nevertheless, the rapid growth of renewable energy sources requires development of large-scale energy storage systems allowing to mitigate instability and unpredictability of power supply [8–11]. For such applications the density and the charge/discharge kinetics of SiBs seem to be sufficient, while low price and availability of sodium are highly advantageous. In the past decade numerous articles about structures capable to intercalate sodium atoms were released. Next to well-investigated layered oxides, such as NaxCoO2 [12–14], NaCrO2 [15,16], Na0.67Fe0.5Mn0.5O2 [17,18] or NaVO2 [19,20], there are threedimensional structures, mostly phosphates, e.g. olivines NaMPO4 (occurring in two types: triphilite [21,22] and maricite [22–24]), NASICON Na3M2(PO4)3 [25–27] or alluaudite Na2M3(PO4)3 [28–36] (M=metal 3d). The most notable properties of the phosphates are their chemical and thermal stability which results from the strong covalent bonding within the PO4 units.
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In yMny(PO4)3
view of the harmless impact on the environmental, alluaudite-NaxFe3are considered as attractive potential cathode materials for Na-ion Batteries
[28–36]. The compounds consist of elements which are not only environmentally friendly but also widely abundant, what allows for significant price reduction of the cathode material. Since the compounds containing manganese [28,30] showed deterioration of electrochemical properties with the increase of manganese only NaxFe3(PO4)3 materials were taken into consideration for further investigation. The previous works present values of the materials’ capacity close to theoretical value of 108mAhg-1 (assuming stoichiometric Na2Fe3(PO4)3 form), however only for relatively small current loads close to C/20 [32,36]. Outstanding values of cycling capacity were presented for composite of highly nanometric NaxFe3(PO4)3 with carbon nanotubes [29] – the obtained values exceeded 140mAhg-1 indicating possibility for the alluaudite structure to hold more than 2 sodium ions per formula. In this paper we report the research results of alluaudite-NaxFe3(PO4)3 used as cathode material for SiBs, which was prepared by conventional solid-state reaction. Presented results show facile middle temperature route of synthesis resulting with finegrained material with improved electrochemical properties. In order to examine the crystal structure and microstructure the XRD measurements, SEM images and particle size measurements were provided. The assembled Na|Na+|NaxFe3(PO4)3 cells were used for electrochemical tests, i.e. cyclic charging/discharging processes and cyclic voltammetry (CV) measurements linked with sodium diffusion coefficient evaluation, allowing for a deeper insight into the electrochemical properties of the obtained alluaudite-NaxFe3(PO4)3.
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1.1.
Material and methods
1.1.1. Preparation of the samples Alluaudite-NaxFe3(PO4)3 powders were obtained by the conventional solid-state reaction. Na2CO3 (Alfa Aesar, anhydrous, 99,997%) and FePO4·2H2O (Sigma-Aldrich) were used as substrates then grounded in a mortar and pressed into pellets. The pellets were heated at 550oC for 12 hours under the 95%Ar-5%H2 gas mixture continuous flow. After that, to avoid the oxidation, the pellets were put into the mBraun glovebox with argon atmosphere (purity <0.1ppm H2O and < 0.1ppm O2). The syntheses were carried out taking into account two assumed compositions: Na2Fe3(PO4)3 and Na1.5Fe3(PO4)3. 1.1.2. Structural and microstructural analysis The crystal structure of the obtained materials was investigated by X-Ray diffraction using the PANAlytical Empyrean diffractometer equipped with Cu Kα source (λ=0.15418 nm). Profile matching of the diffraction data of as-prepared NaxFe3(PO4)3 samples was applied with GSAS+EXPGUI [37] software. The real sodium amount was calculated by iodometric titration method [38] using EM40-BNC Mettler Toledo with 0.05M Na2S2O3 (Alfa Aesar, 99%, anhydrous) as a titrant solution. SEM analysis was performed using FEI Nano SEM200 in cooperation with Faculty of Materials Science and Ceramics, AGH University of Science and Technology in Cracow, Poland. The particle size distribution was provided by laser light diffraction based measurements using the Mastersizer 3000 analyser and distilled water with the refractive index of 1.33 as a dispersant.
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1.1.3. Electrochemical properties For electrochemical tests the cathode layers on the aluminium foil were prepared using 75%wt. NaxFe3(PO4)3 as an active material, 20%wt. high conductive carbon black (Alfa Aesar 99,9+%) and 5%wt. PVDF (Sigma-Aldrich, powder, Mw ~534,000) in NMP (Sigma-Aldrich, 99,5%) solvent. The CR2032 test coin cells were assembled in the argon-filled mBraun glovebox using metallic sodium (Alfa Aesar, 99%) as an anode material and 1M NaPF6 (Sigma-Aldrich, 99,99%) dissolved in a mixture of ethylene carbonate (EC) (Sigma-Aldrich, 99,99%) and diethyl carbonate (DEC) (Sigma-Aldrich, 99,99%) in the 1:1 ratio. The testing cells were galvanostically charged/discharged under various current loads (C/10, C/5, C/2) with the voltage regime from 2.0 to 4.0V. Additionally, to determine the sodium diffusion coefficient, the cyclic voltammetry with diversified scan rates (from 0.075 to 0.5mV/s) was provided. The electrochemical tests were performed using computer controlled BioLogic VMP3 galvanostat/potentiostat.
2.
Results and discussion
2.1.
Structural and microstructural characterization The XRD patterns of as-synthesized samples (with assumed sodium content
x=2.0 and x=1.5) were analyzed using the Rietveld refinement with alluauditeNa2Fe3(PO4)3 taken as initial model. In this structure both alkali and 3d metal occur in two positions: 4a and 4e for sodium and 8f and 4e for iron, what is directly associated with edge-sharing FeO6 octahedrons chains which are cross-connected with PO4 tetrahedrons. The eight-coordinated sodium atoms are located inside channels along [001] directions, what creates the possibility of reversible intercalation/deintercalation processes (Fig. 1). 5
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Fig. 1 Crystal structure of alluaudite-NaxFe3(PO4)3.
Fig. 2a shows the comparison of the XRD patterns of obtained NaxFe3(PO4) samples with assumed compositions x=2.0 and x=1.5. The XRD analysis of the sample with assumed sodium content x=2.0 revealed Na2Fe3(PO4)3 with C2/c symmetry as primary phase, corresponding to the initial model. Further analysis indicated clearly visible additional peaks, most of them assigned to Fe2P2O7 phase (C-1 space group). The reproducibility of this result (four synthesis attempts in the same conditions) allowed to state the hypothesis that the Na2Fe3(PO4)3 is not stable in its stoichiometric form under presented synthesis conditions. The XRD analysis of the sample with assumed sodium content x=1.5 mole per formula showed one-phase Na1.47Fe3(PO4)3 (Fig. 2b) with the real sodium content 1.47 determined by the iodometric titration method described by Olszewska et al. [38]. The amount of sodium in NaxFe3(PO4)3 is equal to the Fe2+ content (the composition can be II
III
schematically written as NaxFe x Fe2 ‒ x(PO4)3), hence the calculations were based on redox reactions in the presence of iodide ions: Fe3+ + 2I- → Fe2+ + I2
(eq. 1)
I2 + 2S2O32- → 2I- + S4O62-
(eq. 2)
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The titration results are listed in Tab. 1. It is noteworthy that the real sodium content was determined three times and the results were highly reproducible. The final value of the real sodium content is an arithmetic average of three measurements and it equals 1.47. Tab. 1 The results of the estimation of real sodium content (by the iodometric titration method). 1st measurement 2nd measurement 3rd measurement Amount of Fe3+[mol]
1.5307
1.5227
1.5279
Amount of Fe2+[mol]
1.4693
1.4773
1.4721
Amount of Na+ [mol]
1.4693
1.4773
1.4721
The Na1.47Fe3(PO4)3 crystal structure was solved using the C2/c Na2Fe3(PO4)3 as an initial model. The cell parameters calculated by Rietveld analysis are in good agreement with literature [36] and are stated in
Tab. 2.
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(a)
(b) Fig. 2 XRD patterns of the obtained samples: comparison of the indexed diffractograms of Na2Fe3(PO4)3 with the contaminations (*) and pure-Na1.47Fe3(PO4)3 (a); the single-phase Na1.47Fe3(PO4)3 with theoretical curve obtained by Rietveld refinement technique (b).
Tab. 2 Cell parameters of C2/c alluaudite-Na1.47Fe3(PO4)3.
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a [Å]
b[Å]
c[Å]
β[o]
V [Å3]
C2/c Na1.47Fe3(PO4)3 11.8577(5) 12.5464(3) 6.501(23) 114.787(3) 879.275(54)
The SEM images of Na1.47Fe3(PO4)3 are shown in Fig. 3. The material consists of small round-shape grains with a tendency to agglomerate. The particle size distribution for five measurements (by laser diffraction method) of Na1.47Fe3(PO4)3 is presented in Fig. 4 and the average particle sizes were gathered in Tab. 3. It can be observed that Na1.47Fe3(PO4)3 possesses mostly submicron-particles (including grains and agglomerates). Taking into consideration the solid-state technique of materials synthesis, it was expected to obtain the larger particles. Unexpectedly, the average particle size is less than 2µm. Considering that observed particles are in fact porous agglomerates of fine grains, the microstructure seems to be suitable for effective sodium ions transport.
(a)
(b)
Fig. 3. SEM images of Na1.47Fe3(PO4)3 with 20 000x (a) and 50 000x magnitude. Tab. 3 The particle size for Na1.47Fe3(PO4)3 obtained after five measurements with average particle size with standard deviation. Dx(50) represents the median diameter of the particle [μm], the Dx(10) and Dx(90) represent the particle diameter [μm] below the 10 and 90 percent of total volume, respectively.
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Dx(10)
Dx(50)
Dx(90)
1
[μm] 0.581
[μm] 2.08
[μm] 37.0
2
0.578
2.00
36.2
3
0.575
1.95
36.6
4
0.572
1.85
35.5
5
0.570
1.86
36.3
average
0.575(5
1.95(9)
36.3(5)
)
Fig. 4 Particle size distribution for Na1.47Fe3(PO4)3.
2.2.
Electrochemical studies In order to investigate the electrochemical performance of alluaudite-
Na1.47Fe3(PO4)3 the testing Na|Na+|Na1.47Fe3(PO4)3 cells were assembled and charged/discharged in the voltage regime of 2.0V to 4.0V with different current loads: C/10, C/5 and C/2 (Fig. 5a). Assuming the possibility that sodium can be intercalated to alluaudite-NaxFe3(PO4)3 structure until x reaches the value of 2.0, it can be observed that discharge capacity of Na2Fe3(PO4)3 under C/10 current load achieved about 86% of Na2Fe3(PO4)3 theoretical capacity, which equals to 108mAh/g. Furthermore, the discharge capacity does not decrease significantly within cycling. At C/2 current rate it achieves about 80mAh/g, which represents ca. 74% of theoretical value. The coulombic 10
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efficiency (Fig. 5b) of charge/discharge processes is nearly 1.0. The abnormally high value of first and second charge capacity may be caused by non-reversible reactions between electrolyte and material surface, what leads to SEI formation [39,40] .
(a)
(b) Fig. 5. The charge/discharge profiles for Na1.47Fe3(PO4)3 under various current loads: voltage in the function of specific capacity (a) and discharge capacity with reversibility of charge/discharge processes in the function of cycle number (b).
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For the further analysis of electrochemical properties of Na1.47Fe3(PO4)3, the cyclic voltammetry tests with various scan rates were applied (Fig. 6a). It is distinctly visible that the values of the peak current rise with the increasing scan rate. The anodic (charge) and cathodic (discharge) peaks are symmetric, slightly shifted from each other and occur near the voltage value of 3V that corresponds to the narrow plateaux at the charge/discharge profiles. To determine the sodium diffusion coefficient, the RandlesSevcik formula was used: 5
3/2
𝐼𝑝 = 2.69 ∙ 10 ∙ 𝑛
∙𝐴∙𝐷
1/2
1/2
∙𝐶∙𝑣
(eq. 3)
where Ip is the peak current [mA] (cathodic or anodic), n is the number of electrons transferred in the electrode reaction (in this paper n = 1, resulting from the Fe2+/Fe3+ redox couple), A is the electrode active surface area [cm2], 𝐷 is the sodium diffusion coefficient [cm2/s], C is the concentration of Na+ ions (1,386·10-3mol/cm3) and v is the CV scanning rate [mV/s]. The diffusion coefficient was determined basing on the linear regression slope of the function of anodic and cathodic current peaks versus square root of the scan rate (Fig. 6b). Obtained values are 5,56·10-14 and 5,05·10-14 cm2/s for anodic and cathodic current peaks, respectively. The relatively low value of the calculated sodium diffusion coefficient indicates the diffusion-limited redox reactions in alluaudite-Na1.47Fe3(PO4)3. Nevertheless, the discussed charge/discharge curves (Fig. 5a) indicate effective kinetics of the electrode processes, what seems to be the result of the fine-grained microstructure of obtained materials.
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(a)
(b) Fig. 6 CV profiles of Na1.47Fe3(PO4)3 at different scan rates (a) with peak current in the function of square root of the scan rate (b).
3.
Conclusions To summarize, the single-phase alluaudite-Na1.47Fe3(PO4)3 was successfully
synthesized by conventional solid-state reaction and its real sodium content was
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determined using the iodometric titration. SEM images and the particle size measurement of the obtained material showed the fine-crystallized microstructure. The further
investigation
exhibited
excellent
electrochemical
performance
of
Na|Na+|Na1.47Fe3(PO4)3 cells which might be directly associated with the fine grains. The cyclic voltammetry at different current rates confirmed the high reversibility of charge/discharge processes through occurrence of the symmetric anodic and cathodic peaks near the 3.0V. In parallel, determination of the diffusion coefficient based on Randles-Sevcik formula indicated that the redox reaction (Fe2+/Fe3+ redox couple) is limited by sodium diffusion during the charge/discharge processes. Nevertheless, this work revealed that the alluaudite-Na1.47Fe3(PO4)3, which consists of the eco-friendly, cheap and widely occurring elements, might be a potential candidate for NiB cathode material.
4.
Acknowledgements This work was funded by the Polish Ministry of Science and Higher Education
(MNiSW) on the basis of the decision number 0020/DIA/2016/45. The work was partly supported by the National Science Centre Poland (NCN) on the basis of the decision number UMO-2016/23/B/ST8/00199. Work was realized by using the infrastructure of the Laboratory of Conversion and Energy Storage Materials in the Centre of Energy AGH. SEM/EDS images were obtained by courtesy of the Faculty of Materials Science and Ceramics, AGH University of Science and Technology in Krakow, Poland.
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NaxFe3(PO4)3 is obtained via conventional solid-state synthesis Iodometric titration was used to determinate real sodium content SEM images and particle size distribution show fine-grained structure Na1.47Fe3(PO4)3 shows desirable discharge capacity
Katarzyna Walczak, Andrzej Kulka, Janina Molenda PII:
S1293-2558(18)30851-3
DOI:
10.1016/j.solidstatesciences.2018.10.017
Reference:
SSSCIE 5782
To appear in:
Solid State Sciences
Received Date:
05 August 2018
Accepted Date:
28 October 2018
Please cite this article as: Katarzyna Walczak, Andrzej Kulka, Janina Molenda, Alluaudite-Na1.47Fe 3
(PO4)3: structural and electrochemical properties of potential cathode material for Na-ion
Batteries, Solid State Sciences (2018), doi: 10.1016/j.solidstatesciences.2018.10.017
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Alluaudite-Na1.47Fe3(PO4)3: structural and electrochemical properties of potential cathode material for Na-ion Batteries Katarzyna Walczak, Andrzej Kulka, Janina Molenda(a),(b) (a)
AGH University of Science and Technology, Faculty of Energy and Fuels, Krakow, Poland
(b)
AGH University of Science and Technology, Centre of Energy, Krakow, Poland [email protected]
Abstract The structural and electrochemical investigation of alluaudite-NaxFe3(PO4)3 obtained by middle temperature solid-state synthesis is presented. XRD analysis with Rietveld refinement revealed contaminations for samples with x = 2.0. Reduction of the initial sodium content to x=1.5 allowed to obtain single-phase Na1.47Fe3(PO4)3 with real x value determined by iodometric titration. SEM images and particle size distribution showed the fine-grained structure, which might have positive impact on kinetics of the charge/discharge processes. For the electrochemical properties investigation, the Na/Na+/Na1.47Fe3(PO4)3 cells were assembled. The cyclic charge/discharge tests were carried out under various current loads showing high discharge capacity values (near the theoretical value 108 mAh/g) under C/10. For further analysis the CV tests with sodium diffusion coefficient evaluation using Randles-Sevcik equation was provided. The 𝐷 value (~10-14 cm2/s) allowed to state that the intercalation/deintercalation processes in alluaudite-NaxFe3(PO4)3 are diffusion-limited. Keywords: Na-ion batteries, intercalation, electrochemistry, alluaudite, Na2Fe3(PO4)3, Na1.47Fe3(PO4)3, Randles-Sevcik equation
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1.
Introduction Taking into consideration extremely fast growth in demand for portable
electronics, hybrid or electric cars and other replaceable power sources, Lithium-ion Batteries (LiBs) became one of the most common energy storage technologies [1,2], what is the major reason of the world lithium sources contraction [3,4] and increase of lithium price [3–5]. It seems that the Sodium-ion Batteries (SiBs) might be the most suitable substitute for lithium-ion technology [6,7], however only in certain, but notable applications. Due to bigger mass and higher ionic radius of sodium comparing to lithium, both the capacity and charge/discharged kinetics of SiBs are limited and thus not suitable for mobile devices and electric vehicles. Nevertheless, the rapid growth of renewable energy sources requires development of large-scale energy storage systems allowing to mitigate instability and unpredictability of power supply [8–11]. For such applications the density and the charge/discharge kinetics of SiBs seem to be sufficient, while low price and availability of sodium are highly advantageous. In the past decade numerous articles about structures capable to intercalate sodium atoms were released. Next to well-investigated layered oxides, such as NaxCoO2 [12–14], NaCrO2 [15,16], Na0.67Fe0.5Mn0.5O2 [17,18] or NaVO2 [19,20], there are threedimensional structures, mostly phosphates, e.g. olivines NaMPO4 (occurring in two types: triphilite [21,22] and maricite [22–24]), NASICON Na3M2(PO4)3 [25–27] or alluaudite Na2M3(PO4)3 [28–36] (M=metal 3d). The most notable properties of the phosphates are their chemical and thermal stability which results from the strong covalent bonding within the PO4 units.
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In yMny(PO4)3
view of the harmless impact on the environmental, alluaudite-NaxFe3are considered as attractive potential cathode materials for Na-ion Batteries
[28–36]. The compounds consist of elements which are not only environmentally friendly but also widely abundant, what allows for significant price reduction of the cathode material. Since the compounds containing manganese [28,30] showed deterioration of electrochemical properties with the increase of manganese only NaxFe3(PO4)3 materials were taken into consideration for further investigation. The previous works present values of the materials’ capacity close to theoretical value of 108mAhg-1 (assuming stoichiometric Na2Fe3(PO4)3 form), however only for relatively small current loads close to C/20 [32,36]. Outstanding values of cycling capacity were presented for composite of highly nanometric NaxFe3(PO4)3 with carbon nanotubes [29] – the obtained values exceeded 140mAhg-1 indicating possibility for the alluaudite structure to hold more than 2 sodium ions per formula. In this paper we report the research results of alluaudite-NaxFe3(PO4)3 used as cathode material for SiBs, which was prepared by conventional solid-state reaction. Presented results show facile middle temperature route of synthesis resulting with finegrained material with improved electrochemical properties. In order to examine the crystal structure and microstructure the XRD measurements, SEM images and particle size measurements were provided. The assembled Na|Na+|NaxFe3(PO4)3 cells were used for electrochemical tests, i.e. cyclic charging/discharging processes and cyclic voltammetry (CV) measurements linked with sodium diffusion coefficient evaluation, allowing for a deeper insight into the electrochemical properties of the obtained alluaudite-NaxFe3(PO4)3.
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1.1.
Material and methods
1.1.1. Preparation of the samples Alluaudite-NaxFe3(PO4)3 powders were obtained by the conventional solid-state reaction. Na2CO3 (Alfa Aesar, anhydrous, 99,997%) and FePO4·2H2O (Sigma-Aldrich) were used as substrates then grounded in a mortar and pressed into pellets. The pellets were heated at 550oC for 12 hours under the 95%Ar-5%H2 gas mixture continuous flow. After that, to avoid the oxidation, the pellets were put into the mBraun glovebox with argon atmosphere (purity <0.1ppm H2O and < 0.1ppm O2). The syntheses were carried out taking into account two assumed compositions: Na2Fe3(PO4)3 and Na1.5Fe3(PO4)3. 1.1.2. Structural and microstructural analysis The crystal structure of the obtained materials was investigated by X-Ray diffraction using the PANAlytical Empyrean diffractometer equipped with Cu Kα source (λ=0.15418 nm). Profile matching of the diffraction data of as-prepared NaxFe3(PO4)3 samples was applied with GSAS+EXPGUI [37] software. The real sodium amount was calculated by iodometric titration method [38] using EM40-BNC Mettler Toledo with 0.05M Na2S2O3 (Alfa Aesar, 99%, anhydrous) as a titrant solution. SEM analysis was performed using FEI Nano SEM200 in cooperation with Faculty of Materials Science and Ceramics, AGH University of Science and Technology in Cracow, Poland. The particle size distribution was provided by laser light diffraction based measurements using the Mastersizer 3000 analyser and distilled water with the refractive index of 1.33 as a dispersant.
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1.1.3. Electrochemical properties For electrochemical tests the cathode layers on the aluminium foil were prepared using 75%wt. NaxFe3(PO4)3 as an active material, 20%wt. high conductive carbon black (Alfa Aesar 99,9+%) and 5%wt. PVDF (Sigma-Aldrich, powder, Mw ~534,000) in NMP (Sigma-Aldrich, 99,5%) solvent. The CR2032 test coin cells were assembled in the argon-filled mBraun glovebox using metallic sodium (Alfa Aesar, 99%) as an anode material and 1M NaPF6 (Sigma-Aldrich, 99,99%) dissolved in a mixture of ethylene carbonate (EC) (Sigma-Aldrich, 99,99%) and diethyl carbonate (DEC) (Sigma-Aldrich, 99,99%) in the 1:1 ratio. The testing cells were galvanostically charged/discharged under various current loads (C/10, C/5, C/2) with the voltage regime from 2.0 to 4.0V. Additionally, to determine the sodium diffusion coefficient, the cyclic voltammetry with diversified scan rates (from 0.075 to 0.5mV/s) was provided. The electrochemical tests were performed using computer controlled BioLogic VMP3 galvanostat/potentiostat.
2.
Results and discussion
2.1.
Structural and microstructural characterization The XRD patterns of as-synthesized samples (with assumed sodium content
x=2.0 and x=1.5) were analyzed using the Rietveld refinement with alluauditeNa2Fe3(PO4)3 taken as initial model. In this structure both alkali and 3d metal occur in two positions: 4a and 4e for sodium and 8f and 4e for iron, what is directly associated with edge-sharing FeO6 octahedrons chains which are cross-connected with PO4 tetrahedrons. The eight-coordinated sodium atoms are located inside channels along [001] directions, what creates the possibility of reversible intercalation/deintercalation processes (Fig. 1). 5
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Fig. 1 Crystal structure of alluaudite-NaxFe3(PO4)3.
Fig. 2a shows the comparison of the XRD patterns of obtained NaxFe3(PO4) samples with assumed compositions x=2.0 and x=1.5. The XRD analysis of the sample with assumed sodium content x=2.0 revealed Na2Fe3(PO4)3 with C2/c symmetry as primary phase, corresponding to the initial model. Further analysis indicated clearly visible additional peaks, most of them assigned to Fe2P2O7 phase (C-1 space group). The reproducibility of this result (four synthesis attempts in the same conditions) allowed to state the hypothesis that the Na2Fe3(PO4)3 is not stable in its stoichiometric form under presented synthesis conditions. The XRD analysis of the sample with assumed sodium content x=1.5 mole per formula showed one-phase Na1.47Fe3(PO4)3 (Fig. 2b) with the real sodium content 1.47 determined by the iodometric titration method described by Olszewska et al. [38]. The amount of sodium in NaxFe3(PO4)3 is equal to the Fe2+ content (the composition can be II
III
schematically written as NaxFe x Fe2 ‒ x(PO4)3), hence the calculations were based on redox reactions in the presence of iodide ions: Fe3+ + 2I- → Fe2+ + I2
(eq. 1)
I2 + 2S2O32- → 2I- + S4O62-
(eq. 2)
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The titration results are listed in Tab. 1. It is noteworthy that the real sodium content was determined three times and the results were highly reproducible. The final value of the real sodium content is an arithmetic average of three measurements and it equals 1.47. Tab. 1 The results of the estimation of real sodium content (by the iodometric titration method). 1st measurement 2nd measurement 3rd measurement Amount of Fe3+[mol]
1.5307
1.5227
1.5279
Amount of Fe2+[mol]
1.4693
1.4773
1.4721
Amount of Na+ [mol]
1.4693
1.4773
1.4721
The Na1.47Fe3(PO4)3 crystal structure was solved using the C2/c Na2Fe3(PO4)3 as an initial model. The cell parameters calculated by Rietveld analysis are in good agreement with literature [36] and are stated in
Tab. 2.
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(a)
(b) Fig. 2 XRD patterns of the obtained samples: comparison of the indexed diffractograms of Na2Fe3(PO4)3 with the contaminations (*) and pure-Na1.47Fe3(PO4)3 (a); the single-phase Na1.47Fe3(PO4)3 with theoretical curve obtained by Rietveld refinement technique (b).
Tab. 2 Cell parameters of C2/c alluaudite-Na1.47Fe3(PO4)3.
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a [Å]
b[Å]
c[Å]
β[o]
V [Å3]
C2/c Na1.47Fe3(PO4)3 11.8577(5) 12.5464(3) 6.501(23) 114.787(3) 879.275(54)
The SEM images of Na1.47Fe3(PO4)3 are shown in Fig. 3. The material consists of small round-shape grains with a tendency to agglomerate. The particle size distribution for five measurements (by laser diffraction method) of Na1.47Fe3(PO4)3 is presented in Fig. 4 and the average particle sizes were gathered in Tab. 3. It can be observed that Na1.47Fe3(PO4)3 possesses mostly submicron-particles (including grains and agglomerates). Taking into consideration the solid-state technique of materials synthesis, it was expected to obtain the larger particles. Unexpectedly, the average particle size is less than 2µm. Considering that observed particles are in fact porous agglomerates of fine grains, the microstructure seems to be suitable for effective sodium ions transport.
(a)
(b)
Fig. 3. SEM images of Na1.47Fe3(PO4)3 with 20 000x (a) and 50 000x magnitude. Tab. 3 The particle size for Na1.47Fe3(PO4)3 obtained after five measurements with average particle size with standard deviation. Dx(50) represents the median diameter of the particle [μm], the Dx(10) and Dx(90) represent the particle diameter [μm] below the 10 and 90 percent of total volume, respectively.
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Dx(10)
Dx(50)
Dx(90)
1
[μm] 0.581
[μm] 2.08
[μm] 37.0
2
0.578
2.00
36.2
3
0.575
1.95
36.6
4
0.572
1.85
35.5
5
0.570
1.86
36.3
average
0.575(5
1.95(9)
36.3(5)
)
Fig. 4 Particle size distribution for Na1.47Fe3(PO4)3.
2.2.
Electrochemical studies In order to investigate the electrochemical performance of alluaudite-
Na1.47Fe3(PO4)3 the testing Na|Na+|Na1.47Fe3(PO4)3 cells were assembled and charged/discharged in the voltage regime of 2.0V to 4.0V with different current loads: C/10, C/5 and C/2 (Fig. 5a). Assuming the possibility that sodium can be intercalated to alluaudite-NaxFe3(PO4)3 structure until x reaches the value of 2.0, it can be observed that discharge capacity of Na2Fe3(PO4)3 under C/10 current load achieved about 86% of Na2Fe3(PO4)3 theoretical capacity, which equals to 108mAh/g. Furthermore, the discharge capacity does not decrease significantly within cycling. At C/2 current rate it achieves about 80mAh/g, which represents ca. 74% of theoretical value. The coulombic 10
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efficiency (Fig. 5b) of charge/discharge processes is nearly 1.0. The abnormally high value of first and second charge capacity may be caused by non-reversible reactions between electrolyte and material surface, what leads to SEI formation [39,40] .
(a)
(b) Fig. 5. The charge/discharge profiles for Na1.47Fe3(PO4)3 under various current loads: voltage in the function of specific capacity (a) and discharge capacity with reversibility of charge/discharge processes in the function of cycle number (b).
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For the further analysis of electrochemical properties of Na1.47Fe3(PO4)3, the cyclic voltammetry tests with various scan rates were applied (Fig. 6a). It is distinctly visible that the values of the peak current rise with the increasing scan rate. The anodic (charge) and cathodic (discharge) peaks are symmetric, slightly shifted from each other and occur near the voltage value of 3V that corresponds to the narrow plateaux at the charge/discharge profiles. To determine the sodium diffusion coefficient, the RandlesSevcik formula was used: 5
3/2
𝐼𝑝 = 2.69 ∙ 10 ∙ 𝑛
∙𝐴∙𝐷
1/2
1/2
∙𝐶∙𝑣
(eq. 3)
where Ip is the peak current [mA] (cathodic or anodic), n is the number of electrons transferred in the electrode reaction (in this paper n = 1, resulting from the Fe2+/Fe3+ redox couple), A is the electrode active surface area [cm2], 𝐷 is the sodium diffusion coefficient [cm2/s], C is the concentration of Na+ ions (1,386·10-3mol/cm3) and v is the CV scanning rate [mV/s]. The diffusion coefficient was determined basing on the linear regression slope of the function of anodic and cathodic current peaks versus square root of the scan rate (Fig. 6b). Obtained values are 5,56·10-14 and 5,05·10-14 cm2/s for anodic and cathodic current peaks, respectively. The relatively low value of the calculated sodium diffusion coefficient indicates the diffusion-limited redox reactions in alluaudite-Na1.47Fe3(PO4)3. Nevertheless, the discussed charge/discharge curves (Fig. 5a) indicate effective kinetics of the electrode processes, what seems to be the result of the fine-grained microstructure of obtained materials.
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(a)
(b) Fig. 6 CV profiles of Na1.47Fe3(PO4)3 at different scan rates (a) with peak current in the function of square root of the scan rate (b).
3.
Conclusions To summarize, the single-phase alluaudite-Na1.47Fe3(PO4)3 was successfully
synthesized by conventional solid-state reaction and its real sodium content was
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determined using the iodometric titration. SEM images and the particle size measurement of the obtained material showed the fine-crystallized microstructure. The further
investigation
exhibited
excellent
electrochemical
performance
of
Na|Na+|Na1.47Fe3(PO4)3 cells which might be directly associated with the fine grains. The cyclic voltammetry at different current rates confirmed the high reversibility of charge/discharge processes through occurrence of the symmetric anodic and cathodic peaks near the 3.0V. In parallel, determination of the diffusion coefficient based on Randles-Sevcik formula indicated that the redox reaction (Fe2+/Fe3+ redox couple) is limited by sodium diffusion during the charge/discharge processes. Nevertheless, this work revealed that the alluaudite-Na1.47Fe3(PO4)3, which consists of the eco-friendly, cheap and widely occurring elements, might be a potential candidate for NiB cathode material.
4.
Acknowledgements This work was funded by the Polish Ministry of Science and Higher Education
(MNiSW) on the basis of the decision number 0020/DIA/2016/45. The work was partly supported by the National Science Centre Poland (NCN) on the basis of the decision number UMO-2016/23/B/ST8/00199. Work was realized by using the infrastructure of the Laboratory of Conversion and Energy Storage Materials in the Centre of Energy AGH. SEM/EDS images were obtained by courtesy of the Faculty of Materials Science and Ceramics, AGH University of Science and Technology in Krakow, Poland.
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NaxFe3(PO4)3 is obtained via conventional solid-state synthesis Iodometric titration was used to determinate real sodium content SEM images and particle size distribution show fine-grained structure Na1.47Fe3(PO4)3 shows desirable discharge capacity