Comparative investigation of Na2FeP2O7 sodium insertion material

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Comparative investigation of Na2FeP2O7 sodium insertion material synthesized by using different sodium sources Jun-chao Zheng, Biyuan Yang, Xiaowei Wang, Bao Zhang, Hui Tong, Wanjing Yu, and Jiafeng Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04516 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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ACS Sustainable Chemistry & Engineering

Comparative investigation of Na2FeP2O7 sodium insertion material synthesized by using different sodium sources Jun-chao Zheng, Bi-yuan Yang, Xiao-wei Wang, Bao Zhang , Hui Tong, Wan-jing Yu, Jia-feng Zhang School of Metallurgy and Environment, Central South University, Changsha 410083, P.R. China

Correspond Author: Bao Zhang Email: [email protected] Add: School of Metallurgy and Environment, Central South University, Lushan Road (south),Changsha city, Hunan Province. PC: 410083



Corresponding author: Bao Zhang, Ph.D; Tel:+86-731-88836357 E-mail address: [email protected] 1

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Na2FeP2O7 powders were synthesized by solid-state method by using different sodium sources (Na2CO3, NaH2PO4and NaOH) and their solid-state reaction mechanism and the electrochemical properties of the synthesized Na2FeP2O7 were studied in details. The results indicate that NaH2PO4 is conducive to the growth of the crystal surface with wider spacing, which contributing to sodium ion intercalation and deintercalation. Na 2FeP2O7 synthesized at 600°C by using NaH2PO4 exhibits the best electrochemical performance among them, showing an initial discharge capacity of 82.8 mAh·g-1, 79.4 mAh·g-1, 77.9 mAh·g-1, 75.3 mAh·g-1, 68.8 mAh·g-1 and 47.8 mAh·g-1 under the cycle rate of C/10, C/5, C/2, 1C, 2C and 5C, respectively. Moreover, a reversible capacity of 81.0 mAh·g-1 at C/10, about 83.5% of the theoretical capacity, can be achieved after different cycle rates. The capacity retention is about 93.3% after 140 cycles under C/2. The results indicate that it is very important to choose the right sodium sources to synthesize Na2FeP2O7 with good performance. Key words： Preferred orientation; Sodium-ion batteries; Sodium iron pyrophosphate; Cathode materials; Different sodium sources; Synthesis mechanism;

Introduction

21st century has witnessed the rapid popularization of automobiles and consumer electrical appliances. As the blood of these small-scale electrical appliances, lithium-ion batteries have been placed great expectations on since its commercialization in 1990s. However, because of the environmental pollution and the increase demand for large-scale applications like grid storage, more attention has been turned to sodium-ion batteries because of its manifold abundance, 2


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environmental friendly and low cost compared with Lithium element. 1-4 Materials based on layered structures like Na xCoO2, NaxVO2, Na0.44MnO2, Nax(Fe1/2Mn1/2)O2, Na(Mn1/3Co1/3Ni1/3)O2 can deliver reversible capacity betwen 100-200 mAh g-1.5-8 In parallel, polyanionic sodium based compounds like NaFePO4, Na2FePO4F, NaVPO4F, NaFeSO4F, Na3V2(PO4)3, Na4M3(PO4)2(P2O7) (M = Fe/Co) have shown remarkable electrochemical properties.9-14 Na2FeP2O7 is one of the latest polyanionic sodium based compound. Its low cost, high earth-abundant elements, high operation 3V (vs Na/Na +), commendable capacity of 90 mAh g-1 with excellent rate and cycle kinetics make it a candidate for the large-scale applications.15 Recently, Na2FeP2O7 are synthesized with different sodium sources and mostly solid-state method.16-20

The performance verified and impurities like Na3PO4 exist, which will have some

bad effects on the crystal structure, morphology and electrochemical properties. As the precursors are proved to play an important part in the electrochemical performance of cathode materials by affecting the morphology and impurity phases, it requires us to select the proper sodium sources for better exploration the performance of Na 2FeP2O7. 21-23 In this paper, Na2FeP2O7 compounds are synthesized by using three kinds of sodium sources (Na2CO3, NaH2PO4 and NaOH). The reaction mechanism of the solid-state method has been studied in details. The contrast of this three sodium sources on the preference orientation of the crystal faces, morphology and electrochemical property of Na 2FeP2O7 are researched carefully.

Experiment

Solid-state method was used to synthesize Na2FeP2O7 compounds. Stoichiometric amount of different sodium sources (Na2CO3, NaH2PO4 and NaOH), NH4H2PO4 (ACS Aladdin) and iron

3



source FeC2O4·2H2O (ACS Aladdin) were mixed by ball milling (rotation speed 300 r/min) for 4 h with ethanol as the dispersant. As the carbon source, 5wt% glucose (99.5% Aldrich) were added to the raw materials as carbon sources for carbon coating. The precursor was dried at 60 °C for 12h in a vacuum oven. Then the precursor was first calcined at 350 °C for 3h and then heated to the temperature between 400-650°C for 10h under argon atmosphere. Finally, the Na 2FeP2O7 compounds were obtained. The Na2FeP2O7 compounds synthesized with Na2CO3, NaH2PO4 and NaOH as sodium sources were marked as NFPO-1, NFPO-2 and NFPO-3, respectively. TG/DSC of the precursor was tested by SDT Q600TG-DTA apparatus at the temperature in the range of 25-800°C in argon atmosphere with 5°C min -1 heating rate. XRD patterns were tested by the X-ray diffraction (Rint-2000, Rigaku) measurement using Cu K radiation. FT-IR spectrums were acquired by the Fourier-transform infrared spectroscopy (Nicolet460). The surface morphology of the powders was acquired by Scanning Electron Microscope (JEOL, JSM-5600LV) and the microstructure were observed by Transmission Electron Microscope (Tecnai G12 t). The surface element’s valence state of sample was detected by XPS (Kratos Model XSAM800) equipped with Mg K achromatic X-ray source (1235.6 eV). Elemental carbon analysis of samples were detected by Carbon-sulfur Analyzer (Eltar, Germany). CR2025 coin-type cell was used to test the electrochemical performance of the Na2FeP2O7 compounds. For positive fabrication, Na2FeP2O7, carbon black and polyvinylidene fluoride (8:1:1 in weight) were mixed in N-methyl pyrrolidinone. Then aluminum foil was used as current collector for the mixture, and the electrodes was dried at 120°C in the vacuum oven for 8h. A disk of sodium metal was used as the counter electrode. The electrolyte was 1 M NaClO 4 in the solution of Ethylene Carbonate and Diethyl carbonate (1:1 in volume) with 5% Fluoroethylene 4


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carbonate. A glass fiber filter was chosen as a membrane. The assembly process of the cells was implemented in a glove box maintaining the H2O and O2 level below 0.1 ppm. Electrochemical tests were implemented with a LAND battery tester, between 2.0-4.0 V versus Na/Na+ electrode at room temperature. The cyclic voltammetry tests (CV) were carried out with an electrochemical analyzer (CHI660D, China). The CV data for the above test cells were recorded in the potential range of 2.0-4.0 V.

Results and discussion

To determine the solid-sate reaction of the precursor and the optimum calcination temperature in the synthetic process. TG-DSC curves of the precursor mixtures with different sodium raw materials before calcination were applied and shown in Figure S1. Many similarities can be observed in the TG-DSC curves of NFPO-1, NFPO-2 and NFPO-3. According to the exothermic/endothermic peaks and weight loss plateaus, all the TG curves can be divided into three main parts with the demarcation points of 350°C, 500°C and 600°C, respectively. The first part range from room temperature to about 350°C can be attributed to the volatilization of absorption and crystal water of the precursor mixture, as long as H2O, CO2 and NH3, the decomposition products of Na2CO3 and NH4H2PO4.24 Reaction between 100°C and 350°C： NaOH + NH4H2PO4 → NaH2PO4 + H2O + NH3 or

Na2CO3 + NH4H2PO4 → NaH2PO4 + H2O + NH3 + CO2

(1) (2)

FeC2O4·2H2O → FeC2O4 + 2H2O

(3)

2NaH2PO4 → Na2H2P2O7 + H2O

(4)

5



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When the temperature raise from 350°C to 500°C ， the main reaction come from the decomposition of FeC2O4.25-26 Reaction between 350°C and 500°C： FeC2O4 → FeCO3 + CO

(5)

FeCO3 → FeO + CO2

(6)

The distinct exothermic/endothermic peaks appeared at about 500°C indicate the formation of Na2FeP2O7. The TG curves become flat around 600°C while the DSC curves still fluctuate. This phenomenon may be attributed to the nucleation and grain growth of Na 2FeP2O7.27 Reaction between 500°C and 700°C： Na2H2P2O7 + FeO → Na2FeP2O7 + H2O

(7)

No obvious weight loss can be observed between 700°C and 800°C, while a peak around 750°C can be seen. This may be attributed to the phase transfer of the material. 27 Based on the above results and analysis, the calcination temperature should be chosen between 500°C and 750°C. However, it is still too wide for further research. To further narrow the calcination scope and confirm the TG-DSC results, XRD analysis is taken on NFPO-2 under five different temperature. In Figure 1, the materials are unformed at the calcination temperature of 400°C and begin to show characteristic peaks of Na2FeP2O7 (PDF#80-2409) at the calcination temperature of 500°C, which is consistent with the analysis of the synthesis reaction of Na2FeP2O7. As the calcination temperature increase from 500°C to 600°C, the diffraction intensity also increase and no peaks of other phases can be observed. The impurity phase of Na 4P2O7 (PDF#01-0356) can be seen in the XRD patterns. Moreover, with the increase of calcination 6


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temperature, the enhancement of diffraction intensity is incommensurate among the different peaks, which means the selectivity of crystal growth can be affected by the temperature. To confirm this, five main peaks corresponding to different crystal facings are marked and comparatively studied, as shown in Figure S2. Among all the crystal facings, the (100), (-110), (01-1) facings can directly affect the electrochemical performance of Na 2FeP2O7 because they form the 3D network channels for sodium-ions.21 However, the crystal faces of (100) and (01-1) cannot be clearly observed from the XRD pattern while crystal face of (-110) has an obvious peak. To figure out the influence on crystal facings caused by different calcining temperature, six main peaks were chosen (-110 included) and each of the intensity ratio were calculated to show the changes. The six main peaks are marked in Figure 2 and named I, II, III, IV, V and VI, corresponding to crystal facings of (-1 -1 1), (-1 -2 2), (-2 0 -1), (-1 -2 1), (-3 -2 2) and (-1 1 0), respectively. The d space of I, II, III, IV, V, and VI are 8.24Å, 5.35 Å, 4.33 Å, 3.86 Å, 2.97 Å and 5.05 Å, respectively. The X-axis represents the calcination temperature and the Y-axis represents the intensity ratio of each peak. As shown in Figure 3, the intensity ratio of different peaks changes obviously with the increase of calcination temperature, which indicate the different preferential growth faces under different temperatures. The line of (-110) shows the highest intensity ratio when the calcination temperature is 600°C, indicating the more channel for sodium-ion transfer are explored and better electrochemical performance can be expected. Moreover, it can be observed that the crystal facings of wider d space (peak I (-1-11), peak II (-1-22), and peak VI (-110)) are preferred orientation under 600°C compared with other crystal facings. The XRD patterns of Na2FeP2O7 synthesized at 600°C with different sodium sources are shown 7



in Figure 2. All the peaks of the XRD patterns can be identified as monoclinic Na 2FeP2O7, which is in line with the literature.17 There were no other peaks observed in all three samples, which indicate that pure phase of synthesized Na 2FeP2O7. However, the intensity ratio of the synthesized samples are different from each other. NFPO-2 synthesized with NaH2PO4 have the largest intensity ratio of crystal face (-110) (12.58%) than NFPO-1 (11.68%) and NFPO-3(12.24%). It can be expected that the NFPO-2 may deliver a better performance. The carbon content of each sample tested and verified by C-S analysis and all the carbon content were around 4%. To further investigate the crystal structure of the synthesized Na2FeP2O7, X-ray diffraction with Rietveld refinement of NFPO-2 was shown in Figure 3. The Na2FeP2O7 assumes a triclinic (space group: P-1) structure with lattice parameters: a=9.7232Å, b=11.0069Å, c=12.2687Å, α=148.6298°, β=121.7213°, γ=68.4386°. Similar results were obtained for the solution synthesized sample. 17 The valence state of Fe and P in Na2FeP2O7 is shown in Figure S3, all the three Fe2p spectra of Na2FeP2O7 synthesized by using Na2CO3, NaH2PO4 and NaOH as sodium sources have two main peaks, which can be allocated to Fe2p3/2 and Fe2p1/2. The three peaks of Fe2p3/2 at 711.50eV, 712.00eV and 711.7eV and the three peaks of Fe2p1/2 at 725.46eV, 725.60eV and 725.03eV are all close to the data of Fe2+ in LiFePO4 (710.6eV and 727.2eV) and Li2FeP2O7 (711.6eV and 725.0eV).28-31 It points out that the state of Fe in the Na 2FeP2O7 synthesized by using Na2CO3, NaH2PO4 and NaOH as sodium sources is +2. The P2p peaks of NFPO-1, NFPO-2 and NFPO-3 located at 133.48eV, 133.59eV and 133.23eV are consistent with the report of the literature mentioned above, which indicates the +5 state of P in NFPO-1, NFPO-2 and NFPO-3. For further examination, Fourier transform infrared (FTIR) spectra is used to figure out the details of chemical bonding of NFPO-1, NFPO-2 and NFPO-3. As shown in Figure S4, a series of 8


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narrow and prominent multiply vibration bands can be observed in the range of 400-1400cm-1 and no obvious differences can be see between the three samples. For NFPP-1, the curves can be divided into three parts. The first part, ranging from 950-1200 cm-1, has three main peaks at 1187, 1105 and 962 cm-1, identical with the P-O stretching vibration frequencies. The p32 peaks observed at 739 and 924 cm-1, belonging to the second part, can be ascribed to the symmetric and asymmetric of P-O-P vibrations. They can be identified as the typical peaks of pyrophosphate group. The peaks of the third part observed at 630, 565 and 526 cm -1 are associated with the vibration of P-O binds of PO4 groups. All these characteristics of vibrational spectra can be researched in the spectra date of NFPO-2 and NFPO-3, indicating the similar connection environment between P-O of them. SEM of the synthesized materials are shown in Figure 4. Big secondary particles composed by small primary particles can be seen in all of the samples. The primary particle size of NFPO-1 and NFPO-2 are similar and smaller than the size of NFPO-3. The small size of NFPO-1 and NFPO-2 can close the distance for sodium diffusion and improve the ion conductivity, which can be hoped to deliver a better electrochemical performance. The structure characteristics of the Na2FeP2O7 samples synthesized by different sodium sources are further studied by TEM. As shown in Figure 5. The TEM images of NFPO-1, NFPO-2, NFPO-3 are list in the left part of Figure 8 a), b) and c). All the three sample have a similar morphology, and HRTEM images of the chose red regions are put at the right side of images. A carbon coating of several nanometers can be observed on the side of particle synthesized with different sodium sources, which can improve the electronic conductivity. Moreover, the FFT of the chosen areas in each HRTEM images investigate the crystal structure of the observed particles. 9



The charge-discharge profiles of the Na2FeP2O7 synthesized of by using different sodium sources under the rate of 0.1C are listed in Figure 6 The specific capacity of NFPO-1, NFPO-2 and NFPO-3 are 82.5 mAh·g-1, 82.8 mAh·g-1 and 81.2 mAh·g-1, respectively. The sample NFPO-2 deliver a higher plateaus and small difference between the plateaus of charge and discharge profiles, showing the smaller polarization of the material. The rate performance of NFPO-2 are shown in Figure 7. The sample NFPO-2 deliver capacities of 82.8 mAh·g-1, 79.4 mAh·g-1, 77.9 mAh·g-1, 75.3 mAh·g-1, 68.8 mAh·g-1 and 47.8 mAh·g-1 under the cycle rate of C/10, C/5, C/2, 1C, 2C and 5C, respectively. It is found that as the c-rate increase from C/10 to 5C, the specific capacity decrease obviously. This relatively inferior capacity may indicate the moderate electric conductivity and suffer from its drawback of particle dimensions. The cycling performance of the Na 2FeP2O7 synthesized of by using different sodium sources measured at the rates range from C/10 to 5C are shown in Figure 8. All the three sample show an excellent repeatable capacity after the cycle under different rates, 96.6%, 97.4% and 97.9% for NFPO-1, NFPO-2 and NFPO-3, respectively. Moreover, the sample NFPO-2 deliver a higher capacity under high rates compared with other two samples. This performance is consistent with the results of the above XRD analysis, which can be attributed to easier insertion and extraction of sodium with the help of larger lattice spacing. For better examination, the cycling performance of NFPO-2 under C/2 are shown in Figure 9. The Y axis on the left side represent capacity and the right one represents coulombic efficiency. A repeatable capacity of 75.3 mAh·g-1 (93.3%) after 140 cycles can be achieved, suggesting that Na insertion/extraction within NFPO-2 is quite reversible. According to our CV analysis Figure 10, four oxidation and four reduction peaks can be 10


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observed on the CV curves in each cycle. The oxidation peaks located at around 2.41V are corresponding to the extraction of sodium from Na 2FeP2O7 to Na1.75FeP2O7. The three oxidation peaks range from 2.80V to 3.20V are consistent with the phase transfer from Na 1.75FeP2O7 to Na1FeP2O7.32 Similarly, the reduction peaks can be divided into two parts, the first ranges from 3.00V to 3.25V represents the insertion of the 0.75Na and the second part around 2.50V represents the insertion of the rest 0.25Na.32 Moreover, a smaller potential deviation of NFPO-2 (V=0.15V) between oxidation peak and reduction peak can be seen, suggesting its better reversibility.33 The CV curves of NFPO-2 also show higher voltage of oxidation peaks, which means higher discharge plateaus and is consistent with the charge-discharge curves in Figure 9. Based on the analysis above, the reasons for different sodium sources can influence the electrochemical property of Na2FeP2O7 are listed as follows: 1）Pure phase of Na2FeP2O7 can be synthesized by Na2CO3, NaH2PO4 and NaOH, which are proved by the XRD patterns. However, different sodium sources can affect the preference orientation of the crystal faces. Na2FeP2O7 with NaH2PO4 as the sodium source is conducive to the growth of crystal surface with wider spacing, which can improve the electrochemical performance. 2）The sodium source can affect the particle size of Na2FeP2O7 and affect the electrochemical performance. NaH2PO4 and Na2CO3 can help synthesize smaller particles, which is beneficial the diffusion of sodium-ions and improve electrochemical property at higher rates.

Conclusion

The Na2FeP2O7 composites were successfully synthesized by using three different sodium sources,

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Na2CO3, NaH2PO4 and NaOH, respectively. The mechanism of the solid-state reaction was proposed. The calcination temperature and sodium sources have both influence on the growth speed of different crystal surfaces, and a wider spacing crystal surface can be observed in the Na2FeP2O7 composites synthesized by using NaH2PO4 as sodium sources. The Na2FeP2O7 synthesized by NaH2PO4 can achieve much higher capacity, higher plateaus and better rates electrochemical properties than others. NaH2PO4 is the best choice for the synthesis of Na2FeP2O7 compared with Na2CO3 and NaOH.

Acknowledgements This study was supported by National Natural Science Foundation of China (Grant No. 51572300) and the Innovation-Driven Project of Central South University (NO.2016CX021). \

Supporting Information Data of TG-DSC curves, the intensity ratio of the main peaks at different sintering temperature, XPS spectra of Fe and P and FTIR spectroscopy.

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Figure 1. XRD patterns of sample which using NaH2PO4 as Na-source sintered at five different temperatures. Figure 2. XRD patterns of Na2FeP2O7 sintered at 600C using different sodium sources. Figure 3. X-ray diffraction with Rietveld refinement of NFPO-2 synthesized at 600C. Figure 4. SEM of the Na2FeP2O7 samples: a) NFPO-1, b) NFPO-2, c) NFPO-3. Figure 5. TEM, HRTEM and FFT images of the Na2FeP2O7 samples: a) NFPO-1, b) NFPO-2, c) NFPO-3. Figure 6. Charge-discharge profile of Na2FeP2O7 synthesized using three kinds of sodium sources at C/10. Figure 7. Discharge curves of the Na2FeP2O7 synthesized using NaH2PO4 at different discharge rates. Figure 8. Cyclic properties of the Na2FeP2O7 synthesized using different sodium sources at different rates. Figure 9. Cycle performance of the Na2FeP2O7 synthesized using NaH2PO4 at C/2. Figure 10. CV curves of the Na2FeP2O7 at a scan rate of 0.1 mV s−1.

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Fig.1

Fig.2

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Fig.3

NFPO-2

Fig.4

17


Rw%=13.5%


Fig.5

a

b

c

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Fig.6

Fig.7

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Fig.8

Fig.9

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Fig.10

21



TABLE OF CONTENTS (TOC) GRAPHIC.

The calcination temperature can affect the preferable faces growth of Na 2FeP2O7, an environmental-friendly cathode material for sodium ion batteries.

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Comparative investigation of Na2FeP2O7 sodium insertion material

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