Narrow band-gap cathode Fe3(PO4)2 for sodium-ion battery with enhanced sodium storage

Journal Pre-proof Narrow band-gap cathode Fe3 (PO4 )2 for sodium-ion battery with enhanced sodium storage Hanqing Dai, Wenqian Xu, Yuanyuan Chen, Min Li, Zhihao Chen, Bobo Yang, Shiliang Mei, Wanlu Zhang, Fengxian Xie, Wei Wei, Ruiqian Guo, Guoqi Zhang

PII:

S0927-7757(20)30154-0

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124561

Reference:

COLSUA 124561

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

7 December 2019

Revised Date:

26 January 2020

Accepted Date:

6 February 2020

Please cite this article as: Dai H, Xu W, Chen Y, Li M, Chen Z, Yang B, Mei S, Zhang W, Xie F, Wei W, Guo R, Zhang G, Narrow band-gap cathode Fe3 (PO4 )2 for sodium-ion battery with enhanced sodium storage, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124561

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Narrow band-gap cathode Fe3(PO4)2 for sodium-ion battery with enhanced sodium storage Hanqing Dai1,a, Wenqian Xu1c, Yuanyuan Chenb, Min Lia,b, Zhihao Chena, Bobo Yanga, Shiliang Meib, Wanlu Zhangb, Fengxian Xieb, Wei Weic*, Ruiqian Guoa,b*, Guoqi Zhanga* a. Institute of Future Lighting, Academy for Engineering and Technology, Fudan University, Shanghai 200433, China. b. Institute for Electric Light Sources, Engineering Research Center of Advanced Lighting Technology, Ministry of Education, Fudan University, Shanghai 200433, China. c. College of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, China. The two authors contributed equally to this work.  Corresponding authors. E-mail addresses: [email protected]; [email protected]; [email protected].

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Graphical abstract

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In this work, a stratiform Fe3(PO4)2 cathode material with narrow band gap of 0.38 eV was favorably fabricated by the solvothermal methods. The electronic conductivity of the Fe3(PO4)2 was investigated by the first-principle. Compared with the literature of the FePO4 materials, the prepared Fe3(PO4)2 possesses higher initial discharge capacity of 442.5 mAh g-1 at the current density of 25 mA g-1. The results reveal that the battery with the Fe3(PO4)2 cathode operates at appropriate voltage window of 0.42-2.35 V, which is crucial for protecting the stability of the Fe3(PO4)2 structure in the charge-discharge process with sodium ions embedded and removed in the Fe3(PO4)2. The results imply that this stratiform narrow band gap Fe3(PO4)2 is of great potential in sodium storage for sodium-ion batteries.

Abstract

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In recent years, the development of cathode materials for sodium-ion batteries is very rapid. This work provides the Fe3(PO4)2 micromaterials fabricated by the solvothermal method for sodium-ion battery cathode. The structure and the electronic conductivity were investigated by the first-principle. Sodium-ion battery

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exhibits excellent capacity of 223.0 mA h g-1 at the current density of 25 mA g-1after eleven cycles. This

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reveals that the Fe3(PO4)2 micromaterials will be a remarkably promising cathode candidate for sodium-ion batteries.

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Keywords: Fe3(PO4)2 micromaterials, first-principle, cathode material, sodium-ion battery

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1. Introduction

Now that the massive consumption of coal and oil involved many fateful consequences such as climate

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change and environmental pollution, clean energy has proliferated all round the world. Since the electricity generation of these energy sources demonstrates a fluctuation with the four seasons, they need cheap and

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efficient fixed energy storage systems when they are redistributed and utilized via the smart grids [1-6]. Thus, the relatively mature lithium-ion batteries (LIBs) have been expected to improve the development of green energy. However, the progressively marvelous market raises widespread concerns about the shortage of minable lithium resources [7-11]. Therefore, sodium-ion batteries (SIBs) have been considered with the intention of furnishing a cost-effective choice that is immune to resource and supply risks due to their

competitive cost effectiveness and abundant resources, exceeding other rechargeable batteries on the market [8, 10, 12, 13]. Recently, extensive cathode materials of SIBs have been reported as mushrooming bamboo shoots after a rain, fluorides and oxyfluoride (FeF3/C [14], Fe-metal organic frameworks (MOFs) [15], and FeO0.7F1.3/C [16]), metal sulfide and sulfur (FeS2 [17] and Na2S [18]), selenium and metal selenide (MoSe2 [19], FeSe2 [20], Cu2Se [21] and Na2Se [22, 23]), and phosphates (Cu3(PO4)2 [24], NaFePO4 [25-28] and FePO4 [29-

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31]), just to name only a few. Unfortunately, the wide band gap and long ion diffusion length, bringing about mediocre ionic and electronic conductivity of mentioned cathode materials above, have seriously restrained their commercial application. Hence, it is difficult that these cathode materials attain the theoretical

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electrochemical properties at room temperature.

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The amorphous FePO4, which has been previously applied for the LIBs, is an acceptable choice for

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SIBs [31]. Although, FePO4 nanocrystals and amorphous FePO4 exhibit the marvelous preponderance of uncomplicated and low-temperature synthetic procedure [32-37], the direct and indirect consequences of the

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previous studies have been insinuated that the battery performance of SIBs with the FePO4-related cathode is poor [31-37]. For all this, the main cause is that the wide band gap (1.323 eV) of the FePO4 brings about

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mediocre ionic and electronic conductivity.

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For alleviating the above issue, a stratiform Fe3(PO4)2 cathode material with narrow band gap of 0.38 eV was favorably fabricated by the solvothermal methods. The electronic conductivity of the Fe3(PO4)2 was investigated by the first-principle. Compared with the literature of the FePO4 materials [29, 31], the prepared Fe3(PO4)2 possesses higher initial discharge capacity of 442.5 mAh g-1 at the current density of 25 mA g -1. The results reveal that the battery with the Fe3(PO4)2 cathode operates at appropriate voltage window of 0.42-2.35 V, which is crucial for protecting the stability of the Fe3(PO4)2 structure in the charge-discharge

process with sodium ions embedded and removed in the Fe3(PO4)2. The results imply that this stratiform narrow band gap Fe3(PO4)2 is of great potential in sodium storage for SIB. 2. Experimental 2.1.

Synthesis of materials The Fe3(PO4)2 micromaterials were successfully designed by the solvothermal methods. Raw materials

comprise (NH4)2Fe(SO4)2·6H2O, H3PO4, urea, sodium dodecyl sulfate and deionized water. Firstly, 0.8 mmol

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(NH4)2Fe(SO4)2·6H2O, 2.8 mmol sodium dodecyl sulfate, 1.6mmol H3PO4 and 64 mmol urea were completely dissolved in 65 mL deionized water. Then the mixture solution was transferred into a 100 mL teflon autoclave, and heated at 100 ℃ for 4 hours. After the autoclave cooled at room temperature, the product

2.2.

Component and morphology characterization

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was washed by ethanol, and dried at 80 ℃ for 24 hours to obtain Fe3(PO4)2 micromaterials.

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The structure of the Fe3(PO4)2 micromaterials was characterized by X-ray diffraction (XRD, Bruker D8

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polycrystalline) with Cu-Kα radiation (V = 30 kV, I = 25 mA, λ= 1.5418 Å) over the 10º to 70º 2θ range. The chemical states of the samples were characterized by X-ray photoelectron spectroscopy (XPS) with the

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Escalab 250Xi system at a pass energy of 150 eV (1 eV/step), using Al-Kα as the exciting X-ray source. The spectra were calibrated with respect to the C 1s peak resulting from the adventitious hydrocarbon, which has

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an energy of 284.8 eV. The morphology of the samples was obtained by S4800 scanning electron microscopy

2.3.

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(SEM) and JEM-2100 transmission electron microscope (TEM) at an accelerating voltage of 100 kV. Calculation Methods

Fe3(PO4)2 is a monoclinic cell, and its space group is P1 21/c1 (14) with experimental lattice parameters a = 8.8828 Å, b = 11.1738 Å, c = 6.1440 Å, β=99.348° and V=601.72 Å3. First-principle calculations were provided by the spin polarized Generalized Gradient Approximation (GGA) using the Perdewe Burkee Ernzerh (PBE) of exchange-correlation parameterization to Density Functional Theory (DFT) utilizing the

Cambridge Sequential Total Energy Package (CASTEP) program. Using Perdew-Wang (PW91) engenders the exchange correlation energy, and also the spin is considered. The influences of different k-points samplings and plane wave cutoff energies were explored in a series of test calculations. The Brillouin Zone integration was approximated using the special k-points sampling scheme of Monkhorst-Pack, and a 3×3×2 k-points grid was used. The cutoff energy of plane wave was 489.8 eV. The maximum root-mean-square convergent tolerance was less than 2.0×10–6 eV/atom. The geometry optimization was stopped when all

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relaxation forces are less than 0.005 eV/nm. The maximum displacement error is within 0.002 nm and the maximum stress was less than 0.1 GPa. 2.4.

Electrochemical Measurement

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The working electrode for electrochemical properties was prepared by a mixture of the Fe3(PO4)2

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micromaterials, polyvinylidene fluoride (PVDF) and acetylene black (8:1:1, mass ratio). In the presence of trace 1-Methyl-2-pyrrolidine (NMP), the above materials were mixed to produce a slurry. Then, it was evenly

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coated on aluminum foil, and dried at 80℃ overnight. Finally, a coin cell of CR 2032 was assembled in an argon-filled glove box with metallic sodium as the counter electrode, a Celgard 2400 membrane as the

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separator, a mixture of NaClO4 (1.0 mol L−1), ethylene carbonate (EC) and Diethyl carbonate (DEC) (1:1:1, volume ratio) as electrolyte. Discharge-charge cycling was performed between 0.0 and 3.0 V on the CT-2001

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LAND battery equipment (Wuhan, China) at room temperature.

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3. Results and discussion

In order to clearly characterize the structure of the prepared materials by X-ray diffraction, the prepared materials were annealed at 700 ℃ for 10 hours under nitrogen conditions. Fig.1a displays the XRD pattern of the prepared materials, demonstrating that it is difficult for the Fe3(PO4)2 to form crystals with good crystallinity. All the obvious diffraction peaks can be assigned to the Fe3(PO4)2 (JCPDS card No. 49-1087),

indicating that the prepared materials were pure Fe3(PO4)2, which slightly coincides with the reference [26]. Fig.1b exhibits the surface morphology and the thickness of large and small stratiform Fe3(PO4)2 micromaterials, owing to the growth of the Fe3(PO4)2 crystals along the (0 0 1) and (0 1 0) crystal plane. The information of the illustration of the TEM images (in Fig.1b) also shows that Fe3(PO4)2 micromaterials is a sheet structure, which is consistent with the result of the SEM image. Conversely, the Fe3(PO4)2 crystals only grow along the (0 0 1) crystal plane (Fig.1c) bringing about the formation of many small flakes, or only grow

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along the (0 1 0) crystal plane (Fig.1d) resulting in the formation of many large flakes.

Fig.1 The XRD pattern (a) and SEM image (b) of the Fe3(PO4)2 micromaterials. The (0 0 1) (c) and (0 1 0) (d) crystal planes, respectively. The illustration is the TEM images of the Fe3(PO4)2 micromaterials.

Afterwards, the composition, chemical and electronic states of elements in the Fe3(PO4)2 micromaterials

were measured. A wide XPS survey of the Fe3(PO4)2 is displayed in Fig. 2a, which betokens that the prepared micromaterials contained Fe, P and O elements with sharp photoelectron peaks appearing at the binding energies of 533.65 eV (O 1s), 135.68 eV (P 2p) and 715.04 eV (Fe 2p), respectively. Then, high-resolution XPS spectra of Fe 2p, P 2p and O 1s were acquired from the Fe3(PO4)2 micromaterials, as shown in Fig. 2b2d. In Fig. 2b, the binding energies of the Fe 2p3/2 and Fe 2p1/2 peaks of the Fe3(PO4)2 micromaterials are located at 712.02 and 725.45 eV respectively, with two characteristic shakeup satellite peaks at 715.23 eV

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(Fe2+ 2p3/2) and 729.92 eV (Fe2+ 2p1/2) for the Fe2+ species, which slightly coincides with the reference value. And the fitted energy difference between the Fe 2p1/2 and Fe 2p3/2 lines is approximately 14.69 eV. In Fig. 2c, the peaks around 131.82, 132.28, 133.33 133.87 and 134.45 eV are consistent with the ionic bindings of

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P. In Fig. 2d, the peaks around 530.21, 531.51 and 533.32 eV are in coincidence with the ionic bindings of

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O. The XPS profile corresponds well to the values of the Fe3(PO4)2 reported in the literatures.

Fig. 2 (a) The survey XPS spectrum of the Fe3(PO4)2 micromaterials. (b-d) High-resolution XPS spectrum of Fe 2p, P 2p

and O 1s acquired from the Fe3(PO4)2 micromaterials. The black line is the experimental line and the red line is the simulated line.

Ulteriorly, in order to comprehend the electronic conductivity of the Fe3(PO4)2, the band structures and the density of states (DOS) of the Fe3(PO4)2 crystal were investigated as shown in Fig. 3a-3e, respectively. The calculated band gap of 0.38 eV depicted in Fig. 3a confirms that the Fe3(PO4)2 is a semiconductor, which is small enough to allow some conduction at room temperature. The DOS for Fe3(PO4)2 (Fig. 3d)

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temperature, which would clearly display the good battery capacity.

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corresponds to an insulator with a band gap of 0.83 eV. Fortunately, this is easy to allow conduction at room

Fig. 3 (a-c) Total band structures of Fe3(PO4) 2. (d) Total and spin density of states of Fe3(PO4) 2. (e) The PDOS of Fe3(PO4)2. (f-i) the acquired skeleton image from the PDOS of Fe3(PO4)2 with Fe, O and P, respectively.

Simultaneously, partial density of states (PDOS) of the Fe3(PO4)2 with Fe, P and O is shown in Fig. 3f3i, respectively. The angular momentum (l-dependent) origin of the various bands is obviously identifiable

from the PDOS. The lowest energy group around -23.1 eV has mainly P-s and O-s states with a small contribution of Fe-d and O-p states. The second group around -20.7 eV has significant contributions from O-s and P-p states with a small contribution of Fe-d and O-p states. The deeper sub-band group around -10.7 eV originates from O-s/p and P-s states with a small contribution of Fe-d states. The groups from -9.7 eV up to fermi energy (EF) originate from O-p and Fe-d states. The groups from EF and above are mainly of P-s/p,

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Fe-s/p/d and O-p states. No similar results have been reported for the time being.

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Fig. 4 Electric charge density difference of the spin states Fe3(PO4) 2 with the crystal plane (-1 0 0).

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Additionally, the difference of charge density of the Fe3(PO4)2 with the crystal plane (-1 0 0) is shown in Fig. 4. It is obvious that the charge density around the Fe and O atoms is higher than that around the P

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atom, and they keep local charge distribution and structural stability, which means that the main contribution of the electronic conductivity of the Fe3(PO4)2 is derived from the Fe and O atoms with fixed positions. And it is pretty obvious that the spin states are mainly composed of the Fe and O atoms.

Fig. 5 (a) Charge–discharge curve of the Fe3(PO4) 2 micromaterials between 0.0 and 3.0 V at a current density of 25 mA g-1. (b) Cycle performance of the Fe3(PO4)2 micromaterials cathode at a current density of 25 mA g -1. (c) Rate performance of the Fe3(PO4)2 micromaterials electrodes.

To further validate the electro-chemical behavior during the insertion and extraction of sodium

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components, charge–discharge curve of the Fe3(PO4)2 micromaterials is performed for comparison as shown in Fig. 5a. Then, the cycle performance of the Fe3(PO4)2 micromaterials cathode at the current density of 25 mA g-1 is given in Fig. 5b. It can be seen from analysis that the Fe3(PO4)2 micromaterials cathode displays a large irreversible capacity of 442.5 mAh g-1 in the first discharge, which primarily stems from the formation

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of solid electrolyte interface (SEI) layer on the surface of the Fe3(PO4)2 micromaterials due to the decomposition of the electrolyte, and sodium ions irreversibly insert into the crystal lattice. Simultaneously,

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it is evidently clear that the reversible capacity of 223.0 mA h g−1 is higher than that of the FePO4 cathode after eleven cycles [29, 31]. The rate performance of the Fe3(PO4)2 micromaterials was investigated as

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illustrated in R3. Fe3(PO4)2 micromaterials demonstrate excellent rate performance and delivers reversible capacities of 420.4, 188.9 and 229.5 mA h g -1 at current densities of 25 and 200 mA g -1, respectively. The

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results disclose that the Fe3(PO4)2 micromaterials display an outstanding rate capability and structure stability even at a very high current density. The results will provide the references for quick charge SIBs application and development in the future. Consequently, we believe that the reversible capacity could be

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improved by some means of modification in the future.

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4. Conclusions

In summary, the Fe3(PO4)2 micromaterials were successfully prepared by the solvothermal methods, and the practical information about the structure and electrical conductivity could be provided by the firstprinciple. The electrochemical characteristics were investigated for application of the cathode in SIB. The Fe3(PO4)2 micromaterials display superior capacity of 223.0 mA h g -1 at the current density of 25 mA g-1 for

the sodium-ion battery. These properties of the Fe3(PO4)2 micromaterials ensure it will be a promising lowcost cathode candidate for SIBs.

Declaration of Interest Statement Please find enclosed our manuscript entitled “Narrow band-gap cathode Fe3(PO4)2 for sodium-ion battery with enhanced sodium storage” which we are submitting for publication in your journal.We confirm that the manuscript has been read and approved by all named authors and that there are no other

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persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with

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respect to intellectual property. In so doing we confirm that we have followed the regulations of our

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institutions concerning intellectual property.

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Signed by all authors as follows: Hanqing Dai, Wenqian Xu, Yuanyuan Chen, Min Lia, Zhihao Chen, Bobo Yang, Shiliang Mei, Wanlu Zhang, Fengxian Xie, Wei Wei, Ruiqian Guoa, Guoqi Zhang

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Acknowledgments

Financial support from the National Natural Science Foundation of China (No. 61675049 and 61377046) is

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gratefully acknowledged.

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Capacitive Deionization for High-Capacity and Fast Removal of NaCl from Water, Chem. Eng. J., (2019).