Monoclinic Phase Na3Fe2(PO4)3: Synthesis, Structure, and

Research Article pubs.acs.org/journal/ascecg

Monoclinic Phase Na3Fe2(PO4)3: Synthesis, Structure, and Electrochemical Performance as Cathode Material in Sodium-Ion Batteries Yao Liu,†,‡ Yirong Zhou,† Junxi Zhang,*,† Yongyao Xia,*,‡ Tong Chen,† and Shiming Zhang§ †

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, Shanghai 200090, People’s Republic of China ‡ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, People’s Republic of China § State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province & Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China. ABSTRACT: Sodium iron phosphate (Na3Fe2(PO4)3) as cathode material for sodium-ion batteries has been synthesized through a simple method of a solid state reaction. It crystallizes in a monoclinic structure in the space group C2/c. The morphology of the as-prepared sample has been investigated by scanning electron microscopy and transmission electron microscopy. The charge/discharge curves show a very flat plateau at about 2.5 V (vs Na/Na+). The initial specific discharge capacity is 61 mAh g−1 and remains at 57 mAh g−1 after 500 cycles at a current rate of 1 C. X-ray photoelectron spectroscopy measurements indicate that not all of the Fe3+ of Na3Fe2(PO4)3 is reduced during the electrochemical process. The ex-situ X-ray diffraction measurements were applied to research the mechanism of sodium-ion storage; the results indicated that the Na3Fe2(PO4)3 compound partly transformed into Na4Fe2(PO4)3 and Na3+xFe2(PO4)3 compound. These results testify to the potential of monoclinic Na3Fe2(PO4)3 as a cathode material in sodium-ion batteries. KEYWORDS: Sodium iron phosphate, NASICON, Solid state synthesis, Sodium-ion diffusion coefficient, Ex-situ X-ray diffraction, X-ray photoelectron spectroscopy

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INTRODUCTION Sodium-ion batteries (SIBs) are considered as potential alternatives to the current lithium-ion batteries (LIBs) due to the natural abundance and low cost of sodium.1,2 However, few electrode materials can meet the requirements for practical applications because the ionic radius of the sodium ion is much larger than that of the lithium ion.3−7 Therefore, we need to find appropriate electrode materials for application in SIBs. Since the olivine LiFePO4 cathode material was first reported by Padhi et al.,8 the search for novel polyanion-based insertion “host” materials has been ongoing.9−11 In a comparison with olivine LiFePO4 cathode material for LIBs, olivine NaFePO4 would be suitable for SIBs.12,13 However, the olivine NaFePO4 cannot be synthesized through a simple solid state reaction and is usually attained by a complicated method of electrochemical Li+/Na+ exchange of olivine LiFePO4.12,14−17 Carlier et al.18 reported Na3Fe3(PO4)4 as the cathode material in SIBs; however, the electrochemical performance of the layered Na3Fe3(PO4)4 compound was not attractive. As we all know, iron-based phosphate compounds have rich and complex structural chemistry. Until now, some iron-based phosphate materials remain untested as cathode material in SIBs. © 2016 American Chemical Society

In this study, Na3Fe2(PO4)3 as the cathode material for SIBs has been reported. The structure of monoclinic Na3Fe2(PO4)3 consists of corner-sharing PO4 tetrahedra and FeO6 octahedra that are like “lantern units”.19−21 The skeleton structures are assembled into a three-dimensional framework defined by M(1) (one position per formula unit) and M(2) (three positions per formula unit) host sites in the rhombohedra. The M(1) and M(2) host sites are usually filled (fully or partially) with “guest” alkali ions (Li, Na). Herein, the Na3Fe2(PO4)3 NASICON-type phase was synthesized through the sample method of the solid state reaction. The structures of samples were investigated by using the methods of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). The electrochemical performance was evaluated through the positive electrode in sodium-ion half coin cells, which exhibited a very long cyclic stability and excellent rate capacity. The Received: July 4, 2016 Revised: December 11, 2016 Published: December 14, 2016 1306

DOI: 10.1021/acssuschemeng.6b01536 ACS Sustainable Chem. Eng. 2017, 5, 1306−1314

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic of synthesis of Na3Fe2(PO4)3 samples.

Figure 2. (a) XRD pattern of Na3Fe2(PO4)3 (the diffraction angle (2θ) was 5−90° with a step size of 0.03° and a counting time of 5 s per step). (b) Crystal structure. TEM (JEM-2100F, Japan) was used to study the microstructure of the Na3Fe2(PO4)3. The XPS measurements were carried out on an RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg Kα radiation (hν = 1253.6 eV). The whole spectra (0−1200 eV) and the narrow spectra of all the elements with much higher resolution were both recorded by using the RBD 147 interface (RBD Enterprises) with AugerScan 3.21 software. Binding energies were calibrated by using the containment carbon (C 1s = 284.6 eV). The XPS data processing was carried out using the XPSPEAK4 program. Electrochemical Measurements. The electrochemical properties were evaluated by using a CR2016-type coin cell. The positive electrode material was prepared by ball-milling with a ratio of active material to conductive material (consisting of acetylene black and graphite with a weight ratio of 6:4) of 62:30 by weight. The average mass of composite loading on the electrode was 15 mg. The area of the electrodes was about 2 cm2. The composite and PTFE (polytetrafluoroethylene, average Mw ∼ 500 000, Sigma-Aldrich Co.) were mixed in a ratio of 92:8 by weight in alcohol through grinding and then coated on the stainless steel mesh, which was welded on the positive electrode shell of the CR2016-type coin cell. The electrode sheet was then dried in a vacuum oven at 120 °C for 24 h. The sodium foil was used as the anode in this work. A 1 mol portion of NaClO4 (AR, purchased from Aladdin Co.) dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:1 was used as the electrolyte, and glass fibers were used as the separator. (Separators were supplied by the Whatman Co.; EC and DMC were purchased from BASF Co.) All coin cells were assembled in an argonfilled glovebox. A galvanostatic cycle was tested in the voltage window 1.5−3.5 V by using a Land CT2001A battery test system (Wuhan Land, China). The cyclic voltammetry (CV) tests were carried out on the electrochemical work station (Chenhua CHI660C, Shanghai, China). CV measurements were made at different scanning rates, and the potential window was 1.5−3.5 V. As-synthesized cathode material

structural transformation during the discharging process was investigated through XPS and ex-situ XRD measurements.

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MATERIALS AND METHODS

Synthesis of Na3Fe2(PO4)3 Samples. Amorphous FePO4 was applied as precursor. First, FePO4 nanoparticles were synthesized by means of coprecipitation. Fe(NO3)3·9H2O (0.1 mol, Sinopharm Chemical Reagent Co.) and NH4H2PO4 (0.1 mol, Sinopharm Chemical Reagent Co.) were dissolved in deionized (DI) water and transferred into a glass reaction kettle. A mixture of DI water (200 mL) and ammonia (20 mL) (ammonia/DI volume ratio of 1:1) was added dropwise to adjust the pH value. Polyvinyl alcohol (5 mL, PVA, Mw: 9000, Sigma-Aldrich Co.) was added as a dispersant. The suspension continued to react at 90 °C for 2 h. After the mixture aged for 2 h, the solution was centrifuged at 8000 rpm for 15 min and washed three times with DI water. The schematic of the synthesis of the Na3Fe2(PO4)3 samples is illustrated in Figure 1. The as-prepared FePO4 nanoparticles were mixed with CH3COONa and NH4H2PO4 based on stoichiometric proportions by high-energy ball-milling for 4 h. Then, the hybrid powder was sintered in a tube furnace at 600 °C for 10 h. All chemicals were analytically pure reagents that were used without further purification. Structural Characterization. The structures of the samples were identified using XRD combined with Rietveld refinements using a Xray diffractometer (Bruker D8 Advance, Germany) with Cu Kα radiation (λ = 0.1506 nm). The diffraction angle (2θ) was 5−90° with a step size of 0.03° and a counting time of 5 s per step. Ex-situ XRD patterns were collected between 5° and 90° (2θ) at a scanning speed of 2° min−1. The refinements were carried out using the GSAS program with the EXPGUI interface and the monoclinic unit-cell in the space group C2/c. The morphology of particles was characterized by using the FE-SEM (HITACH S4800, HITACH Co., Japan). HR1307


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ACS Sustainable Chemistry & Engineering Table 1. Lattice Parameters of Na3Fe2(PO4)3 Samples

a

sample

cryst struct

space group

a (Å)

b (Å)

c (Å)

Na3Fe2(PO4)3a

monoclinic

C2/c

15.3462(9)

8.744(6)

21.664(13)

α (deg)

β (deg) 90.03(4)

γ (deg)

density (Å3) 239.63

Rwp = 5.05%. Rp = 4.01%. χ2 = 1.12.

Figure 3. XPS patterns of Na3Fe2(PO4)3: (a) the whole spectrum; (b−f) the narrow spectra of C 1s, Fe 2p, P 2p, O 1s, and Na 1s, respectively.

c.22 The crystal structure is illustrated in Figure 2b. The framework is built of [Fe2(PO4)3] “lantern units” stacked parallel to the [001] direction. Iron and phosphorus are in octahedral and tetrahedral coordination environments, respectively. The sodium ions are located in the two crystallographic sites, defined as M(1) and M(2) sites, respectively. The point symmetry of the 6-coordinate M(1) that lies along [001], between two [Fe2(PO4)3] lantern units, is 3. The point symmetry of M(2) is 2; it is 8-coordinated and lies at the same z value as the phosphorus atom. It is a NASICON-type structure.

was used as the working electrode, and sodium foil (AR Purchased from Sinopharm Chemical Reagent Co.) was used as the counter electrode and reference electrode. All electrochemical measurements were tested at room temperature (25 °C).

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RESULTS AND DISCUSSION The XRD patterns and the crystal structure of Na3Fe2(PO4)3 are shown in Figure 2a. The refinement used the monoclinic unit-cell, a = 15.128(1) Å, b = 8.721(1) Å, c = 21.564(1) Å, and, β = 90.14(1)° in the space group C2/c, and the results are shown in Table 1. The results indicate that the structure of asprepared Na3Fe2(PO4)3 is monoclinic in the space group C2/ 1308


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FePO4 is of an amorphous structure. A detailed explanation has been published in previous reports.25−29 The TEM images of as-prepared Na3Fe2(PO4)3 are shown in Figure 4b. Under heat treatment, the particle grain size enlarged due to the crystallization process and agglomeration. The HR-TEM image and SAED pattern are shown in Figure 4b. The results indicated that as-synthesized samples were of high crystallinity. The lattice spacing of 2.8 and 3.7 Å could be assigned to the (021) and (022) crystal planes, respectively. The results were in good agreement with the aforementioned XRD results. The electrochemical performance of Na3Fe2(PO4)3 as cathode material in SIBs is shown in Figure 5. The CV test results at various scan rates of 0.02, 0.05, 0.08, 0.1, 0.2, 0.3, 0.4, and 0.5 mV s−1 are displayed in Figure 5a. There is only one pair of peaks, which correspond to the redox of Fe3+/Fe2+. The pair of current peaks is located at 2.4 V/2.6 V at a scan rate of 0.02 mV s−1, and these peaks show outstanding reversibility. The current peaks are still clearly observed at a high scan rate of 0.5 mV s−1. The charge/discharge curves under a rate of 1 C are shown in Figure 5c. The initial discharge/charge specific capacities are about 61 mAh g−1 with very flat voltage plateaus located at about 2.5 V, in good agreement with the results of CV tests. The reactions can be expressed by eq 1. The active masses of electrode material, charge/discharge current density, and specific capacities have been calculated on the basis of Na3+xFe2(PO4)3 (x = 2). The retention of charge/dischargespecific capacity at the rate of 1 C and corresponding Coulombic efficiency are displayed in Figure 5d. The reversible capacity of Na3Fe2(PO4)3 was 61 mAh g−1 and was maintained at 57 mAh g−1 after 500 cycles without obvious fading of the capacity. The initial Coulombic efficiency was up to 95% and remained close to 100% over all of the cycles. The rate capability of Na3Fe2(PO4)3 is shown in Figure 5e,f. The values of reversible capacities were 61, 60, 57, 50, and 40 mAh g−1 at 0.2, 0.5, 1, 2, and 5 C, respectively. When the rate current returned to 1 C, the discharge-specific capacity of 57 mAh g−1 was still recovered. Remarkably, the excellent rate capability and cyclic stability of the Na3Fe2(PO4)3 cathode material clearly prove that Na3Fe2(PO4)3 is a potential cathode material for SIBs.

The XPS spectra of Na3Fe2(PO4)3 are shown in Figure 3. The typical spectra show the whole spectrum (0−1100 eV, Figure 3a) and the narrow spectra of all the elements (Figures 3b−f). The binding energies were calibrated using containment carbon (C 1s = 284.6 eV). As shown in Figure 3c, the Fe 2p spectrum is split into two parts due to spin−orbit coupling, which consists of Fe 2p3/2 and Fe 2p1/2.23 The Na3Fe2(PO4)3 exhibits binding energies located at the values of 712.5 eV for Fe 2p3/2 and 726 eV for Fe 2p1/2. The results confirmed that iron is almost totally in the +3 oxidation state and is in good agreement with the Fe3+ ions’ binding energy.24 The P 2p spectrum shown in Figure 3d is like Fe 2p spectra as it is split into two components of P 2p3/2 and P 2p1/2, respectively. The binding energy of P 2p3/2 is 133.4 eV, and that of P 2p1/2 is 134.3 eV. The observation of only one P 2p doublet corresponds to the appearance of only one environment of phosphorus, which corresponds to the (PO4)3− group of the Na3Fe2(PO4)3 compound. The O 1s spectrum shown in Figure 3e is located at approximately 531 eV and is attributed to the oxygen atoms of the (PO 4 ) 3− group of Na 3 Fe 2 (PO 4 ) 3 compounds. The Na 1s spectrum shown in Figure 3f is located at 1071.7 eV and is attributed to the NaO group of Na3Fe2(PO4). The SEM and TEM images of the precursor FePO4 are shown in Figure 4a. The morphology of FePO4 was ball-like, and the grain size was about 20 nm. The HR-TEM image exhibited no lattice fringe, and the selected area electron diffraction (SAED) images did not show obvious diffraction rings, which proved to be sufficient evidence that the precursor

Na3Fe2(PO4 )3 + x Na + + x e− → Na3 + xFe2(PO4 )3

(1)

To further explain the outstanding rate performance, we calculated the sodium-ion diffusion coefficient (DNa+) using the CV method. The DNa+ value can be calculated from the linear relationship between the peak current (Ip) and the square root of the scan rate (ν1/2) from the CV profiles, according to the following Randles−Sevcik equation (eq 2).30−32 Ip = 2.60 × 105n3/2AD Na +1/2C Na +ν1/2

(2)

Here, n represents the charge-transfer number (here n = 2), A is the surface area of the electrode (2 cm2), and CNa+ is the concentration of sodium ions in the Na3Fe2(PO4)3 electrode (6.85 × 10−3 mol cm−3). The plots of Ip versus ν1/2 are shown in Figure 5b (the value of the slope is shown in the inset). The DNa+ value was calculated to be 2.4 × 10−8 cm2 s−1. Compared to the values for other electrode materials in LIBs, such as 8.3 × 10−14 cm2 s−1 for LiFePO433 or 4.2 × 10−11 cm2 s−1 for Li4Ti5O12,34 the diffusion process of sodium ions becomes easier in Na3Fe2(PO4)3, even though the radius of sodium ions (1.02 Å) is much larger than that of lithium ions (0.76 Å). The

Figure 4. HRTEM images and corresponding SAED images of (a) FePO4 precursor and (b) as-synthesized Na3Fe2(PO4)3 compound. 1309


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Figure 5. Electrochemical performance of Na3Fe2(PO4)3 as cathode material in SIBs: (a) CV curves obtained at different scan rates (potential window 1.5−3.5 V); (b) the slope of the plots of Ip versus ν1/2; (c, d) charge/discharge curves at 1 C and discharge-specific capacity versus cycle number at 1 C; (e, f) different rate performances.

high value of DNa+ is one of the reasons for the outstanding rate capability. After the sodiation process completed, the coin cells were disassembled in the glovebox. The positive electrodes were

characterized by the XPS method. The results are shown in Figure 6. The whole spectrum (0−1100 eV) (Figure 6a) was similar to that of the pristine Na3Fe2(PO4)3 except for the appearance of the F 1s characterization peak, which 1310


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Figure 6. XPS patterns of Na3Fe2(PO4)3 after complete discharge: (a) the whole spectrum of sodiated Na3+xFe2(PO4)3 composite and (b−f) the narrow spectra of Fe 2p, C 1s, O 1s, P 2p, and Na 1s, respectively.

the (PO4)3− group. The results were similar to those of pristine Na3Fe2(PO4)3 apart from the appearance of a new weak peak at 534 eV, which might be attributable to a scant amount of oxygenated material deposited on the surface of the positive electrode. The P 2p spectrum of Na3Fe2(PO4)3 after the discharge process was almost unchanged. The Na 1s spectrum shown in Figure 6f consists of two peaks. The main peak at 1071.7 eV was similar to the peak of pristine Na3Fe2(PO4)3. A weak peak corresponding to NaClO4 was assigned. The results from XPS tests led us to conclude that the Fe3+ in the Na3Fe2(PO4)3 could not be completely reduced during the electrochemical process. Very little substance was deposited on the surface of the positive electrode, and few side-reactions took place during the electrode reaction. To further understand the structural changes in the Na3Fe2(PO4)3 during the sodiation process, ex-situ XRD measurements were performed at different discharge stages, and the results are presented in Figure 7. A wide board peak

corresponded to the component of binder PTFE. The Fe 2p spectrum of completely sodiated Na3Fe2(PO4)3 is displayed in Figure 6b and consists of two segments of spin−orbit coupling (Fe 2p1/2 and Fe 2p3/2). Each component consists of two peaks with the binding energy position related to Fe2+ and Fe3+, respectively.24 The results confirmed that not all of the Fe3+ was reduced after the discharge state. Not all of the Fe3+ in Na3Fe2(PO4)3 compounds showed electrochemical activity, which was in good agreement with the phenomenon of the specific capacity hardly reaching the theoretical specific capacity even though the current density was relatively low. The C 1s spectrum is shown in Figure 6c and consists of four peaks located at 284.6, 286.8, 289.1, and 291.7 eV, respectively. The main peak at 284.6 eV was assigned to the conductive carbon.35 Two peaks at 286.8 and 291.7 eV were attributed to the PTFE binder (CH2 and CF2 groups, respectively).23 The O 1s spectrum of sodiated Na3Fe2(PO4)3 displays only one peak at approximately 531 eV, which was attributed to the O atoms of 1311


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Figure 7. Ex-situ XRD patterns of Na3+xFe2(PO4)3 in different states for Na/Na3Fe2(PO4)3 half cells with magnified views of (2̅02), (4̅04), and (131) diffraction facets [tested at a scanning speed of 2° min−1 between 5° and 90° (2θ)].

appeared at 10°, corresponding to the conductive graphite.36 Several strong peaks appeared beyond the diffraction angle of 60° corresponding to the current collector. The rest of the peaks were attributed to the Na3+xFe2(PO4)3. According to the XRD results, there was formation of a new phase with the insertion of sodium ions because the appearance of obvious peaks and some peaks with growing intensity. The diffraction peaks located at about 20° and 40.5° became stronger, and new peaks appeared at about 40.5° after the insertion of sodium ions. Those diffraction peaks corresponded to the Na4Fe2(PO4)3 phase on the basis of a previous report.37 The results indicated that one phase of Na3Fe2(PO4)3 transformed into another phase of Na4Fe2(PO4)3 during sodium-ion insertion. The XRD patterns illustrate the existing mixtures of two phases of the reaction. We deduce that the part of the slope on the discharge curve might correspond to other processes

and the composition at the discharged state at 1.5 V might correspond to Na5Fe2(PO4)3 compound. We state that the structure of Na5Fe2(PO4)3 might be the same as that of Na4Fe2(PO4)3. Therefore, we think the composition at the discharged state at 1.5 V was the Na 4 Fe 2 (PO 4 ) 3 or Na5Fe2(PO4)3 compound. However, we did not find the standard PDF card of Na5Fe2(PO4)3 composition, for which the crystalline structure needs further research. In summary, Na3Fe2(PO4)3 was successfully synthesized through a simple solid state method, and the structure was characterized by XRD combined with Rietveld refinement. It shows the monoclinic structure in the space group of C2/c. We used the Na3Fe2(PO4)3 as cathode material in SIBs, which showed very high cyclic stability and kept a reversible specific discharge capacity of 40 mAh g−1 at the current rate of 5 C, with very flat voltage plateaus located at about 2.5 V. 1312


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(13) Lee, K. T.; Ramesh, T. N.; Nan, F.; Botton, G.; Nazar, L. F. Topochemical Synthesis of Sodium Metal Phosphate Olivines for Sodium-Ion Batteries. Chem. Mater. 2011, 23, 3593−3600. (14) Wongittharom, N.; Wang, C. H.; Wang, Y. C.; Yang, C. H.; Chang, J. K. Ionic liquid electrolytes with various sodium solutes for rechargeable Na/NaFePO4 batteries operated at elevated temperatures. ACS Appl. Mater. Interfaces 2014, 6, 17564−17570. (15) Oh, S.-M.; Myung, S.-T.; Hassoun, J.; Scrosati, B.; Sun, Y.-K. Reversible NaFePO4 electrode for sodium secondary batteries. Electrochem. Commun. 2012, 22, 149−152. (16) Avdeev, M.; Mohamed, Z.; Ling, C. D.; Lu, J.; Tamaru, M.; Yamada, A.; Barpanda, P. Magnetic structures of NaFePO4 maricite and triphylite polymorphs for sodium-ion batteries. Inorg. Chem. 2013, 52, 8685−8693. (17) Casas-Cabanas, M.; Roddatis, V. V.; Saurel, D.; Kubiak, P.; Carretero-González, J.; Palomares, V.; Serras, P.; Rojo, T. Crystal chemistry of Na insertion/deinsertion in FePO4−NaFePO4. J. Mater. Chem. 2012, 22 (34), 17421−17425. (18) Trad, K.; Carlier, D.; Croguennec, L.; Wattiaux, A.; Lajmi, B.; Ben Amara, M.; Delmas, C. A Layered Iron (III) Phosphate Phase, Na3Fe3(PO4)4: Synthesis, structure, and electrochemical properties as positive electrode in sodium batteries. J. Phys. Chem. C 2010, 114, 10034−10044. (19) d’Yvoire, F; Bretey, E.; de la Rochsre, M. Phase Transitions and ionic cunduction 3D skeketon phosohates. Solid State Ionics 1983, 9, 851−858. (20) Lyubutin, I. S.; Sigaryov, S. E.; Terziev, V. G. PHASE TRANSITIONS IN Na3Fe2(P04): AN INSIDE VIEW. Solid State Ionics 1988, 31, 197−201. (21) Kravchenko, V. V; Sigaryov, S. E. Structure Features of the Superionic Phase Transitions in Na3Fe2(PO4)3. Solid State Commun. 1992, 83, 149−152. (22) Masquelier, C.; Wurm, C.; Rodriguez-Carvajal, J.; Gaubicher, J.; Nazar, L. A Powder Neutron Diffraction Investigation of the Two Rhombohedral NASICON Analogues: γ-Na 3 Fe 2 (PO 4 ) 3 and Li3Fe2(PO4)3. Chem. Mater. 2000, 12, 525−532. (23) Castro, L.; Dedryvére, R.; El Khalifi, M.; Lippens, P.-E.; Bréger, J.; Tessier, C.; Gonbeau, D. The Spin-Polarized Electronic Structure of LiFePO4 and FePO4 Evidenced by in-Lab XPS. J. Phys. Chem. C 2010, 114, 17995−18000. (24) Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254, 2441−2449. (25) Liu, Y.; Xu, S. J.; Zhang, S. M.; Zhang, J. X.; Fan, J. C.; Zhou, Y. R. Direct growth of FePO4/reduced graphene oxide nanosheet composites for the sodium-ion battery. J. Mater. Chem. A 2015, 3, 5501−5508. (26) Xu, S.; Zhang, S.; Zhang, J.; Tan, T.; Liu, Y. A maize-like FePO4@MCNT nanowire composite for sodium-ion batteries via a microemulsion technique. J. Mater. Chem. A 2014, 2, 7221−7228. (27) Liu, Y.; Zhou, Y.; Zhang, J.; Zhang, S.; Ren, P.; Qian, C. Amorphous iron phosphate/carbonized polyaniline nanorods composite as cathode material in sodium-ion batteries. J. Solid State Electrochem. 2016, 20, 479. (28) Liu, Y.; Zhou, Y.; Zhang, J.; Zhang, S.; Xu, S. The transformation from amorphous iron phosphate to sodium iron phosphate in sodium-ion batteries. Phys. Chem. Chem. Phys. 2015, 17, 22144−22151. (29) Liu, Y.; Zhou, Y.; Zhang, J.; Zhang, S.; Ren, P. The relation between the structure and electrochemical performance of sodiated iron phosphate in sodium-ion batteries. J. Power Sources 2016, 314, 1− 9. (30) Parveen; Kant, R. Theory for staircase voltammetry and linear scan voltammetry on fractal electrodes: Emergence of anomalous Randles−Sevik behavior. Electrochim. Acta 2013, 111, 223−233. (31) Churikov, A. V.; Ivanishchev, A. V.; Ushakov, A. V.; Romanova, V. O. Diffusion aspects of lithium intercalation as applied to the development of electrode materials for lithium-ion batteries. J. Solid State Electrochem. 2014, 18, 1425−1441.

Nevertheless, monoclinic Na3Fe2(PO4)3 still has a shortcoming at present, namely, that the specific discharge capacity is relatively low. Once the bottleneck of relatively low specific discharge capacity is broken, the low cost and environmental friendliness of Na3Fe2(PO4)3 might have extensive application prospects in energy storage systems.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Junxi Zhang: 0000-0001-7055-8892 Yongyao Xia: 0000-0001-6379-9655 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Proof-Reading-Service.com (http://www.proofreading-service.com/) for its linguistic assistance. We thank Professor Gain Adair for linguistic assistance. This work was carried out with the financial support of the Project of Shanghai Science and Technology Commission (14DZ2261000) and the National Key Research and Development Plan (2016YFB0901500).

(1) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research development on sodium-ion batteries. Chem. Rev. 2014, 114, 11636− 11682. (2) U.S. Geological Survey (USGS). U.M.Y., Volume I. Metals and Minerals, Mineral Commodity Summaries; January 2012. (3) Van Noorden, R. The rechargeable revolution:A Better Battery. Nature 2014, 507, 26−28. (4) Han, M. H.; Gonzalo, E.; Singh, G.; Rojo, T. A comprehensive review of sodium layered oxides: powerful cathodes for Na-ion batteries. Energy Environ. Sci. 2015, 8, 81−102. (5) Hong, S. Y.; Kim, Y.; Park, Y.; Choi, A.; Choi, N.-S.; Lee, K. T. Charge carriers in rechargeable batteries: Na ions vs. Li ions. Energy Environ. Sci. 2013, 6, 2067−2081. (6) Ellis, B. L.; Nazar, L. F. Sodium and sodium-ion energy storage batteries. Curr. Opin. Solid State Mater. Sci. 2012, 16, 168−177. (7) Kim, S. W.; Seo, D. H.; Ma, X.; Ceder, G.; Kang, K. Electrode Materials for Rechargeable Sodium-Ion Batteries: Potential Alternatives to Current Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 710−721. (8) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phosphoolivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1997, 144, 1188−1194. (9) Morcrette, M.; Wurm, C.; Masquelier, C. On the way to the optimization of Li3Fe2(PO4)3 positive electrode materials. Solid State Sci. 2002, 4, 239−246. (10) Kabbour, H.; Coillot, D.; Colmont, M.; Masquelier, C.; Mentre, O. alpha-Na3M2(PO4)3 (M = Ti, Fe): absolute cationic ordering in NASICON-type phases. J. Am. Chem. Soc. 2011, 133, 11900−11903. (11) David, R.; Pautrat, A.; Kabbour, H.; Mentre, O. Common Building Motifs in Ba 2 Fe 3 (PO 4 ) 4 .2H 2 O, BaFe 3 (PO 4 ) 3 , and Na3Fe3(PO4)4: Labile Fe(2+)/Fe(3+) Ordering and Charge-Dependent Magnetism. Inorg. Chem. 2016, 55, 4354−4361. (12) Fang, Y.; Liu, Q.; Xiao, L.; Ai, X.; Yang, H.; Cao, Y. HighPerformance Olivine NaFePO4 Microsphere Cathode Synthesized by Aqueous Electrochemical Displacement Method for Sodium Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 17977−17984. 1313


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ACS Sustainable Chemistry & Engineering (32) Churikov, A. V.; Ivanishchev, A. V.; Ivanishcheva, I. A.; Sycheva, V. O.; Khasanova, N. R.; Antipov, E. V. Determination of lithium diffusion coefficient in LiFePO4 electrode by galvanostatic and potentiostatic intermittent titration techniques. Electrochim. Acta 2010, 55, 2939−2950. (33) Zhao, Y.; Peng, L.; Liu, B.; Yu, G. Single-crystalline LiFePO4 nanosheets for high-rate Li-ion batteries. Nano Lett. 2014, 14 (5), 2849−2853. (34) Wang, Y.; Liu, H.; Wang, K.; Eiji, H.; Wang, Y.; Zhou, H. Synthesis and electrochemical performance of nano-sized Li4Ti5O12 with double surface modification of Ti(III) and carbon. J. Mater. Chem. 2009, 19, 6789−6799. (35) Bernard, S.; Beyssac, O.; Benzerara, K.; Findling, N.; Tzvetkov, G.; Brown, G. E., Jr XANES, Raman and XRD study of anthracenebased cokes and saccharose-based chars submitted to high-temperature pyrolysis. Carbon 2010, 48 (9), 2506−2516. (36) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 2006, 44 (15), 3342−3347. (37) Hatert, F. Na4Fe2+Fe3+(PO4)3, a new synthetic NASICON-type phosphate. Acta Crystallogr., Sect. E: Struct. Rep. Online 2009, 65 (4), i30.

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Monoclinic Phase Na3Fe2(PO4)3: Synthesis, Structure, and

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