Na2FeP2O7: A Safe Cathode for Rechargeable Sodium-ion Batteries

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Na2FeP2O7: A Safe Cathode for Rechargeable Sodium-ion Batteries Prabeer Barpanda,*,†,‡,⊗ Guandong Liu,†,⊗ Chris D. Ling,§ Mao Tamaru,†,∥ Maxim Avdeev,⊥ Sai-Cheong Chung,† Yuki Yamada,†,‡ and Atsuo Yamada*,†,‡ †

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Unit of Element Strategy Initiative for Catalysts & Batteries, ESICB, Kyoto University, Kyoto 615-8510, Japan § School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia ∥ Mitsubishi Motors Corporation, 1 Nakashinkiri, Hashime-Cho, Okazaki-shi, Aichi 444-8501, Japan ⊥ Bragg Institute, B87, Australian Nuclear Science and Technology Organization, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia ‡

S Supporting Information *

ABSTRACT: Vying for newer sodium-ion chemistry for rechargeable batteries, Na2FeP2O7 pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na2FeP2O7 as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP2O7. High-temperature analyses of the desodiated state NaFeP2O7 show an irreversible phase transition from triclinic (P1̅) to the ground state monoclinic (P21/c) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P2O7)4− anion, which offers better safety than other phosphate-based cathodes. It establishes Na2FeP2O7 as a safe cathode candidate for large-scale economic sodium-ion battery applications. KEYWORDS: sodium-ion battery, cathode, Na2FeP2O7, NaFeP2O7, polymorphism, safety

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Na(Mn1/3Co1/3Ni1/3)O2.8 Based on layered structures (mostly O3-type or P2-type), they deliver reversible capacity in the range of 100−200 mAh/g. In parallel, a suite of polyanionic sodium-based cathode compounds have been unveiled showing commendable electrochemical properties, for example, NaFePO4,9 Na2FePO4F,10 NaVPO4F,11 NaFeSO4F,12 Na3V2(PO4)3,13 Na4M3(PO4)2(P2O7) (M = Fe/Co).14,15 The latest polyanionic insertion system is Na2FeP2O7 pyrophosphate.16−18 Based on the earth-abundant Na−Fe−P−O system, it forms a low cost cathode alternative with a 3 V operation (vs Na/Na+), capacity approaching 90 mAh/g with excellent rate kinetics and cycling stability, making it economically viable for (remote area) large-scale applications. Nevertheless, to realize this goal, the prospective cathode should deliver safe operation without any chemical/thermal breakdown (e.g., excessive heat generation, self-decomposition at high temperature, oxygen evolution, etc.) during its cycling.

INTRODUCTION “Energy” forms the principal challenge for 21st century’s electricity-driven world with widespread usage of consumer electronics, automobiles, and various other devices.1 They are empowered majorly by electrochemical energy storage devices: Li-ion batteries being the major player for over two decades. Since their commercialization in 1991, the Li-ion batteries have gone through leaps and bounds both in research and applications resulting in a multibillion dollar market. Currently, Li-battery stands as a mature device. Although its lightweight and consequent high energy-density has led it to conquer the small-scale portable applications, the growing concern over the cost, safety, and longevity has barred Li-ion batteries from dominating large-scale applications such as automobiles and grid storage. In this scenario, sodium-ion batteries have gained renewed interest especially for large-scale applications owing to the manifold abundance and low cost of Na, the fourth most abundant element in nature.2 Turning their attention from Li-based to Na-based system, electrochemists have developed a plethora of oxide cathode materials with high energy density. Some such systems are NaxCoO2,3,4 NaxVO2,5 Na0.44MnO2,6 Nax(Fe1/2Mn1/2)O2,7 © XXXX American Chemical Society

Received: May 22, 2013 Revised: August 6, 2013

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dx.doi.org/10.1021/cm401657c | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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(Aldrich, 95+%) in acetonitrile media at 60 °C as per the following equation:

Among the different polyanionic frameworks [(XO4)nYm; X = P/ S, Y = F/ OH], the PO4-based systems are arguably the best in terms of thermal/chemical stability. Unlike the SO42− and F− containing systems,10−12,19 which are highly prone to thermal decomposition (above T ≥ 350 °C) and/or moisturerelated degradation, the PO4-based systems have been shown to be robust well above 500 °C.20 The (PO4)3−-based cathodes tend to decompose in the temperature range 500−550 °C to form various pyrophosphate (P2O7) based products such as NaFeP2O7, Fe2P2O7, Na4P2O7, Fe1.5P2O7, and so forth.9,14,20 This is because high-temperature exposure is equivalent to lower chemical potential of oxygen (reducing atmosphere), which is in equilibrium in solid and gas, and leads to thermal decomposition of materials with oxygen (O2 gas) evolution. The loss of oxygen leads to the formation of pyrophosphate [P2O7 or (PO4−x)2] units. Thus, the pyrophosphate anion can be considered energetically more stable than phosphate anions at higher temperature, and thus can be a platform to design thermally stable cathode materials. Fortunately, various transition metal pyrophosphate compounds can be directly synthesized and employed as insertion compounds.21 Tamaru et al. have applied this idea to the Li2−xFeP2O7 cathode22 and established the overall thermodynamic statements with a systematic energy diagram obtained by a combination of differential scanning calorimetry, temperature dependent X-ray diffraction, and electrochemical redox potential.23 This original work has provided a quantitative guideline for the safety and verified the much more stable nature of the charged state of pyrophosphate than that of the phosphate system; a decomposition temperature of 1400 °C for LiFeP2O7 is much higher than that of 700 °C for FePO4. Following this track, we have decided to apply the same methodology to the sodium metal pyrophosphate analogue, which form a novel family of insertion compounds16−18,24−26 for sodium batteries with promising electrochemical performance, Na2FeP2O7 being the front-runner candidate. This Article reports the crystal structure of novel Na2−xFeP2O7 (x = 0, 1) phases. Following, it reports a detail study of the thermal behavior of Na2FeP2O7 and its charged (desodiated) state NaFeP2O7. Very similar to the lithium pyrophosphate case, it delivers excellent stability up to 600 °C (the upper limit in the present experiments) with no thermal decomposition and/or oxygen evolution from the material, simply undergoing an irreversible polymorphic transition above 560 °C. In addition to the easy synthesis, low cost and decent electrochemical properties, the high operational safety establishes Na2FeP2O7 pyrophosphate as a serious contender for practical application in safe and economic large-scale storage systems.

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Na 2Fe IIP2O7 + NO2 BF4 → NaFe IIIP2O7 + NaBF4 + NO2 ↑ The final compound was filtered out and oven-dried at 60 °C for further analyses. Structural Characterization. Synchrotron X-ray diffraction (SXRD) patterns of polycrystalline powder samples were collected on the Powder Diffraction beamline BL-10 of the Australian Synchrotron (Clayton, Australia)27 with a wavelength of λ = 0.82550 Å (calibrated against a LaB6 standard). Samples were loaded into sealed 0.3 mm diameter glass capillaries in a glovebox (considering the often hygroscopic nature of Na-containing phases) for data collection. Rietveld-refinement28 against S-XRD data were carried out using the GSAS program29 with the EXPGUI front-end.30 Scale factors, zeroshifts, background functions, and pseudo-Voigt peak shape parameters were refined in addition to the structural parameters described in the Results and Discussion section below. Temperature-controlled X-ray diffraction (XRD) was performed with a Rigaku RINT-TTR III powder diffractometer (operating at 50 kV, 300 mA) equipped with a Cu−Kα source (λ1 = 1.5405 Å, λ2 = 1.5443 Å). Under steady N2 flow (80 cc/min), the sample was heated (from RT to 600 °C, at an interval of 10−50 °C) inside a Rigaku Reactor-X chamber fitted with a Beryllium window. After keeping the sample at target temperatures for 1 h, diffraction patterns were acquired in the 2θ range of 8−40° (step size of 0.03° s−1). Rietveld refinement28 against temperature-controlled XRD data was performed with the FullProf31 and TOPAS V3.0 program, and the crystal structures were illustrated using the VESTA software.32 Physical Characterization. Mössbauer spectra were acquired with a Topologic System Inc. unit having a 57Co γ-ray source duly calibrated with an α-Fe standard. Typically, ∼0.1 g of powder sample was sealed inside a Pb sample holder by polyethylene films, and the spectra were collected for over 10 h in transmission mode and were analyzed with the MossWinn3.0 software. Thermogravimetric (TG) and differential scanning calorimetry (DSC) studies were conducted with a Rigaku ThermoPlus DSC 8230 unit from RT to 600 °C (heating rate = 5 °C/min) under steady Ar flow (200 cc/min). Electrochemical Characterization. For galvanostatic charge− discharge measurements, the working electrode was formulated by mixing 80 wt % active material Na2FeP2O7, 15 wt % acetylene carbon black, and 5 wt % of polyvinylidene fluoride (PVdF) binder in a minimal amount of NMP (N-methylpyrrolidone) solvent. The resulting slurry was cast on an Al film acting as current collector. Post overnight drying at 120 °C in vacuum, 20 μm thick cathode disks (⌀ = 18 mm) were punched out with a cathode loading of 3 mg cm−2 and were uniaxially pressed at 10 MPa to improve the particle contacts. 2032-type coin cells were assembled inside an Ar-filled glovebox using these cathode disks as working (+ve) electrodes, Na metal foil as counter (−ve) electrodes separated by polypropylene films soaked with 1 M NaClO4 dissolved in propylene carbonate (PC) acting as electrolyte. These coin cells were subjected to galvanostatic cycling (at the rate of C/10) with a TOSCAT-3100 battery tester (Toyo system) in the voltage range of 2.0−4.0 V (at 25 °C).

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION The starting cathode material pyrophosphate Na2FeP2O7 adopts a triclinic (space group = P1̅, #2) three-dimensional framework, isostructural to the previously reported phase of Na3.64Ni2.18(P2O7)2.33 The final Rietveld fit to S-XRD data is shown in Figure 1a. Unit cell parameters are presented in Table 1. Full structural details are presented in Table 2 and deposited as a CIF file, and the structure shown in Figure 2a. Careful Rietveld analysis of the diffraction data for Na2FeP2O7 revealed weak peaks of a secondary phase, which was identified as maricite-type NaFePO434 (∼3.4 wt %), being a thermodynamically stable phase in Na−Fe−P−O systems. The Mössbauer spectrum shown in the inset to Figure 1a further affirms the

Material Synthesis. The starting insertion compound, Na2FeP2O7, was synthesized by conventional solid-state synthesis using a stoichiometric 2:1:2 molar mixture of NaHCO3 (Wako, 99.5%), FeC2O4·2H2O (Junsei, 99+%), and (NH4)2HPO4 (Wako, 99%) precursors. An intimate precursor mixture was prepared by wet planetary ball-milling in acetone media for 3 h (400 rpm) using Crhardened stainless steel (Cr-SS) milling media and container. Post drying the acetone in vacuum, the precursor mixture was ground in agate mortar, pressed into cylindrical pellets (⌀ = 12 mm), and annealed at 600 °C (heating rate = 10 °C/min) for 12 h inside a tube furnace with steady flow of Ar/H2 (95:5) gas to maintain a reducing atmosphere. Upon cooling to ambient temperature, phase-pure Na2FeP2O7 was obtained. The derivative NaFeP2O7 product was synthesized by overnight chemical oxidation with NO2BF4 oxidizer B



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constituent Na atoms and their ordering leads to multistep voltage-capacity profiles. Involving a topotactic Na (de)insertion mechanism with a small volume change (ΔV/V = 2.6%),17 the Na2FeP2O7 efficiently forms the desodiated NaFeIIIP2O7 end-member. A preliminary in situ XRD study reveals a steady and reversible shift in peak positions during electrochemical cycling of the Na2FeP2O7 cathode, indicating the possibility of single-phase redox activity throughout the potential range (Supporting Information, Figure S1). A detailed study of underlying redox mechanism using synchrotron structural analyses is currently under investigation. The pyrophosphate group of compounds has been investigated by mineralogists and crystallographers for over four decades,21 one such compound being α-NaFeP2O7 assuming a monoclinic structure with P21/c (#14) symmetry.35,36 It has an alternate stacking of layers of FeO6 octahedra and layers containing P2O7 units, creating large tunnels for Na atoms. The FeO6 octahedra are separated from each other by PO4 units. The α-NaFeP2O7 polymorph is isostructural to LiFeP2O7 and KFeP2O7.37,38 Nevertheless, here using chemical oxidation of Na2FeP2O7 cathode, we have discovered a new polymorph of NaFeP2O7 (from now on denoted as βNaFeP2O7). In an effort to determine its crystal structure, we synthesized single-phase β-NaFeP2O7 by chemical oxidation of Na2FeP2O7. The completion of 1 Na removal was confirmed by complete Fe2+→Fe3+ transformation by Mössbauer analysis (inset of Figure 3 and Supporting Information, Figure S2a). S-XRD data for β-NaFeP2O7 indicated that it retains triclinic (space group = P1̅, #2) symmetry and the same framework structure as Na2FeP2O7, but with a unit cell volume reduced (as expected) by ∼3.2% (see Table 1). Rietveld-refinement of Na site occupancies led to 3 of the independent sites reducing very close to 0 and one very close to 0.5, with the other 2 remaining close to 1, consistent with the expected stoichiometry. Fractional atomic coordinates for those remaining sites could be freely refined in parallel with their occupancies, and remained stable without moving significantly. No additional peaks could be found in the difference Fourier map. The occupancies were fixed at these values for the final refinement of atomic positions. The final Rietveld fit to S-XRD data of βNaFeP2O7, with trace amount of maricite-NaFePO4 secondary phase, is shown in Figure 3. Unit cell parameters of βNaFeP2O7 are presented in Table 1, with full structural details presented in Table 2 and in the deposited CIF file. The final refined structure of β-NaFeP2O7 is shown in Figure 2b, compared directly to those of α-NaFeP2O7 and Na2FeP2O7. The Na sites are labeled in Figure 2a to illuminate the following discussion of the structural changes observed on desodiation, in addition to the reduced unit cell volume. Comparing the structures of Na2FeP2O7 and β-NaFeP2O7 as presented in Table 2 and Figure 2, we see three key changes. First, the disordered model for Na sites in the smallest channels along the a axis, where partially occupied sites (Na4, Na5, and Na6) contribute a total of 0.5 Na per formula unit in Na2FeP2O7 based on the model of Erragh et al. for Na3.64Ni2.18(P2O7)2,33 resolves to a single fully occupied site (Na4) contributing 0.25 Na per formula unit in β-NaFeP2O7. The reason for the apparent increase in local ordering is unclear, but the 50% reduction in Na density in the channels is unambiguous. Second, the most sterically crowded Na site (Na3) is reduced from full to half occupancy. Third, while Na2FeP2O7 has two independent Na sites occupying opposite sides of the largest channels along the a axis (Na1 and Na2), in

Figure 1. (a) Final Rietveld fit of Na2FeP2O7 to S-XRD data (λ = 0.82550 Å, Rwp = 2.96%, Rp = 2.03%, RF2 = 5.21%). Experimental and calculated profiles and their difference are shown as red crosses and black and green solid lines, respectively. The top and bottom rows of tick marks show the positions of the Na2FeP2O7 and mariciteNaFePO4 phases, respectively. Inset image shows the corresponding Mössbauer spectrum fitted only with two Fe2+-doublets (green and blue lines) assigned to two distinct Fe sites. (b) Galvanostatic charge− discharge profiles of Na2FeP2O7 at a rate of C/10 (at 25 °C) for the first 30 cycles.

Table 1. Comparison of the Lattice Parameters of Na2FeP2O7 and β-NaFeP2O7 Derived from Rietveld Refinement against S-XRD Data a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3)

Na2FeP2O7

β-NaFeP2O7

6.44903(5) 9.48247(8) 10.99331(8) 64.8468(4) 86.2397(5) 73.1333(5) 581.050(8)

6.3297(13) 9.4646(19) 11.100(2) 62.6109(6) 83.2900(7) 72.2418(7) 562.1(3)

majority of Fe2+ species with traces (2.5%) of Fe3+ impurities. The crystal structure is built from corner sharing FeO6−FeO6 [Fe2O11] dimers, which are interconnected by PO4−PO4 [P2O7] units both by corner-sharing and edge-sharing manner to create three-dimensional tortuous zigzag channels for Na+ion migration. It enables the Na2FeP2O7 cathode reversible 1electron Na (de)insertion capacity approaching 90 mAh/g involving an Fe3+/Fe2+ redox activity centered at 3.0 V (Figure 1b). The presence of five distinct crystallographic sites for C


multiplicity

2 2 2 2 2 2 2 2 2 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

site

Fe1 Fe2 P1 P2 P3 P4 Na1 Na2 Na3 Na4 Na5 Na6 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14

0.7220(4) 0.6457(4) 0.5758(8) 0.1229(7) 0.0745(8) 0.2895(7) 0.0283(10) 0.4107(9) 0.2116(9) 1/2 0.1899(17) 0.3778(25) 0.0169(13) 0.6536(13) 0.3037(13) 0.1132(13) 0.7676(13) 0.2845(14) 0.0291(13) 0.7716(13) 0.4278(13) 0.6198(13) 0.3738(13) 0.5543(13) 0.0871(14) 0.0390(14)

x (a) 0.39373(3) −0.0012(3) 0.2050(6) 0.0481(6) 0.3257(6) 0.6900(5) 0.3162(7) 0.6428(7) 0.3464(7) 0 0.0076(15) 0.018(2) 0.2217(10) 0.3548(11) 0.0364(11) 0.4392(10) 0.0452(10) 0.1801(10) 0.0406(10) 0.3973(10) 0.5738(10) 0.1772(10) 0.1965(9) 0.2017(10) 0.7687(10) 0.6042(9)

y (b) 0.21747(22) 0.26610(24) 0.4583(5) 0.3138(5) 0.7885(4) 0.0374(5) 0.4748(6) 0.3381(5) 0.1031(6) 0 0.0432(11) 0.0171(18) 0.3021(8) 0.4345(8) 0.2212(9) 0.8460(8) 0.5381(8) 0.8095(7) 0.6959(8) 0.0366(7) 0.1694(8) 0.0512(8) 0.5358(8) 0.3243(8) 0.1023(8) 0.3441(8)

z (c)

Na2FeP2O7 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.33333 0.5 0.33333 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

occupancy 0.0106(9) 0.0095(9) 0.0143(8) 0.0143(8) 0.0143(8) 0.0143(8) 0.0287(12) 0.0287(12) 0.0287(12) 0.0287(12) 0.0287(12) 0.0287(12) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9) 0.0105(9)

U (Å )

2

0.4054(5) 0.0083(5) 0.2038(9) 0.0562(9) 0.3122(10) 0.6891(9) 0.3239(12) 0.357(2) 0

0.2247(18) 0.3515(18) 0.0476(18) 0.4123(18) 0.0557(19) 0.1606(18) 0.0397(17) 0.3797(18) 0.5486(18) 0.1703(17) 0.1801(18) 0.1823(17) 0.7721(15) 0.6041(19)

0.237(3) 1/2

0.001(2) 0.650(2) 0.310(2) 0.162(2) 0.783(2) 0.271(2) 0.044(2) 0.808(2) 0.413(2) 0.630(2) 0.350(2) 0.568(2) 0.091(2) 0.019(2)

y (b)

0.7196(7) 0.6463(6) 0.5782(13) 0.1289(13) 0.0887(13) 0.2768(12) 0.0112(17)

x (a)

0.2967(14) 0.4060(15) 0.2143(16) 0.8371(15) 0.5320(16) 0.8140(14) 0.6954(15) 0.0468(15) 0.1426(15) 0.0697(15) 0.5495(15) 0.3258(16) 0.1024(14) 0.3454(17)

0.0757(19) 0

0.2128(4) 0.2587(4) 0.4553(8) 0.3114(8) 0.7850(8) 0.0259(7) 0.4868(10)

z (c)

β-NaFeP2O7 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.0 0.5 1.0 0.0 0.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

occupancy

0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15) 0.0081(15)

0.019(3) 0.019(3)

0.0071(11) 0.0045(11) 0.0055(13) 0.0055(13) 0.0055(13) 0.0055(13) 0.019(3)

U (Å2)

Table 2. Atomic Coordinates, Site Occupancies, and Isotropic Atomic Displacement Parameters of Na2FeP2O7 and β-NaFeP2O7 Obtained by Rietveld Refinement against SXRD Data

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likely route via which Na cations were electrochemically removed, this distribution of Na sites in β-NaFeP2O7 suggests an equilibration of electrostatic interactions after desodiation; that is, the Na sites probably all show at least short-range mobility, otherwise sodium ions cannot diffuse to the particle surface to exhibit electrode activity. The charged (desodiated) cathode composition, in the presence of flammable organic solvents/electrolyte, can trigger excessive heat generation, self-decomposition, loss of constituent oxygen, and/or severe parasitic reactions at the electrode− electrolyte interfaces. These can create a potential safety risk during battery operation. Hence, thermal study of charged cathodes is key to know the operational safety of any potential cathode. After the structural evaluation, the thermal behavior of the charged β-NaFeP2O7 phase was examined. Figure 4 shows

Figure 4. Thermal analysis (TG-DSC) curves of the desodiated state NaFeP2O7 under steady Ar flow. A sharp exothermic peak around 560−580 °C is observed associated with no weight change, suggesting a possible phase-transition to a thermodynamically stable polymorph.

Figure 2. Final refined crystal structures of (a) Na2FeP2O7 and (b) βNaFeP2O7 compared to that of (c) α-NaFeP2O7. FeO6 octahedra are brown, PO4 tetrahedra are gray, and Na atoms are yellow. Na sites are labeled for Na2FeP2O7.

the TG-DSC profiles of β-NaFeP2O7 sample prepared ex situ by chemical oxidation of Na2FeP2O7. The DSC curve depicts an exothermic peak with the onset point of 564.2 °C with a moderate heat generation of 16 kJ/mol. The possibility of any chemical/thermal decomposition and/or related weight loss can be eliminated as the TG curve shows no weight change. The combination of an exothermic peak with no weight change points at a possible phase transition of β-NaFeP2O7. To verify this hypothesis, temperature-dependent XRD analysis on β-NaFeP2O7 sample was performed. As shown in Figure 5a, the characteristic diffraction pattern of initial βNaFeP2O7 is retained intact up to 550 °C. In agreement with the DSC data, a transition is noticed in the narrow temperature window of 560−580 °C, with new peaks emerging at the expense of old peaks. Upon the end of heating at 600 °C, the starting β-NaFeP2O7 phase completely disappears to form αNaFeP2O7. Mössbauer spectra of these two end compositions shows no change in valency (i.e., 100% Fe3+ state, Supporting Information, S2, Table S1). Rietveld refinement of the starting (β-NaFeP2O7) and final (α-NaFeP2O7) phase is shown in Figure 5 b−c depicting the absence of any impurities. These results confirm the irreversible (triclinic P1̅ → monoclinic P21/c symmetry) phase transition of β-NaFeP2O7 cathode material. This phase transition was further observed when β-NaFeP2O7 was annealed ex-situ at 600 °C for 4−6 h inside a tube furnace (in air) (Supporting Information, S3). Thus, the β-NaFeP2O7 is metastable, as expected for a material prepared by soft desodiation. It can be rooted to two structural features. First,

Figure 3. Final Rietveld fit of β-NaFeP2O7 to S-XRD data (λ = 0.82550 Å, Rp = 4.14%, Rwp = 6.58%, and RF2 = 7.98%). Experimental and calculated profiles and their difference (plotted in same scale) are shown as red crosses and black and green solid lines, respectively. The top and bottom rows of tick marks show the positions of the Na2FeP2O7 and maricite-NaFePO4 phases, respectively. Inset figure illustrates the corresponding Mössbauer spectrum fitted only with two Fe2+-doublets (green and pink lines) assigned to two distinct constituent Fe sites.

β-NaFeP2O7 the Na1 site remains but Na2 is removed. Thus, we find that the 50% desodiation has occurred evenly throughout the structure, rather than predominantly in the largest channels. Noting that these large channels are the most E



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Figure 5. (a) In-situ high-temperature X-ray diffraction patterns of NaFeP2O7 charged state showing the irreversible phase transition (550 °C < Tt < 590 °C) from β-NaFeP2O7 (triclinic, P1̅) to α-NaFeP2O7 (monoclinic, P21/c). Rietveld refinement of (b) α-NaFeP2O7 and (c) β-NaFeP2O7 polymorphs are shown with the experimental data (red dots), simulated powder pattern (cyan line), their difference (plotted in same scale) (blue line), and Bragg diffraction positions (green ticks). The inset figures illustrate respective structural arrangement of the constituent FeO6 octahedra (green), PO4 tetrahedra (blue), and Na atoms (yellow).

in the β-NaFeP2O7 phase, every alternate FeO6 octahedra and PO4 tetrahedra are connected in edge-sharing fashion. Owing to their dissimilar polyhedral size, the FeO6 octahedra are highly distorted. The degree of octahedral distortion (Δ) in βNaFeP2O7 phase can be quantified by the following formula, where dn and ⟨d⟩ are individual Fe−O bond length and mean Fe−O bond length respectively: 1 Δ= 6

⎧ (dn − ⟨d⟩) ⎫ ⎬ ∑⎨ ⟨d⟩ ⎭ ⎩ n=1 6

2

The distortion values of Fe1 and Fe2 sites are calculated to be Δ(Fe 1) = 5.22353 × 10−3 and Δ(Fe 2) = 1.37801 × 10−3, with an average Δ(Fe) value of 3.3 × 10−3. However, in case of αNaFeP2O7, the distortion Δ(Fe) value is much smaller, about 0.40 × 10−3. It drives the β→α phase transition in NaFeP2O7 upon high-temperature annealing. Second, the Coulombic Fe3+−Fe3+ repulsion between neighboring FeO6−FeO6 is very strong because of their near proximity.39 It makes the overall structure energetically unstable, which upon heating transforms to the stable α-NaFeP2O7 form having well separated symmetric FeO6 octahedral units connected by PO4 tetrahedra exclusively by corner-sharing mode. Similar temperatureinduced phase transition has been recently noticed in the monoclinic LiFeP2 O 7 system,22,40 which undergoes an irreversible polymorphic phase transition from P21/c (#14) to P21 (#4) symmetry.23 The overall energetics of chemical/electrochemical desodiation and β→α phase transition in the Na2−xFeP2O7 system is shown in Figure 6. While the desodiation process with Fe2+→ Fe3+ redox activity (at 2.98 V vs Na/Na+) involves large energy difference (2.98 eV) to form β-NaFeP2O7, it readily transforms to a stable α-NaFeP2O7 polymorph with a negligible 0.2 eV energy change. Attempts to intercalate sodium atoms into αNaFeP2O7 were unsuccessful, making it electrochemically inactive, unlike isostructural LiFeP2O7 showing Li-insertion at

Figure 6. Enthalpy diagram of Na2−xFeP2O7 polymorphs.

2.9 V.38 The larger size of Na and surface energy may be the reason behind such inactivity. Though oxide-based cathodes for sodium batteries date back to 1982, polyanionic cathodes are less explored. Coupling materials abundance/cost perspective and desirable electrochemical performance, a handful of Fe-based cathodes have been proposed for the next generation of Na-ion batteries, which are dominated by PO4-based compositions such as Na2FePO4F, NaFePO4, Na4Fe3(PO4)2(P2O7), and Na2FeP2O7. Among these compounds, the Na2FeP2O7 offers the highest rate capability with a moderate 3 V redox activity. It shows higher thermal stability than any other cathodes with PO4 units as pyrophosphate anions are already reduced products of phosphates and found to be inherently more stable than phosphate anions. In fact, historically these (P2O7)4−-based compounds were obtained by high-temperature annealing of PO4-based precursors, thus were named as “pyrophosphates” (the greek word “pyro” means fire), although their structural nomenclature should be “diphosphate” or “biphosphate”. F



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Overall, the pyrophosphate group of compounds has the potential to form insertion compounds with inherently high thermal stability. This fundamental study can shed light on the design of superior cathode materials for large-scale sodium batteries.

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CONCLUSION To summarize, pyrophosphate Na2FeP2O7 has been shown to be a safe cathode material for sodium-ion batteries. Its charged state (NaFeP2O7) has high thermal stability up to 600 °C (upper limit of the present experiment), which does not involve any decomposition and/or oxygen evolution, but undergoes a polymorphic transition at 560 °C from triclinic (P1̅) to monoclinic (P21/c) phase. This thermal stability far exceeds those of layered oxides and at least is comparable to other polyanionic cathodes such as olivine (NaFePO4). The higher stability can be attributed to the inherent high stability of pyrophosphate (P2O7)4− building blocks. Thus, pyrophosphate cathodes tend to deliver higher operational safety although the net capacity is lower owing to the weight penalty. The combination of low cost materials, moderate capacity (90 mAh/g at 3 V vs Na/Na+), and high rate kinetics with excellent safety makes Na2FeP2O7 a serious contender for large-scale batteries for grid power storage.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format. Further details are given in Figures S1−S3 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*[email protected] (P.B.), [email protected]. u-tokyo.ac.jp (A.Y.). Phone: +81-3-5841-7295. Fax: +81-35841-7488. Author Contributions ⊗

Authors with equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Peter Blanchard for help in acquiring synchrotron X-ray diffraction patterns. Dr. M. Sathiya and Prof. J.M. Tarascon (UPJV, France) are acknowledged for providing in situ XRD data. We gratefully acknowledge the financial support from Mitsubishi Motors Corporation and the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the “Element Strategy Initiative for Catalysts & Batteries” (ESICB) project. P.B. is thankful to the Japan Society for the Promotion of Sciences for a JSPS Fellowship at the University of Tokyo. C.D.L. received support for this work from the Australian Research Council-Discovery Projects.

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Na2FeP2O7: A Safe Cathode for Rechargeable Sodium-ion Batteries

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