770

www.MaterialsViews.comwww.advenergymat.de

FULL

PAPER

Kwang-Ho Ha , Seung Hee Woo , Duckgyun Mok , Nam-Soon Choi , Yuwon Park , Seung M. Oh , Youngshol Kim , Jeongsoo Kim , Junesoo Lee , Linda F. Nazar , * and Kyu Tae Lee *

Na 4− α M 2 + α /2 (P 2 O 7 ) 2 (2/3 ≤ α ≤ 7/8, M = Fe, Fe 0.5 Mn 0.5 , Mn): A Promising Sodium Ion Cathode for Na-ion Batteries

A new polyanion-based compound, Na 3.12 M 2.44 (P 2 O 7 ) 2 (M = Fe, Fe 0.5 Mn 0.5 , Mn) is synthesized and examined as a cathode for Na ion batteries. Off-stoichiometric synthesis induces the formation of a Na-rich phase, Na 3.32 Fe 2.34 (P 2 O 7 ) 2 - a member of the solid solution series Na 4− α Fe 2 + α /2 (P 2 O 7 ) 2 (2/3 ≤ α ≤ 7/8) - which delivers a reversible capacity of about 85 mA h g − 1 at ca. 3 V vs. Na/Na + and exhibits very stable cycle performance. Above all, it shows fast kinetics for Na ions, delivering an almost constant 72% reversible capacity at rates between C/10 and 10C without the necessity for nanosizing or carbon coating. We attribute this to the spacious channel size along the a -axis, along with a single phase transformation upon de/sodiation.

1. Introduction

Sodium ion batteries are an attractive alternative to replace lithium ion batteries for smart grid applications. They have sim-ilar, albeit lower, energy density compared to Li ion batteries, and Na resources are inexhaustible and lower cost. [ 1 , 2 ] This is especially important if the demand for lithium, and hence its price, increases signifi cantly as a result of widespread com-mercialization of automotive lithium ion batteries. Although Na ion batteries have recently received a great deal of attention

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhewileyonlinelibrary.com

K. H. Ha,[+] S. H. Woo,[+] D. Mok, Prof. N. S. Choi, Prof. K. T. LeeInterdisciplinary School of Green Energy and KIER-UNIST Advanced Center for EnergyUlsan National Institute of Science and Technology (UNIST)100 Banyeon-ri, Eonyang-eup, Ulju-gun, Ulsan689-798, South Korea E-mail: ktlee@unist.ac.kr Prof. L. F. NazarDepartment of ChemistryUniversity of Waterloo200 University Avenue west, Waterloo, Ontario, N2L 3G1, CanadaE-mail: lfnazar@uwaterloo.ca Y. Park, Prof. S. M. OhSchool of Chemical and Biological EngineeringSeoul National University599 Gwanangno, Gwanak-gu, Seoul, 151-744, South Korea Dr. Y. Kim, Dr. J. Kim, Dr. J. LeeSK Innovation Co.325 Exporo, Yuseong-gu, Daejeon, 305-712, South Korea [+] K.H.H. and S.H.W. contributed equally to this work.

DOI: 10.1002/aenm.201200825

as a next generation system for grid-storage, Na ion intercalation chemistry is not new. Since the 1980s, many materials have been reported as intercalation hosts for Na ions. For example, Na 0.44 MnO 2 , [ 3 ] λ -MnO 2 , [ 4 ] Na x MO 2 (M = Co, Cr and Ni), [ 5 , 6 ] Na x Mo 2 O 4 , [ 7 ] and hard carbon [ 8–10 ] show reversible de/intercalation of Na ions. Although some initial reports sug-gested slow rate properties, recently, rea-sonably fast sodium de/insertion kinetics have been reported for a few materials. These include Na x Fe 1/2 Mn 1/2 O 2 , [ 11 ] Na 1.0 Li 0.2 Ni 0.25 Mn 0.75 O δ , [ 12 ] Na 1−x Ni 0.5 Mn 0.5 O 2 , [ 13 ] NaNi 1/3 Fe 1/3 Mn 1/3 O 2 , [ 14 ] Na 2 Ti 3 O 7 , [ 15 ] and sodium terephthalate [ 16 ] which have

all shown impressive electrochemical results including high reversible capacity and rate capability.

Although Na ion batteries show promise, they will also face the same problems as Li ion batteries, since the positive elec-trodes have similar chemistry. In particular, as with lithium ion batteries, safety issues are anticipated for sodium metal oxide cathodes. Given this, phosphate-based materials are desirable to improve safety and cycle life–in particular for grid storage - due to the provision of strong P-O bonding as demonstrated for Li ion batteries. Recently, a variety of phosphate-based polyanion materials have been introduced including NaMPO 4 olivines, [ 17–19 ] NaVPO 4 F, [ 20 ] Na 3 V 2 (PO 4 ) 3 F 3 , [ 21 , 22 ] Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), [ 23 ] and Na 2 FePO 4 F, [ 24–27 ] and Na 3 V 2 O 2x (PO 4 ) 2 F 3−2x . [ 28 ]

In 2010, Li 2 FeP 2 O 7 was introduced as a cathode material in lithium ion batteries, and it showed substantially improved kinetics relative to other phosphate-based materials including LiFePO 4 . [ 29 ] Inspired by the facile de/intercalation of pyrophos-phate for lithium, herein, we introduce Na 3.12 M 2.44 (P 2 O 7 ) 2 (M = Fe, Fe 0.5 Mn 0.5 , Mn) as a cathode material for Na ion batteries, which exhibits fast kinetics of intercalation for Na ions. We also report that Na 3.12 M 2.44 (P 2 O 7 ) 2 exhibits solid solution behavior, proceeding in a single-phase reaction during charging and discharging. For reference (added as a note in the proof), we note that independently another pyrophosphate-based material, Na 2 FeP 2 O 7 , has just been reported as a cathode material for sodium ion batteries. [ 30 , 31 ]

2. Results and Discussion

In 1995, Angenault et al. fi rst synthesized a single crystal of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 , and solved its crystal structure. [ 32 ] However,

im Adv. Energy Mater. 2013, 3, 770–776

www.advenergymat.de

FULL P

APER

Figure 1 . (a) XRD patterns of Na 3.12 M 2.44 (P 2 O 7 ) 2 (M = Fe, Fe 0.5 Mn 0.5 , Mn), (b) Rietveld refi nement and the corresponding structure (inset: [100] direction) of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 , (c) Rietveld refi nement of Na 3.12 Fe 1.22 Mn 1.22 (P 2 O 7 ) 2 .

www.MaterialsViews.com

the microcrystalline form of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 and its Mn and Fe 1−x Mn x analogues have not been reported. We synthesized Na 3.12 M 2.44 (P 2 O 7 ) 2 (M = Fe, Fe 0.5 Mn 0.5 , Mn) powders using the conventional solid state method of heating ball-milled precursors with the target stoichiometry at 600 ° C under Ar. The triclinic structure of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 is composed of a centrosymmetrical crown of Fe 2 P 4 O 22 and Fe 2 P 4 O 20 moitiés connected by corner-sharing to form a three-dimensional framework. Each crown unit consists of two FeO 6 octahedra and two P 2 O 7 groups, which is different from monoclinic NaFeP 2 O 7 [ 33 ] and triclinic Na 2 FeP 2 O 7 . [ 30 , 31 ] The corresponding X-ray diffraction (XRD) patterns of the materials shown in Figure 1 a are consistent with the predicted patterns based on solid solution phases of Fe and Mn; as the x value in Na 3.12 Fe 1-

x Mn x (P 2 O 7 ) 2 increases, the peak positions shift to lower angle due to the increase in lattice parameters, as observed for the NaMPO 4 (M = Fe, FeMn, Mn) olivines. Rietveld refi nement of the XRD patterns in the space group P−1 was carried out for Na 3.12 M 2.44 (P 2 O 7 ) 2 (M = Fe, Fe 0.5 Mn 0.5 ) to confi rm their struc-tures (Figure 1 b,c). The lattice parameters, agreement fac-tors, and detailed structural information are summarized in Table 1 and Tables S1-2 in the Supporting Information. The lattice parameters of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 powders (a = 6.4046(8) Å, b = 9.3908(11) Å, c = 10.9729(13) Å, α = 64.4874(13), β = 85.9939(15), γ = 73.1075(14), volume = 568.74(18) Å 3 ) are in complete agreement with the previously reported single crystal data for Na 3.12 Fe 2.44 (P 2 O 7 ) 2 . The new mixed metal phase of Na 3.12 Fe 1.22 Mn 1.22 (P 2 O 7 ) 2 has lattice parameters that are con-sistent with its end members, suggesting it is a solid solution phase with respect to transition metal composition. The SEM image ( Figure 2 a) reveals that the Na 3.12 Fe 2.44 (P 2 O 7 ) 2 microcrys-tals have a broad size distribution ranging from hundreds of nanometers to a few micrometers.

The electrochemical performance of the Na 3.12 Fe 2.44 (P 2 O 7 ) 2 electrode was evaluated at 30 ° C using a half cell with a sodium metal. Cells were constructed using an electrolyte comprised of 0.8 M NaClO 4 in EC:DEC (1:1 vol.%). Figure 2 b shows the charge-discharge profi les of the Na 3.12 Fe 2.44 (P 2 O 7 ) 2 electrode between 4.0 and 1.7 V at a rate of 4.8 mA g − 1 (ca. 0.05C). The theoretical capacity of Na 3.12 Fe II 2.44 (P 2 O 7 ) 2 is 117.6 mA h g − 1 , assuming that 2.44 Na is reversibly de/intercalated by means of oxidation and reduction of 2.44 Fe 2 + /3 + . This is slightly higher than that of Li 2 FeP 2 O 7 (110 mA h g − 1 ), where both materials show a one-electron reaction per Fe atom via Fe 2 + /3 + . The voltage profi les of the Na 3.12 Fe 2.44 (P 2 O 7 ) 2 electrode demonstrate that a reversible capacity of 85 mA h g − 1 (1.76 Na) is delivered with stable cycle performance. It exhibits irreversible capacity during charge on the 1st cycle, however. We believe this is caused by electrolyte decomposition on the surface of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 during charging, resulting in formation of a solid-electrolyte interphase (SEI). This is attributed to reaction of the electro-lyte with sodium carbonate (or products of electrochemically decomposed Na 2 CO 3 [ 34 ] ) on the surface, formed from reaction of the outer layers of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 with CO 2 upon air exposure vis: Na 3.12− α Fe II 2.44− α /2 Fe III α /2 (P 2 O 7 ) 2 + α /2Na 2 CO 3 . A similar reaction has been reported for LiCoO 2 . [ 35 , 36 ] As shown in the XPS profi les ( Figure 3 ), two distinct Na and O peaks were observed on the surface indicating the existence of both Na 2 CO 3 and Na 3.12 Fe 2.44 (P 2 O 7 ) 2 ; however, only one peak for Na and O,

© 2013 WILEY-VCH Verlag GAdv. Energy Mater. 2013, 3, 770–776

respectively, was observed after etching, indicating only the surface is oxidized. This is confi rmed by washing the product with water to remove the Na 2 CO 3 , which greatly reduces the irreversible charge capacity (Figure 2 b). Also, it further con-fi rms that the sodiation capacity is larger than the desodiation

mbH & Co. KGaA, Weinheim 771wileyonlinelibrary.com

77

www.MaterialsViews.comwww.advenergymat.de

FULL

PAPER

Table 1. The lattice parameters of Na 3.12−x M 2.44 (P 2 O 7 ) 2 (M = Fe, Mn, Fe 0.5 Mn 0.5 ) compounds obtained from Rietveld refi nement.

a [Å] α

Volume [Å 3 ] R p [%] R wp [%] R F 2 [%] χ 2 b [Å] β

c [Å] γ

Na 3.12 Fe 2.44 (P 2 O 7 ) 2 6.4046(8) 64.4874(13) 568.74(18) 4.51 6.12 4.41 15.58

9.3908(11) 85.9939(15)

10.9729(13) 73.1075(14)

Na 3.12−x Fe 2.44 (P 2 O 7 ) 2 6.35862(27) 63.6517(22) 565.01(5) 5.14 6.92 3.75 18.69

9.4253(4) 84.8881(27)

11.0016(5) 73.1909(25)

Na 3.12−y Fe 2.44 (P 2 O 7 ) 2 6.32887(34) 63.0667(23) 557.85(5) 5.8 7.63 3.62 23.82

9.3980(5) 83.8493(30)

11.0270(5) 72.6569(29)

Na 3.12 Fe 1.22 Mn 1.22 (P 2 O 7 ) 2 6.4653(5) 64.360(4) 578.50(10) 8.94 12.28 6.28 61.26

9.4305(8) 85.811(5)

11.0295(10) 72.955(5)

Na 3.12 Mn 2.44 (P 2 O 7 ) 2 a) 6.532 64.4160

9.526 85.9592

11.089 73.4145

a) Cell parameters indexed using CRYSFIRE.

capacity on the 1 st cycle due to the reduction of oxidized Fe 3 + in Na 3.12− α Fe II 2.44− α /2 Fe III α /2 (P 2 O 7 ) 2 during discharge.

As a strategy to suppress surface oxidation and simplify material processing, Na 3.12 Fe 2.44 (P 2 O 7 ) 2 was synthesized from an off-stoichiometric mixture of starting materials with a nom-inal composition of Na 3.42 Fe 2.44 (P 2 O 7 ) 2.05 . The off-stoichiometric synthesis yielded a biphasic mixture of the Na-rich phase of the sodium iron phosphate with a small fraction of NaFePO 4 (maricite) as revealed by the XRD pattern ( Figure 4 ). However, mass/charge balance indicates the overall composition cannot be a combination of x Na 3.12 Fe 2.44 (P 2 O 7 ) 2 + y NaFePO 4 , but must be rather the sodium rich, iron poor phase Na 3.32 Fe II 2.34 (P 2 O 7 ) 2 + 0.1NaFePO 4 . This is in accord with previous predictions that var-ious compositions can exist in the range of a continuous solid solution of Na 4− α Fe 2 + α /2 (P 2 O 7 ) 2 (2/3 ≤ α ≤ 1), varying with the occupation factor of the centrosymmetrical site Na(3), based on an investigation of the binary system Na 4 P 2 O 7 -Mg 2 P 2 O 7 . [ 32 ] The composition Na 3.32 Fe 2.34 (P 2 O 7 ) 2 thus corresponds to α = 0.68. The formation of this Na-rich phase was confi rmed through XPS analysis. As shown in the XPS depth profi les ( Figure 5 ), the Fe/Na peak area ratio values of stoichiometric and off-sto-ichiometric samples were constant regardless of the etching depth. This means that both materials have a homogeneous Fe/Na atomic ratio in the bulk. However, the Fe/Na peak area ratio of the off-stoichiometric Na 3.32 Fe 2.34 (P 2 O 7 ) 2 is lower than that of stoichiometric Na 3.12 Fe 2.44 (P 2 O 7 ) 2 , indicating that the overall composition Na 3.32 Fe 2.34 (P 2 O 7 ) 2 has a lower Fe/Na atomic ratio as expected (namely, 2.34Fe/3.32Na vs. 2.44Fe/3.12Na).

Also, from the XPS data, peaks corresponding to Na 2 CO 3 formation were not observed on the surface of the

© 2013 WILEY-VCH Verlag G2 wileyonlinelibrary.com

off-stoichiometric sample ( Figure 6 ). This indicates that the off-stoichiometric sample, Na 3.32 Fe 2.34 (P 2 O 7 ) 2 is more resistant to oxidation by moisture and CO 2 than Na 3.12 Fe 2.44 (P 2 O 7 ) 2 .

The electrochemical performance of Na 3.32 Fe 2.34 (P 2 O 7 ) 2 electrodes were next examined in Na ion batteries. As shown in the voltage profi les ( Figure 7 a), signifi cantly, the irrevers-ible capacity during the 1 st cycle is now negligible due to the change in surface composition. Also, Na 3.32 Fe 2.34 (P 2 O 7 ) 2 delivered a reversible capacity of about 85 mA h g − 1 with good coulombic effi ciency (99.0%), and showed very stable cycle performance with little capacity fading over 60 cycles (Figure 7 b). Theoretically, Na 3.32 Fe 2.34 (P 2 O 7 ) 2 + 0.1 NaFePO 4 ( i.e ., Na 3.42 Fe 2.44 (P 2 O 7 ) 2.05 ) can deliver 109.7 mA h g − 1 , assuming that 2.34 Na is reversibly de/intercalated via one-electron reaction per Fe atom because NaFePO 4 is electrochemically inactive. Above all, the material exhibits excellent rate capability ( Figure 8 ): even at a 10C rate, 72% reversible capacity at C/10 was deliv-ered, despite a range in particle size from hundreds of nm to a few μ m (inset of Figure 8 a). The fast kinetics are attributed to the large diffusion channel along the a -axis and weak Na-O bonding due to the coordination of sodium ions Na(1) and Na(2) to 8 and 7 oxygen atoms, respectively. [ 32 ] It is also notable that the sloping voltage profi les exhibits two steps at 2.5 and 3 V vs. Na/Na + . These arise from the four different Fe ion (redox center) sites which have different structural environments, sim-ilar to the case of the differing redox potential of the isomorphs of LiFeSO 4 F. [ 37 , 38 ] Iron atoms, Fe(1), Fe(2) and Fe(4) are located within distorted octahedra, but Fe(3) is located on a partially occupied fi ve-coordinate site. [ 32 ] The electrochemical perform-ance of both Na 3.12 Fe 1.22 Mn 1.22 (P 2 O 7 ) 2 and Na 3.12 Mn 2.44 (P 2 O 7 ) 2

mbH & Co. KGaA, Weinheim Adv. Energy Mater. 2013, 3, 770–776

www.MaterialsViews.comwww.advenergymat.de

FULL P

APER

Figure 2 . (a) SEM image and (b) voltage profi les of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 . Figure 3 . XPS profi les of stoichiometric Na 3.12 Fe 2.44 (P 2 O 7 ) 2 : (a) Na1s and (b) O1s.

Figure 4 . XRD patterns of stoichiometric Na 3.12 Fe 2.44 (P 2 O 7 ) 2 and off-stoichiometric Na 3.42 Fe 2.44 (P 2 O 7 ) 2.05 .

electrodes were also evaluated. However, as shown in Figures S1-2, FeMn (45 mA h g − 1 ) and Mn (10 mA h g − 1 ) analogues delivered much less reversible capacity than Na 3.12 Fe 2.44 (P 2 O 7 ) 2 due to the poor electrochemical activity of Mn. It is expected that the strategies of nanosizing and doping with Mg or Ca will provide improvement, and related work will be reported in a forthcoming paper.

The electrochemical reaction mechanism of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 was investigated through chemical sodiation and deso-diation. The sequential formation of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 → Na 2.12 Fe 2.44 (P 2 O 7 ) 2 → Na 1.12 Fe 2.44 (P 2 O 7 ) 2 → Na 2.12 Fe 2.44 (P 2 O 7 ) 2 → Na 3.12 Fe 2.44 (P 2 O 7 ) 2 was obtained via repetitive desodiation and sodiation using equimolar NOBF 4 and NaI, respectively. This series form a single phase composition as shown by XRD ( Figure 9 a), indicating that the Na 3.12 Fe 2.44 (P 2 O 7 ) 2 electrode proceeds in a one-phase reaction. The XRD peaks gradually and reversibly shift as x in Na 3.12-x Fe 2.44 (P 2 O 7 ) 2 increases and decreases, and no evidence for a two-phase mixture of the end member phases was observed. The formation of the single phase of Na 3.12−x Fe 2.44 (P 2 O 7 ) 2 was further confi rmed by Rietveld

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 773wileyonlinelibrary.comAdv. Energy Mater. 2013, 3, 770–776

© 2013 WILEY-VCH Verlag G774 wileyonlinelibrary.com

www.MaterialsViews.comwww.advenergymat.de

FULL

PAPER

Figure 5 . XPS depth profi les of stoichiometric Na 3.12 Fe 2.44 (P 2 O 7 ) 2 and off-stoichiometric Na 3.32 Fe 2.34 (P 2 O 7 ) 2 .

Figure 7 . Voltage profi les of (a) off-stoichiometric Na 3.32 Fe 2.34 (P 2 O 7 ) 2 and (b) cycle performance of off-stoichiometric Na 3.32 Fe 2.34 (P 2 O 7 ) 2 .

Figure 6 . XPS profi les off-stoichiometric Na 3.32 Fe 2.34 (P 2 O 7 ) 2 : (a) Na1s and (b) O1s.

refi nement, which gave good agreement factor values as shown in Figure 9 b-c and Tables S3-4. The volume change between the fully desodiated and sodiated samples was 1.9%, which is extremely small compared with other sodium-based phosphate materials including NaFePO 4 and Na 2 FePO 4 F. [ 17 , 26 ] The amount of chemical desodiation and sodiation in Na 3.12−x Fe 2.44 (P 2 O 7 ) 2 as a function of redox state was also confi rmed by ICP analysis (Figure 9 d). This is consistent with the refi nement results. The solid solution behavior was also confi rmed by the appearance of sloping voltage profi les (Figure 7 a).

3. Conclusion

In conclusion, we have demonstrated, for the fi rst time, the syn-thesis of new polyanion-based compounds, Na 3.12 M 2.44 (P 2 O 7 ) 2 (M = Fe, Fe 0.5 Mn 0.5 , Mn) as pure phase microcrystalline pow-ders and their electrochemical performance as a cathode material for Na ion batteries. The deliberate synthesis of the off-stoichiometric phase, Na 3.42 Fe 2.44 (P 2 O 7 ) 2.05 induced the for-mation of a Na-rich phase of Na 3.32 Fe 2.34 (P 2 O 7 ) 2 with NaFePO 4 ,

mbH & Co. KGaA, Weinheim Adv. Energy Mater. 2013, 3, 770–776

www.MaterialsViews.comwww.advenergymat.de

FULL P

APER

Figure 8 . (a) Rate performance of off-stoichiometric Na 3.32 Fe 2.34 (P 2 O 7 ) 2 (inset: SEM image of Na 3.32 Fe 2.34 (P 2 O 7 ) 2 ), and (b) the corresponding voltage profi les.

Figure 9 . (a) XRD patterns of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 via sequential chemical desodiation and sodiation. Rietveld refi nement of (b) Na 2.12 Fe 2.44 (P 2 O 7 ) 2 and (c) Na 1.12 Fe 2.44 (P 2 O 7 ) 2 . (d) Atomic composition (ICP-MS) of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 via sequential chemical desodiation and sodiation.

which showed particularly excellent cycle and rate performance, in spite of the fact that electrochemistry was performed without nanosizing or carbon coating. We attribute this to its surface properties. In addition, Na 3.12 Fe 2.44 (P 2 O 7 ) 2 exhibits a single phase reaction upon de/sodiation. It is expected that this mate-rial will provide an opportunity to gain a more in-depth under-standing of the kinetics of de/intercalation for Na ions.

4. Experimental Section Synthesis : Na 3.12 M 2.44 (P 2 O 7 ) 2 powders (M = Fe, Fe 0.5 Mn 0.5 , Mn)

were synthesized by a solid state reaction. Sodium carbonate (Na 2 CO 3 , ≥ 99.5%, Aldrich), iron oxalate dihydrate (FeC 2 O 4 · 2H 2 O, 99%, Aldrich), manganese acetate tetrahydrate ((CH 3 COO) 2 Mn · 4H 2 O, 99 + %, Aldrich) (as the manganese source), and ammonium hydrogen phosphate ((NH 4 ) 2 HPO 4 , 98 + %, Aldrich) were ball-milled in a stoichiometric molar ratio. The powders were heated at 300 ° C for 6h under an Ar atmosphere, and then at 600 ° C for 12h under an Ar atmosphere.

© 2013 WILEY-VCH Verlag GmAdv. Energy Mater. 2013, 3, 770–776

The off-stoichiometric synthesis (Na 3.42 Fe 2.44 (P 2 O 7 ) 2.05 ) was also performed through the same process as the stoichiometric synthesis, resulting in composites comprised of two phases: Na 3.32 Fe 2.34 (P 2 O 7 ) 2 + 0.1NaFePO 4 .

bH & Co. KGaA, Weinheim 775wileyonlinelibrary.com

776

www.MaterialsViews.comwww.advenergymat.de

FULL

PAPER

Chemical de/sodiation : The chemical oxidation (desodiation) of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 and Na 3.12− α Fe 2.44 (P 2 O 7 ) 2 ( α = ∼ 1) was accomplished using an equimolar amount of nitrosyl tetrafl uoroborate (NOBF 4 , 95%, Aldrich) in tetrahydrofuran (THF) to obtain Na 3.12− α Fe 2.44 (P 2 O 7 ) 2 ( α = ∼ 1) and Na 3.12− α − β Fe 2.44 (P 2 O 7 ) 2 ( α , β = ∼ 1), respectively. The chemical reduction (sodiation) of Na 3.12− α Fe 2.44 (P 2 O 7 ) 2 ( α = ∼ 1) and Na 3.12− α − β Fe 2.44 (P 2 O 7 ) 2 ( α , β = ∼ 1) was accomplished using an equimolar amount of NaI in THF to obtain Na 3.12− α Fe 2.44 (P 2 O 7 ) 2 ( α = ∼ 1) and Na 3.12 Fe 2.44 (P 2 O 7 ) 2 , respectively.

Material characterization : Powder X-Ray diffraction (XRD) data was collected on a Rigaku D/MAX2500V/PC powder diffractometer using Cu-K α radiation ( λ = 1.5405Å) operating from 2 θ = 10 − 80 o . Lattice parameters were determined using full pattern matching in the GSAS platform, followed by Rietveld refi nement. SEM samples were examined in a Quanta 200 fi eld-emission scanning electron microscope (FE-SEM) instrument equipped with an energy dispersive X-Ray spectroscopy (EDX) attachment. The atomic composition of the samples was determined by Varian 720-ES inductively coupled plasma (ICP) spectrometry. Surface analysis was examined with XPS (Thermo Fisher).

Electrochemical characterization : Samples of electrochemically active materials (70 wt.%) were mixed with carbon black (Super P, 15 wt.%) and Polyvinylidene fl uoride (PVdF, 15 wt.%). The electrochemical performance was evaluated using 2032 coin cells with a Na metal anode and 0.8 M NaClO 4 in an ethylene carbonate and diethyl carbonate (1:1 v/v) electrolyte solution. Galvanostatic experiments were performed at a current density of 4.8 mA g − 1 (ca. 0.05C) and 30 ° C. For rate performance, the charging current was fi xed at 0.1C and the discharging current was varied.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the MKE (The Ministry of Knowledge Economy), Korea (NIPA-2012-C1090-1200-0002), and by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2011-0027950). LFN thanks NSERC (Canada) for funding through the Discovery Grant and Canada Research Chair programs.

Received: October 15, 2012Published online: February 18, 2013

[ 1 ] S. W. Kim , D. H. Seo , X. H. Ma , G. Ceder , K. Kang , Adv. Energy Mater. 2012 , 2 , 710 .

[ 2 ] V. Palomares , P. Serras , I. Villaluenga , K. B. Hueso , J. Carretero-Gonzalez , T. Rojo , Energy Environ. Sci. 2012 , 5 , 5884 .

[ 3 ] M. M. Doeff , M. Y. Peng , Y. P. Ma , L. C. Dejonghe , J. Electrochem. Soc. 1994 , 141 , L145 .

[ 4 ] J. M. Tarascon , D. G. Guyomard , B. Wilkens , W. R. McKinnon , P. Barboux , Solid State Ionics 1992 , 57 , 113 .

[ 5 ] J. J. Braconnier , C. Delmas , C. Fouassier , P. Hagenmuller , Mater. Res. Bull. 1980 , 15 , 1797 .

[ 6 ] J. J. Braconnier , C. Delmas , P. Hagenmuller , Mater. Res. Bull. 1982 , 17 , 993 .

[ 7 ] J. M. Tarascon , G. W. Hull , Solid State Ionics 1986 , 22 , 85 .

© 2013 WILEY-VCH Verlag Gwileyonlinelibrary.com

[ 8 ] M. M. Doeff , Y. P. Ma , S. J. Visco , L. C. Dejonghe , J. Electrochem. Soc. 1993 , 140 , L169 .

[ 9 ] D. A. Stevens , J. R. Dahn , J. Electrochem. Soc. 2001 , 148 , A803 . [ 10 ] S. Komaba , W. Murata , T. Ishikawa , N. Yabuuchi , T. Ozeki ,

T. Nakayama , A. Ogata , K. Gotoh , K. Fujiwara , Adv. Funct. Mater. 2011 , 21 , 3859 .

[ 11 ] N. Yabuuchi , M. Kajiyama , J. Iwatate , H. Nishikawa , S. Hitomi , R. Okuyama , R. Usui , Y. Yamada , S. Komaba , Nat. Mater. 2012 , 11 , 512 .

[ 12 ] D. Kim , S. H. Kang , M. Slater , S. Rood , J. T. Vaughey , N. Karan , M. Balasubramanian , C. S. Johnson , Adv. Energy Mater. 2011 , 1 , 333 .

[ 13 ] S. Komaba , N. Yabuuchi , T. Nakayama , A. Ogata , T. Ishikawa , I. Nakai , Inorg. Chem. 2012 , 51 , 6211 .

[ 14 ] D. Kim , E. Lee , M. Slater , W. Q. Lu , S. Rood , C. S. Johnson , Electro-chem. Commun. 2012 , 18 , 66 .

[ 15 ] P. Senguttuvan , G. Rousse , V. Seznec , J. M. Tarascon , M. R. Palacin , Chem. Mater. 2011 , 23 , 4109 .

[ 16 ] Y. Park , D. S. Shin , S. H. Woo , N. S. Choi , K. H. Shin , S. M. Oh , K. T. Lee , S. Y. Hong , Adv. Mater. 2012 , 24 , 3562 .

[ 17 ] K. T. Lee , T. N. Ramesh , F. Nan , G. Botton , L. F. Nazar , Chem. Mater. 2011 , 23 , 3593 .

[ 18 ] P. Moreau , D. Guyomard , J. Gaubicher , F. Boucher , Chem. Mater. 2010 , 22 , 4126 .

[ 19 ] S. M. Oh , S. T. Myung , J. Hassoun , B. Scrosati , Y. K. Sun , Electro-chem. Commun. 2012 , 22 , 149 .

[ 20 ] J. Barker , M. Y. Saidi , J. L. Swoyer , Electrochem. Solid-State Lett. 2003 , 6 , A1 .

[ 21 ] R. K. B. Gover , A. Bryan , P. Burns , J. Barker , Solid State Ionics 2006 , 177 , 1495 .

[ 22 ] R. A. Shakoor , D. H. Seo , H. Kim , Y. U. Park , J. Kim , S. W. Kim , H. Gwon , S. Lee , K. Kang , J. Mater. Chem. 2012 , 22 , 20535 .

[ 23 ] H. Kim , I. Park , D.-H. Seo , S. Lee , S.-W. Kim , W. J. Kwon , Y.-U. Park , C. S. Kim , S. Jeon , K. Kang , J. Am. Chem. Soc. 2012 , 134 , 10369 .

[ 24 ] B. L. Ellis , W. R. M. Makahnouk , W. N. Rowan-Weetaluktuk , D. H. Ryan , L. F. Nazar , Chem. Mater. 2010 , 22 , 1059 .

[ 25 ] N. Recham , J. N. Chotard , L. Dupont , K. Djellab , M. Armand , J. M. Tarascon , J. Electrochem. Soc. 2009 , 156 , A993 .

[ 26 ] B. L. Ellis , W. R. M. Makahnouk , Y. Makimura , K. Toghill , L. F. Nazar , Nat. Mater. 2007 , 6 , 749 .

[ 27 ] Y. Kawabe , N. Yabuuchi , M. Kajiyama , N. Fukuhara , T. Inamasu , R. Okuyama , I. Nakai , S. Komaba , Electrochemistry 2012 , 80 , 80 .

[ 28 ] P. Serras , V. Palomares , A. Goni , I. G. de Muro , P. Kubiak , L. Lezama , T. Rojo , J. Mater. Chem. 2012 , 22 , 22301 .

[ 29 ] S. Nishimura , M. Nakamura , R. Natsui , A. Yamada , J. Am. Chem. Soc. 2010 , 132 , 13596 .

[ 30 ] P. Barpanda , T. Ye , S. Nishimura , S. C. Chung , Y. Yamada , M. Okubo , H. Zhou , A. Yamada , Electrochem. Commun. 2012 , 24 , 116 .

[ 31 ] H. Kim , R. A. Shakoor , C. Park , S. Y. Lim , J.-S. Kim , Y. N. Jo , W. Cho , K. Miyasaka , R. Kahraman , Y. Jung , J. W. Choi , Adv. Funct. Mater. 2012 , doi: 10.1002/adfm.201201589.

[ 32 ] J. Angenault , J. C. Couturier , M. Quarton , F. Robert , Eur. J. Solid State Inorg. Chem. 1995 , 32 , 335 .

[ 33 ] P. Barpanda , S. Nishimura , A. Yamada , Adv. Energy Mater. 2012 , 2 , 841 . [ 34 ] R. Wang , X. Q. Yu , J. M. Bai , H. Li , X. J. Huang , L. Q. Chen ,

X. Q. Yang , J. Power Sources 2012 , 218 , 113 . [ 35 ] A. T. Appapillai , A. N. Mansour , J. Cho , Y. Shao-Horn , Chem. Mater.

2007 , 19 , 5748 . [ 36 ] Z. H. Chen , J. R. Dahn , Electrochem. Solid-State Lett. 2004 , 7 , A11 . [ 37 ] P. Barpanda , M. Ati , B. C. Melot , G. Rousse , J. N. Chotard ,

M. L. Doublet , M. T. Sougrati , S. A. Corr , J. C. Jumas , J. M. Tarascon , Nat. Mater. 2011 , 10 , 772 .

[ 38 ] R. Tripathi , G. Popov , B. L. Ellis , A. Huq , L. F. Nazar , Energy Environ. Sci. 2012 , 5 , 6238 .

mbH & Co. KGaA, Weinheim Adv. Energy Mater. 2013, 3, 770–776