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Synthesis, Crystal Structure, and Properties of the Alluaudite-Type Vanadates Ag2−xNaxMn2Fe(VO4)3 Hamdi Ben Yahia,*,†,‡ Masahiro Shikano,*,‡ Mitsuharu Tabuchi,‡ and Ilias Belharouak† †

Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar Foundation, P.O. Box 5825, Doha, Qatar Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

‡

S Supporting Information *

ABSTRACT: The new members of the Ag2−xNaxMn2Fe(VO4)3 (0 ≤ x ≤ 2) solid solution were synthesized by a solid-state reaction route, and their crystal structures were determined from single-crystal X-ray diffraction data. The physical properties were characterized by Mössbauer and electrochemical impedance spectroscopies, galvanostatic cycling, and cyclic voltammetry. These materials crystallize with a monoclinic symmetry (space group C2/c), and the structure is considered to be a new member of the AA′MM′2(XO4)3 alluaudite family. The A, A′, M, and X sites are fully occupied by Ag+/Na+, Ag+/Na+, Mn2+, and V5+, respectively, whereas a Mn2+/Fe3+ mixture is observed in the M′ site. The Mössbauer spectra confirm that iron is trivalent. The impedance measurements indicate that the silver phase is a better conductor than the sodium phase. Furthermore, these phases exhibit ionic conductivities 2 orders of magnitude higher than those of the homologous phosphates. The electrochemical tests prove that Na2Mn2Fe(VO4)3 is active as positive and negative electrodes in sodium-ion batteries.

1. INTRODUCTION The minerals of the alluaudite group are phosphates with the general formula AA′MM′2(PO4)3. In these natural alluaudites, the large crystallographic A and A′ sites are occupied by Na+, Ca2+, Mn2+, or □ (vacancy), and the distorted octahedral M and M′ sites are occupied by Mn2+, Fe2+, Fe3+, Al3+, or Mg2+. Numerous synthetic alluaudite compounds also exist, and the general formula becomes AA′MM′2(XO4)3 where X = As, Mo, P, or V; M is a large six-coordinated cation such as Ca, Cd, Co, Cu, Fe, In, Li, Mg, Mn, Na, Ni, or Zn; M′ is a small sixcoordinated cation such as Al, Cd, Co, Cu, Fe, In, Mg, Mn, Ni, V, or Zn; A = Ag, Cu, H2O, K, Li, Na, or □; and A′ = Ag, Ca, Cd, Cu, Fe, H, Li, Na, Pd, or □. The alluaudite-type structure that was found for natural minerals was first reported by Fisher in 1955.1 Later on, Moore determined the crystal structure of the alluaudite in the monoclinic system with the C2/c space group.2 The structure consists of infinite chains of edge-sharing MO6 and M′O6 octahedra. These chains are linked by XO4 tetrahedra to form a three-dimensional architecture with two sets of tunnels in which the A and A′ cations are located. When A and A′ stand for Li or Na, the material becomes very interesting as an insertion material for rechargeable batteries. Indeed, the electrochemical properties of several alluaudite compounds have been reported during the past decade. Among those, only phosphates such as Na2M2Fe(PO4)3 (M = Mn, Fe, Co, and Ni)3−5 or sulfates such as Na2+xM2−y(SO4)3 (M = Mn and Fe)6,7 were studied. To our knowledge, no alluaudite vanadates have yet been reported as electrodes for Li- or Naion batteries. It seems that when P5+ or S6+ is replaced by V5+, © XXXX American Chemical Society

which is much larger, the cell volume increases, and the tunnels containing the A and A′ cations become too large for Li (Rionic = 0.92 Å) and Na (Rionic = 1.18 Å). Therefore, we believe that the preparation of alluaudite vanadates would be more successful using larger A and A′ cations such as Ag (Rionic = 1.28 Å) which could be gradually replaced by sodium. In this context, it is worth mentioning that, based on the Inorganic Crystal Structure Database (ICSD), only 14 alluaudite compounds containing silver cations have been reported,8−19 and all of them are phosphates and arsenates (Table S1). On the basis of the aforementioned idea, we successfully prepared the first alluaudite vanadate containing silver, Ag2Mn3(VO4)3,20 and herein we report on the solid-state synthesis of the Ag2−xNaxMn2Fe(VO4)3 solid solution. The samples were characterized by powder and single-crystal X-ray diffraction (XRD), Mössbauer and electrochemical impedance spectroscopies, cyclic voltammetry, and galvanostatic cycling measurements.

2. EXPERIMENTAL SECTION 2.1. Synthesis. The new compounds Ag2−xNaxMn2Fe(VO4)3 were prepared by a solid-state reaction route from a stoichiometric mixture of Ag2O, Na2CO3, MnO, Fe2O3, and V2O5. The mixtures were ground, pelletized, and fired at 600 °C for 8 h under argon. When the pellets were fired at 600 °C for a longer time (100 h), large black single Received: February 28, 2016

A

DOI: 10.1021/acs.inorgchem.6b00486 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. SEM image of the single crystals used for the data collections.

Table 1. Crystallographic Data and Structure Refinement for Ag2−xNaxMn2Fe(VO4)3 (x = 0, 5/4, and 2) Na2Mn2FeV3O12 Mr crystal system, space group temperature (K) a (Å) b (Å) c (Å) β (deg) V (Å3) Z radiation type μ (mm−1) crystal size (mm) diffractometer absorption correction Tmin, Tmax no. of measured reflections no. of independent reflections no. of observed [I > 0σ(I)] reflections Rint (sin θ/λ)max (Å−1) R[F2 > 2σ(F2)], wR(F2), S no. of reflections no. of parameters no. of restraints Δρmax, Δρmin (e Å−3)

Crystal Data 556.5 monoclinic, C2/c 293 12.0067(19) 12.9563(18) 6.8446(11) 111.793(3) 988.7(3) 4 Mo Kα 6.78 0.17 × 0.06 × 0.03 Data Collection SMART APEX Gaussian 0.558, 0.799 2785 1101 1101 0.043 0.656 Refinement 0.028, 0.064, 1.01 1101 94 0 0.58, −0.53

crystals were obtained for the Ag2−xNaxMn2Fe(VO4)3 (x = 0, 5/4, and 2) compositions. 2.2. Electron Microprobe Analysis. Semiquantitative EDX analyses of different single crystals, including the ones investigated on the diffractometer, were carried out with a Genesis (EDAX) analyzer installed on a JSM-500LV (JEOL) scanning electron microscope (SEM). The experimentally observed compositions were close to the ideal ones: Na2Mn2Fe(VO4)3, Ag3/4Na5/4Mn2Fe(VO4)3, and Ag2Mn2Fe(VO4)3 (Figures 1 and S1). 2.3. X-ray Diffraction Measurements. To check the purity of the Ag2−xNaxMn2Fe(VO4)3 powders, routine powder XRD measurements were performed. The data were collected at room temperature over the 2θ angle range 10° ≤ 2θ ≤ 80° with a step size of 0.01° using a RINT2000-TTR (Rigaku) diffractometer operating with Cu Kα radiation. Single crystals of Ag2−xNaxMn2Fe(VO4)3 (x = 0, 5/4, and 2) suitable for XRD were selected on the basis of the size and sharpness of the diffraction spots. Data collection was carried out on a SMART APEX (Bruker) diffractometer using Mo Kα radiation. Data processing

Ag3/4Na5/4Mn2FeV3O12

Ag2Mn2FeV3O12

620.01 monoclinic, C2/c 293 12.0063(13) 12.9364(14) 6.8342(7) 111.7809(16) 985.70(18) 4 Mo Kα 8.21 0.16 × 0.08 × 0.06

726.3 monoclinic, C2/c 293 12.0028(13) 12.9761(14) 6.8355(7) 111.665(2) 989.42(18) 4 Mo Kα 6.78 0.37 × 0.10 × 0.08

SMART APEX Gaussian 0.412, 0.636 2777 1087 1087 0.072 0.656

SMART APEX Gaussian 0.186, 0.453 2714 1042 1042 0.021 0.658

0.028, 0.065, 0.99 1087 96 0 0.57, −0.72

0.021, 0.055, 1.18 1042 95 0 0.43, −0.48

and all refinements were performed with the Jana2006 program package.21 Gaussian-type absorption corrections were applied, and the crystal shapes were determined with the video microscope. For data collection details, see Table 1. 2.4. 57Fe Mö ssbauer Spectroscopy. To confirm the +3 oxidation state of iron in the Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2) samples, the 57Fe Mössbauer spectra were collected with a 57Co γray source. The velocity calibration was done at room temperature with a high-purity α-Fe absorber. The experimental data were fitted to a symmetric doublet consisting of Lorentzian lines using the computer program Mosswinn 3.0. The isomer shift (IS) values are given relative to that of α-Fe at room temperature. 2.5. Electrochemical Cycling. Electrodes were made from mixtures of Na2Mn2Fe(VO4)3 powder, acetylene black (AB), and polyvinylidene fluoride (PVDF) binder dissolved in N-methylpyrrolidone in a weight ratio of 64:30:6. The obtained slurry was mixed for 1 h and coated onto Al and Cu foils. The resulting electrode films were pressed with a twin roller, cut into round plates (14 mm in diameter), and dried at 135 °C for 12 h under vacuum. Na2Mn2Fe(VO4)3/ B


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Inorganic Chemistry Table 2. Crystallographic Data for Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2) Powder Samples composition

T (K)

Ag2Mn2Fe(VO4)3 AgNaMn2Fe(VO4)3 Na2Mn2Fe(VO4)3

rt rt rt

PXRD PXRD PXRD

a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

12.01693(16) 12.0033(4) 11.9940(3)

12.9903(2) 12.9766(5) 12.9517(3)

6.84787(12) 6.8559(3) 6.8517(2)

111.6140(11) 111.693(2) 111.904(3)

993.81(4) 992.26(8) 988.20(5)

Figure 2. Powder XRD patterns of the Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2) samples. NaClO4+PC/Na coin-type cells (CR2032) were assembled in an argon-filled glovebox with polypropylene as a separator. CC testing was performed using the BTS2003H (Nagano Co., Ltd.) battery tester system in a potential ranges of 1.3−4.7 V at a rate of C/10 (that is, one sodium per unit formula in 10 h, 48 mAh/g) and 0.03−2.6 V at a rate of C/10. The cyclic voltammetry (CV) measurements were carried out with a PARSTAT 2263-system in the potential range of 1.3−4.7 V at a scan rate of 0.5 mV/s (at room temperature). 2.6. Electrochemical Impedance Spectroscopy. AC impedance measurements were carried out using a Solartron 1260, covering the frequency range 10−2−107 Hz with an applied voltage of 100 mV. Prior to the measurements, the pellets were coated with Au. The samples were measured over the temperature range 25−300 °C with equilibration periods of 30 min at each temperature. Total conductivity values for Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2), corrected for pellet geometry, were calculated from the intercepts of the low and high frequency electrode spikes on the real axis of the complex impedance plane plots.

should mention, however, that the intensity of the peaks changed significantly (see the (020) peak in Figure 2). 3.1.2. Single-Crystal X-ray Diffraction. The extinction conditions observed for Na2Mn2Fe(VO4)3 and Ag2Mn2Fe(VO4)3 were compatible with space groups C2/c and Cc. The structures were solved in the centro-symmetric space group C2/c. Most of the atomic positions were located using the SIR2004 program.22 The use of difference-Fourier synthesis enabled us to locate the remaining oxygen atomic positions. The refined atomic positions and anisotropic displacement parameters (ADPs) are given in Tables 3 and S2 of the Supporting Information, respectively. One may notice slightly large ADPs for the sodium and silver atoms. This is probably due to the high mobility of the sodium and silver atoms. The refinement of the occupancy of the heavy atoms did not show any significant deviation from the nominal composition; therefore, the ratio of Mn2/Fe2 has been fixed to 0.5/0.5 at the 8f position (Table 3). The final residual factors are given in Table 1. The structure of Na2Mn2Fe(VO4)3 was used as the starting model for the refinement of Ag3/4Na5/4Mn2Fe(VO4)3. As expected, the difference-Fourier synthesis showed large residues close to the Na1 and Na2 atomic positions, indicating the presence of silver atoms. Through the introduction of a Na/Ag disorder in both sites and the application of restrictions (Naocc + Agocc = 1, Naposition = Agposition, and NaADP = AgADP), the final residual factors converged to the values given in Table 1. 3.2. Crystal Structure. The Ag 2−x Na x Mn 2 Fe(VO 4 ) 3 compounds are isostructural with Na2Fe3(PO4)3.23 The structures consist of MnO4 chains of edge-sharing MnO6 octahedra running along [10−1] (Figure 3b). The MnO4 infinite chains are cross-connected by the VO4 tetrahedra, giving rise to channels along [001] in which the eightcoordinated silver and/or sodium atoms are located (Figure 3a). Interatomic distances and bond valence sums (BVSs)24,25 are listed in Table 4. In Na2Mn2Fe(VO4)3, the Mn1−O distances range from 2.163 to 2.214 Å with an average distance of 2.189 Å. This is

3. RESULTS AND DISCUSSION 3.1. Structure Refinement. 3.1.1. Powder X-ray Diffraction. The full pattern-matching refinements were performed with the Jana2006 program package using powder XRD data.21 The backgrounds were estimated by a Legendre function, and the peak shapes were described by a pseudoVoigt function. Evaluation of these data revealed the refined cell parameters listed in Table 2, which are in good agreement with the single-crystal data listed in Table 1. Our analysis confirms that a full-solid solution exists for Ag2−xNaxMn2Fe(VO4)3 (0 ≤ x ≤ 2). The replacement of the eight-coordinated silver (Rionic = 1.28 Å) by sodium (Rionic = 1.18 Å) did not induce significant changes in the cell parameter a, b, or c; however, a slight decrease in the cell volume from 993.81 to 988.20 Å3 was observed (powder XRD data). This is mainly due to the distortion of the β angle, which increases from 111.614° to 111.904° (sin β decreases) for Ag2 Mn2 Fe(VO4 ) 3 and Na2Mn2Fe(VO4)3. This explains why, on the powder patterns, the positions of the diffraction peaks of the sodium phase did not shift much as compared to those of the silver phase. One C


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Inorganic Chemistry Table 3. Atom Positions and Isotopic ADP (Å2) for Ag2−xNaxMn2Fe(VO4)3 (x = 0, 5/4, and 2) atom

Wyckoff

symm

Na1 Na2 Mn1 Mn2/Fe2 V1 V2 O1 O2 O3 O4 O5 O6

4b 4e 4e 8f 4e 8f 8f 8f 8f 8f 8f 8f

−1 2 2 1 2 1 1 1 1 1 1 1

Na1/Ag1 Na2/Ag2 Mn1 Mn2/Fe2 V1 V2 O1 O2 O3 O4 O5 O6

4b 4e 4e 8f 4e 8f 8f 8f 8f 8f 8f 8f

−1 2 2 1 2 1 1 1 1 1 1 1

Ag1 Ag2 Mn1 Mn2/Fe2 V1 V2 O1 O2 O3 O4 O5 O6

4b 4e 4e 8f 4e 8f 8f 8f 8f 8f 8f 8f

−1 2 2 1 2 1 1 1 1 1 1 1

x

occ

Na2Mn2Fe(VO4)39 1/2 1/2 0 0.5/0.5 0.21167(4) 0 0.73062(5) 0.6616(2) 0.1614(2) 0.7172(2) 0.8786(2) 0.0397(2) 0.1104(2) Ag3/4Na5/4Mn2Fe(VO4)3 0.505(3)/0.495(3) 1/2 0.747(3)/0.253(3) 1/2 0 0.5/0.5 0.21096(4) 0 0.73068(4) 0.6630(2) 0.16235(19) 0.71738(19) 0.87797(19) 0.03962(18) 0.1118(2) Ag2Mn2Fe(VO4)3 1/2 1/2 0 0.5/0.5 0.21009(3) 0 0.72982(4) 0.6618(2) 0.16183(17) 0.71711(17) 0.87679(17) 0.03926(16) 0.11280(18)

y

z

Ueq (Å2)

0 0.4938(2) 0.26739(6) 0.15837(4) 0.29125(6) 0.38889(4) 0.50647(18) 0.17165(18) 0.31800(18) 0.40257(19) 0.21951(19) 0.3746(2)

0 1/4 1/4 0.12426(7) 3/4 0.12144(8) 0.1010(4) 0.3810(3) 0.3258(3) 0.1754(4) 0.9801(3) 0.7564(4)

0.0158(6) 0.0497(11) 0.0133(2) 0.01237(17) 0.0127(3) 0.01186(19) 0.0208(9) 0.0170(8) 0.0187(8) 0.0222(9) 0.0157(8) 0.0249(9)

0 0.49065(9) 0.26676(5) 0.15851(3) 0.29190(5) 0.38859(4) 0.50705(17) 0.17221(16) 0.31749(17) 0.40103(17) 0.22013(16) 0.37457(18)

0 1/4 1/4 0.12310(7) 3/4 0.12101(8) 0.1016(4) 0.3807(3) 0.3261(3) 0.1735(4) 0.9803(3) 0.7581(4)

0.0273(3) 0.0317(5) 0.0120(2) 0.01088(17) 0.0110(2) 0.01056(18) 0.0197(8) 0.0161(7) 0.0174(8) 0.0201(8) 0.0147(7) 0.0213(8)

0 0.48995(3) 0.26720(4) 0.15891(3) 0.29312(4) 0.38818(3) 0.50708(14) 0.17310(13) 0.31710(14) 0.40015(15) 0.22099(14) 0.37447(14)

0 1/4 1/4 0.12255(5) 3/4 0.12081(5) 0.1022(4) 0.3808(3) 0.3256(3) 0.1701(3) 0.9798(2) 0.7593(3)

0.02764(15) 0.03462(16) 0.01123(17) 0.01043(14) 0.01010(19) 0.00971(15) 0.0202(7) 0.0158(6) 0.0164(6) 0.0192(6) 0.0146(5) 0.0191(6)

Figure 3. Projection view along [001] of the structure of Na2Mn2Fe(VO4)3 (a), the view along [101] of the MnO4 infinite chains of edge-sharing octahedra (b), and the coordination spheres of the Na1 (c) and Na2 cations (d).

The vanadium tetrahedra are quite regular with distances ranging from 1.698 to 1.736 Å and from 1.684 to 1.732 Å with average values of 1.717 and 1.716 Å for V1 and V2, respectively (Tables 4 and S3). The BVS values of 5.052 and 5.076 for V1 and V2, respectively, are in agreement with the expected value of 5 for V5+. The Na1+ and Na2+ ions are bonded to eight oxygen atoms belonging to six and four different MnO 6 octahedra,

very similar to the results found for Na2MnFe2(PO4)3 in which the Mn1−O distances range from 2.143 to 2.231 Å with an average distance of 2.181 Å.26 The Mn2−O distances range from 2.029 to 2.215 Å with an average distance of 2.089 Å. This is very similar to the average Mn2−O distance of 2.092 Å found in Ag1.5Mn3(AsO4)3.19 The BVS values of 2.072 and 2.62 are in good agreement with the expected values of +2 for Mn12+ (d5) and +2.5 for Mn22+/3+, respectively. D


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Inorganic Chemistry

agreement with the larger ionic radius of silver (Rionic = 1.28 Å) as compared to that of sodium (Rionic = 1.18 Å). This is perhaps why higher ionic conductivity was observed for Ag2Mn2Fe(VO4)3 (see section 3.5). The tunnels running along the c axis containing silver atoms are slightly larger, which enables faster diffusion of the Ag+ ions. One should notice that the increase in silver content induces a decrease in the average Mn1−O and Fe2/Mn2−O distances from 2.189 to 2.182 Å and from 2.089 to 2.083 Å, respectively. This should affect the quadrupolar splitting values obtained from the Mössbauer spectra (see section 3.3 below). 3.3. Mö ssbauer Spectroscopy of Ag2−xNaxMn2Fe(VO4)3. The 57Fe Mössbauer spectra of Ag2−xNaxMn2Fe(VO4)3, measured at room temperature, are given in Figure 4. All of the

Table 4. Interatomic Distances (Å) and BVS Values for Ag2−xNaxMn2Fe(VO4)3 (x = 0, 5/4, and 2)a x = 2 distances Na1/Ag1−O6 (×2) Na1/Ag1−O4 (×2) Na1/Ag1−O4 (×2) Na1/Ag1−O6 (×2) Na1/Ag1 Na2/Ag2−O1 (×2) Na2/Ag2−O1 (×2) Na2/Ag2−O2 (×2) Na2/Ag2−O5 (×2) Na2/Ag2 Mn1−O5 (×2) Mn1−O2 (×2) Mn1−O4 (×2) Mn1 Fe2/Mn2−O6 Fe2/Mn2−O1 Fe2/Mn2−O2 Fe2/Mn2−O5 Fe2/Mn2−O3 Fe2/Mn2−O3 Fe2/Mn2 V1−O5 (×2) V1−O6 (×2) V1 V2−O4 V2−O1 V2−O3 V2−O2 V2

x = 5/4 distances

x = 0 distances

2.398(2)

2.406(2)

2.4201(17)

2.500(2)

2.518(2)

2.547(2)

2.542(3)

2.546(3)

2.5473(18)

2.970(3)

2.970(3)

2.973(2)

⟨2.602⟩ 1.062 [8]b

⟨2.622⟩ 1.108 [8]b

2.465(2)

⟨2.610⟩ 1.036/1.151 [8]b 2.472(2)

2.509(3)

2.525(3)

2.509(3)

2.928(3)

2.970(2)

2.9897(19)

3.129(4)

3.095(2)

3.1041(19)

⟨2.758⟩ 0.782 [8]b

⟨2.766⟩ 0.758/0.842 [8]b 2.154(2) 2.191(2) 2.206(2) ⟨2.183⟩ 2.073 [6]b 2.022(2) 2.032(2) 2.058(3) 2.081(2) 2.083(2) 2.206(2) ⟨2.080⟩ 2.773/2.550 [6]b 1.735(2) 1.702(3) ⟨1.718⟩ 5.037 [4]b 1.677(2) 1.717(2) 1.733(2) 1.729(2) ⟨1.714⟩ 5.102 [4]b

⟨2.769⟩ 0.783 [8]b

2.163(2) 2.192(2) 2.214(3) ⟨2.189⟩ 2.041 [6]b 2.029(3) 2.047(2) 2.067(3) 2.086(2) 2.090(3) 2.215(2) ⟨2.089⟩ 2.706/2.489 [6]b 1.736(2) 1.698(3) ⟨1.717⟩ 5.052 [4]b 1.684(3) 1.715(3) 1.731(3) 1.732(2) ⟨1.716⟩ 5.076 [4]b

2.473(2)

2.153(2) 2.1865(19) 2.206(2) ⟨2.182⟩ 2.085 [6]b 2.023(2) 2.0437(19) 2.059(2) 2.0804(18) 2.086(2) 2.2076(19) ⟨2.083⟩ 2.528/2.749 [6]b 1.7376(17) 1.699(2) ⟨1.718⟩ 5.034 [4]b 1.676(2) 1.728(2) 1.730(2) 1.7324(16) ⟨1.717⟩ 5.062 [4]b

Figure 4. 57Fe Mössbauer spectrum of Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2) at room temperature.

experimental spectra were fitted using one distribution corresponding to a trivalent iron atom in the octahedral environment. The characteristic parameters deduced from this refinement, the isomeric shift (δ), the full width at halfmaximum (Γ), and quadruple splitting (Δ), are given in Table 5. The obtained isomer shift values, which are identical in the Table 5. Fitted 57Fe Mössbauer Parameters Obtained for the Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2) Phasesa compound

a

Average distances are given in brackets. bBond valence sum (BVS) = exp{(r0−r)/b} with the following parameters: b = 0.37, r0 (AgI−O) = 1.842, r0 (NaI−O) = 1.803, r0 (MnII−O) = 1.790, r0 (FeIII−O) = 1.759, and r0 (VV−O) = 1.803 Å.24,25

x

δ (mm s−1)

Γ (mm s−1)

Δ (mm s−1)

Ag2Mn2Fe(VO4)3

0

0.395(4)

0.314(6)

0.494(6)

AgNaMn2Fe(VO4)3

1

0.393(2)

0.322(4)

0.526(3)

Na2Mn2Fe(VO4)3

2

0.394(2)

0.331(5)

0.535(3)

site 3+

Fe [Oh] Fe3+ [Oh] Fe3+ [Oh]

δ is the isomer shift; Γ is the full width at half-maximum, and Δ is the quadrupole splitting.

a

respectively (Figures 3c and d and Figure S2). The BVS values of 1.062 and 0.782 for Na1 and Na2, respectively, are in good agreement with the expected value of 1 for Na+. One should notice that Na2 remains underbonded even after increasing the coordination sphere to 3.2 Å. The full replacement of the eight-coordinated sodium by silver did not induce any significant changes to the average V1− O and V2−O distances or O−V−O angles, which remain almost identical (Tables 4 and S3). However, the average Na1/ Ag1−O and Na2/Ag2−O distances increased from 2.602 to 2.622 Å and from 2.758 to 2.769 Å, respectively. This is in good

three new compounds, are typical values obtained for high-spin Fe3+ in an octahedral site.27 The quadrupole splitting values decrease from 0.535 to 0.494 mm/s in Na2Mn2Fe(VO4)3 and Ag2Mn2Fe(VO4)3. This indicates that deformations of the FeO6 octahedra occur when the silver content increases. Indeed, although the average Fe2−O interatomic distances remain almost identical in Ag2Mn2Fe(VO4)3 and Na2Mn2Fe(VO4)3, E


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Inorganic Chemistry the individual Fe2−O distances increase with the increase in sodium content. 3.4. Electrochemical Properties. The electrochemical behavior of Na2Mn2Fe(VO4)3 as a positive electrode material is depicted in Figure S3. When the sample was charged to 4.7 V in a Na cell, a charge capacity of 199 mAh/g was observed. However, during the first discharge, 1.8 Na ions were intercalated into the structure, providing a discharge capacity of 86 mAh/g. Because only 96 mAh/g was expected for the extraction of two sodium atoms, we conclude that the NaClO4 electrolyte decomposed at 4.7 V. Consequently, no further cycling could be performed. When the sample was discharged first to 1.3 V in a Na cell, a charge capacity of 35 mAh/g was observed. This indicates that the structure is flexible and may intercalate additional sodium atoms, leading to the final composition close to NaMn2/3Fe1/3VO4. Consequently, during the second cycle, more than two sodium atoms could be exchanged, providing charge and discharge capacities of 125 and 100 mAh/g, respectively. One should mention that no optimizations of particle size, morphology, or carbon coating could be performed on Na2Mn2Fe(VO4)3. Therefore, tuning these parameters and the electrolyte composition should significantly improve the electrochemical performance of Na2Mn2Fe(VO4)3. Figure S4 shows the charge/discharge curves of a Na2Mn2Fe(VO4)3/NaClO4−PC/Na half-cell between 0.03 and 2.6 V at a rate of C/10. The first discharge capacity of 440 mAh/g corresponds to the reaction of nine sodium atoms. This capacity is much lower than the theoretical value of 1059 mAh/ g expected for the reduction of one Fe3+ to Fe0, two Mn2+ to Mn0, and three V5+ to V0. Therefore, it remains unclear whether the material undergoes an intercalation/conversion reaction. Further experiments are required to fully understand the insertion/deintercalation mechanisms. This will be the subject of a future work. 3.5. Electrochemical Impedance Spectroscopy of Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2). A typical Nyquist diagram of Na2Mn2Fe(VO4)3 measured on a symmetrical cell under argon at 50 °C is shown in Figure 5. The electrolyte

Figure 6. Arrhenius plots of the ionic conductivities σ of the Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2) materials in argon.

Ag2−xNaxMn2Fe(VO4)3 (x = 0, 1, and 2)/Au) cells. For a comparison to this experiment, the T dependence of the ionic conductivity in terms of the Arrhenius relation σT = B exp(−Ea/kT) quantifies the activation energy Ea. Here, k and B denote the Boltzmann constant and a temperature-independent constant, respectively. In the temperature range 100−300 °C, the three samples exhibit apparent Arrhenius-type T dependencies of Na 2 Mn 2 Fe(VO 4 ) 3 , NaAgMn 2 Fe(VO 4 ) 3 , and Ag2Mn2Fe(VO4)3 conductivities, with Ea values of 0.51, 0.51, and 0.39 eV, respectively. These values are lower than those observed for the homologous phosphates Ag2−xNaxMn2Fe(PO4)3, but the ionic conductivities are 2 orders of magnitude higher (Table 6). When P5+ is replaced by V5+ in Na2Mn2FeTable 6. Activation Energies and Conductivities at 548 K for the Homologous Compounds Ag2−xNaxMn2Fe(VO4)3 and Ag2−xNaxMn2Fe(PO4)3 compound

Ea (eV)

Na2Mn2Fe(VO4)3 NaAgMn2Fe(VO4)3 Ag2Mn2Fe(VO4)3 Na2Mn2Fe(PO4)3 NaAgMn2Fe(PO4)3 Ag2Mn2Fe(PO4)3

0.51 0.51 0.39 0.77 0.58 0.59

σ (548 K) 1.7 2.9 9.7 9.7 3.5 2.9

× × × × × ×

10−4 10−4 10−4 10−7 10−6 10−5

ref this work this work this work 11 11 11

(PO4)3, the cell volume increases from 900.3 to 988.7 Å3, which induces an increase in the average Na1−O and Na2−O distances from 2.5375 and 2.697 Å to 2.602 and 2.758 Å, respectively. From a structural point of view, this corresponds to an enlargement of the channels running along the c axis (Figure 3a), which explains the higher ionic conductivity of the vanadates compared to those of the phosphates.

4. CONCLUSION During our hunt for new electrode materials for sodium-ion batteries, the Ag2O−Na2O−Fe2O3−MnO−V2O5 system was explored by a solid-state reaction route, and the existence of a full solid solution Ag2−xNaxMn2Fe(VO4)3 (0 ≤ x ≤ 2) was confirmed for the first time using single-crystal X-ray diffraction. The gradual replacement of Ag by Na led to the formation of the end member Na2Mn2Fe(VO4)3, which is active as a positive and negative electrode in sodium-ion batteries. Further improvements of the electrochemical properties are possible because no optimizations of particle size,

Figure 5. Typical complex impedance (Z) spectra recorded in air for the asymmetrical cells of (Au/Na2Mn2Fe(VO4)3/Au). The numbers above the solid squares and circles indicate the frequencies in hertz.

resistance was deduced from the low frequency intercept along the real axis of the Cole−Cole plots to calculate the ionic conductivity σ. The relation used for the calculations was σ = l/ (RB × S) where l, S, and RB correspond to the electrolyte thickness, the electrolyte cross-section area, and the bulk resistance, respectively. Figure 6 shows the plots of log(σ) as a function of the inverse absolute temperature T−1 for the (Au/ F


Article

Inorganic Chemistry

(7) Dwibedi, D.; Araujo, R. B.; Chakraborty, S.; Shanbogh, P. P.; Sundaram, N. G.; Ahuja, R.; Barpanda, P. J. Mater. Chem. A 2015, 3 (36), 18564−18571. (8) Dietrich, V.; Pitzschke, D.; Jansen, M. Z. Kristallogr. - New Cryst. Struct. 2011, 226, 7−8. (9) Strelkov, M. A.; Zhizhin, M. G.; Komissarova, L. N. J. Solid State Chem. 2006, 179, 3664−3671. (10) Chouaibi, N.; Daidouh, A.; Pico, C.; Santrich, A.; Veiga, M. L. J. Solid State Chem. 2001, 159, 46−50. (11) Daidouh, A.; Durio, C.; Pico, C.; Veiga, M. L.; Chouaibi, N.; Ouassini, A. Solid State Sci. 2002, 4, 541−548. (12) Kacimi, M.; Ziyad, M.; Hatert, F. Mater. Res. Bull. 2005, 40, 682−693. (13) Leroux, F.; Mar, A.; Guyomard, D.; Piffard, Y. J. Solid State Chem. 1995, 117, 206−212. (14) Guesmi, A.; Driss, A. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2002, 58, i16−i17. (15) Ben Smail, R.; Jouini, T. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2002, 58, i61−i62. (16) Assani, A.; Saadi, M.; Zriouil, M.; El Ammari, L. Acta Crystallogr., Sect. E: Struct. Rep. Online 2011, 67, i5−i5. (17) Stock, N.; Bein, T. Solid State Sci. 2003, 5, 1207−1210. (18) Keller, P.; Riffel, H.; Zettler, F.; Hess, H. Z. Anorg. Allg. Chem. 1981, 474, 123−134. (19) Brahim, A.; Amor, H. Acta Crystallogr., Sect. E: Struct. Rep. Online 2003, 59, i77−i79. (20) Ben Yahia, H.; Shikano, M.; Essehli, R.; Belharouak, I. Z. Kristallogr. - Cryst. Mater. 2016, DOI: 10.1515/zkri-2016-1930. (21) Petricek, V.; Dusek, M.; Palatinus, L. Z. Kristallogr. - Cryst. Mater. 2014, 229, 345−352. (22) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2003, 36, 1103−1103. (23) Yakubovich, O. V.; Simonov, M. A.; Egorov Tismenko, Y. K.; Belov, N. V. Acta Crystallogr., Sect. A: Found. Crystallogr. 1977, 236, 1123−1126. (24) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247. (25) Brese, N. E.; O’Keefe, M. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192−197. (26) Hatert, F.; Rebbouh, L.; Hermann, R. P.; Fransolet, A. M.; Long, G. J.; Grandjean, F. Am. Mineral. 2005, 90, 653−662. (27) Menil, F. J. Phys. Chem. Solids 1985, 46, 763−789.

morphology, carbon coating, or electrolyte composition could be performed during our experiments. These vanadates exhibited higher ionic conductivities compared to those of the homologous phosphates. Indeed, the replacement of P5+ by V5+ induced an increase in the cell volume and an enlargement of the tunnels containing the Na+/Ag+ ions. This improved the diffusion in the tunnels. Na2Mn2Fe(VO4)3 would be the first reported vanadate compound crystallizing with the alluauditetype structure which is active in Na-ion batteries. This opens new perspectives in the search for not only new electrode materials for sodium-ion batteries but also solid electrolytes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00486. Alluaudite compounds containing silver cations (Table S1), anisotropic displacement parameters (Å2) for Ag2−xNaxMn2Fe(VO4)3 (x = 0, 5/4, and 2) (Table S2), angles for Ag2−xNaxMn2Fe(VO4)3 (x = 0, 5/4, and 2) (Table S3), EDX analyses of the single crystal used for the data collection (Figure S1), coordination spheres of the Na1 and Na2 cations in Na2Mn2Fe(VO4)3 (Figure S2), and the charge/discharge and cyclic voltammogram (CV) curves of Na2Mn2Fe(VO4)3 (Figures S3 and S4) (PDF) Crystallographic information for Ag 2 Mn 2 Fe(VO 4 ) 3 (CIF) Crystallographic information for Ag3/4Na5/4Mn2Fe(VO4)3 (CIF) Crystallographic information for Na2 Mn2 Fe(VO 4 ) 3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was supported by the Grant-in-Aid program of the Japan Society for the Promotion of Science (JSPS) KAKENHI Fellows Grant 24-02506. We are also grateful to Maxim Avdeev and Chris D. Ling for the collection of the neutron diffraction and magnetic susceptibility data at ANSTO and the University of Sydney, respectively, which will be published elsewhere.

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REFERENCES

(1) Fisher, D. J. Am. Mineral. 1955, 40, 1100−1109. (2) Moore, P. B. Am. Mineral. 1971, 56, 1955−1975. (3) Essehli, R.; Belharouak, I.; Ben Yahia, H.; Maher, K.; Abouimrane, A.; Orayech, B.; Calder, S.; Zhou, X. L.; Zhou, Z.; Sun, Y. K. Dalton Trans. 2015, 44, 7881−7886. (4) Essehli, R.; Belharouak, I.; Ben Yahia, H.; Chamoun, R.; Orayech, B.; El Bali, B.; Bouziane, K.; Zhou, X. L.; Zhou, Z. Dalton Trans. 2015, 44, 4526−4532. (5) Trad, K.; Carlier, D.; Croguennec, L.; Wattiaux, A.; Ben Amara, M.; Delmas, C. Inorg. Chem. 2010, 49, 10378−10389. (6) Barpanda, P.; Oyama, G.; Nishimura, S.-I.; Chung, S.-C.; Yamada, A. Nat. Commun. 2014, 5, 4358. G


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