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Mar 20, 2016 - High-Temperature Neutron and X‑ray Diffraction Study of Fast. Sodium Transport in Alluaudite-type Sodium Iron ... temperature X-ray/n...
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High-Temperature Neutron and X‑ray Diffraction Study of Fast Sodium Transport in Alluaudite-type Sodium Iron Sulfate Shin-ichi Nishimura,†,‡ Yuya Suzuki,† Jiechen Lu,† Shuki Torii,§ Takashi Kamiyama,§,⊥ and Atsuo Yamada*,†,‡ †

Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8245, Japan § Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK), 203-1, Tokai-mura, Ibaraki 319-1106, Japan ⊥ Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (The Graduate University for Advanced Studies), 203-1, Tokai-mura, Ibaraki 319-1106, Japan S Supporting Information *

ABSTRACT: Sodium-ion battery is a potential alternative to replace lithium-ion battery, the present main actor in electrical energy storage technologies. A recently discovered cathode material Na2.5Fe1.75(SO4)3 (NFS) derives not only high energy density with very high voltage generation over 3.8 V, but also high-rate capability of reversible Na insertion as a result of large tunnels in the alluaudite structure. Here we applied hightemperature X-ray/neutron diffraction to unveil characteristic structural features related to major Na transport pathways. Thermal activation and nuclear density distribution of Na demonstrate one-dimensional Na diffusion channels parallel to [001] direction in full consistence with computational predictions. This feature would be common for the related (sulfo)alluaudite system, forming emerging functional materials group for electrochemical applications.

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INTRODUCTION Rapid and continuous growth of electric energy use requires production of large number of mobile or on-site storage of electric power. Li-ion battery has been used as a main energy storage device for portable electronics, and its use is now rapidly penetrating into much larger applications such as electric vehicles and stationary storage. This situation raises a potential geopolitical risk for stable Li supply due to nonuniform distribution on Earth.1,2 Sodium, with many similar chemical properties with lithium, is free from such risks, owing to superior abundance in the seawater and the Earth’s crust. Thus, sodium-ion battery is expected to be a realistic alternative for energy storage systems. As a consequence, numerous positive electrode materials were reported for the sodium-ion battery in recent 5−10 years3−5 including several layered oxides6−8 and oxyanionic compounds.9−14 Although some combination of cathode and anode material can realize competitive energy density to the lithium-ion batteries, commercialization of the sodium-ion battery has not been achieved to date. One major drawback of the sodium battery system is lower cell voltage due to higher electrodeposition potential of Na than that of Li. As this drawback is simply solved by shifting the working potential of electrodes, developing positive electrode materials with higher working potential is a crucial task to make the sodium system competitive to the © 2016 American Chemical Society

present lithium system. A recently introduced alluauditeNa2+2xFe2−x(SO4)3 (NFS), composed of only Earth abundant elements, is a strong candidate as a high-voltage positive electrode material for sodium-ion batteries and generates 3.8 V versus Na/Na+ on average, which is the highest among Fe-based cathode materials.14 The abnormally high voltage is explained by highly electronegative [SO4]2− anion and the peculiar edgesharing Fe2O10 dimer geometry.14−18 Alluaudite is a natural mineral commonly observed as anhydrous phosphate or arsenate. Fisher first identified the crystal system and symmetry of alluaudite in 1955.19 After 16 years, Moore determined the crystal structure and generalized the chemical composition systematically.20 The alluaudite structure, as shown in Figure 1, has two kinds of large tunnels running along the c-axis. The intriguing tunnel structure has stimulated battery researchers to use the alluaudites as electrode materials for batteries. Richardson first reported electrochemical behaviors of several phospho-alluaudites LixNa2−xMn3−yFey(PO4)3 as candidates of positive electrode material for lithium-ion battery.21 Afterward, Trad and Croguennec et al. systematically studied the Received: February 10, 2016 Revised: March 19, 2016 Published: March 20, 2016 2393

DOI: 10.1021/acs.chemmater.6b00604 Chem. Mater. 2016, 28, 2393−2399

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X-ray and Neutron Diffraction. Powder X-ray diffraction (XRD) patterns were measured by using two kinds of diffractometers with synchrotron radiation. One is a high-resolution diffractometer installed at BL-4B2 of Photon Factory in High-Energy Accelerator Organization, Tsukuba, Japan.24,25 The other is a Debye−Scherrer camera installed at BL02B2 of SPring-8, Harima, Japan. Variable temperature measurements were performed at the Debye−Scherrer camera, while the high-resolution measurement was performed only at ambient temperature. Powder neutron diffraction (ND) measurements were carried out by using a time-of-flight (TOF) type diffractometer SuperHRPD at Japan Proton Accelerator Research Complex (J-PARC).26,27 The powder sample was pelletized and packed into a vanadium cell. Sample temperature was controlled by a vanadium furnace. Rietveld refinements were performed by using a computer program TOPASAcademic V5. Maximum-entropy method (MEM) was applied to reconstruct neutron-scattering-length density (nuclear density) distributions from the neutron diffraction data by using a computer program Dysnomia written by Momma.28 A total of 400 reflections were used for the MEM computation (Q < 5.65 Å−1). The limited memory quasi-Newton method (L-BFGS) was used for the optimization.29 A dj dependent weighing scheme, dj2 where dj is interplanar distance of jth reflection, was applied to the structure factors Fj to suppress undesired artifacts, which are frequently introduced by the MEM process.28,30,31 All the crystallographic images were generated by the VESTA 3.32

Figure 1. Crystal structure of alluaudite Na2.5Fe1.75(SO4)3 projected along [001]. The blue spheres show Na. The green octahedra and the yellow tetrahedra show FeO6 and SO4 units, respectively. The Fe2(SO4)3 framework forms two kind of large channels running along [001].

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electrochemical properties and reaction mechanisms of phosphoalluaudites NaMnFe2(PO4)3 and Li0.5Na0.5MnFe2(PO4)3.22,23 However, extensive research has been limited because intrinsic electrochemical properties do not satisfy the practical requirements. On the contrary, the sulfo-alluaudites provide many advantageous aspects as a positive electrode of batteries: (i) the aforementioned high working potential, (ii) initial state of Fe is divalent, and (ii) Na/Fe ratio is close to 1:1.14 Furthermore, our previous computational evaluations suggest that NFS shows low migration barriers for Na along the tunnel structure.14 Indeed, NFS shows good rate capability in sodium-ion half-cell experimentally: 70% of capacity retention even at 10C rate operation. Here we demonstrate experimental studies on the origin of fast kinetics of NFS by transport measurements and high-temperature X-ray/neutron diffraction.

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RESULT AND DISCUSSION Electrical conductivity of NFS measured with an Au/NFS/Au cell is 1.2 × 10−7 S cm−1 at 300 K and 2.3 S cm−1 at 620 K (Figure 2).

EXPERIMENTAL SECTION

Sample Preparation. The Na2.5Fe1.75(SO4)3 (NFS), x = 0.25 of Na2+2xFe2−x(SO4)3, was synthesized by the solid-state reaction method. Na2SO4 (Wako 99%) and FeSO4·7H2O (Kanto Chemical, 99%) were used as starting materials. Na2SO4 was dried in a glass tube furnace under vacuum and stored in an Ar filled glove box. FeSO4·7H2O was dehydrated under vacuum at 573 K overnight to give anhydrous FeSO4. Stoichiometric amounts of the anhydrous sulfates were mixed with acetone in Cr-hardened steel pot with Cr-hardened steel balls. The milling condition was 400 rpm for 6 h with a planetary mill P6̅ (Fritsch). The milled slurry was dried in vacuum and packed to cylindrical pellets. The pellets were heat treated at 623 K for 24 h. The heat treatment was repeated two times to complete the reaction. Electrical Conductivity Measurement. The NFS powder was pelletized by isostatic pressuring at 300 MPa and then annealed at 623 K for 12 h in Ar atmosphere. After annealing, the samples were polished to form flat surface on both sides. Typical samples have a diameter of about 8 mm and a thickness of about 3 mm. Gold metal was then sputtered on both sides to act as electrodes, blocking the ionic exchange. Alternating current (AC) electrical conductivity measurements were performed within the temperature range of 300−623 K in Ar atmosphere by an impedance analyzer (1296 Dielectric Interface combined with 1260 Impedance/Gain-Phase Analyzer, Solartron) in the frequency range from 106−10−2 Hz.

Figure 2. Arrhenius plot for product of AC electrical conductivity and temperature σT against reciprocal temperature T−1 for Au/NFS/Au symmetrical cell. Inset shows typical complex impedance plot at 472 K.

By assuming the Arrhenius relation, activation energy of the electrical conduction is 58 kJ mol−1 (0.60 eV). Complex impedance spectra show a semicircle at high frequency and Warburg behavior from electrode at low frequency as shown in the inset of Figure 2. As the Au electrodes are expected to be ion blocking, the conductivity is to be dominated by ionic contribution. Further details on the conductivity measurements will be published elsewhere.33 At 620 K, conductivity of NFS reaches to those of sodium superionic conductors, while it is still two orders lower than the highest sodium conductors, Na3Zr2Si2PO1234 and Na β-alumina.35 These results stimulated 2394

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Figure 4. Observed and calculated scattering intensity versus scattering vector Q = 4π sin θ/λ for TOF-ND pattern of NFS at 620 K. Three patterns from different detector banks were used simultaneously to cover wide Q space with best resolution.

any averaged harmonic atomic displacements. Thus, we introduced site-splittings to both of the Na2 and Na3. For more details, see Figure S3 in the Supporting Information. Reliable thermal vibration analysis was not possible without these site splittings. Figure 3, panel a shows powder XRD patterns for NFS collected at temperatures between 300 and 600 K. The 100, 200, and 620 K patterns are omitted due to different exposure conditions and can be found in the Supporting Information. Continuous evolution of the lattice volume suggests that there are no first-order structural transitions nor new phase evolutions against temperature in a range of 100−620 K. Note that, at higher temperature over 620 K, FeSO4 starts to decompose to Fe3O4 and SO2/SO3 gases as mentioned in our previous report.37 Nonconstrained Rietveld refinements for the HT-XRD patterns were carried out to extract structural parameters. The final refinement patterns and resulted parameters are summarized in the Supporting Information. Reliability indices GoF = Rwp/Re and RBragg converged to less than 1.6 and 1.2%, respectively, for all the temperatures (Table S5 in Supporting Information). As expected from the thermal evolution of lattice volume, there are no remarkable change of crystal structure through the whole temperature range. Only atomic displacement parameters (ADP) show obvious evolutions against the temperature (Figure 3c). In general, ADP (B or U) is treated as sum of static component Ustatic and dynamic component Udynamic:

Figure 3. (a) Temperature evolution of powder X-ray patterns recorded between 300 and 600 K, (b) refined lattice constants, and (c) atomic displacement parameters (ADP) B for Na at each temperature.

us to analyze more details of sodium motion at high temperature as previously succeeded in LixFePO4 system.36 Crystal structure of the alluaudite phase Na2.5Fe1.75(SO4)3 was analyzed by Rietveld refinement of high-resolution powder XRD and powder ND at room-temperature with a C2/c structure model.14,37 Off-stoichiometry from the ideal composition Na2Fe2(SO4)3 is important to minimize formation of impurity phases (i.e., Na6Fe(SO4)4, α- and β-FeSO4) during synthesis. Stoichiometric sulfo-alluaudites Na2M2(SO4)3 have not been synthesized to date. Here, the nonstoichiometry x ≈ 0.25 is assumed as one-to-one substitution to Fe by Na and excess Na in Na2 and Na3 sites for charge neutrality. Occupancies gi were refined with the following constraints: gNa1 = 1, gFe + gNa/Fe = 1, gNa2 + gNa3 = 4 − gNa1 − 2 gFe. Thus, free parameters for occupancy were gFe and gNa2. The resulted formula is written as [Na1]1.0[Na2]0.71[Na3]0.54[Fe0.875Na0.125]2(SO4)3. These occupancies were used for all of the subsequent refinements to avoid influences from strong correlations with the atomic displacement parameters. Careful analysis unveiled that the Na2 and Na3 show significant positional disordering, which could not be approximated as

B = 8π 2U = 8π 2(Ustatic + Udynamic) 2395

(1)

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Figure 5. Isosurface of neutron scattering length density ρn at 0.15 barn Å−3 for NFS at (a) room temperature and (b) 620 K reconstructed by the MEM. (c) Corresponding atomic positions are shown as thermal ellipsoids/spheres with 60% probability. (d) Two-dimensional representation of neutronscattering-length density for (100) plane at x = 0.5. The contours are drawn at intervals of 0.04 barn Å−3 up to 0.6 barn Å−3.

Temperature dependence of Udynamic is given in the Debye’s harmonic model as Udynamic =

3ℏ2T ⎡ T ⎢ mkBΘD2 ⎣ ΘD

∫0

ΘD / T

ΘD ⎤ x d x + ⎥ ex − 1 4T ⎦

The temperature-dependent component of ADP is mostly attributed to the thermal vibrations of atoms as expected from eqs 1 and 2. Na3 shows larger slope versus temperature than Na1 and Na2, whereas large fraction of the low-temperature ADPs are ascribed to the static disorder. Furthermore, Na3 shows divergent trend at higher than 500 K, while Na1 and Na2 follow the harmonic approximation even at high temperatures. Such divergence from harmonic approximation is attributed to anharmonic thermal motion of atoms, which is correlated with ion migration to other sites. Rietveld refinement on a high-temperature ND patterns of NFS also confirms that the framework structure maintains the original alluaudite structure. Figure 4 shows the refinement profile for the ND pattern collected at 620 K. The reliability indices Rwp, Rp, and RBragg converged to 2.25%, 3.30%, and 0.84%, respectively. More details of the refinement result can be found in Tables S1−S4 in the Supporting Information. To get further information, we carried out computations based on the maximum entropy method (MEM), where phases and intensity fractions of overlapped reflections are calculated from the refined structure model. Reliability indices for the MEM were 0.58% for RT and 0.56% for 620 K, respectively. More systematic comparisons of the MEM derived structure factors with the observed ones are shown in Figures S16 and S17 in the Supporting Information. Figure 5 represents distributions of the neutron scattering length density ρn for NFS at RT and 620 K. The neutron scattering length density has a linear relation to the nuclear density. The isosurfaces of ρn well trace the surface of thermal ellipsoids/spheres of atoms, which are well reproducing the atomic positions based on the refined original structure

(2)

where m is the atomic mass, kB the Boltzmann constant, ℏ the reduced Planck constant, ΘD the Debye temperature, and T the temperature.38 As the quantum effect is negligible at high temperature, Udynamic is approximated as Udynamic =

3ℏ2T mkBΘD2

(3)

at T > ΘD. The solid lines and broken lines in Figure 3, panel c correspond to eqs 2 and 3, respectively. The ADPs for all the Na sites show deviations from the Debye’s model at low temperatures, even considering low ΘD. Such deviation is a typical sign of static disordering, which is caused by various kind of local disordering of atomic positions: for example, substitution by different size of atoms, atomic vacancies, mixed-valent states, etc. NFS contains three major sources of such local disordering: (1) Na substitution to the Fe site; (2) Na vacancies at the Na2 and Na3 sites; (3) too long Na3O bond length. Although all three factors can contribute to the static disorder, the third feature is intriguing for ionic transport phenomena. Bond valence sum of Na3 (Na3′) is 0.53, which is much smaller than the ideal valence of Na+, while the valences of Na1 and Na2 are close to the ideal value: 1.04 for Na1 and 1.01 for Na2(Na2′), respectively.39 This unusual local environment around Na3 explains the large static disordering and may be suitable for easy migration to other sites. 2396

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Figure 6. Maximum scattering-length density around Na2 and Na3 sites along c-axis (upper) and corresponding effective one-particle potential Veff for Na (lower, see text).

diffusion in NFS. Particularly, the nuclear density around Na3 shows characteristic spreading along the large tunnel structure, while Na1 and Na2 remain localized even at 620 K. Highly anharmonic and anisotropic character of thermal vibration for Na3 indicate the continuous Na3−Na3 conduction pathway along [001] suitable for fast Na ionic transport. These characteristic dynamic features inherent to the new sulfo-alluaudite system should be general for related emerging compounds, some of which may serve as functional material suitable for electrochemical applications.

model. At 620 K, all the constituent atoms shows more spread nuclear density than RT, reflecting the thermal vibrations. Na3 shows remarkable spreading as expected from the thermal evolution of ADP (Figure 3c). Furthermore, the distribution about Na3 shows characteristic evolution along the [001] as shown in Figure 5, panel d. Thus, Na3 preferentially diffuses along the c-axis. Such a one-dimensional feature is similar to nuclear density distribution of LixFePO4 observed by neutron diffraction at high-temperature.36 This is consistent with the fact that the Na3−Na3 migrations have the lowest barrier of 0.28 eV in the sulfo-alluaudite framework as predicted by the computational studies.14,40,41 In contrast, Na2 shows limited evolution even at 620 K. The less mobile character of Na2 is also consistent with higher migration barrier (0.55 eV) as predicted by the computations.14 Once probability density is obtained, we can convert it to effective one-particle potential Veff by assuming the Boltzmann statistics as ⎛ ρ (u ) ⎞ Veff = −kBT log⎜ ⎟ ⎝ ρ(u 0) ⎠

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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.chemmater.6b00604. Details of the Rietveld refiment results of XRD and ND patterns, local environments of Na sites, and convergence of the MEM computations (PDF)

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(4)

where u0 is the zero vector and u the relative displacement vector from the equilibrium position.42,43 Figure 6 demonstrates Veff profile along [001] about Na2 and Na3 calculated from the density distribution of neutron scattering length. Similar to the aforementhioned DFT prediction,14 the Na3−Na3 migration indicates much lower migration barrier than that of Na2−Na2. This contrast also confirms the higher mobility of Na3 more quantitatively. Although the apparent activation energy of Na ionic conductivity is about 0.6 eV, the local effective potential barrier for Na3−Na3 (0.2−0.25 eV) derived from the experimental nuclear density is close to the one obtained by the DFT calculation (0.28 eV).14 This discrepancy may ascribe to the contribution of the other Na sites or grain boundaries.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81 (0)3 5841 7295. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed under a management of “Elements Strategy Initiative for Catalysts and Batteries (ESICB)” supported by a program of Ministry of Education Culture, Sports, Science, and Technology (MEXT), “Elements Strategy Initiative to Form Core Research Center” (since 2012), Japan. The synchrotron XRD experiment at Photon Factory was performed under approval of Photon Factory Program Advisory Committee (Proposal Nos. 2013G670 and 2015G684) and at the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2014A1196, 2015A1503). The neutron diffraction experiments were carried out with the approval of Materials and Life Science Experimental Facility (MLF) in Japan Proton Accelerator Research Complex (J-PARC), Proposal No. 2014E0001.

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CONCLUSION Crystal structure of Na2.5Fe1.75(SO4)3 (NFS) is successfully refined by the C2/c alluaudite structure without any structural transitions between 100 and 620 K. Na3 in the large tunnel shows much larger thermal motion than those of Na1 and Na2. The high-temperature neutron diffraction elucidated thermally activated nuclear density distribution in NFS, which provides the experimental evidence of preferentially one-dimensional Na 2397

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DOI: 10.1021/acs.chemmater.6b00604 Chem. Mater. 2016, 28, 2393−2399

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DOI: 10.1021/acs.chemmater.6b00604 Chem. Mater. 2016, 28, 2393−2399