Combined Experimental and Computational Analyses on the

Article pubs.acs.org/JPCC

Combined Experimental and Computational Analyses on the Electronic Structure of Alluaudite-Type Sodium Iron Sulfate Gosuke Oyama,† Hisao Kiuchi,‡ Sai Cheong Chung,†,∥ Yoshihisa Harada,§,⊥ and Atsuo Yamada*,†,∥ †

Department of Chemical System Engineering, ‡Department of Applied Chemistry, and §Synchrotron Radiation Research Organization, 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 ⊥ Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan S Supporting Information *

ABSTRACT: Alluaudite-type sodium iron sulfate, Na2.56Fe1.72(SO4)3, has the highest redox potential in any Fe3+/Fe2+ environment; here, its electronic structure is investigated by combining X-ray absorption spectroscopy (XAS), semiempirical ligand field multiplet (LFM) simulations, and ab initio calculations. The LFM simulations for the Fe L2,3edge XAS reveals the exceedingly ionic character of the Fe−O bond due to the strong inductive effect of the sulfate group; this is one of the main factors which enhance the electrode potential of Na2.56Fe1.72(SO4)3. Both experimental XAS measurements and ab initio calculations indicate electrochemical charging (desodiation) gives more hybridized iron 3d and oxygen 2p states. The degree of covalency between iron and oxygen changes reversibly along with the change of the iron valence state.

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INTRODUCTION Reduction of the cost is a pressing issue for full-scale adoption of rechargeable batteries in large-scale energy systems (e.g., ongrid storage and electric vehicles). In this pursuit, sodium-ion battery is one of the most promising candidates replacing current lithium-ion battery owing to low cost, high natural abundance, and even geographical distribution of sodium resources.1−3 However, cathode materials have limited both the volumetric and gravimetric energy densities of the sodium-ion battery systems; therefore, considerable efforts have been devoted to identify suitable candidates. To date, a number of layered oxides, NaxMO2 (M: 3d transition metals),4−9 phosphates (e.g., olivine-Na x FePO 4 , 10,11 NASICONNa3−2xV2(PO4)3,12 Na2−xFeP2O7,13 Na4−3xFe3(PO4)2(P2O7)14,15), and fluorophosphates (e.g., Na2−xFePO4F,16 Na1.5‑xVPO4.8F0.7,17,18 Na3−2xV2(PO4)2F319−21) have been reported as active cathode materials, but few of which exhibit comparable electrode performances with the practical lithium-ion battery cathode materials. Inspired by the wide variety of sulfate-based high-voltage cathodes for lithium batteries (e.g., tavorite- and tripliteLi1−xFeSO4F,22,23 layered Li1−xFeSO4OH,24 marinate, orthorhombic Li2−xFe(SO4)225,26), alluaudite-type sodium iron sulfate Na2.56Fe1.72(SO4)3 was discovered recently as a possible sodium battery cathode candidate. It has the highest redox potential (ca. 3.8 V versus Na+/Na, corresponding to 4.1 V versus Li+/Li) among all the reported cathode materials driven by the Fe3+/Fe2+ redox couple.27 The extremely high potential can be rationalized by the strong inductive effect28 of the sulfate © 2016 American Chemical Society

group; consequently, hybridization of Fe 3d orbital and O 2p orbital is expected to be minimal. However, to date, no study has focused on the electronic structure, especially the one near the Fermi level, which is closely related to the redox behaviors of electrodes. In this regard, soft X-ray absorption spectroscopy (XAS) combined with semiempirical simulations or ab initio calculations is a powerful technique for extracting elementselective information on unoccupied Fe 3d and O 2p orbitals. In the present study, the electronic structures of Na2.56Fe1.72(SO4)3 and its desodiated phases are investigated by combining soft XAS, ligand field multiplet (LFM) simulations, and ab initio calculations. Variation of the valence state, crystal field strength, and degree of covalency will be discussed.

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EXPERIMENTAL SECTION Na2.56Fe1.72(SO4)3 compounds were prepared via a solid state synthesis as reported previously.29 For electrochemical tests, 80 wt % active material, 13 wt % carbon black (Ketjen Black, Lion Corp., ECP), and 7 wt % polytetrafluoroethylene (PTFE) binder were mixed to make positive electrode tapes (about 10 mm in diameter) by a pestle and mortar. The tapes were cast onto Al meshes and dried at 393 K for 12 h. 2032-type coin cells were assembled in an Ar-filled glovebox using the aboveprepared cathode tapes, 1 mol dm−3 NaClO4 dissolved in propylene carbonate (PC) electrolyte (Tomiyama Pure Received: June 2, 2016 Revised: September 26, 2016 Published: September 26, 2016 23323

DOI: 10.1021/acs.jpcc.6b05569 J. Phys. Chem. C 2016, 120, 23323−23328

Article

The Journal of Physical Chemistry C Chemical Industries, Ltd.), glass fiber separators, and Na metal anodes. Galvanostatic charge−discharge tests were operated between 2.0 and 4.5 V under a TOSCAT-3100 charge− discharge unit (Toyo System;, the current was set as C/20 (5.3 mA/g), which refers to a current rate to reach theoretical capacity in 20 h. After the charge−discharge experiments, the cells were disassembled in an Ar-filled glovebox. The positive electrode tapes were removed from Al mesh and carefully washed with dimethyl carbonate (DMC) (Kishida Chemical) to remove the electrolytes. Ex situ soft X-ray absorption spectroscopy (XAS) measurements were performed for the charged and discharged electrodes at BL07LSU in SPring-8.30 The samples were attached with carbon tape to the sample holders in an Ar-filled glovebox and were transferred to a vacuum chamber using a transfer vessel without air exposure. The reproducibility of the XAS data is confirmed by measuring two different electrodes at each state of charge (b)−(f). O K-edge XAS was conducted in the bulk-sensitive partial fluorescence yield (PFY) mode. Fe L2,3-edge XAS was measured by inverse partial fluorescent yield (IPFY) mode. This method can suppress self-adsorption and saturation effects using the inverse of integrated O K-edge emission, which is proportional to the X-ray absorption coefficient.31 Energy resolution of the incident beam was approximately 100 meV. All the spectra were recorded at room temperature. Ab initio calculations were conducted with the Vienna ab initio simulation package (VASP). Spin-polarized calculations were conducted employing the GGA (generalized gradient approximation) exchange correlation functional due to Perdew, Burke, and Ernzerhof (PBE). A Hubbard-type correction to the energy functional (the GGA+U method) with a coulomb parameter of U = 4.2 eV was used to describe the localized 3d electrons of the Fe ions. Projector augmented wave (PAW) method as implemented in the VASP program was employed. Energy cutoff of 520 eV for the plane-wave basis and 4 × 4 × 4 Monkhorst−Pack k-point meshes were applied. Convergence of the forces was set to 0.020 eV Å−1. Ferromagnetic and antiferromagnetic orderings of the Fe ions were considered. In the calculation, the off-stoichiometric compositions of Na2.5Fe1.75(SO4)3 (discharged state) and Na0.75Fe1.75(SO4)3 (charged state) were approximated within a × b × 2c supercells. For a direct comparison with the experimental XAS spectra, the calculated partial density of state (p-DOS) curves are manually shifted to 528 and 530 eV for the pristine and desodiated phases, respectively. Partitions of electron charge density were performed with the Bader analysis method.33,34 Unlike the oxygen K-edge where the p-DOS from the Kohn−Sham orbitals can provide a reasonable match with the experimental spectrum, L-edge of transition metals involve strong correlation among the d-electrons and between core holes and the d-electrons, as a result explicit treatment of the many body effects is required. Here, we employed the ligand field multiplet (LFM) calculations to simulate the experimental Fe L2,3-edge XAS of Na2.56Fe1.72(SO4)3. For comparison, a simulation was also done for the olivine-type Li1−xFePO4 (x = 0), of which spectrum was previously reported by Kurosumi et al.35 The simulated spectrum of LiFePO4 is manually shifted by −0.64 eV. The CTM4XAS program,36 based on the Cowan− Butler−Thole code,37−39 was employed. In this method, the relativistic Hartree−Fock atomic calculations are performed for the ground and excited states. Several parameters can be

adjusted to mimic the conditions of an ion in a solid matrix; scaling down of the Slater integrals (Fdd, Fpd, Gpd) from their values for respective free Mn+ ions, crystal field parameters, and charge transfer parameter. Following the previous report by Hibberd et al.,40 the charge transfer parameter was not used for the simulations in the present analyses; the effect is partly taken into account by the reduction of the Slater integrals. Core-hole lifetime and experimental resolution effects were taken into account by broadening the spectra with Lorentzian and Gaussian functions. Γ(L3) = 0.3, Γ(L2) = 0.3, and G = 0.1 eV were applied, where Γ and G represent half-width at halfmaximum values of the Lorentzian and the Gaussian functions, respectively.

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RESULTS AND DISCUSSION Figure 1-i shows the galvanostatic charge−discharge curves of Na2.56−yFe1.72(SO4)3 upon the first two cycles. Some irreversible processes are observed for the first and the second cycles; they mainly involve the irreversible migration of the Fe ion to one of the Na sites, details of which were discussed in ref 27. Here, we focus on the second cycle to elucidate the reversible feature.

Figure 1. (i) Galvanostatic charge−discharge curves of the alluaudite Na2.56Fe1.72(SO4)3 electrode at a C/20 current rate. The electrochemical processes were stopped at different states of charge (a)−(f). (ii) Ex situ Fe L2,3-edge XAS of alluaudite Na2.56Fe1.72(SO4)3 electrode at (a)−(f). (iii) Ex situ O K-edge XAS of the alluaudite electrodes at (a)−(f). Point (a) corresponds to the pristine material while points (b)−(f) are those of the second charge−discharge cycle. 23324

DOI: 10.1021/acs.jpcc.6b05569 J. Phys. Chem. C 2016, 120, 23323−23328

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are shown in Figure 1-iii. For the pristine phase, the relative peak intensity is barely observed below the threshold of the main O K-edge at 534 eV (referred to a pre-edge region). Upon the charge (desodiation) process, the intensity of the pre-edge peak visibly increased and reversibly decreased upon the subsequent discharge (sodiation) process. The intensity increment of the pre-edge peaks is caused by the stronger hybridization at the desodiated phase mainly due to the shorter Fe−O bond length and larger overlap of iron d states with oxygen p states for Fe3+ than that for Fe2+, the details of which will be discussed below. Negligible change is observed above the pre-edge region during the desodiation and sodiation processes. This region is attributed to the hybridized bands between oxygen p and sulfurous p orbitals; thereby, the covalency of S−O bonds marginally changes during the charge−discharge measurements. In order to obtain an overall insight for DOS near the Fermi level and to directly compare with the observed O K-edge XAS spectra, ab initio calculations are performed. The DOS of the pristine alluaudite sodium iron sulfate shows overall band structure similar to those of other oxyanionic compounds, such as olivine-LiFePO4,50 tavorite-, and triplite-LiFeSO4F51 (Figure 3i). The spin-up d states of iron show strong overlap with the

Fe L2,3-edge (2p−3d) XAS at different states of charge (a)− (f) are shown in Figure 1-ii. Peaks at lower (708−712 eV) and higher (719−725 eV) energies correspond to the Fe L3- and L2edges, respectively. Upon the desodiation, intensity ratio of the L3-edge peak at 710 eV to that at 708 eV increases toward the fully charged state (d). As is reported for LixFePO4 system,42 the observed intensity ratio difference can be interpreted as the multiplet structure change due to the oxidation of Fe2+ to Fe3+. In contrast, the intensity ratio toward the discharged state (f) upon the sodiation process, suggesting the reduction of Fe3+ to Fe2+. The observed reversible change for the electronic states of iron is consistent with the previous Mössbauer and X-ray nearedge absorption spectroscopy (XANES) analyses.43 Figure 2 shows LFM simulation for the experimental Fe L2,3edge XAS of the pristine phase with/without the 3d spin−orbit

Figure 2. Experimental Fe L2,3-edge XAS (black lines with points) and LFM calculated spectra without (solid blue lines) and with (dashed blue lines) the 3d spin−orbit couplings (SO) of pristine Na2.56Fe1.72(SO4)3.

coupling. The simulated curve without the 3d spin−orbit coupling shows better agreement with the experimental XAS spectrum, particularly at L2-edge. Better fitting results without the 3d spin−orbit coupling were reported also for the L2,3edges of Fe2SiO4, FeO, and FeF2.43−45 This suggests that the 3d spin−orbit coupling is well suppressed under d6 states. The parameters used for the simulations are listed in Table 1. The Table 1. Parameters Applied for LFM Calculations without the 3d Spin−Orbit Couplings, Which Give the Best Fits to the Experimental Spectra

Na2.56Fe1.72(SO4)3

valence state Fe2+

symmetry

crystal field splitting 10Dq (eV)

Slater integral reduction

Oh

0.5

−5%

Figure 3. Total and partial density of states (DOS) profiles for (i) pristine Na2.5Fe1.75(SO4)3 phase and (ii) desodiated Na0.75Fe1.75(SO4)3 phases calculated with GGA+U (U = 4.2 eV). Total DOS are illustrated as a black dashed line, while partial DOS for Na sp, Fe 3d, S 3p, and O 2p orbitals are shown as green, blue, yellow, and red solid lines, respectively.

obtained crystal field splitting (10Dq = 0.5 eV) is rather small because highly ionic state of iron in Na2.56Fe1.72(SO4)3 may reduce the crystal field splitting. The electronic state of iron will be discussed for the Slater integral parameters. The Slater integral is required to be reduced with 5% from the free Fe2+ ion (see Supporting Information). The reduction of the Slater integral is caused by the quantum mechanical mixing of iron 3d with ligand orbital and is commonly referred to as the “nephelauxetic effect”.40,46,47 Thereby, the 5% reduction of the Slater integral for Na2.56Fe1.72(SO4)3 is small, indicating that iron in Na2.56Fe1.72(SO4)3 is very ionic and closer to the free Fe2+ ion. This is consistent with the high observed average voltage of 3.8 V (4.1 V vs Li) for Na2.56Fe1.72(SO4)3.27 O K-edge (1s−2p) XAS includes direct information about the hybridization of oxygen with other atoms because an ideally ionized O2− ion has no empty 2p orbital for the excitation.49 The obtained spectra at the different states of charge (a)−(f)

oxygen s and p states, with hybridized states extending from −5.92 to −0.76 eV. The spin-down d states are however much more localized. As sodium is removed and iron is oxidized to Fe3+, the spin-up d states of iron move down in energy and appear below the oxygen bands (Figure 3-ii). This is due to the increase in electrostatic charge on iron and the exchange stabilization of the d5 states. Now the spin-down d states show stronger hybridization with the oxygen p states (+0.5 to +2.9 eV), leading to the intensity increment of the O K pre-edge with desodiation. The O K-edge XAS spectra of the pristine and the desodiated phases are compared with calculated partial DOS (p-DOS) in 23325

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Both the oxygen K-edge XAS and ab initio calculations indicate the hybridization between iron 3d and oxygen 2p orbitals increases upon the desodiation process, and the XAS shows that it reversibly decreases upon the subsequent sodiation process. The present spectroscopic analyses are principally applicable for manifold oxyanionic cathode materials to reveal electronic structure changes during charge−discharge processes.

Figure 4. The overall features of oxygen p-DOS curves agree with the O K-edge XAS for both the pristine (Figure 4-i) and

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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.jpcc.6b05569. LFM simulations of the Fe L2,3-edge with various Slater reduction factors (PDF)

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Figure 4. Experimental O K-edge XAS and DFT calculated partial DOS (p-DOS) of unoccupied oxygen orbitals for (i) pristine and (ii) desodiated phases.

*E-mail: [email protected] (A.Y.).

the desodiated (Figure 4-ii) phases. As is discussed above, the pre-edge region is attributed to a hybridized band between iron 3d and oxygen 2p orbitals. The absence of the pre-edge peak for the pristine phase suggests the localized Fe 3d bands, which is consistent with the results of Fe L2,3-edge XAS. The Bader charge analyses (see experimental section) supports the strong inductive effect of the sulfate group further (Table. 2). The mean Bader charge of Fe for the pristine phase

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Masashi Okubo, Dr. Shin-ichi Nishimura (The University of Tokyo, Japan), and Dr. Daisuke Asakura (National Institute of Advanced Industrial Science and Technology, Japan) are thanked for their fruitful discussions. Dr. Jun Miyawaki and Dr. Yusuke Nanba (The University of Tokyo, Japan) are acknowledged for their help in acquiring soft XAS data at the SPring-8 facility. The present work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the “Element Strategy Initiative for Catalysts & Batteries” (ESICB) project. The soft XAS measurements were performed under the joint research of the Synchrotron Radiation Research Organization (SRRO) and the Institute of Solid State Physics (ISSP), the University of Tokyo (Proposal Nos. 2014B7403 and 2015A7403). G.O. and H. K. acknowledge support from JSPS Research Fellowships under “Materials Education Program for the Future Leaders in Research, Industry, and Technology” (MERIT) project.

Table 2. Mean Bader Charges of Each Ions for the Pristine, Desodiated Phases, and Their Differences pristine desodiated difference

Na

Fe

S

O

+0.90 +0.90 0.00

+1.51 +1.90 +0.39

+3.87 +3.81 −0.06

−1.37 −1.29 +0.09

AUTHOR INFORMATION

Corresponding Author

is calculated to be +1.51, which is one of the highest values among the reported polyanionic compounds;52 this reflects the strong ionic character of the Fe−O bond through the inductive effect of the sulfate group. On the other hand, during desodiation the iron ions compensate only 0.39 out of the 0.90 change in charges (corresponding to about 43%); the rest are mainly compensated by the oxygen ions. This suggests the coordinated oxygen ions participate in the electrochemical redox reaction through the Fe−O bonds, despite the exceedingly ionic character of the Fe−O bonds under the inductive effect of the sulfate group. This may due to the increase in the hybridization between Fe and O after the desodiation as is revealed in the oxygen K-edge spectra and the DFT calculations.

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CONCLUSION In summary, the electronic structure of the alluaudite-type Na2.56Fe1.72(SO4)3 is surveyed via combined analyses of ab initio calculations, soft-XAS, and LFM simulations. The Fe L2,3-edge XAS indicates the reversible oxidation/reduction of the iron valence state for the electrochemical desodiation/sodiation processes. The LFM simulation shows the inductive effect of the sulfate group in the pristine Na2.56Fe1.72(SO4)3 is exceedingly strong, and iron is in almost a free Fe2+ state, leading to the highest redox potential among any Fe3+/Fe2+ environments. 23326

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