Communication pubs.acs.org/cm
Na-Excess Cation-Disordered Rocksalt Oxide: Na1.3Nb0.3Mn0.4O2 Kei Sato,† Masanobu Nakayama,‡,§,∥ Alexey M. Glushenkov,⊥ Takahiro Mukai,‡ Yu Hashimoto,‡ Keisuke Yamanaka,# Masashi Yoshimura,# Toshiaki Ohta,# and Naoaki Yabuuchi*,†,∥ †
Department of Applied Chemistry, Tokyo Denki University, 5 Senju Asahi-Cho, Adachi, Tokyo 120-8551, Japan Frontier Research Institute for Materials Science (FRIMS), Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, Japan § Mi2i/GREEN, National Institute of Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan ∥ Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, f1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan ⊥ Institute for Frontier Materials, Deakin University, 75 Pigdons Road, Waurn Ponds, Geelong, Victoria 3216, Australia # SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan ‡
S Supporting Information *
I
electrochemically inactive because of the absence of conductive electrons. Therefore, to induce the electronic conductivity, Mn3+ ions can be substituted for Li/Nb ions according to the binary system, x Na3NbO4 − (1 − x) NaMnO2. However, all our trials failed to synthesize samples by conventional calcination (Figures S1−S3). Phase segregation into Na3NbO4 and NaMnO2 was evidenced, and a narrow solid solution range is expected. Consequently, an alternate route was chosen to synthesize a metastable phase, and mechanical milling was used in this study. Detailed experimental procedures are in the Supporting Information. Structural analysis was carried out using RIETANFP,14 and schematic illustrations of crystal structures were drawn using the VESTA program.15 Figure 1a shows X-ray diffraction patterns of the samples before and after the mechanical milling. A mixture of Na3NbO4 and NaMnO2 gradually changes into a cation-disordered rocksalt structure. Color of powders changes into black after the mechanical milling. Although the size of Na+ is much larger than those of Nb5+ and Mn3+, all cations are located at the same octahedral sites in the ccp lattice. Heating of the sample results in the phase segregation into a mixture of Na3NbO4 and NaMnO2 used as the precursor (Figure S4), indicating this phase is a metastable phase. The successful synthesis of the metastable phase is further supported by the results of scanning electron microscopy (SEM) in Figure S3b and transmission electron microscopy (TEM) in Figure 1b, accompanied by energydispersive X-ray (EDX) elemental mapping. The results of TEM characterization are presented in Figure 1b and Figure S5. From a bright-field image and an electron diffraction pattern (Figure S5), the sample is polycrystalline and represents agglomerates of crystalline nanoparticles. Although some pronounced bright spots (corresponding to relatively larger crystallites) are visible in the diffraction pattern, continuous rings are observed as a more typical feature. These rings originate from fine nanoparticles of 1−4 nm in size, contained in the agglomerates. The sample was beam-sensitive during a
n the past two decades, the research progress in rechargeable Li batteries has contributed to many innovations in modern society through the development of convenient portable electronic devices used in our daily life. Rechargeable Li batteries are now used as power sources of electric vehicles, which potentially allow the sustainable energy development without depending on fossil fuels to happen. Moreover, with the increasing demand for energy storage applications, the development of high-energy and large-scale batteries is an urgent topic.1 Rechargeable Na batteries are a promising candidate,2 but the limited energy density of positive/negative electrodes restricts its practical use. The increase of the energy density is the problem to be solved for Na batteries. For Na batteries, Fe is an essential element because of its abundant resources in the crust. Although NaFeO 2 , 3 Na2/3Fe1/2Mn1/2O2,4 Na2Fe2(SO4)35 etc.6 have been proposed as candidates for positive electrode materials, the available energy density is limited to 300−500 mWh g−1 vs Na, and thus the current technology cannot fully compete with the state-ofthe-art Li technology. Moreover, recent innovations in reaction mechanisms for positive electrodes in the Li system potentially further accelerate the enhancement of energy density. A classical mechanism used in the past three decades is based on the redox reaction of transition metal ions as cationic species. Recent experimental7−9 and theoretical10 studies have revealed that oxide ions, O2−, as anionic species, can also participate in charge compensation. Nb5+ ions, which have no valence electrons, effectively stabilize the redox reaction of O2− coupled with Mn, and negatively charged O2− donate electrons instead of classical cationic species.11 Although Nb is not an ideal element for Na batteries with respect to its abundance, the concept of anion redox is an important strategy to increase the energy density. Indeed, a reversible contribution of O2− for redox is recently evidenced for Na2RuO3.12,13 In this study, Na3NbO4 is targeted as a model host structure for high-capacity positive electrodes. Na and Nb ions are located at octahedral sites in a cubic-close packed (ccp) lattice of oxide ions, and clusters consisting of four Nb ions are surrounded by sodium ions (Figure S1). If all Na ions are extracted/reinserted from/into the crystal lattice, the theoretical capacity reaches 356 mAh g−1. Nevertheless, Na3NbO4 is © 2017 American Chemical Society
Received: January 13, 2017 Revised: May 6, 2017 Published: June 12, 2017 5043
DOI: 10.1021/acs.chemmater.7b00172 Chem. Mater. 2017, 29, 5043−5047
Communication
Chemistry of Materials
Figure 1. Synthesis of Na1.3Nb0.3Mn0.4O2 by mechanical milling; (a) X-ray diffraction patterns of a mixture of Na3NbO4 and NaMnO2 before/after the mechanical milling. Photographs of powders are also shown. (b) Bright-field STEM image and corresponding EDX elemental maps of Mn, Nb, Na and O.
high magnification observation, and it was not possible to record high-resolution images due to an immediate sample evolution under the electron beam. Figure 1b shows a STEM image and EDX elemental maps of the sample. The elemental maps of Mn, Nb, Na and O are effectively identical, indicating the homogeneous distribution of elements and supporting the conclusion about the metastable phase formation by the mechanical milling. Electrochemical properties of the samples prepared by the conventional calcination and mechanical milling are compared in Figure 2. The theoretical capacity reaches 311 mAh g−1 when all Na ions are extracted from Na1.3Nb0.3Mn0.4O2. However, the sample prepared by the conventional calcination delivers only 95 mAh g−1, nearly corresponding to the theoretical capacity based on the Mn3+/Mn4+ redox. This result indicates that O2− redox is not activated. In contrast, the sample obtained by the mechanical milling delivers a large reversible capacity of approximately 200 mAh g−1 at 50 °C, suggesting O2− redox is activated in this system. Electrochemical voltage of the sample increases almost linearly with two different slope angles, which change at 3.2 V (70 mAh g−1). The potential profile of the sample is similar to the case of Li2MnO3-based electrode materials. However, a clear voltage plateau as observed for Li1.3Nb0.3Mn0.4O29,11 is not found for the Na system. Moreover, the cyclability of the sample in the Na cell is not acceptable for battery applications. Reversibility is completely lost after only 20 cycles. Note that its cyclability is effectively improved by the addition of sodium bis(fluorosulfonyl)amide (NaFSA)16 to the electrolyte. The cell delivers a reversible capacity of ca. 100 mAh g−1 after 30 cycles. XRD studies revealed that the sample changes into an amorphous phase when the additive was not added (Figure S6). The results also suggest that electrode
Figure 2. Electrochemical properties of Na1.3Nb0.3Mn0.4O2 at a rate of 10 mA g−1; (a) comparison of electrode performance for the sample prepared by the conventional calcination and mechanical milling, (b) capacity retention in electrolyte with NaPF6 and NaPF6 and NaFSA mixture. Typical sample loading was 2.1 mg cm−2.
performance is potentially further improved by the optimization of electrolyte additives. A detailed study on the origin of the improved reversibility by the NaFSA addition is in progress in our group. The phase stability and electrochemical Na removal process are examined by first-principles calculations for Na21/16Mn6/16Nb5/16O2 (see Figures S7−S11) The calculated voltage profile for Na21/16−xMn6/16Nb5/16O2 is shown in Figure S8b. Two reaction paths, where Nb and Mn ions keep octahedral coordination for oxide ions (path-a) or not (path-b), are considered for the calculation. Figure 3a,b presents changes in density of states (DOS) on charge along path-a, and corresponding partial electronic state densities are shown in Figure 3c−f. Table 1 lists shortest interatomic distances between oxides ions with various compositions. The calculated voltage increases from 2.8 to ∼3.5 V in the compositional range of 0 ≤ x ≤ 0.38, where Mn−O hybridized bands are mainly responsible for charge compensation because Mn 3d eg orbital and O 2p hybridization are visible at an unoccupied band (Figure 3c). In this voltage region, oxygen oxidation is not 5044
DOI: 10.1021/acs.chemmater.7b00172 Chem. Mater. 2017, 29, 5043−5047
Communication
Chemistry of Materials
Figure 3. (a) DOS plots for Na21/16−xNb5/16Mn6/16O2 as a function of composition x, and (b) magnification around Fermi level. Black, green and red lines present partial DOS of Mn, Nb and O ions, respectively. Yellow colored isosurface in panels c−f represent unoccupied electronic state density distribution for the energy range represented by color box in panel b. The electronic structures were derived from HSE06 hybrid functional.
Table 1. Interatomic Distances for the Closest O−O Bonds in A21/16−xNb5/16Mn6/16O2a
a
composition x
0
0.19
0.38
0.56
0.75
0.94
1.13
1.31
A = Na (path-a) A = Na (path-b) A = Li8
2.56 2.56 2.64
2.44 2.44 2.59
2.41 2.35
2.23 (2.41) 2.40
(1.38) 2.25
1.38
1.30 1.31
2.05 2.03
Values in the round brackets indicate unstable structures with respect to the convex hull shown in Figure S8.
Mn L-edge and O K-edge XAS spectra of Na1.3−xNb0.3Mn0.4O2 on charge/discharge processes. Mn L-edge and O K-edge XAS spectra of the reference samples are shown in Figures S12 and S13. An o xid ation st ate of Mn in as-prep ared Na1.3−xNb0.3Mn0.4O2 is assigned as Mn3+, which is the consistent with the oxidation state of Mn in the precursor. On charge to 95 mAh g−1, an XAS spectrum of Mn L-edge distinctly changes, indicating that Mn is oxidized to Mn4+. No change is found on further charge. The spectral evolution on charge clearly shows that Mn is not oxidized beyond Mn4+ after extraction of 0.4 mol of Na ions. This result is consistent with the behavior of the Li counterpart.11 However, no systematic change is observed for the O K-edge XAS spectra of the Li counterpart.11 For the Na system, the presence of carbonate species at 533 eV is evidenced for the fully charged sample (4.0 V vs Na metal) even for the bulk-sensitive fluorescence yield spectra. This fact probably suggests the surface of particles is reactive and reacts with the carbonate solvent. Such side reaction can result in the oxygen loss and accumulation of carbonate species, which is expected to be accelerated for the nanosize Na1.3−xNb0.3Mn0.4O2 prepared by mechanical milling. This observation is also consistent with the fact insufficient cyclability as shown in Figure 2, and the FSA additive probably suppresses such unfavorable side reaction with electrolyte solution and electrolyte consumption on electrochemical cycles. The possibility of oxygen loss is further supported from XAS spectra collected on the discharge process. Clear changes are noted for Mn L-edge XAS spectra on discharge. Reduction of Mn4+ starts for the early discharge process (∼70 mAh g−1) and the formation of Mn2+ is noted for the fully discharged sample.
evidenced. This fact is also confirmed from no oxygen dimerization (Table 1) and unchanged net spin moment (Figure S10) A flat voltage plateau at ∼3.6 V is observed for further charge until x = 0.96, and voltage reaches >4.0 V at that composition (∼220 mAh g−1), which is in good accordance with experimental results (Figure 2a). Electronic structure analysis indicates that two types of hole formation around oxygen (Figure 3d), i.e., oxygen dimerization (Figure 3e)8,17,18 and single O 2p hole formation (Figure 3f).10,19 Net spin moment analysis (Figure S10) at composition x = 0.56 reveals the oxidation of oxygen atom due to nonzero net spin. In detail, net spin moments are close to 0.5 and 0.2, which corresponds to dimerization of oxygen atoms (Figure 3e), and oxygen forming O 2p hole (Figure 3f), respectively. These results suggest that oxygen dimerization20 is more prone to be formed at x > 0.56 in Na21/16−xMn6/16Nb5/16O2 than the Li counterpart (Table 1).11 It is proposed such structural distortions and oxygen dimerization stem from relatively larger ionic radius of Na ion than that of Li ion. Interatomic distances of oxygen and Mn and Nb ions are also longer for the Na system, and thus weaker hybridization is expected. Therefore, rotation and rehybridization for oxygen orbitals10 would be preferred for the Na system. Indeed, crystal structures of desodiated phases (Figure S11) clearly show larger distortion than those of delithiated one. The results of theoretical calculation suggest that oxide ion redox is a reversible process even though oxygen dimerization is rather preferred for the Na system compared with the Li counterpart. Charge compensation mechanisms were further examined by soft X-ray absorption spectroscopy (XAS). Figure 4 compares 5045
DOI: 10.1021/acs.chemmater.7b00172 Chem. Mater. 2017, 29, 5043−5047
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Chemistry of Materials
shown in Figure S5, whereas micrometer size particles prepared by conventional calcination method are used for the Li counterpart. To test this hypothesis, micrometer size and metastable NaxNb0.3Mn0.4O2 was prepared by a different route, i.e., electrochemical Li+/Na+ ion-exchange22 from micrometer size Li1.3Nb0.3Mn0.4O2. As shown in Figure S16, a much higher reversible capacity is obtained for the sample prepared by electrochemical ion-exchange, and a sodium-excess compound with much higher crystallinity is successfully obtained. Therefore, further optimization of particle sizes and chemical compositions is expected to realize the stabilization of the solid-state redox reaction of oxide ions for Na battery applications, like Ru system,12,13 leading to the development of high-energy Na batteries made from the earth-abundant elements. In summary, a metastable cation-disordered rocksalt oxide, Na1.3Nb0.3Mn0.4O2, has been successfully prepared by the mechanical milling. Although oxygen loss is the dominative process on the initial charge for nanosize Na1.3Nb0.3Mn0.4O2, the sample delivers large reversible capacities based on the twoelectron Mn2+/Mn4+ redox. Moreover, expensive Nb ions can be replaced to nonexpensive Ti ions, similar to the lithium counterpart.11,23 Further systematic studies will contribute the development of high-energy rechargeable batteries for energy storage applications in the future.
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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.7b00172. Synthesis of the sample by conventional calcination, its electrochemical properties, SEM/EDX mapping for the samples, heating of Na1.3Nb0.3Mn0.4O2 prepared by mechanical milling, electron diffraction and EDX spectra of Na 1 . 3 Nb 0 . 3 Mn 0 . 4 O 2 , ex situ XRD data of Na1.3Nb0.3Mn0.4O2 after 30 cycle test, the detailed results of first-principles calculations, XAS spectra of the reference samples, Nb L-edge and Mn K-edge XAS spectra of Na1.3Nb0.3Mn0.4O2 (PDF)
Figure 4. Changes in soft XAS spectra of Na1.3−xNb0.3Mn0.4O2−δ on initial charge/discharge processes with fluorescence yield.
No contribution of Nb for charge compensation is also noted in Figure S14. The observed reversible capacity reaches 200 mAh g−1, nearly corresponding to the theoretical capacity of the twoelectron Mn2+/Mn4+ redox (187 mAh g−1). These results suggest, for nanosize Na1.3−xNb0.3Mn0.4O2, Mn2+/Mn4+ redox is a dominant process and the contribution of oxide ion redox is rather small. Such a trend is also supported by the firstprinciples calculations. The formation of unstable superoxide species is expected to be promoted for the Na system (formation of superoxide species was not experimentally evidenced in O K-edge as observed for Li1.3−xNb0.3Fe0.4O221). Although these results are rather disappointing, the metastable phase delivering large reversible capacities has been successfully prepared, and no report for the Mn2+/Mn4+ redox for Na batteries is found in the literature. A similar trend is noted for Mn K-edge (Figure S15). Note the oxygen loss and densification process7 cannot fully explain the large initial reversible capacity. The creation and stabilization of oxygen vacancies in the lattice is possibly responsible for large initial capacities. Nevertheless, crystallinity is drastically reduced on continuous cycles as shown in Figure S6. Moreover, major inconsistency is noted for theoretical and experimental findings for Na1.3−xNb0.3Mn0.4O2. The theoretical results shown in Figure 3 and Table 1 suggest oxygen redox is stable, which is totally different from the experimental observation shown in Figure 4. It is hypothesized this fact probably originates from the difference in surface area of the samples. Particle size of the sample prepared by mechanical milling is nanosize, 1−4 nm, as
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AUTHOR INFORMATION
Corresponding Author
*N. Yabuuchi, e-mail:
[email protected]. ORCID
Alexey M. Glushenkov: 0000-0002-4851-839X Naoaki Yabuuchi: 0000-0002-9404-5693 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported in part by MEXT program “Elements Strategy Initiative to Form Core Research Center”, MEXT; Ministry of Education Culture, Sports, Science and Technology, Japan, and by “Materials research by Information Integration” Initiative (MI2I) project of the Support Program for Starting Up Innovation Hub from Japan Science and Technology Agency (JST). The experiment was performed (The XAS data were obtained) at SR center, Ritsumeikan Universtiy under the approval of Program Review Commitee (S16011). The synchrotron hard X-ray absorption work was 5046
DOI: 10.1021/acs.chemmater.7b00172 Chem. Mater. 2017, 29, 5043−5047
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Chemistry of Materials
(17) McCalla, E.; Abakumov, A. M.; Saubanère, M.; Foix, D.; Berg, E. J.; Rousse, G.; Doublet, M.-L.; Gonbeau, D.; Novák, P.; Van Tendeloo, G.; Dominko, R.; Tarascon, J.-M. Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries. Science 2015, 350, 1516−1521. (18) Saubanère, M.; McCalla, E.; Tarascon, J. M.; Doublet, M. L. The intriguing question of anionic redox in high-energy density cathodes for Li-ion batteries. Energy Environ. Sci. 2016, 9, 984−991. (19) Luo, K.; Roberts, M. R.; Hao, R.; Guerrini, N.; Pickup, D. M.; Liu, Y. S.; Edstrom, K.; Guo, J.; Chadwick, A. V.; Duda, L. C.; Bruce, P. G. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen. Nat. Chem. 2016, 8, 684−691. (20) Du, K.; Zhu, J.; Hu, G.; Gao, H.; Li, Y.; Goodenough, J. B. Exploring reversible oxidation of oxygen in a manganese oxide. Energy Environ. Sci. 2016, 9, 2575−2577. (21) Yabuuchi, N.; Nakayama, M.; Takeuchi, M.; Komaba, S.; Hashimoto, Y.; Mukai, T.; Shiiba, H.; Sato, K.; Kobayashi, Y.; Nakao, A.; Yonemura, M.; Yamanaka, K.; Mitsuhara, K.; Ohta, T. Origin of Stabilization and Destabilization in Solid-State Redox Reaction of Oxide Ions for Rechargeable Lithium Batteries. Nat. Commun. 2016, 7, 13814. (22) Kataoka, R.; Mukai, T.; Yoshizawa, A.; Sakai, T. Development of High Capacity Cathode Material for Sodium Ion Batteries Na0.95Li0.15(Ni0.15Mn0.55Co0.1)O2. J. Electrochem. Soc. 2013, 160, A933−A939. (23) Yabuuchi, N. Solid-state Redox Reaction of Oxide Ions for Rechargeable Batteries. Chem. Lett. 2017, 46, 412−422.
done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2015G529). STEM work was carried out with the support from Deakin Advanced Characterization Facility. A.M.G. acknowledges the funding support from Australian Research Council Discovery grant DP160101178.
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DOI: 10.1021/acs.chemmater.7b00172 Chem. Mater. 2017, 29, 5043−5047
