Sodium-Ion Intercalation Mechanism in MXene Nanosheets Satoshi Kajiyama,† Lucie Szabova,‡ Keitaro Sodeyama,‡,∥ Hiroki Iinuma,† Ryohei Morita,§ Kazuma Gotoh,§,∥ Yoshitaka Tateyama,‡,∥ Masashi Okubo,†,∥ and Atsuo Yamada*,†,∥ †
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan ∥ Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡
S Supporting Information *
ABSTRACT: MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solidstate 23Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na+ electrolyte exhibits reversible Na+ intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na+ is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na+ and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na+ intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability. KEYWORDS: MXene, intercalation, sodium-ion battery, negative electrode, 23Na NMR sodium.21−38 For example, MXene Ti2CTx operates as the highpower negative electrode materials for sodium-ion batteries.38 However, despite their superior electrode performance, the reaction mechanism of the MXene electrodes has not been fully characterized to date. Of primary interest is whether the MXene electrode exhibits ion intercalation between the MXene nanosheets or ion adsorption on the MXene surface. In the present work, we study the reaction mechanism of the MXene Ti3C2Tx in a nonaqueous Na+ electrolyte. Since Ti3C2Tx possesses both higher electronic conductivity and better chemical stability than Ti2CTx,22 Ti3C2Tx can be a suitable option for high-power sodium-ion batteries with long cycle life.33,39 Here, using solid-state 23Na magic angle spinning (MAS) NMR and density functional theory (DFT) calculation, we demonstrate that the Ti3C2Tx electrode exhibits desolvated sodium-ion intercalation between the swelled Ti3C2Tx layers
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evelopment of high-performance electrochemical energy storage devices has attracted increasing attention because of strong social and industrial demands for their widespread use in electric vehicles and smart grids. Sodium-ion batteries are the main potential alternative to the state-of-the-art lithium-ion batteries in terms of abundance and low cost of sodium.1−3 However, sodium-ion intercalation requires ion storage sites specifically suitable to its ionic radius, thereby some typical lithium-ion intercalation compounds such as graphite are not capable of reversible sodium-ion intercalation.4,5 Therefore, exploration for novel sodium-ion intercalation electrode materials is an urgent task to realize advanced sodium-ion batteries.6−15 Layered compounds have been recognized as one of the efficient sodium intercalation electrode materials: the flexible interlayer space can accommodate various ions at high charge/ discharge rate.16−20 In particular, a new family of layered compounds consisting of nanosheets, MXene (Mn+1XnTx: M = Ti, V, Nb, etc.; X = C, N; n = 1−3; Tx is the functional termination group), is an emerging electrode material capable of the electrochemical reaction with various ions including © XXXX American Chemical Society
Received: November 4, 2015 Accepted: February 12, 2016
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to 9.7 Å by HF treatment (Figure 2c,d), which is in good agreement with the XRD data. As Ti3C2Tx is a layered compound consisting of nanosheets, the electrochemical reaction with a nonaqueous Na+ electrolyte may correspond to Na+ intercalation/extraction between the Ti3C2Tx layers and/or Na+ adsorption on the Ti3C2Tx surface. To reveal the electrochemical reaction quantitatively, galvanostatic sodiation/desodiation were conducted (Figure 3). The
without substantial structural change, leading to high cycle stability together with excellent rate capability.
RESULTS AND DISCUSSION Ti3AlC2 was synthesized by high-frequency induction heating at 1300 °C under an argon flow. The powder X-ray diffraction (XRD) pattern confirms the successful synthesis of phase-pure Ti3AlC2 (Figure 1). The (002) peak shifts to a lower angle
Figure 1. Powder X-ray diffraction patterns for Ti3AlC2 and Ti3C2Tx.
upon hydrofluoric acid (HF) solution treatment of Ti3AlC2 for 15 h at room temperature,21 suggesting expansion of the interlayer distance from 9.25 to 9.67 Å. Scanning electron microscopy (SEM) images show an exfoliated layered structure after HF treatment (Figure 2a,b), while energy-dispersive X-ray
Figure 3. (a) Sodiation/desodiation potential profiles and (b) cycle performance for Ti3C2Tx at 20 mA/g. The first sodiation− desodiation potential profiles are shown in the inset of (a).
first sodiation delivers a specific capacity of 270 mAh/g, which corresponds to ca. 2.0 Na+ intercalation/sorption per the formula unit of Ti3C2Tx (provided T = O and x = 2). However, the amount of Na+ detected by EDX is much smaller (0.9 Na+ per Ti3C2Tx), thus the first sodiation may involve the electrolyte decomposition in parallel to Na+ intercalation/ sorption. The electrolyte decomposition explains the irreversible current flow below 0.9 V in the first cyclic voltammetry (Figure S1) and dQ/dV curves (Figure S2). Presumably, the electrolyte decomposition leads to the formation of a stable solid electrolyte interphase (SEI), suppressing further electrolyte decomposition in the subsequent cycles. After several initial cycles, Ti3C2Tx shows good cycle stability with the specific capacity of ca. 100 mAh/g over 100 cycles (Figure 3b). The capacity of 100 mAh/g corresponds to 0.75 Na+ intercalation/ sorption (per formula unit of Ti3C2O2). Having inferred reversible Na+ intercalation/sorption for the Ti3C2Tx electrode, we conducted ex situ XRD measurements (Figure 4) to clarify the structural change during the reaction. During the first sodiation process, the (002) peak shifts to a lower angle, suggesting a significant expansion of the interlayer distance from 9.7 Å (pristine) to 12.5 Å (0.1 V). TEM images also show the multilayered stacking with the expanded interlayer distance upon the first sodiation process (Figure 4). In contrast, only a slight interlayer distance change occurs during the first desodiation and the second sodiation/
Figure 2. SEM images for (a) Ti3AlC2 and (b) Ti3C2Tx and TEM images for (c) Ti3AlC2 and (d) Ti3C2Tx.
(EDX) spectroscopy indicates selective removal of Al (Table S1). All results confirm the formation of exfoliated MXene Ti3C2Tx (Tx = OH, O, and/or F functional termination groups) (Scheme S1).22 The interlayer distance observed by transmission electron microscopy (TEM) is expanded from 9.3 B
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sodiation of Ti3C2Tx accounts for Na+ intercalation between the Ti3C2Tx layers or Na+ adsorption on the Ti3C2Tx layer. To identify the reaction mechanism, we performed ex situ 23Na MAS NMR (Figure 5). All the NMR spectra display a signal ranging from −30 to +20 ppm, which can be deconvoluted into three peaks (Figure 5a; the first sodiation, 0.1 V): peak A (black line in Figure 5a) centered at −9 ppm is assigned to solvated Na+ from the residual electrolyte because of the narrowness of its line width.41 The other two broad peaks centered at −8 and +5 ppm (peaks B and C, blue and red lines in Figure 5a) indicate the existence of two different Na. In general, the MAS NMR peak shift, σ, can be described by σ = σcs + σhf, where σcs is a negative shift from diamagnetic shielding and σhf is a positive shift from the hyperfine interaction with local magnetic moments.42−45 Peak A (Na+ in the electrolyte trace) appears at the lowest frequency (most negative in parts per million) because Na+ in the electrolyte is strongly shielded by the electron donated from the solvent molecules and counteranions.42 As peak B has the similar negative chemical shift, Na for peak B is solvated Na+ as in the liquid electrolyte, whereas the broad line width of peak B suggests the low mobility of solvated Na+. A possible explanation for the less mobile solvated Na+ is that the solvated Na+ is trapped in the SEI layer. Alternatively or in parallel, desolvated Na+ in the SEI layer could have a similar coordination environment as the solvation. For example, Na+ in the poly(ethylene oxide) matrix gives the chemical shift of −9 to −13 ppm.46 Furthermore, solvated Na+, which is adsorbed on the external surface and the edge of particles, may also contribute to peak B, as schematically illustrated in Figure 5a. Part of solvated Na+ for peak B should correspond to the electric double-layer capacitance and/or the pseudocapacitance of the Ti3C2Tx electrode. Meanwhile, peak C exhibits a large positive shift, suggesting the existence of the desolvated Na+. It is most probable that completely or at least partially desolvated Na+ is intercalated into the metallic Ti3C2Tx layers,33 where the Pauli magnetic susceptibility induces the Knight shift. On the basis of the above assignment, ex situ NMR spectra during the initial two cycles (Figure 5 and Figure S5) prove reversible desolvated Na+ intercalation/deintercalation between the Ti3C2Tx layers on sodiation/desodiation. Note that peak B, part of which corresponds to Na+ in the SEI layer, is observed for all samples because the SEI layer exists stably once formed. Therefore, it is difficult to evaluate the adsorption and intercalation capacities, respectively, based on the area ratio
Figure 4. Ex situ XRD patterns and TEM images for Ti3C2Tx upon sodiation and desodiation. The scale bar in the TEM images indicates 5 nm.
desodiation. The TEM images also support that the interlayer distance is kept almost constant after the expansion at the first sodiation. It should be mentioned that the increase in the interlayer distance (2.8 Å) by the first sodiation is much larger than those reported for other Na+ intercalation materials (e.g., 1.0 Å for MoS2 and 1.3 Å for TiS2).16,40 For the electronic state during the sodiation/desodiation, ex situ X-ray photoelectron spectroscopy (XPS) images for oxygen show a reversible spectral change (Figure S3), whereas the ex situ X-ray absorption near-edge structure (XANES) for the Ti K-edge exhibits no change (Figure S4). Therefore, the redox reaction occurs mainly at the functional termination groups rather than at Ti. Although the above results clearly demonstrate reversible sodiation/desodiation of Ti3C2Tx, it is not yet clear whether
Figure 5. 23Na MAS NMR spectra to identify the Na insertion mechanism into Ti3C2Tx. (a) NMR signals during the initial two cycles can be deconvoluted into three peaks (black, blue, and red lines), which originate from different Na+ species, as shown in the schematic illustration. (b) Estimated amount of intercalated Na in the layer based on the area ratio of NMR peaks C/B. C
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ACS Nano of peaks C/B. However, when the NMR signal is normalized by the area of peak B (i.e., normalization by the amount of the solvated Na+), the normalized area of peak C increases/ decreases on sodiation/desodiation continuously (Figure 5b). This result suggests that, rather than the solvated Na+ adsorption, desolvated Na+ intercalation is the main electrochemical reaction accounting for sodiation of Ti3C2Tx. A notable feature of peak C is that the peak position does not shift on sodiation/desodiation. One possible explanation is that the electron density at the Fermi level is constant during sodiation/desodiation, leading to the constant Knight shift. Importantly, the NMR spectra indicate that a certain amount of desolvated Na+ (peak C) remains even after full desodiation up to 3.0 V. The trapped Na+ is also confirmed by the elemental analysis by EDX (Table 1). The trapped Na+ could Table 1. Na Content Change during the Sodiation/ Desodiation Estimated from EDX (Na/Ti Ratio) Na/Ti ratio first sodiation first desodiation second sodiation second desodiation
0.31 0.11 0.30 0.18
Figure 6. (a) XRD patterns for fully sodiated/desodiated Ti3C2Tx before and after the drying process at 200 °C under vacuum for 12 h. The XRD pattern for pristine Ti3C2Tx before the cycle is also shown for comaprison. (b) TEM image for the fully sodiated Ti3C2Tx after the drying process. (c) TEM image for the fully desodiated Ti3C2Tx after the drying process.
explain the constant interlayer distance upon sodiation/ desodiation because it may bahave as a pillar to maintain the interlayer distance. However, as mentioned above, the interlayer distance of Ti3C2Tx expands largely (2.8 Å) by sodiation. Indeed, it has also been reported that Na + intercalation expands the interlayer distance of V2CTx by 2.3 Å, which was explained by Na double-layer formation between the V2CTx sheets.37 Nevertheless, this is not the case for Ti3C2Tx because the amount of reacted Na+ is less than 1.0 per the formula unit of Ti3C2Tx as estimated from EDX (Table 1). Furthermore, completely solvated Na+ intercalation (cointercalation of solvent) is less probable because it should cause the large negative chemical shift in the 23Na MAS NMR, and the intercalation of solvated Na+ increases the interlayer distance by more than 4.0 Å,47,48 which is much larger than the observed value. Therefore, another possible explanation is the swelling effect caused by penetration of the uncoordinated solvent molecules into the interlayer space. The solvent molecules do not spontaneously penetrate into the Ti3C2Tx layers, which is confirmed by the XRD pattern and TEM image of the Ti3C2Tx electrode before the cycle (just immersed in the electrolyte) (Figure 4). However, we assume that the expansion of the interlayer distance by desolvated Na+ intercalation triggers the solvent molecule penetration into the interlayer space. To test this hypothesis, we measured the drying effect on fully sodiated/desodiated Ti3C2Tx. Figure 6a shows the XRD patterns for the sodiated/desodiated Ti3C2Tx before and after the drying process at 200 °C under vacuum for 12 h. Both sodiated/desodiated compounds show the shift of the (002) peak to a higher angle after drying, indicating the decrease in the interlayer distance from 12.5 to10.9 Å. Indeed, the TEM images for both sodiated/desodiated Ti3C2Tx confirm the decrease in the interlayer distance to ca. 11 Å (Figure 6b,c). These results clearly suggest that the solvent molecules evaporate from the interlayer space of both compounds by drying. Therefore, during the sodiation/desodiation processes of the Ti3C2Tx electrode, the penetrated solvent molecules
swell the interlayer space, leading to the constant interlayer distance of 12.5 Å. However, it should also be emphasized that the dried desodiated Ti3C2Tx still has an interlayer distance larger than that of the pristine compound. Therefore, the trapped Na+ also serves as a pillar to expand the interlayer distance. To further examine the origin of the large expansion by Na+ intercalation into Ti3C2Tx layers, we performed the DFT calculations for the equilibrium structures of the pristine and Na+-intercalated Ti3C2Tx with various termination groups (Table 2). Based on the optimized structures with the various termination groups such as −O/−F, −OH/−F, or −OH/−O, the interlayer distance changes from 9.6−10.2 to 10.6−12.9 Å by completely desolvated Na+ intercalation (Scheme 1). It is noteworthy that the existence of the OH termination group largely expands the interlayer distance due to the repulsion force between sodium and hydrogen atoms (Scheme 1b). Although the swelling effect by the solvent molecule penetration is not considered in this calculation and its role is not completely understood, the calculated interlayer distances almost agree with the experimental results (12.5 and 10.9 Å for the swelled and dried samples), suggesting the importance of the Na+ pillaring effect. Certainly, the trapped Na+ and the penetrated solvent molecules cooperate to maintain the largely expanded interlayer space during sodiation/desodiation. Figure 7 summarizes the sodiation mechanism of the Ti3C2Tx electrode. At the beginning of the first sodiation, in parallel with the solvated Na+ adsorption, the electrolyte decomposition leading to the SEI formation occurs, which causes the irreversible capacity at the first cycle. Simultaneously, both desolvated Na+ intercalation and the solvent molecule D
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ACS Nano Table 2. Structural Parameters of Pristine and NaIntercalated Ti3C2Tx Crystals with O, F, and OH Terminationsa a [Å] (±0.02) pristineb pristinec intercalated Nab intercalated Nac F−F O−O OH−OH F−O OH−O OH−F F−Na−F O−Na−O OH−Na−OH F−Na−O OH−Na−O OH−Na−F
c [Å] (±0.1)
interlayer distance [Å]
experimental 3.05 19.86 19.4 3.04 24.55 25.0 pristine 3.10 20.4 3.08 20.2 3.10 23.7 pristine/mixed termination 3.08 20.2−20.4 3.08 19.2−19.4 3.11 20.2 Na intercalated 3.16 21.8 3.06 21.1 3.12 29.3 Na intercalated/mixed termination 3.10 21.5−21.7 3.08 25.4 3.12 25.7−25.8
9.93 9.7 12.28 12.5 10.2 10.1 11.85 10.1−10.2 9.6−9.7 10.1 10.9 10.55 14.65 10.75−10.85 12.7 12.85−12.90
a
With a being the in-plane lattice parameter, c the cell parameter parallel to the planes, and interlayer distance being the distance between two neighboring Ti3C2Tx layers, as described in Scheme 1. b Cell parameters are from a previous report.49 cCell parameters are estimated from experimental data in the present study.
Figure 7. (a) Change in the interlayer distance and the intercalated Na amount during the initial two cycles. (b) Schematic illustration for the proposed mechanism of Na+ insertion into Ti3C2Tx.
high-charge/discharge-rate capability. Indeed, the Ti3C2Tx electrode retains a high specific capacity of 70 mAh/g at a high specific current of 500 mA/g (63% of the capacity at 20 mA/g) (Figure S6). We postulate that the excellent rate capability of Ti3C2Tx results from fast Na+ diffusion in the expanded interlayer space as well as zero-strain sodiation/ desodiation.
penetration occur between the Ti3C2Tx layers to expand the interlayer distance from 9.7 to 12.5 Å (Figure 7a,b, an activation process). After expansion of the interlayer space, reversible intercalation/deintercalation of desolvated Na+ between the Ti3C2Tx layers proceeds as the dominant electrochemical reaction, where the interlayer distance remains practically constant due to the trapped Na+ pillaring and the solvent molecule swelling. When the reaction mechanism proposed in Figure 7 is taken into account, it can be expected that reversible sodiation/ desodiation without structural changes is advantageous to the
CONCLUSION In summary, we have demonstrated that the MXene Ti3C2Tx electrode in a nonaqueous Na+ electrolyte exhibits reversible
Scheme 1. Optimized Structures for (a) Pristine Ti3C2Tx and Na+-Intercalated Ti3C2Tx with the Various Termination Groups: (b) −OH/−F, (c) −O/−F, and (d) −F/−F
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ESPRESSO package.51 We performed spin-unpolarized calculations and included van der Waals interaction contribution empolying the vdW-df2-b86r52−55 functional to describe the interaction between the layers. The energy cutoffs for the plane wave basis set and electron density representation were set up as 60 and 600 Ry, respectively. To correctly capture the electronic structure of Ti, we employed the DFT +U method in the implementation of Cococcioni and de Gironcoli56 with a Hubbard U term of 4.2 eV57 added to the exchange-correlation functional. The core valence interaction was described by plane wave Vanderbilt58 (Ti, O, H) and Rappe, Rabe, Kaxiras, and Joannopoulos59 (C, F, Na) ultrasoft pseudopotentials. The structure calculations were performed using a hexagonal 1 × 1 × 1 cell in the P63/mmc space group composed of two layers of Ti3C2 with F, O, and OH terminations with and without intercalated Na atoms. Integrals in the Brillouin zone were calculated numerically by using a finite Monkhorst−Pack60 12 × 12 × 4 k-point mesh together with Gaussian smearing of 0.005 eV.
desolvated Na+ intercalation/deintercalation into the stacked Ti3C2Tx layers on sodiation/desodiation. The interlayer distance is expanded by the first sodiation process because of desolvated Na+ intercalation and solvent molecule penetration. While the trapped Na+ behaves as a pillar, the penetrated solvent molecules swell the interlayer space, both of which contribute to keep the interlayer distance constant during the sodiation/desodiation processes. Therefore, the electrochemical reaction is not accompanied by any substantial change in the interlayer distance, leading to high cycle stability in addition to fast Na+ diffusion in the expanded interlayer space. Such features extend the MXene-based electrodes as promising potential candidates for advanced energy storage devices.
METHODS Ti3C2Tx Synthesis. Ti3AlC2 was prepared from a precursor mixture of TiC (>99%, High Purity Chemicals, Japan) and Ti2AlC with a molar ratio of 0.8:1. The precursor Ti2AlC was synthesized by following the method in our previous report.38 The precursor mixture placed into a graphite crucible was heated by a high-frequency induction furnace at 1300 °C for 1 h under Ar gas flow condition. MXene Ti3C2Tx was synthesized by treating 1 g of Ti3AlC2 powder in 10 mL of 46% HF aqueous solution (Wako) for 15 h at room temperature. After filtration and washing with deionized water, the HF-treated powder was dried in vacuum at 60 °C for 24 h. Characterization. Powder X-ray diffraction patterns were recorded on a Rigaku RINT-TTR III powder diffractometer with Cu Kα radiation in a step of 0.02° over a 2θ range of 5−80°. Morphologies of resultant materials were observed with scanning electron microscopy (Hitachi, S-4800 operated at 3 kV) and transmission electron microscopy (JEOL, JEM-2100 operated at 200 kV). Elemental analysis was conducted with an energy-dispersive X-ray spectrometer equipped with a JEM-6510LA SEM operated at 15 kV. X-ray photoelectron spectroscopy data were recorded with a ULVAC PHI 5000 VersaProbe spectrometer with monochromatized Al Kα radiation (hν = 1486.6 eV). The pressure in the chamber during XPS measurements was maintained in the 10−6−10−7 Pa range. The sample was placed on conductive carbon, and the peaks were recorded with a constant pass energy mode of 117 eV for survey investigation. Highresolution spectra were taken at pass energy of 23.5 eV, with a step of 0.2 eV. All binding energies were referenced to that of free carbon at 284.5 eV. XANES was performed using synchrotron radiation on beamline BL-9A of the Photon Factory. The spectra were recorded in the transmission mode under ambient condition. The energy of the Xray was calibrated by the metal foil. For 23Na magic angle spinning NMR measurements, samples were sealed into a 3.2 mm ϕ sample rotor in Ar-filled glovebox. 23Na MAS spectra at 133.2 MHz were recorded using an Agilent DD2 spectrometer and a 11.7 T magnet with a 3.2 mm ϕ spinning module at the spinning rate of 18 kHz. The spectra were referenced to 1 M NaCl aqueous solution as 0 ppm. Electrochemical Characterization. For preparation of the working electrode to evaluate the electrochemical performance, 80 wt % Ti3C2Tx, 10 wt % acetylene black, and 10 wt % polyvinylidene difluoride binder were mixed in a minimal amount of Nmethylpyrrolidone (Kanto Chemical) solvent to make a slurry. This slurry was coated on an aluminum foil as current collector, and the asobtained sheet was dried overnight at 120 °C under vacuum. CR2032type coin cells were assembled with Na metal as the counter electrode, a glass fiber membrane as the separator, and 1 M NaPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v, Kishida Chemical) as the electrolyte. These cells were assembled inside an Ar-filled glovebox (Miwa Inc., Japan) (dew point < −100 °C). Sodiation−desodiation potential profiles were recorded at various current rates (20−500 mA/ g) in the range of 0.1−3.0 V (vs Na/Na+). Structural Calculation. The calculations were performed using the DFT method, employing the generalized gradient approximation in the formulation of Perdew−Burke−Ernzerhof50 for the exchangecorrelation functional as implemented in the PWscf code of Quantum
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06958. Additional experimental data for the characterization and electrochemical properties (PDF)
AUTHOR INFORMATION Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grant-in-Aid for Specially Promoted Research No. 15H05701. This work was also supported by MEXT, Japan, under the “Elements Strategy Initiative for Catalysts and Batteries (ESICB)”. M.O. was financially supported by the Ministry of Education, Culture, Science and Technology of Japan, Grant-in-Aid for Scientific Research (B) No. 15H03873. M.O. was also supported by the Murata Science Foundation. The X-ray absorption spectroscopy was carried out under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2014G044). The calculations were carried out on the supercomputers in ISSP, The University of Tokyo and Kyushu University through the HPCI Systems Research Project (Proposal No. hp150068). REFERENCES (1) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636− 11682. (2) Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-Ion Batteries. Adv. Funct. Mater. 2013, 23, 947−958. (3) Yamada, A. Iron-Based Materials Strategies. MRS Bull. 2014, 39, 423−428. (4) Komaba, S.; Murata, W.; Ishikawa, T.; Yabuuchi, N.; Ozeki, T.; Nakayama, T.; Ogata, A.; Gotoh, K.; Fujiwara, K. Electrochemical Na Insertion and Solid Electrolyte Interphase for Hard-Carbon Electrodes and Application to Na-Ion Batteries. Adv. Funct. Mater. 2011, 21, 3859−3867. (5) Tsai, P. C.; Chung, S. C.; Lin, S. K.; Yamada, A. Ab Initio Study of Sodium Intercalation into Disordered Carbon. J. Mater. Chem. A 2015, 3, 9763−9768. F
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DOI: 10.1021/acsnano.5b06958 ACS Nano XXXX, XXX, XXX−XXX
