A Size-Dependent Sodium Storage Mechanism in Li4Ti5O12

Sep 20, 2013 - XANES and EXAFS data were analyzed by ATHENA software package.(49) The ..... Kim , S. W.; Seo , D. H.; Ma , X. H.; Ceder , G.; Kang , K...
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A Size-Dependent Sodium Storage Mechanism in Li4Ti5O12 Investigated by a Novel Characterization Technique Combining in Situ X‑ray Diffraction and Chemical Sodiation Xiqian Yu,†,§ Huilin Pan,‡,§ Wang Wan,† Chao Ma,‡ Jianming Bai,† Qingping Meng,† Steven N. Ehrlich,† Yong-Sheng Hu,*,‡ and Xiao-Qing Yang*,† †

Brookhaven National Laboratory, Upton, New York 11973, United States Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese academy of Sciences, Beijing, 100190, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: A novel characterization technique using the combination of chemical sodiation and synchrotron based in situ X-ray diffraction (XRD) has been detailed illustrated. The power of this novel technique was demonstrated in elucidating the structure evolution of Li4Ti5O12 upon sodium insertion. The sodium insertion behavior into Li4Ti5O12 is strongly size dependent. A solid solution reaction behavior in a wide range has been revealed during sodium insertion into the nanosized Li4Ti5O12 (∼44 nm), which is quite different from the wellknown two-phase reaction of Li4Ti5O12/Li7Ti5O12 system during lithium insertion, and also has not been fully addressed in the literature so far. On the basis of this in situ experiment, the apparent Na+ ion diffusion coefficient (DNa+) of Li4Ti5O12 was estimated in the magnitude of 10−16 cm2 s−1, close to the values estimated by electrochemical method, but 5 order of magnitudes smaller than the Li+ ion diffusion coefficient (DLi+ ∼10−11 cm2 s−1), indicating a sluggish Na+ ion diffusion kinetics in Li4Ti5O12 comparing with that of Li+ ion. Nanosizing the Li4Ti5O12 will be critical to make it a suitable anode material for sodium-ion batteries. The application of this novel in situ chemical sodiation method reported in this work provides a facile way and a new opportunity for in situ structure investigations of various sodium-ion battery materials and other systems. KEYWORDS: Sodium-ion batteries, Li4Ti5O12, in situ X-ray diffraction, size effect, chemical sodiation

I

C,19,20 Na4Fe3(PO4)2P2O7,21 amorphous FePO4,22 and Prussian blue-based compounds23−25 as cathode, and Na2Ti3O7,26 Na2C8H4O4,27 Sn,28 Sb/C,29 SnSb/C,30 and hard carbon31,32 as anode. Unlike what might be assumed that sodium insertion and extraction is similar as lithium, recent detailed structural studies on NaxCoO2,14 NaxVO2,16 and Sb29 show that sodium storage mechanisms in most cathode/anode materials could be quite complicated and significantly different from the corresponding lithium compounds. Unfortunately, for many electrode materials used in Na-ion batteries the sodium storage mechanisms and the relationship between the structural changes and their electrochemical performance are far from well understood. Therefore, completely structural or physical characterization of the materials is quite necessary and the development of new and facile in situ techniques, especially dedicated to sodium storage system, will be very desirable. In this communication, we report our recent progress in developing a novel characterization technique using the

n order to develop new electrode materials for the next generation of energy storage devices such as batteries and supercapacitors with higher energy and power density, longer cycling life and better safety characteristics, the in-depth understanding of the relationship between structural change and electrochemical performance of electrode materials is quite critical. For the current state-of-the-art battery systems, the lithium-ion batteries, various in situ techniques have been developed to study the structural changes relating to the delihiation and lithiation of electrode materials during electrochemical cycling. These in situ techniques include X-ray diffraction (XRD),1,2 X-ray absorption spectroscopy (XAS),3 neutron powder diffraction (NPD),4 nuclear magnetic resonance (NMR),5 transmission X-ray microscopy (TXM),6 and transmission electron microscopy (TEM).7−10 Recently, due to the abundant sodium resources and their potentially low cost in raw materials11−32 research and development of room-temperature sodium (Na)-ion batteries have attracted extensive interest for application in large-scale stationary energy storage and smart grid. Several electrode materials have been investigated, including NaxCoO2,14 NaxMnO2,15 NaxVO2,16 NaxFe1/2Mn1/2O2,17 Na1.0Li0.2Ni0.25Mn0.75O6,18 Na3V2(PO4)3/ © 2013 American Chemical Society

Received: June 20, 2013 Revised: September 12, 2013 Published: September 20, 2013 4721

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reducing solution used in this experiment is sodium-biphenyl1,2-dimethoxyethane (DME) solution.26,35 The reduction potential is proved to be lower than 0.3 V (vs Na+/Na).26 The color of the initial reduction solution is dark green; after reaction with Li4Ti5O12 stoichiometrically, the color of the solution faded to almost colorless and the pristine white Li4Ti5O12 powder turns black (as shown in Figure 1b). For the in situ experiment, over stoichiometric sodium-reducing solution was used to ensure the fully sodiation of the Li4Ti5O12. (Note that the impact of the concentration on redox potential is discussed in Supporting Information.) XRD pattern of the pristine Li4Ti5O12 (nanosized, averaged particle size ∼44 nm) is plotted in Figure 1c. All peaks can be indexed with cubic spinel structure (space group: Fd3m ̅ ), indicating the pure Li4Ti5O12 phase. No other peaks are apparent to make the XRD pattern much cleaner than those collected through in situ electrochemical cells with reflection peaks from cell components and current collectors. The refined lattice constant for the pristine Li4Ti5O12 is a = 8.3526(1) Å, slightly smaller than the reported value in the literature,36 which may be due to the nature of the nanosized Li4Ti5O12 used herein.37 The changes of the XRD patterns as a function of reduction reaction time are plotted in Figure 1d (corresponding conventional XRD patterns are plotted in Supporting Information Figure S1). The intensity of the pattern is represented by the color scale (in the left). With increasing reaction time, the peak intensities for Li4Ti5O12 decrease while a new peak set appears on the left side of the Li4Ti5O12 (400) and (440) reflections, respectively, with increasing intensities. After 2 h of reaction time, no significant changes of the XRD patterns can be observed, indicating the completeness of the sodiation process. Rietveld refinement was performed on the XRD pattern of the final product (Figure 1e). These new peaks can also be indexed with a cubic spinel structure, which was identified in recent work as Na6(16c)LiTi5(16d)O12(32e).35 The refinement results show a ratio of 46.7:53.3 between Li7Ti5O12 and Na6LiTi5O12 phases, in good agreement with the estimated reaction of 2Li4Ti5O12 + 6Na ↔ Li7Ti5O12 + Na6LiTi5O12. (The corresponding refinement details are presented in Supporting Information Figure S2 and Table 1.) To further describe the sodium insertion behavior of the nanosized Li4Ti5O12, the (400) diffraction peaks were fitted with profile shape functions by Jade 6 software package in order to get peak position, full width at half-maximum (FWHM), and peak height; the results are shown in Figure 2a−c, respectively. Although the recent theoretical estimation suggests a Li7Ti5O12 and Na6LiTi5O12 phase separation is more energy favorable than (NaxLi6−x)LiTi5O12 solid solution,35 a continuous peak shift can be observed (Figure 2a), most pronounced at the initial state during our in situ chemical sodiation process, indicating a solid solution behavior (will be further evidenced from the discharge profile as discussed later). Note that the average particle size of the used Li4Ti5O12 is ∼44 nm; this discrepancy could be elucidated with the size-induced enhancement of the solid solution domain thermodynamically. It is well-known that the influence of the interface energy will be enhanced in nanosized material systems.38−41 The strains occurring between the different inserted phases can be considered as a driving force that could alter the phase transition behavior to avoid formation of interface, as evidenced in many cases.40,41 However, the solid solution behavior for sodium insertion into Li4Ti5O12 is much more complicated since two solid solution domain regions (sodium-rich phase

combination of chemical sodiation and in situ X-ray diffraction to study the structure changes of Li4Ti5O12 during sodium insertion. In contrast with most reported in situ XRD experiments (Na-ion batteries) performed during electrochemical cycling, which require special designed electrochemical cells, the chemical sodiation reported herein was carried out in a capillary, making it very easy to be carried out. The chemical method also demonstrates its advantages and uniqueness; very clean and high-quality XRD patterns can be obtained for detailed structural analysis (e.g., no current collector peaks as often seen in in situ electrochemical cell); preferred orientation if existed can be reduced (due to the rotation of the material within the capillary); easy to be combined with other devices; the effects of electronic conduction issues in electrochemical cell such as C-coating, carbon additives can be neglected (kinetic issues), etc. The chemical sodiation of the Li4Ti5O12 and the in situ XRD results reported here clearly demonstrate the power of this in situ technique. Li4Ti5O12 is well-known as a “zero-strain” anode material for lithium-ion batteries, which undergoes two-phase reactions (Li4Ti5O12/Li7Ti5O12) and exhibits superior rate performance during lithium insertion/extraction.33 Recent works show that sodium can also be reversibly inserted/ extracted into/from Li4Ti5O12 framework through a complicated three-phase transition behavior.34,35 The detailed studies reported herein reveal the sodium storage behaviors in Li4Ti5O12 are size dependent. As illustrated in Figure 1a, the in situ experiment was carried out using a glass capillary with 0.7 mm inner diameter. The

Figure 1. The evolution of XRD patterns during in situ chemical sodiation of nanosized (44 nm) Li4Ti5O12. (a) Schematic of the in situ chemical sodiation experiment setup by using a glass capillary; sodiumbiphenyl-1,2-dimethoxyethane (DME) solution was used as reducing agent; (b) A scheme of capillaries before (left) and after (right) a scheme of sodiation; (c) XRD pattern for the Li4Ti5O12 at the beginning of reaction; (d) contour plot of peak intensities as a function of reaction time; (e) XRD pattern for the final sodiated Li4Ti5O12 at the end of reaction. 4722

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Table 1. Summary of the Rietveld Refinement of the Pristine and Fully Sodiated Li4Ti5O12 phase

a (Å)

stain (%)

size (nm)

Li4Ti5O12_44 nm Li4Ti5O12_120 nm Li4Ti5O12_440 nm Li4Ti5O12_sodiated_44 nm Phase1-Li7Ti5O12 Phase2-Na6LiTi5O12 Li4Ti5O12_sodiated_120 nm Phase1-Li7Ti5O12 Phase2-Na6LiTi5O12

8.3526(1) 8.3585(1) 8.3596(3)

0.049 0.002 0

50.9(2.1) 132.5(2.4) 419.9(7.0)

8.4203(1) 8.6696(1)

0.027 0.777

18.3(0.7) 18.7(0.7)

8.3852(4) 8.6985(9)

0.257 0.565

53.8(6.9) 17.8(0.9)

ratio (mol %)

Rwp

Rp

1.534 7.344 8.167

0.903 4.231 4.571

46.71 53.29

3.359

1.086

46.86 53.14

2.014

2.035

100 100 100

phases to be varied with the overall composition. Consequently, the line-width broadening (corresponding to FWHM and peak height) in the diffraction patterns as a function of composition (reaction time) will vary, as clearly observed in Figure 2b,c. Because of the very close lattice constant between Li4Ti5O12 and Li7Ti5O12, we cannot quantitatively distinguish them. However, the evolution of the peak features regarding sodiumrich phase and lithium-rich phase regions can be clearly distinguished. Differentiated from the lithium-rich phase, a very broad peak feature can be observed for the newly formed sodium-rich phase during the entire sodiation process, indicating this newly formed phase has reduced crystallite grain size and/or a significant amount of structure defects and strains compared with original Li4Ti5O12. The Rietveld refinement performed on the final sodiated product (Table 1) further confirms that quite large strain (0.77%) retained within the Na6LiTi5O12 new phase. The large retained strain energy may therefore contribute to the thermodynamics42 of the sodiated/desodiated reaction, which can be directly reflected by the large hysteresis between the discharge and charge curve (Supporting Information Figure S3). The strain energy is estimated using Mott and Nabarro theory43 quantitatively and the detailed results are presented in Supporting Information Table S1. The experimental observation (∼0.2 V) agrees with our theoretical estimation. Finally, it should be noted that the phase evolution observed during in situ chemical sodiation can be well reproduced through in situ

Figure 2. The evolution of peak position (a), FWHM (b), and peak intensity of (400) reflection (c) for both Li4/(Li7)Ti5O12 and Na6LiTi5O12 phases. The first XRD pattern was recorded after 5 min due to the time required for the alignment of the capillary under X-ray beam.

and lithium-rich phase) could be involved. The growth of sodium-rich and lithium-rich phases in expense of the original Li4Ti5O12 phase will cause the crystal domain size of both

Figure 3. Experimental/calculated Ti K-edge (a) XANES spectra and (e) experimental EXAFS spectra of Li4Ti5O12, Li7Ti5O12 (lithiated Li4Ti5O12), and fully sodiated Li4Ti5O12 (Li7Ti5O12 and Na6LiTi5O12 in 1:1 ratio), Na6LiTi5O12 can be proved to be existed in the sodiated product; (b) enlarged view of the XANES pre-edge region as marked by rectangle in panel a; (c) Ti valences of the Li4Ti5O12, Li7Ti5O12 and sodiated Li4Ti5O12 estimated by comparing with standard titanium oxides; (d) crystal structure of Li4Ti5O12 and Na6(Li6)LiTi5O12, Na6LiTi5O12 adopts the same structure of the Li7Ti5O12, with the Na ions occupying the 16c site instead of Li. Because of the large ionic radii of the Na ion, Ti−O and Ti−Ti bond distance are expanded as evidenced by the EXAFS results. 4723

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Figure 4. (a−c) The SEM images and the crystallite size distribution of nanosized (r = 44 nm, r = 120 nm) and submicrosized (r = 440 nm) Li4Ti5O12; (d) in situ XRD patterns collected during chemical sodiation of the nanosized 44 nm (top), 120 nm (middle), and sub-microsized (bottom) Li4Ti5O12, the sodium insertion caused phase transition behavior is strongly related to the particle size of the material. Full sodium insertion can be finished within 2 h for nanosized Li4Ti5O12, while for the submicrosized Li4Ti5O12, no new phase can be observed within the initial 8 h during chemical sodiation; (e) Enlarged view of the (111) reflections for the pristine Li4Ti5O12; (f) XRD patterns of the submicrosized Li4Ti5O12 collected after chemical sodiation of 20, 40, and 80 h, respectively, new Na6LiTi5O12 phase emerged after long time reaction; sodiation completed in 2 and 24 h, respectively, for 44 and 120 nm Li4Ti5O12; (g) the charge−discharge curve (first, dash line; 30th, solid line) of Na storage into Li4Ti5O12 (0.1C, 0.5−3 V).

the threshold energy is somewhat different from the experimental one. But we can still see the same trend in these compounds with sodium insertion. Both experimental and calculated spectra shift to lower energy along with sodium insertion, indicating the reduction of Ti ion during sodium insertion. The pre-edge features shown in Figure 3b also suggest the reduction of the Li4Ti5O12 during Li or Na insertion. The three prepeaks, named as A1−3, correspond to transitions of the titanium 1s-core electrons toward the unoccupied Ti 3d-4s/4p hybridized states, which are semilocalized states surrounding the Ti atom. The A1 peak is due to a bonding t2g band-like bonding-state whereas A2 is the result of nondirectional eg antibonding states. Transition A3 has predominantly Ti-4p character hybridized with the Ti-4s and/ or O-2p orbital. The decrease in intensity of the A1 peak suggests a partial filling of the Ti-3d t2g band by the chargecompensating electrons entering the crystallite accompanying the Li or Na insertion. The average valence state of Ti (in bulk level) is estimated by comparing the half energy position with reference titanium oxides semiquantitatively as shown in Figure 3c (corresponding Ti K-edge XANES spectra are shown in Supporting Information Figure S5b). The valence state of sodiated Li4Ti5O12 is very close to that for Li7Ti5O12.

XRD during electrochemical discharge (Supporting Information Figure S4). These results ensured our confidence of this in situ technique as a new powerful and facile complementary tool for in situ structure studies for Na-ion batteries. The XAS is a well-established technique to obtain fundamental information about electronic structure such as valence state of absorbing atoms. The Ti K-edge spectra of Li− Ti−O compounds originate from the excitation of Ti 1s states to the unoccupied p states that are hybridized with the electronic states of O and Li atoms. Figure 3a shows both experimental and calculated (by density functional theory (DFT) method) Ti K-edge X-ray absorption near edge structure (XANES) spectra. For fully sodiated Li4Ti5O12, the calculated spectrum is averaged with that of Li7Ti5O12 and Na6LiTi5O12 in ratio of 1:1 (simulated Na6LiTi5O12 Ti K-edge spectrum is shown in Supporting Information Figure S5a). It was found that the calculated spectra match the experimental ones very well, especially within 25 eV above the threshold energy, further proving the Na6LiTi5O12 phase formed during sodium insertion. It is well-known that the high-energy unoccupied states above the Fermi level are not accurately treated by the DFT method, which might be the reason that the fine structure of the calculated Ti K-edge beyond 25 eV from 4724

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Numerically it is around Ti3.4+ and is well consistent with the atomic electronic energy loss spectroscopy (EELS) observation, where two Ti ions are Ti4+ while the other three Ti ions are Ti3+, averaged as Ti3.4+ in an unit cell.44 This is further proving that almost equal amount of Na has been inserted to Li4Ti5O12 as in the case of Li insertion. Although no charge carrier density change occurs by substituting the isovalent Na ion with Li ion, local crystalline structure changes do take place upon sodium insertion that results in slight changes in chemical bonding and electronic structure, as shown in Supporting Information Figure S6. The extended X-ray absorption fine structure (EXAFS) spectra analysis was further performed to investigate the local environment around Ti. The Ti−O and Ti−Ti coordination environment are similar for both Li4Ti5O12 and Na6/(Li6)LiTi5O12 (as shown in Figure 3d), except for the larger Ti−O and Ti−Ti bond distance for the Na6LiTi5O12 that is due to the larger ionic radii of Na ions. The Fourier transformed (FT) k2weighted χ(k) spectra of Ti in pristine, lithiated, and sodiated Li4Ti5O12 are compared in Figure 3e. The first peak at around 1.5 Å in the FT spectra is due to the titanium−oxygen interaction in the first coordination shell while the second peak at around 2.9 Å is due to the titanium−titanium interaction in the second coordination shell, as labeled in the spectra and shown schematically in Figure 3d. Compared with Li4Ti5O12 and Li7Ti5O12, the significant non-Gaussian line-shape of the fully sodiated Li4Ti5O12 indicates multiple peaks present in both the first and second FT spectrum peaks. This suggests that there are two different Ti−O and Ti−Ti coordination environment within sodiated Li4Ti5O12, corresponding to the Li7Ti5O12 and Na6LiTi5O12 phases as also evidenced by XRD analysis. Both the XRD and XAS results confirm the new Na6LiTi5O12 phase formed along with Li7Ti5O12 during Na insertion. The Na insertion induced large lattice expansion and the complex three-phase interface35 are expected to hinder the Na+ ion diffusion within the particles. On the basis of the in situ sodiation experiment on the nanosized Li4Ti5O12, the apparent diffusion coefficient can be roughly estimated by the equation

on the 120 and 440 nm Li4Ti5O12 and the results are shown in Figure 4d (middle and bottom respectively). It can be observed that the new Na6LiTi5O12 phase nucleated but the reaction does not complete within the observation time period (12 h) for the 120 nm Li4Ti5O12. The sodiation reaction completed after further reacted 12 h (XRD pattern is displayed in Figure 4f), confirmed by the Rietveld refinement results shown in Table 1. Moreover, unlike the observation of the obvious continuous peak shift at the initial chemical sodiation stage for 44 nm Li4Ti5O12, the peaks’ positions of the 120 nm Li4Ti5O12 phase change slightly upon sodiation (Supporting Information Figure S8). This suggests a very narrow solid solution (NaxLi6−xLiTi5O12) region for 120 nm Li4Ti5O12, which is also found in the case of LiFePO4, where the LixFePO4 solid solution behavior only enhances when LiFePO4 particles down to nanosize.46,47 For 440 nm sample, only slight changes, except for peak intensities decrease without new reflections appear, can be observed from the XRD patterns during the initial chemical sodiation process (8 h). Figure 4f shows the XRD patterns of the 440 nm Li4Ti5O12 after chemical sodiation reaction for 20, 40, and 80 h, respectively. The Na6LiTi5O12 phase became more pronounced as the increase of the sodiation time. However, even after sodiation for 80 h, only 36 wt % of the Li4Ti5O12 has been converted to Na6LiTi5O12 and Li7Ti5O12, further indicating the much more sluggish Na+ ion diffusion kinetics comparing with Li+ ion.35,37 The difference in Na-storage behavior among different particle size is also evident from their electrochemical performance as shown in Figure 4g. For the 440 nm Li4Ti5O12, only a small amount of Na can be inserted (0.27 mol Na per mol Li4Ti5O12), which corresponds to a specific capacity of 16 mAh g−1, while the degree of the reversible Na insertion into 44 nm Li4Ti5O12 can nearly reach its theoretic capacity of 175 mAh g−1 (3 mol Na per 1 mol Li4Ti5O12), consistent with our previous reports.34,35 It is apparent that the electrochemical behavior of Na storage into Li4Ti5O12 is strongly dependent on the particle size due to the very poor Na+ ion diffusion kinetics. Nanosizing the Li4Ti5O12 particles will be quite critical to improve the Na storage performance thus making it a suitable anode material for Na-ion batteries. In addition, compared with the two-phase reaction of Li insertion into Li4Ti5O12, the Na de/insertion and diffusion kinetic will be much more complicated due to the three-phase reaction and it will be further discussed in our future paper. In conclusion, we have developed a novel synchrotron-based in situ XRD technique to study sodium ion battery materials during chemical sodiation. The power of this novel in situ technique has been demonstrated through the investigation of the phase transition behavior of Li4Ti5O12 anode material during sodium insertion. A solid solution reaction behavior in a wide range has been revealed during Na insertion into the nanosized Li4Ti5O12 (∼44 nm). This solid solution reaction behavior, which is quite different from the well-known twophase reaction of Li4Ti5O12/Li7Ti5O12 system during lithium insertion, has not been fully addressed in the literature so far. Size-dependent interface and strain energy-induced thermodynamic property changes are proposed to explain this discrepancy between sodium and lithium insertion. However, we also want to point out that the phase transition of Li4Ti5O12 upon sodium insertion is more complicated than that during lithium insertion, since a new phase Na6LiTi5O12 and threephase reaction are involved. The kinetic governed phase transition pathway changes should also not be neglected and

2

d ( 2π ) τ∼

D Na +

(1)

if assuming the Na insertion kinetic is controlled by sodium diffusion and regardless of particle morphology and diffusion geometry. Considering the average particle size of the investigated nanosized Li4Ti5O12 is 44 nm and the total chemical sodiation reaction time is 2 h (7200 s), the apparent DNa+ is estimated as 2.7 × 10−16 cm2 s−1, 5 order of magnitudes smaller than DLi+ in Li4Ti5O12.45 This value is consistent with our DFT calculation result.35 The apparent Na+ diffusion coefficient is also estimated by electrochemical relaxation method (detailed discussion is presented in Supporting Information and Figure S7), slightly smaller than that estimated from the chemical sodiation. Therefore, the size effect will be expected to be more pronounced for Na storage performance in Li4Ti5O12. To further prove this, two more Li4Ti5O12 samples with larger average particle size of 120 and 440 nm, respectively (Figure 4b,c) were used for comparative studies. The size features are directly reflected from the XRD peak broadening as shown in Figure 4e. The average particle size of these three Li4Ti5O12 samples estimated from the SEM observation agrees well the Rietveld refinement results (Table 1). In situ chemical sodiation experiments were also performed 4725

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worthwhile for further detailed investigations. On the basis of this in situ experiment, the apparent Na+ diffusion coefficient (DNa+) of Li4Ti5O12 was estimated in the magnitude of 10−16 cm2 s−1, 5 order of magnitudes smaller than the Li+ diffusion coefficient (DLi+ ∼ 10−11 cm2 s−1), indicating a sluggish Na+ diffusion kinetics in Li4Ti5O12 comparing with Li+. Nanosizing the Li4Ti5O12 will be critical to make it a suitable anode material for Na-ion batteries. The application of this novel in situ chemical sodiation method provides a facile way and a new opportunity for in situ structure investigations of various Naion battery materials and other systems. Experimental Section. Chemical Sodiation. The nanosized Li4Ti5O12 sample was prepared according to ref 34, and the submicrosized Li4Ti5O12 was supplied by Tianjiao Technology. The morphologies of the materials were observed by scanning electron microscopy (SEM) (Hitachi S-4800). For chemical sodiation experiments, sodium-biphenyl-1,2dimethoxyethane (DME) solution was used as reducing reagent. In a typical process, 230 mg of pure sodium was dissolved into 10 mL of colorless 1 M biphenyl-DME solution, forming a dark-green organic solution as the sodiating reagent. (Results of the chemical sodiation using a lower reducing reagent are shown in Supporting Information Figure S9.) For in situ experiment, 0.6 mg of Li4Ti5O12 powder dispersed with glass wool was loaded into a glass capillary (0.7 mm in diameter), then, 0.01 mL of sodium-biphenyl-DME solution was injected into the capillary. The open end of the capillary was quickly sealed with silicone paste and the capillary was immediately transferred to the synchrotron beamline for in situ experiment. All the operations were carefully performed in the Ar-filled glovebox. In Situ XRD. In situ XRD patterns were collected at beamline X14A of the National Synchrotron Light Source (NSLS) at Brookhaven national laboratory by a linear position sensitive silicon detector. The wavelength used was 0.7788 Å. For in situ XRD experiments in an electrochemical cell, a special designed electrochemical cell was employed. The details for the cell construction were reported in our previous publication.48 XAS and XANES Simulation. XAS experiments were carried out at beamline X18A and X19A (NSLS, BNL) in transmission mode using a Si(111) double-crystal monochromator detuned to the 50% value of its original maximum intensity to eliminate the high order harmonics. XANES and EXAFS data were analyzed by ATHENA software package.49 The calculations for XANES were performed in the framework of DFT via the WIEN2k code. The exchange and correlation effects were treated with generalized gradient approximation. The muffin-tin radii for Li, Na, Ti, and O atoms are 1.8, 1.85, 1.9, and 1.75 atomic units, respectively. RmtKmax was fixed at 7.0 to determine the size of the basis. For all compounds, the experimental lattice parameters were used. Electrochemical Measurements. The Li4Ti5O12 working electrode was prepared by casting the slurry of the active materials (80 wt %), acetylene black (10 wt %), and sodium alginate binder (10 wt %) on Al/Cu foil. The 2032 type coin cells were assembled with pure sodium foil as the counter electrode, and a glass fiber as the separator in an argon-filled glovebox. The discharge/charge measurements were carried out on a Land BT2000 battery test system (Wuhan, China) at a current rate of C/10 under room temperature.

Letter

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S9 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (Y.-S.H.) [email protected]. *E-mail: (X.-Q.Y.) [email protected]. Author Contributions §

X.Y. and H.P. contributed equally to this paper

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work at BNL was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract Number DEAC02-98CH10886. This work at IOP was supported by funding from the“863” Project (2011AA11A235), “973” Projects (2010CB833102, 2009CB220104), NSFC (51222210, 11234013), One Hundred Talent Project of the Chinese Academy of Sciences. The authors acknowledge the technical support from beamline scientists at X14A, X18A, and X19A of National Synchrotron Light Source and 11-BM-B at Advanced Photon Sources (APS, ANL). The support provided by China Scholarship Council (CSC) during a visit of W.W. to Brookhaven National Lab is also acknowledged.

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