In Situ Transmission Electron Microscopy Observation of Sodiation

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In situ TEM Observation of Sodiation-Desodiation in Long Cycle, High Capacity Reduced Graphene Oxide Sodium-Ion Battery Anode Jiayu Wan, Fei Shen, Wei Luo, Lihui Zhou, Jiaqi Dai, Xiaogang Han, Wenzhong Bao, Yue Xu, John Panagiotopoulos, Xiaoxing Fan, Daniel Urban, Anmin Nie, Reza Shahbazian-Yassar, and Liangbing Hu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01959 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 26, 2016

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Chemistry of Materials

In situ TEM Observation of Sodiation-Desodiation in Long Cycle, High Capacity Reduced Graphene Oxide Sodium-Ion Battery Anode Jiayu Wan,1,‡ Fei Shen,1,‡ Wei Luo,1 Lihui Zhou,1 Jiaqi Dai,1 Xiaogang Han,1 Wenzhong Bao,1 Yue Xu,1 John Panagiotopoulos,1 Xiaoxing Fan,1 Daniel Urban,1 Anmin Nie,*,2,3 Reza ShahbazianYassar,*,3 Liangbing Hu*,1 1

Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA

2

Materials Genome Institute, Shanghai University, Shanghai 200444, China

3

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor Street, Chicago, Illinois 60607, United States

‡

These author contributed equally to this work

ABSTRACT: Sodium-ion batteries (SIBs) have attracted broad attention recently as an economic alternative for Li-ion batteries. Cost-efficient reduced graphene oxide (rGO) has been intensively studied as both active material and functional additive in SIBs. However, sodiation-desodiation process in RGO is not fully understood. In this study, we investigate the Na-ion interaction with rGO by in situ Transmission electron microscopy (TEM). For the first time, we observe reversible Na metal cluster (with diameter larger than 10 nm) deposition on rGO surface, which we evidence with atom-resolved HRTEM image of Na metal. This discovery leads to a porous reduced graphene oxide SIB anode with record high reversible specific capacity around 450 mAh/g at 25mA/g, a high rate performance of 200 mAh/g at 250 mA/g, and stable cycling performance up to 750 cycles. In addition, direct observation of irreversible formation of Na2O on rGO unveils the origin of commonly observed low 1st Columbic Efficiency of rGO containing electrodes.

Introduction Sodium-ion batteries (SIBs) have attracted great attention due to their potential applications in grid scale energy storage system.1-6 Advantages of SIBs over lithium-ion batteries (LIBs) are obvious. Sodium resources are abundant and available worldwide, while lithium resources are limited and concentrated only in certain areas around the world.4 Despite the similarities of SIB chemistry to LIB chemistry, great challenges remain for high performance SIBs. One of the major obstacles is that Na-ion is almost 3 times larger than Li-ion in volume, making the pulverization problem more severe. Meanwhile, Na-ion kinetics are much more sluggish.7 In spite of the disadvantages, recent development of cathode materials for SIBs showed excellent promise with high voltage and stable cyclability.2, 8-12 Unfortunately, the anode side remains a challenge, since Si and graphite, the most investigated anode materials in LIBs, cannot be applied in SIBs.13, 14 To date, a number of anode materials for SIBs have been investigated, such as metal/metal alloys,7, 13 metal oxides,15-17 red phosphorus,18-21 and organic compounds.22-24 Despite their advantages, cost-efficiency, stable cyclability, performance and opera-

tion safety issues, still hinder their further applications as SIB anode materials. Carbon materials have been intensively studied as promising materials for energy devices due to their high capacity, abundance, and low cost.25-30 As for SIBs, hard carbon, a non-graphitized carbon, has attracted great attention due to its high specific capacity of ~250 mAh/g.31, 32 However, the rate capability of hard carbon is poor because of its low conductivity and its kinetics limited nanopore-filling Na-ion storage process. Most recently, a number of researchers reported reduced graphite oxide and reduced graphene oxide (rGO) as promising Na-ion storage materials.14,33-36 In contrast to graphite, above-mentioned materials obtain larger interlayer distance (larger than 0.335 nm), allowing Na-ions to intercalate/de-intercalate between each graphitic layer. However, the thermal reduction process of graphite oxide/graphene oxide (GO) requires hours of operation, a high temperature reduction process, and a special atmosphere. More importantly, a trade-off between interlayer distance and conductivity of rGO exists with different reduction temperatures/times. This dilemma results in a

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poor performance of rGO as SIB anode, with reversible specific capacity lower than 300 mAh/g. Thus, a green and simple method for obtaining high power, high specific capacity rGO as SIB anode is highly in demand. Despite solely performed as active electrode materials, rGO is also widely applied as functional additive materials in SIBs anodes,37-39 mostly utilizing its large specific area and high electrical conductivity. In both scenarios, rGO electrochemically interact with Na-ions actively in SIBs, regardless of their physical or chemical properties are considered in the design of experiment process. Consequently, the understanding of sodiation-desodiation process is crucial for rGO containing SIBs. Despite previous attempts to unveil this process, it remains controversial and lack of direct evidence. Herein, we conducted in situ transmission electron microscopy (TEM) to study the sodiation-desodiation process of rGO. Our study provides the first direct observation of reversible Na metal cluster (diameter larger than 10 nm) formation on rapid-rGO surface upon sodiation and desodiation, which reveals an important Na-ion storage mechanism in rGO anodes. The phenomenon is evidenced by both selected area electron diffraction (SAED) pattern and atomic-resolved HRTEM image of the electrode/Na metal, enabled with state-of-the-art K2 summit electron-counting camera with an extremely low electron beam dose rate. The observation supports our record high rGO reversible specific capacity of 450 mAh/g, obtained from the rapid reduced GO (in 2 seconds). Moreover, the rapid reduced GO sustained a highly stable cycling with 750 cycles of around 200 mAh/g at 250 mA/g. Direct observation of irreversible Na2O formation with in situ TEM results also evidenced the capacity loss during the 1st cycle with rGO containing SIB electrodes. Results and Discussion rGO samples are firstly prepared for in situ TEM and electrochemical characterizations. High temperature treatment of GO has long been known as an effective method to separate the graphitic layers, and produce fewlayered reduced graphene flakes.40 During this process, oxygen containing functional groups (such as hydroxyl groups, carboxyl groups and epoxy bridges) are unstable at high temperature (usually above 300 Celsius), thus decomposing into carbon dioxide (CO2) and water (H2O).40, 41 Two types of high temperature reduced GO has been prepared in this work, as illustrated in Figure 1. When stacked GO sheets are reduced at a “regular” process (i.e. hours of reduction at high temperature),36 most functional groups on GO are removed and the interlayer distance are decreased, as shown in Figure 1b. In this process, CO2 and H2O are slowly released, thus rGO layers get restacked after reduction. When it is reduced at a “rapid” process, in which the reduction occurs within minutes or even seconds, the decomposed molecules will be abundantly released, and leave much larger spacing than the “regular” process (Figure 1c). The rapidly released gases not only lead to a larger interlayer spacing and even some

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separated monolayer reduced graphene sheets, but also create a loosely packed structure for rGO.42 A large interlayer spacing contributes to a high specific capacity, while a loosely packed structure is beneficial for high surface functional group Na-ion storage as well as an excellent ionic accessibility, which supports electrochemical reaction at high rate. Thus, the rapid reduced GO (rapid-rGO) is chosen for in situ TEM study and SIB anode. The regular reduced GO is used as a control sample for SIB anode.

Figure 1. Schematics of two types of GO reduction process (a) stacked GO sheet is (b) slowly thermal reduced and (c) rapidly reduced.

Photothermal reduction of GO is chosen here as a simple, controllable, green method, as well as a rapid reduction process. Photons in the infared/visible range absorbed by GO can locally heat up the material, and reduce GO to rGO in an exceedingly short time.29,42 In this study, a Xexon lamp is adopted as light source for the reduction process. When a free-standing GO film is exposed under the Xexon light, the reduction process happens in 2 seconds on average. To understand the photothermal reduction of GO, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM) are applied to study the as-sythesised GO (Figure S1) and rapid-rGO film (Figure S2). As shown in Figure 2b, the orange line and black line describe the XRD patterns of GO and rapid-rGO, respectively. XRD pattern of GO shows a sharp peak, with 2θ at 10.2˚, corresponding to a d-spacing of 8.70 . After rapid reduction, 2θ of rapid-rGO XRD pattern shows a broad distribution from 14.0˚ to 36.9˚, with a peak position at 24.5˚. dspacing of the peak is calculated to be 3.65 , which indicates a successful reduction from GO to rGO. XPS analysis shows the surface chemical composition of GO and rapid-rGO films, respectively.43 As shown in Figure 2c, orange and black curves represent the XPS results of GO and rapid-rGO, where C 1s peak presents at ~ 285 eV and O 1s presents at ~ 533 eV.14 The height of these two peaks provide a qualitative information of C/O ratio of the two materials, so it is obvious that rapid-rGO has a higher C/O ratio than GO. A quantitative analysis of C/O from XPS is shown in Table I, where the ratio percentage increases from 2.54 to 5.94 after rapid reduction. The C1s spectrum of rapid-rGO reveals that the C–C bond at 284.8 eV dominates the carbon bond in rapid-rGO. The small


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Figure 2. Characterization of GO and rapid-rGO. (a) XRD patterns of GO film and rapid-rGO, (b) XPS spectra of GO film and rapid-rGO, (c) C 1s spectrum of rapid-rGO, SEM images of (d) dense GO film and (e) rapid-rGO, inset a higher magnification SEM image of rapid-rGO.

portion of oxygen containing bonds are found to be C–O (286.3 eV), –C=O (287.5 eV), and –O–C=O (291.5 eV) (Figure 2(d)).44, 45 The remaining oxygen containing groups in rapid-rGO are essential to maintaining a relatively large interlayer distance, which enables Na-ion intercalation. Both XRD and XPS results confirm a successful reduction of rapid-rGO by our photothermal method in 2 seconds. Figure 2e shows a typical SEM of tightly packed free standing GO film. After photothermal reduction, the SEM images shows a very loose packing of rapid-rGO layers (Figure 2f), unlike the rGO obtained in a slow thermal reduction process (show in Figure 5c). In quite a few places, the rapid-rGO flakes show a very thin characteristic thickness (few layer or even monolayer rGO), as shown in Figure 2f inset. Table 1. Atomic percentage of carbon and oxygen from XPS analysis. (450 stands for rGO regular reduced at 450 oC, details in supplementary materials) o

XPS analysis

GO

Rapid-rGO

450 C

C/O

2.45

5.94

11.00

In situ TEM is a powerful tool to understand the electrochemical process of a battery material upon ion inser46-50 tion and desertion. Usually, only a solid state electrolyte is in contact with the active materials, which provide an extremely clean and pure environment to study their electrochemical process. Thus, we chose in situ TEM to perform the sodiation-desodiation study with rapid-RGO. As shown in a schematic in Figure 3a, the Na/Na2O is working as counter electrode and solid state electrolyte, while the rapid-rGO is using as working electrode. After applying a negative bias (0.5 V) to the working electrode, the Na migrated into the rGO, as shown in Movie S1. Figure 3b and c show TEM images of the rGO before and after the first sodiation. The morphology of the pristine rGO nanosheet is well presented with a clean surface in Figure 4a. The corresponding SAED, in Figure 3d, clearly shows the (100) and (110) rings of a typical graphite structure (space group: P63mc) with a d-spacing of 2.14 Å and 1.23 Å, respectively. This indicates that the nanosheet is pure rGO without any contamination. Interestingly, it was observed that the rGO nanosheet was curled up and coated with a layer of extraneous material on the surface after the first sodiation, as shown in Figure 3c. The coating layer was determined to be with Na2O (d200=2.80 Å, d220=1.95 Å, and d311=1.67 Å), and Na (d200=2.10 Å, d211=1.75 Å, and d310=1.35 Å), as proved by the corresponding SAED in Figure 3f.



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To further understand this phenomenon, HRTEM images of the rGO after the first sodiation was taken, as shown in Figure 4. Particularly, to avoid the effect of the electron beam, the HRTEM was recorded with an extremely −1 −1 low electron dose rate (2.0 electrons pixel s ) by using a state-of-the-art K2 summit electron-counting camera. As shown in Figure 4a, several nanocrystals with lattice fringes formed on the surface of the rGO. Two of them with good orientations were chosen for a deeper investigation. Figure 4b shows an atomic-scale HRTEM image of one nanocrystal in Figure 4a. The d spacing of two vertical lattice planes was measured to be 3.0 Å, which matches well with that of the {110} lattice planes of body centered cubic (BCC) Na metal, as shown in Figure 4c. This suggests that the Na metal forms on the surface of rGO. To the best of the authors’ knowledge, Figure 4a displays the first published atom-resolved HRTEM image of Na metal. An atomic-scale HRTEM image of another nanocrystal from the green box is shown in Figure 4d. This HRTEM image also shows two vertical lattice planes with an equal d spacing values of 2.8 Å, which are determined to belong to the {200} planes of face centered cubic (FCC) Na2O crystal structure, as shown in Figure 4e. Above analysis indicates that Na metal and Na2O formed after the first sodiation of rGO. Figure 3. In situ TEM study of the first sodiation of rGO. (a) Schematic of in situ TEM set-up (b) TEM image of Na/Na2O as counter electrode/solid electrolyte and rGO as working electrode before sodiation (c) after sodiation (d) Diffraction pattern of rGO before sodiation (e) after sodiation.

Figure 4. HRTEM images of sodiated rGO. (a) Low-mag HRTEM image recording the surface of the sodiated rGO. (b) High-mag HRTEM image taken from the blue box in the panel a. (c) 2x2x2 supercell for BCC Na metal. (d) High-mag HRTEM image taken from the green box in the panel a. (e) 2x2x2 supercell for FCC Na2O. The desodiation process of the rGO was also captured by our in situ TEM, as shown in Figure 5 (Movie S2 and S3 in Supporting Information). Figure 5a shows a HRTEM

image of a domain in the sodiated rGO. The d-spacing of 3 equivalent lattice planes of inset in Figure 5a was measured to be 3.0 Å, belonging to the {110} planes of BCC Na metal.


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The corresponding Fast Fourier Transform (FFT) pattern further confirms that the lattice belongs to the BCC Na metal structure taken with the [1-11] zone axis. Interesting, this Na metal particle was slipping away after applying a positive bias (+2 V) to the rGO nanosheet, as shown in Movie S3. After a total desodiation, we found that the Na2O retained on the surface of the rGO, as indicated by the HRTEM image and corresponding SAED pattern in Figure 5b. However, we did not find the existence of Na metal. The observation of reversible Na metal formation on rGO surface evidences the hypothesis of a Na-ion storage mechanism in rGO and related carbon materials. This result also indicates that the formation of irreversible Na2O on the surface of rGO is a main reason for the first cycle capacity loss.

Figure 5. HRTEM images recording the first desodiation process of rGO. (a) HRTEM image of material on rGO before desodiation. Inset shows atomic scale HRTEM image of a Na metal and the corresponding FFT pattern. (b) HRTEM image of material on rGO after desodiation. Inset shows an SAED pattern of the totally desodiated rGO. The electrochemical properties of the rapid-rGO film in SIB were characterized using coin cells with Na metal as the counter/reference electrode and a 1.0 M NaPF6 solution in ethylene carbonate/diethyl carbonate (EC:DEC = 1:1 by volume) as the electrolyte. Figure 6a shows the potential profiles of rapid-rGO electrode at current densities of 25 mA/g, 50 mA/g, 125 mA/g, 250 mA/g and 500 mA/g in a po+ tential window from 0.01 to 2.0 V vs Na/Na . Figure 6b shows the average desodiation specific capacity is 428 mAh/g, 316 mAh/g, 237 mAh/g, 194 mAh/g and 162 mAh/g for 25 mA/g, 50 mA/g, 125 mA/g, 250 mA/g and 500 mA/g, respectively. Surprisingly, rapid-rGO exhibits an excellent rate performance, which is much better than hard carbon and other 14,34 rGO anodes. The enhanced rate capability in this study is attributed to the greater ionic accessibility in the loosepacked of rGO sheets than tight packed ones. After that, the rapid-rGO electrode is further cycled to 750 cycles at 125 mAh/g, showing a very stable performance of around 200 mAh/g. The capacity loss is only 0.025% per cycle, indicating a very stable cycling performance. The electrochemical characterizations of rapid-rGO demonstrates its great potential as simultaneously high energy density and high power density anode materials for SIBs.

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stored in crystalline graphite (NaC70), due to the weak bind52 ing between Na and C atoms as intercalation compounds. Thus, solvent assist co-intercalation of Na-ion and solvent molecules has been applied for graphite based sodium-ion batteries, albeit these systems usually suffer a relatively low specific capacity (smaller than 150 mAh/g) and high anode 53,54 Hard carbon, also known as non-graphitizable potential. 55-58 carbon, has been intensively studied as SIB anodes. As proposed in the “house of cards” model, the sloping region of the half cell voltage profile correspond to the intercalation of Na-ions in carbon layers and the plateau region correspond 59 to the nanopore filling. Recently, researchers suggested another possibility, which the sloping region correspond to the Na storage on defective sites, while the lower plateau first correspond to an intercalation and then followed with na58 nopore filling. Soft carbon, known as graphitizable carbon, which its interlayer distances can be tuned with different pyrolysis temperatures. The voltage profile of soft carbon SIBs anode usually exhibit a sloping region under 1.0 V (vs. + Na/Na ) without a plateau, and the Na is usually stored in 60 between the carbon layers. With the above summary in mind, we discuss the Na-ion storage mechanism in rapid-rGO electrodes in details. Clearly, from the sodiation potential profile at 25 mA/g (Figure 3a), the sodiation process for our rapid-rGO consists of two parts: (1) a sloping region between 2.0-0.3 V and (2) an inclined potential plateau below 0.3 V. According to recent studies on rGO anodes and other carbon anodes, we propose the Na-ion storage mechanism for our rapid-rGO negative electrode as following: the sloping region between 2.0-0.3 V corresponds to the Na ion storage by functional 33,35,36,58,61 groups/defective sites, the inclined potential plateau below 0.3 V consists of the intercalation of Na-ions be14,35,36,58 tween rGO interlayers (relatively higher voltage) and Na-atom cluster adsorption on rGO surface. Part of the proposed mechanism has been previously verified in recent reports: the functional group/defective sites for reversible Na ion storage from 2.0V to 0.3V has been confirmed with ex situ XPS, Raman combined with electrochemical analy35,36,58 sis, and the Na-ion intercalation in between rGO layers below 0.3V has been proven with ex situ XRD measure30,35,36 ments. Na metal cluster formation, even though as a widely accepted mechanism in hard carbon anodes, is indirectly proven by in situ small angel X-ray scattering analysis, 56,59 and unclear in rGO anodes. Our in situ TEM result, provide the first direct evidence for Na metal cluster formation at low voltages with rGO based SIB anodes. In addition, we noticed that the first cycle Coulombic efficiency (CE) for our rapid-rGO anode is at a low value of 18.5%, just like other 37, 62 rGO or rGO containing anodes and porous carbon an63,64 odes. Such low CE is also revealed to be mainly attributed to the irreversible chemical reaction upon first sodiation with in situ TEM.

To better understand the Na-ion storage mechanism in rapid-rGO electrodes, it is necessary to have a short revisit to the storage mechanism of other carbon based anodes. It is well-known that very limited Na can be reversibly



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Figure 6. Electrochemical performance of rapid-rGO as anode for SIBs: (a) potential profiles of rapid-rGO anode at 500 mA/g, 250 mA/g, 125 mA/g, 50 mA/g and 25 mA/g, (b) rate performance of rapid-rGO at above-mentioned current rates, (c) cycling performance of rapid-rGO. -1 to make a 1 mg mL solution. A free-standing GO film with Conclusion diameter of 35 mm was obtained by filtrating 15 mL of the above solution through a 0.65 μm pore-sized membrane In summary, we applied the first in situ TEM characteriza(Millipore, U.S.A) and dried in air. Photothermal reduction tion to study the sodiation-desodiation mechanism of rGO. of GO film was done by exposing to a 300W Xenon lamp These in situ TEM results provide the first direct evidence of within 2 seconds in air. Thermal reduction was carried out in a reversible metal cluster formation on rGO anodes, as a Na o o a tube furnace from room temperature to 450 C at 5 C storage mechanism for rGO electrode materials. It supports min/s. The reduction process was carried out under H2/Ar our rational designed “2 second” rapid-rGO SIB anode, which (5%/95%) in the tube furnace. exhibits a record high reversible capacity of 450 mAh/g among all carbon based SIBs anodes. In addition, we unveiled the origin of the capacity loss of rGO SIB anodes in the first cycle, evidenced by the side reaction of irreversible formation of Na2O on rGO surface. These discoveries are not only significant in rGO as SIB anode material, but also helpful in understanding the electrochemistry and a rational design of all SIB electrode materials containing rGO, or even benefit to the design of “ultimate” dentrite-free Na metal 65-67 anode. Experimental Section Preparation of Rapid-rGO. Film GO was prepared using an 68 improved Hummer’s method. Firstly, 1.5 g graphite flakes were added into 200 mL mixed acid with H2SO4/H3PO4 ratio 9:1. Then, 9 g KMnO4 was slowly added into the mixture. It o was then heated up to 50 C and stirred overnight. After that, the reaction was cooled to room temperature and poured onto 200 mL ice with 5 mL H2O2. The mixture was further washed with 200 mL 3.0 M HCl once and distilled water several times. The obtained GO was redipersed in distilled water

Material Characterization. XRD patterns were obtained by a Bruker C2/D8 diffractometer using Cu Kα1 radiation (λ = 1.5406 Å). SEM images were taken on a Hitachi SU-70 Schottky field emission gun scanning electron microscope. XPS spectra was collected using a high-sensitivity Kratos AXIS 165 spectrometer with survey pass energy of 160 eV and high-resolution pass energy of 20 eV. Electrochemical measurements. 2025 coin cells were made by using Rapid-rGO free standing films as the working electrode, 1.0 M NaPF6 in (1:1 V/V) ethylene carbonate/ diethyl carbonate (EC/DEC) as the electrolyte, pure sodium metal as the counter electrode and also a 25 μm thick polypropylene separator. The typical mass loading of the rapid-rGO elec-2 trode was 1 mg cm . Galvanostatic cycling was conducted with a Land battery tester at different current densities at a voltage potential between 0.01 V to 2 V. In situ TEM Measurements. The rGO nanosheets were stuck to a gold wire to build an in situ Na-ion cell inside the


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TEM. The gold wire also acted as a current collector on the nanosheet side. Sodium metal,which served as the counter electrode was scratched by a tungsten wire inside a glove box filled with Ar. The two electrodes were mounted onto a Nanofactory Instruments scanning tunneling microscope (STM)-TEM in situ sample holder, which was transferred in an airtight container and loaded into the TEM column. The naturally grown Na2O and NaOH layer on the surface of the sodium metal served as the solid electrolyte for Na transport. The Na/(Na2O+NaOH) (working electrode/solid electrolyte) side was moved forward to contact rGO nanosheet. Once a reliable electrical contact was built, potentials of -0.5 V and 2.0V were applied to the rGO nanosheet to initiate the sodiation and desodiation, respectively. The in situ sodiation experiments were carried out inside JEOL JEM-ARM200CF TEM operated at 80KV and the HRTEM images was recorded by Gatan K2 summit electron-counting camera.

ASSOCIATED CONTENT Supporting Information Additional experimental data and In situ TEM movies on sodiation and desodiation of reduced graphene oxide are shown in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected]; [email protected].

Author Contributions ‡These authors contributed equally. L. Hu, J. Wan, F. Shen conceived the idea. J. Wan, F. Shen, and A. Nie did the experiments. J. Wan, F. Shen, W. Luo, A. Nie, L. Hu and R. Shahbazian-Yassar wrote the manuscript. All authors contributed in the experiments or discussion of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS L. Hu acknowledges the support by NSF grant # CBET1335979 and financial start up support from University of Maryland. We acknowledge the support of the Maryland Nanocenter, its Surface Analysis Center and Nisplab. R. Shahbazian-Yassar acknowledges the financial support from the National Science Foundation (Award No. CMMI1200383). The acquisition of the UIC JEOL JEM-ARM200CF is supported by an MRI-R2 grant from the National Science Foundation (Award No. DMR-0959470). Support from Dr. Robert F. Klie at UIC and Dr. Ana Pakza of Gatan, Inc. is also acknowledged. A. Nie acknowledges the support by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. F. Shen acknowledges the financial support by China Scholarship Council (CSC). References

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In Situ Transmission Electron Microscopy Observation of Sodiation

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