In Situ Raman and Nuclear Magnetic Resonance Study of Trapped

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In-situ Raman and Nuclear Magnetic Resonance Study of Trapped Lithium in the Solid Electrolyte Interface of Reduced Graphene Oxide Wei Tang, Bee-Min Goh, Mary Y. Hu, Chuan Wan, Bingbing Tian, Xuchu Deng, Chengxin Peng, Ming Lin, Jianzhi Hu, and Kian Ping Loh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12551 • Publication Date (Web): 18 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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The Journal of Physical Chemistry

ABSTRACT Motivated by its high surface area and electrical conductivity, reduced graphene oxide (rGO) flakes have been intensively studied as potential anode materials for lithium ion battery (LIB). The high capacity in rGO (600-1000 mAh g-1) compared to graphite (372 mAh g-1) suggest that a different lithiation mechanism may be operational in the former. The high capacity of rGO should be attributed to its high surface area and associated defective sites, however, these may act as trapping sites and undergo side reactions with the solvent and lithium ions to form the solid electrolyte interphase (SEI) upon lithiation, resulting in irreversible capacity loss (ICL) during the initial cycles. Elucidating the temporal evolution of SEI layer on rGO and quantifying the amount of trapped lithium will be useful in developing strategies to mitigate the ICL process. Herein, the Li intercalation mechanism in rGO and graphite was investigated using in-situ Raman spectroscopy and in-situ time-resolved Nuclear Magnetic Resonance (NMR) to provide insights into the origins of the high capacity and ICL loss in rGO. Finally, the dynamic and static SEI passive layer formed on rGO flakes was monitored and a method was developed to quantify the amount of Li+ trapped in the SEI layer. KEYWORDS rGO flakes; in-situ Raman; in-situ NMR; SEI passive layer; Lithium ion storage

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1. INTRODUCTION Following the discovery of mechanically exfoliated graphene by Geim and Novoselov in 2004,1 graphene has attracted intense interests as a potentially high capacity alternative to graphite as anode energy storage material for LIB applications due to its high surface area and conductive properties.2-13 Lithium intercalates and forms compounds with graphite which can be classified as either stage 4’, 3’, 2’, 2, or 1 GIC compounds, corresponding to LiC36, LiC27, LiC18, LiC12 and LiC6, respectively.14 The last stage (Stage 1) dictates a maximum uptake of one Li guest atom per six carbon host atoms (LixC6, x ≤ 1), which corresponds to a theoretical maximum specific capacity of 372 mAh g-1. Lithium intercalated graphite compounds (Li-GICs) are more stable than the Li metal itself as graphite allows reversible intercalation of Li ion at potentials close to that of lithium and affords better cyclability.15

A different lithiation/delithiation behavior may be operational in graphene as compared to graphite. Some experimental and theoretical studies reported that single-layer graphene (SLG) may be able to adsorb lithium on both sides, thus doubling the sorption capacity of lithium as compared to graphite. Furthermore SLG enjoys shorter lithium diffusion distance due to its open 2-D surfaces.16-24 However Pollak et al. reported that the amount of lithium absorbed on SLG seemed to be substantially reduced due to repulsion forces between Li+ on both sides of the graphene layer.21 First principle study of Li absorption on graphene by Lee et al. confirmed that Li capacities on the surface of defect-free SLG and FLG are significantly inferior to that of the


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bulk graphite.25 In this regard, comparing the lithiation mechanism between the two materials will be needed to shed light on this issue. For practical battery applications, the solutionprocessable form of graphene, rather than the ideal fully aromatic graphene, is needed. Reduced graphene oxide (rGO) flakes obtained from the reduction of graphene oxide (GO) have been reported to exhibit reversible capacity in the range of 600-1000 mAh/g, which is higher than the theoretical value of graphite.22,26-28 However the surface chemistry of residual oxygen functional groups such as hydroxyl, epoxy and carboxylic moieties in rGO may dominate the interactions with Li in addition to the usual intercalation chemistry.29 These functional groups may also be involved in side reactions to form the solid electrolyte interphase (SEI) during galvanostatic cycling. The composition of SEI is complex and comprises organic and inorganic electrolyte decomposition products such as lithium carbonate (Li2CO3), lithium oxide (Li2O), lithium hydroxide (LiOH), lithium alkoxides or lithium fluoride (LiF) and electrolyte salt reduction products on graphene surface.29-31 SEI helps prevent further electrolyte decomposition by blocking the electron transport while allowing Li+ to pass through. It therefore ensures good cycling stability of anode.31 On the other hand, the continual growth of this passive layer may lead to gradual reduction of capacity since a portion of lithium might be trapped in the anode material, becomes kinetically inaccessible, resulting in irreversible capacity loss (ICL) during the first few intercalation/de-intercalation cycles.32,33 ICL is particularly serious when rGO is used as anode materials.34-42 The tendency to form SEI layer is in direct correlation with the BET specific surface area in some cases.43-45 Overall, rGO anode endowed with high surface area


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allows enhanced Li+ storage capacity but it also suffers from higher irreversible capacity loss. This double-edge sword has motivated research efforts in passivating Li “traps” in graphene defective sites. For example, controlling the coverage of oxygen on rGO via annealing at different temperatures, surface modification or deposition of metal oxide/ fluoride nanoparticles on rGO surface are widely investigated to solve the trade-off issue between ICL and reversible capacity.29,46 NMR probe, especially 6Li and 7Li NMR,30,47-49 have been used in the past to interrogate the SEI components in graphite and disordered carbon strucutures,50-53 although no studies have been carried out on rGO. Elucidating the temporal evolution of SEI layer on rGO and quantifying the amount of trapped lithium will be useful in developing strategies to mitigate the ICL process.

Herein, we studied and compared the initial lithiation behavior of rGO and graphite by in-situ Raman spectroscopy. In addition, we investigated the eletrochemical process of rGO by in-situ time-resolved NMR probe. The 7Li resonances due to the nano/micro structured Li-metal fibers oriented perpendicular to the flat Li-metal surface, as well as the SEI passive layer, were clearly identified and monitored. Furthermore, the static evolution of SEI in the charged-storage state has also been determined and correlated with the increase of the open-circuit potential.

2. RESULTS AND DISCUSSION


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2.1 Synthesis and characterization of rGO flakes The GO used was synthesized using modified Hummer’s method,54 followed by thermal reduction to produce rGO. Figure 1a shows the exfoliated and crumpled morphology of the rGO. The thermal annealing of GO at 600 oC generated massive defects on rGO planes due to CO2 release. The porous morphology leads to surface area enhancement; and this is confirmed by physisorption analysis (Figure 1b) in which the BET surface area of rGO is determined to be 212 m2 g-1. The visual proof of the defective rGO morphology was provided by scanning tunneling microscopy (STM) image as shown in Figure 1c. Formation of voids and cavities on graphene sheets due to gasification of carbon atoms causes the surface to become porous. Fourier Transmission Infra-red (FTIR) spectrum of rGO in Figure 1d exhibits a characteristic sp3-hybridized C-H peak at 2939 cm−1 and a sp2 peak at 2919 cm−1, it can be seen that the C=C component has increased after annealing, which indicates a partial recovery of the conjugation in the rGO sheets. Although the peak intensities of the oxygenated functionalities (C=O, C-OH and C-O) in rGO decreased, they did not vanish completely. These functional groups may be involved subsequently in the formation of SEI passive layer.29

2.2 Different lithiation behavior between rGO and Graphite studied by cyclic voltammetry and in-situ Raman spectroscopy. Figure 2a shows the galvanostatic first cycle discharge curves for graphite and rGO electrodes, which correspond to the typical shape for graphitic and disordered carbon, respectively. The first discharge capacity for graphite electrode is 320 mAh g-


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(Figure 2a) which agrees well with the typical values reported in the literature.14 Generally, at

potential E > ~0.8 V vs. Li+/Li, decomposition of electrolyte on the external graphite surfaces leads to solid electrolyte interface (SEI) formation. The onset potential of SEI formation is not a fixed value, and has been reported to occur at 2 V,55 1.7 V,56 1 V and 0.8 V.57 At potentials from ~0.8 V to ~0.2 V, the main reaction is due to the solvated Li intercalation. At this potential range, formation of SEI can continue on the external graphite surface if it is insufficiently protected by SEI layers, and solvent-free Li ions can intercalate into the graphite bulk. At voltage range ~0.2 to ~0.005 V vs. Li/Li+, the main reaction is the reversible formation of the binary Li-GIC (graphite intercalation compounds).14 These phenomena can be observed in graphite electrodes where Li intercalation starts at 0.2 V, a plateau is observed in the discharge curve of graphite in Figure 2a. The discharge curve of rGO is different from the archetypical shape of graphite curve (Figure2a). rGO electrode achieves remarkably high discharge capacity, that is, 2355mAh g-1. At ~0.8 V, solvent decomposition and SEI formation commences on rGO. No distinct potential plateau is observed for rGO electrode when it is further discharged to lower voltage. This means that the Li+ intercalation mechanism in rGO should be different from the typical GIC staging phenomenon in graphite electrode. We believe that the Li ions diffuse into the internal surface of rGO sheets and reside within the nanocavities with the concurrent occurrence of solvent decomposition at the edges of rGO sheets throughout the discharging process, resulting in huge polarization discharge curve. Although rGO exhibit superior Li ions uptake compared to


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conventional graphite, they perform poorly in terms of the reversibility of intercalation and deintercalation. The Li+ intercalation/deintercalation behaviour for these two carbonaceous materials can be further probed by using slow scan cyclic voltammetry (CV). Figure 2b shows CV for the first scan which corresponds to the first Li-intercalation/deintercalation cycle of the two electrodes. It is clear that the onset cathodic peak appears at ~0.8 V for both electrodes, which is attributed to the solvent decomposition and SEI film formation.33 The CV of graphite electrode reveals welldefined redox peaks whereby the deintercalation peaks are observed at 0.25 and 0.29 V vs. Li/Li+. This indicates that Li+ could reversibly intercalate and de-intercalate into graphene layers. However, the lack of the well-defined redox peaks in the CV of rGO could be due to irreversible decomposition of solvent and also the irreversible reduction of residual oxygenated groups on rGO.14 Apparently, the CV of rGO electrode reveals more capacitive behavior, which can be ascribed to the presence of ionizable surface oxygenated groups.58 The broad curve suggests that storage of Li ion in rGO may be due to capacitive behavior. Electrochemical intercalation usually proceeds at the interface between the electrode and the electrolyte solution. Laser Raman spectroscopy can be used to examine surface phenomena within its optical skin depth (within 100 nm). It is useful to determine the stage structure of a graphite intercalation compound (GIC), especially, at a particular part of the GIC surface which strongly affects the position, shape, and intensity of the E2g2 band at 1582 cm-1.59,60


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Simultaneous in-situ Raman spectroscopic and electrochemical measurements were performed to investigate the structural and electronic properties of carbonaceous materials during electrochemical doping. Li-GIC is an example of a donor GIC. Intercalation of Li guest into carbonaceous host strongly affects the position, shape and intensity of the G-band at ~1582 cm1 61

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Overall, the Raman spectra in Figure 3a-b reveal that the intensities of G-band for both

electrodes decrease progressively and become unresolved at the end of the discharging process. The decrease in Raman intensity is assigned to the loss of resonance due to Li+ intercalant while the broadened peak is likely due to the disorder and inhomogeneity induced by the intercalation of Li, which may be related to the different phases of GIC stage.21,62 Notably, the G-band positions of graphite (Figure 3a) gradually up-shift from 1578 to 1587 cm-1 as the electrodes are cathodically polarized. This upshift indicates doping effect as Li+ intercalants donate electrons to the carbon host (graphite). At voltage below 0.2 V vs Li/Li+, G-band disappears in graphite electrode. The disappearance of the G-band is attributed to the high electrical conductivity of stage-1 GIC which leads to a reduction in optical skin depth and Raman scattering intensity.63 This result implies the Li+ intercalation mechanism of graphite is due to the formation of stage-1 Li-GIC (LiC6). It is noteworthy to report that G-band signal of rGO electrode weakens and ultimately vanishes at a relatively higher voltage (0.3 V vs. Li/Li+) than that of graphite electrode (Figure 3b). Due to the amorphous nature of rGO, it is unlikely that staged GIC compounds can form. One explanation for the fast vanishing of the G band could be due to the facile interaction of Li+ and its solvated species with the oxygenated group on rGO surface. These Li ions


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randomly accommodate the monolithic defective sites and fully cover the rGO surface at relatively higher voltage, thus causing the early disappearance of G-band relative to graphite. The stages of GIC formation in the electrode can be evidenced by distinct 7Li NMR peaks.19 Discharging electrodes to 0.005 V allow Li ions to be fully inserted to form LiC6 and this can be fingerprinted by a peak appearing around 42-45 ppm.64,65 Consistent with the aforementioned Raman results, the process of Li insertion into graphite is via stages of GIC formation as evident from the peak at 42 ppm (Figure 4a). The spectrum of rGO (Figure 4b) only shows one sharp resonance at 1.8 ppm associated with diamagnetic Li environment. A shoulder peak (indicated by arrow in the inset) at around 2.8 ppm is assignable to Li2O peak. This low frequency chemical shift suggests that Li2O is dominant in the SEI layer,65 which suggests that Li storage on rGO occurs via surface adsorption. Although rGO shows large intrinsic capacity compared to graphite, the presence of large concentration of functional groups in defective rGO results in a greater degree of Li trapping and electrolyte decomposition, contributing to a higher ICL and higher voltage hysteresis. The detailed study of SEI passive layer and ICL in rGO will be performed using in-situ NMR in the following sections.

2.3 In-situ NMR study of dynamic and static evolution of SEI passive layer on rGO flakes The in-situ 7Li NMR experiments were performed using a Varian-Agilent 300 MHz NMR spectrometer, corresponding to 7Li Larmor frequencies of 116.57 MHz. A home made in-situ 7

Li NMR probe with a 5-turn solenoid RF coil (ID of 12 mm), where the excitation magnetic


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field (B1) was perpendicular to the main magnetic field (B0), was used for the measurement.66 The detailed illustrator of the in-situ 7Li NMR is shown in Figure S1. 1 M LiCl in H2O was used as the reference (0 ppm). A discharge/charge current of 0.15mA (according to the current density of 100mA g-1) was used for the in-situ 7Li NMR in order to optimize resolution and collection time. A series of 678 spectra were acquired consecutively over nearly three full discharge-charge cycles with each spectrum acquired using an accumulation number of 512, a recycle delay time of 1 s and a FID length of 30 ms, resulting in 8.74 min for acquiring each spectrum. The results are summarized in Figures 5, 6 and 7. The color mapped stack plots of the spectra corresponding to the 0 ppm and the 242 ppm peak-centered regions along with the charge and discharge cycles are shown in Figure 5a, b and c, respectively. Seven in-situ 7Li NMR spectra corresponding to specific potential stages (i.e., OCV, the end of the discharge, the end of charge, etc) are plotted in Figure 6, where the deconvoluted peaks are superimposed on each spectrum. For the upfield peaks that cover a chemical shift range from -100 to 100 ppm, a broad peak centered at about -3 ppm and a few sharp peaks at about 0 ppm are used to generate a good fit (Figure 6a). The narrow peaks near 0 ppm are from the electrolytes while a broad peak centered at about -3 ppm is attributed to a combination of (i) lithium intercalation in the rGO lattice, and (ii) lithium adsorbed on the rGO via interactions with surface functional groups such as –OH, -H, -COOH, -C=O, etc.30,48,67. In the chemical shift range from 300 to 200 ppm, three peaks located at about 244, 250 and 265 ppm are used to fit the Li-metal electrode signal (Figure 6b). Since the Li-metal electrode surface of the battery was oriented perpendicular to the


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direction of the magnetic field, the peaks located at about 244 and 250 ppm are from the skin layer (10 µm) of the flat Li-metal battery surface while the 265 ppm peak is attributed to the nano/micro structured Li-metal fibers that are oriented perpendicular to the flat Li-metal surface.68,69 The change of peak area of -3ppm board and 265 ppm as a function of the dischargecharge time is presented in Figure 7a and b, respectively. The area of the broad -3.0 ppm peak increases during the first discharge, as can be seen in Figure 7a. Given the spectral resolution associated with static in-situ 7Li NMR for rGO, we are unable to resolve the detailed structures of the Li+ in the SEI layer and the intercalated lithium based on the chemical shifts. However we are able to distinguish between the Li+ in SEI and its counterpart intercalated in rGO electrode based on the temporal evolution of the peak intensity during discharge-charge. The reason is that once Li+ becomes part of the SEI layer, it gets trapped and cannot be recycled. Only the intercalated Li+ in the rGO electrode can be recycled during the discharge-charge process. Meanwhile, the peak area of the -3 ppm resonance (Figure 7a) monotonically increases with the discharge time, reaching a local maxima value at the end of the first discharge. On the other hand, during the first discharge cycle, the peak area of the 265 ppm remains flat and close to zero while lithium ions are peeled off from the surface of lithium metal during the first discharge (Figure 7b). An increase of the 265 ppm accompanied by a decrease of the -3 ppm peak area is clearly observed during the first charge process as a function of the charge time (Figure 7a and b). One important finding is that only a very small portion of the -3 ppm peak is participating in the cycling, indicating the majority of the lithium that has moved to the rGO electrode during the


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discharge process are dead, i.e., not entering into the cycling process. These dead lithium ions are attributed to reactions forming the SEI layers with the rGO surface functional groups such as –H, -OH, -COOH, -C=O, etc. During the 2nd discharge cycle, a decrease of the 265 ppm peak area and an increase of the -3 ppm peak area are clearly observed. This is then followed by an increase of the 265 ppm and a decrease of the -3ppm during the 2nd charge cycle as a function of the charge time. Apparently, during the 2nd charge cycle, the relative percentage of the lithium that is able to recharge back to the Li-metal electrode is significantly increased compared to the first discharge-charge cycle. This result indicates that the SEI is stabilized at the end of the 2nd charge cycle. The stabilization of the SEI layer is further confirmed by the 3rd discharge-charge cycle. This explains the very high capacity of the rGO electrode during the first discharge cycle and the very good recyclability during the subsequent cycles despite the much lower capacity of the second discharge cycle compared to the first. The change in area of the 265 ppm peak can be utilized for quantifying the amount of lithium that are recycled as the peaks are relatively narrow and are well separated from the flat Li-metal signals at 244 and 250 ppm. One practical approach is to use the peak area of the 265 ppm divided by the total peak area of the -3 ppm broad peak at the end of the first discharge cycle. In this way, we find that 0.576E07 / 1.601E07=36% of the intercalated Li+ is recycled back into the Li-anode during first cycle. The amount is consistent with the practical battery test as shown in Figure S2. The -3 ppm peak, on the other hand, is very broad and precise determination of its


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peak area is difficult due to its poor signal-to-noise relative to that of the electrolyte signal (sharp peaks at about 0 ppm). Furthermore, the portion of the -3 ppm corresponding to the recyclable Li-ion is small compared with the SEI. Despite these difficulties, the trend in the change of the peak area associated with the -3 ppm peak is similar to that of the 265 ppm peak, supporting the use of 265 ppm for precise quantification. We like to point out that although a major part of this irreversible capacity is attibuted to the surface growth of SEI layer, a very small amount of Li+ may be trapped in the rGO flakes, which is estimated to be around 2.8% of the initial capacity loss. To further understand the static evolution of SEI passive layer, in-situ 7Li NMR spectra were first acquired using a lower constant discharge current of 0.07 mA, and each spectrum was acquired using the following parameters: an acquisition time of 4.37 minutes, using an accumulation number of 256 and a recycle delay time of 1 s. The battery was first discharged to 0.7V vs. Li/Li+ (at 2753 minutes) to form initial SEI passive layer. At 2753 minutes, the battery circuit was left open while additional in-situ 7Li NMR data were acquired from 2753 minutes to 4370 minutes. Based on the similar fitting method applied before, the time-resolved peak area evolution of reasonances at -3ppm and 265 ppm was summarised in Figure 8a, b and c. Upon discharge, the evolution trends of the resonances at -3ppm and 265 ppm are quite similar to the results mentioned previously (Figure 7 a, b). One interesting observation is the continuous increase of the -3.0 ppm peak area from 2736 to 4370 min; even after the battery circuit was


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open, indicating that lithium ion is continually reacting with the rGO electrode to form SEI. This could be due to the out-diffusion of a small amount of intercalated Li+ from the bulk rGO to the surface to continuously form the SEI layer even without electrochemical driven force. The amount of the self-delithiated Li+ is calculated to be Li0.13C6 based on the delivered capacity when the open-circuit cell (quite for 1600 minutes) was discharged back to 0.7 V (Figure S3).

3. Conclusions In summary, non-destructive analytical tools such as Raman and NMR (either in-situ or ex-situ) were applied to investigate the intercalation mechanism of Li in rGO and graphite. Electrochemical studies coupled with in-situ Raman and ex-situ NMR analyses revealed differences in the Li intercalation mechanisms of rGO and graphite. CV and ex-situ NMR data indicate that Li insertion into disordered rGO sheets occurs via surface adsorption while the Liintercalation mechanism for conventional graphite follows the Li-GIC mechanism. The growth and evolution of the SEI passive layer in rGO during galvanostatic cycling was investigated using in-situ NMR probe. The result indicates that most of SEI passive layer is formed during the first cycle and becomes stabilized at the end of the 2nd charge cycle. Meanwhile a quantitative method was proposed to determine the amount of irreversible Li+ associated with SEI formation using well-separated resonance from the nano/micro structured Li-metal fibers that are oriented perpendicular to the flat Li-metal surface. The amount of intercalated Li+ that was recycled during first cycle was calculated to be 36% based on our NMR studies, which was consistent


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with the practical battery test results. The static growth of SEI passive layer in rGO was observed and analysed, which provided useful insights on the SEI formation mechanism in rGO-based LIBs.


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ASSOCIATED CONTENT

(Word Style “TE_Supporting_Information”). Supporting Information. Detailed synthesis of rGO, materials characterizations, electrochemical characterization and NMR experimental set-up were included in supporting information. The full author list of reference 3 and 30 can be found in

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The following files are available free of charge. brief description (file type, i.e., PDF) brief description (file type, i.e., PDF)

AUTHOR INFORMATION Corresponding Author * Kian Ping Loh, Email: [email protected]; Jian Zhi Hu, Email: [email protected].

Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here.

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. + W.T. and B.M.G. contributed equally to this work.

Funding Sources National Research Foundation mid investigator award “Graphene oxide – A new class of catalytic, ionic and molecular sieving materials NRF-NRF12015-01”. Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT K. P. Loh acknowledges funding support from National Research Foundation mid investigator award “Graphene oxide – A new class of catalytic, ionic and molecular sieving materials NRFNRF12015-01”. In-situ 7Li NMR experiments were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research, and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for the DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. The authors thank Dr. Xilin Chen for helping with the glove box for assembling the in-situ NMR battery and Dr. Xiaolin Li in PNNL for helping making the electrolytes.


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REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. (2) Mukherjee, R.; Thomas, A. V.; Datta, D.; Singh, E.; Li, J.; Eksik, O.; Shenoy, V. B.; Koratkar, N. Defect-Induced Plating of Lithium Metal within Porous Graphene Networks. Nat. Commun. 2014, 5, 3710. (3) Hassoun, J.; Bonaccorso, F.; Agostini, M.; Angelucci, M.; Betti, M. G.; Cingolani, R.; Gemmi, M.; Mariani, C.; Panero, S.; Pellegrini, V., et al. An Advanced Lithium-Ion Battery Based on a Graphene Anode and a Lithium Iron Phosphate Cathode. Nano Lett. 2014, 14, 49014906. (4) Mazar Atabaki, M.; Kovacevic, R. Graphene Composites as Anode Materials in Lithium-ion Batteries. Electron. Mater. Lett. 2013, 9, 133-153. (5) Kucinskis, G.; Bajars, G.; Kleperis, J. Graphene in Lithium Ion Battery Cathode Materials: A Review. J. Power Sources 2013, 240, 66-79. (6) Choi, N.-S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y.-K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges Facing Lithium Batteries and Electrical Double-Layer Capacitors. Angew. Chem. Int. Ed. 2012, 51, 9994-10024.


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Figure1. (a) SEM image of rGO; (b) BET analysis of rGO; (c) STM image of rGO; (d) FTIR spectra of rGO and GO flakes.


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Figure 2. (a) Initial discharge profiles at current density 100 mAg-1 (b) cyclic voltammograms at first cycle (scan rate 58 µV s-1) for graphite and rGO electrodes.


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Figure 3. In-situ Raman spectra of the first discharge of (a) graphite (b) rGO with current density of 100mA g-1.


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Figure 4. Ex-situ 7Li NMR spectra of fully discharged (a) graphite and (b) rGO at a spin rate of 8000 rpm. Isotropic resonances are labeled and the spinning side bands are denoted with *. LiCl (1M) in distilled water was used as an external chemical shift reference at 0 ppm.


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Figure 5. (a) Charge and discharge plot of rGO in the range of 0.005-3V vs. Li/Li+; Colourmapped in-situ 7Li NMR spectra of rGO during the galvanostatic charge/discharge cycles in the range of (b) -100-100ppm and (c)200-300ppm.


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Figure 6. Stacked in-situ 7Li NMR spectra of the rGO composite obtained during the galvanostatic charge/discharge cycles in the range of (a) -150-150ppm and (b) 200-300ppm.


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Figure 7. In-situ NMR Peak area evolution of (a) -3ppm board and (b) 265 ppm during charge and discharge cycles.


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Figure 8. Static growth of SEI passive layer on rGO eletrode. (a) Potential-time plot to demonstrate the potential evolution after remove the electrical contact from rGO cell; In-situ NMR peak area evolution of (b) -3ppm board and (c) 265ppm.


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In Situ Raman and Nuclear Magnetic Resonance Study of Trapped

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