Article pubs.acs.org/cm
Hydrogen-Enriched Reduced Graphene Oxide with Enhanced Electrochemical Performance in Lithium Ion Batteries Dohyeon Yoon,† Kyung Yoon Chung,‡ Wonyoung Chang,‡ Seung Min Kim,§ Mi Jin Lee,∥ Zonghoon Lee,⊥,# and Jaehoon Kim*,†,▽ †
School of Mechanical Engineering, Sungkyunkwan University, 2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 440-746, Republic of Korea ‡ Center for Energy Convergence, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-Gu, Seoul 136-791, Republic of Korea § Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro 92, Bongdong-Eup, Wanju-Gun, Jeonranbuk-Do, Republic of Korea ∥ Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), 50 UNIST-gil, Uljugun, 689-798, Republic of Korea ⊥ School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Uljugun, 689-798, Republic of Korea # Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Uljugun, 689-798, Republic of Korea ▽ Sungkyun Advanced Institute of Nano Technology (SAINT), 2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 440-746, Republic of Korea S Supporting Information *
ABSTRACT: Hydrogen-enriched reduced graphene oxide (RGO) was achieved using double-oxidized graphene oxide (GO2) as an anode in high-performance lithium batteries is reported. GO2 exhibited a much lower carbon-to-oxygen ratio, lower crystallinity, higher Brunauer−Emmett−Teller surface area, higher pore volume, and higher porosity as compared to graphene oxides produced using the typical modified Hummer’s method (GO1). The two forms of GO were reduced using two different reduction methods: supercritical isopropanol (scIPA) and heat treatment. The four types of RGOs synthesized using GO1/GO2 and scIPA/heat treatment exhibited significantly different chemical, morphological, and textural properties. The galvanostatic charge−discharge properties were highly dependent on the physicochemical properties of the RGOs. The scIPA-reduced GO2 exhibited superior electrochemical performance as compared to the thermally reduced GO1/GO2 and scIPA-reduced GO1. Highly reversible capacity (1331 mAh g−1 at 50 mA g−1 after 100 cycles), excellent rate-performance (328 mAh g−1 at 5 A g−1), and good cycling stability up to 1000 cycles even at a current density of 10 A g−1 were observed with the scIPA-reduced GO2 electrode. The characterization results suggested that a large amount of hydrogen-terminated groups, numerous defect sites, and large interlayer spacing have beneficial effects on the electrochemical performance of scIPA-reduced GO2. the bulk layered structure.4 In pursuit of high power LIBs, many strategies have been proposed including (1) downsizing active materials to the nanometer size, thus shortening the lithium diffusion pathway through the electrode material;5 (2) incorporation of conductive layer coatings and/or doping of the active materials with foreign atoms on the surface of electrode materials, thus enhancing the electron conductivity from the current collector to the individual active material;6 and
1. INTRODUCTION Due to current issues such as global warming and fossil fuel depletion, the development of new energy storage materials with high energy and power densities is required to meet challenges in large-scale applications including plug-in hybrid electric vehicles, electric vehicles, and energy storage systems.1−3 Electrode materials for such applications should exhibit a high capacity, long-term cyclability, high power density, and low cost. Graphite, currently used as an anode active material in portable lithium ion batteries (LIBs), exhibits a low theoretical capacity of 374 mAh g−1 and cannot provide a high performance due to slow lithium ion diffusivity through © XXXX American Chemical Society
Received: October 20, 2014 Revised: November 27, 2014
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supercritical alcohols including supercritical methanol, supercritical ethanol, supercritical 1-propanol, supercritical isopropyl alcohol, and supercritical 1-buthanol exhibited high carbon-tooxygen ratios and electrical conductivity values of 14.4 (determined by XPS) and 27 500 S m−1, respectively, which were comparable to those produced via methods utilizing toxic reducing agents (e.g., hydrazine34,48−50 and sodium borohydride51), but much higher than other “green” methods (e.g., Lascorbic acid,52,53 green tea,54 sugar,55 hydrothermal dehydration,56 etc.). The effective removal of the oxygen functionalities could be achieved by the unique hydrogendonating ability of supercritical alcohols.57,58 In addition to effective graphitization, the supercritical alcohol route has many advantages including simplicity, nontoxicity, cost-effectiveness, and fast reaction (requiring only a few seconds to a few hours). Herein, we report the fabrication of a RGO-based anode electrode with a high rate performance and large capacity by a simple, supercritical isopropanol-based (scIPA, Tc = 235 °C, Pc = 4.8 MPa) route. We demonstrate that the control of RGO properties by adjusting the exfoliation and oxidation degree of GO and production of hydrogen-enriched RGO by the reaction of oxygen functionalities on GO with scIPA can remarkably enhance the lithium storage capacity even at high insertion/ deinsertion rates. For example, without heteroatom doping or structure directing chemicals, the produced RGO exhibited an extremely high reversible discharge capacity (1331 mAh g−1 at a current density of 50 mA g−1 after 100 cycles) and high rate performance (328 mAh g−1 at 5 A g−1). More importantly, at a continuous charge/discharge rate of 10 A g−1, the electrode exhibited a stable discharge capacity with almost 100% Coulombic efficiency and delivered 167 mAh g−1 even after 1000 cycles. To the best of our knowledge, these properties are superior to most RGO composite electrodes synthesized by chemical or thermal reduction methods (without doping or using other structure directing chemicals/methods, see SI Table S1). The physicochemical and electrochemical properties of RGO synthesized in scIPA were compared with those of thermally reduced graphene oxide, to elucidate the structure− property relationship of RGO-based electrodes in lithium ion storage.
(3) fabrication of nanostructured active materials, e.g., hierarchically mesoporous morphology, to increase the contact area between electrolytes and active materials.7,8 However, the development of active materials that exhibit stable long-term cyclability at high charge−discharge rates still remains a challenge. In addition, the fabrication of nanostructured materials and sophisticated carbon coating typically require multiple steps, batch-wise running mode, and occasionally expensive structure-directing chemicals. Thus, the fabrication cost is too high for commercial-scale production. Owing to its remarkable electronic conductivity, high surface area, and much higher lithium storage capacity than graphite, graphene has received tremendous interest as a potential active material in LIBs.9−32 Among the various synthetic methods, the most promising and economically viable approach for largescale production of graphene-based materials is the chemical reduction route,33−35 in which graphite powder is first oxidized to produce exfoliated GO, followed by the graphitization of GO to reduced graphene oxide (RGO) using various chemical and/ or thermal routes. Since 2008, many experimental studies have suggested that RGOs,9−11,36,37 doped RGOs,28−31 and RGObased composite materials (such as graphene/Si or grapheneLiFePO4)38 can be promising alternatives for high-performance LIBs. However, the origins of enhanced electrochemical performance and the relationship between the physicochemical properties of RGO and its electrochemical properties are still not clear and are occasionally controversial. For example, a comparison of the electrochemical performance of RGOs revealed that there is a large distribution in the reversible capacities of RGO (between 290 and 1150 mAh g−1) after a few cycles, as well as in the first discharge capacities (540−1600 mAh g−1) at a charge−discharge current density of 50 mA g−1 (see Supporting Information, SI, Table S1).39 This wide discrepancy may be because the electrochemical performances of RGOs depend on a number of different factors including their specific surface area,16 interlayer spacing,9,40 disordered structures/defects/edges,10,23,41−43 chemical functionalities (e.g., cyclic edge ether, phenol, ketone, carbonyl groups),18,20,26,44 hydrogen-terminated sites,45 edge sites,22,23 the presence of heteroatoms (e.g., doping),28−31 textural properties (e.g., pore size distribution, porosity, and/or nanocavities),11,22 and morphological properties (e.g., open pore or hierarchical structure, etc.).24,25,32 These properties are in turn strongly correlated with the properties of the starting GO and deoxygenation techniques used in the production of RGOs. However, there are only few reports (see SI Table S1) on potential correlations between the physicochemical properties of “simple RGOs” and their electrochemical performance (“simple RGOs” indicates RGOs prepared without doping and without the use of structure-directing chemicals).20,44 Furthermore, most studies listed in SI Table S1 utilized conventional GOs produced by a modified version of Hummer’s method and explored different reduction methods and conditions (e.g., thermal reduction at different temperatures from 300 to 900 °C or chemical reduction using different reducing agents and methods). Because the properties of RGO are highly dependent on the properties of GO, rational design and optimization of graphene-based electrodes to obtain highrate performance and long-term cyclability should be considered in the modification of GO properties. Recently, we proposed an environmentally benign, supercritical alcohol-based route to produce RGO without the use of reducing agents.46,47 RGO nanosheets synthesized in various
2. EXPERIMENTAL SECTION 2.1. RGO synthesis. Graphene oxide, prepared using a modified Hummer’s method,59,60 was designated as GO1. Double-oxidized graphene oxide, prepared using GO1 with the same procedure, was designated as GO2. A detailed description of the experimental procedures for GO1 and GO2 synthesis is given in the SI. For the reduction of graphene oxide in scIPA, 1 g of GO was dispersed in 8 mL of IPA using a vortex mixer. The mixed suspension was sonicated for 1 h in a 20 mL glass vial. Then, 3.5 mL of the mixed suspension was introduced into a SUS 316 reactor with an inner volume of 11 mL. After being tightly sealed, the reactor was immersed into a molten salt bath (salt weight ratio of KNO3/NaNO3/Ca(NO3)2 = 46:24:30) and reacted for 1 h at 400 °C. It took less than 2 min to reach 400 °C. After the reaction, the reactor was removed from the salt bath and quenched in a cold water bath. After cooling, the produced RGO suspension was rinsed and filtered through a FP Vericel poly(vinylidene fluoride) membrane filter. Wet RGO powder was dried in a vacuum oven at 70 °C overnight, and sieved using a 75 μm mesh. The reduced graphene oxides synthesized from GO1 and GO2 in scIPA were designated as S-RGO1 and S-RGO2, respectively. In order to prepare thermally reduced graphene oxides, GO1 or GO2 was calcined at 400 °C for 1 h using the SUS 316 reactor and salt bath. Thermally reduced graphene oxide from GO1 and from GO2 was designated as H-RGO1 and H-RGO2, respectively. B
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2.2. Characterization. Functional groups on the surface of the samples were characterized using a NICOLET iS10 Fourier-transform infrared (FT-IR) spectrometer (Thermo Electron Co., NJ, U.S.A.). Prior to the measurements, the samples are dried at 50 °C in a vacuum oven for 24 h and the dried samples were stored in a glovebox with H2O levels below 0.2 ppm and O2 levels below 0.1 ppm. The phase structure of the samples was analyzed using a D/Max-2500 V/PC Rigaku X-ray diffractometer (XRD, Tokyo, Japan) with Cu Kα radiation at 40 kV and 50 mA. X-ray photoelectron spectroscopy (XPS) was performed on the samples using a PHI 5000 Versa Probe spectrometer (ULVAC-PHI Inc., Kanagawa, Japan). Elemental analysis (EA) of samples was carried out using a CHNSO Analyzer (elemental analysis by combustion). C, H, N, and S were analyzed using Thermo Elemental Analyzer Scientific Flash 2000 (Thermo Scientific U.S.A.). O was analyzed using a FISONS-EA 1108 Elemental Analyzer (Thermo Scientific, U.S.A.). The morphologies of the samples were observed using a Hitachi S-4100 field emission scanning electron microscope (FE-SEM, Tokyo, Japan) and a Tecnai-G2 highresolution transmission electron microscope (HR-TEM, FEI Co. Ltd., OR, U.S.A.). For the HR-TEM measurements, the samples were dispersed in dimethylformamide using ultrasonication, and dropped onto a copper grid with a perforated carbon film. An FEI Titan G2 cube 80−300 kV, equipped with a monochromator, an image aberration corrector, and a high brightness gun, was used for TEM experiments at 80 kV of acceleration voltage. This configuration is able to achieve around image resolution of 0.65 Å and energy resolution of 0.12 eV. The RGO and GO sheets were transferred onto Quantifoil TEM grids for TEM analysis. The thermal properties of the samples were examined with a Q500 thermal gravimetric analyzer (TGA, TA Instruments, DE, U.S.A.). The temperature was ramped up to 800 °C at a rate of 10 °C min−1 under a flow of high-purity nitrogen (50 mL min−1). The Brunauer−Emmett−Teller (BET) surface area was measured using a Belsorp-mini II apparatus (BEL Inc., Osaka, Japan). 2.3. Electrochemical Property Measurement. Charge/discharge properties of the prepared samples were measured using a 2032 coin-type cell. The active material (70 wt %), polyvinylidene fluoride (PVDF) as a binder (20 wt %), and carbon black as a conductor in n-methyl-2-pyrolidone (NMP) were carefully mixed. The slurry was then cast on a Cu foil and dried at 80 °C under vacuum for 24 h to remove the solvent. The electrode film was punched into 15 mm-diameter discs (area of 1.77 cm2) and weighed. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate/ ethylmethyl carbonate solvent (volume ratio of EC/DMC/EMC = 1:1:1). All cells were fabricated in a glovebox filled with high purity argon gas. The charge and discharge behavior was monitored using a model WBCS 3000 battery test system (WonATech Corp., Korea) at the voltage range of 0.005−3.0 V (vs Li/Li+) at room temperature. The cyclability was performed at rates of 1, 5, and 10 A g−1 for up to 1000 cycles. The C-rate was varied from 0.05 to 50 A g−1 for rate performance measurements. The electrochemical property test was run in triplicate and average values were reported.
Figure 1. FT-IR spectra of GOs and RGOs.
vibrations,61 was clearly visible (SI Figure S1b). Normalized transmittance of the FT-IR spectra, shown in Figure 1a, revealed that OH groups and the COC group persisted in RGOs. A close inspection of the CH vibration regions (shown in Figure 1b) revealed that the RGO samples retained symmetric CH3 stretching at 2953 cm−1, asymmetric CH2 stretching at 2920 cm−1, and symmetric CH2 stretching at 2850 cm−1, implying that some carbon in RGO was hydrogenated or the oxygenated functional groups were replaced with groups containing CH2 and CH3 groups during the reduction step. More detailed information regarding the chemical composition and oxygenated functionalities on GOs and RGOs was obtained using XPS. A significant difference was observed in the carbon-to-oxygen ratios of RGOs synthesized using GO1 or GO2 under identical reduction conditions. The atomic ratios of carbon to oxygen (C1s/O1s) estimated from XPS are listed in Table 1. Since C1s/O1s represents the bulk composition approximation from the surface technique, C/O ratios estimated using a bulk composition analysis technique, elemental analysis (EA), are also listed in Table 1. Similar trends in the carbon-to-oxygen ratios measured using XPS and EA were observed, although the absolute values of C1s/O1s and C/O were different. The carbon-to-oxygen ratio of GO2 was lower than that of GO1, indicating that a larger number of oxygen functionalities existed in GO2. The RGOs obtained from GO1 exhibited higher carbon-to-oxygen ratios than RGOs obtained from GO2. This was because the initial number of oxygen functionalities in GO2 was larger than that in GO1,
3. RESULTS AND DISCUSSION 3.1. Chemical Properties. Figure 1 shows the FT-IR spectra of GO1, GO2, and RGOs produced using scIPA and thermal reduction. The presence of hydroxyl (ν-COH at ∼3400 cm−1 including vibrations from COOH and H2O), carbonyl (ν-CO at 1731 cm−1), carboxyl (ν-COOH at 1627 cm−1), CH (ν−CH at 1413 cm−1), epoxy (ν-COC at 1220 cm−1), and CO groups (ν-CO at 1050 cm−1) indicated that GO retained several types of oxygen-containing functionalities. As shown in Figure S1a (SI), GO2 exhibited a stronger and broader absorption in the spectral region involving the oxygenated carbons as compared to GO, suggesting that more CO, COOH, COC, and CO groups were present in GO2. Following the reduction, the peaks associated with CO and COOH were greatly reduced, while the CC group (ν-CC at 1550 cm−1), owing to skeletal ring C
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0.412/26.2% 0.420/19.5% 0.412/55.8% 0.384/47.3% 0.860 0.862 0.352/73.8% 0.356/80.5% 0.352/44.2% 0.351/52.7%
25.3 318.5 397.3 100.7 527.2 147.2
8.07 2.25 12.09 13.75 8.73 19.08
0.051 0.179 1.201 0.346 1.150 0.702
9.3 26.4 70.6 40.9 69.7 58.4
NA NA 2052/871 733/337 2121/1078 3350/1521
thus deoxygenation of GO2 under identical reduction conditions resulted in more unremoved oxygenated functionalities. The use of scIPA resulted in higher carbon-to-oxygen ratios in RGOs, as compared to those obtained from the thermal reduction method. The unique reduction ability of scIPA for producing RGOs might be related to the inherent hydrogen donating characteristics of isopropyl alcohol in its supercritical state.57,58 In addition to hydrogen donation, decomposed IPA species in the supercritical state can chemically react with oxygen functionalities on GOs.47 This may cause a higher hydrogen-to-carbon (H/C) ratio of the RGOs produced using scIPA, as compared to RGOs produced using thermal reduction. In particular, the H/C ratio of S-RGO2 was 2.27, a value that was an order of magnitude higher than that of the other RGOs prepared in this study and previously reported values in disordered carbons.45,62,63 Several experimental studies demonstrated that the presence of hydrogen in carbonaceous materials significantly enhanced lithium ion uptake;45,62,64−69 for example, the reversible capacity of disordered carbon was increased from 400 to 900 mAh g−1 with an increase in the H/C ratio from 0.05 to 0.4. This was attributed to lithium binding to hydrogencontaining carbon atoms at the graphene edge sites.62,68 High-resolution XPS C1s spectra of GOs and RGOs are shown in Figure 2. The C1s spectra of GOs and RGOs were
Figure 2. High-resolution XPS spectra and peak deconvolution results of (a) GO1, (b) GO2, (c) H-RGO1, (d) H-RGO2, (e) S-RGO1, and (f) S-RGO2. Area% of different types of carbon estimated from XPS peak deconvolution.
5.11 7.07 0.28 0.72 0.82 2.27
d-spacing of Peak 2 (nm)/area% d-spacing of Peak 1 (nm)/area%
1.67 1.36 15.85 17.73 10.67 12.33
0.95 0.76 10.89 11.15 5.66 9.40
deconvoluted using seven types of carbon with different chemical environments:70 sp2 graphitic carbon (284.5 eV), sp3 carbon (285.5 eV), COH (hydroxyl group, 286.0 eV), C−O/epoxy (ether and epoxy groups, 286.6 eV), CO (carbonyl group, 287.6 eV), OCO (carboxyl group, 288.7 eV), and π−π* shakeup (satellite from sp2 graphitic carbon, 289.9 eV). The area% of the peaks in the C1s spectra of GO and RGOs was quantified, and the results are listed in SI Table S2. In GO1, the major bonding species were sp2 carbon (31%) and carbon−oxygen single-bond species (CO/epoxy group, 36%), while the major bonding species in GO2 were carbon−oxygen double-bond species (CO group, 31%; OCO group, 37%). The oxygenated carbons (COH, CO/epoxy, CO, and OCO, combined) in GO1
GO1 GO2 H-RGO1 S-RGO1 H-RGO2 S-RGO2
C/O by EA H/C by EA C1s/O1s by XPS
Table 1. Chemical, Structural and Charge−Discharge Properties of GOs and RGOs
BET surface area (m2g−1)
average pore size diameter (nm)
total pore volume (cm3/g)
porosity (%)
initial charge/discharge capacities at 50 mAh g−1
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47.3%, respectively, while the area% of Peak 2 in H-RGO1 and S-RGO1 was 26.2% and 19.5%, respectively. This indicated that the average distance between the basal planes in the RGO 2s was greater than that of the RGO 1s (Table 1). At least two plausible explanations exist for the presence of two peaks. First, the remaining oxygen functionalities of the heat-treated GOs at 400 °C may enlarge the interlayer spacing. Second, in addition to the removal of the oxygen functionalities using scIPA, the carbon chain of scIPA would chemically react with the oxygen functionalities on the GOs during the supercritical reduction,47 as discussed in the previous section. Since supercritical alcohol can induce a variety of chemical reactions including hydroxyalkylation, alkylation, and alkoxylation,71−73 it is not clear what types of oxygen functionalities on GOs participated in the reaction with scIPA during the reduction. The larger spacing between the basal planes of the RGO 2s would increase the lithium ion uptake by providing additional lithium storage sites between the graphene layers.9,40 Figure 4 shows the SEM images and N2 adsorption− desorption isotherms of GO1 and GO2. The oxidized graphene
corresponded to 66% of the total carbon bonds, while the amount of oxygenated carbons in GO2 increased to 85%. Thus, the double oxidation resulted in a substantial increase in the number of carbon−oxygen bonds. After the reduction, the four types of RGOs exhibited similar chemical bondings: 65−68% sp2 carbon, 6−10% sp3 carbon, 5−8% hydroxyl groups, 4−5% ether and epoxy groups, 3−4% carbonyl groups, 1−2% carboxyl groups, and 8−12% of π−π* shakeup. Hydroxyl groups were the most abundant oxygen functionalities in RGOs. The presence of hydroxyl and carboxyl groups in RGOs was also observed in the high-resolution O1s spectra, shown in SI Figure S2. 3.2. Structural Properties. The interlayer spacing of GOs and RGOs was characterized by XRD and the results are shown in Figure 3. The XRD patterns of pristine graphite, GO, and
Figure 3. XRD patterns of RGOs.
RGOs with 2θ in the range of 10−90° are shown in SI Figure S3. The 2θ and interlayer spacing values obtained from the XRD patterns are listed in Table 1. Pristine graphite showed a sharp (002) peak at 26.3° with an interlayer spacing of 0.337 nm. Following the oxidation, GO1 exhibited a sharp peak at 2θ = 10.28°, which corresponded to an interlayer spacing of 0.860 nm, while GO2 had a much broader peak at 2θ = 10.26° with an interlayer spacing of 0.862 nm, owing to the presence of the larger amount of oxygenated functional groups on the basal plane of graphene and to a more efficient exfoliation (SI Figure S3). After the reduction, the XRD patterns of RGOs showed relatively broad peaks at 2θ values in the range of 20−25°, with interlayer spacing values of 0.351−0.412 nm, depending on the type of GO and/or reduction methods used. As shown in Figure 3, a precise evaluation of the XRD patterns of the RGOs revealed that the peak at 20−30° consisted of at least two peaks (referred to as Peak 1 and Peak 2), clearly indicating that the RGO sheets retained at least two different interlayer spacings, namely Peak 1 with an interlayer spacing of 0351−0.356 nm and Peak 2 with a much larger interlayer spacing of 0.382− 0.420 nm. The typical interlayer spacing range of RGOs produced using chemical or thermal reduction methods was 0.35−0.38 nm (SI Table S1), which corresponded well with Peak 1 in the RGOs synthesized in this study. As listed in Table 1, Peak 2 of H-RGO2 and S-RGO2 was approximately two times larger than their counterparts synthesized using GO1; the area% of Peak 2 in H-RGO2 and S-RGO2 was 55.8% and
Figure 4. SEM images and N2 absorption−desorption isotherms of GOs.
sheets in GO1 were heavily aggregated and formed large particles (∼50 μm), while much smaller graphene oxide sheets were observed in GO2. This suggested that the incorporation of larger amounts of oxygen functionalities by the double oxidation resulted in superior exfoliation. GO1 exhibited a Type IV isotherm with a distinct hysteresis at P/Po of 0.5−1.0, indicating the presence of a mesoporous structure. The poresize distribution, estimated using the Barrett−Joyner−Halenda (BJH) method, indicated that the pore size of GO1 was very broadly distributed in the range of 2−100 nm, with an average pore size of 8.1 nm. In contrast, GO2 exhibited a Type I isotherm with an average pore size of 2.3 nm, indicating the presence of microspores. As listed in Table 1, the BET surface area of GO2 was an order of magnitude larger than that of GO1. Therefore, the double oxidation induced additional oxygenated functional groups in the basal and the edge sites in individual graphene sheets and thus in a better exfoliation of the graphene sheets. Figure 5 shows the SEM images, TEM images, and the N2 adsorption−desorption isotherms of the RGOs. Depending on the graphene oxide and reduction method chosen, remarkable E
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Figure 6. Aberration-corrected HR-TEM image of (a) H-RGO1, (b) S-RGO1, (c) H-RGO2, and (d) S-RGO2. Figure 5. SEM images, TEM images and N2 absorption−desorption isotherms of (a) H-RGO1, (b) S-RGO1, (c) H-RGO2, and (d) SRGO2.
corresponding diffractogram of inset. As shown in SI Figure S4, topological defects comprised of quasi-amorphous carbon regions were clearly visible. Figure 6b shows a diffractogram from a trilayer S-RGO1 in which three sets of hexagonal patterns were observed. The area of quasi-amorphous carbon regions of S-RGO1 was smaller than that of H-RGO1, suggesting a more effective restoration of the graphitic structure using scIPA (SI Figure S5). When GO2 was used, a striking difference in morphology was observed between H-RGO2 and S-RGO2. In Figure 6c, H-RGO2 exhibited typical nanosized turbostatic carbon sheets, indicating the formation of a larger amount of defects, although hexagonal lattice and diffraction spots are shown in tiny areas (2 × 2 nm2 red square box and the corresponding FFT in small inset of Figure 6c). Overall diffractogram revealed ring pattern due to a large amount of amorphous and randomly oriented small carbon patches. In addition to the atomic-level defects, H-RGO2 also had larger holes with diameters of 2−4 nm (SI Figure S6). The diffractogram of S-RGO2 indicated a relatively incomplete restoration of the graphene structure as compared to S-RGO1, but fewer topological defects were observed as compared to HRGO2 (Figure 6d). It would be more difficult to restore the graphitic structure when GO2 (higher oxidation degree) was reduced under identical conditions as those utilized to reducing GO1 (less oxidation degree). Recent first principle calculation studies indicated that the presence of defects would enhance diffusion and adsorption of Li ions on the graphene layer.41−43 3.3. Electrochemical Properties. Figure 7a shows the cycling performance of RGOs up to 100 cycles in the voltage range of 0.005 to 3.00 V versus Li/Li+ at a current rate of 50 mA g−1. The first charge capacity and the reversible capacity of the RGOs synthesized using GO2 were significantly improved in comparison with those synthesized using GO1. The S-RGO2 and H-RGO2 exhibited first charge capacities of 1521 and 1078 mAh g−1, respectively, and reversible discharge capacities of 1331 and 1009 mAh g−1, respectively, after 100 cycles. These
differences in the microscopic and macroscopic structures were observed. The SEM images showed that H-RGOs retained a highly porous structure, while the scIPA-treated RGOs retained a randomly folded, wrinkled paper-like structure. The low magnification TEM image showed that RGOs formed corrugated, rippled, or wrinkled graphene sheets. The BET surface areas of RGOs were in the order of H-RGO2 (527.2 m2 g−1) > H-RGO1 (397.3 m2 g−1) > S-RGO2 (147.2 m2 g−1) > SRGO1 (100.7 m2 g−1), indicating that the heat-treated RGOs exhibited three-to-four times larger surface areas than the scIPA-reduced RGOs. The surface area of RGO is related to the exfoliation ability of the reduction method or reduction agent. The much larger surface area of the heat-treated RGOs can be due to the sudden increase in temperature, thus the gaseous products produced by decarbonxylation, decarbonylation, and/or hydrodeoxygenation (CO2, CO, H2O, etc.) may induce effective exfoliation of the graphene sheets.74,75 In addition, when scIPA was used, more oxygen functionalities were removed, thus the distance between neighboring graphene sheets was shorter. Under the same reduction conditions, the higher BET surface areas of RGO 2s as compared to RGO 1s may be related to the much larger surface area of the GO 2s. The direct imaging of few-layers RGOs was further analyzed by aberration-corrected HR-TEM at 80 kV (FEI Titan G2 cube 80−300 kV), as shown in Figure 6 and SI Figures S4−S7. The analysis of digital diffractorgram with fast Fourier transformation (FFT) pattern was carried out to identify crystallized regions of graphene that were a single layer to a few layers thick. The digital diffractograms for selective areas of H-RGO1 with different sampling areas and positions indicated the presence of a long-range hexagonal order in the sheet, as shown in Figure 6a. The most graphene lattice is aligned along the crystallized direction in the HR-TEM image and the F
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the capacity at above 0.5 V than that of the RGO1 electrodes. This suggested that Faradaic capacitance was more important Li ion storage mechanism in the RGO2 electrodes, probably owing to the larger amount of defects and residual oxygen functionalities. To demonstrate the rate performance, the RGO electrodes were progressively charged and discharged at 50 mA g−1, 100 mA g−1, 250 mA g−1, 500 mA g−1 1 A g−1, 2.5 A g−1, 5 A g−1, 10 A g−1, 25 A g−1, and 50 A g−1 for 10 cycles, and then again at 50 mA g−1 for 10 cycles, as shown in Figure 8. The charge−
Figure 8. High-rate performance of RGO electrodes at various current densities of 0.05−50Ag−1.
discharge profiles at each current density are shown in SI Figure S9. S-RGO2 exhibited a superior charge−discharge performance and good stability at high current rates up to 5 A g−1. At a current rate of 100 mA g−1 (about 5 h), the reversible discharge capacities were in the order of S-RGO2 (1286 mAh g−1) > HRGO2 (1099 mAh g−1) > H-RGO1 (767 mAh g−1) > S-RGO1 (453 mAh g−1). When the current density was increased to 5 A g−1 (the lithium insertion time was decreased to ca. 1 min), the reversible capacity of S-RGO2 was 328 mAh g−1. These results were superior to other RGO electrodes (199 mAh g−1, HRGO2; 321 mAh g−1, H-RGO1; 104 mAh g−1, S-RGO1). When the current density was further increased to 50 A g−1 (lithium insertion time of a few seconds), H-RGO1 exhibited higher reversible capacities than the other electrodes; the reversible discharge capacities at 50 A g−1 were in the order of H-RGO1 (146 mAh g−1) > S-RGO2 (72 mAh g−1) > H-RGO2 (27 mAh g−1) > S-RGO1 (9 mAh g−1). A plausible reason for the lower discharge capacity of S-RGO2 than that of H-RGO1 at the high current densities will be given in the next section. As shown in SI Figure S10 and listed in SI Table S1, the discharge capacities of S-RGO2 outperformed most of the “simple RGO”-based electrodes reported previously. When the current density was decreased to 50 mA g−1 after the fast charge− discharge process, the reversible discharge capacities of the SRGO2 electrode recovered to the values of the initial 10th cycle, implying a very good integrity of the electrodes. The S-RGO2 electrode exhibited long-term cycling stability at high current densities. Figure 9 shows the capacity retention of the S-RGO2 electrode up to 1000 cycles at current densities of 1, 5, and 10 A g−1. The electrode was first activated by charging and discharging at 100 mA g−1 for the first five cycles. At 1 A g−1, the electrode maintained good capacity retention up to 1000 cycles with a reversible capacity of 376 mAh g−1. At current densities of 5 and 10 A g−1, the electrode maintained its capacity up to 1000 cycles with charge capacities of 193 and
Figure 7. (a) Cycling performance of RGO electrodes at the current density of 50 mAg−1 and (b) capacity potential distribution of RGO electrodes after 100 cycles.
values were much larger than those of the H-RGO1 (871 mAh g−1, first cycle; 734 mAh g−1 after 100 cycles) and of S-RGO1 (337 mAh g−1, first cycle; 307 mAh g−1 after 100 cycles). The initial Coulombic efficiency of the S-RGO2 electrode was low (45.4%) due to the formation of an SEI film during the lithium insertion,76 but reached 94.6% after 10 cycles. The continuous capacity decay during the initial 10 cycles may be due to continuous SEI formation, which may block some of Li+ storage sites. Similar capacity decay during the initial 10 cycles has been observed by others.17−19,21,28,44 As in the case of the S-RGO2 electrode, the Coulombic efficiency was increased during the initial 10 cycles in other RGO electrodes (42.5− 50.8%, initial; 94.4−96.8% after 10 cycles). The first charge capacity and reversible discapacity of S-RGO2 were much higher than those of RGO nanosheets produced using hydrazine,9,10 electron beam irradiation,10 thermal reduction,10,77,78 and N-doped graphanes28,30 (SI Table S1). SI Figure S8 shows the galvanostatic charge and discharge voltage profiles of the RGO electrodes vs Li/Li+. In the first charge curves, the pseudovoltage plateau at around 0.7 V may be due to SEI formation via the decomposition of electrolytes on the electrode surface.76 The presence of defects79 and reactions between lithium ions and oxygenated functional groups80,81 may be responsible for the SEI formation during the Li ion insertion. As reported previously, a capacity below ca. 0.5 V (vs Li/Li+) can originate from Li ion intercalation into the graphene layer, while a capacity above ca. 0.5 V can be attributed to the Faradaic capacitance on the edge sites, defects, and/or surface of the graphene sheets.9,82−84 As shown in Figure 7b, the RGO2 electrodes exhibited a larger fraction of G
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thermally reduced GO; 1030 m2 g−1, mesoporous carbon), but the reversible capacities were quite similar (1013 mAh g−1, thermally reduced GO; 1100 mAh g−1, mesoporous carbon). On the basis of the observations in this study and by others, surface area may not be critical in determining the enhanced reversible capacity of S-RGO2. 3.3.2. Interlayer Spacing. The interlayer spacings of HRGO1 and S-RGO1 were very similar, but initial capacities and reversible capacities of the two electrodes were quite different. Similarly, Pan et al.10 and Oscar et al.39 observed little to no correlation between interlayer spacing and reversible capacity. This may indicate that interlayer spacing is not the dominant factor determining reversible capacity. 3.3.3. Chemical Functionality. In a comparative study on the lithium uptakes of RGOs prepared using heat-treatment at different temperatures (300−1400 °C) with hydrazine, Kuo et al. attributed the enhanced charge capacity of heat-treated RGO at 300 °C to the presence of specific carbon−oxygen single bonds (cyclic edge ether and phenol functionalities), rather than carbonoxygen double bonds (carboxyl, carbonyl, and quinone) and structural factors (surface area, defects).20 Later, Uthaisar et al. studied lithium uptakes of thermally reduced GOs prepared at various temperatures between 250 and 900 °C and suggested that the larger capacity of RGO synthesized at 250 °C compared to the RGOs synthesized using higher temperatures was due to a larger number of carbon−oxygen double bonded ketone groups.20 When the amount of oxygenated groups in H-RGO1 and S-RGO1 and their lithium uptake were compared (Table 1), the oxygenated groups seemed to have a positive effect on the capacity, which agreed well with previous observations. However, in case of H-RGO2 and S-RGO2, the larger amount of oxygenated groups in HRGO2 did not lead to a larger charge/discharge capacity, suggesting that the presence of oxygenated groups is not a key factor in the capacity of RGO 2s. Surface area, interlayer spacing, and defects also may not be responsible for the huge difference in lithium uptake between H-RGO2 and S-RGO2, as discussed in the previous section. The most striking difference between the properties of H-RGO2 and S-RGO2 was the hydrogen content; S-RGO2 exhibited an H/C ratio approximately three times larger than that of H-RGO2. The presence of hydrogen-terminated groups in carbonaceous materials has a positive effect on the reversible capacity by providing additional sites for lithium ion storage, as discussed previously.45,62,64−69 Thus, the key factor in the superior reversible capacity of the SRGO2 electrode as compared to other RGOs electrodes can be related to the larger amount of hydrogen-terminated sites. The incorporation of hydrogen-terminated groups in S-RGO2 was enhanced by the combinatorial use of double-oxidized GO with the larger amount of oxygenated functionalities/larger surface area and supercritical isopropanol that can donate hydrogen and react with oxygenated functionalities in GO2. Lastly, it is not clear what caused the lower reversible charge capacity of S-RGO2 than that of H-RGO1 at current densities >10 A g−1 (Figure 8). Interactions between lithium and hydrogen-terminated sites are known to cause a change in the bonding nature of neighboring carbons.68 S-RGO2 may not have had enough time for its bonding nature to be changed under the ultrahigh charge−discharge rate conditions. A similar trend was observed in the porous carbon monoliths with ordered microstructures; carbon with higher H/C ratios exhibited higher reversible capacities than carbon with lower H/C ratios at current densities below 14.9 A g−1, while at
Figure 9. Long-term cyclability at current densities of 1, 5, and 10 A g−1.
167 mAh g−1, respectively. The excellent cycling stability of the RGOs may be due to the volume variation caused by Li insertion/deinsertion and can be effectively buffered by the large interlayer spacing between the graphene sheets and large porosity.85 As discussed in the Introduction, the lithium ion storage capacity of RGOs is correlated with a number of different structural and chemical parameters (chemical functionalities, interlayer spacing, surface area, pore size, defect sites, etc.), and attempts to change certain parameters have a significant effect on other parameters. In other words, conventional GO and RGO synthetic methods could not decouple one parameter from the others, and thus attempts to generalize graphene properties, which would allow the prediction of lithium storage behaviors of RGOs, seemed to be unsuccessful.39 Without understanding factors that strongly influence the electrochemical response of RGOs-based electrodes, the choice of specific reduction methods (chemical, thermal, electron beam, etc.) for obtaining RGOs with excellent electrochemical performance would not be possible. Therefore, to gain insight into the parameters which determine enhanced reversible capacity of S-RGO2, the first charge and discharge capacity of RGOs and how they are related to their physicochemical properties are listed in Table 1. 3.3.1. Specific Surface Area. As listed in Table 1, larger specific surface areas of RGOs did not always result in a larger lithium ion uptake in “simple RGO” electrodes prepared via conventional methods. For example, the surface area of SRGO2 (147.2 m2 g−1) was much smaller than those of HRGO1 (397.3 m2 g−1) and H-RGO2 (527.2 m2 g−1), but SRGO2 exhibited higher first and reversible charge capacities than H-RGO1 and H-RGO2. When comparing H-RGO1 and S-RGO1, however, the RGO with the larger surface area resulted in larger capacities, even though additional lithium uptake sites were expected with S-RGO1 owing to the presence of additional hydrogen-terminated groups. As shown in SI Figure S11, there did not seem to be any correlation between the specific surface area and first discharge capacities of the RGO electrodes prepared in this study and by others. Even the RGO with a very small surface area of 32.6 m2 g−1 exhibited a large charge capacity of 1255 mAh g−1,20 a value that was much larger than most RGOs with larger surface areas (SI Table S1). Pan et al. compared surface areas and capacities of a thermally reduced GO and a mesoporous carbon with similar interlayer spacings.10 The surface areas were very different (184 m2 g−1, H
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current densities over 18.6 A g−1, these two carbons exhibited similar reversible capacities.63 The hydrogen content of HRGO1 was much lower than that of other RGOs, suggesting that the dominant lithium storage mechanism of H-RGO1 is different from that of S-RGO2. The favorable Li ion transport characteristics of H-RGO1 at high lithium insertion/deinsertion rates may result from its high porosity, high surface area, and the low hydrogen-terminated sites. The high porosity with pore diameters up to 120 nm appeared to render facile electrolyte penetration into the bulk phase of the electrode. The high surface area may have led to a large electrolyte/active material contact area. This may have led to a favorable Li ion transport from the electrolyte to the electrode surface and facile chargetransfer reactions with the large number of absorption sites.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea Grant funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A2061020). Additional supports from Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2012M3A7B4049807) and the Human Resources Development program (no. 20124010203270) of the KETEP grant funded by the Korean government Ministry of Knowledge Economy are also appreciated.
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4. CONCLUSIONS In summary, we demonstrated that the combinatorial use of double-oxidized GO and scIPA reduction produces a hydrogenenriched RGO with an ultrahigh reversible capacity when used as the anode material. The double oxidation of graphite resulted in GO with numerous defects, a large amount of oxygen functionalities, high specific surface area, and high porosity when compared to the GO prepared using the conventional modified Hummer’s method. The unique role of scIPA as a hydrogen-donating agent and as an alkylation reactant produced hydrogen-enriched RGO when doubleoxidized GO was used. The galvanostatic charge−discharge properties results suggested a strong positive correlation between the hydrogen content in RGOs and the reversible capacities. The scIPA reduced-GO2 electrode exhibited a remarkably high reversible capacity (1331 mAh g−1 at 50 mA g−1 after 100 cycles), excellent rate-performance (328 mAh g−1 at 5 A g−1), and good cycling stabilities up to 1000 cycles even at a high current density of 10 A g−1, which outperformed heattreated GOs and scIPA reduced-GO1 electrodes. The formation of hydrogen-enriched sites by scIPA and doubleoxidized GO may play a role in determining the superior electrochemical performance. This study opens up a new possibility for designing high-performance electrodes with ultralarge storage capacity for large-scale lithium ion battery applications.
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ASSOCIATED CONTENT
S Supporting Information *
Materials, synthesis of graphene oxides, synthesis of double oxidized graphene oxides, comparison of synthesis of RGOs and their electrochemical performance, FT-IR spectra of GOs and RGOs, C1s XPS peak deconvolution results of GOs and RGOs, High-resolution XPS spectra of O1s, XRD patterns of graphite, GOs, and RGOs, aberration-corrected HR-TEM images, charge−discharge profiles of H-RGO1, S-RGO1, HRGO2, and S-RGO2 at the current density of 50 mA g−1, charge−discharge profiles of H-RGO1, S-RGO1, H-RGO2, and S-RGO2 at various current densities, comparison of reversible capacities of the “simple RGO”-based electrodes, correlation of 1st charge capacities of “simple RGO” electrodes with the specific surface areas of the RGOs. These materials are available free of charge via the Internet at http://pubs.acs.org.
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