Folding Graphene Film Yields High Areal Energy Storage in Lithium

Jan 19, 2018 - The fold electrodes show stable cycling performance over 500 cycles at a rate of 0.5C rate and much higher rate capability as compared ...
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Folding Graphene Film Yields High Areal Energy Storage in Lithium-Ion Batteries Bin Wang,†,# Jaegeon Ryu,‡,# Sungho Choi,‡ Gyujin Song,‡ Dongki Hong,‡ Chihyun Hwang,‡ Xiong Chen,† Bo Wang,† Wei Li,† Hyun-Kon Song,‡ Soojin Park,*,‡ and Rodney S. Ruoff*,†,§,∥ †

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea § Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ∥ School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We show that a high energy density can be achieved in a practical manner with freestanding electrodes without using conductive carbon, binders, and current collectors. We made and used a folded graphene composite electrode designed for a high areal capacity anode. The traditional thick graphene composite electrode, such as made by filtering graphene oxide to create a thin film and reducing it such as through chemical or thermal methods, has sluggish reaction kinetics. Instead, we have made and tested a thin composite film electrode that was folded several times using a water-assisted method; it provides a continuous electron transport path in the fold regions and introduces more channels between the folded layers, which significantly enhances the electron/ion transport kinetics. A fold electrode consisting of SnO2/graphene with high areal loading of 5 mg cm−2 has a high areal capacity of 4.15 mAh cm−2, well above commercial graphite anodes (2.50−3.50 mAh cm−2), while the thickness is maintained as low as ∼20 μm. The fold electrode shows stable cycling over 500 cycles at 1.70 mA cm−2 and improved rate capability compared to thick electrodes with the same mass loading but without folds. A full cell of fold electrode coupled with LiCoO2 cathode was assembled and delivered an areal capacity of 2.84 mAh cm−2 after 300 cycles. This folding strategy can be extended to other electrode materials and rechargeable batteries. KEYWORDS: folding, graphene composite films, high mass loading, high areal capacity, lithium-ion batteries

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the electron and ion transport kinetics, the capability to buffer a large volume change of active materials during cycling, and the tap density of the electrode material which is associated with the volumetric capacity of the battery. Sponges or aerogels made from CNTs/graphene provide a high porosity for the accommodation of volume change of active materials and fast diffusion of ions in electrolyte, and the continuous carbon backbone normally promises good electron transport paths.7−12 However, the porous structure inevitably lowers the tap density, which results in poor volumetric performance. For example, a freestanding CNT/SnO2 sponge used as an anode in a lithiumion battery (LIB) delivered a high areal capacity of 9.20 mAh cm−2, but the thickness of the electrode was as large as 1.5 mm due to its porous structure, lowering its volumetric capacity.7

echargeable batteries must progress rapidly to meet the ever-growing markets, from portable electronics to gridscale storage.1−3 Effort has been devoted to developing electrode materials and structures to satisfy the requirements for superior performance, such as high energy/power density and stable cycling, but electrode configurations have been more or less the same. In general, electrodes are prepared by loading composite materials on the metal current collectors through a slurry coating and subsequently by roll-pressing. However, the inactive components, which involve binders, conductive carbon, and particularly metal current collectors, greatly lower the practical energy densities of batteries. Freestanding three-dimensional (3D) battery electrodes could be realized by integrating active materials into the matrix of carbon-based frameworks (e.g., interconnected carbon fibers, carbon nanotubes (CNTs) and graphene materials),4−8 which account for a small proportion of weight in the battery and can also be active in the energy storage. The critical properties that should be considered for constructing 3D electrodes include © 2018 American Chemical Society

Received: November 29, 2017 Accepted: January 16, 2018 Published: January 19, 2018 1739

DOI: 10.1021/acsnano.7b08489 ACS Nano 2018, 12, 1739−1746

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Figure 1. (a) Schematic illustration showing the configuration of a “film electrode” and a “fold electrode”. (b) Comparison of normalized “out-of-plane” resistance for film and fold electrodes with various areal densities. For instance, Fold_5 is a fold electrode with areal density of ∼5 mg cm−2.

Figure 2. (a−d) TEM images of rG-O/SnO2 composite and the relevant elemental maps. (e−g) SEM images of the cross section of film electrodes with various areal densities. (h, j) Cross section of different fold electrodes prepared by folding Film_1 (1 mg cm−2), and (i) the fold has a continuous film structure.

To realize a high areal capacity while limiting the electrode weight and volume, freestanding and compact electrode architecture is required. 3D carbon frameworks with higher density are readily prepared by assembling graphene oxide (G-O) sheets so that

they are stacked in a layer-by-layer manner. However, a freestanding composite film such as the rG-O (reduced graphene oxide)/active materials composite paper12−18 was rarely used directly as an electrode, because it suffers from the low electron transport across the film (through the thickness of 1740

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Figure 3. (a) CV curves of Film_1, Film_5, and Fold_5 electrodes at a scan rate of 0.1 mV s−1. Cycle performance of (b) film electrodes and (c) fold electrodes with (d) corresponding Coulombic efficiency curves at 0.2C rate for 100 cycles. (e) Capacity retention plots for the Film_1, Film_5, and Fold_5 electrodes at different C rates. (f) Nyquist plots of the Film_1, Film_5, and Fold_5 electrodes obtained by applying a perturbation voltage of 5 mV in the frequency range of 200 kHz to 0.1 Hz after 1st and 50th cycles.

“film electrode”), in which both were made from the identical composite film for assembling the electrode, were directly used as the electrode for LIBs. The electrode design is illustrated in Figure 1a. In the case of a film electrode, the poor electron/ion transport kinetics in the film hinders the electrochemical reaction between active materials and lithium ions. In contrast, a fold electrode with similar thickness comprised of many continuous layers of the original thin film, where the folds at the edges and thus the overall electrode geometry provides new paths for free electron flow, is a configuration we felt worth studying. To investigate the variation of conductivity for the rG-O/SnO2 film after folding, the resistance of the film before and after introducing folds is compared (Figure S1, Supporting Information). A small difference (almost negligible) of 0.5− 5.6% in resistance after folding was observed for several measurements, suggesting that the “in-plane” electrical conductivity for the fold structure is comparable to the flat

the film) and slow ion diffusion owing to the densely stacked graphene. Using composite film composed of rG-O and SnO2 nanoparticles as an example, such electrodes usually have a capacity lower than 600 mAh g−1 at current density of around 100 mA g−1 and unstable cycling performance.12,13 Nevertheless, this paper-like freestanding electrode is useful as it distributes various active materials uniformly,16,17 accommodates a large volume change of active materials during cycling,19,20 and for its compact, robust, and flexible properties.21 We thus identified that improving the electron/ion transport kinetics in this type of film is critical for boosting its energy storage. We here report a method to make a folded paper-like electrode, made by a water-facilitated folding process. Using SnO2 nanoparticle-decorated rG-O composite films as an example, a folded configuration (namely “fold electrode”), to be compared to a densely stacked thick film structure (namely 1741

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the fold “edge”, and the contact(s) between the film layers was also increased, which are the reasons for the increased electrical conductivity compared to “equivalent” films without folds (that is, equivalent in terms of the total amount of rG-O carbon in the electrodes being compared). The out-of-plane electrical conductivity values of the electrodes, which were also assembled into the cells and then disassembled, were compared by measuring the resistance between top and bottom surfaces as shown schematically in Figure 1b. The normalized R/R0 value (R0 is the resistance measured for Film_1) was used for comparison. The “out-of-plane” resistance of Film_3 and Film_5 was around 2.3 and 4.1 times higher than Fold_3 and Fold_5 electrodes, respectively, indicating an enhanced electron transport for the fold electrodes. The Li storage behavior was first evaluated by cyclic voltammetry (CV) at a scan rate of 0.1 mV s−1 in the potential range of 0.0−2.0 V, as shown in Figure 3a, showing no significant differences in the electrochemical reactions between the films. During the cathodic sweep, a reduction peak at ∼1.0 V is attributed to the formation of the irreversible solidelectrolyte-interphase (SEI) layers, followed by two peaks ascribed to stepwise lithiation of SnO2. The reduction of SnO2 produces amorphous Li2O and metallic Sn (at ∼0.6 V), which can be further lithiated into LixSn (0 < x < 4.4, at 0.16 and 0.25 V).25 From the subsequent anodic sweep, the observed peaks at 0.51 and 0.68 V are dealloying reactions of Li−Sn alloys, while the single broad peak at 1.3 V corresponds to the partial oxidation of Sn. Also, the redox pairs and corresponding peak positions do not depend on the electrode types at such a low scan rate, which can be further corroborated by galvanostatic discharge/charge curves (Figures S9−10). The increased loading levels led to higher overpotentials inside cells. As a result, high loading electrodes (Film_5 and Fold_5) delivered a slightly lower capacity of 1555 mAh g−1 and 1594 mAh g−1 with Coulombic efficiencies of 59.2% and 63.3%, respectively, as compared with the low loading electrode Film_1. When the mass loading was further increased to 6 mg cm−2 (Fold_6), a much lower discharge capacity was observed in Figure S10b, signifying a degraded cycling performance; and thus the maximum mass loading was adjusted to be 5 mg cm−2. The cycling stability and Coulombic efficiency of each film were compared at a rate of 0.2C, as shown in Figure 3b−d and Figure S11. Besides the specific capacity, areal capacity is also compared for the electrodes as it is an important parameter for practical applications when considering the areal limit for batteries. The capacities of thick film electrodes decayed dramatically to have only 32% and 4.4% of retention, respectively, for Film_3 and Film_5 electrodes after 100 cycles, while the fold electrodes afforded more stable cycle life. In particular, the Fold_5 electrode delivered a high areal capacity of 3.67 mAh cm−2 at the 100th cycle thus with 89% capacity retention. During the repeated cycles, the fold electrodes had an average Coulombic efficiency of 99.81% exceeding the practically available value of 99.5%, which sheds light on achieving a high energy density via a simple folding strategy. The additional electron path(s) through the folds enables the electrodes to be activated at an early stage of cycling and the “built-in” gaps between layers can accommodate a huge swelling of typical metal oxide electrodes.26 As a result of such gaps, electrolytes are fully infiltrated into the electrodes, which facilitates fast-charging even while they are in freestanding form. Figure 3e shows the rate-dependent capacity retention for three types of electrodes. The traditional thick

film. Besides electron flow being facilitated by the fold(s), the small gaps between the folded layers after compression into the cell form multiple channels for free diffusion of lithium ions, and the contacted points between the layers provide additional paths for electron flow orthogonal to the plane of the electrode (through the thickness direction). The fold electrodes show stable cycling performance over 500 cycles at a rate of 0.5C rate and much higher rate capability as compared with the film electrodes. More importantly, the fold electrode with high areal loading of 5 mg cm−2 exhibits a higher areal capacity of 4.15 mAh cm−2, well beyond that of commercial graphite anodes (2.5−3.5 mAh cm−2),22,23 while the electrode was readily configured as thin as ∼20 μm. A full cell using LiCoO2 as cathode and a fold electrode as anode was assembled and showed stable cycling over 300 cycles with areal capacity of 2.84 mAh cm−2, which corresponds to a specific and volumetric energy density of 452 Wh kg−1 and 1112 Wh L−1, respectively.

RESULTS AND DISCUSSION SnO2 nanoparticles with size around 5 nm were uniformly distributed on rG-O sheets as shown by the transmission electron microscopy (TEM) images and elemental maps in Figure 2a−d. High-resolution TEM images showed lattice fringes with d-spacing of 0.33 nm, which is assigned to the (110) plane of crystalline SnO2 (JCPDS no. 41-1445). The Xray diffraction (XRD) pattern in Figure S2 also shows the presence of the SnO2 nanocrystals.24 X-ray photoelectron spectroscopy (XPS) spectra in Figures S3−4 further demonstrate that the G-O was reduced to rG-O after heat treatment at 400 °C in argon environment. To improve the energy density of the electrode, the content of SnO2 in the electrode was adjusted to be as high as 85 wt % as shown by thermogravimetric analysis (TGA, Figure S5). The thin rGO/SnO2 film that was selected had an average thickness of 3.02 ± 0.14 μm (measured from scanning electron microscopy (SEM) images of the film cross section) as shown in Figure S6, because it was the maximum thickness which provided stable cycling in our experiments when used as an electrode in LIBs. A folded film was compressed into a cell and then disassembled before cycling for further structural measurement (Figure S7). From these SEM images, the film was found to be composed of relatively compactly stacked layers, and many contact points for electron flow between neighbor layers in the fold electrode when it was finally compressed into cells are obtained. Due to the difficulties in handling the freestanding ultrathin G-O composite film without damage, a folding technique using a water surface was employed to realize the G-O/SnO2 film folds (Figure S8). The process is described in Methods. After reduction at 400 °C in argon, the fold structures persist as shown in the SEM images in Figure 2h−j. As a control experiment, films with thickness of 3.02 ± 0.14 μm (1.01 ± 0.04 mg cm−2, Film_1), 8.91 ± 0.28 μm (2.97 ± 0.06 mg cm−2, Film_3), and 15.1 ± 0.45 μm (5.07 ± 0.14 mg cm−2, Film_5) were prepared (Figure 2e−f). The fold samples with the same mass loadings of ∼3 mg cm−2 (Fold_3) and ∼5 mg cm−2 (Fold_5), respectively, exhibit a continuous architecture of the films (Figure 2h, j); with a magnified image of the fold showing that the flexibility of the composite film (Figure 2i) allows for folding in this manner without breaking the film at the fold. To check the fold region and overall structure of the electrode after compression that occurs from the cell, we disassembled one cell before cycling and obtained SEM images of it that are shown in Figure S7. The fold electrode retains a continuous structure at 1742

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Figure 4. (a) Long-term cycle performance of the Fold_5 electrode at 0.5C rate. (b) The comparison of volumetric capacity versus current density of various freestanding film anodes from the present work and published results (see Table S1). (c) The charge/discharge profiles of the Fold_5-LCO (LiCoO2) full cell at 0.5C rate and (d) the cycling performance at 0.5C rate for 200 cycles with end variation on rates from 1C to 3C.

film electrodes did not retain their initial capacity even at a 1C rate (3.40 mA cm−2). In contrast, the Fold_5 electrode did not show a distinct difference from the Film_1 electrode in capacity retention at a 3C rate (10.2 mA cm−2), with a capacity of 274.5 mAh g−1, which corresponds to an areal capacity of 1.37 mAh cm−2 (Figure S12). To further investigate the durability of the fold electrodes at various current densities, electrochemical impedance spectroscopy (EIS) in the frequency range from 200 kHz to 0.1 Hz at the fully lithiated state (Figure 3f) was done. The semicircle in the impedance spectra indicates the charge transfer resistance (Rct). According to the Nyquist plots, both Film_1 and Fold_5 electrodes show semicircles with a smaller diameter (13.7 Ω and 20.1 Ω, respectively) in the high-medium frequency regions, while the Film_5 electrode has a charge transfer value of 37.7 Ω after the first cycle. The difference between the electrodes increases after 50 cycles. The Film_1 electrode shows the smallest value of 29.3 Ω among the electrodes, followed by Fold_5 for 54.4 Ω and Film_5 for 100.5 Ω, which agrees with capacity retention results. Cycling stability and electron/ion transport kinetics are associated with the structural stability at both the particle and electrode levels. As discussed, SnO2 nanoparticles undergo the conversion and alloying reactions in order and then the oxidation of metallic Sn partially happens in the applied potential ranges. After 100 cycles, SnO2 coexisted with metallic Sn (Figure S13). Given that the graphene composite electrode showed their theoretical capacity, Sn nanoparticles should have a high degree of lithiation (∼Li4.4Sn) with a large volume expansion. Nevertheless, they still retained a good connection to the graphene sheets in the cell. Across the crystalline particles, amorphous lithium oxide was also observed, which

can further enhance the ion kinetics, as has been reported previously.27 Electrode swelling results show significant differences in morphology of the outermost layers as shown in Figures S14− 15. The Film_1 electrode has uniform and dense SEI layers, but the highest amount of swelling of 89.4%, due to the lack of “buffering” spaces. In contrast, irregularly formed byproducts on the top surface of Film_5 are thick and have a porous structure with cracks containing pulverized nanoparticles. However, the Fold_5 film presents better structural integrity which still maintains the fold structure and gaps between the neighboring layers for electrolytes to be infiltrated. As a result, the electrodes expanded as much as 73.1% and 48.9% for Film_5 and Fold_5, respectively. Furthermore, by disassembling the coin cell after cycling for the Film_5 cell, the counter Li foil was found to be damaged by various side reactions, and the Film_5 electrode was pulverized, while the Fold_5 electrode was still in the electrolyte and had a clean surface (Figure S16). In addition, SEI layers are mainly composed of Li2CO3 and LiF from the ex situ XPS results, as also reported by others (Figure S17).28 These results suggest that the folding strategy provides improved structural stability and charge transfer efficiency. To this end, the Fold_5 electrode was further tested at a rate of 0.5C (1.70 mA cm−2) for extended cycles as shown in Figure 4a. The cell shows a cycle life up to 500 cycles with a capacity retention of 73.7% and end areal capacity of 2.53 mAh cm−2 with an average Coulombic efficiency of 99.78%. This electrochemical performance in terms of cycling stability, high current durability, and high areal capacity is compared with previous works related to carbon-based freestanding anodes in the literature (Figure 4b, Table S1).29−35 For a meaningful one1743

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sonicator (Model: WUC-A02H, 60 Hz, Daihan). Afterward, 5 mL NaOH aqueous solution (70 mg mL−1) was dropped into the mixture slowly during sonication; this took about 1 min. The resulting dispersion was stirred at 120 °C for 4 h in a round-bottom flask to obtain a G-O/SnO2 NP dispersion, which was then centrifuged at 8000 rpm and washed with ethanol/water for three times in an attempt to remove the remaining reactants. The resulting dispersion was filtered through a 47 mm diameter anodic aluminum oxide (AAO) membrane with pore size of 0.2 μm (Whatman) to obtain a composite film. Folding the Films. The as-filtered film was delaminated directly onto a water surface (water was in a 1 L glass beaker) and then picked up onto a Cu foil, on which the film could be cut into strips using scissors, and the strips were then transferred onto a sapphire substrate without Cu foil. The folding process is shown in Figure S8. By fixing one end of the film strip on sapphire using tape, the film strip/sapphire was immersed into water. When next pulling it out of the water, the free end of the strip reached the water surface, which led to the first fold of the strip (with help by the use of the tweezer) when it was completely pulled out. After drying it in air for 2 min, the sapphire was reversed and immersed into water for the second fold. The interaction between the first folded two layers evidently increased during the 2 min drying, as it was found that the second fold could be achieved without unfolding the first fold. By repeating this process, folded electrodes with different number of layers were prepared. Characterization. Scanning electron microscopy (SEM, Verios 460, FEI) was used to characterize the surface and cross-sectional structure of each film at an acceleration voltage of 10 kV and current of 0.8 nA. The dimensions and internal structure of rG-O/SnO2 films were determined by transmission electron microscopy (TEM, JEOL2100) and high-resolution TEM (HRTEM, JEOL-2100F) with an acceleration voltage of 200 kV. To investigate the crystallinity of SnO2, X-ray diffraction (XRD) (Smartlab, Rigaku) between 10° and 90° was performed using Cu−Kα radiation (λ = 1.5418 Å). Thermogravimetric analysis (TGA) was done using a TA Instrument Q500 Thermogravimetric Analyzer by increasing the temperature from room temperature to 800 °C at a rate of 10 °C/min in air. X-ray photoelectron spectroscopy (XPS, ThermoFisher, K-alpha) was done to obtain quantitative/qualitative information on the surface oxidation state of film samples. Electrochemistry. All the prepared freestanding films were immersed in the electrolyte for 1 day and then directly used as working electrodes with lithium foil as the counter/reference electrode and a Celgard 2400 membrane as the separator. The electrolyte was 1.3 M LiPF6 in 3:7 v/v ethylene carbonate (EC) and diethyl carbonate (DEC) with 10 wt % fluorinated ethylene carbonate additives included to improve the cycling stability. For galvanostatic cycling tests, cointype half and full cells were fabricated using CR 2032-type cells (Welcos) in an argon-filled glovebox. The mass loading of each electrode was obtained by weight of electrode per unit area and is specified inside the figures (captions). In addition, a slurry was prepared for the LiCoO2 (LCO; from LG Chem.) cathode by mixing the active material, super-P, and polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidinone solvent with a weight ratio of 90:5:5. The LCO slurry was coated onto an Al current collector. The average LCO loading of the electrode was 19.2 mg cm−2. The potential windows for all cycled cells ranged from 0.01 to 2.0 V for the anode half cell and 3.0 to 4.3 V for the cathode half cell. The full cells have a potential window of 1.0−4.1 V for the formation cycle and 1.5−4.0 V for further cycles. The formation cycle is a “precycle” to give a stable SEI formation at a relatively low rate of 0.1C. After cycling, each cell was opened in the glovebox and washed with dimethyl carbonate (DMC) in an attempt to remove residual electrolyte and any other impurities. Then, each of them was dried at room temperature. Galvanostatic discharge/charge test results were made with a Battery Cycler (WBCS3000K8, Wonatech Co., Ltd.). Cyclic voltammograms (CVs) were obtained at 0.1 mV s−1 from 0 to 2.0 V. EIS measurements were carried out between 200 kHz−0.1 Hz with an amplitude of 10 mV at a fully lithiated state (∼0.01 V).

to-one comparison, the capacities are normalized into volumetric capacities, and also current densities are given as the areal current density. From the comparison, the fold electrodes with higher loading density show improved cycling stability and higher volumetric capacity at high current density. The practical viability of the fold electrodes was investigated by assembling and testing a full cell having a commercial LiCoO2 (LCO) cathode, which has a specific capacity of 162 mAh gLCO−1 at 0.1C rate (Figure S18). The N/P ratio (areal capacity ratio of negative to positive electrodes) of the full-cell was set to 1.12. The designed full cell showed an areal capacity of 3.41 mAh cm−2 at the precycle and 2.84 mAh cm−2 for the first cycle at a 0.5C rate (1.43 mA cm−2) (Figure S19, Figure 4c−d). The full cell retained its capacity as high as 89.2% of the initial value after 200 cycles, and all areal capacities delivered in Figure 4 fall in the range of applications for portable devices. When increasing and maintaining the current densities to 2.86, 5.72, and 8.58 mA cm−2 for every 20 cycles, it had areal capacities of 2.14, 1.72, and 1.51 mAh cm−2, respectively (Figure 4d). Afterward, its capacity was fully recovered. Based on the above results, the volumetric energy density of the full cell is estimated to be 1112 Wh L−1 and decreased to 734 Wh L−1 (considering the electrode swelling) after 200 cycles. In addition, specific energy densities are 452 Wh kg−1 at the initial cycle and 403 Wh kg−1 after 200 cycles. The calculations for energy density of the full cells are described in the Supporting Information. The good performance is attributed to the fold structure which provides electron flow paths and built-in pores for accommodation of volume expansion and electrolyte infiltration.

CONCLUSION In summary, folding graphene-based films, (here, a composite film with SnO2 nanoparticles) “back and forth” to create layers separated by gaps, has been realized by a water-facilitated folding method. Compared with traditional freestanding films, folding can provide a continuous electron transport path through the multiple folds and introduce additional channels between the folded layers, which improves the electronic and ionic transport kinetics, and also alleviates the volume expansion of the electrode during cycling. As a result, a fold electrode with high areal loading of 5 mg cm−2 had an areal capacity of 4.15 mAh cm−2, well beyond that of commercial graphite anodes (2.50−3.50 mAh cm−2). It also showed stable cycling over 500 cycles at 1.70 mA cm−2 and improved rate capability compared to the traditional thick electrodes with the same mass loading but without folds. A full cell of such a fold electrode coupled with a commercial LiCoO2 cathode was assembled and delivered an areal capacity of 2.84 mAh cm−2 after 300 cycles, which corresponds to a specific/volumetric energy density of 452 Wh kg−1 and 1112 Wh L−1, respectively. With this approach, freestanding electrodes with different types of active materials can be further applied in other kinds of rechargeable batteries. METHODS Preparation of rG-O/SnO2 NP Composite Film. Graphene oxide (G-O) dispersed with a stated concentration of 4 mg mL−1 was purchased from Graphenea (www.graphenea.com). G-O sheets “decorated” with SnO2 nanoparticles (NP) were synthesized by an in situ hydrolysis process. Specifically, 0.50 g SnCl2 was dispersed in 30 mL ethylene glycol by stirring at 80 °C, then 10 mL G-O dispersion was added, and the mixture was sonicated for 30 min in a bath 1744

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ACS Nano

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08489. Calculation of full cell energy density; measurement of the resistance before and after folding an rG-O/SnO2 film; XRD pattern of rG-O/SnO2 film; XPS measurement of G-O and rG-O films and rG-O/SnO2 film; TGA curve for the rG-O/SnO2 film; SEM images show the cross section of the rG-O/SnO2 film_1 mg cm−2; SEM images of a fold electrode; photos that show achieving two folds of an rG-O/SnO 2 strip; galvanostatic discharge/charge curves of film electrodes and fold electrodes with different loading; cycle performance for film electrodes in terms of areal capacity; discharge curves for the Fold_5 electrode at different rates from 0.2C to 3C; ex situ TEM and XPS for the Fold_5 electrode after 100 cycles; electrode swelling measured by cross-sectional SEM images; photos of Film_5 and Fold_5 electrodes and counter Li foil after cycling; ex situ XPS spectra of C 1s and F 1s for the Fold_5 electrode after 100 cycles; data on the cathode and the precycle performance of the full cell (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: rsruoff@ibs.re.kr, ruoffl[email protected]. ORCID

Bin Wang: 0000-0001-9576-2646 Xiong Chen: 0000-0003-2878-7522 Soojin Park: 0000-0003-3878-6515 Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the Institute for Basic Science (IBSR019-D1) and the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT, and Future Planning as Global Frontier Project (CASE-2015M3A6A5072945). REFERENCES (1) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577−3613. (2) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; CarreteroGonzalez, J.; Rojo, T. Na-Ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884−5901. (3) Lin, M.-C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.-Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B.-J.; Dai, H. An Ultrafast Rechargeable Aluminium-Ion Battery. Nature 2015, 520, 324−328. (4) Yu, M. L.; Wang, Z. Y.; Wang, Y. W.; Dong, Y. F.; Qiu, J. S. Freestanding Flexible Li2S Paper Electrode with High Mass and Capacity Loading for High-Energy Li-S Batteries. Adv. Energy Mater. 2017, 7, 1700018. (5) Xia, L.; Wang, S. Q.; Liu, G. X.; Ding, L. X.; Li, D. D.; Wang, H. H.; Qiao, S. Z. Flexible SnO2/N-Doped Carbon Nanofiber Films as Integrated Electrodes for Lithium-Ion Batteries with Superior Rate Capacity and Long Cycle Life. Small 2016, 12, 853−859. 1745

DOI: 10.1021/acsnano.7b08489 ACS Nano 2018, 12, 1739−1746

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DOI: 10.1021/acsnano.7b08489 ACS Nano 2018, 12, 1739−1746