Flexible binder-free graphene paper cathodes for high-performance Li-O2 batteries

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Flexible binder-free graphene paper cathodes for high-performance Li-O2 batteries Do Youb Kim a,1, Mokwon Kim Yongku Kang a,*

b,1,2

, Dong Wook Kim a, Jungdon Suk a, O Ok Park b,

a

Center for Advanced Battery Materials, Advanced Materials Division, KRICT, 141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea b Department of Chemical and Biomolecular Engineering (BK21+ Graduate Program), KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

In this study, free-standing porous graphene papers for high-capacity and reversible Li-O2

Received 21 January 2015

battery cathodes are investigated. The graphene paper-like films were fabricated by the

Accepted 27 May 2015

assembling of graphene nanoplatelets (GNPs) with the aid of graphene oxides (GOs) as a

Available online 4 June 2015

stabilizer, using a vacuum-assisted filtration method. By using GOs as a stabilizer, the GNP/GO films were fabricated with a paper-like form and they exhibited a highly wrinkled and disordered morphology. Moreover, the use of GNPs as a basic material eliminated the need for a post-annealing to recover the intrinsic electrical conductivity of graphene sheets. Subsequently, the GNP/GO paper could be directly used as a Li-O2 battery cathode without any conducting additives and binders. The GNP/GO paper electrode showed a much higher discharge capacity in comparison to the reduced-GO paper and commercially available carbon papers. We also found that toroidal Li2O2 mainly nucleated and grew on discharge, and decomposed on charge with a relatively high O2 evolution/consumption efficiency of 87%. However, a large number of Li2O2 particles grew inside the GNP/GO paper electrode, resulting in severe volume expansion of the electrode. This volume expansion could be the primary reason for the capacity fading on cycling. Ó 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Recently, lithium (Li)-O2 batteries have attracted much attention as a next-generation electrical energy storage system. This is because they have a higher theoretical energy density than current Li-ion batteries. This property is highly desired for range-anxiety-free electric vehicles and other

high-energy applications [1–5]. Typical non-aqueous Li-O2 batteries consist of a Li metal anode, an aprotic electrolyte containing Li salt, a separator, and a porous air cathode. In order to develop a practical use for Li-O2 batteries, there are many problems to overcome in regard to the electrolytes and Li anode. These problems include the stability of the electrolyte and the effective protection of the Li anode [1–5].

* Corresponding author. E-mail address: [email protected] (Y. Kang). 1 These two authors contributed equally to this work. 2 Current address: Energy Lab., Samsung Advanced Institute of Technology (SAIT), Samsung Electronics, Suwon 443-803, Republic of Korea. http://dx.doi.org/10.1016/j.carbon.2015.05.097 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved.

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The development of air cathodes is also a critical issue, since the air cathode is significantly related to the capacity and reversibility (or cycle life) of the Li-O2 battery [6–8]. Accordingly, there have been many efforts to develop effective cathodes in order to improve the performance of Li-O2 batteries [9–16]. Generally, cathodes for Li-O2 batteries are prepared by spreading a slurry containing a conductive carbon powder (e.g. Ketjen Black, Super P, and Vulcan XC-72), a polymer binder (e.g. polyvinylidene fluoride and polytetrafluoroethylene), and an optional catalyst on a carbon fiber membrane or a metal current collector [9–13]. However, these polymer binders do not have sufficient stability against superoxide radical anions and/or Li2O2, and hence form decomposition products during the discharge/charge cycling [17–19]. To solve this problem with the binder decomposition, many researchers have been driven to explore a binder-free cathode [20–28]. For example, catalytically active metal oxides such as Co3O4 [20], e-MnO2 [21], and NiCo2O4/MnO2 [22] grown on a Ni foam were fabricated and used as binder-free cathodes. Cathodes based on carbon nanotubes (CNTs), grown using a chemical vapor deposition, were also reported [23–25]. These cathodes showed high electrochemical performances as a Li-O2 battery cathode. However, they required relatively complicated and expensive processing conditions and/or were composed of relatively dense materials, which is unsuitable for Li-O2 batteries with high specific capacity and energy density. Graphene has great potential to be used for energy storage because it has excellent electrical conductivity and an extremely high theoretical surface area of 2630 m2/g, which is higher than that of conventional activated carbon (<1500 m2/g) [29–31]. In addition, graphene has potentially copious active sites for nucleating discharge products, which may lead to the higher capacity of Li-O2 batteries [6,32]. Therefore, several reports on high performance cathode materials using graphene as a catalyst in itself or as an effective supporting material for other catalysts in Li-O2 batteries have been reported [6,7,33–39]. However, in most studies, the electrodes were still prepared by a slurry casting method using polymeric binders on current collectors [6,7,33–38]. Studies on binder-free graphene electrodes have scarcely been reported [39]. Graphene has another advantage of high mechanical strength as well as high flexibility, enabling the individual sheets to be arranged into free-standing paper-like structures [40,41]. When graphene paper (GP) is used in energy storage devices, it does not require any conducting additives and binder. This enhances the high energy densities of the devices. Most GPs are fabricated using the following two methods: (i) vacuum filtration of GO dispersion, followed by a thermal annealing [41– 43] or a chemical [44,45] reduction process, and (ii) vacuum filtration of chemically reduced-GO (rGO) dispersion [46,47]. Nevertheless, these methods generally lead to GPs with a densely packed structure similar to graphite. This is attributed to the stacking of the GO sheets during the filtration. Moreover, additional thermal annealing for enhancing the electrical properties often result in brittle GPs, due to thermal shrinking. Recently, we successfully demonstrated the fabrication of highly flexible and porous free-standing GPs. They were composed of graphene nanoplatelet (GNP) and GO, and their application was as a Li-ion battery anode [48]. By using GOs

as a stabilizer, the GNP/GO sheets were routinely fabricated with a paper-like form and exhibited a highly wrinkled and disordered morphology. Moreover, the use of GNPs as a basic material eliminated the need for a post annealing process, which is necessary in order to recover the intrinsic electrical conductivity of the graphene sheets. Subsequently, the GNP/GO paper could be directly used as an electrode without any conducting additives and binders. Features such as high porosity, conductivity, binder-free and flexibility are also thought to be desirable for a Li-O2 battery cathode. Hence, in this study, we investigated the electrochemical performance of the GNP/GO paper as a Li-O2 battery cathode. The GNP/GO paper electrode showed a much higher specific discharge capacity (approaching 10000 mAh/g at 100 mA/g), in comparison to rGO paper and commercially available carbon papers. We systematically investigated the reversibility and cyclability of a Li-O2 battery using the GNP/GO paper electrode. This was achieved with the use of scanning electron microscopy (SEM), X-ray diffraction (XRD), and in situ differential electrochemical mass spectroscopy (DEMS). We found that toroidal Li2O2 mainly nucleated and grew on discharge and decomposed on charge. In addition, the GNP/GO paper electrode showed higher reversibility than the carbon paper electrode with a relatively high O2 efficiency of 87%.

2.

Experimental

2.1.

Preparation of the GNP/GO paper

The GNP/GO papers were prepared following the procedure described in our previous report [48]. The GO powder was synthesized using a modified Hummer’s method [49]. The powder was subsequently dispersed in distilled (DI) water using sonication for 20 min and subsequently centrifuged at 6000 rpm to remove multi-layered species. With the supernatant of dispersion, a stable GO dispersion with a concentration of 0.5 mg/mL was prepared. In the meantime, an aqueous dispersion of GNPs (N002-PDR, XY < 10 lm, average thickness < 1 nm, Angstron Materials) with a concentration of 0.5 mg/mL was prepared by sonication, using a probe sonicator for 1 h with an addition of a small amount of poly(4styrenesulfonic acid) (PSS, Mw = 75,000, 18 wt.% in H2O, Sigma–Aldrich). Approximately 0.2 mL of PSS solution was added to aid the initial wetting of 10 mg of GNP powder in water. These aqueous dispersions of GO and GNP were mixed with a volume ratio of 3:1, followed by the addition of the same amount of DI water. It was then subjected to bath sonication for 10 min, prior to the paper fabrication. The GNP/GO papers were fabricated by a vacuum filtration of the asprepared aqueous dispersion of GO and GNP, through a PVDF filter membrane (Millipore, 0.45 lm pore size and 47 mm diameter). The papers were subsequently dried in ambient air and peeled from the membrane.

2.2.

Characterization

SEM images were obtained using a field-emission scanning electron microscope (Mira III, Tescan), operated at an accelerating voltage of 10 kV. The XRD pattern was obtained with an

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X-ray diffractometer (Ultima IV, Rigaku) using Cu Ka (0.1542 nm) radiation. The XRD samples of discharged/charged electrodes were taken from a cell in a glove box and subsequently enclosed with a Kapton film in order to prevent their exposure to air during testing. X-ray photoelectron spectroscopy (XPS) data were acquired with a Sigma Probe spectrometer (Thermo VG Scientific) using Al Ka X-ray source. The area of analysis was approximately 400 lm diameter for all samples. Fourier transform infrared spectroscopy (FT-IR) data were acquired with a Nicolet 6700 FT-IR spectrometer (Thermo Scientific) using the samples dispersed in KBr pellets.

2.3.

Electrochemical characterization

The electrochemical performance of the GNP/GO paper as a cathode of a Li-O2 battery was tested using a Swagelok-type cell. For comparison, an rGO paper which was prepared by a vacuum filtration of GO dispersion and a thermal reduction [41] was also tested as the cathode. Two commercial carbon papers, AvCarb P50 (AvCarb Material Solution) and Sigracet 35AA (SGL Group) were also applied for the cathode. Li-O2 cell was fabricated in a Ar-filled glove box (MBraun, H2O < 1 ppm), using Li metal as an anode, a glass microfiber filter (GF/C, Whatman) as a separator, and a stainless steel mesh (200 mesh) as a current collector. For cathodes, the as-prepared GNP/GO paper, carbon papers, and rGO paper were directly used after being cut using a punch. They were vacuum dried (150 °C for 3 h) without adding any binder and conducting additives. As an electrolyte, 1.0 M LiNO3 (Sigma–Aldrich) in dimethylacetamide (DMAc, Sigma–Aldrich) was used after drying by freshly ˚ ). Water content was below activated molecular sieves (4 A 10 ppm, which was titrated by a Karl Fischer coulometer (C30, Mettler Toledo). The discharge/charge operations for the assembled Li-O2 cells were conducted on a VMP3 potentiostat (Biologic Science Instrument) at room temperature under 1.5 bar ultrapure (>99.999%) oxygen gas.

2.4. In-situ differential electrochemical mass spectroscopy (DEMS) analysis The total amount of O2 consumed during discharge, in addition to the quantities of O2 and other by-product gases that evolved during charge were measured by in-situ DEMS. This was custom-built, following the set-up reported by IBM [50]. A precise in-line pressure sensor monitored any pressure change in the hermetically sealed Swagelok-type LiO2 cell during discharge, to provide the quantity of consumed O2 using the ideal gas law. After discharge, the O2 in the cell was flushed out and changed to Ar (>99.999%). During charge, any evolved gases in the isolated cell were accumulated and pumped by the Ar carrier gas (during the programmed time (e.g. 20 min)) into a residual gas analyzer (UGA-200, Stanford Research Systems). This measured the quantity and identity of the gases. Subsequently the fractional composition of the each gas was calculated, based on the carrier gas Ar.

3.

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Results and discussions

The GNP/GO papers were prepared by a vacuum filtration method as described in our previous paper (see the Section 2 for the detailed procedure) [48]. Aqueous solutions of GO and GNP with a small amount of poly(4styrenesulfonic acid) were separately prepared using ultrasonication. The mixed solution was subsequently filtered through a polyvinylidene fluoride (PVDF) filter membrane using a vacuum filtration system, followed by drying at room temperature. Finally, the GNP/GO papers peeled from the membrane filter. Fig. 1 shows a photograph and SEM images of the GNP/GO papers. As shown in Fig. 1A, the prepared GNP/GO paper was free-standing and had sufficient flexibility to be bent, rolled, or even folded [48]. Moreover, the GNP/GO paper could be easily cut using a hole punch in order to be readily used as an electrode in a Li-O2 battery cell (see inset in Fig. 1A). Fig. 1B–D shows SEM images of the prepared GNP/GO paper viewed from different angles, demonstrating the microstructure of the paper. As shown, the prepared GNP/GO paper had a wrinkled and disordered structure throughout the paper. This could be attributed to the partial re-aggregation of the GNPs under the holding effect of the GOs during the drying process. These wrinkles could enable the GNPs and GOs to be stacked up into a paper, enabling high porosity within the paper. When compared to an rGO paper with approximately the same mass (which was prepared by a vacuum filtration of GO dispersion and a thermal reduction (Fig. S1 in the Supporting Information)), the GNP/GO paper was much thicker than the rGO paper. This implies that the GNP/GO paper has a larger amount of pores and a larger surface area within the paper, in comparison to the rGO paper. The higher surface area of the GNP/GO paper was also confirmed by N2 adsorption; the Brunauer–Emmett–Teller (BET) specific surface area of the GNP/GO paper was 278.9 m2/g, which was much higher than that of the rGO paper (52.8 m2/g) (Table S1 in the Supporting Information). Since the Li-O2 cathode has to accommodate discharge products for higher capacity, the pores and larger surface area are highly desirable for a Li-O2 battery cathode [7,24–27]. In addition, the thickness of the GNP/GO paper could be readily controlled by varying the concentration of a filtration solution and/or the volume of the solution [48]. In the present study, GNP/GO papers with a thickness of approximately 5 lm were used for all Li-O2 battery testing (Fig. 1D). Since the GNP/GO paper was based on highly conductive GNP, its electrical conductivity was sufficiently high (ca. 164 S/cm for 5 lm-thick paper) [48] without any post thermal annealing step. The high electrical conductivity and structural features (shown in Fig. 1A) enabled the GNP/GO paper to be directly applied as a Li-O2 battery cathode without the addition of conducting additives and binders. The electrochemical performance of the GNP/GO paper electrode as a Li-O2 battery cathode was investigated. Swagelok-type Li-O2 cells were assembled in a Ar-filled glove box using Li foil as a counter electrode, a glass microfiber as a separator, and 1 M LiNO3 in dimethylacetamide (DMAc) as an electrolyte (see the Section 2 for the detailed procedure). For comparison,

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Fig. 1 – (A) Photograph of the as-prepared GNP/GO paper with a diameter of ca. 37 mm, demonstrating its flexibility. Inset shows photograph of the GNP/GO paper (cut using a hole punch with a diameter of 12 mm). (B-D) SEM images of the GNP/GO papers; (B) tilted view, (C) top view, and (D) cross-sectional view. Inset in C is the corresponding SEM image with higher magnification (scale bar: 1 lm). (A color version of this figure can be viewed online.)

we also tested binder-free free-standing electrodes such as rGO paper and two commercially available carbon papers which have been widely used as a cathode in a Li-O2 battery [50–52]. Fig. S2 (in the Supporting Information) shows the SEM images of the two carbon papers, demonstrating that both carbon papers are composed of carbon microfibers as a main frame and that carbon powders and/or platelets

Fig. 2 – Galvanostatic discharge profiles of Li-O2 cells using the GNP/GO paper, rGO paper, and commercially available carbon papers as cathodes, at a current density of 200 mA/g. The current density is based on the weight of the cathode paper. (A color version of this figure can be viewed online.)

partially filled the spaces between carbon microfibers. Fig. 2 shows the galvanostatic discharge profiles of the Li-O2 cells using the GNP/GO paper, rGO paper, and two carbon papers at a current density of 200 mA/g. The two carbon papers exhibited similar specific capacities of 260 mAh/g for P50 and 270 mAh/g for 35AA, with similar discharge plateaus at approximately 2.5 V vs. Li/Li+. In the case of the rGO paper electrode, the specific capacity was ca. 560 mAh/g. To be surprised, the GNP/GO paper electrode exhibited much higher specific capacity of ca. 6910 mAh/g with a ca. 200 mV higher discharge plateau at approximately 2.7 V. Wang et al. reported that GO with sufficient oxygenfunctional groups was favorable for the adsorption of O2. Thus, efficient electrocatalytic activity for the oxygen reduction reaction (ORR) was expected from a theoretical calculation using a density functional theory method [53]. Wang et al. also demonstrated that hybrid cathode materials containing GO exhibited better electrocatalytic activity with a lower overpotential within a hybrid Li-O2 battery. The lower discharge overpotential for the GNP/GO paper electrode, in comparison to the other electrodes, could be attributed to the efficient electrocatalytic activity of the GOs incorporated within the electroconductive GNPs. The larger surface area of the GNP/GO paper also lowered the actual current density in the electrode, resulting in the lower overpotential on discharge. In addition, the significantly higher specific capacity of the GNP/GO paper electrode can be attributed to the larger surface area and the pores inside the GNP/GO paper electrode, in comparison to the surface area and pores of the rGO paper

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and carbon papers (as discussed previously). Cetinkaya et al. recently reported that a free-standing GO paper electrode showed a reversible capacity of ca. 1000 mAh/g at a current density of 0.01 mA/cm2 [54]. In the present study, the GNP/GO paper electrodes were tested at a current density of 200 mA/g; this can be converted to ca. 0.09 mA/cm2, since the nominal area of the electrodes was 1.13 cm2. The GNP/GO paper had a higher specific capacity than that of a GO paper electrode even at a much higher current density. This effect owes to the greater electric conductivity of the GNP/GO paper electrode as well as its larger surface area and higher number of pores. The rate performance of the GNP/GO paper electrode was also investigated. Fig. 3A shows the galvanostatic discharge profiles of the Li-O2 cells at various current densities, using the GNP/GO paper electrode with a cut-off voltage of 2.0 V. As expected, the discharge capacity of the Li-O2 cells decreased with larger overpotentials, as the current density increased. When the current density increased to 300 and 500 mA/g, the full discharge capacity decreased to 4670 and 3600 mAh/g, respectively. When the current density was 100 mA/g, the full discharge capacity reached 9760 mAh/g. The galvanostatic discharge/charge profiles of the GNP/GO paper electrode at various current densities, with a cut-off capacity of 1000 mAh/g, are shown in Fig. 3B. When the capacity was controlled up to 1000 mAh/g, the depth of discharge varied in accordance with the current density. Although both the depth of discharge and the current density increased as the current density increased, the Li-O2 cells exhibited similar discharge/charge profiles with slightly higher overpotentials. This indicates the high rateperformance of the GNP/GO paper electrode. For an ideal non-aqueous Li-O2 battery, the electrochemical reaction occurs according to the following reaction; 2Li + O2 M Li2O2, where the discharge product Li2O2 forms through ORR on a cathode surface during discharge and subsequently decomposes through an oxygen evolution reaction (OER) during charge [1–5]. The formation and decomposition of the discharge products on the GNP/GO paper electrode was investigated using SEM and XRD. Fig. 4A shows the

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galvanostatic full discharge/charge profile of the cell, using the GNP/GO paper electrode at a current density of 200 mA/g. Fig. 4B–F shows the SEM images of the electrode with various discharged or charged states, as marked in Fig. 4A. When the cell was discharged at 30% of its full capacity, relatively small discharge products with a diameter of ca. 210 nm were observed (Fig. 4B). As the cell was further discharged, the discharge products grew larger (Fig. 4C). When the cell was fully discharged, toroidal morphologies of the discharge products, with a diameter greater than 500 nm, were easily recognizable as shown in Fig. 4D. Furthermore, the discharge products decomposed and decreased in size during subsequent charge. When the cell was re-charged at 60% of its full capacity, the diameter of the discharge product decreased to less than 200 nm (Fig. 4E). The discharge products further decomposed with subsequent charge and all the discharge products disappeared. The electrode returned to its pristine state when the cell was fully charged to its full capacity (Figs. 4F and 1C). This observation of the morphology of the discharge products and its growth pattern in accordance with the discharge/charge cycle was consistent with others [17,27,55,56]. The average size of the discharge products during discharge and charge are summarized in Table S2 within the Supporting Information. The discharge products observed in the SEM images were identified as Li2O2 by XRD analysis. Fig. 5 shows the XRD patterns of the GNP/GO paper electrodes recorded at a pristine state and varying levels of discharged or charged states. The XRD pattern obtained from the pristine GNP/GO paper shows no appreciable peaks, except from those of the stainless steel mesh current collector at around 2h = 43.5° and 50.7°, in the range of 30–60°. Along with the discharge, diffraction peaks appeared at around 2h = 32.9°, 35.0°, 40.7°, and 58.8°; (1 0 1), (1 0 2), and (1 1 0) planes of crystalline hexagonal phase Li2O2, respectively (JCPDS #09-0355). The intensities of the Li2O2 diffraction peaks subsequently decreased as the cell charged and they finally disappeared when the cell was fully re-charged. For a better comparison, the normalized peak intensity (which is shown as a relative intensity to that of the stainless steel current collector at around 2h = 43.5°), was plotted (Fig. S3 in the Supporting Information). The

Fig. 3 – Galvanostatic discharge/charge profiles of Li-O2 cells using the GNP/GO paper electrode at various current densities of 100, 200, 300, and 500 mA/g respectively; (A) discharge profiles with a cut-off voltage of 2.0 V vs. Li/Li+ and (B) discharge/ charge profiles with a cut-off capacity of 1000 mAh/g. (A color version of this figure can be viewed online.)

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Fig. 4 – (A) Galvanostatic full discharge/charge profile of a Li-O2 cell, using the GNP/GO paper electrode at a current density of 200 mA/g, and (B–F) top-view SEM images of the GNP/GO paper at the various discharged and charged states, corresponding to the red dots marked in A. (A color version of this figure can be viewed online.)

increase and decrease in the Li2O2 peak intensity in accordance to the discharge and charge respectively, was consistent with observation of discharge products by SEM analysis (Fig. 4). This indicates the reversible formation and decomposition of Li2O2. From these results, we can conclude that the GNP/GO paper electrode in a Li-O2 cell could provide enough pores and sufficient surface area to accommodate a relatively large amount of the desired discharge product, Li2O2, and the ORR and OER occurred reversibly during discharge and charge cycle, respectively. In-situ DEMS is a very powerful tool which can be used to confirm that the reaction at the cathode is overwhelmingly

Fig. 5 – XRD patterns of the GNP/GO paper electrodes recorded at a pristine state and various discharged/charged states. (A color version of this figure can be viewed online.)

due to Li2O2 formation and decomposition. This is achieved by quantifying the amount of O2 consumed and evolved, respectively [15,50,57–59]. The reversibility of the Li-O2 cell using the GNP/GO paper electrode was further investigated by in-situ DEMS (Fig. 6). Fig. 6A shows the galvanostatic discharge/charge profile of the cell using the GNP/GO paper electrode, with a cut-off capacity of 1000 mAh/g at a current density of 200 mA/g. Fig. 6B shows the amount of O2 consumed and evolved during discharge and charge respectively, according to the potential profile shown in Fig. 6A. If the reaction at the cathode follows only the desired electrochemical reaction, 2Li + O2 M Li2O2, the charge that passes per 1 mol of O2 (e /O2), should be 2, both in discharge and charge. When using the GNP/GO paper electrode, the (e /O2)disch and (e /O2)ch were 2.07 and 2.39, respectively. This indicates that the Li2O2 formation and decomposition occurred during discharge and charge, respectively, with some degree of parasitic reactions. Ottakam Thotiyl et al. demonstrated that the electrolyte can decompose during discharge and produce byproducts such as Li2CO3, LiHCO2, and/or LiCO2CH3. This was observed when both dimethyl sulfoxide and tetraglyme were used as a solvent for the electrolytes [60]. Although a different solvent, DMAc, was used as an electrolyte in our experimental system, the slightly higher value for (e /O2)disch (which was greater than 2) could be attributed to the electrolyte decomposition. Since the XRD patterns (Fig. 5) did not show any peaks for the by-products, it appeared that the byproducts were amorphous or their levels were lower than the detection limit of XRD [33,34,61,62]. Furthermore, significant parasitic reactions occurred during subsequent charge. It is believed that the electrolyte and electrode decomposed

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Fig. 6 – (A) Galvanostatic discharge/charge profile of a Li-O2 cell using the GNP/GO paper with a cut-off capacity of 1000 mAh/g at a current density of 200 mA/g. In situ DEMS measurement corresponding to the potential profile in A; (B) O2 consumption and evolution during discharge and charge, respectively, and (C) gas evolution rates for O2, H2, and CO2 during charge (the dotted line in C represents the ideal 2e / O2 process).(A color version of this figure can be viewed online.)

at a relatively high potential. This formed decomposition products including Li2CO3; their oxidative decomposition also concurrently occurred [60,63,64]. Fig. 6C shows the gas evolution rate profiles for O2, CO2, and H2 during charge. As shown, O2 mainly evolved during the whole charge process. However, there was a slight gap

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between the actual O2 evolution profile and the ideal 2e /O2 process (which is represented as a dotted line). Evolution of other gases, including H2, was negligible during the whole charge process; however, some CO2 gas evolved at the end of the charge. The evolution rate of CO2 was negligible until the cell was charged up to nearly 90% of its discharged capacity. This was followed by a sharp increase, indicating that the by-products began to decompose from the relatively higher potential (greater than approximately 4.0 V). For comparison, a Li-O2 cell using the P50 carbon paper electrode was also tested by in-situ DEMS. This was performed at the same current and capacity cut-off conditions as those for the Li-O2 cell, using the GNP/GO paper electrode (Fig. S4 in the Supporting Information). The Li-O2 cell using the GNP/GO paper electrode exhibited superior performance in comparison to the cell using the P50 carbon paper electrode, in almost all respects; (i) it showed lower overpotentials on both discharge and charge, (ii) the discrepancy between the amounts of O2 consumption on discharge and evolution on charge in the cell using the GNP/GO paper electrode, was smaller than that in the cell using the P50 carbon paper electrode, and (iii) the oxygen evolution rate of the GNP/GO paper was consistently higher in the whole range of the charge than that of the P50 carbon paper. Throughout the whole cycle of discharge and charge, the Li-O2 cell using the GNP/GO paper electrode exhibited higher OER/ORR efficiency (87%) than that of the P50 carbon paper electrode (76%). The OER/ORR efficiency of 87% for the GNP/GO paper electrode is quite high in consideration of the fact that no electrolyte is yet capable of giving a OER/ORR efficiency of over 90% in a single cycle [59,63,64]. The lower charge overpotential and higher reversibility of the GNP/GO paper electrode could be attributed to the higher electrochemical activity of graphene, in comparison to carbon fiber and powder (which compose the P50 carbon paper) [32,33–35]. This result is consistent with Sun’s report that graphene nanosheets showed lower overpotentials than the Vulcan XC-72 carbon electrode, in a nonaqueous Li-O2 battery [33]. The DEMS results for the Li-O2 cells using the GNP/GO paper and P50 carbon paper electrodes are also summarized in Table S3 in the Supporting Information. The cyclability of the GNP/GO paper electrode in a Li-O2 cell was also tested. The Li-O2 cells tested in the potential range 2.0 V–4.2 V without capacity limit showed rapid capacity fading on cycling (Fig. S5). In order to improve cyclability, a Li-O2 cell using the GNP/GO paper was tested with a cut-off capacity of 1000 mAh/g at a current density of 200 mA/g (Fig. 7). In the early cycles, the Li-O2 cell showed reversible discharge/charge cycles. However, as the cycle progressed, the overpotentials gradually increased in both discharge and charge. Thus, the reversible discharge/charge cycle continued in less than 20 cycles. The cycling performance of a Li-O2 battery is significantly dependent on the decomposition of the electrolyte and the carbon-based electrode [2,5,15,51]. We further investigated the formation and decomposition of discharge products on the electrode surface during repeated cycles using Fouriertransform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) (Figs. S6 and S7 in the Supporting Information). We found that desired discharge product Li2O2

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Fig. 7 – Cycling performance of a Li-O2 cell using the GNP/GO paper with a cut-off capacity of 1000 mAh/g at a current density of 200 mA/g; (A) galvanostatic discharge–charge profiles and (B) plots of discharge and charge capacity against cycle number. (A color version of this figure can be viewed online.)

mainly formed with small amount of by-product such as Li2CO3 on the initial discharge. Although Li2O2 was fully decomposed on the subsequent charge, Li2CO3 still remained on the surface of the electrode. Since the Li-O2 cell produced CO2 gas at the end of charge (Fig. 6C), some of by-products decomposed at relatively high potentials on charge. However, the elevated potential could also accelerate the decomposition of the electrolyte and the carbon electrode at the same time, which further produced by-products in the end. The accumulation of the by-products interrupted the formation and decomposition of discharge products on following cycles. This result matches with the reports that some of the by-products remained undecomposed and increasingly

accumulated on the electrode surface on repeated cycling, leading to higher polarizations with cycling [60,65,66]. These electrolyte and carbon electrode decomposition, and higher polarizations on cycling could be the reason for the capacity fading of the cell. Interestingly, we found that the thickness of the GNP/GO paper electrode significantly expanded after discharge, in comparison to its pristine state. It may be possible that the GNP/GO paper electrode expanded by absorbing the electrolyte; however we speculated that the formation of discharge products inside the electrode was the main reason for the large volume expansion of the electrode. We closely examined the GNP/GO paper electrode after discharge using

Fig. 8 – (A) Cross-sectional-view SEM images of the GNP/GO paper after discharge. (B)–(D) Magnified SEM images recorded from the top, middle, and bottom side of the GNP/GO paper as marked in A, respectively.

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a SEM. Fig. 8A shows the cross-sectional SEM images of the GNP/GO paper electrode after discharge, at a discharge capacity of 4000 mAh/g. This figure demonstrates that the electrode had a thickness of 75 lm, which is much thicker than the pristine GNP/GO paper (5 lm). Fig. 8B–D shows magnified SEM images recorded from the top, middle, and bottom side of the GNP/GO paper electrode as marked in Fig. 8A, respectively. The toroidal discharge products had an identical morphology to those shown in Fig. 4, and they were evenly distributed inside the electrode. This result indicates that the GNP/GO paper electrode had sufficient surface area for the nucleation of discharge products and enough pores for the pathways of Li ion and O2. However, there was insufficient pore volume to accommodate discharge products. Although the expanded GNP/GO paper electrode had a slight decrease in its thickness after subsequent recharge, it could not be fully restored to its initial state (Fig. S8 in the Supporting Information). This considerable and irreversible volume expansion on discharge was also one of the reasons for the capacity fading of the GNP/GO paper electrode. To improve the cycle life of a Li-O2 battery, studies on the fabrication of GP with larger pore volumes and/or nanocatalysts are currently underway. Furthermore, and most importantly, a more chemically stable electrolyte system has to be discovered to enable the stable cycling of Li-O2 batteries.

4.

Conclusion

In summary, we have fabricated highly flexible free-standing GNP/GO paper and investigated its electrochemical performance as a cathode for a non-aqueous Li-O2 battery. Thanks to the higher surface area arising from the wrinkled and disordered structure of the GNP/GO paper, it demonstrated a much higher discharge capacity than other free-standing electrodes, including rGO paper and commercially available carbon papers. In addition, the GNP/GO paper electrode showed reversible cycles, where the desired discharge product, Li2O2, nucleated and grew on the electrode surface on discharge, and decomposed on charge. Accordingly, O2 mainly evolved during charge, in addition to the decomposition of Li2O2, with a relatively high OER/ORR efficiency of 87%. A large number of Li2O2 particles grew inside the GNP/GO paper electrode, resulting in an irreversible volume expansion of the electrode. This volume expansion could be the primary reason for the capacity fading on cycling, in addition to the electrolyte and/or cathode decomposition at relatively high potentials, as well as Li degradation [67]. We expect that the free-standing GNP/GO paper could be a possible platform for Li-O2 battery research, and that introduction of various catalysts on the GP could improve the performance of Li-O2 batteries.

Acknowledgements This work was financially supported by the GovernmentFunded General Research & Development Program by the Ministry of Trade, Industry and Energy, Republic of Korea. M.K. and O.O.P. acknowledge the support from the World

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Premier Materials (WPM) program (10037689) funded by the Ministry of Knowledge Economy, Republic of Korea.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2015.05.097.

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