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Phenolic resin-grafted reduced graphene oxide as a highly stable anode material for lithium ion batteries Mochen Li, Huaihe Song,* Xiaohong Chen, Jisheng Zhou and Zhaokun Ma A novel and effective route for preparing phenol formaldehyde resin grafted reduced graphene oxide (rGO-g-PF) electrode materials with highly enhanced electrochemical properties is reported. In order to prepare rGO-g-PF, hydroxymethyl-terminated PF is initially grafted to graphene oxide (GO) via esterification reaction. Subsequently, the grafted GO is reduced by the carbonization process under an inert gas atmosphere. The covalent linkage, morphology, thermal stability and electrochemical properties of rGO-g-PF are systematically investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, thermal gravimetric analysis, differential scanning calorimetry and a variety of electrochemical testing techniques. In the constructed architecture, the amorphous carbon shell can inhibit the co-intercalation of solvated lithium ion and avoid partial exfoliation of the graphene

Received 9th October 2014, Accepted 9th December 2014

layers, thus effectively reducing the irreversible capacity and preserving the structural integrity.

DOI: 10.1039/c4cp04556d

conductivity of electrode materials. As a result, the rGO-g-PF electrode exhibits impressive high cycling stability at various large current densities (376.5 mA h g1 at 50 mA g1 for 250 cycles, 337.8 mA h g1

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at 200 mA g1 and 267.8 mA h g1 at 1 A g1 for 200 cycles), in combination with high rate capability.

Meanwhile, the carbon coating layer leading to a decreased thickness of SEI film can improve the

Introduction Graphene, a monolayer of sp2-hybridized carbon atoms arranged in a two-dimensional lattice, has attracted tremendous attention owing to its outstanding chemical and physical properties, such as superior electric and thermal conductivity, flexibility, and mechanical strength.1 Such unique features offer great promise for many potential applications,2 particularly making it an alluring material for electrochemical energy storage,3 graphene can achieve a higher specific capacities compared with other graphite materials.4 The higher specific capacities originate from the unique one-atom-thick planar sheet structure of graphene, which makes it accommodate lithium not only on both sides of the graphene layer, but on the edge sites and other defects or cavities formed within the disorder stacked graphene sheets.5 However, graphene displayed a relative high irreversible capacity and poor stability at large current density according to recent research.6 The high irreversible capacity can be explained as a result of the formation of solid electrolyte interface (SEI) film during the initial cycle and the reaction of lithium ions with some residual

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China. E-mail: [email protected]

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oxygen-containing groups, introduced by the oxidation process and cannot be completely removed during reduction, on the graphene surface. What is worse, based on the present synthesis methods such as the Hummers method, it is quite difficult to acquire fully exfoliated monolayer graphene quantitatively, the productions still contain a large amount of double-, tripleeven multi-layered graphene. Thus during the charge–discharge process, intercalation of solvated lithium ions may cause the exfoliation of graphene layers, thus exposing a new graphene surface in the electrolyte and inducing the formation of SEI film continuously. So, it seems that the modification of graphene or reduced graphene oxide is quite needed in order to acquire better electrochemical properties, especially the cycling stability. However, graphene is chemically stable and has a poor solubility in both polar and apolar solvents, which has limited the application and modification of pristine graphene. Graphene oxide (GO), with hydroxyl and epoxide groups on its basal surface and carbonyl and carboxyl groups located at its edge sites (Lerf–Klinowski model),7 has functioned primarily as a precursor to reduce graphene oxide or chemically modified graphene materials. Recently, grafting polymers on the graphene/graphene oxide surface, as an effective functionalization method, has been widely studied by many groups.8 Extensive research efforts have been devoted to amination or esterification as effective routes for surface modification of graphene. Tang et al. had grafted

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bio-based polyester onto graphene oxide to synthesize a new kind of composite with low threshold percolation of electrical conductivity and high thermal conductivity.8a Polyanilinegrafted graphene hybrid materials through forming amide groups have been synthesized by An and co-workers with a capacitance of 623.1 F g1 at a current density of 0.3 A g1,8b whereas Kumar et al. grafted polyaniline to acylated graphene oxide and found that the composite achieved a capacitance of 250 F g1 with good cycling stability.8c Besides, Wang et al. coated graphite oxide with PAV and incorporated with PVDF using a solution-cast method to improve the dielectric properties of the as-prepared nanocomposites.8d What is more, Yan et al. synthesized porous disordered carbon layers as an energy storage unit for coating on graphene sheets to form interconnected frameworks, which exhibit outstanding volumetric capacitance.9 High impact research published by other groups also utilized graphene (or CNT)/porous carbon to fabricate high level hybrid carbon materials.10 However, none of these literatures reported about using carbonaceous materials to modify and finally encapsulate graphene oxide with the aim of acquiring a structure-stable carbon material to improve its electrochemical properties, e.g., large reversible capacity and high cycling stability. In this article, a novel route for preparing phenolic formaldehyde resin (PF) coated graphene oxide as a carbon material in a two-step process starting from graphene oxide is established. Our strategy involves the functionalization of graphene oxide by grafting PF onto the GO surface via ester linkages, which is subsequently carbonized at elevated temperature and it simultaneously reduces graphene oxide, thereby yielding compact amorphous carbon-coating layers on the graphene surface. Since the product was covalently functionalized and the GO was reduced by carbonization, which C–C bond might form during this process, and hence the PF coating-layer could have a good combination with graphene layers. The formation of the carbon-encapsulated structure and its properties were systematically characterized using various analytical techniques.

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with 2.5 g of NaNO3 and 150 mL of concentrated H2SO4, and the mixture was cooled down to 0 1C. Then, 15 g of KMnO4 was added slowly and stirred for half an hour. After that, the temperature increased to 35 1C and stirred for another 6 hours continuously. The mixture was further diluted with 300 mL deionized water and treated with 30% H2O2 until the color of the mixture changed to brilliant yellow. Finally, graphene oxide was purified by dialysis till the waste water at a neutral pH value. Preparation of GO–NMP dispersion by a solvent-exchange method Since graphene oxide is quite difficult to acquire as its highly dispersed GO–NMP solution by simple sonication treatment, a simple solvent-exchange method is adopted. Typically, a certain amount of NMP was firstly added to the as-prepared GO aqueous solution, and was treated with stirring and sonication. After that, the water was removed by rotary evaporation treatment to obtain GO–NMP dispersion where GO was well dispersed. Synthesis of phenol formaldehyde resin The synthesis of phenol formaldehyde resin was catalyzed by calcium oxide (CaO). Typically, a certain amount of phenol was added to a flask and heated to 40 1C, and then an aqueous solution of formaldehyde was added dropwise with stirring. The molar ratio of formaldehyde and phenol keeps between 1.2–1.3. After the temperature reached 70 1C, CaO was added and mechanically stirred for 3 hours. At last, the products were quickly cooled below 40 1C and CaO was eliminated using a filter. The produced phenol formaldehyde resin was dissolved in NMP to form a PF–NMP solution. Fabrication of rGO-g-PF As illustrated in Scheme 1, the grafted GO with PF covalently bonded on its surface, which was designated GO-g-PF, was prepared from graphene oxide by esterification with EDC/ DMAP as a catalyst. In a typical process, GO–NMP dispersion (0.1 g of GO) and the solution of EDC and DMAP (both the same weight of GO) in NMP were added into a round-bottom flask

Experimental section Materials High purity graphite was purchased from Qing Dao Tai Chang Graphite Co., Ltd. Formaldehyde (37–38%) was purchased from Xi Long Chemical Co. Ltd. N-(3-Dimethylaminopropyl)-N 0 ethylcarbodiimide hydrochloride (EDC, purity of 98.5%) and 4-(dimethylamino)pyridine (DMAP, purity of 99%) were purchased from Aladdin Reagents (Shanghai) Co. Ltd. Phenol, N-methyl-2-pyrrolidone (NMP) and the reagents for the oxidation of graphite including sodium nitrate (NaNO3), concentrated sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2) were analytically pure and used as received. Synthesis of graphene oxide Initially GO was synthesized by a modified Hummers method as described as following. High purity graphite (5 g) was mixed

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Scheme 1

Schematic of synthesis of GO-g-PF.

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with mechanical stirring at 30 1C for 2 hours under a N2 atmosphere. After that, a certain amount of PF was dropwise added (the weight ratio of GO and PF is 4), and the mixture was stirred for 48 hours. To quench the reaction, an equimolar amount of HCl was added to the solution to neutralize DMAP. The produced GO-g-PF was centrifuged at 10 000 rpm for 4 min to remove the NMP and washed with water repeatedly to remove EDC and its hydrolyzed urea derivative (N-ethyl-N 0 -(3dimethylaminopropyl) urea, EDU). The residual water was eliminated by freeze drying. The GO-g-PF was solidified at 150 1C for 2 h and carbonized at 700 1C for 3 h in a nitrogen atmosphere, the GO was reduced during this process and the final product was designated rGO-g-PF. Based on the same procedure, the graphite oxide, which was obtained by freeze drying of GO–H2O dispersion, was also carbonized without grafting treatment and designated rGO. Characterization Fourier transform infrared (FTIR) spectra were recorded on a Nicolet iS50 spectrometer by using the KBr pellets in the frequency range 4000–400 cm1 to characterize and confirm the chemical structure of GO and the formation of covalent bonding between GO and phenolic resin. The morphology of rGO-g-PF was observed by scanning electron microscopy, which was performed on a Zeiss Supratm 55 field emission microscope. High resolution transmission electron microscopy (HR-TEM) measurements were carried out using a JEOL JEM-3010 F microscope. Thermal gravimetric analysis (TG) and differential scanning calorimetry (DSC) were carried out on a NETZSCH STA 449C differential scanning calorimeter. The samples were heated to 700 1C at a heating rate of 5 1C min1 under an argon atmosphere. Electrochemical measurements were carried out by using 2032 coin-type cells. The working electrodes were prepared by mixing electrode materials (rGO-g-PF and rGO), acetylene black, and poly(vinyldifluoride) (PVDF) at a weight ratio of 8 : 1 : 1, and pasting the mixture onto foam nickel (ca. 1.5 mg electrode active material on each foam nickel). A pure lithium sheet was used as the counter electrode. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1 : 1 by volume). The cells were assembled in an argon-filled glove box with the concentrations of moisture and oxygen below 1 ppm. The electrochemical performances were tested at various rates in the voltage range of 0.01–2.50 V. The cyclic voltammetry and the electrochemical impedance spectral (EIS) measurements were carried out on a Zahner Zennium electrochemical workstation. For the cyclic voltammetric measurements, the sweep rate was 0.1 mV s1 and the potential range was 0.01–2.5 V. For the EIS measurements, the frequency range was from 10 MHz to 100 kHz.

Results and discussion Preparation of individually dispersed GO in NMP It is well known that graphene possesses many superior properties compared with other three dimensional materials. Its extraordinary

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Fig. 1 Photos of GO dissolved in NMP by sonication (left) and solventexchange method (right) after centrifugation.

features are based on its two-dimensional structure, hence great importance has been attached to acquiring stable dispersed GO dispersion. Graphene oxide aqueous solution is the most easily prepared GO dispersion, for the large amount of oxygen-containing groups bonded on the GO, making it a highly hydrophilic material.11 Unfortunately, the existence of water will hinder the esterification reactions and affect the catalytic activity of EDC in our study, so the GO needs to be transferred from aqueous solution into organic solution and form a stable dispersion. The traditional method to disperse dried GO into organic solvents, such as N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran (THF),2c,11 was utilizing sonication treatment for a few hours, with the assistance of an ice bath if necessary, which has been widely applied by many researchers and could achieve a quite stable GO dispersion.12 In this study, GO was stably dispersed in NMP by a solventexchange method. As presented in Fig. 1 the GO–NMP dispersion, prepared by sonication, is completely precipitated after centrifugation at 8000 rpm for 15 min. In contrast, for the GO–NMP dispersion obtained by a solvent-exchange method, only a small amount of precipitate is seen at the bottom after the same centrifugation process. It is suggested that GO obtained a better dispersion in NMP by a solvent-exchange method, which may ensure a better grafting effect in the following process, because the stable dispersion state of fewlayered graphene oxide in organic solvent is an important prerequisite for the subsequent grafting of PF uniformly on the surface of GO. Covalent grafting of PF onto the GO surface As schematically illustrated in Scheme 1, GO graft of PF via ester linkages was prepared by the esterification reaction between the hydroxymethyl-terminated PF and GO in NMP solvent with the assistance of EDC and DMAP. GO-g-PF was then reduced by carbonization at elevated temperature. The evidence for the ester bonding between GO and PF is confirmed by FTIR spectra. From Fig. 2, the absorption peaks at 1738 cm1 and 1630 cm1 are assigned to the carbonyl group stretching vibration of –COOH groups at GO edges and the C–C skeleton vibration of graphite oxide,13 respectively. For PF, the absorption peaks at 1508 cm1 and 1611 cm1 are related to the

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Fig. 2

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FTIR spectra of GO, PF, GO-g-PF and rGO-g-PF.

stretching vibration of the aromatic ring, and the peaks at 875, 815 and 751 cm1 demonstrate that the aromatic rings of PF possess a large amount of substituent groups. After esterification reactions, in the spectra of GO-g-PF, the peak at 1738 cm1 (–COOH of GO) almost disappears while a new peak at 816 cm1 assigned to substituent groups on a different position of aromatic rings is observed, suggesting that the esterification reactions occurred successfully. Besides, the CQC absorption peak for GO upshifts to 1635 cm1 from 1630 cm1, which may origin from the interactions of hydrogen bonds.8a Moreover, the hydroxyl (C–OH) stretching vibration peak (1386 cm1) disappears after carbonization, demonstrating the decomposition of oxygencontaining groups and the reduction of GO-g-PF to rGO-g-PF.14 The presence and amount of PF grafting on the graphene oxide surface were further analyzed using thermogravimetric analysis by heating under a nitrogen atmosphere to 700 1C at a rate of 5 1C min1. As shown in Fig. 3, the thermal gravimetric curves of GO and GO-g-PF present obviously weight loss stage at around 216 1C and 200 1C attributing to the pyrolysis of labile oxygen-containing groups such as –OH, –CO and –COOH.8c,15 PF, however, exhibits two stages of weight loss at around 217 1C and 520 1C. The percentage of weight loss of GO at 700 1C was about 52.94%, whereas for PF and GO-g-PF that was 63.13% and 54.23%, respectively. On the basis of this result, the degree

Fig. 3

of functionalization from the gradual mass loss of the GO-g-PF in nitrogen suggests that there is about 87.34 wt% GO and 12.66 wt% PF in the GO-g-PF, which is in accordance with the feed ratio of GO and PF. It is noteworthy that the GO-g-PF material demonstrates a decreased decomposition temperature compared with both GO and PF. The DSC curve of GO-g-PF exhibits a 15.5 1C decrease in the peak of reduction temperature of GO-g-PF (202.3 1C), compared with pure GO (217.8 1C), indicating a chemical reaction between GO and grafted PF. This phenomenon is similar to the recent literature by Barroso-Bujans et al.16 whose PEO–GO intercalated compound displays a much lower decomposition temperature in the thermogravimetric analysis than GO and PEO. They suggested that either the thermal reduction of GO initiated the decomposition of PEO chains or PEO promoted the decomposition of GO. Besides, a recent paper by Dreyer et al.17described the ability of GO sheets to oxidize alcohol to create aldehyde on the GO sheets. Glover et al.18 also speculated a mechanism by which the vinyl acetate groups enhance GO reduction. According to these studies, we propose our results as a similar mechanism that a synergistic interaction takes effect between covalently bonded PF chains and GO. During heating or carbonization, the residual hydroxyl groups located on the PF chains may promote the reduction of GO, thus decreasing the decomposition of GO-g-PF materials. Morphologies The typical morphologies of the rGO-g-PF and rGO electrode materials are observed by SEM. From Fig. 4a and b, the nanosheet structure of rGO-g-PF is clearly observed. The surface of nanosheets appears to be wrinkled and crumped, which is similar to pristine graphene shown in Fig. 4(e) and (f).19 Fig. 4(c) and (d) are high magnification SEM images, it is clearly seen that the rGO-g-PF nanosheets, similar to graphene consisting of single or double layers,20 are transparent. Unfortunately, no significant PF carbon coating layer is directly observed, which may be due to the fact that the coating layer is too thin for the SEM to observe directly. In order to confirm the existence of PF carbon coating layers, the morphology of rGO-g-PF was further characterized by high resolution TEM. The HR-TEM images also exhibit a layered and

TG (a) and DSC (b) curves of GO, PF and GO-g-PF.

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Fig. 4 The SEM images of rGO-g-PF (a–d) and (e and f) rGO.

Fig. 5

The HR-TEM images of rGO-g-PF. Inset image is at lower magnification.

wrinkled structure of rGO-g-PF. As presented in Fig. 5(a) and (b), the amorphous coating of PF carbon is clearly visible, and it could distinguish itself from the graphene edges. Besides, we observed several dark spots on the rGO-g-PF surface in Fig. 5(a) and the inset TEM image. According to previous reports,8a, f in NMP solution the grafted PF chains are extended due to their solubility, however, after drying and elevated temperature carbonization the polymer chains collapse on the surface of

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GO forming nanosized carbon domains corresponding to the dark spots observed. SEM and HR-TEM images of rGO-g-PF reveal that the electrode materials still possess a 2D ultrathin structure, similar to the structure of grapheme, even after PF grafting and coating. According to previous report,5 the Li ion could be stored not only on the two sides of graphene but also between the graphene layers and the vacancies that origin from the stacked graphene

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layers, the ultrathin structure of rGO-g-PF could thus be one of its advantages for improving reversible capacity. Besides, its 2D structure is favorable for the access of the electrolyte into the interior of rGO-g-PF electrodes and accelerates the diffusion of lithium ions into the deep locations,21 contributing to an improved rate performance of rGO-g-PF as will be explained later.

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Electrochemical characterization Galvanostatic charge–discharge experiments were carried out at different current densities in order to characterize the electrochemical performance of rGO-g-PF electrode materials. The first three charge–discharge voltage profiles of rGO-g-PF electrodes vs. Li/Li+ at a current density of 50 mA g1 are shown in Fig. 6(a). The initial discharge capacity (lithium insertion) is as high as 880.5 mA h g1 and the reversible capacity is 457.3 mA h g1. During the first discharge process, the voltage drops fast, and a small voltage plateau, which appears at 0.7 V and disappears in the following cycles, can be attributed to the formation of the solid electrolyte interface (SEI) film and the electrolyte decomposition, contributing to the irreversible capacity of lithium ion batteries.22 Then the voltage decay tends to be gentle. The charge–discharge curves of rGO-g-PF, rising and falling gradually without obvious voltage plateaus with large hysteresis, reveal typical electrochemical characteristics of graphene materials, as previously reported.3a,23 In the CV curves illustrated in Fig. 6(b), it is observed that the redox peaks are not distinguishable, implying the presence of multiple Li storage positions in rGO-g-PF electrodes. The capacity of the potential lower than 0.5 V should be attributed to lithium insertion into the graphene layers, while the capacity above 0.5 V may be due to lithium insertion/extraction from the defects in the rGO-g-PF electrodes, such as vacancies and edges of graphene layers.24 The high irreversible capacity in the first charge–discharge cycle is a common phenomenon for lowtemperature carbon materials, attributing to the formation of SEI film and the decomposition of the electrolyte, corresponding to the voltage of 0.5–0.9 V. The large hysteresis may result from the bonding changes in the host, or the formation of C–H–Li or C–O–Li species in the low-temperature treated carbon materials.25

Fig. 6

Fig. 7(a) presents the cycling performance of rGO-g-PF at a current density of 50 mA g1 for 250 cycles. The initial discharge and charge capacity of rGO-g-PF is 880.5 mA h g1 and 457.3 mA h g1, respectively, exhibiting a Coulombic efficiency of 51.9%. The high irreversible capacity of rGO-g-PF electrode materials arises from many aspects. First, the formation of SEI film, which is composed of Li2O, Li2CO3, alkoxides and polymeric compounds etc., at the electrode/ electrolyte interface or in micro-cracks/defects of rGO-g-PF consumes a large amount of lithium ions during the first discharge process.4,19 Besides, the reaction of lithium ions with residual un-reduced oxygen containing groups also leads a decay of initial discharge capacity.26 After 250 cycles, the reversible capacity of rGO-g-PF maintains 376.5 mA h g1, which is approximately the theoretical capacity of graphite (B372 mA h g1). Cycling performance and cyclic stability under high current densities of 200 mA g1 and 1 A g1 between 0.01 V and 2.5 V were further evaluated, since both of electrode materials are of great importance for applying in lithium ion batteries. As shown in Fig. 7(c), the discharge capacity fades to 333.2 mA h g1 during the 20th cycle and remains at 337.8 mA h g1 during the 150th cycle which are much higher than that of the rGO (282.7 mA h g1). The rGO-g-PF obviously maintains a much higher reversible capacity after 150 cycles, though its capacity is lower than that of rGO in the beginning tens of cycles. At an even higher current density of 1 A g1, the reversible capacity of rGO-g-PF stabilized to 263.4 mA h g1 during the 10th cycle, while the reversible capacity of rGO still fades severely after dozens of cycles and maintains 235 mA h g1 during the 80th cycle when finally getting stabled. Moreover, the reversible capacity and capacity retention of rGO-g-PF can be maintained at 267.8 mA h g1 and 44.7% even after 200 cycles, larger than that of rGO (238.9 mA h g1 and 17.5%). This result clearly suggests that the rGO-g-PF still possesses high cycle stability and specific capacity after hundreds of cycles. As expected, the rGO-g-PF electrode materials exhibit a durable high rate capacity, as plotted in Fig. 7(d). It delivers a reversible capacity of 252.7 mA h g1 when first cycled at 1 A g1 for 50 cycles, 212.3 mA h g1 at 2 A g1 after 10 cycles. When the

(a) Galvanostatic charge–discharge curves of the rGO-g-PF electrode at a current density of 50 mA g1, (b) cyclic voltammetry curve of rGO-g-PF.

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Fig. 7 Cycling performances of rGO-g-PF electrodes at the current density of (a) 50 mA g1, (b) 200 mA g1, (c) 1 A g1, (d) rate performance of rGO-g-PF at various rates for 110 cycles, and the compared cycle data with that of the rGO electrode (b and c).

current density returned to its initial current density of 1 A g1, the capacity still maintained 258 mA h g1. Then the rGO-g-PF depicts tenth-cycle discharge capacities of 301.9, 337.9 and 362.8 mA h g1 at the current density of 0.5 A g1, 0.2 A g1 and 0.1 A g1, respectively. Notably, after 100 cycles at varied current densities, the capacity is retained at 400.8 mA h g1 when the current density is reset to 50 mA g1. Such high rate performance and cycling stability at high discharge/charge rates are significantly higher than the electrochemical properties in previously reported studies on graphene and other carbon anode materials (as shown in Table 1).4,6e,26b,27 In addition to the

Table 1

high reversible capacities, the Columbic efficiency reaches above 97.8% after initial 7 cycles. Compared with other research studies of rGO or graphene, the rGO-g-PF materials exhibit not only substantially larger lithium ion storage capacity but also excellent cycling stability at high current densities. The excellent electrochemical properties of rGO-g-PF could be ascribed as follows. Due to the fact that the GO cannot be fully exfoliated during the synthesis process and may restore some of the layered structures during the carbonization process, both rGO and rGO-g-PF still possess a layered structure when using as electrode materials. During the

Comparison of capacity and rate performance of the rGO-g-PF electrode with those of graphene anodes reported

1st discharge/charge capacity (mA h g1) 1

880/457 at 50 mA g 598/273 at 1 A g1 1233/672 at 0.2 mA cm2 424/328 at 37 mA g1 Discharge: 530 at 150 mA h g1 796/438 at 50 mA g1 1258/255 at 200 mA g1 905.7/462.9 at 100 mA g1 1590/503 at 100 mA g1 Discharge: 930 at 100 mA g1 Discharge: 1021 at 50 mA g1 Current density: 100 mA g1 Current density: 55 mA g1 Current density: 200 mA g1 Current density: 100 mA g1 374/330 at 0.2 C 1967.8/1221.1 at 50 mA g1 Current density: 0.4 mA cm2

Capacity (mA h g1) 374 (30 cycles) 376 (150 cycles) 256 (30 cycles) 267 (200 cycles) 502 (30 cycles) 368 (45 cycles) 238 (40 cycles) ca. 330 (25 cycles) ca. 250 (50 cycles) 292 (50 cycles) 240 (40 cycles) 269 (100 cycles) 308 (15 cycles) 250 (80 cycles) 225 (100 cycles) ca. 250 (40 cycles) 335 (10 cycles) 330 (27 cycles) 1103.2 (100 cycles) 438

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Capacity retention (%) 42.5 44.7 40.7 86.7 45 41.45 19.8 32.2 15.1 28.9 30.1

48.5 62.1

Rate performance (mA h g1) 1

337.8 (200 mA g ) 2

454 (1 mA cm ) 275 (150 mA g1) ca. 200 (300 mA g1)

200 (0.2 A g1) 224 (1 A g1) 235 (0.5 C) 230 (5 A g1)

Ref. This work 4 27a 27b 27c 27d 6e 27e 27f 27g 27h 27i 26b 27j 27k 27l 27m

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Fig. 8 Schematic illustration of the mechanism of PF carbon coating for stabilization of the rGO structure.

first discharge procedure, the formation of SEI film takes place at the electrode/electrolyte interface with the consumption of lithium ions, which is a rather complicated process and the structure and properties of SEI film greatly depend on the type of electrolyte and surface morphology of graphite materials.28 In general, the SEI film formed in the prismatic areas of graphite consists of inorganic unstable compounds with gaseous products raised from solvent reduction, whereas that formed on the basal planes, which is the production of electrolyte anions enriched of organic components, is relatively stable and compact.26a,29 The solvated lithium ion, lithium bonded with carboxyl groups of EC,30 can co-intercalate into the graphene layers from the edge plane and the large solvent molecules induced a sizable strain between the graphene layers adjacent to the intercalates finally leading to the partial exfoliation of the graphite layers and causing an increase of the SEI thickness.31 What is more, electrolyte solution that penetrates into the micro-cracks or fissures on the surface of rGO is irreversibly reduced and forms solid and liquid products. The process induces mechanical stress, as a consequence the micro-cracks or fissures become larger.26a,32 Besides, the solvate molecules stripped from the Li+(EC)4 reacting with graphite edge cause the release of gaseous products (e.g. C2H2) can also induce expansion and further exfoliate graphene layers.33 As a consequence, the partial exfoliated graphene would expose a new surface to the electrolyte and thus the formed surface would further react with the electrolyte to form SEI film as long as it was wetted by the electrolyte.32,34 The mechanism of partial exfoliation of rGO and the formation of extra SEI film are demonstrated in Fig. 8, Route: A. Grafting and subsequent coating of rGO with amorphous carbon, especially on the edge plane, could effectively inhibit the co-intercalation of solvated lithium ions and exfoliation of graphene layers, while the coating layer does not prevent the intercalation and extraction of lithium ions during the charge– discharge processes (Fig. 8, Route: B). Phenolic formaldehyde resin heat-treated below 1000 1C has a ‘‘turbostratic structure’’ with a random graphite layer stacking and forms amorphous carbon, consisting of a diamond like sp3 and graphite like sp2 carbon bonding ratio and possessing excellent mechanical

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resistance.35 The amorphous carbon is composed of graphitic layers stacked turbostratically with many lattice defects and stacking faults. It is likely that the these defects serve to pin the layer together in PF to prevent the layer lattice expansion which would occur during co-intercalation of the electrolyte solvent, thus preventing exfoliation36 and making the electrode material stable even under high current density. In addition, the coating carbon layer provides a buffer phase between rGO and the electrolyte, the decomposition of the electrolyte on the buffer phase to form a SEI passivation layer is much milder than that on rGO.37 We thus speculate that in the case of rGO-g-PF, due to the existence of an amorphous carbon layer, solvated lithium ions can not intercalate into the rGO layers thus preventing the exfoliation and the exposure of a new surface, hence the SEI formation occurs only on the initial surface of electrode materials, but not the prismatic areas even the interlayer of rGO.38 The advantage of amorphous carbon coating on reducing the formation of SEI film on the surface of rGO is evidenced by CV measurement and the initial CV curve of rGO-g-PF and rGO is depicted in Fig. 9. Both CV curves present similar shapes with redox peaks locating at same potential ranges. As illustrated in Fig. 9, the SEI formation peak of rGO-g-PF that located at

Fig. 9

Initial cyclic voltammetry curves of rGO-g-PF and rGO electrodes.

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Fig. 10 (a) AC impedance spectra of rGO-g-PF and rGO electrodes after different cycles, (b) Randles equivalent circuit for a rGO-g-PF electrode.

0.5–0.9 V is rather flat and less obvious compared with that of rGO. According to the test principle, the capacity of the electrode material corresponds to the integral area under the curve, so the less integral area between 0.5–0.9 V suggests a less lithium consumption of SEI formation and reduced irreversible capacity, obviously the area under rGO-g-PF is far more smaller which confirms the advantage of amorphous carbon-coating on the electrode materials. To further demonstrate the effect of PF coating on the electrode performance, the alternating current impedance (AC) measurements are carried out at the voltage of 2.0 V. As shown in Fig. 10(a), all the Nyquist plots for rGO-g-PF consist of two semicircles at a high and medium frequency range and a tail in the low frequency range. The semicircle at high frequency corresponds to the ohmic resistance of the cell, which mainly contributed from the SEI film and contact resistance (Rf). The semicircle in the middle frequency range is mainly related to the complicated electrochemical reactions occurring at the electrolyte/electrode interface, including the charge-transfer resistance (Rct) and corresponding capacitances. Larger the diameter of semicircle indicates the higher interfacial charge transfer resistance and poor conductivity of the electrodes. The inclined line in the low frequency range is attributed to the Warburg impedance, which is associated with solid-state diffusion of Li ions through the bulk of graphite (Re). It can be seen obviously that the diameters of the semicircles of rGO-g-PF electrodes at both high and medium frequency are smaller than that of the rGO electrodes, revealing lower surface film and charge-transfer impedances of rGO-g-PF electrodes. As reported by Gourdin et al. the SEI film will impact the charge transfer reaction involving in the conversion of Li ions to Li atoms due to the more resistant nature of the SEI film.39 The reduced charge transfer resistance could arise from the decrease of SEI film thickness or a large modification of composition of the SEI film.40 Besides, the diameters of the semicircles of rGO-g-PF decrease with increasing cycle numbers, implying depressed Rf and Rct resistance. This phenomenon could be explained as follows: the nanocavities and defects on the rGO-g-PF surface may result in a large irreversible lithium insertion with the cycle process and the electrical conductivity at the rGO-g-PF electrode is thus increased.24a,41

3258 | Phys. Chem. Chem. Phys., 2015, 17, 3250--3260

Table 2 Kinetic parameters of rGO-g-PF and rGO electrodes at the potential of 2.0 V after different cycles

Cycle

Re (O)

Rf (O)

Rct (O)

Cdl (mF)

Cf (mF)

Zw (O)

rGO-g-PF-1st rGO-g-PF-50th rGO-1st

2.25 2.28 2.31

3.96 3.34 5.89

14.59 10.08 32.19

9.37 1.84 11.4

1.97 2.11 2.80

41.36 38.91 58.58

The EIS is modeled by an equivalent circuit shown in Fig. 10 and the obtained kinetic parameters of rGO-g-PF are summarized in Table 2. The Rct and Rf values for the rGO-g-PF electrode (14.59 O and 3.96 O) are much smaller than that of rGO (32.19 O and 5.89 O) in the first cycle, indicating a low charge-transfer resistance and high electronic conductivity of the rGO-g-PF electrodes. What is more, the value of Rct decreased to a smaller value of 10.08 O from the original 14.59 O after 50 cycles, suggesting that the rGO-g-PF is more favorable for the lithiumion transfer at the electrolyte/electrode interface, which are beneficial for the fast insert/extraction of lithium ions and thus results in an improvement of the rate performance of the rGO-g-PF electrodes. The outstanding electrochemical performance of rGO-g-PF electrode materials could be due to the 2D nanostructure and the compact amorphous carbon coating on the surface, especially on the edge plane. First, the ultrathin 2D nanostructure of rGO-g-PF can provide extra lithium storage location on both sides of the graphene surface and nanocavities formed between the graphene layers, which effectively improve the reversible capacity of rGO-g-PF.5 Second, the amorphous carbon, composed of graphitic layers stacked turbostratically with many lattice defects and stacking faults, could suppress the layer lattice expansion occurring during the co-intercalation of solvated lithium ions, thus preventing the partial exfoliation of graphene layers and exposure of a new surface which lead to a decreased consumption of irreversible lithium ions and SEI film formation.21,36,37 Third, the rGO-g-PF electrode materials present a higher electronic conductivity that arises from the decreased thickness of the SEI film formed on the amorphous carbon surface with a smaller chargetransfer resistance.39,40 Finally, the compact PF coating layer with well-defined mechanical can endow the electrode materials with structural integrity and stability and thus lead to superior lithium storage capacity and cycling stability at high rates.21

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Conclusions Highly structure-stable rGO-g-PF electrode material with enhanced properties was prepared in a facile route by functionalizing graphite oxide with phenolic resin via forming ester linkages under the catalyst effect of EDC and DMAP. The forming of a compact PF coating layer and the subsequent carbonization process, during which the reduction of GO takes place, yielded a highly structurestable reduced graphene oxide grafting phenolic resin carbon (rGO-g-PF) electrode material. Initial changes in surface functionalities confirmed that PF was covalently grafted to the reduced GO sheets, thus forming a highly stable coating layer. When used as an anode material for lithium ion batteries, the rGO-g-PF electrode exhibits impressive high cycling stability at various large current densities (376.5 mA h g1 at 50 mA g1 for 250 cycles, 337.8 mA h g1 at 200 mA g1 and 267.8 mA h g1 at 1 A g1 for 200 cycles), in combination with high rate capability (301.9, 337.9, 362.8 and 400.8 mA h g1 at 0.5 A g1, 0.2 A g1, 0.1 A g1 and 50 mA g1, respectively). The functions of the coating layer were investigated and deduced. The PF coating layer, with an amorphous structure, could effectively suppress the co-intercalation of solvated lithium ions and exfoliation of graphene layers, thus maintaining its structure integrity at high current density. Meanwhile, the conductivity increased due to the decreased SEI film thickness. As a result, rGO-g-PF represented great cycle stability and rather large reversible capacity. These extraordinary electrochemical features make it a promising electrode material for lithium ion batteries.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51202009 and 51272016), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).

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Phenolic resin-grafted reduced graphene oxide as a highly stable anode material for lithium ion batteries.

A novel and effective route for preparing phenol formaldehyde resin grafted reduced graphene oxide (rGO-g-PF) electrode materials with highly enhanced...
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