Layered g-C3N4@Reduced Graphene Oxide Composites as Anodes

Aug 17, 2018 - College of Chemistry, National & Local United Engineering Laboratory ... g-C3N4@RGO composite possesses large interlayer distances, ric...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Layered g‑C3N4@Reduced Graphene Oxide Composites as Anodes with Improved Rate Performance for Lithium-Ion Batteries Shuguang Wang, Yanhong Shi, Chaoying Fan, Jinhua Liu, Yanfei Li, Xing-Long Wu, Haiming Xie, Jingping Zhang, and Haizhu Sun* College of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, No. 5268 Renmin Street, Changchun 130024, China ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/08/18. For personal use only.

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ABSTRACT: As important anodes in lithium-ion batteries, graphene is always faced with the aggregation problem that makes most of the active sites lose their function at high current densities, resulting in low Li-ion intercalation capacity and poor rate performance. To address this issue, a layered g-C3N4@ reduced graphene oxide composite (g-C3N4@RGO) was prepared via a scalable and easy strategy. The resultant g-C3N4@RGO composite possesses large interlayer distances, rich N-active sites, and a microporous structure, which largely improves Li storage performance. It shows excellent cycle stability (899.3 mA h g−1 after 350 cycles under 500 mA g−1) and remarkable rate performance (595.1 mA h g−1 after 1000 cycles under 1000 mA g−1). Moreover, the g-C3N4@RGO electrode exhibits desired capacity retention and relatively high initial Coulombic efficiency of 58.8%. Impressively, this result is better than that of RGO (29.1%) and most of RGO-based anode materials reported in the literature. Especially, the g-C3N4@RGO-based electrode is enough to power two tandem red-light-emitting diodes and run a digital watch. Interestingly, the electronic watch can work continuously for more than 20 days. This novel strategy shows the great potential of g-C3N4@RGO composites as energy-storage materials. KEYWORDS: graphene, urea, g-C3N4@RGO, layered structure, rate capability, anode because of their similar layered architecture.12,13 Highly exposed N-active sites on conductive graphene sheets bring such materials with strong synergistic effects between graphene and g-C3N4, thus resulting in improved electrochemical properties. Chen et al. presented g-C3N4-functionalized graphene that exhibited enhanced electrochemical capacitance for supercapacitors (264 F g−1 at 0.4 A g−1).14 Song and coworkers designed porous g-C3N4/graphene nanosheets embedded with Fe2O3 nanoparticles, which showed improved electrochemical behavior for LIBs.15 Graphene modified by gC3N4 will possess abundant N-active sites and holes. Especially, the inner N-defects will be fully used to guarantee reversible Li storage.12,16−19 However, these active sites will lose their function once graphene aggregation occurs. Therefore, it might be an effective way to design a stable twodimensional (2D) structure with rich active sites through the structural excogitation of g-C3N4/graphene-based materials. In this study, we demonstrated a cost-effective way to fabricate layered g-C3N4@reduced graphene oxide composites (g-C3N4@RGO) with micro- and mesopores on their surface.

1. INTRODUCTION Lithium-ion batteries (LIBs) have received extensive interest owing to their broad applications in stationary grids, hybrid electric vehicles, aerospace, and so forth.1 Developing a new electrode material is of importance to satisfy the increasing demand for LIBs. Among various electrode materials, graphene, with a single atomic plane of graphite, has aroused great interest because of its large surface area, high carrier mobility, and excellent electrochemical stability.2−7 However, graphene sheets always tend to aggregate resulting from the strong interlayer van der Waals attractions and high surface energy, which drastically reduces their active sites and slows down the diffusion of lithium ions.8 Consequently, the fabricated LIBs always suffer from a poor reversible and rate capacity. To address these issues, several strategies have been proposed to prevent accumulation of graphene, such as chemical or physical activation, template etching, heteroatomic doping, and so forth.9−11 However, the complicated and tedious synthetic process always leads to a low production yield and high cost. Therefore, developing a simple route to prepare superior graphene materials, with an ideal structure to be used in LIBs, is highly desirable. It is known that graphene can be modified using graphitic carbon nitride (g-C3N4) © XXXX American Chemical Society

Received: June 4, 2018 Accepted: August 17, 2018 Published: August 17, 2018 A

DOI: 10.1021/acsami.8b09219 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces The introduction of g-C3N4 works as a “protective layer” to effectively prevent accumulation of graphene and generates abundant N-active sites for Li+ storage, thus obtaining a high reversible capability. Further, the porous structure provides continuous and interconnected channels to effectively shorten the diffusion distance of lithium ions and electrons. Therefore, the g-C3N4@RGO electrode exhibits improved electrochemical behavior, including higher reversible capacity and better rate performance compared with the RGO anode. Figure 1. Synthesis procedure of g-C3N4@RGO.

2. EXPERIMENTAL SECTION

capacity.21−24 Moreover, Li+ will be easily adsorbed by Nactive sites in g-C3N4, where each N atom alone will attract two Li+ ions to largely improve the storage capacity of LIBs.19 Last, KOH was used to produce pores on these separated RGO sheets. Thus, g-C3N4@RGO shows a 2D structure with wide cracks and pores, which is favored for electrolyte infiltration and rapid Li+ diffusion. As presented in Figure 2a, the obtained g-C3N4@RGO-550 shows a layered structure with very wafery and deep cracks on

2.1. Preparation of Layered g-C3N4@RGO. Graphene oxide (GO) was prepared as previously reported.20 First, 380 mg of GO was immersed in 38 mL of H2O. After complete infiltration, the mixture was added to a Teflon-lined autoclave and kept at 200 °C for 5 h. RGO hydrogel with a black color was obtained. Second, the obtained hydrogel was immersed in 25 mL of saturated urea solution for 12 h to guarantee that urea molecules were completely adsorbed and then dried at 60 °C. Subsequently, the dried products were heated at 450, 550, or 650 °C for 3 h under a N2 atmosphere. The samples denoted as g-C3N4@RGO-X (X = 450, 550, or 650) were obtained. Finally, gC3N4@RGO was blended with KOH (mKOH/mg‑C3N4@RGO = 2:1) and heated to 800 °C for 2 h under a N2 flow. The powder was washed with HCl and H2O until pH = 7.0 and dried at 60 °C. RGO (denoted as RGO-550) was also prepared through the same process except for the adsorption of urea. 2.2. Structural and Morphological Characterization. The morphology and structure of the obtained materials were observed by scanning electron microscopy (SEM, XL 30 ESEM-FEG, FEI Company) and transmission electron microscopy (TEM, JEM2010F). X-ray diffractometer (XRD) patterns and Raman spectra were recorded on a Rigaku SmartLab X-ray diffractometer and a JY HR-800 LabRam confocal Raman microscope, respectively. X-ray photoelectron spectra (XPS) were analyzed by a VG ESCALAB MKII spectrometer, employing Mg Kα excitation (1253.6 eV). Nitrogen adsorption−desorption isotherms for g-C3N4@RGO-550 were carried out via a micromeritics ASAP 2020 system. 2.3. Electrochemical Performance. The working anodes were fabricated by mixing the active materials (80%) with acetylene black (10%) and polyvinylidene fluoride (10%) in N-methyl-2-pyrrolidone, pasted on a Cu foil, and then dried at 60 °C. The mass loading of the active material was about 1.3 mg cm−2. 2032-type coin cells were fabricated in a glovebox filled with Ar. LiPF6 (1 M) in a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1 was used as the electrolyte. The discharge/charge performances of all electrode materials were carried out on a multichannel battery testing system (LAND CT2001A). The cyclic voltammetry (CV) tests as well as electrochemical impedance spectroscopy (EIS) measurements were recorded on an electrochemical workstation (CHI 750 E). CV tests were performed in a potential window of 0.01−3.0 V (vs Li+/Li) with a scanning rate of 0.1 mV s−1.

Figure 2. SEM images of (a) g-C3N4@RGO-550 and (b) RGO-550. (c) Photos of RGO hydrogel and g-C3N4@RGO hydrogel after adsorption of urea. (d) Elemental mapping of g-C3N4@RGO-550.

their surface. When RGO was immersed into the solution containing urea, the urea molecules were easily diffused into the RGO gel, which is a key way for the formation of the interdigitated structure. During the annealing process, abundant gases such as NH3 and CO2 were generated that opened the space between graphene sheets and effectively limited them from stacking together.19 Simultaneously, g-C3N4 was formed and assembled with graphene. Especially, most of the inner defects of graphene can be fully employed to obtain a high capacity, especially at very high current densities. As shown in Figure S1, the increase of the calcination temperature from 450 to 550 °C facilitates the carbonizing degree and enables to get a better papery configuration. To prove the function of urea molecules, RGO-550 was also prepared via the same process without the introduction of urea. As shown in Figure 2b, RGO-550 shows obvious aggregation and forms considerable thick graphene sheets, which may severely prevent electrolyte ion transportation. The interdigitated structure of g-C3N4@RGO will effectively prevent the aggregation of graphene and lead to the expansion of the composite volume. As shown in Figure 2c, the size of g-C3N4@ RGO is much larger than that of RGO. Figure 2d shows the

3. RESULTS AND DISCUSSION 3.1. Morphology and Composition of Layered gC3N4@RGO Composites. As shown in Figure 1, the RGO hydrogel worked as both the carbon source and the supporting framework. Then, the RGO hydrogel was well-immersed into 25 mL of supersaturated urea aqueous solution to ensure the complete adsorption of urea molecules. After drying under vacuum, the RGO hydrogel was subjected to annealing treatment in which urea molecules would decompose to generate g-C3N4 in situ and simultaneously lead to the formation of g-C3N4@RGO composites with an interdigitated structure. This unique structure will limit the aggregation of graphene, shorten the lithium-ion transport pathway, and provide more effective N-active sites, resulting in a high rate B

DOI: 10.1021/acsami.8b09219 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces elemental maps of g-C3N4@RGO-550, which reveal that nitrogen heteroatoms are homogenously distributed in gC3N4@RGO-550. RGO-550 presents a multilayered stacking structure with obvious agglomeration owing to interactions of van der Waals (Figure 3a). High-resolution transmission electron microscopy

Figure 3. TEM and HRTEM images of RGO-550 (a,b) and g-C3N4@ RGO-550 (c,d). The inset of (c) shows the HRTEM image (d) taken from the area indicated by the red square.

Figure 4. (a) XRD patterns and (b) Raman spectra of RGO-550 and g-C3N4@RGO composites. (c) XPS spectra of RGO-550 and gC3N4@RGO-550. (d,e) High-resolution XPS of C 1s and N 1s for gC3N4@RGO-550. (f) Nitrogen adsorption−desorption isotherm. The inset is the corresponding pore size distribution profiles of g-C3N4@ RGO-550.

(HRTEM) image shows the distinct lattices with a spacing of 0.34 nm (Figure 3b) belonging to the (002) facet of graphitic carbon.25 By contrast, however, g-C3N4@RGO-550 in Figure 3c presents a typical voilelike structure with thinner and more transparent layers of graphene, indicating that it is made of single or only few layers of graphene. The dark zonal area with a bandlike structure is g-C3N4, which remains in tight contact with RGO. These results are further confirmed by HRTEM. Figure 3d shows clear lattice fringes of 0.34 nm, which is matched well with the (002) facet of graphitic carbon. In addition, there is a new interplanar spacing of 0.32 nm corresponding to the (002) crystal plane of g-C3N4 because of the interlayer stacking reflection of conjugated aromatic systems.26 It can be more clearly observed that there is an interface (the red dotted line) between RGO and g-C3N4, showing that RGO and g-C3N4 are kept in tight contact with each other. Both RGO-550 and g-C3N4@RGO-450 show a wide diffraction peak at around 25.2° (Figure 4a). This peak belongs to the graphite like (002) lattice planes that are derived from the diffraction of the laminated structure of graphite. Besides, a weak peak at 43° is also observed, which is assigned to the (100) planes of graphitic carbon, indicating the amorphous features of the samples.27 The XRD pattern of gC3N4@RGO-550, however, has a new sharp and strong peak located at 27.3°, ascribed to the (002) peak of g-C3N4.14,26,28,29 The results are consistent with HRTEM images shown in Figure 3d. All results of SEM, TEM, and XRD confirm that the introduction of urea does not destroy the lattice of graphene, and g-C3N4 is successfully generated, which is important for the formation of layered g-C3N4@RGO. Figure 4b shows that the three samples present two characteristic peaks. The peak at 1340 cm−1 belongs to the

D-band, whereas that at 1590 cm−1 is attributed to the Gband.30,31 The ratio of ID/IG for g-C3N4@RGO-450 is 1.48 and that for g-C3N4@RGO-550 is 1.32. These results imply that the graphitization degree has significantly improved with increasing activation temperature. However, the ID/IG ratio of g-C3N4@RGO-550 is slightly higher than that of RGO-550 (1.2), indicating more defects and disordered phases in gC3N4@RGO-550. Although the formation of g-C3N4 results in the decrease of the graphitization degree, it will enhance the surface wettability as well as provide extra defects to enhance Li-ion storage.32 The comprehensive results confirm its positive influence on the resultant materials. The XPS survey spectrum shows a pronounced C 1s peak at about 285 eV for both the samples, along with a weaker O 1s peak at 533 eV (Figure 4c). However, the peak at 400 eV is assigned to N 1s in g-C3N4@RGO-550, indicating that N is successfully doped into the material. High-resolution XPS C 1s spectra can be split into four peaks at 284.6, 285.4, 286.6, and 288.9 eV, which are assigned to C−N, C−C, C−O, and CO groups, respectively (Figure 4d).33 The O 1s spectrum is divided into two peaks at 532.3 and 533.5 eV, corresponding to C−O−H and CO groups, respectively (Figure S2).33 The high-resolution XPS N 1s spectrum for g-C3N4@RGO-550 is fitted to three peaks, 398.2, 399.8, and 401.3 eV, attributed to the pyridinic nitrogen, pyrrole nitrogen, and graphite nitrogen, respectively (Figure 4e).26,34,35 The doping level of nitrogen is 1.72% in g-C3N4@RGO-550. Elemental analysis was further used to measure the atomic compositions of g-C3N4@RGOC

DOI: 10.1021/acsami.8b09219 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

following scans, which is due to the electrolyte passivation in subsequent cycles, and form a reversible and stable state. gC3N4@RGO-450 and RGO-550 show similar CV curves, indicating the similar electrochemical reaction with g-C3N4@ RGO-550 (Figure S3a,b). The g-C3N4@RGO-550 anode shows an initial discharge and charge capacity of 1476.1 and 867.4 mA h g−1, respectively. The irreversible capacity loss mainly results from the irreversible generation of Li2O and SEI layer. Especially, g-C3N4@RGO-550 shows a relatively higher initial Coulombic efficiency (CE, 58.8%) than g-C3N4@RGO450 (51%) and RGO-550 (29.1%) electrodes. The enhanced CE is attributed to its unique layered structure, which will enable uniform formation of the SEI film and limit the unfavorable side reactions of g-C3N4@RGO-550 with the electrolyte. The discharge curve shows a platform at 0.7−0.9 V, further confirming the SEI layer formation (Figure 5b). g-C3N4@RGO-550 electrode shows excellent electrode reversibility and delivers a high capacity of 949.4 mA h g−1 after 50 cycles at 100 mA g−1 (Figure 5c), which is as much as 3 times than that of pure RGO-550 (315.3 mA h g−1). The desired capacity results from extra charges that is stored in the N-sites provided by g-C3N4.19 By contrast, g-C3N4@RGO-450 delivered a lower reversible capacity (745.4 mA h g−1) than gC3N4@RGO-550 (Figure S4). The lower capacity can be attributed to its indistinct hierarchical structure and poor electrical conductivity. By simulating the Nyquist plots (Figure S5), we find that the ohmic resistance (Rs) and the chargetransfer resistance (Rct) of g-C3N4@RGO-550 (Rs = 2.63 Ω, Rct = 44.68 Ω) are much lower than those of g-C3N4@RGO450 (Rs = 3.42 Ω, Rct = 95.41 Ω). This is because the electrical conductivity of samples is enhanced with the increase of temperature. Practical application of g-C3N4@RGO-550 was demonstrated by connecting two as-fabricated coin-type halfbatteries in series and using it to power two tandem red-lightemitting diodes (LEDs) and an electronic watch (Figure 5d). Impressively, the electronic watch can continuously work for more than 20 days (Video S1). A sample made of g-C3N4 only was investigated to better illustrate the high capability of g-C3N4@RGO composites. The first discharge and charge specific capacities of g-C3N4 are 111.5 and 34.2 at 100 mA g−1 (Figure S6a). It is clear to see that the g-C3N4-based electrode exhibits a much lower reversible capacity than g-C3N4@RGO. This is ascribed to the irreversible aggregation of pure g-C3N4 to form a stacking morphology (Figure S6b). As a result, the surface area is largely reduced (6.0633 m2 g−1, Table S2), which always leads to the disappearance of channels and the loss of active sites for Li absorption. In addition, g-C3N4 shows very poor electrochemical conductivity. Rct of g-C3N4@RGO-550 (Rct = 44.68 Ω) was much smaller compared with that of g-C3N4 (Rct = 198.6 Ω), showing that the conductivity of composites is improved because of the combination with graphene (Figure S6c). However, the g-C3N4-based electrode revealed a very good retention capability at a different current density (Figure S6d). Hence, g-C3N4 is considered as a suitable material for improving the rate performance of graphene-based materials. Although there are some shortcomings for unmodified g-C3N4, the combination of g-C3N4 with graphene makes g-C3N4@ RGO composites possess the desired electrochemical properties. Graphene could enhance the electrical conductivity while g-C3N4 brings large number of N-sites to improve the storage of Li+. Therefore, g-C3N4@RGO composites with a layered structure exhibit the desired reversible and rate capacity.

550 (Table S1). g-C3N4@RGO-550 is composed of C (58.223 wt %), H (0.066 wt %), O (38.067 wt %), and N (3.644 wt %). As seen in Figure 4f, g-C3N4@RGO-550 exhibits type IV isotherm with an obvious H3-type hysteresis loop, suggesting that g-C3N4@RGO-550 possesses substantial micropores and mesopores.18 The pore size distribution (inset) was simulated using Barrett−Joyner−Halenda theory and displays a sharp peak at 3.8 nm, suggesting a narrow mesopore size distributed in g-C3N4@RGO-550. Besides, the average pore diameter is about 4.3 nm, implying relatively regular mesopores. Although chemical activation usually results in micropores on the surface of the samples, the large amount of mesopores can be attributed to more gas being released because of the decomposition of urea during the thermal treatment. This mesopore structure will shorten the diffusion path of the electrolyte and be beneficial to the rate performance of LIBs. In addition, g-C3N4@RGO-550 possesses a specific surface area (SSA) of 215 m2 g−1 and a pore volume of 0.199 cm3 g−1 (Table S2). The large SSA will provide more reaction sites for Li absorption. 3.2. Electrochemical Properties of Layered g-C3N4@ RGO Composites. The electrochemical property of g-C3N4@ RGO-550 was first studied by CV. g-C3N4@RGO-550 shows distinct peaks centered at approximately 0.8 and 1.2 V in the first cathodic process, which can be related to the generation of a solid electrolyte interphase (SEI) film accompanied by the decomposition of the electrolyte and the insertion of Li+ into g-C3N4@RGO-550 (Figure 5a).8 In the first round anodic scan, a peak located at 1.2 V can be associated with Li-ion extraction from N-sites. The CV curves nearly overlap in the

Figure 5. (a) CV curves of g-C3N4@RGO-550. (b) Discharge and charge curves of g-C3N4@RGO-550 at 100 mA g−1. (c) Comparison of cycling properties of g-C3N4@RGO-550 with RGO-550. (d) Digital photograph showing that a coin-type half-battery (g-C3N4@ RGO-550) was used to power an electronic watch and two tandem LEDs. (e) Long cycling capability of RGO-550 and g-C3N4@RGO550 electrodes at 1000 mA g−1. D

DOI: 10.1021/acsami.8b09219 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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mA h g−1 at 50, 100, 200, 400, 600, and 800 mA g−1, respectively (Figure S9). Moreover, the capacity recovers to 910 mA h g−1 when it is switched back to 50 mA g−1, proving the structural stability and good electrochemical reversibility of g-C3N4@RGO-550 within a wide voltage range. In comparison, the corresponding capacities of the RGO-550 electrode are only 371.4, 356.2, 279.8, 234.2, 204.1, and 193.7 mA h g−1. Note that g-C3N4@RGO-550 still maintains a capacity as high as 666 mA h g−1 at 800 mA g−1, better than commercial graphite (372 mA h g−1). This enhanced rate capability can be ascribed to its designed hierarchical and porous structural merits, which provide favorable ion diffusion channels and fast transport pathways within the whole electrode, thus resulting in excellent rate capability.23 The electrochemical properties in this work are compared with those reported in the literature [based on porous graphene sheets and carbon nanosheets (CNSs)]. As depicted in Figure 6b, the g-C3N4@RGO-550 electrode shows excellent electrochemical performance. Especially under a high current density, the capacity is almost 2 times higher than other 2D carbon materials such as vertical CNSs, porous CNSs, mesoporous CNSs, and hierarchical porous CNSs.8,22,34,38−42 To shed light on the electrochemical behavior of the gC3N4@RGO-550 electrode, impedance measurements were performed on the g-C3N4@RGO-550 electrode, as-prepared and after cycling. The impedance data are analyzed using ZView software, and the equivalent circuit model is illustrated in Figure 6c. The typical Nyquist plots of g-C3N4@RGO-550 anodes consist of a semicircle in the high-frequency region and an inclined line in low-frequency region, which represent the interfacial impedances, such as the impedance of SEI films (Rs) and charge-transfer impedance (Rct), and Warburg-type diffusion impedance (W), respectively.16,43 It is observed that the two EIS curves are in similar shapes, but Rs and Rct of gC3N4@RGO-550 (Rs = 2.15 Ω, Rct = 22.33 Ω) after cycling are obviously lower than those before cycling (Rs = 2.63 Ω, Rct = 44.68 Ω). The decreased charge/discharge resistance illustrates better contact between the electrolyte and active materials and hence increases the contact zone of the electrolyte as well as fast electron transfer and finally leads to a small internal resistance and good electrode kinetics.16 The relationship between the real resistance (Zre) and the square root of frequency (ω−1/2) is presented in Figure 6d. The σ (Warburg impedance coefficient) value of g-C3N4@RGO-550 after cycling (102.3 Ω rad1/2 s−1/2) is much lower than that before cycling (142.9 Ω rad1/2 s−1/2), suggesting outstanding kinetics and enhanced ion diffusion for g-C3N4@RGO-550 after cycling.43 Its σ value reduces with cycle numbers, leading to a good rate capability for Li+ storage. To sum up, the unique high rate capability and the high reversible capacity are attributed to its novel hierarchical structures and rich N-active sites (Figure 6e). First, the layered 2D structure provides favorable ion diffusion channels to enhance the permeation of the electrolyte, benefiting the fast charging performance. Second, the rich pores can provide shortcuts for quick electron and ion transfer to realize the high rate capacity. Finally, the rich N-active sites offer abundant reaction sites for Li storage to enhance the specific capacity. Li+ will first occupy the defects and then adsorb on the remaining nitrogen sites, thus increasing more active sites for Li+ adsorption. Benefiting from the combined effects of these structures, the g-C3N4@RGO-550 electrode shows improved electrochemical performance, especially a good rate capacity.

g-C3N4@RGO-550 electrode presents a stable cycle performance under a high current density. Its capacity increases gradually and remains as high as 899.3 mA h g−1 even after cycling 350 times at 500 mA g−1 (Figure S7), which is better than those of g-C3N4@RGO-450 (716.3 mA h g−1) and RGO-550 (239.2 mA h g−1). For comparison, we also evaluated the electrochemical properties of g-C3N4@RGO-650 at 500 mA g−1. Figure S8a shows that a low capacity of 483.7 mA h g−1 is obtained after 350 cycles. The poor electrochemical behavior mainly resulted from its obvious aggregation and considerable thick graphene sheets (Figure S8b), which may severely prevent the transportation of electrolyte ions. In addition, g-C3N4@RGO-550 still delivered a high and stable capacity of 595.1 mA h g−1 even after 1000 cycles at 1000 mA g−1 (Figure 5e), showing its good rate performance. This is mainly ascribed to the hierarchical 2D porous structure of gC3N4@RGO-550 that can effectively decrease the Li+/electron diffusion distance and ensure full contact between active materials and the electrolyte.36,37 Besides, the presence of gC3N4 also enhances the capacity by providing more lithiumstorage sites. The good electrochemical stability and a high degree of reversibility of g-C3N4@RGO-550 demonstrate its potential as an advanced electrode for LIBs. The most important advantage of g-C3N4@RGO-550 as an electrode material for LIBs is its superior rate capability. The gC3N4@RGO-550 electrode exhibits a much better rate capability than g-C3N4@RGO-450 and RGO-550 with the charging/discharging currents increasing from 50 to 800 mA g−1 (Figure 6a). The specific capacities of the g-C3N4@RGO550 electrode are 911.5, 920.8, 858.3, 773.4, 709.2, and 666

Figure 6. (a) Rate capability of RGO-550, g-C3N4@RGO-450, and gC3N4@RGO-550 electrodes. (b) Comparison of specific capacities of the g-C3N4@RGO-550 electrode to reported GO electrodes. (c) Nyquist plots for g-C3N4@RGO-550: as-prepared and after the 50th cycle; inset: the equivalent circuit model. (d) Corresponding relationship between Zre and ω−1/2 (where ω = 2πf) of as-prepared and cycled g-C3N4@RGO-550. (e) Schematic illustration of Li storage in g-C3N4@RGO-550. E

DOI: 10.1021/acsami.8b09219 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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4. CONCLUSIONS In this work, the layered g-C3N4@RGO composites were designed and synthesized via a green, easy, and handy approach. The prepared g-C3N4@RGO-550 effectively prevents graphene restacking and simultaneously generates abundant N-active sites. With the unique design, g-C3N4@ RGO-550 exhibits exceptional performance including high reversible capacity, desired rate capability, and ideal cycling life. Moreover, it provides a new tactic for the structural design of modified graphene-based electrodes for LIBs.

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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/acsami.8b09219. SEM image of g-C3N4@RGO-450, O 1s XPS spectra of g-C3N4@RGO-550, elemental analysis of g-C3N4@ RGO-550, summary of nitrogen adsorption isotherm results for g-C3N4@RGO-550, CV plots and charge/ discharge curves at the first three cycles of g-C3N4@ RGO-450 and RGO-550, cycling performance and corresponding CE for g-C3N4@RGO-450 at 100 mA g−1, electrochemical impedance plots for g-C3N4@RGO450 and g-C3N4@RGO-550, cycling performance for gC3N4 at 100 mA g−1, SEM image of g-C3N4, electrochemical impedance plots for g-C3N4 and g-C3N4@ RGO-550, rate performance of g-C3N4, long cycle performance of RGO-550 and g-C3N4@RGO electrodes at 500 mA g−1, cycling performance for g-C3N4@RGO650 at 500 mA g−1, SEM image of g-C3N4@RGO-650, galvanostatic charge/discharge profiles of RGO-550 and g-C3N4@RGO electrodes at different current densities (PDF) Continuous working of the electronic watch for more than 20 days (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-431-85099667. ORCID

Xing-Long Wu: 0000-0003-1069-9145 Haiming Xie: 0000-0002-7653-4071 Jingping Zhang: 0000-0001-8004-3673 Haizhu Sun: 0000-0002-5113-8267 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21574018 and 51433003), the Jilin Provincial Education Department (543), and the Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University).

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REFERENCES

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DOI: 10.1021/acsami.8b09219 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b09219 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX