ABSTRACT: As important anodes in lithium-ion batteries (LIBs

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Energy, Environmental, and Catalysis Applications

Layered g-C3N4@Reduced Graphene Oxide Composites as Anodes with Improved Rate Performance for Lithium-Ion Batteries Shuguang Wang, Yanhong Shi, Chao-Ying Fan, Jinhua Liu, Yanfei Li, Xing-Long Wu, Haiming Xie, Jingping Zhang, and Haizhu Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09219 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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ACS Applied Materials & Interfaces

ABSTRACT: As important anodes in lithium-ion batteries (LIBs), graphene is always faced with the aggregation problem that makes most of active sites lose their function at high current densities, resulting in low Li-ion intercalation capacity and poor rate performance. To address this issue, layered g-C3N4@reduced graphene oxide composite (g-C3N4@RGO) was prepared via a scalable and easy strategy. The resulted g-C3N4@RGO possesses large interlayer distances, rich N-active sites and 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, g-C3N4@RGO electrode exhibits desired capacity retention and relatively high initial Coulombic efficiency (CE) of 58.8%. Impressively, this result is better than that of RGO (29.1%) and most of RGO based anode materials reported in literatures. Especially, 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 more than 20 days. This novel strategy leads to the great potential of g-C3N4@RGO composites as energy storage materials.

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1. INTRODUCTION

Lithium-ion batteries (LIBs) have received extensive interests owing to their broad applications in stationary grids, hybrid electric vehicles (HEV), and aerospaces, etc.1 Developing 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, etc.2-7 However, graphene sheets always tend to aggregate resulted 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 poor reversible and rate capacity.

To address these issues, several strategies were proposed to prevent accumulation of graphene, such as chemical or physical activation, template etching and heteroatomic doping, etc.9-11 However, the complicated and tedious synthetic process always leads to low production yield and high cost. Therefore, developing a simple route to prepare superior graphene materials with ideal structure used as LIBs is highly desirable. It is known that graphene can be modified using graphitic carbon nitride (g-C3N4) because of their similar layered architecture.12,

13

Highly exposed N-active sites on conductive


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graphene sheets bring such materials with strong synergistic effects between graphene and g-C3N4, thus resulting an 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 co-workers designed porous g-C3N4/graphene nanosheets embedded with Fe2O3 nanoparticles, which showed improved electrochemical behaviors for LIBs.15 Graphene modified by g-C3N4 will possess abundant N-active sites and holes. Especially, inner N-defects will be fully used to guarantee reversible Li storage.16-19,

12

However, these active sites will lose their

function once graphene aggregation occurs. Therefore, it might be an effective way to design a stable two-dimensional (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 mecro-porous on their surface. 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 diffusions distance of lithium ions and electron. Therefore, the g-C3N4@RGO electrode exhibits improved electrochemical behaviors including higher reversible capacity and better rate performance compared with RGO anode.



2. EXPERIMENTAL SECTION 2.1. Preparation of Layered g-C3N4@RGO. GO was prepared as previous reported.20 Firstly, 380 mg of GO was immersed in 38 mL H2O. After complete infiltration, the mixture was added to Teflon-lined autoclave and kept at 200°C for 5 h. RGO hydrogel with color of black was obtained. Secondly, the obtained hydrogel was immersed in 25 mL of saturated urea solution for 12 h to guarantee that urea molecular were completely adsorbed, and then dried at 60 °C. Subsequently, the dried products were heated at 450°C, 550°C or 650°C for 3 h under N2 atmosphere. The samples (denoted as g-C3N4@RGO-X, X=450, 550, 650) were obtained. Finally, the g-C3N4@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. The RGO (denoted as RGO-550) was also prepared through the same process except adsorption of urea.

2.2. Structural and Morphological Characterization. The morphology and structure of the obtained materials were observed by scanning electron microscope (SEM, XL 30 ESEM-FEG, FEI Company) and transmission electron microscope (TEM, JEM-2010F). X-ray diffractometer (XRD) and Raman spectra were recorded on a Rigaku SmartLab X-ray diffractometer and JY HR-800 Lab Ramconfocal Raman microscope, respectively. X-ray photoelectron spectra (XPS) was analyzed by a VGESCALAB MKII spectrometer,


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empolying Mg-Kα excitation (1253.6 eV). Nitrogen adsorption-desorption isotherms for g-C3N4@RGO-550 were carried out via a micromeritics ASAP 2020.

2.3. Electrochemical Performance. The working anodes were fabricated by mixing active materials (80%) with acetylene black (10%) and polyvinylidene fluoride (PVDF) (10%), in N-methyl-2-pyrrolidone (NMP), and were pasted on a Cu foils 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 glove box filled with Ar. LiPF6 (1 M) in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with volume ratio of 1:1 was used as the electrolyte. The discharge/charge performances of all electrode materials were carried out on 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 at potential window of 0.01-3.0 V (vs. Li+/Li) with a scanning rate of 0.1 mV s-1.

3. RESULTS AND DISCUSSION

3.1. Morphology and Composition of Layered g-C3N4@RGO Composites. As shown in Figure 1, RGO hydrogel worked as both carbon source and supporting framework. Then, RGO hydrogel was well immersed into 25 mL supersaturated urea aqueous solution to ensure the complete adsorption of urea molecular. After drying under vacuum, 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 interdigitated structure. This unique structure will limit the aggregation of graphene, shorten lithium-ion transport pathway and provide more effective N-active sites, resulting in high rate capacity.21-24 Moreover, Li+ will be easily adsorbed by N-active sites in g-C3N4, where each N atom alone will attract two Li+ 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 a wide cracks and pores, which is favored for the electrolyte infiltration and rapid Li+ diffusion.

Figure 1. Synthesis procedure of g-C3N4@RGO. As presented in Figure 2a, the obtained g-C3N4@RGO-550 shows layered structure with very wafery and deep cracks on 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 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 inner defects of graphene can be fully employed to obtain a high capacity especially at very high current


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densities. As shown in Figure S1, the increase of calcination temperature from 450°C to 550°C facilitates the carbonizing degree and then to get a better papery configuration. To prove the function of urea molecules, RGO-550 was also prepared via the same process without introduction of urea. As shown in Figure 2b, RGO-550 shows obvious aggregation and forms considerable thick graphene sheets, which may severely prevent electrolyte ions transportation. The interdigitated structure of g-C3N4@RGO will effectively prevent the aggregation of the graphene and lead to expand of composite volume. As shown in Figure 2c, the photos obviously show that the size of g-C3N4@RGO is much larger than that of RGO. Figure 2d shows the elemental maps of the g-C3N4@RGO-550, which reveal that nitrogen heteroatoms are homogenously distributed in g-C3N4@RGO-550.

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. RGO-550 presents multilayered stacking structure with obvious agglomeration



owing to interactions of van der Waals (Figure 3a). HRTEM image shows the distinct lattices with a spacing of 0.34 nm (Figure 3b), belonging to the (002) facet of graphitic carbon.25 In contrast, however, g-C3N4@RGO-550 in Figure 3c present typical voile-like 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 band-like structure is g-C3N4, which remains in tight contact with RGO. These results are further confirmed by HRTEM. Figure 3d shows a clear lattice fringes of 0.34 nm, which is matched well with (002) facet of graphitic carbon. In addition, there is a new interplanar spacing of 0.32 nm appeared, corresponding to (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 tight contact with each other.

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


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Both RGO-550 and g-C3N4@RGO-450 show a wide diffraction peak at around 25.2o (Figure 4a). This peak belongs to graphite-like (002) lattice planes that derives from the diffraction of the laminated structure of graphite. Besides, a weak peak at 43o is also observed, which is assigned to the (100) planes of graphitic carbon, indicating amorphous features for samples.27 The XRD pattern of g-C3N4@RGO-550, however, has a new sharp and strong peak located at 27.3o, ascribed to the (002) peak of g-C3N4.28, 14, 29, 26 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 three samples present two characteristic peaks. The peak at 1340 cm-1 belongs to the D-band while that at 1590 cm-1 attributes to G-band, respectively.

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. This results imply that graphitization degree has been significantly improved with increasing activation temperature. However, the ID/IG of g-C3N4@RGO-550 is slightly higher than that of RGO-550 (1.2), indicating more defects and disordered phases in g-C3N4@RGO-550. Although the formation of g-C3N4 results in the decrease of graphitization degree, it will enhance the surface wettability as well as provide extra defects to enhance the Li-ion storage.32 The comprehensive results confirm its positive influence on the resulted materials.



Figure 4. (a) XRD patterns and (b) Raman spectra of RGO-550, g-C3N4@RGO composites. (c) XPS spectra of RGO-550 and g-C3N4@RGO-550. (d), (e) High-resolution XPS of C 1s and N 1s for g-C3N4@RGO-550. (f) Nitrogen adsorption-desorption isotherm. Inset is the corresponding pore size distribution profiles of g-C3N4@RGO-550. XPS survey spectrum shows a pronounced C1s peak at about 285 eV for both the samples, along with a weaker O1s peak at 533 eV (Figure 4c). However, the peak at 400 eV is assigned to N1s in g-C3N4@RGO-550, indicating N is successfully doped into the material. High-resolution XPS C1s spectra can be split into four peaks at 284.6, 285.4, 286.6 and 288.9 eV, which are assigned to the C-N, C-C, C-O and C=O, respectively


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(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 N1s spectrum for g-C3N4@RGO-550 is fitted to three peaks, 398.2, 399.8 and 401.3 eV, attributing to the pyridinic nitrogen, pyrrole nitrogen and graphite nitrogen, respectively (Figure 4e).34,

35,

26

The doping level of nitrogen is 1.72% in

g-C3N4@RGO-550. Elemental analysis was further used to measure the atomic compositions of the g-C3N4@RGO-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, the g-C3N4@RGO-550 exhibits type IV isotherm with an obvious H3-type hysteresis loop, suggesting that the g-C3N4@RGO-550 possesses substantial micropores and mesopores.18 The pore size distribution (inset) was simulated using Barrett-Joyner-Halenda (BJH) and displays 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 the micropores on the surface of the samples, the large amount of mesopores can be attributed to more gas being released due to the decomposition of the urea during the thermal treatment. This mesopore structure will shorten the diffusion path of 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 pore volume of 0.199 cm3 g-1 (Table S2). The large SSA will provide more reaction sites for Li-absorption.



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3.2. Electrochemical properties of Layered g-C3N4@RGO Composites. The electrochemical property of g-C3N4@RGO-550 was firstly studied by CV. G-C3N4@RGO-550 shows distinct peaks centered at ca. 0.8 and 1.2 V in the 1st cathodic process, which can be related to the generation of a solid electrolyte interphase (SEI) film accompanied by the decomposition of 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 following scans, which is due to the electrolyte passivation in subsequent cycles and forms a reversible and stable state. The g-C3N4@RGO-450 and RGO-550 show the similar

CV

curves,

indicating

the

similar

electrochemical

reaction

with

g-C3N4@RGO-550 (Figure S3a, S3b). The g-C3N4@RGO-550 anode shows initial discharge and charge capacity of 1476.1 and 867.4 mA h g-1, respectively. The irreversible capacity loss mainly results from irreversible generation of Li2O and SEI layer. Especially, g-C3N4@RGO-550 shows relatively higher initial Coulombic efficiency (CE, 58.8%) than that of g-C3N4@RGO-450 (51%) and RGO-550 (29.1%) electrodes. The enhanced CE is attributed to its unique layered structure, which will enable uniform formation of SEI film and limits unfavorable side reactions of g-C3N4@RGO-550 with 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 an 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


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much as three 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 In contrast, g-C3N4@RGO-450 delivered a lower reversible capacity (745.4 mA h g-1) than g-C3N4@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 charge transfer resistance (Rct) of g-C3N4@RGO-550 (Rs=2.63 Ω, Rct=44.68 Ω) is much lower than those of g-C3N4@RGO-450 (Rs=3.42 Ω, Rct=95.41 Ω). This is due to the reason that 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 half batteries in series and using it to power two tandem red LEDs and an electronic watch (Figure 5d). Impressively, electronic watch can continuously work more than 20 days (Video S1).

A sample made of g-C3N4-only was investigated to better illustrate high capability of g-C3N4@RGO composites. The first discharge and charge specific capacities of g-C3N4 are 111.5 mA h g-1 and 34.2 mA h g-1 at 100 mA g-1 (Figure S6a). It is clear to see that g-C3N4-based electrode exhibits much lower reversible capacity than g-C3N4@RGO. This is ascribe to the irreversible aggregation of the pure g-C3N4 to form a stacking morphology (Figure S6b). As a result, the surface areas is largely reduced (6.0633 m2 g-1, Table S2), which always leading to the disappearance of channels and the loss of active sites for Li absorbing. In addition, g-C3N4 shows very poor electrochemical conductivity. Rct of g-C3N4@RGO-550 (Rct =44.68 Ω) was much smaller compared with g-C3N4



(Rct=198.6 Ω), showing conductivity of composites is improved due to combination with graphene (Figure S6c). However, g-C3N4-based electrode revealed a very good retention capability at different current density (Figure S6d). Hence, the g-C3N4 is considered as 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 desired electrochemical properties. Graphene could enhance electrical conductivity while g-C3N4 bring large number of N-sites to improve the storage of Li+. Therefore, g-C3N4@RGO composites with layered structure exhibit desired reversible and rate capacity. G-C3N4@RGO-550 electrode presents a stable cycle performance under 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 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 was 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 its hierarchical 2D porous structure of g-C3N4@RGO-550 that can effectively decrease the Li+/electron diffusion distance and ensure a full contact between active materials and electrolyte. 36, 37 Besides,


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the presence of g-C3N4 also enhances the capacity by providing more lithium-storage sites. The good electrochemical stability and a high-degree of reversibility of g-C3N4@RGO-550 demonstrate its promising as advanced electrode for LIBs.

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) The digital photograph shows 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@RGO-550 electrodes at 1000 mA g-1. The most important advantage of g-C3N4@RGO-550 as electrode material for LIBs is its superior rate capability. The g-C3N4@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 g-C3N4@RGO-550 electrode are 911.5, 920.8, 858.3, 773.4, 709.2 and 666 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, respectively. Note that g-C3N4@RGO-550 still remains capacity as high as 666 mA h g-1 at 800 mA g-1, better than that of 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 pathway within the whole electrode, thus resulting 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). As depicted in Figure 6b, g-C3N4@RGO-550 electrode shows more excellent electrochemical performance. Especially under high current density, the capacity is almost two times higher than other 2D carbon materials such as vertically CNSs, porous CNSs, mesoporous CNSs, and hierarchical porous CNSs.8, 22, 34, 38-42 To shed light on electrochemical behavior of the g-C3N4@RGO-550 electrode, impedance measurements were performed on the g-C3N4@RGO-550 electrode as-prepared and after cycled. The impedance data are analyzed using Z-view software and equivalent circuit model is illustrated in Figure 6c. The typical Nyquist plots of


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g-C3N4@RGO-550 anodes consist of a semicircle in high-frequency and an inclined line in low-frequency region, which represents 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 the Rs and Rct of g-C3N4@RGO-550 (Rs=2.15 Ω, Rct =22.33 Ω) after cycling are obviously lower than those of 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 contact zone of electrolyte as well as fast electrons transfer, and finally leads to a small internal resistance and good electrode kinetics.16 The relationship between real resistance (Zre) and 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 of 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 was reducing with cycle numbers, leading to a good rate capability for Li+ storage. To sum up, the unique high-rate capability and high reversible capacity are attributed to its novel hierarchical structures and rich N-active sites (Figure 6e). Firstly, the layered 2D structure provides favorable ion diffusion channels to enhance the permeation of electrolyte, benefiting the fast charging performance. Secondly, the rich pores can provide shortcuts for quick electron and ion transfer to realize high rate capacity. Finally, the rich N-active sites offer abundant reaction sites for Li storage to enhance the specific capacity. Li+ will firstly occupy the defects and then adsorb on remaining nitrogen sites, thus increasing more active sites for Li+ adsorption. Benefiting



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from combined effects of these structures, g-C3N4@RGO-550 electrode shows improved electrochemical performance, especially good rate capacity.

Figure

6.

(a)

Rate

g-C3N4@RGO-550

capability

electrodes.

(b)

of

the

RGO-550,

Comparison

g-C3N4@RGO-450

of specific capacities

and

of the

g-C3N4@RGO-550 electrode to reported GO electrodes. (c) Nyquist plots for the g-C3N4@RGO-550: as-prepared and after 50th cycle, inset: the equivalent circuit model. (d) The 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. 4. CONCLUSIONS In this work, the layered g-C3N4@RGO composites were designed and synthesized


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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 new tactic of structural design of modified graphene-based electrodes for LIBs. ASSOCIATED CONTENT Supporting Information SEM image of g-C3N4@RGO-450, O1s XPS spectra of g-C3N4@RGO-550, elemental analysis of the g-C3N4@RGO-550, summary of nitrogen adsorption isotherm result for the g-C3N4@RGO-550, CV plots and charge/discharge curves at first three cycles of g-C3N4@RGO-450 and RGO-550, cycling performance and corresponding coulombic efficiency for g-C3N4@RGO-450 at 100 mA g-1, electrochemical impedance plots for g-C3N4@RGO-450 and g-C3N4@RGO-550, Cycling performance for the g-C3N4 at 100 mA g-1, SEM image of g-C3N4, electrochemical impedance plots for the 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@RGO-650 at 500 mA g-1, SEM image of g-C3N4@RGO-650, galvanostatic charge-discharge profiles of the RGO-550 and g-C3N4@RGO electrodes at different current densities, Video S1, this material is available free of charge via the internet at http://pubs.acs.org.



AUTHOR INFORMATION Corresponding Author * (H.S.) E-mail: [email protected]. (Haizhu Sun)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

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


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ABSTRACT: As important anodes in lithium-ion batteries (LIBs

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