Superior Cathode Performance of Nitrogen-Doped Graphene

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Superior Cathode Performance of Nitrogen-doped Graphene Frameworks for Lithium Ion Batteries Dongbin Xiong, Xifei Li, Zhimin Bai, Hui Shan, Linlin Fan, Chunxia Wu, Dejun Li, and Shigang Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15872 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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ACS Applied Materials & Interfaces

Superior Cathode Performance of Nitrogen-doped Graphene Frameworks for Lithium Ion Batteries

Dongbin Xionga,b, Xifei Lib,c,d*, Zhimin Baia,*, Hui Shanc, Linlin Fanc, Chunxia Wuc, Dejun Lic, Shigang Lue,* a

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid

Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China. E-mail: [email protected] b

Center for Advanced Energy Materials and Devices, Xi’an University of Technology,

Xi’an 710048, China. E-mail: [email protected] c

Tianjin International Joint Research Centre of Surface Technology for Energy

Storage Materials, College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China. d

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Collaborative Innovation Center of Chemical Science and Engineering, College of Chemistry, Nankai University, Tianjin 300071, China e

R&D Center for Vehicle Battery and Energy Storage, General Research Institute for

Nonferrous Metals, Beijing 100088, PR China. E-mail: [email protected]

Abstract

Development of alternative cathode materials is of highly desirable for sustainable and cost-efficient lithium-ion batteries (LIBs) in energy storage fields. In this study, for the first time, we report tunable nitrogen-doped graphene with active functional 1

ACS Paragon Plus Environment


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groups for cathode utilization of LIBs. When employed as cathode materials, the functionalized graphene frameworks with a nitrogen content of 9.26 at% retain a reversible capacity of 344 mAh g-1 after 200 cycles at a current density of 50 mA g-1. More surprisingly, when conducted at a high current density of 1 A g-1, this cathode delivers a high reversible capacity of 146 mAh g-1 after 1,000 cycles. Our current research demonstrates the effective significance of nitrogen doping on enhancing cathode performance of functionalized graphene for LIBs.

Keywords: Graphene; Oxygenic Functional Groups; Nitrogen-doped; Lithium Ion Batteries; Cathode Materials

1.

Introduction

Currently, lithium-ion batteries (LIBs) have been regarded as one of the most commercioganic

energy

storage/conversion

devices

particularly

for

micro

electro-mechanical devices, electric vehicles and smart grids owing to their high energy density and environmental benignity.1, 2 However, current LIB technique may not well satisfy the growing performance requirements in the aforementioned applications. In general, performance improvement of LIBs is restricted by the cathode materials which invariably show much lower capacities than the anode ones.3, 4

One feasible approach to enhance LIB performance is to employ new classes of

cathode materials that provide high reversible capacity and long cycling life.5,

6

Current inorganic intercalation cathode materials show drawbacks of limited mineral resources, high costs as well as limited reversible capacities.7-10 Hence, developing new cathode materials with high capacity and high stability is one of the most 2


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promising strategies to solve this bottleneck of LIBs. More recently, as reported by our group.11, 12 the graphene has been successfully utilized as a cathode electrode for LIBs based on the surface Faradaic reactions (C=O + Li+ + e- ↔ C-O-Li) of active functional groups in graphene with lithium ions in the voltage window above 1.5 V vs Li/Li+. Some researchers introduced functionalized graphene nano-platelets,13,

14

free-standing reduced graphene oxide (RGO) film,15

hierarchical functionalized multiwalled carbon nanotube (MWNT)/grapheme structures16 to observably obtain higher capacity and better stability than inorganic transition-metal-oxide-based materials.17, 18 Our group reported that the amount of oxygen functional groups highly impacts electrochemical performance of graphene cathode material.11 More importantly, carboxylic functional groups were detected by ex-situ XPS to be the significant active sites to reserve lithium ion via Faradaic reactions.11 Although graphene cathode materials manifest enhanced performance, further improvement of this new cathode in high energy density, high-rate capability and long-term cyclability is required for effective utilization and cost saving. The doping behavior of heteroatoms (e.g., sulfur, nitrogen and boron) into the carbon lattice can significantly enhance the electrochemical properties of carbonaceous host materials,19 because the electronic structure and other intrinsic properties of carbon materials can be effectively altered by molecular and atomic doping. For example, in view of higher electronegativity of nitrogen (3.5) and smaller atomic diameter than carbon (3.0), nitrogen atoms incorporated into carbon lattice effectively facilitate the formation of stronger interactions between the N-doped graphene structure and the Li ions, which are favorable for Li insertion.20,

21

Additionally, N-doped graphene is a good electron-donor. These unique properties make it an attractive material in energy-related areas.22, 23 N-doped graphene has been 3



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wildly utilized as anode materials for LIBs and asymmetric supercapacitors.24-27 Owning to their enhanced electrochemical properties, these modified graphene materials had shown higher specific capacity/capacitance and better cycling stability relative to non-doped graphene materials. As a result, it is possible that the introduction of N-doping could effectively enhance electrochemical performance of graphene as cathode for LIBs. Unfortunately, so far, no research was reported to focus on this important point. In this study, we utilized urea as both nitrogen source and reducing agent to synthesize N-doped graphene nanosheets (NGNS) with considerable residual oxygenic functional groups and tunable nitrogen content via a hydrothermal reaction. The resultant N-doped functionalized graphene shows superior electrochemical performance as cathode in LIBs. Furthermore, the significant effects of N-doping behavior were studied in detail. It is believed that this work can be conducive to the development of this promising new cathode material.

2.

Experimental

2.1 Materials Natural graphite powder (purity≥99.95%) was purchased from Aladdin Chemistry Co., Ltd.. All other chemicals were reagent grade and used received without further purification. Deionized water was used throughout the experiments.

2.2 Synthesis of nitrogen-doped graphene nanosheets (NGNS) Graphite oxide was obtained from natural graphite powder through modified Hummers’ method, as previously reported by our group.12 Then the graphite oxide 4


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was dispersed in deionized water via an ultrasonic to obtain brown graphene oxide (GO) solution (2.0 mg mL-1), followed by centrifugation at 1000 rpm for 30 min to remove some unexfoliated materials. NGNS were synthesized by a general hydrothermal route with GO as original material and urea as reducing-doping agents. In a typical process, the resulting dispersion of GO was divided into three shares with one share of 45 mL, then two of them were mixed with urea in beakers with a mass ratio of 1:20 and 1:300, respectively. After being vigorously stirred for 20 min, the mixtures and another share of GO dispersion without adding urea were sealed in 50 mL Teflon-lined autoclaves, respectively, and maintained at 180 oC for 12 h. Then obtained graphene hydrogel were treated through washing and vacuum filtering, subsequently, the products were freeze-dried under vacuum. The resultant materials were labelled as GNS, NGNS-I and NGNS-II related to the adding of urea with mass ratio of 1:0, 1:20 and 1:300, respectively.

2.3 Materials characterization The microstructures and morphologist of the obtained materials were characterized using scanning emission microscope (SEM, Hitach SU8010) and transmission electron microscope (TEM, JEOL JEM-3000F). The structure was characterized by X-ray diffraction (XRD, Bruker AXS D8Advance) and LabRAM HR800 spectrometer for Raman spectra. X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II) was used to determine the amount of functional groups and the N contents of as-prepared samples. Survey spectra and high-resolution spectra of C1s, O1s and N1s were recorded. All spectra were fitted after a Shirley type background subtraction. 5



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2.4 Electrochemical measurements. Electrochemical properties were characterized using CR2032 coin-type cells, with lithium as the counter and reference electrode. The working electrodes were prepared by coating the mixture of active materials (80 wt.%), acetylene black (10 wt.%) and polyvinylidene fluoride (10 wt.%) dissolved in N-methylpyrrolidone (NMP) onto a Al foil. The mass loading density of the cathode materials is about 0.65 mg cm-2. The electrolyte was 1M LiPF6 in a mixture solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylene methyl carbonate (EMC) in a 1 : 1 : 1 volume ratio. The coin cells were assembled in a dry argon filled glove box. Cyclic voltammetry tests were performed on a Princeton Applied Research VersaSTAT 4 electrochemical workstation at a scan rate of 0.1 mV s-1 at a potential range between 1.5 and 4.5 V (vs. Li/Li+). Galvanostatic discharge–charge cycling was performed on LAND CT2001A test system at room temperature with a potential range of 1.5~4.5 V. The AC impedance spectra were obtained over the frequency ranging from 100 KHz to 0.01 Hz with amplitude of 5.0 mV.

3.

Results and Discussion

The nanostructured morphologies of as-prepared poriferous GNS and NGNS were investigated using SEM and TEM measurements. As displayed in Fig. 1a–c, both GNS and NGNS show a feature of interconnected porous and disordered network with pore walls consisting of thin layers of curly and stacked graphene nanosheets, which is different from GO prior to hydrothermal treatment (see Fig. S1 of ESI). For NGNS-I (Fig. 1b) and NGNS-II (Fig. 1c), the graphene sheets seem 6


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smaller and slighter than that of GNS (Fig. 1a). The novel structure of porous network contributes to the easy access of electrolytes and efficient transfer of lithium ions during the charge/discharge processes.28 Low-magnification TEM images (Fig.1d-f) further exhibit the interconnected porous structure of the GNS and NGNS frameworks composing with the typical graphene sheets, and the NGNS samples reveal relatively smaller hollow compared with GNS. The high-resolution TEM (HRTEM) images (Fig. 1g–i) show that the graphene sheets reveal ultrathin with multilayered structure. The corresponding SAED patterns (see inset of Fig. 1g–i) show the ring-like patterns, indicating the typical feature of hexagonal graphite lattices as reported elsewhere, e.g.(010), (111) and (110).29 The weaker rings of NGNS-I and NGNS-II indicate the disordered structure of the graphene sheets as well as the increasing defects caused by nitrogen doping. 30 Fig. 2a compares the Raman spectra of GNS, NGNS-I and NGNS-II. Two broad peaks of GNS appear at 1602 and 1355 cm-1 are related to the well-defined G band and D band, respectively. Generally, the G band is characteristic of the first order scattering of the E2g mode, and provides the formation of graphitic carbon. The D band corresponds to the mode of the sp2-rings of the disordered graphene layer that is related to various defects (such as hybridization caused by heteroatom doping and structure defects by oxidation/reduction treatment).31 A slight shift (~5 cm-1) of the G band of NGNS samples to higher frequency with respect to that of GNS may be ascribed to the formation of chemical bonds between C and N.32 The intensity ratio of the D band to G band (ID/IG) diagnostically indicates the degree of disorder and defects of carbon materials.32 The ID/IG values of GNS, NGNS-I and NGNS-II are 2.00, 2.18 and 2.51, respectively. An obvious increase of the ID/IG ratio of NGNS-I and GNNS-II visav is GNS indicates that the defects arise due to doping onto the 7



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carbon framework. The XRD patterns of the GO, GNS, NGNS-I and NGNS-II are shown in Fig. S2a. A sharp peak can be observed at 11.8o in GO, indicating the completely oxidation of graphite and formation of abundant oxygenic functional groups on the carbon layer. A weak and board peak at 41o in all samples attributed to the (100) plane of graphite.33 The N-doped samples (NGNS-I and NGNS-II) showed similar diffraction features with a broad peak at approximately 25.2o (smaller than that of GNS at 29.2o), corresponding to the carbon (002) peak that is typical of graphitic carbon materials with a low graphilization degree, which demonstrates that GNS, NGNS-I and NGNS-II were reduced from GO. The smaller 2θ angle of NGNS-I and NGNS-II suggests a larger interlayer spacing, which results from the nitrogen doping.23 X-ray photoelectron spectroscopy (XPS) was performed to investigate chemical composition as well as nitrogen and/or oxygen bonding configurations of the GNS, NGNS-I and NGNS-II. As shown in XPS survey spectra in Fig. 2b, all samples show two remarkable peaks at around 284.5 and 531.5 eV that correspond with C 1s and O 1s spectra, respectively. An additional signal centered at 400.0 eV can be observed for NGNS-I and NGNS-II, which is ascribed to the existence of N 1s. These reveal the formation of oxygen-containing functional groups in all samples, and nitrogen was successfully doped into graphene sheets for NGNS-I and NGNS-II.34 The high-resolution C 1s and O 1s XPS spectra of GNS, NGNS-I and NGNS-II as well as N 1s spectra of NGNS-I and NGNS-II were then collected to gain more insight into oxygenic functional groups and nitrogen doping. As shown in Fig. S2b, the broad C 1s spectra of GNS can be resolved into four peaks with binding energies of 288.7, 288.1, 286.5 and 285.2 eV, corresponding to carboxyl groups (HO-C=O), carbonyl groups (C=O), hydroxyl groups (C-OH) and sp3C–C/sp2 C=C bonds of graphitic 8


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carbon, respectively.35 The C 1s peaks (Fig. S2c and 2d) of NGNS-I and NGNS-II can be fitted with another two components centered at 287.1 and 285.5 eV, which are assigned to sp3 C and sp2 C atoms bonded to N, respectively. The N 1s peak of NGNS-I and NGNS-II can be resolved into three components, representing the graphitic (401.2 eV), pyrrolic (399.8 eV) and pyridinic (398.5 eV) type of N atoms doped in the graphene structure, respectively (Fig.2c).36 In Fig. 2d, the high-resolution spectra of O 1s in all three samples show that there exist three different O components at 531.2, 532.3 and 533.4 eV, corresponding to C=O, C-OH and, HO-C=O, respectively.16 Note that the oxygen percentage of NGNS-I and NGNS-II is lower than that of GNS, that is, the percentage of oxygen for GNS, NGNS-I and NGNS-II is 11.28, 8.41 and 7.37 at%, respectively. The nitrogen atomic percentage of NGNS-I and NGNS-II is 6.82 and 9.26 at%, respectively, indicating that urea successfully functions as both reductant and nitrogen source. The portions of each N-configuration and O-configuration are graphically summarized in Fig. 2e and 2f. One can see that the percentage of graphitic-N remains constant; however, with the increase of the mass ratio of urea to GO, the distribution of the pyrrolic-N obviously increases from 37 at% in NGNS-I to 42 at% in NGNS-II while the pyridinic-N decreases from 45 at% in NGNS-I to 40 at% in NGNS-II. In addition, as shown in Fig. 2f, carboxyl groups is the main component in GNS, while the carboxyl groups of NGNS-I and NGNS-II observably decrease, and both carbonyl groups and hydroxyl groups increase. These trends can be presumably attributed to the tunable dosage of dopant as well as the different kinds of doping at relatively high temperature and high pressure. To explore the electrochemical behavior of NGNS as cathodes for LIBs, the cyclic voltammogram (CV) of GNS, NGNS-I and NGNS-II were compared between 9



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1.5 and 4.5 V (vs. Li/Li+) at a scan rate of 0.1 mV s-1. As show in Fig. 3a, the CV curves in the second cycle of each sample exhibit similar feature with the previous results by our group and others’ reports.11, 37, 38 Notably, the GNS electrodes without N-doping show relative small current density, while increasing doping content of nitrogen in NGNS leads to gradual increases in the gravimetric current density. It is proposed that the introduction of nitrogen atoms in the graphene lattice results in more structure defects combined with oxygen-containing groups, which contribute to the enhanced electrochemical activity of NGNS.20 Fig. 3b displays the charge-discharge profiles of GNS, NGNS-I and NGNS-II electrodes over 1.5 to 4.5 V at 50 mA g-1. All profiles show a similar trend to that observed in the CV curves. The NGNS-I exhibits a substantially higher reversible capacity obtained of approximately 250 mAh g-1, compared to undoped GNS electrode (~195 mAh g-1). More interestingly, NGNS-II with a higher level of N-doping delivers a reversible capacity up to 330 mAh g-1, suggesting that the nitrogen atoms doped in graphene layers play an important role on enhancing the energy storage capability of NGNS as cathode for LIBs. Besides, as observed from Fig. 3b, the charge capacity of each sample is even higher than the discharge capacity, and the coulombic efficiency of the battery is more than 100%. This phenomenon can be due to the previously proposed energy storage mechanism with capacitor-like behavior,8,39,40 that is, the association/disassociation of Li+ (1.5-3 V versus Li/Li+) or the electrolyte anions (PF6-; 3-4.5 V versus Li/Li+) simultaneously occurred with reversible redox reaction. Anions (such as PF6-) can be reversibly adsorbed/desorbed on/from GNS and NGNS materials at high potentials, thereby increasing the specific capacity by storing snions. Usually, carbonaceous materials possess strong anion adsorption, resulting in higher discharge capacity. The cycle performances of the GNS, NGNS-I and NGNS-II electrodes were 10


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evaluated at 50 mA g-1over a range of 1.5~4.5 V, as shown in Fig. 3c. As previously reported by our group, GNS could work stably with cycles, and it delivers relatively low reversible capacity of 202 mAh g-1 in the 200th cycle. Obviously, the doping behavior into GNS significantly enhances the reversible capacity of the cathode materials. The NGNS-II electrode delivers an high reversible capacity of 345 mAh g-1 after 200 cycles, which is obviously much higher than that of undoped GNS and low-level doped GNS-I electrodes with values of 202 and 224 mAh g-1, respectively. The gradual increase of capacities for all three electrodes in the first several cycles can be attributed to the gradual penetration of electrolyte into the porous structure, promoting the activation of electrode materials.8 It is worth noting that the gradually increased capacity of NGNS-II, which is shown in Fig. S3 for the charge-discharge profiles, might owe to the modification of the electronic properties of neighboring carbon atoms by N-doping and/or the proliferation of in-plane defects (eg. carbon vacancies) during the electrochemical cycling.31, 41 The rate capabilities of GNS, NGNS-I and NGNS-II electrodes at different current densities are compared in Fig. 3d. The NGNS-II electrode deliveries average discharge capacities of 315, 327, 290 and 254 mAh g-1 at the current density of 0.05, 0.1, 0.2 and 0.4 A g-1, respectively. These values are higher than those of GNS electrode without N-doping and NGNS-I electrode with low-level N-doping. When the current density was reduced back to 0.05 A g-1, the NGNS-II electrode is able to obtain a reversible capacity of 353 mAh g-1. The rate capacities and retention of these samples at various current densities were summarized in Fig. S4a and 4b. One can see that NGNS-II demonstrated higher capacity retention at various current densities than the NGNS-I and GNS cathodes. These results indicate that GNS-II with high nitrogen-dopingas well as high content of pyridilic nitrogen possesses superior rate 11



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capability in LIBs. To further confirm the important effects of doping behavior on cathode performance, a long-term cycling test has been carried out at a high current density of 1 A g-1 (for the first 3 cycles, a low current density of 0.05 A g-1 was adopted to activate electrodes). As shown in Fig. 3e, both NGNS-I and NGNS-II electrodes show remarkable cycling behavior. During the cycling, the specific capacity of the NGNS-II electrode is always higher than that of the NGNS-I electrode. More excitingly, the NGNS-II electrode can be operated up to 1000 cycles with a reversible capacity of 146 mAh g-1 achieved. Note that nearly 100% Coulombic efficiency of NGNS-II was achieved after several cycles. In addition, NGNS-II shows better performance than the current commercial cathode materials (see Table S1). These results verify that the N-doping behavior profoundly affects functionalized graphene cathode building hextraordinary electrochemical performance. The superior cathode performance of NGNS was further confirmed at an operated voltage range of 2.5-4.5 V. As shown in Fig. S5a, the charge-discharge profiles of GNS, NGNS-I and NGNS-II in the 5th cycle were similar to that of the samples tested at the voltage range of 1.5 to 4.5 V. Importantly, all the samples reveal good cycling performance in Fig. S5b. By contrast, NGNS-II shows better performance than the other two cathode materials, which is similar to the results in Fig. 3c. As expected, the cathode materials deliver lower reversible capacities than that shown in Fig. 3c. However, NGNS-II still delivers high reversible capacity of 147 mAh g-1. As a result, this proposed NGNS materials may display their superiority as cathode for LIBs. Additionally, commercial graphite (CG) was also tested in a potential window (1.5-4.5 V versus Li/Li+). As shown in Fig. S6, CG almost showed no electrochemical activity with specific discharge capacity of less than 3 mAh g-1. 12


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Therefore, the oxygenic functional groups and doped nitrogen atoms combining with the unique 3D porous structure of our proposed NGNS contribute to high reversible capacity for LIBs. The reversible capacity of porous functionalized graphene cathode depends on the content of oxygenic functional groups, as discussed in our previous studies.11, 12 In the current study, the oxygenic functional groups tend to decrease along with the increase of nitrogen content, as described in the above-mentioned XPS results. The reversible capacities significantly increase from 195 mAh g-1 of GNS to 330 mAh g-1 of GNS-II. It suggests that compared to the oxygenic functional groups, N atoms doping in GNS appears to play a more important role on enhancing the electrochemical performance of the graphene cathode. An admitted factor is the enhanced electrical conductivity resulting from N-doping,42 which is proven by electrochemical impedance spectra (See Fig. 4a). The EIS measurements were performed at a discharged state (1.5 V vs. Li+/Li) after 3 cycles. According to the fitting parameters based on an equivalent circuit (Rs represents the resistance of electrolyte, electrode and separator; Rf corresponds to the resistance of SEI film; Rct is the charge transfer resistance43, 44) in the Fig. 4b, NGNS-II electrode possesses lower charge-transfer resistance (318.6 Ω) than that of NGNS-I (355.8 Ω) and GNS (550.2 Ω) electrodes, as displayed in Table S2. Compared to NGNS-I, NGNS-II with more pyrrolic-N and less pyridinic-N delivers great improvement in reversible capacity and rate capacity (the graphitic-N proportions are comparable), thus the enhanced electrochemical performance of NGNS cathodes is more related to the increase of pyrrolic-N. It indicates that the intrinsic nitrogen content effects on the lithium storage performance of the N-doped graphene cathode, moreover, the N-configuration distributions are believed to have an important influence on the electrochemical 13



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activity. As revealed by Wang et al,42 the pyrrolic N easily adsorbs Li+ due to its larger average adsorption energy, resulting in efficient electrochemical interaction between Li atoms and NGNs surface. As can be illustrated in Fig. 5, the improved rate capability and cycling performance of the cathode materials are due to several factors: (1) the hetero-doped N atoms enhance the reactivity and electrical conductivity of graphene because of the larger electronegativity of N (3.04) than C (2.55), which facilitates fast electron transfer;45 (2) the introduction of N atoms cause more edges as well as open and flexible vacancy defects for efficient lithium storage sites; (3) the pyrrolic-N possesses high binding energy with Li, increasing the Li storage capacity; (4) the pyridinic-N and pyrrolic-N formed at the edges and vacancy sites can facilitate the perpendicular diffusion of Li+, and shorten Li+ diffusion distances, significantly leading to the excellent rate capability.46 In addition to above-mentioned reasons, the energy storage mechanism for N-rich cathode materials is another key factor according to previous reports.47, 48 NGNS shows high energy storage performance through the reversible redox reaction accompanied by the association/disassociation of Li+ or electrolyte anions (PF6−):8, 48 [N4]+x(PF6−)x] + xLi+ + xe(N1) + yLi+ + ye-

(N1) + xLi+( PF6−) (1)

[(N1)-y(Li+)y]

(2)

This proposed energy storage mechanism indicates that the N content (pyridinic-N and pyrrolic-N) in the NGNS are responsible for the electrochemical reaction to accommodate Li ions. In addition, N-doping can improve the wettability of graphene with electrolyte,49 which concomitantly contribute to the superior Li-storage performance.

14


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4.

Conclusions

In summary, we present a general hydrothermal strategy for fabricating N-doped functionalized graphene. When employed as new cathode material in LIBs, for the first time, the as-prepared NGNS cathodes were reported to deliver high reversible capacity, outstanding cycling stability and superior rate capability due to their unique 3D porous structure, active oxygenic functional groups and high nitrogen doping. More surprisingly, NGNS-II with 9.26 at% nitrogen content possesses a reversible capacity of 146 mAh g-1 after 1000 cycles at a current density of 1 A g-1. The enhanced electrochemical performance of NGNS results from the redox of surface oxygenic functional groups with lithium ion. Additionally, the unique 3D porous structure acts as efficient transmission channels for sufficient electrolyte infiltration and facile ion diffusion, as can be seen in the high-resolution SEM image of the NGNS electrode in Fig. 6. We demonstrate that NGNS cathodes with higher concentration of doped nitrogen dramatically enhance the electrochemical performance. More importantly, pyrrolic-N provides the most positive effect among the three types of N-configurations (pyrrolic-N, pyridinic-N and graphitic-N). Hence, we believe that this work will open up an opportunity for the development of novel N-doped graphene as cathode materials for high-performance electrochemical storage devices.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (51572194 and 51672189), the Key Project of Tianjin Municipal Natural Science 15



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Foundation of China (14JCZDJC32200), Academic Innovation Funding of Tianjin Normal University (52XC1404), Training Plan of Leader Talent of University in Tianjin, the program of Thousand Youth Talents in Tianjin of China and the Fundamental Research Funds for the Central Universities (2652016114).

ASSOCIATED CONTENT

Supporting Information

SEM images and XRD patterns of GO sample; XRD patterns and C 1s XPS spectra of GNS, NGNS-I, and NGNS-II samples; Galvanostatic charge-discharge profiles of the NGNS-II sample; Electrochemical performances results of GNS, NGNS-I and NGNS-II electrodes; Galvanostatic charge-discharge profiles and cycle performance of CG electrode; The comparison of current commercial cathode materials; Kinetic parameters of GNS, NGNS-I and NGNS-II electrodes. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Xifei Li) *E-mail: [email protected] (Zhimin Bai) *E-mail: [email protected] (Shigang Lu)

Notes 16


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The authors declare no competing financial interest.

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b

a

200 nm

c

200 nm

200 nm

df

e

f

200 nm

200 nm

100 nm

(010)

g (002)

(110)

h

(110)

(002)

(111)

2 layers 5 nm

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(111)

(010)

(111)

(110)

(002)

2 layers 5 nm

5 nm

(010)

i

3 layers

Fig. 1 SEM images of a) GNS, b) NGNS-I and c) NGNS-II; TEM images of d) GNS, e) NGNS-I and f) NGNS-II; HRTEM images (Inset: corresponding SAED patterns) of g) GNS, h) NGNS-I and i) NGNS-II.

24


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a)

D band

b)

G band

C1s O1s

N1s

NGNS-II

Intensity (a.u.)

Intensity(a.u.)

NGNS-II

NGNS-I

NGNS-I

GNS

GNS 1000

1200

1400

1600

1800

2000

700

600

500

Raman Shift (cm-1)

300

200

d)

100

(1)

Pyrrolic N

(3)

Intensity (a.u.)

(2)

Intensity (a.u.)

400

Binding energy (eV)

c)

Pyridinic N Graphitic N

(2)

HO-C=O

(1)

C-OH C=O

406

404

402

400

398

396

394

538

536

e) 10 8 6 4

Graphitic N Pyrrolic N Pyridinic N

6.82%

f) Total oxygen Content / at%

12

9.26% 18%

18%

37%

42%

2 0

45%

NGNS-I

534

532

530

528

526

Binding energy (eV)

Binding energy (eV)

Total nitrogen Content / at%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40%

14 12 10

8.41% 7.37%

8 44%

27%

26%

34%

6 4

34% 32%

2 0

30%

GNS

NGNS-II

HO-C=O C-OH C=O

11.28%

39%

NGNS-I

34%

NGNS-II

Fig. 2 a) Raman spectra and b) XPS survey spectra of GNS, NGNS-I and NGNS-II; c) High-resolution N 1s spectra of NGNS-I (1) and NGNS-II (2); d) High-resolution O 1s spectra of GNS (1), NGNS-I (2) and NGNS-II (3); e) Atomic percentages of three nitrogen species of NGNS-I and NGNS-II; f) Atomic percentages of three oxygen species of GNS, NGNS-I and NGNS-II.

25



a) 0.15

b) 4.5 4.0

Li+ extraction

Voltage /V (vs. Li+/Li)

0.05 0.00 -0.05

Li+ insertion

-0.10

GNS NGNS-I NGNS-II

-0.15

3.5 3.0

1.5

2.0

2.5

3.0

3.5

4.0

GNS NGNS-I NGNS-II

2.5 2.0 1.5

-0.20

Charge

4.5

Discharge

0

50

100

150

200

250

+

Voltage (V vs Li/Li )

350 300 250 200 150

GNS NGNS-I NGNS-II

100 50 0

Capacity /mAh

d) 450 Specific Capacity (mAh g-1)

Specific Capacity (mAh g-1)

c) 400

0

20

40

60

80 100 120 140 160 180 200

400

0.05A/g

350

350

0.05A/g

0.1A/g 0.2A/g

300

0.2A/g 0.4A/g

250 200 150

GNS NGNS-I NGNS-II

100 50 0

0

10

20

30

40

50

60

70

80

Cycle Number

Cycle Number

e)

300

g-1

120

350 300

100 Coulombic efficiency for NGNS-II

250

80

200

1A/g

60

150 40 100 NGNS-II

50

NGNS-I

Charge Discharge

Charge Discharge

0 0

100

200

300

400

500

600

700

800

900

20

Coulombic efficiency / %

Current density / A g-1

0.10

Specific Capacity / mAh g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 1000

Cycle Number

Fig. 3 a) Cyclic voltammogram of GNS, NGNS-I and NGNS-II electrodes in the second cycle at a scan rate of 0.1 mV s-1; b) Galvanostatic charge-discharge profiles and c) cycle performance of GNS, NGNS-I and NGNS-II electrodes at a current density of 0.05 A g-1; d) Rate performance of GNS, NGNS-I and NGNS-II electrodes at various current densities in the range of 0.05-0.4 A g-1; e) Cycle performance of NGNS-I and NGNS-II electrodes (up to 1000 cycles) and the Coulombic efficiency of NGNS-II at a high current density of 1 A g-1. All experiments were in a voltage range of 1.5–4.5 V vs. Li/Li+. 26


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(a) 1000

Experiment datas Fitting datas

GNS NGNS-I NGNS-II

800

-Z'' (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NGS NGNS-I NGNS-II

600 400 200 0 0

200

400

600

800

1000

Z' (Ohm)

(b)

RS

Rf

Rct

CPEf

CPEct

Zw

W

Fig. 4 a) Nyquist plots and fitting data of GNS, NGNS-I and NGNS-II electrodes with amplitude of 5.0 mV over the frequency ranging from 100 KHz to 0.01 Hz; b) Equivalent circuit used to fit an experimental curve.

27



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eLi+

eeLi+ C pyridinic-N

O pyrrolic-N

H graphitic-N

Fig. 5 Schematic illustration of the N-doped functionalized graphene.

28


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Porous network: electrolytes and Li+ conduction channels

Surface functional groups: Faradaic reactions centres

C=O + Li+ + e- ↔ C-O-Li Fig. 6 Schematic illustration of the energy storage mechanism of N-doped functionalized graphene with porous structure.

29



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Graphical Abstract

Porous network: electrolytes and Li+ conduction channels

Surface functional groups: Faradaic reactions centres

C=O + Li+ + e- ↔ C-O-Li


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Superior Cathode Performance of Nitrogen-Doped Graphene

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