Nitrogen-doped mesoporous carbon nanosheets from coal tar as high performance anode materials for lithium ion batteries

Nitrogen-doped mesoporous carbon nanosheets from coal tar as high performance anode materials for lithium ion batteries

NEW CARBON MATERIALS Volume 29, Issue 4, August 2014 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials...

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NEW CARBON MATERIALS Volume 29, Issue 4, August 2014 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2014, 29(4): 280–286.

RESEARCH PAPER

Nitrogen-doped mesoporous carbon nanosheets from coal tar as high performance anode materials for lithium ion batteries Hao-qiang Wang, Zong-bin Zhao*, Meng Chen, Nan Xiao, Bei-bei Li, Jie-shan Qiu* Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

Abstract: Nitrogen-doped mesoporous carbon nanosheets (NMCNs) were prepared from coal tar and melamine using a layered MgO as template. Porous structures and nitrogen doping levels were readily tuned by adjusting experimental parameters. NMCNs show high specific capacities and excellent cyclic stabilities as anode materials for lithium ion batteries. A sample prepared under optimum conditions shows a high reversible capacity of nearly 1 000 mAh/g at a current density of 100 mA/g, which can be ascribed to its unique mesoporous sheet-structure, a high specific surface area of 1 209 m2/g, and a uniform and high bulk nitrogen content of 8.6%. Our work demonstrates that coal tar can act as an excellent carbon source for the production of carbon materials with high performance in lithium-ion batteries. Key Words: Nitrogen-doped; Mesoporous carbon nanosheets; Coal tar; Template; Lithium ion batteries

1

Introduction

Lithium ion batteries (LIBs) have been considered to be the most promising power source of portable electronics for their high energy density and long lifespan. Traditional graphite anode material shows a low theoretical specific capacity of 372 mAh/g and a limited rate performance, thus urging researchers to seek for various anode materials showing enhanced energy densities. Substantial novel carbon forms have been employed, such as carbon nanotubes[1], graphenes[2-6], carbon nanofibers[7], hierarchical porous carbons[8,9], ordered mesoporous carbon[10], hollow carbon capsules[11], carbon nanosheets[8,12,13] and their composites[14,15]. Among these materials, carbon nanosheets are popular because they can offer sufficient electrode/electrolyte interface for lithium adsorption and reduce transport distance of lithium-ion diffusion. Introduction of nitrogen into the carbon frameworks is another way to increase electrical conductivities[7,16,17] and energy densities[16,18]. In the meanwhile, it is most important to use high quality carbon sources as raw materials to synthesize porous carbons as anodes for lithium ion batteries. Coal tar, as a liquid by-product from the pyrolysis of coal to produce coke for the steel industry, is mainly composed of carbonaceous polycyclic aromatic hydrocarbons and has a relatively high residual carbon content. Extraction of coal tar with solvents results in a narrow molecular weight distribution (MWD)[19] and excludes the presence of ash [20],which are desirable properties for the production of high performance carbons. Accordingly, extracts

of coal tar can be used as a promising carbon precursor for synthesizing high quality porous carbons. Herein, we synthesized nitrogen-doped mesoporous carbon nanosheets (NMCNs) by pre-oxidation and carbonization using extracts of coal tar as carbon precursor, melamine as nitrogen/carbon source and MgO layer as template. NMCNs possess sheet structures, high specific surface areas, uniformly-distributed mesopores and high nitrogen contents. When used as anodes for lithium ion batteries, NMCNs exhibit an excellent electrochemical performance.

2 2.1

Experimental Extracts from coal tar

Extracts of coal tar (provided by Ansteel Group Corporation, China) was used as carbon precursor. The extraction process was carried out by using petroleum ether (PE) and toluene. The detailed route was described as follows: 2 g high-temperature coal tar was extracted with 50 mL PE under ultrasonic agitation for 30 min. After filtration, the petroleum ether insoluble (PEI) fraction was dried in a vacuum oven at 50 °C for 24 h and extracted with 50 mL toluene under magnetic stirring at 75 °C for 12 h. The suspension was then filtered and the solute was separated at 60 °C with rotary evaporation to obtain the extracts, marked as petroleum ether insoluble-toluene soluble (PEI-TS). The elemental analysis data of PEI-TS is shown in Table 1.

Received date: 09 June 2014; Revised date: 10 August 2014 *Corresponding author: ZHAO Zong-bin. E-mail: [email protected]; QIU Jie-shan. E-mail: [email protected] Copyright©2014, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(14)60137-2

Hao-qiang Wang et al. / New Carbon Materials, 2014, 29(4): 280–286

2.2

Synthesis of MgO layer template

The porous MgO layer template was synthesized similar to the method reported previously[21]. Briefly, MgO powder was added into deionized water under ultrasonic agitation and subsequently refluxed for 30 h to form Mg(OH)2 with a layer structure. After filtration and drying, the Mg(OH)2 was calcined at 500 °C for 30 min to give rise to the layered MgO. 2.3

Synthesis of nitrogen-doped mesoporous carbon

nanosheets Nitrogen-doped mesoporous carbon nanosheets (NMCNs) were prepared by pre-oxidation and carbonization of the mixtures of PEI-TS, melamine and MgO layer template. The preparation strategy is shown in Fig. 1. Briefly, 0.3 g PEI-TS was dissolved into 20 mL tetrahydrofuran (THF) under intensive magnetic stirring. Then, 0.6 g melamine and 0.8 g MgO template were added into the THF solution under constant stirring to obtain a homogeneous suspension. After that, the mixture was dried at 100 °C overnight. The dried mixture was ground and transferred into a quartz boat and heated in air at a rate of 2 °C /min up to 250 °C and maintained for 2 h, followed by elevating temperature at a rate of 5 °C /min up to 800 °C under nitrogen atmosphere and stayed for 1 h. After cooling down, the composite was washed with excess 1 mol/L HCl solution and deionized water to remove MgO, followed by filtration and drying. The prepared samples were defined as NMCNs-T, where T represents the carbonization temperature, which is 700, 800 or 900 °C. For comparison, the product produced in the absence of melamine under identical condition was denoted as MCNs-800. 2.4

Characterization

The samples were examined using a field-emission scanning electron microscope (FE-SEM, FEI NOVA NanoSEM 450) and a transmission electron microscope (TEM, Philips Tecnai G2 20). The Nitrogen adsorption/desorption isotherms were measured at 77 K on a physical adsorption apparatus (Micromeritics ASAP 2020). Before the measurement, the samples were degassed at 250 °C for 5 h. The surface area (SBET) was calculated by using the Brunauer-Emmett-Teller (BET) method in the relatively pressure (p/p0) range of 0.05-0.24. Total pore volume (Vt) was obtained at the p/p0 of 1. The average pore diameter (Dave) was evaluated from the desorption branch, according to Barrett–Joyner–Halanda (BJH) model. The elemental analysis was carried out by a CHN combustion method on a Elemental Analyzer Vario EL III. X-ray photoelectron spectroscopy (XPS) was obtained on Thermo ESCALAB250 to analyze the surface chemical compositions. Raman analysis was studied on an inVia Raman Microscope (RENISHAW) with a 532 nm-1 laser beam. 2.5

Electrochemical measurements

The working electrodes were prepared by mixing active materials (NMCNs-T and MCNs-800), carbon black and poly (tetrafluoroethylene) in N-methylpyrrolidone with a weight ratio of 80:10:10. The slurry was pasted onto a copper foil,

followed by vacuum drying at 120 °C for 12 h. Li metal foil was used as the counter and reference electrode. The electrolyte used in the work is 1.0 mol/L LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC). The cells were assembled in a glove box filled with Argon. Galvanostatic charge/discharge tests were conducted using a Land Battery workstation with a working voltage window between 0.01 and 3 V. The cyclic voltammograms (CV) were performed with a voltage range of 0.01-3 V at a scan rate of 0.5 mV/s. The electrochemical impedance spectroscopic (EIS) measurements were measured using a CHI660D workstation (CH Instruments Inc., Shanghai, China) with a frequency range from 100 kHz to 0.01 Hz.

3 3.1

Results and discussion Structure and texture characterization

Fig. 2a,b show the FESEM images of NMCNs-800. It can be seen that NMCNs-800 reveals polygonal intersecting nanosheets. It can be seen from the TEM images illustrated in Fig. 2d,e, the nanosheets intersect with each other and their widths are about 200 nm, consistent with the FESEM results. Moreover, the semi-transparent nanosheets form a gap with a distance between two carbon layers of about 10-20 nm (marked by ovals Fig. 2e), which is owing to the removal of MgO layer template. It can be observed from Fig. 2f that there are numerous mesopores of about 5-7 nm on NMCNs-800 derived from the porous MgO layer template (Fig. 2c). Fig. 3 shows the nitrogen adsorption-desorption isotherms and pore size distribution curves of the samples. As shown in Fig. 3a, the type IV isotherms depict obvious hysteresis loops between adsorption and desorption branches, indicating that the samples contain abundant mesopores coming from the porous MgO template. As shown in Fig. 3b, the samples exhibit narrow pore size distributions centered at less than 7 nm with an obvious mesoporosity. In addition, there emerges some large mesopores (> 20 nm) and

Fig. 1 Schematic illustration for preparing NMCNs. Table 1 Elemental analysis of PEI-TS. Carbon precursor PEI-TS Note:* by difference

Element w/ % C

H

S

N

O*

91.54

4.92

1.52

1.12

0.90

Hao-qiang Wang et al. / New Carbon Materials, 2014, 29(4): 280–286

Fig. 2 (a, b) FE-SEM images of NMCNs-800; TEM images of (c) MgO layer template and (d-f) NMCNs-800.

Fig. 3 (a) Nitrogen adsorption-desorption isotherms and (b) pore size distributions of the samples.

macropores, resulting from the space between two carbon layers formed by removing the stacking templates. As listed in Table 2, NMCNs-800 has a BET specific surface area of 1 043 m2/g and a total pore volume of 1.88 cm3/g, which are remarkably higher than those of MCNs-800. This may be caused by the gases produced from melamine decomposition at high temperature and their creation of additional pore structures. NMCNs-700 has a high SBET of 1209 m2/g and a large Vt of 1.90 cm3/g, while SBET and Vt decrease with an increase of carbonization temperature, which is due to the contraction of existing pores during further heat treatment, as a result, NMCNs-900 has the smallest Dave (4.2 nm). The elemental contents of the samples were measured by

the elemental analysis and XPS analysis (Table 3). As determined by elemental analysis, NMCNs-700 has a high nitrogen content of 8.6%, indicating the successful introduction of nitrogen into the porous carbon nanosheets. Table 2 Porous structures of the samples prepared at different temperatures. Samples

Porous structure parameters 2

SBET/m ·g-1

Vt /cm3·g-1

Dave /nm

NMCNs-700

1209

1.90

5.7

NMCNs-800

1043

1.88

6.5

NMCNs-900

658

1.19

4.2

MCNs-800

789

1.52

6.9

Hao-qiang Wang et al. / New Carbon Materials, 2014, 29(4): 280–286

Table 3 Elemental analyses and XPS results of the samples. Elemental analysis C

H

N

O*

w/ %

w/ %

w/ %

NMCNs-700

68.7

3.6

NMCNs-800

72.4

3.3

Samples

XPS N/C

C

N

O#

N/C

w/ %

at/·at

w/ %

w/ %

w/ %

at/·at

8.6

19.1

0.11

83.0

5.0

12.0

0.05

7.5

16.8

0.09

84.1

3.5

12.4

0.04

NMCNs-900

75.8

3.1

5.9

15.2

0.07

85.3

3.2

11.5

0.03

MCNs-800

78.5

3.8

1.3

16.4

0.01

87.2

0.4

12.4

0

Note: *, # by difference.

1.3% of nitrogen content can be also measured resulting from carbon source PEI-TS.

Fig. 4 Raman spectra of the samples.

With the increase of carbonization temperature, the nitrogen content decreases to 7.5% for NMCNs-800 and 5.9% for NMCNs-900. This is due to the release of nitrogen-containing species during high temperature pyrolysis. The reason for the high oxygen contents in the nitrogen-doped carbon materials is attributed to the pre-oxidation process, favoring the incorporation of nitrogen into the carbon matrix[22]. Compared with elemental analyses, the XPS results (Table 3) show relatively lower N-contents which is attributed to the easy elimination of functional groups on the surface during thermal treatment, suggesting a smaller nitrogen content on the surface than in the bulk. As for the MCNs-800,

Fig. 4 shows the Raman spectra of the samples. The peak at 1 350 cm-1 (D band) corresponds to the disordered carbon while the peak at 1 590 cm-1 (G band) is associated with the graphitic layers. High intensity ratio of ID/IG represents a less degree of graphitization or more defective carbon structures. With the increase of carbonization temperature, the ID/IG ratio decreases from 1.12 (NMCNs-700) to 1.11 (NMCNs-800) and 0.99 (NMCNs-900). This is understandable because high temperature helps to increase the graphitization degree and decrease defects originated from the incorporation of nitrogen species. Graphitization makes the samples be more conductive and less porous. Compared with NMCNs-800, MCNs-800 has a lower ID/IG intensity ratio of 0.96 owing to its less amount of nitrogen in the lattice. 3.2

Electrochemical performance

To investigate the electrochemical properties, cyclic voltammogram (CV) of NMCNs-800 at 0.5 mV/s was performed at a voltage window of 0.01-3.0 V, as shown in Fig. 5a. The cathodic peak at 0.5 V in the first cycle is attributed to the decomposition of electrolyte and the formation of the solid electrolyte interphase (SEI) layer. The strong peak disappears in the subsequent cycles, indicating the complete formation of SEI layer in the first cycle. The first discharge-charge curve of NMCNs-800 (Fig. 5b) shows a high

Fig. 5 (a) Cyclic voltammograms at 0.5 mV/s and (b) the charge/discharge profiles at 100 mA/g of NMCNs-800.

Hao-qiang Wang et al. / New Carbon Materials, 2014, 29(4): 280–286

samples decrease, thus reducing the active sites and space for Li adsorption and storage. In addition, the lower nitrogen content at higher temperature can decrease binding and active sites for Li storage. Therefore, all these changes may result in the annealing-induced loss of capacity. MCNs-800 shows lower capacities than the corresponding NMCNs-800 due to its lower SBET, lower Vt and negligible nitrogen content compared with the latter. Fig. 6b shows the cycling behavior of the samples at 100 mA/g in the cut-off window of 0.01-3 V. As can be seen, the samples reveal excellent cycling stability with high specific capacities. After 100 cycles, stabilized capacities of 789 mAh/g for NMCNs-700, 885 mAh/g for NMCNs-800, 559 mAh/g for NMCNs-900 and 648 mAh/g for MCNs-800 were achieved. The decrease of the specific capacity of NMCNs-700 after long-time charge/discharge process may be due to its low graphitization degree and more defects induced by the high nitrogen content at low carbonization temperature, exhibiting the high ID/IG ratio (1.12), thus decreasing the textural stability. With the increasing of annealing temperature, the cycling stability of the samples increases for NMCNs-800 (ID/IG=1.11) and NMCNs-900 (ID/IG=0.99).

Fig. 6 (a) Rate performance, (b) cycling stability and (c) Nyquist plots of the samples.

capacity of 1 986 mAh/g and 990 mAh/g with a 50.2% loss of irreversible capacity, this is consistent with the reduction peak in the first CV curve. Fig. 6a displays the capacity of the samples at various current densities. The coulombic efficiency in the first cycle is 55.4% for NMCNs-700, 49.8% for NMCNs-800 and 47.4% for NMCNs-900, all higher than 45.2% for MCNs-800, indicating that the introduction of nitrogen can restrain reactions causing loss of irreversible capacity in the first discharge/charge cycle[16]. At a low current density of 100 mA/g, the capacities of NMCNs-700, NMCNs-800 and NMCNs-900 are 1 017, 808 and 611 mAh/g, respectively. After cycling at a large current density of 5 A/g for ten cycles, they still restored high reversible capacities of 1 085, 815 and 664 mAh/g when the current rolls back to 100 mA/g. Obviously, with an increase of carbonization temperature, the specific surface areas and pore volumes of the N-doped

Fig. 6c exhibits electrochemical impedance spectroscopic (EIS) results. NMCNs-900 has the smallest diameter of semicircle at high frequency, corresponding to the lowest charge transfer impedance because of its high graphitization degree (ID/IG=0.99) resulting from the high carbonization temperature. However, the charge transfer impedance of NMCNs-700 is lower than that of NMCNs-800. It has been well-demonstrated that the presence of nitrogen enhances the electrical conductivity of carbon materials. Although the graphitization degree of NMCNs-800 is higher than that of NMCNs-700, the conductivity enhancement from the doping N surpasses that from graphitization degree under this condition, as a result, the latter exhibit lower charge transfer impedance than the former[23]. Similarly, the electrical conductivity of NMCNs-800 is higher than that of MCNs-800 resulting from the higher N content in the former. The possible reasons for the superior electrochemical performance of NMCNs-700 are illustrated in Fig. 7. The high BET specific surface area (1 209 m2/g) provides rich interfaces between electrode and electrolyte for Li adsorption and promotion of charge-transfer reaction. The large pore volume (1.90 cm3/g) mainly contributed by uniformly-distributed mesopores supplies sufficient reservoirs for Li storage. In addition, the sheet-like structure and channels between two carbon layers favor the electron penetration and largely shorten the Li penetration distance. Finally, the high nitrogen content of 8.6% adds more active sites for Li adsorption and simultaneously enhances the electrical conductivity of the materials, consequently extra performance can be achieved from these profits.

Hao-qiang Wang et al. / New Carbon Materials, 2014, 29(4): 280–286

Fig. 7 Schematic illustration of Li storage in the NMCNs.

4

Conclusions

Nitrogen-doped carbon nanosheets have been successfully prepared from coal tar with the assistance of the layered MgO template. The coal tar-based porous carbons are featured with sheet-like structures, high specific surface areas, uniformly-distributed mesopores and high nitrogen contents. The specific surface areas and nitrogen contents of the materials can be easily tuned by adjusting the experimental parameters. When used as the anodes for lithium-ion batteries, the carbon nanosheets show high capacities and good cycle stability. The carbon nanosheets with a high specific surface area (1 209 m2/g) and a high-level nitrogen content (8.6%) exhibits a reversible capacity as high as nearly 1 000 mAh/g at a current density of 100 mA/g. The coal tar derived mesoporous carbons are promising candidates of anode materials for high-performance LIBs. Acknowledgements

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