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Cite this: Dalton Trans., 2014, 43, 14931 Received 25th April 2014, Accepted 3rd June 2014
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Porous nitrogen-doped carbon microspheres as anode materials for lithium ion batteries† Taiqiang Chen,a Likun Pan,*a T. A. J. Loh,b D. H. C. Chua,b Yefeng Yao,a Qun Chen,a Dongsheng Li,a Wei Qina and Zhuo Suna
DOI: 10.1039/c4dt01223b www.rsc.org/dalton
Nitrogen-doped carbon microspheres (NCSs) were fabricated via a simple, fast and energy-saving microwave-assisted method followed by thermal treatment under an ammonia atmosphere. NCSs thermally treated at different temperatures were investigated as anode materials for lithium ion batteries (LIBs). The results show that NCSs treated at 900 °C exhibit a maximum reversible capacity of 816 mA h g−1 at a current density of 50 mA g−1 and preserve a capacity of 660 mA h g−1 after 50 cycles, and even at a high current density of 1000 mA g−1, a capacity of 255 mA h g−1 is maintained. The excellent electrochemical performance of NCSs is due to their porous structure and nitrogen-doping. The present NCSs should be promising low-cost anode materials with a high capacity and good cycle stability for LIBs.
1.
Introduction
Currently, lithium ion batteries (LIBs) represent a key class of battery architecture that has emerged as a prime candidate for energy storage devices due to their high energy density and long cycle lifetime.1–7 In recent years, the ever increasing application of LIBs in electric or hybrid electric vehicles has largely promoted the urgent demand for LIBs with a high capacity and excellent rate capability and cycling stability.8–12 However, the most commonly used commercial graphite anode material is limited to a low theoretical capacity of 372 mA h g−1 and cannot meet the increasing demand.13–15 Thus a great deal of effort has been made to explore new carbon anode materials with a high capacity.16–19 Recently, nitrogen-doping has attracted wide attention as an effective method to improve the electrochemical performa Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, China. E-mail: [email protected]; Fax: +86 21 62234321; Tel: +86 21 62234132 b Department of Materials Science and Engineering, National University of Singapore 117574, Singapore † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4dt01223b
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ance of carbon materials. Nitrogen is attractive because it can enhance the interaction, which might be favorable for lithium insertion, between the nitrogen-doped carbon material and lithium, since it has higher electronegativity and smaller atomic diameter than those of carbon. Besides, doping of nitrogen atoms into graphitic networks (e.g. carbon nanotubes) is considered one of the best approaches to produce n-type conductive materials with improved conductivity.20–22 Moreover, nitrogendoping can induce a large number of defects in the carbon structure and offer more active sites for lithium insertion.23–25 To date, different kinds of nitrogen-doped carbon materials, such as nitrogen-doped carbon nanofibers and nanotubes, and graphene, have been developed to show an improved capacity as compared with the undoped counterparts.24,26–31 On the other hand, carbon materials of spherical structure have been demonstrated to be competent for using as anode materials for LIBs, since spherical materials enjoy a high packing density, a low surface to volume ratio, maximal structural stability and ease of preparing electrode films.32–35 Commercial graphitized mesocarbon microbeads are one successful example. In addition, it is well known that nanopores within carbon materials can provide extra high lithiation capability, because the porous structure can not only shorten the lithium ion transport length but also facilitate the charge-transfer reaction on the electrode–electrolyte interface.27,36 Thus one can expect that porous carbon spheres with nitrogen-doping should be very promising high-capacity anode materials for LIBs. Unfortunately, such an exploration on nitrogen-doped carbon spheres for LIBs has seldom been reported so far. In this work, a simple, fast and energy-saving microwaveassisted method was employed to synthesize carbon microspheres (CSs) using sucrose dissolved in a mixed solvent of water and ethylene glycol as a precursor in a microwave system. The as-synthesized carbon microspheres were subjected to a thermal treatment under an ammonia atmosphere to fabricate porous nitrogen-doped carbon microspheres (NCSs). When used as anode materials for LIBs, the NCSs exhibit excellent electrochemical performance due to their porous structure and nitrogen-doping.
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2. Experimental In a typical process, 6.8 g sucrose and 2 g concentrated sulfuric acid were dissolved in 100 ml mixed solvent of water and ethylene glycol (6 : 4 v/v). Subsequently, 20 ml of the solution was sealed in a 35 ml microwave tube and heated at 160 °C with a maximum microwave irradiation power of 100 W for 10 min using a microwave system (Explorer 48, CEM Co.). The resulting precipitate was centrifuged, washed with deionized water, dried in a vacuum oven at 80 °C for 24 h and finally thermally treated under an ammonia atmosphere with a holding time of 2 h. The temperatures of thermal treatment were set at 500, 700 and 900 °C, and the corresponding obtained NCSs are labeled as NCS500, NCS700 and NCS900, respectively. The morphology of as-synthesized NCSs was characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The structure was characterized by X-ray diffraction (XRD, Holland Panalytical PRO PW3040/60) with Cu-Kα radiation (V = 30 kV, I = 25 mA). The nitrogen adsorption isotherm was measured at 77 K with an ASAP 2020 Accelerated Surface Area and Porosimetry System (Micromeritics, Norcross, GA) and the specific surface area, pore volume and mean pore diameter are determined by the Brunauer–Emmett–Teller (BET) method. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Imaging Photoelectron Spectrometer (Axis Ultra, Kratos Analytical Ltd) with a monochromatic Al Kα X-ray source. For electrochemical testing, as-synthesized NCSs, Super-P carbon black and polyvinyldifluoride, with a weight ratio of 80 : 10 : 10, were homogeneously mixed in N-methylpyrrolidone solvent to produce slurry. Then, the resulting slurry was coated on a copper foil using a doctor blading method. Finally, these electrodes were dried at 120 °C under vacuum for 12 h. The active material loading in the electrodes is ∼1.2 mg cm−2. Coin-type cells (CR2032) were assembled in a glove box (MB-10-compact, MBRAUN) under an argon atmosphere with oxygen and water of less than 0.5 ppm using lithium metal foil as the counter and reference electrodes. Celgard 2320 membrane and 1 M LiPF6 electrolyte solution dissolved in a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (1 : 1 : 1 w/w) were used as the separator and the electrolyte, respectively. Galvanostatic charge–discharge cycles were performed using a LAND2001A battery test system in the voltage range of 0.005–3 V. Cyclic voltammetry (CV) was performed using an electrochemical workstation (AUTOLAB PGSTAT302N) within a voltage range of 0.005–3 V at a scan rate of 0.2 mV s−1.
Fig. 1 FESEM image of NCS900. The inset shows a high-magnification image.
XRD patterns of NCS500, NCS700 and NCS900 are displayed in Fig. 2a. In a typical pattern, two broad peaks appear at ∼24° and ∼43.5°, corresponding to (002) and (100) diffraction modes of graphitic structure, which is characteristic of disordered carbon material. When the thermal treatment temperature increases from 500 to 900 °C, the (100) diffraction peak gradually becomes intense, indicating that the carbonization degree increases with the increase of temperature. As illustrated in Fig. S1 (see ESI†), the XRD patterns of CSs thermally treated at 900 °C under a nitrogen atmosphere show two broad peaks similar to that of NCS900, indicating that the ammonia treatment does not modify the graphitic structure of CSs obviously. The specific surface areas, pore volumes and mean pore diameters of NCS500, NCS700 and NCS900 were determined by the BET method and are listed in Table 1. The specific surface area increases with increasing the thermal treatment temperature and the values are 96, 447 and 1630 m2 g−1 for NCS500, NCS700 and NCS900, respectively. NCS900 has a highest pore volume of 0.77 cm3 g−1 with a pore diameter of
3. Results and discussion Fig. 1 shows the FESEM images of NCS900 (the morphologies of NCS500 and NCS700 are similar to that of NCS900 and are not shown here). It can be observed that spherical structure is obtained via the microwave-assisted reaction, and the smooth surfaced NCSs have diameters ranging from 0.5 to 1.5 μm. The
14932 | Dalton Trans., 2014, 43, 14931–14935
Fig. 2 (a) XRD patterns of various samples; XPS N 1s spectra of (b) NCS500, (c) NCS700 and (d) NCS900.
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Table 1 Surface areas, pore volumes and mean pore diameters of NCS500, NCS700 and NCS900
Table 3 The atomic percentages of pyridinic, pyrrolic and graphitic nitrogen determined from the relative areas of corresponding XPS components
Sample
Surface area (m2 g−1)
Pore volume (cm3 g−1)
Mean pore diameter (nm)
Samples
Pyridinic N (at%)
Pyrrolic N (at%)
Graphitic N (at%)
NCS500 NCS700 NCS900
96 447 1630
0.05 0.20 0.77
2.11 1.82 1.88
NCS500 NCS700 NCS900
46.7 42.3 34.0
51.9 51.5 41.3
1.4 6.2 24.7
1.88 nm. CSs thermally treated at 900 °C under a nitrogen atmosphere show a specific surface area of 1025 m2 g−1 with a pore volume of 0.47 cm3 g−1 and a mean pore diameter of 1.82 nm. The results indicate that ammonia treatment can improve the porosity of CSs, which may enhance the lithium storage capacity. An XPS analysis was performed to detect the doping of nitrogen in the NCSs. The carbon, oxygen and nitrogen contents (at%) in NCS500, NCS700 and NCS900 were determined by XPS and are listed in Table 2. The carbon content increases and the oxygen content decreases with the increase of thermal treatment temperature from 500 to 900 °C, indicating that the carbonization degree increases with the increase of temperature. The result is consistent with the XRD analysis. NCS900 has a highest carbon content of 91.5 at% and a lowest oxygen content of 2.2 at%, which may favor a high electrical conductivity, while NCS700 has a nitrogen content of 7.8 at%, higher than those of NCS500 and NCS900. NCS900 shows relatively low nitrogen content because heteroatoms are chemically unstable at such high temperatures.37,38 As shown in Fig. 2b, c and d, the XPS N 1s spectra of the NCSs thermally treated at different temperatures can be deconvoluted into three components located around 398.3, 400.2 and 402.3 eV, which are assigned to pyridinic, pyrrolic and quaternary (graphitic) nitrogen doped in carbon, respectively.23,36 The N 1s spectra are fitted with the Gaussian function and the percentages of pyridinic, pyrrolic and graphitic nitrogen are determined from the relative areas of the corresponding deconvoluted components, as summarized in Table 3. Clearly the percentage of graphitic nitrogen increases with the increase of thermal treatment temperature from 500 to 900 °C, while the percentages of pyridinic and pyrrolic nitrogen decrease. The increase of graphitic nitrogen percentage is understandable, since graphitic nitrogen is more stable than pyridinic and pyrrolic nitrogen.39,40 The graphitic nitrogen percentage in NCS900 was estimated to be 24.7%, much higher than the values (1.4% and 6.2%) for NCS500 and NCS700. CV analysis was conducted to evaluate the electrochemical behaviour of as-prepared NCSs. Fig. 3a shows the CV curves of
Table 2 The carbon, oxygen and nitrogen contents (at%) of NCS500, NCS700 and NCS900 determined by XPS
Samples
C
N
O
NCS500 NCS700 NCS900
86.4 87.9 91.5
7.2 7.8 6.3
6.4 4.3 2.2
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Fig. 3 (a) CV curves and (b) the first and second charge–discharge curves of NCS500, NCS700 and NCS900.
various electrodes in the 3rd scanning cycle. A pronounced cathodic peak near 0 V is observed for all electrodes, which is very common for lithium insertion in carbonaceous materials.41,42 Lithium removal takes place in a wide potential range, resulting in the broad anodic range (0.4–1.5 V) in the CV curves of NCS500 and NCS700.43 In the high potential range from 1.5 to 3.0 V, NCS900 displays a CV curve remarkably different from those of NCS500 and NCS700. Two additional weak anodic peaks around 1.3 and 2.4 V emerge in the curve of NCS900 and the anodic current density in the potential range from 1.5 to 3.0 V is much higher than those of NCS500 and NCS700, which suggests that other lithium storage reactions exist in NCS900. It has been reported that graphitic nitrogen could enhance the kinetic performance of the electrode because of the weakening Li–C interaction.44 NCS900 has a much higher graphitic nitrogen percentage as compared with NCS500 and NCS700. It is likely that the enhanced electrochemical reactivity of NCS900 originates from the high graphitic nitrogen content that improves the electrode reactivity on the carbon surface at high voltage range.
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Fig. 3b illustrates the first and second charge–discharge curves of NCS500, NCS700 and NCS900 at a current density of 50 mA g−1. During the first discharge, a plateau around 0.6 V was found for all electrodes, which was due to the formation of a solid electrolyte interphase and other side reactions, resulting in the large irreversible capacity in the first cycle.27,44 CSs thermally treated at 900 °C under a nitrogen atmosphere show charge–discharge curves similar to those of NCS500 and NCS700 (see Fig. S2a in ESI†) and deliver an initial reversible capacity of 174 mA h g−1. The initial reversible capacities of NCSs increase with the increase of thermal treatment temperature and are 429, 486 and 816 mA h g−1 for NCS500, NCS700 and NCS900, respectively. Therefore, nitrogen-doping has greatly improved the capacity of CSs. It is revealed in the XPS spectra that NCS500 and NCS700 have almost the same nitrogen content and components. As compared with NCS500, the relatively high capacity of NCS700 can mainly be attributed to its higher specific surface area that facilitates the lithium insertion. As for NCS900, because of its much higher graphitic nitrogen percentage and specific surface area, the enhanced reactivity on the carbon surface at high potential range (as discussed above) remarkably improves its lithium storage capacity, resulting in the charge curve at potential above ∼1.6 V being different from those of NCS500 and NCS700. The cycle performances of NCS500, NCS700 and NCS900 at a current density of 50 mA g−1 are shown in Fig. 4a. It can be seen that all the electrodes exhibit excellent cycling stability, indicating that the thermal treatment temperature does not influence the cycle performance of NCSs notably. After 50 cycles, NCS500, NCS700 and NCS900 maintain capacities of 241, 395 and 660 mA h g−1, respectively, much higher than that (155 mA h g−1) of CSs thermally treated at 900 °C under a nitrogen atmosphere (see Fig. S2b in ESI†). It should be noted that the as-prepared NCS900 achieves a capacity much higher than that (300–400 mA h g−1) of nitrogen-doped carbon nanotubes24,28 and comparable to that (684 mA h g−1) of nitrogendoped graphene.26 Furthermore, the rate performances of various NCSs are revealed in Fig. 4b. NCS900 delivers capacities of 500, 411 and 341 mA h g−1 at current densities of 150, 300 and 500 mA g−1, respectively. Even at a high current density of 1000 mA g−1, NCS900 still retains a capacity of 255 mA h g−1, showing a good rate capability. The excellent electrochemical performance of NCS900 can be attributed to its porous structure and nitrogen-doping. Firstly, the large surface area provides a sufficient electrode– electrolyte interface to absorb lithium ions and facilitates the rapid charge-transfer reaction, and large numbers of nanopores on the surface may act as reservoirs for the storage of lithium ions.27,45 Secondly, nitrogen-doping offers more active sites for lithium insertion23 and may contribute to the electrical conductivity.46 Especially, the graphitic nitrogen in the NCSs remarkably enhances the kinetic performance of the electrode, resulting in an enhanced electrochemical performance.44 Moreover, the high carbonization degree of NCS900 should be beneficial to the efficient electron transport.
14934 | Dalton Trans., 2014, 43, 14931–14935
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Fig. 4 (a) Cycle performances and (b) rate performances of NCS500, NCS700 and NCS900. The rate performance was conducted after 50 galvanostatic charge–discharge cycles at a current density of 50 mA g−1.
4.
Conclusions
In summary, NCSs were successfully synthesized via a microwave-assisted reaction followed by thermal treatment under an ammonia atmosphere and investigated as anode materials for LIBs. The results of electrochemical experiments reveal that the NCSs exhibit a high capacity and good cycle stability and rate capability. A maximum capacity of 660 mA h g−1 at a current density of 50 mA g−1 after 50 cycles is achieved for NCSs thermally treated at 900 °C. Even at a high current density of 1000 mA g−1, a capacity of 255 mA h g−1 is maintained. The NCSs should be a very promising candidate for anode materials of LIBs.
Acknowledgements Financial support from the National Natural Science Foundation of China (no. 21276087) and the Shanghai Committee of Science and Technology (11JC1403600) is gratefully acknowledged.
Notes and references 1 Y. Piao, H. S. Kim, Y.-E. Sung and T. Hyeon, Chem. Commun., 2010, 46, 118. 2 S.-K. Park, S.-H. Yu, S. Woo, B. Quan, D.-C. Lee, M. K. Kim, Y.-E. Sung and Y. Piao, Dalton Trans., 2013, 42, 2399.
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Dalton Trans., 2014, 43, 14931–14935 | 14935
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Cite this: Dalton Trans., 2014, 43, 14931 Received 25th April 2014, Accepted 3rd June 2014
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Porous nitrogen-doped carbon microspheres as anode materials for lithium ion batteries† Taiqiang Chen,a Likun Pan,*a T. A. J. Loh,b D. H. C. Chua,b Yefeng Yao,a Qun Chen,a Dongsheng Li,a Wei Qina and Zhuo Suna
DOI: 10.1039/c4dt01223b www.rsc.org/dalton
Nitrogen-doped carbon microspheres (NCSs) were fabricated via a simple, fast and energy-saving microwave-assisted method followed by thermal treatment under an ammonia atmosphere. NCSs thermally treated at different temperatures were investigated as anode materials for lithium ion batteries (LIBs). The results show that NCSs treated at 900 °C exhibit a maximum reversible capacity of 816 mA h g−1 at a current density of 50 mA g−1 and preserve a capacity of 660 mA h g−1 after 50 cycles, and even at a high current density of 1000 mA g−1, a capacity of 255 mA h g−1 is maintained. The excellent electrochemical performance of NCSs is due to their porous structure and nitrogen-doping. The present NCSs should be promising low-cost anode materials with a high capacity and good cycle stability for LIBs.
1.
Introduction
Currently, lithium ion batteries (LIBs) represent a key class of battery architecture that has emerged as a prime candidate for energy storage devices due to their high energy density and long cycle lifetime.1–7 In recent years, the ever increasing application of LIBs in electric or hybrid electric vehicles has largely promoted the urgent demand for LIBs with a high capacity and excellent rate capability and cycling stability.8–12 However, the most commonly used commercial graphite anode material is limited to a low theoretical capacity of 372 mA h g−1 and cannot meet the increasing demand.13–15 Thus a great deal of effort has been made to explore new carbon anode materials with a high capacity.16–19 Recently, nitrogen-doping has attracted wide attention as an effective method to improve the electrochemical performa Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, China. E-mail: [email protected]; Fax: +86 21 62234321; Tel: +86 21 62234132 b Department of Materials Science and Engineering, National University of Singapore 117574, Singapore † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4dt01223b
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ance of carbon materials. Nitrogen is attractive because it can enhance the interaction, which might be favorable for lithium insertion, between the nitrogen-doped carbon material and lithium, since it has higher electronegativity and smaller atomic diameter than those of carbon. Besides, doping of nitrogen atoms into graphitic networks (e.g. carbon nanotubes) is considered one of the best approaches to produce n-type conductive materials with improved conductivity.20–22 Moreover, nitrogendoping can induce a large number of defects in the carbon structure and offer more active sites for lithium insertion.23–25 To date, different kinds of nitrogen-doped carbon materials, such as nitrogen-doped carbon nanofibers and nanotubes, and graphene, have been developed to show an improved capacity as compared with the undoped counterparts.24,26–31 On the other hand, carbon materials of spherical structure have been demonstrated to be competent for using as anode materials for LIBs, since spherical materials enjoy a high packing density, a low surface to volume ratio, maximal structural stability and ease of preparing electrode films.32–35 Commercial graphitized mesocarbon microbeads are one successful example. In addition, it is well known that nanopores within carbon materials can provide extra high lithiation capability, because the porous structure can not only shorten the lithium ion transport length but also facilitate the charge-transfer reaction on the electrode–electrolyte interface.27,36 Thus one can expect that porous carbon spheres with nitrogen-doping should be very promising high-capacity anode materials for LIBs. Unfortunately, such an exploration on nitrogen-doped carbon spheres for LIBs has seldom been reported so far. In this work, a simple, fast and energy-saving microwaveassisted method was employed to synthesize carbon microspheres (CSs) using sucrose dissolved in a mixed solvent of water and ethylene glycol as a precursor in a microwave system. The as-synthesized carbon microspheres were subjected to a thermal treatment under an ammonia atmosphere to fabricate porous nitrogen-doped carbon microspheres (NCSs). When used as anode materials for LIBs, the NCSs exhibit excellent electrochemical performance due to their porous structure and nitrogen-doping.
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2. Experimental In a typical process, 6.8 g sucrose and 2 g concentrated sulfuric acid were dissolved in 100 ml mixed solvent of water and ethylene glycol (6 : 4 v/v). Subsequently, 20 ml of the solution was sealed in a 35 ml microwave tube and heated at 160 °C with a maximum microwave irradiation power of 100 W for 10 min using a microwave system (Explorer 48, CEM Co.). The resulting precipitate was centrifuged, washed with deionized water, dried in a vacuum oven at 80 °C for 24 h and finally thermally treated under an ammonia atmosphere with a holding time of 2 h. The temperatures of thermal treatment were set at 500, 700 and 900 °C, and the corresponding obtained NCSs are labeled as NCS500, NCS700 and NCS900, respectively. The morphology of as-synthesized NCSs was characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The structure was characterized by X-ray diffraction (XRD, Holland Panalytical PRO PW3040/60) with Cu-Kα radiation (V = 30 kV, I = 25 mA). The nitrogen adsorption isotherm was measured at 77 K with an ASAP 2020 Accelerated Surface Area and Porosimetry System (Micromeritics, Norcross, GA) and the specific surface area, pore volume and mean pore diameter are determined by the Brunauer–Emmett–Teller (BET) method. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Imaging Photoelectron Spectrometer (Axis Ultra, Kratos Analytical Ltd) with a monochromatic Al Kα X-ray source. For electrochemical testing, as-synthesized NCSs, Super-P carbon black and polyvinyldifluoride, with a weight ratio of 80 : 10 : 10, were homogeneously mixed in N-methylpyrrolidone solvent to produce slurry. Then, the resulting slurry was coated on a copper foil using a doctor blading method. Finally, these electrodes were dried at 120 °C under vacuum for 12 h. The active material loading in the electrodes is ∼1.2 mg cm−2. Coin-type cells (CR2032) were assembled in a glove box (MB-10-compact, MBRAUN) under an argon atmosphere with oxygen and water of less than 0.5 ppm using lithium metal foil as the counter and reference electrodes. Celgard 2320 membrane and 1 M LiPF6 electrolyte solution dissolved in a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (1 : 1 : 1 w/w) were used as the separator and the electrolyte, respectively. Galvanostatic charge–discharge cycles were performed using a LAND2001A battery test system in the voltage range of 0.005–3 V. Cyclic voltammetry (CV) was performed using an electrochemical workstation (AUTOLAB PGSTAT302N) within a voltage range of 0.005–3 V at a scan rate of 0.2 mV s−1.
Fig. 1 FESEM image of NCS900. The inset shows a high-magnification image.
XRD patterns of NCS500, NCS700 and NCS900 are displayed in Fig. 2a. In a typical pattern, two broad peaks appear at ∼24° and ∼43.5°, corresponding to (002) and (100) diffraction modes of graphitic structure, which is characteristic of disordered carbon material. When the thermal treatment temperature increases from 500 to 900 °C, the (100) diffraction peak gradually becomes intense, indicating that the carbonization degree increases with the increase of temperature. As illustrated in Fig. S1 (see ESI†), the XRD patterns of CSs thermally treated at 900 °C under a nitrogen atmosphere show two broad peaks similar to that of NCS900, indicating that the ammonia treatment does not modify the graphitic structure of CSs obviously. The specific surface areas, pore volumes and mean pore diameters of NCS500, NCS700 and NCS900 were determined by the BET method and are listed in Table 1. The specific surface area increases with increasing the thermal treatment temperature and the values are 96, 447 and 1630 m2 g−1 for NCS500, NCS700 and NCS900, respectively. NCS900 has a highest pore volume of 0.77 cm3 g−1 with a pore diameter of
3. Results and discussion Fig. 1 shows the FESEM images of NCS900 (the morphologies of NCS500 and NCS700 are similar to that of NCS900 and are not shown here). It can be observed that spherical structure is obtained via the microwave-assisted reaction, and the smooth surfaced NCSs have diameters ranging from 0.5 to 1.5 μm. The
14932 | Dalton Trans., 2014, 43, 14931–14935
Fig. 2 (a) XRD patterns of various samples; XPS N 1s spectra of (b) NCS500, (c) NCS700 and (d) NCS900.
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Table 1 Surface areas, pore volumes and mean pore diameters of NCS500, NCS700 and NCS900
Table 3 The atomic percentages of pyridinic, pyrrolic and graphitic nitrogen determined from the relative areas of corresponding XPS components
Sample
Surface area (m2 g−1)
Pore volume (cm3 g−1)
Mean pore diameter (nm)
Samples
Pyridinic N (at%)
Pyrrolic N (at%)
Graphitic N (at%)
NCS500 NCS700 NCS900
96 447 1630
0.05 0.20 0.77
2.11 1.82 1.88
NCS500 NCS700 NCS900
46.7 42.3 34.0
51.9 51.5 41.3
1.4 6.2 24.7
1.88 nm. CSs thermally treated at 900 °C under a nitrogen atmosphere show a specific surface area of 1025 m2 g−1 with a pore volume of 0.47 cm3 g−1 and a mean pore diameter of 1.82 nm. The results indicate that ammonia treatment can improve the porosity of CSs, which may enhance the lithium storage capacity. An XPS analysis was performed to detect the doping of nitrogen in the NCSs. The carbon, oxygen and nitrogen contents (at%) in NCS500, NCS700 and NCS900 were determined by XPS and are listed in Table 2. The carbon content increases and the oxygen content decreases with the increase of thermal treatment temperature from 500 to 900 °C, indicating that the carbonization degree increases with the increase of temperature. The result is consistent with the XRD analysis. NCS900 has a highest carbon content of 91.5 at% and a lowest oxygen content of 2.2 at%, which may favor a high electrical conductivity, while NCS700 has a nitrogen content of 7.8 at%, higher than those of NCS500 and NCS900. NCS900 shows relatively low nitrogen content because heteroatoms are chemically unstable at such high temperatures.37,38 As shown in Fig. 2b, c and d, the XPS N 1s spectra of the NCSs thermally treated at different temperatures can be deconvoluted into three components located around 398.3, 400.2 and 402.3 eV, which are assigned to pyridinic, pyrrolic and quaternary (graphitic) nitrogen doped in carbon, respectively.23,36 The N 1s spectra are fitted with the Gaussian function and the percentages of pyridinic, pyrrolic and graphitic nitrogen are determined from the relative areas of the corresponding deconvoluted components, as summarized in Table 3. Clearly the percentage of graphitic nitrogen increases with the increase of thermal treatment temperature from 500 to 900 °C, while the percentages of pyridinic and pyrrolic nitrogen decrease. The increase of graphitic nitrogen percentage is understandable, since graphitic nitrogen is more stable than pyridinic and pyrrolic nitrogen.39,40 The graphitic nitrogen percentage in NCS900 was estimated to be 24.7%, much higher than the values (1.4% and 6.2%) for NCS500 and NCS700. CV analysis was conducted to evaluate the electrochemical behaviour of as-prepared NCSs. Fig. 3a shows the CV curves of
Table 2 The carbon, oxygen and nitrogen contents (at%) of NCS500, NCS700 and NCS900 determined by XPS
Samples
C
N
O
NCS500 NCS700 NCS900
86.4 87.9 91.5
7.2 7.8 6.3
6.4 4.3 2.2
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Fig. 3 (a) CV curves and (b) the first and second charge–discharge curves of NCS500, NCS700 and NCS900.
various electrodes in the 3rd scanning cycle. A pronounced cathodic peak near 0 V is observed for all electrodes, which is very common for lithium insertion in carbonaceous materials.41,42 Lithium removal takes place in a wide potential range, resulting in the broad anodic range (0.4–1.5 V) in the CV curves of NCS500 and NCS700.43 In the high potential range from 1.5 to 3.0 V, NCS900 displays a CV curve remarkably different from those of NCS500 and NCS700. Two additional weak anodic peaks around 1.3 and 2.4 V emerge in the curve of NCS900 and the anodic current density in the potential range from 1.5 to 3.0 V is much higher than those of NCS500 and NCS700, which suggests that other lithium storage reactions exist in NCS900. It has been reported that graphitic nitrogen could enhance the kinetic performance of the electrode because of the weakening Li–C interaction.44 NCS900 has a much higher graphitic nitrogen percentage as compared with NCS500 and NCS700. It is likely that the enhanced electrochemical reactivity of NCS900 originates from the high graphitic nitrogen content that improves the electrode reactivity on the carbon surface at high voltage range.
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Fig. 3b illustrates the first and second charge–discharge curves of NCS500, NCS700 and NCS900 at a current density of 50 mA g−1. During the first discharge, a plateau around 0.6 V was found for all electrodes, which was due to the formation of a solid electrolyte interphase and other side reactions, resulting in the large irreversible capacity in the first cycle.27,44 CSs thermally treated at 900 °C under a nitrogen atmosphere show charge–discharge curves similar to those of NCS500 and NCS700 (see Fig. S2a in ESI†) and deliver an initial reversible capacity of 174 mA h g−1. The initial reversible capacities of NCSs increase with the increase of thermal treatment temperature and are 429, 486 and 816 mA h g−1 for NCS500, NCS700 and NCS900, respectively. Therefore, nitrogen-doping has greatly improved the capacity of CSs. It is revealed in the XPS spectra that NCS500 and NCS700 have almost the same nitrogen content and components. As compared with NCS500, the relatively high capacity of NCS700 can mainly be attributed to its higher specific surface area that facilitates the lithium insertion. As for NCS900, because of its much higher graphitic nitrogen percentage and specific surface area, the enhanced reactivity on the carbon surface at high potential range (as discussed above) remarkably improves its lithium storage capacity, resulting in the charge curve at potential above ∼1.6 V being different from those of NCS500 and NCS700. The cycle performances of NCS500, NCS700 and NCS900 at a current density of 50 mA g−1 are shown in Fig. 4a. It can be seen that all the electrodes exhibit excellent cycling stability, indicating that the thermal treatment temperature does not influence the cycle performance of NCSs notably. After 50 cycles, NCS500, NCS700 and NCS900 maintain capacities of 241, 395 and 660 mA h g−1, respectively, much higher than that (155 mA h g−1) of CSs thermally treated at 900 °C under a nitrogen atmosphere (see Fig. S2b in ESI†). It should be noted that the as-prepared NCS900 achieves a capacity much higher than that (300–400 mA h g−1) of nitrogen-doped carbon nanotubes24,28 and comparable to that (684 mA h g−1) of nitrogendoped graphene.26 Furthermore, the rate performances of various NCSs are revealed in Fig. 4b. NCS900 delivers capacities of 500, 411 and 341 mA h g−1 at current densities of 150, 300 and 500 mA g−1, respectively. Even at a high current density of 1000 mA g−1, NCS900 still retains a capacity of 255 mA h g−1, showing a good rate capability. The excellent electrochemical performance of NCS900 can be attributed to its porous structure and nitrogen-doping. Firstly, the large surface area provides a sufficient electrode– electrolyte interface to absorb lithium ions and facilitates the rapid charge-transfer reaction, and large numbers of nanopores on the surface may act as reservoirs for the storage of lithium ions.27,45 Secondly, nitrogen-doping offers more active sites for lithium insertion23 and may contribute to the electrical conductivity.46 Especially, the graphitic nitrogen in the NCSs remarkably enhances the kinetic performance of the electrode, resulting in an enhanced electrochemical performance.44 Moreover, the high carbonization degree of NCS900 should be beneficial to the efficient electron transport.
14934 | Dalton Trans., 2014, 43, 14931–14935
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Fig. 4 (a) Cycle performances and (b) rate performances of NCS500, NCS700 and NCS900. The rate performance was conducted after 50 galvanostatic charge–discharge cycles at a current density of 50 mA g−1.
4.
Conclusions
In summary, NCSs were successfully synthesized via a microwave-assisted reaction followed by thermal treatment under an ammonia atmosphere and investigated as anode materials for LIBs. The results of electrochemical experiments reveal that the NCSs exhibit a high capacity and good cycle stability and rate capability. A maximum capacity of 660 mA h g−1 at a current density of 50 mA g−1 after 50 cycles is achieved for NCSs thermally treated at 900 °C. Even at a high current density of 1000 mA g−1, a capacity of 255 mA h g−1 is maintained. The NCSs should be a very promising candidate for anode materials of LIBs.
Acknowledgements Financial support from the National Natural Science Foundation of China (no. 21276087) and the Shanghai Committee of Science and Technology (11JC1403600) is gratefully acknowledged.
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