Elsevier

Carbon

Volume 79, November 2014, Pages 310-320
Carbon

High capacity Li storage in sulfur and nitrogen dual-doped graphene networks

https://doi.org/10.1016/j.carbon.2014.07.072Get rights and content

Abstract

Three-dimensional (3D) networks composing of S and N dual-doped graphene (SNG) were synthesized by a chemical vapor deposition approach using MgSO4-containing whiskers as templates and S source and NH3 as N source. Energy dispersive spectrometer mapping and X-ray photoelectron spectroscopy coupled with Raman analysis have revealed that S and N atoms with concentrations of 5.2 and 1.8 atom%, respectively, have been substitutionally incorporated into the graphene networks via covalent bonds. The SNG, as an anode material for lithium ion batteries (LIBs), exhibits extremely high capacity (3525 mAh/g at the current density of 50 mA/g) and superior rate capability (870 mAh/g at 1000 mA/g) with excellent cycling stability (remaining a reversible capacity of 400 mAh/g at 10 A/g after 2500 cycles). The enhanced conductivity, the 3D porous network with many disorders and the intrinsically high Li storage capacity of S and N-doped carbon segments have led to the excellent electrode performance of the SNG networks. The effects of binder content and calendaring pressure on the electrode performance have been investigated. The full LIB with SNG as anode and LiCoO2 as cathode can afford a high reversible capability (164 mAh/g at 0.2 C) and good cycling stability.

Introduction

Commercialization of Li ion batteries (LIBs) has been realized since graphite is used as anodes instead of Li discs, due to the excellent cycling stability of graphite. However, the limited Li storage capacity of graphite (up to 375 mAh/g) cannot satisfy ever increasing demand in the fields of electronics, vehicles, and storage of renewable energy. Carbon anodes with both high capacity and excellent cycling stability are in urgent need especially for the next-generation high capacity energy storage devices, such as Li–O2 batteries [1], [2] and Li ion capacitors (LIC) [3], [4].

To obtain high performance anodes, reforming carbon materials by designing nanostructures or heteroatom doping has been widely investigated. It has been demonstrated that reduced graphene oxide (RGO) can exhibit reversible capacity up to 1264 mAh/g [5], [6], [7], [8], [9], which is much higher than the capacity of traditional graphite. However, the irreversible stacking of graphene always occurs during direct thermal annealing due to the strong π-interactions. As the results, RGO usually composes of multi-layered graphene, which delivers a much lower Li storage capacity than the single layer graphene [10]. RGO anodes also suffer from low initial coulombic efficiency and fast capacity fading due to the existence of oxygen-containing functional groups [11], [12], [13], [14], [15]. Among the many efforts to fabricating stable porous graphene assemblies, the three dimensional (3D) graphene networks produced by chemical vapor deposition (CVD) processes have been reported to exhibit a highly porous structure and much higher electronic conductivity as compared to RGO. On the other hand, S or N doping of carbon materials has been found to be capable of significantly promoting the Li storage capacity [16], [17], [18]. Heteroatom doped carbons contain more defects through which Li ions can perpendicularly diffuse from outside to inside graphite layers, thus providing more storage regions [18]. Combining both the elaborate structure control and the heteroatom doping technique in CVD synthesis is promising to achieve a graphene-constructed material with extraordinary electrode performance. However, to be used as an electrode material, a scalable synthesis approach is necessary, which is difficult to be realized by the graphene growth on the limited surfaces of Cu foils or Ni foams.

In previous work, we have realized scalable synthesis of two dimensional nanomesh graphene by a CVD process using porous MgO layers as templates [19]. Here, we further present a scalable CVD synthesis of 3D networks composing of S and N dual-doped graphene (SNG), which exhibit an extraordinarily high Li storage capacity. The SNG was produced by a one-step CVD approach using MgSO4-containing whiskers as both templates and S source, and NH3 as N source. 3D networks composing of few-layered graphene were formed due to carbon deposition on the surface of the porous whisker templates. Energy dispersive spectrometer (EDS) mapping and X-ray photoelectron spectroscopy (XPS) analyses coupled with Raman analysis confirm that the S and N atoms have been evenly incorporated into the graphene frameworks via covalent bonds. As anode for LIBs, the SNG exhibits extremely high Li storage capacity (3525 mAh/g at the current density of 50 mA/g) and excellent cycling stability (remaining a reversible capacity of 400 mAh/g at 10 A/g after 2500 cycles), showing much better electrochemical performance as compared to the undoped graphene networks and the sole N or S-doped graphene networks. The reasons for the promotion are discussed. A full cell configuration with the SNG as anode and LiCoO2 as cathode, exhibits a high reversible capacity (164 mAh/g at 0.2 C) and excellent cycling performance.

Section snippets

Experimental

The as-synthesized basic magnesium sulfate whiskers are calcined at 650 °C and 1100 °C for 1 h to obtain porous MgSO4-containing and porous MgO whiskers, respectively. SNG is synthesized by a CVD approach using C2H4, NH3 and the as-produced porous MgSO4-containing whiskers as C source, N source and templates, respectively. In a typical synthesis, 20 g MgSO4 whiskers are fed into a vertical quartz reactor from the top hopper after the reaction temperature reached 650 °C in Ar flow. Then C2H4 and NH3

Results and discussion

The synthesis process of SNG is illustrated in Fig. 1. Cylindrical MgSO4-containing porous whiskers are prepared by calcining of basic magnesium sulfate whiskers, which serve as both templates and S source in a CVD process. 3D graphene networks are formed on the surface of the porous whiskers by C2H4 cracking, followed by an acid washing to remove the templates. Basic magnesium sulfate (5 Mg(OH)2·MgSO4·3H2O) whiskers were prepared by a previously-reported hydrothermal synthesis [20]. As shown in

Conclusions

In summary, 3D networks composing of S and N duel doped few-layered graphene have been synthesized by a CVD approach using the porous MgSO4-containing whiskers as templates and S source. The as-obtained SNG has a high S doping level (5.2 atom%) with a uniform distribution of S and N atoms in the highly porous networks. As anodes for LIBs, the SNG exhibits ultrahigh reversible capacity (3525 mAh/g at the current density of 50 mA/g), excellent rate performance and a long cycle life (more than 2500

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 21206191) and the Science Foundation of China University of Petroleum, Beijing (No. 2462013YXBS007).

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