One step synthesis of Fe2O3/nitrogen-doped graphene composite as anode materials for lithium ion batteries
Introduction
Graphite is widely commercialized as anode material for lithium ion batteries (LIBs) [1]. Nevertheless, the low theoretical capacity of graphite (372 mAh g−1) limits its applications in high-power electric vehicles [2]. New breakthroughs need be pursued intensively in electrode materials to achieve superior performance. Metal oxides, such as SnO2, Fe2O3, Fe3O4, Co3O4, Mn3O4 and so on, have been developed as new alternative anode materials [3], [4], [5], [6], [7]. Among them Fe2O3 with high theoretical specific capacity (1005 mAh g−1), low cost and abundant reserve has been regarded as a potential anode material for LIBs. However, the low electrical conductivity and the poor cycling performance due to huge volume changes occurred during lithium insertion/extraction process of Fe2O3 particles impede its practical applications [8]. Recent researches indicated that nanostructured electrode materials or carbon coating could help to solve the above two problems. Liu and coworkers synthesized Fe2O3 nanorods with diameter of 60–80 nm, which showed a stable specific capacity of 800 mAh g−1 [9]. Zhao et al. creatively coated the Fe2O3 with single-walled carbon nanohorns, which revealed excellent rate performance and cycle stability [10]. Unfortunately, these strategies just delayed the pulverization of the metal oxide in some degree. In a long time of cycling, the crush of the anode materials still happened. Graphene has been used as matrix to combine with metal oxide for LIBs due to its superior electrical and mechanical properties, large specific surface area (∼2600 m2 g−1) and chemical stability [11], [12]. Wang and coworkers successfully obtained Fe2O3/graphene composite (Fe2O3/G) as the anode materials for LIBs, delivering a high discharge capacity of 660 mAh g−1 during up to 100 cycles at the current density of 160 mA g−1 and good rate capability [13].
Recently, people utilized nitrogen-doped graphene for lithium anode battery application and received excellent capacity and cyclic performance [14], [15]. Nitrogen-doped graphene still kept good mechanical properties as pristine graphene. Additionally, the doped nitrogen atoms can provide extra lone pair electrons and as a result, the electron density of graphene will be augmented which makes the electrical conductivity improved than that of the pristine graphene [16], [17]. Moreover, graphene containing nitrogen atoms can induce many surface defects, and as a result the lithium ion can be much easier to get inserted/extracted, leading to the increase of the reversible capacity [14]. Reddy et al. demonstrated that nitrogen-doped graphene prepared by CVD could deliver double capacity compared to that of the pristine graphene [18]. Theoretical studies indicated that the doped nitrogen atoms can be divided into three kinds, pyridinic-like, pyrrolic-like and graphitic-like nitrogen atoms [19], [20], [21]. Li et al. demonstrated that the adsorption energy of lithium ions at the pyridinic-like defects was quite large and the energy barrier for lithium penetrating the defects was very low that the capability for lithium storage would be greatly enhanced with such structure [22]. Besides, the nitrogen-doped graphene provides more active and nucleation sites, which facilitate the morphology and particle size control of the hybrids.
Herein, we utilized one-step hydrothermal method to synthesize the Fe2O3/nitrogen-doped graphene composite (Fe2O3/N-G). TEM revealed that the Fe2O3 particles with diameter of 100–200 nm uniformly decorated on the nitrogen-doped graphene. The XPS results demonstrated that the nitrogen atoms successfully doped in the Fe2O3/N-G after hydrothermal procedure. With the synergistic effect of Fe2O3 and nitrogen-doped graphene, the composite showed a high reversible specific capacity, superior rate capability and outstanding cycling stability as anode materials for LIBs.
Section snippets
Preparation of samples
Graphene oxide (GO) was prepared by the modified Hummers method [3], [23]. In a typical synthesis, 1.6 g of Fe(NH4)2(SO4)2·6H2O was added into 100 mL GO solution (1 mg mL−1) followed by stirring for 10 min. Then, the solution was transferred into a Teflon-lined autoclave and heated at 180 °C for 6 h. Finally, the resulting composite was washed with deionized water for several times, and then dried in a vacuum oven at 60 °C for 10 h. The Fe2O3/G was prepared for comparison in the same procedure while
Results and discussion
The XRD patterns of the GO, Fe2O3, Fe2O3/G and Fe2O3/N-G are shown in Fig. 1a. Due to the presence of oxygenated functional groups, the d-spacing of GO calculated from the (0 0 1) peak is approximately 0.85 nm (2θ = 11°) similar to the reported result [24]. The sharp peaks of Fe2O3, Fe2O3/G and Fe2O3/N-G indicate that the Fe2O3 particles are well crystallized (JCPDS 33-0664). GO has been converted to graphene in the hydrothermal process. The peak of graphene at ∼25° almost overlaps with the main
Conclusions
In summary, Fe2O3/N-G has been synthesized as anode materials for LIBs by one-step hydrothermal procedure. The presence of nitrogen-doped graphene helps Fe2O3 particles formed with smaller size and homogenous distribution. The hybrid composite with good electrical conductivity shows a high reversible specific capacity, superior rate capability and outstanding cycling stability as anode materials for LIBs. After 100 cycles the reversible capacity of Fe2O3/N-G anode still remains 1012 mAh g−1,
Acknowledgements
This work is supported by the National Basic Research Program of China (2012CB932303) and the National Natural Science Foundation of China (Grant No. 51072215 and 51172261).
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