Materials Letters 231 (2018) 47–50 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue Graphene coating magnetite/N-doping carbon hybrid composites and its lithium storage performance Wenjun Xu, Weidong Xue, Yu Zhang, Bin Zhang, Yuan Wang, Rui Zhao ⇑ School of Material and Energy, University of Electronic Science and Technology of China, Chengdu 610054, PR China a r t i c l e i n f o Article history: Received 12 June 2018 Received in revised form 20 July 2018 Accepted 6 August 2018 Available online 6 August 2018 Keywords: Magnetic materials Graphene Nanocomposites Energy storage and conversion Lithium-ion battery a b s t r a c t Here graphene coating magnetite/N-doped carbon hybrid materials (G@Fe3O4/NC) have been synthesized. Structural characterizations revealed that Fe3O4/N-doped carbon was wrapped tightly and uniformly by graphene nanosheets via p-p interactions, which preventing the volume expansion of the material during the charge/discharge processes. G@Fe3O4/NC hybrid composites exhibited a large discharge specific capacity of 958 mAh g1 after 65 cycles at a current density of 200 mA g1. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction In recent years, transition metal oxides such as Co3O4, SnO2, MnO2, ZnO and Fe3O4 have been attracted as LIB anodes because of their high specific capacity (500–1000 mAh g1) in comparison with the commercial graphite (372 mAh g1) [1–3]. Fe3O4-based anode material is one of the most promising candidate anodes in future LIB anodes due to the high theoretical capacity (926 mAh g1), low cost, nature abundance and environmental benignity [4,5]. However, its practical application is still hampered by the pulverization problem, which arises from its large volume change (200%) during lithiation/delithiation . Numerous approaches have been developed to circumvent these shortcomings. Synthesizing nano-structure morphology and coating conductive materials on Fe3O4 surface are two main effective strategies of them [7,8]. In particularly, graphene which has a high surface area, atomicscale thickness, stable physical-chemistry property, high thermal conductivity, excellent mobility of charge carries as well as superior electrical conductivity has been considered as the first choice of a composite material for LIB applications [9,10]. Because of Van der Waals interactions between graphene nanosheets, chemically prepared graphene generally suffers from serious reuniting and restacking after the removal of solvent or drying . Therefore, exploring a facile and feasible way to further reducing the Van der Waals interactions between graphene nanosheets can ⇑ Corresponding author. E-mail address: email@example.com (R. Zhao). https://doi.org/10.1016/j.matlet.2018.08.023 0167-577X/Ó 2018 Elsevier B.V. All rights reserved. effectively enhance the cycling stability of active materialgraphene composites in LIBs. To satisfy the above requirement of active material-graphene composites, we provide a novel method for fabricating graphene coating magnetite/N-doped carbon hybrid materials (G@Fe3O4/ NC) via the p-p interactions. Specifically, the as-prepared G@Fe3O4/NC not only overcomes the large volume change of Fe3O4, but also reduces the reuniting and restacking of graphene nanosheets. Therefore, the G@Fe3O4/NC shows a good electrochemical performance as an anode material in LIBs. 2. Experimental methods Graphene oxide (GO) was synthesized according to the reference with mirror modification . Fe3O4/NC composites were prepared by one-step solvothermal method and annealing process. In a typical process, 4,40 -bis(3,4-discyanophenoxy)biphenyl (Bph) (0.5 g), FeCl36H2O (3.75 g), sodium acetate anhydrous (NaAc) (9.0 g) and ammonium molybdate ((NH4)6Mo7O244H2O) (0.1 g) were added into an ethylene glycol (EG) (100 mL) and polyethylene glycol 2000 (PEG 2000) (2.5 g) mixture. After ultrasonic and vigorously stirred for 30 min, the brown mixture was sealed in a Teflon-lined stainless-steel autoclave and solvothermal treated at 200 °C for 24 h. After cooling to room temperature in air, the black products were obtained by magnetic separation, washed several times with deionized water and ethanol, and dried at 80 °C overnight. Subsequently, the black products were annealed at 300 °C for 5 h and 550 °C for 5 h under argon gas flow with a heating rate 48 W. Xu et al. / Materials Letters 231 (2018) 47–50 of 2 °C min1 to get Fe3O4/NC. Phthalocyanine oligomerfunctionalized Fe3O4 (Fe3O4/Pc) was obtained without annealing process. G@Fe3O4/NC composites were prepared via the p-p interactions, followed by hydrothermal and annealing process. In a typical process, Fe3O4/Pc (0.2 g) was slowly dropped into the undried GO solution (150 mL) (0.4 mg mL1) with magnetic stirring for 24 h. After hydrothermally treated in a Teflon-lined stainless-steel autoclave at 180 °C for 24 h, the precursor was washed, dried and annealing as the same as the above to obtain G@Fe3O4/NC. 3. Results and discussion Fig. 1 shows the SEM and TEM images of Fe3O4/NC nanoparticles and the composite with graphene. Uniform Fe3O4/NC nanoparticles of about 100 nm in size were directly synthesized by solvothermal reaction and annealing process as shown in Fig. 1(a and b). It can be seen that the Fe3O4/NC composites which consisted of Fe3O4 nanoparticles were coated uniformly with amorphous N-doped carbon layer. As shown in Fig. 1(c and d), it is obvious that thin graphene nanosheets had been successfully wrapped around the Fe3O4/NC nanoparticles, which prevented the Fe3O4/NC nanoparticles from aggregating. In addition, the Fe3O4/NC nanoparticles became irregular after wrapped by graphene nanosheets. Fig. 2(a) shows the X-ray diffraction (XRD) pattern of the asprepared G@Fe3O4/NC and Fe3O4/NC composites. All the characteristic peaks are in good agreement with Fe3O4 crystal. The peaks can be ascribed to the standard XRD data (JCPDS card no.19-0629) of Fe3O4. There were no obvious diffraction peaks corresponding to carbon or other impurities. The chemical composition and surface oxidation state of G@Fe3O4/NC were analysized by XPS. Fig. 2(b) showed the survey XPS spectrum of G@Fe3O4/NC that contained Fe, O, C and N. The high-resolution XPS spectrum of Fe 2p (Fig. 2c) showed two main binding energy peaks cantered at 710.7 and 724.5 eV, which were corresponding to the electronic states of Fe 2p3/2 and Fe 2p1/2, respectively. The two peaks could be fitted with two pair spin-orbit doublets those were the characteristic peaks of Fe2+ and Fe3+ . The ratio of Fe3+/Fe2+ calculated from the peak area was 1.9:1, which was close to the theoretical value of 2:1 in Fe3O4. It should be noted that the absent of satellite peaks situated at 718.4 and 732.4 eV excluded the possibility of surface oxidation of Fe3O4 . The main deconvoluted peaks in the O 1 s spec- Fig. 1. SEM and TEM images of (a and b) Fe3O4/NC, (c and d) G@Fe3O4/NC. W. Xu et al. / Materials Letters 231 (2018) 47–50 49 Fig. 2. (a) XRD of G@Fe3O4/NC and Fe3O4/NC, (b–f) XPS of G@Fe3O4/NC. Fig. 3. (a) CV curves of G@Fe3O4/NC. (b) Galvanostatic charge-discharge curves of G@Fe3O4/NC. (c) Cycling performance of G@Fe3O4/NC (red) and Fe3O4/NC (black). (d) Rate performance of G@Fe3O4/NC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 50 W. Xu et al. / Materials Letters 231 (2018) 47–50 trum at 530.1 and 530.7 eV originated from Fe-O bonding in G@Fe3O4/NC, which probably arose from the mixed state of FeO and Fe2O3 in Fe3O4 (Fig. 2d). The peaks at 531.9 and 533.5 eV could be attributed to carbonyl and adsorbed H2O, respectively. In the C 1 s spectrum (Fig. 2e), the four peaks at 284.5, 285, 286.3 and 290 eV were assigned to CAC, C@C, C@N and OAC@O, respectively. In Fig. 2f, the main fitted N 1 s peaks at 397.5, 398.2 and 399.5 eV represented the Fe-N, @NA and pyrrolic-N in the hybrid, which probably arose from the carbonation of phthalocyanine oligomer. The cyclic voltammogram (CV) of G@Fe3O4/NC composites was tested at a scanning rate of 0.1 mV s1 between 0.01 and 3.0 V (vs. Li/Li+). As shown in Fig. 3(a), a small gap was observed at 1.42 V in the first cycle, which is attributed to the irreversible reaction with the electrolyte. The curve of the first cycle was different from the rest with respect to the rapid disappearance of the two obvious cathodic peaks at 0.85 and 0.60 V, which corresponded to insertion of Li+ and reduction of Fe3+ and Fe2+ to Fe0, respectively. The above process involved the lithiation reaction of Fe3O4 and the formation of solid electrolyte interface (SEI) layer. In subsequent cycles, these peaks shifted to 1.18 and 0.78 V, indicating structural variations in the G@Fe3O4/NC electrode after Li-ion insertion in the first cycle. The anodic peak at 1.80 V corresponded to the reversible oxidation of Fe0 to Fe2+ and Fe3+, which was not shifted apparently in subsequent cycles as the cathodic peak. The CV curves were found to be nearly coincided after the first cycle, which indicated a good electrochemical reversibility and stability of the G@Fe3O4/NC composites. The galvanostatic charge-discharge profiles for the initial cycle numbers (1–3 cycles) and the 50th cycle of G@Fe3O4/NC composites over the voltage range 0.01–3.0 V at a current density of 200 mA g1 have been shown in Fig. 3(b). The obvious potential plateau was located at 0.75 V in the first discharge, and thereafter shifted to 0.90 V. The sloping plateaus at 1.50–2.10 V were observed in the charging processes. It can be found that all of the plateaus were corresponding to the peaks of CV curves. In the first cycle, the discharge/charge capacity was 1310 and 863 mAh g1, respectively. The initial capacity loss in the first cycles might be attributed to irreversible lithium loss during the formation of SEI layer and further lithium consumption [14,15]. The cycle performance of Fe3O4/NC and G@Fe3O4/NC composites were also test at a constant current density of 200 mA g1 in the voltage window of 0.01–3.0 V (vs. Li/Li+) (Fig. 3c). The coulombic efficiency of G@Fe3O4/NC was about 65% in the first cycle, then reached 96% in the second cycle and finally remained relatively stable around 98% in all the subsequent cycles. The reversible capacity of G@Fe3O4/NC demonstrated a high capacity of 958 mAh g1 after 65 cycles, which was far superior to the reversible capacity of Fe3O4/NC (about 140 mAh g1 after 65 cycles). The pos- sible reasons are as follows: Firstly, Fe3O4/NC nanoparticles wrapped by graphene nanosheets prevent the volume expansion of the Fe3O4 during charge/discharge processes, which effectively enhance the cycling stability of active material-graphene composites in LIBs. Secondly, the novel synthetic method greatly reduces the serious reuniting and restacking of graphene, which also effectively enhance the cycling stability of G@Fe3O4/NC in LIBs. Thirdly, phthalocyanine oligomer carbonation could enhance the conductivity, which effectively enhance the specific capacity of G@Fe3O4/NC. The rate performance of G@Fe3O4/NC composites was shown in Fig. 3(d). The composite showed a specific capacity of 920, 900, 850, 700 and 550 mAh g1 at the current density of 50, 100, 200, 500, 1000 mA g1, respectively, and can recover to the capacity of 1000 mAh g1 when the current was set back to 50 mA g1. 4. Conclusion In this study, G@Fe3O4/NC composites were successfully synthesized by a novel route based on the p-p interactions. The sample showed high specific reversible capacity (1310 mAh g1 in first cycle), high coulombic efficiency (>97%), and stable rate capability (550 mAh g1 at 1000 mA g1) due to the wrapping by Ndoped carbon and graphene. 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