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C hollow nanospheres
Journal of Electroanalytical Chemistry 855 (2019) 113626
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Interfacial anchoring effect for enhanced lithium storage performance of sesame balls-like Fe3O4/C hollow nanospheres Lanxuan Dai, Wenlong Li, Kehan Zhou, Dongmei Tang, Yue Han, Xiaoyu Wu, Huayu Wu, Guowang Diao, Ming Chen * School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fe3O4/C hollow nanospheres In-situ growth Anchoring effect Thermal decomposition Lithium ion battery
In this paper, hollow mesoporous carbon nanospheres (HMCNs) are used as the growth skeleton. Fe3O4 nanoparticles (Fe3O4 NPs) with a diameter of 10 nm grow in situ on the shell of HMCNs to form a new type of sesame balls-like Fe3O4/C hollow nanospheres (HNSs) by thermal decomposition reaction. The Fe3O4/C HNSs display great cycling stability as anode material in lithium ion battery. At a current density of 1 A g1, the reversible specific capacity of Fe3O4/C HNSs is as high as 946 mA h g1 after 250 cycles. Compared with pure Fe3O4 NPs, the electrochemical properties of the Fe3O4/C HNSs are significantly improved. The anchoring of Fe3O4 NPs on the shell of HMCNs can avoid the aggregation of Fe3O4 NPs, increase the lithiation sites, and accelerate the rapid migration of Li ions. In addition, the framework of Fe3O4/C HNSs, i.e. HMCNs, can promote the conductivity of composite and boost the delivery of electron. This notion and expedient method of construction can be broadened to synthesize other hollow nanostructured material with preferable electrochemistry performance.
1. Introduction With the development of nanotechnology, nanomaterials, especially transition metal oxides nanomaterials, have attracted more attention in the field of lithium ion batteries (LIBs) for energy storage and potential substitutes for graphite owing to their excellent properties [1,2]. As the anode material for LIBs, nanosized particles of transition metal oxides are likely to aggregate and overgrow, leading to the loss of active sites involved in electrochemical reactions to reduce the specific capacity. In addition, the low electrical conductivity can seriously hinder the transport of electrons or ions in batteries or reduction reaction. Among these metal oxides, Fe3O4 has gotten the most extensive focus in virtue of the outstanding theoretical capacity (926 mA h g1), cheap cost, earth plentitude and ecological amity [3–5]. Nevertheless, Fe3O4 as the anode material for lithium batteries still exists some problems (the large irreversible capacity loss, poor cycle stability and low rate capacity, etc) owing to the low electric conductivity and severe volume expansion during Liþ insertion and extraction process [6]. Moreover, Fe3O4 has the similar problems with other transition metal oxides, such as surface instability for the pulverization of active materials, which are attributed to obvious phase and morphological changes associated with typical conversion reactions.
Many strategies have been used to settle the aforesaid troubles. Among those methods, the carbon-coated has been deemed to the most effective way to ameliorate the conductive properties and alleviate the characteristics of volume expansion [7]. The merit of carbon coatings is as follows. First, the carbon layer is propitious to remain intact of Fe3O4 body morphology and prevent from chalking agglomeration. Second, the electrical conductivity of the composite is enhanced by the carbon coating. Third, the outstanding and steady SEI (solid electrolyte interface) film will come into being due to the carbon coating layer. The specific capacity and cycle stability of nitrogen-carbon doped materials can be better enhanced [8]. For example, He et al. reported that Fe3O4 quantum dots (5–10 nm) were embedded porous carbon microspheres via a facile micelle-colloid template method. The Fe3O4@C electrode materials deliver a stable capacity of 601 mA h g1 at 2 A g1 even after 800 cycles [9]. Chen et al. synthesized hollow carbon nanospheres embedded with ultrafine Fe3O4 nanoparticles (Fe3O4@HCNS) by using carboxyl functionalized polystyrene latexes as template and poly dopamine as carbon precursor. Fe3O4@HCNS showed excellent cycling stability and high-rate capability (475 mA h g1 at 5 A g1) [10]. In these carbon substances, hollow mesoporous carbon nanospheres (HMCNs) have aroused the great attention, owing to their good electrical conductivity, perfect mechanical properties, porous shells, accessible
* Corresponding author. E-mail address: [email protected] (M. Chen). https://doi.org/10.1016/j.jelechem.2019.113626 Received 20 August 2019; Received in revised form 1 November 2019; Accepted 4 November 2019 Available online 11 November 2019 1572-6657/© 2019 Elsevier B.V. All rights reserved.
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Cyclic voltammetry (CV) testings were measured through an electrochemical workstation (CHI660E, Chenghua, CHN) with range of the voltage is 0.01–3.0 V and the scanning rate of 0.1 mVs1. The electrochemical impedance spectroscopy (EIS) measurement was performed on the Autolab Electrochemical Analyzer (Ecochemie, Netherlands). The charge and discharge performance and stability of cells were tested with the CT-3008W and the test system (CT-3008W, Xin Wei, CHN) at different current densities and the Voltage range is 0.01–3.0 V.
interior space, and large specific surface area [11]. Moreover, HMCNs use polyporous shells to offer giant hole to accommodate guest compounds, i.e., precursor compounds of iron oxides can be incorporated into HMCNs materials [12–15], which will reduce the aggregation of nanoparticles. The structure in combination with HMCNs has provided a new strategy to solve the shortcomings existing in electrode materials, such as the sharp volume change, low electric conductivity and low rate capacity. In this paper, a simple thermal decomposition method was successfully used to fabricate Fe3O4/C hollow nanospheres (HNSs). The Fe3O4 nanoparticles (NPs) with a diameter of 10 nm on average were anchored on the shell of the HMCNs to form the sesame balls-like Fe3O4/C HNSs. The well structure-designed Fe3O4/C HNSs can provide much more lithium storage sites and accelerate the rapid migration of Li ions due to the interface effects of small nanoparticles. The strong interfacial anchoring of Fe3O4 NPs on the shell of HMCNs has a good constraint effect on Fe3O4 NPs without the nanoparticle aggregation, loss and accumulation, which can maintain the integrity structure of composite. Compared with uncoated Fe3O4 NPs, the electrochemical properties of the sesame balls-like Fe3O4/C HNSs are significantly improved. This strategy can be applied to the design and synthesize of other metal material/hollow nanostructured composites, which are one of suitable materials for energy storage and conversion field with preferable electrochemistry performance.
2.4. Preparation of Fe3O4/C HNSs composite The synthetic strategy for forming the Fe3O4/C HNSs is illustrated in Fig. 1. The particular synthetic method of hollow mesoporous carbon nanospheres (HMCNs) was shown in Supporting Information. 0.1 g HMCNs and 0.353 g ferric acetylacetonate were dispersed in the solution of 5 ml dibenzyl ether and 5 ml oleamine by ultrasonic method. Then the solution was heated at 300 C for 1 h with the protection of argon. After the reaction was finished, the composites were centrifugated and washed by ethanol, and the solid phase was dried. Pure Fe3O4 NPs were synthesized using the same reaction condition without HMCNs (Supporting Information). 3. Results and discussion
2. Experimental section 3.1. Characterization of Fe3O4/C HMCNs 2.1. Materials The morphologies of Fe3O4 NPs and sesame balls-like Fe3O4/C hollow nanospheres are demonstrated in Fig. 2. Fig. 2(a) demonstrates a representative TEM image of Fe3O4 NPs with diameter of about 10 nm. Fig. S1 shows TEM image of Fe3O4 NPs with a lower magnification. Fig. 2(b and c) exhibit the representative TEM images of HMCNs. The diameter of HMCNs is about 300 nm, the thickness of carbon wall is approximately 35 nm. The Brunauer-Emmett-Teller (BET) analysis shows that the specific surface area of HMCNs is about 770.7 m2 g1 and the main pore size is about 2.1 nm in diameter (Figs. S2a and b). Fig. 2 (d, e, f) show the TEM pictures of Fe3O4/C HNSs with distinct magnifications. The reactant ions enter the inner cavity of HMCNs through the mesoporous. Fig. S3 shows the SEM image of Fe3O4/C HNSs with low magnification. In situ growth of Fe3O4 NPs on the inner and outer wall of HMCNs retains the original composition, structure and morphology of Fe3O4 NPs in Fig. 2(a), forming a new type of sesame balls-like Fe3O4/C HNSs. The growth of
All chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents used in this paper were of analytical grade and used without further purification. The electrolyte solution was bought from Guangzhou Tian chi material Science and Technology Ltd. Other commercial solvents and chemicals are reagent level and made use according to the instructions. 2.2. Characterization Field-emission scanning electron microscopy (FESEM) was carried out by Hitachi S-4800 (Japan). Transmission Electron Microscopy (TEM) was underway by JEOL JEM-2100 instrument. High-resolution TEM (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed on FEI Tecnai G2 F30 STWIN (USA) operating at 300 kV. Raman was conducted on Renishaw by Raman spectroscope. X-ray diffraction (XRD) was in progress by a graphite monochromator and Cu Kα radiation (λ ¼ 0.1541 nm) on D8 advance superspeed powder diffractometer (Bruker). Thermogravimetry analysis (TGA, Pyris 1 TGA, PerkinElmer, USA) was tested in the air. X-ray photoelectron spectroscopy (XPS) was conducted by Thermo Escalab 250 system using Al Ka radiation (hν ¼ 1486.6 eV). The pressure of test chamber was maintained below 2 109 Torr during spectral acquisition. After drying at 100 C for 4 h, the specific surface area and pore size distribution were recorded at automatic surface area and porosity analyzer by BET technique (ASAP 2020, HD88) (- 196 C). 2.3. Electrochemical tests Lithium storage performance test was characterized by assembling Fe3O4 NPs and Fe3O4/C HNSs materials into button cell (CR 2032 coin battery). Fe3O4/C HNSs (80%) and acetylene black (10%) as conductive materials, the slurry was dissolved in N-methyl-2-pyrrolidone (NMP) by magnetic stirring for 6 h with 10% PVDF (10%) as binder. Then, the sizing agent was spread on the Cu foil current collector and dried in oven. The semi-cell makes up of an experimental anode with lithium-foil used as cathode, 1 M LiPF6 of EMC/DMC/EC (Volume ratio is 1:1:1) used as electrolyte. It is assembled in a glove box with a high purity argon (Cacuum Atmosphere Co., Ltd.).
Fig. 1. Formation of sesame balls-like Fe3O4/C HNSs. 2
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Fig. 2. TEM images of (a) Fe3O4 NPs, (b, c) HMCNs, (d, e, f) Fe3O4/C HNSs.
HNSs were explicated by X-ray diffraction (XRD) (Fig. 4(a)). The diffraction peaks of two materials emerge at 30.0 (200), 35.4 (311), 43.0 (400), 57.0 (511) and 62.5 (440) are according with the data of PDF # 19–0629 for Fe3O4, which confirms that in-situ growth of Fe3O4 NPs on the wall of HMCNs reserves the original composition of Fe3O4 NPs. HMCNs, Fe3O4 NPs and Fe3O4/C HNSs were characterized by FT-IR absorption spectra (Fig. 4(b)). The chief characteristic peaks of pure Fe3O4 are designated as below: the absorption band at 568 cm1 is designated as the tensile vibration mode of Fe–O at the oxygen octahedral position [17]. In the carbon framework, three strong absorption peaks near 1583, 1200 and 813 cm1 are designated as the stretching vibrations of –OH, C–O and C–H of HMCNs. As a result of the coordination of Fe with C–O via interfacial anchoring, the energy of Fe–O bond increases and the peak shifts to 575 cm1 in Fe3O4/C HNSs. As illustrated in Fig. 4(b), the peaks of –OH, C–O and C–H also have obvious deviation [18–21]. The small displacement of peaks is ascribed to the interaction between Fe3O4 and oxygen-containing groups in HMCNs, which is conducive to the anchoring of Fe3O4 NPs on HMCNs [22]. These results
Fe3O4 NPs on the inner and outer walls of HMCNs can be able to increase the electronic conductivity of Fe3O4 NPs and avert the polymerization of Fe3O4 NPs during the synthesis or charge/discharge processes. In order to display the structure of the nanoparticles, the sample was analyzed by high-resolution TEM and selective electron diffraction. Fig. 3(a and b) show that Fe3O4 NPs grow on the wall of HMCNs homogeneously. Lattice resolved HRTEM images of Fe3O4 are shown in Fig. 3(b). The distinct and consecutive lattice fringes state clearly the superior crystallinity of the composite. The results show that the lattice spacings of 0.25 and 0.29 nm are assigned to the plane of (311) and (220), respectively in Fig. 3(b1) and (b2) [16]. The electron diffraction pattern (SAED) of the selected region is shown in Fig. S4 and the diffraction rings correspond to the plane of (311) and (220). Fig. 3(c) is a high-power dark field image of Fe3O4/C HNSs. Fig. 3(d, e, f) exhibit the distribution of C, Fe, and O. The whole spherical region distinctly shows the presence of O and Fe. Therefore, the sesame balls-like Fe3O4/C HNSs were determined by HRTEM and EDX element mappings. The crystal structure and phase purity of Fe3O4 NPs and Fe3O4/C
Fig. 3. (a, b) HRTEM images of Fe3O4/C HNSs, (b1, b2) the lattice spacings in HRTEM images, (c) HAADF-STEM image of Fe3O4/C HNSs and (d, e, f) element mappings of C, Fe, O. 3
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Fig. 4. (a) XRD patterns of Fe3O4 NPs, Fe3O4/C HNSs and PDF # 19–0629 card. (b) FTIR spectra of HMCNs, Fe3O4 NPs and Fe3O4/C HNSs. (c) Raman spectrum of Fe3O4/C HNSs. (d) TGA curve of Fe3O4/C HNSs.
isothermal curves of the two samples are calculated by BET (BrunancrEmmett-Teller) way and the pore size distribution of the samples are calculated through BJH (Barrett-Joyner-Halenda) way. The result indicates that the specific area of HMCNs is approximately 770.70 m2 g1 and the chief aperture is approximately 2.10 nm (Figs. S2a and b). Pore structure of HMCNs with large specific surface area can supply the position for the deposition of Fe3O4 NPs during hydrothermal process. The N2 adsorption-desorption isotherm curves of Fe3O4 NPs and Fe3O4/C HNSs belong to type IV, and there is an obvious hysteresis ring when P/P0 is between 0.5 and 1.0, which indicates the existence of mesoporous structure in the two samples [25]. The specific area of Fe3O4/C HNSs is calculated to be 400.88 m2 g1 higher than that of Fe3O4 NPs (59.46 m2 g1), which is attributed to the hollow structure of composite. BJH method was used to calculate the aperture distribution curve through the desorption isotherm (see the illustrations in Fig. 5) [26]. The pore distribution of Fe3O4/C HNSs is concentrated in 1.5–6 nm indicating the existence of micropore and mesoporous in the sample. Fe3O4/C HNSs have large BET area and micropore/mesoporous structure, which provide more lithium insertion sites to enhance the lithium storage capacity, and improve the Liþ diffusion path to ensure the rate performance of
indicate that Fe3O4 NPs have been successfully anchored on HMCNs. Raman spectrum is shown in Fig. 4(c). The two absorption bands at 1590 and 1350 cm1 are diagnostic peaks of the typical Raman spectra of carbon, which can be attributable to the G and D bands of carbon, severally. ID/IG (peak strength ratio of D band and G band) can be served to evaluate the graphitization degree of carbon components of composites [8]. The smaller the ID/IG is, the higher the graphitization degree is. The peak strength ratio of D band and G band of Fe3O4/C HNSs is 0.91, which indicates that the degree of carbon shell layer graphitization in Fe3O4/C HNSs is high, which has profit to increase the conductivity of composite [23]. From Fig. 4(d), it can be seen that there are two obvious weight loss stages in the thermogravimetric curve. The first weight loss stage from 30 to 207.4 C is attributed to the dissociation of a small amount of water and hydroxyl groups of carbon shell. As the temperature continues to rise from 207.4 to 503.9 C, the carbon shell in Fe3O4/C HNSs begins to decompose and the mass loss rate is as high as 43.54%. From the thermogravimetric curve calculation of TGA, the mass fraction of Fe3O4 in the tested Fe3O4/C nanoparticles is about 48.23% [24]. The adsorption and desorption isotherms and pore size distribution curves of Fe3O4 NPs and Fe3O4/C HNSs are shown in Fig. 5. The
Fig. 5. N2 adsorption-desorption isotherms and pore size distribution (inset) of (a) Fe3O4 NPs and (b) Fe3O4/C HNSs. 4
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Fig. 6. XPS spectra of the Fe3O4/C HNSs. (a) Survey, high-resolution XPS spectra of (b) Fe 2p, (c) C 1s and (d) O 1s. HMCNs, (e) C 1s, (f) O 1s.
crucial contribution to hold a complete and consistent anchoring structure and promote the contact of Fe3O4 and HMCNs. The strong anchoring effect between Fe3O4 and HMCNs is illustrated in Fig. 7. Large number of oxygen-containing functional groups and defects provide the possibility of a connection with Fe atoms. This is coherent with the result of FT-IR absorption spectra. Additionally, from XPS analysis, the content of Fe3O4 on the surface is about 16%. However, it is concluded that the content of Fe3O4 is about 50% from the TGA data. Therefore, we speculate the growth of Fe3O4 NPs is both on internal and external shells of the HMCNs, which further confirms the interfacial anchoring between Fe3O4 NPs and HMCNs.
lithiation reaction [27]. The surface composition of Fe3O4/C HNSs was explicated through Xray photoelectron spectroscopy (XPS) and EDX spectra (Fig. S5). From Fig. 6(a) and Fig. S5, the existence of element C, O, Fe can be clearly seen. From Fig. 6(b), the spectra of Fe 2p show representative Fe3O4 characteristic peaks. The peak positions are about 711.0 eV and 724.5 eV in response to Fe2þ and Fe3þ apart [28]. There is not satellite peak at 719.0 eV, which confirms the presence of Fe3O4 rather than Fe2O3 in composite samples [29]. The C1s high resolution photoelectron spec– O which troscopy (Fig. 6(c)) can be separately fitted to C–C, C–O and C– correspond to 284.8, 285.8 and 288.8 eV, respectively [28–30]. Fig. 6(d) – O and shows the O 1s spectrum of the Fe3O4/C HNSs is fitted to Fe–O, C– C–O, which correspond to 530.2, 531.8 and 532.1 eV, respectively [31, 32]. Comparing C1s and O1s XPS spectra of Fe3O4/C HNSs with pure HMCNs (Fig. S6), the binding energy of C–O bonds decrease due to the coordination of C–O with Fe to form the interfacial anchoring of Fe–O–C (Fig. 6(e, f)), which is conducive to the decoration of Fe3O4 NPs on carbonaceous matrix. The Fe–O–C linkage is strongly believed to make a
3.2. Electrochemical performance of Fe3O4 NPs and sesame balls-like Fe3O4/C HNSs The electrochemical properties were studied systematically by assembling the fabricated samples of Fe3O4 NPs and Fe3O4/C HNSs with lithium metal into coin-type 2032 cell. The first four cycles of cyclic 5
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voltammetry of Fe3O4/C HNSs and Fe3O4 NPs are shown in Fig. 8 (a) and (b), respectively. The scanning rate is 0.1 mV s1 and the voltage range is 0.01–3.0 V at room temperature. As shown in Fig. 8(a), a faint reduction peak emerging at 1.55 V is attributed to the structural transformation of Fe3O4 (Fe3O4 þ 2Liþ þ 2e → Li2Fe3O4) in the first cycle [33,34]. Another main reduction peak at 0.70 V is the transition of Li2Fe3O4 þ 6Liþ þ 6e→ 3 Fe0 þ 4Li2O together with taking shape in SEI layer [34]. The anodic peak at 1.58 V can be attributed to the oxidation of Fe to Fe3O4 [35,36]. In subsequent cycles, the CV curves basically overlap, indicating that the SEI layer is well formed, and the insertion/extraction reaction of lithium is reversible [37]. In contrast, from the second cycle, the redox peaks of Fe3O4 NPs start falling, which presents the irreversible redox reactions as illustrated in Fig. 8(b) [38]. It makes clear that the specific capacity of Fe3O4 will be a striking reduction as well [39,40]. Fig. 8(c) and (d) show the first, second, third and 64th cycles discharge profiles of Fe3O4 and Fe3O4/C HNSs. Two voltage ranges can
Fig. 7. Description about the anchoring effect of Fe–O–C between Fe3O4 and HMCNs.
Fig. 8. Cyclic voltammograms for the first four cycles of (a) Fe3O4/C HNSs, (b) Fe3O4 NPs. Voltage profiles of (c) Fe3O4/C HNSs, (d) Fe3O4 NPs at 500 mA g1. (e) The rate capability of Fe3O4 NPs and Fe3O4/C HNSs at different current density. (f) Long cycling profiles of the Fe3O4 NPs and Fe3O4/C HNSs electrode at current densities of 1 A g1 after 250 cycles. 6
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Fig. 9. The impedance of Fe3O4 NPs and Fe3O4/C HNSs are measured (a) before the test, (b) after the rate (inset the equivalent circuit for EIS data analysis), (c) after the 50th cycle, and (d) after the 100th cycle.
the density is 0.1, 0.2, 0.5, 1, 2 and 4 A g1. At the time of the density is up to 4 A g1, the discharge specific capacity is still able to attend to 431 mA h g1. Finally, at the time of returning to 0.1 A g1, the discharge specific capacity is able to be recovered to the first 1525 mA h g1. As the contrast compound, the specific discharge capacity of Fe3O4 NPs is only 51 mA h g1 at 4 A g1. The discharge specific capacity is only 384 mA h g1 at 0.1 A g1 [44]. At the time of the current density reach 10 A g1, the reversible specific capacity of Fe3O4/C HNSs is all the same 272 mA h g1 after 100 cycles (Fig. S7). The results show that Fe3O4/C HNSs have excellent rate performance. The rate capability of HMCNs at different current densities is shown in Fig. S8. The contribution of Fe3O4 and HMCNs to the overall capacity in the composite anode is presented in Table S1. Fig. 8(f) shows the long cycle properties of Fe3O4 NPs and Fe3O4/C HNSs after 250 cycles at current density 1 A g1. The reversible specific capacity of Fe3O4/C HNSs is 946 mA h g1 after 250 cycles, while that of Fe3O4 NPs is just 62 mA h g1. The results show an important impact on carbon protective layer to the stability of Fe3O4 [45]. The pure HMCNs have good cyclic stability, but the capacity is merely
be observed in the diagram of Fig. 8(c) and (d). The voltage range of 1.50 V–0.97 V is considered to be that lithium ions are embedded into Fe3O4 matrix to form LixFe3O4 [41,42]. The long voltage platform about 0.85 V is derived from the reduction process of LixFe3O4 to Fe0 [40]. The inverse reaction between Fe0 and Fe3O4 occurs in the first charge curve between the voltage range of 1.20–2.20 V [40,43]. The specific capacity of Fe3O4/C HNSs is 1863 mA h g1 in the primary discharge curve. In Fig. 8(d), Fe3O4 NPs show a lower specific capacity (1059 mA h g1) in the primary discharge curve. The specific capacity of Fe3O4 NPs has a distinct attenuation in subsequent cycles and remains 374 mA h g1 after the 64th test. However, the specific capacity of Fe3O4/C HNSs can still reach 1025 mA h g1 after the 64th test, indicating better cycling stability of Fe3O4/C HNSs. Fig. 8(e) demonstrates the charge-discharge cycle properties of Fe3O4 NPs and Fe3O4/C HNSs at different rates. The material was discharged at the current of 0.1, 0.2, 0.5, 1, 2, 4 A g1 and then returned to the current density 0.1 A g1 for 9 cycles respectively. For Fe3O4/C HNSs the discharge capacity is 1354, 1205, 1076, 999, 617, 431 mA h g1 when
Fig. 10. TEM images of (a) Fe3O4/C HNSs, (b) Fe3O4 NPs after 50 cycles. 7
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380 mA h g1 (Fig. S9). Therefore, Fe3O4 NPs are the main contributors to the capacity of Fe3O4/C HNSs [46]. Furthermore, by comparing to the electrochemical performance of anodes made of other reported Fe3O4-based materials, listed in Table S2, it is clear that our sesame balls-like Fe3O4/C HNSs anode is a highly competitive candidate in terms of capacity output. The electrochemical impedance spectra of two samples are measured before the test, after the 50, 100 cycles and after the rate cycles, and shown in Fig. 9. According to the equivalent fitting circuit (Fig. S10), the specific data are shown in Table S3. The charge-transfer resistances (Rct) of Fe3O4/C HNSs before cycle is 47 Ω smaller than that of Fe3O4 NPs 85 Ω (Fig. 9(a)). With the increase of cycles, the Rct of electrode materials is increased. After 50 cycles, the Rct of Fe3O4/C HNSs is 67 Ω, which is smaller than that of Fe3O4 NPs (130 Ω) (Fig. 9(c)). After 100 cycles, the Rct of Fe3O4/C HNSs is increased to 96 Ω, but the value is still smaller than that of Fe3O4 materials (183 Ω) (Fig. 9(d)). The Rct of Fe3O4/C HNSs is only 58 Ω after the rate cycle, on the contrary, the Rct of pure Fe3O4 NPs is increased to 428 Ω after the rate cycle (Fig. 9(b)). It is proved that Fe3O4/C HNSs have lower charge transfer resistance than pure Fe3O4 NPs, which further indicates that Fe3O4/C HNSs have excellent rate performance. The conclusion is in agreement with above result obtained by rate performance test. The structural changes of Fe3O4 NPs and Fe3O4/C HNSs were studied by TEM. Fig. 10 (a) indicates the TEM pictures of Fe3O4/C HNSs after 50 cycles. Fig. 10(b) shows the TEM pictures of Fe3O4 NPs after 50 cycles. The results show the volume expansion and aggregation of pure Fe3O4 NPs after several charge and discharge process [47]. The size of Fe3O4 NPs on HMCNs is increased due to the volume effect [48]. However, due to the strong interfacial anchoring effect, Fe3O4 NPs are well constrained on HMCNs without loss and accumulation. The super-high capacity of the Fe3O4/C HNSs can be attributed to high specific surface area (400.88 m2 g1) and Fe3O4 NPs with small diameter, which can provide more active sites. The superior rate performance of the Fe3O4/C HNSs is ascribed to HMCNs, which improves the electrical conductivity of composite. At the same time, small Fe3O4 NPs can withstand the stress caused by expansion and enhance the kinetics of Li ion/electron transport. Lastly, the anchoring effect of Fe3O4 on HMCNs maintains the integrity structure of composite and enchanes the stability of Fe3O4/C HNSs.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jelechem.2019.113626. References [1] Y.Z. Su, S. Li, D.Q. Wu, F. Zhang, H.W. Liang, P.F. Gao, C. Cheng, X.L. Feng, Twodimensional carbon-coated graphene/metal oxide hybrids for enhanced lithium storage, ACS Nano 6 (2012) 8349–8356. [2] R.R. Salunkhe, Y.V. Kaneti, Y. Yamauchi, Metal-Organic framework-derived nanoporous metal oxides toward supercapacitor applications: progress and prospects, ACS Nano 11 (2017) 5293–5308. [3] N.A. Zubir, C. Yacou, J. Motuzas, X. Zhang, J.C. Diniz da Costa, Structural and functional investigation of graphene oxide-Fe3O4 nanocomposites for the heterogeneous Fenton-like reaction, Sci. Rep. 4 (2014) 4594. [4] J.S. Chen, Y. Qu, M.G. Wang, J. Han, Y.M. Hu, R. Guo, Aqueous solution-based Fe3O4 seed-mediated route to hydrophilic Fe3O4-Au janus nanoparticles, Langmuir 32 (2016) 4595–4601. [5] M. Aghazadeh, M.R. 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4. Conclusions In conclusion, the sesame balls-like Fe3O4/C HNSs were successfully synthesized by thermal decomposition. From FTIR and XPS, the interfacial anchoring of Fe–O–C promotes the uniform distribution of Fe3O4 NPs on the shells of HMCNs. Electrochemical tests show that Fe3O4/C HNSs have good performance. When the current density is 1 A g1, the reversible specific capacity of Fe3O4/C HNSs is 946 mA h g1 after 250 cycles. The super-high capacity of the Fe3O4/C HNSs can be put down to high specific surface area (400.88 m2 g1) and small size of Fe3O4 NPs. This remarkable improvement of stability performance is attributed to the interfacial anchoring effect between Fe3O4 NPs and HMCNs, which avoids the loss of Fe3O4 NPs and maintains the integrity structure of composite. Therefore, this new type of Fe3O4/C HNSs is an up-andcoming anode material for lithium ion batteries. 5. Declaration of interest statement The authors declare no competing financial interest. Acknowledgements The funding support from the National Natural Science Foundation of China (Grant No. 21773203), Natural Science Foundation of Jiangsu Province (BK20161329), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions is acknowledged. 8
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Interfacial anchoring effect for enhanced lithium storage performance of sesame balls-like Fe3O4/C hollow nanospheres Lanxuan Dai, Wenlong Li, Kehan Zhou, Dongmei Tang, Yue Han, Xiaoyu Wu, Huayu Wu, Guowang Diao, Ming Chen * School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fe3O4/C hollow nanospheres In-situ growth Anchoring effect Thermal decomposition Lithium ion battery
In this paper, hollow mesoporous carbon nanospheres (HMCNs) are used as the growth skeleton. Fe3O4 nanoparticles (Fe3O4 NPs) with a diameter of 10 nm grow in situ on the shell of HMCNs to form a new type of sesame balls-like Fe3O4/C hollow nanospheres (HNSs) by thermal decomposition reaction. The Fe3O4/C HNSs display great cycling stability as anode material in lithium ion battery. At a current density of 1 A g1, the reversible specific capacity of Fe3O4/C HNSs is as high as 946 mA h g1 after 250 cycles. Compared with pure Fe3O4 NPs, the electrochemical properties of the Fe3O4/C HNSs are significantly improved. The anchoring of Fe3O4 NPs on the shell of HMCNs can avoid the aggregation of Fe3O4 NPs, increase the lithiation sites, and accelerate the rapid migration of Li ions. In addition, the framework of Fe3O4/C HNSs, i.e. HMCNs, can promote the conductivity of composite and boost the delivery of electron. This notion and expedient method of construction can be broadened to synthesize other hollow nanostructured material with preferable electrochemistry performance.
1. Introduction With the development of nanotechnology, nanomaterials, especially transition metal oxides nanomaterials, have attracted more attention in the field of lithium ion batteries (LIBs) for energy storage and potential substitutes for graphite owing to their excellent properties [1,2]. As the anode material for LIBs, nanosized particles of transition metal oxides are likely to aggregate and overgrow, leading to the loss of active sites involved in electrochemical reactions to reduce the specific capacity. In addition, the low electrical conductivity can seriously hinder the transport of electrons or ions in batteries or reduction reaction. Among these metal oxides, Fe3O4 has gotten the most extensive focus in virtue of the outstanding theoretical capacity (926 mA h g1), cheap cost, earth plentitude and ecological amity [3–5]. Nevertheless, Fe3O4 as the anode material for lithium batteries still exists some problems (the large irreversible capacity loss, poor cycle stability and low rate capacity, etc) owing to the low electric conductivity and severe volume expansion during Liþ insertion and extraction process [6]. Moreover, Fe3O4 has the similar problems with other transition metal oxides, such as surface instability for the pulverization of active materials, which are attributed to obvious phase and morphological changes associated with typical conversion reactions.
Many strategies have been used to settle the aforesaid troubles. Among those methods, the carbon-coated has been deemed to the most effective way to ameliorate the conductive properties and alleviate the characteristics of volume expansion [7]. The merit of carbon coatings is as follows. First, the carbon layer is propitious to remain intact of Fe3O4 body morphology and prevent from chalking agglomeration. Second, the electrical conductivity of the composite is enhanced by the carbon coating. Third, the outstanding and steady SEI (solid electrolyte interface) film will come into being due to the carbon coating layer. The specific capacity and cycle stability of nitrogen-carbon doped materials can be better enhanced [8]. For example, He et al. reported that Fe3O4 quantum dots (5–10 nm) were embedded porous carbon microspheres via a facile micelle-colloid template method. The Fe3O4@C electrode materials deliver a stable capacity of 601 mA h g1 at 2 A g1 even after 800 cycles [9]. Chen et al. synthesized hollow carbon nanospheres embedded with ultrafine Fe3O4 nanoparticles (Fe3O4@HCNS) by using carboxyl functionalized polystyrene latexes as template and poly dopamine as carbon precursor. Fe3O4@HCNS showed excellent cycling stability and high-rate capability (475 mA h g1 at 5 A g1) [10]. In these carbon substances, hollow mesoporous carbon nanospheres (HMCNs) have aroused the great attention, owing to their good electrical conductivity, perfect mechanical properties, porous shells, accessible
* Corresponding author. E-mail address: [email protected] (M. Chen). https://doi.org/10.1016/j.jelechem.2019.113626 Received 20 August 2019; Received in revised form 1 November 2019; Accepted 4 November 2019 Available online 11 November 2019 1572-6657/© 2019 Elsevier B.V. All rights reserved.
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Cyclic voltammetry (CV) testings were measured through an electrochemical workstation (CHI660E, Chenghua, CHN) with range of the voltage is 0.01–3.0 V and the scanning rate of 0.1 mVs1. The electrochemical impedance spectroscopy (EIS) measurement was performed on the Autolab Electrochemical Analyzer (Ecochemie, Netherlands). The charge and discharge performance and stability of cells were tested with the CT-3008W and the test system (CT-3008W, Xin Wei, CHN) at different current densities and the Voltage range is 0.01–3.0 V.
interior space, and large specific surface area [11]. Moreover, HMCNs use polyporous shells to offer giant hole to accommodate guest compounds, i.e., precursor compounds of iron oxides can be incorporated into HMCNs materials [12–15], which will reduce the aggregation of nanoparticles. The structure in combination with HMCNs has provided a new strategy to solve the shortcomings existing in electrode materials, such as the sharp volume change, low electric conductivity and low rate capacity. In this paper, a simple thermal decomposition method was successfully used to fabricate Fe3O4/C hollow nanospheres (HNSs). The Fe3O4 nanoparticles (NPs) with a diameter of 10 nm on average were anchored on the shell of the HMCNs to form the sesame balls-like Fe3O4/C HNSs. The well structure-designed Fe3O4/C HNSs can provide much more lithium storage sites and accelerate the rapid migration of Li ions due to the interface effects of small nanoparticles. The strong interfacial anchoring of Fe3O4 NPs on the shell of HMCNs has a good constraint effect on Fe3O4 NPs without the nanoparticle aggregation, loss and accumulation, which can maintain the integrity structure of composite. Compared with uncoated Fe3O4 NPs, the electrochemical properties of the sesame balls-like Fe3O4/C HNSs are significantly improved. This strategy can be applied to the design and synthesize of other metal material/hollow nanostructured composites, which are one of suitable materials for energy storage and conversion field with preferable electrochemistry performance.
2.4. Preparation of Fe3O4/C HNSs composite The synthetic strategy for forming the Fe3O4/C HNSs is illustrated in Fig. 1. The particular synthetic method of hollow mesoporous carbon nanospheres (HMCNs) was shown in Supporting Information. 0.1 g HMCNs and 0.353 g ferric acetylacetonate were dispersed in the solution of 5 ml dibenzyl ether and 5 ml oleamine by ultrasonic method. Then the solution was heated at 300 C for 1 h with the protection of argon. After the reaction was finished, the composites were centrifugated and washed by ethanol, and the solid phase was dried. Pure Fe3O4 NPs were synthesized using the same reaction condition without HMCNs (Supporting Information). 3. Results and discussion
2. Experimental section 3.1. Characterization of Fe3O4/C HMCNs 2.1. Materials The morphologies of Fe3O4 NPs and sesame balls-like Fe3O4/C hollow nanospheres are demonstrated in Fig. 2. Fig. 2(a) demonstrates a representative TEM image of Fe3O4 NPs with diameter of about 10 nm. Fig. S1 shows TEM image of Fe3O4 NPs with a lower magnification. Fig. 2(b and c) exhibit the representative TEM images of HMCNs. The diameter of HMCNs is about 300 nm, the thickness of carbon wall is approximately 35 nm. The Brunauer-Emmett-Teller (BET) analysis shows that the specific surface area of HMCNs is about 770.7 m2 g1 and the main pore size is about 2.1 nm in diameter (Figs. S2a and b). Fig. 2 (d, e, f) show the TEM pictures of Fe3O4/C HNSs with distinct magnifications. The reactant ions enter the inner cavity of HMCNs through the mesoporous. Fig. S3 shows the SEM image of Fe3O4/C HNSs with low magnification. In situ growth of Fe3O4 NPs on the inner and outer wall of HMCNs retains the original composition, structure and morphology of Fe3O4 NPs in Fig. 2(a), forming a new type of sesame balls-like Fe3O4/C HNSs. The growth of
All chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents used in this paper were of analytical grade and used without further purification. The electrolyte solution was bought from Guangzhou Tian chi material Science and Technology Ltd. Other commercial solvents and chemicals are reagent level and made use according to the instructions. 2.2. Characterization Field-emission scanning electron microscopy (FESEM) was carried out by Hitachi S-4800 (Japan). Transmission Electron Microscopy (TEM) was underway by JEOL JEM-2100 instrument. High-resolution TEM (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed on FEI Tecnai G2 F30 STWIN (USA) operating at 300 kV. Raman was conducted on Renishaw by Raman spectroscope. X-ray diffraction (XRD) was in progress by a graphite monochromator and Cu Kα radiation (λ ¼ 0.1541 nm) on D8 advance superspeed powder diffractometer (Bruker). Thermogravimetry analysis (TGA, Pyris 1 TGA, PerkinElmer, USA) was tested in the air. X-ray photoelectron spectroscopy (XPS) was conducted by Thermo Escalab 250 system using Al Ka radiation (hν ¼ 1486.6 eV). The pressure of test chamber was maintained below 2 109 Torr during spectral acquisition. After drying at 100 C for 4 h, the specific surface area and pore size distribution were recorded at automatic surface area and porosity analyzer by BET technique (ASAP 2020, HD88) (- 196 C). 2.3. Electrochemical tests Lithium storage performance test was characterized by assembling Fe3O4 NPs and Fe3O4/C HNSs materials into button cell (CR 2032 coin battery). Fe3O4/C HNSs (80%) and acetylene black (10%) as conductive materials, the slurry was dissolved in N-methyl-2-pyrrolidone (NMP) by magnetic stirring for 6 h with 10% PVDF (10%) as binder. Then, the sizing agent was spread on the Cu foil current collector and dried in oven. The semi-cell makes up of an experimental anode with lithium-foil used as cathode, 1 M LiPF6 of EMC/DMC/EC (Volume ratio is 1:1:1) used as electrolyte. It is assembled in a glove box with a high purity argon (Cacuum Atmosphere Co., Ltd.).
Fig. 1. Formation of sesame balls-like Fe3O4/C HNSs. 2
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Fig. 2. TEM images of (a) Fe3O4 NPs, (b, c) HMCNs, (d, e, f) Fe3O4/C HNSs.
HNSs were explicated by X-ray diffraction (XRD) (Fig. 4(a)). The diffraction peaks of two materials emerge at 30.0 (200), 35.4 (311), 43.0 (400), 57.0 (511) and 62.5 (440) are according with the data of PDF # 19–0629 for Fe3O4, which confirms that in-situ growth of Fe3O4 NPs on the wall of HMCNs reserves the original composition of Fe3O4 NPs. HMCNs, Fe3O4 NPs and Fe3O4/C HNSs were characterized by FT-IR absorption spectra (Fig. 4(b)). The chief characteristic peaks of pure Fe3O4 are designated as below: the absorption band at 568 cm1 is designated as the tensile vibration mode of Fe–O at the oxygen octahedral position [17]. In the carbon framework, three strong absorption peaks near 1583, 1200 and 813 cm1 are designated as the stretching vibrations of –OH, C–O and C–H of HMCNs. As a result of the coordination of Fe with C–O via interfacial anchoring, the energy of Fe–O bond increases and the peak shifts to 575 cm1 in Fe3O4/C HNSs. As illustrated in Fig. 4(b), the peaks of –OH, C–O and C–H also have obvious deviation [18–21]. The small displacement of peaks is ascribed to the interaction between Fe3O4 and oxygen-containing groups in HMCNs, which is conducive to the anchoring of Fe3O4 NPs on HMCNs [22]. These results
Fe3O4 NPs on the inner and outer walls of HMCNs can be able to increase the electronic conductivity of Fe3O4 NPs and avert the polymerization of Fe3O4 NPs during the synthesis or charge/discharge processes. In order to display the structure of the nanoparticles, the sample was analyzed by high-resolution TEM and selective electron diffraction. Fig. 3(a and b) show that Fe3O4 NPs grow on the wall of HMCNs homogeneously. Lattice resolved HRTEM images of Fe3O4 are shown in Fig. 3(b). The distinct and consecutive lattice fringes state clearly the superior crystallinity of the composite. The results show that the lattice spacings of 0.25 and 0.29 nm are assigned to the plane of (311) and (220), respectively in Fig. 3(b1) and (b2) [16]. The electron diffraction pattern (SAED) of the selected region is shown in Fig. S4 and the diffraction rings correspond to the plane of (311) and (220). Fig. 3(c) is a high-power dark field image of Fe3O4/C HNSs. Fig. 3(d, e, f) exhibit the distribution of C, Fe, and O. The whole spherical region distinctly shows the presence of O and Fe. Therefore, the sesame balls-like Fe3O4/C HNSs were determined by HRTEM and EDX element mappings. The crystal structure and phase purity of Fe3O4 NPs and Fe3O4/C
Fig. 3. (a, b) HRTEM images of Fe3O4/C HNSs, (b1, b2) the lattice spacings in HRTEM images, (c) HAADF-STEM image of Fe3O4/C HNSs and (d, e, f) element mappings of C, Fe, O. 3
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Fig. 4. (a) XRD patterns of Fe3O4 NPs, Fe3O4/C HNSs and PDF # 19–0629 card. (b) FTIR spectra of HMCNs, Fe3O4 NPs and Fe3O4/C HNSs. (c) Raman spectrum of Fe3O4/C HNSs. (d) TGA curve of Fe3O4/C HNSs.
isothermal curves of the two samples are calculated by BET (BrunancrEmmett-Teller) way and the pore size distribution of the samples are calculated through BJH (Barrett-Joyner-Halenda) way. The result indicates that the specific area of HMCNs is approximately 770.70 m2 g1 and the chief aperture is approximately 2.10 nm (Figs. S2a and b). Pore structure of HMCNs with large specific surface area can supply the position for the deposition of Fe3O4 NPs during hydrothermal process. The N2 adsorption-desorption isotherm curves of Fe3O4 NPs and Fe3O4/C HNSs belong to type IV, and there is an obvious hysteresis ring when P/P0 is between 0.5 and 1.0, which indicates the existence of mesoporous structure in the two samples [25]. The specific area of Fe3O4/C HNSs is calculated to be 400.88 m2 g1 higher than that of Fe3O4 NPs (59.46 m2 g1), which is attributed to the hollow structure of composite. BJH method was used to calculate the aperture distribution curve through the desorption isotherm (see the illustrations in Fig. 5) [26]. The pore distribution of Fe3O4/C HNSs is concentrated in 1.5–6 nm indicating the existence of micropore and mesoporous in the sample. Fe3O4/C HNSs have large BET area and micropore/mesoporous structure, which provide more lithium insertion sites to enhance the lithium storage capacity, and improve the Liþ diffusion path to ensure the rate performance of
indicate that Fe3O4 NPs have been successfully anchored on HMCNs. Raman spectrum is shown in Fig. 4(c). The two absorption bands at 1590 and 1350 cm1 are diagnostic peaks of the typical Raman spectra of carbon, which can be attributable to the G and D bands of carbon, severally. ID/IG (peak strength ratio of D band and G band) can be served to evaluate the graphitization degree of carbon components of composites [8]. The smaller the ID/IG is, the higher the graphitization degree is. The peak strength ratio of D band and G band of Fe3O4/C HNSs is 0.91, which indicates that the degree of carbon shell layer graphitization in Fe3O4/C HNSs is high, which has profit to increase the conductivity of composite [23]. From Fig. 4(d), it can be seen that there are two obvious weight loss stages in the thermogravimetric curve. The first weight loss stage from 30 to 207.4 C is attributed to the dissociation of a small amount of water and hydroxyl groups of carbon shell. As the temperature continues to rise from 207.4 to 503.9 C, the carbon shell in Fe3O4/C HNSs begins to decompose and the mass loss rate is as high as 43.54%. From the thermogravimetric curve calculation of TGA, the mass fraction of Fe3O4 in the tested Fe3O4/C nanoparticles is about 48.23% [24]. The adsorption and desorption isotherms and pore size distribution curves of Fe3O4 NPs and Fe3O4/C HNSs are shown in Fig. 5. The
Fig. 5. N2 adsorption-desorption isotherms and pore size distribution (inset) of (a) Fe3O4 NPs and (b) Fe3O4/C HNSs. 4
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Fig. 6. XPS spectra of the Fe3O4/C HNSs. (a) Survey, high-resolution XPS spectra of (b) Fe 2p, (c) C 1s and (d) O 1s. HMCNs, (e) C 1s, (f) O 1s.
crucial contribution to hold a complete and consistent anchoring structure and promote the contact of Fe3O4 and HMCNs. The strong anchoring effect between Fe3O4 and HMCNs is illustrated in Fig. 7. Large number of oxygen-containing functional groups and defects provide the possibility of a connection with Fe atoms. This is coherent with the result of FT-IR absorption spectra. Additionally, from XPS analysis, the content of Fe3O4 on the surface is about 16%. However, it is concluded that the content of Fe3O4 is about 50% from the TGA data. Therefore, we speculate the growth of Fe3O4 NPs is both on internal and external shells of the HMCNs, which further confirms the interfacial anchoring between Fe3O4 NPs and HMCNs.
lithiation reaction [27]. The surface composition of Fe3O4/C HNSs was explicated through Xray photoelectron spectroscopy (XPS) and EDX spectra (Fig. S5). From Fig. 6(a) and Fig. S5, the existence of element C, O, Fe can be clearly seen. From Fig. 6(b), the spectra of Fe 2p show representative Fe3O4 characteristic peaks. The peak positions are about 711.0 eV and 724.5 eV in response to Fe2þ and Fe3þ apart [28]. There is not satellite peak at 719.0 eV, which confirms the presence of Fe3O4 rather than Fe2O3 in composite samples [29]. The C1s high resolution photoelectron spec– O which troscopy (Fig. 6(c)) can be separately fitted to C–C, C–O and C– correspond to 284.8, 285.8 and 288.8 eV, respectively [28–30]. Fig. 6(d) – O and shows the O 1s spectrum of the Fe3O4/C HNSs is fitted to Fe–O, C– C–O, which correspond to 530.2, 531.8 and 532.1 eV, respectively [31, 32]. Comparing C1s and O1s XPS spectra of Fe3O4/C HNSs with pure HMCNs (Fig. S6), the binding energy of C–O bonds decrease due to the coordination of C–O with Fe to form the interfacial anchoring of Fe–O–C (Fig. 6(e, f)), which is conducive to the decoration of Fe3O4 NPs on carbonaceous matrix. The Fe–O–C linkage is strongly believed to make a
3.2. Electrochemical performance of Fe3O4 NPs and sesame balls-like Fe3O4/C HNSs The electrochemical properties were studied systematically by assembling the fabricated samples of Fe3O4 NPs and Fe3O4/C HNSs with lithium metal into coin-type 2032 cell. The first four cycles of cyclic 5
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voltammetry of Fe3O4/C HNSs and Fe3O4 NPs are shown in Fig. 8 (a) and (b), respectively. The scanning rate is 0.1 mV s1 and the voltage range is 0.01–3.0 V at room temperature. As shown in Fig. 8(a), a faint reduction peak emerging at 1.55 V is attributed to the structural transformation of Fe3O4 (Fe3O4 þ 2Liþ þ 2e → Li2Fe3O4) in the first cycle [33,34]. Another main reduction peak at 0.70 V is the transition of Li2Fe3O4 þ 6Liþ þ 6e→ 3 Fe0 þ 4Li2O together with taking shape in SEI layer [34]. The anodic peak at 1.58 V can be attributed to the oxidation of Fe to Fe3O4 [35,36]. In subsequent cycles, the CV curves basically overlap, indicating that the SEI layer is well formed, and the insertion/extraction reaction of lithium is reversible [37]. In contrast, from the second cycle, the redox peaks of Fe3O4 NPs start falling, which presents the irreversible redox reactions as illustrated in Fig. 8(b) [38]. It makes clear that the specific capacity of Fe3O4 will be a striking reduction as well [39,40]. Fig. 8(c) and (d) show the first, second, third and 64th cycles discharge profiles of Fe3O4 and Fe3O4/C HNSs. Two voltage ranges can
Fig. 7. Description about the anchoring effect of Fe–O–C between Fe3O4 and HMCNs.
Fig. 8. Cyclic voltammograms for the first four cycles of (a) Fe3O4/C HNSs, (b) Fe3O4 NPs. Voltage profiles of (c) Fe3O4/C HNSs, (d) Fe3O4 NPs at 500 mA g1. (e) The rate capability of Fe3O4 NPs and Fe3O4/C HNSs at different current density. (f) Long cycling profiles of the Fe3O4 NPs and Fe3O4/C HNSs electrode at current densities of 1 A g1 after 250 cycles. 6
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Fig. 9. The impedance of Fe3O4 NPs and Fe3O4/C HNSs are measured (a) before the test, (b) after the rate (inset the equivalent circuit for EIS data analysis), (c) after the 50th cycle, and (d) after the 100th cycle.
the density is 0.1, 0.2, 0.5, 1, 2 and 4 A g1. At the time of the density is up to 4 A g1, the discharge specific capacity is still able to attend to 431 mA h g1. Finally, at the time of returning to 0.1 A g1, the discharge specific capacity is able to be recovered to the first 1525 mA h g1. As the contrast compound, the specific discharge capacity of Fe3O4 NPs is only 51 mA h g1 at 4 A g1. The discharge specific capacity is only 384 mA h g1 at 0.1 A g1 [44]. At the time of the current density reach 10 A g1, the reversible specific capacity of Fe3O4/C HNSs is all the same 272 mA h g1 after 100 cycles (Fig. S7). The results show that Fe3O4/C HNSs have excellent rate performance. The rate capability of HMCNs at different current densities is shown in Fig. S8. The contribution of Fe3O4 and HMCNs to the overall capacity in the composite anode is presented in Table S1. Fig. 8(f) shows the long cycle properties of Fe3O4 NPs and Fe3O4/C HNSs after 250 cycles at current density 1 A g1. The reversible specific capacity of Fe3O4/C HNSs is 946 mA h g1 after 250 cycles, while that of Fe3O4 NPs is just 62 mA h g1. The results show an important impact on carbon protective layer to the stability of Fe3O4 [45]. The pure HMCNs have good cyclic stability, but the capacity is merely
be observed in the diagram of Fig. 8(c) and (d). The voltage range of 1.50 V–0.97 V is considered to be that lithium ions are embedded into Fe3O4 matrix to form LixFe3O4 [41,42]. The long voltage platform about 0.85 V is derived from the reduction process of LixFe3O4 to Fe0 [40]. The inverse reaction between Fe0 and Fe3O4 occurs in the first charge curve between the voltage range of 1.20–2.20 V [40,43]. The specific capacity of Fe3O4/C HNSs is 1863 mA h g1 in the primary discharge curve. In Fig. 8(d), Fe3O4 NPs show a lower specific capacity (1059 mA h g1) in the primary discharge curve. The specific capacity of Fe3O4 NPs has a distinct attenuation in subsequent cycles and remains 374 mA h g1 after the 64th test. However, the specific capacity of Fe3O4/C HNSs can still reach 1025 mA h g1 after the 64th test, indicating better cycling stability of Fe3O4/C HNSs. Fig. 8(e) demonstrates the charge-discharge cycle properties of Fe3O4 NPs and Fe3O4/C HNSs at different rates. The material was discharged at the current of 0.1, 0.2, 0.5, 1, 2, 4 A g1 and then returned to the current density 0.1 A g1 for 9 cycles respectively. For Fe3O4/C HNSs the discharge capacity is 1354, 1205, 1076, 999, 617, 431 mA h g1 when
Fig. 10. TEM images of (a) Fe3O4/C HNSs, (b) Fe3O4 NPs after 50 cycles. 7
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380 mA h g1 (Fig. S9). Therefore, Fe3O4 NPs are the main contributors to the capacity of Fe3O4/C HNSs [46]. Furthermore, by comparing to the electrochemical performance of anodes made of other reported Fe3O4-based materials, listed in Table S2, it is clear that our sesame balls-like Fe3O4/C HNSs anode is a highly competitive candidate in terms of capacity output. The electrochemical impedance spectra of two samples are measured before the test, after the 50, 100 cycles and after the rate cycles, and shown in Fig. 9. According to the equivalent fitting circuit (Fig. S10), the specific data are shown in Table S3. The charge-transfer resistances (Rct) of Fe3O4/C HNSs before cycle is 47 Ω smaller than that of Fe3O4 NPs 85 Ω (Fig. 9(a)). With the increase of cycles, the Rct of electrode materials is increased. After 50 cycles, the Rct of Fe3O4/C HNSs is 67 Ω, which is smaller than that of Fe3O4 NPs (130 Ω) (Fig. 9(c)). After 100 cycles, the Rct of Fe3O4/C HNSs is increased to 96 Ω, but the value is still smaller than that of Fe3O4 materials (183 Ω) (Fig. 9(d)). The Rct of Fe3O4/C HNSs is only 58 Ω after the rate cycle, on the contrary, the Rct of pure Fe3O4 NPs is increased to 428 Ω after the rate cycle (Fig. 9(b)). It is proved that Fe3O4/C HNSs have lower charge transfer resistance than pure Fe3O4 NPs, which further indicates that Fe3O4/C HNSs have excellent rate performance. The conclusion is in agreement with above result obtained by rate performance test. The structural changes of Fe3O4 NPs and Fe3O4/C HNSs were studied by TEM. Fig. 10 (a) indicates the TEM pictures of Fe3O4/C HNSs after 50 cycles. Fig. 10(b) shows the TEM pictures of Fe3O4 NPs after 50 cycles. The results show the volume expansion and aggregation of pure Fe3O4 NPs after several charge and discharge process [47]. The size of Fe3O4 NPs on HMCNs is increased due to the volume effect [48]. However, due to the strong interfacial anchoring effect, Fe3O4 NPs are well constrained on HMCNs without loss and accumulation. The super-high capacity of the Fe3O4/C HNSs can be attributed to high specific surface area (400.88 m2 g1) and Fe3O4 NPs with small diameter, which can provide more active sites. The superior rate performance of the Fe3O4/C HNSs is ascribed to HMCNs, which improves the electrical conductivity of composite. At the same time, small Fe3O4 NPs can withstand the stress caused by expansion and enhance the kinetics of Li ion/electron transport. Lastly, the anchoring effect of Fe3O4 on HMCNs maintains the integrity structure of composite and enchanes the stability of Fe3O4/C HNSs.
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4. Conclusions In conclusion, the sesame balls-like Fe3O4/C HNSs were successfully synthesized by thermal decomposition. From FTIR and XPS, the interfacial anchoring of Fe–O–C promotes the uniform distribution of Fe3O4 NPs on the shells of HMCNs. Electrochemical tests show that Fe3O4/C HNSs have good performance. When the current density is 1 A g1, the reversible specific capacity of Fe3O4/C HNSs is 946 mA h g1 after 250 cycles. The super-high capacity of the Fe3O4/C HNSs can be put down to high specific surface area (400.88 m2 g1) and small size of Fe3O4 NPs. This remarkable improvement of stability performance is attributed to the interfacial anchoring effect between Fe3O4 NPs and HMCNs, which avoids the loss of Fe3O4 NPs and maintains the integrity structure of composite. Therefore, this new type of Fe3O4/C HNSs is an up-andcoming anode material for lithium ion batteries. 5. Declaration of interest statement The authors declare no competing financial interest. Acknowledgements The funding support from the National Natural Science Foundation of China (Grant No. 21773203), Natural Science Foundation of Jiangsu Province (BK20161329), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions is acknowledged. 8
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