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Encapsulated Fe3O4 into tubular mesoporous carbon as a superior performance anode material for lithium-ion batteries
Journal of Alloys and Compounds 815 (2020) 152542
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Encapsulated Fe3O4 into tubular mesoporous carbon as a superior performance anode material for lithium-ion batteries Zhijie Cao*, Xiaobo Ma Advanced Energy Storage Materials and Devices Laboratory, School of Physics and Electronic-Electrical Engineering, Ningxia University, Yinchuan, 750021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2019 Received in revised form 17 September 2019 Accepted 2 October 2019 Available online 3 October 2019
A well-defined composite of Fe3O4 encapsulated into tubular mesoporous carbon is prepared as an anode material for lithium-ion batteries. It turns out that up to 67.7 wt% of Fe3O4 nanoparticles are homogeneously embedded into the mesoporous carbon nanotubes. The Fe3O4@tubular mesoporous carbon composites provide effective accommodation for the volume change, fast transport paths for electrons and ions, as well as good contact for nanoparticles, thus exhibiting superior electrochemical performances. The reversible capacity maintains a high specific capacity of ~800 mAh g1 after 1000 cycles at 2000 mA g1, and the electrode still keeps a reasonable level of ~400 mAh g1 even going through 16000 cycles at 10000 mA g1, which makes Fe3O4@tubular mesoporous carbon composite a valuable anode material for lithium-ion batteries. © 2019 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries Composite Fe3O4 Mesoporous carbon Capacity
1. Introduction Rechargeable lithium-ion batteries (LIBs) have been the main power sources for portable electronic devices, and this trend gradually spreads towards the area of large-scale energy storage [1e3]. To satisfy the ever growing requirements of electric market, Li-ion batteries providing superior electrochemical performances like higher energy density, longer cycle life, excellent rate capability, etc., are demanded, which promotes the intensive research of superior electrode materials [4e7]. Among them, Fe3O4 is considered as one of the most favorable candidates considering its outstanding features of large theoretical capacity (~930 mAh g1), low cost, environmental benignancy, etc. [8e10]. However, its massive applications are still restricted by the rapid deterioration of capacity during the long term cycling process, which are associated with the degradation of Fe3O4 anodes mainly due to the large volume expansion/contraction accompanying with the lithium ions insertion/extraction [11e13]. To address these adverse issues, numerous strategies were explored to ameliorate the electrochemical properties of Fe3O4 anodes through stabilizing the structures and/or enhancing the
* Corresponding author. E-mail address: [email protected] (Z. Cao). https://doi.org/10.1016/j.jallcom.2019.152542 0925-8388/© 2019 Elsevier B.V. All rights reserved.
electrical conductivity [14e16]. Of these, designing unique Fe3O4 nanostructured materials is one of the most effective approaches to facilitate the Liþ diffusion and relieve the strain relaxation, thus ameliorating the electrochemical properties of Fe3O4 based electrodes [17e20]. For instance, the specific capacity of Fe3O4 assemblies depositing on Cu nanopillars could sustain 80% after 100 cycles at 8 C [18], and a large capacity of 580 mA h g1 was reversible over 100 cycles at 200 mA g1 in hierarchical hollow Fe3O4 microspheres, which showed a dramatical stability of lithiation/delithiation [20]. Another effective strategy to facilitate the electrochemical performances of Fe3O4 is coated or nanoconfined by carbonaceous materials, which can elastically buffer the huge volume changes, meanwhile to enhance the electronic conductivity [21e24]. Fe3O4 nanospindles coated by a carbon layer of 2e10 nm exhibited a high capacity of 530 mA h g1 during the 80 charge/ discharge cycles at C/2 [21]. While the cycling stability of 2-D ferrite-based hybrid nanosheets was much more stable than that of 3-D composite electrode: the reversible capacities of 2-D structure was approximately 600 mA h g1 while that of 3-D electrodes decreased considerably to 323 mA h g1 over the 50 cycles [22]. Yang et al. [23] prepared the well-design Fe3O4 anode with conformal Fe3O4 nanoparticles on carbon nanotubes, which possessed a delithiation capacity of 800 mA h g1 over 100 cycles at 90 mA g1. Even with substantial improvements, the stability of Fe3O4 anodes during the long-term cycling process still fails to
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satisfy the practical applications due to the huge challenge of restraining pulverization. Inspired by these aforementioned advances, designing Fe3O4/ carbon nanostructured materials could be an effective way to further improve the electrochemical properties of Fe3O4. Our previous work showed that the FeS@tubular mesoporous carbon (FeS@CMK-5) electrode consisting of FeS nanoparticles with tubular mesoporous carbon exhibited large reversible capacity, high cycling stability, excellent rate capability for both LIBS and SIBS because of its structural superiority [25]. Herein to prepare Fe3O4 based electrodes with long-tern duration, well-dispersed Fe3O4 nanoparticles encapsulated in CMK-5 were synthesized to constitute the anode materials of LIBS. These well-defined Fe3O4@CMK-5 anodes offer superior electrochemical performances including large specific capacity, outstanding rate capability and long-tern cycling life, which provides new insights for designing high performance electrodes. 2. Experimental details Materials synthesis. CMK-5 was synthesized by the same method in our previous work [25]. For the preparation of Fe3O4@CMK-5 composite, 5 mL of 0.2 M FeCl3 solution were dropwise added into 0.2 g of CMK-5 with the accompany of continuous stirring. After 4 h, this mixed solution was heated to 80 C and stirred for a period of 8 h under this condition to hydrolyzate FeCl3 to form FeOOH particles. To increase the loading amount of Fe3O4, the above process was repeated for several times, and these products were further washed three times by ethanol. Afterwards, these samples were calcined at 600 C for 4 h under argon atmosphere to form the Fe3O4@CMK-5 composite. The bare Fe3O4 particles were also synthesized under the same conditions for reference. Materials characterization. Crystallographic patterns of these electrode materials were monitored by the X-ray diffraction (XRD, PANalytical X’Pert PRO Alpha-1, Cu/Ka). The nitrogen ad-/desorption behaviors were conducted by the Quantachrome equipment (Autosorb-iQ-MP). The microscopy images were achieved on the ZEISS ULTRA55 (SEM) and JEOL JEM 2100F (TEM) instruments. Thermogravimetric analysis (TGA) was measured at 5 K/min under pure oxygen gas (NETZSCH TG 209 F3 TarsusR). Raman characterizations were recorded on a laser spectromicroscopy (Renishaw RM 1000). Electrochemical characterization. The CR2016 coin-type cells were employed for the measurement of electrochemical properties. The working electrodes were prepared by mixing active materials (Fe3O4@CMK-5, Fe3O4, CMK-5), super P, with carboxymethyl cellulose sodium salt (Na-CMC) at a mass ratio of 80:10:10. The coin
cells was assembled by electrodes, lithium sheets, electrolyte, and polyethylene film. The electrolyte solution was 1 M LiPF6 in a 1:2 (mass ratio) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), with adding 10 wt% fluoroethylene carbonate (FEC). The mass of the active anode material is ~0.38 mg and the electrode surface area is 1.13 cm2. The electrochemical tests were performed on a LAND CT2001A model battery measurement instrument (0.01e3.0 V vs. Li/Liþ). The Electrochemical Impedance Spectroscopy (EIS, 100e0.05 Hz) and Cyclic Voltammetry (CV, 0e3.0 V, 0.5 mV s1) were measured on a Gamry Interface 1000 electrochemical workstation. The amplitude for EIS measurement was 5 mV. 3. Results and discussion Fig. 1 schematically shows the illustration of synthesizing ultrafine Fe3O4 nanoparticles embedded in tubular mesoporous carbon (denoted as Fe3O4@CMK-5). After the stirring procedure at room temperature, the FeCl3 solution would be impregnated into the bimodal channels of CMK-5 due to the mesoporous nature of carbon scaffold. Subsequently, this resulting solution was heated and dried at 80 C, which seems to cause FeCl3 to yield FeOOH nanoparticles encapsulated inside the bimodal channels of CMK-5. Finally, the FeOOH@CMK-5 composite was calcined under Ar to oxidize the FeOOH particles, which led to the formation of discrete Fe3O4 nanoparticles embedded in mesoporous carbon. The loading of Fe3O4 particles into CMK-5 can be confirmed by the XRD, TG, Raman, and nitrogen ad-/desorption analysis. The XRD patterns in Fig. 2a shows that all these sharp reflections of Fe3O4@CMK-5 belong to Fe3O4, indicating the highly crystallized nature of these nanoparticles. While for the two broad signals of CMK-5, they are invisible in the pattern of Fe3O4@CMK-5 probably due to the much lower electron densities of carbon materials in contrast with Fe3O4 [7]. TG curve in the temperature range of 40e800 C is shown in Fig. 2b. Upon heating to 160 C, it presents a weight loss of 0.5 wt% because of the liberation of moisture, and then the following mass increase between 160 C and 340 C is likely to originate from the oxidation of Fe3O4 to Fe2O3 during the calcination process [26]. Finally, carbon is oxidized to CO2 from 340 to 800 C with the accompany of a weight loss of 33.2 wt%. Therefore, the loading fraction of Fe3O4 is 67.7 wt% based on the calculation of residual mass, and such a high original percentage could guarantee the high specific capacity of anode materials. Raman spectra of this Fe3O4@CMK-5 composite and CMK-5 (Fig. 2c) clearly display the distinguishable D- and G-bands of carbon materials peaking at 1346 and 1592 cm1, respectively. Normally, D-band is interrelated with the degree of disorder, edges and other defects in amorphous form, contributing to the activation of zone edge modes
Fig. 1. Illustration of the synthesis process of Fe3O4@CMK-5.
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Fig. 2. (a) XRD patterns, (b) TG curve, (c) Raman spectra, (d) N2 adsorption-desorption isotherms of CMK-5 and Fe3O4@CMK-5.
in carbon materials, while the G-band originates from the E2g stretching vibrations in crystalline carbon [27e29]. With the incorporation of Fe3O4, the D-band intensity increases while the Gband decreases, meanwhile the value of ID/IG (the ratio of peak intensity) increases from 0.96 to 1.60, which indicates an accumulation of structural defects and the generation of nanocrystalline particles [30]. Fig. 2d compares the N2 adsorption-desorption isotherms of Fe3O4@CMK-5 and CMK-5 at 196 C. The specific BET surface area is dramatically decreased from 1633 m2/g to 539 m2/g after the incorporation of Fe3O4, confirming that most of the pore systems of CMK-5 are occupied/blocked by the Fe3O4 nanoparticles. Meanwhile, the Fe3O4@CMK-5 still enjoys a luxurious porosity as well as high specific surface area, thus achieving the dual effects of buffering the volume expansion and facilitating the conductivity of electron/ion during the cycling process of lithium ion insertion/ extraction [31]. Fig. 3a shows the SEM image of Fe3O4 particles, in which a tabular structure with the particle size of ~200 nm presents in a rather homogenous form. While as to Fe3O4@CMK-5, as shown in Fig. 3b, much smaller Fe3O4 particles (white dots) uniformly distribute on the surfaces of CMK-5. The homogeneous distribution of Fe3O4 particles in this composite can be also confirmed by the EDS elemental mapping images (Fig. S1). Further TEM images of Fe3O4@CMK-5 (Fig. 3c and d) demonstrate that Fe3O4 nanoparticles around 5e10 nm (black arrows) are dispersedly embedded in the CMK-5 scaffolds. The interplanar distances for the (311) and (220) planes of Fe3O4 are clearly distinguished (Fig. 3e and f), proving the encapsulated nanoparticles to be Fe3O4. All these evidences verify that the well distributed Fe3O4 nanoparticles on/inside the mesoporous carbon with excellent electrical conductivity may offer an effective accommodation for the volume changes of Fe3O4 particles through the lithiation-delithiation cycles, but also facilitate the transportation of electron and ion, thus enabling Fe3O4@CMK-5 to be a favorable choice for the anodes of LIBs [32e34]. Moreover, high tapping density is important to ensure large volumetric density and
process ability for practical applications [35,36], hence we measured the tapping density of Fe3O4, Fe3O4@CMK-5 and CMK-5 following the method used in Ref. [37]. Results (Fig. S2) show that the pure Fe3O4 nanoparticles have a high tapping density of 1.29 g cm3, while tubular mesoporous carbon CMK-5 displays a quite low value of 0.064 g cm3. After the encapsulation of 67.7 wt% Fe3O4, the Fe3O4@CMK-5 sample enjoys a tapping density of 0.18 g cm3. Although not a large value, it is still higher than that of carbon-coated Fe3O4 nanoparticle (3.0 mg cm3) [38], and nanosized silicon (0.16 g cm3) [39]. The electrochemical performances of Fe3O4@CMK-5 composites were evaluated between 0.01 and 3.0 V. For comparison, the electrochemical performances of pure Fe3O4 and CMK-5 were also measured under the same conditions. Fig. 4a presents the voltagecapacity patterns of the Fe3O4@CMK-5 electrode under 200 mA g1. An apparent discharge plateau at ~0.7 V is observed in the initial cycle, and then slightly shifts to about 0.9 V and stabilizes at this voltage in the following cycles, agreeing well with previous reports [21,40,41]. Meanwhile, it is noted that the plateau at 1.8 V is poorly identified. As shown in Fig. S3c, the cathodic peak of the first cycle corresponds to the reduction of Fe3O4 as well as the reversible formation of SEI film. All these observations above agree well with the typical cyclic voltammetry profiles of the discharge-charge processes of Fe3O4 in other works [42e44]. The Fe3O4@CMK-5 anode reveals a high stable cycling performance over 60 cycles at 200 mA g1 (Fig. 4b). The initial discharge and charge capacities are 1637 mA h g1 and 958 mA h g1, respectively, with an initial Coulombic efficiency of 58.5%. Such an initial irreversible capacity distance originates from the formation of a solid electrolyte interphase (SEI) film [45] and incomplete conversion reaction between Fe3O4 and Fe0 [46]. Then, the electrode at the 2nd cycle delivers a lithiation and delithiation of 950 and 912 mAh g1, respectively, meanwhile the coulombic efficiency increases to 96%. Afterwards, quite little capacity decay is identified in the Fe3O4@CMK-5 electrode, which keeps a quite high specific capacity of around 850
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Fig. 3. SEM images of (a) Fe3O4 and (b) Fe3O4@CMK-5, (c, d) HRTEM images of Fe3O4@CMK-5, (e, f) Magnified images of the Fe3O4 particles reveal the lattice plane (311) and (220).
mAh g1 over through 60 cycles. In contrast, pure Fe3O4 particles fade faster, and the reversible capacity decreases from 925 to 690 mAh g1 after 60 cycles. Pure CMK-5 has an even higher initial discharge capacity (2092 mAh g1), whereas its initial Coulombic efficiency (41.8%) is much lower, and its capacity dramatically declines to 459 mAh g1 within 60 cycles. These advantages demonstrate the benefits by embedding Fe3O4 into mesoporous CMK-5. Further cycling behaviors at rates varying from 100 to 10000 mA g1 indicates the excellent rate performance of Fe3O4@CMK-5 electrodes (Fig. 4c). As shown in Fig. 4d, the discharge capacity of this composite still maintains at a reasonable level of 300 mAh g1 even increasing to a quite high current density of 10000 mA g1, which finally returns to 750 mAh g1 as the rate comes back to 200 mA g1 after more than 70 cycles. Fig. 4e shows the further cycling properties of Fe3O4@CMK-5 at 2000 mA g1 in a long-term of 1000 cycles. The reversible capacity slightly fades from 607 to 520 mAh g1 over 30 cycles, and slowly rises to 800 mAh g1 after 500 cycles, then finally keeps in an impressive level of ~800 mAh g1 starting from 500 to 1000 cycles. Such an extraordinary capacity increase was also identified in amorphous Fe2O3 [47], in which a monotonic increase from 912 to 1621 mAh g1 was observed during the long term tests of 500 cycles under a cycling rate of 1000 mAh g1. Similar phenomenon has also occurred in various metal oxide systems before, like RGO/MnO/RGO
[48], MnOx/Carbon [49], CoO/Li cells [50], etc. All these anomalous capacity rises are considered to be in close connection with the reversible formation of lithium hydroxide and polymeric/gel-like film, the interfacial de-/lithiation of Li-ions, etc. [48e51]. To inspect the longer cycling stability, cycling performance at an even high rate of 10000 mA g1 is presented in Fig. 5. The specific capacity firstly enjoys an obvious drop in the first dozens of cycles, and slowly rises to 500 mAh g1 from the subsequent process to 2000 cycles, similar to the situation above. After that, it starts to drop down, and dramatically decreases to less than 50 mAh g1 after 7000 cycles. To explore the possible reason, coin-type cell after 7000 cycles tests was opened in the glove box (inserted picture), which shows that the electrolyte almost exhausts, indicating the specific capacity loss might be mainly due to the consumption of electrolyte. Then, fresh electrolyte was added into the cell, and the value of capacity immediately returns to ~500 mAh g1. After 11000 cycles, fresh electrolyte was compensated for the second time, and the specific capacity revives again, which keeps at ~400 mAh g1 even after 16000 cycles, which indicates the superior long-term stability of the Fe3O4@CMK-5 anode. To elucidate the superior stability of Fe3O4@CMK-5, SEM images of Fe3O4@CMK-5 and Fe3O4 anodes after different cycles were conducted, and the results are presented in Fig. 6. For the Fe3O4@CMK-5 anode, only few small cracks occur on the surface over the long testing of 7000
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Fig. 4. Electrochemical performances: (a) Voltagecapacity curves of Fe3O4@CMK-5 at 200 mA g1 over the voltage range of 0.01e3.0 V, (a) Cycling performance of Fe3O4@CMK-5, Fe3O4 and CMK-5 at 200 mA g1, (c) Voltagecapacity curves of Fe3O4@CMK-5 at different rates from 100 to 10000 mA g1, (d) Rate capability of Fe3O4@CMK-5 at different rates from 100 to 10000 mA g1, (e) Long cycling performances of Fe3O4@CMK-5 at 2000 mA g1.
Fig. 5. Long cycling performances of Fe3O4@CMK-5 at 10000 mA g1.
cycles (Fig. 6b), and even after 16000 cycles (Fig. 6c and d), the surface is still solid with only some main cracks. For comparison, numerous network cracks are already formed through the Fe3O4 anode only after 200 cycles, which would lead to the drastic peeling of the active material from the matrix (Fig. 6e and f). The excellent electrochemical performances of the Fe3O4@CMK5 electrodes can be attributed to the reasons that: 1) The mesoporous CMK-5 scaffold offers an effective accommodation to mechanically buffer the volume change of Fe3O4 nanoparticles, which ensures the good coulombic efficiency and cycling stability of Fe3O4@CMK-5 anodes. 2) Carbonaceous materials (CMK-5) enjoy excellent electrical conductivity [52], which constructs the conductive backbone for the electron transport (Fig. S5), thus lowering the internal resistance of Fe3O4@CMK-5 anodes (Fig. S4). Meanwhile the ohmic resistance is slightly decreased after 50 cycles, indicating the improved electron transfer ability during the cycling process by the encapsulation into tubular mesoporous carbon [53e55]. Moreover, these 2D mesoporous carbon scaffolds are also perfect transfer channels for the electrolyte, dedicating to accelerating the diffusion process of lithium ion (Fig. S5). All these
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Fig. 6. SEM images of (a) as-prepared Fe3O4@CMK-5 anode, and Fe3O4@CMK-5 anodes after different cycles: (b) 7000 cycles; (c, d) 16000 cycles; (e) as-prepared pure Fe3O4 anode; (f) Fe3O4 anode after 200 cycles.
factors contribute to the excellent rate capability and cycling performance of Fe3O4@CMK-5 anodes. 3) The confined Fe3O4 nanoparticles with small sizes possess high activity, large surface area, as well as short diffusion lengths, meanwhile the mesoporous CMK-5 networks can provide good contact for the ultrafine nanoparticles, and prevent them from aggregating and growing (Figs. 3 and S5), thus ensuring a high capacity and superior rate capability [7,56]. 4. Conclusions In summary, up to 67.7 wt% Fe3O4 was successfully confined into the tubular mesoporous carbon to form the Fe3O4@CMK-5 electrodes for LIBs. Analysis demonstrates that these Fe3O4@CMK-5 anodes offer a large specific capacity of 800 mAh g1 up to 1000 cycles at 2000 mA g1, outstanding rate capability, as well as excellent cycling stability (~400 mAh g1 over 16000 cycles at 10000 mA g1), superior than the corresponding pure Fe3O4 and CMK-5 under the same conditions. These superior electrochemical performances can be attributed to the well-defined Fe3O4@CMK-5 composites, which provide effective volume change accommodation, fast electron transport and short ionic transport lengths for the lithiation/delithiation reaction. Acknowledgements We are thankful for financial supports from the Key Research and Development Projects of Ningxia (2018BEE03003), Natural Science Foundation of Ningxia (2018AAC03052), and National Natural Science Foundation of China Projects (51801107). The part
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Encapsulated Fe3O4 into tubular mesoporous carbon as a superior performance anode material for lithium-ion batteries Zhijie Cao*, Xiaobo Ma Advanced Energy Storage Materials and Devices Laboratory, School of Physics and Electronic-Electrical Engineering, Ningxia University, Yinchuan, 750021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2019 Received in revised form 17 September 2019 Accepted 2 October 2019 Available online 3 October 2019
A well-defined composite of Fe3O4 encapsulated into tubular mesoporous carbon is prepared as an anode material for lithium-ion batteries. It turns out that up to 67.7 wt% of Fe3O4 nanoparticles are homogeneously embedded into the mesoporous carbon nanotubes. The Fe3O4@tubular mesoporous carbon composites provide effective accommodation for the volume change, fast transport paths for electrons and ions, as well as good contact for nanoparticles, thus exhibiting superior electrochemical performances. The reversible capacity maintains a high specific capacity of ~800 mAh g1 after 1000 cycles at 2000 mA g1, and the electrode still keeps a reasonable level of ~400 mAh g1 even going through 16000 cycles at 10000 mA g1, which makes Fe3O4@tubular mesoporous carbon composite a valuable anode material for lithium-ion batteries. © 2019 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries Composite Fe3O4 Mesoporous carbon Capacity
1. Introduction Rechargeable lithium-ion batteries (LIBs) have been the main power sources for portable electronic devices, and this trend gradually spreads towards the area of large-scale energy storage [1e3]. To satisfy the ever growing requirements of electric market, Li-ion batteries providing superior electrochemical performances like higher energy density, longer cycle life, excellent rate capability, etc., are demanded, which promotes the intensive research of superior electrode materials [4e7]. Among them, Fe3O4 is considered as one of the most favorable candidates considering its outstanding features of large theoretical capacity (~930 mAh g1), low cost, environmental benignancy, etc. [8e10]. However, its massive applications are still restricted by the rapid deterioration of capacity during the long term cycling process, which are associated with the degradation of Fe3O4 anodes mainly due to the large volume expansion/contraction accompanying with the lithium ions insertion/extraction [11e13]. To address these adverse issues, numerous strategies were explored to ameliorate the electrochemical properties of Fe3O4 anodes through stabilizing the structures and/or enhancing the
* Corresponding author. E-mail address: [email protected] (Z. Cao). https://doi.org/10.1016/j.jallcom.2019.152542 0925-8388/© 2019 Elsevier B.V. All rights reserved.
electrical conductivity [14e16]. Of these, designing unique Fe3O4 nanostructured materials is one of the most effective approaches to facilitate the Liþ diffusion and relieve the strain relaxation, thus ameliorating the electrochemical properties of Fe3O4 based electrodes [17e20]. For instance, the specific capacity of Fe3O4 assemblies depositing on Cu nanopillars could sustain 80% after 100 cycles at 8 C [18], and a large capacity of 580 mA h g1 was reversible over 100 cycles at 200 mA g1 in hierarchical hollow Fe3O4 microspheres, which showed a dramatical stability of lithiation/delithiation [20]. Another effective strategy to facilitate the electrochemical performances of Fe3O4 is coated or nanoconfined by carbonaceous materials, which can elastically buffer the huge volume changes, meanwhile to enhance the electronic conductivity [21e24]. Fe3O4 nanospindles coated by a carbon layer of 2e10 nm exhibited a high capacity of 530 mA h g1 during the 80 charge/ discharge cycles at C/2 [21]. While the cycling stability of 2-D ferrite-based hybrid nanosheets was much more stable than that of 3-D composite electrode: the reversible capacities of 2-D structure was approximately 600 mA h g1 while that of 3-D electrodes decreased considerably to 323 mA h g1 over the 50 cycles [22]. Yang et al. [23] prepared the well-design Fe3O4 anode with conformal Fe3O4 nanoparticles on carbon nanotubes, which possessed a delithiation capacity of 800 mA h g1 over 100 cycles at 90 mA g1. Even with substantial improvements, the stability of Fe3O4 anodes during the long-term cycling process still fails to
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satisfy the practical applications due to the huge challenge of restraining pulverization. Inspired by these aforementioned advances, designing Fe3O4/ carbon nanostructured materials could be an effective way to further improve the electrochemical properties of Fe3O4. Our previous work showed that the FeS@tubular mesoporous carbon (FeS@CMK-5) electrode consisting of FeS nanoparticles with tubular mesoporous carbon exhibited large reversible capacity, high cycling stability, excellent rate capability for both LIBS and SIBS because of its structural superiority [25]. Herein to prepare Fe3O4 based electrodes with long-tern duration, well-dispersed Fe3O4 nanoparticles encapsulated in CMK-5 were synthesized to constitute the anode materials of LIBS. These well-defined Fe3O4@CMK-5 anodes offer superior electrochemical performances including large specific capacity, outstanding rate capability and long-tern cycling life, which provides new insights for designing high performance electrodes. 2. Experimental details Materials synthesis. CMK-5 was synthesized by the same method in our previous work [25]. For the preparation of Fe3O4@CMK-5 composite, 5 mL of 0.2 M FeCl3 solution were dropwise added into 0.2 g of CMK-5 with the accompany of continuous stirring. After 4 h, this mixed solution was heated to 80 C and stirred for a period of 8 h under this condition to hydrolyzate FeCl3 to form FeOOH particles. To increase the loading amount of Fe3O4, the above process was repeated for several times, and these products were further washed three times by ethanol. Afterwards, these samples were calcined at 600 C for 4 h under argon atmosphere to form the Fe3O4@CMK-5 composite. The bare Fe3O4 particles were also synthesized under the same conditions for reference. Materials characterization. Crystallographic patterns of these electrode materials were monitored by the X-ray diffraction (XRD, PANalytical X’Pert PRO Alpha-1, Cu/Ka). The nitrogen ad-/desorption behaviors were conducted by the Quantachrome equipment (Autosorb-iQ-MP). The microscopy images were achieved on the ZEISS ULTRA55 (SEM) and JEOL JEM 2100F (TEM) instruments. Thermogravimetric analysis (TGA) was measured at 5 K/min under pure oxygen gas (NETZSCH TG 209 F3 TarsusR). Raman characterizations were recorded on a laser spectromicroscopy (Renishaw RM 1000). Electrochemical characterization. The CR2016 coin-type cells were employed for the measurement of electrochemical properties. The working electrodes were prepared by mixing active materials (Fe3O4@CMK-5, Fe3O4, CMK-5), super P, with carboxymethyl cellulose sodium salt (Na-CMC) at a mass ratio of 80:10:10. The coin
cells was assembled by electrodes, lithium sheets, electrolyte, and polyethylene film. The electrolyte solution was 1 M LiPF6 in a 1:2 (mass ratio) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), with adding 10 wt% fluoroethylene carbonate (FEC). The mass of the active anode material is ~0.38 mg and the electrode surface area is 1.13 cm2. The electrochemical tests were performed on a LAND CT2001A model battery measurement instrument (0.01e3.0 V vs. Li/Liþ). The Electrochemical Impedance Spectroscopy (EIS, 100e0.05 Hz) and Cyclic Voltammetry (CV, 0e3.0 V, 0.5 mV s1) were measured on a Gamry Interface 1000 electrochemical workstation. The amplitude for EIS measurement was 5 mV. 3. Results and discussion Fig. 1 schematically shows the illustration of synthesizing ultrafine Fe3O4 nanoparticles embedded in tubular mesoporous carbon (denoted as Fe3O4@CMK-5). After the stirring procedure at room temperature, the FeCl3 solution would be impregnated into the bimodal channels of CMK-5 due to the mesoporous nature of carbon scaffold. Subsequently, this resulting solution was heated and dried at 80 C, which seems to cause FeCl3 to yield FeOOH nanoparticles encapsulated inside the bimodal channels of CMK-5. Finally, the FeOOH@CMK-5 composite was calcined under Ar to oxidize the FeOOH particles, which led to the formation of discrete Fe3O4 nanoparticles embedded in mesoporous carbon. The loading of Fe3O4 particles into CMK-5 can be confirmed by the XRD, TG, Raman, and nitrogen ad-/desorption analysis. The XRD patterns in Fig. 2a shows that all these sharp reflections of Fe3O4@CMK-5 belong to Fe3O4, indicating the highly crystallized nature of these nanoparticles. While for the two broad signals of CMK-5, they are invisible in the pattern of Fe3O4@CMK-5 probably due to the much lower electron densities of carbon materials in contrast with Fe3O4 [7]. TG curve in the temperature range of 40e800 C is shown in Fig. 2b. Upon heating to 160 C, it presents a weight loss of 0.5 wt% because of the liberation of moisture, and then the following mass increase between 160 C and 340 C is likely to originate from the oxidation of Fe3O4 to Fe2O3 during the calcination process [26]. Finally, carbon is oxidized to CO2 from 340 to 800 C with the accompany of a weight loss of 33.2 wt%. Therefore, the loading fraction of Fe3O4 is 67.7 wt% based on the calculation of residual mass, and such a high original percentage could guarantee the high specific capacity of anode materials. Raman spectra of this Fe3O4@CMK-5 composite and CMK-5 (Fig. 2c) clearly display the distinguishable D- and G-bands of carbon materials peaking at 1346 and 1592 cm1, respectively. Normally, D-band is interrelated with the degree of disorder, edges and other defects in amorphous form, contributing to the activation of zone edge modes
Fig. 1. Illustration of the synthesis process of Fe3O4@CMK-5.
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Fig. 2. (a) XRD patterns, (b) TG curve, (c) Raman spectra, (d) N2 adsorption-desorption isotherms of CMK-5 and Fe3O4@CMK-5.
in carbon materials, while the G-band originates from the E2g stretching vibrations in crystalline carbon [27e29]. With the incorporation of Fe3O4, the D-band intensity increases while the Gband decreases, meanwhile the value of ID/IG (the ratio of peak intensity) increases from 0.96 to 1.60, which indicates an accumulation of structural defects and the generation of nanocrystalline particles [30]. Fig. 2d compares the N2 adsorption-desorption isotherms of Fe3O4@CMK-5 and CMK-5 at 196 C. The specific BET surface area is dramatically decreased from 1633 m2/g to 539 m2/g after the incorporation of Fe3O4, confirming that most of the pore systems of CMK-5 are occupied/blocked by the Fe3O4 nanoparticles. Meanwhile, the Fe3O4@CMK-5 still enjoys a luxurious porosity as well as high specific surface area, thus achieving the dual effects of buffering the volume expansion and facilitating the conductivity of electron/ion during the cycling process of lithium ion insertion/ extraction [31]. Fig. 3a shows the SEM image of Fe3O4 particles, in which a tabular structure with the particle size of ~200 nm presents in a rather homogenous form. While as to Fe3O4@CMK-5, as shown in Fig. 3b, much smaller Fe3O4 particles (white dots) uniformly distribute on the surfaces of CMK-5. The homogeneous distribution of Fe3O4 particles in this composite can be also confirmed by the EDS elemental mapping images (Fig. S1). Further TEM images of Fe3O4@CMK-5 (Fig. 3c and d) demonstrate that Fe3O4 nanoparticles around 5e10 nm (black arrows) are dispersedly embedded in the CMK-5 scaffolds. The interplanar distances for the (311) and (220) planes of Fe3O4 are clearly distinguished (Fig. 3e and f), proving the encapsulated nanoparticles to be Fe3O4. All these evidences verify that the well distributed Fe3O4 nanoparticles on/inside the mesoporous carbon with excellent electrical conductivity may offer an effective accommodation for the volume changes of Fe3O4 particles through the lithiation-delithiation cycles, but also facilitate the transportation of electron and ion, thus enabling Fe3O4@CMK-5 to be a favorable choice for the anodes of LIBs [32e34]. Moreover, high tapping density is important to ensure large volumetric density and
process ability for practical applications [35,36], hence we measured the tapping density of Fe3O4, Fe3O4@CMK-5 and CMK-5 following the method used in Ref. [37]. Results (Fig. S2) show that the pure Fe3O4 nanoparticles have a high tapping density of 1.29 g cm3, while tubular mesoporous carbon CMK-5 displays a quite low value of 0.064 g cm3. After the encapsulation of 67.7 wt% Fe3O4, the Fe3O4@CMK-5 sample enjoys a tapping density of 0.18 g cm3. Although not a large value, it is still higher than that of carbon-coated Fe3O4 nanoparticle (3.0 mg cm3) [38], and nanosized silicon (0.16 g cm3) [39]. The electrochemical performances of Fe3O4@CMK-5 composites were evaluated between 0.01 and 3.0 V. For comparison, the electrochemical performances of pure Fe3O4 and CMK-5 were also measured under the same conditions. Fig. 4a presents the voltagecapacity patterns of the Fe3O4@CMK-5 electrode under 200 mA g1. An apparent discharge plateau at ~0.7 V is observed in the initial cycle, and then slightly shifts to about 0.9 V and stabilizes at this voltage in the following cycles, agreeing well with previous reports [21,40,41]. Meanwhile, it is noted that the plateau at 1.8 V is poorly identified. As shown in Fig. S3c, the cathodic peak of the first cycle corresponds to the reduction of Fe3O4 as well as the reversible formation of SEI film. All these observations above agree well with the typical cyclic voltammetry profiles of the discharge-charge processes of Fe3O4 in other works [42e44]. The Fe3O4@CMK-5 anode reveals a high stable cycling performance over 60 cycles at 200 mA g1 (Fig. 4b). The initial discharge and charge capacities are 1637 mA h g1 and 958 mA h g1, respectively, with an initial Coulombic efficiency of 58.5%. Such an initial irreversible capacity distance originates from the formation of a solid electrolyte interphase (SEI) film [45] and incomplete conversion reaction between Fe3O4 and Fe0 [46]. Then, the electrode at the 2nd cycle delivers a lithiation and delithiation of 950 and 912 mAh g1, respectively, meanwhile the coulombic efficiency increases to 96%. Afterwards, quite little capacity decay is identified in the Fe3O4@CMK-5 electrode, which keeps a quite high specific capacity of around 850
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Fig. 3. SEM images of (a) Fe3O4 and (b) Fe3O4@CMK-5, (c, d) HRTEM images of Fe3O4@CMK-5, (e, f) Magnified images of the Fe3O4 particles reveal the lattice plane (311) and (220).
mAh g1 over through 60 cycles. In contrast, pure Fe3O4 particles fade faster, and the reversible capacity decreases from 925 to 690 mAh g1 after 60 cycles. Pure CMK-5 has an even higher initial discharge capacity (2092 mAh g1), whereas its initial Coulombic efficiency (41.8%) is much lower, and its capacity dramatically declines to 459 mAh g1 within 60 cycles. These advantages demonstrate the benefits by embedding Fe3O4 into mesoporous CMK-5. Further cycling behaviors at rates varying from 100 to 10000 mA g1 indicates the excellent rate performance of Fe3O4@CMK-5 electrodes (Fig. 4c). As shown in Fig. 4d, the discharge capacity of this composite still maintains at a reasonable level of 300 mAh g1 even increasing to a quite high current density of 10000 mA g1, which finally returns to 750 mAh g1 as the rate comes back to 200 mA g1 after more than 70 cycles. Fig. 4e shows the further cycling properties of Fe3O4@CMK-5 at 2000 mA g1 in a long-term of 1000 cycles. The reversible capacity slightly fades from 607 to 520 mAh g1 over 30 cycles, and slowly rises to 800 mAh g1 after 500 cycles, then finally keeps in an impressive level of ~800 mAh g1 starting from 500 to 1000 cycles. Such an extraordinary capacity increase was also identified in amorphous Fe2O3 [47], in which a monotonic increase from 912 to 1621 mAh g1 was observed during the long term tests of 500 cycles under a cycling rate of 1000 mAh g1. Similar phenomenon has also occurred in various metal oxide systems before, like RGO/MnO/RGO
[48], MnOx/Carbon [49], CoO/Li cells [50], etc. All these anomalous capacity rises are considered to be in close connection with the reversible formation of lithium hydroxide and polymeric/gel-like film, the interfacial de-/lithiation of Li-ions, etc. [48e51]. To inspect the longer cycling stability, cycling performance at an even high rate of 10000 mA g1 is presented in Fig. 5. The specific capacity firstly enjoys an obvious drop in the first dozens of cycles, and slowly rises to 500 mAh g1 from the subsequent process to 2000 cycles, similar to the situation above. After that, it starts to drop down, and dramatically decreases to less than 50 mAh g1 after 7000 cycles. To explore the possible reason, coin-type cell after 7000 cycles tests was opened in the glove box (inserted picture), which shows that the electrolyte almost exhausts, indicating the specific capacity loss might be mainly due to the consumption of electrolyte. Then, fresh electrolyte was added into the cell, and the value of capacity immediately returns to ~500 mAh g1. After 11000 cycles, fresh electrolyte was compensated for the second time, and the specific capacity revives again, which keeps at ~400 mAh g1 even after 16000 cycles, which indicates the superior long-term stability of the Fe3O4@CMK-5 anode. To elucidate the superior stability of Fe3O4@CMK-5, SEM images of Fe3O4@CMK-5 and Fe3O4 anodes after different cycles were conducted, and the results are presented in Fig. 6. For the Fe3O4@CMK-5 anode, only few small cracks occur on the surface over the long testing of 7000
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Fig. 4. Electrochemical performances: (a) Voltagecapacity curves of Fe3O4@CMK-5 at 200 mA g1 over the voltage range of 0.01e3.0 V, (a) Cycling performance of Fe3O4@CMK-5, Fe3O4 and CMK-5 at 200 mA g1, (c) Voltagecapacity curves of Fe3O4@CMK-5 at different rates from 100 to 10000 mA g1, (d) Rate capability of Fe3O4@CMK-5 at different rates from 100 to 10000 mA g1, (e) Long cycling performances of Fe3O4@CMK-5 at 2000 mA g1.
Fig. 5. Long cycling performances of Fe3O4@CMK-5 at 10000 mA g1.
cycles (Fig. 6b), and even after 16000 cycles (Fig. 6c and d), the surface is still solid with only some main cracks. For comparison, numerous network cracks are already formed through the Fe3O4 anode only after 200 cycles, which would lead to the drastic peeling of the active material from the matrix (Fig. 6e and f). The excellent electrochemical performances of the Fe3O4@CMK5 electrodes can be attributed to the reasons that: 1) The mesoporous CMK-5 scaffold offers an effective accommodation to mechanically buffer the volume change of Fe3O4 nanoparticles, which ensures the good coulombic efficiency and cycling stability of Fe3O4@CMK-5 anodes. 2) Carbonaceous materials (CMK-5) enjoy excellent electrical conductivity [52], which constructs the conductive backbone for the electron transport (Fig. S5), thus lowering the internal resistance of Fe3O4@CMK-5 anodes (Fig. S4). Meanwhile the ohmic resistance is slightly decreased after 50 cycles, indicating the improved electron transfer ability during the cycling process by the encapsulation into tubular mesoporous carbon [53e55]. Moreover, these 2D mesoporous carbon scaffolds are also perfect transfer channels for the electrolyte, dedicating to accelerating the diffusion process of lithium ion (Fig. S5). All these
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Fig. 6. SEM images of (a) as-prepared Fe3O4@CMK-5 anode, and Fe3O4@CMK-5 anodes after different cycles: (b) 7000 cycles; (c, d) 16000 cycles; (e) as-prepared pure Fe3O4 anode; (f) Fe3O4 anode after 200 cycles.
factors contribute to the excellent rate capability and cycling performance of Fe3O4@CMK-5 anodes. 3) The confined Fe3O4 nanoparticles with small sizes possess high activity, large surface area, as well as short diffusion lengths, meanwhile the mesoporous CMK-5 networks can provide good contact for the ultrafine nanoparticles, and prevent them from aggregating and growing (Figs. 3 and S5), thus ensuring a high capacity and superior rate capability [7,56]. 4. Conclusions In summary, up to 67.7 wt% Fe3O4 was successfully confined into the tubular mesoporous carbon to form the Fe3O4@CMK-5 electrodes for LIBs. Analysis demonstrates that these Fe3O4@CMK-5 anodes offer a large specific capacity of 800 mAh g1 up to 1000 cycles at 2000 mA g1, outstanding rate capability, as well as excellent cycling stability (~400 mAh g1 over 16000 cycles at 10000 mA g1), superior than the corresponding pure Fe3O4 and CMK-5 under the same conditions. These superior electrochemical performances can be attributed to the well-defined Fe3O4@CMK-5 composites, which provide effective volume change accommodation, fast electron transport and short ionic transport lengths for the lithiation/delithiation reaction. Acknowledgements We are thankful for financial supports from the Key Research and Development Projects of Ningxia (2018BEE03003), Natural Science Foundation of Ningxia (2018AAC03052), and National Natural Science Foundation of China Projects (51801107). The part
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