A novel shuttle-like Fe₃O₄–Co₃O₄ self-assembling architecture with highly reversible lithium storage

Abstract

We report the synthesis of a novel shuttle-like Fe₃O₄–Co₃O₄ self-assembling architecture through a facile hydrothermal method. The composite manifests quite excellent electrochemical performance as anodes of Li-ion batteries. It delivers a high reversible capacity of ∼1013 mA h g⁻¹ at 100 mA g⁻¹ after 100 cycles (∼1246 mA h g⁻¹ at 0.5 A g⁻¹, 864 mA h g⁻¹ at 1 A g⁻¹ and 625 mA h g⁻¹ at 2 A g⁻¹ after 50 cycles). The rate performance is also outstanding with a reversible capacity of 660 mA h g⁻¹ even at 5 A g⁻¹ and a capacity retention of 94% after 78 cycles. The synergetic effect of Co₃O₄ and Fe₃O₄, as well as the unique self-assembling architecture, may be responsible for the large enhanced electrochemical performance.

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Introduction

Over the past decades, rechargeable lithium-ion batteries (LIBs) have been widely applied to large-scale stationary energy storage, electric vehicles and portable consumer electronics.^1,2 The rapid development of these LIB-powered products demands new-generation LIBs with higher energy density and power density. Traditional low-capacity commercial graphite anodes can hardly meet current demand and novel anode materials urgently need to be developed.^3–5 In recent years, much attention has been paid to transition metal oxides due to their distinctive conversion mechanism of lithium storage, easily tuned morphology and high theoretical capacity, especially Co₃O₄ (890 mA h g⁻¹) and Fe₃O₄ (924 mA h g⁻¹).^6–9 Various pristine Co₃O₄ nanostructures have been intensively investigated as anodes of LIBs, however, the poor electrical conductivity and ion transport kinetics greatly pull down their long-cycle electrochemical performance.^9–12 Therefore, it's necessary to introduce secondary component to modify Co₃O₄, it can help enhance the electrochemical reactivity, electrical/ionic conductivity and mechanical stability.⁹ Compared with Co₃O₄, Fe₃O₄ has not only similar specific capacity but also other unique merits including much better electrical conductivity, abundance, low cost and environmental friendliness.^13–17 So, building novel Fe₃O₄–Co₃O₄ nanostructure is likely to achieve superior long-cycle performance without the sacrifice of capacity. Similar metal oxide composites like Fe₂O₃–SnO₂,¹⁸ CuO–Co₃O₄,¹⁹ Fe₂O₃–Co₃O₄,^20–22 have manifested outstanding electrochemical performance. Wang et al. grew Co₃O₄ nanosheets onto CuO nanowire array through chemical bath deposition, the composite achieved a reversible capacity of 1191 mA h g⁻¹ (90.9% capacity retention) after 200 cycles at 200 mA g⁻¹, and ∼560 mA h g⁻¹ even at high current density of 2.5 A g⁻¹.¹⁹ Hierarchically Fe₂O₃@Co₃O₄ nanowire array was also prepared as anode by hydrothermal synthesis and hydrolysis method, and 1005 mA h g⁻¹ at 200 mA g⁻¹ over 50 cycles was gained.²⁰ Xiong et al. reported a α-Fe₂O₃/Co₃O₄ branched nanowire heterostructure fabricated via hydrothermal synthesis which delivered a capacity of ∼1000 mA h g⁻¹ after 60 cycles.²¹ Also, double-shelled Fe₂O₃/Co₃O₄ hollow microcubes obtained a specific capacity of 500 mA h g⁻¹ after 50 cycles.²²

Herein, we report the synthesis of an unprecedented shuttle-like Fe₃O₄–Co₃O₄ self-assembling architecture through a facile hydrothermal method with ethanol as solvent. In our work, Fe₃O₄ nanoparticles are introduced as structure-building agent, they can gather the produced Co₃O₄ nanoparticles. The formation mechanism and the obtained architecture are distinct from the reported hierarchical heterostructures that big matrices are usually adopted for the synthesis.^{18–20,23,24} This novel self-assembling architecture can offer several remarkable advantages, such as alleviated nanoparticle aggregation due to the regular assembly, short electron/ion diffusion path and enhanced electronic conductivity due to the close contact of nanoparticles. The obtained composite exhibits quite excellent electrochemical performance, which is superior to the pristine Co₃O₄ (ref. 25–33) and many other reported Co₃O₄-based composites.^21,22,34,35

Experimental section

Materials synthesis

Firstly, 3.0 g Co(NO₃)₂·6H₂O and 3.0 g urea were dissolved in 60 mL ethanol under mechanical stirring. Then 0.2 g Fe₃O₄ powder (nanoparticles, cluster size 100–300 nm, Aladdin) was added into the solution above, and 0.5 g hexadecyl trimethyl ammonium chloride (CTAC) was introduced as surfactant. The stirring was continued for 2 hours to form a uniform suspension which was subsequently transferred into a 100 mL Teflon-lined stainless autoclave. After sealed, the autoclave was maintained at 180 °C for 12 h in a drying oven, and then cooled down naturally to room temperature. After that, the precipitate was filtered and washed with ethanol and distilled water for several times, respectively, then dried at 60 °C. Finally, the collected products were calcined at 400 °C for 5 h at air atmosphere to prepare the shuttle-like Fe₃O₄–Co₃O₄ self-assembling architecture (SFC). As comparison, SFC composites and their precursors (before calcination) with 3 h, 6 h, 9 h hydrothermal reaction were obtained, the pristine Co₃O₄ was prepared without the addition of Fe₃O₄ nanoparticles and CTAC.

Electrodes and cells fabrication

Coin-type half cells were fabricated to evaluate the electrochemical properties of prepared materials. The anode slurry, which consists of active materials, conductive carbon blacks (Super P), poly(vinylidene fluoride) (PVDF) at a weight ratio of 7 : 2 : 1, was uniformly coated on Cu foils and then cut into discs as electrodes after drying. N-Methyl-2-pyrrolidone (NMP) was selected as slurry solvent. 1 M LiPF₆ in a solvent mixture of ethylene carbonate and diethylene carbonate (1 : 1 vol%) and lithium metals were used as the electrolyte and the counter electrode, respectively.

Characterization

XRD and XPS characterizations were performed on powder X-ray diffraction (XRD, BRUKER D8 ADVANCE, Cu K radiation (1.5406)) and X-ray Photoelectron Spectroscopy (XPS, Kratos Axis Ultra DLD), respectively. SEM and TEM observations were performed on field-emission scanning electron microscopy (FESEM, ZEISS Ultra 55, 5 kV, Pt-spraying treatment) and transmission electron microscopy (TEM, JEM-2100HR, 200 kV), respectively. EDS characterization was performed on energy-dispersive X-ray spectrometers (EDS, Oxford X-Max 50, FESEM attachment, INCA 4.15; EDS, IET250, TEM attachment, JEMS software package). BET test was performed on Surface Area and Porosity Analyzer (Micromeritics, ASAP 2020). Raman spectra were measured with a microscopic Raman spectrometer (Nippon Optical System Co., Japan, at 632.8 nm).

The charge/discharge test of the assembled cells was performed on the LAND cell test system (Land CT 2001A) at room temperature. And the specific capacity was calculated on the basis of the weight of prepared products contained in the anode. Cyclic voltammetry (CV) experiment was carried out with an electrochemical analyzer (CHI 660D).

Results and discussion

Formation mechanism

To investigate the formation mechanism of SFC, the SEM images of Co₃O₄, Fe₃O₄ and SFC are shown for comparison (Fig. 1). For Co₃O₄ (Fig. 1a), uniform separate nanoparticles of 30–50 nm in diameter can be clearly identified, and serious aggregation occurs. For Fe₃O₄ (Fig. 1b), the separate nanoparticles aggregate more heavily into uniform big clusters with an average size up to ∼200 nm in diameter. Judging from the exposed particles, each nanoparticle is much smaller. For SFC (Fig. 1c), it can be seen that nanoparticles are regularly assembled to form uniform shuttle-like structures. The middle maximum diameter of the shuttle-like structure is ∼50 nm. Evidently, there are several nanoparticles in the middle, so each particle is less than 25 nm in diameter, indicating the refining of Co₃O₄ nanoparticles in SFC. Compared with Co₃O₄ and Fe₃O₄, the nanoparticle aggregation of SFC is greatly relieved and the particles are more closely linked in each shuttle-like unit. This configuration can help improve the mechanical stability and reduce the impedance of the composite.

Fig. 1 SEM images of Co₃O₄ (a), Fe₃O₄ (b) and SFC (c). (The insets are the magnified images.)

Then we further investigate the effect of hydrothermal reaction time and heat treatment on the morphology of SFC. Fig. 2 shows the SEM images of SFC precursors (before calcination) with different reaction time. With 3 h reaction (Fig. 2a), the self-assembling units are small and no obvious shuttle-like structure can be identified. Under such short-time reaction, the amount of alcoholysis Co⁺ is so limited that the self-assembling units can't gather enough products to form obvious shuttle-like structures (Fig. 2a). With a longer reaction time of 6 h, a part of small shuttle-like structures emerge, but there are still many self-assembling units with much smaller size (Fig. 2b). When the reaction time increases to 9 h, the self-assembling units continue to grow bigger, and more obvious shuttle-like structures can be identified (Fig. 2c). With 12 h reaction (Fig. 2d), the composite exhibits more uniform shuttle-like units, the size of the big ones almost stay the same. It can be concluded that the self-assembling units form at the beginning of the alcoholysis reaction and just grow bigger due to the further alcoholysis of Co²⁺ with increased reaction time. Fig. 3 presents the SEM images of SFC (after calcination) with different reaction time. Compared with those before calcination, the unit shape has no obvious change, however, in each unit obvious space occurs between the assembled nanoparticles and thus separate nanoparticles can be easily identified. This is caused by the emission of CO₂ and H₂O during heat treatment.

Fig. 2 SEM images of SFC precursors (before calcination) with different reaction time: 3 h (a), 6 h (b), 9 h (c), 12 h (d).

Fig. 3 SEM images of SFC (after calcination) with different reaction time: 3 h (a), 6 h (b), 9 h (c), 12 h (d).

Based on the analysis above, the possible formation mechanism of the SFC self-assembling architecture can be illustrated as Scheme 1. With the introduction of Fe₃O₄ nanoparticles, a part of Co²⁺ adhere to the surface of Fe₃O₄ nanoparticles under stirring. During hydrothermal process, the alcoholysis products first nucleate on the surface of Fe₃O₄ nanoparticles, these nuclei are regularly assembled due to the magnetic force of Fe₃O₄ under fixed condition. Then the small self-assembling structures grow bigger with the continuous alcoholysis of residual Co²⁺. In this process, the growth of each alcoholysis nucleus is limited by the squeeze from the synchronous growth of other nuclei, leading to the downsizing and shape variation of the alcoholysis particles. After calcination, the alcoholysis product converts to Co₃O₄. Besides, space occurs between the assembled nanoparticles due to the emission of CO₂ and H₂O during heat treatment and thus separate nanoparticles can be easily identified, resulting in the final SFC self-assembling architecture.

Scheme 1 Schematic illustration of the possible formation mechanism of Fe₃O₄–Co₃O₄ self-assembling architecture (SFC).

Composition and structure characterization

The phase composition of SFC is determined by XRD (Fig. 4a). All peaks can be indexed to Co₃O₄ (JCPDS no. 42-1467) and Fe₃O₄ (JCPDS no. 33-0664), indicating that SFC is composed of Co₃O₄ and Fe₃O₄. The main characteristic peak of Co₃O₄ is stronger than that of Fe₃O₄, it can be inferred that Co₃O₄ is the major phase in the composite. The sharp and strong peaks attest the good crystallinity and high crystalline purity, no obvious impurity peaks were detected. Compared with the pattern of pristine Co₃O₄, it can be seen that Co₃O₄ in SFC has no obvious change in crystalline phase. To further identify the amount of two phases in SFC, overall EDS characterization was performed under SEM image (Fig. 4b). The molar ratio of Fe/Co is approximately 1/14, the corresponding molar ratio of Fe₃O₄/Co₃O₄ is 1 : 14, suggesting that Fe₃O₄ mainly acts as the structure-building agent and contributes little to the capacity of SFC. The result is consistent with XRD analysis and the proposed formation mechanism. The theoretical specific capacity of SFC is calculated to be ∼900 mA h g⁻¹. To study the surface chemistry of SFC, X-ray photoelectron spectroscopy (XPS) was performed as shown in Fig. 5. The survey spectrum in Fig. 5a demonstrates the existence of Co, Fe and O elements. Then the core level spectrum of Co is presented in Fig. 5b to analyze the chemical composition and chemical oxidation state of Co in SFC. As can be seen, there are two peaks located at 794.7 eV and 779.9 eV corresponding to Co 2p_1/2 and Co 2p_3/2, respectively, which is attributed to the presence of Co²⁺ and Co³⁺ in the produced Co₃O₄ phase.^36–38 To evaluate the specific surface areas of SFC and pristine Co₃O₄, the nitrogen adsorption–desorption isotherm was obtained (Fig. 6). The BET specific surface area of SFC is determined to be 27.05 m² g⁻¹, this is almost equal to that of pristine Co₃O₄ (25.18 m² g⁻¹).

Fig. 4 (a) XRD patterns of SFC and Co₃O₄; (b) overall EDS of SFC.

Fig. 5 XPS spectra of SFC: survey spectrum (a) and wide spectrum of Co 2p (b).

Fig. 6 N₂ sorption isotherms of SFC and pristine Co₃O₄.

To further understand the structure of SFC, the Raman scattering is recorded in the spectral range 200–1400 cm⁻¹ for SFC and pristine Co₃O₄ (Fig. 7). The Raman spectrum of the pristine Co₃O₄ displays bands at 485, 508, 635 and 693 cm⁻¹.^39,40 For SFC, the peaks detected at around 326, 521, and 715 cm⁻¹ are assigned to the Fe₃O₄, while the other Raman bands are in good agreement with the Co₃O₄ compounds, and have plenty of overlap with that of Fe₃O₄.⁴¹ As can be seen, in this work, adding Fe₃O₄ can increase the intensity of the Raman bands of Co₃O₄, indicating that the existence of a small amount of Fe₃O₄ does favor to the structure of SFC, leads to the synergetic effect of Co₃O₄ and Fe₃O₄, and the Raman spectra is consistent with the XRD and XPS.

Fig. 7 Raman spectra of SFC and pristine Co₃O₄.

To analyze the specific structure of SFC individuals, TEM images of SFC are shown in Fig. 8a. We can perspicuously observe that the shuttle-like units consist of many nanoparticles, further attesting the self-assembling architecture. Also, it can be clearly seen that the average size of the particles is ∼20 nm, which manifests the downsizing of Co₃O₄ nanoparticles. EDS of a SFC individual was also performed (Fig. 8b). The molar ratio of Fe/Co is determined as 1/15, the corresponding molar ratio of Fe₃O₄/Co₃O₄ is 1 : 15, which is in accordance with the overall EDS result. Fig. S1† exhibits the HRTEM image of SFC. The evident crystal lattices and the obscure ones are clearly separated. The d-spacing value of the evident ones are measured to be 0.24 nm and this can be indexed to the (311) plane of Co₃O₄. Apparently, the crystallinity of (311) plane is the best and particles of this crystal orientation are in majority, which agrees with the XRD result. Fig. S2† shows the selected-area electron diffraction (SAED) pattern of SFC. Several legible diffraction rings can be identified, indicating the polycrystalline structure of SFC.

Fig. 8 TEM (the inset is the image after ultrasonic treatment) of SFC (a); EDS of a single SFC unit (b).

Electrochemical performance

Cyclic voltammetry (CV) measurement at a scan rate of 0.2 mV s⁻¹ was carried out to analyze the electrochemical reaction of SFC, Co₃O₄ and Fe₃O₄ (Fig. 9a). During the 1^st cycle, pristine Co₃O₄ and Fe₃O₄ both exhibit only one cathodic peak,^42,43 while SFC exhibits two partially overlapped reduction peaks at ∼0.75 V and ∼0.65 V. Obviously, the two peaks are mainly attributed to the electrochemical reduction of Fe₃O₄, Co₃O₄ to metal Fe, Co and the formation of Li₂O, partly contributed by the decomposition of electrolyte and the formation of SEI film.^19,22,42 Given that Co₃O₄ is the major phase in SFC, the big peak located at ∼0.75 V is related to Co₃O₄ and the small one at ∼0.65 V is associated with Fe₃O₄, indicating that the reduction of Co₃O₄ occurs earlier than that of Fe₃O₄. It means that Fe₃O₄ phase can well maintain the conductivity of SFC with the reduction of Co₃O₄ phase to form Li₂O and metal Co, and then metal Co can act in similar way to promote the electrochemical reduction of Fe₃O₄ phase, so two phases can achieve a synergetic effect.¹⁸ Then during the anodic process, the electrochemical reactions of SFC reverse with the decomposition of Li₂O and the reformation of Fe₃O₄ and Co₃O₄, resulting in the occurrence of two continuous oxidation peaks at ∼1.7 V and ∼2.1 V. Compared with the oxidation peak locations of pristine Co₃O₄ and Fe₃O₄, it can be concluded that the former small oxidation peak is corresponding to Fe₃O₄ (the minor phase) and the latter big one to Co₃O₄ (the major phase). In subsequent cycles, due to the absent of Li-consuming side reactions at low potential, the reduction peaks of three sample all become smaller and shift to higher potential close to theoretical values.^42,44,45 Noting that, as to the reduction peak location difference between the initial and following cycles, the pristine Co₃O₄ shifts from 0.46 V to 0.93 V and Fe₃O₄ from 0.47 V to 0.88 V, while SFC shifts from 0.77 V to 1.02 V for Co₃O₄ phase and 0.66 V to 0.89 V for Fe₃O₄ phase. Obviously, SFC presents smaller shift than the pristine Co₃O₄ and Fe₃O₄. This indicates that the irreversible capacity caused by the side reactions takes smaller proportion in total capacity for SFC in initial discharge process. It can be seen that the electrochemical activity of SFC is large enhanced due to the assembly of Co₃O₄ and Fe₃O₄ nanoparticles. In addition, except for the initial cathodic curve, the CV curves afterwards coincide well, manifesting the excellent cycle stability of SFC.

Fig. 9 (a) CV curves of SFC, Co₃O₄ and Fe₃O₄ tested at scan rate of 0.2 mV s⁻¹. (b) Discharge/charge curves of SFC at 100 mA g⁻¹.

To study the charge/discharge properties of SFC, we obtain the charge/discharge curves at 100 mA g⁻¹ (Fig. 9b). It can be seen that the discharge process can be divided into four stages (denoted as 3.0–2.5 V, 2.5–1.25 V, 1.25–0.75 V, 0.75–0.01 V). In 3.0–2.5 V, there is almost no capacity contribution, 2.5–1.25 V contributes little with a capacity of ∼100 mA h g⁻¹. Known from the CV analysis above, the main electrochemical reactions don't occur in this potential stage, so this small part of capacity may be contributed by some physical lithium storage at the interface. It exists in both initial and later cycles, indicating that this part of capacity is partially reversible.^46,47 Then 1.25–0.01 V contributes the most capacity, suggesting that the major electrochemical reactions occur in this stage. Co₃O₄ and Fe₃O₄ are converted to Co, Fe and Li₂O forms simultaneously, accompanied by the decomposition of electrolyte and the formation of SEI film at lower potential.^19,21 There should be two plateaus in this stage corresponding to Co₃O₄ and Fe₃O₄, however, only one potential plateau can be clearly identified at ∼1.0 V, indicating that two plateaus are continuous and emerge at similar potential. This has been proved by the two highly overlapped reduction peaks in the CV curves above. Since the Li-consuming SEI film formation mainly occurs in 1^st cycle, so the plateaus is longer in 1^st cycle than following cycles. For charge process, the capacity is mainly contributed by 1.25–2.5 V and no evident plateau occurs, this is corresponding to the wide oxidation peak which consists of two continuous peaks. Noting that, discharge/charge paths at different cycles keep nearly the same, the discharge plateau is still evident even at the 40^th cycle, manifesting the excellent discharge/charge stability of SFC.

Fig. 10a exhibits the cycle performance curves of SFC, pristine Co₃O₄ and Fe₃O₄ at a constant current density of 100 mA g⁻¹. Apparently, compared with pristine Co₃O₄ and Fe₃O₄, the cycle performance of SFC has marked improvement. SFC delivers an initial discharge/charge capacity of 1434/1138 mA h g⁻¹, corresponding to a high initial coulombic efficiency of 79.4%, and these values are respectively 1242/882/71% for Co₃O₄ and 1055/757/71.7% for Fe₃O₄, lower than those of SFC. Obviously, three samples have suffered similar irreversible capacity loss (∼300 mA h g⁻¹) in the first cycle. This is mainly caused by Li-consuming side reactions including electrolyte decomposition and SEI film formation which are highly associated with the specific surface area. The higher initial discharge capacity of SFC than pristine Fe₃O₄ and Co₃O₄ may be attributed to more interfacial lithium storage for the novel SFC self-assembling architecture.^46,47 After the initial capacity loss, the capacity of SFC remains stable, leading to a reversible capacity of ∼1013 mA h g⁻¹ after 100 cycles. Even after 200 cycles, the capacity attains ∼867 mA h g⁻¹, which is still close to the theoretical capacity of Fe₃O₄ and Co₃O₄. These may be contributed by several ways. On one hand, due to the emission of CO₂ and H₂O during calcination, small space occurs between the assembled nanoparticles of SFC units, it provides partially reversible interfacial lithium storage.^46,47 On the other hand, the reversible formation/dissolution of a polymeric gel-like film originating from electrolyte decomposition can also contribute part of capacity.^19,48–52 While pristine Co₃O₄ suffers evident capacity decay after only 40 cycles and the capacity finally decreases to ∼469 mA h g⁻¹ after 100 cycles, Fe₃O₄ suffers large capacity degradation even in early stage and the capacity maintains at ∼490 mA h g⁻¹ over 100 cycles. As a whole, SFC obtains a much longer stable stage, also degrades much slower later, and finally gets a higher reversible capacity than the pristine Fe₃O₄ and Co₃O₄. It can be inferred that the unique shuttle-like self-assembling architecture has take a critical promoting role in the cycling. Due to the synergetic effect of two integrated phases, the reversibility of electrochemical reactions for SFC has marked enhancement. On one way, the introduction of more conductive Fe₃O₄ phase and the close contact of two phases effectively promote electrochemical reactivity of Co₃O₄ phase. On another way, the porous shuttle-like shape effectively restrains the agglomeration of nanoparticles and provides more space for volume variation of electrodes, so as to better maintain the integrity of electrode in long cycle, also it can promote the electrolyte infiltration of electrode.^53–55

Fig. 10 (a) Cycle performance curves of SFC, pristine Co₃O₄ and Fe₃O₄ at 100 mA g⁻¹ (the inset is the cycle performance of SFC about 200 cycles); (b) rate performance curves of SFC and Co₃O₄; (c) long-cycle performance curves of SFC at 0.5 A g⁻¹, 1 A g⁻¹, 2 A g⁻¹.

Fig. 10b presents the rate performance curves of SFC and pristine Co₃O₄ nanoparticles at varied current density from 0.1 A g⁻¹ to 5 A g⁻¹. At ≤1 A g⁻¹, both manifest excellent performance in each current stage and the capacity can return to the initial level after 60 cycles with the current density returning to 100 mA g⁻¹, but SFC shows better stability and higher reversible capacity. When the current density increases to 2 A g⁻¹ and 5 A g⁻¹, the capacity damping occurs for both samples, however, SFC decreases much slower and still maintains a high capacity of ∼1000 mA h g⁻¹ at 2 A g⁻¹ over 66 cycles and ∼660 mA h g⁻¹ at 5 A g⁻¹ over 72 cycles. When the current density again returns to 100 mA g⁻¹, both samples still can come back to the original capacity level, indicating that the electrodes aren't destroyed even at 5 A g⁻¹. The capacity of SFC is stable and reaches ∼1320 mA h g⁻¹ (94% capacity retention vs. initial discharge capacity) after 78 cycles. To further evaluate the cycle performance of SFC under large current density, its long-cycle performance curves at 0.5 A g⁻¹, 1 A g⁻¹ and 2 A g⁻¹ were obtained (Fig. 10c). As can be seen, the capacity of SFC maintains steadily at ∼1246 mA h g⁻¹ at 0.5 A g⁻¹ over 50 cycles. At 1 A g⁻¹ and 2 A g⁻¹, it is stable in early stage and decreases slowly to lower level. After 50 cycles, SFC still obtains a high reversible capacity of 864 mA h g⁻¹ at 1 A g⁻¹ and 625 mA h g⁻¹ at 2 A g⁻¹. The large enhanced rate performance of SFC should be mainly attributed to the marked improvement of conductivity. The uniformly dispersed Fe₃O₄ nanoparticles enhance the whole conductivity of the composite. Then the nanoparticles are refined and close linked, so that the ion/electron diffusion paths are greatly shorten.^19,20 Moreover, the porous shuttle-like shape can better relieve the rapid volume variation than dispersed nanoparticles at large current.

To evaluate the function of Fe₃O₄ and the novel structure on the impedance of SFC, electrochemical impedance spectroscopy (EIS) was conducted for SFC, pristine Co₃O₄ and Fe₃O₄ (Fig. 11). In the Nyquist plots, the occurrence of high frequency semicircle can be attributed to SEI film impedance; the spectra in the medium-frequency region are related to the charge-transfer resistance on the interface of the electrolyte and electrode, and the sloping lines result from the diffusion behavior of Li⁺ inside the electrode material.^56,57 As can be seen, SFC exhibits much smaller semicircles than pristine Co₃O₄ in high- and medium-frequency range, indicating a smaller SEI resistance R_SEI and lower charge transfer resistance R_ct for SFC. The difference between the two sloping lines also manifests the lower Li⁺ diffusion resistance for SFC.⁵⁸ Also, the Fe₃O₄ has the smallest charge transfer resistance, implying it has better conductivity and can reduce the charge transfer resistance of SFC.

Fig. 11 Nyquist plots of SFC, Co₃O₄ and Fe₃O₄.

All the results show that the electrochemical performance of SFC has large enhancement compared with pristine Co₃O₄ and Fe₃O₄. It is contributed by several ways. Firstly, the introduction of more conductive Fe₃O₄ improves the conductivity of composite. Secondly, the novel self-assembling architecture enables the close contact of nanoparticles and alleviates the aggregation, which can significantly improve the electrochemical stability and interfacial impedance. Thirdly, the small space between the assembled nanoparticles provides more interfacial active sites for lithium storage. Moreover, the refining of Co₃O₄ nanoparticles greatly shortens the transport path of electrons and ions.

Conclusions

We report the successful synthesis of an unprecedented shuttle-like Fe₃O₄–Co₃O₄ self-assembling architecture through a facile hydrothermal method with Fe₃O₄ nanoparticles as structure-building agent and ethanol as solvent. The composite exhibits extremely better electrochemical performance than pristine Co₃O₄. The novel self-assembling architecture, as well as the synergistic effect, may be responsible for the enhanced electrochemical performance. The superior lithium-storage performance along with the facile synthesis method surely makes this unique composite a promising anode material for lithium ion batteries.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (No. 11204090), the Project of DEGP (No. 2013KJCX0050), the Scientific and Technological Plan of Guangdong Province, Guangzhou City and its Yuexiu District, China (No. 2014B040404067, 2014A040401005, 201508030033, 2012J2200031, 2013-CY-007), a project funded by the China Postdoctoral Science Foundation (No. 2014M552190), and the Undergraduates' Innovating Experimentation Project of China, Guangdong Province and South China Normal University (No. 2014083, 2014087, 2014089, 201510574056).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12306b

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