Introduction

The development of high-performance lithium-ion batteries (LIBs) is an important topic in the field of energy research (Li et al. 2018). Transition metal oxides as promising anode materials of LIBs have attracted much attention due to their high specific capacities and low cost (Goodenough and Park 2013; Wang et al. 2018a). The calcination of metal-organic frameworks (MOFs) is an effective and facile strategy for the preparation of metal oxides or carbon (Ji et al. 2016; Feng et al. 2018). This can be ascribed to the large specific surface areas and suitable channels of MOFs, which facilitate the transmission of lithium ions during the cycles. Among them, metal oxides derived from Prussian blue (PB) with the formula FeFe(CN)6 or PB analogues (PBAs) were considered to be one of the candidate anodes because of the advantages of low cost and abundant resources as well as environmental friendliness (Catala and Mallah 2017; Nie et al. 2014). However, metal oxides generally have the inherent disadvantages such as poor conductivities and large volume changes during the discharge/charge cycles (Wang et al. 2018b). Therefore, it is necessary to combine metal oxides with the conductive and flexible carbonaceous substrates such as porous carbon (Chen et al. 2016), graphene (Ji et al. 2017; Liao et al. 2018; Shao et al. 2019), and carbon nanotubes (Huang et al. 2015). As we all know, carbon cloth (CC) has been widely employed as a flexible and conductive substrate to composite with active components, and the resulting composites displayed excellent electrochemical properties (Zhao et al. 2016b).

Based on above considerations, the combination of CC and PB (or PBAs)-derived metal oxides will exhibit excellent lithium storage abilities (Zhang et al. 2015), due to the synergistic effect of CC and metal oxides. On the one hand, CC has good flexibility and high conductivity, which can provide rapid channels for the diffusion and migration of electrons. On the other hand, the large specific surface area will afford abundant sites for the nucleation and growth of active materials, and then PB or PBA particles can be easily and uniformly dispersed on the surface of CC. In the present contribution, PB particles in this contribution were firstly grown on CC by a solution immersion strategy (Scheme 1). Subsequently, the Fe2O3/CC composite was obtained after high-temperature calcination of PB/CC precursor. To the best of our knowledge, the composites based on CC and PB (PBAs)-derived metal oxides have not been documented to date. The resulting Fe2O3/CC composite was directly employed as self-standing anodes and delivered superior electrochemical performances than pure Fe2O3 and CC.

Scheme 1
scheme 1

The preparation process of the Fe2O3/CC composite

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Experimental details

Chemical reagents

All chemicals of analytical grade were purchased from Sigma-Aldrich. CC was treated with the concentrated nitric acid, washed several times with deionized water, and then dried at 60 °C overnight.

Syntheses of PB and Fe2O3

PB was synthesized according to the reported procedure with slight modifications (Pramudita et al. 2014). 1.646 g of potassium ferricyanide was dispersed in 50 mL of deionized water to form solution A. Solution B was obtained by adding 1.622 g of ferric chloride in deionized water (100 mL). Solution A was slowly poured into solution B under stirring. After being stirred for several minutes, the resulting mixture was placed in an oven and dried for 6 h. The precipitates were separated by centrifuging, washed with ethanol/water (v/v = 1/1), and finally dried overnight. PB products were further calcined at 350 °C for 6 h in a muffle furnace under an air atmosphere. The red-brown Fe2O3 products were obtained when the furnace was cooled naturally to room temperature.

Syntheses of PB/CC and Fe2O3/CC composites

The pretreated CC (1 cm × 1 cm) and 2 mL of solution A were firstly placed in a small glass vial. After an immersion of 6 h, 2 mL of solution B was added into the above vial. The PB/CC composite was obtained upon shaking gently the vial for several minutes and further standing for 6 h. The obtained sample was washed several times with deionized water and dried. It can be obviously observed that the color of CC changed to blue, demonstrating the successful loading of PB particles. The Fe2O3/CC composite was prepared by the calcination of PB/CC precursor by the same method as pure Fe2O3 mentioned above. Based on the thermogravimetric (TG) analysis (Fig. S1), the weight fraction of Fe2O3 in the composite is calculated to be 13.00% with a loading density of 1.72 mg cm−2, which is higher than those in reported metal oxide/GF composites (Huang et al. 2017; Lin et al. 2016; Wang et al. 2016). In fact, we have carried out considerable synthesis experiments to study the effect of Fe2O3 mass loading on the electrochemical performance. However, it is very difficult to further increase the loading ratio of Fe2O3 in the composite, because excessive PB particles will easily off from the CC surface upon increasing the concentration of reactants during the preparation process.

Characterizations

The phase structures of all products were performed on a Shimadzu XRD-6000 X-ray diffractometer (XRD). The Raman analyses were conducted on a HR800 Raman spectroscope. Elemental and valence states were analyzed using a Thermo-VG Scientific ESCALAB 250 X-ray photoelectron spectrometer (XPS). TG analyses were conducted on a Pyris Diamond TGA analyzer at a ramp rate of 5 °C min−1 under air atmosphere. Scanning electron microscopy (SEM, ZEISS Merlin Compact) was employed to investigate the microstructures, whereas the energy-dispersive spectrometer (EDS, Oxford X-Max) was used to analyze the compositions of materials.

Electrochemical measurements

The as-prepared CC and Fe2O3/CC materials were directly used as self-standing anodes without adding any conductive agent and binder. The mass specific capacity of the composite was based on the total weight of GF and Fe2O3, because both components contribute the capacity for the electrode. Accordingly, the areal capacity of the composite is the product of mass specific capacity and weight per area of electrode. For the comparative experiments, pure Fe2O3 was mixed with super-P and PVDF with a mass ratio of 7:2:1 in the NMP. The slurry was then coated on copper foil. The battery was assembled and developed using the LAND CT 2001A battery test system. The electrolyte was 1 mol L−1 of LiPF6 solution and the test environment was performed at a constant temperature of 25 °C. The materials were subjected to cyclic voltammetry, constant current charge and discharge, and different current multi-rate performance tests. The impedance tests were conducted on an Autolab PGSTAT302N instrument with a test range of 0.01 Hz–100 kHz at an open-circuit voltage.

Results and discussion

As shown in Fig. 1a, the peaks of CC at 2θ = 26.2° and 43.1° corresponded to the (002) and (100) crystal lattice planes of the graphitized carbon. The characteristic peaks of PB were fully consistent with those in the literature (Zhang et al. 2012) and JCPDS card 73-0687, indicating the preparation of the same material. For the PB/CC composite, the diffraction patterns comprised all characteristic peaks of CC and PB, confirming the successful loading of PB particles. The diffraction patterns of Fe2O3 obtained by calcining PB precursors were in perfect agreement with the monoclinic phase (JCPDS card 02-1047) (Fig. 1b). In addition, no other peaks were found in the patterns, revealing that PB particles have been completely converted to Fe2O3 after the high-temperature treatment. Similarly, the diffraction patterns of Fe2O3/CC also contained the characteristic peaks of CC and Fe2O3, demonstrating the complete conversion of PB to Fe2O3.

Fig. 1
figure 1

XRD patterns of the a PB/CC and b Fe2O3/CC systems

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SEM images of CC, PB, Fe2O3, PB/CC, and Fe2O3/CC are displayed in Fig. 2. The CC substrate exhibited a self-standing and flexible structure (Fig. 2a), where carbon fibers formed a dense crossover network by the interconnection way. This type of structure facilitates the nucleation and growth of active materials. The PB particles exhibited a square shape with a side length of about ~ 148 nm (Fig. 2b), typical characteristics of PB (Zhu et al. 2018). The particle surface of PB-derived Fe2O3 became rough (Fig. 2c), which can be reasonably attributed to the loss of organic species (C, H) in the framework of PB during the calcination process (Hu et al. 2012). After the loading of PB particles, the color of CC changed from black to blue (Fig. 2d), confirming that the surface of CC was densely wrapped with PB particles. It can be seen from the morphology of PB/CC that PB particles had a similar shape to pure PB. Moreover, these PB particles were densely and uniformly dispersed into the whole network of CC (Fig. 2e, f), which can be ascribed to the large specific surface area of CC and the strong interaction between CC and PB.

Fig. 2
figure 2

SEM images of a CC, b PB, c Fe2O3, d–f PB/CC, and g–i Fe2O3/CC; j elemental mapping distribution; k SAED pattern; and l HR-TEM image of Fe2O3/CC

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The color of CC changed from blue to reddish brown after the calcination of PB/CC precursor (Fig. 2g), confirming that PB particles were completely converted to metal oxides. After a careful observation, the CC substrate still maintained a self-standing structure, and the entire skeleton did not undergo any collapse and destruction after the high-temperature treatment, which can be attributed to the high mechanical strength and flexibility of CC. In addition, Fe2O3 particles were also densely loaded on CC and did not fall off from the CC substrate (Fig. 2h). From SEM images of the cross section of Fe2O3/CC, the thickness of Fe2O3 layer and the diameter of carbon fiber in the composite are 8.37 μm and 0.34 μm, respectively (Fig. S2). Further analysis revealed that the average size and morphology of Fe2O3 particles were similar to those of pure Fe2O3 (Fig. 2i). Selected area electron diffraction (SAED) pattern (Fig. 2k) indicated the crystallinity of Fe2O3 with the rings corresponding to the (440), (400), and (311) planes. High-resolution TEM (HR-TEM) image (Fig. 2l) revealed fringe d-spacings of 0.265 and 0.218 nm, corresponding to the (310) and (321) lattice planes of Fe2O3, respectively, in accordance with the result of XRD. According to the elemental mapping distribution, the composite contained the elements of C, O, and Fe. These species were uniformly dispersed into the entire network of CC, further proving the successful loading of Fe2O3 (Fig. 2j). The close interaction between Fe2O3 particles and the CC substrate plays a crucial role in improving the electrical conductivity and cycle stability of the electrode (Luo et al. 2017).

In the Raman spectrum of PB/CC (Fig. 3a), the Fe–C peaks appeared at 267 cm−1 and 532 cm−1, while the C≡N peaks were located at 2093 cm−1 and 2151 cm−1. This is consistent with the results reported previously (Jiang et al. 2017). The characteristic peak of CC was also observed, indicating the combination of PB and CC components. The peaks at 200 cm−1 and 700 cm−1 in the spectra of Fe2O3/CC represented the existence of A1g and Eg, respectively, demonstrating the conversion of PB to Fe2O3. Similar to PB/CC, the characteristic peak assigned to CC was also observed in the Raman spectrum of Fe2O3/CC.

Fig. 3
figure 3

Raman spectra of the a PB/CC and b Fe2O3/CC systems

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The elemental valence states of Fe2O3/CC were investigated by XPS (Fig. 4). The XPS survey spectrum indicated the existence of C, Fe, and O elements. The high-resolution Fe 2p XPS spectrum revealed two strong peaks at 710.9 eV (Fe 2p3/2) and 724.8 eV (Fe 2p1/2), which were further decomposed into three peaks of 710.8, 712.5, and 724.8 eV, respectively, indicating the formation of Fe–OH, C–O–Fe, and Fe–O bonds in Fe2O3 (Zheng et al. 2018). In addition, both major peaks were accompanied by two weak satellites at 733.1 and 719.2 eV. The O 1s spectrum of the sample exhibited two peaks of 530.1 and 531.6 eV, which are assigned to the O2− forming oxide with iron and OH−, respectively. The C 1s spectrum revealed the binding energy peaks of 284.5 eV (C–C and C=C) and 285.4 eV (C–O). The XPS results confirmed the successful combination of CC and Fe2O3 components.

Fig. 4
figure 4

XPS spectra of the Fe2O3/CC composite: a survey spectrum, b O 1s spectrum, c Fe 2p spectrum, and d C 1s spectrum

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Figure 5 illustrates CV curves for the first three cycles of CC, Fe2O3, and Fe2O3/CC electrodes at a sweep speed of 0.2 mV s−1 with a voltage window of 0.01 to 3.0 V. It can be found that CV curves of Fe2O3/CC contained the characteristic peaks of CC and Fe2O3 components. For the composite electrode, a sharp reduction peak at around 0.58 V in the first cathodic scan was ascribed to the conversion reaction between Fe2O3 and Li to form Fe and Li2O, together with the formation of a solid electrolyte interface (SEI) film caused by the decomposition of electrolyte (Zhu et al. 2017). Two broad and weak anodic peaks in the anodic scan at around 1.55 and 1.86 V correspond to the oxidation reaction of Fe and the decomposition of Li2O to Fe3+ (Kobayashi et al. 2018). From the subsequent cycle, this reduction peak potential shifts to a higher voltage of about 0.74 V, indicating the polarization of active material and the existence of irreversible processes during the first cycle. CV curves of the second and third scans overlapped substantially, indicating good redox reversibility and high stability for the composite electrode during the cycles (Zhu et al. 2013).

Fig. 5
figure 5

CV curves of the a CC, b Fe2O3, and c CC/Fe2O3 electrodes

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Figure 6 shows the charge/discharge curves of CC, Fe2O3, and Fe2O3/CC electrodes at 100 mA g−1 in the range of 0.01–3.0 V. The CC substrate exhibited very low capacities (Fig. 6a) of below 90 mA h g−1. For pure Fe2O3, the charge and discharge capacities in the first lap were 780 and 1143 mA h g−1, respectively, with the Coulomb efficiency of about 68% (Fig. 6b). The irreversible loss of the initial capacity is due to some irreversible reactions such as the decomposition of electrolyte and the formation of solid electrolyte interphase (SEI) layers (Lv et al. 2017). Subsequent to the 50th lap, the charge/discharge capacity was only 180 and 187 mA h g−1, respectively. For the Fe2O3/CC composite, the initial charge/discharge specific capacities were 402 and 544 mA h g−1, respectively, with the Coulomb efficiency of about 74% (Fig. 6b). At the 50th lap, the charge/discharge capacity was 383 and 385 mA h g−1, respectively, with a Coulomb efficiency of 98%. The results showed that the Fe2O3/CC composite exhibited a higher charge/discharge efficiency and specific capacity than those observed for pure CC and Fe2O3 (Xie et al. 2017).

Fig. 6
figure 6

Charge/discharge curves of the a CC, b Fe2O3, and c CC/Fe2O3 electrodes at 100 mA g−1

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Figure 7 a shows the cycling performances of CC, Fe2O3, and Fe2O3/CC at 100 mA g−1. The low specific capacity (~ 63 mA h g−1) for pure CC remained almost unchanged, indicating high cycling stability. For pure Fe2O3 electrode, the specific capacity rapidly decayed from the initial 1143 to 81 mA h g−1 at 135 cycles with a capacity retention rate of only 7%. The sharp decline in specific capacity may be ascribed to the significant stacking and structural collapse of Fe2O3 particles during the intercalation/delithiation process, as commonly observed in metal oxides (Jiang et al. 2017). In sharp contrast, the capacity of Fe2O3/CC remained almost unchanged from the first few laps to the 135th lap, and the capacity at the 135th cycle was still ~ 400 mA h g−1. Notably, the composite delivered a high areal capacity of 5.1 mA h cm−2, while pure Fe2O3 only delivered a value of below 0.1 mA h cm−2. In addition, the Coulomb efficiency of the composite was near 100% during the whole cycle process. Based on the above analysis, the composite electrode exhibited a significantly superior cycle performance and capacity retention to pure CC and Fe2O3. The significant enhancement in electrochemical performances could be ascribed to the synergistic effect between CC and metal oxide (Zhao et al. 2016a). First, the close combination of Fe2O3 nanoparticles with the highly conductive and flexible CC substrate could improve the diffusion kinetics by shortening diffusion pathway for the rapid electron/ion transport. Second, the nanostructured Fe2O3 particles could facilitate electrolyte penetration and Li+ ion diffusion during the cycles. Third, the integrated composite electrode avoids the use of polymer binders and conductive additives, significantly reducing the inactive interface and improving the rate capability. The lithium storage properties of Fe2O3/CC can be comparable with those reported Fe2O3- or CC-based anode materials as listed in Table 1 (Meng et al. 2017; Kobayashi et al. 2018; Xie et al. 2017; Wei et al. 2015; Tjandra et al. 2016; Cao et al. 2014; Shan et al. 2018; Xu et al. 2017). It is worth mentioning that the areal specific capacities of several graphene-based composites are calculated to be much smaller than our case, although these materials have higher mass specific capacities, which was a common problem associated with lightweight graphene (Ji et al. 2016; Jiang et al. 2017). For example, Fe2O3/graphene aerogel displayed a low areal capacity of 0.6 mA h cm−2 despite a high mass capacity of 745 mA h g−1 at 100 mA g−1 after 100 cycles (Meng et al. 2017).

Fig. 7
figure 7

a Cycling performances at 100 mA g−1 and b the specific capacities at different current densities for the CC, Fe2O3, and Fe2O3/CC electrodes

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Table 1 Comparison of the lithium storage properties of Fe2O3/CC with those of reported Fe2O3-based or CC-based materials
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Figure 7 b shows the charge/discharge curves of the CC, Fe2O3, and Fe2O3/CC electrodes at different current densities. At 20, 50, 100, 200, 500, and 20 mA g−1, the specific capacities of Fe2O3/CC were 578, 490, 427, 244, 121, and 557 mA h g−1 respectively. The specific capacities of pure Fe2O3 decreased significantly, reaching the values of 1425, 541, 315, 99, 22, and 138 mA h g−1 at the same current densities, respectively. The CC substrate showed the capacities of 142, 123, 104, 76, 28, and 126 mA h g−1 at 20, 50, 100, 200, 500, and 20 mA g−1, respectively. The superior rate capability of Fe2O3/CC to pure Fe2O3 can be attributed to the high conductivity and flexibility of CC, providing an efficient channel and path for the rapid electron transfer during the electrochemical reaction process (Qiu et al. 2016).

Figure 8 shows the Nyquist diagrams of Fe2O3 and Fe2O3/CC before the cycles. The equivalent circuit plot of the material is shown in Fig. 8 (inset), and the fitting values with ZView software of the resistance components in the simplified equivalent circuit are given in Table S1. The depressed semicircle in the high-frequency region presents the charge transfer resistance (Rct), while Re is the lithium-ion diffusion resistance in the electrolyte. Obviously, the Re and Rct values of the composite were much smaller than those observed for pure Fe2O3. The analysis from EIS indicated that the introduction of CC effectively improved the lithium-ion diffusion and charge transfer process during the Li+ insertion/extraction reaction, resulting in excellent cycle stability (Xiong et al. 2013). SEM images (Fig. S3) of the Fe2O3/CC electrode after the cycles revealed that the composite remained intact free-standing framework. The Fe2O3 particles were well preserved and still uniformly coated on the surface of CC except that the particle boundary became smooth. Above result revealed the tight attachment between Fe2O3 and CC in the composite during the cycles, which facilitated the enhancement of cycle stability.

Fig. 8
figure 8

Nyquist plots of Fe2O3 and Fe2O3/CC (inset: the simplified equivalent circuit of the electrode material)

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Conclusion

In summary, the Fe2O3/CC composite was prepared through a solution impregnation method followed by high-temperature calcination. The composite could be used directly as self-standing electrodes for LIBs. Compared with pure CC and Fe2O3, the composite exhibited a higher mass and areal specific capacity and better cycle stability and rate capability. The outstanding electrochemical properties are ascribed to the synergistic effect between Fe2O3 and CC components, where metal oxides contribute a high lithium storage capacity, whereas CC provides excellent conductivity and flexibility. In addition, the close contact between Fe2O3 and CC also improves the cycle stability of electrode. This is the first case of composites based on CC and PB (or PB analogues)-derived metal oxides. More importantly, the present research provides some ideas for the design of high-performance integrated electrodes based on the CC substrate.