Introduction

Lithium-ion batteries (LIBs) as the most popular energy storage device have attracted widespread attentions because of their high storage capacities, stable cycling and high safety [1,2,3]. However, the low theoretical capacity of conventional graphite anode material (372 mA h g−1) has inhibited the further application for next-generation high-energy LIBs. Iron oxides, such as Fe2O3 and Fe3O4, are attractive anode materials for LIBs due to their natural abundance, eco-friendly and low cost. In addition, the theoretical capacities of Fe2O3 and Fe3O4 are as high as 1007 and 926 mA h g−1, respectively (Fe2O3 + 6Li ↔ 3Li2O + 2Fe and Fe3O4 + 8Li ↔ 4Li2O + 3Fe) [4,5,6]. However, iron oxides exhibit rapid capacity degradation and inferior cycling stability because of the low intrinsic electrical conductivity (below 10−6 Ω−1 cm−1) [7], the formation of unstable SEI film and the electrode collapse caused by the large volume expansion during the charge/discharge process (over 200%) [8]. Generally, two effective methods have been employed to surmount the above issues. One is to minimize the particles size down to the nanoscale. Taroscon’s group has reported that nano-sized Fe2O3 electrode exhibits the significant improvement in electrochemical performance compared with micro-sized Fe2O3 electrode [9, 10]. The nano-sized particles can shorten the diffusion path of Li ions and minimize the strain to alleviate the absolute volume expansion. Another strategy is to incorporate carbon materials to form Fe2O3/carbon composites, which has been considered as the most effective method to overcome the above issues. Carbon materials including carbon nanotubes [11], carbon aerogel [12], carbon nanofiber [13], grapheme [14] and N-doped graphene aerogel [15] can not only release the residual stress arising from volume expansion of Fe2O3 particles, but also enhance the electrical conductivity and restrain the particle agglomeration. Xiao et al. [16] reported a facile hydrothermal method to prepare Fe2O3/graphene composite, which showed a high reversible capacity up to 1069 mA h g−1 at a current density of 50 mA g−1 after 50 cycles. Nevertheless, the irreversible aggregation and restacking caused by the strong π–π stacking and van der Waals force can easily occur and severely affect the electrochemical performance [17]. Wang et al. developed a one-pot solvothermal method to fabricate carbon coated Fe2O3 composites [18]. However, the thin carbon film with a thickness of about 6 nm leads to a reduction of the contact interface between Fe2O3 and the electrolyte and the core–shell structure does not have appropriate void space to accommodate the volume expansion of Fe2O3 particles during cycling [19, 20]. Therefore, the challenge in obtaining reasonable structure and high-performance Fe2O3/carbon anode materials for LIBs still remains.

Metal organic frameworks (MOFs) as emerging porous material have received growing attentions because of their unique structure, tunable pore sizes, and extensive applications in gas storage and separation, catalysts, drug delivery and so on [21]. In addition, the various pore size and large specific surface area of MOFs make them feasible as templates to prepare porous carbon materials, such as Ni-MOF, MOF-2, MOF-5, ZIF-8, etc. [22,23,24,25,26]. Furthermore, heteroatoms (e.g., B, S, N, and P) existed in the organic ligands of MOFs are vital to synthesize heteroatoms doping in the MOFs-derived carbonaceous materials [27]. Among them, N-doping in carbon materials is considered as the most effective approach to improve electronic conductivity of carbon materials by creating extrinsic defects [28]. ZIF-8 is the most popular precursors to prepare carbon materials because of the high content of N in 2-methylimidazolate (≈ 34 wt%) [29]. However, only conventional assembly processes can be employed for most of those MOFs-derived powdery carbons materials. The polymer binder and conductive additives will increase the contact resistance and lower the rate capability due to their insulativity and electrochemical inactivity. Therefore, the design and fabrication of flexible and free-standing electrode materials have been considered as the trend of development in LIBs. In this regard, researchers have elaborately designed kinds of Fe2O3/carbon electrode materials that eradicate the usage of binder and current collectors in traditional battery electrode configuration, such as Fe2O3@ carbon/carbon cloth [18], Fe2O3/graphene/carbon nanotubes [30], Fe2O3/nanotubes [31], and Fe2O3/carbon nanofiber [19] and so on. But the usage of nano-sized MOFs-derived carbons as basic architectural units in 1D to construct flexible and free-standing Fe2O3/carbon electrode is rarely explored. As a convenient and inexpensive industry-viable technology, electrospinning has been widely employed to prepare 1D nanomaterials with controllable morphology and compositions [32].

In this paper, a novel strategy that combines the electrospinning technology with nonaqueous sol–gel method is developed to incorporate Fe2O3 nanoparticles into ZIF-8-derived nitrogen-doped highly porous carbon fibers (NPCFs). Benefiting from the smaller size of Fe2O3 nanoparticles (about 5 nm), robust 1D structure and superior electrical conductivity of NPCFs as well as the strong covalent bond interactions (Fe–O–C bond) between Fe2O3 and NPCFs, the electrochemical performance of Fe2O3 nanoparticles has been distinctively improved after incorporating it into NPCFs. We also explore the effect of different loading densities on the electrochemical performance of the composites. The Fe2O3/NPCFs-66.9% exhibits the best rate (337 mA h g−1 at 5000 mA g−1) and cycling performances (1106 mA h g−1 after 100 cycles at 100 mA g−1). In addition, the preparation processes are convenient and time-saving, and the raw materials are natural abundance, eco-friendliness and low cost. Therefore, the flexible and free-standing Fe2O3/NPCFs-66.9% composite shows promising applications as anodes material for LIBs.

Experimental section

Synthesis of the ZIF-8 nanoparticles

All chemicals were purchased from Sigma-Aldrich and used without further purification. Zn(NO3)2·6H2O (0.3 g) was dissolved in methanol (20 mL) to form a solution. 2-methylimidazole (0.6 g) was dissolved in methanol (10 mL) to form another clear solution. Then, the two solutions were mixed together, and stirred for 2 h in an ice bath. After that, the white powers were washed with methanol for five times by centrifugation (10,000 rpm, 10 min) and dried at 90 °C under vacuum for 24 h.

Synthesis of ZIF-8-derived nitrogen-doped porous carbon fibers (NPCFs)

In a typical synthesis, 0.8 g ZIF-8 power was mixed with 5 mL N,N-dimethylformamide (DMF) under ultrasonic until it was well dispersed. Then, 0.4 g polyacrylonitrile (PAN, Mw = 150,000 g mol−1) was added into the above solution and stirred at 65 °C for 6 h to prepare the electrospinning precursor. The electrospinning process was executed at a high positive voltage 18 kV and the collect distance was 15 cm between the electrospinning jet and alumina foil collector. The flow rate was 0.5 mL h−1. The as-prepared fibers were peeled off from the alumina foil and then carbonized in a furnace at 800 °C for 2 h with a heating rate of 2 °C min−1 in N2 flow. The obtained sample was washed with dilute HCl and deionized water for several times to remove the remanent Zn component. The ZIF-8-derived NPCFs was finally synthesized.

Synthesis of Fe2O3 nanoparticles and Fe2O3/NPCFs composites

The Fe2O3–NPCFs hybrid was fabricated via a facile one-step nonaqueous sol–gel coassembly with controllable mass ratio that can be directly used as free-standing anodes. In briefly, 0.423 g ferric trichloride hexahydrate (FeCl3·6H2O) was dissolved in 10 mL tetrahydrofuran (THF) with the help of ultrasonication to get the Fe-based precursor solution. Then, 0.029 g of NPCFs was immerged into the above Fe-based precursor solution for 24 h at room temperature. Finally, the above mixed suspension was pyrolyzed in a flask at 250 °C and kept 2 h to convert the Fe-based precursor to Fe2O3. The Fe2O3/NPCFs mats were obtained by washing with THF five times and absolute ethanol five times, and drying at 90 °C for 20 h in vacuum. For comparison, the controllable mass ratio was prepared by the same process except changing the FeCl3·6H2O to 0.232 and 0.725 g, respectively. The as-synthesized composites were labeled as Fe2O3/NPCFs-66.9%, Fe2O3/NPCFs-41.1% and Fe2O3/NPCFs-79.2%, respectively. Fe2O3 bulk counterpart was synthesized by the same process as the Fe2O3–NPCFs samples. The only difference was that NPCFs was not added in the above Fe-based precursor.

Materials characterization

The crystalline structures of the as-synthesized products were examined by X-ray diffraction (XRD, Bruker D2 Phaser X-ray Diffractometer, Cu Kα radiation). X-ray photoelectron spectroscopy (XPS) experiments were carried out on Thermo ESCALAB 250XI using an Al Kα X-ray source. Raman scattering spectra were recorded with a Renishaw System 2000 spectrometer. Thermogravimetry (TG) measurement was carried out on a TA Instruments Hi-Res TGA 2950 from 30 to 700 °C at a rate of 10 °C min−1 in flowing air. Nitrogen adsorption/desorption measurements were carried out on a Micromeritics ASAP 2460 analyzer. The pore size distributions and specific surface areas and were studied by Barrett–Joyner–Halenda model and Brunauer–Emmett–Teller (BET) equation, respectively. The morphologies of the products were examined by field-emission scanning electron microscopy (FE-SEM, Hitachi S4800) and transmission electron microscopy (TEM, Hitachi H-800).

Electrochemical measurements

The electrochemical tests were conducted with coin cells (CR2032). Three different loadings of Fe2O3-NPCFs samples and pure NPCFs were directly used as the working electrodes. The density of the active material is about 1.5–1.8 mg cm−2. A lithium foil was used as the counter electrode, and Celgard 2400 was used as the separator. The electrolyte was LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 vol %). In addition, the Fe2O3 electrode was made by mixing active materials, Super-P carbon black and polyvinylidene fluoride (PVDF) with a weight ratio of 80:10:10. The coin cells were assembled in an argon-filled glove box (H2O, O2 < 0.1 ppm, MBraun, Germany). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out by a CHI660D electrochemical workstation. CV was recorded at a scan rate of 0.1 mV s−1 within the voltage range of 0.01–3.0 V, and EIS was recorded at open potential over the frequency range from 100 to 0.01 Hz. Galvanostatic charge/discharge measurements were performed on NEWARE CT-3008 instrument between 0.01 and 3.0 V vs Li/Li+ at room temperature.

Results and discussion

In this work, Fe2O3 nanoparticles about 5 nm were prepared through a nonaqueous sol–gel method. In comparison with the complex aqueous sol–gel process, nonaqueous processes can enable the nanomaterials with good crystallinity and uniform morphologies [33]. The formation mechanism of Fe2O3 nanoparticles during the sol process is depicted as follows:

In the following pyrolysis process, the gel molecules break down and change into Fe2O3 nanoparticles. The XRD pattern of ZIF-8 is shown in Fig. S1a and the positions of reflection peaks are consistent with the simulated pattern, demonstrating that ZIF-8 is successfully prepared. Fig. S1b exhibits the XRD pattern of NPCFs and shows two broad humps at 2θ of about 28° and 43°, suggesting the amorphous nature of carbon [34]. Figure 1a shows XRD patterns of Fe2O3/NPCFs composites. As can be seen, the diffraction peaks of three nanocomposites can be assigned to Fe2O3 without any impurities (JCPDS 33-0664). The full XPS spectrum of Fe2O3/NPCFs-66.9% in Fig. 1b was employed to confirm that the composite is only composed of C, O, N and Fe elements. In addition, two distinct peaks located at 724.9 eV and 711.1 eV are observed in Fig. 1c, corresponding to Fe 2p1/2 and Fe 2p3/2 of Fe2O3, respectively [35, 36]. What is more, a broad satellite peak located at about 718.1 eV can further indicate it belongs to Fe2O3. Based on the XRD and XPS results, it can be confirmed that the Fe2O3/NPCFs composites have been synthesized successfully through this facile method. The flexible test results of the Fe2O3/NPCFs films are shown in Fig. 1d. A full film without any apparent defects in bending state can be seen. To further explore the flexibility of Fe2O3/NPCFs films, we attached the composites film on a pen by winding it closely. However, no obvious structural failure can be observed even after rolling, suggesting that the Fe2O3/NPCFs films have great potential for freestanding anodes with their outstanding flexibility.

Figure 1
figure 1

a XRD patterns of Fe2O3/NPCFs composites. b Full XPS spectrum of Fe2O3/NPCFs-66.9%. c High-resolution XPS spectrum of Fe 2p in Fe2O3/NPCFs-66.9% d Digital photographs showing the flexibility of the Fe2O3/NPCFs-66.9% product

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The interfacial interaction in Fe2O3/NPCFs hybrids was investigated by Raman measurement, which has been testified a useful tool to observe the modification of carbon and their derivatives. As shown in Fig. 2a, Raman spectra of NPCFs show the regular D peak (ca. 1342 cm−1) and G peak (ca. 1573 cm−1). It is commonly accepted that the chemical link can change the electronic structures of carbon materials due to the change of hybridization of the carbon atoms. In addition, the charge transfer between carbon materials and nanocrystals can cause the shift of the G peak [37, 38]. Therefore, Raman measurement is an ideal probe to investigate the interaction between Fe2O3 and NPCFs. In this work, the observed shift by 11 cm−1 from 1573 (NPCFs) to 1584 cm−1 (Fe2O3/NPCFs composite) demonstrates the presence of a charge transfer between NPCFs and Fe2O3 nanoparticles. The interaction can also be confirmed by TG measurement in the air. Because the types of contact, such as loose contact and tight contact, between the metal oxide catalysts and carbon materials are the crucial factor for oxidation reactivity of carbon materials [39]. As shown in Fig. 2b, the oxidation temperature for Fe2O3/NPCFs composite is about 280 °C, while NPCFs is about 470 °C. Such a large drop in temperature suggested that there was a tight contact between Fe2O3 and NPCFs. In addition, the Fe2O3/NPCFs composites exhibit a total weight loss of about 33.1, 20.8 and 58.9% between 280 and 400 °C, which is attributed to the evaporation of adsorbed water molecules and the combustion of NPCFs (Fig. S2a). Therefore, the content of Fe2O3 in the composites is 66.9, 79.2 and 41.1% for Fe2O3/NPCFs-66.9%, Fe2O3/NPCFs-41.1% and Fe2O3/NPCFs-79.2% products. The binding between NPCFs and Fe2O3 was further identified by XPS. Figure S2b exhibits the full-scan XPS spectra of NPCFs, suggesting the existence of C, N, and O. In Fig. 2c, the C 1s spectra of the NPCFs was deconvoluted into four peaks, which corresponds to O–C=O, C–O, C–N and C–C [40]. Compared with NPCFs, the peak intensity of C–O and O–C=O becomes lower after incorporating Fe2O3 nanoparticles into NPCFs (Fig. 2d). It can be attributed to the new oxygen-containing groups and some O–C=O groups on the NPCFs were broken down to form the Fe–O–C bonds during the in situ nonaqueous sol–gel process, respectively [41]. Furthermore, high-resolution XPS spectra of O 1s in Fig. 2e, f can also prove the conclusion. The new peak at 531.3 eV can be attributed to the Fe–O–C bond between Fe2O3 and NPCFs. It can also be confirmed from the previous literature that the binding energy of O 1s in Fe–O–C bond can exist in the line of 531–533 eV [42]. All of this analysis confirmed the conclusion that the Fe–O–C bonds were formed between NPCFs and Fe2O3. The strong interfacial interaction can not only enhance the structural stability of the whole electrode, but also can withstand the volume expansion and agglomeration of Fe2O3 nanoparticles during cycling.

Figure 2
figure 2

Raman spectra (a) and TG curves (b) of NPCFs and Fe2O3/NPCFs-66.9%. High-resolution XPS spectra of C 1s (c, d) and O 1s (e, f) of NPCFs and Fe2O3/NPCFs-66.9%

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The morphology and microstructure of the as-prepared NPCFs and Fe2O3/NPCFs hybrids were characterized by FE-SEM. As shown in Fig. 3a–c, the obtained NPCFs present a 1D structure with a diameter of about 200 nm and smooth surface. Many typical mesoporous formed by the carbonization of ZIF-8 nanoparticles can also be observed. In addition, NPCFs weave into an interconnected network with an interspace of about hundreds of nanometers, which can provide efficient channels for electrolyte penetration and electron transfer. The as-synthesized ZIF-8 is also characterized by SEM, and the crystalline size is about 50 nm (Fig. S3). From Fig. 3d–l, it is more clearly observed that no obvious morphological changes in the Fe2O3/NPCFs composites after decorating with Fe2O3 nanoparticles, still showing 1D architecture. However, the roughness of the three NPCFs surfaces increases with the increase in Fe2O3 loading from 41.1 to 79.2%. When the Fe2O3 content was increased to 79.2%, the nanoparticles will agglomerate on the surface of NPCFs due to the lack of sufficient space for infiltration. Moreover, it would affect the transfer of Li+ across the electrolyte and Fe2O3/NPCFs interface. Therefore, exploring the mass ratio between Fe2O3 and NPCFs to an optimal composition is crucial for getting superior electrochemical performance.

Figure 3
figure 3

SEM images of NPCFs (a–c), Fe2O3/NPCFs-41.1% (d–f), Fe2O3/NPCFs-66.9% (g–i) and Fe2O3/NPCFs-79.2% (j–l)

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The high-resolution TEM image (HRTEM) of Fe2O3 counterpart is shown in Fig. 4a. The sizes of the Fe2O3 nanoparticles are about 5 nm, which is much smaller than that of previous reports [35, 43]. However, the Fe2O3 nanoparticles are clustered together due to the lack of the NPCFs matrix. The TEM images in Fig. 4b, c reveal that Fe2O3 nanoparticles are distributed in the hollow carbon cages without agglomeration. In addition, rich mesopores existed in the 1D structure can also be observed clearly. These mesopores can not only shorten the Li+ diffusion distance, insure the efficient Li+ flux, but also can accommodate the possible volume change of Fe2O3 nanoparticles during cycling. The HRTEM image in Fig. 4f showed that the interplanar spacing was 0.27 nm corresponding to (104) planes of tetragonal Fe2O3. Elemental mapping test was used to determine the distribution of Fe2O3 nanoparticles in the Fe2O3/NPCFs-66.9% sample. As shown in Fig. 4e–i, a highly uniform distribution of C, N, Fe, and O species can be seen, further confirming that Fe2O3 nanoparticles evenly distributed in the NPCFs matrix.

Figure 4
figure 4

High-resolution TEM images of Fe2O3 (a). TEM (b, c) and high-resolution TEM (d) images of Fe2O3/NPCFs-66.9% hybrid. TEM image of Fe2O3/NPCFs-66.9% (e) and its corresponding element mapping of C (f), Fe (g), N (h), O (i). The inset of d selected area electron diffraction pattern of Fe2O3/NPCFs-66.9% hybrid

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The porous and textural properties of the as-prepared NPCFs and Fe2O3/NPCFs-66.9% are further investigated by nitrogen adsorption/desorption analyses. As shown in Fig. 5, these two isotherms exhibit type IV isotherms with distinct hysteresis loops in the medium to high relative pressure, which illustrates the mesoporous feature of the two materials [44]. The specific surface areas of NPCFs, Fe2O3/NPCFs-66.9% and bare Fe2O3 (Fig. S4) are 424.7, 125.4 and 10.1 m2 g−1, respectively. The corresponding pore size distribution curves indicate that mesopores dominate in these samples. The porous structure in the bare Fe2O3 may be associated with the textural mesoporosity between individual particles in the aggregates. The large specific surface areas and the meso-sized transporting pores of Fe2O3/NPCFs-66.9% can not only promote Li ions into the electrode material, but also increase the contact areas between the active materials and electrolyte.

Figure 5
figure 5

Nitrogen adsorption/desorption isotherms and the corresponding pore size distribution curves of NPCFs (a) and Fe2O3/NPCFs-66.9% (b)

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The electrochemical behavior of the Fe2O3/NPCFs-66.9% material was examined by cyclic voltammetry (CV) at a scan rate of 0.1 mV s−1 in the 0.01–3.0 V, and the curves are shown in Fig. 6a. In the first cathodic sweep, a weak peak at 1.56 V is attributed to lithium intercalation in the crystal structure of Fe2O3 nanoparticles. Another main cathodic peak at 0.66 V is assigned to the stepwise reduction of Fe3+ to Fe2+ and the complete reduction of Fe2+ to Fe0 as well as the formation of a solid electrolyte interface (SEI) layer because of the degradation of the electrolyte [45, 46]. In the anodic process, a broad peak centered at about 1.6 V is corresponded to the oxidation of Fe0 to Fe3+ [47]. In addition, a long tail (green elliptical region) at between 0.25 and 0.01 V can be observed in the CV curves, which is corresponded to the insertion of Li ions into the NPCFs matrix [48]. Moreover, the curves almost overlapped in the following cycles, suggesting the superior reversibility of the Fe2O3/NPCFs-66.9% electrode. The electrochemical reaction mechanism of the electrode can be described by the following equation [46]:

$$ {\text{Fe}}_{2} {\text{O}}_{3} + 6{\text{Li}}^{ + } + 6{\text{e}} \leftrightarrow 2{\text{Fe}} + 3{\text{Li}}_{2} {\text{O}} $$
Figure 6
figure 6

a CV curves of Fe2O3/NPCFs-66.9%. Charge–discharge profiles of Fe2O3 (b) and Fe2O3/NPCFs-66.9% (c) at 50 mA g−1. d Rate performance curves Fe2O3 and Fe2O3/NPCFs composites. e Cycling performance of Fe2O3 and Fe2O3/NPCFs at 100 mA g−1. f Electrochemical impedance spectra of Fe2O3, NPCFs and Fe2O3/NPCFs-66.9% electrodes

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As a comparison, the CV curves of Fe2O3 anode are shown in Fig. S5. The shape is consistent with the Fe2O3/NPCFs-66.9% electrode, suggesting that the similar electrochemical reactions have happened in these two electrodes during lithium insertion/extraction processes.

The first five galvanostatic charge/discharge curves of Fe2O3 and Fe2O3/NPCFs-66.9% anodes tested at 50 mA g−1 are shown in Fig. 6b, c, respectively. In the first discharge curves, Fe2O3 and Fe2O3/NPCFs-66.9% anodes show two similar potential plateaus at about 1.6 and 0.7 V, which are caused by lithium inserting into crystalline structure of Fe2O3, and the reduction of Fe3+ to Fe0 by metallic Li as well as the formation of SEI film. The reverse conversion reaction from Fe0 to Fe3+ results in the sloping plateau from 1.50 to 2.0 V. All these observations are consistent with the CV results. The initial discharge and charge capacities of Fe2O3/NPCFs-66.9% electrode are 1778 and 1323 mA h g−1 with a Coulombic efficiency (CE) of 74.4%, which is higher than that of Fe2O3 nanoparticles (48.9%). The large capacity loss can be attributed to the formation of SEI film and the irreversible decomposition of electrolyte [49, 50]. In the following four cycles, the charge–discharge curves of the Fe2O3/NPCFs-66.9% electrode almost overlap, demonstrating better cyclic stability. (1313 mA h g−1 at the fifth cycles) However, for the Fe2O3 electrode, the specific capacity is dramatically reduced due to the lack of NPCFs matrix to accommodate the volume expansion during cycling. (339 mA h g−1 at the fifth cycles).

Figure 6d exhibits the rate performance of the Fe2O3 and Fe2O3/NPCFs electrodes at different current densities from 50 to 5000 mA g−1 and 5 cycles for each step. We can observe that the Fe2O3 electrode delivers fast capacity fading compared with Fe2O3/NPCFs electrodes. The discharge specific capacity decreases to 8.4 mA h g−1 at 5000 mA g−1. When the current density returns to 50 mA g−1, the capacity can only recover to 224 mA h g−1. On the contrary, all the three Fe2O3/NPCFs electrodes show better rate performance than Fe2O3 bulk electrode and the Fe2O3/NPCFs-66.9% electrode has the highest reversible capacity at different current densities. The average discharge capacities (removing the first cycle) of Fe2O3/NPCFs-66.9% electrode are 1351, 1211, 1044, 866, 666, 540, and 337 mA h g−1 at a current density from 50 to 5000 mA g−1, respectively. Notably, when the current density returns to 50 mA g−1, the discharge specific capacities can recover to 1310 mA h g−1, corresponding to a nearly 97% capacity retention. This implies that the NPCFs matrix plays an important role to overcome the intrinsic disadvantages of Fe2O3 bulk materials. In addition, the superior rate cycling capability of Fe2O3/NPCFs composites is mainly attributed to the stability of 1D structure, improved electronic conductivity and the strong interfacial interaction Fe–O–C bond between Fe2O3 nanoparticles and NPCFs.

The cycling stability of the Fe2O3 and Fe2O3/NPCFs electrodes were investigated by galvanostatic charge–discharge processes between 0.01 and 3.0 V at 100 mA g−1 for 100 cycles, as shown in Fig. 6e. Fe2O3 electrode exhibits a high initial capacity, whereas Fe2O3/NPCFs a rapid decrease. When the stability test reached 50 cycles, the specific capacity decreased from 1482 to 123 mA h g−1 (only 8% retained). The main reason for the rapid capacity fading is the pulverization of the electrode caused by the large volume expansion of Fe2O3 nanoparticles during the charge–discharge processes. On the contrary, the capacity of the Fe2O3/NPCFs electrodes leveled off from the 40th cycle. The reversible capacities after 100 cycles are about 1106, 820, and 563 mA h g−1 for Fe2O3/NPCFs-66.9%, Fe2O3/NPCFs-44.1% and Fe2O3/NPCFs-79.2%, respectively. Three reasons could be contributed to the significantly increased electrochemical performance: (1) The small size of Fe2O3 nanoparticles about 5 nm can shorten the diffusion distance of Li ions and enhance the reaction efficiency. (2) The N-doping of porous carbon fibers can increase the conductivity and provide lots of active sites, thus realizing the rapid charge transfer and the high reversibility capacity. (3) The robust 1D structure of NPCFs and the strong interfacial interaction Fe–O–C bond can not only restrain the aggregation of Fe2O3 nanoparticles, but also can accommodate the volume change to effectively maintain the integrity of the whole electrode during the long-term cycles. In addition, when the mass ratio of Fe2O3 is 66.9 wt%, the Fe2O3/NPCFs-66.9% composite exhibits the best cycling stability, suggesting an optimized mass ratio is also important for improving the electrochemical performance. Moreover, the Fe2O3/NPCFs-79.2% electrode shows a lower rate performance and cycling stability as compared with Fe2O3/NPCFs-66.9%, Fe2O3/NPCFs-44.1%. This phenomenon can be attributed to the reduced electrical conductivity and porous architecture for the composite. Because pure Fe2O3 material has a relatively low intrinsic electrical conductivity, the conductivity of the whole material decreases drastically as the loading density increases. In addition, when the mass ratio was further increased from 66.9 to 79.2%, most of the Fe2O3 nanoparticles exposed on the outer surface of the NPCFs network, which was consistent with observation by SEM in Fig. 3i, j. These nanoparticles blocked the void space of NPCFs matrix and hindered the infiltration of electrolyte into the whole Fe2O3/NPCFs hybrids. Therefore, the Fe2O3/NPCFs-79.2% electrode exhibited a poor electrochemical performance compared with Fe2O3/NPCFs-66.9%, Fe2O3/NPCFs-44.1%.

To reveal the charge transfer kinetics of the Fe2O3/NPCFs composites, EIS measurement was performed (Fig. 6f). Apparently, each plot consists of one compressed semicircle in the high-middle frequency regions that is assigned to the charge transfer resistance (Rct) and a straight sloping line in the low-frequency region corresponding to the Warburg impedance (ZW), which is influenced by ion diffusion [51, 52]. The Rct of the Fe2O3/NPCFs-66.9% electrode is 628 Ω, which is higher than that of the NPCFs (423 Ω) and much lower than Fe2O3 (1335 Ω) electrodes. The smaller Rct value of Fe2O3/NPCFs-66.9% hybrid means that it has a rapid electron transport rate during the electrochemical lithium insertion/extraction reaction. The excellent electrical conductivity and the unique architecture of the composite collectively contribute to the superior electrochemical performance.

The morphologies of the Fe2O3/NPCFs-44.1%, Fe2O3/NPCFs-66.9% and Fe2O3/NPCFs-79.2% electrodes after 100 cycles were further examined by SEM and shown in Fig. 7a–c, respectively. It can be observed clearly that the 1D architecture of NPCFs was preserved without the obvious damage in Fig. 7a, b. However, the structure of NPCFs was almost destroyed, and a lot of agglomerations can be found easily. This phenomenon may be caused by the increase in Fe2O3 content. Because of the lack of a NPCFs matrix to accommodate the serious volume expansion of excess Fe2O3 nanoparticles during Li+ intercalation/deintercalation process, this can result in structural distortion and crack. These results are in good agreement with the cycling performance tests. Fe2O3/NPCFs-66.9% and Fe2O3/NPCFs-44.1% electrodes exhibit higher reversible specific capacity than Fe2O3/NPCFs-79.2% electrode.

Figure 7
figure 7

SEM images of Fe2O3/NPCFs-44.1% (a), Fe2O3/NPCFs-66.9% (b) and Fe2O3/NPCFs-79.2% (c) electrodes after 100 charge/discharge cycles at 100 mA g−1

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Conclusion

In summary, free-standing and flexible electrode Fe2O3/NPCFs were successfully prepared through electrospinning technology and nonaqueous sol–gel process, followed by pyrolysis. The resultant 1D NPCFs were formed of mesopores and macropores and exhibited a large surface area and high porosity. We also explored the impact of Fe2O3 loading amount on electrochemical performance in this paper and the optimal content in Fe2O3/NPCFs hybrids is 66.9 wt%. In addition, the results of TG curves, Raman spectra, XPS spectra indicated that the strongly interfacial interaction Fe–O–C bonds were formed between NPCFs and Fe2O3. When used as an anode material for LIBs, the Fe2O3/NPCFs-66.9% hybrid exhibited the smaller size of Fe2O3 nanoparticles (about 5 nm), 1D architecture and excellent electrical conductivity of NPCFs as well as the strong interfacial interaction. Moreover, the convenient preparation processes, natural abundance raw materials and low cost exhibit a promising application for Fe2O3/NPCFs-66.9% composite as electrode material for LIBs.