Background

As a high-performance green chemical power source, lithium-ion batteries (LIBs) have been widely used in portable mobile electronics markets and in electric vehicles due to its high energy density, long cycle life, low self-discharge, and lack of a memory effect [1]. However, with the development of the time, the traditional LIBs based on a graphite material cannot satisfy the growing requirements of high energy density and power density because of the low theoretical capacity (372 mAh g−1) of graphite material [2]. Transition metal oxides (TMOs) have been thriving over the past decades with the purpose of achieving superior specific capacities to commercial graphite [3, 4]. Typically, Fe2O3 has been regarded as one of the most promising anode candidate due to its high theoretical capacity (1007 mAh g−1), environmentally friendly nature, non-toxicity, and natural abundance [5, 6]. Despite its tremendous potential, however, its commercial application in LIBs is still hindered by some serious disadvantages such as the fast capacity fading and the volume expansion [7] during discharge/charge process.

To overcome the above issues and improve the electrochemical performance, various optimization strategies have been proposed. A well-accepted strategy [8] is design of nanostructured composite electrode, which not only better accommodates large strains but also provides short diffusion paths for lithium-ion insertion/extraction. To date, lots of nanostructured Fe2O3 materials including nanoparticles, nanorods, nanowires, and nanotubes have been designed and fabricated by different methods [9,10,11,12,13,14,15]. With the help of nanostructure, the volume expansion of Fe2O3 can be effectively accommodated. Furthermore, TMO-based LIB performance has been further improved by introducing nanostructured TMOs into conductive matrices recently [15,16,17,18,19]. For instance, the introduction of carbon coating layers onto Fe2O3 core has been widely explored due to the capacity of the carbon layer to enhance electrical conductivity effectively and restrain the cracking and crumbling of the Fe2O3 anode upon cycling. Zhao et al. [20] prepared Fe2O3 nanoparticles and graphene oxides through hydrothermal and Hummers’ [21] method respectively. Then, graphene-Fe2O3 composites were obtained by freeze drying process. Some Fe2O3–C core-shell composites such as carbon nanotube@Fe2O3@C, Fe2O3@C hollow spheres, and Fe2O3@graphite nanoparticles were fabricated by two-step synthesis methods containing hydrothermal reactions and high-temperature calcination processes [22,23,24]. These composites have shown excellent Li storage properties. However, the complicated preparation process, long treatment time, and high cost of these composites restrict their further applications. Therefore, developing a simpler approach for Fe2O3–C core-shell structure is urgently needed.

Herein, we report a synthesis of the Fe2O3/carbon core-shell nano-composite via a simple one-step hydrothermal process. The resultant Fe2O3/C nano-composite possesses pomegranate-like structure in which Fe2O3 was capsuled in carbon shells and every core-shell connects with each other as a pomegranate. This unique porous pomegranate structure can not only ensure good electrical conductivity for active Fe2O3, but also accommodate huge volume change during cycles as well as facilitate the fast diffusion of Li ion. As a result, the anodes exhibited a remarkable performance improvement when they were used in LIBs.

Methods

Iron nitrate nonahydrate (Fe3(NO3)3·9H2O), anhydrous dextrose (C6H12O6), anhydrous ethanol (CH3CH2OH), polyvinylidene difluoride (PVDF), and N-methyl-2-pyrrolidinone (NMP) were purchased from Tianjin Fuchen Chemical Reagents Factory, China. Deionized water (H2O) was provided by Hebei University of Technology.

The pomegranate-shaped Fe2O3/C nano-composite was prepared by a hydrothermal method. Firstly, 1.212 g Fe3(NO3)3·9H2O and 0.9 g C6H12O6 were dissolved in 40 mL of deionized water by magnetic stirring for 30 min, the ratio of carbon in the C6H12O6 to iron in the Fe3(NO3)3·9H2O is 10:1. Secondly, the solution was sealed in a capacity of 100 ml Teflon-lined autoclave and heated to 190 °C for 9 h and cooled naturally to room temperature. Then, the hydrothermal synthesis products were taken out and centrifugally separated with deionized water. Last, the products were dried in the thermostatic drying chamber at 60 °C for 12 h.

The phase composition of the samples was investigated by powder XRD on a Rigaku D/Max 2500 V/pc X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å) with scan range (2θ) 20~70° and the scan step of 0.02°. Raman spectra were attained with an Ar-ion laser of 532 nm using the in Via Reflex Raman imaging microscope system. The carbon content of pomegranate-shaped Fe2O3/C nano-composites was estimated by the thermogravimetric analysis (TGA; TA Instruments, SDTQ600) method [22, 24], which showed weight change after heating up. The weight ratio of carbon was calculated as 45.2 wt%. The morphology of the samples was performed by scanning electron microscopy (SEM) (JEOL JSM-6700F). The microstructure was characterized with a JEOL JEM-2100F transmission electron microscope (TEM), and the elemental composition of the samples was analyzed by energy-dispersive X-ray spectroscopy (EDS). The elements and its valence states were analyzed by X-ray photoelectron spectroscopy (XPS; VG ESCALAB MK II, VG Scientific).

In order to investigate the electrochemical performance, active materials (80 wt%), Super-P (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) were mixed in N-methyl-2-pyrrolidinone (NMP) to form a slurry. Then, the slurry was coated onto a Cu foil substrate and dried at 100 °C for 6 h. The active materials were used as the working electrode and Li metal foil was used as the counter electrode, 1 mol L−1 LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) was used as the electrolyte, Celgard 2300 was used as the separator, and CR2025 coin cells were assembled in an argon-atmosphere glove box. Cycling tests were tested at 25 °C using a CT-4008 battery cycler system between 0.01 and 3.00 V at a current density of 100 mA g−1 for 100 cycles. The rate testing at different current densities (10 cycles each at 100 mA g−1, 200 mA g−1, 500 mA g−1, and 2000 mA g−1) was followed by an additional cycle test at 100 mA g−1. Cyclic voltammetry (CV) was performed on an electrochemical workstation (Zahner Im6e) at a scanning rate of 0.5 mV s−1 in a potential range of 0.01~3 V (vs. Li/Li+) at room temperature. For comparison, the electrochemical performance of Fe2O3 nanospheres (25~50 nm, CAS no. 1309-37-1, purchased from Shanghai Aladdin Biochemical Technology Co. Ltd.) was also tested using a same measuring parameter.

Results and Discussion

The crystallographic structures of Fe2O3/C nano-composite are confirmed by XRD, and the result is shown in Fig. 1a. It can be seen that XRD pattern of Fe2O3/C nano-composite could be indexed as the hematite crystal structure of Fe2O3 (JPDS No. 33-0664). Diffraction peaks of Fe2O3 in (012), (104), (110), (006), (113), (024), (116), (018), (214), (300), and (208) crystalline plane can be clearly observed. No diffraction peaks of carbon are detected due to the low hydrothermal reaction temperature (190 °C) that is below the crystallization temperature of carbon.

Fig. 1
figure 1

a XRD patterns of Fe2O3/C nano-composite. b Raman spectra of Fe2O3/C nano-composite

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Raman measurement is used to verify the formation of Fe2O3/C nano-composite. As shown in Fig. 1b, Raman spectra exhibit the peak located around 1306 cm−1 associated with the hematite two-magnon scattering that are the feature of Fe2O3. Due to that Fe2O3 was coated with carbon, the peak of Fe2O3 is not obvious [25]. The peaks at 1396 cm−1 and 1571 cm−1 are characteristic carbon D-band and G-band peaks, respectively. The former corresponds to the disordered carbon, while the later assigns to 2D-graphite. The low value of intensity ratio between D and G bands (ID/IG) implies high relative amount of graphitic carbon and good electrical conductivity of carbon layer, which is beneficial for the conductivity of Fe2O3/C nano-composite.

XPS survey spectra are shown in Fig. 2 for further evaluating the chemical compositions and valence states of the product. Figure 2a presents an XPS fully scanned spectra of Fe2O3/C nano-composite. The C 1s, O 1s, and Fe 2p core photoionization signals and Fe Auger and O Auger signals can be clearly found. An XPS high-resolution scan of the Fe 2p core level is shown in Fig. 2b. It is shown that the peaks at 711.6 and 725.2 eV correspond to Fe 2p3/2 and Fe 2p1/2 in the Fe 2p spectrum, respectively. The binding energy difference is 13.6 eV which is consistent with the trivalent oxidation state of Fe [26]. The C 1s spectrum of Fe2O3/C (Fig. 3c) suggests three carbon-containing functional groups: C–C/C=C (284.2 eV), C=O (287.3 eV), and O–C=O (290.4 eV) groups. The presence of the Fe–O–C bond (533.4 eV) in the O 1s spectrum (Fig. 3d) indicates the presence of strong interfacial interactions (Fe–O–C bonds) between Fe2O3 and carbon-based matrix.

Fig. 2
figure 2

a XPS survey spectra of Fe2O3/C, b Fe 2p, c C 1s, and d O 1s spectra

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Fig. 3
figure 3

a, b SEM images of Fe2O3/C nano-composite; insets: the pore size distribution of Fe2O3/C composites. c, d TEM images of Fe2O3/C nano-composite. e High-resolution TEM image and f corresponding SAED patterns of Fe2O3/C

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SEM images of Fe2O3/C nano-composite are shown in Fig. 3a, b. It is clear shown that spherical nanoparticles with uniform size between 30 and 40 nm are homogeneously dispersed. There is lots of space left between particles, forming a 3D conductive structure. The average diameter of the Fe2O3/C particles was found to be 34.3 nm, as shown in Fig. 3a as inset.

More in-depth information about the Fe2O3/C nano-composite is further monitored by TEM images (Fig. 3). As shown in Fig. 3c, d, the Fe2O3 nanoparticles are well-enclosed within carbon shells, implying a pomegranate core-shell structure. According to high-resolution transmission electron microscopy (HRTEM) analysis of Fe2O3/C core-shell nanoparticle (Fig. 3e), crystalline planes of Fe2O3 (104), (012) with a distance spacing of 0.33 nm and 0.27 nm can be clearly found, which is in agreement with the above XRD test results. It can also clearly be seen that the Fe2O3 nanoparticles are well-covered by a carbon layer with a thickness of about 1.75 nm. The corresponding selected area electron diffraction (SAED) pattern confirms that the polycrystalline diffraction ring of the sample corresponds to the Fe2O3 planes, as shown in Fig. 3f.

Figure 4a depicts the CV plots with a voltage range between 0.01 and 3.0 V at a scan rate of 0.1 mV s−1. In the first cycle, the cathodic peak at about 0.7 V was believed to the conversation of Fe3+ to Fe0 as well as the formation of solid electrolyte interphase (SEI) film, while the broad peak near 0.1 V may be related to the Li+ ion insertion in carbon and the formation of LiC6 [27]. A dominant anodic peak at 1.75 V can be attributed to the oxidation of Fe0 to Fe3+. The related reaction can be described by the Eq. (1) [27]:

$$ {\mathrm{Fe}}_2{\mathrm{O}}_3+6{\mathrm{Li}}^{+}+6{\mathrm{e}}^{\hbox{-}}\leftrightarrow 2\mathrm{Fe}+3{\mathrm{Li}}_2\mathrm{O} $$
(1)
Fig. 4
figure 4

The cyclic voltammogram (a) and voltage profiles (b) of the Fe2O3/C composite at the first, second, and third cycle. c Cycle performance of Fe2O3/C and Fe2O3 nanoparticles at 100 mA g−1. d Rate capability of Fe2O3/C and Fe2O3 nanoparticles with a current density ranging from 100 to 2000 mA g−1

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In the following cycles, both positions of cathodic and anodic peaks shifted to a higher potential (0.8 and 1.78 V respectively), which can be ascribed to the improved kinetics of the Fe2O3 electrode after the structure realignment and electrochemical activation. Meanwhile, the intensities of CV curves dropped slightly, which may resulted from a better electrical contact between electrodes with electrolyte and the formation of a stable SEI film. In addition, the overlapped CV curves in the following cycles implied a good electrochemical reversibility.

The initial three charge/discharge cycling results of Fe2O3/C electrodes at a constant current density of 100 mA g−1 are shown in Fig. 4b. The first discharge capacity of Fe2O3/C was 917 mAh g−1 and was only 760 mAh g−1 during charging. The loss of capacity may be caused by the inevitable formation of solid electrolyte interphase (SEI) film. The reversible capacity of the second and third cycle is 776 and 763 mAh g−1 respectively. It exhibits the excellent cyclic stability.

The cycling performance of the electrode at a current density of 100 mA g−1 is shown in Fig. 4c. The second discharge capacity of the Fe2O3/C is 776 mAh g−1, and after 100 cycles, the electrode retained a specific capacity of 705 mAh g−1, which is about 90% of the second discharge capacity, indicating a good cycling performance. And the coulombic efficiency is almost 100% after 100 cycles, further confirming the superior electrochemical performance. The rate performance of the Fe2O3/C at current density ranging from 100 to 2000 mA g−1 is displayed in Fig. 4d. It showed good rate capability, with a charging capacity of 710 mAh g−1, 620 mAh g−1, 580 mAh g−1, and 480 mAh g−1 at 100 mA g−1, 200 mA g−1, 500 mA g−1, and 2000 mA g−1, respectively. When the rate was returned to 100 mA g−1, the capacity of the electrode was back to 680 mAh g−1, which showed excellent rate capability. The excellent electrochemical performance is mainly attributed to the enhanced core-shell structural stability, and carbon improves the electric conductivity. Every core-shell structural connects as pomegranate which also can improve electron transfer to improve the electric conductivity and enhance structural stability.

Figure 4c, d also shows the cycling performance of the Fe2O3 nanoparticle anode at 100 mA g−1. The first discharge capacity of the Fe2O3 nanoparticles is about 720.9 mAh g−1, but after 100 cycles, it only retained a specific capacity of 396.5 mAh g−1. And the rate performance of the Fe2O3 nanoparticles at current rates ranging from 100 to 2000 mA g−1 is shown in Fig. 4d. The capacity of the Fe2O3 anode is 570 mAh g−1, 505 mAh g−1, 450 mAh g−1, and 345 mAh g−1 at 100 mA g−1, 200 mA g−1, 500 mA g−1, and 2000 mA g−1, respectively. When the rate was returned to 100 mA g−1, the capacity of the electrode was back to 395 mAh g−1. Therefore, the electrochemical rate and cycling performance of Fe2O3 nanoparticle anode is not a 60% as good as Fe2O3/C anode, which is mainly because of the volume expansion of Fe2O3 nanoparticles during the charge and discharge process.

The theoretical capacity (Ctheo.) of the as-obtained pomegranate-shaped Fe2O3/C anode is Ctheo. = CFe2O3,theo. × Fe2O3% + Ccarbon,theo. × Carbon% = 1007 × 54.8% + 372 × 45.2% = 720 mAh g−1. After charge/discharge cycling at 100 mA g−1 for 100 cycles, the discharge capacity remained at about 705 mAh g−1, which is slightly lower than the theoretical capacity. These high capacities may result from the synergistic interactions between Fe2O3 and carbon.

Figure 5 shows the electrochemical impendence spectroscopy (EIS) of the Fe2O3 and Fe2O3/C electrodes before and after 100 cycles. The high-frequency semicircle in the Nyquist plot is connected with the charge transfer resistance of the electrode, whereas the slope line at the low frequency is an indication of Warburg impedance of Li ion into active material diffusion. It is well known that smaller semicircle represents a lower charge transfer resistance of an electrode. Obviously, the diameter of the semicircle for the core-shell pomegranate-shaped Fe2O3/C composite before and after cycles is much smaller than that of the Fe2O3 contrast material in the corresponding state, indicating the fact that the core-shell pomegranate-shaped Fe2O3/C composite electrode possesses lower contact and charge transfer impedances when used as anode materials than the bare Fe2O3 sample. This result can be attributed to the porous pomegranate-shaped structure of the Fe2O3/C anode, which can provide more space to adapt the change of volume and promote Li+ ion diffusion during lithiation and delithiation processes.

Fig. 5
figure 5

Nyquist plots of Fe2O3 and Fe2O3/C electrodes

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Li storage performances of the as-obtained core-shell pomegranate-shaped Fe2O3/C anode and related Fe2O3/C materials reported in the previous literature are summarized in Table 1 [27,28,29,30,31,32,33,34]. It can be seen from the table that the pomegranate-shaped Fe2O3/C anode shows higher capacity after cycling than most of reported anodes. The excellent performance of the material in Li-ion storage can be attributed to the unique structure of macroscopical pomegranate shape with plentiful porosity as well as microscopic core-shell Fe2O3–C structure, which can ensure good electrical conductivity for active Fe2O3, accommodate huge volume change during cycles, and facilitate the fast diffusion of Li ion.

Table 1 The comparison of Li storage performances between this work and the previous literature
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Conclusions

In summary, we have successfully designed and synthesized pomegranate-shaped Fe2O3/C to realize industrialization. The Fe2O3 nanoparticles are well-enclosed within carbon shells, and every core-shell structure is connected to each other as pomegranate, which not only improves the stability of the anode during discharging/charging process but also leads to the improvement of the lithium reaction kinetics. This structure greatly reduces the volume expansion and provides good electrolyte diffusion. So the Fe2O3/C composites as the anode of LIB exhibit superior lithium ion storage performance.