Introduction

In recent years, lithium-ion batteries (LIBs) have become the dominant power sources in portable electronic devices and electric vehicles due to their high gravimetric and volumetric capacity, long cycle life, less environmental pollution, etc. (Dunn et al. 2011; Bruce et al. 2008; Xu et al. 2014). Although graphite has served as a reliable anode material for commercial LIBs with high reversibility, its low capacity (372 mAh g−1) can hardly meet the increasing demand for high energy/power density of next-generation LIBs (Zhang et al. 2012a, b). To improve the electrochemical performance of LIBs, considerable efforts have been made recently in finding new electrode materials, which should have higher energy density compared with the existing system (Ji et al. 2011). Transition metal oxides have been widely investigated as anode materials for LIBs because they have higher specific capacity and volumetric energy density than graphite (Zhou et al. 2014; Wang et al. 2011; Zhao et al. 2008; Zhang et al. 2014a, b; Tummala et al. 2012). However, transition metal oxides suffer from large volume change during lithiation/delithiation process, which leads to poor reversibility and rapid capacity degradation. For example, MnO delivered a capacity of 650 mAh g−1 in the first cycle but it decreased quickly to 250 mAh g−1 after 20 cycles (Liu et al. 2012); Fe3O4 showed a discharge capacity of more than 949 mAh g−1 in the first cycle that declined to 200 mAh g−1 after 20 cycles (Yoon et al. 2011). To circumvent the poor cycle and high rate problems of transition metal oxides, tremendous efforts have been made to improve their electrochemical performance by optimizing the size of particles, constructing special architectures, coating with electronically conductive layers, or doping metals into compounds (Xu et al. 2014; Zhang et al. 2012a, b; Wang et al. 2011; Issac et al. 2011). Recently, ternary metal oxides have received considerable attention due to their good cyclic stability due to the possible synergistic effect, which enhances the intrinsic properties of each component. For example, MnCo2O4 quasi-hollow microsphere electrodes exhibited a long cycle life of 610 mAh g−1 after 100 cycles (Li et al. 2013). ZnFe2O4/C electrodes showed a high specific capacity of 840 mAh g−1 after 30 cycles (Deng et al. 2011). However, the ternary metal oxides still cannot tolerate high current density. ZnFe2O4 electrode only maintained a discharge capacity of 86 mAh g−1 at high current density of 1600 mA g−1 (Xie et al. 2014).

Carbon coating is a simple and effective way to alleviate the above issues (Deng et al. 2011). On one hand, carbon materials are very stable anode materials in LIBs due to their small volume change during Li+ insertion/extraction. The SEI films on carbon surface are also relatively stable, which further ensure the structural stability upon cycling (Xiao et al. 2013; Hu et al. 2006; Needham et al. 2006; Li et al. 2009). On the other hand, carbon has high conductivity, which greatly enhances the rapid electron transport during the electrochemical Li+ insertion/extraction reaction (Geng et al. 2014; Zhi et al. 2008). In particular, carbon coatings can serve as perfect barriers to protect the inner active materials and maintain their high capacities (Su et al. 2013; Wang et al. 2010; Lou et al. 2008; Liu et al. 2009; Su et al. 2012).

MnFe2O4 is an important ternary metal oxide and has been widely investigated in drug delivery (Shah et al. 2013), environmental remediation (Yao et al. 2014), and LIBs (Lin et al. 2013) for its excellent physical and chemical properties. During the past few years, Xiao et al. synthesized MnFe2O4–graphene nanocomposites by hydrothermal method, which showed an enhanced reversible capacity and an excellent rate capacity (Xiao et al. 2013). Lin et al. successfully synthesized MnFe2O4 particles with different morphologies including cube, truncated cube, polyhedron, and octahedron (Lin et al. 2013). The polyhedral MnFe2O4 particles exhibited a large initial discharge capacity of 980 mAh g−1 and remained about 428 mAh g−1 after 50 cycles, which was large than the other MnFe2O4 particles. Zhang et al. successfully synthesized mesoporous MnFe2O4 microspheres which displayed a capacity of 410.3 mAh g−1 after 50 cycles at 0.2 C (Zhang et al. 2013a, b). Permien et al. synthesized nanocrystalline MnFe2O4 by conventional refluxing method and investigated the reaction mechanism of Li+ insertion into anode materials (Permien et al. 2013). To the best of our knowledge, there are no reports on the studies of carbon-coated MnFe2O4 (MnFe2O4@C) composites as anode materials for LIBs. In this paper, we report a facile method for the preparation of MnFe2O4@C composites, which show good reversible capacity, remarkable rate, and cycling performance for LIBs.

Experimental

Synthesis of MnFe2O4 nanospheres

All the reagents were of analytical purity and were used without further purification. MnFe2O4 was prepared by a hydrothermal method. 2.5 mmol manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), 5.0 mmol iron chloride hexahydrate (FeCl3·6H2O), 1.0 g polyethylene glycol (PEG), and 3.6 g CH3COONa were mixed and dissolved in 40 ml of ethylene glycol (EG) to form a uniform solution. The mixture was transferred into a Teflon-lined autoclave and maintained at 200 °C for 10 h. Finally, the MnFe2O4 microspheres were obtained after centrifugation and drying at 60 °C overnight.

Synthesis of MnFe2O4@C composites

The as-prepared MnFe2O4 nanospheres (0.2 g) were dispersed in 25 mL deionized water by ultrasonication to form a suspension. 0.5 g of glucose was added to the solution under gentle stirring. The resulting suspension was transferred to a Teflon autoclave (40 mL), which was heated at 200 °C for 12 h. The solid products were separated by centrifugation and dried at 60 °C overnight. The MnFe2O4@C composites were obtained after calcination at 500 °C in Ar atmosphere for 4 h.

Characterization

X-ray diffraction (XRD) patterns were obtained by a diffractometer (Philips PW3040/60) with Cu Ka radiation (λ = 1.5418 Å). Raman spectrum was recorded by a Renishaw RM2000 Raman system with He–Ne (632.8 nm) laser excitation. The morphologies of the samples were examined using scanning electron microscopy (SEM, Hitachi S4800), and the microstructures were investigated using high-resolution TEM (HRTEM, JEOL-2100F) performed at an acceleration voltage of 200 kV. The thermogravimetric analysis (TG) measurement of the sample was performed using Netzsch STA 449C thermal analyzer. The carbon percentage in MnFe2O4@C composites was determined by a Flash EA1112 element analysis analyzer (Thermo Fisher Scientific Inc).

Electrochemical measurement

Electrochemical performances of the as-prepared composites were investigated with two-electrode coin-type cells (CR2025). The working electrodes were prepared by a slurry coating procedure. The slurry consisted of the as-synthesized MnFe2O4@C composite or pure MnFe2O4 as active material, acetylene black as conducting additive, and polyvinylidene fluorides (PVDF) as binder with a weight ratio of 75:10:15 in N-methyl pyrrolidinone (NMP); the slurry was spread on a nickel foam and dried at 80 °C for 6 h in vacuum. The CR2025-type coin cells were assembled in an argon-filled glove box. The electrolyte solution was 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1. The CV experiment was tested on a CHI604D electrochemistry workstation from 0.01 to 3.0 V (vs. Li+/Li) at a scan rate of 0.1 mV s−1. The galvanostatic charge and discharge measurements were tested at a current density of 100 mA g−1, and the rate performance was conducted at different current densities (100, 200, 500, 1000 mA g−1) with 10 charge–discharge cycles at each step on a battery test system (Neware Co. Ltd., Shenzhen) in the voltage range from 0.005 to 3 V. AC impedance measurements were carried out using a CHI604D electrochemical workstation with a 5 mV sinusoidal voltage in the frequency range from 1 MHz to 0.1 Hz at room temperature. The impedance data were fitted using the ZsimpWin computer program.

Results and discussion

Figure 1 shows the typical XRD patterns of the pristine MnFe2O4 nanospheres and MnFe2O4@C composites. All the diffraction peaks in the XRD pattern of pristine MnFe2O4 can be perfectly indexed to the (220), (311), (400), (422), (333), (440), and (533) planes of face-centered cubic MnFe2O4 with a lattice constant of a = 8.511 Å (JCPDS No. 74-2403). The size of MnFe2O4 nanoparticles estimated from the Debye–Scherrer formula is about 20 nm. The characteristic peaks of the MnFe2O4@C composites can also be well indexed as cubic MnFe2O4. No obvious reflection peaks from other impurities were detected in the as-prepared MnFe2O4@C composites.

Fig. 1
figure 1

XRD patterns of the MnFe2O4 nanospheres and MnFe2O4@C composites

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The existence of carbon in the MnFe2O4@C composites is further confirmed by Raman spectroscopic analysis. As shown in Fig. 2, the two characteristic D and G bands at about 1371 and 1585 cm−1 belong to the disordered and graphitic carbon, respectively (Yuan et al. 2011). The peak intensity ratio between D and G (I D/I G) can be used to estimate the degree of crystallinity of carbon materials. Here, the I D/I G value is calculated to be 0.85, indicating that the carbon in these composites is partially graphitized (Zhao et al. 2013).

Fig. 2
figure 2

Raman spectrum of the as-prepared MnFe2O4@C composites

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The carbon content of the as-synthesized MnFe2O4@C composites was evaluated by thermal gravimetric analysis (TGA) in air (Fig. 3). The TG curves demonstrate that the weight loss of the pristine MnFe2O4 microspheres is about 3.13 wt% below 100 °C, which is ascribed to the desorption of physically adsorbed H2O, while the weight loss of 3.27 wt% from 100 to 600 °C is due to the decomposition of organic species (e.g., EG and PEG) adsorbed on the sample surface. The slight weight increase of ~1 wt% from 100 to 250 °C in the TG curve of MnFe2O4@C sample can be attributed to the rapid formation of C=O functional groups on carbon shells due to the pre-oxidation reaction (Wu et al. 2013), which is similar to the TG result of RGO–MnFe2O4 nanocomposites (Zhang et al. 2014a, b). The rapid weight loss of MnFe2O4@C composites appears between 300 and 450 °C, which is attributed to the oxidation of carbon. On the basis of the weight losses, the amount of carbon in the MnFe2O4@C composites is around 11.18 wt%. To further determine the carbon content of MnFe2O4@C composites, the elemental analysis experiment was also performed. The carbon content is revealed to be 12.01 wt%, which is well consisted with the TG result.

Fig. 3
figure 3

TG curve of the MnFe2O4 nanospheres and MnFe2O4@C composites

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The morphology and microstructure of the as-prepared products were measured using SEM and TEM. It is very clear that the pristine MnFe2O4 has a spherical morphology with diameters ranging from 300 to 400 nm (Fig. 4a, b). An HRTEM image (Fig. 4c) of the MnFe2O4 nanospheres confirms the high crystallinity of MnFe2O4 nanoparticles. The lattice fringe with a spacing of 0.30 nm is assigned to the (220) plane of cubic MnFe2O4. As shown in Fig. 4d and e, the spherical morphology is well preserved after pyrolysis treatment for the preparation of MnFe2O4@C. An HRTEM image of the MnFe2O4@C nanosphere and its corresponding FFT pattern are shown in Fig. 4f; the lattice fringes of MnFe2O4 show a spacing of 0.26 nm, which agrees well with the d-spacing of the (311) plane of MnFe2O4. Also, the carbon layers can be clearly seen in the HRTEM image with a uniform thickness of 3–5 nm, confirming the successful wrapping of thin carbon shells on MnFe2O4 microspheres.

Fig. 4
figure 4

SEM image (a), TEM image (b), and HRTEM (c) image of MnFe2O4 nanospheres; SEM image (d), TEM (e), and HRTEM (f), images of MnFe2O4@C composites and the corresponding FFT pattern (inset)

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The formation process and mechanism of MnFe2O4@C composites is proposed. In the first stage, MnFe2O4 is nucleated in the presence of Fe3+ and Mn2+ ions under solvothermal condition via the reaction (2Fe3+ + Mn2+ + 4H2O → MnFe2O4 + 8H+), which leads to the formation of nanosized crystalline MnFe2O4 (10–15 nm). Then, in the second stage, these nanosized crystallites are self-assembled into large secondary nanospheres, driven by a need to reduce their surface energy. CH3COONa plays a crucial role as a mineralizer for the formation of MnFe2O4 nanospheres with a narrow size distribution (Zhang et al. 2012a, b). Ethylene glycol acts as a structure-directing agent to regulate the surface state of the nanosized crystalline particles and influence their nucleation and aggregation process, which are finally assembled into nanospheres (Zhang et al. 2013a, b). Subsequently, carbonaceous materials are produced by hydrothermal carbonization of glucose at low temperature (180–200 °C), which is a well-established method to create hydrophilic carbon materials (Wang et al. 2001). The surfaces of MnFe2O4 nanosphere with rich hydroxyl groups can induce the in situ precipitation of carbonaceous units to form a uniform carbon shell by carbonization of glucose. In order to improve the stability and electrical conductivity of the carbon shells, they are further graphitized at 500 °C for 4 h in Ar atmosphere. Eventually, the MnFe2O4@C nanospheres are produced.

The electrochemical reactivity of MnFe2O4@C composites for lithium storage was evaluated by CV measurement. Figure 5a shows the CV curves of the as-prepared MnFe2O4@C composite electrode from the first to the fourth cycle between 0.01 and 3.0 V at a scan rate of 0.1 mV s−1. There is a broad peak centered at 0.5 V in the first cathodic process, which is associated with the reduction of Mn2+ and Fe3+ to Mn0 and Fe0, and the formation of SEI film due to the decomposition of the electrolyte (Xiao et al. 2013). Recently, in situ TEM has been used to understand the electrochemical conversion mechanism of MnFe2O4 in LIBs (Liu et al. 2014). It was found that single-crystalline MnFe2O4 was converted into polycrystalline Li2O/Mn/Fe with large volume expansion during the first discharge and subsequently transformed to polycrystalline MnO/Fe3O4 with volume shrinkage during charge. Reversible conversion between MnO/Fe3O4 and Li2O/Mn/Fe occurred during the subsequent cycles with small volume changes. The main cathodic peak in Fig. 5a shifts to about 0.8 V from the second cycles, distinguishing the later conversion mechanism from that in the first cycle. During the anodic process, two peaks are present at 1.60 and 1.75 V, which are associated with the oxidation of Mn and Fe to MnO and Fe3O4, respectively. Compared with the first discharge process, the peak current and the integrated area of this process decrease, indicating a large capacity loss during the charge process. In the subsequent charging process, two anodic peaks shift to 1.7 and 1.8 V because of the polarization of electrode materials (Tao et al. 2011). The conversion reaction could be described as follows (Liu et al. 2014):

Fig. 5
figure 5

a CV curves of MnFe2O4@C composites at a scan rate of 0.1 mV s−1. b Voltage profiles of MnFe2O4@C composites for the selected galvanostatic cycles at a current density of 100 mA g−1. c Cycling performance of the prepared MnFe2O4 nanospheres and MnFe2O4@C composites at a current density of 100 mA g−1. d Rate capability of the MnFe2O4 nanospheres and MnFe2O4@C composite electrode between 100 and 1000 mA g−1

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In the first cycle,

$${\text{MnFe}}_{ 2} {\text{O}}_{ 4} + {\text{ 8Li}}^{ + } + {\text{ 8e}}^{ - } \to {\text{ Mn }} + {\text{ 2Fe }} + {\text{ 4Li}}_{ 2} {\text{O}}$$
(1)
$${\text{Mn }} + {\text{ Li}}_{ 2} {\text{O }} \to {\text{ MnO }} + {\text{ 2Li}}^{ + } + {\text{ 2e}}^{ - }$$
(2)
$$3 {\text{Fe }} + {\text{ 4Li}}_{ 2} {\text{O }} \to {\text{ Fe}}_{ 3} {\text{O}}_{ 4} + {\text{ 8Li}}^{ + } + {\text{ 8e}}^{ - }$$
(3)

In the subsequent charge–discharge cycles,

$${\text{Mn }} + {\text{ 3Fe }} + {\text{ 5Li}}_{ 2} {\text{O }} \leftrightarrow {\text{ MnO }} + {\text{ Fe}}_{ 3} {\text{O}}_{ 4} + { 1}0{\text{Li}}^{ + } + { 1}0{\text{e}}^{ - }$$
(4)

The discharge–charge voltage profiles of the MnFe2O4@C composites at a current density of 100 mA g−1 are shown in Fig. 5b. In the first discharge process, the constant slopes between 0.7 and 0.2 V indicate the conversion reaction from MnFe2O4 to Mn/Fe and the SEI formation. The discharge plateau shifts to 0.7–1 V from the second cycle, which agrees well with the CV analysis. A plateau at 1.5–2 V during all the charge cycles was recorded. These voltage plateaus were formed due to the reversible oxidation/reduction conversion between Mn/Fe and MnO/Fe3O4 during the Li+ insertion/extraction.

The cycling performance of pristine MnFe2O4 nanospheres and MnFe2O4@C composites was measured at a constant current of 100 mA g−1 and is shown in Fig. 5c. The MnFe2O4@C composite electrode delivers a discharge capacity of 1222 mAh g−1 and a charge capacity of 938 mAh g−1 in the first cycle, which is much larger than the theoretical value of MnFe2O4 (917 mAh g−1). The initial capacity loss of 23.2 % mainly results from the diverse irreversible process, such as the irreversible phase conversion (Liu et al. 2014), the interfacial lithium storage, and the inevitable formation of SEI layer, which are common for most anode materials (Mai et al. 2011; Xu et al. 2012; Chen et al. 2012; Tang et al. 2014). However, the Coulombic efficiency reaches 95.5 % after the 5th cycle and remains relatively stable at 98 % in the subsequent cycles. In addition, the MnFe2O4@C composite retains a capacity of 646 mAh g−1 after 50 cycles, which is larger than the capacities of MnFe2O4 with different morphologies in literature (Lin et al. 2013; Permien et al. 2013). Even though a higher capacity (1335 mAh g−1) can be observed at the first cycle for the pristine MnFe2O4 microspheres, the capacity decreases quickly to 300 mAh g−1 after 50 cycles, showing an obviously poorer performance than MnFe2O4@C composite.

To investigate the rate capacity of MnFe2O4@C composites, the electrode was cycled at different current densities. As shown in Fig. 5d, the MnFe2O4@C composite electrode exhibits a surprising enhancement in rate capacity compared with pristine MnFe2O4 microspheres. The capacity of MnFe2O4@C composites is maintained at about 252 mAh g−1 at a high current density of 1000 mA g−1 after 40 cycles and recovers to 626 mAh g−1 when the current density is reduced back to 100 mA g−1. However, the pristine MnFe2O4 nanospheres deliver only ~40 mAh g−1 at the current density of 1000 mA g−1. The poor rate performance is mainly caused by their low electronic conductivity and large volume variations during the charge/discharge process. The carbon shells in MnFe2O4@C composite can not only enhance the conductivity of the electrode but also serve as protective layers to prevent the inner active materials from pulverizing, and thus maintain their high capacities (Su et al. 2013). The results indicate that the as-prepared MnFe2O4@C composites could tolerate high current charge–discharge cycling.

To further understand the good rate performance for MnFe2O4@C composites, electrochemical impedance spectra (EIS) of the pristine MnFe2O4 nanospheres and MnFe2O4@C composite electrodes were measured after 50 cycles and are shown in Fig. 6a. The impedance data were fitted using the ZsimpWin computer program. A semicircle followed by a straight line is observed in the EIS spectrum, which is a typical blocking-type behavior of thin-film electrode (Abe et al. 2004). The semicircle at the high-frequency region is assigned to the charge-transfer process of Li+ ion at the MnFe2O4/electrolyte interface, while the line in the lower-frequency region is ascribed to the limited diffusion of Li+ ions into MnFe2O4. As shown in Fig. 6a, the MnFe2O4@C electrode exhibits a smaller semicircle compared with the pristine MnFe2O4, indicating a lower charge-transfer resistance of the MnFe2O4@C electrode. The equivalent circuit model of the studied system is shown in Fig. 6b to represent the internal resistance of the test battery. The circuit component R e represents the internal resistance of the test battery, R sl and C sl are designated for the resistance of the migration and capacity of the layer in the high-frequency semicircle, R ct and C dl are associated with the charge-transfer resistance and a double-layer capacitance in the medium-frequency semicircle, Z W is associated with a Warburg element corresponding to the lithium-ion finite diffusion process (Su et al. 2012; Yang et al. 2010). The fitted impedance parameters are listed in Table 1. It can be seen that R sl of the MnFe2O4@C electrode is 3.05 Ω, which is significantly lower than that of the pristine MnFe2O4 (153.5 Ω). This can be attributed to the incorporation of carbon, which has high conductivity, and greatly enhances the electron transport during the electrochemical lithium insertion/extraction reaction. Carbon shell can also prevent the aggregation of MnFe2O4, resulting in significantly short diffusion path. The charge-transfer resistance R ct of the MnFe2O4@C electrode is 94.2 Ω, while the pristine MnFe2O4 electrode is 325 Ω. Since the charge-transfer procedure occurs on the contact interface between active material and electrolyte, it is reasonable that MnFe2O4@C electrode with larger contact interface exhibits smaller R ct. From the above discussion, it is concluded that the total resistance of MnFe2O4@C electrode is obviously lower than that of the pristine MnFe2O4 electrode, resulting in the significant improvement in the electrochemical performance.

Fig. 6
figure 6

a Electrochemical impedance spectra of the pristine MnFe2O4 nanospheres and MnFe2O4@C composite electrodes after the 50th discharge/charge cycle. b Equivalent circuit for MnFe2O4 nanospheres and MnFe2O4@C composite electrode/electrolyte interface

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Table 1 Impedance parameters derived using equivalent circuit model for MnFe2O4 and MnFe2O4@C electrodes
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Conclusion

In summary, we have successfully fabricated the MnFe2O4@C composites by a scalable solvothermal process with subsequent calcination. The thin carbon shells with a thickness of 3–5 nm are well wrapped on MnFe2O4 nanospheres. The as-prepared MnFe2O4@C composites are proved to have a greatly improved electrochemical performance as comparison with the pristine MnFe2O4, showing excellent cyclic stability and rate performance. This can be attributed to the uniform and continuous carbon layers, which can prevent the aggregation and maintain the integrity of particles, and greatly improve the electronic conductivity of electrodes. Our results reveal a new approach to MnFe2O4@C composite with great potential as superior anode materials for LIBs.