Polyaniline coated Fe₃O₄ hollow nanospheres as anode materials for lithium ion batteries

Abstract

Polyaniline (PANI) coated Fe₃O₄ hollow nanospheres (h-Fe₃O₄@PANI) have been successfully synthesized and investigated as anode materials for lithium ion batteries (LIBs). The structure and composition analyses have been performed by employing X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show a good combination between h-Fe₃O₄ and PANI with a conductive state. When evaluated as anode materials for LIBs, the h-Fe₃O₄@PANI nanocomposites exhibit excellent LIB performance with enhanced reversible capacity, good cycling performance and rate capability as compared to h-Fe₃O₄ and solid Fe₃O₄ (s-Fe₃O₄) nanospheres. The improved electrochemical performance of the nanocomposite is considered due to the hollow nature of the products and the coated PANI layers.

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1. Introduction

Owing to their high energy density, long cycling life, and environmental friendliness, rechargeable lithium ion batteries (LIBs) have achieved great commercial success in portable electronic devices and stationary energy storage systems.^1,2 However, the specific energy of current LIBs is still insufficient for many applications due to the low capacity of electrode materials (the capacity of commercial graphite anode is 372 mA h g⁻¹).^3,4 In recent years, transition metal oxides have been widely studied as anodes for their high theoretical capacities.^5,6 Among these materials, iron oxide, especially Fe₃O₄, is considered as one of the most promising anodes due to its high specific capacity (∼926 mA h g⁻¹), nontoxicity, natural abundance, low cost, and high electronic conductivity.^7–10 Iron oxide anodes also show safety benefits compared with graphite because of their higher potential, which reduces lithium dendrite deposition on the anodes during cycling.¹¹ When matched with suitable materials with a higher voltage platform than commercial LiCoO₂ cathodes (∼3.6 V), iron oxides show potential application in electric vehicle or hybrid electric vehicle fields.¹² However, the practical using of Fe₃O₄ material mainly suffers from drastic volume change during charging/discharging cycles, which results in pulverization, subsequent electrical connectivity loss, and thus rapid capacity fading.

Many approaches have been proposed to overcome the limitations, such as construction of nanostructured materials with a suitable morphology, anchoring or embedding active materials in a conducting medium (carbon nanotube, reduced graphene oxide, or conducting polymers). The nanostructure facilitates electron and ion transportation, provides more active sites for the storage of lithium ions than its bulk counterpart, and accommodates the local volume change upon charging/discharging cycles.^1,13 The incorporation of conducting materials not only lowers the charge-transfer resistance during electrochemical reactions, but also prevents cracking and the breakdown of active materials caused by the volume variation.^14–18 Among different conducting materials, polyaniline (PANI) is attractive because of its high conductivity, good environmental stability, and commercial viability with low production cost. Anodes based on metal oxides and PANI usually possess excellent lithium storage properties. For example, an urchin-like Fe₂O₃ structure coated with PANI shows a large reversible capacity, high rate capability, and excellent cycling stability after 100 cycles.¹⁹ Fe₂O₃ nanorods capped with PANI possess a high capacity of 778 mA h g⁻¹ at a current of 1.0 Ag⁻¹ after 100 cycles.²⁰ Although a wide range of methods including macromolecule-induced self-assembly,²¹in situ surface polymerization,^22,23 plasma polymerization,²⁴ and electropolymerization²⁵ have been employed to synthesize PANI modified Fe₃O₄ nanostructures, which show excellent electromagnetic wave absorption and pseudocapacitive properties, to the best of our knowledge, there is no report on lithium storage properties of Fe₃O₄/PANI anodes.

In this work, we synthesized PANI layer coated Fe₃O₄ hollow nanospheres (h-Fe₃O₄@PANI) via a facile one-pot hydrothermal and subsequent in situ polymerization method. The as-prepared nanocomposite presents two structural superiorities, in which the hollow inner can reduce the internal stress during the electrochemical process, provide void spaces for active materials from the electrode during volume expansion and the PANI layer can effectively enhance the conductivity. In this way the h-Fe₃O₄@PANI nanocomposite exhibits excellent cycling performance, reversible capacity, and rate capability as compared with h-Fe₃O₄ and solid Fe₃O₄ (s-Fe₃O₄) nanospheres, making it promising for application in LIBs.

2. Experimental

2.1 Synthesis of h-Fe₃O₄ nanospheres and h-Fe₃O₄@PANI composites

All the chemicals were used as received without further purification. The h-Fe₃O₄ nanospheres were synthesized by a facile one-pot hydrothermal method. In a typical synthesis, iron(III) chloride hexahydrate (FeCl₃·6H₂O, 0.75 g), sodium citrate (Na₃C₆H₅O₇, 1.8 g), and urea (CO(NH₂)₂, 0.45 g) were dissolved in 50 mL Milli-Q water. Sodium polyacrylate ((C₃H₃NaO₂)_n, 0.45 g) was added into the solution under continuous stirring for 1.5 h and sealed in a 100 mL Teflon-lined stainless-steel autoclave at 200 °C for 5 h. The product was then collected by centrifugation and dried in a vacuum oven at 60 °C for 12 h. h-Fe₃O₄@PANI composites were synthesized by using in situ polymerization of the aniline monomer in the presence of h-Fe₃O₄ nanospheres. Typically, h-Fe₃O₄ nanospheres (0.8 g) were dispersed in ethanol (500 mL) by sonication. Subsequently, aniline (C₆H₅NH₂, 0.8 mL) was injected into the above suspension. Then HCl aqueous solution (25.0 mL, 1.21 M) containing ammonium peroxydisulfate ((NH₄)₂S₂O₈, APS) (2.48 g) was added into the ethanol suspension. For full polymerization of aniline, the mixture was mechanically stirred at 0 °C for 24 h. After washing with distilled water and ethanol several times, the product was dried under vacuum at 50 °C for 8 h.

2.2 Materials characterization

A powder X-ray diffractometer (XRD) with Cu Kα irradiation (λ = 1.5418 Å, Bruker Model D8) was used to detect the crystallographic information for the samples. The morphology and microstructure of the products were examined using field-emission scanning electron microscopy (FESEM; Hitachi-S5500, 5 kV; Zeiss Merlin, 5 kV) and transmission electron microscopy (TEM; JEOL, JEM-2100, 200 kV; FEI, Tecnai G² 20, 200 kV). X-ray photoelectron spectroscopy (XPS, Escalab 250, Al Kα) was employed to probe the surface chemical states. Thermal gravimetric analysis (TGA, Netzsch-STA 449C) was measured from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ under an air flow. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a PE (Spotlight 3000) spectrometer in the 400–4000 cm⁻¹ region to investigate the functional groups of the samples. The Brunauer–Emmett–Teller (BET) surface area of the products was analyzed by using nitrogen adsorption–desorption isotherms at 77 K in a Micromeritics ASAP 2010 system. The adsorption isotherm was used to determine the pore size distribution via the Barret–Joyner–Halender (BJH) method. The average pore size was determined based on the nitrogen adsorption volume at arelative pressure of 0.994.

2.3 Electrochemical measurements

The working electrodes with a typical thickness of 30–40 μm were prepared by mixing the active materials, conductive carbon black and carboxymethyl cellulose in a weight ratio of 8 : 1 : 1 in the N-methyl-2-pyrrolidone solvent, with the following addition of a suitable amount of deionized water. The obtained slurry was then coated on a Cu foil (current collector) and dried under vacuum at 80 °C overnight. The obtained electrodes were punched and dried again at 120 °C for 6 h. The loading mass of the active materials on the Cu foil was about 2.5–3 mg cm⁻² for each electrode. Testing cells (CR 2032 coin-type half cells) were assembled inside an Ar-filled glove box by using a lithium metal foil as the counter electrode and the reference electrode and microporous polypropylene as the separator. The electrolyte used was 1 M lithium hexafluorophosphate (LiPF₆) dissolved in a mixture of ethylene carbonate, propylene carbonate, and diethyl carbonate (3 : 1 : 1 in volume ratio). Cyclic voltammogram (CV) curves were recorded between 0.05 and 3 V (vs. Li⁺/Li) at a scan rate of 0.5 mV s⁻¹. Galvanostatic charge and discharge of the cells were performed at a current density of 100 mA g⁻¹ between a voltage window of 0.05 and 3 V (vs. Li⁺/Li). For the rate performance test, the discharge current gradually increased from 50 mA g⁻¹ to 100, 500, 1000, 2000, 4000, and 5000 mA g⁻¹, and then decreased to 50 mA g⁻¹. Electrochemical impedance spectroscopy was conducted at open circuit potential (OCP) by applying an alternating current voltage of 5 mV over a frequency range of 100 kHz to 0.01 Hz. All the electrochemical tests were performed symmetrically at room temperature.

3. Results and discussion

The XRD patterns of the as-prepared h-Fe₃O₄@PANI and h-Fe₃O₄ are shown in Fig. 1. All diffraction peaks of h-Fe₃O₄ and the h-Fe₃O₄@PANI composite are in good agreement with the standard pattern of magnetite Fe₃O₄ with a cubic inverse spinel structure (JCPDS card No. 19-0629). No other peaks from possible impurities are observed, indicating the high purity of the products. It is worth noting that no distinct diffraction peaks corresponding to PANI are detected which may due to its amorphous nature. The FTIR spectra (Fig. 2) show a similar trend for the h-Fe₃O₄@PANI composite and PANI. For PANI, the characteristic bands located at 1572 and 1477 cm⁻¹ are ascribed to the C=C stretching vibration of the quinoid and benzene rings, demonstrating the presence of a conducting state of PANI (emeraldine salt).²⁶ The bands located at 1300, 1123, and 832 cm⁻¹ correspond to the C–N stretching vibrations of the secondary aromatic amine, C=N stretching vibrations (–N=quinoid=N–), and out-of-plane bending vibrations of the C–H band in the aromatic ring, respectively.²⁷ In addition, the bands between 800 and 500 cm⁻¹ are attributed to the C–H vibration of benzene rings.²⁸ Remarkably, the FTIR spectrum of the h-Fe₃O₄@PANI composite shows combined characteristics of both PANI and h-Fe₃O₄.²⁹ The intense vibration band for PANI (∼1123 cm⁻¹) related to the C=N stretching in aromatic amines decreases. The possible reason is that the benzoid ring structure in the initial PANI polymer is transferred into semiquinoid or quinoid ones upon the compositing with h-Fe₃O₄ nanospheres.³⁰

Fig. 1 XRD patterns of the as-prepared h-Fe₃O₄, h-Fe₃O₄@PANI and the standard pattern of the Fe₃O₄ phase.

Fig. 2 FTIR spectra of PANI and h-Fe₃O₄@PANI samples.

The thermal behavior of the as-prepared h-Fe₃O₄@PANI composite and pure PANI is investigated by TGA and derivative thermogravimetric (DTG) curves (Fig. S1†). The small weight loss before 100 °C is attributed to the evaporation of moisture, gaseous content and trace ethanol in the sample. As the temperature rises, the rapid mass loss of about 40.5 wt% taking place in the range of 100–400 °C is ascribed to the combustion of PANI and oxidation of Fe₃O₄ to Fe₂O₃ (4Fe₃O₄ + O₂ = 6Fe₂O₃) in the air atmosphere.^31–33 According to the change in weight and the reaction equation, the estimated content of Fe₃O₄ and PANI in the composite is about 48.8 wt% and 51.2 wt%, respectively.

The low and high magnification FESEM images (Fig. 3a and b) of the as-prepared h-Fe₃O₄@PANI composite confirm the formation of a large number of nanospheres with a rough surface. From the typical low magnification TEM images (Fig. 3c–f), it can be found that the nanospheres show a hollow nature with an average diameter in the range of 200–300 nm. The core–shell structure is unambiguously resolved from the magnified TEM images (Fig. 3g and h). The PANI with a lower contrast shell is coated on to the h-Fe₃O₄ nanospheres. It is further showed that the nanospheres are assembled by primary particles with a size of 10–20 nm and there exists a porous gap between each particle. The selected area electron diffraction (SAED) pattern (inset in Fig. 3c) shows the polycrystalline nature of the h-Fe₃O₄@PANI.

Fig. 3 (a and b) FESEM images, (c–h) TEM images, (i and j) HRTEM images of the h-Fe₃O₄@PANI sample. The inset in (c) shows the SAED pattern of the h-Fe₃O₄@PANI sample. The white and blue arrows in (b) and (g) show the existence of PANI coatings.

Fig. 3i and j show HRTEM images of an individual nanosphere. The lattice spacings of d ∼ 2.55 Å, 2.93 Å and 4.87 Å correspond to the (311), (220) and (111) planes of Fe₃O₄. The PANI layer is amorphous because no lattice fringe is observed. This is in agreement with the XRD results. The high conductive PANI coated layer can be a key factor to enhance the electrochemical performance not only because the conductivity of the composites is improved but they can also play a role as a buffering material due to its flexibility.^18,19 Typical FESEM and TEM images of the h-Fe₃O₄ sample are shown in Fig. S2.† The EDS spectra (Fig. S3 and S4†) and the elemental mapping images (Fig. S5†) of the two samples further confirm the distribution of Fe, O, N and agree well with the TEM results.

The specific surface areas and the porous nature of the samples are determined by measuring nitrogen adsorption–desorption isotherms at 77 K (Fig. 4a and b). Both samples exhibit type II adsorption–desorption isotherms according to IUPAC classification, indicating that the samples are composed of nanoparticles.³⁴ It is also shown that only a large adsorption is observed at relative pressures close to 1. Therefore the adsorption mainly occurs in the interparticle voids with a mesopore range. The corresponding BET specific surface areas are ∼109.4 and 124.8 m² g⁻¹ for h-Fe₃O₄@PANI and h-Fe₃O₄, respectively. The adsorption isotherm was used to determine pore size distribution based on the BJH method (Fig. 4b). The pore size of the h-Fe₃O₄@PANI and h-Fe₃O₄ samples distribute in 5–30 nm, which reveals the existence of mesopores. The mesopores mainly originate from the aggregation of primary nanoparticles within h-Fe₃O₄@PANI and h-Fe₃O₄ nanospheres as shown by the TEM results. The surface chemical states of the h-Fe₃O₄@PANI composite and h-Fe₃O₄ are determined by XPS (Fig. 5a–d, S6–S8†). The survey spectrum of the h-Fe₃O₄@PANI composite (Fig. 5a) shows the presence of Fe, O, N, and C peaks, which is in agreement with previous EDS results.

Fig. 4 (a) Nitrogen adsorption–desorption isotherms and ((b) & inset) the corresponding pore size distribution curves of h-Fe₃O₄ (black) and h-Fe₃O₄@PANI (blue) samples.

Fig. 5 (a) XPS survey spectra of the h-Fe₃O₄@PANI sample, (b–d) high-resolution XPS spectra of the Fe 2p, O 1s, and N 1s regions, respectively.

The de-convoluted scan of Fe 2p (Fig. 5b) shows two distinct binding energy peaks located at 711.64 and 724.72 eV corresponding to the electronic states of Fe 2p3/2 and Fe 2p1/2 respectively, which can be fitted with two spin–orbit doublets and a shakeup satellite. The doubles are characteristic of the peaks of Fe²⁺ and Fe³⁺. The results are consistent with the reported values of Fe₃O₄.^35–37 The Fe³⁺/Fe²⁺ ratio calculated from the peak area is 2.04, which is close to 2 : 1 in Fe₃O₄. Moreover, the satellite peak situated at about 719.4 eV is a characteristic peak of Fe³⁺ in γ-Fe₂O₃, suggesting that the surface of h-Fe₃O₄ is partly oxidized.^35,36 The O 1s peak (Fig. 5c) is deconvoluted into three components with peaks at 530.38, 531.66 and 533.12 eV, which can be assigned to lattice oxygen, oxygen vacancies or defects related OH-group, and molecular water in the sample or adsorbed on the surface. The present results are in good agreement with those of the previous report.³⁸ The de-convolution of the N 1s peak (Fig. 5d) shows two distinct curves, related to different nitrogen forms. The peak at 399.2 eV corresponds to pyridine-N (=N–),³⁹ and the peak at 400.1 eV may be assigned to pyrrolic-N (–NH–).⁴⁰ The results are also consistent with the data as demonstrated in the literature.^41,42

The electrochemical properties of the assembled cell are investigated by CV between 0.05 and 3 V at a scan rate of 0.5 mV s⁻¹. The CV curves of the first three cycles for the h-Fe₃O₄@PANI cell are shown in Fig. S9.† In the first cycle, there is a large reduction peak at 0.47 V, which can be ascribed to the irreversible conversion from Fe₃O₄ to Fe/Li₂O upon Li uptake.^43,44 Moreover, the anodic peak at 1.72 V is attributed to the oxidation of the metallic iron into Fe₃O₄.^43,44 In the subsequent cycles, both the cathodic and anodic peaks are shifted towards a higher potential due to the irreversible change of Fe₃O₄ from the spinel to the rock salt type structure during the initial insertion of lithium ions.⁴⁵ In addition, the current of the cathodic and anodic peaks decreased obviously, which is due to the formation of a solid electrolyte interphase (SEI) film.⁴⁵ In the CV graph of the h-Fe₃O₄ cell (Fig. S10†), a pair of cathodic and anodic peaks appears at 0.63 and 1.68 V in the first scan. In the following second and third cycles, the cathodic peak shifts to 0.75 V, while the anodic peak position does not show any obvious change. The galvanostatic charge and discharge measurements of the assembled cells are performed at a rate of 100 mA g⁻¹ in the voltage range of 0.05–3 V (versus Li/Li⁺) at room temperature. Fig. 6a shows the charge–discharge voltage profiles of the h-Fe₃O₄@PANI cell for the first three cycles. The first discharge profile exhibits three plateaus (∼1.44 V, 0.91 V and 0.77 V) and a following slope. The three plateaus represent a three-step lithium ion insertion process. The slope is attributed to the formation of an SEI film and lithium ion insertion into the polymer material. The characters of multiple discharge plateaus have also been observed in the Fe₃O₄/graphene sheet composite and spinel transition metal oxide anodes.^37,46 The plateau around 0.77 V confirms the existence of a conversion process for the lithiation of the h-Fe₃O₄@PANI electrode. In the charging state, the reactions occur in the reverse direction and Li⁺ is released at a voltage of ∼1.5 V. The initial discharge and charge capacities are 1800 and 1350 mA h g⁻¹ for h-Fe₃O₄@PANI electrodes respectively. The values are found to be much higher than those of h-Fe₃O₄ nanospheres and the results recently reported by other groups, such as 1300–990 mA h g⁻¹ for hierarchical Fe₃O₄@PPy nanocages,⁴⁷ 972–880 mA h g⁻¹ for hollow Fe₃O₄ microspheres^48–50 and 1550–1080 mA h g⁻¹ for other nanostructured iron oxide anodes.^51,52 The irreversible capacity loss arising during the first cycle is due to the incomplete conversion reaction and the formation of the SEI layer. The charge–discharge voltage profiles of the h-Fe₃O₄ cell for the first three cycles are also shown in Fig. S11.† In the first discharge curve, there is one clear potential plateau at ∼0.69 V due to the conversion from Fe₃O₄ to Fe, followed by a sloping curve down to the cutoff voltage, which is in agreement with that of the reported results for Fe₃O₄ anodes.^53,54Fig. 6b shows the comparison of cycling performance of the h-Fe₃O₄@PANI, h-Fe₃O₄, and s-Fe₃O₄ cells at a current density of 100 mA g⁻¹ in a voltage range of 0.05–3 V (versus Li⁺/Li) up to 50 cycles. It can be seen that the h-Fe₃O₄@PANI cell exhibits much better cycling performance. It is also found that the h-Fe₃O₄@PANI cell possesses the highest lithium storage capacity, and the reversible capacity reaches ∼1090 mA h g⁻¹ after 50 cycles, which decreases to ∼956 mA h g⁻¹ for the h-Fe₃O₄ cell and 335 mA h g⁻¹ for the s-Fe₃O₄ cell. It is noticed that the reversible capacity of the h-Fe₃O₄@PANI and h-Fe₃O₄ cells is higher than the theoretical value of Fe₃O₄. We first evaluated the contribution of PANI to the capacity of h-Fe₃O₄@PANI and tested the capacity of pristine PANI at the same current density. The result shows that the capacity is only 38 mA h g⁻¹ for PANI, which is obviously lower than the theoretical capacity of Fe₃O₄ and comparable with the values reported elsewhere.^20,55–58 Therefore the effect of PANI addition on the overall specific capacity can be considered as negligible, and the additional reversible capacity beyond the theoretical capacity originates from the Fe₃O₄ electrode. Actually, this phenomenon is generally observed in other conversion based electrodes.⁵⁹ There are several different mechanisms that have been proposed to reveal the possible origin of the extra capacity. For example, Laruelle et al. showed that the polymeric gel-like film formed at the grain boundaries between nanoparticles at a low potential range enables extra lithium storage on its surface through a capacitive way.⁶⁰ Maier et al. proposed that the interfaces between the dispersively distributed metal nanoparticles and lithium binary compound matrix allow for the storage of lithium ions on the lithium compound side, and electrons are localized on the metallic side.^61,62 Hu and co-workers demonstrated that the contribution to the extra capacity comes from the reversible reaction of LiOH and Li to form Li₂O and LiH.⁶³ Apart from those, the reactions of the Li₂O phase⁶⁴ and the further evolution of the SEI film⁶⁵ are also found to be related with the additional capacities. In our case, the extra capacity was maintained with the cell cycling, which is in agreement with Laruelle's work. Considering the similar reaction processes of Fe₃O₄ and CoO⁶⁰ electrodes, we think that the formation of a polymeric gel-like film is responsible for the extra capacity. Moreover, the contributions from other reversible side reactions cannot be neglected.^63,66 It should be noted that a full understanding of the extra capacity relies on the experimental ability to in situ identify nanoscale structures present at the electrode/salt interfaces under real working conditions.

Fig. 6 Electrochemical performance of the h-Fe₃O₄@PANI sample: (a) galvanostatic charge/discharge curves of h-Fe₃O₄@PANI for the first three cycles between 0.05 V and 3.00 V (vs. Li⁺/Li) at a current density of 100 mA g⁻¹, (b) cycling performance and (c) rate capability of the h-Fe₃O₄@PANI (blue lines), h-Fe₃O₄ (black lines) and s-Fe₃O₄ (red lines) electrodes at different current densities between 50 and 5000 mA g⁻¹.

Compared to h-Fe₃O₄ and s-Fe₃O₄ cells, the improved capacity and cycle life of the h-Fe₃O₄@PANI cell may be considered due to the large specific surface area and porous nature of nanospheres which is more convenient and accessible for electrolyte diffusion and intercalation of lithium ions into the active phases. The presence of the PANI layers also reduces the exposed surface of active material to the electrolyte and results in a more conductive and stable SEI film, which is favorable to the reversible capacity improvement.¹ The subsequent coulombic efficiency quickly increases to 98%, 96% and 90% for h-Fe₃O₄@PANI, h-Fe₃O₄ and s-Fe₃O₄ electrodes, respectively. The electrochemical performance of the electrodes at different current densities between 50 and 5000 mA g⁻¹ is shown in Fig. 6c. The charge/discharge rates are programmable modified from 50 mA g⁻¹ to 100, 500, 1000, 2000, 4000, 5000 and then back to 50 mA g⁻¹ for 10 cycles. The reversible capacity of h-Fe₃O₄@PANI cell varies from 1220.7 mA h g⁻¹ to 714.6 mA h g⁻¹ at current densities of 50 mA g⁻¹ and 1000 mA g⁻¹, respectively, while the reversible capacity of the h-Fe₃O₄ cell drops from 1139.4 mA h g⁻¹ to 670.4 mA h g⁻¹, which are lower than but still close to those of the h-Fe₃O₄@PANI cell. However, at higher current densities of 2000, 4000, and 5000 mA g⁻¹, the h-Fe₃O₄@PANI cell exhibits remarkably higher capability than that of the h-Fe₃O₄ cell. The reversible capacities of the h-Fe₃O₄@PANI cell are 642 and 549.5 mA h g⁻¹ at the current densities of 2000 and 4000 mA g⁻¹. Even at a high rate of 5000 mA g⁻¹, the h-Fe₃O₄@PANI cell still shows a capacity as high as 439 mA h g⁻¹. In contrast, the capacity retention of the h-Fe₃O₄ cell at high current densities (2000 to 5000 mA g⁻¹) is inferior. For example, the capacity is 408, 169.2, and 32 mA h g⁻¹ at the current densities of 2000, 4000, and 5000 mA g⁻¹, respectively. When the rate returns to the initial 50 mA g⁻¹ after 80 cycles, the h-Fe₃O₄@PANI and h-Fe₃O₄ cells can go back to their original capacities to some extent as illustrated in Fig. 6c. For the s-Fe₃O₄ cell, the rate performance is inferior to the h-Fe₃O₄@PANI and h-Fe₃O₄ cells in the whole current density range. The results show the positive influence of PANI addition and the importance of hollowed nanostructures inrate performance enhancement, especially at higher current densities. Specifically, the PANI layer offers a rapid conductive network connecting the h-Fe₃O₄ spheres, while the porous structure can reduce the effective diffusion distance for lithium ions and electrons, thus leading to good rate capability. The reversible capacity and rate capability of the present h-Fe₃O₄@PANI composite anodes is found to be much higher than the values recently reported by other groups as shown in Table S1.†

EIS is carried out at OCP to determine the lithium ion transfer behavior in the cells after the 1^st and 50^th cycles. As shown in Fig. 7, all the Nyquist plots of the h-Fe₃O₄@PANI, h-Fe₃O₄ and s-Fe₃O₄ cells exhibit a single semicircle in the high to medium frequency region and an inclined line in the low frequency region, which are attributed to the charge-transfer process and the lithium ion diffusion process inside the electrode material.^{15–17,51,52} The semicircle diameter of the h-Fe₃O₄@PANI cell is found to be much smaller than those of the h-Fe₃O₄ and s-Fe₃O₄ cells, indicating that the addition of PANI enhances the charge transfer process, which is beneficial for improving rate capability. This is in good agreement with the existing reports that PANI can greatly reduce electron transfer resistance in the composite.⁵⁸ This will lead to high utilization of the cell even under high rate discharge conditions.⁶⁷ After the 50^th cycle, the charge-transfer resistances increase, which are attributed to the formation of SEI films,⁶⁸ inducing the lower conductivity of the electrodes. The morphology and structure of the h-Fe₃O₄@PANI electrodes after the rate capability testing (50 cycles) were also characterized by TEM observations (Fig. S12†). The sample still maintains the initial morphology and structure after the cycling test, demonstrating the good stability of the nanocomposite structures during charge/discharge cycling.

Fig. 7 Nyquist plots of the h-Fe₃O₄@PANI (blue), h-Fe₃O₄ (black) and s-Fe₃O₄ (red) electrodes after the first (solid) and the 50^th (hollow) cycles at the current density of 100 mA g⁻¹ with an amplitude of 5 mV over the frequency range of 100 kHz and 0.01 Hz.

The good lithium storage properties of h-Fe₃O₄@PANI nanocomposite can be attributed to the following reasons. (1) The hollow nature of the Fe₃O₄ nanospheres provides extra active sites for the storage of lithium ions, which is beneficial for enhancing the specific capacity. (2) The addition of PANI can accommodate the local volume change upon charge/discharge cycling and is able to alleviate the problem of pulverization and aggregation of the electrode material, hence improving the cycling performance. (3) The PANI has a good electrical conductivity and serves as the conductive channels between h-Fe₃O₄, which decrease the inner resistance of LIBs. Moreover, the porous structure can reduce the effective diffusion distance for lithium ions and electrons, also resulting in good rate capabilities.

4. Conclusions

In summary, h-Fe₃O₄@PANI nanocomposites have been successfully synthesized. When used as the anode materials for LIBs, the as-prepared h-Fe₃O₄@PANI nanocomposites delivered high capacity, good cycling stability and high rate capability as compared to h-Fe₃O₄ and s-Fe₃O₄ nanospheres. The good electrochemical performance can be attributed to the hollow nature of the products, the addition of PANI, and the unique assembled architecture. We believe that the lithium storage properties of the h-Fe₃O₄@PANI nanocomposites can be further optimized by tailoring the inner space and PANI thickness. The excellent electrochemical performance combined with the low-cost Fe element endows h-Fe₃O₄@PANI nanocomposites with high potential in next generation LIBs.

Acknowledgements

The authors would like to appreciate the financial support from the Chinese National Natural Science Foundation (51401114 and 51571054). This work made use of the resources of the National Center for Electron Microscopy in Beijing.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: TGA and DTG plots, additional SEM and TEM images, XPS spectra, electrochemical results, and Table. See DOI: 10.1039/c7se00139h

‡ These authors contributed equally to this work.

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