A particle–carbon matrix architecture for long-term cycle stability of ZnFe₂O₄ anode

Abstract

ZnFe₂O₄/C with a unique compound structure was in situ synthesized through a facile one-step route using glycine as complexing agent and carbon source. ZnFe₂O₄ nanoparticles are embedded in a carbon matrix to form submicron particles. The carbon matrix in the composite material is divided into surface carbon and inner carbon, not only facilitates the electronic conduction, but also inhibits the aggregation of ZnFe₂O₄ nanoparticles, which largely accommodates the mechanical stresses caused by the volume change of ZnFe₂O₄ during charge/discharge process. ZnFe₂O₄/C could maintain their integrity and provide excellent properties. The obtained ZnFe₂O₄/C has a specific capacity of 2055 mA h g⁻¹ at 1000 mA g⁻¹ and a capacity retention of 54% with the current density increasing from 100 to 5000 mA g⁻¹. The excellent performance is derived from the unique compound structure and this facile fabrication method also has good prospects in synthesizing other materials.

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1. Introduction

Nowadays, lithium ion batteries (LIBs) are considered as a strong driving force for the development of portable electronic devices.^1–3 LIBs are also essential to future advanced electric vehicles (EVs) over traditional rechargeable battery systems.⁴ However, the development of LIBs essentially relies on new electrode materials with significantly higher properties than the traditional ones.⁵

Ternary Fe-based oxides (MFe₂O₄, M = Mg, Zn, Mn, Co and Ni) are seemed as important alternative candidates to replace graphite (372 mA h g⁻¹ in theory), owing to their high theoretical specific capacity (about 1000 mA h g⁻¹), nature abundant, nontoxic and low price.⁶ In MFe₂O₄, two active components Fe and M oxides have different lithiation potentials, which lead to the volume change taking place in a stepwise manner and thus this material shows more stable cycling performance.⁷ In spite of the remarkable lithium-storage capabilities, the development of MFe₂O₄ is usually hampered by fast capacity fading and poor rate capability, caused by the inherent poor electronic conduction, server agglomeration and large volume change during charge/discharge process.⁸ In related studies, progresses have been achieved through the design of electrode material to form nanocomposites or introduce conductive agents to solve those issues.^9,10 For instance, ZnO/ZnFe₂O₄/C octahedra with hollow interiors maintains at 837 mA h g⁻¹ at 1000 mA g⁻¹ and mesoporous ZnFe₂O₄/C composite microspheres exhibiting initial discharge capacities of 1551 mA h g⁻¹.^11,12 However, these fabrication processes are always complex and low yield, sometimes even require chemical hazardous operations. Meanwhile, nanomaterials with too large specific surface area increase the interface reaction of anode and electrolyte, leading to the dissolution of electrode materials and the decomposition of electrolyte during cycling.¹³ Moreover, it is more difficult to prepare electrode using nanomaterials than micromaterials.

In order to solve the above problems, we design a submicron architecture of ZnFe₂O₄/C anode material, in which the ZnFe₂O₄ nanoparticles are embedded in a carbon matrix. Such ZnFe₂O₄/C anode material could retain a reversible capacity of 2055 mA h g⁻¹ after 200 cycles at 1000 mA g⁻¹, as well as a superior performance at high current densities (882 mA h g⁻¹ capacity retention after 1000 cycles at 5000 mA g⁻¹). The excellent performance owes to the unique compound structure. This composite architecture not only improves the conductivity of the materials, but also increases the interface between ZnFe₂O₄ and carbon. Meanwhile the carbon matrix largely accommodates internal mechanical stresses and inhibits the aggregation of ZnFe₂O₄ nanoparticles. The proposed composite architecture and sample synthesis route are effectively applied to build a variety of other materials, such as CoFe₂O₄/C.

2. Experimental section

2.1. Materials synthesis

Zn(NO₃)₂·6H₂O (0.005 mol), Fe(NO₃)₃·9H₂O (0.01 mol) and glycine (3 g used as ligand) were dissolved in 40 mL distilled water and then stirred for 1 h at 80 °C to form sol–gel. After that, the sol–gel was calcined at 750 °C in N₂ for 2 h, which was denoted as ZFC7. In contrast, pure ZnFe₂O₄ was made by heating the sol–gel in air, which was named as ZFO. And the preparation processes of them are schematically illustrated in Fig. 1. During the pyrolysis process in N₂, the glycine transforms to carbon and the metal ions in situ transform into oxide particles, generating the final composite material.

Fig. 1 Schematic illustration of the fabrication of ZFC7 and ZFO.

2.2. Materials characterization

X-ray diffraction (XRD) was carried out to identify the phase composition of the synthesized samples by Bruker AXS D8 using Cu Kα radiation (λ = 0.1541 nm). X-ray photoelectron spectroscopy analysis (XPS) was determined by a Kratos Axis Ultra spectrometer with a monochromatic Al Kα radiation (hν = 1486.6 eV). Thermal gravimetric analysis (TGA) was used to measure the content of carbon on a NETZSCH thermal analyzer (STA 449 F3) with a heating rate of 10 °C min⁻¹ in flowing air atmosphere. Fourier transform infrared spectrometer (FTIR) spectra of the samples were collected by Themo Nicolet 670FT-IR from 400 to 4000 cm⁻¹. A JSM-6700F scanning electron microscope (FESEM) was performed to observe the morphologies of the synthesized samples. The elemental composition was examined by energy dispersive spectroscopy (EDS). Transmission electron microscope (TEM, JEOL JEM-2010) was used to characterize the microstructures of samples at the active voltage of 200 kV.

2.3. Electrochemical measurements

For electrochemical evaluation, the test electrodes were composed of active powder materials (ZnFe₂O₄/C and ZnFe₂O₄, 60 wt%), conductive carbon black (30 wt%) and sodium alginate (SA, 10 wt%) based on the whole electrode. The mixtures were dispersed in deionized water and ball milled for 12 h to form a slurry coating on a copper foil substrate. After drying in vacuum for 24 h, the electrode was cut into a series of discs with the diameter of 14 mm and pressed at 6 MPa. The active material mass loading of the electrode was about 1.6 mg cm⁻². The Celgard 2400 and Li foils were used as the separator and counter electrode, separately. 1 mol L⁻¹ LiPF₆ dissolved in a mixture solution (EC : DMC = 1 : 1 v/v) was employed as the electrolyte. CR2032 type coin cells were fabricated in an Ar-filled glove-box and cycled galvanostatically at different currents between 0.005 and 3.0 V (vs. Li/Li⁺) on a LAND Cell test system (CT2001A, Wuhan, China). The specific capacities of ZnFe₂O₄/C were calculated by the total weights of the composites including carbon. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) of the test cells were measured by an electrochemical workstation (CHI660B, Shanghai, China). The data of EIS were collected with the frequency ranging from 0.1 Hz to 100 kHz.

3. Results and discussion

XRD patterns of ZFO and ZFC7 is listed in Fig. 2a. The crystal structures of ZFO and ZFC7 are in accordance with the cubic ZnFe₂O₄ phase (JCPDS Card No. 22-1012). There are also some peaks of ZnO phase (JCPDS Card No. 21-1486) in ZFC7. Compared with ZFO, the particles of ZFC7 coated with homogeneous carbon layer are smaller, as shown in Fig. 2b and c. In addition, nitrogen adsorption/desorption isotherms and pore size distribution plots of ZFC7 and ZFO are proved in Fig. S1.† Both of them with little pore reduce the dissolution of electrode materials and the decomposition of electrolyte during cycling. The EDS results of ZFC7 in Fig. 2d demonstrate that the atom ratio of Zn, Fe and O approximates the amount of the stoichiometric ratio of ZnFe₂O₄ and the weight fraction (wt%) of carbon is about 17.1%. ZFC7 was subjected for characterization the nature of carbon coating by the Raman spectroscopy. In Fig. S2,† two peaks at wave numbers of 1334 and 1589 cm⁻¹ are appeared, which correspond to the D band (disordered graphitic structure) and G band (graphitic structure) of carbon. The ratio of I_D/I_G for ZFC7 is 1.44, which suggests that the carbon is mainly in amorphous state.

Fig. 2 XRD patterns of ZFO and ZFC7 (a), FESEM images of ZFO (b) and ZFC7 (c), EDS mapping images of ZFC7 (d).

The result of TG shows 18.5 wt% content of carbon (Fig. S3†). The first weight loss of ZFC7 is about 6.5 wt% below 150 °C, which is recorded for the evaporation of absorbed water. The following two obvious weight loss in the TG curve of ZFC7 from 150 to 600 °C belongs to the combustion reaction of carbon, which is different with the one step weight loss of common carbon. Here, the carbon matrix in the composite material is divided into surface carbon and inner carbon. The unique structure of ZFC7 is schematically illustrated in Fig. 1. The weight loss corresponding to the surface carbon is about 8.5 wt% in the range of 150 and 380 °C. In order to survey the inner carbon, ZFC7 was treated in air from room temperature to 400 °C, which was denominated as ZFC7-4. From the FESEM images of ZFC7-4 in Fig. S4b† and 4d, the exposed particles and carbon between particles are discovered (the surface carbon has been combusted). With the further increase of temperature, another notable weight loss about 10 wt% is in consistent with the EDS result of ZFC7-4 (Fig. S4b†), which is contributed by inner carbon. This unique composite architecture is the key reason to the high performance of ZFC7. The surface carbon protects the active materials from the etching of electrolyte, and ensures the integrality of the particles during the charge/discharge process. The inner carbon connects the nanosized ZnFe₂O₄ particles, which increases the conductivity of the materials and inhibits the electrochemical aggregation of ZnFe₂O₄ nanoparticles during long-term cycling.

XPS measurements are employed to further confirm the surface elemental composition and metal oxidation states of ZFC7 composites. The Fe 2p_3/2 and Fe 2p_1/2 peaks at 710.7 and 724.6 eV indicate the existence of Fe³⁺ in the composites.¹⁴ The peaks centering at 1021.4 eV and 1044.4 eV are ascribed to Zn 2p_3/2 and Zn 2p_3/2, which confirm the oxidation state of Zn is Zn²⁺.¹⁵ O 1s core level spectrums are fitted into three distinct peaks locating at 529.8 eV, 531.0 eV, and 532.6 eV, respectively. The strong peak at 529.8 eV is in accordance with the oxygen atoms in the oxide of ZnFe₂O₄. Other peaks at higher energy are related to chemisorbed, dissociated oxygen or OH species on the surface of ZnFe₂O₄.¹⁶ The peak associating with C–C bond (284.4 eV) is intensive, and C–O at 285.4 eV and C=O at 288.7 eV are also found (Fig. 3d).^17,18 FTIR is utilized to verify the difference of chemical bond between ZFC7 and ZFO (Fig. 3e). A broad absorption peak at ∼3500 cm⁻¹ belongs to the asymmetric and symmetric stretching vibrations of the –OH group in absorbing water molecules. Other absorption peaks of ZFO and ZFC7 are assigned to metal–O bonding.¹⁹ The difference of metal–O bond between ZFC7 and ZFO is caused by the carbon, as it influences their bond energy slightly.²⁰

Fig. 3 High-resolution XPS spectra of Fe 2p (a), Zn 2p (b), O 1s (c) and C 1s (d) for ZFC7, FTIR spectra of ZFC7 and ZFO (e).

The morphologies of ZFC7, etched ZFC7 and ZFO were characterized by TEM and HR-TEM tests, and the results are illustrated in Fig. 4a–f. The low magnification TEM image of ZFC7 (Fig. 4a) shows the submicron particles are coated by carbon, which clearly encapsulates lots of small grains. Particularly, typical inner carbon between the particles is presented in Fig. 4b. And the surface carbon on the particles is exhibited in Fig. 4c. The TEM images of etched ZFC7 (immerging in concentrated nitric acid for 2 h) are employed to survey the combination of carbon and ZnFe₂O₄. From Fig. 4d, many holes in the carbon are observed. And it is worth to mention that there are still many ZnFe₂O₄ nanoparticles “dissolving” in the carbon matrix (Fig. 4f). The microstructures of ZFO are different from that of ZFC7, as shown in Fig. 4g–i. There are clear edges of single phase in ZFO.

Fig. 4 TEM and HRTEM images of ZFC7 (a–c), etched ZFC7 (d–f) and ZFO (g–i).

Charge/discharge cycling performances of ZFC7 and ZFO were tested under the same condition (Fig. 5). The discharge capacity of ZFO at 1000 mA g⁻¹ is 1604 mA h g⁻¹, which is 78% of ZFC7 (2055 mA h g⁻¹ over 200 times at 1000 mA g⁻¹). In order to compare the electrochemical performance of ZFC7 and ZFO further, the rate behaviors of them are presented in Fig. 5b. ZFC7 displays a satisfied performance at high rate. The microstructures of ZFC7 and ZFO after 200 charge/discharge cycles are shown in Fig. 5c–f. The particle of ZFC7 is still integrity, while many small particles break away from ZFO particle. More different from ZFC7, complex crystalline grains emerge on the edge of the ZFO particle, which is various from the original one in Fig. 4i. The change of ZFO particle may lead to the poor electrochemical performance.

Fig. 5 Cycling performance at 1000 mA g⁻¹ (a) and rate performance of ZFC7 and ZFO (b), TEM and HRTEM images of ZFC7 (c and d) and ZFO (e and f) after 200 cycles at 1000 mA g⁻¹.

In order to probe the electrochemical performance of these samples, the long cycle performances were performed at a rigorously large rate of 5000 mA g⁻¹. From Fig. 6, ZFC7 presents amazing discharge capacities of 882 mA h g⁻¹ after 1000 cycles. The test results strongly confirm its excellent electrochemical stability at high current. On the contrary, the capacities of ZFO fade quickly.

Fig. 6 Cycling performances of ZFC7 and ZFO at 5000 mA g⁻¹.

The high electrochemical performance of ZFC7 is attributed to the designed unique architecture. Firstly, the inner and surface carbon afford excellent protection around ZnFe₂O₄ and thus act as the buffer that improves the mechanical property of the electrode. Secondly, the inner carbon inhibits the electrochemical aggregation of ZnFe₂O₄ nanoparticles effectively. Thirdly, the external highly elastic and stable carbon networks improve the electrical conductivity of overall electrode and Li⁺ could be stored in accessible interstitial sites between the oxide and carbon.²¹ In addition, SA plays an indispensable role in demonstrating the materials' electrochemical performance, which is superior than using PVDF binder (Fig. S5†). The existence of the functional groups, including carboxyl and hydroxyl in SA, could promote the chemical bonding among the active materials, resulting in the stronger binding ability.²²

EIS was performed on the ZFC7 and ZFO cells after 0, 200, 500 and 1000 cycles at 5000 mA g⁻¹. The shape of EIS is semicircle in the high medium frequency region and an inclined line in the low frequency region.²³ Typical Nyquist plot is shown in Fig. 7a. The semicircle is related to charge-transfer resistances of lithium ions at the interface among the electrode and electrolyte (R_p), the electronic resistivity of the active material and ionic conductivity in the electrode (R_b). The straight line is assigned to the Warburg impedance (W_o) during the lithium diffusion process, and R_s represents the internal resistance of the test battery.²⁴ The radius of semicircle and slope barely change with the increase of cycle number, due to its excellent electrochemical stability (Fig. 7a). Compared with ZFC7, the radius of semicircle of the ZFO is larger and the line is far from Y axis at the same cycles (Fig. 7b). These evidences prove that carbon can encapsulate active materials and permute fast transportation of Li⁺ and electron during continuous charge/discharge processes.²⁵

Fig. 7 Nyquist plots of ZFC7 (a) and ZFO (b) electrodes after 0, 200, 500 and 1000 cycles, the inset in (a) is the corresponding equivalent circuit.

CV curves of ZFC7 and ZFO electrodes are shown in Fig. 8a and b. The first curve is obviously different from the following ones. Upon the initial cathodic sweep of ZFC7, three small reduction peaks could be observed at about 1.7 V, 1.4 V and 1.0 V. On the base of Bresser et al.' analyses, ZnFe₂O₄ would be initially lithiated to Li_xZnFe₂O₄ (Li_0.4ZnFe₂O₄, Li_0.9ZnFe₂O₄ and Li_1.45ZnFe₂O₄). When 1.45 ≤ x ≤ 2, the partially lithiated Li_1.45ZnFe₂O₄ is decomposed to Li₂O and a new rock-salt phase metal oxide, in which all the iron is reduced to Fe²⁺. Zn_xFe_yO, having an x : y ratio of 1 : 2 (and x + y = 1) is proposed. Followed these small peaks, the main sharp reduction peak at around 0.7 V is found, which corresponds to the complete decomposition of ZnFe₂O₄ to LiZn, Fe⁰, Li₂O and formation a solid electrolyte interphase.²⁶ The values of the above agree well with the electrochemical lithiation of ZnFe₂O₄ is proposed by Bresser et al.²⁷ There is a pronounced peak near the main peak at 0.5 V which is belonged to ZnO.²⁸ Meanwhile, on the first anodic potential sweep, one broad peak around 1.6–1.7 V involving the oxidation of Zn⁰ and Fe⁰ is observed.²⁹ During the second cycle, the values of those peaks shift a little. The voltammogram does not change significantly upon further sweeps, which confirms the highly reversible uptake and release of lithium once the initial structural changes are completed. Remarkably, the initial reduction peaks of ZFO are less complex than that of ZFC7 (Fig. 8b), due to the lithiation extent (steps A to C) is slight, which leads the low capacity of ZFO.²⁷ And the values of the related peaks shift a little, suggesting a lager polarization and a poor electrical conductivity of ZFO. In addition, the larger area in Fig. 8a shows the higher specific capacity of ZFC7.

Fig. 8 CV spectra of ZFC7 (a) and ZFO (b) for the first five cycles, charge/discharge voltage profiles of ZFC7 (c) and ZFO (d) for the 1st, 2nd, 3rd, 100th and 200th at 1000 mA g⁻¹.

According to previous literatures and the above analyses,^30–32 the involved electrochemical reactions in the discharge and charge processes are listed as follows:

Discharge process:

(A) ZnFe₂O₄ + 0.4Li⁺ + 0.4e⁻ → Li_0.4ZnFe₂O₄

(B) Li_0.4ZnFe₂O₄ + 0.5Li⁺ + 0.5e⁻ → Li_0.9ZnFe₂O₄

(C) Li_0.9ZnFe₂O₄ + 0.55Li⁺ + 0.55e⁻ → Li_1.45ZnFe₂O₄

Li_1.45ZnFe₂O₄ + 0.55Li⁺ + 0.55e⁻ → Zn_xFe_yO₃ + Li₂O (x : y = 1 : 2)

(D) Zn_xFe_yO₃ + 6Li⁺ + 6e⁻ → 3Li₂O + xFe + yZn

(E) Zn + Li⁺ + e⁻ → LiZn (alloy)

Charge process:

(F) 3Li₂O + 2Fe → 6Li⁺ + Fe₂O₃ + 6e⁻

(G) Li₂O + Zn → ZnO + 2Li⁺ + 2e⁻

(H) LiZn (alloy) → Li⁺ + Zn + e⁻

The first, second, third, one hundredth and two hundredth charge and discharge curves of ZFC7 and ZFO were obtained at 1000 mA g⁻¹ (Fig. 8c and d). The initial charge/discharge capacity of ZFC7 are 1331 and 1744 mA h g⁻¹, much higher than that of ZFO. The capacity performance of ZFC is nearly the same or even higher than the theoretical capacity of ZnFe₂O₄, suggesting that the carbon phase significantly affects the overall electrochemical performance of the composite. Li⁺ could be stored in accessible interstitial sites between the oxide and carbon.³³ From Fig. 8c and d, the slope of ZFC7 is longer than that of ZFO, which is related to the interfacial storage mechanism.³⁴

The XRD patterns of the CoFe₂O₄ using Co instead of Zn are in accordance with CoFe₂O₄ (JCPDS Card, No. 03-0864) calcinated at 750 °C (Fig. 9a), which is different from that of ZFC7, because of the higher decomposition temperature. The obtained CoFe₂O₄/C composite presents good capacity, high coulombic efficiency and superior high-rate capability. CoFe₂O₄/C delivers the first charge and discharge capacities of 1822 and 1330 mA h g⁻¹ (Fig. 9b) with an initial coulombic efficiency of 73%. The reversible capacity of CoFe₂O₄/C retains approximately 1639 mA h g⁻¹ at the current density of 1000 mA g⁻¹ even after 150 cycles (about 99% of the capacity retention). The electrochemical performance of ZFC7 is superior to that of CoFe₂O₄/CNT nanocomposites (1045 mA h g⁻¹ after 100 cycles at the current density 200 mA g⁻¹) and Co₃O₄/CoFe₂O₄ nanocomposite (initial specific capacity of 1353.9 mA h g⁻¹).^35,36 The CoFe₂O₄/C electrode has the capacities of 1587, 1350, 1160, 1097, 1019 and 897 mA h g⁻¹ at the current densities of 50, 100, 200, 500, 1000 and 5000 mA g⁻¹, respectively. During the low current density area, the capacity returns to the corresponding initial value. The result indicates that this method is an effective, simple and general way to prepare high performance MFe₂O₄/C.

Fig. 9 X-ray diffraction patterns of CoFe₂O₄/C (a), plot of capacity and Coulomb efficiency vs. cycle number for CoFe₂O₄/C (0.005–3.0 V, 1000 mA g⁻¹) (b), the 1st, 2nd, 3rd, 100th, and 150th charge/discharge voltage profiles of CoFe₂O₄/C at the current density of 1000 mA g⁻¹ (c), rate performance of CoFe₂O₄/C (d).

4. Conclusions

The unique composite architecture is effective to obtain the higher capacity and long-term cycle stability of ZnFe₂O₄/C, due to the synergistic effect by introducing nanoparticles into carbon matrix. Such a unique geometric confinement of electrochemically active materials within a carbon matrix could effectively enhance the electrode stability and alleviate the volume changes during the Li insertion/extraction process. ZFC7 could retain a reversible capacity of 2055 mA h g⁻¹ after 200 cycles at 1000 mA g⁻¹, as well as a superior performance at high current densities (882 mA h g⁻¹ capacity retention after 1000 cycles at 5000 mA g⁻¹). ZnFe₂O₄ particle–carbon matrix architecture anode materials were synthesized through a simple method, which might open a novel avenue for fast synthesis of materials embedded in carbon network. In this generic strategy, CoFe₂O₄/C with good electrochemical performance was also synthesized.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (No. U1504211, No. 21501049, No. 21471049), the Key Project of Science and Technology department of Henan Province (No. 142102210449, No. 142102210452), the Key Project of Science and Technology of Henan Educational Committee, China (No. 14B150007).

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Footnotes

† Electronic supplementary information (ESI) available: Nitrogen adsorption/desorption isotherms and pore size distribution plots of ZFC7 and ZFO, Raman spectra of ZFC7, TG curves of as-synthesized ZFC7 with a heating rate of 10 °C min⁻¹ in flowing air atmosphere; FESEM images of ZFC7 (a) and (c) and ZFC7-4 (b) and (d); the atom ratio of Zn, Fe, O and C of them is shown inset and cycling performances of ZFC7 and ZFO at 1000 mA g⁻¹ with PVDF binder. See DOI: 10.1039/c6ra04382h

‡ These authors contributed equally.

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