Qiuxian Wang‡abc,
Hongyun Yue‡
abc,
Ting Duabc,
Wanli Zhanga,
Yun Qiaoabc,
Hongyu Dongabc,
Yanhong Yinabc and
Shuting Yang*abc
aSchool of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China. E-mail: shutingyang@foxmail.com; Tel: +86-373-3326439
bNational and Local Joint Engineering Laboratory of Motive Power and Key Materials, Xinxiang, Henan 453007, P. R. China
cCollaborative Innovation Center of Henan Province for Motive Power and Key Materials, Henan Normal University, Xinxiang, Henan 453007, China
First published on 31st March 2016
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.
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.
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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 2p3/2 and Fe 2p1/2 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 2p3/2 and Zn 2p3/2, 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 CO 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
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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.
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.
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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.
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 (Rp), the electronic resistivity of the active material and ionic conductivity in the electrode (Rb). The straight line is assigned to the Warburg impedance (Wo) during the lithium diffusion process, and Rs 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
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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 LixZnFe2O4 (Li0.4ZnFe2O4, Li0.9ZnFe2O4 and Li1.45ZnFe2O4). When 1.45 ≤ x ≤ 2, the partially lithiated Li1.45ZnFe2O4 is decomposed to Li2O and a new rock-salt phase metal oxide, in which all the iron is reduced to Fe2+. ZnxFeyO, 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.
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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− → Li0.4ZnFe2O4 |
(B) Li0.4ZnFe2O4 + 0.5Li+ + 0.5e− → Li0.9ZnFe2O4 |
(C) Li0.9ZnFe2O4 + 0.55Li+ + 0.55e− → Li1.45ZnFe2O4 |
Li1.45ZnFe2O4 + 0.55Li+ + 0.55e− → ZnxFeyO3 + Li2O (x![]() ![]() ![]() ![]() |
(D) ZnxFeyO3 + 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.
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. |
This journal is © The Royal Society of Chemistry 2016 |