MFe2O4 (M = Ni, Co) nanoparticles anchored on amorphous carbon coated multiwalled carbon nanotubes as anode materials for lithium-ion batteries

Carbon 123 (2017) 448e459

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MFe2O4 (M ¼ Ni, Co) nanoparticles anchored on amorphous carbon coated multiwalled carbon nanotubes as anode materials for lithiumion batteries Rencheng Jin*, Qingyao Wang, Yuming Cui, Shaohua Zhang School of Chemistry & Materials Science, Ludong University, Yantai, 264025, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 May 2017 Received in revised form 27 July 2017 Accepted 29 July 2017 Available online 31 July 2017

One dimensional CNTs@C@MFe2O4 (M ¼ Ni, Co) composites have been rationally designed and fabricated via a facile solvothermal method and a calcination process. In the unique structure, amorphous carbon served as binder can increase the loading of MFe2O4 on CNTs and strengthened the binding of MFe2O4 and CNTs. As anode materials for lithium ion batteries, the CNTs@C@MFe2O4 delivers a high specific capacity, excellent cycling stability and high rate capacity. The enhanced electrochemical performance can be attributed to the uniform dispersion of MFe2O4 nanoparticles on the amorphous carbon coated CNTs, which can improve contact area between the MFe2O4 nanoparticles and the electrolyte, enhance electrical conductivity, buffer the volume change maintain the structural integrity of the electrodes. Meanwhile, the lithiation and delithiation processes are systematically investigated by X-ray photoelectron spectroscopy analysis and transmission electron microscope technique. Furthermore, the efficient synthesis process developed here can also be extended to design and synthesize other transition metal oxides/carbon nanotube functional materials. © 2017 Published by Elsevier Ltd.

1. Introduction Rechargeable lithium-ion batteries (LIBs), as one of promising electrical energy storage devices, have attracted intensive interest due to their desirable properties including high energy density, long lifespan, and environmental benignity [1e3]. With the increasing requirement of high energy density and safety of the LIBs, high power and high-capacity LIBs are needed. Therefore, seeking of novel anode materials with enhanced performances is necessary. As representative cases, transition metal oxides based materials are considered as the optimal choice for the electrode materials of LIBs on account of their high theoretical capacity (2e3 times higher than that of graphite) and low cost [4e9]. The major issue inhibited their application still lies in the fact of their rapid capacity fading, large volume changes and pulverization during lithium insertion/extraction [9,10]. Consequently, various approaches including optimizing particle size and morphology [11e14], surface modification with electronic and/or ionic conductive layers [15e17], and doping with alien ions [4,18] have

* Corresponding author. E-mail address: [email protected] (R. Jin). http://dx.doi.org/10.1016/j.carbon.2017.07.092 0008-6223/© 2017 Published by Elsevier Ltd.

been developed to overcome these shortcomings. One of the most effective strategies is to fabricate transition metal oxide/carbon nanotube (CNT) anodes, which shows enhanced cycling performance and a high rate capability. Metal ferrite materials, one of promising alternative anode materials for LIBs, have been widely investigated. In the past decades, ZnFe2O4 nano-octahedrons [19], NiFe2O4 nanofibers [20], NiFe2O4 and CoFe2O4 ball-in-ball nanostructures [21] and metal ferrite with carbon substrate such as CoFe2O4/graphene [22,23], ZnFe2O4/graphene [24], NiFe2O4/graphene [25,26], and NiFe2O4/ carbon [27,28] are prepared to improve the electrochemical performance. For the research of metal ferrite/carbon composites, the main efforts have been concentrated on increasing electrical conductivity, while less works are aimed to clearly show the electrochemical reaction mechanism of metal ferrite in such electrodes. Herein, MFe2O4 (M ¼ Ni, Co) nanoparticles anchored on amorphous carbon coated multiwalled carbon nanotubes (CNTs) were fabricated by a facile solvothermal route combined with hightemperature calcination treatment. In such composites, the amorphous carbon acted as a linker was fabricated through a hydrothermal procedure, which enhances the adhesion between MFe2O4 and CNTs and increased the loading of MFe2O4 on CNTs. The CNTs as

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Scheme 1. Schematic illustration of the proposed formation mechanism of CNTs@C@MFe2O4.

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an electrical conductive component can effectively promote rapid Liþ supplies and limit the aggegation of the electrode, which makes each MFe2O4 nanoparticle to be an effective microreactor for fast electrochemical reactions and keeps the structural integrity during cycling, and thereby, benefits for its cycling stability and rate performance. The electrochemical test results showed that the CNTs@C@MFe2O4 composites exhibited superior rate capability (786 mAh g1 at 10 A g1 for CNTs@C@NiFe2O4, and 580 mAh g1 at 10 A g1 for CNTs@C@CoFe2O4) and cycle stability (1159 mAh g1 at 0.2 A g1 for CNTs@C@NiFe2O4, 1031 mAh g1 at 0.2 A g1 for CNTs@C@CoFe2O4). To deep understand the electrochemical reaction mechanism of the CNTs@C@MFe2O4, the ex situ TEM and XPS

Fig. 1. (a, b) SEM images, (c) TEM image, (d) SAED pattern, and (e) HRTEM image of the one dimensional CNTs@C@NiFe2O4.

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analysis were explored.

2. Experimental section 2.1. Synthesis of CNTs@C@MFe2O4 composites The multiwalled carbon nanotubes were purchased from Shenzhen Nanotech Port Co., Ltd. All other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. In a typical process, 50 mg of multiwalled carbon nanotubes (CNTs) was dispersed in 15 mL of distilled water containing 0.15 g of glucose. Then the resulting solution was

transferred into a 25 mL Teflon-lined autoclave and held at 160  C for 12 h. The amorphous carbon coated MWCNTs (CNTs@C) were collected through centrifugation and washed with distilled water for several times. 0.5 mmol of NiCl2.6H2O, 1 mmol of FeCl3.6H2O combined with 0.2 g of glucose were dissolved into the mixture of the 1, 2-propanediol (10 mL) and distilled water (8 mL). Then 50 mg of CNTs@C was added into the above solution and continued stirring for another 10 min. After that, the solution was transferred into Teflon-lined autoclave and sonicated for 30 min. Subsequently, 2 mL of distilled water containing 0.2 g of sodium hydroxide was introduced into the autoclave. The autoclave was sealed and heated at 180  C for 12 h, and then cooled naturally to room temperature.

Fig. 2. (a, b) SEM images, (c) TEM image, (d) SAED pattern, and (e) HRTEM image of the one dimensional CNTs@C@CoFe2O4.

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The obtained black precipitates were washed thoroughly with distilled water and ethanol, and dried under vacuum at 80  C overnight. The CNTs@C@NiFe2O4 powder was obtained after calcination treatment under argon atmosphere at 500  C for 6 h. The CNTs@C@CoFe2O4 was prepared by using CoCl2.6H2O instead of NiCl2.6H2O without changing other parameters. For comparison purpose, the CNTs@MFe2O4 was prepared through the same solvothermal process by using CNTs instead of CNTs@C. 2.2. Materials characterization The obtained products were determined on Rigaku D/Max2550pc X-ray diffractometer with Cu Ka radiation. The structure and morphology of the samples were characterized on an FEI Quanta 200 F field emission scanning electron microscope and an FEI Tecnai G2 S-Twin transmission electron microscope, respectively. The surface analysis was performed on a PHI-5702 multifunctional X-ray photoelectron spectrometer with a pass energy of 29.35 eV and a Mg Kaline excitation source. 2.3. Electrochemical measurements The working electrodes were prepared by pasting the slurry of CNTs@C@MFe2O4, carbon black, and poly(vinyl difluoride) (PVDF) in the weight ratio of 80:10:10 on a piece of copper foil. Then the electrode plates were dried in the vacuum at 120  C overnight. The cells were assembled in an argon-filled glovebox, in which the pure lithium foil was used as the counter and reference electrode, Celgard 2400 film as the separator, and 1.0 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 v/v) as the electrolyte. Cyclic voltammetry was performed on a CHI660E (Shanghai Chenhua Instrument Company, China) electrochemical workstation in the potential range from 3 to 0.01 V at a scan rate of 0.2 mV s1. Electrochemical impedance spectra (EIS) were characterized by the same instrument over a frequency range of 100 kHz to 0.01 Hz. Galvanostatic charge and discharge tests were performed on a NEWARE BTS-3008 (Neware Co., Ltd, China) battery tester at a voltage window of 0.01e3.00 V. 3. Results and discussion The synthesis process of CNTs@C@MFe2O4 is presented in Scheme 1. In the first step, the CNTs@C with the diameter ranging from 20 to 40 nm can be obtained by using hydrothermal technique. The corresponding characterizations (X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)) are presented in Figs. S1eS2 (Supporting information). In step II, the surface of the CNTs@C were coated with MFe2O4 nanoparticles in the mixed solution of 1, 2propanediol and distilled water via a solvothermal process. During the solvothermal process, amorphous carbon serves as binder and glucose as a surfactant can promote the precipitation of Ni2þ, Co2þ and Fe3þ on the sidewall of CNTs@C, which leads to in situ formation of MFe2O4. For comparison, the CNTs@MFe2O4 is fabricated by using CNTs instead of CNTs@C and the SEM iamges are presented in Fig. S3. The results indicate that the amorphous carbon with many functional groups can increase the loading of MFe2O4 on CNTs. In addition, the CNTs@C@MFe2O4 was fabricated by calcinating under argon atmophere at 500  C (Step III). The morphology of the obtained CNTs@C@NiFe2O4 is presented in Fig. 1. According to the SEM images (Fig. 1aeb), the CNTs@C@NiFe2O4 nanostructure retains the one-dimensional nanostructure of the CNT while the surface becomes coarse because of the deposition of the NiFe2O4 nanoparticles. The NiFe2O4 nanoparticles anchored on the nanotubes are further illustrated by

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transmission electron microscopy (TEM) in Fig. 1c. Fig. 1d displays the SAED pattern of the CNTs@C@NiFe2O4. The d-values calculated from the concentric rings are in accordance with (002) of CNT and (220), (311), (400), (511), and (440) of cubic NiFe2O4. The lattice fringe spacings of 0.299 nm and 0.255 nm match well with the (220) and (311) planes of the NiFe2O4 (Fig. 1e). When CoCl2.6H2O is introduced in the reaction system instead of NiCl2.6H2O, the morphology of the sample is almost unchanged (Fig. 2aec). The SAED pattern represents the facets of the CoFe2O4 (Fig. 2d) and a lattice spacing of 0.253 nm corresponds to the (311) planes of the cubic CoFe2O4 (Fig. 2e). For comparison, the MFe2O4 nanoparticles were fabricated without CNTs@C or CNTs and the SEM and TEM images were exhibited in Fig. S4. The samples after solvothermal process and calcination treatment are characterized by XRD (Fig. 3). When no CNTs@C is introducted into the reaction system, the diffraction peaks of the obtained samples can be indexed to the cubic phase of NiFe2O4 and CoFe2O4. Whereas new peaks assigned to the (002) and (101) planes of CNTs can be observed for CNTs@C@NiFe2O4 and CNTs@C@CoFe2O4. As presented in Fig. 4, the surface composition and the oxidation state of the CNTs@C@NiFe2O4 are determined by X-ray photoelectron spectroscopy (XPS). Four elements including C, O, Ni, and Fe are showed in the survey XPS spectrum (Fig. 4a). For O 1s XPS spectrum (Fig. 4b), the peak located at about 532.1 eV is attributed to the presence of the adsorbed O2, H2O, and CO2 [4]. The other two peaks at 530.0 and 528.8 eV are ascribed to the metaloxygen bond. In the Ni 2p spectrum, the major peaks at 872.0, 871.0, 854.3 and 853.5 eV with the two shakeup satellites at 878.1 and 860.3 eV match well with the Ni 2p3/2 and Ni 2p1/2, respectively (Fig. 4c) [29,30]. As shown in Fig. 4d, two strong peaks at 723.2 and 710.1 eV and two shakeup satellites at 731.7 and 717.2 eV can be assigned to Fe 2p3/2 and Fe 2p1/2 of Fe3þ, respectively [28,31,32]. Fig. 5adisplays the wide XPS spectra of CNTs@C@CoFe2O4, demonstrating the presence of C, O, Co, and Fe elements. The O 1s spectrum ascribed to the metal-oxygen bond appears at 530.4 and 528.9 eV (Fig. 5b). The binding energy at 795.9 eV and 779.6 eV are ascribed to Co 2p3/2 and Co 2p1/2 of Co2þ, respectively. Meanwhile, two shakeup satellites at 801.8 and 785.2 eV can also be observed (Fig. 5c). Compared with Fig. 4d, the binding energy attributed to Fe 2p3/2 and Fe 2p1/2 of Fe3þ has little change (Fig. 5d). The Li storage performance of the CNTs@C@MFe2O4 electrodes is evaluated by Cyclic voltammetry (CV) and galvanostatic tests. Fig. 6a depicts the CV curves of the CNTs@C@NiFe2O4 upon initial

Fig. 3. XRD patterns of NiFe2O4, CoFe2O4, CNTs@C@NiFe2O4 and CNTs@C@CoFe2O4. (A colour version of this figure can be viewed online.)

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Fig. 4. (a) XPS survey of CNTs@C@NiFe2O4, (b, c, d) high resolution XPS spectra of O 1s, Ni 2p, and Fe 2p for CNTs@C@NiFe2O4. (e, f) high resolution XPS spectra of Ni 2p, and Fe 2p after 5 times discharge (cut off voltage: 0.01 V), (g, h) high resolution XPS spectra of Ni 2p, and Fe 2p after 100th cycles(cut off voltage: 3.0 V). (A colour version of this figure can be viewed online.)

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Fig. 5. (a) XPS survey of CNTs@C@CoFe2O4, (b, c, d) high resolution XPS spectra of O 1s, Co 2p, and Fe 2p for CNTs@C@CoFe2O4. (e, f) high resolution XPS spectra of Co 2p, and Fe 2p after 5 times discharge (cut off voltage: 0.01 V), (g, h) high resolution XPS spectra of Co 2p, and Fe 2p after 100th cycles (cut off voltage: 3.0 V). (A colour version of this figure can be viewed online.)

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Fig. 6. Electrochemical properties: a) CV curves of CNTs@C@NiFe2O4 in the first three cycles at a scan rate of 0.2 mV s1, b) charge/discharge profiles of CNTs@C@NiFe2O4 at 0.2 A g1, c) CV curves of NiFe2O4 in the first three cycles at a scan rate of 0.2 mV s1, d) charge/discharge profiles of NiFe2O4 at 0.2 A g1, e) cycling stabilities combined with Coulombic effiiency at 0.2 A g1, and f) rate capabilities from 0.2 to 10 A g1 of NiFe2O4, CNTs@NiFe2O4 and CNTs@C@NiFe2O4 electrodes. (A colour version of this figure can be viewed online.)

three cycles at 0.2 mV s1. In the first cycle, the cathodic peak at 1.12 and 0.52 V can be ascribed to the conversion of NiFe2O4 to Li2O and metallic Ni and Fe [27]. For the anodic scan, the peak at 1.57 V can be attributed to the oxidation of Ni and Fe to Ni2þ and Fe3þ. In the subsequent sweeps, two reduction peaks located at 1.55 and 0.71 V correspond to the reduction of NiO and Fe2O3 to metallic Ni and Fe, respectively [26,28], while the oxidation peak shifts to 1.63 V. The anodic peaks shift may be attributed to the partial polarization of the electrodes [26]. For comparison purpose, the CV of the pure phase NiFe2O4 and CNTs@NiFe2O4 are also evaluated and presented in Fig. 6c and Fig. S5a. In the discharge process, two peaks related to the reduction of NiO and Fe2O3 to metallic Ni and Fe can also be observed. However, the peak located at ~1.55 V (reduction of NiO to Ni) is weak. Meanwhile, the larger difference between the two anodic peaks (0.15 V for NiFe2O4, 0.19 V for CNTs@NiFe2O4) indicates the worse polarization of the NiFe2O4 and CNTs@NiFe2O4 electrodes. To determine the lithiation and delithiation processes of

the electrode, the XPS spectra of the cycled electrodes are performed. The Ni 2p and Fe 2p spectra of the electrodes after 5 times discharge (cut off potential: 0.01 V) are shown in Fig. 4eef. Compared with the Ni 2p spectrum of the CNTs@C@NiFe2O4 in Fig. 4c, two new peaks located at 868.2 and 851.0 can be observed, which correspond to Ni0 state (Fig. 4e) [33,34]. According to the Fe 2p spectrum in Fig. 4f, one weak peak at 704.9 eV assigned to the Fe0 can be found [35]. The result proves that NiFe2O4 can convert into metallic Ni and Fe when the electrode is discharged to 0.01 V. After cycles for 100 times (charged to 3.0 V), the peaks correspond to Ni0 and Fe0 state disappear while the peaks ascribed to Ni2þ and Fe3þ maintain (Fig. 4geh). Based on the above analysis, the following reactions are proposed to illustrate the discharge-charge cycles. In the first cycle of discharge, the NiFe2O4 is reduced by lithium metal and the metallic Ni and Fe combined with amorphous Li2O appear (Eq. (1)). And then metallic Ni and Fe convert to the respective metal oxides during the charge process, as presented

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in Eqs. (2)e(3). NiFe2O4 þ 8Liþ þ 8e / Ni þ 2Fe þ 4Li2O

(1)

Ni þ Li2O 4 NiO þ 2Liþ þ 2e

(2)

2Fe þ 3Li2O 4 Fe2O3 þ 6Liþ þ 6e

(3)

To further identify the lithiation and delithiation processes, the ex situ HRTEM analysis on the fully discharged (0.01 V) and charged (3.0 V) electrode materials is carried out. Fig. 7a and c shows the TEM and HRTEM images of the discharge electrode (5 cycles). After 5 cycles, the nanoparticles anchored on the CNTS can be obviously observed (Fig. 7a). The HRTEM image of the discharged electrode presents that the small amorphous nanoparticles disperse around the CNTs (Fig. 7c), indicating the NiFe2O4 can be reduced to the amorphous Ni and Fe, respectively. When the electrode is charged for 100 times, the morphology is still preserved (Fig. 7b). Such a stable morphology may be benificial for enhancing the cycle stability. Furthermore, the lattice fringe spacings confirm the presence of NiO (0.243 nm) and Fe2O3 (0.209 nm), rather than NiFe2O4 (Fig. 7d). The charge-discharge performances for the first three times of the CNTs@C@NiFe2O4, NiFe2O4, and CNTs@NiFe2O4 are investigated at the current density of 0.2 A g1 with the potential range of 0.01e3.0 V. The first discharge and charge capacities of CNTs@C@NiFe2O4, NiFe2O4, and CNTs@NiFe2O4 are 1431 and 1264 mAh g1 (Fig. 6b), 1480 and 1092 mAh g1 (Fig. 6d), 1487 and 1120 mAh g1 (Fig. S5b), leads to the Coulombic efficiency of 88.3%, 73.8% and 75.3%, respectively. The result indicates that the addition of CNTs can effectivley enhance the reversibility of the electrodes. The capacity fading of the electrode is attributed to the irreversible formation of the solid electrolyte interface (SEI) layer on electrode surface or the decomposition of the electrolyte [36e38]. Fig. 6e presents the cycle performance of the CNTs@C@NiFe2O4,

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NiFe2O4@C and NiFe2O4 at a rate of 0.2 A g1. As expected, the CNTs@C@NiFe2O4 delivers the better cycle stability than that of CNTs@NiFe2O4. After 100 cycles at 0.2 A g1, the CNTs@C@NiFe2O4 still maintains a reversible discharge capacity as high as 1160 mAh g1, which is higher than those of CNTs@NiFe2O4 (923 mAh g1), NiFe2O4 (234 mAh g1) and the previous work in the literature [20,21,25,27]. To further evaluate the superior cyclability and rate capacity of the CNTs@C@NiFe2O4, the chargedischarge capacities are investigated at the current densities ranging from 0.2, 0.5, 1, 2, 5, 10 A g1 for every 10 successive cycles and then back to 0.2 A g1 for 20 cycles. With increasing the current density, the CNTs@C@NiFe2O4 electrode delivers superior specific capacities (from 1176 to 786 mAh g1), and the capacity recovers to 1125 mAh g1 when the current density returns to 0.2 A g1 (Fig. 6d). Fig. 8a presents the CV profiles of the obtained the CNTs@C@CoFe2O4. The wide cathodic peak at 0.34 V in the first cycle can be assigned to the formation of SEI in the electrode surface and the reduction of CoFe2O4 to Co and Fe. In the subsequent cathodic cycle, two obvious reduction peaks at 0.73 and 1.44 V can be observed, which are attributed to the reduction of Fe2O3 and CoO to metallic Fe and Co, respectively [39]. During anodic process, the peak located at 1.60 V is associated with the oxidation of metal Co and Fe to Co2þ and Fe3þ, and this peak shifts to 1.65 V in the second and subsequent cycles, implying the partial polarization of the CoFe2O4 sample. The CoFe2O4 and CNTs@CoFe2O4 electrodes display the similar phenomena, as presented in Fig. 8c and Fig. S6a. To ascertain the above reaction mechanism during the chargedischarge process, the XPS spectra of the sample collected at different stages are detected. Except the binding energies assigned to Co2þ and Fe3þ, new peaks located at 791.6, 776.7 and 705.5 eV are ascribed to the metallic Co and Fe, respectively (Fig. 5eef) [40e42]. After Liþ extraction in the charge process, the Co and Fe nanoclusters vanish along with the electrochemical oxidation of Co and Fe to CoO and Fe2O3. As the result, the binding energy belonged

Fig. 7. a, c) TEM and HRTEM images of the CNTs@C@NiFe2O4 electrode after 5 times discharge (cut off voltage: 0.01 V), b, d) TEM and HRTEM images of the CNTs@C@NiFe2O4 electrode after 100th cycles (cut off voltage: 3.0 V).

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Fig. 8. Electrochemical properties: a) CV curves of CNTs@C@CoFe2O4 in the first three cycles at a scan rate of 0.2 mV s1, b) charge/discharge profiles of CNTs@C@CoFe2O4 at 0.2 A g1, c) CV curves of CoFe2O4 in the first three cycles at a scan rate of 0.2 mV s1, b) charge/discharge profiles of @CoFe2O4 at 0.2 A g1, e) cycling stabilities combined with Coulombic effiiency at 0.2 A g1, and f) rate capabilities from 0.2 to 10 A g1 of CoFe2O4, CNTs@CoFe2O4 and CNTs@C@CoFe2O4 electrodes. (A colour version of this figure can be viewed online.)

to Co2þ and Fe3þ can be observed in Fig. 5geh. The TEM and HRTEM images after discharge and charge process are provided to further determined the reaction process. The TEM images in Fig. 9a and b indicate that the morphology of the CNTs@C@CoFe2O4 is stable during the Liþ insertion and extraction process. Fig. 9c demonstrates the corresponding HRTEM image of the electrode material after 5 times discharge, only amorphous nanoparticles can be observed, similar to the CNTs@C@NiFe2O4 electrode. After the 100th charge, the lattice spacings of 0.262 nm and 0.210 nm are assigned to the CoO and Fe2O3. The above experimental results indicate that the electrochemical full lithiation reaction of the CoFe2O4 consists of three main stages: (1) lithium insertion in CoFe2O4, (2) reduction reaction of CoFe2O4 forming amorphous nano-Co, Fe and Li2O, and (3) the nano-Co and Fe dispersed in Li2O matrix turn to be CoO and Fe2O3. CoFe2O4 þ 8Liþ þ 8e /Co þ 2Fe þ 4Li2O

(4)

Co þ Li2O 4 CoO þ 2Liþ þ 2e

(5)

2Fe þ 3Li2O 4 Fe2O3 þ 6Liþ þ 6e

(6)

Fig. 8b demonstrates the discharge-charge profiles for the 1st, 2nd, and 3rd cycle at the current density of 0.2 A g1. The initial discharge and charge capacities of the electrode are 1221, and 1003 mA h g1, leads to the Coulombic efficiency of 82.1%, which is higher than that of CoFe2O4 (68.7%, Fig. 8d) and CNTs@CoFe2O4 (72.3%, Fig. S6b). The improved Coulombic efficiency for the CNTs@C@CoFe2O4 might be attributed to the increased reversibility of the involved electrochemical reactions. The cycling performance for CNTs@C@CoFe2O4, CNTs@CoFe2O4, and CoFe2O4 was evaluated by the discharge-charge experiments at the current density of 0.2 A g1 (Fig. 8c). For CNTs@CoFe2O4, the specific capacity is 930 mAh g1 at the second cycle and drops to 712 at the 20th cycle, and then maintains at 719 after 100 cycles. While the capacity for

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Fig. 9. a, c) TEM and HRTEM images of the CNTs@C@CoFe2O4 electrode after 5 times discharge (cut off voltage: 0.01 V), b, d) TEM and HRTEM images of the CNTs@C@CoFe2O4 electrode after 100th cycles (cut off voltage: 3.0 V).

pure phase CoFe2O4 decrease sharply to 322 mAh g1 after 100 cycles. The cycling performance of the CNTs@C@CoFe2O4 is better than those of pure phase CoFe2O4 and CNTs@CoFe2O4 and shows a significant increase after 40th cycle, which may due to the formation of a reversible polymeric/gel film on the nanocomposite. Similar phenomena can be also observed in previous works [43]. Furthermore, the rate capability of CNTs@C@CoFe2O4 and CNTs@CoFe2O4 is also investigated at various rates from 0.2 to 10 A g1. As shown in Fig. 8d, the reversible capacity of CNTs@C@CoFe2O4 keeps at 920, 831, 758, 667, 639, and 580 mA h g1 at the current density of 0.2, 0.5, 1, 2, 5, 10 A g1, whereas, the CNTs@CoFe2O4 elelctrode displays the reversible capacity of 736, 640, 534, 440, 368, and 288 mA h g1 at the same current density, implying that the rate capacity of CNTs@C@CoFe2O4 has been effectively improved. The experimental results indicate that the CNTs@C@CoFe2O4 shows the better or comparable stability and rate performance compared to the previously reported CoFe2O4 [39,44e47]. The excellent performance of CNTs@C@MFe2O4 can be

understood using electrochemical impedance spectra (EIS). As shown in Fig. 10, the spectra of CNTs@C@MFe2O4 before and after cycles consist of a depressed semicircle in the high and middle frequencies, and a slope in the low frequency. The high and middlefrequency semicircles are related to interphase resistance (Rf) and charge transfer resistance (Rct), respectively. And the slope in the low frequency reflects the ion diffusion inside electrode. As can be seen in Fig. 10a and b, the CNTs@C@MFe2O4 electrodes display little variation after 100 cycles, indicating the stability of the electrodes. In addition, the total impedance of the pure phase MFe2O4 and CNTs@MFe2O4 (Fig. S7) electrodes are higher than those of CNTs@C@MFe2O4 electrode, implying the reduced electrical resistance for the CNTs@C@MFe2O4 based electrode and the enhanced charge transfer at the interface. In addition, the charge transfer for the pure phase MFe2O4 after 100 cycles is much bigger than that of the fresh cell, further reflecting the rapid fading of the electrodes. The results indicate that the CNTs can effectively increase the electrical conductivity and enhance the reaction kinetics of the eletrodes, which improves the rate capacity of the electrodes.

Fig. 10. Nyquist plots of the fresh cells and the cells after 100 cycles: a) CNTs@C@NiFe2O4, b) CNTs@C@CoFe2O4. (A colour version of this figure can be viewed online.)

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Based on the above experimental results, the superior Li-storage property and improved coulombic efficiency may result from the cooperation of the unique structure and the rich oxygenous groups of amorphous carbon. The MFe2O4 nanoparticles anchored on the CNT backbone can effectively shorten the diffusion distance of electrons and Li ions and ensure their rapid transport. The CNT backbone can promote the rapid charge transfer and thus reduce the electrode reaction resistance. Moreover, the CNTs serve as a flexible backbone anchoring and stabilizing MFe2O4 particles to inhibit the agglomeration and provide a large electrolyte/electrode contact area during cycles. The amorphous carbon on CNTs can strengthen the binding of MFe2O4 to CNTs and prevent the detachment of MFe2O4 from CNTs, which can be ascertained by TEM images after 100 cycles (Figs. 7b and 9b). For comparison, the pure phase MFe2O4 electrodes after 100 cycles are decomposed and the morphologies are detected by TEM. Due to the large volume variation and pulverization of the pure phase MFe2O4 upon repeated charging-discharging process, the aggregated nanoparticles and some irregular particles can be observed (Figs. S8a and S8c), indicating the serious destruction of the electrodes. The corresponding HRTEM images display that the crystalline nanoparticles almost become amorphous and some crystalline region can be occassionally observed. The lattice fringes of 0.243 nm (Fig. S8b) and 0.262 nm (Fig. S8d) can be attributed to NiO and CoO, respectively. The results further prove that the CNTs can not only accommodate volume expansion/contraction, but also prohibit the aggregation of the active materials during the lithiation/delithiation processes. 4. Conclusions In summary, CNTs@C@MFe2O4 hybrid has been designed and fabricated through a solvothermal process accompanied by a hightemperature calcnation. The amorphous carbon on the surface of CNTs serves a linker to strengthen the binding of MFe2O4 on CNTs. Such CNTs@C@MFe2O4 is composed of numerous nanoparticles anchored on the CNTs, which can be employed as a potential anode material for LIBs. The electrochemical measurements results demonstrate that the unique CNTs@C@MFe2O4 delivers high specific capacity and better cycle performance. More importantly, the CNTs@C@MFe2O4 exhibits an outstanding high rate capacity, displaying 786 and 580 mAh g1 for CNTs@C@NiFe2O4 and CNTs@C@CoFe2O4 at high rate of 10 A g1, respectively. The superior performance originates from the amorphous carbon and one dimensional CNTs. The amorphous carbon can not only hinders the detachment of MFe2O4 from CNTs, but also accommodate volume varation during the lithiation/delithiation processes. The CNTs can improve the electronic conductivity and also limit the aggregation of the active materials. In addition, the XPS analysis and ex situ HRTEM technique are introduced to deeper understand the reaction mechanism of the electrodes. Moreover, in view of the easy fabrication of CNTs@C@MFe2O4, the synthesis process could be extended to prepare many other transition metal oxides with these or similar structures. Acknowledgments The authors are grateful for the financial support of the Natural Science Foundation of China (Project no. 21301086), and Natural Science Foundation of Shandong Province (Project no. ZR2013BQ008). Appendix A. Supplementary data Supplementary data related to this article can be found at http://

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