Ferroferric oxide nanoclusters decorated Ti3C2Tx nanosheets as high performance anode materials for lithium ion batteries

Electrochimica Acta 329 (2020) 135146

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Ferroferric oxide nanoclusters decorated Ti3C2Tx nanosheets as high performance anode materials for lithium ion batteries Fuyi Jiang, Rong Du, Xinsheng Yan, Ming Zhang, Qi Han, Xueqin Sun, Xiaoyu Zhang, Yanli Zhou* School of Environmental and Material Engineering, Yantai University, Yantai, 264005, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 September 2019 Received in revised form 24 October 2019 Accepted 24 October 2019 Available online xxx

Titanium carbide MXene as a new 2D anode material has been used for lithium ion batteries, due to its high electrical conductivity, low Liþ diffusion barriers and excellent chemical stability. Nevertheless, the low specific capacity of titanium carbide limits its application. Herein, we used facile ultrasonic and freeze drying methods to fabricate the ferroferric oxide (Fe3O4) modified Ti3C2Tx hybrids for lithium storage. In the hybrid, Fe3O4 nanoclusters are homogeneously anchored on the surface of the single or few layered Ti3C2Tx nanosheets by electrostatic interactions. Due to the synergistic effect of Fe3O4 nanoclusters and Ti3C2Tx nanosheets, all the prepared hybrids show superior lithium storage performance to the single titanium carbide and Fe3O4 nanoclusters. The hybrid with weight ratio of Ti3C2Tx nanosheets and Fe3O4 (1:1) exhibits a high lithium storage capacity of 437.6 mA h g1 after 100 cycles at 100 mA g1. Even at a high current density of 2 A g1, it still retains a stable capacity of 326.6 mA h g1 after 1000 long cycles. The excellent lithium storage performance is proved to be the combination of battery and capacitance behaviors based on the kinetics analysis. This work demonstrates that Fe3O4 nanoclusters decorated Ti3C2Tx nanosheets hybrids are promising high performance anode materials for lithium ion batteries. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Ti3C2Tx nanosheets Fe3O4 nanoclusters Ultrasonic Freeze drying Lithium storage

1. Introduction As the rapid development of science technology and serious pollution of environment, energy storage systems are urgently demanded to replace conventional non-renewable energy resources [1e3]. During these energy storage devices, lithium-ion batteries (LIBs) have been widely used in portable electronics because of its high theoretical energy density, high working voltage and ultra-long cycle life [4,5]. Two-dimensional (2D) materials such as graphene, nitrides, and transition metal sulfides have already been used as electrode materials due to their unique physical and chemistry properties [6e9]. A new 2D transition metal carbides and nitrides named MXenes have greatly aroused researcher’s attention owing to their large surface areas, good electrical conductivity and abundant surface functional groups [10e13]. MXene can be prepared by etching the A layers from MAX crystal structure in the HF acid solution [14e16]. MAX, whose chemical formula is Mnþ1AXn

* Corresponding author. E-mail address: [email protected] (Y. Zhou). https://doi.org/10.1016/j.electacta.2019.135146 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

(n ¼ 1, 2, or 3), where M refers to transition metal (V, Ti, Cr, Nb, etc), A is a ⅧA or ⅥA element (such as Si, Al, Sn, Ge, In, etc.), and X refers to carbon or nitrogen. MXenes is Mnþ1XnTx, T is some terminal groups such as eOH, eF or eO [17e19]. Ti3C2Tx as the most common MXene was commonly prepared by etching Al atoms from Ti3AlC2 in a HF solution. Ti3C2Tx has been widely used as anode materials due to its good electrical conductivity, low Liþ diffusion impedance and stable structure. However, the re-stacking structure and instinct low capacity of Ti3C2Tx limit its practical application [20e23]. Introducing some interlayer spacers, such as metal ion and carbon materials, is an effective strategy to suppress the restacking of Ti3C2Tx sheets and improve the lithium storage performance of Ti3C2Tx electrodes [24e27]. For instance, J. Luo et al. reported that the Sn4þ decorated Ti3C2Tx composites showed a high mass specific capacity of 635 mA h g1 at 100 mA g1 [24]. The reversible capacity of CNTs@Ti3C2Tx composites could reach 175 mA h g1 at 10 A g1 [25]. The Ti3C2Tx/CNFs hybrids prepared by X. Yan et al. showed a lithium storage capacity of 320 mA h g1 at 1C [26]. Although above prepared Ti3C2Tx based hybrids have improved the capacities to some extent, the capacities increase of Ti3C2Tx MXene is not enough to meet the increasing requirement

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for high-performance LIBs based energy systems. It is more effective to prepare a hybrid of Ti3C2Tx and some active materials with high specific capacities to improve the lithium storage performance. Transition metal oxides as new lithium storage electrodes have gained wide attentions owing to their high theoretical capacities. Thus, some transition metal oxides have been introduced in the Ti3C2Tx layers to increase the specific capacity of the electrodes for LIBs [20,28,29]. Among numerous transition metal oxides, Fe3O4 has been regarded as the most promising anode materials due to its very-high theoretical capacity (924 mA h g1), low cost and environmental friendly features [30e32]. For example, Fe3O4 modified multi-layered Ti3C2Tx hybrids delivered good lithium storage performance [29]. However, the multi-layered Ti3C2Tx MXenes limit the loading of Fe3O4 to its outer surface, and more active sites in the inner surface between the Ti3C2Tx layers are neglected, which will affect the improvement of lithium storage capacities. Thus, to design and construct a hybrid consisting of few-layered Ti3C2Tx nanosheets and Fe3O4 might be an effective strategy to improve the electrochemical performance of Ti3C2Tx-based electrodes for LIBs. Herein, we designed the hybrids of Ti3C2Tx nanosheets (sTi3C2Tx) and Fe3O4 nanoclusters (s-Ti3C2Tx/Fe3O4) by facile ultrasonic treatment and freeze drying methods. The multi-layered structure of Ti3C2Tx was first exfoliated to single or few layers with the insertion of tetrabutyl ammonium hydroxide followed by the assistance of ultrasonic treatment. Then, the Fe3O4 nanoclusters were attached onto the surface of s-Ti3C2Tx nanosheets dependent on the electrostatic interactions. Four s-Ti3C2Tx/Fe3O4 hybrids were obtained by changing the amount of Fe3O4. These hybrids as lithium storage anode materials exhibit eminent electrochemical performance in comparison with pristine s-Ti3C2Tx and Fe3O4, benefiting from the synergistic effect of s-Ti3C2Tx and Fe3O4 nanoclusters. Among them, the s-Ti3C2Tx/Fe3O4 hybrid with a weight ratio of 1:1 shows the best lithium storage performance.

ratio of 1:0.5, 1:0.7 and 1:1.3 (s-Ti3C2Tx/Fe3O4-1:0.5, s-Ti3C2Tx/ Fe3O4-1:0.7, and s-Ti3C2Tx/Fe3O4-1:1.3) were also prepared only changing the content of Fe3O4 and the content of s-Ti3C2Tx is fixed at 0.07 g. 2.4. Characterization of materials The crystal structure and morphology features of the products were analyzed by X-ray diffractometer (Shimadzu XRD-7000, Japan), Raman spectrometer (Horiba LabRAM HR Evolution, France) using a laser (l ¼ 532 nm), TEM (JEOL-1400 Plus, Japan) and FESEM (JSM-7610F, Japan). The valence states of different elements for typical s-Ti3C2Tx/Fe3O4 hybrid were performed by an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, USA). 2.5. Electrochemical measurements The working electrode was prepared by mixing the electrode material (s-Ti3C2Tx/Fe3O4, s-Ti3C2Tx or Fe3O4), acetylene black and binder (carboxyl methyl cellulose sodium, CMC) with the mass ratio of 7:2:1 in the deionized water to form a sticky slurry, then pasted onto a copper foil, and dried in vacuum. The loading mass of electrode material is 1.0e1.5 mg cm2. The CR-2032 typed coin cell was assembled using lithium foil as counter electrode, Celgard 2400 polypropylene as separator, and the electrolyte is a solution of

2. Experimental section 2.1. Synthesis of Ti3C2Tx nanosheets 6 g Ti3AlC2 was added into 60 mL HF solution and stirred for 48 h, then the precipitate was collected by washed with alcohol and deionized water until the PH approached 6. The sediment was dried at 60  C in vacuum oven. After that, 0.5 g Ti3C2Tx was dispersed in 10 mL tetrabutyl ammonium hydroxide and stirred for 18 h, the above dispersion was filtrated, washed with deionized water, and finally dried at 60  C in vacuum oven to get s-Ti3C2Tx black powder.

Fig. 1. Schematic illustration of preparation of s-Ti3C2Tx/Fe3O4 hybrids.

2.2. Synthesis of Fe3O4 nanoclusters The Fe3O4 nanoclusters were prepared according to our precious work [33]. Typically, 2 mmol FeCl3 was added into 20 mL ethylene glycol and stirred until completely dissolved. Then 0.9 g polyacrylic acid was dipped into above solutions and stirred for half an hour. After that, 5 mL ammonium hydroxide was added and stirred for another half an hour. The above obtained solution was transferred to Teflon-lined stainless steel autoclave and kept at 230  C for 2 h, the sediment was collected, washed with deionized water and alcohol for several times, and finally dried in oven at 60  C. 2.3. Synthesis of s-Ti3C2Tx/Fe3O4 hybrids 0.07 g s-Ti3C2Tx and 0.07 g Fe3O4 were dispersed in 40 mL deionized water and ultrasound for 6 h. After that, the above obtained mixture was freeze dried for 24 h to obtain the black powder (s-Ti3C2Tx/Fe3O4-1:1). The other s-Ti3C2Tx/Fe3O4 hybrids with mass

Fig. 2. XRD patterns of Ti3AlC2, Ti3C2Tx, s-Ti3C2Tx and s-Ti3C2Tx/Fe3O4-1:1 hybrid.

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1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v). Cycling performance, rate capabilities and galvanostatic charge-discharge curves were measured by battery testing system (Land CT2001A, China) at 25  C in the voltage range of 0.01e3 V. Cyclic voltammetry (CV) curves were obtained from an electrochemical workstation (CHI660e, China) in a range of 0.01e3 V. Electrochemical impedance spectra (EIS) were carried out on an electrochemical workstation (AUTOLAB PGSTAT302N, Switzerland) with a frequency range of 100 kHze0.01 Hz and an amplitude of 10 mV. 3. Result and discussion Fig. 1 shows the schematic preparation process of s-Ti3C2Tx/ Fe3O4 hybrids. First, Ti3AlC2 was immersed in HF solution. The Ti3AlC2 will react with HF to obtain the multi-layered Ti3C2Tx, along with the formation of some hydrophilic terminal groups such as

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eOH, eF or eO. Then multi-layered Ti3C2Tx was further dispersed in the tetrabutyl ammonium hydroxide solution after which the layer distance of Ti3C2Tx was enlarged. Afterwards, both s-Ti3C2Tx and small-sized Fe3O4 nanoclusters with an average diameter of 50 nm were mixed with ultrasonic treatment. In this step, the multi-layered Ti3C2Tx will be exfoliated into single or few layered Ti3C2Tx nanosheets due to the increase of layer distance. Meanwhile, numerous Fe3O4 nanoclusters will uniformly attach onto the surface of Ti3C2Tx nanosheets owing to the electrostatic interaction. Finally, the above dispersion was freeze dried and the s-Ti3C2Tx/ Fe3O4 hybrids can be successfully obtained. The XRD patterns of Ti3AlC2, Ti3C2Tx, s-Ti3C2Tx, Fe3O4 and typical Ti3C2Tx/Fe3O4 with a weight ratio of 1:1 are presented in Fig. 2. All the diffraction peaks of Ti3AlC2 are well matched with hexagonal Ti3AlC2 (JCPDS NO. 52e0875). After etched by HF, the (104) crystal plane of Ti3AlC2 disappears, and some (00l) peaks, such as (002), (004), (006) and (008) planes of Ti3C2Tx are found in the XRD

Fig. 3. (a) Raman spectra of Ti3C2Tx, s-Ti3C2Tx, and s-Ti3C2Tx/Fe3O4, XPS spectra of (b) survey spectrum, high resolution spectra of (c) C 1s, (d) Fe 2p, (e) O1s, and (f) Ti 2p of s-Ti3C2Tx/ Fe3O4-1:1 hybrid.

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pattern, indicating the successful removal of Al atoms. After inserted by tetrabutyl ammonium hydroxide and further ultrasonic treatment, the (002) diffraction peak of Ti3C2Tx shifted to a lower 2q angle, which demonstrates that the interlayer spacing is enlarged, suggesting the generation of s-Ti3C2Tx. As observed from the XRD pattern of typical s-Ti3C2Tx/Fe3O4 hybrid, the peak located at 2q z 7 corresponding to (002) plane is shifted to lower angle in contrast with that of s-Ti3C2Tx, implying that the interplanar spacing of s-Ti3C2Tx are further enlarged after the intercalation of Fe3O4 nanoclusters. Besides the diffraction peaks of s-Ti3C2Tx, the other diffraction peaks located at 30.1, 35.5 , 43.1, 57 and 62.9 are ascribed to (020)/(114), (122)/(212), (008)/(220), (0110)/(232), (325)/(235) crystal planes of orthorhombic phase Fe3O4 (JCPDS card NO. 76e0956). For the other s-Ti3C2Tx/Fe3O4 hybrids, the similar XRD patterns were obtained (Fig. S1). Fig. 3a shows the Raman spectra of Ti3C2Tx, s-Ti3C2Tx and sTi3C2Tx/Fe3O4. The three samples show similar Raman shifts. The peak at 148 cm1 is attributed to the A1g symmetry out-of-plane vibrations of Ti atoms, while the peaks at 298, 400, and 601 cm1 are the Eg vibrations, including in-plane shear modes of C, Ti and surface functional groups, which is in accordance with the previous reports [34,35]. The XPS survey spectrum of typical s-Ti3C2Tx/Fe3O4 hybrid shown in Fig. 3b reveals the signals of Fe, O, Ti, N and C elements, demonstrating the successful synthesis of the s-Ti3C2Tx/ Fe3O4 hybrid. Fig. 3c shows the high-resolution XPS spectrum of C1s, which can be fitted with four peaks centered at 281.2, 284.6, 286.2 and 288.3 eV. The peak at 281.2 eV can be assigned to the CeTi bond. The peak at 284.6, 286.2 and 288.3 eV can be indexed to CeC, CeH/CeO and C]O, respectively [34]. Fe 2p XPS spectrum of the samples corresponding to Fe 2p3 and Fe 2p1 are located at 710.4 and 724.6 eV, respectively (Fig. 3d). As for the Fe 2p3, it can be deconvolved into two peaks centered at 710.1 and 711.1 eV, while the Fe 2p1 peak can be fitted with two peaks located at 724.1 and 725.0 eV. The peak situated at 710.1 eV can be assigned to the Fe (II) in Fe3O4, the other peak located at 711.1 eV can be ascribed to Fe (III) [29,35]. Another small wide peak at 718.9 eV is caused by the slight

oxidation of Fe3O4 [36]. As for the O 1s spectrum (Fig. 3e), two peaks centered at 529.3 and 532 eV correspond to the oxygen in metal oxides [36]. The other peaks located at 530.3, 530.9 and 533.1 eV are assigned as CeTi-Ox/FeeO (II), CeTi-(OH)x and oxygen in H2O [29,36]. Two peaks at 454.6 and 461.0 eV in the Ti 2p spectrum (Fig. 3f) are ascribed to TieC 2p3 and 2p1, respectively. Besides, the other four peaks located at 455.7, 456.1, 458.0 and 464.0 eV are corresponded as Ti (II) 2p3, Ti (III) 2p3, Ti (IV) 2p3, and Ti (IV) 2p1, respectively [28,37,38]. The above analysis verifies the successful formation of the s-Ti3C2Tx/Fe3O4 hybrid. The microstructure of Ti3C2Tx and s-Ti3C2Tx are shown in Fig. 4. Fig. 4a and b shows the SEM images of HF-treated Ti3C2Tx. The low magnified SEM image of Ti3C2Tx shows an accordion-like multilayered structure, which is different from the morphology of Ti3AlC2 (Fig. S2). This result shows that the Al atoms were successfully removed. The morphologies of s-Ti3C2Tx treated by tetrabutyl ammonium hydroxide are presented in Fig. 4c and d. It can found that the multi-layered structure disappears and a thinner sTi3C2Tx nanosheet with a smooth surface can be obtained. The corresponding SEM images of typical s-Ti3C2Tx/Fe3O4 hybrid are shown in Fig. 5a and b. As observed from the magnified SEM image, the surface of s-Ti3C2Tx becomes rough in comparison with single sTi3C2Tx, and no obvious redundant Fe3O4 aggregates appear in the product, suggesting that all the Fe3O4 nanoclusters are successfully loaded onto the s-Ti3C2Tx. The SEM mapping further shows the uniform distribution of Ti, C, Fe and O elements (Fig. 5c). The related side-view and top-view TEM images are shown in Fig. 5d and e. As observed, the surface of s-Ti3C2Tx is coated by so many compact Fe3O4 nanoparticles. Fig. 5e clearly shows that some Fe3O4 nanoclusters with a particle size of 50 nm are uniformly distributed on the s-Ti3C2Tx nanosheets, which is in agreement with SEM and TEM images of Fe3O4 (Fig. S3). The other s-Ti3C2Tx/Fe3O4 hybrids with different weight ratios of s-Ti3C2Tx and Fe3O4 show the similar morphologies (Fig. S4). With the amount of Fe3O4 nanoclusters increases, the coverage ratio on the surface of s-Ti3C2Tx nanosheets gradually boosts. When the ratio of Fe3O4 and s-Ti3C2Tx increases to

Fig. 4. SEM images of (a) and (b) Ti3C2Tx, (c) and (d) s-Ti3C2Tx.

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Fig. 5. (a) Low-magnification and (b) magnified SEM images, (c) SEM mapping, (d) side-view and (e) top-view TEM images of s-Ti3C2Tx/Fe3O4-1:1 hybrid.

1.3:1, the aggregation of Fe3O4 nanoclusters becomes obvious, this is resulted from the insufficient surface area of s-Ti3C2Tx nanosheets for Fe3O4 loading. All of above obtained samples were employed as anode materials to study their lithium storage performance. The CV curves of sTi3C2Tx/Fe3O4-1:1 are shown in Fig. 6a. A strong reduction peak at 0.64 V can be observed in the first cathodic scan, which is attributed to the conversion reaction of Fe3O4 to Fe and formation of solid electrolyte interface film [29]. Besides, a weak cathodic peak appears at 0.9 V, which is resulted from the generation of LixFe3O4. There are two anode peaks at 1.6 V and 1.8 V, corresponding to the oxidation of Fe0 to Fe2þ/Fe3þ, In the subsequent two scans, the cathodic peak shifts positively to 0.76 V, due to the structure rearrangement [39e41]. While the position of anodic peaks are almost not changed, indicating the good reversibility of s-Ti3C2Tx/Fe3O41:1 [42]. Due to the strong signals peaks of Fe3O4, the corresponding cathodic and anodic peaks of pure s-Ti3C2Tx nanosheets

are not obvious (Fig. 6a) [43]. Fig. 6b shows the charge/discharge profiles of s-Ti3C2Tx/Fe3O4-1:1 at 0.1 A g1. The initial discharge and charge specific capacity of s-Ti3C2Tx/Fe3O4-1:1 is 565.3 mA h g1 and 367.6 mA h g1, respectively. The columbic efficiency is only 65.03%, which is resulted from the formation of SEI film and irreversible reactions [31]. Additionally, the columbic efficiency for the second and fifth cycles increases to 94% and 96.9%, respectively. Fig. 6c shows the rate capability of s-Ti3C2Tx/Fe3O4-1:1 hybrid from 0.1 A g1 to 2 A g1. It is obviously observed that the rate capacities of s-Ti3C2Tx/Fe3O4-1:1 is higher than that of s-Ti3C2Tx nanosheets and Fe3O4, and it also exhibits the best cycling stability among all the hybrids (Fig. S6c). When it goes back to 0.1 A g1, the capacity of s-Ti3C2Tx/Fe3O4-1:1 can recover to 400 mA h g1, exhibiting good reversibility. The cycling performance of s-Ti3C2Tx nanosheets, Fe3O4 and s-Ti3C2Tx/Fe3O4-1:1 hybrid at 0.1 A g1 is presented in Fig. 6d, respectively. The specific capacity of s-Ti3C2Tx/Fe3O4-1:1 can reach to 450 mA h g1 after 100 cycles, which is much higher

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Fig. 6. (a) CV curves, (b) charge/discharge curves of s-Ti3C2Tx/Fe3O4-1:1 at 0.1 A g1, (c) rate capabilities at different current densities, (d) cycling performance at 0.1 A g1, (e) highrate cycling performance at 2 A g1 for 1000 cycles of Fe3O4, Ti3C2Tx and s-Ti3C2Tx/Fe3O4-1:1 hybrid.

than that of pure s-Ti3C2Tx nanosheets and Fe3O4. However, the specific capacity of s-Ti3C2Tx/Fe3O4-1:0.5, s-Ti3C2Tx/Fe3O4-1:0.7 and s-Ti3C2Tx/Fe3O4-1:1.3 at the same testing conditions is only 267.3 mA h g1, 376.5 mA h g1 and 381.1 mA h g1, respectively (Fig. S6d), which are much lower than that of s-Ti3C2Tx/Fe3O4-1:1. Fig. 6e shows the long cycling performance of s-Ti3C2Tx/Fe3O41:1 at 2 A g1, its specific capacity can sustain at 326.6 mA h g1 after 1000 cycles, which is higher than that of Fe3O4, s-Ti3C2Tx nanosheets and other s-Ti3C2Tx/Fe3O4 hybrids (Fig. S6e). The difference of electrochemical results for four hybrids can be resulted from the limitation of Fe3O4 loading content. With the Fe3O4 amount increases, the specific capacity of electrode will rise gradually, but when the mass ratio of s-Ti3C2Tx and Fe3O4 content is 1:1.3, the specific capacity decreases, which is lower than that of sTi3C2Tx/Fe3O4-1:1. The phenomenon can be explained that the limited surface area of s-Ti3C2Tx is insufficient to load the excess Fe3O4, and redundant Fe3O4 nanoclusters will aggregate together on the surface of s-Ti3C2Tx, which can be observed from TEM and SEM data (Fig. S4). Therefore, s-Ti3C2Tx/Fe3O4-1:1 is the optimized product in this case. The excellent lithium storage performance is better than most of previous reported Ti3C2Tx MXene related papers (Table S1). EIS techniques were used to explore the kinetic process of the

electrode. Fig. 7a compares the Nyquist plots of s-Ti3C2Tx/Fe3O4-1:1 hybrid, Fe3O4 and s-Ti3C2Tx (Table S2). As observed, the diameter of the semicircle of s-Ti3C2Tx/Fe3O4-1:1 is much smaller than that of pure Fe3O4, which indicates the small charge transfer impedance upon cycling, the steeper inclined line implies the rapid ion diffusion behavior of s-Ti3C2Tx/Fe3O4 electrode. Meanwhile, the lithiumion transport behavior can also be discussed according to the Warburg factor (s) relevant to the slope of the linear fittings in the low-frequency region [44]. As seen from Z’ vs. u1/2 curves (Fig. 7b), the s-Ti3C2Tx/Fe3O4-1:1 electrode exhibits an distinctly lower slope than that of Fe3O4, illustrating its faster lithium-ion transport kinetics inside the electrode (Table S3). To better comprehend the outstanding lithium storage performance of s-Ti3C2Tx/Fe3O4-1:1 electrode, we further investigated its electrochemical mechanism. Fig. 7c shows the CV curves at different scan rates. They show similar shapes containing one cathodic peak and one wide anodic peak. The charge storage mechanism can be derived from the relationship between current (i) and scan rate (n) on the basis of the following equations:

i ¼ avb

(1)

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Fig. 7. (a) EIS plots (the inset is the equivalent circuit) and (b) relationship between Z0 and u1/2 in the low frequency region of s-Ti3C2Tx, Fe3O4 and s-Ti3C2Tx/Fe3O4-1:1 electrodes, (c) CV curves of s-Ti3C2Tx/Fe3O4-1:1 at different scan rates, (d) relationship between log i and log n of cathodic and anodic peaks, (e) capacitive contribution rates of s-Ti3C2Tx/Fe3O41:1 at different scan rates, (f) CV curves with capacitance contribution to the charge storage at 1.0 mV s1.

log i ¼ b log n þ log a

(2)

Both a and b are adjustable parameters, where the b value can describe the charge storage behavior of the materials. Generally, the charge storage of electrodes includes the ion-diffusion controlled battery behavior (b ¼ 0.5) and surface-controlled pseudocapacitive behavior (b ¼ 1.0) [45]. The linear relationship between log i and log n is shown in Fig. 7d. The calculated b values according to the currents of oxidation and reduction peaks are 0.92 and 0.62. The b values manifest that the charge storage process is a mixture of battery behavior and capacitance contribution. Moreover, equation (3) is employed to quantify the capacitance contribution (k1n) and diffusion controlled battery contribution (k2n):

iðVÞ ¼ k1 n þ k2 n0:5

(3)

As shown in Fig. 7e, the rate of the capacitance contribution increases with the scan rate, the value ranges from 74.6 to 86.3% at the scan rates of 0.2e1 mV s1. Fig. 7f shows the CV curve of s-

Ti3C2Tx/Fe3O4-1:1 electrode at 1.0 mV s1. The shaded area represents the proportion of capacitance, revealing that the Li-storage capacitance contribution dominates the overall capacity storage at high rates. The high reversible capacity and long cycle life of s-Ti3C2Tx/ Fe3O4 electrode for LIBs is mainly dependant on the synergistic effect of few-layered Ti3C2Tx nanosheets and Fe3O4 nanoclusters. First, the high conductive Ti3C2Tx nanosheets can facilitate the electron transfer, and meanwhile suppress the particle pulverization of Fe3O4 induced by the volume changes. Moreover, the single or few layered Ti3C2Tx nanosheets can load more high capacity Fe3O4 nanoclusters to its inner and outer surface, which can improve the availability of Ti3C2Tx and enhance the lithium storage capacities. Second, the small-sized hierarchical Fe3O4 nanoclusters can effectively shorten the Liþ diffusion distance and enhance Liþ diffusion kinetics. Third, the void between Fe3O4 primary nanoparticles can provide enough space to inhibit the pulverization of active material, keeping the structure integrity. More importantly, the large available surface area and rich active sites of s-Ti3C2Tx/

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Fe3O4 electrode induce the occurrence of surface-controlled pseudocapacitive behavior, which contributes to the higher lithium storage capacities. All the above results suggest that the s-Ti3C2Tx/ Fe3O4 hybrids are potential promising anode materials for future LIBs. 4. Conclusions In summary, s-Ti3C2Tx/Fe3O4 hybrids have been fabricated by a facile ultrasonic followed by freeze drying method. Four s-Ti3C2Tx/ Fe3O4 hybrids can be obtained by changing the weight ratio of sTi3C2Tx and Fe3O4 nanoclusters. As anode materials for LIBs, all these hybrids exhibit higher specific capacities and better rate capability than pure s-Ti3C2Tx and Fe3O4. Among these hybrids, sTi3C2Tx/Fe3O4-1:1 shows the best electrochemical performance for LIBs. The reversible lithium storage capacity can maintain at 326.6 mA h g1 after 1000 cycles at a high current density of 2 A g1. The excellent performance is attributed to the synergistic effect of s-Ti3C2Tx and Fe3O4 nanoclusters as well as the surface-controlled capacitance behavior based on the kinetics analysis. The excellent performances of these hybrids suggest that s-Ti3C2Tx/Fe3O4 hybrids prepared by this facile method are promising anode materials for future LIBs. Declaration of competing interest There are no conflicts to declare. Acknowledgments This work was financially supported by the National Natural Science Foundation of China of China (No. 51772257), the Major Basic Research Project of Shandong Natural Science Foundation (ZR2018ZC1459) and Doctor Foundation of Shandong Province (No. ZR2017BB081). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135146. References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 469e499. [2] P.L. Taberna, S. Mitra, P. Poizot, P. Simon, J.M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nat. Mater. 5 (2006) 567e573. [3] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652e657. [4] H. Ibrahim, A. Ilinca, J. Perron, Energy storage systemsdcharacteristics and comparisons, Renew. Sustain. Energy Rev. 12 (2008) 1221e1250. [5] B. Xu, A. Oudalov, A. Ulbig, G. Andersson, D.S. Kirschen, Modeling of lithiumIon battery degradation for cell life assessment, IEEE Trans. Smart Grid 9 (2018) 1131e1140. [6] F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, V. Pellegrini, Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage, Science 347 (2015) 1246501. [7] L. Shi, T. Zhao, Recent advances in inorganic 2D materials and their applications in lithium and sodium batteries, J. Mater. Chem. 5 (2017) 3735e3758. [8] Y. Sun, S. Gao, F. Lei, C. Xiao, Y. Xie, Ultrathin two-dimensional inorganic materials: new opportunities for solid state nanochemistry, Accounts Chem. Res. 48 (2014) 3e12. [9] T.F. Zhou, W.K. Pang, C.F. Zhang, J.P. Yang, Z.X. Chen, H.K. Liu, Z.P. Guo, Enhanced sodium-ion battery performance by structural phase transition from two-Dimensional hexagonal-SnS2 to orthorhombic-SnS, ACS Nano 8 (2014) 8323e8333. [10] M. Naguib, J. Come, B. Dyatkin, V. Presser, P.L. Taberna, P. Simon, M.W. Barsoum, Y. Gogotsi, MXene: a promising transition metal carbide anode for lithium-ion batteries, Electrochem. Commun. 16 (2012) 61e64.

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