MXene interlayer anchored Fe3O4 nanocrystals for ultrafast Li-ion batteries

Chemical Engineering Science xxx (xxxx) xxx

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MXene interlayer anchored Fe3O4 nanocrystals for ultrafast Li-ion batteries Da Xu a,b, Kun Ma a, Ling Chen b, Yanjie Hu b, Hao Jiang a,b,⇑, Chunzhong Li a,b,⇑ a

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China b

h i g h l i g h t s  The Fe3O4 nanocrystals are well-distributed between Ti3C2 interlayers.  The 2D confinement effects stabilize the hybrids with high exposure of active sites.  The Fe3O4@Ti3C2 hybrids possess a rapid and stable charge/discharge capability.

a r t i c l e

i n f o

Article history: Received 15 September 2019 Received in revised form 28 October 2019 Accepted 5 November 2019 Available online xxxx Keywords: Fe3O4 nanocrystals MXene 2D space confinement High-rate Li-ion batteries

a b s t r a c t Balancing energy density and charging rate has been identified as a great challenge for Li-ion batteries (LIBs), which mainly hinges on developing high-performance electrode materials. Herein, we have developed novel Fe3O4@Ti3C2 hybrids, in which the Fe3O4 nanocrystals are well-anchored between Ti3C2 interlayers with the assistance of the synergistic effects of 2D physical confinement and TiAOAFe covalent bonds. Such structural design can address the dispersion and volume change of nanocrystals during continuous charge/discharge with the enhancement of structural stability and the exposure of abundant active sites for each component. Meantime, the charge polarization caused by the TiAOAFe covalent bonds greatly accelerates the lithiation reaction kinetics and electrons transfer. These advantages endow the Fe3O4@Ti3C2 hybrids with a very high specific capacity of 1172 mAh g 1 and a rapid charging capability of 366 mAh g 1 in 66 s. A 90% capacity retention can be maintained even through 1000 cycles at 5 A g 1. More impressively, we can also achieve a free-standing electrode by a simple vacuum filtering, exhibiting a high areal capacity of 4.2 mAh cm 2 at 4.4 mg cm 2 almost without sacrificing gravimetric capacity. The present 2D confined strategy provides a new notion to construct satisfactory electrodes for energy storage. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The cruising range and charging rate are the two major bottlenecks of developing electric vehicles. With the breakthrough of high-energy LIBs technology, the range of electric vehicles has basically solved, e.g. which is gradually improved from 150 to 400 km for passenger cars (Armand and Tarascon, 2008; Ma et al., 2018; Li et al., 2018; Liu et al., 2018). Charging has instead become the pivotal. However, energy density and charging rate are not compatible for LIBs. To balance them, it is indispensable to exploit novel electrode materials with high ions/electrons dual-conductivity function. The low-cost Fe3O4 has always ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Jiang), [email protected] (C. Li).

received a lot of attention mainly due to better electrical conductivity and high theoretical specific capacity (926 mAh g 1) caused by the narrow band gap (0.1 eV) and the involved eight electrons transfer reactions (Lu et al., 2017; He et al., 2013; Liu et al., 2016; Xu et al., 2018; Lee et al., 2013). Nevertheless, the intrinsic semiconductor property and slow Li+ diffusion kinetics greatly limit its fast charging capacity. Additionally, the conspicuous volume change during continuous Li+ intercalation/deintercalation leads to serious structure deterioration, giving rise to rapid capacity fading. According to the diffusion equation of Li+ in electrode materials: s = Lion2/Dion, where s is the diffusion time, L is the diffusion length, and D is the diffusivity in the surface/bulk, the particle size has significant effects on their electrochemical performance (Zhu et al., 2017; Malik et al., 2010; Li et al., 2019; Tian et al., 2019). When

https://doi.org/10.1016/j.ces.2019.115342 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: D. Xu, K. Ma, L. Chen et al., MXene interlayer anchored Fe3O4 nanocrystals for ultrafast Li-ion batteries, Chemical Engineering Science, https://doi.org/10.1016/j.ces.2019.115342

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the size drops nanocrystal scale (<10 nm), the Li+ diffusion distance will be greatly shortened with rapid reaction kinetics (Li et al., 2019; Peng et al., 2018). The large specific surface area enables high exposure of active sites. These features are also beneficial for contributing abundant pseudocapacitance (Simon et al., 2014; Wang et al., 2017). Additionally, the small size of nanocrystals can effectively release structural stress change before and after Li+ insertion (Wang et al., 2013; Xu and Zhu, 2012; Xiong et al., 2012). However, the high surface energy and magnetic property easily cause the coalescence and aggregation of Fe3O4 nanocrystals. Coupling with high conductive substrates has been recognized as a promising strategy to restrain the above phenomenon, and meantime accelerate the electron transfer rate (Liu et al., 2016; Sathish et al., 2012; Chen et al., 2011). For example, Tour et al. reported the in situ growth of Fe3O4 nanocrystals on the surface of graphene nanoribbons, achieving a high lithium storage capacity of 926 mAh g 1 (Li et al., 2015). After 100 cycles, only 442 mAh g 1 was maintained owing to the agglomeration of Fe3O4 nanocrystals caused by the weak bonding force between the two components. Recently, in situ synchrotron X-ray absorption spectroscopy was applied to unveil pulverization mechanism of Fe3O4 based nanomaterials, discovering that strongly anchored Fe3O4 on conductive substrate is the pivotal to alleviating the capacity fading (Yun et al., 2019). However, it is highly desired to get a clue to realize the aforementioned interactions to withstand the volume effects during the repeated charge and discharge process. Two-dimensional (2D) Ti3C2 MXene has been extensively investigated for supercapacitors application (Yan et al., 2017; Hu et al., 2018). The rich surface functional groups, tunable interlayer distance and high electrical conductivity have inspired us to build novel Fe3O4@Ti3C2 hybrids, in which the Fe3O4 nanocrystals have been well-anchored between Ti3C2 interlayers by the unique 2D physical confinement and TiAOAFe covalent bonds effects. Such impressive hybrids can greatly strengthen the structural stability with an improved active sites exposure for each component. The formation of TiAOAFe covalent bonds induces charge polarization between them with electrons-enriched Fe atoms, accelerating the lithiation reaction and electrons transfer. Consequently, the asobtained Fe3O4@Ti3C2 hybrids deliver a very high reversible capacity of 1172 mAh g 1 at 0.1 A g 1 with superior charging capability (366 mAh g 1 in 66 s) and long cycle life (>90% capacity retention after 1000 cycles at 5 A g 1). More impressively, a thicknesstunable self-standing film can be also obtained by vacuum filtering the above hybrids, which demonstrates a superior areal capacity of 4.2 mAh cm 2 without sacrificing gravimetric capacity even at 4.4 mg cm 2.

2. Experimental section 2.1. Synthesis of the Fe3O4@Ti3C2 hybrids The Ti3C2 nanosheets were firstly obtained by etching of the Ti3AlC2 powders and the subsequent washing with deionized water. After ultrasonic dispersion and centrifugal separation, the Ti3C2 nanosheets colloidal solution was achieved by collecting the upper dark green supernatant. The polyetherimide (PEI)-modified Fe3O4 nanocrystals were obtained by dissolving the iron (Ⅱ) acetylacetonate (Fe(acac)2) and PEI in diethylene glycol (DEG) for a while, followed with washing and dispersion in deionized water. The PEImodified Fe3O4 nanocrystals suspension was then dropped into the Ti3C2 nanosheets colloidal solution under stirring for 1 h. The agglomerates were formed and settled down to the bottom, which were collected by removing the supernatant. After freeze-drying, the Fe3O4@Ti3C2 hybrids were obtained. The self-standing film was obtained by vacuum filtering the Fe3O4@Ti3C2 hybrids. As a

control, we also prepared the Fe3O4/Ti3C2 mixture by mechanically mixing the Ti3C2 nanosheets and the Fe3O4 nanocrystals. 2.2. Characterization The morphology and microstructure of samples were investigated via transmission electron microscope (TEM, JEOL-2100), field-emission scanning electron microscope (FESEM, Hitachi S4800), X-ray powder diffractometer (XRD, Bruker D8 Advance, Cu Ka radiation), X-ray photoelectron spectroscopy (XPS) spectra (AXIS Ultra DLD, Al Ka radiation). Nitrogen adsorption/desorption curves were tested by Brunauer-Emmett-Teller (BET) measurement by Micromeritics ASAP 2010 surface area analyzer. The Zeta potential distribution was tested by Zetasizer Nano (Malvern, ZEN3600). 2.3. Electrochemical Measurement: Electrochemical measurements were performed by using cointype 2016 cells. The working electrode was prepared by mixing the active materials, carbon black and poly(vinyl difluoride) (PVDF, 5%) with weight ratio of 8:1:1 and pasted on Cu foil. The loading mass of the active materials is around 1.0 mg cm 2. The selfstanding film is directly applied as the working electrode. The counter electrode is pure lithium foil and the separator was polypropylene membrane (Celgard 2400). The electrolyte is 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume) with 5 wt% fluoroethylene carbonate (FEC). The cells were assembled in an argon-filled glove box. Cyclic voltammogram (CV) experiments from 0.01 to 3 V and impedance spectra over the frequency from 100 kHz to 0.01 Hz were performed on an Autolab PGSTAT302N. Galvanostatic charge/discharge tests were carried out on LANDCT2001A battery tester at different current densities at room temperature. 3. Results and discussion The synthesis process of the Fe3O4@Ti3C2 hybrids is schematically illustrated in Fig. 1. The negatively charged Ti3C2 nanosheets colloidal solution is firstly obtained by selectively etching Al layers of Ti3AlC2 and the subsequent delamination and sonication dispersion. The Zeta potential value is about 28.6 mV (Fig. S1). Fig. S2 shows the morphology and thickness (about 1–2 nm). The positively charged PEI-modified Fe3O4 nanocrystals suspension (41.6 mV) was then dropped into Ti3C2 nanosheets colloidal solution. With the help of electrostatic interaction, the Fe3O4 nanocrystals have been well-attached on the surface of few-layer Ti3C2 nanosheets. The Zeta potential value of the solution has been changed into 25.2 mV. In such hybrids, the few-layered Ti3C2 nanosheets are reckoned to anchor the volume change of Fe3O4 nanocrystals during charge/discharge process with highly structural integrity, which also effectively refrains their agglomeration and restacking. Meantime, the strong covalent interaction (TiAOAFe) can not only further strengthen the structure, but also can be applied as rapid electrical transfer bridge. These advantages will endow the Fe3O4@Ti3C2 hybrids with high-rate and superior cycling performances for LIBs. Fig. 2(a) provides the SEM image of the Fe3O4@Ti3C2 hybrids. The ultrathin and flexible nanostructure has been wellmaintained without the Fe3O4 nanocrystals aggregation, implying their homogeneous dispersion. After ultrasonic treatment for about 40 min, the corresponding TEM image of the Fe3O4@Ti3C2 hybrids is shown in Fig. 2(b), revealing that the Fe3O4 nanocrystals are indeed highly distributed on the surface of few-layer Ti3C2 nanosheets with strong interaction between them. The size is in

Please cite this article as: D. Xu, K. Ma, L. Chen et al., MXene interlayer anchored Fe3O4 nanocrystals for ultrafast Li-ion batteries, Chemical Engineering Science, https://doi.org/10.1016/j.ces.2019.115342

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Fig. 1. Schematic illustration for the fabrication and the corresponding structural change after intercalating lithium of the Fe3O4@Ti3C2 hybrids.

Fig. 2. (a) SEM and (b) TEM images of the Fe3O4@Ti3C2 hybrids (inset showing the corresponding SAED pattern); (c) XRD patterns of the Fe3O4@Ti3C2 hybrids, the Fe3O4/Ti3C2 mixture, the Ti3C2 nanosheets and the Fe3O4 nanocrystals; (d) Nitrogen adsorption–desorption isotherms and (e–f) high-resolution XPS spectra of Ti 2p and Fe 2p for the Fe3O4@Ti3C2 hybrids and the Fe3O4/Ti3C2 mixture, respectively.

Please cite this article as: D. Xu, K. Ma, L. Chen et al., MXene interlayer anchored Fe3O4 nanocrystals for ultrafast Li-ion batteries, Chemical Engineering Science, https://doi.org/10.1016/j.ces.2019.115342

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range of 4.0–6.0 nm. The conspicuous diffraction ring (inset of Fig. 2(b)) confirms the high crystalline of Fe3O4 nanocrystals and the regular lattices are assigned to the layered Ti3C2 (Ma et al., 2018; Li et al., 2019). On the contrary, we can observe some separated Fe3O4 nanoparticles and even aggregations for the Fe3O4/ Ti3C2 mixture (Fig. S3). Fig. 2(c) gives the XRD patterns of the Fe3O4@Ti3C2 hybrids, the Fe3O4/Ti3C2 mixture, the Ti3C2 nanosheets and the Fe3O4 nanocrystals. All of them show a good crystallinity. For the Fe3O4@Ti3C2 hybrids, the diffraction peaks can be only indexed to the inverse spinel Fe3O4 and layered Ti3C2. In addition, we can see that the Ti3C2 (0 0 2) characteristic peak at 7.0° disappears while can be seen in the Fe3O4/Ti3C2 mixture, strongly indicating the Fe3O4 nanocrystals in the Ti3C2 interlayer effectively refrain the restacking of Ti3C2 nanosheets. This result can be further implied by their relatively higher Brunauer-Emmett-Teller (BET) specific surface area of 212.8 m2 g 1 compared to the Fe3O4/Ti3C2 mixture (121.4 m2 g 1), as shown in Fig. 2(d), which is also identified with the above observations. Meanwhile, the Fe3O4@Ti3C2 hybrids possess an obvious pore-size distribution of about 2–10 nm while a big pore about 30 nm for the Fe3O4/Ti3C2 mixture (Fig. S4). The XPS technique is used to further analyze the elemental compositions and electronic structure of the samples. The XPS survey spectra in Fig. S5 demonstrate both the Fe3O4@Ti3C2 hybrids and the Fe3O4/Ti3C2 mixture are mainly composed of Fe, O, Ti and C (Yan et al., 2017; Wang et al., 2018). In high-resolution Ti 2p spectra of Fig. 2(e), the Fe3O4@Ti3C2 hybrids exhibit a remarkably increased TiAOAFe covalent bond located at 458.9 eV compared with the Fe3O4/Ti3C2 mixture, verifying the strong interaction between them (Yan et al., 2017; Meng et al., 2018). Notably, Fig. 2(f) gives an increased Fe2+ (710.2 eV) content in the Fe3O4@Ti3C2 hybrids (Tan et al., 2019; Liu et al., 2018), indicating an obvious charge polarization with higher electron density, which is beneficial for accelerating the electrochemical reaction kinetic.

The optimization of the Fe3O4@Ti3C2 hybrids is conducted by changing the amount of the Fe3O4 nanocrystals suspension. The Fe3O4@Ti3C2 hybrids with 81.2% Fe3O4 content, estimated by the ICP-MS results, show a highest lithium storage capacity, as shown in Fig. S6. Further increasing the Fe3O4 content will lead to the agglomeration of partial Fe3O4 nanocrystals (Fig. S7). The cyclic voltammetry (CV) curves of the Fe3O4@Ti3C2 hybrids is offered in 0.01–3.0 V at 0.2 mV s 1 in Fig. 3(a). Only one peak at 0.48 V is observed in the first cathodic scan, which comes from a conversion reaction of Fe3O4 into Fe0 and Li2O with the irreversible solid electrolyte interface (SEI) (Yun et al., 2019; Zhang et al., 2015). In initial anodic scan, two peaks (1.65 and 1.83 V) correspond to the reversible conversion of Fe0 to Fe2+ and Fe3+ can be found (Zhou et al., 2013). Impressively, another peak at 1.0 V representing Li+ intercalation to form LixFe3O4 disappears but can be observed in the Fe3O4/Ti3C2 mixture in the subsequent cathodic scans (Fig. 3(b)) (Wang et al., 2018; Zhou et al., 2013). This phenomenon indicates the Li+ intercalation into Fe3O4 is negligible in the Fe3O4@Ti3C2 hybrids, instead by the interfacial storage mechanism with a capacitive-controlled behavior. Such charge storage behavior can significantly improve kinetic and durability of electrode materials. Additionally, the CV curves of the Fe3O4@Ti3C2 hybrids almost overlap in the subsequent cycles, implying the high reversibility. To further shed light on the Li+ storage mechanisms and the phase evolution of the Fe3O4@Ti3C2 hybrids, ex situ XRD measurements are performed at electrochemical states of the initial cycle (Fig. 3 (c) and (d)). The (3 1 1) peak of Fe3O4 at 35.4° gradually shifts to lower angles during discharge process, indicating the expansion of the lattice parameter of Fe3O4 during consecutive lithiation process. When the potential reaches to 0.9 V, the peak shifts to 35.1°, corresponding to the (3 1 1) peak of Li2Fe3O4. After further discharging to 0.5 V, the (3 1 1) peak disappears with the appearance of a new diffraction peak at 44.7° belonging to the (1 1 0) plane of Fe. During the charge process, the (1 1 0) peak of Fe disappears

Fig. 3. (a, b) The initial three CV curves at 0.2 mV s 1 of the Fe3O4@Ti3C2 hybrids and the Fe3O4/Ti3C2 mixture; (c, d) the initial charge and discharge curves and the corresponding ex-situ XRD patterns, (e) the ex situ Fe 2p3/2 XPS spectra at different charge/discharge states of the Fe3O4@Ti3C2 hybrids.

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gradually but the diffraction peak of Fe3O4 cannot be observed, indicating the metal Fe has been oxidized to Fe3O4 with amorphous structure. Ex situ XPS results in Fig. 3(e) further declare the lithiation reaction process of the Fe3O4@Ti3C2 hybrids. In the gradual discharge process, the pristine Fe3+ 2p3/2 (712.2 eV) disappears while Fe0 2p3/2 (706.8 eV) emerges (Tan et al., 2019). When the potential reaches to 0.01 V, it only shows the peak of Fe0 with the full conversion reaction, which is consistent with ex situ XRD analysis. During charging, the Fe2+ 2p3/2 peak appears at 1.6 V. After fully charging to 3.0 V, the XPS curve almost recovers the initial state, indicating the highly reversible conversion reaction. Fig. 4(a) provides the first three charge/discharge curves of the Fe3O4@Ti3C2 hybrids at 0.1 A g 1. The initial discharge and charge capacities are 1501 and 1065 mAh g 1 with Coulombic efficiency (CE) of 71%. As for the subsequent two cycles, the curves well overlap with the rapidly increased CE of 98%. In view of the strong coupling interaction between high power Ti3C2 nanosheets and high energy Fe3O4 nanocrystals, the Fe3O4@Ti3C2 hybrids exhibit higher capacity than the Fe3O4/Ti3C2 mixture with the same Fe3O4 content at various current densities, as shown in Fig. 4(b). Impressively, when the current density increases to 20 A g 1, the Fe3O4@Ti3C2 hybrids can still maintain a high stable capacity of 366 mAh g 1 at 20 A g 1, which is comparable to the theoretical value of the traditional graphite (372 mAh g 1). As the current density returns to 0.1 A g 1 again, a reversible capacity of 1172 mAh g 1 is achieved. The Fe3O4@Ti3C2 hybrids also possess a superior cyclic stability. A specific capacity of 1093 mAh g 1 for the Fe3O4@Ti3C2 hybrids is delivered at 0.2 A g 1 for 100 cycles without degradation (Fig. 4 (c)). As for the Fe3O4/Ti3C2 mixture, a low capacity of 455 mAh g 1 is delivered after 100 cycles. The microstructure of the Fe3O4@Ti3C2 hybrids after 100 cycles is also depicted in Fig. S8, giving a relatively good structure retention. No obvious aggregations can be observed. More impressively, the Fe3O4@Ti3C2 hybrids maintain a high capacity retention (>90%) even at 1.0 and 5.0 A g 1 for 1000 cycles (Fig. 4(d)). To our knowledge, the comprehensive LIBs performance is among the best reports of iron oxides-based hybrids

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so far, e.g. Fe3O4/conductive polymers (Shi et al., 2017) and Fe3O4/carbon nanofibers (Fu et al., 2015), and is much better than the Fe3O4/graphene hybrids (Zhao et al., 2015), the Fe3O4/CNTs hybrids (Du et al., 2017), and the hierarchical Fe3O4 hollow spheres (Wang et al., 2013). Table S1 summarized the detailed comparison by choosing some typical Fe3O4-based electrode materials. To clarify the excellent electrochemical performance, Fig. 5(a) and (b) provided the log i versus log v profiles of the both samples according to the law of i = avb, where i represents peak current, v is scanning rate and b value approaches to 0.5 or 1.0 representing diffusion or capacitance controlled processes, respectively (Chao et al., 2016; Muller et al., 2015; Liu et al., 2018). Fig. S9 provides the corresponding CV curves from 0.4 to 2.0 mV s 1. The b values of the Fe3O4@Ti3C2 hybrids for cathodic (0.80) and anodic (0.82) peaks are higher than the Fe3O4/Ti3C2 mixture (0.63 and 0.76), suggesting the Fe3O4@Ti3C2 hybrids are capacitance-dominated storage mechanism featured by fast reaction kinetics. The capacitive contribution can be quantificationally separated by the current separation equation of i = k1v + k2v1/2 developed by Dunn et al (Chao et al., 2016; Brezesinski et al., 2010). The capacitive contribution of the Fe3O4@Ti3C2 hybrids gradually increases to 80.3% with the scanning rate increasing from 0.4 to 2.0 mV s 1 (Fig. 5 (c)). Fig. 5(d) shows capacitive capacity contribution (grid area) to the total capacity of the Fe3O4@Ti3C2 hybrids at 0.4 mV s 1. In addition, the Nyquist plots (Fig. S10) for electrochemical impedance spectra (EIS) indicate that the charge transfer resistance of the Fe3O4@Ti3C2 hybrids (146.1 X) is lower than the Fe3O4/Ti3C2 mixture (213.7 X). These results indicate the Fe3O4@Ti3C2 hybrids possess high ion and electron conductivity, hence showing a superior rate capability. In view of the large size and good mechanical properties of Ti3C2 nanosheets, the self-standing Fe3O4@Ti3C2 films can be also obtained by vacuum filtering the Fe3O4@Ti3C2 hybrids (Fig. S11). The film thickness is linearly tunable in the range of 10.6– 36.0 lm with the weight from 1.3 to 4.4 mg cm 2 just by changing the amount of the hybrids. The surface conductivity is about 45.7 S

Fig. 4. (a) The initial three charge and discharge curves at 0.1 A g 1 for the Fe3O4@Ti3C2 hybrids; (b) rate performance at 0.1–20 A g 1 and (c) capacity retention at 0.2 A g for 100 cycles of the Fe3O4@Ti3C2 hybrids and the Fe3O4/Ti3C2 mixture; (d) capacity retention at 1 and 5.0 A g 1 of the Fe3O4@Ti3C2 hybrids for 1000 cycles.

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Please cite this article as: D. Xu, K. Ma, L. Chen et al., MXene interlayer anchored Fe3O4 nanocrystals for ultrafast Li-ion batteries, Chemical Engineering Science, https://doi.org/10.1016/j.ces.2019.115342

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Fig. 5. (a, b) The log(i) versus log(v) plots of cathodic and anodic peaks in CV curves for the Fe3O4@Ti3C2 hybrids and the Fe3O4/Ti3C2 mixture; (c) normalized contribution of the capacitive and diffusion-controlled capacities at different sweep rates; (d) a typical capacitive contribution (grid area) to the total capacity at 0.4 mV s 1 of the Fe3O4@Ti3C2 hybrids.

Fig. 6. Electrochemical performance of the self-standing film electrode for LIBs. (a) Rate performance at 0.2–6.4 mA cm 2 (inset showing the corresponding cross-section SEM image); (b) capacity retention for 500 cycles at 1.6 mA cm 2 (inset showing the corresponding CE); (c) gravimetric/areal specific capacities at 0.2 mA cm 2 versus the film weight, and their cycling stability for 100 cycles.

cm 1 at initial stage without a big change even after 200 times bending test, indicating the excellent mechanical strength (Fig. S12). The self-standing Fe3O4@Ti3C2 films can be directly

employed as the LIBs electrode. Fig. 6(a) provides the rate performance of the Fe3O4@Ti3C2 film at 1.3 mg cm 2, which delivers high reversible capacity of 982, 904, 807, 728, 627, and 560 mAh g 1 at

Please cite this article as: D. Xu, K. Ma, L. Chen et al., MXene interlayer anchored Fe3O4 nanocrystals for ultrafast Li-ion batteries, Chemical Engineering Science, https://doi.org/10.1016/j.ces.2019.115342

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0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 mA cm 2, respectively. Furthermore, the capacity of 692 mAh g 1 can still maintained after 500 cycles at 1.6 mA cm 2 with 95% capacity retention (Fig. 6(b)). The CE almost approaches to 100% during the cycles (inset of Fig. 6(b)). Fig. 6(c) shows the gravimetric/areal specific capacities at 0.2 mA cm 2 versus the film weight and their cycling stability for 100 cycles. It can be observed that the gravimetric capacities are almost constant with the increase of film weight while their areal capacities linearly increase from 1.3 to 4.2 mAh cm 2. In addition, all the films display superior stability with almost no capacity loss after 100 cycles. As shown in Fig. S13, the freestanding electrode also delivers an outstanding volumetric capacity (1226.4 mAh cm 3 at 1.3 mg cm 2) without obvious decrease even at 4.4 mg cm 2. The surface and cross-section morphologies at different lithiation states of the films with the weight of 2.5 mg cm 2 further verify the structural durability. As shown in the Fig. S14, there is no obvious fracture occurred even through 100 cycles. The thickness increases from 20.5 to 23.0 lm at the initial and first lithiation state. Even after 100 cycles, the film is almost intact without obvious thickness change, indicating a highly structural stability. Such excellent film electrode with stable and high gravimetric/areal capacities shows a big potential in lightweight electrical vehicles and other fields.

4. Conclusions In summary, the Fe3O4@Ti3C2 hybrids have been synthesized by a simple 2D confined reaction, in which the Fe3O4 nanocrystals are well-anchored between Ti3C2 interlayers with abundant active sites exposure for each component. The synergistic effects of physical confinement and TiAOAFe covalent bonds can well-address the dispersion and volume change of nanocrystals in the process of continuous charge/discharge. The lithiation reaction kinetics and electrons transfer are also accelerated by the charge polarization in the interface of the two components with the assistance of TiAOAFe covalent bonds. These advantages simultaneously enable the Fe3O4@Ti3C2 hybrids with rapid charging performance and long cycle life. Resultantly, they deliver quite high reversible capacities of 1172 mAh g 1 at 0.1 A g 1 and 366 mAh g 1 even at 20 A g 1 (charging for 66 s), respectively. After 1000 cycles at 5 A g 1, about 90% capacity retention can be achieved. Furthermore, the Fe3O4@Ti3C2 hybrids can be filtered into a free-standing film electrode, which delivers a relatively higher areal capacity of 4.2 mAh cm 2 at 4.4 mg cm 2 almost without sacrificing the gravimetric capacity. The lithium storage mechanism has also been revealed. This work provides a novel 2D confinement strategy that may be generally applicable to other nanomaterials toward high-performance energy storage devices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (21975074, 91534202, and 91834301), the Basic Research Program of Shanghai (17JC1402300), the Shanghai Scientific and Technological Innovation Project (18JC1410500), the National Program for Support of Top-Notch Young Professionals, and the Fundamental Research Funds for the Central Universities (222201718002).

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