15November2019

Nano-Micro Letters

Plate-to-layer Bi2MoO6/MXene Heterostructure as High Performance Anode Material for Lithium Ion Batteries

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Peng Zhang1, Danjun Wang1, 2, Qizhen Zhu1, *, Ning Sun1, Feng Fu2, *, Bin Xu1, *

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Nano-Micro Lett. (2019) 11: 81

First Online: 25 September 2019 (Communication)

DOI:10.1007/s40820-019-0312-y

*Corresponding author. E-mail: binxumail@163.com (B. Xu); Zhuqz@mail.buct.edu.cn (Q. Zhu); yadxfufeng@126.com (F. Fu)

 

Abstract

 


Toc

Bi2MoO6 is a potentially promising anode material for lithium ion batteries (LIBs) on account of its high theoretical capacity coupled with low desertion potential. Due to low conductivity and large volume expansion/contraction during charge/discharge cycling of Bi2MoO6, effective modification is indispensable to address these issues. In this study, a plate-to-layer Bi2MoO6/ Ti3C2Tx (MXene) heterostructure is proposed by electrostatic assembling positive-charged Bi2MoO6 nanoplates on negative-charged MXene nanosheets. MXene nanosheets in the heterostructure act as a highly conductive substrate to load and anchor the Bi2MoO6 nanoplates, so to improve electronic conductivity and structural stability. When the mass ratio of MXene is optimized to 30%, the Bi2MoO6/MXene heterostructure exhibits high specific capacities of 692 mAh g-1 at 100 mA g-1 after 200 cycles and 545.1 mAh g-1 with 99.6% coulombic efficiency at 1 A g-1 after 1000 cycles. The results provide not only a high-performance lithium storage material, but also an effective strategy that could address the intrinsic issues of various transition metal oxides by anchoring them on MXene nanosheets to form heterostructures and use as anode materials for LIBs.


 

Keywords

Bi2MoO6; MXene; Electrostatic self-assembly; Heterostructure; Lithium-ion batteries

 

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1 Introduction

    Lithium ion batteries (LIBs), with advantages of high energy density, environmental benignity, and no memory effect, have occupied unparalleled markets of electric vehicles and portable electronics [1-3]. However, the low theoretical capacity (372 mAh g-1) of the commercial graphite anode material actually limits the further development of LIBs due to the booming demand for higher energy density in applications, which urges researchers to explore high-performance anode materials for next-generation LIBs [4, 5]. On this account, a variety of transition metal oxides (TMOs) have been investigated as anode materials for LIBs because of their high theoretical capacity and moderate cost [6-8], e.g., SnO2 [9, 10], Fe3O4 [11], and Mn3O4 [12]. Nevertheless, owing to their alloying or conversion storage mechanism, the huge volume expansion/contraction of TMOs during lithiation/delithiation processes results in the instability of the electrode structure and the repeated cracking/forming of the solid electrolyte interphase (SEI), leading to the capacity fading together with continuous consumption of electrolyte [13-16]. For example, SnO2 can store up to 4.4 units of Li in one unit of Sn, endowing with a high theoretical capacity of 790 mAh g-1 as well as a volume charge of more than 200% [17]. Decreasing the particle size of TMOs to nano-scale coupled with morphological control (e.g., nanotubes [18], nanospheres [19], and nano-flowers [20]) provides a promising direction to improve the cycling stability of TMOs [16, 21, 22]. In addition, the intrinsic low electrical and ionic conductivity renders the TMOs with unacceptable rate capability, which also obstructs its application as electrode material of LIBs [6, 16, 21]. An efficient strategy to overcome this issue is to rationally design composites combining TMOs and highly conductive materials, such as graphene [23], carbon nanotubes [24], and amorphous carbon [25].
    Among various TMOs, bismuth molybdate (Bi2MoO6) with a structure of alternate [Bi2O2]2+ layers and [MoO4]2- perovskite layers can be potentially used as an anode material for LIBs due to its high theoretical capacity (791 mAh g-1) as well as low desertion potential (<1.0 V) [26, 27]. Up to date, most reports about Bi2MoO6 focused on its photocatalyst properties, while its application in LIBs, which has great significance, still needs further development. When used as anode material, the main restricts of Bi2MoO6 are similar to those of other TMOs, i.e., the large volume change during lithiation/delithiation and the intrinsic low conductivity. Thus, the strategies of rational design on the structure and combination with conductive materials can also be employed for improving electrochemical performances of Bi2MoO6 electrode material [27-31]. For example, Zhai et al. [28] reported that a Bi2MoO6/reduced graphene oxide (rGO) composite based on the in-situ growth of Bi2MoO6 on the rGO substrate, in which the rGO provides high conductivity and is beneficial for exposing the active sites and alleviating the volume change of Bi2MoO6.
    More recently, two-dimensional (2D) transition metal carbides and nitrides known as MXenes have been widely studied because of their high metallic conductivity, tailorable surface chemistries, and mechanical flexibility [32-35]. Typically, MXenes are synthesized by selectively etching the A layer (group IIIA or group IVA element) from the ternary precursors known as MAX phases and have a general formula of Mn+1XnTx, where M is an early transition metal; X stands for C and/or N; n=1, 2, or 3; Tx represents the surface functional groups, such as -O, -F, and -OH [36, 37]. According to the calculation result, Ti3C2Tx, one of the most studied MXenes, possesses lower lithium diffusion barrier (~0.07 eV) compared with that of the graphite carbons (~0.3 eV), indicating faster Li+ transport and higher lithiation/delithiation rate than the commercial graphite anode [38, 39]. In this regard, Ti3C2Tx MXene could be an ideal substrate to combine with various TMOs (e.g., SnO2 [40, 41], Fe3O4 [42], Sb2O3 [43], LiMn2O4 [44]) to fabricate high performance electrode materials for LIBs. For example, Zhao et al. [45] fabricated a flexible Ti3C2Tx/NiCo2O4 hybrid film via an in-situ growth method. As the Ti3C2Tx facilitated fast ion transport and electron transfer, the electrochemical performance of the film electrode was effectively enhanced. Therefore, integrating with the highly conductive Ti3C2Tx MXene is expected to significantly improve the lithium storage performances of Bi2MoO6.
    In this work, for the first time, Bi2MoO6/MXene (Ti3C2Tx) composites with a plate-to-layer heterostructure have been fabricated through a simple electrostatic self-assembly followed by freeze-drying method. Based on the electrostatic interaction, the positive-charged Bi2MoO6 nanoplates are uniformly anchored on the surface of the negative-charged MXene nanosheets, leading to a synergistic effect between the Bi2MoO6 nanoplates and the MXene nanosheets with several merits: (i) The Bi2MoO6 nanoplates distributed on the MXene can sufficiently expose the active sites for high capacity and simultaneously serve as spacers to prevent the MXene nanosheets from restacking, so could greatly enhance the ion accessibility. (ii) The MXene can effectively prevent the Bi2MoO6 nanoplates from aggregation and alleviate their huge volume change in lithiation/delithiation processes, effectively avoiding the loss of the active sites and ensuring the cycle stability. (iii) The MXene nanosheets contact closely with the Bi2MoO6 nanoplates, which can enhance the electronic conductivity and facilitate Li+ diffusion, in favor of the rate performance. Consequently, acting as an anode material for LIBs, the Bi2MoO6/MXene heterostructures exhibit high capacity, superior rate capability, and excellent cycling stability, demonstrating not only a promising LIBs anode material, but also the effectiveness of MXene substrate in place to enhance the electrochemical performance of TMOs.

2 Experimental

2.1 Materials Syntheses
2.1.1 Synthesis of Ti3C2Tx MXene
    The MXene nanosheets were obtained as reported previously [40, 46, 47]. Typically, 0.99 g of lithium fluoride (LiF) was added to 10 mL of 12 M hydrochloric acid (HCl) under stirring for dissolving. Then, 1 g of Ti3AlC2 powder (400 mesh, purchased from 11 Technology Co. Ltd) was slowly added to the above solution. The mixture was stirred at 35 °C for 24 h to ensure the complete etching. After that, the residue was washed with deionized water for several times until the pH of the supernatant is above 6. The precipitate was then re-dispersed in deionized water followed by sonication for 1 h under Ar atmosphere. After centrifugation at 5200 rpm for 1 h, the supernatant was collected as the MXene aqueous solution. In order to determine the concentration of the MXene aqueous solution, 5 mL of MXene aqueous solution was filtered and vacuum dried to obtain a pristine MXene film. After weighting the film, the concentration was obtained. The concentration of MXene aqueous solution was diluted to 2 mg mL-1.
2.1.2 Synthesis of the Bi2MoO6 Nanoplates
    The Bi2MoO6 nanoplates were synthesized as reported previously [48]. Typically, 2 mmol of Bi(NO3)3·5H2O was added to 10 mL of HNO3, while 0.143 mmol of (NH4)6Mo7O24·4H2O was dissolved in 10 mL of deionized water, respectively. After stirring for 30 min, the Bi(NO3)3·5H2O solution was added to the (NH4)6Mo7O24·4H2O solution dropwise under stirring for 60 min. Once approached to pH neutral (adjusted by ammonium hydroxide), the mixture was transferred into a Teflon-lined stainless steel autoclave and then heating at 180 °C for 12 h. Subsequently, the product was washed with deionized water and absolute ethanol for several times and dried under vacuum to obtain the pristine Bi2MoO6 nanoplates.
2.1.3 Synthesis of the Bi2MoO6/MXene Heterostructures
    The composite Bi2MoO6/MXene heterostructures were prepared via an electrostatic self-assembly method. The positive-charged Bi2MoO6 nanoplates was added to deionized water and sonicated for 30 min to form the Bi2MoO6 suspension (1 mg mL-1). Then, the Bi2MoO6 suspension was mixed with the MXene solution (1 mg mL-1) under continuous stirring for 30 min with various ratios. The mixture was freeze-dried for 48 h to obtain the Bi2MoO6/MXene heterostructures (denoted as Bi2MoO6/MXene-x,where x stands for the mass ratio of MXene in the heterostructures).
2.2 Materials Characterization
    The morphology of the prepared Bi2MoO6, MXene nanosheets, and Bi2MoO6/MXene heterostructures were observed through scanning electron microscope (SEM, Hitachi S4800), transmission electron microscope (TEM, Hitachi HT7700), and scanning transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping (STEM, Hitachi HT7700). Zeta potentials were measured by a Marlvern laser particle size analyzer (ZS980). XRD patterns were performed using X’Pert-Pro MPD (PANalytical, the Netherlands) diffractometer with monochromatic Cu Ka radiation (λ =1.5418Å, with scan speed of 4° min-1). Raman spectra were conducted through Raman spectrometer (Renishaw 1000) with a 1 mW He-Ne laser (633nm) as an irradiation source. XPS analysis was performed by ESCALAB 250 (ThermoFisher Scientific, USA). AFM image was collected using atomic force microscope (Dimension ICON).
2.3 Electrochemical Measurements
    All the electrochemical measurements were tested by assembling CR2025 coin-type cells in Ar-filled glove box at room temperature. The working electrodes were fabricated by mixing active materials (Bi2MoO6/MXene heterostructures or Bi2MoO6) conductive agent (Super-P) and binder (carboxymethylcellulose sodium, CMC) in deionized water with a mass ratio of 70:20:10 followed by coating the mixed slurry onto copper foil. The mass loading of the active materials on the current collector were fixed to 0.8 mg cm-2 in order to standardize the test results. Lithium foil, Celgard 3500 membrane, and 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) with an addition of 5 wt% fluoroethylene carbonate (FEC) were employed as counter electrode, separator, and electrolyte for LIBs, respectively. The amount of electrolyte used in each coin cell is 120 μL. The galvanostatic charge/discharge tests were carried out on Land BT2000 battery tester (Wuhan, China) in the voltage range of 0.01-3 V. Cyclic voltammetry (CV) measurements were performed on the VSP electrochemical workstation (Bio-Logic, France) with a potential window of 0.01-3 V at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) was tested in the frequency range of 100 kHz to 0.1 Hz with amplitude of 10 mV. The galvanostatic intermittent titration technique (GITT) was carried out with current pulse (100 mA g-1 for LIBs) for 30 min followed by 1 h relaxation on a Land BT2000 battery tester (Wuhan, China). The diffusion coefficients were calculated from the GITT potential profiles according to the Fick’s second law with the following Eq. 1:

11 81 gs1

where  stands for the duration of current pulse; mB represents the mass of active material in the pole; VM and MB is the molar volume and the molar mass of the active materials, respectively; S is the geometric area of the electrode; means the potential variation during the current pulse;  represents the quasi-thermodynamic equilibrium potential variation before and after the current pulse.

3 Results and Discussion

    Figure 1 illustrates the synthetic process for the Bi2MoO6/MXene heterostructure. Firstly, the delaminated Ti3C2Tx MXene nanosheets were produced by etching aluminum (Al) layers from Ti3AlC2 precursor with a solution of LiF and HCl, followed by sonication and the subsequent centrifugation. As shown in Fig. S1, the SEM image indicates the Ti3AlC2 MAX phase has a layered structure, and the XRD pattern is consistent with that of the previous reports [32, 33]. Single or few-layered MXene nanosheets can be readily obtained through such a mild delamination route with LiF/HCl, which is conducive to loading the Bi2MoO6 nanoplates. The prepared MXene have a lateral size of 1-3 μm (Fig. 2a) with a thickness below 1.83 nm, indicating 1 or 2 layers on a substrate (Fig. S2). The Bi2MoO6 nanoplates were synthesized from Bi(NO3)3·5H2O and (NH4)6Mo7O24·4H2O via a hydrothermal method, leading to the presence of amino group on the surface of Bi2MoO6 nanoplates and thus positively charged. The synthesized Bi2MoO6 displays irregular plate-like morphology (Figs. 2b, S3a, b) with a lattice spacing of 0.277 nm, corresponding to the (200) plane of orthorhombic Bi2MoO6 (Fig. S3c) [31]. Its structure was further confirmed by the EDS mappings, which show a rough proportion of Bi, Mo, and O elements, coupled with the uniform distribution (Fig. S4). The Bi2MoO6 crystal structure intrinsically consisting of alternate [Bi2O2]2+ layers and [MoO4]2- perovskite layers offer open and stable channels for Li+ insertion/extraction, endowing Bi2MoO6 with greatly improved rate capability compared with other TMOs. However, the intrinsic low conductivity and the large volume change during lithiation/delithiation of Bi2MoO6 still requires the modification with conductive materials [28, 30]. The assembly of the Bi2MoO6 nanoplates with the Ti3C2Tx MXene nanosheets was achieved by adding the Bi2MoO6 dispersion into the MXene solution and mixing thoroughly under continuous stirring. In order to confirm the successful electrostatic self-assembly of the Bi2MoO6/MXene heterostructure, the zeta potentials of the as-prepared heterostructures coupled with the two components, i.e., the pristine Ti3C2Tx nanosheets and the Bi2MoO6 nanoplates, were measured. As shown in Fig. S5, the as-prepared MXene nanosheets are negatively charged with a zeta potential of -40.5 mV, which is ascribed to the presence of surface functional groups (e.g., -O, -F, and -OH) [32, 33]. When the positive-charged Bi2MoO6 (10.1 mV) mixed with the negative-charged MXene, Bi2MoO6 nanoplates were anchored on the MXene nanosheets based on an electrostatic interaction. Therefore, the zeta potentials of the Bi2MoO6/MXene heterostructures shift down with the increase of the MXene content, demonstrating the effective electrostatic process. In the assembled heterostructure, the highly conductive MXene nanosheets are expected to significantly enhance the conductivity and alleviate the volume expansion/contraction of Bi2MoO6 during lithiation/delithiation, thus improving the rate performance and cycling stability.

Fig. 1 Schematic diagram for the simple electrostatic self-assembly of positive-charged Bi2MoO6 nanoplates on the negative-charged MXene nanosheets

Fig. 1 Schematic diagram for the simple electrostatic self-assembly of positive-charged Bi2MoO6 nanoplates on the negative-charged MXene nanosheets

    The morphologies of the Bi2MoO6/MXene heterostructures are shown in Fig. 2c-h. The composite samples were named with the mass ratio of Bi2MoO6/MXene: 50:50 (Bi2MoO6/MXene-50%, Fig. 2c, d), 70:30 (Bi2MoO6/MXene-30%, Fig. 2e, f), and 90:10 (Bi2MoO6/MXene-10%, Fig. 2g, h). It is apparent that with increasing Bi2MoO6 content, more Bi2MoO6 nanoplates cover on the MXene nanosheets. For the Bi2MoO6/MXene-50%,the Bi2MoO6 nanoplates are sparsely wrapped with the MXene nanosheets (Fig. 2c,  d). When the Bi2MoO6/MXene mass ratio increases to 70:30, the uniform and dense distribution of Bi2MoO6 is observed, almost covering the entire surface of the MXene nanosheets (Fig. 2e, f), implying an optimized mass ratio. As the Bi2MoO6 content continues to rise to 90 wt%, the Bi2MoO6 nanoplates evidently aggregate, as the insufficient surface area of the MXene nanosheets cannot load and anchor so many Bi2MoO6 nanoplates (Fig. 2g, h). TEM images were employed to further observe the structures of the Bi2MoO6/MXene heterostructures. It can be seen that the Bi2MoO6 nanoplates anchored on the MXene nanosheets have an irregular structure. The Bi2MoO6/MXene-50% (Fig. 3a, b), Bi2MoO6/MXene-30% (Fig. 3c, d) and Bi2MoO6/MXene-10% (Fig. 3e, f) display sparse, moderate and dense distribution of Bi2MoO6 on the MXene nanosheets, respectively, in accordance with the corresponding SEM images. The STEM and corresponding element mappings (Fig. 3f) reveal similar images of Bi, Mo, Ti, and C elements, demonstrating the homogeneous distribution of Bi2MoO6 in the composite Bi2MoO6/MXene-30%.

Fig. 2 SEM images of a MXene nanosheets, b Bi2MoO6 nanoplates, c, d Bi2MoO6/MXene-50%, e, f Bi2MoO6/MXene-30%, and g, h Bi2MoO6/MXene-10%

Fig. 2 SEM images of a MXene nanosheets, b Bi2MoO6 nanoplates, c, d Bi2MoO6/MXene-50%, e, f Bi2MoO6/MXene-30%, and g, h Bi2MoO6/MXene-10%

Fig. 3 TEM images of a, b Bi2MoO6/MXene-50%, c, d Bi2MoO6/MXene-30%, and e, f Bi2MoO6/MXene-10%. g STEM and corresponding element (Bi, Mo, Ti, and C) mapping images of the Bi2MoO6/MXene-30%

Fig. 3 TEM images of a, b Bi2MoO6/MXene-50%, c, d Bi2MoO6/MXene-30%, and e, f Bi2MoO6/MXene-10%. g STEM and corresponding element (Bi, Mo, Ti, and C) mapping images of the Bi2MoO6/MXene-30%

    XRD analysis was conducted to identify the composition of the samples, as shown in Fig. 4a. The XRD pattern of the as-prepared Bi2MoO6 nanoplates consists of strong (020) peak at 10.9° and (131) peak at 28.2° as well as several weak peaks corresponding to the (200), (060), (151), (202), and (062) planes of pristine Bi2MoO6 [50, 51]. After the assembly process, the (002) peak of MXene occurs in the patterns of the composites, indicating the successful combination of Bi2MoO6 and MXene. Moreover, Fig. 4b displays the regional XRD profiles of the Bi2MoO6/MXene-10%, Bi2MoO6/MXene-30%, Bi2MoO6/MXene-50%, and pristine MXene, which is marked by a purple frame in Fig. 4a. The (002) peak of MXene down-shifts as the mass ratio of the MXene decreases in the composites, which is attributed to the gradually increased lattice spacing of MXene expanded by the Bi2MoO6 nanoplates. The structures of the Bi2MoO6/MXene heterostructures were also characterized by Raman spectroscopy, and the results were shown in Fig. 4c. The pristine Bi2MoO6 displays distinct phonon modes in the range of 200-1000 cm-1, signifying the vibrational modes of orthorhombic Bi2MoO6. Specifically, the bands located at 202, 285, 324, 355, and 404 cm-1 can be ascribed to the bending, wagging, and twisting modes of Mo-O bonds, while the bands at 713, 796, and 848 cm-1 correspond to the stretching modes of Mo-O bond [28, 31]. Notably, besides the band situated at 621 cm-14) corresponding to Eg in-plane vibration of surface functional group atoms, the bands of the pristine MXene show similar locations to the Bi2MoO6, including the bands located at 193 cm-12) and 711 cm-13) for A 1g symmetry out-plane vibrations of Ti and C atoms, and those at 284 cm-15) and 356 cm-15) for Eg in-plane vibration of Ti and C atoms [52, 53]. The Bi2MoO6/MXene heterostructures display all the characteristic peaks belonging to Bi2MoO6 (e.g., 713, 796, and 848 cm-1) and MXene (621 cm-1), and the increase of MXene content in the heterostructures leads to the stronger peaks for MXene together with the weaker peaks for Bi2MoO6.

Fig. 4 Characterization of Bi2MoO6/MXene heterostructures: a, b XRD patterns, c Raman speatra of the Bi2MoO6/MXene heterostructures. High-resolution of d Bi 4f, e Mo 3d, f O 1s, and g Ti 2p XPS spectrum of Bi2MoO6/MXene-30%

Fig. 4 Characterization of Bi2MoO6/MXene heterostructures: a, b XRD patterns, c Raman speatra of the Bi2MoO6/MXene heterostructures. High-resolution of d Bi 4f, e Mo 3d, f O 1s, and g Ti 2p XPS spectrum of Bi2MoO6/MXene-30%

    XPS was carried out to evaluate the surface chemical properties of the Bi2MoO6 nanoplates, pristine MXene, and Bi2MoO6/MXene-30%. The XPS survey of the Bi2MoO6/MXene-30% comprises the characteristic peaks of Bi2MoO6 and MXene, such as Ti 2p peak for Ti3C2Tx as well as Bi 4f and Mo 3d peaks for Bi2MoO6 (Fig. S6), confirming the strong integration between the Bi2MoO6 nanoplates and the MXene nanosheets. High-resolution XPS spectra of Bi 4f and Mo 3d core levels of Bi2MoO6/MXene-30% and Bi2MoO6 are shown in Fig. 4d, e. The Bi 4f core level of Bi2MoO6/MXene-30% displays two peaks at 159.4 and 164.7 eV, related to Bi 4f7/2 and Bi 4f5/2 of the Bi3+, respectively. Meanwhile, the Mo 3d core level of Bi2MoO6/MXene-30% could be divided into two peaks situated at 232.8 and 235.9 eV, which can be assigned to Mo 3d5/2 and Mo 3d3/2 of Mo6+, respectively. It is noteworthy that the peaks of Bi 4f and Mo 3d core levels of Bi2MoO6/MXene-30% have higher binding energies than those of the Bi2MoO6, implying that the Bi2MoO6 nanoplates become more electrochemically active with the assistance of MXene. As shown in Fig. 4f, the O 1s core level of Bi2MoO6 is fitted with two components centered at 529.5 and 530.3 eV, which could be ascribed to Bi-O and Mo-O bonds, respectively [30, 47, 48]. Different from the Bi2MoO6, the Bi2MoO6/MXene-30% shows an additional peak at 531.6 eV corresponding to Ti-OH bond due to the presence of the MXene. Ti 2p XPS spectra were also showed in Fig. 4g to identify the chemical composition of the MXene. The Ti 2p core level was fitted with seven components, including three doublets (Ti 2p3/2-Ti 2p1/2) and a single peak located at 460.8 eV. The three doublets centered at 454.7/462.2, 455.9/464.4, and 458.7/466.1 eV could be assigned to Ti-C, Ti(II), and Ti-O, respectively, while the single peak might result from TiO2, implying the partial oxidation of the MXene during the construction of the Bi2MoO6/MXene heterostructure [40, 50].

Fig. 5 Electrochemical performance of the Bi2MoO6/MXene electrodes: a CV curves of Bi2MoO6/MXene-30% for the first three cycles at 0.1 mV s-1; b charge/discharge profiles of Bi2MoO6/MXene-30% at 100 mA g-1 at different cycles; c cycling performance of Bi2MoO6/MXene-50%, Bi2MoO6/MXene-30%, Bi2MoO6/MXene-10%, and pristine Bi2MoO6 electrodes at 100 mA g-1 for 200 cycles; d charge/discharge profiles of Bi2MoO6/MXene-30% at different current rates; e comparison of rate capabilities of Bi2MoO6/MXene-30% at various current rates from 50 to 2000 mA g-1; f long-term cycling performance of Bi2MoO6/MXene-30% in 1000 cycles at 1 A g-1

Fig. 5 Electrochemical performance of the Bi2MoO6/MXene electrodes: a CV curves of Bi2MoO6/MXene-30% for the first three cycles at 0.1 mV s-1; b charge/discharge profiles of Bi2MoO6/MXene-30% at 100 mA g-1 at different cycles; c cycling performance of Bi2MoO6/MXene-50%, Bi2MoO6/MXene-30%, Bi2MoO6/MXene-10%, and pristine Bi2MoO6 electrodes at 100 mA g-1 for 200 cycles; d charge/discharge profiles of Bi2MoO6/MXene-30% at different current rates; e comparison of rate capabilities of Bi2MoO6/MXene-30% at various current rates from 50 to 2000 mA g-1; f long-term cycling performance of Bi2MoO6/MXene-30% in 1000 cycles at 1 A g-1

    The lithium ion storage behaviours of the Bi2MoO6/MXene composite electrodes and the Bi2MoO6 electrode were first explored by cyclic voltammetry (CV) for the first three cycles at a scan rate of 0.1 mV s-1 within a voltage range of 0.01-3 V, as shown in Figs. 5a and S7. All the electrodes show irreversible peaks at around 1.65, 1.26, and 0.58 V in the first cathodic scan, which could be attributed to the insertion of Li+ into layer-structure of the Bi2MoO6 crystal, the irreversible transformation from Bi2MoO6 to Bi and Mo metal (Eq. 2), the electrolyte decomposition and formation of solid electrolyte interface (SEI) coupled with the alloying transformation from Bi to Li3Bi, respectively [29-31]. Especially, the board peak at 0.58 V was subsequently split into several minor peaks, corresponding to the lithiation process of Bi to LiBi (Eq. 3) and Li3Bi (Eq. 4). In the anodic scan, a strong peak at 0.98 V was observed which is caused by the de-alloying reaction of Li3Bi. Besides, several board peaks at 1.31, 1.63, and 2.36 V are related to the oxidation of Mo (Eq. 5) and Bi (Eq. 6), respectively [28, 31]. The electrochemical reactions of the Bi2MoO6 are listed as follows:

11 81 gs26

    The galvanostatic charge/discharge curves of the electrodes were conducted at a current density of 100 mA g-1 in the voltage range of 0.01-3 V (Fig. 5b and S8), which show the electrochemical behaviours in accordance with the CV curves. The initial charge and discharge capacity of Bi2MoO6/MXene-30% is 615.5 and 844.2 mAh g-1, respectively, leading to an initial coulombic efficiency (ICE) of 72.9%. The capacity loss in the first cycle might be attributed to the formation of SEI layer coupled with the irreversible reactions as mentioned above [31, 54, 55]. The ICE of Bi2MoO6/MXene-30% (72.9%) is lower than those of Bi2MoO6/MXene-10% (76.6%) and Bi2MoO6 (82.6%), but higher than that of Bi2MoO6/MXene-50% (71.2%). The possible reason for this phenomenon is that with higher MXene content, the increased surface area of MXene results in more irreversible reactions between Li+ and the functional groups on the MXene surface, such as -O, -F, and -OH [47, 56].
    The comparison of the cycling performance of the Bi2MoO6/MXene composite electrodes and the Bi2MoO6 electrode is given in Fig. 5c at a current density of 100 mA g-1. The Bi2MoO6/MXene-30% delivers a stable capacity of 692 mAh g-1 after 200 cycles, much higher than those of Bi2MoO6/MXene-50% (617.5 mAh g-1), Bi2MoO6/MXene-10% (497.6 mAh g-1), and the pristine Bi2MoO6 (416.1 mAh g-1). It indicates that the optimized ratio of the composite is beneficial for maximizing the lithium storage capacity of the Bi2MoO6/MXene. Furthermore, a capacity fading occurs in the initial dozens of cycles followed by a capacity reactivation process and an eventual capacity stabilization. The capacity fading results from the mechanical degradation of the electrode structure and the formation of an unstable SEI layer, while the subsequent capacity reactivation and stabilization could be ascribed to a restructuring process as well as stable SEI formation with cycling [57].
    Figures 5d and S9 show the charge/discharge curves of Bi2MoO6/MXene-30% and the pristine Bi2MoO6 at various current densities from 0.05 to 2 A g-1. As seen, the pristine Bi2MoO6 has a slightly higher capacity of 629.4 mAh g-1 at the small current density while it could only deliver capacities of 375.6, 323.7, and 279.8 mAh g-1 at 0.5, 1, and 2 A g-1, respectively. By contrast, the specific capacity of Bi2MoO6/MXene-30% is measured to be 626.5, 580.6, 522.1, 477, 414.8, and 328.2 mAh g-1 at 0.05, 0.1, 0.2, 0.5, 1, and 2 A g-1, respectively. It demonstrates that the conductive MXene effectively enhance the rate performance of the composite electrode. As displayed in Fig. 5e, the Bi2MoO6/MXene-30% exhibits the best rate capability from 0.05 to 2 A g-1 compared with the Bi2MoO6/MXene-50%, Bi2MoO6/MXene-10% and Bi2MoO6 electrodes. When the current density is turned back from 2 to 0.1 A g-1, the specific capacity of Bi2MoO6/MXene-30% are recovered up to 566.3 mAh g-1, signifying the best reversibility and structural stability compared with other electrodes. Furthermore, to estimate the long-term cycling stability, Bi2MoO6/MXene-30% was charged/discharged for 1000 cycles at a high current density of 1 A g-1 (Fig. 5f). After the first three cycles at 0.1 A g-1 for activation, the Bi2MoO6/MXene-30% exhibits a reversible capacity of 507.2 mAh g-1 at the fifth cycle and retains a capacity of 545.1 mAh g-1 with 99.6% coulombic efficiency after 1000 cycles. The delivered capacity of Bi2MoO6/MXene-30% gradually increases with cycling, which is caused by the restructuring process and the subsequent formation of a stable structure during repeated lithiation/delithiation [57]. The SEM images of Bi2MoO6/MXene-30% before and after 1000 cycles at 1 A g-1 were explored to show the structural stability of the electrode as displayed in Fig. S10. It could be clearly seen that the Bi2MoO6 were wrapped by the MXene nanosheets, which are consistent with the SEM images of the Bi2MoO6/MXene-30% materials (Fig. 2e, f). After 1000 charge/discharge cycles, the surface of the electrode turns rough but no obvious crack was observed, indicating the excellent structural stability of the Bi2MoO6/MXene-30% electrode during lithiation/delithiation.
    To study the origins of the better rate capability of Bi2MoO6/MXene-30%, the lithium diffusion coefficients of Bi2MoO6/MXene-30% and Bi2MoO6 were calculated by GITT according to Eq. 1, as shown in Figs. 6a, b and S10. Compared with the Bi2MoO6, Bi2MoO6/MXene-30% exhibits lower overpotential and higher diffusion coefficients during the lithiation/delithiation process, implying better reaction kinetics [58, 59]. The enhanced reaction kinetics of Bi2MoO6/MXene-30% can be associated with the decoration of the highly conductive MXene nanosheets, which support and contact with the Bi2MoO6 nanoplates sufficiently to efficiently improve the charge transport. Additionally, the EIS spectra show that the Bi2MoO6/MXene-30% has the lowest Rct resistance of 105.9 Ω and Rs resistance of 3.08 Ω among these four samples, verifying its enhanced reaction kinetics for lithium ion storage (Fig. S12a,  b, Table S1). The impedance behaviors of the Bi2MoO6/MXene-30% and Bi2MoO6 electrodes were explored by the complex model of capacity to confirm the promotion of MXene for rapid diffusion and transportation of lithium ions (Fig. S12c, d). After the decoration of MXene, the Bi2MoO6/MXene-30% displays much lower minimal characteristic relaxation time constant τ0 (919 ms) and downtrend of normalized C′(ω) than the pristine Bi2MoO6 electrodes, implying faster transport and diffusion of electrolyte ions in the Bi2MoO6/MXene-30% electrode [52].
    CV measurements were performed to unravel the charge storage kinetics of Bi2MoO6/MXene-30%. Figure 6a depicts the CV curves of Bi2MoO6/MXene-30% at various scan rates from 0.2 to 3 mV s-1. The charge storage mechanism could be analysed by the following formula, which shows the relationship between the measured current (i) and the scan rate (v):

11 81 gs7

where b could be calculated from the slope of the fitted log(i)-log(v) curves [60, 61], distinguishing the electrochemical behaviour as a diffusion-controlled process (b=0.5) or a non-diffusion-controlled behaviour (b=1). As shown in Fig. 6d, the b value of Bi2MoO6/MXene-30% is 0.8987 at 0.58 V in the cathodic scan and 0.8662 at 0.97 V in the anodic scan, indicating a fast charge storage kinetic dominated by pseudocapacitive behaviour. This behaviour can also be confirmed by quantifying the pseudocapacitive contribution based on Eq. 8:

11 81 gs8

where i(v), k1v, k2v1/2 and v is the measured current at a certain potential, the capacitive-dominated current, the diffusion-controlled current and the corresponding scan rate, respectively. The capacitive-dominated current at a certain scan rate could be obtained via calculating the value of k1 [60, 61]. Figures 6e and S13 depict the CV curves of Bi2MoO6/MXene-30% at various scan rates, in which the shaded portion stands for the capacitive-dominated region, while the non-shaped portion means the diffusion-controlled region. It can be seen that up to 72.8% of the charge is contributed by the pseudocapacitive behaviour at 1 mV s-1. Moreover, the pseudocapacitive contribution of the Bi2MoO6/MXene-30% electrode enhances with the scan rate increasing (Fig. 6f). The capacitive-dominated mechanism coupled with the highly conductive MXene can offer ultrafast lithium ion storage, endowing Bi2MoO6/MXene-30% with enhanced rate capability and cycling stability.

Fig. 6 Electrochemical kinetic analysis of Li storage behavior of Bi2MoO6/MXene-30%. a GITT profiles (current pulse at 100 mA g-1 for 30 min followed by 1 h relaxation); b diffusion coefficients calculated from GITT profiles according to overpotential; c CV curves at various scan rates from 0.2 to 3 mV s-1 in the voltage range of 0.01-3 V (vs. Li+/Li); d log(i)-log(v) curves; e CV profile measured at 1 mV s-1 with shaded area displaying the pseudocapacitive contribution; f normalized proportions of capacitive and diffusion-controlled contribution at various scan rates

Fig. 6 Electrochemical kinetic analysis of Li storage behavior of Bi2MoO6/MXene-30%. a GITT profiles (current pulse at 100 mA g-1 for 30 min followed by 1 h relaxation); b diffusion coefficients calculated from GITT profiles according to overpotential; c CV curves at various scan rates from 0.2 to 3 mV s-1 in the voltage range of 0.01-3 V (vs. Li+/Li); d log(i)-log(v) curves; e CV profile measured at 1 mV s-1 with shaded area displaying the pseudocapacitive contribution; f normalized proportions of capacitive and diffusion-controlled contribution at various scan rates

    The above results suggest that the lithium storage property of Bi2MoO6 could be effectively enhanced by introducing highly conductive MXene as a substrate to fabricate a plate-to-layer Bi2MoO6/MXene heterostructure. In the heterostructure, the MXene nanosheets can promote the charge transport and alleviate the volume change of the Bi2MoO6, leading to high specific capacity, superior rate capability and excellent long-term cycling stability. The Bi2MoO6/MXene-30% exhibits competitive performance compared with other TMOs-based electrodes, indicating its promising potential as an anode material of LIBs.

4 Conclusions

    To solve the problems of Bi2MoO6 as an electrode material for LIBs, i.e., low electronic conductivity and huge volume change, we have fabricated Bi2MoO6 nanoplates on highly conductive Ti3C2Tx MXene nanosheets to form a plate-to-layer heterostructure via a simple electrostatic self-assembled method. In the Bi2MoO6/MXene heterostructure, the MXene nanosheets can not only promote the electron transfer and facilitate the Li+ transport, but also accommodate the volume expansion/contraction of Bi2MoO6 during lithiation/delithiation, endowing the composite electrodes with high conductivity, good structural stability and excellent electrochemical durability. As a result, the Bi2MoO6/MXene-30% exhibits remarkably enhanced lithium storage properties, presenting a specific capacity of 692 mAh g-1 at 100 mA g-1 after 200 cycles, a superior rate capability of 328.2 mAh g-1 at 2 A g-1 as well as an outstanding cycling durability with a capacity of 545.1 mAh g-1 and 99.6% coulombic efficiency at 1 A g-1 after 1000 cycles. The Bi2MoO6/MXene heterostructure with competitive performance is conceivable to be a promising high-performance anode material for LIBs. Furthermore, various TMOs/Ti3C2Tx composites were suggested to be explored using Ti3C2Tx MXene nanosheets as a conductive substrate to achieve good electrochemical performance and application in energy storage.

Acknowledgments

    This work was financially supported by the National Natural Science Foundation of China (Nos. 51572011 and 51802012) and the National Key Research and Development Program of China (2017YFB0102204).

 

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Citation Information

Peng Zhang, Danjun Wang, Qizhen Zhu, Ning Sun, Feng Fu, Bin Xu, Plate-to-Layer Bi2MoO6/MXene-Heterostructured Anode for Lithium-Ion Batteries. Nano-Micro Lett.(2019) 11: 81. https://doi.org/10.1007/s40820-019-0312-y

History

Received: 11 August 2019 / Accepted: 01 September 2019 / Published online: 25 September 2019


Additional Info

  • Type of Publishing: JOUR - Journal
  • Title: Plate-to-layer Bi2MoO6/MXene Heterostructure as High Performance Anode Material for Lithium Ion Batteries
  • Author: Peng Zhang, Danjun Wang, Qizhen Zhu, Ning Sun, Feng Fu, Bin Xu
  • Year: 2019
  • Volume: 11
  • Journal Name: Nano-Micro Letters
  • ISSN: 2150-5551
  • URL: http://dx.doi.org/10.1007/s40820-019-0312-y
  • Abstract: Bi2MoO6 is a potentially promising anode material for lithium ion batteries (LIBs) on account of its high theoretical capacity coupled with low desertion potential. Due to low conductivity and large volume expansion/contraction during charge/discharge cycling of Bi2MoO6, effective modification is indispensable to address these issues. In this study, a plate-to-layer Bi2MoO6/ Ti3C2Tx (MXene) heterostructure is proposed by electrostatic assembling positive-charged Bi2MoO6 nanoplates on negative-charged MXene nanosheets. MXene nanosheets in the heterostructure act as a highly conductive substrate to load and anchor the Bi2MoO6 nanoplates, so to improve electronic conductivity and structural stability. When the mass ratio of MXene is optimized to 30%, the Bi2MoO6/MXene heterostructure exhibits high specific capacities of 692 mAh g-1 at 100 mA g-1 after 200 cycles and 545.1 mAh g-1 with 99.6% coulombic efficiency at 1 A g-1 after 1000 cycles. The results provide not only a high-performance lithium storage material, but also an effective strategy that could address the intrinsic issues of various transition metal oxides by anchoring them on MXene nanosheets to form heterostructures and use as anode materials for LIBs.
  • Publish Date: Wednesday, 25 September 2019
  • Start Page: 81
  • DOI: 10.1007/s40820-019-0312-y