Yuxiang Luo, Pei Zhang, Xunhui Xiong,*, Haikuo Fu

1 Guangzhou Key Laboratory of Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China

2 Qingyuan Jiazhi New Material Research Institute Co. Ltd., Qingyuan 511500, China

Keywords:Sodium ion batteries Nanoflower structure Expanded interlayer spacing MoS2 nanosheets

A B S T R A C T Two-dimensional (2D) MoS2 nanomaterials have been extensively studied due to their special structure and high theoretical capacity, but it is still a huge challenge to improve its cycle stability and achieve superior fast charge and discharge performance. Herein, a facile one-step hydrothermal method is proposed to synthetize an ordered and self-assembled MoS2 nanoflower (MoS2/C NF) with expanded interlayer spacing via embedding a carbon layer into the interlayer. The carbon layer in the MoS2 interlayer can speed the transfer of electrons, while the nanoflower structure promotes the ions transport and improves the structural stability during the charging/discharging process.Therefore,MoS2/C NF electrode exhibits exceptional rate performance(318.2 and 302.3 mA·h·g-1 at 5.0 and 10.0 A·g-1,respectively)and extraordinary cycle durability (98.8% retention after 300 cycles at a current density of 1.0 A·g-1). This work provides a simple and feasible method for constructing high-performance anode composites for sodium ion batteries with excellent cycle durability and fast charge/discharge ability.

1. Introduction

Recently,the sodium ion batteries(SIBs)have been extensively researched due to the sodium resources are almost inexhaustible,environmental friendliness, and especially the electrochemical similarity between lithium and sodium[1-8].Unfortunately,a larger Na ion diameter and a higher standard electrochemical potential of Na+/Na (-2.71 Vvs.SHE) than that of Li+/Li (-3.04 V versus SHE) lead to lower energy and power density for SIBs. As a result,most of the traditional lithium ion batteries(LIBs)electrode materials are not suitable for SIBs and exhibit poor electrochemical performances. Accordingly, developing an ideal anode with large specific capacity, fast charge-discharge behavior as well as long cycling life for SIBs is urgently desired [9,10].

Various materials have been broadly reported as potential anodes for SIBs in recent years, such as carbon-based materials[11-14], metals and alloys [9,15,16], metal sulfides and oxides[17-21]. Among them, 2D transition metal chalcogenides (TMCs)have been widely reported due to their unique framework and remarkable electrochemical behaviors [22-25]. As a typical 2D TMC, MoS2possesses a large layer distance of 0.62 nm, in which Mo atom layer is sandwiched between two S atom layersviacovalent bonds.This unique structure endows MoS2with fast reversible Na+diffusion and an ideal theoretical reversible capacity (～670.0 mA·h·g-1).However,the critical challenges of MoS2anode applied in SIBs are the serious volume variations during Na+insertion/extraction and the low electronic and ionic conductivity, which will lead to a unsatisfactory electrochemical performance and limit the practical applications [26,27]. Then constructing an ordered MoS2nanostructure with expanded interlayer spacing has been proposed to overcome the abovementioned problems [28-30].Firstly,the ordered nanostructure can effectively shorten the diffusion pathway for Na ions and allow effective strain relaxation[31,32]. Besides, a larger interlayer spacing can easily intercalation/de-intercalation sodium ions and maintain the structure stable during the sodiation/desodiation process [33,34]. For example, MoS2-PEO nanocomposite can effectively buffer the volume change and prevent the aggregation of MoS2nanosheets by introducing the macromolecular polymer PEO into the interlayer spacing [35]. It is worth noting the polymer impedes the transfer of electrons, and introducing a carbon layer into the interlayer can effectively solve this problem. The carbon layer can enhance the electronic conductivity of the composite and accommodates volume change during cycling. More importantly, the enlarged interlayer spacing caused by the introduction of the carbon layer facilitates rapid intercalation/de-intercalation of Na ions. Inspired by previous accumulations,designing an ordered MoS2nanostructure with expanded interlayer spacing by introducing carbon matrix is a promising strategy to overcome the above-mentioned challenges of MoS2-based anode for extraordinary SIBs.

Herein, a simple one-step hydrothermal method is designed to synthesize an ordered and self-assembled MoS2nanoflower structures (MoS2/C NF) with expanded interlayer spacing via embedding a carbon layer into the interlayer. Assisted by hexadecyltrimethy ammonium bromide (CTAB) and glucose additive during hydrothermal process, 2D MoS2nanosheets are selfassembled into an ordered 3D nanoflower structure with an expanded interlayer spacing. The unique structure brings higher electronic conductivity, shorten the diffusion pathway for Na+as well as improves structural stability in the sodiation/desodiation process. Therefore, the as-fabricated MoS2/C NF anode delivered a reversible specific capacity of 478.6 mA· h· g-1at 0.1 A· g-1, a desirable rate behavior of 302.3 mA·h·g-1at 10.0 A·g-1and superior cycling retention rate (98.8% after 300 cycles at 1.0 A· g-1).

2. Experimental

2.1. Synthesis of MoS2/C NF composite

First, completely dissolved 0.75 g CTAB in 60 ml deionized water. After that, 0.6 g Na2MoO4·2H2O, 0.75 g thioacetamide and 0.75 g glucose were dissolved in the solution.And then the solution was transferred into high pressure reactor and then maintained at 200 °C for 24 hours. Ultimately, the powders were collected by centrifugation and named it MoS2/C NF. The obtained MoS2/C NF precursor was maintained at 700 °C (heating rate: 5 °C· min-1)for 2 h in argon atmosphere. For comparison, MoS2/C and MoS2NF composites were obtained by similar preparation methods without CTAB and glucose additive, respectively.

2.2. Materials characterization

Raman measurement were performed by Raman spectrophotometer (Horiba JobinYvon, HR800, France). The X-ray diffraction(XRD) were conducted by Rigaku D/max 2500. SEM (JEOL JSM07600F), TEM (JEM-2010 JEOL) and HRTEM were applied to characterize the morphology and structure features including crystalline lattice parameters of these composites, respectively. XPS was got by using Thermo K-Alpha XPS spectrometer.Thermogravimetric analysis(TGA)of the composites were collected on Nitzsch STA 449C thermal analyze to confirm the composition of composites.

2.3. Electrochemical testing

The cycling and rate performances of all composites were measured with CR2032 coin cells on a LAND-BT2013A battery tester.All the operations were carried out in an argon-filled glove box.A sodium metal and a glass fiber were treated as the counter electrode and the separator. The testing electrodes were obtained by mixed 80 % (mass) active anode materials, 10 % (mass) CMC and 10 % (mass) acetylene black in deionized water. Generally, the mass loading of the active material is about areal density of～0.5 mg· cm-2.The electrolyte is composed of 1.0 mol·L-1NaClO4dissolved in the mixture of EC/PC(1/1,by volume)containing with 5% FEC additive. Cyclic voltammetry (CV) curve testes were got at different scan rates on CHI 660E workstation.The impedance spectroscopy was tested on the same device.

3. Results and Discussion

The synthetic steps for MoS2/C NF is schematically shown in Scheme 1. CTAB acts as a ‘‘magnet”, attracting irregular MoS2nanosheets to self-assemble into an ordered 3D nanoflower structure. At the same time, the glucose additive entered the MoS2interlayer spacing during the hydrothermal process and forms a stable carbon layer after annealing.

Scheme 1. Schematic diagram for the fabricated of MoS2/C NF.

The XRD patterns for MoS2/C NF and MoS2NF were firstly used to investigate the crystalline phase. As demonstrated by Fig. 1a,MoS2NF shows three major diffractions located at 14.4°, 32.3°,and 59.5°, which can be perfectly index to hexagonal 2H-MoS2(PDF No. 37-1492) [36]. In contrast, the (002) characteristic peak of MoS2/C NF shifts to a low angle to ～8.96°, which corresponds to a crystal plane distance of 1.1 nm (Bragg equation:2dsin θ ＝nλ).The enlarged interlayer spacing of MoS2/C NF proves that the interlayer effect of the glucose additive on the S-Mo-S layer during the hydrothermal synthesis[37].Fig.1b demonstrates the Raman spectra of samples MoS2/C NF and MoS2/NF, all the composites have two sharp bands at 377 and 402 cm-1, which should be attributed to E2gand A1gphonon modes 2H phase MoS2, respectively [28]. Compared with MoS2NF, MoS2/C NF displays two signals appeared at 1353 and 1595 cm-1, representing D and G bands of the C layer derived by glucose [38]. Then so as to measure the mass ratio of carbon in MoS2/C NF, thermogravimetric analysis (TGA) was carried out (Fig. 1c). The significant quality loss should be assigned to the conversion reaction of MoS2to MoO3(Fig.S1)and the oxidation of C layer.Thus,the mass percentage of C in MoS2/C NF is launched to be about 11%(mass).

To further analyze the chemical interactions of the composites,the XPS characterization was also performed (Fig. S2). The Mo 3d spectrum of MoS2/C NF was displayed in Fig. 1d, a pair of peaks at 232.4 and 229.3 eV pointed to Mo 3d5/2and Mo 3d3/2[28].Meanwhile, two evident peaks observed at 162.1 and 163.2 eV in the S 2p spectrum (Fig. 1e), pointing to the S 2p3/2and S 2p1/2[39]. It is worth noting that the characteristic of peaks of Mo 3d and S 2p for MoS2/C NF shift to a lower binding energy when compared with MoS2NF, demonstrating an increased electron cloud density around MoS2[40,41]. The bias of electron cloud leads to a strong electron interaction between MoS2NF and carbon layers,which will cause a strong coupling between MoS2NF and carbon layers and then accelerates the diffusion speed of Na ions and improves the structural stability. The C 1s spectrum (Fig. 1f) exhibits the peaks centered at 284.8,286.3 and 288.8 eV,which should be associated to the C sp2, C sp3and O—C＝O, respectively[20,42].The sp2 hybridized graphitic structure can effectively boost the conductivity of the composites and allows the electrons within the entire structural frame to be quickly transferred [43].

Fig. 1. (a) XRD patterns and (b) Raman spectra of MoS2/C NF and MoS2 NF. (c) TGA curve of MoS2/C NF heated from 30℃ to 700℃ in air atmosphere. High-resolution elemental (d) Mo 3d and (e) S 2p region of MoS2/C NF and MoS2 NF, (g) High-resolution elemental C 1s region of MoS2/C NF.

Fig.2. (a,b)High and low magnification SEM images of MoS2/C NF;(c,d)TEM images and(e)HRTEM image of MoS2/C NF;(f)SAED pattern of MoS2/C NF;(g-k)TEM image with corresponding EDS elemental mappings of MoS2/C NF.

The morphology and structural features were studied by scanning electron microscopy (SEM). As obviously demonstrated in Fig. 2a and b, SEM images distinctly indicate that MoS2/C NF nanoflowers with about diameter of 200 nm are uniformly distributed. In sharp contrast, MoS2/C composites prepared without CTAB or excessive CTAB are disordered and stacked(Fig.S3).Therefore,suitable amount of CTAB in the hydrothermal process plays a critical to construct ordered structure. As displayed in the transmission electron microscopy (TEM) and high-resolution TEM(HRTEM) images (Fig. 2c-e), MoS2/C NF display an ordered 3D nanoflower structure composed of ultrathin nanosheets with an expanded interlayer spacing of 1.1 nm.However,MoS2NF without introducing carbon layer show that the interlayer spacing of nanosheets is only 0.62 nm (Fig. S4). The SAED pattern in Fig. 3f proves that MoS2/C NF is well-crystallized. The energy dispersive X-ray spectroscopy(EDS)element mapping indicates that the uniform distribution of Mo, S, and C (Fig. 2g-j and Fig. S5). Such an ordered 3D nanoflower structure with expanded interlayer spacing is expected to restrict the agglomeration of MoS2nanosheets for an extended cycling life.

Fig.3. (a)CV curves of MoS2/C NF at a scanning rate 0.1 mV·s-1;(b)Galvanostatic charge/discharge curves of MoS2/C NF;(c)Cycling performance at the current density of 1 A·g-1 and (d) Rate capability of MoS2/C NF, MoS2/C and MoS2 NF; (e) long cycling at a high current density of MoS2/C NF.

To unveil the electrochemical performances of MoS2/C NF anode for SIBs, half SIB cells are assembled and tested within the voltage range of 0.01-3 V.The initial three cyclic voltammetry(CV)profiles of all the composites are displayed in Fig.3a and Fig.S6.During the initial discharge progress, two obvious reduction peaks were centered at 0.86 and 0.53 V are attributed to the formation of NaxMoS2by the embedded of Na+into MoS2and the solid-electrolyteinterphase (SEI) film produced during the first discharge process,respectively [35,44]. The sharp peak at 0.1 V should be associated to the phase conversion between Mo and Na2S [45]. In the first charging process, it can be observed that there is a oxidation peak centered at 1.78 V,which can be corresponded to the reverse reaction of Mo with Na2S [46]. The CV curves show good reproducibility in the subsequent cycles,revealing that the sodium ions storage in the MoS2/C NF anode has good reversibility.The sodium storage capabilities of all the composites are investigated in the voltage window of 0.01-3 V.As demonstrated in Fig.3b,the first discharge and charge capacities of MoS2/C NF are 556.3 and 478.6 mA·h·g-1with an initial coulomb efficiency (ICE) of 86.1%, which is much higher than those of MoS2/C (74.2%) and MoS2NF (83.9%)(Fig.S7).The ordered nanoflower structures can shorten ion diffusion path and the carbon layer can facilitate the transfer of electrons to the active materials, which are the main reasons for the increase in the initial coulombic efficiency of MoS2/C NF.The large irreversible capacity occurred in the initial sodiation/desodiation progress caused by the formation of solid electrolyte interface(SEI) film [47]. Besides, MoS2/C NF shows the highest capacity at 0.1 A g-1among the three kinds of MoS2-based anodes (Fig. S8),which is attributed to a significant increased active sites for Na storage provided by the ordered 3D nanoflower structure and expanded interlayer spacing. In addition, at a current density of 1.0 A· g-1, MoS2/C NF demonstrates a superior cycle performance after 300 cycles with a capacity retention rate of 98.8% (Fig. 3c).Whereas the capacities of MoS2/C and MoS2NF exhibit a huge decay only after 100 cycles.All the electrodes display excellent rate performances (Fig. 3d), especially MoS2/C NF shows the highest capacities of 430.5, 407.8, 378.8, 357.4, 338.9, 318.2 and 302.3 mA· h· g-1at different current density of 0.1, 0.2, 0.5, 1.0, 2.0,5.0 and 10.0 A·g-1, respectively. Impressively, when the current density returned to 0.1 A· g-1, the reversible discharge capacity of 444.1 mA· h· g-1can be fully recovered and shows no attenuation in the subsequent 150 cycles (Fig. S9). The fully recovered capacity at 0.1 A· g-1may be attributed to the optimized nanostructure of active materials and SEI layers lead to a dramatically reactivated sodium storage capacity as well we extra defects generated upon cycling.When MoS2/C NF is cycled at high densities 5 A·g-1and 10 A·g-1(Fig.3e),no obvious decays are observed during the 150 cycles.

To fully illustrate the excellent rate and outstanding cycling performance of MoS2/C NF composite,CV profiles of different scanning rates were carried out to investigate the kinetics of sodiation/desodiation progress.All the CV curves show similar shapes at different scan rates were displayed in Fig. 4a. The charge storage behavior can be calculated according to the formula:lg（i）＝blg v（ ）+lg（a）, whereiis the peak currents of the cathodic/anodic peaks and v is the different scan rates [29]. The value ofbcan be launched by the slopes of lg（i）and lg v（ ）[48].Normally,b＝0.5 indicates the charge storage is controlled by diffusion, andb＝1 represents an ideal pseudocapacitive process.All the b values at different peaks of the MoS2/C NF are illustrated in Fig. 4b. It is clearly seen that all thebvalues are higher than 0.9, which indicates that the surface pseudocapacitive in MoS2/C NF plays a leading role in the sodiation/desodiation progress. The surface pseudocapacitive contribution to the total charge storage can be analyzed by the formula:i＝k1v +k2, wherek1v represents the pseudocapacitive effect, which can be got through calculatingk1value. The ratio of pseudocapacitive contribution for MoS2/C NF can reach 86.1%,88.9%,91.6%,92.4%and 93.6%at the scan rates of 0.2, 0.4, 0.6, 0.8 and 1.0 mV· s-1, respectively (Fig. 4c). At the same time,both MoS2/C and MoS2NF possess very high capacitive contributions at different scan rates (Figs. S10 and S11). These results indicate that both the introduction of the C layer in the MoS2interlayer spacing or the design of an ordered 3D nanoflower structure have high sodium storage capacity,but their cycling stability are very poor. Therefore, the C layer and the 3D nanoflower structure are introduced at the same time to obtain MoS2/C NF,which further improves the pseudocapacitance contribution and enhances the cycling stability.

To further explore the reason that the enhanced electrochemical performance of MoS2/C NF,electrochemical impedance spectra were carried out. Fig. S12 demonstrates the Nyquist plots of the MoS2/C NF, MoS2/C and MoS2NF after 100th cycle at 1 A·g-1. The semicircle in the high-to-medium frequency is called the charge transfer resistance(Rct),which occurs between the active material and the liquid electrolyte,and the straight line is associated to the Warburg diffusion of Na ions into the materials[49].Obviously,the Rct of MoS2/C NF is much smaller than that of MoS2/C and MoS2NF,which means that MoS2/C NF has a faster charge transfer speed.As shown in Fig.S13,the morphology of MoS2/C NF after 200 cycles at 1 A·g-1still maintains the dispersed nanoflower shape, almost no volume expansion and maintains a uniform element distribution.The results fully illustrate that the ordered 3D nanoflower structure and the expanded interlayer spacing caused by the introduction of C layer can well buffer the huge volume expansion during the cycling and maintain structural stability.

Fig.4. (a)CV profiles at different scan rates for the MoS2/C NF;(b)b values at different cathodic/anodic peaks for the MoS2/C NF;(c)Pseudocapacitive contribution at a scan rate of 0.6 mV· s-1 for the MoS2/C NF; (d) Capacitance contribution of MoS2/C NF at different scan rates;

4. Conclusions

In conclusion, we designed a facile one-step hydrothermal method to compose ordered 3D nanoflower structure with expanded interlayer spacing through CTAB and glucose additives.This unique structure brings higher electronic conductivity,speeds up the charge transfer speed and shortens the diffusion distance of Na ions. At the same time, The C layer in the interlayer spacing of MoS2nanosheets is an important reason of stabilizing the electrode system structure to withstand repeated sodiation/desodiation progress. Therefore, the MoS2/C NF demonstrates superior reversible capacity(478.6 mA·h·g-1at 0.1 A·g-1),remarkable rate performance (318.2 mA· h· g-1and 302.3 mA·h· g-1at high current density of 5 and 10 A·g-1, respectively) and extraordinary cycling stability(98.8%retention after 300 cycles at 1 A·g-1).Overall, the present study shows the reasonable combination of C matrix and nanostructure is a feasible solution to achieve long cycling life and rapid charge/discharge capability SIBs, as well as can be applied to other energy storage fields.

Acknowledgements

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.

Acknowledgements

We gratefully acknowledge the financial support from National Natural Science Foundation of China (51874142), Pearl River S&T Nova Program of Guangzhou (201806010031), the Fundamental Research Funds for the Central Universities (2019JQ09), Guangdong Innovative and Entrepreneurial Research Team Program(2016ZT06N569), Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program(2019TQ05L903) and Young Elite Scientists Sponsorship Program by CAST (2019QNRC001).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.04.013.