Background

Due to their high energy density, long cycle life, and environmental friendliness, lithium ion batteries (LIBs) are widely utilized in portable electronic devices [1] (e.g., mobile phones and watches), electric vehicles [2, 3], and renewable energy storage [4,5,6,7,8]. Graphite is the most widely used anode materials in commercial LIBs, benefiting from its low working voltage, good conductivity, and low cost [9,10,11]. However, the characteristic structure of graphite leads to feasible generation of LiC6, allowing only one lithium ion intercalation in every six carbon atoms which results in a low theoretical specific capacity of 372 mAh g− 1, which is far away from the current business requirements [12].

Currently, it is preferred to obtain appropriate electrode materials in LIBs for higher battery capacity, longer cycle life, and better rate capability. Consequently, Li-alloy-based anode materials [13], transition metal oxides [14], oxysalts, and transition metal sulfides [15] are often served as the anode materials in LIBs, since these materials display all the necessary properties for appropriate electrode materials. Among these materials, transition metal sulfides (e.g., CuS2 [16], WS2 [17], and MoS2 [18,19,20] have been an exciting topic in research as they are earth-abundant and show high specific capacity when used as anode materials in LIBs [21]. As a typical representative, MoS2 has gained a lot of attention due to its particular S-Mo-S layered structure [22], high theoretical specific capacity compared to traditional graphite anode, and there is a transfer reaction of four electrons when used as anode materials in LIBs [23, 24]. In addition, the van der Waals forces between the MoS2 layers are very weak, allowing lithium-ion diffusion without causing significant volume change [25, 26]. However, MoS2 is still an unsatisfactory anode material due to its low electrical conductivity, leading to the poor cycling and rate performance [27]. To solve this problem, a number of strategies have been developed to improve its electrical conductivity such as the incorporation of MoS2 with carbon materials [28,29,30].

To date, a variety of MoS2/carbon composites have been synthesized as anode materials in LIBs, namely, layered MoS2/graphene composites [31], MoS2/C multilayer nanospheres [32], MoS2-CNT composite [33], multilayered graphene/MoS2 heterostructures [34], or petal-like MoS2 nanosheets space-confined in hollow mesoporous carbon spheres [35]. Despite gratifying progress in electrical conductivity, cycling, and rate performance of the electrode, some other conflicts in the synthesis method have persisted. At present, the most commonly used synthetic method is hydrothermal approach followed by an annealing process, which can introduce carbon matrix with some surfactants such as sodium oleate or oleyamine and sulfur element with some L-cysteine in the first procedure. Moreover, expensive and toxic organic reagents were always indispensable and unavoidable during the synthesis process when compared with co-precipitation method. Currently, co-precipitation method is just beginning to gain popularity in the synthesis of inorganic nanostructured materials due to its cost-effective, non-toxic, trustworthy, and stable [36, 37]. To the best of our knowledge, there has been little report on the synthesis of MoS2/C nanocomposite by co-precipitation/calcination process, especially with potassium humate.

Potassium humate, a sort of aromatic hydroxy carboxylate, which consisted of a wide variety of oxygen-containing functional groups, can be considered as functionalized graphene candidate [38]. In general, a great deal of researches have been made to use potassium humate as carbon source to synthesize carbon materials under extremely harsh conditions [38, 39]. Huang [38] reported that potassium humate can be straightforward carbonization to prepare reduced graphite oxide materials. In this paper, MoS2/C nanocomposites were synthesized via a co-precipitation/calcination route, by employing an organic matter (potassium humate) and an inorganic substance ((NH4)6Mo7O24) as reagents. The electrochemical performance of the samples as a LIBs anode was measured, and the results showed that the sample calcinated at 700 °C (MoS2/C-700) exhibited better cycling ability and rate behavior. The discharge capacity of the sample remained at 554.9 mAh g− 1 after 50 cycles at the current density of 100 mA g− 1, which is much better than the other two samples calcinated at 600 °C and 800 °C, respectively. Meanwhile, the as-prepared MoS2/C-700 displays a comparable electrochemical performance [40, 24].

Methods/Experimental

Potassium humate was obtained from Double Dragons Humic Acid Co., Ltd. Xinjiang (China), and the composition analysis of potassium humate was shown in Additional file 1: Table S1. All of the chemical reagents (except potassium humate) were of pure analytical grade and used without further purification.

Synthesis of MoS2/C

The precursor was prepared by co-precipitation from (NH4)6Mo7O24 and potassium humate in the presence of HNO3 followed by a freeze-dried process for 2 days. In a typical procedure, 4 g of potassium humate were dissolved in 40 mL of 0.25 M (NH4)6Mo7O24 solution. Subsequently, the above-mentioned solution was added dropwise to 100 mL of 0.5 M HNO3 solution with vigorous magnetic stirring. The duration of the magnetic stirring was for several hours. The lower precipitation was then separated from the mixture solution, freeze-dried, and labeled as Mo-HA precursor. The precursor was mixed with anhydrous Na2SO4 (with a proportion of 1:10) and ground in a mortar to form a homogeneous mixture. The mixture was then calcinated at 700 °C for 3 h (with a heating rate of 10 °C min− 1) and then naturally cooled down to room temperature. Finally, the products were washed with deionized water and ethanol three times followed by a freeze-dried procedure to obtain the MoS2/C powder. In parallel, the samples calcinated at 600 and 800 °C were synthesized as well.

Characterization

The surface organic functional groups of potassium humate were measured by Fourier transform spectrophotometer (FT-IR, VERTEX 70, Bruker) with KBr as the reference sample. The structure and morphology of different samples were characterized by X-ray diffraction (XRD, BRUKER D8 Advance) with Cu Kα radiation (λ = 1.54178 Å), transmission electron microscopy (TEM, Hitachi H-600), high-resolution transmission electron microscopy (HRTEM, JEM-2100F), LEO 1450VP scanning electron microscope (SEM), energy-dispersive X-ray spectrometer (EDX), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi spectrometer). Thermogravimetric analyses (TGA) were conducted on a thermogravimetric analyzer (Netzsch TGA 409). Raman spectrum was carried out on Bruker Senterra with 532 nm wavelength.

Electrochemical Measurements

Electrochemical measurements were performed on coin cells. The working electrodes were fabricated by mixing 80 wt.% of the as-prepared MoS2/C active materials, 10 wt.% of acetylene black, and 10 wt.% of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidinone (NMP) solvent to form a homogeneous slurry. The slurry was coated on the copper foil and dried in a vacuum at 110 °C for 12 h. The coin cells were assembled in an argon-filled glovebox. In the measurement, lithium foil was used as the counter electrode and reference electrode, and a polypropylene film (Celgard-2400) was used as a separator. The electrolyte solution was 1 mol L− 1 LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (EC/DMC/DEC, 1:1:1, volume ratio). The galvanostatic charge-discharge measurements were performed in a potential range of 0.01–3.0 V by using a LAND CT2001A battery testing instrument (Wuhan) at room temperature. Cyclic voltammetry (CV) measurements were performed on an electrochemical work-station (CHI 660D) at a scanning rate of 0.1 mV s− 1 between 0.01 and 3.0 V.

Results and Discussion

The surface chemistry of potassium humate was studied using FTIR spectrum. In Fig. 1a, the broad peaks centered at 3400 cm− 1 were ascribed to the stretching vibrations of −OH, −COOH, and H2O bonds, The peaks at 1627, 1413, and 1050 cm− 1 were attributed to the stretching vibrations of the −COO groups and −CH, −OH and so on [41], respectively, indicating the rich oxygen-containing functional groups on the surface of pure potassium humate, which is beneficial to complexation reaction or adsorption. TGA curve of the homogeneous mixture of Mo-HA precursor and anhydrous Na2SO4 (with a proportion of 1:10) in an argon atmosphere with a heating rate of 10 °C min− 1 is shown in Fig. 1b. It can be seen that there are three steps of weight loss in the TGA curve. The first weight loss is 1.59% from room temperature to 250 °C, which may be due to decomposition of the water in the surface of the Mo-HA precursors. There are another two consecutive steps of weight loss, with a weight loss of 1.35% from 250 to 500 °C, and a weight loss of 3.17% from 500 to 800 °C, and then the mass remains constant, indicating that the precursor has been decomposed completely at 800 °C. For such a system, we choose those three temperatures for calcination as 600, 700, and 800 °C, denoted as MoS2/C-600, MoS2/C-700, and MoS2/C-800, respectively.

Fig. 1
figure 1

a FT-IR spectra of pure potassium humate. b TGA curve of the homogeneous mixture of Mo-HA precursor and anhydrous Na2SO4 (with a proportion of 1:10)

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According to the literature [34], a possible mechanism of the reaction process has been proposed and schematically depicted in Scheme 1. Moreover, the corresponding formulas are listed in Additional file 1: Equations 1–5. In these equations, potassium humate was abbreviated as K-HA. There might be a complexation when potassium humate was dissolved in (NH4)6Mo7O24 solution, with the participation of HNO3 solution, which leads to the generation of Mo-HA. After heating the mixture of the Mo-HA precursor and anhydrous Na2SO4 in an argon atmosphere at a relatively high temperature, the Mo-HA precursor would be carbonized to form the intermediate of amorphous carbon, and then the intermediate would reduce anhydrous Na2SO4 to generate Na2S, further hydrolyzed to hydrogen sulfur. Finally, hydrogen sulfur may react with MoOx, leading to the formation of MoS2/C nanocomposites.

Scheme 1
scheme 1

Schematics depicting the fabrication procedure of MoS2/C nanocomposite

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Figure 2a–b show the XRD patterns and Raman spectra of the MoS2/C nanocomposites calcinated at different temperatures. Figure 2a shows that almost all the diffraction peaks of MoS2/C-600 and MoS2/C-700 can be well indexed to the hexagonal MoS2 phase (JCPDS card no. 86-2308), which is consistent with those of previous report [42]. There are still some other peaks mismatching the standard card in the MoS2/C-800 sample. We speculate that the crystalline of MoS2/C has been destroyed at high temperature. From the Raman spectra (Fig. 2b), it can be seen that the peaks located in between 379 and 400 cm− 1 belonged to E1 2g (the in-plane displacement of Mo and S atoms) and A 1g (out-of-plane symmetric displacement of Mo and S atoms) Raman modes, respectively [24, 43]. The bands appeared at 1347 and 1589 cm− 1 were the characteristic D- and G-band, and the value of I D /I G were 0.96, 0.91, and 0.94 as the temperature goes from 600 to 800 °C. The former corresponds to the amorphous carbon or sp3-hybridized carbon (D-band), and the latter assigned to the sp2-hybridized carbon (G-band) [44]. Although there is no great distinction between the degree of graphitization, the MoS2/C-700 sample is still a little higher than the other two samples to a certain extent, indicating that the carbon in this sample is not only in the form of amorphous carbon, but also some graphitic carbon. Therefore, we focused on the MoS2/C-700 sample in the following investigations.

Fig. 2
figure 2

a XRD patterns. b Raman spectra of MoS2/C nanocomposites calcinated at different temperatures. c Survey XPS spectra of MoS2/C-700. d High-resolution XPS spectra of Mo 3d. e S 2p. f C 1 s

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To further study the chemical composition and chemical bonds of MoS2/C-700, X-ray photoelectron spectroscopy (XPS) analysis was carried out. The survey XPS spectrum (Fig. 2c–f) reveals the presence of Mo, S, C, and O elements in the MoS2/C-700 nanocomposite. The high-resolution XPS spectra of Mo 3d and S 2p are shown in Fig. 2d, e, respectively. The peaks at 229.4 and 232.6 eV are assigned to the Mo 3d5/2 and Mo 3d3/2, confirming the existence of Mo in MoS2/C-700 [45, 46]. The presence of another XPS peak at 226.5 eV is indexed to S 2 s, which is resulted from the surface of the MoS2/C-700 [47]. Moreover, the XPS peaks at 162.3 and 163.4 eV in S 2p spectra are characteristic peaks of the S 2p3/2 and S 2p1/2 of MoS2, respectively. Figure 2f shows that the C1 s spectrum can be divided into three peaks, denoted as C–C, C–O, and C=O groups, respectively.

The EDX spectrum indicates that the sample calcinated at 700 °C contains Mo, S, and C elements, as shown in Fig. 3a. Figure 3b, c show the SEM images of the sample of MoS2/C-700. For comparison, the SEM images of MoS2/C-600 nanocomposite and MoS2/C-800 nanocomposite were also shown in Additional file 1: Figure S1. In order to explore the corresponding element distribution in the sample of MoS2/C-700, the corresponding elemental mapping analysis were carried out. As shown in Fig. 4a–d, the elemental mapping images of MoS2/C-700 demonstrated the uniform distribution of Mo, S, and C all over the MoS2/C-700 nanocomposite, which is consistent with the EDX and XPS results.

Fig. 3
figure 3

a EDX spectrum of MoS2/C-700. b, c SEM images of MoS2/C-700 nanocomposite

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Fig. 4
figure 4

a-d Elemental mapping images of MoS2/C-700; (e) TEM image, (f) the SAED and (g) High resolution TEM image of MoS2/C-700 nanocomposite, (h) Enlarged HR-TEM image of the marked area in figure (g)

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As displayed in Fig. 4e–h, the morphology and structure of the as-synthesized MoS2/C nanocomposites were investigated by transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). The TEM image (Fig. 4e) and the SEM images (Fig. 3b, c) clearly show that the structure of MoS2/C-700 nanocomposite is wrinkled two-dimensional nanosheets with the width of ~ 800 nm and the thickness of ~ 20 nm. SAED pattern in Fig. 4f shows that the hexagonal lattice structure of MoS2 is well crystallized. Furthermore, the crystal lattices of the sample were shown at HRTEM profiles ((Fig. 4g, h) and Additional file 1: Figure S2). The profiles showed highly crystalline MoS2 nanosheets with an interlayer distance of 0.27 nm corresponding to (100) plane of hexagonal MoS2 [24, 34]. In addition, Additional file 1: Figure S2 clearly reveals that the carbon nanosheets were decorated with MoS2 nanosheets.

Figure 5a shows the CV curves of the first 3 cycles of MoS2/C-700 electrode at a scan rate of 0.1 mV s− 1 in the potential window of 0.01–3.00 V vs. Li+/Li. During the first cycle, the reduction peak at 1.0 V indicates the lithium insertion mechanism, which is due to the insertion of lithium ions into the MoS2 layers to form LixMoS2. At the same time, there has been a phase transition from 2H (trigonal prismatic) to 1T (octahedral) [48]. Another reduction peak at 0.4 V is attributed to the conversion of LixMoS2 into metallic Mo and Li2S. The broad oxidation peak located at 2.35 V represents the deintercalation of Li2S to S. During the subsequent cycles, the two cathodic peaks at 1.0 and 0.4 V disappear with appearance of three new peaks at 2.0, 1.2, and 0.3 V, indicating the reduction of MoS2 and the conversion from S8 to polysulfides and then to Li2S [24].

Fig. 5
figure 5

a CV curves of the first three cycles of MoS2/C-700 electrode at a scan rate of 0.1 mV s− 1. b Discharge and charge curves of the first 3 cycles of MoS2/C-700 electrode at a current density of 100 mA g− 1. c Cycling performance MoS2/C electrode and the pristine MoS2 electrode at a current density of 100 mA g− 1, and Coulombic efficiency of MoS2/C-700 electrode. d Rate performance of MoS2/C and the pristine MoS2 electrode at the current densities ranging from 100 to 1000 mA g− 1

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The discharge and charge curves of the first 3 cycles of MoS2/C-700 electrode were recorded, and the corresponding results were shown in Fig. 5b. In the first cycle, the discharge and charge capacities of MoS2/C-700 electrode are 802.8 and 651.4 mAh g− 1, respectively, with a Coulomb efficiency of 81.14%. The irreversible capacity loss may be due to some irreversible reaction such as the decomposition of electrolyte and the formation of solid electrolyte interface (SEI) film [49, 50].

The cycle stability of whole MoS2/C electrode and the pristine MoS2 electrode at a current density of 100 mA g− 1 are presented in Fig. 5c. At the same time, the Coulomb efficiency of MoS2/C-700 is also recorded. After 50 cycles, the discharge capacities of MoS2/C-600, MoS2/C-700, MoS2/C-800, and pristine MoS2 electrode at a current density of 100 mA g− 1 remain at 399.7, 554.9, 245.7, and 332.9 mAh g− 1, respectively. As shown in Additional file 1: Table S1, it has summarized the discharge capacities after 50 cycles of MoS2-based electrode presented in other literature, the as-prepared MoS2/C-700 display a comparable electrochemical performances compared to the previous work. It is concluded that the MoS2/C-700 electrode shows the most outstanding cycle performance and the Coulomb efficiency of the sample maintained a high level at about 100% after the first 3 cycles. It may benefit from the small amount of graphitic carbon in this sample, leading to enhanced electrical conductivity of the nanocomposite.

In addition to the cycling stability, the high-rate performance is also an important factor for high-power applications. Figure 5d shows the rate performance of MoS2/C and the pristine MoS2 electrode at the current densities ranging from 100 to 1000 mA g− 1. At 1000 mA g− 1, the discharge capacity of MoS2/C-700 can still maintain at a relatively high value of ~ 450 mAh g− 1, which is higher than the other MoS2/C electrodes and pristine MoS2 electrode we have prepared at the same current density. When the current density is changed back to 100 mA g− 1, the capacity of MoS2/C-700 sample can recover up to ~ 500 mAh g− 1 after 50 cycles at different current densities, revealing the good rate capability of the sample.

The electrochemical impedance spectra (EIS) measurements on the MoS2/C and the pristine MoS2 electrode were conducted in order to gain a further understanding about the excellent electrochemical performance of the MoS2/C-700 sample (Fig. 6). There is a semicircle at the high frequency region followed by a slope line at the low frequency region on these Nyquist plots. It can be seen that the semicircle at the high frequency region of the MoS2/C-700 sample is evidently smaller than that of the other three samples, which is related with the charge transfer resistance (Rct) occurred at the electrolyte and electrodes interfaces. Therefore, this result further implies that the incorporation of potassium humate markedly improve the conductivity of MoS2, leading to further improvement in the electrochemical performances.

Fig. 6
figure 6

Nyquist plots of the MoS2/C electrode and the pristine MoS2 electrode tested in a frequency range of 0.01 Hz to 100 kHz

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Conclusions

In this work, two-dimensional MoS2/C nanosheets were successfully synthesized via a co-precipitation/calcination route by employing an organic matter (potassium humate) and an inorganic substance ((NH4)6Mo7O24) as reagents. Structural characterizations show that as-prepared MoS2/C-700 nanocomposite is two-dimensional (2D) MoS2/C nanosheets with irregular shape. The 2D MoS2/C nanosheets exhibited improved electrochemical performance when fabricated as anode material for LIBs. Furthermore, a possible reaction process was proposed. The current synthesis strategy may be expanded into the synthesis of other nanocomposite that can be served as anode materials for high-performance lithium-ion batteries.