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N plasma treatment on graphene oxide-MoS2 composites for improved performance in lithium ion batteries
Materials Chemistry and Physics 240 (2020) 122169
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N plasma treatment on graphene oxide-MoS2 composites for improved performance in lithium ion batteries Jingxuan Jiao a, Kai Du b, Yuanting Wang a, Pingping Sun c, Huihui Zhao a, Peijuan Tang a, Qi Fan d, *, He Tian b, Qi Li a, **, Qingyu Xu a, e, *** a
School of Physics, Southeast University, Nanjing, 211189, China Center of Electron Microscopy, State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China College of Physics and Electronic Engineering, Jiangsu Second Normal University, Nanjing, 210013, China d College of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, China e National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� N plasma treatment is applied on gra phene oxide (GO)-MoS2 composites (NGO-MoS2). � N has been successfully doped into both GO and MoS2. � The structure of MoS2 transforms from the metallic phase (1T) to semi conducting phase (2H). � The electrochemical performance of NGO-MoS2 anode has been significantly improved due to better electronic conductivity.
A R T I C L E I N F O
A B S T R A C T
Keywords: MoS2 Lithium-ion battery Plasma
Petal-like graphene oxide-MoS2 (GO-MoS2) composites are prepared by the hydrothermal method. The structural characterizations confirm the metallic phase (1T) of MoS2 sheets with a few layers, and the uniform distribution of graphene oxide (GO) and MoS2. However, the functional groups on the surface of GO and the intrinsic low conductivity of MoS2 lead to the poor electrochemical performance of GO-MoS2 in lithium ion batteries. N-doped GO-MoS2 (N-GO-MoS2) composites are prepared by N plasma treatment on GO-MoS2 composites at room tem perature. N is clearly confirmed to be doped into both graphene oxide and MoS2 uniformly, and MoS2 changes to semiconducting phase (2H). In lithium ion batteries, it is found that the initial capacity increases from 561.4 mAh g 1 for GO-MoS2 to 726.9 mAh g 1 for N-GO-MoS2 at 1 C. After 100 cycles, N-GO-MoS2 still exhibits capacity of 592.7 mAh g 1 (1.34 mAh g 1 loss per cycles and 81.5% capacity retention rate), which is much better than that of GO-MoS2 (only 31.6 mAh g 1 after 100 cycles). The significantly improved high-rate cycling electrochemical performance for N-GO-MoS2 can be attributed to the improved stability of MoS2 by the phase transition from 1T to 2H, the enhanced electron transportation by the reduction of graphene oxide to graphene and the simulta neous N doping in MoS2 and graphene oxide.
* Corresponding author. ** Corresponding author. *** Corresponding author. School of Physics, Southeast University, Nanjing, 211189, China. E-mail addresses: [email protected] (Q. Fan), [email protected] (Q. Li), [email protected] (Q. Xu). https://doi.org/10.1016/j.matchemphys.2019.122169 Received 4 June 2019; Received in revised form 29 August 2019; Accepted 10 September 2019 Available online 11 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 240 (2020) 122169
In this paper, we propose a plasma-assisted synthesis of nitrogendoped graphene oxide-MoS2 composite (N-GO-MoS2). Through N plasma treatment on graphene oxide-MoS2 composite (GO-MoS2), the reduction of graphene oxide (GO), the phase transformation of MoS2 and the doping of N can be simultaneously achieved. Furthermore, the ca pacity and cycle performance have been significantly improved for the LIB using N-GO-MoS2 as anode, in comparison with GO-MoS2.
1. Introduction Advanced energy storage technologies such as rechargeable lithium ion batteries (LIB), supercapacitors, Li-air batteries, etc., are considered as the most promising technologies to overcome the challenges of shortage of fossil energy in the future [1–4]. Among these technologies, LIB systems due to their high-energy density, long life, quick recharge ability and environmental friendliness, have been widely studied [5]. However, there are still some obstacles to be solved for the energy storage, such as cost of production, selection of electrode materials, reversible capacity, and so on [6]. As the current commercial LIB anode, graphite only has a low theoretical specific capacity of 372 mAh g 1 [7], which seriously hinders its further development and applications. Although some nanostructured carbon materials, like 0D nanoparticle [8], 1D nanofibers or nanotubes [9,10], 2D graphene [11] and unique 3D architecture nanostructure [12], etc., have the potential to surpass the theoretical specific capacity, the cost and large-scale production will be the main obstacles. Thus, much effort has been devoted to find new electrode materials, e.g. 2D materials, which have been explosively applied in LIB. As a promising typical 2D material, MoS2 benefit from its unique S–Mo–S layers separated by weak van der Waals attractions, the large intrinsic interlayer spacing (0.62 nm) and high theoretical lithium storage capacity (～669 mAh g 1), has been widely studied in lithium energy storage field [13–15]. However, there are some disadvantages, such as intrinsic poor electronic conductivity of sulfides, irreversible restack and volume variation, low active material utilization [16,17], resulting in significant capacity fading and inferior rate performance. Various strategies have been proposed to overcome these challenges. Generally, there are two strategies which are often applied: combination with carbon materials to improve the intrinsic poor electronic conduc tivity of MoS2 and modification of the morphology of MoS2 to reduce irreversible volume variation [18–21]. Typically, combination with carbon materials is based on various nanostructured carbon materials such as 3D architecture nanostructure [22,23], 2D graphene [24–27], 1D carbon nanotube or nanofiber [28,29], and 0D carbon nanoparticle [30,31]. Modification of morphology depends on some materials (the most typical material is carbon) those have unique nanostructures or by adding some chemicals such as PVP [32], or changing the concentration of the solution such as ethanol during the synthesis [33]. In addition, N doping [34] or various crystalline phases [35] have been applied for MoS2 in LIB. However, most strategies require high sintering carbon ization temperature and complicated processes, resulting in a low yield, which is not conducive to the commercialization of MoS2 as anode in LIB.
2. Experimental details 2.1. Synthesis of GO-MoS2 and N-GO-MoS2 The GO was prepared by modified Hummer’s method [36,37]. The GO-MoS2 was synthesized by hydrothermal method. Typically, 30 mg GO was added to 60 mL deionized water and sonicated for 2 h in an ice bath. 1.7655 g (NH4)6Mo7O24⋅4H2O and 1.5225 g thiourea were dis solved in GO dispersion, then the mixture was transferred into a 100 mL Teflon-lined stainless -steel autoclave, kept in an oven at 200 � C for 24 h, and then cooled to room temperature. The obtained black powder was then washed repeatedly with distilled water and absolute ethanol and dried in a vacuum oven at 80 � C for 10 h, labeled as GO-MoS2. N plasma bombarded 0.4 g GO-MoS2 for 40 min in a glass tube, with a power of 100 W under a N2 flux of 100 mL/min under pressure of 10 Pa at room temperature, and the products was labeled as N-GO-MoS2. 2.2. Structural characterizations The crystalline structure was studied by powder X-ray diffraction (XRD, Rigaku Smartlab 3) using Cu-Kα radiation (λ ¼ 1.5406 Å). Raman measurements were performed on Horiba Jobin Yvon LabRAM HR800 spectrometer with excitation laser beam wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Prevac spectrometer employing Al Kα radiation. Scanning electron microscope (SEM) images were taken on an FEI Inspect F50. Trans mission electron microscope (TEM) and energy-dispersive X-ray spec troscopy (EDS) elemental mapping were performed on an FEI Titan G2 80–200 ChemiSTEM. 2.3. Electrochemical measurements The working electrodes was first prepared by mixing active materials (GO-MoS2 or N-GO-MoS2) with carbon black (super P and CNTs) and binder [polyvinylidene fluoride (PVDF)] in a weight ratio of 80:10:5:5 into suitable 1-Methyl-2-pyrrolidinone (NMP) to form slurry. Then the slurry was uniformly coated on a Cu foil with a scraper, dried at 120 � C in a vacuum oven overnight to form the electrode film. The dried
Fig. 1. Structural characterization of GO-MoS2 and N-GO-MoS2, (a) XRD patterns, (b) Raman spectra. 2
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Fig. 2. High-resolution XPS spectra of GO-MoS2 and N-GO-MoS2 for (a) C 1s, (b) N 1s, (c) Mo 3d, (d) S 2p, with the fitted results.
positive electrode sheet was cut into discs with diameter of 12 mm, and then pressed under a fixed pressure. The coin cells were assembled in an argon-filled glove box. Pure lithium foil was used as the counter and reference electrode, a polypropylene separator (Celgard 2400) was used to separate the cathode and anode with 1 mol l 1 LiPF6 in ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1 v/v ratio) as the elec trolyte. Galvanostatic charging/discharging tests were conducted in a voltage cut-off of 0–3.0 V (vs. Liþ/Li) at different C rates on a Neware Battery Testing System at room temperature. The specific capacity was calculated based on the mass of active material. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on CHI 660b electrochemical analyzer workstation (Chenhua, Shanghai, China). For the EIS measurements, the amplitude of AC voltage on cells was 5 mV and the frequency range was between 0.1 Hz and 100 kHz.
spacing. The properly reducing layer spacing facilitates the transfer of electrons on the surface and improves the conductivity of MoS2 [40]. It is worth noting that after N plasma treatment, (102) and (006) peaks disappear, (103) and (105) peaks appear [35,41], may be caused by crystalline phase conversion. To further make clear the crystalline structure transition after the N plasma treatment, the Raman spectra of both samples are obtained, as shown in Fig. 1(b). Raman spectrum of GO-MoS2 has seven peaks at around 145, 193, 234, 281, 334, 374 and 402 cm 1. The peak at 281 cm 1 is the typical E1g mode of the metallic octahedral coordinated MoS2 (1T-MoS2) [42]. Furthermore, J1, J2 and J3 modes are observed at 145, 234 and 334 cm 1, respectively, further confirming the 1T structure [35]. The peak at 193 cm 1 arises from the different layers of MoS2 [35], 374 and 402 cm 1 are the typical peaks of semiconducting trigonal MoS2 (2H–MoS2) [43,44], which may be caused by the transformation of 1T MoS2 under Raman illumination and the drying process during synthesis [35]. Theoretical calculations indi cate that 1T phase is inherently unstable, may lead to a transition to 2H phase under heating conditions [45,46]. Therefore, the N plasma has high energy, and bombardment on the MoS2 surface can easily lead to the transition from 1T to 2H. There are only two sharp peaks around 376 and 401 cm 1 for N-GO-MoS2, which correspond to typical E12g and A1g peaks of 2H MoS2, confirming the structure transformation from 1T to 2H by the N plasma treatment. The D and G bands of GO appear at about 1354 and 1580 cm 1 after the N plasma treatment, indicating the reduction of GO to graphene [39]. The relative intensity of D band to G band (ID/IG) in N-GO-MoS2 is 1.27, which is higher than that in GO-MoS2 (ID/IG � 1.02), indicating that GO in N-GO-MoS2 has a high disordered structure, which is conducive to the transport of Liþ ions [47]. XPS spectra for GO-MoS2 and N-GO-MoS2 were obtained to detect
3. Results and discussion Fig. 1(a) shows the XRD patterns of GO-MoS2 and N-GO-MoS2. Due to the low concentration of GO (less than 2% of mass ratio, calculated by the weight of GO added and the final product), the diffraction peaks from GO is hardly seen in XRD pattern. The observed peaks in XRD patterns can be assigned to the diffraction from (002), (100), (102), (103), (006), (105), and (110) planes of MoS2 (JCPDS card No. 37–1492) [38,39]. After the N plasma treatment, only (002), (100) and (110) peaks can be observed for N-GO-MoS2, (102) and (006) peaks disappear. The (002) peak of GO-MoS2 locates at 12.62� with a 4.75� half width, while N-GO-MoS2 has a slight right shift of 1.25� at 13.87� with a much smaller half width of 3.09� , indicating that plasma treat ment can improve crystalline structure with shrinkage of interlayer 3
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bond [50]. The N 1s peak can be deconvoluted to peaks locating at 398.4 and 400.0 eV, which correspond to pyridinic N and pyrrolic N, respec tively [35,51,52]. Through the analysis of Mo 3d spectrum of GO-MoS2 in Fig. 2(c), the peaks at 228.5 eV and 231.8 eV correspond to the 3d5/2 and 3d3/2 of Mo–S bonding in 1T MoS2 [41]. The Mo 3d peaks at 229.2 eV and 233.0 eV can be attributed to 2H phase [53]. In addition, a weak peak is observed at around 225.8 eV, which can be assigned to 2s peak of S [54]. After N plasma treatment, besides the observation of both Mo 3d5/2 and 3d3/2, a new peak at around 235.7 eV is observed, which can be attributed to Mo6þ due to the N doping [55]. The Mo6þ ions in the N-GO-MoS2 may improve the oxidation and reduction between Mo and Mo6þ during discharge and charge processes, thus the reversible ca pacity during the electrochemical cycling [30]. Additionally, in N-GO-MoS2, the Mo 3d5/2 and 3d3/2 of N-GO-MoS2 only appear at around 229.2 and 232.5 eV, confirming that there is a phase trans formation by N plasma treatment. The XPS spectra of S 2p3/2 and 2p1/2 for GO-MoS2 are at 161.3 and 162.3 eV, shown in Fig. 2(d), corre sponding to those of 1T MoS2. Additionally, the new S 2P peaks at 162.0 eV and 163.0 eV can be attributed to 2H phase. After N plasma treatment, these S 2p peaks are only at 162.0 and 163.0 eV, corre sponding to 2H MoS2. Fig. 3 shows the SEM and TEM images of GO-MoS2 and N-GO-MoS2. As shown in Fig. 3(a), the SEM image of GO-MoS2 shows a flower-like appearance that is stacked by a few layers of MoS2 petals. After N plasma treatment, no significant morphology change of MoS2 petals is observed. However, GO cannot be clearly observed in the SEM images. TEM images in Fig. 3(b) and (f) show clearly the uniform mixing of GO and MoS2 sheets. This can be further clearly observed in high-resolution TEM (HRTEM) images, shown in Fig. 3(c) and (g). The clear layered structure is from MoS2 sheets, and GO shows clearly flocculating morphology, surrounding the MoS2 sheets. The interlayer distance of MoS2 is measured, which is 0.68 nm for GO-MoS2, and 0.65 nm for NGO-MoS2, corresponding to the (002) planes of MoS2. The slight smaller layer distance after N plasma treatment for N-GO-MoS2 is consistent with the results of XRD, which might be due to the structure trans formation and better crystallinity [40]. Though the layer distance is slight smaller after N plasma treatment, the ion radius of Liþ is only 0.076 nm [56], which is much smaller than the interlayer distance of GO-MoS2 and N-GO-MoS2. This indicates that the interlayer distance should not be the main cause of the difference in electrochemical per formance between N-GO-MoS2 and GO-MoS2. Furthermore, the energy disperse spectroscopic (EDS) elemental mapping images (Fig. 3(d) and (h)) of GO-MoS2 and N-GO-MoS2 provide clear evidence of uniform distribution of GO, which wraps the MoS2 sheet. Furthermore, the much stronger N signal in the N elemental mapping for N-GO-MoS2 indicates the efficient doping of N by N plasma treatment. And also the uniform intensity of N not only in MoS2, but also in C, indicating that N has been uniformly doped into both MoS2 and GO. In order to investigate the chemical reaction in LIB delithiation/ interlithiation processes, cyclic voltammetry (CV) was measured be tween 0 and 3 V at 0.2 mV s 1 scan rate. Fig. 4(a) shows the CV curves of the 1st, 2nd and 3rd cycles of GO-MoS2. The first discharge of GO-MoS2 exhibits two peaks at 1.5 V and 0.44 V. The peak at 1.5 V is ascribed to the intercalation of Liþ into 1T-MoS2 layer to form LixMoS2, and the peak at 0.44 V is attributed to the conversion from LixMoS2 to Li2S and Mo nanoparticles [57]. During the first delithiation process, two remarkable peaks are present at 1.79 and 2.24 V, which associate with the reaction from Mo to MoS2 and the oxidation of Li2S to sulfur [58]. Fig. 4(b) show the CV curves of the 1st, 2nd and 3rd cycles of N-GO-MoS2. The first discharge of N-GO-MoS2 exhibits two peaks at 1.07 and 0.44 V. Compared with GO-MoS2, the different peak position at 1.07 V is ascribed to the intercalation of Liþ into 2H MoS2 layer, forming LixMoS2, with the phase transformation of 2H to 1T structure [59,60]. Like GO-MoS2, two remarkable peaks during the first delithiation pro cess are present at 1.79 and 2.24 V, which associate with the Mo reaction and the oxidation of Li2S. The first CV cycle of GO-MoS2 and N-GO-MoS2
Fig. 3. SEM images of (a) GO-MoS2, and (e) N-GO-MoS2. TEM images of (b) GO-MoS2 and (f) N-GO-MoS2. HRTEM images of (c) GO-MoS2 and (g) N-GOMoS2. Elemental mapping images of S, Mo, C, and N corresponding to (d) the selected area in (c), and (h) the selected area in (f).
the modification of the valence state of each element after N plasma treatment, as shown in Fig. 2. Fig. 2(a) is the typical C1s peak of GOMoS2 and N-GO-MoS2, which is deconvoluted into four types of carbon bonds at 284.6eV (C–C), 285.7eV (C–N/C–S), 286.6eV (C–O) and – O) [48]. After N plasma treatment, the peak intensities of 288.6eV (C– oxygen functional groups in N-GO-MoS2 are much weaker while the peak intensities of C–N/C–S are much higher. The unstable oxygen functional groups in GO are removed and some nitrogen functional groups are formed, confirming the N doping in GO [49]. N 1s XPS spectrum is overlapped with Mo 3p peak, as can be seen in Fig. 2(b). After the deconvolution, the N 1s peak can be clearly seen. The intensity of N 1s peak significantly increases after the N plasma treatment, further confirming the doping of N in N-GO-MoS2. The N 1s peak appears at about 400 eV in N-GO-MoS2, which is due to the formation of Mo–N 4
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Fig. 4. Cyclic voltammetry curves of (a) GO-MoS2 and (b) N-GO-MoS2 at a scanning rate of 0.2 mV s
1
for the initial three scans.
Fig. 5. Charge/discharge profiles for the first three cycle at 0.5C and the first cycle at the different current density of 1, 5, 10 C of (a) GO-MoS2 and (b) N-GO-MoS2. (c) Rate performance with different current density and (d) Cycling performance at 1 C of GO-MoS2 and N-GO-MoS2.
are consistent with the results of Raman, that GO-MoS2 and N-GO-MoS2 are in 1T and 2H structure, respectively [35]. For the first discharge curve, N-GO-MoS2 exhibits a new peak between 0.01 and 0.25 V, which can be ascribed to the formation of a solid electrolyte interface (SEI) layer or an irreversible reaction of Liþ with the surface functional groups in N-GO. In the subsequent discharge cycles, two small reduction peaks appear at 1.36 and 2.00 V for GO-MoS2, 1.24 and 1.98 V for N-GO-MoS2, respectively. The peaks at 1.36 V or 1.24 V correspond to the oxidation of Mo to Mo4þ and/or Mo6þ, the peaks at 2.00 V or 1.98 V correspond to the reduction of sulfur [61]. The redox peaks intensity at 2.00/2.24 V of
N-GO-MoS2 is higher than GO-MoS2, indicating that the lithium-sulfur battery effect provides more significant effect on the capacity contri bution. The 2nd and 3rd cycle curves are nearly overlapped, which re flects the excellent cycling stability of GO-MoS2 and N-GO-MoS2 at subsequent cycles. It is worth noting that there are some differences in voltage of Mo oxidation and sulfur reduction, which can be explained by the atomic structure of MoS2. 2H MoS2 owns a trigonal prism structure, while 1T MoS2 has octahedral coordination of metal atoms. Compared with 2H MoS2, this metallic phase can accelerate charge transfer [35, 62]. 5
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Fig. 6. (a) EIS spectra, (b) the linear fits of Z0 and ω
1/2
of GO-MoS2 and N-GO-MoS2. The inset image in (a) is the equivalent circuit.
To further study the electrochemical performance of N-GO-MoS2, rate and cycling performance are shown in Fig. 5. Fig. 5(a) and (b) are the charge-discharge profiles of initial three cycles at 0.5C and the subsequent cycles under various rates, the corresponding capacities have been summarized in Fig. 5(c). When discharge and charge at a low rate current of 0.5 C, N-GO-MoS2 delivers a high initial discharge and charge capacities of 967.4 and 798.6 mAh g 1, and a Coulombic effi ciency of 82.5%. This result means that a few Li ions are trapped by MoS2 or consumed for SEI formation during the first cycle. During the first discharge and charge profiles, the charge and discharge platform that appears is the same as the first circle in the CV curve. The curves of the second circle and the third circle almost coincide, indicating that the charge and discharge of the battery from the second run is quite stable. Also, the charge and discharge platform of the curve is also the same as the corresponding runs of the CV curve. Although discharge and charge capacities decrease gradually with increasing rate current, specific ca pacity can still be maintained at a high value. Compared with N-GOMoS2, the charge/discharge profiles of GO-MoS2 display much inferior performance, especially at high rate, with only 108.5 and 119.7 mAh g 1 at 10 C, respectively. Rate performance is shown in Fig. 5(c), with increasing the rate from 0.5C to 10 C, and then returning to 0.5 C. It is noted that the cells run five cycles at different rate current, except for the first run with six cycles at 0.5 C, for the formation of SEI. N-GO-MoS2 electrode delivers high discharge specific capacities of 967.4 mAh g 1 at first cycle, and 884.9, 857.3, 696.1, 590.8 mAh g 1 at 0.5, 1, 5, 10 C, respectively. When the current returns to 0.5 C, the specific capacity completely recovers and remain stably around 924.9 mA g 1 Compared with N-GO-MoS2, GO-MoS2 electrode displays much inferior capacities under the same condition, which delivers discharge specific capacities of 627.3, 605.7, 394.7, 119.7 mAh g 1 at 0.5, 1, 5, 10 C, respectively. Also, after high rate current, GO-MoS2 cells are no longer stable. When charging/discharging at a low rate current of 0.1 C and then high rate current of 1 C for 100 cycles, as shown in Fig. 5(d), N-GO-MoS2 exhibits a stable cycling behavior with a starting specific capacity of 726.9 mAh g 1 and 592.7 mAh g 1 after 100 cycles, with only 1.34 mAh g 1 loss per cycle and 81.5% capacity retention rate. GO-MoS2 exhibits much poorer cycle stability, with a start capacity of 561.4 mAh g 1 and 31.6 mAh g 1 after 100 cycles. The electrochemical impedance spectroscopy (EIS) was applied to further investigate the electrochemical performance. Fig. 6 shows the EIS spectra and the fitting curves for GO-MoS2 and N-GO-MoS2. The equivalent circuit is shown in the inset of Fig. 6(a), which contains three units, solution resistance (Rs), charge transfer resistance (Rct) and Warburg impedance (Zw) [63]. In LIB systems, Rs is the intercept impedance at high frequencies, Rct is the semicircle in the high-middle frequency region, and Zw stands for the line inclined in the
low-frequency region. As shown in Fig. 6(a), Rs of N-GO-MoS2 (3.9 Ω) and GO-MoS2 (4.6 Ω) are almost same, due to the same cell structure with same electrolyte. Rct of N-GO-MoS2 is 79.6 Ω, which is much smaller than that of GO-MoS2 (164.2 Ω), indicating that N-GO-MoS2 has a larger charge transfer ability than GO-MoS2. According to the fitting equivalent circuit, the Warburg coefficient (σ) of N-GO-MoS2 (34.17) is 0.66 times of that of GO-MoS2 (51.39), corresponding to a 2.26 times higher Li-ion diffusion coefficient [38], which reveals that N plasma treatment significantly facilitates the Liþ ion diffusion. As is shown in Fig. 7(a) and (e), the CV measurements at different scan rate from 0.1 to 2 mV s 1 were carried out to investigate the kinetic origin of improved rate capability of N-GO-MoS2. There is a relationship of i ¼ aνb, where i is the peak current associated with a particular scan rate ν, a and b are both fitting parameters [64]. b can be determined from the slop of log(i) versus log(v). b ¼ 0.5 implies that the current is proportional to the square root of the scan rate, which is consistent with traditional diffusion dominated charge storage, while b ¼ 1 indicates that the current is linearly proportional to the scan rate, which is characteristic of a capacitor–like charge storage mechanism [64]. Thus, it can be applied to analyze the degree of capacitive effect. By plotting log(i) vs. log(ν), b of GO-MoS2 and N-GO-MoS2 at four peaks are 0.94324, 0.78373, 0.72327, 0.67174 and 1.05428, 0.89043, 0.89933, 0.83597, respectively, indicating that N-GO-MoS2 has a superior capacitive kinetics than GO-MoS2. The equation i(V) ¼ k1νþk2ν1/2, where k1 and k2 are both determined, and k1ν represents the capacitive and k2ν1/2 the diffusion contribution [64,65]. This can be used to quantitatively analyze the percentage of capacitive and diffusion con tributions at a certain scan rate. This capacitive contribution is a pseu docapacitive contribution, stemming from the delithiation/interlithiation between the MoS2 layers [64]. Fig. 7(c) and (d) show the capacitive contribution at 1 mV s 1 and normalized contribution ratio of capacitive and diffusion capacities at different scan rates of GO-MoS2. The capacitive contributions are 44.34%, 46.82%, 48.51%, 51.04%, 53.54% and 63.95% at the scan rates of 0.2, 0.4, 0.6, 0.8, 1.0 and 2.0 mV s 1, respectively. Fig. 7(g) and (h) show the capacitive contribution at 1 mV s 1 and normalized contribution ratio of capacitive and diffusion capacities at different scan rates of N-GO-MoS2. Compared with GO-MoS2, N-GO-MoS2 has higher capacitive contribu tions, which are 64.62%, 66.61%, 68.64%, 70.49%, 71.68% and 71.81% at the scan rates of 0.2, 0.4, 0.6, 0.8, 1.0 and 2.0 mV s 1, respectively. These results suggest that N-GO-MoS2 has favorable electrochemical kinetics, leading to the excellent rate capability. On the basis of above result, it can be concluded that N plasma treatment significantly improves the rate capability and cycling per formance of MoS2. Firstly, the high-energy N plasma bombards the surface of MoS2 and breaks the Mo–S bonds to form S vacancies. The S 6
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Fig. 7. (a) and (e) are CV curves at various scan rate from 0.1 to 2 mV s 1, (b) and (f) are relationship between log(ν) and log(i), (c) and (g) are capacitive contribution at 1 mV s 1; (d) and (h) are normalized contribution ratio of capacitive and diffusion capacities at different scan rate of GO-MoS2 and N-GO-MoS2.
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vacancies modify the work function of MoS2 by some suspended Mo bonds combining with N to form Mo–N bonds, which is confirmed by the results of XPS. The Mo–N bonds can increase the electronic conductivity compared to the Mo–S bonds, which is better for MoS2 used in electro chemical performance. Secondly, after plasma treatment, the structure of MoS2 transforms from unstable 1T phase to stable 2H phase. Thirdly, the N plasma also acts on GO to convert it to N-doped graphene [49], which improves the electrical conductivity [66]. Graphene is often used as an excellent conductive material to combine with active materials, but it is very easy to agglomerate and is not suitable for water dispersion, which limits its application. Generally GO is dispersed in water to mix with the active material, and then GO is reduced by high temperature to form reduced GO. However, high-temperature treatment may modify the crytstalline structure of MoS2, and it is difficult to maintain few-layered structure. By using N plasma treatment, in addition to the in situ reduction of GO, the doping of N ions in both GO and MoS2 is simultaneously achieved. Furthermore, N plasma treatment at room temperature does not influence the few layer structure of MoS2. The above mentioned advantages synergistically contribute to the high performance of N-GO-MoS2 electrode.
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4. Conclusion In summary, we successfully develop a facile method to synthesize the N-doped graphene-MoS2 composites for high performance anode in LIB. The mixtures of GO and MoS2 are prepared by hydrothermal method. And then the composites are further treated in N plasma at room temperature. The N plasma treatment not only converts MoS2 from the 1T structure to the 2H structure, reduces the GO to graphene, but also efficiently dopes N into both GO and MoS2. These factors syner gistically contribute to the significant improved electrochemical per formance of MoS2. N-GO-MoS2 exhibits a stable cycling behavior with an initial specific capacity of 726.9 mAh g 1 and 592.7 mAh g 1 after 100 cycles, which is only 1.34 mAh g 1 loss per cycle and 81.5% ca pacity retention rate. This is much better than that of GO-MoS2 without N plasma treatment, which exhibits poor cycle stability, with an initial capacity of 561.4 mAh g 1 and 31.6 mAh g 1 after 100 cycles. The plasma treatment can operate at room temperature to reduce the GO to graphene and simultaneously dope selected atoms in composites, which provides an efficient technique for the synthesis and modification of electrode materials in LIB. Acknowledgement This work is supported by the National Natural Science Foundation of China (51771053, 51471085), the National Key Research and Development Program of China (2016YFA0300803), the Natural Sci ence Foundation of Jiangsu Province of China (BK20151400), the Fundamental Research Funds for the Central Universities, and the open research fund of Key Laboratory of MEMS of Ministry of Education, Southeast University. References [1] M. Armand, J.-M. Tarascon, Building better batteries:Researchers must find a sustainable way of providing the power our modern lifestyles demand, Nature 451 (2008) 652–657. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [3] N. Mahmood, T. Tang, Y. Hou, Nanostructured anode materials for lithium ion batteries: progress, challenge and perspective, Adv. Energy Mater. 6 (2016) 1600374. [4] W. Zhang, Y. Sun, Q. Liu, J. Guo, X. Zhang, Vanadium and nitrogen co-doped CoP nanoleaf array as pH-universal electrocatalyst for efficient hydrogen evolution, J. Alloy. Comp. 791 (2019) 1070–1078. [5] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176.
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N plasma treatment on graphene oxide-MoS2 composites for improved performance in lithium ion batteries Jingxuan Jiao a, Kai Du b, Yuanting Wang a, Pingping Sun c, Huihui Zhao a, Peijuan Tang a, Qi Fan d, *, He Tian b, Qi Li a, **, Qingyu Xu a, e, *** a
School of Physics, Southeast University, Nanjing, 211189, China Center of Electron Microscopy, State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China College of Physics and Electronic Engineering, Jiangsu Second Normal University, Nanjing, 210013, China d College of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, China e National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� N plasma treatment is applied on gra phene oxide (GO)-MoS2 composites (NGO-MoS2). � N has been successfully doped into both GO and MoS2. � The structure of MoS2 transforms from the metallic phase (1T) to semi conducting phase (2H). � The electrochemical performance of NGO-MoS2 anode has been significantly improved due to better electronic conductivity.
A R T I C L E I N F O
A B S T R A C T
Keywords: MoS2 Lithium-ion battery Plasma
Petal-like graphene oxide-MoS2 (GO-MoS2) composites are prepared by the hydrothermal method. The structural characterizations confirm the metallic phase (1T) of MoS2 sheets with a few layers, and the uniform distribution of graphene oxide (GO) and MoS2. However, the functional groups on the surface of GO and the intrinsic low conductivity of MoS2 lead to the poor electrochemical performance of GO-MoS2 in lithium ion batteries. N-doped GO-MoS2 (N-GO-MoS2) composites are prepared by N plasma treatment on GO-MoS2 composites at room tem perature. N is clearly confirmed to be doped into both graphene oxide and MoS2 uniformly, and MoS2 changes to semiconducting phase (2H). In lithium ion batteries, it is found that the initial capacity increases from 561.4 mAh g 1 for GO-MoS2 to 726.9 mAh g 1 for N-GO-MoS2 at 1 C. After 100 cycles, N-GO-MoS2 still exhibits capacity of 592.7 mAh g 1 (1.34 mAh g 1 loss per cycles and 81.5% capacity retention rate), which is much better than that of GO-MoS2 (only 31.6 mAh g 1 after 100 cycles). The significantly improved high-rate cycling electrochemical performance for N-GO-MoS2 can be attributed to the improved stability of MoS2 by the phase transition from 1T to 2H, the enhanced electron transportation by the reduction of graphene oxide to graphene and the simulta neous N doping in MoS2 and graphene oxide.
* Corresponding author. ** Corresponding author. *** Corresponding author. School of Physics, Southeast University, Nanjing, 211189, China. E-mail addresses: [email protected] (Q. Fan), [email protected] (Q. Li), [email protected] (Q. Xu). https://doi.org/10.1016/j.matchemphys.2019.122169 Received 4 June 2019; Received in revised form 29 August 2019; Accepted 10 September 2019 Available online 11 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 240 (2020) 122169
In this paper, we propose a plasma-assisted synthesis of nitrogendoped graphene oxide-MoS2 composite (N-GO-MoS2). Through N plasma treatment on graphene oxide-MoS2 composite (GO-MoS2), the reduction of graphene oxide (GO), the phase transformation of MoS2 and the doping of N can be simultaneously achieved. Furthermore, the ca pacity and cycle performance have been significantly improved for the LIB using N-GO-MoS2 as anode, in comparison with GO-MoS2.
1. Introduction Advanced energy storage technologies such as rechargeable lithium ion batteries (LIB), supercapacitors, Li-air batteries, etc., are considered as the most promising technologies to overcome the challenges of shortage of fossil energy in the future [1–4]. Among these technologies, LIB systems due to their high-energy density, long life, quick recharge ability and environmental friendliness, have been widely studied [5]. However, there are still some obstacles to be solved for the energy storage, such as cost of production, selection of electrode materials, reversible capacity, and so on [6]. As the current commercial LIB anode, graphite only has a low theoretical specific capacity of 372 mAh g 1 [7], which seriously hinders its further development and applications. Although some nanostructured carbon materials, like 0D nanoparticle [8], 1D nanofibers or nanotubes [9,10], 2D graphene [11] and unique 3D architecture nanostructure [12], etc., have the potential to surpass the theoretical specific capacity, the cost and large-scale production will be the main obstacles. Thus, much effort has been devoted to find new electrode materials, e.g. 2D materials, which have been explosively applied in LIB. As a promising typical 2D material, MoS2 benefit from its unique S–Mo–S layers separated by weak van der Waals attractions, the large intrinsic interlayer spacing (0.62 nm) and high theoretical lithium storage capacity (～669 mAh g 1), has been widely studied in lithium energy storage field [13–15]. However, there are some disadvantages, such as intrinsic poor electronic conductivity of sulfides, irreversible restack and volume variation, low active material utilization [16,17], resulting in significant capacity fading and inferior rate performance. Various strategies have been proposed to overcome these challenges. Generally, there are two strategies which are often applied: combination with carbon materials to improve the intrinsic poor electronic conduc tivity of MoS2 and modification of the morphology of MoS2 to reduce irreversible volume variation [18–21]. Typically, combination with carbon materials is based on various nanostructured carbon materials such as 3D architecture nanostructure [22,23], 2D graphene [24–27], 1D carbon nanotube or nanofiber [28,29], and 0D carbon nanoparticle [30,31]. Modification of morphology depends on some materials (the most typical material is carbon) those have unique nanostructures or by adding some chemicals such as PVP [32], or changing the concentration of the solution such as ethanol during the synthesis [33]. In addition, N doping [34] or various crystalline phases [35] have been applied for MoS2 in LIB. However, most strategies require high sintering carbon ization temperature and complicated processes, resulting in a low yield, which is not conducive to the commercialization of MoS2 as anode in LIB.
2. Experimental details 2.1. Synthesis of GO-MoS2 and N-GO-MoS2 The GO was prepared by modified Hummer’s method [36,37]. The GO-MoS2 was synthesized by hydrothermal method. Typically, 30 mg GO was added to 60 mL deionized water and sonicated for 2 h in an ice bath. 1.7655 g (NH4)6Mo7O24⋅4H2O and 1.5225 g thiourea were dis solved in GO dispersion, then the mixture was transferred into a 100 mL Teflon-lined stainless -steel autoclave, kept in an oven at 200 � C for 24 h, and then cooled to room temperature. The obtained black powder was then washed repeatedly with distilled water and absolute ethanol and dried in a vacuum oven at 80 � C for 10 h, labeled as GO-MoS2. N plasma bombarded 0.4 g GO-MoS2 for 40 min in a glass tube, with a power of 100 W under a N2 flux of 100 mL/min under pressure of 10 Pa at room temperature, and the products was labeled as N-GO-MoS2. 2.2. Structural characterizations The crystalline structure was studied by powder X-ray diffraction (XRD, Rigaku Smartlab 3) using Cu-Kα radiation (λ ¼ 1.5406 Å). Raman measurements were performed on Horiba Jobin Yvon LabRAM HR800 spectrometer with excitation laser beam wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Prevac spectrometer employing Al Kα radiation. Scanning electron microscope (SEM) images were taken on an FEI Inspect F50. Trans mission electron microscope (TEM) and energy-dispersive X-ray spec troscopy (EDS) elemental mapping were performed on an FEI Titan G2 80–200 ChemiSTEM. 2.3. Electrochemical measurements The working electrodes was first prepared by mixing active materials (GO-MoS2 or N-GO-MoS2) with carbon black (super P and CNTs) and binder [polyvinylidene fluoride (PVDF)] in a weight ratio of 80:10:5:5 into suitable 1-Methyl-2-pyrrolidinone (NMP) to form slurry. Then the slurry was uniformly coated on a Cu foil with a scraper, dried at 120 � C in a vacuum oven overnight to form the electrode film. The dried
Fig. 1. Structural characterization of GO-MoS2 and N-GO-MoS2, (a) XRD patterns, (b) Raman spectra. 2
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Materials Chemistry and Physics 240 (2020) 122169
Fig. 2. High-resolution XPS spectra of GO-MoS2 and N-GO-MoS2 for (a) C 1s, (b) N 1s, (c) Mo 3d, (d) S 2p, with the fitted results.
positive electrode sheet was cut into discs with diameter of 12 mm, and then pressed under a fixed pressure. The coin cells were assembled in an argon-filled glove box. Pure lithium foil was used as the counter and reference electrode, a polypropylene separator (Celgard 2400) was used to separate the cathode and anode with 1 mol l 1 LiPF6 in ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1 v/v ratio) as the elec trolyte. Galvanostatic charging/discharging tests were conducted in a voltage cut-off of 0–3.0 V (vs. Liþ/Li) at different C rates on a Neware Battery Testing System at room temperature. The specific capacity was calculated based on the mass of active material. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on CHI 660b electrochemical analyzer workstation (Chenhua, Shanghai, China). For the EIS measurements, the amplitude of AC voltage on cells was 5 mV and the frequency range was between 0.1 Hz and 100 kHz.
spacing. The properly reducing layer spacing facilitates the transfer of electrons on the surface and improves the conductivity of MoS2 [40]. It is worth noting that after N plasma treatment, (102) and (006) peaks disappear, (103) and (105) peaks appear [35,41], may be caused by crystalline phase conversion. To further make clear the crystalline structure transition after the N plasma treatment, the Raman spectra of both samples are obtained, as shown in Fig. 1(b). Raman spectrum of GO-MoS2 has seven peaks at around 145, 193, 234, 281, 334, 374 and 402 cm 1. The peak at 281 cm 1 is the typical E1g mode of the metallic octahedral coordinated MoS2 (1T-MoS2) [42]. Furthermore, J1, J2 and J3 modes are observed at 145, 234 and 334 cm 1, respectively, further confirming the 1T structure [35]. The peak at 193 cm 1 arises from the different layers of MoS2 [35], 374 and 402 cm 1 are the typical peaks of semiconducting trigonal MoS2 (2H–MoS2) [43,44], which may be caused by the transformation of 1T MoS2 under Raman illumination and the drying process during synthesis [35]. Theoretical calculations indi cate that 1T phase is inherently unstable, may lead to a transition to 2H phase under heating conditions [45,46]. Therefore, the N plasma has high energy, and bombardment on the MoS2 surface can easily lead to the transition from 1T to 2H. There are only two sharp peaks around 376 and 401 cm 1 for N-GO-MoS2, which correspond to typical E12g and A1g peaks of 2H MoS2, confirming the structure transformation from 1T to 2H by the N plasma treatment. The D and G bands of GO appear at about 1354 and 1580 cm 1 after the N plasma treatment, indicating the reduction of GO to graphene [39]. The relative intensity of D band to G band (ID/IG) in N-GO-MoS2 is 1.27, which is higher than that in GO-MoS2 (ID/IG � 1.02), indicating that GO in N-GO-MoS2 has a high disordered structure, which is conducive to the transport of Liþ ions [47]. XPS spectra for GO-MoS2 and N-GO-MoS2 were obtained to detect
3. Results and discussion Fig. 1(a) shows the XRD patterns of GO-MoS2 and N-GO-MoS2. Due to the low concentration of GO (less than 2% of mass ratio, calculated by the weight of GO added and the final product), the diffraction peaks from GO is hardly seen in XRD pattern. The observed peaks in XRD patterns can be assigned to the diffraction from (002), (100), (102), (103), (006), (105), and (110) planes of MoS2 (JCPDS card No. 37–1492) [38,39]. After the N plasma treatment, only (002), (100) and (110) peaks can be observed for N-GO-MoS2, (102) and (006) peaks disappear. The (002) peak of GO-MoS2 locates at 12.62� with a 4.75� half width, while N-GO-MoS2 has a slight right shift of 1.25� at 13.87� with a much smaller half width of 3.09� , indicating that plasma treat ment can improve crystalline structure with shrinkage of interlayer 3
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bond [50]. The N 1s peak can be deconvoluted to peaks locating at 398.4 and 400.0 eV, which correspond to pyridinic N and pyrrolic N, respec tively [35,51,52]. Through the analysis of Mo 3d spectrum of GO-MoS2 in Fig. 2(c), the peaks at 228.5 eV and 231.8 eV correspond to the 3d5/2 and 3d3/2 of Mo–S bonding in 1T MoS2 [41]. The Mo 3d peaks at 229.2 eV and 233.0 eV can be attributed to 2H phase [53]. In addition, a weak peak is observed at around 225.8 eV, which can be assigned to 2s peak of S [54]. After N plasma treatment, besides the observation of both Mo 3d5/2 and 3d3/2, a new peak at around 235.7 eV is observed, which can be attributed to Mo6þ due to the N doping [55]. The Mo6þ ions in the N-GO-MoS2 may improve the oxidation and reduction between Mo and Mo6þ during discharge and charge processes, thus the reversible ca pacity during the electrochemical cycling [30]. Additionally, in N-GO-MoS2, the Mo 3d5/2 and 3d3/2 of N-GO-MoS2 only appear at around 229.2 and 232.5 eV, confirming that there is a phase trans formation by N plasma treatment. The XPS spectra of S 2p3/2 and 2p1/2 for GO-MoS2 are at 161.3 and 162.3 eV, shown in Fig. 2(d), corre sponding to those of 1T MoS2. Additionally, the new S 2P peaks at 162.0 eV and 163.0 eV can be attributed to 2H phase. After N plasma treatment, these S 2p peaks are only at 162.0 and 163.0 eV, corre sponding to 2H MoS2. Fig. 3 shows the SEM and TEM images of GO-MoS2 and N-GO-MoS2. As shown in Fig. 3(a), the SEM image of GO-MoS2 shows a flower-like appearance that is stacked by a few layers of MoS2 petals. After N plasma treatment, no significant morphology change of MoS2 petals is observed. However, GO cannot be clearly observed in the SEM images. TEM images in Fig. 3(b) and (f) show clearly the uniform mixing of GO and MoS2 sheets. This can be further clearly observed in high-resolution TEM (HRTEM) images, shown in Fig. 3(c) and (g). The clear layered structure is from MoS2 sheets, and GO shows clearly flocculating morphology, surrounding the MoS2 sheets. The interlayer distance of MoS2 is measured, which is 0.68 nm for GO-MoS2, and 0.65 nm for NGO-MoS2, corresponding to the (002) planes of MoS2. The slight smaller layer distance after N plasma treatment for N-GO-MoS2 is consistent with the results of XRD, which might be due to the structure trans formation and better crystallinity [40]. Though the layer distance is slight smaller after N plasma treatment, the ion radius of Liþ is only 0.076 nm [56], which is much smaller than the interlayer distance of GO-MoS2 and N-GO-MoS2. This indicates that the interlayer distance should not be the main cause of the difference in electrochemical per formance between N-GO-MoS2 and GO-MoS2. Furthermore, the energy disperse spectroscopic (EDS) elemental mapping images (Fig. 3(d) and (h)) of GO-MoS2 and N-GO-MoS2 provide clear evidence of uniform distribution of GO, which wraps the MoS2 sheet. Furthermore, the much stronger N signal in the N elemental mapping for N-GO-MoS2 indicates the efficient doping of N by N plasma treatment. And also the uniform intensity of N not only in MoS2, but also in C, indicating that N has been uniformly doped into both MoS2 and GO. In order to investigate the chemical reaction in LIB delithiation/ interlithiation processes, cyclic voltammetry (CV) was measured be tween 0 and 3 V at 0.2 mV s 1 scan rate. Fig. 4(a) shows the CV curves of the 1st, 2nd and 3rd cycles of GO-MoS2. The first discharge of GO-MoS2 exhibits two peaks at 1.5 V and 0.44 V. The peak at 1.5 V is ascribed to the intercalation of Liþ into 1T-MoS2 layer to form LixMoS2, and the peak at 0.44 V is attributed to the conversion from LixMoS2 to Li2S and Mo nanoparticles [57]. During the first delithiation process, two remarkable peaks are present at 1.79 and 2.24 V, which associate with the reaction from Mo to MoS2 and the oxidation of Li2S to sulfur [58]. Fig. 4(b) show the CV curves of the 1st, 2nd and 3rd cycles of N-GO-MoS2. The first discharge of N-GO-MoS2 exhibits two peaks at 1.07 and 0.44 V. Compared with GO-MoS2, the different peak position at 1.07 V is ascribed to the intercalation of Liþ into 2H MoS2 layer, forming LixMoS2, with the phase transformation of 2H to 1T structure [59,60]. Like GO-MoS2, two remarkable peaks during the first delithiation pro cess are present at 1.79 and 2.24 V, which associate with the Mo reaction and the oxidation of Li2S. The first CV cycle of GO-MoS2 and N-GO-MoS2
Fig. 3. SEM images of (a) GO-MoS2, and (e) N-GO-MoS2. TEM images of (b) GO-MoS2 and (f) N-GO-MoS2. HRTEM images of (c) GO-MoS2 and (g) N-GOMoS2. Elemental mapping images of S, Mo, C, and N corresponding to (d) the selected area in (c), and (h) the selected area in (f).
the modification of the valence state of each element after N plasma treatment, as shown in Fig. 2. Fig. 2(a) is the typical C1s peak of GOMoS2 and N-GO-MoS2, which is deconvoluted into four types of carbon bonds at 284.6eV (C–C), 285.7eV (C–N/C–S), 286.6eV (C–O) and – O) [48]. After N plasma treatment, the peak intensities of 288.6eV (C– oxygen functional groups in N-GO-MoS2 are much weaker while the peak intensities of C–N/C–S are much higher. The unstable oxygen functional groups in GO are removed and some nitrogen functional groups are formed, confirming the N doping in GO [49]. N 1s XPS spectrum is overlapped with Mo 3p peak, as can be seen in Fig. 2(b). After the deconvolution, the N 1s peak can be clearly seen. The intensity of N 1s peak significantly increases after the N plasma treatment, further confirming the doping of N in N-GO-MoS2. The N 1s peak appears at about 400 eV in N-GO-MoS2, which is due to the formation of Mo–N 4
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Fig. 4. Cyclic voltammetry curves of (a) GO-MoS2 and (b) N-GO-MoS2 at a scanning rate of 0.2 mV s
1
for the initial three scans.
Fig. 5. Charge/discharge profiles for the first three cycle at 0.5C and the first cycle at the different current density of 1, 5, 10 C of (a) GO-MoS2 and (b) N-GO-MoS2. (c) Rate performance with different current density and (d) Cycling performance at 1 C of GO-MoS2 and N-GO-MoS2.
are consistent with the results of Raman, that GO-MoS2 and N-GO-MoS2 are in 1T and 2H structure, respectively [35]. For the first discharge curve, N-GO-MoS2 exhibits a new peak between 0.01 and 0.25 V, which can be ascribed to the formation of a solid electrolyte interface (SEI) layer or an irreversible reaction of Liþ with the surface functional groups in N-GO. In the subsequent discharge cycles, two small reduction peaks appear at 1.36 and 2.00 V for GO-MoS2, 1.24 and 1.98 V for N-GO-MoS2, respectively. The peaks at 1.36 V or 1.24 V correspond to the oxidation of Mo to Mo4þ and/or Mo6þ, the peaks at 2.00 V or 1.98 V correspond to the reduction of sulfur [61]. The redox peaks intensity at 2.00/2.24 V of
N-GO-MoS2 is higher than GO-MoS2, indicating that the lithium-sulfur battery effect provides more significant effect on the capacity contri bution. The 2nd and 3rd cycle curves are nearly overlapped, which re flects the excellent cycling stability of GO-MoS2 and N-GO-MoS2 at subsequent cycles. It is worth noting that there are some differences in voltage of Mo oxidation and sulfur reduction, which can be explained by the atomic structure of MoS2. 2H MoS2 owns a trigonal prism structure, while 1T MoS2 has octahedral coordination of metal atoms. Compared with 2H MoS2, this metallic phase can accelerate charge transfer [35, 62]. 5
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Fig. 6. (a) EIS spectra, (b) the linear fits of Z0 and ω
1/2
of GO-MoS2 and N-GO-MoS2. The inset image in (a) is the equivalent circuit.
To further study the electrochemical performance of N-GO-MoS2, rate and cycling performance are shown in Fig. 5. Fig. 5(a) and (b) are the charge-discharge profiles of initial three cycles at 0.5C and the subsequent cycles under various rates, the corresponding capacities have been summarized in Fig. 5(c). When discharge and charge at a low rate current of 0.5 C, N-GO-MoS2 delivers a high initial discharge and charge capacities of 967.4 and 798.6 mAh g 1, and a Coulombic effi ciency of 82.5%. This result means that a few Li ions are trapped by MoS2 or consumed for SEI formation during the first cycle. During the first discharge and charge profiles, the charge and discharge platform that appears is the same as the first circle in the CV curve. The curves of the second circle and the third circle almost coincide, indicating that the charge and discharge of the battery from the second run is quite stable. Also, the charge and discharge platform of the curve is also the same as the corresponding runs of the CV curve. Although discharge and charge capacities decrease gradually with increasing rate current, specific ca pacity can still be maintained at a high value. Compared with N-GOMoS2, the charge/discharge profiles of GO-MoS2 display much inferior performance, especially at high rate, with only 108.5 and 119.7 mAh g 1 at 10 C, respectively. Rate performance is shown in Fig. 5(c), with increasing the rate from 0.5C to 10 C, and then returning to 0.5 C. It is noted that the cells run five cycles at different rate current, except for the first run with six cycles at 0.5 C, for the formation of SEI. N-GO-MoS2 electrode delivers high discharge specific capacities of 967.4 mAh g 1 at first cycle, and 884.9, 857.3, 696.1, 590.8 mAh g 1 at 0.5, 1, 5, 10 C, respectively. When the current returns to 0.5 C, the specific capacity completely recovers and remain stably around 924.9 mA g 1 Compared with N-GO-MoS2, GO-MoS2 electrode displays much inferior capacities under the same condition, which delivers discharge specific capacities of 627.3, 605.7, 394.7, 119.7 mAh g 1 at 0.5, 1, 5, 10 C, respectively. Also, after high rate current, GO-MoS2 cells are no longer stable. When charging/discharging at a low rate current of 0.1 C and then high rate current of 1 C for 100 cycles, as shown in Fig. 5(d), N-GO-MoS2 exhibits a stable cycling behavior with a starting specific capacity of 726.9 mAh g 1 and 592.7 mAh g 1 after 100 cycles, with only 1.34 mAh g 1 loss per cycle and 81.5% capacity retention rate. GO-MoS2 exhibits much poorer cycle stability, with a start capacity of 561.4 mAh g 1 and 31.6 mAh g 1 after 100 cycles. The electrochemical impedance spectroscopy (EIS) was applied to further investigate the electrochemical performance. Fig. 6 shows the EIS spectra and the fitting curves for GO-MoS2 and N-GO-MoS2. The equivalent circuit is shown in the inset of Fig. 6(a), which contains three units, solution resistance (Rs), charge transfer resistance (Rct) and Warburg impedance (Zw) [63]. In LIB systems, Rs is the intercept impedance at high frequencies, Rct is the semicircle in the high-middle frequency region, and Zw stands for the line inclined in the
low-frequency region. As shown in Fig. 6(a), Rs of N-GO-MoS2 (3.9 Ω) and GO-MoS2 (4.6 Ω) are almost same, due to the same cell structure with same electrolyte. Rct of N-GO-MoS2 is 79.6 Ω, which is much smaller than that of GO-MoS2 (164.2 Ω), indicating that N-GO-MoS2 has a larger charge transfer ability than GO-MoS2. According to the fitting equivalent circuit, the Warburg coefficient (σ) of N-GO-MoS2 (34.17) is 0.66 times of that of GO-MoS2 (51.39), corresponding to a 2.26 times higher Li-ion diffusion coefficient [38], which reveals that N plasma treatment significantly facilitates the Liþ ion diffusion. As is shown in Fig. 7(a) and (e), the CV measurements at different scan rate from 0.1 to 2 mV s 1 were carried out to investigate the kinetic origin of improved rate capability of N-GO-MoS2. There is a relationship of i ¼ aνb, where i is the peak current associated with a particular scan rate ν, a and b are both fitting parameters [64]. b can be determined from the slop of log(i) versus log(v). b ¼ 0.5 implies that the current is proportional to the square root of the scan rate, which is consistent with traditional diffusion dominated charge storage, while b ¼ 1 indicates that the current is linearly proportional to the scan rate, which is characteristic of a capacitor–like charge storage mechanism [64]. Thus, it can be applied to analyze the degree of capacitive effect. By plotting log(i) vs. log(ν), b of GO-MoS2 and N-GO-MoS2 at four peaks are 0.94324, 0.78373, 0.72327, 0.67174 and 1.05428, 0.89043, 0.89933, 0.83597, respectively, indicating that N-GO-MoS2 has a superior capacitive kinetics than GO-MoS2. The equation i(V) ¼ k1νþk2ν1/2, where k1 and k2 are both determined, and k1ν represents the capacitive and k2ν1/2 the diffusion contribution [64,65]. This can be used to quantitatively analyze the percentage of capacitive and diffusion con tributions at a certain scan rate. This capacitive contribution is a pseu docapacitive contribution, stemming from the delithiation/interlithiation between the MoS2 layers [64]. Fig. 7(c) and (d) show the capacitive contribution at 1 mV s 1 and normalized contribution ratio of capacitive and diffusion capacities at different scan rates of GO-MoS2. The capacitive contributions are 44.34%, 46.82%, 48.51%, 51.04%, 53.54% and 63.95% at the scan rates of 0.2, 0.4, 0.6, 0.8, 1.0 and 2.0 mV s 1, respectively. Fig. 7(g) and (h) show the capacitive contribution at 1 mV s 1 and normalized contribution ratio of capacitive and diffusion capacities at different scan rates of N-GO-MoS2. Compared with GO-MoS2, N-GO-MoS2 has higher capacitive contribu tions, which are 64.62%, 66.61%, 68.64%, 70.49%, 71.68% and 71.81% at the scan rates of 0.2, 0.4, 0.6, 0.8, 1.0 and 2.0 mV s 1, respectively. These results suggest that N-GO-MoS2 has favorable electrochemical kinetics, leading to the excellent rate capability. On the basis of above result, it can be concluded that N plasma treatment significantly improves the rate capability and cycling per formance of MoS2. Firstly, the high-energy N plasma bombards the surface of MoS2 and breaks the Mo–S bonds to form S vacancies. The S 6
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Fig. 7. (a) and (e) are CV curves at various scan rate from 0.1 to 2 mV s 1, (b) and (f) are relationship between log(ν) and log(i), (c) and (g) are capacitive contribution at 1 mV s 1; (d) and (h) are normalized contribution ratio of capacitive and diffusion capacities at different scan rate of GO-MoS2 and N-GO-MoS2.
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vacancies modify the work function of MoS2 by some suspended Mo bonds combining with N to form Mo–N bonds, which is confirmed by the results of XPS. The Mo–N bonds can increase the electronic conductivity compared to the Mo–S bonds, which is better for MoS2 used in electro chemical performance. Secondly, after plasma treatment, the structure of MoS2 transforms from unstable 1T phase to stable 2H phase. Thirdly, the N plasma also acts on GO to convert it to N-doped graphene [49], which improves the electrical conductivity [66]. Graphene is often used as an excellent conductive material to combine with active materials, but it is very easy to agglomerate and is not suitable for water dispersion, which limits its application. Generally GO is dispersed in water to mix with the active material, and then GO is reduced by high temperature to form reduced GO. However, high-temperature treatment may modify the crytstalline structure of MoS2, and it is difficult to maintain few-layered structure. By using N plasma treatment, in addition to the in situ reduction of GO, the doping of N ions in both GO and MoS2 is simultaneously achieved. Furthermore, N plasma treatment at room temperature does not influence the few layer structure of MoS2. The above mentioned advantages synergistically contribute to the high performance of N-GO-MoS2 electrode.
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4. Conclusion In summary, we successfully develop a facile method to synthesize the N-doped graphene-MoS2 composites for high performance anode in LIB. The mixtures of GO and MoS2 are prepared by hydrothermal method. And then the composites are further treated in N plasma at room temperature. The N plasma treatment not only converts MoS2 from the 1T structure to the 2H structure, reduces the GO to graphene, but also efficiently dopes N into both GO and MoS2. These factors syner gistically contribute to the significant improved electrochemical per formance of MoS2. N-GO-MoS2 exhibits a stable cycling behavior with an initial specific capacity of 726.9 mAh g 1 and 592.7 mAh g 1 after 100 cycles, which is only 1.34 mAh g 1 loss per cycle and 81.5% ca pacity retention rate. This is much better than that of GO-MoS2 without N plasma treatment, which exhibits poor cycle stability, with an initial capacity of 561.4 mAh g 1 and 31.6 mAh g 1 after 100 cycles. The plasma treatment can operate at room temperature to reduce the GO to graphene and simultaneously dope selected atoms in composites, which provides an efficient technique for the synthesis and modification of electrode materials in LIB. Acknowledgement This work is supported by the National Natural Science Foundation of China (51771053, 51471085), the National Key Research and Development Program of China (2016YFA0300803), the Natural Sci ence Foundation of Jiangsu Province of China (BK20151400), the Fundamental Research Funds for the Central Universities, and the open research fund of Key Laboratory of MEMS of Ministry of Education, Southeast University. References [1] M. Armand, J.-M. Tarascon, Building better batteries:Researchers must find a sustainable way of providing the power our modern lifestyles demand, Nature 451 (2008) 652–657. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [3] N. Mahmood, T. Tang, Y. Hou, Nanostructured anode materials for lithium ion batteries: progress, challenge and perspective, Adv. Energy Mater. 6 (2016) 1600374. [4] W. Zhang, Y. Sun, Q. Liu, J. Guo, X. Zhang, Vanadium and nitrogen co-doped CoP nanoleaf array as pH-universal electrocatalyst for efficient hydrogen evolution, J. Alloy. Comp. 791 (2019) 1070–1078. [5] J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176.
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