Nitrogen-Doped Carbon Composite

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Novel Amorphous MoS2/MoO3/Nitrogen-doped Carbon Composite with Excellent Electrochemical Performance for Lithium Ion Batteries and Sodium Ion Batteries Kunjie Zhu, Xiaofeng Wang, Jun Liu, Site Li, Hao Wang, Linyu Yang, Sailin Liu, and Tian Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01595 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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ACS Sustainable Chemistry & Engineering

Novel Amorphous MoS2/MoO3/Nitrogen-doped Carbon Composite with Excellent Electrochemical Performance for Lithium Ion Batteries and Sodium Ion Batteries

⊥

Kunjie Zhu,† Xiaofeng Wang, ∗† Jun Liu, ∗‡ Site Li, § Hao Wang,† Linyu Yang, †, Sailin Liu,† Tian Xie† †

School of Materials Science and Engineering, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, China. E-mail: [email protected]. ‡ School of Materials Science and Engineering, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, China. E-mail: [email protected]. § Carnegie Mellon University Department of Chemistry, 4400 Fifth Ave Pittsburgh, PA 15213 USA. ⊥ School of physics and technology, Xin Jiang University, Urumqi, Xinjiang 830000, China. ABSTRACT A novel amorphous MoS2/MoO3/Nitrogen-doped Carbon composite has been successfully synthesized for the first time. The synthesis strategy only involves a facile reaction that partially sulfurizing organic-inorganic hybrid material Mo3O10 (C2H10N2) (named as MoOx/ethylenediamine) nanowire precursors under low temperature (300 ℃). It is more interesting that such amorphous composites as LIBs and SIBs anode electrodes showed much better electrochemical properties than that of most previously reported molybdenum-based materials with crystal structure. Such as, the amorphous composite electrode for LIBs can reach up to 1253.3 mA h g-1 at a current density of 100 mA g-1 after 50 cycles, and still retain 887.5 mA h g-1 at 1000 mA g-1 after 350 cycles. Similarly, for SIBs, it also retains 538.7 mA h g-1 after 200 cycles at 300 mA g-1and keeps 339.9 mA h g-1 at 1000 mA g-1 after 220 cycles, corresponding to the capacity retentions of nearly 100%. In addition, the amorphous composite electrode exhibits superior rate performance for LIBs and SIBs. Such superior electrochemical performance may be attributed to: (1), Carbonaceous matrix can enhance the conductivity of the amorphous composite. (2), Heteroatom, such as N, doping within this unique compositional features can increase the active ions absorption sites on the amorphous composite surface which benefitted the insertion / extraction of lithium /sodium ions. (3), The hybrid nanomaterials could provide plenty of diffusion channels for ions during the insertion/extraction process. (4) 1D chain structure reduces the transfer distance of lithium/sodium ions into/from the electrode. KEYWORDS: Amorphous; MoS2/MoO3; Lithium ions batteries; Sodium ion batteries. INTRODUCTION In recent years, owing to the pollution problems and the fossil energy crisis on the earth, clean energy, especially the electrochemical energy storage system has attracted more and more attention1. The core of the electrochemical energy system is various energy materials. Since commercially applied, lithium ion batteries (LIBs) have captured the market of mobile power supply, greatly improving living standard 2 . However, graphite as the commercial electrode material of LIBs only has a theory specific capacity of 372 mA h g-1, which is unsatisfactory to

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meet the requirements of electric vehicle3. So researchers began to explore alternative materials to satisfy the needs of society, namely higher specific capacity, more stable cycle performance, higher initial discharge specific capacity, higher efficiency and lower cost, etc4-6. Based on this background, researchers mainly study various metal oxide materials, metal sulfide materials and metal selenide materials, etc7-9. Under electrochemical reaction, these active materials suffer from the problems such as volume expansion and low conductivity, making their cycling stability very poor and limiting their practical application despite their much higher theoretical specific capacity than carbon material10-11. To overcome these shortcomings, researchers carry out massive efforts to improve the crystal structure stability and conductivity of active materials, such as element doping and surface coating12-13. In consideration of the cost of electrochemical energy storage system, sodium ion batteries (SIBs) have attracted great attention owing to the abundance, wide distribution and low cost of sodium resources. However, the radius of sodium ion (1.02 Å) is larger than that of the lithium ion (0.69 Å), which means Na+ is more difficult to extract/insert from/in the electrode materials, further deteriorating the cycling performance and rate performance of the SIBs14-16. Thus, searching for advanced active materials becomes one of the key factors of large-scale commercial application of LIBs/SIBs. Recently, molybdenum-based materials (such as MoO2, MoO3, MoS2 and MoSe2) are of great interest to researchers to synthesize a wide array of substances as electrochemical energy storage active materials. They show many desirable properties including good chemical stability and a high theoretical specific capacity. In practical studies, however, researchers find that this kind of materials are not as perfect as they expect, because their poor electrical conductivity and poor ions kinetics of diffusion reduce the cycle stability of the battery 17 . To resolve these issues, molybdenum-based materials are incorporated with good electronic conductivity substrate to form hierarchical structure or synthesized as nanostructure to lessen the ions diffusion path lengths. For example, Yang et al. prepared ultrafine MoO2/graphene composites firstly used as anode materials for LIBs18. Lu et al. adopted hydrothermal process to synthesize α-MoO3/graphene composites, which are able to retain a reversible specific capacity of up to 823 mA h g−1 after 70 cycles at 200 mA g−1 for LIBs19. Noticeably, Xu et al. fabricated hybrid nanostructure of MoS2 nanosheet@TiO2 nanotube for LIBs anode material by four-steps 20 . Yang et al. synthesized MoSe2 nanosheets@porous hollow carbon spheres for LIBs and SIBs anode materials by three-step process21. Although high cycling properties and rate performance for LIBs or SIBs are obtained by conductivity modification and synthesizing nanostructures of molybdenum-based materials, the complex synthetic procedure and low yield restrict their large scale application. Additionally, most of the reported molybdenum-based materials for LIBs/SIBs are highly crystalline, which are often synthesized under high temperature or high pressure. Therefore, it is still remain great challenge to exploring a facile and low-temperature method to synthesize molybdenum-based materials with excellent electrochemical performance for LIBs/SIBs. In this work, a novel amorphous MoS2/MoO3/N-doped C composite (a-MM/NCc) has been successfully synthesized for the first time. Different from most of other reported active substances, the as synthesized substance is an amorphous composite due to the low-temperature reaction. Due to their unique architecture characteristics, the electrochemical test results show those amorphous composite materials have a high specific capacity, superior rate capability and excellent cycling stability in both LIBs and SIBs. As was stated above, there are very few reports of amorphous


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material explored as LIBs or SIBs, so this work may contribute to further research. EXPERIMENTAL Synthesis of organic-inorganic hybrid material of Mo3O10(C2H10N2) nanowire All chemicals and solvents involved in this work were purchased from commercial sources and used without further purification. Previous method has been used to synthesize the organic-inorganic hybrid material Mo3O10(C2H10N2) nanowire22-24. In a typical experiment, 1.24g of (NH4)6Mo7O24·4H2O and 1.0ml EDA was dissolved in 15 ml of deionized water firstly, and then 1M HCl aqueous solution was cautiously added, dropwise with magnetic stirring at room temperature until a white precipitate appeared ( pH around 4-5). After a reaction at 50 ℃ for 2 h, the product was collected and washed several times with deionized water and then dried at 70℃ overnight. Synthesis of the amorphous MoS2/MoO3/N-doped C composite In a typical synthesis, thiourea was used as sulfur source. The as-prepared organic-inorganic hybrid material of MoOx/EDA nanowire was placed in a quartz boat, filled with thiourea. Subsequently, the sample was heated at 300℃ for 2 h under H2/Ar flow (5% of H2) with a heating rate of 2 ℃ min-1, and then cooled to ambient temperature under argon. Characterization The collected amorphous composite product were characterized by X-ray diffraction (XRD) on a Rigaku D/max 2500 XRD diffractometer (Cu-Kα radiation, λ=1.15178 Å) and Raman spectroscopy (Jobin Yvon LabRAM Hr800, Longjumeau, France). Element content analysis (CHNSO) was conducted by an elemental analyzer (Vario Micro Cube, Elementar, Germany). The X-ray photoelectron spectroscopy (XPS) experiments were carried out on an ESCALAB 250Xi System (ThermoFisher) equipped with a monochromatic Al-Kα (1486.6 eV) source and a concentric hemispherical energy analyzer. Scanning electron microscope (SEM) was characterized using FEI Nova Nano SEM 230 scanning electron microscope. Transmission electron microscope (TEM JEOLJEM-2100) examination equipped with energy dispersive system (EDX) was used to investigate the structure of amorphous composite product. Electrochemical Measurements The anode electrodes were prepared by mixing a-MM/NCc, conductive agent (Super P) and binder (PAA) together in a weight ratio of 7:2:1. Then the mixture was dissolved in deionized water and magnetically stirred for 24 h. Subsequently, the obtained slurry was coated on Cu foils, and then dried at 95 ℃ in vacuum oven for 12 hours to evaporate the dispersants. The coin cells were laboratory-assembled by a CR2016 press in a glove box (Mbraun, Germany) filled with ultrahigh purity argon. As for LIBs, polyethylene membrane was used as separator and lithium foil was used as the anode. The electrolyte used for LIBs was 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume). As for SIBs, the glass microfiber filters was used as separator and sodium foil was used as the anode. The electrolyte used for SIBs was 1M NaClO4 in ethylene carbonate (EC) and diethylcarbonate (DEC) (1:1 in volume) with 1% fluoroethylenecarbonate (FEC) as additive. The electrochemical measurements were conducted by a Land Battery Tester (Land CT 2001A, Wuhan, China) at room temperature. Cyclic voltammetry (CV) measurements were carried out on a Chi604e electrochemical workstation at a scan rate of 0.05 mV s-1.



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RESULTS AND DISCUSSION Structural characterization Firstly, XRD was used to detect the phase purity and crystallinity of the precursors. As shown in Fig. S1, the XRD patterns of the organic-inorganic hybrid material of Mo3O10(C2H10N2) can be assigned to the pure MoOx/EDA(JCPDS#00-058-1318) with no detected impurity24. To examine the morphology of as-prepared organic-inorganic hybrid material MoOx/EDA, the SEM images were performed. From the Fig. S2, it can be clearly seen that the MoOx/EDA shows uniform wire-like morphology with lengths of about 10-50 µm and diameters of about several nanometers. In order to determine the final products, several different detection techniques were conducted. First, XRD detection method was used to detect the phase. From the XRD pattern of the final products in Fig. 1, no obvious diffraction peaks can be observed and only two wider drums can be seen, indicating the as prepared materials are amorphous. Interestingly, according to previous literature reports, amorphous MoO3 has a wide drum at around 2θ=10°, in agreement with the former drum in Fig. 125. Similarly, amorphous MoS2 has a wide drum between 30° to 50°, which corresponds to the latter drum in Fig. 126-28. Then, the element content analysis (CNOSH) and ICP-OES were used to detect the element content of the products. The results are shown in Table 1. It can be seen the products mainly contain Mo, S, O, C and N elements. For further determining the valence state of each element, XPS test was conducted on the products as exhibited in Fig. 2a. The peaks of Mo 3d, S 2p, O 1s, C 1s and N 1s can be clearly seen, corresponding to the product of element content analysis (CNOSH) and ICP-OES. From Fig. 2b, the binding energies of Mo (3d5/2) and Mo (3d3/2) in the composite are observed at 229.7 eV and 232.46 eV, respectively, which are attributed to Mo (℃) (typical of MoS2)29-30. Simultaneously, XPS (Fig. 2b) showed peak at 235.58 eV is for Mo (℃) (typical of MoO3)31-32. From Fig. 2c, the S 2p3/2 peak at 162.0eV and S 2p1/2 peak at 163.2 eV indicate the presence of S2-, with a binding energy typical of MoS225-26. From Fig. 2d, the XPS O 1s core level spectrum is dominated by one peak at about 533.1 eV typical of O2-33. From Fig. 2e and Fig. 2f, it can be observed that the binding energies of 285 eV, 287.8 eV and 398.4 eV are typical of the C 1s, C=O and N 1s spectral lines of C-N, C=O and C-N in EDA, CO2 and EDA34-36, where CO2 is from air and EDA is from the residual organic substance. Those observations are in agreement with previous reports37-40. The composition of the amorphous composite could also be determined by Raman spectra as exhibited in Fig. S3. Two characteristic bands of MoS2 are observed at 373.8 cm-1 (E12g) and 400.2 cm-1 (A1g)41-42. In addition, three typical peaks at 662, 816.3 and 990.9 cm-1 can be attributed to the asymmetrical stretching vibration of O-Mo-O bonds, the symmetric stretch of the terminal oxygen atoms and the asymmetric stretch of the terminal oxygen atoms, respectively43-44. The broad peak in the 1200-1500 cm-1 is attributed amorphous carbon45. Those observations are in agreement with previous reports.46-47 Thus, considering these experimental results together, it can be eventually determined the amorphous composite mainly consists of MoS2 and MoO3, namely MoS2/MoO3 (the mole ratio is 3:2). Table 1, the results of the element content analysis (CNOSH) and ICP-OES. Element

Mo

S

O

C

N

Wt%

47.87

20.56

10.99

9.34

10.80

Wt%

49.87

20.01

9.98

9.34

10.80


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( theoretical) To further characterize the morphology and detailed structure of a-MM/NCc, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed. As shown in Fig. 3a, the morphology of a-MM/NCc is consistent with the organic-inorganic hybrid material of Mo3O10(C2H10N2) precursor, namely, uniform wire-like morphology. It should be noted that the former is shorter than the latter in length, which is caused by the subsequent heat treatment process. Fig. 3b and c show the diameters of a-MM/NCc are of around 450 nm. That is to say, the precursors have not become coarse in the subsequent heat treatment process and still maintained the morphology of 1D chain structure. Typical TEM images from Fig. 4a intuitively reveal the morphology of a-MM/NCc is 1D wire-like, in agreement with the SEM results. From Fig. 4b, the HRTEM images also demonstrate that the products are amorphous, which is consistent with XRD results. Besides, the EDX analysis results were showed in Fig. 4c, indicating the presence of Mo, S, O, C and N elements in the products, in agreement with the element content analysis (CNOSH) and ICP-OES results. Additionally, the HRTEM image of a-MM/NCc was showed in Fig. 4d. To further determine the space distribution of compositional elements in a-MM/NCc, EDX element mapping was carried out. The corresponding elemental mapping of Mo, S, O, C and N elements are displayed in Fig. 4d-i respectively, indicating the uniform distribution of the elements. Electrochemical performance In this work, the fundamental electrochemical properties of the amorphous composite anode materials were investigated in both LIB and SIB half-cell batteries. The cyclic voltammetry (CV) curves of the electrode for LIBs are firstly shown in Fig. 5. The CV measurement is demonstrated in the voltage range of 0.01-3.0 V at a scan rate of 0.05 mV s-1 and the lithium plate was acted as counter electrode. The CV shape of a-MM/NCc electrode was significantly different from the pure electrode of MoS2 or MoO3. As shown in Fig. 5, there was a clear cathodic peak at around 1.875 V in the first discharge cycle, corresponding to the intercalation of Li+ into the interlayer spacing between the Mo-O octahedron layer and Mo-O octahedron interlayer of the amorphous composite to form LixMoO3, which is related to the reaction of MoO3+ x Li+ + x e- LixMoO3 ( )48,49. The weak cathodic peak at around 1.0 V subsequently was corresponding to insertion of Li+ ions into the interlayer spacing between the Mo-S layer of the amorphous composite to form LixMoS2, which is assigned to the reaction of MoS2 + x Li+ + x e- LixMoS2 ( )50,51. The broad weak cathodic peak below 0.5 V is associated with the reaction of LixMoO3 and LixMoS2 decompositions respectively, namely, LixMoO3 + (6-x) Li+ + (6-x) e- Mo+3Li2O ( ) and LixMoS2 + (4-x) Li+ + (4-x) e- Mo+2Li2S ( )41-44. It is necessary to point out that the cathodic peak at 0.1 V is corresponding to the inevitable formation of a solid–electrolyte interface (SEI) layer on the surface of the amorphous composite electrode, which disappeared in the following two cycles. During the first charge process, there are two obvious anodic plateaus at 1.47 V and 2.35 V which can be ascribed to the reversible reactions of and , respectively. Similarly, the broad weak anodic peaks between 0 V and 1.0 V can be attributed to the reversible reactions of and 41-45. In the following two CV loops, it can be clearly observed the CV curves perfectly overlap together, suggesting that a-MM/NCc electrode reveals a significant capability and superior cycling stability. Thus, it can be concluded that the mechanism of lithium storage in a-MM/NCc electrode is that the Li+ insertion/extraction into/from the interlayer spacing between the Mo-O



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layer and Mo-S layer respectively. The electrochemical cycling performance of a-MM/NCc electrode as LIBs anode material at a current density of 100 mA g-1 in the voltage range of 0.01–3.0 V vs. Li/Li + was presented at the Fig. 6a. In the first electrochemical cycle, the discharge specific capacity of a-MM/NCc was as high as 1816.1 mA h g-1, significantly exceeding the theoretical specific capacity of MoO3 and MoS2 electrodes (1111 mA h g-1 and 670 mA h g-1, respectively.), which may be owing to this unique structure and compositional features within the amorphous composite providing massive insertion sites for lithium ions. Relatively, it delivered a charge specific capacity of 1369.3 mA h g-1. It should be noted that the irreversible specific capacity mainly results from the formation of SEI layer, corresponding to an initial coulombic efficiency of 75.4%. In the next two charge/discharge cycles, the discharge specific capacity gradually tended to be stable with a slight decline and it can be intuitively observed that the discharge specific capacity of a-MM/NCc almost showed no decline from 4th cycle to 50th cycle. Improbably, the remaining discharge specific capacity of 50th cycle of a-MM/NCc electrode reaches up to 1253.3 mA h g-1, nearly 3.7 times more than the theoretical specific capacity of current commercial graphite electrode (372 mA h g-1). The coulombic efficiency from the 4th cycle to the 50th cycle is nearly 100% with excellent cycling stability. Meanwhile, the voltage-specific capacity of a-MM/NCc electrode was shown in Fig. 6b. Two pairs of charge/discharge platforms were obviously observed in the curve, namely 2.35 V/1.875 V and 1.47 V/0.5 V, which is supported by the CV curves. Apparently the charge/discharge voltage-specific capacity curves of the 3rd, 5th, 10th, 20th and 50th cycles almost overlap together, also indicating that a-MM/NCc electrode delivers an excellent cycling stability. In order to further detect the electrochemical performance of a-MM/NCc electrode, rate capacity is also tested at different current densities, namely, 50, 100, 200, 300, 500, 1000 and 2000 mA g-1. Quite evidently, the results of the rate capacity test were exhibited in Fig. 6c, revealing excellent rate capability and stable cycling performance, too. More specifically, the discharge specific capacity of a-MM/NCc electrode at a large current density of 2000 mA g-1 can still reach at 497.6 mA h g-1 and the discharge specific capacity of a-MM/NCc electrode has no obvious decline at each circulation of the rate capacity test. More delightfully, a-MM/NCc electrode exhibits an exceptionally long-term cycling stability of LIBs. As can be seen from Fig. 6d, the electrode achieves a marked high stable specific capacity of 887.5 mA h g-1 at a current density of 1000 mA g-1 after 350 cycles. Noticeably, the coulombic efficiency of a-MM/NCc electrode are nearly 100% during the entire long-term cycling test except for the initial several cycles, which has not been previously reported as far as we know. Similarly, the electrochemical properties of a-MM/NCc electrode used as SIBs anode material were also explored. Fig. 7 shows the CV curve of a-MM/NCc electrode for SIBs where the counter electrode was acted by the sodium tablets with a voltage range of 0.01-3.0 V at a scan rate of 0.05 mV s-1. The first cathodic peak observed in the initial discharge semi-circle was located at around 2.10 V, which is corresponding to the reaction of MoO3 + x Na+ + x eNaxMoO3 ( )52,53. The next cathodic plateau at around 1.25 V is according to the reaction of MoS2+ x Na+ + x e- NaxMoS2 ( )54-55. With the further intercalation of the sodium ions into the active material, the subsequent reactions below 1.0 V are NaxMoO3 + (6-x) Na+ + (6-x) e- Mo + 3Na2O ( ) and NaxMoS2 + (4-x) e- Mo + 2 Na2S ( )45-48. Moreover, it must be noted that the cathodic reaction at 0.1V is to form the compact SEI films, which could inhibit further decomposition of electrolyte and avoid the exfoliation of the active material from copper foil in


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the electrolyte during the entire charge–discharge cycling test. Correspondingly, the next charge semi-circle is corresponding to the sodium ions extraction from a-MM/NCc electrode. The broad weak anodic peak around 0.45 V was due to the Na+ extraction from the sodium oxide or sodium sulfide to form the NaxMoO3 or NaxMoS2, that is, the inverse reaction of and occurred respectively. Meanwhile, the anodic peaks at 1.41 V and 1.65 V were assigned to the reversible reactions of and during the first charge semi-circle45-48. Similarly, corresponding to the above LIBs CV curves, the SIBs CV curves of the following cycles can overlap together perfectly, indicating superior stable cycling performance of the active material. Hence, it can be concluded that the mechanism of a-MM/NCc electrode for SIBs is the Na+ insertion/extraction into/from the interlayer spacing between the Mo-O layer and Mo-S layer respectively, which is similar to LIBs. Also, the electrochemical characteristics of the amorphous composite electrode for SIBs were examined by the constant current charge-discharge circling method in the voltage range of 0.01–3.0 V vs. Na/Na+. As described in the Fig. 8a, the discharge specific capacity of a-MM/NCc electrode at a current density of 300 mA g-1 was up to 827.9 mA h g-1 in the first discharge process. That is to say, this unique structure and compositional features in the amorphous active material can provide enough space for the insertion/extraction of not only lithium ions but also sodium ions. As such, the subsequent charge specific capacity of the active material can still reach to 553.4 mA h g-1, displaying the initial coulombic efficiency of 66.8%, in spite of the irreversible reaction to the formation of the SEI film. Meanwhile, it can be observed that the trend of the discharge specific capacity curve in the early 40 cycles slightly decreases and then increases gradually to be steady. Significantly, after 200 charge/discharge cycles, the discharge specific capacity of a-MM/NCc electrode for SIBs can reach 538.7 mA h g-1 with a capacity retention of 97% except for the first cycle, indicating that the active material has excellent cycle stability. As shown in Fig. 8b, the voltage-specific capacity curve of a-MM/NCc electrode clearly demonstrates two pairs of stable charge and discharge platforms, namely 0.45/1.25 V and1.41/ 2.10 V, which are consistent with the CV curves. At this point, it can be concluded that the active material has superior electrochemical performance by observing the charge/discharge voltage-specific capacity curves of the 3rd, 10th, 50th, 100th and 200th cycles which almost overlap together. The result of rate capacity test to a-MM/NCc electrode for SIBs at different current densities was shown in Fig. 8c. From the curves of rate performance it can be seen that a-MM/NCc electrode exhibits high rate stability. To be specific, when tested at the initial low current density of 50 mA g-1, the discharge specific capacity of the active material can amazingly reach 907.4 mA h g-1 in the first cycle and around 655.9 mA h g-1 in the next nine cycles with a coulombic efficiency of 72.2%. Beyond that, an ideal discharge specific capacity of 310.1 mA h g-1 was still retained when the current density was up to 1000 mA g-1. Additionally, the discharge specific capacity was well-recovered when the current density returned to100 mA g-1 after the high current density of 1000 mAg-1, which suggests that a-MM/NCc electrode has superior rate capacity. Fig. 8d shows that a-MM/NCc electrode has superior performance of long-term cycling stability at a large current density of 1000 mA g-1. The discharge specific capacity of a-MM/NCc electrode for SIBs can reach 339.9 mA h g-1 after 220 cycles, nearly 100% retention of its initial discharge specific capacity except for the first cycle, which indicates that the active material exhibits excellent cycling stability. Compared with recently reported molybdenum-based materials with crystal structure for both LIBs and SIBs, it can be clearly seen that the a-MM/NCc in our work shows much better



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electrochemical performance, as listed in Tables 2 and 3. The excellent electrochemical performance of a-MM/NCc may be resulted from a series of aspects as follows: (1), Carbonaceous matrix can enhance the conductivity of the amorphous composite. (2), Heteroatom, such as N, doping within this unique compositional features can increase the active ions absorption sites on the amorphous composite surface which benefitted the insertion / extraction of lithium /sodium ions. (3), The hybrid nanomaterials could provide plenty of diffusion channels for ions during the insertion/extraction process 56 - 57 . (4) 1D chain structure reduces the transfer distance of lithium/sodium ions into/from the electrode. Table 2 A performance comparison for molybdenum-based materials with crystal structure for LIBs with crystal structure Specific capacity at lower current density mA h g-1(A g-1)

Specific capacity at higher current density mA h g-1(A g-1)

Capacity retention after certain cycles %(cycle)

Coulombic efficiency at first cycle %

Amorphous or crystal phase

Present work

1253.3(0.1)

887.5(1)

75.4

Amorphous

MoO2/GO18

63.7

Crystal

Materialsref

963.4(0.06)

-

Nearly 100% (350) 79.4%(40)

19

953(0.2)

754.6(1)

83.3%(200)

72.1

Crystal

MoO3@C58

696(0.2)

502(1)

79.9%(100)

66.6

Crystal

MoS2/CMK-350

893(0.1)

391(8)

97.4%(150)

69

Crystal

TiO2/MoS255

421(0.1)

112(20)

83.1(100)

71.6

Crystal

MoS2/GO59 MoSe2@PHCS21

890(0.1) 820(0.5)

545(2) 640(3)

71%(100) 98%(100)

76 57.4

Crystal Crystal

MoSe2/C60

650(0.1)

450(2)

93%(50)

72.2

Crystal

MoO3/GO

Table 3 A performance comparison for molybdenum-based materials with crystal structure for SIBs with crystal structure

Materialsref

Present work

Specific capacity at lower current density mA h g-1(A g-1)

Specific capacity at higher current density mA h g-1(A g-1)

Capacity retention after certain cycles %(cycle)

Coulombic efficiency at first cycle %

Amorphous or crystal phase

655.9(0.05)

310.1(1)

97%(220)

66.8

Amorphous


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α-MoO353

213(0.1117)

100(1.117)

55%(500)

53.2

Crystal

MoO3/GO52

875(0.058)

330(1.117)

71%(100)

88

Crystal

TiO2/MoS255

214(0.02)

48(4)

82.3%(100)

62.2

Crystal

MoS2/RGO22

450(0.1)

300(2)

81.1%(160)

64

Crystal

MoS2/C61

610(0.05)

370(2.5)

80%(200)

80

Crystal

MoSe2@PHCS21

575(0.2)

400(1.5)

90%(100)

65.9

Crystal

MoSe262

442(0.1)

315(1.5)

96%(50)

85

Crystal

CONCLUSIONS In summary, a novel amorphous MoS2/MoO3/Nitrogen-doped Carbon composite (a-MM/NCc) has been synthesized for the first time. The synthesis strategy only involves partially sulfurizing organic-inorganic hybrid material Mo3O10 (C2H10N2) nanowire precursors under low temperature (300 ℃). More interestingly, a-MM/NCc electrodes show excellent electrochemical performance for LIBs and SIBs. Such as, a-MM/NCc electrode for LIBs can reach up to 1253.3 mA h g-1 at a current density of 100 mA g-1 after 50 cycles. Similarly, for SIBs, it also retains 538.7 mA h g-1 after 200 cycles at 300 mA g-1. The a-MM/NCc electrode has superior performance of long-term cycling stability at a large current density of 1000 mA g-1 both for LIBs and SIBs, which both capacity retentions of their corresponding initial discharge specific capacity are near up to 100%. Such superior performance may be attributed to nitrogen and carbon co-doping within this unique amorphous composite which not only enhances the conductivity but also increases the active ions absorption sites on the amorphous composite surface. Besides, the hybrid nanomaterials could provide plenty of diffusion channels for ions during the insertion/extraction process. And 1D chain structure reduces the transfer distance of lithium/sodium ions into/from the electrode. As far as we know, such novel amorphous composite has much better electrochemical performance than that of most previously reported molybdenum-based materials with crystal structure. Therefore, our work may shed some light on the exploration of the next generation energy transfer and storage devices. Supporting Information XRD patterns and SEM images of the Mo3O10 (C2H10N2) nanowire, and Raman spectra of the as-prepared amorphous composite ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (grant Nos. 51472271, 61376018, and 51174233), the Project of Innovation-driven Plan in Central South University (2016CX002) and the National Basic Research Program of China (973 Program) grant No. 2013CB932901.



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Figure 1 X-ray diffraction (XRD) patterns of the amorphous composite.

Figure 2 XPS spectra for the amorphous composite: (a) survey spectrum, and the high-resolution spectra of (b) Mo 3d, (c) S 2p, (d) O 1s, (e) C 1s, (f) N 1s.


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Figure 3 (a b and c) SEM images of the amorphous composite.

Fig 4 (a) TEM images of the amorphous composite. (b) HRTEM images of the amorphous composite. (c) The EDX analysis results of the amorphous composite. (d, e, f, g, h and i) STEM image and the corresponding elemental mapping of Mo (purple), S (blue), O (yellow), C (red) and N (green) in the amorphous composite.



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Figure 5 Cyclic voltammetry curves of the electrode of the amorphous composite for LIBs case at a scan rate of 0.05 mV s-1 from 0-3.0V.


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Figure 6 Electrochemical performance of the electrode of the amorphous composite for LIBs case. (a) Cyclic performances of the amorphous composite electrode at 100 mA g-1. (b) The charge/discharge voltage-specific capacity curves of the 1st, 3rd, 5th, 10th, 20th and 50th cycles. (c) Rate performances of the amorphous composite electrode form 50 to 2000 mA g-1. (d) Cycling performance of the electrode at a high current density 1000 mA g-1.



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Figure 7 Cyclic voltammetry curves of the electrode of the amorphous composite for SIBs case at a scan rate of 0.05 mV s-1 from 0-3.0V.


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Figure 8 Electrochemical performance of the electrode of the amorphous composite for SIBs case. (a) Cyclic performances of the amorphous composite electrode at 300 mA g-1. (b) The charge/discharge voltage-specific capacity curves of the 1st, 3rd, 5th, 10th, 20th and 50th cycles. (c) Rate performances of the amorphous composite electrode form 50 to 1000 mA g-1. (d) Cycling performance of the electrode at a high current density 1000 mA g-1.

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TOC GRAPHIC

For Table of Contents Use Only Synopsis: The amorphous MoS2/MoO3/Nitrogen-doped Carbon composites showed excellent electrochemical properties both for LIBs and SIBs, which is better than that of most previously reported molybdenum-based materials with crystal structure.


Nitrogen-Doped Carbon Composite

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