Fabrication and lithium storage performances of a composite of tin disulfide and ordered mesoporous carbon

SnS2 has been investigated as anode material for lithium ion batteries. To obtain anode material having high capacity and long-term lifespan, in the present work, a nanostructured composite comprising SnS2 nanosheets and CMK-3 ordered mesoporous carbon has been designed. The CMK-3/SnS2 composite is fabricated via incipient wetness impregnation, followed by chemical reduction and chemical conversion in an inert gas at high temperature. The obtained composite exhibits boosted lithium storage behaviors involving high specific capacity, fast rate response and stable cyclability. At a discharge-charge rate of 100 mA g-1, the CMK-3/SnS2 electrode delivers a specific capacity of 985.2 mA h g-1 with the utilization efficiency of SnS2 present in the composite of 90.5 %. Even, after 500 cycles testing at higher discharge-charge rates of 0.5 and 1 A g-1, CMK-3/SnS2 still can maintain specific capacities of 556.2 and 402.9 mA h g-1, respectively, much higher than the theoretical specific capacity of commercialized graphite anode material. This work demonstrates considerable application potential of the CMK-3/SnS2 anode in Li-ion batteries.

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Cite this paper: Vietnam J. Chem., 2020, 58(5), 622-629 Article DOI: 10.1002/vjch.202000050 622 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Fabrication and lithium storage performances of a composite of tin disulfide and ordered mesoporous carbon Le Thi Thu Hang * , Hoang Thi Bich Thuy, Dang Viet Anh Dung, Dang Trung Dung School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung district, Hanoi 10000, Viet Nam Submitted March 31, 2020; Accepted April 28, 2020 Abstract SnS2 has been investigated as anode material for lithium ion batteries. To obtain anode material having high capacity and long-term lifespan, in the present work, a nanostructured composite comprising SnS2 nanosheets and CMK-3 ordered mesoporous carbon has been designed. The CMK-3/SnS2 composite is fabricated via incipient wetness impregnation, followed by chemical reduction and chemical conversion in an inert gas at high temperature. The obtained composite exhibits boosted lithium storage behaviors involving high specific capacity, fast rate response and stable cyclability. At a discharge-charge rate of 100 mA g -1 , the CMK-3/SnS2 electrode delivers a specific capacity of 985.2 mA h g -1 with the utilization efficiency of SnS2 present in the composite of 90.5 %. Even, after 500 cycles testing at higher discharge-charge rates of 0.5 and 1 A g -1 , CMK-3/SnS2 still can maintain specific capacities of 556.2 and 402.9 mA h g -1 , respectively, much higher than the theoretical specific capacity of commercialized graphite anode material. This work demonstrates considerable application potential of the CMK-3/SnS2 anode in Li-ion batteries. Keywords. Tin disulfide, lithium storage, composite, cyclability. 1. INTRODUCTION Since the first commercialization in 1991, lithium ion batteries (LIBs) have been regarded as the most popular storage technology for mobile and portal applications including mobile phones, laptops, digital cameras because of their low self-discharge, high energy density, nearly zero-memory effect, and long lifespan. [1] Graphite is established as a main anode active material for commercial LIBs since it possesses some advantages such as low working potential, low cost, and high stability. Nevertheless, it also exhibits some disadvantages such as high lithium ion diffusion resistance, harmful lithium dendrite growth on the graphite surface, and low theoretical specific capacity (372 mA h g -1 ). [2] To meet the growing demand on high power density and high energy density, the development of new electrode active materials with outstanding electrochemical properties is indispensable. Much effort has been dedicated to developing alternatives to graphite anode for LIBs. In this context, conversion-alloying anode materials have recently received great attention, especially, SnS2, a tin based anode material since it possesses outstanding properties such as high specific capacity (1230 mA h g -1 ), low cost, low working potential, environmental friendliness, and ready availability. [3] Unfortunately, this material also suffers from large stress caused by huge volume expansion during lithiation process, which is also found for other alloy-type anode materials. This leads to the pulverization phenomenon of the active material, electrical disconnection between active material particles and a current collector, deterioration of the solid electrolyte interphase (SEI) layer on the electrode, and, eventually, significant degradation of the electrochemical performance. So far, some strategies have been supposed to solve the drawbacks of SnS2. They include fabrication of nanostructure, [4] fabrication of composites of SnS2 with other active/inactive materials, [5] fabrication of doped SnS2 [6] and structure design for SnS2-based materials. [7] In this work, we report a facile routine to synthesize the composite of SnS2 and CMK-3, a kind of ordered mesoporous carbon, with the aim of application as high performance-anode material for LIBs. For this designed composite, SnS2 nanosheets are filled inside and deposited partially outside mesopores of CMK-3 framework, which works as nanocage to confine the SnS2 nanosheets. Owing to high porosity and high stable structure of CMK-3, the volume expansion of SnS2 nanosheets during Vietnam Journal of Chemistry Le Thi Thu Hang et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 623 lithiation process can be alleviated. Accordingly, the composite exhibits remarkable lithium storage behaviors in term of cyclability, rate capability, and capacity. 2. MATERIALS AND METHODS 2.1. Synthesis of CMK-3/SnS2 composite CMK-3/SnS2 composite was fabricated via incipient wetness impregnation technique, following by heat treatment at high temperature. Where, CMK-3 as supporting material was synthesized by hard template method, which was described in the literature. [8] In detail, CMK-3 powder (0.5 g) was added in 5 mL of a 50v/50v ethanol/water mixture containing SnCl4 and ultrasonicated for 30 mins. After drying at 80 C for 5 h, the resultant powder continued to be mixed well with 2 g of NaBH4, and then storage in the humid air for 48 h. Because NaBH4 is a highly hygroscopic substance in combination with the presence of oxygen in the air, the CMK-3/SnO2 composite could be easily formed according to chemical reactions as follows: [9] SnCl4 + 4NaBH4 → Sn +4NaCl +2B2H6 +2H2 (1) Sn + O2 → SnO2 (2) After that, the CMK-3/SnO2 sample was rinsed, and dried at 100 o C, followed by a heat treatment process in nitrogen atmosphere at 350 o C for 3 h. Prior to the heat treatment, CMK-3/SnO2 powder was put into a small alumina crucible, which was surrounded by a certain amount of thiourea in a large covered alumina crucible. At such a high temperature, thiourea was completely decomposed to convert SnO2 to SnS2 via the following reactions: [10,11] (NH2)2CS → NH3 + HNCS (3) (NH2)2CS → H2S + C(NH)2 (4) SnO2 + 2H2S → SnS2 + 2H2O (5) Next, the sample was washed thoroughly by mixture of ethanol and deionized water to remove the undesired byproducts. Finally, the CMK-3/SnS2 composite was obtained by drying at 120 o C for 24 h under vacuum. 2.2. Microstructure characterizations Morphological, chemical and structural characterizations were performed on a scanning electron microscope (SEM, Hitachi S-4700/EX-200, Japan), a X-ray photoelectron spectrometer (XPS, Multilab 2000, VG, UK) and a high-resolution X-ray diffractometer (D/MAX Ultima III, Japan), respectively. The thermal stability of the sample was measured in a temperature range, up to 900 o C in the air atmosphere. This thermogravimetric analysis (TGA) was conducted on a thermogravimetric analyzer (TGA-50, Shimadzu, Japan). 2.3. Electrochemical characterization Lithium storage behaviors of the CMK-3/SnS2 composite were measured using CR2032-coin cells. The working electrodes were fabricated via casting method. A mixture of CMK-3/SnS2 (electrode active material), carbon super P (conducting agent) and lithium polyacrylate (binder) with a mass ratio of 8:1:1 was mixed well in deionized water to form a black slurry, which was accompanied by a casting process using a doctor blade on a copper foil. After being dried at 100 o C at a vacuum oven, the foil was cut into dishes with a 14-mm diameter. Each coin cell was comprised of a working electrode, a Li metal foil as counter/reference electrode and a separator. The electrolyte used in the cell was an organic 50v/50v mixture solution of dimethyl carbonate (DMC) and ethylene carbonate (EC) containing 1 M LiPF6 and 5 wt% fluoride ethylene carbonate (FEC) additive. The cell assembly process was implemented in an argon gas-tight glove box. The loading mass of the working electrodes was fixed around 1.0 mg cm -2 . The specific capacities of the electrodes were calculated, based on the mass of the active material. Two electrochemical measurement techniques including cyclic voltammetry (CV) and galvanostatic charging-discharging (GCD) were employed to investigate the lithiation/delithiation mechanism as well as discharge-charge response of the synthesized materials. The CV measurements were preceded on a Gamry Potentiostat instrument while GCD measurements were conducted on a battery cycler (WonATech, WBCS 3000). 3. RESULTS AND DISCUSSION To verify the crystalline structure as well as phase purity of the synthesized CMK-3/SnS2 composite, XRD analysis was used. Figure 1 showed XRD pattern of CMK-3/SnS2. For comparison, the XRD pattern of CMK-3 supporting material was included. Obviously, the reflection peaks of CMK-3/SnS2 all matched well to the standard lines of SnS2 phase with the hexagonal structure (JCPDS card No. 01- 089-2358, space group: P-3m1). It is noted that both two typical diffraction peaks located at 24 o and 44 o of CMK-3 [12] were not recognized in the XRD pattern of CMK-3/SnS2. This probably stems from Vietnam Journal of Chemistry Fabrication and lithium storage performances © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 624 the covering of SnS2 nanosheets on the surficial CMK-3, as well as the low content of CMK-3 present in the composite, all of which were further confirmed by SEM and TGA results as below. To investigate the morphology of CMK/3-SnS2, the samples were examined by SEM. Figure 2 presents SEM images of the resultant CMK-3/SnS2 composite. As seen in figures 2a,b, CMK-3 was composed of uniformly short nanorods. The length of the nanorods was around 1 µm. Each nanorod was supposed to be constructed by primary carbon particles with sizes of ~10 nm. These particles were arranged in a well-ordered structure (inset in figure 1b) to form a typical hexagonal shape, which is regarded as the reversible replication of SBA-15 hard template. [13] Figure 1: XRD patterns of CMK-3 and CMK-3/SnS2 composite Figure 2: Low- and high-resolution SEM images of CMK-3 and CMK-3/SnS2 composite After loading SnS2, the surface of nanorods seem to be covered by SnS2 nanosheets, which possessed the thickness of about 30 nm (figures 2c,d). Due to the presence of SnS2 nanosheets, the voids between the adjacent nanorods were formed to be relatively large. This suggests that such a structure of the CMK-3/SnS2 induce the contact of electrolyte with the active substance to become easier. [14] In order to determine the thermal stability as well as the practical composition of CMK-3/SnS2, TGA was implemented in the air atmosphere at a heating rate of 10 o C min -1 . The obtained TGA result is displayed in figure 3. It is clear to observe that there were four main weight losses. The first weight loss of 3.2 wt.%, occurred at around the temperature of 95 o C. This process is associated to the vaporization of water molecules absorbing physically at the surface of the sample. The second weight loss started at 415 o C, corresponding to the oxidation process of SnS2 in the CMK-3/SnS2 composite. [3,15] The further weight losses recorded at an onset temperature range of 527 o C was assigned to the combustion of CMK-3 present in the CMK- 3/SnS2 composite. [16] At a temperature range of 527- 660 o C, in addition to the combustion reaction of CMK-3, the oxidation process of SnS2 still occurred to form SnO2. [3] At temperatures above 660 o C, the combustion and oxidation processes completed, and the weight of the sample almost unchanged. Figure 3: TGA plot of CMK-3/SnS2 composite in the air atmosphere C + O2 → CO2 (6) SnS2 +3O2→ SnO2 + 2SO2↑ (7) Based on the TGA result, the content of SnS2 present in the composite was calculated to be 73.41 wt.%. XPS is a powerful method widely used for the surface analysis of materials. In the present work, XPS enabled verifying the chemical state of the synthesized CMK-3/SnS2 composite. As shown in figure 4a, wide survey XPS spectrum, the presence of the constituent elements of CMK-3/SnS2 such as C, Sn, and S was detected. Remarkably, the appearance of O and N elements was recorded as well. The existence of O element can be explained by the surface functionalization of CMK-3 during Vietnam Journal of Chemistry Le Thi Thu Hang et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 625 the synthesis process. Meanwhile, the presence of N element was caused by the doping N process occurring when thiourea was decomposed at the high temperature in the synthesis process of the CMK-3/SnS2 composite. The result obtained in this study is totally consistent with those reported previously. [17,18] In figure 4b, the Sn 3d-core-level XPS spectrum was deconvoluted into two peaks of Sn 3d5/2 at 487.0 eV and Sn 3d3/2 at 495.5 eV, which are characteristic of Sn 4+ . Furthermore, a satellite peak was observed at 497.5 eV. On the other hand, the S 2p spectrum in figure 4c illustrates the appearance of S 2p3/2 at a binding energy of 162.1 eV and that of S 2p1/2 at a binding energy of 163.3 eV. These results proved the presence of SnS2 component in the synthesized composite. [14] In figure 4d, the C 1s-core-level XPS spectrum was separated into three components at binding energies of 284.6, 286.4 and 288.7 eV. These peaks represent C=C, C–N/C–O, and C=N/C=O functional groups.[12,19] This is evidence of the doping N atoms into C networks of CMK-3 frame work. According to the deconvoluted N 1s XPS in figure 4e, there were three bonding types between N and C. In particular, the peaks at 399.1 eV and 400.6 eV are associated to pyridinic N and pyrrolic/pyridonic N Figure 4: XPS spectra of CMK-3/SnS2 composite: (a) Wide survey, (b) Sn3d, (c) S 2p, (d) C 1s, (e) N 1s, (f) Illustration of N bond types in the C network of CMK-3 bonds, respectively (figure 4f). [20] According to the previous reports, [21-22] doping N heteroatoms into C networks would increase the number of active sites for lithium storage as well as the electrical conductivity for the carbonaceous materials. Hence, the doping N was expected to benefit for the electrochemical performances of the synthesized CMK-3/SnS2 composite. For investigation of Li insertion/extraction behavior, the CMK-3 and CMK-3/SnS2 composite electrodes were tested by CV method at the potential between 0.01 and 3 V vs. Li + /Li at a scan rate of 0.1 mV s -1 for five cycles, with open circuit potential (OCP) as a starting point. The measured results are presented in Figure 5. As seen in Figure 5a, in the first cycle, the CMK-3 electrode has two cathodic peaks at 1.65 V and 0.7 V vs. Li + /Li. In the following cycles, these peaks totally disappeared, suggesting the irreversible formation of a SEI layer on the surface of the electrode as well as the presence of other irreversible parasitic reactions. [12] From the second cycle onwards, the shapes of the CV plots appeared similar. In addition, these CV plots in this work are the same to those of the previous report. [19] In particular, the discharge capacity of CMK-3 attained came from two parts: (i) The discharge capacity is related to lithium storage in cavities formed naturally, boundaries between graphite crystallite domains in turbostratic structure as well as in the diversity defects of pores, nanorods of CMK-3 carbon. [23] This part was attained at the potential above 0.45 V vs. Li + /Li; (ii) The another came from the underpotential deposition process of Li metal. [12] This contribution was gained Vietnam Journal of Chemistry Fabrication and lithium storage performances © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 626 at the potential below 0.45 V vs. Li + /Li. As for the anodic scan direction, delithiation process occurred within a wide potential range from 0.01 to 3.0 V vs. Li + /Li with the appearance of two broadening anodic peaks located at 0.14 and 1.16 V vs. Li + /Li. The former was characteristic of delithiation from nanosized graphite-like domains. The latter represented delithiation from pore structure and/or defect sites within the carbon wall. Figure 5: CV plots of CMK-3 and CMK-3/SnS2 composite Figure 6: Discharge-charge voltage profiles of (a) CMK-3 and (b) CMK-3/SnS2 at a current density of 100 mA g -1 . (c) Cyclability and (d) rate response of CMK-3 and CMK-3/SnS2 composite. (e) Long-term cyclability of CMK-3/SnS2 at different rates of discharge-charge for 500 cycles As for the CMK-3/SnS2 composite electrode, the Li insertion/extraction was complex. At the first cycle, five cathodic peaks appeared (figure 5b). The first peak at 2.53 V vs. Li + /Li is assigned to the formation of the SEI layer on the electrode surface. Four next peaks at 1.85, 1.29, 0.32 and 0.13 V vs. Li + /Li correspond to insert Li into SnS2 of the composite as described below: [24] Li + electrolyte + xe - → SEI layer (8) SnS2 + xLi + + xe - → LixSnS2 (9) LixSnS2 + 4xLi + + (4-x)e - ⇋ Sn + 2Li2S) (10) Sn + 4.4Li + + 4.4e - ⇋ Li4.4Sn (11) In the following cycles, the peaks corresponding to the formation of the SEI layer and LixSnS2 compound disappeared. This indicates that these processes were irreversible and only occurred at the first discharge. Meanwhile, the rest cathodic peaks exhibited a slight shift because of the structural change of the active material after the first cycle. For the anodic scan direction, two distinct anodic peaks (0.52 V, and 1.89 V vs. Li + /Li) were recorded for the five cycles of scanning. These peaks imply Vietnam Journal of Chemistry Le Thi Thu Hang et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 627 the dealloying of Li4.4Sn to form Sn, and the oxidation of Sn to SnS2. [25] In addition, in comparison with figure 5a, two characteristic anodic peaks of CMK-3 were found in the CV plot of the CMK-3/SnS2 composite. Figures 6a,b illustrate the characteristic galvanostatic discharge-charge curves of the CMK-3 and CMK-3/SnS2 composite electrodes at a current density of 100 mA g -1 for 100 cycles. As seen in Figure 6a, as for the first cycle, the CMK-3 electrode delivered specific discharge/charge capacities of 1276.8/628.8 mA h g -1 . Accordingly, its initial coulombic efficiency (CE) was calculated to be 49.2 %. This obtained low CE resulted from the irreversible formation of the SEI layer and parasitic reactions. Within first ten cycles, the specific capacity had tendency to decrease rapidly for both discharge/charge processes due to the marginal structure deterioration of porous CMK-3. [12] In the subsequent cycles, the microstructure stabilized gradually. Accordingly, the discharge-charge voltage curves almost overlapped, demonstrating highly stable cyclability of the CMK-3. Noticeably, no discharge/charge plateaus were observed for the CMK-3 electrode. In contrast, the CMK-3/SnS2 composite electrode exhibited two discharge plateaus (~1.4 V and ~0.4 V) and two charge plateaus (~0.5 V and ~1.8 V). The obtained results are accordance with the CV analysis results above. In the first cycle, the CMK- 3/SnS2 could offered discharge/charge capaciti