A Sulfur@microporous Carbon Cathode With a ... - ACS Publications

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C: Physical Processes in Nanomaterials and Nanostructures

A Sulfur@microporous Carbon Cathode With a High Sulfur Content for Magnesium-Sulfur Batteries With Nucleophilic Electrolytes Weiqin Wang, Hancheng Yuan, Yanna Nuli, Jingjing Zhou, Jun Yang, and Jiulin Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09003 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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The Journal of Physical Chemistry

A Sulfur@microporous Carbon Cathode with a High Sulfur Content for Magnesium-sulfur Batteries with Nucleophilic Electrolytes Weiqin Wang, Hancheng Yuan, Yanna NuLi,* Jingjing Zhou, Jun Yang, Jiulin Wang School of Chemistry and Chemical Engineering, Shanghai Electrochemical Energy Devices Research Center, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. E-mail: [email protected]

ABSTRACT. To enhance the long-term cycling stability of sulfur cathode for high-energydensity magnesium-sulfur (Mg-S) batteries with easily prepared (PhMgCl)2-AlCl3/THF based nucleophilic electrolytes, a sulfur@microporous carbon (S@MC) composite with a high sulfur content (64.7 wt%) was prepared by impregnating sulfur into microporous carbon. S@MC cathode combines the merits of both small chain-like S2-4 molecules and large ring-like S8 molecules with S2-4 existing as a highly dispersed state inside the micropores of carbon and S8 on the outside surface of carbon. MC as a host improves the electrode reaction kinetics and physically adsorbs sulfur and polysulfides. Copper sulfides formed when sulfur is coated on a Cu current collector at 50 C provide a strong chemical interaction between Cu and sulfur, which protects sulfur from the electrolyte and relieves the dissolution of polysulfides. Based on the

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advantages of the microporous carbon and Cu current collector, S@MC exhibits an initial discharge capacity of approximate 979.0 mAh g-1, and maintains a capacity of 368.8 mAh g-1 after 200 cycles at a rate of 0.1 C, demonstrating improved sulfur utilization and cycle stability. When the rate increases to 0.2 C, the composite can still deliver a capacity of about 200 mAh g-1 with 100% Coulombic efficiency. This strategy of stabilization of S2-4 small molecules in MC and chemical bond of Cu to S8 large molecules provides an effective approach to improve the cycling stability, rate performance, and sulfur content for Mg-S batteries with nucleophilic electrolytes. 1. INTRODUCTION The rapid development of wearable devices, electric vehicles and renewable energy applications are in urgent need for exploring high-energy-density, green and sustainable energy storage systems. It is now necessary for batteries to possess low-cost, good safety characteristics and high volumetric energy density for space available to mount battery packs.1, 2 Lithium-sulfur (Li-S) batteries, which utilize light-weight elemental S as the active cathode material to reversibly react with Li via a two-electron reaction corresponding to a conversion class as opposed to an intercalation class, have attracted increasing attention for natural abundance, environmental friendliness, high theoretical capacity (1675 mAh g-1 or 3459 mAh cm-3) and energy density (2500 Wh kg-1 or 2800 Wh L-1 based on a complete reaction to Li2S) 3 of sulfur. It offers the possibility of high gravimetric capacities and theoretical energy densities ranging up to a factor of five beyond conventional Li-ion systems. Due to a relatively lower price ($2700 and $64800/ton for Mg and Li, respectively), 2, 4 a higher theoretical volumetric capacity (3832 and 2062 mAh cm-3 for Mg and Li),2 and higher expected safety (the dendritic morphologies of magnesium deposits are lower than that of lithium) 5 and safer operation, magnesium-sulfur (Mg-


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S) batteries are promising candidates, yielding a theoretical volumetric energy density exceeding 4000 Wh L-1. 5 However, the development of rechargeable Mg-S batteries is seriously hindered by the lack of a suitable electrolyte that is compatible with both Mg and electrophilic S. A nonnucleophilic 3HMDSMgCl-AlCl3/THF (HMDS=hexamethyldisilazide) electrolyte and the first prototype of a rechargeable Mg-S battery were suggested by Muldoon et al.2, 5 However, the Mg system is substantially plagued by the poor cycle stability resulted from sulfur and polysulfides dissolution due to the THF solvent, which corresponds to a crucial component of the electrochemically active species [Mg2(μ-Cl)3·6THF].2 Additionally, on cycling, Mg-S batteries also suffer from low overall capacity, high self-discharge and low Coulombic efficiency which results from low active material utilization, a low sulfur loading amount due to the electrically and ionically insulating nature of sulfur, and a large volume change in sulfur during the reaction. A few extant studies have been focused on developing non-nucleophilic electrolytes, such as (HMDS)2Mg-AlCl3 in ethereal and ionic liquid solvents,6-11 (HMDS)2Mg-MgCl2/THF,12 (HMDS)2Mg-AlCl3-MgCl2/THF,13

borate-centered

anion-based

magnesium

(BCM)

electrolytes,14-16 MgB[(hfip)4]2/DME (hfip=hexafluoroisopropyloxy),17 MgCl2-AlCl3, AlEtCl2, AlEt2Cl, AlEt3 or AlPh3 in ethereal and ionic liquid solvents,18-25 Lewis acid-free MgCl2(DTBP)MgCl/THF (DTBP=2,6-di-tert-butylphenolate),26 Mg(TFSI)2 based electrolytes.27-30 Lithium salts as an additive were proposed to improve the reversibility of Mg-S batteries by activating electrochemically inactive MgS and/or MgS2, which extensively precipitates on the surface of the cathode during discharge, through Li+.10 On the other hand, a variety of strategies have also been explored to address the dissolution of magnesium polysufides (MgSx, x≥4) by designing separator 13 and sulfur-carbon cathodes with various carbon matrices 8, 9, 13, 15, 16, 17, 27, 29 or fabricating new elemental selenium (Se)31 and selenium-sulfur solid solution (SeS2)31


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cathodes to physically restrain polysulfides, facilitate electron transport, and provide free-space to accommodate the volume change in S/polysulfides.15,

31-32

We firstly reported on the

feasibility of a sulfur cathode in a typical nucleophilic electrolyte, (PhMgCl)2-AlCl3/THF (called as APC), which can be easily prepared and possesses appropriate anodic stability and high reversibility of Mg deposition-dissolution, by simply using Cu as the cathode current collector.33 Copper sulfides formed when sulfur is coated on a Cu current collector at 50 C provide a strong chemical interaction between Cu and sulfur, which protects sulfur and increases its compatibility with the electrolyte. Reversible discharge-charge of the Mg-S cell was demonstrated for over 20 cycles with specific discharge-charge capacities and high Coulombic efficiency. The dissolution of polysulfides led to a rapid capacity fade in the Mg-S system. However, the study proved the concept of rechargeable Mg-S batteries with nucleophilic electrolytes by rationally designing a metal-stabilized sulfur electrode. The shuttle effect and the consequent loss of active sulfur should be still settled fundamentally. A sulfur cathode based on sulfide graphdiyne (SGDY), which is composed of a conducting carbon skeleton with high Li+ mobility and short sulfur energy-storing unites, can essentially avoid polysulfide dissolution and be compatible with nucleophilic APC based electrolytes.34 Although these developments are encouraging, using nucleophilic electrolytes in Mg-S batteries is still a challenge. Furthermore, very few sulfur/carbon materials have been exploited as cathodes of Mg-S batteries with nucleophilic electrolytes. To enable a reversible electrochemical reaction at high current rates, the sulfur must maintain intimate contact with an electrically conductive additive to improve the conductivity and mitigate polysulfide dissolution. As one important component of sulfur materials, the carbon host plays a key role in the improved electrochemical performance of Mg-S batteries. Especially, high sulfur content in sulfur/carbon


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cathodes generally make polarization more severe and lead to poor cycling stability, lower capacity, and decreased Coloumbic efficiency. However, sulfur/carbon cathodes would need to have relatively high sulfur content in carbon in order to achieve high volumetric capacity. In this paper, S@MC composite with 64.7 wt% sulfur content incorporating small S2-4 molecules with large S8 molecules was prepared by melt-diffusion of sublimed sulfur into commercial microporous carbon with high electrical conductivity and large pore volume. S@MC electrode using Cu as the current collector was used as a novel cathode for advanced Mg-S batteries with APC based nucleophilic electrolytes. Mesoporous carbon as a host improves the electrode reaction kinetics, provides free-space to accommodate the volume change and physically adsorbs of sulfur and polysulfides. Furthermore, chemical interaction between sulfur and Cu current collector enables uniform distribution of sulfur in the electrode, thereby promoting good electrical contact, and can effectively confine polysulfides in the cathode by restraining polysulfide dissolution. Taking advantage of the merits of both S2-4 and S8, S@MC composite shows admirable electrochemical properties in terms of specific capacity, cycling stability, and rate capability and can be promised for the application in practical Mg-S batteries with high energy density. 2. MATERIALS AND METHODS 2. 1 Preparation of S@MC composite Sulfur@microporous carbon (S@MC) composite was prepared by a melt-diffusion method. Sublimed sulfur (S, 99.99%, Sigma-Aldrich) and commercial microporous carbon (MC, ACS Material LLC, USA) with a weight ratio of 4:1 were mixed by ball-milling for 1 h at 350 rmp. Subsequently, the mixture was heated at 155 °C for 8h with heating rate of 0.5 °C min-1


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followed by 300 °C for 1h with heating rate of 5°C min-1 in a tubular furnace under argon atmosphere, causing the molten sulfur to flow into the pores of the MC to achieve S@MC composite. 2.2 Preparation of electrolytes The preparation was conducted in an argon-filled glove box (Mbraun, Unilab, Germany) containing less than 2 ppm H2O and O2. Pure magnesium electrolyte 0.4 mol L-1 (PhMgCl)2AlCl3/THF and hybrid Mg2+/Li+ electrolyte 0.4 mol L-1 (PhMgCl)2-AlCl3 + 1.0 mol L-1 LiCl/THF were obtained by dissolving the predetermined amount of PhMgCl (Sigma-Aldrich, 95%), AlCl3 (Sigma-Aldrich, 99.99%) and LiCl (Alfa Aesar, ultra dry, 99.9%) in tetrahydrofuran (THF) (Aladdin, further dried using a 3 Å molecular sieve) under stirring for at least 24h, respectively. 2.3 Characterization X-ray powder diffraction (XRD) analysis was performed on a Rigaku diffractometer D/MAX-2200/PC equipped with Cu Kα radiation. Thermogravimetric analysis (TGA) was performed using a Thermo Gravimetric Analyzer (TGA/Pyris 1 TGA). X-ray Photoelectron Spectroscopy (XPS) analysis was performed on an AXIS UltraDLD. The spectra were acquired using monochromatic Al Kα (1486.6 eV) radiation. The C 1s line with a binding energy of 284.8 eV was used as a reference. Scanning electron microscopy (SEM) was conducted on a JEOL field-emission microscope (JSM-7401F) and transmission electron microscopy (TEM) was performed on a JEOL high-resolution electron microscope (JEM-2010). After dischargingcharging to different states, the electrodes were washed with THF solvent to remove soluble residue in an argon-filled glove box and then transferred out of the glove box without exposure to the atmosphere.


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2.4 Electrochemical Measurements. Additionally, 70 wt % S@MC composite, 20 wt % Super-P carbon powder (Timcal), and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone were mixed with a magnetic stirrer under an air atmosphere for 6 h at room temperature, coated on Cu foil current collector using a scraper, and dried by heating under vacuum at 50 C for 12 h. The electrode possessed 12 mm diameter and 100 μm layer thickness and contained approximately 0.6-0.7 mg active material. The electrochemical performance was examined via CR2016 coin cells with a Mg counter electrode, an Entek PE membrane separator (ET 20-60, 37% porosity, 20 μm thickness), and 0.4 mol L-1 (PhMgCl)2-AlCl3/THF or 0.4 mol L-1 (PhMgCl)2-AlCl3 + 1.0 mol L-1 LiCl/THF electrolyte. The cells were assembled in an argon-filled glove box. Galvanostatic dischargecharge (magnesiumation-demagnesiumation) measurements were conducted at an ambient temperature on a Land battery measurement system (Wuhan, China) with a cut-off voltage corresponding to 0.5-1.7 V vs. Mg. The electrochemical impedance spectroscopy (EIS) tests were carried out on an Autolab PGSTAT302N (Metrohm Autolab, Switzerland). 3. RESULTS AND DISCUSSION X-ray diffraction (XRD) patterns of sulfur (S), microporous carbon (MC) and S@MC composite powders are shown in Fig. 1a. the pattern of sublimed sulfur exhibits main peaks centered at 2θ = 23.1°, 25.8°, 26.7° and 27.7°, which match well with the (222), (026), (311) and (206) reflections of the Fddd orthorhombic phase of S8 molecules (JCPSD no. 78-1889). The sharp diffraction peaks demonstrate that sulfur exists in a crystalline state. The broad peaks at around 31o and 43o in the pattern of MC imply the amorphous feature of the carbon material. It


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has been reported that the sharp diffraction peaks of bulk crystalline sulfur disappear entirely after encapsulating sulfur into the micropores of carbon, demonstrating that sulfur exists as a highly dispersed state with a low-molecular monolayered coverage inside the micropores of carbon.35 For S@MC composite, some diffraction peaks related to S8 are observed and the peak intensity obviously decreases. This means that not all the sulfur are confined within the micropores and some of them exist outside of the micorpores in a crystal form. The components of S and S@MC electrodes that use Cu as the current collector are also confirmed by XRD measurements. In addition to S, a few new peaks appear in the electrodes (Fig. 1b), which can be assigned to CuS (JCPDS no. 06-0464) and Cu2S (JCPDS no. 83-1462) for the S electrode, and Cu1.92S (JCPDS no. 83-1462) and Cu2S (JCPDS no. 84-0209) for S@MC electrode, respectively. Copper sulfides, which are resulted from a chemical interaction between sulfur and Cu current collector, form at 50 C during the electrode drying process. The valence of copper tends to lower values for the S@MC electrode due to the reduction of carbon in the composite. Compared with the corresponding powders, the peak intensity of sulfur in the electrodes decreases and sulfur peaks in the S@MC electrode almost disappear, suggesting the elemental sulfur exists as an amorphous or dispersed state in the S@MC electrode due to the chemical bonding between sulfur active material and Cu current collector.


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Figure 1. XRD results of S powder, MC powder, S@MC composite powder (a), S electrode and S@MC electrode with Cu current collector (b).


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Figure 2. Overall XPS spectra of S@MC powder and S@MC electrode (a). The high resolution S 2p spectra of S@MC powder and S@MC electrode (b), and Cu 2p spectrum of S@MC electrode (c).

In order to further investigate the chemical bonds of S@MC electrode, X-ray photoelectron spectroscopy (XPS) measurements were conducted on S@MC powder and S@MC electrode, respectively. The overall XPS spectra indicate that the synthesized S@MC composite mainly consists of C, O and S elements (Fig. 2a). For the high resolution S 2p spectrum of S@MC powder (upper figure in Fig. 2b), the peaks located at 164.1 and 165.3 eV can be mainly attributed to S 2p3/2 and S 2p1/2 of cyclo-S8.36 Furthermore, the additional peak at a higher binding energy of 168.5 eV arises from sulfur atoms located at the chain end of small S2-4 molecules.36 A substantial ring scission of cyclo-S8 could take place above 155 ºC due to the low viscosity of liquid sulfur.37 Therefore, chain-like sulfur molecules could migrate into the carbon micropores. Based on above analysis, sulfur has been confined into the micropores of MC to form small chain-like S2-4 molecules during the heat-treatment. Once the sulfur molecules diffuse into the carbon micropores, they could not go back to S8 rings due to the space confinement of carbon micropores, but maintain as small S2-4 molecules. In addition, there is still some elemental sulfur outside the micropores in the S@MC composite powder. Since the normal XPS measurements only detect several nanometers deep into the sample surface, S8 outside the micropores masks and/or decrease the signals of S2-4 inside the micropores. Fig. 2b also presents the S 2p binding energy spectrum of S@MC electrode with Cu current collector. The peaks at 169.1 and 168.1 eV are related to the sulfur atoms located at the chain end of small S2-4 molecules.36, 38 Those located at 163.8 and 165.0 eV can be attributed to S 2p3/2 and S 2p1/2 of


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sulfur atoms in the middle of chain/cycle molecules, respectively.38, 39 This means ring-like S8 in the S@MC powder has been changed to short-chain Sx (x ≤⁓8) in the S@MC electrode probably duo to the presence of chemical bonds.34 Two peaks located at 161.9 and 162.7 eV with a peak separation of 0.8 eV, which are consistent with the expected values (160-164 eV) of S in sulfide phases.40 The Cu 2p core level spectrum shown in Fig. 2c exhibits two peaks at approximately 932.6 eV and 952.5 eV, which are assigned to 2p3/2 and 2p1/2 core levels of Cu1+, respectively.41 It is reported that the monolayer of S enhances surface self-diffusion on Cu surface by several orders of magnitude.42 Mobile Cu and S containing moieties can be responsible for the significant material transport observed during the adsorption of sulfur on Cu.43 The XRD and XPS results demonstrate the occurrence of chemical bonds between elemental sulfur in the S@MC material and copper of the current collector. For the S@MC electrode, it can be determined that most of elemental sulfur migrates into the micropores of carbon in the form of chain-like S2-4 molecules (called as internal sulfur), and others outside the micropores of the carbon (called as external sulfur) change into the middle of chain/cycle molecules and/or chemically interacts with the Cu substrate by forming of copper sulfides. Highly dispersed state of internal sulfur inside the carbon micropores and chemical bonding of external sulfur with Cu current collector enable uniform and tight distribution of sulfur in the electrode, thereby promoting good electrical contact of the composite material with the current collector.


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Figure 3. TGA curves of S and S@MC composite (a), Raman spectra of S, MC and S@MC composite (b), N2 adsorption-desorption isotherms (c) and pore size distribution plots (d) of MC and S@MC calculated by DFT method, inset of (d) is the pore size distribution plots calculated by H-K method.

Thermogravimetric analysis (TGA) was performed on elemental sulfur and S@MC composite in Ar atmosphere to compare the thermal behavior of elemental sulfur and S@MC composite and confirm the sulfur content in the S@MC composite. As shown in Fig. 3a, elemental sulfur shows a rapid weight loss starting from 150 oC and ending at 290 oC. In


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contrast, the onset and offset temperatures of S@MC, especially the latter, shift toward much higher values and the rate of weight loss decreases due to the interaction between the carbon matrix and sulfur, suggesting the thermal stability of the composite is substantially improved in comparison with pure sulfur. There is weight loss about 24.4 wt% below 290 oC for S@MC composite, indicating elemental sulfur exists in the composite in the form of external sulfur covered on the surface of MC. The weight loss starting at above 290 oC and ending at 450 oC is related to the release of internal sulfur within the micropores of MC. All sulfur content in the S@MC composite is calculated to be 64.7 wt% according to the weight loss. Raman spectra of sulfur, MC and S@MC composite are shown in Fig. 3b. For MC and S@MC composite, two strong peaks located at ~1350 and ~1580 cm-1 can be assigned to the disordered carbon (D band) and graphite-like carbon (G band). The ratio of ID/IG is 0.88 and 0.98 for MC and S@MC composite, respectively. The incremental ID/IG value of S@MC indicates that more defects emerge after sulfur infusion. The peak at ~2700 cm-1 in MC and S@MC corresponds to the two phonon lattice vibration (2D band), which suggest the existence of graphitic carbon in favour of the electron transport for electrochemical reactions. Though it is hard to capture the Raman signals of the confined S2-4 molecules due to the interference of the carbon matrix, the disappearance of the typical peaks below 500 cm-1 corresponding to the strong S vibration for cyclo-S8 confirm that sulfur exists in different forms in S@MC composite. XPS and TGA results have shown that most of sulfur molecules in S@MC composite are small sulfur allotropes with linear chain configuration in the low-molecular forms. The specific surface area analysis of mesoporous carbon and S@MC composite was performed by nitrogen adsorption-desorption measurements. A Type I isotherm (a Langmuirtype isotherm) is shown in Fig. 3c, indicating the characteristic microporous structure of MC.


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The DFT pore size distribution shows that MC possesses relatively narrow pore diameters (

A Sulfur@microporous Carbon Cathode With a ... - ACS Publications

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