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Graphene Sandwiched by Sulfur-Confined Mesoporous Carbon Nanosheets: A Kinetically Stable Cathode for Li-S Batteries Sen Xin, Ya You, Hui-Qin Li, Weidong Zhou, Yutao Li, Leigang Xue, and Huai-Ping Cong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12142 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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ACS Applied Materials & Interfaces

Graphene Sandwiched by Sulfur-Confined Mesoporous Carbon Nanosheets: A Kinetically Stable Cathode for Li-S Batteries Sen Xina,b‡, Ya Youb,‡, Hui-Qin Lia,‡, Weidong Zhoub, Yutao Lib, Leigang Xueb, and Huai-Ping Conga,*

a

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei,

230009, P. R. China. b

Materials Science and Engineering Program and Texas Materials Institute, The University of

Texas at Austin, Austin, Texas 78712, United States.

Corresponding Author * [email protected]. ‡

These authors contributed equally to this work.

KEYWORDS: lithium-sulfur battery, graphene, mesoporous carbon, kinetics, rate performance.

ACS Paragon Plus Environment

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ABSTRACT: The practical use of lithium-sulfur batteries for the next-generation energy storage, especially the automobiles, was hindered by low electronic conductivity of sulfur and the resulted poor rate capabilities. Here, we report a sulfur-carbon composite by confining S into a graphene sandwiched in mesoporous carbon nanosheets with two-dimensional ultrathin morphology, suitable mesopore size and large pore volume, and excellent electronic conductivity. Served as cathode material for Li-S battery, the elaborately designed S/C composite leads to “kinetically stable” transmissions of Li ions and electrons, triggering a stable electrochemistry and a record-breaking rate performance. In this way, the S/C composite has been proved a promising cathode material for high-rate Li-S batteries targeted at automobile storage.


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INTRODUCTION Facing the ever-growing energy demand on emerging storage applications, such as wearable devices and grid, rechargeable lithium-chalcogen batteries, as exemplified by Li-S and Li-Se batteries, become particularly attractive due to their ultrahigh energy output (~2600 W h kg-1) and cost advantages.1-11 However, the application of Li-S batteries, especially in automobiles, is still hindered by the insulation and poor retaining of cathode sulfur, which result in low capacity, short cycling life and poor rate capability in the battery.12-17 In solving these critical problems, considerable efforts have been made to improving the sulfur conduction and retention by confining sulfur into conductive nanostructures such as porous carbons.5, 13, 18-25 Many porous carbons, such as microporous carbon,13 mesoporous carbon,23, 24 and macroporous carbon,25 were employed, and the cathode performance was found directly related to the pore size of the carbon matrix.4 Though provided with the largest pore volume to achieve the highest S loading content, macroporous carbon has been reported highly ineffectual in activating enclosed sulfur and restricting soluble polysulfides owing to its large size and open architecture.4 The microporous carbon has been reported with an optimized polysulfide confinement, yet a low S loading deteriorates its practicality in battery use.4 Hence, the mesoporous carbons with sizes ranging from 2 to 50 nm are considered as an optimized choice due to its combined advantage in S loading and confinement. The nanocasting method, especially those performed on hard templates such as ordered silicate templates,26,

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has been reported effective to prepare mesoporous

carbons due to a precise duplication of template body and an easy control of porous structure and other parameters. For example, Nazar’s Group has reported the first cyclable Li-S battery, in which S was restricted into an ordered mesoporous carbon (CMK-3) matrix synthesized from an


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mesoporous silicate template (SBA-15) via nanocasting, so that the active S loss from the cathode can be prevented to improve the cycling performance of battery.23 Other conductive substrates, such as graphitic carbons including graphene,28, 29 carbon nanotubes15, 30, 31 or carbon black,32 conductive polymers,7, 33, 34 and newly reported graphitic carbon nitride,35, 36 were also employed to improve the cathode performance. Though promising results are witnessed on improving the cycle life of Li-S battery, challenges still exist in building a high-power battery for automobile use due to a sluggish electrochemical reaction on the cathode side, which originates from low ionic and electronic conductivity of S. For example, the porous carbons, especially those derived from pyrolysis of carbohydrates or polymers, though benefit a uniform S dispersion to improve the Li+ conductivity in the composite, suffer from a poor electron conduction for electrochemical reaction between Li and S due to a low graphitization degree. The graphitic carbons, such as graphene or carbon nanotubes, are known for their high electronic conductivity, yet suffer from a low ionic conductivity after S loading since it cannot effectively disperse bulk S. Noted that the kinetics of cathode reaction is primarily determined by the both the electron transmission and the ionic transportation, we have shown a promising strategy to improve the rate performance of the cathode, that is, encapsulation of nano-S into a graphene sandwiched in ultrathin mesoporous carbon nanosheets (G@MC).37 On the one hand, the mesoporous carbon sheets on both sides of the G@MC ensure a uniform dispersion of nano-S to prevent any agglomeration to form bulk S, and help to achieve a highly-efficient trapping of S into the carbon host. On the other hand, the two-dimensional ultrathin morphology ensures a short ionic diffusion path for a rapid Li+ access to active S, and the embedded graphene provides a fast planar transmission of electrons for lithiation/delithiation of S.38-40 In this way, a mixed


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conductive network is formed, which brings remarkable Li storage performance especially at high discharge-charge rates, and make the composite cathode an appealing choice to realize high-rate Li-S batteries for transportation use.

EXPERIMENTAL SECTION Synthesis of G@MC. A template-nanocasting method was employed to prepare the G@MC (Figure 1), in which all the chemical reagents used were of analytical grade and were used without further purification. The synthesis started from the preparation of GO from the oxidization of graphite powder through a modified Hummers’ method.41 And then, 1 g of the assynthesized GO was dispersed in 10 mL of de-ionized water under ultrasonic treatment for 3 h, followed by addition of 40 mg of NaOH, 1 g of CTAB and 1 mL of TEOS to form a solution. The solution was then stirred at 40 oC for 12 h, and filtered to yield a brown solid powder. This powder, consisting of GO coated by mesoporous silicate (MS) on both sides of it, was denoted as GO@MS. After that, the GO@MS was successively washed by de-ionized water, dried and calcined at 800 oC in argon to remove the CTAB and reduce the GO to yield the G@MS template. In the subsequent nanocasting process, 0.1 g of sucrose was pre-dissolved in a solution mixture containing 200 µL of de-ionized water and 200 µL of alcohol. The sucrose solution was then slowly and dropwise added to 0.1 g of the G@MS to enable a complete impregnation of carbohydrate into the mesopores of the template. The mixture was then pre-carbonized at 160 oC for 12 h in air and further carbonized at 800 oC for 3 h in Ar so that conductive carbon could form in the mesopores G@MS template. The resultant, denoted as the G@(MS/C), was soaked in an aqueous solution containing 5 wt-% hydrofluoric acid over the night to remove the silicate


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template, then collected by centrifugation, washed by de-ionized water and finally dried at 80 oC to yield the G@MC. Synthesis of control carbon materials. Two control carbon materials were prepared. The first carbon material was prepared from a mesoporous silicate template, which was synthesized by annealing the GO@MS at 800 oC in air to remove the CTAB and GO. After that, it was loaded with the water-alcohol solution of sucrose (with the same amount as that in the preparation of G@MC), and then the same carbonization and template removal process was performed to yield a mesoporous carbon material denoted as MC. The second carbon material, denoted as G@C, was prepared from the direct carbonization of GO and sucrose mixture at the same mass ratio as that in the preparation of G@MC. Synthesis of S/C composite. S was physically mixed with each of the above carbon materials in a mass ratio of 6:4. The mixture was then sealed in Ar and heated at 155 oC for 12 h to enable a full impregnation of S into the mesoporous carbon, yielding the S/C composite. Morphological and structural characterizations. The morphology of all the samples was investigated using a field-emission scanning electron microscopy (SEM, Zeiss Merlin Compact) operated at an accelerating voltage of 5 kV. Transmission electron microscopic (TEM) images were taken on a Hitachi H-7650 operated at 100 kV. High-resolution TEM, energy-dispersive Xray (EDX), scanning transmission electron microscopy (STEM) and element mapping analysis were performed on JEM-2100F at 200 kV with Oxford Inca. X-ray diffraction (XRD) patterns of all the samples were collected on an X’Pert PRO MPD diffractometer equipped with CuKα radiation. Raman spectra were collected by using a LABRAM-HR confocal laser microRaman spectrometer. X-ray photoelectron spectroscopic (XPS) spectra were collected on an ESCALAB250Xi X-ray photoelectron spectrometer equipped with a monochromatic X-ray


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source. Thermogravimetic analysis was performed in N2 atmosphere on a thermal analyzer (STA449F3) with a heating rate of 10 K min-1. Atomic force microscopic (AFM) images were collected on a Veeco DI Nano-scope MultiMode V system. N2 sorption isotherms were conducted on a Quantachrome Autosorb-IQ. The degassing of bare carbon material was carried out at 120 oC, and for the S/C composite material, the process was performed at room temperature to avoid any sublimation of S into the instrument, and a long degas duration of 12 h was applied to ensure a complete removal of air from the sample. Electrochemical characterization. The as-synthesized S/C composite was mixed with Ketjen black and poly-(vinyl difluoride) dissolved in N-methyl-pyrrolidone at a weight ratio of 80:10:10 to form a slurry. The slurry was then pasted on an aluminum foil (Goodfellow), dried under normal pressure at 80 ºC overnight and cut into slices to yield the S cathodes. The electrode had a diameter of 12 mm and an active material load of ~2 mg cm–2. Swagelok-type Li-S batteries were assembled in an Ar-filled glovebox (with oxygen and water contents < 0.1 ppm) by pairing the above working electrode with Li foils as the anodes. A ether electrolyte consisting of 1 M bis (trifluoromethane) sulfonamide lithium salt dissolved in a mixture of 1,3-dioxolane and dimethoxymethane (v:v = 1:1) was used as the electrolyte. 20 µL of the electrolyte was added for each battery during its assembly. For separator, a microporous membrane (Celgard, USA) was employed. Galvanostatic discharge-charge (GDC) cycling tests of the assembled batteries were carried out on an Arbin BT-1 system in the voltage range of 1-3 V (vs. Li+/Li). The current density was calculated based on the weight of active sulfur on the cathode. Cyclic voltammograms (CVs) were collected on an Autolab PG302N electrochemical workstation with a scan rate of 0.1 mV s–1 in the potential range of 1-3 V (vs. Li+/Li). Nyquist plots were also collected on the Autolab PG302N workstation within a frequency range of 0.1-100000 Hz.


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RESULTS AND DISCUSSIONS Figure 1 shows the preparation process of the cathode material. As a starting material, graphene oxide (GO) was prepared according to a modified Hummers’ method and then coated with mesoporous silicate on its both sides to yield the GO@MS template. After that, hightemperature reduction, nanocasting and carbonization were successively applied to the template, enabling the carbon formation in the silicate mespores (G@(MS/C)). After etching by the HF solution, the G@MC material was obtained, which was then employed as the substrate for S loading to yield the target cathode material denoted as G@(MC/S). Figure 2 summarizes the morphological characterization results of during the preparation of the G@MC substrate. During the template preparation, the surfactant, cetyltrimetyl ammonium bromide (CTAB) is quickly absorbed onto both sides of GO by electrostatic interactions, forming a uniform micelle layer for subsequent growth of silica nuclei on GO’s surface with slow hydrolysis of tetraethylorthosilicate (TEOS). This results in a homogenous coating of CTABmesoporous silicate onto both sides of GO, leading to GO@MS sheets with a two-dimensional sandwich-like morphology with an average longitudinal length of > 1 µm and a thickness of 17 nm as confirmed by the SEM, TEM and AFM (Figure 1, 2a, 2b and Figure S1a). After the removal of CTAB and reduction of GO, the resultant with graphene sandwiched in mesoporous silicate (G@MS), maintains its pristine sheet-like morphology and sandwich cross section, with no apparent change in length and thickness (Figure 2c, 2d, and Figure S1b). From the highresolution TEM image in the inset of Figure 2d, it is clearly observed that the silicate layer consists of numerous mesopores with an average diameter of 4-5 nm. In the subsequent synthesis of G@MC, sucrose was loaded as the carbon precursor into the mesoporous silicate, and a


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nanocasting approach was employed to pass the above structural features of the G@MS down to the G@MC (Figure 1). As can be seen from Figure 2e and 2f, the G@MC still maintains its sheet-like morphology, with ~8 nm carbon layer coated on both sides of the graphene. A highresolution TEM image further reveals the porous structure of carbon layer (inset of Figure 2f). Figure 3a confirms the amorphous nature of the G@MC since the characteristic (002) XRD peak of graphene at 26.6o has been replaced by a large and broad diffraction peak. This result is further verified by the Raman spectra (Figure 3b), in which the D-band at 1350 cm-1 is more intensive than the G-band at 1600 cm-1 (The intensity ratio of D band to G band is 1.05), suggesting an disordered structure of the [email protected], 43 The porous structure of the G@MC was further studied by nitrogen adsorption/desorption measurements. The carbon substrate shows a Type IV isotherms with significant adsorptions at low relative pressure region (

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