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High Performance Lithium–Sulfur Batteries with a Sustainable and Environmentally Friendly Carbon Aerogel Modified Separator Lin Zhu, Liangjun You, Penghui Zhu, Xiangqian Shen, Lezhi Yang, and Kesong Xiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02322 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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ACS Sustainable Chemistry & Engineering

High Performance Lithium–Sulfur Batteries with a Sustainable and Environmentally Friendly Carbon Aerogel Modified Separator Lin Zhu*,†, Liangjun You†, Penghui Zhu†, Xiangqian Shen*,†, Lezhi Yang‡, Kesong Xiao‡ †

School of Materials Science & Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, P. R. China

‡

Changsha Research Institute of Mining and Metallurgy Co., Ltd, 966 South Lushan Road, Changsha, 410012, P.R. China. Tel.: +86 511 88780191, fax: +86 511 88781947, E-mail: [email protected] (L. Zhu), [email protected] (X. Shen)

ABSTRACT Carbon–based aerogel prepared via direct conversion of natural biomass has wide application prospect in the field of environment and energy. Herein, the sustainable and environmentally friendly porous carbon aerogel is prepared through the hydrothermal treatment, freeze–drying and postpyrolysis process using sweet potato as the precursors. The as–prepared carbon aerogel is used to modify the commercial separator of lithium–sulfur batteries to solve the problems of poor cycle life and low utilization rate of active substances. The carbon aerogel coating can not only suppress the shuttle effect of the polysulfide intermediates during cycling and reduce the cell resistance, but also act as an upper current collector to increase the utilization rate of sulfur. The cell with carbon aerogel modified separator exhibits high electrochemical performance. The results show that the initial discharge capacity is 1216 mAh/g at 0.1 C and the reversible discharge capacity retains 431 mAh/g after 1000 cycles at 1 C with the coulombic efficiency over 95.3 %. KEYWORDS: Lithium–sulfur batteries, Modified separator, Lithium polysulfide, Carbon aerogel, Biomass

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ACS Paragon Plus Environment


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■ INTRODUCTION The fast development of electric vehicles and mobile electronics puts forward higher requirements for the performance of the battery, so it is very exigent and significant to develop high specific energy and environmentally friendly novel energy storage lithium ion battery.1,2 Among the next generation of energy storage systems, lithium–sulfur (Li–S) batteries are acknowledged as the most typical and promising candidates due to their high theoretical specific capacity (1675 mAh/g) and energy density (2600 Wh/kg). The values are about five times greater than conventional lithium ion batteries.3–6 Moreover, the inherent characteristics of their active material sulfur, such as low toxicity, abundant resources, low cost, environmentally friendly and wide operational temperature range, act as an extra advantage for achieving the large–scale commercial applications.7 Despite these advantageous features, the widespread practical applications of Li–S batteries are still largely restricted plagued by its own issues: i) the natural insulation of sulfur and lithium sulfide limit the electron transfer during the electrochemical reactions, leading to poor utilization rate of active substances, and ii) the dissolution of lithium polysulfide (LiPS) intermediates (Li2Sx, x=4–8) in the liquid electrolytes cause the well–known “shuttling effect”, resulting in self–discharge, the loss of active substances, fast capacity fading, and low coulombic efficiency, and iii) the significant volume expansion during charge/discharge process may also result in the fast capacity fading and poor cycling performance.8,9 To address these issues, many approaches have focused on structure design and development of the cathode. The traditional approach adopted by researchers is to introduce carbonaceous materials into sulfur cathode, such as activated carbon,10 mesoporous carbon,11,12 acetylene black,13 carbon nanotubes,14 carbon nanofiber,15 graphene,16-18 etc. These carbon materials not only improve the electrical conductivity of the cathode, but also capture and accommodate the active materials. Moreover, they can not only adsorb and immobilize LiPS, but also supply lithium ion diffusion channels, thereby enhancing rate performance and cyclic stability. However, the addition of carbon materials reduces the sulfur loading amount in the cathode, which decreases the energy density of Li–S batteries. Furthermore, these cathodes involve 2


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rigorous preparation process and high cost, which limit commercialization of Li–S batteries. Thus, the easier, more efficient and cost–effective approaches are needed for commercialized applications of Li–S batteries. More recently, various types of interlayers, including porous carbon,19-21 conductive polymer,22,23 oxide,24 were inserted in between cathode and separator for intercepting the diffusing LiPS. Moreover, inserting the interlayer can not only localize the active substances within the cathode side and strengthen the electronic conductivity of sulfur electrode, but also provides more reactive sites for the reaction and buffer the volume change of the cathode during cycling. To date, the carbon–based materials derived from natural biomass are attracting increasing interest with the advantages of wide source, sustainable, environmentally friendly, and low cost. Compared with other carbon–based materials, the biomass carbon not only has a strong adsorption capacity, chemical stability and regeneration ability, but also has evolutionary pore structure, high specific surface area, rich surface functional groups and stable aromatic structure. These advantages make it has broad application prospects in the environment and energy field.25,26 In particular, the carbon aerogels derived from biomass have been receiving distinctive attention because of their excellent physical properties such as surface hydrophobicity, high porosity, large specific surface area, low apparent density, and high electrical conductivity. The carbon aerogels derived from different biomass, such as winter melon,27 cotton,28 pomelo peel,29 sodium carboxymethyl cellulose,30 watermelon,31 leonardite fulvic acid,32 and bacterial cellulose,33 have been developed for pollutants adsorption,27-30 supercapacitors,30-32 and battery.33 These outstanding characteristics inspired us to develop a high–performance interlayer between cathode and separator using carbon aerogels derived from biomass. Sweet potato, as a renewable resource, is widely cultivated in china for making starch and extracting pectin. In this study, sweet potato was chosen as a carbon precursor to prepare porous carbon aerogels (SP–CA) by the simple hydrothermal carbonization, freeze–drying and pyrolysis process for the first time. One side of the routine Celgard separator was coated by a coating layer composed of SP–CA and conductive agent (Super–P). Higher capacity retention and better reversible capacity of the Li–S batteries 3



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were achieved by employing SP–CA modified separator. ■ EXPERIMENTAL SECTION Materials. The sweet potato was collected from local supermarket which was picked in

Shandong

Province,

China.

Elemental

sulfur

(S,

99.98

wt.

%)

and

N–methyl–2–pyrrolidone (NMP, 99 wt. %) were obtained from Aldrich. Polyvinylidene difluoride (PVDF, Solef 21216) and super P carbon (SPC, TIMCAL) were obtained from Solvay and Alfa Aesar, respectively. All of the chemicals were used as received without any further purification. Synthesis of the Sweet Potato Derived Carbon Aerogel. As shown in Figure S1, the raw sweet potato (SP) was peeled, cut into small pieces with dimension of about 4×2.5×1 cm3. Then they were put into a Teflon–lined stainless–steel autoclave and hydrothermal treated for 10 h at 180 °C. Subsequently, the spongy sweet potato hydrogels (SP–H) was taken out and washed repeatedly with hot water (about 70 °C) to remove soluble impurities. After that, the brown sweet potato aerogel (SP–A) was prepared by freezeing at -20 °C for 6 h, followed by freeze–drying for 10 h at -80 °C. Then, the sweet potato derived carbon aerogel (SP–CA) was obtained by the pyrolysis carbonization process at 800 °C in a tubular furnace under a flow of nitrogen at a heating rate of 5 °C/min. The tubular furnace was held at 800 °C for 1 h to allow complete pyrolysis. Then it was naturally cooled down to ambient temperature to obtain the black SP–CA. In order to study the adsorption ability of SP–CA for polysulfides, 0.05 g of the SP–CA powder was putted into a glass bottle containing 10 ml of 0.1 M Li2S4 solution. The polysulfide Li2S4 solution was prepared by chemically reacting Li2S and a stoichiometric amount of elemental sulfur in DME:DOL (1:1 by volume) solution. The glass bottle was shaken after the addition of the SP–CA powder, and then the glass bottle was automatically deposited in a glove box for later observation. As shown in Figure S2, it could be seen that the color of the solution changed from dark red to bright yellow after 30 minutes. With time increasing, the color changed from bright yellow to nearly transparent after 2 h. The results of adsorption experiment of polysulfides demonstrated that the SP–CA had excellent adsorption capability for polysulfides due to its highly porous 4


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structure. In addition, the content of elemental sulfur in the polysulfide solution before and after adding the SP–CA powder was determined by Plasma Emission Spectrometer (ICP-OES). The ICP results of sulfur in the polysulfide solution were summarized in the Table S1. The results showed that the content of sulfur element in the polysulfide solution decreased from 9.862 g/L to 0.435 g/L after the addition of the SP–CA powder, further demonstrating the excellent adsorption capacity of the SP–CA powder. Preparation of the SP-CA Modified Separator. Figure S3 schematically illustrated the fabrication process of the sweet potato derived carbon aerogel (SP–CA) modified separator (Celgard 2400). The slurry for the SP–CA modified separator was obtained by mixing SP–CA, Super–P, and polyvinylidene difluoride with N–methyl–2–pyrrolidone as the dispersant. The mass ratio of SP–CA/PVDF/SP was 6: 3: 1, and they were stirred in a blender for 3 minutes to obtain the slurry. Then one side of Celgard 2400 was coated using the slurry by an automatic coating machine, and it was dried in a vacuum oven at 30 °C for 24 h. Finally the SP–CA modified separator was sliced into the disc with the diameter of 19 mm which was showed in Figure S4a, b. The thickness of the SP–CA modified layer was approximately 10–13 µm which detected by the micrometer as shown in Figure S4c, d, and the average weight of the carbon coatings on the separator was estimated to be 0.4–0.6 mg/cm-2 after calculated. The SEM images of the surfaces of routine separator and SP–CA modified separator were shown in Figure S5a, b, c, d, the cross–sectional morphology of SP–CA modified separator was displayed in Figure S5e. Preparation of Sulfur Cathode and Assemble of Li-S Cell. The sulfur cathodes were prepared by melt diffusion method and the slurry coating procedure. The material of cathode was carbon/sulfur (C/S) composite which was blended in the mass ratio of 1: 4 and ball milled for 5 h, and then the grated mixture of ketjen black and sulfur powder was covered in a reactor and heated for 12 h at 155 °C under vacuum. The cathodic electrode for the Li–S cell was composed of C/S composite, super–P and PVDF. They were dissolved in NMP at the mass ratio of 80: 5: 15. An automatic coating machine, which could control the coating speed, was used to coat the slurry of the cathodic electrode onto an aluminum foil. Then the cathodic electrode was dried under vacuum at for 12 h 60 °C. 5



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After that the film was sliced into discs with a diameter of 12 mm for the cathodic electrode of Li–S cell, and the loading mass of the sulfur cathodes used for the experimental tests was 1.1–1.4 mg/cm-2. CR2025–type Li–S coin cells were assembled in a glove box which filled with Ar atomosphere, in which the water and oxygen contents were less than 0.1 ppm. The routine separator (Celgard 2400) and the SP–CA modified separator were used as the separator, and the electrolyte was1,2–dimethoxymethane (DME) and 1,3–dioxolane (DOL) mixed by the volume ratio of 1:1 with 1mol dm-3 LiTFSI and 0.1mol dm-3 LiNO3 additive, and the amount of the electrolyte in this work was about 60 µL in each cell. The structure diagram of the cell with the SP–CA modified separator was shown in Figure 1.

Figure 1. The structure diagram of the cell with the SP-CA modified separator.

Characterization. The surface morphology and microstructure of SP, SP derivatives, and different separators was performed by means of field emission scanning electron microscopy (JSM–7001F) and transmission electron microscopy (JEM–2100HR). The crystal structure was investigated using X–ray powder diffraction (XRD) (Rigaku D/Mmax 2500PC diffractometer) with Cu–Kα radiation. The chemical functional groups present on the SP and SP derivatives were characterized with a NEXUS 670 FTIR Spectrometer using KBr method in the wave number range 400–4000 cm-1. The content of elemental carbon and oxygen was determined by elemental analyzer (EA-1112). Nitrogen adsorption–desorption experiments were measured using a pore size and surface area 6


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analyzer (Quanta Chrome Corporation, USA) at 77 K to investigate the textural characteristics of the SP–CA samples. The vibrational characteristic of the samples was investigated on a Renishaw Microscope System RM2000 with a 20 mW Ar+ laser at 532 nm. Cyclic voltammetry (CV) was carried out on the AUTOLAB electrochemical work station in a voltage range of 1.7–2.8 V (vs. Li/Li+) at the scan rate of 0.1 mV/s. The assembled cells were galvanostatically tested using a CT2001A cell test instrument (LAND model, Wuhan RAMBO testing equipment, Co. Ltd) by cycling in the voltage range of 1.7–2.8 V (vs. Li/Li+). The electrochemical impedance spectrometry (EIS) measurements of the cells were carried out over the frequency range between 100 kHz to 100 mHz by using a VMP2 electrochemical work station (DHS Instruments Co. Ltd). ■ RESULTS AND DISCUSSION

Figure 2. SEM images of the SP–A sample (a) and SP–CA sample (b), and TEM images of the SP–CA sample (c, d).

Figure 2 illustrated the scanning electron microscope (SEM) images of SP–A and SP–CA. It could be shown that SP–A displayed agglomerate ball–like structure with a large amount of regular shape arranged together, indicating that the low temperature hydrothermal treatment could not cause great damage of the original structure. After pyrolysis treatment under N2 atmosphere at 800 °C, an amorphous interconnected porous 7



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structure was observed on the surface of SP–CA. Figure S5 showed the SEM images of the surface of the routine separator and the SP–CA modified separator at different magnification. It could be observed that the surface of the routine separator displayed a uniformly distributed porous structure. While the surface morphology of the SP–CA modified separator still retained the interconnected porous structure after the SP–CA was coated on the routine separator, which was similar to the morphology of the SP–CA sample. However, its surface became smooth and compact. Transmission electron microscopy (TEM) was also utilized to analyze the structures and morphologies of the SP–CA sample, and the results were also shown in Figure 2. These results indicated that the porous structure of SP–CA could not only speed up the ion transmission rate, but also improve the sufficient contact with the positive electrode and the electrolyte which could lead to an excellent electrochemical performance.

Figure 3. XRD patterns (a) and FTIR spectra (b) of raw SP, SP–A, and SP–CA, Raman spectra (c) of SP–A and SP–CA, and N2 adsorption/desorption isotherms and BJH pore size distribution profiles (d, inset) of SP–CA.

8


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X–ray diffraction (XRD) was utilized to analyze the microstructure and the graphitization degree of the corresponding sample, and the results were shown in Figure 3a. It could be observed that all the samples displayed the strong diffraction peak at around 23°, which corresponded to the (002) reﬂection of the disordered carbon layer and the graphite carbon structure. The raw SP presented diffraction peak at 22.9°, which corresponded to graphitic carbon structure of the biomass precursor.34 After hydrothermal treatment (SP–A) and calcination (SP–CA), the peak became broader and its position shifted to the higher angle, which indicated that the degree of amorphization of the samples increased. In addition, a new weak diffraction peak in the higher angle region (around 43°) corresponded to (100) planes of carbon structure was found in the XRD pattern of SP–CA, which indicated further destruction of graphitic carbon structure during the calcination process. The FTIR spectra of raw SP, SP–A, and SP–CA were shown in Figure 3b. It demonstrated that the intensity of absorption peaks around 3420 cm-1, 2920 cm-1, 1639 cm-1, 1038 cm-1, and 670 cm-1 corresponded to stretching vibrations of –OH, C–H, C=C, and –COOH, etc. It was also found that although the patterns of three samples were similar, the intensity of some peaks of SP–A and SP–CA samples decreased or even disappeared and the position of some peaks changed. The content of elemental carbon and oxygen was determined by elemental analyzer, and the results were summarized in the Table S2. The results indicated that the content of elemental carbon in the sample increased after hydrothermal treatment and postpyrolysis process, while the content of elemental oxygen decreased. These results indicated that the raw SP was rich in the functional groups like hydroxyl, alkenes, aromatics and carbonyls, etc., while these functional groups reduced or disappeared after pyrolysis carbonization process. Raman spectroscopy is an effective technique for showing the structure and quality information of the graphitized carbon materials, such as defects density, disorder, defect structures and doping levels, etc. Raman spectra of the samples SP–A and SP–CA materials were shown in Figure 3c. The raw SP displayed almost a horizontal line which was not showed. The Raman spectra of SP–A and SP–CA materials showed two 9



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fingerprint peaks at 1350 cm-1 and 1588 cm-1 that corresponded to the D and G bands, respectively. The D band was denoted as the presence of disordered carbon or defective graphitic structures, while the G band was attributed to the crystalline graphite structure and the tangential vibration of the sp2–bond carbon atoms in the graphitic layers. It could be seen that the intensity of D band of SP–CA increased, indicated the carbon–based lattice defects increased after pyrolysis treatment at 800 °C. In addition, the peak height ratio of ID/IG (R) showed that there was a negative correlation between the graphitization degree and the intensity ratio of D band to G band. The values of ID/IG for the SP–CA materials was calculated to be 0.972, demonstrated that a large number of defective graphitic structures and highly porous structure were developed in the carbon materials after the pyrolysis carbonization process, which was actually beneficial to the electronic conductivity of the SP–CA sample.35-42 In addition, the existence of the defects is good for lithium storage.42 To investigate the textural properties, the nitrogen gas adsorption–desorption isotherms and pore size distribution profiles of the SP–CA were illustrated in Figure 3d. Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface areas. According to IUPAC classification, the N2 adsorption–desorption isotherm of SP–CA could be defined to a type IV adsorption curve with a hysteresis loop at relative high pressures, indicating the characteristic mesoporous structure of SP–CA. Figure 3d (inset) showed the pore size distributions curve, which could be seen that the sharp peak was found at 3.9 nm and most of the pores were below 10 nm which also indicated the mesoporous structure in the SP–CA. The specific surface area, average pore size, and total pore volume of the SP–CA calculated by BJH method were 590.13 m2/g, 2.09 nm, and 0.31 cm3/g, respectively. These results indicated that the mesopores could not only provide transport channels for lithium ions, but also effectively limit the shuttle effect of the polysulfide intermediates, thus reducing the loss of the active substances.

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Figure 4. The cyclic voltammogram scans of the Li–S cell with routine separator (a) and SP–CA modified separator (b).

Cyclic voltammograms (CV) of the cells with routine separator and SP–CA modified separator within a cutoff voltage window of 1.7–2.8 V at the scan rate of 0.1 mV s-1 for the first five cycles were executed. As shown in Figure 4a,b, two reduction peaks were emerged at about 2.0 V and 2.3 V, which corresponded to the reduction process of elemental sulfur to long chain lithium polysulfides and the conversion of long chain lithium polysulfides to short chain lithium polysulfides, respectively.43 Moreover, it could be observed that the CV curve of the cell with SP–CA modified separator presented sharper reduction and oxidation peaks and displayed a larger covering area during the first five cycles. In addition, it could be found that the reduction peak of the cell with SP–CA modified separator (2.04V) was slightly larger than that of the cell with routine separator (1.99V). The sharper reduction and oxidation peaks, higher reduction cell potential and larger covering area indicated good chemical reaction kinetics because of the good excellent conductivity and strong polysulfides adsorption of the SP–CA coated layer on the separator. The anodic and cathodic peaks of CV curves for the cell with SP–CA modified separator maintained stable in the five cycles and these peaks were overlapped in terms of peak positions and peak currents, indicating better reversibility and smaller polarization than the cell with routine separator. The result was similar to some previous studies using biomass–derived activated carbon as the barrier layer of polysulfides.44

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Figure 5. The discharge/charge voltage profiles of the Li–S cell with routine separator (a) and SP–CA modified separator (b).

Figure 5a,b showed the discharge–charge voltage profiles of different cycles at the rate of 1 C for the cells with routine separator and SP–CA modified separator, respectively. The discharge–charge capacities were calculated by the weight of elemental sulfur. It was obvious that there were two separate discharge plateaus during the process of discharge, which indicated the two complete reduction reactions occurred. The lower discharge plateau was around 2.0 V and the upper discharge plateau was around 2.3 V, which was consistent well with the cathodic peaks in CV curves. The discharge plateau of 2.3 V represented the first reaction of the transformation of sulfur into long–chain polysulfides (Li2Sx, 4

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