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Preparation of lamellar carbon matrix for sulfur as cathode material of lithium-sulfur batteries
Electrochimica Acta 143 (2014) 374–382
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Preparation of lamellar carbon matrix for sulfur as cathode material of lithium-sulfur batteries Xue-Bing Yang, Wen Zhu ∗ , Ke Qin, Hui-Yong Wang State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
a r t i c l e
a b s t r a c t
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Article history: Received 1 May 2014 Received in revised form 13 July 2014 Accepted 14 July 2014 Available online 30 July 2014 Keywords: Lamellar carbon Carbon-sulfur Cathode materials Lithium-sulfur batteries
Sulfur is a promising cathode material for lithium batteries as it has high theoretical specific capacity and low cost. However, practical electrochemical performance of lithium-sulfur batteries needs to be improved. In this work, a new method is described to prepare carbon matrix for sulfur to improve electrochemical properties of sulfur electrodes. The carbon matrix is prepared by deoxidizing carbon precursor synthesized by carbonizing sucrose with concentrated sulfuric acid. Carbon matrix-sulfur composite has been characterized by scanning electron microscopy, transmission electron microscopy and Fourier transform infrared. Results indicate that carbon matrix-sulfur composite is composed of lamellas. The lamella contains a layer of carbon coating on the outside and chemical bonds of C-S. The formation of C-S bonds is promoted by deoxidizing carbon precursor. The carbon matrix-sulfur electrode exhibits improved discharge properties, which results from the appropriate structure. Carbon coating and C-S bonds confine sulfur and maintain contact between sulfur species and conductive carbon matrix. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Development of mobile multifunctional electric devices requires better performance of lithium batteries such as higher specific capacity. Used as cathode materials for commercial rechargeable lithium batteries, transition-metal oxides and phosphate have relatively low theoretical specific capacity [1,2]. As a cathode material for lithium batteries, sulfur has high theoretical specific capacity of 1672 mAh g-1 . And theoretical energy density of lithium-sulfur batteries reaches 2500 Wh Kg-1 [3,4]. Moreover, sulfur is abundant, inexpensive and nontoxic. Thus, sulfur is a promising cathode material for secondary lithium batteries [5–7]. However, practical specific capacity of lithium-sulfur batteries is not high and cycle life is not long. There are several reasons for the low utilization rate of sulfur. The first reason is dissolution of lithium polysulfide (Li2 Sn (2 < n ≤ 8)) into organic liquid electrolyte, which leads to loss of sulfur species [8–11]. The second reason is agglomeration of insulated sulfur species. As products of discharge reactions, Li2 S2 and Li2 S are nonconductive and insoluble in organic liquid electrolyte. They precipitate onto surface of sulfur electrodes and agglomerate [12–14]. The third reason is corrosion of lithium electrodes. The dissolved Li2 Sn passes through separator and
∗ Corresponding author. E-mail address: [email protected] (W. Zhu). http://dx.doi.org/10.1016/j.electacta.2014.07.080 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
diffuses to the surface of lithium anodes. It reacts with lithium, which results in corrosion of lithium anodes [15]. Different solutions have been reported to improve the performance of lithium-sulfur batteries. The first kind of methods is to confine sulfur species to cathodes through adsorption. For example, porous carbon with different pore size is used to adsorb sulfur species to its surface [16–18]. Electrochemical tests have indicated that porous carbon increases the utilization rate of sulfur. The second type of means is to restrict sulfur species through envelopment. Conductive substances are utilized to wrap sulfur or sulfur-carbon composite. For instance, carbon materials such as carbon particles [19] and graphene [20–22] are used to envelop sulfur or sulfur-carbon composite to hinder the dissolution of polysulfide. Conductive polymers such as polypyrrole [23,24] and polyaniline [25] are utilized to wrap sulfur or sulfur-carbon composite to hinder the dissolution of sulfur species. Besides, nafion is used to envelop the entire sulfur electrode [26]. Results have shown that the method of wrapping is effective in improving electrochemical performance of lithium-sulfur batteries. The third kind of methods is to confine sulfur species through chemical bonds. Conductive substances are utilized to heat with sulfur to form chemical bonds. For example, carbyne is used to heat with sulfur to form C-S bonds. C-S bonds hinder the dissolution of sulfur species. The carbon-sulfur composite with C-S bonds exhibits good cycle stability [27]. In this work, envelopment and chemical bonding are combined to increase the utilization rate of sulfur. Lamellar carbon matrix has
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been prepared to wrap sulfur and form C-S bonds with sulfur. The structure is promising in improving the electrochemical properties of lithium-sulfur batteries.
2. Experimental 2.1. Synthesis of carbon-sulfur composite and characterization The synthesis process of carbon-sulfur composite is shown in Fig. 1. Firstly, carbon precursor was prepared by carbonizing 0.8 g sucrose (C12 H22 O11 ) with 5 mL concentrated sulfuric acid (H2 SO4 , 98 wt %). Then, 0.3 mol dm-3 sodium thiosulfate (Na2 S2 O3 ) solution was dropped into the carbon precursor sol to react with H2 SO4 to synthesize sulfur. After filtration, the product of carbon precursorsulfur was washed and then dispersed in solvent. Subsequently, sodium hydroxide (NaOH) solution and hydrazine hydrate (N2 H4 • H2 O) were dropped into the carbon precursor-sulfur sol to deoxidize carbon precursor to carbon matrix. Afterwards, the carbon matrix-sulfur sol was filtered. After washing for several times, the carbon matrix-sulfur composite was dried at 50 ◦ C for 10 h. Carbon matrix was obtained through no addition of Na2 S2 O3 solution to the carbon precursor sol. Mixture of acetylene black and sulfur was prepared for comparison. At first, 0.35 g acetylene black and 0.65 g sulfur were mixed and ground for 2 h. Subsequently, the mixture was heated at 155 ◦ C for 4 h in the atmosphere of argon. Morphology, element components and structure of materials were characterized by field emission scanning electron microscopy (FE-SEM, Sirion 200), energy dispersive X-ray spectroscopy (EDS, GENESIS) and X-ray diffraction (XRD, X Pert PRO). Microstructure of synthesized materials was analyzed through field emission transmission electron microscopy (FE-TEM, Tecnai G2 F30). Chemical bonds of materials were characterized by Fourier transform infrared spectroscopy (FT-IR, VERTEX 70) and Raman spectroscopy (LabRAM HR800) with laser wavelength of 532 nm. Content of sulfur in composite was obtained through thermo-gravimetric analysis (TG, Pyrisl TGA). Samples were tested in atmosphere of nitrogen and the heating rate was 10 ◦ C min-1 .
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2.2. Assembly of batteries and tests of electrochemical properties The first step was preparation of sulfur electrodes. Carbon-sulfur composite was mixed with acetylene black and polyvinylidene fluoride (PVDF) in mass ratio of 70:20:10 with N-methyl-pyrrolidone (NMP) as dispersant. The mixed slurry was coated onto aluminum foil (20 m in thickness) and was then dried at 60 ◦ C for 24 h. Subsequently, the prepared electrode was punched into circular disks. Mass density of materials for circular electrodes lay between 1.8 mg cm-2 and 2.3 mg cm-2 .The sulfur electrodes were used as cathodes of lithium-sulfur batteries. The second step was assembly of batteries. Electrolyte consisted of lithium bis(trifluoromethanesulfonyl)imide (1 mol dm-3 ) as solute and mixture of 1, 3-dioxolane (DOL) and 1, 2dimethoxyethane (DME) with volume ratio of 1:1 as solvent. Lithium nitrate (LiNO3 , 0.1 mol dm-3 ) was used as an additive in the electrolyte. Adopted anodes were lithium foils with thickness of 100 m. Separator between cathode and anode was microporous polypropylene film (Celgard 2300). Coin cells (CR2032) were assembled in a vacuum glove box (Lab2000) filled with purified argon. The third step was tests of assembled lithium-sulfur batteries. Cyclic voltammetry tests were performed through electrochemical workstation with scanning rate of 0.1 mV s-1 . Cyclic discharge and charge capacity were tested by LAND CT2001A battery test system in galvanostatic mode. Current rates and specific capacity were calculated according to the mass of sulfur in sulfur electrodes. Alternating current impedance tests were carried out through electrochemical workstation with voltage amplitude of 5 mV and frequency between 0.1 Hz and 100 KHz. 3. Results and Discussion X-ray diffraction patterns of carbon precursor, carbon matrix and carbon matrix-sulfur composite are shown in Fig. 2. In Fig. 2 (a), acetylene black exhibits obvious diffraction peak at 24.95◦ . The peak corresponds to the crystal plane of (002) [28]. The sharp peak indicates similar distance between graphitic layers. In comparison with acetylene black, both carbon precursor and carbon matrix exhibit wider diffraction peaks and lower diffraction angles. Wider
Fig. 1. Schematic illustration of the synthesis process of carbon-sulfur composite.
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Fig. 2. XRD patterns of acetylene black, carbon precursor, carbon matrix (a), sulfur and carbon matrix-sulfur composite (b).
diffraction peaks correspond to broader distribution of distance between graphitic layers. Lower diffraction angles indicate longer distance between graphitic layers according to Bragg diffraction equation (2dsin=n). Therefore, distance between graphitic layers in carbon precursor and carbon matrix is longer and more irregular than that in acetylene black. The diffraction pattern of carbon precursor is similar to that of carbon matrix, which indicates that the process of reduction hardly affects the distance between graphitic layers in carbon precursor. In Fig. 2 (b), sulfur exhibits the highest diffraction peak at 23.3◦ . The peak corresponds to the crystal plane of (222). Diffraction peaks of carbon matrix-sulfur composite are similar to those of sulfur, which evidences the existence of sulfur in carbon matrix-sulfur composite. Besides, there is a wide diffraction peak with top at angle of 23.3◦ . The wide peak is different from that of carbon matrix in diffraction angle. And it ought to be related to the diffraction of sulfur nanoparticles in carbon matrix-sulfur composite. Fig. 3 (a) and Fig. 3 (b) show the morphology and the element components of carbon precursor. In Fig. 3 (a), carbon precursor is composed of many lamellas which are small in area and thin. In Fig. 3 (b), the EDS pattern shows that carbon precursor consists of element of C and element of O. Carbon precursor is the product of carbonizing sucrose (C12 H22 O11 ) with concentrated H2 SO4 . The EDS pattern indicates that the element of O in sucrose has not been removed completely after carbonization. Fig. 3 (c) and Fig. 3 (d) show the morphology and the element components of carbon matrix. In comparison with carbon precursor, carbon matrix comprises larger lamellas in Fig. 3 (c). In Fig. 3 (d), the EDS pattern indicates that carbon matrix mainly consists of element of C. Compared with carbon precursor, carbon matrix contains less element of O according to relative intensity of elements. The EDS pattern evidences that carbon precursor has been effectively deoxidized by N2 H4 • H2 O. FE-TEM images of carbon matrix are shown in Fig. 3 (e) and Fig. 3 (f). In Fig. 3 (e), the lamella is laminated. The thick lamella is composed of several thin graphitic layers. There is no obvious crystal lattice in the graphitic layers according to the magnification image (Fig. 3 (f)). Fig. 4 (a) and Fig. 4 (b) show the morphology of carbon matrixsulfur composite. In Fig. 4 (a), carbon matrix-sulfur composite is laminated and is similar to carbon matrix. Sulfur is synthesized through precipitation reaction between H2 SO4 and Na2 S2 O3 . The reaction is fast and sulfur is easy to agglomerate to particles, which has been reported [19]. In Fig. 4 (a), there are no sulfur particles on the surface of lamellas. Sulfur ought to exist inside the lamellas. Fig. 4 (b) shows edge morphology of lamellas in carbon matrix-sulfur composite. The lamellas are thin. EDS maps of carbon
matrix-sulfur composite in Fig. 4 (c) and Fig. 4 (d) indicate that carbon and sulfur are uniformly distributed. Fig. 4 (e) and Fig. 4 (f) are FE-TEM images of carbon matrix-sulfur composite. In Fig. 4 (e), the dark zones in the lamella correspond to sulfur particles as different elements result in contrast in images. Fig. 4 (e) indicates that sulfur disperses in the lamella. Fig. 4 (f) is the magnification image of the region marked with red frame in Fig. 4 (e). As is shown in Fig. 4 (f), there is a layer of carbon coating on the outside of the lamella. Thickness of the carbon coating is about 10 nm. The structure of carbon matrix-sulfur composite corresponds to the schematic illustration described in Fig. 1. Fig. 5 (a) and Fig. 5 (b) show the morphology of acetylene black and acetylene black-sulfur mixture respectively. As is shown in Fig. 5 (a), acetylene black exhibits morphology of particles with diameter of about 100 nm. And acetylene black particles are separated, which indicates low degree of aggregation. In regard to acetylene black-sulfur mixture, it exhibits particle diameter of about 150 nm in Fig. 5 (b). Acetylene black-sulfur particles are bigger than acetylene black particles, which results from the existence of sulfur on the outside of acetylene black particles. Fig. 6 shows FT-IR spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur. In Fig. 6 (a), acetylene black exhibits two absorption bands at wavenumber of 1623 cm-1 and 3437 cm-1 . The band at 1623 cm-1 is a resonance absorption band of vibration of C-C in graphitic zones and vibration of hydroxyl (O-H) in water (H2 O) adsorbed by potassium bromide (KBr) [29]. The other absorption band at 3437 cm-1 corresponds to stretching vibration of O-H in H2 O adsorbed by KBr [30]. Carbon precursor exhibits absorption bands at 1597 cm-1 , 1708 cm-1 and 3437 cm-1 . The absorption band at 1597 cm-1 corresponds to vibration of C-C in graphitic zones [29]. The absorption band at 1708 cm-1 is related to vibration of C=O [31]. The band at 3437 cm-1 is similar to that of acetylene black. In contrast with acetylene black, carbon precursor contains chemical bonds of C=O, which accounts for the high peak of O in Fig. 3 (b). Carbon matrix exhibits absorption bands at 1623 cm-1 and 3437 cm-1 . The bands are similar to those of acetylene black. In contrast to carbon precursor, carbon matrix does not contain chemical bonds of C=O, which is consistent with the low peak of O in Fig. 3 (d). The chemical bonds of C=O in carbon precursor are removed through reduction reaction. In Fig. 6 (b), carbon precursor-sulfur composite exhibits absorption bands at 1603 cm-1 , 1709 cm-1 and 3437 cm-1 . The bands are similar to those of carbon precursor. Chemical bonds of C-S do not obviously exist in carbon precursor-sulfur composite. In regard to carbon matrix-sulfur composite, it exhibits absorption bands at 667 cm-1 , 990 cm-1 , 1115 cm-1 , 1623 cm-1 and 3437 cm-1 .
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Fig. 3. FE-SEM images of carbon precursor (a) and carbon matrix (c); EDS patterns of carbon precursor (b) and carbon matrix (d); FE-TEM images of carbon matrix (e, f).
The bands at 1623 cm-1 and 3437 cm-1 are similar to those of carbon matrix while bands at 667 cm-1 , 990 cm-1 and 1115 cm-1 are different from those of carbon matrix. The band at 667 cm-1 corresponds to vibration of chemical bonds of C-S. Bands at 990 cm-1 and 1115 cm-1 are related to symmetric and asymmetric vibration of S-C-S bonds respectively. In comparison with carbon precursorsulfur composite, chemical bonds of C-S apparently exist in carbon matrix-sulfur composite. And the formation of C-S bonds is promoted by deoxidizing carbon precursor with N2 H4 • H2 O. Fig. 7 shows Raman spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur.
In Fig. 7 (a), acetylene black exhibits two Raman bands at Raman shift of 1348 cm-1 and 1590 cm-1 . The Raman bands at about 1348 cm-1 and about 1590 cm-1 are called D band and G band. The D band is related to disordered carbon such as carbon around defects [32]. And the G band corresponds to carbon sites containing sp2 bonds in hexagonal aromatic rings [33]. Intensity ratio between D band and G band indicates the degree of disorder [34]. Lower ratio corresponds to lower degree of disorder and higher degree of graphitization. The D band and the G band of carbon precursor are at 1374 cm-1 and 1594 cm-1 . The D band of carbon precursor is different from that of acetylene black in Raman shift, which
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Fig. 4. FE-SEM images (a, b) and EDS maps (c, d) of carbon matrix-sulfur composite; FE-TEM images (e, f) of carbon matrix-sulfur composite.
may result from different size of graphitic layers. Carbon matrix exhibits D band at 1359 cm-1 and G band at 1579 cm-1 . D band and G band of carbon matrix differ from those of carbon precursor in Raman shift, which may be related to the removal of element of oxygen in carbon precursor. The intensity ratios of ID / IG for carbon
precursor and carbon matrix are both lower than that for acetylene black, which indicates that the degree of disorder in carbon precursor and carbon matrix is lower than that in acetylene black. In Fig. 7 (b), carbon precursor-sulfur composite exhibits D band at 1374 cm-1 and G band at 1586 cm-1 . The G band of carbon
Fig. 5. FE-SEM images of acetylene black (a) and acetylene black-sulfur mixture (b).
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Fig. 6. FT-IR spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur.
Fig. 7. Raman spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur.
precursor-sulfur differs from that of carbon precursor in Raman shift, which results from addition of sulfur into carbon precursor. In regard to carbon matrix-sulfur, it exhibits D band at 1370 cm-1 and G band at 1564 cm-1 . D band and G band of carbon matrixsulfur obviously differ from those of carbon matrix in Raman shift. The apparent difference results from chemical bonding between carbon matrix and sulfur, which is consistent with the result from FT-IR spectra in Fig. 6.
Fig. 8 shows EDS pattern of carbon matrix-sulfur composite and TG curves of carbon matrix, carbon matrix-sulfur , acetylene black-sulfur and sulfur. Fig. 8 (a) indicates that carbon matrix-sulfur consists of element of C and element of S. The high intensity of sulfur peak corresponds to high content of sulfur in carbon matrix-sulfur composite. The content of sulfur is obtained through TG analysis. As is shown in Fig. 8 (b), mass of sulfur hardly changes when temperature is below 200 ◦ C. When temperature
Fig. 8. EDS pattern (a) of carbon matrix-sulfur composite and TG curves (b) of carbon matrix, carbon matrix-sulfur, acetylene black-sulfur and sulfur.
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Fig. 9. Cyclic voltammograms of carbon matrix-sulfur electrode.
increases from 200 ◦ C to 300 ◦ C, mass of sulfur decreases gradually, which results from the evaporation of sulfur. In regard to acetylene black-sulfur composite, it exhibits similar change trend of mass rate to that of sulfur, which indicates that the mass change of acetylene black-sulfur results from the evaporation of sulfur. Through calculation, the mass rate of sulfur in acetylene black-sulfur mixture is 63.1%. In comparison with acetylene black-sulfur, mass rate of carbon matrix-sulfur composite decreases slower. Corresponding reason is that the evaporation of sulfur is hindered by carbon coating and C-S bonds in carbon matrix-sulfur composite. In Fig. 8 (b), mass losses of carbon matrix and carbon matrix-sulfur are 7.9% and 60.7% at the turn temperature of mass rate for carbon matrix-sulfur. Through calculation, the content of sulfur in carbon matrix-sulfur composite is 57.3%. Fig. 9 shows cyclic voltammograms of carbon matrix-sulfur electrode at scanning rate of 0.1 mV s-1 . In the curve of each cycle, there are three peaks in the reduction process and one peak in the oxidation process. The first reduction peak at about 2.34 V corresponds to reaction from S8 to Li2 S8 . And the second peak at around 2.05 V is related to reduction reaction from Li2 Sm (2 < m < 8) to Li2 S2 or Li2 S. The third reduction peak at about 2.12 V is related to decrease in length of S-S chain which chemically bonds with carbon matrix. In regard to the oxidation process, the peak at around 2.38 V corresponds to increase in the length of S-S chain. In the curves of four cycles, peaks are at similar potential, which indicates good reversibility of electrochemical reactions for carbon matrix-sulfur electrode.
Fig. 10 shows charge and discharge curves for carbon matrixsulfur electrode and acetylene black-sulfur electrode at current rate of 0.1 C (1 C = 1672 mA g-1 ). In Fig. 10 (a), each discharge curve exhibits three plateaus of potential. The first plateau between 2.3 V and 2.4 V corresponds to the first reduction peak at about 2.34 V in Fig. 9. The second plateau at around 2.1 V corresponds to the second reduction peak at about 2.05 V in Fig. 9. The third plateau between 2.1 V and 2.2 V corresponds to the third reduction peak at around 2.12 V in Fig. 9. And the third plateau is related to the reduction of sulfur species with C-S bonds. The plateau links the first plateau and the second one, which makes the discharge curve gentle. In comparison with the first discharge curve, the following three discharge curves exhibit higher potential for the first plateau, which corresponds to the increase in potential of the first reduction peak in Fig. 9. The increase in discharge potential is related to activation of sulfur in carbon matrix-sulfur electrode. In regard to the charge process, the plateau between 2.15 V and 2.3 V corresponds to the oxidation peak at about 2.38 V in Fig. 9. In Fig. 10 (b), the discharge curves exhibit two plateaus of potential. One plateau between 2.3 V and 2.4 V is similar to the first plateau in Fig. 10 (a). The other plateau at about 2.1 V is similar to the second plateau in Fig. 10 (a). In contrast to carbon matrix-sulfur electrode, acetylene blacksulfur electrode does not exhibit the discharge plateau between 2.1 V and 2.2 V, which results from the difference in the connection condition of sulfur. In regard to the charge process, there is a potential plateau at around 2.4 V. Fig. 11 shows cyclic discharge capacity of carbon matrix-sulfur and acetylene black-sulfur electrodes. As is shown in Fig. 11 (a), acetylene black-sulfur electrode exhibits discharge capacity of 968 mAh g-1 in the first cycle and 334 mAh g-1 in the hundredth cycle. Discharge capacity of acetylene black-sulfur electrode decreases fast when cycle number increases. The reason of degradation is that sulfur exists on the surface of acetylene black particles and is not confined. Corresponding morphology has been described in Fig. 5. Thus, formed polysulfide in acetylene black-sulfur electrode dissolves into electrolyte easily. As more polysulfide dissolves into electrolyte, more products of insulated Li2 S2 and Li2 S precipitate on the surface of sulfur electrode and agglomerate. The higher agglomeration degree for insulated sulfur species leads to lower utilization rate of sulfur and lower discharge capacity. In regard to carbon matrix-sulfur electrode, it exhibits discharge capacity of 1266 mAh g-1 in the first cycle and 886 mAh g-1 in the hundredth cycle. In comparison with acetylene black-sulfur electrode, carbon matrixsulfur electrode exhibits higher discharge capacity and more stable discharge property. The better discharge properties result from the appropriate structure of carbon matrix-sulfur composite. As is described above, sulfur is confined to carbon matrix by carbon
Fig. 10. Charge and discharge curves of carbon matrix-sulfur electrode (a) and acetylene black-sulfur electrode (b).
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Fig. 11. Cyclic discharge capacity of carbon matrix-sulfur and acetylene black-sulfur electrodes at current rate of 0.1 C (a); Discharge capacity of carbon matrix-sulfur electrode at current rate of 0.2 C (b).
Fig. 12. Electrochemical impedance spectra (Nyquist plots) of carbon matrix-sulfur electrode (a) and acetylene black-sulfur electrode (b).
coating and C-S bonds. Thus, polysulfide is hindered from dissolving into electrolyte, which contributes to alleviating the agglomeration degree for insulated sulfur species. Coulombic efficiency of carbon matrix-sulfur electrode is also shown in Fig. 11 (a). The efficiency lies between 90% and 100%, which indicates that self-discharge reaction is not obvious. The suppression of self-discharge reaction results from protection of lithium electrode through LiNO3 . The reaction between lithium electrode and polysulfide leads to low coulombic efficiency. It has been reported that LiNO3 contributes to the formation of protection film on the surface of lithium electrode [35]. The protection film prevents the reaction between lithium and polysulfide, which results in high coulombic efficiency. As is shown in Fig. 11 (b), carbon matrix-sulfur electrode exhibits discharge capacity of 787 mAh g-1 in the hundredth cycle at current rate of 0.2 C. And the cycle stability of discharge is relatively good. In regard to the coulombic efficiency, it lies between 90% and 100%, which indicates that self-discharge reaction is not apparent. Fig. 12 shows electrochemical impedance spectra of carbon matrix-sulfur electrode and acetylene black-sulfur electrode from 100 kHz to 0.1 Hz. Before discharge, each spectrum curve consists of a semicircle in high frequency region and an inclined line in low frequency region. The semicircle corresponds to chargetransfer resistance and longer diameter indicates higher resistance [36,37]. Intercept of the semicircle at the axis of Z (re) is related to combination of resistance for active materials, resistance for electrolyte and contact resistance [38]. The inclined line is related to diffusion of ions. As is shown in Fig. 12 (a), carbon matrixsulfur electrode exhibits lower charge-transfer resistance after 100
cycles than before cycle, which is related to activation of sulfur species in carbon matrix-sulfur electrode. In Fig. 12 (b), acetylene black-sulfur electrode exhibits two semicircles after 100 cycles. The semicircle in higher frequency region corresponds to resistance of solid interface layers between electrodes and electrolyte. And the other semicircle in lower frequency region is related to charge-transfer resistance [38]. In contrast to carbon matrix-sulfur electrode, acetylene black-sulfur electrode exhibits the semicircle in higher frequency region, which is related to agglomeration of insulated sulfur species.
4. Conclusions A new method has been described to prepare carbon matrix for sulfur as cathode material of lithium-sulfur batteries. Prepared carbon matrix is lamellar and has relatively high degree of graphitization. Synthesized carbon matrix-sulfur composite is laminated. On the outside of the lamella, there is a layer of carbon coating with thickness of about 10 nm. In the lamella, there are C-S bonds which form in the process of deoxidizing carbon precursor. The structure increases the utilization rate of sulfur and improves discharge properties. In the carbon matrix-sulfur electrode, carbon coating and C-S bonds confine sulfur to conductive carbon matrix and maintain contact, which hinders the dissolution of polysulfide and alleviates agglomeration degree for insulated sulfur species. The synthesized carbon matrix can also be used to form homogeneous composite with other electrode materials.
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Acknowledgements This work was financed by the Special Fund for Strategic Emerging Industry Development of Shenzhen (JCYJ20120618100557119), Independent Research Fund of State Key Laboratory of Materials Processing and Die & Mould Technology, Program for Changjiang Scholars and Innovative Research Team in University (IRT1014), and the Fundamental Research Funds for the Central Universities (2013NY009). Technical assistance from the Analytical and Testing Center of HUST is gratefully acknowledged. References [1] T.N.L. Doan, M. Ghaznavi, Y. Zhao, Y.G. Zhang, A. Konarov, M. Sadhu, R. Tangirala, P. Chen, Binding mechanism of sulfur and dehydrogenated polyacrylonitrile in sulfur/polymer composite cathode, J. Power Sources 241 (2013) 61–69. [2] B.C. Duan, W.K. Wang, A.B. Wang, Z.B. Yu, H.L. Zhao, Y.S. Yang, A new lithium secondary battery system: the sulfur/lithium-ion battery, J. Mater. Chem. A 2 (2014) 308–314. [3] N.A. Canas, S. Wolf, N. Wagner, K.A. Friedrich, In-situ X-ray diffraction studies of lithium-sulfur batteries, J. Power Sources 226 (2013) 313–319. [4] J. Zhang, J.Y. Xiang, Z.M. Dong, Y. Liu, Y.S. Wu, C.M. Xu, G.H. Du, Biomass derived activated carbon with 3 D connected architecture for rechargeable lithiumsulfur batteries, Electrochim. Acta 116 (2014) 146–151. [5] C.F. Zhang, H.B. Wu, C.Z. Yuan, Z.P. Guo, X.W. Lou, Confining sulfur in doubleshelled hollow carbon spheres for lithium-sulfur batteries, Angew. Chem. Int. Ed. 51 (2012) 9592–9595. [6] W. Ahn, K.B. Kim, K.N. Jung, K.H. Shin, C.S. Jin, Synthesis and electrochemical properties of a sulfur-multi walled carbon nanotubes composite as a cathode material for lithium sulfur batteries, J. Power Sources 202 (2012) 394–399. [7] H.S. Ryu, J.W. Park, J. Park, J.P. Ahn, K.W. Kim, J.H. Ahn, T.H. Nam, G.X. Wang, H.J. Ahn, High capacity cathode materials for Li-S batteries, J. Mater. Chem. A 1 (2013) 1573–1578. [8] S. Xin, L. Gu, N.H. Zhao, Y.X. Yin, L.J. Zhou, Y.G. Guo, L.J. Wan, Smaller sulfur molecules promise better lithium-sulfur batteries, J. Am. Chem. Soc. 134 (2012) 18510–18513. [9] Z.B. Wang, S.C. Zhang, L. Zhang, R.X. Lin, X.M. Wu, H. Fang, Y.B. Ren, Hollow spherical carbonized polypyrrole/sulfur composite cathode materials for lithium/sulfur cells with long cycle life, J. Power Sources 248 (2014) 337–342. [10] L.X. Miao, W.K. Wang, A.B. Wang, K.G. Yuan, Y.S. Yang, A high sulfur content composite with core-shell structure as cathode material for Li-S batteries, J. Mater. Chem. A 1 (2013) 11659–11664. [11] L.Q. Lu, L.J. Lu, Y. Wang, Sulfur film-coated reduced graphene oxide composite for lithium-sulfur batteries, J. Mater. Chem. A 1 (2013) 9173–9181. [12] C. Barchasz, F. Molton, C. Duboc, J.C. Lepretre, S. Patoux, F. Alloin, Lithium/sulfur cell discharge mechanism: an original approach for intermediate species identification, Anal. Chem. 84 (2012) 3973–3980. [13] C. Barchasz, J.C. Lepretre, F. Alloin, S. Patoux, New insights into the limiting parameters of the Li/S rechargeable cell, J. Power Sources 199 (2012) 322–330. [14] C. Barchasz, F. Mesguich, J. Dijon, J.C. Lepretre, S. Patoux, F. Alloin, Novel positive electrode architecture for rechargeable lithium/sulfur batteries, J. Power Sources 211 (2012) 19–26. [15] Z.Q. Jin, K. Xie, X.B. Hong, Z.Q. Hu, Capacity fading mechanism in lithium sulfur cells using poly(ethylene glycol)-borate ester as plasticizer for polymer electrolytes, J. Power Sources 242 (2013) 478–485. [16] X.F. Wang, X.P. Fang, X.W. Guo, Z.X. Wang, L.Q. Chen, Sulfur in hierarchically pore-structured carbon pillars as cathode material for lithium-sulfur batteries, Electrochim. Acta 97 (2013) 238–243. [17] M.Y. Gao, X. Xiong, W.K. Wang, S.R. Zhao, C.M. Li, H. Zhang, Z.B. Yu, Y.Q. Huang, Discharge-charge process of the porous sulfur/carbon nanocomposite cathode for rechargeable lithium sulfur batteries, J. Power Sources 248 (2014) 1149–1155.
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Preparation of lamellar carbon matrix for sulfur as cathode material of lithium-sulfur batteries Xue-Bing Yang, Wen Zhu ∗ , Ke Qin, Hui-Yong Wang State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 1 May 2014 Received in revised form 13 July 2014 Accepted 14 July 2014 Available online 30 July 2014 Keywords: Lamellar carbon Carbon-sulfur Cathode materials Lithium-sulfur batteries
Sulfur is a promising cathode material for lithium batteries as it has high theoretical specific capacity and low cost. However, practical electrochemical performance of lithium-sulfur batteries needs to be improved. In this work, a new method is described to prepare carbon matrix for sulfur to improve electrochemical properties of sulfur electrodes. The carbon matrix is prepared by deoxidizing carbon precursor synthesized by carbonizing sucrose with concentrated sulfuric acid. Carbon matrix-sulfur composite has been characterized by scanning electron microscopy, transmission electron microscopy and Fourier transform infrared. Results indicate that carbon matrix-sulfur composite is composed of lamellas. The lamella contains a layer of carbon coating on the outside and chemical bonds of C-S. The formation of C-S bonds is promoted by deoxidizing carbon precursor. The carbon matrix-sulfur electrode exhibits improved discharge properties, which results from the appropriate structure. Carbon coating and C-S bonds confine sulfur and maintain contact between sulfur species and conductive carbon matrix. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Development of mobile multifunctional electric devices requires better performance of lithium batteries such as higher specific capacity. Used as cathode materials for commercial rechargeable lithium batteries, transition-metal oxides and phosphate have relatively low theoretical specific capacity [1,2]. As a cathode material for lithium batteries, sulfur has high theoretical specific capacity of 1672 mAh g-1 . And theoretical energy density of lithium-sulfur batteries reaches 2500 Wh Kg-1 [3,4]. Moreover, sulfur is abundant, inexpensive and nontoxic. Thus, sulfur is a promising cathode material for secondary lithium batteries [5–7]. However, practical specific capacity of lithium-sulfur batteries is not high and cycle life is not long. There are several reasons for the low utilization rate of sulfur. The first reason is dissolution of lithium polysulfide (Li2 Sn (2 < n ≤ 8)) into organic liquid electrolyte, which leads to loss of sulfur species [8–11]. The second reason is agglomeration of insulated sulfur species. As products of discharge reactions, Li2 S2 and Li2 S are nonconductive and insoluble in organic liquid electrolyte. They precipitate onto surface of sulfur electrodes and agglomerate [12–14]. The third reason is corrosion of lithium electrodes. The dissolved Li2 Sn passes through separator and
∗ Corresponding author. E-mail address: [email protected] (W. Zhu). http://dx.doi.org/10.1016/j.electacta.2014.07.080 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
diffuses to the surface of lithium anodes. It reacts with lithium, which results in corrosion of lithium anodes [15]. Different solutions have been reported to improve the performance of lithium-sulfur batteries. The first kind of methods is to confine sulfur species to cathodes through adsorption. For example, porous carbon with different pore size is used to adsorb sulfur species to its surface [16–18]. Electrochemical tests have indicated that porous carbon increases the utilization rate of sulfur. The second type of means is to restrict sulfur species through envelopment. Conductive substances are utilized to wrap sulfur or sulfur-carbon composite. For instance, carbon materials such as carbon particles [19] and graphene [20–22] are used to envelop sulfur or sulfur-carbon composite to hinder the dissolution of polysulfide. Conductive polymers such as polypyrrole [23,24] and polyaniline [25] are utilized to wrap sulfur or sulfur-carbon composite to hinder the dissolution of sulfur species. Besides, nafion is used to envelop the entire sulfur electrode [26]. Results have shown that the method of wrapping is effective in improving electrochemical performance of lithium-sulfur batteries. The third kind of methods is to confine sulfur species through chemical bonds. Conductive substances are utilized to heat with sulfur to form chemical bonds. For example, carbyne is used to heat with sulfur to form C-S bonds. C-S bonds hinder the dissolution of sulfur species. The carbon-sulfur composite with C-S bonds exhibits good cycle stability [27]. In this work, envelopment and chemical bonding are combined to increase the utilization rate of sulfur. Lamellar carbon matrix has
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been prepared to wrap sulfur and form C-S bonds with sulfur. The structure is promising in improving the electrochemical properties of lithium-sulfur batteries.
2. Experimental 2.1. Synthesis of carbon-sulfur composite and characterization The synthesis process of carbon-sulfur composite is shown in Fig. 1. Firstly, carbon precursor was prepared by carbonizing 0.8 g sucrose (C12 H22 O11 ) with 5 mL concentrated sulfuric acid (H2 SO4 , 98 wt %). Then, 0.3 mol dm-3 sodium thiosulfate (Na2 S2 O3 ) solution was dropped into the carbon precursor sol to react with H2 SO4 to synthesize sulfur. After filtration, the product of carbon precursorsulfur was washed and then dispersed in solvent. Subsequently, sodium hydroxide (NaOH) solution and hydrazine hydrate (N2 H4 • H2 O) were dropped into the carbon precursor-sulfur sol to deoxidize carbon precursor to carbon matrix. Afterwards, the carbon matrix-sulfur sol was filtered. After washing for several times, the carbon matrix-sulfur composite was dried at 50 ◦ C for 10 h. Carbon matrix was obtained through no addition of Na2 S2 O3 solution to the carbon precursor sol. Mixture of acetylene black and sulfur was prepared for comparison. At first, 0.35 g acetylene black and 0.65 g sulfur were mixed and ground for 2 h. Subsequently, the mixture was heated at 155 ◦ C for 4 h in the atmosphere of argon. Morphology, element components and structure of materials were characterized by field emission scanning electron microscopy (FE-SEM, Sirion 200), energy dispersive X-ray spectroscopy (EDS, GENESIS) and X-ray diffraction (XRD, X Pert PRO). Microstructure of synthesized materials was analyzed through field emission transmission electron microscopy (FE-TEM, Tecnai G2 F30). Chemical bonds of materials were characterized by Fourier transform infrared spectroscopy (FT-IR, VERTEX 70) and Raman spectroscopy (LabRAM HR800) with laser wavelength of 532 nm. Content of sulfur in composite was obtained through thermo-gravimetric analysis (TG, Pyrisl TGA). Samples were tested in atmosphere of nitrogen and the heating rate was 10 ◦ C min-1 .
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2.2. Assembly of batteries and tests of electrochemical properties The first step was preparation of sulfur electrodes. Carbon-sulfur composite was mixed with acetylene black and polyvinylidene fluoride (PVDF) in mass ratio of 70:20:10 with N-methyl-pyrrolidone (NMP) as dispersant. The mixed slurry was coated onto aluminum foil (20 m in thickness) and was then dried at 60 ◦ C for 24 h. Subsequently, the prepared electrode was punched into circular disks. Mass density of materials for circular electrodes lay between 1.8 mg cm-2 and 2.3 mg cm-2 .The sulfur electrodes were used as cathodes of lithium-sulfur batteries. The second step was assembly of batteries. Electrolyte consisted of lithium bis(trifluoromethanesulfonyl)imide (1 mol dm-3 ) as solute and mixture of 1, 3-dioxolane (DOL) and 1, 2dimethoxyethane (DME) with volume ratio of 1:1 as solvent. Lithium nitrate (LiNO3 , 0.1 mol dm-3 ) was used as an additive in the electrolyte. Adopted anodes were lithium foils with thickness of 100 m. Separator between cathode and anode was microporous polypropylene film (Celgard 2300). Coin cells (CR2032) were assembled in a vacuum glove box (Lab2000) filled with purified argon. The third step was tests of assembled lithium-sulfur batteries. Cyclic voltammetry tests were performed through electrochemical workstation with scanning rate of 0.1 mV s-1 . Cyclic discharge and charge capacity were tested by LAND CT2001A battery test system in galvanostatic mode. Current rates and specific capacity were calculated according to the mass of sulfur in sulfur electrodes. Alternating current impedance tests were carried out through electrochemical workstation with voltage amplitude of 5 mV and frequency between 0.1 Hz and 100 KHz. 3. Results and Discussion X-ray diffraction patterns of carbon precursor, carbon matrix and carbon matrix-sulfur composite are shown in Fig. 2. In Fig. 2 (a), acetylene black exhibits obvious diffraction peak at 24.95◦ . The peak corresponds to the crystal plane of (002) [28]. The sharp peak indicates similar distance between graphitic layers. In comparison with acetylene black, both carbon precursor and carbon matrix exhibit wider diffraction peaks and lower diffraction angles. Wider
Fig. 1. Schematic illustration of the synthesis process of carbon-sulfur composite.
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Fig. 2. XRD patterns of acetylene black, carbon precursor, carbon matrix (a), sulfur and carbon matrix-sulfur composite (b).
diffraction peaks correspond to broader distribution of distance between graphitic layers. Lower diffraction angles indicate longer distance between graphitic layers according to Bragg diffraction equation (2dsin=n). Therefore, distance between graphitic layers in carbon precursor and carbon matrix is longer and more irregular than that in acetylene black. The diffraction pattern of carbon precursor is similar to that of carbon matrix, which indicates that the process of reduction hardly affects the distance between graphitic layers in carbon precursor. In Fig. 2 (b), sulfur exhibits the highest diffraction peak at 23.3◦ . The peak corresponds to the crystal plane of (222). Diffraction peaks of carbon matrix-sulfur composite are similar to those of sulfur, which evidences the existence of sulfur in carbon matrix-sulfur composite. Besides, there is a wide diffraction peak with top at angle of 23.3◦ . The wide peak is different from that of carbon matrix in diffraction angle. And it ought to be related to the diffraction of sulfur nanoparticles in carbon matrix-sulfur composite. Fig. 3 (a) and Fig. 3 (b) show the morphology and the element components of carbon precursor. In Fig. 3 (a), carbon precursor is composed of many lamellas which are small in area and thin. In Fig. 3 (b), the EDS pattern shows that carbon precursor consists of element of C and element of O. Carbon precursor is the product of carbonizing sucrose (C12 H22 O11 ) with concentrated H2 SO4 . The EDS pattern indicates that the element of O in sucrose has not been removed completely after carbonization. Fig. 3 (c) and Fig. 3 (d) show the morphology and the element components of carbon matrix. In comparison with carbon precursor, carbon matrix comprises larger lamellas in Fig. 3 (c). In Fig. 3 (d), the EDS pattern indicates that carbon matrix mainly consists of element of C. Compared with carbon precursor, carbon matrix contains less element of O according to relative intensity of elements. The EDS pattern evidences that carbon precursor has been effectively deoxidized by N2 H4 • H2 O. FE-TEM images of carbon matrix are shown in Fig. 3 (e) and Fig. 3 (f). In Fig. 3 (e), the lamella is laminated. The thick lamella is composed of several thin graphitic layers. There is no obvious crystal lattice in the graphitic layers according to the magnification image (Fig. 3 (f)). Fig. 4 (a) and Fig. 4 (b) show the morphology of carbon matrixsulfur composite. In Fig. 4 (a), carbon matrix-sulfur composite is laminated and is similar to carbon matrix. Sulfur is synthesized through precipitation reaction between H2 SO4 and Na2 S2 O3 . The reaction is fast and sulfur is easy to agglomerate to particles, which has been reported [19]. In Fig. 4 (a), there are no sulfur particles on the surface of lamellas. Sulfur ought to exist inside the lamellas. Fig. 4 (b) shows edge morphology of lamellas in carbon matrix-sulfur composite. The lamellas are thin. EDS maps of carbon
matrix-sulfur composite in Fig. 4 (c) and Fig. 4 (d) indicate that carbon and sulfur are uniformly distributed. Fig. 4 (e) and Fig. 4 (f) are FE-TEM images of carbon matrix-sulfur composite. In Fig. 4 (e), the dark zones in the lamella correspond to sulfur particles as different elements result in contrast in images. Fig. 4 (e) indicates that sulfur disperses in the lamella. Fig. 4 (f) is the magnification image of the region marked with red frame in Fig. 4 (e). As is shown in Fig. 4 (f), there is a layer of carbon coating on the outside of the lamella. Thickness of the carbon coating is about 10 nm. The structure of carbon matrix-sulfur composite corresponds to the schematic illustration described in Fig. 1. Fig. 5 (a) and Fig. 5 (b) show the morphology of acetylene black and acetylene black-sulfur mixture respectively. As is shown in Fig. 5 (a), acetylene black exhibits morphology of particles with diameter of about 100 nm. And acetylene black particles are separated, which indicates low degree of aggregation. In regard to acetylene black-sulfur mixture, it exhibits particle diameter of about 150 nm in Fig. 5 (b). Acetylene black-sulfur particles are bigger than acetylene black particles, which results from the existence of sulfur on the outside of acetylene black particles. Fig. 6 shows FT-IR spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur. In Fig. 6 (a), acetylene black exhibits two absorption bands at wavenumber of 1623 cm-1 and 3437 cm-1 . The band at 1623 cm-1 is a resonance absorption band of vibration of C-C in graphitic zones and vibration of hydroxyl (O-H) in water (H2 O) adsorbed by potassium bromide (KBr) [29]. The other absorption band at 3437 cm-1 corresponds to stretching vibration of O-H in H2 O adsorbed by KBr [30]. Carbon precursor exhibits absorption bands at 1597 cm-1 , 1708 cm-1 and 3437 cm-1 . The absorption band at 1597 cm-1 corresponds to vibration of C-C in graphitic zones [29]. The absorption band at 1708 cm-1 is related to vibration of C=O [31]. The band at 3437 cm-1 is similar to that of acetylene black. In contrast with acetylene black, carbon precursor contains chemical bonds of C=O, which accounts for the high peak of O in Fig. 3 (b). Carbon matrix exhibits absorption bands at 1623 cm-1 and 3437 cm-1 . The bands are similar to those of acetylene black. In contrast to carbon precursor, carbon matrix does not contain chemical bonds of C=O, which is consistent with the low peak of O in Fig. 3 (d). The chemical bonds of C=O in carbon precursor are removed through reduction reaction. In Fig. 6 (b), carbon precursor-sulfur composite exhibits absorption bands at 1603 cm-1 , 1709 cm-1 and 3437 cm-1 . The bands are similar to those of carbon precursor. Chemical bonds of C-S do not obviously exist in carbon precursor-sulfur composite. In regard to carbon matrix-sulfur composite, it exhibits absorption bands at 667 cm-1 , 990 cm-1 , 1115 cm-1 , 1623 cm-1 and 3437 cm-1 .
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Fig. 3. FE-SEM images of carbon precursor (a) and carbon matrix (c); EDS patterns of carbon precursor (b) and carbon matrix (d); FE-TEM images of carbon matrix (e, f).
The bands at 1623 cm-1 and 3437 cm-1 are similar to those of carbon matrix while bands at 667 cm-1 , 990 cm-1 and 1115 cm-1 are different from those of carbon matrix. The band at 667 cm-1 corresponds to vibration of chemical bonds of C-S. Bands at 990 cm-1 and 1115 cm-1 are related to symmetric and asymmetric vibration of S-C-S bonds respectively. In comparison with carbon precursorsulfur composite, chemical bonds of C-S apparently exist in carbon matrix-sulfur composite. And the formation of C-S bonds is promoted by deoxidizing carbon precursor with N2 H4 • H2 O. Fig. 7 shows Raman spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur.
In Fig. 7 (a), acetylene black exhibits two Raman bands at Raman shift of 1348 cm-1 and 1590 cm-1 . The Raman bands at about 1348 cm-1 and about 1590 cm-1 are called D band and G band. The D band is related to disordered carbon such as carbon around defects [32]. And the G band corresponds to carbon sites containing sp2 bonds in hexagonal aromatic rings [33]. Intensity ratio between D band and G band indicates the degree of disorder [34]. Lower ratio corresponds to lower degree of disorder and higher degree of graphitization. The D band and the G band of carbon precursor are at 1374 cm-1 and 1594 cm-1 . The D band of carbon precursor is different from that of acetylene black in Raman shift, which
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Fig. 4. FE-SEM images (a, b) and EDS maps (c, d) of carbon matrix-sulfur composite; FE-TEM images (e, f) of carbon matrix-sulfur composite.
may result from different size of graphitic layers. Carbon matrix exhibits D band at 1359 cm-1 and G band at 1579 cm-1 . D band and G band of carbon matrix differ from those of carbon precursor in Raman shift, which may be related to the removal of element of oxygen in carbon precursor. The intensity ratios of ID / IG for carbon
precursor and carbon matrix are both lower than that for acetylene black, which indicates that the degree of disorder in carbon precursor and carbon matrix is lower than that in acetylene black. In Fig. 7 (b), carbon precursor-sulfur composite exhibits D band at 1374 cm-1 and G band at 1586 cm-1 . The G band of carbon
Fig. 5. FE-SEM images of acetylene black (a) and acetylene black-sulfur mixture (b).
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Fig. 6. FT-IR spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur.
Fig. 7. Raman spectra of acetylene black, carbon precursor, carbon matrix, carbon precursor-sulfur and carbon matrix-sulfur.
precursor-sulfur differs from that of carbon precursor in Raman shift, which results from addition of sulfur into carbon precursor. In regard to carbon matrix-sulfur, it exhibits D band at 1370 cm-1 and G band at 1564 cm-1 . D band and G band of carbon matrixsulfur obviously differ from those of carbon matrix in Raman shift. The apparent difference results from chemical bonding between carbon matrix and sulfur, which is consistent with the result from FT-IR spectra in Fig. 6.
Fig. 8 shows EDS pattern of carbon matrix-sulfur composite and TG curves of carbon matrix, carbon matrix-sulfur , acetylene black-sulfur and sulfur. Fig. 8 (a) indicates that carbon matrix-sulfur consists of element of C and element of S. The high intensity of sulfur peak corresponds to high content of sulfur in carbon matrix-sulfur composite. The content of sulfur is obtained through TG analysis. As is shown in Fig. 8 (b), mass of sulfur hardly changes when temperature is below 200 ◦ C. When temperature
Fig. 8. EDS pattern (a) of carbon matrix-sulfur composite and TG curves (b) of carbon matrix, carbon matrix-sulfur, acetylene black-sulfur and sulfur.
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Fig. 9. Cyclic voltammograms of carbon matrix-sulfur electrode.
increases from 200 ◦ C to 300 ◦ C, mass of sulfur decreases gradually, which results from the evaporation of sulfur. In regard to acetylene black-sulfur composite, it exhibits similar change trend of mass rate to that of sulfur, which indicates that the mass change of acetylene black-sulfur results from the evaporation of sulfur. Through calculation, the mass rate of sulfur in acetylene black-sulfur mixture is 63.1%. In comparison with acetylene black-sulfur, mass rate of carbon matrix-sulfur composite decreases slower. Corresponding reason is that the evaporation of sulfur is hindered by carbon coating and C-S bonds in carbon matrix-sulfur composite. In Fig. 8 (b), mass losses of carbon matrix and carbon matrix-sulfur are 7.9% and 60.7% at the turn temperature of mass rate for carbon matrix-sulfur. Through calculation, the content of sulfur in carbon matrix-sulfur composite is 57.3%. Fig. 9 shows cyclic voltammograms of carbon matrix-sulfur electrode at scanning rate of 0.1 mV s-1 . In the curve of each cycle, there are three peaks in the reduction process and one peak in the oxidation process. The first reduction peak at about 2.34 V corresponds to reaction from S8 to Li2 S8 . And the second peak at around 2.05 V is related to reduction reaction from Li2 Sm (2 < m < 8) to Li2 S2 or Li2 S. The third reduction peak at about 2.12 V is related to decrease in length of S-S chain which chemically bonds with carbon matrix. In regard to the oxidation process, the peak at around 2.38 V corresponds to increase in the length of S-S chain. In the curves of four cycles, peaks are at similar potential, which indicates good reversibility of electrochemical reactions for carbon matrix-sulfur electrode.
Fig. 10 shows charge and discharge curves for carbon matrixsulfur electrode and acetylene black-sulfur electrode at current rate of 0.1 C (1 C = 1672 mA g-1 ). In Fig. 10 (a), each discharge curve exhibits three plateaus of potential. The first plateau between 2.3 V and 2.4 V corresponds to the first reduction peak at about 2.34 V in Fig. 9. The second plateau at around 2.1 V corresponds to the second reduction peak at about 2.05 V in Fig. 9. The third plateau between 2.1 V and 2.2 V corresponds to the third reduction peak at around 2.12 V in Fig. 9. And the third plateau is related to the reduction of sulfur species with C-S bonds. The plateau links the first plateau and the second one, which makes the discharge curve gentle. In comparison with the first discharge curve, the following three discharge curves exhibit higher potential for the first plateau, which corresponds to the increase in potential of the first reduction peak in Fig. 9. The increase in discharge potential is related to activation of sulfur in carbon matrix-sulfur electrode. In regard to the charge process, the plateau between 2.15 V and 2.3 V corresponds to the oxidation peak at about 2.38 V in Fig. 9. In Fig. 10 (b), the discharge curves exhibit two plateaus of potential. One plateau between 2.3 V and 2.4 V is similar to the first plateau in Fig. 10 (a). The other plateau at about 2.1 V is similar to the second plateau in Fig. 10 (a). In contrast to carbon matrix-sulfur electrode, acetylene blacksulfur electrode does not exhibit the discharge plateau between 2.1 V and 2.2 V, which results from the difference in the connection condition of sulfur. In regard to the charge process, there is a potential plateau at around 2.4 V. Fig. 11 shows cyclic discharge capacity of carbon matrix-sulfur and acetylene black-sulfur electrodes. As is shown in Fig. 11 (a), acetylene black-sulfur electrode exhibits discharge capacity of 968 mAh g-1 in the first cycle and 334 mAh g-1 in the hundredth cycle. Discharge capacity of acetylene black-sulfur electrode decreases fast when cycle number increases. The reason of degradation is that sulfur exists on the surface of acetylene black particles and is not confined. Corresponding morphology has been described in Fig. 5. Thus, formed polysulfide in acetylene black-sulfur electrode dissolves into electrolyte easily. As more polysulfide dissolves into electrolyte, more products of insulated Li2 S2 and Li2 S precipitate on the surface of sulfur electrode and agglomerate. The higher agglomeration degree for insulated sulfur species leads to lower utilization rate of sulfur and lower discharge capacity. In regard to carbon matrix-sulfur electrode, it exhibits discharge capacity of 1266 mAh g-1 in the first cycle and 886 mAh g-1 in the hundredth cycle. In comparison with acetylene black-sulfur electrode, carbon matrixsulfur electrode exhibits higher discharge capacity and more stable discharge property. The better discharge properties result from the appropriate structure of carbon matrix-sulfur composite. As is described above, sulfur is confined to carbon matrix by carbon
Fig. 10. Charge and discharge curves of carbon matrix-sulfur electrode (a) and acetylene black-sulfur electrode (b).
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Fig. 11. Cyclic discharge capacity of carbon matrix-sulfur and acetylene black-sulfur electrodes at current rate of 0.1 C (a); Discharge capacity of carbon matrix-sulfur electrode at current rate of 0.2 C (b).
Fig. 12. Electrochemical impedance spectra (Nyquist plots) of carbon matrix-sulfur electrode (a) and acetylene black-sulfur electrode (b).
coating and C-S bonds. Thus, polysulfide is hindered from dissolving into electrolyte, which contributes to alleviating the agglomeration degree for insulated sulfur species. Coulombic efficiency of carbon matrix-sulfur electrode is also shown in Fig. 11 (a). The efficiency lies between 90% and 100%, which indicates that self-discharge reaction is not obvious. The suppression of self-discharge reaction results from protection of lithium electrode through LiNO3 . The reaction between lithium electrode and polysulfide leads to low coulombic efficiency. It has been reported that LiNO3 contributes to the formation of protection film on the surface of lithium electrode [35]. The protection film prevents the reaction between lithium and polysulfide, which results in high coulombic efficiency. As is shown in Fig. 11 (b), carbon matrix-sulfur electrode exhibits discharge capacity of 787 mAh g-1 in the hundredth cycle at current rate of 0.2 C. And the cycle stability of discharge is relatively good. In regard to the coulombic efficiency, it lies between 90% and 100%, which indicates that self-discharge reaction is not apparent. Fig. 12 shows electrochemical impedance spectra of carbon matrix-sulfur electrode and acetylene black-sulfur electrode from 100 kHz to 0.1 Hz. Before discharge, each spectrum curve consists of a semicircle in high frequency region and an inclined line in low frequency region. The semicircle corresponds to chargetransfer resistance and longer diameter indicates higher resistance [36,37]. Intercept of the semicircle at the axis of Z (re) is related to combination of resistance for active materials, resistance for electrolyte and contact resistance [38]. The inclined line is related to diffusion of ions. As is shown in Fig. 12 (a), carbon matrixsulfur electrode exhibits lower charge-transfer resistance after 100
cycles than before cycle, which is related to activation of sulfur species in carbon matrix-sulfur electrode. In Fig. 12 (b), acetylene black-sulfur electrode exhibits two semicircles after 100 cycles. The semicircle in higher frequency region corresponds to resistance of solid interface layers between electrodes and electrolyte. And the other semicircle in lower frequency region is related to charge-transfer resistance [38]. In contrast to carbon matrix-sulfur electrode, acetylene black-sulfur electrode exhibits the semicircle in higher frequency region, which is related to agglomeration of insulated sulfur species.
4. Conclusions A new method has been described to prepare carbon matrix for sulfur as cathode material of lithium-sulfur batteries. Prepared carbon matrix is lamellar and has relatively high degree of graphitization. Synthesized carbon matrix-sulfur composite is laminated. On the outside of the lamella, there is a layer of carbon coating with thickness of about 10 nm. In the lamella, there are C-S bonds which form in the process of deoxidizing carbon precursor. The structure increases the utilization rate of sulfur and improves discharge properties. In the carbon matrix-sulfur electrode, carbon coating and C-S bonds confine sulfur to conductive carbon matrix and maintain contact, which hinders the dissolution of polysulfide and alleviates agglomeration degree for insulated sulfur species. The synthesized carbon matrix can also be used to form homogeneous composite with other electrode materials.
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