Selenium [email protected] carbon aerogel composite for rechargeable lithium batteries with good electrochemical performance

Selenium [email protected] carbon aerogel composite for rechargeable lithium batteries with good electrochemical performance

Journal of Power Sources 284 (2015) 95e102 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/loc...

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Journal of Power Sources 284 (2015) 95e102

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Selenium sulfi[email protected] carbon aerogel composite for rechargeable lithium batteries with good electrochemical performance Zhian Zhang*, Shaofeng Jiang, Yanqing Lai, Junming Li, Junxiao Song, Jie Li* School of Metallurgy and Environment, Central South University, Changsha, Hunan, 410083, China

h i g h l i g h t s  The  The  The  The

SeS2 was encapsulated into mesoporous carbon aerogels for lithium batteries. [email protected] composite showed good electrochemical performance. possibility of discharge reaction model of the composite cathode was showed. [email protected] composite would be a promising cathode material.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 January 2015 Received in revised form 27 February 2015 Accepted 4 March 2015 Available online 5 March 2015

Selenium sulfide (SeS2) encapsulated into 3D interconnected mesoporous carbon aerogels (MCA) as a selenium sulfide/carbon composite material was prepared for lithium batteries. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations show the mesoporous structures of the carbon aerogels and the homogeneous distribution of selenium sulfide in the composite. The electrochemical performances of the selenium sulfi[email protected] carbon aerogel ([email protected]) composite cathode was evaluated using cyclic voltammetry, galvanostatic chargeedischarge, and electrochemical impedance spectroscopy. It is found that the [email protected] cathode shows a better electrochemical performance than the pristine SeS2 cathode. The [email protected] composite with selenium sulfide content of 49.3 wt.% displays an initial discharge capacity of 1150 mAh g 1 at 50 mA g 1 and a reversible discharge capacity of 601 mAh g 1 after 10 cycles at 500 mA g 1. The better electrochemical performance benefit from the high electron conductivity and 3D interconnected porous structures of the carbon aerogels, which contribute to dispersing SeS2 and trapping polysulfide and polyselenide intermediates within the skeleton structure of the mesoporous carbon aerogels. © 2015 Elsevier B.V. All rights reserved.

Keywords: Lithium batteries Mesoporous carbon aerogels Selenium sulfi[email protected] carbon aerogel composite Good electrochemical performance

1. Introduction High energy density rechargeable batteries have received great attention in recent years because of their potential applications, such as the power source for electric vehicles, energy storage devices and smart grids [1]. In current technology, the energy density of lithium-ion batteries is mainly limited by the cathode material [2e4]. Therefore, development of high specific capacity cathode for lithium-ion batteries is critical for the success in their applications.

* Corresponding authors. E-mail addresses: [email protected] (Z. Zhang), [email protected] (J. Li). http://dx.doi.org/10.1016/j.jpowsour.2015.03.019 0378-7753/© 2015 Elsevier B.V. All rights reserved.

Sulfur has been considered as the next generation potential cathode for high energy density lithium batteries due to high theoretical specific capacity (1675 mAh g 1) and energy density (2600 Wh kg 1) [5e7]. However, lithiumesulfur (LieS) batteries suffer from the electronically and ionically insulating nature of sulfur and the solubility of reductive polysulfides in organic electrolyte, resulting in poor cycling performance. The most effective method is to employ conductive porous carbon as a host to constrain polysulfide intermediates and enhance the conductivity of the sulfur cathode [8e10]. As a congener of sulfur, selenium also has been considered as a prospective candidate for cathode material in high energy density lithium batteries for specific applications because of its higher electrical conductivity than sulfur and its comparable volumetric capacity (3253 Ah L 1) to sulfur

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Fig. 2. (a) XRD patterns and (b) Raman spectra of pristine SeS2, MCA, [email protected] composite. Fig. 1. (a) N2 adsorptionedesorption isotherms and (b) pore size distributions of MCA and [email protected] composite.

(3467 Ah L 1) [11e14]. Lately, Wang et al. [15] reported that [email protected] carbon composite exhibits excellent capacity retentions. However selenium has a lower theoretical specific capacity (675 mAh g 1) than sulfur. Since selenium possesses high cycling stability, but low reversible capacity, and sulfur has high reversible capacity, but poor cycling stability, it is desirable to develop a cathode material that combines the advantages of S and Se. As a consequence, selenium sulfide (SeS2) has been explored as a promising cathode material for lithium batteries, but only several literature were reported at present [12,16]. Since SeS2 has similar chemical properties with sulfur and selenium, it is believed that the carbon materials used to stabilize sulfur and selenium should be also effective for SeS2. Abouimrane et al. [12] conducted pioneering work on the use of Se2S/CNT composite as a cathode material with a reversible capacity of more than 500 mAh g 1 using ether-based electrolyte after 50 cycles, but they were not investigated for the distribution and the state of selenium sulfide in the Se2S/CNT composite in detail. In addition, it is reported that a carbonized polyacrylonitrile (CPAN) is served as the scaffold for the composites with excellent cycling stability, but active material of SeSx is only 33% in the composites due to the limited pore volume of CPAN [16]. Carbon aerogels (CA), as a novel and special type of carbon material, have been recognized as promising electrode materials due to their three-dimensional (3D)

interconnected structure, high surface area, large pore volume, high electrical conductivity and controllable pore size [17e20]. However, selenium sulfi[email protected] aerogel composite has been rarely reported in rechargeable lithium batteries. Herein, we synthesized selenium sulfi[email protected] carbon aerogel cathode material by a melt-diffusion strategy which selenium sulfide was encapsulated a three-dimensional interconnected mesoporous carbon aerogels (MCA). The selenium sulfi[email protected] carbon aerogel composite with selenium sulfide sulfur content of 49.3 wt.% exhibits an initial discharge capacity of 1150 mAh g 1 at 50 mA g 1 and a reversible discharge capacity of 601 mAh g 1 after 10 cycles at 500 mA g 1, which are better than that of the pristine SeS2 cathode. The better electrochemical performance are attributed to the unique [email protected] electrode architecture facilitating charge transport, and the trapping of polysulfide and polyselenide intermediates within the skeleton structure of the mesoporous carbon aerogels. In addition, [email protected] composite was investigated for the distribution and the state of selenium sulfide in the [email protected] composite by analyzing accurately through N2 adsorption/ desorption, XRD, Raman, FESEM, TEM-EDX, and high resolution transmission electron microscopy (HRTEM). Our results demonstrate that selenium sulfi[email protected] carbon aerogel composite is a promising cathode material for rechargeable lithium batteries.

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Fig. 3. High-magnification SEM images of (a) MCA and (b) [email protected] composite. (c) TGA curves of MCA and [email protected] composite.

2. Experimental

2.3. Electrochemical measurements

2.1. Materials preparation

The composite cathode slurry was made by mixing 80 wt.% [email protected] composite material, 10 wt.% carbon black and 10 wt.% sodium alginate (SA) binder in deionized water solvent. Then, the slurry was spread onto aluminum foil (20 mm), and dried at 60  C under vacuum overnight. The pristine SeS2 cathode containing 40 wt.% SeS2, 50 wt.% carbon black, and 10 wt.% sodium alginate binder was prepared in the same way for comparison. The dried electrodes were punched into round discs with a diameter of 1.0 cm and an active material load of about 1.5e2 mg cm 2. Coin-type (CR2025) cells were assembled in an argon-filled glove box (Universal 2440/750) in which oxygen and water contents were less than 1 ppm. The electrolyte used was 1 M bis(trifluoromethane) sulfonamide lithium salt (LiTFSI, Sigma Aldrich) in a mixed solvent of 1,3-dioxolane (DOL, Acros Organics) and 1,2-dimethoxyethane (DME, Acros Organics) with a volume ratio of 1: 1, including 0.1 M LiNO3 as an electrolyte additive. Lithium metal was used as counter electrode and reference electrode and Celgard 2400 was used as separator. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted using PARSTAT 2273 electrochemical measurement system. CV tests were performed at a scan rate of 0.2 mV s 1 in the voltage range of 0.8e4.0 V. EIS measurements were carried out at open-circuit potential in the frequency range between 100 kHz and 10 mHz with a perturbation amplitude of 5 mV. Galvanostatic charge/discharge tests were performed in the potential range of 0.8e4.0 V at 25  C by using a LAND CT2001A battery-testing instrument. All the electrochemical tests were conducted at room temperature.

A 6:4 weight ratio mixture of mesoporous carbon aerogels (MCA) and SeS2 (Sigma Aldrich) was mixed. Then, the [email protected] mixture was heated at 160  C for 12 h with heating rate of 5  C/ min in a tubular furnace under argon atmosphere. After cooling down to room temperature, the [email protected] composite was obtained. The [email protected] composite was collected and dried in an oven at 50  C for 24 h. The SeS2 content in the composite was determined by thermogravimetric analyzer (TGA, SDTQ600) at a heating rate of 5  C/min from 20 to 800  C with N2.

2.2. Material characterization The morphologies of the samples were investigated by field emission scanning electron microscopy (FESEM, Nova Nano SEM 230) and transmission electron microscopy (TEM, Tecnai G2 20ST). The elements on the surface of sample were identified by energydispersive X-ray spectroscopy (EDS) and scanning transmission electron microscopy (STEM, Tecnai G2 F20)/energy dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD, Rigaku3014) measurements were made with Cu Кa radiation. N2 adsorption/ desorption measurements were performed by using Quantachrome instrument (Quabrasorb SI-3MP) at 77 K. The structure of the samples was tested by Raman spectrometer (Jobin-Yvon LabRAM HR-800, Horiba).

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Fig. 4. TEM images of (a) MCA, (c) [email protected] composite. HRTEM images of (b) MCA and (d) [email protected] composite.

3. Results and discussion Fig. 1 shows the (a) N2 adsorptionedesorption isotherms and (b) pore size distributions of MCA and the [email protected] composite. From Fig. 1a, it is shown that the curve of the MCA exhibits a typical type IV isotherm with a hysteresis loop, indicating the existence of well-developed porous structure. From Fig. 1b, the average pore size in MCA is about 5.1 nm. After SeS2 loading, the

BrunauereEmmetteTeller (BET) surface area and pore volume of MCA decrease markedly, accompanied by a significant reduction in pore size distribution in the mesoporous region (8e12 nm), which suggests the impregnation of SeS2 into the mesoporous channels of MCA. The XRD patterns and Raman spectra of the [email protected] composite are shown in Fig. 2a and b. For selenium sulfide as shown in Fig. 2a, it possesses a crystal structure which are in good accordance

Fig. 5. EDS elemental mapping images of carbon, sulfur and selenium in [email protected] composite.

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Fig. 6. CV curve of the [email protected] composite cathode at 0.2 mV s 1.

with the diffraction peaks of the phase of selenium sulfide [16]. For pure MCA, the presence of the broad (002) and (100) peaks in XRD pattern of MCA sample suggests an amorphous state [19,21]. All of the diffraction peaks of the pristine SeS2 disappear after heating in [email protected] composite, which may be due to the highly dispersed state of SeS2 inside the 3D interconnected mesoporous carbon aerogels. Similar to the XRD pattern, characteristic Raman peaks of SeS2 are not observed in [email protected] composite, and only two broad carbon peaks at 1327 cm 1 and 1591 cm 1 representing the disordered graphite (D band) and crystalline graphite (G band), respectively, appear in the Raman spectrum of [email protected] composite (as shown in Fig. 2b). Both XRD and Raman measurements confirm that SeS2 molecules are constrained in MCA to form an amorphous structure, which is in good agreement with the results of Fig. 1. The high-magnification field emission scanning electron microscopy (FESEM) images of (a) MCA and (b) [email protected] composite are shown in Fig. 3, respectively. From Fig. 3a, it can be seen that the MCA materials are composed of carbon particle interconnecting each other, forming a 3D network architectural structure [19]. Fig. 3b presents the morphology of the [email protected] composite in which SeS2 particles are uniformly distributed. As shown in Fig. 3b, it is obviously indicated that the [email protected] composite still keeps three-dimensional porous structure after SeS2 encapsulated into 3D interconnected mesoporous carbon aerogels. In addition, the SeS2 content in the composite was determined to be 49.3 wt % by thermogravimetric (TG) analysis (Fig. 3c). Furthermore, in order to study the morphological and structural characterization of the MCA and the [email protected] composite, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed at various magnifications. Fig. 4 shows TEM and HRTEM images of MCA (a and b) and [email protected] composite (c and d). From Fig. 4a, it can be observed that the average size of MCA particles (about 20 nm) are connected into a three dimensional conductive network, which is consistent with the FESEM images. The porous structure of MCA has typically mesoporous pore dimensions, which are consistent with the pore size distributions pattern as shown in Fig. 1b. Graphene-like few layers of carbon surrounding the pores of carbon can be seen at HRTEM (Fig. 4b) [22]. Fig. 4c indicates that the [email protected] composite can still maintain a typical interconnected product with a homogeneous distribution for SeS2 and MCA components. Compared with the pure MCA sample (Fig. 4a and b), it is clearly revealed from Fig. 4c and d that some of the SeS2 are

Fig. 7. Galvanostatic dischargeecharge voltage profiles of (a) the pristine SeS2 cathode and (b) [email protected] composite cathode. (c) the first cycle discharge curves of the [email protected] composite cathode at the current density of 50, 100, 250 and 500 mA g 1.

entrapped within porous channels of the MCA matrices, and almost all the mesopores are filled with SeS2 nanoparticles for the [email protected] composite. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping images (Fig. 5) reveal that the carbon elemental mapping image overlaps with selenium and sulfur mapping images, demonstrating the uniform distribution of SeS2 in

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Fig. 9. Rate capability of the [email protected] composite and the pristine SeS2 cathodes as the current rate increased from 250 mA g 1 to 2000 mA g 1.

Fig. 8. (a) Cycling performances of the pristine SeS2 cathode and [email protected] composite cathode. (b) EIS of the pristine SeS2 and [email protected] composite cathodes before the first discharge.

the skeleton structure of the mesoporous carbon aerogels. In order to understand the reduction/oxidation reactions in LieSeS2 batteries, the CV for the [email protected] composite cathode was recorded at a scan rate of 0.2 mV/s in the voltage range of 4.0e0.8 V as shown in Fig. 6. The CV curve shows four reduction peaks and two oxidation peaks, which are consistent with the result of Abouimrane reported [11,12]. The first reduction peak at around 2.35 V corresponds to the conversion of high-order polysulfides (due to the introduction of sulfur) to low-order polysulfides. As the reduction proceeds, low-order polysulfides converts to Li2S2 and Li2S, which should be responsible for the third reduction peak at around 2.04 V. The second reduction peak at around 2.18 V and fourth reduction peak at around 1.80 V can be attributed to the conversion of Se to polyselenides and then to Li2Se, respectively. Correspondingly, the first oxidation peak at around 2.46 V and the second oxidation peak at around 2.38 V during the charge sweep are attributed to the oxidation of Li2S and Li2Se, respectively [12]. These results are different from the previous report by Wang, due to different reaction mechanisms in different electrolytes [16]. In order to gain further insight on the electrochemical performance of the cells, galvanostatic discharge-charge test was carried out at the current density of 500 mA g 1 between 0.8 V and 4.0 V. All capacities in this study were calculated based on SeS2 mass. Galvanostatic discharge-charge voltage profiles of the cells with the

pristine SeS2 cathode and the [email protected] composite cathode are shown in Fig. 7a and b, respectively. Consistent with the result of cyclic voltammetry measurement, four discharge plateaus are exhibited in discharge curves for both of the LieSeS2 batteries [12,16]. After the 10th cycle, the pristine SeS2 cathode discharge capacity is 396 mAh g 1, showing a poor cycle performance. In contrast, the [email protected] composite cathode discharge capacity is 601 mAh g 1 after the 10th cycle having a retention rate of 71%. The better electrochemical performance are attributed to the unique [email protected] electrode architecture facilitating charge transport, and the trapping of polysulfide and polyselenide intermediates within the skeleton structure of the mesoporous carbon aerogels. It is found that the cell with the [email protected] composite cathode shows complete and stable plateaus, indicating the excellent electrochemical stability of the SeS[email protected] composite cathode [23,24]. Due to the better electrochemical performance, the first cycle discharge curves of the [email protected] composite cathode at the current density of 50, 100, 250 and 500 mA g 1 were studied as shown in Fig. 7c. All of the four curves show the typical four-plateau behavior of a SeS2 cathode, which is consistent with the results of CV (Fig. 6) and the galvanostatic discharge-charge test (Fig. 7a and b). At current density of 50 mA g 1, the first cycle discharge capacity is 1150 mAh g 1, which is considerably greater than the theoretical specific capacity of LieSe batteries (675 mAh g 1). The first cycle discharge capacity decrease to 1129, 1012 and 846 mA g 1 when the current density increased to 100, 250 and 500 mA g 1, respectively. With the increase of the discharge current density from 50 to 500 mA g 1, the first discharge capacity slightly fades, and the discharge plateaus gradually drop, but the typical four plateaus in the discharge curves still maintain during all the cycles even at the very high current rates (500 mA g 1), suggesting little kinetic barrier in the electrode process and good rate capability. Cycling performances of the pristine SeS2 and [email protected] composite cathodes are presented in Fig. 8a. The initial discharge capacity of the pristine SeS2 cathode is 816 mAh g 1 at a current density of 500 mA g 1, which is similar to the report of Wang et al. [16]. For the [email protected] composite cathode, it displays a higher utilization of active materials than the pristine SeS2 cathode with an initial discharge capacity of 846 mAh g 1. After 130 cycles, the [email protected] composite cathode retains a reversible capacity of 308 mA h g 1, almost six times than the pristine SeS2, which can be attributed to the good electrical of MCA and the good dispersion of

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Fig. 10. Scheme of the heating-melt process and the possibility of discharge reaction model of the [email protected] composite cathode.

SeS2 in the porous channels of MCA. In addition, electrochemical impedance spectra (Fig. 8b) of the pristine SeS2 cathode and the [email protected] composite cathode indicate that the resistant of [email protected] cathode is much smaller than pristine SeS2 electrode, implying that the conductivity of the [email protected] composite cathode has been enhanced. Before the first discharge, the semicircle corresponds to the sum resistance of surface layers, like the interphase electronic contact resistance on the cathode and the interface resistance of the lithium anode [25]. The rate capability of the [email protected] composite and the pristine SeS2 cathodes as the current rate increased from 250 mA g 1 to 2000 mA g 1 were presented in Fig. 9. The reversible capacity of the [email protected] composite and the pristine SeS2 cathodes remain about 1074 mA h g 1 and 961 mA h g 1 at 250 mA g 1 in the first cycle discharge. After 5 cycles at a current density of 500 mA g 1, the [email protected] composite and the pristine SeS2 cathodes show the reversible capacity 731 mAh g 1 and 395 mAh g 1, respectively. Even after 15 cycles at a current density of 2000 mA g 1, the [email protected] composite cathode shows good rate reversible capacity 371 mAh g 1, almost four times than the pristine SeS2 cathode. Then, after 20 cycles, as the current density returns to 250 mA g 1, the reversible specific capacity of [email protected] composite cathode can even remains at values of 512 mAh g 1, while the pristine SeS2 cathode shows a poor reversible specific capacity 121 mAh g 1. The better rate capability of the [email protected] composite cathode are attributed to the unique [email protected] composite cathode architecture facilitating charge transport, and the trapping of polysulfide and polyselenides within the skeleton structure of the mesoporous carbon aerogels. To better understand the synthetic process of [email protected] composite and the reduction/oxidation reaction in LieSeS2 batteries, the scheme of the heating-melt process and the possibility of discharge reaction model of the [email protected] composite cathode are shown in Fig. 10. The three dimensionally networked MCA matrices in the [email protected] composite serves as a useful electronic framework for SeS2 homogenous dispersion. In heating-melt process, SeS2 nanoparticles occupy the sites of pores; this directly leads to a rapid decrease of the number of the micropores, which is consistent with the results of Fig. 1. In the discharge process, it may include four steps reduction reactions in LieSeS2 batteries which are consistent with the results of CV and galvanostatic dischargecharge test. The first (discharge to 2.29 V) and third (discharge to 2.03 V) steps can be attributed to the conversion of high-order

polysulfides (due to the introduction of sulfur) to low-order polysulfides and then to Li2S2 and Li2S. The second (discharge to 2.14 V) and fourth (discharge to 1.85 V) steps can be assigned to the conversion Se to polyselenides and then to Li2Se. 4. Conclusion We synthesized [email protected] composite with high dispersed SeS2 inside the porous channel of MCA matrix by a melt-diffusion strategy. The selenium sulfi[email protected] carbon aerogel composite with sulfur content of 49.3 wt.% exhibits an initial discharge capacity of 1150 mAh g 1 at current density of 50 mA g 1 and a reversible discharge capacity of 601 mAh g 1 after 10 cycles at current density of 500 mA g 1, which are higher than that of the pristine SeS2 cathode. The better electrochemical performance are attributed to the unique [email protected] electrode architecture facilitating charge transport, and the trapping of polysulfide and polyselenide intermediates within the skeleton structure of the mesoporous carbon aerogels. Our results demonstrate that selenium sulfi[email protected] carbon aerogel composite is a promising cathode material for rechargeable lithium batteries. Acknowledgments The authors acknowledge the financial support of the Teacher Research Fund of Central South University (2013JSJJ027) and the Strategic Emerging Industries Program of Shenzhen, China (JCYJ20140509142357195) and the Fundamental Research Funds for the Central Universities of Central South University (No. 2014zzts180) and Undergraduates Free Exploration Program of Central South University (No. 2282014bks211). References [1] [2] [3] [4] [5] [6] [7] [8]

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